An aluminum-based material surface protective coating and a method for preparing the same

Abstract

The application relates to the technical field of coating, in particular to an aluminum-based material surface protective coating and a preparation method thereof, which comprises the following steps: S1, performing surface pretreatment on an aluminum-based material, performing anodic oxidation treatment on the surface of the pretreated aluminum-based material to form an anodic oxidation bottom layer; S2, preparing and solidifying a sol-gel transition layer on the surface of the anodic oxidation bottom layer; and S3, coating a copolymer coating solution on the surface of the sol-gel transition layer to form a polymer surface layer after solidification. The application constructs a composite coating system on the surface of the aluminum-based material, utilizes the synergistic effect of the components of each layer, and makes the coating exhibit good adhesion, high hardness and excellent corrosion resistance, and is suitable for the aluminum-based material with high surface protection requirements.

Description

Technical Field

[0001] This invention relates to the field of coating technology, and in particular to a protective coating for aluminum-based materials and its preparation method. Background Technology

[0002] Aluminum-based materials, as important lightweight structural materials, are widely used in aerospace, transportation, electronics, and architectural decoration. However, aluminum and its alloys are prone to pitting corrosion and intergranular corrosion in environments containing corrosive media such as chloride ions. Furthermore, their relatively low surface hardness and insufficient wear resistance significantly limit their long-term reliable use under harsh conditions. Traditional surface protection technologies mainly include anodizing, chemical conversion coatings, and electroplating. Anodizing technology generates a dense, porous alumina film on the aluminum surface through electrochemical methods, exhibiting good corrosion resistance, and the porous structure provides a basis for subsequent sealing or coloring treatments. Chemical conversion coatings (such as chromate conversion) are simple to process, resulting in a film with strong adhesion to the substrate and good self-healing capabilities. Electroplating deposits a metallic coating on the aluminum surface, significantly improving appearance, conductivity, or wear resistance.

[0003] Chinese patent application CN101008098A discloses a two-step electrochemical method for preparing a protective coating on the surface of aluminum-based composite materials. The method involves pre-treating the in-situ titanium diboride particle-reinforced aluminum-based composite material sequentially with polishing, degreasing, and acid activation. Then, anodizing is performed using an acid mixture as the anodic oxidation solution to generate an aluminum oxide film on the material surface. Subsequently, a uniform rare earth compound film is obtained through constant current cathodic polarization deposition using a rare earth salt mixture. This invention employs a two-step electrochemical surface treatment method to construct a dense protective film of a certain thickness on the surface of the composite material, significantly improving its corrosion resistance, and the film preparation process is simple.

[0004] In existing technologies, anodic oxide films consist of numerous vertical nanoscale micropores. While this is beneficial for adsorbing dyes or sealing agents, it also provides a rapid channel for corrosive media such as water and chloride ions to reach the aluminum substrate, resulting in fundamental defects in long-term corrosion resistance, especially in harsh salt spray environments. Furthermore, the oxide film is essentially a hard and brittle ceramic layer with poor impact and bending deformation resistance, making it prone to microcracks under mechanical or thermal stress. Once cracked, its protective performance drops sharply. Simultaneously, the adhesion between the oxide film and subsequent organic coatings is limited. Although its surface is porous, the bonding with organic coatings mainly relies on physical anchoring and limited hydrogen bonds, lacking strong chemical bonds. In humid and hot environments, the interface is prone to blistering and peeling, leading to failure. Summary of the Invention

[0005] To address the problems mentioned in the background section, this invention provides a protective coating for aluminum-based materials and its preparation method.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] In a first aspect, the present invention provides a protective coating for the surface of an aluminum-based material. The coating is disposed on the surface of the aluminum substrate and comprises, from the inside out:

[0008] Anodized underlayer bonded to the surface of the aluminum substrate;

[0009] A sol-gel transition layer formed on the anodized substrate;

[0010] And a polymer surface layer formed on the sol-gel transition layer.

[0011] Furthermore, the anodized underlayer is a porous alumina layer generated in situ on the surface of the aluminum substrate through anodizing process.

[0012] Furthermore, the sol-gel transition layer is an inorganic-organic hybrid layer, which includes a silicon-oxygen network formed by the hydrolysis and condensation of a silane precursor, and a corrosion inhibitor uniformly distributed in the network; the silane precursor includes at least tetraethyl orthosilicate and an epoxy-containing silane coupling agent; the corrosion inhibitor is preferably a cerium salt.

[0013] Furthermore, the polymer surface layer comprises a copolymer and silica nanoparticles dispersed therebetween; the copolymer is formed by free radical copolymerization of 4-distyrene structural units and dimethyl itaconic acid structural units.

[0014] Furthermore, the thickness of the anodized underlayer is 5-20 micrometers, the thickness of the sol-gel transition layer is 1-5 micrometers, and the thickness of the polymer surface layer is 15-50 micrometers.

