A method for preparing a magnesium alloy surface micro-arc oxidation film layer based on segmentation frequency

Abstract

The application discloses a preparation method of a magnesium alloy surface micro-arc oxidation film layer based on segmented frequency, and adopts the segmented frequency method to perform micro-arc oxidation treatment on the magnesium alloy, and the treatment time is divided into three stages of an initial stage, a middle stage and a later stage; the initial stage is 50-150 Hz of pulse frequency, the middle stage is 500-1000 Hz of pulse frequency, and the later stage is 2000-5000 Hz of pulse frequency. Through the three different stages of the initial stage, the middle stage and the later stage of the micro-arc oxidation treatment, lower frequency pulses, medium frequency pulses and higher frequency pulses are respectively adopted, so that the impact of the larger peak current in the initial stage on the power supply equipment elements is reduced, meanwhile, the obtained film layer has good corrosion resistance, and the problem that high performance and low power supply equipment requirements cannot be met during the preparation of the magnesium alloy micro-arc oxidation film layer is effectively solved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of magnesium alloy surface treatment, and particularly relates to a preparation method of a magnesium alloy surface micro-arc oxidation film layer based on segmented frequency. BACKGROUND

[0002] Compared with other metal materials, magnesium alloy has some advantages, such as better heat dissipation, fatigue resistance, easy cutting and electromagnetic shielding, and thus has a good application prospect. However, since magnesium is relatively active, magnesium and magnesium alloy are easily corroded in use, and thus improving the corrosion resistance thereof becomes the most urgent requirement for promoting large-scale application of magnesium alloy. Compared with other surface treatment technologies, micro-arc oxidation technology is a relatively new surface treatment technology and is one of the most effective treatment methods for improving the corrosion resistance of magnesium alloy.

[0003] Micro-arc oxidation is developed from traditional anodic oxidation technology. Micro-arc oxidation converts the anode reaction from the Faraday oxidation reaction zone to the high-voltage discharge zone by using a high voltage far exceeding the working range of anodic oxidation, so that a large number of micro-area breakdown discharges occur on the surface of magnesium alloy, and thus substances in the electrolyte react with magnesium alloy, thereby generating a layer of ceramic oxide film on the surface of magnesium alloy. During the micro-arc oxidation process, the thickness of the film layer continuously increases with the continuous micro-area breakdown discharges on the surface of magnesium alloy. The power supply equipment used in micro-arc oxidation has various types such as direct current, alternating current and pulse, and among them, the pulse power supply has become the mainstream because the film layer formed by the pulse power supply has good surface quality and performance.

[0004] Patent CN104894628A discloses a method for preparing a magnesium alloy micro-arc oxidation ceramic layer by stage voltage boosting. The magnesium alloy sample is used as an anode, and a stainless steel tank is used as a cathode. Stage 1 has an electrolyte temperature of 35-40℃, a voltage of 260-290V, a working frequency of 500-700Hz and a duty cycle of 20-40%. Stage 2 has a voltage of 300-330V. Stage 3 has a voltage of 340-380V. The method improves the defect of insufficient discharge breakdown capacity in the later stage of micro-arc oxidation under a single voltage mode, and improves the corrosion resistance of the film layer. However, in the micro-arc oxidation process, constant voltage and variable voltage are two processing modes of micro-arc oxidation. Regardless of which mode is used, the voltage in the later stage of micro-arc oxidation must be kept at a high level to maintain the continuous occurrence of discharge breakdown and the continuous growth of the film layer. This is a characteristic of the micro-arc oxidation technology, and thus changing the voltage (i.e. continuously increasing the voltage) under the variable voltage mode is a natural thing.

