Dedicated to Dr. U. Heubner on the occasion of his 65th birthdayThe corrosion behavior of eight low and high alloy steels was investigated under simulating the conditions at the cathode of a molten carbonate fuel cell at 650 "C. Different Li-containing iron oxides (LiFe02 and LiFe,O,) were formed in contact with the eutectic (Li, K)-carbonate melt depending on the Cr-content of the steel. These oxides show low solubility in the melt and protect the metallic material against further corrosive attack. Fast growing scales of Fe,O, and LiFe,O, were observed on the low alloy ferritic steel 10 CrMo 9 10. Higher alloy steels form LiFeO, in contact with the melt and mixed Fe-Cr-spinels underneath. Steels with Cr-contents over 20 wt.% Cr form a mixed I,iCr,r,Fe,O, and LiCrO, layer in contact with the metal. Marker experiments on the commercial steel 1.4404 (X2 CrNiMo 17 13 2) show that the outer LiFeO, layer grows mainly by outward diffusion of iron ions (Fe3-), whereas the inner (Fe,Ni)Cr20, spinel layer grows inward. After 500 hours, LiFe,O, was formed between the spinel and the LiFeO, layer, but it had disappeared after several thousand hours of exposure as it was fully transformed to LiFeO,. Co-containing LiFeO, was found after 500 hours on the high Co-containing steel 1.4971 (X12 CrCoNi 21 20), but is not stable after several thousand hours exposure. Co diffuses outward to form a protective LiCoO, layer of a few microns in thickness. Protective Cr20, layers were not observed on steels with high Cr-content (2 25 wt.% Cr) due to peroxide ions in the melt, which cause oxidation Cr,O, and flux to chromate, which is highly soluble in the melt. Further quantitative investigations on total corrosion considering the chromate formation have shown that high alloy steels with high amounts of Cr form mainly K,CrO,.In Auslagerungsexperimenten wurde das Korrosionsverhalten von acht niedrig-und hochlegierten Stahlen unter Simulation der Bedingungen an der Kathodenseite von Schmelzkarbonat-Brennstoffzellen bei 650 "C untersucht. In Abhangigkeit vom Cr-Gehalt des gepriiften Stahles bilden sich im Kontakt mit der Schmelze verschiedene Li-haltige Eisenoxide, die das metallische Material vor weiterein korrosiven Angriff der eutektischen (Li,K)-Karbonat-Schmelze schutzen. Schnell wachsende Korrosionsschichten wurden auf dem niedriglegierten ferritischen Stahl 10 CrMo 9 10 beobachtet, wobei die Korrosionsschicht aus Fe,O, und LiFe,O, besteht. Korrosionsbestandigere, hoherlegierte Stahle bilden eine aul3ere LiFe0,-Schicht und darunter chromhaltige Spinelle. Auf Stahlen mit hohen Cr-Gehalten (2 20 Gew.% Cr) werden keine Fe-Cr-Spinelle mehr gebildet, sondern LiCrO, und das Mischoxid LiCr,.,Fe,O, werden als innere Korrosionsschichten nachgewiesen. Markerexperimente am Werkstoff 1.4404 (X2 CrNi-Mo 17 13 2) haben gezeigt, dal3 die LiFeO, als aul3ere Schicht durch Auswiirtsdiffusion von Fe-Kationen wachst, wahrend die (Fe, Ni)Cr20,-Spinellschicht offenbar nach innen wachst. Das nach 500 h gebildete, intermediare Oxid LiFe50, wird nach 5000 Versu...
The corrosion behavior of commercial Fe and Co base alloys and Fe-Cr model alloys with different contents of Co and/or Mn was investigated by continuous exposure tests in the presence of a thin carbonate film. All alloys studied form multi-layered corrosion scales consisting of outer Li containing oxides and inner Cr rich oxides, i.e. spinels or LiCrO,. The LiCrO, is formed on alloys with high Cr contents ( 2 20 wt.%), whereas mixed (Fe,M)3-.rCrx0, spinels (M = Co, Mn, Ni) were found on alloys with lower Cr content (15-20 wt.7i). Insoluble Cr containing oxides occur only in the inner layers of the corrosion scale, whereas on the surface of corroded specimens soluble chromates were detected. Alloys with Mn contents greater than 15 wt.% form Mn,03 in the initial stages of the experiments, this oxide reacts with the melt and formation of Li,MnO, takes place. In exposure tests up to 500 h Fe-Cr alloys with low contents of Mn and Co (10 wt.% Co or Mn) form iron rich oxides (LiFeO? and LiFe,08) with varying amounts of dissolved Mn or co. In the later corrosion stages outward diffusion of Mn and/or Co takes place and LiCoO, and Li,MnO, are formed on top of LiFeO,, whereby the concentration of Mn and/or Co in the inner layers (LiFeO, and spinel) decreases. The outer Li containing oxides LiFeO,, LiCoO, and Li,MnO, are nearly insoluble in the melt and when present at the surface protect the metallic material from further corrosive attack. Fe-Cr model alloys containing Co and Mn form multi-layered corrosion layers after 2000 h of exposure. These layers consist of four oxides in the following sequence from the metal-scale to the scale-melt interface: (Fe,Cr,Co,Mn),O, spinel, LiFeO,, Li,MnO, and LiCoO,, Das Korrosionsverhalten von verschiedenen kommerziellen Werkstoffen und Fe-Cr Modellegierungen mit verschiedenen Zusatzen an Co undoder Mn wurde in kontinuierlichen Auslagerungsversuchen in Anwesenheit eines dunnen Schmelzfilmes untersucht. Alle Werkstoffe bilden eine mehrphasige Korrosionsschicht aus, die aus Li-haltigen Oxiden (LiFeO,, LiCoO, und Li,MnO,) auBen und chromhaltigen Oxiden (Spinelle und LiCrO,) innen besteht. LiCrO, wird auf allen hoch Cr haltigen Werkstoffen (220 Gew.% Cr) gebildet, warend gemischte (Fe,M),-,Cr,O, Spinelle (M = Co, Mn, Ni) auf Legierungen rnit geringeren Cr Gehalten zwischen 15 und 20 Gew.% Cr beobachtet werden. In der Schmelze unliisliche, feste chromhaltige Oxide werden nur im Inneren der Korrosionsschicht beobachtet, wahrend Cr,O, im Kontakt mit der Schmelze in losliches Chromat uberfuhrt wird. Bei Mn-Gehalten uber 15 Gew.% Mn wird zunachst die Bildung von Mn,O, beobachtet, welches durch die Schmelze in Li2Mn0, umgewandelt wird. Bei kleineren Gehalten an Co und Mn (10 Gew.% Co oder Mn) werden nach kurzen Versuchszeiten (max. 500 h) uberwiegend Eisenoxide (LiFeO, und LiFe,O,) gebildet, die griilJere Mengen an geltistem Mn oder Co enthalten. Im weiteren Verlauf des Versuches entstehen durch Auswartsdiffusion von Co und Mn zusatzlich die Oxide LiCoO, und Li,MnO, auf LiFeO,, wobei der Gehalt an ...
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