CN101545116A - Method for electroplating inorganic molten salt on surface of magnesium and magnesium alloy with aluminum - Google Patents

Abstract

The invention relates to a method for electroplating inorganic molten salt on surface of magnesium and magnesium alloy with aluminum, which belongs to the technical field of metal surface treatment. The method comprises a step of preplating a zinc layer as a substrate and a step of electroplating inorganic molten salt with aluminum. The step of preplating the zinc layer as the substrate comprises a step of immersion-plating the zinc layer and a step of electroplating the zinc layer. The step of electroplating the zinc layer is performed in an inorganic molten salt system, wherein anhydrous aluminum chloride is main salt, and sodium chloride, potassium chloride or a mixture of the two is a main complexing agent. An aluminum coating obtained by the method is complete, dense, uniform and bright, is firmly combined with a matrix, can be individually used as a protective layer, and can be prepared into a surface functional layer with better corrosion resistance, better wear resistance and high hardness through subsequent treatment. The whole process of the method has the advantages of simple process, cheap equipment, friendliness to environment, and the like.

Description

A method for electroplating aluminum with inorganic molten salt on the surface of magnesium and magnesium alloys

Technical field:

The invention relates to a method for electroplating aluminum with inorganic molten salt on the surface of magnesium and magnesium alloys, belonging to the technical field of metal surface treatment.

Background technique:

Magnesium alloy has the advantages of low density, high specific strength, and good electromagnetic shielding performance. It is known as a green engineering material in the 21st century and has a wide range of application values in the automotive industry, aerospace industry and electronics industry. However, poor corrosion resistance, poor wear resistance, and low hardness have become bottlenecks restricting the application of magnesium alloys, limiting their large-area use. Therefore, in order to enable magnesium alloys to be used in harsher environments, it is necessary to develop corresponding surface treatment technologies for magnesium alloys. The surface treatment technologies currently developed on magnesium alloys mainly include chemical conversion, electrochemical anodic oxidation or micro-arc oxidation, electroplating or electroless metal coating, etc.

The chemical conversion film of magnesium alloy has strong bonding ability to the substrate, good insulation and optical properties, but the film layer is too thin and soft, so it can only be used as a coating base and is rarely used alone. The coating obtained by electrochemical oxidation or micro-arc oxidation has a certain thickness, which greatly improves the corrosion resistance of the magnesium alloy surface, but the film layer is loose and porous, and cannot completely and effectively block the erosion of the corrosion medium on the magnesium alloy; And because the brittleness of the oxide film is very large, the film layer is easy to fall off, so it cannot achieve a good protective effect. Electroplating or electroless metal coating on the surface of magnesium alloy is also an important means of protection. It not only improves the corrosion resistance and wear resistance and hardness of the surface of magnesium alloy, but also has electrical conductivity and decoration. At present, electroplating or electroless nickel plating is mainly performed on the surface of magnesium alloys, but because the coating is cathodic, there is a danger of accelerating the corrosion of the magnesium alloy of the substrate when the coating itself is defective or damaged during service, and the presence of nickel coating It also brings difficulties to the recycling of magnesium alloys.

In the preparation of magnesium alloy surface coatings, people are committed to researching a protective coating suitable for magnesium alloys with various requirements such as good corrosion and wear resistance, high hardness, easy recycling, and environmental friendliness. Among them, coating aluminum coating on the surface of magnesium alloy is considered to be the most ideal coating to meet this requirement. The reasons are as follows: 1) the electrode potential of aluminum is not much different from that of magnesium, and it will not cause serious galvanic corrosion to magnesium alloy; 2) The surface treatment of aluminum is much more mature than that of magnesium alloys, and aluminum can be further treated (such as anodizing, micro-arc oxidation, etc.) to obtain a protective coating with high hardness, good wear resistance and corrosion resistance; 3) the aluminum element is magnesium It is an important alloying element of the alloy, and the preparation of the aluminum coating has no adverse effect on the recycling of the material. At present, there are many methods for preparing aluminum coatings on the surface of magnesium alloys, including physical vapor deposition, chemical vapor deposition, magnetron sputtering, cold spraying, laser surface modification, and aluminum plating. However, there are various problems in these preparation technologies, such as expensive equipment, which cannot be widely used, or the obtained coating has poor bonding force, is not dense and incomplete, and is difficult to do further surface treatment, which is far from meeting the needs of magnesium in practical applications. Alloys are in high demand.

