Table of contents
(1)Degreasing
(2) Pickling
(3) Immersion flux
(4)Drying and preheating
(5)Hot-dip galvanizing
(6)Organization
(7) Passivation
(8)Cooling
(9) Inspect coating
Hot-dip galvanizing is one of the most effective means of delaying environmental corrosion of steel materials. It immerses the cleaned and activated steel products in molten zinc liquid. Through the reaction and diffusion between iron and zinc, the steel products are The surface is coated with zinc alloy coating with good adhesion. Compared with other metal protection methods, the hot-dip galvanizing process combines the physical barrier and electrochemical protection of the coating, the bonding strength between the coating and the substrate, the density, durability, maintenance-freeness and economy of the coating. It has unparalleled advantages in terms of durability and adaptability to the shape and size of products. Here is a brief description of the hot-dip galvanizing process for easy differentiation.
Hot-dip galvanizing process workpiece → degreasing → water washing → pickling → water washing → dipping in plating solvent → drying and preheating → hot dip galvanizing → finishing → cooling → passivation → rinsing → drying → inspection
Partial process description
(1)Degreasing
Chemical degreasing or water-based metal degreasing cleaning agents can be used to remove oil until the workpiece is completely soaked in water.
(2) Pickling
You can use H2SO4 15%, thiourea 0.1%, 40~60℃ or HCl 20%, methenamine 3~5g/L, 20~40℃ for pickling. Adding corrosion inhibitors can prevent excessive corrosion of the substrate and reduce the hydrogen absorption of the iron substrate, while adding fog suppressors can inhibit the escape of acid mist. Poor degreasing and pickling treatments will result in poor adhesion of the coating, failure to coat with zinc or the zinc layer falling off.
(3) Immersion flux
Also called a solvent, it can keep the workpiece active to a certain extent before immersion plating to avoid secondary oxidation and enhance the bonding between the coating and the substrate. NH4Cl 100-150g/L, ZnCl2 150-180g/L, 70~80℃, 1~2min. And add a certain amount of explosion-proof agent.
(4)Drying and preheating
In order to prevent the workpiece from deforming due to a sharp increase in temperature during immersion plating, remove residual moisture, and prevent zinc explosion and zinc liquid splashing, the preheating is generally 80 to 140°C.
(5)Hot-dip galvanizing
It is necessary to control the zinc liquid temperature, immersion plating time and the speed at which the workpiece is drawn out of the zinc liquid. The extraction speed is generally 1.5 meters/min. If the temperature is too low, the fluidity of the zinc liquid will be poor, and the coating will be thick and uneven, prone to sagging and poor appearance quality; if the temperature is high, the fluidity of the zinc liquid will be good, and the zinc liquid will easily separate from the workpiece, reducing the occurrence of sagging and wrinkles, and the adhesion will be reduced. Strong, thin coating, good appearance, high production efficiency; but if the temperature is too high, the workpiece and zinc pot will suffer serious iron loss, and a large amount of zinc slag will be produced, which will affect the quality of the zinc-dip layer and easily cause color difference, making the surface color ugly and causing high zinc consumption.
The thickness of the zinc layer depends on the zinc liquid temperature, zinc immersion time, steel material and zinc liquid composition. In order to prevent high-temperature deformation of the workpiece and reduce zinc slag caused by iron loss, general manufacturers use 450~470℃, 0.5~1.5min. Some factories use higher temperatures for large workpieces and cast iron parts, but they must avoid the temperature range where iron loss peaks. However, we recommend adding an alloy with iron removal function and lowering the eutectic temperature to the zinc solution and lowering the galvanizing temperature to 435-445°C.
(6)Organization
The main purpose of cleaning the workpiece after plating is to remove residual zinc and zinc tumors on the surface, which is accomplished by using a special vibrator for hot-dip galvanizing.
(7) Passivation
The purpose is to improve the atmospheric corrosion resistance of the workpiece surface, reduce or extend the appearance time of white rust, and maintain the good appearance of the coating. All use chromate passivation, such as Na2Cr2O7 80~100g/L, sulfuric acid 3~4ml/L, but this passivation solution seriously affects the environment, so it is best to use chromium-free passivation.
(8)Cooling
Generally, water cooling is used, but the temperature should not be too low or too high, generally not lower than 30°C and not higher than 70°C.
(9) Inspect coating
The appearance is bright and delicate, with no sagging or wrinkled skin. The coating thickness gauge can be used for thickness inspection, and the method is relatively simple. The thickness of the coating can also be obtained by converting the zinc adhesion amount. To determine the bonding strength, a bending press can be used to bend the sample at 90 to 180°. There should be no cracks or coating peeling off. You can also use a heavy hammer to test, and conduct salt spray tests and copper sulfate etching tests in batches.
