JP7237108B2 - Corrosion resistant neodymium iron boron magnet, surface treatment method and use of hydroxyl compound - Google Patents

Description

The present invention relates to corrosion-resistant neodymium-iron-boron magnets, surface treatment methods and uses of hydroxyl compounds.

In recent years, Neodymium Iron Boron (NdFeB) magnets with ultra-high energy density have been widely used in fields such as electronic products, electric/hybrid vehicles, home appliances, industrial motors, wind power generation, nuclear magnetic resonance, etc., and have strong market demand. There is In addition, the technological innovation of rare earth magnets is progressing day by day, and the production volume and performance are constantly improving, which strongly promotes the development of modern technology and information industry. However, Nd and Fe atoms on the surface _of neodymium _- iron-boron magnets react with _oxygen in the air to form _Nd2O3 , FeO, _Fe2O3 , _Fe3O4 . Neodymium iron boron magnets are more susceptible to corrosion in humid environments. Therefore, it is necessary to perform anti-corrosion treatment in order to improve corrosion resistance.

Currently, metal plating, plating-electroless plating, electrodeposition coating, and passivation are widely used as methods for forming a protective layer on the surface of neodymium-iron-boron magnets. Since neodymium-iron-boron magnets have a porous structure, the plating solution penetrates into the material during plating, causing internal intergranular corrosion, resulting in uneven thickness of the protective layer and a short service life. Not suitable for deep holes and complex parts. The protective layer obtained by the plating-electroless plating method has poor adhesion to the substrate, is easily peeled off, and generates sewage. The thickness of the coating layer produced by the electrodeposition method is non-uniform.

Passivation is the process of forming a stable, dense film on the surface of a metal with strong adhesion to the substrate. This membrane separates the metal matrix from corrosive media, thereby preventing further corrosion of the metal. Such a film is called a passivation film.

In CN102084438A, an R-Fe-B based sintered magnet is subjected to an oxidizing heat treatment in a temperature range of 450°C to 900°C in a humidity-changing environment, then a treatment solution is applied to the surface, and then dried to remove at least A method for producing a corrosion-resistant magnet is disclosed in which the corrosion resistance of neodymium-iron-boron is improved by forming a chemical conversion film containing Fe, Zr, Nd, fluorine, and oxygen. However, with this method, it is difficult to stably store the processing solution, and on-site preparation is required, which increases the complexity of the work. In addition, the treatment liquid is sensitive to temperature, and if the temperature at which the treatment liquid is applied exceeds 80° C., it affects the stability of the treatment liquid and further affects the corrosion resistance of neodymium-iron-boron.

In CN101809690B, the magnet is heat-treated at 200-600°C, oxygen partial pressure is 10 ^{2-10 5} Pa, and water vapor partial pressure is 0.1-1000 Pa, and the surface layer is made mainly of hematite. A method of manufacturing a rare earth sintered magnet including layers is disclosed. The corrosion resistance of rare earth sintered magnets modified by such a method is greatly affected by oxygen partial pressure and water vapor partial pressure. In order to create the aforementioned partial pressure environment in the processing chamber, it is necessary to introduce an oxidizing gas, which not only increases the manufacturing cost but also imposes strict requirements on the hermeticity of the apparatus.
For CN105839045A, a neodymium iron boron magnet is placed in a vacuum furnace, evacuated to 20Pa or less, filled with nitrogen gas of 0.1 to 0.2MPa, and then evacuated 2 to 3 times. The temperature is raised to 400 to 750°C, nitrogen gas is introduced until the pressure reaches 1×10 ³ to 1×10 ⁵ Pa, and the treatment is carried out for 2 to 24 hours. It is disclosed to form a corrosion resistant layer of the containing compound. This method has demanding equipment requirements, increases equipment investment costs, and takes long processing times.

CN111441017A discloses a method of producing an anti-corrosion coating on the surface of a neodymium iron boron magnet by depositing a composite coating on the surface of the neodymium iron boron magnet. Although this method can improve the adhesion between the plating layer and the substrate, it has drawbacks such as a large equipment investment, poor production efficiency, and inability to process complicated parts.

EP0326088A3 describes the steps of washing the magnets in an alkaline solution, washing the washed magnets first with water, then with an acid washing solution, and finally with water, and cleaning the washed magnets with a plating solution containing zinc phosphate. forming a zinc phosphate protective layer to inhibit surface corrosion; and coating the surface of the zinc phosphate protective layer with an amide-imide coating layer to provide a corrosion-resistant and durable coating layer to protect against corrosion. A method for providing sufficient corrosion protection to neodymium-bonded magnets is disclosed that includes steps. This method is complicated to operate and has low production efficiency, and cannot operate neodymium iron boron magnets with complicated structures.

