A magnesium alloy surface corrosion-resistant composite coating and a preparation method thereof

Abstract

The application discloses a kind of magnesium alloy surface corrosion-resistant composite coating and preparation method thereof, belong to magnesium alloy surface treatment technical field.The corrosion-resistant composite coating of the application includes transition primer layer, ceramic corrosion-resistant intermediate layer and dense sealing surface layer formed in magnesium alloy substrate surface sequentially from inside to outside;The three-layer synergistic structure of the composite coating is transition primer layer using rare earth modified chemical conversion, micro-arc oxidation ceramic corrosion-resistant intermediate layer, organic-inorganic nanometer composite sealing surface layer, which can effectively improve the bonding strength of the coating and magnesium alloy matrix, reduce the porosity of the coating, the porosity is not higher than 0.32%, significantly prolong the salt spray corrosion time, all above 168h, overcome the technical defects of poor adhesion, many pores and insufficient corrosion resistance of existing magnesium alloy coating, and have good industrial application prospect.

Description

Technical Field

[0001] This invention belongs to the field of magnesium alloy surface treatment technology, and more specifically, relates to a corrosion-resistant composite coating for magnesium alloy surfaces and its preparation method. Background Technology

[0002] Magnesium alloys, as the lightest metallic structural materials currently available, possess numerous advantages such as high specific strength, excellent specific stiffness, good damping and vibration reduction properties, excellent thermal and electrical conductivity, easy machining, and recyclability. They are widely used in automotive parts, aerospace components, electronic device housings, and marine fittings, making them key materials for achieving lightweight design and energy conservation and emission reduction. However, magnesium alloys are extremely chemically reactive and have very low standard electrode potentials. Under conditions such as humid air, salt solutions, and acidic or alkaline environments, they are prone to electrochemical corrosion, leading to pitting corrosion, intergranular corrosion, and general corrosion. This shortens the service life of components and limits the large-scale application of magnesium alloys in high-end equipment and harsh environments.

[0003] Currently, commonly used protective technologies for magnesium alloy surfaces include chemical conversion treatment, anodizing, micro-arc oxidation, electroplating, and organic coating. However, single surface treatment processes have limitations in application: chemical conversion films are usually thin and have limited density, resulting in insufficient protection under long-term corrosive environments; while anodized and micro-arc oxidation films have high hardness and excellent wear resistance, they are prone to inherent defects such as micropores and microcracks, allowing corrosive media to penetrate to the substrate through these defect channels and induce substrate corrosion; the adhesion between organic coatings and magnesium alloy substrates needs improvement, as they are prone to peeling and flaking under humid or mechanical load conditions, and their high-temperature resistance is limited, making them unsuitable for high-temperature applications. It is difficult for a single film or coating to achieve a synergistic optimization of density, adhesion, corrosion resistance, and mechanical properties.

[0004] Existing composite coating technologies mostly employ simple film layer stacking, lacking chemical bonding between layers, resulting in weak interlayer adhesion and easy delamination failure. Furthermore, the preparation process is cumbersome, energy-intensive, and environmentally unfriendly, with some processes using toxic and harmful substances such as chromium and fluorine, which does not meet the requirements of green production.

[0005] Therefore, there is an urgent need to provide a corrosion-resistant composite coating for magnesium alloy surfaces that has strong interlayer bonding, high density, excellent corrosion resistance, green and environmentally friendly processing, and strong adaptability, as well as its preparation method. Summary of the Invention

[0006] To address the aforementioned problems in the existing technology, the technical problem to be solved by this invention is to provide a corrosion-resistant composite coating for magnesium alloy surfaces. This composite coating possesses advantages such as strong interlayer bonding, high density, and excellent corrosion resistance. Another technical problem to be solved by this invention is to provide a method for preparing the above-mentioned corrosion-resistant composite coating for magnesium alloy surfaces, wherein the preparation steps are green and environmentally friendly, and the process parameters are controllable.

[0007] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows: A method for preparing a corrosion-resistant composite coating on a magnesium alloy surface includes the following steps: 1) The pretreated magnesium alloy substrate is immersed in the conversion solution and reacted under the conversion treatment conditions to form a transition layer on the surface of the magnesium alloy substrate; the transition layer is a rare earth modified chemical conversion layer. 2) Using a magnesium alloy substrate with a transition underlayer as the anode, micro-arc oxidation is performed in a micro-arc oxidation electrolyte to form a ceramic corrosion-resistant intermediate layer on the surface of the transition underlayer. 3) The magnesium alloy substrate with a transition underlayer and a ceramic corrosion-resistant intermediate layer is immersed in a sealing liquid. After dip coating and segmented heat curing treatment, an organic-inorganic nanocomposite sealing surface layer is formed on the surface of the ceramic corrosion-resistant intermediate layer, thus obtaining a corrosion-resistant composite coating on the magnesium alloy surface.

