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 humidity͒ 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. Corrosion resistant alloys such as stainless steels rely on their passive films for the maintenance of their characteristically low dissolution rates. Under most conditions, this film provides outstanding protection against uniform corrosion. Corrosion resistant alloys do suffer from localized corrosion when discrete locations lose this protection. It is generally accepted that such local loss of protection occurs spontaneously in the presence of aggressive ions, although in most cases, the alloy surface is able to repassivate the failed oxide film, thereby limiting the damage. These metastable events have been the subject of many investigations 1-4 to understand the underlying processes that lead to the more damaging, stable pits. Models have been constructed that relate the frequency of metastable pitting to the likelihood of stable pitting. [1][2][3]5 Such models are of use for predicting the likelihood of corrosion damage occurring in service if the material and exposure conditions are known, although they generally do not predict the extent of damage, although there are exceptions.5 In many applications of corrosion resistant alloys, only stable pitting creates sufficient damage to be of concern. Others have fit exposure data to develop empirical power laws for pit propagationwhere d pit is the maximum pit depth measured, t is the exposure time, and A and n are constants that depend on the material and environment. While useful for the environments and materials used in the exposure testing, such approaches are difficult to extend to other material/environment systems as the underlying factors controlling pit size are usually not clearly delineated.In applications where the intended service life is very long, it may be considered unwise to extrapolate empirical power laws to times orde...

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