The Bureau of Mines, U.S. Department of the Interior, is conducting atmospheric corrosion tests on five alloys and two coated-steel products at five sites as part of the National Acid Precipitation Assessment Program. Tests have been conducted on samples boldly exposed to the atmosphere, sheltered from the atmosphere, and facing skyward and groundward. Details of the corrosion process related to orientation and sheltering and involving particulates, corrosion film chemistry and morphology, and the dissolution/reprecipitation process were established. The corrosion film on zinc saturates with sulfur at around 7 wt pct with increasing ambient sulfur dioxide concentration. Zinc corrosion on the skyward side appears to be cathodically protected in two-sided old exposures. Only large particulates are present on the surface of sheltered copper and zinc; small particulates dissolve and disperse into the corrosion film. The dissolution/reprecipitation process occurs primarily during the final stages of drying. The information is essential to the development of damage models for the effects of acid deposition of metallic materials.