Preparation method of magnesium alloy high-corrosion-resistance coating with environment-friendly pretreatment

Abstract

The invention belongs to the technical field of magnesium alloy surface treatment, and discloses a pretreatment environment-friendly magnesium alloy high-corrosion-resistance coating preparation method which comprises the following steps: step 1, magnesium alloy zinc immersion; step 1.1, preparing a base material: polishing the base material; step 1.2, acid pickling; step 1.3, activation: adopting a mixed solution of ammonium bifluoride and phosphoric acid at room temperature, and forming a layer of protective film on the surface while removing an oxide layer; 1.4, zinc immersion is conducted, specifically, a zinc layer is deposited on the surface of the magnesium alloy through a replacement reaction; step 2, preparing an ionic liquid; step 2.1, preparation of an additive-free ionic liquid; 2.2, preparing an ionic liquid for electroplating an aluminum-manganese alloy coating; and step 3, post-treatment. Citric acid pickling and ammonium bifluoride-phosphoric acid activation liquid are adopted for pretreatment, and toxic chemicals are avoided; alCl-EMIC ionic liquid is used as a base, MnCl is added, an amorphous aluminum-manganese alloy coating is electroplated at the room temperature, and the corrosion resistance is remarkably improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of magnesium alloy surface treatment, and in particular relates to a method for preparing a magnesium alloy high-corrosion-resistant coating with environmentally friendly pre-treatment. Background Art

[0002] Magnesium alloy has the lowest density among structural metals and their alloy materials, about 1.74g / cm 3 , which is 2 / 3 of aluminum and 1 / 4 of iron. In addition, magnesium alloys have a series of advantages, such as: (1) low elastic modulus. (2) easy to process structural metals. (3) strong thermal and electrical conductivity. (4) good electromagnetic shielding, etc. However, magnesium is very active, with a standard hydrogen electrode potential of -2.37V, and has a strong tendency to thermodynamic corrosion, making it extremely corrosion-resistant. The poor corrosion resistance of magnesium alloys seriously restricts their application. Therefore, without sacrificing the mechanical properties of magnesium alloys themselves, with the help of surface engineering technology, the purpose of corrosion protection can be achieved and a certain aesthetic appearance can be made.

[0003] Surface treatment of magnesium alloys primarily involves conversion and coating. While chemical conversion provides strong adhesion to the substrate, the film is thin and soft, typically serving only as a coating substrate. While anodic oxidation and micro-arc oxidation offer excellent adhesion to the substrate, providing excellent insulation, decorative properties, and wear resistance, they are complex and unstable processes, require significant equipment investment, are inefficient, and are costly. Among coating methods, the primary challenge is the adhesion between the coating and the substrate. Currently, only zinc and nickel can deposit coatings with excellent adhesion on magnesium alloy surfaces. Zinc is typically used to coat magnesium alloys, due to concerns about galvanic corrosion between the coating and the substrate.

[0004] Galvanizing magnesium alloys utilizes the chemical potential difference between the metals to cause a replacement reaction between zinc and magnesium, depositing a thin zinc layer on the magnesium substrate. Different series of magnesium alloys have different potentials between the two phases, and their deposition conditions are also different. Therefore, it is difficult to obtain a uniform zinc layer on the surface of magnesium alloys, such as AZ91D magnesium alloy. The process flow of zinc immersion for magnesium alloys is: polishing → degreasing → pickling → activation → zinc immersion → post-treatment. The pre-treatment solution for zinc immersion, i.e., degreasing, pickling, and activation, is relatively complex and has certain typical toxicity. In addition, because the zinc immersion layer is thin, it is difficult to use as a protective layer and is often used as a base layer.

[0005] Ionic liquid electroplating of aluminum-manganese alloys typically produces nanoscale amorphous structures, resulting in dense, highly defect-free aluminum-manganese alloys with excellent corrosion resistance. Numerous methods exist for preparing aluminum-manganese alloy coatings, including inorganic molten salt systems: AlCl₃-KCl-MnCl₂, AlCl₃-NaCl-MnCl₂, and AlCl₃-NaCl-KCl-MnCl₂. However, these methods operate at temperatures exceeding 100°C, resulting in severe corrosion to the equipment and impacting the mechanical properties of the substrate.

