Corrosion-resistant coating for improving coercivity of NdFeB permanent magnet, preparation method thereof, and permanent magnet

Abstract

The present invention relates to a corrosion-resistant coating for improving the coercivity of Nd-Fe-B permanent magnets, a preparation method thereof, and an Nd-Fe-B permanent magnet, belonging to the technical field of surface anti-corrosion treatment, and solving the problems of poor protective ability of the surface coating of sintered Nd-Fe-B permanent magnets in the prior art and weakening the magnetic properties of the magnets. Along the direction of entering the interior of the Nd-Fe-B matrix, the corrosion-resistant coating sequentially includes a surface co-permeation layer and an interface diffusion layer; the surface co-permeation layer is a metal layer composed of zinc elements and dysprosium elements; the interface diffusion layer is a metal layer composed of zinc elements, dysprosium elements and iron elements. The technical solution provided by the present invention can improve the coercivity of Nd-Fe-B magnets, reduce the magnetic loss of Nd-Fe-B magnets, and enhance the corrosion resistance of the corrosion-resistant coating.

Description

Technical Field

[0001] The present invention belongs to the technical field of surface anti-corrosion treatment, and particularly relates to a corrosion-resistant coating for improving the coercivity of a neodymium-iron-boron permanent magnet, a preparation method thereof, and a permanent magnet. Background Art

[0002] As the third-generation rare-earth permanent magnet material, sintered neodymium-iron-boron (NdFeB) is widely used in the fields of automotive industry, electrical engineering, aerospace, household appliances, communication, etc. due to its excellent magnetic properties.

[0003] The neodymium-iron-boron material contains rare-earth neodymium (Nd) with relatively high chemical activity, and its internal structure presents a multi-phase structure, including the main phase Nd2Fe 14 B, Nd-rich phase Nd4Fe, B-rich phase Nd 1+x Fe4B4, etc. Due to the large difference in electrochemical properties between phases, internal corrosion is likely to occur. At the same time, the neodymium-iron-boron permanent magnet is prepared by powder metallurgy, and there are many pores in the material, creating conditions for the erosion of media such as oxygen and water. Due to the above two reasons, improving the surface protection technology of the neodymium-iron-boron permanent magnet and enhancing its corrosion resistance have become the key to breaking through the engineering application of rare-earth permanent magnet materials. In addition, the development of neodymium-iron-boron permanent magnet materials is closely related to the level of its surface protection technology. Due to the harsh application environment, corrosion resistance is particularly important.

[0004] Common methods for preparing surface corrosion-resistant coatings mainly include chemical conversion coatings, electroplating, electroless plating, electrophoresis, physical vapor deposition, etc. To improve the intrinsic coercivity of sintered neodymium-iron-boron permanent magnets, common methods include adding dysprosium (Dy) element to the sintered neodymium-iron-boron permanent magnet for melting and ingot casting. In addition, there is also a double-alloy process, in which metal Dy or Dy2O3 is used as a secondary alloy to prepare powder, mixed with the main alloy powder, and then formed and sintered. A method of high-temperature infiltration of dysprosium using a dysprosium fluoride nano-dispersion is adopted.

[0005] In the prior art, in order to improve the coercivity of NdFeB permanent magnets and prepare a corrosion-resistant coating, the magnetron sputtering process in physical vapor deposition is adopted. First, a metal layer is deposited on the surface of the NdFeB permanent magnet, and then a heavy rare earth or heavy rare earth alloy layer is continuously deposited on the surface of the metal layer. Through heat treatment methods such as high-temperature treatment and tempering, the metal and the heavy rare earth or heavy rare earth alloy penetrate into the interior of the NdFeB matrix to obtain a corrosion-resistant coating that improves the coercivity of the NdFeB permanent magnet. Before depositing the coating on the permanent magnet surface, the surface of the permanent magnet needs to be cleaned with dilute nitric acid first. The pickling process, on the one hand, dissolves a small amount of magnetic phase, weakening the performance of the permanent magnet, and on the other hand, also increases the risk of material corrosion. At the same time, due to the shielding effect of the surface coating, the performance of the permanent magnet will be further weakened. In addition, multiple physical vapor depositions are required in the process, and diffusion is carried out at a high temperature not lower than 700 °C. The process is complex and costly. Summary of the Invention

[0006] In view of the above analysis, the implementation of the present invention aims to provide a corrosion-resistant coating for improving the coercivity of NdFeB permanent magnets, its preparation method, and NdFeB permanent magnets, so as to solve the problems of poor surface coating protection ability and weakened magnet performance of sintered NdFeB permanent magnets in the prior art.

[0007] In the first aspect, the present invention provides a corrosion-resistant coating for improving the coercivity of NdFeB permanent magnets. Along the direction of entering the interior of the NdFeB matrix, the corrosion-resistant coating sequentially includes a surface co-permeation layer and an interface diffusion layer;

[0008] The surface co-permeation layer is a metal layer composed of zinc element and dysprosium element; the interface diffusion layer is a metal layer composed of zinc element, dysprosium element and iron element.

[0009] Further, the corrosion-resistant coating is obtained by co-permeation of zinc powder and dysprosium oxide.

[0010] Further, the thickness of the surface co-permeation layer is 8 μm - 10 μm, and the thickness of the interface diffusion layer is 3 μm - 5 μm.

[0011] Further, in the surface co-permeation layer, the content of zinc is 99.7 - 99.9 wt%, and the content of dysprosium is 0.1 - 0.3 wt%; in the interface diffusion layer, the content of zinc is 80.0 - 83.0 wt%, the content of dysprosium is 0.08 - 0.10 wt%, and the rest is iron.

[0012] Further, in terms of mass parts, the co-permeation materials for preparing the corrosion-resistant coating include:

[0013] 20.0 - 50.0 parts of zinc powder, 1.0 - 5.0 parts of dysprosium oxide, 2.0 - 8.0 parts of activator, and 30.0 - 50.0 parts of inert medium.

[0014] Second aspect, an embodiment of the present invention provides a method for preparing a corrosion-resistant coating for improving the coercivity of a neodymium-iron-boron permanent magnet, comprising the following steps:

[0015] Step 1, clean the neodymium-iron-boron permanent magnet and set aside after drying;

[0016] Step 2, sequentially load the co-permeation material and the permanent magnet into the vacuum furnace inside the heat treatment furnace, and close the hatch of the vacuum furnace; the co-permeation material includes 20.0 - 50.0 parts of zinc powder, 1.0 - 5.0 parts of dysprosium oxide, 2.0 - 8.0 parts of activator, and 30.0 - 50.0 parts of inert medium.

