1Electrochemistry Laboratory, Faculty of Engineering, Hokkaido University, Sapporo
A theory is presented which illustrates thermodynamically the local and the partial breakdown of passivity as a mechanical failure of the passive film. The electrostriction pressure due to a high electric field in the film produces a compressive stress which could surmount the breakdown stress of the film. The surface tension stabilizes the film absorbing the electrostriction pressure, but this surface effect decreases with increasing film thickness. Hence, there is a critical film thickness beyond which mechanical breakdown could occur. Anion adsorption lowering the surface tension decreases the critical film thickness for breakdown.
The net stress in the film is shown as a function of electric field, dielectric constant, surface tension and film thickness, and the breakdown potential to depend on the anion concentration in solution, at which the film thickness is critical for breakdown. The theory is also shown to explain as its consequence the observed effect of anion concentration on the breakdown potential and the effect of inhibitive anions on the film breakdown for aluminum and stainless steel in solution containing halogen ion.
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