The effect of aeration on the corrosion process of magnetized Nd-Fe-B magnets

Abstract

The effects of aeration upon the corrosion process of magnetized Nd-Fe-B based sintered magnets have been studied. The tests were carried out in naturally aerated and in deaerated 3.5 (wt.%) sodium chloride solutions at various testing times by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization methods. In deaerated solution the magnets exhibit higher impedance values (one order of magnitude) than in aerated one. Bode phase angle diagrams obtained in the former solution presented a single broad time constant, indicating the existence of a single process in the investigated frequency range; moreover, the impedance behaviour was maintained fairly constant from 5 days of immersion throughout the rest of the testing period. On the other hand, the sample immersed in the aerated solution presented a constant and slow diminution of the impedance all long the immersion time. In this case, Bode phase angle diagrams presented a HF shoulder interpreted as the influence of the magnetic field on the transport of oxygen to the interface. Larger icorr values and more noble potentials were also obtained for the magnet in aerated solution. In the presence of oxygen, the corrosive attack penetrated deeply into the specimens and, consequently, the magnetic properties diminished with immersion time. The magnetic properties most influenced by corrosion were the remanence (Br) and the maximum energy product (BHmax).