In industrial cities with a high concentration of smoke, the air contains a significant amount of sulfur dioxide and solid particles (which include 30% water-insoluble residue, 33% combusted fuel residue, 20% iron oxide, and 8% water-soluble sulfates, etc.). When sulfur dioxide dissolves in rainwater, it becomes highly acidic, leading to intense corrosion of the zinc coating. Even if a protective layer of zinc oxide, zinc hydroxide, and zinc carbonate has formed on the surface of the zinc coating, it can transform into soluble zinc sulfate in this strongly acidic aqueous solution and be washed away, accelerating the corrosion rate. Therefore, the corrosion rate of the zinc coating is almost directly proportional to the sulfur dioxide content in the air.
Solid particles can also cause localized corrosion on the surface of the zinc coating. Some of these solid particles are hygroscopic, meaning they can attract and hold moisture, dissolving sulfur components to form acidic solutions, which then corrode the zinc coating at those locations. Generally, the corrosion rate of zinc coating in industrial air containing sulfur is approximately 420 to 770 milligrams per square decimeter per year.
