Effect of thermal cycling and phase transformations on chemical life of ferritic stainless steels

Thin foils (0.3 mm thick) of ferritic stainless steels Crofer 22 APU and Crofer 22 H typically used for SOFC/SOEC applications were isothermally and discontinuously oxidized in air for up to 1000 h at 1000 °C. Both steel grades suffered from breakaway oxidation, Crofer 22 APU failing faster (70 h) than Crofer 22 H (700 h). Counterintuitively, isothermal exposure of Crofer 22 APU resulted in a more severe corrosion attack compared to discontinuous exposures with intermediate cooling steps. In contrast, Crofer 22 H was not affected by the thermal history of exposure. Breakaway oxidation of both steel grades is related to the α-to-γ transformation occurring above 900 °C. Crofer 22 H is less susceptible to austenitization. • Thin foils of Crofer 22 APU and Crofer 22 H suffer from premature breakaway oxidation at 900-1000 °C. • Crofer 22 APU is more susceptible to premature breakaway compared to Crofer 22 H. • Alloying with ferrite stabilizers such as W, Nb, Si, etc. prevents or postpones premature breakaway at 900-1000 °C. • Martensite was detected in the Crofer 22 APU foils after oxidation at 1000 °C but not in Crofer 22 H. • Austenitization is the main trigger of premature breakaway oxidation • A novel lifetime model accounting for austenitization and geometrical constraints is proposed and validated.