Surface protective coating for die cast magnesium alloys and method of making and use thereof

Abstract

The application discloses a preparation method of a die-casting magnesium alloy surface protective coating, and the die-casting magnesium alloy substrate is immersed into pure water to perform a hydrothermal reaction, so that the die-casting magnesium alloy with a grown LDH coating is obtained. By adopting pure water as an in-situ growth liquid, the protective coating prepared through one-step hydrothermal reaction has excellent bonding force with the die-casting magnesium alloy substrate, the formed protective coating has a sheet structure, the sheets are in a two-dimensional spreading state, are closely attached to the surface of the substrate, and are arranged in a "tiled" manner along the surface, have excellent compactness, can effectively prevent the corrosion of corrosive ions in the outside world on the magnesium alloy, and further effectively improve the corrosion resistance of the die-casting magnesium alloy, thereby further promoting the wide application of the magnesium alloy in the fields of aviation, aerospace, automobiles, electronics and the like. In the whole preparation process, no reagent is involved, only pure water is adopted, and the cost is low and no pollution is caused to the environment.

Description

Technical Field

[0001] This invention belongs to the field of metal surface treatment technology, specifically relating to a protective coating for die-cast magnesium alloys, its preparation method, and its application. Background Technology

[0002] Magnesium alloys, as green engineering materials of the 21st century, possess characteristics such as low density, high specific strength, good processing performance, good damping properties, excellent electromagnetic shielding, high recyclability, and abundant reserves. They demonstrate enormous application prospects and practical value in numerous fields, including aerospace, automotive, medical devices, electronic communications, and sporting goods. However, magnesium's low electrode potential (-2.37 V) and high chemical reactivity make it prone to corrosion in humid air or saline environments, severely limiting its widespread application in complex industrial environments.

[0003] High purification, alloying, and surface coating are three main methods for improving the corrosion resistance of magnesium alloys. High purification, by minimizing the content of harmful elements and impurities during alloy smelting, reduces the number of easily corroded phases, thus effectively reducing the alloy's corrosion tendency. However, high purification processes are complex, involving zone melting and vacuum remelting, and require expensive equipment and high energy consumption. Therefore, high purification has a low cost-effectiveness ratio, making it difficult to achieve large-scale industrial applications. Alloying, on the other hand, involves adding one or more alloying elements to pure magnesium to regulate the metal's microstructure, second-phase distribution, and electrochemical behavior, making the alloy's reaction to corrosive media more stable, slower, or possessing self-passivation capabilities, thereby preventing corrosion. However, while alloying can improve corrosion resistance, it may also have impacts on other aspects of the metal material, such as its mechanical properties. In other words, alloying cannot simultaneously improve both corrosion resistance and mechanical properties of the metal material.

[0004] Surface coating is one of the most common and effective corrosion protection methods in engineering practice. It effectively delays or even blocks the corrosion process through isolation, sacrificial anodic protection, or intelligent response mechanisms. Furthermore, compared to high-purity materials and alloying, surface coatings offer advantages such as low cost, ease of operation, short processing cycle, and flexible and diverse processes. They can also achieve multifunctionality by constructing different coating systems, making them widely used not only in industry but also a hot research area in materials science. Nevertheless, to date, coatings prepared on magnesium alloy surfaces still face key challenges such as high cost, weak adhesion to the substrate, and insufficient mechanical durability, preventing their widespread use in practical engineering applications. Summary of the Invention

[0005] This invention aims to at least partially solve one of the technical problems in related technologies. Therefore, the main objective of this invention is to provide a method for preparing a protective coating on the surface of die-cast magnesium alloys that is low-cost, easy to operate, low-pollution or even pollution-free, easy to industrialize, and capable of in-situ growth.

[0006] The objective of this invention is achieved through the following technical solution: A method for preparing a protective coating on the surface of a die-cast magnesium alloy involves immersing a die-cast magnesium alloy substrate in an in-situ growth solution for a hydrothermal reaction to obtain a die-cast magnesium alloy with an LDH coating, wherein the in-situ growth solution is pure water.

