White micro-arc oxidation film on surface of magnesium alloy and preparation method of white micro-arc oxidation film

Abstract

The invention belongs to the technical field of magnesium alloy surface treatment, and particularly relates to a magnesium alloy surface white micro-arc oxidation film and a preparation method thereof. Comprising the following steps: preparing an electrolyte containing inorganic base, a film-forming agent, a fluorine-containing coloring agent, a discharge regulator, an auxiliary additive and a surfactant; a magnesium alloy base material serves as an anode, stainless steel serves as a cathode, micro-arc oxidation treatment is carried out in electrolyte, and a magnesium oxide / magnesium fluoride composite film layer, namely the magnesium alloy surface white micro-arc oxidation film, is formed on the surface of the base material by controlling the current density. Through the unique electrolyte formula design and the low-voltage micro-arc oxidation process and the synergistic effect of all the components, the oxidation film prepared on the surface of the magnesium alloy is white, the L value is larger than 85, the solar absorptivity is smaller than 0.3, and application scenes with strict requirements for the whiteness and the solar absorptivity can be met.

Description

Technical Field

[0001] This invention belongs to the field of magnesium alloy surface treatment technology, specifically relating to a white micro-arc oxide film on the surface of magnesium alloy and its preparation method. Background Technology

[0002] Magnesium alloys are widely used in aerospace, electronic devices, and other fields due to their low density, high specific strength, good electromagnetic shielding properties, and recyclability. However, magnesium alloy surfaces easily form a loose oxide film, resulting in poor corrosion resistance. Corrosion is even more pronounced in salt spray and humid environments, severely limiting the further development of magnesium alloys. To overcome this deficiency, surface treatment of magnesium alloys to generate a corrosion-resistant and aesthetically pleasing film is crucial.

[0003] Common surface treatment methods include spraying / electrophoresis and micro-arc oxidation. Spraying / electrophoresis can prepare white organic coatings on magnesium alloy surfaces for surface decoration or aerospace thermal control applications. However, organic coatings are prone to aging and discoloration in harsh environments, especially in the extreme conditions of space, leading to a decrease in whiteness and making it difficult to meet long-term stability requirements. In recent years, the preparation of white inorganic ceramic coatings on light metal surfaces such as aluminum alloys using micro-arc oxidation has attracted increasing attention. Patent application CN201611103201.X provides a method for preparing a white thermal control coating with high solar reflectivity on aluminum alloy surfaces. Micro-arc oxidation can produce white films with an L value higher than 85 on aluminum and titanium alloy surfaces. However, due to the higher reactivity of magnesium alloys, it is difficult to prepare white films with high L values ​​and low solar absorption, failing to meet the application requirements of magnesium alloys in fields with high requirements for appearance and thermal control performance. Summary of the Invention

[0004] To address the challenge of existing micro-arc oxidation methods in achieving a white film on magnesium alloys that balances high L-value and low solar absorption, this invention provides a method for preparing a white micro-arc oxidation film on magnesium alloy surfaces.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A method for preparing a white micro-arc oxidation film on the surface of a magnesium alloy includes the following steps: Using a magnesium alloy substrate as the anode, micro-arc oxidation is performed in an electrolyte to form a white micro-arc oxidation film with a magnesium oxide / magnesium fluoride composite film structure on the surface of the magnesium alloy substrate. The electrolyte is prepared by mixing 10 g / L to 70 g / L of inorganic alkali, 5 g / L to 30 g / L of film-forming agent, 10 g / L to 100 g / L of fluorinated colorant, 5 mL / L to 20 mL / L of discharge regulator, 0.5 g / L to 10 g / L of auxiliary additives, and 0.1 g / L to 1 g / L of surfactant. The voltage during the micro-arc oxidation process is 60 V to 120 V, and the current density is 2 A / dm³. 2 ~10A / dm 2 .

[0006] In the electrolyte design of this invention, the raw materials work synergistically: inorganic alkali (such as sodium hydroxide) provides a stable alkaline environment with pH>11, laying the foundation for the controllability of the micro-arc oxidation reaction; film-forming agent (such as sodium phosphate) improves the conductivity of the electrolyte, ensuring that the magnesium alloy can effectively ignite and form a basic film layer under low voltage; fluorinated colorant (such as sodium fluoride) reacts with the matrix to generate MgF2, and MgF2 and MgO form a gradient refractive structure, significantly enhancing the light reflection capability of the film layer. At the same time, the wide bandgap characteristic of MgF2 reduces visible light absorption, helping to improve whiteness; discharge regulator (such as ethylene glycol) makes the micro-arc discharge more uniform, avoids local sintering, and can also neutralize active oxygen atoms, inhibiting the formation of dark manganese oxides, further ensuring whiteness; auxiliary additives (such as sodium tetraborate) enter the MgO lattice to form point defects through boron and phosphorus elements, inhibiting the generation of oxygen vacancies and reducing the light absorption effect; surfactant (such as sodium dodecylbenzenesulfonate) reduces the surface tension of the electrolyte, improves the wettability of the matrix, eliminates the "edge effect", avoids local dark spots, and ensures the consistency of film whiteness. In terms of process, it is combined with 60V~120V low-voltage micro-arc oxidation to precisely control 2A / dm 2 ~10A / dm 2 Parameters such as current density and electrolyte temperature are optimized to avoid excessive sintering and carbonization of the film layer caused by high voltage. This invention, through raw material synergy and process optimization, prepares a white micro-arc oxidation film with an L-value > 85 and a solar absorptivity < 0.3 in one step, balancing performance, cost, and industrial feasibility.

[0007] Furthermore, the cathode material used in the micro-arc oxidation process is stainless steel.

[0008] Furthermore, the inorganic base is one or a mixture of two of sodium hydroxide, potassium hydroxide, and sodium carbonate.

[0009] Furthermore, the film-forming agent is one or a mixture of two of sodium phosphate, sodium silicate, and sodium aluminate.

[0010] Furthermore, the fluorinated colorant is one or a mixture of two of sodium fluoride, potassium fluoride, and ammonium fluoride.

[0011] Furthermore, the discharge regulator is one or a mixture of two of ethylene glycol, propylene glycol, and glycerol.

