During hot-dip galvanizing production, a layer of zinc ash (primarily zinc oxide) floats on the surface of the molten zinc and becomes thicker over time. If not removed, it can lead to uncoated black spots on galvanized steel pipes, resulting in defective products. Therefore, hot-dip galvanizing operators frequently skim the zinc ash from the surface of the molten zinc. Generally, in processes using zinc chloride + ammonium chloride as solvents, zinc ash is skimmed every half-hour or so; whereas in processes using hydrochloric acid as a solvent, zinc ash needs to be skimmed every 5 to 6 minutes. This is due to the fact that the former solvent produces less zinc ash, while the latter generates more.
In hot-dip galvanizing production, the formation of iron-zinc alloys, i.e., the ability to coat the steel pipe substrate with a zinc layer, requires a clean surface free from contamination by iron oxide layers and other impurities. Therefore, solvents (fluxes) are used to protect and clean the surface of the acid-washed steel pipe substrate from oxidation by oxygen in the air. After the steel pipe is immersed in the molten zinc, the solvent can also disperse dirt on the surface of the molten zinc. At the same time, zinc ash and solvent residue are produced. When a large amount of zinc ash is present at the steel pipe entrance on the surface of the molten zinc, the solvent is first burned away. Even if it is not burned, it cannot disperse a large amount of zinc ash in the molten zinc. Therefore, the pure iron steel pipe substrate is immediately exposed to the air or zinc ash, and an oxide layer or solvent residue will quickly form on its surface, resulting in uncoated black spots. Therefore, it is required to frequently skim the zinc ash on the surface of the molten zinc to keep the zinc liquid surface at the steel pipe entrance always metallic and shiny. As long as the previous processes are well-executed, during the hot-dip galvanizing process, it can be ensured that a zinc layer is coated without uncoated areas due to zinc ash and other factors.
However, excessively frequent skimming also has certain drawbacks. After skimming the zinc ash, the bright metal surface is exposed on the surface of the molten zinc. This pure metal surface, being at high temperatures, will quickly combine with oxygen in the air to form zinc ash. If aluminum-added molten zinc is used, aluminum oxide will also be produced. Therefore, the more frequently zinc ash is skimmed, the greater the amount of zinc ash generated, leading to a significant increase in zinc consumption and costs. Therefore, experienced operators have mastered the optimal number of skimming times through practice.
