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

In searching for new working fluids for power generation, supercritical CO2 (sCO(2)) offers some attractive features for efficient cycles. However, compatibility with structural alloys is a concern. NiCr-based alloys have excellent compatibility at 600 degrees-800 degrees C at 20-30 MPa sCO(2). However, conventional steels have restrictions in temperature because of carburization and accelerated oxidation in sCO(2), 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 degrees-650 degrees C. Only highly alloyed advanced austenitic steels are resistant to carburization at 650 degrees-750 degrees C, suggesting that Cr-rich oxide scales are good barriers to C ingress. Above 800 degrees 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.