Causes and Control Methods of Hot-Dip Galvanizing Surface Defects
Hot dip galvanizing is a metallurgical reaction process. From a microscopic perspective, the hot-dip galvanizing process is composed of two dynamic balances: thermal balance and zinc-iron exchange balance. When a steel workpiece is immersed in molten zinc liquid at about 450°C, the workpiece at room temperature absorbs the heat of the zinc liquid. When it reaches above 200°C, the interaction between zinc and iron gradually becomes obvious, and zinc penetrates into the surface of the iron workpiece.
As the temperature of the workpiece gradually approaches the temperature of the zinc liquid, alloy layers containing different proportions of zinc and iron are formed on the surface of the workpiece, forming a layered structure of the zinc coating. As time goes by, different alloy layers in the coating show different growth rates. From a macro perspective, the above process is manifested as the workpiece being immersed in the zinc liquid and the zinc liquid surface boiling. When the zinc-iron reaction gradually becomes balanced, the zinc liquid surface gradually calms down. The workpiece is lifted out of the zinc liquid level, and when the temperature of the workpiece gradually drops below 200°C, the zinc-iron reaction stops, the hot-dip galvanized coating is formed, and the thickness is determined.
Hot-dip galvanizing coating thickness requirements
The factors that affect the thickness of the zinc coating mainly include: base metal composition, surface roughness of the steel, content and distribution of active elements silicon and phosphorus in the steel, internal stress of the steel, geometric dimensions of the workpiece, and hot-dip galvanizing process.
The current international and Chinese hot-dip galvanizing standards are divided into sections according to the thickness of the steel. The average thickness and local thickness of the zinc coating should reach the corresponding thickness to determine the anti-corrosion performance of the zinc coating. Workpieces with different steel thicknesses require different times to achieve thermal balance and zinc-iron exchange balance, and the thickness of the coating formed is also different. The average thickness of the coating in the standard is based on the industrial production experience value of the above-mentioned galvanizing mechanism, and the local thickness is the empirical value required to take into account the uneven thickness distribution of the zinc coating and the requirements for the corrosion resistance of the coating.
Therefore, ISO standards, American ASTM standards, Japanese JIS standards and Chinese standards have slightly different zinc coating thickness requirements, but they are mostly the same.
The role and influence of hot coating coating thickness
The thickness of the hot-dip galvanized coating determines the anti-corrosion performance of the plated parts.
For thin steel plates with smooth surfaces below 3mm, it is difficult to obtain thicker coatings in industrial production. In addition, the thickness of the zinc coating that is not commensurate with the thickness of the steel will affect the bonding force between the coating and the base material and the appearance quality of the coating. Excessively thick coating will cause the coating to have a rough appearance and peel easily, and the plated parts will not be able to withstand the collision during transportation and installation.
If there are more active elements silicon and phosphorus in the steel, it is very difficult to get a thin coating in industrial production, this is because the silicon content in the steel affects the growth mode of the alloy layer between zinc and iron, which will make the zinc-iron alloy layer grow rapidly and push the zinc-iron phase to the surface of the coating, resulting in the coating surface rough and dull, forming a gray coating with poor adhesion.
Therefore, as discussed above, there is uncertainty in the growth of galvanized layer, and it is often difficult to obtain a certain range of coating thickness in actual production. The thickness specified in the hot-dip galvanized standard is the empirical value generated after a large number of experiments, taking into account various factors and requirements, which is more reasonable.
Causes and control methods of surface defects of hot-dip galvanized layer
1. Surface pretreatment
Poor surface pretreatment is the main reason for missing plating (iron exposure):
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.
2. The zinc layer falls off
The main causes of zinc layer shedding are: Surface oxidation, silicon compounds, cold binding emulsion is too dirty, NOF oxidation atmosphere and protective gas dew point is too high, air fuel ratio is unreasonable, hydrogen flow rate is low, furnace aerobic infiltration, strip steel into the pot temperature is low, RWP section furnace pressure is low and furnace door suction, NOF section furnace temperature is low, grease evaporation is not enough, the zinc pot aluminum content is low, the unit speed is too fast, the reduction is insufficient, the residence time in the zinc liquid is too short, and the coating is thick.
