Magnesium alloy spraying pretreatment process

Abstract

The invention relates to the technical field of alloys, in particular to a magnesium alloy spraying pretreatment process. According to the process, after alkali washing and acid activation are conducted on magnesium alloy, pre-adsorption and main conversion treatment are conducted in sequence. Both the pre-adsorption liquid A and the main conversion liquid B contain polyether / zwitterionic modified colloidal silicon dioxide mother liquor, the mother liquor is obtained through amination, polyether, tertiary amination and zwitterionic modification of colloidal silicon dioxide, and the dispersion stability and interfacial compatibility of the colloidal silicon dioxide in an acidic fluorine-containing environment are remarkably improved. A uniform and compact conversion film is formed on the surface of the treated magnesium alloy, and the coating is high in binding force, high in corrosion resistance, excellent in appearance and suitable for high-end application such as 3C product shells.

Description

Technical Field

[0001] This invention relates to the field of alloy technology, and in particular to a pretreatment process for magnesium alloy spraying. Background Technology

[0002] Magnesium alloys, as lightweight structural materials, are widely used in the casings of 3C products and other fields. However, their high reactivity and poor corrosion resistance often necessitate surface conversion treatments to improve coating adhesion and protective performance. Traditional chromate conversion processes have been phased out due to environmental restrictions. Current mainstream chromium-free conversion technologies mostly employ systems such as fluorozirconates and molybdates, providing basic protection by forming an inorganic conversion film. However, such conversion films are prone to uneven thickness and micropores on complex structural components (such as ribs and chamfers), leading to defects in the coating appearance such as orange peel and gloss fluctuations after spraying.

[0003] To improve film uniformity, existing technologies attempt to introduce colloidal silica as a filler in the conversion solution, using its nanoscale effect to fill microscopic defects in the conversion film. However, colloidal silica is prone to aggregation or sedimentation in conversion solution systems containing fluorine-containing weak acids and high salt ions due to electrolyte action. This not only results in the loss of its nano-filling function but also causes localized particle accumulation and increased roughness in the conversion film. Especially on large flat surfaces such as laptop casings, this non-uniformity amplifies differences in gloss and reflectivity after coating, making it difficult to meet high-end aesthetic requirements.

[0004] Furthermore, even with simple modification of colloidal silica using silane coupling agents, its surface chemical properties remain ill-suited to the dynamic adsorption-film formation process in acidic conversion environments. Unmodified colloidal silica lacks interfacial compatibility and struggles to disperse stably in the conversion solution, resulting in uneven pre-adsorption layer coverage, disordered orientation of the subsequent conversion film growth, and decreased interfacial adhesion. Therefore, achieving stable dispersion and interfacial directional anchoring of colloidal silica in complex chemical environments while maintaining its nano-functionality has become a key bottleneck in improving the reliability of magnesium alloy pretreatment processes. Summary of the Invention

[0005] In view of this, the purpose of this invention is to propose a pretreatment process for magnesium alloy spraying, which addresses the problems of traditional chromate processes being environmentally unfriendly, and existing chromium-free technologies (such as fluorozirconate systems) having uneven thickness and numerous micropores in the conversion film formed on complex magnesium alloy structures, leading to defects such as orange peel and gloss differences in subsequent coatings.

[0006] To achieve the above objectives, the present invention provides a pretreatment method for magnesium alloy spraying, comprising the following steps: (1) The magnesium alloy substrate is subjected to alkaline washing and acid activation to obtain a pretreated workpiece; (2) Immerse the pretreated workpiece in pre-adsorption solution A for pre-adsorption treatment and then wash it with water; (3) Immerse the workpiece treated in step (2) into the main conversion solution B for conversion treatment and wash it with water to obtain a magnesium alloy workpiece with a conversion film formed on the surface; The pre-adsorption solution A comprises, by mass, 20,000 parts of deionized water, 50-70 parts of diammonium hydrogen citrate, 8-12 parts of ammonium hydrogen fluoride, 15-25 parts of (3-glycidyl etheroxypropyl)trimethoxysilane, and 160-240 parts of the polyetherified / ampholyte modified colloidal silica mother liquor; the pH of the pre-adsorption solution A is 3.9-4.1.

[0007] The main conversion solution B comprises, by mass, 20,000 parts of deionized water, 100-140 parts of diammonium hydrogen citrate, 30-50 parts of sodium molybdate dihydrate, 30-50 parts of ammonium hydrogen fluoride, 15-25 parts of ammonium fluorozirconate, 60-100 parts of (3-glycidyl etheroxypropyl)trimethoxysilane, and 120-200 parts of the polyetherified / ampholyte modified colloidal silica mother liquor; and additionally, 80-100 parts of deionized water are dissolved with 8-12 parts of potassium permanganate and then added; the pH of the main conversion solution B is 3.9-4.1.

[0008] Furthermore, the preparation process of the polyetherified / zwitterionic modified colloidal silica mother liquor includes at least the following: A: Amination of colloidal silica was performed using (3-aminopropyl)triethoxysilane; B: The aminated colloidal silica was polyetherified by methoxy polyethylene glycol-epoxy end-capping. C: Tertiary amine groups were introduced onto the surface of polyetherified colloidal silica using (N,N-dimethyl-3-aminopropyl)trimethoxysilane; D: An amphoteric group was constructed by reacting 1,3-propanesulfonyl lactone with a tertiary amine group.

[0009] Preferably, the solid content of the polyetherified / ampholyte modified colloidal silica mother liquor is 11wt%-13wt%.

[0010] Furthermore, in the preparation of the polyetherified / zwitterionic modified colloidal silica mother liquor, the mass ratio of (3-aminopropyl)triethoxysilane, colloidal silica, methoxy polyethylene glycol-epoxy end-capped, (N,N-dimethyl-3-aminopropyl)trimethoxysilane to 1,3-propane sulfonyl lactone is 4-8:1000:15-25:4-8:4-8.

