Acid-alkali-resistant high-corrosion-resistant composite protective coating for hardware fittings and preparation method of composite protective coating

Abstract

The invention discloses a composite protective coating for an acid-alkali-resistant high-corrosion-resistant hardware fitting and a preparation method of the composite protective coating. The composite coating structure comprises a zinc coating and an epoxy coating which are formed on the surface of the iron-based tool. The preparation method comprises the following steps: firstly, carrying out alkaline electroplating treatment on an iron-based tool to form a zinc coating with the thickness of 5-7 microns; then completely immersing the galvanized fitting into the epoxy coating, uniformly coating by adopting a dip-coating centrifugal mode, and baking and curing at 200-240 DEG C to form an epoxy coating; and repeatedly coating to enable the total thickness of the epoxy coating to reach more than 10 microns, thereby obtaining the composite protective coating. Through the synergistic effect of alkaline electroplating and the functional epoxy coating and in combination with a specific dip-coating process, the coating has excellent adhesive force, impact resistance, high hardness, long-acting corrosion resistance and acid and alkali resistance, can replace a stainless steel fitting, remarkably reduces the manufacturing cost of the communication fitting, and is suitable for long-acting protection in a harsh corrosion environment.

Description

Technical Field

[0001] This invention relates to the field of functional protective coating materials, and in particular to a composite protective coating for acid and alkali resistant and highly corrosion-resistant hardware and its preparation method. Background Technology

[0002] Communication fittings are critical structural components in communication networks used to connect, secure, and protect cables and equipment. Exposed to outdoor atmospheres and various corrosive environments for extended periods, they must possess excellent corrosion and acid / alkali resistance. Currently, most commercially available fittings are made of stainless steel, which offers good corrosion resistance but is expensive. To reduce costs, the industry is gradually shifting towards iron-based materials, but long-term protection requirements must be met through surface protective coatings. Therefore, developing high-performance composite coatings suitable for iron-based fittings has become a technological focus.

[0003] Among existing protective technologies, electroplating zinc is a commonly used anti-corrosion method. However, acidic electroplating processes suffer from poor deep plating capability, easy hydrogen embrittlement and corrosion of the substrate, and poor coating adhesion. In contrast, cyanide-free alkaline zinc plating processes use sodium hydroxide as a complexing agent, offering advantages such as stable plating solution, environmental friendliness, uniform and dense coating, strong deep plating capability, and good adhesion to the substrate. However, its single-coat chemical corrosion resistance and long-term protection capability are still insufficient. Another commonly used technology is epoxy coating, which has strong coating adhesion and outstanding chemical resistance. However, traditional spraying methods are costly, have difficulty controlling uniformity, and offer limited long-term protective reliability when used alone on iron-based surfaces.

[0004] Therefore, there is a need in the existing technology for a preparation method that can combine the advantages of alkaline electroplating and epoxy coating, and achieve uniform and stable composite coating through an efficient and low-cost coating process, so as to truly achieve the goal of cost reduction and efficiency improvement by "replacing stainless steel with iron" while meeting the service requirements of harsh environments. Summary of the Invention

[0005] To address the aforementioned problems, this invention proposes a composite protective coating for acid and alkali resistant, high-corrosion-resistant hardware and its preparation method. A dense zinc plating layer and a chemically resistant epoxy layer are sequentially formed on the surface of the iron-clad hardware, achieving long-lasting composite protection with high corrosion resistance and acid and alkali resistance, significantly reducing the manufacturing cost of communication hardware. This process employs repeated dip coating and high-temperature curing to ensure a uniform and dense coating, with overall performance reaching and exceeding the protective level of stainless steel hardware.

[0006] This invention can be achieved through the following technical solutions: A method for preparing a composite protective coating for acid and alkali resistant and highly corrosion-resistant hardware includes the following steps: Step 1: Perform alkaline electroplating treatment on the surface of the iron metal fittings to form a zinc plating layer with a thickness of 5-7μm, thus obtaining zinc-plated metal fittings; Step 2: Immerse the galvanized fittings completely in the epoxy coating, apply it evenly using a dip-coating centrifugal method, and bake the coated fittings to cure and form an epoxy coating. Step 3: Repeat step 2 to make the total thickness of the epoxy coating reach more than 10μm, and finally obtain a composite protective coating for acid and alkali resistant and highly corrosion resistant hardware.

