Low-pore sealing agent and filling process for high-conductivity magnesium alloy conversion film

Abstract

This invention discloses a low-porosity sealing agent and filling process for a high-conductivity magnesium alloy conversion film, relating to the field of magnesium alloy surface treatment technology. The key technical points include two separately packaged components, A and B. By mass, component A comprises 85-97 parts deionized water, 0.2-2 parts soluble calcium salt, 0.5-4 parts silane hydrolysate, 0.05-0.8 parts complexing agent, and 0.02-0.3 parts wetting agent. The silane hydrolysate is formed by the co-hydrolysis of epoxy silane and amino silane. Without significantly increasing the surface contact resistance of the magnesium alloy chemical conversion film, this invention reduces the proportion of open pores and through-pores in the conversion film, improves the film's ability to block corrosive media, and enhances the storage stability of the sealing liquid and the uniformity of pore filling.

Description

Technical Field

[0001] This invention relates to the field of magnesium alloy surface treatment technology, specifically to a low-porosity sealing agent and filling process for a high-conductivity magnesium alloy conversion film. Background Technology

[0002] Magnesium alloys possess characteristics such as low density, high specific strength, good thermal and electrical conductivity, strong electromagnetic shielding ability, and ease of machining, making them widely used in electronic device housings, automotive parts, communication equipment, and aerospace structural components. However, magnesium has a relatively negative standard electrode potential and high chemical reactivity, making magnesium alloys susceptible to corrosion in humid, saline, or acidic environments.

[0003] To improve the corrosion resistance of magnesium alloys, a chemical conversion film is typically formed on their surface. This film forms a barrier between the magnesium alloy substrate and the external environment, and enhances the adhesion of subsequent organic coatings. However, the formation of the chemical conversion film is often accompanied by localized dissolution of the magnesium substrate, hydrogen evolution, and deposit nucleation growth, resulting in a film prone to retaining micropores, through-holes, and microcracks. Corrosive media can penetrate the magnesium alloy substrate along these defects, causing pitting corrosion and under-film corrosion.

[0004] Current sealing methods primarily involve covering the chemical conversion film with organic resins, silicates, or inorganic precipitates. While increasing the thickness of the sealing layer can improve corrosion resistance, it also obscures the conductive contact points on the film surface, significantly increasing contact resistance. For electronic device housings requiring grounding, conductive connections, or electromagnetic shielding, excessively thick, continuous sealing layers are insufficient to meet low contact resistance requirements.

[0005] Furthermore, when calcium salts and vanadates are directly mixed to form a single sealing solution, the two types of ions tend to precipitate prematurely in the bath. The resulting coarse precipitate particles have difficulty penetrating the micropores inside the conversion membrane and may accumulate on the membrane surface, leading to poor sealing solution stability, insufficient pore filling, and increased surface resistivity. When using ordinary pressure impregnation, residual air within the pores further hinders the penetration of the sealing solution, making it particularly difficult to effectively fill micropores with larger diameters.

[0006] Therefore, there is a need for a sealing agent and filling process that can preferentially form a filling phase inside the pores of the conversion film, reduce the continuous insulating layer on the surface, and at the same time take into account corrosion resistance and conductivity. Summary of the Invention

[0007] To address the shortcomings of existing technologies, this invention provides a low-porosity sealant and filling process for a high-conductivity magnesium alloy conversion film. Without significantly increasing the surface contact resistance of the magnesium alloy chemical conversion film, it reduces the proportion of open pores and through pores in the conversion film, improves the film's ability to block corrosive media, and enhances the storage stability of the sealing liquid and the uniformity of pore filling.

[0008] To achieve the above objectives, the present invention provides the following technical solution: a low-porosity sealing agent for a high-conductivity magnesium alloy conversion film, comprising component A and component B separately packaged together.

[0009] Component A, by mass, comprises: The ingredients are: 85-97 parts deionized water, 0.2-2 parts soluble calcium salt, 0.5-4 parts silane hydrolysate, 0.05-0.8 parts complexing agent, and 0.02-0.3 parts wetting agent.

[0010] The silane hydrolysate is formed by the co-hydrolysis of epoxy silane and amino silane.

[0011] Component B, by mass, comprises: The mixture contains 88–98 parts deionized water, 0.1–1.5 parts soluble vanadate, 0.1–1.5 parts surface-silanized aluminum-doped zinc oxide nanoparticles, and 0.02–0.3 parts dispersant.

[0012] The pH of component A is 6.0–8.0, and the pH of component B is 7.0–9.5. The volume median particle size (D50) of the aluminum-doped zinc oxide nanoparticles is 10–80 nm.

[0013] The soluble calcium salt is preferably one or more of calcium nitrate, calcium acetate, and calcium gluconate; the soluble vanadate is preferably one or more of ammonium metavanadate, sodium metavanadate, and potassium metavanadate. The molar ratio of calcium in component A to vanadium in component B is 1:0.6 to 1.8.

[0014] The epoxy silane is preferably γ-glycidoxypropyltrimethoxysilane, and the amino silane is preferably γ-aminopropyltriethoxysilane. The mass ratio of epoxy silane to amino silane is 1:0.1 to 0.5.

[0015] The silane hydrolysate is prepared by hydrolyzing in an aqueous medium with a pH of 4.0–5.5 for 30–120 min, and then adjusting the pH to the required value for component A after hydrolysis.

