The Effect of Minor Alloying Elements in Ferritic Steels for ...
The Effect of Minor Alloying Elements in Ferritic Steels for ...
The Effect of Minor Alloying Elements in Ferritic Steels for ...
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P0804-Schuisky<br />
<strong>The</strong> <strong>Effect</strong> <strong>of</strong> <strong>M<strong>in</strong>or</strong> <strong>Alloy<strong>in</strong>g</strong> <strong>Elements</strong> <strong>in</strong> <strong>Ferritic</strong> <strong>Steels</strong><br />
<strong>for</strong> Interconnects <strong>in</strong> SOFCs<br />
Mikael Schuisky 1 , Andreas Rosberg 1 , Lars Mikkelsen 2 ,<br />
Peter Vang Hendriksen 2 , Niels Christiansen 3 and Jørgen Gutzon Larsen 3<br />
1 AB Sandvik Materials Technology<br />
SE-81181 Sandviken / Sweden<br />
Tel.: +46-26-264083<br />
Fax: +46-26-264177<br />
Mikael.schuisky@sandvik.com<br />
2 Fuel Cell & Solid State Chemistry Department<br />
Risø National Laboratory<br />
DK-4000 Roskilde / Denmark<br />
3 Topsoe Fuel Cells A/S, Nymølley 55<br />
DK-2800 Lyngby / Denmark<br />
Abstract<br />
<strong>The</strong> <strong>in</strong>fluence on oxidation resistance as well as conductivity, by the addition <strong>of</strong> m<strong>in</strong>or<br />
alloy<strong>in</strong>g elements <strong>in</strong> ferritic chromium steels, have been <strong>in</strong>vestigated. This was done by<br />
produc<strong>in</strong>g several different steel melts <strong>in</strong> which the chemical content <strong>of</strong> alloy<strong>in</strong>g elements<br />
such as Si, Al, Mn, Ti, Nb, Zr, Ni, and Mo were varied. <strong>The</strong> addition <strong>of</strong> rare earth elements<br />
such as Ce was also briefly studied. <strong>The</strong> steels <strong>in</strong>vestigated were produced <strong>in</strong> a<br />
conventional steel mak<strong>in</strong>g process. <strong>The</strong> materials were further processed by hot-roll<strong>in</strong>g<br />
and cold roll<strong>in</strong>g <strong>in</strong>clud<strong>in</strong>g anneal<strong>in</strong>g, down to sta<strong>in</strong>less steel strips with a thickness <strong>of</strong> 0,2<br />
mm. <strong>The</strong> weight ga<strong>in</strong>s <strong>of</strong> coupons dur<strong>in</strong>g oxidation <strong>in</strong> air <strong>of</strong> the different alloys were<br />
studied by. <strong>The</strong> oxidation was also studied by means <strong>of</strong> a thermal balance to <strong>in</strong>vestigate<br />
the <strong>in</strong>itial oxidation mechanisms <strong>of</strong> the different steels. <strong>The</strong> oxide scales <strong>of</strong> the oxidized<br />
samples were further <strong>in</strong>vestigated by means <strong>of</strong> X-ray diffraction, scann<strong>in</strong>g electron<br />
microscopy comb<strong>in</strong>ed with energy dispersive spectroscopy. <strong>The</strong> electrical <strong>in</strong>terface<br />
resistance between the alloys and a LSM plate was measured <strong>in</strong> air at 750°C. <strong>M<strong>in</strong>or</strong><br />
addition <strong>of</strong> Al has a positive effect on the high temperature corrosion properties due to<br />
<strong>for</strong>mation <strong>of</strong> a well protected alum<strong>in</strong>ium oxide scale. However, as expected this scale is<br />
well electrical <strong>in</strong>sulat<strong>in</strong>g and will there<strong>for</strong>e <strong>in</strong>crease the area specific resistance at the<br />
