1, the introduction

Hot dip galvanizing, also known as hot dip galvanizing, is a method by which steel members are immersed in molten zinc to obtain a metal coating. In recent years, with the rapid development of high voltage power transmission, transportation and communication, the protection requirements for steel parts are getting higher and higher, and the demand for hot dip galvanizing is also increasing.

2, hot dip galvanizing layer protection performance

Usually the thickness of the electrogalvanizing layer is 5 ~ 15μm, and the hot galvanizing layer is generally more than 35μm, or even as high as 200μm. Hot dip galvanizing has good covering ability, compact coating and no organic inclusions. It is well known that the mechanism of resisting atmospheric corrosion of zinc includes mechanical protection and electrochemical protection. Under atmospheric corrosion conditions, zinc layer has ZnO, Zn(OH)2 and basic zinc carbonate protective film on its surface, which slows down the corrosion of zinc to a certain extent. This protective film (also known as white rust) will form a new film after it is damaged. When the zinc layer is seriously damaged and the iron substrate is endangered, zinc produces electrochemical protection to the substrate, the standard potential of zinc is -0.76V, and the standard potential of iron is -0.44V. When the zinc and iron form a microcell, zinc is dissolved as the anode and iron is protected as the cathode. It is obvious that hot dip galvanizing has better atmospheric corrosion resistance than electric dip galvanizing.

3, hot dip galvanizing layer formation process

Galvanized layer formation process is the most out of pure iron substrate and formed between the zinc layer of iron, zinc alloy, the process of workpiece surface formed in hot dip plating of iron - zinc alloy layer, and makes good combination between iron and pure zinc layer, the process can be simply described as follows: when the blacksmith a immersed in molten zinc liquid, zinc first formed in the interface and alpha (heart) solid solution. This is the matrix metal iron in the solid state dissolved with zinc atoms to form a crystal, the two metal atoms are fused, the attraction between the atoms is relatively small. So when zinc in solid solution after reaching saturation, two kinds of zinc iron atom diffusion, spread to (or called infiltration) iron matrix of zinc atom in the lattice matrix migration, gradually formed and iron alloy, and spread to molten zinc liquid iron and zinc in the formation of intermetallic compounds FeZn13, sank into the hot dip galvanized pot is zinc slag. When the workpiece is removed from the zinc leaching solution, the surface forms a pure zinc layer, which is a hexagonal crystal. It contains no more than 0.003% iron.

4, hot dip galvanizing process and related instructions

4.1 Process

Workpiece → degreasing → washing → pickling → washing → dip-assisted plating solvent → drying preheating → hot-dip galvanizing → finishing → cooling → passivation → rinsing → drying → inspection

4.2 Technical process description

(1) skim

Chemical degreasing or water-based metal degreasing cleaning agent can be used to degrease the workpiece until it is completely saturated by water.

(2) the pickling

H2SO4 15%, thiourea 0.1%, 40 ~ 60℃ or HCl 20%, ullotropine 3 ~ 5g/L, 20 ~ 40℃ for pickling. The addition of corrosion inhibitor can prevent the substrate from over-corrosion and reduce the amount of hydrogen absorption in iron substrate, and the addition of antifogging agent can restrain the acid mist escaping. Defatting and pickling treatment will cause the coating adhesion is not good, not zinc plating or zinc layer off.

(3) immersion aid plating agent, also known as solvent, can keep the workpiece has a certain activity before immersion plating to avoid secondary oxidation, in order to enhance the coating and matrix. NH4Cl 100-150g/L, ZnCl2 150-180g/L, 70 ~ 80℃, 1 ~ 2min. And add a certain amount of explosion-proof agent.

(4) drying preheating

In order to prevent the workpiece in the dip plating due to the rapid rise of temperature and deformation, and remove residual moisture, prevent the generation of zinc explosion, resulting in zinc liquid splash, preheating is generally 80 ~ 140℃.

(5) hot dip galvanized

The temperature of zinc solution, dip time and the speed of workpiece extraction from zinc solution should be controlled well. The extraction speed is generally 1.5 m /min. The temperature is too low, the fluidity of zinc liquid is poor, the coating is thick and uneven, easy to produce hanging, poor appearance quality; High temperature, good fluidity of zinc liquid, zinc liquid is easy to separate from the workpiece, reduce the flow and wrinkle phenomenon, strong adhesion, thin coating, good appearance, high production efficiency; But the temperature is too high, the workpiece and the zinc pot iron damage is serious, produce a lot of zinc slag, affect the quality of zinc coating and easy to cause color difference to make the surface color ugly, zinc consumption is high.

The thickness of zinc layer depends on the temperature of zinc solution, zinc dipping time, steel material and zinc solution composition.

