Magnesium alloy double-layer composite coating and preparation method and application thereof

Abstract

The invention relates to a magnesium alloy double-layer composite coating and a preparation method and application thereof. The preparation method comprises the following steps: (1) polishing a magnesium alloy matrix to obtain a polished matrix, carrying out ultrasonic cleaning on the polished matrix by using ethanol, and then naturally airing to obtain a treated magnesium alloy matrix; (2) the Na2SiO3 solution and the Ca (HCO3) 2 solution are mixed, a mixed solution is obtained, and the concentration of Ca (HCO3) 2 in the mixed solution is supersaturated; and putting the treated magnesium alloy matrix into the mixed solution, carrying out hydrothermal treatment, and drying to obtain the double-layer composite coating on the treated magnesium alloy matrix. The double-layer composite coating is prepared on the surface of the magnesium alloy matrix, and good protection application prospects are shown. And the structure is optimized through simulation analysis, and the problem that the coating is not completely wrapped is successfully solved, so that the corrosion durability of the magnesium alloy double-layer composite coating is remarkably improved.

Description

Technical Field

[0001] The present invention relates to the technical field of magnesium alloys, and particularly relates to a double-layer composite coating for magnesium alloys, a preparation method thereof, and an application thereof. Background Art

[0002] Magnesium alloys are important lightweight materials. With their high strength, excellent thermal conductivity, good damping and shock absorption performance, and excellent biocompatibility, they have broad application prospects in the fields of aerospace, electronic equipment, and biomedicine. However, due to their low electrode potential and easy corrosion, their industrial applications are severely restricted. At present, the methods for improving the corrosion resistance of magnesium alloys mainly include alloying, surface modification, and surface coating technology.

[0003] Surface coating technology is one of the important ways to improve the corrosion resistance of magnesium alloys. By preparing a coating on the surface of magnesium alloys, the contact between the corrosion medium and the alloy surface can be effectively blocked, oxidation and corrosion can be delayed, and the corrosion resistance of the alloy can be significantly improved. The coating acts as a protective barrier by isolating the alloy from direct contact with air and moisture. Common coating preparation technologies include micro-arc oxidation, magnetron sputtering, electroplating, chemical vapor deposition, and hydrothermal method, etc. Among them, the hydrothermal synthesis method is regarded as a method with broad prospects for in-situ synthesis of corrosion-resistant coatings due to its economy, environmental protection, and high efficiency.

[0004] In recent years, significant progress has been made in the preparation of corrosion-resistant coatings for magnesium alloys by the hydrothermal method. For example, Farasat Iqbal et al. deposited a magnesium phosphate coating by a one-step hydrothermal method, significantly improving the corrosion performance. Huanlin Zhang et al. successfully prepared a hydroxyapatite / lauric acid composite coating on AZ31 magnesium alloy. Through electrochemical measurements and long-term corrosion resistance tests of simulated body fluid (SBF), it was proved that the corrosion resistance of the composite coating was significantly enhanced. Jing Yuan et al. prepared rGO / Mg(OH)2 composite films on AZ61 alloy by the hydrothermal method in an alkaline solution containing deionized water and graphene oxide. It was found that as the content of GO in the hydrothermal solution increased, the corrosion resistance of the composite film first increased and then decreased. However, the coatings prepared by the hydrothermal method still have defects in encapsulation, and usually cannot completely encapsulate the samples. The overall encapsulation of the coating is the key to the practical application of magnesium alloys, but this problem is often ignored in research. It was found in the research that when the coating thickness increases, the coating at the corners of the sample often cannot completely cover the substrate.

[0005] From a practical and economic perspective, metal and its alloy coatings are not usually widely used in the corrosion protection of magnesium alloys. This is because it is very difficult to grow metal coatings such as nickel, copper, stainless steel, and titanium on the surface of magnesium alloys, and usually costly techniques such as spraying, magnetron sputtering, and laser cladding are required. In addition, there is a large potential difference between these metal coatings and magnesium alloys, which is likely to cause galvanic corrosion and further increase the corrosion risk. In contrast, calcium carbonate (CaCO3), as an inexpensive mineralization material, has advantages such as pH-sensitive release, wear resistance, compatibility, and degradability. CaCO3 is stable under neutral or alkaline conditions and decomposes only in acidic environments, so it shows good potential in the development of long-term corrosion-resistant coatings. In addition, silicate can also quickly form a wollastonite layer on the alloy surface and is often used in the preparation of magnesium alloy coatings. Although CaCO3 and silicate are of great research significance as corrosion-resistant coatings, there are currently few studies on the combined application of CaCO3 and silicate to inhibit corrosion. Summary of the Invention

[0006] To solve the above technical problems, the object of the present invention is to provide a double-layer composite coating for magnesium alloy, its preparation method and application. The double-layer composite coating for magnesium alloy prepared by the present invention has a simple method, and the prepared double-layer composite coating for magnesium alloy shows good prospects for protective application.

[0007] The technical solution of the present invention to solve the above technical problems is as follows:

[0008] The first object of the present invention is to provide a preparation method of a double-layer composite coating for magnesium alloy, including the following steps:

[0009] (1) Polish the magnesium alloy substrate to obtain a polished substrate, ultrasonically clean the polished substrate with ethanol, and then air-dry it naturally to obtain a treated magnesium alloy substrate;

[0010] (2) Mix the Na2SiO3 solution and the Ca(HCO3)2 solution to obtain a mixed solution, and the concentration of Ca(HCO3)2 in the mixed solution is supersaturated; place the treated magnesium alloy substrate in the mixed solution, perform hydrothermal treatment at 180°C to 200°C, and then dry it to obtain a double-layer composite coating on the treated magnesium alloy substrate, that is, a double-layer composite coating for magnesium alloy.

[0011] Principle description: Place the magnesium alloy substrate in the mixed solution for hydrothermal reaction. Since Ca(HCO3)2 is in a supersaturated state, the undissolved Ca(HCO3)2 will precipitate on the surface of the sample. During the heating process, Ca(HCO3)2 decomposes according to the following reaction formula:

[0012] Ca(HCO3)2 = CaCO3 + H2O + CO2↑;

[0013] Meanwhile, an in-situ film-forming reaction occurs on the AZ91D substrate. Due to the high chemical activity of the magnesium alloy, it will react rapidly in an aqueous solution, and the reaction formula is as follows

[0014] Mg + 2H2O = Mg(OH)2 + H2↑;

[0015] The essence of in-situ film formation on the surface of the magnesium alloy is the reaction of α-Mg with OH- ions. A dense Mg(OH)2 film formed in a high-temperature and high-pressure environment has excellent anti-corrosion effects. In addition, free Ca 2+ ions in a small amount of dissolved Ca(HCO3)2 and SiO3 2+ ions will react to form new precipitates, forming a new thin-film protection. The reaction formula is as follows, and finally a double-layer composite coating is obtained.

[0016] Ca 2+ + SiO3 2- = CaSiO3↓;

[0017] The beneficial effects of the present invention are as follows: The present invention uses the hydrothermal method to develop a double-layer composite coating on the magnesium alloy, forming an overall wrapped structure, which effectively inhibits the corrosion of the magnesium alloy.

[0018] Furthermore, the magnesium alloy substrate in step (1) was also chamfered before polishing.

[0019] The beneficial effect of adopting the above further scheme is that the chamfering treatment can effectively resist the stress concentration at the corners of the magnesium alloy substrate, achieving the full wrapping of the magnesium alloy, thereby significantly improving the corrosion durability of the coating.