Contents of the invention

The purpose of the present invention is to overcome the deficiencies of the prior art and provide a method for preparing an aluminum coating on the surface of magnesium and magnesium alloys. The aluminum coating of the present invention should not only be firmly bonded to the substrate, but also be complete and compact with a certain thickness, so that the prepared aluminum coating can not only be used as a protective layer alone, but also can be further converted into a more corrosion-resistant, wear-resistant and high-hardness coating through subsequent processing. film layer to improve the comprehensive performance of the magnesium alloy surface.

The present invention is implemented through the following technical solutions:

A method for electroplating aluminum with inorganic molten salt on the surface of magnesium and magnesium alloys, comprising the steps of:

(1) After pretreatment such as grinding, polishing, degreasing, etc., magnesium or magnesium alloys are etched and activated in a certain etching solution or activation solution; Or electrogalvanizing, the thickness of the zinc layer is 2-20um, and the pre-prepared galvanized layer or galvanized layer that is well combined with the magnesium alloy substrate is used as the transition layer (bottom layer) before aluminum plating.

On the one hand, the transition zinc layer of the present invention protects the magnesium alloy and prevents the corrosion of the magnesium alloy in the inorganic molten salt; Active, so that the electrodeposited aluminum coating is firmly combined with the substrate.

(2) Electroplated aluminum adopts inorganic molten salt system, with anhydrous aluminum chloride as the main salt, sodium chloride, potassium chloride or a mixture of the two as complexing agent, halides (such as manganese chloride, tin chloride , sodium bromide, potassium bromide, sodium iodide, potassium iodide, etc.) or tetramethylammonium chloride are additives, and their composition weight ratio is: main salt accounts for 65-90%; complexing agent accounts for 5-30%; The additive accounts for 0-5%; the preferred range is as follows: the main salt accounts for 75-85%; the complexing agent accounts for 15-25%; the additive accounts for 0-3%.

During the aluminum electroplating process, the operating temperature is 100-250°C, the current density is 0.5-5A/dm ² , the electroplating time is 5-180 minutes, the electroplating method includes DC electroplating or pulse electroplating, and the thickness of the obtained aluminum coating is 5-60um.

The operating temperature of the invention is 100-250 DEG C, and has no influence on the microstructure and mechanical properties of the magnesium alloy. By adjusting the current density and plating time to prepare the required thickness of the aluminum coating. Use high-purity argon or nitrogen to protect the molten salt to prevent the volatilization of anhydrous aluminum chloride. Adding additives in the molten salt can effectively inhibit the formation of Al dendrites and expand the range of current density. Pulse electroplating can effectively improve the compactness of the coating. Stirring the molten salt can increase the crystallization rate and effectively prevent the growth of aluminum dendrites.

The method for electroplating aluminum with inorganic molten salt on the surface of magnesium and magnesium alloys of the present invention, the specific production process is: grinding → degreasing → etching → activation → pre-galvanizing (dip galvanizing or electro-galvanizing) → light emission → activation → melting Salt electroplating aluminum → drying, adding tap water and deionized water to clean between each step. Among them, the etching and activation before galvanizing can be carried out according to the specific process; the light emitting and activation treatment of the zinc coating before aluminum plating can be selected according to requirements.

Compared with the prior art, the present invention has the following significant advantages:

1. The aluminum coating of the present invention is complete, dense, uniform and bright, and well combined with the substrate. It can be used alone as a protective layer, and can also be prepared into a surface functional layer that is more corrosion-resistant, wear-resistant and high-hardness after subsequent processing.

