28. What are the main factors that determine the corrosion rate of zinc coating?

The corrosion rate of zinc coatings varies depending on the properties of zinc salts (corrosion products) formed on the surface under different conditions. When zinc salts like zinc hydroxide are poorly soluble in the corrosive solution, they can effectively protect the underlying zinc layer. Conversely, if the generated zinc salts are highly soluble, they may fail to provide adequate protection, leading to accelerated corrosion.
Therefore, the main factors that increase the corrosion rate include: high sulfur content in the air, excessive humidity, too many solid particles such as dust, frequent thunderstorms, and high oxygen content in the aqueous solution on the zinc layer, with a pH value between 1 to 6 and 12.5-14.5. Impurities in the zinc layer generally have little effect under atmospheric conditions, but they accelerate the corrosion of the zinc layer in strong acidic solutions due to the formation of micro-batteries. In strong alkaline solutions, the corrosion rate of the zinc layer also increases.