A multilayer composite structure zinc-nickel alloy plating film structure and a preparation process thereof

Abstract

The application relates to the technical field of metal surface treatment, and discloses a multilayer composite structure zinc-nickel alloy plating film structure and a preparation process. The application realizes significant technical progress through organic integration of seven steps of multistage pretreatment, electrodeposition, gradient heat treatment, trivalent chromium passivation, multistage sealing, hydrogen removal and quality detection. The multistage pretreatment completely removes surface defects of a steel base material, obtains a clean and activated surface, and improves the bonding strength of a plating layer. The electrodeposition forms a zinc-nickel alloy layer containing gamma phase and delta phase, and the zinc-nickel alloy layer has excellent corrosion resistance and mechanical properties. The gradient heat treatment eliminates internal stress and promotes uniform distribution of alloy phases. The trivalent chromium passivation replaces the hexavalent chromium process, meets environmental protection requirements, and can flexibly select various appearances. The multistage sealing treatment chemically bonds nanometer silicon dioxide to fill micropores to construct a gradient composite sealing system. The hydrogen removal treatment completely eliminates the risk of hydrogen embrittlement. Comprehensive quality detection realizes whole-process control.

Description

Technical Field

[0001] This invention relates to the field of metal surface treatment technology, and in particular to a multilayer composite zinc-nickel alloy coating structure and its preparation process. Background Technology

[0002] Currently, zinc-nickel alloy coatings, as a high-performance anti-corrosion coating, have been widely used in the automotive, military, and aerospace industries. Existing zinc-nickel alloy coating technologies typically employ a single electrodeposition process to form a zinc-nickel alloy layer on the surface of a steel substrate, followed by a simple chromate passivation treatment. However, with increasingly harsh industrial environments, especially for critical safety components such as automotive brake calipers, which operate under high temperatures and are subject to severe conditions such as mudslide impacts, extremely high requirements are placed on the corrosion resistance, adhesion, and impact resistance of the coating.

[0003] However, existing technologies for traditional zinc-nickel alloy coatings have the following drawbacks: First, the coating structure is simple, typically consisting of only a zinc-nickel alloy layer and a simple passivation film, lacking effective sealing layer protection. This results in the passivation film's micropores not being completely sealed, allowing corrosive media to easily penetrate into the substrate through these micropores. Second, the internal stress generated during electrodeposition is not effectively eliminated, making the coating prone to cracking or peeling during service. Third, the pretreatment process is simple, making it difficult to obtain a uniform and clean activated surface for substrates with rough surfaces and numerous casting defects such as sand holes and pores, such as high-carbon die-cast iron, thus affecting the quality of subsequent coatings. Fourth, the passivation process often uses hexavalent chromium passivation, posing environmental risks, and the passivation film has weak adhesion to the substrate. Fifth, there is a lack of systematic hydrogen removal treatment; hydrogen atoms penetrating into the steel substrate during electrodeposition can easily cause hydrogen embrittlement, reducing the mechanical properties and safety of components. Sixth, the quality inspection methods are limited, making it difficult to achieve comprehensive and precise control over coating thickness, adhesion, corrosion resistance, and appearance quality. Summary of the Invention

[0004] The purpose of this invention is to provide a multi-layer composite zinc-nickel alloy coating structure and preparation process, which solves the problems of existing zinc-nickel alloy coating structures being simple, having insufficient corrosion resistance, poor adhesion, being prone to hydrogen embrittlement, and being environmentally unfriendly.

[0005] To achieve the above objectives, in a first aspect, the present invention provides a process for preparing a multilayer composite zinc-nickel alloy coating structure, characterized by comprising the following steps: The steel substrate undergoes multi-stage pretreatment to remove surface oxide layers, grease, and casting defects, resulting in a clean and activated surface. A zinc-nickel alloy layer is electrodeposited on the surface of a pretreated steel substrate; Gradient heat treatment is applied to the zinc-nickel alloy layer to eliminate internal stress in the coating and promote uniform distribution of the alloy phase. Trivalent chromium passivation treatment is performed on the surface of the zinc-nickel alloy layer to form a dense passivation film; The trivalent chromium passivation film is subjected to multi-stage sealing treatment to form a sealing layer; Hydrogen embrittlement risk is eliminated by subjecting the multilayer composite coating after sealing treatment to hydrogen removal treatment; Quality inspection is performed on the finished multilayer composite coating.

