Magnesium alloy low-cost high-corrosion-resistance micro-arc oxidation film layer and preparation method thereof

Abstract

The invention relates to the technical field of metal surface treatment, in particular to a magnesium alloy low-cost high-corrosion-resistance micro-arc oxidation film layer and a preparation method thereof.The preparation method comprises the steps that firstly, a magnesium alloy workpiece is pretreated, then an electrolyte used in the micro-arc oxidation process is prepared, the pretreated magnesium alloy workpiece serves as an anode, stainless steel serves as a cathode, and the magnesium alloy workpiece is prepared; and placing in the electrolyte, and carrying out a micro-arc oxidation process to finally obtain a compact oxidation film layer. The compact oxidation film layer is finally obtained through an electrical parameter external control process of online monitoring regulation and control and composite carrier in cooperation with a low-energy-consumption electrolyte system. The main components of the oxidation film are magnesium silicate, magnesium phosphate, magnesium oxide, magnesium fluoride and zirconium oxide, so that the hardness of the film layer is higher, the wear resistance of the film layer is improved, the neutral salt spray test is greater than or equal to 480h (10 microns) 9 grade, the performance requirements of vehicle parts are met, the oxidation film is suitable for large-scale industrial production, and the comprehensive energy consumption is reduced by 50-60%.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of metal surface treatment, and relates to a low-cost high-corrosion-resistance micro-arc oxidation film layer of a magnesium alloy and a preparation method thereof, in particular to a low-cost high-corrosion-resistance micro-arc oxidation film layer preparation method suitable for magnesium alloy parts. BACKGROUND

[0002] Under the development trend of automobile lightweight, magnesium alloy is widely used in seat supports, steering wheel skeletons, chassis parts and other components due to its high specific strength and small density. However, magnesium alloy has poor corrosion resistance due to its active chemical properties, and needs to be treated on the surface to improve its performance. The micro-arc oxidation process is one of the mainstream surface treatment technologies for magnesium alloy at present, which can significantly improve the hardness and wear resistance of the material by forming an oxide film layer containing crystalline components on the metal surface.

[0003] The traditional micro-arc oxidation process uses a single pulse power source, which has low energy conversion efficiency and needs to maintain a high film-forming voltage of 350-450V and a large current density, resulting in high energy consumption. At the same time, the conventional electrolyte is mainly composed of a single film-forming agent, which lacks efficient active ingredients, has a slow film-forming rate, and takes a long processing time of 30-60 minutes (thickness of 5-20 microns), resulting in low production efficiency. In addition, high energy consumption and long working hours directly increase production costs, and the traditional power equipment has a problem of waste, which further aggravates energy waste and makes it difficult to adapt to the large-scale production demand of magnesium alloy. Therefore, developing a low-energy, low-cost and high-efficiency micro-arc oxidation process has become a key to promoting the large-scale application of magnesium alloy in the automotive field. SUMMARY

[0004] The present application aims to provide a low-cost high-corrosion-resistance micro-arc oxidation film layer of a magnesium alloy and a preparation method thereof, to solve the problems of high energy consumption, high cost and low efficiency of the traditional process, while ensuring that the magnesium alloy micro-arc oxidation film layer has excellent corrosion resistance and high hardness, and ensuring that the magnesium alloy can be applied in the automotive field.

[0005] The present application achieves performance breakthrough through the synergistic innovation of micro-arc oxidation power supply external control technology innovation and electrolyte optimization, and the specific technical scheme is as follows: A preparation method of a low-cost high-corrosion-resistance micro-arc oxidation film layer of a magnesium alloy, comprising the following steps: (1) Pretreating the magnesium alloy workpiece; (2) Configuring the electrolyte used in the micro-arc oxidation process, the electrolyte composition is: sodium silicate 60-80 g / L, sodium phosphate 10-20 g / L, potassium fluoride 25-35 g / L, sodium fluoride 5-10 g / L, glycerol 5-10 ml / L, acrylic acid emulsion 5-20 g / L, sodium fluorozirconate 10-20 g / L, sodium hydroxide 1-5 g / L, and the rest is water; (3) taking the pretreated magnesium alloy workpiece as an anode, taking stainless steel as a cathode, and placing them in the electrolyte to perform a micro-arc oxidation process, so as to finally obtain a dense oxide film layer.

[0006] Further, the pretreatment in step (1) is that the surface of the magnesium alloy workpiece is sequentially subjected to oil removal, rust removal, activation, and drying.

