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Hot-dip galvanizing principle and process description

Apr. 27, 2022

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1 Hot-dip galvanizing is a method of obtaining a metal cover by immersing steel components in a molten zinc solution. In recent years, with the rapid development of high-voltage power transmission, transportation and communication, the demand for protection of steel parts has been increasing, and the demand for hot-dip galvanizing has been growing.

 

2 Hot-dip galvanizing layer protection function  

General electroplated zinc layer thickness of 5 to 15μm, while the hot dip galvanized layer is generally more than 35μm, and even up to 200μm. hot dip galvanized cover ability, fine coating, no organic matter adulteration. As we all know, the mechanism of zinc's resistance to atmospheric corrosion has mechanical and electrochemical maintenance, in the atmospheric corrosion conditions on the exterior of the zinc layer ZnO, Zn (OH) 2, and alkaline zinc carbonate maintenance film, to a certain extent, to slow down the corrosion of zinc, this layer of maintenance film (also known as white rust) is destroyed and will constitute a new film layer. When the zinc layer is severely damaged, endangering the iron substrate, zinc on the substrate onset of electrochemical maintenance, the standard potential of zinc -0.76V, the standard potential of iron -0.44V, zinc, and iron constitute a microcell when the zinc as the anode is dissolved, iron as the cathode is maintained. It is clear that hot-dip galvanizing of the base metal iron resistance to atmospheric corrosion is better than electroplating zinc.

 

3 Hot-dip galvanizing layer composition process   

The process of hot-dip galvanizing is the process of forming an iron-zinc alloy between the iron substrate and the pure zinc layer outside the Z. The iron-zinc alloy layer is formed on the exterior of the workpiece during hot-dip galvanizing to make an excellent connection between the iron and the pure zinc layer, and the process can be briefly described as follows: When the iron workpiece is immersed in the molten zinc solution, the zinc and alpha iron (body core) solid melt is first formed at the interface. This is the base metal iron in a solid-state dissolved with zinc atoms to form a crystal, the two metal atoms are intermingled, and the gravitational force between the atoms is relatively small. Thus, when zinc reaches saturation in the solid melt, the atoms of the two elements of zinc and iron disperse from each other, and the zinc atoms dispersed into (or infiltrated into) the iron matrix relocate in the matrix lattice and gradually form an alloy with iron, while the iron dispersed into the molten zinc liquid forms an intermetallic compound with zinc, FeZn13, which sinks to the bottom of the hot-dip galvanizing pot, i.e. zinc slag. When the workpiece is removed from the zinc dip, it forms a pure zinc layer, which is a hexagonal crystal. Its iron content is no more than 0.003%.

 

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4 Hot-dip galvanizing technology process and related clarification  

 

4.1 Technical process  

 

Workpiece → degreasing → washing → pickling → washing → dipping auxiliary plating solvent → drying preheating → hot dip galvanizing → packing → cooling → passivation → rinsing → drying → inspection

 

4.2 Technical process clarification  

 

(1)Degreasing

Chemical degreasing or water-based metal degreasing cleaning agent can be used to remove oil, to reach the workpiece completely by water wetting stopped.   

 

(2)Acid cleaning  

Can choose H2SO4 15%, thiourea 0.1%, 40 ~ 60 ℃ or with HCl 20%, urotropine 3 ~ 5g / L, 20 ~ 40 ℃ for pickling. Participating in the corrosion inhibitor can avoid the substrate over corrosion and reduce the amount of iron substrate hydrogen absorption, together with the participation of fog suppressant to press the acid mist escape.  Poor degreasing and pickling treatment will constitute poor adhesion of the plating layer, and the zinc layer will not be plated or fall off.   

 

(3) Dipping aid plating agent  

NH4Cl 100-150g/L, ZnCl2 150-180g/L, 70~80℃, 1~2min. and participate in a certain amount of explosion-proof agents.   

 

(4)Drying preheating

In order to avoid deformation of the workpiece due to the sharp increase in temperature during dipping and plating, and to remove residual moisture and avoid the onset of zinc explosion, which constitutes an explosion of zinc splash, preheating is generally 80 to 140 ℃.  

 

(5) Hot-dip galvanizing  

It is necessary to control the temperature of the zinc solution, the time of dipping and the speed of the workpiece being drawn out of the zinc solution. If the temperature is too low, the zinc liquid will not flow well, the coating will be thick and uneven, easy to hang, and the appearance will be poor; if the temperature is high, the zinc liquid will flow well, and the zinc liquid will be easy to leave the workpiece, which will reduce hanging and wrinkling, and the adhesion will be strong, the coating will be thin, the appearance will be good, and the production efficiency will be high; however, if the temperature is too high, the iron loss of the workpiece and the zinc pot will be severe, and a lot of zinc slag will occur, which will affect the quality of the zinc dip layer. It is easy to constitute chromatic aberration and make the appearance ugly, and the zinc consumption is high. The thickness of the zinc layer depends on the temperature of the zinc solution, the time of zinc dipping, the raw material of the steel and the composition of the zinc solution. In order to avoid high-temperature deformation of the workpiece and to reduce the zinc slag due to iron loss, manufacturers usually use 450-470°C and 0.5-1.5 min. Some factories use higher temperatures for large workpieces and cast iron parts, but avoid the temperature scale of the peak of iron loss. However, we advocate adding an alloy with iron removal and lower eutectic temperature to the zinc solution and lowering the galvanizing temperature to 435-445℃.

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(6) Cleaning up

The main purpose of post-plating cleaning is to remove the residual zinc and zinc tumor, which is done by using a special boiler for hot-dip galvanizing.

 

(7) Passivation

The intention is to improve the appearance of the workpiece to resist atmospheric corrosion, reduce or extend the time of white rust, and adhere to the outstanding appearance of the plating. All with chromate passivation, such as Na2Cr2O7 80 ~ 100g / L, sulfuric acid 3 ~ 4ml / L, but this passivation solution has a severe impact on the environment, the Z best choice of chromium-free passivation.

 

(8) Cooling  

General water cooling, but the temperature is not too low and not too high, generally not less than 30 ℃ not higher than 70 ℃

 

(9) Inspection

The appearance of the coating is bright, and detailed, with no hanging, wrinkled skin scene. A thickness check can use the coating thickness gauge, the method is relatively simple. The thickness of the coating can also be obtained by converting the amount of zinc adhesion. Contact strength can be used to bending press, in the sample for 90 ~ 180 ° bending, there should be no cracks and plating off. It can also be checked by hammering and doing the salt spray test and copper sulfate leaching test in batches.

 

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Founded in 1998, Hebei Annuo Automation Technology Co., Ltd. has more than 20 years experience in Galvanizing field. We have built more than 30 complete Hot dip galvanizing plants in China and abroad.If you don't understand anything about galvanizing, you can contact our team of experts for help.


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