alloy-LSM <strong>in</strong>terface. <strong>The</strong> effect <strong>of</strong> Al on the corrosion properties <strong>of</strong> the ferritic chromium<br />
alloy as well as the effect <strong>of</strong> the other m<strong>in</strong>or alloy<strong>in</strong>g elements will be presented.<br />
Introduction<br />
In the last decade ferritic chromium steels have become the strongest candidate materials<br />
<strong>for</strong> long life time <strong>in</strong>terconnects <strong>in</strong> solid oxide fuel cells (SOFC) [1,2]. In SOFC life times<br />
over 40 000 operat<strong>in</strong>g hours are required <strong>for</strong> the components <strong>in</strong> the stack, <strong>in</strong>clud<strong>in</strong>g the<br />
<strong>in</strong>terconnects. <strong>The</strong> ma<strong>in</strong> features which make chromia <strong>for</strong>m<strong>in</strong>g ferritic steels the prime<br />
candidate material <strong>for</strong> <strong>in</strong>terconnects is: a good corrosion stability, alloys are cheap to<br />
produce but are also more ductile and flexible <strong>in</strong> comparison to ceramic <strong>in</strong>terconnects, and<br />
a thermal expansion close to the thermal expansion <strong>of</strong> the ceramic materials used <strong>in</strong><br />
1
SOFC’s [3]. Thus <strong>in</strong> the last decade, SOFC developers and steel producers have shown<br />
an <strong>in</strong>creased <strong>in</strong>terest to develop new suitable steel grades <strong>for</strong> the application as<br />
<strong>in</strong>terconnect <strong>in</strong> SOFC [1-8]. <strong>The</strong> first generation <strong>of</strong> ferritic chromium steel <strong>for</strong> the use as<br />
<strong>in</strong>terconnect <strong>in</strong> SOFC developed at Sandvik <strong>in</strong> the 1990s was called 0YC44. <strong>The</strong> 0YC44<br />
grade is <strong>in</strong> pr<strong>in</strong>ciple a simple iron-chromium steel with a small addition <strong>of</strong> the rare earth<br />
metal cerium. <strong>The</strong> grade 0YC44 has previously been <strong>in</strong>vestigated <strong>for</strong> the application as<br />
<strong>in</strong>terconnect <strong>in</strong> SOFC [3, 7-11]. <strong>The</strong> grade 0YC44 was referred to as a Fe 0.78 Cr 0.22 alloy <strong>in</strong><br />
ref. [9]. <strong>The</strong> 0YC44 grade has successfully been tested <strong>in</strong> a 13 000 hours SOFC stack test<br />
at Topsoe Fuel A/S [11]. <strong>The</strong> second generation <strong>of</strong> <strong>in</strong>terconnect materials, i.e. grade<br />
1C44Mo20, developed at Sandvik is a ferritic chromium steel alloyed with Mo and Nb as<br />
well as some m<strong>in</strong>or addition <strong>of</strong> Ti and Zr [10]. In the present paper the <strong>in</strong>fluence on<br />
oxidation resistance <strong>in</strong> air as well as conductivity, by the addition <strong>of</strong> m<strong>in</strong>or alloy<strong>in</strong>g<br />
elements <strong>in</strong> the second generation ferritic chromium steels, have been compared with the<br />
0YC44 grade. This was done by produc<strong>in</strong>g 0,2 mm steel strips from several steel melts <strong>in</strong><br />
which the chemical content <strong>of</strong> alloy<strong>in</strong>g elements such as Si, Al, Mn, Ni, Nb, N, and Mo<br />
were varied. <strong>The</strong> addition <strong>of</strong> Ce was also briefly studied. <strong>The</strong> oxidation resistance <strong>of</strong> the<br />
