阅读说明：本技术 耐腐蚀的钕铁硼磁体及表面处理方法和羟基化合物的用途 (Corrosion-resistant neodymium-iron-boron magnet, surface treatment method and application of hydroxyl compound ) 是由 张雅文 胡占江 冀平 白继华 张明鑫 董义 袁易 陈雅 袁文杰 于 2020-12-30 设计创作，主要内容包括：本发明公开了耐腐蚀的钕铁硼磁体及表面处理方法和羟基化合物的用途。所述羟基化合物含有C2～C15的烃基,其能够提高钕铁硼磁体的耐腐蚀性。(The invention discloses a corrosion-resistant neodymium iron boron magnet, a surface treatment method and application of a hydroxyl compound. The hydroxyl compound contains C2-C15 hydrocarbyl, and can improve the corrosion resistance of the neodymium iron boron magnet.)

1. The application of the hydroxyl compound in improving the corrosion resistance of the neodymium iron boron magnet is characterized in that the hydroxyl compound has the structure shown in the formula (I):

wherein R is selected from C2-C15 alkyl, and n is a natural number of 1-3.

2. The use according to claim 1, wherein the hydroxy compound is a mono-or di-alcohol, R is selected from C2-C15 alkyl, and n is 1 or 2.

3. The use according to claim 1, wherein the hydroxy compound is a monohydric alcohol, R is selected from the group consisting of C3-C6 alkyl groups, and n is 1.

4. Use according to claim 1, wherein the hydroxy compound is ethanol, n-propanol, isopropanol, n-butanol, isobutanol, n-pentanol, isopentanol, neopentyl alcohol, n-hexanol or isohexanol.

5. A surface treatment method of a neodymium iron boron magnet is characterized by comprising the steps of forming a hydroxyl compound layer on the surface of the neodymium iron boron magnet to obtain a prefabricated product; the hydroxy compound has a structure represented by formula (I):

wherein R is selected from C2-C15 alkyl, and n is a natural number of 1-3.

6. The surface treatment method according to claim 5, wherein the hydroxyl compound is a monohydric alcohol or a dihydric alcohol, R is selected from C2-C15 alkyl groups, and n is 1 or 2.

7. The surface treatment method according to claim 5, wherein the neodymium-iron-boron magnet is soaked in the hydroxyl compound, taken out and dried to obtain a preform.

8. A surface treatment method according to any one of claims 5 to 7, further comprising the step of heat treating the preform to form an oxide layer on the surface of the NdFeB magnet.

9. The surface treatment method according to claim 8, wherein the heat treatment comprises: placing the prefabricated product in a tunnel heating furnace, and heating for 25-50 min at 380-450 ℃; and introducing nitrogen into the tunnel heating furnace in the heating process, wherein the flow rate of the nitrogen is controlled to be 25-100L/min.

10. A corrosion-resistant neodymium-iron-boron magnet, which is obtained by the surface treatment method according to claim 8 or 9, wherein the thickness of the oxide layer is 0.6 to 3.5 μm; the time for the corrosion-resistant neodymium iron boron magnet to start to corrode is more than 45 minutes measured under the conditions of 35 ℃ and 5 wt% NaCl according to GB/T10125-2012 salt spray test, and the time for the corrosion-resistant neodymium iron boron magnet to start to corrode is more than 1.2 hours measured under the conditions of 85 ℃ and 85% RH according to GB/T2423.3-2006.

Technical Field

The invention relates to a corrosion-resistant neodymium-iron-boron magnet and a surface treatment method thereof, and also relates to application of a hydroxyl compound.

Background

In recent years, neodymium iron boron (NdFeB) magnets with ultra-high energy density have been widely used in the fields of electronic products, electric/hybrid vehicles, household appliances, industrial motors, wind power generation, nuclear magnetic resonance, and the like, and have strong market demands. Meanwhile, the rare earth magnet technology is innovated day by day, the yield and the performance are continuously improved, and the development of modern science and technology and information industry is powerfully promoted. However, Nd atoms and Fe atoms on the surface of the ndfeb magnet react with oxygen in the air to form Nd₂O₃、FeO、Fe₂O₃、Fe₃O₄. In a humid environment, neodymium iron boron magnets are more susceptible to corrosion. Therefore, it is necessary to perform an anti-corrosion treatment to improve its anti-corrosion performance.

At present, the protection layer is formed on the surface of the neodymium iron boron magnet by methods of metal plating, electroplating + chemical plating metal, electrophoretic coating, passivation and the like. Because the neodymium iron boron magnet is of a porous structure, plating liquid can enter the material in the electroplating process by the electroplating metal method, so that internal intergranular corrosion is caused, and the obtained protective layer is uneven in thickness, short in service life and not suitable for parts with deep holes and complexity. The protective layer obtained by the electroplating and chemical plating metal method has poor binding force with a substrate, is easy to peel and fall off, and generates sewage. The thickness of the coating obtained by the electrophoresis method was not uniform.

