Preparation method of magnesium-aluminum alloy material loaded with steam hydrotalcite coating on surface
The dense steam hydrotalcite coating was prepared on the surface of magnesium-aluminum alloy by alkali heat treatment and in-situ steam method, which solved the problem of poor corrosion resistance of magnesium alloy, achieved the improvement of the high adhesion and corrosion resistance of the coating, and was suitable for marine atmospheric environments.
Patent Information
- Application Number
- CN202510347272.7
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-03-24
- Publication Date
- 2025-07-22
AI Technical Summary
The corrosion resistance of existing magnesium alloys is poor, especially in corrosive environments, and local corrosion is prone to occur. The adhesion of conventional hydrotalcite coatings is poor and the thickness is thin, resulting in limited service life of magnesium alloys.
The steam hydrotalcite coating was prepared on the surface of magnesium-aluminum alloy by alkali heat treatment combined with in-situ steam method. The inhibitory effect of the aluminum-rich phase was weakened through alkali heat treatment, the hydrotalcite formation was promoted, and the dense coating was formed by water vapor reaction under high temperature and high pressure.
The prepared steam hydrotalcite coating is dense and has strong adhesion, which significantly improves the corrosion resistance of magnesium alloys and is especially suitable for marine atmospheric environments with strong corrosiveness.
Smart Images
Figure CN120350338A_ABST
Abstract
Description
Technical Field
[0001] The present invention relates to the technical field of the preparation of surface coatings for magnesium-aluminum alloys, and specifically relates to a method for preparing a magnesium-aluminum alloy material with a surface-loaded steam hydrotalcite coating. Background Art
[0002] Magnesium alloys are alloys composed of magnesium as the base and other elements added. They have excellent physical and mechanical properties, such as light weight, high specific strength, damping capacity, and dimensional stability. Due to their excellent properties, they are widely used in transportation, aerospace, and the 3C industries (communications, computers, consumer electronic products). In addition, magnesium alloy products can be 100% recycled after simple treatment, making them an environmentally friendly material. However, due to the relatively low electrode potential of magnesium alloys and poor corrosion resistance, their huge application potential is limited. In addition, its oxide film is loose and porous, unable to form an effective and stable protective film, and is prone to local corrosion (i.e., pitting and galvanic corrosion) in erosive environments. This reality greatly limits its long-term service life.
[0003] Currently, enhancing the corrosion resistance of magnesium alloys mainly includes the following two aspects: controlling metallurgical factors and surface treatment technologies. Controlling metallurgical factors includes alloy components, impurity elements, microstructure, and phase components. Alloying elements can improve the corrosion resistance of magnesium alloys by refining grains, changing the composition and structure of the surface oxide film and corrosion product film. In magnesium alloys with a high aluminum content, in addition to partial solid solution of aluminum in the magnesium matrix, it also exists in the form of β -Mg 17 Al 12 and AlMn phase (the second phase). Although the second-phase particles provide the Al 3+ ions required for the growth of hydrotalcite in the steam coating, they also inhibit the formation of hydrotalcite to a certain extent. Controlling metallurgical factors alone cannot meet the requirements of magnesium alloy applications. To further improve corrosion resistance, various surface treatment methods have been studied, mainly including chemical conversion treatment, anodic oxidation, physical vapor deposition technology, metal plating, sol-gel method, laser heat treatment, etc.
[0004] As a protective coating after surface treatment, the hydrotalcite coating has a layered flake structure with cross-arrangement. Destructive ions in the corrosive medium can invade the interior of the coating through the pores in the outer layer, thereby damaging the substrate. The hydrotalcite coating prepared by the conventional two-step method has the disadvantages of poor adhesion, relatively thin coating thickness, and time-consuming and laborious. Therefore, it is necessary to improve the denseness and thickness of the hydrotalcite coating, enhance the barrier effect and ion exchange effect of the hydrotalcite coating, so as to improve the corrosion resistance of the substrate.
[0005] Existing problems such as high treatment cost and complex process generally exist on the surface of magnesium-aluminum alloy. Therefore, to further improve the corrosion resistance of the hydrotalcite coating and extend the protection time of the coating for the magnesium-aluminum alloy substrate, we need a method that can not only increase the density and thickness of the hydrotalcite coating surface but also has the characteristics of being simple and easy to control, low cost, and environmentally friendly to prepare the hydrotalcite coating. Summary of the Invention
[0006] The object of the present invention is to provide a preparation method of a magnesium-aluminum alloy material with a steam hydrotalcite coating loaded on its surface. Its process is environmentally friendly, simple and easy to control. The prepared magnesium-aluminum alloy material has a steam coating loaded on its surface, the coating structure is dense and has strong adhesion, making the material have good corrosion resistance.
