17. What are the causes of corrosion of zinc coating in outdoor? What are the corrosion products?

The corrosion of zinc coatings outdoors primarily depends on air quality, humidity, and rainfall. Outdoor air can be categorized as follows: (1) Rural air in clean environments is relatively good, as it contains minimal harmful pollutants. In such cases, rainwater becomes the main corrosive agent. During thunderstorms, nitrogen oxides form in the air, causing rainwater to contain nitrates, nitrites, nitric acid, and nitrous acid. These acidic components accelerate the corrosion of zinc coatings.
(2) The ambient air in the metropolitan area contains a large amount of carbon dioxide, which is within the range of 0.03-0.07% in volume. Although the content is small, it can form carbonic acid, and there are also waste gas produced by the combustion of domestic coal, such as sulfur dioxide, which is easy to cause corrosion to the zinc layer.
(3) The ambient air in the industrial zone contains a large amount of sulfur dioxide; it also contains sulfur-containing compounds in the form of sodium sulfate and calcium sulfate, and a large amount of soot, dust and other particles. All these will accelerate the corrosion rate of the zinc layer.
(4) The coastal area's ambient air is primarily composed of chloride ions. Since seawater contains salt, rainwater also contains sodium chloride and a small amount of iodine. Field measurements indicate the presence of sulfates, all of which accelerate the corrosion of the zinc coating.
(5) The environment of the tropical zone is high temperature, long dry period and often rainy, so a thick zinc oxide protective layer will be formed on the surface of the zinc layer. However, if the rainy season is too long and the temperature range is large, this protective layer will not have obvious effect on corrosion protection.
The main corrosion products of zinc layer outdoors are zinc oxide and zinc hydroxide. Secondly, it contains basic zinc carbonate and zinc sulfate.