In the production of hot-dip galvanized steel pipes, operators often spend considerable effort scraping off the zinc ash from the molten zinc surface at the tail end of the pipes. When steel pipes are hot-dip galvanized, they are initially dipped into the molten zinc at an angle from the head, gradually immersing the tail end. This allows air inside the pipe and gases generated from the reaction between flux and zinc to be expelled from the tail. Consequently, the molten zinc can enter the interior of the pipe without obstruction, completing the galvanizing process of the inner wall. As the molten zinc enters the pipe, it carries out zinc ash and flux residue formed from the reaction between the flux and molten zinc on the entire inner surface of the pipe, along with the tail end. This results in a significant amount of zinc ash appearing on the molten zinc surface at the tail end. In contrast, the zinc ash and flux residue formed from the reaction between the flux and molten zinc on the head and outer surface of the pipe are distributed along the entire length of contact, making the zinc ash appear less prominent on the molten zinc surface.
Furthermore, after acid pickling treatment, iron salts and carbon particles adhering to the inner wall of the steel pipe are more difficult to remove compared to those on the outer surface. When coated with flux, these impurities are carried into the molten zinc. Iron salts react with the molten zinc to produce zinc slag and flux residue. The zinc slag sinks to the bottom of the molten zinc, while small carbon particles and flux residue float along with zinc ash (ZnO) on the surface of the molten zinc. Therefore, there is more zinc ash and other waste components on the surface of the molten zinc at the tail end of the galvanized steel pipe in the galvanizing pot compared to any other location.
Another reason is that the aluminum content on the molten zinc surface inside the steel pipe is much lower than that on the outer surface in contact with the molten zinc. Consequently, the protective alumina film is reduced or absent, leading to an increased production of zinc ash.