[0011] Preferably, the number average molecular weight of the methoxy polyethylene glycol-epoxy end-capped material is 10,000.

[0012] Preferably, the colloidal silica has a silica content of 20%-21%, a pH of 2.8-3.2, and an average particle size of 13-16 nm.

[0013] Preferably, in step (2), the pre-adsorption treatment is carried out at 38-42℃ for 45-75s, the workpiece oscillation frequency during the treatment is 8-12 times / min, and the water washing time after the pre-adsorption treatment is 8-12s.

[0014] Preferably, in step (3), the conversion treatment is carried out at 38-42℃ for 160-240s, and the workpiece oscillation frequency during the treatment is 8-12 times / min; after the conversion treatment, the workpiece is immersed in three deionized water tanks for 25-35s each.

[0015] Preferably, after step (3), a sealing treatment step is further included: the magnesium alloy workpiece obtained in step (3) is immersed in the sealing liquid for 45-75s and then taken out, rinsed with deionized water for 8-12s and spun dry, and then placed in a hot air oven at 75-85℃ for 8-12min to dry, and cooled to room temperature to obtain the pre-treatment workpiece for spraying; wherein, by mass parts, the sealing liquid contains 80-120 parts of ethanol, 25-35 parts of (3-glycidyl etheroxypropyl)trimethoxysilane and 15-25 parts of (3-aminopropyl)triethoxysilane, and the pH is 3.9-4.1.

[0016] The beneficial effects of this invention are: This invention utilizes a stepwise surface modification of colloidal silica, sequentially introducing amino groups, polyether segments, tertiary amine groups, and zwitterionic structures, thereby endowing it with excellent steric hindrance and charge stabilization in acidic conversion solutions. The hydration layer formed by the polyethylene glycol segments effectively blocks the tendency for electrolyte-induced aggregation, while the zwitterionic structure adapts to different pH environments through dynamic charge balance, significantly improving the dispersion uniformity of particles in the fluorinated zirconium salt and molybdate composite system.

[0017] During the pre-adsorption stage, the modified colloidal silica forms a strong chemical anchor with the magnesium alloy substrate through its multi-functional active groups on the surface, providing uniform nucleation sites for subsequent conversion film growth. Its nano-size effect effectively fills the interfacial micropores, reducing the porosity of the conversion film. At the same time, it promotes the directional deposition of components such as fluorozirconate and molybdate in the main conversion solution through interfacial energy regulation, forming a dense and continuous composite film layer.

[0018] The introduction of permanganate into the main conversion solution and the modified colloidal silica produce a synergistic oxidation effect, further enhancing the crosslinking density and barrier properties of the film. The flexible characteristics of the polyether segments alleviate the internal stress of the conversion film, preventing a decrease in the coating's impact resistance due to excessive brittleness, while the hygroscopic inhibition effect of the zwitterionic structure reduces the probability of interfacial water film formation, thereby delaying the penetration of corrosive media.

[0019] By employing a stepwise process of pre-adsorption and main conversion, a gradient design for the conversion membrane structure was achieved, from bottom-layer anchoring to surface growth, significantly improving the uniformity of membrane coverage on complex structures. The resulting conversion membrane not only exhibits excellent salt spray resistance but also provides a smooth and highly active interfacial base for subsequent powder coating, enabling the coating to possess high gloss, high reflectivity, and long-term environmental reliability. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments. Example 1:

