In the production of hot-dip galvanized steel pipes, operators often have to make great efforts to scrape off the zinc oxide (zinc ash) that accumulates on the zinc bath surface at the tail end of the pipes. When steel pipes undergo hot-dip galvanizing, they are initially dipped into the zinc bath at an angle from the head, gradually immersing the tail end. This method allows air inside the steel pipe and gases produced by the reaction between flux (or molten flux) and zinc to be expelled from the tail end. Consequently, the zinc bath can enter the interior of the steel pipe unobstructed, completing the galvanizing process of the inner wall. As the zinc bath enters the steel pipe, the zinc oxide and flux residue formed by the reaction between the flux (or molten flux) and zinc bath on the entire inner surface of the steel pipe are expelled from the tail end together. This results in a significant amount of zinc oxide appearing on the zinc bath surface at the tail end. In contrast, the zinc oxide and flux residue formed by the reaction between the flux (or molten flux) and zinc bath on the head and outer surface of the steel pipe are distributed across the entire contact surface of the steel pipe, making them appear sparse on the zinc bath surface.
Furthermore, after pickling treatment, iron salts and carbon particles adhering to the inner wall of the steel pipe are more difficult to remove than those on the outer surface. After applying the flux (or molten flux), these impurities are carried into the zinc bath. The iron salts react with the zinc bath to produce zinc slag and flux residue. The zinc slag sinks to the bottom of the zinc bath, while small carbon particles and flux residue float together with zinc oxide (ZnO) on the surface of the zinc bath. Therefore, there is more zinc oxide and other waste components on the surface of the zinc bath at the tail end of the galvanizing steel pipe than anywhere else in the zinc pot.
Another reason is that the aluminum content on the zinc bath surface inside the steel pipe is much lower than that on the zinc bath in contact with the outer surface. This results in a reduced or absent protective alumina film, leading to an increased production of zinc oxide.
