Passivation oxide film on Nd–Fe–B permanent magnets in borate buffer solution by ellipsometry

Abstract

The passivation of Nd–Fe–B permanent magnet was investigated in neutral borate solution at pH 8.4. The thickness of the passive oxide film on the magnets was measured by ellipsometry and the composition was estimated by glow discharge optical emission spectroscopy (GD-OES). The passivation of the magnets takes place in the potential range between −0.2 and 1.0 V vs. Ag/AgCl/Sat. KCl. In the potential range, current density decays to the lower than 10 −6 A cm −2 after potentiostatic oxidation for 1800 s. The passive oxide film growth is assumed to be optically simulated from a model with a homogeneous film with complex refractive index, N = 2.1 − j0.086. The thickness estimated from the refractive index linearly increases with potential from 3.6 nm at −0.2 V to 7.8 nm at 1.0 V. The passive film growth follows the ionic migration model under high electric field, i.e., the Cabrera–Mott growth model. The ionic conductivity estimated from the model is about κ = 1.7 × 10 −16 Ω −1 cm −1. The passive oxide film is preferentially composed of iron oxide/hydroxide. Boron and neodymium are, respectively, concentrated at the surface of the oxide film and at the inner layer in the oxide film.