Interconnect Corrosion in Steam Containing Fuel Gas

Abstract

Iron or chromium based alloys are currently the most preferred materials for SOFC interconnects. The poisoning of the cathode of the membrane electrode assembly (MEA) due to evaporation of chromium species from the metallic interconnect (MIC) and the oxidation of the interconnect surface during stack operation at elevated (800-900 °C) temperatures are regarded to be the major mechanisms affecting the degradation behaviour of the SOFC stack. The oxidation at the cathode side of the interconnect can cause up to one third of the total stack degradation. Despite intensive investigations on corrosion stability of MIC for SOFC application over the past decades, the main attention was focused mainly on the material properties in the cathode side gas conditions (ambient air). The growth behavior and the composition of oxides grown in the SOFC anode gas environment as well as the influence of the oxides on the contact resistance between MIC and anode contacting are still not investigated comprehensively. The MIC materials have to be stable over more than 40,000 hours to satisfy a demand for a continuous increase of operation times of the SOFC stacks. Real time tests over such long time scales are cost intensive. A modification of standard material test procedures, with the aim to accelerate material degradation, is therefore necessary. In order to realize this, three ferritic steels (Crofer 22 APU, Crofer 22 H, AISI 441) and a chromium-based alloy were tested in SOFC anode gas environment in a temperature range between 725 – 875°C. The experiments were carried out under variation of the water vapor content in the gas mixture for different exposure times in order to create the accelerated degradation testing conditions for MIC and to investigate the behavior of these materials caused by the formation of growing chromium oxide based scales. Both gravimetric measurements and FESEM/EDX data, as well as polished microimage sections were analyzed to characterize the oxidation kinetics and the microstructure of the oxide scales and interconnect materials. A clear correlation between increasing temperatures and increasing oxide growth rate constants kp,w can be demonstrated in all materials. This interrelation results in thicker surface oxide scales. A comparison of surface oxide thicknesses after 1000 h oxidation in reducing atmosphere show an increase between 725°C and 875°C with a maximum factor of about 3.8 in ferritic steels and 2.4 for CFY. The comparison of kp,w values at temperatures between 725 and 875°C show acceleration factors of 8,2-11.9 in ferritic steels and 2.3 for CFY. No significant dependence of the oxide growth rate in ferritic steels and CFY by variation of the water content in fuel gas for concentrations within H2/H2O=87/13 vol.% and H2/H2O= 50/50 vol.% was found. The structures of the oxide layers are specific for each material and consist mainly of Cr2O3 (CFY) and Cr2O3/(Cr,Mn)3O4 in ferritic steels with different element distributions and thickness ratios. Beside this, the zone of an inner oxidation of MI...

Effective yttrium based coating for steel interconnects of solid oxide cells: Corrosion evaluation in steam-hydrogen atmosphere

“…Corrosion exposures in high steam content (90% H2O/10% H2) at 750 °C revealed that the uncoated Crofer 22 APU does not "survive" a 2000 h exposure to this environment. In literature, the number of publications investigating steel corrosion in gases with such a high steam content is limited [ [31,42]].…”

Co-Electrolysis with CFY-Stacks

“…In parallel degradation study (18) the interface between the CFY and the nickel mesh was investigated by dual gas samples with 4 probe resistance measurement at 875°C in 50% H 2 O in H 2 . The area specific resistance was ASR=1.2 mΩcm² after 100 h with minor increase of ∆ASR=0.4 mΩcm²/1000 h. Normed to the stack this value will be the half and have a negligible amount on the stack degradation (∆ASR=0.2 mΩcm²/1000 h).…”