Mechanisms of Copper Corrosion in Aqueous Environments
Mechanisms of Copper Corrosion in Aqueous Environments
Mechanisms of Copper Corrosion in Aqueous Environments
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<strong>Mechanisms</strong> <strong>of</strong> <strong>Copper</strong> <strong>Corrosion</strong><strong>in</strong> <strong>Aqueous</strong> <strong>Environments</strong>The Swedish National Councilfor Nuclear Waste ReportReport 2009:4
Scientific workshop on<strong>Mechanisms</strong> <strong>of</strong> <strong>Copper</strong> <strong>Corrosion</strong> <strong>in</strong><strong>Aqueous</strong> <strong>Environments</strong>November 16, 2009, Conference Center <strong>of</strong> the Confederation <strong>of</strong>Swedish Enterprise (När<strong>in</strong>gslivets hus), Stockholm.IntroductionIn 2010 the Swedish Nuclear Fuel and Waste Management Company,SKB, plans to submit an application for a f<strong>in</strong>al repository forspent nuclear fuel. The proposed method is called KBS and <strong>in</strong>volvesdeposit<strong>in</strong>g the nuclear waste deep down <strong>in</strong> the bedrock <strong>in</strong>the municipality <strong>of</strong> Östhammar on the Swedish east coast. It isbased on three protective barriers: copper canisters encapsulat<strong>in</strong>gthe spent nuclear fuel, bentonite clay surround<strong>in</strong>g the copper canisters,and f<strong>in</strong>ally the rock itself.The geological environment surround<strong>in</strong>g the copper canisterswill be nearly oxygen-free, and one <strong>of</strong> the premises on which theKBS-3 rests is the assumption that copper cannot corrode <strong>in</strong> suchan environment. The scientific f<strong>in</strong>d<strong>in</strong>gs <strong>of</strong> a small group <strong>of</strong> researchersat KTH (the Royal Institute <strong>of</strong> Technology) <strong>in</strong>Stockholm have therefore been met with widespread <strong>in</strong>terest anddebate. Their f<strong>in</strong>d<strong>in</strong>gs are <strong>in</strong> short that copper actually can corrode<strong>in</strong> pure water, free from oxygen as well as from complex<strong>in</strong>g ions.S<strong>in</strong>ce the release <strong>of</strong> the KTH f<strong>in</strong>d<strong>in</strong>gs, the long-term safety <strong>of</strong>the KBS-3 method has been questioned. SKB has, however, refutedthe f<strong>in</strong>d<strong>in</strong>gs, and the matters were thoroughly discussed at a scientificworkshop <strong>in</strong> Stockholm on November 16, 2009. The workshopwas organized around two ma<strong>in</strong> themes:3
Introduction1. A fundamental enquiry <strong>in</strong>to the corrosion characteristics <strong>of</strong>copper <strong>in</strong> oxygen-free environments.2. What additional <strong>in</strong>formation is needed to confirm this specificcorrosion process and to assess the importance <strong>of</strong> the processfor the f<strong>in</strong>al repository?The workshop was hosted by the Swedish National Council forNuclear Waste, with a panel <strong>of</strong> <strong>in</strong>ternationally recognized expertsfrom the fields <strong>of</strong> chemistry and materials science and eng<strong>in</strong>eer<strong>in</strong>g,chosen by SKB, KTH, the Swedish Radiation Safety Authority, andthe Swedish National Council for Nuclear Waste.The purpose <strong>of</strong> this report is to expla<strong>in</strong> the controversy and presentthe outcome <strong>of</strong> the workshop. The report is primarily aimed atexperts <strong>in</strong> the field <strong>of</strong> nuclear waste management, such as <strong>of</strong>ficialsat regulatory authorities and people <strong>in</strong> the nuclear <strong>in</strong>dustry and scientificcouncils.We would also like to refer to the newsletter published on thewebsite <strong>of</strong> the Swedish National Council for Nuclear Waste(www.karnavfallsradet.se). We would also like to refer to the publicationNuclear Waste State <strong>of</strong> the Art Report 2010 – challenges for thef<strong>in</strong>al repository programme (SOU 2010:6e).For those readers who would like a more detailed scientific discussion<strong>of</strong> the various topics raised <strong>in</strong> this report, we would like torefer to the bibliography at the end <strong>of</strong> the report.The report is structured <strong>in</strong> the follow<strong>in</strong>g way:• Executive summary: A general overview <strong>of</strong> the issue, <strong>in</strong>tendedfor anyone <strong>in</strong>terested <strong>in</strong> the management <strong>of</strong> nuclear waste disposal.• Topic 1: A fundamental understand<strong>in</strong>g <strong>of</strong> the corrosion characteristics<strong>of</strong> copper <strong>in</strong> oxygen-free environments – a scientificguide to the controversy <strong>of</strong> copper corrosion <strong>in</strong> oxygen freeenvironments.• Topic 2: What additional <strong>in</strong>formation is needed to confirm thisspecific corrosion process and to assess the importance <strong>of</strong> theprocess for the f<strong>in</strong>al repository? – a summary <strong>of</strong> recommendationsfrom the members <strong>of</strong> the expert panel at the scientificworkshop on November 16, 2009.4
Introduction• Written statements by the panel members: A publication <strong>of</strong> thewritten statements prepared by the panel members after the scientificworkshop.• Edited Transcripts (# sv. redigerade utskrifter) from the scientificworkshop.The members <strong>of</strong> the expert panel:Dr. Ron LatanisionDr. Latanision is Corporate Vice Presidentand Director <strong>of</strong> Exponent’sMechanics and Materials Practice. He isa member <strong>of</strong> the National Academy <strong>of</strong>Eng<strong>in</strong>eer<strong>in</strong>g and an elected Fellow <strong>of</strong>ASM International, NACE International,and the American Academy <strong>of</strong>Arts and Sciences. Dr. Latanision is amember <strong>of</strong> the International <strong>Corrosion</strong>Council and serves as a Co-Editor-<strong>in</strong>-Chief <strong>of</strong> <strong>Corrosion</strong> Reviews.Dr. Gaik Khuan ChuahDr. Chuah earned her PhD <strong>in</strong> Chemistryfrom Texas A&M University,USA. She holds a position at theNational University <strong>of</strong> S<strong>in</strong>gapore whereshe teaches and supervises students <strong>in</strong>the area <strong>of</strong> heterogeneous catalysis. Dr.Chuah has various responsibilities as thechairperson for safety <strong>in</strong> the ChemistryDepartment at NUS, and as a member<strong>of</strong> the Faculty <strong>of</strong> Science Safety.5
IntroductionPr<strong>of</strong>. Digby D. MacdonaldDr. MacDonald earned his PhD <strong>in</strong> Chemistryfrom the University <strong>of</strong> Calgary <strong>in</strong> Canada. He isDist<strong>in</strong>guished Pr<strong>of</strong>essor <strong>of</strong> Materials Science andEng<strong>in</strong>eer<strong>in</strong>g at Pennsylvania State University.Pr<strong>of</strong>essor MacDonald is an elected fellow <strong>of</strong>NACE International, the Electrochemical Society,the Royal Society <strong>of</strong> Canada, the Royal Society<strong>of</strong> New Zealand, ASM International, theWorld Innovation Foundation, the Institute <strong>of</strong><strong>Corrosion</strong>, and the International Society <strong>of</strong>Electrochemistry.Pr<strong>of</strong>. David ShoesmithDr. Shoesmith is a Pr<strong>of</strong>essor <strong>in</strong> the Department<strong>of</strong> Chemistry at the University <strong>of</strong> WesternOntario. He specializes <strong>in</strong> research on the electrochemistry<strong>of</strong> materials and corrosion science.Pr<strong>of</strong>essor Shoesmith holds the Natural Sciencesand Eng<strong>in</strong>eer<strong>in</strong>g Research Council and OntarioPower Generation Industrial Research Chair <strong>in</strong>Nuclear fuel Disposal Chemistry. He is anelected fellow <strong>of</strong> NACE International and theCanadian Society for Chemistry.Moderator:Pr<strong>of</strong>. David J. DuquetteDr. Duquette is the John Tod Horton Pr<strong>of</strong>essor<strong>of</strong> Materials Science and Eng<strong>in</strong>eer<strong>in</strong>g at RensselaerPolytechnic Institute <strong>in</strong> Troy, New York. Hisresearch <strong>in</strong>terests <strong>in</strong>clude all aspects <strong>of</strong> corrosionwith a special emphasis on localized corrosion andchemo-mechanical <strong>in</strong>teractions such as stress corrosioncrack<strong>in</strong>g, corrosion fatigue, and hydrogen<strong>in</strong>ducedcrack<strong>in</strong>g. Pr<strong>of</strong>essor Duquette is anelected Fellow <strong>of</strong> NACE International, ASMInternational and the Electrochemical Society.6
Executive SummaryThe scientific controversyIn the Swedish high-level waste management programme, it hasbeen assumed that copper metal cannot be corroded by pure oxygen-freewater (i.e. water without any complex<strong>in</strong>g ions such as sulphidesand chlorides). This assumption has, however, been challengedby experimental results published by Hultqvist andSzakalos, researchers at KTH (the Royal Institute <strong>of</strong> Technology).They also challenge the assumption <strong>of</strong> thermodynamic immunity<strong>of</strong> copper <strong>in</strong> water at elevated temperatures.Hultqvist and Szakalos refer to both experimental and theoreticalobservations and argue that these observations are consistentwith their theory that pure oxygen-free water corrodes copper. Inthe process, hydrogen ions are reduced to hydrogen atoms and acorrosion product is formed whose identity and exact compositionare not yet known. Accord<strong>in</strong>g to the research team, the hydrogenatoms will either form hydrogen gas molecules or be absorbed byand diffuse <strong>in</strong>to the copper metal.The researchers also concluded that when the partial pressure <strong>of</strong>hydrogen reaches a level <strong>of</strong> 1 mbar the copper corrosion ceased.The corrosion process cont<strong>in</strong>uous only if hydrogen is removedfrom the system so that the hydrogen pressure is reduced underthis critical level.SKB do not f<strong>in</strong>d the scientific evidence for the proposed reactionmechanism conv<strong>in</strong>c<strong>in</strong>g concludes that no conv<strong>in</strong>c<strong>in</strong>g evidenceexists that water oxidizes copper. They will nevertheless <strong>in</strong>cludethe effect <strong>of</strong> such a corrosion mechanism <strong>in</strong> the safety assessment,even though their op<strong>in</strong>ion is that the actual corrosion mechanismwill not limit the lifetime <strong>of</strong> the canisters <strong>in</strong> the f<strong>in</strong>al repository.But what is the basis <strong>of</strong> this controversy? One important elementis oppos<strong>in</strong>g views with respect to the <strong>in</strong>terpretation <strong>of</strong> ther-7
Executive Summarymodynamic data. Hultqvist, Szakalos and others claim that thermodynamicdata prove that copper is not immune <strong>in</strong> pure oxygenfreewater. Hence, their result is actually thermodynamicallyexpected, although at a slow reaction rate. One weakness <strong>in</strong> thework <strong>of</strong> Hultqvist and Szakalos is, however, that the corrosionproduct <strong>of</strong> their proposed reaction has not been identified. SKBargue that the results <strong>of</strong> Hultqvist and Szakalos challenge somebasic pr<strong>in</strong>ciples <strong>of</strong> thermodynamics which constitute the scientificfoundation <strong>of</strong> both physics and chemistry. They assert that onesuch pr<strong>in</strong>ciple is the change <strong>in</strong> Gibbs free energy (G), which predictswhether a reaction is probable or not.As far as a specific chemical reaction is concerned (e.g. coppercorrosion <strong>in</strong> water), its feasibility is determ<strong>in</strong>ed by the change <strong>in</strong>Gibbs energy (ΔG). A spontaneous reaction is characterized by adecrease <strong>in</strong> Gibbs energy (ΔG
Executive Summaryand the extent <strong>of</strong> copper corrosion would still be determ<strong>in</strong>ed bythe amount <strong>of</strong> sulphide and chloride ions.Different types <strong>of</strong> experimentsThere are different types <strong>of</strong> studies that can be used to explore thedifferent aspects <strong>of</strong> corrosion: laboratory studies, <strong>in</strong>-situ experimentsand analogues (i.e., natural or man-made artefacts). The <strong>in</strong>itialstate <strong>of</strong> a laboratory experiment is well-known, and control <strong>of</strong>the environment <strong>in</strong> the experiments is very good. On the negativeside, their representativeness is less, due to the simplified systemand the short time scale. It is good to get quick results, but it isharder to evaluate the long-term effects. In-situ experiments are<strong>in</strong>vestigations <strong>in</strong> realistic environments, and therefore closer toreality than laboratory results (the real system is represented <strong>in</strong> the<strong>in</strong>-situ experiment). In this case the <strong>in</strong>itial state is quite well-knownand representativeness is rather good. It is possible to performboth short-term and medium-term experiments. In the case <strong>of</strong> theanalogues, the <strong>in</strong>itial state is unknown (e.g., the sample thickness isnot known ????) and the environment cannot be controlledbecause the reaction has already happened when you look at it.However, the analogue entails a long reaction time, some parameterscan be measured and representativeness can range from poorto good, depend<strong>in</strong>g on what aspect is be<strong>in</strong>g considered. Allexperiments have weaknesses. In weight loss experiments it is possibleto determ<strong>in</strong>e how much has disappeared after a certa<strong>in</strong> time,but it is not possible to dist<strong>in</strong>guish between what happened <strong>in</strong> the<strong>in</strong>itial phase, which might have occurred quickly, and whatoccurred over a longer period <strong>of</strong> time. Another weakness is associatedwith measur<strong>in</strong>g corrosion depths and corrosion rates <strong>in</strong>specimens, s<strong>in</strong>ce it is not possible to differentiate between differentmechanisms that may have operated over different lengths <strong>of</strong> time.This makes it difficult to extrapolate experimental results for use <strong>in</strong>the safety assessment.11
Executive SummaryThe experimental evidence <strong>of</strong> Hultqvist and SzakalosIn one <strong>of</strong> the experiments performed by Hultqvist, two glass vesselswere used <strong>in</strong> which copper foils were kept <strong>in</strong> pure oxygen freewater for 15 years. One glass vessel had a membrane <strong>of</strong> plat<strong>in</strong>um sothat H 2 was not evacuated. In the other glass vessel, H 2 wasremoved through a membrane <strong>of</strong> palladium. The latter vessel showssigns <strong>of</strong> corrosion: the foils turn black <strong>in</strong> colour. Hultquist arguesthat if H 2 is removed, which is always the case <strong>in</strong> an open system,this must be expected to happen. Hultqvist and Szakalos argue thatthe atomic hydrogen that is formed also can be absorbed <strong>in</strong>to thecopper metal. Hultqvist et al. have published pro<strong>of</strong>s us<strong>in</strong>g twomethods. Hydrogen was detected both by secondary ion massspectrometry and by a quantitative study on out gass<strong>in</strong>g <strong>in</strong> vacuum.Secondary ion mass spectrometry is sensitive to hydrogen.It is concluded by Hultqvist and Szakalos that the process <strong>of</strong>copper corrosion <strong>in</strong> water has been verified by experimental results,such as the formation <strong>of</strong> hydrogen, <strong>in</strong>crease <strong>of</strong> weight, hydrogen <strong>in</strong>the copper metal, chemical analysis <strong>of</strong> the corrosion product, aswell as by visual <strong>in</strong>spection and metallographic exam<strong>in</strong>ation.SKB’s corrosion studiesThe purpose <strong>of</strong> SKB’s studies <strong>of</strong> copper corrosion is to understandcopper behaviour <strong>in</strong> water <strong>in</strong> greater detail. SKB’s work on coppercorrosion <strong>in</strong>cludes literature reviews and various k<strong>in</strong>ds <strong>of</strong> experiments,both short-term and more long-term studies (gas measurements,some simpler glass conta<strong>in</strong>er experiments as well as electrochemistry).Moreover, theoretical calculations are performed andthey look <strong>in</strong>to equilibrium reactions <strong>in</strong> water.SKB have found that the corrosion rate decreases with time <strong>in</strong>the short-term electrochemical and laboratory experiments. Veryfew, if any, results <strong>in</strong>dicate that the corrosion rate <strong>in</strong>creases withtime. In the <strong>in</strong>-situ experiments, copper(II) corrosion products are<strong>of</strong>ten found, <strong>in</strong>dicat<strong>in</strong>g that the copper <strong>in</strong> the experiment hasundergone periods <strong>of</strong> oxidiz<strong>in</strong>g conditions. The analogue experimentsshow that copper <strong>in</strong> its native form has rema<strong>in</strong>ed stable for avery long time <strong>in</strong> both the natural state and eng<strong>in</strong>eered artefacts.One <strong>of</strong> SKB’s ongo<strong>in</strong>g studies <strong>of</strong> copper corrosion <strong>in</strong>cludesfirst-pr<strong>in</strong>ciple computer calculations <strong>of</strong> the thermodynamic prop-12
Executive Summaryerties <strong>of</strong> Cu-O-H phases. The ma<strong>in</strong> objective <strong>of</strong> the calculationshas been to look for a stable product (or phase) between copper,oxygen and hydrogen that could be the f<strong>in</strong>al product <strong>of</strong> thesupposed reaction between copper and water that is suggested byHultqvist and Szakalos. Other objectives are (i) to calculate, fromfirst pr<strong>in</strong>ciples, the thermodynamic properties <strong>of</strong> knownCu(I)compounds with oxygen and hydrogen and (ii) to analyzethe thermodynamic stability <strong>of</strong> copper and its compounds <strong>in</strong> oxygen-freewater environment.The compounds <strong>of</strong> copper(I) with oxygen and hydrogen arecopper oxide (also known as cuprite Cu 2 O) and copper hydride,respectively. Cuprite is a stable and well known substance withregard to both its chemical and electrical properties. <strong>Copper</strong>hydride, on the other hand, is a less studied phase. It does exist, butit is very unstable and loses hydrogen quickly with time. In thestudy, computer calculations were performed <strong>in</strong> an attempt toreproduce the experimental <strong>in</strong>formation on cuprite and copperhydride. They show that the reaction with copper and oxygen isenergetically favourable and that the obta<strong>in</strong>ed thermodynamicproperties <strong>of</strong> cuprite are <strong>in</strong> quite good agreement with exist<strong>in</strong>gexperimental data. More or less good agreement with experimentwas seen also for the hydride, although the reaction is shown to beunfavourable thermodynamically. Hence, it is known that the copperoxide is stable but the hydride is unstable.The next part <strong>of</strong> the study was to search for other possible stableCu-O-H phases. The study showed that copper oxyhydride isnot a stable configuration. It also showed that copper hydroxide isa quite unstable species and its formation energy (ΔG) is experimentallyknown to be positive. If the copper hydroxide is condensed<strong>in</strong>to a solid phase, it was found that the most stable structurewas a comb<strong>in</strong>ation <strong>of</strong> the structure <strong>of</strong> cuprite and the structure<strong>of</strong> ice, “cuprice”. The hydroxide has a reasonable electronic spectrumcompared with the spectra <strong>of</strong> cuprite and copper hydride.However, if the stability <strong>of</strong> copper hydroxide is compared withthat <strong>of</strong> cuprite and water, it is found to be unstable. Thermodynamicallyit should decompose <strong>in</strong>to cuprite and water. Hence,accord<strong>in</strong>g to SKB’s study cuprite is still the most stable <strong>of</strong> thesecompounds. <strong>Copper</strong> hydroxide may exist as a meta-stable phase,but its thermodynamic properties <strong>in</strong>dicate that it is unstable comparedwith cuprite and water.13
Executive SummaryThe use <strong>of</strong> archaeological analoguesOne argument that is used by Hultqvist et al. to support their ideathat water can corrode copper are the copper co<strong>in</strong>s from the warshipWasa. The co<strong>in</strong>s have been exposed to water for over 330 yearsand have been reduced <strong>in</strong> size. Hultqvist argues that this is due tothe fact that copper is corroded by the water itself and not by sulphide,s<strong>in</strong>ce copper sulphide has extremely low solubility.Hultqvist’s <strong>in</strong>terpretation has, however, been criticized by SKB andothers, who claim it is the presence <strong>of</strong> sulphide that has caused thecorrosion <strong>of</strong> the copper co<strong>in</strong>s.SKB also use archaeological analogues <strong>in</strong> their argumentation.They claim that the bronze cannons from the warship Kronan,which sank <strong>in</strong> 1678 and were raised <strong>in</strong> 1986, are good objects tostudy, because the environment – the sediment <strong>of</strong> the Baltic Sea,considered to be oxygen-free and with brackish water – is “similarto what the copper canisters will be exposed to” <strong>in</strong> Swedishrepositories. This analogue is, however, criticized by Szakalos s<strong>in</strong>cethe corrosion <strong>of</strong> bronze differs fundamentally from that <strong>of</strong> copper.Szakalos argues that because the formation <strong>of</strong> passivat<strong>in</strong>g t<strong>in</strong> onthe bronze surface greatly reduces the corrosion rate <strong>in</strong> aqueousenvironments, the corrosion rate on these cannons is around 1,000times slower than can be expected with pure copper.What additional <strong>in</strong>formation is needed to confirm thisspecific corrosion process and to assess the importance<strong>of</strong> the process for the f<strong>in</strong>al repository?Szakálos argues that the situation at the planned repository <strong>in</strong>Forsmark is complex and threaten<strong>in</strong>g from both corrosion andembrittlement po<strong>in</strong>ts <strong>of</strong> view. The copper canisters will <strong>in</strong>itially beexposed to atmospheric corrosion until the oxygen is consumed.Then there is corrosion by water, sulphide, salt, stress corrosioncrack<strong>in</strong>g, <strong>in</strong>tergranular corrosion and evaporation-<strong>in</strong>duced corrosion.Szakálos means that before the KBS-3 concept can beaccepted, it needs to be tested <strong>in</strong> realistic conditions. He cites anSKI report from 1996: “<strong>Copper</strong> <strong>of</strong> identical composition as thefuture canisters should be placed <strong>in</strong> a future site environment, withartificial heat<strong>in</strong>g at about 80 degrees, with bentonite, etc. Such anexperiment could be monitored for several decades. ” He concludes14
Executive Summaryby say<strong>in</strong>g that the problem with copper is that it reacts slowly witheveryth<strong>in</strong>g.The experts <strong>in</strong> the panel po<strong>in</strong>t to the necessity <strong>of</strong> furtherresearch on this topic to be able to assess whether the results <strong>of</strong>Hultqvist and Szakálos are realistic or not. Both Pr<strong>of</strong>essorLatanision and Dr. Chuah argued that hydrogen may be producedby corrosion <strong>of</strong> copper <strong>in</strong> oxygen-free water, but that it is essentialto know that the corrosion product is thermodynamically stableand that it can be identified and characterized. Latanision refers tothe fact that there are a number <strong>of</strong> sophisticated surface analyticaltechniques that should be used to demonstrate that the proposedreaction products are <strong>in</strong>deed formed.Pr<strong>of</strong>essor Macdonald emphasizes that the k<strong>in</strong>etics must also beexam<strong>in</strong>ed, i.e. both the corrosion mechanism and how fast thereaction occurs. He also stresses the importance <strong>of</strong> know<strong>in</strong>g thatthe water <strong>in</strong> the experiment is pure, s<strong>in</strong>ce even very small amounts<strong>of</strong> monovalent copper ions and dissolved hydrogen gas <strong>in</strong> the waterare <strong>of</strong> great importance. Considerable care must be exercised, whendesign<strong>in</strong>g experiments aimed at demonstrat<strong>in</strong>g copper corrosion toensure that corrosion is spontaneous upon <strong>in</strong>itiation <strong>of</strong> the experiment.Dr. Chuah suggests that more experiments should be carriedout with the aim <strong>of</strong> (i) confirm<strong>in</strong>g or disprov<strong>in</strong>g the formation <strong>of</strong>hydrogen through direct detection by mass spectrometry;(ii) study<strong>in</strong>g the experimental conditions <strong>of</strong> Hultquist andSzakálos under which hydrogen is formed; (iii) exam<strong>in</strong><strong>in</strong>g thereaction products formed on copper us<strong>in</strong>g <strong>in</strong>-situ methods to avoidany phase transformation on exposure to atmospheric conditions;(iv) measur<strong>in</strong>g the strength <strong>of</strong> the exposed copper; (v) quantify<strong>in</strong>gthe thickness <strong>of</strong> the corrosion layers as a function <strong>of</strong> time; andother relevant tests.Latanision suggests that well-controlled experiments (the reactantsCu and H 2 O must be completely specified and controlled)that are def<strong>in</strong>itive should be carried out at a third-party laboratory.He also suggests that the susceptibility <strong>of</strong> copper to embrittlementcaused by absorbed hydrogen should be exam<strong>in</strong>ed on copper tensilespecimens that are electrolytically charged with hydrogen atcathodic current densities that correspond to the corrosion ratesassociated with the measurements reported by the KTH team. Thisresearch should also be performed by an objective third-party<strong>in</strong>stitution. Further, copper is an obvious material to consider for15
Executive Summarydisposal <strong>in</strong> chemically reduc<strong>in</strong>g environments. If copper corrosion<strong>in</strong> anoxic environments is observed and confirmed as described <strong>in</strong>the research, the conditions which have led to corrosion need to beclarified and then either controlled or eng<strong>in</strong>eered out <strong>of</strong> therepository environment.Macdonald argues for more research on the common contam<strong>in</strong>antbisulphide ions, HS - , <strong>in</strong> groundwater and the corrosion that itcauses. He says that a much more detailed analysis is warranted t<strong>of</strong>ully def<strong>in</strong>e the conditions under which immunity <strong>of</strong> copper mightbe expected to exist. He also recommends what such an analysisshould <strong>in</strong>clude.Shoesmith po<strong>in</strong>ts to the fact that some <strong>of</strong> the references citedby Szakálos do not stand up to scrut<strong>in</strong>y, and that Hultqvist andSzakálos have made an <strong>in</strong>complete analysis <strong>of</strong> the available literatureon corrosion <strong>of</strong> copper cool<strong>in</strong>g systems. He states that areview <strong>of</strong> the literature provided for the workshop, the presentationsmade, and a personal search <strong>of</strong> additional literature <strong>in</strong>dicatethat there is no evidence that significant corrosion <strong>of</strong> Cu can besusta<strong>in</strong>ed by water reduction.The workshop resulted <strong>in</strong> the follow<strong>in</strong>g conclusionsIt can be concluded that <strong>in</strong> theory copper may be corroded by pureoxygen-free water with respect to the follow<strong>in</strong>g reaction:2 Cu(s) + H 2 O ↔ Cu 2 O + H 2 .Hydrogen atoms or molecules must be a reaction product, <strong>in</strong> additionto the possible formation <strong>of</strong> copper hydroxide or copper oxidespecies, because only protons can possibly accept electrons fromcopper atoms. Hence, the key issue is how far to the right thereaction above can proceed and the partial pressure <strong>of</strong> hydrogenthat is obta<strong>in</strong>ed at equilibrium. Accord<strong>in</strong>g to Hultqvist et al. thehydrogen pressure is 1 mbar, which is much higher than the naturalpartial pressure <strong>in</strong> air. Then the corrosion reaction will proceeduntil the equilibrium pressure is atta<strong>in</strong>ed, and if the hydrogen iscont<strong>in</strong>uously removed the corrosion may be extensive. However,thermodynamic calculations result <strong>in</strong> an extremely low hydrogenpressure at equilibrium, which is the theoretical justification for thegeneral assumption that copper is resistant to corrosion <strong>in</strong> pureanoxic water.16
Executive SummaryIf it is assumed that the system is open such that H 2 can becont<strong>in</strong>uously removed from the reaction area, the reaction will beshifted to the right and corrosion will be favoured. There are howevera number <strong>of</strong> questions that need to be answered regard<strong>in</strong>g theexperiment and results presented by Szakalos and Hultqvist. Onesuch issue is that the proposed corrosion product needs to beidentified and characterized. If the corrosion product is chemicallycharacterized the formation energy can be computed and the overallspontaneity <strong>of</strong> the corrosion reaction calculated. Then a newsubstance has to be added to the thermodynamic data tables.Moreover, based on available <strong>in</strong>formation presented by Szakalosand Hultqvist, it is not possible to state what the proposed reactionmeans for a copper canister <strong>in</strong> the repository environment. It is for<strong>in</strong>stance unclear what the presence <strong>of</strong> the bentonite buffer willmean for the removal <strong>of</strong> H 2 . It is also unclear how complex<strong>in</strong>g ionssuch as sulphide and chloride that are present <strong>in</strong> the repositoryenvironment will <strong>in</strong>fluence the proposed reaction.Hence, there are a number <strong>of</strong> issues that need to be addressed.The yet unknown corrosion product needs to be identified, thecorrosion rate needs to be determ<strong>in</strong>ed, and corrosion <strong>in</strong> a realisticrepository environment needs to be studied.The members <strong>of</strong> the panel also stated that further research isrequired to clarify the experimental results and the analyticalmethods used by Szakalos and Hultqvist.17
Written statements by the panel members19
Written statements by the panel members20
Written statements by the panel members<strong>Mechanisms</strong> <strong>of</strong> <strong>Copper</strong> <strong>Corrosion</strong> <strong>in</strong> <strong>Aqueous</strong> <strong>Environments</strong>Summary StatementG. K. ChuahNational University <strong>of</strong> S<strong>in</strong>gapore1. Based on the presentations at the workshop and the literaturesent to us, I would like to make the follow<strong>in</strong>g summary statement.The f<strong>in</strong>d<strong>in</strong>gs <strong>of</strong> Hultquist [1–3] and Szakálos [4] thatcopper can corrode <strong>in</strong> oxygen-free water is controversial as itappears to contradict present-day knowledge derived fromthermodynamics that copper is <strong>in</strong>ert <strong>in</strong> an oxygen-free environment.This f<strong>in</strong>d<strong>in</strong>g, if applicable to repository conditions,has implications for the lifetime <strong>of</strong> the copper canisters to beused for the storage <strong>of</strong> spent nuclear wastes under SKB’s KBS-3concept. The workshop convened on 16 November 2009 servesto give all parties a chance to present their f<strong>in</strong>d<strong>in</strong>gs to an audiencewith a wide range <strong>of</strong> backgrounds.2. In order to be <strong>in</strong> conformance with the known thermodynamics<strong>of</strong> the copper-water system, Hultquist and Szakálos have proposedthe formation <strong>of</strong> an amorphous copper hydroxideH x CuO y . However, such a hydroxide has yet to be directlydetected. The authors report that powder x-ray analyses on anumber <strong>of</strong> different copper samples from the anoxic experimentsshowed spectra <strong>in</strong>dicative <strong>of</strong> CuO and Cu 2 O with distortions.3. In contrast to these results, experiments conducted by twoother groups <strong>of</strong> <strong>in</strong>vestigators failed to show the presence <strong>of</strong>hydrogen. The experimental setups differed from that <strong>of</strong>Hultquist. In the study by Simpson and Schenk [5], copperspecimens were immersed <strong>in</strong> 8000 mg/l chloride solutions at50°C and 80°C and a flow <strong>of</strong> nitrogen was passed over the samples.A gas chromatograph was used with an assumed detectionlimit <strong>of</strong> 1 vppm H 2 . Weight ga<strong>in</strong>s and losses were measured forthe copper foils, but no hydrogen was detected. The authorsconcluded that water cannot be an oxidant for copper <strong>in</strong> purewater or dilute chloride media. Eriksen et al. [6, 7] also reportedno hydrogen evolution dur<strong>in</strong>g the exposure <strong>of</strong> copper to distilledwater for 61 days. However, their gas chromatogramsshowed the presence <strong>of</strong> oxygen <strong>in</strong> the system, which would havereacted with any hydrogen formed. The authors did note that21