Passivation is a process of forming a stable, dense film on the surface of a metal that is strongly bonded to the substrate. This film isolates the metal substrate from the corrosive medium, thereby preventing further corrosion of the metal. Such a film is called a passivation film.

CN102084438A discloses a method for producing a corrosion-resistant magnet, which comprises subjecting an R-Fe-B sintered magnet to an oxidation heat treatment at a temperature in the range of 450 to 900 ℃ in a humidity-varying environment, applying a treatment liquid to the surface of the R-Fe-B sintered magnet, and drying the R-Fe-B sintered magnet to form a chemical conversion coating film containing at least Fe, Zr, Nd, fluorine, and oxygen as constituent elements, thereby improving the corrosion resistance of neodymium iron boron. However, the treatment solution used in this method is not easy to be stably stored, and needs to be prepared as it is, which increases the complexity of the work. In addition, the treatment liquid is sensitive to temperature, and when the temperature of the coating treatment liquid exceeds 80 ℃, the stability of the treatment liquid is affected, and the corrosion resistance of the neodymium iron boron is further affected.

CN101809690B discloses a method for producing a surface-modified rare earth sintered magnet, which comprises subjecting a magnet to a heat treatment at 200 to 600 ℃ under conditions in which the oxygen partial pressure is controlled to be 102 to 105Pa and the water vapor partial pressure is controlled to be 0.1 to 1000Pa, thereby obtaining a surface-modified layer mainly containing hematite on the surface layer. The corrosion resistance of the rare earth sintered magnet modified by this method is greatly affected by the partial pressure of oxygen and the partial pressure of water vapor. In order to create the above-mentioned partial pressure environment in the processing chamber, it is necessary to introduce an oxidizing gas, which increases the production cost and also imposes strict requirements on the sealing property of the equipment.

CN105839045A discloses a method for improving the corrosion resistance of a sintered neodymium iron boron magnet, wherein the neodymium iron boron magnet is placed in a vacuum furnace, when the vacuum furnace is vacuumized to be below 20Pa, nitrogen gas with pressure of 0.1-0.2 MPa is filled, the vacuum furnace is vacuumized again, and the steps are repeated for 2-3 times; then, the temperature of the vacuum furnace is increased to 400-750 ℃, and nitrogen is filled until the pressure is 1 multiplied by 10³～1×10⁵Pa, treating for 2-24 h to form a compound corrosion-resistant layer containing nitrogen element with the thickness of 1-50 mu m on the surface of the magnet. The method has strict requirements on equipment, increases the equipment investment cost and has longer treatment time.

CN111441017A discloses a method for preparing an anti-corrosion coating on the surface of a neodymium iron boron magnet, wherein a composite coating is evaporated on the surface of the neodymium iron boron magnet. Although the method can improve the binding force between the coating and the substrate, the method has the defects of large equipment investment, low production efficiency, incapability of processing complex parts and the like.

EP0326088A3 discloses a method of providing adequate corrosion protection for a neodymium boron iron magnet, comprising: cleaning the magnet in an alkaline solution; cleaning the cleaned magnet with water, then cleaning with an acidic cleaning solution, and finally cleaning with water; the washed magnet is treated and cleaned in a plating solution containing zinc phosphate to form a zinc phosphate protective layer to inhibit surface corrosion; and an amide imide coating is applied to the surface of the zinc phosphate protective layer and a durable corrosion resistant coating is applied to provide further corrosion protection. The method has the disadvantages of complex operation and low production efficiency, and cannot operate the neodymium boron iron magnet with a complex structure.

US4917778B discloses an anticorrosion method for a sintered magnet of neodymium iron boron group, which comprises immersing a sintered magnet of neodymium iron boron group in an oxidizing acid to activate the surface thereof; internal stress of not more than 1000kgf/cm²The nickel-plated magnet of (1); to which a cationic electrodeposition coating is applied. The coating obtained by the method has uneven thickness, poor binding force between the coating and the substrate, complex operation and high equipment investment cost.

In conclusion, the method has the technical problems of complex process, long period, high equipment requirement, environmental pollution, increased production cost, low batch production efficiency, uneven thickness of the prepared coating, poor binding force with the substrate, easy falling and the like.

Disclosure of Invention

In view of the above, an object of the present invention is to provide an application of a hydroxy compound in improving corrosion resistance of a neodymium iron boron magnet. The invention finds that the hydroxyl compound can improve the corrosion resistance of the neodymium iron boron magnet.

Another object of the present invention is to provide a surface treatment method for an ndfeb magnet. The method has simple process, short treatment period and difficult environmental pollution.

