Preparation method of rare earth conversion coating on surface of AZ91D magnesium alloy material
By optimizing the combination of rare earth salts and hydrogen peroxide solution and the soaking process, the problems of insufficient film formation quality and adhesion of rare earth conversion films were solved, and the high corrosion resistance of AZ91D magnesium alloy was achieved.
Patent Information
- Application Number
- CN202511007513.X
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-07-22
- Publication Date
- 2025-10-17
AI Technical Summary
The existing rare earth passivation process for forming rare earth conversion films on the surface of AZ91D magnesium alloy has insufficient film quality, density, and adhesion to the substrate, resulting in poor corrosion resistance.
A rare earth passivation solution composed of rare earth salts (cerium chloride and lanthanum chloride) and hydrogen peroxide solution was used to immerse AZ91D magnesium alloy. The passivation temperature, time and number of immersions were optimized to form a high-quality rare earth conversion film.
It significantly improves the uniformity and substrate adhesion of rare earth conversion films, enhances the corrosion resistance of AZ91D magnesium alloy, avoids localized corrosion weak points, and forms high-quality rare earth conversion films.
Smart Images
Figure CN120796960A_ABST
Abstract
Description
TECHNICAL FIELD
[0001] The application relates to the technical field of corrosion resistance of metal materials, and particularly relates to a preparation method of a rare earth conversion film on the surface of AZ91D magnesium alloy material. BACKGROUND
[0002] The AZ91D magnesium alloy is widely applied in the fields of aviation, automobiles, electronics and sports because of good casting performance and high yield strength. However, the AZ91D magnesium alloy is easy to be corroded in the marine environment or the humid environment containing Cl - , and has poor protection capability, so corrosion resistance of the alloy needs to be studied. At present, the methods for studying the corrosion resistance of the AZ91D magnesium alloy include an anodic oxidation method, a metal coating method, a laser treatment method and a surface conversion method. The anodic oxidation method can obtain the alloy with good wear resistance and corrosion resistance, and has good bonding force, electrical insulation and impact resistance, and is one of the commonly used surface treatment technologies for the AZ91D magnesium alloy. However, the anodic oxidation generally contains chromium and fluorine, pollutes the environment and damages human health; the metal coating method can obtain superior corrosion resistance, but most of the plating layers contain heavy metals; the laser treatment method can treat the surface with complex geometry, but the AZ91D magnesium alloy is easy to be oxidized, evaporated and produce vaporization, pores and thermal stress during the laser treatment, which is not conducive to the experiment. The surface conversion method is widely used in the study of the corrosion resistance of the AZ91D magnesium alloy, and the most remarkable one is that the rare earth elements are added in the chemical conversion treatment process by using the chemical immersion method to form the rare earth conversion film. The rare earth conversion film has the advantages of non-toxicity, harmlessness, environmental protection, simple treatment process and good protection performance, and is an important research object for surface protection of the AZ91D magnesium alloy. However, the existing rare earth passivation process still needs to be further optimized in terms of the film forming quality (such as uniformity, compactness and bonding force with the matrix) and the final corrosion resistance. SUMMARY
[0003] The application aims to provide a preparation method of a rare earth conversion film on the surface of AZ91D magnesium alloy material, so as to improve the film forming quality and corrosion resistance of the rare earth conversion film.
[0004] In order to achieve the above-mentioned application purposes, the application provides the following technical scheme.
[0005] The application provides a preparation method of a rare earth conversion film on the surface of AZ91D magnesium alloy material, which comprises the following steps.
[0006] The AZ91D magnesium alloy is placed in a rare earth passivation solution for immersion, so as to complete the preparation of the rare earth conversion film on the surface of the AZ91D magnesium alloy.
[0007] The rare earth passivation solution is a rare earth salt, a hydrogen peroxide solution and water.
[0008] The rare earth salt is one or both of cerium chloride and lanthanum chloride.
[0009] Optionally, the mass fraction of the rare earth salt in the rare earth passivation solution is 2-5wt%; the mass fraction of the hydrogen peroxide solution in the rare earth passivation solution is 1-4wt%, and the mass fraction of water in the rare earth passivation solution is 90-97wt%.
[0010] Optionally, when the rare earth salt is a mixture of cerium chloride and lanthanum chloride, the mass ratio of cerium chloride to lanthanum chloride is 1:1.
[0011] Optionally, when the rare earth salt is cerium chloride, the temperature of the soaking is 30-50℃, the time is 35-45min, and the soaking times are 3-5 times.
[0012] Optionally, when the rare earth salt is cerium chloride, the temperature of the soaking is 50℃, the time is 45min, and the soaking times are 3 times.
[0013] Optionally, when the rare earth salt is lanthanum chloride, the temperature of the soaking is 30-40℃, the time is 25-45min, and the soaking times are 2-5 times.
[0014] Optionally, when the rare earth salt is lanthanum chloride, the temperature of the soaking is 40℃, the time is 40min, and the soaking times are 2 times.
