Anti-corrosion coating on the surface of NdFeB permanent magnets and its preparation method

Abstract

The application discloses a neodymium-iron-boron permanent magnet surface anticorrosion coating and a preparation method thereof, and the preparation method comprises the following steps: pretreating the neodymium-iron-boron permanent magnet, depositing an aluminum-based alloy coating on the surface of the pretreated neodymium-iron-boron permanent magnet through arc spraying, and performing vacuum heat treatment and coating of a sealing agent, wherein the temperature of the vacuum heat treatment is 400 DEG C to 600 DEG C, so that the neodymium-iron-boron permanent magnet surface anticorrosion coating is obtained. The neodymium-iron-boron permanent magnet surface anticorrosion coating of the application can greatly improve the bonding strength and the anticorrosion performance through the construction of a composite protection system of a "metallurgical bonding diffusion layer (bottom layer) + Al sacrificial anode protection layer (middle layer) + sealing agent barrier layer (surface layer)", which has important industrial value for improving the service life of the neodymium-iron-boron magnet in severe environments and widening the application scenarios of the neodymium-iron-boron magnet.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of surface protection of rare earth permanent magnet materials, and particularly relates to a surface corrosion-resistant coating of a neodymium-iron-boron permanent magnet and a preparation method thereof. BACKGROUND

[0002] Nd2Fe 14 B) permanent magnet, as the third generation of rare earth permanent magnet materials, is called "magnetic king" due to its extremely high magnetic energy product (>50 MGOe) and coercive force (>20 kOe), and is widely used in new energy vehicle drive motors, wind power generators, and precision instruments. However, the multi-phase grain boundary structure (main phase Nd2Fe 14 B coexists with Nd-rich phase and B-rich phase) of the neodymium-iron-boron permanent magnet leads to a serious corrosion failure problem. Specifically, in an environment with humidity > 60% or temperature > 80℃, the grain boundary Nd-rich phase is preferentially oxidized to form Nd(OH)3, which causes volume expansion and initiates coating cracking. In particular, in a salt spray or acidic medium, micro-batteries are formed by Fe and Nd to accelerate electrochemical corrosion, and the corrosion rate can reach 0.5 mm / year. In addition, micron-sized pores (pore diameter 1-5 μm) generated by the sintering process will become a penetration channel for the corrosion medium, further accelerating the overall failure.

[0003] Currently, electroplating, physical vapor deposition, and organic coating are widely used in the field of neodymium-iron-boron corrosion protection. However, traditional protection technologies (such as electroplated nickel / zinc layers) are difficult to meet the requirements of harsh working conditions due to high porosity (>3%) and cyanide pollution of the environment; organic coatings (such as epoxy resin) have poor temperature resistance (<120℃) and are prone to carbonization and peeling at high temperatures; physical vapor deposition has high equipment costs and small equipment cavities, and has low deposition efficiency, making it difficult to produce on a large scale. In addition, whether it is electroplating, organic coating, or physical vapor deposition, the combination of the coating with the substrate is mainly mechanical or simple physical adhesion, which is prone to cracking or even peeling under the action of thermal stress or mechanical stress. Once the coating is damaged, the corrosion medium will penetrate directly, causing the magnet to fail rapidly. For example, the existing Chinese patent with publication number CN120556109A discloses a method for plating aluminum on the surface of a neodymium-iron-boron magnet, which can obtain a high-density, corrosion-resistant green coating, and can significantly extend the corrosion resistance of the neodymium-iron-boron magnet. However, the maximum bonding force of the coating is only 0.2 MPa. Therefore, there is an urgent need to find a preparation method for a neodymium-iron-boron permanent magnet surface corrosion-resistant coating that can fundamentally solve the problem of the bonding force between the coating and the substrate, which has important industrial value for improving the service life of neodymium-iron-boron magnets in harsh environments and broadening their application scenarios. SUMMARY

[0004] The technical problem to be solved by the present application is to overcome the shortcomings of the prior art and provide a surface corrosion-resistant coating of a neodymium-iron-boron permanent magnet and a preparation method thereof.

[0005] To solve the above-mentioned technical problems, the present invention adopts the following technical solution.

[0006] A method for preparing an anti-corrosion coating on the surface of a neodymium iron boron permanent magnet includes the following steps:

[0007] S1. Pre-treatment of neodymium iron boron permanent magnets;

[0008] S2. Using aluminum-containing materials as raw materials, an aluminum-based alloy coating is deposited on the surface of a pretreated neodymium iron boron permanent magnet by arc spraying to obtain a coating substrate;

[0009] S3. The coating substrate is subjected to vacuum heat treatment to form an alloy diffusion layer; the temperature of the vacuum heat treatment is 400℃~600℃.

