Premature breakaway oxidation of ferritic stainless steels triggered by austenitization

Thin foils (0.2–0.3 mm thick) of commercially available ferritic stainless steels Crofer 22 APU and Crofer 22 H have been oxidized in air for up to 2000 h at 900, 950, 1000, and 1050 °C. Both steel grades suffered from premature breakaway oxidation, despite the remnant Cr content in the foils being 18–20 wt% at the oxide-metal interface. Crofer 22 APU was found to be more susceptible to breakaway oxidation compared to Crofer 22 H. The classical lifetime prediction tools systematically overestimated the time to breakaway. Premature breakaway oxidation above 900 °C is demonstrated to be triggered by austenitization (the α-to-γ transformation in the Fe-Cr system) occurring in the Cr-depleted corners and edges of the foils. A novel lifetime model accounting for austenitization and geometrical constraints was introduced and validated. • Thin foils of Crofer 22 APU and Crofer 22 H suffer from premature breakaway oxidation at 900 °C and above. • 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 oxidation. • Austenitization is the main trigger of premature breakaway oxidation. • A novel lifetime model accounting for austenitization and geometrical constraints is proposed and validated.