Stability of Chromia (Cr2O3)-Based Scales Formed during Corrosion of Austenitic Fe-Cr-Ni Alloys in Flowing Oxygenated Supercritical Water

The comparative corrosion resistance of two high-chromium austenitic Fe-Cr-Ni alloys, namely Type 310S stainless steel (UNS S31008) and Alloy 33 (UNS R20033), was examined after exposure in supercritical water (25 MPa and 550°C), using a flow-loop autoclave testing facility operated at a flow rate of 200 mL/min. Electron microscopy techniques were used to determine links between the composition and structure of the Cr2O3-based oxide scale formed on both alloys and the difference in corrosion resistance observed. The weight change kinetics was distinctly different: progressively positive (weight gain) for Type 310S stainless steel and progressively negative (weight loss) for Alloy 33. The descaled weight loss was lower for Alloy 33, indicating improved corrosion resistance. This improved corrosion resistance was attributed to the improved stability of the Cr2O3-based scale that formed on Alloy 33 despite the negative weight change kinetics. The suitability of these alloys as candidate fuel cladding for the Generation IV supercritical water-cooled reactor concept is discussed in light of the findings presented.