[0010] S4. After vacuum heat treatment, a sealing agent is applied to the surface of the coating substrate to obtain an anti-corrosion coating for the surface of the NdFeB permanent magnet.

[0011] In a further improvement to the above preparation method, in step S3, the temperature of the vacuum heat treatment is 500℃~600℃.

[0012] In a further improvement to the above preparation method, in step S3, the temperature of the vacuum heat treatment is 550℃~600℃.

[0013] In a further improvement to the above preparation method, in step S3, the thickness of the alloy diffusion layer is 1 μm to 10 μm.

[0014] In a further improvement of the above preparation method, in step S2, the aluminum-containing material comprises the following components by mass percentage: 70%–99% Al element and 1%–30% X element; wherein the X element is one or more of Cu element, Zn element, Ga element, Ti element, Y element, La element, and Ce element.

[0015] In a further improvement to the above preparation method, in step S2, the thickness of the aluminum-based alloy coating is 50μm to 300μm; the voltage of the arc spraying is 24V to 40V; and the spraying distance of the arc spraying is 100mm to 300mm.

[0016] In a further improvement to the above preparation method, in step S3, the vacuum heat treatment time is 60 min to 240 min, the heating rate during the vacuum heat treatment is 1 °C / min to 12 °C / min, and the vacuum degree of the vacuum heat treatment is ≤5 × 10⁻⁶. -2 Pa.

[0017] In a further improvement to the above preparation method, in step S4, the sealing agent is one or more of epoxy resin, siloxane, silicate and phosphate, and the phosphate is sodium polyphosphate.

[0018] In a further improvement to the above preparation method, in step S1, the pretreatment is: sandblasting, cleaning, and drying the neodymium iron boron permanent magnet; the cleaning is ultrasonic cleaning.

[0019] As a general technical concept, the present invention also provides a surface anti-corrosion coating for NdFeB permanent magnets prepared by the above-mentioned method for preparing the surface anti-corrosion coating of NdFeB permanent magnets.

[0020] Compared with the prior art, the advantages of the present invention are as follows:

[0021] This invention proposes a method for preparing an anti-corrosion coating on the surface of NdFeB permanent magnets. First, an aluminum-based alloy coating is deposited on the surface of the NdFeB permanent magnet by arc spraying. Then, vacuum heat treatment is performed at 400℃~600℃, followed by coating with a sealing agent, resulting in an anti-corrosion coating with high bonding strength and good corrosion resistance. Specifically, under vacuum and high-temperature conditions, aluminum and other metal elements in the aluminum-based alloy coating undergo interdiffusion with the metal elements on the surface of the NdFeB permanent magnet. This interdiffusion forms an in-situ, dense alloy layer with a gradient composition at the interface between the coating and the substrate. The atomic percentage of aluminum (Al) in this alloy layer continuously decreases from the coating side to the substrate side; correspondingly, the atomic percentage of major elements in the substrate, such as iron (Fe) and neodymium (Nd), continuously increases from the substrate side to the coating side, thus forming a gradient transition in composition and effectively alleviating the interfacial stress concentration caused by abrupt changes in composition. This alloy layer not only achieves a metallurgical bond between the coating and the substrate, with a bonding strength far exceeding that of traditional mechanical bonding, but also effectively welds together the microscopic pores and defects on the substrate surface, completely blocking the path of corrosive media penetration along the pores. The anti-corrosion coating on the surface of the NdFeB permanent magnet of this invention, through the construction of a composite protective system of "metallurgical bonding diffusion layer (bottom layer) + Al sacrificial anode protective layer (middle layer) + sealing agent barrier layer (top layer)," can greatly improve the bonding strength and anti-corrosion performance. The metallurgical diffusion layer provides a fundamental physical barrier, aluminum, as a sacrificial anode, provides long-term electrochemical protection, and Al oxidation products Al2O3 and the sealing agent provide a stable physical barrier. The synergistic effect of these three components results in a qualitative leap in the anti-corrosion performance of the NdFeB permanent magnet. The preparation method of this invention has the advantages of low process cost and simple operation, making it suitable for large-scale production. Attached Figure Description

[0022] Figure 1 This is a flowchart illustrating the preparation process of the anti-corrosion coating on the surface of the neodymium iron boron permanent magnet in Example 1 of the present invention. Detailed Implementation

[0023] The present invention will be further described below with reference to the accompanying drawings and specific preferred embodiments, but this does not limit the scope of protection of the present invention. All materials and instruments used in the following embodiments are commercially available.