[0015] Optionally, when the rare earth salt is a mixture of cerium chloride and lanthanum chloride, the temperature of the soaking is 50-55℃, the time is 38-42min, and the soaking times are 1-2 times.
[0016] Optionally, when the rare earth salt is a mixture of cerium chloride and lanthanum chloride, the temperature of the soaking is 50℃, the time is 50min, and the soaking times are 1 time.
[0017] Optionally, the mass concentration of the hydrogen peroxide solution is 25-35%.
[0018] Compared with the prior art, the present application has the following beneficial effects:
[0019] The present application replaces the highly toxic chromate salt with cerium chloride, lanthanum chloride or cerium chloride + lanthanum chloride; by improving the key process parameters such as passivation temperature, time, concentration and soaking method, the present application realizes precise control of the film thickness, density and composition distribution, significantly improves the uniformity and substrate adhesion of the conversion film, avoids local corrosion weak points, and forms a high-quality rare earth conversion film, and through the synergistic effect of rare earth, the corrosion resistance of the AZ91D magnesium alloy is improved. BRIEF DESCRIPTION OF DRAWINGS
[0020] Figure 1 The process flow chart for preparing the rare earth conversion film on the surface of the AZ91D magnesium alloy material of the present application;
[0021] Figure 2 Circuit diagram used for impedance fitting of the present application;
[0022] Figure 3 Polarization curve plots for products prepared for Examples 1-8;
[0023] Figure 4 Electrochemical impedance spectroscopy plots for products prepared for Examples 1-8;
[0024] Figure 5 Polarization curve plots for products prepared for Examples 1, Comparative Examples 1-9;
[0025] Figure 6 Electrochemical impedance spectroscopy plots for products prepared for Examples 1, Comparative Examples 1-9;
[0026] Figure 7 Polarization curve plots for products prepared for Examples 9-16;
[0027] Figure 8 Electrochemical impedance spectroscopy plots for products prepared for Examples 9-16;
[0028] Figure 9 Polarization curve plots for products prepared for Examples 9, Comparative Examples 10-18;
[0029] Figure 10 Electrochemical impedance spectroscopy plots for products prepared for Examples 9, Comparative Examples 10-18;
[0030] Figure 11 Polarization curve plots for products prepared for Examples 17, 18, Comparative Examples 19-27;
[0031] Figure 12 Electrochemical impedance spectroscopy plots for products prepared for Examples 17, 18, Comparative Examples 19-27;
[0032] Figure 13 Polarization curve plots for products prepared for Examples 17, Comparative Examples 28-34;
[0033] Figure 14 Electrochemical impedance spectroscopy plots for products prepared for Examples 17, Comparative Examples 28-34;
[0034] Figure 15 SEM images of surface morphology for blank and CeCl3at different passivation temperatures, where (a) is blank AZ91D magnesium alloy (b) is Comparative Example 6; (c) is Example 1; (d) is Comparative Example 7;
[0035] Figure 16The surface morphology SEM images of the blank sample and the LaCl3 at different passivation temperatures, wherein (a) is the blank AZ91D magnesium alloy, (b) is the comparative example 15; (c) is the embodiment 9; (d) is the comparative example 16;
[0036] Figure 17 The surface morphology SEM images of blank samples and mixed rare earths with different mass ratios, where (a) is blank AZ91D magnesium alloy; (b) is Example 17; (c) is Comparative Example 23; (d) is Comparative Example 27;
[0037] Figure 18 These are the element distribution diagrams of Example 17: (a) is the SEM diagram; (b) is the distribution diagram of Al; (c) is the distribution diagram of Mg; (d) is the distribution diagram of Mn; (e) is the distribution diagram of Cl; (f) is the distribution diagram of Zn; (g) is the distribution diagram of La; and (h) is the distribution diagram of Ce. DETAILED DESCRIPTION
[0038] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as limiting the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.
[0039] It should be understood that the terms described herein are intended only to describe particular embodiments and are not intended to limit the present invention. In addition, for numerical ranges herein, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Each smaller range between any intermediate value within a stated value or stated range and any other stated value or intermediate value within the stated range is also encompassed by the present invention. The upper and lower limits of these smaller ranges may be independently included or excluded within the scope.
[0040] Unless otherwise indicated, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art. Although only preferred methods and materials are described herein, any methods and materials similar or equivalent to those described herein may also be used in the practice or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials associated with the documents. In the event of any conflict with any incorporated document, the contents of this specification shall prevail.
[0041] It will be apparent to those skilled in the art that various modifications and variations may be made to the specific embodiments of the present invention without departing from the scope or spirit of the invention. Other embodiments will be apparent to those skilled in the art from the present invention. The present description and examples are intended to be illustrative only.
[0042] As used herein, the terms "comprise", "include", "have", "contain", and the like are open-ended terms, i.e., meaning "including but not limited to".
[0043] The raw materials used in the present application can be obtained commercially or prepared by the prior art.
