JP2007258698A - Method for manufacturing rare earth magnet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a manufacturing method capable of obtaining a rare earth magnet having excellent corrosion resistance. <P>SOLUTION: The method for manufacturing a rare earth magnet includes a process that makes a treating liquid containing acid having oxidizing property and boric acid and/or boric acid ion contact a surface of a magnetic element containing a rare earth element. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method for producing a rare earth magnet.

  Although a rare earth magnet containing a rare earth element has an excellent magnetic force, it contains a rare earth element that is easily oxidized as a main component, and therefore tends to have low corrosion resistance. Therefore, rare earth magnets are often configured such that a protective layer made of resin, plating, or the like is provided on the surface of a magnet body containing a rare earth element.

For the purpose of further improving the corrosion resistance of the rare earth magnet provided with this protective layer, the magnet body before forming the protective layer is treated with an acidic solution, specifically nitric acid ( For example, see Patent Document 1). By such treatment, the deteriorated layer, the oxide layer, and the like generated during processing of the surface of the magnet body are removed, and the magnet body is hardly deteriorated. As a result, the corrosion resistance of the rare earth magnet is improved.
JP-A-9-270346

  However, in recent years, the range of application of rare earth magnets has expanded more than ever, and even when the treatment with an acidic solution such as nitric acid as described above is performed, sufficient corrosion resistance cannot be obtained depending on the application. There was a case.

  Then, this invention is made | formed in view of such a situation, and it aims at providing the manufacturing method which can obtain the rare earth magnet which has the outstanding corrosion resistance.

  When the present inventors examined the cause of the progress of corrosion even in a rare earth magnet treated with nitric acid, the following was found. That is, the magnet body has a structure composed of a main phase and a grain boundary between the main phases, but the grain boundary may be etched by the treatment with nitric acid, and this etching forms voids in the magnet body. It was confirmed that In the rare-earth magnet, corrosive components such as moisture easily enter the inside of the magnet body due to the pores, and this contributes to corrosion of the magnet body such as dropping of the main phase. .

  Therefore, based on such knowledge, the present inventors can obtain a rare earth magnet with better corrosion resistance by using a treatment liquid that hardly causes etching of the grain boundary part during the surface treatment of the magnet body. As a result, the present invention has been completed.

  That is, the method for producing a rare earth magnet of the present invention includes a step of bringing an oxidizing acid and a treatment liquid containing boric acid and / or borate ions into contact with the surface of a magnet body containing a rare earth element. It is characterized by. In this step, the surface portion of the magnet body is removed by contact with the treatment liquid.

  As described above, in the method for producing a rare earth magnet according to the present invention, the magnet body is treated with a treatment liquid containing an oxidizing acid and boric acid and / or borate ions. It is possible to remove the work-affected layer or the like on the surface while suppressing. The cause is not necessarily clear, but is presumed as follows.

  That is, usually, when the acid comes into contact with the magnet body, the grain boundary portion tends to be selectively eluted. When an oxidizing acid such as nitric acid is used, dissolution of the main phase part may also occur, so the selective elution of the grain boundary part is reduced, but the grain boundary part is slightly easier to elute, so this part A depression may be formed. Once such a depression is formed, the pH of the depression is lower than that of the surrounding area. As a result, even when an oxidizing acid is used, the grain boundary portion may be etched in the depth direction from the surroundings. On the other hand, since the treatment liquid of the present invention is a combination of an oxidizing acid and boric acid or borate ions, it is considered that a buffering action can be exhibited. Therefore, by using such a treatment liquid, a decrease in pH at the recessed portion as described above is suppressed. As a result, it is considered that the etching at the grain boundary portion is suppressed and the surface of the magnet body is etched uniformly. However, the action is not limited to this.

  For the reasons as described above, in the production method of the present invention, a work-affected layer and the like can be satisfactorily removed, and a magnet body with few holes on the surface can be obtained. And by forming a protective layer on the surface of such a magnet body, a rare earth magnet having excellent corrosion resistance can be obtained.

