Abstract
A model is proposed for the oxidation of metals including both bulk crystal lattice diffusion and short-circuit diffusion paths. Assuming local equilibrium between point defects in the bulk and in grain boundaries, we obtain an effective parabolic rate constant keff=kbulk [1+θ exp-(ΔH/RT],where ΔH is the enthalpy for the reaction between point defects in the bulk and in short circuits, θ is the fraction of the short-circuit area, and T is temperature. In the case of the high-temperature oxidation of nickel coated with a thin film of vanadium pentoxide, this model yields to a rate law with a critical oxygen pressure Pc. The nickel vanadium oxide located in the short circuits of the growing oxide NiO is liquid below Pc, leading to a rapid short-circuit diffusion. Above this value, a vanadate precipitates and acts as diffusion blocks for migrating Ni ions.
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