Hot dipped galvanized steel pipes are made by reacting the molten metal with the iron matrix to produce an alloy layer, so that the matrix and the coating are combined. Hot-dip galvanizing is to first pickle the steel pipe to remove iron oxide on the surface of the steel pipe. After pickling, it is cleaned in a tank of ammonium chloride or zinc chloride aqueous solution or a mixed aqueous solution of ammonium chloride and zinc chloride, and then sent to a hot dip plating tank. http://www.ruijiesteelpipe.com/galvanized-steel-pipe/hot-dipped-galvanized-steel-pipe/