[0024] Example 1

[0025] A method for preparing an anti-corrosion coating on the surface of a neodymium iron boron permanent magnet according to the present invention, such as... Figure 1 As shown, it includes the following steps:

[0026] S1, The purchased component is (PrNd) 27.5 Ce 4.0 Co 0.3 Cu 0.2 Al 0.6 Ga 0.2 Zr 0.3 Fe bal B 0.89 Using sintered NdFeB permanent magnets as the substrate, the substrate was sandblasted with 120-mesh α-alumina to control the surface roughness Ra≈4.5μm; then, it was ultrasonically cleaned in anhydrous ethanol and acetone for 5 minutes each, and then dried to obtain the pretreated substrate.

[0027] S2. Using aluminum-based powder-core wire as raw material, an aluminum-based alloy coating is deposited on the pretreated substrate surface through an arc spraying process, resulting in an aluminum-based alloy coating thickness of 100μm. The specific parameters for arc spraying are: voltage 32V, current 180A, and spraying distance 150mm, resulting in a substrate with a deposited aluminum-based alloy coating. The aluminum-based powder-core wire, by mass percentage, contains the following components: Al 90%, Zn 5%, Cu 3%, and Ti 2%.

[0028] S3. Place the above-deposited aluminum-based alloy coating substrate into a vacuum furnace for vacuum heat treatment, i.e., evacuate to 5×10⁻⁶. -2 Pa, heating to 580℃ at 5℃ / min and holding for 120min, allows the elements in the aluminum-based alloy coating to interdiffused with the elements in the matrix, forming a dense alloy diffusion layer at the interface with a thickness of 5-8μm. Then, air-cooled to room temperature under an argon atmosphere; sodium polyphosphate sealing agent is applied to obtain an anti-corrosion coating on the surface of the neodymium iron boron permanent magnet.

[0029] Control group: based on component (PrNd) 27.5 Ce 4.0 Co 0.3 Cu 0.2 Al 0.6 Ga 0.2 Zr 0.3 Fe bal B0.89 Sintered NdFeB permanent magnets were used as blank control samples.

[0030] Example 2

[0031] The preparation method of the anti-corrosion coating on the surface of the neodymium iron boron permanent magnet of the present invention is basically the same as that of the preparation method of the anti-corrosion coating on the surface of the neodymium iron boron permanent magnet in Example 1, except that the temperature of the vacuum heat treatment in step S3 is 440°C.

[0032] The anti-corrosion coating on the surface of the neodymium iron boron permanent magnet prepared in this embodiment has an alloy diffusion layer thickness of 2-4 μm.

[0033] Comparative Example 1

[0034] A method for preparing an anti-corrosion coating on the surface of a neodymium iron boron permanent magnet includes the following steps:

[0035] S1, The purchased component is (PrNd) 27.5 Ce 4.0 Co 0.3 Cu 0.2 Al 0.6 Ga 0.2 Zr 0.3 Fe bal B 0.89 Using sintered NdFeB permanent magnets as the substrate, the substrate was sandblasted with 120-mesh α-alumina to control the surface roughness Ra≈4.5μm; then, it was ultrasonically cleaned in anhydrous ethanol and acetone for 5 minutes each, and then dried to obtain the pretreated substrate.

[0036] S2. Using aluminum-based powder-core wire as raw material, an aluminum-based alloy coating is deposited on the pretreated substrate surface through an arc spraying process, resulting in an aluminum-based alloy coating thickness of 100μm. The specific parameters for arc spraying are: voltage 32V, current 180A, and spraying distance 150mm, resulting in a substrate with a deposited aluminum-based alloy coating. The aluminum-based powder-core wire, by mass percentage, contains the following components: Al 90%, Zn 5%, Cu 3%, and Ti 2%.

[0037] S3. Apply sodium polyphosphate sealing agent to the above-mentioned deposited aluminum-based alloy coating substrate to obtain a NdFeB permanent magnet surface anti-corrosion coating.

[0038] Neutral salt spray tests and adhesion tests were conducted on the anti-corrosion coatings on the surfaces of NdFeB permanent magnets in Examples 1 and 2, the NdFeB permanent magnets in the control group, and the NdFeB permanent magnets in Comparative Example 1. The results are shown in Table 1. The neutral salt spray test conditions were: continuous spraying, nozzle pressure of 78.5–137.3 kPa, 5% NaCl solution used as the salt spray solution, constant chamber temperature of 35 ± 2℃, humidity greater than 95%, pH range of 6.6–7.1, and spray density of 100 cm³ / s. 2 The horizontal collection area mist rate was 1–2 mL / h, and the time when obvious red rust appeared on the sample surface was recorded. The bonding strength test conditions were: universal testing machine, tensile rate of 1 mm / min.