Written statements by the panel membersthe lack <strong>of</strong> hydrogen evolution notwithstand<strong>in</strong>g, “the surface <strong>of</strong>exposed copper foils were unevenly corroded with smaller areasclearly discoloured whilst large areas were seem<strong>in</strong>gly unaffected.”4. It is stated <strong>in</strong> the draft report by SKB’s consultant, F. K<strong>in</strong>g [8],and <strong>in</strong> the presentation by C. Lilja <strong>of</strong> SKB [9] at the 16 thNovember Workshop that an attempt by Möller <strong>in</strong> 1995 toreproduce the experiment <strong>of</strong> Hultquist showed that no visualdifference <strong>in</strong> copper strips could be discerned from Pd-and Ptsealedvessels. However, at the same Workshop, Szakálos<strong>in</strong>formed the participants that Möller was able to reproduce theexperiment after contact with Hultquist on the experimentaldetails. If true, then it is regrettable that the same observationby a third party has not been disclosed <strong>in</strong> published reports.Instead, Möller’s <strong>in</strong>itial f<strong>in</strong>d<strong>in</strong>gs have <strong>of</strong>ten been cited <strong>in</strong> support<strong>of</strong> the claim that the observations <strong>of</strong> Hultquist have notbeen reproduced by others.5. In view <strong>of</strong> the claim by Hultquist and Szakálos <strong>of</strong> what seems tobe a violation <strong>of</strong> known thermodynamics, it is only prudent thatmore experiments be carried with the aim <strong>of</strong> (i) confirm<strong>in</strong>g ordisprov<strong>in</strong>g the formation <strong>of</strong> hydrogen through direct detectionby mass spectrometry; (ii) study<strong>in</strong>g the experimental conditions<strong>of</strong> Hultquist and Szakálos under which hydrogen is formed;(iii) exam<strong>in</strong><strong>in</strong>g the reaction products formed on the copper by<strong>in</strong>-situ methods to avoid any phase transformation on exposureto atmospheric conditions; (iv) measur<strong>in</strong>g the strength <strong>of</strong> theexposed copper; (v) quantify<strong>in</strong>g the thickness <strong>of</strong> the corrosionlayers as function <strong>of</strong> time; and other relevant tests. Moredetailed experiments have also been suggested by Pr<strong>of</strong>essor(Emeritus) Latanision, a member <strong>of</strong> the panel.6. There are a number <strong>of</strong> volum<strong>in</strong>ous reports provid<strong>in</strong>g criticalanalyses <strong>of</strong> the work by Hultquist and Szakálos. It is my op<strong>in</strong>ionthat all these critiques and reports are neither helpful norfruitful <strong>in</strong> the furtherance <strong>of</strong> our understand<strong>in</strong>g <strong>of</strong> copper corrosion<strong>in</strong> aqueous environments. Without more studies <strong>of</strong> thereaction conditions lead<strong>in</strong>g to copper corrosion <strong>in</strong> seem<strong>in</strong>glyanoxic conditions and a careful analysis <strong>of</strong> reaction productsformed, any hypothesis put forth can only be speculative. In thecourse <strong>of</strong> such an <strong>in</strong>vestigation, different detection techniques22
Written statements by the panel members<strong>in</strong>clud<strong>in</strong>g those <strong>of</strong> surface science are important. Surface sciencestudies are relevant for a fundamental understand<strong>in</strong>g <strong>of</strong> howreactions occur, and to be dismissive <strong>of</strong> its use based on a subjectiveop<strong>in</strong>ion that “the fundamental conditions and the processesthat operate <strong>in</strong> the gas phase are totally different fromthose that occur <strong>in</strong> solution” [8] would be to presume priorknowledge <strong>of</strong> the outcome.7. A jo<strong>in</strong>t study <strong>in</strong>volv<strong>in</strong>g the KTH <strong>in</strong>vestigators, SKB and anunbiased third party should be conducted to come to a fullunderstand<strong>in</strong>g <strong>of</strong> the observations. I suggest that Hultquist andSzakálos be <strong>in</strong>volved <strong>in</strong> the <strong>in</strong>vestigation solely because theyshould be able to advise on the experimental conditions thathave led them to the observations made.8. The results obta<strong>in</strong>ed from such a jo<strong>in</strong>t <strong>in</strong>vestigation may or maynot have implications for the f<strong>in</strong>al disposal <strong>of</strong> nuclear waste <strong>in</strong>copper canisters, but unless and until we know more, we reallycan say noth<strong>in</strong>g (although a lot has been said already). Theknowledge that can be ga<strong>in</strong>ed from further work, whether thereare some experimental artifacts that have been overlooked orwhether copper <strong>in</strong>deed corrodes under certa<strong>in</strong> conditions, canonly help <strong>in</strong> our understand<strong>in</strong>g <strong>of</strong> the copper-water system.9. Although this may not be <strong>in</strong> the terms <strong>of</strong> reference for the summarystatement, I would like to comment on the lack <strong>of</strong> criticalcomments and feedback on SKB’s published reports. I feel thatthere should be documentation <strong>of</strong> the comments/queries/feedbacktogether with the responses by the authors <strong>of</strong> the reports.As an example, <strong>in</strong> the Posiva report (Work<strong>in</strong>g Report 2003-45),an attempt was made to detect hydrogen <strong>in</strong> the gas phase. As nohydrogen was found, the authors commented that this could bedue to the limited sensitivity <strong>of</strong> the method, and suggested howthe sensitivity <strong>of</strong> detection could be <strong>in</strong>creased by decreas<strong>in</strong>g theheadspace volume, decreas<strong>in</strong>g the <strong>in</strong>itial pressure <strong>of</strong> nitrogenand/or decreas<strong>in</strong>g the length <strong>of</strong> the experiment. However, thereis no <strong>in</strong>dication <strong>of</strong> any follow-up experiments based on the suggestions.Furthermore, when the experiment was extended from6 to 30 days, it is stated <strong>in</strong> the report that “the analysis <strong>of</strong> thegas phase for hydrogen could not be performed because <strong>of</strong> afailure <strong>in</strong> tak<strong>in</strong>g the gas sample at the end <strong>of</strong> the test”. As theseare simple experiments which do not <strong>in</strong>volve an extensive length23
Written statements by the panel members<strong>of</strong> time, I am surprised that the experiment had not beenrepeated. One <strong>of</strong> the conclusions <strong>in</strong> the report was that “thecorrosion <strong>of</strong> copper at room temperature virtually stops after60–80 h due to anoxic condition established <strong>in</strong> the experiments”.This is at odds with the results presented <strong>in</strong> the samereport where the authors show a l<strong>in</strong>e with a positive slope <strong>in</strong>dicat<strong>in</strong>g<strong>in</strong>creas<strong>in</strong>g resistance <strong>of</strong> the Cu-wire probe with time (i.e.,cont<strong>in</strong>u<strong>in</strong>g corrosion) (Fig. 10a <strong>of</strong> report) and also with thesolution analysis for dissolved copper where the copper concentrationafter 30 days was higher than for 1–25 days. In view<strong>of</strong> such contradictions, I wonder if anyone reads the reportsgenerated and <strong>of</strong>fers critical comments.10. I am <strong>of</strong> the op<strong>in</strong>ion that there should be a mechanism <strong>in</strong> placefor a critical review <strong>of</strong> the publications to ensure that mean<strong>in</strong>gfulexperiments are conducted and repeated if necessary so thatthe reports published on the SKB website are scientificallysound and <strong>in</strong>formative.11. Lastly, I would like to thank the Swedish National Council forNuclear Waste for their k<strong>in</strong>d <strong>in</strong>vitation to serve as a member <strong>of</strong>the panel.References1. G. Hultquist, Corros. Sci. 26 (1986) 173.2. G. Hulquist, G.K. Chuah, K.L.Tan, Corros. Sci. 29 (1989) 1371.3. G. Hultquist, P. Szakálos, M.J. Graham, G.I. Sproule, G.Wikmark, <strong>in</strong> Proc. 17 th Int. <strong>Corrosion</strong> Congress, Las Vegas,USA, 2008, Paper 3884.4. P. Szakálos, G. Hultquist, G.Wikmark, Electrochem. Solid StateLetts, 10, (2007) C63.5. J.P. Simpson, R. Schenk, Corros. Sci. 27 (1087) 1365.6. T.E. Eriksen, P. Ndamlaba, I. Grenthe, SKB Technical Report88-17.7. T.E. Eriksen, P. Ndalamba, I. Grenthe, Corros. Sci. 29 (1989)1241.8. F. K<strong>in</strong>g, Critical Review <strong>of</strong> the Literature on the <strong>Corrosion</strong> <strong>of</strong><strong>Copper</strong> by Water (Draft), November 2009.9. C. Lilja, Presentation at Workshop on <strong>Mechanisms</strong> <strong>of</strong> <strong>Copper</strong><strong>Corrosion</strong> <strong>in</strong> <strong>Aqueous</strong> <strong>Environments</strong>, Stockholm, 16 Nov 2009.24
Written statements by the panel members<strong>Mechanisms</strong> <strong>of</strong> <strong>Copper</strong> <strong>Corrosion</strong> <strong>in</strong> <strong>Aqueous</strong> <strong>Environments</strong>Summary StatementR.M. LatanisionPr<strong>of</strong>essor (Emeritus), MITMember, U.S. Nuclear Waste Technical Review BoardFirst, I do need to emphasize that the comments which follow arem<strong>in</strong>e and do not represent the views <strong>of</strong> MIT or <strong>of</strong> the US NuclearWaste Technical Review Board. I want to thank the SwedishNational Council for Nuclear Waste for the <strong>in</strong>vitation to take part<strong>in</strong> this important Workshop. Nuclear electric generation is animportant part <strong>of</strong> the energy mix <strong>of</strong> many nations <strong>of</strong> the world andit is important that the wastes which are produced be disposed <strong>of</strong><strong>in</strong> a reliable manner. Officials <strong>in</strong> many <strong>of</strong> those nations are consider<strong>in</strong>ggeologic repositories as one approach to handl<strong>in</strong>g high-levelnuclear waste and spent nuclear fuel. In some <strong>in</strong>stances, as <strong>in</strong>Sweden, repositories are contemplated <strong>in</strong> rock formations that leadto reduc<strong>in</strong>g chemical environments and the material <strong>of</strong> choice <strong>in</strong>terms <strong>of</strong> the construction <strong>of</strong> waste packages is copper. The claimsthat have emerged over a period <strong>of</strong> more than two decades fromthe KTH research group led by Hultquist, which purport to showthat copper corrodes <strong>in</strong> oxygen-free water at room temperature,have generated global concern. These claims were contrary to theunderstand<strong>in</strong>g <strong>of</strong> the thermodynamics <strong>of</strong> the copper-water systemfrom the very beg<strong>in</strong>n<strong>in</strong>g as they are today.The fact that these claims have been taken seriously, as theyshould, by both the developer <strong>of</strong> the Swedish repository, SKB, andthe Swedish regulatory body, SSM, is reflected <strong>in</strong> the StockholmWorkshop. Representatives from the parties <strong>in</strong> Sweden that have<strong>in</strong>terest <strong>in</strong> this matter presented summaries <strong>of</strong> their researchand/or commissioned studies, statements were made by a Panel <strong>of</strong>which I was a member, and an extended dialogue occurred amongthe participants. The above was well documented at the workshop,and I do not wish to repeat all <strong>of</strong> that. In the follow<strong>in</strong>g, I havesummarized my views on the work that was presented <strong>in</strong>Stockholm <strong>in</strong> the form <strong>of</strong> a series <strong>of</strong> op<strong>in</strong>ions that I consider to becompletely objective. In addition, I have <strong>in</strong>cluded four recommendationsfor follow-up actions that I believe should be pursued as ameans <strong>of</strong> resolv<strong>in</strong>g this matter.25
Written statements by the panel members(1.) The ThermodynamicsThere is much to argue that from a thermodynamics perspectivethe reaction proposed by the KTH team does not proceed.Cu + H 2 O H 2 (g) + H x CuO yThe hydrogenated copper oxide reaction product is not known <strong>in</strong>nature and, thus, this reaction is unknown <strong>in</strong> the thermodynamics<strong>of</strong> copper corrosion. I have seen no compell<strong>in</strong>g data to suggest thatthis reaction product has been produced and identified <strong>in</strong> any <strong>of</strong>the KTH team publications.Recommendation: If the above reaction has any merit, then it must bedemonstrated that the reaction products that are proposed are <strong>in</strong>deedproduced. There are a host <strong>of</strong> sophisticated surface analytical techniquesthat should be committed to such a demonstration.(2.) Experimental ConfirmationThere have been many episodes <strong>in</strong> the history <strong>of</strong> science <strong>in</strong> whichseem<strong>in</strong>gly remarkable observations have ultimately been shown tobe (a) associated with an artifact <strong>of</strong> the experiment, or (b) contraryto scientific understand<strong>in</strong>g at that moment <strong>in</strong> history but subsequentlydemonstrated by researchers around the world to begenu<strong>in</strong>e. Polywater is an example <strong>of</strong> the former. The observationby Coriou and his colleagues <strong>in</strong> France that high purity water couldcause <strong>in</strong>tergranular stress corrosion crack<strong>in</strong>g <strong>of</strong> Inconel 600 is anexample <strong>of</strong> the latter that is now well known to anyone who has<strong>in</strong>terest <strong>in</strong> nuclear electric generation. The fact that some researchershave attempted to confirm the f<strong>in</strong>d<strong>in</strong>gs <strong>of</strong> the KTH team withoutsuccess suggests that there may be some peculiarity <strong>in</strong> thedesign <strong>of</strong> the KTH experiments that has escaped identification.There is a need for a clear understand<strong>in</strong>g <strong>of</strong> the experimental detailsassociated with the KTH work.Recommendation: While scientists may debate the merits <strong>of</strong> oppos<strong>in</strong>gviews as expressed <strong>in</strong> the literature and <strong>in</strong> public debate, the most productiveapproach <strong>in</strong> my view is a well-controlled experiment that isdef<strong>in</strong>itive. In the present case, the reactants Cu and H 2 O must becompletely specified and controlled. The products must be identifiedwith certa<strong>in</strong>ty. This attempt at experimental confirmation should becarried out at a third-party laboratory.26
Written statements by the panel membersThe recommended research should be done to replicate as closelyas possible the methodology and apparatus <strong>of</strong> the research work atKTH that has led to the present controversy. It is important thatthe work be done by capable <strong>in</strong>dividuals at an <strong>in</strong>stitution that is<strong>in</strong>dependent <strong>of</strong> the researchers and any entity with a stake <strong>in</strong> theSwedish repository project and that the fund<strong>in</strong>g for the researchcome from an <strong>in</strong>dependent source. The chemistry and microstructure<strong>of</strong> the copper must be well characterized. The water must becharacterized, verifiably deoxygenated at the outset <strong>of</strong> the experimentand monitored for oxygen dur<strong>in</strong>g the course <strong>of</strong> the experiment.The conductivity <strong>of</strong> the water must be measured at the outsetand monitored dur<strong>in</strong>g the experiment. Likewise, the pH <strong>of</strong> thewater should be monitored: it would be expected to <strong>in</strong>crease ashydrogen is evolved. Such monitor<strong>in</strong>g would allow an assessment<strong>of</strong>, for example, potential leach<strong>in</strong>g <strong>of</strong> ionic species from theexperimental flask and whether the membranes were effectivelysealed, thereby provid<strong>in</strong>g a barrier to oxygen <strong>in</strong>gress from theatmosphere dur<strong>in</strong>g the term <strong>of</strong> the experiment. In short, a carefulmaterials balance must be performed. There is no <strong>in</strong>dication <strong>in</strong> theKTH work that a materials balance was performed on key elements,which represents the omission <strong>of</strong> a useful diagnostic.It is my op<strong>in</strong>ion that the above research can and should be doneexpeditiously.(3.) Hydrogen Embrittlement <strong>of</strong> <strong>Copper</strong>The KTH team has claimed evidence that some <strong>of</strong> the hydrogenthat is produced by copper corrosion may embrittle the copper.There appears to be no metallographic or fractographic evidencedto support this claim nor does there appear to be any systematicevaluation <strong>of</strong> the mechanical properties <strong>of</strong> cupper which has been<strong>in</strong>fused with hydrogen. It is known that hydrogen will degrade themechanical properties <strong>of</strong> copper which is not oxygen-free and thatsolute segregation to gra<strong>in</strong> boundaries <strong>in</strong> copper may lead to <strong>in</strong>tergranularfailure. However, I know <strong>of</strong> no work that has identifiedhydrogen embrittlement as a failure mode <strong>in</strong> the case <strong>of</strong> OFHCcopper.Recommendation: The susceptibility <strong>of</strong> copper to embrittlement byabsorbed hydrogen should be exam<strong>in</strong>ed on copper tensile specimenselectrolytically charged with hydrogen at cathodic current densitiesthat correspond to the corrosion rates associated with the measure-27
Written statements by the panel membersments reported by the KTH team. Once aga<strong>in</strong>, this research should beperformed by an objective third party <strong>in</strong>stitution.(4.) Implications with Respect to the RepositoryThe objectives <strong>of</strong> the workshop were <strong>in</strong> part to air the various dataand hypotheses that have emerged regard<strong>in</strong>g copper corrosion <strong>in</strong>oxygen-free water as a step along the path <strong>of</strong> resolution <strong>of</strong> theissues <strong>of</strong> the mean<strong>in</strong>g and importance <strong>of</strong> the corrosion data, particularlywith respect to the potential corrosion <strong>of</strong> KBS-3 coppercanisters <strong>in</strong> a repository environment. I am <strong>of</strong> the op<strong>in</strong>ion that therate <strong>of</strong> transport <strong>of</strong> water through the proposed bentonite bufferthat is <strong>in</strong>tended to surround the emplaced copper canisters is likelyto be extremely slow. Thus, even if a corrosion reaction does occur,the amount <strong>of</strong> corrosion per unit time would be very low s<strong>in</strong>ce it isdiffusion controlled. If it is determ<strong>in</strong>ed by means <strong>of</strong> the confirm<strong>in</strong>gresearch recommended above that copper does <strong>in</strong> fact corrode <strong>in</strong>water, as unlikely as that seems to me, I would then look to aneng<strong>in</strong>eer<strong>in</strong>g solution to bound the problem <strong>of</strong> copper corrosionwith those particular conditions that may have given rise to coppercorrosion clearly identified so that the conditions where<strong>in</strong> corrosionmight occur could be elim<strong>in</strong>ated.Recommendation: <strong>Copper</strong> is an obvious material to consider for disposal<strong>in</strong> chemically reduc<strong>in</strong>g environments. If copper corrosion <strong>in</strong>anoxic environments is observed and confirmed as described <strong>in</strong> theabove research, the conditions which have led to corrosion need to beclearly identified and then either controlled or eng<strong>in</strong>eered out <strong>of</strong> therepository environment.28
Written statements by the panel membersIs <strong>Copper</strong> Immune to <strong>Corrosion</strong> When <strong>in</strong> Contact With Water?Digby D. MacdonaldCenter for Electrochemical Science and TechnologyDepartment <strong>of</strong> Materials Science and Eng<strong>in</strong>eer<strong>in</strong>gPennsylvania State UniversityUniversity Park, PA 16802Tel: (814) 863-7772; E-mail: ddm2@psu.edu.Sweden’s SKB-III plan for the disposal <strong>of</strong> high level nuclear waste(HLNW) is predicated upon the condition that copper, the materialfrom which the canisters will be fabricated, is thermodynamicallyimmune to corrosion when <strong>in</strong> contact with pure water. In theimmune state, corrosion cannot occur because any oxidation process<strong>of</strong> the copper is characterized by a positive change <strong>in</strong> the Gibbsenergy, rather than a negative change, as demanded by the SecondLaw <strong>of</strong> Thermodynamics for a spontaneous process. Accord<strong>in</strong>gly,“immunity” is a thermodynamic state that must be characterizedupon the basis <strong>of</strong> thermodynamic arguments.Consider the lowest corrosion reaction <strong>in</strong> the copper/watersystem:Cu + H + = Cu + + 1/2H 2 (1)The change <strong>in</strong> Gibbs energy for this reaction can be written aswhich, upon rearrangement yields(2)(3)where is the change <strong>in</strong> standard Gibbs energy; i.e., the change<strong>in</strong> Gibbs energy when all components <strong>of</strong> the reaction are <strong>in</strong> theirstandard state with the fugacity <strong>of</strong> hydrogen, , and the activity<strong>of</strong> cuprous ion, , be<strong>in</strong>g equal to one. At equilibrium, ,and designat<strong>in</strong>g the equilibrium values <strong>of</strong> and with superscripts“e” we may write(4)29
Written statements by the panel membersWe now def<strong>in</strong>e two quantities, P and P e , as followsand(5)The condition for spontaneity <strong>of</strong> Reaction (1) then becomes P P e .The quantity P e has been calculated for Reaction (1) us<strong>in</strong>gEquation (4) and is plotted as a function <strong>of</strong> pH <strong>in</strong> Figure 1. Theseplots divide the P versus pH doma<strong>in</strong> <strong>in</strong>to regions <strong>of</strong> immunity(upper region) and corrosion (lower region). Accord<strong>in</strong>gly, P e versuspH divides the doma<strong>in</strong> <strong>in</strong>to regions <strong>of</strong> thermodynamic immunity(upper region) and corrosion (lower region). These plotsclearly demonstrate that whether copper is immune (thermodynamicallystable) depends sensitively upon the value <strong>of</strong> P and henceupon the <strong>in</strong>itial conditions <strong>in</strong> the system. Thus, if P is small (e.g., atPo<strong>in</strong>t a, Figure 1), P < P e and the corrosion <strong>of</strong> copper is spontaneousas written <strong>in</strong> Equation (1). On the other hand, if the system islocated at Po<strong>in</strong>t (b), Figure 1), P > P e and corrosion is not possible,thermodynamically, and hence the metal is “immune”.Return<strong>in</strong>g now to the case described by Po<strong>in</strong>t a, we note that as thecorrosion reaction proceeds, the concentration <strong>of</strong> Cu + and thefugacity <strong>of</strong> hydrogen at the <strong>in</strong>terface will <strong>in</strong>crease, particularly <strong>in</strong> amedium <strong>of</strong> restricted mass transport, such that P will steadily<strong>in</strong>crease with time until it meets the value <strong>of</strong> P e at the correspond<strong>in</strong>gtemperature. At this po<strong>in</strong>t, the metal may be classified as be<strong>in</strong>g“quasi-immune”; “quasi” only because transport <strong>of</strong> Cu + and H 2away from the canister surface, through the bentonite overpackmust be matched by corrosion, <strong>in</strong> order to ma<strong>in</strong>ta<strong>in</strong> P = P e at themetal surface. Accord<strong>in</strong>gly, the corrosion rate ultimately becomescontrolled by the diffusion <strong>of</strong> Cu + and H 2 through the adjacentbentonite overpack. Thus, we conclude that, for any system start<strong>in</strong>gat a po<strong>in</strong>t below the Pe versus pH for the relevant temperature,copper metal is not thermodynamically immune and will corrode <strong>in</strong>the repository at a rate that is governed by the rate <strong>of</strong> transport <strong>of</strong>the corrosion products away from the metal surface. Of course,this rate is readily predicted by solv<strong>in</strong>g the diffusion equation, ifthe diffusivities <strong>of</strong> Cu + and H 2 <strong>in</strong> bentonite are known.(6)30
Written statements by the panel membersFigure 1<strong>Corrosion</strong> doma<strong>in</strong> diagram for copper <strong>in</strong> water as a function <strong>of</strong>temperature0-5-10P e<strong>Corrosion</strong> Not PossiblebLog(P)-15-201/T /K^-1 vs Log(K)<strong>Corrosion</strong> PossibleTemperature /K433.15333.15-25a273.15-300 5 10 15 20pHAs noted above, for any system whose <strong>in</strong>itial conditions (value <strong>of</strong>P) lie above the relevant P e versus pH l<strong>in</strong>e, copper is unequivocallyimmune and corrosion cannot occur as it would violate the SecondLaw <strong>of</strong> Thermodynamics. It is evident, that the conditions forimmunity may be eng<strong>in</strong>eered <strong>in</strong> advance by dop<strong>in</strong>g the bentonitewith a Cu(I) salt and a suitable reduc<strong>in</strong>g agent to simulate hydrogen,such that the <strong>in</strong>itial conditions lie above P e versus pH. It issuggested that cuprous sulfite, Cu 2 SO 3 , might be a suitable material.Of course, the dopant will slowly diffuse out <strong>of</strong> the bentoniteand <strong>in</strong>to the external environment, but it might be sufficiently slowthat the conditions <strong>of</strong> immunity may be ma<strong>in</strong>ta<strong>in</strong>ed for a considerableperiod. Thus, <strong>in</strong> a “back-<strong>of</strong>-the-envelope” calculation,we choose L = 10 cm and D = 10 -9 cm/s to yield a diffusion time<strong>of</strong> 1011 seconds or 316,456 years. At a time <strong>of</strong> this order, the value<strong>of</strong> P at the canister surface will have been reduced to P e and corrosionwill have <strong>in</strong>itiated at a rate that is determ<strong>in</strong>ed by the transport<strong>of</strong> Cu + and H 2 through the bentonite overpack. It is important to(7)31
Written statements by the panel membersnote that the above calculation is only a rough estimate and that amore accurate value can be obta<strong>in</strong>ed by solv<strong>in</strong>g the diffusion equationwith experimentally determ<strong>in</strong>ed values for the diffusivities <strong>of</strong>Cu + and H 2 . The important po<strong>in</strong>t is that immunity may be ma<strong>in</strong>ta<strong>in</strong>edfor a sufficiently long period that the more active components<strong>of</strong> the HLNW will have decayed away.Table 1Prediction <strong>of</strong> copper corrosion <strong>in</strong> pure water as a function <strong>of</strong>solution composition at 273.15 K and at pH = 7. Activitycoefficients assumed to be one. Log(P e ) = -16.895.Case aCu+ ppbCu+ fH2 ppbH2 Log(P) <strong>Corrosion</strong> possible?1 10 -6 63 10 -6 0.002 -9 No2 10 -6 63 10 -12 2 x10 -9 -12 No3 10 -6 63 10 -18 2 x10 -15 -15 No4 10 -6 63 10 -24 2 x10 -21 -18 Yes5 10 -6 63 10 -30 2 x10 -27 -21 Yes6 10 -6 63 10 -18 2 x10 -15 -15 No7 10 -12 63 x 10 -6 10 -18 2 x10 -15 -21 Yes8 10 -18 63 x 10 -12 10 -18 2 x10 -15 -27 Yes9 10 -24 63 x 10 -18 10 -18 2 x10 -15 -33 Yes10 10 -30 63 x 10 -24 10 -18 2 x10 -15 -39 YesTo complete the discussion with regard to copper immunity, when<strong>in</strong> contact with pure water, we give <strong>in</strong> Table 1 various comb<strong>in</strong>ations<strong>of</strong> the fugacity <strong>of</strong> hydrogen, , and the activity <strong>of</strong> cuprous ion,, and the calculated values <strong>of</strong> P, together with a judgment onwhether corrosion is possible or whether immunity should prevail.As can be seen from the values conta<strong>in</strong>ed <strong>in</strong> Table 1, the values <strong>of</strong>P e = [Cu + ]. , for equilibrium, assum<strong>in</strong>g unit activity and fugacitycoefficients, are very low and hence it should be relatively easyto ensure immunity by dop<strong>in</strong>g as <strong>in</strong>dicated above. However, it isalso noted that experiments that have been performed to detect thecorrosion <strong>of</strong> copper <strong>in</strong> contact with pure water have yielded contradictoryresults, with some experiments <strong>in</strong>dicat<strong>in</strong>g that corrosionoccurs while others <strong>in</strong>dicate that copper is immune. It is suggestedby the present author that this unsatisfactory state <strong>of</strong> affairs stemsfrom a poor def<strong>in</strong>ition <strong>of</strong> the <strong>in</strong>itial conditions <strong>of</strong> the experiment,and <strong>in</strong> some cases the concentration <strong>of</strong> Cu + and H 2 may be such32
Written statements by the panel membersthat P exceeds the equilibrium P e versus pH correlation and hencethe system pre-exists <strong>in</strong> the immune condition. In this case, no corrosionwould be expected to occur as the metal is thermodynamicallyimmune. On the other hand, <strong>in</strong> other experiments, the <strong>in</strong>itial[Cu + ] and may be such that the system is characterized by a Pvalue that lies below the P e versus pH correlation shown <strong>in</strong> Figure1, <strong>in</strong>dicat<strong>in</strong>g that corrosion is spontaneous. In this <strong>in</strong>stance, corrosionwill occur until the concentrations <strong>of</strong> corrosion products buildup <strong>in</strong> a closed system to render P = P e . At that po<strong>in</strong>t, corrosionwill cease. Clearly, considerable care must be exercised whendesign<strong>in</strong>g experiments to demonstrate copper corrosion to ensurethat corrosion is spontaneous upon <strong>in</strong>itiation <strong>of</strong> the experiment.The analysis presented above is restricted to the corrosion <strong>of</strong>copper <strong>in</strong> contact with pure water. However, groundwater is farfrom pure and a common contam<strong>in</strong>ant is bisulfide ion, HS - . Thisspecies arises from dissolution <strong>of</strong> sulfide m<strong>in</strong>erals <strong>in</strong> the host rock<strong>of</strong> the repository, from dissolution <strong>of</strong> pyrite <strong>in</strong> the bentonite, andeven from the decomposition <strong>of</strong> organic (plant) material. It is fairto conclude that bisulfide, and other sulfur-conta<strong>in</strong><strong>in</strong>g species areubiquitous <strong>in</strong> groundwater environments at concentration rang<strong>in</strong>gup to a few ppm, at least. It is also well-known that sulfide,<strong>in</strong>clud<strong>in</strong>g bisulfide, activates copper by giv<strong>in</strong>g rise to the formation<strong>of</strong> Cu 2 S at potentials that are significantly more negative that thepotential for the formation <strong>of</strong> Cu 2 O. Thus, <strong>in</strong> the presence <strong>of</strong>bisulfide, the lowest corrosion reaction <strong>of</strong> copper may be written as2Cu + HS - + H + = Cu 2 S + H 2 (8)for which the change <strong>in</strong> Gibbs energy is written asAs before, we def<strong>in</strong>e an equilibrium value <strong>of</strong> P aswhere(9)(10)(11)33
Written statements by the panel membersValues <strong>of</strong> P e versus pH are plotted <strong>in</strong> Figure 2 as a function <strong>of</strong> temperaturefor temperatures rang<strong>in</strong>g from 0 o C to 160 o C <strong>in</strong> steps <strong>of</strong>20 o C. Aga<strong>in</strong>, P e versus pH divides the diagram <strong>in</strong>to two regionscorrespond<strong>in</strong>g to spontaneous corrosion (lower region) andimmunity (upper region). The reader will note that the P e valuesfor the l<strong>in</strong>es are more positive than those for the Cu – pure watercase by a factor <strong>of</strong> about 10 27 , demonstrat<strong>in</strong>g that immunity ismuch more difficult to achieve <strong>in</strong> the presence <strong>of</strong> bisulfide.Figure 2<strong>Corrosion</strong> doma<strong>in</strong> diagram for copper <strong>in</strong> water + HS - as afunction <strong>of</strong> temperature.2015P e<strong>Corrosion</strong> Not Possible (Immunity)10Log(P)5<strong>Corrosion</strong> Possible (Active)Temperature / K273.15333.150433.15-50 5 10 15 20pHIn order to illustrate the difficulties posed by small amounts <strong>of</strong>bisulfide <strong>in</strong> the environment, different comb<strong>in</strong>ations <strong>of</strong> the fugacity<strong>of</strong> hydrogen, , and the activity <strong>of</strong> cuprous ion, ,together with the correspond<strong>in</strong>g value <strong>of</strong> P e for T = 273.15 K andpH = 7 ate given <strong>in</strong> Table 2. Not<strong>in</strong>g aga<strong>in</strong> that immunity isachieved only if P > P e , it is evident that the desired immune conditioncould only be achieved by hav<strong>in</strong>g an extraord<strong>in</strong>arily lowconcentration <strong>of</strong> HS - and/or an extraord<strong>in</strong>arily high partial pressure<strong>of</strong> hydrogen. For example, the two conditions that are listed <strong>in</strong>Table 2 that are predicted to yield immunity are Cases 4 and 9,34
Written statements by the panel memberswhich have [HS - ] and comb<strong>in</strong>ations <strong>of</strong> 3.3 x10 -10 ppm and 10 -6atm and 0.033 ppm and 10 10 atm, respectively. In the first case, theconcentration <strong>of</strong> HS - is orders <strong>of</strong> magnitude lower that the sulfideconcentration <strong>in</strong> groundwater (a few ppb to a few ppm), particularly<strong>in</strong> the presence <strong>of</strong> bentonite, which commonly conta<strong>in</strong>spyrite, FeS 2 . In the second case, the required partial pressure <strong>of</strong>hydrogen (10 10 atm) is impossibly high to be achieved and ma<strong>in</strong>ta<strong>in</strong>edpractically <strong>in</strong> the repository. Accord<strong>in</strong>gly, the prospects forachiev<strong>in</strong>g immunity <strong>of</strong> copper <strong>in</strong> a repository <strong>in</strong> which the groundwaterconta<strong>in</strong>s a significant concentration <strong>of</strong> bisulfide must bejudged to be remote. Of course, these predictions can easily bechecked by experiments, and experiments to do so should be performedat the earliest opportunity.Table 2Prediction <strong>of</strong> copper corrosion <strong>in</strong> pure water conta<strong>in</strong><strong>in</strong>g bisulfideion as a function <strong>of</strong> solution composition at 273.15 K. Activitycoefficients assumed to be one. Log(P e ) = 10.21.Case aHS- /m ppmHS- fH2 /atm ppbH2 Log(P) <strong>Corrosion</strong> possible?1 10 -4 3.3 10 -6 0.002 1 Yes2 10 -6 0.033 10 -6 0.002 3 Yes3 10 -8 0.00033 10 -6 0.002 5 Yes4 10 -10 0.0000033 10 -6 0.002 7 Yes5 10 -12 3.3 x 10 -8 10 -6 0.02 9 Yes6 10 -14 3.3 x 10 -10 10 -6 0.02 11 No5 10 -6 0.033 10 -4 0.2 4 Yes6 10 -6 0.033 10 -2 20 5 Yes7 10 -6 0.033 1 2000 6 Yes8 10 -6 0.033 10 2 2 x 10 5 7 Yes9 10 -6 0.033 10 4 2 x 10 7 8 Yes10 10 -6 0.033 10 6 2 x 10 9 9 Yes11 10 -6 0.033 10 8 2 x 10 11 10 Yes12 10 -6 0.033 10 10 2 x 10 13 11 No35
