Oxidation kinetics study of the iron-based steel for solid oxide fuel cell application

Kinetics of oxidation of Fe-Cr steel containing 25 wt.-percent Cr was studied as a function of temperature ( 1023 - 1173 K) for up to 480 h in flowing air, which corresponds to SOFC cathode environment operating conditions. The oxidation process was found to be a parabolic, suggesting that the diffusion of ionic defects in the scale is the slowest, rate determining step and it occurs predominantly by short-circuit diffusion paths. Comparison of the determined activation energy of oxidation of the studied steel with literature data indicates that at 1098 - 1173 K the chromia scale grows by the outward solid-state diffusion of chromium interstitials, whereas at 1023 - 1098 K - through a significant contribution of counter-current oxygen/chromium diffusion along Cr2O3 grain boundaries. The oxide scales were composed mainly of Cr2O3 with a continuous thin Mn1.5Cr1.5O4 spinel layer on top of the chromia scale. The oxidation test results on Fe-25Cr steel demonstrate the applicability of the commercial type DIN 50049 stainless steel as interconnect for SOFC.