Formation and dissolution behaviour of niobium oxide in phosphoric acid solutions

The effect of phosphoric acid concentration and temperature on the formation and dissolution process of niobium oxide was investigated using capacitance, potential and galvanostatic measurements. The formation rate of the niobium oxide increases with increasing phosphoric acid concentration and decreases with increasing temperature. The dissolution rate of the niobium oxide is accelerated by increasing phosphoric acid concentration and temperature. The activation energy was calculated for both the formation and dissolution process and found to be 8.93 and 16.65 kJ/mol respectively. The effect of formation voltage on the dissolution process of niobium oxide was also investigated. The oxide film formed at high-formation voltage has a more defective character than that formed at lower voltage. This enhances the dissolution process of the oxide. The effect of current density on the formation rate and the thickness during the oxide film growth was measured. (C) 1997 Elsevier Science S.A.