(1) Defatting
Chemical degreasing or water-based metal degreasing cleaning agents can be used to degrease the workpiece until it is completely soaked in water. (2) Pickling can be carried out with H2SO4 15%, thiourea 0.1%, 40~60 ℃ or HCl 20%, hexamethylene tetramine 1~3g/L, 20~40 ℃. Adding corrosion inhibitors can prevent excessive corrosion of the matrix and reduce hydrogen absorption of the iron matrix.
Poor degreasing and pickling treatment can result in poor adhesion of the coating, failure to apply zinc or detachment of the zinc layer.
(3) Dipping auxiliary plating agent
Also known as a binder, it can maintain a certain degree of activity of the workpiece before immersion, in order to enhance the bonding between the coating and the substrate. NH4Cl 15%~25%, ZnCl2 2.5%~3.5%, 55~65 ℃, 5-10 minutes. To reduce the volatilization of NH4Cl, glycerol can be added appropriately.
(4) Drying preheating
In order to prevent deformation of the workpiece during immersion plating due to a sharp increase in temperature, remove residual moisture, and prevent the occurrence of zinc explosion, resulting in zinc liquid splashing, the preheating temperature is generally 120~180 ℃.
(5) Hot dip galvanizing
It is necessary to control the temperature of the zinc solution, the immersion time, and the speed at which the workpiece is removed from the zinc solution. Low temperature, poor fluidity of zinc solution, thick and uneven coating, easy to produce sagging, and poor appearance quality; High temperature, good fluidity of zinc liquid, easy detachment of zinc liquid from the workpiece, reduced sagging and wrinkling, strong adhesion, thin coating, good appearance, and high production efficiency; However, if the temperature is too high, the workpiece and zinc pot iron are severely damaged, resulting in a large amount of zinc slag, which affects the quality of the zinc dipping layer, resulting in high zinc consumption and even inability to apply zinc. At the same temperature, the immersion time is long and the coating is thick. When different temperatures require the same thickness, high-temperature immersion plating requires a long time. In order to prevent high-temperature deformation of workpieces and reduce zinc slag caused by iron loss, general manufacturers use 450~470 ℃ for 0.5~1.5min. Some factories use higher temperatures for large workpieces and cast iron castings, but they need to avoid the temperature range of peak iron loss. In order to improve the flowability of the hot dip plating solution at lower temperatures, prevent excessive coating thickness, and improve the appearance of the coating, pure aluminum of 0.01% to 0.02% is often added. Aluminum needs to be added in small quantities multiple times.
(6) Organize
After plating, the main task of organizing the workpiece is to remove residual zinc and zinc nodules on the surface, which can be done using vibration or manual methods.
(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 a good appearance of the coating. They are passivated with Chromate and dichromate, such as Na2Cr2O7 80 ~ 100g/L, sulfuric acid 3 ~ 4ml/L.
(8) Cooling
Generally, water cooling is used, but the temperature should not be too low to prevent the workpiece, especially the casting, from cracking the matrix structure due to quenching and shrinkage.
(9) Inspection
The appearance of the coating is bright, delicate, and free from sagging and wrinkling. The thickness inspection can be carried out using a coating thickness gauge, which is a relatively simple method. The coating thickness can also be obtained by converting the zinc adhesion amount. The bonding strength can be achieved by using a bending press to bend the sample at 90-180 °, and there should be no cracks or peeling of the coating. It can also be inspected by striking with a heavy hammer.