19. Why does zinc coating corrode faster in sulfur containing industrial air?

In industrial cities with high concentrations of flue gas, the air contains substantial amounts of sulfur dioxide and solid particles, including 30% insoluble residues, 33% charred fuel residues, 20% iron oxides, and 8% water-soluble sulfates. When rainwater absorbs sulfur dioxide, it becomes highly acidic, causing severe corrosion to zinc coatings. Even when protective layers of zinc oxide, zinc hydroxide, and zinc carbonate form on the surface, these can still dissolve into soluble zinc sulfate in the acidic solution, accelerating corrosion. Consequently, the corrosion rate of zinc coatings is nearly directly proportional to the sulfur dioxide concentration in the air.
Solid particles can also cause localized corrosion on the zinc coating surface. Some of these solid particles are hygroscopic, which can absorb moisture and dissolve sulfur components into an acidic solution, leading to corrosion of the zinc layer at that location. The corrosion rate of zinc coatings in sulfur-containing industrial air is generally around 420 to 770 milligrams per square decimeter per year.