Preparation method of gradient coating for enhancing corrosion resistance of magnesium-based alloy through carbon embedding

Abstract

The invention belongs to the technical field of metal material surface modification, and discloses a preparation method of a gradient coating for enhancing corrosion resistance of a magnesium-based alloy by carbon embedding, which comprises the following specific steps: step 1, pretreating a magnesium-based alloy matrix, and sequentially carrying out mechanical polishing, oil removal, acid pickling and drying treatment to obtain a clean magnesium-based alloy matrix with an activated surface; the carbon content of the prepared gradient coating is gradually increased from a magnesium-based alloy matrix to a surface layer, so that the thermal expansion coefficient difference and internal stress of the coating and the matrix are effectively relieved, cracking and falling are avoided, and the bonding stability is improved; the carbon-based coating has excellent chemical stability and barrier property, can efficiently isolate corrosive media, has better protection effect than the prior art, solves the core problem of poor corrosion resistance of the magnesium-based alloy, adopts 300 DEG C low-temperature treatment, avoids the influence of high temperature on matrix performance and heat storage phase change related application adaptability, does not use heavy metals and discharge harmful substances in the whole process, and is suitable for industrial production. The carbon source is easy to obtain, the cost is low, and environmental protection and economical efficiency are both considered.

Description

Technical Field

[0001] This invention belongs to the field of surface modification technology of metallic materials, specifically a method for preparing a gradient coating that enhances the corrosion resistance of magnesium-based alloys by carbon embedding. Background Technology

[0002] Carbon encapsulation is an important composite modification process in the field of materials science. It refers to the material preparation technology that uses physical or chemical means to encapsulate, embed, or load active functional materials as core components into the pores, interlayers, or surfaces of carbon-based carriers to form core-shell, embedded, or loaded composite structures.

[0003] Magnesium and magnesium alloys have broad application prospects in aerospace, automotive, biomedical, and electronic products due to their low density, high specific strength, good biocompatibility, and biodegradability. However, magnesium's extremely reactive chemical properties and very low standard electrode potential result in poor corrosion resistance, which severely restricts its widespread application. Existing surface treatment technologies for improving the corrosion resistance of magnesium alloys mainly include micro-arc oxidation, chemical conversion coatings, electroplating / chemical plating, vapor deposition, and laser surface treatment. However, these technologies have certain limitations: micro-arc oxidation coatings have many micropores and cracks, making them prone to failure in long-term corrosive environments; chemical conversion coatings have low mechanical strength and limited protective lifespan; electroplating / chemical plating processes have poor adhesion between the coating and the magnesium substrate, and electroplating processes generate wastewater containing heavy metals, causing environmental pollution; vapor deposition and laser surface treatment suffer from high equipment costs, complex processes, and difficulty in large-area processing. Therefore, there is an urgent need to develop a low-temperature, high-efficiency, environmentally friendly, and highly protective carbon-based strengthening surface modification technology suitable for the characteristics of magnesium alloys to meet the application needs of thermal storage phase change materials and other related applications. Summary of the Invention

[0004] The purpose of this invention is to provide a method for preparing a gradient coating that enhances the corrosion resistance of magnesium-based alloys with carbon embedding, so as to solve the problem of poor corrosion resistance of magnesium-based alloys and make them better suited for the application scenarios of thermal storage phase change materials.

