ZnO-ZnAl composite coating taking sintered neodymium iron boron as matrix and preparation method of ZnO-ZnAl composite coating

Abstract

The invention provides a ZnO-ZnAl composite coating with sintered neodymium iron boron as a matrix and a preparation method of the ZnO-ZnAl composite coating. The ZnO-ZnAl composite coating is composed of a ZnAl coating and ZnO nanoparticles, and the ZnO-ZnAl composite coating is prepared through surface pretreatment of a sintered neodymium-iron-boron magnet, brushing and curing of ZnAl coating liquid and preparation of the ZnO-ZnAl composite coating. According to the ZnO-ZnAl composite coating, the corrosion degree of the sintered neodymium-iron-boron magnet can be remarkably reduced, the surface of the sintered neodymium-iron-boron magnet is free of rust spots after the neutral salt mist resistance time can reach 656 h, the aging resistance is excellent, the corrosion current density is as low as 1.65 * 10 <-8 > A / cm < 2 >, and the influence on the magnetic performance of the sintered neodymium-iron-boron magnet is extremely small.

Description

Technical Field

[0001] This invention belongs to the field of magnet surface protection technology, specifically relating to a ZnO-ZnAl composite coating based on sintered NdFeB and its preparation method. Background Technology

[0002] Neodymium iron boron (NdFeB) permanent magnets are widely used in various fields such as new energy vehicles, electronic information, wind power generation, and intelligent manufacturing due to their excellent magnetic energy product and coercivity. However, NdFeB materials themselves have poor corrosion resistance and are extremely susceptible to intergranular corrosion, pulverization, and surface oxidation in humid or corrosive environments, leading to performance degradation, structural damage, and shortened lifespan. Therefore, improving the surface protection capabilities of NdFeB magnets is crucial for extending their service life and ensuring stable device operation.

[0003] Currently, electroplated Ni or Ni-Cu-Ni three-layer structures are commonly used in industry as anti-corrosion coatings. Although they can improve protective performance to some extent, the process of such metal plating involves a large amount of waste liquid and heavy metal emissions, making it difficult to meet the development trend of prioritizing both green environmental protection and high reliability. At the same time, with the development of functional materials, composite coatings that combine good density, adhesion, and environmental performance are gradually becoming a hot research direction in surface protection.

[0004] The hexavalent chromium contained in traditional Dacromet coatings is being gradually restricted or even banned globally due to its potential carcinogenic risks and serious environmental pollution. Therefore, developing chromium-free green protective coatings has become a current research hotspot. ZnAl-based chromium-free Dacromet coatings, as an environmentally friendly alternative, show broad application prospects. However, the ZnAl coatings currently used for sintering NdFeB magnets still suffer from poor corrosion resistance, a problem that needs to be addressed.

[0005] Sintered NdFeB magnets are a multiphase composite material, Nd2Fe 14The B-phase is the source of the magnet's magnetic properties and is relatively stable, but it may corrode slowly under extreme conditions. The Nd-rich grain boundary phase, due to its high chemical activity, is considered the most susceptible to corrosion within the magnet. Secondary phases (Nd1Fe4B4) and impurities have a certain impact on the overall corrosion resistance of the magnet and may induce localized corrosion in corrosive environments. Chinese patent CN 106700701 A discloses a high-temperature resistant, environmentally friendly Dacromet coating. By replacing some zinc powder with magnesium manganese powder and water with anhydrous ethanol, the high-temperature corrosion resistance and thermal shock resistance of the coating are improved. However, its resistance to rapid ammonium nitrate corrosion is only 162 minutes, indicating poor corrosion resistance. Reference 1 reports a chromium-free Zn-Al coating. By controlling the content of Zn and Al and the heat treatment temperature, the corrosion resistance of the coating is improved, but its coating density and wear resistance are limited (Jiang Jie, Jiang Jianjun, Yang Lijing, et al. Preparation and corrosion resistance mechanism of Zn-Al coating on sintered NdFeB surface [J]. Chinese Journal of Corrosion and Protection, 2023, 43(01):104-110.). Summary of the Invention

[0006] The purpose of this invention is to provide a ZnO-ZnAl composite coating based on sintered NdFeB and its preparation method. The composite coating is composed of ZnAl coating and ZnO nanoparticles, which solves the problem of poor corrosion resistance of existing sintered NdFeB magnets.

