CN114892238A - Method for improving corrosion resistance of magnesium alloy micro-arc oxidation film layer by pretreatment process - Google Patents
Method for improving corrosion resistance of magnesium alloy micro-arc oxidation film layer by pretreatment process Download PDFInfo
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- 229910000861 Mg alloy Inorganic materials 0.000 title claims abstract description 85
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- 238000004140 cleaning Methods 0.000 claims abstract description 17
- 238000001035 drying Methods 0.000 claims abstract description 14
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- 238000005520 cutting process Methods 0.000 claims abstract description 8
- 238000002791 soaking Methods 0.000 claims abstract description 7
- 238000000227 grinding Methods 0.000 claims abstract description 3
- 239000003792 electrolyte Substances 0.000 claims description 38
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- WMFOQBRAJBCJND-UHFFFAOYSA-M Lithium hydroxide Chemical class [Li+].[OH-] WMFOQBRAJBCJND-UHFFFAOYSA-M 0.000 claims description 30
- KWYUFKZDYYNOTN-UHFFFAOYSA-M Potassium hydroxide Chemical compound [OH-].[K+] KWYUFKZDYYNOTN-UHFFFAOYSA-M 0.000 claims description 18
- 239000011777 magnesium Substances 0.000 claims description 15
- LFQSCWFLJHTTHZ-UHFFFAOYSA-N Ethanol Chemical compound CCO LFQSCWFLJHTTHZ-UHFFFAOYSA-N 0.000 claims description 13
- PUZPDOWCWNUUKD-UHFFFAOYSA-M sodium fluoride Chemical compound [F-].[Na+] PUZPDOWCWNUUKD-UHFFFAOYSA-M 0.000 claims description 12
- 239000008367 deionised water Substances 0.000 claims description 10
- 229910021641 deionized water Inorganic materials 0.000 claims description 10
- CSCPPACGZOOCGX-UHFFFAOYSA-N Acetone Chemical compound CC(C)=O CSCPPACGZOOCGX-UHFFFAOYSA-N 0.000 claims description 6
- 239000004115 Sodium Silicate Substances 0.000 claims description 6
- 229910052782 aluminium Inorganic materials 0.000 claims description 6
- 239000011775 sodium fluoride Substances 0.000 claims description 6
- 235000013024 sodium fluoride Nutrition 0.000 claims description 6
- NTHWMYGWWRZVTN-UHFFFAOYSA-N sodium silicate Chemical compound [Na+].[Na+].[O-][Si]([O-])=O NTHWMYGWWRZVTN-UHFFFAOYSA-N 0.000 claims description 6
- 229910052911 sodium silicate Inorganic materials 0.000 claims description 6
- 238000004506 ultrasonic cleaning Methods 0.000 claims description 6
- XAGFODPZIPBFFR-UHFFFAOYSA-N aluminium Chemical compound [Al] XAGFODPZIPBFFR-UHFFFAOYSA-N 0.000 claims description 4
- 238000012545 processing Methods 0.000 claims description 4
- 239000010935 stainless steel Substances 0.000 claims description 4
- 229910001220 stainless steel Inorganic materials 0.000 claims description 4
- 244000137852 Petrea volubilis Species 0.000 claims description 3
- 235000019441 ethanol Nutrition 0.000 claims description 3
- BPQQTUXANYXVAA-UHFFFAOYSA-N Orthosilicate Chemical compound [O-][Si]([O-])([O-])[O-] BPQQTUXANYXVAA-UHFFFAOYSA-N 0.000 claims description 2
- 239000010410 layer Substances 0.000 description 67
- FAPWRFPIFSIZLT-UHFFFAOYSA-M Sodium chloride Chemical compound [Na+].[Cl-] FAPWRFPIFSIZLT-UHFFFAOYSA-M 0.000 description 18
- UFHFLCQGNIYNRP-UHFFFAOYSA-N Hydrogen Chemical compound [H][H] UFHFLCQGNIYNRP-UHFFFAOYSA-N 0.000 description 14
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- 239000002105 nanoparticle Substances 0.000 description 2
- 229910052760 oxygen Inorganic materials 0.000 description 2
- 238000002203 pretreatment Methods 0.000 description 2
- 229910052761 rare earth metal Inorganic materials 0.000 description 2
- 150000002910 rare earth metals Chemical class 0.000 description 2
- 238000011160 research Methods 0.000 description 2
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- C—CHEMISTRY; METALLURGY
- C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
- C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
- C25D11/00—Electrolytic coating by surface reaction, i.e. forming conversion layers
- C25D11/02—Anodisation
- C25D11/30—Anodisation of magnesium or alloys based thereon
-
- C—CHEMISTRY; METALLURGY
- C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
- C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
