Corrosion protection provided by electrolytic nickel and tin coatings for Nd–Fe–B magnets

Abstract Nd–Fe–B magnets are prone to corrosion due their complex multiphase microstructure. The primary corrosion protection method is to apply a coating to isolate the magnet from the environment. Two electrodeposited metallic coatings, nickel and tin, and their ability to protect Nd–Fe–B magnet from corrosion were studied in this work. Two Nd–Fe–B magnet grades with different corrosion resistance were used as substrates. Both accelerated exposure and electrochemical tests were performed to compare the coatings and uncoated magnets. Nickel coatings provided good protection for Nd–Fe–B magnets against corrosion in heat-humidity tests. Tin coatings provided better corrosion protection than nickel coatings in the presence of defects and in salt spray test; in the case of both coated magnets, corrosion clearly started from coating defects in the form of pitting and the amount of corrosion damage was related to the presence of defects in the coating. The ability of tin coating to protect the Nd–Fe–B magnet in environments with elevated temperatures was poorer than that of nickel coating. Depending on the corrosive environment, also the magnet grade of the substrate had an effect on the corrosion rate. The magnets with no cobalt addition, i.e., compromised corrosion resistance, experiencing higher damage rates. Electrochemical impedance spectroscopy measurements of the coated magnets confirmed that development of pitting corrosion was the key damage mode in immersion.