Chromium Depletion in a Ni-30Cr Alloy During High-Temperature Oxidation

Ni-30Cr alloy samples were oxidized at temperatures between 500 and 900 °C to investigate the link between the evolution of the microstructure and the chemical composition in the alloy substrate beneath a growing chromia layer. Before oxidation, a layer of ultrafine grains was observed between the surface and a thick lamellar layer. This structure was replaced by some larger recrystallized grains after oxidation. The growth kinetics of the recrystallized grains was described by a parabolic law with a kinetic constant following an Arrhenius law from 500 to 700 °C. For samples oxidized at 800 and 900 °C, all Cr profiles showed a gradient close to the shape predicted by Wagner models. In the samples oxidized between 500 and 700 °C, many Cr profiles showed a two-step shape, with the smaller Cr fraction in the step closer to the alloy/oxide interface. By considering a fast diffusion accelerated by grain boundaries in the zone of recrystallized grains, the two-step shape can be simulated by numerical resolution of diffusion problem.