[0020] Furthermore, the total concentration of Na2SiO3 and Ca(HCO3)2 in the mixed solution is 40 - 100 mM.

[0021] The beneficial effect of adopting the above further scheme is that the concentration of the mixed solution directly affects the thickness of the CaCO3 and CaSiO3 composite coating. As the concentration increases, the deposition amounts of CaCO3 and CaSiO3 are more. The composite coatings prepared with the solution within the concentration range of the present invention can effectively inhibit the corrosion of the magnesium alloy.

[0022] Furthermore, the total concentration of Na2SiO3 and Ca(HCO3)2 in the mixed solution is 80 mM.

[0023] The beneficial effect of adopting the above further scheme is that the coating samples prepared at an electrolyte concentration of 80 mM show the lowest corrosion current and the best anti-corrosion performance.

[0024] Further, the volume ratio of the Na2SiO3 solution to the Ca(HCO3)2 solution is 1-2:1-2; the concentration of the Na2SiO3 solution is 40-100 mol / L; the concentration of the Ca(HCO3)2 solution is 40-100 mol / L.

[0025] Further, the time of the hydrothermal treatment in step (2) is 50 min to 70 min.

[0026] Further, the temperature of the drying in step (2) is 90°C to 110°C, and the time is 50 min to 70 min.

[0027] Further, when the treated magnesium alloy substrate is placed in the mixed solution in step (2), the treated magnesium alloy substrate is horizontally placed in the mixed solution.

[0028] The beneficial effect of adopting the above further scheme is that: the magnesium alloy substrate is horizontally placed in the mixed solution, there is a vortex in the mixed solution system, most of the precipitates flow with the vortex, and will fall on the surface of the magnesium alloy substrate when passing through the magnesium alloy substrate, and form a film / block under high temperature conditions, forming a protective coating.

[0029] The second object of the present invention is to provide a magnesium alloy double-layer composite coating, which successively includes a Mg(OH)2 coating and a CaSiO3 / CaCO3 composite coating from the inside to the outside.

[0030] The beneficial effect of the present invention is that: in the present invention, first an Mg(OH)2 coating is in-situ generated on the surface of the magnesium alloy, and then wrapped with a CaSiO3 / CaCO3 composite coating. The corrosion current of the prepared coating sample is reduced by 4 to 5 orders of magnitude, which can completely wrap the magnesium alloy substrate, has excellent corrosion durability, and shows a good protective application prospect.

[0031] Further, the thickness of the composite coating is 50 μm to 1 mm.

[0032] The third object of the present invention is to provide an application of the magnesium alloy double-layer composite coating, and the magnesium alloy double-layer composite coating is used for the protection of magnesium alloys. Description of the Drawings

[0033] Figure 1 It is the preparation flow chart of MCS40 in Example 1 of the present invention;

[0034] Figure 2SEM images of Examples 1-4 of the present invention; among them, (a) is the SEM image of MCS40 of Example 1 at low magnification resolution; (b) is the SEM image of MCS40 of Example 1 at high magnification resolution; (c) is the SEM image of MCS60 of Example 2 at low magnification resolution; (d) is the SEM image of MCS60 of Example 2 at high magnification resolution; (e) is the SEM image of MCS80 of Example 3 at low magnification resolution; (f) is the SEM image of MCS80 of Example 3 at high magnification resolution; (g) is the SEM image of MCS100 of Example 4 at low magnification resolution; (h) is the SEM image of MCS100 of Example 4 at high magnification resolution;

[0035] Figure 3 XRD patterns and EDS images of Examples 1-4 and Comparative Example 1 of the present invention; among them, (a) is the XRD pattern of the surfaces of Examples 1-4 and Comparative Example 1, where AZ91D is the AZ91D magnesium alloy matrix of Comparative Example 1, 40 mM is MCS40 of Example 1, 60 mM is MCS60 of Example 2, 80 mM is MCS80 of Example 3, and 100 mM is MCS100 of Example 4; (b) is the element proportion on the surface of MCS60 of Example 2; (c) is the EDS image of MCS80 of Example 3; (d) is the weight proportion of element C in MCS80 of Example 3; (e) is the weight proportion of element O in MCS80 of Example 3; (f) is the weight proportion of element Mg in MCS80 of Example 3; (g) is the weight proportion of element Ca in MCS80 of Example 3; (h) is the weight proportion of element Si in MCS80 of Example 3;

[0036] Figure 4 XPS spectrum of the surface of MCS60 of Example 2 of the present invention; among them, (a) is the full XPS spectrum; (b) is the high-resolution XPS spectrum of C1s; (c) is the high-resolution XPS spectrum of O1s; (d) is the high-resolution XPS spectrum of Mg 1s; (e) is the high-resolution XPS spectrum of Ca2p; (f) is the high-resolution XPS spectrum of Si 2p;

[0037] Figure 5 Detection result diagrams of MCS40 of Example 1, MCS60 of Example 2, MCS80 of Example 3, MCS100 of Example 4, and AZ91D of Comparative Example 1 of the present invention in 3.5 wt% NaCl solution; among them, (a) is the open circuit potential; (b) is the potentiodynamic polarization curve; (c) is the cathodic Tafel constant (B a ) and the anodic Tafel constant (B c ); (d) is the corrosion potential (E corr ) and the corrosion current density (I corr); (e) Stern-Geary coefficient and polarization resistance (Rp); (f) corrosion rate (V corr );

[0038] Figure 6 EIS test results of MCS40 of Example 1, MCS60 of Example 2, MCS80 of Example 3, MCS100 of Example 4, and AZ91D of Comparative Example 1 in 3.5 wt% NaCl solution; where (a) is the Nyquist plot; (b) is the Bode impedance plot; (c) is the Bode phase angle plot; (d) is the equivalent circuit of AZ91D of Comparative Example 1 before the inductive reactance disappears; (e) is the equivalent circuit of Examples 1 to 4;

[0039] Figure 7 is the equivalent circuit of AZ91D of Comparative Example 1 after the inductive reactance disappears;

[0040] Figure 8 Fitting curves of MCS40 of Example 1, MCS60 of Example 2, MCS80 of Example 3, MCS100 of Example 4, and AZ91D of Comparative Example 1 in the EIS test; where (a) is C eff ; (b) is d eff ; (c) is R total ;

[0041] Figure 9 Nyquist plots, Bode impedance plots, and Bode phase angle plots of MCS40 of Example 1, MCS60 of Example 2, MCS80 of Example 3, MCS100 of Example 4, and AZ91D of Comparative Example 1 after soaking in 3.5 wt% NaCl solution for different times; where (a) is the Nyquist plot of AZ91D; (b) is the Bode impedance plot of AZ91D; (c) is the Bode phase angle plot of AZ91D; (d) is the Nyquist plot of MCS40; (e) is the Bode impedance plot of MCS40; (f) is the Bode phase angle plot of MCS40; (g) is the Nyquist plot of MCS60; (h) is the Bode impedance plot of MCS60; (i) is the Bode phase angle plot of MCS60; (j) is the Nyquist plot of MCS80; (k) is the Bode impedance plot of MCS80; (l) is the Bode phase angle plot of MCS80; (m) is the Nyquist plot of MCS100; (n) is the Bode impedance plot of MCS100; (o) is the Bode phase angle plot of MCS100;

