Kinetic theory of inhibition and passivation in electrochemical reactions

Abstract

A kinetic theory of inhibition and passivation in anodic organic and metal dissolution reactions is given. Two types of cases arise. The first is one in which the inhibition results from effects of an inhibiting species, produced in a competing reaction, on the free available surface of the electrode and on the activation energy of the primary anodic reaction; the second case is one of self-inhibition which can arise in certain decompositions of adsorbed radicals, e.g., in the oxidation of the formate ion. A reversal of the direction of the log [current]-potential relation is predicted, and the negative Tafel slope observed in the inhibition of an anodic reaction depends on its mechanism, on the isotherm for the inhibiting species, and on the number of electrons required for its production. Generally, the width, in units of electrode potential, of the region of normal and reverse Tafel behavior is determined by the magnitude of the interaction parameter f(θ) in a Temkin type of isotherm for the adsorbed species. The sharpest current-potential relation is observed under Langmuir conditions. Application to several kinds of experimental results including passivation behavior at stainless steel indicates a sufficiently satisfactory agreement between theory and experiment that the general basis of the calculations is supported.