  More specifically, it is preferable to remove from the surface of the magnet body to a depth region of 5 μm or more by contact with the treatment liquid. Under such conditions, it is easy to obtain a magnet body having a very uniform surface, and the work-affected layer on the surface of the magnet body is more reliably removed. As a result, a rare earth magnet having further excellent corrosion resistance can be obtained.

  ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to provide the manufacturing method which can obtain the rare earth magnet which has the outstanding corrosion resistance.

  Hereinafter, preferred embodiments of the present invention will be described.

  FIG. 1 is a diagram showing a rare earth magnet obtained by a manufacturing method according to a preferred embodiment of the present invention. Moreover, FIG. 2 is a figure which shows typically the cross-sectional structure which follows the II-II line | wire of the rare earth magnet shown in FIG. As illustrated, the rare earth magnet 1 is composed of a magnet body 2 and a protective layer 4 formed on the surface thereof.

  The magnet body 2 is a permanent magnet containing a rare earth element. In this case, rare earth elements refer to scandium (Sc), yttrium (Y), and lanthanoid elements belonging to Group 3 of the long-period periodic table. Examples of the lanthanoid element include lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium. (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), lutetium (Lu) and the like are included.

  Examples of the constituent material of the magnet body 2 include a material containing the rare earth element and a transition element other than the rare earth element in combination. In this case, the rare earth element is preferably at least one element selected from the group consisting of Nd, Sm, Dy, Pr, Ho, and Tb. These elements include La, Ce, Gd, Er, Eu, Tm, Yb, and More preferably, it further contains at least one element selected from the group consisting of Y.

  As transition elements other than rare earth elements, iron (Fe), cobalt (Co), titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), nickel (Ni), copper (Cu) At least one element selected from the group consisting of zirconium (Zr), niobium (Nb), molybdenum (Mo), hafnium (Hf), tantalum (Ta) and tungsten (W) is preferable, and Fe and / or Co are more preferable. preferable.

  More specifically, examples of the constituent material of the magnet body 2 include R—Fe—B and R—Co materials. In the former constituent material, R is preferably a rare earth element mainly composed of Nd. In the latter constituent material, R is preferably a rare earth element mainly composed of Sm.

  As a constituent material of the magnet body 2, an R—Fe—B constituent material is particularly preferable. When the magnet body 2 is of the R-Fe-B type, when the treatment liquid is brought into contact in the manufacturing method described later, the surface alteration layer and the like can be removed well, and the formation of pores is particularly effective. Can be reduced.

  The R-Fe-B-based magnet element body 2 has a main phase having a substantially tetragonal crystal structure, and a rare earth-rich phase having a high blending ratio of rare earth elements in the grain boundary portion of the main phase, and The structure has a boron-rich phase with a high boron atom content. These rare earth-rich phase and boron-rich phase are nonmagnetic phases that do not have magnetism. Such a nonmagnetic phase is usually contained in an amount of 0.5 to 50% by volume in the magnet constituting material. Moreover, the particle size of the main phase is usually about 1 to 100 μm.

  In such R—Fe—B-based constituent materials, the rare earth element content is preferably 8 to 40 atomic%. When the rare earth element content is less than 8 atomic%, the crystal structure of the main phase becomes almost the same as that of α-iron, and the coercive force (iHc) tends to decrease. On the other hand, if it exceeds 40 atomic%, a rare earth-rich phase is excessively formed, and the residual magnetic flux density (Br) tends to be small.

  The Fe content is preferably 42 to 90 atomic%. When the Fe content is less than 42 atomic%, the residual magnetic flux density decreases, and when it exceeds 90 atomic%, the coercive force tends to decrease. Furthermore, the B content is preferably 2 to 28 atomic%. If the B content is less than 2 atomic%, a rhombohedral structure is likely to be formed, and this tends to reduce the holding force. On the other hand, if it exceeds 28 atomic%, a boron-rich phase is excessively formed, and this tends to reduce the residual magnetic flux density.