[0006] The section on "performing multi-stage pretreatment on the steel substrate to remove surface oxide layers, grease, and casting defects to obtain a clean and activated surface" includes the following steps: Ultrasonic degreasing treatment is performed on the steel substrate to remove surface grease and organic contaminants; Electrolytic degreasing treatment is performed on the degreased steel substrate to remove deep oil stains and oxide scale; The steel substrate after electrolytic degreasing is subjected to pickling and activation treatment to remove the surface oxide layer and expose the fresh metal substrate; The pickled steel substrate is subjected to multi-stage countercurrent rinsing to remove residual acid and impurity ions from the surface.

[0007] The step of "electrodepositing a zinc-nickel alloy layer on the surface of a pretreated steel substrate" includes the following steps: Prepare a zinc-nickel alloy electroplating solution containing zinc salt, nickel salt, conductive salt, buffer, and composite additives, and perform electrodeposition by rack plating or barrel plating. The changes in ion concentration in the plating solution are monitored in real time using online detection methods, and an automatic replenishment system is used to maintain the ion concentration balance. After electrodeposition, the coating is rinsed in multiple stages of countercurrent to remove residual plating solution from the surface.

[0008] The step of "performing gradient heat treatment on the zinc-nickel alloy layer to eliminate internal stress in the coating and promote uniform distribution of the alloy phase" includes the following steps: The electrodeposited parts are placed in a heat treatment furnace and heated to the target temperature range at a set heating rate and held at that temperature. Slowly cool to room temperature at the set cooling rate to avoid cracking or peeling of the coating due to excessive temperature difference.

[0009] The step of "performing trivalent chromium passivation treatment on the surface of the zinc-nickel alloy layer to form a dense passivation film" includes the following steps: A trivalent chromium passivation solution containing trivalent chromium salt, complexing agent, oxidant and pH buffer is prepared, and the plated parts are immersed in the passivation solution to form a dense passivation film; Choose one of the following passivation methods based on the product's appearance requirements: silver-white passivation, blue-white passivation, colored passivation, or black passivation. After passivation, multi-stage countercurrent rinsing is performed to remove residual passivation solution from the surface.

[0010] The step of "multi-stage sealing treatment of trivalent chromium passivation film to form a sealing layer" includes the following steps: The passivated plated parts are immersed in an alcohol-water solution containing a silane coupling agent, so that the silane coupling agent forms a chemical bond on the surface of the passivation film. The plated part after the first sealing is immersed in an aqueous sealing agent containing nano-silica particles to fill the micropores of the passivation film and enhance the surface density. The plated parts after the second sealing are dried with hot air to completely cure the sealing layer.

[0011] In a second aspect, a multi-layer composite zinc-nickel alloy coating structure is prepared using the preparation process of the multi-layer composite zinc-nickel alloy coating structure described in the first aspect.

[0012] This invention discloses a fabrication process for a multi-layer composite zinc-nickel alloy coating structure. The invention achieves significant technological advancements through the organic integration of seven steps: multi-stage pretreatment, electrodeposition, gradient heat treatment, trivalent chromium passivation, multi-stage sealing, hydrogen removal, and quality inspection. The multi-stage pretreatment thoroughly removes the oxide layer, grease, and casting defects from the steel substrate surface, resulting in a clean and activated surface and improved coating adhesion strength. Electrodeposition forms a zinc-nickel alloy layer containing γ and δ phases, exhibiting excellent corrosion resistance and mechanical properties, while online monitoring ensures compositional stability. Gradient heat treatment eliminates internal stress and promotes uniform distribution of the alloy phases. This invention improves the density of the coating; trivalent chromium passivation replaces hexavalent chromium, meeting environmental protection requirements and improving corrosion resistance, while allowing for flexible selection of various appearance types; multi-level sealing treatment uses silane coupling agents to chemically bond nano-silica to fill micropores, constructing a gradient composite sealing system to form a synergistic barrier effect; hydrogen removal treatment completely eliminates the risk of hydrogen embrittlement, ensuring the mechanical properties of key safety components; comprehensive quality testing achieves full-process quality control; in summary, this invention constructs a structurally stable and high-performance multi-layer composite coating system, achieving a balance of high corrosion resistance, high adhesion, low hydrogen embrittlement risk, and environmental friendliness. Attached Figure Description

[0013] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below.