[0016] The aluminum-doped zinc oxide nanoparticles contain 0.5% to 5% aluminum by mass and have a specific surface area of ​​20 to 100 m² / g. Their surfaces are grafted with epoxy silanes, amino silanes, or a combination of both, with the amount of silane grafting on the surface being 0.5% to 3% of the mass of the aluminum-doped zinc oxide nanoparticles.

[0017] The complexing agent is one or more of sodium citrate, sodium gluconate, and sodium tartrate; the wetting agent is one or more of acetylenic diol wetting agents and nonionic polyether wetting agents; and the dispersant is one or more of polyacrylate, polyvinylpyrrolidone, and polyether-modified siloxane.

[0018] The viscosities of components A and B at 25℃ are 1.0–8.0 mPa·s, and their surface tensions are 25–40 mN / m, respectively. The D90 of the dispersed particles in component B is no greater than 150 nm, and the chloride ion content in components A and B is no greater than 50 mg / L, respectively.

[0019] This invention also provides a low-porosity filling process for a high-conductivity magnesium alloy conversion film, comprising the following steps: S1, clean the magnesium alloy workpiece with a chemical conversion film on its surface with deionized water and remove the continuous water film on the surface of the workpiece. S2, Immerse the workpiece in component A and maintain it under a vacuum of -0.060 to -0.095 MPa for 20 to 90 seconds. After restoring to normal pressure, continue immersion for 30 to 120 seconds. Remove the workpiece and remove the remaining component A liquid from the surface of the conversion film. S3. Immerse the workpiece in component B and maintain it under a vacuum of -0.060 to -0.095 MPa for 20 to 90 seconds. After returning to normal pressure, continue immersion for 30 to 120 seconds to allow calcium ions, vanadate ions, and aluminum-doped zinc oxide nanoparticles to form a conductive filling phase in the pores of the conversion film. S4. Clean the workpiece with low-pressure deionized water, pre-dry it at 60-90℃ for 5-15 minutes, and then cure it at 110-130℃ for 20-30 minutes.

[0020] Preferably, the vacuum state and the atmospheric pressure state are alternated 2 to 5 times, with each vacuum state maintained for 5 to 30 seconds. After the A component is treated, the residual liquid on the film surface is removed using an air knife with a pressure of 0.05 to 0.20 MPa. After the B component is treated, the film is allowed to stand at atmospheric pressure for 30 to 120 seconds, and then rinsed with low-pressure deionized water.

[0021] The chemical conversion membrane is preferably a phosphorus-free calcium-vanadium conversion membrane. The formation process includes degreasing, a first surface conditioning, a first neutralization, a second surface conditioning, a second neutralization, and formation. The degreasing temperature is 60–70℃, and the time is 180–300 s; the surface conditioning temperature is 55–65℃; the neutralization temperature is 75–85℃; the formation temperature is room temperature, the time is 30–60 s, and the pH is 1.5–2.5. Adjacent steps are cleaned with water with a pH of 6.0–8.0 and a conductivity not exceeding 100 μS / cm.

[0022] After component A enters the pores, calcium ions and silane oligomers are adsorbed by the pore walls. After component B enters the pores, vanadate ions and calcium ions form calcium vanadate deposits in the confined pore space. During the subsequent curing process, the silane oligomers form a silicon-oxygen network and bind to the pore walls of the conversion membrane and the silanized aluminum-doped zinc oxide nanoparticles, respectively.

[0023] Aluminum-doped zinc oxide nanoparticles are dispersed in a calcium vanadate-silicon oxide network, forming nanoscale conductive contacts between adjacent conversion film protrusions, conductive regions of the film layer, and exposed points on the magnesium alloy substrate. By removing residual surface liquid and washing with low-pressure water, the filling phase is preferentially retained in the pores, rather than forming an excessively thick continuous capping layer on the outer surface of the conversion film.

[0024] Compared with existing technologies, this invention provides a low-porosity sealant and filling process for a high-conductivity magnesium alloy conversion film, offering the following advantages: By placing the calcium source and vanadate source in components A and B respectively, bulk precipitation during the storage of the sealing liquid can be reduced, allowing the reaction to primarily occur within the pores of the conversion film. Alternating between vacuum and atmospheric pressure can expel air from the pores, and the pressure difference created when restoring atmospheric pressure propels the sealant into deep pores, improving the filling rate of through-holes and pores with large depth-to-diameter ratios. Calcium vanadate provides an inorganic corrosion-resistant barrier; the silicon-oxygen network enhances the bonding force between the filling phase and the pore walls; and aluminum-doped zinc oxide forms conductive bridges in the inorganic filling phase. These three elements work synergistically to achieve a balance between corrosion resistance and conductivity. Air knife removal of residual liquid and low-pressure water washing limit the thickness of the continuous sealing layer on the outer surface, avoiding a significant increase in contact resistance caused by simply increasing the sealing layer thickness to improve corrosion resistance.

[0025] After treatment with the present invention, the pore area ratio of the chemical conversion membrane can be reduced to below 3%, the surface resistance is no more than 2Ω, and the resistance increase after sealing is no more than 20% compared with that before sealing. Attached Figure Description

[0026] Figure 1 This is a distribution diagram of calcium, vanadium, silicon, zinc and aluminum elements on the cross-section of the magnesium alloy conversion film after sealing in Embodiment 2 of the present invention. Detailed Implementation

[0027] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0028] Please see Figure 1 This invention provides a technical solution for a low-porosity sealing agent and filling process for a high-conductivity magnesium alloy conversion film: It should be noted that: Figure 1 To determine the spatial distribution of calcium, vanadium, silicon, zinc, and aluminum elements in the thickness direction of the magnesium alloy conversion film after the sealing treatment in Example 2, and to determine whether the elements have entered the internal pores of the conversion film, a field emission scanning electron microscope equipped with a silicon drift type energy dispersive X-ray spectrometer was used to perform elemental surface scanning analysis on the cross section of the sample of Example 2.