[0042] Figure 10Optical photographs of MCS40 of Example 1, MCS60 of Example 2, MCS80 of Example 3, MCS100 of Example 4, and AZ91D of Comparative Example 1 after being immersed in 3.5 wt% NaCl solution for different times;

[0043] Figure 11 Data graphs of the immersion test and hydrogen evolution test of MCS40 of Example 1, MCS60 of Example 2, MCS80 of Example 3, MCS100 of Example 4, and AZ91D of Comparative Example 1 in 3.5 wt% NaCl solution; where (a) is the weight loss curve within 24 h of the immersion test; (b) is the average rate within 24 h of the immersion test; (c) is a schematic diagram of the hydrogen evolution test, (d) is the hydrogen release curve and average rate within 24 h of the hydrogen evolution experiment; (e) is the average rate within 24 h of the hydrogen evolution experiment; (f) is the hydrogen release rate and weight loss within 24 h of the hydrogen evolution experiment;

[0044] Figure 12 Surface XPS spectra of MCS60 of Example 2 after being immersed for 168 h; where (a) is the full XPS spectrum; (b) is the high-resolution XPS spectrum of C1s; (c) is the high-resolution XPS spectrum of O1s; (d) is the high-resolution XPS spectrum of Mg1s; (e) is the high-resolution XPS spectrum of Ca 2p; (f) is the high-resolution XPS spectrum of Si 2p;

[0045] Figure 13 Sample morphology diagrams and simulation analysis diagrams of Example 2 and Comparative Example 3 of the present invention; where (a) is the sample morphology diagram of Comparative Example 3; (b) is the sample morphology diagram of Example 2; (c) is the simulation analysis diagram of Comparative Example 3; (d) is the simulation analysis diagram of Example 2;

[0046] Figure 14 Mechanical analysis diagrams of Comparative Example 1 and Comparative Example 2 of the present invention at 190 °C; where (a) is the mechanical analysis diagram of AZ91D with chamfering treatment in Comparative Example 2; (b) is the mechanical analysis diagram of AZ91D without chamfering treatment in Comparative Example 1;

[0047] Figure 15 Transient structures of Comparative Example 1 and Comparative Example 2 of the present invention at 190 °C; where (a) is the total deformation; (b) is the equivalent stress data graph;

[0048] Figure 16 Optical photographs of MCS40 of Example 5, MCS60 of Example 6, MCS80 of Example 7, MCS100 of Example 8, and AZ91D of Comparative Example 2 after being immersed in 3.5 wt% NaCl solution for different times. Detailed implementation manners

[0049] The principles and features of the present invention are described below. The examples given are only for explaining the present invention and are not intended to limit the scope of the present invention. For those without specific technical details or conditions in the examples, the techniques or conditions described in the literature in this field or according to the product specifications are followed. For reagents or instruments without indicating the manufacturer, they are all conventional products that can be purchased through regular channels.

[0050] Example 1: Preparation of a magnesium alloy double-layer composite coating I

[0051] (1) The AZ91D magnesium alloy substrate with dimensions of 25mm * 25mm * 5mm was polished using sandpaper with a grit size of 1000 to 2000 meshes to obtain the polished substrate. The polished substrate was ultrasonically cleaned with ethanol for 10 min and then air-dried naturally to obtain the treated AZ91D magnesium alloy substrate;

[0052] (2) 65 mL of a mixed solution with a concentration of 40 mM (a solution of Na2SiO3 and Ca(HCO3)2 with a concentration and volume ratio of 1:1) was poured into the PTFE inner lining of the autoclave. Subsequently, the treated AZ91D magnesium alloy substrate was horizontally placed into the PTFE inner lining of the autoclave and hydrothermally treated at 190 °C for 60 min to obtain a corrosion-resistant coating sample. Finally, the coating sample was dried in an oven at 100 °C for 60 min. The preparation process is as Figure 1 shown, and a magnesium alloy double-layer composite coating named MCS40 was obtained.

[0053] Example 2: Preparation of a magnesium alloy double-layer composite coating II.

[0054] (1) The AZ91D magnesium alloy substrate with dimensions of 25mm * 25mm * 5mm was polished using sandpaper with a grit size of 1000 to 2000 meshes to obtain the polished substrate. The polished substrate was ultrasonically cleaned with ethanol for 10 min and then air-dried naturally to obtain the treated AZ91D magnesium alloy substrate;

[0055] (2) 65 mL of a mixed solution with a concentration of 60 mM (a solution of Na2SiO3 and Ca(HCO3)2 with a concentration and volume ratio of 1:1) was poured into the PTFE inner lining of the autoclave. Subsequently, the treated AZ91D magnesium alloy substrate was horizontally placed into the PTFE inner lining of the autoclave and hydrothermally treated at 190 °C for 60 min to obtain a corrosion-resistant coating sample. Finally, the coating sample was dried in an oven at 100 °C for 60 min. The preparation process is as Figure 1 shown, and a magnesium alloy double-layer composite coating named MCS60 was obtained.

[0056] Example 3: Preparation of a magnesium alloy double-layer composite coating III

[0057] (1) The AZ91D magnesium alloy substrate with dimensions of 25mm * 25mm * 5mm was polished using sandpaper with a grit size of 1000 to 2000 meshes to obtain the polished substrate. The polished substrate was ultrasonically cleaned with ethanol for 10 minutes and then air-dried naturally to obtain the treated AZ91D magnesium alloy substrate;

[0058] (2) 65 mL of a mixed solution with a concentration of 80 mM (a solution of Na2SiO3 and Ca(HCO3)2 with a concentration and volume ratio of 1:1) was poured into the PTFE inner lining of the autoclave. Subsequently, the treated AZ91D magnesium alloy substrate was horizontally placed into the PTFE inner lining of the autoclave and hydrothermally treated at 190 °C for 60 minutes to obtain a corrosion-resistant coating sample. Finally, the coating sample was dried in an oven at 100 °C for 60 minutes. The preparation process is as Figure 1 shown, and a magnesium alloy double-layer composite coating named MCS80 was obtained.

[0059] Example 4: Preparation of Magnesium Alloy Double-Layer Composite Coating Four

[0060] (1) The AZ91D magnesium alloy substrate with dimensions of 25mm * 25mm * 5mm was polished using sandpaper with a grit size of 1000 to 2000 meshes to obtain the polished substrate. The polished substrate was ultrasonically cleaned with ethanol for 10 minutes and then air-dried naturally to obtain the treated AZ91D magnesium alloy substrate.

[0061] (2) 65 mL of a mixed solution with a concentration of 100 mM (a solution of Na2SiO3 and Ca(HCO3)2 with a concentration and volume ratio of 1:1) was poured into the PTFE inner lining of the autoclave. Subsequently, the treated AZ91D magnesium alloy substrate was horizontally placed into the PTFE inner lining of the autoclave and hydrothermally treated at 190 °C for 60 minutes to obtain a corrosion-resistant coating sample. Finally, the coating sample was dried in an oven at 100 °C for 60 minutes. The preparation process is as Figure 1 shown, and a magnesium alloy double-layer composite coating named MCS100 was obtained.

[0062] Example 5: Preparation of Magnesium Alloy Double-Layer Composite Coating Five

[0063] This example is the same as Example 1, except that in step (1), the AZ91D magnesium alloy substrate was chamfered, and the rest of the preparation process, conditions, and raw materials are the same as those in Example 1.