[0005] It has been shown that the pulse frequency has a great influence on the growth and quality of the film layer in pulse mode, and also has a great influence on the power supply equipment. Generally speaking, the surface quality of the film layer formed under high frequency pulse is better, but the growth rate is relatively small. At the initial stage of micro-arc oxidation film, the thin film layer makes the average load current larger. The low capacitance caused by high frequency meets the larger average load current, which will cause a large charging and discharging transient current (i.e. peak current), thereby impacting the power supply equipment components, and even causing serious damage. A lower pulse frequency can greatly reduce the impact of the peak current on the power supply equipment components, but the quality of the film layer is often reduced. Therefore, the current single frequency processing method cannot balance the high-performance film layer and the low power supply equipment impact, which seriously hinders the large-scale application of magnesium alloy to some extent. SUMMARY

[0006] In view of the above prior art, the purpose of the present application is to provide a preparation method of magnesium alloy surface micro-arc oxidation film layer based on segmented frequency.

[0007] To achieve the above purpose, the technical scheme adopted by the present application is as follows:

[0008] The present application provides a preparation method of magnesium alloy surface micro-arc oxidation film layer based on segmented frequency, comprising the following steps:

[0009] (1) surface pretreatment of magnesium alloy;

[0010] (2) connecting the magnesium alloy pretreated in step (1) with a fine aluminum rod, then immersing the magnesium alloy in an electrolyte, and connecting the other end of the fine aluminum rod with the positive electrode of a power supply;

[0011] (3) using the segmented frequency method, micro-arc oxidation treatment is performed on the magnesium alloy in step (2), which is divided into three stages of initial stage, middle stage and late stage according to the treatment time:

[0012] Initial stage: pulse frequency 50-150Hz, pulse voltage 250-450V, duty cycle 10-50%, treatment time 1-5 minutes;

[0013] Middle stage: pulse frequency 500-1000Hz, pulse voltage 250-450V, duty cycle 10-50%, treatment time 2-30 minutes;

[0014] Late stage: pulse frequency 2000-5000Hz, pulse voltage 250-450V, duty cycle 10-50%, treatment time 2-60 minutes.

[0015] Preferably, the surface pre-treatment step in step (1) is: polishing the magnesium alloy with 150# and 400# silicon carbide water sandpaper, cleaning for 1-5 minutes, and blowing dry.

[0016] Preferably, the electrolyte in step (2) is one of a phosphate system, a silicate system, and an aluminate system.

[0017] More preferably, the electrolyte in step (2) comprises the following components: 10-15 g / L sodium silicate, 10-15 g / L potassium fluoride, 2-5 g / L sodium hydroxide, and 5-10 mL / L glycerol.

[0018] Preferably, the pulse power mode of the micro-arc oxidation treatment in step (3) is one of a unipolar pulse mode and a bipolar pulse mode.

[0019] More preferably, the pulse power mode of the micro-arc oxidation treatment in step (3) is a unipolar pulse mode.

[0020] Advantages of the present application:

[0021] The present application can significantly weaken the impact of large peak current on power supply equipment elements by using low-frequency pulses in the initial stage of micro-arc oxidation treatment, and also enables a larger area of magnesium alloy sample to be treated. The reason is that the film layer generated in the initial stage of micro-arc oxidation is very thin and has very small resistance, and the average current load is large, while a lower frequency increases the capacitive reactance of the load, thereby significantly reducing the peak current.

[0022] The present application enables the film layer to grow at a relatively fast speed while also having good quality by using medium-frequency pulses in the middle stage of micro-arc oxidation treatment. Compared with high frequency, the working time of a single pulse is longer at low frequency, and the substances participating in film formation in the electrolyte have sufficient time to migrate to the magnesium alloy surface to participate in the film formation reaction, and the film formation efficiency is higher. However, at the same time, the long time of breakdown state is prone to cause continuous breakdown of some micro-regions on the sample surface, resulting in large micro-cracks in the film layer, and even defects such as ablation pits and large amounts of ablated substances, thereby seriously damaging the quality of the film layer. The use of medium frequency can well balance the growth rate and quality of the film layer.

[0023] The present application can further modify and improve the surface of the film layer by using high-frequency pulses in the later stage of micro-arc oxidation treatment, so that the obtained film layer has more uniform pores and no obvious micro-cracks, and the surface quality of the film layer is better.