[0008] Preferably, in step 1), the pretreatment process of the magnesium alloy is as follows: the surface of the magnesium alloy substrate is polished step by step, and after cleaning, the magnesium alloy substrate is subjected to alkaline washing and acid washing activation treatment in sequence, and then cleaned to neutral.

[0009] Preferably, the magnesium alloy substrate is an AZ31B magnesium alloy sheet.

[0010] Preferably, the progressive grinding uses 400#, 800#, 1200#, and 2000# silicon carbide sandpaper; the cleaning time is 12~20 min; the alkaline washing temperature is 45~55℃ and the time is 6~10 min; the acid washing temperature is 28~35℃ and the time is 1.5~2.5 min.

[0011] Preferably, in step 1), the conversion solution is composed of the following components by mass fraction: 6%~10% sodium silicate, 0.5%~2% rare earth salt, 1%~3% boric acid, 0.2%~0.5% phytic acid, and the balance is deionized water; the conversion solution temperature is 35~45℃, and the treatment time is 10~20min.

[0012] Preferably, the rare earth salt is selected from at least one of cerium nitrate and lanthanum nitrate.

[0013] Preferably, in step 2), the micro-arc oxidation electrolyte is composed of the following components by mass fraction: 8%~12% potassium silicate, 2%~5% sodium aluminate, 0.5%~1% sodium hydroxide, 0.3%~0.8% polyethylene glycol, and the balance is deionized water.

[0014] Preferably, in step 2), the micro-arc oxidation process adopts a constant voltage mode, and the process parameters are set as follows: voltage 350~450V, frequency 400~600Hz, pulse width 200~400μs, and oxidation time 8~15min.

[0015] Preferably, in step 3), the sealing liquid is composed of the following components by mass fraction: 8%~10% γ-glycidyl etheroxypropyltrimethoxysilane, 2%~4% nano titanium dioxide, 1%~3% nano silica, 10%~15% deionized water, 0.5%~1% glacial acetic acid, and the balance being anhydrous ethanol.

[0016] Preferably, in step 3), the dipping time is 1-3 minutes, and the coating is left to air dry at room temperature for 5-10 minutes after dipping; the segmented thermosetting includes a first stage of holding at 60-85°C for 15-25 minutes, and a second stage of holding at 120-140°C for 25-40 minutes.

[0017] The corrosion-resistant composite coating on the magnesium alloy surface is prepared by the method described above.

[0018] The aforementioned corrosion-resistant composite coating on the magnesium alloy surface comprises, from the inside out, a transition underlayer, a ceramic corrosion-resistant intermediate layer, and a dense sealing surface layer formed sequentially on the surface of the magnesium alloy substrate; wherein, the thickness of the transition underlayer is 5~15μm, the thickness of the ceramic corrosion-resistant intermediate layer is 20~30μm, and the thickness of the dense sealing surface layer is 10~20μm.

[0019] Beneficial effects: Compared with the prior art, the present invention has the following advantages: 1) The transition layer of the present invention forms chemical adsorption with the magnesium alloy substrate, and at the same time forms Si-O-Mg and Si-O-Al covalent bonds with the ceramic intermediate layer. The dense sealing surface layer forms chemical bonds with the ceramic intermediate layer through a silane coupling agent. The three-layer structure is gradient fused, and the interlayer bonding force is far greater than that of conventional physical superposition composite coatings, with no peeling, flaking, or delamination. 2) The magnesium alloy surface corrosion-resistant composite coating prepared by the present invention has high hardness of ceramic corrosion-resistant intermediate layer, high overall hardness of composite coating, and excellent wear resistance. At the same time, the transition layer and sealing layer have a certain toughness to avoid coating cracking and peeling, taking into account both hardness and toughness, and adapting to the mechanical wear requirements under complex working conditions. 3) This invention employs a three-layer synergistic structure: a rare earth modified chemical conversion transition base layer, a micro-arc oxidation ceramic corrosion-resistant intermediate layer, and an organic-inorganic nanocomposite sealing surface layer. The rare earth conversion layer not only provides a base bonding but also contains rare earth ions that can participate in the subsequent micro-arc oxidation film formation process, refining the ceramic layer grains and reducing microcracks. The silane coupling agent in the organic-inorganic nano sealing layer undergoes a condensation reaction with the hydroxyl groups on the surface of the ceramic layer to form covalent bonds. At the same time, nano-TiO2 and SiO2 particles physically fill the micropores and construct a cross-linked network, thereby effectively improving the bonding strength between the coating and the magnesium alloy substrate, reducing the coating porosity, and significantly extending the salt spray corrosion resistance time. This overcomes the technical defects of existing magnesium alloy coatings, such as poor bonding strength, numerous pores, and insufficient corrosion resistance. 4) The preparation steps of this invention are green and environmentally friendly, the process parameters are controllable, and it can be adapted to industrial production. Attached Figure Description

[0020] Figure 1 This is a process flow diagram for preparing a corrosion-resistant composite coating on a magnesium alloy surface. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of this invention clearer, the invention is further described below with reference to specific embodiments. Unless otherwise specified, the technical means used in the following embodiments are all conventional means well known to those skilled in the art. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer are followed. Where the manufacturers of reagents or instruments are not specified, they are all conventional products that can be purchased commercially.

