A low-cost, high-corrosion-resistant micro-arc oxidation film on magnesium alloy and its preparation method

Abstract

This invention relates to the field of metal surface treatment technology, specifically to a low-cost, high-corrosion-resistant micro-arc oxidation film for magnesium alloys and its preparation method. First, the magnesium alloy workpiece is pretreated. Then, an electrolyte for the micro-arc oxidation process is prepared. The pretreated magnesium alloy workpiece is used as the anode, and stainless steel as the cathode, placed in the electrolyte for micro-arc oxidation, ultimately yielding a dense oxide film. This invention utilizes an externally controlled electrical parameter process of "online monitoring and regulation control + composite carrier wave," combined with a low-energy-consumption electrolyte system, to achieve the desired dense oxide film. The main components of the oxide film are magnesium silicate, magnesium phosphate, magnesium oxide, magnesium fluoride, and zirconium oxide, resulting in high film hardness and improved wear resistance. It achieves a neutral salt spray test rating of ≥480h (10μm) at level 9, meeting the performance requirements of automotive parts and suitable for large-scale industrial production, with a comprehensive energy consumption reduction of 50%~60%.

Description

Technical Field

[0001] This invention relates to the field of metal surface treatment technology, and to a low-cost, high-corrosion-resistant micro-arc oxidation film layer for magnesium alloys and its preparation method, specifically to a low-cost, high-corrosion-resistant micro-arc oxidation film layer preparation method suitable for magnesium alloy parts. Background Technology

[0002] With the trend towards lightweighting in the automotive industry, magnesium alloys, due to their high specific strength and low density, are widely used in components such as seat brackets, steering wheel frames, and chassis parts. However, magnesium alloys are chemically reactive and have poor corrosion resistance, requiring surface treatment to improve their performance. Micro-arc oxidation is currently one of the mainstream surface treatment technologies for magnesium alloys. By forming an oxide film layer containing crystalline components on the metal surface, it can significantly improve the material's hardness and wear resistance.

[0003] Traditional micro-arc oxidation processes employ a single pulse power supply, resulting in low energy conversion efficiency. Maintaining a high film-forming voltage of 350-450V and a large current density leads to persistently high energy consumption. Simultaneously, conventional electrolytes are mostly based on a single film-forming agent, lacking highly efficient active components, resulting in slow film formation rates and processing times as long as 30-60 minutes (thickness 5-20μm), leading to low production efficiency. Furthermore, high energy consumption and long processing times directly increase production costs, and traditional power supply equipment often suffers from idle energy consumption, further exacerbating energy waste and making it difficult to meet the demands of large-scale magnesium alloy production. Therefore, developing a low-energy, low-cost, and high-efficiency micro-arc oxidation process is crucial for promoting the large-scale application of magnesium alloys in the automotive industry. Summary of the Invention

[0004] This invention aims to provide a low-cost, high-corrosion-resistant micro-arc oxidation film layer for magnesium alloys and its preparation method, solving the problems of high energy consumption, high cost, and low efficiency of traditional processes. At the same time, it ensures that the magnesium alloy micro-arc oxidation film layer has excellent corrosion resistance and high hardness, ensuring the engineering application of magnesium alloys in the automotive field.

[0005] This process achieves a performance breakthrough through synergistic innovation in micro-arc oxidation power supply external control technology and electrolyte optimization. The specific technical solution is as follows:

[0006] A method for preparing a low-cost, highly corrosion-resistant micro-arc oxidation film on magnesium alloy includes the following steps:

[0007] (1) Pretreatment of magnesium alloy workpieces;

[0008] (2) Prepare the electrolyte used in the micro-arc oxidation process. The electrolyte composition is: sodium silicate 60~80g / L, sodium phosphate 10~20g / L, potassium fluoride 25~35g / L, sodium fluoride 5~10g / L, glycerol 5~10ml / L, acrylic emulsion 5~20g / L, sodium fluorozirconate 10~20 g / L, sodium hydroxide 1~5g / L, and the remainder is water;

[0009] (3) Take the pretreated magnesium alloy workpiece as the anode and stainless steel as the cathode, place them in the above electrolyte, and carry out micro-arc oxidation process to finally obtain a dense oxide film layer.

[0010] Further, the pretreatment described in step (1) is as follows: the surface of the magnesium alloy workpiece is sequentially degreased, derusted, activated and dried.