[0006] The pretreatment solution for zinc immersion of magnesium alloys is complex in composition and is often non-environmentally friendly. For example, degreasing often uses a mixed solution of sodium hydroxide, sodium carbonate and sodium phosphate or sodium dodecylsulfonate, and the degreasing temperature is higher than room temperature; pickling solutions often use non-environmentally friendly treatment solutions such as chromic acid, hydrochloric acid, hydrofluoric acid and their mixed solutions; activation solutions often use non-environmentally friendly solutions such as hydrofluoric acid, hydrochloric acid and their mixed solutions. Summary of the Invention

[0007] The purpose of the present invention is to provide a method for preparing a magnesium alloy high corrosion-resistant coating with environmentally friendly pre-treatment to solve the above-mentioned technical problems.

[0008] In order to solve the above technical problems, the specific technical solution of the method for preparing a magnesium alloy high corrosion-resistant coating with environmentally friendly pretreatment is as follows: A method for preparing a magnesium alloy high corrosion-resistant coating with environmentally friendly pre-treatment comprises the following steps: Step 1, zinc immersion of magnesium alloy; Step 1.1, substrate preparation: mechanically polish the magnesium alloy substrate with sandpaper to remove the oxide layer; Step 1.2, pickling: further remove the oxide layer; Step 1.3, activation: using a mixed solution of ammonium hydrogen fluoride and phosphoric acid at room temperature to remove the oxide layer while forming a protective film on the surface; Step 1.4, zinc immersion: depositing a zinc layer on the surface of the magnesium alloy through a replacement reaction; Step 2, preparation of ionic liquid; Step 2.1, preparation of additive-free ionic liquid; Step 2.2, preparing ionic liquid for electroplating aluminum-manganese alloy coating; Step 3: Post-processing.

[0009] Furthermore, the step 1.1 adopts pickling solution for pickling, and the pickling solution components also include citric acid, oxalic acid, acetic acid, acetic anhydride, phosphoric acid, nitrate and a mixture of the above solutions. The concentration of the pickling solution is 10~30g / L, the temperature is room temperature, and the pickling time is 10~60s.

[0010] Furthermore, the concentrations of the mixed solution of ammonium bifluoride and phosphoric acid in step 1.3 are 100-200 g / L and 100-150 ml / L, respectively, and the time is 1-3 min.

[0011] Furthermore, the zinc leaching solution in step 1.4 is composed of a pyrophosphate system, including zinc sulfate, potassium / sodium pyrophosphate, sodium fluoride / potassium / lithium and sodium carbonate, with concentrations of ZnSO4·7H2O: 30-40 g / L, K4P2O7: 120-150 g / L, NaF: 5-7 g / L, Na2CO3: 5-7 g / L, temperature 40-80°C, and time 3-15 min.

[0012] Furthermore, the step 2.1 includes the following steps: Take anhydrous AlCl3 and EMIC as raw materials, and prepare the liquid in a glove box so that the molar ratio of AlCl3 and EMIC is 1.5~2:1. The temperature is controlled at ≤60°C during liquid preparation.

[0013] Furthermore, the step 2.2 includes the following steps: Take anhydrous manganese chloride with a content of 0.01~0.2mol / L, add it to the prepared additive-free ionic liquid at room temperature, and stir thoroughly until the two are completely complexed; Step 2.2.1, process parameters for aluminum-manganese alloy coating: The electrodeposition experiment was carried out under inert atmosphere to ensure that the water and oxygen content was ≤0.01ppm. The ionic liquid was AlCl3:EMIC=1.5~2:1, the additive content was 0.01~0.2mol / L, and the electrodeposition current density was 5~20mA / cm 2 The electrodeposition time depends on the actual required thickness. The electrodeposition temperature is room temperature ~ 60 ° C. The electrodeposition experiment is carried out with a high-purity aluminum plate as the anode and a zinc-impregnated magnesium alloy as the cathode. Step 2.2.2 Cleaning: The aluminum coating obtained by electrodeposition needs to be removed from the inert atmosphere. Its surface contains residual ionic liquid. After taking it out of the glove box, it needs to be immediately placed in anhydrous ethanol for cleaning. The cleaning time is 1~2 minutes, the cleaning temperature is room temperature, and the number of cleanings is 2 times. At the same time, deionized water is used for cleaning at room temperature, the cleaning time is 1~2 minutes, and the number of cleanings is 2 times.