It is still another object of the present invention to provide a corrosion-resistant ndfeb magnet obtained by the above method. After the neodymium iron boron magnet is treated by the method, a compact oxide layer is formed on the surface and even the holes of the neodymium iron boron magnet, and oxygen and the like are effectively isolated, so that the neodymium iron boron magnet has higher corrosion resistance.

In one aspect, the invention provides an application of a hydroxyl compound in improving corrosion resistance of a neodymium iron boron magnet, wherein the hydroxyl compound has a structure shown in a formula (I):

wherein R is selected from C2-C15 alkyl, and n is a natural number of 1-3.

According to the application of the invention, preferably, the hydroxyl compound is a monohydric alcohol or a dihydric alcohol, R is selected from C2-C15 alkyl, and n is 1 or 2.

According to the application of the invention, preferably, the hydroxyl compound is a monohydric alcohol, R is selected from alkyl of C3-C6, and n is 1.

According to the use of the present invention, preferably, the hydroxy compound is ethanol, n-propanol, isopropanol, n-butanol, isobutanol, n-pentanol, isopentanol, neopentyl alcohol, n-hexanol or isohexanol.

In another aspect, the present invention provides a surface treatment method of a neodymium iron boron magnet, including a step of forming a hydroxyl compound layer on a surface of the neodymium iron boron magnet to obtain a preform; the hydroxy compound has a structure represented by formula (I):

wherein R is selected from C2-C15 alkyl, and n is a natural number of 1-3.

According to the surface treatment method of the present invention, preferably, the hydroxyl compound is a monohydric alcohol or a dihydric alcohol, R is selected from C2-C15 alkyl groups, and n is 1 or 2.

According to the surface treatment method of the present invention, preferably, the neodymium iron boron magnet is soaked in the hydroxyl compound, taken out and dried to obtain the preform.

According to the surface treatment method of the present invention, it is preferable that the method further includes a step of heat-treating the preform to form an oxide layer on the surface of the neodymium iron boron magnet.

According to the surface treatment method of the present invention, preferably, the heat treatment includes: placing the prefabricated product in a tunnel heating furnace, and heating for 25-50 min at 380-450 ℃; and introducing nitrogen into the tunnel heating furnace in the heating process, wherein the flow rate of the nitrogen is controlled to be 25-100L/min.

On the other hand, the invention also provides a corrosion-resistant neodymium-iron-boron magnet which is obtained by the surface treatment method, wherein the thickness of the oxide layer is 0.6-3.5 microns; the time for the corrosion-resistant neodymium iron boron magnet to start to corrode is more than 45 minutes measured under the conditions of 35 ℃ and 5 wt% NaCl according to GB/T10125-2012 salt spray test, and the time for the corrosion-resistant neodymium iron boron magnet to start to corrode is more than 1.2 hours measured under the conditions of 85 ℃ and 85% RH according to GB/T2423.3-2006.

Hydroxyl compounds are typically used as organic solvents, but have been found to improve the corrosion resistance of neodymium iron boron magnets. The neodymium iron boron magnet attached with the hydroxyl compound layer is heated to form a compact oxide layer mainly composed of ferroferric oxide on the surface of the neodymium iron boron magnet. The compact oxide layer has strong bonding force with the magnet. Salt spray tests and constant-humidity heat tests show that the oxide layer has better corrosion resistance than a thin film formed by a hydroxyl compound layer which is not attached. The method utilizes the hydroxyl compound to treat the surface of the neodymium iron boron magnet, has simple process, can control the temperature and the nitrogen flow, has short production period and does not pollute the environment.

Detailed Description

The present invention will be further described with reference to the following specific examples, but the scope of the present invention is not limited thereto.

Neodymium iron boron magnets are susceptible to oxidation corrosion. The corrosion resistance of the magnet is improved by adopting a conventional method, the cost is high, and the process is complex. Although the passive film can also improve the corrosion resistance of the magnet, the effect is still not ideal. The inventors of the present application have surprisingly found that a hydroxy compound, which is generally used as an organic solvent, can improve the corrosion resistance of a neodymium iron boron magnet, thereby completing the present invention.

< uses of the hydroxy Compound >

The invention provides application of a hydroxyl compound in improving the corrosion resistance of a neodymium iron boron magnet. The ndfeb magnet may also be referred to as a ndfeb permanent magnet or a ndfeb permanent magnet material. The neodymium iron boron magnet mainly comprises Nd, Fe and B, and can also comprise other transition metal elements, other rare earth elements and inevitable impurities, such as C, O and N. These are well known in the art and will not be described in detail herein. The neodymium iron boron magnet may be a bonded magnet or a sintered magnet, preferably a sintered magnet. In view of the greater likelihood of the porous structure of sintered magnets, the present invention is particularly suitable for sintered neodymium iron boron magnets. According to a preferred embodiment of the present invention, the present invention provides a use of a hydroxy compound for improving corrosion resistance of a sintered neodymium-iron-boron magnet.