- C25D11/00—Electrolytic coating by surface reaction, i.e. forming conversion layers
- C25D11/02—Anodisation
- C25D11/026—Anodisation with spark discharge
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Abstract
The invention discloses a method for improving corrosion resistance of a magnesium alloy micro-arc oxidation film layer by a pretreatment process, which comprises the steps of cutting a magnesium alloy into blocks, grinding and polishing the blocks, and then cleaning and drying the blocks to obtain a magnesium alloy sample; soaking a magnesium alloy sample in a saturated alkaline solution, carrying out water bath treatment, and then cleaning and drying to obtain a magnesium alloy sample after pretreatment; and (3) carrying out micro-arc oxidation treatment on the magnesium alloy sample after pretreatment, and then cleaning and drying to form a micro-arc oxidation film layer on the magnesium alloy. The method combines the pretreatment of the magnesium alloy by adopting the saturated alkaline solution and the micro-arc oxidation to prepare the film with excellent corrosion resistance, has important significance for improving the application range of the magnesium alloy, and has wide application prospect.
Description
Technical Field
The invention relates to the field of surface treatment, in particular to a method for improving corrosion resistance of a magnesium alloy micro-arc oxidation film layer by a pretreatment process.
Background
Magnesium alloy has become an indispensable material in the industrial fields of aerospace, automobiles, electronics and the like and the biomedical field, but the poor corrosion resistance of magnesium alloy is always a main factor for restricting the further popularization and application of the magnesium alloy. The surface treatment is one of the most economic and effective methods for improving the corrosion resistance of the magnesium alloy, and is a hot spot for researching the protection of the magnesium alloy at present.
Micro-arc oxidation (MAO) has the advantages of simple process, environmental protection, high film hardness, excellent corrosion resistance, capability of forming ceramic oxide films and the like, is widely concerned by a plurality of researchers, and is one of the most potential treatment technologies for magnesium alloy surfaces in recent years. However, in practical application, the single micro-arc oxidation film cannot meet the use requirement due to the porous characteristic, and the application of the micro-arc oxidation film is limited. Aiming at the problem, many scholars optimize the film layer, and the existing method for improving the micro-arc oxidation film mainly comprises the steps of adjusting operating parameters (such as working voltage, reaction time, frequency, duty ratio and the like), changing the electrolyte formula, performing post hole sealing treatment and the like.
Few people explore the effect of pretreating a micro-arc oxidation sample, and the current scholars convert the surface layer of a metal material into nano-particles by performing surface nano-treatment technologies such as ultrasonic rolling, ultrasonic shot blasting, ultrasonic cold forging and the like on the surface of the magnesium alloy, and use the nano-particles as a micro-arc oxidation pretreatment means for surface modification, so that the corrosion resistance of the micro-arc oxidation sample is improved. However, the surface nano-treatment technology has certain requirements on equipment and higher experimental cost. The researchers also adopt molybdate to prepare a molybdate chemical conversion film on the surface of the magnesium alloy, and then prepare the micro-arc oxidation film, and the inventors find that the corrosion resistance of the composite film is improved, but the long-term corrosion resistance is not further evaluated. The rare earth coating obtained by chemical conversion can effectively isolate magnesium alloy from a corrosion medium, the rare earth pretreatment process is also used for being combined with a micro-arc oxidation technology by people to research a series of performances of a composite film layer, and the corrosion resistance of the composite film layer is found to be improved, but the research is not carried out systematically.