[0064] Example 6: Preparation of Magnesium Alloy Double-Layer Composite Coating Six

[0065] This example is the same as Example 2, except that in step (1), the AZ91D magnesium alloy substrate was chamfered, and the rest of the preparation process, conditions, and raw materials are the same as those in Example 2.

[0066] Example 7: Preparation of Double-Layer Composite Coating on Magnesium Alloy VII

[0067] This example is the same as Example 3, except that the AZ91D magnesium alloy substrate was chamfered in step (1), and the remaining preparation processes, conditions, and raw materials are the same as those in Example 3.

[0068] Example 8: Preparation of Double-Layer Composite Coating on Magnesium Alloy VIII

[0069] This example is the same as Example 4, except that the AZ91D magnesium alloy substrate was chamfered in step (1), and the remaining preparation processes, conditions, and raw materials are the same as those in Example 4.

[0070] Comparative Example 1: AZ91D Magnesium Alloy Substrate

[0071] This comparative example uses the same AZ91D magnesium alloy substrate as in Example 1 without any treatment.

[0072] Comparative Example 2: AZ91D Magnesium Alloy Substrate

[0073] This comparative example is the same as Comparative Example 1, except that the AZ91D substrate was chamfered in this comparative example.

[0074] Comparative Example 3: Preparation of Double-Layer Composite Coating on Magnesium Alloy

[0075] This comparative example is the same as Example 2, except that the treated AZ91D magnesium alloy substrate was vertically placed in the polytetrafluoroethylene reactor liner in step (2), and the remaining preparation processes, conditions, and raw materials are the same as those in Example 2.

[0076] Comparative Example 4: Preparation of Double-Layer Composite Coating on Magnesium Alloy

[0077] This comparative example is the same as Example 6, except that the treated AZ91D magnesium alloy substrate was vertically placed in the polytetrafluoroethylene reactor liner in step (2), and the remaining preparation processes, conditions, and raw materials are the same as those in Example 6.

[0078] Test Example 1:

[0079] 1. Characterization:

[0080] 1.1 Surface Morphology

[0081] The surface morphologies of the MCS40, MCS60, MCS80, and MCS100 coatings prepared in Examples 1 to 4 were observed by a field emission scanning electron microscope (SEM, Reglus 8100), and the results are as Figure 2 shown.

[0082] From Figure 2It can be obtained that:

[0083] (1) Figure 2 The surface morphologies of MCS40, MCS60, MCS80, and MCS100 prepared in Examples 1 to 4 are respectively shown. From the low-magnification resolution images, that is Figure 2 a, Figure 2 c, Figure 2 e, and Figure 2 g, it can be observed that the surface microstructure of the magnesium alloy double-layer composite coating is clearly divided into two layers. The first layer is relatively flat, and the surface scratches are the marks left during the grinding process. The second layer shows a disordered granular structure, randomly arranged on the first layer. This may be the result of the deposition of CaCO3 particles and the formation of wollastonite / calcium silicate (CaSiO3). In addition, from Figure 2 a, Figure 2 c, Figure 2 e, and Figure 2 g, it can be seen that as the concentration of the mixed solution increases, the structure of the second layer of the magnesium alloy double-layer composite coating tends to be denser.

[0084] (2) The high-magnification resolution images show that, that is Figure 2 b, Figure 2 d, Figure 2 f, and Figure 2 h, it can be obtained that when the concentration increases, the outermost surface is covered by a loose porous flocculent structure. This phenomenon is directly related to the increase in the concentration of SiO3 2- , and it may be the direct manifestation of wollastonite / calcium wollastonite covering the sample surface.

[0085] 1.2 Phase composition

[0086] The phase compositions of the AZ91D magnesium alloy substrate of Comparative Example 1 and the magnesium alloy double-layer composite coatings of Examples 1 to 4 were detected by X-ray diffraction (XRD, smartlab9, Rigaku Corporation, Japan), and the results are as Figure 3 (a) shown.

[0087] From Figure 3 (a), it can be obtained that:

[0088] Figure 3 (a) shows the XRD patterns of the AZ91D magnesium alloy substrate of Comparative Example 1 and the surfaces of MCS40, MCS60, MCS80, and MCS100 of Examples 1 to 4. It can be seen that the XRD pattern of the magnesium alloy substrate is mainly composed of the Mg phase and a small amount of Mg 12 Al 17 alloy phase. As the concentration of the mixed solution increases, the peak intensity of the Mg phase significantly weakens, and Mg 12 Al 17The peak positions of the alloy phases completely disappear, while the peak positions of the Mg(OH)2 phase are strengthened. In addition, the CaSiO3, aragonite (CaCO3), and calcite (CaCO3) phases also show an increasing trend. This indicates that the increase in the concentration of the mixed solution leads to the further formation and encapsulation of the surface coating on the AZ91D magnesium alloy substrate.

[0089] 1.3 Surface chemical composition

[0090] The chemical composition of the surface of the magnesium alloy double-layer composite coating MCS60 in Example 2 was characterized by energy-dispersive X-ray spectroscopy (EDS, Bruker, Karlsruhe, Germany), and the results are as Figure 3 (b), Figure 3 (c), Figure 3 (d), Figure 3 (e), Figure 3 (f), Figure 3 (g), Figure 3 (h) shown.

[0091] From Figure 3 (b) to (h), it can be obtained that:

[0092] After EDS characterization, Figure 3 (b) is the weight percentage of the five main elements C, O, Mg, Ca, and Si on the coating surface of MCS60 in Example 2, Figure 3 (c) to (h) are the distribution of the five elements C, O, Mg, Ca, and Si. The results show that the peak intensity of the Mg element is the highest, indicating the presence of the magnesium matrix and Mg(OH)2. The weight percentage of the O element is the highest, reaching 52.34 wt%, which is mainly due to the fact that O elements are contained in Mg(OH)2, CaCO3, and CaSiO3 in the coating. In addition, the loose microstructure is basically consistent with the distribution profiles of C, Ca, O, and Si elements, indicating that the main components of the second-layer loose microstructure are CaCO3 and CaSiO3. And Mg(OH)2 is mainly distributed in the flat area of the first layer.

[0093] 1.4 Chemical composition and valence state

[0094] The chemical composition and valence state of the surface of the magnesium alloy double-layer composite coating MCS60 in Example 2 were characterized by X-ray photoelectron spectroscopy (XPS, Escalab 250Xi, Thermo Fisher Scientific Inc.), and the results are as Figure 4 shown.

[0095] As Figure 4 shown:

[0096] Figure 4(a)-(f) show the XPS survey spectrum of sample MCS60 and the high-resolution XPS spectra of C 1s, O 1s, Mg 1s, Ca 2p and Si 2p. As Figure 4 shown in (a), the presence of five elements, namely C, O, Mg, Ca and Si, is clearly shown in the survey spectrum, which is consistent with Figure 3 the results of XRD and EDS in

[0097] 2 Electrochemical measurements

[0098] Each material was electrochemically tested using a standard three-electrode system and 3.5 wt% NaCl solution with an electrochemical workstation (CS2350H, CorrTest Instruments Ltd, Wuhan, China). The auxiliary electrode was a platinum electrode, and the reference electrode was an Ag / AgCl (saturated potassium chloride) electrode. The working area was 1 cm 2 of Examples 1-4, MCS40, MCS60, MCS80, MCS100 of Comparative Example 1, and the AZ91D magnesium alloy substrate were used as the working electrodes.

