Neodymium-iron-boron magnet surface treatment method and neodymium-iron-boron magnet

Abstract

The invention provides a neodymium-iron-boron magnet surface treatment method and a neodymium-iron-boron magnet, and belongs to the technical field of rare earth permanent magnet material preparation. The surface treatment method comprises the steps that a to-be-electroplated magnet with the clean and rough surface is prepared; connecting with a power supply, soaking in a plating solution, and forming a corrosion-resistant plating layer on the surface of the to-be-electroplated magnet through an ion exchange electrodeposition method; wherein the ion exchange electrodeposition method comprises a first stage of synchronously applying a constant magnetic field and a variable-frequency electric field and a second stage of synchronously applying a rotating magnetic field and a variable-frequency electric field, the intensity of the constant magnetic field and the rotating magnetic field is 1000-3000 gs, and the rotating speed of the rotating magnetic field is 20-100 rpm / min. According to the neodymium-iron-boron magnet surface treatment method provided by the invention, a variable-frequency electric field and magnetic field synergistic enhanced ion exchange deposition technology is adopted to improve the deposition efficiency and corrosion resistance of a plating layer, and the neodymium-iron-boron magnet with an excellent corrosion resistance effect is efficiently prepared.

Description

Technical Field

[0001] The invention belongs to the technical field of rare earth permanent magnetic material preparation, and particularly relates to a surface treatment method for a neodymium iron boron magnet and the neodymium iron boron magnet. Background Art

[0002] NdFeB permanent magnet material has the characteristics of high remanence, high magnetic energy product, and high intrinsic coercivity. It is one of the permanent magnet materials with the strongest magnetic properties. Rare earth permanent magnet materials are widely used in new energy vehicles, maglev trains, wind power generation, energy-saving home appliances and other fields. At present, the performance and stability requirements of rare earth permanent magnet materials are getting higher and higher. Due to the special composition and structure of NdFeB permanent magnet materials, their corrosion resistance is poor. Therefore, the study of the corrosion resistance of NdFeB permanent magnet materials is one of the keys to solving the performance, stability and safety of NdFeB permanent magnet materials.

[0003] At present, the surface treatment of NdFeB magnetic materials in industrial applications usually adopts electrodeposition technology. Generally, electrodeposition technology only uses current density as a control factor, but it is often restricted by the diffusion rate of ions in the solution, which in turn affects the deposition efficiency. An external magnetic field helps to increase the ion diffusion rate and deposition efficiency, but the magnetoelectric effect generated by the continuous magnetic field will affect the consistency of the coating. Summary of the invention

[0004] In view of this, the object of the present invention is to provide a surface treatment method for NdFeB magnets and NdFeB magnets, aiming to solve at least one technical problem in the background technology.

[0005] The present invention is achieved in that: A first aspect of the present invention provides a method for surface treatment of a NdFeB magnet, the method comprising the following steps: Pre-treating the NdFeB magnets to obtain magnets with clean and rough surfaces to be electroplated; Connecting the magnet to be electroplated to a power source and immersing it in a plating solution, and forming a corrosion-resistant coating on the surface of the magnet to be electroplated by an ion exchange electrodeposition method; Among them, the ion exchange electrodeposition method includes two stages. In the first stage, a constant magnetic field and a variable frequency electric field are applied synchronously; in the second stage, a rotating magnetic field and a variable frequency electric field are applied synchronously. The intensities of the constant magnetic field and the rotating magnetic field are 1000gs~3000gs, and the rotation speed of the rotating magnetic field is 20rpm / min~100rpm / min.

[0006] Furthermore, the variable frequency electric field conditions include: the current density of the forward pulse is 8A / dm 2 , the operating frequency is 400Hz~800Hz, the duty cycle is 20%~60%; the current density of the reverse pulse is 0.2A / dm 2, the operating frequency is 400Hz and the duty cycle is 20%.

[0007] Furthermore, the first stage processing time is 10 minutes to 15 minutes; the second stage processing time is 10 minutes to 15 minutes.

[0008] Furthermore, the processing temperature of the ion exchange electrodeposition method is 45°C±5°C.