The following focuses on the description of hydroxy compounds having the structure shown in formula (I):

wherein R is selected from C2-C15 alkyl, and n is a natural number of 1-3. The hydroxyl compounds of the present invention may have a melting point of less than 15 deg.C, preferably less than 10 deg.C, more preferably less than 5 deg.C. The hydroxyl compounds of the present invention may have a boiling point above 35 deg.C, preferably above 75 deg.C, more preferably above 80 deg.C. The hydroxy compound of the present invention is liquid at normal temperature, for example, at 15 to 30 ℃. Thus, the method is favorable for forming a uniform hydroxyl compound layer on the surface of the neodymium iron boron magnet, is not easy to completely volatilize, and is also favorable for the hydroxyl compound to permeate into holes on the surface of the neodymium iron boron magnet, thereby improving the corrosion resistance of the neodymium iron boron magnet. The conventional heating treatment cannot ensure that the holes on the surface of the neodymium iron boron magnet form a corrosion-resistant layer. Just because the hydroxyl compound permeates into the holes on the surface of the neodymium iron boron magnet, the corrosion resistance requirement which cannot be met by conventional heating treatment is realized.

In the present invention, the hydroxyl compound may be an alcohol, including but not limited to a monohydric alcohol, a dihydric alcohol, or a trihydric alcohol. The invention finds that the alcohol is beneficial to improving the corrosion resistance of the magnet, and particularly the monohydric alcohol has better effect. n is a natural number of 1 to 3, preferably n is 1 or 2, and more preferably n is 1.

In the present invention, R may be selected from hydrocarbon groups of C2 to C15. The hydrocarbyl group may be an alkyl, alkenyl or alkynyl group, preferably an alkyl group. According to one embodiment of the invention, R is selected from C2-C15 alkyl, preferably C3-C9 alkyl, more preferably C3-C6 alkyl. The alkyl group may be a straight chain alkyl group or a branched chain alkyl group. Examples of alkyl groups include, but are not limited to, methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, and the like.

According to one embodiment of the invention, the hydroxyl compound is a mono-or di-alcohol, R is selected from C2-C15 alkyl, and n is 1 or 2. According to another embodiment of the invention, the hydroxy compound is a monohydric alcohol, R is selected from the group consisting of C3-C6 alkyl groups, and n is 1.

Examples of hydroxy compounds include, but are not limited to, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, n-pentanol, isopentanol, neopentyl alcohol, n-hexanol, isohexanol, n-heptanol, n-octanol, n-nonanol, and the like. According to one embodiment of the invention, the hydroxy compound is ethanol, n-propanol, isopropanol, n-butanol, isobutanol, n-pentanol, isopentanol, neopentyl alcohol, n-hexanol or isohexanol. According to one embodiment of the invention, the hydroxy compound is isopropanol. Therefore, the corrosion resistance of the neodymium iron boron magnet can be obviously improved, the film can be formed at normal temperature, the cost is reduced, and the process is simplified.

According to the application of the invention, the surface of the neodymium iron boron magnet can be treated by adopting the following steps:

(1) pretreating an iron boron magnet blank to form a neodymium iron boron magnet;

(2) forming a hydroxyl compound layer on the surface of the neodymium-iron-boron magnet to obtain a prefabricated product; and

(3) and carrying out heat treatment on the prefabricated product to form an oxide layer on the surface of the neodymium iron boron magnet.

The detailed steps and process parameters are the same as those described below and are not described herein again.

< method of surface treatment >

The surface treatment method of the neodymium iron boron magnet of the invention can comprise the steps of forming a hydroxyl compound layer on the surface of the neodymium iron boron magnet to obtain a prefabricated product; the method also comprises the step of carrying out heat treatment on the prefabricated product to form an oxide layer on the surface of the neodymium iron boron magnet. Furthermore, the method of the present invention may include a pre-treatment step of the neodymium iron boron magnet. As described in detail below.

Step of pretreatment

The neodymium iron boron magnet blank is subjected to pretreatment steps of chamfering and polishing, chemical degreasing, acid washing, ultrasonic cleaning, water washing and the like to obtain the neodymium iron boron magnet for the step of forming the prefabricated product. The neodymium iron boron magnet mainly comprises Nd, Fe and B, and can also comprise other transition metal elements, other rare earth elements and inevitable impurities, such as C, O and N. These are well known in the art and will not be described in detail herein. In view of the greater likelihood of the sintered magnet having a porous structure, the neodymium iron boron magnet of the present invention is preferably a sintered neodymium iron boron magnet.

And in the chamfering and polishing process, polishing and chamfering are carried out on the neodymium iron boron magnet blank by adopting grinding processing. The chamfer polishing is performed by conventional processes and will not be described herein.