[0017] Step 3, start rotating the vacuum furnace to mix the co-permeation material and the permanent magnet evenly;

[0018] Step 4, evacuate the vacuum furnace, fill it with inert gas, and keep the vacuum degree in the furnace at 2.0×10-2 - 8.0×10-2 Pa;

[0019] Step 5, start heating, after heating to the target temperature, keep it warm and maintain the vacuum degree at 2.0×10-2 - 8.0×10-2 Pa;

[0020] Step 6, stop heating, keep the pressure and cool to room temperature, and take out the permanent magnet.

[0021] Further, in the step 1, the neodymium-iron-boron permanent magnet is a formed neodymium-iron-boron permanent magnet product.

[0022] Further, in the step 5, the target temperature is 350 - 390 °C, and the heat preservation time is 3 - 4 h.

[0023] Further, in the step 1, place the neodymium-iron-boron permanent magnet in a degreasing solution for cleaning, and the components of the degreasing solution include one or several of NaOH, Na2CO3, Na3PO4, OP-10, and sodium dodecylbenzenesulfonate.

[0024] Further, the particle size of the zinc powder is 15 μm - 30 μm; the particle size of the dysprosium oxide is 15 μm - 30 μm; the activator is ammonium chloride, and the particle size of the ammonium chloride is 15 μm - 30 μm; the inert medium is quartz sand, and the particle size of the quartz sand is 2 cm.

[0025] Third aspect, an embodiment of the present invention provides a neodymium-iron-boron permanent magnet, the surface of the neodymium-iron-boron permanent magnet contains a corrosion-resistant coating, and the corrosion-resistant coating is the corrosion-resistant coating on the surface of the neodymium-iron-boron permanent magnet described in the first aspect or the corrosion-resistant coating prepared by the preparation method described in the second aspect.

[0026] Compared with the prior art, the present invention can at least achieve one of the following beneficial effects:

[0027] (1) The process of the present invention combines dysprosium element and zinc element to prepare a thermal diffusion co-deposited coating through a single technological process, obtaining an interfacial diffusion layer composed of zinc element, dysprosium element and metallic iron. Compared with the interfacial diffusion layer (Fe5Zn 21 ) of the prior art, due to the relatively high activity of the dysprosium element and the fact that it is also a rare earth element like the neodymium element in neodymium iron boron, the bonding ability between the interfacial diffusion layer and the substrate in the present invention is increased, thereby reducing the interfacial stress between the interfacial diffusion layer and the substrate, making the interfacial diffusion layer more stable and not prone to phenomena such as cracking and peeling. At the same time, the activity of the dysprosium element also increases the diffusion rate of metal atoms, reducing the technological time.

[0028] (2) The pickling process is eliminated, reducing the risk of weakening the coercivity and corrosion resistance of the permanent magnet.

[0029] (3) There is a metallurgical bond between the co-deposited layer and the substrate of the present invention, improving the poor bubble formation and protection ability of the magnet surface coating. At the same time, the doped Dy element in the coating diffuses into the magnet structure, forming intermetallic compounds at the interface with the substrate, improving the coercivity of the magnet and slowing down the magnetic shielding effect of the coating.

[0030] (4) By controlling the mass fraction of zinc powder and the co-deposition temperature, the thickness of the surface co-deposited layer is adjusted to ensure that the surface co-deposited layer has sufficient corrosion resistance; by adjusting the mass fraction of dysprosium oxide and the co-deposition temperature, the thickness of the interfacial diffusion layer is adjusted to reduce the interfacial stress between the interfacial diffusion layer and the substrate.

[0031] (5) In the embodiment of the present invention, by controlling the co-deposition temperature at 350 °C to 390 °C, the diffusion rate can be accelerated while reducing the magnetic loss of the substrate during co-deposition.

[0032] (6) The experimental results show that the corrosion-resistant coating prepared by the present invention does not rust in a neutral salt spray environment for no less than 500 h and can be up to 1200 h. The surface remanence loss of the neodymium iron boron magnet does not exceed 1.0%, and the coercivity is increased by 5 - 10%, solving the problem of the influence of surface treatment process and surface coating shielding on the magnetic properties of permanent magnet materials.

[0033] (7) The processing method provided by the present invention can perform dysprosium infiltration on the formed neodymium iron boron permanent magnet product without adding a dysprosium infiltration step during the production of the neodymium iron boron permanent magnet product, or processing the dysprosium element into a nanomaterial first, thus simplifying the process of producing neodymium iron boron permanent magnet products and improving the production efficiency.

[0034] In the present invention, the above technical solutions can also be combined with each other to achieve more preferred combined solutions. Other features and advantages of the present invention will be described in the following specification. Moreover, some advantages can be made obvious from the specification or understood by implementing the present invention. The objectives and other advantages of the present invention can be realized and obtained from the content specifically pointed out in the specification and the drawings. Description of the Drawings

[0035] The drawings are only for the purpose of showing specific embodiments and are not considered as a limitation to the present invention. Throughout the drawings, the same reference signs denote the same components.

[0036] Figure 1 It is a schematic structural diagram of a permanent magnet with a zinc-diffused dysprosium corrosion-resistant coating provided by an embodiment of the present invention;

[0037] Figure 2 It is a microstructural morphology diagram of a permanent magnet with a zinc-diffused dysprosium corrosion-resistant coating provided by an embodiment of the present invention;

[0038] Figure 3 It is an Auger electron spectroscopy diagram of a permanent magnet with a zinc-diffused dysprosium corrosion-resistant coating provided by an embodiment of the present invention;

[0039] Figure 4 It is a cross-sectional morphology and EPMA micro-area composition diagram of a permanent magnet with a zinc-diffused dysprosium corrosion-resistant coating provided by an embodiment of the present invention;

[0040] Figure 5 It is a physical diagram of the corrosion morphology of a zinc-diffused neodymium-iron-boron permanent magnet in the prior art after being tested by a cyclic salt spray test to simulate the marine environmental atmosphere;

[0041] Figure 6 It is a physical diagram of the corrosion morphology of a neodymium-iron-boron permanent magnet with a zinc-diffused dysprosium corrosion-resistant coating provided by an embodiment of the present invention after being tested by a cyclic salt spray test to simulate the marine environmental atmosphere.

[0042] Reference Signs:

[0043] 101 Neodymium-iron-boron substrate, 102 is the interfacial diffusion layer, and 103 is the co-permeation layer. Detailed Embodiments

[0044] The prior art corrosion-resistant coatings and their preparation processes have the following technical problems:

[0045] 1. In the prior art, the zinc-iron diffusion boundary layer obtained by zinc diffusion plating is fragile and easy to fall off, resulting in an increase in the boundary stress between the existing diffusion boundary layer and the substrate.