[0007] Preferably, the alkaline electroplating in step 1 is a cyanide-free alkaline zinc plating process, and the plating solution composition includes: 8-15 g / L zinc oxide, 80-150 g / L sodium hydroxide, 0.5-5 mL / L special gloss additive, 4-12 mL / L auxiliary additive, and the balance is deionized water.

[0008] Preferably, the alkaline electroplating treatment in step 1 specifically involves electroplating for 15-25 minutes at a current density of 2-4 A / dm² and a temperature of 25-35°C.

[0009] Preferably, the epoxy coating in step 2 comprises the following components by weight: 30-35 parts modified epoxy resin, 25-30 parts amino resin, 25-30 parts diluent, 2-5 parts zinc powder, 12-16 parts aluminum paste, 2-4 parts BYK additive, and 6-8 parts other additives.

[0010] Preferably, the baking temperature in step 2 is 200-240℃ and the baking time is 35-45 minutes.

[0011] Preferably, the total thickness of the composite coating is greater than 15 μm.

[0012] Preferably, the BYK additive is a mixture of BYK-306 leveling agent and BYK-141 defoamer, with a weight ratio of 1:1.

[0013] Preferably, the diluent is a mixed solvent of propylene glycol methyl ether acetate and xylene in a weight ratio of 1:1.

[0014] Preferably, the other additives are a mixed solvent of KH-560, zinc phosphate, and synthetic silica matting powder, in a weight ratio of 1:1:1.

[0015] The beneficial effects of this invention are: This invention achieves high corrosion resistance and acid / alkali resistance on the surface of iron-based hardware through a composite design of alkaline electroplating and functional epoxy coating, along with a specific dip-coating centrifugal process, significantly reducing the manufacturing cost of communication hardware. In this composite coating system, the alkaline zinc plating layer not only has strong adhesion to the iron substrate and excellent deep-plating capability, but also acts as a sacrificial anode to provide electrochemical protection, effectively delaying substrate corrosion. Meanwhile, the epoxy coating containing active fillers such as zinc powder and aluminum paste forms a dense and stable physicochemical barrier after high-temperature curing, synergistically enhancing the coating's resistance to penetration, chemical corrosion, and mechanical properties. Through repeated dip-coating and high-temperature curing processes, the coating is ensured to be uniform, defect-free, and with controllable overall thickness, thus maintaining excellent adhesion, impact resistance, and hardness even in harsh corrosive environments. The comprehensive protective performance reaches or even exceeds that of stainless steel hardware, achieving the cost reduction and efficiency improvement goal of "replacing stainless steel with iron." Attached Figure Description

[0016] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used in conjunction with embodiments of the invention to explain the invention and do not constitute a limitation thereof. In the drawings: Figure 1 The thickness of the composite protective coating. Detailed Implementation

[0017] The following provides a detailed description of the embodiments of the present invention: These embodiments are implemented based on the technical solution of the present invention, and provide detailed implementation methods and processes. However, the scope of protection of the present invention is not limited to the following embodiments. Experimental methods in the following embodiments that do not specify specific conditions are generally performed under conventional conditions.