[0015] Secondly, the present invention provides a method for preparing the above-mentioned protective coating on the surface of aluminum-based materials, comprising the following steps:

[0016] S1. Perform surface pretreatment on the aluminum substrate, and then perform anodizing treatment on the surface of the pretreated aluminum substrate to form an anodized underlayer;

[0017] S2. Prepare and solidify a sol-gel transition layer on the surface of the anodized substrate;

[0018] S3. Coating the copolymer coating solution onto the surface of the sol-gel transition layer, and curing it to form a polymer surface layer.

[0019] Furthermore, in step S1, the surface pretreatment includes mechanical polishing, cleaning, and chemical activation; mechanical polishing involves using sandpaper of different grits to polish the aluminum substrate step by step; cleaning involves placing the polished aluminum substrate in acetone, anhydrous ethanol, and deionized water in sequence for ultrasonic cleaning; chemical activation includes alkaline washing to remove oil and acid etching activation.

[0020] Further, in step S1, the anodizing process uses the pretreated aluminum substrate as the anode and applies a DC voltage to the electrolyte for oxidation. The electrolyte contains sulfuric acid, oxalic acid, and boric acid. Specifically, the anodizing process involves applying a DC voltage of 10-20V for 20-40 minutes while maintaining the electrolyte temperature at 20±2℃. The electrolyte, by concentration, contains 180-200g / L of sulfuric acid, 15-18g / L of oxalic acid, and 5-6g / L of boric acid. Preferably, the DC voltage is applied by increasing the voltage from 0V to a constant voltage of 10-20V at a rate of 0.5-2V / min.

[0021] Furthermore, in step S2, the preparation of the sol-gel transition layer includes the following sub-steps:

[0022] S2.1. Tetraethyl orthosilicate, an epoxy-containing silane coupling agent, and a corrosion inhibitor are mixed and subjected to a hydrolysis-condensation reaction at 60-70°C in the presence of an acidic catalyst and water. The mixture is then aged at room temperature for 48-96 hours to obtain a stable silica sol. The acidic catalyst is selected from at least one of hydrochloric acid, nitric acid, or sulfuric acid, and its addition is such that the pH of the reaction system is maintained at 2-4. The epoxy-containing silane coupling agent is preferably γ-glycidoxypropyltrimethoxysilane. The corrosion inhibitor is preferably cerium nitrate. The mass ratio of tetraethyl orthosilicate to the epoxy-containing silane coupling agent is (4-6):1. The amount of corrosion inhibitor added is 1-5% of the total mass of tetraethyl orthosilicate and the epoxy-containing silane coupling agent.

[0023] S2.2. Apply silica sol to the surface of the anodic oxide substrate, and then perform step-by-step thermal curing on the wet film to obtain a sol-gel transition layer. The specific coating method can be spin coating, for example, first spin coating at a speed of 400-600 rpm for 5-15 seconds, and then spin coating at a speed of 2500-3500 rpm for 20-40 seconds. The step-by-step thermal curing procedure includes: first curing at 80-85℃ for 1-2 hours, then curing at 100-110℃ for 2-3 hours, and finally curing at 110-120℃ for 1-2 hours.

[0024] Furthermore, after the curing step, an oxygen plasma treatment step is included on the surface of the sol-gel transition layer, with a treatment power of 80-150W and a treatment time of 2-8 minutes.

[0025] Furthermore, step S3 includes the following sub-steps:

[0026] S3.1 Add 4-di-styrene and dimethyl itaconic acid to N-methylpyrrolidone at a molar ratio of (1.5-2.5):1. In another formulation, the mass ratio of 4-di-styrene to dimethyl itaconic acid is (1.7-2.9):1. Add 0.8-2.0% of azobisisobutyronitrile as an initiator. React in an oil bath at 68-78℃, with stirring at 100-200 rpm and under nitrogen protection for 6-18 hours. After the reaction is complete, precipitate the reaction solution dropwise into a precipitant (such as methanol). After separation and purification, obtain the copolymer. The total mass concentration of 4-di-styrene and dimethyl itaconic acid in N-methylpyrrolidone is 200-350 g / L. The purification step includes redissolving the precipitate in tetrahydrofuran and then redeprecipitating it in methanol. This process can be repeated 1-3 times.

[0027] S3.2 Dissolve the copolymer in tetrahydrofuran or N,N-dimethylformamide to prepare a solution with a solid content of 16-20 wt%; then add 15-20 wt% silica nanoparticles, 1.5-2.0 wt% leveling agent, and 2.0-3.0 wt% photoinitiator to the solution based on the copolymer mass, and sonicate for 10-30 min to obtain a uniformly dispersed copolymer coating solution; the silica nanoparticles are preferably surface-hydrophobic modified particles with an average particle size of 10-30 nm.

[0028] S3.3. Coating the copolymer coating solution onto the surface of the sol-gel transition layer, and then performing a curing process to form a polymer surface layer; the curing process includes: firstly pre-curing at 60-80℃ for 5-15 min, then irradiating with ultraviolet light with a wavelength of 300-400 nm for 5-30 s, and finally post-curing at 120-130℃ for 20-40 min.