[0014] Figure 1 This is a flowchart of the preparation process of a multi-layer composite zinc-nickel alloy coating structure provided by the present invention.

[0015] Figure 2 This is a flowchart of a multi-stage pretreatment process for steel substrates to remove surface oxide layers, grease, and casting defects, thereby obtaining a clean and activated surface.

[0016] Figure 3This is a flowchart of the process of electrodepositing a zinc-nickel alloy layer on the surface of a pretreated steel substrate.

[0017] Figure 4 This is a flowchart of a process for performing gradient heat treatment on a zinc-nickel alloy layer to eliminate internal stress in the coating and promote uniform distribution of the alloy phase.

[0018] Figure 5 This is a flowchart of a process for performing trivalent chromium passivation treatment on the surface of a zinc-nickel alloy layer to form a dense passivation film.

[0019] Figure 6 This is a flowchart of a multi-stage sealing process for a trivalent chromium passivation film to form a sealing layer.

[0020] Figure 7 This is a flowchart of a process for hydrogen removal treatment of a multilayer composite coating that has undergone sealing treatment to eliminate the risk of hydrogen embrittlement.

[0021] Figure 8 This is a flowchart for quality inspection of finished multilayer composite coatings. Detailed Implementation

[0022] The embodiments of the present invention are described in detail below. Examples of the embodiments are shown in the accompanying drawings. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain the present invention, but should not be construed as limiting the present invention.

[0023] Please see Figures 1 to 8 In a first aspect, the present invention provides a process for preparing a multilayer composite zinc-nickel alloy coating structure, characterized by comprising the following steps: S1 performs multi-stage pretreatment on the steel substrate to remove the surface oxide layer, grease and casting defects, and obtain a clean and activated surface. S11 performs ultrasonic degreasing on steel substrates to remove surface grease and organic contaminants; S12 performs electrolytic degreasing treatment on degreased steel substrates to remove deep oil stains and oxide scale; S13 involves pickling and activating the steel substrate after electrolytic degreasing to remove the surface oxide layer and expose the fresh metal substrate. S14 performs multi-stage countercurrent rinsing on the pickled steel substrate to remove residual acid and impurity ions from the surface.

[0024] Specifically, the steel substrate is first placed in an ultrasonic degreasing tank. The cavitation effect of ultrasound causes the degreasing solution to vibrate at high frequency, peeling away grease and organic contaminants from the steel substrate surface and dispersing them in the degreasing solution, achieving thorough removal of surface grease. The degreased steel substrate is then transferred to an electrolytic degreasing tank, where the steel substrate acts as the cathode or anode. Under the influence of current, the electrolyte undergoes an electrolytic reaction, generating a large number of hydrogen or oxygen bubbles. The mechanical peeling action of these bubbles effectively removes deep-seated oil and oxide scale from the surface and pores of the steel substrate. Next, the electrolytically degreased steel substrate is immersed in an acid pickling and activation tank. The pickling solution chemically reacts with the oxide layer on the surface of the steel substrate, dissolving and removing the oxide layer to expose a chemically active, fresh metal substrate, providing a good bonding interface for subsequent electrodeposition. Finally, the pickled steel substrate is sent to a multi-stage countercurrent rinsing tank. Utilizing the countercurrent rinsing principle, clean water flows from the last stage to the first stage, and the steel substrate moves from the first stage to the last stage, achieving efficient removal of residual acid and impurity ions, and obtaining a clean and activated surface.

