Preparation method of compact micro-arc oxidation film layer on magnesium surface

Abstract

The invention relates to the field of metal surface protection treatment, and discloses a preparation method of a magnesium surface compact micro-arc oxidation film layer, which comprises the following steps: (1) pretreating a magnesium alloy matrix; (2) carrying out micro-arc oxidation treatment on the surface of the pretreated matrix, wherein a micro-arc oxidation electrolyte contains silicate, fluoride and hydroxide; (3) the sample subjected to micro-arc oxidation treatment is placed in an alkaline solution for hydrothermal hole sealing treatment, the treatment temperature ranges from 100 DEG C to 120 DEG C, and the time ranges from 12 h to 24 h; the nano-scale magnesium hydroxide generated by the hydrothermal reaction effectively seals the pores of the micro-arc oxidation layer to form a double-sealing mechanism of physical filling and chemical bonding; according to the method, through the synergistic effect of micro-arc oxidation and a hydrothermal hole sealing process, a compact composite protective layer is successfully constructed on the surface of the magnesium alloy, the synergistic protection effect provides an innovative solution for magnesium alloy surface treatment, and the method has both process controllability and environmental friendliness.

Description

Technical Field

[0001] The invention relates to the technical field of metal surface protection treatment, in particular to a method for preparing a dense micro-arc oxidation film layer on a magnesium surface. Background Art

[0002] Magnesium and its alloys are widely used in aerospace, lightweight automotive applications, consumer electronics (e.g., mobile phones / laptops), and biomedical applications due to their low density, high specific strength, excellent thermal conductivity, and electromagnetic shielding properties. However, due to its extremely high chemical activity (standard electrode potential of -2.37 V), magnesium is susceptible to electrochemical corrosion, leading to a rapid decline in mechanical properties and severely limiting its practical application. Surface treatment of magnesium alloys is an effective method for improving corrosion resistance. Micro-arc oxidation (MAO) can produce a ceramic layer with excellent wear resistance, corrosion resistance, and insulation properties on the surface of magnesium alloys. This technology has been widely used in the surface protection of light metals. However, the transient high temperature and rapid cooling of the plasma discharge during MAO lead to stress concentration within the coating. This results in micropores and cracks in MAO coatings, resulting in insufficient long-term corrosion resistance. Pitting corrosion is particularly prevalent in aggressive environments such as those containing chloride ions. Therefore, sealing MAO coatings is an effective method to further improve their corrosion resistance.

[0003] There are many sealing methods, including constructing a hydrophobic LDH film on the micro-arc oxidation coating and densifying the film with a sealant, which can effectively improve the corrosion resistance of the micro-arc oxidation film on magnesium alloys. This technology uses a one-step alkaline hydrothermal method to seal the micro-arc oxidation holes on the surface of the magnesium alloy, producing a dense micro-arc oxidation film. This effectively reduces the access of the corrosive medium in the micro-arc oxidation layer to the magnesium substrate. It also provides an excellent foundation for the further construction of composite coatings, effectively improving the corrosion resistance of magnesium alloys in harsh environments, extending the service life of the material, and reducing maintenance costs. Summary of the Invention

[0004] In response to the shortcomings of the existing technology, the present invention provides a method for preparing a dense micro-arc oxidation film on the surface of magnesium. A simple one-step alkaline hydrothermal technology is used to seal the pores on the surface of the magnesium alloy micro-arc oxidation ceramic layer to prepare a dense micro-arc oxidation film. This solves the problem that the micro-arc oxidation coating of the magnesium alloy fails and the corrosion resistance is reduced due to the corrosive medium reaching the surface of the magnesium substrate through the surface pores and cracks.

[0005] To achieve the above objectives, the present invention is implemented through the following technical solutions: A method for preparing a dense micro-arc oxidation film on a magnesium surface, comprising the following steps:

[0006] (1) Pretreatment of the magnesium alloy substrate;

[0007] (2) performing micro-arc oxidation treatment on the pretreated substrate surface, the micro-arc oxidation electrolyte comprising silicate, fluoride and hydroxide;

[0008] (3) placing the micro-arc oxidation treated sample in an alkaline solution for hydrothermal sealing treatment, the treatment temperature being 100-120℃ and the treatment time being 12-24h.