[0014] Furthermore, the step 3 includes the following steps: Step 3.1 Sandblasting; Step 3.2, using chromate passivation; Step 3.3: Conduct a neutral salt spray test.

[0015] Furthermore, the neutral salt spray test in step 3.3 includes the following parameters: 5% NaCl aqueous solution, temperature 35°C±1°C, saturated air barrel temperature 35~40°C, relative humidity 85%, spray collection volume 1~2ml / 80cm*1h, brine barrel temperature 35°C±1°C, brine pH 6.5~7.2.

[0016] The method for preparing a magnesium alloy high corrosion-resistant coating with environmentally friendly pretreatment according to the present invention has the following advantages: 1. Environmental protection: The pre-treatment uses citric acid pickling and ammonium bifluoride-phosphoric acid activation solution to avoid toxic chemicals; 2. Optimize the zinc immersion process: The pyrophosphate zinc immersion solution forms a uniform and dense zinc layer (thickness of about 1μm) on the surface of AZ91D magnesium alloy; 3. Ionic liquid plating: Based on AlCl3-EMIC ionic liquid, MnCl2 is added to electroplate amorphous aluminum-manganese alloy coating at room temperature, which significantly improves corrosion resistance (no corrosion in neutral salt spray for 504 hours). BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 This is the SEM image of AZ91D magnesium alloy immersing zinc; Figure 2 This is the SEM image of the aluminum-manganese alloy coating surface of AZ91D magnesium alloy; Figure 3 This is the cross-sectional SEM image of the aluminum-manganese alloy coating on AZ91D magnesium alloy; Figure 4 This is the appearance of the aluminum-manganese passivation coating on AZ91D magnesium alloy after 504h neutral salt spray test. DETAILED DESCRIPTION

[0018] In order to better understand the purpose, structure and function of the present invention, the following is a further detailed description of a method for preparing a magnesium alloy high corrosion-resistant coating with environmentally friendly pretreatment in conjunction with the accompanying drawings.

[0019] The present invention provides a method for preparing a magnesium alloy high corrosion-resistant coating with environmentally friendly pre-treatment, comprising the following steps: Step 1, zinc dipping of magnesium alloy; Step 1.1, substrate preparation; The AZ91D magnesium alloy substrate was mechanically polished with sandpaper to remove the thick oxide layer formed by long-term environmental storage.

[0020] Step 1.2, pickling; Use an acid wash solution containing citric acid, oxalic acid, acetic acid, acetic anhydride, phosphoric acid, nitrates, or a mixture of these solutions. Use a concentration of 10-30 g / L at room temperature for 10-60 seconds to further remove the oxide layer.

[0021] Step 1.3, activation; A mixed solution of ammonium hydrogen fluoride and phosphoric acid is used with concentrations of 100-200 g / L and 100-150 ml / L respectively, at room temperature for 1-3 minutes to remove the oxide layer while forming a protective film on the surface to prevent oxidation before the next step.

[0022] Step 1.4, zinc dipping; A zinc layer is deposited on the surface of a magnesium alloy via a replacement reaction. The zinc immersion bath is primarily composed of a pyrophosphate system, containing zinc sulfate, potassium / sodium pyrophosphate, sodium / potassium / lithium fluoride, and sodium carbonate. The concentrations are 30-40 g / L ZnSO₄·7H₂O, 120-150 g / L K₄P₂Oₐ, 5-7 g / L NaF, and 5-7 g / L Na₂CO₃. The temperature is 40-80°C, and the duration is 3-15 minutes.

