Effect of Water Vapor on the High-Temperature Oxidation of Pure Ni
The high-temperature oxidation behavior of pure Ni in air and Ar with and without 30 vol%H2O at 1,000 °C was investigated to understand the effects of water–vapor on the resulting oxidation kinetics and scale structures. It was found that water–vapor significantly affected the morphology and scale structure of NiO. A duplex NiO scale with a powder-like outer and dense inner NiO layer developed when the Ni was oxidized in atmospheres containing water–vapor. The grain size of the dense inner NiO layer was much smaller than that formed in dry atmospheres. The growth of the powder-like NiO required outward diffusion of Ni and its continued formation occurred at the interface between the powder and dense NiO layers. The dense inner NiO layer grew outward and incorporated the powder-like NiO particles and the resulting grain size of the inner layer was smaller in the presence of water–vapor. The water–vapor is speculated to have prevented sintering of NiO particles during growth of the NiO scale.