[0009] Furthermore, the NdFeB magnet pretreatment specifically includes: First, place the NdFeB magnet in an alkaline cleaning solution at 65℃±5℃, stir to remove the oil stains on the NdFeB magnet, and then clean it with water; Then the NdFeB magnet is placed in HNO 3 Soak in the solution for 30s±5s, then rinse with water; Then put the NdFeB magnet into H 2 SO 4 Soak in the solution for 30s±5s, clean with water and then ultrasonically clean with anhydrous ethanol to obtain a magnet to be electroplated with a clean and rough surface.

[0010] Furthermore, the alkaline washing solution includes NaOH, Na 2 CO 3 、Na 3 PO 4 , emulsifier and sodium lauryl sulfate.

[0011] The second aspect of the present invention provides a NdFeB magnet obtained by surface treatment using the above method.

[0012] Compared with the prior art, the present invention has the following beneficial effects: The present invention provides a surface treatment method for NdFeB magnets, which uses variable frequency electric field and magnetic field to synergistically strengthen ion exchange deposition technology to improve the deposition efficiency and corrosion resistance of the coating, and efficiently prepares NdFeB magnets with excellent corrosion resistance. The present invention uses enhanced ion exchange deposition technology to apply a magnetic field and an electric field to the magnet to be plated in two stages, and applies a constant magnetic field and an electric field in the first stage to improve the coating quality and deposition efficiency, and applies a rotating magnetic field and a variable frequency electric field in the second stage to improve the coating flatness. DETAILED DESCRIPTION In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific implementation cases described herein are only used to explain the present invention and are not used to limit the present invention.

[0013] A method for treating the surface of a NdFeB magnet, the method comprising steps 1 to 4: Step 1, alkali washing pretreatment of NdFeB magnets; The NdFeB magnets in the present invention are sintered NdFeB magnets, including but not limited to sintered NdFeB magnets of grades N40, N45, N45H, and N38SH; First, place the NdFeB magnet in an alkaline cleaning solution at 65℃±5℃, stir to remove the oil stains on the NdFeB magnet, and then clean it with water; the alkaline cleaning solution includes NaOH, Na 2 CO 3 、Na 3 PO 4 , emulsifier and sodium lauryl sulfate, not limited to the alkaline washing liquid formula listed; the alkaline washing liquid can also be other liquids allowed in the art; Step 2, pre-treating the NdFeB magnet by acid leaching to obtain a magnet to be electroplated with a clean and rough surface; (1) Place the NdFeB magnet in 6wt% HNO 3 Soak in the solution for 30s±5s, then rinse with water; (2) Place the NdFeB magnet in 3wt% H 2 SO 4 Soak in the solution for 30s±5s, clean with water and then ultrasonically clean with anhydrous ethanol to obtain a magnet to be electroplated with a clean and rough surface.

[0014] Step 3, connecting the magnet to be electroplated to a power source and immersing it in a plating solution, and forming a corrosion-resistant coating on the surface of the magnet to be electroplated by an ion exchange electrodeposition method; The ion exchange electrodeposition method includes two stages. In the first stage, a constant magnetic field with a magnetic field strength of 1000gs~3000gs and a variable frequency electric field are applied simultaneously, and the processing time is 10min~15min; in the second stage, a rotating magnetic field (magnetic field strength of 1000gs~3000gs, rotation speed of 20rpm / min~100rpm / min) and a variable frequency electric field are applied, and the processing time is 10min~15min; the processing temperature of the ion exchange electrodeposition method is 45℃±5℃.

[0015] In the specific implementation, the conditions of the variable frequency electric field include: the current density of the forward pulse is 8A / dm 2 , the operating frequency is 400Hz~800Hz, the duty cycle is 20%~60%; the current density of the reverse pulse is 0.2A / dm 2 , the operating frequency is 400Hz and the duty cycle is 20%.

[0016] The plating solution in step 3 can be any plating solution that can form a corrosion-resistant coating as allowed in the art, including but not limited to nickel-based plating solution, zinc-based plating solution, and lanthanum-based plating solution. The following embodiment is illustrated by a nickel-based plating solution, which contains at least the following components in concentration: nickel sulfate 180g / L~280g / L; nickel chloride 20g / L~40g / L; boric acid 30g / L~40g / L; sodium dodecyl sulfate 0.05g / L~0.1g / L; pH value 2~6 is adjusted by ammonia water. The plating solution specifically uses: nickel sulfate 220g / L; nickel chloride 30g / L; boric acid 35g / L; sodium dodecyl sulfate 0.07g / L; pH=4 is adjusted by ammonia water, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0017] Step 4: After the ion exchange electrodeposition is completed, the NdFeB magnet is taken out, cleaned with water and dried to obtain a NdFeB magnet with a corrosion-resistant coating formed on the surface.