[0007] To achieve the above object, the present invention adopts the following technical solutions: A preparation method of a magnesium-aluminum alloy material with a steam hydrotalcite coating loaded on its surface, comprising the following steps: (1) Subject the polished magnesium-aluminum alloy substrate to alkali heat treatment to obtain a magnesium-aluminum alloy precursor; the purpose is to weaken the inhibition of the aluminum-rich phase on the growth of the coating. (2) Prepare a steam hydrotalcite coating on the surface of the magnesium-aluminum alloy precursor by the in-situ steam method to obtain a magnesium-aluminum alloy material with a steam hydrotalcite coating loaded on its surface.
[0008] Preferably, the alkali heat treatment is specifically: immerse the magnesium-aluminum alloy substrate in a sodium hydroxide solution, take it out after immersion, wash and dry to obtain the magnesium-aluminum alloy precursor; the concentration of the sodium hydroxide is 2.5-3.5 wt%, the immersion temperature is 50 °C, and the immersion time is 15-25 min.
[0009] Preferably, the alkali heat treatment is specifically: immerse the magnesium-aluminum alloy substrate in an alkali mixture of sodium hydroxide and sodium carbonate or sodium hydroxide and sodium bicarbonate, take it out after immersion, wash and dry to obtain the magnesium-aluminum alloy precursor; the concentration of sodium hydroxide in the alkali mixture is 1.5-2.5 wt%, the concentration of sodium carbonate or sodium bicarbonate is 9-11 wt%, the immersion temperature is 50 °C, and the immersion time is 15-25 min. Ensure that insufficient corrosion products are formed at too low a temperature and complex corrosion products are formed at too high a temperature. Take out, wash with pure water and dry. More preferably, the concentration of sodium hydroxide in the alkali mixture is 2%, and the concentration of sodium bicarbonate (sodium carbonate) is 10%.
[0010] Preferably, the step (2) is specifically as follows: suspend the magnesium-aluminum alloy precursor inside a reaction vessel filled with deionized water, with the height of the lower surface of the magnesium-aluminum alloy precursor being higher than the liquid level of the deionized water; under a closed environment, heat the reaction vessel to evaporate the deionized water and cause a chemical reaction with the magnesium-aluminum alloy precursor to obtain a magnesium-aluminum alloy material with a steam coating loaded on its surface; the heating temperature is 150-160 °C, and the heating time is 4-6 hours.
[0011] Preferably, the aluminum content in the magnesium-aluminum alloy matrix is 5-9 wt%.
[0012] Compared with the prior art, the technical solution of the present invention has the following beneficial effects: (1) In the present invention, a Mg-Al series magnesium-aluminum alloy with a high aluminum content is selected. Since its actual crystallization process mostly occurs under non-equilibrium conditions, aluminum mainly exists in the form of β phase, and a small amount exists in the form of aluminum solid solution. β The electrode potential of the β phase is much higher than that of magnesium, which makes it difficult for the aluminum element in the 3+ phase to generate Al β to participate in the reaction. On the one hand, alkali heat treatment can utilize the electrode potential difference between the α -Mg phase and the β phase to accelerate the corrosion of α -Mg around the β phase, so that the β phase is covered by magnesium hydroxide, reducing the galvanic corrosion in subsequent reactions, which is beneficial for the 3+ phase to be converted into the Al
[0013] ions necessary for the growth of hydrotalcite and promoting the formation of hydrotalcite; on the other hand, alkali heat treatment weakens the inhibition of the aluminum phase on the steam reaction, increases the activity of the aluminum-rich phase on the surface of the magnesium-aluminum alloy, makes the coating thicker and the surface more dense, which is beneficial for blocking the contact with corrosive media and slowing down the corrosion rate.
[0014] (3) The present invention selects the in-situ steam method to prepare the steam hydrotalcite coating. Under high temperature and high pressure, water vapor will penetrate through the porous magnesium hydroxide "shell" and continue to react with the substrate to form an alkaline pH environment suitable for the growth of hydrotalcite. This growth mode from the inside out will greatly improve the adhesion of the coating. The present invention controls parameters such as the reaction time, temperature, and the distance between the sample and the liquid surface to achieve the improvement of the corrosion resistance of the sample. Compared with the prior art, the in-situ steam method only requires water as the reaction source, and the elements required during the growth of hydrotalcite also come from the surface of the magnesium-aluminum alloy substrate. No other chemical reagents need to be added during the whole reaction process. It is simple to operate, environmentally friendly, and inexpensive; (4) The steam hydrotalcite coating obtained by the present invention has the characteristics of a dense structure, good corrosion resistance, and a long service life. This surface-modified magnesium-aluminum alloy used as a structural material is particularly suitable for the marine atmospheric environment with strong corrosion. Brief Description of the Drawings
[0015] The present invention will be further described below in conjunction with the drawings: Figure 1 (a) in it is the scanning electron microscope photograph of the steam hydrotalcite coating prepared without alkali heat treatment in Comparative Example 1; (b) is the scanning electron microscope photograph of the steam hydrotalcite coating prepared with alkali heat treatment in Example 1 (the magnification is 10,000 times for both); Figure 2 (a) in it is the cross-sectional scanning electron microscope photograph of the steam hydrotalcite coating prepared without alkali heat treatment in Comparative Example 1; (b) is the cross-sectional scanning electron microscope photograph of the steam hydrotalcite coating prepared with alkali heat treatment in Example 1 (the magnification is 2,500 times for both); Figure 3 is the X-ray diffraction (XRD) pattern of the steam hydrotalcite coating prepared without alkali heat treatment in Comparative Example 1 and the steam hydrotalcite coating prepared with alkali heat treatment in Example 1.