[0021] (1) Weigh 1000g of colloidal silica sol (Nissan Chemical Corporation, model SNOWTEX ST-O, silica content 20.8%, pH 3.0, average particle size 14.5nm) and add it to a beaker, then add 1000g of deionized water and 20g of ethanol; under magnetic stirring conditions of 23℃ and 300r / min, add 25wt% ammonia water dropwise to adjust the pH to 8.5, stir for 8min, and obtain an alkalized and activated colloidal silica dispersion; (2) Under the condition of maintaining the temperature of the dispersion obtained in step (1) at 38°C, weigh 4g of (3-aminopropyl)triethoxysilane and add it dropwise over 8 minutes; continue to stir the reaction at 38°C and 300r / min for 45 minutes to obtain a surface-aminated colloidal silica dispersion. (3) Under the condition of maintaining the temperature of the dispersion obtained in step (2) at 48℃, weigh 15g of methoxy polyethylene glycol-epoxy end-capped (JenKem Beijing Jiankai Technology, model M-PEG-EPOX, number average molecular weight 10000) and add it at once, and continue to stir the reaction at 48℃ and 300r / min for 2h; when the pH is lower than 8.5 during the reaction, add 25% ammonia water to adjust the pH to 8.5 to obtain a surface polyetherified colloidal silica dispersion; (4) Cool the dispersion obtained in step (3) to 38°C and maintain pH 7.8 (adjust with glacial acetic acid or 25wt% ammonia water), weigh 4g of (N,N-dimethyl-3-aminopropyl)trimethoxysilane and add it dropwise over 4min, then stir the reaction at 38°C and 300r / min for 45min to obtain a polyetherified colloidal silica dispersion with tertiary amine groups on the surface; (5) Under the condition of maintaining the dispersion obtained in step (4) at 38°C, weigh 4g of 1,3-propanesulfonyl lactone and add it in 3 portions (each 8min apart). Then raise the temperature to 48°C and continue stirring for 2h. After the reaction is completed, adjust the pH of the system to 3.9 with glacial acetic acid and add deionized water to a total mass of 2100g to obtain polyetherified / ampholyte modified colloidal silica mother liquor with a solid content of 11.12wt%. (6) Take 20,000 g of deionized water and add it to the corrosion-resistant polypropylene tank. Add 50 g of diammonium hydrogen citrate and 8 g of ammonium hydrogen fluoride in sequence and stir until completely dissolved. Then add 15 g of (3-glycidyl etheroxypropyl)trimethoxysilane and continue stirring for 15 min to complete the pre-hydrolysis. Then add 160 g of the polyetherified / ampholyte modified colloidal silica mother liquor obtained in step (5), stir for 8 min, adjust the pH to 3.9 with 25 wt% ammonia water and keep the temperature at 38 ℃. Let it stand for 15 min to obtain the pre-adsorption liquid A. (7) Preparation of main conversion solution B: Take 20,000 g of deionized water and add it to the main conversion tank. Then add 100 g of diammonium citrate, 30 g of sodium molybdate dihydrate, 30 g of ammonium fluoride and 15 g of ammonium fluorozirconate and stir until clear. Then add 60 g of (3-glycidyl etheroxypropyl)trimethoxysilane and stir for 15 min for pre-hydrolysis. Then add 120 g of the polyetherified / ampholyte modified colloidal silica mother liquor obtained in step (5), stir for 8 min, and adjust the pH to 3.9 with 25 wt% ammonia water and keep the temperature at 38 ℃. Take another 80 g of deionized water to dissolve 8 g of potassium permanganate to prepare potassium permanganate concentrate, add it and stir for 1 min to obtain main conversion solution B. (8) Select 10 magnesium alloy AZ91D die castings (single part dimensions 300mm×200mm), first dry grind with 600 grit sandpaper to remove surface burrs and obvious tool marks, then rinse with deionized water for 60s and spin dry; add 400g sodium hydroxide and 200g sodium carbonate to 19400g deionized water and stir to dissolve, heat to 45℃ to obtain alkaline washing solution; immerse the die castings in alkaline washing solution for 150s and gently agitate, then take them out and rinse with deionized water for 25s; then slowly add 600g 65% nitric acid to 19400g deionized water and stir evenly, cool to 23℃ to obtain nitric acid activation solution; immerse the alkaline washed die castings in activation solution for 25s and take them out immediately, then immerse them in two tanks of deionized water for 25s each; obtain pretreated workpieces; (9) Immerse the pretreated workpiece obtained in step (8) in the pre-adsorption solution A obtained in step (6) at 38°C for 45s. During the immersion process, keep the workpiece agitated 8 times / min. After taking it out, rinse it quickly in deionized water for 8s, and then transfer it to the main conversion solution B obtained in step (7) at 38°C for 160s. During the immersion process, keep the workpiece agitated 8 times / min. After the treatment is completed, take it out and rinse it in deionized water in three tanks for 25s each. Then spin dry to obtain the surface modified workpiece. (10) Add 80g of ethanol to 4850g of deionized water and stir evenly. Then add 25g of (3-glycidyl etheroxypropyl)trimethoxysilane and 15g of (3-aminopropyl)triethoxysilane and stir for 8min. Then add glacial acetic acid to adjust the pH to 3.9 and continue stirring for 15min to complete the hydrolysis and ripening to obtain the sealing liquid. Immerse the surface-modified workpiece obtained in step (9) in the sealing liquid for 45s and take it out. Rinse it with deionized water for 8s and spin dry. Then place it in a 75℃ hot air oven to dry for 8min and cool it to room temperature to obtain the pre-treatment workpiece for spraying. Example 2:

