Exploring Material Solutions for Supercritical CO₂ Applications above 800 °C

There has been recent interest in exploring revolutionary supercritical CO₂ (sCO₂) power cycles, and this exploratory investigation was seeking materials with CO₂ compatibility at up to 1200 °C. Initial exposures were conducted at 0.1 and 2 MPa CO₂ for up to 1000 h at 900–1200 °C. As expected, specimens of Mo and W that might be used as matrix materials in cermets were rapidly attacked under these conditions. Even an alumina-forming FeCrAlMo alloy showed high mass gains in less than 100 h at 1200 °C due to the formation of Fe-rich oxide. However, at 900–1100 °C, more protective behavior was observed for FeCrAlMo specimens, with or without pre-oxidation, in 0.1 MPa CO₂, but increased attack was observed in 2 MPa CO₂. In contrast, most Ni-based alloys exposed at 900–1100 °C showed higher mass gains and thicker reaction products than formed in air. Thus, Ni-based alloys appear less compatible with CO₂ environments above 800 °C compared to lower temperatures. Low mass gains were observed for CVD SiC at 900–1200 °C, but MoSi₂ and Mo(Si,Al)₂ specimens did not form protective scales under these conditions at 1000 and 1100 °C.