The transition from internal oxidation to external oxidation of Ni-Al alloy and its critical Al content

The isothermal oxidation behavior of Ni-xAl (x=9, 10, 11, 12, 13, 14 at.%) alloy in pure oxygen at 900℃ was studied. The scales of Ni-xAl (x=9, 10, 11 at.%) with lower aluminum content are composed of an outer layer of NiO and an inner layer of NiO+NiAl ₂ O ₄ . Internal oxidation of Al occurs beneath the scales, resulting in a faster corrosion rate. The outer layer structure of the scales of Ni-xAl (x=13,14 at.%)with higher aluminum content is similar to that of the Ni-Al alloys with lower aluminum content, but the innermost layer forms a continuous and dense Al ₂ O ₃ protective layer, providing good protection for the alloy without the internal oxidation of Al. Ni-12Al is very unique, with both internal and external oxidation characteristics, and its oxidation behavior falls between protective and non-protective. Due to slight differences in the composition of the alloy in different regions, its oxidation behavior is similar to Ni-9Al, Ni-10Al, and Ni-11Al in areas with slightly lower aluminum content, while its oxidation behavior is similar to Ni-13Al and Ni-14Al in areas with slightly higher aluminum content. It can be inferred that 12 at.% Al is the critical concentration of the alloy for the transition from internal oxidation to external oxidation.