1 Introduction

Hot-dip galvanizing, also known as hot-dip galvanizing, is a method in 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 communication, the requirements for the protection of steel parts are getting higher and higher, and the demand for hot-dip galvanizing is also increasing. The following will take you to understand the principle and process description of hot-dip galvanizing.

2. Protective performance of hot-dip galvanized layer

Usually, the thickness of the electro-galvanized layer is 5-15 μm, while the hot-dip galvanized layer is generally above 35 μm, even as high as 200 μm. Hot-dip galvanizing has good coverage, dense coating and no organic inclusions. As we all know, the mechanism of zinc’s resistance to atmospheric corrosion includes mechanical protection and electrochemical protection. Under atmospheric corrosion conditions, the surface of the zinc layer has ZnO, Zn(OH)2 and basic zinc carbonate protective films, which slow down the corrosion of zinc to a certain extent. When the protective film (also known as white rust) is damaged, a new film will be formed. When the zinc layer is seriously damaged and endangers the iron matrix, zinc produces electrochemical protection for the matrix, 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, and iron Protected as cathodic. Obviously, the atmospheric corrosion resistance of hot-dip galvanizing to the base metal iron is better than that of electro-galvanizing.

3 The formation process of hot-dip galvanized layer

The formation process of the hot-dip galvanized layer is the process of forming an iron-zinc alloy between the iron matrix and the outermost pure zinc layer. The iron-zinc alloy layer is formed on the surface of the workpiece during hot-dip plating, so that the iron and the pure zinc layer are very close. The process can be simply described as follows: when the iron workpiece is immersed in the molten zinc solution, a solid solution of zinc and α iron (body-centered) is first formed on the interface. This is a crystal formed by dissolving zinc atoms in the base metal iron in a solid state. The two metal atoms are fused, and the gravitational force between the atoms is relatively small. Therefore, when zinc reaches saturation in the solid solution, the atoms of the two elements of zinc and iron diffuse into each other, and the zinc atoms diffused into (or infiltrated) the iron matrix migrate in the matrix lattice, and gradually form alloys with iron, while diffusion The iron in the molten zinc solution forms an intermetallic compound FeZn13 with zinc, which sinks to the bottom of the hot-dip galvanizing pot, which is zinc slag. When the workpiece is removed from the zinc immersion solution, a pure zinc layer is formed on the surface, which is a hexagonal crystal. Its iron content is not more than 0.003%.

4. Hot-dip galvanizing process and related instructions

4.1 Process

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

4.2 Description of relevant technological process

(1) Degreasing

The oil can be removed by chemical degreasing or water-based metal degreasing cleaning agent until the workpiece is completely soaked by water.

(2) Pickling

It can be acid washed with H2SO4 15%, thiourea 0.1%, 40～60℃ or HCl 25%, urotropine 3～5g/L, 20～40℃. The addition of corrosion inhibitor can prevent the matrix from over-corrosion and reduce the hydrogen absorption of the iron matrix, and at the same time, the addition of fog inhibitor can inhibit the escape of acid mist.

Poor degreasing and pickling treatment will result in poor coating adhesion, failure to coat zinc or the zinc layer falling off.

(3) Dipping flux

Also known as solvent, it can maintain a certain activity of the workpiece before immersion plating to avoid secondary oxidation, so as to enhance the bonding between the coating and the substrate. NH4Cl 100-150g/L, ZnCl2 150-180g/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 a sharp rise in temperature during immersion plating, and to remove residual moisture to prevent zinc explosion, resulting in explosion of zinc liquid, the preheating is generally 80 to 140 °C. However, explosion-proof agents are generally added now, and the explosion-proof agents of Liaoyang Haoyan Hot-Dip Galvanizing Additives Co., Ltd. are good.

(5) Hot dip galvanizing

It is necessary to control the temperature of the zinc solution, the dipping time and the speed at which the workpiece is drawn out of the zinc solution. The extraction speed is generally 1.5 m/min

If the temperature is too low, the fluidity of the zinc liquid is poor, the coating is thick and uneven, it is easy to sag, and the appearance quality is poor; if the temperature is high, the liquid zinc has good fluidity, and the zinc liquid is easy to separate from the workpiece, reducing the occurrence of sagging and wrinkling, and the adhesion Strong, thin coating, good appearance and high production efficiency; but if the temperature is too high, the iron loss of the workpiece and the zinc pot is serious, resulting in a large amount of zinc slag, which affects the quality of the dipped zinc layer and easily causes color difference to make the surface color ugly and high zinc consumption.

The thickness of the zinc layer depends on the temperature of the zinc solution, the dipping time, the material of the steel and the composition of the zinc solution. In addition, galvanized alloy is also very important. Haoyan's special multi-component alloy for hot-dip galvanizing was rated as the best alloy in the current market by many experts at the National Green Batch Hot-dip Galvanizing Forum in 2008! Product (Patent No. 200710139349.3)".

In order to prevent the high temperature deformation of the workpiece and reduce the zinc slag caused by iron loss, the general manufacturers use 450 ~ 470 ℃, 0.5 ~ 1.5min. Some factories use higher temperatures for large workpieces and iron castings, but avoid the temperature range where the iron loss peaks. However, we recommend adding an alloy with iron removal function and lowering eutectic temperature to the zinc solution and reducing the galvanizing temperature to 435-445 °C.