[0023] Step 2, preparation of ionic liquid; Step 2.1, preparation of additive-free ionic liquid; Prepare a liquid solution of anhydrous AlCl₃ and EMIC in a glove box (water and oxygen content ≤ 0.01 ppm) to a molar ratio of 1.5 to 2:1. Maintain the temperature ≤ 60°C during solution preparation. Ionic liquids for aluminum-manganese electroplating include alkylimidazoles, alkylpyridines, and quaternary ammonium salts.

[0024] Step 2.2, preparing ionic liquid for electroplating aluminum-manganese alloy coating; Take anhydrous manganese chloride with a content of 0.01~0.2mol / L, add it to the prepared additive-free ionic liquid at room temperature, and stir thoroughly until the two are completely complexed.

[0025] Step 2.2.1, process parameters for aluminum-manganese alloy coating; The electrodeposition experiment was carried out under inert atmosphere conditions, that is, the water and oxygen content must be ≤0.01ppm. The ionic liquid of AlCl3:EMIC=1.5~2:1, the additive content was 0.01~0.2mol / L, and the electrodeposition current density was 5~20mA / cm 2 , electrodeposition thickness (depending on the actual required thickness), electrodeposition temperature is room temperature ~ 60 ° C. In the experiment, high-purity aluminum plate is used as anode and zinc-impregnated magnesium alloy is used as cathode for electrodeposition experiment.

[0026] Step 2.2.2, cleaning: The aluminum coating obtained by electrodeposition needs to be removed from the inert atmosphere. Its surface contains residual ionic liquid. After taking it out of the glove box, it needs to be immediately placed in anhydrous ethanol for cleaning. The cleaning time is 1~2 minutes, the cleaning temperature is room temperature, and the number of cleanings is 2 times. At the same time, deionized water needs to be used for cleaning at room temperature, the cleaning time is 1~2 minutes, and the number of cleanings is 2 times.

[0027] Step 3, post-processing; Step 3.1, sandblasting: make the coating surface uniform.

[0028] Step 3.2, using chromate passivation; Step 3.3, neutral salt spray test: 5% NaCl aqueous solution, temperature 35°C±1°C, saturated air barrel temperature 35~40°C, relative humidity 85%, spray collection volume 1~2ml / 80cm*1h, brine barrel temperature 35°C±1°C, brine pH 6.5~7.2.

[0029] Example 1: 1. Mechanical polishing The samples were polished with 600-grit and 800-grit sandpapers respectively.

[0030] 2. Degreasing Degreasing was performed using anhydrous ethanol and ultrasonic treatment at room temperature for 15 min.

[0031] 3. Pickling Use citric acid, concentration 20g / L, rinse at room temperature for 20s 4. Deionized water cleaning Two rinses of deionized water were performed, each at room temperature for 15 seconds, to remove residual acid solution.

[0032] 5. Activation Use a mixed solution of ammonium bifluoride and phosphoric acid with concentrations of 100 g / L and 100 ml / L respectively, and soak at room temperature for 1.5 minutes.

[0033] 6. Deionized water cleaning Two rinses of deionized water were performed, each at room temperature for 15 seconds, to remove the residual activation solution.

[0034] 7. Zinc Dipping ZnSO4·7H2O: 35 g / L, K4P2O7: 140 g / L, NaF: 5 g / L, Na2CO3: 5 g / L, temperature 70°C, time 10 min, stirring method is gas stirring.

[0035] 8. Deionized water cleaning Use two deionized water rinses, each at room temperature for 15 seconds, to remove residual zinc immersion solution.

[0036] 9. Clean with anhydrous ethanol Rinse with anhydrous ethanol at room temperature for 15 seconds and blow dry for later use.

[0037] 10. Electroplating aluminum-manganese alloy coating The molar ratio of AlCl3 to EMIC was 2:1, the additive was 0.13 mol / L, the current density was 10 mA / cm2, the electrodeposition time was 60 min, and the electrodeposition temperature was 35°C. In the experiment, the electrodeposition experiment was carried out with a high-purity aluminum plate as the anode and a zinc-impregnated magnesium alloy as the cathode.

