Method for preparing one-step in-situ self-hole-sealing micro-arc oxidation coating on surface of magnesium alloy

Abstract

The invention discloses a method for preparing a one-step in-situ self-sealing hole micro-arc oxidation coating on the surface of a magnesium alloy, which comprises the following steps: immersing the magnesium alloy serving as an anode and stainless steel serving as a cathode into a modified micro-arc oxidation electrolyte, and applying direct-current pulse electricity to start a micro-arc oxidation reaction, namely generating the self-sealing hole micro-arc oxidation coating on the surface of the magnesium alloy in situ, the modified micro-arc oxidation electrolyte is prepared from 4 g / L to 8 g / L of sodium phosphate, 8 g / L to 12 g / L of sodium fluoride, 2 g / L to 5 g / L of sodium silicate, 6 g / L to 12 g / L of potassium hydroxide, 2 g / L to 8 g / L of sodium tetraborate, 1 g / L to 3 g / L of sodium molybdate, 1 g / L to 4 g / L of trisodium citrate dihydrate and 0.5 g / L to 1.5 g / L of MXene. According to the method, the MXene is added into the composite electrolyte containing the sodium molybdate and the trisodium citrate, the self-sealing hole micro-arc oxidation film is generated on the surface of the magnesium alloy in situ in one step by applying the pulse current, rapid degradation of the magnesium alloy is effectively inhibited, and the service life of the magnesium alloy is prolonged.

Description

Technical Field

[0001] This invention relates to the field of micro-arc oxidation technology for corrosion-resistant metallic materials, and more specifically, to a method for preparing a one-step in-situ self-sealing micro-arc oxidation coating on the surface of magnesium alloys. Background Technology

[0002] Magnesium alloys are the lightest structural metallic materials, possessing a range of advantages including low density, vibration absorption, high stiffness, good biocompatibility, good machinability, and excellent battery shielding. They are widely used in aerospace, medical devices, and new energy vehicles. However, their highly reactive chemical properties, with a standard electrode potential as low as -2.37V, indicate a strong tendency to corrode. This makes them extremely susceptible to corrosion in harsh environments such as acids, alkalis, seawater, and humidity, severely limiting their widespread application.

[0003] To expand the application scope of magnesium alloys, researchers have conducted extensive research. Micro-arc oxidation (MAO), as a simple and effective surface treatment technology, has gained popularity because it can generate a metal oxide ceramic layer with high hardness and high bonding strength on the surface of magnesium alloys without expensive equipment or complex operations, and effectively improves its surface properties.

[0004] The high porosity of micro-arc oxidation (MAO) films is the most fatal drawback of coating applications. Modification methods for MAO films on magnesium alloys can be summarized into three categories: first, by adjusting the electrolyte composition or process parameters to change the phase composition and microstructure of the film, thereby improving its corrosion resistance; second, by introducing nanoscale pore-blocking materials during film formation to reduce the porosity of the film through physical filling and block the penetration channels of corrosive media; and third, by performing post-sealing treatment on the prepared MAO film to further optimize its protective effect. However, even with adjustments to the electrolyte composition and electrical parameters during micro-arc oxidation, film porosity is still difficult to eliminate, leaving pathways for corrosive media intrusion. While post-sealing can reduce porosity and improve film density, its complex operation process and high experimental costs limit its practical application.

[0005] A previous patent application, CN111172577A, disclosed a method for preparing a low-porosity micro-arc oxidation film on a magnesium alloy surface. However, this method produces a low-porosity micro-arc oxidation film, which cannot seal pores and therefore cannot fundamentally eliminate the porosity and crack channels in the film. These pore defects become rapid penetration channels for corrosive media. This directly leads to significant deficiencies in the electrochemical performance of the film, with a significantly low electrochemical impedance, failing to form a stable corrosion protection barrier, and a significantly high corrosion current density, accelerating the electrochemical reaction process and drastically increasing the corrosion rate of the magnesium alloy.

