Hot dip galvanizing, also known as hot dip zinc coating, is a method of obtaining a metallic coating by dipping steel components in molten zinc.
In recent years, with the rapid development of high-voltage power transmission, transportation and communication industries, the demand for protection of steel components has increased, resulting in a steady increase in the demand for hot-dip galvanizing.
I. Protective performance of hot-dip galvanized layer
Typically, the thickness of a galvanized zinc layer ranges from 5 to 15μm, while the thickness of a hot-dip galvanized layer is generally greater than 65μm, and can even reach up to 100μm. Hot dip galvanizing provides good coverage, the coating is compact and free from organic impurities.
It is widely known that zinc's mechanism for resisting atmospheric corrosion involves mechanical and electrochemical protection.
Under conditions of atmospheric corrosion, the surface of the zinc layer presents a protective film composed of ZnO, Zn(OH)2 and basic zinc carbonate.
This, to some extent, slows down the corrosion of zinc. This protective film (also known as white rust) will form a new layer if damaged. When the zinc layer is severely damaged, putting the iron base at risk, zinc provides electrochemical protection to the base.
With zinc's standard potential of -0.76 V and iron's standard potential of -0.44 V, when zinc and iron form a microcell, the zinc acts as the anode and dissolves, while the iron acts as the cathode , is protected.
Clearly, hot-dip galvanizing provides superior resistance to atmospheric corrosion of the underlying iron metal compared to zinc electroplating.
II. Hot dip galvanized layer forming process
The process of forming a hot-dip galvanized layer involves the formation of an iron-zinc alloy between the iron substrate and the outer layer of pure zinc.
The surface of the part forms a layer of iron-zinc alloy during hot-dip galvanizing, which allows an excellent bond between the iron and the pure zinc layer.
This process can be simply described as follows: when the iron part is immersed in molten zinc, a solid solution of zinc and iron α (body centered) forms at the interface.
This is a crystal formed by dissolving zinc atoms in the iron base metal in its solid state. The atoms of both metals are fused with relatively weak atomic forces.
Therefore, when zinc saturation is reached in the solid solution, zinc and iron atoms begin to diffuse. The zinc atoms that diffuse (or infiltrate) the iron substrate migrate within the base metal structure, gradually forming an alloy with the iron.
The iron that diffuses into the molten zinc forms an intermetallic compound with zinc, FeZn13, which sinks to the bottom of the hot-dip galvanizing vessel, becoming zinc slag.
When the part is removed from the zinc immersion liquid, a layer of pure zinc is formed on the surface, which is a hexagonal crystal. Its iron content does not exceed 0.003%.
III. Hot dip galvanizing process and related descriptions
1. Process Flow
Part → Degreasing → Washing → Pickling → Washing → Soaking pre-coating → Drying and preheating → Hot dip galvanizing → Finishing → Cooling → Passivation → Rinsing → Drying → Inspection
2. Explanation of Process Flow
(1) Degreasing
Chemical degreaser or water-based metal degreasing detergent can be used to remove oil until the workpiece is completely wetted by water.
(2) Pickling
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 excessive corrosion of the substrate and reduce the amount of hydrogen absorbed by the iron substrate.
Poor degreasing and pickling treatment can result in poor coating adhesion, inability to galvanize the zinc, or delamination of the zinc layer.
(3) Pre-coating soaking
Also known as binder, it can maintain a certain activity of the part before dip coating to improve the bond between the coating and the substrate. NH4Cl 15%~25%, ZnCl2 2.5%~3.5%, 55~65°C, 5~10min. Glycerin can be added appropriately to reduce the volatilization of NH4Cl.
(4) Drying and preheating
In order to prevent the workpiece from deforming due to a drastic increase in temperature during dip coating and to remove residual moisture to prevent zinc explosion and zinc liquid splashing, preheating is generally 120 ~ 180℃.
(5) Hot dip galvanizing
It is necessary to control the temperature of the zinc liquid, the immersion time and the speed at which the part is removed from the zinc liquid. If the temperature is too low, the zinc liquid has poor fluidity, the coating is thick and uneven, and there is a tendency to drip, resulting in poor appearance quality.
If the temperature is high, the zinc liquid has good fluidity, and it is easy for the zinc liquid to separate from the workpiece, reducing dripping and wrinkling, strong adhesion, fine coating, good appearance and high production efficiency.
However, if the temperature is too high, the loss of iron from the zinc workpiece and pot is severe, a large amount of zinc slag is generated, which affects the quality of the zinc layer, the consumption of zinc is large, and may even be impossible to plate.
At the same temperature, the longer the immersion time, the thicker the coating. At different temperatures, the higher the temperature, the longer the soaking time required for the same thickness.
In general, manufacturers use 450 ~ 470 ℃, 0.5 ~ 1.5 min to avoid high-temperature deformation of the workpiece and reduce zinc slag caused by iron loss.
Some factories use higher temperatures for large parts and cast iron parts, but should avoid the peak temperature range of high iron loss.
In order to improve the fluidity of the hot-dip galvanizing solution at a lower temperature, prevent the coating from being too thick, and improve the appearance of the coating, 0.01% ~ 0.02% pure aluminum is often added. Aluminum must be added in small quantities several times.
(6) Finish
After galvanizing, the part is finished mainly to remove excess zinc and zinc nodules on the surface, which can be done by vibration or manual methods.
(7) Passivation
The objective is to improve the surface resistance of the part to atmospheric corrosion, reduce or delay the appearance of white rust and maintain a good appearance of the coating.
Chromate passivation is used, such as Na2Cr2O7 80~100g/L, sulfuric acid 3~4ml/L.
(8) Cooling
Water cooling is generally used, but the temperature should not be too low to prevent workpieces, especially castings, from breaking due to shrinkage caused by quenching.
(9) Inspection
The coating should be shiny, thin, without stains or wrinkles. The coating thickness can be measured by a coating thickness gauge, which is relatively simple.
Coating thickness can also be obtained by calculating the amount of zinc adhesion.
The bond strength can be tested by bending it with a pressure bending machine, bending the sample 90~180°, there should be no cracking or falling off of the coating. Hammering can also be used for inspection.
