Influence of potentiostatic aging, temperature and pH on the diffusivity of a point defect in the passive film on Nb in an HCl solution

This work mainly primarily focuses on investigating the transport property of the point defect in the passive film on Nb in an HCl solution via electrochemical measurement techniques based on the point defect model (PDM). The transport property of a point defect in the passive film was characterized by the diffusivity of the point defect (D0) and the steady-state current density (iss). The influences of the potentiostatic aging, temperature and pH on the D0 and iss were analyzed. The results demonstrated that the passive film on Nb was an n-type semiconductor with a donor density of 1020 cm−3. The steady-state passive current (iss) was dependent on the formation potential, this behavior was, to some extent, different from that of some other n-type semi-conductive passive films but similar to that of the passive film on Ti. The diffusivity of the point defect (D0) in the passive film on Nb at 3 V for 24 h in a 1 M HCl solution was calculated to be 7.45 × 10−18 cm2 s−1, the D0 increased with the decreased potentiostatic aging, pH and with the increment of temperature.