Hot-dip galvanizing, also called hot-dip galvanizing, is a method of obtaining a metallic coating by immersing steel components in molten zinc. With the rapid development of high-voltage power transmission, transportation, and communications in recent years, the requirements for protection of iron and steel parts have become higher and higher, and the demand for hot-dip galvanizing has also increased.

Protective properties of hot-dip galvanized layer

Usually the thickness of the galvanized layer is 5-15um, while the hot-dip galvanized layer is generally above 65um, even as high as 100um. Hot-dip galvanizing has good covering ability, dense plating and no organic inclusions. As we all know, the mechanism of zinc’s resistance to atmospheric corrosion includes mechanical protection and electrochemical protection. Under the condition of atmospheric corrosion, there are protective films of Zn0, Zn (OH) 2 and basic zinc carbonate on the surface of zinc layer, which can slow down the corrosion of zinc to a certain extent. If the protective film (also called white rust) is damaged, a new layer will be formed.

When the zinc layer is severely damaged and the iron matrix is ​​endangered, zinc generates electrochemical protection for the matrix. The standard potential of zinc is -0.76V, the standard potential of iron is -0.44V. When zinc and iron form a microbattery, zinc is dissolved as an anode, and iron is dissolved. Protected as a cathode. Obviously, the hot-dip galvanizing ability of the base metal iron is better than that of electro-galvanizing.

Formation process of hot-dip galvanized layer

The process of hot-dip galvanizing layer formation is the process of forming an iron-zinc alloy between the iron substrate and the outermost pure zinc layer. The surface of the workpiece is formed with an iron-zinc alloy layer during hot dip plating, which makes the iron and pure zinc layer very Good combination, the process can be simply described as follows: when an iron workpiece is immersed in molten zinc liquid, zinc and a iron (body-centered) solid solution are first formed on the interface. This is a crystal formed by dissolving zinc atoms in the solid state of the base metal iron. 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 elements of zinc and iron diffuse into each other, and the zinc atoms diffused into (or infiltrated into) the iron matrix migrate in the matrix of the matrix, gradually form an alloy with iron, and diffuse The iron in the molten zinc solution forms an intermetallic compound FeZn13 with zinc, which sinks into the bottom of the hot-dip galvanizing pot, which is zinc slag. When the workpiece is removed from the zinc bath, a pure zinc layer is formed on the surface, which is a hexagonal crystal with an iron content of not more than 0.003%.

application

Crafting process

Workpiece degreasing → water washing → pickling → water washing → dip assisting plating solvent → drying and preheating → hot galvanizing → finishing → cooling → passivation → rinsing → drying → inspection

Process description

(1) Skim

Chemical degreasing or water-based metal degreasing cleaning agents can be used to remove the oil until the workpiece is completely wetted with water.

(2) Pickling

It can be pickled with H2S0415%, thiourea 0.1%, 40 ~ 60 ℃ or HC120%, hexamethylenetetramine 1 ~ 3g / L, 20 ~ 40 ℃. Adding a corrosion inhibitor can prevent over-corrosion of the substrate and reduce the hydrogen absorption of the iron substrate. Poor degreasing and pickling treatment will cause poor adhesion of the coating, and zinc will not be plated or the zinc layer will fall off.

(3) Dip plating agent

Also called bonding agent, it can keep the workpiece active before immersion plating to enhance the bonding between the coating and the substrate.

NH4C115% ~ 25%, ZnC122.5% ~ 3.5%, 55 ~ 65 ℃, 5 ~ 10min. In order to reduce the volatilization of NH4C1, glycerin can be added appropriately.

(4) Drying and preheating

In order to prevent the workpiece from being deformed due to the sharp rise in temperature during immersion plating, and to remove the residual moisture, to prevent the occurrence of zinc explosion, which will cause the zinc liquid to splash, the preheating is generally 120 ~ 180 ° C.

(5) Hot-dip galvanizing

It is necessary to control the temperature of the zinc solution, the immersion time and the speed of removing the workpiece from the zinc solution. Too low temperature, poor liquidity of zinc liquid, thick and uneven coating, easy to produce sagging, poor appearance quality; high temperature, good liquidity of zinc liquid, zinc liquid easily detached from the workpiece, reducing the occurrence of sagging and wrinkling, adhesion Strong, thin coating, good appearance, and high production efficiency; but the temperature is too high, the workpiece and zinc pot iron damage is serious, a large amount of zinc slag is generated, which affects the quality of the zinc immersion layer, the zinc consumption is large, and even can not be applied.

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, high temperature dip plating takes longer.

