Here, we will not discuss the causes of missed plating spots due to pickling, solvents, and drying, but only focus on the reasons for missed plating spots during hot-dip galvanizing.
(1) Aluminum added to the zinc liquid reacts with oxygen in the air to form aluminum oxide. Tests have shown that the zinc ash at the entrance where the steel pipe enters the zinc liquid contains about 15.2% aluminum oxide. With a melting point of 2050°C and a low density of only 3.9-4.0 kg/L, aluminum oxide floats on top, while zinc oxide has a melting point of 1975°C and a density of 5.606 kg/L. At the operating temperature of 480-510°C, the density of zinc liquid is 6.54-6.79 kg/L. Therefore, the lowest-density aluminum oxide is always on top. If the solvent-coated steel pipe is not dry or has been exposed to air for a long time after drying, the solvent will become damp again. When the steel pipe enters the zinc liquid, it first contacts aluminum oxide and then zinc oxide (zinc ash). These substances adhere to the surface of the steel pipe, burning off the solvent and resulting in missed plating spots.
(2) During startup and reproduction, aluminum with low density floats on the surface of the zinc liquid due to prolonged stillness. When a solvent-coated steel pipe contacts it, the following reaction occurs immediately:
2Al + 3ZnCl₂ → 2AlCl₃ + 3Zn
As seen, the reactive aluminum immediately replaces zinc in the solvent compound, forming aluminum chloride (AlCl₃), which sublimates at 178°C. Similarly, aluminum reacts with ammonium chloride in the solvent to form AlCl₃·NH₃, which boils and evaporates at around 400°C. These reactions result in the loss of chlorine, which aids in galvanizing, leading to missed plating spots.
(3) The zinc liquid temperature is generally higher during initial startup. When the solvent contacts the zinc liquid, it does not have enough time to complete its reaction process of physical adsorption and compounding, forming degraded solvent residue that loses its effectiveness, resulting in missed plating spots.
(4) When a solvent-coated steel pipe is forced into the zinc liquid using clamps or turntables for dipping, these tools can damage the solvent film on the steel pipe to varying degrees. Therefore, when in contact with the zinc liquid, this area loses its galvanizing ability, causing missed plating spots.
(5) Starting production before reaching the process temperature, with a lower zinc liquid temperature, not extending the zinc dipping time, and a large concentration of aluminum on the surface, the reaction between iron and zinc is slower. An iron-zinc alloy layer cannot be formed in a short time, so uncoated areas may be found on the steel pipe after dipping.
(6) If the aluminum content in the galvanizing pot is excessive and the zinc liquid temperature is unstable, a large number of solid particles of Fe-Al-Zn compounds will suspend in the zinc liquid. When the steel pipe passes through, these solid particles adhere to the surface of the steel pipe, causing surface roughness defects.
Solutions:
(1) During startup, the aluminum content in the zinc liquid should be lower than during normal production. Gradually increase it to the specified process level as production normalizes.
(2) Frequently scrape the zinc ash on the surface of the zinc liquid at the steel pipe entrance.
(3) The solvent coated on the steel pipe should be dry and not damp or undried.
(4) The temperature of the zinc liquid in the galvanizing pot should not be too high or too low.
(5) Avoid scratching the solvent coated on the steel pipe during transportation.
(6) The steel pipe should be dipped into the zinc liquid at a large angle to avoid rolling on the zinc liquid surface.