In US4917778B, a neodymium-iron-boron sintered magnet is impregnated with an oxidizing acid to activate the surface, the magnet is plated with nickel having an internal stress of 1000kgf/ ^cm2 or less, and cation electrodeposition coating is applied. A method for producing a neodymium iron boron based sintered magnet is disclosed. In this method, the thickness of the resulting coating film is uneven, the adhesion between the coating film and the substrate is poor, the work is complicated, and the equipment investment is high.

As mentioned above, the above method has complicated process, long cycle, high equipment requirements, environmental pollution, increased production cost, low mass production efficiency, and uneven thickness of the produced coating. It has many technical problems, such as its weak bonding force with the matrix and its tendency to fall off.

In view of this, it is an object of the present invention to provide the use of hydroxyl compounds to improve the corrosion resistance of neodymium iron boron magnets. The present invention found that hydroxyl compounds can improve the corrosion resistance of neodymium iron boron magnets.

Another object of the present invention is to provide a surface treatment method for neodymium iron boron magnets. The method has a simple process, a short treatment cycle, and is less polluting to the environment.

Another object of the present invention is to provide a corrosion resistant neodymium iron boron magnet obtained by the above method. After being treated by the above method, the neodymium-iron-boron magnets are effectively blocked from oxygen gas and have higher corrosion resistance due to the formation of a dense oxide layer from the surface to the pores.

（そのうち、RはC2～C15の炭化水素基から選ばれたものであり、nは1～3の自然数である。） On the other hand, the present invention provides the use of a hydroxyl compound having the structure shown in formula (I) for improving the corrosion resistance of neodymium iron boron magnets.

(Among them, R is selected from C2-C15 hydrocarbon groups, and n is a natural number from 1 to 3.)

Preferably, according to the use of the present invention, said hydroxyl compound is a monohydric alcohol or a dihydric alcohol, R is selected from C2-C15 alkyl and n is 1 or 2.

Preferably, according to the use of the present invention, said hydroxyl compound is a monohydric alcohol, R is selected from C3-C6 alkyl and n is one.

According to the use of the present invention, said hydroxyl compound may be ethanol, N-propanol, isopropanol, n-butanol, isobutanol, n-pentanol, isoamyl alcohol, neopentyl alcohol, n-hexanol or isohexanol. preferable.

（そのうち、RはC2～C15の炭化水素基から選ばれたものであり、nは1～3の自然数である。） The present invention also provides a surface treatment method for a neodymium iron boron magnet, comprising the step of forming a layer of a hydroxyl compound having a structure represented by formula (I) on the surface of a neodymium iron boron magnet to obtain a preform.

(Among them, R is selected from C2-C15 hydrocarbon groups, and n is a natural number from 1 to 3.)

According to the surface treatment method of the present invention, it is preferable that the hydroxyl compound is a monohydric alcohol or a dihydric alcohol, R is selected from C2 to C15 alkyls, and n is 1 or 2. .

According to the surface treatment method of the present invention, it is preferable to obtain a preform by impregnating a neodymium-iron-boron magnet with the hydroxyl compound, taking it out, and then blow-drying it.

Preferably, the surface treatment method according to the present invention further includes the 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 according to the present invention, the heat treatment includes placing the preform in a tunnel heating furnace, heating at 380 to 450 ° C. for 25 to 50 minutes, flowing nitrogen gas into the tunnel heating furnace during heating, and flowing the nitrogen gas. is preferably controlled to 25 to 100 L/min.

In the present invention, the oxide layer has a thickness of 0.6 to 3.5 μm, and was measured under the conditions of 35° C. and 5 wt% NaCl according to GB/T 10125-2012 <<artificial atmosphere corrosion test/salt spray test>>. The time until the corrosion-resistant neodymium-iron-boron magnet starts to corrode exceeds 45 minutes, and the time until the corrosion-resistant neodymium-iron-boron magnet starts to corrode measured under conditions of 85°C and 85% RH according to GB/T2423.3-2006 provides a corrosion-resistant neodymium-iron-boron magnet obtained by the above-described method in which the time of holding is more than 1.2 hours.