[0044] The present application provides a preparation method of a rare earth conversion film on the surface of an AZ91D magnesium alloy material, comprising the following steps:
[0045] The AZ91D magnesium alloy is placed in a rare earth passivation solution for immersion to complete the preparation of the rare earth conversion film on the surface of the AZ91D magnesium alloy.
[0046] The present application first pretreats the AZ91D magnesium alloy, and in the embodiments of the present application, the pretreatment method comprises:
[0047] The AZ91D magnesium alloy is customized into a rectangular thin die casting piece with a size of (10mm x 10mm x 1mm) for surface morphology and element distribution detection and a size of (30mm x 10mm x 1mm) for polarization curve and impedance spectrum detection; the AZ91D magnesium alloy is polished by using water-based sandpaper (1000 mesh) to polish off the surface layer; the AZ91D magnesium alloy is polished by using a polishing cloth to spray a polishing agent to obtain a preliminary sample; the AZ91D magnesium alloy is degreased by using acetone; the acetone is washed away by using a large amount of deionized water for multiple times; and the pretreatment of the AZ91D magnesium alloy is completed by using an oven to dry at a temperature of 120℃ for 1h.
[0048] After the AZ91D magnesium alloy of the present application is immersed in the rare earth passivation solution, it also needs to be cleaned and dried, specifically, the AZ91D magnesium alloy sample after passivation treatment is sequentially cleaned with anhydrous ethanol and deionized water; and the AZ91D magnesium alloy sample is naturally air-dried.
[0049] In the present application, the rare earth passivation solution is one or both of a rare earth salt, a hydrogen peroxide solution and water.
[0050] The rare earth salt is one or both of cerium chloride and lanthanum chloride.
[0051] In the present application, the preparation method of the rare earth passivation solution is to mix the rare earth salt, the hydrogen peroxide solution and water and stir them uniformly to obtain the rare earth passivation solution.
[0052] In the present application, the mass concentration of the hydrogen peroxide solution is 25-35%, preferably 27-34%, further preferably 28-32%, and more further preferably 29-30%.
[0053] In the present application, the mass fraction of the rare earth salt in the rare earth passivation solution is 2-5wt%, preferably 3-4wt%; the mass fraction of the hydrogen peroxide solution in the rare earth passivation solution is 1-4wt%, preferably 2-3wt%, further preferably 2.5wt%; the mass fraction of water in the rare earth passivation solution is 90-97wt%, preferably 92-96wt%, further preferably 94-95wt%.
[0054] In the present application, when the rare earth salt is a mixture of cerium chloride and lanthanum chloride, the mass ratio of cerium chloride to lanthanum chloride is 1:1.
[0055] In the present application, when the rare earth salt is cerium chloride, the temperature of the immersion is 30-50℃, for example, 30℃, 35℃, 40℃, 45℃, 50℃, etc., and the time is 35-45min, for example, 35min, 40min, 45min, etc.; the number of immersions is 1-5, for example, 1, 2, 3, 4, 5.
[0056] In the present application, when the rare earth salt is cerium chloride, the temperature of the immersion is 50℃, the time is 45min, and the number of immersions is 3.
[0057] In the present application, when the rare earth salt is lanthanum chloride, the temperature of the immersion is 30-40℃, for example, 30℃, 35℃, 40℃, etc.; the time is 25-45min, for example, 25min, 30min, 35min, 40min, 45min, etc.; and the number of immersions is 1-5, for example, 1, 2, 3, 4, 5.
[0058] In the present application, when the rare earth salt is lanthanum chloride, the temperature of the immersion is 40℃, the time is 40min, and the number of immersions is 2.
[0059] In the present application, when the rare earth salt is a mixture of cerium chloride and lanthanum chloride, the temperature of the immersion is 50-55℃, for example, 50℃, 55℃, etc.; the time is 38-42min; and the number of immersions is 1-4, for example, 1, 2, 3, 4.
[0060] In the present application, when the rare earth salt is a mixture of cerium chloride and lanthanum chloride, the temperature of the immersion is 50℃, the time is 40min, and the number of immersions is 1.
[0061] The technical solutions provided by the present application will be described in detail below in conjunction with the examples, but they should not be understood as limiting the scope of protection of the present application.
[0062] In the examples of the present application, the concentration of the hydrogen peroxide solution is all 30wt.%.