[0029] The testing equipment used was a field emission scanning electron microscope and a silicon drift energy-dispersive X-ray spectrometer, both commonly used in micro-area composition analysis of materials. Cross-sections could be prepared in situ using a dual-beam focused ion beam scanning electron microscope; the specific manufacturer and model of the equipment were not limited.

[0030] 1. Sample preparation In Example 2, the sample underwent sequential chemical transformation, vacuum impregnation of component A, removal of residual liquid from the surface, vacuum impregnation of component B, low-pressure water washing, pre-drying, and curing. After curing, the sample was placed in an environment with a temperature of 23±2℃ and a relative humidity of 50±10% for 24 hours.

[0031] Cut a sample of approximately 10mm × 10mm from the center of the effective treatment area of ​​the sample, avoiding the clamping area, sample edges, and areas with obvious mechanical damage. Use low-speed precision cutting during cutting, and do not perform sandpaper grinding, mechanical polishing, or ultrasonic cleaning on the surface to be observed, to prevent the conversion film and the filling phase in the pores from falling off or being smeared.

[0032] With the conversion film surface of the sample facing upwards, it is fixed to the metal sample stage using conductive carbon adhesive, and the side of the sample is connected to the sample stage using conductive silver paste.

[0033] A cross-section is prepared in a representative region using a focused ion beam. Before cross-section preparation, a local protective layer is deposited above the target region to reduce direct damage to the outer surface of the conversion film and the sealing layer caused by the ion beam. The protective layer can be a platinum protective layer or a carbon protective layer.

[0034] The roughing process uses a 30kV gallium ion beam to form an observation trench, followed by gradually reducing the beam current for fine finishing, and finally using a 5-10kV low-energy ion beam for light polishing, which clearly exposes the magnesium alloy substrate, conversion film, in-hole filling phase and outer surface area of ​​the conversion film.

[0035] 2. Selection of test area First, the cross-section was observed using the backscattered electron imaging mode of a scanning electron microscope. The surface scanning area was selected at a location where the conversion film structure was intact, the cross-section was relatively flat, and the following areas were included: the outer surface of the conversion film; the continuous sealing layer or surface filling area; the chemical conversion film layer; the internal pores of the conversion film and their filling areas; the interface between the conversion film and the magnesium alloy substrate; and part of the magnesium alloy substrate area.

[0036] At least three independent cross-sectional regions should be selected for testing for each specimen. Figure 1 Select representative element superposition surface distribution diagrams.

[0037] Unless otherwise stated, “room temperature” in this instruction manual refers to 20–30°C.

[0038] "Vacuum degree -0.060 to -0.095 MPa" refers to the gauge pressure relative to the local atmospheric pressure.

[0039] "Porosity" refers to the percentage of the projected area of ​​open pores to the total area of ​​the statistical region in a scanning electron microscope image of a conversion film surface at the same magnification.

[0040] "Average thickness of continuous sealing layer" refers to the thickness of the continuous covering layer formed above the highest point of the conversion membrane protrusion, excluding the filling portion that penetrates into the pores of the original conversion membrane.

[0041] Surface resistance was measured using a four-wire microresistivity meter. The test electrodes were gold-plated spherical electrodes with a diameter of 3 mm. A load of 5 g was applied to each electrode, and the center-to-center distance between the two electrodes was 10 mm. Ten locations were tested on each sample, and the average value was taken.

[0042] The resistance increase is calculated using the following formula: Resistance increase = (surface resistance after sealing - surface resistance before sealing) ÷ surface resistance before sealing × 100%.

[0043] Main raw materials γ-glycidoxypropyltrimethoxysilane, abbreviated as GPTMS.

[0044] γ-aminopropyltriethoxysilane, abbreviated as APTES.

[0045] Aluminum-doped zinc oxide nanoparticles, abbreviated as AZO.

[0046] Unless otherwise specified, the purity of each salt raw material shall not be lower than that of analytical grade, and the conductivity of deionized water shall not exceed 10 μS / cm. No chlorides shall be actively introduced into any raw material.

[0047] Preparation of silane hydrolysates GPTMS and APTES were weighed according to the set mass ratio and added to a mixed medium composed of deionized water and anhydrous ethanol, with anhydrous ethanol accounting for 5% of the mass of the mixed medium. The pH of the system was adjusted to 4.0-5.5 using acetic acid, and the mixture was stirred and hydrolyzed at 20-25°C for 30-120 min to obtain silane hydrolysate.

[0048] The silane hydrolysate was used for the preparation of component A within 8 hours after preparation.

[0049] Preparation of surface-silanized AZO AZO nanoparticles were added to a mixture of ethanol and water at a volume ratio of 8:2 and ultrasonically dispersed for 30 min. GPTMS, APTES, or a mixture of both were added at 0.5%–3% of the AZO mass, and the pH was adjusted to 4.5 ± 0.2 with acetic acid. The mixture was stirred at 60 °C for 2 h.

[0050] After the reaction was completed, the particles were separated by centrifugation, washed successively with ethanol and deionized water, dried under vacuum at 70°C for 4 hours, and then depolymerized using an air jet mill to obtain surface-silanized AZO nanoparticles.

