Comparative oxidation behaviour of Fe-9Cr steel in CO2 and H2O at 550 °C: Detailed analysis of the inner oxide layer

Abstract Fe-9Cr steel was oxidized in pure water vapour and in CO 2 at 550 °C. In both environments the Fe–Cr spinel layer was composed of small equiaxed grains which stoichiometry evolved from Fe 2.7 Cr 0.3 O 4 to Fe 2 CrO 4 in H 2 O and from Fe 2.6 Cr 0.4 O 4 to Fe 1.9 Cr 1.1 O 4 in CO 2 . In both cases the mean stoichiometry was Fe 2.3 Cr 0.7 O 4 . Single oxidation mechanism was proposed: the “available space model”. Carbon for oxidation in CO 2 and hydrogen for oxidation in H 2 O were detected at metal/oxide interface within the steel. These atoms could be responsible for nanometric cavities formation by trapping vacancies created by outward cationic diffusion.