2. Both the transition layer zinc and the protective layer aluminum in the coating system of the present invention are alloy elements of magnesium alloys, which is beneficial to the recycling of materials.

3. The whole preparation process of the present invention has the characteristics of industrial practicability such as low-cost equipment, easy-to-obtain raw materials, simple process, convenient operation and environmental friendliness.

Detailed ways:

The present invention will be further described below in conjunction with the examples.

Example 1

1. Material preparation: Pure magnesium is cleaned and degreased by ultrasonic cleaning in acetone solution after cutting and grinding.

2. Etching: 85% H ₃ PO ₄ 5ml/L, NH ₄ HF ₂ 100g/L, the rest is water, the temperature is 10-35°C. Treat for 0.5 to 5 minutes, wash with tap water for 1 to 2 minutes immediately after taking it out, and then wash with deionized water for 1 to 2 minutes.

3. Activation: Na ₄ P ₂ O ₇ 40g/L, Na ₂ B ₄ O ₇ 70g/L, NaF 20g/L, and the rest is water. The temperature is 70-80°C, and it is treated for 2-5 minutes. After taking it out, it is immediately washed with tap water for 1-2 minutes, and then with deionized water for 1-2 minutes.

4. Dip zinc: ZnSO _4· 7H ₂ O 30g/L, Na ₄ P ₂ O ₇ 120g/L, NaF 5g/L, LiF 3g/L, Na ₂ CO ₃ 5g/L, and the rest is water. pH = 10.0-10.6, temperature 75-85°C, treatment for 30-90 minutes, immediately after taking out, wash with tap water for 1-2 minutes, and then wash with deionized water for 1-2 minutes. The thickness of the zinc layer in this embodiment is 3-10um .

5. Electroplating aluminum: by weight percentage, AlCl ₃ 75%, NaCl 25%, fully melted into molten salt at 200°C after mixing under argon protection, electroplating current density 1.5A/dm ² , time 30 minutes, after taking out Immediately wash with tap water for 1-2 minutes, then wash with deionized water for 1-2 minutes, and finally dry. The thickness of the electroplated aluminum layer obtained in this embodiment is about 9um.

The relevant performance data of the present embodiment are as follows:

The bonding force between the immersion coating and the substrate is greater than 30MP; the hardness of the aluminum coating is 60-70HV, and the self-corrosion current density of the aluminum coating in 3.5wt% sodium chloride solution is less than 1e-6A/cm ² , which is 1-2 orders of magnitude lower than that of the matrix magnesium .

Example 2

1. Material preparation: After AZ31B magnesium alloy is cut and polished, use ultrasonic cleaning in acetone solution to remove oil.

2. Etching: 85% H ₃ PO ₄ 2～5ml/L, temperature 40～70℃, the rest is water. Treat for 0.5 to 5 minutes, wash with tap water for 1 to 2 minutes immediately after taking it out, and then wash with deionized water for 1 to 2 minutes.

3. Galvanizing: ZnSO ₄ 7H ₂ O 36g/L, Na ₄ P ₂ O ₇ 10H ₂ O 134g/L, C ₆ H ₅ Na ₃ O 7 _{2H 2} O 10g/L, KF 2H ₂ O 10g/L L, sodium lauryl sulfate 0.1g/L, the rest is water. pH=9.0~10.0, temperature 40~70℃, cathode current density 0.1~0.5A/dm ² , time 50~60 minutes, wash with tap water for 1~2 minutes immediately after taking it out, and then wash with deionized water for 1~2 minutes , The thickness of the zinc layer in this embodiment is 5-8um.

4. Idemitsu: HNO ₃ 30ml/L, HCl 10ml/L, the rest is water. The temperature is 10-35°C, and the treatment is for 5-30 seconds. Immediately after taking it out, wash it with tap water for 1-2 minutes, and then wash it with deionized water for 1-2 minutes.

