WO1999002337A1 - Passivation haute temperature d'aimants a base de terres rares - Google Patents

Passivation haute temperature d'aimants a base de terres rares Download PDF

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Publication number
WO1999002337A1
WO1999002337A1 PCT/US1998/014468 US9814468W WO9902337A1 WO 1999002337 A1 WO1999002337 A1 WO 1999002337A1 US 9814468 W US9814468 W US 9814468W WO 9902337 A1 WO9902337 A1 WO 9902337A1
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WIPO (PCT)
Prior art keywords
rare earth
approximately
magnet
magnets
neodymium
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PCT/US1998/014468
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English (en)
Inventor
Alexander N. Savich
Olga Gennadievna Ospennicova
Vadim Petrovich Piskorskiy
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Aura Systems, Inc.
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Publication date
Priority claimed from RU97111723/09A external-priority patent/RU97111723A/ru
Application filed by Aura Systems, Inc. filed Critical Aura Systems, Inc.
Priority to AU83991/98A priority Critical patent/AU8399198A/en
Publication of WO1999002337A1 publication Critical patent/WO1999002337A1/fr

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    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C8/06—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases
    • C23C8/08—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases only one element being applied
    • C23C8/10—Oxidising
    • C23C8/16—Oxidising using oxygen-containing compounds, e.g. water, carbon dioxide
    • C23C8/18—Oxidising of ferrous surfaces
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
    • H01F1/00—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties
    • H01F1/01—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials
    • H01F1/03—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity
    • H01F1/032—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of hard-magnetic materials
    • H01F1/04—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of hard-magnetic materials metals or alloys
    • H01F1/047—Alloys characterised by their composition
    • H01F1/053—Alloys characterised by their composition containing rare earth metals
    • H01F1/055—Alloys characterised by their composition containing rare earth metals and magnetic transition metals, e.g. SmCo5
    • H01F1/057—Alloys characterised by their composition containing rare earth metals and magnetic transition metals, e.g. SmCo5 and IIIa elements, e.g. Nd2Fe14B
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
    • H01F1/00—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties
    • H01F1/01—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials
    • H01F1/03—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity
    • H01F1/032—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of hard-magnetic materials
    • H01F1/04—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of hard-magnetic materials metals or alloys
    • H01F1/047—Alloys characterised by their composition
    • H01F1/053—Alloys characterised by their composition containing rare earth metals
    • H01F1/055—Alloys characterised by their composition containing rare earth metals and magnetic transition metals, e.g. SmCo5
    • H01F1/057—Alloys characterised by their composition containing rare earth metals and magnetic transition metals, e.g. SmCo5 and IIIa elements, e.g. Nd2Fe14B
    • H01F1/0571—Alloys characterised by their composition containing rare earth metals and magnetic transition metals, e.g. SmCo5 and IIIa elements, e.g. Nd2Fe14B in the form of particles, e.g. rapid quenched powders or ribbon flakes
    • H01F1/0572—Alloys characterised by their composition containing rare earth metals and magnetic transition metals, e.g. SmCo5 and IIIa elements, e.g. Nd2Fe14B in the form of particles, e.g. rapid quenched powders or ribbon flakes with a protective layer
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
    • H01F41/00—Apparatus or processes specially adapted for manufacturing or assembling magnets, inductances or transformers; Apparatus or processes specially adapted for manufacturing materials characterised by their magnetic properties
    • H01F41/02—Apparatus or processes specially adapted for manufacturing or assembling magnets, inductances or transformers; Apparatus or processes specially adapted for manufacturing materials characterised by their magnetic properties for manufacturing cores, coils, or magnets
    • H01F41/0253—Apparatus or processes specially adapted for manufacturing or assembling magnets, inductances or transformers; Apparatus or processes specially adapted for manufacturing materials characterised by their magnetic properties for manufacturing cores, coils, or magnets for manufacturing permanent magnets
    • H01F41/026—Apparatus or processes specially adapted for manufacturing or assembling magnets, inductances or transformers; Apparatus or processes specially adapted for manufacturing materials characterised by their magnetic properties for manufacturing cores, coils, or magnets for manufacturing permanent magnets protecting methods against environmental influences, e.g. oxygen, by surface treatment

