Mechanism of high temperature oxidation of Inconel 625 superalloy with various solution and ageing heat treatment processes

The high-temperature oxidation behaviour of the Inconel 625 alloy at 950 °C was investigated after different ageing treatments. The effect of heat treatment on the oxidation behaviour of the alloy was analysed by characterizing the structure and elemental distribution before and after oxidation. The results reveal that the two ageing treatments at 650 °C for 500 h and at 750 °C for 400 h both reduced the oxidation mass gain. After oxidation at 950 °C, an outer Cr₂O₃ layer and inner Al₂O₃ are identified as the main oxidation products. Moreover, Nb₂O₅ and δ(Ni₃Nb) phases precipitated after oxidation. The ageing treatments cause the rapid generation of a dense Cr₂O₃ layer on the surface, which prevents the diffusion of oxygen into the matrix, reduce the Al₂O₃ inward growth depth, and improve the oxidation resistance of the alloy.