Micro-arc oxidation electrolyte, magnesium alloy hard ceramic membrane and preparation method of magnesium alloy hard ceramic membrane

Abstract

The invention discloses a magnesium alloy micro-arc oxidation electrolyte which comprises sodium metaaluminate, sodium hydroxide, sodium fluoroaluminate, aluminum fluoride and glycerol. The content of sodium metaaluminate is 5-25 g / L, the content of sodium hydroxide is 1-3 g / L, the content of sodium fluoroaluminate is 0.3-0.7 g / L, the content of aluminum fluoride is 0.2-0.3 g / L, and the content of glycerol is 1 g / L. The magnesium alloy micro-arc oxidation electrolyte comprises potassium metaaluminate, potassium hydroxide, potassium fluoroaluminate, aluminum fluoride and glycerol, the content of potassium metaaluminate is 10-20 g / L, the content of potassium hydroxide is 3-5 g / L, the content of potassium fluoroaluminate is 0.5-0.7 g / L, the content of aluminum fluoride is 0.3-0.5 g / L, and the content of glycerol is 1 g / L. The invention further discloses the magnesium alloy hard ceramic membrane and a preparation method thereof. According to the preparation method of the magnesium alloy hard ceramic film, the components and the microstructure of the magnesium alloy micro-arc oxidation film layer are changed, and meanwhile the hardness and the corrosion resistance of the magnesium alloy micro-arc oxidation film layer are improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of light metal surface treatment, and in particular relates to a micro-arc oxidation electrolyte. The present invention also relates to a magnesium alloy hard ceramic film and a preparation method thereof. Background Art

[0002] Magnesium alloys are widely used in aerospace, underwater equipment, and transportation due to their low density, high specific strength, and excellent machinability. However, their poor wear resistance and corrosion resistance are two bottlenecks that hinder their potential performance advantages. Magnesium is chemically active, and its surface is prone to corrosion in air and humid environments, forming corrosion pits and black magnesium oxide corrosion products, which can cause failure of magnesium alloy parts and limit their application in specialized environments. Therefore, research on magnesium alloy surface treatment technologies is one of the effective methods to promote its development and application.

[0003] Micro-arc oxidation, also known as plasma electrolytic oxidation, is a surface treatment technology that electrochemically grows a ceramic layer in situ on metal surfaces (aluminum, magnesium, titanium, and their alloys). This technology induces micro-arc discharges under a high-voltage electric field, causing complex physical and chemical reactions on the metal surface to form a ceramic oxide film with high hardness, corrosion resistance, and wear resistance. Micro-arc oxidation of magnesium alloys has long been a popular topic, with extensive research conducted on both electrolyte formulation and electrical parameter control. However, when micro-arc oxidized magnesium alloy (AZ41M) in phosphate and silicate electrolytes, the ceramic film is primarily composed of MgO and Mg2SiO4 phases, with a hardness of only 200-400 HV. Furthermore, the in-situ ceramic film is loose and porous, with a high porosity, making it more permeable to corrosive media and reducing corrosion resistance.

[0004] The micro-arc oxidation film on aluminum alloy has high hardness (1000~1500HV) and excellent corrosion resistance (neutral salt spray resistance time is >500h). This is because its micro-arc oxidation ceramic layer is mainly Al2O3. The Mohs hardness of aluminum oxide is as high as 9 (second only to diamond). Although the micro-arc oxidation film on aluminum alloy is also porous, these pores will form a closed layer due to the self-passivation effect of aluminum oxide in a corrosive environment, further blocking corrosion. Therefore, many scholars have adopted different methods, hoping to in-situ generate or add nano-alumina particles in the micro-arc oxidation film on magnesium alloy to improve the quality and performance of the micro-arc oxidation ceramic film on magnesium alloy. Summary of the Invention

[0005] The first object of the present invention is to provide a method for preparing a magnesium alloy hard ceramic film, which changes the composition and microstructure of the magnesium alloy micro-arc oxidation film layer and improves the hardness and corrosion resistance of the magnesium alloy micro-arc oxidation film layer.

