Influence of the chemical composition of the plating solution on the ability of nickel coatings to protect Nd2Fe14B magnets against corrosion

The commercial application of rare earth magnets has been in part held back by their poor corrosion resistance. As part of e!orts to overcome this problem two types of nickel coatings were electroplated on the surface of Nd 2 Fe 14 B substrates, one from an acidic plating solution and the other from an alkaline solution. The corrosion characteristics of both coatings were evaluated both by electrochemical potentiodynamic polarisation and by physical exposure to high-humidity environments. The results showed that both coatings enhanced the corrosion resistance of substrate. However, both the degree of protection a!orded by the coating and the strength of its adhesion to the substrate strongly depended on the chemical composition of the original plating solution. The coating plated from the acidic solution provided a high degree of protection against both aqueous and atmospheric corrosion, whilst that plated from the alkaline solution behaved only marginally better than the bare substrate. It was postulated that the poor performance of the coatings plated from the alkaline bath could have been due to the spontaneous formation of a passive oxide "lm between the Nd 2 Fe 14 B substrate and the nickel coating. ( 2001 Elsevier Science B.V. All rights reserved.