[0022] This application provides a method for preparing a corrosion-resistant composite coating on a magnesium alloy surface, such as... Figure 1 As shown, AZ31B magnesium alloy sheet was selected as the substrate. The substrate underwent sequential surface pretreatment to remove oxide scale, processing defects, oil stains, and impurities, providing a clean and activated surface for subsequent coating preparation and ensuring the initial bonding stability between the coating and the substrate. A rare earth silicate transition underlayer was prepared on the pretreated AZ31B magnesium alloy substrate surface. This transition underlayer effectively improves the bonding force between the substrate and the subsequent ceramic intermediate layer, while also enhancing the corrosion resistance of the composite coating. A ceramic corrosion-resistant intermediate layer was prepared on the transition underlayer surface using a micro-arc oxidation process. The high temperature and pressure of micro-arc oxidation formed a dense and hard ceramic layer on the substrate surface, serving as the main corrosion barrier for the composite coating. Finally, a silane-nanocomposite dense sealing surface layer was prepared on the ceramic corrosion-resistant intermediate layer surface to seal the tiny pores on the ceramic intermediate layer surface, further isolating the external corrosive medium from contact with the substrate and improving the overall corrosion resistance of the composite coating.

[0023] In the following examples, the alkaline washing solution used in the pretreatment of the magnesium alloy substrate consists of the following raw materials by mass fraction: 5% sodium carbonate, 1.5% sodium bicarbonate, 0.5% nonionic surfactant, and the balance being deionized water. The pickling solution is a 15% (w / w) mixed solution of dilute phosphoric acid and citric acid, wherein the mass ratio of dilute phosphoric acid to citric acid is 3:1, and the balance is deionized water. The molecular weight of the polyethylene glycol is 4000. Example 1

[0024] A method for preparing a corrosion-resistant composite coating on a magnesium alloy surface includes the following steps: 1. Magnesium alloy substrate pretreatment 1) AZ31B magnesium alloy sheet is selected as the base. The surface of the magnesium alloy sheet is polished step by step with silicon carbide sandpaper of 400#, 800#, 1200# and 2000# to remove the surface oxide scale and processing defects. After polishing, it is rinsed clean with high pressure deionized water. 2) Place the polished magnesium alloy sheet in anhydrous ethanol and ultrasonically clean it for 15 minutes at 400W ultrasonic power to remove surface oil and dust. After removing it, dry it with cold air. 3) Immerse the ultrasonically cleaned magnesium alloy sheet in an alkaline cleaning solution and treat it with alkaline cleaning at 50°C for 8 minutes. After alkaline cleaning, rinse it with deionized water until the surface is neutral. Then immerse it in an acid cleaning solution and treat it with acid cleaning activation at 30°C for 2 minutes. After acid cleaning, rinse it with deionized water until neutral. Dry it with cold air for later use. 2. Transitional underlay preparation 1) Preparation of conversion solution: Weigh out 8% sodium silicate, 1% cerium nitrate, 0.5% lanthanum nitrate, 2% boric acid, and 0.3% phytic acid by mass fraction, with the remainder being deionized water. Add the above components to a container in sequence; stir at room temperature until completely dissolved, and then adjust the pH to 10 to obtain the conversion solution. 2) Immerse the pretreated magnesium alloy in the conversion solution and convert it at 40℃ for 15 min with a stirring speed of 80 r / min. After taking it out, rinse it with deionized water and dry it at 70℃ for 12 min to form a 10 μm transition underlayer. 3. Preparation of Ceramic Corrosion-Resistant Intermediate Layer 1) Preparation of micro-arc oxidation electrolyte: Weigh out 10% potassium silicate, 3% sodium aluminate, 0.8% sodium hydroxide, 0.5% polyethylene glycol, and the remainder deionized water by mass fraction. Stir until completely dissolved, adjust the pH of the electrolyte to 11.5, and control the electrolyte temperature at 20℃. 2) A magnesium alloy plated with a transition layer was used as the anode, and a stainless steel plate as the cathode. Both were immersed in a micro-arc oxidation electrolyte and micro-arc oxidation was performed in a constant voltage mode. The process parameters were set as follows: voltage 400V, frequency 500Hz, pulse width 300μs, and oxidation time 10min. After the micro-arc oxidation treatment, the sample was removed and repeatedly rinsed with deionized water to remove residual electrolyte from the surface. It was then placed in a 110℃ oven and dried for 25min to form a 25μm ceramic corrosion-resistant intermediate layer on the surface of the transition layer. 4. Preparation of dense sealing surface layer 1) Preparation of sealing fluid: Weigh out 9% γ-glycidyl oxypropyltrimethoxysilane, 3% nano titanium dioxide, 2% nano silica, 12% deionized water, 0.8% glacial acetic acid, and the remainder is anhydrous ethanol by mass fraction. Add them to the container and stir until initially mixed. Disperse the mixture by ultrasonic power at 500W for 35 minutes to ensure uniform dispersion of nanoparticles and obtain the sealing fluid. 2) Immerse the magnesium alloy sample with the transition underlayer and ceramic intermediate layer after step 3 into the sealing liquid, dip it at a lifting speed of 6 cm / min for 2 min, and let it air dry at room temperature for 8 min to allow the surface solvent to evaporate initially. 3) The obtained sample was placed in an oven for segmented heat curing: first, it was kept at 80℃ for 20 min, then at 130℃ for 30 min, and after naturally cooling to room temperature, a 15μm sealed surface layer was formed, and a corrosion-resistant composite coating on the magnesium alloy surface was obtained. Example 2

