(Invited) The Morphological Stability of Passive Oxide Films

Abstract

Understanding the kinetics of passive oxide formation and breakdown has been an ongoing problem for corrosion scientists for several decades. A model for the formation of a passive oxide film on a metal that is an extension of the Point Defect Model (PDM) will be presented. The potential description of the PDM is replaced with a boundary value problem that ensures Gauss' law is satisfied at the oxide-solution interface by considering the presence of the Helmholtz layer in solution. Our model predicts the observed linear variation of the steady-state film thickness with anode potential, and an increasing steady-state film thickness with increasing pH. A linear stability analysis of the moving metal-oxide and oxide-solution interfaces shows that, depending on the parameters, the passive film may be unstable to morphological perturbations of the film interfaces, leading to non-planar films and potentially the formation of a pit in the oxide. This also implies that one-dimensional models of oxide growth, that assume planar interfaces, can be inapplicable in broad classes of corrosion processes. The analysis shows that a morphological instability exists if the oxide dissolution mechanism is such that an increasing Helmholtz layer potential drop leads to an increasing dissolution rate. The instability behavior is consistent with the literature on breakdown of passivity in the presence of chloride ions. The theory provides insights on the initiation of passivity breakdown leading to pitting corrosion and the role that interfacial energy plays in determining stability.