Oxygen transport during the high temperature oxidation of pure nickel

The high temperature oxidation of nickel has been investigated in air under atmospheric pressure in the temperature range 600-900 degrees C. The oxidation kinetic curves deviate from the parabolic law for temperatures over 800 degrees C. The observation of scale morphologies and the use of two stage oxidation experiments under O-16(2)/O-18(2) atmospheres showed that oxygen transport through the NiO scale had to be taken into consideration during the oxidation process. Despite the main outward diffusion of Ni species through the oxide scale, the inward oxygen diffusion at lower temperatures (< 800 degrees C) or the oxygen transport, probably as molecular species, via pores or micro-cracks were found to play a major role in the formation of duplex oxide scales, made of small equiaxed oxide grains at the metal/oxide interface overgrown by larger columnar grains at the gas/oxide interface. Oxygen diffusion coefficients into thermally grown NiO scales were determined and compared to the values of Ni diffusion coefficients from the literature.