Rare earth metal-based permanent magnets, for instance, R—Fe—B based permanent magnets represented by a Nd—Fe—B based permanent magnet, or R—Fe—N based permanent magnets represented by a Sm—Fe—N based permanent magnet, etc., utilize inexpensive materials abundant in resources and possess high magnetic properties, and particularly among them, R—Fe—B based permanent magnets are employed today in various fields.
However, since rare earth metal-based permanent magnets contain a highly reactive rare earth element: R, they are apt to be oxidized and corroded in ambient, and in case they are used without applying any surface treatment, corrosion tends to proceed from the surface in the presence of small acidic or alkaline substance or water to generate rust, and this brings about the degradation and the fluctuation in magnetic properties. Moreover, in case such a rusty magnet is embedded in a magnetic circuit and a like device, there is fear of scattering rust as to contaminate peripheral components.
Although there are numerous methods for imparting corrosion-resistance to rare earth metal-based permanent magnets, known is a method for forming a corrosion-resistant film using a silicon compound as the starting raw material on the surface of the rare earth metal-based permanent magnet. Recently, various studies are being made with an aim to further improve the performance of such corrosion-resistant films.
For instance, in Japanese Patent Laid-Open No. 2000-182813 (Patent literature 1) is proposed a method comprising coating the surface of the rare earth metal-based permanent magnet with a treating solution comprising an aqueous solution of alkali silicate having dispersed therein fine zinc particles, and then applying a heat treatment thereto to obtain a corrosion-resistant film containing fine zinc particles dispersed therein. This method utilizes the anti-corrosive function of the film using alkali silicate as the starting raw material and the sacrificial anti-corrosive function of the fine zinc particles that is potentially base, and is expected as a method capable of imparting high corrosion resistance to the rare earth metal-based permanent magnets. However, in order to form the corrosion-resistant film containing fine zinc particles dispersed therein, this method requires the treating solution to be kept alkaline in order to maintain the fine zinc particles uniformly dispersed in the treating solution; thus, in case the treating solution is applied to the surface of the rare earth metal-based permanent magnet, hydroxides of the metal which constitutes the magnet generate on the surface of the magnet, and, due to the layer comprising such metal hydroxides covering the surface of the magnet, there occur problems such as a difficulty in forming a corrosion-resistant film containing fine zinc particles dispersed therein with high adhesive strength, a tendency in generating cracks due to the poor flexibility of the film using alkali silicate as the starting raw material, a troublesome waste liquid treatment, and the like.
Further, in Japanese Patent Laid-Open No. 2001-143949 (Patent literature 2) is proposed a method comprising coating the surface of the rare earth metal-based permanent magnet with a treating solution containing silico-organic compound and inorganic fine particles having an average particle diameter of 1 nm to 100 nm, and then applying a heat treatment thereto to obtain a corrosion-resistant film containing fine inorganic particles dispersed therein. This method is proposed in order to form a thin and dense corrosion-resistant film on the surface of the rare earth metal-based permanent magnet using a silico-organic compound as the starting raw material; the internal stress inside the film which forms during the process of the film formation can be relaxed by dispersing fine inorganic particles having the specified average particle diameter in the film component, and hence, the generation of physical defects such as cracks can be thereby prevented from occurring. This method is also expected as a method capable of imparting high corrosion resistance to the rare earth metal-based permanent magnets. However, by taking into consideration that it is difficult to uniformly disperse fine inorganic particles in the order of nanometer size in water, it is necessary for preparing a treating solution to use an organic solvent such as lower alcohol and the like as the main component with a small amount of water added therein, and to prepare a sol solution by subjecting silico-organic compound to a hydrolysis polymerization reaction under an acidic condition while simultaneously dispersing fine inorganic particles. Since the thus prepared treating solution is acidic, in case the treating solution is applied to the surface of the rare earth metal-based permanent magnet, there occur problems such as inducing corrosion of the magnet, or causing the fluctuation in the composition of the treating solution due to the volatilization of the organic solvent; furthermore, there are problems such as causing unfavorable influences to the environment, or requiring a troublesome waste solution treatment.
Accordingly, the objectives of the present invention are to provide a stable and simple method for producing a rare earth metal-based permanent magnet having on the surface thereof a corrosion-resistant film containing fine zinc particles dispersed therein, a corrosion-resistant rare earth metal-based permanent magnet produced by the method, a dip spin coating method being suitable for forming a coating film on thin type work pieces having various shapes, and a method for forming a coating film on a work piece.