[0006] Previous patent application CN113862751A disclosed a method for preparing a self-sealing micro-arc oxidation film on a magnesium alloy surface. This method achieves self-sealing of the micro-arc oxidation film by adding titanium dioxide sol to an electrolyte. However, this method has significant limitations in terms of process implementation. Titanium dioxide sol itself is a thermodynamically unstable system. Under the strong electric field of the micro-arc oxidation process, the sol particles are prone to agglomeration, leading to uneven distribution of effective film-forming particles in the electrolyte and affecting the uniformity of sealing. Furthermore, the self-sealing micro-arc oxidation film prepared by this method has poor overall performance, with limited improvement in electrochemical corrosion resistance. Its impedance modulus is only on the order of 10⁵, and the corrosion current density is 6.152 × 10⁻⁶. -7 A·cm -2 It is difficult to achieve long-term corrosion protection.

[0007] Existing patent application CN120384293A discloses a method for preparing a base coating on a magnesium alloy surface using micro-arc oxidation, followed by sealing to prepare an intermediate coating and a deposition method to prepare a top coating. The sealing solution includes indium antimony oxide nanoparticles, zinc aluminum oxide nanoparticles, MXene nanosheets, and OES-70 surfactant. This sealing system suffers from poor synergy, unstable interfacial bonding, and insufficient corrosion resistance and durability. The prepared film cannot form chemical bonds, making it difficult to form a uniform and dense sealing layer, resulting in poor interfacial bonding. Furthermore, the indium antimony oxide and zinc aluminum oxide nanoparticles themselves do not possess excellent corrosion protection functions, only acting as physical barriers. Moreover, the sealing and micro-arc oxidation coating preparation are performed in two separate steps, failing to achieve one-step in-situ self-sealing and preventing the direct addition of MXene to the electrolyte, thus failing to address the issues of MXene aggregation and interfacial compatibility in the electrolyte. As a two-dimensional nanomaterial, MXene, due to its high surface energy, is susceptible to van der Waals forces in aqueous electrolyte systems, leading to layer stacking and aggregation, which prevents it from effectively filling the micropores on and inside the micro-arc oxidation film. Furthermore, unmodified MXene exhibits poor interfacial compatibility with other components in the electrolyte and with the micro-arc oxidation film layer, making it difficult to form a stable and uniform dispersion system, thus significantly reducing the uniformity of the sealing layer. Summary of the Invention

[0008] To address the problem of corrosive media penetration caused by micropores in MAO (micro-arc oxidation) coatings, the present invention aims to provide a method for preparing a MAO coating on magnesium alloy surfaces in a one-step in-situ self-sealing process without post-treatment. This method can prepare a low / non-porosity MAO coating on magnesium surfaces in a single operation, effectively inhibiting the rapid degradation of magnesium alloys and extending their service life.

[0009] The objective of this invention is achieved through the following technical solution: This invention provides a method for preparing a one-step in-situ self-sealing micro-arc oxidation coating on the surface of a magnesium alloy, comprising: using a magnesium alloy as the anode and stainless steel as the cathode, immersing the mixture in a modified micro-arc oxidation electrolyte, and applying a DC pulse to initiate the micro-arc oxidation reaction, thereby generating a self-sealing micro-arc oxidation coating in situ on the surface of the magnesium alloy. The modified micro-arc oxidation electrolyte comprises: 4-8 g / L sodium phosphate (Na3PO4), 8-12 g / L sodium fluoride (NaF), 2-5 g / L sodium silicate (Na2SiO3), 6-12 g / L potassium hydroxide (KOH), 2-8 g / L sodium tetraborate (Na2B4O7), 1-3 g / L sodium molybdate (Na2MoO4), 1-4 g / L trisodium citrate dihydrate (C6H5Na3O7·2H2O), and 0.5-1.5 g / L MXene.