5. Activation: HF 15-20ml/L, the rest is water. The temperature is 10-35°C, and the time is 5-30 seconds. Immediately after taking it out, wash it with tap water for 1-2 minutes, then wash it with deionized water for 1-2 minutes, and then blow it dry.

6. Electroplating aluminum: by weight percentage, AlCl ₃ 79%, NaCl 10%, KCl 10%, NaI 1%, fully melted into molten salt at 160°C after mixing under the protection of argon, and mixed the molten salt with a magnetic stirrer Stir, the cathode current density is 1.2A/dm ² , and the time is 60 minutes. Immediately after taking it out, wash it with tap water for 1-2 minutes, then wash it with deionized water for 1-2 minutes, and finally dry it. The electroplated aluminum layer obtained in this example The thickness is about 15um.

The relevant performance data of the present embodiment are as follows:

The binding force between the galvanized layer and the substrate is greater than 50MP; the hardness of the aluminum coating is 80-90HV, and the self-corrosion current density in 3.5wt% sodium chloride solution is less than 1e-6A/cm ² , which is 1-2 orders of magnitude lower than that of the substrate magnesium alloy .

Example 3

1. Material preparation: After AZ31B magnesium alloy is cut and polished, use ultrasonic cleaning in acetone solution to remove oil.

2. Etching: Na ₄ P ₂ O _7· 10H ₂ O 40g/L, Na ₂ CO ₃ 3.7g/L, NaNO ₃ 5g/L, NaH ₂ PO _4· 2H ₂ 0 5g/L, the rest is water. The temperature is 50-70°C, and it is treated for 0.5-5 minutes. After taking it out, it is immediately washed with tap water for 1-2 minutes, and then with deionized water for 1-2 minutes.

3. Activation: HF 30ml/L, ZnO 20g/L, the rest is water. The temperature is 10-35°C, and it is treated for 0.5-5 minutes. After taking it out, it is immediately washed with tap water for 1-2 minutes, and then with deionized water for 1-2 minutes.

4. Zinc plating: ZnSO ₄ 7H ₂ O 35.6g/L, Na ₄ P _{2 O 7} 10H ₂ O 134g/L, C ₆ H ₅ Na ₃ O _{7 2H 2} O 10g/L, KF.2H ₂ O 10g/L, sodium lauryl sulfate 0.1g/L, and the rest is water. pH=9.0～10.0, temperature 40～70℃, cathode current density 0.2～0.5A/dm ² , time is 50 minutes, wash with tap water for 1～2 minutes immediately after taking it out, then wash with deionized water for 1～2 minutes, In this embodiment, the thickness of the zinc layer is about 6um.

5. Electroplating aluminum: by weight percentage, AlCl ₃ 85%, NaCl 8%, KCl 7%, fully melted into liquid molten salt at 180°C after mixing under argon protection, cathode current density 1A/dm ² , time 180 After taking it out, wash it with tap water for 1-2 minutes immediately, then wash it with deionized water for 1-2 minutes, and finally dry it. The thickness of the electroplated aluminum layer obtained in this embodiment is about 40um.

The relevant performance data of the present embodiment are as follows:

The binding force between the galvanized layer and the substrate is greater than 50MP; the hardness of the aluminum coating is 60-80HV, and the self-corrosion current density in 3.5wt% sodium chloride solution is less than 1e-6A/cm ² , which is 1-2 orders of magnitude lower than that of the substrate magnesium alloy .

Example 4

1. Material preparation: After AZ91D magnesium alloy is cut and polished, use ultrasonic cleaning in acetone solution to degrease.

2. Etching: 85% H ₃ PO ₄ 2～5ml/L, the rest is water. The temperature is 40-70°C, and it is treated for 0.5-5 minutes. After taking it out, it is immediately washed with tap water for 1-2 minutes, and then with deionized water for 1-2 minutes.