Although hydroxyl compounds are usually used as organic solvents, the present invention has found that they can improve the corrosion resistance of neodymium-iron-boron magnets. A neodymium-iron-boron magnet to which a hydroxyl compound layer is attached forms a dense oxide layer containing iron oxide as a main component on its surface by heat treatment. This dense oxide layer has strong adhesion to the magnet. Salt spray tests and constant humidity thermal tests have shown that the oxide layer has better corrosion resistance than films formed without the hydroxyl compound layer applied. The present invention uses a hydroxyl compound to treat the surface of the neodymium iron boron magnet, the process is simple, the temperature and nitrogen flow rate are properly controlled, the production period is short, and the environment is not polluted.

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

Neodymium iron boron magnets are prone to corrosion due to oxidation. Conventional methods for improving the corrosion resistance of magnets are costly and complicated. Although the passivation protective film can improve the corrosion resistance of the magnet, the effect is not sufficient. The present inventors have surprisingly found that a hydroxyl compound, which is usually used as an organic solvent, improves the corrosion resistance of neodymium-iron-boron magnets, and have completed the present invention.

<Use of hydroxyl group>
The present invention provides the use of hydroxyl compounds to improve the corrosion resistance of neodymium iron boron magnets. Neodymium iron boron magnets are called neodymium iron boron permanent magnets or neodymium iron boron permanent magnetic materials. Neodymium-iron-boron magnets are mainly composed of Nd, Fe and B, and also contain other transition metal elements, other rare earth elements and inevitable impurities such as C, O and N. Since these are well known in the art, detailed description thereof will be omitted here. Neodymium iron boron magnets may be bonded magnets or sintered magnets, preferably sintered magnets. In particular, the present invention is particularly suitable for sintered neodymium-iron-boron magnets, since sintered magnets are more likely to have a porous structure. According to a preferred embodiment of the present invention, there is provided the use of hydroxyl compounds to improve the corrosion resistance of sintered neodymium-iron-boron magnets.

（そのうち、RはC2～C15の炭化水素基から選ばれたものであり、nは1～3の自然数である。） Hereinafter, hydroxyl compounds having a structure represented by general formula (I) will be mainly described.

(Among them, R is selected from C2-C15 hydrocarbon groups, and n is a natural number from 1 to 3.)

The hydroxyl compound according to the present invention may have a melting point of less than 15°C, preferably less than 10°C, more preferably less than 5°C. The hydroxyl compound according to the present invention may have a boiling point above 35°C, preferably above 75°C, more preferably above 80°C. The hydroxyl compound according to the present invention is liquid at room temperature, for example, at 15 to 30°C. This is advantageous for forming a uniform hydroxyl compound layer on the surface of the neodymium-iron-boron magnet, and because it is difficult to volatilize completely, the hydroxyl-group compound soaks into the pores on the surface of the neodymium-iron-boron magnet, improving corrosion resistance. is also advantageous. The conventional heat treatment cannot ensure the formation of a corrosion-resistant layer in the pores on the surface of the neodymium-iron-boron magnet. Since the hydroxyl group compound penetrated into the pores on the surface of the neodymium-iron-boron magnet, corrosion resistance that could not be achieved by conventional heat treatment can be achieved.

The hydroxyl compounds of the present invention may be alcoholic, including but not limited to monohydric alcohols, dihydric alcohols or trihydric alcohols. The present invention has found that alcohols are advantageous for improving the corrosion resistance of magnets, and monohydric alcohols are particularly effective. n is a natural number of 1 to 3, preferably 1 or 2, more preferably 1.

In the present invention, R may be selected from C2-C15 hydrocarbon groups. The hydrocarbon group may be alkyl, alkenyl or alkynyl, preferably alkyl. According to one embodiment of the present invention, R is selected from C2-C15 alkyl, preferably C3-C9 alkyl, more preferably C3-C6 alkyl. Alkyl may be straight chain alkyl or branched chain alkyl. Examples of alkyl 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 present invention, said hydroxyl compound is a monohydric or dihydric alcohol, R is C2-C15 alkyl and n is 1 or 2. According to another embodiment of the present invention, said hydroxyl compound is a monohydric alcohol, R is selected from C3-C6 alkyl and n is 1.

Examples of hydroxyl compounds include ethanol, n-propanol, isopropanol, n-butanol, isobutanol, n-pentanol, isoamyl alcohol, neopentyl alcohol, n-hexanol, isohexanol, n-heptanol, n-octanol, n- Including but not limited to nonanol and the like. According to one embodiment of the invention, the hydroxyl compound is ethanol, n-propanol, isopropanol, n-butanol, isobutanol, n-pentanol, isoamyl alcohol, neopentyl alcohol, n-hexanol or isohexanol. According to one specific embodiment of the invention, said hydroxyl compound is n-propanol. As a result, the corrosion resistance of the neodymium-iron-boron magnet can be remarkably improved, the film can be formed at room temperature, the cost can be reduced, and the process can be simplified.