Disclosure of Invention
The invention aims to provide a method for improving corrosion resistance of a magnesium alloy micro-arc oxidation film by a pretreatment process, and overcomes the defects in the prior art.
In order to achieve the purpose, the invention adopts the following technical scheme:
a method for improving corrosion resistance of a magnesium alloy micro-arc oxidation film layer by a pretreatment process comprises the following steps:
1) cutting the magnesium alloy into blocks, grinding and polishing, and then cleaning and drying to obtain a magnesium alloy sample;
2) soaking a magnesium alloy sample in a saturated alkaline solution, carrying out water bath treatment, and then cleaning and drying to obtain a magnesium alloy sample after pretreatment;
3) and (3) carrying out micro-arc oxidation treatment on the magnesium alloy sample after pretreatment, and then cleaning and drying to form a micro-arc oxidation film layer on the magnesium alloy.
Further, the magnesium alloy in the step 1) adopts AZ91D magnesium alloy;
the cutting of the magnesium alloy into blocks specifically comprises the following steps: the AZ91D magnesium alloy was cut into cylinders of size Φ 8x6mm with wire cutting.
Further, the polishing in the step 1) is specifically as follows: sequentially polishing with 150#, 400#, 1200#, 2000# water sand paper; the cleaning method specifically comprises the following steps: ultrasonic cleaning with acetone for 5min, and then ultrasonic cleaning with ethanol for 5 min.
Further, in the step 2), the saturated alkaline solution is a saturated LiOH solution, saturated Ca (OH) 2 Solutions or saturated Mg (OH) 2 And (3) solution.
Further, the water bath treatment in the step 2) is specifically as follows: carrying out thermostatic water bath for 24h in a water bath kettle at the temperature of 25 ℃.
Further, the electrolyte in the step 3) adopts single-component silicate alkaline electrolyte, specifically: sodium silicate, potassium hydroxide and sodium fluoride are respectively weighed at room temperature and sequentially dissolved in deionized water to obtain the electrolyte.
Furthermore, the electrolyte contains 8g/L of sodium silicate, 2g/L of potassium hydroxide and 0.5g/L of sodium fluoride.
Further, the micro-arc oxidation treatment specifically comprises: and (3) tightly connecting the magnesium alloy sample after pretreatment with the sheathed aluminum wire, fixing the magnesium alloy sample on a positive plate of a power supply, then placing the magnesium alloy sample and a stainless steel plate in a prepared electrolyte, and setting electrical parameters for micro-arc oxidation treatment.
Further, the electrical parameters are specifically: under the constant voltage mode, the positive voltage is 450V, the negative voltage is 30V, the frequency is 600kHz, the duty ratio is 10 percent, and the processing time is 15 min.
Further, the cleaning in the step 3) is specifically as follows: ultrasonic cleaning with deionized water and absolute ethyl alcohol respectively.
Compared with the prior art, the invention has the following beneficial technical effects:
1) the method combines the pretreatment process of the saturated alkaline solution with the micro-arc oxidation technology to prepare the micro-arc oxidation film layer, thereby providing a reference value for the subsequent pretreatment process.
2) The micro-arc oxidation film prepared by the invention has excellent corrosion resistance, and the self-corrosion current density of the film in the polarization test is reduced by one order of magnitude compared with that of the film without a pretreatment process and is reduced by three orders of magnitude compared with that of a magnesium alloy matrix. In the hydrogen evolution experiment process, a magnesium alloy sample is soaked in a NaCl solution with the mass fraction of 3.5% for 360 hours, and the hydrogen evolution amount per unit area of the sample after the pretreatment process is reduced by two orders of magnitude compared with the sample without the pretreatment process, so that the magnesium alloy sample shows excellent corrosion resistance.
3) Compared with the prior magnesium alloy pretreatment process, the method has the advantages that the influence of the pretreatment process on the appearance, the components and the corrosion resistance of the film is more systematically and comprehensively discussed.
Drawings
In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required in the embodiments will be briefly described below, it should be understood that the following drawings only illustrate some embodiments of the present invention and therefore should not be considered as limiting the scope, and those skilled in the art can also obtain other related drawings based on the drawings without inventive efforts.