[0099] Before the electrochemical test, each group needed to conduct an open-circuit potential (OPC) test for 30 minutes to reduce the influence of the surface instability of each group of materials on the electrochemical test results. The results are shown in Figure 5 (a).

[0100] It can be obtained from Figure 5 (a) that:

[0101] Figure 5(a) shows the open circuit potential test results, demonstrating the changes in the OCP of AZ91D of Comparative Example 1, MCS40 of Example 1, MCS60 of Example 2, MCS80 of Example 3, and MCS100 of Example 4 within 1800 seconds. The initial OCP value is due to the penetration process of the test solution into the coating. Subsequently, the stabilization of the OCP value implies a relatively stable surface state, and the corrosion products enhance the corrosion resistance. The OCP value of AZ91D in Comparative Example 1 gradually increases with the increase of the immersion time and finally stabilizes at about -1.55V. The increase in the OCP value reflects the dissolution of Mg and the formation of corrosion products on the alloy surface, reaching a dynamic equilibrium. The OCP values of MCS40 of Example 1, MCS60 of Example 2, MCS80 of Example 3, and MCS100 of Example 4 are significantly higher than that of AZ91D, indicating that the coating has excellent anti-corrosion effects. The finally stabilized OCP values are in the order of: AZ91D (-1.55V) < MCS40 (-0.72V) < MCS100 (-0.70V) < MCS60 (-0.68V) < MCS80 (-0.52V). Generally, coatings prepared at higher electrolyte concentrations usually have higher OCP values, indicating that these coatings have a weaker corrosion tendency. However, the OCP value of the coating sample prepared at 100 mM electrolyte concentration is at a lower level, which may be due to the porous defects on its surface.

[0102] 2.1 Potentiodynamic polarization

[0103] The electrochemical tests were carried out at room temperature. The potentiodynamic polarization was scanned from -0.5V to 1.5V at a scanning rate of 1 mV / s, and the potentiodynamic polarization curves were obtained as shown in Figure 5 (b). The potentiodynamic polarization curves are used to evaluate the corrosion resistance. By fitting the potentiodynamic polarization curves, the corrosion potential (E corr ), corrosion current density (I corr ), anodic Tafel constant (B a ), cathodic Tafel constant (B c ), corrosion rate (V corr ), Stern-Geary coefficient (B), and polarization resistance (R p ) of MCS40 of Example 1, MCS60 of Example 2, MCS80 of Example 3, MCS100 of Example 4, and AZ91D of Comparative Example 1 can be obtained to compare the corrosion resistance of Examples 1 - 4 and Comparative Example 1. Among them, the detection and calculation methods of the polarization resistance (R p ) and the corrosion current density I corr refer to ASTM G59 - 23, as shown in Table 1 and Figure 5 (b) - Figure 5 (f).

[0104] Table 1

[0105]

[0106] From Table 1, Figure 5 (b) to Figure 5 (f), it can be obtained that:

[0107] (1) The dynamic potential polarization curves of MCS40, MCS60, MCS80, MCS100 of Examples 1 to 4 and the AZ91D magnesium alloy matrix of Comparative Example 1 in 3.5 wt% NaCl solution are as shown in Figure 5 (b). It can be seen from the figure that as the potential increases, the corrosion current densities of MCS40, MCS60, MCS80, and MCS100 are significantly lower than that of AZ91D, which intuitively indicates that the coated samples have better corrosion resistance. Figure 5 (c) shows that the numerical gap between the cathode and anode slopes of the AZ91D magnesium alloy matrix is relatively large, while the gap of MCS40, MCS60, MCS80, and MCS100 is small, which is the main reason for the larger Stern-Geary coefficient B of the AZ91D magnesium alloy matrix ( Figure 5 e).

[0108] (2) From Figure 5 (d) and Table 1, it can be seen that the corrosion potential (E corr ) of the coated samples shifts forward, and the corrosion current density (I corr ) is reduced by at least three orders of magnitude or more, indicating that the corrosion resistance of the coated samples has been significantly improved. Among them, the corrosion current densities (I corr ) of the MCS60 and MCS80 coated samples are both reduced by 5 orders of magnitude. Usually, a smaller Stern-Geary coefficient B value and a larger R p value indicate better corrosion performance. As shown in Figure 5 (e), relative to the AZ91D magnesium alloy matrix, the Stern-Geary coefficient B value of the coated samples decreases, and the R p value increases. The Rp values of the MCS60, MCS80, and MCS100 coated samples are in the same order of magnitude, and the value of MCS80 is the largest. At the same time, the corrosion rate of the MCS80 coated sample is the lowest ( Figure 5 f). Through the comparative analysis of the above potentiodynamic polarization curve data, it can be clearly concluded that the MCS80 coated sample has the best corrosion performance.

[0109] 2.2 Electrochemical impedance spectroscopy (EIS) test

[0110] (1) The EIS tests were performed on AZ91D of Comparative Example 1, MCS40 of Example 1, MCS60 of Example 2, MCS80 of Example 3, and MCS100 of Example 4 using an electrochemical workstation. The tests were carried out at the open circuit potential (OCP), with a sinusoidal voltage amplitude of 5 mV and a frequency range from 100 kHz to 0.01 Hz. In the frequency range from 0.01 Hz to 100,000 Hz, the overall corrosion resistance of AZ91D of Comparative Example 1, MCS40 of Example 1, MCS60 of Example 2, MCS80 of Example 3, and MCS100 of Example 4 was analyzed through the EIS results to obtain the Nyquist plot and Bode plot. The results are as shown in Figure 6 (a), Figure 6 (b), Figure 6 (c).

[0111] (2) The impedance spectrum data after testing for AZ91D of Comparative Example 1, MCS40 of Example 1, MCS60 of Example 2, MCS80 of Example 3, and MCS100 of Example 4 were fitted with an equivalent circuit (EEC). The results are as shown in Figure 6 (d), Figure 6 (e). The scattered points in the figure represent the measured data, and the fitted data are shown as curves. The fitted data of each component are shown in Table 2, and the effective capacitance C eff , effective thickness d eff , and total resistance R total related to corrosion protection were calculated. The results are shown in Table 3 and Figure 8 . The specific calculation methods for each data are as follows:

[0112] In the fitting circuit of AZ91D, R s (solution resistance), and the low-frequency region consists of an inductor (L) and a resistor (R L ), indicating that pitting corrosion has occurred on the alloy. When the surface of AZ91D is coated with a coating, the inductor loop disappears. This is because of the defects and heterogeneity in AZ91D. The CEP f / R f represents the constant phase element of the corrosion film and the corrosion film resistance, while in the coated sample, it is related to the penetration resistance of the corrosive electrolyte through the coating.

[0113] CPE dl / R ct is the double-layer capacitance at the substrate / coating interface and the charge transfer resistance at the substrate / coating.

[0114] CPE i / R i and CPE f / R fParallel combination, representing the second constant related to the capacitive and resistive responses within the dense interior of the coating. A constant phase element (CPE) is used instead of a pure capacitor to describe the behavior of an ideal capacitor. Its impedance is expressed as follows:

[0115]

[0116] where ω is the angular frequency, j is the imaginary unit, Y0 is the CPE admittance, and n is the CPE empirical exponent. The value of n ranges from 0 to 1, where n = 0 represents a pure resistor and n = 1 represents a pure capacitor.