[0038] 11. Cleaning: Ultrasonic cleaning with anhydrous ethanol for 2 minutes at room temperature, twice. Ultrasonic cleaning with deionized water at room temperature, 2 minutes at room temperature, twice.

[0039] 12. Sandblasting White corundum, pressure 0.1~0.2MPa, time 1~2min, number of times 1~2 times, until the overall surface is uniform.

[0040] 13. Passivation Chromate passivation, concentration 10g / L, temperature room temperature, time 25s 14. Neutral salt spray test A 5% NaCl aqueous solution was tested at a temperature of 35°C ± 1°C, a saturated air tank temperature of 35-40°C, a relative humidity of 85%, a spray collection volume of 1-2 ml / 80 cm² / hour, a brine tank temperature of 35°C ± 1°C, and a brine pH of 6.5-7.2. No significant corrosion was observed on samples subjected to a neutral salt spray test for 2500 hours.

[0041] Example 2 1. Mechanical polishing The samples were polished with 600-grit and 800-grit sandpapers respectively.

[0042] 2. Degreasing Degreasing was performed using anhydrous ethanol and ultrasonic treatment at room temperature for 15 min.

[0043] 3. Pickling Use citric acid, concentration 20g / L, rinse at room temperature for 20s 4. Deionized water cleaning Two rinses of deionized water were performed, each at room temperature for 15 seconds, to remove residual acid solution.

[0044] 5. Activation Use a mixed solution of ammonium bifluoride and phosphoric acid with concentrations of 100 g / L and 100 ml / L respectively, and soak at room temperature for 1.5 minutes.

[0045] 6. Deionized water cleaning Two rinses of deionized water were performed, each at room temperature for 15 seconds, to remove the residual activation solution.

[0046] 7. Zinc Dipping ZnSO4·7H2O: 35g / L, K4P2O7: 140g / L, NaF: 5g / L, Na2CO3: 5g / L, temperature 70°C, time 10min, after 5min ultrasonic treatment, switch to gas stirring method for zinc immersion.

[0047] 8. Deionized water cleaning Use two deionized water rinses, each at room temperature for 15 seconds, to remove residual zinc immersion solution.

[0048] 9. Clean with anhydrous ethanol Rinse with anhydrous ethanol at room temperature for 15 seconds and blow dry for later use.

[0049] 10. Electroplating aluminum-manganese alloy coating The molar ratio of AlCl3 to EMIC was 2:1, the additive was 0.13 mol / L, the current density was 8 mA / cm2, the electrodeposition time was 60 min, and the electrodeposition temperature was 35°C. In the experiment, the electrodeposition experiment was carried out with a high-purity aluminum plate as the anode and a zinc-impregnated magnesium alloy as the cathode.

[0050] 11. Cleaning: Ultrasonic cleaning with anhydrous ethanol for 2 minutes at room temperature, twice. Ultrasonic cleaning with deionized water at room temperature, 2 minutes at room temperature, twice.

[0051] 12. Sandblasting White corundum, pressure 0.1~0.2MPa, time 1~2min, number of times 1~2 times, until the overall surface is uniform.

[0052] 13. Passivation Chromate passivation, concentration 10g / L, temperature room temperature, time 25s 14. Neutral salt spray test A 5% NaCl aqueous solution was tested at a temperature of 35°C ± 1°C, a saturated air tank temperature of 35-40°C, a relative humidity of 85%, a spray collection volume of 1-2 ml / 80 cm² / hour, a brine tank temperature of 35°C ± 1°C, and a pH of 6.5-7.2. No significant corrosion was observed on samples subjected to a neutral salt spray test for 2550 hours.

[0053] Example 3 Experimental Procedure 1. Mechanical polishing The samples were polished with 600-grit and 800-grit sandpapers respectively.

[0054] 2. Degreasing Degreasing was performed using anhydrous ethanol and ultrasonic treatment at room temperature for 15 min.

[0055] 3. Pickling Use citric acid, concentration 20g / L, rinse at room temperature for 20s 4. Deionized water cleaning Two rinses of deionized water were performed, each at room temperature for 15 seconds, to remove any residual acid solution.