[0022] (1) Weigh 1000g of colloidal silica sol (Nissan Chemical Corporation, model SNOWTEX ST-O, silica content 20.8%, pH 3.0, average particle size 14.5nm) and add it to a beaker, then add 1000g of deionized water and 20g of ethanol; under magnetic stirring conditions of 25℃ and 300r / min, add 25wt% ammonia water to adjust the pH to 9, stir for 10min, and obtain an alkalized and activated colloidal silica dispersion; (2) Under the condition of maintaining the temperature of the dispersion obtained in step (1) at 40°C, 6g of (3-aminopropyl)triethoxysilane was weighed and added dropwise over 10min; the reaction was continued at 40°C and 300r / min for 60min to obtain a surface-aminated colloidal silica dispersion. (3) Under the condition of maintaining the temperature of the dispersion obtained in step (2) at 50℃, weigh 20g of methoxy polyethylene glycol-epoxy end-capped (JenKem Beijing Jiankai Technology, model M-PEG-EPOX, number average molecular weight 10000) and add it at once, and continue to stir the reaction at 50℃ and 300r / min for 3h; when the pH is lower than 9 during the reaction, add 25% ammonia water to adjust the pH to 9, and obtain a surface polyetherified colloidal silica dispersion; (4) Cool the dispersion obtained in step (3) to 40°C and maintain pH 8 (adjust with glacial acetic acid or 25wt% ammonia water), weigh 6g of (N,N-dimethyl-3-aminopropyl)trimethoxysilane and add it dropwise over 5min, then stir the reaction at 40°C and 300r / min for 60min to obtain a polyetherified colloidal silica dispersion with tertiary amine groups on the surface; (5) Under the condition of maintaining the dispersion obtained in step (4) at 40°C, weigh 6g of 1,3-propanesulfonyl lactone and add it in 3 portions (each 10min apart). Then raise the temperature to 50°C and continue stirring for 3h. After the reaction is completed, adjust the pH of the system to 4 with glacial acetic acid and add deionized water to a total mass of 2100g to obtain polyetherified / ampholyte modified colloidal silica mother liquor with a solid content of 11.58wt%. (6) Take 20,000 g of deionized water and add it to the corrosion-resistant polypropylene tank. Add 60 g of diammonium hydrogen citrate and 10 g of ammonium hydrogen fluoride in sequence and stir until completely dissolved. Then add 20 g of (3-glycidyl etheroxypropyl)trimethoxysilane and continue stirring for 20 min to complete the pre-hydrolysis. Then add 200 g of the polyetherified / ampholyte modified colloidal silica mother liquor obtained in step (5), stir for 10 min, adjust the pH to 4 with 25 wt% ammonia water and keep the temperature at 40 °C. Let it stand for 20 min to mature and obtain the pre-adsorption liquid A. (7) Preparation of main conversion solution B: Take 20,000 g of deionized water and add it to the main conversion tank. Then add 120 g of diammonium hydrogen citrate, 40 g of sodium molybdate dihydrate, 40 g of ammonium hydrogen fluoride and 20 g of ammonium fluorozirconate and stir until clear. Then add 80 g of (3-glycidyl etheroxypropyl)trimethoxysilane and stir for 20 min for pre-hydrolysis. Then add 160 g of the polyetherified / ampholyte modified colloidal silica mother liquor obtained in step (5), stir for 10 min, and adjust the pH to 4 with 25 wt% ammonia water and keep the temperature at 40 ℃. Take another 90 g of deionized water to dissolve 10 g of potassium permanganate to prepare potassium permanganate concentrate, add it and stir for 2 min to obtain main conversion solution B. (8) Select 10 magnesium alloy AZ91D die castings (single part dimensions 300mm×200mm), first dry grind with 600 grit sandpaper to remove surface burrs and obvious tool marks, then rinse with deionized water for 60s and spin dry; add 400g sodium hydroxide and 200g sodium carbonate to 19400g deionized water and stir to dissolve, heat to 50℃ to obtain alkaline washing solution; immerse the die castings in alkaline washing solution for 180s and gently agitate, then take them out and rinse with deionized water for 30s; then slowly add 600g 65% nitric acid to 19400g deionized water and stir evenly, cool to 25℃ to obtain nitric acid activation solution; immerse the alkaline washed die castings in activation solution for 30s and take them out immediately, then immerse them in two tanks of deionized water for 30s each; obtain pretreated workpieces; (9) Immerse the pretreated workpiece obtained in step (8) in the pre-adsorption solution A obtained in step (6) at 40°C for 60s. During the immersion process, keep the workpiece swinging 10 times / min. After taking it out, rinse it quickly in deionized water for 10s, and then transfer it to the main conversion solution B obtained in step (7) at 40°C for 200s. During the immersion process, keep the workpiece swinging 10 times / min. After the treatment is completed, take it out and rinse it in three deionized water tanks for 30s each. Then spin dry to obtain the surface modified workpiece. (10) Add 100g of ethanol to 4850g of deionized water and stir evenly. Then add 30g of (3-glycidyl etheroxypropyl)trimethoxysilane and 20g of (3-aminopropyl)triethoxysilane and stir for 10min. Then add glacial acetic acid to adjust the pH to 4 and continue stirring for 20min to complete the hydrolysis and ripening to obtain the sealing liquid. Immerse the surface-modified workpiece obtained in step (9) in the sealing liquid for 60s and take it out. Rinse it with deionized water for 10s and spin dry. Then place it in an 80℃ hot air oven to dry for 10min and cool it to room temperature to obtain the pre-treatment workpiece for spraying. Example 3:

[0023] (1) Weigh 1000g of colloidal silica sol (Nissan Chemical Corporation, model SNOWTEX ST-O, silica content 20.8%, pH 3.0, average particle size 14.5nm) and add it to a beaker, then add 1000g of deionized water and 20g of ethanol; under magnetic stirring conditions of 27℃ and 300r / min, add 25wt% ammonia water dropwise to adjust the pH to 9.5, stir for 12min, and obtain an alkalized and activated colloidal silica dispersion; (2) Under the condition of maintaining the temperature of the dispersion obtained in step (1) at 42°C, 8g of (3-aminopropyl)triethoxysilane was weighed and added dropwise over 12 minutes; the reaction was continued at 42°C and 300r / min for 75 minutes to obtain a surface-aminated colloidal silica dispersion. (3) Under the condition of maintaining the temperature of the dispersion obtained in step (2) at 52℃, weigh 25g of methoxy polyethylene glycol-epoxy end-capped (JenKem Beijing Jiankai Technology, model M-PEG-EPOX, number average molecular weight 10000) and add it at once, and continue to stir the reaction at 52℃ and 300r / min for 4h; when the pH is lower than 9.5 during the reaction, add 25% ammonia water to adjust the pH to 9.5 to obtain a surface polyetherified colloidal silica dispersion; (4) Cool the dispersion obtained in step (3) to 42°C and maintain pH 8.2 (adjust with glacial acetic acid or 25wt% ammonia water), weigh 8g of (N,N-dimethyl-3-aminopropyl)trimethoxysilane and add it dropwise over 6min, then stir the reaction at 42°C and 300r / min for 75min to obtain a polyetherified colloidal silica dispersion with tertiary amine groups on the surface; (5) Under the condition of maintaining the dispersion obtained in step (4) at 42°C, weigh 8g of 1,3-propanesulfonyl lactone and add it in 3 portions (each 12min apart). Then raise the temperature to 52°C and continue stirring for 4h. After the reaction is completed, adjust the pH of the system to 4.1 with glacial acetic acid and add deionized water to a total mass of 2100g to obtain polyetherified / ampholyte modified colloidal silica mother liquor with a solid content of 12.05wt%. (6) Take 20,000 g of deionized water and add it to the corrosion-resistant polypropylene tank. Add 70 g of diammonium citrate and 12 g of ammonium bifluoride in sequence and stir until completely dissolved. Then add 25 g of (3-glycidyl etheroxypropyl)trimethoxysilane and continue stirring for 25 min to complete the pre-hydrolysis. Then add 240 g of the polyetherified / ampholyte modified colloidal silica mother liquor obtained in step (5), stir for 12 min, adjust the pH to 4.1 with 25 wt% ammonia water and keep the temperature at 42 ℃. Let it stand for 25 min to mature and obtain pre-adsorption liquid A. (7) Preparation of main conversion solution B: Take 20,000 g of deionized water and add it to the main conversion tank. Then add 140 g of diammonium citrate, 50 g of sodium molybdate dihydrate, 50 g of ammonium fluoride and 25 g of ammonium fluorozirconate and stir until clear. Then add 100 g of (3-glycidyl etheroxypropyl)trimethoxysilane and stir for 25 min for pre-hydrolysis. Then add 200 g of polyetherified / ampholyte modified colloidal silica mother liquor obtained in step (5), stir for 12 min, and adjust the pH to 4.1 with 25 wt% ammonia water and keep the temperature at 42 ℃. Take another 100 g of deionized water to dissolve 12 g of potassium permanganate to prepare potassium permanganate concentrate, add it and stir for 3 min to obtain main conversion solution B. (8) Select 10 magnesium alloy AZ91D die castings (single part dimensions 300mm×200mm), first dry grind with 600 grit sandpaper to remove surface burrs and obvious tool marks, then rinse with deionized water for 60s and spin dry; add 400g sodium hydroxide and 200g sodium carbonate to 19400g deionized water and stir to dissolve, heat to 55℃ to obtain alkaline washing solution; immerse the die castings in alkaline washing solution for 210s and gently agitate, then take them out and rinse in deionized water for 35s; then slowly add 600g 65% nitric acid to 19400g deionized water and stir evenly, cool to 27℃ to obtain nitric acid activation solution; immerse the alkaline washed die castings in activation solution for 35s and take them out immediately, then immerse them in two tanks of deionized water for 35s each; obtain pretreated workpieces; (9) Immerse the pretreated workpiece obtained in step (8) in the pre-adsorption solution A obtained in step (6) at 42°C for 75s. During the immersion process, keep the workpiece swinging 12 times / min. After taking it out, rinse it quickly in deionized water for 12s, and then transfer it to the main conversion solution B obtained in step (7) at 42°C for 240s. During the immersion process, keep the workpiece swinging 12 times / min. After the treatment is completed, take it out and rinse it in three deionized water tanks for 35s each. Then spin dry to obtain the surface modified workpiece. (10) Add 120g of ethanol to 4850g of deionized water and stir evenly. Then add 35g of (3-glycidyl etheroxypropyl)trimethoxysilane and 25g of (3-aminopropyl)triethoxysilane and stir for 12min. Then add glacial acetic acid to adjust the pH to 4.1 and continue stirring for 25min to complete the hydrolysis and ripening to obtain the sealing solution. Immerse the surface-modified workpiece obtained in step (9) in the sealing solution for 75s and take it out. Rinse it with deionized water for 12s and spin dry. Then place it in an 85℃ hot air oven to dry for 12min and cool it to room temperature to obtain the pre-treatment workpiece for spraying.