Written statements by the panel membersFigure 3 Volt-Equivalent diagram for the S/H2O system at 25 o C, pH = 0.3H 2S 2O 4HS 2O 4(-)Volt Equivalent, V210S(2-)HS(-)S 2(2-)S 3(2-)S 4(2-)SS 2O 4(2-)S 2O 3(2-)SO3(2-)HSO 3(-)H 2SO 4HSO 4(-)H 2SO 3SO 4(2-)S 4O 6(2-)H 2SS 5(2-)-1-4 -2 0 2 4 6 8Average Sulfur Oxidation StateVolt Equivalent Diagram for S/H2O System at pH=0 and at 25 CAs noted above, the calculations presented above are <strong>of</strong> a “back-<strong>of</strong>the-envelope”nature and a much more detailed analysis is warrantedto fully def<strong>in</strong>e the conditions under which immunity <strong>of</strong>copper might be expected. The analysis should <strong>in</strong>volve the follow<strong>in</strong>gactivities:1. The analysis should <strong>in</strong>clude the full range <strong>of</strong> sulfur species, asdef<strong>in</strong>ed <strong>in</strong> the volt-Equivalent diagram (e.g. Figure 3). All <strong>of</strong> thespecies plotted <strong>in</strong> this diagram, except sulfite and sulfate, candonate atomic sulfur to the copper surface and hence, potentially,are strong activators <strong>of</strong> the metal, thereby mak<strong>in</strong>g immunityharder to achieve. Other activat<strong>in</strong>g species should be<strong>in</strong>cluded <strong>in</strong> the analysis, <strong>in</strong>clud<strong>in</strong>g chloride, bromide, and otheranions. It is important to note that the sulfur species used toconstruct the Volt-Equivalent Diagrams are very labile andhence readily change from one to the other as conditions change<strong>in</strong> the system. Accord<strong>in</strong>gly, a prime objective will be to ascerta<strong>in</strong>which <strong>of</strong> the species are the most effective at destroy<strong>in</strong>g immu-36
Written statements by the panel membersnity on copper. This can only be done by compar<strong>in</strong>g the <strong>Corrosion</strong>Doma<strong>in</strong> Diagrams (e.g., Figures 1 and 2) for the reaction<strong>of</strong> copper with each <strong>of</strong> the sulfur species.2. The system should be modeled along the corrosion evolutionarypath, which is def<strong>in</strong>ed by the variation <strong>of</strong> temperature, pH, [HS -], and as the repository ages; note that these four quantitiesare the primary <strong>in</strong>dependent variables for Reactions (1) and (8).The time dependences <strong>of</strong> pH, [HS - ], and must be modeledby solv<strong>in</strong>g the transport equations for the transfer <strong>of</strong> H + , HS - ,and H 2 across the bentonite layer, recogniz<strong>in</strong>g the existence <strong>of</strong> asource term for bisulfide <strong>in</strong> the bentonite (dissolution <strong>of</strong> FeS 2 ).Solution <strong>of</strong> the thermal diffusion equation will yield the temperatureas a function <strong>of</strong> distance from the copper surface andtime. Because the diffusivities <strong>of</strong> H + , HS - , and H 2 are temperature-dependent,as is the rate constant for FeS 2 dissolution, thesystem <strong>of</strong> equations that will describe the evolution <strong>of</strong> therepository and hence will <strong>in</strong>dicate whether and under what conditionsimmunity may be achieved, will be highly non-l<strong>in</strong>ear andmust be solved numerically.3. The possibility <strong>of</strong> dop<strong>in</strong>g the bentonite with a Cu(I) salt, suchas Cu 2 SO 3 , should be explored to determ<strong>in</strong>e whether immunitymight be ma<strong>in</strong>ta<strong>in</strong>ed over extended periods. Thus, the “back-<strong>of</strong>the-envelope”calculations reported above suggest that immunitymight be susta<strong>in</strong>ed over periods <strong>of</strong> several hundreds <strong>of</strong>thousands <strong>of</strong> years. Given that the performance horizon <strong>of</strong> therepository is 10,000 years, it may well be possible to imposeimmunity on the system over the entire planned storage period.Practically, this issue could be explored by <strong>in</strong>sert<strong>in</strong>g sourceterms <strong>in</strong> the model outl<strong>in</strong>ed <strong>in</strong> 2 above for Cu + and SO 3 2- fromthe bentonite overpack.37
Written statements by the panel membersMECHANISM OF COPPER CORROSION IN AQUEOUSENVIRONMENTSD.W. ShoesmithDepartment <strong>of</strong> Chemistry,The University <strong>of</strong> Western Ontario,London, ON, N6A 5B7, CanadaINTRODUCTIONSzakalos and Hultquist have proposed that the corrosion <strong>of</strong> copper<strong>in</strong> water leads to the production <strong>of</strong> hydrogen and a previouslyunreported H x CuO y phase [1–8]. It is claimed that the mechanism<strong>of</strong> this reaction <strong>in</strong>volves the dissociation <strong>of</strong> water to produce thisphase and H atoms. Subsequently, this surface species may dehydrateto yield the known phase Cu 2 O and the H atoms either comb<strong>in</strong>eto evolve H 2 or absorb <strong>in</strong>to the copper. Even <strong>in</strong> the presence<strong>of</strong> dissolved O 2 it is claimed that corrosion occurs via this mechanismand that the O 2 present is consumed, not by a direct reactionto corrode copper, but by reaction with the H atoms produced bywater decomposition, thereby lead<strong>in</strong>g to further corrosion <strong>of</strong> thecopper by water decomposition and reaction with H atoms.This mechanism represents a radical new view <strong>of</strong> the process <strong>of</strong>copper corrosion which contradicts our present understand<strong>in</strong>g <strong>of</strong>the thermodynamics <strong>of</strong> copper corrosion as well as propos<strong>in</strong>g anew reaction pathway at odds with a wealth <strong>of</strong> published <strong>in</strong>formation.These results are then used to estimate rates <strong>of</strong> corrosion <strong>of</strong>copper nuclear waste conta<strong>in</strong>ers which are orders <strong>of</strong> magnitudegreater than those calculated by SKB and other national nuclearwaste disposal programs.THE WORKSHOP PRESENTATIONSAt this meet<strong>in</strong>g Hultquist, Szakalos, and SKB had the opportunityto present their research results and/or their approach to <strong>in</strong>vestigat<strong>in</strong>gthe copper corrosion process as it perta<strong>in</strong>ed to buriednuclear waste conta<strong>in</strong>ers. SKB (Lilja) chose to present its widerang<strong>in</strong>g and extensive program on copper corrosion and also theresults <strong>of</strong> first pr<strong>in</strong>ciples calculations <strong>of</strong> the thermodynamic properties<strong>of</strong> Cu-O-H, the phase claimed as a corrosion product by38
Written statements by the panel membersHultquist et al. Hultquist presented the results <strong>of</strong> his research conductedover the last 23 years, which was already well documented<strong>in</strong> the available published papers provided before the workshop.The presentation by Szakalos, however, merits more detailedcomment, s<strong>in</strong>ce it started with the claim that the <strong>in</strong>stability <strong>of</strong> copper<strong>in</strong> pure O 2 -free water was undisputed among thermodynamicexperts, which rather pre-empted the purpose <strong>of</strong> the meet<strong>in</strong>g. Acase was made that many <strong>of</strong> the rates measured <strong>in</strong> the Swedish(SKB) and other national programs (Canada, F<strong>in</strong>land, Japan) were<strong>in</strong> the range measured by Hultquist et al., imply<strong>in</strong>g, withoutexplicitly stat<strong>in</strong>g, that even the SKB measurements justified theirclaims.In the subsequent discussion period a number, but not all, <strong>of</strong>these assertions were challenged. For <strong>in</strong>stance, Fraser K<strong>in</strong>g (a consultantto SKB) po<strong>in</strong>ted out the <strong>in</strong>consistency <strong>in</strong> compar<strong>in</strong>g predictedcorrosion rates <strong>in</strong> the Swedish/F<strong>in</strong>nish programs (0.33 mm<strong>in</strong> 10 6 years) to the results <strong>of</strong> conservative estimates based on massbalance calculations <strong>in</strong> the Japanese program (18–26 mm <strong>in</strong> 10 3years) (Quoted from SKB report TR-01-23). Other referencesquoted <strong>in</strong> this presentation also do not stand up to scrut<strong>in</strong>y. Forexample, it is implied that the observation <strong>of</strong> <strong>in</strong>tergranular corrosionon copper <strong>in</strong> aerated sulphide-conta<strong>in</strong><strong>in</strong>g salt water by Al.Kharafi et al. [9] <strong>in</strong>dicates this process will occur under anoxicrepository conditions, but does not acknowledge the positive electrochemicalpotentials used <strong>in</strong> the experiments described <strong>in</strong> thepaper. Such redox conditions are only achievable under fully aeratedconditions <strong>in</strong> the presence <strong>of</strong> sulphide. The authors’ [9] claimthat “The present results are immediately relevant to the discussion<strong>of</strong> the proposed use <strong>of</strong> copper conta<strong>in</strong>ers for the disposal <strong>of</strong>Swedish, F<strong>in</strong>nish and Canadian high-level waste deep <strong>in</strong> graniteenvironments” is <strong>in</strong>correct, but accepted by Szakalos.The claim that the corrosion <strong>of</strong> copper cool<strong>in</strong>g systems is anexample <strong>of</strong> copper corrosion <strong>in</strong> O 2 -free water is a further example<strong>of</strong> what can only be described as an <strong>in</strong>complete analysis <strong>of</strong> theavailable literature. The corrosion processes occurr<strong>in</strong>g <strong>in</strong> thesesystems are well characterized and known to be due to the <strong>in</strong>-leakage<strong>of</strong> oxygen [10]. To quote from Park et al. [10], “These problemshave been understood from the relationship between the corrosionrate <strong>of</strong> copper and a DO (dissolved oxygen) concentration.The experimental results showed a bell-shaped relationship (3 referencesgiven). The corrosion rate <strong>in</strong> LOWC (low-oxygen water39
Written statements by the panel memberschemistry) is satisfactorily low ow<strong>in</strong>g to its (Cu) thermodynamicstability, and is reduced significantly <strong>in</strong> HOWC (high-oxygenwater chemistry) ow<strong>in</strong>g to passive oxide formation. However,<strong>in</strong>termediate oxygen water chemistry (IOWC) (50 ppb < DO < 2ppm) results <strong>in</strong> much higher corrosion rates. The IOWC mayoccur when air leaks <strong>in</strong>to the system.”In fact, contrary to support<strong>in</strong>g the claim that the extensive corrosionand f<strong>in</strong>ely divided corrosion product observed <strong>in</strong> the 15-yeartest [7] is due to Cu corrosion by H 2 O to produce H 2 , theseobservations suggest the copious corrosion observed was due tothe ma<strong>in</strong>tenance <strong>of</strong> IOWC conditions or a cycl<strong>in</strong>g between LOWCand HOWC conditions. As <strong>in</strong> the Cu cool<strong>in</strong>g systems, the ma<strong>in</strong>tenance<strong>of</strong> totally anoxic conditions would have been very unlikely.K<strong>in</strong>g [11] makes a similar po<strong>in</strong>t <strong>in</strong> his review <strong>of</strong> the experiments <strong>of</strong>Hultquist et al.REVIEW OF THE CORROSION MECHANISM OFCOPPER UNDER ANOXIC CONDITIONSThe two key processes <strong>in</strong>volved if the corrosion mechanism proposedby Hultquist et al. is to occur are the production <strong>of</strong> H 2 andthe formation <strong>of</strong> a stable corrosion product at potentials wellbelow those presently accepted <strong>in</strong> thermodynamic calculations.Hydrogen ProductionThe key feature <strong>of</strong> the corrosion process proposed by Hultquist etal. is the production <strong>of</strong> H 2 . In their <strong>in</strong>itial publication [1], H 2 wasdetected us<strong>in</strong>g a solid-electrolyte H 2 probe, pre-calibrated over arange <strong>of</strong> H 2 /N 2 mixtures. In other experiments (with the muchmore readily corrodible Zn [2]) the response <strong>of</strong> this probe wasvalidated aga<strong>in</strong>st manometric measurements. They also used an ionpump to measure pressure build-up, assumed to be due to H 2 formation,and eventually thermal out-gass<strong>in</strong>g coupled to mass spectrometryto determ<strong>in</strong>e hydrogen present <strong>in</strong> corrosion products andthe Cu itself. Except <strong>in</strong> this last case, there are no grounds tounequivocally dispute that H 2 was formed, despite the number <strong>of</strong>experimental uncerta<strong>in</strong>ties noted by K<strong>in</strong>g [11].40
Written statements by the panel membersExcept for the <strong>in</strong>itial study the authors claim that the development<strong>of</strong> a H 2 pressure with<strong>in</strong> a sealed vessel will suppress corrosionas thermodynamic equilibration is approached. This argument isused to expla<strong>in</strong> apparent decreases <strong>in</strong> H 2 generation rates with time,and to account for the major differences <strong>in</strong> the extents <strong>of</strong> corrosionobserved on specimens corroded over a 15-year period <strong>in</strong> vesselssealed with Pd (through which, they claim, H 2 can escape andextensive corrosion is possible) and with Pt (through which, theyclaim, H 2 cannot escape and corrosion should be suppressed). It isalso assumed, to account for apparent imbalances <strong>in</strong> the extent <strong>of</strong>corrosion and the amount <strong>of</strong> H 2 detected, that considerableamounts <strong>of</strong> H are absorbed <strong>in</strong>to the Cu. This last conclusion mustbe treated with suspicion, s<strong>in</strong>ce the solubility <strong>of</strong> H <strong>in</strong> Cu is knownto be small (as po<strong>in</strong>ted out by K<strong>in</strong>g [11]) and the only analyticalevidence provided by Hultquist et al. comes from out-gass<strong>in</strong>gexperiments on specimens covered <strong>in</strong> corrosion products whosedegree <strong>of</strong> hydration is unknown. A similar reservation applies tothe SIMS analyses <strong>of</strong> Hultquist et al., and the claim that the heavilycorroded specimen from the 15-year experiment is H embrittleds<strong>in</strong>ce it cracked on bend<strong>in</strong>g is speculative at best. The reference<strong>of</strong>fered <strong>in</strong> support [12] is not relevant s<strong>in</strong>ce it <strong>in</strong>volved hydrogencharg<strong>in</strong>g <strong>of</strong> Cu at currents greater than 10 mA·cm -2 , which is manyorders <strong>of</strong> magnitude greater than could possibly be susta<strong>in</strong>ed bycorrosion.The <strong>Corrosion</strong> ProductFor a corrosion process to be susta<strong>in</strong>ed by H 2 O reduction, theformation <strong>of</strong> an anodic corrosion product must occur. This <strong>in</strong>troducesa dilemma s<strong>in</strong>ce, with the exception <strong>of</strong> copper sulphides(Cu x S with x ~ 2), there are no known stable corrosion products,accord<strong>in</strong>g to accepted thermodynamic reason<strong>in</strong>g, if the corrosionpotential is ma<strong>in</strong>ta<strong>in</strong>ed <strong>in</strong> the range where water reduction is thermodynamicallypossible. The corrosion product formed <strong>in</strong> theirexperiments [5] was analyzed by X-Ray Diffraction (XRD) andSecondary Ion mass Spectrometry (SIMS). XRD <strong>in</strong>dicated thepresence <strong>of</strong> CuO and Cu 2 O and the SIMS showed a range <strong>of</strong> productswith vary<strong>in</strong>g O and H contents. Consistent with earlier observations[3], the H content <strong>of</strong> the products was higher <strong>in</strong> tests performed<strong>in</strong> the absence <strong>of</strong> O 2 . Despite the XRD results, the authors41
Written statements by the panel membersclaimed the formation <strong>of</strong> a new phase, H x CuO y . Unfortunately,both techniques are ex-situ and the phases observed may not bethose formed by corrosion, but a comb<strong>in</strong>ation <strong>of</strong> the oxides presentat the start <strong>of</strong> the experiment, the corrosion products, andtheir conversion products when exposed to air prior to analysis.Consequently, these analyses neither confirm nor disprove theformation <strong>of</strong> H x CuO y . In response to comments that oxides (particularlyCu 2 O) <strong>in</strong>itially present on Cu specimens at the start <strong>of</strong>their experiments were ignored, Szakalos et al. [8] claimed thatthey would have been reduced by the H 2 formed <strong>in</strong> their experiment.This argument is unconv<strong>in</strong>c<strong>in</strong>g s<strong>in</strong>ce there is considerableevidence to show that air-formed films on Cu are notoriously difficultto fully reduce, even electrochemically [13, 14].To justify their claim that the phase, H x CuO y , was energeticallypossible, Hultquist et al. [8] used molecular dynamics simulationsto obta<strong>in</strong> a free energy <strong>of</strong> formation for the reactionCu + H 2 O → CuOH + H 2<strong>of</strong> -311 kJ/mol. Such a value <strong>in</strong>dicates this reaction is energeticallyfavourable. As noted by K<strong>in</strong>g [11], this not consistent with thevalue <strong>of</strong> + 9kJ/mol calculated by Protopop<strong>of</strong>f and Marcus [15].Even if the formation <strong>of</strong> such a surface adsorbed species were energeticallypossible (Protopop<strong>of</strong>f and Marcus give a value <strong>of</strong> -228kJ/mol for the free energy <strong>of</strong> formation <strong>of</strong> Cu(OH) ads based on aconsiderable experimental database), it does not demonstrate thatthe formation <strong>of</strong> a three dimensional H x CuO y is, therefore, energeticallyfeasible.In fact, the first pr<strong>in</strong>ciples calculations <strong>of</strong> Korzhavji (presentedat the meet<strong>in</strong>g) showed HCuO to be an unstable phase withrespect to Cu 2 O. His calculations also showed that the corrosion<strong>of</strong> Cu by H 2 O, tak<strong>in</strong>g <strong>in</strong>to account the three dimensional nature <strong>of</strong>the corrosion product (as opposed to the two dimensional nature<strong>of</strong> a surface reaction) via the reactionsH 2 O + 2Cu → Cu 2 O + H 22H 2 O + 2Cu → 2CuOH + H 2were both energetically unfavourable. The values <strong>of</strong> ΔH and ΔG forthese reactions were calculated to be large and positive, leav<strong>in</strong>g littleleeway to dispute the validity <strong>of</strong> the calculations.42
Written statements by the panel membersPotential-pH DiagramsSzakalos et al. [5] then proposed a modified potential-pH diagramto <strong>in</strong>clude the H x CuO y phase, assumed to be stable over a potentialrange from the stability l<strong>in</strong>e for Cu 2 O formation to below the stabilityl<strong>in</strong>e for H 2 O. There is well documented literature based onelectrochemical measurements coupled to sensitive <strong>in</strong>-situ surfaceanalytical techniques such as surface-enhanced Raman spectroscopy(SERS) [16], atomic force microscopy (AFM) [17], glanc<strong>in</strong>g<strong>in</strong>cidence X-ray diffractometry (XRD) [18] and ellipsometry [19]demonstrat<strong>in</strong>g the formation <strong>of</strong> monolayer to sub-monolayer levels<strong>of</strong> a Cu(OH) ads species with<strong>in</strong> the thermodynamically forbiddenregion <strong>of</strong> the potential-pH diagram. Two sets <strong>of</strong> authors [15,17] have proposed modification <strong>of</strong> the standard potential-pH diagramto <strong>in</strong>clude these surface states.The calculations performed by Protopop<strong>of</strong>f and Marcus [15] areparticularly thorough and based on an extensive experimental database.They note that Cu shows an <strong>in</strong>termediate behavior betweenthe transition metals (e.g., Fe and Ni), which strongly adsorb OHand H, and the noble metals (e.g., Au and Ag) which do not.Accord<strong>in</strong>g to their calculations there is a narrow underpotentialdeposition region where adsorbed H and OH could coexist, mak<strong>in</strong>git feasible that, on the monolayer scale, the decomposition <strong>of</strong>H 2 O to produce H ads and OH ads , lead<strong>in</strong>g to the production <strong>of</strong> H 2and Cu(OH) ads could occur. This would be m<strong>in</strong>or <strong>in</strong> extent, s<strong>in</strong>ceit would be limited by the ability to form only a monolayer <strong>of</strong> whatwould be the corrosion product, Cu(OH) ads . S<strong>in</strong>ce their thoroughreview <strong>of</strong> literature could f<strong>in</strong>d no experimental evidence for theformation <strong>of</strong> H ads , they assumed it would have a bond energy comparableto that for H ads at the metal/gas <strong>in</strong>terface.Only one study [20], performed <strong>in</strong> alkal<strong>in</strong>e solutions us<strong>in</strong>gSERS, has shown evidence <strong>of</strong> the formation <strong>of</strong> Cu(OH) ads via H 2 Oreduction,CuO ads + H 2 O + e - → Cu(OH) ads + OH - (X)the key reaction accord<strong>in</strong>g to Hultquist et al. It is clear from theirRaman results that a pre-oxidized surface state (CuO ads ) is requiredfor this reaction to progress. Maurice et al. [21] could f<strong>in</strong>d no evidencefor this reaction us<strong>in</strong>g electrochemical techniques coupled to<strong>in</strong>-situ AFM. The latter authors noted that the key differencebetween these two studies was the state <strong>of</strong> the surface; electro-43
Written statements by the panel memberspolished and smooth <strong>in</strong> their case, but slightly roughened by anelectrochemical oxidation-reduction cycle (necessary to activatethe SERS signal) <strong>in</strong> the experiments <strong>of</strong> Hart<strong>in</strong>ger et al. [20]. Activation<strong>of</strong> a SERS signal requires the formation <strong>of</strong> nanosized surfaceCu particles, and the Raman results <strong>of</strong> Hart<strong>in</strong>ger et al. [21] couldbe expla<strong>in</strong>ed by the presence <strong>of</strong> CuO ads surface species present dueto the <strong>in</strong>complete reduction <strong>of</strong> the oxide formed dur<strong>in</strong>g the oxidation-reductioncycle. It was speculated than they could be subsurface,i.e., on the underside <strong>of</strong> the Cu nanoparticles.The essential feature <strong>of</strong> this discussion is the formation <strong>of</strong>nanoparticulate material dur<strong>in</strong>g the oxidation (to bulk oxide) –reduction cycle on Cu, as well as the possibility that mixed Cu oxidationstates (Cu/Cu I Cu II ) coexist on the surface. Such states areknown to be catalytic for H 2 O, but especially O 2 , reduction [22].This raises the real possibility that, <strong>in</strong> an experiment started with anoxide-covered surface and a solution conta<strong>in</strong><strong>in</strong>g dissolved O 2 , theconsumption <strong>of</strong> O 2 could lead first to the exhaustion <strong>of</strong> O 2 andthen the reduction <strong>of</strong> the oxide. If, as <strong>in</strong> the case <strong>of</strong> the Cu cool<strong>in</strong>gsystems discussed above [10], this led to the formation <strong>of</strong> partiallyreduced Cu surface species (i.e., CuO ads ) then reaction X could beactivated lead<strong>in</strong>g to the production <strong>of</strong> H 2 . Of course, such a reactionis only susta<strong>in</strong>able on nano-particulate material until availablesurface O states are consumed. An ongo<strong>in</strong>g process is only susta<strong>in</strong>ablewith a cont<strong>in</strong>u<strong>in</strong>g supply <strong>of</strong> O 2 , as <strong>in</strong>dicated <strong>in</strong> the heatexchanger studies. Such a mechanism could expla<strong>in</strong> the apparentlimited production <strong>of</strong> H2 <strong>in</strong> some <strong>of</strong> the experiments with H andS. (I am <strong>in</strong>debted to Roger Newman, University <strong>of</strong> Toronto, for thesuggestion than the “violation <strong>of</strong> thermodynamics” would require thesurface energy generated by the <strong>in</strong>volvement <strong>of</strong> nanoparticles <strong>in</strong> the 2to 3 nm size range).The <strong>Corrosion</strong> MechanismDespite the fact that dissolved O 2 will be unavoidably present, atleast <strong>in</strong>itially, <strong>in</strong> their experiments, the possibility that dissolved O 2was <strong>in</strong>volved <strong>in</strong> the corrosion <strong>of</strong> Cu was not considered byHultquist et al. Instead, O 2 was assumed to be consumed by reactionwith H atoms produced by the reduction <strong>of</strong> H 2 O. While thismechanism could apply if the metal was exposed only to the aqueousvapour phase with no condensed liquid phase present, it is at44
Written statements by the panel membersodds with the very extensive literature on the reduction <strong>of</strong> O 2 dissolved<strong>in</strong> aqueous solutions. This literature encompasses not onlystudies <strong>in</strong> corrosion science and eng<strong>in</strong>eer<strong>in</strong>g but also energy systemssuch as batteries and, <strong>in</strong> particular, fuel cells. This apparentdismissal <strong>of</strong> a very large body <strong>of</strong> published experimental evidencesuggests a fundamental misunderstand<strong>in</strong>g <strong>of</strong> the aqueous corrosionprocess. As discussed above, and <strong>in</strong> detail for Cu by K<strong>in</strong>g [11], theO 2 reduction reaction is strongly catalyzed on oxide/metal <strong>in</strong>terfaceswith mixed oxidation states (<strong>in</strong> this case Cu/Cu I /Cu II ) atpotentials where surface Cu(OH) ads species exist, but at potentialstoo positive for H 2 O reduction. The adoption <strong>of</strong> a reaction mechanismthat applies <strong>in</strong> the vapour phase ignores this well provenelectrochemical mechanism <strong>in</strong>volv<strong>in</strong>g the utilization <strong>of</strong> solutionconnectedanodes and cathodes. It would appear that this conceptualmisunderstand<strong>in</strong>g is a key premise <strong>in</strong> their <strong>in</strong>sistence that H 2 Orather than O 2 reduction dom<strong>in</strong>ates the corrosion process.Anoxic <strong>Corrosion</strong> <strong>in</strong> the Presence <strong>of</strong> Other AnionsThere is a wealth <strong>of</strong> available literature to show that many otheranions besides OH - , chemisorb on Cu surfaces. Particularlystrongly adsorbed are the halides (I - , Br - , Cl - ) [23–26] and especiallySH - . Of particular <strong>in</strong>terest <strong>in</strong> the present context are Cl - andSH - both <strong>of</strong> which are anticipated <strong>in</strong> the Swedish groundwaters towhich waste conta<strong>in</strong>ers would eventually be exposed. Theseadsorption processes occur <strong>in</strong> the potential region close to thewater reduction region. Commonly Cl - is more strongly adsorbedthan OH - and has been observed electrochemically to catalyzeH 2 O reduction to H 2 [25]. However, there is no evidence to showthat stable chloride-conta<strong>in</strong><strong>in</strong>g oxide/hydroxide phases can beformed by H 2 O reduction. On the contrary, Cl - has been shown,by <strong>in</strong>-situ STM <strong>in</strong>vestigations, to be absorbed and desorbedreversibly without <strong>in</strong>terfer<strong>in</strong>g with the surface structure <strong>of</strong> Cu[24]. Phase formation only occurs at considerably more positivepotentials where H 2 O reduction to H 2 is thermodynamicallyimpossible.Despite these published studies, Szakalos (<strong>in</strong> correspondenceafter the workshop) has claimed that the Cu hydroxyl chloride,paratacamite (Cu 2 (OH) 3 Cl), conta<strong>in</strong><strong>in</strong>g Cu II ) can form by waterreduction under anoxic conditions. Offered <strong>in</strong> support <strong>of</strong> this45
Written statements by the panel membersclaim is a statement from the UK Environment Agency Report[27] claim<strong>in</strong>g this is the case. This statement is not referenced <strong>in</strong>the report. Also <strong>of</strong>fered <strong>in</strong> support <strong>of</strong> this claim is evidence from astudy <strong>of</strong> historical artefacts from which it is concluded that paratacamiteis more stable than Cu 2 O [28]. This claim is unjustified(and not made by the authors <strong>of</strong> the paper). Secondly, the conclusionsdrawn by Szakalos (that the evidence <strong>in</strong> this paper justifieshis claim that paratacamite, and hence all the corrosion products <strong>in</strong>the 5-year LOT exposure test [29], were formed under anoxic conditions)are the opposite <strong>of</strong> those drawn by the authors. It is worthquot<strong>in</strong>g the conclusions from the paper <strong>of</strong> Domenech-Carbo et al.[28]: “Thus “green’ samples <strong>of</strong> C1-13 and C1-14 <strong>in</strong> which a significantamount <strong>of</strong> CuCl accompanies copper trioxychloride can tentativelybe attributed to a corrosion process under wet aerobicconditions, while samples C1-9 conta<strong>in</strong><strong>in</strong>g copper trioxyhydoxychloride(a category which <strong>in</strong>cludes paratacamite) plus malachiteprobably corresponds to a region <strong>of</strong> the buried helmet <strong>in</strong> contactwith wet oxygenated soil, then exposed to relatively high carbonateconcentrations. F<strong>in</strong>ally “reddish” deposits <strong>in</strong> sample C1-11 areformed by cuprite. <strong>Copper</strong> trihydroxychlorides accompany cuprite<strong>in</strong> sample C1-12 while CuCl is absent. These features suggest thatthe corrosion process <strong>in</strong> this portion <strong>of</strong> the helmet occurred <strong>in</strong> arelatively dry and aerobic environment.” One can only concludethat the observations and conclusions <strong>in</strong> this paper strongly supportthe claims made <strong>in</strong> the 5-year LOT report [28] that theoxide/hydroxide/hydroxychloride phases observed are the products<strong>of</strong> Cu corrosion <strong>in</strong>volv<strong>in</strong>g O 2 reduction.The only anion for which there is thermodynamic and experimentalevidence to show it can lead to the formation <strong>of</strong> bulk corrosionproducts by H 2 O reduction is SH - /S 2- . The available <strong>in</strong>formationhas been comprehensively calculated, reviewed and discussed<strong>in</strong> SKB reports [30-32]. More recent studies have shown that thecorrosion product is exclusively Cu x S (with x between 1.8 and 2),with no evidence for the simultaneous formation <strong>of</strong>oxide/hydroxide/hydroxychloride corrosion products under anoxicconditions [33–36]. Even <strong>in</strong> solutions conta<strong>in</strong><strong>in</strong>g 5 mol/L chloridethere was no evidence for any product other than Cu x S [37].46
Written statements by the panel membersSUMMARY AND CONCLUSIONS• A review <strong>of</strong> the literature provided for this workshop, the presentationsmade at the workshop, and a personal search <strong>of</strong> additionalliterature shows there is no evidence that significant corrosion<strong>of</strong> Cu can be susta<strong>in</strong>ed by water reduction.• The claims that H x CuO y and/or Cu 2 (OH) 3 Cl are formed as stablecorrosion products by the anoxic corrosion <strong>of</strong> copper cannotbe justified. In the case <strong>of</strong> H x CuO y no experimental characterizationis available and theoretical calculations show thephase to be unstable. In the case <strong>of</strong> Cu 2 (OH) 3 Cl the publishedliterature shows it is a product <strong>of</strong> the aerobic, not the anoxic,corrosion <strong>of</strong> copper. Its observation <strong>in</strong> long-term tests <strong>in</strong> whichthe exclusion <strong>of</strong> oxygen cannot be guaranteed is to be expected.• It may be possible that the reduction <strong>of</strong> oxides/hydroxides presentat the start <strong>of</strong> experiments could lead to nanoparticulatecopper able to temporarily support water reduction and theproduction <strong>of</strong> hydrogen. However, for such a process to lead tothe accumulation <strong>of</strong> mean<strong>in</strong>gful corrosion damage wouldrequire the presence <strong>of</strong> dissolved oxygen at least <strong>in</strong>termittently.• There is evidence to show that the ma<strong>in</strong>tenance <strong>of</strong> low levels <strong>of</strong>dissolved oxygen (50 ppb to 2 ppm) could lead to much moreextensive corrosion than lower or higher oxygen levels. This<strong>of</strong>fers a potential explanation for the extensive corrosionobserved by Hultquist et al. <strong>in</strong> one <strong>of</strong> their 15-year experiments.• The only anionic species for which there is thermodynamic andexperimental evidence to show it can susta<strong>in</strong> the anoxic corrosion<strong>of</strong> copper is sulphide. In this case the stability <strong>of</strong> the corrosionproduct (Cu x S; 1.8 < x ≤ 2) is well characterized.• It can be concluded that the anoxic corrosion <strong>of</strong> Cu can only besusta<strong>in</strong>ed <strong>in</strong> the presence <strong>of</strong> sulphide.REFERENCES1. G. Hultquist, Corros. Sci., 26, 173 (1986)2. M. Seo, G. Hulquist, L. Grasjo and N. Sato, Proc 10 th Int. Corr.Congress, Paper 2.31, 481 (1987)47
