Method for inhibiting blackening of zinc-aluminum-magnesium coating through alloy phase optimization

Abstract

The invention belongs to the technical field of zinc-aluminum-magnesium coating steel plate production, and discloses a method for inhibiting blackening of a zinc-aluminum-magnesium coating through alloy phase optimization, which comprises the following steps: 1) a steel strip enters a zinc pot, and is subjected to hot dipping in a plating solution; the chemical components of the plating solution comprise, by mass, 47%-57% of Al, 1.8%-2% of Mg, 1%-2% of Si, 0.02%-0.15% of microelements and the balance Zn and inevitable impurities. And (2) after the steel strip leaves the plating solution, the steel strip is purged by a pulse pressure air knife and then is subjected to segmented cooling, and the zinc-aluminum-magnesium plating steel strip is obtained.The reasonable proportion and ideal distribution of alloy phases in the zinc-aluminum-magnesium plating are achieved by controlling the hot dipping and post-plating processes, the risk of phase boundary corrosion is remarkably reduced, and the blackening resistance of the zinc-aluminum-magnesium plating is improved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of zinc-aluminum-magnesium plated steel plate production, and particularly relates to a method for inhibiting blackening of zinc-aluminum-magnesium plated layer by optimizing alloy phases. BACKGROUND

[0002] The zinc-aluminum-magnesium plated steel plate is a new type of plated steel plate formed by adding magnesium to the original aluminum-zinc plated layer. The zinc-aluminum-magnesium plated product has excellent flat corrosion resistance and notch corrosion resistance, and the corrosion resistance is 5-10 times that of zinc plated products. In addition, the welding, phosphating, painting and forming performance of the zinc-aluminum-magnesium plated layer are compatible with the existing processes, and it is widely considered to be the best choice for the comprehensive performance of the front steel corrosion resistant plated layer. In addition to being applied in the automobile, home appliance, building and other industries, the zinc-aluminum-magnesium plated steel plate can also be used as a substrate to produce color coated steel plates, and as a pre-coated product in the home appliance and building fields.

[0003] However, due to the Mg contained in the component system of the zinc-aluminum-magnesium plated layer, the MgZn2 phase and the Mg2Si phase that improve corrosion resistance are added to the plated layer structure, but there is a potential difference between different phases in the plated layer. The existing fingerprint-resistant film cannot completely isolate water vapor, and the high-aluminum zinc-aluminum-magnesium plated layer forms a micro-battery in a humid environment, causing electrochemical corrosion. The coverage of the corrosion product causes the light to weaken and refract after passing through the transparent coating film, and even interfere, thereby forming blackening. Blackening is a dense oxide film that has no effect on corrosion resistance and service life, but seriously affects the appearance. The existing technology mainly adjusts the plated layer composition or performs surface treatment to inhibit blackening, which to some extent alleviates the blackening phenomenon, but still does not achieve good results. SUMMARY

[0004] The technical problem to be solved by the present application is to provide a method for inhibiting blackening of zinc-aluminum-magnesium plated layer by optimizing alloy phases, which solves the problems existing in the prior art. By controlling hot dipping and post-plating processes, the reasonable proportion and ideal distribution of each alloy phase in the zinc-aluminum-magnesium plated layer are realized, the risk of phase boundary corrosion is significantly reduced, and the blackening resistance of the zinc-aluminum-magnesium plated layer is improved.

[0005] To solve the technical problems proposed in the present application, the present application provides a method for inhibiting blackening of zinc-aluminum-magnesium plated layer by optimizing alloy phases, comprising the following steps: 1) The steel strip enters the zinc pot and is hot-dipped in the plating solution; 2) After the steel strip leaves the plating solution, it is first blown by an air knife, and then is subjected to segmented cooling to obtain a zinc-aluminum-magnesium plated steel strip.

[0006] In the above scheme, the steel strip is subjected to conventional pretreatment and heat treatment before entering the zinc pot. The pretreatment mainly includes cleaning and degreasing, and the heat treatment mainly includes annealing heating and cooling processes.

[0007] In the scheme, the temperature of the plating solution is 575-610 ℃, and the Mg content in the plating solution is 1.8-2%.

[0008] In the scheme, the chemical components of the plating solution include, in terms of mass percentage, Al 47-57%, Mg 1.8-2%, Si 1-2%, trace elements 0.02-0.15%, and the rest is Zn and inevitable impurities.

[0009] Further, the trace elements are one or more of Zr, Ca, and Cr.

