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

Protection performance of hot dip galvanized layer

The thickness of the electroplated zinc layer is usually 5 ~ 15um, while the hot-dip galvanized layer is generally above 35um, and even up to 200um. The hot-dip galvanizing has good covering ability, the coating is dense, and no organic matter is included. As we all know, the mechanism of zinc’s resistance to atmospheric corrosion is mechanical protection and electrochemical protection. Under atmospheric corrosion conditions, the zinc layer has Zn0, Zn (oH) 2 and basic zinc carbonate protective film to slow down the corrosion of zinc to a certain extent. The protective film (also called white rust) will be damaged and a new film will be formed.

When the zinc layer is seriously damaged and endangers the iron substrate, 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 zinc and iron form a microbattery, zinc is dissolved as an anode, iron It is protected as a cathode. Obviously, the hot-dip galvanizing has better atmospheric corrosion resistance to the base metal iron than electro-galvanizing.

Hot galvanized layer formation process

The process of forming a hot-dip galvanized layer is the process of forming an iron-zinc alloy between the iron substrate and the outermost pure zinc layer. The iron-zinc alloy layer is formed on the surface of the workpiece during hot dip plating, which makes the iron and pure zinc layer very Good combination, the process can be simply described as: when the iron workpiece is immersed in the molten zinc solution, the solid solution of zinc and a iron (body center) is first formed on the interface.

This is a kind of crystal formed by dissolving zinc atoms in the solid state of the metal matrix iron. The two metal atoms are fused and the attraction between the atoms is relatively small. Therefore, when zinc reaches saturation in the solid melt, the two element atoms of zinc and iron diffuse with each other, and the zinc atoms diffused (or infiltrated) in the iron matrix migrate in the matrix lattice, gradually forming an alloy with iron, and diffusing The iron in the molten zinc solution forms an intermetallic compound FeZn13 with zinc and sinks into the bottom of the hot-dip galvanizing pot, which is zinc slag. When the workpiece is removed from the zinc dipping solution, a pure zinc layer is formed on the surface, which is hexagonal crystal. Its iron content is not more than 0.003%.

Hot galvanizing process

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1. Process

Workpiece → degreasing → water washing → acid washing → water washing → dipping auxiliary plating solvent → drying preheating → hot galvanizing → finishing → cooling → passivation → rinse → drying → inspection

2. Process description

(1) Degreasing: chemical degreasing or water-based metal degreasing cleaning agent can be used to degrease until the workpiece is completely wetted by water.

(2) Pickling: H2S0415%, thiourea 0.1%, 40 ~ 60 ℃ or HC125%, urotropine 3 ~ 5g / L, 20 ~ 40 ℃ can be used for pickling. The addition of corrosion inhibitor can prevent the matrix from over-corrosion and reduce the amount of hydrogen absorbed by the iron matrix. At the same time, the fog inhibitor is added to suppress the escape of acid mist. Poor degreasing and pickling treatment will result in poor adhesion of the coating, zinc plating or zinc layer fall off.

(3) Dip plating aid: also known as solvent, it can keep the workpiece with certain activity before immersion plating to avoid secondary oxidation, so as to enhance the combination of the plating layer and the substrate. NH4C1100-150g / L, ZnC12150-180g / L, 70 ~ 85 ℃, 1 ~ 2min. And add a certain amount of surfactant.

(4) Drying and preheating: In order to prevent the workpiece from being deformed due to the rapid temperature rise during immersion plating, and to remove residual moisture, to prevent the occurrence of zinc explosion and the zinc liquid to splash, preheating is generally 80 ~ 140 ℃. But now generally anti-explosion agent is added.

(5) Hot-dip galvanizing: It is necessary to control the temperature of the zinc solution, the time of immersion plating and the speed of the work pieces drawn from the zinc solution. The lead-out speed is generally 1.5 m / min. If the temperature is too low, the fluidity of the zinc solution is poor, the coating is thick and uneven, it is easy to produce sagging, and the appearance quality is poor. Wrinkle phenomenon occurs, strong adhesion, thin coating, good appearance, and high production efficiency; but the temperature is too high, the iron loss of the workpiece and the zinc pot is serious, a large amount of zinc slag is produced, affecting the quality of the zinc dipping layer and easily causing color difference to make the surface color Unsightly, high zinc consumption.

The thickness of the zinc layer depends on the temperature of the zinc solution, the immersion time, the steel material and the composition of the zinc solution. In addition, galvanized alloy is also very important. In order to prevent high temperature deformation of the workpiece 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 parts, but to avoid the temperature range of the peak iron loss. However, we recommend adding an alloy with iron removal function and lower eutectic temperature to the zinc solution and lower the galvanizing temperature to 435-445 ℃.

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(6) Finishing: The finishing of the workpiece after plating is mainly to remove the residual zinc and zinc tumors on the surface, which is completed by using a special galvanized vibrator.

(7) Passivation: The purpose is to improve the surface corrosion resistance of the workpiece, reduce or prolong the appearance of white rust, and maintain a good appearance of the coating. All are passivated with chromate, such as Na2Cr20780 ~ 100g / L, sulfuric acid 3 ~ 4ml / L, but this passivation liquid seriously affects the environment, it is best to use chromium-free passivation.

(8) Cooling: generally water cooling, but the temperature should not be too low or too high, generally not lower than 30 ℃ not higher than 70 ℃.

(9) Inspection: The appearance of the coating is bright, detailed, without sagging and wrinkling. Thickness inspection can use coating thickness gauge, the method is relatively simple. The thickness of the coating can also be converted from the amount of zinc adhesion. For the bonding strength, a bending press can be used to bend the sample 90 ~ 180 °, and there should be no cracks and coating falling off. It can also be tested by hammering, and the salt spray test and copper sulfate corrosion test are done in batches.

The formation and control of zinc ash and zinc slag

Zinc ash and zinc slag not only seriously affect the quality of the zinc immersion layer, causing the coating to be rough and producing zinc nodules. Moreover, the cost of hot dip galvanizing is greatly increased. Generally, the zinc consumption per 1t workpiece is 40 ~ 100kg. If the zinc dust is serious, the zinc consumption will be as high as 140 ~ 200kg. The control of zinc slag is mainly to control the temperature and reduce the scum produced by the oxidation of the surface of the zinc liquid, so it is more necessary to use alloys with iron removal and anti-oxidation functions and use small thermal conductivity, high melting point, small specific gravity, and not with zinc liquid .The reaction can not only reduce the heat loss but also prevent the oxidation of ceramic beads or glass balls. This kind of ball is easy to be pushed away by the workpiece and has no adhesion to the workpiece.

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For the formation of zinc slag in the zinc solution, the main reason is that the iron content dissolved in the zinc solution exceeds the solubility at this temperature. The zinc-iron alloy with extremely poor fluidity is formed. The zinc content in the zinc slag can be as high as 94%, which is hot plating The key to the high cost of zinc.

It can be seen from the solubility curve of iron in zinc solution: at different temperatures and different holding times, the amount of iron dissolved or iron loss is different. At around 500 ℃, 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 ℃ malignant dissolution zone.

In this temperature range, the zinc solution has the most serious corrosion to the workpiece and the zinc pot, and the iron loss exceeds 560 ℃, and the iron loss increases significantly. Above 660 ℃, zinc is a destructive attack on the iron matrix, and the zinc slag will increase sharply, and the plating cannot be performed . Therefore, the current plating is mostly carried out in the range of 430 ~ 450 ℃.

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