[0007] This invention employs a one-step hydrothermal method to grow a dense and highly protective coating on the surface of die-cast magnesium alloys.

[0008] In some specific embodiments, the process parameters of the hydrothermal reaction are: hydrothermal reaction at 110-160℃ for 2-7 hours.

[0009] Furthermore, the process parameters for the hydrothermal reaction are: hydrothermal reaction at 120-150℃ for 3-6 hours.

[0010] Furthermore, the process parameters for the hydrothermal reaction are: hydrothermal reaction at 150°C for 3 hours.

[0011] In some specific embodiments, the die-cast magnesium alloy matrix before hydrothermal reaction is further subjected to water grinding.

[0012] As part of the same inventive concept, the present invention also provides a protective coating for the surface of die-cast magnesium alloys.

[0013] As part of the same inventive concept, this invention also provides the application of the method for preparing the protective coating on the surface of the die-cast magnesium alloy in the surface treatment of magnesium alloys.

[0014] Furthermore, the method for preparing the protective coating of the present invention is also applicable to wrought magnesium alloys, aluminum alloys, titanium alloys, steel, etc., and has good versatility.

[0015] As part of the same inventive concept, this invention also provides the application of the protective coating on the surface of the die-cast magnesium alloy in the fields of aerospace, automotive, and electronics.

[0016] Compared with the prior art, the present invention has at least the following advantages: 1) The preparation method of the present invention uses pure water as the in-situ growth liquid. The protective coating prepared by the one-step hydrothermal reaction has excellent adhesion to the die-cast magnesium alloy substrate. The protective coating formed has a sheet-like structure, and these sheets are in a two-dimensional spreading state, tightly attached to the substrate surface, and arranged in a "flat" manner along the surface. It has excellent density and can effectively prevent external corrosive ions from eroding the magnesium alloy, thereby effectively improving the corrosion resistance of the die-cast magnesium alloy and further promoting the widespread application of magnesium alloy in aerospace, automotive, electronics and other fields.

[0017] 2) The preparation method of the present invention does not involve any reagents in the entire preparation process, only pure water is used, the cost is low, there is no pollution to the environment, the whole process is simple, efficient and green and environmentally friendly, which helps to realize the engineering application of protective coatings on magnesium alloy surfaces, can meet the needs of industrial development and mass production, and can further expand the application range of magnesium alloys. Attached Figure Description

[0018] To more clearly illustrate the specific embodiments of the present invention, the accompanying drawings used in the description of the specific embodiments or the prior art will be briefly introduced below.

[0019] Figure 1 This is a scanning electron microscope image of the protective coating (120ºC, 3 hours) on the surface of the die-cast magnesium alloy in Example 1; Figure 2 This is the X-ray diffraction pattern of the protective coating (120ºC, 3 hours) on the surface of the die-cast magnesium alloy in Example 1; Figure 3 These are the Tafel polarization curves of AM50 die-cast magnesium alloy and protective coating (120ºC, 3 hours) magnesium alloy in Example 1 on the same coordinate system. Figure 4 These are scanning electron microscope images of the protective coating (120ºC, 6 hours) on the surface of the die-cast magnesium alloy in Example 2; Figure 5 This is the X-ray diffraction pattern of the protective coating (120ºC, 6 hours) on the surface of the die-cast magnesium alloy in Example 2; Figure 6 These are the Tafel polarization curves of AM50 die-cast magnesium alloy and protective coating (120ºC, 6 hours) magnesium alloy in Example 2 on the same coordinate system. Figure 7 This is a scanning electron microscope image of the protective coating (150ºC, 3 hours) on the surface of the die-cast magnesium alloy in Example 3; Figure 8 This is the X-ray diffraction pattern of the protective coating (150ºC, 3 hours) on the surface of the die-cast magnesium alloy in Example 3; Figure 9 These are the Tafel polarization curves of AM50 die-cast magnesium alloy and protective coating (150ºC, 3 hours) magnesium alloy in Example 3 on the same coordinate system. Figure 10 This is an adhesion test diagram of the protective coating (150ºC, 3 hours) on the surface of the die-cast magnesium alloy in Example 3. Detailed Implementation

[0020] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. The following embodiments are merely descriptive and not limiting, and should not be construed as limiting the scope of protection of the present invention.

