Oxidation behavior of PM-HIPed Alloy 625 in high-temperature supercritical carbon dioxide

• The oxidation behavior of PM-HIPed Alloy 625 in S-CO 2 environments was studied. • The ground surface oxidized faster than the polished surface. • δ -phase precipitation occurred in the sub-surface layer due to element segregation. • A greatly denser δ -phase was observed on ground surface due to accelerated Cr diffusion. • NbC particles tend to form hollow-shell oxides when exposed on the surface. Oxidation behavior of PM-HIPed Alloy 625 with polished and ground surfaces in supercritical CO₂ was investigated. The polished surface formed an outer Fe-rich oxide layer and an inner Cr-rich oxide layer, whilst the ground surface developed a thicker, continuous chromia layer. δ -phase precipitation occurred in the sub-surface layer due to element segregation caused by oxidation. A greatly denser δ -phase was observed on the ground surface due to accelerated Cr diffusion by the deformation layer. In addition, NbC particles tend to be fully oxidized when exposed on the surface, forming a hollow-shell structure.