Preparation method of neodymium iron boron sintered permanent magnet surface coating

Abstract

The invention discloses a preparation method of a neodymium-iron-boron sintered permanent magnet surface coating, which comprises the following technological processes: oil removal, water washing, acid pickling, water washing, ultrasonic dust removal, water washing, phosphorization, water washing, sealing, water washing and blow-drying and curing. A phosphating solution adopted in phosphating is formed by compounding a first phosphating solution, a second phosphating solution and a nano dispersing agent, the mass ratio of the first phosphating solution to the second phosphating solution to the nano dispersing agent is (0.5-1.5): (0.5-1.5): (1-3), the first phosphating solution comprises zinc dihydrogen phosphate, manganese acid phosphate, phosphoric acid, zinc nitrate, cobalt salt, a surfactant, nickel sulfate and a corrosion inhibitor, and the second phosphating solution comprises phosphoric acid, zinc oxide, cobalt salt, fluoride and a complexing agent. According to the method, high-efficiency and high-quality preparation of the surface coating of the permanent magnet can be achieved, the corrosion resistance of the permanent magnet is remarkably improved on the premise that the magnetic performance of the permanent magnet is not affected and the production cost is not increased, and meanwhile the unfilled corner area and the unfilled corner proportion of a product are effectively reduced.

Description

Technical Field

[0001] The invention belongs to the field of NdFeB sintered permanent magnets, and in particular relates to a method for preparing a surface coating of a NdFeB sintered permanent magnet. Background Art

[0002] As a key functional material, NdFeB sintered permanent magnets are widely used in high-tech fields such as electronics and information technology, medical equipment, new energy vehicles, household appliances, and robotics. In most applications, NdFeB sintered permanent magnets are installed in the rotors of compressors and motors. These devices often operate in harsh environments such as high temperature, high humidity, and weak acidity or salinity. Furthermore, due to the high brittleness of NdFeB sintered permanent magnets, they are prone to hydrogen embrittlement after pickling, resulting in significant chipping. While minor chipping will not significantly affect the overall performance of compressor and motor rotors, excessive chipping or excessive number of chippings can still negatively impact the equipment's operational reliability and service life. Therefore, effectively improving the corrosion resistance of NdFeB sintered permanent magnets and reducing the area and proportion of chipping are crucial.

[0003] Sintered NdFeB permanent magnets are porous and loose, and become hard and brittle after high-temperature sintering. This makes it difficult to meet the requirements for high corrosion resistance and zero chipping. The industry currently uses two main technical approaches to improve this problem: optimizing the surface coating process to enhance corrosion resistance, and increasing the powder particle size to reduce chipping defects. However, both approaches have significant limitations. The coating process is often subject to customer-specified technical specifications and involves cost factors, making major changes difficult. While increasing the powder particle size can improve the chipping problem, it can lead to fluctuations in the magnetic properties of the NdFeB sintered permanent magnets (primarily manifested as performance degradation). This performance fluctuation can cause the product to fail to meet design specifications, especially in NdFeB sintered permanent magnets that require the GBD process. Therefore, how to effectively improve the corrosion resistance of NdFeB sintered permanent magnets and reduce the area and ratio of chipping defects, while ensuring stable magnetic properties, has become a technical bottleneck that urgently needs to be overcome in process optimization.

[0004] To address the insufficient corrosion resistance of NdFeB sintered permanent magnets, a Chinese invention patent (application number: 201110411826.3) proposes a method for preparing a particle-reinforced organic anti-corrosion coating on the surface of NdFeB permanent magnets. This method first deposits a phosphate film on the permanent magnet surface as a base protective layer. Subsequently, a particle-reinforced organic composite coating is constructed on top of the phosphate film via a cathodic electrophoresis process, creating a dual-protection system. This technical solution requires a secondary coating, which increases production cycles, complicates material turnover, significantly increases management costs, and requires high employee expertise. Furthermore, this solution does not address the hydrogen embrittlement of NdFeB sintered permanent magnets after pickling, nor does it improve the problem of chipped corners. Summary of the Invention

[0005] In order to achieve the feasibility of large-scale production while taking into account cost-effectiveness and magnetic property stability, the present invention proposes a method for preparing a surface coating of NdFeB sintered permanent magnets. By using a specific phosphating solution and combining it with a specific preparation process, the surface coating of NdFeB sintered permanent magnets can be prepared with high efficiency and high quality through a single phosphating operation. Without affecting the magnetic properties of the NdFeB sintered permanent magnets and without increasing production costs, the corrosion resistance of the NdFeB sintered permanent magnets is significantly improved, and the area and proportion of the missing corners of the product are effectively reduced.