Definitions

  • the present invention relates to permanent magnets manufactured from rare earth elements having corrosion resistant surfaces and a method for the preparation thereof. More particularly, the invention relates to permanent magnets formed from alloys primarily comprising neodymium, iron and boron, which are formed with a high temperature passivating film that is highly resistant to corrosion and oxidation.
  • Rare earth magnets are composed of metal alloys and mixtures that are well known in the art and the literature.
  • Rare earth magnets generally contain various percentages of at least one rare earth element from the lanthanides group of elements.
  • Other components include non-rare earth elements such as iron, cobalt, nickel and boron. All of these commonly used magnetic ingredients may be formed into alloys or mixtures containing one or more rare earth elements that are generally known to exhibit good magnetic properties.
  • rare earth magnets are often described and identified according to their relative compositions of rare earth elements and other constituents.
  • commonly preferred types of rare earth magnets contain alloys of neodymium or praseodymium in combination with iron and boron. These magnetic alloys may further include a wide range of other elements such as aluminum, gallium, copper, iron, zirconium, titanium, cobalt, carbon, palladium, uranium and the like.
  • Other rare earth magnets may be formed from alloys of samarium, cobalt and iron.
  • Still other rare earth magnets may include alloys containing appropriate proportions of dysprosium and cobalt, and other suitable magnetic materials.
  • rare earth magnets or magnetic materials are readily oxidized and eventually corrode. This undesirable effect reduces the magnetic retentive capacity of the magnets, and is particularly significant at elevated temperatures above approximately 125- 150°C.
  • the use of such reactive rare earth elements in these magnets frequently limits their application to surrounding temperatures under 100 °C.
  • rare earth magnets especially in their basic powdered form, are highly reactive when exposed to oxygen, moisture, salt-containing environments, and chemical agents containing chloride ions. The rare earth elements react similarly in these conditions as members of the same chemical group.
  • rare earth magnets are well known, and their extensive applications greatly realized, there exists a continuous need to improve their resistance to oxidation and corrosion stability when exposed to various operating environments. For this reason, it has been proposed to provide an oxidation-resistant protective layer on the surfaces of rare earth magnets.
  • a wide variety of protective layers for rare earth magnets may be formed from many conventional materials including oxidation resistant compositions and mixtures, or corrosion inhibitors that passivate or substantially abate the reactivity of magnets. It is generally understood and acknowledged that a binding reaction occurs between the oxidation resistant composition or the corrosion inhibitor with the surface of the rare earth magnet. As a result of this surface interaction, the oxidation resistant composition or corrosion inhibitor is attached or adhered to the surface of the particle in the form of a protective layer or coating. These coatings substantially encapsulate or surround the magnetic bodies, and to some degree, partially neutralize the reactivity of their magnetic constituents. In general, these types of passivating agents are added and mixed with the magnetic material during processing. Basic magnetic material often exists in the form of larger particles that are reduced in size through grinding or other conventional methods. An effective amount of the oxidation resistant composition and/or corrosion inhibitor may be immediately added upon breaking or shearing of the magnet particles.
  • passivating agents have been found to generally render inert the otherwise exposed surfaces of magnetic particles or aggregations thereof. Before any considerable oxidation can occur, the passivating material is applied to minimize or immediately arrest corrosion.
  • Rare earth magnets and magnetic materials may also be formed into larger sized magnets by a process known as sintering.
  • particles of a rare earth magnet or magnet material may be sintered through standard techniques into a much larger article with the desired density and configuration for an end use product.
  • passivating agents, or other suitable antidegradant agents may be applied to the surface of the sintered rare earth article in any conventional manner such as by spraying, brushing or dipping.
  • Various configurations for the protected magnetic material may be achieved that are suitable for use in a wide variety of electromechanical devices including motors, coils, actuators, etc.
  • Magnetic articles may also be formed by mixing the rare earth magnet or magnetic materials with binder agents that assist in forming the magnetic bodies into a desired configuration.
  • NdFeB magnets Permanent rare earth magnets consisting of neodymium, iron and boron, hereinafter referred to as the NdFeB magnets, have several distinct advantages over other rare earth magnets that may be based on samarium or cobalt. NdFeB magnets generally provide a high degree of magnetic performance and exhibit magnetic properties that are often far better than those of conventional samarium or cobalt based rare earth permanent magnets. It has been observed that less neodymium is required for rare earth magnets as compared to these other conventional permanent magnets. Neodymium is also naturally more abundant than sarmarium, and is available in sufficient quantities as a substantial magnet constituent.
