Study On The Preparing Technique Of Anti-corrosive Coatings On Sintered NdFeB Permanent Magnet

The sintered NdFeB (neodymium-iron-boron) permanent magnet as the third-era permanent magnet, has been used for many applications in various fields such as automobile, aviation, microwave communications, instruments and apparatuses, audios, magnetic separations, computers and health care in recent several decades, these all benefit from its excellent magnetic properties such as high coercivity, high saturation magnetization, large maximum energy product, rich resources reserves and comparatively low price. But the previous research shows that the sintered NdFeB magnet exists in multiphase structure, the Eθ of Nd-rich phase is-2.431V, Nd is very easy to be oxidized as one of the most reactive metals. At the same time, the potential difference of different phases accelerates the corrosion rate of substrate, and then the decline of the magnetism. This severely limits the application of sintered NdFeB magnet in various fields. Hence, improving the corrosion resistance of NdFeB magnet is a major issue to solve. And the research on surface protection technology of sintered NdFeB magnet is significant.In this dissertation, the improvement of anticorrosive properties and preparation technologies of surface coatings for sintered NdFeB permanent magnet were investigated. On that basis, the cyclic voltammetry was firstly employed to the preparation of coatings on sintered NdFeB magnet, and Ni-Co alloy coating were successfully prepared on the surface of sintered NdFeB magnet. Simultaneously, in order to reduce the cost and simplify the preparation process, we successfully prepared two kinds of cooperating coatings with excellent anticorrosive properties, by the composition of electrodeposition and sol-gel technique. The following contents were mainly researched in the present study.(1) Cyclic voltammetry (CV) possesses many advantages in the preparation of alloy coatings than other techniques. In order to fill the vacuum that the using of CV in preparation of surface coatings or films on sintered NdFeB magnet, the CV was employed to prepare the Ni-Co alloy coating, and make a comparison with that prepared by electrodeposion technique in our previous work. The comparison results shown that the introduced of CV made the grain refining and the more uniform and dense of surface morphologies. The long-term immersion test indicated that the Ni-Co alloy coating prepared by cyclic voltammetry (CV) exhibited more outstanding long-term corrosion protection properties for sintered NdFeB magnet.(2) Given environment friendly, simplifying preparation technics and reducing the cost, a cooperating coating (CC) consisted of a bottomed cathodic nickel coating and a topped sol-gel coating was prepared on sintered NdFeB permanent magnet by electrodeposition and sol-gel combined technique. The electrochemical measurements including electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization shown that CC exhibited considerable corrosion protection properties for sintered NdFeB permanent magnet. In order to investigate the practical corrosion protection properties of CC for sintered NdFeB magnet, the electrochemical impedance spectroscopy (EIS) technique was employed to study the changes in electrochemical corrosion behaviors of CC in3.5wt.%NaCl corrosive media. The results of long-term immersion test shown that there were three partially overlapped time constants could be observed in the Bode plots (phase angle as a function of frequency) during the immersion time ranging from1h to624h. The impedance modulus|Z|of CC remained at a very high value, indicating the excellent protective properties provided for sintered NdFeB magnet in this stage. As the immersion time extended to792h, there were two well-defined time constants could be observed, and the impedance modulus|Z|of CC decreased distinctly, but still approximately three orders higher than that of bare substrate, suggesting that CC exhibited considerable long-term corrosion protection for sintered NdFeB magnet.(3) On the basis of ensure the reliability of coatings’ corrosion protection properties, taking into account environment friendly, preparation process simplification and low cost, a cooperating coating (SZC) consisted of a bottomed anodic zinc coating and a topped sol-gel coating was prepared on sintered NdFeB permanent magnet by electrodeposition and sol-gel combined technique. The single Zn coating (UZC) was also studied for a comparison. The corrosion characteristics of SZC and UZC in neutral3.5wt.%NaCl solution were evaluated using electrochemical measurements including open circuit potential (OCP), electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization test. At the same time, a long-term immersion test was carried out in neutral3.5wt.%NaCl solution in order to further investigate the changes of anticorrosive properties of SZC during the service process. The results of electrochemical tests shown that due to the blocking effect of sol coating, SZC could suppress the corrosion process by holding back the transfer or diffusion of corrosive media, and therefore shown the more excellent anticorrosive properties than single Zn coating (UZC), and could provide good long-term corrosion protection for sintered NdFeB magnet.