20. How does the zinc coating corrode in cold tap water?

Water-gas galvanized steel pipes (referred to as "Galvanized Welded Steel Pipes for Low-Pressure Fluid Transportation" in the new national standard GB3091-82) are primarily used as water supply pipes. Therefore, it is essential to study the corrosion behavior of the zinc coating in tap water. To understand this corrosion process, we must first examine the composition of tap water. Typically, 1 liter of tap water contains 10 milligrams of dissolved oxygen. This oxygen reacts with the zinc coating to form zinc hydroxide, which exists as a corrosive byproduct rather than a protective layer. Soft water, containing higher levels of dissolved oxygen, carbon dioxide, and sodium salts (from chemical softening processes), accelerates zinc coating corrosion. In contrast, hard water contains aluminum hydroxide, silicic acid, phosphates, magnesium salts, and calcium carbonate, which form protective layers on the zinc coating. Consequently, galvanized steel pipes exhibit better corrosion resistance in hard water compared to soft water.
The pH value of tap water is generally in the range of 7.5-9.5. When the calcium bicarbonate, sulfide, chloride and nitride are within the allowable content, the zinc layer is in a stable state and protected because of the formation of insoluble carbonate layer.
Tap water is treated with liquid chlorine for disinfection, which is highly corrosive to zinc coatings. When zinc coatings contain tin exceeding 0.28%, they may develop pitting corrosion in tap water. Therefore, hot-dip galvanized steel pipes used for water supply should not intentionally incorporate tin into their coatings. Generally, tin-free zinc coatings corrode at a rate of approximately 0.66 mg/dm²·d in cold tap water, while tin-containing zinc coatings corrode at a rate of about 2.03 mg/dm²·d.