1.Degreasing: Chemical degreasing or water-based metal degreasing cleaner is used to remove oil until the workpiece is completely wetted by water.
2.Water rinsing.
3.Pickling: Pickling is performed using H₂SO₄ 15%, thiourea 0.1%, at 40–60°C; or HCl 20%, urotropine 3–5g/L, at 20–40°C. Corrosion inhibitors are added to prevent excessive corrosion of the substrate and reduce hydrogen absorption by the iron matrix, while mist suppressants are added to inhibit acid fume escape. Poor degreasing and pickling treatment will result in poor coating adhesion, inability to plate zinc, or zinc layer detachment.
4.Water rinsing.
5.Flux dipping: Also known as solvent treatment, this maintains workpiece activity before immersion to prevent secondary oxidation and enhance coating-substrate bonding. A certain amount of anti-explosion agent is also added.
6.Drying and preheating: To prevent workpiece deformation due to rapid temperature rise during immersion and to remove residual moisture to prevent zinc explosion and splashing. Preheating temperature is generally 80–140°C.
7.Hot-dip galvanizing: Zinc bath temperature, immersion time, and workpiece withdrawal speed must be controlled. Withdrawal speed is generally 1.5 m/min. Low temperature results in poor zinc fluidity, thick and uneven coating, easy dripping and poor appearance; high temperature improves zinc fluidity, easier zinc separation from workpiece, reduced dripping and wrinkling, strong adhesion, thin coating, good appearance, and high production efficiency. However, excessive temperature causes severe iron loss of workpiece and zinc pot, producing large amounts of zinc dross, affecting immersion zinc layer quality, easily causing color difference and poor surface appearance and high zinc consumption. Most manufacturers use 450–470°C for 0.5–1.5 min to prevent high-temperature deformation and reduce zinc dross from iron loss. However, it is recommended to add alloys with iron removal function and reduced eutectic temperature to the zinc bath, and lower the galvanizing temperature to 435–445°C.
8.Finishing: Post-plating finishing mainly involves removing surface excess zinc and zinc nodules using a hot-dip galvanizing dedicated vibrator.
9.Cooling: Generally water cooling, but temperature should not be too low or too high, generally not lower than 30°C and not higher than 70°C.
10.Inspection: Inspect coating appearance for brightness, fineness and absence of dripping or wrinkling.
Author: Bo Wang, Engineering Department.