Environmental Effects in the Atmospheric Corrosion of Zinc

The Bureau of Mines, U.S. Department of the Interior, as part of the National Acid Precipitation Assessment Program, is conducting a field exposure program to measure the corrosion damage on commonly used structural metals and the incremental effects of acidic deposition and other environmental variables. Corrosion damage, air quality, rain chemistry, and meteorology are being continuously measured at five sites in the Eastern and Midwest United States for periods up to 84 months. Corrosion damage on zinc for periods up to 36 months was determined from weight-loss measurements, analysis of corrosion film chemistry, and of precipitation runoff chemistry. The results suggest the long-term zinc corrosion film consists of a stable inner layer sensitive to atmospheric sulfur dioxide (SO2) concentrations and a nonprotective outer layer. A synergistic reaction involving SO2 and nitrogen dioxide (NO2) may occur. Dry deposition of SO2 and NOx on the corrosion film is substantial. Zinc losses from the corrosion film are proportional to hydrogen ion loading. Covariance between many air quality and rain chemistry variables may be low enough to avoid, to a considerable degree, problems in developing linear regression models of corrosion damage.