Written statements by the panel members3. G. Hulquist, G.K. Chuah, K.I. Tan, Corros. Sci., 29, 1371(1989)4. L. Grasjo, G. Hultquist, Q. Lu and M. Seo, Mats Sci. Forum,185–188, 703 (1995)5. P. Szakalos, G. Hultquist and G. Widmark, Electrochem. SolidState Letts, 10, C63, (2007)6. P. Szakalos, G. Hultquist and G. Widmark, Electrochem. SolidState Letts, 11, S2 (2008)7. G. Hultquist, P. Szakalos, M.J. Graham, G.I. Sproule and G.I.Widmark, Proc. 17 th Int. Corr. Congress, Oct 6–10,2008, LasVegas, NV, USA, Paper 3884 (2008)8. G. Hultquist, P. Szakalos, M.J. Graham, A.B. Belonoshko, G.I.Sproule, L. Grasjo, P. Dorogokupets, B. Danilov, T. Aastrup, G.Widmark, G.K. Chuah, J.C. Eriksen and A. Rosengren, Catal.Lett., DOI 10.1007/s10562-009-0113-x, published onl<strong>in</strong>e; 28July (2009)9. Al. Kharafi, I.M. Ghayad and B.G. Ateya; Electrochem.SolidState Letts, 11, G15 (2008)10. B.S. Park, I.S. Hwang, I.H. Rhee, K.-T. Kim, B.C. Syrett and J.Ste<strong>in</strong>; <strong>Corrosion</strong>, 61, 559 (2005)11. F. K<strong>in</strong>g, Critical Review <strong>of</strong> the Literature on the <strong>Corrosion</strong> <strong>of</strong><strong>Copper</strong> by Water, report supplied prior to the meet<strong>in</strong>g12. C.N. Panagopoulos and N. Zachavopoulos; J. Mater. Sci., 29,3843 (1994)13. U. Bertocci, Electrochim. Acta, 49, 1831 (2004)14. M.V. Vazquez, S.R. de Sanchez, E.J. Calvo and D.J. Schiffr<strong>in</strong>, J.Electroanal. Chem., 374, 179 (1994)15. E. Protopop<strong>of</strong>f and P. Marcus; Electrochimica Acta, 51, 408(2005)16. H.Y. Chan, C.G. Takoudis and M.J. Weaver, J.Phys. Chem. B,103, 357 (1999)17. J.R. LaGraff and A.A. Gewirth, Surf. Sci., 326, L461 (1995)18. Y.S. Chu, I.K. Rob<strong>in</strong>son and A.A. Gewirth, J. Chem. Phys. 110,(1999)19. J.M.M. Droog, C.A. Alderliesten, P.T. Alderliesten and G.A.Bootsma, J. Electroanal. Chem., 111, 61 (1980)20. S. Hart<strong>in</strong>ger, B. Pett<strong>in</strong>ger and K. Doblh<strong>of</strong>er, J. Electroanal.Chem., 397, 335 (1995)21. V. Maurice, H.-H. Strehblow and P. Marcus, Surf. Sci., 458, 185(2000)48
Written statements by the panel members22. E.J.M. O’Sullivan and E.J. Calvo, In “Comprehensive ChemicalK<strong>in</strong>etics”, Vol.27, “Electrode K<strong>in</strong>etics”, editor, R.G. Compton,Elsevier, Amsterdam, 247 (1987)23. B. Obliers, M. Anastasescu, P. Brockman and K. Wandelt, Surf.Sci., 573, 47 (2004)24. P. Brockman, M. Wilms, M. Kruft, C. Stuhlmann and K.Wandelt, J. Electroanal. Chem., 467, 307 (1999)25. B. Wohlmann, Z. Park, M. Kruft, C. Stuhlmann and K. Wandelt,Colloids and surfaces A, 134, 15 (1998)26. D. Irish, L. Stohlberg and D.W. Shoesmith, Surf. Sci. 158, 238(1985)27. Environment Agency report on technical issues associated withdeep reposditories for radioactive waste <strong>in</strong> different environments,Science report: SC060054/SR1 (2009);http://publications.environmentagency.gov.uk/pdf/SCHO0809BQVU-e-e.pdf28. A. Donenech-Carbo, M.T. Domenech-Carbo and I. Mart<strong>in</strong>ez-Lazaro, Microchim. Acta 162, 351 (2008)29. O Karnland, S.I. Olsson, A.I. Dueck, M.I. Birgersson, U.I.Nilsson, T.I. Hernan-Hakansson, K. Pedersen, S. Nilsson, T.E.Eriksen and B. Rosborg; “Long term test <strong>of</strong> Buffer Material atthe Aspo Hard Rock laboratory, LOT Project. F<strong>in</strong>al Report onthe A2 Test Parcel”, SKB TR-09-29 (2009)30. I. Puigomenech and C. Taxen; “Thermodynamic Data for <strong>Copper</strong>:Implications for the <strong>Corrosion</strong> <strong>of</strong> <strong>Copper</strong> under RepositoryConditions”, SKB TR-00-13 (2000)31. F. K<strong>in</strong>g, L. Ahonen, C. Taxen, U. Vuor<strong>in</strong>en and L. Werme,“<strong>Copper</strong> <strong>Corrosion</strong> under Expected Conditions <strong>in</strong> a DeepGeologic Repository”; SKB TR-01-2332. F. K<strong>in</strong>g, “<strong>Corrosion</strong> <strong>of</strong> <strong>Copper</strong> <strong>in</strong> Alkal<strong>in</strong>e Chloride <strong>Environments</strong>”,SKB TR-02-25 (2002)33. J.M. Smith, Z. Q<strong>in</strong>, L. Werme and D.W. Shoesmith; Mat. Res.Soc. Symp. Proc. Vol. 932,869 (2006)34. J.M. Smith, Z. Q<strong>in</strong>, F. K<strong>in</strong>g, L. Werme and D.W. Shoesmith;<strong>Corrosion</strong>, 63, 135 (2007)35. J.M. Smith, J.C. Wren, M. Odziemkowski and D.W. Shoesmith;J. Electrochem. Soc.; 154, C431 (2007)36. J.M. Smith, Z. Q<strong>in</strong> and D.W. Shoesmith; Proc.17 th Int. Corr.Congress, Oct 6–10, 2008, Las Vegas, NV, USA (2008)49
Written statements by the panel members37. J.M. Smith, Z. Q<strong>in</strong>, F. K<strong>in</strong>g and D.W. Shoesmith, Proc. Of theWorkshop on Sulphur-Assisted <strong>Corrosion</strong> <strong>of</strong> Nuclear WasteDisposal Systems. Brussels, Belgium, Oct 21–23 (2008)50
Edited transcripts from the workshop51
Edited transcripts from the workshop52
Edited transcripts from the workshop1. Questions, hypotheses and facts related to corrosion<strong>of</strong> copper <strong>in</strong> water, by Associate Pr<strong>of</strong>essor GunnarHultquist, Royal Institute <strong>of</strong> Technology, KTHThe Swedish method <strong>of</strong> copper conta<strong>in</strong>ment <strong>of</strong> spent nuclear fuelwas launched <strong>in</strong> early 1980s dur<strong>in</strong>g political turmoil <strong>in</strong> Sweden andthe model was claimed to be based on proven science. The methodwas also <strong>in</strong>tended to be used by other countries. At that time a planfor the f<strong>in</strong>al disposal <strong>of</strong> spent nuclear fuel had to be presentedbefore any new power plant could be commissioned. One millionyears <strong>of</strong> safe conta<strong>in</strong>ment was orig<strong>in</strong>ally required, but today a life<strong>of</strong> “only” 100,000 years is claimed with a canister thickness <strong>of</strong> 50mm <strong>of</strong> copper. Even with this thickness, a risk <strong>of</strong> local attack isconsidered to exist <strong>in</strong> the Swedish method, and general corrosion isassumed to be zero <strong>in</strong> the presence <strong>of</strong> pure water.The KBS-3 method was developed 30 years ago and consists <strong>of</strong> acopper canister surrounded by clay deposited 500 m down <strong>in</strong> theSwedish bedrock. It differs from other countries’ concepts <strong>in</strong> thatit assumes thermodynamic immunity <strong>in</strong> water, i.e. copper isassumed to withstand water even at elevated temperatures. But,says Hultquist, this assumption <strong>of</strong> immunity is disputed.<strong>Corrosion</strong> reactionsHultquist presents the ma<strong>in</strong> corrosion reactions that take place <strong>in</strong>the repository environment:Cu + O 2 (dissolved <strong>in</strong> ground water) ⇒ Cu oxideWhen the oxygen is consumed:2) Cu + sulphide ions <strong>in</strong> water ⇒ Cu sulphide3) Cu + water molecules ⇒ Cu hydroxides +H 2 +hydrogen <strong>in</strong> CumetalHe says that there is a consensus regard<strong>in</strong>g reactions 1 and 2.Reaction 3, however, is disputed and is the topic <strong>of</strong> discussion <strong>of</strong>this sem<strong>in</strong>ar.“What happens with copper <strong>in</strong> water?” he asks, and answers:53
Edited transcripts from the workshop“We have proved that we have a reaction product, which is solidand porous. Molecular hydrogen can be measured <strong>in</strong> the gas phase.We also have hydrogen <strong>in</strong> the metal, which can be measured.”The question is how copper is corroded by pure water?In Hultquist´s experiment, the pressure <strong>of</strong> hydrogen is 10 -3 bar at atemperature <strong>of</strong> 20–80°C. S<strong>in</strong>ce the natural H 2 pressure <strong>in</strong> air is 5 • 10 -7 bar, and Hultquist measures a higher hydrogen pressure <strong>in</strong> theexperiment, it follows that copper is corroded by water. He shows:(H 2 O ⇒ OH - + H + ) ⇒ CuOH + H 2 + hydrogen <strong>in</strong> Cu where H 2≈ 10 -3 bar at 20–80°C.Hultquist and his colleagues have been criticized for the waythey have dealt with the issue <strong>in</strong> a Pourbaix diagram. In response tothis criticism, he says that the hydrogen content <strong>of</strong> the air must beconsidered. In the diagram, the hydrogen pressure is normally setto approximately zero and this, he argues, does not represent thecomplete truth. In the Pourbaix diagram presented by Hultquist,this “new” situation is shown by displac<strong>in</strong>g the “hydrogen borderl<strong>in</strong>e”and thereby adjust<strong>in</strong>g it to the actual hydrogen concentration<strong>in</strong> air.If copper is immersed <strong>in</strong> water, a solid product can be observedto form. A by-product is also created by this: hydrogen <strong>in</strong> atomicform, not molecular, which can react <strong>in</strong> different ways. Either thehydrogen is absorbed <strong>in</strong>to the copper metal or it is accumulatedand exerts a pressure <strong>in</strong> the experimental glass jar.To demonstrate the experiment theoretically, Hultquist calculatesthe bond between copper and, <strong>in</strong> this case, OH, which isalways present <strong>in</strong> water. First pr<strong>in</strong>ciples simulation results <strong>in</strong> thestrong bond<strong>in</strong>g <strong>of</strong> an OH-group to a Cu-(100) surface, lead<strong>in</strong>g tothe formation <strong>of</strong> a 3-dimensional copper hydroxide.“Depend<strong>in</strong>g on the strength <strong>of</strong> the bond, we th<strong>in</strong>k this showsthat a three-dimensional growth <strong>of</strong> monovalent copper hydroxideis possible.”Experiments <strong>in</strong> practiceThe figure below shows that hydrogen produced by copper corrosionat room temperature exceeds the exist<strong>in</strong>g hydrogen pressure.“Well, it is up to you if you believe that this difference <strong>in</strong> copperco<strong>in</strong> size is due to reaction with sulphide,” says Hultqvist. “I don’t54
Edited transcripts from the workshopbelieve it myself, because copper sulphide has extremely low solubility<strong>in</strong> air.”Figure xHydrogen from copper corrosion at room temperature exceedsthe exist<strong>in</strong>g hydrogen pressure.Hultquist presents data from the experiment and says that it is asimple but carefully planned set-up, where materials and jo<strong>in</strong>tsneed to be corrected. The experiment is based on well-knownultra-high-vacuum components. Two sta<strong>in</strong>less steel parts are used.In the upper one, which is <strong>in</strong>itially evacuated, the hydrogen pressureis measured. It has a membrane <strong>of</strong> palladium that only permitsthe passage <strong>of</strong> hydrogen.When the experiment is started, the air conta<strong>in</strong>ed <strong>in</strong> the water isremoved as quickly as possible by pump<strong>in</strong>g. The available counteract<strong>in</strong>gpressure <strong>of</strong> H 2 for copper corrosion by water, 5 • 10 -7 bar, is<strong>in</strong>dicated on the slide. After some time, the hydrogen pressure <strong>in</strong>the experiment builds up and exceeds 5 • 10 -7 bar. The question isnow how far the corrosion process cont<strong>in</strong>ues.To ensure that the hydrogen evolves from corrosion, Hultquistevacuates the hydrogen from the system, seals it and permits theprocess to start aga<strong>in</strong>. The pressure rises to 1 millibar, where it isshown that the corrosion process levels out and stops. No morehydrogen is evolved above this pressure. The higher pressure that55
Edited transcripts from the workshophas evolved equalizes <strong>in</strong> the whole system, <strong>in</strong> both the lower andupper parts <strong>of</strong> the experimental set-up.An illustration <strong>of</strong> another experiment shows the two glass vesselswhere copper foils <strong>in</strong> pure oxygen-free water have been keptfor 15 years. In one glass vessel, the H 2 was not evacuated due toits membrane <strong>of</strong> plat<strong>in</strong>um. In the other one, with a membrane <strong>of</strong>palladium, H 2 was actually removed. The latter shows signs <strong>of</strong> corrosion:the foils have turned black. Actually, different k<strong>in</strong>ds <strong>of</strong> corrosioncan be seen.Hultquist argues that if you remove H 2 , which nature alwaysdoes, we must expect this to happen. It is not enough to have copperisolated from molecular oxygen or air.The researchers at KTH also analyzed hydrogen that has diffused<strong>in</strong>to the metal. Hydrogen is transported everywhere, it is justa matter <strong>of</strong> time, Hultquist says. They have published pro<strong>of</strong>s, us<strong>in</strong>gtwo methods: Hydrogen was detected both <strong>in</strong> secondary ion massspectrometry and <strong>in</strong> a quantitative study on out-gass<strong>in</strong>g <strong>in</strong> vacuum.Secondary ion mass spectrometry is sensitive to hydrogen. Theresult is sometimes difficult to <strong>in</strong>terpret, but Hultquist states thatthis is not a reason not to use it.Evidence <strong>of</strong> copper corrosion by water has been verified byvarious experiments with results such as: hydrogen formation,<strong>in</strong>crease <strong>in</strong> weight, hydrogen <strong>in</strong> the copper metal, chemical analysis<strong>of</strong> the corrosion product, visual <strong>in</strong>spection and metallographicexam<strong>in</strong>ation.Hultquist shows a picture <strong>of</strong> two copper co<strong>in</strong>s from the warshipWasa that were exposed to water for over 330 years. DagensIndustri published this picture <strong>in</strong> 1984. “Well, it is up to you if youbelieve that this difference <strong>in</strong> copper co<strong>in</strong> size is due to reactionwith sulphide. I don’t believe it myself, because copper sulphidehas extremely low solubility.”Hultquist published evidence <strong>of</strong> copper corrosion back <strong>in</strong> 1986<strong>in</strong> a peer-reviewed journal (<strong>Corrosion</strong> Science), but it was not until2009, after new publications, that the scientific debate started <strong>in</strong>earnest. One hypothesis that is consistent with all experimentalobservations is that copper is corroded by water itself, whichmeans that copper corrosion takes place even without molecularoxygen. We cannot blame molecular oxygen all the time.56
Edited transcripts from the workshopImplications and suggestionsHultquist illustrates what might happen to copper if it wereexposed to water for a very long time, such as 100.000 years. After1,000 years the general corrosion depth could be 10 mm.“I don’t say that this will be the case. But there is a substantialrisk that this could happen. In my op<strong>in</strong>ion we have to try to f<strong>in</strong>dsome solution to this problem.”2. Thermodynamics and k<strong>in</strong>etics <strong>of</strong> copper corrosion <strong>in</strong>oxygen-free water, by Peter Szakálos, Royal Institute<strong>of</strong> Technology, KTHPeter Szakálos concludes that it was already known 30 years agothat copper was not thermodynamically immune <strong>in</strong> pure O 2 -freewater, and that this fact is undisputed among thermodynamicexperts. In fact, this contention is disputed with<strong>in</strong> the scientificcommunity. The research done by the KTH group does not changethe known thermodynamics <strong>of</strong> water corrosion <strong>of</strong> copper. Theresults can be expla<strong>in</strong>ed by the formation <strong>of</strong> an amorphous copperhydroxide. Several scientific publications suggest the existence <strong>of</strong>different amorphous hydroxides, both monovalent and bivalent,which can easily be converted to oxides. This makes them moredifficult to study.Szakálos has looked at papers that SKB usually refers to whencompany representatives argue that the KTH group’s f<strong>in</strong>d<strong>in</strong>gs arewrong (see Christ<strong>in</strong>a Lilja, who spoke before Szakálos at the sem<strong>in</strong>ar).Two were published <strong>in</strong> <strong>Corrosion</strong> Science, <strong>in</strong> 1987 and 1989,and both <strong>of</strong> the experiments had O 2 , oxygen gas, <strong>in</strong> the setup, andit was easily seen. One experiment was performed with nitrogengas, which always conta<strong>in</strong>s a ppm level <strong>of</strong> oxygen. Szakálos statesthat no one today would use that k<strong>in</strong>d <strong>of</strong> experiment for theseexposures.In the other paper, an oxygen peak was detected.“This is important. Only one or attempt has been made dur<strong>in</strong>gthe last 23 years to repeat Gunnar Hultquist´s <strong>in</strong>itial experiment.The experiment was carried out at the Swedish National Test<strong>in</strong>gand Research Institute, SP, <strong>in</strong> Borås and the results were <strong>in</strong>correctlydescribed as show<strong>in</strong>g that Hultquist´s observations couldnot be repeated. The truth is that the only experiment that was car-57
Edited transcripts from the workshopried out accord<strong>in</strong>g to his <strong>in</strong>structions did <strong>in</strong> fact <strong>in</strong>dicate that coppercorrodes <strong>in</strong> O 2 -free water. However, no follow-up was everdone, Szakálos says.It has been claimed that copper canisters should be corrosionresistants<strong>in</strong>ce native copper is found at a few locations <strong>in</strong> theworld. However, Szakálos argues, the situation is the same fornative iron and nickel, z<strong>in</strong>c, etc, but no one uses this argument toclaim that iron is corrosion-resistant <strong>in</strong> groundwater conta<strong>in</strong><strong>in</strong>gchlorides, sulphides, sulphates and methane/acetate, etc.There has also been a lot <strong>of</strong> discussion about “archaeologicalanalogues” recently. It has been stated <strong>in</strong> the news that bronzecannons from the warship Kronan, wrecked 1678 and retrieved <strong>in</strong>1986, are good objects to study, because the environment is“astonish<strong>in</strong>gly similar to what the copper canisters will be exposedto,” <strong>in</strong> other words the environment <strong>in</strong> Swedish repositories.Szakálos questions the word “environment”. What is it?“It refers to the sediment <strong>of</strong> the Baltic Sea. It is assumed thatthe sediment will be O 2 -free and conta<strong>in</strong> brackish water. A lot <strong>of</strong>studies have been conducted on these bronze cannons. But theproblem is, <strong>of</strong> course, that corrosion <strong>of</strong> bronzes differs fundamentallyfrom that <strong>of</strong> copper. An accumulation <strong>of</strong> passivat<strong>in</strong>g t<strong>in</strong> formson the bronze surface, which greatly reduces the corrosion rate <strong>in</strong>aqueous environments. It was documented 30 years ago that a layer<strong>of</strong> t<strong>in</strong> was found on the cannons. The corrosion rate on these cannonsis around 1,000 times slower than you can expect from copper,which is confirmed by these reports. A possible explanationfor why mar<strong>in</strong>e copper artefacts have not been used as “archaeologicalanalogues” might be that they have corroded too much andobviously much more than bronze artefacts.“Lots <strong>of</strong> copper artefacts have been found <strong>in</strong> the Baltic Seasediment, but they are <strong>in</strong> quite bad conditions. If that is your analogue,you should be nervous.”He displays a copper compass from the same warship withsevere corrosion.“This represents a different corrosion process. Here it is obviouslya sulphide situation. And the <strong>in</strong>terest<strong>in</strong>g part is that the copperco<strong>in</strong>s on “Kronan” were more corroded than the “Wasa” co<strong>in</strong>s,and several “Kronan” co<strong>in</strong>s had no rema<strong>in</strong><strong>in</strong>g metal core left.”A document published <strong>in</strong> 1965 shows a study by Ol<strong>of</strong>Arrhenius, the son <strong>of</strong> the famous Nobel Prize w<strong>in</strong>ner SvanteArrhenius. He made a corrosion study <strong>of</strong> about 3,000 copper co<strong>in</strong>s58
Edited transcripts from the workshopfrom the Wasa warship. He concluded that copper does corrode tothe same extent as Hultquist reported <strong>in</strong> 1984 and 2009. Thesek<strong>in</strong>ds <strong>of</strong> mar<strong>in</strong>e copper artefacts have never been studied as “analogues”for the KBS-3 model. Instead, SKB and others have studiedbronze.Accord<strong>in</strong>g to Szakálos, the KTH group has discovered hydrogenthroughout the system <strong>in</strong> the course <strong>of</strong> their corrosion studies:<strong>in</strong> the metal, <strong>in</strong> the corrosion product, and possibly dissolved <strong>in</strong>water and hydrogen <strong>in</strong> the gas phase.<strong>Copper</strong> corrosion <strong>in</strong> O 2 -free water is a well known problem <strong>in</strong><strong>in</strong>dustry. All cool<strong>in</strong>g systems for power generators and accelerators,such as CERN <strong>in</strong> Switzerland, corrode several micrometersper year. This occurs <strong>in</strong> closed systems (no contact with air) conta<strong>in</strong><strong>in</strong>gwater that is deionized and degassed. The <strong>in</strong>dustry tries toreduce these corrosion rates and to achieve oxygen-tight metal fitt<strong>in</strong>gs,such as UHV fitt<strong>in</strong>gs. Nevertheless, the corrosion rate stillreaches micrometers per year.He illustrates the problem with partial plugg<strong>in</strong>g <strong>of</strong> the cool<strong>in</strong>gsystems by corrosion products such as oxides and hydroxides.These systems clog with<strong>in</strong> a few years’ time. The environmentmakes the copper hot, around 70° to 90° degrees, the same temperaturethat the copper canister will atta<strong>in</strong>. In the <strong>in</strong>dustrial systemsthere is, <strong>of</strong> course, no groundwater, the water be<strong>in</strong>g much lessaggressive, i.e. pure water.Another <strong>in</strong>dicative example is an <strong>in</strong>vestigation cover<strong>in</strong>g fiveyears <strong>of</strong> the copper corrosion rates <strong>in</strong> Swedish clay and soils, forexample anoxic clay and sulphide clay. The redox potentials wererecorded for some Swedish sites, at least some <strong>of</strong> which wereregarded as O 2 -free, basically due to biological activity; bacteriaconsume the available oxygen. The corrosion rates are still quitehigh, aga<strong>in</strong> <strong>in</strong> the micrometer per year range, <strong>of</strong>ten between 10 and20 micrometers, despite the low average temperature <strong>of</strong> theSwedish soil.Szakálos gives examples <strong>of</strong> measured corrosion rates <strong>in</strong> differentstudies by SKB as well as <strong>in</strong> studies <strong>in</strong> other countries. SKB and theF<strong>in</strong>nish nuclear power <strong>in</strong>dustry assume very low corrosion rates <strong>in</strong>their calculations, for example 0.33 mm <strong>in</strong> one million years.“This is equivalent to a corrosion rate <strong>of</strong> 0.33 nanometer peryear, or put another way, only two copper atom layers per year (!).This is an astonish<strong>in</strong>gly low corrosion rate. Not even titanium or59
Edited transcripts from the workshopthe best high-alloy materials could ever reach this range. TheJapanese use rates that are closer to what we believe to be true.”He demonstrates that the KTH researchers, as well as otherresearchers, arrive at corrosion rates <strong>in</strong> the micrometer range.“Some examples: Our results and a lot <strong>of</strong> other researchers’results are <strong>in</strong> the micrometer per year range. Whatever you do withthe water environment, you cannot avoid those copper corrosionrates. Still we have SKB´s safety analysis <strong>in</strong>dicat<strong>in</strong>g a corrosion rate<strong>of</strong> 3 ångströms per year. This value is totally new for me; they previouslyclaimed around 3 nm. But even 3 nm per year is roughly1,000 to 10,000 times lower than the corrosion rates measured byus and SKB and reported <strong>in</strong> other <strong>in</strong>vestigations. If you believe <strong>in</strong>an Ångström-level corrosion rate, then it could be up to 100,000times lower than measured corrosion rates, see for <strong>in</strong>stance thecorrosion rates <strong>in</strong> Japanese groundwater/deep repository.”Szakálos refers to a study <strong>of</strong> copper corrosion <strong>in</strong> bentonite 1 . Thetwo-year study <strong>in</strong>dicates that the corrosion rate is quite high. Thecorrosion rate starts to level <strong>of</strong>f after two years, but the rate is stillabout 10 to 20 μm per year. It <strong>in</strong>volves a dissolution-precipitationprocess <strong>in</strong> contact with bentonite clay. The bentonite participatesactively <strong>in</strong> the corrosion process.“You have a solubility <strong>of</strong> copper <strong>in</strong> the bentonite pore water. Itprecipitates there as corrosion products, which is shown <strong>in</strong> thestudy. It is also stated that the oxygen transport was not ratelimit<strong>in</strong>g.The corrosion did cont<strong>in</strong>ue <strong>in</strong>dependently <strong>of</strong> the oxygenflux. This is not a surprise for us at KTH.”He reads from the report: “Precipitation <strong>of</strong> copper <strong>in</strong>evitablyoccurred <strong>in</strong> all <strong>of</strong> the tests, with usually more than half <strong>of</strong> the totalcopper corroded be<strong>in</strong>g <strong>in</strong> the form <strong>of</strong> precipitate, rather than be<strong>in</strong>gsorbed on the clay”.“I guess that it was <strong>in</strong>itially thought that some copper could besorbed on the clay particles. But we can see that it is really corrosionwe’re talk<strong>in</strong>g about here.”Significant corrosion is also confirmed by the LOT project <strong>in</strong>the Äspö laboratory, 500 meters down <strong>in</strong> the ground, under realisticconditions as can be seen <strong>in</strong> appendix 6 to the LOT report (<strong>of</strong>ficialSKB report published <strong>in</strong> November 2009). The German1<strong>Corrosion</strong> Science, Vol 33, No 12, pp. 1979–1995, 1992 Pr<strong>in</strong>ted <strong>in</strong> Great Brita<strong>in</strong> AMechanistic Study <strong>of</strong> the Uniform <strong>Corrosion</strong> <strong>of</strong> <strong>Copper</strong> <strong>in</strong> Compacted Na-Montmorillonite/Sand Mixtures, F. K<strong>in</strong>g, C.D. Litke and S.R. Ryan.60
Edited transcripts from the workshopresearchers 2 f<strong>in</strong>d an astonish<strong>in</strong>gly high copper content <strong>in</strong> the bentoniteimmediately adjacent to the copper tube. A significant fraction<strong>of</strong> the copper <strong>in</strong> the bentonite is precipitated <strong>in</strong> the form <strong>of</strong>copper and copper-iron-sulphides, represent<strong>in</strong>g a corrosion rate <strong>of</strong>about 4 micrometers per year. Consider<strong>in</strong>g the total amount <strong>of</strong>copper <strong>in</strong> the corrosion product, bentonite and groundwater, it ismost likely that the copper corrosion rate is at least 10 micrometersper year, not tak<strong>in</strong>g pitt<strong>in</strong>g corrosion <strong>in</strong>to account. This is stillvery high.Szakálos refers to work with the MICROBE laboratory <strong>in</strong>Äspö, because he believes it is <strong>in</strong>terest<strong>in</strong>g background <strong>in</strong>formation.The Swedish groundwater conta<strong>in</strong>s lots <strong>of</strong> sulphates, which <strong>in</strong> therepository will be converted to sulphides by bacterial activity.Acetate, produced by dissolved carbon dioxide and dissolvedhydrogen gas, also occurs. The hydrogen gas could also be consumedby bacteria with formation <strong>of</strong> acetate. This activity affectsthe chemistry and redox potential <strong>of</strong> the groundwater as well ascopper corrosion. Furthermore, acetate and sulphide are known tocause stress corrosion crack<strong>in</strong>g <strong>in</strong> copper metal. There is a balancebetween sulphate and sulphide, ma<strong>in</strong>ly regulated by bacterial activity.The addition <strong>of</strong> copper to the system, which acts as a sulphides<strong>in</strong>k (CuS formation), causes more sulphate to be converted tosulphide by bacterial activity, accelerat<strong>in</strong>g the sulphide-<strong>in</strong>ducedcopper corrosion. <strong>Copper</strong> corrosion is consequently accelerated bythe precipitation <strong>of</strong> copper sulphides and copper-iron sulphides <strong>in</strong>the bentonite. The SKB LOT project confirms that the precipitation<strong>of</strong> those corrosion products cont<strong>in</strong>ues on the bentonite clayparticles.Szakálos has constructed a corrosion model expla<strong>in</strong><strong>in</strong>g theobservations from the LOT project, see Figure 1.2Bundesanstalt für Geowissenschaften und Rohst<strong>of</strong>fe.61
Edited transcripts from the workshopFigure 1<strong>Corrosion</strong> model expla<strong>in</strong><strong>in</strong>g the observations from the LOTproject where copper was exposed <strong>in</strong> groundwater-saturatedbentonite<strong>Copper</strong> solubility <strong>in</strong> sal<strong>in</strong>e water at 80°C: 2300μg/L (POSIVA 2003:45)Cu oxides, mostlyCu 2 O(LOT, Rosborg)Cu hydroxides,mostly Cu hydroxidechlorides(LOT, Rosborg)CuS and (Cu,Fe) sulphidesprecipitated irreversibly on thebentonite particles(LOT, BGR <strong>in</strong> Berl<strong>in</strong>)He describes that the sulphides precipitate <strong>in</strong> a gradient, whichmeans that the sulphide activity decreases to a very low level, possiblydown to zero, but at least very low at the canister surface. Theredox potential <strong>in</strong>creases if the sulphide level is low. The result isformation <strong>of</strong> copper hydroxides and copper oxides, which is confirmedby, for <strong>in</strong>stance, Rosborg (SKB LOT report Nov. 2009).This means that water molecules and chloride ions control coppercorrosion at the canister surface. However, corrosion proceeds atseveral places simultaneously, as shown <strong>in</strong> the figure. <strong>Copper</strong> ionsolubility is found to be several orders <strong>of</strong> magnitudes higher thanthermodynamically expected, especially <strong>in</strong> salt water at elevatedtemperatures. So copper “dissolves” <strong>in</strong> the bentonite pore waterand precipitates as different corrosion products on the bentoniteparticles. In presence <strong>of</strong> sulphide, the precipitation will be dom<strong>in</strong>atedby CuS, as shown <strong>in</strong> the figure. Bentonite represents a hugeactive surface for precipitation <strong>of</strong> copper corrosion products, thusdriv<strong>in</strong>g the corrosion process forward.62
Edited transcripts from the workshop“Here we have anoxic, or oxygen-free, corrosion, but we stillget hydroxides and oxides. I th<strong>in</strong>k that it is our contribution to abetter understand<strong>in</strong>g <strong>of</strong> the corrosion processes that occur <strong>in</strong> suchenvironments and <strong>of</strong> what happens with the copper canister. Ourresearch shows that <strong>in</strong> pure anoxic water, you still have corrosion.”There are also other processes, such as <strong>in</strong>tergranular corrosion,which is a more aggressive type <strong>of</strong> corrosion at the gra<strong>in</strong> boundaries<strong>in</strong> the copper. This is documented by a study <strong>in</strong> Japan <strong>in</strong> arealistic environment (anoxic). The same phenomenon also occurs<strong>in</strong> an oxic environment. It was noted back <strong>in</strong> 2003 that coppermaterials conta<strong>in</strong><strong>in</strong>g phosphorus have been found to be highly susceptibleto stress corrosion crack<strong>in</strong>g. In 2008, quite new resultswere found: sulphide does <strong>in</strong>deed <strong>in</strong>duce stress corrosion crack<strong>in</strong>g<strong>in</strong> copper. The threshold sulphide concentration for the <strong>in</strong>itiation<strong>of</strong> stress corrosion crack<strong>in</strong>g is likely to be <strong>in</strong> the range <strong>of</strong> 0.005-0.01 M.“This is somewhat higher than what you normally f<strong>in</strong>d <strong>in</strong>Swedish groundwater, but not very much. This is a warn<strong>in</strong>g thatthere is a clear risk <strong>in</strong> the “Forsmark situation,” with hot copperand groundwater evaporation. Because the copper surface is hot,we have a k<strong>in</strong>d <strong>of</strong> evaporation situation – salt/sulphide will beenriched. A concentration <strong>of</strong> those substances will result <strong>in</strong> stresscorrosion crack<strong>in</strong>g, especially if you have a phosphorus-alloyedcopper. That is exactly what we have <strong>in</strong> the KBS-3 model.”There are many mechanisms <strong>of</strong> corrosion <strong>in</strong> a deep repository.Initially there is atmospheric corrosion, which can be very severe.Another mechanism can be referred to as O 2 -depleted gaseous corrosion<strong>in</strong> comb<strong>in</strong>ation with high concentrations <strong>of</strong> moisture andsalts. A third mechanism is the evaporation-<strong>in</strong>duced salt/sulphidecorrosion that could contribute with a corrosion rate <strong>of</strong> severalmicrometer per year as general corrosion, but it could also bemillimeters or centimeters per year if it <strong>in</strong>duces <strong>in</strong>tergranular corrosionor stress corrosion crack<strong>in</strong>g. The situation where thegroundwater takes up to 1,000 years to saturate the bentonite could<strong>in</strong>volve another mechanism. As long as the copper is hot, the corrosionrate is high <strong>in</strong> a repository. There is also the problem <strong>of</strong>hydrogen embrittlement. Gunnar Hultquist showed 20 years agothat hydrogen is absorbed by copper dur<strong>in</strong>g corrosion.<strong>Corrosion</strong> <strong>of</strong> copper by oxygen-free water is a well-knownmechanism <strong>in</strong> <strong>in</strong>dustrial copper cool<strong>in</strong>g systems and <strong>in</strong> synchro-63