[0011] Further, the alkaline degreasing agent for degreasing consists of: sodium hydroxide 50-60 g / L, sodium carbonate 10-16 g / L, sodium phosphate 10-15 g / L, and sodium dodecyl sulfate 0.5-1 g / L, ultrasonically cleaned at 60-80℃ for 10-30 min; the rust removal solution for derusting consists of: 98 wt.% concentrated sulfuric acid 50 ml / L and 65 wt.% nitric acid 20 ml / L, soaked at room temperature for 10-15 s; the activation solution for activation consists of: 40 wt.% hydrofluoric acid 10 ml / L, soaked at room temperature for 2-3 min; the drying conditions are drying at 80-100℃ for 20-30 min; and water washing is performed after the degreasing, derusting, and activation steps.

[0012] Furthermore, the electrolyte described in step (3) is maintained at 30-40°C during the micro-arc oxidation process.

[0013] Furthermore, step (3) specifically involves:

[0014] A composite carrier voltage, consisting of a DC fundamental frequency and a pulse carrier frequency, is applied to the anode and cathode. The ratio of the DC voltage to the peak value of the pulse voltage is set to 1:1 to 1:3, and the average current density is 0.3 to 0.7 A / dm². 2 Constant current boost for 5-20 minutes; during constant current boost, the initial pulse frequency is 5000-6000Hz and the duty cycle is 45-50%;

[0015] During the constant current boost process, the composite carrier voltage is turned off every 2-10 seconds, while a 500-600ms triangular wave is applied to the anode and cathode. The impedance is analyzed and Rp=ΔE / (Δ i / S), Rp: discharge resistance, ΔE: transient transition voltage, Δi: transient transition current, S: oxide film surface area;

[0016] When Rp is greater than the threshold, the composite carrier frequency and duty cycle are dynamically adjusted to adapt to the film state and continue the micro-arc oxidation process. Specifically, the composite carrier frequency and duty cycle are reduced simultaneously or only one of them is reduced.

[0017] Furthermore, the composite carrier frequency in the adapted film layer state is 500~2000Hz, the voltage is 200~300V, the duty cycle is 10-30%, and the oxidation time is 5~15min.

[0018] Furthermore, the magnesium alloy is one of AZ, AM, and cast magnesium alloy.

[0019] An oxide film prepared according to the method of the present invention comprises magnesium silicate, magnesium phosphate, magnesium fluoride, magnesium oxide, and zirconium oxide; the film thickness is 5~20 μm; the corrosion resistance meets the requirements of ≥480h neutral salt spray test and ≥350Hv Vickers hardness. 0.05 .

[0020] The technological principle of this invention is as follows:

[0021] (1) Electrical parameter control process level: This invention monitors the discharge resistance of the micro-arc oxidation system online by applying a 500-600ms triangular wave (consistent with the peak value and frequency of the composite carrier power supply voltage) every 2-10s, and dynamically adjusts the composite carrier frequency and duty cycle to adapt to the film state. The method of this invention can avoid strong spark eruption, reduce ineffective energy loss, and evenly distribute energy to avoid local concentration; intelligent frequency conversion accurately supplies energy by regulating the pulse high-level time, reducing waste, at 0.3~0.7A / dm 2 Efficient film formation can be achieved with an average current density.

[0022] (2) Electrolyte level: Fluorides, as passivating agents for magnesium, can react with Mg 2+ The formation of stable precipitates lowers the activation energy of the oxidation reaction, directly reducing the film-forming voltage. Glycerol combines dispersibility and stability, inhibiting bubble aggregation in the electrolyte, avoiding current waste caused by "arc interruption" in micro-arc discharge, increasing current efficiency by more than 20%, accelerating oxide film growth, and shortening processing time. Simultaneously, its hydroxyl groups can combine with ions in the electrolyte, optimizing the oxide film growth rate, shortening processing time by 50%, and indirectly reducing energy consumption. The concentration should be controlled at 5-10 ml / L to balance stability and film-forming efficiency. Sodium phosphate has a complexation thickening mechanism that can affect the discharge characteristics of micro-arc oxidation, helping to reduce energy consumption. Sodium silicate ionizes into SiO3 in aqueous solution. 2- and Na +Ions enhance the conductivity of the electrolyte. When a high voltage is applied, these ions participate in the electrode reaction, forming an insulating oxide film on the magnesium alloy surface. When the film thickness reaches a certain level and the local electric field strength exceeds a critical value, electrical breakdown occurs, generating micro-arc discharge. The high temperature and high pressure environment of the micro-arc is crucial for the subsequent formation of a dense ceramic film, modulating the film layer and thus affecting the discharge characteristics of micro-arc oxidation, thereby reducing energy consumption. Sodium fluorozirconate ionizes into ZrF6 in the electrolyte. 2- Under the high temperature and high pressure environment of micro-arc discharge, zirconium ions participate in film formation, generating ceramic phases such as ZrO2, which possess extremely high hardness and good chemical stability. This reduces the energy density of the micro-arc discharge and improves film density. Furthermore, sodium fluorozirconate reacts with magnesium ions in alkaline electrolytes to form insoluble fluoride or zirconate passivation films, covering the magnesium alloy surface and thus inhibiting magnesium substrate corrosion and stabilizing the oxidation process. Sodium hydroxide can reduce energy consumption per unit area, improve film quality, and control the average current density during micro-arc oxidation to 1 A / dm². 2 the following.