[0025] A method for preparing a corrosion-resistant composite coating on a magnesium alloy surface includes the following steps: 1. Magnesium alloy substrate pretreatment 1) AZ31B magnesium alloy sheet is selected as the base. The surface of the magnesium alloy sheet is polished step by step with silicon carbide sandpaper of 400#, 800#, 1200# and 2000# to remove the surface oxide scale and processing defects. After polishing, it is rinsed clean with high pressure deionized water. 2) Place the polished magnesium alloy sheet in anhydrous ethanol and ultrasonically clean it for 20 minutes at 350W ultrasonic power to remove surface oil and dust. After removing it, dry it with cold air. 3) Immerse the ultrasonically cleaned magnesium alloy sheet in an alkaline cleaning solution and treat it with alkaline cleaning at 55°C for 6 minutes. After alkaline cleaning, rinse it with deionized water until the surface is neutral. Then immerse it in an acid cleaning solution and treat it with acid cleaning activation at 35°C for 2 minutes. After acid cleaning, rinse it with deionized water until neutral. Dry it with cold air for later use. 2. Transitional underlay preparation 1) Preparation of conversion solution: Weigh out 7% sodium silicate, 1.2% cerium nitrate, 0.4% lanthanum nitrate, 2.5% boric acid, and 0.2% phytic acid by mass fraction, with the remainder being deionized water. Add the above components to a container in sequence; stir at room temperature until completely dissolved, and then adjust the pH to 9.5 to obtain the conversion solution. 2) Immerse the pretreated magnesium alloy in the conversion solution and convert it at 45°C with a stirring speed of 80 r / min for 12 min. After taking it out, rinse it with deionized water and dry it at 75°C for 10 min to form an 8 μm transition underlayer. 3. Preparation of Ceramic Corrosion-Resistant Intermediate Layer 1) Preparation of micro-arc oxidation electrolyte: Weigh out 9% potassium silicate, 3.5% sodium aluminate, 0.6% sodium hydroxide, 0.6% polyethylene glycol, and the remainder deionized water by mass fraction. Stir until completely dissolved, adjust the pH of the electrolyte to 11, and control the electrolyte temperature at 25℃. 2) A magnesium alloy plated with a transition layer was used as the anode, and a stainless steel plate as the cathode. Both were immersed in a micro-arc oxidation electrolyte and micro-arc oxidation was performed in a constant voltage mode. The process parameters were set as follows: voltage 380V, frequency 450Hz, pulse width 350μs, and oxidation time 12min. After the micro-arc oxidation treatment, the sample was removed and repeatedly rinsed with deionized water to remove residual electrolyte from the surface. It was then placed in a 105℃ oven and dried for 30min to form a 22μm ceramic corrosion-resistant intermediate layer on the surface of the transition layer. 4. Preparation of dense sealing surface layer 1) Preparation of sealing fluid: Weigh out 8% γ-glycidyl etheroxypropyltrimethoxysilane, 3.5% nano titanium dioxide, 1.5% nano silica, 10% deionized water, 0.7% glacial acetic acid, and the remainder anhydrous ethanol by mass fraction. Add them to the container and stir until initially mixed. Disperse the mixture by ultrasonic power at 450W for 40 minutes to ensure uniform dispersion of the nanoparticles and obtain the sealing fluid. 2) Immerse the magnesium alloy sample with the transition underlayer and ceramic intermediate layer after step 3 into the sealing liquid, dip it at a lifting speed of 5 cm / min for 3 min, and let it air dry at room temperature for 10 min. 3) The obtained sample was placed in an oven for segmented heat curing: first, it was kept at 75℃ for 25 min, then at 125℃ for 35 min, and after naturally cooling to room temperature, a 13μm sealed surface layer was formed, and a corrosion-resistant composite coating on the magnesium alloy surface was obtained. Example 3

