(1) Degreasing
Chemical degreasing or water-based metal degreasing cleaning agent can be used to remove oil until the workpiece is completely soaked with water. (2) Pickling: H2SO4 15%, thiourea 0.1%, 40～60℃ or HCl 20%, hexamethylenetetramine 1～3g/L, 20～40℃ can be used for pickling. In addition, adding a corrosion inhibitor can prevent the matrix from over-corrosion and reduce the hydrogen absorption of the iron matrix.
Poor degreasing and pickling treatment will result in poor coating adhesion, failure to coat zinc, or the zinc layer falling off.
(3) Dip Flux
Also known as a binder, it can maintain a certain activity of the workpiece before immersion plating to enhance the bonding between the coating and the substrate. NH4Cl 15%～25%, ZnCl2 2.5%～3.5%, 55～65℃, 5～10min. In order to reduce the volatilization of NH4Cl, glycerol can be appropriately added.
(4) Drying preheat
In order to prevent the workpiece from deforming due to a sharp rise in temperature during immersion plating and to remove residual moisture to prevent the explosion of zinc, resulting in an explosion of zinc liquid, the preheating is generally 120 ~ 180 ° C.
(5) Hot dip galvanised steel sheet metal
It is necessary to control the temperature of the zinc bath, the dipping time, and the speed at which the workpiece is removed from the zinc bath. If the temperature is too low, the fluidity of the zinc liquid is poor, the coating layer is thick and uneven, easy to sag, and the appearance quality is poor; if the temperature is high, the liquid zinc has good fluidity, and the zinc liquid is easy to separate from the workpiece, reducing the occurrence of sagging and wrinkling, and the adhesion Strong, thin coating, good appearance, and high production efficiency; but if the temperature is too high, the iron loss of the workpiece and the zinc pot is serious, and a large amount of zinc slag is generated, which affects the quality of the dipped zinc layer. The zinc consumption is large and even cannot be plated. The immersion plating time is long at the same temperature, and the plating layer is thick. When the same thickness is required at different temperatures, the high-temperature immersion plating takes a long time. To prevent high-temperature deformation of the workpiece and reduce zinc slag caused by iron loss, manufacturers generally use 450 ~ 470 ℃, 0.5 ~ 1.5min. Some factories use higher temperatures for large workpieces and iron castings but avoid the temperature range where the iron loss peaks. To improve the fluidity of the hot-dip plating solution at a lower temperature, prevent the coating from being too thick, and improve the appearance of the coating, 0.01% to 0.02% of pure aluminum is often added. Aluminum should be added in small amounts and multiple times.
(6) Organize
After plating, I finish the workpiece mainly to remove the residual zinc and zinc nodules on the surface, which can be done by vibration or manual methods.
(7) Passivation
The purpose is to improve the atmospheric corrosion resistance of the workpiece surface, reduce or prolong the appearance time of white rust, and keep the coating with a good appearance. All are passivated with chromate, such as Na2Cr2O7 80～100g/L, and sulfuric acid 3～4ml/L.
(8) Cooling
It is generally cooled with water, but the temperature should not be too low to prevent the workpiece, especially the casting, from cracking the matrix structure due to the chilling shrinkage.
(9) Inspection
The appearance of the coating is bright and detailed, without sagging and wrinkling. Thickness inspection can use a coating thickness gauge; the method is relatively simple. The coating thickness can also be obtained by converting the amount of zinc adhesion. The bonding strength can be determined by using a bending press to bend the sample at 90 to 180°, and there should be no cracks and coating peeling off. It can also be tested by hitting it with a heavy hammer.