Influence of temperature on the oxidation behaviour of a ferritic-martensitic steel in supercritical water

The oxidation behaviour of a ferritic-martensitic steel exposed to deaerated supercritical water at 560-650 °C and 25 MPa was investigated. A double-layered oxide formed on all samples, consisting of an Fe-rich outer magnetite layer and a Cr-rich inner oxide layer. The temperature had a strong influence on the oxidation rate and morphology of the oxide films. The oxidation kinetics showed nearly parabolic behaviour. The calculated activation energy indicated that the oxidation rate-controlling step was the outward diffusion of Fe2+ and Fe3+ ions through the spinel lattice. The mechanism behind the temperature dependence of the oxidation is further discussed.