[0029] The beneficial effects of this invention are:

[0030] 1. In the technical solution of this invention, a multi-layered composite protective system with functional gradients is constructed through a multi-layered structure design consisting of an anodized underlayer, a sol-gel transition layer, and a polymer top layer. The anodized underlayer grows in situ on the surface of the aluminum substrate, forming a porous alumina layer with good adhesion, providing a stable substrate for subsequent coatings. The sol-gel transition layer forms an inorganic-organic hybrid network through the hydrolysis and condensation reaction of a silane precursor. Its low-viscosity sol can effectively penetrate and fill the micropores of the anodized layer, transforming the originally interconnected channels into tortuous barriers, which helps to block the direct path of corrosive media. The copolymer top layer, by introducing 4-diphenylstyrene rigid units and dimethyl itaconic acid flexible crosslinkable units, constructs a copolymer chain structure that combines steric hindrance effect and reactivity. During the UV curing stage, it forms a dense three-dimensional covalent crosslinked network through free radical-initiated carbon-carbon double bond crosslinking, significantly reducing the internal free volume of the coating and improving the resistance to media penetration. Uniformly dispersed silica nanoparticles fill the gaps between polymer chains and enhance cohesive strength and adhesion through interfacial hydrogen bonding between surface silanol groups and polymer polar groups, ultimately forming a composite coating with low porosity, high crosslinking density, and good interfacial stability. The multi-layered synergistic structural design enhances the bonding force between the coating layers. The epoxy-containing silane coupling agent used in the sol-gel transition layer plays a crucial bridging role. Its silane group at one end can hydrolyze and chemically bond with the active groups on the anodic oxide layer surface, while the epoxy group at the other end can react with active components (such as carboxylic acid ester groups) in the upper polymer surface layer during curing. This interlayer bonding method, primarily based on chemical bonding, significantly improves the interfacial stability of the coating system compared to traditional physical anchoring and weak hydrogen bonding, thus helping to improve its resistance to blistering and peeling under harsh environments such as humid heat and salt spray.

[0031] 2. In the technical solution of this invention, the polymer surface layer is formed by copolymerizing rigid biphenyl units with crosslinkable units and curing them under ultraviolet light to create a crosslinked network. Simultaneously, silica nanoparticles are incorporated, giving the surface layer a combination of rigidity, toughness, and wear resistance. When external mechanical or thermal stress exists, the relatively flexible yet tough polymer surface layer can deform and dissipate energy before the underlying brittle anodic oxide layer, helping to buffer and disperse stress, thus positively impacting the reduction of the risk of cracking due to stress concentration. The introduction of nanoparticles may further improve the mechanical properties of the coating through interfacial effects.

[0032] 3. In the technical solution of this invention, the corrosion inhibitor loaded in the sol-gel transition layer provides the coating system with potential active protection capabilities. When the coating is locally damaged or corrosive media penetrates to the interface area, the corrosion inhibitor can be slowly released and form a protective precipitation film on the metal surface, which helps to inhibit local corrosion and may extend the protective life of the coating. Detailed Implementation

[0033] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. 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.

[0034] Example 1

[0035] A method for preparing a protective coating on the surface of an aluminum-based material includes the following steps:

[0036] S1. Select a 6061 aluminum alloy plate (size: 50mm×50mm×2mm). Use 400-grit, 800-grit, and 1200-grit sandpaper to progressively polish the surface until it becomes mirror-smooth. After polishing, place the plate sequentially in acetone, anhydrous ethanol, and deionized water, and ultrasonically clean each for 10 minutes to remove grease and particulate contaminants. Dry the surface after cleaning. Immerse the plate in a 70℃, 50g / L sodium hydroxide (NaOH) aqueous solution for 5 minutes, then rinse thoroughly with deionized water for 30 minutes. Immerse the plate in a mixed acid solution of 10% nitric acid and 0.5% hydrofluoric acid (HF) for 30 seconds to thoroughly remove the surface oxide layer and achieve micro-roughening. Rinse repeatedly with deionized water until neutral and dry the surface. Finally, prepare a solution of 180g / L sulfuric acid, 15g / L oxalic acid, and 5g / L hydrofluoric acid. g / L boric acid was dissolved in deionized water, stirred evenly, and cooled to 20℃; the pretreated aluminum plate was used as the anode and the graphite plate as the cathode (cathode-to-cathode area ratio 1:3), and oxidation was performed using a DC regulated power supply; the voltage was increased from 0V to 15V at a rate of 1V / min, and then oxidized at a constant voltage of 15V for 30min; after oxidation, the aluminum plate was immediately removed and rinsed with deionized water to obtain the anodized bottom layer;

