Electrochemical corrosion characterisation of intermetallic phases occurring in Nd–Fe–B type magnets

Abstract

The corrosion behaviour of Nd–Fe–B type magnets was characterised by conducting gravimetric and electrochemical studies with synthetic preparations of the three maiN intermetallic phases of these materials. Tests were conducted in deaerated sulphate solutIOns at pH 0·3–9; some accelerated atmospheric corrosion tests were also carried out. In acidified solutions the neodymium rich (Nd₄Fe) synthetic phase was the most active with a high corrosion rate, which could be further accelerated by cathodic hydrogenation; the boron rich (NdFe₄B₄) phase was the most noble and had the lowest corrosion rates. In neutral solutions corrosion of the magnets is inhibited by the protective properties of neodymium oxide-hydroxide layers formed on the surface of the neodymium rich phase. The protective properties of these layers disappeared at pH < 4.