Specimens of galvanized steel sheet were exposed to polluted and clean air in controlled environmental chambers having the capability to simulate diurnal conditions. Corrosion of the zinc films was essentially a linear function of time for each exposure condition. The range of weight losses for the polluted and clean air environments were unexpected approximately the same. However, uniform corrosion of the zinc occurred in the polluted exposures whereas pitting corrosion of the zinc was observed in the clean air exposures.
Scanning electron microscopy/microprobe analysis of the galvanized zinc surface was used to study the mechanism of initiation and propagation of pitting corrosion. Corrosion products were observed to localize and form a barrier that could accentuate an acidic condition within the pit. Pitting corrosion is thus accelerated by the difference in pH within and outside the pit. During the pollutant exposures pitting corrosion is prevented because the drastic difference in pH within and outside the pit. During the pollutant exposures pitting corrosion is prevented because the drastic difference in pH did not occur.