According to the use of the present invention, the neodymium iron boron magnet surface may be treated by the following steps.
(1) pretreating a neodymium iron boron magnet material 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 preform;
(3) A process of heat-treating the preform to form an oxide layer on the surface of the neodymium-iron-boron magnet.
The detailed steps and process parameters are the same as below and are omitted here.

<Surface treatment method>
A surface treatment method for a neodymium-iron-boron magnet according to the present invention includes a step of forming a hydroxyl compound layer on the surface of the neodymium-iron-boron magnet to obtain a preform. A step of forming an oxide layer may be included. The method according to the invention may also include a pretreatment step for the neodymium iron boron magnet. Details will be described later.

Pretreatment Process A neodymium iron boron magnet material is subjected to pretreatment processes such as chamfer polishing, chemical degreasing, pickling, ultrasonic cleaning, and water washing to obtain a neodymium iron boron magnet, which is subjected to a preform forming process. Neodymium-iron-boron magnets are mainly composed of Nd, Fe, and B, and may contain inevitable impurities such as other transition metal elements, other rare earth elements, and C, O, and N. These are well known in the art and will not be described in detail here. From the viewpoint that a sintered magnet is likely to have a porous structure, the neodymium iron boron magnet according to the present invention is preferably a sintered neodymium iron boron magnet.

In the chamfering and polishing step, polishing and chamfering are performed by lapping the neodymium-iron-boron magnet material. The chamfering process may be performed by a normal method, and the description thereof is omitted here.

In the chemical degreasing step, an alkaline degreasing agent is used to remove oil stains on the surface of the neodymium-iron-boron magnet material. The alkaline degreasing agent is one or more selected from sodium hydroxide, sodium carbonate, trisodium phosphate and sodium silicate. According to one embodiment of the invention, the alkaline degreasing agent is a solution consisting of sodium hydroxide, trisodium phosphate, sodium bicarbonate and water. Specifically, alkaline degreasing agents include 20-30 g/L sodium hydroxide, 20-30 g/L sodium bicarbonate and 3-10 g/L trisodium phosphate. According to one embodiment of the invention, the alkaline degreasing agent is a solution consisting of sodium hydroxide, sodium carbonate, trisodium phosphate, sodium silicate and water. Specifically, alkaline degreasing agents include 10-15 g/L sodium hydroxide, 20-30 g/L sodium carbonate, 50-70 g/L trisodium phosphate and 1-5 g/L sodium silicate. When the alkaline degreasing agent described above is used, the degreasing effect is high, the formation of the hydroxyl group compound layer is promoted, and the corrosion resistance is improved.

In the pickling step, the degreased neodymium-iron-boron magnet material is washed with water and then pickled to remove rust. The acid solution used in the pickling process may be a hydrochloric acid solution or a nitric acid solution, but a nitric acid solution is preferred. The nitric acid solution may have a concentration of 1-10 wt%, preferably 2-8 wt%, more preferably 3-6 wt%. This makes it possible to effectively remove rust, which is advantageous for forming a hydroxyl group compound layer and improving corrosion resistance.

The ultrasonic cleaning process can use conventional ultrasonic cleaning equipment to sufficiently remove rust and residual acid solution. The water washing process is the process of further removing the remaining acidic solution to obtain the neodymium iron boron magnet for the preform forming process.

（そのうち、RはC2～C15の炭化水素基であり、nは1～3の自然数である。）
水酸基化合物は、融点が15℃未満であってもよく、好ましくは10℃未満であり、より好ましくは5℃未満である。水酸基化合物は、沸点が35℃超えてもよく、好ましくは75℃超え、より好ましくは80℃超える。水酸基化合物は、常温で、例えば、15～30℃で液状である。これにより、ネオジム鉄ホウ素磁石表面に均一な水酸基化合物層を形成し、水酸基化合物は、完全に揮発しにくいため、ネオジム鉄ホウ素磁石表面の空孔に浸漬し、耐食性をさらに向上された。水酸基化合物は、ネオジム鉄ホウ素磁石表面の空孔に浸漬するため、耐食性をさらに向上できる。 Preform forming step A preform is obtained by forming a hydroxyl compound layer on the surface of a neodymium iron boron magnet. The hydroxyl compound layer may contain other substances as long as they do not affect corrosion resistance. Preferably, the hydroxyl compound layer consists of only the hydroxyl compound. This ensures sufficient corrosion resistance. The hydroxyl compound according to the present invention has a structure represented by formula (I) below.