3. Electro-galvanizing: ZnSO ₄ 7H ₂ O 36g/L, Na ₄ P _{2 O 7} 10H ₂ O 134g/L, C ₆ H ₅ Na ₃ O _{7 2H 2} O 10g/L, KF.2H ₂ O 10g /L, sodium lauryl sulfate 0.1g/L, and the rest is water. pH=9.0～10.0, temperature 40～70℃, cathode current density 0.1～0.5A/dm ² , time 60 minutes, wash with tap water for 1～2 minutes immediately after taking it out, and then wash with deionized water for 1～2 minutes. Embodiment The thickness of the zinc layer is about 6-8um.

4. Idemitsu: HNO ₃ 30ml/L, HCl 10ml/L, the rest is water. The temperature is 16-35°C, and the treatment is for 5-30 seconds. Immediately after taking it out, wash it with tap water for 1-2 minutes, and then wash it with deionized water for 1-2 minutes.

5. Activation: HF 15-20ml/L, the rest is water. The temperature is 16-35°C, and the time is 5-10 seconds. Immediately after taking it out, wash it with tap water for 1-2 minutes, then wash it with deionized water for 1-2 minutes, and then dry it.

6. Electroplating aluminum: by weight percentage, AlCl ₃ 79%, NaCl 10%, KCl 10%, KBr 1%, fully melted into molten salt at 160°C after mixing under the protection of argon, and the molten salt was heated by magnetic stirring Stir; current density 1A/dm ² , time 60 minutes, wash with tap water for 1 to 2 minutes immediately after taking it out, then wash with deionized water for 1 to 2 minutes, and finally dry, the thickness of the electroplated aluminum layer obtained in this example is 15um .

The relevant performance data of the present embodiment are as follows:

The binding force between the zinc coating and the substrate is greater than 50MP, the hardness of the aluminum coating is 60-80HV, and the self-corrosion current density in 3.5wt% sodium chloride solution is less than 1e-6A/cm ² , which is 1-2 orders of magnitude lower than that of the substrate magnesium alloy.

Example 5

1. Material preparation: After AZ91D magnesium alloy is cut and polished, use ultrasonic cleaning in acetone solution to degrease.

2. Etching: Na ₄ P ₂ O _7· 10H ₂ O 40g/L, Na ₂ CO ₃ 3.7g/L, NaNO ₃ 5g/L, NaH ₂ PO _4· 2H ₂ O 5g/L, the rest is water. The temperature is 50-70°C, and it is treated for 0.5-5 minutes. After taking it out, it is immediately washed with tap water for 1-2 minutes, and then with deionized water for 1-2 minutes.

3. Activation: HF 30ml/L, ZnO 30g/L, the rest is water. The temperature is 10-35°C, and it is treated for 0.5-5 minutes. After taking it out, it is immediately washed with tap water for 1-2 minutes, and then with deionized water for 1-2 minutes.

4. Zinc plating: ZnSO ₄ 7H ₂ O 35.6g/L, Na ₄ P _{2 O 7} 10H ₂ O 134g/L, C ₆ H ₅ Na ₃ O _{7 2H 2} O 10g/L, KF.2H ₂ O 10g/L, sodium lauryl sulfate 0.1g/L, and the rest is water. pH=9.0～10.0, temperature 40～70℃, cathode current density 0.1～0.5A/dm ² , time is 50 minutes, wash with tap water for 1～2 minutes immediately after taking it out, then wash with deionized water for 1～2 minutes, In this embodiment, the thickness of the zinc layer is about 7um.

5. Electroplating aluminum: by weight percentage, AlCl ₃ 78.5%, NaCl 10%, KCl 10%, tetramethylammonium chloride 1.5%, fully melted into liquid molten salt at 160°C after mixing under the protection of argon, using Pulse electroplating, frequency 1000Hz, duty cycle 20-35%, cathode current density 0.8A/dm ² , time 180 minutes, wash with tap water for 1-2 minutes immediately after taking it out, then wash with deionized water for 1-2 minutes, and finally After drying, the thickness of the electroplated aluminum layer obtained in this embodiment is about 35um.