[0006] The second object of the present invention is to provide an electrolyte for preparing magnesium alloy hard ceramic film.

[0007] The third object of the present invention is to provide a magnesium alloy hard ceramic film prepared by the above preparation method.

[0008] The first technical solution adopted by the present invention is: a magnesium alloy micro-arc oxidation electrolyte, comprising sodium metaaluminate, sodium hydroxide, sodium fluoroaluminate, aluminum fluoride, and glycerol; the sodium metaaluminate content is 5-25 g / L, the sodium hydroxide content is 1-3 g / L, the sodium fluoroaluminate content is 0.3-0.7 g / L, the aluminum fluoride content is 0.2-0.3 g / L, and the glycerol content is 1 g / L; Or it contains potassium metaaluminate, potassium hydroxide, potassium fluoroaluminate, aluminum fluoride, and glycerol; the potassium metaaluminate content is 10~20g / L, the potassium hydroxide content is 3~5g / L, the potassium fluoroaluminate content is 0.5~0.7g / L, the aluminum fluoride content is 0.3~0.5g / L, and the glycerol is 1g / L.

[0009] The second technical solution adopted by the present invention is: a method for preparing a magnesium alloy hard ceramic film, the specific method is as follows: S1. Grinding and polishing the surface of the magnesium alloy, and then cleaning and drying it to obtain a magnesium alloy sample; S2. Prepare electrolyte: Weigh various components according to any of the above proportions to prepare electrolyte, and test the pH value and conductivity value of the solution to obtain electrolyte; S3. Using the magnesium alloy sample as the anode and the stainless steel as the cathode, micro-arc oxidation treatment is performed in an electrolyte by increasing the current in stages, and water circulation cooling is performed to obtain a micro-arc oxidation sample; S4. Clean and dry the micro-arc oxidation sample to obtain a magnesium alloy hard ceramic film.

[0010] The present invention is also characterized in that: The specific method of S2 is as follows: Add the weighed aluminum fluoride and sodium fluoroaluminate respectively to deionized water, stir to dissolve, then add sodium aluminate, sodium hydroxide, and glycerol in sequence. After the previous component is fully dissolved, add the next component. During the entire process of preparing the solution, introduce compressed air to circulate and stir the solution for 50 to 60 minutes until the solution is clear and transparent.

[0011] The pH value of the electrolyte in S2 is 12.5~13, and the solution conductivity is 25~30mS / cm.

[0012] The process of controlling the electrical parameters of the bidirectional pulsed DC power supply in S3 is as follows: During the micro-arc oxidation process of magnesium alloy, a bidirectional pulsed DC power supply and a constant current transformer control method are used. The power supply current is 1~5A: voltage: 450~550V, the pulse frequency is 500~1000HZ, the duty cycle is 10%~40%, the positive and negative pulse ratio is 1:0, 2:1 or 4:1, the micro-arc oxidation time is 1h~2h, and the electrolyte temperature is controlled below 50℃.

[0013] In S3, during the electrical parameter control process of micro-arc oxidation of magnesium alloy, an initial current of 1~2A is given until arcing occurs on the surface of the magnesium alloy sample. Subsequently, the voltage value gradually increases at a rate of 1V / min. When the power supply voltage remains unchanged within 30 minutes, the power supply current is increased by increasing the current through the power supply control panel by 0.5A each time until the voltage value reaches the set value.

[0014] The third technical solution adopted by the present invention is: the magnesium alloy hard ceramic film prepared according to the above preparation method, the micro-arc oxidation ceramic film is off-white in color, the thickness of the micro-arc oxidation ceramic film is 15~50μm, the hardness of the micro-arc oxidation ceramic film is higher than 700HV, and the neutral salt spray resistance time is greater than 400h.