(Among them, R is a C2-C15 hydrocarbon group and n is a natural number from 1 to 3.)
The hydroxyl compound may have a melting point of less than 15°C, preferably less than 10°C, more preferably less than 5°C. The hydroxyl compound may have a boiling point above 35°C, preferably above 75°C, more preferably above 80°C. The hydroxyl group compound is liquid at room temperature, for example, 15 to 30°C. As a result, a uniform hydroxyl compound layer is formed on the surface of the neodymium iron boron magnet, and since the hydroxyl compound is difficult to volatilize completely, it is immersed in the pores on the surface of the neodymium iron boron magnet, further improving corrosion resistance. Since the hydroxyl compound is immersed in the pores on the surface of the neodymium-iron-boron magnet, the corrosion resistance can be further improved.

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

R may be selected from C2-C15 hydrocarbon groups. The hydrocarbon group may be alkyl, alkenyl or alkynyl, preferably alkyl. According to an embodiment of the present invention, R may be selected from C2-C15 alkyl, preferably C3-C9 alkyl, more preferably C3-C6 alkyl. Alkyl may be straight chain alkyl or branched chain alkyl. Examples of alkyl 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 one embodiment, 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 another embodiment, said hydroxyl compound is a monohydric alcohol, R is selected from C3-C6 alkyl and n is 1.

Illustrative examples of hydroxyl compounds according to the invention are ethanol, n-propanol, isopropanol, n-butanol, isobutanol, n-pentanol, isoamyl alcohol, neopentyl alcohol, n-hexanol, isohexanol, n-heptanol, n-octanol. , n-nonanol, and the like. According to one embodiment, the hydroxyl compound is ethanol, n-propanol, isopropanol, n-butanol, isobutanol, n-pentanol, isoamyl alcohol, neopentyl alcohol, n-hexanol or isohexanol. According to another embodiment, said hydroxyl compound is n-propanol. This is advantageous for forming a hydroxyl compound layer on the surface and pores of the neodymium-iron-boron magnet, and enables cost reduction and process simplification.

A hydroxyl compound layer can be formed on the surface of the neodymium bond magnet by various methods such as thermal spraying and impregnation. The impregnation step is more suitable for the present invention to improve corrosion resistance because the hydroxyl compound can be sufficiently adhered to the neodymium iron boron magnet and the gaps of the neodymium iron boron magnet can also be filled with the hydroxyl compound. I found out. Therefore, the surface treatment method according to the present invention is extremely suitable for improving the corrosion resistance of sintered neodymium-iron-boron magnets.

According to one embodiment of the present invention, a neodymium-iron-boron magnet is impregnated with the hydroxyl compound, taken out and then blown dry to obtain a preform. The impregnation time may be any time necessary for sufficiently bonding the hydroxyl compound to the surface of the neodymium-iron-boron magnet and sufficiently immersing it in the pores of the neodymium-iron-boron magnet. The neodymium iron boron magnet should be completely impregnated with the hydroxyl compound. The impregnation time may be 5-60 min, preferably 10-50 min, more preferably 20-30 min. As a result, the hydroxyl compound can be sufficiently bonded to the surface of the neodymium iron boron magnet, and the pores of the neodymium iron boron magnet can be sufficiently immersed, and the productivity is improved. Blow-drying is carried out by conventional methods, but it is not recommended to use a vacuum operation in order to avoid destruction of the hydroxyl group compound layer.

Oxide Layer Forming Step The preform is heat treated to form an oxide layer on the surface of the neodymium-iron-boron magnet. The thickness of the oxide layer may be 0.6-3.5 μm, preferably 1-3 μm, more preferably 2-3 μm.
The preform is placed in a tunnel heating furnace for heat treatment, and in the heating process, nitrogen gas is flowed through the tunnel heating furnace. As the tunnel heating furnace, equipment commonly used in the industry may be adopted, and the explanation thereof is omitted here.

The tunnel heating furnace may have a heat treatment temperature of 380-450°C, preferably 400-450°C, more preferably 420-450°C. Too high a heat treatment results in increased manufacturing costs and reduced uniformity of the oxide layer. If the heat treatment temperature is too low, the oxide layer formed will have a large porosity, an uneven thickness, a weak magnetic coupling force, and a tendency to fall off. The heat treatment time may be 25-50 min, preferably 27-40 min, more preferably 30-40 min. Too long a heat treatment time results in increased manufacturing costs and reduced thickness uniformity of the oxide layer. If the heat treatment time is too short, no oxide layer is formed in the pores, resulting in deterioration of corrosion resistance.

