1. Surface pretreatment
Poor surface pretreatment is the main reason for missing plating (exposed iron):
Steel is stained with paint or mineral grease during the process of leaving the factory, storage, transportation, and processing. The flaw detection surface of the welding part needs to be coated with special grease that is difficult to clean. However, there is no degreasing process. Surface impurities are removed by pickling, resulting in leakage of plating (iron exposure). ).
Another situation is lack of pickling and line clamping, or the pickling concentration is too high, causing acid salts to precipitate on the surface and in the grooves. Failure to wash with water or incomplete washing with water can easily lead to leakage and virtual plating.
In addition, when solvent hot-dip galvanizing is used, when the ratio of the most commonly used mixed aqueous solution of ZnCl2 and NH4Cl is incorrect, no eutectic point is formed, especially when the concentration is too high, in the circular hole of the support plate welding corner seam It is very easy to cause ash inclusion and leakage plating.
2. The zinc layer peels off
The main reasons causing the zinc layer to fall off are: surface oxidation, silicon compounds, cold rolling emulsion too dirty, NOF section oxidation atmosphere and protective gas dew point too high, unreasonable air-fuel ratio, low hydrogen flow, oxygen penetration into the furnace, with The temperature of the steel entering the pot is low, the furnace pressure in the RWP section is low and the furnace door sucks air, the furnace temperature in the NOF section is low, the grease cannot be evaporated, the aluminum content of the zinc pot is low, the unit speed is too fast, the reduction is insufficient, and the zinc liquid is in the The residence time is too short and the coating is too thick.
3. Other ingredients in zinc solution
Too many other metal components or harmful elements contained in the zinc liquid may cause defects such as zinc slag particles adhering to the coating surface and some abnormal patterns, cracks and other defects:
(1)Iron
After hot-dip galvanizing for a period of time, zinc accumulation and tiny zinc slag particles will appear on the surface of steel parts, resulting in a rough coating surface and a decrease in smoothness. Such tiny particles are generally Fe-Zn alloy particle slag.
The sources of iron in the zinc liquid generally include corrosion of the zinc pot, dissolution of the parts, iron ions in the plating flux, and iron salts on the parts. Therefore, in order to obtain a flat and smooth coating, it is necessary to strictly control the iron content in the zinc liquid, reduce the introduction of iron ions, control the temperature of the zinc liquid, avoid high and low temperatures of the zinc liquid, and reduce the corrosion rate of the zinc pot. Generally, when the iron content in the zinc liquid should be 0.20%, it must be cooled and left to stand and the zinc slag must be salvaged.
(2)Aluminum
Aluminum is the most commonly used added element in hot-dip galvanizing. Adding different concentrations of aluminum to the zinc solution can produce zinc coatings with different properties.
It is generally believed that when hot-dip galvanizing, adding <1% (mass fraction, the same below) aluminum to the zinc liquid can play the following roles: ① Improve the brightness of the coating; ② Reduce the oxidation of the zinc liquid surface; ③ Inhibit brittle Fe-Zn Phase formation to obtain a coating with good adhesion. In actual production, the zinc liquid containing 0.005% to 0.020% Al can achieve the purpose of bright coating, and can reduce the oxidation of the zinc liquid surface and the production of zinc ash.
The adding method of Zn-Al master alloy is often due to the quality problem of the added master alloy and the improper or too fast method of adding aluminum and zinc-aluminum alloy, resulting in the formation of a large amount of aluminum and iron compounds in the zinc liquid. This zinc-aluminum-iron three Elemental "dregs" or particles are suspended on the surface of the zinc liquid and are very viscous. They can easily adhere to steel parts and seriously damage the quality of the coating. At this time, it is useless to just cool down and remove the slag.
In this case, the addition of alloy should be stopped immediately, and zinc liquid purification measures should be taken to reduce the aluminum content. When the situation is not serious, the air cooling time can be appropriately extended to prevent bubbles and wrinkles from appearing on the coating surface after rapid water cooling.
(3) Tin, lead
Under normal circumstances, zinc ingots do not contain tin and only trace amounts of lead. In recent years, some zinc alloy suppliers have added tin and lead to so-called multi-component alloys in order to lower the galvanizing temperature and obtain a bright white coating.
When hot-dipping steel parts, the zinc liquid level will appear on the surface of the coating at 430°C. Feather-like spots and small ugly white spots will appear. Once the addition amount of the alloy containing tin and lead reaches 0.5%, zinc flowers will appear. Zinc There is a significant increase in ash.
When the content of lead and tin is too high, it is easy to form coarse crystals and cracks during the cooling process of the zinc layer, which affects the surface smoothness and corrosion resistance of the zinc layer.
Intergranular corrosion is most sensitive to impurity lead. Intergranular corrosion will occur when the amount of lead reaches 0.02%. The coating will break along the grain boundaries and lose adhesion, and even bubbles the size of soybean grains will appear on the surface. When using zinc-aluminum alloys or multi-component alloys, it is necessary to understand the contents of lead, tin, and aluminum to decide whether to add them to the zinc liquid.
(4)Nickel
Adding zinc-nickel alloy to the zinc liquid can effectively reduce the diffusion rate of zinc and iron atoms in the zeta phase, thus controlling the growth of the thickness of the immersion coating. When the Ni content in the zinc liquid is 0.06%, the Ni content can reach 0.8% , the thickness growth is obviously controlled after adding Ni, so the growth of the thickness of the immersion coating can be effectively controlled and the flow properties of the zinc liquid can be improved. Therefore, the coating thickness of hot-dip galvanizing using zinc-nickel alloy is more uniform, the surface is brighter, and there is less zinc splatter.
In short, the surface defects of hot-dip galvanized steel strips are more complex and have many causes. The solutions are multifaceted and require a lot of difficult and meticulous work to improve the hot-dip galvanizing technology.