different ferritic chromium alloys <strong>in</strong> air was <strong>in</strong>vestigated by compar<strong>in</strong>g the area specific<br />
weight ga<strong>in</strong> <strong>of</strong> each sample. <strong>The</strong> electrical <strong>in</strong>terface resistances between the alloys and<br />
La 0.85 Sr 0.15 MnO 3 (LSM) were measured dur<strong>in</strong>g ag<strong>in</strong>g at 750°C.<br />
Experimetal<br />
In this studied the two commercial Sandvik alloys 0YC44 and 1C44Mo20 as well as<br />
several model alloys were produced by ord<strong>in</strong>ary steel manufactur<strong>in</strong>g processes. <strong>The</strong><br />
materials were further processed to cold rolled strips by first hot roll<strong>in</strong>g the alloys down to<br />
<strong>in</strong>termediate sizes, followed by further cold roll<strong>in</strong>g down to a f<strong>in</strong>al strip thickness <strong>of</strong> 0,2<br />
mm. <strong>The</strong> nom<strong>in</strong>al chemical compositions <strong>of</strong> all alloys referred to <strong>in</strong> this study are listed <strong>in</strong><br />
table 1 below, together with the designated names.<br />
Cyclic oxidations studies <strong>of</strong> the material were done <strong>in</strong> air <strong>in</strong> a conventional furnace at spec.<br />
temperature range. <strong>The</strong> samples were cooled to room temperature be<strong>for</strong>e measur<strong>in</strong>g the<br />
weight ga<strong>in</strong>s at 168 hours <strong>in</strong>tervals. <strong>The</strong> samples were oxidized <strong>for</strong> a total <strong>of</strong> 1008 hours.<br />
Initial oxidation mechanisms <strong>of</strong> some <strong>of</strong> the model alloys were also studied by thermal<br />
balance us<strong>in</strong>g a <strong>The</strong>rmo Cahn <strong>The</strong>rmax 700 equipment. <strong>The</strong> <strong>in</strong>vestigations were done <strong>in</strong><br />
air at 850°C <strong>for</strong> 72 hours with a ramp up time <strong>of</strong> 5° /m<strong>in</strong>.<br />
For phase identification studies <strong>of</strong> the <strong>for</strong>med oxide scales, x-ray diffraction (XRD) was<br />
per<strong>for</strong>med us<strong>in</strong>g a Bruker D8 Discover diffractometer. <strong>The</strong> surface morphologies as well<br />
as the cross sectional micrographs <strong>of</strong> the oxide scales were <strong>in</strong>vestigated utiliz<strong>in</strong>g a Leo<br />
DSM 962 scann<strong>in</strong>g electron microscope (SEM) equipped with Energy Dispersive<br />
Spectroscopy (EDS) <strong>for</strong> elemental analysis.<br />
<strong>The</strong> electrical <strong>in</strong>terface resistances between LSM plates and the alloys were measured by<br />
a DC four-po<strong>in</strong>t method <strong>in</strong> air at 750°C over a period <strong>of</strong> 1000 h. A contact layer <strong>of</strong><br />
(La,Sr)(Mn,Co)O 3 was applied between the alloys and LSM plates. Experimental details<br />
are given <strong>in</strong> [12].<br />
2
Table 1. Chemical composition <strong>of</strong> the ferritic grades discussed <strong>in</strong> this paper (<strong>in</strong> wt. %) the<br />
roman number <strong>of</strong> the 1C44Mo20 denoted different melts<br />
Name Cr Mn Si Ni Mo Add. Fe<br />
0YC44 21,6 0,3 0,3 0,2 0 Ce Bal.<br />
1C44Mo20 - I 22,2 0,1 0,1 0,2 1,0 Nb, Ti, Zr Bal.<br />
1C44Mo20 - II 22,3 0,1 0,2 0,3 1,0 Nb, Ti, Zr Bal.<br />
1C44Mo20 - III 22,3 0,1 0,1 0,09 1,0 Nb, Ti, Zr Bal.<br />
#514 21,9 0,3 0,1 0,07 0,6 Nb, Ti, Zr Bal.<br />