[0039] Table 1. Relevant performance data of the anti-corrosion coating on the surface of NdFeB permanent magnets in Examples 1-2 and Comparative Example 1.

[0040]

[0041] As shown in Table 1, compared to the control group (unprotected substrate), the anti-corrosion coatings on the NdFeB permanent magnet surfaces prepared in Examples 1 and 2 exhibit significantly improved bonding strength and salt spray corrosion resistance. Compared to Comparative Example 1, the bonding strength of the anti-corrosion coating on the NdFeB permanent magnet surface prepared in Example 1 increased from 9.37 MPa to 25.8 MPa, and the salt spray resistance time increased from 360 hours to 600 hours, demonstrating a significant performance improvement. Compared to Comparative Example 1, the bonding strength and salt spray corrosion resistance of the anti-corrosion coating on the NdFeB permanent magnet surface prepared in Example 2 also showed a very significant improvement, while compared to the anti-corrosion coating on the NdFeB permanent magnet surface prepared in Example 1, its bonding strength and salt spray corrosion resistance decreased slightly. Obviously, in the preparation method of this invention, if the vacuum heat treatment temperature is below 400℃, the driving force for atomic diffusion is insufficient, resulting in the inability to form an effective alloy diffusion layer; if the vacuum heat treatment temperature is above 600℃, the formed alloy layer will melt and generate a liquid phase, which will flow or even peel off from the substrate during the heat treatment process, leading to sample rejection. Therefore, the anti-corrosion coating on the surface of the NdFeB permanent magnet prepared by this invention has excellent bonding strength and anti-corrosion performance. In particular, its bonding strength and anti-corrosion performance are optimal when the vacuum heat treatment temperature is between 550℃ and 600℃.

[0042] In summary, the method for preparing the anti-corrosion coating on the surface of NdFeB permanent magnets of the present invention, through the synergistic effect of "arc spraying of aluminum-based alloy coating + vacuum heat treatment at 400℃~600℃", not only densifies the coating but also generates a dense alloy diffusion layer in situ at the interface with the substrate, realizing the metallurgical bond between the coating and the substrate. This fundamentally solves the problems of weak adhesion and easy peeling of traditional protective coatings, and provides a multi-layered long-term anti-corrosion mechanism that combines active and passive methods, thereby greatly improving the bonding strength and long-term anti-corrosion performance of the coating. This has important industrial value for improving the service life of NdFeB magnets in harsh environments and expanding their application scenarios.

[0043] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make many possible variations and modifications to the technical solutions of the present invention using the methods and techniques disclosed above, or modify them into equivalent embodiments with equivalent changes, without departing from the spirit and technical essence of the present invention. Therefore, any simple modifications, equivalent substitutions, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solutions of the present invention shall still fall within the protection scope of the technical solutions of the present invention.

Claims

1. A method for preparing a corrosion resistant coating on the surface of a neodymium-iron-boron permanent magnet, characterized in that The method comprises the following steps: S1, pretreating the Nd-Fe-B permanent magnet; the pretreatment is: sandblasting, cleaning and drying the Nd-Fe-B permanent magnet; The cleaning is ultrasonic cleaning; S2, depositing an aluminum-based alloy coating on the surface of the pretreated Nd-Fe-B permanent magnet by arc spraying with an aluminum-containing material as raw material to obtain a coating substrate; the aluminum-containing material contains the following components by mass percentage: Al 90%, Zn 5%, Cu 3%, Ti 2%; the voltage of the arc spraying is 32 V, the spraying distance of the arc spraying is 150 mm; the thickness of the aluminum-based alloy coating is 100 μm; S3, vacuum heat treating the coating base to form an alloy diffusion layer; the vacuum heat treatment has a vacuum degree of 5*10 - 2 Pa; the vacuum heat treatment has a temperature of 580℃, a time of 120min, and a heating rate of 5℃ / min; the alloy diffusion layer has a thickness of 5-8μm; the atomic percentage content of aluminum in the alloy diffusion layer presents a continuous downward trend from the coating side to the base side, while the atomic percentage content of the main elements iron and neodymium in the base presents a continuous upward trend from the base side to the coating side. S4, coating a sealing agent on the surface of the coating substrate after vacuum heat treatment to obtain a corrosion-resistant coating on the surface of the Nd-Fe-B permanent magnet; the sealing agent is a phosphate; the phosphate is sodium polyphosphate.

2. A corrosion-resistant coating on the surface of a Nd-Fe-B permanent magnet prepared by the method according to claim 1.

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