[0056] 5. Activation Use ammonium bifluoride solution with a concentration of 200g / L and soak at room temperature for 1.5 minutes.

[0057] 6. Deionized water cleaning Two rinses of deionized water were performed, each at room temperature for 15 seconds, to remove the residual activation solution.

[0058] 7. Zinc Dipping ZnSO4·7H2O: 35 g / L, K4P2O7: 140 g / L, NaF: 5 g / L, Na2CO3: 5 g / L, temperature 50°C, time 10 min, stirring method is gas stirring.

[0059] 8. Deionized water cleaning Use two deionized water rinses, each at room temperature for 15 seconds, to remove residual zinc immersion solution.

[0060] 9. Clean with anhydrous ethanol Rinse with anhydrous ethanol at room temperature for 15 seconds and blow dry for later use.

[0061] 10. Electroplating aluminum-manganese alloy coating The molar ratio of AlCl3 to EMIC was 2:1, the additive was 0.13 mol / L, the current density was 10 mA / cm2, the electrodeposition time was 60 min, and the electrodeposition temperature was 35°C. In the experiment, the electrodeposition experiment was carried out with a high-purity aluminum plate as the anode and a zinc-impregnated magnesium alloy as the cathode.

[0062] 11. Cleaning: Ultrasonic cleaning with anhydrous ethanol for 2 minutes at room temperature, twice. Ultrasonic cleaning with deionized water at room temperature, 2 minutes at room temperature, twice.

[0063] 12. Sandblasting White corundum, pressure 0.1~0.2MPa, time 1~2min, number of times 1~2 times, until the overall surface is uniform.

[0064] 13. Passivation Chromate passivation, concentration 10g / L, temperature room temperature, time 25s 14. Neutral salt spray test A 5% NaCl aqueous solution was tested at a temperature of 35°C ± 1°C, a saturated air tank temperature of 35-40°C, a relative humidity of 85%, a spray collection volume of 1-2 ml / 80 cm² / hour, a brine tank temperature of 35°C ± 1°C, and a pH of 6.5-7.2. No significant corrosion was observed on samples subjected to a neutral salt spray test for 2800 hours.

[0065] Experimental results: Figure 1 This is the SEM image of AZ91D magnesium alloy after zinc immersion. The zinc layer completely covers the magnesium matrix. Figure 2 This is a SEM image of the aluminum-manganese alloy coating after zinc immersion on magnesium alloy. The aluminum-manganese coating is very dense and has an amorphous structure. Figure 3 This is a cross-sectional SEM image of the zinc-plated aluminum-manganese alloy coating on magnesium alloy. It can be seen that the zinc layer and the substrate, as well as the aluminum-manganese layer and the zinc-plated layer are well bonded, and there are no potential microcracks. Figure 4 After 504 hours of neutral salt spray test, no corrosion or corrosion spots were found on the surface of the passivated aluminum-manganese coating, indicating excellent corrosion resistance.

[0066] It will be understood that the present invention is described by way of some embodiments, and it will be appreciated by those skilled in the art that various changes or equivalent substitutions may be made to these features and embodiments without departing from the spirit and scope of the present invention. In addition, under the teachings of the present invention, these features and embodiments may be modified to adapt to specific circumstances and materials without departing from the spirit and scope of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed herein, and all embodiments falling within the scope of the claims of this application are intended to be protected by the present invention.

Claims

1. A method for preparing a magnesium alloy high corrosion-resistant coating with environmentally friendly pre-treatment, characterized in that: The steps include: Step 1, zinc immersion of magnesium alloy; Step 1.1, substrate preparation: mechanically polish the magnesium alloy substrate with sandpaper to remove the oxide layer; Step 1.2, pickling: further remove the oxide layer; Step 1.3, activation: using a mixed solution of ammonium hydrogen fluoride and phosphoric acid at room temperature to remove the oxide layer while forming a protective film on the surface; Step 1.4, zinc immersion: depositing a zinc layer on the surface of the magnesium alloy through a replacement reaction; Step 2, preparation of ionic liquid; Step 2.1, preparation of additive-free ionic liquid; Step 2.2, preparing ionic liquid for electroplating aluminum-manganese alloy coating; Step 3: Post-processing.

