Passive behaviour of niobium

The passive behaviour of niobium at electrode potentials from −1 to 4 V (sce) in aqueous solutions of pH 0–13 at 25°C has been studied by quasi-steady (20 h) and transient polarization measurements, dc capacitance determinations and some AES depth profile investigations. Niobium exhibits a quasi-steady passive current varying little with the electrode potential and pH. After potential (or current) steps, transient superpolarization occurs. The transient data accord with the Cabrera-Mott equation and fit reasonably with λ = 1.5 Å and z = 5 for metal ion transfer at the metal—oxide interface. The capacitance data satisfy a simple insulator model, reveal the main charge distribution in the passive electrode, and combine with film thickness data to give ε = 46 for the static dielectric constant of the passive film. The anodization ratio is 37 Å V⁻¹ under quasi-steady conditions. The passive niobium electrode seems mostly to carry a net negative charge. Anodic oxygen evolution does not occur.