[0010] This invention employs a sodium molybdate-sodium citrate electrolyte complex system. The carboxyl groups of molybdate and citric acid can form hydrogen bonds and coordination interactions with the hydroxyl and terminal oxygen groups on the MXene surface, improving the dispersion stability of MXene in the electrolyte. Furthermore, the magnesium molybdate precipitate formed by molybdate ions synergistically fills the discharge channels with the negatively charged MXene, generating a one-step in-situ self-sealing MAO coating, thereby solving the problems of MXene aggregation and interfacial compatibility in the electrolyte.

[0011] As some specific embodiments of the present invention, the parameters of the micro-arc oxidation reaction include: an initial given voltage of 50 V, a duty cycle of 30%-100%, a frequency of 30-200 Hz, and a voltage increase of 10 V per minute to 150 V after the voltage is turned on, followed by an increase of 5 V per minute to 230 V until the end. During the micro-arc oxidation process, continuous stirring is required to maintain the homogeneity of the electrolyte, and the experiment is conducted in a constant-temperature cooling environment to ensure that the micro-arc oxidation process takes place under stable thermodynamic conditions.

[0012] As some specific embodiments of the present invention, after the micro-arc oxidation reaction is completed, the magnesium alloy is further washed with distilled water and then dried, and the drying temperature is 50-60°C.

[0013] As some specific embodiments of the present invention, the magnesium alloy needs to be pretreated before use, specifically including: cutting and grinding the magnesium alloy, immersing it in an alkaline degreasing solution for degreasing treatment, cleaning and drying.

[0014] As some specific embodiments of the present invention, the alkaline degreasing solution includes 15-20 g / L NaOH, 20-30 g / L Na2CO3 and 15-20 g / L Na3PO4; the degreasing treatment temperature is 50-60℃ and the time is 1-2 minutes.

[0015] As some specific embodiments of the present invention, after degreasing treatment, the cleaning includes first rinsing the magnesium alloy surface with deionized water to remove residual degreasing liquid; and then performing ultrasonic cleaning with anhydrous ethanol and deionized water respectively.

[0016] As some specific embodiments of the present invention, the MXene is prepared by using Ti3AlC2 as a precursor and a mixed solution of LiF and HCl as an etchant. MXene is an excellent 2D material. In this invention, when added to a composite electrolyte, its negative surface charge and abundant functional groups ensure good compatibility and bonding strength with the anode interface. Moreover, its impermeability and tortuous structure endow the coating with dense physical barrier properties, effectively extending the diffusion path of corrosive media.

[0017] As some specific embodiments of the present invention, the preparation method of MXene includes: reacting LiF and HCl at 30-40℃ for 15-20 minutes, then slowly adding Ti3AlC2 powder, and continuing to react at 30-40℃ for 36-48 hours; after the reaction is completed, the mixture obtained from the reaction is centrifuged and washed multiple times until the pH of the supernatant is neutral; the precipitate after washing is dispersed in water and ultrasonically peeled off in an ice-water bath; the ultrasonically dispersed dispersion is then centrifuged to obtain a dark green dispersion, which is the MXene.

[0018] Furthermore, the mixing ratio of LiF, HCl and Ti3AlC2 is 3-4 g: 20-30 mL: 2-3 g.

[0019] Furthermore, in the centrifugal washing process, the centrifugation speed is 3500-4000 rpm, the process is repeated 7-9 times, and the detergent used is water.

[0020] Furthermore, when centrifuging the ultrasonically dispersed solution, the centrifugation speed is 3000-4000 rpm and the time is 1-1.5 hours.

[0021] Furthermore, after the reaction is complete, the resulting dark green dispersion is purged with nitrogen and stored in a refrigerator at 0-3°C.

[0022] As some specific embodiments of the present invention, the sodium hydroxide, sodium carbonate, sodium phosphate, sodium silicate, sodium fluoride, potassium hydroxide, sodium tetraborate, sodium molybdate, trisodium citrate, lithium fluoride, hydrochloric acid, aluminum carbide and ethanol are all chemically pure or higher purity.