[0010] Preferably, the trace elements are Zr, Ca, and Cr, and the mass percentage in the plating solution is Zr 0.013-0.018%, Ca 0.002-0.005%, and Cr 0.001-0.007%.

[0011] In the scheme, the air knife blowing adopts pulse pressure, and the pulse pressure is an alternating cycle of two pressure sections: the pressure of the first pressure section is 30-40 kPa, and the single duration is 1-3 s; the pressure of the second pressure section is 20-30 kPa, and the single duration is 1-3 s.

[0012] In the scheme, the segmented cooling is two-stage cooling: the first stage is air blowing cooling, and the cooling rate is 80-100 ℃ / s, and the cooling is to 150-180 ℃; the second stage is water quenching, and the cooling rate is 10-20 ℃ / s, and the cooling is to room temperature.

[0013] Further, the temperature of the water quenching tank is ≤45 ℃, and the pH value of the water quenching tank is ≤9.

[0014] In the scheme, the microstructure of the zinc-aluminum-magnesium coating includes an aluminum-rich phase (α-Al), a zinc-rich phase (η-Zn), a Mg2Si phase, a Zn-based solid solution containing Al and Mg, and a MgZn2 phase.

[0015] Further, the average grain size of the aluminum-rich phase is ≤20 μm.

[0016] Further, the zinc-rich phase is distributed in the form of isolated particles, and the average grain size is ≤10 μm.

[0017] Further, the Mg2Si phase is finely and dispersively distributed, the average grain size is ≤5 μm, and the strong cathode phase is avoided.

[0018] Further, the MgZn2 phase is dispersively precipitated in the form of fine particles at the grain boundaries, and the average grain size is ≤500 nm.

[0019] Further, the microstructure and volume fraction of the zinc-aluminum-magnesium coating layer include: 60-70% of the aluminum-rich phase, 20-30% of the zinc-rich phase, 5-8% of the Mg2Si phase, 3-5% of the Zn-based solid solution containing Al and Mg, and ≤2% of the MgZn2 phase.

[0020] Further, the volume ratio of the aluminum-rich phase to the zinc-rich phase is 1.6-2.6.

[0021] Further, the volume ratio of the Mg2Si phase to the total volume of the aluminum-rich phase and the zinc-rich phase is 0.06-0.1.

[0022] Further, the volume ratio of the MgZn2 phase to the Zn-based solid solution containing Al and Mg is ≤0.4.

[0023] In the above scheme, the zinc-aluminum-magnesium coating layer steel strip is coated with a chromium-free passivation solution, dried, and a chromium-free passivation film is formed.

[0024] Further, the zinc-aluminum-magnesium coating layer steel strip is subjected to conventional cleaning and degreasing before being coated with the chromium-free passivation solution.

[0025] Further, the thickness of the chromium-free passivation film is 100-300 nm.

[0026] Further, the chromium-free passivation solution is composed of zirconium salt, titanium salt, fluoride, and water.

[0027] Further, the chromium-free passivation solution includes the following raw materials: 2-5 g / L of zirconium salt, 1-3 g / L of titanium salt, 0.5-2 g / L of fluoride, and the rest is water.

[0028] Further, the zirconium salt is one or more of zirconium oxychloride, zirconium sulfate, zirconium carbonate, zirconium ammonium carbonate, sodium fluorozirconate, potassium fluorozirconate, ammonium fluorozirconate, and zirconium nitrate.

[0029] Further, the titanium salt is one or more of titanium sulfate, fluorotitanic acid, potassium fluorotitanate, ammonium fluorotitanate, titanium trichloride, titanyl sulfate, and potassium titanate.

[0030] Further, the fluoride is one or more of sodium fluoride, ammonium fluoride, zinc fluoride, potassium fluorosilicate, and sodium fluorosilicate.

[0031] Further, the steel plate temperature during drying is 90-130°C, and the drying time is 1-3 s.

[0032] Further, the surface of the chromium-free passivation film is coated with an acrylic resin coating, and the acrylic resin coating is baked and cured to form an acrylic resin coating layer.

[0033] Further, the thickness of the acrylic resin coating layer is 5-10 μm.

[0034] Further, the acrylic resin paint is composed of water-based acrylic resin, diacetone acrylamide, nano-silica particles and water.

[0035] Further, the acrylic resin paint includes the following mass percentage of raw materials: water-based acrylic resin 40~60%, diacetone acrylamide 3~5%, nano-silica particles 0.5~1%, and the rest is water.