[0025] S2 involves electrodepositing a zinc-nickel alloy layer on the pretreated steel substrate surface; S21 is prepared with a zinc-nickel alloy electroplating solution containing zinc salt, nickel salt, conductive salt, buffer, and composite additives, and electrodeposition is carried out by rack plating or barrel plating. S22 monitors the changes in ion concentration in the plating solution in real time through online detection and uses an automatic replenishment system to maintain the ion concentration balance. After S23 electrodeposition is completed, the coating is subjected to multi-stage countercurrent rinsing to remove residual plating solution from the surface.

[0026] Specifically, a zinc-nickel alloy electroplating solution containing zinc salts, nickel salts, conductive salts, buffers, and composite additives is prepared in the electroplating tank. Zinc and nickel salts serve as the source of metal ions for the plating layer, conductive salts improve the conductivity of the plating solution, buffers stabilize the pH value, and composite additives improve the fineness and dispersion of the plating layer. The pretreated steel substrate is used as the cathode, and the zinc-nickel alloy anode or a soluble anode is used as the anode. The substrate is immersed in the electroplating solution using either rack plating or barrel plating methods. Rack plating is suitable for parts with complex shapes and large dimensions, while barrel plating is suitable for mass-produced parts such as small fasteners. Electrodeposition is carried out under controlled current density, plating solution temperature, and stirring rate, causing zinc-nickel alloy ions to be reduced and deposited on the surface of the steel substrate to form a plating layer. During electrodeposition, the concentration changes of zinc and nickel ions in the plating bath are monitored in real time using online detection methods, such as atomic absorption spectrophotometers or electrochemical sensors. The detection signals are fed back to an automatic replenishment system. When the ion concentration deviates from the set range, the automatic replenishment system precisely replenishes the corresponding metal salt solution to maintain ion concentration balance and ensure that the nickel content in the coating remains stable within the design range. This guarantees that the coating has a stable γ-phase and δ-phase composition and excellent corrosion resistance. After electrodeposition, the plated part is removed from the plating tank and sent to a multi-stage countercurrent rinsing tank. The multi-stage countercurrent rinsing thoroughly removes residual plating solution from the coating surface, preventing water stains or corrosion spots from forming on the coating surface, resulting in a clean zinc-nickel alloy coating.

[0027] S3 performs gradient heat treatment on the zinc-nickel alloy layer to eliminate internal stress in the coating and promote uniform distribution of the alloy phase. S31 places the electrodeposited part in a heat treatment furnace, raises it to the target temperature range at a set heating rate, and holds it at that temperature. S32 cools slowly to room temperature at a set cooling rate to avoid cracking or peeling of the coating due to excessive temperature difference.

[0028] Specifically, the electrodeposited workpiece is placed stably on the conveyor belt of the mesh belt furnace. The furnace heating system is then activated, gradually raising the temperature from room temperature to the target temperature range at a set rate. This rate is optimized to ensure a uniform temperature rise within the coating while avoiding excessive thermal stress caused by a large difference in thermal expansion between the coating and the substrate. Once the target temperature range is reached, the furnace enters a holding phase, where the workpiece remains within this temperature range for a set duration. This allows the γ-phase and δ-phase zinc-nickel alloy within the coating to diffuse and distribute evenly. Simultaneously, the internal stress generated during electrodeposition is fully released and eliminated at this temperature through atomic diffusion and dislocation movement, significantly improving the density of the coating and its bonding strength with the substrate. After the heat preservation is completed, the mesh belt furnace is slowly cooled to room temperature at a set cooling rate. This cooling rate is optimized so that the coating and the substrate shrink synchronously, avoiding cracking of the coating due to tensile stress caused by excessive temperature difference or rapid cooling, or peeling of the coating due to rapid shrinkage of the substrate. In the end, a zinc-nickel alloy coating with low internal stress, uniform alloy phase distribution and dense structure is obtained.

[0029] S4 undergoes trivalent chromium passivation treatment on the surface of the zinc-nickel alloy layer to form a dense passivation film. S41 is configured with a trivalent chromium passivation solution containing trivalent chromium salts, complexing agents, oxidants and pH buffers. The plated parts are immersed in the passivation solution to form a dense passivation film. S42 can choose one of the following passivation methods based on the product's appearance requirements: silver-white passivation, blue-white passivation, colored passivation, or black passivation. After S43 passivation treatment, multi-stage countercurrent rinsing is performed to remove residual passivation solution from the surface.

