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Controlled field studies were conducted at material exposure sites at Research Triangle Park; NC, and Steubenville, OH, between April 25, 1987, and December 28, 1987. Automatic covering/spray systems located at each site were used to expose panels of galvanized steel and paints under the following conditions: (1) dry deposition only; (2) dry plus ambient wet deposition; and (3) dry deposition plus deionized water (Dl). Ambient and Dl-runoff samples were collected on an event basis and underwent detailed chemical analyses. The results from the analyses of the Dl-runoff samples from galvanized steel and oil-based paint panels from each site are presented in this paper. The runoff results show that the dry deposition of acid gases at the sites increased the dissolution rate of galvanized steel corrosion products. At

A unique laboratory exposure system for exposing materials to complex air mixtures that simulate atmospheric conditions has been developed for studying the aerodynamic and chemical processes controlling atmospheric degradation of materials. Runoff studies using covering-spray devices were also developed to provide insight into the effects of atmospheric acidity on materials in the early stage of field exposure. Using these facilities, exposure studies of galvanized steel panels have shown that the deposition of sulfur dioxide (SO2) is controlled by the gas phase resistance of the atmosphere during periods of surface wetness. The delivery of SO2 to the galvanized steel surface is controlled by the atmosphere. Each molecule of SO2 that deposits to the wet surface will react with one zinc (Zn) atom to form zinc sulfate (ZnSO4). During periods of surface dryness, the surface density of SO2 corresponds to the formation of a saturated monolayer of SO2. The deposition of nitrogen dioxide (NO2) is an order of magnitude less than SO2 during periods of surface wetness and appears to be controlled by the surface resistance of galvanized steel. The delivery of NO2 is limited to its solubility within the aqueous surface film. Of the atmospheric gases, nitric acid (HNO3) readily deposits during periods of surface wetness and dryness. Rain acidity contributes to the corrosion of zinc according to the stoichiometric relationship 2(H+) = Zn2+. The corrosion film that forms on zinc is soluble in rain due to dissolved atmospheric carbon dioxide (CO2). The dissolution of the corrosion film depends on the residence time of rainwater on the surface of the galvanized steel panel.