(Olin Palladium Award) Some Critical Issues in the Breakdown of Passive Films
Authors: Macdonald Digby D. 
Published: February 2020

Passivity breakdown is the precursor to most forms of localized corrosion, including pitting, stress corrosion cracking, corrosion fatigue, and crevice corrosion, and also results in the phenomenon of depassivation, including electrochemical polishing and trans-passive dissolution, corresponding to passivity breakdown over macroscopic dimensions. In this OLIN Palladium Medal address, I will review my work in this area in terms of developing the Point Defect Model (PDM) for passivity and passivity breakdown and will identify principal challenges, as I see them. Starting with the rate equation for barrier layer growth, as predicted by the PDM, and by using phase-space analysis, I derive the condition that must be met for passivity to exist; the rate of growth of the barrier layer at the metal/barrier layer interface, due to the generation of oxygen vacancies at zero barrier layer thickness, must exceed the dissolution rate of the barrier layer. If this condition is met, the barrier layer may exist as a meta-stable phase and passivate the surface. In this regard, it is important to note that passivity is a kinetic state, not a equilibrium thermodynamic phenomenon, as has been so often assumed in the past. This fact has been used to resolve Faraday’s famous paradox arising from his iron-in-nitric acid experiments of 1833.

Violation of this condition implies that the barrier layer cannot exist on the surface, even as a meta-stable phase and, hence, that the surface depassivates (loss of the barrier layer). If depassivation occurs over a macroscopic area, this phenomenon accounts for acid depassivation, transpassive dissolution, resistive depassivation, and electrochemical polishing. However, if depassivation occurs on a microscopic area, localized corrosion phenomena, including pitting, stress corrosion cracking, corrosion fatigue, and crevice corrosion occur, resulting in rapid penetration of a cavity into the metal. This unified theory of corrosion (UTC) accounts for all known forms of corrosion, except for hydrogen embrittlement, which lies outside of the realm of the PDM. After having established the basis of the UTC, I will then proceed to discuss the mechanism by which both general and localized depassivation occurs, which has been identified as being due to cation vacancy condensation at the m/bl interface, such that the barrier layer separates from the metal and film growth into the metal via oxygen vacancy generation cannot occur. This condition assures the depassivation inequality stated above, because the rate of growth of the barrier layer into the metal is reduced to zero and because all oxides exhibit a finite dissolution rate. The lecture will then proceed to show how the UTC predicts the critical breakdown voltage and induction time for a single breakdown site, as a function of the chloride activity, pH, and potential. It will then be shown that, by postulating that the breakdown sites are normally distributed with respect to the cation vacancy diffusivity, it is possible to derive the distribution functions for the breakdown voltage and the induction time for a large population of potential breakdown sites on the surface and to identify the critical characteristic of a breakdown site (high cation vacancy diffusivity). From there, I will demonstrate a variety of phenomena, including photo-inhibition of passivity breakdown, the impact of other halides (F-, Br-, I-), anion inhibition via competitive absorption into surface oxygen vacancies, the role of the precipitated outer layer, and the predicted impact of alloying elements on passivity breakdown. Where possible and appropriate, these predictions will be compared with experiment.

As expected the PDM contains many model parameters that are poorly-known. However, values for all of the parameters may be obtained by optimizing the expressions for the impedance of the barrier layer as a function of frequency and film formation potential, as derived from the PDM, on experimental electrochemical impedance spectroscopic (EIS) data and optimization of the expressions for passivity breakdown (potential and induction time) on the corresponding experimental data. Finally, I will demonstrate how the PDM has led to the development of a deterministic protocol, Damage Function Analysis (DFA), for predicting the evolution of localized corrosion damage on metal and alloy surfaces, as a system evolves along a pre-determined corrosion evolutionary path, which is defined as the path taken by the system in terms of those properties of the system (temperature, potential, pH, [Cl-], etc) that have a significant impact of the damage accumulation rate. Time permitting, I will demonstrate how DFA may be used to predict the evolution of general and localized corrosion damage in practical systems.