[0007] Further, the alkaline oil removal agent used for oil removal is 50-60 g / L of sodium hydroxide, 10-16 g / L of sodium carbonate, 10-15 g / L of sodium phosphate, and 0.5-1 g / L of sodium dodecyl sulfate, and ultrasonic cleaning is performed at 60-80°C for 10-30 min; the rust removal solution used for rust removal is 50 ml / L of 98 wt.% sulfuric acid and 20 ml / L of 65 wt.% nitric acid, and immersion is performed at room temperature for 10-15 s; the activation liquid used for activation is 10 ml / L of 40 wt.% hydrofluoric acid, and immersion is performed at room temperature for 2-3 min; the drying condition is drying at 80-100°C for 20-30 min; and water washing is performed after the oil removal, rust removal, and activation steps.

[0008] Further, the electrolyte in step (3) is maintained at 30-40°C during the micro-arc oxidation process.

[0009] Further, step (3) is specifically as follows. A composite carrier voltage obtained by superimposing a direct current base wave and a pulse carrier wave is applied to the anode and the cathode, wherein the direct current voltage value and the pulse voltage peak value are set to 1:1-1:3, and the average current density is 0.3-0.7 A / dm 2 , and the constant current boosting is performed for 5-20 min; during the constant current boosting, the initial pulse frequency is 5000-6000 Hz, and the duty cycle is 45-50%; During the constant current boosting, the composite carrier voltage is turned off every 2-10 s, and a 500-600 ms triangular wave is applied to the anode and the cathode at the same time, the impedance is resolved, and Rp=ΔE / (Δ i / S) is calculated, where Rp is the discharge resistance, ΔE is the transient transition voltage, Δi is the transient transition current, and S is the surface area of the oxide film. When Rp is greater than a threshold value, the composite carrier frequency and the duty cycle are dynamically adjusted to adapt to the film layer state to continue the micro-arc oxidation process, and specifically, the composite carrier frequency and the duty cycle are simultaneously reduced or only one of them is reduced.

[0010] Further, the composite carrier frequency adapted to the film layer state is 500-2000 Hz, the voltage is 200-300 V, and the duty cycle is 10-30%; and the oxidation time is 5-15 min.

[0011] Further, the magnesium alloy is one of AZ, AM, and cast magnesium alloy.

[0012] An oxide film layer prepared by the preparation method of the application, the oxide film layer is composed of magnesium silicate, magnesium phosphate, magnesium fluoride, magnesium oxide, zirconium oxide; the film layer thickness is 5-20 microns; the corrosion resistance meets the neutral salt spray test ≥480h, and the Vickers hardness ≥350Hv 0.05 .

[0013] The process principle of the application is as follows: (1) The electric parameter control process layer: the application can avoid strong spark eruption, reduce invalid energy loss, uniformly distribute energy to avoid local concentration by monitoring the discharge resistance of the micro-arc oxidation system every 2-10s, dynamically adjusting the composite carrier frequency and duty cycle to adapt to the film layer state, and applying 500-600ms triangular wave (consistent with the composite carrier power voltage peak value and frequency) for 0.3-0.7A / dm 2 of average current density can realize efficient film formation.

[0014] (2) Electrolyte layer: fluoride as a passivator of magnesium, can form stable precipitate with Mg 2+ , reduce the activation energy of oxidation reaction, and directly reduce the film forming voltage; glycerol has dispersibility and stability, can inhibit the aggregation of bubbles in the electrolyte, avoid the current waste caused by micro-arc discharge "arc breaking", improve the current efficiency by more than 20%, accelerate the growth of oxidation film, and shorten the processing time; at the same time, its hydroxyl group can combine with ions in the electrolyte, optimize the growth rate of the oxidation film, shorten the processing time by 50%, indirectly reduce the energy consumption, and need to control 5-10ml / L to balance the stability and film forming efficiency; sodium phosphate has complex thickening mechanism, can affect the discharge characteristics of micro-arc oxidation, and is helpful to reduce energy consumption; sodium silicate ionizes SiO3 2- and Na + ions in aqueous solution, enhances the conductivity of the electrolyte, when high voltage is applied, these ions participate in the electrode reaction, and form an insulating oxide film on the surface of magnesium alloy. When the film thickness reaches a certain degree, the local electric field strength exceeds the critical value, and electric breakdown occurs, micro-arc discharge occurs, the high temperature and high pressure environment of micro-arc is the key to the subsequent formation of dense ceramic film, which modulates the film layer, and then affects the discharge characteristics of micro-arc oxidation, reduces the energy consumption; sodium fluorozirconate ionizes ZrF6 2-Ions, in the high temperature and high pressure environment of micro-arc discharge, zirconium will participate in film layer formation, generating ceramic phases such as ZrO2, which has extremely high hardness and good chemical stability, can reduce the energy density of micro-arc discharge, improve the compactness of the film layer, and sodium fluorozirconate can react with magnesium ions in an alkaline electrolyte to generate a difficultly soluble passivation film of fluoride or zirconate covering the surface of the magnesium alloy, thereby inhibiting the corrosion of the magnesium matrix and stabilizing the oxidation process; sodium hydroxide can reduce the energy consumption per unit area, improve the film layer quality, and control the average current density during the micro-arc oxidation process to 1A / dm 2 The following.