#515 21,9 0,3 0,1 0,07 1,5 Nb, Ti, Zr Bal.<br />
#517 21,9 0,3 0,2 0,06 0 Nb, Ti, Zr Bal.<br />
#518 22,1 0.4 0,2 0,06 1,0 Nb,Ti,Zr, Ce Bal.<br />
#521 22,1 0,4 0,1 0,06 1,0 Nb, Ti, Zr, N Bal.<br />
#522 22,2 0,4 0,2 1,0 1,0 Nb, Ti, Zr Bal.<br />
#525 22,1 0,3 0,2 0,06 1,0 Nb, Ti, Zr ,Al Bal.<br />
#527 22,0 0,4 0,2 0,06 0 Ce Bal.<br />
Results and Discussion<br />
An oxidation study was done on coupons <strong>of</strong> all the model alloys at 850°C <strong>in</strong> air <strong>for</strong> 1008<br />
hours. <strong>The</strong> weight ga<strong>in</strong>s <strong>of</strong> two coupons <strong>of</strong> each alloy were measured with <strong>in</strong>tervals <strong>of</strong> 168<br />
hours and the area specific weight is plotted versus time <strong>in</strong> Fig. 1. In the figure it can be<br />
seen that 3 model alloys are behav<strong>in</strong>g differently that the rest <strong>of</strong> the model alloys. <strong>The</strong> 3<br />
alloys stand<strong>in</strong>g out are the model alloys #525, #521 and #527. <strong>The</strong> rest <strong>of</strong> the model alloys<br />
are all hav<strong>in</strong>g similar oxidation behaviour and end up with a weight ga<strong>in</strong> <strong>of</strong> less than 1,0<br />
mg/cm 2 after 1000 hours <strong>of</strong> oxidation at 850°C <strong>in</strong> air. <strong>The</strong> model alloy #525 with a small<br />
addition <strong>of</strong> Al (0,3 wt-%) shows the by far best oxidation resistance. <strong>The</strong> SEM-EDS<br />
analysis <strong>of</strong> the cross section shows that, despite the low Al-content, a th<strong>in</strong> well adherent<br />
alum<strong>in</strong>ium oxide scale has <strong>for</strong>med expla<strong>in</strong><strong>in</strong>g the low growth rate. <strong>The</strong> second alloy to<br />
stand out is #521, which shows a higher weight ga<strong>in</strong> than the majority <strong>of</strong> the model alloys.<br />
<strong>The</strong> reason <strong>for</strong> the dramatic <strong>in</strong>crease <strong>of</strong> the weight ga<strong>in</strong> curve <strong>of</strong> the #521 is due to<br />
spallation and crack<strong>in</strong>g <strong>of</strong> the oxide scale lead<strong>in</strong>g to breakaway oxidation. This behaviour<br />
<strong>of</strong> the #521 alloy can be expla<strong>in</strong>ed by the fact that #521 was alloyed with a high N content<br />
i.e. 0,08 wt-%. <strong>The</strong> N will consume all the Ti and Zr which are added to give a well<br />
adherent oxide scale. <strong>The</strong> last alloy to stick out is the model alloy #527, which was a new<br />
melt <strong>of</strong> the first generation <strong>of</strong> Sandvik materials <strong>for</strong> <strong>in</strong>terconnects <strong>in</strong> SOFC i.e. the 0YC44<br />
grade. It is appears that a small addition <strong>of</strong> a rare earth metals such as Ce to a ferritic<br />
chromium steel will not have as positive effect on the corrosion resistance as the addition<br />
<strong>of</strong> group IV and V metals. <strong>The</strong> #527 alloy had the by far worst corrosion resistance <strong>of</strong> all<br />
the model alloys with a weight ga<strong>in</strong> <strong>of</strong> more than 1,6 mg/cm 2 after 1000 hours oxidation at<br />
850°C <strong>in</strong> air.<br />
3
Figure 1<br />
Weight ga<strong>in</strong> <strong>of</strong> the different model alloys as a function <strong>of</strong> time while oxidized<br />
<strong>in</strong> air at 850°C<br />
