Corrosion (presentation)
Corrosion (presentation) Corrosion (presentation)
Corrosion
- Page 2 and 3: Type of Corrosion • Uniform • G
- Page 4 and 5: Effect of pH • Magnetite (for exa
- Page 6 and 7: Reactions At the Anode: Fe Fe ++ +
- Page 8 and 9: Factors Affecting Galvanic Corrosio
- Page 10 and 11: Stress Corrosion Cracking • Three
- Page 12 and 13: Example of Erosion Corrosion
- Page 14 and 15: General Corrosion Control • Elimi
- Page 16 and 17: pH Control • Add LiOH for low pH
- Page 18 and 19: Conductivity HTS • Used for troub
- Page 20 and 21: Dissolved O 2 Secondary Side •O2i
- Page 22 and 23: Moderator Normal • Stainless stee
- Page 24 and 25: Moderator Chemistry Control • Pro
- Page 26: For You to Do • Read the section
<strong>Corrosion</strong>
Type of <strong>Corrosion</strong><br />
• Uniform<br />
• Galvanic<br />
• Pitting and Crevice <strong>Corrosion</strong><br />
• Stress <strong>Corrosion</strong> Cracking<br />
•Erosion<br />
• Microbiologically Induced <strong>Corrosion</strong>
Uniform <strong>Corrosion</strong><br />
• Can be a good or a bad thing<br />
• <strong>Corrosion</strong> occurs evenly over the surface<br />
• Oxide layers can be very tough<br />
– Magnetite<br />
•Fe 3 O 4
Effect of pH<br />
• Magnetite (for example)<br />
– Low pH no oxides form<br />
– High pH porous Fe(OH) 3 .xH 2 O forms<br />
– pH 10-12 ideal range for carbon steel<br />
• Different metals require different<br />
conditions for uniform corrosion<br />
• pH is controlled to control uniform<br />
corrosion
Galvanic <strong>Corrosion</strong><br />
• Chemical reactions<br />
• Electrons removed from one reactant travel<br />
through an external circuit<br />
• On material tends to disappear
Reactions<br />
At the Anode:<br />
Fe Fe ++ + 2e -<br />
At the Cathode:<br />
2e - + 2H + H2 (bubbles)<br />
In the Electrolyte:<br />
H2O OH - + H +<br />
Galvanic Cell<br />
H2 Bubbles<br />
CATHODE<br />
(Cu)<br />
ELECTROLYTE<br />
(H2O + NaCl)<br />
Fe ++ + 2 OH - Fe (OH)2 (corrosion product)<br />
-<br />
e<br />
-<br />
e<br />
Fe ++ ions<br />
-<br />
e<br />
ANODE<br />
(Fe)<br />
<strong>Corrosion</strong><br />
Product
Cathodic Protection<br />
• Each metal has a different potential to<br />
donate electrons<br />
• Anode always corrodes<br />
• To protect a metal<br />
– Select a metal that more easily donates<br />
electrons<br />
– Build a cell with metal to be protected as<br />
cathode
Factors Affecting Galvanic<br />
<strong>Corrosion</strong><br />
• Oxygen concentration of the electrolyte<br />
• Temperature<br />
• Conductivity of the electrolyte<br />
• Cathode/anode surface area
Pitting and Crevice <strong>Corrosion</strong><br />
Electrolyte (H 2 O plus dissolved oxygen)<br />
Scale<br />
Metal<br />
Region where pit will form
Stress <strong>Corrosion</strong> Cracking<br />
• Three conditions required for most common kind<br />
– Metal under tensile stress<br />
– Dissolved Oxygen<br />
– Chloride Ion<br />
• Brittle cracks form at the sites of stress<br />
• Failure can be fast<br />
• Failure can occur at stress loads far below yield<br />
strength
Erosion <strong>Corrosion</strong><br />
• Flow removes protective layer<br />
• New protective layer forms using up metal<br />
• Promoted by<br />
– High flow<br />
– Turbulent conditions<br />
– Particulates in fluid<br />
• Concern with feeders in HTS
Example of Erosion <strong>Corrosion</strong>
Microbiologically Induced<br />
• Bacteria in water<br />
<strong>Corrosion</strong><br />
– Can be in presence of oxygen or not<br />
– Bacteria form a nodule<br />
• Similar to pitting corrosion.
General <strong>Corrosion</strong> Control<br />
• Eliminate oxygen<br />
• Eliminate chloride ion<br />
• Maintain pH levels<br />
• Prevent stagnation<br />
• Chlorinate
SOLUBILITY OF Fe 3O 4 (µg Fe/kg H 2O)<br />
10<br />
8<br />
6<br />
4<br />
2<br />
HTS pH Control<br />
pH 9.3<br />
0<br />
225 240 255 270 285 300<br />
TEMP. ( °C )<br />
inner<br />
zone<br />
PNGS BNGS-A<br />
pH 9.7<br />
outer<br />
zone<br />
Reactor Inlet Header<br />
pH 10.2<br />
PNGS BNGS-A<br />
Reactor Outlet Header
pH Control<br />
• Add LiOH for low pH<br />
• Ion exchange columns for high pH<br />
• Lithium tends to collect in the pressurizer<br />
– pH goes up if water goes to main system<br />
– Called lithium hide out<br />
– Possible pH excursions during cool down
Dissolved Oxygen HTS<br />
• Oxygen is corrosive<br />
– More so in hot water<br />
• Attacks zirconium, iron, their protective<br />
oxide coatings<br />
• Continuous source through radiolysis of<br />
D2O • Controlled by adding excess D2 or H2 • Explosion Hazard
Conductivity HTS<br />
• Used for troubleshooting<br />
• Not controlled
pH Secondary Side<br />
• pH controlled by adding chemicals<br />
– Ammonia (from decomposed hydrazine)<br />
– Morpholine<br />
• Chemicals must be volatile
Dissolved O 2 Secondary Side<br />
•O2is corrosive particularly if Cl- is present<br />
• Three levels of removal<br />
– Air extraction<br />
– Deaerator<br />
– Hydrazine addition<br />
• Environmental hazard<br />
– chemicals discharged in boiler blow off
Conductivity Secondary Side<br />
• Conductivity of boiler water shows the<br />
presence of bad boiler things<br />
• Specifically cation conductivity is<br />
measured<br />
– Measures the conductivity of anions Cl - , SO 4 - ,<br />
HCO 3 -<br />
– IX is used to replace cations with H +<br />
– Conductivity sensitive to the anions
Moderator Normal<br />
• Stainless steel system<br />
• Requires neutral pH and low conductivity<br />
• Deuterated IX columns remove all<br />
impurities<br />
• Conductivity monitored<br />
• pH is used for troubleshooting
Moderator Over Poisoned<br />
• pH 4-6 to prevent gadolinium nitrate<br />
precipitation<br />
• pH is monitored
Moderator Chemistry Control<br />
• Proper use of IX’s<br />
• Prevent air in-leakage
Flakes<br />
Sludge and Scale<br />
Scale<br />
Boiler Tubesheet or<br />
Tube Support Plate<br />
HT Fluid<br />
Boiler Tube<br />
Boiler Water<br />
Deposits/Sludge Pile<br />
<strong>Corrosion</strong><br />
occurs here
For You to Do<br />
• Read the section on corrosion pp. 19-37<br />
• Answer questions p. 38