Atmospheric Corrosion Model for Galvanized Steel Structures

This report develops a model for predicting the corrosion of galvanized steel structures based on two competing mechanisms: the formation and dissolution of the basic zinc-carbonate film that forms on zinc surfaces. The model consists of a diffusivity term that describes film growth and a dissolution term that describes the rate of film removal. Dissolution becomes the rate-determining process for predicting the long-term corrosion behavior of galvanized steel structures. Components of the dissolution term were evaluated with data collected from field exposure experiments that were designed to separate the effects of wet and dry acidic deposition from the effects of normal weathering of galvanized steel specimens. The model’s dissolution term predicted the long-term corrosion of galvanized steel with reasonable accuracy. For further evaluation, the dissolution model was applied to historical, long-term corrosion data of galvanized steel products, taking into account their sizes and shapes. The field data used in this evaluation were consistent with corrosion rates predicted by the model, within the limits of uncertainty of the environmental data. Thus, the model can be used with reasonable confidence to predict corrosion behavior of different structures if environmental conditions can be properly described.