A method of inhibiting galvanic corrosion of a magnesium alloy part with a dissimilar fastener

Abstract

The application provides a method for inhibiting galvanic corrosion of a magnesium alloy part and a dissimilar fastener, comprising magnesium alloy substrate and galvanized steel bolt nut or rivet connection, at least one of the magnesium alloy substrate or the galvanized steel bolt nut is coated with an organic layer. In the embodiments of the application, the galvanized steel bolt nut is coated with a polyamide coating and the magnesium substrate is electrostatically sprayed with an organic layer. Compared with other solutions for inhibiting galvanic corrosion of a magnesium alloy part, such as using aluminum bolts, the comprehensive performance of the solution is excellent, the cost is low, the operation is simple, and the solution has good industrial application value.

Description

Technical Field

[0001] This application belongs to the field of electro-corrosion protection technology, and in particular relates to a method for inhibiting galvanic corrosion between magnesium alloy parts and dissimilar fasteners. Background Technology

[0002] Magnesium alloys possess advantages such as low density, high specific strength, excellent casting properties, good biocompatibility, and low cost, making them promising for applications in aerospace, defense, automotive manufacturing, and biomedical industries. However, magnesium has a low standard electrode potential, making it prone to galvanic corrosion as an anode when in contact with other metals. Furthermore, the spontaneously formed oxide film on the surface of magnesium alloys is not dense enough to provide the same level of protection as aluminum alloys. Therefore, galvanic corrosion caused by contact between magnesium alloys and other metals is one of the main factors limiting their engineering applications.

[0003] In recent years, the reliability and durability of bolted contact structures have been a focus of attention in the fields of mechanical engineering and materials corrosion science. In service environments, bolted contacts, due to their inherent mechanical and geometric characteristics, become weak points in the structure. On the one hand, the assembly preload and external alternating loads introduce significant stress concentration at the threaded pairs and contact interfaces. This not only directly induces fretting wear and damages surface integrity but also has a synergistic effect with corrosive media, significantly accelerating the localized corrosion process, manifesting as stress corrosion cracking and corrosion fatigue, which are extremely harmful. On the other hand, mechanical friction and scraping during assembly easily damage the protective film on the surface of bolts and nuts, exposing the base metal and forming galvanic corrosion cells, thus triggering abnormally rapid localized corrosion at the pits.

[0004] In the assembly of magnesium alloys with dissimilar metal fasteners (such as steel bolts and nuts), galvanic corrosion has always been a key bottleneck restricting long-term reliability. Currently, the mainstream choices in engineering applications include aluminum bolts and nuts, Teflon (polytetrafluoroethylene, PTFE) coated bolts and nuts, and galvanized steel bolts and nuts. Aluminum bolts and nuts are lightweight and resistant to atmospheric corrosion, but lack strength, their passivation film is easily damaged in acids and alkalis, and contact with magnesium alloys can trigger galvanic corrosion; moreover, they are relatively expensive. Galvanized steel bolts and nuts provide protection through sacrificial anodes, are low in cost, but their zinc layer corrodes faster in chloride or acidic environments, resulting in insufficient durability. Teflon coatings have excellent chemical inertness and impermeability, but their mechanical strength is low and they are easily damaged. From the perspective of the feasibility of bolt and nut surface treatment processes, spraying organic powder onto the surface of bolts and nuts presents the following problems: on the one hand, the threads are subjected to great shear stress during assembly, making the coating easily damaged; on the other hand, the deposition state of the organic powder on the threads and the root of the threads is different, affecting the assembly accuracy of the threads. Summary of the Invention

[0005] This application is made in view of the above-mentioned problems, and its purpose is to provide a method for suppressing galvanic corrosion between magnesium alloy parts and dissimilar fasteners.

[0006] This application provides a method for inhibiting galvanic corrosion between magnesium alloy parts and dissimilar fasteners, characterized by comprising the following steps: An organic layer is coated on at least one surface of a magnesium alloy substrate and a galvanized steel bolt and nut. After coating, the magnesium alloy substrate is connected to the galvanized steel bolts and nuts or riveted.

[0007] In any embodiment, the magnesium alloy substrate is a magnesium alloy substrate with surface phosphating treatment, and some substrates are further coated with polyester powder.

[0008] In any embodiment, the organic layer is a Teflon layer or a polyamide layer.

[0009] In any implementation, it further includes: Silicone rubber is spin-coated into the holes where the magnesium alloy substrate is connected to the galvanized steel bolts, nuts, or rivets.

[0010] In any embodiment, the polyamide layer has a thickness of 100-200 μm; the Teflon layer has a thickness of 20-30 μm.

[0011] In any embodiment, the thickness of the polyester powder layer electrostatically sprayed on the surface of the magnesium alloy substrate is 50-150 μm.

[0012] In any embodiment, the step of coating the surface of the galvanized steel bolt and nut with a polyamide layer specifically includes: S11. Prepared into a polyamide colloid; S12. After the colloid completely covers the bolt and nut, use a spin coater to spin coat at a rate of 1000 rpm for 60 seconds to obtain a uniform coating on the surface of the bolt and nut. S13. Heat and cure the polyamide coating applied to the surface of the bolt and nut at 80℃-120℃ for 10 minutes, and then heat at 150℃ for no less than 10 minutes until the coating is completely hardened.

