A method for one-step preparation of a colored surface coating on a magnesium alloy, the obtained magnesium alloy and its application

Abstract

The present invention belongs to the technical field of magnesium alloy surface treatment, and specifically relates to a method for one-step preparation of a colored surface coating on a magnesium alloy, the obtained magnesium alloy and its application. The method comprises the following steps: 1) preparing a mixed aqueous solution of NaAlO2 and / or NaOH; 2) filling the mixed aqueous solution of NaAlO2 and / or NaOH obtained in step 1) into a corrosion-resistant reaction vessel, and then immersing a clean AZ91D magnesium alloy sample into the mixed aqueous solution of NaAlO2 and / or NaOH; 3) closing the bottle mouth of the reactor, and carrying out a heating reaction at 95-105 °C under magnetic stirring, and the heating time is 3.5-4.5 h; 4) after the heating reaction is completed, washing the AZ91D magnesium alloy sample to obtain a magnesium alloy with a colored surface coating. The present invention successfully prepares a corrosion-resistant colored coating by a one-step hydrothermal method, and the method has the advantages of simplicity, environmental protection and low cost.

Description

Technical Field

[0001] The present invention belongs to the technical field of surface treatment of magnesium alloys, and particularly relates to a method for one-step preparation of a colored surface coating for magnesium alloys, the obtained magnesium alloy and its applications. Background Art

[0002] Due to its advantages such as high specific strength, good damping characteristics, good hot formability, and good electromagnetic shielding performance, magnesium alloys are widely used in industries such as automobiles, electronics, aerospace, and chemical engineering. However, affected by the characteristics of easy corrosion and monotonous color of magnesium alloys, their further applications are severely affected. Therefore, studying coatings for magnesium alloys with corrosion resistance and colored surfaces is of great significance for the application of magnesium alloys.

[0003] Coloring based on optical interference microstructures is a unique method of artificial coloring, which has attractive advantages in terms of resolution, chromaticity, chemical stability, and environmental friendliness. Structural coloring is caused by the physical interaction between visible light and the surface of micro / nano structures, where the surface color changes significantly with the viewing angle. The structural colors on metal surfaces are obtained using many surface treatment techniques, such as sputtering, electrolysis, laser, plasma, anodic oxidation, and machining, etc. At the same time, surface coatings are widely used to improve the corrosion resistance of magnesium alloys due to their economy and effectiveness. Wu et al. proposed an environmentally friendly colored coating by micro-arc oxidation of a magnesium-lithium alloy, which has excellent corrosion resistance and an optical camouflage protective layer. Yi et al. added tannic acid to a solution containing titanium and zirconium ions and successfully prepared a golden yellow Ti / Zr conversion coating on an AZ91D substrate. Hoche et al. prepared a corrosion-resistant PVD coating of TiMgAlN and a PVD color-tuning film composed of Nb through plasma and chemical polishing pretreatment. However, the existing preparation methods all have disadvantages such as high preparation cost, environmental unfriendliness, and single color change. So far, it is still a huge challenge to prepare a coating on magnesium alloys that simultaneously has a colored micro / nano structure surface and anti-corrosion performance. Summary of the Invention

[0004] To solve the deficiencies of the prior art, the present invention provides a method for one-step preparation of a colored surface coating for magnesium alloys, the obtained magnesium alloy and its applications. By changing the types and concentrations of reagents, the present invention can change the surface microstructure size and coating thickness, and finally prepare a coating surface with various colors such as yellow, cyan, blue, and red based on the interference effect of light. There is an excellent bonding force between the coating and the magnesium alloy substrate, and the corrosion current density is reduced by three orders of magnitude compared with the magnesium alloy substrate. This preparation method is economical, environmentally friendly, and simple, which helps to promote the industrial application of surface modification of magnesium alloys.