[0015] The beneficial effects of the present invention are: (1) The preparation method of the magnesium alloy hard ceramic film of the present invention draws on the characteristics of high hardness and high corrosion resistance of aluminum oxide. By adding a multi-component aluminum metal salt (aluminate, fluoroaluminate and aluminum fluoride) to the magnesium alloy micro-arc oxidation electrolyte, a bidirectional pulsed DC power supply and a step constant current method are adopted to control the growth rate of the ceramic film layer in each stage, maintain the slow and uniform growth of the ceramic film layer, prevent the initial current from being too large so that high voltage breakdown occurs in the local area of ​​the basic surface, forming holes or even pits, and increase the density and corrosion resistance of the ceramic film layer. In the process of micro-arc oxidation, a uniform and dense alloy film containing aluminum oxide is grown in situ on the surface of the substrate of the magnesium alloy, which changes the composition and microstructure of the magnesium alloy micro-arc oxidation film layer, and at the same time improves the hardness and corrosion resistance of the magnesium alloy micro-arc oxidation film layer. In addition, the electrolyte has the characteristics of simple composition, low cost and green environmental protection, which greatly improves the surface protection ability of the magnesium alloy and extends the service life of the magnesium alloy. (2) The magnesium alloy micro-arc oxidation electrolyte of the present invention uses aluminum-containing metal salt as the main salt. A large amount of free Al ions obtained by hydrolysis participate in the micro-arc oxidation reaction, and a metal composite ceramic film containing a certain volume fraction of aluminum oxide and magnesium oxide is in situ grown on the surface of the substrate to form a ceramic layer with high hardness, high wear resistance and high corrosion resistance. Compared with adding hard nano-ceramic particles to silicate and phosphate solutions, the electrolyte of the present invention has the advantages of simple composition, low cost, easy waste liquid treatment, green environmental protection and sustainability. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1This is a scanning electron microscope image of the surface of the micro-arc oxidation ceramic film on AZ41M magnesium alloy prepared in Example 3 of the present invention; Figure 2 The scanning electron microscope image of the cross section of the AZ41M magnesium alloy micro-arc oxidation ceramic film prepared in Example 3 of the present invention and the EDS elemental analysis image of the substrate and the ceramic film layer; Figure 3 This is an EDS surface scanning electron microscopy element analysis diagram of the surface of the AZ41M magnesium alloy micro-arc oxidation ceramic film prepared in Example 3 of the present invention; Figure 4 Statistical histogram of Vickers hardness of Examples 1 to 3 of the present invention and Comparative Example 1. DETAILED DESCRIPTION

[0017] The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0018] The magnesium alloy micro-arc oxidation electrolyte of the present invention comprises sodium metaaluminate, sodium hydroxide, sodium fluoroaluminate, aluminum fluoride, and glycerol; the sodium metaaluminate content is 5-25 g / L, the sodium hydroxide content is 1-3 g / L, the sodium fluoroaluminate content is 0.3-0.7 g / L, the aluminum fluoride content is 0.2-0.3 g / L, and the glycerol content is 1 g / L.

[0019] The magnesium alloy micro-arc oxidation electrolyte of the present invention comprises potassium metaaluminate, potassium hydroxide, potassium fluoroaluminate, aluminum fluoride, and glycerol in another ratio; the potassium metaaluminate content is 10-20 g / L, the potassium hydroxide content is 3-5 g / L, the potassium fluoroaluminate content is 0.5-0.7 g / L, the aluminum fluoride content is 0.3-0.5 g / L, and the glycerol content is 1 g / L.

[0020] The method for preparing a magnesium alloy hard ceramic film of the present invention uses any of the above-mentioned electrolytes. On the one hand, the method reselects the electrolyte components and ratios; on the other hand, the micro-arc oxidation electrical parameters are regulated. The specific method is as follows: S1. Grinding and polishing the surface of the magnesium alloy, and then cleaning and drying it to obtain a magnesium alloy sample; S2. Prepare electrolyte: Weigh various components according to the proportions of claim 1 or 2 to prepare electrolyte, and test the pH value and conductivity value of the solution to obtain electrolyte; the specific method is as follows: Add the weighed aluminum fluoride and sodium fluoroaluminate respectively to deionized water, stir to dissolve, then add sodium aluminate, sodium hydroxide, and glycerol in sequence. After the previous component is fully dissolved, add the next component. During the entire process of preparing the solution, introduce compressed air to circulate and stir the solution for 50 to 60 minutes until the solution is clear and transparent.