[0037] S2. In a flask, add 50 mL of tetraethyl orthosilicate (TEOS), 10 mL of γ-glycidyl etheroxypropyltrimethoxysilane (GLYMO), and 2.5 g of cerium nitrate hexahydrate in sequence. While stirring vigorously at 200 rpm, slowly add 50 mL of deionized water with pH adjusted to 3.5 using 0.1 M hydrochloric acid. The mixture is continuously magnetically stirred at 65 °C for 24 h, and then aged at room temperature for 72 h to obtain a stable, transparent, pale yellow silica sol. The silica sol is then coated onto the surface of an anodized substrate using a spin-coating method. The spin-coating program is set to two stages: the first stage is 500 rpm for 10 s; the second stage is 3000 rpm for 30 s. After coating, the wet film is subjected to stepwise thermal curing: first, it is cured at 80 °C for 1 h, then at 110 °C for 2 h, and finally at 120 °C for 1 h to obtain a sol-gel transition layer.

[0038] S3. Add 10.0 g of 4-distyrene and 5.0 g of dimethyl itaconic acid to a flask, and add 35 mL of N-methylpyrrolidone (NMP) as a solvent; add 0.15 g of azobisisobutyronitrile (AIBN) as an initiator; under nitrogen protection, react in an oil bath at 70 °C with a stirring speed of 150 rpm for 18 h; after the reaction is complete, slowly add the viscous reaction solution dropwise to 500 mL of ice-cold methanol to obtain a white flocculent precipitate, which is purified by filtration and washing with cold methanol: the product is redissolved in tetrahydrofuran (THF), and then added dropwise to methanol to precipitate again, this process is repeated twice; the final product is vacuum dried at 45 °C for 36 h to obtain a white solid copolymer; weigh 6.0 g of the above copolymer, add 34 g of tetrahydrofuran (THF), and prepare a solution with a solid content of approximately 15 wt%; add 15 wt% silica nanoparticles (average particle size 20 nm) and 1.5 g of other components based on the copolymer mass to the solution. The copolymer coating solution was prepared by mechanically stirring for 1 hour and then ultrasonically treating for 20 minutes. The coating solution was applied to the surface of the sol-gel transition layer by dip-coating method, with the dipping speed controlled at 100 mm / min. Then, a stepped thermosetting process was performed: first, flash evaporation was carried out at room temperature for 15 minutes to allow the solvent to evaporate initially; then, curing was carried out at 60℃ for 15 minutes; then, irradiation with ultraviolet light at a wavelength of 365 nm was carried out for 20 seconds; finally, curing was carried out at 130℃ for 40 minutes. After curing, the oven was closed and the sample was allowed to cool naturally to room temperature to obtain the protective coating on the aluminum-based material surface.

[0039] Example 2

[0040] This embodiment is basically the same as Embodiment 1, with the main differences as follows:

[0041] The electrolyte in S1 consists of 190 g / L sulfuric acid, 16 g / L oxalic acid, and 5.5 g / L boric acid. The voltage is increased from 0 V to 18 V at a rate of 1.5 V / min, and oxidation is carried out at a constant voltage of 18 V for 25 min.

[0042] Add 60 mL of tetraethyl orthosilicate (TEOS), 12 mL of GLYMO, and 3.0 g of cerium nitrate hexahydrate sequentially to S2, and add 60 mL of deionized water with pH adjusted to 3.0 using nitric acid. Stir the mixture at 65 °C for 20 h, then age at room temperature for 60 h. The stepwise thermosetting process is as follows: first, cure at 85 °C for 1.5 h, then at 110 °C for 2.5 h, and finally at 115 °C for 1.5 h.

[0043] Add 12.0g of 4-distyrene and 5.5g of dimethyl itaconic acid to S3, add 40mL of N-methylpyrrolidone, add 0.21g of AIBN, and react for 18h under nitrogen protection, 75℃ oil bath and stirring at 180rpm. Weigh 7.2g of the above copolymer and add it to 32.8g of N,N-dimethylformamide (DMF) to prepare a solution with a solid content of approximately 18wt%. Add 18wt% silica nanoparticles (average particle size 15nm), 2.0wt% leveling agent (BYK-333), and 2.5wt% photoinitiator 1173 to the solution based on the copolymer mass. Stepwise thermosetting is performed as follows: first, cure at 70℃ for 10min; then irradiate with ultraviolet light at a wavelength of 360nm for 15s; finally, post-cur at 130℃ for 30min.

[0044] Example 3

[0045] This embodiment is basically the same as Embodiment 1, with the main differences as follows:

[0046] The electrolyte in S1 consists of 200 g / L sulfuric acid, 18 g / L oxalic acid, and 6 g / L boric acid. The voltage is increased from 0 V to 12 V at a rate of 0.8 V / min, and oxidation is carried out at a constant voltage of 12 V for 35 min.

[0047] Add 55 mL TEOS, 9 mL GLYMO and 2.0 g cerium nitrate hexahydrate sequentially to S2; add 55 mL of deionized water with pH adjusted to 3.8 using nitric acid; stir the mixture at 62 °C for 26 h, then age at room temperature for 84 h; the stepwise thermosetting process is as follows: first cure at 82 °C for 1 h, then cure at 105 °C for 3 h, and finally cure at 120 °C for 1 h.

