Multilayer ceramic coating for impeding corrosion of sintered NdFeB magnets

Sintered NdFeB magnets have complex microstructure that makes them susceptible to corrosion in active environments. The current paper evaluated the anticorrosion characteristics of multilayer titanium nitride ceramic coating applied through cathodic arc physical vapour deposition (CAPVD) for protection of sintered NdFeB permanent magnets. The performance of ceramic coating was compared to the electrodeposited nickel coating having a copper interlayer. Electrochemical impedance spectroscopy (EIS) and cyclic polarization in simulated marine environment were employed to determine the rates of coatings degradation and passivation behaviour respectively. The coating morphologies and surface chemistry were studied with scanning electron microscope (SEM). X-ray diffraction (XRD) was used for identification of component phases in the coatings and the substrate. The results showed that the polarization resistance of ceramic coating increased with the exposure time. The rate of degradation of Rp for the ceramic coating had an extraordinary negative slope followed by a stable duration, before declining towards the coating failure. In comparison the nickel coating with copper interlayer degraded sharply. The vapour deposited ceramic coating was found to have permeable defects that tended to “re-passivate” during exposure providing prolonged corrosion protection to the NdFeB substrate. The magnetic properties were unaffected and remained at par with the nickel coating having copper interlayer.