In the chemical oil removal process, an alkaline oil removal agent is used for removing oil stains on the surface of the neodymium iron boron magnet blank. The alkaline degreasing agent may be one or more selected from sodium hydroxide, sodium carbonate, trisodium phosphate and sodium silicate. According to one embodiment of the present invention, the alkaline degreasing agent is a solution consisting of sodium hydroxide, trisodium phosphate, sodium bicarbonate and water. Specifically, the alkaline degreasing agent contains 20-30 g/L of sodium hydroxide, 20-30 g/L of sodium bicarbonate and 3-10 g/L of trisodium phosphate. According to one embodiment of the present invention, the alkaline oil remover is a solution consisting of sodium hydroxide, sodium carbonate, trisodium phosphate, sodium silicate and water. Specifically, the alkaline oil removing agent contains 10-15 g/L of sodium hydroxide, 20-30 g/L of sodium carbonate, 50-70 g/L of trisodium phosphate and 1-5 g/L of sodium silicate. By adopting the alkaline degreasing agent, the degreasing effect is good, and the formation of a hydroxyl compound layer is facilitated, so that the corrosion resistance is improved.

In the acid cleaning process, the deoiled neodymium iron boron magnet blank is washed by water and then is subjected to acid cleaning and rust removal. The acidic solution used in the pickling process may be a hydrochloric acid solution or a nitric acid solution, preferably a nitric acid solution. The concentration of the nitric acid solution may be 1 to 10 wt%, preferably 2 to 8 wt%, and more preferably 3 to 6 wt%. This can effectively remove rust, thereby facilitating the formation of a hydroxyl compound layer to improve corrosion resistance.

In the ultrasonic cleaning process, conventional ultrasonic cleaning equipment can be used to sufficiently remove rust and remove residual acidic solution. The water washing process further removes the remaining acidic solution, thereby obtaining a neodymium iron boron magnet for use in the preform forming step.

Preform forming step

And forming a hydroxyl compound layer on the surface of the neodymium-iron-boron magnet to obtain a prefabricated product. The hydroxyl compound layer may contain other substances as long as corrosion resistance is not affected. Preferably, the hydroxyl compound layer is formed of only a hydroxyl compound. This can sufficiently ensure corrosion resistance. The hydroxy compounds of the present invention have the structure shown in formula (I):

wherein R is selected from C2-C15 alkyl, and n is a natural number of 1-3.

The melting point of the hydroxy compound may be less than 15 deg.C, preferably less than 10 deg.C, more preferably less than 5 deg.C. The boiling point of the hydroxy compound may be above 35 deg.c, preferably above 75 deg.c, more preferably above 80 deg.c. The hydroxy compound is liquid at room temperature, for example, at 15 to 30 ℃. Thus, the method is favorable for forming a uniform hydroxyl compound layer on the surface of the neodymium iron boron magnet, is not easy to completely volatilize, and is also favorable for the hydroxyl compound to permeate into holes on the surface of the neodymium iron boron magnet, thereby improving the corrosion resistance of the neodymium iron boron magnet. The hydroxyl compound permeates into the holes on the surface of the neodymium iron boron magnet, so that the corrosion resistance is further improved.

The hydroxyl compound is preferably an alcohol, for example a monohydric, dihydric or trihydric alcohol. Alcohols are very suitable for the present invention, especially monohydric alcohols, with better results. n is a natural number of 1 to 3, preferably n is 1 or 2, and more preferably n is 1.

R may be selected from C2-C15 hydrocarbon groups. The hydrocarbyl group may be an alkyl, alkenyl or alkynyl group, preferably an alkyl group. In certain embodiments, R may be selected from C2 to C15 alkyls, preferably C3 to C9 alkyls, more preferably C3 to C6 alkyls. The alkyl group may be a straight chain alkyl group or a branched chain alkyl group. Examples of alkyl groups include, but are not limited to, methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, and the like. In certain embodiments, the hydroxy compound is a mono-or diol, R is selected from the group consisting of C2-C15 alkyl, and n is 1 or 2. In other embodiments, the hydroxy compound is a monohydric alcohol, R is selected from the group consisting of C3-C6 alkyl, and n is 1.

Examples of the hydroxy compound of the present invention include, but are not limited to, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, n-pentanol, isopentanol, neopentanol, n-hexanol, isohexanol, n-heptanol, n-octanol, n-nonanol, and the like. In certain embodiments, the hydroxy compound is ethanol, n-propanol, isopropanol, n-butanol, isobutanol, n-pentanol, isopentanol, neopentyl alcohol, n-hexanol, or isohexanol. In other embodiments, the hydroxy compound is isopropanol. Therefore, the hydroxyl compound layer is formed on the surface and the holes of the neodymium iron boron magnet, the cost is reduced, and the process is simplified.