[0005] The technical solutions provided by the present invention are as follows:

[0006] A method for one-step preparation of a colored surface coating on a magnesium alloy, comprising the following steps:

[0007] 1) Prepare an aqueous mixed solution of NaAlO2 and / or NaOH;

[0008] 2) Pour the aqueous mixed solution of NaAlO2 and / or NaOH obtained in step 1) into a corrosion-resistant reaction vessel, and then immerse a clean AZ91D magnesium alloy sample into the aqueous mixed solution of NaAlO2 and / or NaOH;

[0009] 3) Seal the mouth of the reactor, and carry out a heating reaction at 95 - 105 °C under magnetic stirring, and the heating time is 3.5 - 4.5 h;

[0010] 4) After the heating reaction, wash the AZ91D magnesium alloy sample to obtain a magnesium alloy with a colored surface coating.

[0011] The above technical solution:

[0012] The hydrothermal treatment of the AZ91D magnesium alloy with the NaAlO2 solution can form a colored coating on the surface of the AZ91D magnesium alloy, which can improve the corrosion resistance of the AZ91D magnesium alloy. Moreover, the color of the coating can be adjusted by adjusting the concentration of NaAlO2;

[0013] The hydrothermal treatment of the AZ91D magnesium alloy with the NaOH solution can form a colored coating on the surface of the AZ91D magnesium alloy, which can improve the corrosion resistance of the AZ91D magnesium alloy. Moreover, the color of the coating can be adjusted by adjusting the concentration of NaOH;

[0014] NaAlO2 and NaOH together have a synergistic effect on the formation of the colored coating on the surface of the AZ91D magnesium alloy, and have a synergistic effect on increasing the corrosion resistance of the colored coating on the surface of the AZ91D magnesium alloy. The mass ratio of NaAlO2 to NaOH is (1 - 3):1.

[0015] Specifically, the concentration of NaAlO2 is 0.004 - 0.167 g / ml.

[0016] Specifically, the concentration of NaOH is 0.004 - 0.167 g / ml.

[0017] The present invention also provides a magnesium alloy with a colored surface coating prepared by the above method.

[0018] For the magnesium alloy with a colored surface coating provided by the present invention, the coating and the substrate have excellent bonding strength, the color of the coating can be controlled by changing the type of reagent and adjusting the solution concentration, and the prepared coating has good corrosion resistance.

[0019] Specifically, in the magnesium alloy with a colored surface coating, the surface of the magnesium alloy matrix has alloy phases Al 12 Mg 17 protrusions, and is covered with a Mg(OH)2 layer with a thickness of 6.8 - 12.7 μm.

[0020] Specifically, in terms of the corrosion rate, the corrosion resistance of the magnesium alloy with a colored surface coating is 4 - 752 times that of the AZ91D magnesium alloy.

[0021] For the magnesium alloy with a colored surface coating of the present invention, the corrosion current is reduced by three orders of magnitude compared to the matrix, the EIS capacitance radius is increased by four orders of magnitude, and the charge transfer resistance Rct is increased by three orders of magnitude compared to the matrix.

[0022] The present invention also provides the application of the above magnesium alloy with a colored surface coating as a body material or a casing material. Due to its low density, magnesium alloy is very suitable for use as the body of a vehicle. The corrosion-resistant colored magnesium alloy surface provided by the present invention makes it more advantageous in terms of quality and appearance. In addition, it is also suitable for casing materials, such as casings of mobile phones and other casings that require light weight and variable colors. Description of the Drawings

[0023] Figure 1 is the experimental flowchart of the present invention.

[0024] Figure 2 is the SEM surface microstructure morphology of the samples MSA, MSH, and MSAH-Q prepared in the examples. Among them, parts (a) and (b) are MSA; parts (c) and (d) are MSH; parts (e) and (f) are MSAH-QSEM.

[0025] Figure 3 is the SEM surface microstructure morphology and EDS element distribution of the samples MSA, MSH, and MSAH-Q at low magnification. Among them, parts (a) and (b) are MSA; parts (c) and (d) are MSH; parts (e) and (f) are MSAH-QSEM.

[0026] Figure 4 is the XRD diffraction pattern of MSA, MSH, and MSAH-Q.

