Oxidation of austenitic and ferritic/martensitic alloys in supercritical water

• Oxidation of austenitic SS and Ni base alloys in aerated SCW, at 400 °C and 500 °C and 25 and 30 MPa.
• The influence of temperature and pressure on oxide characteristics (weight gain, morphology and composition) has been determined.
• Density related water properties may account for the differences observed in the oxide layer composition.

Supercritical Water Reactors (SCWRs), one of the concepts considered by the Generation IV International Forum, are an attractive option due to their high thermal efficiency, 45% vs. 33% for current Light Water Reactors (LWR) and their more simple design. The reference design for the European SCWR is a direct cycle system operating at 25 MPa with core inlet temperatures of 280 and average core outlet temperature of 500 °C. In this range of temperatures, shifting from subcritical to supercritical conditions, there is a sharp change in water density as well as in chemical properties, such as dielectric constant and ionic product. These changes in properties could influence the behavior of austenitic alloys and other materials to oxidation and stress corrosion cracking, providing unexpected responses in the light of the available knowledge for LWRs. Oxide layer characteristics are relevant to both stress corrosion cracking and corrosion product transport.Oxidation experiments on austenitic alloys, 316 L SS, Alloy 600 and Alloy 625, and F/M T91 were performed at two temperatures, 400 °C and 500 °C, and 25 and 30 MPa in order to explore the influence of the chemical properties of supercritical water on the oxide layers characteristics. Results from the characterization of the oxide layers by optical, SEM/EDX and Auger spectroscopy as well as the oxidation kinetics are discussed.

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