The Role of the Surface Oxide Nanostructure in Passivity Breakdown and Localized Corrosion

Models of passivity and passivity breakdown generally consider the passive film as a uniform and homogeneous oxide It is also often thought that passive films are amorphous. However it has been shown for several metals and alloys that the passive film is crystalline and exhibits (nano)grains and inter-granular boundaries.
The aim of this work was to develop a model of passivity breakdown and pit initiation including explicitly the role of the inter-granular boundaries within a crystalline passive film.
The potential drops at an inter-granular boundary are modified with respect to the oxide matrix. Consequently the potential drop within the oxide barrier layer is locally smaller, and the potential drops at the metal/oxide and oxide/electrolyte interfaces are therefore also locally modified. Passivity breakdown and pit initiation are thus favoured at the inter-granular boundaries of the passive oxide film.
The key experimental observations at the nanometer scale, mainly by STM, and validating the model, will be reviewed: crystallinity of the passive film, existence of nanograins and inter-granular boundaries, evidence of localized depassivation/repassivation at the oxide inter-granular boundaries without chloride ions, voiding at the metal/oxide interface, competitive adsorption of Cl^- and OH^-, enhanced dissolution of hydroxy-chloride metal complexes.
Corrosion Science 50(2008) 2698-2704