What are the types and processes of galvanizing?

What are the types and processes of galvanizing

Technological process

Workpiece → degreasing → water washing → pickling → water washing → immersion assisted plating solvent → drying and preheating → hot dip galvanizing → finishing → cooling → passivation → rinsing → drying → inspection

Related process description

(1) degreasing

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

(2) pickling

It can be pickled by 15% H2SO4, 0.1% thiourea,40~60℃ or 20% HCl, 1~3g/L hexamethyltetramine,20~40℃. The addition of corrosion inhibitor can prevent overcorrosion and reduce hydrogen absorption of iron matrix.

Poor degreasing and pickling treatment will cause poor adhesion of the coating, zinc plating or zinc layer off.

(3) dipping aid plating agent

Also known as bonding agent, can maintain a certain activity of the workpiece before dipping, in order to enhance the coating and substrate bonding. NH4Cl 15%-25%,ZnCl2 2.5%-3.5%,55~65℃,5 ~10min. Glycerol can be added appropriately to reduce NH4Cl volatilization.

(4) drying and preheating

In order to prevent the workpiece from deformation due to a sharp rise in temperature when dipping, and remove residual moisture, prevent zinc explosion, resulting in zinc liquid splash, preheating is generally 120~180℃.

(5) hot dip galvanizing

It is necessary to control the temperature of zinc solution, the dipping time and the speed of workpiece removal from zinc solution.

The temperature is too low, the fluidity of zinc liquid is poor, the coating is thick and uneven, easy to produce flow hanging, poor appearance quality; High temperature, good fluidity of zinc liquid, zinc liquid easy to leave the workpiece, reduce the flow and wrinkle phenomenon, strong adhesion, thin coating, good appearance, high production efficiency; But the temperature is too high, the workpiece and zinc pot iron damage is serious, produce a lot of zinc slag, affect the quality of zinc coating, zinc consumption is large, even can not be plated.

At the same temperature, the dipping time is long and the coating is thick. Different temperature, require the same thickness, high temperature dip plating time is long.

In order to prevent high-temperature deformation of the workpiece and reduce zinc slag caused by iron damage, manufacturers generally use 450~470℃, 0.5~1.5min. Some factories use higher temperatures for large parts and cast iron, but to avoid the temperature range of peak iron damage.

In order to improve the fluidity of hot dip bath at low temperature, prevent the coating from being too thick, and improve the appearance of the coating, 0.01%~0.02% pure aluminum is often added. Aluminum is added in small amounts many times.

(6) Sorting

After plating, the finishing of the workpiece is mainly to remove the residual zinc and zinc nodulation on the surface, with vibration or manual method.

(7) passivation

The purpose is to improve the atmospheric corrosion resistance of the workpiece surface, reduce or prolong the appearance of white rust, and maintain a good appearance of the coating. Chromate passivation is used, such as Na2Cr2O7 80~100g/L, sulfuric acid 3~4ml/L.

(8) Cooling

Generally use water cooling, but the temperature can not be too low, to prevent the workpiece, especially the casting due to cold shrinkage matrix structure cracking.

(9) Inspection

Coating appearance bright, fine, no flow hanging, wrinkle phenomenon. Thickness can be tested by coating thickness measuring instrument, the method is relatively simple. The coating thickness can also be obtained by converting the amount of zinc adhesion. Binding strength can be used bending press, the sample for 90~180° bending, there should be no cracks and coating falling off. It can also be tested by percussion with a heavy hammer.