The relevant performance data of the present embodiment are as follows:

The binding force between the zinc coating and the substrate is greater than 50MP; the hardness of the aluminum coating is 70-85HV, and the self-corrosion current in 3.5wt% sodium chloride solution is less than 1e-6A/cm ² , which is 1-2 orders of magnitude lower than that of the substrate magnesium alloy.

Example 6

1. Material preparation: After AM60 magnesium alloy is cut and polished, it is cleaned and degreased by ultrasonic wave in acetone solution.

2. Etching: Na ₄ P ₂ O _7· 10H ₂ O 40g/L, Na ₂ CO ₃ 3.7g/L, NaNO ₃ 8g/L, NaH ₂ PO _4· 2H ₂ O 5g/L, the rest is water. The temperature is 50-70°C, and it is treated for 0.5-5 minutes. After taking it out, it is immediately washed with tap water for 1-2 minutes, and then with deionized water for 1-2 minutes.

3. Activation: HF 20ml/L, NH ₄ HF ₂ 10g/L, ZnO 30g/L, and the rest is water. The temperature is 10-35°C, and it is treated for 0.5-5 minutes. After taking it out, it is immediately washed with tap water for 1-2 minutes, and then with deionized water for 1-2 minutes.

4. Electro-galvanizing: ZnSO ₄ ·7H ₂ O 20g/L, Na ₄ P ₂ O ₇ ·10H ₂ O 100g/L, NaF 5g/L, sodium lauryl sulfate 0.1g/L, and the rest is water. pH=9.0~9.6, temperature 40~70°C, cathode current density 0.1~0.5A/dm ² , time 50 minutes, wash with tap water for 1~2 minutes immediately after taking it out, and then wash with deionized water for 1~2 minutes. Embodiment The thickness of the zinc layer is 6um.

6. Electroplating aluminum: by weight percentage, AlCl ₃ 70%, NaCl 15%, KCl 15%, fully melted into molten salt at 160°C after mixing under argon protection, electroplating current density 1A/dm ² , time 60 minutes , immediately after taking it out, wash it with tap water for 1-2 minutes, then wash it with deionized water for 1-2 minutes, and finally dry it. The thickness of the electroplated aluminum layer obtained in this embodiment is 15um.

The relevant performance data of the present embodiment are as follows:

The binding force between the zinc coating and the substrate is greater than 50MP; the hardness of the aluminum coating is 60-70HV, and the self-corrosion current density in 3.5wt% sodium chloride solution is less than 1e-6A/cm ² , which is 1-2 orders of magnitude lower than that of the substrate magnesium alloy.

Example 7

1. Material preparation: After AM60 magnesium alloy is cut and polished, it is cleaned and degreased by ultrasonic wave in acetone solution.

2. Etching: 85% H ₃ PO ₄ 2～5ml/L, the rest is water. The temperature is 40-70°C, and it is treated for 0.5-5 minutes. After taking it out, it is immediately washed with tap water for 1-2 minutes, and then with deionized water for 1-2 minutes.

4. Electro-galvanizing: in two steps (1) Pre-galvanizing: ZnSO _4· 7H ₂ O 36g/L, Na ₄ P ₂ O _7· 10H _{2 O} 134g/L, C ₆ H ₅ Na ₃ O _7· 2H ₂ O 10g/L, KF.2H ₂ O 10g/L, sodium lauryl sulfate 0.1g/L, and the rest is water. pH=9.0~10.0, temperature 40~70°C, cathode current density 0.1~0.5A/dm ² , time 5~30 minutes, wash with tap water for 1~2 minutes immediately after taking it out, and then wash with deionized water for 1~2 minutes minute. (2) Bright galvanizing: ZnO 17g/L, NaOH 135g/L, Merlin gloss agent 1.0ml/L, Merlin pot opener 20ml/L, and the rest is water. The temperature is 10-35°C, the cathode current density is 1.5-2.5A/dm ² , and the time is 10-60 minutes. Immediately after taking it out, wash it with tap water for 1-2 minutes, and then wash it with deionized water for 1-2 minutes. In this embodiment, the thickness of the zinc layer is about 5-20um.