[0005] To achieve the above objectives, the present invention provides the following technical solution: a method for preparing a gradient coating that enhances the corrosion resistance of a magnesium-based alloy by carbon embedding, the specific steps of which are as follows: Step 1: Pretreatment of magnesium-based alloy matrix, which involves mechanical grinding, degreasing, pickling and drying in sequence to obtain a clean and surface-activated magnesium-based alloy matrix; Step 2: Gradient configuration of carbon-based precursors. Three carbon-based precursor slurries with high, medium and low carbon contents were prepared respectively. The carbon source mass fraction in the high carbon content precursor was 30%~50%, the carbon source mass fraction in the medium carbon content precursor was 15%~30%, and the carbon source mass fraction in the low carbon content precursor was 5%~15%. Step 3: Gradient embedding and laying. The magnesium-based alloy matrix treated in Step 1 is placed in an embedding container. First, a low-carbon content carbon-based precursor slurry with a thickness of 50 μm is laid on the surface of the matrix. Then, medium-carbon content and high-carbon content carbon-based precursor slurries are laid in sequence, with each layer having a thickness of 50 μm, to form a gradient embedding system. Step 4: Low-temperature embedding treatment. Place the container containing the gradient embedding system in a heating furnace and raise the temperature to 300°C at a rate of 5°C / min under inert gas protection. Hold the temperature for 2-4 hours to complete the carbon embedding process. Step 5: Post-processing. After the furnace temperature drops to room temperature, remove the magnesium-based alloy substrate, remove excess precursor residue from the surface, and perform cleaning and drying processes in sequence to obtain a magnesium-based alloy workpiece with a gradient carbon embedded coating on the surface. Step Six: Performance Testing. The prepared gradient coating is tested for corrosion resistance, adhesion, and microhardness to ensure that the coating performance meets the application requirements.

[0006] Preferably, the magnesium-based alloy matrix in step one includes one of the AZ31, AZ91, and WE43 series magnesium alloys.

[0007] Preferably, in step one, the mechanical polishing is performed using 400-grit, 800-grit, and 1200-grit sandpaper sequentially until the surface roughness Ra of the substrate is ≤0.8μm; the degreasing is performed using an alkaline degreasing agent, which is soaked at 60-80℃ for 10-20 minutes; the alkaline degreasing agent is composed of 50-80g / L sodium hydroxide, 30-50g / L sodium carbonate, 20-40g / L trisodium phosphate, and 5-10g / L surfactant; the pickling is performed using a 5%-10% hydrochloric acid solution, which is soaked at room temperature for 3-5 minutes; and the drying process is performed in an oven at 80-100℃ for 1-2 hours.

[0008] Preferably, the carbon-based precursor slurry in step two further includes a binder, a dispersant, and a solvent. The binder is one of phenolic resin and epoxy resin, with a mass fraction of 5% to 10%. The dispersant is one of polyethylene glycol and sodium dodecylbenzenesulfonate, with a mass fraction of 1% to 3%. The solvent is a mixed solution of ethanol and deionized water, with a volume ratio of ethanol to deionized water of 1:1 to 3:1.

[0009] Preferably, the carbon source in step two is selected from one or more of graphite powder, activated carbon, carbon nanotubes, and graphene, and the carbon source particle size is 50~200nm.

[0010] Preferably, the laying process in step three is carried out by spraying or scraping to ensure that the precursor slurry is evenly covered in each layer and that there are no air bubbles or cracks between the layers.

[0011] Preferably, the inert gas in step four is argon or nitrogen, and the gas flow rate is 0.5~1.5L / min; the precursor is stirred every 30 minutes during the heat preservation process to ensure uniform distribution of the carbon source.

[0012] Preferably, the cleaning in step five is ultrasonic cleaning, the cleaning medium is ethanol, and the cleaning time is 10-15 minutes; the drying treatment is carried out in a vacuum oven at 60-80℃ for 1-2 hours with a vacuum degree of 0.05-0.1MPa.

[0013] Preferably, the total thickness of the gradient carbon embedding coating in step five is 150 μm, and the interface between the coating and the magnesium-based alloy substrate is a composite of metallurgical bonding and mechanical bonding.

[0014] Preferably, the corrosion resistance test in step six is ​​conducted using a neutral salt spray test, with test conditions of 5% NaCl solution, temperature of 35℃, atomization pressure of 0.07~0.1MPa, and test time of not less than 72 hours; the bonding strength test is conducted using the cross-cut test with a cross-cut spacing of 1mm; and the microhardness test is conducted using a Vickers hardness tester with a load of 50g and a holding time of 10 seconds.