The hydroxyl compound layer can be formed on the surface of the neodymium iron boron magnet by adopting various processes, such as spraying, soaking and the like. The invention finds that the soaking process is more suitable for the invention because the hydroxyl compound can be fully combined with the neodymium iron boron magnet through soaking, and the hydroxyl compound can be filled in the holes of the neodymium iron boron magnet, thereby being beneficial to improving the corrosion resistance. Therefore, the surface treatment method is very suitable for improving the corrosion resistance of the sintered neodymium-iron-boron magnet.

According to one embodiment of the invention, the neodymium iron boron magnet is soaked in the hydroxyl compound, taken out and dried to obtain the prefabricated product. The soaking time needs to be guaranteed only if the hydroxyl compound can be sufficiently combined with the surface of the neodymium iron boron magnet and can sufficiently permeate into the holes of the neodymium iron boron magnet. It is necessary to completely immerse the neodymium iron boron magnet in the hydroxyl compound. The soaking time may be 5-60 min, preferably 10-50 min, and more preferably 20-30 min. Therefore, the hydroxyl compound can be fully combined with the surface of the neodymium iron boron magnet and can fully permeate into the holes of the neodymium iron boron magnet, and the production efficiency is improved. In addition, blow-drying is carried out by a conventional method, but a vacuum operation is not recommended to avoid destruction of the hydroxyl compound layer.

Oxide layer formation step

And carrying out heat treatment on the prefabricated product to form an oxide layer on the surface of the neodymium iron boron magnet. The thickness of the oxide layer may be 0.6 to 3.5 μm, preferably 1 to 3 μm, and more preferably 2 to 3 μm.

Placing the prefabricated product in a tunnel heating furnace for heat treatment; nitrogen was introduced into the tunnel furnace during the heating. The tunnel furnace may be any conventional one in the art and will not be described herein.

In the tunnel heating furnace, the heat treatment temperature can be 380-450 ℃, preferably 400-450 ℃, and more preferably 420-450 ℃. The heat treatment is too high, resulting in an increase in production cost and deterioration in uniformity of the oxide layer; the heat treatment temperature is too low, the formed oxide layer has large porosity, uneven thickness and poorer magnet binding force and is easy to fall off. The heat treatment time may be 25 to 50min, preferably 27 to 40min, and more preferably 30 to 40 min. The heat treatment time is too long, resulting in an increase in production cost and deterioration in thickness uniformity of the oxide layer; the heat treatment time is too short, and an oxide layer cannot be formed in the holes, resulting in deterioration of corrosion resistance.

And introducing nitrogen into the tunnel heating furnace in the heating process, wherein the flow rate of the nitrogen is controlled to be 25-100L/min. Preferably, the flow rate of the nitrogen is controlled to be 40-60L/min. More preferably, the flow rate of the nitrogen is controlled to be 45-55L/min. The oxygen concentration in the atmosphere surrounding the neodymium iron boron magnet is too low due to too high flow of nitrogen, and a compact oxide layer cannot be formed; the flow of nitrogen is too low, so that the oxygen concentration around the neodymium iron boron magnet is too high, and the oxidation reaction speed is too fast to form a uniform oxide layer.

< Corrosion-resistant NdFeB magnet >

The corrosion-resistant neodymium iron boron magnet is obtained by the surface treatment method. The thickness of the oxide layer may be 0.6 to 3.5 μm, preferably 1 to 3.5 μm, and more preferably 1.2 to 3.5 μm. Therefore, the neodymium iron boron magnet can be ensured to have higher corrosion resistance. If the thickness of the oxide layer is too low, the thickness is not uniform, the porosity is large, and the corrosion prevention period of the oxide layer is short because the thickness of the oxide layer is thin. If the thickness of the oxide layer is excessively large, the heat treatment time needs to be prolonged, increasing the production cost.

The time for the corrosion-resistant neodymium iron boron magnet to start to corrode is more than 45 minutes, preferably more than 1 hour, more preferably more than 1.5 hours, for example 1.5 to 2 hours, measured under the conditions of 35 ℃ and 5 wt% NaCl according to GB/T10125-.

The corrosion resistant neodymium iron boron magnet starts to corrode for more than 1.2 hours, preferably more than 1.5 hours, more preferably more than 2 hours, for example 2-3 hours, measured at 85 ℃ and 85% RH according to GB/T2423.3-2006.

The following examples and comparative examples use the starting materials.

Alkaline degreasing agent: an aqueous solution containing 25g/L sodium hydroxide, 25g/L sodium bicarbonate and 5g/L trisodium phosphate.

The test methods used in the following examples and comparative examples are described below.

Salt spray test: the time at which the corrosion of the neodymium iron boron magnet begins to occur is recorded according to GB/T10125-2012 salt fog test for Artificial atmosphere corrosion test at 35 ℃ and 5 wt% NaCl.

Constant temperature and humidity test: the time to onset of corrosion of the neodymium iron boron magnet was recorded as measured according to GB/T2423.3-2006 at 85 ℃ and 85% RH.