[0013] In any embodiment, the preparation of the surface-phosphating treated magnesium alloy substrate includes the following steps: S21. Drill screw holes in the magnesium alloy substrate and polish the surface to a smooth finish using 5000-grit sandpaper; S22. Perform alkaline degreasing: Use a weak alkaline solution mainly composed of sodium carbonate, trisodium phosphate and surfactants to immerse or spray the magnesium alloy substrate at 45-55℃ for 5-8 minutes to thoroughly remove grease and dirt from the surface; then rinse thoroughly with running water to ensure no cleaning agent residue remains, and obtain a degreased magnesium alloy substrate. S23. Phosphating treatment: The phosphating solution uses ammonium dihydrogen phosphate as the main salt with a concentration of 15-25 g / L, and adds a small amount of sodium nitrate as a film-forming promoter. Citric acid is also added. The pH value of the phosphating solution is controlled at 3.0-3.8. The degreased magnesium alloy substrate is treated at 40-50℃ for 3-6 min. S24. Immediately rinse the phosphated magnesium alloy substrate with multiple water passes to terminate the reaction and remove impurities, and finally rinse with deionized water; then bake in hot air at 110-130℃ for 10-15 minutes to obtain the phosphated magnesium alloy substrate.

[0014] In any embodiment, the step of spraying polyester powder specifically includes: S31. Provides a phosphated magnesium alloy substrate; S32. Polyester powder is transported by air and uniformly adsorbed onto the surface of a grounded magnesium alloy substrate under a high voltage of 60-80kV. By precisely controlling the powder output, atomizing air pressure and spray gun movement, a uniform powder layer is formed on all surfaces, edges and screw holes of the magnesium alloy substrate in one go. S33. Stabilize the surface temperature of the magnesium alloy substrate at 180-200℃ and maintain it for 15-20 minutes; after cooling, a semi-solid AZ91 magnesium alloy substrate with electrostatic spraying is obtained.

[0015] In any embodiment, a magnesium alloy fastener capable of suppressing galvanic corrosion includes: A magnesium alloy substrate and galvanized steel bolts and nuts, wherein the magnesium alloy substrate and galvanized steel bolts and nuts are connected or riveted together, and at least one surface of the magnesium alloy substrate and galvanized steel bolts and nuts is coated with an organic layer.

[0016] The beneficial effects of this application are as follows: The present invention provides an embodiment in which a polyamide coating is applied to galvanized steel bolts and nuts and an organic layer is electrostatically sprayed onto a magnesium substrate. Compared to a double-layer organic coating, the combination of galvanization and coating results in an atomically deposited galvanized layer that, under stress, flakes off rather than completely breaks down. Even when damage occurs, the undamaged areas continue to provide protection. This combination reduces galvanic corrosion to some extent and is cost-effective. Compared to other methods for inhibiting galvanic corrosion of magnesium alloy components (such as using aluminum bolts), this invention offers superior overall performance, lower cost, and simpler operation, making it highly valuable for industrial applications. Attached Figure Description

[0017] Figure 1 The image shows the corrosion state of Test Example 1 after 240 hours of testing in a 5% NaCl salt spray environment.

[0018] Figure 2 The image shows the corrosion state of Test Example 2 after 240 hours of testing in a 5% NaCl salt spray environment.

[0019] Figure 3 The image shows the corrosion state of Test Example 3 after 240 hours of testing in a 5% NaCl salt spray environment.

[0020] Figure 4 The image shows the corrosion state of Test Example 4 after 240 hours of testing in a 5% NaCl salt spray environment.

[0021] Figure 5 This is a diagram showing the corrosion status of Comparative Example 1 after 240 hours of testing in a 5% NaCl salt spray environment.

[0022] Figure 6 This is a diagram showing the corrosion status of Comparative Example 2 after 240 hours of testing in a 5% NaCl salt spray environment.

[0023] Figure 7 This is a diagram showing the corrosion status of Comparative Example 3 after 240 hours of testing in a 5% NaCl salt spray environment.

[0024] Figure 8 The corrosion status diagrams for Test Example 5 and Comparative Example 4 after 480 hours of testing in a 5% NaCl salt spray environment are shown.

[0025] Figure 9 The image shows the corrosion state of Test Example 6 after 480 hours of testing in a 5% NaCl salt spray environment.

[0026] Figure 10 The corrosion status diagrams for Test Example 7 and Comparative Example 5 after 480 hours of testing in a 5% NaCl salt spray environment are shown.

[0027] Figure 11 This is a cross-sectional SEM image of the organic coating electrostatically sprayed onto the magnesium alloy substrate of this application.

[0028] Figure 12 This is a schematic diagram illustrating the principle of protection and failure of organic coating and zinc plating treatment for bolts and nuts in this application.