  In the constituent materials described above, a part of Fe in the R—Fe—B system may be substituted with Co. Thus, if a part of Fe is replaced by Co, the temperature characteristics can be improved without deteriorating the magnetic characteristics. In this case, it is desirable that the amount of substitution of Co not be larger than the content of Fe. If the Co content exceeds the Fe content, the magnetic properties of the magnet body 2 tend to deteriorate.

  A part of B in the constituent material may be substituted with an element such as carbon (C), phosphorus (P), sulfur (S), or copper (Cu). By replacing a part of B in this way, the magnet body can be easily manufactured and the manufacturing cost can be reduced. At this time, the substitution amount of these elements is desirably an amount that does not substantially affect the magnetic properties, and is preferably 4 atomic% or less with respect to the total amount of constituent atoms.

  Further, from the viewpoint of improving the holding power and reducing the manufacturing cost, in addition to the above-described structure, aluminum (Al), titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), bismuth (Bi ), Niobium (Nb), tantalum (Ta), molybdenum (Mo), tungsten (W), antimony (Sb), germanium (Ge), tin (Sn), zirconium (Zr), nickel (Ni), silicon (Si) ), Gallium (Ga), copper (Cu), hafnium (Hf), or other elements may be added. These addition amounts are also preferably in a range that does not affect the magnetic properties, and are preferably 10 atomic% or less with respect to the total amount of constituent atoms. In addition, oxygen (O), nitrogen (N), carbon (C), calcium (Ca), and the like are conceivable as other components that are inevitably mixed. It may be contained in an amount of.

  The protective layer 4 can be applied without particular limitation as long as it is usually formed as a layer protecting the surface of the rare earth magnet. Examples thereof include a resin layer formed by painting or vapor deposition polymerization method, a metal layer formed by plating or vapor phase method, an inorganic layer formed by coating method or vapor phase method, an oxide layer, a chemical conversion treatment layer, and the like.

  As the resin layer, both a layer made of a thermosetting resin or a layer made of a thermoplastic resin can be applied. Thermosetting resins include phenolic resin, epoxy resin, urethane resin, silicone resin, melamine resin, epoxy melamine resin, thermosetting acrylic resin, polyimide resin, polyamideimide resin, polyparaxylylene resin, polyurea resin, epoxy A silicone resin, an acrylic silicone resin, etc. are mentioned. Moreover, as a thermoplastic resin, the vinyl resin which uses vinyl compounds, such as acrylic acid, ethylene, styrene, vinyl chloride, vinyl acetate, as a raw material is mentioned.

Examples of the metal layer include Ni, Ni-P, Cu, Zn, Cr, Sn, Al, or a combination of a plurality of layers made of these. The inorganic layer, alkali _silicates, oxides such as _{SiO 2, Al 2 O 3,} TiN, include those made of nitride or the like such as AlN.

  Further, examples of the oxide layer include a layer formed by oxidizing the surface of the magnet body 2. Such an oxide layer is mainly formed from an oxide derived from the element of the magnet body 2. For example, the rare earth element oxide or transition element oxide in the magnet body 2 is included.

  Furthermore, the chemical conversion treatment layer is a layer formed by subjecting the surface of the magnet body 2 to chemical conversion treatment or the like. For example, the layer which consists of metal phosphates, such as zinc phosphate, manganese phosphate, iron phosphate, zinc calcium phosphate, molybdate, chromate, etc. is mentioned.

  Next, a preferred method for manufacturing the rare earth magnet 1 having the above configuration will be described.

First, the magnet body 2 can be manufactured by a powder metallurgy method. In this method, first, an alloy having a desired composition is manufactured by a known alloy manufacturing process such as a casting method or a strip casting method. Next, this alloy is pulverized to a particle size of 10 to 100 μm using a coarse pulverizer such as a jaw crusher, a brown mill, or a stamp mill. Thereafter, the particle size is further adjusted to 0.5 to 5 μm by a fine pulverizer such as a jet mill or an attritor. The powder thus obtained is preferably molded at a pressure of 0.5 to ⁵ t / cm ² in a magnetic field having a magnetic field strength of 600 kA / m or more.

