High-Temperature Oxidation of Commercial Alloys in Supercritical CO2 and Related Power Cycle Environments

The oxidation behavior of several Fe and Ni commercial alloys was evaluated in high-temperature environments expected in next-generation supercritical carbon dioxide power cycles. The alloys were exposed to pure CO₂ at 0.1 and 20 MPa and to CO₂ containing H₂O, O₂, with and without SO₂ at 0.1 MPa, at temperatures of 550°C (Fe alloys) and 700-750°C (Ni alloys). For comparison, the alloys were also exposed to air and supercritical steam. Low-Cr ferritic Fe alloys formed Fe-rich oxides leading to high oxidation rates in pure CO₂, while high-Cr austenitic Fe alloys formed Cr-rich oxides leading to low oxidation rates. H₂O and O₂ impurities had negligible effect on the oxidation behavior of all Fe alloys. The addition of SO₂ had negligible effect on the oxidation of ferritic Fe alloys, but significantly enhanced the rate for austenitic Fe alloys, where S in the oxide and underlying alloy contributed to the transition from Cr-rich to Fe-rich oxide growth. Ni alloys formed Cr-rich oxides and showed low oxidation rates in all CO₂-containing environments which were very similar to those obtained in air and supercritical steam. Little or no effect of pressure was observed for any of the alloys exposed to pure CO₂.