For the new melt <strong>of</strong> the commercial alloy 1C44Mo20-III the oxidation was per<strong>for</strong>med at 5<br />
different temperatures from 700 up to 900°C on coupons with the size 30x40x0,2 mm. <strong>The</strong><br />
weight ga<strong>in</strong> <strong>of</strong> the alloy at 850°C was 0.9 mg/cm 2 which are very similar to the weight<br />
ga<strong>in</strong>s shown <strong>in</strong> figure 1 <strong>for</strong> the majority <strong>of</strong> the model alloys. At the highest temperature<br />
breakaway corrosion occurred after 840 hours giv<strong>in</strong>g rise to a <strong>in</strong>crease <strong>of</strong> the weight ga<strong>in</strong><br />
<strong>for</strong> these coupons. For the lower oxidation temperatures very low weight ga<strong>in</strong>s were<br />
recorded below 0,1 mg/cm 2 at 700°C after 1000 hours exposure to air. <strong>The</strong> thicknesses <strong>of</strong><br />
the <strong>for</strong>med oxide scales were also measured with cross sectional SEM after the oxidation<br />
study and the measured thicknesses are given <strong>in</strong> table 2. <strong>The</strong> oxide scale thicknesses are<br />
<strong>in</strong> good agreement with the obta<strong>in</strong>ed weight ga<strong>in</strong>s.<br />
Figure 2<br />
<strong>The</strong> recorded weight ga<strong>in</strong> dur<strong>in</strong>g oxidation <strong>of</strong> 0.2 mm thick coupons <strong>of</strong> the<br />
alloy 1C44Mo20 at different temperatures <strong>in</strong> air<br />
4
From the weight ga<strong>in</strong>s recorded <strong>in</strong> Fig. 2 <strong>in</strong> can be seen that k<strong>in</strong>etics <strong>of</strong> the oxidation <strong>of</strong><br />
1C44Mo20 is fairly good follow<strong>in</strong>g the parabolic law. To illustrate this parabolic behaviour<br />
<strong>of</strong> 1C44Mo20 the square <strong>of</strong> the weight ga<strong>in</strong> per unit area (∆W/A) was plotted as a function<br />
<strong>of</strong> the time, t <strong>for</strong> the five different oxidation temperatures This should then follow the<br />
parabolic law and give a straight l<strong>in</strong>e when plotted:<br />
(∆W/A) 2 = Kp * t Equation 1<br />
Where Kp is the parabolic rate constant equal to the slope <strong>of</strong> the l<strong>in</strong>e shown <strong>in</strong> Fig. 3 <strong>for</strong><br />
the sample oxidized at 750°C.<br />
Figure 3<br />
Parabolic plot <strong>of</strong> the oxidation <strong>of</strong> 1C44Mo20-III at 750°C <strong>in</strong> air<br />
<strong>The</strong> calculated parabolic rate constants <strong>for</strong> the different temperatures are given <strong>in</strong> Table 2,<br />
below.<br />
Table 2<br />
Calculated parabolic rate constants, oxide scale thickness and estimated life<br />
times be<strong>for</strong>e breakaway corrosion <strong>for</strong> 1C44Mo20 alloy <strong>in</strong> the temperature<br />
range 700 - 900°C<br />
Temperature (°C) Kp (mg 2 cm -4 h -1 ) Scale thickness (µm) Est. life time (h)<br />
700 6,6 x 10 -6 0,8 1 100 000<br />
750 4,8 x 10 -5 2 150 000<br />
800 2,4 x 10 -4 3 29 000<br />
850 7,8 x 10 -4 6 9 000<br />
900 8,4 x 10 -3 16 840<br />
Estimated life times <strong>of</strong> the 0,2 mm thick 1C44Mo20 alloy can be calculated from equation<br />
1 at the different temperatures assum<strong>in</strong>g that the mass ga<strong>in</strong> recorded after 840 hours,<br />
when the breakaway corrosion was observed <strong>for</strong> the sample oxidized at 900°C would be<br />