FIG. 1 is a surface topography of various pre-treated saturated alkaline solutions prepared in accordance with examples, wherein (a) LiOH; (b) mg (OH) 2 ；(c)Ca(OH) 2 ；
FIG. 2 is a micro-arc oxidized surface topography after pre-treatment of a base electrolyte and different saturated alkaline solutions prepared in an example, wherein (a) the base electrolyte; (b) LiOH; (c) mg (OH) 2 ；(d)Ca(OH) 2 ；
FIG. 3 is a polarization curve of MAO membrane in 3.5% NaCl solution after treatment of the substrate prepared in the example, MAO and different saturated alkaline solutions;
FIG. 4 is the electrochemical impedance spectrum of the MAO membrane in 3.5% NaCl solution after treatment of the substrate, MAO and various saturated alkaline solutions prepared in the examples;
FIG. 5 shows the hydrogen evolution per unit area of MAO membrane in 3.5% NaCl solution after treatment of the substrate, MAO and various saturated alkaline solutions prepared in the examples;
FIG. 6 is a current density-time curve without pretreatment and treated with saturated LiOH solution, with the inset being an enlarged view from 0 to 50 s;
FIG. 7 is a graph showing the potential of a magnesium alloy substrate and a sample treated with a saturated LiOH solution in a base electrolyte.
Detailed Description
Embodiments of the invention are described in further detail below:
a method for improving corrosion resistance of a magnesium alloy micro-arc oxidation film layer by a pretreatment process comprises the following specific steps:
using linear cutting to cut AZ91D magnesium alloy into cylinders with the size of phi 8x6mm, and punching the side edge parts of the samples
And (3) tapping the blind holes, sequentially polishing the surface of the sample by using No. 150, No. 400, No. 1200 and No. 2000 water sand paper, ultrasonically cleaning for 5min by using acetone after mechanical polishing, ultrasonically cleaning for 5min by using ethanol, and drying for later use. Respectively preparing saturated LiOH and saturated Ca (OH) 2 Saturated Mg (OH) 2 500ml of each solution, placing a polished AZ91D magnesium alloy sample in the solution, carrying out thermostatic water bath in a water bath kettle at 25 ℃ for 24h, taking out the sample, washing with deionized water and drying for later use. Respectively weighing sodium silicate, potassium hydroxide and sodium fluoride at room temperature, sequentially dissolving in deionized water to obtain a basic electrolyte, and before micro-arc oxidation treatment, closely connecting a magnesium alloy sample subjected to pretreatment with a sheathed aluminum wire and fixing the magnesium alloy sample on a positive plate of a power supply. And placing the sample and the stainless steel plate in the prepared electrolyte, setting experiment parameters through a power supply display screen after the samples are completely connected, switching on positive and negative voltages, and clicking an operation key. The specific electrical parameters are: under the constant voltage mode, the positive voltage is 450V, the negative voltage is 30V, the frequency is 600kHz, the duty ratio is 10 percent, and the processing time is 15 min. Most preferablyAnd then ultrasonically cleaning the sample subjected to micro-arc oxidation treatment by using deionized water and absolute ethyl alcohol respectively, and then drying the sample in an oven for later use.
The technical solutions of the present invention are described below clearly and completely with reference to the following embodiments, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
Examples
Respectively preparing saturated LiOH and saturated Ca (OH) 2 Saturated Mg (OH) 2 500ml of each solution, placing a polished AZ91D magnesium alloy sample in the solution, carrying out thermostatic water bath in a water bath kettle at 25 ℃ for 24h, taking out the sample, washing with deionized water and drying for later use. The surface appearance and the cross-section appearance of the magnesium alloy film layer are characterized by using a scanning electron microscope with the model number of SU6600, and the element composition and content in the magnesium alloy film layer are detected by using an EDS analysis module equipped with the scanning electron microscope, wherein the acceleration voltage is 15kV, and the beam size is 32 muA.
