Electro-deposition and hydrothermal transition of alumina sol film on sintered NdFeB magnets

Alumina film is effective for improving the corrosion resistance and protecting the magnetic properties of NdFeB magnets, but the complicated preparation method limits its application. In this study, we prepare alumina sol (AS) film on NdFeB magnets with an electric chemistry method from modified alumina sol. And a hydrothermal treatment (HTT) improves the combination and protection ability of AS film on NdFeB. Microstructural investigation suggests that the uniform AS film can be prepared on NdFeB under optimized electric conditions and the hydrothermal treatment helps to repair defects on the AS film. With the formation of electro-deposited AS film, the corrosion current density value reduces more than an order of magnitude and the intergranular corrosion of NdFeB is obviously suppressed in corrosive environments. The thermal stability of NdFeB magnets is also improved by the AS film. The morphology, composition and phase transition of the AS film are systematically investigated. In addition, the underlying formation mechanisms for electro-deposited AS film have been analyzed by zeta potential measurement and cathode polarization measurement at the sol–metal interface. These results have important implications for the preparation of protection film to prevent magnetic loss on sintered NdFeB magnets.