[0021] When a quantity, concentration, or other value or parameter is described as a range, preferred range, or preferred upper and lower limits, it should be understood that it is equivalent to specifically disclosing any range by combining any pair of upper or preferred values ​​with any lower or preferred values, regardless of whether the range is specifically disclosed. Unless otherwise stated, the numerical range values ​​listed herein include the endpoints of the range and all integers and fractions within that range.

[0022] Unless otherwise stated, all percentages, parts, ratios, etc. in this document are by weight.

[0023] The materials, methods, and embodiments described herein are exemplary and should not be construed as limiting unless otherwise stated.

[0024] In the following embodiments, the die-cast magnesium alloy substrate is subjected to a water-grinding process as follows: the die-cast magnesium alloy substrate is ground with 150, 600, 800, 1200 and 2000# sandpaper respectively, then ultrasonically cleaned with alcohol for 5 minutes and dried to obtain the water-grinded die-cast magnesium alloy substrate.

[0025] Example 1 This embodiment provides a method for preparing a protective coating on the surface of a die-cast magnesium alloy, which includes the following steps: Pure water was poured into the inner liner of the reactor, and then the water-milled die-cast magnesium alloy AM50 sample was placed into the pure water. After sealing the reactor, it was placed in an electric oven for in-situ growth. The reaction temperature was 120ºC, and the constant temperature reaction time was set to 3 hours to obtain a die-cast magnesium alloy with a protective coating on the surface.

[0026] This application conducts performance tests on the die-cast magnesium alloy with a protective coating grown on its surface, prepared in this embodiment, specifically as follows: 1) Microscopic morphology The die-cast magnesium alloy with a protective coating grown on its surface in this embodiment was observed using a scanning electron microscope (SEM), and its SEM image is shown below. Figure 1As shown in the figure, the sample surface is covered with a large number of sheet-like structures. These sheets exhibit a two-dimensional spreading state, tightly attached to the substrate surface, and arranged in a "lay-out" pattern along the surface. Compared with the vertically oriented nanosheet array morphology exhibited by typical layered bimetallic hydroxide (LDH) materials, this structure shows significantly different orientation characteristics: the sheets do not stack and grow in a vertical direction, but extend along the substrate surface, forming a highly oriented in-plane spreading structure.

[0027] The die-cast magnesium alloy with a protective coating grown on its surface in this embodiment was characterized by XRD, and its X-ray diffraction pattern is shown below. Figure 2 As shown in the figure, the coating is mainly composed of two phases: LDH and Mg(OH)2. The XRD pattern shows diffraction peaks of the (003) and (006) crystal planes of LDH at 2θ ≈ 11.5° and 23.2°, respectively, indicating that the LDH structure was successfully formed in the coating and possesses a certain degree of crystal order. Meanwhile, characteristic diffraction peaks of Mg(OH)2 can be observed at 2θ ≈ 18.5°, 38.0°, and 50.8°, indicating that magnesium hydroxide products are generated during the coating growth process.

[0028] 2) Electrochemical performance This embodiment also tested the electrochemical performance of AM50 die-cast magnesium alloy and protective-coated die-cast magnesium alloy. Specifically, the polarization curves of the die-cast magnesium alloy sample and the magnesium alloy sample with a protective coating grown on its surface were tested in 3.5 wt.% sodium chloride solution using an electrochemical workstation (Princeton 4000A).

[0029] In this embodiment, the Tafel polarization curves of AM50 die-cast magnesium alloy (AM50) and protective coated die-cast magnesium alloy (LDH film) under the same coordinate system are shown in the figure. Figure 3 As shown in the figure, the corrosion voltage of the exposed AM50 die-cast magnesium alloy is -1.47 V, while the corrosion voltage of the protective-coated die-cast magnesium alloy is -1.45 V. Therefore, compared to the exposed die-cast magnesium alloy substrate, the preparation of the protective coating increases its corrosion voltage, and from a thermodynamic perspective, the corrosion tendency of the magnesium alloy substrate decreases.