[0023] This process can be widely applied to the surface treatment of magnesium alloy components such as automotive seat brackets, steering wheel frames, and chassis parts. It not only meets the material performance requirements of automotive lightweighting but also adapts to the needs of large-scale mass production due to its low energy consumption and low cost advantages. As the automotive industry continues to increase its requirements for energy conservation, emission reduction, and cost control, this process is expected to become one of the mainstream technologies for magnesium alloy surface treatment, promoting the large-scale application of magnesium alloys in the automotive field.

[0024] Advantages and benefits of the present invention

[0025] 1. Low Energy Consumption: This invention utilizes an external electrical parameter control process combining "online monitoring and regulation control + composite carrier wave" to suppress the formation of large sparks during micro-arc oxidation, resulting in ultra-fine sparks and reducing energy waste. Combined with a low-energy electrolyte system, it synergistically reduces the film-forming voltage and the required current density, ultimately yielding a dense oxide film. Overall energy consumption is reduced by 50%~60%.

[0026] 2. Low cost: Low energy consumption directly reduces equipment operating costs; glycerol improves electrolyte stability and reduces raw material loss; processing time is shortened by more than 50%, reducing labor costs; and overall costs are reduced by 30-50%.

[0027] 3. High performance: The main components of the oxide film are magnesium silicate, magnesium phosphate, magnesium oxide, magnesium fluoride and zirconium oxide, which makes the film layer hard and improves the wear resistance of the film layer. The neutral salt spray test is ≥480h (10μm) level 9, which meets the performance requirements of automotive parts and is suitable for large-scale industrial production. Attached Figure Description

[0028] Figure 1This is a schematic diagram of the micro-arc oxidation system in this invention. Detailed Implementation

[0029] Example 1

[0030] 1. Pretreatment: Take an AZ91D magnesium alloy seat bracket workpiece with a surface area of ​​0.5m². 2 Place the sample in an alkaline degreasing agent (50 g / L sodium hydroxide, 10 g / L sodium carbonate, 10 g / L sodium phosphate, 0.5 g / L sodium dodecyl sulfate) and ultrasonically clean it at 60°C for 10 min. After rinsing with water, immerse it in a rust removal solution (50 ml / L concentrated sulfuric acid and 20 ml / L 65 wt.% nitric acid) at room temperature for 10 s. After rinsing with water, immerse it in an activation solution (10 ml / L hydrofluoric acid) at room temperature for 2 min. After removing it, rinse it three times with deionized water and dry it in a 100°C oven for 20 min.

[0031] 2. Micro-arc oxidation treatment:

[0032] (1) Prepare the electrolyte by using sodium silicate 60g / L and sodium phosphate 10g / L as the base system, then add potassium fluoride 25g / L, sodium fluoride 5g / L, glycerol 5ml / L, then add a mixed solution of acrylic emulsion (E0503 Shenzhen Yoshida Chemical Co., Ltd.) 5g / L and sodium fluorozirconate 10g / L, stir evenly, then add sodium hydroxide 1g / L, and the rest is deionized water.

[0033] (2) Micro-arc oxidation process: The micro-arc oxidation system in this embodiment is as follows: Figure 1 As shown, it mainly consists of a working circuit and a detection circuit. An electrolyte with the above-mentioned components is placed in an oxidation tank. The workpiece serves as the anode, and 316L stainless steel serves as the cathode, with a spacing of 5 cm. A series-connected pulse power supply and a DC power supply constitute the composite carrier power supply of this embodiment. The positive and negative terminals of the composite carrier power supply are connected to the anode and cathode, respectively, to form the working circuit. A triangular wave detection signal source (a power supply that can output a triangular waveform voltage) is connected in parallel with the composite carrier power supply. The positive and negative terminals of the triangular wave detection signal source are connected to the anode and cathode, respectively, to form the detection circuit.