[0007] The objective of this invention can be achieved through the following technical solutions:

[0008] A method for preparing a ZnO-ZnAl composite coating based on sintered NdFeB substrate includes the following steps:

[0009] Step 1: Perform surface pretreatment on the sintered NdFeB magnets to serve as the substrate;

[0010] Step 2: Using Zn-based coating liquid as the bottom layer and Al-based coating liquid as the top layer, a two-coat, two-bake brush coating process is adopted, followed by preheating and curing treatment to obtain a ZnAl composite coating.

[0011] Step 3: Introduce nano-ZnO particles into the Al-based coating solution. The nano-ZnO particles account for 6 wt.% of the total mass of the coating solution. The ZnO-ZnAl composite coating is obtained by sintering at a temperature of 210~240℃ and using the two-coat, two-bake brush coating and curing process of Step 2.

[0012] Furthermore, in step 1, the specific method for surface pretreatment is as follows: the sintered NdFeB magnet is mechanically ground and polished in sequence, then acid-washed and activated, ultrasonically washed with anhydrous ethanol, dried at 100~120℃ for 20~30 minutes, and then allowed to cool naturally to room temperature.

[0013] Furthermore, in step 2, the Zn-based coating liquid, by mass percentage, consists of: 55% zinc powder with a particle size of 20-40 μm, 1% cellulose ether thickener, 1.2% SiO2 additive, 1% silane coupling agent, 5% titanium dioxide, 0.2% defoamer BykVP-040, 1% emulsifier Byk073, and 2% polytetrafluoroethylene. The organic solvent is titanium isooctanol (IV), and chloromethane is used as the diluent. The Al-based coating liquid, by mass percentage, consists of: 20% aluminum powder with a particle size of 20-40 μm, 2% cellulose ether, 3% silane coupling agent, 0.5% titanium dioxide, and 32% polytetrafluoroethylene. Dimethylaminoethanol is used as the solvent, and methylal is used as the diluent.

[0014] Furthermore, in step 2, the two-coat, two-bake brush coating process is as follows: first, the Zn-based coating liquid is evenly brushed onto the substrate, then preheated in a tube furnace at 70~90℃ for 10~15 minutes, followed by curing at 200~250℃ for 25~45 minutes. After naturally cooling to room temperature, the Al-based coating liquid is brushed on and subjected to the same preheating, curing, and cooling process to finally obtain the ZnO-ZnAl composite coating.

[0015] The present invention provides a ZnO-ZnAl composite coating with sintered NdFeB as the matrix prepared by the above preparation method.

[0016] Compared with the prior art, the present invention has the following advantages:

[0017] (1) The protective coating on the surface of the sintered NdFeB magnet provided by the present invention is a ZnO-ZnAl composite coating. This composite coating can significantly reduce the corrosion degree of the sintered NdFeB magnet. After a neutral salt spray test time of up to 656 hours, there are no rust spots on the surface. It has excellent anti-aging ability and a corrosion current density as low as 1.65×10 -8 A / cm 2 Furthermore, it has little impact on the magnetic properties of sintered NdFeB magnets, and the ZnAl-based chromium-free Dacromet coating is low-cost and environmentally friendly, with no waste emissions during the preparation process.

[0018] (2) The nano-ZnO particles introduced into the Al-based coating solution in this invention have a significant reinforcing effect on the anti-corrosion coating. On the one hand, the nano-ZnO particles have good antioxidant properties and barrier effect, which can effectively block moisture and corrosive media, and improve the corrosion resistance of the coating. Their nanoscale particle size allows them to be uniformly dispersed in the coating, enhancing the structural density, improving hardness and wear resistance, thereby improving scratch resistance and abrasion resistance. At the same time, the synergistic effect between the nano-ZnO particles and the coating components helps to enhance the adhesion to the metal substrate and reduce peeling. Studies have also shown that the nano-ZnO particles have certain self-healing potential and good thermal stability, which helps to improve the structural stability of the coating under high temperature environment, delay the thermal aging process, and ensure its long-term service performance. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the cross-sectional structure of the ZnO-ZnAl composite coating, showing the relationship between the bottom layer Zn, the top layer Al, the nano ZnO particles and the substrate.

[0020] Figure 2 This is a flow chart of the preparation process of the ZnO-ZnAl composite coating.

