Galvanized steel pipes for water gas (new national standard GB3091-82, also known as galvanized welded steel pipes for low-pressure fluid transportation) are mostly used as tap water pipes. Therefore, it is necessary to study the corrosion behavior of zinc coatings in tap water. To investigate the corrosion in this environment, we must first understand the composition of tap water. Typically, 1 liter of tap water contains 10 milligrams of dissolved oxygen, which reacts with the zinc coating to form zinc hydroxide. This zinc hydroxide does not provide protection and exists as a corrosion product. Soft water, which contains higher levels of dissolved oxygen, carbon dioxide, and sodium salts (water softened through chemical methods), accelerates the corrosion rate of zinc coatings. Hard water, which contains aluminum hydroxide, silicic acid, phosphates, magnesium salts, and calcium carbonate, protects the zinc coating, making galvanized steel pipes more resistant to corrosion in hard water than in soft water.
The pH value of tap water generally ranges from 7.5 to 9.5. When calcium bicarbonate, sulfides, chlorides, and nitrides are within permissible levels, the zinc coating becomes stable and protected by the formation of a non-soluble carbonate layer.
The addition of liquid chlorine for disinfection and sterilization in tap water is extremely harmful to zinc coatings, causing intense corrosion. When the zinc coating contains more than 0.28% tin, pitting corrosion may occur in tap water. Therefore, it is advisable not to intentionally add tin to the coating of hot-dip galvanized steel pipes used for tap water. Generally speaking, the corrosion rate of tin-free zinc coatings in cold tap water is approximately 0.66 milligrams per square decimeter per day, while the corrosion rate of tin-containing zinc coatings is about 2.03 milligrams per square decimeter per day.