  • Rare earth permanent magnets containing neodymium as a rare earth element are especially favored, and are continuously replacing the samarium or cobalt based rare earth permanent magnets in relatively smaller types of magnetic circuits.
  • the economical advantages of shifting to neodymium based rare earth magnets has even motivated their use in various applications that previously relied upon less effective ferrite materials or costly electromagnets.
  • the demand for NdFeB magnets has rapidly grown, along with their expansion in applications, as high performance magnets in many common devices such as electric motors, actuators and sensors. Because of their excellent magnetic properties and relatively low cost, these types of magnets are also used extensively in the electric and electronic industrial fields, particularly in electrical parts for the automobile industry. The constantly progressing technology in these fields demands further improvements in the performance of these magnets.
  • NdFeB magnets possess a relatively high standard electrode potential (E298)- For all the rare earth metals, this value (E298) ranges from approximately -
  • NdFeB magnets are highly sensitive to electrochemical corrosion. Neodymium alone has an unfavorable tendency to also easily oxidize in air, and particularly, in moist air.
  • This oxidation not only gives rise to the formation of an oxide layer on the surfaces of the magnet, but it also proceeds inwardly into the magnetic body to cause intergranular corrosion which typically develops along the grain boundary of the alloy. This phenomenon is the most noticeable in the neodymium based magnets because of the very active neodymium enriched phase that often exists in the grain boundary of these structures.
  • the function of the cathode is performed by grains of the main magnetic phase of the alloy, Nd2 Fei4 B.
  • the anodic function is performed by the intergranular neodymium enriched phase of the alloy and the isolates of the NdFe4B4 phase.
  • the amount of NdFe4B4 phase alloy in conventional commercial magnets typically ranges from approximately 80 to 85% in volume.
  • the relative proportions of Nd2 ⁇ 3 is approximately 1-5% in volume, and that of the Nd Fe4 B4 phase may be up to 8% in volume.
  • the remaining composition of the alloy mainly consists of the neodymium enriched intergranular phase alloy.
  • the formation of the resultant oxide on the magnet surfaces degrades the output of the magnetic circuit and impinges upon its performance.
  • the oxide also has a fairly loose structure and sheds from the magnet surface thereby smearing other devices disposed nearby.
  • the resulting oxidation and corrosion presents various problems that only proliferate over time.
  • NdFeB magnets which include their lack of resistance to corrosion or oxidation as with most rare earth magnets, there is a continuous need to develop a highly corrosive resistant NdFeB magnet.
  • Several methods have been proposed for improving the corrosion resistance of NdFeB magnets including the formation of a protective film by electrolytic or electroless nickel plating, or aluminum-ion chromating.
  • Other proposed techniques for providing metallic articles with corrosion proof or rust proof layers include the spray coating or application of various epoxy resins.
  • a conventional method for protecting active metals involves the application of chemically stable galvanic coatings of transition metal elements such as nickel, zinc, chromium, or other suitable protective elements.
  • Application of these protective coatings often presents severe disadvantages that weigh against their utility. For example, many problems arise when attempting to form coatings of metals such as zinc over magnetic bodies. The devices used to form these types of coatings are often complicated and bulky. The method itself is also deficient in operational efficiency, incapable of mass-production, and a relatively expensive process. While the addition of these coatings may be effective in improving resistance to corrosion, they are also often detrimental to the magnetic properties of rare earth magnets, including NdFeB magnets, and tends to dilute their strength. The relative amounts of these protective materials surrounding a magnetic article is therefore limited to a very small quantity.
  • NdFeB magnets The inherent pores and cracks in NdFeB magnets also complicate the process of applying galvanic coatings to their surfaces.
  • protective layers for these rare earth magnets include an electroplated nickel layer, an aluminum ion-plated layer, or films formed of other suitable material.
  • the process of nickel electroplating is advantageous in that the resulting surface protective layer is excellent in mechanical strength, and the layer will not in itself appreciably absorb humidity.
  • the plating current tends to concentrate on outer peripheral portions of irregular surfaces since it is difficult for the plating current to pass through inner holes and inner peripheral portions.