[0048] In S3, 8.0 g of 4-distyrene and 4.5 g of dimethyl itaconic acid were added, along with 30 mL of N-methylpyrrolidone and 0.10 g of AIBN. The mixture was reacted for 16 h under nitrogen protection, in a 68 °C oil bath, and with stirring at 120 rpm. 4.8 g of the copolymer was weighed and dissolved in 25.2 g of tetrahydrofuran (THF) to prepare a solution with a solid content of approximately 16 wt%. Hydrophobic silica nanoparticles (average particle size 25 nm), 1.8 wt% leveling agent (BYK-333), and 2.2 wt% photoinitiator were added to the solution based on the copolymer mass. The stepwise thermosetting process was as follows: pre-curing at 65 °C for 12 min, followed by irradiation with 385 nm ultraviolet light for 10 s, and finally post-curing at 125 °C for 35 min.

[0049] Example 4

[0050] This embodiment is basically the same as Embodiment 1, with the main differences as follows:

[0051] The electrolyte in S1 consists of 180 g / L sulfuric acid, 15 g / L oxalic acid, and 5 g / L boric acid. The voltage is increased from 0 V to 10 V at a rate of 2 V / min, and oxidation is carried out at a constant voltage of 10 V for 40 min.

[0052] Add 40 mL of TEOS, 10 mL of GLYMO, and 0.5 g of cerium nitrate hexahydrate sequentially to S2, and add 50 mL of deionized water with pH adjusted to 2.0 using hydrochloric acid. Stir the mixture at 60 °C for 24 h, and then age it at room temperature for 48 h. The stepwise thermosetting process is as follows: first, cure at 80 °C for 2 h, then at 100 °C for 3 h, and finally at 110 °C for 2 h.

[0053] Add 8.5g of 4-distyrene and 5.0g of dimethyl itaconic acid to S3, add 45mL of N-methylpyrrolidone, add 0.108g of initiator AIBN, and react for 18h under nitrogen protection, 68℃ oil bath and stirring at 100rpm; weigh 8.0g of the above copolymer and add it to 42.0g of tetrahydrofuran to prepare a solution with a solid content of about 16wt%; add 15wt% of hydrophobic silica nanoparticles (average particle size 30nm), 1.5wt% of leveling agent (BYK-333) and 2.0wt% of photoinitiator 184 to the solution based on the copolymer mass; the step-curing process is as follows: first pre-curing at 60℃ for 15min, then irradiating with ultraviolet light at a wavelength of 300nm for 30s, and finally post-curing at 120℃ for 40min.

[0054] Comparative Example 1

[0055] The difference between this comparative example and Example 1 is that tetraethyl orthosilicate is not added; the remaining steps are the same as in Example 1.

[0056] Comparative Example 2

[0057] The difference between this comparative example and Example 2 is that GLYMO is not added; the remaining steps are the same as in Example 2.

[0058] Comparative Example 3

[0059] The difference between this comparative example and Example 3 is that cerium nitrate hexahydrate is not added; the remaining steps are the same as in Example 3.

[0060] Comparative Example 4

[0061] The difference between this comparative example and Example 4 is that 4-distyrene is not added; the remaining steps are the same as in Example 4.

[0062] Comparative Example 5

[0063] The difference between this comparative example and Example 1 is that dimethyl itaconic acid is not added; the remaining steps are the same as in Example 1.

[0064] (I) Coating Adhesion Test: Referring to GB / T 9286-2021 "Cross-cut Test for Paints and Varnishes", the coating samples to be tested prepared in Examples 1-4 and Comparative Examples 1-5 were placed in a standard laboratory environment (temperature 23±2℃, relative humidity 50±5%) for 24 hours. Using a cross-cutting cutter with a blade spacing of 1mm, six parallel lines were cut in two mutually perpendicular directions to form a grid pattern containing 25 1mm×1mm squares. A soft brush was used to lightly brush along the diagonal direction of the grid several times to remove loose coating debris generated during cutting. Pressure-sensitive adhesive tape (width ≥25mm) meeting the standard requirements was tightly applied to the grid area. Within 60±30s after application, the tape was peeled off, and the coating peeling in the grid area was observed. According to GB / T 9286-2021, adhesion is classified into 0 to 5 levels: Level 0 (completely smooth cut edges, no peeling); Level 1 (minor coating peeling at cut intersections, affected area ≤ 5%); Level 2 (significant coating peeling at cut edges and / or intersections, 5% < affected area ≤ 15%); Level 3 (coating partially or completely peeling off along the cut edge in large fragments, and / or partially or completely peeling off at different locations within the squares, 15% < affected area ≤ 35%); Level 4 (coating peeling off in large fragments along the cut edge, and / or partially peeling off entire squares, 35% < affected area ≤ 65%); Level 5 (peeling degree exceeds Level 4). The results are shown in Table 1.

