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In order to improve the corrosion resistance of the sintered NdFeB permanent magnets, Al 2 O 3 ceramic coatings were prepared by a two-step micro-arc oxidation process in an aluminate solution. During the micro-arc oxidation process, the voltage-time curve could be divided into four regions consistent with the valve metals. Results show that the coating exhibits a typical micro-arc oxidation porous surface with the thickness of ∼5 μm, and is only composed of Al 2 O 3 crystalline phase with some Fe elements and a few Nd and P elements. The surface roughness of coated NdFeB magnets is higher than that of uncoated sample, while the corrosion resistance is enhanced by one order of magnitude. Unfortunately, the magnetic properties (remanence and maximum energy product) of coated NdFeB sample decrease in different degrees. [ABSTRACT FROM AUTHOR]

Corrosion behaviors of sintered NdFeB magnets in H 2 SO 4 solution, HCl solution and HNO 3 solution of different concentrations were investigated by pickling, immersion test and electrochemical test. The results show that the corrosion mechanism of sintered NdFeB magnets in HNO 3 solution is uniform corrosion, while the one in H 2 SO 4 solution and HCl solution is selectively intergranular corrosion. The corrosion rate of NdFeB magnets increases with increasing of the concentrations of the three acid solutions, and the corrosion rate of NdFeB magnets in the HNO 3 solution is the smallest among the three acid solutions both in immersion test and electrochemical test. Therefore, the HNO 3 solution is more suitable to be used as the pickling solution for the surface pretreatment of sintered NdFeB magnets. [ABSTRACT FROM AUTHOR]