[0063] Example 1
[0064] (1) Pretreatment of AZ91D magnesium alloy: The AZ91D magnesium alloy was customized into a rectangular thin compression cast piece with a size of (10 mm x 10 mm x 1 mm, for surface morphology and element distribution detection) and (30 mm x 10 mm x 1 mm, for polarization curve and impedance spectrum detection); the AZ91D magnesium alloy was polished by using water-based sandpaper (1000 mesh) to polish off the surface layer; the AZ91D magnesium alloy was polished by using a polishing cloth to spray a polishing agent, to obtain a preliminary sample; the AZ91D magnesium alloy was degreased by using acetone; the acetone was washed away by using a large amount of deionized water for multiple times; the pretreatment was completed by using an oven to dry at a temperature of 120°C for 1 h;
[0065] (2) 3 g of CeCl3, 2.5 g of hydrogen peroxide and 94.5 g of water were mixed, and the reagents were stirred to be uniformly mixed, to obtain a CeCl3-containing passivation solution; then the pretreated AZ91D magnesium alloy was immersed in the CeCl3-containing passivation solution, and the immersion was performed at 50°C for 45 min, for a total of 3 times; the AZ91D magnesium alloy sample after passivation treatment was sequentially cleaned with anhydrous ethanol and deionized water, and the AZ91D magnesium alloy sample was naturally dried, to complete the preparation of the rare earth conversion film on the surface of the AZ91D magnesium alloy material.
[0066] Example 2
[0067] The difference from Example 1 is that 4 g of CeCl3, 2.5 g of hydrogen peroxide and 93.5 g of water were mixed, and the reagents were stirred to be uniformly mixed, to obtain a CeCl3-containing passivation solution.
[0068] Example 3
[0069] The difference from Example 1 is that the immersion time is 35 min.
[0070] Example 4
[0071] The difference from Example 1 is that the immersion time is 40 min.
[0072] Example 5
[0073] The difference from Example 1 is that the immersion temperature is 30°C.
[0074] Example 6
[0075] The difference from Example 1 is that the immersion temperature is 40°C.
[0076] Example 7
[0077] The difference from Example 1 is that the immersion times is 4 times.
[0078] Example 8
[0079] The difference from Example 1 is that the immersion time is 25 min.
[0080] Comparative Example 1
[0081] The difference from Example 1 is that 1 g of CeCl3, 2.5 g of hydrogen peroxide, and 96.5 g of water are mixed, and the reagents are stirred to mix uniformly, to obtain a CeCl3-containing passivation solution.
[0082] Comparative Example 2
[0083] The difference from Example 1 is that 2 g of CeCl3, 2.5 g of hydrogen peroxide, and 95.5 g of water are mixed, and the reagents are stirred to mix uniformly, to obtain a CeCl3-containing passivation solution.
[0084] Comparative Example 3
[0085] The difference from Example 1 is that the immersion time is 25 min.
[0086] Comparative Example 4
[0087] The difference from Example 1 is that the immersion time is 30 min.
[0088] Comparative Example 5
[0089] The difference from Example 1 is that the immersion time is 50 min.
[0090] Comparative Example 6
[0091] The difference from Example 1 is that the immersion temperature is 20°C.
[0092] Comparative Example 7
[0093] The difference from Example 1 is that the immersion temperature is 60°C.
[0094] Comparative Example 8
[0095] The difference from Example 1 is that the immersion time is 1 time.
[0096] Comparative Example 9
[0097] The difference from Example 1 is that the immersion time is 2 times.
[0098] The AZ91D magnesium alloy materials obtained in Examples 1 to 8 and Comparative Examples 1 to 9 were subjected to electrochemical workstation testing. A three-electrode system configuration was used, and electrochemical testing was performed in a 3.5 wt.% NaCl solution: the working electrode used the passivated magnesium alloy material, the auxiliary electrode was a platinum electrode, and the reference electrode was a saturated calomel electrode (SCE), and the system was used to simulate a corrosion environment. The test conditions were: alternating current impedance high frequency 10 kHz, low frequency 1 Hz, and the circuit diagram used for impedance fitting was as shown in Figure 2The polarization curve initial potential setting parameter is -1.4V, the terminal potential setting parameter is 0V, the scanning section number is 1s, and the scanning speed is 0.01V / s.
[0099] The polarization curve shows that the more positive the corrosion voltage and the smaller the corrosion current, the better the corrosion resistance of the coating. The diameter of the capacitive reactance arc of the impedance spectrogram is large, the corresponding constant is small, the impedance value of the Faraday current produced is large, indicating that the film layer plays a large resistance role, and it is difficult for the reaction to occur on the electrode surface, and the corrosion resistance is better.
[0100] The experimental results are shown in Figures 3-6 and Figure 15 , the results of Figures 3-6 and Figure 15 show that, according to the data of Comparative Examples 1 and 2, when the addition amount of cerium chloride is less than 3g, the rare earth ions are insufficient to cover the substrate; when the addition amount of cerium chloride of Example 2 is greater than 3g, Ce 3+ over-accumulation leads to film layer cracks. Positive effects: when the addition amount of CeCl3 of Example 1 is 3g, the impedance value is increased to 2281.2Ω / cm 2 , and the self-corrosion potential is positively shifted to -0.666V. According to the data comparison of Examples 1, 3, 4 and Comparative Example 5, with the increase of immersion time, the impedance spectrum value and the self-corrosion potential of the magnesium alloy show a trend of increasing and then decreasing, and the impedance spectrum value is the largest at 2281.2Ω / cm 2 at 45min (Example 1), and the self-corrosion potential is -0.666V, and combined with Figure 15 (SEM image) it can be seen that a film structure is formed. According to the data of Examples 1, 5, 6 and Comparative Examples 6, 7, with the increase of temperature, the impedance spectrum value and the self-corrosion potential show a trend of increasing and then decreasing, and the best corrosion resistance is obtained at 50℃ (Example 1), and H2O2 is decomposed at 50℃, which is accelerated, thereby promoting the conversion of Ce 3+ → Ce 4+ which is conducive to the formation of the film structure. According to the data of Examples 1, 7, 8 and Comparative Examples 8, 9, with the increase of immersion times, the best corrosion resistance is obtained when the immersion times is 3 times.
