A review is presented of the point defect model (PDM) for the growth and breakdown of passive films on metal and alloy surfaces in contact with aqueous solutions. The model provides a reasonable account of the steady-state properties of cation-conducting and anion-conducting barrier layers on nickel and tungsten, respectively, in phosphate buffer solutions; of the impedance characteristics of passive films on nickel; of the breakdown of passive films on a wide range of metals and alloys; of the distributions in the breakdown parameters (breakdown voltage and induction time); of the role of alloying elements in enhancing the resistance of alloys to passivity breakdown; of transpassive dissolution and electropolishing; of erosion-corrosion; and of photoinhibition of pit nucleation. Additionally, the PDM has allowed us to formulate a set of principles for designing new alloys and has led to the development of a deterministic method for predicting

Humankind has been able to develop a metals-based civilization primarily because the reactive metals (Fe, Ni, Cr, Al, Ti, Zr, . . .) exhibit extraordinary kinetic stabilities in oxidizing environments. From the time of Schonbein and Faraday (1830s), the reason for this stability has been attributed to the existence of a thin reaction product ®lm on the metal (or alloy) surface. This ®lm effectively isolates the metal from the corrosive environment. However, attempts to elucidate the mechanisms of the formation of passive oxide ®lms, which generally comprise bilayer structures consisting of a defective oxide that grows directly into the metal and an outer, precipitated hydroxide (or oxyhydroxide on even oxide) layer, have yielded only a rudimentary understanding of the chemistry and physics of the growth and breakdown processes. In this paper, selected aspects of passivity and passivity breakdown are reviewed, with emphasis on the physical models that have been proposed to account for the experimental observations. One such model, the Point Defect Model, is shown to account for most, if not all, experimental observations, and to provide a robust basis for predicting the occurrence of passivity breakdown in any given system. By combining the Point Defect Model with deterministic models for pit growth and crack growth, it is now possible to predict the evolution of localized corrosion damage in a wide range of systems.

A model based on the movement of point defects in an electrostatic field is proposed to interpret the growth behavior of a passive film on a metal surface. This model results in a logarithmic growth law. The theoretical equations derived from the model readily account for experimental data for the growth of a passive film on iron. It is found that the field strength of the film is 1.11×106V/normalcm . The dependence of film/solution interface potential difference on the applied potential (α) was found to be 0.743, and is independent of the identity of the anion in solution. However, the dependence of the potential difference across the film/solution interface on the solutionpH (β) is strongly dependent on the identity of the solution anion.

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A theory is developed for the steady-state properties of passive films that form on metals and alloys in aqueous environments. This theory is based on the point defect model developed earlier, and predicts that the steady-state thickness of the barrier film and the log of the steady-state current will vary linearly with applied voltage. These relationships may be used to estimate empirical parameters that describe the dependencies of the potential drop across the barrier film/ environment interface on the applied voltage and pH and to estimate kinetic parameters for dissolution of the film. If dissolution at the film-solution interface occurs very slowly, the primary passive film is envisaged to consist of a rigid oxide sublattice that transmits cations from the metal to a gel-like, precipitated upper layer. If dissolution at the barrier film/ environment interface occurs rapidly, then a steady-state thickness is achieved by a balance between the rate of dissolution of the film at the film-solution interface and the rate of growth of the film into the underlying metal phase, due to the outward movement of oxygen vacancies (i.e., inward movement of oxygen ions) through the barrier layer. The model is