The influence of oxygen partial pressure on the base oxide of chromia forming steels: The story prior to breakaway oxidation

The influence of pO2 on the base oxide formed on 304 L stainless steel has been investigated at 600 °C. The alloy was exposed in 5%O2-95%N2 and 10%H2-20%H2O-Ar atmospheres and the initial stages of oxidation were analysed by TEM and EDX. In both environments, the thin scale consists of a Cr-rich oxide overlaid by a FeCrMn oxide. However, the subscale formed in H2-H2O is richer in Cr compared to the 5%O2-95%N2 case (∼90 cation% and ∼70 cation% respectively). The findings are in good agreement with thermodynamic calculations and can explain breakaway oxidation of marginal chromia forming steels in H2-H2O.