[0012] Furthermore, the auxiliary additive is one or a mixture of two of sodium tetraborate, sodium hexametaphosphate, and sodium pyrophosphate.

[0013] Furthermore, the surfactant is one or a mixture of two of sodium dodecyl sulfate, sodium dodecylbenzene sulfonate, and sodium polyacrylate.

[0014] In the above preparation method, the amount of each additive must be within a suitable range. Too high or too low a concentration will have adverse effects: too high an inorganic alkali concentration will easily lead to film cracking, while too low a concentration will make it difficult to effectively ignite the arc; too high a concentration of film-forming agent will make the film rough, while too low a concentration will result in uneven film formation; too high a concentration of fluorinated colorant will easily agglomerate and affect the quality of the film, while too low a concentration will result in insufficient whiteness; too high a concentration of discharge regulator will affect the conductivity of the electrolyte, while too low a concentration will result in uneven discharge; too high a concentration of auxiliary additives will easily generate impurities, while too low a concentration will make it difficult to suppress defects such as oxygen vacancies; too high a concentration of surfactant will generate too much foam, while too low a concentration will result in poor substrate wettability and easy appearance of local dark spots.

[0015] Furthermore, the micro-arc oxidation process employs a DC pulse power supply and includes precise control of voltage, current density, and solution temperature to ensure uniform film growth. The voltage ranges from 60V to 120V, the frequency from 200Hz to 800Hz, and the duty cycle from 20% to 60%. The current density during the micro-arc oxidation process is controlled to stabilize the voltage at a low level that enables effective arc initiation and film growth, avoiding excessively high voltage that could lead to a decrease in film quality and color deviation.

[0016] Furthermore, during the micro-arc oxidation process, the current density is 2 A / dm². 2 ~10A / dm 2 The process involves adjusting the current density to induce micro-arc discharge on the magnesium alloy surface and generate a film layer. The electrolyte temperature is maintained between 10℃ and 30℃ to prevent the thermal decomposition and failure of additives, ensuring the consistency and repeatability of the film layer thickness. The electrolyte temperature must be strictly controlled between 10℃ and 30℃; temperatures exceeding 30℃ will lead to poor film uniformity and decreased whiteness.

[0017] Furthermore, the micro-arc oxidation treatment time is 20 min to 40 min.

[0018] Using the above method, magnesium alloy surfaces can be prepared with an L value > 85 and a solar absorptivity (α) of [missing value]. s A white micro-arc oxide film with a diameter of <0.3.

[0019] Compared with the prior art, the present invention has the following beneficial effects: (1) The present invention adopts a multi-component synergistic fluorine-containing alkaline electrolyte system and combines it with a low-voltage micro-arc oxidation process. Using magnesium alloy as the anode, a magnesium oxide / magnesium fluoride composite film is formed on the surface of magnesium alloy under specific process parameters to achieve the preparation of an inorganic film with high whiteness, low solar absorptivity and long-term stability. The electrolyte contains 10 g / L to 70 g / L of inorganic alkali to provide a stable alkaline environment; 5 g / L to 30 g / L of film-forming agent to ensure effective arc initiation and film formation at low voltage; 10 g / L to 100 g / L of fluorinated colorant reacting with the magnesium alloy substrate to generate MgF2, which forms a gradient refractive structure with MgO, enhancing the film's light reflection and reducing visible light absorption; 5 mL / L to 20 mL / L of discharge regulator to ensure uniform discharge during micro-arc oxidation and suppress the formation of dark-colored compounds; 0.5 g / L to 10 g / L of boron-phosphorus auxiliary additive to suppress oxygen vacancies and reduce light absorption; and 0.1 g / L to 1 g / L of surfactant to ensure consistent film whiteness. The synergistic effect of these components solves the problem of preparing white films with high L-values ​​and low solar absorptivity caused by the high activity of magnesium alloys. Furthermore, this invention also employs a low voltage of 60V to 120V and a current of 2A / dm². 2 ~10A / dm 2 Stable micro-arc discharge is achieved through control of current density and DC pulse power supply parameters, avoiding sintering and carbonization of the film. Compared with organic coatings, the inorganic composite film prepared by this invention has stable chemical properties and is not prone to aging and discoloration in the extreme environment of space, meeting the requirements for long-term stability.

[0020] (2) Through a unique electrolyte formulation design and a low-voltage micro-arc oxidation process, the components of this invention work synergistically to produce a white oxide film on the surface of magnesium alloys. The higher the whiteness of the film, the larger its L value and the lower its light absorption capacity. The film prepared by this invention has an L value greater than 85 and a solar absorptivity of less than 0.3, filling the gap in the preparation of white films with high L values ​​and low absorptivity on the surface of magnesium alloys. This can meet the requirements of some application scenarios with strict requirements for whiteness and solar absorptivity.

[0021] (3) The preparation method of the present invention is simple to operate, the raw materials are readily available, the cost is low and the environment is environmentally friendly, and it is suitable for large-scale industrial production. By precisely controlling the micro-arc oxidation process parameters, especially by using a special low-voltage micro-arc oxidation method, high-quality white micro-arc oxidation films can be stably prepared.

[0022] (4) Compared with traditional high-voltage micro-arc oxidation, the low-voltage micro-arc oxidation process of the present invention can be processed using ordinary DC pulse power supply, which greatly reduces energy consumption and is conducive to energy conservation and emission reduction in industrial applications. Furthermore, it does not require customized high-voltage micro-arc oxidation power supply, thus reducing equipment costs. Attached Figure Description

[0023] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0024] Figure 1 Comparative photographs of the micro-arc oxidation films prepared in Example 1, Comparative Example 1, and Comparative Example 2 are shown. In Comparative Example 1, a is the micro-arc oxidation film prepared under low voltage conditions of AZ91; b is the white micro-arc oxidation film prepared in Example 1; and c is the micro-arc oxidation film prepared under high voltage conditions of AZ91 in Comparative Example 2.

[0025] Figure 2 These are comparative photographs of the micro-arc oxidation films prepared in Example 2 and Comparative Example 3. In Example 3, a is the micro-arc oxidation film prepared on the surface of AZ31 magnesium alloy under low voltage conditions, and b is the white micro-arc oxidation film prepared in Example 2.