[0046] 2. The prior art corrosion-resistant coatings weaken the magnetism of the substrate.

[0047] 3. During the process of fabricating the existing corrosion-resistant coatings, pickling and high temperatures can cause magnetic attenuation of the substrate.

[0048] To at least solve one of the above technical problems, the present invention provides a corrosion-resistant coating for improving the coercivity of NdFeB permanent magnets. Along the direction of entering the interior of the NdFeB substrate, the corrosion-resistant coating sequentially includes a surface co-permeation layer and an interface diffusion layer;

[0049] The surface co-permeation layer is a metal layer composed of zinc and dysprosium elements; the interface diffusion layer is a metal layer composed of zinc, dysprosium, and iron elements.

[0050] Among them, the corrosion-resistant coating is obtained by co-permeating zinc powder and dysprosium oxide. Specifically, due to the relatively high activity of dysprosium and its similar properties to neodymium, another rare earth element, during the thermal diffusion process, dysprosium oxide (dysprosium oxide) first reacts with the NdFeB substrate and diffuses at the interface to form an interface diffusion layer. Due to the high activity of dysprosium, on the one hand, this interface diffusion layer can effectively improve the thermal diffusion rate of zinc elements into the NdFeB substrate and the bonding force between the surface co-permeation layer and the substrate, and on the other hand, it can improve the coercivity of the permanent magnetic material to weaken the influence of the corrosion-resistant coating on the coercivity of the substrate.

[0051] Specifically, in terms of parts by mass, the raw materials for the co-permeation include: 20.0 - 50.0 parts of zinc powder, 1.0 - 5.0 parts of dysprosium oxide, 2.0 - 8.0 parts of activator, and 30.0 - 50.0 parts of inert medium.

[0052] Among them, zinc powder is the zinc provider, and dysprosium oxide is the dysprosium provider. It should be noted that dysprosium is relatively active and easy to oxidize, so generally, the commercially available product is dysprosium oxide. The present invention uses common dysprosium oxide to save costs. The activator is ammonium chloride or aluminum chloride. In the examples of the present invention, ammonium chloride can decompose to produce trace amounts of hydrogen chloride gas within the co-permeation temperature range, and this gas can keep the surface of the permanent magnet in an activated state, which is helpful for the diffusion of metal atoms at the interface; the inert medium is one or several of quartz sand, brown fused alumina sand, and ceramic sand. The inert medium plays a buffering role and can prevent the co-permeation materials from being knocked and chipped during the tumbling process in the co-permeation furnace. In the examples of the present invention, quartz sand is selected because it has a relatively high melting point and will not have an adverse impact on the substrate and the co-permeation layer during the thermal diffusion process. At the same time, quartz sand has a relatively high hardness, is wear-resistant and not easy to break, and can play a good buffering role.

[0053] In the examples of the present invention, the mass fraction of zinc powder is the main factor affecting the thickness of the surface co-permeation layer, and the mass fraction of dysprosium oxide is the main factor affecting the thickness of the interface diffusion layer. In view of the technical requirements of the corrosion-resistant coating, the thicker the surface co-permeation layer, the better, to increase the corrosion resistance. And the thinner the interface diffusion layer, the better, to reduce the boundary stress. Therefore, the mass fraction of zinc powder should be much larger than that of dysprosium oxide.

[0054] In the embodiment of the present invention, the thickness of the surface co-permeation layer is 8 μm - 10 μm, and the thickness of the interface diffusion layer is 3 μm - 5 μm. Specifically, in the surface co-permeation layer, the content of zinc is 99.7 - 99.9 wt%, and the content of dysprosium is 0.1 - 0.3 wt%; in the interface diffusion layer, the content of zinc is 80.0 - 83.0 wt%, the content of dysprosium is 0.08 - 0.10 wt%, and the rest is iron.

[0055] The present invention also provides a method for preparing a corrosion-resistant coating for improving the coercivity of a neodymium-iron-boron permanent magnet, comprising the following steps:

[0056] Step 1: Clean the neodymium-iron-boron permanent magnet and set it aside after drying.

[0057] In the embodiment of the present invention, the specific cleaning process is to place the neodymium-iron-boron permanent magnet in a degreasing solution of 10 - 13 wt%, and the components of the degreasing solution include one or more of NaOH, Na2CO3, Na3PO4, OP-10, and sodium dodecylbenzenesulfonate.

[0058] Step 2: Load the co-permeation raw materials and the permanent magnet into the vacuum furnace in sequence, and close the furnace door of the vacuum furnace;

[0059] In the embodiment of the present invention, after closing the furnace door of the vacuum furnace, two symmetrically distributed left and right heating tanks on the outer layer also need to be closed. The co-permeation raw materials include 20.0 - 50.0 parts of zinc powder (for example, 25.0 parts, 30.0 parts, 35.0 parts, 40.0 parts, and 45.0 parts), 1.0 - 5.0 parts of dysprosium oxide (2.0 parts, 3.0 parts, 4.0 parts), 2.0 - 8.0 parts of activator, and 30.0 - 50.0 parts of inert medium.

[0060] It should be noted that the permanent magnets in the embodiments of the present invention are usually formed products, such as rotors or stators in motors, magnets in audio speakers, magnetic heads in hard disks, and magnet components in nuclear magnetic resonance equipment. Since neodymium-iron-boron is relatively brittle, collisions between formed products are likely to cause product fragmentation, so the feeding must be carried out in the order of step 2.

[0061] Step 3: The heat treatment furnace starts to rotate, and the co-permeation raw materials and the permanent magnet are mixed evenly;

[0062] In the embodiment of the present invention, the rotation speed of the heat treatment furnace is 10 - 20 r / min (for example, 12 r / min, 15 r / min, 18 r / min). Within this rotation speed range, a uniform and consistent coating can be obtained quickly, and severe collisions between the substrates will not occur.

[0063] Step 4: Evacuate the vacuum furnace, fill it with an inert gas, and keep the vacuum degree in the furnace at 2.0×10 -2 ~8.0×10-2 Pa;

[0064] In the embodiments of the present invention, the activator removes the oxide layer on the surface of dysprosium oxide, thereby increasing the vacuum degree to prevent the substrate from being oxidized and the dysprosium element from being oxidized again.

[0065] Step 5: Start heating, heat to the target temperature and then hold the temperature, maintaining the vacuum degree at 2.0×10 -2 ~8.0×10 - 2 Pa.