[0065] Table 1. Results of cross-cut adhesion test for each coating sample

[0066]

[0067] (II) Pencil Hardness Test for Coatings: Referring to GB / T 6739-2006 "Determination of Hardness of Paints and Varnishes by Pencil Method", select a set of pencils of 4H, 3H, 2H, H, HB, B, 2B, 3B, and 4B. Expose approximately 5-6 mm of the pencil lead and grind it vertically flat on 400-grit sandpaper. Insert the selected pencil into the hardness tester fixture, making it at a 45° angle to the coating surface, ensuring the flat surface of the lead is in contact with the coating. Fix the coating sample to be tested on the moving platform of the tester, and make the pencil draw a uniform line of 6.5 mm across the coating surface at a speed of approximately 0.5-1 mm / s, applying a load of 750 ± 10 g. After drawing, gently wipe away any graphite residue at the scratch with a soft cloth or eraser and observe the coating surface. If the scratch causes "plowing" or "cutting through" the coating (i.e., irreversible plastic damage or exposure of the underlying layer), it is determined that the pencil of that hardness has scratched the coating. The pencil hardness value of the coating is defined as the next level after the highest hardness grade of the unscratched coating. The results are shown in Table 2:

[0068] Table 2. Pencil hardness test results for each coated sample

[0069]

[0070] (III) Chemical immersion test: Prepare the following three test media: a) 3.5% sodium chloride (NaCl) aqueous solution; b) 0.1 mol / L sulfuric acid aqueous solution; c) 0.1 mol / L sodium hydroxide (NaOH) aqueous solution.

[0071] The sample was suspended vertically or fixed on a support, with approximately half of its area immersed in the aforementioned chemical medium. After immersion for 1 hour, 6 hours, 24 hours, and 72 hours, the sample was removed and gently rinsed with running deionized water to remove residual medium, then blotted dry with filter paper. Changes in the immersion area and the coating above the liquid line were observed and recorded, noting any blistering, discoloration, loss of gloss, softening, shrinkage, or peeling of the coating; and whether corrosion spots, rust, or discoloration appeared on the base metal. The results are shown in Table 3.

[0072] Table 3. Observation results of chemical immersion test for each coating sample (72h)

[0073]

[0074] (IV) Neutral Salt Spray Test: Referring to GB / T 10125-2021 "Artificial Atmosphere Corrosion Test - Salt Spray Test", prepare a test solution of (5.0±0.5)% sodium chloride (NaCl, analytical grade) solution, prepared with deionized water, and adjust the pH to 6.8. Place each sample in the salt spray chamber at a 30° angle to the vertical direction, on the sample rack, with the coating facing upwards, and maintain sufficient distance between samples for continuous spraying. The test cycle is set to 500 hours. During the test, ensure that the salt spray deposition rate is within 1.0-2.0 mL of solution collected per hour per 80 cm² horizontal collection area. After the 500-hour test cycle, remove all samples, clean and dry them. Observe whether defects such as blistering, cracking, peeling, rust, and discoloration appear on the coating surface. Rating is performed according to GB / T 6461-2002 "Rating of Specimens and Specimens of Metallic and Other Inorganic Coatings on Metallic Substrates After Corrosion Testing". The rating system consists of two parts: Protection Rating (R): based on the percentage of the total area corroded by the base metal; and Appearance Rating (A): based on the type, size, and density of defects appearing on the coating surface. It is expressed as an RA value. The results are shown in Table 4-5.

[0075] Table 4. Observation results of neutral salt spray test (500h) on coating samples of Examples 1-4

[0076]

[0077] Table 5. Observation results of neutral salt spray test (500h) on coating samples of Comparative Examples 1-5

[0078]

[0079] As shown in Table 1, the adhesion rating of Examples 1-4 is all 0, indicating that the cut edges are completely smooth and no squares have fallen off. In these examples, the anodized underlayer provides good roughness and a chemically active surface, which is beneficial for the adhesion of the sol-gel transition layer. The silica sol component in the sol-gel transition layer forms a chemical bond with the underlayer, and its structure is compact, providing a stable foundation for the upper copolymer coating. The copolymer coating solution is applied by dip-coating, which bonds tightly with the transition layer. The synergistic effect between the layers results in extremely strong adhesion between the coating and the aluminum-based material.