[0029] Figure 13 This is a schematic diagram illustrating the principle of improving the corrosion resistance of bolts and nuts through zinc plating and organic coating treatment. Detailed Implementation

[0030] The following detailed description, with appropriate reference to the accompanying drawings, discloses embodiments of the method for suppressing galvanic corrosion between magnesium alloy parts and dissimilar fasteners according to this application. However, unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of practically identical structures may be omitted. This is to avoid unnecessarily lengthy descriptions and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided for the purpose of enabling those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0031] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0032] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0033] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0034] Unless otherwise specified, all steps in this application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the mention that the method may also include step (c) indicates that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0035] Unless otherwise specified, the terms "comprising" and "including" as used in this application can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other components not listed may also be included, or that only the listed components may be included.

[0036] Unless otherwise specified, the term "or" is inclusive in this application. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, the condition "A or B" is satisfied by any of the following conditions: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0037] Based on the protective design concept of isolating dissimilar metals between magnesium alloy parts and fasteners, this application proposes several different types of technical solutions: one is to coat the surface of galvanized bolts and nuts with a coating, which is simple, efficient and easy to implement and promote, and suitable for standardized assembly scenarios; the second is to apply electrostatic powder coating to the surface of magnesium alloy components; and the third is to combine the two. Through a complete process system of pretreatment, spraying and curing, a strong, continuous and dense isolation layer is formed on the substrate surface, thereby achieving comprehensive protection for magnesium alloys.

[0038] Against this backdrop, this application further discovered through systematic experiments that applying polyamide coatings to galvanized bolts and nuts and spraying organic layers onto magnesium alloy components can serve as solutions for the corrosion of magnesium alloy components and bolts. Both methods aim to interrupt the corrosion current path. Although they differ in process and applicable conditions, they together constitute complementary solutions within this patent system. They can be flexibly selected or combined according to the reliability requirements, process conditions, and cost factors of specific application scenarios, providing reliable and flexible technical support for improving the long-term corrosion resistance of magnesium alloy structural components.

[0039] In one embodiment of this application, a method for suppressing galvanic corrosion between magnesium alloy parts and dissimilar fasteners is proposed, comprising connecting a magnesium alloy substrate to a galvanized steel bolt and nut or riveting, wherein at least one surface of the magnesium alloy substrate or the galvanized steel bolt and nut is coated with an organic layer.

[0040] In some embodiments, the magnesium alloy substrate is a semi-solid AZ91D magnesium alloy substrate, which is a semi-solid AZ91D magnesium alloy substrate with surface phosphating treatment.

[0041] In some embodiments, the galvanized steel bolts and nuts are coated with a Teflon layer or a polyamide layer.

[0042] In some embodiments, the magnesium alloy substrate is electrostatically coated with polyester powder. The polyester powder is polyurethane resin with an acid value of approximately 30.

[0043] In some embodiments, the magnesium alloy substrate is connected to galvanized steel bolts and nuts coated with a Teflon layer or a polyamide layer.

[0044] In some embodiments, the magnesium alloy substrate with a surface electrostatically sprayed with polyester powder is connected to galvanized steel bolts and nuts, rivets, galvanized steel bolts and nuts with a Teflon coating, or galvanized steel bolts and nuts with a polyamide coating.

[0045] In some embodiments, a rubber gasket is provided between the magnesium alloy substrate with polyester powder electrostatically sprayed on its surface and the rivet.

[0046] In some embodiments, silicone rubber is spin-coated into the holes where the magnesium alloy substrate is connected to the galvanized steel bolts, nuts, or rivets.

[0047] The silicone rubber used is commercially available 704 silicone rubber, used to isolate corrosive media.

[0048] In some embodiments, the galvanized steel bolt and nut surface is coated with a polyamide layer, including the following steps: S11. Prepared into a polyamide colloid; S12. After the colloid completely covers the bolt and nut, use a spin coater to spin coat at a rate of 1000 rpm for 60 seconds to obtain a uniform coating on the surface of the bolt and nut. S13. Heat and cure the polyamide coating applied to the surface of the bolt and nut at 80℃-120℃ for 10 minutes, and then heat at 150℃ for no less than 10 minutes until the coating is completely hardened.

[0049] In some embodiments, the specific steps of coating curing in step three are as follows: In any embodiment, the phosphating treatment of the semi-solid AZ91 magnesium alloy substrate includes the following steps: S21. Drill screw holes in the semi-solid AZ91 magnesium alloy substrate and polish the surface to a smooth finish using 5000-grit sandpaper; S22. Perform alkaline degreasing: Use a weak alkaline solution mainly composed of sodium carbonate, trisodium phosphate and surfactants to immerse or spray the magnesium alloy substrate at 45-55℃ for 5-8 minutes to thoroughly remove grease and dirt from the surface; then rinse thoroughly with running water to ensure no cleaning agent residue remains, and obtain a degreased magnesium alloy substrate. S23. Phosphating treatment: The phosphating solution uses ammonium dihydrogen phosphate as the main salt with a concentration of 15-25 g / L, and adds a small amount of sodium nitrate as a film-forming promoter. Citric acid is also added. The pH value of the phosphating solution is controlled at 3.0-3.8. The degreased magnesium alloy substrate is treated at 40-50℃ for 3-6 min. S24. Immediately rinse the phosphated magnesium alloy substrate with multiple water passes to terminate the reaction and remove impurities, and finally rinse with deionized water; then bake in hot air at 110-130℃ for 10-15 minutes to obtain a phosphated semi-solid AZ91 magnesium alloy substrate.