[0024] The micro-arc oxidation film layer prepared on the surface of the magnesium alloy has a high growth rate and good surface quality in the growth process by stages, and under the combined influence of the above factors, the film layer has very good corrosion resistance. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 Waveform diagram of load voltage and current at the initial stage of magnesium alloy micro-arc oxidation process in embodiment 1 and comparative example 1 of the present application;

[0026] Figure 2 Scanning electron microscope photos of magnesium alloy micro-arc oxidation film layers in embodiment 2 and comparative example 2 of the present application;

[0027] Figure 3 Polarization curve comparison diagram of the film layers obtained in embodiment 3 and comparative example 3, comparative example 4 and comparative example 5 of the present application. DETAILED DESCRIPTION

[0028] It should be noted that the following detailed description is exemplary and is intended to provide further explanation of the present application. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs.

[0029] As described in the background, the high film layer performance and the low power supply equipment requirements cannot be met when the magnesium alloy micro-arc oxidation film layer is prepared. The present application not only greatly improves the corrosion resistance of the film layer, but also significantly reduces the impact of the pulse peak current on the power supply equipment, thereby improving the service life of the equipment.

[0030] Changing the frequency during the micro-arc oxidation process is not only a process selection problem, but also relates to whether the power supply equipment has this function. The power supply equipment used in the present application is independently developed and has this function.

[0031] Based on this, the present application provides a preparation method of magnesium alloy surface micro-arc oxidation film layer based on segmented frequency, which comprises the following steps:

[0032] (1) The magnesium alloy sample is pretreated on the surface; it is polished with 150# and 400# silicon carbide water sandpaper in turn, washed for 1-5 minutes and dried.

[0033] (2) The pretreated magnesium alloy sample in step (1) is connected with a thin aluminum rod, and then the magnesium alloy sample is immersed in an electrolyte, and the other end of the thin aluminum rod is connected with the positive electrode of a power supply; the electrolyte is one of a phosphate system, a silicate system and an aluminate system, and preferably the electrolyte comprises the following components: 10-15 g / L of sodium silicate, 10-15 g / L of potassium fluoride, 2-5 g / L of sodium hydroxide and 5-10 mL / L of glycerol.

[0034] (3) The magnesium alloy sample in step (2) is subjected to micro-arc oxidation treatment by using a segmented frequency method, and the treatment time is divided into three stages: an initial stage, an intermediate stage and a later stage; the pulse power supply mode of the micro-arc oxidation treatment is one of a unipolar pulse mode and a bipolar pulse mode, and is preferably a unipolar pulse mode.

[0035] Initial stage: pulse frequency 50-150Hz, pulse voltage 250-450V, duty cycle 10-50%, processing time 1-5 minutes;

[0036] Middle stage: pulse frequency 500-1000Hz, pulse voltage 250-450V, duty cycle 10-50%, processing time 2-30 minutes;

[0037] Late stage: pulse frequency 2000-5000Hz, pulse voltage 3250-450V, duty cycle 10-50%, processing time 2-60 minutes.

[0038] In order for those skilled in the art to more clearly understand the technical solutions of the present application, the technical solutions of the present application will be described in detail below in conjunction with specific embodiments.

[0039] The test materials used in the embodiments of the present application are all conventional test materials in the art and can be purchased through commercial channels.

[0040] Example 1

[0041] A preparation method of a magnesium alloy surface micro-arc oxidation film layer based on segmented frequency, specifically as follows:

[0042] (1) The AZ91D magnesium alloy sample was subjected to certain surface pretreatment, and was polished with 150# and 400# silicon carbide abrasive papers in turn, and then was placed in an ultrasonic cleaner for cleaning for 2 minutes, and was blown dry after being taken out.

[0043] (2) The magnesium alloy sample after pretreatment in step (1) was connected with a fine aluminum rod, and then the magnesium alloy sample was completely immersed in an electrolyte composed of 10g / L sodium silicate, 10g / L potassium fluoride, 3g / L sodium hydroxide and 5mL / L glycerol, and the other end of the fine aluminum rod was connected with the positive electrode of a power supply.