  • Magnets having peculiar shapes, such as a cylindrical magnet often have inner peripheral portions that are hardly coated with the electroplated nickel layer. As a result, the film thickness becomes relatively thin in these regions and a sufficient degree of uniformity in the film thickness cannot be achieved by nickel electroplating alone.
  • coatings derived from materials such as phosphate or chromate may deteriorate the physical properties of substrate metals and diminish their metallic properties.
  • Other relatively complicated procedures which include vapor plating methods, vacuum deposition, ion spattering, and ion plating, are also costly and are often not effective in coating surfaces of magnetic bodies that are formed with irregularities such as holes, cracks and grooves.
  • all known metal coatings are generally cathodic due to their relatively lower standard electrode potential values (E298) comparison to rare earth metals, any disturbance in the coating continuity (e.g., micropores) may intensify the process of corrosion as compared to a noncoated magnet. This typically leads to complete deterioration of the protective coating and may interfere with the performance of the magnet and its host apparatus.
  • NdFeB magnet when a NdFeB magnet is provided with metal plating, its corrosion resistance greatly depends on the surface condition of the magnet body. Its resistance to corrosion is severely decreased when an oxidized or degraded layer is already formed on the magnet surface which has poor magnetic properties, or is formed with pores or any other type of surface irregularity. Permanent magnets formed with the aforementioned corrosion proof coatings provide limited effectiveness in resisting corrosion. The passivation of magnetic surfaces by these involved and complicated methods therefore involve the addition of a non-magnetic layer, the possible deterioration of magnetic properties, and the needless etching or otherwise altering of magnetic body surfaces which may weaken the overall strength of the magnet.
  • these oxidation resistant compositions may include epoxy silanes and/or an epoxy resin compound.
  • Forming resinous coatings generally requires a relatively large layer thickness because the applied layer of resin often has a particularly porous texture. The thick coating of resin consequently formed may also hinder the application for which the coated article is intended. For example, when the magnet is used as a rotor magnet for a motor, the motor would tend to have an undesirable large air gap between the coated rotor magnet and the stator which significantly reduces the potential torque capacity of a motor.
  • the present invention provides rare earth magnets with corrosive resistant films that are formed through high temperature thermal treatment.
  • the methods disclosed for producing high temperature passivating films may be conducted in a vacuum environment. Except as specifically described herein, the equipment used for application of these high temperature passivating films are that of a standard design ordinarily used in the manufacture of magnets. Of course, the present invention may be carried out in other suitable equipment that achieves the conditions and intended goals with rare earth magnets as described herein.
  • the treatment process itself is sufficiently efficient and productive to be carried out on a relatively large scale.
  • the present invention is directed to the passivation of rare earth magnets in order to greatly reduce their tendency to corrode without adversely affecting their magnetic properties, and their structural and chemical integrity.
  • the rare earth magnet if formed with a passivating layer when subjected to high temperature thermal treatment and regulated pressure.
  • the corrosion resistance of NdFeB magnets may be increased by the disclosed methods of high temperature passivation in a vacuum environment where the passivated layer formed on magnets may range from approximately 10 to 30 microns in thickness. The thickness of the passivated layer may vary beyond this particular range.
  • Rare earth magnets formed with varying thicknesses of this passivated layer achieve protection against corrosion in accordance with the present invention.
  • Another variation of the present invention includes a method of forming a corrosion resistant passivating layer on the surface of NdFeB magnets by high temperature thermal processing in a controlled environment.
  • the process of forming these protective films presents no stringent requirements as to the surface quality of magnets. Magnets manufactured with existing cleavages and microcracks are permissible, and demonstrate a high resistance to corrosion upon treatment according to disclosed methods.
  • This flexible method of passivating rare earth magnets such as NdFeB magnets is also directed to avoiding complicated inspection procedures for selecting suitable magnets, and limiting the percentage of rejected magnets. These methods of high temperature passivation do not place strict requirements for a high finish surface on the magnet bodies.
  • the passivating film may be readily formed on nonuniform surfaces having cleavages, cracks or other deformities.
  • passivating films are formed onto surfaces of rare earth magnets that do not worsen their magnetic and structural properties. The magnetic characteristics of the formed structure remain intact after passivation, and there is no separation of soft magnetic phases in the material. Application of passivating films by the disclosed methods of high temperature passivation produces no adverse effect on the magnetic characteristics of the structure.