[0101] Table 1 Impedance value and polarization corrosion potential value of AZ91D magnesium alloy material obtained by Examples 1-8 and Comparative Examples 1-9
[0102]
[0103]
[0104] As shown in Table 1, when the CeCl3 solution is selected as the passivation solution, the AZ91D magnesium alloy material has the best impedance value and self-corrosion potential when the mass fraction of CeCl3 in the dilute passivation solution is 3wt.%, the immersion temperature is 50℃, the immersion time is 45min, and the immersion is performed for 3 times.
[0105] Example 9
[0106] (1) Pretreatment of the AZ91D magnesium alloy: the AZ91D magnesium alloy was customized into a rectangular thin die-casting piece with a size of (10mm x 10mm x 1mm, for surface morphology and element distribution detection) and (30mm x 10mm x 1mm, for polarization curve and impedance spectrum detection); the AZ91D magnesium alloy was polished by using water-based sandpaper (1000 mesh) to polish off the surface layer; the AZ91D magnesium alloy was polished by using a polishing cloth to spray a polishing agent, thereby obtaining a preliminary sample; the AZ91D magnesium alloy was degreased by using acetone; the acetone was washed away by using a large amount of deionized water for multiple times; and the pretreatment was completed by using an oven to dry at a temperature of 120℃ for 1h;
[0107] (2) 4g of LaCl3, 2.5g of hydrogen peroxide and 93.5g of water were mixed to obtain a passivation solution containing LaCl3, and then the pretreated AZ91D magnesium alloy was immersed in the passivation solution containing LaCl3 at 40℃ for 40min for a total of 2 times; the AZ91D magnesium alloy sample after the passivation treatment was sequentially cleaned with anhydrous ethanol and deionized water; and the preparation of the rare earth conversion film on the surface of the AZ91D magnesium alloy material was completed after the AZ91D magnesium alloy sample was naturally dried.
[0108] Example 10
[0109] The difference from Example 9 is that 5g of LaCl3, 2.5g of hydrogen peroxide and 92.5g of water were mixed to obtain a passivation solution containing LaCl3.
[0110] Example 11
[0111] The difference from Example 9 is that the immersion time is 35min.
[0112] Example 12
[0113] The difference from Example 9 is that the immersion time is 45min.
[0114] Example 13
[0115] The difference from Example 9 is that the immersion temperature is 30℃.
[0116] Example 14
[0117] The difference from Example 9 is only that the number of immersions is 3 times.
[0118] Example 15
[0119] The difference from Example 9 is only that the number of immersions is 4 times.
[0120] Example 16
[0121] The difference from Example 9 is only that the number of immersions is 5 times.
[0122] Comparative Example 10
[0123] The difference from Example 9 is only that 1 g of LaCl3, 2.5 g of hydrogen peroxide and 96.5 g of water are mixed, and the reagents are stirred to mix uniformly, to obtain a passivation liquid containing LaCl3.
[0124] Comparative Example 11
[0125] The difference from Example 9 is only that 2 g of LaCl3, 2.5 g of hydrogen peroxide and 95.5 g of water are mixed, and the reagents are stirred to mix uniformly, to obtain a passivation liquid containing LaCl3.
[0126] Comparative Example 12
[0127] The difference from Example 9 is only that 3 g of LaCl3, 2.5 g of hydrogen peroxide and 94.5 g of water are mixed, and the reagents are stirred to mix uniformly, to obtain a passivation liquid containing LaCl3.
[0128] Comparative Example 13
[0129] The difference from Example 9 is only that the immersion time is 25 min.
[0130] Comparative Example 14
[0131] The difference from Example 9 is only that the immersion time is 30 min.
[0132] Comparative Example 15
[0133] The difference from Example 9 is only that the immersion temperature is 20°C.
[0134] Comparative Example 16
[0135] The difference from Example 9 is only that the immersion temperature is 50°C.
[0136] Comparative Example 17
[0137] The difference from Example 9 is only that the immersion temperature is 60°C.
[0138] Comparative Example 18
[0139] The difference from Example 9 is only that the number of immersions is 1 time.
