Localized corrosion on metal surfaces exposed to atmospheric conditions of high relative humidity, dust accumulation and some precipitation can be significantly limited by the capacity of the external cathode to provide the current required to sustain corrosion. The present state of modeling localized corrosion is based primarily on fully immersed electrodes, with only limited studies of localized corrosion in moist layers of dust, particulates and deposits. This study quantifies the extent to which crevice corrosion can be sustained on metal surfaces under thin electrolyte films.
Numerical solutions were developed to simulate and characterize the crevice corrosion system elucidating the effects of film and crevice geometry, presence of particles, mass transport limitations, pH variation, solution properties and electrode kinetics. The presence of particles was accounted through its effects of solution blockage and electrode coverage. These effects were quantitatively characterized by simplified correlations which can be used to analyze particles containing systems in terms of equivalent homogeneous systems.
Analytical solutions were developed for the cathodic current distribution and cathode capacity of extended electrodes covered by a thin electrolyte film. These models were combined to provide detailed current distribution on the cathode with overlapping kinetics regimes. Conditions for the saturation of the cathodic current and trends in the variation of the cathode capacity with electrode length were identified.
Surface roughness between the crevice former and the metal surface was modeled, yielding a constriction factor which can be used to represent the rough surface as a smooth walled crevice with an equivalent crevice gap. The effect of corrosion product accumulation at the anodic site on the damage evolution profile was modeled using time-stepped moving boundary simulations. A non-symmetrical tear-shaped evolution of the corroding site is indicated, that preferentially grows towards the crevice opening.
The parametric analysis and diagnostic models developed in this study would aid in identifying the effects of the physical and chemical parameters that control crevice corrosion in thin electrolyte films. Based on this quantitative understanding, the robustness of metals/alloys that are exposed to atmospheric conditions, and hence are susceptible to crevice corrosion under thin electrolyte films, can be evaluated.