79. Why does the zinc ash content at the tail end of the steel pipe's zinc bath exceed that in other areas during hot-dip galvanizing?

In hot-dip galvanizing production, operators must meticulously scrape off zinc ash from the molten zinc surface at the pipe's tail end. During the process, steel pipes are first tilted into the zinc bath at the head end, gradually allowing the tail end to immerse. This design enables the tail end to expel gases formed by the reaction between dissolved flux and zinc, allowing the molten zinc to flow unimpeded into the pipe's interior for complete inner wall coating. As the zinc bath penetrates the pipe, the zinc ash and flux residues generated from the reaction between the molten zinc and the inner surface are expelled through the tail end, resulting in a significant accumulation of zinc ash on the tail end's surface. Meanwhile, the zinc ash and flux residues from the reaction between the molten zinc and the head end as well as all exterior surfaces of the pipe are distributed across the entire contact area, leaving the zinc bath surface relatively clean.
Furthermore, iron salts and carbon particles adhering to the inner walls of steel pipes awaiting galvanizing are more difficult to remove than those on the outer surfaces after acid pickling. When the solution (melting) agent is applied, these residues are carried into the zinc bath. The reaction between iron salts and zinc liquid produces zinc slag and solution (melting) agent residues. The zinc slag settles beneath the zinc bath, while small carbon particles and solution (melting) agent residues float on the surface along with zinc ash (ZnO). Consequently, the zinc ash and other waste components on the zinc bath surface in the tail section of galvanized steel pipes are significantly more abundant than in any other area.
Another reason is that the aluminum content on the inner surface of the zinc bath is far less than that on the outer surface in contact with it, thus reducing or eliminating the protective film of aluminum oxide, and increasing the amount of zinc ash.