| The sintered NdFeB (neodymium-iron-boron) permanent magnet as the third-era permanent magnet, which exhibits excellent magnetic properties such as high remanence, high coercivity, large energy product, good machinability and comparatively low price, has been used for many applications in various fields such as acoustics, aviation, electronics, automation, biomedical and communications. However, the anticorrosive properties of sintered NdFeB permanent magnet are very poor, and easily form the galvanic corrosion in corrosive media and humid environment, which results in the changes in the composition and microstructure of magnet, and then the decline of the magnetism. This severely impedes and limits the extensive application of NdFeB magnet in various fields. The anticorrosive properties of NdFeB magnet have become a key index to estimate its application properties. Therefore, it is very important to improve the corrosion resistance of NdFeB permanent magnets.So far, most of the researches on the corrosion protection of sintered NdFeB permanent magnet were emphasizing particularly on the investigation on technical study and tests of the short-term protective properties of coatings or films prepared on sintered NdFeB permanent magnet in corrosive media. Unfortunately, few works were focusing on the tests of the long-term anticorrosive properties and the changes in electrochemical corrosion behaviors in corrosive media, as well as the failure mechanism, which may be an indicator of the actual corrosion protection properties of coatings or films provided for sintered NdFeB permanent magnet.Based on the problems mentioned above, different films or coatings possessed excellent anticorrosive properties were prepared by dint of large numbers of orthogonal experiments, which were on the basis of the investigation on the corrosion protection of sintered NdFeB permanent magnet and other metals. The potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) techniques and immersion tests were employed to study the changes in the actual corrosion protection properties of films or coatings provided for permanent magnet in service environment. Sealed Ni-TiO2 composite coating, Ni-Co alloy coating, Ni-Co-TiO2 composite coating and silicane treated Zn-TiO2 grade film were firstly prepared on sintered NdFeB permanent magnet by the combination of electrodeposition and sol-gel methods. The major research works in the present study are as follows:(1) Taking into account environment friendly and simplifying operation technics, a protective sealed Ni-TiO2 composite coating (SCC) was prepared on sintered NdFeB permanent magnet by pulse current electrodeposition and sol-gel combined technique. For a comparison, unsealed Ni-TiO2 composite coating was also investigated. The potentiodynamic polarization and electrochemical impedance spectroscopy tests showed that SCC exhibited more excellent anticorrosive properties than UCC, and therefore provided better corrosion protection for sintered NdFeB permanent magnet. The electrochemical impedance spectroscopy (EIS) technique was employed to study the changes in electrochemical corrosion behaviors and failure mechanism of films or coatings in 3.5 wt.% NaCl corrosive media. The results of corrosion tests indicated that the evolution of electrochemical corrosion behavior of SCC could be divided into three stages. The first stage was the immersion time ranging from 1 h to 240 h in which three partially overlapped time constants could be observed and the impedance modulus|Z| of SCC remained at a very high value, suggesting the excellent protective properties provided for the substrate. The second stage was the immersion time ranging from 240 h to 264 h in which two well-defined time constants could be observed, and the impedance modulus |Z| of SCC decreased sharply. At the second stage, the SCC still provided good protection for the substrate. The third stage was the immersion time of 288 h in which only one well-defined time constant could be observed and the impedance modulus|Z| of SCC decreased more, and then the protective properties of SCC for substrate practically disappeared.(2) A protective Ni-Co alloy coating was electrodeposited on sintered NdFeB permanent magnet applying electrodeposition technique. The microstructure, surface morphologies and chemical composition of coatings were investigated using X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS), respectively. The corrosion protection properties of alloy coating for sintered NdFeB permanent magnet in neutral 3.5 wt.% NaCl corrosive media were evaluated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques. Microstructure analysis showed that the microstructure of Ni-Co alloy coating was substitutional solid solution with the Co as solute and Ni as solvent, respectively; the addition of cobalt element into matrix metal Ni altered the preferential orientation of matrix metal Ni from (20 0) crystal face to (111) crystal face. Surface morphologies tests indicated that, compared with pure Ni coating, the surface morphologies of Ni-Co alloy coating were more compact and uniform due to the grain-refining. The results of potentiodynamic polarization test suggested that, compared with pure nickel coating, Ni-Co alloy coating exhibited much nobler corrosion potential (Econ) and lower corrosion current density (icon), indicating better anticorrosive properties. The long-term immersion test by dint of EIS indicated that the Ni-Co alloy coating still presented high impedance value of 1.9×1O5Ω·cm2 with the immersion time of 576 h suggesting the excellent anticorrosive properties. The corrosion protection properties of pure nickel coating provided for sintered NdFeB permanent magnet practically disappeared with the immersion time of 144 h, which also indicated that the Ni-Co alloy coating provided better corrosion protection properties for the sintered NdFeB permanent magnet compared with pure nickel coating.(3) In order to further enlarge the application fields of sintered NdFeB permanent magnet, a protective Ni-Co-TiO2 composite coating was prepared on the sintered NdFeB permanent magnet by composite electrodeposition technique. The electrochemical corrosion behaviours of the composite coating in 0.5 mol L-1 H2SO4,0.6 mol L-1 NaOH,0.6 mol L-1 Na2SO4 and neutral 3.5 wt.% NaCl corrosive media were evaluated by potentiodynamic polarization measurements and electrochemical impedance spectroscopy (EIS), showing good corrosion, protection for sintered NdFeB permanent magnet in diverse corrosive media. In order to further investigate the protective properties of the composite coating for sintered NdFeB permanent magnet and the practicability of the composite coating, the long-term immersion test was carried out in neutral 3.5 wt.% NaCl corrosive media using EIS. The results of long-term corrosion test showed that the Ni-Co-TiO2 composite coating could provide long-term protection in neutral 3.5 wt.% NaCl corrosive media for sintered NdFeB permanent magnet due to the formation of passive film.(4) Taking into account environment friendly, simplifying operation technics and reducing the technical cost, a anodic Zn-TiO2/Si sol gradient film (TZT), whose role in corrosion protection for sintered NdFeB permanent magnet was the combination of mechanical isolation and electrochemical protection, was prepared on sintered NdFeB permanent magnet by electrodeposition and sol-gel combined technique. The anticorrosive properties of TZT in neutral 3.5 wt.% NaCl corrosive media were evaluated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) technique. In order to further study the corrosion protection of film, the immersion tests were employed in neutral 3.5 wt.% NaCl corrosive media. Comparing the surface morphlogies of TZT with the immersion time of 336 h with that of the Zn-TiO2 composite coating with the immersion time of 264 h, it can be seen that, due to the blocking effect of sealing layer, TZT could suppress the corrosion process by holding back the transfer or diffusion of corrosive media, and therefore showed the excellent corrosion protection properties for sintered NdFeB permanent magnet.
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