[0050] In some embodiments, the electrostatic spraying of the magnesium alloy substrate surface includes the following steps: S31. Obtain a phosphating-treated semi-solid AZ91 magnesium alloy substrate; S32. Polyester powder is transported by air and uniformly adsorbed onto the grounded substrate surface under a high voltage of 60-80kV; by precisely controlling the powder output, atomizing air pressure and spray gun movement, a uniform powder layer is formed on all surfaces, edges and screw holes of the magnesium alloy substrate in one go. S33. Stabilize the surface temperature of the magnesium alloy substrate at 180-200℃ and maintain it for 15-20 minutes; after cooling, a semi-solid AZ91 magnesium alloy substrate with electrostatic spraying is obtained.

[0051] In some embodiments, the polyamide layer coating on the surface of the galvanized steel bolt and nut has a thickness of 100-200 μm; the Teflon layer coating on the surface of the galvanized steel bolt and nut has a thickness of 20-30 μm.

[0052] In some embodiments, the thickness of the polyester powder layer electrostatically sprayed on the surface of the magnesium alloy substrate is 50-150 μm.

[0053] Improper coating thickness directly affects corrosion protection and assembly reliability. If the coating is too thin, it is prone to penetration, failing to completely cover the metal surface, allowing electrolyte to seep in and causing corrosion failure; it is also not wear-resistant, easily worn through during assembly, leaving it unprotected. If the coating is too thick, it is prone to cracking, with high curing stress, causing the coating to easily crack and peel, becoming a corrosion initiation point; assembly is difficult, potentially affecting thread fit and torque control; it is uneconomical, increasing costs while potentially decreasing performance, and making it difficult to achieve uniform process consistency.

[0054] Test case The following describes test examples of this application. The test examples described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the test examples, they shall be performed in accordance with the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all commercially available conventional products.

[0055] Test Example 1 At room temperature, perform the following steps: Step 1: Put the semi-solid AZ91D magnesium alloy substrate with surface phosphating treatment into contact with Teflon-plated galvanized steel bolts and nuts, ensuring that the two are in close contact, to obtain the substrate-bolt contact component; Step 2: Place the contact component in a 35°C 5% sodium chloride solution salt spray atmosphere and observe the surface corrosion morphology in situ for 240 hours to evaluate its corrosion resistance. Step 3: After the 240-hour salt spray test, remove the contact parts to remove corrosion products and record the surface morphology. After testing, its corrosion status under a 240-hour continuous spray environment of 35℃ and 5% NaCl is shown below. Figure 1 .

[0056] Test Example 2 At room temperature, perform the following steps: Step 1: Put the semi-solid AZ91D magnesium alloy substrate with surface phosphate treatment into contact with the polyamide galvanized steel bolt and nut, ensuring that the two are in close contact, to obtain the substrate-bolt contact component; Step 2: Place the contact component in a 35°C 5% sodium chloride solution salt spray atmosphere and observe the surface corrosion morphology in situ for 240 hours to evaluate its corrosion resistance. Step 3: After the 240-hour salt spray test, remove the contact parts to remove corrosion products and record the surface morphology. After testing, its corrosion status under a 240-hour continuous spray environment of 35℃ and 5% NaCl is shown below. Figure 2 .

[0057] Test Example 3 At room temperature, perform the following steps: Step 1: Mill mounting screw holes on the surface of the semi-solid AZ91D magnesium alloy substrate. Then, perform electrostatic powder coating on the magnesium alloy substrate. Assemble the prepared substrate with Teflon steel bolts and nuts and galvanized steel bolts and nuts. After ensuring that the two are in close contact, use epoxy resin to fill and seal the exposed bolts and nuts at the contact point to obtain the substrate-bolt contact.

[0058] Step 2: Place the contact component in a 35°C 5% sodium chloride solution salt spray atmosphere and observe the surface corrosion morphology in situ for 240 hours to evaluate its corrosion resistance. After testing, its corrosion status under a 240-hour continuous spray environment of 35℃ and 5% NaCl is shown below. Figure 3 .

[0059] Test Example 4 At room temperature, perform the following steps: Step 1: Mill mounting screw holes on the surface of the semi-solid AZ91D magnesium alloy substrate. Then, perform electrostatic powder spraying on the magnesium alloy substrate. Assemble the prepared substrate with Teflon-plated galvanized steel bolts and nuts and polyamide-plated galvanized steel bolts and nuts. After ensuring that the two are in close contact, use epoxy resin to fill and seal the exposed bolts and nuts at the contact point to obtain the substrate-bolt contact.

[0060] Step 2: Place the contact component in a 35°C 5% sodium chloride solution salt spray atmosphere and observe the surface corrosion morphology in situ for 240 hours to evaluate its corrosion resistance. After testing, its corrosion status under a 240-hour continuous spray environment of 35℃ and 5% NaCl is shown below. Figure 4 .

[0061] Test Example 5 At room temperature, perform the following steps: Step 1: Mill mounting screw holes on the surface of the semi-solid AZ91D magnesium alloy substrate. Then, perform electrostatic powder spraying on the magnesium alloy substrate. Assemble the prepared substrate with Teflon steel bolts and nuts. Before assembly, apply 704 silicone rubber to the screw holes and nuts to achieve an isolation effect and ensure that the two are in close contact, thus obtaining the substrate-bolt contact.