[0015] The process can be widely applied to the surface treatment of magnesium alloy components such as automobile seat supports, steering wheel skeletons and chassis parts, which can not only meet the requirements of automobile lightweighting on material performance, but also adapt to large-scale production demand through the advantages of low energy consumption and low cost. With the increasing requirements of the automobile industry for energy saving and emission reduction and cost control, the process is expected to become one of the mainstream technologies for magnesium alloy surface treatment, and to promote the large-scale application of magnesium alloy in the automobile field.

[0016] Advantages and benefits of the present application 1. Low energy consumption: the present application can inhibit the formation of large sparks in the micro-arc oxidation process through the electric parameter external control process of "online monitoring and regulation + composite carrier", form ultra-fine sparks, and reduce energy waste. The low energy consumption electrolyte system is matched to cooperatively reduce the film forming voltage and the required current density, and finally a dense oxide film layer is obtained. The comprehensive energy consumption is reduced by 50%~60%; 2. Low cost: low energy consumption directly reduces the operating cost of the equipment, glycerol improves the stability of the electrolyte and reduces the loss of raw materials, the processing time is shortened by more than 50%, the labor cost is reduced, and the overall cost is reduced by 30~50%; 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 have high hardness and improves the wear resistance of the film layer. The neutral salt spray test is ≥480h (10μm) 9 level, which meets the performance requirements of vehicle parts and adapts to large-scale industrial production. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 The figure is a schematic diagram of the micro-arc oxidation system in the present application. DETAILED DESCRIPTION

[0018] Example 1 1. Pretreatment: take AZ91D magnesium alloy seat support workpieces, the surface area is 0.5m 2, put into alkaline degreasing agent (sodium hydroxide 50g / L, sodium carbonate 10g / L, sodium phosphate 10g / L, sodium dodecyl sulfate 0.5g / L), ultrasonic cleaning at 60°C for 10min, after water washing, put into rust removal solution (98wt.% concentrated sulfuric acid 50ml / L, 65wt.% nitric acid 20ml / L) at room temperature for 10s, after water washing, put into activation solution (40wt.% hydrofluoric acid 10ml / L) at room temperature for 2min, after taking out, rinse with deionized water for 3 times, dry in 100°C oven for 20min; 2. Micro-arc oxidation treatment: (1) The electrolyte is configured, sodium silicate 60g / L, sodium phosphate 10g / L as the basic system, then add potassium fluoride 25g / L, sodium fluoride 5g / L, glycerol 5ml / L, then add acrylic emulsion (E0503, Shenzhen Jitian Chemical Co., Ltd.) 5g / L and sodium fluorozirconate 10g / L mixed solution, stir uniformly, then add sodium hydroxide 1g / L, the rest is deionized water.

[0019] (2) Micro-arc oxidation process: the micro-arc oxidation system in this embodiment is shown in Figure 1 , mainly composed of working circuit and detection circuit, put the electrolyte of the above components into the oxidation tank, the workpiece as anode, 316L stainless steel as cathode, the distance is 5cm. The series pulse power supply and direct current power supply form the composite carrier power supply of this embodiment, the positive and negative electrodes of the composite carrier power supply are connected with the anode and cathode respectively to form the working circuit; the triangular wave detection signal source (a power supply that can output triangular wave voltage) is connected in parallel with the composite carrier power supply; the positive and negative electrodes of the triangular wave detection signal source are connected with the anode and cathode respectively to form the detection circuit.