  Thereafter, the obtained compact is preferably sintered in an inert gas atmosphere or under vacuum at 1000 to 1200 ° C. for 0.5 to 10 hours and then rapidly cooled. Further, the sintered body is subjected to heat treatment at 500 to 900 ° C. for 1 to 5 hours in an inert gas atmosphere or vacuum, and the sintered body is processed into a desired shape (practical shape) as necessary. A magnet body 2 is obtained.

  In the production of the rare earth magnet 1, an acid treatment is then performed in which the surface of the magnet body 2 is brought into contact with an oxidizing acid and a treatment liquid containing boric acid and / or borate ions. Specifically, an aqueous solution is preferable as the treatment liquid. The contact of the treatment liquid with the magnet body 2 can be performed by, for example, a method of immersing the magnet body 2 in the treatment liquid or a method of spraying the treatment liquid onto the magnet body 2.

  Examples of the acid having an oxidizing property contained in the treatment liquid include an acid having an oxidizing property such as nitric acid, perchloric acid, and chromic acid. A combination of non-oxidizing acids such as sulfuric acid, hydrochloric acid, phosphoric acid, and acetic acid with oxidizing agents such as nitric acid or nitrate, nitrous acid or nitrite, hydrogen peroxide, and permanganate is also oxidizable. It can be applied as an acid having. Among these, nitric acid that can favorably remove the altered layer on the surface of the magnet body 2 is preferable. Here, as the acid used for the treatment liquid, for example, when an acid having no oxidizing property such as sulfuric acid or hydrochloric acid is used, the grain boundary tends to be dissolved more easily than the main phase of the magnet body 2, and the deteriorated layer or the like is used. It cannot be removed sufficiently. In addition, hydrogen generated by the acid may be occluded on the surface of the magnet body 2 and the occlusion site may become brittle. On the other hand, by using an acid having an oxidizing property as described above, the surface of the magnet body 2 can be uniformly dissolved while avoiding embrittlement, and the altered layer on the surface can be removed well. It becomes possible. The boric acid contained in the treatment liquid is preferably at least one of oxygen acids generated by hydrating diboron trioxide, such as orthoboric acid, metaboric acid, and tetraboric acid. Further, the borate ion is preferably at least one of ions generated by ionizing these acids in water.

  Such a treatment liquid is preferably obtained by dissolving an acid having oxidizing properties and a compound containing boron such as boric acid or a boric acid salt in a solvent (preferably water). Examples of the compound containing boron include boric acid (orthoboric acid), metaboric acid, tetraboric acid, sodium metaborate, potassium metaborate, ammonium metaborate, sodium tetraborate, potassium tetraborate, ammonium tetraborate, Examples thereof include lithium tetraborate, sodium pentaborate, potassium pentaborate, ammonium pentaborate and hydrates thereof.

  In particular, in the case of nitric acid, the concentration of the oxidizing acid in the treatment liquid is preferably 2 mol / L or less, more preferably 0.01 to 1 mol / L, and 0.05 to 0.5 mol / L. Is more preferable. When the nitric acid concentration in the treatment liquid is less than 0.01 mol / L, the amount of dissolution of the surface of the magnet body 2 becomes insufficient, and it tends to be difficult to sufficiently remove the deteriorated layer and the like. On the other hand, if it exceeds 2 mol / L, the dissolution rate of the magnet body 2 becomes too high, and it becomes difficult to control the amount of dissolution, and it tends to be difficult to maintain the dimensional accuracy of the magnet body 2.

