A comprehensive protection method for preventing magnesium alloy galvanic corrosion

Abstract

The present application belongs to the technical field of magnesium alloy galvanic corrosion protection, and discloses a comprehensive protection method for preventing magnesium alloy galvanic corrosion. Since magnesium alloy and metal bolt fasteners are connected as dissimilar metals, magnesium alloy is accelerated to corrode. The method comprises the following steps: 1. reducing the potential difference between dissimilar metals; 2. coating the surface of the cathode and anode metals with an insulating coating to prevent direct contact between dissimilar metals. For magnesium alloy, first, phosphate conversion film treatment is performed, and then organic epoxy zinc phosphate primer coating is performed. For metal screws and gaskets, surface electrophoretic coating is performed. 3. using a larger gasket for assembly, increasing the contact area of the gasket and the magnesium alloy surface coating, reducing the pressure of the gasket on the coating when the screw is tightened, thereby avoiding damage to the magnesium alloy surface coating. The present application greatly reduces galvanic corrosion through the above method, and the process is simple and reliable.

Description

Technical Field

[0001] This invention relates to the field of magnesium alloy corrosion protection technology, specifically to a comprehensive protection method for preventing magnesium alloy corrosion by galvanic corrosion. Background Technology

[0002] Magnesium alloys are currently the lightest metallic structural materials used in engineering applications. They possess a range of advantages, including high specific strength, high specific stiffness, good damping and vibration reduction properties, excellent electromagnetic shielding performance, and ease of recycling. They hold broad application prospects in aerospace, rail transportation, automotive lightweighting, electronic communications, and biomedicine. Achieving large-scale application of magnesium alloys is crucial for reducing equipment weight, lowering energy consumption, and improving product performance. However, the extremely high chemical reactivity of magnesium alloys and their most negative standard electrode potential among all structural metals result in poor corrosion resistance, particularly resistance to galvanic corrosion. This has become the most significant technical bottleneck restricting their further widespread application.

[0003] Currently, the industry's protection technologies against galvanic corrosion of magnesium alloys mainly employ insulating gaskets, sealants, and surface treatments to reduce or even prevent galvanic corrosion between dissimilar metals. Existing research, such as patents CN200710161317.3, CN201610757330.4, and 202111540984.9, reduces galvanic corrosion by metallurgically bonding the magnesium alloy and steel connectors and adding an intermediate insulating metal (aluminum alloy or zinc alloy) to lower the potential difference. This method introduces an intermediate buffer metal and may even require metallurgical bonding, making it complex, costly, and unsuitable for large-scale industrial production. Furthermore, it lacks insulation treatment for the anode and cathode, resulting in limited galvanic protection effectiveness. Patents CN202410409155.4 and CN202311344276.7 reduce the potential difference between the anode and cathode by performing surface treatments such as zinc plating and aluminum plating on carbon steel screws, followed by phosphating and electrophoresis to isolate them from direct contact with magnesium alloys. This method involves a three-electrode surface treatment (zinc plating, phosphating, and electrophoresis), making the process complex. Patent CN202422193609.7 only studies the surface organic coating of aluminum alloy screws. Although aluminum alloys have low electrode potentials, their inherent strength is insufficient, limiting their industrial applications. Patents CN202411689115.6 and CN201020128416.9 avoid direct contact between dissimilar metals by preparing a micro-arc oxidation coating on the surface of magnesium alloys. However, micro-arc oxidation is energy-intensive and expensive, and a single micro-arc oxidation coating has limited effectiveness in preventing galvanic corrosion of magnesium alloys. Patent No. CN201810349758.4 describes a three-stage coating process for magnesium alloys—film, electrophoresis, and powder coating—to protect them from corrosion. This three-stage coating process is complex, time-consuming, and increases costs. While the above patents propose many methods to prevent magnesium alloy galvanic corrosion, none involve concurrent neutral salt spray testing for performance verification. Patent No. 202411880546.0 describes a method involving adhesive sealing of the mating surfaces of connectors, along with the use of ceramic insulating gaskets and fasteners. However, this adhesive sealing requires manual labor, hindering automated industrial production, increasing labor costs, and making it difficult to control the amount of sealant used. It also complicates the disassembly and reassembly of fasteners, and ceramic connectors are relatively expensive. Although this method allows magnesium alloy galvanic specimens to remain uncorroded for 500 hours in a 3.5% wt sodium chloride neutral salt spray test, the overall cost is high.