[0036] Further, the water-based acrylic resin is a water-soluble acrylic resin containing carboxyl groups.

[0037] Further, the particle size of the nano-silica particles is 50~100nm.

[0038] Further, the steel plate temperature during baking curing is 232~240℃, and the baking curing time is 10~20s.

[0039] The key technical concept of the present application is as follows: The present application mainly improves the blackening resistance of zinc-aluminum-magnesium coating through the proportion and distribution of each alloy phase in the coating: 1) In the chemical composition of the zinc-aluminum-magnesium coating, the Mg content is limited to 1.8~2%, and the precise limitation of Mg content is beneficial to refining the grain size of MgZn2 phase. Further, one or more of trace elements Zr, Ca and Cr is added, with a total mass percentage of 0.02~0.15%. Adding trace element Zr can further refine the grain size. Zr acts as a heterogeneous nucleation core to inhibit the precipitation of coarse MgZn2 phase, thereby improving the uniformity of the coating structure. Adding trace element Cr can form a diffusion barrier at the interface of the coating and the substrate, inhibiting the excessive growth of Fe-Zn alloy layer. Adding trace element Ca can reduce the contact angle between the alloy liquid and the substrate, and promote the uniform spreading of the coating.

[0040] 2) Due to the poor wettability of Zn and Al, the zinc-rich phase is easy to spread along the grain boundary of the aluminum-rich phase during hot dipping, forming a continuous network, which is difficult to maintain isolated particle shape, and will lead to the intensification of "micro-battery corrosion" of the zinc-rich phase, triggering the selective oxidation of zinc. In high-aluminum zinc-aluminum-magnesium coating, the aluminum-rich phase (main phase) is the cathode, and the zinc-rich phase is the anode. When the zinc-rich phase forms a continuous network, it is equivalent to building a "through anode network" in the coating, forming a large number of micro-batteries with the aluminum-rich phase cathode. When the zinc-rich phase is in a continuous network, the corrosion of zinc is "large area and through", and a large amount of Zn 2+In surface enrichment, and in the local alkaline environment of high aluminum matrix (Al hydrolysis produces weak alkaline), it is easy to form black ZnO microcrystalline (particle size <100 nm is black, not white of macro ZnO). These black ZnO densely cover the surface of the coating, that is, it shows "blackening". The existence of continuous network rich zinc phase will cut off the continuity of the aluminum-rich phase, making it difficult for the Al2O3 passivation film to completely cover the surface of the coating; at the same time, the rapid corrosion of the network rich zinc phase will continuously damage the local passivation film, causing black corrosion products to continuously generate and accumulate, and eventually forming a stable black appearance. For this, a pulse air knife is used, when the zinc-rich phase is broken into isolated particles of 5-10 microns by the pulse air knife, the anode (zinc-rich phase) is isolated by the cathode (aluminum-rich phase), the micro-battery effect is weakened, the corrosion rate of zinc is reduced and uniformly distributed, the corrosion product is mainly off-white, the coating can maintain a silver-white appearance for a long time, and blackening is avoided.

[0041] 3) In the structure of zinc-aluminum-magnesium coating, because the MgZn2 phase is easy to gather at the phase boundary, it is easy to cause the proportion to be too large, so after plating, segmented cooling is adopted, first fast cooling to 150-180 DEG C, inhibiting Mg diffusion, and then slow cooling to room temperature, avoiding stress cracking; the solubility of Mg in Al increases with the decrease of temperature, and fast cooling can make Mg "freeze" in the aluminum-rich phase, reducing the diffusion amount to the zinc-rich phase. The aluminum-rich phase / zinc-rich phase ratio (alpha / eta) needs to be controlled in 1.6-2.6, when alpha / eta<1.6 (i.e. the zinc-rich phase is relatively too much), the anode area is too large, the corrosion rate exceeds the passivation film repair rate, and blackening is inevitably caused; when alpha / eta>2.6 (the aluminum-rich phase is relatively too much), the toughness of the coating decreases, and the coating is easy to crack during processing, leading to blackening. The Mg2Si phase / (aluminum-rich phase+zinc-rich phase) ratio (beta / (alpha+eta)) needs to be controlled in 0.06-0.1, which reflects the filling efficiency of Mg2Si to the phase boundary; when <0.06, the phase boundary is insufficiently filled, and the corrosion medium diffuses fast; when >0.1, the risk of Mg2Si agglomeration increases. The MgZn2 phase / Zn base Al and Mg containing solid solution ratio (gamma / delta) needs to be controlled in ≤0.4, and the Zn base solid solution can dissolve part of MgZn2, and the solubility is about 0.5%, when gamma / delta>0.4, the solid solution is saturated, and MgZn2 will inevitably precipitate at the phase boundary, causing phase boundary corrosion.