[0026] A method for preparing a corrosion-resistant composite coating on a magnesium alloy surface includes the following steps: 1. Magnesium alloy substrate pretreatment 1) AZ31B magnesium alloy sheet is selected as the base. The surface of the magnesium alloy sheet is polished step by step with silicon carbide sandpaper of 400#, 800#, 1200# and 2000# to remove the surface oxide scale and processing defects. After polishing, it is rinsed clean with high pressure deionized water. 2) Place the polished magnesium alloy sheet in anhydrous ethanol and ultrasonically clean it for 12 minutes at 450W ultrasonic power to remove surface oil and dust. After cleaning, dry it with cold air. 3) Immerse the ultrasonically cleaned magnesium alloy sheet in an alkaline cleaning solution and treat it with alkaline cleaning at 45°C for 10 minutes. After alkaline cleaning, rinse it with deionized water until the surface is neutral. Then immerse it in an acid cleaning solution and treat it with acid cleaning activation at 28°C for 3 minutes. After acid cleaning, rinse it with deionized water until neutral. Dry it with cold air for later use. 2. Transitional underlay preparation 1) Preparation of conversion solution: Weigh out 9% sodium silicate, 0.8% cerium nitrate, 0.6% lanthanum nitrate, 1.8% boric acid, and 0.4% phytic acid by mass fraction, with the remainder being deionized water. Add the above components to a container in sequence; stir at room temperature until completely dissolved, and adjust the pH to 10.5 to obtain the conversion solution. 2) Immerse the pretreated magnesium alloy in the conversion solution and convert it at 38°C for 18 min with a stirring speed of 90 r / min. After taking it out, rinse it with deionized water and dry it at 65°C for 15 min to form a 12 μm transition underlayer. 3. Preparation of Ceramic Corrosion-Resistant Intermediate Layer 1) Preparation of micro-arc oxidation electrolyte: Weigh out 11% potassium silicate, 2.5% sodium aluminate, 1% sodium hydroxide, 0.4% polyethylene glycol, and the remainder deionized water by mass fraction. Stir until completely dissolved, adjust the pH of the electrolyte to 12, and control the electrolyte temperature at 18℃. 2) A magnesium alloy plated with a transition layer was used as the anode, and a stainless steel plate as the cathode. Both were immersed in a micro-arc oxidation electrolyte and micro-arc oxidation was performed in a constant voltage mode. The process parameters were set as follows: voltage 420V, frequency 550Hz, pulse width 280μs, and oxidation time 8min. After the micro-arc oxidation treatment, the sample was removed and repeatedly rinsed with deionized water to remove residual electrolyte from the surface. It was then placed in a 115℃ oven and dried for 20min to form a 28μm ceramic corrosion-resistant intermediate layer on the surface of the transition layer. 4. Preparation of dense sealing surface layer 1) Preparation of sealing fluid: Weigh 10% γ-glycidyl oxypropyltrimethoxysilane, 2.5% nano titanium dioxide, 2.5% nano silica, 13% deionized water, 0.9% glacial acetic acid, and the remainder anhydrous ethanol by mass fraction. Add them to the container and stir until initially mixed. Disperse the mixture by ultrasonic power at 550W for 30 minutes to ensure uniform dispersion of the nanoparticles and obtain the sealing fluid. 2) Immerse the magnesium alloy sample with the transition underlayer and ceramic intermediate layer after step 3 into the sealing liquid, dip it at a lifting speed of 7 cm / min for 2 min, and let it air dry at room temperature for 6 min to allow the surface solvent to evaporate initially. 3) The obtained sample was placed in an oven for segmented heat curing: first, it was kept at 85℃ for 18 min, then at 135℃ for 25 min, and after naturally cooling to room temperature, a 17μm sealed surface layer was formed, and a corrosion-resistant composite coating on the magnesium alloy surface was obtained. Example 4