[0044] (3) The magnesium alloy sample in step (2) was subjected to micro-arc oxidation treatment by using a unipolar pulse mode, and the process parameters were as follows:

[0045] Initial stage: pulse frequency 150Hz, pulse voltage 390V, duty cycle 20%, processing time 2 minutes;

[0046] Middle stage: pulse frequency 700Hz, pulse voltage 390V, duty cycle 20%, processing time 3 minutes;

[0047] Late stage: pulse frequency 2000Hz, pulse voltage 390V, duty cycle 20%, processing time 5 minutes.

[0048] Example 2

[0049] A preparation method of magnesium alloy surface micro-arc oxidation film layer based on segmented frequency, specifically as follows:

[0050] (1) The AZ91D magnesium alloy sample was pretreated on the surface, polished with 150# and 400# silicon carbide water sandpaper in turn, then placed in an ultrasonic cleaner for cleaning for 2 minutes, and dried with a hair dryer after taking out.

[0051] (2) The magnesium alloy sample after pretreatment in step (1) was connected with a fine aluminum rod, then the magnesium alloy sample was completely immersed in an electrolyte composed of 10 g / L sodium silicate, 10 g / L potassium fluoride, 3 g / L sodium hydroxide and 5 mL / L glycerol, and the other end of the fine aluminum rod was connected with the positive electrode of a power supply.

[0052] (3) The magnesium alloy sample in step (2) was treated by micro-arc oxidation in a unipolar pulse mode, and the process parameters were as follows:

[0053] Initial stage: pulse frequency 100 Hz, pulse voltage 350 V, duty cycle 20%, treatment time 2 minutes;

[0054] Middle stage: pulse frequency 800 Hz, pulse voltage 350 V, duty cycle 20%, treatment time 8 minutes;

[0055] Later stage: pulse frequency 3700 Hz, pulse voltage 350 V, duty cycle 20%, treatment time 5 minutes.

[0056] Example 3

[0057] A preparation method of magnesium alloy surface micro-arc oxidation film layer based on segmented frequency, specifically as follows:

[0058] (1) The AZ91D magnesium alloy sample was pretreated on the surface, polished with 150# and 400# silicon carbide water sandpaper in turn, then placed in an ultrasonic cleaner for cleaning for 2 minutes, and dried with a hair dryer after taking out.

[0059] (2) The magnesium alloy sample after pretreatment in step (1) was connected with a fine aluminum rod, then the magnesium alloy sample was completely immersed in an electrolyte composed of 10 g / L sodium silicate, 10 g / L potassium fluoride, 3 g / L sodium hydroxide and 5 mL / L glycerol, and the other end of the fine aluminum rod was connected with the positive electrode of a power supply.

[0060] (3) The magnesium alloy sample in step (2) was treated by micro-arc oxidation in a unipolar pulse mode, and the process parameters were as follows:

[0061] Initial stage: pulse frequency 60 Hz, pulse voltage 400 V, duty cycle 25%, treatment time 2 minutes;

[0062] Middle stage: pulse frequency 700 Hz, pulse voltage 400 V, duty cycle 25%, processing time 6 minutes;

[0063] Late stage: pulse frequency 3500 Hz, pulse voltage 400 V, duty cycle 25%, processing time 4 minutes.

[0064] Example 4

[0065] A magnesium alloy surface micro-arc oxidation film layer preparation method based on segmented frequency, the specific steps are as follows:

[0066] (1) The AZ91D magnesium alloy sample was pretreated on the surface, and was polished with 150# and 400# silicon carbide water sandpaper in turn, and then was placed in an ultrasonic cleaner for cleaning for 2 minutes, and was dried with a hair dryer after being taken out.

[0067] (2) The magnesium alloy sample pretreated in step (1) was connected with a fine aluminum rod, and then the sample was completely immersed in an electrolyte composed of 10 g / L sodium silicate, 10 g / L potassium fluoride, 3 g / L sodium hydroxide and 5 mL / L glycerol, and the other end of the fine aluminum rod was connected with the positive electrode of the power supply.