[0006] The present invention solves the above-mentioned technical problem by adopting a technical solution: a method for preparing a surface coating of a sintered NdFeB permanent magnet. The process flow of the preparation method comprises: degreasing → water washing → pickling → water washing → ultrasonic deashing → water washing → phosphating → water washing → sealing → water washing → air drying and curing. The phosphating solution used in the phosphating is prepared by compounding a first phosphating solution, a second phosphating solution and a nano-dispersant. The mass ratio of the first phosphating solution, the second phosphating solution and the nano-dispersant is (0.5-1.5): (0.5-1.5): (1-3). The first phosphating solution comprises zinc dihydrogen phosphate, acid manganese phosphate, phosphoric acid, zinc nitrate, a cobalt salt, a surfactant, nickel sulfate and a corrosion inhibitor. The second phosphating solution comprises phosphoric acid, zinc oxide, a cobalt salt, a fluoride and a complexing agent.

[0007] The present invention utilizes a specific phosphating solution in conjunction with a specific preparation process to achieve efficient, high-quality production of surface coatings for NdFeB sintered permanent magnets through a single phosphating operation. Testing has shown that the method can effectively reduce defects during the production of NdFeB sintered permanent magnets, reducing the proportion of yellowing and chipping in the product to below 2%. Furthermore, without compromising the magnetic properties of the NdFeB sintered permanent magnets or increasing production costs, it significantly improves the corrosion resistance of the magnets while effectively reducing the area and proportion of chipped corners.

[0008] The method of the present invention not only reduces the number of production steps in coating preparation and the risk of human error, but also maintains the stability of product performance, providing a more competitive technical solution for the industrial production of NdFeB sintered permanent magnets. The method of the present invention is simple, low-cost, and produces no waste gas or waste residue. The surface coating produced is high-quality, requires no post-processing, and is environmentally friendly.

[0009] Preferably, the concentration of the phosphating solution is 15%±1%, the phosphating temperature is 40-60°C, and the phosphating time is 1000-1200 s.

[0010] Preferably, the first phosphating solution is a product with a commercial model of FX-022, the second phosphating solution is a product with a commercial model of NPS-05C, and the nano-dispersant is a product with a commercial model of SFT-1388.

[0011] Preferably, the mass ratio of the first phosphating solution, the second phosphating solution and the nano-dispersant is 1:1:2.

[0012] Preferably, the washing between the phosphating and sealing steps is performed with deionized water, with the conductivity of the deionized water being ≤2 μS / cm. The deionized water washing after phosphating can remove residual phosphating solution from the permanent magnet surface, ensuring the quality of the permanent magnet surface coating, improving corrosion resistance, and compatibility with subsequent processes.

[0013] Preferably, the deionized water washing is performed twice to further enhance the phosphating effect and ensure the coating quality.

[0014] Preferably, degreasing powder is added to the degreasing liquid used for degreasing, and the addition ratio of the degreasing powder to the degreasing liquid is 1.00%±0.05%.

[0015] Preferably, the pickling adopts nitric acid with a concentration of 3%±0.1% as the pickling solution.

[0016] Preferably, a deashing agent is added to the deashing liquid used in the ultrasonic deashing, and the addition ratio of the deashing agent to the ultrasonic deashing liquid is 3%±0.1%. The ultrasonic frequency used in the ultrasonic deashing is 28 kHz to 32 kHz.

[0017] Preferably, a sealing agent is added to the sealing liquid used for sealing, and the proportion of the sealing agent in the sealing liquid is 10% ± 1%. The function of the sealing process is to fill the micropores of the phosphate film, forming a dense protective layer, significantly improving the consistency of the coating, thereby improving the corrosion resistance and wear resistance of the permanent magnet, and reducing the watermark yellowing and chipping rate of the product appearance.

[0018] Compared with the existing technology, the present invention has the following advantages: The method for preparing the surface coating of NdFeB sintered permanent magnets utilizes a specific phosphating solution and a specific preparation process flow. This allows for efficient, high-quality preparation of the surface coating of NdFeB sintered permanent magnets through a single phosphating operation. This significantly improves the corrosion resistance of the NdFeB sintered permanent magnets without compromising their magnetic properties or increasing production costs, while effectively reducing the area and proportion of chipped corners in the product. Testing has shown that the method can effectively reduce defects in the production process of NdFeB sintered permanent magnets, reducing the yellowing rate and chipped corner ratio of the product to below 2%. This method reduces the number of production steps involved in coating preparation, lowering the risk of operator error while maintaining product performance stability, providing a more competitive technical solution for the industrial production of NdFeB sintered permanent magnets. The method is simple, low-cost, produces no waste gas or waste residue, produces a high-quality surface coating, requires no post-processing, and is environmentally friendly. DETAILED DESCRIPTION

[0019] The present invention is described in further detail below with reference to the examples.

