Oxidation of Tungsten

Two oxide layers form during the oxidation of tungsten between 700° and 1000°C. The outer layer is porous, powdery, yellow tungstic oxide, , and the inner layer is a dense, thin, dark‐blue, tightly adherent oxide of uncertain composition. The oxidation reaction follows initially the parabolic rate law, but eventually there is a transition to the linear rate law. The rate of formation of the inner oxide is presumably inversely proportional to its thickness. The inner oxide seems to transform to the outer oxide at a constant rate. Upon combining the rate laws of the two individual processes, an overall rate equation covering the whole range is obtained. The thickness of the inner layer tends to a limiting value when the rate of its formation is equal to the rate of transformation to the outer layer.