[0021] Figure 3 The images show the macroscopic morphology of the ZnO-ZnAl composite coatings after 656 hours of salt spray corrosion in Examples 1-4.

[0022] Figure 4 The images show the macroscopic morphology of the ZnO-ZnAl composite coatings from Examples 1-4 after a 14-day PCT aging test.

[0023] Figure 5 The graphs show the electrochemical corrosion performance of the ZnO-ZnAl composite coatings in Examples 1-4.

[0024] Figure 6 This is a comparison of the magnetic properties (BH curves) of the ZnO-ZnAl composite coating and the NdFeB substrate in Example 3. Detailed Implementation

[0025] The present invention will now be described in further detail with reference to specific embodiments and accompanying drawings.

[0026] In the following examples, the Zn-based and Al-based coating solutions used were purchased from Amet (China) Chemical Co., Ltd., Shanghai Qingpu Branch, and were designated as ZincMet 200 (DR) and Tektronix Rust Silver S-HT, respectively. The diluents used for the Zn-based and Al-based coating solutions were also purchased from the aforementioned company, and were designated as ZincMet Diluent 50 and Tektronix Rust Diluent HT, respectively.

[0027] In the following examples, the Zn-based coating liquid, by mass percentage, consists of: 55% zinc powder with a particle size of 20-40 μm, 1% cellulose ether thickener, 1.2% SiO2 additive, 1% silane coupling agent, 5% titanium dioxide, 0.2% defoamer BykVP-040, 1% emulsifier Byk073, and 2% polytetrafluoroethylene. The organic solvent is titanium isooctanol (IV), and chloromethane is used as the diluent. The Al-based coating liquid, by mass percentage, consists of: 20% aluminum powder with a particle size of 20-40 μm, 2% cellulose ether, 3% silane coupling agent, 0.5% titanium dioxide, and 32% polytetrafluoroethylene. Dimethylaminoethanol is used as the solvent, and methylal is used as the diluent.

[0028] Example 1

[0029] A method for preparing a ZnO-ZnAl composite coating based on sintered NdFeB substrate includes the following steps:

[0030] Step 1: The surface of the sintered NdFeB magnet sample after cutting and processing is mechanically ground and polished. Then, the oil is removed by acid washing. The treated sample is ultrasonically cleaned with anhydrous ethanol. Then, it is placed in an oven and heated at 100~120℃ for 20~30 minutes to dry. Then, it is allowed to cool naturally to room temperature for later use as a substrate.

[0031] Step 2: Using a "two-coat, two-bake" brush coating process, first, the Zn-based coating liquid is evenly brushed onto the substrate surface, preheated at 80℃ for 10 minutes and cured at 200℃ for 30 minutes. After it cools naturally to room temperature, the Al-based coating liquid is evenly brushed on and the preheating and curing process is repeated to obtain the ZnAl coating.

[0032] Step 3: Using a "two-coat, two-bake" brush coating process, an Al-based coating liquid containing nano-ZnO particles is uniformly brushed onto the ZnAl coating surface. The nano-ZnO particles are 50nm in size and account for 2wt.% of the total mass of the coating liquid. The ZnO-ZnAl composite coating is obtained by using a sintering temperature of 240℃ and the "two-coat, two-bake" brush coating and curing process from Step 2.

[0033] The macroscopic morphology of the corrosion surface of the ZnO-ZnAl composite coating based on sintered NdFeB in a neutral salt spray corrosion test is shown below. Figure 3 As shown in a), the figure shows that corrosion is more severe in some local areas, and corrosion products, solvents or some solutions remain on the surface in the form of small droplets, indicating severe corrosion.

[0034] The surface morphology of the ZnO-ZnAl composite coating based on sintered NdFeB substrate in the PCT high-pressure accelerated aging test is as follows: Figure 4 As shown in (a), it can be seen from the figure that the ZnAl composite coating with 2wt.% ZnO content only shows discrete micron-sized cheongsams in local areas, indicating that the densification effect of ZnO on the substrate oxide film is limited at low concentrations and it fails to effectively inhibit the local gas escape caused by the penetration of corrosive media.

[0035] Electrochemical corrosion performance of ZnO-ZnAl composite coating on sintered NdFeB surface in electrodeization test, such as Figure 5 As shown, from Figure 5 It can be seen that the ZnAl composite coating with a ZnO content of 2 wt.% has the fastest corrosion rate. The low ZnO content is not conducive to the formation of a dense and continuous protective layer, and the protective ability is relatively insufficient.

