Corrosion resistance of Nd-Fe-B magnets coated with polypyrrole films

Abstract

Nd-Fe-B magnets present outstanding magnetic properties. However, due to their low corrosion resistance, their applications are limited to non-corrosive environments. Nowadays, significant efforts are underway to increase the corrosion resistance of these materials, through the use of coatings. Herein are presented the results of a study on the corrosion resistance of Nd-Fe-B magnets coated with polypyrrole (PPY). The electrochemical behavior of coated and uncoated magnets has been studied by Electrochemical Impedance spectroscopy (EIS) in synthetic saliva. The results were compared to previous investigations, which were carried out under similar conditions, in Na2SO 4 and NaCl solutions. In sulphate solution, the corrosion resistance of the PPY-coated magnet was 3 times larger (1600 Ω.cm2) than that of uncoated magnet (500 Ω.cm2). In NaCl solution, however, the corrosion resistance of coated and uncoated magnets were very similar (250 Ω.cm2). In synthetic saliva, both the uncoated and coated magnets presented good corrosion performance (1940 Ω.cm2). Such behavior can be attributed to the phosphate ions in saliva, which play a role as corrosion inhibitor, producing phosphating, at least partially, of the magnet surface. The PPY-coated magnets presented a strong diffusional control from moderate to low frequencies, caused by the polypyrrole film. The thicker PPY film increased the corrosion resistance of the magnet in synthetic saliva.