[0030] Specifically, a trivalent chromium passivation solution containing trivalent chromium salts, complexing agents, oxidants, and pH buffers is prepared in the passivation tank. Trivalent chromium salts are the main film-forming substance of the passivation film; the complexing agent stabilizes trivalent chromium ions and controls the film formation rate; the oxidant promotes the distribution of chromium oxidation states in the passivation film; and the pH buffer maintains a stable pH value in the passivation solution. The plated parts, after gradient heat treatment, are immersed in the passivation solution. Under controlled passivation temperature and time conditions, the zinc and nickel elements on the surface of the zinc-nickel alloy layer react chemically with the trivalent chromium ions in the passivation solution, depositing a dense trivalent chromium passivation film on the coating surface. This passivation film effectively isolates the corrosive medium from contact with the zinc-nickel alloy layer, significantly improving the corrosion resistance of the coating. Based on the downstream customers' product appearance requirements, the passivation solution formulation can be adjusted by modifying the type and concentration of the complexing agent, the amount of oxidant added, and the pH buffer ratio to select one of the following passivation appearance types: silver-white passivation, blue-white passivation, colored passivation, or black passivation. Silver-white passivation presents a silver-white metallic luster, blue-white passivation presents a slightly bluish silver-white, colored passivation presents iridescent or golden-yellow colors, and black passivation presents a uniform black. After passivation, the plated parts are removed from the passivation tank and sent to a multi-stage countercurrent rinsing tank. Multi-stage countercurrent rinsing thoroughly removes residual passivation solution from the surface of the plated parts, preventing flow marks or discoloration on the passivation film surface, resulting in a clean and uniform trivalent chromium passivation film.

[0031] S5 performs multi-stage sealing treatment on the trivalent chromium passivation film to form a sealing layer; S51 immerses the passivated plated part in an alcohol-water solution containing a silane coupling agent, so that the silane coupling agent forms a chemical bond on the surface of the passivation film; S52 immerses the plated part after the first sealing into an aqueous sealant containing nano-silica particles to fill the micropores of the passivation film and enhance the surface density. S53 performs hot air drying on the plated parts after the second sealing process to completely cure the sealing layer.

[0032] Specifically, the passivated workpiece is first immersed in a first sealing tank, which contains an alcohol-water solution containing a silane coupling agent. One end of the silane coupling agent molecule contains a silane group that can condense with the hydroxyl groups on the surface of the trivalent chromium passivation film, and the other end contains an organic functional group. When the workpiece is immersed in the first sealing solution, the silane coupling agent undergoes a hydrolysis-condensation reaction on the passivation film surface, forming a strong chemical bond, thereby constructing an organosilane molecular layer on the passivation film surface. This molecular layer is hydrophobic and chemically stable, effectively blocking the penetration of corrosive media. Subsequently, the workpiece after the first sealing is transferred to a second sealing tank, which contains an aqueous sealing agent containing nano-silica particles. The particle size of the nano-silica particles is optimized to fill the micropores on the surface of the first sealing layer, as well as the micropores and defects present in the passivation film itself. During the filling process, the nanoparticles form a dense nanoparticle surface layer through physical accumulation and surface adsorption, further enhancing the density and barrier effect of the sealing layer. After the second sealing is completed, the plated part is sent into a hot air drying device. Under the process conditions of controlling the drying temperature and drying time, the silane coupling agent of the first sealing layer is fully cross-linked and cured, while the nano-silica particles of the second sealing layer are firmly bonded to the organic matrix to form a stable, dense and complete sealing layer. This sealing layer forms a chemical bond with the trivalent chromium passivation film below and a physical intercalation with the nanoparticle surface layer above, thus constructing a composite sealing system with a gradient structure.

[0033] S6 performs hydrogen removal treatment on the multilayer composite coating after sealing to eliminate the risk of hydrogen embrittlement. S61 places the plated parts, which have undergone sealing treatment, in a hydrogen-removing oven with circulating air and keeps them at a set temperature range, so that the hydrogen atoms adsorbed in the coating and the steel substrate can diffuse out. During the hydrogen removal process of S62, a temperature recorder records and prints the temperature curve in real time to ensure that the hydrogen removal process meets the requirements.

