A study on the surface and crack tip oxidation of alloy 600 through high-resolution characterization

Surface, stagnant grain boundary (GB), and crack tip oxidation of Alloy 600 were studied. The mirror-finished surface oxide has a duplex structure and the formation mechanisms of each layer are discussed. A more compact oxide film is observed on the cold-worked surface, while the nano-GBs are preferentially oxidized. The crack tip oxidation rate is around four orders of magnitude faster than that of the stagnant GB, and their oxidation mechanisms are believed to be the same. Diffusion-induced grain boundary migration (DIGM) is observed around the crack and oxidized GB tips. The role of Ni expulsion on the DIGM is discussed.