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. Results on a ferritic–martensitic steel and conventional and advanced austenitic steels at 450–650°C in 30 MPa supercritical CO2 (sCO2) with and without 1% O2 and 0.1% H2O additions have indicated that carburizing sCO2 environments will have lower maximum operating temperatures compared to steam plants. Pack Al and Cr coatings were evaluated at 600o and 650°C and showed some benefit for up to 2000 h. With the addition of impurities in the sCO2, the coatings were less protective at 650°C.