Oxidation and Carburization of Alloys Exposed to Impure Supercritical CO₂

A laboratory study was performed by exposing seven candidate heat-exchanger alloys to simulated conditions of advanced open supercritical CO₂ (sCO₂) Brayton power cycles. The alloys, consisting of ferritic steels, austenitic stainless steels, and nickel-base alloys, were exposed to impure CO₂ containing initially 3.6% O₂ and 5.3% H₂O at a constant pressure of 200 bar. The test temperatures varied from 650 to 750°C. The total exposure time of each test was 1000 hours, with alloy coupons removed from the reaction retort after approximately 300, 600, and 1000 hours. Metallurgical analyses were performed on the exposed samples using optical microscopy, SEM/EDS, and a micro-hardness tester to characterize the scale morphologies and the extent of carburization in the alloys. Results indicate that the oxidation kinetics were significantly influenced by the test temperature and alloy composition. In general, a change of 50°C in temperature can lead to a change in oxide thickness by a factor of up to 4. By comparison, the ferritic steels suffered the highest oxidation and carburization attack, followed by austenitic stainless steels and nickel-base alloys. Alloys containing a high combined Cr and Ni content offered the best resistance to oxidation and carburization. However, only the nickel-base alloys exhibited adequate carburization resistance to the laboratory conditions after the exposure time of 1000 hours.