Mechanistic understanding for the formation of Cr-rich haematite layer on the T12 ferritic steel in supercritical water

Abstract

The oxides formed on T12 ferritic steel surface after exposure to 560 °C/25 MPa supercritical water with dissolved oxygen contents of 300 ppb was characterised by various techniques. It was found that the weight gain and thickness of the oxide scale increase with the increasing exposure time and the oxidation kinetics of T12 followed with parabolic law. The Cr content at the outermost layer increases while the Cr content at the inner/outer layer interface decreases with the increasing of exposure time. The Cr-rich haematite layer oxide was observed on the specimen surface when the test duration extended to 1000 h. The mechanism for the formation of the Cr-rich haematite layer was proposed based on the Cr can diffuse outward through the magnetite layer by short-circuits of the oxide layer and reach the magnetite/haematite interface.