  The concentration of boric acid and / or borate ions in the treatment liquid is preferably 0.01 to 0.7 mol / L, more preferably 0.05 to 0.7 mol / L, as the boron concentration. preferable. If this concentration is less than 0.01 mol / L, it tends to be difficult to suppress etching of the grain boundary portion by acid. On the other hand, when it exceeds 0.7 mol / L, precipitation of undissolved material tends to occur, and the processability tends to decrease. The treatment liquid may contain both boric acid and borate ions. In this case, it is preferable that the total concentration of these be in the above range.

  By such treatment with the treatment liquid, the surface portion of the magnet body 2 is dissolved and removed. This removal amount is preferably 5 μm or more in average thickness from the surface of the magnet body 2, and more preferably 10 to 15 μm or more. Thereby, the surface altered layer and the oxide layer generated during the processing of the magnet body 2 and the like can be reliably removed. However, if the removal amount is too large, the magnet size becomes too small and it becomes difficult to obtain sufficient magnetic properties. Therefore, the removal thickness by dissolution of the magnet body 2 is preferably 100 μm or less. In order to obtain such a good dissolution amount, for example, when the magnet body 2 is immersed in the treatment liquid, the immersion time is preferably 30 seconds to 30 minutes, and preferably 1 minute to 10 minutes. More preferred.

  After the treatment with the treatment liquid, in order to remove undissolved substances and residual acid components adhering to the surface of the magnet body 2, it is preferable to wash the magnet body 2, preferably using ultrasonic waves. This ultrasonic cleaning is preferably performed in pure water with very few chlorine ions that generate rust on the surface of the magnet body 2. Further, before and after the washing and in each process of the treatment with the treatment liquid, washing with water may be performed as necessary.

  Then, the rare earth magnet 1 is obtained by forming the protective layer 4 on the surface of the magnet body 2. For example, in the case of a resin layer, the protective layer 4 is prepared by dissolving the above-described resin in a solvent to prepare a coating solution, which is applied to the surface of the magnet body 2 by dip coating, dip spin coating, spray coating, or the like. After application, it can be formed by baking or drying. The resin layer can also be formed by a so-called polymerization vapor deposition method in which a monomer for forming the resin layer is vaporized and vapor deposited on the surface of the magnet body and polymerized at the same time.

  The metal layer can be formed by subjecting the magnet body 2 to electroplating, electroless plating, or the like. In addition, gas phase methods such as ion plating, sputtering, and vapor deposition can also be applied. In addition, the inorganic layer can be formed by applying an alkali silicate solution or a metal alkoxide solution to the surface of the magnet body 2 and then baking or drying it. In addition, an ion plating method, a CVD method, etc. It can also be formed using a vapor phase method.

  Furthermore, the oxide layer can be formed, for example, by treating the magnet body 2 in an oxidizing atmosphere. Examples of the oxidizing atmosphere include air, an oxygen atmosphere, and a water vapor atmosphere. For example, the oxide layer is satisfactorily formed by heating the magnet body 2 under these oxidizing atmospheres.

  Furthermore, when the chemical conversion treatment layer forms a layer made of a metal phosphate, for example, the surface of the magnet base 2 is treated with a chemical conversion treatment liquid containing phosphoric acid, an acid, and a metal. It can be formed by precipitating a metal phosphate on the surface of 2.

  In such a method of manufacturing the rare earth magnet 1, the alteration layer or the oxide layer on the surface generated during the processing of the magnet body 2 is dissolved and removed by the treatment of the magnet body 2 with the treatment liquid described above. In addition, the surface of the magnet body 2 is smoothed by uniformly dissolving the surface. As a result, the deterioration of the magnet body 2 itself is less likely to occur than when a deteriorated layer or the like remains on the surface of the magnet body 2. Further, since the smoothness of the surface of the magnet body 2 is increased, the entire surface of the magnet body 2 can be satisfactorily covered even when a thin protective layer is formed. As a result, the rare-earth magnet 1 having a small number of pinholes in the protective layer 4 and excellent corrosion resistance can be obtained.