[0034] Specifically, the plated parts, after being sealed, are placed stably on the racks or trays of the hydrogen removal oven. The oven door is closed, and the heating and circulating air systems are activated. The circulating air inside the oven ensures uniform temperature distribution, preventing localized overheating or underheating. The oven heats up to the target hydrogen removal temperature range at a set rate. This temperature range is optimized to ensure sufficient diffusion driving force for hydrogen atoms in the steel substrate and coating, while preventing aging of the sealing layer or phase transformation of the zinc-nickel alloy layer due to excessive temperature. Once the set temperature range is reached, the oven enters a holding phase, where the plated parts remain within this temperature range for a set duration. Hydrogen atoms adsorbed or diffused into the steel substrate and coating during the electrodeposition process gain sufficient diffusion activation energy, desorbing from trap sites such as lattice interstices, dislocations, and grain boundaries, and diffusing out to the coating surface and the interior of the steel substrate. This completely eliminates the risk of hydrogen embrittlement, ensuring the mechanical properties and service safety of the components. During the hydrogen removal process, the temperature recorder collects temperature data in real time through thermocouples placed in the oven and prints out the temperature curve in real time. Operators can monitor the actual execution of the hydrogen removal process through the temperature curve to ensure that the hydrogen removal temperature always meets the process requirements. If the temperature deviates from the set range, the system will automatically alarm and adjust the heating power.

[0035] S7 performs quality inspection on the finished multilayer composite coating.

[0036] S71 uses non-destructive thickness measurement methods to detect the thickness of the zinc-nickel alloy layer, trivalent chromium passivation film, and sealing layer; S72 uses adhesion testing equipment and salt spray corrosion testing equipment to test the coating adhesion and corrosion resistance, respectively; S73 uses visual inspection and color difference detection methods to inspect the appearance quality of the coating.

[0037] Specifically, the finished multilayer composite coating is first placed in a non-destructive thickness measurement station. Using non-destructive methods such as X-ray fluorescence thickness gauges or eddy current thickness gauges, the thickness of the zinc-nickel alloy layer, the trivalent chromium passivation film, and the sealing layer are measured point-by-point. The detection probe scans along a set path on the surface of the coated part to obtain spatial distribution data of each layer's thickness, ensuring that the thickness of each layer meets the design tolerance range. Subsequently, the coated part is transferred to an adhesion testing station. Using a friction coefficient tester or a cross-cut adhesion tester, the bonding strength between the coating and the steel substrate is quantitatively or qualitatively tested. By measuring the peel force or observing the detachment in the cross-cut area, it is determined whether the coating adhesion meets the usage requirements. Simultaneously, samples from the same batch are placed in a salt spray corrosion test equipment and continuously sprayed according to the neutral salt spray test standard. The appearance time of white rust and red rust on the sample surface is observed and recorded periodically to evaluate the corrosion resistance of the multilayer composite coating in a simulated corrosive environment. Finally, the plated parts undergo visual quality inspection. Inspectors visually examine the plating surface for defects such as bubbles, pinholes, cracks, flow marks, and color differences. A color difference detection device is used to quantitatively measure the plating color, and the measured values ​​are compared with a standard color chart or the color difference tolerance standard provided by the customer to determine whether the appearance quality is acceptable. Based on the above inspection results, the plated parts are graded according to quality. Qualified products proceed to the packaging and warehousing process, while unqualified products are returned to the corresponding upstream process for rework or disposed of as scrap, depending on the type of defect.

[0038] In a second aspect, a multi-layer composite zinc-nickel alloy coating structure is prepared using the preparation process of the multi-layer composite zinc-nickel alloy coating structure described in the first aspect.

[0039] The above-disclosed embodiments are merely one or more preferred embodiments of this application and should not be construed as limiting the scope of this application. Those skilled in the art can understand that all or part of the processes for implementing the above embodiments and equivalent changes made in accordance with the claims of this application still fall within the scope of this application.