[0066] In the embodiments of the present invention, the target temperature is 350~390 °C (for example, 360 °C, 370 °C, 380 °C), and the holding time is 3~4 h. Among them, the Curie temperature of neodymium iron boron is 390 °C. Therefore, co-permeation at the target temperature set in the present invention can effectively reduce the magnetic loss of neodymium iron boron and improve the metal penetration rate at the same time.

[0067] Step 6: Stop heating, keep the pressure and cool to room temperature, and take out the permanent magnet.

[0068] In the embodiments of the present invention, after taking out the permanent magnet, ultrasonically clean the permanent magnet to remove the surface grit.

[0069] In the embodiments of the present invention, the particle size of zinc powder is 10 μm~25 μm (for example, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, 19 μm, 20 μm, 21 μm, 22 μm, 23 μm, 24 μm); the particle size of dysprosium oxide is 10~25 μm (for example, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, 19 μm, 20 μm, 21 μm, 22 μm, 23 μm, 24 μm); the particle size of ammonium chloride is 10~25 μm (for example, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, 19 μm, 20 μm, 21 μm, 22 μm, 23 μm, 24 μm); the quartz sand is 1 - 5 cm (for example, 2 cm, 3 cm, 4 cm).

[0070] In the present invention, by controlling the particle sizes of zinc powder and dysprosium oxide, the metal diffusion rate can be effectively increased. Controlling the particle size of ammonium chloride facilitates the decomposition of ammonium chloride into ammonia and hydrogen chloride gases more easily, enabling the surface of the permanent magnet to quickly enter the activation state, which helps the diffusion of metal atoms at the interface. Controlling the particle size of quartz sand can increase its buffering performance, make the substrate heat evenly, and prevent the quartz sand from causing wear or breaking the substrate surface during the stirring process.

[0071] An embodiment of the present invention provides a neodymium-iron-boron permanent magnet, which is prepared by using the above-mentioned corrosion-resistant coating preparation method for improving the coercivity of the neodymium-iron-boron permanent magnet. By treating the permanent magnet with the method of the present invention, a powder zinc infiltration coating with improved coercivity of the neodymium-iron-boron magnet and excellent corrosion resistance can be obtained.

[0072] The following describes the specific implementation manners of the present invention in conjunction with embodiments. The following embodiments are only used to more clearly illustrate the technical solutions of the present invention, so that those skilled in the art can well understand and utilize the present invention. However, it should be understood that these descriptions are only exemplary and do not intend to limit the scope of the present disclosure. In addition, in the following description, the descriptions of well-known structures and technologies are omitted to avoid unnecessarily confusing the concepts of the present disclosure.

[0073] Example 1

[0074] This example is used to prepare a corrosion-resistant coating composed of the following components in parts by mass. The co-permeation materials include:

[0075] (1) Zinc powder (10 - 25 μm), 20.0 parts; (2) Dysprosium oxide powder (10 - 25 μm), 1.0 part; (3) Ammonium chloride (10 μm), 2.0 parts; (4) Quartz sand (SiO2, 1 cm), 30.0 parts, (5) Neodymium-iron-boron permanent magnet, 47.0 parts.

[0076] It includes the following steps:

[0077] 1) Place 47.0 parts of the neodymium-iron-boron permanent magnet in a degreasing solution composed of one or several of 5 g / L NaOH, 10 g / L Na2CO3, 2 g / L Na3PO4, 0.3 ml / L OP-10, and 0.1 ml / L sodium dodecylbenzenesulfonate, ultrasonically clean at room temperature for 5 min, then rinse with deionized water, and finally place in an oven at 80 °C for drying for later use;

[0078] 2) Load 20.0 parts of zinc powder, 1.0 part of dysprosium oxide powder, 2.0 parts of ammonium chloride, and 30.0 parts of quartz sand into the vacuum furnace body, and then load 47.0 parts of the neodymium-iron-boron magnet;

[0079] 3) Start the rotating device, the vacuum furnace rotates at a low speed, and the rotation speed of the furnace body is 10 r / min to ensure uniform mixing of the powder and the permanent magnet;

[0080] 4) Evacuate the vacuum furnace, and then fill it with N2 gas. The vacuum degree in the vacuum furnace body is maintained at 2.0×10 -2 ~8.0×10 -2 Pa;

[0081] 5) Turn on the heating device, wait for the temperature in the furnace to rise to 385 °C, and keep it warm for 3.0 h;

[0082] 6) Turn off the heating switch. Wait for the furnace temperature to drop to room temperature, stop the rotating device, and open the exhaust valve. After the pressure inside the furnace drops to atmospheric pressure, open the furnace door, separate the furnace charge, and take out the permanent magnet.

[0083] 7) Place the permanent magnet after thermal diffusion co-permeation treatment in deionized water and ultrasonically clean it for 1 minute to remove surface grit. Then place the permanent magnet in an oven at 80 °C for drying to prevent the surface of the permanent magnet from being oxidized again, obtaining a neodymium-iron-boron permanent magnet with a zinc-dysprosium corrosion-resistant coating. The structural schematic diagram is shown in Figure 1 .

[0084] Repeat Example 1 multiple times. Extract 3 specimens for neutral salt spray test to evaluate the protective performance of the corrosion-resistant coating. The concentration of the NaCl solution in the neutral salt spray experiment is 5 wt%, the pH is 6.5 - 7.2, the test temperature is 35 ± 2 °C, and the sedimentation rate is 1 - 2 ml / 80 cm 2 ·h. Observe the appearance time and area of white rust and red rust on the surface of the magnet. The later the appearance time of rust and the smaller the area, the stronger the protective ability. The thickness of the surface co-permeation layer is 8.2 μm, and the thickness of the interface diffusion layer is 3.1 μm. In the surface co-permeation layer, the zinc content is 99.7 wt%, and the dysprosium content is 0.1 wt%. In the interface diffusion layer, the zinc content is 80.0 wt%, the dysprosium content is 0.08 wt%, and the rest is iron. The neutral salt spray test lasts for more than 500 h, and no red rust, bubbling or peeling appears on the surface of the magnet. The magnetic attenuation ratio is 5.8%, and the coercivity increases by 5.3%.

