The corrosion of zinc coatings outdoors primarily depends on factors such as the nature of the air, humidity, and rainfall. Therefore, outdoor air can be classified into the following categories:
(1) Rural Environment
Clean rural air is relatively benign because it contains little, if any, harmful pollutants. Consequently, rainwater is the primary factor causing corrosion to zinc coatings. During thunderstorms, nitrogen oxides form in the air, resulting in rainwater containing nitrate, nitrite, nitric acid, and nitrous acid. These acidic rainwater accelerate the corrosion of zinc coatings.
(2) Urban Environment
The air contains a higher concentration of carbon dioxide, ranging from 0.03% to 0.07% by volume. Although the concentration is low, it can form carbonic acid. Additionally, waste gases from coal combustion for daily living, such as sulfur dioxide, are present, which readily cause corrosion to zinc coatings.
(3) Industrial Environment
The air contains higher levels of sulfur dioxide and sulfur compounds in the form of sodium sulfate and calcium sulfate. There are also particles such as soot and dust. These factors accelerate the corrosion rate of zinc coatings.
(4) Coastal Environment
The air mainly contains chloride ions. Additionally, because seawater contains salt, rainwater contains sodium chloride and a small amount of iodine. According to measurements, it also contains a certain amount of sulfate. These components accelerate the corrosion of zinc coatings.
(5) Tropical Environment
Due to the high temperatures and long dry periods in tropical regions, coupled with frequent rainfall, a thick protective layer of zinc oxide forms on the surface of zinc coatings. However, if the rainy season is too long and the temperature fluctuates widely, this protective layer will not effectively prevent corrosion.
The primary corrosion products of zinc coatings outdoors are zinc oxide and zinc hydroxide. They also contain basic zinc carbonate and zinc sulfate to a lesser extent.