FIG. 1 shows the surface morphology of different saturated alkaline solution pretreatments. Table 1 results of the energy spectrum (at%) of the membrane layer pretreated with different saturated alkaline solutions.
TABLE 1 energy spectrum results (at%) of pre-treated membrane layers of different saturated alkaline solutions
As can be seen from FIG. 1, the pretreated surface of the saturated LiOH solution is uniformly distributed with a large number of highly dense thin layers with the length of about 500nm, which are grown in a staggered manner on the surface, and saturated Mg (OH) 2 The surface appearance after the solution pretreatment is similar to that after the saturated LiOH solution pretreatment, the length of a highly compact thin sheet layer uniformly distributed on the surface is about 200-300 nm, and the surface appearance is more compact than that of the LiOH. Saturated Ca (OH) 2 The surface appearance after the pretreatment of the solution is distributed with a large number of lengthsThe flaky substances with the size of about 100nm are staggered on the surface, the shapes of the flaky substances are not as uniform and compact as those of the flaky substances, and in addition, white spherical particles with the size of about 200-500 nm are generated on the surface. LiOH, Mg (OH) can be seen in Table 1 2 And Ca (OH) 2 The pretreatment film layers all contain Mg, O and Al elements, LiOH and Mg (OH) 2 The element contents are not very different, compared with Ca (OH) 2 The pretreatment film layer also contains Ca element, which proves that Ca is 2+ Participates in the reaction. Content of Mg element in combination with LiOH and Mg (OH) 2 The pre-treated film layer is reduced, the Al element content is increased, the atomic percentage is 26.45%, and the Al element content is probably related to white spherical particles generated in the film layer.
Respectively weighing sodium silicate, potassium hydroxide and sodium fluoride at room temperature, sequentially dissolving in deionized water to obtain a basic electrolyte, and before micro-arc oxidation treatment, closely connecting a magnesium alloy sample subjected to pretreatment with a sheathed aluminum wire and fixing the magnesium alloy sample on a positive plate of a power supply. And placing the sample and the stainless steel plate in the prepared electrolyte, setting experiment parameters through a power supply display screen after the samples are completely connected, switching on positive and negative voltages, and clicking an operation key. The specific electrical parameters are: under the constant voltage mode, the positive voltage is 450V, the negative voltage is 30V, the frequency is 600kHz, the duty ratio is 10 percent, and the processing time is 15 min. Finally, ultrasonically cleaning the sample subjected to micro-arc oxidation treatment by using deionized water and absolute ethyl alcohol respectively, and then drying the sample in an oven for later use. The surface morphology and the cross-sectional morphology of the magnesium alloy film layer are characterized by using a scanning electron microscope with the model number of SU6600, and the element composition and the content in the magnesium alloy film layer are detected by using an EDS analysis module equipped with the scanning electron microscope, wherein the acceleration voltage is 15kV, and the beam size is 32 muA.
FIG. 2 shows the surface morphology of the micro-arc oxidation film after pretreatment of the basic electrolyte and different saturated alkaline solutions, and Table 2 shows the energy spectrum results of the micro-arc oxidation film after pretreatment of the basic electrolyte and different saturated alkaline solutions.
