Study Of Surface Coating On The Anti-corrosion Property Of Bonded NdFeB Magnets

NdFeB magnets are widely applied in electron, motor and communication because of their outstanding magnetic properties. The chemical property of NdFeB is very active, which will be easily oxidized and corroded in high temperatures or high humidity, thus limiting the application of magnets. Therefore, studying on the corrosion resistance of NdFeB permanent magnet has a positive signification. In order to improve the corrosion resistance of bonded NdFeB magnets, the nano-TiO2/fluorocarbon composite coating is coated on the surface of bonded NdFeB magnets in the thesis. The main results of the thesis are as follows:Due to their high surface energy, nano-TiO2 tends to strongly agglomerate, which seriously limits their applications, and degrades the performance of nanoparticles. Therefore, appropriate surface modification of nanoparticles is necessary. Different content of KH550 grafting surface treatment process is developed in this study. The results of sedimentation experiment show that the dispersion is improved in different degree. And the 1.8 g KH550 modified TiO2 nanoparticles is the best. The properties of 1.8 g KH550 modified TiO2 nanoparticles are studied by infrared spectroscopy (IR), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM). It is found that KH550 is successfully bonded to the surface of nano-TiO2. The graft ratio is 4.14% and the modified nano-TiO2 has a good dispersability in the dispersion medium.Nano-TiO2 fluorocarbon composite coating is prepared by incorporating the modified TiO2 nanoparticles into fluorocarbon matrix and is characterized by SEM and contact angle. The modified nano-TiO2 has a good dispersability in fluorocarbon matrix. After coated with fluorocarbon coating, the bonded NdFeB magnet exhibits a large contact angle, increasing from 78° to 86°. However, the bonded NdFeB magnet exhibits a hydrophobic property with contact angle of 113° after coated with nano-TiO2/fluorocarbon composite coating. The electrochemical technique such as open circuit potential, polarization curves and electrochemical impedance spectroscopy (EIS) are used to investigate the corrosion behavior of the bonded NdFeB magnet in NaCl, HCl and NaOH solution, respectively. The results show that the corrosion resistance of nano-TiO2/fluorocarbon composite coating is the best in NaCl, HCl and NaOH solution, respectively. In NaOH solution, the hydroxide passivation film formed on the bonded NdFeB magnet can protect the matrix away from attack. In NaCl solution, Cl ions would not only destroy the fluorocarbon coating, but also accelerate active dissolution easily. In HCl solution, the bonded NdFeB magnet occurs activation dissolution easily. Therefore, the corrosion rate is higher than those in other solutions.The corrosion behavior of samples in NaCl solution is characterized by the electrochemical noise analysis technique. The results show that the samples are inclined to localized corrosion. It is found that the corrosion resistance of nano-TiO2/fluorocarbon composite coating is the best due to the minimum value of σI and W, and the maximum value of Rn. By domain analysis of nano-TiO2/fluorocarbon composite coating, the corrosion behavior process can be divided into four stages:a passivation stage, a metastable pitting stage, a stable pitting corrosion stage and a later pitting corrosion stage. The current white noise levels W can reflect the corrosion process. When the corrosion occurs, W tends to increase.