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 better as it does not contain harmful pollutants, or only contains minimal amounts. Therefore, the main factor causing corrosion to zinc coatings is rainwater. During thunderstorms, nitrogen oxides form in the air, resulting in rainwater containing nitrate and nitrite, nitric acid and nitrous acid. These acidic rainwater accelerate the corrosion of zinc coatings.
(2) Urban Environment
The air contains a relatively high concentration of carbon dioxide, ranging from 0.03% to 0.07% by volume. Although the concentration is low, it can form carbonic acid. Additionally, exhaust gases from the burning of coal for daily use, such as sulfur dioxide, are also present. These substances can easily cause corrosion to zinc coatings.
(3) Industrial Environment
The air contains a relatively high concentration of sulfur dioxide, as well as sulfur compounds in the form of sodium sulfate and calcium sulfate. There are also many particles such as soot and dust. These factors accelerate the corrosion rate of zinc coatings.
(4) Coastal Environment
The air mainly contains chloride ions, and since seawater contains salt, rainwater contains sodium chloride and a small amount of iodine. According to measurements, it also contains a certain amount of sulfates. These components accelerate the corrosion of zinc coatings.
(5) Tropical Environment
Due to the high temperatures and long dry periods in tropical regions, there is often abundant rainfall. This results in the formation of a thick protective layer of zinc oxide on the surface of zinc coatings. However, if the rainy season is too long and the temperature fluctuates widely, this protective layer will not have a significant effect on corrosion prevention.
The main 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.