[0009] Preferably, the pretreatment in step (1) comprises:

[0010] The magnesium alloy is cut into a sample, which is polished with 400#, 600#, 800#, 1500# and 2000# sandpaper in sequence, cleaned with deionized water and acetone and dried.

[0011] Preferably, the micro-arc oxidation electrolyte in step (b) comprises:

[0012] sodium silicate 10-20g / L, potassium fluoride 5-10g / L and sodium hydroxide 8-12g / L.

[0013] Preferably, the micro-arc oxidation treatment parameters in step (b) comprise:

[0014] pulse frequency 500-1000Hz, end voltage 300-400V, duty cycle 2-5% and treatment time 8-15min.

[0015] Preferably, the micro-arc oxidation is performed in constant voltage mode, with the peak current controlled in the range of 4-6A.

[0016] Preferably, the alkaline solution in step (c) is a sodium hydroxide solution with a concentration of 0.8-1.2mol / L.

[0017] Preferably, the hydrothermal sealing treatment is performed in a polytetrafluoroethylene-lined high-pressure reaction kettle, with the solution filling amount being 60-80% of the volume of the reaction kettle.

[0018] Preferably, step (c) further comprises:

[0019] the sample is sequentially cleaned with deionized water and anhydrous ethanol under ultrasonic wave and dried in vacuum at 60-80℃ for 2-4h.

[0020] Preferably, the micro-arc oxidation treatment in step (b) comprises:

[0021] the sample is hung in the electrolytic cell with the magnesium alloy sample as the anode and stainless steel as the cathode.

[0022] Preferably, the magnesium alloy substrate is selected from AZ31, AZ91 or AM60 magnesium alloy.

[0023] The application provides a preparation method of a dense micro-arc oxidation film layer on a magnesium surface.

[0024] 1、The application successfully constructs a dense composite protective layer on the surface of the magnesium alloy through the synergistic effect of micro-arc oxidation and hydrothermal sealing process.

[0025] 2、The application adopts a simple one-step alkaline hydrothermal technology to seal the holes on the surface of the micro-arc oxidation ceramic layer of the magnesium alloy, and prepares a dense micro-arc oxidation film layer, thereby solving the problem of the failure of the micro-arc oxidation ceramic layer and the decline of the corrosion resistance of the magnesium alloy micro-arc oxidation coating due to the fact that the corrosion medium reaches the surface of the magnesium substrate through the surface holes and cracks. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1 It is a method flowchart of the application;

[0027] Figure 2 It is an SEM diagram of the micro-arc oxidation film layer before and after the hydrothermal treatment of Example 1 of the application, wherein (a) is before the hydrothermal treatment, and (b) is after the hydrothermal treatment;

[0028] Figure 3 It is an XRD diagram of the micro-arc oxidation film layer before and after the hydrothermal treatment of Example 1 of the application. DETAILED DESCRIPTION

[0029] The technical solutions in the embodiments of the application will be clearly and completely described below with reference to the accompanying drawings of the application. Obviously, the described embodiments are only part of the embodiments of the application, rather than all the embodiments of the application. Based on the embodiments in the application, all the other embodiments obtained by those skilled in the art without creative work fall within the protection scope of the application.

[0030] Please refer to the accompanying drawings of the application Figure 1 - the accompanying drawings of the application Figure 3 The application provides a preparation method of a dense micro-arc oxidation film layer on a magnesium surface, and a composite protective layer with a gradient structure is constructed on the surface of the magnesium alloy through the synergistic effect of micro-arc oxidation and hydrothermal sealing. The method comprises the following steps:

[0031] S1, pretreating the magnesium alloy substrate;

[0032] S2, performing micro-arc oxidation treatment on the pretreated substrate surface;

[0033] S3, placing the micro-arc oxidation treated sample in an alkaline solution for hydrothermal sealing treatment.

[0034] In step S1, the interface of the magnesium alloy substrate is optimized through a multi-stage surface treatment process.

