During hot-dip galvanizing production, a layer of zinc oxide (primarily ZnO) floats on the surface of the zinc bath and gradually thickens. If not removed, it can lead to ungalvanized black spots on the galvanized steel pipes, resulting in defective products. Therefore, hot-dip galvanizing operators frequently scrape the zinc oxide off the surface of the zinc bath. In processes using zinc chloride + ammonium chloride as the flux, zinc oxide is typically scraped every half-hour or so. In processes using hydrochloric acid as the flux, however, zinc oxide needs to be scraped every 5 to 6 minutes due to the higher rate of zinc oxide formation. This is because the former flux generates less zinc oxide, while the latter generates more.
In hot-dip galvanizing production, the formation of iron-zinc alloy, or the ability to galvanize the steel pipe substrate, requires a clean surface free from iron oxide layers and other impurities. Therefore, a flux is used to protect and clean the acid-pickled steel pipe substrate surface from oxidation by atmospheric oxygen. Once the steel pipe is immersed in the zinc bath, the flux also helps disperse dirt on the surface of the zinc bath, simultaneously generating zinc oxide and flux residue. When a large amount of zinc oxide is present at the steel pipe entrance on the zinc bath surface, the flux is first burned away. Even if it is not burned, it cannot disperse a large amount of zinc oxide in the zinc bath. As a result, the pure iron steel pipe substrate is immediately exposed to air or zinc oxide, and an oxide layer or flux residue quickly forms on its surface, leading to ungalvanized black spots. Therefore, it is necessary to frequently scrape the zinc oxide off the zinc bath surface to ensure that the zinc bath surface at the steel pipe entrance always maintains a metallic shine. As long as the previous processes are well executed, the galvanizing process can ensure the application of a zinc layer without ungalvanized areas due to zinc oxide or other factors.
However, excessively frequent scraping also has drawbacks. Scraping exposes a shiny metal surface on the zinc bath, which, being a pure metal and exposed to high temperatures, quickly combines with oxygen in the air to form zinc oxide. If aluminum is added to the zinc bath, aluminum oxide (Al2O3) may also be produced. Therefore, the more frequently zinc oxide is scraped, the more zinc oxide is generated, significantly increasing zinc consumption and costs. Consequently, experienced operators have mastered the optimal scraping frequency through practice.
