Effects of bias voltage on coating structures and anticorrosion performances of PA-PVD Al coated NdFeB magnets

Abstract

Al coated NdFeB magnets were prepared by plasma-assisted-physical-vapor-deposition (PA-PVD) method for enhancing the corrosion resistance. Morphologies and structures were characterized by an X-ray diffractometer and a scanning electron microscope. Corrosion behaviors of the samples with different bias voltages were studied by electrochemical methods and neutral salt spray test, respectively. The bias voltage during the deposition process was optimized according to the coating structures and corrosion resistances. The density and flatness of Al coating increase with increasing the bias voltage, and at the same time the thickness decreases. The coating density and thickness turn to be stable when bias voltage is 1800 V. Also, the corrosion behaviors of PA-PVD Al coatings in different media, such as NaOH, HNO3 and NaCl solutions, were studied. The self-corrosion potentials (Ecorr) of PA-PVD Al coatings almost keep constant in NaOH and HNO3 solutions with different concentrations. However, the self-corrosion current densities (Jcorr) decrease with the increasing concentrations. Ecorr shifts to negative potential and Jcorr decreases gradually when increasing the concentration of NaCl solution. The corrosion mechanisms of Al coatings are discussed based on the corrosion behaviors in different media.