[0035] The magnesium alloy substrate of the selected grade is cut into a rectangular sample, preferably controlling the length direction size of the sample to be 40-60 mm, the width direction to be 15-25 mm, and the thickness direction to be 3-7 mm. A through hole with a diameter of 1-3 mm is processed in the middle of the upper end of the sample for subsequent process suspension and electrical connection. This structure design can avoid the problem of uneven current density caused by edge effect.

[0036] The surface of the cut sample is subjected to gradient polishing treatment, and 400# to 2000# mesh sandpaper is used for step-by-step fine polishing. During the polishing process, the polishing direction of adjacent mesh sandpaper should be orthogonal, which can effectively eliminate the one-way grinding marks generated in the previous process. When the surface roughness is reduced to Ra≤0.2μm, visual inspection shows no visible macroscopic scratches, and at this time the surface active site distribution tends to be uniform, laying a foundation for the uniform distribution of discharge channels in the subsequent micro-arc oxidation process.

[0037] After mechanical polishing, the sample is transferred to an ultrasonic cleaning device for multi-stage cleaning. First, deionized water is used for ultrasonic cleaning for 10-15 min, with a frequency controlled in the range of 40-60 kHz, to remove surface residual grinding particles. Subsequently, acetone solvent is used for secondary ultrasonic cleaning for 5-8 min, which can effectively dissolve and remove oil contaminants attached to the substrate surface during processing. The cleaned sample is immediately placed in a 50-80℃ air drying oven for 20-30 min, with the drying air flow speed controlled at 2-4 m / s, to ensure rapid surface dehydration while avoiding oxidation.

[0038] The pretreatment stage realizes the dual goals of surface energy regulation and contamination layer removal through the synergistic effect of physical polishing and chemical cleaning.

[0039] In step S2, a porous ceramic layer is constructed on the pretreated magnesium alloy surface through plasma discharge reaction under the excitation of a pulse power supply.

[0040] The pretreated sample is suspended as an anode in an electrolytic cell, forming a reaction system with a stainless steel cathode, and the inter-electrode distance is preferably controlled in the range of 50-100 mm. This distance design can balance the uniformity of electric field distribution and the ohmic pressure drop of electrolyte.

[0041] The electrolyte system adopts a silicate-fluoride-hydroxide composite formula, wherein the addition amount of sodium silicate is preferably 10-20g / L, and the SiO3 produced by its hydrolysis is 2- During the discharge process, the ions react with the magnesium element in the matrix to form Mg2SiO4 ceramic phase, which constitutes the skeleton structure of the coating. The concentration of potassium fluoride is controlled in the range of 5-10g / L, and the F - Ions reduce the discharge breakdown voltage through complexation and promote uniform distribution of discharge channels. The amount of sodium hydroxide added is preferably 8-12 g / L to maintain the pH value of the electrolyte in the range of 12.0-13.5. This alkaline environment can inhibit the interference of the hydrogen evolution side reaction on the film formation process.

[0042] The power supply system utilizes a bipolar pulse mode with a set frequency of 500-1000Hz. This frequency range ensures a sufficient number of discharges per unit time to form a continuous ceramic layer while avoiding energy loss caused by high frequency. The termination voltage is controlled in the range of 300-400V, a threshold that both triggers effective micro-arc discharges and prevents coating ablation caused by excessive voltage. The duty cycle parameter is preferably 2-5%. By adjusting the ratio of pulse on-time to off-time, the instantaneous energy input of the plasma discharge is controlled, thereby adjusting the balance between coating growth rate and thermal stress accumulation.

[0043] The treatment time is set to 8-15 minutes, which covers the three stages of the micro-arc oxidation process:

[0044] The initial anodic oxide layer formation period, the stable discharge growth period and the coating densification period.

[0045] In constant voltage mode, the peak current is dynamically stabilized in the range of 4-6A. This current characteristic reflects the dynamic equilibrium state between discharge intensity and coating impedance.

[0046] During the treatment process, the electrolyte temperature is maintained at 25-35°C through a circulating cooling system to avoid decomposition of solution components due to excessive temperature rise.