Claims

1. A fabrication process for a multilayer composite zinc-nickel alloy coating structure, characterized in that, Includes the following steps The steel substrate undergoes multi-stage pretreatment to remove surface oxide layers, grease, and casting defects, resulting in a clean and activated surface. A zinc-nickel alloy layer is electrodeposited on the surface of a pretreated steel substrate; Gradient heat treatment is applied to the zinc-nickel alloy layer to eliminate internal stress in the coating and promote uniform distribution of the alloy phase. Trivalent chromium passivation treatment is performed on the surface of the zinc-nickel alloy layer to form a dense passivation film; The trivalent chromium passivation film is subjected to multi-stage sealing treatment to form a sealing layer; Hydrogen embrittlement risk is eliminated by subjecting the multilayer composite coating after sealing treatment to hydrogen removal treatment; Quality inspection is performed on the finished multilayer composite coating.

2. The preparation process of the multilayer composite zinc-nickel alloy coating structure as described in claim 1, characterized in that, The section on "multi-stage pretreatment of steel substrate to remove surface oxide layers, grease, and casting defects to obtain a clean and activated surface" includes the following steps: Ultrasonic degreasing treatment is performed on the steel substrate to remove surface grease and organic contaminants; Electrolytic degreasing treatment is performed on the degreased steel substrate to remove deep oil stains and oxide scale; The steel substrate after electrolytic degreasing is subjected to pickling and activation treatment to remove the surface oxide layer and expose the fresh metal substrate; The pickled steel substrate is subjected to multi-stage countercurrent rinsing to remove residual acid and impurity ions from the surface.

3. The preparation process of the multilayer composite zinc-nickel alloy coating structure as described in claim 1, characterized in that, The process of "electrodepositing a zinc-nickel alloy layer on a pretreated steel substrate" includes the following steps: Prepare a zinc-nickel alloy electroplating solution containing zinc salt, nickel salt, conductive salt, buffer, and composite additives, and perform electrodeposition by rack plating or barrel plating. The changes in ion concentration in the plating solution are monitored in real time using online detection methods, and an automatic replenishment system is used to maintain the ion concentration balance. After electrodeposition, the coating is rinsed in multiple stages of countercurrent to remove residual plating solution from the surface.

4. The preparation process of the multilayer composite zinc-nickel alloy coating structure as described in claim 1, characterized in that, The step of "gradient heat treatment of zinc-nickel alloy layers to eliminate internal stress in the coating and promote uniform distribution of alloy phases" includes the following steps: The electrodeposited parts are placed in a heat treatment furnace and heated to the target temperature range at a set heating rate and held at that temperature. Slowly cool to room temperature at the set cooling rate to avoid cracking or peeling of the coating due to excessive temperature difference.

5. The preparation process of the multilayer composite zinc-nickel alloy coating structure as described in claim 1, characterized in that, The process of "performing trivalent chromium passivation treatment on the surface of a zinc-nickel alloy layer to form a dense passivation film" includes the following steps: A trivalent chromium passivation solution containing trivalent chromium salt, complexing agent, oxidant and pH buffer is prepared, and the plated parts are immersed in the passivation solution to form a dense passivation film; Choose one of the following passivation methods based on the product's appearance requirements: silver-white passivation, blue-white passivation, colored passivation, or black passivation. After passivation, multi-stage countercurrent rinsing is performed to remove residual passivation solution from the surface.

6. The preparation process of the multilayer composite zinc-nickel alloy coating structure as described in claim 1, characterized in that, The process of "multi-stage sealing treatment of trivalent chromium passivation film to form a sealing layer" includes the following steps: The passivated plated parts are immersed in an alcohol-water solution containing a silane coupling agent, so that the silane coupling agent forms a chemical bond on the surface of the passivation film. The plated part after the first sealing is immersed in an aqueous sealing agent containing nano-silica particles to fill the micropores of the passivation film and enhance the surface density. The plated parts after the second sealing are dried with hot air to completely cure the sealing layer.

7. A multi-layer composite zinc-nickel alloy coating structure, characterized in that, The multilayer composite zinc-nickel alloy coating structure is prepared using the preparation process described in any one of claims 1-6.

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