[0016] Figure 4 is the potentiodynamic polarization (tafel) curve of the steam hydrotalcite coating prepared without alkali heat treatment in Comparative Example 1, the steam hydrotalcite coating prepared with alkali heat treatment in Example 1, and the AZ91D magnesium-aluminum alloy substrate; Figure 5 is the Bode diagram of the steam hydrotalcite coating prepared without alkali heat treatment in Comparative Example 1, the steam hydrotalcite coating prepared with alkali heat treatment in Example 1, and the AZ91D magnesium-aluminum alloy substrate; Figure 6 is the electrochemical Nyquist diagram of the steam hydrotalcite coating prepared without alkali heat treatment in Comparative Example 1, the steam hydrotalcite coating prepared with alkali heat treatment in Example 1, and the AZ91D magnesium-aluminum alloy substrate; Figure 7Figure (a) shows SEM images of AZ91D magnesium alloy at different time intervals under neutral salt spray conditions in 5 wt.% NaCl solution; Figure (b) shows SEM images of the magnesium alloy material prepared in Comparative Example 1 (with a steam hydrotalcite coating prepared without alkali heat treatment on its surface) at different time intervals under neutral salt spray conditions in 5 wt.% NaCl solution; Figure (c) shows SEM images (magnification 2000 and 8000 times) of the magnesium alloy material prepared in Example 1 (with a steam hydrotalcite coating prepared by alkali heat treatment on its surface) under neutral salt spray conditions in 5 wt.% NaCl solution. Detailed Embodiments
[0017] The present invention provides a method for preparing a magnesium alloy material with a steam hydrotalcite coating on its surface. To make the advantages and technical solutions of the present invention clearer and more definite, the present invention will be described in detail below with reference to specific embodiments and the accompanying drawings.
[0018] The raw materials required for the following examples and comparative examples can all be obtained through commercial channels. Example 1
[0019] The substrate material is AZ91D magnesium alloy (about 9% aluminum, about 1% zinc, and about 90% magnesium). The steps for preparing a steam hydrotalcite coating on the surface of AZ91D magnesium alloy are as follows: (1) Cut the AZ91D magnesium alloy into a cuboid of 20×20×5 mm 3 and drill holes, then polish it successively with 150#, 400#, 800#, 1500#, and 2500# SiC sandpapers until there are no obvious scratches on the surface of the magnesium alloy substrate. Subsequently, rinse it with distilled water and anhydrous ethanol respectively, and dry it with cold air. (2) Prepare an alkali mixture containing 2% sodium hydroxide and 10% sodium bicarbonate per liter of pure water and pour it into a small beaker for standby. Then, place the small beaker in a water bath and heat it to 50°C. Immerse the polished AZ91D magnesium alloy in the alkali solution for 20 minutes and then take it out, wash it with pure water and dry it to obtain a magnesium alloy precursor. (3) Measure 20 mL of pure water with a measuring cylinder and add it to a polytetrafluoroethylene liner with a volume capacity of 100 mL. Hang the magnesium alloy precursor at the top of the liner so that the lower surface of the magnesium alloy sample is about 2 cm away from the liquid level, and ensure that the sample is hung properly and will not fall. Place the liner in a high-temperature and high-pressure reaction kettle, and then put the reaction kettle into an electrothermal constant-temperature forced-air drying oven preheated to 150°C. Heat it at 150°C for 5 hours. After the reaction is completed, take it out and cool it naturally to room temperature to obtain a magnesium alloy material with a steam hydrotalcite coating. Example 2
[0020] The substrate material is AZ91D magnesium alloy, and the steps for preparing a steam hydrotalcite coating on the surface of AZ91D magnesium alloy are as follows: (1) Cut the AZ91D magnesium alloy into a cuboid of 20×20×5 mm 3 and drill holes. Then, use SiC sandpapers of 150#, 400#, 800#, 1500#, and 2500# to polish in sequence until there are no obvious scratches on the surface of the magnesium alloy substrate. Subsequently, rinse with distilled water and anhydrous ethanol respectively, and dry with cold air; (2) Prepare an alkali mixture containing 2% sodium hydroxide and 10% sodium bicarbonate per liter of pure water, pour it into a small beaker for standby. Then, place the small beaker in a water bath and heat it to 50°C. Immerse the polished AZ91D magnesium alloy in the alkali solution for 15 minutes and take it out. Then, wash it with pure water and dry it to obtain a magnesium alloy precursor; (3) Measure 20 mL of pure water with a measuring cylinder and add it to a polytetrafluoroethylene liner with a volume capacity of 100 mL. Hang the magnesium alloy precursor at the top of the liner so that the lower surface of the magnesium alloy sample is about 2 cm away from the liquid level, and ensure that the sample is hung well and will not fall. Place the liner into a high-temperature and high-pressure reactor, and then put the reactor into an electrothermal constant-temperature forced-air drying oven preheated to 150°C. Heat it at 150°C for 5 hours. After the reaction is completed, take it out and cool it naturally to room temperature to obtain a magnesium alloy material loaded with a steam hydrotalcite coating. Example 3