Example 1

50 sintered NdFeB magnet blanks (with the length of 55mm, the width of 21.4mm and the height of 1.8mm) are chamfered and polished by grinding. And degreasing and deoiling the surface of the neodymium iron boron magnet blank by adopting an alkaline deoiling agent. And washing the deoiled neodymium iron boron magnet blank with water, and then removing surface oxides by using 3 wt% nitric acid solution for acid washing. And finally, carrying out ultrasonic cleaning and water washing for 2 times to obtain the neodymium-iron-boron magnet.

And soaking the neodymium iron boron magnet in isopropanol for 20 minutes, taking out the neodymium iron boron magnet and drying the neodymium iron boron magnet by blowing to obtain a prefabricated product. And placing the prefabricated products on a copper net in order, starting a tunnel heating furnace to heat to 380 ℃, starting a nitrogen valve, controlling the nitrogen flow in the furnace to be 50L/min, placing the copper net containing the prefabricated products at a conveying belt feeding port after 1 minute, conveying the copper net containing the prefabricated products into the tunnel heating furnace through the conveying belt, and heating for 25 minutes to obtain the neodymium-iron-boron magnet with the oxide layer on the surface. The oxide layer thickness was 1 μm.

Example 2

50 sintered NdFeB magnet blanks (with the length of 55mm, the width of 21.4mm and the height of 1.8mm) are chamfered and polished by grinding. And degreasing and deoiling the surface of the neodymium iron boron magnet blank by adopting an alkaline deoiling agent. And washing the deoiled neodymium iron boron magnet blank with water, and then removing surface oxides by using 3 wt% nitric acid solution for acid washing. And finally, carrying out ultrasonic cleaning and water washing for 2 times to obtain the neodymium-iron-boron magnet.

And soaking the neodymium iron boron magnet in isopropanol for 20 minutes, taking out the neodymium iron boron magnet and drying the neodymium iron boron magnet by blowing to obtain a prefabricated product. And placing the prefabricated products on a copper net in order, starting a tunnel heating furnace to heat to 400 ℃, starting a nitrogen valve, controlling the flow of nitrogen in the furnace to be 50L/min, placing the copper net containing the prefabricated products at a conveying belt feeding port after 1 minute, conveying the copper net containing the prefabricated products into the tunnel heating furnace through a conveying belt, and heating for 30 minutes to obtain the neodymium-iron-boron magnet with the oxide layer on the surface. The oxide layer was 2 μm thick.

Example 3

50 sintered NdFeB magnet blanks (with the length of 55mm, the width of 21.4mm and the height of 1.8mm) are chamfered and polished by grinding. And degreasing and deoiling the surface of the neodymium iron boron magnet blank by adopting an alkaline deoiling agent. And washing the deoiled neodymium iron boron magnet blank with water, and then removing surface oxides by using 3 wt% nitric acid solution for acid washing. And finally, carrying out ultrasonic cleaning and water washing for 2 times to obtain the neodymium-iron-boron magnet.

And soaking the neodymium iron boron magnet in isopropanol for 20 minutes, taking out the neodymium iron boron magnet and drying the neodymium iron boron magnet by blowing to obtain a prefabricated product. And placing the prefabricated products on a copper net in order, starting a tunnel heating furnace to heat to 420 ℃, starting a nitrogen valve, controlling the flow of nitrogen in the furnace to be 45L/min, placing the copper net containing the prefabricated products at a conveying belt feeding port after 1 minute, conveying the copper net containing the prefabricated products into the tunnel heating furnace through a conveying belt, and heating for 40 minutes to obtain the neodymium-iron-boron magnet with the oxide layer on the surface. The oxide layer was 3 μm thick.

Example 4

50 sintered NdFeB magnet blanks (with the length of 55mm, the width of 21.4mm and the height of 1.8mm) are chamfered and polished by grinding. And degreasing and deoiling the surface of the neodymium iron boron magnet blank by adopting an alkaline deoiling agent. And washing the deoiled neodymium iron boron magnet blank with water, and then removing surface oxides by using 3 wt% nitric acid solution for acid washing. And finally, carrying out ultrasonic cleaning and water washing for 2 times to obtain the neodymium-iron-boron magnet.

And soaking the neodymium iron boron magnet in isopropanol for 20 minutes, taking out the neodymium iron boron magnet and drying the neodymium iron boron magnet by blowing to obtain a prefabricated product. And placing the prefabricated products on a copper net in order, starting a tunnel heating furnace to heat to 450 ℃, starting a nitrogen valve, controlling the flow of nitrogen in the furnace to be 55L/min, placing the copper net containing the prefabricated products at a conveying belt feeding port after 1 minute, conveying the copper net containing the prefabricated products into the tunnel heating furnace through a conveying belt, and heating for 35 minutes to obtain the neodymium-iron-boron magnet with the oxide layer on the surface. The oxide layer thickness was 3.5 μm.