TABLE 2 energy spectrum results of micro-arc oxidation film layer after pretreatment of basic electrolyte and different saturated alkaline solutions
As can be seen from FIG. 2, the micro-arc oxide film layer pretreated by the three saturated alkaline solutions has island-shaped protrusions and holes which are mutually interwoven and microcracks which are the same as the micro-arc oxide film layer treated by the basic electrolyte. The micro-arc oxidation film layer after the LiOH pretreatment has more uniform holes compared with the film layer after the base electrolyte treatment; mg (OH) 2 The pre-treated micro-arc oxidation film layer has fewer holes, more island-shaped protrusions and a smoother surface compared with the micro-arc oxidation film layer treated by the basic electrolyte; ca (OH) 2 The aperture of the micro-arc oxidation film layer after the pretreatment is enlarged, the island-shaped protrusions are increased, the surface of the micro-arc oxidation film layer is rough, and obvious micro cracks appear, which is probably caused by stress formed in the film layer by a large amount of heat released by the reaction in the process of micro-arc oxidation. As can be seen from Table 2, the base electrolyte and LiOH, Mg (OH) 2 、Ca(OH) 2 The pretreated micro-arc oxidation film layer consists of four elements including Mg, O, Al and Si, and compared with the micro-arc oxidation film layer of the basic electrolyte, the pretreated micro-arc oxidation film layer has the advantages that the content of the Mg element is reduced, the content of the O element is increased, the content of the Si element is reduced, and the fact that the film layer generated by pretreatment influences the growth of the subsequent micro-arc oxidation film layer can be proved to result in the increase of the content of the oxide of the micro-arc oxidation film layer. It is noteworthy that although Ca (OH) 2 The Al content in the film layer after pretreatment reaches 26.45 percent, but the Al content in the micro-arc oxidation film layer is not much different from that of the micro-arc oxidation film layer of the basic electrolyte, and Mg (OH) 2 The content of Al element in the micro-arc oxidation film layer after pretreatment is increased to 8.54 percent.
The electrochemical performance of the micro-arc oxidation film layer is tested by adopting a Princeton Versa STAT-3F type electrochemical workstation, and a NaCl solution with the mass fraction of 3.5 wt% is used as a corrosion medium. A standard three-electrode corrosion test system is adopted, wherein a working electrode is a magnesium alloy sample, an auxiliary electrode is a Pt sheet, a reference electrode is a saturated calomel electrode, and specific potential values in subsequent experiments are all referred to the saturated calomel electrode. Not testing the working electrodePolishing the surface of the substrate to expose the substrate, connecting the substrate with a copper wire by tin soldering, sealing the joint by acrylic resin AB glue, and reserving 1cm 2 The area to be measured. Before detection, a sample is immersed into a 3.5 wt% NaCl aqueous solution to test after the open circuit potential OCP is stable. All tests were performed at room temperature.
1. Electrochemical Impedance Spectroscopy (EIS) testing: the test frequency range is 100 kHz-0.01 Hz, and the amplitude of the applied disturbance voltage is 10 mV. And after the test is finished, adopting Zview analysis software to perform equivalent circuit fitting on the impedance data, and selecting a proper equivalent circuit diagram according to the physical structure of the micro-arc oxidation film layer.
2. And (3) testing a potentiodynamic polarization curve: the scanning potential (relative open circuit potential OCP) is-0.25V-0.5V, and the scanning speed is 1 mV/s. The self-corrosion potential (Ecorr) and the self-corrosion current density (icorr) of the sample were obtained by Tafel linear extrapolation, and the polarization resistance was calculated according to the formula (1).
In the formula: r p Polarization resistance (k Ω cm 2); beta is a a -anode Tafel slope; beta is a c -cathode Tafel slope; i.e. i corr -corrosion current density (. mu.A. cm-2).
FIG. 3 is a polarization curve of MAO membrane in 3.5% NaCl solution after treatment of substrate, MAO and different saturated alkaline solutions, and the self-corrosion potential, corrosion current density and polarization resistance obtained by fitting the polarization curve are shown in Table 3.
TABLE 3 self-corrosion potential, corrosion current density and polarization resistance obtained by polarization curve fitting
From fig. 3, it can be seen that the self-corrosion potentials of the micro-arc oxidation film layers treated by different saturated alkaline solutions are all increased, and the potentials are shifted by about 200mV compared with the potential of the micro-arc oxidation film layer treated by the basic electrolyte, which indicates that the corrosion tendency is small, and the self-corrosion potentials of the film layers after the pretreatment of LiOH, Mg (OH)2 and ca (OH)2 are not greatly different. As can be seen from the graph, the corrosion current density of the micro-arc oxidation film layer treated by different saturated alkaline solutions is reduced by three orders of magnitude compared with that of the substrate, and is reduced by one order of magnitude compared with that of the micro-arc oxidation film layer treated by the basic electrolyte; the self-corrosion potential is shifted by 50-70 mV positively compared with the matrix; the polarization resistance and the micro-arc oxidation film layer processed by the basic electrolyte are improved by one order of magnitude.