[0062] Step 2: Place the contact component in a 35°C 5% sodium chloride solution salt spray atmosphere and observe the surface corrosion morphology in situ for 480 hours to evaluate its corrosion resistance. After testing, its corrosion status under a 480-hour continuous spray environment at 35℃ and 5% NaCl is shown below. Figure 8 .

[0063] Test Example 6 At room temperature, perform the following steps: Step 1: Mill mounting screw holes on the surface of the semi-solid AZ91D magnesium alloy substrate. Then, perform electrostatic powder spraying on the magnesium alloy substrate. Assemble the prepared substrate with Teflon-plated galvanized steel bolts and nuts and polyamide-plated galvanized steel bolts and nuts. Before assembly, apply 704 silicone rubber to the screw holes and nuts to achieve an isolation effect and ensure that the two are in close contact, thus obtaining the substrate-bolt contact component.

[0064] Step 2: Place the contact component in a 35°C 5% sodium chloride solution salt spray atmosphere and observe the surface corrosion morphology in situ for 480 hours to evaluate its corrosion resistance. After testing, its corrosion status under a 480-hour continuous spray environment at 35℃ and 5% NaCl is shown below. Figure 9 .

[0065] Test Example 7 At room temperature, perform the following steps: Step 1: Mill mounting screw holes on the surface of the semi-solid AZ91D magnesium alloy substrate, then perform electrostatic powder coating on the magnesium alloy substrate, and install Dacromet rivet screws + aluminum washers ( Figure 8 c) Assemble with an electrostatically sprayed magnesium alloy plate to ensure close contact between the two, resulting in a substrate-rivet contact.

[0066] Step 2: Place the contact component in a 35°C 5% sodium chloride solution salt spray atmosphere and observe the surface corrosion morphology in situ for 240 hours to evaluate its corrosion resistance. After testing, its corrosion status under a 240-hour continuous spray environment of 35℃ and 5% NaCl is shown below. Figure 10 .

[0067] Comparative Example 1 At room temperature, perform the following steps: Step 1: Assemble the semi-solid AZ91D magnesium alloy substrate with surface phosphating treatment with commercial aluminum bolts and nuts, ensuring that the two are in close contact, to obtain the magnesium substrate-bolt contact component; Step 2: Place the contact component under a continuous spray atmosphere of 35℃ 5% sodium chloride solution and observe the surface corrosion morphology for 240 hours to evaluate its corrosion resistance. Step 3: After the 240-hour salt spray test, remove the contact parts to remove corrosion products and record the surface morphology. After testing, its corrosion status under a 240-hour continuous spray environment of 35℃ and 5% NaCl is shown below. Figure 5 .

[0068] Comparative Example 2 At room temperature, perform the following steps: Step 1: Put the semi-solid AZ91D magnesium alloy substrate with surface phosphate treatment into contact with commercial galvanized steel bolts and nuts, ensuring that the two are in close contact, to obtain a magnesium substrate-bolt contact. Step 2: Place the contact component in a 35°C 5% sodium chloride solution salt spray atmosphere and observe the surface corrosion morphology in situ for 240 hours to evaluate its corrosion resistance. Step 3: After the 240-hour salt spray test, remove the contact parts to remove corrosion products and record the surface morphology. After testing, its corrosion status under a 240-hour continuous spray environment of 35℃ and 5% NaCl is shown below. Figure 6 .

[0069] Comparative Example 3 At room temperature, perform the following steps: Step 1: Put the semi-solid AZ91D magnesium alloy substrate with phosphate treatment into contact with Teflon steel bolts and nuts, ensuring that the two are in close contact, to obtain substrate-bolt contact 1; Put the semi-solid AZ91D magnesium alloy substrate with phosphate treatment into contact with polyamide steel bolts and nuts, ensuring that the two are in stable contact, to obtain substrate-bolt contact 2. Step 2: Place the contact component in a 35°C 5% sodium chloride solution salt spray atmosphere and observe the surface corrosion morphology in situ for 240 hours to evaluate its corrosion resistance. Step 3: After the 240-hour salt spray test, remove the contact parts to remove corrosion products and record the surface morphology. After testing, its corrosion status under a 240-hour continuous spray environment of 35℃ and 5% NaCl is shown below. Figure 7 .

[0070] Comparative Example 4 At room temperature, perform the following steps: Step 1: Mill mounting screw holes on the surface of the semi-solid AZ91D magnesium alloy substrate. Then, perform electrostatic powder spraying on the magnesium alloy substrate. Assemble the prepared substrate with galvanized steel bolts and nuts. Before assembly, apply 704 silicone rubber to the screw holes and nuts to achieve an isolation effect and ensure that the two are in close contact, thus obtaining the substrate-bolt contact component.

[0071] Step 2: Place the contact component in a 35°C 5% sodium chloride solution salt spray atmosphere and observe the surface corrosion morphology in situ for 480 hours to evaluate its corrosion resistance. After testing, its corrosion status under a 480-hour continuous spray environment at 35℃ and 5% NaCl is shown below. Figure 8 .