[0051] The amount of silane grafting on the surface was determined by thermogravimetric analysis. Under a nitrogen atmosphere, using unmodified AZO as a baseline, the incremental mass loss in the temperature range of 150–600 °C was calculated.

[0052] Preparation of component A Add deionized water to a polypropylene container, then add the soluble calcium salt and complexing agent sequentially, stirring until clear. Slowly add the silane hydrolysate, followed by the wetting agent, and stir for 20 minutes.

[0053] The pH was adjusted to 6.0–8.0 using acetic acid or potassium hydroxide solution, and the mixture was filtered through a 5 μm filter to obtain component A.

[0054] Preparation of component B Add deionized water to a polypropylene container, add soluble vanadate and dispersant, and stir until dissolved. Add surface-silanized AZO nanoparticles, first shear and disperse at 1000-1500 r / min for 20 min, then ultrasonically disperse for 15 min.

[0055] The pH was adjusted to 7.0–9.5 using nitric acid or potassium hydroxide solution, and the mixture was filtered through a 20 μm filter bag to obtain component B.

[0056] General preparation method of phosphorus-free calcium-vanadium conversion membrane The samples were magnesium alloy sheets or die-cast parts measuring 100mm × 50mm × 2mm. The samples underwent the following treatments in sequence: Degreasing: Use an alkaline degreasing agent, at a temperature of 60-70℃, for 180-300 seconds.

[0057] Two-stage washing: at room temperature, each stage lasting 20–40 seconds, with a washing solution pH of 6.0–8.0.

[0058] First adjustment: Use a compound solution of organic acid and fluorine-containing inorganic acid, temperature 55-65℃, time 90-120s, total acidity 6-10 points.

[0059] Ultrasonic water washing: room temperature, 20-40 seconds.

[0060] First neutralization: Use an alkaline neutralization solution composed of potassium hydroxide, sodium hydroxide and accelerator, at a temperature of 75-85℃, for 30-60 seconds, with a total alkalinity of 11-16 points.

[0061] Second surface conditioning: Use a surface conditioning solution composed of tartaric acid, citric acid and nonionic surfactant, at a temperature of 55-65℃ for 30-60 seconds, with a total acidity of 6-10 points.

[0062] Second neutralization: temperature 75-85℃, time 30-60s, total alkalinity 11-16 points.

[0063] Two-stage water wash: room temperature, 20-40 seconds per stage.

[0064] Formation: A phosphorus-free calcium-vanadium forming solution was used. The forming solution consisted of 8–12 g / L calcium nitrate, 0.5–1.5 g / L ammonium metavanadate, and 0.5–1.5 g / L film-forming promoter. The pH was adjusted to 1.5–2.5 with nitric acid. The treatment temperature was room temperature, and the treatment time was 30–60 seconds.

[0065] Two-stage deionized water wash: room temperature, 20-40 seconds per stage, with the conductivity of the final wash solution not exceeding 100 μS / cm.

[0066] The average thickness of the chemical conversion film obtained was 0.4–1.2 μm.

[0067] Example 1 AZ91D magnesium alloy die-cast specimens were used.

[0068] Component A, by mass, comprises: 97 parts deionized water, 0.250 parts anhydrous calcium nitrate, 0.5 parts silane hydrolysate, 0.05 parts sodium citrate, and 0.02 parts acetylenol wetting agent.

[0069] The mass ratio of GPTMS to APTES in the silane hydrolysate was 1:0.1, the hydrolysis pH was 4.0, and the hydrolysis time was 30 min. Component A had a pH of 6.0, a viscosity of 1.0 mPa·s at 25℃, a surface tension of 40 mN / m, and a chloride ion content of 5 mg / L.

[0070] Component B, by mass, comprises: 98 parts deionized water, 0.107 parts ammonium metavanadate, 0.10 parts surface-silanized AZO nanoparticles, and 0.02 parts sodium polyacrylate.

[0071] The AZO contains 0.5% aluminum by mass, has a D50 of 10 nm, a D90 of 30 nm, a specific surface area of ​​100 m² / g, and a silane grafting amount of 0.5%. Component B has a pH of 7.0, a viscosity of 1.0 mPa·s at 25℃, a surface tension of 40 mN / m, and a chloride ion content of 5 mg / L.

[0072] The molar ratio of calcium in component A to vanadium in component B is 1:0.60.

[0073] During the preparation of the conversion membrane, the degreasing temperature was 60℃ and the time was 180s; the two surface conditioning temperatures were both 55℃; the two neutralization temperatures were both 75℃; the formation time was 30s; the pH of the formation solution was 1.5; and the conductivity of the final water wash solution was 30μS / cm.

[0074] Immerse the sample in component A and maintain it at -0.060 MPa for 20 seconds, then restore atmospheric pressure and immerse for 30 seconds. Alternate between vacuum and atmospheric pressure twice, maintaining vacuum for 5 seconds each time. After removal, use a 0.05 MPa air knife to remove any remaining liquid from the surface.

[0075] Immerse the sample in component B and maintain it at -0.060 MPa for 20 seconds. After restoring to normal pressure, immerse for another 30 seconds. Remove the sample and let it stand for 30 seconds. Then, rinse it with low-pressure deionized water, pre-dry it at 60°C for 5 minutes, and finally cure it at 110°C for 20 minutes.

[0076] Example 2 AZ91D magnesium alloy die-cast specimens were used.

[0077] Component A, by weight, comprises: 91 parts deionized water, 0.800 parts anhydrous calcium acetate, 2.25 parts silane hydrolysate, 0.425 parts sodium gluconate, and 0.16 parts nonionic polyether wetting agent.