[0027] A method for preparing a corrosion-resistant composite coating on a magnesium alloy surface includes the following steps: 1. Magnesium alloy substrate pretreatment 1) AZ31B magnesium alloy sheet is selected as the base. The surface of the magnesium alloy sheet is polished step by step with silicon carbide sandpaper of 400#, 800#, 1200# and 2000# to remove the surface oxide scale and processing defects. After polishing, it is rinsed clean with high pressure deionized water. 2) Place the polished magnesium alloy sheet in anhydrous ethanol and ultrasonically clean it for 16 minutes at 400W ultrasonic power to remove surface oil and dust. After cleaning, dry it with cold air. 3) Immerse the ultrasonically cleaned magnesium alloy sheet in an alkaline cleaning solution and treat it with alkaline cleaning at 52°C for 7 minutes. After alkaline cleaning, rinse it with deionized water until the surface is neutral. Then immerse it in an acid cleaning solution and treat it with acid cleaning activation at 32°C for 2 minutes. After acid cleaning, rinse it with deionized water until neutral. Dry it with cold air for later use. 2. Transitional underlay preparation 1) Preparation of conversion solution: Weigh out 8.5% sodium silicate, 1% cerium nitrate, 0.5% lanthanum nitrate, 2.2% boric acid, and 0.3% phytic acid by mass fraction, with the remainder being deionized water. Add the above components to a container in sequence; stir at room temperature until completely dissolved, and adjust the pH to 10.2 to obtain the conversion solution. 2) Immerse the pretreated magnesium alloy in the conversion solution and convert it at 42℃ for 14 min with a stirring speed of 85 r / min. After taking it out, rinse it with deionized water and dry it at 72℃ for 13 min to form an 11 μm transition underlayer. 3. Preparation of Ceramic Corrosion-Resistant Intermediate Layer 1) Preparation of micro-arc oxidation electrolyte: Weigh out 10.5% potassium silicate, 3.2% sodium aluminate, 0.9% sodium hydroxide, and 0.5% polyethylene glycol by mass fraction, with the remainder being deionized water. Stir until completely dissolved, adjust the pH of the electrolyte to 11.8, and control the electrolyte temperature at 22℃. 2) A magnesium alloy plated with a transition layer was used as the anode, and a stainless steel plate as the cathode. Both were immersed in a micro-arc oxidation electrolyte and micro-arc oxidation was performed in a constant voltage mode. The process parameters were set as follows: voltage 410V, frequency 520Hz, pulse width 320μs, and oxidation time 9min. After the micro-arc oxidation treatment, the sample was removed and repeatedly rinsed with deionized water to remove residual electrolyte from the surface. It was then placed in a 110℃ oven and dried for 22min to form a 26μm ceramic corrosion-resistant intermediate layer on the surface of the transition layer. 4. Preparation of dense sealing surface layer 1) Preparation of sealing fluid: Weigh out 9.5% γ-glycidyl oxypropyltrimethoxysilane, 3.2% nano titanium dioxide, 1.8% nano silica, 12.5% ​​deionized water, and 0.8% glacial acetic acid by mass fraction, with the remainder being anhydrous ethanol. Add these to a container and stir until initially mixed. Then, ultrasonically disperse the mixture at 500W for 33 minutes to ensure uniform dispersion of the nanoparticles and obtain the sealing fluid. 2) Immerse the magnesium alloy sample with the transition underlayer and ceramic intermediate layer after step 3 into the sealing liquid, dip it at a lifting speed of 7 cm / min for 2 min, and let it air dry at room temperature for 9 min to allow the surface solvent to evaporate initially. 3) The obtained sample was placed in an oven for segmented heat curing: first, it was kept at 82℃ for 22 min, then at 132℃ for 28 min, and after naturally cooling to room temperature, a 16μm sealed surface layer was formed, and a corrosion-resistant composite coating on the magnesium alloy surface was obtained. Example 5

[0028] A method for preparing a corrosion-resistant composite coating on a magnesium alloy surface includes the following steps: 1. Magnesium alloy substrate pretreatment 1) AZ31B magnesium alloy sheet is selected as the base. The surface of the magnesium alloy sheet is polished step by step with silicon carbide sandpaper of 400#, 800#, 1200# and 2000# to remove the surface oxide scale and processing defects. After polishing, it is rinsed clean with high pressure deionized water. 2) Place the polished magnesium alloy sheet in anhydrous ethanol and ultrasonically clean it for 18 minutes at 380W ultrasonic power to remove surface oil and dust. After removing it, dry it with cold air. 3) Immerse the ultrasonically cleaned magnesium alloy sheet in an alkaline cleaning solution and treat it with alkaline cleaning at 48°C for 9 minutes. After alkaline cleaning, rinse it with deionized water until the surface is neutral. Then immerse it in an acid cleaning solution and treat it with acid cleaning activation at 29°C for 2 minutes. After acid cleaning, rinse it with deionized water until neutral. Dry it with cold air for later use. 2. Transitional underlay preparation 1) Preparation of conversion solution: Weigh out 7.5% sodium silicate, 1.1% cerium nitrate, 0.5% lanthanum nitrate, 2.3% boric acid, and 0.3% phytic acid by mass fraction, with the remainder being deionized water. Add the above components to a container in sequence; stir at room temperature until completely dissolved, and then adjust the pH to 9.8 to obtain the conversion solution. 2) Immerse the pretreated magnesium alloy in the conversion solution and convert it at 43℃ for 16 min with a stirring speed of 75 r / min. After taking it out, rinse it with deionized water and dry it at 68℃ for 14 min to form a 9 μm transition underlayer. 3. Preparation of Ceramic Corrosion-Resistant Intermediate Layer 1) Preparation of micro-arc oxidation electrolyte: Weigh out 9.5% potassium silicate, 3.3% sodium aluminate, 0.7% sodium hydroxide, and 0.5% polyethylene glycol by mass fraction, with the remainder being deionized water. Stir until completely dissolved, adjust the pH of the electrolyte to 11.2, and control the electrolyte temperature at 23℃. 2) A magnesium alloy plated with a transition layer was used as the anode, and a stainless steel plate as the cathode. Both were immersed in a micro-arc oxidation electrolyte and micro-arc oxidation was performed in a constant voltage mode. The process parameters were set as follows: voltage 390V, frequency 480Hz, pulse width 340μs, and oxidation time 11min. After the micro-arc oxidation treatment, the sample was removed and repeatedly rinsed with deionized water to remove residual electrolyte from the surface. It was then placed in a 108℃ oven and dried for 28min to form a 24μm ceramic corrosion-resistant intermediate layer on the surface of the transition layer. 4. Preparation of dense sealing surface layer 1) Preparation of sealing fluid: Weigh out 8.5% γ-glycidyl oxypropyltrimethoxysilane, 3.3% nano titanium dioxide, 2.2% nano silica, 11% deionized water, 0.8% glacial acetic acid, and the remainder anhydrous ethanol by mass fraction. Add them to the container and stir until initially mixed. Disperse the mixture by ultrasonic power at 480W for 37 minutes to ensure uniform dispersion of the nanoparticles and obtain the sealing fluid. 2) Immerse the magnesium alloy sample with the transition underlayer and ceramic intermediate layer after step 3 into the sealing liquid, dip it at a lifting speed of 6 cm / min for 2 min, and let it air dry at room temperature for 7 min to allow the surface solvent to evaporate initially. 3) The obtained sample was placed in an oven for segmented heat curing: first, it was kept at 78℃ for 24 min, then at 128℃ for 32 min, and after naturally cooling to room temperature, a 14μm sealed surface layer was formed, and a corrosion-resistant composite coating on the magnesium alloy surface was obtained.