the same <strong>for</strong> other temperatures. <strong>The</strong>se estimated life times <strong>of</strong> the 0,2 mm thick strip <strong>of</strong><br />
the 1C44Mo20 alloy are listed above <strong>in</strong> table 2. <strong>The</strong>se estimated life times <strong>for</strong> the<br />
1C44Mo20 are <strong>in</strong> good agreement on earlier presented results on similar commercial<br />
alloys [14].<br />
5
<strong>The</strong> SEM cross section <strong>of</strong> the <strong>for</strong>med oxide scale on the model alloys after 1008 hours<br />
oxidization were also analysed below the SEM pictures <strong>for</strong> the model alloys #522 and<br />
#527 are shown <strong>in</strong> Fig. 4.<br />
Figure 4 SEM cross section <strong>of</strong> the <strong>for</strong>med oxide scale on the two model alloys #522<br />
(right) and #527 (left) after 1008 hours oxidation at 850°C <strong>in</strong> air<br />
From the SEM cross section micrographs above it can be seen that the thickness <strong>of</strong> the<br />
#527 model alloy i.e. approx. 13 µm is more than twice as thick as the oxide scale <strong>for</strong>med<br />
on the model alloy #522 i.e. approx. 5 µm. <strong>The</strong> EDS mapp<strong>in</strong>g <strong>of</strong> all alloys show an<br />
<strong>in</strong>creas<strong>in</strong>g Mn content <strong>in</strong> the outer part <strong>of</strong> the oxide scale <strong>in</strong>dicat<strong>in</strong>g the <strong>for</strong>mation <strong>of</strong><br />
MnCr 2 O 4 sp<strong>in</strong>el which was verified by X-ray diffraction. At the chromia scale - alloy<br />
<strong>in</strong>terface small particles <strong>of</strong> silica was <strong>for</strong>med. Under the chromia scale <strong>in</strong>ternal oxides <strong>of</strong><br />
titanium and alum<strong>in</strong>ium can be observed <strong>in</strong> both alloys. <strong>The</strong> <strong>in</strong>itial oxidation mechanism <strong>of</strong><br />
the model alloys were also studied by thermal balance and below <strong>in</strong> Fig. 5. the recorded<br />
plots <strong>of</strong> weight vs. time <strong>for</strong> the alloys #527 and #524 are shown.<br />
Figure 5<br />
<strong>The</strong>rmal balance studies <strong>of</strong> alloys #527 and 1C44Mo20 - I<br />
<strong>The</strong> area specific weight ga<strong>in</strong>s were calculated <strong>for</strong> the samples analysed with thermal<br />
balance from the weight recorded at t = 3 hours until t = 72 hours. <strong>The</strong> time t = 3 hours<br />
was chosen s<strong>in</strong>ce at this time the temperature had stabilized at 850°C <strong>for</strong> the samples.<br />
<strong>The</strong> weight difference was denoted ∆m and found to be 1,297 mg/cm 2 /69 hours or 0,019<br />
mg/cm 2 /h <strong>for</strong> the #527 and 0,485 mg/cm 2 / 69 hours or 0,007 mg/cm 2 /h <strong>for</strong> the 1C44Mo20.<br />
<strong>The</strong> much lower ∆m weight ga<strong>in</strong> <strong>of</strong> the 1C44Mo20 alloy compared to the #527 alloy once a<br />
ga<strong>in</strong> shows a much better oxidation resistance <strong>of</strong> the 1C44Mo20 type <strong>of</strong> alloys which is<br />
expla<strong>in</strong>ed by the addition <strong>of</strong> Mo and Nb to the alloy.<br />
6
<strong>The</strong> area specific resistance (ASR) values <strong>of</strong> the alloy-LSM plate <strong>in</strong>terfaces were<br />