[0020] Six examples and 12 comparative examples of NdFeB sintered permanent magnets were selected and phosphated with different phosphating solutions to prepare surface coatings. For ease of comparison, the preparation methods of the coatings in the examples and comparative examples differ only in the formulation of the phosphating solution. There are four types of phosphating solutions, each of which contains the same concentration of A, B, C, or D. A is a phosphating solution product of model FX-022 provided by Ningbo Haishu Gaoqiao Yongrun Surface Treatment Agent Business Department, which is a purple-red transparent liquid with a pH value of 1.0-2.0 and a specific gravity of 1.35 g / cm 3 , its composition includes zinc dihydrogen phosphate, acid manganese phosphate, phosphoric acid, zinc nitrate, cobalt salt, surfactant, nickel sulfate and corrosion inhibitor; B is a phosphating liquid product model NPS-05C provided by Ningbo Haishu District Yongfu Chemical Co., Ltd., which has a light brown liquid appearance, a pH value ≥1.5, a relative density relative to water of 1.20, and a boiling point >115°C. Its composition includes phosphoric acid, zinc oxide, cobalt salt, fluoride and a complexing agent; C is a nano-dispersant product model SFT-1388 provided by Dongguan Fit Cleaning Technology Co., Ltd., which has a liquid appearance, a pH value of 6.5~8.5, a boiling point of 150~200°C, a volatility point >55°C, a slight odor, a melting point <-10°C, a combustion point >200°C, and a water solubility at 20°C <2wt%; D is compounded by A, B, and C in a mass ratio of 1:1:2.

[0021] The six embodiments are Examples 1-1, 1-2, 1-3, 1-4, 1-5 and 1-6 using a phosphating solution with the addition of D; the 12 comparative examples are Comparative Examples 1-1, 1-2, 1-3 and 1-4 using a phosphating solution with the addition of A, Comparative Examples 2-1, 2-2, 2-3 and 2-4 using a phosphating solution with the addition of B, and Comparative Examples 3-1, 3-2, 3-3 and 3-4 using a phosphating solution with the addition of C.

[0022] The process flow of the coating preparation method of each embodiment and comparative example includes: degreasing → water washing (1 time) → acid washing → water washing (2 times) → ultrasonic deashing → water washing (1 time) → phosphating → water washing (2 times) → sealing → water washing (2 times) → blow drying and curing. The specific process flow and process parameters are shown in Table 1.

[0023] Table 1: Coating preparation process and process parameters of each embodiment and comparative example

[0024] The degreasing powder used in each comparative example and embodiment is a product provided by Ganzhou Yingteng Environmental Protection Technology Co., Ltd., which has the appearance of a powder and is composed of soda ash, a phosphoric acid chelating agent, an ammonium salt rust inhibitor and a surfactant; the deashing agent is a product model DS-550 provided by Yantai Deshan Chemical Co., Ltd., which is composed of sodium hydroxide (<7.0%), soda ash (<20.0%), triethanolamine (<40.0%) and acrylate polymer (<2.0%); the sealing agent is a product model BW-608 provided by Beijing Ailsmu Technology Co., Ltd., which has the appearance of a light yellow translucent liquid and a specific gravity of 1.150±0.05 g / cm 3 , pH value is 12.0~13.0, no flash point, non-flammable, its composition includes 40% silica gel, 1% nano silicon dioxide, 5% sodium benzoate, 6% soda ash, and 8% triethanolamine.

[0025] The surface-coated NdFeB sintered permanent magnets of the six examples and twelve comparative examples were inspected for yellowing of watermarks and chipping using a CCD (Computerized Device for Appearance and Dimensions) instrument. The yellowing ratio and chipping ratio were calculated. The inspection criteria were as follows: Yellowing of watermark: Through CCD image analysis, the color difference ΔE ≥ 5 (based on a standard white plate) and the area ≥ 1 mm 2 The area is determined to be yellow watermark; Corner chipping: Single corner chipping area ≥ 0.2 mm 2 (Based on CCD profile analysis), and the number of missing corners is counted for each product.