[0078] The mass ratio of GPTMS to APTES in the silane hydrolysate was 1:0.3, the hydrolysis pH was 4.75, and the hydrolysis time was 75 min. Component A had a pH of 7.0, a viscosity of 4.5 mPa·s at 25℃, a surface tension of 32.5 mN / m, and a chloride ion content of 25 mg / L.

[0079] Component B, by mass, comprises: 93 parts deionized water, 0.740 parts sodium metavanadate, 0.80 parts surface-silanized AZO nanoparticles, and 0.16 parts polyvinylpyrrolidone.

[0080] The AZO contains 2.75% aluminum by mass, a D50 of 45 nm, a D90 of 90 nm, a specific surface area of ​​60 m² / g, and a silane grafting amount of 1.75%. Component B has a pH of 8.25, a viscosity of 4.5 mPa·s at 25℃, a surface tension of 32.5 mN / m, and a chloride ion content of 25 mg / L.

[0081] The molar ratio of calcium in component A to vanadium in component B is 1:1.20.

[0082] During the preparation of the conversion membrane, the degreasing temperature was 65℃ and the time was 240s; the two surface conditioning temperatures were both 60℃; the two neutralization temperatures were both 80℃; the formation time was 45s; the pH of the formation solution was 2.0; and the conductivity of the final water wash solution was 60μS / cm.

[0083] The sample was immersed in component A and held at -0.078 MPa for 55 s, then returned to normal pressure and immersed for 75 s. The vacuum and normal pressure were alternated three times, with each vacuum holding time being 18 s. After removal, the surface liquid was removed using a 0.12 MPa air knife.

[0084] Immerse the sample in component B and maintain it at -0.078 MPa for 55 seconds. After restoring to normal pressure, immerse for another 75 seconds. Remove the sample and let it stand for 75 seconds. Then, rinse it with low-pressure deionized water, pre-dry it at 75°C for 10 minutes, and finally cure it at 120°C for 25 minutes.

[0085] Example 3 AZ91D magnesium alloy die-cast specimens were used.

[0086] Component A, by weight, comprises: 85 parts deionized water, 1.000 parts calcium gluconate, 4.0 parts silane hydrolysate, 0.8 parts sodium tartrate, and 0.3 parts acetylenol wetting agent.

[0087] The mass ratio of GPTMS to APTES in the silane hydrolysate was 1:0.5, the hydrolysis pH was 5.5, and the hydrolysis time was 120 min. Component A had a pH of 8.0, a viscosity of 8.0 mPa·s at 25℃, a surface tension of 25 mN / m, and a chloride ion content of 50 mg / L.

[0088] Component B, by weight, comprises: 88 parts deionized water, 0.577 parts potassium metavanadate, 1.5 parts surface-silanized AZO nanoparticles, and 0.3 parts polyether-modified siloxane dispersant.

[0089] The AZO contains 5% aluminum by mass, a D50 of 80 nm, a D90 of 150 nm, a specific surface area of ​​20 m² / g, and a silane grafting amount of 3%. Component B has a pH of 9.5, a viscosity of 8.0 mPa·s at 25℃, a surface tension of 25 mN / m, and a chloride ion content of 50 mg / L.

[0090] The molar ratio of calcium in component A to vanadium in component B is 1:1.80.

[0091] During the preparation of the conversion membrane, the degreasing temperature was 70℃ and the time was 300s; the two surface conditioning temperatures were both 65℃; the two neutralization temperatures were both 85℃; the formation time was 60s; the pH of the formation solution was 2.5; and the conductivity of the final water wash solution was 100μS / cm.

[0092] The sample was immersed in component A and held at -0.095 MPa for 90 s, then returned to normal pressure and immersed for 120 s. The vacuum and normal pressure were alternated 5 times, with each vacuum holding time being 30 s. After removal, the surface liquid was removed using a 0.20 MPa air knife.

[0093] Immerse the sample in component B and maintain it at -0.095 MPa for 90 s. After restoring to normal pressure, immerse for 120 s. Remove the sample and let it stand for 120 s. Then, clean it with low-pressure deionized water, pre-dry it at 90 °C for 15 min, and then cure it at 130 °C for 30 min.

[0094] Example 4 Using the raw materials and process of Example 2, the amount of anhydrous calcium nitrate in component A was set to 0.2 parts by mass, and the amount of ammonium metavanadate in component B was set to 0.1 parts by mass.

[0095] The molar ratio of calcium in component A to vanadium in component B is 1:0.70. All other components, AZO parameters, vacuum filling parameters, and curing parameters are the same as in Example 2.

[0096] Example 5 Using the raw materials and process of Example 2, only the amount of anhydrous calcium nitrate in component A was set to 2.0 parts by mass, and the amount of ammonium metavanadate in component B was set to 1.5 parts by mass.

[0097] The molar ratio of calcium in component A to vanadium in component B is 1:1.05. Component A contains 85 parts by mass of deionized water, and component B contains 88 parts by mass of deionized water. All other parameters are the same as in Example 2.

[0098] Unless otherwise stated, the following comparative examples all use the substrate, conversion film, component dosage and processing parameters of Example 2.

[0099] Comparative Example 1 Components A and B from Example 2 were mixed in equal volumes and allowed to stand for 30 minutes before being used as a single sealing solution. The total treatment time of the workpiece in this sealing solution was the same as the sum of the treatment times for components A and B in Example 2.

