Patent ID: 8105444

Claim:
A method for preparing a highly corrosion resistant rare earth permanent magnet, comprising the sequential steps of: casting an alloy, said alloy containing R which is a rare earth element or a combination of two or more rare earth elements, T which is Fe or Fe and Co, and B as main components, and specifically consisting essentially of 26.8 to 33.5% by weight of R, 0.78 to 1.25% by weight of B, 0.05 to 3.5% by weight in total of at least one element selected from the group consisting of Ni, Ga, Zr, Nb, Hf, Ta, Mn, Sn, Mo, Zn, Pb, Sb, Al, Si, V, Cr, Ti, Cu, Ca, and Mg, and the balance of T and incidental impurities; pulverizing the alloy in an oxygen-free atmosphere of argon, nitrogen or vacuum; finely pulverizing, compacting in a magnetic field, sintering, and aging the finely pulverized, compacted and sintered alloy, thereby producing a sintered magnet, the magnet having an oxygen concentration of up to 0.6% by weight and magnetic properties, Br of 12.0 kG to 14.8 kG and iHc of 11 kOe to 35 kOe; machining and/or grinding the magnet for surface finishing; pretreating the magnet, which is machined and/or ground, with mineral acid, nickel electroplating the magnet, which is pretreated, to form a plating of a predetermined thickness; immersing the magnet, which is nickel electroplated, in a phosphate-containing aqueous solution, washing with water; and further heat treating the magnet in an atmosphere having an oxygen partial pressure of at least 1.3×10 3 Pa (10 Torr) at 150 to 400° C. for 1 to 24 hours for thereby forming a thin nickel oxide layer in a surface region of the plating.