Corrosion stability of ferritic stainless steels for solid oxide electrolyser cell interconnects

Long-term oxidation behaviour of eight ferritic steels with 20–29 wt.% chromium (F 20 T, TUS 220 M, AL 453, Crofer 22 APU, Crofer 22 H, Sanergy HT, E-Brite and AL 29-4C) has been studied. The samples were cut into square coupons, ground and annealed for 140–1000 h at 1173 K in flowing, wet hydrogen, air and pure oxygen. The reaction kinetics was followed by mass increase of individual samples over time. Parabolic rate law was observed for most measurements. The respective rate constants have been evaluated and compared. The chemical composition of the oxide scale was investigated by XRD and SEM/EDXS. The major constituent is chromium oxide. Other oxides, such as (Mn, Cr)3O4, MnTiO3, SiO2 or Al2O3, are also present in different amounts depending on the chemical composition of the steel. The oxidation rate increases with increasing oxygen partial pressure and decreasing chromium concentration. Chromium diffusion coefficients in Cr2O3 and parabolic rate constants are compared. The reaction mechanism for the chromia formation is suggested. The results are discussed with respect to the applications of the steels in a working solid oxide electrolyser cell stack. Furthermore, suggestions for the development of a superior alloy composition are given.