[0068] (3) The magnesium alloy sample in step (2) was treated by micro-arc oxidation by using a unipolar pulse mode, and the process parameters were as follows:

[0069] Initial stage: pulse frequency 80 Hz, pulse voltage 410 V, duty cycle 20%, processing time 3 minutes;

[0070] Middle stage: pulse frequency 900 Hz, pulse voltage 410 V, duty cycle 20%, processing time 9 minutes;

[0071] Late stage: pulse frequency 4000 Hz, pulse voltage 410 V, duty cycle 20%, processing time 4 minutes.

[0072] Example 5

[0073] A magnesium alloy surface micro-arc oxidation film layer preparation method based on segmented frequency, the specific steps are as follows:

[0074] (1) The AZ91D magnesium alloy sample was pretreated on the surface, and was polished with 150# and 400# silicon carbide water sandpaper in turn, and then was placed in an ultrasonic cleaner for cleaning for 2 minutes, and was dried with a hair dryer after being taken out.

[0075] (2) The magnesium alloy sample pretreated in step (1) was connected with a fine aluminum rod, and then the magnesium alloy sample was completely immersed in an electrolyte composed of 10 g / L sodium silicate, 10 g / L potassium fluoride, 3 g / L sodium hydroxide and 5 mL / L glycerol, and the other end of the fine aluminum rod was connected with the positive electrode of the power supply.

[0076] (3) The magnesium alloy sample in step (2) is subjected to micro-arc oxidation treatment in a unipolar pulse mode, and the process parameters are as follows:

[0077] Initial stage: pulse frequency 70 Hz, pulse voltage 380 V, duty cycle 15%, treatment time 3 minutes;

[0078] Middle stage: pulse frequency 600 Hz, pulse voltage 380 V, duty cycle 15%, treatment time 7 minutes;

[0079] Later stage: pulse frequency 4100 Hz, pulse voltage 380 V, duty cycle 15%, treatment time 5 minutes.

[0080] Example 6

[0081] A preparation method of a magnesium alloy surface micro-arc oxidation film layer based on segmented frequency, specifically as follows:

[0082] (1) The AZ91D magnesium alloy sample is subjected to certain surface pretreatment, and is polished with 150# and 400# silicon carbide abrasive papers in turn, and then is placed in an ultrasonic cleaner for cleaning for 2 minutes, and is blown dry after being taken out.

[0083] (2) The magnesium alloy sample after pretreatment in step (1) is connected with a fine aluminum rod, and then the magnesium alloy sample is completely immersed in an electrolyte composed of 10 g / L sodium silicate, 10 g / L potassium fluoride, 3 g / L sodium hydroxide and 5 mL / L glycerol, and the other end of the fine aluminum rod is connected with the positive electrode of a power supply.

[0084] (3) The magnesium alloy sample in step (2) is subjected to micro-arc oxidation treatment in a unipolar pulse mode, and the process parameters are as follows:

[0085] Initial stage: pulse frequency 60 Hz, pulse voltage 420 V, duty cycle 25%, treatment time 3 minutes;

[0086] Middle stage: pulse frequency 750 Hz, pulse voltage 420 V, duty cycle 25%, treatment time 15 minutes;

[0087] Later stage: pulse frequency 4300 Hz, pulse voltage 420 V, duty cycle 25%, treatment time 10 minutes.

[0088] Comparative Example 1

[0089] A preparation method of a magnesium alloy surface micro-arc oxidation film layer based on segmented frequency, specifically as follows:

[0090] (1) The AZ91D magnesium alloy sample was subjected to certain surface pretreatment, and was polished with 150# and 400# silicon carbide water sandpaper in turn, and then was placed in an ultrasonic cleaner for cleaning for 2 minutes, and was dried by a hair dryer after being taken out.

[0091] (2) The magnesium alloy sample after the pretreatment in step (1) was connected with a fine aluminum rod, and then the magnesium alloy sample was completely immersed in an electrolyte composed of 10 g / L sodium silicate, 10 g / L potassium fluoride, 3 g / L sodium hydroxide and 5 mL / L glycerol, and the other end of the fine aluminum rod was connected with the positive pole of a power source.

