Hot-dip galvanizing process and related instructions

Hot-dip galvanizing process

Workpiece → degreasing → water washing → pickling → water washing → immersion in the auxiliary plating solution → drying and preheating → hot-dip galvanizing → finishing → cooling → passivation → rinsing → drying → inspection

Hot dip galvanizing process description

(1) Degreasing

Chemical degreasing or water-based metal degreasing cleaning agent can be used for degreasing until the workpiece is completely wetted by water.

(2) Pickling

It can be extracted with 15% sulfuric acid, 0.1% thiourea, 40～60℃ or 20% hydrochloric acid, 3～5g/L urotropine, and 20～40℃. The addition of corrosion inhibitor can prevent excessive corrosion of the matrix and reduce the hydrogen absorption of the iron matrix. At the same time, adding a mist suppressant can inhibit the escape of acid mist. Improper degreasing and pickling treatments can lead to poor coating adhesion, zinc-free coating or zinc layer peeling.

(3) Immersion flux

Also known as solvent, it can keep the workpiece active before immersion plating, avoid secondary oxidation, thereby enhancing the bonding force between the plating layer and the substrate. NH4Cl 100-150g/L, ZnCl2 150-180g/L, 70～80℃, 1～2min, add a certain amount of antiknock agent.

(4) Drying and preheating

In order to prevent the workpiece from deforming due to the sharp rise in temperature during immersion plating, and to remove residual moisture, to prevent zinc explosion and cause zinc liquid explosion, the preheating temperature is generally 80-140°C.

(5) Hot dip galvanizing

It is necessary to control the temperature of the zinc liquid, the immersion time and the speed of drawing the workpiece from the zinc liquid. The extraction speed is generally 1.5m/min. If the temperature is too low, the fluidity of the zinc liquid is poor, the coating thickness is uneven, it is prone to sag, and the appearance quality is poor; the temperature is high, the fluidity of the zinc liquid is good, and the zinc liquid is easy to separate from the workpiece, which reduces Phenomenon of sagging and wrinkles. Strong, thin coating, good appearance, high production efficiency; but too high temperature will seriously damage the workpiece and the zinc pot, produce a large amount of zinc dross, affect the quality of the zinc dipping layer, easily cause chromatic aberration, make the surface color unsightly, and high zinc consumption .

The thickness of the zinc layer depends on the temperature of the zinc solution, the immersion time, the material of the steel and the composition of the zinc solution.

In order to prevent the high temperature deformation of the workpiece and reduce the zinc slag caused by the iron loss, the general manufacturers use 450-470℃, 0.5-1.5min. Some manufacturers use higher temperatures for large workpieces and cast iron parts, but avoid the peak iron loss. temperature range. However, we recommend adding an alloy with iron removal function to lower the eutectic temperature of the zinc solution and reduce the galvanizing temperature to 435-445°C.

(6) Finishing

The finishing of the workpiece after electroplating is mainly to remove the surface zinc and zinc nodules, and use a special vibrator for hot-dip galvanizing.

(7) Passivation

Its purpose is to improve the atmospheric corrosion resistance of the workpiece surface, reduce or prolong the appearance of white rust, and maintain a good coating appearance. They are all passivated with chromate, such as Na2Cr2O7 80-100g/L and sulfuric acid 3-4ml/L, but this kind of passivation solution has a serious impact on the environment, and it is best to use chromium-free passivation.

(8) Cooling

Generally water-cooled, but the temperature should not be too low or too high, generally not lower than 30℃, not higher than 70℃,

(9) Check

The appearance of the coating is bright, meticulous, without sagging, and without wrinkles. Thickness detection can use coating thickness gauge, the method is relatively simple. The thickness of the coating can also be obtained by converting the amount of zinc adhesion. The bond strength can be bent with a bending press, and the sample should be bent 90-180° without cracks or peeling of the coating. The heavy hammer percussion test can also be used to conduct the salt spray test and copper sulfate corrosion test in batches.