[0023] Compared with the prior art, the present invention has the following beneficial effects: 1) This invention optimizes the electrolyte composition by adding MXene to the sodium molybdate-sodium citrate electrolyte compound system, allowing MXene to directly participate in the MAO reaction, which significantly reduces the difficulty of operation and production costs. 2) This invention utilizes MXene in synergy with molybdate and sodium citrate. The carboxyl groups of molybdate and citric acid can form hydrogen bonds and coordination with the hydroxyl and terminal oxygen groups on the surface of MXene, improving the dispersion stability of MXene in the electrolyte and solving the problems of MXene agglomeration and interfacial compatibility in the electrolyte. The magnesium molybdate precipitate formed by molybdate ions, together with the negatively charged MXene, fills the pores of the discharge channel, thus preparing an in-situ self-sealing MAO coating on the magnesium alloy surface in one step. This achieves simultaneous improvement in the coating's corrosion resistance and mechanical properties, taking into account both the protective effect and service durability of the magnesium alloy. 3) This invention can prepare an in-situ self-sealing MAO coating on the surface of magnesium alloy in one step without post-processing, which greatly reduces the difficulty of operation and improves the preparation efficiency. 4) The low / non-porosity MAO coating prepared in one step on the surface of magnesium alloy by this invention can effectively inhibit the rapid degradation of magnesium alloy and extend its service life. 5) The one-step in-situ self-sealing MAO coating prepared by this invention achieves efficient sealing of micropores on the coating surface through an in-situ sealing mechanism, endowing the coating with excellent electrochemical corrosion resistance at the structural level. Test data shows that the impedance modulus of this coating can reach 1.5 × 10⁻⁶. 7 Ω·cm 2 The corrosion current density is 1.136 × 10⁻⁶. -8 A·cm -2 Significantly improved performance: The electrochemical impedance of the low-porosity micro-arc oxidation film prepared on the surface of magnesium alloy in the previous patent application CN111172577A is 3.0 × 10⁻⁶. 5 Ω·cm 2 The corrosion current density is 2.994 × 10⁻⁶. -7 A·cm -2 The earlier patent application CN113862751A demonstrated that the electrochemical impedance of the self-sealing micro-arc oxidation film prepared on the surface of magnesium alloy was on the order of 10⁵, and the corrosion current density was 6.152 × 10⁻⁶. ‑7 A·cm -2 In comparison, the electrochemical impedance of the coating of this invention is increased by two orders of magnitude, and the corrosion current density is reduced by one order of magnitude, which significantly slows down the corrosion process of magnesium alloys and solves the technical problem of insufficient corrosion resistance caused by micropore defects in traditional MAO coatings.

[0024] 6) The sodium molybdate-sodium citrate system used in this invention allows sodium citrate to form a stable bond with the hydroxyl and oxygen atoms on the MXene surface through coordination. MXene can be anchored in the pores of the film through the coordination of sodium citrate, significantly improving the interfacial bonding strength of the film. Simultaneously, sodium molybdate can hydrolyze during the sealing process to generate molybdate precipitate, forming a synergistic filling effect with MXene and significantly improving the compactness of the sealing layer. In the sodium molybdate-sodium citrate system of this invention, molybdate can form a passivation film in a corrosive environment, actively inhibiting the occurrence of corrosion reactions. The sealing layer formed synergistically with MXene not only provides physical barrier but also has active protection functions, significantly improving the long-term corrosion resistance of the coating. Attached Figure Description

