Corrosion Resistance Of Electroless Plating Ni-P Alloy On Sintered NdFeB Magnets

NdFeB type magnet, which possesses excellent magnetic properties, is a new type of permanent magnet material, which is discovered by Japan in 1983. Permanent magnetic material is rapidly widely used in the electronics industry, communication, information, defense, medicine and health, environmental protection and oil fields etc. in a short period of several decades because of its rich source and low price. But the strong chemical activity, poor anticorrosion and easy being oxidized of NdFeB magnets material seriously affect its scope of application.To improve the corrosion resistances of NdFeB type magnets, lots of attempts have been done, such as enhancing the intrinsic corrosion property of material by the addition of elements and applying various protective coating on the surface of the magnets. Among all surface coatings amorphous Ni-P coating, which possesses more mechanical, magnetic and anti-corrosion performance over the crystal materials, may be one of the most excellent. In this thesis, reports on corrosion mechanism and anticorrosion process of NdFeB permanent magnet are reviewed. On this basis, electroless deposition of Ni-based alloy was employed to NdFeB magnet to enhance its corrosion resistance.Surface loose, lots hole and Nd element very active(E=-2.4eV) and vivacity of the sintered NdFeB permanent magnet, the pretreatment processes and apply plating process which is difficult to apply plating whether in acidic or alkali electroless plating bath, it is the key to get ideal coating that alleviate corrupt. The optimized pretreatment processes include burnish process, plug process, oil removing process, picking and activation process, and precoppered process. Oil-removed by supersonic, and acidwashing must be carried in 30-40mL/L of nitric acid (65%) with the addition 0.5g/L of thiourea, in pH adjusted to 4–5 by ammonia liquor, in limited time of 20-30s.Experiments find that we can't direct plat with acidic solution in the sintered NdFeB magnets'surface, otherwise magnet corrosion. In this paper, the composite deposit coating include first pre-copperplating coating, second litmusless plating coating and last alkaline plating coating. The composite deposit coating has a better corrosion resistance and little porosity than ultrasonic electroless plating. The pre-copperplating can improve coating adhesion strength and corrosion resistance, effectively reduce the porosity of the coating. Microstructure analysis performed by means of SEM, the results show that the coating and magnet surface has a stronger adhesion, and the same size of cellular particles composite of the coating.The results have shown that the composite complexing agnet has more stability solution than single complexing agnet, and has a higher deposit rate. So the composite coating has a more compactability and fewer porosity.The experiment indicate that the alkaline electrolessly plating Ni-P deposit ,which were prepared for the sintered NdFeB magnets under 40 kHz frequency ultrasonic field, has better fine microstructure and adhesion force than the Ni-P deposit without ultrasound field.