[0018] Example 1 This embodiment 1 is a method for surface treatment of NdFeB magnets, using unmagnetized N40 commercial sintered NdFeB magnets as prepared magnets, and specifically includes the following steps: 1. First, place the N40 magnet in an alkaline cleaning solution at 65℃±5℃ and stir to remove oil for 10 minutes. After cleaning and removing oil, rinse it with deionized water. The alkaline cleaning formula is NaOH 7.5g / L; Na 2 CO 3 45g / L; Na 3 PO 4 50g / L; emulsifier OP-10 2g / L; sodium lauryl sulfate 0.2g / L; 2. Place the magnet treated in step 1 into 6wt% HNO 3 The solution was soaked for 30 seconds, and then cleaned with deionized water; 3wt% H 2 SO 4 The surface of the sintered NdFeB magnet was pretreated by immersing it in the solution for 30 seconds, washing it with deionized water, and then ultrasonically washing it with anhydrous ethanol for 3 minutes. 3. Place the pretreated sintered NdFeB magnet in a nickel-based electrolyte, connect the magnet to the negative electrode, and use the nickel sheet as the positive electrode. Control the electrolyte temperature to 45°C ± 5°C, and perform two-stage ion exchange deposition to plate nickel on the surface of the sintered NdFeB magnet. (1) In the first stage of ion exchange deposition, an external constant magnetic field and a variable frequency electric field are applied, and the magnetic field strength is 3000 gs; the parameters of the variable frequency electric field include: the current density of the forward pulse is 8 A / dm 2 , the operating frequency is 600Hz, the duty cycle is 40%, and the current density of the reverse pulse is 0.2A / dm 2, the operating frequency is 400Hz, the duty cycle is 20%, and the processing time is 10min; (2) In the second stage of ion exchange deposition, the constant magnetic field is adjusted to a rotating magnetic field, that is, a rotating magnetic field and a variable frequency electric field are applied simultaneously. The magnetic field parameters include: the magnetic field strength is 3000gs, the rotation speed is 60rpm / min; the parameters of the variable frequency electric field include: the current density of the forward pulse is 8A / dm 2 , the operating frequency is 600Hz, the duty cycle is 40%; the current density of the reverse pulse is 0.2A / dm 2 , the operating frequency is 400Hz, the duty cycle is 20%, and the processing time is 10min; 4. Finally, the sintered NdFeB magnet with nickel plating on the surface is placed in deionized water at 50°C for cleaning, and then dried in an environment of 60°C to obtain a NdFeB magnet with a corrosion-resistant coating formed on the surface.

[0019] Example 2 This embodiment 2 provides a surface treatment method for NdFeB magnets, which differs from embodiment 1 only in that the strengths of the constant magnetic field and the rotating magnetic field in step 3 ion exchange deposition are both adjusted to 2000 gs, and other parameters and conditions are the same as those in embodiment 1.

[0020] Example 3 This embodiment 3 provides a surface treatment method for NdFeB magnets, which differs from embodiment 1 only in that the strengths of the constant magnetic field and the rotating magnetic field in step 3 ion exchange deposition are both adjusted to 1000 gs, and other parameters and conditions are the same as those in embodiment 1.

[0021] Example 4 This embodiment 4 provides a surface treatment method for NdFeB magnets, which differs from embodiment 1 only in that the rotation speed of the rotating magnetic field in step 3 ion exchange deposition is adjusted to 20 rpm / min, and other parameters and conditions are the same as those in embodiment 1.

[0022] Example 5 This embodiment 5 provides a surface treatment method for NdFeB magnets, which differs from embodiment 1 only in that the rotation speed of the rotating magnetic field in step 3 ion exchange deposition is adjusted to 100 rpm / min, and other parameters and conditions are the same as those in embodiment 1.