[0020] The temperature of the electrolyte during the micro-arc oxidation treatment process is maintained at 30°C.

[0021] The specific process is as follows: set the composite carrier power supply electrical parameters of the working circuit, the peak value ratio of the direct current voltage value to the carrier (pulse) voltage is set to 1:1, the average current density is 0.3A / dm 2 , the initial carrier (pulse) frequency is set to 5000Hz, the duty cycle is 50%, then the discharge resistance of the micro-arc oxidation system is monitored online: every 10s, the voltage output of the composite carrier power supply is turned off, a triangular wave of 500ms is applied to the workpiece, the peak value and frequency of the triangular wave are consistent with the voltage peak value and frequency of the composite carrier power supply before turning off, the impedance is analyzed and Rp=ΔE / (Δ i / S) is calculated, Rp is the discharge resistance, ΔE: transient jump voltage (voltage instantaneous change amount), Δi: transient jump current (current instantaneous change amount) S: surface area of the oxidation film; when Rp is greater than the set threshold value 6Ωm 2At the same time, the frequency and duty cycle of the composite carrier power supply are reduced to adapt to the film layer state, and the micro-arc oxidation process continues until Rp does not exceed the threshold value; the dynamic adjustment in this embodiment is: the frequency gradually changes from 5000 Hz to 2000 Hz (each adjustment reduces 100 Hz based on the last time the composite carrier power supply is turned off), and the duty cycle decreases from 50% to 30% (each adjustment reduces 5% based on the last time the composite carrier power supply is turned off), and finally the cutoff voltage is 200 V, the frequency is maintained at 2000 Hz, the duty cycle is 30%, and the voltage is 200 V, and the oxidation continues for 5 min, and the total oxidation time is 10 min. A high-density micro-arc oxidation film is formed.

[0022] 3. Post-processing: rinsed with deionized water 3 times, oven dried at 100°C for 20 min, obtained a workpiece with a micro-arc oxidation film thickness of 5 μm, the main components of the film layer were magnesium silicate, magnesium phosphate, magnesium fluoride, magnesium oxide, and zirconium oxide by XRD test, the neutral salt spray test was tested according to GB / T10125-2012 for 240h (level 9), and the Vickers hardness was tested according to GB / T 4340.1-2021 300Hv 0.05 , the energy consumption per unit area was 0.4 kWh / (μm•m 2 ).

[0023] Example 2 1. Pretreatment: take AZ91D magnesium alloy steering wheel skeleton workpiece, the surface area is 0.3m 2 , put it into an alkaline degreasing agent (sodium hydroxide 50g / L, sodium carbonate 10g / L, sodium phosphate 10g / L, sodium dodecyl sulfate 0.5g / L), ultrasonic cleaning at 60°C for 10min, after water washing, put it into a rust removal solution (98wt.% concentrated sulfuric acid 50ml / L, 65wt.% nitric acid 20ml / L) for immersion at room temperature for 10s, after water washing, put it into an activation solution (40wt.% hydrofluoric acid 10ml / L) for immersion at room temperature for 2min, take out and rinse with deionized water 3 times, and dry in an oven at 100°C for 20min; 2. Micro-arc oxidation treatment: (1) Configure electrolyte, sodium silicate 80g / L, sodium phosphate 20g / L as the basic 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 uniformly, then add sodium hydroxide 5g / L, and the rest is deionized water.

[0024] (2) Micro-arc oxidation process: the detailed process is the same as in example 1, the difference is: The average current density is 0.5A / dm 2, the dynamic adjustment in the embodiment is as follows: the frequency gradually changes from 5000 Hz to 1000 Hz (the step is 100 Hz), the duty cycle decreases from 50% to 20% (the step is 5%), and finally the cutoff voltage is 260 V, the frequency is maintained at 1000 Hz, the duty cycle is maintained at 20%, and the voltage is maintained at 260 V, the oxidation is continued for 10 min, the total oxidation time is 20 min, and the high-density micro-arc oxidation film is formed.

[0025] 3. Post-processing: rinsing with deionized water 3 times, drying in a 100°C oven for 20 min, obtaining a workpiece with a micro-arc oxidation film thickness of 10 μm, testing the main components of the film layer by XRD, which are magnesium silicate, magnesium phosphate, magnesium fluoride, magnesium oxide, and zirconium oxide, testing the neutral salt spray test for 480 h (level 9) according to GB / T10125-2012, and testing the Vickers hardness of 350 Hv according to GB / T 4340.1-2021 0.05 , and the energy consumption per unit area is 0.3 kWh / (μm·m 2 ).

