81. What are the hazards of zinc ash to hot-dip galvanizing?

As analyzed in the zinc ash composition section, zinc ash contains not only the primary component zinc oxide but also other oxides such as aluminum oxide, antimony oxide, chlorides, iron, and acid-insoluble impurities. These components pose significant risks to galvanized steel pipes, manifesting in four key ways: (1) When exposed to high-temperature zinc ash on the zinc liquid surface, steel pipes must pass through the ash layer before immersion, which may burn away the solvent and cause leakage. (2) The ash traps solvent residues. If these residues adhere to the pipe surface, they not only prevent zinc coating but also encapsulate the pure zinc layer. When moisture from the air is absorbed, the chlorides react with water to form corrosive media, accelerating the corrosion of the pure zinc layer and causing tear-like droplets that damage the pipe. (3) During zinc liquid discharge, solvent residues on the pipe surface absorb moisture, triggering localized corrosion. (4) The ash contains carbon particles from steel pipe pickling and dissolution, creating false leakage on the zinc layer. Although the inner iron-zinc alloy layer remains, it becomes thinner than the surrounding pure zinc layer, reducing service life. (5) Excessive zinc ash thickens the zinc liquid surface, degrading its quality. (6) When pipes exit the galvanizing pot, accumulated zinc ash forms yellow spots or patches on the surface. Without timely internal and external cleaning, these spots solidify into visible yellow marks. If galvanized steel pipes are discharged from the galvanizing pot immediately via the roller conveyor, the zinc ash, due to the weight of the pipes, will be pressed into the pure zinc layer. Combined with the pipes 'jumping motion on the conveyor, this creates a linear distribution of yellow zinc ash spots on the surface. If these spots remain unremoved, the acidic impurities (e.g., chlorides) in the zinc ash will absorb moisture from the air and corrode the galvanized layer. Removing these spots results in pitting, thinning or even eliminating the pure zinc layer, significantly shortening the product's lifespan. (6) The zinc ash also contains iron. When mixed with the pure zinc layer on the galvanized surface, it may form yellow rust spots under certain humidity conditions and cause false endpoint readings during testing, compromising the galvanizing quality.