Neodymium-iron-boron surface multilayer coating and preparation method thereof

Abstract

The invention provides a neodymium iron boron surface multilayer coating and a preparation method thereof, and belongs to the technical field of rare earth permanent magnet materials. A first Ni plating layer, a second Cu plating layer and a third Ni plating layer are sequentially formed on the surface of the neodymium-iron-boron magnet, and the magnet with the plating layers is obtained; and sequentially carrying out vacuum thermal diffusion treatment and thermal oxidation treatment on the magnet with the plating layer to obtain the neodymium iron boron surface multi-layer plating layer. In the vacuum thermal diffusion treatment process, thermal diffusion can occur between every two of the first Ni coating, the second Cu coating and the third Ni coating, mutual injection of atoms among the multiple coatings is achieved, a multi-coating diffusion layer (namely a Ni-Cu diffusion layer) is formed, the electrochemical property and impedance are improved, and the electrochemical performance of the device is improved. The corrosion resistance and the coating binding force are improved, meanwhile, the magnetic performance cannot be reduced, a layer of stable NiO passive film can be formed through thermal oxidation treatment, and the corrosion resistance is improved.

Description

Technical Field

[0001] The invention relates to the technical field of rare earth permanent magnetic materials, and in particular to a NdFeB surface multilayer coating and a preparation method thereof. Background Art

[0002] NdFeB permanent magnet materials belong to powder metallurgy materials and are composed of multiple phases with large potential differences. In particular, the neodymium-rich phase has a low potential and is prone to intergranular corrosion. In order to improve the corrosion resistance of NdFeB magnets, electroplating of zinc, electroplating of nickel and electroplating of copper and nickel are usually used. The nickel-copper-nickel coating on the surface of NdFeB magnets is becoming more and more popular due to its coating hardness, wear resistance, corrosion resistance and good coating bonding strength. However, there is still a problem of poor corrosion resistance. Summary of the invention

[0003] In view of this, the object of the present invention is to provide a NdFeB surface multilayer coating and a preparation method thereof. The present invention improves the corrosion resistance of the multilayer coating by vacuum thermal diffusion and thermal oxidation treatment.

[0004] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions: The present invention provides a method for preparing a multilayer coating on the surface of NdFeB, comprising the following steps: Forming a first Ni coating, a second Cu coating and a third Ni coating in sequence on the surface of the NdFeB magnet to obtain a magnet with coatings; The magnet with coating is subjected to vacuum thermal diffusion treatment and thermal oxidation treatment in sequence to obtain the NdFeB surface multilayer coating.

[0005] Preferably, the vacuum thermal diffusion is carried out at a temperature of 150-500° C. and for a time of 30-300 min.

[0006] Preferably, the thermal oxidation treatment is carried out at a temperature of 150-500° C. and for a time of 30-300 min.

[0007] Preferably, the thermal oxidation treatment is performed in an oxygen atmosphere.

[0008] Preferably, the forming of the first Ni plating layer, the second Cu plating layer and the third Ni plating layer are all carried out under the conditions of a constant magnetic field and a variable frequency electric field.

[0009] Preferably, the parameters of the variable frequency electric field include: forward current density of 6-10A / dm 2 , operating frequency is 400~800Hz, duty cycle is 40~80%; reverse current density is 0.2~1A / dm 2 , the operating frequency is 400~800Hz, and the duty cycle is 40~80%.

[0010] Preferably, the magnetic field strength of the constant magnetic field is 0.4~0.8T.

[0011] Preferably, the nickel plating solution for forming the first Ni plating layer and the third Ni plating layer independently includes the following components in concentration: 180~280g / L of nickel sulfate, 20~40g / L of nickel chloride, 30~40g / L of boric acid, 0.6~1g / L of o-benzoylsulfonyl imide, 0.2~0.5g / L of 1,4-butynediol, 0.1~0.2g / L of coumarin, 0.1~0.2g / L of formaldehyde, and 0.05~0.1g / L of sodium dodecyl sulfate, and the pH values ​​of the first nickel plating solution and the second nickel plating solution are independently 2~6.