[0018] Example 1: A method for preparing a composite protective coating for acid and alkali resistant and highly corrosion-resistant hardware, comprising the following steps: Step 1: Add 16g of zinc oxide, 160g of sodium hydroxide, 1mL of anisaldehyde, and 8mL of diethanolamine to a mixing container, then add deionized water to make up to 2L, stir well, and obtain the electroplating solution. Step 2: Weigh 30 parts of modified epoxy resin, 25 parts of amino resin, 25 parts of diluent (propylene glycol methyl ether acetate and xylene mixed in a 1:1 weight ratio), 2 parts of zinc powder, 12 parts of aluminum paste, 2 parts of BYK additive (BYK-306 leveling agent and BYK-141 defoamer mixed in a 1:1 weight ratio), and 6 parts of other additives (KH-560, zinc phosphate and synthetic silica matting powder mixed in a 1:1:1 weight ratio), add them to a mixing container, stir evenly, and obtain epoxy coating; Step 3: Place the iron metal fittings in the electroplating solution and electroplat them for 25 minutes at a current density of 2A / dm2 and a temperature of 25℃ to form a zinc plating layer with a thickness of 5μm, thus obtaining zinc-plated metal fittings; Step 4: Immerse the galvanized fittings completely in the epoxy coating, keep for 10 seconds, then remove them at a constant speed and centrifuge at 800 rpm for 30 seconds to remove excess coating and obtain a uniform wet film. Bake the coated fittings at 200℃ for 45 minutes to cure and form an epoxy coating. Step 5: Repeat step 4 to make the total thickness of the epoxy coating reach more than 10μm, and finally obtain a composite protective coating for acid and alkali resistant and highly corrosion resistant hardware.

[0019] Example 2: A method for preparing a composite protective coating for acid and alkali resistant and highly corrosion-resistant hardware, comprising the following steps: Step 1: Add 23g of zinc oxide, 230g of sodium hydroxide, 5.5mL of anisaldehyde, and 16mL of diethanolamine to a mixing container, then add deionized water to make up to 2L, stir well, and obtain the electroplating solution. Step 2: Weigh 32.5 parts of modified epoxy resin, 27.5 parts of amino resin, 27.5 parts of diluent (propylene glycol methyl ether acetate and xylene mixed in a 1:1 weight ratio), 3.5 parts of zinc powder, 14 parts of aluminum paste, 3 parts of BYK additive (BYK-306 leveling agent and BYK-141 defoamer mixed in a 1:1 weight ratio), and 7 parts of other additives (KH-560, zinc phosphate and synthetic silica matting powder mixed in a 1:1:1 weight ratio), add them to a mixing container, stir evenly, and obtain epoxy coating; Step 3: Place the iron metal fittings in the electroplating solution and electroplat them for 20 minutes at a current density of 3A / dm2 and a temperature of 30℃ to form a zinc plating layer with a thickness of 6μm, thus obtaining zinc-plated metal fittings; Step 4: Immerse the galvanized fittings completely in the epoxy coating, keep for 10 seconds, then remove them at a constant speed and centrifuge at 800 rpm for 30 seconds to remove excess coating and obtain a uniform wet film. Bake the coated fittings at 220℃ for 40 minutes to cure and form an epoxy coating. Step 5: Repeat step 4 to make the total thickness of the epoxy coating reach more than 10μm, and finally obtain a composite protective coating for acid and alkali resistant and highly corrosion resistant hardware.

[0020] Example 3: A method for preparing a composite protective coating for acid and alkali resistant and highly corrosion-resistant hardware, comprising the following steps: Step 1: Add 30g of zinc oxide, 300g of sodium hydroxide, 10mL of anisaldehyde, and 24mL of diethanolamine to a mixing container, then add deionized water to make up to 2L, stir well, and obtain the electroplating solution. Step 2: Weigh 35 parts of modified epoxy resin, 30 parts of amino resin, 30 parts of diluent (propylene glycol methyl ether acetate and xylene mixed in a 1:1 weight ratio), 5 parts of zinc powder, 16 parts of aluminum paste, 4 parts of BYK additive (BYK-306 leveling agent and BYK-141 defoamer mixed in a 1:1 weight ratio), and 8 parts of other additives (KH-560, zinc phosphate and synthetic silica matting powder mixed in a 1:1:1 weight ratio), add them to a mixing container, stir evenly, and obtain epoxy coating; Step 3: Place the iron metal fittings in the electroplating solution and electroplat them for 15 minutes at a current density of 4A / dm2 and a temperature of 35℃ to form a zinc plating layer with a thickness of 7μm, thus obtaining zinc-plated metal fittings; Step 4: Immerse the galvanized fittings completely in the epoxy coating, keep for 10 seconds, then remove them at a constant speed and centrifuge at 800 rpm for 30 seconds to remove excess coating and obtain a uniform wet film. Bake the coated fittings at 240℃ for 35 minutes to cure and form an epoxy coating. Step 5: Repeat step 4 to make the total thickness of the epoxy coating reach more than 10μm, and finally obtain a composite protective coating for acid and alkali resistant and highly corrosion resistant hardware.

