Galvanized steel pipes for water gas (new national standard GB3091-82, 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 of zinc coating in tap water. To investigate the corrosion in this environment, it is first essential to understand the composition of tap water. Generally, 1 liter of tap water contains 10 milligrams of dissolved oxygen. This dissolved oxygen reacts with the zinc coating to form zinc hydroxide, which does not provide protection but exists as a corrosion product. Soft water contains more dissolved oxygen, carbon dioxide, and sodium salts (water softened by chemical methods), which accelerate the corrosion rate of the zinc coating. Hard water contains aluminum hydroxide, silicic acid, phosphates, magnesium salts, and calcium carbonate, which have a protective effect on the zinc coating. Therefore, the zinc coating of galvanized steel pipes exhibits better corrosion resistance in hard water than in soft water.
The pH value of tap water generally ranges from 7.5 to 9.5. When the contents of calcium bicarbonate, sulfides, chlorides, and nitrides are within allowable limits, the zinc coating is stabilized and protected by the formation of a non-soluble carbonate layer.
Tap water is treated with liquid chlorine for disinfection and sterilization, which is extremely harmful to the zinc coating and has a strong corrosive effect. 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 coating in cold tap water is approximately 0.66 milligrams per square decimeter per day, while the corrosion rate of tin-containing zinc coating is about 2.03 milligrams per square decimeter per day.
