Passivity of Metals in the Point Defect Model: Effect of Chloride [Part-III]

Abstract: In this paper we study the effect of addition of chloride on the stability of the compact oxide layer pre-existing on a metal surface at a given impressed potential and . The variant of the point defect model (PDM)advanced by us recently is used to construct a theory for the chloride-induced build-up of metal vacancies at the metal-film interface and the chloride-induced dissolution of the compact oxide layer. Under the quasi-steady-state approximation the relevant moving boundary value problem is solved exactly and analytical expressions are found for the incubation time, oxide dissolution time, and critical pitting potential, the time evolution of the passive current density and the metal vacancy concentration and the dependence of these quantities on the chloride ion concentration. Some diagnostics are also provided. The replacement of a reaction of the original PDM by another reaction in the variant is shown to have important consequences for the chloride-ion effect. While the original PDM invoked the Schottky-pair reaction to couple the chloride ion to the cation flux and to the metal vacancy generation, the present model couples the chloride ion directly to the metal vacancy annihilation without invoking the Schottky-pair reaction. The anion flux-pinning by chloride is shown to be sufficient to destabilize the oxide layer. An interesting conclusion of the present work is that thicker oxide layers are in general more susceptible to pitting due to the chloride ion and thinner oxide layers destabilize by simple dissolution in the presence of chloride.