[0027] Figure 5 is the XPS spectrum of MSAH-Q. Among them, part (a) is the full-spectrum scan, part (b) is the O 1s fine spectrum, part (c) is the Mg 1s fine spectrum, and part (d) is the Al 2p fine spectrum.

[0028] Figure 6This is a diagram of the surface color changes of samples prepared using different concentrations of NaAlO2 or NaOH. In the figure, NaAlO2: (a) 0.25g, (b) 0.50g, (c) 2.00g, (d) 4.00g, (e) 10.00g; NaOH: (f) 0.25g, (g) 0.50g, (h) 2.00g, (i) 4.00g, (j) 10.00g.

[0029] Figure 7 These are the surface color diagrams of samples prepared using combinations of NaAlO2 and NaOH with different concentrations, including the color of 0.25g NaAlO2, (a) 0.25g NaOH, (b) 0.50g, and (c) 2.0g samples; the color of 0.25g NaOH, (d) 0.5g NaAlO2, (e) 1.0g, and (f) 2.0g samples; and the color of equal amounts of the two drugs, (g) 0.5g, (h) 1.0g, and (i) 2.0g samples.

[0030] Figure 8 (a) CIE1931 chromaticity coordinates and (b) color change direction of the color surface.

[0031] Figure 9 The coating thickness and macroscopic surface color of different samples, including (a) (d) 1.0g NaOH, (b) (e) 3.0g NaAlO2, (c) (f) 0.5g NaOH and 1.5g NaAlO2.

[0032] Figure 10 The process of color surface coating formation is shown in the figure, where (a) OH - (b) Growth of Mg(OH)2 coating as the ion concentration decreases.

[0033] Figure 11 This is a diagram of the surface coating color mechanism analysis, including (a) AZ91D substrate, (b) thin coating with small structure, and (c) thick coating with large structure.

[0034] Figure 12 This is the MSH Cross-cut tape test image of the sample.

[0035] Figure 13 is the dynamic potential polarization curve of the coating sample in 3.5wt% NaCl solution.

[0036] Figure 14These are the Nyquist plots, impedance modulus curves and phase angle diagrams of the coating sample in 3.5wt% NaCl solution, among which, (a) Nyquist plots (c) Bode plots of |Z| vs. frequency (d) bode plots of phase angle vs. frequency; (b) is a local enlarged view of (a).

[0037] Figure 15 This is an equivalent circuit model diagram, where (a) AZ91D magnesium alloy substrate and (b) color coating. DETAILED DESCRIPTION

[0038] The principles and features of the present invention are described below. The examples given are only used to explain the present invention and are not used to limit the scope of the present invention.

[0039] 1.1 Materials and Reagents

[0040] AZ91D magnesium alloy (composition: 89.674wt% Mg, 9.3wt% Al, 0.63wt% Zn, 0.32wt% Mn, 0.05wt% Si, 0.003wt% Fe, 0.021wt% Cu, 0.001wt% Ni, 0.001wt% Be) was used as the matrix and purchased from Dongguan Kuangyu Metal Materials Co., Ltd. Anhydrous ethanol (AR, 99.7%) was purchased from Tianjin Fuyu Fine Chemical Co., Ltd. Sodium hydroxide (NaOH) was purchased from Sichuan Xilong Science Co., Ltd. Sodium aluminate (NaAlO2) was purchased from Aladdin. Sodium chloride (NaCl) was supplied by Shantou Xilong Technology Co., Ltd., China. All chemicals used in the present invention were of analytical grade and did not require further purification.

[0041] 1.2 Sample pretreatment

[0042] AZ91D magnesium alloy was cut into 20 mm × 20 mm × 5 mm sections and a 2.5 mm diameter hole was drilled near the top edge. The magnesium alloy sample was polished using 1000, 1500, 2000, and 5000 grit SiC sandpaper to remove the oxide layer. The sample was then ultrasonically cleaned in anhydrous ethanol for 10 minutes to remove surface dirt and fat-soluble substances.