In order to prevent high-temperature deformation of workpieces and reduce zinc slag due to iron loss, general manufacturers use 450 ~ 470 ℃, 0.5 ~ 1.5min. Some factories use higher temperatures for large workpieces and cast iron, but avoid the temperature range of the peak iron loss. In order to improve the fluidity of the hot-dip plating solution at a lower temperature, 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 quantities.

(6) Finishing

The finishing of the workpiece after plating is mainly to remove the residual zinc and zinc knobs on the surface, either by vibration or manual methods.

(7) Passivation

The purpose is to improve the surface corrosion resistance of the workpiece surface, reduce or prolong the appearance of white rust, and maintain the good appearance of the coating. All are passivated with chromate, such as Na2Cr20780 ~ 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 structure due to chilling and shrinking.

(9) Inspection

The appearance of the coating is bright, detailed, and free from sagging and wrinkling. Coating thickness gauge can be used for thickness inspection. The method is relatively simple, and the thickness of the coating can also be obtained through conversion of zinc adhesion. The bonding strength can be bent with a bending press, and the sample is bent at 90 ~ 180 °. There should be no cracks and coatings falling off, and it can also be tested by hammering.

Control of zinc ash and zinc slag

application

(1) Formation of zinc ash and zinc slag

Zinc ash and zinc slag not only seriously affect the quality of the zinc coating, cause the coating to be rough, and produce zinc knobs. Moreover, the cost of hot-dip galvanizing is greatly increased. Normally, the workpiece consumes 80 ~ 120kg zinc per 1t of plating. If the zinc ash and zinc slag are serious, its zinc consumption will reach 140 ~ 200kg. Zinc-carbon control is mainly to control the temperature and reduce the dross produced by the surface oxidation of the zinc solution.

Some manufacturers in China are covered with refractory sand and charcoal. Foreign countries use ceramics or glass balls with small thermal conductivity, high melting point, small specific gravity, and no reaction with zinc liquid, which can reduce heat loss and prevent oxidation. Such balls are easily pushed away by the workpiece and are non-sticky to the workpiece Attached effect.

For the formation of zinc slag in the zinc solution, the zinc-iron alloy with extremely poor fluidity is formed when the iron content dissolved in the zinc solution exceeds the solubility at this temperature. The key to high zinc costs.

From the solubility curve of iron in zinc solution, it can be seen that the amount of dissolved iron, ie, the amount of iron loss, is different at different temperatures and different holding times. When the temperature is around 500 ° C, the iron loss increases sharply with the heating and holding time, and it is almost linear. Below or above the range of 480 ~ 510 ℃, iron loss increases slowly with time. Therefore, people call 480 ~ 510 ℃ the malignant dissolution zone. In this temperature range, the zinc solution has the most severe corrosion on the workpiece and the zinc pot. The iron loss has increased significantly above 560 ° C. Above 660 ° C, zinc is a destructive attack on the iron substrate. . Therefore, plating is currently performed in two regions of 450 to 480 ° C and 520 to 560 ° C.

(2) Control of the amount of zinc slag

To reduce zinc slag, we must reduce the iron content in the zinc solution, which is to start from the factors that reduce the dissolution of iron:

a. As far as possible, the material of zinc pot should be welded with carbon steel and low silicon content. The high carbon content will accelerate the corrosion of the iron pot by the zinc liquid, and the high silicon content can also promote the corrosion of the zinc liquid to the iron. At present, 08F high-quality carbon steel plates are mostly used. It contains 0.087% carbon (0.05% ~ 0.11%), silicon ≤ 0.03%, and contains elements such as nickel and chromium that can inhibit iron from being etched.

Do not use ordinary carbon steel, otherwise the zinc consumption is large and the life of the zinc pot is short. It has also been proposed to use silicon carbide to make a molten zinc bath. Although the iron loss can be solved, the modeling process is also a problem.

b. Always slag. First raise the temperature to the upper limit of the process temperature so that the zinc slag is separated from the zinc liquid, and then drop below the process temperature. After the zinc slag sinks to the bottom of the tank, it is fished with a spoon. Plating parts that have fallen into the zinc solution must be salvaged in time.

c. To prevent the iron in the flux from being brought into the zinc bath with the workpiece, the red-brown iron-containing compound will be generated after the flux is used for a certain period of time, and it should be filtered off regularly. It is better to maintain the pH of the flux at about 5.

d. Less than 0.01% of aluminum in the plating solution will accelerate the formation of slag. Appropriate amount of aluminum will not only improve the fluidity of the zinc solution, increase the brightness of the coating, but also help reduce zinc slag and zinc ash. A small amount of aluminum floating in the liquid is beneficial to reduce oxidation, too much affects the quality of the coating, and produces defects.

e. Heating and heating should be uniform to prevent explosion and local overheating.

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