[0015] The beneficial effects of this invention are as follows: The carbon content of the prepared gradient coating gradually increases from the magnesium alloy substrate to the surface, effectively alleviating the difference in thermal expansion coefficients and internal stress between the coating and the substrate, preventing cracking and peeling, and improving bonding stability. The excellent chemical stability and barrier properties of the carbon-based coating can effectively isolate corrosive media, and the protective effect is superior to existing technologies. It solves the core problem of poor corrosion resistance of magnesium alloys. The process adopts a low-temperature treatment at 300℃ to avoid the impact of high temperature on the substrate performance and the adaptability of heat storage phase change related applications. There is no use of heavy metals or emission of harmful substances throughout the process, and the carbon source is readily available and inexpensive, taking into account both environmental protection and economy. Attached Figure Description

[0016] Figure 1 This is a process flow diagram of the preparation method of the present invention. Detailed Implementation

[0017] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0018] like Figure 1 As shown in the figure, this invention provides a method for preparing a gradient coating that enhances the corrosion resistance of a magnesium-based alloy by carbon embedding. The specific steps are as follows: Step 1: Pretreatment of magnesium-based alloy matrix, which involves mechanical grinding, degreasing, pickling and drying in sequence to obtain a clean and surface-activated magnesium-based alloy matrix; Step 2: Gradient configuration of carbon-based precursors. Three carbon-based precursor slurries with high, medium and low carbon contents were prepared respectively. The carbon source mass fraction in the high carbon content precursor was 30%~50%, the carbon source mass fraction in the medium carbon content precursor was 15%~30%, and the carbon source mass fraction in the low carbon content precursor was 5%~15%. Step 3: Gradient embedding and laying. The magnesium-based alloy matrix treated in Step 1 is placed in an embedding container. First, a low-carbon content carbon-based precursor slurry with a thickness of 50 μm is laid on the surface of the matrix. Then, medium-carbon content and high-carbon content carbon-based precursor slurries are laid in sequence, with each layer having a thickness of 50 μm, to form a gradient embedding system. Step 4: Low-temperature embedding treatment. Place the container containing the gradient embedding system in a heating furnace and raise the temperature to 300°C at a rate of 5°C / min under inert gas protection. Hold the temperature for 2-4 hours to complete the carbon embedding process. Step 5: Post-processing. After the furnace temperature drops to room temperature, remove the magnesium-based alloy substrate, remove excess precursor residue from the surface, and perform cleaning and drying processes in sequence to obtain a magnesium-based alloy workpiece with a gradient carbon embedded coating on the surface. Step Six: Performance Testing. The prepared gradient coating is tested for corrosion resistance, adhesion, and microhardness to ensure that the coating performance meets the application requirements.

[0019] First, the magnesium-based alloy substrate undergoes pretreatment including mechanical grinding, degreasing, pickling, and drying to activate the surface. Then, carbon-based precursor slurries with high, medium, and low carbon contents are prepared. These slurries are then gradient-laid onto the substrate surface in the order of low carbon, medium carbon, and high carbon to form an embedding system. Next, under inert gas protection, the temperature is increased to 300℃ at a rate of 5℃ / min and held for 2-4 hours to complete the low-temperature embedding. After cooling, the coating undergoes cleaning and drying. Finally, the coating's corrosion resistance, adhesion, and microhardness are tested, completing the preparation of the gradient coating. Simultaneously, the gradient structure reduces the internal stress between the coating and the substrate, improving bonding stability and preventing cracking and detachment. The carbon-based coating blocks corrosive media, significantly enhancing corrosion resistance. The low-temperature process does not damage the substrate properties and is simple, environmentally friendly, and produces no pollutants, with low carbon source costs.

[0020] In step one, the magnesium-based alloy matrix includes one of the AZ31, AZ91, and WE43 series magnesium alloys.

[0021] Magnesium alloys of the AZ31, AZ91, and WE43 series are highly adaptable and possess excellent specific strength and biocompatibility. When combined with gradient coatings, they can maximize the material's potential. At the same time, these alloys have mature and readily available processes, which can reduce the difficulty of industrialization and meet the corrosion resistance modification requirements of magnesium-based alloy components in different scenarios.

[0022] In step one, mechanical polishing is performed using 400-grit, 800-grit, and 1200-grit sandpaper sequentially until the surface roughness Ra of the substrate is ≤0.8μm. Degreasing is done using an alkaline degreasing agent, which is soaked at 60-80℃ for 10-20 minutes. The alkaline degreasing agent consists of 50-80g / L sodium hydroxide, 30-50g / L sodium carbonate, 20-40g / L trisodium phosphate, and 5-10g / L surfactant. Pickling is done using a 5%-10% dilute hydrochloric acid solution, soaked at room temperature for 3-5 minutes. Drying is performed in an oven at 80-100℃ for 1-2 hours.

