Surface film electrochemistry of austenitic stainless steel and its main constituents in supercritical water

Supercritical water is used as a solvent and reagent for the decomposition of toxic and hazardous wastes, and is also considered as a prospective coolant in fossil fuel and new generation nuclear power plants. In that respect, mechanisms of corrosion of candidate reactor materials in supercritical water remain to be elucidated in order to devise criteria on the most suitable alloy for a specific application in a more deterministic way. In the present paper, results on the oxidation behaviour of UNS S31600 stainless steel and its main constituent elements (Fe, Cr and Ni) in supercritical water (500–700 °C, 30 MPa) stemming from both in situ oxide film resistance and impedance measurements and ex situ microscopic and spectroscopic analyses are presented. They are discussed within the frames of an approach combining thermodynamic and kinetic considerations from both aqueous electrochemistry at subcritical temperatures and gas/steam oxidation of compatible materials in the relevant temperature range. Tentative conclusions concerning the temperature limits of applicability of Pourbaix and Ellingham diagrams for the thermodynamic description of corrosion have been drawn.