(6) finishing

After plating, the finishing of the workpiece is mainly to remove the residual zinc and zinc nodules on the surface, which is completed by using a special vibrator for hot-dip galvanizing.

(7) Passivation

The purpos is to improve the atmospheric corrosion resistance of the workpiece surface, reduce or prolong the appearance time of white rust, and keep the coating with a good appearance. All are passivated with chromate, such as Na2Cr2O7 80~100g/L, sulfuric acid 3~4ml/L, but this passivation solution seriously affects the environment, and 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 ℃ and 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 coating thickness can also be obtained by converting the amount of zinc adhesion. The bonding strength can be determined by using a bending press to bend the sample at 90 to 180°, and there should be no cracks and coating peeling off. It can also be tested by heavy hammer knocking, and salt spray test and copper sulfate corrosion test can be done in batches.

5 Formation and control of zinc ash and zinc slag

5.1 Formation of zinc ash and zinc dross

The zinc ash slag not only seriously affects the quality of the galvanized layer, but also causes the coating to be rough and zinc nodules. Moreover, the cost of hot-dip galvanizing is greatly increased. Usually, 40-100kg of zinc is consumed per 1t of plated workpiece. If the zinc ash and zinc slag are serious, the zinc consumption will be as high as 140-200kg. The main purpose of controlling zinc slag is to control the temperature and reduce the scum caused by the oxidation of the zinc liquid surface. Therefore, it is necessary to use alloys with iron removal and anti-oxidation functions, and use alloys with low thermal conductivity, high melting point, and small specific gravity, which are incompatible with zinc liquid. When the reaction occurs, the ceramic beads or glass balls that can reduce heat dissipation and prevent oxidation can be covered. Such balls are easily pushed away by the workpiece and have no adhesion to the workpiece.

For the formation of zinc slag in the zinc solution, it is mainly a zinc-iron alloy with extremely poor fluidity formed when the iron content dissolved in the zinc solution exceeds the solubility at this temperature. The zinc content in the zinc slag can be as high as 94%. The key to the high cost of zinc.

It can be seen from the solubility curve of iron in zinc solution that the amount of dissolved iron, that is, the amount of iron loss, is different at different temperatures and different holding times. At around 500°C, the amount of iron loss increases sharply with heating and holding time, almost in a linear relationship. Below or above the range of 480 to 510 °C, the iron loss increases slowly with time. Therefore, people will 480 ~ 510 ℃ known as malignant dissolution zone. In this temperature range, the zinc liquid erodes the workpiece and the zinc pot most seriously, and the iron loss increases significantly when the temperature exceeds 560 °C. When the temperature exceeds 660 °C, the zinc is destructive to the iron matrix, and the zinc slag will increase sharply, and the plating cannot be carried out. . Therefore, plating is currently carried out in the range of 430 to 450°C.

5.2 Control of Zinc Slag Amount

To reduce the zinc slag, it is necessary to reduce the content of iron in the zinc solution, that is, to reduce the factors of iron dissolution:

⑴ Plating and heat preservation should avoid the peak area of iron dissolution, that is, do not operate at 480 ~ 510 ℃.

(2) The zinc pot material should be welded with steel plates containing carbon and low silicon content as much as possible. 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 iron by the zinc liquid. At present, 08F/XG08/WKS high-quality steel plates are mostly used. , and contains elements such as nickel, chromium, etc., which can inhibit the corrosion of iron. Ordinary carbon steel cannot be used, otherwise the zinc consumption will be large and the life of the zinc pot will be short. It has also been proposed to use silicon carbide to make a molten zinc tank. Although it can solve the iron loss, the modeling process is a difficult problem. At present, the zinc pot made of industrial ceramics can only be made into a cylindrical shape and is small in size, although it can meet the requirements of small parts plating. Zinc requirements but cannot guarantee the galvanization of large workpieces.

⑶ It is necessary to scour the slag frequently. First raise the temperature to the upper limit of the process temperature to separate the zinc slag from the zinc liquid, make the zinc slag settle on the bottom of the tank, and then use a zinc scoop or a special slag scoop to remove it. The plated parts that fall into the zinc solution should be salvaged in time.

⑷To prevent the iron in the flux from being brought into the zinc bath with the workpiece, the flux should be regenerated and recycled on-line, the ferrous content should be strictly controlled, and the PH value should always be kept at 4.5-5.

⑹ Heating and heating should be uniform to prevent local overheating.

Hot-dip galvanized layers protect steel much better than paint or plastic layers. During hot-dip galvanizing, diffusion occurs between zinc and steel to form a zinc-iron intermetallic compound layer, that is, an alloy layer. The metallurgical bond between the alloy layer and steel and zinc is stronger than the bond between paint and steel. The hot-dip galvanized layer exposed to the atmospheric environment will not fall off for decades until it is completely corroded naturally.

The steel anti-corrosion measures selected for the photovoltaic support system are hot-dip galvanizing after the components are processed. First, the original steel structure profiles are processed and formed, and then under the strict quality management and the whole process of modern computer monitoring, after surface cleaning, heat treatment, hot-dip continuous plating, accelerated cooling and other complex processes, they are plated with zinc layers to produce seamless Defects, no leakage of plated steel structure finished products. In this process, it is ensured that each part of the steel component can be protected by the zinc layer, to avoid bending, punching or trimming after galvanizing, resulting in no galvanizing and other processing defects on the trimming and punching edges, so as to prevent the components from being corroded in advance. .