Long term oxidation resistance of alloys for gas-cooled reactors

Long term oxidation resistance of Alloy 617, Alloy 230, and Alloy X was investigated at 950 °C up to 5000 h in impure helium which simulates the environment of a high temperature reactor (HTR). Post-treatment examination showed that all materials oxidized during the tests with the growth of a surface chromium oxide that includes manganese or titanium. Oxidation induced the formation of a subsurface carbide-depleted zone. In addition, aluminum-containing alloys exhibited internal oxidation. Corrosion kinetics was globally parabolic corresponding to the growth of the external oxide as well as to internal oxidation. The oxidation behaviour of Alloy 617 is in good agreement with the data published in the 1980s. Alloy 230 and Alloy X showed smaller corrosion rate than Alloy 617. This higher oxidation of Alloy 617 was mainly attributed to a more intensive internal oxidation. Extrapolation of the measured kinetics over 20 years calls for the formation of a 50 μm-thick surface Cr-oxide and for in-depth decarburization and penetration of alumina.