[0024] Comparative Example 1: The difference between Comparative Example 1 and Example 2 is that in steps (6) and (7) of Example 2, the silica mass of 200g of polyetherified / ampholyte modified colloidal silica mother liquor and 160g of polyetherified / ampholyte modified colloidal silica mother liquor was replaced with colloidal silica sol (Nissan Chemical Corporation, model SNOWTEX ST-O) and deionized water was added to keep the total water volume of the tank solution consistent with that of Example 2. In step (6), 95.2g of colloidal silica sol replaced 200g of mother liquor and 104.8g of deionized water was added. In step (7), 75.9g of colloidal silica sol replaced 160g of mother liquor and 84.1g of deionized water was added. The other conditions were the same as those of Example 2.

[0025] Comparative Example 2: The difference between Comparative Example 2 and Example 2 is that in step (3) of Example 2, methoxy polyethylene glycol-epoxy end-capping is not added and the ring-opening reaction between methoxy polyethylene glycol-epoxy end-capping and surface amino groups does not occur. Instead, the colloidal silica dispersion obtained in step (2) is kept at 50°C and 300 r / min under magnetic stirring for 3 h. When the pH is lower than 9, 25 wt% ammonia water is added to adjust the pH to 9. Then, it directly enters steps (4) and (5) of Example 2 to introduce tertiary amine groups and construct zwitterionic groups through the ring-opening reaction of 1,3-propanesulfonyl lactone. Finally, the pH of the system is adjusted to 4 with glacial acetic acid and deionized water is added to a total mass of 2100 g to obtain the mother liquor. The other conditions are the same as in Example 2.

[0026] Comparative Example 3: The difference between Comparative Example 3 and Example 2 is that (N,N-dimethyl-3-aminopropyl)trimethoxysilane is not added and tertiary amine groups are not introduced in step (4) of Example 2. Instead, the colloidal silica dispersion obtained in step (3) is cooled to 40°C and pH is maintained at 8. The mixture is then stirred under vacuum for 60 min at 40°C and 300 r / min magnetic stirring. Then, step (5) of Example 2 is performed to construct zwitterionic groups by ring-opening reaction of 1,3-propanesulfonyl lactone with the residual amino group in the system. The pH of the system is adjusted to 4 with glacial acetic acid and deionized water is added to a total mass of 2100 g to obtain the mother liquor. The other conditions are the same as in Example 2.

[0027] Comparative Example 4: The difference between Comparative Example 4 and Example 2 is that: in step (5) of Example 2, 1,3-propanesulfonyl lactone is not added and the ring-opening reaction of 1,3-propanesulfonyl lactone with tertiary amine to construct zwitterionic groups does not occur. Instead, after the reaction in step (4), the pH of the system is directly adjusted to 4 with glacial acetic acid and deionized water is added to a total mass of 2100g to obtain the mother liquor; the other conditions are the same as in Example 2.

[0028] Comparative Example 5: The difference between Comparative Example 5 and Example 2 is that the pre-adsorption liquid A treatment is omitted in step (9) of Example 2. Instead, the pre-treated workpiece obtained in step (8) is directly immersed in the main conversion liquid B obtained in step (7) at 40°C for 200s. During the immersion process, the workpiece is kept oscillating 10 times / min. After the treatment is completed, it is taken out and rinsed in three deionized water tanks for 30s each. Then it is spun dry to obtain the surface-modified workpiece. The other conditions are the same as those in Example 2.