measured dur<strong>in</strong>g ag<strong>in</strong>g at 750°C. <strong>The</strong> ASR-values <strong>for</strong> some <strong>of</strong> the alloys are shown <strong>in</strong> Fig.<br />
6. <strong>The</strong> ASR-values <strong>for</strong> the model alloy #525, with a small addition <strong>of</strong> Al showed the by far<br />
highest degradation rate well over 100 mΩcm 2 /10 3 h <strong>of</strong> all the model alloys. <strong>The</strong> second<br />
largest degradation rate was found <strong>for</strong> the #527 alloy with a degradation <strong>of</strong> more than 7<br />
mΩcm 2 /10 3 h. <strong>The</strong> much thicker oxide scale <strong>of</strong> the #527 alloy is likely the cause <strong>of</strong> this<br />
<strong>in</strong>creased ASR degradation rate. However, this is still lower than the earlier reported ASR<br />
degradation rate <strong>for</strong> the similar 0YC44 alloy <strong>of</strong> 45 mΩcm 2 /10 3 h [10]. This could be<br />
expla<strong>in</strong>ed by the difference <strong>in</strong> silicon content <strong>of</strong> the two 0YC44 type alloys. <strong>The</strong><br />
commercial alloy had a twice as high Si content as the model alloy #527. <strong>The</strong> rest <strong>of</strong> the<br />
model alloys all had similar degradation rates, <strong>of</strong> below 3 mΩcm 2 /10 3 h. <strong>The</strong> new melt <strong>of</strong><br />
the commercial alloy 1C44Mo20 had a very low degradation rate <strong>of</strong>
Conclusions<br />
<strong>The</strong> first general conclusion to be drawn <strong>for</strong> this study is that the alum<strong>in</strong>ium content <strong>in</strong> the<br />
alloy used <strong>for</strong> <strong>in</strong>terconnects <strong>in</strong> SOFC should be kept low preferably below 0,3 wt-%.<strong>M<strong>in</strong>or</strong><br />
additions <strong>of</strong> Al to the alloy <strong>in</strong>creases the corrosion resistance, but it also <strong>in</strong>creases the<br />
degradation rate <strong>of</strong> the ASR by a factor <strong>of</strong> 100.<br />
<strong>The</strong> second conclusion to be made is that the second generation <strong>of</strong> steel i.e. the<br />
1C44Mo20 type <strong>of</strong> alloys have much better corrosion resistance as well as conductivty <strong>of</strong><br />
the oxide scale compared to the 0YC44 type <strong>of</strong> alloys. Especially, the new melt which has<br />
a very low degradation rate <strong>of</strong> the ASR < 1 mΩcm 2 /10 3 h and 2,5 times lower weight ga<strong>in</strong><br />
that the 0YC44 type <strong>of</strong> alloys.<br />
Furthermore, the additon <strong>of</strong> small amounts <strong>of</strong> rare earth metals such a Ce to the<br />
1C44Mo20 alloy did not greatly <strong>in</strong>crease the oxidation resistance <strong>of</strong> the alloy nor the<br />
conductivity <strong>of</strong> the <strong>for</strong>med oxide scale.<br />
<strong>The</strong> Si content seems to have a very important role <strong>in</strong> the electrical resistance and should<br />
there<strong>for</strong>e be kept as low as possible preferably below 0,1 %.<br />
Nitrogen should be kept low to avoid spallation <strong>of</strong> the <strong>for</strong>med oxide scale.<br />
<strong>The</strong> parabolic rate constant <strong>for</strong> the 1C44Mo20 alloy was calculated to be 6,6 x 10 -6 mg 2<br />
cm -4 h -1 at 700°C, 2,4 x 10 -4 mg 2 cm -4 h -1 at 800°C and 8,4 x 10 -3 mg 2 cm -4 h -1 at 900°C,<br />
respectively.<br />
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