Effects of dissolved oxygen on the oxide scales formed on ferritic-martensitic steel T12 exposed to supercritical water

•The dissolved oxygen (DO) had significant influences on the oxide film.•Weight gain increases while oxide resistance decreases with increasing DO content.•The oxide was multi-layer with Fe-rich outer layer and Cr-rich inner layer.•The Cr-rich hematite layer oxide was observed in high DO level environment.•Cr can diffuse outward and reach the outermost layer through the defects in oxide.