  In particular, the treatment liquid contains a combination of an acid having an oxidizing property and boric acid and / or borate ions, so that the grain boundary portion of the magnet body 2 is compared with the case where the treatment is performed using only the acid. Etching can be reduced. As a result, the magnet body 2 has few holes in the depth direction from the surface, and even when a corrosive substance such as moisture penetrates the protective layer 4, internal magnetism hardly occurs. . Due to the above factors, according to the embodiment described above, the rare earth magnet 1 having excellent corrosion resistance can be obtained.

EXAMPLES Hereinafter, although an Example demonstrates this invention still in detail, this invention is not limited to these Examples.
[Manufacture of rare earth magnets]

Example 1
By powder metallurgy, an ingot having a composition of 27.6Nd-4.9Dy-0.5Co-0.4Al-0.07Cu-1.0B-remaining Fe (numbers represent weight percentages) is produced. This was coarsely pulverized. Thereafter, jet milling with an inert gas was performed to obtain a fine powder having an average particle size of about 3.5 μm. The obtained fine powder was filled in a mold and molded in a magnetic field. Next, after sintering in vacuum, heat treatment was performed to obtain a sintered body. The obtained sintered body was cut into a size of 20 mm × 10 mm × 2 mm and processed to obtain a magnet body.

  Next, the obtained magnet body was subjected to acid cleaning under the condition of immersing in a treatment solution in which 0.4 mol / L of nitric acid and 0.1 mol / L of boric acid were dissolved in pure water for 2 minutes. Sonic water washing was performed.

  Thereafter, the magnet body was dried, and after spraying an epoxy resin paint on its surface, it was baked at 180 ° C. for 30 minutes to form a protective layer made of a resin having a thickness of 10 μm to obtain a rare earth magnet. .

(Example 2)
A rare earth magnet was prepared in the same manner as in Example 1 except that the acid cleaning was performed under the condition of immersing in a treatment solution in which 0.4 mol / L of nitric acid and 0.4 mol / L of boric acid were dissolved in pure water for 2 minutes. Obtained.

(Example 3)
Rare earth in the same manner as in Example 1 except that the acid cleaning was performed for 2 minutes in a treatment solution in which 0.4 mol / L nitric acid and 0.1 mol / L sodium tetraborate were dissolved in pure water. A magnet was obtained.

Example 4
A rare earth magnet was prepared in the same manner as in Example 1 except that the acid cleaning was performed under the condition of immersing in a treatment solution in which 0.4 mol / L nitric acid and 0.05 mol / L boric acid were dissolved in pure water for 2 minutes. Obtained.

(Example 5)
A rare earth magnet was prepared in the same manner as in Example 1 except that the acid cleaning was performed under the condition of immersing in a treatment solution in which 0.2 mol / L of nitric acid and 0.01 mol / L of boric acid were dissolved in pure water for 3 minutes. Obtained.

(Example 6)
A rare earth magnet was prepared in the same manner as in Example 1 except that the acid cleaning was performed under the condition of immersing in a treatment solution in which 0.1 mol / L nitric acid and 0.005 mol / L boric acid were dissolved in pure water for 5 minutes. Obtained.

(Comparative Example 1)
A rare earth magnet was obtained in the same manner as in Example 1 except that acid washing was not performed.

(Comparative Example 2)
A rare earth magnet was obtained in the same manner as in Example 1 except that the acid cleaning was performed under the condition that the nitric acid was 0.4 mol / L and immersed in a treatment solution not containing boric acid for 2 minutes.

(Comparative Example 3)
A rare earth magnet was obtained in the same manner as in Example 1 except that the acid cleaning was performed under the condition that the boric acid was 0.2 mol / L and was immersed in a treatment solution containing no nitric acid for 30 minutes.

(Comparative Example 4)
A rare earth magnet was obtained in the same manner as in Example 1 except that the acid washing was performed for 2 minutes in a treatment solution in which oxalic acid was dissolved at the same concentration instead of boric acid.