[0047] After the micro-arc oxidation treatment, the sample is removed from the electrolytic cell and rinsed with deionized water in multiple stages to remove any electrolyte residue adhering to the surface. The rinsed sample is then transferred to a drying apparatus and treated at 50-80°C for 20-40 minutes to ensure sufficient drying of the coating's internal pore structure. The porous ceramic layer formed during this stage features a three-dimensional, through-hole structure with a pore size distribution ranging from 0.5 to 5 μm, providing a material transport pathway and reactive sites for the subsequent hydrothermal sealing reaction.

[0048] In step S3, the pore structure of the micro-arc oxidation layer is in-situ sealed by a high-temperature and high-pressure hydrothermal reaction.

[0049] The MAO treated sample is vertically placed in a corrosion resistant reaction vessel, preferably a polytetrafluoroethylene lined high pressure reactor with volume adapted to the sample size, typically 30-100 mL size is chosen. A pre-configured alkaline solution is injected into the reactor volume to 60-80% of the reactor volume, this filling volume is designed to reserve appropriate gas phase space to buffer the thermal expansion pressure while ensuring sufficient solution to wet the sample surface.

[0050] The alkaline solution is preferably a sodium hydroxide aqueous solution with concentration of 0.8-1.2 mol / L, this concentration range synergistically works through the following mechanisms: the moderate alkaline environment promotes the controlled corrosion reaction of the magnesium substrate, the released Mg2+ ions 2+ ions combine with OH - ions in the solution to form the precursor; meanwhile the solution ionic strength is maintained above the critical nucleation threshold to induce the heterogeneous nucleation of nanoparticles preferentially inside the pores. After sealing the reaction vessel, it is transferred to a programmed temperature heating device to raise the system temperature to the range of 100-120 °C at a heating rate of 3-5 °C / min, this heating rate can balance the reaction kinetics requirement and the thermal inertia effect of the equipment.

[0051] The isothermal reaction stage is maintained at the set temperature for 12-24 h, this time range covers the complete crystallization process of nanoparticle nucleation, growth and ripening. The high temperature and high pressure environment (autogenous pressure about 0.1-0.3 MPa) significantly increases the surface activity of the magnesium alloy, leading to selective corrosion reaction of the substrate (Mg + 2H2O → Mg(OH)2+ H2↑), the generated Mg(OH)2nanocrystalline grains are deposited along the inner wall of the micro-arc oxidation layer pores. The three-dimensional confinement effect of the pore structure guides the nanoparticles to form an interlocking filling mode, the size is preferably controlled in the range of 50-200 nm, realizing the multi-level closure of the micron-sized channels.

[0052] After the reaction is completed, the system temperature is reduced to below 50 °C using the furnace cooling method, the cooling rate is controlled at ≤2 °C / min, this slow cooling process can reduce the risk of coating cracking caused by thermal stress. After the sample is taken out, it is sequentially subjected to multi-stage cleaning: first, ultrasonic treatment with deionized water for 3-5 min, the frequency is preferably 40-60 kHz, to remove loose surface attachments; then use anhydrous ethanol for 2-4 min with ultrasonic assistance, use the low surface tension characteristics of organic solvents to replace the residual water inside the pores.

[0053] The cleaned sample is transferred to a vacuum drying device, and is treated at 60-80℃ for 2-4h, with the vacuum degree being maintained at -0.08 to -0.1 MPa. The drying condition realizes the mild removal of trace moisture in the coating by reducing the vaporization temperature of water molecules, and avoids the nanoparticle agglomeration phenomenon caused by traditional high-temperature drying. The finally obtained composite film layer presents gradient structure characteristics: the surface layer is a nanocrystalline dense layer generated by hydrothermal treatment, the middle transition zone is a micro-arc oxidation layer filled with Mg(OH)2, and the bottom layer is a ceramic layer metallurgically combined with the substrate, and a continuous protection system is formed between the layers through chemical bonding.

[0054] In order to better understand the present application, the above method is described in detail below in combination with specific examples.