[0021] The substrate material is AZ91D magnesium alloy, and the steps for preparing a steam hydrotalcite coating on the surface of AZ91D magnesium alloy are as follows: (1) Cut the AZ91D magnesium alloy into a cuboid of 20×20×5 mm 3 and drill holes. Then, use SiC sandpapers of 150#, 400#, 800#, 1500#, and 2500# to polish in sequence until there are no obvious scratches on the surface of the magnesium alloy substrate. Subsequently, rinse with distilled water and anhydrous ethanol respectively, and dry with cold air; (2) Prepare an alkali mixture containing 2% sodium hydroxide and 10% sodium bicarbonate per liter of pure water, pour it into a small beaker for standby. Then, place the small beaker in a water bath and heat it to 50°C. Immerse the polished AZ91D magnesium alloy in the alkali solution for 25 minutes and take it out. Then, wash it with pure water and dry it to obtain a magnesium alloy precursor; (3) Measure 20 mL of pure water with a graduated cylinder and add it to a polytetrafluoroethylene inner liner with a volume capacity of 100 mL. Hang the magnesium-aluminum alloy precursor at the top of the inner liner so that the lower surface of the magnesium-aluminum alloy sample is about 2 cm away from the liquid level, and ensure that the sample is hung properly and will not fall. Place the inner liner into a high-temperature and high-pressure reaction kettle, and then put the reaction kettle into an electrothermal constant-temperature forced-air drying oven preheated to 150 °C. Heat it at 150 °C for 5 hours. After the reaction is completed, take it out and let it cool naturally to room temperature to obtain a magnesium-aluminum alloy material loaded with a steam hydrotalcite coating. Example 4
[0022] The matrix material is AZ80 magnesium-aluminum alloy (about 8% aluminum, about 0.4 - 1.0% zinc, about 0.20 - 0.60% manganese, a small amount of silicon and iron, and the balance is magnesium). The steps for preparing a steam hydrotalcite coating on the surface of AZ80 magnesium-aluminum alloy are as follows: (1) Grind the surface of the AZ80 magnesium-aluminum alloy: Cut the AZ80 magnesium-aluminum alloy into a cuboid of 20×20×5 mm 3 and drill holes, and then use 150#, 400#, 800#, 1500#, and 2500# SiC sandpapers to polish it in sequence until there are no obvious scratches on the surface of the magnesium-aluminum alloy matrix. Subsequently, rinse it with distilled water and anhydrous ethanol respectively, and dry it with cold air; (2) Prepare an alkali mixture containing 2% sodium hydroxide and 10% sodium bicarbonate per liter of pure water, pour it into a small beaker for standby. Then place the small beaker in a water bath and heat it to 50 °C. Immerse the polished AZ80 magnesium-aluminum alloy in the alkali solution for 20 min and take it out, and then wash it with pure water and dry it to obtain a magnesium-aluminum alloy precursor; (3) Measure 20 mL of pure water with a graduated cylinder and add it to a polytetrafluoroethylene inner liner with a volume capacity of 100 mL. Hang the magnesium-aluminum alloy precursor at the top of the inner liner so that the magnesium-aluminum alloy sample is about 2 cm away from the liquid level, and ensure that the sample is hung properly and will not fall. Place the inner liner into a high-temperature and high-pressure reaction kettle, and then put the reaction kettle into an electrothermal constant-temperature forced-air drying oven preheated to 150 °C. Heat it at 150 °C for 5 hours. After the reaction is completed, take it out and let it cool naturally to room temperature to obtain a magnesium-aluminum alloy material loaded with a steam hydrotalcite coating. Example 5
[0023] The matrix material is AM50 magnesium-aluminum alloy (about 5% aluminum, about 0.05 - 0.5% manganese, a small amount of lead, zinc, and copper, and the balance is magnesium). The steps for preparing a steam hydrotalcite coating on the surface of AM50 magnesium-aluminum alloy are as follows: (1) Grind the surface of the AM50 magnesium-aluminum alloy: Cut the AM50 magnesium-aluminum alloy into a cuboid of 20×20×5 mm 3A cuboid is punched, and then polished successively with 150#, 400#, 800#, 1500#, and 2500# SiC sandpapers until there are no obvious scratches on the surface of the magnesium alloy matrix. Subsequently, it is rinsed with distilled water and anhydrous ethanol respectively, and dried with cold air; (2)Prepare an alkali mixture containing 2% sodium hydroxide and 10% sodium bicarbonate per liter of pure water, pour it into a small beaker for standby. Subsequently, place the small beaker in a water bath and heat it to 50°C. Immerse the polished AM50 magnesium alloy in the alkali solution for 20 minutes and then take it out, and then wash and dry it with pure water to obtain a