[0117] For the evaluation of the double-layer capacitance represented by the CPE, the changes in Y (constant phase element coefficient) and n related to the oxide layer are used to evaluate the change in the effective capacitance (C eff ) of the coating during electrochemical experiments. The calculation is performed using the Brug equation, as shown below:

[0118]

[0119] The Brug equation evaluates the C eff (F·cm -2 ) of a single time constant. Therefore, for the CPE dl -related C eff calculation, the resistance value at the start of the compatible arc for each sample under study is used. Then, the effective thickness d eff of the innermost layer of the coating is calculated through the value of C eff :

[0120]

[0121] where ε0 is the vacuum permittivity (ε0 = 8.85×10 -14 F·cm -1 ), A is the exposed test area (1 cm 2 ), and d eff (nm) is the effective thickness. d eff (nm) is the thickness of the corrosion product film. In the NaCl solution system, the corrosion process of the AZ91D magnesium alloy substrate and its coating samples is essentially a reaction between H2O and Mg. Therefore, the corrosion products are mostly Mg(OH)2, with a small amount of MgO present. So, the relative permittivity ε of the corrosion products is taken as (ε Mg(OH)2 = 8.2).

[0122] In addition, to comprehensively evaluate the protective performance of the sample from the entire EEC system, R total is used to represent the total resistance related to corrosion protection, and its calculation formula is as follows:

[0123] R total = R i + Rf +R ct ;

[0124] Table 2

[0125]

[0126]

[0127] Table 3

[0128] Example 1 Example 2 Example 3 Example 4 Comparative Example 1 <![CDATA[C eff，i (F·cm -2 )]]> <![CDATA[4.44×10 -9 > <![CDATA[4.81×10 -9 > <![CDATA[3.06×10 -10 > <![CDATA[4.73×10 -6 > / <![CDATA[C eff，f (F·cm -2 )]]> <![CDATA[8.96×10 -10 > <![CDATA[1.05×10 -9 > <![CDATA[1.06×10 -9 > <![CDATA[7.01×10 -10 > <![CDATA[1.32×10 -5 > <![CDATA[C eff，dl (F·cm -2 )]]> <![CDATA[1.41×10 -6 > <![CDATA[9.16×10 -8 > <![CDATA[1.80×10 -6 > <![CDATA[2.51×10 -6 > <![CDATA[9.51×10 -7 > <![CDATA[d eff，i (mm)]]> <![CDATA[1.66×10 -4 > <![CDATA[1.51×10 -4 > <![CDATA[2.37×10 -3 > <![CDATA[1.53×10 -7 > / <![CDATA[d eff，f (mm)]]> <![CDATA[8.10×10 -4 > <![CDATA[6.94×10 -4 > <![CDATA[6.86×10 -4 > <![CDATA[1.03×10 -3 > <![CDATA[5.51×10 -8 > <![CDATA[R total > <![CDATA[1.54×10 6 > <![CDATA[3.64×10 6 > <![CDATA[4.48×10 6 > <![CDATA[3.28×10 6 > 1621.7

[0129] From Figures 6 - 7 Tables 2 - 3, it can be obtained that:

[0130] (1) Figure 6 (a)-(c) show the Nyquist plots, Bode impedance plots and Bode phase angle plots of AZ91D of Comparative Example 1, MCS40 of Example 1, MCS60 of Example 2, MCS80 of Example 3, and MCS100 of Example 4. The charge transfer in the Nyquist plot is related to the radius of the capacitive reactance arc. Generally speaking, the larger the radius of the capacitive arc, the greater the resistance to electron transfer and the lower the electron exchange rate. For metal materials, a low electron exchange rate usually means a low corrosion rate and better corrosion resistance.

[0131] The Nyquist plot of AZ91D of Comparative Example 1 shows two capacitive loops, and the semicircle diameter of the capacitor is about 120 Ω·cm 2 . The semicircle diameters of Examples 1 - 4 and Comparative Example 1 are significantly increased, showing the excellent protective effect of the coating. Examples 1 - 4 show two capacitive loops at medium and low frequencies, corresponding to the double-layer structure in the SEM morphology diagram. The capacitive loop at high frequencies is related to the corrosion product, as shown in Figure 6 (a). In the Bode plot, the phase angles in the low-frequency and high-frequency regions reflect the integrity and corrosion resistance of the film. Generally, the higher the impedance modulus |Z|, the better the corrosion resistance of the sample. As can be seen from Figure 6 b, the |Z| values of all examples are higher than those of AZ91D of Comparative Example 1, and the |Z| values of MCS40, MCS60, MCS80, and MCS100 of Examples 1 - 4 are more than three orders of magnitude higher than those of AZ91D. Figure 6 (c) shows that within the entire measurement frequency range, the phase angles of Examples 1 - 4 and Comparative Example 1 are all lower than 90°, indicating that the behavior of the capacitor is not completely ideal. At low and high frequencies, the phase angles of Examples 1 - 4 are higher than those of the AZ91D magnesium alloy substrate of Comparative Example 1, further proving the protective effect of the coating.

[0132] (2) From Figure 8It can be seen that the anti-corrosion performance ranking is: Comparative Example 1 < Example 1 < Example 4 < Example 2 < Example 3, and MCS80 of Example 3 shows the best anti-corrosion performance.

[0133] 3. Immersion and hydrogen evolution tests

[0134] 3.1 Immersion test

[0135] (1) The immersion test is used to evaluate the continuous corrosion behavior of the samples. Under normal temperature conditions, MCS40, MCS60, MCS80, MCS100 of Examples 1 to 4 and AZ91D of Comparative Example 1 were weighed, and the weighed MCS40, MCS60, MCS80, MCS100 of Examples 1 to 4 and AZ91D of Comparative Example 1 were immersed in a 3.5 wt% NaCl solution. Samples were taken out at 0 h, 24 h, 72 h, 120 h, and 168 h, rinsed with a high-pressure water gun filled with deionized water, and then processed again in an ultrasonic cleaning environment. The total immersion time was 168 h. After the materials in each group were air-dried at room temperature, surface optical photos were taken as Figure 10 shown, and weighed to obtain the weight loss Δm (unit: g);

[0136] MCS60 of Example 2 after 168 h of immersion was characterized by X-ray photoelectron spectroscopy, and the results are as Figure 12 shown.

[0137] (2) A continuous EIS test was carried out using an electrochemical workstation to analyze the corrosion behavior of the samples at 12 h, 24 h, 48 h, 72 h, and 96 h of immersion, and the equivalent circuit (EEC) in Figure 6 、 Figure 7 was used for EIS data analysis, and the results are as Figure 9 、Table 4 and Table 5 shown.

[0138] 3.2 Hydrogen evolution test

[0139] In the hydrogen evolution test, hydrogen is generated by the cathodic hydrogen evolution reaction. Therefore, the amount of hydrogen evolution can be used as an index of the sample dissolution rate. The funnel was inverted above the sample, and the end of the funnel was connected to a 25 mL dropper. The corrosion rates of AZ91D magnesium alloy and coated samples in a 3.5 wt% NaCl solution were studied by collecting the volume of hydrogen. The mass loss rate (V G ) and hydrogen evolution rate (V H ) of the specimen were calculated according to the following formula:

[0140]

[0141] where M i represents the initial weight (unit: mg), M f represents the final weight (unit: mg), Hi Represents the initial liquid level of the dropper (unit: mL), H f Represents the final liquid level of the dropper (unit: mL), A is the area of the immersion zone (mm 2 ), and T is the immersion time (h).