[0043] 1.3 Preparation of color coating

[0044] Take 60ml of deionized water and put it into a 100ml beaker. Then add NaAlO2 and NaOH solid particles and stir in a magnetic stirrer until they are completely dissolved. Finally, pour the prepared solution into the polytetrafluoroethylene cup with the sample suspended. Use glass fiber wire to hang the sample in the polytetrafluoroethylene reactor lining, such asFigure 1 As shown. Place in a magnetic stirrer and heat at 100°C. During the heating process, slowly stir the rotor to keep the solution flowing. Remove the sample after 4 hours and ultrasonically clean it in anhydrous ethanol for 10 minutes to obtain a clean sample. The sample prepared by adding 10.00g of sodium aluminate (NaAlO2) and sodium hydroxide was named MSA; the sample prepared by adding 10.00g of sodium hydroxide (NaOH) was named MSH; the sample prepared by adding 2.00g of NaAlO2 and 2.00g of NaOH was named MSAH-Q.

[0045] 1.4 Characterization

[0046] The surface morphology of the samples was measured using a field-emission scanning electron microscope (SEM, Reglus8100), and the surface chemical composition of the samples was characterized using energy-dispersive X-ray spectroscopy (EDS, Bruker, Karlsruhe, Germany). The crystal microstructure of the samples was analyzed using an X-ray diffractometer (XRD, D8 Advance, Bruker AXS, Karlsruhe, Germany). The chemical composition and valence state of the samples were measured using an X-ray photoelectron spectrometer (XPS, Escalab, 250Xi, Thermo Scientific, Waltham, Massachusetts, America) with an Al Kα x-ray source (hv = 1486.6 eV). The macroscopic color of the sample surface was captured using a video microscope (G1200, Kailiwei, Guamgdong, China).

[0047] 1.5 Corrosion performance evaluation

[0048] Electrochemical corrosion tests were performed on the samples using a CS2350H electrochemical workstation (WuHan corrtest instruments Corp, Ltd, WuHan, China). A conventional three-electrode measurement system was used, with a platinum electrode as the auxiliary electrode and an Ag / AgCl (saturated KCl) electrode as the reference electrode. The working area was 1 cm 2 The test sample is used as the working electrode. Before the electrochemical test, the test sample needs to be tested for 30 minutes of open circuit potential to reduce the influence of sample surface instability on the electrochemical test results. The electrochemical test is carried out at room temperature, and the potential polarization is scanned at a speed of 1mv / s from -0.5v to

[0049] 1.5 V range scan. EIS tests were performed under OCP with a sinusoidal voltage amplitude of 5 mV and a frequency range of 100 kHz to 0.01 Hz.

[0050] 2. Results and Analysis

[0051] 2.1 Surface Microtopography

[0052] As Figure 2 shown are the SEM images of MSA, MSH, and MSAH-Q at different magnifications. The surface microstructure of MSA has the largest size and is evenly distributed in the form of flakes on the sample surface ( Figure 2 part a), and there are also dense granular structures of different sizes in the interlayers of the flake structures ( Figure 2 part b). The surface of the sample MSH is composed of nanosheets arranged in a fish-scale pattern, with sparse coral-like structures distributed in some areas and some small holes ( Figure 2 parts c, d). Compared with the other two samples, the surface microstructure of MSAH-Q is the smallest and the most dense ( Figure 2 part e), and the observed microstructure is needle-shaped and irregularly distributed on the sample surface ( Figure 2 part f). Strip-shaped scratches left during polishing can be observed on the surface of all samples.

[0053] 2.2 Coating Composition Analysis

[0054] As Figure 3 shown are the EDS analysis results. It can be seen from the figure that the weight percentage of Mg element in the three samples of MSA, MSH, and MSAH-Q is the highest, and the O element on the sample surface is between 10% and 15%. This may be because the OH - in the alkaline solution reacts with the AZ91D substrate to form hydroxides. The content of Al element on the surface of sample MSA is the lowest, and the content of Al element on the surface of sample MSH is the highest. Moreover, the higher the concentration of OH - , the higher the content of Al element on the sample surface, and the Al element is concentrated in the convex parts in the SEM images. The weight percentages of Na element in the three samples are not very different. The Na element may be experimental residues, and the residual weight percentage is between 1.5% and 1.8%.

