Galvanized steel pipes are divided into two types: hot-dip galvanized steel pipes and cold galvanized steel pipes.
Hot-dip galvanized pipes are made by reacting molten metal with the iron substrate to form an alloy layer, thus combining the substrate and the coating. In the hot-dip galvanizing process, the steel pipes are first acid-washed to remove the iron oxide on the surface. After acid washing, they are cleaned in a solution of ammonium chloride or zinc chloride or a mixed solution of ammonium chloride and zinc chloride, and then sent into the hot-dip galvanizing bath. Hot-dip galvanizing has the advantages of uniform coating, strong adhesion, and long service life. In the hot-dip galvanizing process, the steel pipe substrate and the molten coating undergo complex physical and chemical reactions to form a corrosion-resistant, closely structured zinc-iron alloy layer. The alloy layer, pure zinc layer, and steel pipe substrate are integrated. Therefore, it has strong corrosion resistance.
Cold galvanizing is also known as electro-galvanizing, with a very low zinc coating weight of only 10-50g/m2. Its inherent corrosion resistance is much inferior to that of hot-dip galvanized pipes. The zinc coating on cold-galvanized steel pipes is an electroplated layer, which is independently stratified from the steel pipe base. The zinc coating is thin and simply adheres to the steel pipe base, making it prone to detachment. Therefore, its corrosion resistance is poor. In newly built residences, the use of cold-galvanized steel pipes as water supply pipes is prohibited.