[0025] Other features, objects, and advantages of the present invention will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings: Figure 1 SEM images of magnesium alloy and the MAO coating formed on the surface of magnesium alloy, where a is the SEM image of magnesium alloy AZ91D, b is the SEM image of MAO coating formed on the surface of magnesium alloy in Comparative Example 1, and c is the SEM image of MAO coating formed on the surface of magnesium alloy in Example 1. Figure 2 EDS elemental diagram of the in-situ self-sealing MAO coating obtained in Example 1; where (a) is the morphology diagram of the element collection site, (b) is the element spectrum, (c) is Si, (d) is Ti, (e) is Mo, (f) is Mg, (g) is O, (h) is C, and (i) is P. Figure 3 Electrochemical impedance spectroscopy and potentiodynamic polarization curves of magnesium alloy AZ91D, Comparative Example 1 and Example 1 with MAO coating attached to the surface of magnesium alloy are shown. Among them, a, b and c are the electrochemical impedance spectra of magnesium alloy AZ91D, Comparative Example 1 and Example 1 with MAO coating attached to the surface of magnesium alloy, respectively, and d is the potentiodynamic polarization curve. Figure 4 The image shows the salt spray test results of magnesium alloy AZ91D, Comparative Example 1, and Example 1 with MAO coatings attached to the surface of the magnesium alloy. Detailed Implementation

[0026] The present invention will now be described in detail with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention. These all fall within the scope of protection of the present invention.

[0027] Example 1 (1) Magnesium alloy pretreatment AZ91 magnesium alloy substrates were cut into 30 mm × 30 mm × 2 mm samples, and then polished with metallographic sandpaper at grades #180, #600, and #1000 to remove corrosion spots and oxides from the substrate surface. The polished magnesium alloy substrates were then immersed in an alkaline degreasing solution (20 g / L NaOH, 30 g / L Na2CO3, and 20 g / L Na3PO4) at 60°C for 100 seconds. After degreasing, the magnesium alloy surface was immediately rinsed with deionized water to remove any residual degreasing solution. Subsequently, ultrasonic cleaning was performed in a CNC ultrasonic cleaning machine using anhydrous ethanol and deionized water, with each step lasting 10 minutes. After ultrasonic cleaning, the sample surface was rinsed with deionized water after each step. Finally, the magnesium alloy was placed in a drying oven to dry for later use.

[0028] (2) Preparation of MXene materials MXene was prepared using Ti3AlC2 as a precursor and a LiF / HCl mixed solution as an etchant.

[0029] First, weigh 1.6 g of LiF and 20 mL of 9M HCl and put them into a PTFE reactor. After reacting at 40°C for 15 minutes, weigh 2 g of Ti3AlC2 powder and slowly add it in. Then react at 40°C for 48 hours.

[0030] After the reaction is terminated, the reactants are transferred to centrifuge tubes and the resulting mixture is centrifuged and washed multiple times at 3500 rpm using distilled water as the washing agent. The operation is repeated 7-8 times until the pH of the supernatant is neutral. The washed precipitate is then redispersed in 100 ml of deionized water and ultrasonically peeled in an ice-water bath.

[0031] Finally, the ultrasonically dispersed solution was centrifuged at 3000 rpm for 1 hour to obtain the final dark green dispersion, which was then filled with nitrogen and stored in a refrigerator at 3°C.

[0032] (3) Preparation of MAO coating The pretreated magnesium alloy was used as the anode and the stainless steel was used as the cathode. They were both immersed in a prepared modified micro-arc oxidation electrolyte. Then, a DC pulse was applied to start the micro-arc oxidation reaction to prepare a magnesium alloy MAO coating.

[0033] The electrolyte consists of: 4 g / L Na3PO4, 10 g / L NaF, 2 g / L Na2SiO3, 6 g / L KOH, 4 g / L Na2B4O7, 1 g / L Na2MoO4, 2 g / L C6H5Na3O7·2H2O, and 1 g / L MXene.

[0034] The experimental parameters for micro-arc oxidation are as follows: the initial given voltage is 50 V, the duty cycle is 40%, the frequency is 50 Hz, and after the voltage is turned on, it increases by 10 V per minute to 150 V, and then increases by 5 V per minute to 230 V until the end of the experiment.

[0035] During the micro-arc oxidation process, continuous stirring is required to maintain the homogeneity of the electrolyte, and the experiment must be conducted in a constant temperature and cooling environment to ensure that the micro-arc oxidation process is carried out under a stable thermodynamic environment.

[0036] After the micro-arc oxidation is completed, the AZ91 magnesium alloy is rinsed with distilled water and dried in a 50 ℃ forced-air drying oven to obtain a one-step in-situ self-sealing micro-arc oxidation coating on the surface of the magnesium alloy.