[0026] Figure 3 The surface microstructures of the white micro-arc oxide films prepared in Examples 1 and 2 are shown, where a is the surface microstructure of the white micro-arc oxide film on the surface of AZ91 magnesium alloy in Example 1, and b is the surface microstructure of the white oxide film on the surface of AZ31 magnesium alloy in Example 2.

[0027] Figure 4 The XRD pattern of the white oxide film prepared in Example 1.

[0028] Figure 5 Comparison of simulated solar reflectance curves of the micro-arc oxidation films prepared in Example 1, Comparative Example 1, and Comparative Example 2.

[0029] Figure 6 The images show a comparison of macroscopic photographs of the white micro-arc oxide film prepared in Example 1 before and after 15 days of ultraviolet irradiation. In the image, a is the photograph before irradiation and b is the photograph after irradiation. Detailed Implementation

[0030] The specific embodiments of the present invention are described in detail below, but it should be understood that the scope of protection of the present invention is not limited to the specific embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Unless otherwise specified, the experimental methods described in the embodiments of the present invention are conventional methods, and the materials and reagents used in the following embodiments are commercially available unless otherwise specified.

[0031] To achieve the preparation of a white film with a high L-value and low solar absorption ratio on the surface of magnesium alloys, the existing solutions mainly include: (1) adding Zr(NO3)4 to a phosphate-based electrolyte to form a coating containing MgO, t-ZrO2 and Mg-O-Zr compounds through micro-arc oxidation, thereby reducing the absorption-emission ratio (α). s The absorptivity is 0.405 and the emissivity is 0.873 at the optimal concentration of 10 g / L. Alternatively, adding ZnSO4 or nano-ZnO particles can generate a ZnO phase (band gap 3.37 eV), which can reduce α to some extent. s(Minimum 0.35) and increase ε (0.88). However, due to the complex composition of the film layer, the absorption ratio is still relatively high. (2) Sodium aluminate, strong alkali, sodium fluoride, potassium titanate, sodium stannate and triethanolamine are selected as electrolytes and combined with bipolar pulse constant current micro-arc oxidation process to make the ceramic film prepared on the magnesium alloy surface appear bright white, and the sum of the absolute values ​​of L*, a* and b* of the film layer is ≥90. However, potassium titanate and sodium stannate are easily hydrolyzed in the electrolyte, and the pH value fluctuation affects the stability of the complexing agent (such as sodium citrate), which easily leads to uneven coating color or insufficient whiteness. (3) Potassium fluorozirconate (K2ZrF6) is used as the main salt, combined with hydrofluoric acid (HF) and hydrogen peroxide (H2O2) stabilizers to form a white and smooth micro-arc oxidation film in an acidic environment (pH 1~6), and its L value can reach more than 90. However, the raw material cost of the fluorozirconate system used in this process is high, and hydrofluoric acid poses serious safety risks and causes significant corrosion and damage to equipment. (4) By preparing a composite coating on the magnesium alloy substrate with a micro-arc oxidation Si salt layer, Zr salt layer, and Zn salt layer as the inner layer and a magnetron sputtered Si film as the outer layer, the reflectivity of the magnesium alloy substrate is improved so that the solar absorptivity is ≤0.20, and it has excellent stability and adaptability to the space environment. However, this preparation process is complex, costly, and inefficient, making it difficult to achieve large-scale application. In view of the above problems, this invention provides a method for preparing a white micro-arc oxidation film on the surface of magnesium alloy. Its core lies in the fact that the electrolyte system used is a multi-component synergistic system. Each component achieves functional complementarity through precise proportioning, and at the same time, stable micro-arc discharge (60V~120V) under low voltage conditions is achieved, which is significantly different from the traditional micro-arc oxidation technology (usually requiring more than 200V). The inorganic alkaline component in the electrolyte system provides the necessary alkaline environment (pH>11), making the micro-arc oxidation process of magnesium alloy more stable and controllable. Sodium phosphate, sodium silicate, or sodium aluminate are used as the main film-forming agents to improve the conductivity of the solution, enabling the magnesium alloy to generate a film layer through micro-arc discharge under low voltage conditions. Fluorides, as key fluorine-containing colorants in the electrolyte, participate in the formation of magnesium fluoride compounds (such as MgF2) during the micro-arc discharge process. MgF2 has a band gap of 10.8 eV, far exceeding the 7.8 eV of MgO. This wide band gap characteristic makes MgF2 almost non-absorbent of visible light, and its refractive index (n=1.38) forms a gradient refractive structure with MgO (n=1.74), significantly enhancing the light reflectivity of the film layer. The addition of polyol discharge modifiers such as ethylene glycol, propylene glycol, and glycerol makes the discharge process more uniform and avoids local sintering. Furthermore, polyols can generate in-situ hydrogen gas during discharge, effectively neutralizing active oxygen atoms in the discharge region and inhibiting the formation of dark-colored compounds such as high-valence manganese oxides (such as Mn2O3) in manganese-containing magnesium alloys (such as AZ31 and AZ91), thus contributing to whiteness control.Boron and phosphorus elements in polyanionic additives such as sodium tetraborate, sodium hexametaphosphate, and sodium pyrophosphate in the electrolyte can enter the MgO lattice, forming a point defect structure that effectively suppresses the generation of oxygen vacancies and further reduces the light absorption effect caused by oxygen vacancies. Adding surfactants such as sodium dodecylbenzenesulfonate and sodium polyacrylate can reduce the surface tension of the electrolyte, improve substrate wettability, eliminate the "edge effect," avoid local dark spots, and improve the uniformity of apparent whiteness. The low-voltage micro-arc oxidation mode used in this invention is another key to achieving high-whiteness film preparation. Low voltage allows for precise control of discharge energy, avoiding excessive sintering and component carbonization problems caused by high-energy discharge. Under low-voltage operation, the micro-arc discharge is more gentle and uniform, which is conducive to generating a high-purity, fine-grained MgO-MgF2 composite system, resulting in a uniform bright white film with an L value greater than 85 and a solar absorptivity less than 0.3.

