A Variant of the Point Defect Model for Passivity of Metals

TL;DR

This paper proposes a modified point defect model for metal passivity, replacing empirical parameters with physical ones, accurately describing vacancy annihilation, volume conservation, and stress-induced failure phenomena.

Contribution

It introduces a new variant of the point defect model with physical parameters, improving the description of defect dynamics and passivity behavior.

Findings

Replaces empirical parameters with physical resistances.

Accurately models vacancy annihilation and volume conservation.

Discusses stress-induced failure and pitting mechanisms.

Abstract

A variant of the point defect model originally enunciated by Macdonald and co-workers is advanced and its theoretical implications for the steady state current density, barrier layer thickness and the concentration of metal vacancy at the metal/film interface are deduced. The differences between the original point defect model and the present variant are also highlighted. The empirical parameters alpha and beta in the original point defect model are replaced with two physical parameters Rcont and Rhof which represent respectively the electronic contact resistance at the metal/film interface and the electronic resistivity of the oxide film. The present variant correctly describes the annihilation of the metal vacancies at the metal/film interface and also enforces the conservation of particle and defect volumes during the solid-state reactions leading to the natural inclusion of the famous Pilling-Bedsworth ratio RPB into the model. Diagnostics which help to check the model predictions with experiments are given. Use of this variant to describe stress-induced failure of the barrier oxide leading to pitting is also discussed.