[0029] Comparative Example 1 In preparing the magnesium alloy surface coating, micro-arc oxidation was directly performed on the pretreated AZ31B magnesium alloy surface. All other parameters and preparation processes were exactly the same as in Example 1, resulting in a double-layer coating without a transition underlayer.

[0030] Comparative Example 2 When preparing the magnesium alloy surface coating, after the ceramic corrosion-resistant intermediate layer is prepared, no sealing treatment is performed. All other parameters and preparation processes are exactly the same as in Example 1, resulting in a double-layer coating without a sealing surface layer.

[0031] Comparative Example 3 In the preparation of the magnesium alloy surface coating, the composition of the conversion solution in step 2 is as follows: by mass fraction, sodium silicate 8%, boric acid 2%, phytic acid 0.3%, and the balance is deionized water; the remaining parameters and preparation process are exactly the same as in Example 1, resulting in a composite coating without rare earth modified transition layer.

[0032] Comparative Example 4 In the preparation of the magnesium alloy surface coating, the composition of the sealing liquid in step 4 is: 9% γ-glycidyl etheroxypropyltrimethoxysilane, 12% deionized water, 0.8% glacial acetic acid, and the balance is anhydrous ethanol; the remaining parameters and preparation process are exactly the same as in Example 1, and a sealing layer composite coating without nanoparticles is obtained.

[0033] The coatings prepared in Examples 1-5 and Comparative Examples 1-4 were subjected to performance tests, and the methods were as follows: 1) Neutral Salt Spray Test: The neutral salt spray accelerated corrosion test was conducted according to GB / T10125-2021 "Artificial Atmosphere Corrosion Test - Salt Spray Test". Test conditions: 5% NaCl solution (mass fraction), pH value 6.5–7.2, test temperature (35±2)℃, continuous spraying. The time when obvious white corrosion products, blistering, or rust area appears on the coating surface is recorded as the neutral salt spray test time.

[0034] 2) Self-corrosion potential test: The test was conducted using an electrochemical workstation. The test system was a three-electrode system: the working electrode was the test sample, the reference electrode was a saturated calomel electrode (SCE), and the auxiliary electrode was a platinum sheet electrode; the electrolyte was a 3.5% (w / w) NaCl aqueous solution. After the open-circuit potential of the sample stabilized for 30 min, the stable potential value was recorded, which is the self-corrosion potential, in V vs. SCE.

[0035] 3) Coating adhesion test: The adhesion was tested according to GB / T5210-2006 "Paints and Varnishes - Pull-off test", using a pull-off tester. A special test column was bonded to the coating surface with a strong adhesive. After complete curing at room temperature, a tensile force was applied at a uniform rate, and the maximum tensile force at which the coating interface peeled off was recorded. The coating adhesion was then calculated in MPa.

[0036] 4) Coating porosity test: The porosity was determined using the potassium ferricyanide spot drop method. The test solution formulation was: 10 g / L potassium ferricyanide + 5 g / L sodium chloride + deionized water. The test solution was evenly added to multiple points on the coating surface and kept for 5 minutes. The proportion of the area with blue spots to the total test area was observed and counted, which is the coating porosity (%). The test results are shown in Table 1.

[0037] Table 1. Coating performance test results of Examples 1-5 and Comparative Examples 1-4 As shown in Table 1, the corrosion-resistant composite coatings on magnesium alloy surfaces prepared in Examples 1-5 of this invention all have a neutral salt spray test time of more than 168 hours, a significant positive shift in self-corrosion potential, a coating adhesion of not less than 5.7 MPa, and a porosity of not more than 0.32%, demonstrating excellent overall corrosion resistance.