[0012] Preferably, the copper plating solution for forming the second Cu plating layer comprises components with the following concentrations: 180-280 g / L copper sulfate, 40-80 g / L concentrated sulfuric acid, 0.006-0.01 g / L ethylene thiourea, 0.08-0.18 g / L sodium dodecyl sulfate, 0.001-0.1 g / L 2-mercaptobenzimidazole, 0.05-0.1 g / L polyethylene glycol, 0.06-0.15 g / L sodium chloride, 0.018-0.18 g / L sodium polydisulfide dipropane sulfonate, and the mass fraction of the concentrated sulfuric acid is 98%.

[0013] The present invention also provides a multi-layer coating on the surface of NdFeB obtained by the preparation method described in the above technical solution.

[0014] The invention provides a method for preparing a multilayer coating on the surface of a neodymium iron boron magnet, comprising the following steps: sequentially forming a first Ni coating, a second Cu coating and a third Ni coating on the surface of a neodymium iron boron magnet to obtain a magnet with coatings; and sequentially performing vacuum thermal diffusion treatment and thermal oxidation treatment on the magnet with coatings to obtain the multilayer coating on the surface of the neodymium iron boron magnet.

[0015] Compared with the prior art, the present invention has the following beneficial effects: The present invention sequentially performs vacuum thermal diffusion and thermal oxidation treatment on the coated magnet. During the vacuum thermal diffusion process, the first Ni coating layer, the second Cu coating layer and the third Ni coating layer undergo thermal diffusion, thereby achieving mutual injection of atoms between the multiple coating layers to form a NiCu alloy transition layer (i.e., a Ni-Cu diffusion layer, such as Figure 2 ), improve electrochemical properties and impedance, increase corrosion resistance and coating adhesion while magnetic properties will not decrease significantly (such as Figure 3 ), the thermal oxidation treatment can form a stable NiO passivation film on the surface of the coating after vacuum thermal diffusion treatment, thereby improving the corrosion resistance. The multilayer coating on the surface of NdFeB obtained by the present invention is a multilayer diffusion coating with high corrosion resistance.

[0016] Furthermore, the present invention adopts electric-magnetic multi-field synergy, and utilizes the effects of pulsed electric field and magnetic field to regulate the composition and uniformity of the plating solution, improve the quality of the coating, and enhance the corrosion resistance of the coating.

[0017] The data of the embodiment show that the corrosion current density of the multilayer coating on the surface of NdFeB prepared by the present invention is as low as 0.23×10 -6 A.cm -2 The corrosion potential is as low as -243mV, the neutral salt spray corrosion time is as high as 264h, the pressure immersion time is as high as 312h, and the maximum oxidation weight gain of the coating is 0.29g. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 A flow chart of pre-treatment of NdFeB magnets before electrodeposition; Figure 2 This is the cross-sectional morphology of the coating obtained by sequence number 15; Figure 3 is the element distribution diagram of Cu in the Ni-Cu diffusion layer; Figure 4 is the element distribution diagram of Ni in the Ni-Cu diffusion layer; Figure 5 This is the cross-sectional morphology of the coating obtained by sequence number 18; Figure 6 is the element distribution diagram of Cu in the Ni-Cu diffusion layer; Figure 7 This is the element distribution diagram of Ni in the Ni-Cu diffusion layer. DETAILED DESCRIPTION

[0019] The present invention provides a method for preparing a multilayer coating on the surface of NdFeB, comprising the following steps: Forming a first Ni coating, a second Cu coating and a third Ni coating in sequence on the surface of the NdFeB magnet to obtain a magnet with coatings; The magnet with coating is subjected to vacuum thermal diffusion treatment and thermal oxidation treatment in sequence to obtain the NdFeB surface multilayer coating.

[0020] Unless otherwise specified, the raw materials used in the present invention are all commercially available products in the art.

[0021] The present invention preferably immerses the NdFeB magnet in a first nickel plating solution, performs a first electrodeposition to form the first Ni plating layer, then immerses the NdFeB magnet in a copper plating solution, performs a second electrodeposition to form the second Cu plating layer, and then immerses the NdFeB magnet in a second nickel plating solution, performs a third electrodeposition to form the third Ni plating layer, and obtains the coated magnet.