[0034] The electrolyte temperature was maintained at 30℃ during the micro-arc oxidation process.

[0035] The specific process is as follows: Set the electrical parameters of the composite carrier power supply in the working circuit, with the peak value ratio of the DC voltage to the carrier (pulse) voltage set to 1:1, and the average current density to be 0.3A / dm³. 2The initial constant current voltage boost lasted for 5 minutes. During the constant current voltage boost process, the initial carrier (pulse) frequency was set to 5000Hz and the duty cycle to 50%. Then, the discharge resistance of the micro-arc oxidation system was monitored online. The voltage output of the composite carrier power supply was turned off every 10 seconds, and a 500ms triangular wave was applied to the workpiece. The peak value and frequency of this triangular wave were consistent with the peak value and frequency of the composite carrier power supply before the turn-off. The impedance was analyzed and Rp=ΔE / (Δ i / S), Rp is the discharge resistance, ΔE: transient transition voltage (instantaneous change in voltage), Δi: transient transition current (instantaneous change in current), S: oxide film surface area; when Rp is greater than the set threshold of 6Ωm 2 Simultaneously, the frequency and duty cycle of the composite carrier power supply are reduced to adapt to the film state and the micro-arc oxidation process continues until Rp does not exceed the threshold. Specifically, in this embodiment, the dynamic adjustment is as follows: the frequency gradually changes from 5000Hz to 2000Hz (each adjustment reduces the frequency by 100Hz based on the previous shutdown of the composite carrier power supply), the duty cycle decreases from 50% to 30% (each adjustment reduces the duty cycle by 5% based on the previous shutdown of the composite carrier power supply), and finally the cutoff voltage is 200V. The frequency is maintained at 2000Hz, the duty cycle at 30%, and the voltage at 200V, and oxidation continues for 5 minutes, for a total oxidation time of 10 minutes. A highly dense micro-arc oxidation film is formed.

[0036] 3. Post-treatment: Rinse three times with deionized water, dry in a 100℃ oven for 20 minutes to obtain a workpiece with a micro-arc oxide film thickness of 5μm. XRD analysis showed that the main components of the film were magnesium silicate, magnesium phosphate, magnesium fluoride, magnesium oxide, and zirconium oxide. A neutral salt spray test (level 9) was conducted for 240 hours according to GB / T10125-2012, and the Vickers hardness was tested to 300 Hv according to GB / T 4340.1-2021. 0.05 Energy consumption per unit area: 0.4 kWh / (μm•m 2 ).

[0037] Example 2

[0038] 1. Pre-treatment: Take an AZ91D magnesium alloy steering wheel frame workpiece with a surface area of ​​0.3 m². 2 Place the sample in an alkaline degreasing agent (50 g / L sodium hydroxide, 10 g / L sodium carbonate, 10 g / L sodium phosphate, 0.5 g / L sodium dodecyl sulfate) and ultrasonically clean it at 60°C for 10 min. After rinsing with water, place it in a rust removal solution (50 ml / L concentrated sulfuric acid and 20 ml / L 65 wt.% nitric acid) and soak it at room temperature for 10 s. After rinsing with water, place it in an activation solution (10 ml / L hydrofluoric acid) and soak it at room temperature for 2 min. Remove the sample and rinse it three times with deionized water. Dry it in an oven at 100°C for 20 min.

[0039] 2. Micro-arc oxidation treatment:

[0040] (1) Prepare the electrolyte with sodium silicate 80g / L and sodium phosphate 20g / L as the base system, then add potassium fluoride 35g / L, sodium fluoride 10g / L, glycerol 10ml / L, then add a mixed solution of acrylic emulsion 20g / L and sodium fluorozirconate 20g / L, stir evenly and then add sodium hydroxide 5g / L, the rest is deionized water.

[0041] (2) Micro-arc oxidation process: The detailed process is the same as in Example 1, except that:

[0042] The average current density is 0.5 A / dm. 2 In the initial constant current boosting process, the dynamic adjustment in this embodiment is as follows: the frequency gradually changes from 5000Hz to 1000Hz (step size 100Hz), the duty cycle decreases from 50% to 20% (step size 5%), and the final cutoff voltage is 260V. The frequency is maintained at 1000Hz, the duty cycle at 20%, and the voltage at 260V, and oxidation continues for another 10 minutes, with a total oxidation time of 20 minutes, forming a highly dense micro-arc oxide film.

