21. How does the zinc coating corrode in hot water?

Zinc-coated steel pipes are used not only for water supply but also for transporting hot water, boiling water, and steam. The corrosive effects of these media differ from those of cold tap water. For instance, calcium bicarbonate, which is beneficial to zinc coatings in cold water, becomes harmful in hot water. This occurs because calcium bicarbonate decomposes into calcium carbonate and free carbon dioxide when heated, resulting in carbon dioxide and air bubbles adhering to the zinc surface, thereby accelerating corrosion. Additionally, the iron-zinc potential difference reverses in hot water, leading to increased oxygen accumulation on the zinc surface, further accelerating its corrosion.