Hot-dip galvanizing, also known as hot-dip galvanizing, melts zinc ingots at high temperatures, then puts some auxiliary materials, and then immerses the metal structural parts in a galvanizing bath to attach a layer of zinc to the metal members.
  
Crafting process 

Workpiece → degreasing → washing → pickling → washing → immersion plating solvent → drying preheating → hot galvanizing → finishing → cooling → passivation → rinsing → drying → inspection
  
1, Degreasing
  
Chemical degreasing or water-based metal degreasing cleaning agents can be used to remove oil until the workpiece is completely wetted by water.
  
2, Pickling
  
H2SO4 15%, thiourea 0.1%, 40-60 ° C or HCl 20%, hexamethylenetetramine 1-3 g / L, 20 ~ 40 ° C for pickling; adding corrosion inhibitor can prevent matrix over-corrosion And reduce the amount of hydrogen absorbed by the iron matrix.
  
Poor degreasing and pickling treatment will result in poor adhesion of the coating and no plating of zinc or zinc.
  
3. Immersion plating agent
  
Also known as a bonding agent, the workpiece can be held to have a certain activity before immersion plating to enhance the bonding of the coating to the substrate. NH4Cl 15% to 25%, ZnCl2 2.5% to 3.5%, 55 to 65 ° C, 5 to 10 minutes. To reduce the volatilization of NH4Cl, glycerol may be added as appropriate.
  
4, Drying preheating
  
In order to prevent the workpiece from being deformed due to a sharp rise in temperature during immersion plating, and to remove residual moisture, it is prevented from generating zinc, which causes the zinc liquid to splatter, and the preheating is generally 120 to 180 °C.
  
5, Hot-dip galvanizing

It is necessary to control the temperature of the zinc bath, the immersion time and the speed at which the workpiece is removed from the zinc bath.
  
6, Finishing

After the plating, the finishing of the workpiece is mainly to remove the surface residual zinc and zinc tumor, either by vibration or manual method.
  
7, Passivation
  
The purpose is to improve the surface corrosion resistance of the workpiece surface, reduce or prolong the appearance of white rust, and maintain a good appearance of the coating. Both are passivated with chromate, such as Na2Cr2O7 80 ~ 100g / L, sulfuric acid 3 ~ 4ml / L.
  
8, Cooling

It is generally cold in water, but the temperature should not be too low to prevent cracking of the substrate due to the retraction of the workpiece, especially the casting.
  
9, Inspection

The appearance of the coating is fine, bright, no sag, wrinkle; the thickness test can be coated with a thickness gauge, which is relatively simple. The thickness of the plating layer can also be obtained by conversion of the amount of zinc adhesion. The bonding strength can be bent by a bending machine, and the sample is bent at 90 to 180°, and there should be no crack and the coating peeling off. It can also be tested with a heavy hammer.
  
The advantage of hot-dip galvanizing is that it has strong anti-corrosion ability, and the adhesion and hardness of the galvanized layer are good. The disadvantage is that the price is high, and a large amount of equipment and site are required. The steel structural parts are too large to be placed in the galvanizing tank, the steel structural parts are too thin, and the hot plating is easily deformed.