[0055] As Figure 4 shown is the XRD diffraction analysis. The intensity of the characteristic peak of Mg at 57.6° (PDF#35-0821) of MSA is much higher than that of other samples, which is completely consistent with the result that the proportion of Mg element in this sample is the highest in the EDS ( Figure 3 b). The characteristic peaks of Mg at 70.7° and 78.3° (PDF#35-0821) of MSH are the most obvious, indicating that the (004) and (202) crystal planes are related to the concentration of OH - . The sample shows Al 12 Mg 17Characteristic peaks (PDF 01 - 1128), which is consistent with the EDS analysis result ( Figure 3 d). Characteristic peaks of Mg(OH)2 appear at 18.3° and 33.0° for the sample (PDF 07 - 0239). The heights and intensities of these two diffraction peaks are relatively low and the peak shapes are relatively wide, probably because the grains of the Mg(OH)2 substance contained in the coating are smaller. The peak position fluctuation of Mg(OH)2 in the MSH sample is more obvious than that of the other two samples, indicating that more Mg(OH)2 appears on the surface with the increase of OH - ions.

[0056] To further confirm the chemical composition on the sample surface, the sample MSAH - Q was selected for XPS analysis. Figure 5 (a) is the full - scan spectrum of the sample. The main elements on the sample surface are O, Mg, and Al, etc. There are two characteristic peaks in O1s (as Figure 5 b), and the binding energies are located at 530.4 eV and 531.9 eV respectively, corresponding to metal oxides and hydroxides. It can be known from the XRD analysis that the hydroxide is mainly Mg(OH)2. Characteristic peaks of Mg 1s appear at the binding energy positions of 1304.4 eV and 1306.2 eV (as Figure 5 c), corresponding to Mg metal and Mg 2+ , Mg comes from the AZ91D magnesium alloy substrate. According to the XRD results, it is inferred that Mg 2+ comes from Mg(OH)2. The characteristic peak intensity of Mg metal is higher than that of Mg 2+ , indicating that the content of Mg(OH)2 is lower compared with the substrate, that is, the coating is very thin. From the Al2p fine spectrum, the characteristic peak appearing at the binding energy position of 74.4 eV corresponds to the peak value of Al 3+ . No Al 3+ compounds were detected in the XRD pattern, indicating that Al 3+ only exists in the surface layer and has a low content. This is very likely to be generated after the hydrolysis of NaAlO2, and the alloy phase Al 12 Mg 17 is covered by Mg(OH)2 and thus does not appear. The main component of the prepared coating is Mg(OH)2.

[0057] 2.3 Macroscopic color change

[0058] Keeping the reaction time at 4 h and the solution volume at 60 ml unchanged, the two drugs were added according to the mass gradients of 0.25 g, 0.50 g, 2.00 g, 4.00 g, and 10.00 g. The results are as Figure 6 shown. It can be seen from the figure that with the increase of the mass of NaAlO2, the color of the sample changes from yellow ( Figure 6 a) → red - yellow ( Figure 6 b) → purple - yellow (Figure 6 c) → Cyan( Figure 6 d) → Blue( Figure 6 e) as a series of transformations. As the mass of NaOH gradually increases, the color of the sample changes from green( Figure 6 f) → Yellowish green( Figure 6 g, h, i) → Red( Figure 6 j).

[0059] Three groups of experiments were set with reaction conditions of 100 °C and 4 h. In the first group, the mass of NaAlO2 was kept at 0.25 g unchanged, and the amount of NaOH added was gradually increased. The macroscopic color change on the surface of the sample was as Figure 7 shown. As the concentration increased, the areas of yellow and red in the sample color gradually increased( Figure 7 a - c). In the second group, the mass of NaOH was kept at 0.25 g, and the amount of NaAlO2 added was changed. The color change of the sample was slow( Figure 7 d - f), indicating that NaOH played a major role in the color change. In the third group, the two drugs were increased synchronously and equally. The color changed from cyan( Figure 7 g) → Yellowish green( Figure 7 h) → Red( Figure 7 i). Through the above analysis, it can be seen that the two drugs have a synergistic effect on changing the surface color of the magnesium alloy, and the macroscopic color of the sample is denser and more uniform at low - concentration ratios.