[0055] Example 1:

[0056] The embodiment of the present application provides a preparation method of a dense micro-arc oxidation film layer on a magnesium surface, comprising the following steps:

[0057] 1. Substrate preparation

[0058] An AZ31 magnesium alloy plate is selected and processed into a 50mm×20mm×5mm rectangular sample by wire cutting.

[0059] A 2mm diameter through hole is drilled at the center of the upper end of the sample for suspension and conduction.

[0060] A step-by-step sanding process is adopted: 400#, 600#, 800#, 1500# and 2000# silicon carbide sandpaper is used in sequence to sand along the orthogonal direction, and each sanding time is 3min, and the final surface roughness Ra is 0.08μm.

[0061] The sanded sample is placed in a 40kHz ultrasonic cleaning machine, first cleaned with deionized water for 10min to remove swarf, then cleaned with acetone for 5min for degreasing, and finally dried in a 60℃ air drying oven for 30min.

[0062] 2. Micro-arc oxidation film formation

[0063] A 3L electrolyte is configured: 45g of sodium silicate nonahydrate (Na2SiO3·9H2O), 24g of potassium fluoride (KF) and 30g of sodium hydroxide (NaOH) are accurately weighed and dissolved in deionized water and stirred until completely dissolved.

[0064] The pretreated sample is suspended as an anode in the center of the electrolytic tank, and an annular stainless steel cathode (inner diameter 120mm) is kept at an interelectrode distance of 80mm.

[0065] A bipolar pulse power supply is turned on, and the output parameters are set as frequency 700Hz, constant voltage mode 350V, duty cycle 3%, and treatment time 10min.

[0066] During the process, the peak current was monitored to be stable in the range of 5.0±0.2A, and the electrolyte temperature was controlled at 28±2℃ through the external circulation cooling system.

[0067] After the treatment, the sample was taken out, the surface was rinsed with deionized water until neutral, and dried at 60°C for later use.

[0068] 3. Hydrothermal sealing treatment

[0069] Measure 35 ml of 0.9 mol / L sodium hydroxide solution (prepared by dissolving 3.6 g of NaOH in 100 ml of deionized water) and inject it into a 50 ml polytetrafluoroethylene reactor.

[0070] The micro-arc oxidation sample was fixed vertically on the reaction rack to ensure that it was completely immersed in the solution. The sealed reactor was placed in a muffle furnace and heated to 110°C at a rate of 4°C / min. After holding at this temperature for 18 hours, it was cooled to below 40°C in the furnace.

[0071] After the sample was taken out, the following steps were performed: ultrasonic cleaning with deionized water (40 kHz, 5 min) - dehydration with anhydrous ethanol (3 min) - vacuum drying at 80°C (vacuum degree -0.09 MPa, time 3 h).

[0072] Example 2:

[0073] The present invention provides a method for preparing a dense micro-arc oxidation film on a magnesium surface, comprising the following steps:

[0074] 1. Matrix Preparation

[0075] AZ91 magnesium alloy ingots were selected and processed by wire cutting into 60mm×25mm×7mm specimens.

[0076] The surface pretreatment includes: 2000# sandpaper end grinding to Ra=0.12 μm, deionized water / acetone graded cleaning (time is the same as in Example 1), and hot air drying at 70° C. for 25 min.

[0077] 2. Micro-arc oxidation film formation

[0078] Prepare the electrolyte: dissolve 60 g of sodium silicate nonahydrate, 30 g of potassium fluoride and 36 g of sodium hydroxide in 3 L of deionized water and stir magnetically for 30 min.

[0079] The inter-electrode spacing was 100 mm, and the power supply parameters were set to a frequency of 1000 Hz, a constant voltage of 400 V, a duty cycle of 5%, and a processing time of 15 min.

[0080] The peak current was monitored in real time at 6.0A, and the electrolyte temperature was controlled at 32±1℃ by increasing the cooling water flow.

[0081] 3. Hydrothermal sealing treatment

[0082] Into a 100ml reactor, 80ml 1.2mol / L NaOH solution (prepared from 9.6g NaOH) was injected, and the sample was placed horizontally to ensure maximum contact area.

[0083] The muffle furnace program was set as follows: 5℃ / min to 120℃, constant temperature for 24h, and then 1℃ / min to room temperature.