  • the passivating layer may consist of oxides derived from various rare earth metals and other alloying elements present in the magnet.
  • the protective film adheres strongly to the magnet surface and stands well against various mechanical effects such as scratching without detriment to its protective properties against corrosion and oxidation.
  • Another variation of the present process includes the formation of a high temperature passivating layer that arrests the process of corrosion and prevents its proliferation. If corrosion centers develop on the rare earth magnet, the corrosion process still does not take on an avalanche-like nature as with other coatings. The formation of any corrosive products does not include coarse or loosely dispersed particles or contaminants. Pursuant to these disclosed methods, corrosion is not permitted to spread over the entire surface of a magnetic body.
  • FIG. 1(a) is a photograph of a magnet formed in accordance with the principles of the present invention generated by a scanning electron microscope (SEM) when viewed perpendicular relative to the high temperature passivated surface.
  • FIG. 1(b) is a photo micrograph of another magnet formed in accordance with the principles of the present invention generated by a scanning electron microscope (SEM) when viewed perpendicular relative to the high temperature passivated surface.
  • FIG. 2 is a graphical description of temperature and pressure variations during another sample heat passivation process described in this embodiment of the invention.
  • FIG. 3 is a graphical description of the temperature and pressure variations during a sample heat passivation process described in this embodiment of the invention.
  • the concepts of the present invention may be applied to rare earth magnets generally regardless of their chemical composition or geometrical shape.
  • the concepts of the present invention may of course be evenly applied to other rare earth magnets with varying dimensions and configurations.
  • the magnets described herein were manufactured according to a wet forming method, including milling in a C2H5OH medium,
  • the magnets were basically divided into two groups differing in alloy composition, charge materials and manufacturing methods.
  • Group A Group A
  • the alloy was melted with metallic neodymium.
  • Group B magnets were made from the base alloy (65%) with additions of dysprosium, neodymium and boron. Metallic neodymium was used for melting the base alloy only. The additions were melted from the charge of an approximate composition based on the alloy 30%Nd-l%B-Fe.
  • a chemically homogeneous film may be produced on the magnet surface by high temperature passivation in a vacuum environment. Elevated temperatures are generally known to promote the occurrence of diffusive processes.
  • the homogeneous passivating film may be formed by ensuring a dynamic equilibrium between the reactions of formation and the reactions of decomposition of the passivating film. These competing reactions are controlled by selecting appropriate conditions including preferable temperatures, surrounding pressure and time of exposure. High temperature passivation of the NdFeB test magnets were carried out in standard electric vacuum resistance furnaces such as a CHB3 type apparatus that is typically used for baking and heat treatment applications.
  • the basic process for high temperature passivation of these magnets may consist of heat treatment at a temperature of approximately 1000°C for about 30 minutes in a vacuum environment where the surrounding pressure ranges from approximately 1.0 x 10"5 to 1.0 x 10"" mm Hg. It should be noted that these selected values were the particular maximum limit for the furnace used in the passivating process, and does not in any way restrict the scope of the present invention which may achieve a higher temperature passivated layer beyond these particular temperature or pressure values.
  • the surfaces of magnets were prepared in advance by degreasing and chemical pickling. Having selected preferable methods of surface preparation, the accompanying conditions of high temperature passivation were varied which included changing the temperature, time of exposure, and surrounding pressure.
  • the resistance of the NdFeB magnets studied were subjected to various forms of accelerated testing methods.
  • the method of accelerated testing used with the present invention did not change the corrosion mechanism of magnets that is encountered in actual service.
  • These methods comply with the typical working modes for NdFeB magnets, and are relatively simple to reproduce for purposes of testing.
  • the magnets intended for service in a relatively humid climate should be tested in the presence of NaCl, while the simulation of an industrial environment should include S02 which is a common contaminant in that type of surrounding. It is further known that the presence of NaCl speeds up anodic reactions since Cl" ions aggravate the decomposition of existing passivating films.
  • a preferable method of accelerated testing involves soaking magnet specimens in an aqueous solution of NaCl at a concentration of approximately 3% by weight at room temperature.
