25. How does the zinc coating corrode on buildings?

The corrosion of zinc coatings on buildings is determined by the properties of construction materials. For example, cement and lime slurry exhibit high alkalinity, which significantly corrodes zinc layers. During drying, evaporated moisture condenses on the zinc surface while oxygen-rich air is introduced, creating an oxygen-free environment that intensifies corrosion. Magnesium chloride in cement further accelerates corrosion by disrupting the protective film. Roof asphalt, when exposed to sunlight, generates organic acids that, upon contact with rainwater, cause severe corrosion.
The galvanized steel pipe used for transporting hot water and steam is usually wrapped with a layer of insulation material. If the moisture in the insulation material is not easily evaporated, it can easily cause corrosion. For example, if slag wool is used as the insulation material, it contains not only a certain amount of moisture but also sulfides, which accelerates the corrosion rate of the zinc coating.