3. Other ingredients in the zinc solution
Excessive inclusion of other metal components or harmful elements in the zinc liquid may cause defects such as zinc slag particles adhering to the surface of the coating and some abnormal patterns, turtle cracks and other defects:
(1) Iron
After hot dip galvanizing for a period of time, zinc accumulation and small zinc slag particles will appear on the surface of the steel, resulting in rough surface of the coating and a decrease in smoothness. Such small particles are generally Fe – Zn alloy granular slag.
The source of iron in the zinc liquid is generally the corrosion of the zinc pot, the dissolution of the parts, the iron ion in the plating agent and the iron salt on the parts. Therefore, in order to obtain a 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 the sudden rise and fall of the temperature of the zinc liquid, and reduce the corrosion rate of the zinc pan. When the iron content in the general zinc liquid should be 0.20%, it must be cooled down and the zinc slag salvaging.
(2) Aluminum
Aluminum is the most commonly used additive element in hot dip galvanizing. Zinc coating with different properties can be obtained by adding different concentration of Al to zinc solution.
It is generally believed that when hot dip galvanizing, adding <1%(mass fraction, the same below) of aluminum to the zinc solution can play the following roles: ① improve the brightness of the coating; ② Reduce zinc surface oxidation; ③ The formation of brittle Fe-Zn phase is inhibited to obtain a coating with good adhesion. In the actual production, the zinc liquid containing 0.005% ~ 0.020%Al can achieve the purpose of bright coating, and can reduce the surface of the zinc liquid oxidation and zinc ash generation.
The addition method of Zn-Al intermediate alloy m is often due to the quality problems of the added intermediate alloy and the improper or too fast method of adding aluminum and zinc-aluminum alloy, resulting in the formation of a large number of aluminum and iron compounds in the zinc liquid. The zinc-aluminum-iron terpolymer “surface slag” or particles are suspended on the surface of the zinc liquid and are very viscous and easy to adhere to the steel parts, which seriously damages the quality of the coating. At this point, it is useless to only rely on cooling and dregs.
In this case, the addition of alloy should be immediately stopped, and the zinc liquid purification measures should be taken to reduce the aluminum content, and the air cooling time can be properly extended when the situation is not serious to prevent bubbles and wrinkles after the rapid water cooling of the coating surface.
(3) Tin, aluminum
In general, 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 the so-called multi-component alloy in order to reduce the galvanized temperature and obtain a white bright coating.
In hot plating steel parts, the zinc liquid surface at 430℃ coating surface will appear feathery spots and small ugly white spots, containing tin, lead alloy added once 0.5%, there will be zinc flowers, zinc ash increased significantly.
When the content of lead and tin is too high, it is easy to form coarse crystals and turtle cracks in the cooling process of the zinc layer, which affects the surface smoothness and corrosion resistance of the zinc layer.
Intergranular corrosion is the most sensitive to lead impurity, and when the amount of lead reaches 0.02%, intergranular corrosion will occur, and the adhesion will be broken along the grain boundary in the coating, and even the surface of the soybean grain size bubble will appear. When applying zinc-aluminum alloy or multi-component alloy, it is necessary to understand the content of lead, tin and aluminum, and decide whether to add it in the zinc liquid.
(4) Nickel
Adding zinc-nickel alloy to zinc liquid can effectively reduce the diffusion rate of zinc and iron atoms in the ζ phase, so it can control the growth of the thickness of the dipped coating. When the Ni content in zinc liquid is 0.06%, the Ni content can reach 0.8%, and the thickness growth after adding Ni is obviously controlled, so it can effectively control the growth of the dipped coating thickness and improve the flow performance of zinc liquid. Therefore, the thickness of the coating is more uniform, the surface is more bright, and the zinc flowers are less.
In short, the surface defects of hot dip galvanized strip steel are more complex, the causes are more, the solution is multi-faceted, and a lot of difficult and meticulous work is needed to improve the hot plating technology one step.