Hot dip galvanizing process and relevant instructions

Technological process workpiece → degreasing → washing → pickling → washing → dipping auxiliary plating solvent → drying and preheating → hot galvanizing and hot galvanizing → finishing → cooling → passivation → bleaching and hot galvanizing and washing → drying → inspection of relevant technological process instructions (1) degreasing can be carried out by chemical degreasing or water-based metal degreasing detergent until the workpiece is completely soaked by water. (2) H2SO4 15%, thiourea 0.1%, 40-60 ℃ or HCl 20%, hexamethylenetetramine 1-3g / L, 20-40 ℃ can be used for pickling. Adding corrosion inhibitor can prevent the matrix from over corrosion and reduce the hydrogen absorption of iron matrix. Poor degreasing and pickling will result in poor adhesion of the coating, failure of zinc plating or falling off of zinc layer. (3) The dip aid, also known as binder, can keep the workpiece active before the dip to enhance the combination of 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, glycerin can be added properly. (4) In order to prevent the workpiece from deforming due to the sharp rise of temperature during the dip process, and to remove the residual moisture and prevent the occurrence of zinc explosion, resulting in the zinc liquid splashing, the preheating is generally 120-180 ℃. (5) Hot dip galvanizing should control the temperature of zinc bath, the time of dip galvanizing and the moving speed of workpiece from zinc bath.

Low temperature, poor fluidity of zinc liquid, thick and uneven coating, easy to produce sagging, poor appearance quality; high temperature, good fluidity of zinc liquid, easy to separate the zinc liquid from the workpiece, reduce sagging and wrinkling phenomenon, strong adhesion, thin coating, good appearance, high production efficiency; but high temperature, serious iron damage of workpiece and zinc pot, resulting in a large number of zinc slag, affecting the quality of zinc dipping layer, high zinc consumption , or even cannot be plated. At the same temperature, the plating time is long and the coating thickness is thick. When the same thickness is required at different temperatures, the high temperature dip coating takes 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 ℃, 0.5-1.5min. Some factories use higher temperature for large workpieces and iron castings, but should avoid the temperature range of iron loss peak. In order to improve the fluidity of hot-dip solution at low temperature, prevent the coating from being too thick and improve the coating appearance, 0.01% - 0.02% pure aluminum is often added. Aluminum should be added in small amount and many times. (6) After finishing and plating, the finishing of workpiece is mainly to remove the residual zinc and zinc nodules on the surface, either by vibration or by hand. (7) The purpose of passivation is to improve the atmospheric corrosion resistance of the workpiece surface, reduce or extend the occurrence time of white rust, and maintain the good appearance of the coating. All of them are passivated by chromate, such as na2cr2o7 80-100g / L, sulfuric acid 3-4ml / L. (8) Generally, water-cooling is used for cooling, but the temperature should not be too low, so as to prevent the matrix structure cracking of the workpiece, especially the casting due to chilling shrinkage. (9) The appearance of the coating shall be bright, meticulous, free of sagging and wrinkling. The coating thickness gauge can be used for thickness inspection, and the method is relatively simple. The coating thickness can also be obtained by conversion of zinc adhesion. Bending press can be used to bend the sample 90-180 ° with no crack and coating falling off. It can also be tested by striking with a heavy hammer.