[0023] Gradual sanding with multi-grit sandpaper ensures precise control of the substrate surface roughness to Ra≤0.8μm, removing impurities and oxide scale while creating a uniformly rough surface, providing a good foundation for coating adhesion. A scientifically formulated alkaline degreasing agent, through high-temperature immersion, efficiently removes oil stains, with each component synergistically enhancing the degreasing effect. Mild hydrochloric acid pickling quickly removes residual oxide layers and slightly roughens the surface, preventing excessive corrosion. Drying in an 80~100℃ oven thoroughly removes moisture, preventing it from affecting coating adhesion. This comprehensive pretreatment process ensures a clean and activated substrate surface, significantly improving the adhesion between the subsequent coating and the substrate.

[0024] In step two, the carbon-based precursor slurry also includes a binder, a dispersant, and a solvent. The binder is either phenolic resin or epoxy resin, with a mass fraction of 5% to 10%. The dispersant is either polyethylene glycol or sodium dodecylbenzenesulfonate, with a mass fraction of 1% to 3%. The solvent is a mixed solution of ethanol and deionized water, with a volume ratio of ethanol to deionized water of 1:1 to 3:1.

[0025] Phenolic resin and epoxy resin, as binders, enhance the moldability of the slurry and the adhesion of the coating; polyethylene glycol and sodium dodecylbenzene sulfonate can effectively prevent carbon source agglomeration and ensure uniform dispersion of the slurry; the mixed solvent of ethanol and deionized water can adapt to each component and promote dissolution and dispersion; the scientific ratio of each component and their synergistic effect make the slurry stable and coatable, laying the foundation for the uniformity and density of the gradient coating, while improving the adhesion between the coating and the substrate, ensuring the corrosion resistance and mechanical properties of the final coating.

[0026] In step two, the carbon source is selected from one or more of graphite powder, activated carbon, carbon nanotubes, and graphene, and the carbon source particle size is 50~200nm.

[0027] Carbon sources such as graphite powder and activated carbon possess excellent chemical stability and barrier properties, which can effectively improve the corrosion resistance of coatings. Multiple options are available to suit different application scenarios. The particle size design of 50~200nm results in a large specific surface area and high reactivity of the carbon source, making it easy to disperse evenly in the slurry and form a dense coating. Carbon nanotubes and graphene can also enhance the mechanical properties of the coating and work synergistically with other components to ensure the structural integrity of the gradient coating and strengthen its protective and mechanical effects.

[0028] In step three, the laying process uses spraying or scraping to ensure that the precursor slurry is evenly covered in each layer and that there are no air bubbles or cracks between layers.

[0029] Spraying or scraping are mature and easy-to-operate coating methods that are suitable for industrial production needs. They can precisely control the thickness of each layer to 50μm, ensuring the uniformity of the gradient structure. Both methods can achieve full coverage of the substrate by the precursor slurry, avoiding missed coatings. At the same time, they can effectively expel interlayer air, reduce defects, and ensure tight interlayer bonding without bubbles or cracks, making the gradient coating structure dense. This reduces internal stress and blocks the penetration of corrosive media, providing a guarantee for the coating's excellent corrosion resistance and structural stability.

[0030] In step four, the inert gas is argon or nitrogen, and the gas flow rate is 0.5~1.5L / min. During the heat preservation process, the precursor is stirred every 30 minutes to ensure uniform distribution of carbon source.

[0031] Argon and nitrogen are chemically stable and can be used as protective gases to prevent oxidation of magnesium-based alloys at high temperatures, ensuring a clean environment for coating preparation. The flow rate design of 0.5~1.5L / min effectively isolates air and does not waste gas. Stirring the precursor every 30 minutes during heat preservation can prevent carbon source sedimentation and agglomeration, ensuring its uniform distribution in the coating. This makes the composition and properties of each area of ​​the gradient coating consistent, the structure more compact, and thus improves the adhesion between the coating and the substrate, as well as the overall corrosion resistance and mechanical stability, ensuring the modification effect.