[0140] The AZ91D magnesium alloy materials obtained in Examples 9-16 and Comparative Examples 10-18 were subjected to electrochemical workstation testing. Electrochemical testing was performed in a 3.5 wt.% NaCl solution using a three-electrode system: the working electrode was a passivated magnesium alloy material, the auxiliary electrode was a platinum electrode, and the reference electrode was a saturated calomel electrode (SCE). This system was used to simulate a corrosive environment. Test conditions: AC impedance high frequency 10 kHz, low frequency 1 Hz. The polarization curve initial potential setting parameters were -1.4 V, the end potential setting parameters were 0 V, the scan segment number was 1 s, and the scan rate was 0.01 V / s.
[0141] The experimental results are as follows Figures 7-10 As shown in Table 2, the impedance spectra of Examples 9 to 16 and Comparative Examples 10 to 18 have the largest impedance spectrum of Example 9 at 3148.7 Ω / cm 2 , corresponding to the least film defects, the self-corrosion potential shifts positively to -0.639V, indicating that the anodic reaction is significantly suppressed, indicating that it has the best corrosion resistance.
[0142] According to the data comparison of Examples 9, 13, and Comparative Examples 15 to 17, it can be seen that the temperature increase accelerates La 3+ Replacement reaction with magnesium matrix (Mg+2La 3+ →Mg 2+ +2La 2+ ), while promoting the OH produced by the decomposition of H2O2 - with La 3+ A dense La(OH)3 film was formed (the diameter of the capacitive arc was the largest when the temperature was 50°C in Example 17). The positive shift of the self-corrosion potential was -0.670 V (maximum value), which proved that the conversion film inhibited the anodic dissolution (Mg→Mg 2+ +2e - According to Examples 9, 14 to 16 and Comparative Example 18, it can be seen that the single immersion (Comparative Example 18) process has problems such as uneven film layer and poor bonding strength (combination Figure 16 ), 2 times of immersion is the best process parameter, forming a protective layer containing rare earth, and the impedance value is increased to 3148.6Ω / cm 2 , the self-corrosion potential is -0.639 V. According to the data of Examples 9, 11, 12 and Comparative Examples 13, 14, when the passivation time is 40 min (Example 9), the capacitive reactance value of the impedance presents a maximum value, and the self-corrosion potential of the polarization curve is more positive.
[0143] Table 2 Impedance values and polarization corrosion potential values of AZ91D magnesium alloy materials obtained in Examples 9 to 16 and Comparative Examples 10 to 18
[0144]
[0145]
[0146] From Table 2, when LaCl3solution is selected as the passivation solution, the mass fraction of LaCl3in the dilute passivation solution is set to 4wt.%, the immersion temperature is 40℃, the immersion time is 40min, and the AZ91D magnesium alloy material is treated twice, the treated AZ91D magnesium alloy material has the best impedance value and self-corrosion potential.
[0147] Example 17
[0148] (1) Pretreatment of AZ91D magnesium alloy: the AZ91D magnesium alloy was custom-made into a rectangular thin die-casting piece with a size of (10mm x 10mm x 1mm, for surface morphology and element distribution detection) and (30mm x 10mm x 1mm, for polarization curve and impedance spectrum detection); the AZ91D magnesium alloy was polished by using water-based sandpaper (1000 mesh) to grind off the surface layer; the AZ91D magnesium alloy was polished by using a polishing cloth to spray a polishing agent, obtaining a preliminary sample; the AZ91D magnesium alloy was degreased by using acetone; the acetone was washed away by using a large amount of deionized water for multiple times; the pretreatment was completed by using an oven to dry at a temperature of 120℃ for 1h;
[0149] (2) 1g of CeCl3, 1g of LaCl3, 2.5g of hydrogen peroxide and 95.5g of water were mixed, and the reagents were stirred to mix uniformly, obtaining a passivation solution containing CeCl3and LaCl3, then the pretreated AZ91D magnesium alloy was immersed in the passivation solution containing CeCl3and LaCl3, at a temperature of 50℃, for 50min, for a total of 1 time, then the passivated AZ91D magnesium alloy sample was cleaned with anhydrous ethanol and deionized water in sequence, and the AZ91D magnesium alloy sample was naturally air-dried, completing the preparation of the rare earth conversion film on the surface of the AZ91D magnesium alloy material.
[0150] Example 18
[0151] The difference from Example 17 is only that the immersion temperature is 55℃.
[0152] Comparative Example 19
[0153] The difference from Example 17 is only that the immersion temperature is 30℃.
[0154] Comparative Example 20
[0155] The difference from Example 17 is only that the immersion temperature is 35℃.
[0156] Comparative Example 21
[0157] The difference from Example 17 is only that the immersion temperature is 40℃.
[0158] Comparative Example 22
[0159] The difference from Example 17 is that the immersion temperature is 45°C.
[0160] Comparative Example 23
[0161] The difference from Example 17 is that 1 g of CeCl3, 3 g of LaCl3, 2.5 g of hydrogen peroxide, and 93.5 g of water are mixed, and the reagents are stirred to mix uniformly, to obtain a passivation solution containing CeCl3and LaCl3.