Comparative example 1

50 sintered NdFeB magnet blanks (with the length of 55mm, the width of 21.4mm and the height of 1.8mm) are chamfered and polished by grinding. And degreasing and deoiling the surface of the neodymium iron boron magnet blank by adopting an alkaline deoiling agent. And washing the deoiled neodymium iron boron magnet blank with water, and then removing surface oxides by using 3 wt% nitric acid solution for acid washing. And finally, carrying out ultrasonic cleaning and water washing for 2 times to obtain the neodymium-iron-boron magnet.

And (3) neatly placing the neodymium iron boron magnet on a copper net, starting a tunnel heating furnace to heat to 380 ℃, starting a nitrogen valve, controlling the flow of nitrogen in the furnace to be 50L/min, placing the copper net containing the prefabricated product at a conveying belt feeding port after 1 minute, conveying the copper net containing the prefabricated product into the tunnel heating furnace through the conveying belt, and heating for 25 minutes to obtain the neodymium iron boron magnet with an oxide layer on the surface. The oxide layer thickness was 0.8 μm.

ComparisonExample 2

50 sintered NdFeB magnet blanks (with the length of 55mm, the width of 21.4mm and the height of 1.8mm) are chamfered and polished by grinding. And degreasing and deoiling the surface of the neodymium iron boron magnet blank by adopting an alkaline deoiling agent. And washing the deoiled neodymium iron boron magnet blank with water, and then removing surface oxides by using 3 wt% nitric acid solution for acid washing. And finally, carrying out ultrasonic cleaning and water washing for 2 times to obtain the neodymium-iron-boron magnet.

And soaking the neodymium iron boron magnet in isopropanol for 20 minutes, taking out the neodymium iron boron magnet and drying the neodymium iron boron magnet by blowing to obtain a prefabricated product. And placing the prefabricated products on a copper net in order, starting a tunnel heating furnace to heat to 350 ℃, starting a nitrogen valve, controlling the nitrogen flow in the furnace to be 40L/min, placing the copper net containing the prefabricated products at a conveying belt feeding port after 1 minute, conveying the copper net containing the prefabricated products into the tunnel heating furnace through the conveying belt, and heating for 20 minutes to obtain the neodymium-iron-boron magnet with the oxide layer on the surface. The oxide layer thickness was 0.5 μm.

Comparative example 3

50 sintered NdFeB magnet blanks (with the length of 55mm, the width of 21.4mm and the height of 1.8mm) are chamfered and polished by grinding. And degreasing and deoiling the surface of the neodymium iron boron magnet blank by adopting an alkaline deoiling agent. And washing the deoiled neodymium iron boron magnet blank with water, and then removing surface oxides by using 3 wt% nitric acid solution for acid washing. And finally, carrying out ultrasonic cleaning and water washing for 2 times to obtain the neodymium-iron-boron magnet.

And soaking the neodymium iron boron magnet in isopropanol for 20 minutes, taking out the neodymium iron boron magnet and drying the neodymium iron boron magnet by blowing to obtain a prefabricated product. And placing the prefabricated products on a copper net in order, starting a tunnel heating furnace to heat to 500 ℃, starting a nitrogen valve, controlling the nitrogen flow in the furnace to be 50L/min, placing the copper net containing the prefabricated products at a conveying belt feeding port after 1 minute, conveying the copper net containing the prefabricated products into the tunnel heating furnace through the conveying belt, and heating for 25 minutes to obtain the neodymium-iron-boron magnet with the oxide layer on the surface. The oxide layer thickness was 0.7 μm.

TABLE 1

As can be seen from table 1, the corrosion resistance of the neodymium iron boron magnet of the present invention is significantly improved. As is clear from example 1 and comparative example 1, the neodymium-iron-boron magnet (comparative example 1) which was not soaked in isopropyl alcohol was inferior in corrosion resistance after heat treatment. The corrosion resistance of the iso-propanol soaked neodymium iron boron magnet (example 1) is significantly improved after heat treatment.

The corrosion resistance of the neodymium iron boron magnet can be further improved by adjusting the heat treatment conditions. According to embodiments 1 to 3, the thickness of the oxide layer can be increased by appropriately increasing the heat treatment temperature and the heat treatment time, and the corrosion resistance of the neodymium iron boron magnet can be further improved. Comparing example 4 with example 3, it can be seen that increasing the heat treatment temperature requires a corresponding increase in nitrogen flow to avoid excessive oxidation, which would otherwise result in reduced corrosion resistance.

It is understood from the comparison between example 1 and comparative examples 2 to 3 that too low or too high heat treatment temperature is not favorable for improving the corrosion resistance of the neodymium iron boron magnet.

The present invention is not limited to the above-described embodiments, and any variations, modifications, and substitutions which may occur to those skilled in the art may be made without departing from the spirit of the invention.