[0022] The present invention preferably performs chamfering, oil removal, oxide layer removal and ultrasonic cleaning on the NdFeB magnet in sequence to obtain a magnet to be plated, and then immerses the magnet to be plated in the first nickel plating solution.

[0023] The present invention preferably uses a centrifugal finishing machine or a centrifugal vortex finishing machine to chamfer the NdFeB magnets according to different sizes. The present invention has no special limitation on the specific parameters of the chamfering, and a method well known to those skilled in the art can be used.

[0024] In the present invention, the degreasing agent used for degreasing preferably includes the following components in concentrations: 5-10 g / L sodium hydroxide, 30-45 g / L sodium carbonate, 40-60 g / L sodium phosphate, 0.1-1 g / L OP-10 emulsifier, and 0.1-0.2 g / L sodium dodecyl sulfate.

[0025] In the present invention, the degreasing agent is preferably neutral or weakly alkaline, and the weakly alkaline preferably has a pH value of 8.5 to 10.5.

[0026] In the present invention, the deoxidation layer preferably comprises the following steps: first pickling in an acid wash solution for 20 to 30 seconds, then 2 SO 4 Activate in the solution for 5 to 10 seconds to remove the rust layer.

[0027] In the present invention, the pickling solution used in the pickling is preferably prepared from nitric acid solution, thiourea and water, and the pickling solution contains HNO 3 The concentration of thiourea in the pickling solution is preferably 0.5 g / L.

[0028] In the present invention, the ultrasonic frequency of the ultrasonic cleaning is preferably 20-60 Hz, specifically 20, 30, 40, 50 or 60 Hz; the ultrasonic cleaning can ensure that the surface of the magnet is clean and tidy to prevent contamination of the next process.

[0029] Figure 1 Flow chart of pre-treatment of NdFeB magnets before electrodeposition.

[0030] In the present invention, the first electrodeposition, the second electrodeposition and the third electrodeposition are preferably carried out under the conditions of a constant magnetic field and a variable frequency electric field. The present invention adopts a constant magnetic field and a variable frequency electric field to coordinate and comprehensively act, thereby regulating the composition and uniformity of the plating solution, improving the quality of the coating, and optimizing the magnetic properties.

[0031] In the present invention, the magnetic field strength of the constant magnetic field is preferably 0.4-0.8T, specifically 0.4, 0.5, 0.6, 0.7 or 0.8T.

[0032] In the present invention, the parameters of the variable frequency electric field preferably include: forward current density of 6-10A / dm 2 , specifically 6, 7, 8, 9 or 10A / dm 2 The operating frequency is 400~800Hz, specifically 400, 500, 600, 700 or 800Hz; the duty cycle is 40~80%, specifically 40%, 50%, 60%, 70% or 80%; the reverse current density is 0.2~1A / dm 2 , specifically 0.2, 0.4, 0.6, 0.8 or 1A / dm 2 The operating frequency is 400~800Hz, specifically 400, 500, 600, 700 or 800Hz, and the duty cycle is 40~80%, specifically 40%, 50%, 60%, 70% or 80%.

[0033] In the present invention, the variable frequency electric field is preferably a bidirectional pulse electric field.

[0034] In the present invention, the first electrodeposition, the second electrodeposition and the third electrodeposition are preferably carried out under ultrasonic conditions, and the frequency of the ultrasound is independently preferably 20-60 Hz, specifically 20, 30, 40, 50 or 60 Hz.

[0035] In the present invention, the temperature of the first electrodeposition, the second electrodeposition and the third electrodeposition is preferably independently 45±5°C, specifically 40, 45 or 50°C.