[0021] Among them, the pH value of the electrolyte is 12.5~13, and the solution conductivity is 25~30mS / cm.

[0022] S3. Using the magnesium alloy sample as the anode and the stainless steel as the cathode, micro-arc oxidation treatment is performed in an electrolyte by increasing the current in stages, and water circulation cooling is performed to obtain a micro-arc oxidation sample; Among them, the process of controlling the electrical parameters of the bidirectional pulsed DC power supply is as follows: During the micro-arc oxidation process of magnesium alloy, a bidirectional pulsed DC power supply and a constant current transformer control method are used. The power supply current is 1~5A: voltage: 450~550V, the pulse frequency is 500~1000HZ, the duty cycle is 10%~40%, the positive and negative pulse ratio is 1:0, 2:1 or 4:1, the micro-arc oxidation time is 1h~2h, and the electrolyte temperature is controlled below 50℃.

[0023] During the electrical parameter control process of micro-arc oxidation of magnesium alloy, an initial current of 1~2A is given until arcing occurs on the surface of the magnesium alloy sample. At this time, the voltage is 200V~300V. As the film thickness increases, the voltage value gradually increases at a rate of 1V / min. When the power supply voltage remains basically unchanged within 30 minutes, the power supply current is increased by increasing the current through the power supply control panel, increasing it by 0.5A each time until the voltage value reaches the set value.

[0024] S4. Clean and dry the micro-arc oxidation sample to obtain a magnesium alloy hard ceramic film.

[0025] The present invention draws on the characteristics of high hardness and high corrosion resistance of aluminum oxide. By adding multi-component aluminum metal salts (metaaluminate, fluoroaluminate and aluminum fluoride) to the magnesium alloy micro-arc oxidation electrolyte, and adopting a bidirectional pulsed DC power supply and a step constant current method, a uniform and dense alloy film containing aluminum oxide is grown in situ on the surface of the substrate of the magnesium alloy during the micro-arc oxidation process, which changes the composition and microstructure of the magnesium alloy micro-arc oxidation film layer, and at the same time improves the hardness and corrosion resistance of the magnesium alloy micro-arc oxidation film layer. In addition, the electrolyte has the characteristics of simple composition, low cost and green environmental protection, which greatly improves the surface protection ability of the magnesium alloy and extends the service life of the magnesium alloy.

[0026] The method of the present invention reselects the components and proportions of the electrolyte; by adding a multi-component aluminum metal salt (metaaluminate, fluoroaluminate and aluminum fluoride) to the magnesium alloy micro-arc oxidation electrolyte, a large amount of free Al ions are generated in the solution during the electrolysis process. During the micro-arc oxidation process, arc discharge causes the surface of the magnesium alloy to partially melt, and a large amount of free Al ions in the electrolyte react with the matrix metal under high temperature and high pressure to generate an alumina ceramic phase in situ on the surface of the magnesium alloy, thereby increasing the hardness and corrosion resistance of the film layer.

[0027] On the other hand, the electrical parameters of micro-arc oxidation are regulated: a bidirectional pulsed DC power supply and a step constant current method are used to control the growth rate of the micro-arc oxidation film, thereby improving the density of the film and effectively reducing the porosity on the surface of the film, thereby achieving the purpose of improving the hardness and corrosion resistance of the micro-arc oxidation film on magnesium alloy.

[0028] The present invention provides a magnesium alloy micro-arc oxidation electrolyte with an aluminum-containing metal salt as its primary salt. The hydrolysis of the metal salt produces a large amount of free Al ions that participate in the micro-arc oxidation reaction, in situ growing a metal composite ceramic film containing a certain volume fraction of aluminum oxide and magnesium oxide on the substrate surface, forming a ceramic layer with high hardness, high wear resistance, and high corrosion resistance. Compared to adding hard nano-ceramic particles to silicate and phosphate solutions, the present electrolyte offers advantages such as simple composition, low cost, easy wastewater treatment, and environmental sustainability.

[0029] The magnesium alloy hard ceramic film prepared according to the above preparation method has a grayish white color, a thickness of 15-50 μm, a hardness higher than 700 HV, and a neutral salt spray resistance time greater than 400 h.

