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process
Workpiece → degreasing → water washing → pickling → water washing → immersion in assist plating solvent → drying and preheating → hot-dip galvanizing → finishing → cooling → passivation → rinsing → drying → inspection
(1) Degreasing
Chemical degreasing or water-based metal degreasing cleaning agent can be used to degreasing until the workpiece is completely wetted by water.
(2) Pickling
It can be pickled with H2SO4 15%, thiourea 0.1%, 40~60℃ or HCl 20%, hexamethylenetetramine 1~3g/L, 20~40℃. The addition of corrosion inhibitor can prevent the matrix from over-corrosion and reduce the hydrogen absorption of the iron matrix. Poor degreasing and pickling treatments will cause poor adhesion of the coating, no zinc coating or peeling of the zinc layer.
(3) Immersion flux
Also known as bonding agent, it can keep the work piece active before immersion plating to enhance the bonding between the plating layer and the substrate. NH4Cl 15%~25%, ZnCl2 2.5%~3.5%, 55~65℃, 5~10min. In order to reduce NH4Cl volatilization, glycerin can be added appropriately.
(4) Drying and preheating
In order to prevent the workpiece from deforming due to the sharp rise in temperature during the immersion plating, and to remove residual moisture, to prevent zinc explosion, resulting in zinc liquid explosion, the preheating is generally 120-180°C.
(5) Hot-dip galvanizing
It is necessary to control the temperature of the zinc solution, the time of dipping and the speed at which the workpiece is removed from the zinc solution. The temperature is too low, the fluidity of the zinc liquid is poor, the coating is thick and uneven, it is easy to produce sagging, and the appearance quality is poor; the temperature is high, the fluidity of the zinc liquid is good, the zinc liquid is easy to separate from the workpiece, and the phenomenon of sagging and wrinkles is reduced. Strong, thin coating, good appearance, high production efficiency; however, if the temperature is too high, the workpiece and the zinc pot will be severely damaged, and a large amount of zinc dross will be produced, which will affect the quality of the zinc dipping layer and consume large amounts of zinc. At the same temperature, the immersion plating time is long and the plating layer is thick. When the same thickness is required at different temperatures, it takes a long time for high-temperature immersion plating. In order to prevent the high temperature deformation of the workpiece and reduce the zinc dross caused by iron loss, the general manufacturer adopts 450~470℃, 0.5~1.5min. Some factories use higher temperatures for large workpieces and iron castings, but avoid the temperature range of peak iron loss. In order to improve the fluidity of the hot dip plating solution at lower temperatures, 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 multiple times.
(6) finishing
Finishing the workpiece after plating is mainly to remove the surface zinc and zinc nodules, either by shaking or manual methods.
(7) Passivation
The purpose is to improve the resistance to atmospheric corrosion on the surface of the workpiece, reduce or prolong the appearance of white rust, and maintain a good appearance of the coating. They are all passivated with chromate, such as Na2Cr2O7 80~100g/L, sulfuric acid 3~4ml/L.
(8) Cooling
It is generally water-cooled, but the temperature should not be too low to prevent the workpiece, especially the casting, from cracking in the matrix due to chilling and shrinking.
(9) Inspection
The appearance of the coating is bright, detailed, without sagging or wrinkles. Thickness inspection can use coating thickness gauge, the method is relatively simple. The thickness of the coating can also be obtained by converting the amount of zinc adhesion. The bonding strength can be bent by a bending press, and the sample should be bent at 90-180°, and there should be no cracks or peeling of the coating. It can also be tested by hitting with a heavy hammer.
2. Hot-dip galvanized layer formation process The hot-dip galvanized layer formation process is the process of forming an iron-zinc alloy between the iron matrix and the outermost pure zinc layer. The iron-zinc alloy layer is formed on the surface of the workpiece during hot-dip galvanizing. The iron and pure zinc layer are well combined, and the process can be simply described as: when the iron workpiece is immersed in molten zinc, a solid solution of zinc and alpha iron (body core) is first formed on the interface. This is a crystal formed by dissolving zinc atoms in the base metal iron in a solid state. The two metal atoms are fused, and the attraction between the atoms is relatively small. Therefore, when zinc reaches saturation in the solid solution, the two element atoms of zinc and iron diffuse each other, and the zinc atoms that have diffused (or infiltrated) into the iron matrix migrate in the matrix lattice, and gradually form an alloy with iron, and diffuse The iron and zinc in the molten zinc form an intermetallic compound FeZn13, which sinks into the bottom of the hot-dip galvanizing pot, which is called zinc dross. When the workpiece is removed from the zinc immersion solution, a pure zinc layer is formed on the surface, which is a hexagonal crystal. Its iron content is not more than 0.003%.