Edited transcripts from the workshoptrons. Thermodynamically this is expected, says Szakálos.Although it says noth<strong>in</strong>g about the corrosion k<strong>in</strong>etics.It has been found experimentally that the corrosion rate <strong>of</strong> coppercaused by water itself is <strong>in</strong> the order <strong>of</strong> one to ten micrometersper year. The same rates can be found <strong>in</strong> bentonite or soil, whichmeans that the values are about 1,000 to 10,000 times higher thanthe theoretical assumptions <strong>in</strong> the safety analysis.The canisters are exposed to elevated temperatures for at least1,000 years. The situation at the planned repository <strong>in</strong> Forsmark iscomplex and very severe from both a corrosion and an embrittlementpo<strong>in</strong>t <strong>of</strong> view. The copper canisters will <strong>in</strong>itially be exposedto atmospheric corrosion until the oxygen is consumed, whichcould take some months or years. Then there is corrosion by water,sulphide, salt, stress corrosion crack<strong>in</strong>g, <strong>in</strong>tergranular corrosion,evaporation-<strong>in</strong>duced corrosion and dissolution-precipitation <strong>in</strong> thebentonite, and naturally they should all be added together <strong>in</strong> orderto get the total corrosion over time.Szakálos says:“Before we can accept the KBS-3 solution, we need to test itunder realistic conditions.”He cites an SKI report (96:38) from 1996: “<strong>Copper</strong> <strong>of</strong> identicalcomposition as the future canisters should be placed <strong>in</strong> a future siteenvironment, with artificial heat<strong>in</strong>g at about 80 degrees, with bentonite,etc. Such an experiment could be monitored for several decades.Even 10–30 years is a short period <strong>of</strong> time <strong>in</strong> the presentcontext.”“I th<strong>in</strong>k that must be done before we can accept this solution,”he says.F<strong>in</strong>ally, he uses an example <strong>of</strong> copper canisters stored 18 years<strong>in</strong> a moist cellar to po<strong>in</strong>t out that copper does not corrode fast, butcopper reacts with every k<strong>in</strong>d <strong>of</strong> pollutant. This is a known phenomenon.“Everyone that works with atmospheric corrosion knows it.That is the problem with copper: it actually reacts with everyth<strong>in</strong>g,but slowly.”64
Edited transcripts from the workshop3. <strong>Copper</strong> corrosion processes <strong>in</strong> the Cu-O-H system,and their role <strong>in</strong> long-term safety assessments, byChrist<strong>in</strong>a Lilja, SKBChrist<strong>in</strong>a Lilja opens by present<strong>in</strong>g the scientific body <strong>of</strong> knowledgethat SKB uses as a basis for the performance assessment, andwhere thermodynamics is one <strong>of</strong> the cornerstones. She shows thePourbaix-diagram, with its two types <strong>of</strong> copper oxides. There areother species as well, such as the <strong>in</strong>termediate phases <strong>of</strong> adsorbedcopper hydroxide, that could form a sub monolayer.“It’s a precursor to the copper monoxide, and when you get amonolayer, it will convert to the stable oxide,” she says.The electrochemical approach is also very important to SKB. Avoltammogram shows reactions <strong>in</strong>clud<strong>in</strong>g electron transfer. Withthese data it is possible to make Tafel diagrams, which can be usedto identify the different reaction steps <strong>in</strong> a corrosion process.Impedance spectra where the frequency is varied provide <strong>in</strong>formationon, for example, rate-limit<strong>in</strong>g steps as well as, if they are frequent.Electrochemistry can be used to make a probe and measurethe corrosion directly by measur<strong>in</strong>g the change <strong>in</strong> metal thickness.If it corrodes the metal thickness decreases, which changes theresistance, says Lilja.Accord<strong>in</strong>g to Lilja, a large body <strong>of</strong> knowledge and a wide range<strong>of</strong> methods are available for study<strong>in</strong>g corrosion. There are twowell-known types <strong>of</strong> stable oxides, and SKB has performed severalreviews and compilations to obta<strong>in</strong> state-<strong>of</strong>-the-art <strong>of</strong> the knowledge<strong>of</strong> copper.Different types <strong>of</strong> experiments are used to study corrosionSKB uses different types <strong>of</strong> experiments to study the differentaspects <strong>of</strong> corrosion, all <strong>of</strong> which are valuable. Lilja po<strong>in</strong>ts out thatthere is no s<strong>in</strong>gle experiment that expla<strong>in</strong>s everyth<strong>in</strong>g perfectly. Allhave their pros and cons. She takes the laboratory studies as anexample, but SKB has also used <strong>in</strong>-situ experiments as well as analogues,which can be natural or man-made artefacts.The <strong>in</strong>itial state <strong>of</strong> a laboratory experiment is well-known, andcontrol <strong>of</strong> the environment <strong>in</strong> experiments is very good. There aredisadvantages, however. Representativeness is poorer due to the65
Edited transcripts from the workshopsimplified system, and the time scale is short. Quick results are anadvantage, but it is more difficult to evaluate long-term effects.She cont<strong>in</strong>ues with <strong>in</strong>-situ experiments, which are <strong>in</strong>vestigations<strong>in</strong> realistic environments and are therefore closer to reality than labexperiments.“Here you have an <strong>in</strong>itial state that is quite well-known. Youcan control it, s<strong>in</strong>ce you can determ<strong>in</strong>e the environment for theexperiment. Representativeness is rather good and you can representthe actual system <strong>in</strong> the <strong>in</strong>-situ experiment. Both short-termand medium-term experiments can be conducted.In the case <strong>of</strong> analogues, i.e. studies <strong>of</strong> what happens <strong>in</strong> nature,the situation is the opposite <strong>of</strong> that <strong>in</strong> the <strong>in</strong>itial state <strong>of</strong> the labexperiment.“In the analogues, you have no knowledge <strong>of</strong> what the systemlooked like <strong>in</strong> the beg<strong>in</strong>n<strong>in</strong>g, or how thick the sample was. Youcannot control the environment, s<strong>in</strong>ce the reaction has alreadyoccurred. You can measure some parameters. Representativenesscould anywhere from poor to good, depend<strong>in</strong>g on what you’relook<strong>in</strong>g at. She says it takes a long time to get the results, but theanalogue represents a long reaction time.What k<strong>in</strong>d <strong>of</strong> results does SKB have? A typical and expectedtype <strong>of</strong> result <strong>in</strong> short-term electrochemical and laboratory experimentsis that the corrosion rate decreases with time. Very few, ifany, results <strong>in</strong>dicate that the corrosion rate <strong>in</strong>creases.In-situ experiments <strong>of</strong>ten result <strong>in</strong> copper(II) corrosion products.The copper <strong>in</strong> the experiment has undergone periods <strong>of</strong> oxidiz<strong>in</strong>gconditions.“You can do <strong>in</strong>-situ studies too, but you have to bear the previoushistory <strong>of</strong> the artefacts <strong>in</strong> m<strong>in</strong>d when evaluat<strong>in</strong>g the results,”says Lilja.The analogue experiments show that copper <strong>in</strong> the native formhas been preserved stable for a very long time <strong>in</strong> both natural andman-made artefacts.All experiments have weaknesses. In weight loss experiments –which are usually done <strong>in</strong> the laboratory, but may also be <strong>in</strong>-situexperiments – it is possible to determ<strong>in</strong>e how much metal has beenlost after a given time.“But you can’t dist<strong>in</strong>guish between what happened <strong>in</strong> the veryfirst phase, which might have occurred rapidly, and what happenedover a longer period. And if you measure corrosion depth and corrosionrates <strong>in</strong> specimens, for example <strong>in</strong>-situ experiments, there66
Edited transcripts from the workshopcould have been different mechanisms operat<strong>in</strong>g dur<strong>in</strong>g differentperiods. That’s why it is so difficult to use extrapolate fromexperimental results for the safety assessment.”How SKB studies copper corrosionThe purpose <strong>of</strong> the SKB studies is to obta<strong>in</strong> a more detailed understand<strong>in</strong>g<strong>of</strong> the behaviour <strong>of</strong> copper <strong>in</strong> water. Accord<strong>in</strong>g to Lilja,SKB carries literature reviews. The state-<strong>of</strong>-the-art report on coppercorrosion from 2001 is be<strong>in</strong>g updated and will probably becompleted by the end <strong>of</strong> this year.A review <strong>of</strong> what has been published on the corrosion <strong>of</strong> copper<strong>in</strong> water is be<strong>in</strong>g prepared by Fraser K<strong>in</strong>g. A study by PavelKorzhavyi on the properties <strong>of</strong> the copper(I)oxide will also bepublished. He has also reviewed the literature on the properties <strong>of</strong>the copper(I)oxide.SKB is plann<strong>in</strong>g and carry<strong>in</strong>g out certa<strong>in</strong> experiments <strong>in</strong>volv<strong>in</strong>ggas measurements as well as some simpler glass conta<strong>in</strong>er experiments.“We are also conduct<strong>in</strong>g electrochemical experiments. We havesome short-term results ready for publication very soon. But weare also plann<strong>in</strong>g more long-term experiments that will take abouta year. We are also perform<strong>in</strong>g theoretical calculations and exam<strong>in</strong><strong>in</strong>gequilibrium reactions <strong>in</strong> water,” Lilja says.SKB’s conclusions from the copper corrosion studiesShe goes on to present some <strong>of</strong> the conclusions from the review <strong>of</strong>the studies <strong>of</strong> copper corrosion <strong>in</strong> water and says that a lot <strong>of</strong> conclusionscan be drawn from the different published papers andreports. But it is not easy to obta<strong>in</strong> a s<strong>in</strong>gle consistent picture; theresults are contradictory <strong>in</strong> certa<strong>in</strong> respects.“For example, the study from 1987 on electrochemistry publishedby Hultquist and co-workers gives a copper corrosionpotential that is 155 mV more positive than the equilibrium potentialat 1 atmosphere. If you compare some earlier papers with thelater one, the reported hydrogen generation rate varies by a factor<strong>of</strong> 3,000 under apparently the same conditions. And temperaturehas no effect.67
Edited transcripts from the workshopShe says that these observations have not been reproduced byother researchers. There have been some studies, early <strong>in</strong> 1987 and1989 on H2 evolution. One study was <strong>in</strong>itiated by the authorities<strong>in</strong> 1995 with palladium and plat<strong>in</strong>um-sealed vessels, and measurementswere made <strong>of</strong> corrosion <strong>in</strong> oxychloride water <strong>in</strong> 2003.“We also found conclusions regard<strong>in</strong>g the adsorbed copperhydroxide species. It is known to form below the potential forcopper(I)oxide <strong>in</strong> water. It’s hard f<strong>in</strong>d an account <strong>of</strong> the evolvedH2. In contrast with this proposed mechanism, there is no evidence<strong>in</strong> aqueous systems that oxygen gas is consumed by a reactionwith hydrogen atoms produced by the reduction <strong>of</strong> water.This casts some doubts on whether evidence from gas phase studiescan be used to <strong>in</strong>fer mechanisms <strong>in</strong> aqueous phases.”She emphasises that whenever new results emerge they are takenseriously by SKB and are analyzed <strong>in</strong> the context <strong>of</strong> earlier results.SKB is conduct<strong>in</strong>g additional theoretical and experimental studiesto learn more about these corrosion processes.“Our conclusion is that there is no conv<strong>in</strong>c<strong>in</strong>g evidence thatwater oxidizes copper.”Safety assessment and safety assessment methodologyTwo th<strong>in</strong>gs are needed for a safety assessment: scientific knowledgeand methodology. The SKB methodology, which has 10 steps,was used <strong>in</strong> the SR-Can safety assessment and will be used <strong>in</strong> theupcom<strong>in</strong>g SR-Site safety assessment. To compile all exist<strong>in</strong>gknowledge, a structured, hierarchical documentation is needed that<strong>in</strong>cludes all features, events and processes that exist <strong>in</strong> the repositorysystem.“Each process is then described and put <strong>in</strong>to a process report,and then you describe how you’re go<strong>in</strong>g to handle it <strong>in</strong> the safetyassessment. We need to have a multidiscipl<strong>in</strong>ary description <strong>of</strong> theevolution <strong>of</strong> the repository system. We need to look at the geology,the hydrogeology, the chemistry, the climate, bentonite, thecopper canister, radionuclides, all <strong>of</strong> this.”A strategy is needed to do calculations. An appropriate mix <strong>of</strong>pessimistic assumptions and more realistic descriptions is used,along with a mix <strong>of</strong> simplified and complicated models. Whenevaluat<strong>in</strong>g the results, Lilja po<strong>in</strong>ts out the necessity <strong>of</strong> <strong>in</strong>tegrat<strong>in</strong>gdifferent discipl<strong>in</strong>es.68
Edited transcripts from the workshop“Be<strong>in</strong>g pessimistic can mean that you only look at the amount<strong>of</strong> material that could react and disregard the fact that it needs tobe transported to the reaction site, the copper canister, to be ableto react. Or you only count the rate <strong>of</strong> one process and disregardthe possibility that there could be other rate-limit<strong>in</strong>g processes.Another way to be pessimistic is to assign very high values to thedriv<strong>in</strong>g forces.”When this pr<strong>in</strong>ciple is applied to the calculation strategy for thecorrosion calculations, SKB starts by perform<strong>in</strong>g a mass balanceand then looks at how much material is available for a reaction, disregard<strong>in</strong>gtransport. For example, this method is used for thepyrite <strong>in</strong> the bentonite, and for oxygen which is <strong>in</strong>itially entrapped<strong>in</strong> the bentonite.The next step is to look at a mass transport-limited process,which is used for the sulphide <strong>in</strong> groundwater. Mass transport<strong>in</strong>volves consider<strong>in</strong>g advective transport with groundwater to thebuffer, which is how the sulphide is transported by the water. Masstransport also <strong>in</strong>cludes diffusion over the groundwater-buffer<strong>in</strong>terface and then diffusion through the buffer. Diffusion refers totransport through the water.SKB disregards k<strong>in</strong>etics, the rate <strong>of</strong> the reaction itself, for copperreactions and <strong>in</strong>stead assumes that they occur <strong>in</strong>stantly. Liljasays that this is one <strong>of</strong> the pessimistic assumptions that she talkedabout.Tools are also needed to do the safety assessment. For mass balance,simple multiplication can be used. For diffusive transport,simple equations are <strong>of</strong>ten used. More complex mass transport <strong>in</strong>the repository requires the use <strong>of</strong> the concept <strong>of</strong> equivalent flow,which was primarily developed for radionuclide transport and sulphidecorrosion, but tool is also applicable to other corrosives orcorrosion products.“You use the same models but adjust the concentration gradientsand the diffusivity.It is also necessary to look at the site and to know how thewater is mov<strong>in</strong>g. Mass transport calculations have resulted <strong>in</strong> transportproperties or flow conditions for 6,000 positions at Forsmark.”Lilja also <strong>in</strong>cludes data from modell<strong>in</strong>g hydrogeology, climateevolutions and chemistry.69
Edited transcripts from the workshop<strong>Copper</strong> corrosion <strong>in</strong> safety assessmentIt is, <strong>of</strong> course, necessary to <strong>in</strong>clude copper corrosion by oxygen <strong>in</strong>the safety assessment, Lilja says. First there is the <strong>in</strong>itial atmosphericcorrosion, which is assumed to have a very limited duration,from fabrication until the canister is deposited. SKB uses experimentaldata on copper corrosion <strong>in</strong> the atmosphere. Pessimisticmass balance calculations are used for the <strong>in</strong>itially entrapped oxygen<strong>in</strong> the bentonite.“We have to look at the possible penetration <strong>of</strong> glacial meltwaterafter a glaciation. This meltwater could conta<strong>in</strong> more oxygenthan the groundwater. We also look at mass transport. All copperoxygen reactions and their effects should be added to the corrosion<strong>of</strong> copper by sulphide, which is the most dom<strong>in</strong>ant process <strong>in</strong> thesafety assessment results. And the reaction is mass transportlimited.She also po<strong>in</strong>ts out that when she has performed the corrosioncalculations, she feeds them <strong>in</strong>to the radionuclide transport calculations,which provide a measure <strong>of</strong> the risk, which <strong>in</strong> turn is comparedto the limit set by the authorities.In the safety assessment, SKB also formulates a reference evolutionand scenarios that are assumed to represent the most probablestate <strong>of</strong> affairs. The company also uses “what-if” calculations,which are not <strong>in</strong>cluded <strong>in</strong> the risk assessment, but can be used toanalyze the effect <strong>of</strong> any process. To evaluate the corrosion processsuggested by Hultqvist and Szakálos, SKB makes a what-if calculationus<strong>in</strong>g a one mbar equilibrium pressure and puts this <strong>in</strong>to themass transport calculations.The results <strong>of</strong> the calculations are divided <strong>in</strong>to two stages. In the<strong>in</strong>itial stage, the bentonite has not yet been saturated by water.“We make a pessimistic assumption that we fill all the unsaturatedvoid volumes <strong>in</strong> the bentonite backfill, with hydrogen gas to1 mbar and then let it diffuse outwards through the water. There isalways water outside, as the repository is situated below thegroundwater table. If we make the pessimistic assumption that ittakes 1,000 years for the buffer to become saturated, we get a corrosion<strong>of</strong> less than 1 mm.”In the next stage, water saturates the repository. In a pessimisticassumption, SKB disregards the hydrogen gas <strong>in</strong> the groundwater.Accord<strong>in</strong>g to Lilja, it has been measured and most probably correspondsto a pressure <strong>of</strong> about 1 mbar <strong>in</strong> the groundwater. If there70
Edited transcripts from the workshopis hydrogen <strong>in</strong> the water, the reaction will stop. It is the transportrate <strong>of</strong> the hydrogen that determ<strong>in</strong>es the corrosion rate. Hydrogengas has a higher diffusivity than sulphide and is therefore transportedmore easily. SKB exam<strong>in</strong>es hydrogen transport both <strong>in</strong> thebuffer and <strong>in</strong> a partly eroded buffer. With the flow conditions atForsmark it is assumed that all <strong>of</strong> the canisters are <strong>in</strong>tact after 10million years.In other words, Lilja states that the copper corrosion caused bysulphide is small, and any corrosion due to the proposed mechanismwould be even smaller.ConclusionsSKB asserts that there is no conv<strong>in</strong>c<strong>in</strong>g scientific evidence <strong>of</strong> areaction mechanism driven by a proposed new phase that is morestable than the known copper oxides. With<strong>in</strong> the framework <strong>of</strong> thesafety assessment methodology, SKB argues that the process couldbe handled as any other process. “What-if calculations” <strong>of</strong> theeffect <strong>of</strong> such a corrosion mechanism will be <strong>in</strong>cluded <strong>in</strong> theassessment. It is the view <strong>of</strong> SKB that the corrosion caused bythese mechanisms will not limit the lifetime <strong>of</strong> the canisters <strong>in</strong> thef<strong>in</strong>al repository.4. Thermodynamic properties <strong>of</strong> Cu–O–H phases fromfirst-pr<strong>in</strong>ciple calculations, by Pavel Korzhavyi, RoyalInstitute <strong>of</strong> Technology (KTH) and consultant to SKBThe goal <strong>of</strong> Pavel Korzhavyi´s computer calculations, which hedescribes <strong>in</strong> his presentation, has been to look for a stable product,a stable phase between copper, oxygen and hydrogen that would bea good candidate for the f<strong>in</strong>al product <strong>of</strong> a supposed reactionbetween copper and water. He has tried to f<strong>in</strong>d the f<strong>in</strong>al product <strong>of</strong>the corrosion reaction that the KTH group has been present<strong>in</strong>g. Ifthe reaction had occurred, there should be some ”rust”, i.e. the corrosionproduct, and it should be stable.“This product is not very well-known to those who work withcorrosion <strong>of</strong> copper, whether it is an oxyhydride, a hydroxide, orwhatever else forms there. It is not very well studied, and it’s quite71
Edited transcripts from the workshop<strong>in</strong>terest<strong>in</strong>g to f<strong>in</strong>d some unexplored area there, because corrosion<strong>of</strong> copper is an old story,” Korzhavyi says.His goals are also to calculate, from first pr<strong>in</strong>ciples, the thermodynamicproperties <strong>of</strong> known and unknown Cu(I)compounds withoxygen and hydrogen. He also <strong>in</strong>tends to analyze the thermodynamicstability <strong>of</strong> copper and its compounds <strong>in</strong> the oxygen-freewater environment.He describes the approach us<strong>in</strong>g a simple example <strong>of</strong> a hydrogenmolecule. In the computer exercise, his research group first positionstwo nuclei <strong>of</strong> this molecule at some distance from oneanother, and s<strong>in</strong>ce the nuclei are heavy and the electrons are light,they f<strong>in</strong>d the equilibrium configurations <strong>of</strong> the electrons and computethe energy <strong>of</strong> the system.“But this is not yet the energy <strong>of</strong> the whole system. Until nowwe have solved quantum-mechanical equations only for the electrons.We can compute this energy as a function <strong>of</strong> the distancebetween the protons and what we f<strong>in</strong>d is the energy <strong>of</strong> a stablearrangement <strong>of</strong> the electrons <strong>in</strong> the field <strong>of</strong> static nuclei. However,we still treat the nuclei as classical particles. We must solve anotherSchröd<strong>in</strong>ger equation, now for the nuclei, <strong>in</strong> order to get the f<strong>in</strong>alenergy <strong>of</strong> the system <strong>of</strong> electrons and nuclei altogether.He presents some results <strong>of</strong> a real calculation for the H 2 dimer,O 2 dimer, copper(II), etc. He optimizes or calculates the energiesas a function <strong>of</strong> <strong>in</strong>ter-atomic distance <strong>in</strong> the molecules. He saysthat the m<strong>in</strong>imum po<strong>in</strong>t here is not the energy <strong>of</strong> the molecule, orthe equilibrium.“We have to <strong>in</strong>clude the zero-po<strong>in</strong>t energy, because nuclei cannotbe determ<strong>in</strong>ed at a fixed position. The uncerta<strong>in</strong>ty pr<strong>in</strong>ciplemakes them move, and the energy <strong>of</strong> their motion is the zero-po<strong>in</strong>tenergy, which can be calculated <strong>in</strong> the follow<strong>in</strong>g way: We treat this[dependence <strong>of</strong> energy on the <strong>in</strong>ternuclear distance] as an effectivepotential, and then we solve the Schröd<strong>in</strong>ger equation for thateffective potential. When the ma<strong>in</strong> quantum number is zero, we getzero-po<strong>in</strong>t energy, which is related to the vibration frequency <strong>of</strong>this molecule. We get the harmonic part, the anharmonic part,summ<strong>in</strong>g up all together, and then we get these energies <strong>in</strong> thesevery strange units, which are <strong>in</strong>verse centimetres.These units are very familiar to spectroscopists, but for chemistsand other specialists it is necessary to convert these energies, orfrequencies, <strong>in</strong>to someth<strong>in</strong>g more familiar such as electronvolts:1,000 <strong>in</strong>versed centimetres is equal to 0.12 electronvolts or 1272
Edited transcripts from the workshopkilojoules per mole. These figures correspond to the accuracy weare aim<strong>in</strong>g to achieve <strong>in</strong> the calculations. It is quantum mechanics,but it is not completely free from approximations. These approximationsgive rise to large errors for objects like hydrogen or oxygenmolecules. The accuracy <strong>of</strong> exist<strong>in</strong>g approximations, as well astheir strictness, <strong>in</strong>creases cont<strong>in</strong>uously. But it is not yet possible toreproduce, for <strong>in</strong>stance, the dimerization energy <strong>of</strong> an oxygenmolecule. In order to elim<strong>in</strong>ate that big error from the very beg<strong>in</strong>n<strong>in</strong>g,we can use reliable experimental data, which fortunately exist,on dimerization energies <strong>of</strong> O 2 and H 2 .Korzhavyi says that he will show that for known phases <strong>of</strong> copperwith oxygen and hydrogen, the methods can provide the accuracythat SKB is aim<strong>in</strong>g for. The known compounds <strong>of</strong> copper(I)with oxygen and hydrogen are copper(I) oxide and copper(I)hydride. <strong>Copper</strong>(I) oxide (cuprite) is a well-known and well studiedmaterial. It’s thermodynamic properties, electrical properties,etc. are well-established. It was the precursor <strong>of</strong> semiconductortechnology. <strong>Copper</strong> hydride is a less known phase. It does exist,but it is very unstable. By perform<strong>in</strong>g a computer calculation, hehad tried to reproduce the experimental <strong>in</strong>formation on the cupriteand on the hydride, a stable phase and an unstable phase, to seehow it works.He skips the electronic part, and goes directly to the atomicmotion <strong>in</strong> the cuprite, or copper(I) oxide, structure. The phononspectrum is plotted <strong>in</strong> Figure y. This is the spectrum <strong>of</strong> the energies<strong>of</strong> the atomic vibrations, dependent on the way the atomsmove. Sound waves <strong>in</strong> the lower part are the acoustic part <strong>of</strong> thespectrum. The upper part reflects the vibration <strong>of</strong> an oxygen ion <strong>in</strong>the cage <strong>of</strong> the copper cations <strong>in</strong> the structure.“It is a very high frequency, and <strong>in</strong> order to ma<strong>in</strong>ta<strong>in</strong> the accuracy<strong>of</strong> the calculations all these phonon spectra and phonon energieshave to be taken <strong>in</strong>to account, <strong>in</strong> addition to the energies <strong>of</strong>the electrons.”He shows the dots, which are the experimental data. The l<strong>in</strong>esare <strong>in</strong> agreement with experimental observations.Korzhavyi performs these calculations as a function <strong>of</strong> volumeas well.“Under compression the lattice becomes stiffer. It is normallyobserved that the frequency <strong>of</strong> vibration <strong>in</strong>creases upon compression.This is however not valid for all the vibrations <strong>in</strong> the cuprite.73
Edited transcripts from the workshop“It is valid for the oxygen <strong>in</strong> the cage. But some other peaks thatcan be seen <strong>in</strong> the density <strong>of</strong> states, for example these lowfrequencyacoustic modes, get s<strong>of</strong>ter when the lattice is compressed.“You can see now, when I compress the lattice, this peak movesup <strong>in</strong> the frequency, and that one goes down.”He also shows the peak go<strong>in</strong>g down upon compression, andsays that it is an anomalous peak. The peak whose frequency<strong>in</strong>creases upon compression corresponds to the vibration <strong>of</strong> anoxide ion <strong>in</strong>side the tetrahedron <strong>of</strong> copper cations.“We believe that the anomaly occurs when these tetrahedralunits are rotat<strong>in</strong>g with respect to each other. They can rotate moreeasily if the lattice shr<strong>in</strong>ks a little bit. And so it does, when thesevibration modes are excited at low temperatures.”Figure 2Phonon spectrum <strong>of</strong> cuprite Cu2O. The dots correspond toexperimental data and the l<strong>in</strong>es to experimental observations.Both <strong>in</strong> experiments and calculations, the result <strong>of</strong> a negative thermalexpansion <strong>of</strong> cuprite can be seen at low temperatures. Ashigher and higher energy vibration modes are excited, they behavenormally and restore normal (positive) thermal expansion.74
Edited transcripts from the workshopIf the phonon spectra are known, then many properties <strong>of</strong> thematerial can be calculated. In order to obta<strong>in</strong> the thermodynamicproperties, the heat capacity can be accurately calculated from thephonon spectra. He shows a comparison with experimental datafor the stable phase <strong>of</strong> cuprite.“In the case <strong>of</strong> the hydride, you have a more extreme frequency<strong>of</strong> hydrogen vibrations. It’s very high, about 1,000 <strong>in</strong>verse centimetres.Hydrogen also vibrates <strong>in</strong> a tetrahedral cage. <strong>Copper</strong>hydride has been synthesized <strong>in</strong> aqueous solution. Its optical spectrahave been measured, and this hydrogen l<strong>in</strong>e has been detected.The hydride is a very unstable phase. It loses hydrogen veryquickly with time, and the <strong>in</strong>tensity <strong>of</strong> the hydrogen l<strong>in</strong>e quicklydecreases.”He emphasizes that other thermodynamic properties <strong>of</strong> copperhydride also are <strong>in</strong> quite good agreement with exist<strong>in</strong>g experimentaldata: the crystal structure and the lattice parameters. Thermalexpansion is normal <strong>in</strong> this case, and the correct structure isobta<strong>in</strong>ed. The hexagonal form <strong>of</strong> copper hydride is found to bemore stable than the cubic form.The heat capacity is <strong>in</strong> good agreement with experimental datafor temperatures below 170 K. What happens above is that thehydrogen lattice most probably undergoes a superionic transition,it becomes liquid. This can be seen from the very high amplitude <strong>of</strong>hydrogen vibrations <strong>in</strong> the hydride structure, Korzhavyi argues.Summary <strong>of</strong> thermodynamic properties“For the Cu 2 O, these are the calculated results at 0 K. We can calculatethe properties at f<strong>in</strong>ite temperatures, us<strong>in</strong>g the phononspectra and <strong>in</strong>tegrat<strong>in</strong>g them. This is how it compares with thethermodynamic tables, show<strong>in</strong>g that this reaction between copperand oxygen [to form Cu 2 O] is energetically favourable,” Korzhavyisays.His approach has been tested on the other oxide, copper(II)oxide. The research group can also see more or less good agreementwith experimental data for the hydride.“We get the reaction [between Cu and H 2 to form CuH] to beunfavourable thermodynamically, therefore copper hydride cannotbe formed spontaneously from hydrogen and copper, which is also75
Edited transcripts from the workshopknown experimentally. It has to be formed from a solution. Thecalculated energy is with<strong>in</strong> the experimental accuracy.”He says that it is very difficult to perform experiments on thismaterial, and starts describ<strong>in</strong>g the search for the other possible stablephase. Some suggestions come from the researchers; somehydrogen, some oxygen and some copper(I). Korzhavyi considersvarious possibilities. Hydrogen can be <strong>in</strong> the form <strong>of</strong> a hydride ion,H − , as <strong>in</strong> the hydride, or <strong>in</strong> the form <strong>of</strong> a proton, H + , as <strong>in</strong> thehydroxide. Oxygen is present as O 2- and copper as Cu + .“We require global charge neutrality. We distribute these atomsover various candidate structures, relax the structures, and thencompute the energy. We also compute the phonon spectrum to getthe zero-po<strong>in</strong>t energy and f<strong>in</strong>ite-temperature properties.”He describes the first expedition <strong>in</strong>to the unknown, which he is<strong>in</strong>vestigat<strong>in</strong>g.“We know that oxide is stable, hydride is unstable, but maybethere is someth<strong>in</strong>g <strong>in</strong> between these two, which could be more stablethan either one. We distribute all the ions <strong>in</strong> various configurations,and then relax the result<strong>in</strong>g structures. In Configuration 1,the anions, H - and O 2- , are too close. The anions are negativelycharged and they repel each other. We don’t expect this structureto be stable. In Configuration 2, two hydride ions H - are too close,first neighbours. We don’t expect this configuration to be stable,either. But with Configuration 3, we have a chance. All the anionsare second neighbours <strong>in</strong> that structure. Let’s compute the energies.“The phonon spectrum <strong>of</strong> Configuration 3 is almost okay.There are some <strong>in</strong>stabilities <strong>in</strong> the structure, show<strong>in</strong>g that it is notcompletely healthy. But this phonon spectrum can be used, afterresett<strong>in</strong>g the imag<strong>in</strong>ary frequencies to zero, <strong>in</strong> order to computethe energies. The results are shown <strong>in</strong> Table 4: the first configurationwas found to be very unstable, so we did not compute thephonon spectrum from it, but for the second and the third configurationswe could compute the free and total energy. We f<strong>in</strong>dthat the oxyhydride is not a stable configuration. It decomposesreadily <strong>in</strong>to the stable phase (oxide) and even forms the unstablephase (hydride).The second proposal concerns the hydroxide, H + , O 2- , Cu + ,which is a molecule known to exist and its geometry and dissociationenergies are known experimentally.76