[0023] Comparative Example 1 The difference between this comparative example 1 and example 2 is that no magnetic field is applied during the entire ion exchange electrodeposition stage in step 3, and other parameters and conditions are the same as those in example 2.

[0024] Step 3 of this comparative example 1 is specifically as follows: Place the pretreated sintered NdFeB magnet in a nickel-based electrolyte, connect the magnet to the negative electrode, and use the nickel sheet as the positive electrode. Control the electrolyte temperature to 45℃±5℃ and set the current density of the forward pulse to 8A / dm 2 , the operating frequency is 600Hz, the duty cycle is 40%, and the current density of the reverse pulse is 0.2A / dm 2 , the operating frequency is 400Hz, the duty cycle is 20%, and the processing time is 20min.

[0025] Comparative Example 2 The difference between this comparative example 2 and example 2 is that a constant magnetic field is applied during the entire ion exchange electrodeposition stage in step 3, and other parameters and conditions are the same as those in example 2.

[0026] Step 3 of this comparative example 2 is specifically as follows: Place the pretreated sintered NdFeB magnet in a nickel-based electrolyte, connect the magnet to the negative electrode, and use the nickel sheet as the positive electrode. Control the electrolyte temperature to 45°C ± 5°C, and apply an external magnetic field and variable frequency electric field. The magnetic field strength is 2000gs; the frequency conversion electric field parameters include: the current density of the positive pulse is 8A / dm 2 , the operating frequency is 600Hz, the duty cycle is 40%, and the current density of the reverse pulse is 0.2A / dm 2 , the operating frequency is 400Hz, the duty cycle is 20%, and the total processing time is 20min.

[0027] Comparative Example 3 The difference between this comparative example 3 and example 2 is that a rotating magnetic field is applied during the entire ion exchange electrodeposition stage in step 3, and other parameters and conditions are the same as those in example 2.

[0028] Step 3 of this comparative example 3 is specifically as follows: Place the pretreated sintered NdFeB magnet in a nickel-based electrolyte, connect the magnet to the negative electrode, and use the nickel sheet as the positive electrode. Control the electrolyte temperature to 45°C ± 5°C, and apply an external rotating magnetic field and variable frequency electric field. The parameters of the rotating magnetic field include: the magnetic field strength is 2000gs, the speed is 60rpm / min; the parameters of the variable frequency electric field include: the current density of the forward pulse is 8A / dm 2 , the operating frequency is 600Hz, the duty cycle is 40%, and the current density of the reverse pulse is 0.2A / dm 2 , the operating frequency is 400Hz, the duty cycle is 20%, and the total processing time is 20min.

[0029] Comparative Example 4 The difference between Comparative Example 4 and Example 2 is that a constant magnetic field is applied only in the second stage of ion exchange electrodeposition in step 3, and other parameters and conditions are the same as those in Example 2.

[0030] Step 3 of this comparative example 4 is specifically as follows: The pretreated sintered NdFeB magnet was placed in a nickel-based electrolyte, with the magnet connected to the negative electrode and the nickel sheet as the positive electrode. The electrolyte temperature was controlled at 45°C ± 5°C, and two stages of ion exchange deposition were performed; (1) In the first stage of ion exchange deposition, only a variable frequency electric field is applied, and its parameters are: the current density of the forward pulse is 8A / dm 2 , the operating frequency is 600Hz, the duty cycle is 40%; the current density of the reverse pulse is 0.2A / dm 2 , the operating frequency is 400Hz, the duty cycle is 20%, and the processing time is 10min; (2) In the second stage of ion exchange deposition, an external constant magnetic field and a variable frequency electric field are applied, and the magnetic field strength is 2000 gs; the parameters of the variable frequency electric field include: the current density of the forward pulse is 8A / dm 2 , the operating frequency is 600Hz, the duty cycle is 40%, and the current density of the reverse pulse is 0.2A / dm 2 , the operating frequency is 400Hz, the duty cycle is 20%, and the processing time is 10min.

[0031] Comparative Example 5 The difference between this comparative example 5 and example 2 is that a rotating magnetic field is applied only in the second stage of ion exchange electrodeposition in step 3, and other parameters and conditions are the same as those in example 2.