[0036] In the present invention, the first nickel plating solution and the second nickel plating solution independently preferably include components with the following concentrations: 180-280 g / L of nickel sulfate, 20-40 g / L of nickel chloride, 30-40 g / L of boric acid, 0.6-1 g / L of o-benzoylsulfonyl imide, 0.2-0.5 g / L of 1,4-butynediol, 0.1-0.2 g / L of coumarin, 0.1-0.2 g / L of formaldehyde, and 0.05-0.1 g / L of sodium dodecyl sulfate, and the pH value of the nickel plating solution is 2-6.

[0037] In the present invention, the concentration of nickel sulfate in the first nickel plating solution and the second nickel plating solution can be 180, 200, 220, 240, 260 or 280 g / L independently. Controlling the concentration of nickel sulfate within the above range can ensure that there is an appropriate amount of Ni in the plating solution. 2+ , maintaining a suitable deposition rate.

[0038] In the present invention, the concentration of nickel chloride in the first nickel plating solution and the second nickel plating solution can be 20, 25, 30, 35 or 40 g / L independently. Controlling the concentration of nickel chloride within the above range can adjust the anode dissolution efficiency and the conductivity of the plating solution.

[0039] In the present invention, the concentration of boric acid in the first nickel plating solution and the second nickel plating solution can specifically be 30, 35 or 40 g / L. Controlling the concentration of boric acid within the above range can maintain the stability of the pH value of the plating solution, reduce cathode polarization, and improve deposition uniformity.

[0040] In the present invention, the concentration of o-benzoylsulfonyl imide in the first nickel plating solution and the second nickel plating solution can be 0.6, 0.8 or 1 g / L independently. Controlling the concentration of o-benzoylsulfonyl imide within the above range can act as a secondary brightener, refine grains, enhance ductility of the coating, and improve leveling.

[0041] In the present invention, the concentration of 1,4-butynediol in the first nickel plating solution and the second nickel plating solution can be 0.2, 0.3, 0.4, or 0.5 g / L independently. Controlling the concentration of 1,4-butynediol within the above range can act as a secondary brightener to improve the mirror gloss of the coating and enhance the leveling property.

[0042] In the present invention, the concentration of coumarin in the first nickel plating solution and the second nickel plating solution can be 0.1, 0.15 or 0.2 g / L independently. Controlling the concentration of coumarin within the above range can refine the grains of the nickel plating layer and make the plating layer bright and smooth.

[0043] In the present invention, the concentration of formaldehyde in the first nickel plating solution and the second nickel plating solution can be 0.1, 0.15 or 0.2 g / L independently. Controlling the concentration of formaldehyde within the above range can accelerate the nickel ion reduction deposition rate and improve the cathode current efficiency.

[0044] In the present invention, the concentration of sodium dodecyl sulfate in the first nickel plating solution and the second nickel plating solution can be 0.05, 0.06, 0.07, 0.08, 0.09 or 0.1 g / L independently. Controlling the concentration of sodium dodecyl sulfate within the above range can reduce the surface tension of the plating solution and effectively prevent the generation of pinholes and pitting.

[0045] In the present invention, the solvents of the first nickel plating solution and the second nickel plating solution are preferably water.

[0046] In the present invention, the pH values ​​of the first nickel plating solution and the second nickel plating solution may be 2, 3, 4, 5 or 6 independently.

[0047] The present invention preferably uses ammonia water to adjust the pH value of the first nickel plating solution and the second nickel plating solution to 2-6. The present invention has no special limitation on the concentration and dosage of the ammonia water, and a solution familiar to those skilled in the art can be used.

[0048] In the present invention, the copper plating solution preferably includes components with the following concentrations: 180-280 g / L copper sulfate, 40-80 g / L concentrated sulfuric acid, 0.006-0.01 g / L ethylene thiourea, 0.08-0.18 g / L sodium dodecyl sulfate, 0.001-0.1 g / L 2-mercaptobenzimidazole, 0.05-0.1 g / L polyethylene glycol, 0.06-0.15 g / L sodium chloride, 0.018-0.18 g / L sodium polydisulfide dipropane sulfonate, and the concentration of the concentrated sulfuric acid is 98 wt %.