[0085] Example 2

[0086] This example is used to prepare a corrosion-resistant coating composed of the following components in parts by mass: (1) zinc powder (10 - 25 μm), 30.0 parts; (2) dysprosium oxide powder (10 - 25 μm), 2.0 parts; (3) ammonium chloride (13 μm), 4.0 parts; (4) quartz sand (SiO2, 2 cm), 35.0 parts, (5) neodymium-iron-boron permanent magnet, 29.0 parts. It includes the following steps:

[0087] 1) Place 29.0 parts of neodymium-iron-boron permanent magnet in a degreasing solution composed of one or several of 5 g / L NaOH, 10 g / L Na2CO3, 2 g / L Na3PO4, 0.3 ml / L OP-10 and 0.1 ml / L sodium dodecylbenzenesulfonate, ultrasonically clean it at room temperature for 5 minutes, then rinse it with deionized water, and finally place it in an oven at 80 °C for drying for use;

[0088] 2) Load 30.0 parts of zinc powder, 2.0 parts of dysprosium oxide powder, 4.0 parts of ammonium chloride and 35.0 parts of quartz sand into the vacuum furnace body, and then load 29.0 parts of neodymium-iron-boron magnet;

[0089] 3) Start the rotating device, and the vacuum furnace rotates at a low speed. The rotation speed of the furnace body is 12 r / min to ensure uniform mixing of the powder and the permanent magnet;

[0090] 4) Evacuate the vacuum furnace, and then fill it with N2 gas. The vacuum degree in the vacuum furnace body is maintained at 2.0×10 -2 ~8.0×10 -2 Pa;

[0091] 5) Turn on the heating device. Wait until the temperature in the furnace rises to 385 °C and keep it warm for 3.0 h;

[0092] 6) Turn off the heating switch. Wait until the furnace temperature drops to room temperature, stop the rotating device, and open the exhaust valve; wait until the air pressure in the furnace drops to atmospheric pressure, then open the furnace door, separate the furnace charge, and take out the permanent magnet;

[0093] 7) Place the permanent magnet after the thermal diffusion co-permeation treatment in deionized water and ultrasonically clean it for 1 min to remove the surface grit. Then place the permanent magnet in an oven at 80 °C to dry, and obtain a neodymium-iron-boron permanent magnet coated with a zinc-dysprosium corrosion-resistant coating. The structural schematic diagram is shown in Figure 1 .

[0094] Repeat Example 2 multiple times, and extract 3 specimens for neutral salt spray test to evaluate the protective performance of the corrosion-resistant coating. The concentration of the NaCl solution in the neutral salt spray experiment is 5 wt%, the pH is 6.5 - 7.2, the test temperature is 35 ± 2 °C, and the sedimentation rate is 1 - 2 ml / 80 cm 2 ·h; Observe the appearance time and area of white rust and red rust on the surface of the magnet. The later the appearance time of rust and the smaller the area, the stronger the protection ability. The thickness of the surface co-permeation layer is 8.8 μm, and the thickness of the interface diffusion layer is 3.4 μm. In the surface co-permeation layer, the zinc content is 99.7 wt%, and the dysprosium content is 0.1 wt%; in the interface diffusion layer, the zinc content is 80.2 wt%, and the dysprosium content is 0.09 wt%, and the rest is iron. The neutral salt spray test lasts for more than 500 h, and no red rust, bubbling or peeling appears on the surface of the magnet; the magnetic attenuation ratio is 5.8%, and the coercivity increases by 5.3%.

[0095] Example 3

[0096] This example is used to prepare a corrosion-resistant coating composed of the following components in parts by mass: (1) zinc powder (10 - 25 μm), 40.0 parts; (2) dysprosium oxide powder (10 - 25 μm), 3.0 parts; (3) ammonium chloride (16 μm), 6.0 parts; (4) quartz sand (SiO2, 3 cm), 40.0 parts, (5) neodymium-iron-boron permanent magnet, 14.0 parts. The steps include:

[0097] 1) Place 14.0 parts of neodymium iron boron permanent magnet in a degreasing solution composed of one or more of 5 g / L NaOH, 10 g / L Na2CO3, 2 g / L Na3PO4, 0.3 ml / L OP-10 and 0.1 ml / L sodium dodecylbenzenesulfonate, ultrasonically clean at room temperature for 5 min, then rinse with deionized water, and finally place in an oven at 80 °C for drying for later use;

[0098] 2) Load 40.0 parts of zinc powder, 3.0 parts of dysprosium oxide powder, 6.0 parts of ammonium chloride and 40.0 parts of quartz sand into the vacuum furnace body, and then load 14.0 parts of neodymium iron boron magnet;

[0099] 3) Start the rotating device, rotate the vacuum furnace at a low speed, and the rotation speed of the furnace body is 15 r / min to ensure uniform mixing of the powder and the permanent magnet;

[0100] 4) Evacuate the vacuum furnace, and then fill it with N2 gas. The vacuum degree in the vacuum furnace body is maintained at 2.0×10 -2 ~8.0×10 -2 Pa;

[0101] 5) Turn on the heating device, wait until the temperature in the furnace rises to 385 °C, and keep it warm for 3.0 h;

[0102] 6) Turn off the heating switch, wait until the furnace temperature drops to room temperature, stop the rotating device, and open the exhaust valve; wait until the air pressure in the furnace drops to atmospheric pressure, then open the furnace door, separate the furnace charge, and take out the permanent magnet;

[0103] 7) Place the permanent magnet after thermal diffusion co-permeation treatment in deionized water and ultrasonically clean for 1 min to remove the surface grit, and then place the permanent magnet in an oven at 80 °C for drying to obtain a neodymium iron boron permanent magnet coated with a zinc-dysprosium corrosion-resistant coating. The structural schematic diagram is shown in Figure 1 .

[0104] Repeat Example 3 multiple times, extract 3 specimens for neutral salt spray test to evaluate the protective performance of the corrosion-resistant coating. The concentration of the NaCl solution in the neutral salt spray experiment is 5 wt%, the pH is 6.5 - 7.2, the test temperature is 35 ± 2 °C, and the sedimentation rate is 1 - 2 ml / 80 cm 2 ·h; Observe the appearance time and area of white rust and red rust on the surface of the magnet. The later the appearance time of rust and the smaller the area, the stronger the protective ability. The thickness of the surface co-permeation layer is 9.2 μm, and the thickness of the interface diffusion layer is 3.8 μm. In the surface co-permeation layer, the zinc content is 99.8 wt%, and the dysprosium content is 0.1 wt%; in the interface diffusion layer, the zinc content is 80.3 wt%, and the dysprosium content is 0.09 wt%, and the rest is iron. The neutral salt spray test lasts for more than 500 h, and no red rust, bubbling or peeling appears on the surface of the magnet; the magnetic attenuation ratio is 5.8%, and the coercivity increases by 5.3%.

