Study On The Protective Electroless Plating Of Sintered NdFeB Magnets

In this dissertation, corrosion mechanisms and current anticorrosion treatments of NdFeB magnets were reviewed. Electroless deposition of Ni-P alloys on the sintered NdFeB magnets was employed to enhance their corrosion resistances. Utilizing electrochemical workstation, X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive spectrometer (EDS), the effects of compositions and processing parameters on the microstructures and properties of the coatings were studied systematically.The pretreatment processes were necessary before electroless plating on sintered NdFeB magnets. These processes include plug process, oil removing process, pickling and activation process. The function of plug process was increasing the binding force of electroless coatings and substrate. However, the experiment result showed that the effect of plug process can't be exhibited until a period of time. Saponification and emulsification in the oil removing process can bring a clean surface for subsequent processes without decompose the magnets after ultrasonic cleaning 2 to 3 minutes in the pH range of 9 to 10 and at a temperature of 65℃. The pickling and activation process of the sintered NdFeB should not use the strong acid because it will corrode the magnets, so the weak acid was adopted.Parameters of electroless Ni-P coatings were optimized by an orthogonal experiment. Effects of pre-plating in ultrasonic field on the adhesion of electroless Ni-P coatings to sintered NdFeB magnets were investigated. The results showed that the adhesion of Ni-P coating to Nd-Fe-B magnet was enhanced with increasing ultrasonic power. When increasing ultrasonic power up to 150W, the adhesion strength reached the maximum. Synchronously, a uniform and compact deposit can be obtained by pre-plating in ultrasonic field. It is suggested that the ultrasonic pre-plating contributes to the improvement of adhesion through two ways: stirring and cavitation action.Ni-P coatings were deposited on sintered NdFeB magnets with the addition of Nd³⁺ in the electroless plating solution. The results showed that the addition of Nd³⁺ could significantly improve the adhesion and corrosion resistance of Ni-P coatings on magnets. When increasing Nd³⁺ up to 2.5g·L^-1, the adhesion strength of the Ni-P coating to NdFeB magnet was enhanced from 6.4 to 25.2MPa. Synchronously, the corrosion potential E_corr of the coated magnets in 3.5% NaCl solution increased from -0.382 to -0.148V, the corrosion current density I_corr decreased from 4.52 to 0.07μA·cm^-2, and the coated magnets could be as long as 256h free of corrosion in the neutral salt spray, reflecting the improvement of corrosion resistance.