[0060] To express the surface color more precisely, the chromaticity coordinates (x, y) of the color coating were plotted in the CIE 1931 chromaticity diagram, as Figure 8 shown. Nine samples with the largest color differences among all samples were marked in the chromaticity coordinates. For the surface of the sample with miscellaneous colors, the part with the largest color - occupied area was selected as the marking point in the chromaticity coordinates. Among them, points A, B, and C correspond to Figure 6 (a), (c), (e) respectively, and the chromaticity coordinates are (0.38, 0.44), (0.22, 0.23), and (0.15, 0.16). As the concentration of NaOH increases, the chromaticity coordinates move to the lower left corner, that is, the x - axis and y - axis coordinates decrease. Points D, E, and F correspond to Figure 6 (f), (h), (j) respectively, and the chromaticity coordinates are (0.33, 0.56), (0.31, 0.54), and (0.53, 0.36). Points G, H, and I correspond to Figure 7 (a), (c), (i) respectively, and the chromaticity coordinates are (0.23, 0.35), (0.34, 0.64), and (0.50, 0.46). Color - change arrows were used to more precisely represent the color change( Figure 7b) The colors of the six samples D, E, F, G, H, and I all change from cyan to green to yellow to red as the amount of drug added increases.

[0061] 2.4 Effects of different solutions on coatings

[0062] Three solutions with a pH of 13.2 were prepared using NaOH, NaAlO2 alone or NaAlO2 and NaOH (mass ratio 2:1) and kept at 100℃ for 4h. The surface coating thickness of the sample prepared in NaAlO2 solution was the thickest, while the thickness of the other two samples was not much different ( Figure 9 a, b, c). The surface colors of the samples are also very different. The surface of the sample with NaOH added is red and yellow. The sample with NaAlO2 added has the most blue part, with some yellow areas. The surface color of the sample prepared in the mixed solution is uniform green ( Figure 9 d, e, f). This shows that during the experiment, AlO - It not only provides OH continuously - The effect of ions also has a synergistic enhancement effect, making the color of the sample surface more uniform.

[0063] 2.5 Analysis of surface coating formation mechanism

[0064] EDS, XRD and XPS analysis show that the coating is mainly composed of Mg and O, with Al as the main element. 12 Mg 17 The alloy phase exists, and the surface coating is mainly composed of Mg(OH)2. The reaction equation is as follows:

[0065] Mg→Mg 2+ +2e - (1)

[0066] Mg 2+ +2OH - →Mg(OH)2↓(2)

[0067] During the formation of Mg(OH)2, OH - Ions play an important role, OH - The ions mainly come from NaAlO2 and NaOH, and the water in the solution also provides a small amount of OH - ions. The reaction equation is as follows:

[0068] H2O→H + +OH - (3)

[0069] NaOH → Na + +OH - (4)

[0070] AlO2 - +2H2O→OH - +Al(OH)3↓(5)

[0071] Use NaOH and NaAlO2 to prepare a mixed solution. When the amount of NaOH added is large enough, the OH required for the reaction - There is NaOH provided; when OH - When the concentration decreases to a certain level, AlO2 - Then the reaction of formula (5) is made ( Figure 10 a). Through the above analysis, it can be determined that Mg(OH)2 is the main component of the sample color coating. Combining the analysis results of EDS and XRD, it can be determined that the convex part of the coating is a stable Al 12 Mg 17 Alloy phase, from Figure 10 (b) SEM observation of MSAH-Q shows that its surface is also covered with a layer of needle-like Mg(OH)2. + With OH - The reaction generates Mg(OH)2, which is deposited on the surface of the substrate to form a protective film ( Figure 10 b).