[0105] Example 4

[0106] This example is used to prepare a corrosion-resistant coating composed of the following components in parts by mass: (1) zinc powder (10 - 25 μm), 40.0 parts; (2) dysprosium oxide powder (10 - 25 μm), 3.0 parts; (3) ammonium chloride (19 μm), 6.0 parts; (4) quartz sand (SiO₂, 4 cm), 40.0 parts, (5) neodymium iron boron permanent magnet, 14.0 parts. It includes the following steps:

[0107] 1) Place 14.0 parts of neodymium iron boron permanent magnet in a degreasing solution composed of one or several of 5 g / L NaOH, 10 g / L Na₂CO₃, 2 g / L Na₃PO₄, 0.3 ml / L OP-10 and 0.1 ml / L sodium dodecylbenzenesulfonate, ultrasonically clean at room temperature for 5 min, then rinse thoroughly with deionized water, and finally place in an oven at 80 °C for drying for later use;

[0108] 2) Load 40.0 parts of zinc powder, 3.0 parts of dysprosium oxide powder, 6.0 parts of ammonium chloride and 40.0 parts of quartz sand into the vacuum furnace body, and then load 14.0 parts of neodymium iron boron magnet;

[0109] 3) Start the rotating device, rotate the vacuum furnace at a low speed, with the rotation speed of the furnace body being 18 r / min, to ensure uniform mixing of the powder and the permanent magnet;

[0110] 4) Evacuate the vacuum furnace, then fill it with N₂ gas, and keep the vacuum degree in the vacuum furnace body at 2.0×10 -2 ~8.0×10 -2 Pa;

[0111] 5) Turn on the heating device, wait until the temperature in the furnace rises to 390 °C, and keep it warm for 3.0 h;

[0112] 6) Turn off the heating switch, wait until the furnace temperature drops to room temperature, stop the rotating device, and open the exhaust valve; wait until the pressure in the furnace drops to atmospheric pressure, then open the furnace door, separate the furnace charge, and take out the permanent magnet;

[0113] 7) Place the permanent magnet after thermal diffusion co-permeation treatment in deionized water and ultrasonically clean for 1 min to remove the surface grit, then place the permanent magnet in an oven at 80 °C for drying, to obtain a neodymium iron boron permanent magnet with a corrosion-resistant coating prepared on its surface. The structural schematic diagram is shown in Figure 1 .

[0114] Repeat Example 4 multiple times, extract 3 specimens for neutral salt spray test to evaluate the protective performance of the corrosion-resistant coating. The concentration of the NaCl solution in the neutral salt spray experiment is 5 wt%, the pH is 6.5 - 7.2, the test temperature is 35 ± 2 °C, and the sedimentation amount is 1 - 2 ml / 80 cm 2·h; Observe the appearance time and area of white rust and red rust on the magnet surface. The later the appearance time of rust and the smaller the area, the stronger the protection ability. The thickness of the surface co-permeation layer is 9.4 μm, and the thickness of the interface diffusion layer is 3.8 μm. In the surface co-permeation layer, the zinc content is 99.8 wt%, and the dysprosium content is 0.1 wt%; in the interface diffusion layer, the zinc content is 80.3 wt%, the dysprosium content is 0.09 wt%, and the rest is iron. The neutral salt spray test lasts for more than 500 h, and no red rust, bubbling or peeling appears on the magnet surface; the magnetic attenuation ratio is 5.8%, and the coercivity increases by 5.3%.

[0115] Example 5

[0116] This example is used to prepare a corrosion-resistant coating composed of the following components in parts by mass: (1) zinc powder (10 - 25 μm), 40.0 parts; (2) dysprosium oxide powder (10 - 25 μm), 3.0 parts; (3) ammonium chloride (22 μm), 6.0 parts; (4) quartz sand (SiO₂, 5 cm), 40.0 parts, (5) neodymium iron boron permanent magnet, 14.0 parts. It includes the following steps:

[0117] 1) Place 14.0 parts of neodymium iron boron permanent magnet in a degreasing solution composed of one or several of 5 g / L NaOH, 10 g / L Na₂CO₃, 2 g / L Na₃PO₄, 0.3 ml / L OP-10 and 0.1 ml / L sodium dodecylbenzene sulfonate, ultrasonically clean at room temperature for 5 min, then rinse with deionized water, and finally place in an oven at 80 °C for drying for later use;

[0118] 2) Load 40.0 parts of zinc powder, 3.0 parts of dysprosium oxide powder, 6.0 parts of ammonium chloride and 40.0 parts of quartz sand into the vacuum furnace body, and then load 14.0 parts of neodymium iron boron magnet;

[0119] 3) Start the rotating device, rotate the vacuum furnace at a low speed, and the rotation speed of the furnace body is 20 r / min to ensure uniform mixing of the powder and the permanent magnet;

[0120] 4) Evacuate the vacuum furnace, then fill it with N₂ gas, and keep the vacuum degree in the vacuum furnace body at 2.0×10 -2 ~8.0×10 -2 Pa;

[0121] 5) Turn on the heating device, wait until the temperature in the furnace rises to 390 °C, and keep it warm for 4.0 h;

[0122] 6) Turn off the heating switch, wait until the furnace temperature drops to room temperature, stop the rotating device, and open the exhaust valve; wait until the air pressure in the furnace drops to atmospheric pressure, then open the furnace door, separate the furnace charge, and take out the permanent magnet;

[0123] 7) After the heat diffusion co-permeation treatment, the permanent magnet is placed in deionized water and ultrasonically cleaned for 1 min to remove surface grit. Then, the permanent magnet is dried in an oven at 80 °C to obtain a neodymium iron boron permanent magnet coated with a zinc-dysprosium corrosion-resistant coating. The structural schematic diagram is shown in Figure 1 .

[0124] Example 5 was repeated multiple times, and 3 specimens were selected for the neutral salt spray test to evaluate the protective performance of the corrosion-resistant coating. The concentration of the NaCl solution in the neutral salt spray experiment was 5 wt%, the pH was 6.5 - 7.2, the test temperature was 35 ± 2 °C, and the sedimentation rate was 1 - 2 ml / 80 cm 2 ·h; Observe the appearance time and area of white rust and red rust on the surface of the magnet. The later the appearance time of rust and the smaller the area, the stronger the protective ability. The thickness of the surface co-permeation layer is 9.4 μm, and the thickness of the interfacial diffusion layer is 3.8 μm. In the surface co-permeation layer, the zinc content is 99.8 wt%, and the dysprosium content is 0.1 wt%; in the interfacial diffusion layer, the zinc content is 80.3 wt%, the dysprosium content is 0.09 wt%, and the rest is iron. The neutral salt spray test lasted for more than 500 h, and no red rust, bubbling or peeling occurred on the surface of the magnet; the magnetic attenuation ratio was 5.8%, and the coercivity increased by 5.3%.

