Corrosion of alumina-forming austenitic steel Fe–20Ni–14Cr–3Al–0.6Nb–0.1Ti in supercritical water

Abstract

The oxidation behavior of alumina-forming austenitic (AFA) steel Fe–20Ni–14Cr–3Al–0.6Nb–0.1Ti has been examined after exposure to supercritical water (SCW) at 500 °C and 25 MPa pressure and with a dissolved oxygen content of 25 wppb, for exposure periods of 1300 h, 1700 h and 3000 h. A double layer oxide structure developed on all samples, consisting specifically of a Fe-rich outer hematite layer and an inner layer of Al–Cr–Fe-rich oxide containing a small fraction of FCC Ni-rich metal. Additionally, a thin Ni-rich layer in the bulk alloy close to the oxide/metal interface was also observed. Based on weight gain measurements, the oxidation resistance of the AFA steel was superior to other austenitic alloys, 800H, D9, and 316 stainless steel, tested under similar conditions. The formation of the protective Al–Cr–Fe-rich oxide layer may provide good long-term protection against corrosion in SCW environment.