[0072] 2.6 Surface coating color mechanism

[0073] Microstructure and chemical composition can cause light interference and thus coloring. The color change is determined by the microstructure, composition and coating thickness of the surface coating. The coating color is determined by the wavelength of light. The visible range is 380-780nm. The color corresponding to each wavelength range is as follows: Figure 11 (a). When a light beam enters a high-density medium from a low-density medium, the wavelengths of both the reflected and refracted light beams decrease compared to the incident light beam. Figure 11 b, c). This is due to the difference in the absorbance and thickness of the material itself, which will directly lead to changes in the absorption coefficient. Ideally, the path of the light beam on the flat coating changes as follows Figure 11 As shown in (b), the reflection of the light makes the wavelength shorter, but the reflected light of the refracted beam is superimposed, which increases the wavelength of the light. 12 Mg 17 In the coating where the alloy phase is exposed and Mg(OH)2 protrusions are formed, the reflection interface of the light beam changes due to the difference in the size and shape of the protrusion microstructure. This will cause a significant change in the direction of the reflected light velocity, such as Figure 11 As shown in (c), the color will change when viewed at different angles.

[0074] 2.7 Adhesion test

[0075] The adhesion of the coating is an important indicator to measure the quality of the coating. It is tested using a Bonder knife designed and manufactured according to the ISO2409-1992 standard. The tape used for the test is the 600-HC33 test tape produced by 3M Company in the United States, and the ASTM D3359-09 test method is adopted. Generally speaking, the thicker the coating, the lower the adhesion. Therefore, the sample MSH with the largest thickness is selected for the adhesion test. As can be seen from the figure, the edge of the incision is completely smooth and there is no peeling at the edge of the grid ( Figure 12 ). According to the adhesion test standard, the ASTM grade of the sample MSH is 5B. This means that there is excellent adhesion between the color coating and the AZ91D substrate.

[0076] 2.8 Corrosion protection

[0077] Electrochemical tests are used to evaluate the corrosion resistance of the experimental samples. The corrosion potential (E corr ) and corrosion current density (I corr ) of the coating under open-circuit conditions are determined by extrapolating the Tafel linear polarization curves on the anodic and cathodic sides. Figure 13 Shows the dynamic potential polarization curves of the AZ91D bare substrate, MSA, MSH, and MSAH-Q samples in a 3.5 wt% NaCl solution. It can be clearly seen from the figure that the samples with the color coating have all moved a certain distance to the lower right compared with the AZ91D substrate. Table 1 shows the detailed data of the measured corrosion potential (E corr ) and corrosion current density (I corr ). The corrosion inhibition rate η is calculated by Equation (6), where i 0corr is the corrosion current density of the Mg substrate, and i corr is the corrosion current density of the sample.

[0078]

[0079] From the polarization curves and detailed parameters, it can be seen that the value of #MSAH-Q is the smallest, which is 3 orders of magnitude lower than that of the substrate. According to electrochemical theory, for materials with better corrosion resistance, both the corrosion current density and the positive corrosion voltage are relatively low. The corrosion inhibition rates of MSA, MSH, and MSAH-Q are higher than that of the substrate, and they have better anti-corrosion effects compared with the bare samples.

[0080] Table 1 Detailed parameters of the polarization curves of the AZ91D substrate and the coating samples

[0081]

[0082] To further evaluate the anti-corrosion performance of the AZ91D substrate and the coating, Figure 14(a, c, and d) respectively present the Nyquist plots, impedance modulus curves, and phase angle plots of the AZ91D substrate and the coatings in a 3.5 wt% sodium chloride aqueous solution. In the Nyquist plot ( Figure 14 a), the scatter points are the measured data, and the fitting data are connected by curves. Therefore, it can be seen from the figure that the radii of MSH and MSAH-Q are much larger than those of the AZ91D substrate and MSA, increasing by about four orders of magnitude compared to the substrate. Among them, the corrosion protection effect of MSA is poor, probably related to its relatively thin Mg(OH)2 coating. From the Bode plot ( Figure 14 c), it can be seen that the |Z| values of the three experimental samples are all higher than that of the AZ91D substrate. The phase angles of the obtained color-coated samples are all higher than that of the AZ91D substrate ( Figure 14 d). Among them, the sample MSAH-Q shows a high phase angle both at high and low frequencies, indicating its excellent corrosion protection performance and also proving the synergistic effect of NaAlO2 and NaOH in corrosion protection.