[0042] The application also further inhibits blackening by adopting a chromium-free passivation solution and an acrylic resin coating for post-treatment of the zinc-aluminum-magnesium plated steel strip. The chromium-free passivation solution comprises 2-5 g / L of zirconium salt, 1-3 g / L of titanium salt and 0.5-2 g / L of fluoride, and the rest is water. The titanium and zirconium ions can generate nanoscale oxide particles through hydrolysis, fill the film layer pores, and make the passivation film density increase by more than 40%, effectively blocking Cl -corrosive medium such as H2O permeates. The water-based acrylic resin is used as a matrix to form hydrogen bonds and coordination with the amino and hydroxyl groups of the chromium-free passivation film, so as to realize strong combination of the "passivation film-coating" interface, and the nano-silicon dioxide forms Mg-O-Si bonds with the Mg 2+ and further seals the defects of the plating layer to cooperatively inhibit the corrosion of the micro-battery.

[0043] Compared with the prior art, the present application has the following beneficial effects: (1) The present application innovatively proposes a method for inhibiting blackening of a zinc-aluminum-magnesium plating layer by optimizing alloy phases, and by controlling hot dipping and post-plating processes, a reasonable proportion and ideal distribution of the alloy phases are realized, the risk of phase boundary corrosion is significantly reduced, and the blackening resistance of the zinc-aluminum-magnesium plating layer is improved.

[0044] (2) The post-treatment method of the present application further improves the blackening resistance of the zinc-aluminum-magnesium plating layer by using chromium-free passivation and water-based organic coating, the materials used meet the environmental protection requirements, the pollution to the environment is reduced, and by using the process, a relatively dense post-treatment film can be formed, water vapor can be effectively prevented from penetrating, and the risk of blackening during outdoor storage in rainy and humid weather is further reduced.

[0045] (3) The plating layer treated by the method of the present application has excellent blackening resistance and no blackening occurs in 500 hours of laminated sheet humid heat test, which is significantly improved compared with the traditional process. BRIEF DESCRIPTION OF DRAWINGS

[0046] Figure 1 is the alloy phase diagram of the high-aluminum zinc-aluminum-magnesium plating layer of Example 1 of the present application.

[0047] Figure 2 is the surface macrograph of the high-aluminum zinc-aluminum-magnesium commercial material of Example 1 of the present application.

[0048] Figure 3 is the surface macrograph of the high-aluminum zinc-aluminum-magnesium commercial material of Comparative Example 1 of the present application. DETAILED DESCRIPTION

[0049] In order to better understand the present application, the content of the present application will be further illustrated below in combination with examples, but the content of the present application is not limited to the following examples only.

[0050] EXAMPLE The following examples adopt the production method for inhibiting blackening of a zinc-aluminum-magnesium plating layer by optimizing alloy phases, which includes the following steps: 1) After the steel strip is subjected to conventional cleaning, degreasing pretreatment and annealing heat treatment, it enters a zinc pot for hot dipping in a plating solution; the chemical composition of the plating solution includes, by mass percentage, Al 47-57%, Mg 1.8-2%, Si 1-2%, Zr+Ca+Cr 0.02-0.15%, and the rest is Zn and unavoidable impurities, and the plating solution temperature is 575-610℃. 2) After the steel strip leaves the plating solution, it is first subjected to air knife blowing, the air knife blowing uses pulse pressure, the pulse pressure is the alternating circulation of two pressure sections: the pressure of the first pressure section is 30-40 kPa, the single duration is 1-3 s; the pressure of the second pressure section is 20-30 kPa, the single duration is 1-3 s; Then two-stage cooling is performed, the first stage is air blowing cooling, the cooling rate is 80-100 ℃ / s, and the cooling is to 150-180 ℃; the second stage is water cooling in a quenching tank, the temperature of the quenching tank is ≤45 ℃, the pH value of the quenching tank is ≤9, the cooling rate is 10-20 ℃ / s, and the cooling is to room temperature, to obtain a zinc-aluminum-magnesium plated steel strip; 3) A chromium-free passivation solution is coated on the surface of the zinc-aluminum-magnesium plated steel strip, the chromium-free passivation solution comprises the following raw materials: zirconium salt 2-5 g / L, titanium salt 1-3 g / L, fluoride 0.5-2 g / L, and the rest is water; after coating, drying is performed, the plate temperature during drying is 90-130 ℃, the drying time is 1-3 s, and a chromium-free passivation film with a thickness of 100-300 nm is formed; 4) An acrylic resin coating is coated on the surface of the chromium-free passivation film, the acrylic resin coating comprises the following raw materials in mass percentage: water-based acrylic resin 40-60%, diacetone acrylamide 3-5%, nano-silicon dioxide particles with a particle size of 50-100 nm 0.5-1%, and the rest is water; after coating, baking and curing are performed, the plate temperature during curing is 232-240 ℃, the curing time is 10-20 s, and an acrylic resin coating with a thickness of 5-10 μm is formed.