[0043] 3. Post-treatment: Rinse three times with deionized water, dry in a 100℃ oven for 20 minutes to obtain a workpiece with a micro-arc oxide film thickness of 10μm. XRD analysis showed that the main components of the film were magnesium silicate, magnesium phosphate, magnesium fluoride, magnesium oxide, and zirconium oxide. A neutral salt spray test (level 9) was performed for 480 hours according to GB / T10125-2012, and the Vickers hardness was tested to 350 Hv according to GB / T 4340.1-2021. 0.05 Energy consumption per unit area: 0.3 kWh / (μm•m 2 ).

[0044] Example 3

[0045] 1. Pretreatment: Take an AZ91D magnesium alloy chassis component workpiece with a surface area of ​​1.5m². 2 Place the sample in an alkaline degreasing agent (50 g / L sodium hydroxide, 10 g / L sodium carbonate, 10 g / L sodium phosphate, 0.5 g / L sodium dodecyl sulfate) and ultrasonically clean it at 60°C for 10 min. After rinsing with water, place it in a rust removal solution (50 ml / L concentrated sulfuric acid and 20 ml / L 65 wt.% nitric acid) and soak it at room temperature for 10 s. After rinsing with water, place it in an activation solution (10 ml / L hydrofluoric acid) and soak it at room temperature for 2 min. Remove the sample and rinse it three times with deionized water. Dry it in an oven at 100°C for 20 min.

[0046] 2. Micro-arc oxidation treatment:

[0047] (1) Prepare the electrolyte with sodium silicate 70g / L and sodium phosphate 15g / L as the base system, then add potassium fluoride 30g / L, sodium fluoride 8g / L, glycerol 8ml / L, then add a mixed solution of acrylic emulsion 10g / L and sodium fluorozirconate 15g / L, stir evenly and then add sodium hydroxide 3g / L, the rest is deionized water, the workpiece is used as the anode and 316L stainless steel is used as the cathode, with a spacing of 5cm; during the micro-arc oxidation process, the electrolyte temperature is maintained at 30℃ by the cooling system.

[0048] (2) Micro-arc oxidation process: The detailed process is the same as in Example 1, except that:

[0049] The ratio of DC voltage to carrier voltage is set to 1:2, and the average current density is 0.7 A / dm². 2 The constant current voltage boosting process lasts for 20 minutes. In this embodiment, the dynamic adjustment is as follows: the frequency gradually changes from 5000Hz to 500Hz (step size 100Hz), the duty cycle decreases from 50% to 10% (step size 5%), and the final cutoff voltage is 300V (step size 10V). The frequency is maintained at 500Hz, the duty cycle at 10%, and the voltage at 300V. The oxidation continues for 10 minutes, with a total oxidation time of 30 minutes, forming a highly dense micro-arc oxide film.

[0050] 3. Post-treatment: Rinse three times with deionized water, dry in a 100℃ oven for 20 min to obtain a workpiece with a micro-arc oxide film thickness of 20 μm. XRD analysis showed that the main components of the film were magnesium silicate, magnesium phosphate, magnesium fluoride, magnesium oxide, and zirconium oxide. A neutral salt spray test (level 9) was performed according to GB / T10125-2012 for 960 h, and the Vickers hardness was 380 Hv. 0.05 Energy consumption per unit area: 0.4 kWh / (μm•m 2 ).

[0051] Comparative Example 1

[0052] 1. Pretreatment: Take an AZ91D magnesium alloy chassis component workpiece with a surface area of ​​1.5m². 2 Place the sample in an alkaline degreasing agent (50 g / L sodium hydroxide, 10 g / L sodium carbonate, 10 g / L sodium phosphate, 0.5 g / L sodium dodecyl sulfate) and ultrasonically clean it at 60°C for 10 min. After rinsing with water, place it in a rust removal solution (50 ml / L concentrated sulfuric acid and 20 ml / L 65 wt.% nitric acid) and soak it at room temperature for 10 s. After rinsing with water, place it in an activation solution (10 ml / L hydrofluoric acid) and soak it at room temperature for 2 min. Remove the sample and rinse it three times with deionized water. Dry it in an oven at 100°C for 20 min.

[0053] 2. Micro-arc oxidation treatment:

[0054] (1) Prepare the electrolyte using the traditional electrolyte composition: 10 g / L sodium silicate, 10 g / L potassium fluoride, stir evenly and then add 3 g / L sodium hydroxide, the remainder being deionized water.

