Effect of Water Vapor on the Oxidation Behavior of Ni-Based Superalloys Under the Coupling Conditions of Thermal Shock and Creep

The oxidation behavior of an equiaxed Ni-based superalloy (MS4716CC2) under coupled gas thermal shock (950 °C), creep stress (200 MPa), and water vapor (0% and 0.2%) was investigated. The oxide scale exhibited a typical three-layer structure: an outer NiO layer, an intermediate complex spinel layer, and an inner α-Al₂O₃ layer. Quantitative kinetic analysis revealed that the introduction of 0.2% water vapor accelerated the internal γ' phase depletion rate. (The parabolic rate constant for the depletion layer thickening, K_h, increased from 0.35 to 0.61.) Paradoxically, the net mass gain rate decreased (K_p dropped from 3.73 to 1.20) due to severe scale spallation. This was mechanistically attributed to water vapor inducing proton incorporation and increasing cation vacancy concentration, which facilitated the anomalous outward migration of metal ions (particularly Mo). This process resulted in the formation of porous Mo-dominated volatile oxides/hydroxides, severely compromising the structural integrity and adhesion of the protective scale.