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

Fe-9Cr steel was oxidized in pure water vapour and in CO2 at 550 °C. In both environments the Fe-Cr spinel layer was composed of small equiaxed grains which stoichiometry evolved from Fe2.7Cr0.3O4 to Fe2CrO4 in H2O and from Fe2.6Cr0.4O4 to Fe1.9Cr1.1O4 in CO2. In both cases the mean stoichiometry was Fe2.3Cr0.7O4. Single oxidation mechanism was proposed: the “available space model”. Carbon for oxidation in CO2 and hydrogen for oxidation in H2O 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.