  • the stability criterion for passivating films include the time of emergence, and the number and nature of corrosion centers.
  • the corrosion stability of metals are assessed by changes in the weight of selected specimens. Depending on the particular method of measurement selected, the recorded change in weight may be increased or reduced. It has been observed that electrochemical corrosion is substantially influenced by both the process factors, i.e. surface finish, porosity, etc., and by the specific composition of the material. For example, an increase in the neodymium component of an alloy from 34% to 37% by weight may reduce its corrosion rate by approximately one half. This has been observed under varied test conditions where the surrounding temperature was 150°C or 20°C, the relative humidity was 15% or 95%, and the surrounding pressure was approximately 5 psi.
  • the rate of corrosion may also be affected by varying the composition of the alloy to include small additions of elements such as iron, chromium, titanium, zirconium and lead.
  • elements such as iron, chromium, titanium, zirconium and lead.
  • the addition of chromium produced the best effect in minimizing electrochemical corrosion.
  • salt fog tests the typical weight change of a specimen after 10 hours of soaking was about one and a half times lower than samples containing titanium. It should be noted that the rate of electrochemical corrosion in these conditions was assessed by the changes in weight of the test magnets. Accordingly, many alloying combinations promote the improvement of their relative resistance to corrosion, which continues nonetheless, but at different rates with varying compositions.
  • the method of accelerated testing used involved soaking the magnet specimens in an aqueous solution of NaCL at a concentration of approximately 3% by weight at room temperature. Corrosion resistance was measured at the time, t5Q ; that 50% of the sample surface
  • the surfaces of rare earth magnets may be preliminarily prepared for improved high temperature passivation. Since the surface of magnets after grinding is often soiled with grinding waste, products of interaction with the cutting fluid, etc., the magnets are preferably degreased in advance by rinsing them in organic solvents such as freon or acetone. Next, the surface may be cleaned by pickling techniques in aqueous solutions of suitable acids including HNO3, H3PO4 and HF. The concentration of these acids preferably range from approximately 0.6, 1.25, 2.5, 5.0, 10.0, or 100 percent by volume. The pickling time preferably ranged from about 0.5 to 25 minutes. During testing, some control specimens were tested without pickling.
  • a preferable method of surface preparation involves pickling in a 2.5% solution of H3PO4 for about 2 minutes.
  • the positive effect of pickling may be due to the increased homogeneity of the magnetic surface.
  • the results of investigations of group A magnets on the SUPERPROBE-733 apparatus before and after pickling in 2.5% solutions of either H3PO4 or HNO3 (for about 2 minutes) demonstrated that the size and number of surface pores diminished successively while shifting from the unpickled specimen to that of a pickled sample in solutions of either HNO3 or H3PO4.
  • Figure 2 is a graphical description of temperature and pressure variations during a sample heat passivation process described in this embodiment of the invention.
  • the surrounding pressure during heat passivation is controlled in two phases.
  • the surrounding pressure ranged from approximately 1.0 x 10 " 4 to 1.0 x 10 " " mm Hg.
  • the range of temperatures for this heat treatment ranged from approximately 500°C to 1100°C.
  • the time of exposure for the specimens at these pressure and temperature conditions ranged from about 10 to 120 minutes.
  • the surrounding pressure ranged from approximately 1.0 x 10"2 to 1.0 x 10"4 mm Hg, while the temperature remained in the range of approximately 500°C to
  • the time of exposure for the specimens during the second phase ranged from 10 to 120 minutes. Thereafter, the specimens were permitted to cool with under vacuum conditions where the surrounding pressure was varied between approximately 1.0 x 10 " 4 to 1.0 x 10 ⁇ " mm Hg. The duration of the cooling process ranged from 5 to 30 minutes. The specimens were then removed and exposed to air and ambient conditions.
  • Figure 3 is a graphical description of the temperature and pressure variations during a sample heat passivation process described in this embodiment of the invention.
  • the microinhomogeneities (e.g., microcracks) of the protective layer can be filled by immersing the magnet into a polymer solution characterized by effective adhesive properties.
  • the high temperature passivation of rare earth magnets are performed in preferable heating and cooling conditions.
  • the effects of high temperature passivation were observed at microsections of the magnet surfaces in photographs generated by a scanning electron microscope (SEM) that were taken perpendicular relative to the magnet coating surface.
  • SEM scanning electron microscope
  • the passivating coating is clearly visible when viewed from the edge of the magnet body. It was observed that the coating thickness was approximately 30 microns in thickness for Group A magnets as shown in FIG. 1(a).
  • the thickness of the passivating layer was approximately 10 microns for Group B magnets.
