Niobium specimens with chemical polishing were anodized in a phosphoric acid solution galvanostatically at i = 1 ,1 0 ,1 0 0and 1 00 0Am up to E = 10 0V,and then potentiostatically at E = 100V for t = 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 t = 7.2ks was higher at lower i in the range of0 .02to 0 .2 5Am . The spectra

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... f Rutherford backscattering spectroscopy(RBS)and glow discharge optical emission spectroscopy ( GD-OES)showed that higher i causes film thickness to decrease and the amount of incorporated phosphorus to increase. M icro 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 i . Parallel equivalent capacitance,C ,and the dielectric dissipation factor,tan δ ,of anodic oxide films decreased with increasing i . The mechanism of decrease in the number of imperfections in anodic oxide with increasing i is discussed in term of film thickness, phosphorus incorporation and Nb transport number.