How does zinc coating corrode in hot water?

Galvanized steel pipes are not only used for tap water but also for transporting lukewarm water, boiling water, steam, and other media, which have different corrosive effects on the zinc coating compared to cold tap water. For instance, calcium bicarbonate in cold tap water is beneficial to the zinc coating, but it becomes harmful in hot water. This is because calcium bicarbonate decomposes into calcium carbonate and free carbon dioxide when heated. Consequently, carbon dioxide and air bubbles adhere to the zinc coating, causing the most severe corrosion. Additionally, the iron-zinc potential difference behaves oppositely in hot water, making it easier for oxygen to accumulate on the surface of the zinc coating, which also accelerates the corrosion of the zinc coating.