Galvanized steel pipes for water gas supply (now referred to as "galvanized welded steel pipes for low-pressure fluid transport" under the new Chinese national standard GB3091-82) are predominantly used as water supply pipelines, making it essential to study the corrosion behavior of zinc coatings in tap water environments. To investigate this corrosion mechanism, it is first necessary to understand the composition of tap water. Typically, 1 liter of tap water contains 10 mg of dissolved oxygen, which reacts with the zinc coating to form non-protective zinc hydroxide that exists as a corrosion product. Soft water, containing elevated levels of dissolved oxygen, carbon dioxide, and sodium salts (chemically softened water), accelerates zinc corrosion. Conversely, hard water contains aluminum hydroxide, silicic acid, phosphates, magnesium salts, and calcium carbonate, which form protective layers on the zinc surface, resulting in better corrosion resistance for galvanized steel pipes in hard water compared to soft water.
The pH of tap water generally ranges from 7.5 to 9.5. When calcium bicarbonate, sulfides, chlorides, and nitrides are within permissible concentration limits, the zinc coating remains stable and protected due to the formation of an insoluble carbonate layer.
Chlorine addition for water disinfection purposes poses a significant corrosion risk to zinc coatings. When the zinc coating contains more than 0.28% tin, pitting corrosion occurs in tap water environments. Therefore, tin should not be deliberately added to the galvanized coating of steel pipes intended for water supply applications. Generally, tin-free zinc coatings exhibit a corrosion rate of approximately 0.66 mg/dm²·day in cold tap water, while tin-containing coatings demonstrate significantly accelerated corrosion at around 2.03 mg/dm²·day under the same conditions.
