Oxide Film Growth Kinetics on Metals and Alloys

Abstract

Oxide layers play a crucial role in the corrosion resistance of metals and alloys and the growth kinetics of these films is of major interest. To express the oxide growth kinetics, three main models are available: the Cabrera-Mott model, the Fehlner-Mott model and the Point Defect Model (PDM). These models are reviewed in the first part of the paper. Among these models, the PDM is the only one that takes into account the interfacial potential drops during the oxide growth. However, in this model: (i) no parameters relative to the substrate alloy are taken into account, and (ii) the growth is limited by the flow of oxygen vacancies through the film (transport via both cation interstitial and vacancy positions are not taken into account). Here we present a "generalized model" for the kinetics of oxide growth in which the evolution of the interfacial potential drops during oxide growth is included, as well as the variation of the electric field in the oxide during film growth. This new model allows us to describe the growth of oxide films on alloys under non-steady-state conditions. The link between oxide growth and cation release into the solution is also included. Passivity of metals is a central issue in corrosion science. Oxide films play a key role in the corrosion resistance of metals and alloys. The understanding of their growth kinetics requires a detailed knowledge of the involved interfacial reactions and transport mechanisms. Modeling oxide growth is a major issue for lifetime prediction in corrosion engineering. The phenomena identified to be involved in the formation of oxide films are: (i) the growth of the oxide layer, (ii) the dissolution of the oxide film and (iii) the precipitation of dissolving species from the solution, (ii) and (iii) being relevant for anodic oxide films only. For the last sixty years, considerable efforts have been made in order to understand the elementary processes and the role of point defects (interstitials or vacancies) in the growth of oxide films. Three main models, all based on the Wagner theory, 1 have been proposed to express the growth kinetics of compact oxide film formed by solid-state diffusion on metals: (i) the Cabrera-Mott model, 2 (ii) the Fehlner-Mott model, 3 both initially developed to treat the case of oxide films formed thermally in air, and (iii) the Point Defect Model 4-6 (PDM), developed for electrochemically-formed oxide films. More recently, two additional models describing the growth of oxide films on alloys, have been published: the Mixed Conduction Model (MCM) 7,8 (exten-sion of the PDM model allowing determining the electronic properties of the oxide layer) and an atomistic model 9