A key problem in the application of a supercritical carbon dioxide (CO2) turbine cycle to a fast breeder reactor is the corrosion of structural materials by supercritical CO2 at high temperature. In this study, corrosion tests on the candidate materials, high-chromium martensitic steel (12Cr-steel) and FBR grade type 316 stainless steel (316FR), were performed for up to approximately 2000h at 400–600°C in supercritical CO2 pressurized at 20MPa. Corrosion due to the high temperature oxidation was measured in both steels. Results showed that the behavior differed greatly. For 12Cr-specimens, weight gain showed parabolic growth as exposure time increased at each temperature. The oxidation coefficient could be estimated by the Arrhenius function. The specimens were covered by two successive layers of oxide, an Fe-Cr-O layer and an Fe-O layer. A partial thin oxide diffusion layer appeared between the base metal and the Fe-Cr-O layer. The corrosion behavior was equivalent to that in supercritical CO2 at 10MPa, and no effects of CO2 pressure were observed in this study. For 316FR specimens, the weight gain was significantly lower than that of 12Cr-specimen, and good resistance against corrosion was observed. No dependency of temperature or immersed time on weight gain was observed under the test conditions, and the value of all tested specimens was within 2g/m2. Some nodule shape oxide was observed on the surface of the 316FR specimen. Carburizing, known as a factor in the occurrence of breakaway corrosion and/or the degradation of ductility, was observed on the surface of both steels.

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