[0049] In the present invention, the concentration of copper sulfate in the copper plating solution can specifically be 180, 200, 220, 240, 260 or 280 g / L. Controlling the concentration of copper sulfate within the above range can ensure that there is an appropriate amount of copper ions in the plating solution and maintain a suitable deposition rate.

[0050] In the present invention, the concentration of concentrated sulfuric acid in the copper plating solution can be specifically 40, 50, 60, 70 or 80 g / L. Controlling the concentration of concentrated sulfuric acid within the above range can improve the conductivity of the plating solution and inhibit the hydrolysis of copper ions.

[0051] In the present invention, the concentration of ethylene thiourea in the copper plating solution can be specifically 0.006, 0.007, 0.008, 0.009 or 0.01 g / L. Controlling the concentration of ethylene thiourea within the above range can effectively refine the grains of the copper plating layer and enhance the flatness, brightness and density of the plating layer.

[0052] In the present invention, the concentration of sodium dodecyl sulfate in the copper plating solution may specifically be 0.08, 0.10, 0.12, 0.14, 0.16 or 0.18 g / L.

[0053] In the present invention, the concentration of 2-mercaptobenzimidazole in the copper plating solution can be specifically 0.001, 0.005, 0.01, 0.05 or 0.1 g / L. Controlling the concentration of 2-mercaptobenzimidazole within the above range can reduce the surface tension of the plating solution and effectively prevent the generation of pinholes and pitting.

[0054] In the present invention, the concentration of polyethylene glycol in the copper plating solution can specifically be 0.05, 0.06, 0.07, 0.08, 0.09 or 0.1 g / L. Controlling the concentration of polyethylene glycol within the above range can reduce the surface tension of the plating solution and inhibit the deposition rate of copper ions in the high current density area.

[0055] In the present invention, the concentration of sodium chloride in the copper plating solution can specifically be 0.06, 0.09, 0.12 or 0.15 g / L. Controlling the concentration of sodium chloride within the above range can optimize the anode dissolution efficiency and improve the conductivity of the plating solution.

[0056] In the present invention, the concentration of sodium polydisulfide dipropane sulfonate in the copper plating solution can be specifically 0.018, 0.02, 0.05, 0.1, 0.15 or 0.18 g / L. Controlling the concentration of sodium polydisulfide dipropane sulfonate within the above range can accelerate the deposition of copper ions, refine the grains and improve the flatness of the coating, and synergistically enhance the brightening effect and ductility with chloride ions.

[0057] In the present invention, the solvent of the copper plating solution is preferably water.

[0058] In the present invention, the first nickel plating solution and the second nickel plating solution preferably include components with the following concentrations: 200 g / L nickel sulfate, 30 g / L nickel chloride, 35 g / L boric acid, 0.7 g / L o-benzoylsulfonyl imide, 0.35 g / L 1,4-butynediol, 0.1 g / L coumarin, 0.1 g / L formaldehyde, 0.1 g / L sodium dodecyl sulfate, and the pH value of the first nickel plating solution and the second nickel plating solution is 5.

[0059] In the present invention, the copper plating solution preferably includes components with the following concentrations: 230 g / L copper sulfate, 60 g / L concentrated sulfuric acid, 0.01 g / L ethylene thiourea, 0.10 g / L sodium dodecyl sulfate, 0.005 g / L 2-mercaptobenzimidazole, 0.05 g / L polyethylene glycol, 0.1 g / L sodium chloride, and 0.15 g / L sodium polydisulfide propane sulfonate, and the concentration of the concentrated sulfuric acid is 98 wt.%.

[0060] In the present invention, the thickness of the first Ni plating layer is preferably 4-5 μm, the thickness of the second Cu plating layer is preferably 7-10 μm, specifically 7, 8, 9 or 10 μm, and the thickness of the third Ni plating layer is preferably 4-5 μm.

[0061] In the present invention, the total thickness of the first Ni plating layer, the second Cu plating layer and the third Ni plating layer is preferably 15-20 μm, specifically 15, 16, 17, 18, 19 or 20 μm.

[0062] After obtaining the magnet with coating, the present invention sequentially performs vacuum thermal diffusion and thermal oxidation treatment on the magnet with coating to obtain the multi-layer coating on the surface of NdFeB.