[0032] Example 1 A method for preparing a white micro-arc oxidation film on the surface of a magnesium alloy includes the following steps: In deionized water, an inorganic alkali (sodium hydroxide and sodium carbonate in a 1:1 mass ratio), a film-forming agent (sodium phosphate and sodium silicate in a 4:1 mass ratio), a fluorinated colorant (sodium fluoride and ammonium fluoride in a 3:1 mass ratio), a discharge regulator (ethylene glycol and glycerol in a 1:1.5 volume ratio), an auxiliary additive (sodium tetraborate), and a surfactant (sodium dodecylbenzene sulfonate) were added and stirred thoroughly until completely dissolved to obtain a micro-arc oxidation electrolyte (inorganic alkali concentration of 60 g / L, film-forming agent concentration of 25 g / L, fluorinated colorant concentration of 50 g / L, discharge regulator concentration of 5 mL / L, auxiliary additive concentration of 4 g / L, and surfactant concentration of 0.8 g / L). An AZ91 magnesium alloy sample, ultrasonically cleaned with anhydrous ethanol for 5 min and then dried, was used as the anode, and a stainless steel plate as the cathode. The sample was placed in the prepared micro-arc oxidation electrolyte, and the cooling water circulation system was turned on. A standard DC pulse power supply was used, with the maximum voltage set to 105 V and the current density at 5 A / dm³. 2 The sample was subjected to micro-arc oxidation treatment for 28 minutes at a frequency of 500 Hz, a duty cycle of 40%, and a temperature of 20 °C. Immediately after the micro-arc oxidation treatment, the sample was removed and rinsed in flowing deionized water for 2 minutes to remove electrolyte residue, and then air-dried at room temperature. Measurements showed that the white micro-arc oxidation ceramic film on the magnesium alloy surface obtained in this embodiment had a film thickness of 58 μm, an L value of 91.8, and a solar absorptivity of 0.21.

[0033] Example 2 A method for preparing a white micro-arc oxidation film on the surface of a magnesium alloy includes the following steps: In deionized water, inorganic alkali (sodium hydroxide and potassium hydroxide in a mass ratio of 1:1.2), film-forming agent (sodium phosphate and sodium aluminate in a mass ratio of 3:1), fluorinated colorant (potassium fluoride and ammonium fluoride in a mass ratio of 1:1.5), discharge regulator (propylene glycol and glycerol in a volume ratio of 1:1), auxiliary additive (sodium pyrophosphate), and surfactant (sodium polyacrylate) were added and stirred thoroughly until completely dissolved to obtain a micro-arc oxidation electrolyte (inorganic alkali concentration of 40 g / L, film-forming agent concentration of 16 g / L, fluorinated colorant concentration of 30 g / L, discharge regulator concentration of 15 mL / L, auxiliary additive concentration of 7 g / L, and surfactant concentration of 0.5 g / L). An AZ31 magnesium alloy sample, ultrasonically cleaned with anhydrous ethanol for 5 min and dried, was used as the anode, and a stainless steel plate as the cathode, placed in the prepared micro-arc oxidation electrolyte. The cooling water circulation system was turned on, and a standard DC pulse power supply was used, with the maximum voltage set to 90 V and the current density controlled at 3.5 A / dm³. 2 The sample was subjected to micro-arc oxidation treatment for 20 minutes at a frequency of 700 Hz, a duty cycle of 20%, and a temperature of 15 °C. Immediately after the micro-arc oxidation treatment, the sample was removed and rinsed in flowing deionized water for 2 minutes to remove electrolyte residue. It was then air-dried at room temperature. Measurements showed that the white micro-arc oxidation ceramic film on the magnesium alloy surface obtained in this embodiment had a film thickness of 45 μm, an L value of 90.2, and a solar absorptivity of 0.24.

[0034] Example 3 A method for preparing a white micro-arc oxidation film on the surface of a magnesium alloy includes the following steps: In deionized water, an inorganic alkali (potassium hydroxide and sodium carbonate in a mass ratio of 2:1), a film-forming agent (sodium phosphate), a fluorinated colorant (sodium fluoride and potassium fluoride in a mass ratio of 1:2), a discharge regulator (ethylene glycol and propylene glycol in a volume ratio of 2:1), an auxiliary additive (sodium hexametaphosphate), and a surfactant (sodium dodecylbenzenesulfonate) were added and stirred thoroughly until completely dissolved to obtain a micro-arc oxidation electrolyte (inorganic alkali concentration of 30 g / L, film-forming agent concentration of 30 g / L, fluorinated colorant concentration of 60 g / L, discharge regulator concentration of 9 mL / L, auxiliary additive concentration of 0.5 g / L, and surfactant concentration of 0.3 g / L). An AZ91 magnesium alloy sample, ultrasonically cleaned with anhydrous ethanol for 5 min and then dried, was used as the anode, and a stainless steel plate as the cathode. The sample was placed in the prepared micro-arc oxidation electrolyte, and the cooling water circulation system was turned on. A standard DC pulse power supply was used, with the maximum voltage set to 82 V and the current density controlled at 2 A / dm³. 2The sample was subjected to micro-arc oxidation treatment for 25 minutes at a frequency of 200 Hz, a duty cycle of 50%, and a temperature of 25 °C. Immediately after the micro-arc oxidation treatment, the sample was removed and rinsed in flowing deionized water for 2 minutes to remove electrolyte residue, and then air-dried at room temperature. Measurements showed that the white micro-arc oxidation ceramic film on the magnesium alloy surface obtained in this embodiment had a film thickness of 52 μm, an L value of 87.2, and a solar absorptivity of 0.27.

