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

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.