[0029] Comparative Example 6: The difference between Comparative Example 6 and Example 2 is that in step (7) of Example 2, potassium permanganate concentrate is not prepared or added. Instead, the pH of the main conversion solution B is adjusted to 4 with 25wt% ammonia water and kept at a constant temperature of 40°C. Then, 90g of deionized water is added and stirred for 2min to directly obtain the main conversion solution B. The other conditions are the same as in Example 2.

[0030] Sample preparation: Pre-treatment workpieces were prepared according to the examples and comparative examples. Ten magnesium alloy AZ91D die-cast parts (single part dimensions 300mm×200mm, including ribs / beveled structure) were selected for each group, consistent with step (8) of Example 2. The batch of substrate, grinding (600 grit sandpaper), alkaline washing, nitric acid activation, deionized water immersion, water volume of the bath solution, bath temperature 40℃, oscillation frequency 10 times / min, and time of each step were strictly consistent with Example 2. When used for coating application performance testing, the pre-treatment workpieces of each group were preheated in an 80℃ hot air oven for 5 minutes and then electrostatic powder spraying was performed (the powder was polyester-TGIC powder coating, model AkzoNobel Interpowder). The spraying voltage was 60kV, the distance between the spray gun and the workpiece was 250mm, the spray gun moving speed was 400mm / s, and two reciprocating sprays were applied. The coating was then cured in a 180℃ hot air circulating oven for 20 minutes to form a powder coating. After curing, all samples were conditioned for 24 hours at 23℃ and 50% relative humidity before various tests were performed. For electrochemical testing, 20mm × 20mm samples were cut from the planar area of ​​the pre-treated workpiece in each group, with only 1.0cm exposed. 2 The effective area is defined, and the remaining areas are sealed with insulating encapsulation material.

[0031] Application Example 1: Powder Coating Thickness and Thickness Uniformity The thickness of the samples after spraying and curing in the examples and comparative examples was measured using the eddy current method without damage. Before the thickness measurement, the eddy current thickness gauge was calibrated at two points using a standard foil with metrological traceability. For each sample, 8 measuring points were selected in the planar area and 8 measuring points were selected in the rib area for measurement and recording. The average thickness of the planar area Tplanar, the average thickness of the rib area Trib, and the maximum thickness difference ΔT = |Trib - Tplanar| were calculated.

[0032] Application Example 2: Surface roughness Ra and waviness Wt of powder coating For the samples of the examples and comparative examples after spraying, curing, and conditioning, roughness and waviness were measured using a stylus-type surface profilometer. The stylus tip radius was 2 μm, the measuring force was 0.75 mN, the sampling length was 0.8 mm, the evaluation length was 4.0 mm, and the travel speed was 0.5 mm / s. Five non-overlapping measurement lines were measured on each sample in the planar area, and Ra and Wt were calculated respectively. The arithmetic mean was taken as the result of that sample. The average of 10 samples in the same group was then calculated and the standard deviation was recorded.

[0033] Application Example 3: Powder Coating with 60° Gloss For the samples of the examples and comparative examples after spraying, curing and conditioning, the specular gloss was measured using a 60° geometric gloss meter. Before testing, the gloss was calibrated to the nominal value using a black glass standard plate. Five measuring points were evenly selected in the planar area for each sample, and each point was measured once. The arithmetic mean of the five points was taken as the gloss value of the sample. The average value was then calculated for 10 samples in the same group.

[0034] Application Example 4: Powder Coating DOI (Distinctness of Image) According to HG / T 4570-2013, the DOI measurement mode of the orange peel meter was used to test the sharpness of the samples after spraying, curing and conditioning of the examples and comparative examples. Before the test, the instrument was calibrated according to the standard plate. Three measuring points were selected in the plane area for each sample and the measurement was repeated three times and the average was taken to obtain the DOI value of the sample. The average value was then calculated for 10 samples in the same group.

[0035] Application Example 5: Neutral Salt Spray Corrosion and Coating Failure Rating According to GB / T 10125-2021, for the samples of the examples and comparisons after spraying, curing, and conditioning, a 100mm long scribing line was prepared on the flat area and cut through to the substrate. The back of the sample and the non-test edges were sealed with salt spray resistant tape to ensure that corrosion entered from the exposed surface. The samples were placed at a 20° angle in a salt spray chamber for a neutral salt spray test. The sodium chloride concentration of the spray solution was 50g / L, the chamber temperature was 35℃, the solution pH was 6.8, and the sedimentation rate was 1.5mL / (80cm). 2 •h), continuous test for 480h; after the test, gently rinse with deionized water and place at 23℃ for 2h, rate the failure morphology such as blistering and rusting according to GB / T 1766-2008, and measure the maximum corrosion expansion width W on both sides of the scribing.

[0036] Application Example 6: Surface Potential Dynamics Polarization in Pre-treatment Before Spraying According to GB / T 24196-2009, 20mm × 20mm samples were cut from the pre-treatment workpieces (unpowder coated) of the examples and comparative examples, and sealed to ensure an effective exposed area of ​​1.0cm². 2 A 3.5% sodium chloride aqueous solution was used as the corrosive medium at a solution temperature of 25°C. Potentiodynamic polarization testing was performed using a three-electrode system (a saturated calomel electrode as the reference electrode and a platinum sheet electrode as the counter electrode). The sample was scanned after its open-circuit potential stabilized in the solution for 30 minutes. The scanning range was from a negative shift of 250 mV to a positive shift of 750 mV relative to the open-circuit potential, with a scan rate of 1 mV / s. The corrosion potential E was obtained using Tafel extrapolation. corr With corrosion current density i corr。

[0037] Application Example 7: Powder Coating Impact Resistance According to GB / T 1732-2020, the samples of the examples and comparative examples after spraying, curing and conditioning were subjected to impact tests using a fixed mass hammer and punch. The impact method was direct impact, and the drop height of the hammer was gradually increased until the coating cracked or peeled off. The maximum drop height H that did not cause damage to the coating was recorded.

