Formation Mechanism Of Hot-Dip Galvanized Coating

Hot dip galvanizing is a metallurgical reaction process. From a microscopic point of view, the process of hot dip galvanizing is two dynamic balances: thermal balance and zinc-iron exchange balance. When the iron and steel workpiece is immersed in molten zinc solution at about 450°C, the workpiece at room temperature absorbs the heat of the zinc solution, and when it reaches above 200°C, the interaction between zinc and iron is gradually obvious, and zinc penetrates into the surface of the iron workpiece.

As the temperature of the workpiece gradually approaches the temperature of the molten zinc, alloy layers with different ratios of zinc and iron are formed on the surface of the workpiece, forming a layered structure of the zinc coating. As time goes on, different alloy layers in the coating show different growth rates. From a macroscopic point of view, the above process shows that the workpiece is immersed in the zinc liquid, and the zinc liquid surface boils. When the zinc-iron reaction gradually balances, 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, and the hot-dip galvanized coating is formed and the thickness is determined.
Hot-dip galvanized coating thickness requirements

The main factors affecting the thickness of the zinc coating are: the composition of the base metal, the surface roughness of the steel, the content and distribution of the active elements silicon and phosphorus in the steel, the internal stress of the steel, the geometric size of the workpiece, and the hot-dip galvanizing process.

The current international and Chinese hot-dip galvanizing standards are divided into sections according to the thickness of the steel, and the flat thickness and local thickness of the zinc coating should reach the corresponding thickness to determine the corrosion resistance of the zinc coating. For workpieces with different steel thicknesses, the time required to achieve thermal equilibrium and zinc-iron exchange equilibrium is different, and the thickness of the formed coating 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 unevenness of the thickness distribution of the zinc coating and the requirements for the corrosion resistance of the coating.

Therefore, the ISO standard, the American ASTM standard, the Japanese JIS standard and the Chinese standard have slightly different requirements on the thickness of the zinc coating, which are similar.

Next, several different hot-dip galvanizing and coating characteristics are explained.
Hot-dip pure zinc-coated steel sheet (GI sheet)

At present, 0.2% Al element is added to the zinc solution used in the production of GI plates. The effect of adding AI is to improve the fluidity of the zinc solution. Aluminum and iron elements react preferentially to form an iron-aluminum alloy phase layer, which inhibits the reaction between zinc and the steel plate substrate, thereby improving the adhesion of the coating.

At the same time, a small amount of aluminum in the zinc solution is added to zinc. The aluminum oxide film is formed on the surface of the liquid, which prevents the oxidation reaction of zinc on the surface of the zinc liquid and reduces the consumption of zinc. On the surface of the coating, aluminum can first react with oxygen to form an aluminum oxide protective film, which prevents the surface of the coating from oxidizing and increases the surface brightness.

GI plate is a typical cathodic protection mechanism that uses sacrificial anode (zinc) to protect the steel matrix from corrosion, and GI plate is the most common and widely used coated steel plate. It is used in construction, home appliances, automobiles, transportation, agriculture and other industries Lots of applications.

It can be seen from Figures 1a and 1b that the coating surface of the GI plate is composed of plateaus and uniform pits, and the morphology is mainly formed by a smooth roll with a certain roughness after galvanizing to form a surface with a roughness of about 1 μm on the coating surface. As can be seen from Figure 1c, the GI plate is composed of the outermost pure zinc layer, the steel substrate and a thin inhibitory layer in the middle, namely the iron-aluminum alloy phase layer FeAl3 or Fe2Al5 phase. The iron-aluminum alloy phase prevents the diffusion of iron to the zinc layer. , to avoid the formation of a relatively brittle zinc-iron alloy phase in the coating, thereby ensuring the adhesion of the coating.
Hot dip galvanized alloy coated steel sheet (GA sheet)

GA plate is a plated steel plate with a zinc-iron alloy phase layer formed by heat treatment at 500-550 °C after galvanizing, so that iron and zinc in the steel matrix diffuse each other, and the iron content on the surface of the coating is about 10% (mass fraction).

The GA plate needs to be heat treated after galvanizing to make the zinc and iron interdiffused to form a zinc-iron alloy phase. Therefore, in order to reduce the alloying time and reduce the blocking effect of the inhibition layer, the Al content in the zinc solution will be higher than that in the GI plate. It should be lower, generally around 0.13%. The iron element in the coating improves the overall corrosion potential and weldability of the coating, but the formation of the zinc-iron alloy phase layer increases the brittleness of the coating, and the coating is prone to powdering or peeling during the deformation process, which affects the service life of the molding die.

