(1) When aluminum is added to molten zinc, it reacts with oxygen in the air to form aluminum oxide. Tests have shown that the zinc ash at the entrance where steel pipes enter the molten zinc contains about 15.2% aluminum oxide. Aluminum oxide has a melting point of 2050°C and a low density of only 3.9-4.0 kg/L, whereas zinc oxide has a melting point of 1975°C and a density of 5.606 kg/L. The density of molten zinc at operating temperatures of 480-510°C is 6.54-6.79 kg/L. It is evident that aluminum oxide, with the lowest density, always floats on top. When steel pipes coated with flux are not dry or have been exposed to air for a long time after drying, the flux may become damp again. When the steel pipes enter the molten zinc, they first come into contact with aluminum oxide and then with zinc oxide (zinc ash). These substances adhere to the surface of the steel pipes, burning off the flux and resulting in uncoated spots.
(2) During startup and re-production, due to prolonged inactivity, aluminum with low density floats to the surface of the molten zinc. When steel pipes coated with flux come into contact with it, the following reaction occurs immediately:
2Al + 3ZnCl₂ → 2AlCl₃ + 3Zn
From the equation, it is clear that the more reactive aluminum immediately replaces zinc in the flux compound, forming aluminum chloride (AlCl₃), which sublimates at 178°C. Similarly, aluminum reacts with ammonium chloride in the flux to produce a compound of AlCl₃·NH₃, which boils and evaporates around 400°C. Therefore, these reactions result in the complete loss of chlorine, which aids in galvanizing, leading to uncoated spots.
(3) When production just begins, the temperature of the molten zinc is generally higher. After the flux comes into contact with the molten zinc, it does not have enough time to complete its reaction process, physical adsorption, and chemical combination, resulting in degraded flux residue that loses its function. This leads to uncoated spots.
(4) When steel pipes coated with flux are immersed in molten zinc for galvanizing, tools such as pliers and turntables are used to force them into the molten zinc. These tools can damage the flux film on the steel pipes to varying degrees at the points of contact. Therefore, when in contact with the molten zinc, this area loses its galvanizing ability, resulting in uncoated spots.
(5) When production begins before reaching the process temperature, the reaction between iron and zinc is relatively slow due to the lower temperature of the molten zinc, the lack of extended immersion time, and the concentration of aluminum on the surface. An iron-zinc alloy layer cannot be formed in a short time. Therefore, once removed, uncoated areas may be found on the steel pipes.
(6) If there is excess aluminum in the galvanizing pot and the temperature of the molten zinc is unstable, a large number of solid particles of Fe-Al-Zn compounds will suspend in the molten zinc. When steel pipes pass through, these solid particles adhere to the surface of the steel pipes, causing surface roughness defects.
Solutions:
(1) During startup production, the aluminum content in the molten zinc should be lower than that during normal production. As production normalizes, gradually increase it to the specified process level.
(2) Frequently scrape the zinc ash from the surface of the molten zinc at the steel pipe entrance.
(3) The flux applied to steel pipes should be dry and free from moisture or incomplete drying.
(4) The temperature of the molten zinc in the galvanizing pot should not be too high or too low.
(5) Avoid scratching the flux coated on steel pipes during transportation.
(6) Steel pipes should be immersed in the molten zinc at a large angle to minimize rolling on the surface of the molten zinc.
