Niobium specimens with chemical polishing were anodized in a phosphoric acid solution galvanostatically at ia,ini=1, 10, 100 and 1000Am−2 up to Ea=100V, and then potentiostatically at Ea=100V for tpa=7.2ks. During galvanostatic anodizing, the anode potential increased almost linearly with time on all the specimens, while, during potentiostatic anodizing, the anodic current decreased with time. The current density at tpa=7.2ks was higher at lower ia,ini in the range of 0.02 to 0.25Am−2. The spectra of Rutherford backscattering spectroscopy (RBS) and glow discharge optical emission spectroscopy (GD-OES) showed that higher ia,ini causes film thickness to decrease and the amount of incorporated phosphorus to increase. Micro imperfections were formed in the film at the ridge of the convex network structure produced by chemical polishing, and the number of imperfections decreased with increasing ia,ini. Parallel equivalent capacitance, Cp, and the dielectric dissipation factor, tan δ, of anodic oxide films decreased with increasing ia,ini. The mechanism of decrease in the number of imperfections in anodic oxide with increasing ia,ini is discussed in term of film thickness, phosphorus incorporation and Nb5+ transport number.