Edited transcripts from the workshop“If you take that experimental <strong>in</strong>formation, then <strong>of</strong> course it is aquite unstable molecule, with respect to hydrogen, oxygen, andcopper <strong>in</strong> their standard states. Its formation energy is experimentallyknown to be positive. But maybe if we condense it <strong>in</strong>to somesolid phase we can obta<strong>in</strong> energy benefits.”Korzhavyi cont<strong>in</strong>ues:“When we <strong>in</strong>vestigated that, we found a stable, solid phase,composed <strong>of</strong> these molecules. In order to describe the structurethat we get <strong>in</strong> the result, the most stable structure, we exploredmany possibilities. The most stable was some comb<strong>in</strong>ation <strong>of</strong> thestructure <strong>of</strong> cuprite and the structure <strong>of</strong> ice.”In order to <strong>in</strong>troduce these structures, he carries out an exercise,go<strong>in</strong>g from the cuprite structure to the structure <strong>of</strong> ice. The structure<strong>of</strong> cuprite, the Cu 2 O is composed <strong>of</strong> two lattices. With<strong>in</strong> eachlattice there are balls connected by sticks, and sticks are the chemicalbonds. However, there are no bonds connect<strong>in</strong>g the blue andthe red lattices together. They are completely identical, and areheld together by physical, not chemical <strong>in</strong>teractions.“In order to change to ice, we elim<strong>in</strong>ate one <strong>of</strong> the identical sublattices,and now we are left with the anti-cristobalite structure. Tochange to ice, hydrogen is needed <strong>in</strong> place <strong>of</strong> copper, but hydrogendoesn’t like to sit <strong>in</strong> between the oxygens like copper does. Inwater or <strong>in</strong> ice hydrogen moves closer to one <strong>of</strong> the oxygen atoms,form<strong>in</strong>g a water molecule. We have the freedom to distribute thewater molecules <strong>in</strong> various ways; it will all be the same structure.”Korzhavyi shows two variants <strong>of</strong> the copper hydroxide structure,drawn from the calculations. It is an <strong>in</strong>termediate structurebetween those <strong>of</strong> cuprite and ice which Korzhavyi calls “cuprice.”“We have l<strong>in</strong>early coord<strong>in</strong>ated copper cations, and hydrogenssitt<strong>in</strong>g like they sit <strong>in</strong> the ice. The bonds and the atoms are <strong>in</strong> thesame environment as <strong>in</strong> the stable phases <strong>in</strong> water [ice] and cuprite.Hydrogen bonds connect<strong>in</strong>g the O 2- and H + species are shown bybroken l<strong>in</strong>es. We f<strong>in</strong>d actually not only one structure, but a wholefamily <strong>of</strong> structures with similar energies.”The hydroxide has a reasonable electronic spectrum comparedto the spectra <strong>of</strong> Cu 2 O and copper hydride. Both the stable(Cu 2 O) and unstable (CuH) materials are semiconductors, which“cuprice” also is. It has a band gap separat<strong>in</strong>g the occupied andunoccupied electronic states. It has a normal, all positive vibrationspectrum.77
Edited transcripts from the workshop“If some experimentalists wish to use all this data <strong>in</strong> order t<strong>of</strong><strong>in</strong>d these species as corrosion products, they are welcome. Wehave hydrogen l<strong>in</strong>es, oxygen l<strong>in</strong>es, OH stretch<strong>in</strong>g and bend<strong>in</strong>gl<strong>in</strong>es and whatever <strong>in</strong> these <strong>in</strong>frared spectra and it all looks reasonable.”Korzhavyi po<strong>in</strong>ts to the most important th<strong>in</strong>g: Thermodynamicproperties.“The negative enthalpy <strong>of</strong> formation means that the formation<strong>of</strong> this substance from the elements <strong>in</strong> their standard states is energeticallyfavourable. The free energy <strong>of</strong> formation <strong>of</strong> CuOH is anegative number. If we now compare the stability <strong>of</strong> this copperhydroxide relative to cuprite and water, it’s unstable. Thermodynamically,it should decompose onto cuprite and water. If weevaluate the enthalpies or free energies <strong>of</strong> the reactions <strong>of</strong> copperwith water (crystall<strong>in</strong>e copper and liquid water) to form eithercuprite or copper(I) hydroxide, we get positive numbers. These areenergetically, or thermodynamically, unfavourable reactions. Thisis the conclusion.*”From all these exercises, Korzhavyi concludes that cuprite isstill the champion <strong>of</strong> stability among the considered compounds.<strong>Copper</strong>(I) hydroxide may exist as a metastable phase, but its thermodynamicproperties are such that it is unstable with respect tothe stable phases, which are cuprite and water.Korzhavyi acknowledges his collaborators Pr<strong>of</strong>essor BörjeJohansson, Inna Soroka and Mats Boman at Uppsala University,and Evyas Isaev at the University <strong>of</strong> L<strong>in</strong>köp<strong>in</strong>g.He also thanks SKB for f<strong>in</strong>ancial support, and the SwedishNational Infrastructure for Comput<strong>in</strong>g for provid<strong>in</strong>g the computerresources.5. Hear<strong>in</strong>g and panel discussionTopic 1: Fundamental understand<strong>in</strong>g <strong>of</strong> the corrosioncharacteristics <strong>of</strong> copper <strong>in</strong> oxygen-free environmentsGaik Khuan Chuah: Do you plan to conduct more experiments onthe evolution <strong>of</strong> hydrogen, especially with other than aqueoussolutions, aqueous water or pure water?Peter Szakálos, KTH: Not at the moment.78
Edited transcripts from the workshopGunnar Hultquist, KTH: An experiment like the one we havealready performed is be<strong>in</strong>g conducted at this moment, and theresults will be published <strong>in</strong> 2010. There will be another pro<strong>of</strong> forhydrogen <strong>in</strong> copper metal, based on long-time exposure, where wecont<strong>in</strong>uously measure hydrogen <strong>in</strong> the gas phase. There will also beanalysis <strong>of</strong> hydrogen <strong>in</strong> the metal. Detection <strong>of</strong> the gas phase cont<strong>in</strong>ues.We can see that hydrogen must come <strong>in</strong>to the metal. It isdifficult to expla<strong>in</strong> how it is possible, but you will be able to see it.Gaik Khuan Chuah: When I read your published paper, I wouldlike to have a more detailed description <strong>of</strong> the experiments, thevalves, the gauges, the names, and even the gas analysis, so I can besure, rather than be satisfied with an <strong>in</strong>direct explanation, by means<strong>of</strong> iron pump current or pressure gauge. I th<strong>in</strong>k that would reallyleave me with no room to wonder what else there is that canexpla<strong>in</strong> this, <strong>in</strong> terms <strong>of</strong> artefacts.Peter Szakálos: It is true. We should cont<strong>in</strong>ue to analyze the product,which is not characterized very well. If it is an amorphousproduct that easily converts to oxides, that is not easy. But weperform experiments with a critical eye, and really try to sort outerrors. Someth<strong>in</strong>g still happens. The evidence <strong>of</strong> hydrogen <strong>in</strong> themetal compared with the hydrogen content <strong>in</strong> the beg<strong>in</strong>n<strong>in</strong>g isreally strik<strong>in</strong>g. Someth<strong>in</strong>g unknown is go<strong>in</strong>g on. But more detailedstudies on this product and how this corrosion actually occurs areneeded.Digby Macdonald: I was part <strong>of</strong> the National Academy <strong>of</strong> Sciences’review <strong>of</strong> the SKB-2 plan. Implicit <strong>in</strong> that was the idea that youcould control the electrochemical properties <strong>of</strong> the bentonite toensure that copper would rema<strong>in</strong> immune over a long period <strong>of</strong>time, forever. Is the same concept be<strong>in</strong>g carried through <strong>in</strong> theSKB-3 plan, and exactly what is be<strong>in</strong>g done to ensure that?Christ<strong>in</strong>a Lilja, SKB: The concept is the same, but the description<strong>of</strong> the evolution <strong>of</strong> the repository is now more detailed regard<strong>in</strong>gthe mechanisms. In the beg<strong>in</strong>n<strong>in</strong>g, it was a simplified description <strong>of</strong>the concept.Digby Macdonald: Does pyrite exists with<strong>in</strong> the bentonite? If thebentonite has got pyrite <strong>in</strong> it, the pyrite will react with oxygen andproduce all those nasty sulphur compounds that I showed <strong>in</strong> thevolt equivalent diagrams.79
Edited transcripts from the workshopChrist<strong>in</strong>a Lilja: There will be pyrite <strong>in</strong> the bentonite. We have setlimits on how much we could allow and it is <strong>in</strong>cluded <strong>in</strong> the safetyassessment, by mass balance calculations.Digby Macdonald: One way <strong>of</strong> gett<strong>in</strong>g rid <strong>of</strong> the pyrite, is to givethe clay a hydrogen peroxide wash, and you oxidize the pyrite upto sulphate.Christ<strong>in</strong>a Lilja: That is maybe one possible way, but it must be aproduct which is possible to make blocks out <strong>of</strong>, and then to selland manufacture. If there are very special treatments, it may not bethe best way to go.Digby Macdonald: As I showed, as time goes on and the copper oneplus concentration builds up at the <strong>in</strong>terface, and the hydrogenconcentration as well, it will move the po<strong>in</strong>t P up until you meetthe equilibrium l<strong>in</strong>e, at which po<strong>in</strong>t you have achieved immunity.One way <strong>of</strong> do<strong>in</strong>g that is to put a copper(I) salt <strong>in</strong>to the bentonite.Preferably a salt that has a reduc<strong>in</strong>g anion. You keep the redoxpotential low, which will simulate the addition <strong>of</strong> hydrogen. If youuse cupruous nitrate, for example, you might be able to ma<strong>in</strong>ta<strong>in</strong>copper <strong>in</strong> the immune state, right from the beg<strong>in</strong>n<strong>in</strong>g.Christ<strong>in</strong>a Lilja: Theoretically that may be possible, but that willmake the system more complicated. We strive for as simple a systemas possible, with the copper and the bentonite <strong>in</strong> the rock. I donot th<strong>in</strong>k that will be a solution for the repository.Fraser K<strong>in</strong>g, consultant for SKB: I have two comments. Firstly, byadd<strong>in</strong>g copper(I) to the bentonite to achieve immunity. My commentthere would be: It is not necessary, there is enough <strong>of</strong> a corrosionbarrier there. Even with the oxygen present <strong>in</strong> the bentonite,we use so little <strong>of</strong> the corrosion barrier, that it’s not necessaryto add any copper(I).Digby Macdonald: No, nitrite, a reduc<strong>in</strong>g anion.Fraser K<strong>in</strong>g: I don’t th<strong>in</strong>k we should add nitrite, because it causesstress corrosion. Pick another anion, if you want, but don’t usethat.Digby Macdonald: Details.Ron Latanision: In the case <strong>of</strong> the experiment with the palladiummembrane, the argument is that hydrogen is be<strong>in</strong>g produced by thecorrosion reaction with copper, and the hydrogen is penetrat<strong>in</strong>g80
Edited transcripts from the workshopthrough the membrane and be<strong>in</strong>g released. As the hydrogen isbe<strong>in</strong>g produced by the reduction <strong>of</strong> protons, the pH <strong>of</strong> the solutionshould simultaneously <strong>in</strong>crease. I wonder whether you haveexam<strong>in</strong>ed the pH change dur<strong>in</strong>g the course <strong>of</strong> those experiments?Peter Szakálos, KTH. In the closed vessels we haven’t had theopportunity to measure the pH, but <strong>in</strong> the other experiment wehave. Gunnar has also published a paper that shows the pH differencefor different metals: copper, iron and z<strong>in</strong>c.Gunnar Hultquist, KTH: In 1986, I measured and published thepH. It was 6.9. That means, that the pH doesn’t change significantly.My <strong>in</strong>terpretation is that both OH - and H + were consumed.This means, <strong>in</strong>directly, that water was consumed, because there’s aconsumption <strong>of</strong> water by a production <strong>of</strong> H + . The ion product,you can’t avoid that.Ron Latanision: Your suggestion is that both hydrogen ions andhydroxide ions are consumed at the same rate?Gunnar Hultquist: If it’s exactly the same pH, or about the same,<strong>of</strong> course. But we have to calculate, we have to look <strong>in</strong>to the system.In the experiment, there was a lot <strong>of</strong> water.Ron Latanision: This is <strong>in</strong> the context <strong>of</strong> experiments that may becritical to understand<strong>in</strong>g. I would be concerned that perhaps withthe volume <strong>of</strong> water, whatever change <strong>in</strong> the hydrogen ion andhydroxyl ion concentrations may occur may be overwhelmed bythe volume <strong>of</strong> water. I would suggest a very useful experiment: youcan use a smaller volume <strong>of</strong> H 2 O, measure the hydrogen which isreleased through the palladium membrane, and perform a mass balance,because that hydrogen that has been produced by the reduction<strong>of</strong> protons should allow you to determ<strong>in</strong>e how many hydroxylions were produced. That should equate to a change <strong>in</strong> pH. It’s one<strong>of</strong> the pieces <strong>of</strong> a puzzle that would hopefully fit, or not fit.Gunnar Hultquist: I fully agree. I th<strong>in</strong>k this is a critical experiment.Dave Shoesmith: I didn’t follow the details <strong>of</strong> the calculation thatyou presented, Gunnar. It looked like it was a calculation <strong>of</strong> an<strong>in</strong>terfacial reaction and that it was a copper surface with s<strong>in</strong>gleatoms adsorbed, or coord<strong>in</strong>ated, to the surface.Gunnar Hultquist: I didn’t say anyth<strong>in</strong>g about numbers. I onlyconcluded that this calculation showed that the bond between cop-81
Edited transcripts from the workshopper, <strong>in</strong> that case (100)- surface was enough. It is possible, that athree-dimensional monovalent copper hydroxide is grow<strong>in</strong>g.Dave Shoesmith: My questions are about the details <strong>of</strong> that calculation.That was an <strong>in</strong>terface with a s<strong>in</strong>gle atom? It didn’t <strong>in</strong>corporateany questions <strong>of</strong> whether that species adsorbed on the surfacewould be <strong>in</strong>teract<strong>in</strong>g with the medium, which it would if it waswater.Gunnar Hultquist: We discussed this with Anders Rosengren andagreed that the best th<strong>in</strong>g to mimic the situation about pure wateris to do it that way. It’s very difficult to have OH - approach<strong>in</strong>g. Wealways have this OH - <strong>in</strong> pure water. In the simulation, OH - wasfirst at an <strong>in</strong>f<strong>in</strong>ite distance from the copper surface, and then itwent down.Dave Shoesmith: But was the counterbalanc<strong>in</strong>g <strong>in</strong>fluence <strong>of</strong> the<strong>in</strong>teraction <strong>of</strong> the OH- with the water matrix <strong>in</strong>cluded <strong>in</strong> that calculation?Anatoly Belonoshko, KTH: Those calculations have been donewithout account<strong>in</strong>g for the water medium.Pavel Korzhavyi, KTH, consultant for SKB: Yes, the water that wehave dissociates <strong>in</strong>to H + and OH - fragments <strong>of</strong> the water molecule.Whereas <strong>in</strong> the calculation, an OH • molecule is considered, withoutany surround<strong>in</strong>g water. That molecule, if it is electrically neutral, isknown to be radical, a very strongly reactive species, which islack<strong>in</strong>g one electron. There is one vacancy <strong>in</strong> the oxygen p-shell.The radical wants to stick to anyth<strong>in</strong>g that has a more or lessloosely bound electron. To me, there is no surprise that this moleculeadsorbs to anyth<strong>in</strong>g, <strong>in</strong>clud<strong>in</strong>g gold. I would accept this calculationas a valid calculation <strong>of</strong> an adsorption energy <strong>of</strong> the OHradical to a metallic surface. But, it can’t serve as an estimate <strong>of</strong> theheat <strong>of</strong> formation or the free energy <strong>of</strong> formation <strong>of</strong> copper(I)hydroxide.Ron Latanision: I’m <strong>in</strong>terested <strong>in</strong> your “cuprice.” We’ve heard anumber <strong>of</strong> comments about adsorption. I’m just wonder<strong>in</strong>gwhether you have chemical adsorption <strong>of</strong> either electropositive orelectronegative species onto the surface <strong>of</strong> “cuprice.” Could youdeterm<strong>in</strong>e from your calculations whether that would <strong>in</strong>crease ordecrease its stability, by <strong>in</strong>ject<strong>in</strong>g electron density <strong>in</strong>to the density<strong>of</strong> states diagram? Is that possible?82
Edited transcripts from the workshopPavel Korzhavyi: You see the density <strong>of</strong> states <strong>in</strong> the bottom panel.This is the density <strong>of</strong> states <strong>of</strong> the “cuprice.” There, for thehydroxide. The presence <strong>of</strong> the band gap, at the Fermi level, tells usthat all the bonds <strong>in</strong> this compound are chemically saturated.Ron Latanision: I accept that. Suppose you had an electronegativeadsorbate, which <strong>in</strong> fact withdrew charge from the density <strong>of</strong>states. Would that destabilize “cuprice” from your po<strong>in</strong>t <strong>of</strong> view?Pavel Korzhavyi: Normally, when the bonds are saturated, this isthe most stable configuration you can have. If you add someth<strong>in</strong>gor remove some charge, you also <strong>in</strong>duce an imbalance, which makesthe system metallic. That usually destabilizes the th<strong>in</strong>g. I tried tocreate as stable configuration <strong>of</strong> the ions as possible. To me, thepresence <strong>of</strong> the band gap, which is quite comparable to the bandgaps <strong>of</strong> cuprite and copper hydride, is evidence that if someth<strong>in</strong>g isgo<strong>in</strong>g to be more stable than that with this composition (CuOH),it’s not go<strong>in</strong>g to be much more stable. I am confident that all theatoms <strong>in</strong> this structure are feel<strong>in</strong>g well.Ron Latanision: I’m not so comfortable. I’ll just repeat. You have alot <strong>of</strong> density <strong>of</strong> states illustrated. If they’re fully saturated, then Iwould agree with you. But if there is some opportunity to <strong>in</strong>jectelectron density <strong>in</strong>to the p-level, for example, that would presumably<strong>in</strong>crease its stability, unless it’s fully saturated. You’re confidentthat you’ve got fully saturated density <strong>of</strong> states. Is that correct?Pavel Korzhavyi: You can go <strong>in</strong> several directions. For <strong>in</strong>stance, thecopper(II) hydroxide could be formed, by putt<strong>in</strong>g <strong>in</strong> more OHgroups.That would also be metastable. You can also desorb, youcan remove hydrogen from here, or water molecules from thiscompound. It will become more stable, because then the cupritewill form. It is a stable situation, or at least the configuration is stable.But you can remove species from there, and you can add speciesthere, to form more stable compounds.Ron Latanision: One way <strong>of</strong> do<strong>in</strong>g that would be through chemicaladsorption <strong>of</strong> adsorbates, whether they are electropositive or electronegative.Isn’t that correct?Pavel Korzhavyi: Yes.Ron Latanision: I th<strong>in</strong>k this is very <strong>in</strong>terest<strong>in</strong>g. My po<strong>in</strong>t is not toput you <strong>in</strong> a difficult position. I’m just <strong>in</strong>terested <strong>in</strong> what you’ve83
Edited transcripts from the workshopsaid. I th<strong>in</strong>k there may be some implications that could be important,and we should not dismiss them.Mart<strong>in</strong> Boj<strong>in</strong>ov, VTT Technical Research Centre One po<strong>in</strong>t thatyou wrongly po<strong>in</strong>ted out is that you have to consider that it hasbeen shown by Marcuse and co-workers, that copper OH, or OHadsorbed, is more stable as a surface adsorbate than copper two O,as a treaty(?) compound. They are actually different types <strong>of</strong> calculations.I wouldn’t say that they can be compared directly.Gunnar Hultquist, KTH: This is more or less a surface scienceapproach at a very low temperature. It’s only a question about howfar, at a certa<strong>in</strong> temperature, by diffusion these species move. Thisapproximation is not relevant for a long-term discussion <strong>of</strong> whatwill happen. We saw water already, partial dissociation <strong>of</strong> waterproduce OH and H + . In some way, we have to consider both; Ith<strong>in</strong>k some people here realized that. So, we can’t forget this H, wecan’t leave it out, it must be <strong>in</strong>troduced <strong>in</strong>to the discussion. Wheredoes it go? My experiments show that it goes <strong>in</strong>to the metal.Marcuse, he does the experiment at room temperature, and waitsonly hours. Over years it’s another th<strong>in</strong>g.Mart<strong>in</strong> Boj<strong>in</strong>ov: You really can’t extrapolate th<strong>in</strong>gs from ten hoursto 10,000 years. But this is a different situation to make surfacecalculations and also [???] calculations. The Marcuse approach isconsistent with both the STM and thermodynamics, and I tend tobelieve more <strong>in</strong> that.Gaik Khuan Chuah: You mentioned the fact that there was noconv<strong>in</strong>c<strong>in</strong>g evidence for the corrosion <strong>of</strong> copper <strong>in</strong> water, and thatthis was proven. I also read <strong>in</strong> the draft report that was given to meon Friday, by Fraser K<strong>in</strong>g, that there was a mention by Möller <strong>in</strong>1995 that he repeated the experiments with plat<strong>in</strong>um and palladiumfoil, and found no corrosion. May I know whether such reports <strong>of</strong>the work that was carried out, are actually available <strong>in</strong> the publicdoma<strong>in</strong>, or is this exclusive to SKB?Christ<strong>in</strong>a Lilja, SKB: It has been available at the authority’s websitefor at least ten years.Gaik Khuan Chuah: Is there any reason why there is a discrepancy?Was there any dialogue between SKB and the researchers to f<strong>in</strong>dout why there was a difference?84
Edited transcripts from the workshopChrist<strong>in</strong>a Lilja: Yes, as far as I know. Gunnar was <strong>in</strong>volved <strong>in</strong> theexperiment that was required by the authorities.Gunnar Hultquist, KTH: Yes, I tried to. It was me aga<strong>in</strong>st ten people.There are two th<strong>in</strong>gs that are not exam<strong>in</strong>ed <strong>in</strong> these experiments.One: Choice <strong>of</strong> glass. It’s well-known that quartz glasstransports molecules. Actually quartz is used for calibration <strong>of</strong>helium. The second was, that there was, also said by Möller, that ifhe changed these tubes to sodium glass, then he saw someth<strong>in</strong>gthat I could see. This is left out <strong>in</strong> your report.Christ<strong>in</strong>a Lilja: It’s not left out, but he didn’t draw any conclusionsabout that. There were m<strong>in</strong>or effects that he couldn’t draw conclusionsfrom.Gunnar Hultquist: Partly left out.Digby Macdonald: What is <strong>in</strong>terest<strong>in</strong>g is the accumulated corrosiondamage. How much <strong>of</strong> the copper canister is go<strong>in</strong>g to be lost <strong>in</strong> amillion years, or whatever the number might be, 100,000 years. Inorder to do such a calculation, you have to specify the corrosionevolutionary path, which is the path that the system takes <strong>in</strong> terms<strong>of</strong> those <strong>in</strong>dependent variables that have a significant impact on thecorrosion rate: pH, hydrogen, etc. What work has been done ondef<strong>in</strong><strong>in</strong>g the corrosion evolutionary path? SKB probably have totake <strong>in</strong>to account ice ages and other th<strong>in</strong>gs as well.Christ<strong>in</strong>a Lilja: The SR-Can safety assessment is an extensivedescription <strong>of</strong> the evolution <strong>of</strong> the repository, over the long-term.It is opposed[?] to the hydrology, the chemistry, and the climate<strong>in</strong>fluence.Digby Macdonald: Did you try to predict how the chemistry isgo<strong>in</strong>g to change over the storage period, and if so, has it been published?Christ<strong>in</strong>a Lilja: Yes.Allan Hed<strong>in</strong>, SKB, responsible for the long-term safety assessment:I’m not sure. Our corrosion calculations are based on mass transport.We identify the species that cause corrosion, and we limit andestimate the mass transport <strong>of</strong> those. We are not try<strong>in</strong>g to establisha corrosion rate based on the chemical conditions. We just establishwhich species could cause corrosion and limit it by the mass85
Edited transcripts from the workshoptransport <strong>of</strong> those species. I th<strong>in</strong>k you had the impression that wewere do<strong>in</strong>g someth<strong>in</strong>g different.Digby Macdonald: Any factor that affects the corrosion rate def<strong>in</strong>esthe corrosion evolutionary path. Then to calculate the accumulateddamage is simply to <strong>in</strong>tegrate over that path.Allan Hed<strong>in</strong>: Essentially, that is what we are do<strong>in</strong>g.Fraser K<strong>in</strong>g, consultant to SKB: What is described <strong>in</strong> SR-Can is avery detailed description <strong>of</strong> what I would describe as the far fieldand how that changes. We have this state-<strong>of</strong>-the-art report, whichalso <strong>in</strong>cludes a description <strong>of</strong> how the near field environmentchanges, which is more important for the corrosion behaviour. Inparticular, I th<strong>in</strong>k SKB, perhaps by adsorption, has used <strong>in</strong>formationthat was developed <strong>in</strong> the Canadian programme, where we’vedeveloped a mixed potential model, which takes <strong>in</strong>to account thevarious reactions <strong>in</strong> the bentonite buffer. Through that model, wepredict the time-dependant evolution <strong>of</strong> the concentration pr<strong>of</strong>iles<strong>of</strong> the eleven species we <strong>in</strong>clude <strong>in</strong> that model, which <strong>in</strong>cludes chloride,oxygen, iron species, adsorbed copper(II), precipitated copper(II),and so on. We calculate a corrosion potential, which is <strong>of</strong>more <strong>in</strong>terest. That’s our detailed analysis <strong>of</strong> how the corrosionbehaviour would change over time. What’s <strong>in</strong> the SR-Can report isa detailed description <strong>of</strong> how the groundwater chemistry changes,and also the buffer chemistry, <strong>in</strong> terms <strong>of</strong> changes <strong>in</strong> the pore waterchemistry and <strong>in</strong> the buffer. We have all that <strong>in</strong>formation, <strong>in</strong>reports.Ron Latanision: Peter, there is clearly a corrosion product that youcan see <strong>in</strong> the metallographic section. Have you identified it?Peter Szakálos, KTH: Yes, we know that the most <strong>of</strong> it is themonovalent oxide, Cu 2 O. There was also some hydroxide, on thetop, identified by ESCA. In this case, with the palladium membrane,the hydrogen pressure will not be 1 mbar <strong>in</strong>side. It would be<strong>in</strong>f<strong>in</strong>itely low. Theoretically, you could form any k<strong>in</strong>d <strong>of</strong> oxide. Itdoesn’t have to be a monovalent hydroxide.Ron Latanision: Are your f<strong>in</strong>d<strong>in</strong>gs stable copper compounds?Peter Szakálos: Yes.Ron Latanision: It appears to me that there is some evidence <strong>of</strong><strong>in</strong>tergranular penetration. Am I see<strong>in</strong>g that correctly?86
Edited transcripts from the workshopPeter Szakálos: Yes. As the general corrosion is visible, <strong>in</strong> the bottoml<strong>in</strong>e. There are some parts that go deeper.Ron Latanision: Is this a precursor? You also mentioned <strong>in</strong> yourpresentation concern about hydrogen embrittlement and mechanicalbehaviour <strong>of</strong> copper, when hydrogen has been adsorbed. Is thisa manifestation <strong>of</strong> some <strong>of</strong> the concern you have about mechanicalbehaviour? Are those cracks, or what are we look<strong>in</strong>g at?Peter Szakálos: They are not actually cracks. In detail we could seethat the gra<strong>in</strong> boundaries are k<strong>in</strong>d <strong>of</strong> corroded. The basis for thishydrogen embrittlement th<strong>in</strong>k<strong>in</strong>g is that the bulk metal conta<strong>in</strong>s alot <strong>of</strong> hydrogen after this exposure. I don’t know exactly howmuch hydrogen you must have to get true embrittlement. But <strong>in</strong>the literature you can f<strong>in</strong>d that all <strong>of</strong> the mechanical properties dodeteriorate because <strong>of</strong> hydrogen.Ron Latanision: Do you mean hydrogen embrittlement, or do youmean that there is some perceptible effect on mechanical properties,broadly?Peter Szakálos: We haven’t measured any deterioration <strong>in</strong> mechanicalproperties <strong>in</strong> a scientific way, but we can see that it is done <strong>in</strong>the literature. There are studies that have shown, by <strong>in</strong>creas<strong>in</strong>g thehydrogen content <strong>in</strong> the copper metal, that it is susceptible tohydrogen embrittlement. That is not big news.Ron Latanision: I have to admit I haven’t seen that literature. Whatsources are you th<strong>in</strong>k<strong>in</strong>g <strong>of</strong>?Peter Szakálos: I have referred to them <strong>in</strong> a draft report’s commentsregard<strong>in</strong>g the BRITE-report. The last paper, number 44, S.Nakahara, “Scripta Metallurgica”, which is submitted to you. Thereit is stated that hydrogen embrittlement is observed <strong>in</strong> copper, silver,and gold. These metals are quite <strong>in</strong>sensitive, but it does happen.Ron Latanision: Is the embrittlement <strong>in</strong>tergranular? Have theylooked at the fracture surfaces? What do we know about thisembrittlement?Peter Szakálos: The references can be read <strong>in</strong> “Scripta Metallurgica”.Ron Latanision: This is an important issue. I f<strong>in</strong>d it hard to acceptthe concept that copper is embrittled by hydrogen without see<strong>in</strong>g87
Edited transcripts from the workshopevidence <strong>of</strong> either fractured surface, or mechanical data, or whatever.Peter Szakálos: If you do it by gas phase, you’re totally correct. Youneed a huge pressure. But dur<strong>in</strong>g electrochemical charg<strong>in</strong>g, it doeshappen to a degree, it’s measurable. This is relative hydrogen content,and not the best paper. You can see that it starts to deterioratedirectly when you charge it. In this case, it is a relative embrittlementratio. It is a problem, but it’s not ppm levels here. It’s hardto tell when it starts, when it becomes dangerous. But it does haveeffects. This was also “Scripta Metallurgica”, with two or more references.Fraser K<strong>in</strong>g, consultant to SKB: The current density there, 25 milliampsper square centimetre: That’s enormous.Ron Latanision: Mr moderator: Do you remember whether theembrittlement occurs as <strong>in</strong>tergranular or not? Do you remember<strong>of</strong>f-hand?David Duquette: In ultra pure copper it’s transgranular, and <strong>in</strong>commercial copper it can be a comb<strong>in</strong>ation <strong>of</strong> both. The hydrogengoes to the gra<strong>in</strong> boundaries, because it acts like a s<strong>in</strong>k, basically. Inorder to get it electrochemically, we have to charge at almost a1,000 millivolts through a reversible potential. The amount <strong>of</strong>hydrogen we were charg<strong>in</strong>g <strong>in</strong>to it was <strong>in</strong>credible. Anyth<strong>in</strong>g belowthat didn’t get much embrittlement. It’s very difficult to embrittlecopper with hydrogen.Peter Szakálos, KTH: The other problem is a hydrogen sickness.That is not the same as the normal embrittlement effect. This isfrom a friction stir weld. It is coupled to oxide <strong>in</strong>clusions. It is alsostated that the hydrogen could precipitate <strong>in</strong> these locations. It’s acomplex situation.Dave Shoesmith: In the present discussion <strong>of</strong> how hydrogen gets<strong>in</strong>to copper, we have a discussion about the formation <strong>of</strong> the corrosionproduct, and the discussion about how hydrogen gets <strong>in</strong>.But the diffusion rate <strong>of</strong> the proton, generally speak<strong>in</strong>g, <strong>in</strong> anoxide-hydroxide film, is 10 -17 centimetre 2 per second. It’s not clearto me, that you don’t have a barrier.Peter Szakálos: We have looked at this <strong>in</strong> electron microscope, andunder anoxic conditions, it is really a porous product.88