[0032] Step 3 of this comparative example 5 is specifically as follows: The pretreated sintered NdFeB magnet was placed in a nickel-based electrolyte, with the magnet connected to the negative electrode and the nickel sheet as the positive electrode. The electrolyte temperature was controlled at 45°C ± 5°C, and two stages of ion exchange deposition were performed; (1) In the first stage of ion exchange deposition, only a variable frequency electric field is applied, and its parameters include: the current density of the forward pulse is 8A / dm 2 , the operating frequency is 600Hz, the duty cycle is 40%; the current density of the reverse pulse is 0.2A / dm 2 , the operating frequency is 400Hz, the duty cycle is 20%, and the processing time is 10min; (2) In the second stage of ion exchange deposition, an external rotating magnetic field and a variable frequency electric field are applied. The magnetic field parameters include: magnetic field strength of 2000 gs, rotation speed of 60 rpm / min; the parameters of the variable frequency electric field include: the current density of the forward pulse is 8 A / dm 2 , the operating frequency is 600Hz, the duty cycle is 40%, and the current density of the reverse pulse is 0.2A / dm 2 , the operating frequency is 400Hz, the duty cycle is 20%, and the processing time is 10min.

[0033] Comparative Example 6 The difference between this comparative example 6 and example 2 is that the strength of the constant magnetic field and the rotating magnetic field in step 3 ion exchange electrodeposition is adjusted to 4000 gs, and the other parameters and conditions are the same as those in example 2.

[0034] At room temperature, the corrosion resistance of the NdFeB magnets prepared in Examples 1 to 5 and Comparative Examples 1 to 6 was tested in a 3.5% NaCl solution using an electrochemical workstation. The thickness of the coating was tested using an X-ray fluorescence multilayer thickness gauge and the deposition efficiency was calculated, where deposition efficiency = coating thickness / total deposition time. The parameter data are shown in Table 1. Table 1

[0035] Self-corrosion potential E corr The more negative, the higher the activity and the faster the electrochemical corrosion. v and the self-corrosion current density I corr The relationship between the two can be referred to by the formula v =A×I corr / (n×F), where A is the atomic weight of the metal to be corroded; n is the chemical valence of the metal; F is the Faraday constant, and the self-corrosion current density I corr The larger the value, the faster the electrochemical corrosion; the coating impedance value is related to the charge transfer resistance R in the equivalent circuit ct The greater the impedance value, the better the corrosion resistance of the coating.

[0036] From the data in Table 1, it can be seen that the corrosion-resistant coating of more than 20 μm can be quickly prepared in Examples 1 to 5 of the present invention in 20 minutes, which greatly improves the deposition efficiency and has strong corrosion resistance. corr Increase, self-corrosion current density decreases, charge transfer resistance R ct Increase, indicating its excellent corrosion resistance.

[0037] From the data of Examples 1 to 3, it can be seen that as the magnetic field strength increases, the corrosion current density I corr It first decreases and then increases. With the increase of magnetic field strength, the corrosion resistance of the coating first increases and then decreases, and the deposition rate increases. The reason is that applying a magnetic field in the same direction as the migration direction of nickel ions during nickel deposition can improve the deposition efficiency of nickel ions, but excessive magnetic field strength may affect the quality of the coating.

[0038] From the data of Examples 1, 4 and 5, it can be seen that as the magnetic field speed increases, the corrosion current density I corrThe deposition rate increases by first decreasing and then increasing; the reason is that increasing the magnetic field rotation speed can improve the deposition efficiency, but too high a rotation speed will interfere with the electric field, resulting in a decrease in coating performance.

[0039] From the comparison between Comparative Example 1 and Example 2, it can be seen that when no magnetic field is applied, the deposition efficiency of the electroplating layer is greatly reduced, and the corrosion resistance is reduced, which shows that a suitable magnetic field can help improve the deposition efficiency and performance of the coating.

[0040] By comparing Comparative Example 2 and Example 2, it can be seen that when a constant magnetic field is applied throughout the entire electrodeposition stage, the efficiency of the electroplating layer deposition increases, but the flatness deteriorates and the corrosion resistance decreases. The reason is that applying a magnetic field throughout the entire stage can help nickel ions migrate to the electrode plating surface, but has an adverse effect on the surface leveling process of current reversal.