[0004] In summary, existing protective measures generally suffer from drawbacks such as complex processes, high costs, and limited effectiveness. Therefore, there is an urgent need in this field for a new technical solution that is efficient, long-lasting, environmentally friendly, easy to implement, and low-cost, capable of fundamentally inhibiting or significantly slowing down the galvanic corrosion process when magnesium alloys are joined with dissimilar metals. This would break through the technical barriers to magnesium alloy applications and promote their large-scale application in downstream key sectors. Summary of the Invention

[0005] The purpose of this invention is to solve the problem of galvanic corrosion when magnesium alloys are coupled with metal screw fasteners, and to propose a comprehensive protection method to prevent galvanic corrosion of magnesium alloys.

[0006] A comprehensive protection method for preventing galvanic corrosion of magnesium alloys includes the following steps: Step 1: Select a metal screw with a potential close to that of the magnesium alloy according to the galvanic series. Test the open circuit potential and polarization curve of all the metals used. Select a metal screw that can reduce the potential difference with the magnesium alloy by using the open circuit potential and polarization curve. Step 2: Apply a phosphate conversion film treatment to the surface of the magnesium alloy to form a uniform and dense phosphate conversion film on the surface of the magnesium alloy. Step 3: Apply an organic epoxy zinc phosphate primer to the surface of the magnesium alloy phosphate conversion film to create a composite coating of "conversion film + organic coating" on the magnesium alloy surface. Step 4: Perform surface electrophoretic treatment on the metal screws and washers to give the metal screws and washers an electrophoretic coating. Step 5: Using the two standard shims, replace the shim closest to the magnesium alloy composite coating with a larger shim. Using two standard shims reduces damage to the coating caused by the shims rotating relative to the magnesium alloy surface when tightening the screws. The larger shim increases the contact area, reduces the pressure of the shim on the organic paint on the magnesium alloy surface, and prevents the organic paint from being damaged by the shim in a humid environment due to its lower performance.

[0007] The metal screws are carbon steel screws, 304 stainless steel screws, and aluminum alloy screws; for example, carbon steel galvanized screws and carbon steel Dacromet screws.

[0008] The gaskets are standard gaskets and non-standard oversized gaskets.

[0009] The thickness of the phosphate conversion membrane is 10µm-20µm.

[0010] The aqueous solution for phosphate conversion membrane treatment consists of: 35 g / L ammonium dihydrogen phosphate, 35 g / L manganese sulfate monohydrate, 2 g / L sodium nitrate, 2 g / L tetrasodium ethylenediaminetetraacetate, and 1 g / L benzalkonium chloride.

[0011] The phosphate conversion membrane has a double-layer structure, with the inner layer being magnesium hydrogen phosphate and the outer layer being manganese hydrogen phosphate.

[0012] The total thickness of the composite coating is 90µm-100µm.

[0013] The thickness of the electrophoretic coating is 10µm-20µm.

[0014] The organic coating method includes manual brushing or electrostatic powder spraying.

[0015] Compared with the prior art, the present invention has the following beneficial effects: This invention, based on the principle of reducing potential difference, selects screws and washers made of carbon steel, galvanized carbon steel, Dacromet carbon steel, and aluminum alloy. A phosphate conversion film treatment is applied to the magnesium alloy, followed by coating with an organic epoxy zinc-phosphorus primer, achieving the preparation of an "inorganic + organic" composite coating. Both the phosphate conversion film and zinc-phosphorus primer coating are low-cost processes, and the prepared composite coating itself possesses high corrosion resistance. Electrophoretic coating is then applied to the screws and washers; electrophoretic coating is also a mature and inexpensive process. By simply enlarging the washer assembly method, a significant improvement in galvanic corrosion resistance is achieved.