[0036] Example 2

[0037] This embodiment is basically the same as that of embodiment 1, except that in step 3, the nano ZnO particles account for 4 wt.% of the total mass of the coating liquid.

[0038] The macroscopic morphology of the ZnO-ZnAl composite coating with sintered NdFeB as the matrix prepared in Example 2 during the neutral salt spray corrosion test is as follows: Figure 3 As shown in b), the number of droplets on the corroded surface of the sample increases, the surface becomes more uniform, and there are fewer corrosion product spots. This indicates that as the ZnO content increases to 4 wt.%, the oxide component increases, which makes it easier to form a stable passivation layer in the corrosive environment, reducing the diffusion and dissolution of corrosion products, and the degree of corrosion is moderate.

[0039] The surface morphology of the ZnO-ZnAl composite coating with sintered NdFeB as the matrix prepared in Example 2 during the PCT high-pressure accelerated aging test is as follows. Figure 4 As shown in (b), the number of bubbles increases and they exhibit uneven aggregation, reflecting the intensified interfacial reaction induced by ZnO at a concentration of 4 wt.% ZnO, which leads to the propagation of microcracks within the oxide and the formation of gas release channels.

[0040] The electrochemical corrosion performance of the ZnO-ZnAl composite coating prepared in Example 2, based on sintered NdFeB, was as follows in the electrodeization test: Figure 5 As shown, from Figure 5 It can be seen that the corrosion rate of the ZnO-ZnAl composite coating with a ZnO content of 4 wt.% is slower. Increasing the ZnO content is conducive to forming a dense and continuous protective layer, and its protective ability is better than that of the ZnAl composite coating with a ZnO content of 2 wt.% is better.

[0041] Example 3

[0042] This embodiment is basically the same as that of embodiment 1, except that in step 3, the nano ZnO particles account for 6 wt.% of the total mass of the coating liquid.

[0043] The macroscopic morphology of the ZnO-ZnAl composite coating with sintered NdFeB as the matrix prepared in Example 3 during the neutral salt spray corrosion test is as follows: Figure 3 As shown in c), the number of spots and droplets on the sample surface has been further reduced, resulting in a relatively smooth and uniform gray surface with only a very small amount of fine corrosion traces. ZnO in a corrosive environment helps to form a denser and more stable corrosion product film, hindering the invasion of Cl ions and effectively reducing the degree of corrosion. The visible corrosion traces on the surface are further reduced, and the protective performance is superior to ZnO-ZnAl composite coatings with 2wt.%, 4wt.%, and 8wt.% ZnO content, exhibiting the best corrosion resistance.

[0044] The surface morphology of the ZnO-ZnAl composite coating with sintered NdFeB as the matrix prepared in Example 3 was obtained in the PCT high-pressure accelerated aging test as follows: Figure 4 As shown in (c), the ZnAl composite coating with a ZnO content of 6 wt.% has densely and uniformly distributed microbubbles (3-5 μm in diameter) on its surface, with a coverage of 100 wt.%. This indicates that ZnO at this concentration optimizes the compactness of the oxide film through uniform dispersion, but the excessive bubble generation reflects that the interfacial stress concentration has not been completely eliminated.

[0045] The electrochemical corrosion performance of the ZnO-ZnAl composite coating prepared in Example 3, based on sintered NdFeB, was as follows in the electrodeization test: Figure 5 As shown, from Figure 5 It can be seen that the ZnO-ZnAl composite coating with a ZnO content of 6 wt.% has the slowest corrosion rate. Increasing the ZnO content is conducive to forming a dense and continuous protective layer, and the protective ability is relatively the best.

[0046] The ZnO-ZnAl composite coating prepared in Example 3, based on sintered NdFeB, was compared with the NdFeB substrate in a magnetic property test. The obtained BH curves are shown below. Figure 6 As shown in the figure, the BH curves of the two almost completely overlap. The relevant magnetic properties obtained in the experiment are shown in Table 1.

[0047] Table 1

[0048]

[0049] Table 1 shows that compared with the NdFeB matrix, the remanence, intrinsic coercivity, coercivity, and maximum energy product of the NdFeB magnets with the ZnO-ZnAl composite coating based on sintered NdFeB matrix show minimal changes, all within negligible ranges. This indicates that the ZnO-ZnAl composite coating with 6 wt.% ZnO content has minimal impact on the magnetic properties of NdFeB magnets, significantly improves their corrosion resistance, and provides an efficient and reliable protective barrier for sintered NdFeB magnets.

