Effect of pressure on high-temperature oxidation of Ni alloys in supercritical CO2 containing impurities

Direct supercritical CO2 (sCO2) power cycles require structural materials compatible with hot sCO2 containing high levels of water and oxygen impurities. Chromia-forming Ni-based superalloys, which are leading candidates at temperatures > 600 degrees C, have shown good compatibility in pure sCO2, however potential impurity effects are not well understood. Herein we exposed several commercial Ni-based alloys to 95%CO2-4%H2O-(1-3)%O2 at 750 degrees C and 0.1 or 20 MPa for 2500 h. Protective chromia scales formed in all cases. Nevertheless, water vapor -assisted oxide volatilization clearly occurred at high pressure. The volatilization rates decreased with time, but still represent a potentially serious compatibility issue in real, high-velocity, sCO2 systems.