[0038] Comparative Example 1, without the preparation of a rare earth-modified transition underlayer, showed a significant decrease in the adhesion between the coating and the substrate, a marked increase in porosity, and a substantial reduction in salt spray resistance time. Comparative Example 2, without the preparation of an organic-inorganic nano-dense sealing surface layer, resulted in the inability to effectively seal the pores of the micro-arc oxidation ceramic layer, allowing corrosive media to easily penetrate the substrate along the defects, resulting in the worst corrosion resistance. Comparative Example 3, lacking the addition of cerium nitrate and lanthanum nitrate rare earth components in the transition layer conversion solution, exhibited insufficient density, bonding performance, and protective capability in the transition layer. Comparative Example 4, without the addition of nano-TiO2 and nano-SiO2 in the sealing layer, showed a decrease in the density of the sealing layer, significantly inferior to the embodiments of the present invention.

[0039] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing a corrosion-resistant composite coating on a magnesium alloy surface, characterized in that, Includes the following steps: 1) The pretreated magnesium alloy substrate is immersed in the conversion solution and reacted under the conversion treatment conditions to form a transition layer on the surface of the magnesium alloy substrate; the transition layer is a rare earth modified chemical conversion layer. 2) Using a magnesium alloy substrate with a transition underlayer as the anode, micro-arc oxidation is performed in a micro-arc oxidation electrolyte to form a ceramic corrosion-resistant intermediate layer on the surface of the transition underlayer. 3) The magnesium alloy substrate with a transition underlayer and a ceramic corrosion-resistant intermediate layer is immersed in a sealing liquid. After dip coating and segmented heat curing treatment, an organic-inorganic nanocomposite sealing surface layer is formed on the surface of the ceramic corrosion-resistant intermediate layer, thus obtaining a corrosion-resistant composite coating on the magnesium alloy surface.

2. The method for preparing a corrosion-resistant composite coating on a magnesium alloy surface according to claim 1, characterized in that, In step 1), the pretreatment process of the magnesium alloy is as follows: the surface of the magnesium alloy substrate is polished step by step, and after cleaning, the magnesium alloy substrate is subjected to alkaline washing and acid washing activation treatment in sequence, and then cleaned to neutral.

3. The method for preparing a corrosion-resistant composite coating on a magnesium alloy surface according to claim 1, characterized in that, In step 1), the conversion solution is composed of the following components by mass fraction: sodium silicate 6%~10%, rare earth salt 0.5%~2%, boric acid 1%~3%, phytic acid 0.2%~0.5%, and the balance is deionized water; the conversion solution temperature is 35~45℃, and the treatment time is 10~20min.

4. The method for preparing a corrosion-resistant composite coating on a magnesium alloy surface according to claim 3, characterized in that, The rare earth salt is selected from at least one of cerium nitrate and lanthanum nitrate.

5. The method for preparing a corrosion-resistant composite coating on a magnesium alloy surface according to claim 1, characterized in that, In step 2), the micro-arc oxidation electrolyte is composed of the following components by mass fraction: potassium silicate 8%~12%, sodium aluminate 2%~5%, sodium hydroxide 0.5%~1%, polyethylene glycol 0.3%~0.8%, and the balance is deionized water.

6. The method for preparing a corrosion-resistant composite coating on a magnesium alloy surface according to claim 1, characterized in that, In step 2), the micro-arc oxidation process adopts a constant voltage mode, and the process parameters are set as follows: voltage 350~450V, frequency 400~600Hz, pulse width 200~400μs, and oxidation time 8~15min.

7. The method for preparing a corrosion-resistant composite coating on a magnesium alloy surface according to claim 1, characterized in that, In step 3), the sealing liquid is composed of the following components by mass fraction: 8%~10% γ-glycidyl etheroxypropyltrimethoxysilane, 2%~4% nano titanium dioxide, 1%~3% nano silica, 10%~15% deionized water, 0.5%~1% glacial acetic acid, and the balance being anhydrous ethanol.

8. The method for preparing a corrosion-resistant composite coating on a magnesium alloy surface according to claim 1, characterized in that, In step 3), the dipping time is 1-3 minutes, and after dipping, the material is left to dry at room temperature for 5-10 minutes. The segmented thermosetting includes a first stage of holding at 60-85°C for 15-25 minutes and a second stage of holding at 120-140°C for 25-40 minutes.

9. The magnesium alloy surface corrosion-resistant composite coating prepared by the method of any one of claims 1-8.

10. The corrosion-resistant composite coating on the magnesium alloy surface according to claim 9, characterized in that, It includes a transition underlayer, a ceramic corrosion-resistant intermediate layer, and a dense sealing surface layer formed sequentially from the inside to the outside on the surface of a magnesium alloy substrate; wherein the thickness of the transition underlayer is 5~15μm, the thickness of the ceramic corrosion-resistant intermediate layer is 20~30μm, and the thickness of the dense sealing surface layer is 10~20μm.

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