Effect of grain boundaries on the formation of oxide scale in Fe–Cr alloy for SOFCs

The effect of grain boundaries on the formation of oxide scale was investigated for two different Fe–Cr alloys at 1073 K in H2–H2O atmosphere: typical stainless steel (SUS430) and Laves-phase forming Fe–Cr alloy. Identified oxide scale phases were Cr–Mn–(Fe) spinel and Cr2O3 on both surfaces of the Fe–Cr alloys. A relatively fast diffusion of elements was observed at the grain boundaries of SUS430 with a formation of ridges on the oxide scale surface. On the other hand, the diffusion of elements was controlled by precipitating a Laves-phase (Fe2(Nb, Mo) phase) at the alloy grain boundaries in the Laves-phase forming alloy. The oxide scale growth rates of the Laves-phase forming alloy was 9.8 × 10− 15 cm2 s− 1, which was much smaller than that of SUS430 (4.5 × 10− 14 cm2 s− 1) in the same condition. This was due to the lower diffusivity of elements at the grain boundaries of alloy. The control of elemental diffusivity at the alloy grain boundary (Laves-phase forming alloy) is effective to form stable and thin oxide scales on the Fe–Cr alloy surface in H2–H2O.