[0026] Example 3 1. Pretreatment: taking AZ91D magnesium alloy bottom part workpieces with a surface area of 1.5 m 2 , putting them into an alkaline degreasing agent (sodium hydroxide 50 g / L, sodium carbonate 10 g / L, sodium phosphate 10 g / L, and sodium dodecyl sulfate 0.5 g / L), ultrasonic cleaning at 60°C for 10 min, washing with water, then putting them into a rust removal solution (98wt.% concentrated sulfuric acid 50 ml / L, 65wt.% nitric acid 20 ml / L) for immersion at room temperature for 10 s, washing with water, then putting them into an activation solution (40wt.% hydrofluoric acid 10 ml / L) for immersion at room temperature for 2 min, taking out and rinsing with deionized water 3 times, and drying in a 100°C oven for 20 min; 2. Micro-arc oxidation treatment: (1) Configuring electrolyte: sodium silicate 70 g / L, sodium phosphate 15 g / L as the basic system, then adding potassium fluoride 30 g / L, sodium fluoride 8 g / L, glycerol 8 ml / L, then adding a mixed solution of acrylic emulsion 10 g / L and sodium fluorozirconate 15 g / L, stirring uniformly, then adding sodium hydroxide 3 g / L, and the rest is deionized water; the workpiece is used as the anode, and 316L stainless steel is used as the cathode with a distance of 5 cm; the electrolyte temperature is maintained at 30°C through a cooling system during the micro-arc oxidation treatment.

[0027] (2) Micro-arc oxidation process: the detailed process is the same as that in Example 1, except that: The ratio of direct current voltage to carrier voltage is set to 1:2, and the average current density is 0.7 A / dm 2, the constant current boosting process, the dynamic adjustment in this embodiment is as follows: the frequency gradually changes from 5000 Hz to 500 Hz (step length 100 Hz), the duty cycle decreases from 50% to 10% (step length 5%), and finally the cutoff voltage is 300 V (step length 10 V). The frequency, the duty cycle, and the voltage are maintained at 500 Hz, 10%, and 300 V respectively, and the oxidation is continued for 10 min. The total oxidation time is 30 min, and a high-density micro-arc oxidation film is formed.

[0028] 3. Post-processing: rinsed with deionized water for 3 times, dried in an oven at 100 ℃ for 20 min, and a workpiece with a micro-arc oxidation film thickness of 20 μm was obtained. XRD test showed that the main components of the film layer were magnesium silicate, magnesium phosphate, magnesium fluoride, magnesium oxide, and zirconium oxide. The neutral salt spray test was tested according to GB / T10125-2012 for 960 h (level 9), and the Vickers hardness was 380 Hv. 0.05 , and the energy consumption per unit area was 0.4 kWh / (μm·m 2 ).

[0029] Comparative Example 1 1. Pretreatment: AZ91D magnesium alloy bottom part workpieces with a surface area of 1.5 m 2 were taken, and were placed in an alkaline degreasing agent (sodium hydroxide 50 g / L, sodium carbonate 10 g / L, sodium phosphate 10 g / L, and sodium dodecyl sulfate 0.5 g / L) for ultrasonic cleaning at 60 ℃ for 10 min. After water washing, the workpieces were placed in a rust removal solution (98 wt.% concentrated sulfuric acid 50 ml / L and 65 wt.% nitric acid 20 ml / L) for immersion at room temperature for 10 s. After water washing, the workpieces were placed in an activation solution (40 wt.% hydrofluoric acid 10 ml / L) for immersion at room temperature for 2 min, and were taken out and rinsed with deionized water for 3 times, and were dried in an oven at 100 ℃ for 20 min. 2. Micro-arc oxidation treatment: (1) The electrolyte was configured, and the traditional electrolyte composition was used: sodium silicate 10 g / L, potassium fluoride 10 g / L, and sodium hydroxide 3 g / L were added, and the rest was deionized water.