[0055] (2) Micro-arc oxidation process: The detailed process is the same as in Example 3, except that:

[0056] To obtain an oxide film of the same thickness as in Example 3, the average current density was set to 1.5 A / dm². 2 ;

[0057] During the constant current boost process, the dynamic adjustment in this embodiment is as follows: the frequency gradually changes from 5000Hz to 500Hz, the duty cycle decreases from 50% to 10%, the final cutoff voltage is 350V, the frequency is maintained at 500Hz, the duty cycle is 10%, the voltage is 350V, and oxidation continues for 10 minutes, with a total oxidation time of 30 minutes.

[0058] 3. Post-treatment: Rinse three times with deionized water, dry in a 100℃ oven for 20 min to obtain a workpiece with a micro-arc oxide film thickness of 20 μm. XRD analysis showed that the main components of the film were magnesium silicate, magnesium oxide, and magnesium fluoride. A neutral salt spray test (level 9) was performed according to GB / T10125-2012 for 240 h, yielding a Vickers hardness of 320 Hv. 0.05 Energy consumption per unit area: 1.0 kWh / (μm•m 2 ).

[0059] Comparative Example 2

[0060] 1. Pretreatment: Take an AZ91D magnesium alloy chassis component workpiece with a surface area of ​​1.5m². 2 Place the sample in an alkaline degreasing agent (50 g / L sodium hydroxide, 10 g / L sodium carbonate, 10 g / L sodium phosphate, 0.5 g / L sodium dodecyl sulfate) and ultrasonically clean it at 60°C for 10 min. After rinsing with water, place it in a rust removal solution (50 ml / L concentrated sulfuric acid and 20 ml / L 65 wt.% nitric acid) and soak it at room temperature for 10 s. After rinsing with water, place it in an activation solution (10 ml / L hydrofluoric acid) and soak it at room temperature for 2 min. Remove the sample and rinse it three times with deionized water. Dry it in an oven at 100°C for 20 min.

[0061] 2. Micro-arc oxidation treatment:

[0062] (1) Prepare the electrolyte with sodium silicate 70g / L and sodium phosphate 15g / L as the base system, then add potassium fluoride 30g / L, sodium fluoride 8g / L, glycerol 8ml / L, then add a mixed solution of acrylic emulsion 10g / L and sodium fluorozirconate 15g / L, stir evenly and then add sodium hydroxide 3g / L, the rest is deionized water, the workpiece is used as the anode and 316L stainless steel is used as the cathode; the electrolyte temperature is maintained at 30℃ during the micro-arc oxidation process.

[0063] (2) Micro-arc oxidation process: A traditional single-pulse power supply is used, and the electrical parameters of the pulse power supply are set as follows: current frequency 2000Hz, duty cycle 30%; average current density 0.7A / dm 2 The initial constant current voltage was increased for 30 minutes, and the film formation voltage (cutoff voltage) was controlled at 300V. The voltage was maintained at 300V for another 10 minutes, and the total oxidation time was 40 minutes, forming a micro-arc oxide film.

[0064] 3. Post-treatment: Rinse three times with deionized water, dry in a 100℃ oven for 20 min to obtain a workpiece with a micro-arc oxide film thickness of 20 μm. XRD analysis showed that the main components of the film were magnesium silicate, magnesium phosphate, magnesium oxide, magnesium fluoride, and zirconium oxide. A neutral salt spray test (level 9) was performed according to GB / T10125-2012 for 480 h, yielding a Vickers hardness of 350 Hv. 0.05 Energy consumption per unit area: 0.8 kWh / (μm•m 2 ).

[0065] Comparative Example 3

[0066] Comparative Example 3 describes the preparation of a micro-arc oxidation film using a traditional sodium silicate electrolyte system. This method requires high voltage for oxidation when using a traditional pulse power supply.

[0067] 1. Pretreatment: Take an AZ91D magnesium alloy chassis component workpiece with a surface area of ​​1.5m². 2 Place the sample in an alkaline degreasing agent (50 g / L sodium hydroxide, 10 g / L sodium carbonate, 10 g / L sodium phosphate, 0.5 g / L sodium dodecyl sulfate) and ultrasonically clean it at 60°C for 10 min. After rinsing with water, place it in a rust removal solution (50 ml / L concentrated sulfuric acid and 20 ml / L 65 wt.% nitric acid) and soak it at room temperature for 10 s. After rinsing with water, place it in an activation solution (10 ml / L hydrofluoric acid) and soak it at room temperature for 2 min. Remove the sample and rinse it three times with deionized water. Dry it in an oven at 100°C for 20 min.