[0037] Comparative Example 1 Based on Example 1, sodium molybdate, sodium citrate, and MXene were removed from the electrolyte formulation, while the remaining steps were performed according to Example 1. Specifically, the steps are as follows: (1) Pretreatment of magnesium alloy AZ91 magnesium alloy substrates were cut into 30 mm × 30 mm × 2 mm samples, and then polished with metallographic sandpaper at grades #180, #600, and #1000 to remove corrosion spots and oxides from the substrate surface. The polished magnesium alloy substrates were then immersed in an alkaline degreasing solution (20 g / L NaOH, 30 g / L Na2CO3, and 20 g / L Na3PO4) at 60°C for 100 seconds. After degreasing, the magnesium alloy surface was immediately rinsed with deionized water to remove any residual degreasing solution. Subsequently, ultrasonic cleaning was performed in a CNC ultrasonic cleaning machine using anhydrous ethanol and deionized water, with each step lasting 10 minutes. After ultrasonic cleaning, the sample surface was rinsed with deionized water after each step. Finally, the magnesium alloy was placed in a drying oven to dry for later use.

[0038] (2) Preparation of MAO coating The pretreated magnesium alloy was used as the anode and the stainless steel was used as the cathode. They were both immersed in a prepared basic micro-arc oxidation electrolyte. Then, a DC pulse was applied to start the micro-arc oxidation reaction to prepare a magnesium alloy MAO coating.

[0039] The electrolyte consists of: 4 g / L Na3PO4, 10 g / L NaF, 2 g / L Na2SiO3, 6 g / L KOH, and 4 g / L Na2B4O7.

[0040] The experimental parameters for micro-arc oxidation are as follows: the initial given voltage is 50 V, the duty cycle is 40%, the frequency is 50 Hz, and after the voltage is turned on, it increases by 10 V per minute to 150 V, and then increases by 5 V per minute to 230 V until the end of the experiment.

[0041] During the micro-arc oxidation process, continuous stirring is required to maintain the homogeneity of the electrolyte, and the experiment must be conducted in a constant temperature and cooling environment to ensure that the micro-arc oxidation process is carried out under a stable thermodynamic environment.

[0042] After micro-arc oxidation, the AZ91 magnesium alloy is rinsed with distilled water and dried in a 50 ℃ forced-air drying oven to obtain the MAO coating.

[0043] Effect Example 1. SEM Image Representation like Figure 1 As shown in Figures 1 and 2, these are SEM images of magnesium alloy AZ91D, the MAO layer formed on the surface of magnesium alloy in Comparative Example 1, and the in-situ self-sealing MAO coating formed on the surface of magnesium alloy in Example 1, respectively. It can be clearly observed that the AZ91D substrate surface is smooth, the MAO coating formed in Comparative Example 1 has a large number of dense micropores, while the surface of the in-situ self-sealing MAO coating in Example 1 has a large number of molten particles attached to it, which are adsorbed into the micropores of micro-arc oxidation. The large-diameter micropores of micro-arc oxidation are almost completely sealed, indicating that the in-situ self-sealing MAO coating prepared on the surface of magnesium alloy using the electrolyte of Example 1 has excellent pore-sealing effect.

[0044] 2. EDS elemental characterization The in-situ self-sealing MAO coating prepared on the magnesium alloy surface in Example 1 was characterized by EDS elemental analysis, such as... Figure 2 As shown, (a) is the morphology of the element collection site, (b) is the element spectrum, (c) is Si, (d) is Ti, (e) is Mo, (f) is Mg, (g) is O, (h) is C, and (i) is P.

[0045] EDS characterization showed that the elements Ti, O, P, Si, C, Mo, K, B, F, and Na in magnesium and electrolyte all participated in the formation of the coating. The Ti element came from the added modifier MXene, and all elements were uniformly distributed on the surface of the magnesium alloy.

