Computational Modeling of Bounding Conditions for Pit Size on Stainless Steel in Atmospheric Environments

Abstract

This paper presents analytic expressions for calculating bounding conditions for pitting under atmospheric conditions. These expressions allow the prediction of the maximum pit size that can develop under known atmospheric conditions by considering the factors that can control the inherent galvanic coupling between a circular pit under a thin electrolyte layer surrounded by a concentric cathodic area. Expressions are developed for the maximum cathodic current and the minimum anodic current required for pit stability. An analytic expression for the maximum cathodic current that the surrounding area can supply to the pit is developed and validated by comparison to calculations using the finite element method. The effects of the controlling environmental parameters (deposition density and relative humidity) on the cathode bounding parameters are explored, as is the effect of the size of the pit. The analytical expression for the maximum cathodic current is then coupled to the Galvele pit stability product to estimate the maximum pit size that could develop for a given set of environmental conditions. Those results are then compared to data available in the literature from outdoor exposures of stainless steels for up to 26 years.