During heating, nitrogen gas is passed through the tunnel heating furnace, and the flow rate of nitrogen gas is controlled at 25 to 100 L/min. Preferably, the flow rate of the nitrogen gas is controlled at 40-60 L/min. More preferably, the flow rate of the nitrogen gas is controlled at 45-55 L/min. If the nitrogen gas flow rate is too high, the oxygen concentration around the neodymium-iron-boron magnet becomes too low to form a dense oxide layer. If the nitrogen gas flow rate is too low, the oxygen concentration around the neodymium-iron-boron magnet will be too high, and the oxidation reaction rate will be too fast to form a uniform oxide layer.

<Corrosion-resistant neodymium-iron-boron magnet>
A corrosion-resistant neodymium-iron-boron magnet was obtained by the surface treatment method described above. The thickness of the oxide layer may be 0.6-3.5 μm, preferably 1-3.5 μm, more preferably 1.2-3.5 μm. Thereby, the corrosion resistance of the neodymium-iron-boron magnet can be further enhanced. If the thickness of the oxide layer is too thin, the thickness will be non-uniform and the porosity will be large. If the thickness of the oxide layer is too thick, a long heat treatment time is required, resulting in high production costs.

The time until the corrosion-resistant neodymium-iron-boron magnet begins to corrode, measured at 35°C and 5 wt% NaCl according to GB/T 10125-2012 <<artificial atmosphere corrosion test/salt spray test>>, is more than 45 minutes, preferably More than 1 hour, more preferably more than 1.5 hours, for example 1.5 to 2 hours.

The time until the corrosion-resistant neodymium-iron-boron magnet begins to corrode, measured under conditions of 85°C and 85% RH according to GB/T2423.3-2006, is more than 1.2 hours, preferably more than 1.5 hours, and more preferably more than 2 hours. , for example 2-3 hours.

The raw materials used in the following examples and comparative examples are described below.
Alkaline degreaser: aqueous solution containing 25 g/L sodium hydroxide, 25 g/L sodium bicarbonate and 5 g/L trisodium phosphate.

The measurement methods used in Examples and Comparative Examples are described below.
Salt spray test: According to GB/T 10125-2012 <<artificial atmosphere corrosion test/salt spray test>>, the time until the neodymium-iron-boron magnet started to corrode under conditions of 35°C and 5 wt% NaCl was measured and recorded.

Constant temperature and humidity test: According to GB/T2423.3-2006, the time until the neodymium iron boron magnet starts to corrode was measured and recorded under the conditions of 85°C and 85% RH.

Example 1
50 pieces of sintered neodymium iron boron magnet material (length 55mm, width 21.4mm, height 1.8mm) were chamfered and polished using polishing process. The surface of the neodymium-iron-boron magnet material was degreased using an alkaline degreasing agent. After degreasing, the neodymium-iron-boron magnet material was washed with water and pickled with a 3 wt% nitric acid solution to remove surface oxides. Finally, ultrasonic cleaning and washing with water were repeated twice to obtain a neodymium-iron-boron magnet.
A neodymium iron boron magnet was immersed in isopropanol for 20 minutes, taken out and blown dry to obtain a preform. Arrange the preforms neatly on the copper mesh, turn on the tunnel heating furnace and heat it to a temperature of 380 ° C, open the nitrogen valve, control the nitrogen gas flow rate in the furnace to 50 L / min, and after 1 minute, preform A copper net was placed at the feed port of the belt, sent from the belt into a tunnel heating furnace, and heated for 25 minutes to obtain a neodymium-iron-boron magnet having an oxide layer on the surface. The thickness of the oxide layer was 1 μm.

Example 2
50 pieces of sintered neodymium iron boron magnet material (length 55mm, width 21.4mm, height 1.8mm) were chamfered and polished using polishing process. The surface of the neodymium-iron-boron magnet material was degreased using an alkaline degreasing agent. After degreasing, the neodymium-iron-boron magnet material was washed with water and pickled with a 3 wt% nitric acid solution to remove surface oxides. Finally, ultrasonic cleaning and washing with water were repeated twice to obtain a neodymium-iron-boron magnet.
A neodymium iron boron magnet was immersed in isopropanol for 20 minutes and then removed and blown dry to obtain a preform. Arrange the preforms neatly on the copper mesh, turn on the tunnel heating furnace and heat it to a temperature of 400 ° C, open the nitrogen valve, control the nitrogen gas flow rate in the furnace to 50 L / min, and after 1 minute, preform A copper net was placed at the feed port of the belt, fed from the belt into a tunnel heating furnace, and heated for 30 minutes to obtain a neodymium-iron-boron magnet having an oxide layer on the surface. The thickness of the oxide layer was 2 μm.

