Stock material or miscellaneous articles – Composite – Of metal
Reexamination Certificate
1999-08-25
2002-09-03
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
Composite
Of metal
C428S450000, C428S702000
Reexamination Certificate
active
06444328
ABSTRACT:
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The present invention relates to an Fe—B—R based permanent magnet having an excellent corrosion-resistant film, and a process for producing the same. More particularly, the present invention relates to an Fe—B—R based permanent magnet which has, on its surface, an excellent corrosion-resistant film having an excellent adhesion to the surface of the magnet; which has a thermal shock resistance enough to resist even a heat cycle for a long period of time in a temperature range of −40° C. to 85° C.; which can exhibit a stable high magnetic characteristic that cannot deteriorate even if the magnet is left to stand under high-temperature and high-humidity conditions of a temperature of 80° C. and a relative humidity of 90%; and in which the film is free from hexa-valent chromium, and to a process for producing the same.
DESCRIPTION OF THE RELATED ART
An Fe—B—R based permanent magnet, of which an Fe—B—Nd based permanent magnet is representative, is practically used in various applications, because it is produced of an inexpensive material rich in natural resources and has a high magnetic characteristic.
However, the Fe—B—R based permanent magnet is liable to be corroded by oxidation in the atmosphere, because it contains highly reactive R and Fe. When the Fe—B—R based permanent magnet is used without being subjected to any treatment, the corrosion of the magnet is advanced from its surface due to the presence of a small amount of acid, alkali and/or water to produce rust, thereby bringing about the degradation and dispersion of the magnetic characteristic. Further, when the magnet having the rust produced therein is assembled into a device such as a magnetic circuit, there is a possibility that the rust is scattered to pollute surrounding parts or components.
There is a already proposed magnet which has a corrosion-resistant metal-plated film on its surface, which is formed by a wet plating process such as an electroless plating process and an electroplating process in order to improve the corrosion resistance of the Fe—B—R based permanent magnet with the above-described point in view (see Japanese Patent Publication No.3-74012). In this process, however, an acidic or alkaline solution used in a pretreatment prior to the plating treatment may remain in pores in the magnet, whereby the magnet may be corroded with the passage of time in some cases. In addition, the magnet is poor in resistance to chemicals and for this reason, the surface of the magnet may be corroded during the plating treatment. Further, even if the metal-plated film is formed on the surface of the magnet, as described above, if the magnet is subjected to a corrosion test under conditions of a temperature of 60° C. and a relative humidity of 90%, the magnetic characteristic of the magnet may be degraded by 10% or more from an initial value after lapse of 100 hours.
There is also a conventionally proposed process in which a corrosion-resistant film such as a phosphate film or a chromate film is formed on the surface of an Fe—B—R based permanent magnet (see Japanese Patent Publication No.4-22008). The film formed in this process is excellent in adhesion to the surface of the magnet, but if it is subjected to a corrosion test under conditions of a temperature of 60° C. and a relative humidity of 90%, the magnetic characteristic of the magnet may be degraded by 10% or more from an initial value after lapse of 300 hours.
In a process conventionally proposed in order to improve the corrosion resistance of the Fe—B—R based permanent magnet, i.e., in a so-called aluminum-chromate treating process (see Japanese Patent Publication No.6-66173), a chromate treatment is carried out after formation of an aluminum film by a vapor deposition process. This process remarkably improves the corrosion resistance of the magnet. However, the chromate treatment used in this process uses hexa-valent chromium which is undesirable for the environment and for this reason, a waste-liquid treating process is complicated. It is feared that a film formed in this process influences a human body during handling of the magnet, because it contains just a small amount of hexa-valent chromium.
On the other hand, in recent years, the field of application of the Fe—B—R based permanent magnet is not limited to the electric industry and the domestic electric appliance industry, and it has been expected that the Fe—B—R based permanent magnet can be applied to fields where it is used in a hard condition. In correspondence to this fact, it is regarded as important that the Fe—B—R based permanent magnet has required characteristics including not only an excellent corrosion resistance under given conditions, but also an excellent thermal shock resistance relative to a variation in temperature. For example, a magnet assembled into parts such as a motor for an automobile must resist a large variation in temperature. To meet such demand, a corrosion-resistant film itself formed on the magnet must be prevented from being cracked or peeled off due to a variation in temperature.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide an Fe—B—R based permanent magnet which has, on its surface, an excellent corrosion-resistant film having an excellent adhesion to the surface of the magnet; which has a thermal shock resistance enough to resist even a heat cycle for a long period of time in a temperature range of −40° C. to 85° C.; which can exhibit a stable high magnetic characteristic that cannot deteriorate even if the magnet is left to stand under high-temperature and high-humidity conditions of a temperature of 80° C. and a relative humidity of 90%; and in which the film is free from hexa-valent chromium, and to a process for producing the same.
The present inventors, in a course of various zealous studies made with the above points in view, have paid their intention to the fact that a metal film is formed on the surface of an Fe—B—R based permanent magnet, and a metal oxide film having less influencing the human body and the environment is formed on the metal film. A process for forming a primary coat layer on the surface of an Fe—B—R based permanent magnet using a metal as a main component, and forming a glass layer on the surface of the primary coat layer has been already proposed (see Japanese Patent Application Laid-open No.1-165105). Japanese Patent Application Laid-open No.1-165105 describes that it is difficult to form a glass layer uniformly, when the glass layer has a thickness of less than 1 &mgr;m. However, as a result of further studies made by the present inventors, surprisingly it has been found that if the metal film is formed on the surface of the Fe—B—R based permanent magnet, and the metal oxide film having a thickness of 1 &mgr;m or less is formed on the metal film, the metal oxide film is firmly closely adhered to the metal film on the magnet to exhibit an excellent effect not only in the corrosion resistance under given conditions, but also in thermal shock resistance with respect to a variation in temperature.
The present invention has been accomplished based on such knowledge. To achieve the above object, according to a first aspect and feature of the present invention, there is provided an Fe—B—R based permanent magnet having a metal oxide film having a thickness of 0.01 &mgr;m to 1 &mgr;m on the surface thereof with a metal film interposed therebetween.
According to a second aspect and feature of the present invention, in addition to the first feature, the metal film is formed of at least one metal component selected from the group consisting of Al, Sn, Zn, cu, Fe, Ni, Co and Ti.
According to a third aspect and feature of the present invention, in addition to the first feature, the metal film has a thickness in a range of 0.01 &mgr;m to 50 &mgr;m.
According to a fourth aspect and feature of the present invention, in addition to the first feature, the metal oxide film is formed of at least one metal oxide component selected from the group consist
Kikui Fumiaki
Nishiuchi Takeshi
Yoshimura Kohshi
Piziali Andrew T
Sumitomo Special Metals Co. Ltd.
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