magnesium alloy precursor; (3)Measure 20 mL of pure water with a measuring cylinder and add it to a polytetrafluoroethylene lining with a volume capacity of 100 mL. Hang the magnesium alloy precursor at the top of the lining, keeping a distance of about 2 cm between the magnesium alloy sample and the liquid surface, and ensure that the sample is hung properly and will not fall. Place the lining into a high-temperature and high-pressure reaction kettle, and then put the reaction kettle into an electrothermal constant-temperature forced-air drying oven preheated to 150°C. Heat it at 150°C for 5 hours. After the reaction is completed, take it out and cool it naturally to room temperature to obtain a magnesium alloy material coated with steam hydrotalcite. Example 6
[0024] The matrix material is AZ91D magnesium alloy, and it is taken from the same magnesium alloy plate as in Example 1. The difference between this Example 6 and Example 1 is that the alkali heat treatment solution in this example is a 3% sodium hydroxide solution, and the rest are the same as in Example 1. Example 7
[0025] The matrix material is AZ91D magnesium alloy, and it is taken from the same magnesium alloy plate as in Example 1. The difference between this Example 7 and Example 1 is that the alkali heat treatment solution in this example is a 2.5% sodium hydroxide solution, and the alkali heat treatment time is 15 minutes, and the rest are the same as in Example 1. Example 8
[0026] The matrix material is AZ91D magnesium alloy, and it is taken from the same magnesium alloy plate as in Example 1. The difference between this Example 8 and Example 1 is that the alkali heat treatment solution in this example is a 3.5% sodium hydroxide solution, and the alkali heat treatment time is 25 minutes, and the rest are the same as in Example 1. Example 9
[0027] The matrix material is AZ91D magnesium alloy, and it is taken from the same magnesium alloy plate as in Example 1. The difference between this Example 9 and Example 1 is that the reaction kettle in this example is heated at 160°C, and the rest are the same as in Example 1. Example 10
[0028] The substrate material is AZ91D magnesium alloy, and it is taken from the same magnesium alloy sheet as that in Example 1. The difference between this Example 10 and Example 1 is that the reaction kettle in this example is heated for 4 hours, and the rest is the same as in Example 1. Example 11
[0029] The substrate material is AZ91D magnesium alloy, and it is taken from the same magnesium alloy sheet as that in Example 1. The difference between this Example 11 and Example 1 is that the reaction kettle in this example is heated for 6 hours, and the rest is the same as in Example 1. Example 12
[0030] The substrate material is AZ91D magnesium alloy, and it is taken from the same magnesium alloy sheet as that in Example 1. The difference between this Example 12 and Example 1 is that the sodium bicarbonate in the alkali heat treatment solution in this example is replaced with sodium carbonate of the same concentration, and the rest is the same as in Example 1. Example 13
[0031] The substrate material is AZ91D magnesium alloy, and it is taken from the same magnesium alloy sheet as that in Example 1. The difference between this Example 13 and Example 1 is that the composition of the alkali heat treatment solution in this example is 1.5% sodium hydroxide and 9% sodium bicarbonate, and the rest is the same as in Example 1. Example 14
[0032] The substrate material is AZ91D magnesium alloy, and it is taken from the same magnesium alloy sheet as that in Example 1. The difference between this Example 14 and Example 1 is that the composition of the alkali heat treatment solution in this example is 2.5% sodium hydroxide and 11% sodium bicarbonate, and the rest is the same as in Example 1. Comparative Example 1
[0033] The substrate material is AZ91D magnesium alloy, and it is taken from the same magnesium alloy sheet as that in Example 1. The steps for preparing a steam hydrotalcite coating on the surface of AZ91D magnesium alloy are as follows: (1) Cut the AZ91D magnesium alloy into a cuboid of 20×20×5 mm 3 and drill holes, then polish it successively with 150#, 400#, 800#, 1500#, and 2500# SiC sandpapers until there are no obvious scratches on the surface of the magnesium alloy substrate, and then rinse it with distilled water and absolute ethanol respectively, and dry it with cold air; (2) Measure 20 mL of pure water with a graduated cylinder and add it to a polytetrafluoroethylene liner with a volume capacity of 100 mL. Suspend the polished magnesium alloy on the top of the liner so that the lower surface of the magnesium alloy sample is about 2 cm away from the liquid level, and ensure that the sample is well suspended and will not fall. Place the liner into a high-temperature and high-pressure reactor, and then put the reactor into an electrothermal constant-temperature forced-air drying oven preheated to 150 °C. Heat at 150 °C for 5 hours. After the reaction is completed, take it out and cool it naturally to room temperature to obtain a magnesium alloy material coated with steam hydrotalcite. Comparative Example 2