[0092] (3) The magnesium alloy sample in step (2) was subjected to micro-arc oxidation treatment in a unipolar pulse mode, and the process parameters were as follows:

[0093] The pulse frequency was 2000 Hz, the pulse voltage was 390 V, the duty cycle was 20%, and the treatment time was 10 minutes.

[0094] Comparative Example 1 was compared with Example 1, and a single and high-frequency pulse was used.

[0095] Comparative Example 2

[0096] A preparation method of a magnesium alloy surface micro-arc oxidation film layer based on segmented frequency, and the specific steps are as follows:

[0097] (1) The AZ91D magnesium alloy sample was subjected to certain surface pretreatment, and was polished with 150# and 400# silicon carbide water sandpaper in turn, and then was placed in an ultrasonic cleaner for cleaning for 2 minutes, and was dried by a hair dryer after being taken out.

[0098] (2) The magnesium alloy sample after the pretreatment in step (1) was connected with a fine aluminum rod, and then the magnesium alloy sample was completely immersed in an electrolyte composed of 10 g / L sodium silicate, 10 g / L potassium fluoride, 3 g / L sodium hydroxide and 5 mL / L glycerol, and the other end of the fine aluminum rod was connected with the positive pole of a power source.

[0099] (3) The magnesium alloy sample in step (2) was subjected to micro-arc oxidation treatment in a unipolar pulse mode, and the process parameters were as follows:

[0100] The pulse frequency was 100 Hz, the pulse voltage was 350 V, the duty cycle was 20%, and the treatment time was 15 minutes.

[0101] Comparative Example 2 was compared with Example 2, and a single and low-frequency pulse was used.

[0102] Comparative Example 3

[0103] A preparation method of a magnesium alloy surface micro-arc oxidation film layer based on segmented frequency, and the specific steps are as follows:

[0104] (1) The AZ91D magnesium alloy sample was subjected to certain surface pretreatment, and was polished with 150# and 400# silicon carbide water sandpaper in turn, and then was placed in an ultrasonic cleaner for cleaning for 2 minutes, and was dried by a hair dryer after being taken out.

[0105] (2) The magnesium alloy sample after the pretreatment in step (1) was connected with a fine aluminum rod, and then the magnesium alloy sample was completely immersed in an electrolyte composed of 10 g / L sodium silicate, 10 g / L potassium fluoride, 3 g / L sodium hydroxide and 5 mL / L glycerol, and the other end of the fine aluminum rod was connected with the positive pole of a power source.

[0106] (3) The magnesium alloy sample in step (2) was subjected to micro-arc oxidation treatment in a unipolar pulse mode, and the process parameters were as follows:

[0107] The pulse frequency was 60 Hz, the pulse voltage was 400 V, the duty cycle was 25%, and the treatment time was 12 minutes.

[0108] Comparative Example 3 was compared with Example 3, and a single and low-frequency pulse was used.

[0109] Comparative Example 4

[0110] A preparation method of a magnesium alloy surface micro-arc oxidation film layer based on segmented frequency, and the specific steps are as follows:

[0111] (1) The AZ91D magnesium alloy sample was subjected to certain surface pretreatment, and was polished with 150# and 400# silicon carbide water sandpaper in turn, and then was placed in an ultrasonic cleaner for cleaning for 2 minutes, and was dried by a hair dryer after being taken out.

[0112] (2) The magnesium alloy sample after the pretreatment in step (1) was connected with a fine aluminum rod, and then the magnesium alloy sample was completely immersed in an electrolyte composed of 10 g / L sodium silicate, 10 g / L potassium fluoride, 3 g / L sodium hydroxide and 5 mL / L glycerol, and the other end of the fine aluminum rod was connected with the positive pole of a power source.

[0113] (3) The magnesium alloy sample in step (2) was subjected to micro-arc oxidation treatment in a unipolar pulse mode, and the process parameters were as follows:

[0114] The pulse frequency was 700 Hz, the pulse voltage was 400 V, the duty cycle was 25%, and the treatment time was 12 minutes.

