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 efforts to overcome this pr oblem two types of nickel coatings were electroplated on the surface of Nd2 Fe14B substrates, one from an acidic plating solution and the other from an alkaline solution. The corrosion characteristics of both coatings were eva luated both by electrochemical potentiodynamic polarisation and by physical exposure to high-humidity environments. The results showed that both coati ngs enhanced the corrosion resistance of substrate. However, both the degre e of protection afforded by the coating and the strength of its adhesion to the substrate strongly depended on the chemical composition of the origina l plating solution. The coating plated from the acidic solution provided a high degree of protection against both aqueous and atmospheric corrosion. w hilst that plated from the alkaline solution behaved only marginally better than the bare substrate. It was postulated that the poor performance of th e coatings plated from the alkaline bath could have been due to the spontan eous formation of a passive oxide film between the Nd2Fe14B substrate and t he nickel coating. (C) 2001 Elsevier Science B.V. All rights reserved.