[0034] The substrate material is AZ91D magnesium alloy, and it is taken from the same magnesium alloy sheet as that in Example 1. The difference between this Comparative Example 2 and Example 1 is that the alkali heat treatment time in the preparation of the steam hydrotalcite coating in this comparative example is 5 min, and the rest are the same as those in Example 1. Comparative Example 3
[0035] The substrate material is AZ91D magnesium alloy, and it is taken from the same magnesium alloy sheet as that in Example 1. The difference between this Comparative Example 3 and Example 1 is that the alkali heat treatment time in the preparation of the steam hydrotalcite coating in this comparative example is 50 min, and the rest are the same as those in Example 1. Comparative Example 4
[0036] The substrate material is AZ91D magnesium alloy, and it is taken from the same magnesium alloy sheet as that in Example 1. The difference between this Comparative Example 4 and Example 1 is that the alkali heat treatment temperature in the preparation of the steam hydrotalcite coating in this comparative example is room temperature, and the rest are the same as those in Example 1.
[0037] Select Example 1 as the representative example, and conduct the following Figures 1-7 relevant tests on Example 1 and Comparative Example 1. The test results and analysis are as follows.
[0038] Figure 1 (a) and (b) are the scanning electron microscope photos (magnification: 10,000 times) of the steam hydrotalcite coating prepared without alkali heat treatment in Comparative Example 1 and the hydrotalcite coating prepared with alkali heat treatment in Example 1, respectively. As Figure 1 shown in (a) and (b), the hydrotalcite coating prepared on the surface of the AZ91D magnesium alloy with alkali heat treatment in Example 1 is denser than the hydrotalcite coating prepared on the surface of the AZ91D magnesium alloy without alkali heat treatment in Comparative Example 1, and its gaps are getting smaller and smaller. This is related to the increase in the hydrotalcite content in the coating, which can play a "sealing hole" role on the microscopic surface of the sample.
[0039] Figure 2(a) and (b) are the cross-sectional scanning electron microscope photos (magnification: 2500 times) of the steam hydrotalcite coating prepared without alkali heat treatment in Comparative Example 1 and the steam hydrotalcite coating prepared with alkali heat treatment in Example 4, respectively. As Figure 2 shown in (a) and (b), it can be found that the thickness of the hydrotalcite coating prepared on the surface of the AZ91D magnesium alloy after alkali heat treatment in Example 1 increased from 24.13 µm to 55.86 µm compared with the coating prepared on the surface of the AZ91D magnesium alloy without alkali heat treatment in Comparative Example 1, indicating that the coating grows faster after alkali heat treatment and the coating thickness increases significantly. At the same time, it also further shows that after alkali heat treatment, to a certain extent, the problem of β phase inhibiting corrosion is overcome, more magnesium hydroxide is generated, and the coating becomes thicker.
[0040] Figure 3 are the X-ray diffraction (XRD) patterns of the steam hydrotalcite coating prepared without alkali heat treatment in Comparative Example 1 and the steam hydrotalcite coating prepared with alkali heat treatment in Example 1. As Figure 3 shown, characteristic peaks of the (003) and (006) planes of the hydrotalcite layered structure appear at diffraction angles of 11.27 o and 23. The characteristic diffraction peaks of hydrotalcite in the hydrotalcite coating prepared on the surface of the AZ91D magnesium alloy after alkali heat treatment in Example 1 are stronger, indicating that the hydrotalcite content in the hydrotalcite coating prepared after alkali heat treatment is higher.
[0041] Figure 4 is a comparison of the potentiodynamic polarization (tafel) curves of the steam hydrotalcite coating prepared without alkali heat treatment in Comparative Example 1, the steam hydrotalcite coating prepared with alkali heat treatment in Example 1, and the AZ91D magnesium alloy substrate. From Figure 4 it can be seen that the current densities of the anodic and cathodic branches of the coated samples are both inhibited, indicating that the steam coating improves the corrosion resistance of the substrate. Compared with the hydrotalcite coating without alkali heat treatment, the self-corrosion potential of the hydrotalcite coating prepared on the surface of the AZ91D magnesium alloy after alkali heat treatment increased from -1350 mV to -1061 mV, and the self-corrosion current density decreased from 1.71×10 -6 A·cm -2 to 1.19×10 -8 A·cm -2 , a decrease of two orders of magnitude, further indicating that alkali heat treatment plays a positive role in the process of preparing the hydrotalcite coating, and the corrosion resistance of the magnesium alloy material prepared by alkali heat treatment is stronger.
