71. Why do missed coating spots and zinc particles frequently occur during aluminum-zinc alloy potting of steel pipes, especially at startup? What are the solutions?

This paper does not discuss the leaching spots caused by pickling, solvent and drying, but only discusses the causes of leaching spots in hot-dip galvanizing.
(1) The aluminum in the zinc-aluminum alloy reacts with air to form aluminum oxide. Laboratory tests show that the zinc ash at the steel pipe's entry point contains approximately 15.2% aluminum oxide. Aluminum oxide has a melting point of 2050°C and a density of 3.9-4.0 kg/L, while zinc oxide melts at 1975°C with a density of 5.606 kg/L. At the operating temperature of 480-510°C, the zinc liquid's density ranges from 6.54 to 6.79 kg/L. This density gradient causes aluminum oxide to remain at the top. When the steel pipe isn't properly dried or stays in air for too long after drying, moisture from the solvent reabsorbs. As the pipe enters the zinc bath, it first contacts aluminum oxide before zinc oxide (zinc ash). These substances adhere to the pipe surface, burning off the solvent and resulting in mottled coating defects.
(2) During the initial and subsequent production phases, aluminum with low density and prolonged static time floats to the surface of molten zinc. When the steel pipe coated with solvent comes into contact with it, the following reaction immediately occurs: 2Al + 3ZnCl₂ → 2AlCl₃ + 3Zn. As shown in the equation, the more reactive aluminum instantly displaces zinc from the solvent compound, forming aluminum trichloride (AlCl₃). However, AlCl₃ sublimates at 178°C. Similarly, aluminum reacts with ammonium chloride in the solvent to form AlCl₈NH₃, which boils and evaporates at around 400°C. Consequently, these reactions completely deplete the chlorine content essential for plating assistance, resulting in missed plating spots.
(3) The temperature of zinc liquid is usually high at the beginning of the operation. When the solvent comes into contact with the zinc liquid, the physical adsorption and combination of the solvent cannot be completed in time, and the solvent residue is formed. The solvent loses its function, and the leakage of the plating mottling is produced.
(4) When the steel pipe coated with solvent is put into the zinc bath for plating, it is necessary to force it into the zinc bath with pliers and turntable. The contact between these tools and the steel pipe will destroy the solvent film to varying degrees, so the plating ability of the contact area will be lost and the plating spot will be produced.
(5) When the production starts, the process temperature is not reached yet, and the zinc bath temperature is low, the zinc immersion time is not extended, and the aluminum bath is concentrated on the surface, the reaction between iron and zinc is slow, and the iron-zinc alloy layer cannot be formed in a short time, so once the group comes out, there will be some unzinced parts on the steel pipe.
(6) Excessive aluminum content in the galvanizing bath combined with unstable zinc temperature may cause Fe-Al-Zn compound particles to suspend in the zinc bath. When steel pipes pass through, these particles adhere to the pipe surfaces, resulting in surface roughness defects. Solutions: (1) During initial production, the aluminum content in the zinc bath should be lower than normal production levels, gradually increasing to the specified process standard as operations normalize; (2) Regularly scrape off zinc ash from the zinc bath surface at the pipe inlet; (3) Ensure the solvent applied to steel pipes is dry, avoiding moisture or incomplete drying; (4) Maintain the zinc bath temperature within optimal range; (5) Prevent solvent damage to steel pipes during transportation; (6) Immerse steel pipes at a steep angle into the zinc bath, minimizing rolling on the surface.