[0072] Comparative Example 5 At room temperature, perform the following steps: Step 1: Mill mounting screw holes on the surface of the semi-solid AZ91D magnesium alloy substrate, then perform electrostatic powder coating on the magnesium alloy substrate, and install Dacromet rivet screws (without washers). Figure 8 a) Dacromet rivet screws + silicone washers ( Figure 8 b) Assemble with an electrostatically sprayed magnesium alloy plate to ensure close contact between the two, resulting in a substrate-rivet contact.

[0073] Step 2: Place the contact component in a 35°C 5% sodium chloride solution salt spray atmosphere and observe the surface corrosion morphology in situ for 240 hours to evaluate its corrosion resistance. After testing, its corrosion status under a 240-hour continuous spray environment of 35℃ and 5% NaCl is shown below. Figure 10 .

[0074] The test results are as follows: The Teflon-coated galvanized steel bolts and nuts used in Test Example 1, in addition to the zinc plating layer reducing the galvanic corrosion potential, also benefited from the Teflon coating acting as a barrier. Due to the small area of ​​coating damage and the protection of the zinc plating layer, the galvanic corrosion effect was reduced. Observation revealed only shallow corrosion pits on the magnesium substrate surface. The actual corrosion condition of the magnesium substrate was improved compared to Comparative Example 1.

[0075] The polyamide-coated galvanized steel bolts and nuts used in Test Example 2, in addition to the zinc plating layer which reduces the galvanic corrosion potential, also benefit from a stronger polyamide coating as a barrier. Due to the smaller damaged area of ​​the coating, coupled with the protection of the zinc plating layer, the galvanic corrosion effect is significantly reduced. Observation revealed only shallow corrosion pits on the magnesium substrate surface. Under the dual protection of the polyamide film and the zinc plating layer, corrosion propagation was very limited. The actual corrosion condition of the magnesium substrate was significantly improved compared to Comparative Example 1.

[0076] The electrostatically sprayed magnesium substrate used in Test Example 3 exhibited excellent isolation performance, effectively preventing contact between the magnesium substrate and the Teflon-coated steel bolts and nuts, as well as the galvanized steel bolts and nuts, thus avoiding galvanic corrosion. Observation revealed almost no corrosion products on the magnesium substrate surface. The galvanized steel nuts showed significant corrosion, likely due to the high surface tension of the epoxy resin. The epoxy resin droplets coated on the nuts contracted and failed to form an effective coating, leading to corrosion of the galvanized steel screws in the salt spray environment. This indicates that the galvanized steel bolts and nuts corroded in a corrosive environment, and the magnesium substrate did not provide cathodic protection. The electrostatically sprayed powder layer effectively protected and isolated the substrate, confirming the main concept of this invention. No corrosion was observed on the magnesium substrate, demonstrating better overall corrosion resistance compared to Comparative Example 1.

[0077] The commercially available aluminum bolts and nuts used in Comparative Example 1 form electrical couples upon contact with the magnesium substrate, accelerating the corrosion of the magnesium substrate. Observation revealed deep corrosion pits at the contact points. Since they have already been practically used in magnesium alloy electric drive housings of some new energy vehicles, the corrosion degree of the magnesium substrate under this condition was selected as the control group for this experiment. In Comparative Example 2, commercially available galvanized steel bolts and nuts formed galvanic couples with the magnesium substrate. Due to the large potential difference between steel and magnesium, this significantly accelerated the corrosion of the magnesium substrate. Observation revealed deep corrosion pits and even localized perforation at the contact points. This indicates that galvanizing the steel bolts and nuts alone cannot effectively solve the galvanic corrosion problem during assembly. The actual corrosion of the magnesium substrate was more severe than in Comparative Example 1. The organic layers on the Teflon steel bolts and nuts and the polyamide steel bolts and nuts used in Comparative Example 3 could theoretically isolate the magnesium substrate from the steel bolts and nuts. However, due to the damage to the organic layers during assembly and tightening, galvanic corrosion between the magnesium substrate and the bolts and nuts could not be avoided. Observation revealed deep corrosion pits at the contact points between both types of bolts and nuts and the magnesium substrate. Because the polyamide coating is more resilient than the Teflon coating, the galvanic corrosion at the contact points of the polyamide steel bolts and nuts was slightly weaker. This indicates that simply applying an organic coating to the steel bolts and nuts cannot effectively solve the galvanic corrosion problem during assembly. The actual corrosion of the magnesium substrate was slightly more severe compared to Comparative Example 1.

[0078] In Test Example 5 and Comparative Example 4, when assembling electrostatically powder-coated magnesium substrates with Teflon-coated steel screws and galvanized steel screws, 704 silicone rubber was used to coat the bolts and nuts. This better prevented contact between the magnesium substrate and the bolts and nuts, reduced friction during assembly, protected the integrity of the coating, and thus avoided galvanic corrosion. After 480 hours of salt spray testing, no significant corrosion was observed on the surface of the magnesium substrate, while the galvanized steel bolts and nuts themselves showed significant rust, and no significant corrosion was observed on the Teflon-coated steel screws. With the addition of Teflon or galvanized coating to protect the steel bolts and nuts on top of electrostatic powder coating, the magnesium alloy substrate showed no significant corrosion. However, under long-term testing, the galvanized steel bolts and nuts themselves experienced severe corrosion, jeopardizing the overall reliability. Compared to Comparative Example 1, the overall corrosion resistance was better.