FIG. 4 shows the electrochemical impedance spectra of MAO membrane in 3.5% NaCl solution after treatment of substrate, MAO and different saturated alkaline solutions. As can be seen in the figure, the capacitive arc resistance diameter of the micro-arc oxidation film layer treated by the saturated alkaline solution is far larger than that of the micro-arc oxidation film layer treated by the basic electrolyte, and the diameter is increased by at least one order of magnitude. The alternating-current impedance spectrum of the MAO film layer after pretreatment of different saturated alkaline solutions is mainly generated by two sections of overlapped capacitive reactance arcs, and the disappearance of the inductive reactance arcs in a low-frequency area proves that the corrosion resistance of the magnesium alloy matrix is improved by the treatment of the saturated alkaline solutions.
And (3) characterizing the corrosion rate of the AZ91D magnesium alloy substrate and the micro-arc oxidation film layer thereof in a 3.5% NaCI solution by using a hydrogen evolution method. In an aqueous solution environment of 3.5 wt.% NaCl, a sample is placed under an inverted funnel, the surface to be measured is kept in complete contact with the solution, gas generated by reaction can be discharged out of the solution in the burette with good air tightness, and the change of the concave scale of the liquid surface in the burette represents the amount of hydrogen evolution in a certain time.
V H ＝V 0 /S (2)
In the formula V H Hydrogen evolution corrosion rate (ml/cm) 2 )；V 0 -gas evolution volume (ml); s-area of test specimen (cm) 2 )。
FIG. 5 shows the hydrogen evolution per unit area of the micro-arc oxide film layer in 3.5% NaCI solution after the AZ91D magnesium alloy substrate, the basic electrolyte and different saturated alkaline solutions are treated. As can be seen from the figure, the hydrogen evolution amount of the AZ91D magnesium alloy matrix is stable all the time during the soaking processThe hydrogen evolution quantity per unit area is the largest and is obviously higher than that of other groups of samples, and the average hydrogen evolution rate reaches 9.7 multiplied by 10 -2 mL/(cm 2 H). At the initial stage of soaking, the hydrogen evolution amount of the micro-arc oxidation film layer treated by the basic electrolyte and the micro-arc oxidation film layer pretreated by different saturated alkaline solutions is basically not existed, so that the micro-arc oxidation film layer has a certain protection effect on magnesium alloy, the protection effects of the four film layers are obviously higher than that of a magnesium alloy matrix along with the increase of the soaking time, after the soaking time is about 210h, the hydrogen evolution amount per unit area of the micro-arc oxidation film layer treated by the basic electrolyte is obviously increased, which indicates that the film layer begins to be damaged, and the hydrogen evolution amount per unit area is 1.3 multiplied by 10 -2 mL/(cm 2 H); the hydrogen evolution amount per unit area of the micro-arc oxidation film layer treated by the saturated alkaline solution is slowly increased and is obviously lower than that of the MAO film layer treated by the basic electrolyte, and the pretreatment of the saturated alkaline solution has obvious effect on the corrosion resistance of the subsequent micro-arc oxidation film layer.
As can be seen from FIG. 6, the process of starting the arc of the LiOH-MAO film is more stable within the first 50s, there are no two prominent small peaks without pretreatment, and the current density is lower, because a layer of film grows on the surface of the magnesium alloy after the treatment of the saturated LiOH solution, which reduces the conductivity of the sample and makes the micro-arc oxidation arc starting more difficult, thereby reducing the current density.
And (3) placing the sample which is only pretreated by the saturated LiOH solution in an electrochemical workstation for testing, wherein the solution is a basic electrolyte for preparing the micro-arc oxidation film layer. The potential change of the sample placed in the cell was observed and compared with the sample without pretreatment.
As can be seen from FIG. 7, the potential of the magnesium alloy matrix in the basic electrolyte is-1.613V, while the potential of the sample treated by the saturated LiOH solution in the basic electrolyte is-1.543V, which is 70mV higher than that of the sample without pretreatment, thus indicating that the sample with a film layer grown after the pretreatment of the saturated LiOH solution is thermodynamically more stable, and simultaneously, the difference in arcing can be proved to be more difficult due to the fact that the micro-arc oxidation film layer grows by applying the voltage.