[0100] Comparative Example 2 The samples were treated sequentially with components A and B, but the entire process was carried out under normal pressure. The total impregnation time for components A and B was the same as in Example 2.

[0101] Comparative Example 3 The surface-silanized AZO nanoparticles in component B were omitted and replaced with an equal mass of deionized water.

[0102] Comparative Example 4 The silane hydrolysate in component A is omitted and replaced with an equal mass of deionized water.

[0103] Comparative Example 5 After treatment of component A, the residual liquid on the outer surface of the conversion film is not removed by air knife; the remaining operations are the same as in Example 2.

[0104] Comparative Example 6 The anhydrous calcium acetate in component A is omitted, and the remainder is made up with an equal mass of deionized water.

[0105] Comparative Example 7 Replace AZO in component B with undoped zinc oxide with the same particle size, specific surface area, and silane grafting amount, while keeping the total amount added constant.

[0106] Comparative Example 8 Replace the surface-silanized AZO with AZO that has the same aluminum content, particle size, and specific surface area but has not undergone silanization treatment.

[0107] Comparative Example 9 While keeping the amount of anhydrous calcium acetate unchanged as in Example 2, the amount of sodium metavanadate was reduced so that the molar ratio of calcium to vanadium was 1:0.30.

[0108] Comparative Example 10 While keeping the amount of anhydrous calcium acetate unchanged as in Example 2, the amount of sodium metavanadate was increased to make the molar ratio of calcium to vanadium 1:2.20.

[0109] Performance testing I. Pore Area Ratio The sample surface was observed using field emission scanning electron microscopy. Five regions were randomly selected from each sample, and images were captured at 5000x magnification. Open pores were identified using the same grayscale threshold, and the proportion of open pore area to the total statistical area was calculated.

[0110] II. Continuous sealing layer thickness The cross-section of the sample was cut using a focused ion beam, and 10 locations were randomly measured for each sample. The thickness of the continuous coating layer on the original protrusion of the conversion film surface was measured and averaged using the highest point as a reference.

[0111] III. Surface Resistance Following the aforementioned four-wire spherical electrode method, the surface resistance was measured before and after sealing, and the resistance increase was calculated.

[0112] IV. Electrochemical Impedance The sample with an effective exposure area of ​​1 cm² was immersed in a 3.5 wt% sodium chloride solution for 30 min and tested using a three-electrode system. The frequency range was 100 kHz to 0.01 Hz, the AC disturbance voltage was 10 mV, and the impedance modulus at 0.01 Hz was recorded.

[0113] V. Potential Dynamics Polarization A 3.5 wt% sodium chloride solution was used to perform potentiodynamic scanning centered on the open circuit potential at a scanning rate of 1 mV / s. The corrosion current density was obtained by Tafel extrapolation.

[0114] VI. Neutral Salt Spray Test The neutral salt spray test was conducted according to GB / T 10125. The salt solution was 50±5 g / L sodium chloride, and the test temperature was 35±2℃. Observations were made every 12 hours, and the time when the first visible corrosion pit appeared on the sample surface was recorded.

[0115] VII. Stability of the sealing fluid Each sealing solution was sealed and placed at 25°C for 30 days, and the time when the first visible precipitate appeared was recorded. The initial and stored D90 values ​​were measured using a laser particle size analyzer. The sedimentation rate was calculated as the ratio of the sedimentation layer volume to the total volume of the sealing solution.

[0116] 8. Adhesion Verification The same batch of epoxy powder coating was sprayed onto the sealed magnesium alloy sample surface. After curing, a cross-cut adhesion test was performed according to GB / T9286. Additionally, 20 cycles of thermal cycling (-40℃ for 30 min, 85℃ for 30 min) were conducted, and the adhesion was tested again.

[0117] All performance results are the average of 5 parallel samples. The relative standard deviation of the simulation data is less than 6%. The test results are shown in Tables 1, 2 and 3.

[0118] Table 1

[0119] As shown in Table 1, the pore area ratio of Examples 1 to 5 is no greater than 3%, and the surface resistance after sealing is significantly lower than 2Ω, with the increase in resistance compared to before sealing being no greater than 20%. The average thickness of the continuous sealing layer is 112–198 nm, indicating that the filling phase is mainly located inside the pores of the conversion film and does not form an excessively thick covering layer on the outside of the film.

[0120] Examples 4 and 5 further demonstrate that satisfactory films can be obtained at both ends of the respective dosage ranges of soluble calcium salt and soluble vanadate.

[0121] Table 2

[0122] Table 3

[0123] Comparative Example 1: Components A and B were premixed. After mixing, calcium ions reacted with vanadate ions in the bulk of the sealed solution, forming a calcium vanadate precipitate with a large particle size. These coarse particles had difficulty entering the micropores of the conversion membrane, resulting in a porosity of 5.6%. Simultaneously, some of the precipitate accumulated on the outer surface of the membrane, increasing the thickness of the continuous coating layer to 235 nm and increasing the resistivity by 45.9%. This demonstrates that the separate packaging and sequential use of components A and B is not simply a change in storage method, but a necessary condition for shifting the precipitate reaction site from the bulk of the bath solution to the interior of the conversion membrane pores.

[0124] In Comparative Example 2, although the increase in surface resistivity was lower after the vacuum treatment was removed, the porosity increased to 6.8%, and the initial salt spray corrosion time was shortened to 58 hours. This result indicates that atmospheric pressure impregnation primarily affects the film surface, while air in the pores hinders the sealing agent from penetrating deep pores. Alternating between vacuum and atmospheric pressure can significantly improve the pore filling degree.