[0016] Compared to other methods that use insulating gaskets, insulating sleeves, sealants, applying sealant to joints, or assembling first and then applying an organic coating for sealing, this invention achieves a comprehensive anti-galvanic corrosion technology that is efficient, long-lasting, environmentally friendly, easy to implement, and low-cost. Attached Figure Description

[0017] Figure 1 The electric galvanic sequence of different metals in artificial seawater; Figure 2 The open-circuit potentials of the different metals used in this invention; Figure 3 Polarization curves of different metals used in this invention; Figure 4 The images show the sSEM cross-sectional morphology of galvanized and Dacromet-coated carbon steel screws: (a1) SEM cross-sectional image of the galvanized layer on the head of a galvanized carbon steel screw; (a2) SEM cross-sectional image of the galvanized layer between the threads of a galvanized carbon steel screw; (a3) ​​SEM cross-sectional image of the galvanized layer between the threads of a galvanized carbon steel screw; (b1) SEM cross-sectional image of the galvanized layer on the head of a Dacromet-coated carbon steel screw; (b2) SEM cross-sectional image of the galvanized layer between the threads of a Dacromet-coated carbon steel screw; (b3) SEM cross-sectional image of the galvanized layer between the threads of a Dacromet-coated carbon steel screw. Figure 5 The electrochemical impedance of the metal + coating in this invention; Figure 6 This is a statistical analysis of the neutral salt spray lifetime of the examples; Figure 7 This is a statistical analysis of the neutral salt spray lifetime of Comparative Example 1; Figure 8 This is a statistical analysis of the neutral salt spray lifetime of Comparative Example 2; Figure 9 This is a statistical analysis of the neutral salt spray lifetime of Comparative Example 3; Figure 10 This is a statistical analysis of the neutral salt spray lifetime of Comparative Example 4; Figure 11 This is a statistical analysis of the neutral salt spray lifetime of Comparative Example 5; Figure 12 The following are schematic diagrams of screw assembly: (a) Assembly method: 1 standard washer, (b) Assembly method: 2 standard washers, (c) Assembly method: 1 standard washer + 1 large washer. Detailed Implementation

[0018] The technical solutions in the embodiments of the present invention will be clearly and completely described below.

[0019] Example 1 This embodiment exhibits optimal resistance to galvanic corrosion. A "phosphate conversion film + organic coating" is prepared on the magnesium alloy surface. The metal screws and washers undergo surface electrophoretic treatment. A double-washer assembly method is adopted, with a standard washer on top and an enlarged washer on the bottom.

[0020] Process parameters: Magnesium alloy has a composite coating; screws and washers have an electrophoretic coating; assembly method is one standard washer + one large washer.