[0030] Example 1 The method for preparing the magnesium alloy hard ceramic film in this embodiment is as follows: S1. Cut AZ41M magnesium alloy into Φ55×5mm discs, punch Φ5 holes on the edges, grind and polish with sandpaper, and then clean with alcohol for later use; S2. Weigh potassium metaaluminate, potassium hydroxide, potassium fluoroaluminate, aluminum fluoride and glycerol respectively, add aluminum fluoride and potassium fluoroaluminate into deionized water, stir and dissolve, then add potassium metaaluminate, potassium hydroxide and glycerol in sequence, and add the next ingredient after the previous ingredient is fully dissolved; during the whole process of preparing the solution, introduce compressed air to circulate and stir the solution for 50-60 minutes until the solution is clear and transparent, thereby obtaining an electrolyte. The electrolyte contained potassium metaaluminate at a concentration of 15 g / L, potassium hydroxide at a concentration of 3 g / L, potassium fluoroaluminate at a concentration of 0.5 g / L, aluminum fluoride at a concentration of 0.2 g / L, and glycerol at a concentration of 1 g / L. The pH value of the electrolyte was measured using a PHS-3G pH meter produced by Shanghai Yidian Scientific Instrument Co., Ltd. and was 12.65. The conductivity was measured using a PE382 conductivity meter produced by Mettler-Toledo Instruments Co., Ltd. and was 26.32 mS / cm. S3. The polished magnesium alloy sample was placed in the electrolyte as the anode, and the stainless steel was used as the cathode. A bidirectional pulsed DC power supply was used, and the electrical parameters were controlled in constant current mode with a current of 1-5A, a pulse frequency of 500Hz, a duty cycle of 20%, a positive-negative pulse ratio of 1:0, and a termination voltage of 570V. The micro-arc oxidation treatment time was 70min, and the electrolyte temperature was controlled at 48°C. During the electrical parameter control process of micro-arc oxidation of magnesium alloy, an initial current of 1~2A is given until arcing occurs on the surface of the magnesium alloy sample. As the film thickness increases, the voltage value gradually increases at a rate of 1V / min. When the power supply voltage remains basically unchanged within 30 minutes, the power supply current is increased by increasing the current through the power supply control panel, increasing it by 0.5A each time until the voltage reaches the set value.

[0031] S4. Clean and dry the micro-arc oxidation sample to obtain a magnesium alloy hard ceramic film.

[0032] The film thickness of the test piece was tested using a Time 2822 coating thickness gauge produced by Beijing Times Peak Technology Co., Ltd. The thickness was 45~50μm, and the Vickers hardness of the sample surface was tested using a Qness microhardness tester, which was 700HV.

[0033] Example 2 The method for preparing the magnesium alloy hard ceramic film in this embodiment is as follows: S1. Cut AZ41M magnesium alloy into Φ55×5mm discs, punch Φ5 holes on the edges, grind and polish with sandpaper, and then clean with alcohol for later use; S2. Weigh sodium metaaluminate, sodium hydroxide, sodium fluoroaluminate, aluminum fluoride and glycerol respectively, add aluminum fluoride and sodium fluoroaluminate into deionized water, stir and dissolve, then add sodium metaaluminate, sodium hydroxide and glycerol in sequence, and add the next ingredient after the previous ingredient is fully dissolved; during the whole process of preparing the solution, introduce compressed air to circulate and stir the solution for 50-60 minutes until the solution is clear and transparent, thereby obtaining an electrolyte. The electrolyte contained sodium metaaluminate at a concentration of 20 g / L, sodium hydroxide at a concentration of 2 g / L, sodium fluoroaluminate at a concentration of 0.5 g / L, aluminum fluoride at a concentration of 0.3 g / L, and glycerol at a concentration of 1 g / L. The pH value of the electrolyte was measured using a PHS-3G pH meter produced by Shanghai Yidian Scientific Instrument Co., Ltd. and was 12.77. The conductivity was measured using a PE382 conductivity meter produced by Mettler-Toledo Instruments Co., Ltd. and was 26.88 mS / cm. S3. The polished magnesium alloy sample was placed in the electrolyte as the anode, and the stainless steel was used as the cathode. A bidirectional pulsed DC power supply was used, and the electrical parameters were controlled in constant current mode with a current of 1-3A, a pulse frequency of 500Hz, a duty cycle of 15%, a positive-negative pulse ratio of 1:0, and a termination voltage of 504V. The micro-arc oxidation treatment time was 60min, and the electrolyte temperature was controlled at 49°C. During the electrical parameter control process of micro-arc oxidation of magnesium alloy, an initial current of 1~2A is given until arcing occurs on the surface of the magnesium alloy sample. As the film thickness increases, the voltage value gradually increases at a rate of 1V / min. When the power supply voltage remains basically unchanged within 30 minutes, the power supply current is increased by increasing the current through the power supply control panel, increasing it by 0.5A each time until the voltage reaches the set value.