[0042] Figure 5 is a comparison of the Bode diagrams of the steam hydrotalcite coating prepared without alkali heat treatment in Comparative Example 1, the steam hydrotalcite coating prepared with alkali heat treatment in Example 1, and the AZ91D magnesium alloy substrate. FromFigure 5 It can be seen that, compared with the magnesium-aluminum alloy matrix, the highest impedance modulus of the hydrotalcite coating prepared on the surface of the alkali-heat-treated AZ91D magnesium-aluminum alloy and the hydrotalcite prepared on the surface of the AZ91D magnesium-aluminum alloy without alkali heat treatment increases in the low-frequency region, indicating that the presence of the hydrotalcite coating effectively improves the corrosion resistance of the magnesium-aluminum alloy matrix; compared with the hydrotalcite coating after alkali heat treatment, the highest impedance modulus of the hydrotalcite coating prepared on the surface of the alkali-heat-treated AZ91D magnesium-aluminum alloy significantly increases in the low-frequency region, which further affirms the positive role of alkali heat treatment in the process of preparing the hydrotalcite coating and can effectively improve the corrosion resistance of the magnesium-aluminum alloy material, which is consistent with Figure 4 the tafel curve results in
[0043] Figure 6 Figure 1 shows the comparison of the electrochemical Nyquist diagrams of the steam hydrotalcite coating prepared without alkali heat treatment in Comparative Example 1, the steam hydrotalcite coating prepared with alkali heat treatment in Example 1, and the AZ91D magnesium-aluminum alloy matrix. As Figure 6 shown, compared with the magnesium-aluminum alloy matrix, the capacitive arcs of the hydrotalcite coating prepared on the surface of the alkali-heat-treated AZ91D magnesium-aluminum alloy and the hydrotalcite prepared on the surface of the AZ91D magnesium-aluminum alloy without alkali heat treatment are significantly larger, indicating that the presence of the hydrotalcite coating effectively improves the corrosion resistance of the magnesium-aluminum alloy matrix; compared with the hydrotalcite coating after alkali heat treatment, the capacitive arc and impedance value of the hydrotalcite coating prepared on the surface of the alkali-heat-treated AZ91D magnesium-aluminum alloy are much larger than those of other samples in the low-frequency region, which shows that the magnesium-aluminum alloy material prepared by alkali heat treatment has higher electrochemical capacitance and better corrosion resistance, which is consistent with Figure 4 the tafel curve results in
[0044] The unmodified AZ91D magnesium-aluminum alloy and the samples obtained in Example 1 and Comparative Example 1 above were tested by salt spray experiment: a continuous spray salt spray experiment was carried out using a 5wt.% sodium chloride solution. The temperature of the salt spray test chamber was 35°C, the temperature of the saturator was 47°C, there were 6 samples in each group, and the samples were photographed at certain time intervals to observe the corrosion condition and records were made. The experimental results are shown in Figure 7 Figure 7 (a), (b), and (c) are scanning electron microscope photos (magnification factors of 2000 and 8000 times) of the AZ91D magnesium-aluminum alloy, the magnesium-aluminum alloy material prepared in Comparative Example 1 (with a steam hydrotalcite coating prepared without alkali heat treatment on its surface), and the magnesium-aluminum alloy material prepared in Example 1 (with a hydrotalcite coating prepared with alkali heat treatment on its surface) placed under neutral salt spray conditions of 5wt.% NaCl solution at different time intervals. As Figure 7 As shown in (a), the surface shows a "gully-ridden" morphology, and the surface of the magnesium-aluminum alloy matrix is completely covered by corrosion products; as Figure 7 shown in (b), compared with the AZ91D magnesium-aluminum alloy, Figure 7 the flatness of the surface of the hydrotalcite coating in (b) is improved, the outermost coating is cracked, and the surface of the inner coating is covered by corrosion products in the shape of "flower buds". The hydrotalcite morphology of the hydrotalcite coating prepared on the surface of the AZ91D magnesium-aluminum alloy without alkali heat treatment basically disappears; as Figure 7 shown in (c), the surface of the hydrotalcite coating prepared on the surface of the AZ91D magnesium-aluminum alloy after alkali heat treatment has high integrity, indicating that the hydrotalcite coating prepared on the surface of the AZ91D magnesium-aluminum alloy after alkali heat treatment has excellent corrosion protection effect on the magnesium-aluminum alloy matrix.
[0045] Table 1 shows the comparison of the potentiodynamic polarization (tafel) curve data and the coating thickness of all comparative examples and example samples. Through the comparison of experimental data (Example 1, Example 6, Comparative Example 1), it can be intuitively seen that alkali heat treatment is beneficial to promoting the formation of hydrotalcite, and makes the coating thicker, which is beneficial to blocking the contact with corrosive media and slowing down the corrosion rate.