[0063] In the present invention, the temperature of the vacuum thermal diffusion is preferably 150-500°C, specifically 150, 200, 250, 300, 350, 400, 450 or 500°C, and the time is preferably 30-300min, specifically 30, 150 or 300min. Controlling the time and temperature within the above range is beneficial to controlling the thickness of the diffusion layer, avoiding complete mutual diffusion leading to Cu precipitation on the surface of the coating, thereby causing a decrease in the corrosion resistance of the coating, and avoiding a significant decrease in magnetic properties during the heat treatment process; during the vacuum thermal diffusion process, the first Ni coating, the second Cu coating and the third Ni coating diffuse with each other to form a Ni-Cu diffusion layer, thereby significantly improving the corrosion resistance and coating bonding of the coating, and the magnetic properties will not decrease.

[0064] In the present invention, the vacuum thermal diffusion is preferably performed in a vacuum thermal diffusion furnace under a protective atmosphere, and the protective atmosphere is preferably argon.

[0065] In the present invention, the temperature of the thermal oxidation treatment is preferably 150-500°C, specifically 150, 200, 250, 300, 350, 400, 450 or 500°C, and the time is preferably 30-300min, specifically 30, 150 or 300min. Controlling the time and temperature within the above range is beneficial to controlling the thickness and integrity of the formed surface passivation film, while avoiding a significant decrease in magnetic properties during the thermal oxidation treatment. The thermal oxidation treatment can form a stable NiO passivation film, thereby improving corrosion resistance.

[0066] In the present invention, the thermal oxidation treatment is preferably performed in an oxygen atmosphere.

[0067] The present invention also provides a multi-layer coating on the surface of NdFeB obtained by the preparation method described in the above technical solution.

[0068] The technical solutions in the present invention will be described clearly and completely below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0069] Test methods in examples and comparative examples 1: Electrochemical test (corrosion current density and corrosion potential) The electrochemical performance of the magnet was tested using a Shanghai Chenhua CHI760E electrochemical workstation in a 3.5wt.% NaCl solution. First, the open circuit potential (OCPT) of the pulsed electrodeposition magnet was measured for 1200s until the open circuit potential stabilized. Then the potentiodynamic polarization curve (Tafel) of the sample was measured, and the measurement interval near the open circuit potential was selected for testing, with a scan rate of 1mV / s.

[0070] 2: Neutral salt spray test Carry out in accordance with the provisions of GB / T 10125.

[0071] 3: Plating oxidation weight gain Carry out according to the provisions of GB / T 6461-2002.

[0072] 4: Pressure immersion corrosion test The sample was suspended in a pressure tank filled with 3.5wt% NaCl aqueous solution, and air was introduced and constant pressure (0.2MPa) was maintained for a certain period of time.

[0073] 5: Coating thickness Proceed according to the provisions of GB / T 13452.2-200 Method 4A or Method 4B.

[0074] Example 1 (1) The NdFeB magnet is chamfered, degreased, deoxidized and ultrasonically cleaned in sequence to obtain a magnet to be plated. The degreaser used for degreasing includes the following components: 10 g / L sodium hydroxide, 45 g / L sodium carbonate, 60 g / L sodium phosphate, 1 g / L OP-10 emulsifier, and 0.2 g / L sodium dodecyl sulfate. The deoxidation layer includes the following steps: first, in the pickling solution (HNO 3 The samples were acid washed for 30 s in 10 wt % H 2 SO 4 The samples were activated in the solution for 10 s and the ultrasonic frequency of ultrasonic cleaning was 20 Hz.