[0034] S4. Clean and dry the micro-arc oxidation sample to obtain a magnesium alloy hard ceramic film.

[0035] The film thickness of the test piece was measured using a Time 2822 coating thickness gauge produced by Beijing Times Peak Technology Co., Ltd. The Vickers hardness of the sample surface was 773 HV using a Qness microhardness tester.

[0036] Example 3 The method for preparing the magnesium alloy hard ceramic film in this embodiment is as follows: S1, same as Example 1; S2. Weigh sodium metaaluminate, sodium hydroxide, sodium fluoroaluminate, aluminum fluoride and glycerol respectively, add aluminum fluoride and sodium fluoroaluminate into deionized water, stir and dissolve, then add sodium metaaluminate, sodium hydroxide and glycerol in sequence, and add the next ingredient after the previous ingredient is fully dissolved; during the whole process of preparing the solution, introduce compressed air to circulate and stir the solution for 50-60 minutes until the solution is clear and transparent, thereby obtaining an electrolyte. The electrolyte contained sodium metaaluminate at a concentration of 25 g / L, sodium hydroxide at a concentration of 1 g / L, sodium fluoroaluminate at a concentration of 0.7 g / L, aluminum fluoride at a concentration of 0.3 g / L, and glycerol at a concentration of 1 g / L. The pH value of the electrolyte was measured using a PHS-3G pH meter produced by Shanghai Yidian Scientific Instrument Co., Ltd. and was 12.85. The conductivity was measured using a PE382 conductivity meter produced by Mettler-Toledo Instruments Co., Ltd. and was 27.12 mS / cm. S3. The polished magnesium alloy sample was placed in the electrolyte as the anode, and the stainless steel was used as the cathode. A bidirectional pulsed DC power supply was used. The electrical parameters were controlled in constant current mode with a current of 1-3.5A, a pulse frequency of 800Hz, a duty cycle of 15%, a positive-negative pulse ratio of 4:1, and a termination voltage of 550V. The micro-arc oxidation treatment time was 100min. The electrolyte temperature was controlled at 49°C. During the electrical parameter control process of micro-arc oxidation of magnesium alloy, an initial current of 1~2A is given until arcing occurs on the surface of the magnesium alloy sample. As the film thickness increases, the voltage value gradually increases at a rate of 1V / min. When the power supply voltage remains basically unchanged within 30 minutes, the power supply current is increased by increasing the current through the power supply control panel, increasing it by 0.5A each time until the voltage reaches the set value.

[0037] S4. Clean and dry the micro-arc oxidation sample to obtain a magnesium alloy hard ceramic film.

[0038] The film thickness of the test piece was measured using a Time 2822 coating thickness gauge produced by Beijing Times Peak Technology Co., Ltd. The Vickers hardness of the sample surface was measured using a Qness microhardness tester and was 867 HV.

[0039] Figure 1 a is a 500x scanning electron microscope photograph of the AZ41M magnesium alloy micro-arc oxidation ceramic membrane prepared in this example, and b is a 1000x scanning electron microscope photograph of the AZ41M magnesium alloy micro-arc oxidation ceramic membrane. It can be seen from the figure that there are a large number of "crater-shaped" micropores with a diameter of 1~5μm on the surface of the ceramic membrane, with a porosity of 10%. The pores are interconnected or independent, and there are fine cracks on the surface.