(Comparative Example 5)
A rare earth magnet was obtained in the same manner as in Example 1 except that the acid cleaning was performed under the condition of immersing in a treatment solution in which phosphoric acid was dissolved at the same concentration instead of boric acid for 2 minutes.

(Comparative Example 6)
A rare earth magnet was obtained in the same manner as in Example 1 except that the acid cleaning was performed under the condition of immersing in a treatment solution in which acetic acid was dissolved at the same concentration instead of boric acid for 2 minutes.

(Comparative Example 7)
A rare earth magnet was obtained in the same manner as in Example 1 except that the acid washing was performed for 2 minutes in a treatment solution in which trisodium citrate was dissolved at the same concentration instead of boric acid.
[Characteristic evaluation]

(Measurement of removal amount of magnet body by acid cleaning)
In the production of the rare earth magnets of Examples 1 to 6 and Comparative Examples 1 to 7, when the magnet body was subjected to acid cleaning, the amount of the magnet body dissolved and removed by the acid cleaning was determined as the magnet element before acid cleaning. It was evaluated by the thickness (μm) of the part removed from the body. The thickness value was obtained by calculating the removal thickness on one side from the change in thickness dimension before and after acid cleaning measured with a micrometer. And the value of removal thickness was made into the average value of the value obtained by measuring in 5 points | pieces. The obtained results are shown in Table 1.

(Evaluation of protective layer peeling, measurement of weight loss and tape test)
The rare earth magnets of Examples 1 to 6 and Comparative Examples 1 to 7 were each subjected to a pressure cooker test (PCT test) under the conditions of 120 ° C., 0.2 MPa, 100% RH, and 100 hours, and the appearance after the test. While observing (presence or absence of peeling of the protective layer), the amount of decrease in weight after the test relative to before the test was measured. In addition, for each rare earth magnet after the PCT test, in accordance with a grid surface tape test specified in JIS K5600 (1999), cuts penetrating the layer are formed in a grid pattern on the protective layer, and an adhesive tape is applied thereon. The test which evaluates the presence or absence of peeling of a protective layer when peeling after extending | stretching was done. The results obtained are summarized in Table 2.

In Table 2, the evaluation of peeling of the protective layer was described in accordance with A: no peeling, B: partial peeling only at corners, and C: peeling at corners and portions other than corners. Moreover, evaluation of the tape test was performed according to the classification based on JISK5600. In the evaluation, a six-stage evaluation is performed from classification 0 in which no peeling was observed to classification 5 in which most of the peeling occurred, and the smaller the number, the smaller the peeling.

  From Table 1, since the rare earth magnets of Examples 1 to 6 had no peeling of the protective layer after the PCT test and no weight reduction was observed, the deterioration due to the PCT test was small, and the corrosion resistance was excellent. Turned out to be. Moreover, since the peeling of the protective layer was not observed in the cross-cut tape test, it was confirmed that the adhesion between the magnet body and the protective layer was also good. On the other hand, in the rare earth magnets of Comparative Examples 1 to 7, peeling of the protective layer and weight reduction occurred after PCT, and it was confirmed that the corrosion resistance was inferior to that of the example.

It is a figure which shows the rare earth magnet obtained by the manufacturing method which concerns on suitable embodiment. It is a figure which shows typically the cross-sectional structure which follows the II-II line | wire of the rare earth magnet shown in FIG.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Rare earth magnet, 2 ... Magnet base body, 4 ... Protective layer.

Claims (3)

  A method for producing a rare earth magnet, comprising a step of bringing an oxidizing acid and a treatment liquid containing boric acid and / or borate ions into contact with a surface of a magnet body containing a rare earth element.   The method for producing a rare earth magnet according to claim 1, wherein in the step, a surface portion of the magnet body is removed by contact with the treatment liquid.   3. The method for producing a rare earth magnet according to claim 1, wherein, in the step, a part of the magnet body is removed from the surface to a depth region of 5 μm or more by contact with the treatment liquid.

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