[0046] 3. Corrosion resistance test The corrosion resistance of the prepared coatings was analyzed using a CHI760 electrochemical workstation. This experiment employed a traditional three-electrode system (a saturated calomel electrode as the reference electrode, magnesium alloys and the magnesium alloys with MAO coatings prepared in Examples 1 and Comparative Example 1 as working electrodes, and a platinum sheet as the auxiliary electrode), and was conducted at room temperature in a 3.5 wt% NaCl electrolyte. The electrochemical impedance spectroscopy scan frequency range was 10 Hz. 5 HZ-10 -2 HZ, sinusoidal voltage amplitude of 10 mV, polarization curve scan range is open circuit potential EOCP ±300 mV, scan rate 10 mV / s. After power is turned on and the open-circuit voltage stabilizes, the prepared coating is tested.

[0047] The results are as follows Figure 3 As shown, a, b, and c are the electrochemical impedance spectra of the magnesium alloy, the magnesium alloy covered with MAO coating in Comparative Example 1, and the magnesium alloy covered with in-situ self-sealing MAO coating in Example 1, respectively, and d is the potentiodynamic polarization curve.

[0048] Figure 3 The results show that, in Example 1, after adding the modified substance MXene to the electrolyte formulation, the electrochemical impedance spectroscopy of the in-situ self-sealing MAO coating on the magnesium alloy surface reached 1.5 × 10⁻⁶. 7 Ω·cm 2 The impedance of Comparative Example 1, after being coated with MAO, was 1.2 × 10⁻⁶. 5 Ω·cm 2 The impedance of the magnesium alloy AZ91D matrix is ​​2×10. 3 Ω·cm 2 It can be seen that the in-situ self-sealing MAO coating of Example 1 improves by two and four orders of magnitude compared with the MAO coating and magnesium alloy substrate of Comparative Example 1, respectively.

[0049] Figure 3 The polarization curve results of d show that the corrosion potential of the in-situ self-sealing MAO coating prepared in Example 1 is -1.363 V, and the corrosion current density is 1.136 × 10⁻⁶ V. -8 A·cm -2 The MAO coating prepared in Comparative Example 1 had a corrosion potential of -1.472 V and a corrosion current density of 4.335 × 10⁻⁶ V. -6 A·cm -2 The corrosion potential of the magnesium alloy AZ91D matrix is ​​-1.529 V, and the corrosion current density is 3.302 × 10⁻⁶. -5 A·cm -2 Therefore, the corrosion potential of the in-situ self-sealing MAO coating prepared by this invention shifted positively by 0.166 V, and the corrosion current density was reduced by two and three orders of magnitude, respectively, compared with the MAO coating and AZ91D substrate of Comparative Example 1, resulting in a significant improvement in corrosion resistance.

[0050] 4. Salt spray test The salt spray test was conducted using the following method: A 5 wt.% sodium chloride aqueous solution was used as the salt spray test medium. Salt spray corrosion tests were conducted on AZ91D, magnesium alloy with MAO coating prepared in Comparative Example 1, and magnesium alloy with in-situ self-sealing MAO coating prepared in Example 1 using a salt spray test chamber. The test adopted a cycle of 12 hours of continuous spraying followed by 12 hours of rest. Every 24 hours, the surface morphology of the above samples was photographed, recorded, and compared.

[0051] Salt spray test results are as follows Figure 4 As shown: Analysis of the salt spray test results showed that the MAO coatings of AZ91D and Comparative Example 1 exhibited significant corrosion pits on the 2nd and 4th days, respectively, after a certain period of cyclic salt spray testing. In contrast, the self-sealing MAO coating of Example 1 only showed corrosion pits after 14 days of salt spray testing. This indicates that, under salt spray corrosion conditions, the in-situ self-sealing modified MAO coating prepared using the composite electrolyte of this invention can provide longer-lasting protection for the magnesium alloy substrate. The mechanism of action lies in the synergistic filling effect of magnesium molybdate precipitation and MXene in the electrolyte system, which blocks the discharge channels, forming a dense protective barrier to prevent the intrusion of corrosive media, significantly improving the corrosion resistance and durability of the MAO coating.