[0038] The test results for the above application examples are shown in Table 1.

[0039] Table 1 Application Example Test Results

[0040] Data Analysis: As can be seen from the data of Examples 1-3 in Table 1, the pretreatment process for magnesium alloy spraying of the present invention can obtain a relatively stable film thickness distribution after electrostatic powder spraying. The difference in film thickness between the plane and the rib is kept within a small range. At the same time, the surface roughness and waviness of the coating are generally at a low level, so that the gloss and reflectivity are maintained at the good state required for a high-gloss appearance. After the neutral salt spray test, the blistering level of the coating is low and the corrosion expansion at the scribing is controlled. The potentiodynamic polarization test also shows that the corrosion potential shifts in the positive direction and is accompanied by a decrease in corrosion current density, indicating that the pretreatment film has both interfacial bonding and barrier protection functions. The reason is speculated to be that colloidal silica modified by (3-aminopropyl)triethoxysilane, methoxy polyethylene glycol-epoxy end-capping, (N,N-dimethyl-3-aminopropyl)trimethoxysilane and 1,3-propane sulfonyl lactone can uniformly anchor and regulate the surface micro-energy distribution during the pre-adsorption stage. Subsequently, in the main conversion system containing (3-glycidyl etheroxypropyl)trimethoxysilane, sodium molybdate dihydrate and ammonium fluorozirconate, the continuous nucleation and dense growth of the conversion film are promoted, and the interfacial bonding is further stabilized during the ammonia sealing process. Thus, both appearance and corrosion resistance reliability are taken into account in complex magnesium alloy structural parts such as laptop shells.

[0041] As can be seen from the data in Table 1 for Example 2 and Comparative Example 1, when unmodified colloidal silica is used instead of colloidal silica synergistically modified with (3-aminopropyl)triethoxysilane, methoxy polyethylene glycol-epoxy end-capping, (N,N-dimethyl-3-aminopropyl)trimethoxysilane, and 1,3-propane sulfonyl lactone, the surface roughness and waviness of the powder coating increase significantly, leading to a decrease in gloss and sharpness. Blistering and scratch corrosion propagation after salt spraying are also more easily aggravated, and electrochemical polarization manifests as an increase in corrosion current density. The main reason for this is that unmodified colloidal silica is more prone to agglomeration and deposition in a weakly acidic primary conversion environment, resulting in discontinuous distribution of the pre-adsorption layer and inducing local defects in the subsequent conversion film. In contrast, the multi-component grafting in Example 2 simultaneously provides steric hindrance stability and charge balance on the same particle surface, forming an unpredictable synergistic interface control effect, achieving a synergistic effect of 1+1 greater than 2 in terms of appearance and corrosion resistance.

[0042] As can be seen from the data in Table 1 for Example 2 and Comparative Example 2, when the modified colloidal silica lacks methoxy polyethylene glycol-epoxy end-capping, the coating appearance-related indicators deteriorate, accompanied by a decrease in salt spray and electrochemical corrosion resistance indicators. This may be because the hydrophilic flexible segments provided by the methoxy polyethylene glycol-epoxy end-capping can form a stable hydration layer and enhance the dispersion stability of particles in the conversion solution, thereby constructing a more uniform coverage during the pre-adsorption stage. Without this segment, particles are more prone to flocculation in systems with high ionic strength, resulting in uneven nucleation and defect penetration in the conversion film, making it difficult to simultaneously achieve both appearance and corrosion resistance. This further highlights the synergistic advantages of the flexible segments and charge regulation in Example 2.

[0043] As can be seen from the data in Table 1 for Example 2 and Comparative Example 3, when the modified colloidal silica lacks (N,N-dimethyl-3-aminopropyl)trimethoxysilane, the overall appearance and corrosion resistance tend to decrease. After salt spraying, defects are more likely to extend from the punctured areas to both sides, and electrochemical polarization also manifests as a weakening of corrosion control capabilities. The reason is speculated to be that (N,N-dimethyl-3-aminopropyl)trimethoxysilane provides a key reaction site for the subsequent introduction of zwitterionic structures into 1,3-propanesulfonyl lactone. Its absence weakens the charge balance and salt tolerance of the particle surface, making the pre-adsorption layer more susceptible to damage in the acidic main conversion system. Example 2 achieves dual inhibition of interfacial water film and ion migration through the combination of reactive aminosilane and zwitterionicity.

[0044] As can be seen from the data in Table 1 for Example 2 and Comparative Example 4, when 1,3-propane sulfonyl lactone is missing from the modified colloidal silica, although there is still some improvement due to silane grafting and methoxy polyethylene glycol-epoxy end-capping, the performance is significantly weakened in terms of salt spray and electrochemical corrosion resistance, accompanied by a simultaneous decline in appearance indicators. The main reason is that without 1,3-propane sulfonyl lactone, it is difficult to form a stable zwitterionic structure on the particle surface, making it more prone to charge shielding and adsorption rearrangement in the weakly acidic main conversion system, thus causing the pre-adsorption layer to be less dense and the micropores of the conversion film to increase. Example 2, on the other hand, uses the combined effect of zwitterionic structure and flexible segments to both inhibit particle aggregation and reduce the probability of interfacial hygroscopicity and ion channel formation.