General manufacturers in order to prevent high temperature deformation of the workpiece and reduce zinc slag caused by iron loss, 450 ~ 470℃, 0.5 ~ 1.5min. Some factories use higher temperatures for large parts and cast iron, but avoid the temperature range of peak iron loss. However, it is recommended to add an alloy with the function of removing iron and reducing the eutectic temperature in the zinc solution and to lower the galvanizing temperature to 435-445℃.

(6)

After plating on the workpiece finishing is mainly to remove the surface of zinc and zinc tumors, with the use of hot dip galvanizing special vibrator to complete.

(7) passivation

The purpose is to improve the surface of the workpiece anti-atmospheric corrosion performance, reduce or prolong the appearance of white rust, keep the coating has a good appearance. Chromate passivation is used, such as Na2Cr2O7 80 ~ 100g/L, sulfuric acid 3 ~ 4ml/L, but this passivation solution seriously affects the environment, the best use of chromium-free passivation.

To cool (8)

Generally use water cooling, but the temperature should not be too low or too high, generally not less than 30 degrees Celsius not higher than 70 degrees Celsius,

(9) test

The coating appearance is bright, meticulous, no flow hanging, wrinkle phenomenon. The coating thickness tester can be used to test the thickness and the method is relatively simple. The coating thickness can also be converted by the amount of zinc attachment. Bending press can be used for binding strength. The sample can be bent at 90 ~ 180° without cracking and coating peeling off. Also can be used to hammer the test, and batches of salt spray test and copper sulfate etching test.

Formation and control of zinc ash and zinc slag

5.1 Formation of zinc ash and zinc slag

Zinc ash and zinc slag not only seriously affect the quality of zinc dipping layer, resulting in rough coating and zinc tumor. And the cost of hot dip galvanizing greatly increased. Usually each 1T workpiece zinc consumption 40 ~ 80kg, if the zinc ash zinc slag is serious, the zinc consumption will be as high as 100 ~ 140kg. Zinc slag is mainly control the temperature, reduce zinc liquid surface oxidation and the scum, so more to adopt iron removal function and antioxidation function of the alloy and with low thermal conductivity, high melting point, density is small, do not react with zinc liquid, can reduce the heat lost and can prevent the oxidation of ceramic or glass ball cover, the ball is easy to be pushed away, artifacts, and the workpiece without adhesion effect.

The formation of zinc slag in zinc liquid is mainly dissolved in zinc liquid iron content exceeds the solubility of the temperature formed by the fluidity of zinc ferroalloy, zinc content in zinc slag can be as high as 94%, which is the key to the high cost of hot dip galvanizing.

It can be seen from the solubility curve of iron in zinc solution that the amount of dissolved iron, namely the amount of lost iron, is different at different temperature and holding time. When the temperature is near 500℃, the iron loss increases sharply with the heating and holding time, almost in a linear relationship. When the temperature is lower than 480 ℃ or higher than 510℃, the iron loss increases slowly with time. Therefore, 480 ~ 510℃ is called the malignant dissolution zone. In this temperature range, the corrosion of zinc solution on the workpiece and the zinc pot is the most serious, and the iron loss increases obviously when the temperature is above 560℃. When the temperature is above 660℃, the zinc corrosion on the iron matrix is destructive, and the zinc slag will increase sharply, so the plating can not be done. Therefore, the current plating is mostly carried out in the two areas of 430 ~ 450℃ and 540 ~ 600℃.

5.2 Control of zinc slag

In order to reduce zinc slag, it is necessary to reduce the iron content in liquid zinc, which is to start from reducing the factors of iron dissolution:

(1) plating and heat preservation should avoid the peak area of iron dissolution, that is, do not work at 480 ~ 510℃.

(2) the zinc pot material should be welded with steel plate with low carbon and silicon content as far as possible. With high carbon content, zinc solution will accelerate the corrosion of iron pot, and high silicon content can also promote the corrosion of zinc solution to iron. At present, the use of 08F/XG08/WKS high quality steel plate. And contain elements such as nickel and chromium which can inhibit the corrosion of iron. Can not use ordinary carbon steel, otherwise the consumption of zinc, zinc pot life is short. It is also proposed to use silicon carbide to make molten zinc trough, although it can solve the amount of iron loss, but the modeling process is a difficult problem, the zinc pot produced by industrial ceramics can only be made into a cylindrical type and small volume, although it can meet the requirements of small galvanized but can not guarantee the galvanized large workpiece.

(3) often fishing slag. First, the temperature is raised to the upper limit of the process temperature so that the zinc slag and zinc liquid can be separated, and then the zinc slag is sunk in the bottom of the tank and retrieved with zinc scoop or special slag dredger. Falling into the zinc plating parts should be timely salvage.

(4) to prevent the iron in the plating agent with the workpiece into the zinc tank, the plating agent to carry out online regeneration cycle treatment, strict control of ferrous content, not allowed to be higher than 4G/L, pH value is always maintained at 4.5-5.5.

(5) heating and heating should be uniform to prevent local overheating.