[0075] (2) The magnet to be plated is immersed in a first nickel plating solution for a first electrodeposition to form a first Ni plating layer, and then immersed in a copper plating solution for a second electrodeposition to form a second Cu plating layer, and then immersed in a second nickel plating solution for a third electrodeposition to form a third Ni plating layer, thereby obtaining a magnet with a coating, wherein the first nickel plating solution and the second nickel plating solution have the same composition, and both include the following components in concentrations: 200 g / L nickel sulfate, 30 g / L nickel chloride, 35 g / L boric acid, 0.7 g / L o-benzoylsulfonyl imide, 0.35 g / L 1,4-butynediol, 0.1 g / L coumarin, 0.1 g / L formaldehyde, 0.1 g / L sodium dodecyl sulfate, and the first nickel plating solution and the pH value of the second nickel plating solution is 5, the solvents of the first nickel plating solution and the second nickel plating solution are both water, the copper plating solution includes the following components in concentration: copper sulfate 230g / L, concentrated sulfuric acid (concentration is 98wt%) 60g / L, ethylene thiourea 0.01g / L, sodium dodecyl sulfate 0.10g / L, 2-mercaptobenzimidazole 0.005g / L, polyethylene glycol 0.05g / L, sodium chloride 0.1g / L, poly (sodium disulfide propane sulfonate) 0.15g / L, the solvent of the copper plating solution is water, the first electrodeposition, the second electrodeposition and the third electrodeposition are all carried out under the conditions of a constant magnetic field and a bidirectional pulsed electric field, and the parameters of the bidirectional pulsed electric field include: a current density of 6A / dm 2 , the operating frequency is 0, 400, 600 or 800 Hz, the duty cycle is 0, 40%, 60% or 80%, the magnetic field strength of the constant magnetic field is 0, 0.4, 0.6 or 0.8 T, the temperature of the first electrodeposition, the second electrodeposition and the third electrodeposition is 40°C, the first electrodeposition, the second electrodeposition and the third electrodeposition are carried out under ultrasonic conditions, and the frequency of the ultrasound is 0, 20, 40 or 60 Hz.

[0076] Table 1 shows the corrosion resistance results of coated magnets obtained with different magnetic field intensities, bidirectional pulsed electric fields and ultrasonic frequencies during electrodeposition. It can be seen that the corrosion resistance of coated magnets obtained with different magnetic field intensities, bidirectional pulsed electric field parameters and ultrasonic frequencies is different. The best corrosion resistance is achieved when the magnetic field intensity is 0.6T, the working frequency of the bidirectional pulsed electric field is 600Hz, the duty cycle is 80% and the ultrasonic frequency is 20Hz.

[0077] Table 1 Corrosion resistance of coated magnets obtained with different magnetic field intensities, parameters of bidirectional pulsed electric fields and ultrasonic frequencies

[0078] (3) The coated magnet obtained in No. 5 was subjected to vacuum thermal diffusion and argon gas was introduced. The temperature of the vacuum thermal diffusion was 0, 150, 300 or 450°C and the time was 0, 30, 150 or 300 min. The corrosion resistance of the coating after different vacuum thermal diffusions is shown in Table 2. It can be seen that the corrosion resistance of the coating after different vacuum thermal diffusions is different. The corrosion resistance of the coating after the vacuum thermal diffusion temperature is 300°C and the vacuum thermal diffusion time is 300 min is the best.

[0079] Table 2 Corrosion resistance of coatings after different vacuum thermal diffusion

[0080] (4) The coating of No. 15 after vacuum thermal diffusion is introduced into oxygen for thermal oxidation treatment. The thermal oxidation treatment temperature is 0, 150, 300 or 450°C, and the time is 0, 30, 150 or 300 min to obtain the NdFeB surface multilayer coating. The corrosion resistance test of the NdFeB surface multilayer coating after different thermal oxidation treatments is shown in Table 3. It can be seen that the corrosion resistance of the NdFeB surface multilayer coating after different thermal oxidation treatments is different. The corrosion resistance of the NdFeB surface multilayer coating after the thermal oxidation treatment temperature is 150°C and the thermal oxidation treatment time is 300 min is the best.