[0045] As can be seen from the data in Table 1 for Example 2 and Comparative Example 5, when the pre-adsorption solution A step is omitted and the main conversion solution B is directly applied, the film thickness uniformity, salt spray corrosion resistance, and electrochemical protection capabilities all show adverse changes, and continuous corrosion propagation channels are more likely to form at the marked areas. The presumed reason is that the diammonium citrate and ammonium bifluoride in the pre-adsorption solution A can form a controllable activation / complexation state on the magnesium alloy surface, providing more uniform nucleation and anchoring points for the modified colloidal silica and the subsequent ammonium fluorozirconate-molybdate system. Without this pre-adsorption step, the main conversion film relies more on local micro-region differences in the substrate, exhibiting non-uniform growth, and defects are difficult to completely repair in the sealing step.

[0046] As can be seen from the data in Example 2 and Comparative Example 6 in Table 1, when potassium permanganate is lacking in the main conversion solution B, the salt spray and electrochemical corrosion resistance related indicators decrease significantly, but the appearance indicators and impact performance do not deteriorate simultaneously. This may be because potassium permanganate participates in oxidation promotion and defect repair during the main conversion process, which is beneficial for forming a denser conversion film and reducing ion permeation channels. Its absence results in a more porous film layer, leading to decreased corrosion resistance. However, at the same time, due to the reduced degree of oxidative cross-linking and decreased interfacial residual stress, the powder coating is more likely to release energy through microscopic deformation when subjected to impact, resulting in abnormally high impact height values.

[0047] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the invention is limited to these examples; within the framework of the invention, the technical features of the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations of the different aspects of the invention as described above, which are not provided in detail for the sake of brevity.

Claims

1. A magnesium alloy pre-spraying treatment method, characterized by, The method comprises the following steps: (1) alkali washing and acid activation of the magnesium alloy substrate to obtain a pretreated workpiece; (2) pre-adsorption treatment of the pretreated workpiece in pre-adsorption solution A and water washing; (3) conversion treatment of the workpiece treated in step (2) in main conversion solution B and water washing to obtain a magnesium alloy workpiece with a conversion film on the surface; wherein the pre-adsorption solution A comprises, by mass fraction: deionized water 20000 parts, diammonium hydrogen citrate 50-70 parts, ammonium hydrogen fluoride 8-12 parts, (3-glycidyloxypropyl)trimethoxysilane 15-25 parts, and the polyetherized / zwitterionized modified colloidal silica mother liquor 160-240 parts; the main conversion solution B comprises, by mass fraction: deionized water 20000 parts, diammonium hydrogen citrate 100-140 parts, sodium molybdate dihydrate 30-50 parts, ammonium hydrogen fluoride 30-50 parts, ammonium fluorozirconate 15-25 parts, (3-glycidyloxypropyl)trimethoxysilane 60-100 parts, the polyetherized / zwitterionized modified colloidal silica mother liquor 120-200 parts; and, another 80-100 parts of deionized water is taken to dissolve 8-12 parts of potassium permanganate and then added; the preparation process of the polyetherized / zwitterionized modified colloidal silica mother liquor at least comprises: A: aminoization of colloidal silica with (3-aminopropyl)triethoxysilane; B: polyetherization of the aminoized colloidal silica with methoxypolyethylene glycol-epoxy end-capped; C: introduction of tertiary amine groups on the surface of the polyetherized colloidal silica with (N,N-dimethyl-3-aminopropyl)trimethoxysilane; D: construction of zwitterionic groups by reaction of 1,3-propane sulfolane with the tertiary amine groups.

2. The magnesium alloy spray pretreatment method according to claim 1, characterized by, The pH of the pre-adsorption solution A is 3.9-4.1; the pH of the main conversion solution B is 3.9-4.

1.

3. The magnesium alloy spray pretreatment method according to claim 1, characterized by, The solid content of the polyetherized / zwitterionized modified colloidal silica mother liquor is 11wt%-13wt%.

4. The magnesium alloy spray pretreatment method according to claim 1, characterized by, The mass ratio of (3-aminopropyl)triethoxysilane, colloidal silica, methoxypolyethylene glycol-epoxy end-capped, (N,N-dimethyl-3-aminopropyl)trimethoxysilane, and 1,3-propane sulfolane in the raw materials for preparing the polyetherized / zwitterionized modified colloidal silica mother liquor is 4-8:1000:15-25:4-8:4-8.

5. The magnesium alloy spray pretreatment method according to claim 1, characterized by, The number average molecular weight of the methoxypolyethylene glycol-epoxy end-capped is 10000.

6. The magnesium alloy spray pretreatment method according to claim 1, characterized by, The silica content of the colloidal silica is 20%-21%, the pH is 2.8-3.2, and the average particle size is 13-16nm.

7. The magnesium alloy spray pretreatment method according to claim 1, characterized by, The pre-adsorption treatment in step (2) is carried out at 38-42℃ for 45-75s, and the workpiece swings at a frequency of 8-12 times / min during the treatment; the water washing time after the pre-adsorption treatment is 8-12s.

8. The magnesium alloy spray pretreatment method according to claim 1, characterized by, The conversion treatment in step (3) is carried out at 38-42℃ for 160-240s, and the workpiece swings at a frequency of 8-12 times / min during the treatment; the workpiece is immersed in three tanks of deionized water for 25-35s each for washing after the conversion treatment.

9. The magnesium alloy spray pretreatment method according to claim 1, characterized by, After step (3), a sealing treatment step is further included: the magnesium alloy workpiece obtained in step (3) is immersed in a sealing liquid for 45-75 s, then taken out, rinsed with deionized water for 8-12 s and spun dry, and then placed in a hot air oven at 75-85 ℃ for drying for 8-12 min, and cooled to room temperature, to obtain a pretreated workpiece before spraying.

10. The magnesium alloy spray pretreatment method according to claim 9, characterized by, The sealing liquid contains, by mass fraction, 80-120 parts of ethanol, 25-35 parts of (3-glycidyloxypropyl)trimethoxysilane and 15-25 parts of (3-aminopropyl)triethoxysilane, and has a pH of 3.9-4.1.

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