Scale formed upon molybdenum consists of either two or three oxide layers., depending upon temperature and time. The outermost layer is the volatile MoO3. The inner layers of oxide have a composition very close to MoO2 but do not exhibit an X-ray pattern corresponding to MoO2. The rate of evaporation of MoO3 and the mechanism of molybdenum oxidation were determined. It was found that after an initial parabolic dependence of weight change upon time the rate of oxidation became linear. This is due to the fact that the outer MoO3 layer is non-protective so that the rate of oxidation of the suboxide to the trioxide at the suboxide-trioxide interface is equal to the rate of diffusion of oxygen through the suboxide film to the metal-suboxide interface. It was also found that the presence of liquid MoO3 increased the rate of oxidation of molybdenum. 6.3.9

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