[0115] Comparative Example 4 was compared with Example 3, and a single and medium-frequency pulse was used.

[0116] Comparative Example 5

[0117] A preparation method of a magnesium alloy surface micro-arc oxidation film layer based on segmented frequency, and the specific steps are as follows:

[0118] (1) The AZ91D magnesium alloy sample was pretreated on the surface, polished with 150# and 400# silicon carbide sandpaper in turn, and then placed in an ultrasonic cleaner for cleaning for 2 minutes. After being taken out, the sample was dried with a hair dryer.

[0119] (2) The pretreated magnesium alloy sample in step (1) was connected with a fine aluminum rod, and then the magnesium alloy sample was immersed in an electrolyte composed of 10 g / L sodium silicate, 10 g / L potassium fluoride, 3 g / L sodium hydroxide and 5 mL / L glycerol. The other end of the fine aluminum rod was connected with the positive pole of a power supply.

[0120] (3) The magnesium alloy sample in step (2) was subjected to micro-arc oxidation treatment in a unipolar pulse mode, and the process parameters were as follows:

[0121] pulse frequency 3500 Hz, pulse voltage 400 V, duty cycle 25%, and treatment time 12 minutes;

[0122] Compared with Example 3, a single and high-frequency pulse was used in Comparative Example 5.

[0123] The waveforms of the initial load voltage and current in the micro-arc oxidation process of Example 1 and Comparative Example 1 were collected by an oscilloscope, and the results are shown in Figure 1 . Figure 1 In the figure, a is the voltage and current waveform diagram at the initial stage of the micro-arc oxidation of Comparative Example 1, and b is the voltage and current waveform diagram at the initial stage of the micro-arc oxidation of Example 1. Compared with the waveform at a frequency of 2000 Hz in Comparative Example 1, the voltage and current waveforms in Example 1 are obviously different. The pulse voltage has a relatively sufficient charging and discharging time at a frequency of 150 Hz, so that the peak current is significantly reduced, and its value is only 1 / 16 of the maximum peak current in Comparative Example 1. It can be seen that the use of a lower pulse frequency at the initial stage of micro-arc oxidation can significantly reduce the peak current, thereby greatly reducing the impact on the power supply equipment elements.

[0124] The film layers prepared in Example 2 and Comparative Example 2 were characterized by a scanning electron microscope, and the results are shown in Figure 2 . Figure 2 In the figure, a is the film layer obtained in Comparative Example 2, and b is the film layer obtained in Example 2. When the frequency is always 100 Hz in Comparative Example 2, there are many micro-cracks on the surface of the obtained film layer. The existence of these micro-cracks greatly reduces the surface quality of the film layer. In comparison, the magnesium alloy micro-arc oxidation film layer prepared in Example 2 has no obvious micro-cracks on the surface, and the surface quality is better. It can be seen that the use of a higher pulse frequency at the later stage of micro-arc oxidation, especially at the later stage, can significantly reduce the generation of micro-cracks on the surface of the film layer and improve the surface quality of the film layer.

[0125] The film layers prepared by example 3, comparative example 3, comparative example 4, comparative example 5 and the magnesium alloy substrate are tested by using an electrochemical workstation to obtain electrochemical polarization curves, the test uses a three-electrode system, the test solution is a 3.5% by mass fraction NaCl aqueous solution, a saturated calomel electrode is selected as a reference electrode, and the scanning rate of the electrode potential is 1 mV / s, and the results are shown in Figure 3 FIG. 3, and the polarization curves in FIG. 3 are fitted and analyzed, and the results are shown in Table 1. Figure 3

[0126] Table 1

[0127] Type Corrosion current density / (A cm -2 ) polarization resistance / (Ω·cm 2 )]]> Magnesium alloy matrix 1.23 x 10 -5 ]] 2.55 x 10 3 ]] Example 3 4.45 x 10 -8 ]] 9.40 x 10 5 ]] Comparative Example 3 1.25 x 10 -7 ]]> 3.25 x 10 5 ]] Comparative Example 4 5.10 x 10 -7 ]]> 1.15 x 10 5 ]]> Comparative Example 5 1.05 x 10 -7 ]]> 3.77 x 10 5 ]]>

