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 H-2-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 (Fe-2(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 x 10(-15) cm(2) s(-1), which was much smaller than that of SUS430 (4.5 x 10(-14) cm(2) 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 H-2-H2O. (c) 2008 Elsevier B.V. All rights reserved.