  • the amount of oxygen present may be so significant that it becomes comparable with that of boron.
  • the processed magnets may eventually contain as much as 0.63 percent by weight of oxygen.
  • Oxygen is observed to be predominantly present in the intergranular neodymium enriched phase of the rare earth alloy. It has been observed that the neodymium enriched phase consists of at least two phases of different composition and structure. One phase contains approximately 70 percent of neodymium and 15 percent of oxygen, respectively, by weight.
  • the composition of this phase may be expressed as an oxide with the formula Nd2 ⁇ 3 5 the resultant ratio would be approximately 85.7 percent by weight of neodymium and 14.3 percent by weight of oxygen. It should be observed that the sum of weight percentages of elements in TABLE 2 for the intergranular and white phases of coating approaches very closely this value. Furthermore, when subjected to electrization with an electron beam source, additional data indirectly demonstrates that these phases are highly resistant.
  • the high temperature passivating layer formed on the surfaces may therefore consist of a stable neodymium oxide that is apparently dose in composition to Nd2 ⁇ 3.
  • the passivating coating contains additional elements described above. A possible formation mechanism for the passivating films is further detailed in the present disclosure. Generally, in most types of vacuum systems, the neodymium oxidation reaction (1) will be shifted sharply to the right:
  • oxygen dissociation pressure in reaction may be readily determined in reaction (1) to be approximately equal to 1.0 x 10"38 atmosphere at equilibrium. However, there are no vacuum systems presently available that can attain such conditions. When a direct reaction occurs similar to reaction (1) that is thermodynamically possible, the neodymium oxide layer formed would still only provide a relatively loose layer. If this formed oxide layer were to function as a passivating film, protection against electrochemical corrosion would be extremely ineffectual. Other thermodynamic possibilities may be assessed involving other reactions with the rare earth metals present in a magnet and oxygen. Additionally, the reactive properties of nitrogen may also be evaluated to determine its impact on the formation of the corrosion resistant passivating layer. This and other types of additional information may be obtained upon a complete microscopic analysis of the passivating coating in model experiments with controllable atmosphere of all magnet constituents including light elements.
  • the positive effect of resistance to corrosion may be based on the chemical and the structural homogeneity of the magnet surface after high temperature passivation. As explained above, this type of coating consistency reduces the effect of electrochemical corrosion and contributes to its stability under various operating conditions. In the event there are minor inconsistencies in the thickness of the formed protective layer, the magnet surface may be further treated after high temperature passivation with a very thin layer or coating of varnish to mend these minor film defects. It has even been observed that the use of varnish on the magnet surfaces has made it possible to raise the stability of passivating films even further during autoclaving tests.
  • NdFeB magnets that are initially formed by caking methods to attain more than 90% of theoretical density. After the magnets are polished to required sizes, their surfaces are washed with water and degreased with organic solvents such as pure alcohol. The magnet specimens are placed in 5.0 - 99.0% solution of orthophosphoric acid, H3PO4, or nitric acid, HNO3, and water for about 0.5 -20 minutes. The NdFeB magnets are then washed in water and permitted to dry which may be further assisted by compressed air. The magnets are thereafter subjected to high temperature thermal conditioning in accordance with the present invention at approximately 500°C-1000°C for approximately 10 to 120 minutes in a controlled atmosphere environment. The components of this controlled atmosphere are listed in TABLE 3 according to percentage by volume:
  • Component Composition vol.-% hydrogen 0.001-0.00001 oxygen 0.0001-21.0 nitrogen 0.0005-78.0 carbon monoxide 0.0001-0.003 carbon dioxide 0.0001-0.03 hydrocarbons 0.0001-0.001 water vapors 0.0005-0.05 helium or argon the remaining portion of 100%
  • the pressure conditions in this controlled atmosphere is set at an initial pressure of approximately 1100 mm Hg and is reduced to approximately 1.0 x 10"" mm Hg.
  • a surface-active substance such as olein acid may be added to the solution in various amounts ranging from approximately 0.1-5.0% by volume. Accordingly, as a result of the aforementioned process, a thin, dense and strong passivating film is formed on the surfaces of these rare earth magnets that protect them from corrosion.
  • this protective layer may contain stable oxides, phosphoric compounds of the elements, or other products derived from the magnet material.
  • the precise structure and composition of the protective layer phases may of course be determined by roentgen and microroentgenospectral analytical methods, or by other accepted procedures.
  • the rare earth magnet and the heat passivation process described herein show 15-30% improvement of intrinsic coercive force after the thermal procedure.