Third, the protective performance of the hot-dip galvanized layer The thickness of the electro-galvanized layer is usually 5-15μm, and the hot-dip galvanized layer is generally above 65μm, even as high as 100μm. Hot-dip galvanizing has good coverage, dense coating and no organic inclusions. As we all know, the anti-atmospheric corrosion mechanism of zinc includes mechanical protection and electrochemical protection. Under atmospheric corrosion conditions, there are protective films of ZnO, Zn(OH)2 and basic zinc carbonate on the surface of the zinc layer, which can slow down the corrosion of zinc to a certain extent. The protective film (also known as white rust) is damaged and a new film is formed. When the zinc layer is seriously damaged and the iron matrix is ​​endangered, zinc will produce electrochemical protection for the matrix. The standard potential of zinc is -0.76V, and the standard potential of iron is -0.44V. When zinc and iron form a microbattery, zinc is dissolved as an anode. It is protected as a cathode. Obviously, hot-dip galvanizing has better atmospheric corrosion resistance to base metal iron than electro-galvanizing.
Fourth, the formation control of zinc ash and zinc slag during hot-dip galvanizing
Zinc ash and zinc dross not only seriously affects the quality of the zinc immersion layer, but also causes the coating to be rough and produce zinc nodules. Moreover, the cost of hot-dip galvanizing is greatly increased. Usually, zinc consumption is 80-120kg per 1 ton workpiece. If the zinc ash and dross are serious, the zinc consumption will be as high as 140-200kg. Controlling the zinc carbon is mainly to control the temperature and reduce the scum produced by the oxidation of the zinc liquid surface. Some domestic manufacturers use refractory sand, charcoal ash, etc. Foreign countries use ceramic or glass balls with low thermal conductivity, high melting point, low specific gravity, and no reaction with zinc liquid, which can reduce heat loss and prevent oxidation. This kind of ball is easy to be pushed away by the workpiece, and it is not sticky to the workpiece. Side effect. For the formation of zinc dross in zinc liquid, it is mainly a zinc-iron alloy with extremely poor fluidity formed when the iron content dissolved in the zinc liquid exceeds the solubility at this temperature. The zinc content in the zinc dross can be as high as 95%, which is hot-dip galvanizing. The key to the high cost of zinc. It can be seen from the solubility curve of iron in zinc liquid that the amount of dissolved iron, that is, the amount of iron loss, is different at different temperatures and different holding times. At around 500°C, the iron loss increases sharply with heating and holding time, almost in a linear relationship. Below or above the range of 480~510℃, the iron loss increases slowly with time. Therefore, people call 480~510℃ the malignant dissolution zone. In this temperature range, the zinc liquid will corrode the workpiece and the zinc pot the most serious. The iron loss will increase significantly when the temperature is above 560℃, and the zinc will destructively etch the iron matrix when the temperature is above 660℃. . Therefore, plating is currently carried out in the two regions of 450-480°C and 520-560°C.
5. Control of the amount of zinc dross
To reduce the zinc dross, it is necessary to reduce the iron content in the zinc solution, which is to start with reducing the factors of iron dissolution:
⑴Plating and heat preservation should avoid the peak area of ​​iron dissolution, that is, do not operate at 480~510℃.
⑵ As far as possible, the zinc pot material should be welded with steel plates with carbon and low silicon content. The high carbon content will accelerate the corrosion of the iron pan by the zinc liquid, and the high silicon content can also promote the corrosion of the iron by the zinc liquid. At present, 08F high-quality carbon steel plates are mostly used. Its carbon content is 0.087% (0.05%~0.11%), silicon content is ≤0.03%, and it contains elements such as nickel and chromium that can inhibit iron from being corroded. Do not use ordinary carbon steel, otherwise the zinc consumption will be large and the life of the zinc pot will be short. It was also proposed to use silicon carbide to make a zinc melting tank, although it can solve the iron loss, but the modeling process is also a problem.
⑶Removing slag frequently. The temperature is first raised to the upper limit of the process temperature to separate the zinc slag from the zinc liquid, and then lowered to below the process temperature, so that the zinc slag sinks to the bottom of the tank and then is picked up with a spoon. The plated parts that fall into the zinc liquid should also be salvaged in time.
⑷It is necessary to prevent the iron in the plating agent from being brought into the zinc tank with the workpiece. The reddish-brown iron-containing compound will be formed when the plating agent is used for a certain period of time, and it must be filtered out regularly. It is better to maintain the pH value of the plating agent around 5.
⑸ Less than 0.01% aluminum in the plating solution will accelerate the formation of dross. A proper amount of aluminum will not only improve the fluidity of the zinc solution and increase the brightness of the coating, but also help reduce zinc dross and zinc dust. A small amount of aluminum floating on the liquid surface is beneficial to reduce oxidation, and too much affects the quality of the coating, causing spot defects.
⑹ Heating and heating should be uniform to prevent explosion and local overheating.

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Post time: Sep-30-2021