[0079] In Test Example 4, the electrostatically powder-coated magnesium substrate was assembled with Teflon-plated galvanized steel screws and polyamide-plated galvanized steel screws. The dual protection of the organic coating and zinc plating essentially prevented contact between the magnesium substrate and the steel bolts and nuts, thus avoiding galvanic corrosion. Observation revealed that there were almost no corrosion products on the surface of the magnesium substrate. Compared with Test Example 5, the corrosion of the bolts and nuts was significantly improved after galvanizing. In Test Example 6, when the electrostatically powder-coated magnesium substrate was assembled with Teflon-plated galvanized steel screws and polyamide-plated galvanized steel screws, 704 silicone rubber was used to coat the bolts and nuts. This reduced friction during assembly, protected the integrity of the coating, and further prevented contact between the magnesium substrate and the Teflon-plated galvanized steel bolts and nuts, thus avoiding galvanic corrosion. Observation revealed that there were almost no corrosion products on the surface of the magnesium substrate, indicating the best overall protective performance.

[0080] Test Example 7 and Comparative Example 5 use a simpler riveting assembly process, but when the rivets are stretched and deformed, they can easily damage the coating on the substrate connection section, while having no significant effect on the substrate contact surface. Figure 8 (a) without gasket Figure 8 (b) is a silicone pad, Figure 8 (c) shows the condition of the aluminum gasket after 240 hours of salt spray testing. The results indicate that the main function of the gasket is to prevent corrosion from spreading to the back side (gasket), while having no significant effect on corrosion of the expanded cross-section of the riveting. Adding an aluminum gasket during the connection of the electrostatically sprayed magnesium plate (Test Example 7) resulted in almost no corrosion after 20 days (the best protective effect). The main functions of the gasket are: 1. to isolate the nut from the substrate, reducing the probability of mechanical damage; simultaneously, the aluminum gasket can reduce the potential difference of the materials, inhibiting galvanic corrosion; 2. to reduce the impact of assembly stress on the substrate surface; 3. to reduce deformation and expansion to minimize damage to the hole wall coating. In contrast, although silicone gaskets theoretically provide insulation, they are difficult to achieve in practice and may introduce risks such as moisture absorption and impurities, making their protection less reliable than that of aluminum gaskets.

[0081] The protective principle of galvanized steel bolts and nuts ( Figure 12 To reduce galvanic corrosion, a uniform, dense, low-potential zinc layer is deposited onto steel bolts and nuts to prevent direct contact between the electrolyte, the steel bolts and nuts, and the magnesium alloy substrate. This reduces the potential difference between the steel bolts and nuts and the magnesium substrate. The protective principle of organic coatings on steel bolts and nuts is to form a waterproof and insulating barrier between the bolts / nuts and the magnesium alloy substrate; however, this barrier is easily damaged during assembly, and its protective performance is greatly reduced once damaged. The protection and failure mechanisms of zinc plating and coatings are discussed. Figure 12The methods are as follows: Organic coatings mainly improve barrier and insulation effects, but the insulation performance is often difficult to achieve ideal results due to assembly damage; zinc plating mainly provides barrier and reduces potential difference. Both types of coatings are damaged by mechanical stress during assembly, and a certain potential difference still exists between zinc and the magnesium substrate. Therefore, galvanic corrosion can still easily occur between the magnesium alloy and the bolts, and the size of the area involved in galvanic corrosion (exposed area) becomes a key factor in its occurrence. In Test Examples 1 and 2, the organic coating method effectively reduced the cathode area where galvanic corrosion occurs. The polyamide coating, with its excellent toughness and density, further reduced the exposed area, significantly reducing galvanic corrosion of the magnesium substrate during service. Electrostatic spraying technology can provide excellent insulation protection for the substrate. Since the substrate experiences relatively low stress during bolt and nut assembly, the sprayed coating maintains good integrity, essentially avoiding galvanic corrosion and providing good protection for the magnesium substrate.

[0082] During riveting assembly, the deformation and scraping of the rivet can damage the organic coating of the substrate at the rivet hole, causing the exposed magnesium substrate to come into direct contact with the rivet metal, which has a higher potential, resulting in galvanic corrosion. At the same time, the damaged area is much smaller than the area covered by the surrounding intact coating, forming a "small anode / large cathode" pattern. This leads to an extremely high anodic dissolution current density and a sharp increase in the local corrosion rate, making the connection point a weak point in the corrosion of the entire component and significantly reducing its corrosion resistance.

[0083] The technical solutions proposed in this invention for addressing galvanic corrosion occurring between magnesium alloy structural components and steel bolts and nuts (Test Examples 1, 2, and 3) can significantly reduce galvanic corrosion in contact between semi-solid injection molded AZ91D magnesium substrates and steel bolts and nuts. While the riveting process is convenient, adding aluminum washers during assembly can effectively reduce damage to the magnesium substrate coating, providing good protection. This demonstrates the feasibility of the design approach proposed in this invention, which involves coating the surface of galvanized steel bolts and nuts or magnesium alloy substrates with an organic layer to effectively isolate the magnesium substrate from the bolts and nuts, thereby reducing the galvanic corrosion effect of magnesium alloys.

