80. What are the advantages and disadvantages of frequent zinc ash scraping?

During hot-dip galvanizing, the zinc ash layer (primarily zinc oxide) on the molten zinc surface thickens progressively. Failure to remove this residue results in black spots from missed coating on galvanized steel pipes, leading to defective products. Consequently, operators routinely scrape off the zinc ash from the molten zinc surface. In processes using zinc chloride and ammonium chloride as solvents, the ash is scraped every 30 minutes. For processes employing hydrochloric acid as the solvent, the scraping interval is reduced to 5-6 minutes. This difference stems from the solvent's varying ash generation rates: the former produces less ash, while the latter generates more.
In hot-dip galvanizing production, the formation of iron-zinc alloy requires a clean steel pipe substrate free from iron oxide layers and impurities to achieve effective zinc coating. A flux is used to protect and clean the acid-washed pipe surface, preventing oxidation caused by atmospheric oxygen. When the pipe is immersed in molten zinc, the flux disperses surface contaminants while producing zinc ash and residual solvent. If significant zinc ash accumulates at the pipe inlet, the flux will burn off first. Even if partially burned, the zinc ash cannot be dispersed in the molten zinc, leaving the pure iron substrate exposed to air or ash. This leads to rapid oxidation layer formation or solvent residue accumulation, resulting in black spots from missed coating. Therefore, regular removal of zinc ash from the molten zinc surface is essential to maintain a metallic luster at the pipe inlet. By ensuring proper execution of initial processes, the hot-dip galvanizing procedure guarantees consistent zinc coating without missed coating due to zinc ash or other contaminants.
However, excessive ash scraping poses certain drawbacks. When zinc ash is scraped off, the exposed metallic surface of the zinc liquid becomes rapidly exposed to oxygen in the air under high-temperature conditions, forming zinc ash. In the case of aluminum-zinc liquid, aluminum oxide is also produced. Consequently, the more frequently the ash is scraped, the greater the amount of zinc ash generated, leading to significantly increased zinc consumption and higher costs. Therefore, experienced operators have developed the optimal ash-scraping frequency through practical experience.