[0021] Example 4: A method for preparing a composite protective coating for acid and alkali resistant and highly corrosion-resistant hardware, comprising the following steps: Step 1: Add 30g of zinc oxide, 300g of sodium hydroxide, 10mL of anisaldehyde, and 8mL of diethanolamine to a mixing container, then add deionized water to make up to 2L, stir well, and obtain the electroplating solution. Step 2: Weigh 35 parts of modified epoxy resin, 30 parts of amino resin, 25 parts of diluent (propylene glycol methyl ether acetate and xylene mixed in a 1:1 weight ratio), 5 parts of zinc powder, 16 parts of aluminum paste, 4 parts of BYK additive (BYK-306 leveling agent and BYK-141 defoamer mixed in a 1:1 weight ratio), and 8 parts of other additives (KH-560, zinc phosphate and synthetic silica matting powder mixed in a 1:1:1 weight ratio), add them to a mixing container, stir evenly, and obtain epoxy coating; Step 3: Place the iron metal fittings in the electroplating solution and electroplat them for 25 minutes at a current density of 4A / dm2 and a temperature of 35℃ to form a zinc plating layer with a thickness of 7μm, thus obtaining zinc-plated metal fittings; Step 4: Immerse the galvanized fittings completely in the epoxy coating, keep for 10 seconds, then remove them at a constant speed and centrifuge at 800 rpm for 30 seconds to remove excess coating and obtain a uniform wet film. Bake the coated fittings at 240℃ for 45 minutes to cure and form an epoxy coating. Step 5: Repeat step 4 to make the total thickness of the epoxy coating reach more than 10μm, and finally obtain a composite protective coating for acid and alkali resistant and highly corrosion resistant hardware.

[0022] Comparative Example 1: The difference between this comparative example and Example 1 is that there is no alkaline electroplating underlayer, only an epoxy coating.

[0023] A method for preparing a composite protective coating for acid and alkali resistant and highly corrosion-resistant hardware includes the following steps: Step 1: Weigh 30 parts of modified epoxy resin, 25 parts of amino resin, 25 parts of diluent (propylene glycol methyl ether acetate and xylene mixed in a 1:1 weight ratio), 2 parts of zinc powder, 12 parts of aluminum paste, 2 parts of BYK additive (BYK-306 leveling agent and BYK-141 defoamer mixed in a 1:1 weight ratio), and 6 parts of other additives (KH-560, zinc phosphate and synthetic silica matting powder mixed in a 1:1:1 weight ratio). Add them to a mixing container and stir evenly to obtain epoxy coating. Step 2: Immerse the iron metal tool completely in the epoxy coating, keep it for 10 seconds, then remove it at a constant speed and rotate it in a centrifuge at 800 rpm for 30 seconds to remove excess coating and obtain a uniform wet film. Bake the coated metal tool at 200℃ for 45 minutes to cure and form an epoxy coating. Step 3: Repeat step 2 to make the total thickness of the epoxy coating reach more than 10μm, and finally obtain a composite protective coating for acid and alkali resistant and highly corrosion resistant hardware.

[0024] Comparative Example 2: The difference between this comparative example and Example 1 is that step 5 is omitted, and only a single application of epoxy coating is performed.