[0083] In the equivalent circuit models (ECs) of the AZ91D magnesium alloy substrate and the coating samples ( Figure 15 a, b), Rs is the solution resistance, Rf is the coating resistance, and Rct is the charge transfer resistance. The constant phase element (CPE) is used to replace the pure capacitor, which is a non-ideal capacitor set for circuit fitting. The constant phase element has a better fitting effect than the pure capacitor in the equivalent circuit. As can be seen from the fitting data in Table 2, the Rct of the AZ91D magnesium alloy substrate is 322.00 Ω, and the Rcts of the samples MSA, MSH, and MSAH-Q are 4230 Ω, 12350 Ω, and 311740 Ω respectively. Among them, MSAH-Q is three orders of magnitude higher than the magnesium alloy substrate. This shows that the coating has better corrosion protection performance than the magnesium alloy substrate and can effectively prevent Cl - from corroding the AZ91D substrate.

[0084] Table 2 Equivalent circuit fitting data of the AZ91D substrate and the coating samples

[0085]

[0086] The present invention successfully prepared a corrosion-resistant color coating by a one-step hydrothermal method, which has the advantages of simplicity, environmental friendliness, and low cost. The prepared coating and the substrate have excellent adhesion. The coating color can be controlled by changing the types of reagents and adjusting the solution concentration. The prepared coating has good corrosion protection performance, with the corrosion current reduced by three orders of magnitude compared to the substrate, the EIS capacitance radius increased by four orders of magnitude, and the charge transfer resistance Rct increased by three orders of magnitude compared to the substrate. This method can provide new methods and means for the color diversification and corrosion protection of magnesium alloys.

[0087] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for one-step preparation of a colored surface coating on a magnesium alloy, characterized in that, It includes the following steps: 1) Prepare a mixed aqueous solution of NaAlO2 and NaOH, and the mass ratio of NaAlO2 to NaOH is (1 - 3):1; 2) Pour the mixed aqueous solution of NaAlO2 and NaOH obtained in step 1) into a corrosion-resistant reaction vessel, and then immerse a clean AZ91D magnesium alloy sample into the mixed aqueous solution of NaAlO2 and NaOH; 3) Seal the mouth of the reactor, and carry out a heating reaction at 95 - 105 °C under magnetic stirring, and the heating time is 3.5 - 4.5 h; 4) After the heating reaction, wash the AZ91D magnesium alloy sample to obtain a magnesium alloy with a colored surface coating.

2. The method for preparing a magnesium alloy colored surface coating in one step according to claim 1, characterized in that: The concentration of NaAlO2 is 0.004 - 0.167 g / ml.

3. The method for one-step preparation of a colored surface coating on a magnesium alloy according to claim 1 or 2, characterized in that: The concentration of NaOH is 0.004 - 0.167 g / ml.

4. A magnesium alloy with a colored surface coating prepared by the method according to any one of claims 1 to 3.

5. The magnesium alloy with a colored surface coating according to claim 4, wherein: In the magnesium alloy with a colored surface coating, the surface of the magnesium alloy matrix has alloy phases Al 12 Mg 17 protrusions and is covered with a Mg(OH)2 layer.

6. The magnesium alloy with a colored surface coating according to claim 4, characterized in that: The corrosion resistance of the magnesium alloy with the colored surface coating is 4 - 752 times that of the AZ91D magnesium alloy.

7. Use of a magnesium alloy with a colored surface coating according to any one of claims 4 to 6, characterized in that: As a body material or as a chassis material.

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