[0081] Table 3 Corrosion resistance of multilayer coatings on NdFeB surface after different thermal oxidation treatments

[0082] Figure 2 This is the cross-sectional morphology of the coating obtained with No. 15. Figure 3 is the element distribution diagram of Cu in the Ni-Cu diffusion layer, Figure 4 is the element distribution diagram of Ni in the Ni-Cu diffusion layer, Figure 5 This is the cross-sectional morphology of the coating (without Ni-Cu diffusion layer) obtained in sequence number 18. Figure 6 is the element distribution diagram of Cu in the Ni-Cu diffusion layer. Figure 7 is the element distribution diagram of Ni in the Ni-free Cu diffusion layer. The results show that after vacuum heat treatment, thermal diffusion will occur in the first Ni plating layer, the second Cu plating layer and the third Ni plating layer, realizing the mutual injection of atoms between the multi-layer plating layers to form a Ni-Cu diffusion layer.

[0083] The magnetic properties and bonding strength of the multilayer coatings on the surface of NdFeB obtained from No. 18 (without vacuum thermal diffusion and thermal oxidation treatment) and No. 21 were tested. See Table 4. It can be seen that there is no significant change in the magnetic properties before and after the heat treatment, but the bonding strength of the coating is improved after the thermal oxidation treatment.

[0084] Table 4 Performance test results of NdFeB surface multilayer coatings obtained from No. 18 and No. 21

[0085] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any form. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be regarded as the protection scope of the present invention.

Claims

1. A method for preparing a multilayer coating on the surface of NdFeB, characterized in that: The following steps are involved: Forming a first Ni coating, a second Cu coating and a third Ni coating in sequence on the surface of the NdFeB magnet to obtain a magnet with coatings; The magnet with coating is subjected to vacuum thermal diffusion treatment and thermal oxidation treatment in sequence to obtain the NdFeB surface multilayer coating.

2. The preparation method according to claim 1, characterized in that: The vacuum thermal diffusion is carried out at a temperature of 150-500° C. and for a time of 30-300 min.

3. The preparation method according to claim 1, characterized in that: The temperature of the thermal oxidation treatment is 150-500° C. and the time is 30-300 min.

4. The preparation method according to claim 1 or 3, characterized in that: The thermal oxidation treatment is performed in an oxygen atmosphere.

5. The preparation method according to claim 1, characterized in that: The forming of the first Ni plating layer, the second Cu plating layer and the third Ni plating layer is all carried out under the conditions of a constant magnetic field and a variable frequency electric field.

6. The preparation method according to claim 5, characterized in that: The parameters of the variable frequency electric field include: forward current density of 6~10A / dm 2 , operating frequency is 400~800Hz, duty cycle is 40~80%; reverse current density is 0.2~1A / dm 2 , the operating frequency is 400~800Hz, and the duty cycle is 40~80%.

7. The preparation method according to claim 5, characterized in that: The magnetic field strength of the constant magnetic field is 0.4~0.8T.

8. The preparation method according to claim 1, characterized in that: The nickel plating solution for forming the first Ni plating layer and the third Ni plating layer independently includes the following components in concentration: 180-280 g / L of nickel sulfate, 20-40 g / L of nickel chloride, 30-40 g / L of boric acid, 0.6-1 g / L of o-benzoylsulfonyl imide, 0.2-0.5 g / L of 1,4-butynediol, 0.1-0.2 g / L of coumarin, 0.1-0.2 g / L of formaldehyde, and 0.05-0.1 g / L of sodium dodecyl sulfate. The pH values ​​of the first nickel plating solution and the second nickel plating solution are independently 2-6.

9. The preparation method according to claim 1, characterized in that: The copper plating solution for forming the second Cu plating layer includes components of the following concentrations: 180-280 g / L copper sulfate, 40-80 g / L concentrated sulfuric acid, 0.006-0.01 g / L ethylene thiourea, 0.08-0.18 g / L sodium dodecyl sulfate, 0.001-0.1 g / L 2-mercaptobenzimidazole, 0.05-0.1 g / L polyethylene glycol, 0.06-0.15 g / L sodium chloride, and 0.018-0.18 g / L sodium polydisulfide dipropane sulfonate, wherein the mass fraction of the concentrated sulfuric acid is 98%.

10. The multi-layer coating on the surface of NdFeB prepared by the preparation method according to any one of claims 1 to 9.

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