Study On The Electrochemical Fabrication And The Related Theories Of Ni-P Coatings On NdFeB Permanent Magnet Substrates

Nd-Fe-B type magnet, which possesses excellent magnetic properties, is a new type of permanent magnet material, which is discovered by Japan and the USA in 1983. Since its discovery, its research and application has been hot spots of the study of magnet materials. However, its poor corrosion resistance limits its application range.To improve the corrosion resistances of Nd-Fe-B type magnets, lots of attempts have been done, such as alloy additions and surface coatings. Among all surface coatings, amorphous Ni-P coating, which possesses more excellent mechanical, magnetic and anti-corrosion performance over the crystal materials, may be one of the most excellent. In general, the Ni-P coatings can be fabricated by electroless-deposition and electrodeposition techniques. And electrodeposition technique has proven to be less energy consumption, lower cost and easier to control. Therefore, the electroplating of Ni-P deposits, especially the amorphous Ni-P deposits on NdFeB based magnetic substrates are worthy of further detailed research. But the problem existing is that, the latter is reported on literature less often, especially about NdFeB type magnets,So in this paper, it is decided to prepare Ni-P alloy, one of amorphous materials, on NdFeB sintered magnets by DC electro-deposition. First, the technological parameters of electroplating Ni-P amorphous alloy on NdFeB sintered magnets, such as current density, temperature, pH value and content of phosphorous acid of the deposition solution, were studied by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques in conjunction with X-ray diffraction (XRD), scanning transmission electron microscopy (SEM) and X-ray energy-dispersive spectrometry (EDX). The optimized amorphous Ni-P coated NdFeB , which has excellent anti-corrosion, can stand for c.a. 180 h against neutral 3.0 wt. % NaCl salt spray without any pitting corrosion. Meanwhile, the results also show that large phosphorous content is the precondition for Ni-P coatings to possessthe amorphous structure, but too much high phosphorous content can damage the amorphous structure due to the formation of new P-rich phase.Secondly, the mechanism of electrodepositing Ni - P alloy on NdFeB sintered magnets and the influence of phosphorus on the behavior of electroplating Ni were investigated by cyclic voltammetry (CV), cathode potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), chronoamperometry (CA). H3PO3 accelerates the electrodeposition of Ni, and change the mechanism of electrodepositing Ni. In the case of low deposition overpotential, the electroplating of Ni and Ni - P alloy coating followed 3D nucleating and subsequent progressional grain growth mechanism controlled by the electrochemical step;in the case of high deposition overpotential, the electroplating of Ni coating followed 3D nucleating and subsequent instantaneous grain growth mechanism controlled by the diffusion step, while the electroplating of Ni - P alloy coating followed 3D nucleating and subsequent progressional grain growth mechanism controlled by the diffusion step.At last, the electrochemical corrosion behaviors of uncoated and Ni-P coated NdFeB sintered magnets in neutral 3.0 wt. % NaCl solutions at different time were primarily investigated by electrochemical impedance spectroscopy (EIS). In neutral 3.0 wt. % NaCl solution, inter-granular corrosion happened on uncoated NdFeB sintered magnets, which resulted in the inter-granular destroyed so as to falling off and accelerating the corrosion of the base. But during the initial stage, only a little general corrosion happened on Ni-P coated NdFeB, and the rate of corrosion decreased because of self-recovering. That can make out that, Ni-P coating possesses excellent anti-corrosion for NdFeB sintered magnets.