Corrosion protection of NdFeB permanent magnets

Abstract

Neodymium-iron-boron (Nd-Fe-B), the most powerful permanent magnet material developed to date, derives its attractiveness from its desirable magnetic properties. However, the poor corrosion resistance of Nd-Fe-B in humid and/or chloride containing environments seriously limits its applications. The present project aims at developing feasible and effective coating systems or methods for improving the corrosion resistance of the Nd-Fe-B magnet so as to widen its scope of applications. Three coating systems or methods have been successfully developed and their efficacy has been evaluated by the salt spray test and electrochemical tests, in comparison with commercial coated samples. (1) The first system developed consists of a duplex Ni(undercoat) / Cr(topcoat) coating electrodeposited by DC plating. It was found that for a Ni(20um) / Cr(1um) coating, the magnet did not show signs of corrosion in the salt spray test for more than 360 hours. (2) To further enhance the performance of the Ni layer, pulsed current plating of Ni has been attempted. The corrosion resistance of pulse-Ni plated samples was found to be at least doubled as compared with the DC plated ones of similar thickness. (3) As an alternative to metallic coatings, an organic coating system consisting of a commercial primer and a BMI (bismaleimide) topcoat has been applied on the Nd-Fe-B magnet. For an exposure time of 360 hours in the salt spray test, there was no sign of failure for samples protected with a primer(5um) / BMI(5um) coating. All these coatings exhibited excellent adhesion strength as indicated by the scotch tape test and were reasonably scratch-resistant.