[0021] Step 1: Select screws and corresponding washers made of carbon steel, galvanized carbon steel, Dacromet carbon steel, or aluminum alloy according to the thermocouple sequence. Step 2: Perform electrophoretic surface treatment on the screws and washers to coat their surfaces with an electrophoretic coating. Step 3: Apply a phosphate conversion film treatment to the magnesium alloy surface to form a uniform and dense phosphate conversion film. The phosphate conversion film treatment process is as follows: First, wet grind the magnesium alloy sample to 600#; then, perform surface conditioning pretreatment—immerse it in an 80℃, 0.07mol / L Na2HPO4 solution for 1 min; finally, immerse it in a 60℃ film-forming aqueous solution for 10 min, rinse with deionized water and dry. The film-forming aqueous solution formula is: NH4H2PO4: 35g / L, MnSO4: 35g / L, NaNO3: 2g / L, EDTA4Na: 2g / L, benzalkonium chloride: 1g / L. Step 4: Building upon Step 3, apply an organic epoxy zinc phosphate primer to the surface of the magnesium alloy phosphate conversion film. The coating method involves preparing the epoxy zinc phosphate primer with a ratio of primer:curing agent:diluent of 6:1:1. First, mix the zinc phosphate primer and diluent according to the ratio and stir thoroughly. Then, add the curing agent according to the ratio and stir evenly. Let the prepared organic epoxy zinc phosphate primer cure at room temperature for 30 minutes. Seal the through holes on the sample with rubber stoppers to prevent the organic epoxy zinc phosphate primer from entering the threaded holes. Apply the prepared organic epoxy zinc phosphate primer with a first brush, evenly brushing it onto the sample surface. Place the sample in an oven at 60℃ to dry for 30 minutes. After drying, remove it and apply a second brush to achieve the desired coating thickness. Place the coated sample in an oven at 60℃ to dry for 24 hours. After curing, remove it and cool to room temperature, then place it in a desiccator for later use.

[0022] Step 5: The screws that have undergone surface electrophoresis are assembled with the magnesium alloy to obtain the galvanic couple sample.

[0023] Comparative Example 1 Process parameters: Magnesium alloy has a composite coating; screws and washers have no electrophoretic coating; assembly method is 1 standard washer; The operating steps are the same as in Example 1.

[0024] Comparative Example 2 Process parameters: Magnesium alloy has a composite coating; screws and washers have no electrophoretic coating; assembly method is 2 standard washers; The operating steps are the same as in Example 1.

[0025] Comparative Example 3 Process parameters: Magnesium alloy has a composite coating; screws and washers have no electrophoretic coating; assembly method is 1 standard washer + 1 large washer; The operating steps are the same as in Example 1.

[0026] Comparative Example 4 Process parameters: Magnesium alloy has a composite coating; screws and washers have an electrophoretic coating; assembly method is 1 standard washer; The operating steps are the same as in Example 1.

[0027] Comparative Example 5 Process parameters: Magnesium alloy has a composite coating; screws and washers have an electrophoretic coating; assembly method: 2 standard washers; The operating steps are the same as in Example 1.

[0028] Experimental Section The screws used in this invention are m6×12mm in size, the standard washers are m6×12mm in size, the large washers are m6×16mm in size, and the assembly torque of all screws is 8N•m.

[0029] (1) Electrochemical testing; In this experiment, electrochemical measurements were performed using a Type A electrochemical workstation. A three-electrode system was employed, and the potentiodynamic polarization curves were scanned from -0.3 V vs OCP to 1.6 V vs Ref at a scan rate of 0.333 mV / s. The electrochemical impedance spectroscopy (EIS) measurement frequency range was 10... 5 ~ 10 -2 The impedance data was fitted at Hz with a sinusoidal perturbation of 50 mV. The electrochemical impedance spectroscopy test frequency range was 10 Hz. 5 Up to 10 -2 The frequency was Hz, and the perturbation amplitude was 50 mV. Polarization curves and electrochemical impedance spectroscopy were performed entirely under isothermal conditions of 30±1℃. For electrochemical measurements, to ensure repeatability, three parallel samples were tested under each condition.

[0030] (2) Salt spray test; Salt spray tests were conducted on the samples according to the salt spray test standard (ASTM B117-03) at a temperature of 35±1℃. The original macroscopic morphology of the samples was recorded. After the start of the test, samples were taken out and photographed every 24 hours. The degree of corrosion and corrosion resistance were compared based on the changes in the morphology of the samples before and after the test. Five parallel samples were tested for each type of sample to ensure the accuracy of the experimental results.

