Growth of Porous Anodic Films on Niobium in Hot Phosphate-Glycerol Electrolyte

摘要

Porous anodic films have been developed on niobium at constant formation voltages between 5 and 50 V in glycerol electrolyte containing 0.8 mol dm~(-3) dibasic potassium phosphate at 433 K. Except at the formation voltage of 5 V, at which the current density is approximately constant during anodizing, the current shows the minimum and then maximum. After the current maximum, the current density decreases gradually, which follows the Cottrell equation. Thus, diffusion of some species in the pores should control the growth rate of the anodic films. The porous films developed are practically Nb_2O_5 with a small amount of carbon contamination. Incorporation of phosphorus species, which often occurs in aqueous electrolytes, is negligible at all the formation voltages in the present electrolyte. Based on the results obtained, the growth process of the porous anodic films on niobium in this hot organic electrolyte is discussed.