[0035] Example 4 A method for preparing a white micro-arc oxidation film on the surface of a magnesium alloy includes the following steps: Inorganic alkali (sodium hydroxide), film-forming agent (sodium silicate and sodium aluminate in a mass ratio of 3:2), fluorinated colorant (sodium fluoride), discharge regulator (ethylene glycol), auxiliary additive (sodium tetraborate and sodium hexametaphosphate in a mass ratio of 2:1), and surfactant (sodium dodecyl sulfate) were added to deionized water and stirred thoroughly until completely dissolved to obtain a micro-arc oxidation electrolyte (inorganic alkali concentration of 50 g / L, film-forming agent concentration of 30 g / L, fluorinated colorant concentration of 70 g / L, discharge regulator concentration of 15 mL / L, auxiliary additive concentration of 7.5 g / L, and surfactant concentration of 1 g / L). An AZ91 magnesium alloy sample, ultrasonically cleaned with anhydrous ethanol for 5 min and then dried, was used as the anode, and a stainless steel plate as the cathode. The sample was placed in the prepared micro-arc oxidation electrolyte, and the cooling water circulation system was turned on. A standard DC pulse power supply was used, with the maximum voltage set to 60 V and the current density controlled at 2.5 A / dm³. 2 The sample was subjected to micro-arc oxidation treatment for 35 minutes at a frequency of 400 Hz, a duty cycle of 60%, and a temperature of 20 °C. Immediately after the micro-arc oxidation treatment, the sample was removed and rinsed in flowing deionized water for 2 minutes to remove electrolyte residue, and then air-dried at room temperature. Measurements showed that the white micro-arc oxidation ceramic film on the magnesium alloy surface obtained in this embodiment had a film thickness of 39 μm, an L value of 86.5, and a solar absorptivity of 0.28.

[0036] Example 5 A method for preparing a white micro-arc oxidation film on the surface of a magnesium alloy includes the following steps: In deionized water, an inorganic alkali (sodium hydroxide and sodium carbonate in a mass ratio of 3:2), a film-forming agent (sodium aluminate), a fluorinated colorant (potassium fluoride), a discharge regulator (glycerol), an auxiliary additive (sodium pyrophosphate), and a surfactant (sodium dodecyl sulfate) were added and stirred thoroughly until completely dissolved to obtain a micro-arc oxidation electrolyte (inorganic alkali concentration: 50 g / L, film-forming agent concentration: 20 g / L, fluorinated colorant concentration: 100 g / L, discharge regulator concentration: 20 mL / L, auxiliary additive concentration: 10 g / L, surfactant concentration: 0.4 g / L). An AZ31 magnesium alloy sample, ultrasonically cleaned with anhydrous ethanol for 5 min and then dried, was used as the anode, and a stainless steel plate as the cathode. The sample was placed in the prepared micro-arc oxidation electrolyte, and the cooling water circulation system was turned on. A standard DC pulse power supply was used, with the maximum voltage set to 120 V and the current density controlled at 8 A / dm³. 2 The sample was subjected to micro-arc oxidation treatment for 20 minutes at a frequency of 200 Hz, a duty cycle of 50%, and a temperature of 20 °C. Immediately after the micro-arc oxidation treatment, the sample was removed and rinsed in flowing deionized water for 2 minutes to remove electrolyte residue. It was then air-dried at room temperature. Measurements showed that the white micro-arc oxidation ceramic film on the magnesium alloy surface obtained in this embodiment had a film thickness of 61 μm, an L value of 89.8, and a solar absorptivity of 0.25.

[0037] Example 6 A method for preparing a white micro-arc oxidation film on the surface of a magnesium alloy includes the following steps: In deionized water, an inorganic alkali (potassium hydroxide and sodium carbonate in a mass ratio of 5:2), a film-forming agent (sodium silicate), a fluorinated colorant (sodium fluoride and potassium fluoride in a mass ratio of 1:2), a discharge regulator (propylene glycol), auxiliary additives (sodium tetraborate and sodium pyrophosphate in a mass ratio of 2:1), and a surfactant (sodium polyacrylate) were added and stirred thoroughly until completely dissolved to obtain a micro-arc oxidation electrolyte (inorganic alkali concentration of 70 g / L, film-forming agent concentration of 10 g / L, fluorinated colorant concentration of 30 g / L, discharge regulator concentration of 12 mL / L, auxiliary additive concentration of 6 g / L, and surfactant concentration of 0.6 g / L). An AZ31 magnesium alloy sample, ultrasonically cleaned with anhydrous ethanol for 5 min and then dried, was used as the anode, and a stainless steel plate as the cathode. The sample was placed in the prepared micro-arc oxidation electrolyte, and the cooling water circulation system was turned on. A standard DC pulse power supply was used, with the maximum voltage set to 110 V and the current density controlled at 3.5 A / dm³. 2The sample was subjected to micro-arc oxidation treatment for 40 minutes at a frequency of 800 Hz, a duty cycle of 60%, and a temperature of 25°C. Immediately after the micro-arc oxidation treatment, the sample was removed and rinsed in flowing deionized water for 2 minutes to remove electrolyte residue. It was then air-dried at room temperature. Measurements showed that the white micro-arc oxidation ceramic film on the magnesium alloy surface obtained in this embodiment had a film thickness of 56 μm, an L value of 90.5, and a solar absorptivity of 0.24.

[0038] The concentrations of each component in the electrolytes of Examples 1-6 are shown in Table 1.

[0039] Table 1. Concentrations of electrolyte components in Examples 1-6 of the white micro-arc oxidation film on magnesium alloys. Comparative Example 1 A method for preparing a micro-arc oxidation film includes the following steps: Inorganic alkali (sodium hydroxide), film-forming agent (sodium silicate), and auxiliary additive (sodium tetraborate) were added to deionized water and stirred thoroughly until completely dissolved to obtain a micro-arc oxidation electrolyte (inorganic alkali concentration: 20 g / L, film-forming agent concentration: 75 g / L, auxiliary additive concentration: 7 g / L). An AZ91 magnesium alloy sample, ultrasonically cleaned with anhydrous ethanol for 5 min and then dried, was used as the anode, and a stainless steel plate as the cathode. The sample was placed in the prepared micro-arc oxidation electrolyte. The cooling water circulation system was turned on, and a standard DC pulse power supply was used, with the maximum voltage set to 120 V and the current density controlled at 4 A / dm³. 2 The sample was subjected to micro-arc oxidation treatment for 12 minutes at a frequency of 500 Hz, a duty cycle of 40%, and a temperature of 20 ℃. Immediately after the micro-arc oxidation treatment, the sample was removed and rinsed in flowing deionized water for 2 minutes to remove electrolyte residue. Then, it was dried with air at room temperature to obtain the final product.

