A damage function for predicting the corrosion of galvanized steel structures by wet and dry deposition has been developed from the thermodynamics and kinetics of atmospheric corrosion chemistry. The function mathematically expresses the competing reactions for the buildup and dissolution of the basic zinc-carbonate corrosion film with exposure time. Major findings as expressed by the theoretical function are as follows: 1. During periods of surface wetness, SO2 reaching the surface reacts stoichiometrically with the zinc. 2. Rain acidity reacts stoichiometrically with the zinc. 3. The corrosion film of basic zinc carbonate is soluble in clean rain. The dissolution depends on the residence time of rain on the galvanized steel surface. 4. Deposition velocity controls the rate of corrosion of galvanized steel structures by gaseous SO2 during periods of wetness.
This paper recommends testing the applicability of the proposed damage function with field corrosion data that is being acquired by the Bureau of Mines and the U.S. Environmental Protection Agency and with corrosion data from prior field exposure studies. In this manner a validated damage function should be developed for conducting an assessment of acid deposition to galvanized steel structures.