Corrosion behavior of ferritic–martensitic steel T91 in supercritical water

Abstract

The corrosion behavior of a ferritic–martensitic steel T91 exposed to supercritical water at 500 °C and at two different dissolved oxygen concentrations, 25 ppb and 2 ppm, for exposure times up to 505 h was characterized. In this corrosion experiment, all the exposed samples formed a stable oxide layer, but of varying thicknesses. The dissolved oxygen concentration played a significant role in the oxidation behavior of this alloy. For lower oxygen concentration exposure, phase distribution analysis showed a duplex oxide structure, consisting of layered magnetite and spinel phases. Higher oxygen concentration resulted in an additional hematite enriched layer in the outer oxide region. Furthermore, samples exposed to high oxygen content supercritical water exhibited a more porous oxide scale with weaker adhesion to the substrate. Samples of T91 steel whose surface was modified by oxygen ion implantation were tested in the lower oxygen supercritical water environment and were noted to exhibit a lower oxide layer thickness compared to the untreated alloy. The formation of nanometer-sized oxides precipitates at the surface due to oxygen ion implantation appears to influence the nucleation and growth texture of the oxide scale, which may play a role in reducing the oxide layer thickness.