[0040] Measurements showed that the thickness of the magnesium alloy micro-arc oxide film prepared in this comparative example was 25 μm, the L value was 77.9, and the solar absorptivity was 0.46. Compared with Example 1, this comparative example also prepared an oxide film on the surface of AZ91 magnesium alloy under low voltage conditions, but the electrolyte contained only inorganic alkali, film-forming agent, and auxiliary additives, resulting in a thinner film, a lower L value, and a higher solar absorptivity.

[0041] Comparative Example 2 A method for preparing a micro-arc oxidation film includes the following steps: Inorganic alkali (sodium hydroxide or potassium hydroxide), film-forming agent (sodium silicate), and auxiliary additive (sodium pyrophosphate) were added to deionized water and stirred thoroughly until completely dissolved to obtain a micro-arc oxidation electrolyte (potassium hydroxide or sodium hydroxide concentration of 2 g / L, film-forming agent sodium silicate concentration of 18 g / L, and auxiliary additive sodium pyrophosphate concentration of 5 g / L). An AZ91 magnesium alloy sample, ultrasonically cleaned with anhydrous ethanol for 5 min and then dried, was used as the anode, and a stainless steel plate as the cathode. The sample was placed in the prepared micro-arc oxidation electrolyte, and the cooling water circulation system was turned on. A bipolar pulsed micro-arc oxidation power supply was used, with the maximum voltage set to 500V and the current density controlled at 6 A / dm³. 2 The sample was subjected to micro-arc oxidation treatment for 20 minutes at a frequency of 800 Hz, a duty cycle of 20%, and a temperature of 20 °C. Immediately after the micro-arc oxidation treatment, the sample was removed and rinsed in running deionized water for 2 minutes to remove electrolyte residue. It was then dried with air at room temperature to obtain the final product.

[0042] Measurements showed that the thickness of the magnesium alloy micro-arc oxidation ceramic film prepared in this comparative example was 23 μm, the L value was 79.8, and the solar absorptivity was 0.43. Compared with Example 1, this comparative example also prepared an oxide film on the surface of AZ91 magnesium alloy, but the electrolyte contained only inorganic alkali, film-forming agent, and auxiliary additives at low concentrations, requiring high-voltage conditions for film formation. The resulting film was thinner, had a lower L value, and a higher solar absorptivity.

[0043] Comparative Example 3 A method for preparing a micro-arc oxidation film includes the following steps: Inorganic alkali (sodium hydroxide), film-forming agent (sodium silicate), and fluorine-containing colorant (potassium fluoride) were added to deionized water and stirred thoroughly until completely dissolved to obtain a micro-arc oxidation electrolyte (inorganic alkali concentration 50 g / L, film-forming agent concentration 20 g / L, fluorine-containing colorant concentration 20 g / L). An AZ31 magnesium alloy sample, ultrasonically cleaned with anhydrous ethanol for 5 min and then dried, was used as the anode, and a stainless steel plate as the cathode. The sample was placed in the prepared micro-arc oxidation electrolyte. The cooling water circulation system was turned on, and a standard DC pulse power supply was used, with the maximum voltage set to 100V and the current density controlled at 5 A / dm³. 2 The sample was subjected to micro-arc oxidation treatment for 15 minutes at a frequency of 800 Hz, a duty cycle of 20%, and a temperature of 25 ℃. Immediately after the micro-arc oxidation treatment, the sample was removed and rinsed in flowing deionized water for 2 minutes to remove electrolyte residue. Then, it was dried with air at room temperature to obtain the final product.

[0044] Measurements showed that the thickness of the magnesium alloy micro-arc oxide film prepared in this comparative example was 30 μm, the L value was 79.6, and the solar absorptivity was 0.40. Compared with Example 2, this comparative example also prepared an oxide film on the surface of AZ31 magnesium alloy under low voltage conditions, but the electrolyte contained only inorganic alkali, film-forming agent, and fluorinated colorant with a lower concentration of fluorinated colorant. The resulting film was thinner, had a lower L value, and a higher solar absorptivity.

[0045] Comparative Example 4 A method for preparing a micro-arc oxidation film includes the following steps: Inorganic alkali (sodium hydroxide), film-forming agent (sodium silicate), and fluorine-containing colorant (potassium fluoride) were added to deionized water and stirred thoroughly until completely dissolved to obtain a micro-arc oxidation electrolyte (inorganic alkali concentration 30 g / L, film-forming agent concentration 30 g / L, fluorine-containing colorant concentration 40 g / L). An AZ31 magnesium alloy sample, ultrasonically cleaned with anhydrous ethanol for 5 min and then dried, was used as the anode, and a stainless steel plate as the cathode. The sample was placed in the prepared micro-arc oxidation electrolyte. The cooling water circulation system was turned on, and a standard DC pulse power supply was used, with the maximum voltage set to 120 V and the current density controlled at 3.5 A / dm³. 2 The sample was subjected to micro-arc oxidation treatment for 25 minutes at a frequency of 500 Hz, a duty cycle of 40%, and a temperature of 20 ℃. Immediately after the micro-arc oxidation treatment, the sample was removed and rinsed in flowing deionized water for 2 minutes to remove electrolyte residue. Then, it was dried with air at room temperature to obtain the final product.

[0046] Measurements showed that the thickness of the magnesium alloy micro-arc oxide film prepared in this comparative example was 42 μm, the L value was 79.6, and the solar absorptivity was 0.38. Compared with Example 2, this comparative example also prepared an oxide film on the surface of AZ31 magnesium alloy under low voltage conditions, but the electrolyte contained only inorganic alkali, film-forming agent, and fluorinated colorant with a lower concentration of fluorinated colorant. The resulting film thickness was similar, but the L value was lower and the solar absorptivity was higher.