[0026] The types and concentrations of the phosphating solutions used in the coatings of the embodiments and comparative examples, as well as the yellowing ratio and chipping ratio of watermarks on the product surfaces are shown in Table 2.

[0027] Table 2: Type and concentration of phosphating solution used in the coatings of various examples and comparative examples and the ratio of yellowing and chipping of watermarks

[0028] As can be seen from Table 2, the method of the present invention can effectively reduce defects in the production process of NdFeB sintered permanent magnets, reducing the proportion of watermark yellowing in products from 2.25%~5.12% to 0.94%~1.66%, and reducing the proportion of chipped corners in products from 3.03%~11.24% to 1.05%~1.86%.

[0029] For comparative examples 1-2, 2-2, 3-2 and example 1-3, which added phosphating solutions of the same concentration but different formulations, the overall process flow hours, the number of workers required for overall process, and the product production efficiency of their coating preparation are shown in Table 3.

[0030] Table 3: Overall flow time, number of workers required, and product production efficiency for coating preparation in different comparative examples and embodiments

[0031] Combining Tables 2 and 3, it can be seen that the method of the present invention can achieve high-efficiency, high-quality preparation of surface coatings for NdFeB sintered permanent magnets, effectively improving product production efficiency and labor efficiency, and reducing product production costs. Testing has shown that after using the surface coating prepared using the method of the present invention, the magnetic properties of the NdFeB sintered permanent magnets are not affected, the corrosion resistance is significantly improved, and the area and ratio of the missing corners are effectively reduced.

Claims

1. A method for preparing a surface coating of a NdFeB sintered permanent magnet, characterized in that: The preparation method has a process flow comprising: degreasing → water washing → pickling → water washing → ultrasonic deashing → water washing → phosphating → water washing → sealing → water washing → air drying and curing. The phosphating solution used in the phosphating is prepared by compounding a first phosphating solution, a second phosphating solution and a nano-dispersant. The mass ratio of the first phosphating solution, the second phosphating solution and the nano-dispersant is (0.5-1.5): (0.5-1.5): (1-3). The first phosphating solution comprises zinc dihydrogen phosphate, acid manganese phosphate, phosphoric acid, zinc nitrate, cobalt salt, surfactant, nickel sulfate and corrosion inhibitor. The second phosphating solution comprises phosphoric acid, zinc oxide, cobalt salt, fluoride and a complexing agent.

2. The method for preparing a surface coating of a sintered NdFeB permanent magnet according to claim 1, characterized in that: The concentration of the phosphating solution is 15%±1%, the phosphating temperature is 40-60°C, and the phosphating time is 1000-1200 s.

3. The method for preparing a surface coating of a sintered NdFeB permanent magnet according to claim 1, characterized in that: The first phosphating solution is a product with a commercial model of FX-022, the second phosphating solution is a product with a commercial model of NPS-05C, and the nano-dispersant is a product with a commercial model of SFT-1388.

4. The method for preparing a surface coating of a sintered NdFeB permanent magnet according to claim 1, wherein: The mass ratio of the first phosphating solution, the second phosphating solution and the nano-dispersant is 1:1:

2.

5. The method for preparing a surface coating of a sintered NdFeB permanent magnet according to claim 1, wherein: The water washing between the phosphating and the sealing is carried out with deionized water, and the conductivity of the deionized water used is ≤2 μS / cm.

6. The method for preparing a surface coating of a sintered NdFeB permanent magnet according to claim 5, characterized in that: The number of times of washing with deionized water is 2 times.

7. The method for preparing a surface coating of a sintered NdFeB permanent magnet according to claim 1, characterized in that: The degreasing liquid used for the degreasing is added with degreasing powder, and the addition ratio of the degreasing powder to the degreasing liquid is 1.00%±0.05%.

8. The method for preparing a surface coating of a sintered NdFeB permanent magnet according to claim 1, characterized in that: The pickling adopts nitric acid with a concentration of 3%±0.1% as the pickling liquid.

9. The method for preparing a surface coating of a sintered NdFeB permanent magnet according to claim 1, wherein: The ultrasonic deashing liquid used in the ultrasonic deashing contains a deashing agent, the addition ratio of the deashing agent in the ultrasonic deashing liquid is 3%±0.1%, and the ultrasonic frequency used in the ultrasonic deashing is 28 kHz to 32 kHz.

10. The method for preparing a surface coating of a sintered NdFeB permanent magnet according to claim 1, characterized in that: The blocking solution used in the blocking process contains a blocking agent, and the addition ratio of the blocking agent to the blocking solution is 10%±1%.

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