The formation process of the zinc-iron alloy phase layer increases the surface roughness of the coating, reduces the color and luster, and the surface of the coating becomes gray and darkened. The greater roughness increases the paintability of the coating. At the same time, the coating surface with greater hardness and surface roughness has better sand and gravel impact resistance. Therefore, GA board has better corrosion resistance, welding performance, coating performance and sand impact resistance than GI board, but the forming performance is relatively poor, and it is not as bright as GI board.

GA plates are used as automotive panels, mainly used in Japanese and Korean cars. The production of GA plates has strict production control requirements for the plated zinc-iron alloy phase layer, so the process control requirements are relatively high, and domestic production has been mature.

As shown in Figure 2a, the surface of the GA plate coating is composed of rough zinc-iron alloy phase δ1p and a small amount of ξ phase. As shown in Figure 2b, the outermost layer is relatively loose columnar crystal δ1p and a denser δ1k layer close to the substrate. There is a Г phase layer with a thickness of about 1 μm at the interface between the coating and the substrate.

Hot-dip Al-Zn-Coated Steel Sheet (GL Sheet)

Hot-dip Al-Zn coating generally includes Galfan or GF coating containing 5% Al and Galvalume or GL coating containing 55% Al. At present, the hot-dip Al-Zn coating of continuous strip generally refers to containing 55.0% Al and 43.4% Zn. , 1.6% Si Galvalume steel plate is GL plate.

Due to the high content of aluminum in the coating of the GL plate, the coating has the corrosion resistance and high temperature oxidation resistance of aluminum, and the presence of zinc makes the coating have cathodic protection properties. At present, GL boards are generally used as color-coated boards in construction, automobiles, home appliances, agriculture and other industries, and are directly used in mufflers, exhaust pipes, refrigerator backplanes, electronic microwave ovens, heat exchangers, etc.

The diameter of the spangle on the surface of the GL coating is generally 1-3mm. Due to the many factors affecting the size of the spangle, with the difference in the content of alloying elements in the zinc solution or the cooling rate after plating, the spangle of the coating will also have a large range of changes. , Generally speaking, the spangle size is also allowed within a larger range of 0.2 to 5.0 mm.

It can be seen from Figure 3 that the GL coating is composed of two layers, and the outer layer is an aluminum-zinc alloy layer, which is composed of an aluminum-rich solid solution of dendrites and a zinc-rich phase between dendrites. The inner layer is an aluminum-zinc-iron intermetallic compound layer, which is located between the aluminum-zinc alloy layer and the steel substrate. Similar in structure to the GI coating, the inner aluminum-zinc-iron intermetallic compound layer of the GL coating prevents iron from entering the aluminum-zinc alloy. layer and increase the adhesion of the coating, while the addition of silicon limits the growth of the brittle Al-Zn-Fe intermetallic layer.

Hot-dip galvanized aluminum-magnesium coated steel sheet (ZnAlMg sheet)

ZnAlMg board originated in Japan. Due to the marine climate of the island country and many typhoons and earthquakes, high strength and high corrosion resistance of steel is required. At the same time, Japan’s resources are depleted and the supply and demand of raw materials for production are tight. Therefore, ZnAlMg board was developed, which improves corrosion resistance and is effective. Utilize resources to reduce costs and protect the environment.

Due to the different compositions of aluminum and magnesium in ZnAlMg sheets developed by different companies, many ZnAlMg sheets with different composition ratios have appeared. For example, Nissin Steel has developed ZAM (Zn-6%Al-3%Mg) products, SuperDyma plate (Zn-11%Al-3%Mg) product, Thyssen’s ZnMgEcoprotect (Zn-1%Al-1%Mg) product, voestalpine’s Corrender (Zn-2%Al-2%Mg) product, ArcelorMittal’s Magnelis (Zn-3.7%Al-3.0%Mg) products and so on.

The ZAM plate coating is composed of the outer layer and the alloy layer at the interface between the outer layer and the substrate. The outer layer of the coating is composed of aluminum-rich phase and Zn/Al/Zn2Mg ternary eutectic phase, as shown in Figure 4a and Figure 4b. The corrosion resistance of ZnAlMg coating is greatly improved than that of GI coating. For example, the corrosion resistance of ZAM plate coating can even reach 16 times that of GI coating.

In the ZnAlMg coating, Mg2Zn11 or MgZn2 is distributed in the crystal boundary and dendrite gap, which plays a key role in improving the corrosion resistance. It has good edge corrosion resistance and good processability, and is suitable for building materials. Due to the high hardness of its outer surface, it can also resist surface wear during the forming process, which is beneficial to use as an environmentally friendly and resource-saving coated steel product.

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