[0125] The microscopic tissue morphology of the NdFeB material prepared by the present invention is shown in Figure 2 , and the macroscopic organizational structure is shown in Figure 4 . Figure 4 In, a is the overall image of the corrosion-resistant coating and the neodymium iron boron matrix in the specimen, b is the distribution of zinc elements in the specimen, c is the distribution of metallic iron in the specimen, and d is the distribution of dysprosium elements in the specimen, where the specimen is a neodymium iron boron permanent magnet coated with the protective coating provided in the embodiment of the present invention. It can be seen that in the technical solution of the present invention, dysprosium elements can indeed form a relatively thin interfacial diffusion layer in the neodymium iron boron matrix. At the same time, the dysprosium elements are evenly distributed on both sides of the interfacial diffusion layer with the interfacial diffusion layer as the axis to increase the similarity of the physical and chemical properties of the surface co-permeation layer, the interfacial diffusion layer, and the matrix, thereby making the interfacial diffusion layer more stable and not prone to cracking, peeling and other phenomena.

[0126] In the embodiment of the present invention, the Auger electron detection method is used to detect the distribution of each element in the neodymium iron boron matrix, and the Auger electrons sequentially pass through the surface penetration layer, the boundary diffusion layer and the matrix. The detection results are as Figure 3 shown. At 100 min, the zinc content suddenly drops sharply and the iron content rises sharply, indicating that the Auger electrons have penetrated the surface penetration layer, thus proving that the surface penetration layer does indeed form a layered structure on the surface of the body. At 120 min, the iron content is the largest, indicating that the Auger electrons have completely entered the matrix, and the dysprosium content shows a small peak at about 110 min, indicating that the interfacial diffusion layer formed by dysprosium is between the surface penetration layer and the matrix.

[0127] The experimental results show that the corrosion resistance time of the protective coating prepared by the present invention in a neutral salt spray environment is not less than 500 h without rusting. The surface remanence loss of the co-permeation corrosion-resistant coating prepared by the present invention for the NdFeB magnet does not exceed 1.0%, and the coercivity is increased by 5-10%. The problems of the surface treatment process and the shielding of the surface coating affecting the magnetic properties of the permanent magnet material are solved.

[0128] Example 6

[0129] The particle size of zinc powder is 15 μm, and the particle size of dysprosium oxide is 12 μm. The remaining components, production steps, and process conditions are the same as those in Example 1. The obtained surface co-permeation layer is 9.1 μm.

[0130] Example 7

[0131] The particle size of zinc powder is 20 μm, and the particle size of dysprosium oxide is 15 μm. The remaining components, production steps, and process conditions are the same as those in Example 1. The obtained surface co-permeation layer is 8.6 μm.

[0132] Example 8

[0133] The particle size of zinc powder is 24 μm, and the particle size of dysprosium oxide is 20 μm. The remaining components, production steps, and process conditions are the same as those in Example 1. The obtained surface co-permeation layer is 8.0 μm.

[0134] Comparative Example 1

[0135] This example is used to prepare a corrosion-resistant coating composed of the following components in parts by mass: (1) zinc powder (0-25 μm), 40.0 parts; (2) ammonium chloride (10-25 μm), 6.0 parts; (3) quartz sand (SiO2, 1-5 cm), 40.0 parts, (4) NdFeB permanent magnet, 14.0 parts. It includes the following steps:

[0136] 1) Place 14.0 parts of NdFeB permanent magnet in a degreasing solution composed of one or several of 5 g / L NaOH, 10 g / L Na2CO3, 2 g / L Na3PO4, 0.3 ml / L OP-10, and 0.1 ml / L sodium dodecylbenzenesulfonate, ultrasonically clean at room temperature for 5 min, then rinse with deionized water, and finally place in an oven at 80 °C for drying for later use;

[0137] 2) Load 40.0 parts of zinc powder, 6.0 parts of ammonium chloride, and 40.0 parts of quartz sand into the vacuum furnace body, and then load 14.0 parts of NdFeB magnet;

[0138] 3) Start the rotating device, rotate the vacuum furnace at a low speed, and the rotation speed of the furnace body is 18 r / min to ensure uniform mixing of the powder and the permanent magnet;

[0139] 4) Evacuate the vacuum furnace, and then fill it with N2 gas. The vacuum degree in the vacuum furnace body is maintained at 2.0×10 -2 ~8.0×10 -2 Pa;

[0140] 5) Turn on the heating device. When the temperature in the furnace rises to 390 °C, keep it warm for 3.0 h;

[0141] 6) Turn off the heating switch. When the furnace temperature drops to room temperature, stop the rotating device and open the exhaust valve. When the air pressure in the furnace drops to atmospheric pressure, open the furnace door, separate the furnace charge, and take out the permanent magnet;

[0142] 7) Place the permanent magnet after thermal diffusion co-permeation treatment in deionized water and ultrasonically clean it for 1 min to remove the surface grit. Then place the permanent magnet in an oven at 80 °C to dry, and obtain a neodymium iron boron permanent magnet with an anti-corrosion coating prepared on the surface.

[0143] As can be seen from Examples 6-8, under the same process conditions, the smaller the particle sizes of zinc powder and dysprosium oxide, the thicker the surface co-permeation layer obtained by co-permeation. This means that reducing the powder particle size can increase the symbiotic rate, shorten the co-permeation treatment time, so as to improve production efficiency and reduce production costs. At the same time, the obtained coating is more uniform and dense, thus enhancing the protective performance of the coating. Therefore, on the premise of meeting the process technical requirements, using small particle size powder for co-permeation treatment not only has higher economic benefits, but also has higher corrosion resistance of the coating.

[0144] To illustrate the performance of the corrosion-resistant coating in the embodiments of the present invention, the embodiments of the present invention are described from four aspects: corrosion resistance, magnetic properties, magnetic attenuation, and particle size. Specifically, see Tables 1-4. Figure 5 - Figure 6 . The comparison results of corrosion resistance are shown in Table 1, the comparison of magnetic properties is shown in Table 2, and the comparison results of magnetic attenuation are shown in Table 3.

[0145] Table 1 Comparison of Corrosion Resistance between Examples 1-5 and Comparative Example 1

[0146]

[0147] As can be seen from Table 1, compared with the prior art, the corrosion-resistant coating provided by the present invention can significantly extend the test time of the permanent magnet in the corrosion resistance test and effectively inhibit the corrosion degree of the permanent magnet.