[0050] Example 4

[0051] This embodiment is basically the same as that of embodiment 1, except that in step 3, the nano ZnO particles account for 8 wt.% of the total mass of the coating liquid.

[0052] The macroscopic morphology of the ZnO-ZnAl composite coating with sintered NdFeB as the matrix prepared in Example 4 during the neutral salt spray corrosion test is as follows: Figure 3 As shown in d), the surface of the 8 wt.% ZnO sample is relatively uniform and smooth, with minimal surface droplet residue. However, corrosion spots are relatively... Figure 3c) Increase: The introduction of excessive ZnO may cause uneven dispersion or stress concentration in the coating microstructure, resulting in micro-defects or pores.

[0053] The surface morphology of the ZnO-ZnAl composite coating with sintered NdFeB as the matrix prepared in Example 4 was obtained in the PCT high-pressure accelerated aging test as follows: Figure 4 As shown in (d), the ZnO-ZnAl composite coating with a content of 8 wt.% has the lowest foaming density, with only a few scattered microbubbles (diameter <3 μm) on the surface and no tendency to aggregate. This indicates that the high concentration of ZnO significantly inhibits the evolution of defects in the oxide film through the self-passivation effect, resulting in the lowest degree of aging.

[0054] The electrochemical corrosion performance of the ZnO-ZnAl composite coating prepared in Example 4, based on sintered NdFeB, was as follows in the electrodeization test: Figure 5 As shown, from Figure 5 It can be seen that the ZnO-ZnAl composite coating with an 8wt.% ZnO content has a faster corrosion rate, indicating that the excessive addition of ZnO leads to an increase in coating porosity or microcracks, thereby weakening the medium shielding effect.

[0055] In summary, the brush-coating-sintering composite coating based on ZnAl modified with 6wt.% ZnO content shows good application potential in improving the corrosion resistance of NdFeB magnets. After 656 hours of neutral salt spray testing, the surface remains rust-free, and the corrosion current density is as low as 1.65 × 10⁻⁶. -8 A / cm 2 It has excellent anti-aging ability and has minimal impact on the magnetic properties of sintered NdFeB magnets.

[0056] The specific details of the test experiments conducted on the ZnO-ZnAl composite coating based on sintered NdFeB are as follows:

[0057] (1) Neutral Salt Spray Test (NSS): A 3.5 wt.% NaCl solution was converted into fine salt spray particles and evenly distributed in the test chamber using an atomizing device. The test environment parameters were controlled as follows: the temperature in the test chamber was 35 ± 2 ℃, the pH value of the salt spray solution was maintained between 6.5 and 7.2, and the relative humidity was maintained above 95%. The samples were placed in the chamber at the specified angle (15°-30° with respect to the vertical direction) and an appropriate distance was maintained between the samples to ensure uniform misting. The test duration was maintained at more than 28 days. During the test, the changes on the sample surface were observed regularly, with a focus on whether there were signs of corrosion such as blistering, peeling, and rust on the protective layer.

[0058] (2) PCT high pressure accelerated aging test: The high pressure accelerated aging chamber of model EHS-211M was used for the test. The experimental conditions were set as follows: temperature maintained at 120℃±2℃, steam pressure at 2 bar, and relative humidity maintained at 100% (saturated steam state).

[0059] (3) Electrochemical performance testing (Tafel polarization curve): Tafel polarization curves were tested using an electrochemical workstation in 3.5% NaCl solution. The electrochemical workstation used in the experiment was a Reference 600+ model manufactured by Gamry. A typical three-electrode system was used for the test, in which a saturated calomel electrode was used as the reference electrode, a platinum sheet was used as the auxiliary electrode, and the sample was used as the working electrode. Before the test, the sample was immersed in the test electrolyte and allowed to stand for 2 hours under open-circuit potential conditions to ensure that the system reached an electrochemically stable state. Subsequently, the electrochemical workstation was started to perform a potential scan on the working electrode. The scan rate was set to 1 mV / s, and the scan range covered the area from the negative corrosion potential to the positive overpotential range. Throughout the process, the polarization behavior of the sample was recorded in real time and presented in the form of a Tafel curve.

