In industrial cities with high emissions, the air contains a large amount of sulfur dioxide and solid particles (including 30% water-insoluble residue, 33% burned-out fuel residue, 20% iron oxides, and 8% water-soluble sulfates, etc.). When sulfur dioxide is dissolved in rainwater, it becomes highly acidic, causing intense corrosion to 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 highly acidic aqueous solution and be washed away, accelerating the corrosion rate. Therefore, the corrosion rate of the zinc coating is nearly 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 that corrode the zinc coating at those sites. Generally, the corrosion rate of zinc coatings in sulfur-containing industrial atmospheres is approximately 420 to 770 milligrams per square decimeter per year.