[0148] Table 2 Comparison of Magnetic Properties between Examples 1-5 and Comparative Example 1

[0149]

[0150]

[0151] As can be seen from Table 2, compared with the pure galvanizing technology of the prior art, the corrosion-resistant coating of the embodiment of the present invention can effectively increase the coercivity (Hcb). It should be noted that the intrinsic coercivity (Hcj) is different from the coercivity. When the reverse magnetic field H = Hcb, although the external magnetic induction intensity is zero, the residual magnetization intensity (Br) of the magnet itself is not zero at this time, but the action of the applied reverse magnetic field and Br cancels each other out. When the reverse magnetic field H = Hcj, the residual magnetization intensity of the magnet drops to 0. Therefore, although the residual magnetization intensity (Br) of Comparative Example 1 is slightly higher than that of Examples 1-5, its Hcj is much lower than that of Examples 1-5. It can be seen from this that the Hcb of Examples 1-5 is greater than that of Comparative Example 1. Therefore, the corrosion-resistant coating of the present invention can increase the coercivity of the permanent magnet.

[0152] Table 3 Comparison of magnetic decay between Examples 1-5 and Comparative Example 1

[0153] Group Magnetic moment before baking (μVs.com) Magnetic moment after baking (μVs.com) Attenuation ratio Comparative Example 1 9.15 8.08 11.7％ Example 1 9.08 8.55 5.8％

[0154] As can be seen from Table 3, the attenuation ratio of the permanent magnet in Comparative Example 1 is 11.7%, while the attenuation ratio of the permanent magnets in Examples 1-5 of the present invention is 5.8%. Therefore, the corrosion-resistant coating provided by the embodiments of the present invention can reduce the magnetic decay of the permanent magnet.

[0155] In summary, the corrosion-resistant coating for improving the coercivity of neodymium-iron-boron permanent magnets and its preparation method of the present invention can be applied to the surface protection process of sintered neodymium-iron-boron permanent magnets, can eliminate the pickling process in the pretreatment, and obtain a protective coating with firm adhesion, excellent corrosion resistance and capable of improving the coercivity of the magnet. This coating significantly improves problems such as coating blistering on the magnet surface, poor protection ability, and weakening of the magnet performance, thereby prolonging the service life of the magnet and improving the thermal stability of the magnet, and ensuring the reliability of the magnet device.

[0156] The embodiments disclosed in the present invention have been described above. However, these embodiments are for illustrative purposes only and are not intended to limit the scope of the present disclosure. The scope of the present disclosure is defined by the appended claims and their equivalents. Without departing from the scope of the present disclosure, those skilled in the art can make various substitutions and modifications, and these substitutions and modifications should all fall within the scope of the present disclosure.

Claims

1. A corrosion-resistant coating for improving the coercivity of NdFeB permanent magnets, characterized in that, Along the direction into the interior of the NdFeB matrix, the corrosion-resistant coating sequentially includes a surface co-permeation layer and an interface diffusion layer; The surface co-permeation layer is a metal layer composed of zinc element and dysprosium element; the interface diffusion layer is a metal layer composed of zinc element, dysprosium element and iron element, wherein the dysprosium element is uniformly distributed on both sides of the interface diffusion layer with the interface diffusion layer as the axis; The corrosion-resistant coating is obtained by co-permeating zinc powder and dysprosium oxide, and the temperature of the co-permeation is 350 - 390 °C; The thickness of the surface co-permeation layer is 8 μm - 10 μm; The thickness of the interface diffusion layer is 3 μm - 5 μm.

2. The corrosion-resistant coating for improving the coercivity of the NdFeB permanent magnet according to claim 1, wherein In the surface co-permeation layer, the content of zinc is 99.7 - 99.9 wt%, and the content of dysprosium is 0.1 - 0.3 wt%; in the interface diffusion layer, the content of zinc is 80.0 - 83.0 wt%, the content of dysprosium is 0.08 - 0.10 wt%, and the rest is iron.

3. The corrosion-resistant coating for improving the coercivity of the NdFeB permanent magnet according to claim 1, wherein By mass, the co-permeation materials for preparing the corrosion-resistant coating include: 20.0 - 50.0 parts of zinc powder, 1.0 - 5.0 parts of dysprosium oxide, 2.0 - 8.0 parts of activator, and 30.0 - 50.0 parts of inert medium.

4. A method for preparing a corrosion-resistant coating for improving the coercivity of a neodymium-iron-boron permanent magnet according to claim 1, characterized in that, It includes the following steps: Step 1, clean the NdFeB permanent magnet and set it aside after drying; Step 2, load the co-permeation materials and the permanent magnet into the vacuum furnace in sequence, and close the furnace door of the vacuum furnace; the co-permeation materials include 20.0 - 50.0 parts of zinc powder, 1.0 - 5.0 parts of dysprosium oxide, 2.0 - 8.0 parts of activator, and 30.0 - 50.0 parts of inert medium; Step 3, start the rotation of the vacuum furnace to mix the co-permeation materials and the permanent magnet evenly; Step 4: Evacuate the vacuum furnace and fill it with inert gas to keep the vacuum degree in the furnace at 2.0×10 -2 ~8.0×10 -2 Pa; Step 5: Start heating up. After heating up to the target temperature, keep the temperature constant and maintain the vacuum degree at 2.0×10 -2 ~8.0×10 -2 Pa; Step 6, stop heating, keep the pressure and cool to room temperature, and take out the permanent magnet.

5. The preparation method of the corrosion-resistant coating for improving the coercivity of the Nd-Fe-B permanent magnet according to claim 4, characterized in that, In Step 1, the NdFeB permanent magnet is a formed NdFeB permanent magnet product.

6. The preparation method of the corrosion-resistant coating for improving the coercivity of the NdFeB permanent magnet according to claim 4, wherein In Step 5, the target temperature is 350 - 390 °C, and the heat preservation time is 3 - 4 h.

7. The corrosion-resistant coating for improving the coercivity of Nd-Fe-B permanent magnets according to claim 4, characterized in that, The particle size of the zinc powder is 15 μm - 30 μm; the particle size of the dysprosium oxide is 15 μm - 30 μm; the activator is ammonium chloride, and the particle size of the ammonium chloride is 15 μm - 30 μm; the inert medium is quartz sand, and the particle size of the quartz sand is 2 cm.

8. A neodymium iron boron permanent magnet, characterized in that, The surface of the NdFeB permanent magnet is provided with a corrosion-resistant coating, and the corrosion-resistant coating is the corrosion-resistant coating described in any one of claims 1 to 3 or the corrosion-resistant coating prepared by the preparation method described in any one of claims 4 to 7.

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