Phosphating passivation of vacuum evaporated Al/NdFeB magnets boosting high anti-corrosion performances

Abstract A phosphating passivation is conducted on the surface of Al coated NdFeB magnets by a gaseous process. The surface morphology, structure and elemental compositions of the samples are characterized by X-ray diffraction, scanning electron microscopy and X-ray photoelectron spectroscopy, respectively. The anti-corrosion performances of the phosphating passivated Al/NdFeB magnets are analyzed by neutral salt spray test, potentiodynamic polarization curve and electrochemical impedance spectroscopy measurements. The corrosion process and corrosion mechanism of the samples are studied by analyzing the coating morphologies, potentiodynamic polarization curves and surface elemental compositions during the immersion test in 3.5 mass% NaCl solution for different time. The surface passivation of Al coated NdFeB magnets can be realized by gaseous phosphating technology, which can effectively improve the corrosion resistance of Al coating. The optimized sample obtained with phosphating temperature at 300 °C achieves the improvement of NSS time from 96 h to 288 h. Gaseous phosphating can effectively seal the pores in vacuum evaporated Al coatings, and form a phosphating passivation film on the coating surface, which contribute to the enhancement of anti-corrosion performances of Al coated NdFeB magnets.