High-Temperature Compatibility of Structural Alloys with Supercritical and Subcritical CO₂

In searching for new working fluids for power generation, supercritical CO₂ (sCO₂) offers some attractive features for efficient cycles. However, compatibility with structural alloys is a concern. NiCr-based alloys have excellent compatibility at 600°-800°C at 20-30 MPa sCO₂. However, conventional steels have restrictions in temperature because of carburization and accelerated oxidation in sCO₂, similar to observations in CO₂. To assess the impact of carburization on steel mechanical properties, small (25mm long) dogbone tensile bars are being exposed and tested after exposure at 450°-650°C. Only highly alloyed advanced austenitic steels are resistant to carburization at 650°-750°C, suggesting that Crrich oxide scales are good barriers to C ingress. Above 800°C, it is only possible to conduct subcritical evaluations at this time, but initial results suggest most conventional high-temperature Fe- and Ni-based alloys are more rapidly degraded by CO₂ at higher temperatures. Coatings are a potential solution that require more study.