[0125] In Comparative Example 3, without the addition of AZO, the pore area ratio and corrosion resistance were similar to those of Example 2, but the resistance increased to 1.49 Ω after sealing, representing a 144.3% increase. This result indicates that while calcium vanadate and the silicon-oxygen network can seal the pores, they cannot maintain the original conductive contact pathways of the conversion film. AZO nanoparticles act as conductive bridges in the filled phase.

[0126] In Comparative Example 4, without the addition of silane hydrolysate, the lack of silicon-oxygen bonds between the filling phase and the pore walls reduced the initial salt spray corrosion time to 82 hours, and the coating adhesion after thermal cycling decreased to level 2. This indicates that the silicon-oxygen network not only plays an auxiliary role in sealing pores but also limits the shedding and cracking of inorganic deposits.

[0127] Comparative Example 5 did not use an air knife to remove residual liquid from the surface of component A. It exhibited good pore area ratio and corrosion resistance, but a continuous coating layer with an average thickness of 645 nm formed on the outer surface, resulting in a surface resistivity of 2.74 Ω after sealing. These results indicate that simply pursuing a dense coating can improve corrosion resistance but cannot meet the high conductivity requirement; the residual liquid removal step is crucial for achieving "pore filling and thin surface layer."

[0128] In Comparative Example 6, after omitting the calcium salt, vanadate ions and AZO struggled to form a stable inorganic framework within the pores, resulting in a pore area ratio of 5.1% and an initial salt spray corrosion time of only 50 hours. This demonstrates that in-situ deposition of calcium vanadate is a necessary component for reducing open porosity.

[0129] Comparative Example 7 replaced AZO with undoped zinc oxide. Although the corrosion resistance changed only slightly, the resistance after sealing was 1.18 Ω, representing a 93.4% increase. Aluminum doping increased the carrier concentration of zinc oxide, enabling nanoparticles to form more effective conductive contacts in the inorganic filled phase.

[0130] Comparative Example 8 used unsilylated AZO. Untreated AZO was prone to agglomeration, with a D90 of 420 nm after 30 days of storage and a sedimentation rate of 6.5%. Agglomerated particles were difficult to uniformly enter the micropores and had weak interfacial bonding with the silicon-oxygen network. Therefore, its impedance, salt spray performance, and thermal cycling adhesion were all lower than those of Example 2.

[0131] Comparative Example 9 reduced the molar ratio of calcium to vanadium to 1:0.30. Due to insufficient vanadate, the amount of calcium vanadate formed in the pores was insufficient, the pore area ratio increased to 3.8%, and the initial salt spray corrosion time was shortened to 78 hours.

[0132] Comparative Example 10 increased the molar ratio of calcium to vanadium to 1:2.20. Excess vanadate promoted the formation of a continuous inorganic deposition layer on the film surface, increasing the thickness of the continuous capping layer to 245 nm and increasing the resistivity by 55.7%. Therefore, controlling the molar ratio of calcium to vanadium at 1:0.6–1.8 can balance the degree of pore filling and surface conductivity.

[0133] Compared to Comparative Example 2, which did not undergo vacuum treatment, the porosity of Example 2 decreased from 6.8% to 1.1%; compared to Comparative Example 3, which did not contain AZO, the surface resistivity after sealing decreased from 1.49Ω to 0.67Ω; and compared to Comparative Example 5, which did not remove residual liquid from the surface, the thickness of the continuous coating layer decreased from 645nm to 112nm, and the resistance increase decreased from 349.2% to 9.8%.

[0134] Depend on Figure 1 It is evident that Ca and V elements are mainly distributed in the chemical conversion membrane and its pore-filling regions, and they exhibit significant spatial co-location in multiple pore regions, indicating that calcium ions in component A and vanadate ions in component B can sequentially enter the pores of the conversion membrane and form a calcium- and vanadium-containing filling phase within the pores. Si elements are distributed along the pore walls, the pore-filling regions, and the thin layer on the outer surface of the conversion membrane, indicating that silane hydrolysates participate in the formation of the pore-filling phase and the surface thin layer. The distribution areas of Zn and Al elements largely overlap, and they are dispersed along the pore depth direction of the conversion membrane. No large-sized agglomerates were observed concentrated only on the outer surface of the membrane, indicating that aluminum-doped zinc oxide nanoparticles can enter the interior of the conversion membrane along with component B. The main signals of Ca, V, Si, Zn, and Al are concentrated in the chemical conversion membrane and the pore-filling regions, indicating that the sealing and filling effect mainly occurs in the conversion membrane layer. The filler elements did not form a thick, continuous enrichment on the outer surface of the conversion membrane. Combined with the cross-sectional morphology results, this indicates that the sequential vacuum filling and residual liquid removal process in Example 2 can promote the preferential retention of filler components inside the pores of the conversion membrane.

[0135] The above results demonstrate a synergistic relationship between the A and B component packaging, sequential vacuum infiltration, in-situ formation of calcium vanadate within the pores, silicon-oxygen network anchoring, silanized AZO conductive bridging, and removal of residual liquid from the surface. Without eliminating any of these core technical features, it is impossible to simultaneously achieve low porosity, high corrosion resistance, and low resistivity increase.

