Preparation, microstructure and corrosion resistance of Al2O3 coatings by cathodic plasma electrolytic deposition on sintered NdFeB permanent magnet

Abstract

The Al2O3 ceramic coatings were prepared on the sintered NdFeB magnets by cathodic plasma electrolytic deposition (CPED) method in aluminum nitrate alcohol solution. The surface and cross-sectional morphologies, element and phase composition, corrosion resistance and magnetic properties of the coatings were analyzed by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), thermal shock, hydrothermal weight loss, electrochemical corrosion and physical property measurement system (PPMS), respectively. The results show that the coatings mainly consist of γ-Al2O3 and α-Al2O3 phases, and the volume of α-Al2O3 increases with increasing the voltages. The surface of the coatings shows a typical coral reef structure, and the thickness of the coatings increases from 29 µm to 53 µm with increasing the voltages. The Al2O3 coatings have excellent thermal shock resistance as well as the excellent corrosion resistance in hydrothermal conditions. The corrosion resistance of the coatings enhances with increasing the voltages, reaching the strongest at 470 V, and then decreases. This study provides a new method for forming ceramic coatings on the surface of sintered NdFeB magnets to improve its corrosion resistance.