Metal oxidation: An electrochemical perspective
A general electrochemical theory for the oxidation of metals is presented. The oxidizing metal is viewed as a self-driving electrochemical cell in which the oxide acts as both ionic and electronic conductor. The irreversibility of each of the charge-transfer steps in the reaction mechanism is expressed in terms of an overpotential, the sum of these being equal to the Gibbs free-energy change accompanying the reaction. From this equality, it is possible to calculate the reaction rate as a function of the change in free energy and of such system parameters as oxide conductivity. It is shown that the Wagner theory and the Mott-Cabrera theory can be derived as special cases of the general electrochemical theory. The common basis for the Wagner theory and the Hoar-Price theory of parabolic oxidation is also demonstrated.
Key wordsmetal oxidation Wagner theory Mott-Cabrera theory
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