[0047] Comparative Example 5 A method for preparing a micro-arc oxidation film includes the following steps: Inorganic alkali (potassium hydroxide), film-forming agent (sodium silicate), and fluorine-containing colorant (potassium fluoride) were added to deionized water and stirred thoroughly until completely dissolved to obtain a micro-arc oxidation electrolyte (inorganic alkali concentration 1 g / L, film-forming agent concentration 15 g / L, fluorine-containing colorant concentration 5 g / L). An AZ31 magnesium alloy sample, ultrasonically cleaned with anhydrous ethanol for 5 min and then dried, was used as the anode, and a stainless steel plate as the cathode. The sample was placed in the prepared micro-arc oxidation electrolyte, and the cooling water circulation system was turned on. A bipolar pulsed micro-arc oxidation power supply was used, with the maximum voltage set to 450 V and the current density controlled at 6 A / dm³. 2The sample was subjected to micro-arc oxidation treatment for 20 minutes at a frequency of 1000 Hz, a duty cycle of 15%, and a temperature of 20 ℃. Immediately after the micro-arc oxidation treatment, the sample was removed and rinsed in flowing deionized water for 2 minutes to remove electrolyte residue. Then, it was dried with air at room temperature to obtain the final product.

[0048] Measurements showed that the thickness of the magnesium alloy micro-arc oxide film prepared in this comparative example was 28 μm, the L value was 81.4, and the solar absorptivity was 0.36. Compared with Example 2, this comparative example also prepared an oxide film on the surface of AZ31 magnesium alloy, but the electrolyte contained only inorganic alkali, film-forming agent, and fluorinated colorant at low concentrations, requiring high-voltage conditions for film formation. The resulting film was thinner, had a lower L value, and a higher solar absorptivity.

[0049] Performance testing Figure 1 Comparative photographs show the micro-arc oxide film prepared on the surface of AZ91 magnesium alloy under low voltage conditions in Comparative Example 1, the white micro-arc oxide film prepared in Example 1, and the micro-arc oxide film prepared under high voltage conditions in Comparative Example 2. In the photographs, a is the micro-arc oxide film prepared under low voltage conditions (Comparative Example 1), b is the white micro-arc oxide film prepared in this invention (Example 1), and c is the micro-arc oxide film prepared under high voltage conditions (Comparative Example 2). Figure 2 Comparative photos show the micro-arc oxide film prepared on the surface of AZ31 magnesium alloy under low voltage conditions in Comparative Example 3 and the white micro-arc oxide film prepared in Example 2. In the photos, a is the micro-arc oxide film prepared on the surface of AZ31 magnesium alloy under low voltage conditions (Comparative Example 3), and b is the white micro-arc oxide film prepared in this invention (Example 2).

[0050] Figure 3 The figures show the surface microstructures of the white micro-arc oxide films prepared on the surfaces of AZ91 and AZ31 magnesium alloys according to Examples 1 and 2, respectively. Figure a shows the surface microstructure of the white micro-arc oxide film on the AZ91 magnesium alloy surface, and figure b shows the surface microstructure of the white micro-arc oxide film on the AZ31 magnesium alloy surface. As can be seen from the figures, the white micro-arc oxide films prepared on the surfaces of AZ91 and AZ31 magnesium alloys in the embodiments of the present invention have similar structures, exhibiting an uneven surface with a small number of micropores.

[0051] Figure 4 The XRD pattern shows the white micro-arc oxidation film prepared on the surface of AZ91 magnesium alloy according to Example 1. As can be seen from the figure, diffraction peaks of three phases—Mg, MgO, and MgF2—are present in the XRD pattern. The Mg diffraction peak originates from the magnesium alloy substrate, indicating that the film is mainly composed of MgO and MgF2.

[0052] The solar absorptivity of the samples was tested according to the method specified in GJB 2502.2-2006. Since the micro-arc oxide films are all opaque materials, the solar absorptivity cannot be directly measured, but the solar reflectance spectrum can be directly measured. A spectrophotometer (PerkinElmer Lambda 950) was used to measure the simulated solar reflectance of the samples prepared in Example 1, Comparative Example 1, and Comparative Example 2. During the test, the samples were placed at the center of the integrating sphere, and the light reflectance spectrum of the samples in the wavelength range of 200 nm to 2500 nm was measured. Figure 5 Comparison of simulated solar reflectance spectra of the micro-arc oxide film prepared under low voltage conditions on the surface of AZ91 magnesium alloy in Comparative Example 1, the white micro-arc oxide film prepared in Example 1, and the micro-arc oxide film prepared under high voltage conditions in Comparative Example 2. (a) shows the micro-arc oxide film prepared under low voltage conditions (Comparative Example 1), (b) shows the white micro-arc oxide film prepared according to this invention (Example 1), and (c) shows the micro-arc oxide film prepared under high voltage conditions (Comparative Example 2). The simulated solar reflectance of the samples was calculated using the following formula:

[0053] ; In the formula, ρ s This represents the total simulated solar reflectance. ρλ i wavelength λ i Reflectance within the range; Δλ i For (1 / 2) λ i +1− λ i ); E s ( λi The simulated solar spectral irradiance within the wavelength range λi is expressed in W / m². 2 ·μm; n This represents the number of test points.

[0054] The solar absorptivity of the sample is calculated using the following formula: ; In the formula, a s The solar absorptivity is the ratio of solar energy absorbed. The comparison of solar absorptivity is consistent with the descriptions in the above embodiments and comparative examples.

[0055] UV340 lamps were used as the ultraviolet light source for the ultraviolet irradiation experiment, with an irradiation intensity of 0.55 W / m². 2 The irradiation time is 15 days. Figure 6Macroscopic images of the white micro-arc oxide film prepared in Example 1 on the surface of AZ91 magnesium alloy before and after 15 days of ultraviolet irradiation are compared. In the image, a is the image before irradiation, and b is the image after irradiation. The comparison shows that the white micro-arc oxide film did not change color after 15 days of ultraviolet irradiation, indicating that the film layer has excellent resistance to ultraviolet radiation.

