Preparation and corrosion resistance of epoxy resin coating for bonded NdFeB magnet

Abstract

The epoxy resin (ER) coatings were coated on ring-shaped bonded NdFeB magnet (RSM) using air spraying and cathodic electrophoresis deposition processes. The morphologies and microstructures of the bare RSM (B-RSM), sprayed ER/RSM (S-ER/RSM) and electrophoretic ER/RSM (E-ER/RSM) were observed using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), 3D laser confocal scanning microscopy (LSCM) and fourier infrared spectrometer (FTIR). The corrosion behaviors of the ER coatings were analyzed by potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and neutral salt spray test (NSS). The results showed that ER coatings covered the pores of B-RSM well and played a good sealing role. Moreover, E-ER/RSM had a more uniform film layer and better surface leveling than S-ER/RSM. Compared with B-RSM and S-ER/RSM, the corrosion current density (Icorr) of E-ER/RSM reached 1.904 × 10−5 A/cm2, one order of magnitude lower than that of B-RSM (1.033 × 10−4 A/cm2) and about twice lower than that of S-ER/RSM (4.728 × 10−5 A/cm2). The value of charge transfer resistance (Rct) of E-ER/RSM is 1671.6 Ω·cm2, which is higher than that of both B-RSM (236.96 Ω·cm2) and S-ER/RSM (425.46 Ω·cm2). The NSS time of E-ER/RSM reached 144 h, longer than that of S-ER/RSM (72 h) and B-RSM (24 h). The corrosion resistance of bonded NdFeB magnets was improved due to the good shielding properties and physical barrier effect of the ER coatings. In addition, the corrosion mechanisms of ER coatings were discussed in detail.