[0162] Comparative Example 24
[0163] The difference from Example 17 is that 1 g of CeCl3, 2 g of LaCl3, 2.5 g of hydrogen peroxide, and 94.5 g of water are mixed, and the reagents are stirred to mix uniformly, to obtain a passivation solution containing CeCl3and LaCl3.
[0164] Comparative Example 25
[0165] The difference from Example 17 is that 1 g of CeCl3, 0.5 g of LaCl3, 2.5 g of hydrogen peroxide, and 96 g of water are mixed, and the reagents are stirred to mix uniformly, to obtain a passivation solution containing CeCl3and LaCl3.
[0166] Comparative Example 26
[0167] The difference from Example 17 is that 3 g of CeCl3, 1 g of LaCl3, 2.5 g of hydrogen peroxide, and 93.5 g of water are mixed, and the reagents are stirred to mix uniformly, to obtain a passivation solution containing CeCl3and LaCl3.
[0168] Comparative Example 27
[0169] The difference from Example 17 is that 0.5 g of CeCl3, 0.5 g of LaCl3, 2.5 g of hydrogen peroxide, and 96.5 g of water are mixed, and the reagents are stirred to mix uniformly, to obtain a passivation solution containing CeCl3and LaCl3.
[0170] Comparative Example 28
[0171] The difference from Example 17 is that the number of immersions is 2.
[0172] Comparative Example 29
[0173] The difference from Example 17 is that the number of immersions is 3.
[0174] Comparative Example 30
[0175] The difference from Example 17 is that the number of immersions is 4.
[0176] Comparative Example 31
[0177] The difference from Example 17 is that AZ91D magnesium alloy is replaced by AZ31B magnesium alloy.
[0178] Comparative Example 32
[0179] The difference from Example 17 is that AZ91D magnesium alloy is replaced by 301 stainless steel.
[0180] Comparative Example 33
[0181] The difference from Example 17 is that 1 g of CeCl3, 1 g of LaCl3, 5 g of hydrogen peroxide and 93 g of water are mixed, and the reagents are stirred to be uniformly mixed to obtain a passivation solution containing CeCl3and LaCl3.
[0182] Comparative Example 34
[0183] The difference from Example 17 is that 10 g of citric acid, 10 g of hydrogen peroxide solution, 2.06 g of sodium molybdate, 2.22 g of CeCl3and 75.72 g of deionized water are used to configure a rare earth passivation solution.
[0184] The AZ91D magnesium alloy materials obtained in Examples 17-19 and Comparative Examples 19-34 are detected by an electrochemical workstation. A three-electrode system is configured, and electrochemical tests are carried out in a 3.5 wt.% NaCl solution: the working electrode uses the passivated magnesium alloy material, the auxiliary electrode is a platinum electrode, and the reference electrode is a saturated calomel electrode (SCE), which is used to simulate the corrosion environment. The test conditions are: alternating current impedance high frequency 10 kHz, low frequency 1 Hz. The initial potential setting parameter of the polarization curve is -1.4 V, the terminal potential setting parameter is 0 V, the scanning section number is 1 s, and the scanning speed is 0.01 V / s.
[0185] The experimental results are shown in Table 1. Figures 11-14 As shown in Table 1, according to the data of Examples 17, 18 and Comparative Examples 19-22, at low temperature (such as 30°C of Comparative Example 19), the diffusion rate of rare earth ions is slow, and the conversion film does not grow fully, and there are many micropores or defects. At 50°C (Example 17), the diffusion of rare earth ions and the film formation rate reach a balance, and a uniform and dense composite conversion film (containing CeO2, La2O3and other oxides) is formed, which effectively isolates the substrate from the corrosion medium (such as Cl - ). High temperature (such as 55°C of Example 18) may cause excessive dissolution or coarse crystallization of the film layer, thereby reducing the protection ability. According to the data of Examples 17 and Comparative Examples 23-27, different concentration ratios of mixed rare earths show different capacitive values of impedance spectrum, and the maximum value is 3324.2 Ω / cm 2, the self-corrosion potential is most positive at -0.670V, indicating that the corrosion resistance is the best at this time. According to the data of Example 17 and Comparative Examples 28-30, with the increase of the immersion times, the corrosion resistance of the material decreases, and multiple immersions (more than once) can cause the excessive growth of the conversion film, the loose and cracking of the film layer, or the uneven distribution of the components, thereby reducing the protective performance. However, single immersion (Example 17) can make the rare earth ions quickly and uniformly react with the alloy surface at 50°C to form a dense and continuous conversion film (mainly composed of CeO2, La2O3 and other oxides), effectively isolate the corrosive medium (such as Cl-), and make the corrosion resistance optimal. According to Example 17 and Comparative Examples 31 and 32, the same passivation solution has different corrosion resistances for different base materials, and the impedance value of the 301 stainless steel (Comparative Example 32) is 1906.1 Ω / cm 2 , the self-corrosion potential is most positive at -0.670V; and the impedance value of the AZ31B magnesium alloy (Comparative Example 31) is 2353.4 Ω / cm 2 , the self-corrosion potential is most positive at -0.709V; and the impedance value of the AZ91D (Example 17) is 3324.2 Ω / cm 2 , the self-corrosion potential is most positive at -0.670V; indicating that the AZ91D magnesium alloy (Example 17) has the best corrosion resistance. According to Example 17 and Comparative Examples 33 and 34, the use of different passivation solutions for passivation treatment has the best corrosion resistance. According to the polarization impedance results, the use of different types of metals for passivation with the same passivation solution is not as effective as the use of AZ91D magnesium alloy. The use of different formulations of passivation solutions for passivation treatment of AZ91D magnesium alloy is not as effective as the use of the passivation solution of the present application.