Edited transcripts from the workshopDave Shoesmith: That’s a po<strong>in</strong>t I have. I have not seen a characterization,or a visualization <strong>of</strong> the corrosion product, that I couldevaluate, <strong>in</strong> what you have published.Peter Szakálos: If you look at the ECC paper, there are picturestaken with SEM. I can also show a paper from Kunze, “<strong>Corrosion</strong>Science,” 2004. In this simple manner, it might be shown that it isporous. It is a layer <strong>of</strong>, <strong>in</strong> this case, bivalent hydroxide.Dave Shoesmith: Isn’t the diffusion barrier one nanometre Cu 2 O?Peter Szakálos: Yes, but I wouldn’t call that a perfect diffusion barrier.These are a k<strong>in</strong>d <strong>of</strong> porous products. It looks nice <strong>in</strong> thisshape, but, as you see…Dave Shoesmith: Are you tell<strong>in</strong>g me that the Cu 2 O is porous? Itgenerally isn’t on copper.David Duquette: I’m not sure why KTH, or the work that has beendone by Gunnar and Peter, really requires that hydrogen embrittlecopper, except to say that you got hydrogen <strong>in</strong> the copper and thefact that it might embrittle is evidence for hydrogen <strong>in</strong> it. It doesnot strengthen your argument, and those <strong>of</strong> us who’ve done a lot<strong>of</strong> cathodic charg<strong>in</strong>g <strong>of</strong> copper, and other metals over the years,would argue that it’s a very difficult process to embrittle. Theamount <strong>of</strong> hydrogen that you are produc<strong>in</strong>g is not large. I th<strong>in</strong>k itwould be difficult to support.Peter Szakálos: These k<strong>in</strong>ds <strong>of</strong> crystals are Cu 2 O. This is the oxide.These crystals are quite dense. Someth<strong>in</strong>g else is grow<strong>in</strong>g here,probably hydroxides. Also, deeper down. This is not a crystall<strong>in</strong>eproduct; it’s a k<strong>in</strong>d <strong>of</strong> porous sponge structure. It’s not a protectiveproduct. It looks bad. If you do this <strong>in</strong> an anoxic environmentthat differs from an oxic, if you have dissolved O2, I th<strong>in</strong>k theproduct looks much better. It gets, as you said, more <strong>of</strong> a passivelayer. In this situation it is porous. How much hydroxide, eventhough it would be a monolayer <strong>of</strong> hydroxide, I th<strong>in</strong>k it could bequite much, but I’m not sure <strong>of</strong> it.Dave Shoesmith: I th<strong>in</strong>k the conventional wisdom <strong>of</strong> a passive layerwould accept that 10 microns isn’t enough to see what we perceiveto be a 1 nanometre barrier layer. There is no evidence <strong>in</strong> that particularpicture that that system is porous through to the metal surface.89
Edited transcripts from the workshopPeter Szakálos: No, but the corrosion rate implies that. The corrosionrate is <strong>in</strong> the micrometer per year range, and it couldn’t beprotective.Ron Latanision: Christ<strong>in</strong>a Lilja mentioned the use <strong>of</strong> electrochemicaltools. You talked about voltammetry, and so on. Much <strong>of</strong> whatwe’ve talked about today, <strong>in</strong> assess<strong>in</strong>g the hydrogen fugacity, forexample, at various stages <strong>of</strong> proposed process, has been based onequilibrium thermodynamics. But corrosion is not an equilibriumprocess, and it would seem to me to make sense to apply potentialsand drive the reactions that are concerned – whether you’re look<strong>in</strong>gat it from KTH’s po<strong>in</strong>t <strong>of</strong> view or from your po<strong>in</strong>t <strong>of</strong> view – toassess whether or not, by driv<strong>in</strong>g the potential, you can <strong>in</strong>crease therate <strong>of</strong> hydrogen evolution, if that’s occurr<strong>in</strong>g. Or did you, by suppress<strong>in</strong>gthe potentials even further – if you don’t see hydrogenevolution, that would be equally <strong>in</strong>terest<strong>in</strong>g. Why has no one chosen,<strong>in</strong> this debate, to look at the application <strong>of</strong> a driv<strong>in</strong>g force, apolarization experiment?Mart<strong>in</strong> Bojonov, VTT Technical Research Centre: We have anongo<strong>in</strong>g study on that. We have been us<strong>in</strong>g polarization curves, orcurrent versus potential curves, and even spectroscopy to followprocesses <strong>in</strong> the vic<strong>in</strong>ity. An example: If the corrosion potential isat m<strong>in</strong>us 200 millivolts, <strong>in</strong> a system that has been anoxic, andclosed for up to three weeks, then we haven’t observed any evidence<strong>of</strong> hydrogen evolution.Ron Latanision: You see no evidence?Mart<strong>in</strong> Boj<strong>in</strong>ov: If you polarize the system to m<strong>in</strong>us one volt, youwill <strong>of</strong> course get a hydrogen evolution, but this was, so far, for theclosed system. Now we have a system <strong>in</strong> which we have the palladiummembrane <strong>in</strong>stalled. We have been follow<strong>in</strong>g this for sometime, but I won’t say more for the moment, because these areongo<strong>in</strong>g experiments.Ron Latanision: Let me understand the experiment, though. Youbegan your experiment at an open circuit potential, which is above,or below, the reversible hydrogen equilibrium?Mart<strong>in</strong> Boj<strong>in</strong>ov: We beg<strong>in</strong> our experiment at the corrosion potential.We pump gas that conta<strong>in</strong>s 5 ppb <strong>of</strong> oxygen <strong>in</strong> the gas phase.Ron Latanision: It’s an oxidiz<strong>in</strong>g process?90
Edited transcripts from the workshopMart<strong>in</strong> Boj<strong>in</strong>ov: It’s not oxidiz<strong>in</strong>g, it’s an anoxic environment. It is5 ppb <strong>in</strong> the gas phase, but then it is consumed for a couple <strong>of</strong>hours <strong>in</strong> the system. After that, it’s an anoxic environment. Wefollow the potential, and then we reduce the system, but mildly,not to go <strong>in</strong>to the “hydrogen evolution region”. We follow theprocess by several electrochemical means, <strong>in</strong>clud<strong>in</strong>g impedancespectroscopy and measurements <strong>of</strong> the concentration <strong>of</strong> copper <strong>in</strong>the solution, with an ion-selective electrode. We will be present<strong>in</strong>gthese results publicly rather soon, and then we’ll see what the scientificcommunity has to say about it.Ron Latanision: Peter, have you looked at polarization phenomena<strong>in</strong> your experiments?Peter Szakálos, KTH: That is not my way <strong>of</strong> do<strong>in</strong>g research. Ihaven’t done that. I don’t work with electrochemical methods atall.Gunnar Hultquist, KTH: There is a paper published by me andsomeone named Håkan Herö <strong>in</strong> “<strong>Corrosion</strong> Science” 1984. I knowthat the <strong>in</strong>terpretations <strong>of</strong> these potentials are very difficult. Youcan get very low potential only due to adsorption on gold. For oneand the same sulphur content, sulphur adsorption on gold is lowerthan on copper. How to study this? The <strong>in</strong>terpretation is very difficult.It may be useful to conduct these k<strong>in</strong>ds <strong>of</strong> experiments, butit will never stand on its own. It has to be verified, aga<strong>in</strong>, by othermeasures.Digby McDonald: Do you know the nature <strong>of</strong> that localized attack?Is that classical pitt<strong>in</strong>g corrosion?Peter Szakálos: It as a k<strong>in</strong>d <strong>of</strong> pitt<strong>in</strong>g corrosion, but, if you look atthe literature, some conditions need to be met <strong>in</strong> order to get pitt<strong>in</strong>gcorrosion. It’s strange that we can monitor, or see, this <strong>in</strong> avery clean environment.Digby McDonald: I would f<strong>in</strong>d pitt<strong>in</strong>g corrosion to be a phenomenonwhere differential aeration has been established. Whereas youget the cathodic reaction occurr<strong>in</strong>g on the external surface, and theanodic reaction occurr<strong>in</strong>g <strong>in</strong>side the cavity. Putt<strong>in</strong>g it another way:Do you see pitt<strong>in</strong>g corrosion <strong>in</strong> any <strong>of</strong> your experiments?Peter Szakálos: We have seen that <strong>in</strong> this 15-year exposure, so it is aslow process, if it’s ongo<strong>in</strong>g. This took 15 years to produce. But <strong>in</strong>a shorter exposure you just see even, smooth, general corrosion.91
Edited transcripts from the workshopDigby McDonald: You don’t see pitt<strong>in</strong>g at all?Peter Szakálos: No, not at all.Gunnar Hultquist, KTH: There’s no pitt<strong>in</strong>g corrosion <strong>in</strong> the normalsense. I myself call it localized corrosion. There’s no crevice,obviously, but it’s local. For some reason, the corrosion preferredto take place locally on different spots. Sometimes along gra<strong>in</strong>boundaries.Dave Shoesmith: I wanted to question this mechanism for oxygenconsumption. We assess that the primary consumption <strong>of</strong> oxygenis due to the decomposition products <strong>of</strong> water. What evidence doyou have to support that? It’s not the conventional wisdom.Gunnar Hultquist: No, it’s not and that’s why we are here. If werely on what we know already, we don’t need to be here.Dave Shoesmith: But when we can see oxygen reduction manyhundreds <strong>of</strong> millivolts more positive than what we can see waterreduction, how can you draw that conclusion?Gunnar Hultquist: Because <strong>of</strong> the use <strong>of</strong> isotopes. When you useisotope 18 O you have molecular oxygen present, it comes fromwater, still. The pattern is better.Dave Shoesmith: I don’t th<strong>in</strong>k that proves that it’s a hydrogen atomthat’s scaveng<strong>in</strong>g oxygen.Peter Szakálos, KTH: It is shown by isotope studies. If you have anatmosphere with water and oxygen molecules, for example moistair, we f<strong>in</strong>d that the corrosion product is preferentially built upwith oxygen from the water molecule – this is not unique for copper,it also happens to iron and z<strong>in</strong>c and other metals. By isotopesensitive analyses (i.e. SIMS) <strong>of</strong> the corrosion product you can seethat most <strong>of</strong> the oxygen, <strong>in</strong>clud<strong>in</strong>g <strong>in</strong> the copper oxide, actuallyorig<strong>in</strong>ates from the water molecule. The released hydrogen atomswill be consumed by the fast catalytic reaction with O 2 with theformation <strong>of</strong> “new” water molecules. Thus, you can’t detect anyhydrogen production caused by corrosion if O 2 is present. Oxygenis, actually, a k<strong>in</strong>d <strong>of</strong> catalytic activator here. But the corrosionproduct is built up ma<strong>in</strong>ly from water.Dave Shoesmith: What you’re say<strong>in</strong>g is that there’s no role for oxygen<strong>in</strong> this corrosion process, exceptive <strong>of</strong> that route, or is this analternative?92
Edited transcripts from the workshopPeter Szakálos: I would say it’s a catalytic.Dave Shoesmith: Is this the only role?Peter Szakálos: No, you also f<strong>in</strong>d O from oxygen molecules <strong>in</strong> theproduct, but the majority comes from the water. Both are ongo<strong>in</strong>g.You can also f<strong>in</strong>d out that the new water molecules are formedwith the isotope, let’s say that is the isotope 18-18 O 2 <strong>in</strong> theexperiment, then you f<strong>in</strong>d that <strong>in</strong> the newly formed water. However,oxygen from the O 2 molecule will also form corrosion products.As Gunnar showed <strong>in</strong> his paper, most <strong>of</strong> the oxygen <strong>in</strong> theproduct comes from water molecules. This was shown 15 yearsago. There has been a lot <strong>of</strong> <strong>in</strong>terest <strong>in</strong> this paper, and it’s <strong>in</strong>terest<strong>in</strong>gthat the community hasn’t adopted this yet.Dave Shoesmith: What is the condition <strong>of</strong> this experiment?Gunnar Hultquist: The condition is published <strong>in</strong> “<strong>Corrosion</strong> Science.”You asked about the role <strong>of</strong> oxygen. It’s to keep the activity<strong>of</strong> hydrogen low. That’s why it’s necessary to avoid that. That isdone by remov<strong>in</strong>g or only start<strong>in</strong>g up experiments where we arecerta<strong>in</strong> we have no O 2 .Fraser K<strong>in</strong>g, consultant to SKB: This work was done <strong>in</strong> the gasphase. The partial pressure <strong>of</strong> water is very low, where you havesub-monolayer quantities adsorb<strong>in</strong>g on the surface <strong>in</strong> the gasphase. There’s no aqueous electrochemistry happen<strong>in</strong>g here at all.Dave Shoesmith: There’s no possibility <strong>in</strong> this experiment that youcould couple separate anodes and cathodes through a watermedium?Fraser K<strong>in</strong>g: Correct.Gaik Khuan Chuah: Basically, these k<strong>in</strong>ds <strong>of</strong> studies with isotopelabell<strong>in</strong>g have also been done by others than Dr. Hultquist. Datahave been published, and even <strong>in</strong> the presence <strong>of</strong> oxygen, the surfacescience studies show that water would preferentially adsorbonto the copper sites. There are references on this. Secondly, Dr.K<strong>in</strong>g mentioned that these are gas phase studies. I would just liketo po<strong>in</strong>t out that <strong>in</strong> 2007 pr<strong>of</strong>essor Gerhard Ertl got the NobelPrize for study<strong>in</strong>g reactions occurr<strong>in</strong>g on solid surfaces. Heshowed that the data from the gas phase or the surface sciencestudy are relevant to <strong>in</strong>dustrial processes, which <strong>in</strong>volve very highpressures as well. You can see the equivalence <strong>in</strong> terms <strong>of</strong> gas phase93
Edited transcripts from the workshopto the aqueous phase. Basically, there is no pressure gap. For that,he got the Nobel Prize.Topic 2: What additional <strong>in</strong>formation is needed to confirm thisspecific corrosion process and to assess the importance <strong>of</strong> theprocess for the f<strong>in</strong>al repository?David Duquette: The ma<strong>in</strong> reason why we’re here is to understandwhether or not you can put copper <strong>in</strong>to the earth for a millionyears and have it to last for some reasonable period <strong>of</strong> time. Wewill focus on: What additional <strong>in</strong>formation is needed to confirmthe specific corrosion processes as it relates to the vault, and thecopper <strong>in</strong> the vault.Dave Shoesmith: This is not the time for specific comments,because there’s a lot <strong>of</strong> discussion which is eye-open<strong>in</strong>g, or different,to what we read <strong>in</strong> the papers. But, one th<strong>in</strong>g that bothers meis the distance between some <strong>of</strong> the studies and some <strong>of</strong> the claims.Sometimes the answer to these solutions is detailed experiments at<strong>in</strong>credibly complicated levels. But the problems that we’re deal<strong>in</strong>gwith are ones at specified, very general levels. Expla<strong>in</strong><strong>in</strong>g those verygeneral phenomena <strong>in</strong> complicated systems by detailed experimentsis extremely difficult. It’s not clear to me that the right techniquesare necessarily always used <strong>in</strong> all experiments. That’s not just a referenceto one or other side <strong>of</strong> this discussion. That is not an easyproblem. We’ve come down to argu<strong>in</strong>g <strong>in</strong>terpretations <strong>of</strong> <strong>in</strong>dividualpieces <strong>of</strong> data, and whether we believe this mechanism or thatmechanism. If there is go<strong>in</strong>g to be a resolution (solution?), thensome improved experiments are required, probably on both sides.David Duquette: The real question is whether the results that havebeen generated both by the SKB and by the KTH team are relevantto the vault. In other words, if you’re primarily look<strong>in</strong>g at whetherit’s go<strong>in</strong>g to be a safe disposal <strong>of</strong> copper conta<strong>in</strong>ers or not. Arethese the experiments, that should be be<strong>in</strong>g performed? Are thesethe appropriate experiments to justify that? Are we gett<strong>in</strong>g the<strong>in</strong>formation that we need to conv<strong>in</strong>ce ourselves as a communitythat you can put copper <strong>in</strong>to bentonite, and bentonite <strong>in</strong>to granite,for a hundred thousand or a million years?Dave Shoesmith: It isn’t simply a corrosion issue. The multi-barriersystem alone is meant to make sure that we don’t rely on one bar-94
Edited transcripts from the workshoprier. However, that doesn’t mean to say that you shouldn’t make areally good attempt to determ<strong>in</strong>e the specific mechanisms andappropriate experiments for <strong>in</strong>dividual barriers. When you makethe extrapolation from whether we have the right issue on the tablehere to whether there is a solution to long-term conta<strong>in</strong>ment <strong>of</strong>radioactive waste, you’re ask<strong>in</strong>g a much bigger question. Such as:Do we accept the <strong>in</strong>terpretations <strong>of</strong> these corrosion processes? Idon’t th<strong>in</strong>k the experiments are <strong>of</strong>f-base, no. I th<strong>in</strong>k they are f<strong>in</strong>e,but it will always be difficult to bridge that gap between someth<strong>in</strong>gthat you want to get, where you have a practical number from thelarge system, and then the experimental system which will generatethe data. It will always end up <strong>in</strong> some computational model. It willalways be obscure.David Duquette: The last hour was supposed to focus on the science.We’ve discussed whether they made sense from a thermodynamicallyk<strong>in</strong>etic po<strong>in</strong>t <strong>of</strong> view. The question now, is: Do you needthat level <strong>of</strong> science, is it relevant to safe disposal <strong>of</strong> copper? Wehave two different concepts <strong>of</strong> the mechanisms that exist. What hasto be done to support one side or the other? Or, even if it’s relevant,given the fact that it’s not pure water and absolutely purecopper. What do you th<strong>in</strong>k these two proponents for their mechanismshave to do, to advance their case?Dave Shoesmith: If I understand the SKB position correctly, sulphidedom<strong>in</strong>ates the system, which is what I agree with. I don’tth<strong>in</strong>k the KTH team thought about sulphide, and therefore there’sa bridge, or rather a gap, between those two approaches. The questionis: Why haven’t you considered sulphide? Or: Why do youth<strong>in</strong>k that sulphide is totally dom<strong>in</strong>ant? If sulphide is totally dom<strong>in</strong>ant,then there’s no po<strong>in</strong>t <strong>in</strong> try<strong>in</strong>g to resolve this argument.Christ<strong>in</strong>a Lilja, SKB: Why we th<strong>in</strong>k sulphide is dom<strong>in</strong>ant? Weth<strong>in</strong>k that these corrosion processes are mass transport-limited. Ifyou look at the mass transport, then it’s as simple as look<strong>in</strong>g at thegradients and the diffusivities. Then you f<strong>in</strong>d that sulphide isdom<strong>in</strong>ant.Peter Szakálos, KTH: There is a mass transport model, but itdoesn’t cover your long time exposure. The only long-term exposureexperiment presented is the LOT project. It’s a five year project.We understand that it doesn’t fit with your model. The corrosionrate is very high. What you can observe is copper oxide. I95
Edited transcripts from the workshopth<strong>in</strong>k this was done at KTH. Actually, it’s the LOT project, but itidentifies oxides and copper hydroxides, or copper hydroxide chlorides.The German researchers at BGR have shown that <strong>in</strong> thebentonite there is a lot <strong>of</strong> formation <strong>of</strong> copper sulphides and copperiron sulphides. It means that the nature is more complex thanthe <strong>in</strong>itial model. One way to see it is that if you take care <strong>of</strong> all thesulphides, the redox potential will be somewhat higher. If theredox potential is somewhat higher, still anoxic and still withoutoxygen gas, it will allow these products to form, <strong>in</strong> other words,corrosion by copper oxide and copper hydroxychloride formation.The situation is much more complex than previously thought. Thisis based on the results from the LOT research. I believe they aregood results, because if you get a lot <strong>of</strong> sulphides here, it reallymeans that you have reached an anoxic, or O 2 free, environment. Imean, if you have a model that is thirty years old, it is not strangethat it has to be changed because <strong>of</strong> new results. That is normal.But from what I understand from the safety analysis, this is notdiscussed.Fraser K<strong>in</strong>g, consultant to SKB: I am not exactly sure what thequestion is, but let me talk and then you can direct me if I’m go<strong>in</strong>g<strong>in</strong> the wrong direction. In the overall analysis, as Christ<strong>in</strong>a said,most damage occurs due to sulphide. Orig<strong>in</strong>ally, there was anassumption that that was a transport-limited process. We now haveexperimental evidence to show that that’s the case. Even under arelatively high rate <strong>of</strong> mass transport, with a relatively high sulphideconcentration, the reaction is sulphide transport-limited.When you go to a bentonite system, you drop the diffusion coefficientby two orders <strong>of</strong> magnitude. Compared to these experimentsyou were <strong>in</strong>creas<strong>in</strong>g the diffusion layer thickness by two, or three,orders <strong>of</strong> magnitude. In the repository, the flux <strong>of</strong> sulphides isabout five orders <strong>of</strong> magnitude lower than <strong>in</strong> the experiments. Inthe repository it’s virtually certa<strong>in</strong> that the reaction with sulphide isgo<strong>in</strong>g to be transport-limited. This is sort <strong>of</strong> what you are show<strong>in</strong>ghere. In terms <strong>of</strong> modell<strong>in</strong>g this whole scenario, we have a currentprogram where we’re look<strong>in</strong>g at develop<strong>in</strong>g an extension to thismodel, that I was talk<strong>in</strong>g about previously, where we predict theevolution <strong>of</strong> the environment. Up to this po<strong>in</strong>t, we’ve just done it<strong>in</strong> the oxygen-conta<strong>in</strong><strong>in</strong>g system; also <strong>in</strong>volves copper(II) be<strong>in</strong>gformed, and so on. We predicted <strong>in</strong> that case how the corrosionpotential evolves over time, to the po<strong>in</strong>t that now we are deal<strong>in</strong>g96
Edited transcripts from the workshopwith the sulphide. We did some experiments many years ago <strong>in</strong>Canada where we simulated this on a very small scale. This potential– the corrosion potential, not the redox potential – as you addthe sulphide, as the sulphide takes effect, the potential drops by900 millivolts, because at that stage, water becomes an oxidant.We’ve always taken that <strong>in</strong>to account, and that’s why 95 percent <strong>of</strong>the damage that SKB takes <strong>in</strong>to account is due to sulphide. We’vehad that analysis for a long time. We are at the stage <strong>of</strong> develop<strong>in</strong>ga reactive transport model to predict that the calculations we’vedone to date simulate almost exactly the experimental data that wedeveloped <strong>in</strong> Canada ten or fifteen years ago, where the potentialgoes through this very dramatic drop, when sulphide reaches thecopper surface. That’s because we’re chang<strong>in</strong>g the mechanism. Theoxidant is chang<strong>in</strong>g from consumption <strong>of</strong> oxygen to the reduction<strong>of</strong> copper(II), to the reduction <strong>of</strong> water. You can just do a simpleEvans diagram. When those rate-controll<strong>in</strong>g steps change, and thatpotential comes down 900 millivolts, it’s a process we’ve understoodfor a long time. It’s a process we’re now <strong>in</strong>clud<strong>in</strong>g <strong>in</strong> ourdetailed reactive transport model.Peter Szakálos, KTH: I’m not satisfied with that discussion. If youfigure out that you can produce oxides and hydroxides, and that isnot <strong>in</strong>cluded <strong>in</strong> the model? It was believed that sulphide ions producea copper sulphide. Now we can see that, because we havesome solubility <strong>of</strong> copper, you produce a lot <strong>of</strong> copper ions, andthey go <strong>in</strong>to the bentonite and will be consumed there when theyencounter the sulphides. The diffusion distance will be changed,and you will have a cont<strong>in</strong>uous precipitation <strong>of</strong> sulphides <strong>in</strong> thebentonite. There is a reaction with water and chlorides, and that isnot <strong>in</strong>cluded.Fraser K<strong>in</strong>g, consultant to SKB: You’re correct up to a po<strong>in</strong>t, whereI wouldn’t agree with the last po<strong>in</strong>t. But, you’re exactly right, thesulphide is precipitated <strong>in</strong> the bentonite, with the reaction with theiron(II) – there’s always iron(II) <strong>in</strong> the bentonite – so you get a lot<strong>of</strong> that sulphide precipitat<strong>in</strong>g as amorphous FeS, which may t<strong>of</strong>urther to change <strong>in</strong>to pyrite. But a lot <strong>of</strong> that sulphide won’t reachthe conta<strong>in</strong>er surface, because it’s precipitated.Peter Szakálos: Yes, it takes care <strong>of</strong> it earlier.Fraser K<strong>in</strong>g: But the potential, <strong>in</strong> that case, does not lower thewater reduction potential. It stays poised at a potential, as we’ve97
Edited transcripts from the workshopseen <strong>in</strong> our experiments, <strong>of</strong> m<strong>in</strong>us 400 millivolts, versus calomel.It’s nowhere near the water reduction potential. Only when thatsulphide reaches the surface does that potential drop. That’sbecause <strong>in</strong> the presence <strong>of</strong> sulphide, water becomes an oxidant andthen the potential drops. That’s when water will be reduced,hydrogen is evolved, and that’s the reaction we’ve taken <strong>in</strong>toaccount through these mass transport sulphide calculations. Evers<strong>in</strong>ce KBS-3.Peter Szakálos: We are just look<strong>in</strong>g at the results here, and, we cansee that it forms oxides and hydroxide chlorides.Fraser K<strong>in</strong>g: In the LOT experiments, he’s got oxygen <strong>in</strong> there, <strong>in</strong>those experiments. They are formed under aerated conditions,they’re a remnant <strong>of</strong> the fact that he’s had oxygen <strong>in</strong> there.Peter Szakálos: In the LOT five-year exposure, oxides and copperhydroxide chlorides formed on the surface <strong>of</strong> your samples. I can’tunderstand why you have oxygen there, and not here. Because ifyou form sulphides here, how could you have a high oxygen partialpressure here? Where is that com<strong>in</strong>g from?Bo Rosborg, Studsvik Water Chemistry, consultant to SKB: A greatpart <strong>of</strong> the exposure time is an oxic period.Peter Szakálos: Is that true? Here you see that you form sulphidesimmediately. How oxic is it?Bo Rosborg: I cannot tell how long the oxic period lasts. But <strong>of</strong>course, the ultimate experiment is to measure the corrosion potentialhere, to see if you have plus 200, or around zero, or m<strong>in</strong>us 400,500. Then you know where you are. It’s easy to say, but difficult todo. We have made an effort, but we have too many stray currentsaround <strong>in</strong> the laboratory.Peter Szakálos: It’s k<strong>in</strong>d <strong>of</strong> a good experiment. It’s five years. Weshould approach a k<strong>in</strong>d <strong>of</strong> thermodynamic situation here that iscorrect. If there was some oxygen from the beg<strong>in</strong>n<strong>in</strong>g, I don’tbelieve that it dom<strong>in</strong>ates this corrosion for five years. There arereports show<strong>in</strong>g that bentonite itself consumes oxygen. A lot <strong>of</strong>reports <strong>in</strong>dicate that’s it anoxic with<strong>in</strong> five days or so.Dave Shoesmith: We’re study<strong>in</strong>g the transformation <strong>of</strong> oxides bysulphide, both <strong>in</strong> the copper system and <strong>in</strong> the iron oxide system.If it’s Cu(I) oxide, then the sulphide would convert it chemically98
Edited transcripts from the workshopby an anion exchange process. You don’t see the transition to thelow potentials, until you break through. You see only the metalsurface. If it’s copper(II), or <strong>in</strong> the iron case, it’s iron(III), then thesulphide is not nearly as reactive, because it needs a redox process.Therefore it leaves beh<strong>in</strong>d residues <strong>of</strong> the oxidized form. Youwould expect to see residues, irrespective <strong>of</strong> whether the sulphidebroke through to the surface or not. You would expect to see residues<strong>of</strong> copper(II) oxide hydroxide, just as on gas pipel<strong>in</strong>es andth<strong>in</strong>gs we see residues <strong>of</strong> iron(III) oxide <strong>in</strong> the presence <strong>of</strong> sulphide.The fact that it is there could just be a residue <strong>of</strong> what hasbeen suggested: a pre-period, and the conversion hasn’t takenplace.Peter Szakálos: If you know the situation here, with heated copper.This is k<strong>in</strong>d <strong>of</strong> a high value measured. What is the copper ion <strong>in</strong> saltwater, chloride water? Then you have a process with copper ionsleav<strong>in</strong>g this copper surface and this process will cont<strong>in</strong>ue. I can’tsee how it goes down to ångströms per year, or nanometres peryear. That should be proven, before we can accept this model. Isn’tthat true?Dave Shoesmith: My po<strong>in</strong>t is that sulphide reduces the potential,localized size, and leaves those residues beh<strong>in</strong>d. Then they becomeisolated <strong>in</strong>sulators, not <strong>in</strong>volved <strong>in</strong> the corrosion process. The corrosionprocess is tak<strong>in</strong>g place with the sulphide on the conductiveregions <strong>of</strong> the surface. But just see<strong>in</strong>g an oxidized residue leftbeh<strong>in</strong>d doesn’t mean that sulphide isn’t do<strong>in</strong>g anyth<strong>in</strong>g.Peter Szakálos: Do you feel comfortable with this situation, <strong>in</strong> thatwe have a theoretical model, and when we measure it, it’s like tento twenty micrometres per year <strong>in</strong> corrosion rate. It’s not evenclose to the model. Or, do you believe that it, eventually, goesdown to the ångström level, or the nanometre level?Dave Shoesmith: Well, the rates are another issue. At 5x10 -5 molarsulphide, under diffusion control, our rate is hundreds <strong>of</strong> nanometresper year, and we have no <strong>in</strong>termediate bentonites, no <strong>in</strong>termediatediffusion barriers. We’re already down to hundreds <strong>of</strong>nanometres per year, with sulphide alone.Peter Szakálos: Yes, with sulphide alone. But just consider<strong>in</strong>g thissituation with the high solubility <strong>in</strong> sal<strong>in</strong>e water, you produce copperions at k<strong>in</strong>d <strong>of</strong> a high rate, and you get this form <strong>of</strong> precipita-99
Edited transcripts from the workshoption here <strong>in</strong> the bentonite. You can’t just look at it as a simple sulphidediffusion, produc<strong>in</strong>g a sulphide there. It is not happen<strong>in</strong>g.Wouldn’t you agree that there is some question here?Mart<strong>in</strong> Boj<strong>in</strong>ov, VTT Technical Research Centre: A comment onthe value that you have shown, 2,300 micrograms per litre. It’s ourvalue, and I will tell you that we didn’t believe <strong>in</strong> it much, becausethese are experiments that have been performed <strong>in</strong> sta<strong>in</strong>less steelautoclaves with six-day exposure. We first performed the measurementwithout Oxisorb, which has been commented on by thepr<strong>of</strong>essor from S<strong>in</strong>gapore. We mentioned that we didn’t achieveour mass balance, due to the fact that some <strong>of</strong> the copper wasdeposited on the autoclave walls. We didn’t f<strong>in</strong>d a proper way toclean up this system, so the fact that these concentrations arehigher <strong>in</strong> anoxic conditions is simply an artefact <strong>of</strong> the fact that wedidn’t achieve our mass balance. This data shouldn’t be somehowused to produce theoretical calculations. This is mentioned <strong>in</strong> ourreport.Peter Szakálos: The solubility is quite high anyway, if you look atthe literature. It is extremely high. I was just stat<strong>in</strong>g that it is not apassivat<strong>in</strong>g metal, that gives you this problem. You produce ions,and the corrosion will cont<strong>in</strong>ue. If I saw that the corrosion rategoes down to nanometres per year, I would be happy, but first thismust be shown.Digby Macdonald: You’ve drawn the redox potential as go<strong>in</strong>g up. Isthat based on anyth<strong>in</strong>g scientific, or is that just artistic licence?Peter Szakálos: No, it’s because we have some sulphide activity <strong>in</strong>the repository, and clearly no sulphides are formed on the coppersurface, but <strong>in</strong>stead a lot <strong>of</strong> copper sulphides precipitate on thebentonite particles at a distance from the copper canister. Thismeans that the sulphide activity is lower and the redox potential ishigher close to the copper surface. Thus we can understand theformation <strong>of</strong> copper oxides and hydroxychlorides close to the coppersurface, caused by the anoxic and sal<strong>in</strong>e water, as observed <strong>in</strong>the LOT project.Digby Macdonald: But the redox potential is go<strong>in</strong>g to be dom<strong>in</strong>atedby the hydrogen that’s produced. In which case, it should go theother way.100
