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

Abstract

There is considerable interest in increasing the working temperature of both open and closed supercritical CO2 (sCO2) cycles to ≥700°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 sCO2 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 650–750°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 + O2 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 O2 or H2O impurities could be observed in these exposures.