Corrosion protection of Nd–Fe–B magnets by silanization

Abstract

Nd–Fe–B type permanent magnets present excellent magnetic properties. However, they are highly sensitive to the attack of corrosive environment. The aim of the present work is to describe the improvement of corrosion resistance of such magnets by dodecyl trimethoxysilane (DTMS) treatment. Electrochemical impedance spectroscopy (EIS) tests showed that, after treated by either DTMS, phosphate or chromate precursor, the protectiveness of Nd–Fe–B alloys was apparently increased. Corrosion performance of magnet treated with silane only was slightly inferior to those of phosphatized or chromatized ones. However, significant improvement in corrosion protection was achieved after two-step treatments, i.e. by top-coating silane films on phosphate or chromate under-layer. The depth profiles of element concentrations on surfaces measured by secondary ion mass spectroscopy (SIMS) indicate a two-layer surface structure for two-step treated samples. Co-existing of silane and phosphate or chromate was detected in the top-layer, suggesting the interaction between the two components. This results in the formation of more stable structure of hybrid organ–inorganic coatings against the attack of corrosive species.