Example 3
50 pieces of sintered neodymium iron boron magnet material (length 55mm, width 21.4mm, height 1.8mm) were chamfered and polished using polishing process. The surface of the neodymium-iron-boron magnet material was degreased using an alkaline degreasing agent. After degreasing, the neodymium-iron-boron magnet material was washed with water and pickled with a 3 wt% nitric acid solution to remove surface oxides. Finally, ultrasonic cleaning and washing with water were repeated twice to obtain a neodymium-iron-boron magnet.
A neodymium iron boron magnet was immersed in isopropanol for 20 minutes and then removed and blown dry to obtain a preform. Arrange the preforms neatly on the copper mesh, turn on the tunnel heating furnace to heat to a temperature of 420 ° C, open the nitrogen valve, control the nitrogen gas flow rate in the furnace to 45 L / min, and after 1 minute, preform A copper net was placed at the feed port of the belt, sent from the belt into a tunnel heating furnace, and heated for 40 minutes to obtain a neodymium-iron-boron magnet having an oxide layer on the surface. The thickness of the oxide layer was 3 μm.

Example 4
50 pieces of sintered neodymium iron boron magnet material (length 55mm, width 21.4mm, height 1.8mm) were chamfered and polished using polishing process. The surface of the neodymium-iron-boron magnet material was degreased using an alkaline degreasing agent. After degreasing, the neodymium-iron-boron magnet material was washed with water and pickled with a 3 wt% nitric acid solution to remove surface oxides. Finally, ultrasonic cleaning and washing with water were repeated twice to obtain a neodymium-iron-boron magnet.
A neodymium iron boron magnet was immersed in isopropanol for 20 minutes and then removed and blown dry to obtain a preform. Arrange the preforms neatly on the copper mesh, turn on the tunnel heating furnace to heat to a temperature of 450 ° C, open the nitrogen valve, control the nitrogen gas flow rate in the furnace to 55 L / min, and after 1 minute, preform A copper net was placed at the feed port of the belt, sent from the belt into a tunnel heating furnace, and heated for 35 minutes to obtain a neodymium-iron-boron magnet having an oxide layer on the surface. The thickness of the oxide layer was 3.5 µm.

Comparative example 1
50 pieces of sintered neodymium iron boron magnet material (length 55mm, width 21.4mm, height 1.8mm) were chamfered and polished using polishing process. The surface of the neodymium-iron-boron magnet material was degreased using an alkaline degreasing agent. After degreasing, the neodymium-iron-boron magnet material was washed with water and pickled with a 3 wt% nitric acid solution to remove surface oxides. Finally, ultrasonic cleaning and washing with water were repeated twice to obtain a neodymium-iron-boron magnet.
Arrange the neodymium iron boron magnets neatly on the copper net, turn on the tunnel heating furnace and heat it to a temperature of 380 ° C, open the nitrogen valve, control the nitrogen gas flow rate in the furnace to 50 L / min, and after 1 minute A preformed copper net was placed at the feed port of the belt, sent from the belt into a tunnel heating furnace, and heated for 25 minutes to obtain a neodymium-iron-boron magnet having an oxide layer on the surface. The thickness of the oxide layer was 0.8 µm.

Comparative example 2
50 pieces of sintered neodymium iron boron magnet material (length 55mm, width 21.4mm, height 1.8mm) were chamfered and polished using polishing process. The surface of the neodymium-iron-boron magnet material was degreased using an alkaline degreasing agent. After degreasing, the neodymium-iron-boron magnet material was washed with water and pickled with a 3 wt% nitric acid solution to remove surface oxides. Finally, ultrasonic cleaning and washing with water were repeated twice to obtain a neodymium-iron-boron magnet.
A neodymium iron boron magnet was immersed in isopropanol for 20 minutes and then removed and blown dry to obtain a preform. Arrange the preforms neatly on the copper mesh, turn on the tunnel heating furnace and heat it to a temperature of 350 ° C, open the nitrogen valve, control the nitrogen gas flow rate in the furnace to 40 L / min, and after 1 minute, preform A copper net was placed at the feed port of the belt, fed from the belt into a tunnel heating furnace, and heated for 20 minutes to obtain a neodymium-iron-boron magnet having an oxide layer on the surface. The thickness of the oxide layer was 0.5 μm.