The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents, improvements and the like that fall within the spirit and principle of the present invention are intended to be included therein.
Claims (10)
1. A method for improving corrosion resistance of a magnesium alloy micro-arc oxidation film layer by a pretreatment process is characterized by comprising the following steps:
1) cutting the magnesium alloy into blocks, grinding and polishing, and then cleaning and drying to obtain a magnesium alloy sample;
2) soaking a magnesium alloy sample in a saturated alkaline solution, carrying out water bath treatment, and then cleaning and drying to obtain a magnesium alloy sample after pretreatment;
3) and (3) carrying out micro-arc oxidation treatment on the magnesium alloy sample after pretreatment, and then cleaning and drying to form a micro-arc oxidation film layer on the magnesium alloy.
2. The method for improving the corrosion resistance of the magnesium alloy micro-arc oxidation film layer by the pretreatment process according to claim 1, wherein the magnesium alloy in the step 1) is AZ91D magnesium alloy;
the cutting of the magnesium alloy into blocks specifically comprises the following steps: the AZ91D magnesium alloy was cut into cylinders of size Φ 8x6mm with wire cutting.
3. The method for improving the corrosion resistance of the magnesium alloy micro-arc oxidation film layer by the pretreatment process according to claim 1, wherein the polishing in the step 1) is specifically as follows: sequentially polishing with 150#, 400#, 1200#, 2000# water sand paper; the cleaning method specifically comprises the following steps: ultrasonic cleaning with acetone for 5min, and then ultrasonic cleaning with ethanol for 5 min.
4. The method for improving the corrosion resistance of the magnesium alloy micro-arc oxide film layer by the pretreatment process as claimed in claim 1, wherein the saturated alkaline solution in the step 2) is a saturated LiOH solution, a saturated Ca (OH) 2 Solutions or saturated Mg (OH) 2 And (3) solution.
5. The method for improving the corrosion resistance of the magnesium alloy micro-arc oxidation film layer by the pretreatment process according to claim 1, wherein the water bath treatment in the step 2) is specifically as follows: carrying out thermostatic water bath for 24h in a water bath kettle at the temperature of 25 ℃.
6. The method for improving the corrosion resistance of the magnesium alloy micro-arc oxidation film layer by the pretreatment process according to claim 1, wherein the electrolyte in the step 3) is a single-component silicate alkaline electrolyte, and specifically: sodium silicate, potassium hydroxide and sodium fluoride are respectively weighed at room temperature and sequentially dissolved in deionized water to obtain the electrolyte.
7. The method for improving the corrosion resistance of the magnesium alloy micro-arc oxidation film layer by the pretreatment process according to claim 6, wherein the electrolyte comprises 8g/L of sodium silicate, 2g/L of potassium hydroxide and 0.5g/L of sodium fluoride.
8. The method for improving the corrosion resistance of the magnesium alloy micro-arc oxidation film layer by the pretreatment process according to claim 1, wherein the micro-arc oxidation treatment specifically comprises the following steps: and (3) tightly connecting the magnesium alloy sample after pretreatment with the sheathed aluminum wire, fixing the magnesium alloy sample on a positive plate of a power supply, then placing the magnesium alloy sample and a stainless steel plate in a prepared electrolyte, and setting electrical parameters for micro-arc oxidation treatment.
9. The method for improving the corrosion resistance of the magnesium alloy micro-arc oxidation film layer by the pretreatment process according to claim 1, wherein the electrical parameters are as follows: under the constant voltage mode, the positive voltage is 450V, the negative voltage is 30V, the frequency is 600kHz, the duty ratio is 10 percent, and the processing time is 15 min.
10. The method for improving the corrosion resistance of the magnesium alloy micro-arc oxidation film layer by the pretreatment process according to claim 1, wherein the cleaning in the step 3) is specifically as follows: ultrasonic cleaning with deionized water and absolute ethyl alcohol respectively.
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