[0040] Figure 2 The following are scanning electron microscope (SEM) images of the cross-section of the magnesium alloy micro-arc oxidation ceramic film prepared in this example, as well as EDS elemental analysis images of the substrate and ceramic film layer. It can be seen from the figure that the ceramic film layer after micro-arc oxidation of AZ41M magnesium alloy is continuous and uniform with good bonding to the substrate. EDS analysis found that the main element compositions of the ceramic film layer are 32.82% Mg, 24.5% Al, and 42.68% O, respectively. The oxygen content is the highest, and the aluminum content is significantly increased compared to that of the substrate, from 3.42% in the substrate to 24.50% in the ceramic film, indicating that the aluminum ions in the electrolyte participate in the growth of the ceramic film and form metal oxides therein.

[0041] Figure 3 This is an EDS surface scanning element analysis diagram of the magnesium alloy micro-arc oxidation ceramic film prepared in this example. It can be seen from the figure that Mg, Al, and O elements are evenly distributed in the ceramic film layer, with their contents of 27.8%, 25.8%, and 45.7%, respectively, indicating that a composite metal oxide ceramic film is formed on the surface of the magnesium alloy, improving the quality and performance of the magnesium alloy micro-arc oxidation film layer.

[0042] Comparative Example 1 The preparation method of magnesium alloy hard ceramic film is as follows: S1. Cut AZ41M magnesium alloy into Φ55×5mm discs, punch Φ5 holes on the edges, grind and polish with sandpaper, and then clean with alcohol for later use; S2. Prepare magnesium alloy micro-arc oxidation electrolyte: 15g / L sodium silicate, 1g / L sodium hydroxide, 10g / L sodium fluoride, 3g / L sodium tungstate, and deionized water as solvent; the pH value of the electrolyte is 12.50, and the conductivity value tested by a conductivity meter is 29.85mS / cm; S3. Place the magnesium alloy specimen as the anode in the electrolyte, and the stainless steel plate as the cathode. Use a bidirectional pulsed DC power supply with constant current mode to control the electrical parameters. The power supply electrical parameter settings are: current 2A, pulse frequency 500Hz, duty cycle 20%, positive-negative pulse ratio 1:0, termination voltage 328V, and micro-arc oxidation treatment time 120min. S4. Take out the micro-arc oxidation test piece, rinse it with deionized water and then dry it.

[0043] The film thickness of the test piece was measured using a Time 2822 coating thickness gauge produced by Beijing Times Peak Technology Co., Ltd. The Vickers hardness of the sample surface was measured using a Qness microhardness tester and was 310 HV.

[0044] Figure 4 The following is a statistical histogram of the Microhardness distribution for Examples 1-3 and Comparative Example 1. As can be seen from the figure, in Comparative Example 1, where no aluminum salt-containing electrolyte was added, the Vickers hardness of the ceramic layer was only 310 HV. With the addition of metaaluminate and aluminum fluoride, the hardness of the ceramic layer increased to 700 HV. This is due to the fact that during the micro-arc oxidation process, aluminum ions in the solution combined with oxygen ions to form an in-situ aluminum oxide film with corundum and spinel structures on the substrate surface, improving the film's quality and performance. As the concentrations of sodium metaaluminate and aluminum fluoride in the electrolyte increased, the Vickers hardness of the magnesium alloy micro-arc oxidation ceramic layer also increased, from 700 HV to 867 HV, significantly improving the film's hardness.

[0045] The present invention adds aluminum / fluorine-containing metal salts to the magnesium alloy micro-arc oxidation electrolyte, adopts a bidirectional pulsed DC power supply and a step-by-step current control method, so that an alloy film containing aluminum oxide is in situ grown on the surface of the magnesium alloy during the micro-arc oxidation process, thereby changing the composition and microstructure of the magnesium alloy micro-arc oxidation film layer, and at the same time improving the hardness and corrosion resistance of the magnesium alloy micro-arc oxidation film layer. In addition, the electrolyte has the characteristics of simple composition, low cost and green environmental protection, which greatly improves the surface protection ability of the magnesium alloy and extends the service life of the magnesium alloy.