[0032] In step five, ultrasonic cleaning is used, with ethanol as the cleaning medium, and the cleaning time is 10-15 minutes. The drying process involves keeping the product in a vacuum oven at 60-80℃ for 1-2 hours, with a vacuum degree of 0.05-0.1MPa.

[0033] Ultrasonic cleaning combined with ethanol medium can efficiently remove residual precursors and impurities from the surface, while being gentle and non-damaging to the coating. The 10-15 minute cleaning time ensures thorough cleaning. Drying in a 60-80℃ vacuum oven with a vacuum level of 0.05-0.1MPa can quickly remove residual moisture and ethanol, preventing coating oxidation or bubble formation. The entire post-treatment process can ensure the cleanliness of the coating surface and maintain the integrity of the gradient structure, further improving the coating density.

[0034] In step five, the total thickness of the gradient carbon embedding coating is 150 μm, and the interface between the coating and the magnesium-based alloy substrate is a composite of metallurgical and mechanical bonding.

[0035] The total thickness of 150μm forms an effective corrosion barrier layer to resist media penetration without causing excessive internal stress and cracking due to excessive thickness. The composite interface of metallurgical and mechanical bonding significantly improves the bonding strength between the coating and the substrate, preventing peeling during use. The synergy between composite bonding and appropriate thickness gives the coating both good structural stability and protective capabilities, ensuring the long-term corrosion resistance of the gradient coating and enhancing its mechanical reliability, fully meeting the usage requirements of magnesium-based alloy components in multiple scenarios.

[0036] In step six, the corrosion resistance test is conducted using a neutral salt spray test. The test conditions are 5% NaCl solution, temperature 35℃, atomization pressure 0.07~0.1MPa, and test time not less than 72 hours. The bonding strength test is conducted using the cross-cut test with a cross-cut spacing of 1mm. The microhardness test is conducted using a Vickers hardness tester with a load of 50g and a holding time of 10 seconds.

[0037] The neutral salt spray test conditions closely resemble actual corrosive environments. Parameters such as 5% NaCl solution and 35℃ can accurately assess the coating's corrosion resistance, and a duration of over 72 hours can effectively verify its long-term protective performance. The cross-cut test is simple to operate and provides intuitive results; a 1mm spacing can scientifically assess the bonding strength between the coating and the substrate. The Vickers hardness tester detects the characteristics of the coating; a 50g load and a 10-second holding time can accurately obtain microhardness data. These three testing methods are mature, reliable, and highly targeted, covering the core performance indicators of the coating. They can comprehensively verify the stability of the preparation process, ensuring that the corrosion resistance, adhesion, and mechanical properties of the gradient coating meet the usage requirements, providing strong assurance for product quality.

[0038] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus.

[0039] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a gradient coating that enhances the corrosion resistance of a magnesium-based alloy by carbon embedding, characterized in that, The specific steps are as follows: Step 1: Pretreatment of magnesium-based alloy matrix, which involves mechanical grinding, degreasing, pickling and drying in sequence to obtain a clean and surface-activated magnesium-based alloy matrix; Step 2: Gradient configuration of carbon-based precursors. Three carbon-based precursor slurries with high, medium and low carbon contents were prepared respectively. The carbon source mass fraction in the high carbon content precursor was 30%~50%, the carbon source mass fraction in the medium carbon content precursor was 15%~30%, and the carbon source mass fraction in the low carbon content precursor was 5%~15%. Step 3: Gradient embedding and laying. The magnesium-based alloy matrix treated in Step 1 is placed in an embedding container. First, a low-carbon content carbon-based precursor slurry with a thickness of 50 μm is laid on the surface of the matrix. Then, medium-carbon content and high-carbon content carbon-based precursor slurries are laid in sequence, with each layer having a thickness of 50 μm, to form a gradient embedding system. Step 4: Low-temperature embedding treatment. Place the container containing the gradient embedding system in a heating furnace and raise the temperature to 300°C at a rate of 5°C / min under inert gas protection. Hold the temperature for 2-4 hours to complete the carbon embedding process. Step 5: Post-processing. After the furnace temperature drops to room temperature, remove the magnesium-based alloy substrate, remove excess precursor residue from the surface, and perform cleaning and drying processes in sequence to obtain a magnesium-based alloy workpiece with a gradient carbon embedded coating on the surface. Step Six: Performance Testing. The prepared gradient coating is tested for corrosion resistance, adhesion, and microhardness to ensure that the coating performance meets the application requirements.