[0068] 2. Micro-arc oxidation treatment:

[0069] (1) Prepare the electrolyte by adding 10 g / L sodium silicate and 10 g / L potassium fluoride, stirring evenly, then adding 3 g / L sodium hydroxide and the remainder deionized water. The workpiece is used as the anode and 316L stainless steel is used as the cathode, with a spacing of 5 cm. The electrolyte temperature is maintained at 30℃ during the micro-arc oxidation process.

[0070] (2) Micro-arc oxidation process: A traditional pulse power supply is used, and the electrical parameters of the pulse power supply are set as follows: current frequency 2000Hz, duty cycle 30%; average current density is 2A / dm³. 2 The constant current voltage was increased for 10 minutes in the early stage, and the film formation voltage (cutoff voltage) was controlled at 400V. The voltage was maintained at 400V for another 30 minutes of oxidation, and the total oxidation time was 40 minutes to form a micro-arc oxide film.

[0071] 3. Post-treatment: Rinse three times with deionized water, dry in a 100℃ oven for 20 min to obtain a workpiece with a micro-arc oxide film thickness of 20 μm. XRD analysis showed that the main components of the film were magnesium silicate, magnesium oxide, and magnesium fluoride. A neutral salt spray test of 96 h (level 9) was performed according to GB / T10125-2012, and the Vickers hardness was tested to 320 Hv according to GB / T 4340.1-2021. 0.05 Energy consumption per unit area: 1.2 kWh / (μm•m 2 ).

[0072] Comparative Example 4

[0073] Comparative Example 4 shows the preparation of micro-arc oxidation films using a traditional sodium phosphate electrolyte system. This method requires a higher voltage for oxidation when using a traditional pulse power supply.

[0074] 1. Pretreatment: Take an AZ91D magnesium alloy chassis component workpiece with a surface area of ​​1.5m². 2 Place the sample in an alkaline degreasing agent (50 g / L sodium hydroxide, 10 g / L sodium carbonate, 10 g / L sodium phosphate, 0.5 g / L sodium dodecyl sulfate) and ultrasonically clean it at 60°C for 10 min. After rinsing with water, place it in a rust removal solution (50 ml / L concentrated sulfuric acid and 20 ml / L 65 wt.% nitric acid) and soak it at room temperature for 10 s. After rinsing with water, place it in an activation solution (10 ml / L hydrofluoric acid) and soak it at room temperature for 2 min. Remove the sample and rinse it three times with deionized water. Dry it in an oven at 100°C for 20 min.

[0075] 2. Micro-arc oxidation treatment:

[0076] (1) Prepare the electrolyte. Another traditional electrolyte is used: 5 g / L sodium phosphate is used as the base system, and then 20 g / L potassium fluoride and 5 g / L sodium fluorozirconate are added. After stirring evenly, 1 g / L sodium hydroxide is added, and the rest is deionized water. The workpiece is used as the anode and 316L stainless steel is used as the cathode. The electrolyte temperature is maintained at 30℃ during the micro-arc oxidation process.

[0077] (2) Micro-arc oxidation process: A traditional pulse power supply is used, and the electrical parameters of the pulse power supply are set as follows: current frequency 1000Hz, duty cycle 30%; current density 2A / dm 2 The initial constant current voltage was increased for 50 minutes, the film formation voltage was controlled at 450V, and the voltage was maintained at 450V for another 10 minutes of oxidation. The total oxidation time was 60 minutes, forming a micro-arc oxide film.

[0078] 3. Post-treatment: Rinse three times with deionized water, dry in a 100℃ oven for 20 min to obtain a workpiece with a micro-arc oxidation film thickness of 20 μm. XRD analysis showed that the main components of the film were magnesium phosphate, magnesium oxide, and magnesium fluoride. The neutral salt spray test was performed at level 9 for 120 h, and the Vickers hardness was 350 Hv. 0.05 Energy consumption per unit area: 1.3 kWh / (μm•m 2 ).

[0079] Examples 1-3 describe methods for preparing micro-arc oxidation films of different thicknesses on magnesium alloy parts for automobiles. By changing the concentration of the electrolyte and the voltage and time of the micro-arc oxidation process, oxide films of different thicknesses can be obtained.

[0080] Compared with Comparative Example 1, the oxide film prepared in Example 3 has increased magnesium phosphate and zirconium oxide, and the film layer is denser, with a microhardness of 320 Hv. 0.05 Increased to 380Hv 0.05 It improves corrosion resistance by more than 100% and reduces energy consumption by 60%.

[0081] Compared with Comparative Example 2, the oxide film prepared in Example 3 had the same composition, but the film was denser and its corrosion resistance was improved by 3 times, from 350 Hv. 0.05 Increased to 380Hv 0.05 Power consumption has been reduced by 50% compared to the previous version.