[0030] (2) Micro-arc oxidation process: the detailed process was the same as that in Example 3, except that: In order to obtain an oxidation film layer with the same thickness as in Example 3, the average current density was set to 1.5 A / dm 2 . In the constant current boosting process, the dynamic adjustment in this embodiment is as follows: the frequency gradually changes from 5000 Hz to 500 Hz, the duty cycle decreases from 50% to 10%, and finally the cutoff voltage is 350 V. The frequency, the duty cycle, and the voltage are maintained at 500 Hz, 10%, and 350 V respectively, and the oxidation is continued for 10 min. The total oxidation time is 30 min.

[0031] 3. Post-processing: rinse with deionized water 3 times, oven drying at 100℃ for 20 min, get the workpiece with micro-arc oxidation film thickness of 20 μm, the main components of the film layer are magnesium silicate, magnesium oxide, magnesium fluoride, according to GB / T10125-2012 test neutral salt spray test 240h (9 level), Vickers hardness 320Hv 0.05 , energy consumption per unit area 1.0 kWh / (μm•m 2 ).

[0032] Comparative Example 2 1. Pretreatment: take AZ91D magnesium alloy bottom part workpiece, surface area is 1.5m 2 , put into alkaline degreasing agent (sodium hydroxide 50g / L, sodium carbonate 10g / L, sodium phosphate 10g / L, sodium dodecyl sulfate 0.5g / L), ultrasonic cleaning at 60℃ for 10min, after water washing, put into rust removal solution (98wt.% concentrated sulfuric acid 50ml / L, 65wt.% nitric acid 20ml / L) at room temperature for 10s, after water washing, put into activation solution (40wt.% hydrofluoric acid 10ml / L) at room temperature for 2min, take out and rinse with deionized water 3 times, oven drying at 100℃ for 20min; 2. Micro-arc oxidation treatment: (1) Configure electrolyte, sodium silicate 70g / L, sodium phosphate 15g / L as the basic system, then add potassium fluoride 30g / L, sodium fluoride 8g / L, glycerol 8ml / L, then add the mixed solution of acrylic emulsion 10g / L and sodium fluorozirconate 15g / L, after stirring uniformly, add sodium hydroxide 3g / L, the rest is deionized water, the workpiece is used as anode and 316L stainless steel is used as cathode; the electrolyte temperature is maintained at 30℃ during the micro-arc oxidation treatment process.

[0033] (2) Micro-arc oxidation process: use traditional single pulse power source, set pulse power parameters: current frequency 2000Hz, duty cycle 30%; average current density is 0.7A / dm 2 , constant current boosting for 30min, control the film forming voltage (cut-off voltage) to be 300V, continue to oxidize for 10min at 300V, total oxidation time is 40min, form micro-arc oxidation film.

[0034] 3. Post-processing: rinse with deionized water 3 times, oven drying at 100℃ for 20 min, get the workpiece with micro-arc oxidation film thickness of 20 μm, the main components of the film layer are magnesium silicate, magnesium oxide, magnesium fluoride, according to GB / T10125-2012 test neutral salt spray test 240h (9 level), Vickers hardness 320Hv 0.05 , energy consumption per unit area 1.0 kWh / (μm•m 2 ).

[0035] Comparative Example 3 Comparative Example 3 is to prepare micro-arc oxidation film using traditional sodium silicate electrolyte system. When using traditional pulse power, high voltage is needed for oxidation.

[0036] 1. Pretreatment: take AZ91D magnesium alloy bottom part workpiece, the surface area is 1.5 m 2 , put into alkaline degreasing agent (sodium hydroxide 50 g / L, sodium carbonate 10 g / L, sodium phosphate 10 g / L, sodium dodecyl sulfate 0.5 g / L), ultrasonic cleaning at 60°C for 10 min, after water washing, put into rust removal solution (98wt.% concentrated sulfuric acid 50 ml / L, 65wt.% nitric acid 20 ml / L) at room temperature for 10 s, after water washing, put into activation solution (40wt.% hydrofluoric acid 10 ml / L) at room temperature for 2 min, take out and rinse with deionized water for 3 times, dry in 100°C oven for 20 min; 2. Micro-arc oxidation treatment: (1) Configure electrolyte, sodium silicate 10 g / L, add potassium fluoride 10 g / L, after stirring uniformly, add sodium hydroxide 3 g / L, the rest is deionized water, workpiece as anode, 316L stainless steel as cathode, distance 5 cm; the temperature of electrolyte is maintained at 30°C during micro-arc oxidation treatment.