[0084] It should be noted that the substrate material in this invention is not limited to the scope disclosed in the above test examples. As long as the metal surface is uniformly coated with the polyamide coating or the organic layer is sprayed with a thickness of not less than 100 μm, the system has good corrosion resistance.

[0085] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A method for inhibiting galvanic corrosion between magnesium alloy parts and dissimilar fasteners, characterized in that, Includes the following steps: An organic layer is coated on at least one surface of a magnesium alloy substrate and a galvanized steel bolt and nut. After coating, the magnesium alloy substrate is connected to the galvanized steel bolts and nuts or riveted.

2. The method for inhibiting galvanic corrosion between magnesium alloy parts and dissimilar fasteners according to claim 1, characterized in that, The magnesium alloy substrate is a magnesium alloy substrate with surface phosphating treatment, and its surface is sprayed with polyester powder.

3. The method for inhibiting galvanic corrosion between magnesium alloy parts and dissimilar fasteners according to claim 2, characterized in that, The organic layer is a Teflon layer or a polyamide layer.

4. The method for inhibiting galvanic corrosion between magnesium alloy parts and dissimilar fasteners according to claim 2, characterized in that, It also includes: Silicone rubber is spin-coated into the holes where the magnesium alloy substrate is connected to the galvanized steel bolts, nuts, or rivets.

5. The method for inhibiting galvanic corrosion between magnesium alloy parts and dissimilar fasteners according to claim 3, characterized in that, The polyamide layer has a thickness of 100-200 μm; the Teflon layer has a thickness of 20-30 μm.

6. The method for inhibiting galvanic corrosion between magnesium alloy parts and dissimilar fasteners according to claim 3, characterized in that, The thickness of the polyester powder layer electrostatically sprayed on the surface of the magnesium alloy substrate is 50-150 μm.

7. The method for inhibiting galvanic corrosion between magnesium alloy parts and dissimilar fasteners according to claim 3, characterized in that, The step of coating the surface of the galvanized steel bolts and nuts with a polyamide layer specifically includes: S11. Prepared into a polyamide colloid; S12. After the colloid completely covers the bolt and nut, use a spin coater to spin coat at a rate of 1000 rpm for 60 seconds to obtain a uniform coating on the surface of the bolt and nut. S13. Heat and cure the polyamide coating applied to the surface of the bolt and nut at 80℃-120℃ for 10 minutes, and then heat at 150℃ for no less than 10 minutes until the coating is completely hardened.

8. The method for inhibiting galvanic corrosion between magnesium alloy parts and dissimilar fasteners according to claim 3, characterized in that, The preparation of the surface-phosphating treated magnesium alloy substrate includes the following steps: S21. Drill screw holes in the magnesium alloy substrate and polish the surface to a smooth finish using 5000-grit sandpaper; S22. Perform alkaline degreasing: Use a weak alkaline solution mainly composed of sodium carbonate, trisodium phosphate and surfactants to immerse or spray the magnesium alloy substrate at 45-55℃ for 5-8 minutes to thoroughly remove grease and dirt from the surface; then rinse thoroughly with running water to ensure no cleaning agent residue remains, and obtain a degreased magnesium alloy substrate. S23. Phosphating treatment: The phosphating solution uses ammonium dihydrogen phosphate as the main salt with a concentration of 15-25 g / L, and adds a small amount of sodium nitrate as a film-forming promoter. Citric acid is also added. The pH value of the phosphating solution is controlled at 3.0-3.

8. The degreased magnesium alloy substrate is treated at 40-50℃ for 3-6 min. S24. Immediately rinse the phosphated magnesium alloy substrate with multiple water passes to terminate the reaction and remove impurities, and finally rinse with deionized water; then bake in hot air at 110-130℃ for 10-15 minutes to obtain the phosphated magnesium alloy substrate.

9. The method for inhibiting galvanic corrosion between magnesium alloy parts and dissimilar fasteners according to claim 8, characterized in that, The step of spraying polyester powder specifically includes: S31. Provides a phosphated magnesium alloy substrate; S32. Polyester powder is transported by air and uniformly adsorbed onto the surface of a grounded magnesium alloy substrate under a high voltage of 60-80kV; by precisely controlling the powder output, atomizing air pressure and spray gun movement, a uniform powder layer is formed on all surfaces, edges and screw holes of the magnesium alloy substrate in one go. S33. Stabilize the surface temperature of the magnesium alloy substrate at 180-200℃ and maintain it for 15-20 minutes; after cooling, a semi-solid AZ91 magnesium alloy substrate with electrostatic spraying is obtained.

10. A magnesium alloy fastener capable of suppressing galvanic corrosion, characterized in that, include: A magnesium alloy substrate and galvanized steel bolts and nuts, wherein the magnesium alloy substrate and galvanized steel bolts and nuts are connected or riveted together, and at least one surface of the magnesium alloy substrate and galvanized steel bolts and nuts is coated with an organic layer.

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