In order to improve the corrosion resistance of steel pipes, general steel pipes are galvanized. Galvanized welded pipe is divided into hot galvanizing and cold galvanizing. Cold galvanizing is generally used less. Hot dip galvanized pipe is to make molten metal react with iron substrate to produce alloy layer, so that the substrate and coating can be combined. Hot galvanizing refers to pickling the steel pipe first. In order to remove iron oxide on the steel pipe surface, after pickling, the steel pipe is cleaned in ammonium chloride or zinc chloride aqueous solution or mixed ammonium chloride and zinc chloride aqueous solution tank, and then sent to the hot dip tank. Hot dip galvanizing has the advantages of even coating, strong adhesion and long service life.