[0060] (4) Electrochemical impedance spectroscopy (EIS): The surface-treated samples were subjected to EIS in 3.5% NaCl solution using an electrochemical workstation to evaluate their corrosion resistance. The classic three-electrode system was used during the test, with a saturated calomel electrode (SCE) as the reference electrode, a platinum sheet as the auxiliary electrode, and the test sample as the working electrode. Before the test, the sample was allowed to stand at an open-circuit voltage (OCP) for two hours to ensure the system reached a stable state. Under open-circuit voltage conditions, the sample was characterized, and an amplitude of 5 mV and a frequency range of 0.01 to 10 mV were applied. 5 A sinusoidal potential signal X at Hz is input into the electrochemical system, generating a sinusoidal current signal Y. Through calculations within the electrochemical workstation, the system impedance values ​​at each frequency are obtained, forming an electrochemical impedance spectrum.

[0061] The magnetic properties of the ZnO-ZnAl composite coating based on sintered NdFeB in Example 3 were tested separately, and the details are as follows:

[0062] Magnetic Performance Testing: This experiment employed the NIM-200C Precision Measurement Device for Permanent Magnet Materials, developed by the National Institute of Metrology, China, to perform BH curve testing. During the testing process, a standard-sized cylindrical sample was selected, with its outer surface coated with the material to be tested. The sample was accurately placed into the measurement magnetic circuit, ensuring good circuit closure to minimize testing errors caused by air gap effects. The experiment was conducted at room temperature. The instrument controlled the change in the applied magnetic field and collected magnetic induction intensity data in real time, ultimately obtaining a complete hysteresis loop to evaluate key magnetic performance parameters of the coating material, such as remanent magnetic induction (Br), coercivity (Hc), and maximum energy product ((BH)max).

Claims

1. A method for producing a ZnO-ZnAl composite coating on a sintered neodymium-iron-boron substrate, characterized in that The method comprises the following steps: Step 1, surface pretreatment of sintered Nd-Fe-B magnet as a substrate; Step 2, using two-coat two-bake brushing coating process, preheating and curing treatment are carried out to obtain ZnAl composite coating with Zn-based coating liquid as the bottom layer and Al-based coating liquid as the top layer; Step 3, introducing nano-ZnO particles into the Al-based coating liquid, the mass fraction of nano-ZnO particles in the total coating liquid is 6wt.%, and the ZnO-ZnAl composite coating is obtained by the sintering temperature of 210-240℃ and the two-coat two-bake brushing coating and curing process of step 2.

2. The production method according to claim 1, characterized by, In step 1, the specific method of surface pretreatment is: the sintered Nd-Fe-B magnet is sequentially subjected to mechanical polishing and polishing treatment, then subjected to acid pickling activation, ultrasonic water washing with anhydrous ethanol, drying at 100-120℃ for 20-30 minutes, and then naturally cooling to room temperature.

3. The production method according to claim 1, characterized by, In step 2, the Zn-based coating liquid comprises, by mass percentage: zinc powder with a particle size of 20-40μm 55%, cellulose ether thickening agent 1%, SiO2 additive 1.2%, silane coupling agent 1%, titanium white powder 5%, defoaming agent Byk VP-0400.2%, emulsifier Byk073 1%, polytetrafluoroethylene 2%, organic solvent is titanium (IV) isooctanol, chloromethane is a diluent medium; the Al-based coating liquid comprises, by mass percentage: aluminum powder with a particle size of 20-40μm 20%, cellulose ether 2%, silane coupling agent 3%, titanium white powder 0.5%, polytetrafluoroethylene 32%, dimethylaminoethanol as a solvent, and methylal as a diluent.

4. The method of claim 1, wherein, In step 2, the two-coat two-bake brushing coating process is as follows: the Zn-based coating liquid is uniformly brushed on the substrate, then preheating treatment is carried out at 70-90℃ for 10-15 minutes in a tube furnace, then curing treatment is carried out at 200-250℃ for 25-45 minutes, after naturally cooling to room temperature, the Al-based coating liquid is brushed and the same preheating, curing and cooling treatment is carried out, and finally the ZnO-ZnAl composite coating is obtained.

5. The ZnO-ZnAl composite coating prepared by the method according to any one of claims 1-4 and taking sintered Nd-Fe-B as a substrate.

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