[0025] A method for preparing a composite protective coating for acid and alkali resistant and highly corrosion-resistant hardware includes the following steps: Step 1: Add 16g of zinc oxide, 160g of sodium hydroxide, 1mL of anisaldehyde, and 8mL of diethanolamine to a mixing container, then add deionized water to make up to 2L, stir well, and obtain the electroplating solution. Step 2: Weigh 30 parts of modified epoxy resin, 25 parts of amino resin, 25 parts of diluent (propylene glycol methyl ether acetate and xylene mixed in a 1:1 weight ratio), 2 parts of zinc powder, 12 parts of aluminum paste, 2 parts of BYK additive (BYK-306 leveling agent and BYK-141 defoamer mixed in a 1:1 weight ratio), and 6 parts of other additives (KH-560, zinc phosphate and synthetic silica matting powder mixed in a 1:1:1 weight ratio), add them to a mixing container, stir evenly, and obtain epoxy coating; Step 3: Place the iron metal fittings in the electroplating solution and electroplat them for 25 minutes at a current density of 2A / dm2 and a temperature of 25℃ to form a zinc plating layer with a thickness of 5μm, thus obtaining zinc-plated metal fittings; Step 4: Immerse the galvanized fittings completely in the epoxy coating, keep for 10 seconds, then remove at a constant speed and centrifuge at 800 rpm for 30 seconds to remove excess coating and obtain a uniform wet film. Bake the coated fittings at 200℃ for 45 minutes to cure and obtain a composite protective coating for acid and alkali resistant and highly corrosion resistant fittings.

[0026] Comparative Example 3: The difference between this comparative example and Example 1 is that conventional epoxy coatings are used instead of the zinc-containing epoxy coating prepared in this invention.

[0027] A method for preparing a composite protective coating for acid and alkali resistant and highly corrosion-resistant hardware includes the following steps: Step 1: Add 16g of zinc oxide, 160g of sodium hydroxide, 1mL of anisaldehyde, and 8mL of diethanolamine to a mixing container, then add deionized water to make up to 2L, stir well, and obtain the electroplating solution. Step 2: Weigh 30 parts of modified epoxy resin, 25 parts of amino resin, 25 parts of diluent (propylene glycol methyl ether acetate and xylene mixed in a 1:1 weight ratio), and 2 parts of BYK additive (BYK-306 leveling agent and BYK-141 defoamer mixed in a 1:1 weight ratio), add them to a mixing container, stir evenly, and obtain a conventional epoxy coating. Step 3: Place the iron metal fittings in the electroplating solution and electroplat them for 25 minutes at a current density of 2A / dm2 and a temperature of 25℃ to form a zinc plating layer with a thickness of 5μm, thus obtaining zinc-plated metal fittings; Step 4: Immerse the galvanized fittings completely in conventional epoxy coating, keep for 10 seconds, then remove them at a constant speed and centrifuge at 800 rpm for 30 seconds to remove excess coating and obtain a uniform wet film. Bake the coated fittings at 200℃ for 45 minutes to cure and form an epoxy coating. Step 5: Repeat step 4 to make the total thickness of the epoxy coating reach more than 10μm, and finally obtain a composite protective coating for acid and alkali resistant and highly corrosion resistant hardware.

[0028] Performance testing 1. Neutral Salt Spray Test According to GB / T 10125-2021 standard, a neutral salt spray test was conducted on the composite protective coating to observe the time of red rust appearance, blistering on both sides of the scratch, and the coating condition in the unscratched area.

[0029] Table 1 Results of Neutral Salt Spray Test

[0030] As shown in Table 1, the example group showed no red rust within 1200 hours, mainly due to the synergistic protective mechanism of the zinc plating layer and the functional epoxy coating: the dense zinc plating layer at the bottom provided excellent electrochemical sacrificial anodic protection and mechanical barrier; the epoxy coating containing zinc powder and aluminum paste at the top enhanced the coating's density, shielding properties, and resistance to chemical corrosion; the repeated dip-coating process ensured a uniform and defect-free coating, thus achieving long-term corrosion protection. In contrast, Comparative Example 1 lacked a zinc plating underlayer, and the epoxy coating was in direct contact with the iron substrate, lacking sacrificial anodic protection and effective barrier, allowing corrosive media to easily penetrate into the substrate, leading to rapid rusting. Comparative Example 2 had insufficient coating thickness and poor uniformity, with local weak points becoming the preferred sites for corrosion, significantly shortening the protective life. Comparative Example 3 lacked the reinforcing and shielding effects of active fillers such as zinc powder and aluminum paste, resulting in decreased coating resistance to penetration and chemical corrosion, failing to effectively delay media erosion.