[0030] From a kinetic perspective, corrosion rate and corrosion resistance are directly related to corrosion current density. The corrosion current density of exposed AM50 die-cast magnesium alloy is 1.52 × 10⁻⁶. -5 A cm -2 The corrosion current density of the protective coating on die-cast magnesium alloy is 6.63 × 10⁻⁶. -7 A cm -2Compared to the exposed die-cast magnesium alloy substrate, the corrosion current density of the sample with the protective coating was reduced by two orders of magnitude, indicating that the protective coating has a significant corrosion protection effect on AM50 die-cast magnesium alloy. In other words, the protective coating on the surface of die-cast magnesium alloy involved in this invention has superior corrosion resistance and can effectively protect the die-cast magnesium alloy substrate from corrosion.

[0031] Example 2 This embodiment provides a method for preparing a protective coating on the surface of a die-cast magnesium alloy, which includes the following steps: Pure water was poured into the inner liner of the reactor, and then the water-milled die-cast magnesium alloy AM50 sample was placed into the pure water. After sealing the reactor, it was placed in an electric oven for in-situ growth. The reaction temperature was 120 ºC, and the constant temperature reaction time was set to 6 hours to obtain a die-cast magnesium alloy with a protective coating on the surface.

[0032] This application conducts performance tests on the die-cast magnesium alloy with a protective coating grown on its surface, prepared in this embodiment, specifically as follows: 1) Microscopic morphology The die-cast magnesium alloy with a protective coating grown on its surface in this embodiment was observed using a scanning electron microscope (SEM), and its SEM image is shown below. Figure 4 As shown in the figure, the sample surface is covered with a large number of sheet-like structures. These sheets exhibit a two-dimensional spreading state, tightly adhering to the substrate surface and arranged in a "lay-out" pattern along its surface. Compared with the vertically oriented nanosheet array morphology exhibited by typical LDH materials, this structure displays significantly different orientation characteristics: the sheets do not stack and grow vertically, but extend along the substrate surface, forming a highly oriented in-plane spreading structure. However, compared with... Figure 1 Compared to the nanosheet structure in the previous figure, the lamellar structure observed in this figure is significantly larger in size. This indicates that with the extension of hydrothermal time, the lamellars gradually expand inward, increase in size, and tend to become flatter at the edges. In some areas, local overlap or stacking even occurs, and the overall structure tends to become wider and more flat.

[0033] The die-cast magnesium alloy with a protective coating grown on its surface in this embodiment was characterized by XRD, and its X-ray diffraction pattern is shown below. Figure 5 As shown in the figure, the coating is mainly composed of two phases: LDH and Mg(OH)2. The XRD pattern shows diffraction peaks of the (003) and (006) crystal planes of LDH at 2θ ≈ 11.5° and 23.2°, respectively, indicating that the LDH structure was successfully formed in the coating and possesses a certain degree of crystal order. Meanwhile, characteristic diffraction peaks of Mg(OH)2 can be observed at 2θ ≈ 18.5°, 38.0°, and 50.8°, indicating that magnesium hydroxide products are generated during the coating growth process.

[0034] 2) Electrochemical performance This embodiment also tested the electrochemical performance of AM50 die-cast magnesium alloy and protective-coated die-cast magnesium alloy. Specifically, the polarization curves of the die-cast magnesium alloy sample and the magnesium alloy sample with a protective coating grown on its surface were tested in 3.5 wt.% sodium chloride solution using an electrochemical workstation (Princeton 4000A).

[0035] Tafel polarization curves of AM50 die-cast magnesium alloy (AM50) and protective coated die-cast magnesium alloy (LDH film) on the same coordinate system are shown in the following figure. Figure 6 As shown in the figure, the corrosion voltage of the exposed AM50 die-cast magnesium alloy is -1.47 V, while the corrosion voltage of the protective-coated die-cast magnesium alloy is -1.41 V. Therefore, compared to the exposed die-cast magnesium alloy substrate, the preparation of the protective coating significantly increases its corrosion voltage, and from a thermodynamic perspective, the corrosion tendency of the magnesium alloy substrate decreases.