[0080] Comparative Example 1, without tetraethyl orthosilicate, achieved an adhesion grade of 4. The lack of tetraethyl orthosilicate prevented the effective formation of the sol-gel transition layer, failing to provide a good adhesion base for the upper coating. This resulted in a significant decrease in the adhesion between the coating and the substrate, causing the coating to flake off in large fragments along the cut edges, with many square sections peeling off completely. Comparative Example 2, without GLYMO, achieved an adhesion grade of 3. The lack of GLYMO deteriorated the performance of the transition layer, leading to partial coating detachment along the cut edges and partial foliar peeling of the square sections. Comparative Example 3, without cerium nitrate hexahydrate, achieved an adhesion grade of 2. The lack of cerium nitrate hexahydrate may have reduced the bonding strength between the transition layer and the upper and lower layers, with noticeable coating detachment visible at the cut edges and intersections. Comparative Example 4, without 4-distyrene, achieved an adhesion grade of 3. The lack of 4-distyrene may have affected the properties of the copolymer, thereby affecting the adhesion between the copolymer coating and the transition layer, leading to partial coating detachment along the cut edges and partial foliar peeling of the square sections. Comparative Example 5, without dimethyl itaconic acid, achieved an adhesion grade of 5. The lack of itaconic acid alters the properties of the copolymer, preventing it from bonding well with the transition layer. This results in large-area peeling of the coating, with over 65% of the grid area showing separation of the coating from the substrate and extremely poor adhesion.

[0081] As shown in Table 2, the pencil hardness value of Example 1 is 2H, while that of Examples 2-4 is 3H. This indicates that the copolymer coating has high hardness. The monomers in the copolymer form a network structure with a certain degree of crosslinking through copolymerization, and the added silica nanoparticles play a reinforcing role, thus improving the hardness of the coating.

[0082] Comparative Example 1 had a pencil hardness of HB. The lack of tetraethyl orthosilicate prevented the proper formation of the sol-gel transition layer, resulting in an incomplete coating structure, insufficient support for the copolymer coating, and a significant decrease in hardness. Comparative Example 2 had a pencil hardness of H. The absence of GLYMO affected the performance of the transition layer, consequently impacting the hardness and overall performance of the copolymer coating, leading to a lower hardness than the example. Comparative Example 3 had a pencil hardness of 2H. The absence of cerium nitrate hexahydrate had some impact on the transition layer, but compared to Comparative Examples 1 and 2, the impact on coating hardness was smaller, though still lower than the example, indicating that cerium nitrate hexahydrate also plays a role in improving coating hardness. Comparative Examples 4 and 5 both had a pencil hardness of H. The absence of 4-distyrene or dimethyl itaconic acid affected the structure and properties of the copolymer, leading to a decrease in coating hardness, lower than the example.

[0083] As shown in Table 3, after immersion in the three test media for 72 hours, the coatings in Examples 1-4 generally performed well, with only slight changes. The anodized underlayer provided a certain degree of corrosion resistance, the sol-gel transition layer further enhanced the density and chemical stability of the coating, and the hydrophobic components and silica nanoparticles in the copolymer coating improved the chemical resistance of the coating. The synergistic effect of each layer effectively prevented the chemical media from eroding the substrate.

[0084] Comparative Example 1 showed rapid and severe damage in all three media. The lack of tetraethyl orthosilicate prevented the formation of the sol-gel transition layer, resulting in an incomplete coating system. Chemical media easily penetrated the substrate, leading to blistering, peeling, and substrate corrosion. Comparative Example 2 also showed significant coating damage in all three media. The absence of GLYMO affected the performance of the transition layer, reducing the coating's chemical resistance. Chemical penetration caused blistering, peeling, and substrate corrosion. Comparative Example 3 showed significant loss of gloss in the acidic medium and slight whitening, loss of gloss, and softening in the alkaline medium. The absence of cerium nitrate hexahydrate had some impact on the transition layer, causing a decrease in coating performance under chemical action, but the damage was less severe compared to Comparative Examples 1 and 2. Comparative Example 4 showed blistering and softening in all three media. The absence of 4-distyrene affected the copolymer's performance, reducing the coating's chemical resistance and causing damage under chemical action. Comparative Example 5 showed rapid and severe swelling, wrinkling, and softening in all three media. Without the addition of dimethyl itaconic acid, the properties of the copolymer are altered, making it unable to effectively resist chemical erosion, and the coating is quickly damaged.

[0085] As shown in Tables 4-5, the RA values ​​of Examples 1-4 were all 10 / 9, with only slight changes on the surface; no blistering, cracking, or peeling was observed. The multilayer coating structure exhibits excellent resistance to salt spray corrosion. The anodic oxide underlayer, sol-gel transition layer, and copolymer coating work together to effectively prevent corrosive substances such as chloride ions in the salt spray from eroding the substrate, thus protecting the aluminum-based material.