[0031] 2. Acid rain cyclic corrosion test Acid rain cyclic corrosion tests were conducted on the composite protective coating according to GB / T 10125-2021 standard. Each cycle included the following steps: salt spray stage (5% sodium chloride, 35℃, 4h); drying stage (60℃, relative humidity ≤30%, 2h); and wet stage (50℃, relative humidity ≥95%, 2h). The number of cycles in which red rust appeared on the composite protective coating was recorded.

[0032] Table 2 Results of acid rain cyclic corrosion test

[0033] As shown in Table 2, all embodiments exhibited excellent corrosion resistance in the acid rain cyclic corrosion test (no red rust appeared after 16 cycles). This is mainly because the alkaline electroplated zinc layer prevents the corrosive medium from directly contacting the iron substrate and provides a smooth and active adhesion interface for the upper epoxy coating. The epoxy coating effectively blocks the penetration of corrosive ions such as H+ and Cl-. The zinc layer in the epoxy coating preferentially corrodes when the coating is locally damaged or has micropores, continuously providing cathodic protection for the iron substrate and delaying the occurrence of substrate corrosion.

[0034] 3 Impact Test and Failure Hardness Impact tests were conducted on the composite protective coating according to GB / T 1732-2020 standard to observe whether the coating peeled off or cracked in the impact area, and whether the substrate was exposed (50 kg·cm without damaging the substrate). The destructive hardness of the composite protective coating was tested according to GB / T 6739-2022 standard.

[0035] Table 3 Impact test and destructive hardness test results

[0036] As shown in Table 3, the composite protective coatings prepared in all examples exhibit both excellent impact resistance and high hardness. This is attributed to the alkaline electroplating underlayer enhancing adhesion, the dense epoxy coating forming -Zn-O- chemical bonds with the underlayer to enhance interfacial adhesion, and the repeated coating-curing process allowing the epoxy resin to fully crosslink, resulting in increased coating hardness (up to 5H) while maintaining a certain degree of toughness. In contrast, the comparative examples, lacking any of the aforementioned key elements, suffered from decreased adhesion, density, or reinforcing effect, resulting in poor performance in impact and hardness tests.

[0037] 4. Acid and alkali resistance tests Prepare a sulfuric acid solution with pH 2.0 and a sodium hydroxide solution with pH 12.0. Immerse the composite protective coating completely in the solution and keep it for 24 hours. Observe whether the coating blisteres, changes color, or peels off.

[0038] Table 4 Results of acid and alkali resistance tests

[0039] As shown in Table 4, all composite coatings prepared in these examples exhibited excellent performance with "no change" in acid and alkali resistance tests. The fundamental reason lies in the dual synergistic protection mechanism of the "zinc plating layer + epoxy coating": the alkaline electroplated zinc layer has strong adhesion to the iron substrate, providing a first uniform and defect-free physical barrier layer; the epoxy coating (≥10μm) has a continuous and uniform structure with low porosity, effectively blocking the penetration and diffusion of H+ and OH- ions. Furthermore, the modified epoxy resin and amino resin in the coating formulation cross-link and cure, forming a highly chemically stable three-dimensional network structure. The stable COC and CN bonds on its molecular chains effectively resist hydrolytic corrosion from acids and alkalis. In contrast, the epoxy coating in Comparative Example 1 is directly coated on the iron substrate, lacking the cathodic protection of the zinc plating layer and a dense underlayer. In acidic media, the iron substrate is prone to hydrogen evolution reaction, and hydrogen gas accumulates at the coating interface, leading to blistering. In alkaline media, loose ferric hydroxide easily forms on the iron substrate surface, causing a decrease in coating adhesion and even peeling. Comparative Example 2 shows that the epoxy coating formed by a single application is generally less thick and dense than that formed by multiple applications. Under long-term immersion, the medium can more easily penetrate to the coating interface, causing localized corrosion, manifested as discoloration and slight blistering. Comparative Example 3 uses conventional epoxy coatings that lack the reinforcing effect of zinc powder and aluminum paste, resulting in a loss of gloss and softening of the coating surface.