[0036] From a kinetic perspective, corrosion rate and corrosion resistance are directly related to corrosion current density. The corrosion current density of exposed AM50 die-cast magnesium alloy is 1.52 × 10⁻⁶. -5 A cm -2 The corrosion current density of the protective coating on die-cast magnesium alloy is 4.96 × 10⁻⁶. -7 A cm -2 Compared to the exposed die-cast magnesium alloy substrate, the corrosion current density of the sample with the protective coating was reduced by two orders of magnitude, indicating that the protective coating has a significant corrosion protection effect on AM50 die-cast magnesium alloy. In other words, the protective coating on the surface of die-cast magnesium alloy involved in this invention has superior corrosion resistance and can effectively protect the die-cast magnesium alloy substrate from corrosion.

[0037] Example 3 This embodiment provides a method for preparing a protective coating on the surface of a die-cast magnesium alloy, which includes the following steps: Pure water was poured into the inner liner of the reactor, and then the water-milled die-cast magnesium alloy AM50 sample was placed into the pure water. After sealing the reactor, it was placed in an electric oven for in-situ growth. The reaction temperature was 150 ºC, and the constant temperature reaction time was set to 3 hours to obtain a die-cast magnesium alloy with a protective coating on the surface.

[0038] This application conducts performance tests on the die-cast magnesium alloy with a protective coating grown on its surface, prepared in this embodiment, specifically as follows: 1) Microscopic morphology The die-cast magnesium alloy with a protective coating grown on its surface in this embodiment was observed using a scanning electron microscope (SEM), and its SEM image is shown below. Figure 7 As shown in the figure, the sample surface is covered with a large number of sheet-like structures. These sheets exhibit a two-dimensional spreading state, tightly adhering to the substrate surface and arranged in a "lay-out" pattern along its surface. Compared with the vertically oriented nanosheet array morphology exhibited by typical LDH materials, this structure displays significantly different orientation characteristics: the sheets do not stack and grow vertically, but extend along the substrate surface, forming a highly oriented in-plane spreading structure. However, compared with... Figure 4 Compared to the nanosheet structures in the previous diagram, the lamellar structures observed in this figure exhibit stronger adhesion to the substrate surface. The lamellars tend to spread along the substrate surface, oriented in-plane, with a more compact and continuous overall morphology, forming a larger contact area with the substrate. This morphological feature indicates that, under the current synthetic conditions, the lamellar growth process is more likely to achieve preferentially oriented stacking along the substrate direction.

[0039] The die-cast magnesium alloy with a protective coating grown on its surface in this embodiment was characterized by XRD, and its X-ray diffraction pattern is shown below. Figure 8 As shown in the figure, the coating is mainly composed of two phases: LDH and Mg(OH)2. The XRD pattern shows diffraction peaks of the (003) and (006) crystal planes of LDH at 2θ ≈ 11.5° and 23.2°, respectively, indicating that the LDH structure was successfully formed in the coating and possesses a certain degree of crystal order. Meanwhile, characteristic diffraction peaks of Mg(OH)2 can be observed at 2θ ≈ 18.5°, 38.0°, and 50.8°, indicating that magnesium hydroxide products are generated during the coating growth process.

[0040] 2) Electrochemical performance This embodiment also tested the electrochemical properties of AM50 die-cast magnesium alloy (AM50) and protective-coated die-cast magnesium alloy. Specifically, the polarization curves of the die-cast magnesium alloy sample and the magnesium alloy sample with a protective coating grown on its surface were tested in 3.5 wt.% sodium chloride solution using an electrochemical workstation (Princeton 4000A).

[0041] In this embodiment, the Tafel polarization curves of AM50 die-cast magnesium alloy (AM50) and protective coated die-cast magnesium alloy (LDH film) under the same coordinate system are shown in the figure. Figure 9 As shown in the figure, the corrosion voltage of the exposed AM50 die-cast magnesium alloy is -1.47 V, while that of the die-cast magnesium alloy with protective coating is -1.36 V. Therefore, compared to the exposed die-cast magnesium alloy substrate, the preparation of the protective coating significantly increases its corrosion voltage, and from a thermodynamic perspective, the corrosion tendency of the magnesium alloy substrate is significantly reduced.