[0037] (2) Micro-arc oxidation process: use traditional pulse power, set pulse power parameters, current frequency 2000 Hz, duty cycle 30%; average current density 2 A / dm 2 , constant current voltage rise 10 min in early stage, control film forming voltage (cut-off voltage) 400 V, continue oxidation at 400 V for 30 min, total oxidation time 40 min, form micro-arc oxidation film.

[0038] 3. Post-treatment: rinse with deionized water for 3 times, dry in 100°C oven for 20 min, get workpiece with micro-arc oxidation film thickness 20 μm, test main components of film layer by XRD, magnesium silicate, magnesium oxide, magnesium fluoride, test 9 level of neutral salt spray test for 96 h according to GB / T10125-2012, test Vickers hardness 320Hv according to GB / T 4340.1-2021 0.05 , unit area energy consumption 1.2 kWh / (μm·m 2 ).

[0039] Comparative Example 4 Comparative Example 4 is to prepare micro-arc oxidation film using traditional sodium phosphate electrolyte system. When using traditional pulse power, higher voltage is needed for oxidation.

[0040] 1. Pretreatment: take AZ91D magnesium alloy bottom part workpiece, the surface area is 1.5 m2 , put into alkaline degreasing agent (sodium hydroxide 50 g / L, sodium carbonate 10 g / L, sodium phosphate 10 g / L, sodium dodecyl sulfate 0.5 g / L), ultrasonic cleaning at 60°C for 10 min, after water washing, put into rust removal solution (98wt.% concentrated sulfuric acid 50 ml / L, 65wt.% nitric acid 20 ml / L) at room temperature for 10 s, after water washing, put into activation solution (40wt.% hydrofluoric acid 10 ml / L) at room temperature for 2 min, take out and rinse with deionized water for 3 times, dry in 100°C oven for 20 min; 2. Micro-arc oxidation treatment: (1) Configure electrolyte, adopt another traditional electrolyte: a mixed solution of 5 g / L of sodium phosphate as the basic system, 20 g / L of potassium fluoride, and 5 g / L of sodium fluorozirconate, after stirring uniformly, add 1 g / L of sodium hydroxide, and the rest is deionized water, the workpiece is used as an anode, and 316L stainless steel is used as a cathode, and the electrolyte temperature is maintained at 30°C during the micro-arc oxidation treatment process.

[0041] (2) Micro-arc oxidation process: adopt a traditional pulse power source, set the pulse power source electrical parameters: current frequency 1000 Hz, duty cycle 30%; current density 2 A / dm 2 , constant current voltage rise for 50 min in the early stage, control the film forming voltage to be 450 V, continue to oxidize for 10 min at 450 V, the total oxidation time is 60 min, and the micro-arc oxidation film is formed.

[0042] 3. Post-treatment: rinse with deionized water for 3 times, dry in a 100°C oven for 20 min, obtain the workpiece with a micro-arc oxidation film thickness of 20 μm, the main components of the film layer are magnesium phosphate, magnesium oxide, and magnesium fluoride, the neutral salt spray test is 9 levels for 120 h, and the Vickers hardness is 350 Hv 0.05 , the energy consumption per unit area is 1.3 kWh / (μm•m 2 ).

[0043] Examples 1-3 are preparation methods of micro-arc oxidation films with different thicknesses on magnesium alloy parts on automobiles, and different thicknesses of oxidation films can be obtained by changing the concentration of the electrolyte of the application and the voltage and time of the micro-arc oxidation process.

[0044] Compared with Comparative Example 1, the oxidation film prepared in the example has increased magnesium phosphate and zirconium oxide, and the film layer is more dense, the microhardness is increased from 320 Hv 0.05 to 380 Hv 0.05 , the corrosion resistance is increased by more than 1 times, and the energy consumption is reduced by 60%.

[0045] Compared with Comparative Example 2, the oxidation film prepared in Example 3 has no change in composition, but the film layer is more dense, and the corrosion resistance is increased by 3 times, from 350 Hv0.05 Increased to 380Hv 0.05 Power consumption has been reduced by 50% compared to the previous version.

[0046] 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 them in the above electrolyte, and carry out micro-arc oxidation process to finally obtain a dense oxide film layer.

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, Step (3) is as follows: 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%; 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 simultaneously or only one of them is reduced.

6. The method for preparing a low-cost, high-corrosion-resistant micro-arc oxidation film on magnesium alloy according to claim 5, characterized in that, 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.

7. 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.

8. 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 1-7, 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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