[0051] The specific process parameters of each embodiment are shown in Tables 1-5.

[0052] Table 1 Chemical composition and temperature of plating solution

[0053] Table 2 Air knife blowing parameters

[0054] Table 3 Two-stage cooling parameters

[0055] Table 4 Chromium-free passivation film parameters

[0056] Table 5 Acrylic resin coating parameters

[0057] The microstructure detection results of the zinc-aluminum-magnesium plating layer of each embodiment are shown in Tables 6 and 7. Figure 1The high-aluminum zinc-aluminum-magnesium plated alloy phase diagram of Example 1 is also shown, and the microstructure of the zinc-aluminum-magnesium plating layer includes an aluminum-rich phase (α-Al), a zinc-rich phase (η-Zn), a Mg2Si phase, a Zn-based solid solution containing Al and Mg, and a MgZn2 phase, wherein: the average grain size of the aluminum-rich phase is ≤20 μm; the zinc-rich phase is distributed in the form of isolated particles, and the average grain size is ≤10 μm; the Mg2Si phase is finely and diffusely distributed, and the average grain size is ≤5 μm; and the MgZn2 phase is diffusely precipitated in the form of fine particles at the grain boundaries, and the average grain size is ≤500 nm.

[0058] Table 6 Microstructure of zinc-aluminum-magnesium plating layer

[0059] Table 7 Proportion between microstructures in zinc-aluminum-magnesium plating layer

[0060] The final products of each example were tested for blackening resistance, using the laminated sheet wet heat method in the SH / T 0692-2000 standard, with a test time of 500 h, a humidity of 95% RH, and a temperature of 50°C. The total color difference change value ΔE of the front and back of the sample before and after the test was measured. At the same time, boiling water cooking tests were carried out, with boiling water cooking for 1 h, and the total color difference change value ΔE of the front and back of the sample before and after the test was measured. The comparative example was a commercially available high-aluminum zinc-aluminum-magnesium anti-fingerprint commercial material. The blackening results of each example and the comparative example are shown in Table 8.

[0061] Table 8 Test results for blackening resistance

[0062] The above examples are merely illustrative for the sake of clarity and are in no way limiting on the scope of the application. Other variations and modifications of the embodiments can occur to those skilled in the art upon reading the foregoing description. Such variations and modifications are intended to fall within the scope of the application. Accordingly, the application is not limited to that precisely as shown and described.

Claims

1. A method for suppressing blackening of a zinc-aluminum-magnesium coating by alloy phase optimization, characterized in that The method comprises the following steps: 1) the steel strip enters a zinc pot to be hot-dip plated in a plating solution; the plating solution comprises, in terms of mass percentage, Al 47-57%, Mg 1.8-2%, Si 1-2%, trace elements 0.02-0.15%, and the rest Zn and inevitable impurities; the trace elements are one or more of Zr, Ca and Cr; 2) after the steel strip leaves the plating solution, the steel strip is blown by a pulse pressure air knife, the pulse pressure is an alternating cycle of two pressure sections, the pressure of the first pressure section is 30-40 kPa, and the single duration is 1-3 s; the pressure of the second pressure section is 20-30 kPa, and the single duration is 1-3 s; then the steel strip is subjected to segmented cooling to obtain a zinc-aluminum-magnesium plated steel strip.

2. The method of inhibiting black tarnish of zinc-aluminum-magnesium coatings by alloy phase optimization according to claim 1, characterized in that, The segmented cooling is two-section cooling, the first section is air blowing cooling, the cooling rate is 80-100 ℃ / s, and the steel strip is cooled to 150-180 ℃; the second section is water cooling in a quenching tank, the cooling rate is 10-20 ℃ / s, and the steel strip is cooled to room temperature.

