Role of Grain Growth in the Oxidation of Nickel

The isothermal growth of a scale upon nickel 270 at 800°–1000°C is accompanied by a coordinated growth of the grains of the oxide, wherein the mean intercept of the grains (measured parallel with the scale surface) is directly proportional to the thickness of the scale. The average grain volume of the columnar grains of the scale is directly proportional to the time of oxidation. Correspondingly, the scale thickness and the mean intercept of the grains increase in proportion to the cube root of the time. Markers maintain the same relative positions in the scale as the scale thickens, showing that the scale grows by swelling. These findings are explained by the formation of new oxide, mainly along grain boundaries, where oxygen diffusing inward upon grain boundaries meets nickel diffusing outward through the grains. The sweeping of the grain boundaries that accompanies grain growth serves to distribute the new oxide uniformly throughout the volume of the scale, resulting in growth by swelling.