[0046] Comparing the samples of Example 1, Example 2, and Example 3 with Comparative Example 2 and Comparative Example 3 shows that too long or too short alkali heat treatment time is not conducive to improving the corrosion resistance. Too long alkali heat treatment time will consume raw materials excessively and increase the surface corrosion products, which is not conducive to the subsequent formation of hydrotalcite. Too short alkali heat treatment time cannot make the generated corrosion products surround β , and cannot weaken its inhibition of corrosion. By optimizing the alkali heat treatment time, the consumption of aluminum-containing phases during the alkali heat treatment process can be reduced and ideal samples with high corrosion resistance can be formed, so that the subsequent coating can be made more dense while ensuring the integrity of the raw materials.
[0047] Comparing the samples (Example 6, Example 7, Example 8) generated after single sodium hydroxide alkali heat treatment with Comparative Example 1, their self-corrosion potential increases and the self-corrosion current density decreases significantly, indicating that the corrosion resistance of the samples is improved after single sodium hydroxide alkali heat treatment; but compared with the samples of Example 1, Example 2, and Example 3, their self-corrosion potential decreases and the self-corrosion current density increases. Moreover, the effects of the samples of Example 12, Example 13, and Example 14 are similar to those of the sample of Example 1, indicating that the use of an alkali mixture containing sodium carbonate or sodium bicarbonate for alkali heat treatment plays a positive role in the process of preparing the hydrotalcite coating. Since carbonate is an essential anion for the formation of hydrotalcite, it helps to further improve the corrosion resistance of the coating.
[0048] For other magnesium-aluminum alloys with high aluminum content and complex second phases (Examples 4 and 5), alkali heat treatment can also effectively improve their corrosion resistance. It can be seen from Comparative Example 4 that the temperature of alkali heat treatment is also crucial, and the temperature of alkali heat treatment must be strictly controlled to ensure the improvement of the corrosion resistance of the subsequent coating.
[0049]
[0050] The parts not mentioned above can be realized by referring to the prior art.
[0051] Certainly, the above description is not a limitation of the present invention, and the present invention is not limited to the above examples. Changes, modifications, additions or substitutions made by those skilled in the art within the scope of the essence of the present invention should also fall within the protection scope of the present invention.
Claims
1. A preparation method of a magnesium-aluminum alloy material with a steam hydrotalcite coating on the surface, characterized in that, It includes the following steps: (1) Subject the polished magnesium-aluminum alloy substrate to alkali heat treatment to obtain a magnesium-aluminum alloy precursor; (2) Prepare a steam hydrotalcite coating on the surface of the magnesium-aluminum alloy precursor by the in-situ steam method to obtain a magnesium-aluminum alloy material with a steam hydrotalcite coating loaded on the surface.
2. The preparation method of the magnesium-aluminum alloy material with a surface-loaded steam hydrotalcite coating according to claim 1, characterized in that, For the alkali heat treatment, the steps are as follows: Immerse the magnesium-aluminum alloy substrate in a sodium hydroxide solution, take it out after immersion, wash and dry it to obtain a magnesium-aluminum alloy precursor; the concentration of the sodium hydroxide is 2.5-3.5 wt%, the immersion temperature is 50 °C, and the immersion time is 15-25 min.
3. The preparation method of the magnesium-aluminum alloy material with a surface-loaded steam hydrotalcite coating according to claim 1, characterized in that, For the alkali heat treatment, the steps are as follows: Immerse the magnesium-aluminum alloy substrate in an alkali mixture of sodium hydroxide and sodium carbonate or sodium hydroxide and sodium bicarbonate, take it out after immersion, wash and dry it to obtain a magnesium-aluminum alloy precursor; the concentration of sodium hydroxide in the alkali mixture is 1.5-2.5 wt%, the concentration of sodium carbonate or sodium bicarbonate is 9-11 wt%, the immersion temperature is 50 °C, and the immersion time is 15-25 min.
4. The preparation method of the magnesium-aluminum alloy material with a surface-loaded steam hydrotalcite coating according to claim 1, characterized in that For the preparation of the steam hydrotalcite coating on the surface of the magnesium-aluminum alloy precursor by the in-situ steam method, the steps are as follows: Suspend the magnesium-aluminum alloy precursor inside a reaction vessel filled with deionized water, and the height of the lower surface of the magnesium-aluminum alloy precursor is higher than the liquid level of the deionized water; under a closed environment, heat the reaction vessel to evaporate the deionized water and cause a chemical reaction with the magnesium-aluminum alloy precursor to obtain a magnesium-aluminum alloy material with a steam coating loaded on the surface; the heating temperature is 150-160 °C, and the heating time is 4-6 hours.
5. The preparation method of the magnesium-aluminum alloy material with a surface-loaded steam hydrotalcite coating according to claim 1, characterized in that, The aluminum content in the magnesium-aluminum alloy substrate is 5-9 wt%.