[0041] By comparing Comparative Example 3 and Example 2, it can be seen that when a rotating magnetic field is applied during the entire electroplating stage, the deposition efficiency of the electroplating layer decreases and the corrosion resistance is reduced, indicating that applying a rotating magnetic field during the entire stage is likely to cause defects in the coating and affect the coating performance.

[0042] By comparing Comparative Example 4 and Example 2, it can be seen that when only a constant magnetic field is applied in the second stage of electrodeposition, the efficiency of electroplating layer deposition is reduced, the flatness is significantly reduced, and the corrosion resistance is reduced. The reason is that the application of the magnetic field may cause a certain interference to the electric field, resulting in a decrease in the flatness of the coating surface, and the application of the magnetic field in the second stage will cause the decrease in the flatness of the coating to be unable to be leveled by the reverse electric field.

[0043] It can be seen from the comparison between comparative example 5 and embodiment 2 that the deposition efficiency of the electroplated layer decreases significantly when only the rotating magnetic field is applied in the second stage of electrodeposition, which is not much different from the deposition efficiency without applying the magnetic field in comparative example 1. At the same time, the low thickness of the coating is also likely to lead to a decrease in coating performance.

[0044] By comparing Comparative Example 6 with Example 2, it can be seen that when the magnetic field strength increases to 4000gs, the efficiency of electroplating deposition increases, the flatness decreases, and the corrosion resistance decreases. The reason is that excessive variable frequency magnetic field strength will cause certain interference to the electric field, resulting in a decrease in plating performance.

[0045] The above-mentioned embodiments only express several implementation methods of the present invention, and the description thereof is relatively specific and detailed, but it cannot be understood as limiting the scope of the patent of the present invention. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached claims.

Claims

1. A method for surface treatment of NdFeB magnets, characterized in that: The method comprises the following steps: Pre-treating the NdFeB magnets to obtain magnets with clean and rough surfaces to be electroplated; Connecting the magnet to be electroplated to a power source and immersing it in a plating solution, and forming a corrosion-resistant coating on the surface of the magnet to be electroplated by an ion exchange electrodeposition method; Among them, the ion exchange electrodeposition method includes two stages. In the first stage, a constant magnetic field and a variable frequency electric field are applied synchronously; in the second stage, a rotating magnetic field and a variable frequency electric field are applied synchronously. The intensities of the constant magnetic field and the rotating magnetic field are 1000gs~3000gs, and the rotation speed of the rotating magnetic field is 20rpm / min~100rpm / min.

2. A method for surface treatment of NdFeB magnets according to claim 1, characterized in that: The conditions of the variable frequency electric field include: the current density of the forward pulse is 8A / dm 2 , the operating frequency is 400Hz~800Hz, the duty cycle is 20%~60%; the current density of the reverse pulse is 0.2A / dm 2 , the operating frequency is 400Hz and the duty cycle is 20%.

3. A method for surface treatment of NdFeB magnets according to claim 1, characterized in that: The first stage treatment time is 10min to 15min; the second stage treatment time is 10min to 15min.

4. A method for surface treatment of NdFeB magnets according to claim 1, characterized in that: The processing temperature of the ion exchange electrodeposition method is 45°C±5°C.

5. The method for surface treatment of NdFeB magnets according to claim 1, characterized in that: The NdFeB magnet pretreatment specifically includes: First, place the NdFeB magnet in an alkaline cleaning solution at 65℃±5℃, stir to remove the oil stains on the NdFeB magnet, and then clean it with water; Then soak the NdFeB magnet in HNO3 solution for 30s±5s, and then clean it with water; Then, the NdFeB magnet is immersed in the H2SO4 solution for 30s±5s, cleaned with water and then ultrasonically cleaned with anhydrous ethanol to obtain a magnet to be electroplated with a clean and rough surface.

6. A method for surface treatment of NdFeB magnets according to claim 5, characterized in that: The alkaline washing solution comprises NaOH, Na2CO3, Na3PO4, an emulsifier and sodium dodecyl sulfate.

7. NdFeB magnet, characterized in that: The surface treatment is carried out by the surface treatment method for NdFeB magnets according to any one of claims 1 to 6.

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