[0042] From a kinetic perspective, corrosion rate and corrosion resistance are directly related to corrosion current density. The corrosion current density of exposed AM50 die-cast magnesium alloy is 1.52 × 10⁻⁶. -5 A cm -2 The corrosion current density of the protective coating on die-cast magnesium alloy is 9.32 × 10⁻⁶. -8 A cm -2 Compared to the bare die-cast magnesium alloy substrate, the corrosion current density of the sample with the protective coating was reduced by three orders of magnitude, indicating that the protective coating has a significant corrosion protection effect on AM50 die-cast magnesium alloy. In other words, the protective coating on the surface of die-cast magnesium alloy involved in this invention has superior corrosion resistance and can effectively protect the die-cast magnesium alloy substrate from corrosion.

[0043] 3) Bonding strength test results To assess the bonding strength between the coating and the substrate, this embodiment employs the cross-cut adhesion test to examine the die-cast magnesium alloy with a protective coating grown on its surface. The test method is conducted in accordance with the national standard GB / T 9286. A grid of intersecting lines is drawn on the coating surface at 1 mm intervals, and then peeled off using standard tape. The extent of coating detachment in the intersecting areas is observed to determine the degree of bonding between the coating and the substrate.

[0044] The results are as follows Figure 10 As shown, no obvious peeling or flaking occurred in the defined areas of the coating, and the boundaries were clear, with a rating of 0, indicating that the coating has good adhesion to the magnesium alloy substrate. This strong adhesion not only contributes to the long-term stable service of the coating but also provides structural protection for its corrosion resistance.

[0045] In summary, the protective coating on the surface of die-cast magnesium alloy prepared by this invention exhibits excellent corrosion resistance and shows great application potential. This invention uses a one-step hydrothermal method, employing pure water, to grow layered double hydroxide (LDH) in situ on the surface of AM50 die-cast magnesium alloy. The surface of this LDH protective coating is covered with numerous nano-layered structures, avoiding the classic LDH nanoporous structure, and thus possessing superior stability and corrosion resistance. The in-situ growth of a dense, non-porous LDH coating on the die-cast magnesium alloy surface effectively isolates the die-cast magnesium alloy substrate from external corrosive media, improving the corrosion resistance of the magnesium alloy. The improvement effect is comparable to that of the superhydrophobic classic LDH coatings in our previous work. Therefore, this invention effectively reduces production costs, uses environmentally friendly pure water, employs a shorter hydrothermal time, and eliminates the need for low surface energy modification of the coating to achieve a superhydrophobic effect, thus constructing a protective coating with particularly excellent corrosion resistance on the surface of die-cast magnesium alloy.

[0046] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention, and they should all be covered within the scope of the claims and specification of the present invention.

Claims

1. A method for preparing a protective coating on the surface of a die-cast magnesium alloy, characterized in that, A die-cast magnesium alloy substrate is immersed in an in-situ growth solution for hydrothermal reaction to obtain a coating of die-cast magnesium alloy with a lamellar structure composed of two phases, LDH and Mg(OH)2, and the lamellar structure is arranged in a flat pattern along the surface of the magnesium alloy. The in-situ growth solution is pure water, the die-cast magnesium alloy is AM50, and the process parameters of the hydrothermal reaction are: hydrothermal reaction at 150 ºC for 3 hours. The coating obtained is mainly composed of two phases, LDH and Mg(OH)2.

2. The method for preparing a protective coating on the surface of die-cast magnesium alloy according to claim 1, characterized in that, It also includes water grinding of the die-cast magnesium alloy matrix before hydrothermal reaction.

3. A protective coating on the surface of a die-cast magnesium alloy obtained by the preparation method according to claim 1 or 2.

4. The application of the protective coating on the surface of die-cast magnesium alloy according to claim 3 in the fields of aerospace, automobile and electronic products.

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