[0040] 5. Film thickness test The film thickness of the composite protective coating samples was measured using a metallographic microscope. At least 10 points were measured for each sample, and the average value was taken.

[0041] Table 5. Film thickness test results of composite protective coating

[0042] As shown in Table 5, the total thickness of the composite coating in all embodiments is significantly higher than that in the comparative example, and is greater than 15 μm. This indicates that the alkaline electroplating and epoxy repeated dip-coating process used in this invention can effectively construct a protective system of sufficient thickness.

[0043] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for preparing a composite protective coating for acid and alkali resistant and highly corrosion-resistant hardware, characterized in that, Includes the following steps: Step 1: Perform alkaline electroplating treatment on the surface of the iron metal fittings to form a zinc plating layer with a thickness of 5-7μm, thus obtaining zinc-plated metal fittings; Step 2: Immerse the galvanized fittings completely in the epoxy coating, apply it evenly using a dip-coating centrifugal method, and bake the coated fittings to cure and form an epoxy coating. Step 3: Repeat step 2 to make the total thickness of the epoxy coating reach more than 10μm, and finally obtain a composite protective coating for acid and alkali resistant and highly corrosion resistant hardware.

2. The method for preparing the composite protective coating for acid and alkali resistant and highly corrosion-resistant hardware according to claim 1, characterized in that, The alkaline electroplating in step 1 is a cyanide-free alkaline zinc plating process. The plating solution consists of: 8-15 g / L zinc oxide, 80-150 g / L sodium hydroxide, 0.5-5 mL / L special gloss additive, 4-12 mL / L auxiliary additive, and the balance is deionized water.

3. The method for preparing the composite protective coating for acid and alkali resistant and highly corrosion-resistant hardware according to claim 1, characterized in that, The alkaline electroplating treatment in step 1 specifically involves electroplating for 15-25 minutes at a current density of 2-4 A / dm² and a temperature of 25-35°C.

4. The method for preparing the composite protective coating for acid and alkali resistant and highly corrosion-resistant hardware according to claim 1, characterized in that, The epoxy coating in step 2 comprises the following components by weight: 30-35 parts modified epoxy resin, 25-30 parts amino resin, 25-30 parts diluent, 2-5 parts zinc powder, 12-16 parts aluminum paste, 2-4 parts BYK additive, and 6-8 parts other additives.

5. The method for preparing the composite protective coating for acid and alkali resistant and highly corrosion-resistant hardware according to claim 1, characterized in that, In step 2, the baking temperature is 200-240℃ and the time is 35-45 minutes.

6. The method for preparing the composite protective coating for acid and alkali resistant and highly corrosion-resistant hardware according to claim 1, characterized in that, The total thickness of the composite coating is greater than 15 μm.

7. The method for preparing the composite protective coating for acid and alkali resistant and highly corrosion-resistant hardware according to claim 4, characterized in that, The BYK additive is a mixture of BYK-306 leveling agent and BYK-141 defoamer, with a weight ratio of 1:

1.

8. The method for preparing the composite protective coating for acid and alkali resistant and highly corrosion-resistant hardware according to claim 4, characterized in that, The diluent is a mixed solvent of propylene glycol methyl ether acetate and xylene in a weight ratio of 1:

1.

9. The method for preparing the composite protective coating for acid and alkali resistant and highly corrosion-resistant hardware according to claim 4, characterized in that, The other additives are a mixture of KH-560, zinc phosphate, and synthetic silica matting agent in a weight ratio of 1:1:1.

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