[0084] The post-treatment used an enhanced cleaning program: ultrasonic extension to 8min in deionized water, and vacuum drying temperature increased to 85℃.

[0085] Example 3:

[0086] The embodiment of the present application provides a preparation method of a dense micro-arc oxidation film layer on a magnesium surface, comprising the following steps:

[0087] 1. Preparation of the substrate

[0088] The AM60 magnesium alloy was stamped into a 40mm×15mm×3mm thin sheet sample.

[0089] The polishing process was simplified to a single-stage fine grinding with 2000# sandpaper (Ra=0.15μm), and the cleaning and drying conditions were the same as in Example 1.

[0090] 2. Micro-arc oxidation film formation

[0091] The amount of electrolyte was halved: 15g of sodium silicate nonahydrate, 7.5g of potassium fluoride and 12g of sodium hydroxide were dissolved in 1.5L of deionized water.

[0092] The inter-electrode distance was shortened to 50mm, the low-frequency parameters (500Hz, 300V, duty cycle 2%) were set, and the treatment time was compressed to 8min.

[0093] The peak current was maintained at 4.0A during the process, and the electrolyte temperature was maintained at 25℃ by natural heat dissipation.

[0094] 3. Hydrothermal sealing treatment

[0095] A 30ml micro-reactor was used, and 18ml of 0.8mol / L NaOH solution (1.92g of NaOH) was injected.

[0096] A stepwise heating strategy was adopted: first, 2℃ / min to 80℃, then 1℃ / min to 100℃, and then 12h of constant temperature.

[0097] During the cooling stage, the furnace door was opened to accelerate heat dissipation, and the total cooling time was controlled within 8h. The drying conditions were adjusted to 60℃ low vacuum (-0.06MPa) drying for 4h.

[0098] Performance characterization test:

[0099] 1. Surface topography analysis (SEM)

[0100] To study the effect of hydrothermal treatment on the sealing performance of micro-arc oxidation (MAO) coating, the surface of the sample before and after treatment was observed by scanning electron microscope (SEM). As shown in Figure 2 Figure 2 (a) is the micro-arc oxidation ceramic film layer before hydrothermal treatment of Example 1, and a large number of micron-sized holes are distributed on the surface, the hole morphology is not uniform, and the porosity is high. Figure 2 (b) is the coating after hydrothermal treatment of Example 1, and it can be observed that the surface becomes obviously dense and the holes basically disappear, indicating that the hydrothermal treatment effectively seals the pores on the surface of the coating, and the good sealing effect is preliminarily embodied in the morphology.

[0101] 2. Phase composition analysis (XRD)

[0102] Figure 3 The X-ray diffraction (XRD) patterns of the coating before and after hydrothermal treatment of Example 1 are shown. The results show that the main phases of the coating before and after hydrothermal treatment are magnesium oxide (MgO) and magnesium hydroxide [Mg(OH)2], and a small amount of base metal magnesium (Mg) exists. Among them, after hydrothermal treatment, the diffraction peak intensity of Mg and MgO is obviously reduced, while the peak intensity of Mg(OH)2 is enhanced, indicating that part of MgO undergoes hydration reaction under hydrothermal conditions and is converted into Mg(OH)2. This phase change helps to densify the coating and effectively improves the sealing performance.

[0103] 3. Electrochemical performance analysis

[0104] Through polarization curve test, the effect of hydrothermal treatment on the corrosion resistance of the coating was further evaluated. The related polarization parameters are shown in Table 1. In Example 1, compared with before hydrothermal treatment, the corrosion current density of the sample after treatment decreased significantly from 8.32×10 -2 μA / cm 2 to 2.19×10 -3 μA / cm 2 ; the corrosion potential shifted obviously to the positive direction; the polarization resistance increased from 440.49kΩ / cm 2 to 14547.29kΩ / cm 2 . In addition, the protection efficiency also increased from 0.9985 to 0.9999, fully indicating that the hydrothermal treatment significantly enhances the densification and corrosion resistance of the coating.

