Abstract
An expression has been derived for the initial oxidation rates of metals based upon the model of inward movement of oxygen through the lattice and along low resistance paths within the superficial oxide film. The equation involves three parameters: the parabolic rate constant, the ratio of the diffusion constants for short circuit and lattice diffusion, and the fraction of available oxygen sites lying within low resistance paths. The latter is assumed to decay in time as a first order rate process. It is demonstrated that this equation adequately represents the oxidation kinetics of titanium, zirconium and hafnium in the temperature range 300°–600°C. The empirically determined activation energies for decay of low resistance sites lie between 7 and 12 kcal/mole. The ratios of the activation energies for short circuit and lattice diffusion in the dioxides of these metals are found to be in the range of 0.80 while the initial fraction of oxygen sites lying within defective material is estimated to be of the order of 0.1. This corresponds to a dislocation density of 10 12/cm 2.
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