Oxidation of 9 % Cr ferritic-martensitic steels in supercritical water: role of Co and Cu addition

The oxidation behavior of P92 (Cr-Mo-V steel) and G115 (modified steel containing Co and Cu) were compared in deoxygenated supercritical water (SCW) at 650 °C and 30 MPa. The oxidation resistance of G115 steel was significantly enhanced by alloying elements (Co and Cu) and its microstructural characteristics. In the early oxidation stage (0–156 h) of oxidation, oxidation-resistant Co diffused into the residual matrix of G115, partially hindering the complete oxidation of the internal oxide layer, resulting in a thicker internal oxide layer compared to P92. Simultaneously, Cu-rich particles precipitated within the internal oxide layer at the interface between oxide/residual matrix. By the mid-term oxidation stage (215 h), the Co-enriched residual matrix layer and high-angle grain boundaries in G115 facilitated the formation of a continuous, dense Cr-rich oxide layer. In contrast, P92 developed a discontinuous Cr-rich layer. The dense Cr-rich oxide layer in G115 effectively reduced corrosion rates by hindering ion diffusion. In the long-term oxidation stage (800 h), hindered Fe diffusion beneath the Cr-rich oxide layer increased the local oxygen activity, accelerating the transformation of Fe₃O₄ to Fe₂O₃. Meanwhile, Cu migrated beneath the upper Cr-rich oxide layer, forming a protective Cu-rich sublayer.