[0056] In existing technologies, white films with an L value > 85 can be prepared on the surfaces of aluminum and titanium alloys through micro-arc oxidation. However, it is difficult to obtain white ceramic films with a high L value (whiteness) on magnesium alloy surfaces, which cannot meet the requirements of decorative or thermal control applications where high whiteness is required. Furthermore, the solar absorptivity of existing magnesium alloy micro-arc oxidation films is typically high (> 0.35), which is insufficient to meet the requirements of aerospace and other fields for low solar absorptivity (α). s The invention addresses the stringent requirement of <0.3% whiteness; furthermore, traditional organic white coatings (such as spraying / electrophoresis processes) are prone to aging and discoloration under the extreme environment of space (temperature cycling, ultraviolet radiation, atomic oxygen), leading to a decrease in whiteness. Existing technologies for achieving low absorptivity or high whiteness, such as multi-step micro-arc oxidation + magnetron sputtering composite processes, are complex, energy-intensive (requiring high-voltage power supplies >180V), and partially rely on high-cost / high-safety-risk electrolytes or dedicated power supply equipment (such as bipolar pulse power supplies), increasing industrial production costs and operational difficulty. This invention employs an alkaline system containing specific additives and a low-voltage micro-arc oxidation process (<120V) to prepare a high L-value (≥85) and low solar absorptivity (α) on magnesium alloy surfaces in a single step. s This invention produces a white ceramic film with a solar absorptivity of <0.3%, improving process economy and meeting the needs of aerospace, 3C electronics, and other fields for high-performance thermal control and decorative coatings on lightweight material surfaces. The micro-arc oxidation electrolyte of this invention is an alkaline solution composed of inorganic alkali, fluorides, and additives. A white film layer can be prepared on magnesium alloy surfaces using a common DC power supply under low voltage (<120V) conditions. The electrolyte of this invention has better component stability, is non-corrosive to equipment, and has lower raw material costs and energy consumption. Furthermore, this invention requires only one step of micro-arc oxidation to prepare a white film layer with a low solar absorptivity, eliminating the need for multiple steps and simplifying the process.

[0057] Patent application number 201911383550.5 discloses a micro-arc oxidation electrolyte composed of a strong alkali, fluoride, and a small amount of additives, and uses a low voltage (<120V) for micro-arc oxidation treatment. In contrast, the electrolyte in this patent is an acidic solution composed of potassium fluorozirconate, hydrofluoric acid, and hydrogen peroxide, with an oxidation voltage of 180~750V. The electrolyte of this invention is an alkaline solution, which is non-corrosive to equipment and has lower raw material costs and energy consumption. Patent application number 202310168300.X discloses a micro-arc oxidation electrolyte composed of a strong alkali, fluoride, and a small amount of additives, and uses a common DC power supply under low voltage (<120V) conditions to prepare a white film on the magnesium alloy surface. In contrast, the comparative patent uses sodium aluminate, a strong alkali, sodium fluoride, potassium titanate, sodium stannate, and triethanolamine as the electrolyte, and requires a dedicated bipolar pulse power supply to prepare a bright white ceramic film on the magnesium alloy surface. The electrolyte of this invention exhibits better stability and lower equipment requirements. Patent application number 202510062988.2 requires multiple steps of micro-arc oxidation and magnetron sputtering to prepare a multilayer film, resulting in a complex process. This invention requires only a single micro-arc oxidation step to prepare a white film with low solar absorptivity, eliminating the need for multiple steps and simplifying the process.

[0058] Although preferred embodiments of the present invention have been described, those skilled in the art, upon learning the inventive concept of the present invention, can make other changes and modifications to these embodiments, all of which fall within the scope of the present invention.

[0059] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. If such modifications and variations fall within the scope of equivalents of this invention, then this invention also intends to include these modifications and variations.

Claims

1. A method for preparing a white micro-arc oxidation film on a magnesium alloy surface, characterized by, The method comprises the following steps: The white micro-arc oxidation film with magnesium oxide / magnesium fluoride composite film structure is formed on the surface of the magnesium alloy substrate by micro-arc oxidation treatment in an electrolyte with the magnesium alloy substrate as an anode; The electrolyte is obtained by mixing inorganic alkali 10-70 g / L, film forming agent 5-30 g / L, fluorine-containing colorant 10-100 g / L, discharge regulator 5-20 mL / L, auxiliary additive 0.5-10 g / L and surfactant 0.1-1 g / L; The voltage in the micro-arc oxidation treatment process is 60V-120V, and the current density is 2A / dm 2 ~10A / dm 2 .

2. The method for preparing a white micro-arc oxide film on the surface of a magnesium alloy according to claim 1, characterized in that, During the micro-arc oxidation treatment, the temperature of the electrolyte is 10-30 ℃, and the micro-arc oxidation treatment time is 20-40 min.

3. The method for preparing a white micro-arc oxide film on the surface of a magnesium alloy according to claim 1, characterized in that, The fluorine-containing colorant is one or more of sodium fluoride, potassium fluoride and ammonium fluoride.

4. The method for preparing a white micro-arc oxide film on the surface of a magnesium alloy according to claim 1, characterized in that, The inorganic alkali is one or more of sodium hydroxide, potassium hydroxide and sodium carbonate.

5. The method for preparing a white micro-arc oxide film on the surface of a magnesium alloy according to claim 1, characterized in that, The film forming agent is one or more of sodium phosphate, sodium silicate and sodium aluminate.

6. The method for preparing a white micro-arc oxide film on the surface of a magnesium alloy according to claim 1, characterized in that, The discharge regulator is one or more of ethylene glycol, propylene glycol and glycerol.

7. The method for preparing a white micro-arc oxidation film on the surface of a magnesium alloy according to claim 1, characterized in that, The auxiliary additive is one or more of sodium tetraborate, sodium hexametaphosphate and sodium pyrophosphate.

8. The method for preparing a white micro-arc oxide film on the surface of a magnesium alloy according to claim 1, characterized in that, The surfactant is one or more of sodium dodecyl sulfate, sodium dodecyl benzene sulfonate and sodium polyacrylate.

9. The method of claim 1, wherein the white micro-arc oxidation film is prepared by the following steps: (1) cleaning the magnesium alloy surface; (2) coating the magnesium alloy surface with a white micro-arc oxidation film; (3) drying the white micro-arc oxidation film; and (4) polishing the white micro-arc oxidation film. The micro-arc oxidation treatment adopts a direct current pulse power source with a frequency of 200-800 Hz and a duty cycle of 20-60%.

10. A white micro-arc oxidation film on a surface of a magnesium alloy, characterized by, The white micro-arc oxidation film on the surface of the magnesium alloy is prepared by the method for preparing the white micro-arc oxidation film on the surface of the magnesium alloy according to any one of claims 1-9.

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