3. The method of inhibiting black tarnish of zinc-aluminum-magnesium coatings by alloy phase optimization according to claim 2, characterized in that, The temperature of the quenching tank is ≤45 ℃, and the pH value of the quenching tank is ≤9.

4. The method of suppressing blackening of zinc-aluminum-magnesium coating by alloy phase optimization according to claim 1, characterized in that, The microstructure of the zinc-aluminum-magnesium plated layer comprises an Al-rich phase, a Zn-rich phase, an Mg2Si phase, a Zn-based solid solution containing Al and Mg, and an MgZn2 phase; the average grain size of the Al-rich phase is ≤20 μm; the Zn-rich phase is distributed in the form of isolated particles, and the average grain size is ≤10 μm; the Mg2Si phase is finely and diffusely distributed, and the average grain size is ≤5 μm; the MgZn2 phase is diffusely precipitated in the form of fine particles at the grain boundaries, and the average grain size is ≤500 nm.

5. The method of suppressing blackening of zinc-aluminum-magnesium coating by alloy phase optimization according to claim 1, characterized in that, The microstructure of the zinc-aluminum-magnesium plated layer and the volume fraction of the microstructure comprise: the Al-rich phase 60-70%, the Zn-rich phase 20-30%, the Mg2Si phase 5-8%, the Zn-based solid solution containing Al and Mg 3-5%, and the MgZn2 phase ≤2%; the volume ratio of the Al-rich phase to the Zn-rich phase is 1.6-2.6; the volume ratio of the Mg2Si phase to the total volume of the Al-rich phase and the Zn-rich phase is 0.06-0.1; and the volume ratio of the MgZn2 phase to the Zn-based solid solution containing Al and Mg is ≤0.

4.

6. The method of suppressing blackening of zinc-aluminum-magnesium coating by alloy phase optimization according to claim 1, characterized in that, The trace elements are Zr, Ca and Cr, and the mass percentage of the trace elements in the plating solution is: Zr 0.013-0.018%, Ca 0.002-0.005%, and Cr 0.001-0.007%; and the temperature of the plating solution is 575-610 ℃.

7. The method of inhibiting black tarnish of zinc-aluminum-magnesium coatings by alloy phase optimization according to claim 1, characterized in that, The zinc-aluminum-magnesium plated steel strip is coated with a chromium-free passivation solution, dried, and a chromium-free passivation film is formed; the chromium-free passivation solution comprises the following raw materials: zirconium salt 2-5 g / L, titanium salt 1-3 g / L, fluoride 0.5-2 g / L, and the rest water; and the thickness of the chromium-free passivation film is 100-300 nm.

8. The method of inhibiting black tarnish of zinc-aluminum-magnesium coatings by alloy phase optimization according to claim 7, characterized in that, The zirconium salt is one or more of zirconium oxychloride, zirconium sulfate, zirconium carbonate, zirconium carbonate ammonium, sodium fluorozirconate, potassium fluorozirconate, ammonium fluorozirconate, zirconium nitrate; the titanium salt is one or more of titanium sulfate, fluorotitanic acid, potassium fluorotitanate, ammonium fluorotitanate, titanium trichloride, titanium sulfate, potassium titanate; the fluoride is one or more of sodium fluoride, ammonium fluoride, zinc fluoride, potassium fluorosilicate, sodium fluorosilicate; the steel plate temperature during drying is 90-130 DEG C, and the drying time is 1-3 s.

9. The method of inhibiting black tarnish of zinc-aluminum-magnesium coatings by alloy phase optimization according to claim 7, characterized in that, The surface of the chromium-free passivation film is coated with acrylic resin paint, and baked and cured to form an acrylic resin coating layer; the acrylic resin paint is composed of water-based acrylic resin, diacetone acrylamide, nano-silicon dioxide particles and water; the thickness of the acrylic resin coating layer is 5-10 microns.

10. The method of suppressing blackening of zinc-aluminum-magnesium coatings by alloy phase optimization according to claim 9, characterized in that The acrylic resin paint includes the following mass percentage of raw materials: 40-60% of water-based acrylic resin, 3-5% of diacetone acrylamide, 0.5-1% of nano-silicon dioxide particles, and the rest is water; the particle size of the nano-silicon dioxide particles is 50-100 nm; the steel plate temperature during baking and curing is 232-240 DEG C, and the baking and curing time is 10-20 s.

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