The corrosion rate of zinc coatings varies depending on the nature of the zinc salts (corrosion products) formed on the surface of the zinc coating under various conditions. If the zinc salts are not very soluble in the corrosive solution, such as zinc hydroxide, they can protect the underlying zinc layer. Conversely, if the zinc salts formed are highly soluble, they provide little or no protection to the underlying zinc layer, resulting in a faster corrosion rate.
Therefore, the main factors that increase the corrosion rate include: high sulfur content in the air, high air humidity, excessive solid particles such as dust, frequent thunderstorms, high oxygen content in the aqueous solution on the zinc coating, and a solution pH value within the range of 1 to 6 and 12.5 to 14.5. Impurities in the zinc coating generally have little effect under atmospheric conditions, except in strong acid solutions where they can accelerate the corrosion of the zinc layer by forming micro-batteries. In strong alkaline solutions, the corrosion rate of the zinc layer will also increase.