2. The method for preparing a magnesium alloy high corrosion-resistant coating with environmentally friendly pretreatment according to claim 1, characterized in that: In step 1.1, pickling is performed using an acid wash solution, wherein the pickling solution further comprises citric acid, oxalic acid, acetic acid, acetic anhydride, phosphoric acid, nitrate, and a mixture of the above solutions. The concentration of the pickling solution is 10-30 g / L, the temperature is room temperature, and the pickling time is 10-60 s.

3. The method for preparing a magnesium alloy high corrosion-resistant coating with environmentally friendly pretreatment according to claim 1, characterized in that: The concentrations of the mixed solution of ammonium bifluoride and phosphoric acid in step 1.3 are 100-200 g / L and 100-150 ml / L, respectively, and the mixing time is 1-3 minutes.

4. The method for preparing a magnesium alloy high corrosion-resistant coating with environmentally friendly pre-treatment according to claim 1, characterized in that: The zinc leaching solution in step 1.4 is composed of a pyrophosphate system, including zinc sulfate, potassium / sodium pyrophosphate, sodium / potassium / lithium fluoride, and sodium carbonate, with concentrations of ZnSO4·7H2O: 30-40 g / L, K4P2O7: 120-150 g / L, NaF: 5-7 g / L, and Na2CO3: 5-7 g / L. The temperature is 40-80°C and the time is 3-15 min.

5. The method for preparing a magnesium alloy high corrosion-resistant coating with environmentally friendly pretreatment according to claim 1, characterized in that: The step 2.1 includes the following steps: Take anhydrous AlCl3 and EMIC as raw materials, and prepare the liquid in a glove box so that the molar ratio of AlCl3 and EMIC is 1.5~2:

1. The temperature is controlled at ≤60°C during liquid preparation.

6. The method for preparing a magnesium alloy high corrosion-resistant coating with environmentally friendly pre-treatment according to claim 1, characterized in that: The step 2.2 includes the following steps: Take anhydrous manganese chloride with a content of 0.01~0.2mol / L, add it to the prepared additive-free ionic liquid at room temperature, and stir thoroughly until the two are completely complexed; Step 2.2.1, process parameters for aluminum-manganese alloy coating: The electrodeposition experiment was carried out under inert atmosphere to ensure that the water and oxygen content was ≤0.01ppm. The ionic liquid was AlCl3:EMIC=1.5~2:1, the additive content was 0.01~0.2mol / L, and the electrodeposition current density was 5~20mA / cm 2 The electrodeposition thickness depends on the actual required thickness. The electrodeposition temperature is room temperature ~ 60 ° C. The electrodeposition experiment is carried out with a high-purity aluminum plate as the anode and a zinc-impregnated magnesium alloy as the cathode. Step 2.2.2 Cleaning: The aluminum coating obtained by electrodeposition needs to be removed from the inert atmosphere. Its surface contains residual ionic liquid. After taking it out of the glove box, it needs to be immediately placed in anhydrous ethanol for cleaning. The cleaning time is 1~2 minutes, the cleaning temperature is room temperature, and the number of cleanings is 2 times. At the same time, deionized water is used for cleaning at room temperature, the cleaning time is 1~2 minutes, and the number of cleanings is 2 times.

7. The method for preparing a magnesium alloy high corrosion-resistant coating with environmentally friendly pre-treatment according to claim 1, characterized in that: The step 3 comprises the following steps: Step 3.1 Sandblasting; Step 3.2, using chromate passivation; Step 3.3: Conduct a neutral salt spray test.

8. The method for preparing a magnesium alloy high corrosion-resistant coating with environmentally friendly pretreatment according to claim 7, characterized in that: The neutral salt spray test in step 3.3 includes the following parameters: 5% NaCl aqueous solution, temperature 35°C±1°C, saturated air barrel temperature 35~40°C, relative humidity 85%, spray collection volume 1~2ml / 80cm*1h, brine barrel temperature 35°C±1°C, brine pH 6.5~7.2.

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