Evaluation of coated steels in supercritical CO₂

The carburizing supercritical CO₂ (sCO₂) environment limits the use of lower cost steels in the lower temperature (450–650°C) portions of the sCO₂ Brayton cycle because of concerns about internal carburization and embrittlement. Results on a ferritic–martensitic steel and conventional and advanced austenitic steels at 450–650°C in 30 MPa sCO₂ with and without 1% O₂ and 0.1% H₂O additions have indicated that sCO₂ environments will have lower maximum operating temperatures compared to steam plants. Pack Al and Cr coatings were evaluated at 650°C on T91 and 316H substrates and showed some benefit for up to 2000 h at 650°C, especially without impurities. However, characterization indicated Al₂O₃ was not formed and Cr-rich carbides formed in the Cr coatings. With the addition of impurities in the sCO₂, the coatings were less protective at 650°C. Subsequent exposures at 600°C in sCO₂ showed similar behavior. Postexposure evaluations included measuring the bulk C content and room temperature tensile properties. Improvements were indicated but the tensile results were complicated by the high temperature pack coating process affecting the substrate properties.