Effect of H2O Addition on Corrosion and Carburization Behavior in High-Chromium Alloys Exposed to Supercritical CO2 at 500 °C

To clarify the role of H 2 O addition on corrosion and carburization behavior, four chromia-forming alloys (SS 347H, Alloy 800HT, SS 310S, and Alloy 625) containing 18–25 wt% Cr were exposed to supercritical–CO 2 (S–CO 2 ) containing 3.1 vol.% H 2 O at 500 °C and 25 MPa for 1000 h. All alloys exhibited very low weight gains and formed thin, continuous oxide scales without spallation or the formation of thick Fe-rich nodular oxides. The oxide scales consisted primarily of chromia (Cr 2 O 3 ) with accompanying (Mn,Fe) 3-x Cr x O 4 spinel. Minor and discontinuous SiO 2 , Al 2 O 3 , and TiO 2 were locally observed depending on alloy composition. Compared with exposures in high-purity S–CO 2 , the presence of H 2 O moderately increased corrosion rates at 500 °C. In the Fe-based alloys, Cr-rich carbides were observed within subsurface Cr-depleted regions. However, no carbon enrichment was detected at the oxide and matrix interface, indicating that these carbides originated from intrinsic carbon rather than carbon ingress from S–CO 2 . Thermodynamic calculations further demonstrate that H 2 O addition increases the oxygen partial pressure while significantly reducing carbon activity. This combination stabilizes chromia formation and effectively suppresses carburization at this intermediate temperature.