[0186] According to the above, when the mixed solution of CeCl3 and LaCl3 is selected as the passivation solution, the mass ratio of CeCl3 and LaCl3 is set to 1g:1g, the immersion temperature is 50°C, the immersion time is 50min, and the immersion is 1 time, the treated AZ91D magnesium alloy material has the best impedance value and self-corrosion potential, and is better than the effect of treating the magnesium alloy with CeCl3 solution and LaCl3 solution respectively, which reveals that the synergistic mechanism of Ce-La double rare earth ions achieves better protective performance than single rare earth at a specific ratio.
[0187] Table 3 Impedance value and polarization corrosion potential value of AZ91D magnesium alloy material obtained in Example 17-19 and Comparative Example 19-30
[0188] Sample Impedance values (Ω / cm 2 )]]> Self-corrosion potential (V) Example 17 3324.2 -0.670 Example 18 1475.2 -0.682 Comparative Example 19 890.7 -0.693 Comparative Example 19 1396.8 -0.698 Comparative Example 20 1261.5 -0.686 Comparative Example 21 1261.5 -0.686 Comparative Example 22 1337.6 -0.697 Comparative Example 23 1638.5 -0.681 Comparative Example 24 868.9 -0.683 Comparative Example 25 996.1 -0.697 Comparative Example 26 1590.5 -0.694 Comparative Example 27 1039.5 -0.674 Comparative Example 28 1055.3 -0.694 Comparative Example 29 1331.9 -0.697 Comparative Example 30 1527.6 -0.682 Comparative Example 31 2353.4 -0.709 Comparative Example 32 1906.1 -0.707 Comparative Example 33 1775.9 -0.697 Comparative Example 34 3023.3 -0.692
[0189] The above merely describes the preferred embodiments of the present application, and it should be pointed out that, for those skilled in the art, several improvements and refinements can be made without departing from the principles of the present application, and these improvements and refinements should also be considered as falling within the protection scope of the present application.
Claims
1. A method for preparing a rare earth conversion film on the surface of an AZ91D magnesium alloy material, characterized in that: The following steps are involved: The AZ91D magnesium alloy is placed in a rare earth passivation solution for immersion to complete the preparation of a rare earth conversion film on the surface of the AZ91D magnesium alloy; The rare earth passivation solution comprises rare earth salt, hydrogen peroxide solution and water; The rare earth salt is one or both of cerium chloride and lanthanum chloride.
2. The preparation method according to claim 1, characterized in that The mass fraction of the rare earth salt in the rare earth passivation solution is 2-5 wt %; the mass fraction of the hydrogen peroxide solution in the rare earth passivation solution is 1-4 wt %; and the mass fraction of water in the rare earth passivation solution is 90-97 wt %.
3. The preparation method according to claim 1, characterized in that When the rare earth salt is a mixture of cerium chloride and lanthanum chloride, the mass ratio of cerium chloride to lanthanum chloride is 1:
1.
4. The preparation method according to claim 1, characterized in that When the rare earth salt is cerium chloride, the soaking temperature is 30-50° C., the soaking time is 35-45 minutes, and the soaking times are 1-5 times.
5. The preparation method according to claim 1, characterized in that When the rare earth salt is cerium chloride, the soaking temperature is 50° C., the soaking time is 45 minutes, and the soaking times are 3 times.
6. The preparation method according to claim 1, characterized in that When the rare earth salt is lanthanum chloride, the soaking temperature is 30-40° C., the soaking time is 25-45 minutes, and the soaking times are 1-5 times.
7. The preparation method according to claim 1, characterized in that When the rare earth salt is lanthanum chloride, the soaking temperature is 40°C, the soaking time is 40 minutes, and the soaking times are 2 times.
8. The preparation method according to claim 1, characterized in that When the rare earth salt is a mixture of cerium chloride and lanthanum chloride, the soaking temperature is 50-55° C., the soaking time is 38-42 minutes, and the soaking times are 1-4 times.
9. The preparation method according to claim 1, characterized in that When the rare earth salt is a mixture of cerium chloride and lanthanum chloride, the soaking temperature is 50° C., the soaking time is 40 minutes, and the soaking frequency is 1 time.
10. The preparation method according to claim 1 or 2, characterized in that: The mass concentration of the hydrogen peroxide solution is 25-35%.