Oxidation and carburizing of FBR structural materials in carbon dioxide

•No breakaway corrosion was observed in the specimens at 600 °C for 8000 h.•Oxide structure was clarified by using EBSD, TEM and Raman technics.•In the carburized region, the concentration was limited to max 1 mass %.

A key problem in the application of a supercritical CO2 turbine cycle to a fast breeder reactor is the corrosion of structural materials brought about by supercritical CO2 at high temperatures. In this study, metallurgical analysis of the ferritic–martensitic steels (12Cr-steel and T91) exposed at 400–600 °C for up to 8000 h in supercritical CO2 pressurized at 20 MPa and at 550 °C for up to 5000 h in CO2 gas at atmospheric pressure has been carried out.Two successive oxide layers formed on the steels were kept under the maximum testing conditions (at 600 °C for 8000 h at 20 MPa), and no breakaway corrosion was observed. Each oxide layer thickness increased with exposure time, due to parabolic oxide growth. By using the electron backscattering diffraction technique, the outer oxide layer could be separable into thin Fe2O3 and thick Fe3O4, and the formation behavior was clarified. Carburizing was observed in the base metal just under the oxide layer, and the crystal structure was mainly M23C6.