Corrosion protection of NdFeB magnets by surface coatings - Part 2: electrochemical behaviour in various solutions

Various surface modification techniques, including single and bimetallic electroplating, and chemical vapour deposition (CVD), have been applied to improve the poor corrosion resistance of NdFeB magnets. The corrosion resistance of the surface-treated samples has been compared with that of some commercially available coated samples including nickel, zinc, gold coatings. Potentiokinetic polarisation behaviour in 1N H2SO4, 10% H3PO4 and Ringer's solution was studied. It was found that in the 1N H2SO4 solution, the corrosion resistance of gold coating was the best, whereas in chloride-contaminated solution, Ni/Cr and gold coatings have comparable corrosion resistance. The results of salt spray tests of the coated specimens (in Part 1) were compared with the electrochemical data. In a simulated marine environment, epoxy coating was found to be slightly better than Ni/Cr coatings. Zn coating provided a certain degree of protection to the magnets in various solutions studied. The choice between cathodic, anodic and polymer coatings was found to be governed by the service environment and application. (orig.)