Stability of Fe–Cr alloy interconnects under CH4–H2O atmosphere for SOFCs

The chemical stability of Fe–Cr alloys (ZMG232 and SUS430) was examined under humidified CH4 gases at 1073 K to simulate the real anode atmosphere in SOFC operation. Surface microstructure change and oxide scale layer formation were observed on the oxidized Fe–Cr alloy surfaces. The main reaction products were Mn–Cr–(Fe) spinels for both alloys. Secondary ion mass spectrometry (SIMS) was applied to measure the elemental distribution of minor and major elements around the oxide scale/alloy interface. A high concentration of Mn on the oxide scale surface suggested the fast diffusion of Mn in the oxide scale to form the spinels. Annealing in CH4–H2O made the oxide scale thicker with duration time on the alloy surface. The parabolic growth rates (kp) of oxide scale layer were evaluated from the thickness of oxide scales by secondary ion mass spectrometry (SIMS) depth profiles, which were calculated to the following: kp=6.25×10−6 μm2/s for SUS430 and kp=4.42×10−6 μm2/s for ZMG232. The electrical conductivity of oxidized alloys showed the semi-conductor temperature dependence for both alloys. The electrical conductivity of oxidized ZMG232 alloy was higher than that of oxidized SUS430.