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Abstract

L'invention concerne un aimant permanent à base de terres rares, ayant une surface résistant à la corrosion, qui comprend un corps d'aimant ayant une surface externe formée d'au moins un alliage de métaux rares et une couche passivée à haute température entourant sensiblement la surface externe du corps d'aimant. Un procédé de passivation d'un aimant formé de terre rare à base de néodyme par conditionnement thermique à haute température, dans un environnement à atmosphère contrôlée.
PCT/US1998/014468 1997-07-11 1998-07-10 Passivation haute temperature d'aimants a base de terres rares WO1999002337A1 (fr)

Priority Applications (1)

Application Number Priority Date Filing Date Title
AU83991/98A AU8399198A (en) 1997-07-11 1998-07-10 High temperature passivation of rare earth magnets

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
RU97111723 1997-07-11
RU97111723/09A RU97111723A (ru) 1997-07-11 Редкоземельный постоянный магнит и способ его получения и пассивации

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Cited By (6)

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Publication number Priority date Publication date Assignee Title
EP1744331A1 (fr) * 2004-03-31 2007-01-17 TDK Corporation Aimant de terres rares et procede de fabrication de celui-ci
US9491911B2 (en) 2014-02-19 2016-11-15 Dennis Jason Stelmack Method for modifying environmental conditions with ring comprised of magnetic material
CN113257508A (zh) * 2021-05-13 2021-08-13 中钢天源股份有限公司 一种高综合性能钕铁硼的制作方法
CN114420439A (zh) * 2022-03-02 2022-04-29 浙江大学 高温氧化处理提高高丰度稀土永磁抗蚀性的方法
CN114574806A (zh) * 2022-03-02 2022-06-03 浙江大学 一种稀土永磁材料表面耐蚀涂层及其制备方法
CN114717511A (zh) * 2022-03-30 2022-07-08 北矿磁材（阜阳）有限公司 一种烧结钕铁硼磁体表面Al薄膜的制备方法

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US5173206A (en) * 1987-12-14 1992-12-22 The B. F. Goodrich Company Passivated rare earth magnet or magnetic material compositions
EP0571002A2 (fr) * 1989-08-25 1993-11-24 Dowa Mining Co., Ltd. Alliage pour aimant permanent à résistance contré l'oxydation améliorée et procédé de fabrication
US5282904A (en) * 1990-04-10 1994-02-01 Crucible Materials Corporation Permanent magnet having improved corrosion resistance and method for producing the same
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US5173206A (en) * 1987-12-14 1992-12-22 The B. F. Goodrich Company Passivated rare earth magnet or magnetic material compositions
US5122203A (en) * 1989-06-13 1992-06-16 Sps Technologies, Inc. Magnetic materials
US5470400A (en) * 1989-06-13 1995-11-28 Sps Technologies, Inc. Rare earth anisotropic magnetic materials for polymer bonded magnets
EP0571002A2 (fr) * 1989-08-25 1993-11-24 Dowa Mining Co., Ltd. Alliage pour aimant permanent à résistance contré l'oxydation améliorée et procédé de fabrication
US5282904A (en) * 1990-04-10 1994-02-01 Crucible Materials Corporation Permanent magnet having improved corrosion resistance and method for producing the same

Cited By (13)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP1744331A4 (fr) * 2004-03-31 2010-06-02 Tdk Corp Aimant de terres rares et procede de fabrication de celui-ci
US20120112862A1 (en) * 2004-03-31 2012-05-10 Tdk Corporation Rare earth magnet and method for manufacturing same
US9903009B2 (en) 2004-03-31 2018-02-27 Tdk Corporation Rare earth magnet and method for manufacturing same
EP1744331A1 (fr) * 2004-03-31 2007-01-17 TDK Corporation Aimant de terres rares et procede de fabrication de celui-ci
US9491911B2 (en) 2014-02-19 2016-11-15 Dennis Jason Stelmack Method for modifying environmental conditions with ring comprised of magnetic material
CN113257508B (zh) * 2021-05-13 2023-09-01 中钢天源股份有限公司 一种钕铁硼的制作方法
CN113257508A (zh) * 2021-05-13 2021-08-13 中钢天源股份有限公司 一种高综合性能钕铁硼的制作方法
CN114420439A (zh) * 2022-03-02 2022-04-29 浙江大学 高温氧化处理提高高丰度稀土永磁抗蚀性的方法
CN114420439B (zh) * 2022-03-02 2022-12-27 浙江大学 高温氧化处理提高高丰度稀土永磁抗蚀性的方法
CN114574806A (zh) * 2022-03-02 2022-06-03 浙江大学 一种稀土永磁材料表面耐蚀涂层及其制备方法
JP2023129176A (ja) * 2022-03-02 2023-09-14 浙江大学 高温酸化処理により高濃縮の希土類永久磁石の耐腐食性を増加させる方法
CN114717511A (zh) * 2022-03-30 2022-07-08 北矿磁材（阜阳）有限公司 一种烧结钕铁硼磁体表面Al薄膜的制备方法
CN114717511B (zh) * 2022-03-30 2023-08-04 北矿磁材（阜阳）有限公司 一种烧结钕铁硼磁体表面Al薄膜的制备方法

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