Claims

1. A magnesium alloy micro-arc oxidation electrolyte, characterized in that: The invention comprises sodium metaaluminate, sodium hydroxide, sodium fluoroaluminate, aluminum fluoride and glycerol; the content of sodium metaaluminate is 5-25 g / L, the content of sodium hydroxide is 1-3 g / L, the content of sodium fluoroaluminate is 0.3-0.7 g / L, the content of aluminum fluoride is 0.2-0.3 g / L, and the content of glycerol is 1 g / L.

2. Magnesium alloy micro-arc oxidation electrolyte, characterized in that: The invention comprises potassium metaaluminate, potassium hydroxide, potassium fluoroaluminate, aluminum fluoride and glycerol; the content of potassium metaaluminate is 10-20 g / L, the content of potassium hydroxide is 3-5 g / L, the content of potassium fluoroaluminate is 0.5-0.7 g / L, the content of aluminum fluoride is 0.3-0.5 g / L, and the content of glycerol is 1 g / L.

3. A method for preparing a magnesium alloy hard ceramic film, using the electrolyte according to any one of claims 1 or 2, characterized in that: The specific method is as follows: S1. Grinding and polishing the surface of the magnesium alloy, and then cleaning and drying it to obtain a magnesium alloy sample; S2. Prepare an electrolyte: Weigh various components according to the proportions of claim 1 or 2 to prepare an electrolyte, and test the pH value and conductivity of the solution to obtain an electrolyte; S3. Using the magnesium alloy sample as the anode and the stainless steel as the cathode, micro-arc oxidation treatment is performed in an electrolyte by increasing the current in stages, and water circulation cooling is performed to obtain a micro-arc oxidation sample; S4. Clean and dry the micro-arc oxidation sample to obtain a magnesium alloy hard ceramic film.

4. The method for preparing a magnesium alloy hard ceramic film according to claim 3, characterized in that: The specific method of S2 is as follows: Add the weighed aluminum fluoride and sodium fluoroaluminate respectively to deionized water, stir to dissolve, then add sodium aluminate, sodium hydroxide, and glycerol in sequence. After the previous component is fully dissolved, add the next component. During the entire process of preparing the solution, introduce compressed air to circulate and stir the solution for 50 to 60 minutes until the solution is clear and transparent.

5. The method for preparing a magnesium alloy hard ceramic film according to claim 4, characterized in that: The pH value of the electrolyte in S2 is 12.5-13, and the conductivity of the solution is 25-30 mS / cm.

6. The method for preparing a magnesium alloy hard ceramic film according to claim 3, characterized in that: The process of controlling the electrical parameters of the bidirectional pulsed DC power supply in S3 is as follows: During the micro-arc oxidation process of magnesium alloy, a bidirectional pulsed DC power supply and a constant current transformer control method are used. The power supply current is 1~5A: voltage: 450~550V, the pulse frequency is 500~1000HZ, the duty cycle is 10%~40%, the positive and negative pulse ratio is 1:0, 2:1 or 4:1, the micro-arc oxidation time is 1h~2h, and the electrolyte temperature is controlled below 50℃.

7. The method for preparing a magnesium alloy hard ceramic film according to claim 6, characterized in that: In the electrical parameter control process of magnesium alloy micro-arc oxidation in S3, an initial current of 1~2A is given until arcing occurs on the surface of the magnesium alloy sample. Subsequently, the voltage value is gradually increased at a rate of 1V / min. When the power supply voltage remains unchanged within 30 minutes, the power supply current is increased by increasing the current by 0.5A each time through the power supply control panel until the voltage value reaches the set value.

8. The magnesium alloy hard ceramic film prepared according to the preparation method of claim 3, characterized in that: The micro-arc oxidation ceramic film is off-white in color, has a thickness of 15 to 50 μm, has a hardness higher than 700 HV, and has a neutral salt spray resistance time greater than 400 h.

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