Characterization of initial intergranular oxidation processes in alloy 600 at a sub-nanometer scale

Intergranular oxidation in Alloy 600 exposed to 480 °C hydrogenated steam was examined at the nano-scale level with analytical electron microscopy and atom probe tomography. The fundamental processes of minor element oxidation and diffusion-induced grain boundary migration (DIGM) were explored. Intergranular oxidation was observed, with Ti and Al oxidation preceding Cr oxidation. Extensive DIGM is observed, with concentrations of minor elements far exceeding bulk values. Calculations are performed which support that the mechanism of DIGM causes large-scale segregation of minor elements. Also, discrete oxide particles ahead of the oxide front provide evidence for classical intergranular internal oxidation at temperatures below 500 °C.