[0086] Comparative Example 1 had an RA value of 2 / 2S. Within 100 hours, the coating exhibited extensive blistering and peeling, with widespread corrosion of the base metal. The lack of tetraethyl orthosilicate resulted in an incomplete coating system, failing to effectively resist salt spray corrosion, leading to rapid coating damage and severe base corrosion. Comparative Example 2 had an RA value of 5 / 4. The coating showed numerous medium-sized blisters, some of which ruptured, causing base corrosion. The absence of GLYMO affected the performance of the transition layer, reducing the coating's salt spray corrosion resistance and causing damage in salt spray environments. Comparative Example 3 had an RA value of 7 / 6. The coating was generally intact, but significant base corrosion pits were observed. The absence of cerium nitrate hexahydrate had a certain impact on the transition layer. Although the coating did not blister or peel, salt spray could still penetrate the base, leading to base corrosion. Comparative Example 4 had an RA value of 4 / 3. The coating severely lost its gloss and yellowed, exhibiting numerous small, dense bubbles and a few cracks. Localized areas showed coating softening and loss of adhesion. The absence of 4-diphenyl styrene affects the copolymer's properties, reducing the coating's resistance to salt spray corrosion and leading to various damage phenomena in salt spray environments. In Comparative Example 5, the RA value was 1 / 1S, and the coating completely swelled, wrinkled, and peeled off the substrate within 72 hours, resulting in comprehensive and uniform corrosion of the substrate metal. The absence of dimethyl itaconic acid altered the copolymer's properties, rendering it unable to resist salt spray corrosion, causing rapid coating damage and severe substrate corrosion.

[0087] In summary, this invention constructs a composite coating system on the surface of an aluminum substrate, utilizing the synergistic effect of each component to achieve a coating with good adhesion, high hardness, and excellent corrosion resistance, making it suitable for aluminum substrates with high surface protection requirements.

[0088] In the description of this specification, the reference to terms such as "embodiment," "various embodiments," etc., indicates that a specific feature, structure, material, or characteristic described in connection with that embodiment or preparation example is included in at least one embodiment of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments.

[0089] 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 protective coating on the surface of an aluminum-based material, characterized in that, Includes the following steps: S1. Perform surface pretreatment on the aluminum substrate, and then perform anodizing treatment on the surface of the pretreated aluminum substrate to form an anodized underlayer; S2. Prepare and solidify a sol-gel transition layer on the surface of the anodized substrate; S3. The copolymer coating solution is applied to the surface of the sol-gel transition layer and cured to form a polymer surface layer; The copolymer coating solution in step S3 is prepared by the following steps: 4-Bistyrene and dimethyl itaconic acid were added to N-methylpyrrolidone, and azobisisobutyronitrile was added. The mixture was reacted in an oil bath with stirring for 6-18 hours. After the reaction was completed, the precipitate was separated and purified to obtain a copolymer. The copolymer was added to tetrahydrofuran or N,N-dimethylformamide to prepare a solution with a solid content of 16-20 wt%. Based on the copolymer mass, 15-20 wt% silica nanoparticles, 1.5-2 wt% leveling agent, and 2-3 wt% photoinitiator were added. The mixture was ultrasonically treated for 10-30 minutes to obtain a copolymer coating solution. The mass ratio of 4-distyrene to dimethyl itaconic acid is (1.7-2.9):

1. The total mass concentration of 4-distyrene and dimethyl itaconic acid in N-methylpyrrolidone is 200-350 g / L, and the amount of azobisisobutyronitrile added is 0.8-2.0% of the total mass of 4-distyrene and dimethyl itaconic acid. The oil bath temperature is 68-78℃, and the stirring speed is 100-200rpm.

2. The preparation method according to claim 1, characterized in that, The anodizing process in step S1 is as follows: using an aluminum substrate as the anode, applying a DC voltage of 10-20V to the electrolyte for 20-40 minutes, and maintaining the electrolyte temperature at 20±2℃. The electrolyte, by concentration, contains: sulfuric acid 180-200 g / L, oxalic acid 15-18 g / L and boric acid 5-6 g / L.

3. The preparation method according to claim 1, characterized in that, The sol-gel transition layer in step S2 is specifically prepared by the following steps: Tetraethyl orthosilicate, an epoxy-containing silane coupling agent, and a corrosion inhibitor are subjected to a hydrolysis-condensation reaction under the action of an acidic catalyst, followed by aging to obtain a silica sol. The silica sol is then coated onto the surface of an anodized substrate, and the coated wet film is cured to obtain a sol-gel transition layer.

4. The preparation method according to claim 1, characterized in that, The curing process in step S3 includes: pre-curing at 60-80℃ for 5-15 minutes, curing with ultraviolet light with a wavelength of 300-400nm for 5-30 seconds, and post-curing at 120-130℃ for 20-40 minutes.

5. The preparation method according to claim 3, characterized in that, The epoxy-containing silane coupling agent is selected from γ-glycidyl etheroxypropyltrimethoxysilane, the corrosion inhibitor is selected from cerium salt, and the curing procedure is as follows: first, cure at 80-85℃ for 1-2 hours, then cure at 100-110℃ for 2-3 hours, and finally cure at 110-120℃ for 1-2 hours.

6. The preparation method according to claim 3, characterized in that, The hydrolysis-condensation reaction is carried out at a temperature of 60-70℃; the aging conditions are: aging at room temperature for 48-96 hours; the acid catalyst is selected from at least one of hydrochloric acid, nitric acid or sulfuric acid, and the amount added is such that the pH value of the reaction system is maintained at 2-4.

7. A protective coating on the surface of an aluminum-based material prepared by the preparation method according to any one of claims 1-6.

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