[0128] According to the corrosion-related theory, the smaller the corrosion current density and the greater the polarization resistance, the better the corrosion resistance of the material. As shown in Table 1, compared with the magnesium alloy substrate, the corrosion current density of the film layer obtained in example 3 is reduced by 3 orders of magnitude, which indicates that the film layer in example 3 has very excellent corrosion resistance; compared with the film layers obtained in comparative examples 3, 4 and 5, the corrosion current density of example 3 is significantly smaller than that of the three comparative examples, and the value is reduced by one order of magnitude.

[0129] In addition, as shown by the comparison of the polarization resistance, compared with the magnesium alloy substrate, the polarization resistance of example 3 is increased by 2 orders of magnitude, and compared with the film layers obtained in comparative examples 3, 4 and 5, the polarization resistance of example 3 is also significantly greater than that of the three comparative examples, even greater than the sum of the polarization resistances of the three comparative examples, i.e., 8.17*10 5 Ω·cm 2 , which fully indicates that the segmented frequency of low frequency, medium frequency and high frequency used in the present application is much better than the sum of the low frequency alone, the medium frequency alone and the high frequency alone, and therefore the segmented frequency of low frequency, medium frequency and high frequency used in the present application has a significant synergistic effect on improving the corrosion resistance of the micro-arc oxidation film layer.

[0130] In summary, the present application provides a preparation method of a magnesium alloy surface micro-arc oxidation film layer based on a segmented frequency. The prepared film layer has excellent corrosion resistance, significantly reduces the impact damage of the pulse peak current on the power supply equipment, and improves the service life of the equipment.

[0131] The above only describes the preferred embodiments of the present application and is not intended to limit the present application. Those skilled in the art can make various modifications and changes to the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the present application shall be included in the protection scope of the present application.​

Claims

1. A method for preparing a magnesium alloy surface micro-arc oxidation film layer based on segmented frequency, characterized in that, The method comprises the following steps: (1) surface pretreatment of the magnesium alloy; (2) connecting the pretreated magnesium alloy in step (1) with a thin aluminum rod, and then immersing the magnesium alloy in an electrolyte, with the other end of the thin aluminum rod connected to the positive pole of a power supply; (3) micro-arc oxidation treatment of the magnesium alloy in step (2) by using a segmented frequency method, which is divided into three stages of initial stage, middle stage and later stage according to treatment time: initial stage: pulse frequency 50-150 Hz, treatment time 1-5 minutes; middle stage: pulse frequency 500-1000 Hz, treatment time 2-30 minutes; later stage: pulse frequency 2000-5000 Hz, treatment time 2-60 minutes; During the micro-arc oxidation treatment, the pulse voltage and duty cycle are constant, the pulse voltage is 250-450 V, and the duty cycle is 10-50%.

2. The method according to claim 1, wherein the method is characterized by, The surface pretreatment step in step (1) is: polishing the magnesium alloy with 150# and 400# silicon carbide water sandpaper, cleaning for 1-5 minutes, and blowing dry.

3. The method according to claim 1, wherein the method is characterized by, The electrolyte in step (2) is one of a phosphate system, a silicate system and an aluminate system.

4. The method according to claim 3, wherein the method is characterized by, The electrolyte in step (2) comprises the following components: 10-15 g / L sodium silicate, 10-15 g / L potassium fluoride, 2-5 g / L sodium hydroxide and 5-10 mL / L glycerol.

5. The method according to claim 1, wherein the method is characterized by: The pulse power mode of the micro-arc oxidation treatment in step (3) is one of a unipolar pulse mode and a bipolar pulse mode.

6. The method according to claim 5, wherein the method is characterized by, The pulse power mode of the micro-arc oxidation treatment in step (3) is a unipolar pulse mode.

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