Edited transcripts from the workshopPeter Szakálos: I just show that because <strong>of</strong> a lower<strong>in</strong>g <strong>of</strong> the sulphide,you could have a situation here with sal<strong>in</strong>e water and notmuch else. This sal<strong>in</strong>e water could produce this k<strong>in</strong>d <strong>of</strong> corrosion.Digby Macdonald: Have you tried calculat<strong>in</strong>g the redox potentials?Peter Szakálos: No, I’m just fitt<strong>in</strong>g the data to what has been publishedabout this LOT project. It’s just to show what they havereported about the LOT project, and what the model predicts.There is a big difference.Bo Rosborg, Studsvik Water Chemistry, consultant to SKB: Themodel used <strong>in</strong> the LOT project is that you have divided time <strong>in</strong>toan oxic period and an anoxic period. The model states that dur<strong>in</strong>gthe oxic period, you should have rates below seven microns a year.The last figures po<strong>in</strong>t to 4, or 2.4, or 3.5, which is well below themodel values. Then the anoxic period starts, which we are discuss<strong>in</strong>g.Ron Latanision: I like to turn the conversation a step back. As aneducator, I taught my corrosion eng<strong>in</strong>eer<strong>in</strong>g students at MIT thatone <strong>of</strong> the roles that they play, is to choose construction materialfor eng<strong>in</strong>eer<strong>in</strong>g systems, whatever those systems may be, based onthe service environment <strong>in</strong> which those systems are <strong>in</strong>tended toperform. I th<strong>in</strong>k that if we look at this better, <strong>in</strong> terms <strong>of</strong> the actualemplacement <strong>of</strong> a package, a waste conta<strong>in</strong>er, <strong>in</strong> the repository,we’re deal<strong>in</strong>g with a circumstance <strong>in</strong> which we do believe the environmentto be reduc<strong>in</strong>g. We do believe that sulphur, and sulphides<strong>in</strong> particular, would be present <strong>in</strong> a reduc<strong>in</strong>g environment. There issome evidence, that chloride would play a role. Now there’s aquestion, obviously water plays a role from the very beg<strong>in</strong>n<strong>in</strong>g. It isthere to solvate species which carry charge, without which therewould be no corrosion reaction. If this were absolutely dry, no onewould be concerned. We know it’s not absolutely dry, and thereforethere is a question <strong>of</strong> what role hydrogen may play. We have aroughly thirty year old <strong>in</strong>vestigation which shows that water playsa role. That’s a bit unusual, <strong>in</strong> terms <strong>of</strong> our historical understand<strong>in</strong>g<strong>of</strong> the <strong>in</strong>fluence <strong>of</strong> sulphides and <strong>of</strong> chlorides, etc. That’s important.But I th<strong>in</strong>k the ultimate question is whether or not the k<strong>in</strong>d<strong>of</strong> role, that is be<strong>in</strong>g proposed, <strong>in</strong> the case <strong>of</strong> <strong>in</strong>fluence <strong>of</strong> water,overwhelms the effect that we know historically to be the case forchlorides and sulphides, and so on. At this early stage, <strong>in</strong> terms <strong>of</strong>my <strong>in</strong>volvement <strong>in</strong> this conversation, I haven’t been conv<strong>in</strong>ced that101
Edited transcripts from the workshopit plays an overwhelm<strong>in</strong>g role. It may play a role, but I do believethat the historical data base would suggest that sulphide concernsand chloride concerns are first-order. I’m not yet conv<strong>in</strong>ced thatthe questions <strong>of</strong> copper corrosion <strong>in</strong> a reduc<strong>in</strong>g environment toproduce hydrogen is on the same first-order. I just don’t know.Maybe. That’s the first po<strong>in</strong>t I want to make. The second po<strong>in</strong>t isthat corrosion eng<strong>in</strong>eers always ask: “What is the mode <strong>of</strong> corrosion?Is it uniform, is it localized, an embrittlement phenomenon?”Based on all the th<strong>in</strong>gs I’ve read <strong>in</strong> preparation for this meet<strong>in</strong>g,and all <strong>of</strong> the th<strong>in</strong>gs that I’ve heard today, what would concern memost were if someone were to be conv<strong>in</strong>ced that hydrogen is be<strong>in</strong>gproduced and embrittl<strong>in</strong>g the copper. As a personal judgement,that would concern me more than the rate <strong>of</strong> uniform corrosion orthe rate <strong>of</strong> localized corrosion, because <strong>of</strong> the loss <strong>of</strong> <strong>of</strong>f-load<strong>in</strong>gcapacity and the loss <strong>of</strong> the <strong>in</strong>tegrity <strong>of</strong> the package. There aresome critical issues that we still need to resolve. What role is thelicens<strong>in</strong>g authority play<strong>in</strong>g <strong>in</strong> this conversation? In the UnitedStates, the Nuclear Regulatory Commission, the equivalent <strong>of</strong> yourlicens<strong>in</strong>g SSM, plays a pretty significant role <strong>in</strong> terms <strong>of</strong> licenseapplication, once it’s submitted. Maybe even prior to its submission,<strong>in</strong> terms <strong>of</strong> conversations. Have SKB or the people from theuniversities had conversations with the licens<strong>in</strong>g authority? Whatk<strong>in</strong>d <strong>of</strong> response are you gett<strong>in</strong>g?Bo Strömberg, SSM: We have been follow<strong>in</strong>g these discussions andhave had meet<strong>in</strong>gs with both Dr. Hultquist, his group <strong>of</strong> researchers,and SKB. And we’ve made also a sort <strong>of</strong> review <strong>of</strong> the experiment,which was actually done by Timo Sario. That document was<strong>in</strong>cluded <strong>in</strong> the material that you have been look<strong>in</strong>g at. We’re alsodo<strong>in</strong>g literature reviews, and are contemplat<strong>in</strong>g do<strong>in</strong>g some k<strong>in</strong>d <strong>of</strong>experiments <strong>of</strong> our own <strong>in</strong> this area. But our real sort <strong>of</strong> judgementwill not count until we get the license application.Ron Latanision: That’s what I would expect.Peter Szakálos, KTH: What do you th<strong>in</strong>k about these new resultsfrom Japanese researchers that sulphide does by itself <strong>in</strong>duce stresscorrosion crack<strong>in</strong>g?Ron Latanision: <strong>Copper</strong> is an unusual metal, and there is a lot <strong>of</strong>evidence, that, unlike most metallic alloy systems, relatively purecopper can experience stress corrosion crack<strong>in</strong>g <strong>in</strong> sulphides, <strong>in</strong>102
Edited transcripts from the workshopchlorides, <strong>in</strong> other media, typically under oxidiz<strong>in</strong>g conditions.That’s well-known.Peter Szakálos: But this was reduc<strong>in</strong>g.Ron Latanision: I haven’t had an opportunity to exam<strong>in</strong>e this.When I look at fracture surfaces, I would like to know what’s onthe fracture surface. The fact that sulphide is <strong>in</strong> the environment,maybe it plays a role, maybe it does not. Maybe this problem isassociated with segregated species along a gra<strong>in</strong> boundary, whichare <strong>in</strong>teract<strong>in</strong>g <strong>in</strong> some other way. I can’t make a comment on thatwithout know<strong>in</strong>g much more about it. It’s <strong>in</strong>trigu<strong>in</strong>g, it would beunusual. But I th<strong>in</strong>k it does need to be explored.Fraser K<strong>in</strong>g, consultant to SKB: The species that cause stress corrosion<strong>of</strong> copper are nitrite, ammonia, and acetate. Not chloride, notsulphide. Chloride is good and promotes active dissolution. I likechloride.There’s a huge long list for brass, but for pure copper, it’s just thenitrite, acetate and ammonia. This issue with the sulphide. I haveseen this paper, and it <strong>in</strong>volved slow stra<strong>in</strong> rate experiments <strong>in</strong> arange <strong>of</strong> sulphide concentrations <strong>in</strong> deaerated sea water, underanaerobic conditions. They were s<strong>in</strong>gle, slow stra<strong>in</strong> rate experiments.There’s no obvious trend. There’s some variability <strong>in</strong> theresults. They’re look<strong>in</strong>g at stra<strong>in</strong> to failure <strong>in</strong> these slow stra<strong>in</strong> rateexperiments. There isn’t a consistent trend with concentration. Butthey did conclude that, based on this fractographic evidence andthe elongation to failure, there was this threshold concentration <strong>of</strong>five to ten millimolar <strong>of</strong> sulphide where they didn’t see any stresscorrosion. Even if you assume that they’re right, and this is stresscorrosion, these concentrations – the groundwater concentrationaway from the canister is 10 -5 molar sulphide. As we’ve heard it’stransport-limited through the bentonite, and the <strong>in</strong>terfacial concentration<strong>of</strong> sulphide is two or three orders <strong>of</strong> magnitude belowthat. Let’s say we’re down to 10 -6 molar sulphide at the canistersurface. We’re four orders <strong>of</strong> magnitude below this value. If webelieve this special value for the canister, and that’s what we’re sort<strong>of</strong> focuss<strong>in</strong>g on <strong>in</strong> this session. In terms <strong>of</strong> the canister, it’s goodnews.Peter Szakálos: I just saw that the sulphide concentration could beclose to the range <strong>in</strong> the F<strong>in</strong>nish repository. It was published.103
Edited transcripts from the workshopFraser K<strong>in</strong>g: Somewhat higher sulphide concentration than <strong>in</strong> some<strong>of</strong> the F<strong>in</strong>ish groundwater, but aga<strong>in</strong>, it’s <strong>in</strong> the groundwater.We’re concerned about what it is at the canister surface. A lot <strong>of</strong>that sulphide is precipitated <strong>in</strong> the bentonite before it gets to thecanister. The <strong>in</strong>terfacial concentration <strong>of</strong> sulphide at the canister’ssurface, once it tries to diffuse, is very low. These experiments weredone <strong>in</strong> bulk solutions. They put <strong>in</strong> 10 millimolar sulphide. It’s an<strong>in</strong>credible concentration, compared to what we have <strong>in</strong> thegroundwater.Peter Szakálos: Yes, but we have a problem here, with the“Forsmark situation.” There we have water enter<strong>in</strong>g the system,and cont<strong>in</strong>uously you will have a production <strong>of</strong> high ion rates <strong>of</strong> allthe salts that we have there.Fraser K<strong>in</strong>g: But this hot period here corresponds to the aerobicperiod. By the time you establish anaerobic conditions, the canister’scooled.David Duquette: Part <strong>of</strong> what we’re supposed to be do<strong>in</strong>g <strong>in</strong> thissection is identify<strong>in</strong>g th<strong>in</strong>gs that have to be followed up on.“Where are some unanswered questions that are showstoppers?”This is among them. I’m quite familiar with this work. Theseresults were from relatively very low stra<strong>in</strong> rate experiments. Thereweren’t large differences <strong>in</strong> the ductility, there was active corrosiongo<strong>in</strong>g on the copper. They reported some evidence <strong>of</strong> stress corrosioncrack<strong>in</strong>g, but it wasn’t strong evidence. This is the k<strong>in</strong>d <strong>of</strong>th<strong>in</strong>g you have to be careful <strong>of</strong>. I would admonish you, Peter,about mak<strong>in</strong>g presentations where you have s<strong>in</strong>gle quotes from apaper. If you look at the whole paper, it wasn’t very much stresscorrosion crack<strong>in</strong>g. It wasn’t like you went from 30 percent ductilityto 10 percent; it was 30 percent to 24 percent, or 26 percent. Itis work that I th<strong>in</strong>k has to be followed up on, and I th<strong>in</strong>k what thissession is supposed to be do<strong>in</strong>g is identify<strong>in</strong>g that k<strong>in</strong>d <strong>of</strong> th<strong>in</strong>g.Fraser K<strong>in</strong>g: SKB is sponsor<strong>in</strong>g a study at the University <strong>of</strong>Toronto on this very subject.Digby Macdonald: In the case <strong>of</strong> the iron system, the reactivity <strong>of</strong>these sulphur species depends upon the ease with which they candonate atomic sulphur to the iron surface. For example, thiosulphatewill <strong>in</strong>duce stress corrosion crack<strong>in</strong>g and sensitize 304sta<strong>in</strong>less steel. It’s directly related to the ability to donate atomic104
Edited transcripts from the workshopsulphur. Many <strong>of</strong> those species that I showed you <strong>in</strong> that list dohave the ability <strong>of</strong> donat<strong>in</strong>g atomic sulphur. If there were a rationalprogramme to look <strong>in</strong>to this, there would be a systematic test doneon each <strong>of</strong> these sulphur species to determ<strong>in</strong>e which ones are themost deleterious to copper. I’ll have to echo Ron’s comments.What I th<strong>in</strong>k is lack<strong>in</strong>g is that I haven’t heard a del<strong>in</strong>eation <strong>of</strong> thecritical issues <strong>in</strong> this whole technology. You know, what is it that’sreally important, that needs to be done? Maybe that’s our task thisafternoon, to del<strong>in</strong>eate this. I’m a bit surprised that somebodyhasn’t put up a list <strong>of</strong> critical issues that need to be addressed <strong>in</strong>develop<strong>in</strong>g the technology.Saida Laârouchi Engström, SKB: This is someth<strong>in</strong>g we do for all theeng<strong>in</strong>eered barriers <strong>in</strong> our system. Every third year, we produce aresearch development report that we submit to the government.All the issues that need more research are mentioned <strong>in</strong> this report.It’s divided <strong>in</strong>to different barriers. There’s the copper canister,there’s the bentonite issues, and so forth. Each year these issues arediscussed with the scientific community. A programme that is preparedafter the regulatory review and the decision <strong>of</strong> the governmentdeterm<strong>in</strong>es the research to be conducted for the com<strong>in</strong>g threeyears. We’ve produced n<strong>in</strong>e <strong>of</strong> these reports over the years. Thescientific community at large and SKB’s research team have been<strong>in</strong>timately <strong>in</strong>volved <strong>in</strong> identify<strong>in</strong>g critical issues. These <strong>in</strong>clude corrosion,bentonite erosion and many other areas. The work proceedscont<strong>in</strong>uously.Digby Macdonald: I’m surprised we haven’t seen a list <strong>of</strong> issues,though.Saida Laârouchi Engström, SKB: They are <strong>in</strong> my reports. You arevery welcome to them.Digby Macdonald: I’m surprised we haven’t seen a list here, dur<strong>in</strong>gthis debate that is go<strong>in</strong>g on.Peter Szakálos, KTH: Maybe the panel also knows someth<strong>in</strong>g abouta Japanese concept. This is described <strong>in</strong> the SKB report. There is anastonish<strong>in</strong>gly big difference between these situations, <strong>in</strong> theJapanese groundwater, compared to Sweden/F<strong>in</strong>land.Digby Macdonald: A huge difference.105
Edited transcripts from the workshopPeter Szakálos: We’re talk<strong>in</strong>g about a corrosion rate <strong>of</strong> two copperatom layers per year here. In the Japanese case, they assume a30,000 to 60,000 times higher corrosion rate.Fraser K<strong>in</strong>g: That’s taken out <strong>of</strong> context. The Japanese have takenall sulphates <strong>in</strong> the clay – and for those people who know bentonite,there’s a huge amount <strong>of</strong> gypsum and anhydrite <strong>in</strong> thisbentonite. They’ve done a mass balance calculation, and they’vetaken all <strong>of</strong> the sulphate and assumed that it, by some process, presumablymicrobial activity, has turned <strong>in</strong>to sulphide. Then theyapply all that sulphide to the canister. That Japanese number <strong>of</strong> 9to 13 millimetres is a measure <strong>of</strong> how much sulphate, how muchgypsum is <strong>in</strong> the bentonite. I don’t want to be critical <strong>of</strong> theJapanese, but, <strong>in</strong> my op<strong>in</strong>ion, that’s an extremely conservative th<strong>in</strong>gto have done. I wouldn’t have done it.Peter Szakálos: Do you mean that the Japanese people are quiteconservative, and they go for safety first?Allan Hed<strong>in</strong>, responsible for the safety assessments at SKB: I don’tth<strong>in</strong>k we have to say so much more about this. You can be conservativeand you can be super-conservative, and I th<strong>in</strong>k Fraserexpressed it quite well, that the Japanese approach <strong>in</strong> this particularcalculation could be regarded as super-conservative, <strong>in</strong> the sensethat it doesn’t really mean anyth<strong>in</strong>g. This is noth<strong>in</strong>g that onewould, for any reason, expect would happen <strong>in</strong> the system.Whereas the way we handle it, the numbers we are compar<strong>in</strong>g forthe Swedish and F<strong>in</strong>nish systems, are aim<strong>in</strong>g at someth<strong>in</strong>g that issomewhat more reasonable, but still pessimistic.Peter Szakálos: If your model was right, I would agree with that.But, then you must demonstrate that with full-scale experiments.That should be done.Ron Latanision: I noticed <strong>in</strong> that the Japanese data there’s discussion<strong>of</strong> a pitt<strong>in</strong>g factor. Are we dist<strong>in</strong>guish<strong>in</strong>g pitt<strong>in</strong>g from uniformcorrosion <strong>in</strong> terms <strong>of</strong> the SKB approach and the Japaneseapproach? What is the importance <strong>of</strong> that pitt<strong>in</strong>g factor, from yourperspective?Christ<strong>in</strong>a Lilja, SKB: We haven’t been us<strong>in</strong>g the concept <strong>of</strong> the traditionalpitt<strong>in</strong>g factor, as the late analysis show that it’s more <strong>of</strong> anuneven corrosion than real pits.106
Edited transcripts from the workshopFraser K<strong>in</strong>g, consultant to SKB: Go<strong>in</strong>g back to 1978, when SKB-TR78-90, came out. In that analysis, SKB assumed uniform corrosion,and at that stage there were some question about whether there’dbe localized corrosion. No experimental evidence under laboratoryconditions was available at that time.Ron Latanision: No evidence, then, <strong>of</strong> localized corrosion?Fraser K<strong>in</strong>g: Correct. In order to ma<strong>in</strong>ta<strong>in</strong> conservatism – because<strong>of</strong> the well-known pitt<strong>in</strong>g <strong>of</strong> copper hot water pipes – it wasdecided to take <strong>in</strong>to account some possibility <strong>of</strong> localized corrosion.The treatment that was used was this pitt<strong>in</strong>g factor. To get avalue for that pitt<strong>in</strong>g factor, they did an analysis <strong>of</strong> the NBS’s, theNational Bureau <strong>of</strong> Standards, corrosion data, the undergrounddisposal data. From that, they came out with various measures <strong>of</strong>the pitt<strong>in</strong>g factor, based on that old data. At one stage it was 25,and that was the extreme value. That was <strong>in</strong> some k<strong>in</strong>d <strong>of</strong> strangeUS ash type soil. A more realistic value is 5. Recently we’ve usedthe value <strong>of</strong> 3. Now, that still assumes that there’s some classicalpitt<strong>in</strong>g mechanism that separates anodic and cathodic sites. Nowwe understand from the experiments we’ve done under simulatedrepository conditions that that’s not what we see. What we see iscorrosion <strong>of</strong> the whole surface. If you look at the surface, it’sroughened. It’s not perfectly atomically uniform, there’s a roughen<strong>in</strong>g.In SR-Can, the most recent analysis, SKB no longer uses apitt<strong>in</strong>g factor. We use a surface roughen<strong>in</strong>g, and that’s plus orm<strong>in</strong>us 40 or 50 microns, and that’s based on experimental data. In achloride-dom<strong>in</strong>ated system, copper dissolves actively <strong>in</strong> the presence<strong>of</strong> oxygen. We have no evidence that we have permanent separation<strong>of</strong> anodic and cathodic sites. The Japanese are still us<strong>in</strong>g apitt<strong>in</strong>g factor. SKB have now gone away from that, to this surfaceroughen<strong>in</strong>g technique.Ron Latanision: I th<strong>in</strong>k that’s a useful perspective, <strong>in</strong> terms <strong>of</strong>what’s on this slide.David Duquette: Do you agree that the pitt<strong>in</strong>g factor is unimportant?Peter Szakálos: I th<strong>in</strong>k we have some k<strong>in</strong>d <strong>of</strong> pitt<strong>in</strong>g corrosion,s<strong>in</strong>ce there are bivalent corrosion products. It should be <strong>in</strong>cluded.Gaik Khuan Chuah: When you do the experiments at SKB, afterthat you write a report. Who reads the report and, after that, looks107
Edited transcripts from the workshopat it critically and suggests further th<strong>in</strong>gs to do? When this reportf<strong>in</strong>ally reaches the government, who <strong>in</strong> the government reads thereport?Saida Laârouchi Engström, SKB: By law, SKB is completely <strong>in</strong>charge <strong>of</strong> putt<strong>in</strong>g forward a proven and safe solution for a f<strong>in</strong>alrepository. We have to provide the Government with evidence andsubmit an application, which will happen <strong>in</strong> one year. The researchreport I talked about is also required by law. It’s gett<strong>in</strong>g dissem<strong>in</strong>atedto 30 or 40 organisations like universities, people at KTHand others. Everybody will review and give their po<strong>in</strong>t <strong>of</strong> views onwhat SKB should be putt<strong>in</strong>g their effort <strong>in</strong>to solv<strong>in</strong>g. That goes foreng<strong>in</strong>eered barriers, safety analyses, and so on. Once we’ve donethat, the authorities will compile the viewpo<strong>in</strong>ts <strong>of</strong> all the differentorganisations and submit them, along with their evaluation, to theGovernment. The Government issues directives regard<strong>in</strong>g to whatextent the research we are conduct<strong>in</strong>g is okay, and what otherth<strong>in</strong>gs we should be do<strong>in</strong>g more <strong>of</strong>. This has been done every thirdyear, and we have submitted n<strong>in</strong>e <strong>of</strong> these reports. The researchreports that we publish are publicly available. We are 100 percentopen with our research. You can download most <strong>of</strong> our reports –which are published and quality assured – on our website. And wehave been do<strong>in</strong>g this for the last twenty years. SKB is required bylaw to come up with a solution. The authorities will review it andthen make their recommendation to the Government. This will besubmitted along with a rul<strong>in</strong>g by an environmental court that willdo the same th<strong>in</strong>g, subject to the provisions <strong>of</strong> the EnvironmentalCode. The Government will thus receive two proposals. One fromSSM, with their op<strong>in</strong>ion <strong>of</strong> the application that SKB will be submitt<strong>in</strong>g<strong>in</strong> one year, and one under the provisions <strong>of</strong> the EnvironmentalCode. Then the Government will make their own rul<strong>in</strong>g.But nobody expects politicians to be corrosion chemists, or radiophysicists.They will have to rely on the environmental court andtheir competent authorities <strong>in</strong> the field.David Duquette: Do you get a technical response about the details<strong>of</strong> your research, with comments about what you should do, toimprove the quality <strong>of</strong> the research, and/or its reproducibility?Saida Laârouchi Engström, SKB: Yes, from the competent authority,SSM. They have their own experts <strong>in</strong> the field that critique usand <strong>in</strong>dicate the areas where they th<strong>in</strong>k SKB is not do<strong>in</strong>g enough.108
Edited transcripts from the workshopBut this is actually secondary to the judgements <strong>of</strong> our own <strong>in</strong>houseexperts. We are actually our own best critics, <strong>in</strong> our ownorganization. Our experts will also suggest what should be done <strong>in</strong>different areas <strong>in</strong> order to be able compile sufficient evidence whenthe application is submitted to the Government.David Duquette: But proposal and prescription are two differentth<strong>in</strong>gs.Bo Strömberg, SSM: Maybe I can clarify. The difference betweenthe Swedish system and the US system is that we take a less prescriptiverole. We make recommendations to SKB, but these arerather general and cover the overall direction <strong>of</strong> the programme,progress<strong>in</strong>g <strong>in</strong> sort <strong>of</strong> the right direction. We don’t always commenton the technical details. But we have our own experts, withwhom we discuss these matters, and we write review statements.The <strong>in</strong>tention <strong>of</strong> be<strong>in</strong>g less prescriptive is that we should not go s<strong>of</strong>ar that we start to design the repository from the regulatory side.Then we would have difficulty <strong>in</strong> review<strong>in</strong>g it objectively when weget the license application.Gaik Khuan Chuah: I didn’t quite understand. Didn’t SKB itselfhave technical experts, who would look through the results, toanswer questions that have been raised? Are these consultants?Saida Laârouchi Engström, SKB: Yes, we have our own <strong>in</strong>-houseexperts, but we have also an external expert group that reviews ourwork and gives us suggestions, or recommendations, on how tocarry out our research. That group <strong>in</strong>cludes experts not only fromSweden, but also from outside Sweden.Gaik Khuan Chuah: Could you give us an idea <strong>of</strong> how, over then<strong>in</strong>e reports, the questions have changed? What is the trend?Saida Laârouchi Engström: These reports deal with the whole issue<strong>of</strong> a f<strong>in</strong>al repository. What we are discuss<strong>in</strong>g here is the process <strong>in</strong>one <strong>of</strong> the eng<strong>in</strong>eered barriers, which is a very important one forour system. The research that we carry out is actually <strong>in</strong>tended toprove the safety <strong>of</strong> our f<strong>in</strong>al repository, which you cannotextrapolate directly from one process. Those reports cover all k<strong>in</strong>ds<strong>of</strong> issues, such as fuel dissolution, bentonite behaviour, copper andmechanical issues, from the viewpo<strong>in</strong>t <strong>of</strong> corrosion. The reports arenot only about canisters, they deal with what research, <strong>in</strong> general,and <strong>in</strong> detail, should be done to prove the safety <strong>of</strong> the f<strong>in</strong>al109
Edited transcripts from the workshoprepository. When we get the Government’s rul<strong>in</strong>g on the reportand our research, they either confirm what we are do<strong>in</strong>g – “Goahead, this is okay for this area” – or generally, as has been the case,they will say, “Well, this looks good, but do more <strong>of</strong> that work onthis area or that area.” These reports have been very important tous, just to confirm that the work we are do<strong>in</strong>g, with our ownexperts, and the experts outside the organization, is on the righttrack. This is not research that has been done <strong>in</strong> a laboratory <strong>in</strong>splendid isolation; the scientific community, at a very high level,has been very much <strong>in</strong>volved.Ron Latanision: It’s not unusual <strong>in</strong> our <strong>in</strong>teractions <strong>in</strong> the UnitedStates, as the Yucca Mounta<strong>in</strong> project evolved, that there were lots<strong>of</strong> questions. There was a lot <strong>of</strong> research that was identified that ledto some concerns about the viability <strong>of</strong> the waste package <strong>in</strong> oxidiz<strong>in</strong>genvironments. They were always presented <strong>in</strong> a relativelytimely way. Why has it taken so long for the question <strong>of</strong> hydrogento surface here? It has been go<strong>in</strong>g on for thirty years, has it not?Am I miss<strong>in</strong>g a po<strong>in</strong>t?Christ<strong>in</strong>a Lilja, SKB: It’s not new; it’s been <strong>in</strong> the safety assessmentfor a long time. I mentioned the process reports that <strong>in</strong>cludea structured list <strong>of</strong> every process. It’s been dwelt on earlier. But ithas been discussed more now that we’ve taken a deeper look <strong>in</strong>tothe area.Allan Hed<strong>in</strong>, SKB: This issue was raised for the first time <strong>in</strong> 1986.Some attempts to reproduce the results were made, and as thosefailed, the question faded away. In recent years, there have beennew publications. We have, for the last couple <strong>of</strong> years, beenwork<strong>in</strong>g actively <strong>in</strong> this area. It has been a two-stage th<strong>in</strong>g. Rightor wrong, but this is how it has evolved.David Duquette: What if Peter and Gunnar are correct, and thecorrosion rate is 10 microns per year? Where do you go?Christ<strong>in</strong>a Lilja, SKB: We’re go<strong>in</strong>g to the mass transport calculations,because we th<strong>in</strong>k the process is mass transport. It is already<strong>in</strong>cluded <strong>in</strong> the safety assessment.Gaik Khuan Chuah: Did SKB contact Gunnar when you couldn’treproduce the results? You did your experiments. You couldn’tprove it, but s<strong>in</strong>ce you have the advantage that you are both <strong>in</strong> thesame country, wouldn’t it be easy to get Gunnar over and plan and110
Edited transcripts from the workshopdo the experiments together? Just to prove, once and for all, thisway or that way.Allan Hed<strong>in</strong>, SKB: I th<strong>in</strong>k it is too early to say today. We havebeen <strong>in</strong> contact with Gunnar and his colleagues dur<strong>in</strong>g this phasethat we have seen now. But there has been no attempt to do a jo<strong>in</strong>texperiment. I’m not sure we will do that <strong>in</strong> the future either. Itneeds to be further looked <strong>in</strong>to, experimentally. Even if the resultsthat we have seen are right, even if hydrogen is produced to anequilibrium pressure <strong>of</strong> 1 mbar, it is fully possible to make thesafety case, based on mass transport considerations, that this willnot limit the lifetime <strong>of</strong> the canisters.Peter Szakálos: Only theoretically.Allan Hed<strong>in</strong>: The calculation is theoretical. Any prediction aboutthe future is theoretical. So are yours.Peter Szakálos: You must fit the data to what everyone can measure.We’re talk<strong>in</strong>g about corrosion rates <strong>of</strong> micrometers per yearthat are the weak po<strong>in</strong>t. You must do some full-scale tests andshow how it works.Allan Hed<strong>in</strong>: We are proceed<strong>in</strong>g with this issue. We have to. Whateverwe measure <strong>in</strong> the laboratory, to make any sense <strong>of</strong> that, <strong>in</strong> therepository environment, it has to be adapted to the conditions thatprevail <strong>in</strong> the repository environment. That’s exactly what we do <strong>in</strong>the safety assessment. You cannot just measure a corrosion rate <strong>in</strong>the laboratory and then take that and multiply it by the number <strong>of</strong>years and get the actual amount <strong>of</strong> corrosion. I th<strong>in</strong>k that is alsovery well accepted by everyone that reviews us.Peter Szakálos: We haven’t seen those results, from a full-scale test.That is what we are look<strong>in</strong>g for.Allan Hed<strong>in</strong>: We have discussed the LOT results, for example,today.Peter Szakálos: They don’t fit your model.111
In 2010 the Swedish Nuclear Fuel and Waste Management Company,SKB, plans to submit its license application for the f<strong>in</strong>al repository<strong>of</strong> spent nuclear fuel. The proposed method is the so-called KBS-3method and implies plac<strong>in</strong>g the spent nuclear fuel <strong>in</strong> copper canisters,surrounded by a buffer <strong>of</strong> bentonite clay, at 500 m depth <strong>in</strong> thebedrock. The site selected by SKB to host the repository is located <strong>in</strong>the municipality <strong>of</strong> Östhammar on the Swedish east coast.The copper canister plays a key role <strong>in</strong> the design <strong>of</strong> the repositoryfor spent nuclear fuel <strong>in</strong> Sweden. The long-term physical and chemicalstability <strong>of</strong> copper <strong>in</strong> aqueous environments is fundamental for thesafety evolution <strong>of</strong> the proposed disposal concept. However, the corrosionresistance <strong>of</strong> copper has been questioned by results obta<strong>in</strong>edunder anoxic conditions <strong>in</strong> aqueous solution. These observationscaused some head-l<strong>in</strong>es <strong>in</strong> the Swedish newspapers as well as publicand political concerns. Consequently, the Swedish National Councilfor Nuclear Waste organized a scientific workshop on the issue“<strong>Mechanisms</strong> <strong>of</strong> <strong>Copper</strong> <strong>Corrosion</strong> <strong>in</strong> <strong>Aqueous</strong> <strong>Environments</strong>”.The purpose <strong>of</strong> the workshop was to address the fundamentalunderstand<strong>in</strong>g <strong>of</strong> the corrosion characteristics <strong>of</strong> copper regard<strong>in</strong>goxygen-free environments, and to identify what additional <strong>in</strong>formationis needed to assess the validity <strong>of</strong> the proposed corrosionmechanism and its implication on the conta<strong>in</strong>ment <strong>of</strong> spent nuclearfuel <strong>in</strong> a copper canister.This sem<strong>in</strong>ar report is based on the presentations and discussionsat the workshop. It also <strong>in</strong>cludes written statements by the members<strong>of</strong> the expert panel.