5. Idemitsu: HNO ₃ 30ml/L, HCl 10ml/L, the rest is water. The temperature is 10-35°C, and the treatment is for 5-30 seconds. Immediately after taking it out, wash it with tap water for 1-2 minutes, and then wash it with deionized water for 1-2 minutes.

6. Activation: HF 15-20ml/L, the rest is water. The temperature is 10-35°C, and the time is 5-30 seconds. Immediately after taking it out, wash it with tap water for 1-2 minutes, then wash it with deionized water for 1-2 minutes, and then blow it dry.

7. Electroplating aluminum: by weight percentage, AlCl ₃ 78.5%, NaCl 10%, KCl 10%, tetramethylammonium chloride 1.5%, fully melted into molten salt at 190°C after mixing under the protection of argon, and use magnetic force Stir to stir the molten salt, electroplating current density 1A/dm ² , time 60 minutes, wash with tap water for 1 to 2 minutes immediately after taking it out, then wash with deionized water for 1 to 2 minutes, and finally dry it. The thickness of the electroplated aluminum layer is 15um.

The relevant performance data of the present embodiment are as follows:

The binding force between the zinc coating and the substrate is greater than 50MP, the hardness of the aluminum coating is 65-75HV, and the self-corrosion current density in 3.5wt% sodium chloride solution is less than 1e-6A/cm ² , which is 1-2 orders of magnitude lower than that of the substrate magnesium alloy.

Claims (6)

1. A method for electroplating aluminum with inorganic molten salt on the surface of magnesium and magnesium alloys, characterized in that: the surface of magnesium or magnesium alloy is primed with a pre-galvanized layer, and then aluminum is electroplated in inorganic molten salt. 2. The method for electroplating aluminum with inorganic molten salt on the surface of magnesium and magnesium alloys according to claim 1, characterized in that the pre-galvanized layer includes galvanized layer or electro-galvanized layer with a thickness of 2-20um. 3. The method for electroplating aluminum with inorganic molten salt on the surface of magnesium and magnesium alloys according to claim 1, characterized in that the aluminum electroplating adopts an inorganic molten salt system, and the inorganic molten salt system contains: main salt, complex agents and additives; among them, Main salt: anhydrous aluminum chloride, accounting for 65-90%; Complexing agent: sodium chloride, potassium chloride or a mixture of both, accounting for 5-30%; Additives: halides or tetramethylammonium chloride, 0-5%; During the aluminum electroplating process, the operating temperature is 100-250°C, the electroplating method includes DC electroplating or pulse electroplating, the electroplating time is 5-180 minutes, the current density is 0.5-5A/dm ² , and the thickness of the electroplated aluminum layer obtained is 5-60um. 4. The method for electroplating aluminum on the surface of magnesium and magnesium alloys with inorganic molten salt according to claim 3, characterized in that the halide is manganese chloride, tin chloride, sodium bromide, potassium bromide, sodium iodide or potassium iodide . 5. The method for electroplating aluminum with inorganic molten salt on the surface of magnesium and magnesium alloys according to claim 3, characterized in that: a protective atmosphere is used to prevent the volatilization of the plating solution, and the protective atmosphere used is argon or nitrogen to anhydrous chlorination. Aluminum Chemically inactive gas. 6. The method for electroplating aluminum on the surface of magnesium and magnesium alloys with inorganic molten salt according to claim 3, characterized in that: during the electroplating process, solution stirring is used to improve the crystallization speed and surface quality of the coating.