[0105] Table 1 Polarization calculation data

[0106] Sample Ecorr (V) RP (kQ / cm 2 )]]> icoor (pA / cm 2 )]]> PE (%) Magnesium alloy substrate -1.51 197.56 16.8 / Before hydrothermal treatment -1.47 440.49 8.32 x 10 -2 ]]> 0.9985 After hydrothermal treatment -1.07 14547.29 2.19 x 10 -3 ]]> 0.9999

[0107] ​Microstructure analysis shows that the nanoscale magnesium hydroxide generated by hydrothermal reaction effectively seals the pores of the micro-arc oxidation layer, forming a dual sealing mechanism of physical filling and chemical bonding. Phase evolution confirms that the sealing process promotes the directional conversion of magnesium oxide to magnesium hydroxide, forming a dense structure with preferred orientation. Electrochemical tests show that the composite film layer significantly improves the corrosion resistance, and its synergistic protection effect provides an innovative solution for magnesium alloy surface treatment, with controllable process and environmental friendliness.

[0108] While embodiments of the application have been shown and described, it is to be understood that the embodiments described are merely divergences of the principles and spirit of the application and that numerous modifications, changes, substitutions, and alterations can be made thereto without departing from the spirit and scope of the application as defined by the appended claims and their equivalents.

Claims

1. A method for preparing a dense micro-arc oxidation film on a magnesium surface, characterized in that: The following steps are involved: (1) Pretreatment of the magnesium alloy substrate; (2) performing micro-arc oxidation on the pretreated substrate surface, wherein the micro-arc oxidation electrolyte comprises silicate, fluoride and hydroxide; (3) The sample after micro-arc oxidation treatment is placed in an alkaline solution for hydrothermal sealing treatment at a temperature of 100-120°C for 12-24 hours.

2. The method for preparing a dense micro-arc oxidation film on a magnesium surface according to claim 1, characterized in that: The pretreatment in step (1) includes: The magnesium alloy was cut into specimens, polished with 400#, 600#, 800#, 1500#, and 2000# sandpaper in sequence, cleaned with deionized water and acetone, and then dried.

3. The method for preparing a dense micro-arc oxidation film on a magnesium surface according to claim 1, characterized in that: The micro-arc oxidation electrolyte in step (b) comprises: Sodium silicate 10-20g / L, potassium fluoride 5-10g / L, sodium hydroxide 8-12g / L.

4. The method for preparing a dense micro-arc oxidation film on a magnesium surface according to claim 1, characterized in that: The micro-arc oxidation treatment parameters of step (b) include: The pulse frequency is 500-1000 Hz, the termination voltage is 300-400 V, the duty cycle is 2-5%, and the processing time is 8-15 minutes.

5. The method for preparing a dense micro-arc oxidation film on a magnesium surface according to claim 4, characterized in that: The micro-arc oxidation adopts a constant voltage mode, and the peak current is controlled in the range of 4 to 6A.

6. The method for preparing a dense micro-arc oxidation film on a magnesium surface according to claim 1, characterized in that: The alkaline solution in step (c) is a sodium hydroxide solution with a concentration of 0.8 to 1.2 mol / L.

7. The method for preparing a dense micro-arc oxidation film on a magnesium surface according to claim 1, characterized in that: The hydrothermal sealing treatment is carried out in a polytetrafluoroethylene-lined high-pressure reactor, and the solution filling amount is 60-80% of the reactor volume.

8. The method for preparing a dense micro-arc oxidation film on a magnesium surface according to claim 1, characterized in that: After step (c), the method further comprises: The samples were ultrasonically cleaned with deionized water and anhydrous ethanol in sequence, and vacuum dried at 60-80°C for 2-4 h.

9. The method for preparing a dense micro-arc oxidation film on a magnesium surface according to claim 1, characterized in that: The micro-arc oxidation treatment of step (b) comprises: It is suspended in an electrolytic cell, with a magnesium alloy sample as the anode and stainless steel as the cathode.

10. The method for preparing a dense micro-arc oxidation film on a magnesium surface according to claim 1, characterized in that: The magnesium alloy matrix is ​​selected from AZ31, AZ91 or AM60 magnesium alloys.

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