[0031] For the corrosion problem that occurs when assembling magnesium alloy components with metal screws and fasteners, existing technologies are mostly complex, ineffective, and expensive. To address these issues, this invention first treats the magnesium alloy with a phosphate conversion film, forming a uniform, dense, and corrosion-resistant phosphate conversion film. Then, an organic epoxy zinc phosphate primer is applied, achieving the preparation of an "inorganic + organic" composite coating. This composite coating itself exhibits excellent corrosion resistance, achieving a neutral salt spray test life of 2000 hours.

[0032] The electric galvanic series of different metals in seawater are as follows Figure 1 As shown, considering the reduction of potential difference and cost, this invention selected carbon steel screws, aluminum alloy screws, and stainless steel screws for experimentation. The carbon steel screws included carbon steel, galvanized carbon steel, and Dacromet carbon steel, based on open circuit potential... Figure 2 It can be seen that the presence of the zinc plating layer and Dacromet coating on the carbon steel surface can reduce the potential of the carbon steel surface. The zinc plating and Dacromet coating processes are inexpensive, so they can reduce the potential difference and weaken the tendency of galvanic corrosion at a relatively low cost.

[0033] In an embodiment of the present invention, the thickness of the magnesium alloy phosphate conversion film is 10-20 µm, the total thickness of the composite coating is 90-100 µm, and the thickness of the electrophoretic coating is 10-20 µm.

Claims

1. A comprehensive protection method for preventing galvanic corrosion of magnesium alloys, characterized in that, Includes the following steps: Step 1: Select a metal screw with a potential close to that of the magnesium alloy according to the galvanic series. Test the open circuit potential and polarization curve of all the metals used. Select a metal screw that can reduce the potential difference with the magnesium alloy by using the open circuit potential and polarization curve. Step 2: Apply a phosphate conversion film treatment to the surface of the magnesium alloy to form a uniform and dense phosphate conversion film on the surface of the magnesium alloy. Step 3: Apply an organic epoxy zinc phosphate primer to the surface of the magnesium alloy phosphate conversion film to create a composite coating of "conversion film + organic coating" on the magnesium alloy surface. Step 4: Perform surface electrophoretic treatment on the metal screws and washers to give the metal screws and washers an electrophoretic coating. Step 5: Based on the two gaskets, replace the gasket below that is close to the magnesium alloy composite coating with a larger gasket.

2. The comprehensive protection method for preventing galvanic corrosion of magnesium alloys according to claim 1, characterized in that, The metal screws are carbon steel screws, 304 stainless steel screws, or aluminum alloy screws.

3. The comprehensive protection method for preventing electrolytic corrosion of magnesium alloys according to claim 1, characterized in that, The gaskets are standard gaskets and non-standard oversized gaskets.

4. The comprehensive protection method for preventing galvanic corrosion of magnesium alloys according to claim 1, characterized in that, The thickness of the phosphate conversion membrane is 10µm-20µm.

5. The comprehensive protection method for preventing electrolytic corrosion of magnesium alloys according to claim 1, characterized in that, The aqueous solution for phosphate conversion membrane treatment consists of: 35 g / L ammonium dihydrogen phosphate, 35 g / L manganese sulfate monohydrate, 2 g / L sodium nitrate, 2 g / L tetrasodium ethylenediaminetetraacetate, and 1 g / L benzalkonium chloride.

6. The comprehensive protection method for preventing galvanic corrosion of magnesium alloys according to claim 4 or 5, characterized in that, The phosphate conversion membrane has a double-layer structure, with the inner layer being magnesium hydrogen phosphate and the outer layer being manganese hydrogen phosphate.

7. The comprehensive protection method for preventing galvanic corrosion of magnesium alloys according to claim 1, characterized in that, The total thickness of the composite coating is 90µm-100µm.

8. The comprehensive protection method for preventing galvanic corrosion of magnesium alloys according to claim 1, characterized in that, The thickness of the electrophoretic coating is 10µm-20µm.

9. The comprehensive protection method for preventing galvanic corrosion of magnesium alloys according to claim 1, characterized in that, The organic coating method includes manual brushing or electrostatic powder spraying.

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