[0052] The specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art can make various modifications or variations within the scope of the claims, which do not affect the essence of the present invention.

Claims

1. A method for preparing a one-step in-situ self-sealing micro-arc oxidation coating on a magnesium alloy surface, characterized in that, include: Magnesium alloy is used as the anode and stainless steel as the cathode. The mixture is immersed in a modified micro-arc oxidation electrolyte and a DC pulse is applied to start the micro-arc oxidation reaction, thereby generating a self-sealing micro-arc oxidation coating in situ on the surface of the magnesium alloy. The modified micro-arc oxidation electrolyte comprises: 4-8 g / L sodium phosphate, 8-12 g / L sodium fluoride, 2-5 g / L sodium silicate, 6-12 g / L potassium hydroxide, 2-8 g / L sodium tetraborate, 1-3 g / L sodium molybdate, 1-4 g / L trisodium citrate dihydrate, and 0.5-1.5 g / L MXene.

2. The method according to claim 1, characterized in that, The parameters of the micro-arc oxidation reaction include: an initial given voltage of 50 V, a duty cycle of 30%-100%, a frequency of 30-200 Hz, and a voltage increase of 10 V per minute to 150 V after the voltage is turned on, followed by an increase of 5 V per minute to 230 V until the end.

3. The method according to claim 1 or 2, characterized in that, After the micro-arc oxidation reaction is completed, the magnesium alloy is washed with distilled water and then dried at a temperature of 50-60°C.

4. The method according to claim 1, characterized in that, The magnesium alloy needs to be pretreated before use, specifically including: cutting and grinding the magnesium alloy, immersing it in an alkaline degreasing solution for degreasing, cleaning and drying.

5. The method according to claim 4, characterized in that, The alkaline degreasing solution contains 15-20 g / L sodium hydroxide, 20-30 g / L sodium carbonate, and 15-20 g / L sodium phosphate; the degreasing treatment is carried out at a temperature of 50-60°C for 1-2 minutes.

6. The method according to claim 4, characterized in that, After degreasing, the cleaning process includes first rinsing the magnesium alloy surface with deionized water to remove residual degreasing solution; then performing ultrasonic cleaning with anhydrous ethanol and deionized water respectively.

7. The method according to claim 1, characterized in that, The MXene was prepared using Ti3AlC2 as a precursor and a mixed solution of LiF and HCl as an etchant.

8. The method according to claim 7, characterized in that, The preparation method of MXene includes: reacting LiF and HCl at 30-40℃ for 15-20 minutes, then slowly adding Ti3AlC2 powder, and continuing to react at 30-40℃ for 36-48 hours; after the reaction, the mixture obtained is centrifuged and washed multiple times until the pH of the supernatant is neutral; the washed precipitate is dispersed in water and ultrasonically peeled off in an ice-water bath; the ultrasonically dispersed solution is then centrifuged to obtain a dark green dispersion, which is the MXene.

9. The method according to claim 8, characterized in that, The mixing ratio of LiF, HCl and Ti3AlC2 is 3-4g: 20-30 mL: 2-3 g; And / or, in the centrifugal washing, the centrifugation speed is 3500-4000 rpm, the number of repetitions is 7-9, and the detergent used is water; And / or, when centrifuging the ultrasonically dispersed solution, the centrifugation speed is 3000-4000 rpm and the time is 1-1.5 hours; And / or, after the reaction is complete, the resulting dark green dispersion is purged with nitrogen and stored in a refrigerator at 0-3°C.

10. The method according to any one of claims 1-9, characterized in that, The sodium phosphate, sodium fluoride, sodium silicate, potassium hydroxide, sodium tetraborate, sodium molybdate, trisodium citrate, sodium hydroxide, sodium carbonate, lithium fluoride, hydrochloric acid, aluminum carbide, or ethanol are of chemical purity or higher.

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