2. The method for preparing a gradient coating to enhance the corrosion resistance of a carbon-embedded magnesium-based alloy according to claim 1, characterized in that: The magnesium-based alloy matrix mentioned in step one includes one of the AZ31, AZ91, and WE43 series magnesium alloys.

3. The method for preparing a gradient coating to enhance the corrosion resistance of a carbon-embedded magnesium-based alloy according to claim 1, characterized in that: In step one, mechanical polishing is performed using 400-grit, 800-grit, and 1200-grit sandpaper sequentially until the surface roughness Ra of the substrate is ≤0.8μm. Degreasing is performed using an alkaline degreasing agent, soaking the substrate at 60-80℃ for 10-20 minutes. This alkaline degreasing agent consists of 50-80g / L sodium hydroxide, 30-50g / L sodium carbonate, 20-40g / L trisodium phosphate, and 5-10g / L surfactant. Acid washing uses a 5%-10% (w / w) dilute hydrochloric acid solution, soaking the substrate at room temperature for 3-5 minutes. Drying is performed in an oven at 80-100℃ for 1-2 hours.

4. The method for preparing a gradient coating to enhance the corrosion resistance of a carbon-embedded magnesium-based alloy according to claim 1, characterized in that: The carbon-based precursor slurry in step two also includes a binder, a dispersant, and a solvent. The binder is one of phenolic resin and epoxy resin, with a mass fraction of 5% to 10%. The dispersant is one of polyethylene glycol and sodium dodecylbenzenesulfonate, with a mass fraction of 1% to 3%. The solvent is a mixed solution of ethanol and deionized water, with a volume ratio of ethanol to deionized water of 1:1 to 3:

1.

5. The method for preparing a gradient coating to enhance the corrosion resistance of a carbon-embedded magnesium-based alloy according to claim 1, characterized in that: The carbon source mentioned in step two is selected from one or more of graphite powder, activated carbon, carbon nanotubes, and graphene, and the carbon source particle size is 50~200nm.

6. The method for preparing a gradient coating to enhance the corrosion resistance of a carbon-embedded magnesium-based alloy according to claim 1, characterized in that: The laying process described in step three uses spraying or scraping to ensure that the precursor slurry is evenly covered in each layer and that there are no air bubbles or cracks between layers.

7. The method for preparing a gradient coating to enhance the corrosion resistance of a carbon-embedded magnesium-based alloy according to claim 1, characterized in that: The inert gas mentioned in step four is argon or nitrogen, with a gas flow rate of 0.5~1.5L / min; the precursor is stirred every 30 minutes during the heat preservation process to ensure uniform distribution of the carbon source.

8. The method for preparing a gradient coating to enhance the corrosion resistance of a carbon-embedded magnesium-based alloy according to claim 1, characterized in that: The cleaning in step five is performed using ultrasonic cleaning with ethanol as the cleaning medium, and the cleaning time is 10-15 minutes. The drying process is carried out in a vacuum oven at 60-80℃ for 1-2 hours with a vacuum degree of 0.05-0.1MPa.

9. The method for preparing a gradient coating to enhance the corrosion resistance of a carbon-embedded magnesium-based alloy according to claim 1, characterized in that: The gradient carbon embedding coating described in step five has a total thickness of 150 μm, and the interface between the coating and the magnesium-based alloy substrate is a composite of metallurgical and mechanical bonding.

10. The method for preparing a gradient coating to enhance the corrosion resistance of a carbon-embedded magnesium-based alloy according to claim 1, characterized in that: The corrosion resistance test in step six uses a neutral salt spray test with 5% NaCl solution, temperature 35℃, atomization pressure 0.07~0.1MPa, and test time not less than 72 hours; the bonding strength test uses the cross-cut test with a cross-cut spacing of 1mm; the microhardness test uses a Vickers hardness tester with a load of 50g and a holding time of 10 seconds.

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