Comparative example 3
50 pieces of sintered neodymium iron boron magnet material (length 55mm, width 21.4mm, height 1.8mm) were chamfered and polished using polishing process. The surface of the neodymium-iron-boron magnet material was degreased using an alkaline degreasing agent. After degreasing, the neodymium-iron-boron magnet material was washed with water and pickled with a 3 wt% nitric acid solution to remove surface oxides. Finally, ultrasonic cleaning and washing with water were repeated twice to obtain a neodymium-iron-boron magnet.
A neodymium iron boron magnet was immersed in isopropanol for 20 minutes and then removed and blown dry to obtain a preform. Arrange the preforms on the copper mesh in an orderly manner, turn on the tunnel heating furnace to heat it to a temperature of 500 ° C, open the nitrogen valve, control the nitrogen gas flow rate in the furnace to 50 L / min, and after 1 minute, preform A copper net was placed at the feed port of the belt, sent from the belt into a tunnel heating furnace, and heated for 25 minutes to obtain a neodymium-iron-boron magnet having an oxide layer on the surface. The thickness of the oxide layer was 0.7 μm.

As can be seen from Table 1, the neodymium-iron-boron magnets according to the present invention have significantly improved corrosion resistance. As can be seen from Example 1 and Comparative Example 1, the neodymium iron boron magnet not impregnated with isopropyl alcohol (Comparative Example 1) was inferior in corrosion resistance after heat treatment. The neodymium-iron-boron magnet impregnated with isopropyl alcohol (Example 1) had significantly improved corrosion resistance after heat treatment.

By adjusting the heat treatment conditions, the corrosion resistance of the neodymium-iron-boron magnet can be further improved. According to Examples 1 to 3, the heat treatment temperature was moderately increased and the heat treatment time was increased, thereby increasing the thickness of the oxide layer and improving the corrosion resistance of the neodymium-iron-boron magnet. As can be seen from the comparison between Example 4 and Example 3, increasing the heat treatment temperature requires correspondingly increasing the nitrogen gas flow rate to avoid excessive oxidation, otherwise the corrosion resistance decreases.

As can be seen from the comparison between Example 1 and Comparative Examples 2 and 3, both too low and too high heat treatment temperatures are disadvantageous in improving the corrosion resistance of neodymium-iron-boron magnets.
The present invention is not limited to the above-described embodiments, and any modifications, improvements, replacements, etc. conceived by those skilled in the art shall be included in the scope of the present invention without departing from the spirit of the present invention.

Claims (9)

A corrosion resistance improver for neodymium iron boron magnets, comprising a hydroxyl compound, wherein the hydroxyl compound is represented by formula (I):

[In the above formula, R is selected from C2 to C15 hydrocarbon groups, and n is a natural number of 1 to 3]
An improver characterized by having a structure shown in The improver according to claim 1, wherein the hydroxyl compound is a monohydric alcohol or a dihydric alcohol, R is selected from C2-C15 alkyl, and n is 1 or 2. The improver according to claim 1, wherein the hydroxyl compound is a monohydric alcohol, R is selected from C3-C6 alkyl, and n is 1. The improver according to claim 1, wherein the hydroxyl compound is ethanol, n-propanol, isopropanol, n-butanol, isobutanol, n-pentanol, isoamyl alcohol, neopentyl alcohol, n-hexanol or isohexanol. forming a hydroxyl compound layer on the surface of a neodymium iron boron magnet to obtain a preform, wherein the hydroxyl compound has the formula (I):

[In the above formula, R is selected from C2 to C15 hydrocarbon groups, and n is a natural number of 1 to 3]
A surface treatment method for a neodymium iron boron magnet, characterized by having a structure shown in. The surface treatment according to claim 5, wherein the hydroxyl compound is a monohydric alcohol or a dihydric alcohol, R is selected from C2 to C15 alkyls, and n is 1 or 2. Method. 6. The surface treatment method according to claim 5, wherein a neodymium-iron-boron magnet is impregnated with the hydroxyl compound, taken out, and then blown dry to obtain a preform. 8. The surface treatment method according to any one of claims 5 to 7, comprising the step of heat-treating the preform to form an oxide layer on the surface of the neodymium-iron-boron magnet. The heat treatment is a step of placing the preform in a tunnel heating furnace and heating it at 380° C. or higher and 450° C. or lower for 25 minutes or more and 50 minutes or less. 9. The surface treatment method according to claim 8, comprising a step of controlling the flow rate to 100 L/min or less.