[0142] The total corrosion rate (CR1) is calculated from the weight loss and is calculated in accordance with ASTM G1-03 "Standard Practice for Preparation, Cleaning, and Evaluation of Corrosion Test Specimens", with the unit of mm / year. For the hydrogen evolution experiment, the total weight loss is calculated according to the conversion relationship (1 mL of H2 gas = 0.001083 g of consumed Mg). CR1 is determined by the following formula

[0143]

[0144] where Δm is the weight loss (g); A is the exposed area of the sample in the NaCl solution (cm 2 ); t is the total immersion time of the sample (h); ρ is the density of the AZ91D magnesium alloy (1.82 g / cm 3 ), and the results are as Figure 11 shown.

[0145] Table 4

[0146]

[0147]

[0148] Table 5

[0149]

[0150] The results are as Figures 9 - 11 shown in Tables 4-5:[[]]END]]

[0151] (1) Figure 9 Shows the Nyquist and Bode plots of MCS40 of Example 1, MCS60 of Example 2, MCS80 of Example 3, MCS100 of Example 4, and AZ91D of Comparative Example 1. As the immersion time increases, the protective effect of the sample is improved, mainly due to the accumulation of corrosion products in the coating channels, effectively preventing the further entry of corrosive media into the substrate interface. In AZ91D of Comparative Example 1, as the immersion time prolongs, the inductive loop in the Nyquist plot gradually disappears ( Figure 9 a), indicating the weakening of pitting corrosion. At the same time, in the Bode plot, the increase in the impedance modulus |Z| and the phase angle in the low-frequency region also reflect the decrease in the corrosion rate ( Figure 9 b-c). For all coated samples, the capacitance loop, |Z| value, and phase angle all increase significantly during the immersion process ( Figure 9(d - o) further demonstrated the reduction of the corrosion rate. This reduction in the corrosion rate is related to the formation of corrosion products, which effectively block the contact between the corrosive medium and the substrate through the coating.

[0152] (2) In the long - term immersion of MCS40 in Example 1, MCS60 in Example 2, MCS80 in Example 3, MCS100 in Example 4, and AZ91D in Comparative Example 1, the EIS data analysis adopted Figure 6 (d), Figure 6 (e), Figure 7 the equivalent circuit (EEC) in Figure 9 a). It can be clearly observed from the Nyquist plot of the AZ91D sample in Comparative Example 1 that as the immersion time increases, the inductive reactance disappears at the 48 - h test ( Figure 9 a). Before the disappearance of the inductive reactance, the equivalent circuit in Figure 6 (d) was used for fitting analysis, and after the disappearance of the inductive reactance, the equivalent circuit in Figure 7 was used. For Examples 1 - 4, the equivalent circuit Figure 6 (e) was always used for fitting. It can be seen from the fitting result Table 5 that the total resistance of all samples increased after immersion, indicating that the protective performance of the samples was further improved during the immersion process.

[0153] (3) The immersion test can intuitively reflect the corrosion behavior of the samples in 3.5 wt% NaCl solution. After immersion of MCS40 in Example 1, MCS60 in Example 2, MCS80 in Example 3, MCS100 in Example 4, and AZ91D in Comparative Example 1, obvious corrosion differences were observed in the optical images ( Figure 10 ). AZ91D completely lost its metallic luster after 24 h of immersion, and obvious corrosion points appeared at the corners. As the immersion time extended, the area of the corrosion region significantly expanded, showing a trend of spreading from the edge to the center. Corrosion points were observed on MCS40, MCS60, and MCS100 after 48 h, 96 h, and 24 h of immersion respectively, and these corrosion points were all concentrated at the corner positions. As time passed, the corrosion region did not significantly expand. No corrosion points were observed on MCS80 during the entire test process, indicating that MCS80 has excellent corrosion durability. In addition, although pits were formed at the corners of MCS40, MCS60, and MCS80 after corrosion, these pits were filled with corrosion products during the subsequent immersion process, while this phenomenon did not occur in the samples of MCS100. This is probably because the corrosive medium has not completely eroded the CaCO3 layer, and the corrosion products filling the corrosion points are Mg(OH)2 formed on the Mg substrate. This phenomenon may be related to the increase in the deposition thickness of the CaCO3 coating caused by the high - concentration mixed solution. Due to pitting corrosion during the corrosion process, the coating becomes loose and the local bonding force decreases, resulting in coating peeling.

[0154] (2) Combining the mass loss in the immersion test and the hydrogen evolution in the hydrogen evolution experiment can more accurately analyze the behavior of the sample during continuous corrosion. Figure 11 Shows the mass loss data in the 168h immersion test and the hydrogen evolution data in the hydrogen evolution test of MCS40 of Example 1, MCS60 of Example 2, MCS80 of Example 3, MCS100 of Example 4, and AZ91D of Comparative Example 1 in a 3.5wt% NaCl solution. The schematic diagram of the hydrogen evolution experiment is as Figure 11 shown in c. The curve slope in the line graph represents the weight loss rate and hydrogen evolution rate of each group during the exposure time. The larger the slope, the stronger the corrosion reaction ( Figure 11 a and d). Figure 11 (b) and Figure 11 (e) show the average mass loss rate and hydrogen evolution rate of each group within 168h. The results show that the V G and V H values of AZ91D are greater than those of the coated samples. This indicates that MCS40 of Example 1, MCS60 of Example 2, MCS80 of Example 3, and MCS100 of Example 4 have better corrosion resistance than AZ91D of Comparative Example 1. As Figure 11 shown in f, the corrosion rate calculated by the mass loss test is lower than that obtained by the hydrogen evolution test. This lower mass loss corrosion rate may be due to incomplete removal of corrosion products and errors during the weighing process. The result comparison shows that the mass loss and hydrogen evolution values of MCS80 of Example 3 are the lowest, indicating that the sample formed in the 80mM mixed solution has more excellent anti-corrosion performance.

[0155] (3) As Figure 12 shown, the XPS spectra of the surface of the MCS60 coated sample after 168h immersion were compared with Figure 4 the uncorroded sample. In the full spectrum, the peak intensity of the Si 2p orbital completely disappeared in the immersed sample ( Figure 12 a). In addition, no characteristic peak of CaSiO3 was seen in the fine spectrum of Ca 2p ( Figure 12 e). In the Si 2p fine spectrum, no obvious characteristic peak appeared ( Figure 12 f), indicating that CaSiO3 had been dissolved during the immersion process. In contrast, the characteristic peak intensities of metal carbides in the C 1s fine spectrum ( Figure 12 b) and metal oxides in the O1s fine spectrum ( Figure 12 c) increased. Only one peak position appeared in the Mg 1s and Ca 2p fine spectra, corresponding to the characteristic peaks of Mg(OH)2 and CaCO3 respectively ( Figure 12In Figures d and e), since it is difficult to generate CaCO3 in a 3.5 wt% NaCl solution environment, it can be inferred that during the immersion process, CaSiO3 on the surface of the coated sample was dissolved.

[0156] Test Example 2:

[0157] Optical photos of the magnesium alloy double-layer composite coating materials prepared in Example 1 and Comparative Example 3 were taken, and SEM tests were carried out. The results are as Figure 13 (a), Figure 13 (b) shown. In order to accurately analyze the differences in the placement methods of the two samples, the fluent module of Workbench was used to perform a simulation analysis on the preparation process. The results are as Figure 13 (c) and Figure 13 (d) shown.

