Initial Assessment of Ni-Base Alloy Performance in 0.1 MPa and Supercritical CO2

There is considerable interest in increasing the working temperature of both open and closed supercritical CO2 (sCO(2)) cycles to a parts per thousand yen700A degrees C. At these temperatures, it is unlikely that Fe-base alloys have suitable strength and therefore the focus is on Ni-base alloys for this application. To begin addressing the lack of sCO(2) materials compatibility data under these conditions, initial work exposed a wide range of candidate alloys for 500 h at 20 MPa (200 bar) CO2 at 650-750A degrees C in high-purity CO2. In general, the reaction products were thin and protective in these exposures. A smaller group of alloy coupons focusing on chromia- and alumina-forming alloys was exposed for 500 h in 0.1 MPa (1 bar) air, CO2, CO2 + O-2 and CO2 + H2O for comparison. The thin surface oxides formed were very similar to those formed at high pressure and no clear detrimental effect of CO2 oxidation or O-2 or H2O impurities could be observed in these exposures.