[0082] Compared with Comparative Examples 3-4 (traditional method), the oxide film prepared in Example 3 changed the power supply mode and electrolyte system. XRD test showed that the main components of the film in Example 3 were magnesium silicate, magnesium phosphate, magnesium fluoride, magnesium oxide, and zirconium oxide. The film was denser, the corrosion resistance was improved by more than 6 times, and the power consumption was reduced by 65-70%.

Claims

1. A method for preparing a low-cost, high-corrosion-resistant micro-arc oxidation film on magnesium alloy, characterized in that, Includes the following steps: (1) Pretreatment of magnesium alloy workpieces; (2) Prepare the electrolyte used in the micro-arc oxidation process. The electrolyte composition is: sodium silicate 60~80g / L, sodium phosphate 10~20g / L, potassium fluoride 25~35g / L, sodium fluoride 5~10g / L, glycerol 5~10ml / L, acrylic emulsion 5~20g / L, sodium fluorozirconate 10~20 g / L, sodium hydroxide 1~5g / L, and the remainder is water; (3) Take the pretreated magnesium alloy workpiece as the anode and stainless steel as the cathode, place it in the above electrolyte, and carry out micro-arc oxidation process to finally obtain a dense oxide film layer. The micro-arc oxidation process specifically involves applying a composite carrier voltage, consisting of a DC fundamental wave and a pulse carrier wave, to the anode and cathode. The ratio of the DC voltage to the peak pulse voltage is set to 1:1 to 1:3, and the average current density is 0.3 to 0.7 A / dm³. 2 Constant current boost for 5-20 minutes; during constant current boost, the initial pulse frequency is 5000-6000Hz and the duty cycle is 45-50%; During the constant current boost process, the composite carrier voltage is turned off every 2-10 seconds, while a 500-600ms triangular wave is applied to the anode and cathode. The impedance is analyzed and Rp=ΔE / (Δ i / S), Rp: discharge resistance, ΔE: transient transition voltage, Δi: transient transition current, S: oxide film surface area; When Rp is greater than the threshold, the composite carrier frequency and duty cycle are dynamically adjusted to adapt to the film state and continue the micro-arc oxidation process. Specifically, the composite carrier frequency and duty cycle are reduced at the same time or only one of them is reduced. The composite carrier frequency in the adapted film layer state is 500~2000Hz, the voltage is 200~300V, the duty cycle is 10-30%, and the oxidation time is 5~15min.

2. The method for preparing a low-cost, high-corrosion-resistant micro-arc oxidation film on magnesium alloy according to claim 1, characterized in that, The pretreatment described in step (1) is as follows: the surface of the magnesium alloy workpiece is sequentially degreased, derusted, activated and dried.

3. The method for preparing a low-cost, high-corrosion-resistant micro-arc oxidation film on magnesium alloy according to claim 2, characterized in that, The alkaline degreasing agent is composed of: sodium hydroxide 50-60 g / L, sodium carbonate 10-16 g / L, sodium phosphate 10-15 g / L, and sodium dodecyl sulfate 0.5-1 g / L, ultrasonically cleaned at 60-80℃ for 10-30 min; the rust removal solution is composed of: 98 wt.% concentrated sulfuric acid 50 ml / L and 65 wt.% nitric acid 20 ml / L, soaked at room temperature for 10-15 s; the activation solution is composed of: 40 wt.% hydrofluoric acid 10 ml / L, soaked at room temperature for 2-3 min; the drying conditions are drying at 80-100℃ for 20-30 min; and water washing is performed after the degreasing, rust removal, and activation steps.

4. The method for preparing a low-cost, high-corrosion-resistant micro-arc oxidation film on magnesium alloy according to claim 1, characterized in that, The electrolyte in step (3) is maintained at 30-40°C during the micro-arc oxidation process.

5. The method for preparing a low-cost, high-corrosion-resistant micro-arc oxidation film on magnesium alloy according to claim 1, characterized in that, The magnesium alloy is one of AZ, AM and cast magnesium alloy.

6. An oxide film prepared by a method for preparing a low-cost, high-corrosion-resistant micro-arc oxide film layer of magnesium alloy according to any one of claims 1-5, characterized in that: The oxide film is composed of magnesium silicate, magnesium phosphate, magnesium fluoride, magnesium oxide, and zirconium oxide; the film thickness is 5~20μm; the corrosion resistance meets the requirements of a neutral salt spray test ≥480h and a Vickers hardness ≥350Hv. 0.05 .

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