[0158] From Figure 14 it can be obtained that:

[0159] (1) Figure 13 (a) and Figure 13 (b) show the optical photos and SEM images of the magnesium alloy double-layer composite coating materials prepared in Example 1 and Comparative Example 3 in a 60 mM mixed solution. It can be clearly seen from the optical photos that the vertically placed magnesium alloy double-layer composite coating material is black, which is the result of the reaction between the magnesium alloy substrate and water; while the surface of the horizontally placed magnesium alloy double-layer composite coating material is milky white, indicating that CaCO3 and CaSiO3 are deposited on the AZ91D magnesium alloy substrate.

[0160] (2) In the SEM images, the surface of the sample prepared by vertical placement is relatively flat, while the surface of the horizontally placed sample shows a disordered particle arrangement. Through Figure 3 XRD and EDS analysis results and Figure 4 XPS analysis, it was found that the main components of the particles deposited in the vertically placed sample are CaCO3 and CaSiO3, and these substances are not deposited on the sample prepared by vertical placement. This phenomenon is directly related to the change in the fluid flow direction inside the reaction kettle during the heating process.

[0161] (3) Figure 13 (c) and Figure 13 (d) show the velocity vector diagrams of the two models. It can be clearly observed that there are vortices in the solution system under both placement methods, and the direction of the arrow is the flow direction of the vortices. The precipitate flows with the vortices and will fall on the surface and around the sample when passing through the sample. The vertically placed sample directly obstructs the vortices, causing the precipitate in the vortices to be pushed to the surroundings and making it difficult to deposit on the sample surface under the influence of gravity, so it is difficult to form a coating. On the contrary, for the horizontally placed sample, most of the precipitate is deposited on the sample surface and forms a film / block under high-temperature conditions, forming a coating with protective performance.

[0162] Test Example 3:

[0163] 1. Use the transient structural module of the workbench structure to perform mechanical analysis and transient structural analysis on the AZ91D magnesium alloy matrix of Comparative Example 1 and Comparative Example 2 at 190 °C. The results are as shown in Figure 14 and 15 .

[0164] From Figure 14 and 15 it can be obtained that:

[0165] During the immersion process, corrosion points initially appear at the corners, as shown in Figure 10 . In addition, during the coating formation process, under high-temperature and long-time treatment conditions, the phenomenon that the coating cannot completely wrap the sample may occur.

[0166] For the AZ91D magnesium alloy matrix without chamfering treatment, its deformation and stress are concentrated at the corner positions ( Figure 14 a). In actual sample preparation and testing, the sample usually does not show obvious deformation, and the deformation generated by the simulation analysis is mainly used to offset the influence caused by stress concentration. Stress concentration makes the sample more fragile, and damage usually occurs in the area of stress concentration. In order to reduce the influence of stress concentration on the sample, chamfering treatment is performed on the AZ91D. The simulation results are as shown in Figure 14 (b). The result comparison shows that although there is still stress concentration at the corner positions of the chamfered sample, the stress value is significantly reduced. During the heating process, the maximum and average values of the total deformation of the untreated sample are smaller than those of the chamfered sample ( Figure 15 a), and the maximum and average values of the equivalent stress are higher than those of the chamfered sample ( Figure 15 b). This indicates that the chamfered sample is more prone to deformation, thus effectively resisting stress concentration.

[0167] 2 Immersion test

[0168] Under normal temperature conditions, MCS40 of Example 5, MCS60 of Example 6, MCS80 of Example 7, MCS100 of Example 8, and AZ91D of Comparative Example 2 are immersed in a 3.5 wt% NaCl solution. Each group of materials is taken out at 0 h, 24 h, 72 h, 120 h, and 168 h, rinsed with a high-pressure water gun filled with deionized water, and then processed again in an ultrasonic cleaning environment. After each group of materials is air-dried at room temperature, surface optical photos are taken, as shown in Figure 16 .

[0169] From Figure 16 it can be obtained that:

[0170] Immersion experiments were carried out to observe the corrosion conditions of each group, and the results verified the accuracy of the workbench simulation results.

[0171] Compared with before the chamfering treatment, the corrosion degree was significantly reduced. The pitting corrosion at the corners of the AZ91D magnesium alloy substrate in Comparative Example 2 was significantly weakened, and the overall corrosion was also more uniform than before chamfering. Pitting corrosion occurred at one corner of the MCS60 sample in Example 6 and the MCS100 sample in Example 8 due to insufficient grinding, while no corrosion points appeared in the areas with smooth chamfers. These phenomena indicate that the chamfering treatment effectively reduces stress concentration and significantly improves the formation and performance of the coating.

[0172] Although the embodiments of the present invention have been shown and described above, it can be understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those of ordinary skill in the art can make changes, modifications, substitutions, and variations to the above embodiments within the scope of the present invention.

Claims

1. A method for preparing a double-layer composite coating on a magnesium alloy, characterized in that, The preparation method comprises the following steps: (1) The magnesium alloy substrate is polished to obtain a polished substrate, and the polished substrate is ultrasonically cleaned with ethanol and then naturally dried to obtain a treated magnesium alloy substrate; (2) The Na2SiO3 solution and the Ca(HCO3)2 solution are mixed to obtain a mixed solution, and the concentration of Ca(HCO3)2 in the mixed solution is supersaturated; the treated magnesium alloy substrate is placed in the mixed solution, and hydrothermal treatment is carried out at 180 °C to 200 °C, and then dried to obtain a double-layer composite coating on the treated magnesium alloy substrate, namely the magnesium alloy double-layer composite coating.

2. The preparation method of a magnesium alloy double-layer composite coating according to claim 1, characterized in that, The magnesium alloy substrate in step (1) is also chamfered before polishing.

3. The preparation method of a magnesium alloy double-layer composite coating according to claim 1, wherein, The total concentration of Na2SiO3 and Ca(HCO3)2 in the mixed solution is 40-100 mM.

4. The preparation method of a magnesium alloy double-layer composite coating according to claim 3, characterized in that, The volume ratio of the Na2SiO3 solution to the Ca(HCO3)2 solution is 1-2:1-2; the concentration of the Na2SiO3 solution is 40-100 mol / L; the concentration of the Ca(HCO3)2 solution is 40-100 mol / L.

5. The preparation method of a magnesium alloy double-layer composite coating according to claim 1, wherein The time of the hydrothermal treatment in step (2) is 50 min to 70 min.

6. The preparation method of a magnesium alloy double-layer composite coating according to claim 5, characterized in that, The drying temperature in step (2) is 90 °C to 110 °C, and the time is 50 min to 70 min.

7. The preparation method of a magnesium alloy double-layer composite coating according to claim 6, characterized in that, When the treated magnesium alloy substrate is placed in the mixed solution in step (2), the treated magnesium alloy substrate is horizontally placed in the mixed solution.

8. A magnesium alloy double-layer composite coating, characterized in that, Prepared by the preparation method according to any one of claims 1 to 7.

9. A magnesium alloy double-layer composite coating according to claim 8, wherein The thickness of the composite coating is 50 μm to 1 mm.

10. Application of a double-layer composite coating for magnesium alloy, characterized in that, The magnesium alloy double-layer composite coating according to any one of claims 8 to 9 is used for the protection of magnesium alloys.

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