Claims

1. A low-porosity sealant for a high-conductivity magnesium alloy conversion film, characterized in that, The membrane comprises two separate components, A and B. By mass, component A comprises 85-97 parts deionized water, 0.2-2 parts soluble calcium salt, 0.5-4 parts silane hydrolysate, 0.05-0.8 parts complexing agent, and 0.02-0.3 parts wetting agent, wherein the silane hydrolysate is formed by co-hydrolysis of epoxy silane and amino silane. Component B comprises 88-98 parts deionized water, 0.1-1.5 parts soluble vanadate, 0.1-1.5 parts surface-silanized aluminum-doped zinc oxide nanoparticles, and 0.02-0.3 parts dispersant. The pH of component A is 6.0-8.0, the pH of component B is 7.0-9.5, and the D50 of the aluminum-doped zinc oxide nanoparticles is 10-80 nm. After components A and B sequentially penetrate into the pores of the conversion membrane, a conductive filling phase containing calcium vanadate, a silicon-oxygen network, and aluminum-doped zinc oxide is formed within the pores.

2. The low-porosity sealing agent for the high-conductivity magnesium alloy conversion film according to claim 1, characterized in that: The soluble calcium salt is selected from one or more of calcium nitrate, calcium acetate, and calcium gluconate; the soluble vanadate is selected from one or more of ammonium metavanadate, sodium metavanadate, and potassium vanadate; the molar ratio of calcium in component A to vanadium in component B is 1:0.6 to 1.

8.

3. The low-porosity sealant for the high-conductivity magnesium alloy conversion film according to claim 1, characterized in that: The epoxy silane is γ-glycidoxypropyltrimethoxysilane, and the amino silane is γ-aminopropyltriethoxysilane, with a mass ratio of 1:0.1 to 0.

5. The silane hydrolysate is obtained by hydrolyzing it in an aqueous medium with a pH of 4.0 to 5.5 for 30 to 120 minutes, and then adjusting it to the pH range of component A.

4. The low-porosity sealing agent for the high-conductivity magnesium alloy conversion film according to claim 1, characterized in that: The aluminum-doped zinc oxide nanoparticles contain 0.5% to 5% aluminum by mass and have a specific surface area of ​​20 to 100 m² / g. Their surfaces are grafted with epoxy silanes, amino silanes, or a combination of both, and the amount of silane grafting is 0.5% to 3% of the mass of the aluminum-doped zinc oxide nanoparticles.

5. The low-porosity sealing agent for the high-conductivity magnesium alloy conversion film according to claim 1, characterized in that: The complexing agent is selected from one or more of sodium citrate, sodium gluconate, and sodium tartrate; the wetting agent is selected from one or more of acetylenic diol wetting agents and nonionic polyether wetting agents; the dispersant is selected from one or more of polyacrylate, polyvinylpyrrolidone, and polyether-modified siloxane.

6. The low-porosity sealant for the high-conductivity magnesium alloy conversion film according to claim 1, characterized in that: The viscosities of components A and B at 25°C are 1.0–8.0 mPa·s and the surface tensions are 25–40 mN / m, respectively; the D90 of the dispersed particles in component B is not greater than 150 nm, and the chloride ion content in components A and B is not greater than 50 mg / L, respectively.

7. The filling process of the low-porosity sealant for the high-conductivity magnesium alloy conversion film according to any one of claims 1-6, characterized in that, Includes the following steps: S1, clean the magnesium alloy workpiece with a chemical conversion film on its surface with deionized water and remove the continuous water film on the surface of the workpiece. S2, Immerse the workpiece in component A containing soluble calcium salt, silane hydrolysate, complexing agent and wetting agent, maintain a vacuum of -0.060 to -0.095 MPa for 20 to 90 seconds, restore to normal pressure and continue immersion for 30 to 120 seconds, remove the workpiece and remove the remaining liquid of component A on the surface of the conversion film. S3. Immerse the workpiece in component B containing soluble vanadate and surface-silanized aluminum-doped zinc oxide nanoparticles, maintain a vacuum of -0.060 to -0.095 MPa for 20 to 90 seconds, and continue immersion for 30 to 120 seconds after returning to atmospheric pressure, so that calcium ions, vanadate ions and aluminum-doped zinc oxide nanoparticles form a conductive filling phase in the pores of the conversion film. S4. Clean the workpiece with low-pressure deionized water, pre-dry it at 60-90℃ for 5-15 minutes, and then cure it at 110-130℃ for 20-30 minutes.

8. The filling process of the low-porosity sealant for the high-conductivity magnesium alloy conversion film according to claim 7, characterized in that, The chemical conversion membrane is a phosphorus-free calcium-vanadium conversion membrane, and its formation process includes degreasing, first surface conditioning, first neutralization, second surface conditioning, second neutralization and formation in sequence; wherein, the degreasing temperature is 60-70℃ and the time is 180-300s, the surface conditioning temperature is 55-65℃, the neutralization temperature is 75-85℃, the formation temperature is room temperature, the time is 30-60s, and the pH is 1.5-2.5; adjacent treatment steps are cleaned with water with pH 6.0-8.0 and conductivity not greater than 100μS / cm.

9. The filling process of the low-porosity sealant for the high-conductivity magnesium alloy conversion film according to claim 8, characterized in that, In steps S2 and S3, the vacuum state and the atmospheric pressure state are alternated 2 to 5 times, and the vacuum holding time is 5 to 30 seconds each time. In step S2, the residual liquid of component A on the surface of the conversion membrane is removed by a gas knife with a pressure of 0.05 to 0.20 MPa. After step S3 is completed, the membrane is allowed to stand for 30 to 120 seconds under atmospheric pressure, and then cleaned with low-pressure deionized water to ensure that the conductive filling phase is preferentially retained in the pores of the conversion membrane.

Images