Study On Corrosion Resistance And Kinetics Of Electroless Deposition Of Ni-based Alloy On NdFeB Magnets

NdFeB alloy as the third-era permanent magnet is widely used in many industries because of its outstanding properties, such as excellent magnetic properties and machinability, and comparatively low price. Unfortunately, it is prone to oxidation in working environments, especially in damp heat atmosphere, which leads to a severe electrochemical erosion. Poor corrosion resistance is the obstruction to broaden applications of NdFeB alloy. In this thesis, reports on corrosion mechanism and anticorrosion treatment of NdFeB permanent magnet are reviewed. On this basis, electroless deposition of Ni-based alloy was employed to NdFeB magnet to enhance its corrosion resistance.Parameters of electroless deposition of Ni-P alloy were optimized by an orthogonal experiment. The structure and morphology of the Ni-P layer on magnets deposited from an acidic bath and from an alkaline bath were studied and compared. The corrosion rate of Ni-P deposited magnets in 3.5wt% NaCl, 10vol% HC1 and 20wt%NaOH solutions were measured. The corrosion behaviors of both types of Ni-P deposits were evaluated by electrochemical potentiodynamic polarization and impedance techniques. The results show that both deposits enhance the corrosion resistance of the substrate. Ni-P deposited from the acidic bath provides a high degree of protection. The difference in the corrosion behavior of the two deposits is attributed to their distinct structure and density.The experimental analysis of activation energy of electroless deposition reaction shows that acidic bath reaction system possesses a low activation energy and a high deposition rate. The reaction kinetics of electroless deposition of Ni-P alloy in acidic bath was discussed. Reaction kinetic parameters were obtained and an empirical equation of deposition rate was derived.Electroless Ni-Co-P ternary alloy deposits were prepared by optimizing the bath composition and operating conditions to improve the corrosion resistance of NdFeB permanent magnets. The effects of pH and metallic ion ratio ([Co~2+]/[Ni~2++Co~2+]) on plating rate and deposit composition were investigated. Polarization curves of NdFeB matrix with and without Ni-Co-P deposits in 3.5wt%NaCl solution were examined. It is found that the plating rate and the cobaltcontent of the deposit increase with increasing pH, while the nickel and phosphorus contents decrease. With increasing metallic ratio, the plating rate decreases and the cobalt content of the deposit increases with a simultaneous decrease in the nickel and phosphorus contents. The polarization curves exhibit that the corrosion rate of NdFeB magnets decreases by two orders of magnitude in 3.5wt%NaCl solution after electroless plating with the Ni-Co-P deposits. A metallic ion ratio at 0.3 in the bath solution achieved the best corrosion resistance.Effects of pre-plating in ultrasonic field on the adhesion between deposit and NdFeB matrix were investigated. The results show that a uniform and compact deposit is prepared by pre-plating with ultrasonic, and the adhesion is improved effectively. It is suggested that ultrasonic pre-plating contributes to the improvement of adhesion through three aspects: activation, cavitation and stirring action.