The effects of Niobium on the tructure and properties(especially electric properties) of passive film of Zirconium alloys in pH 8.5 buffer solution are examined by the photo-electrochemical analysis. For Zr-xNb alloys (x=0, 0.45, 1.5, 2.5 wt%), photocurrent began to increase at the incident energy of 3.5 3.7 eV and exhibited the 1st peak at 4.3 eV and the 2nd peak at 5.7 eV. From $(i_{ph}\nu)1/2$ vs.$\nu$ plot, indirect band gap energies $E_{g1}$=3.01∼3.47 eV , $E_{g2}$ =4.44∼4.91 eV were obtained. With increasing Nb content, the relative photocurrent intensity of 1st peak significantly increased. Compared with photocurrent spectrum of thermal oxide of Zr-2.5Nb, It was revealed that 1st peak in photocurrent spectrum for the passive film formed on Zr-Nb alloy was generated by two types of electron transitions; the one caused by hydrous $ZrO_2$ and the other created by Nb. Two electron transition sources were overlapped over the same range of incident photon energy. In the photocurrent spectrum for passive film formed on Zr-2.5Nb alloy in which Nb is dissolved into matrix by quenching, the relative photocurrent intensity of 1st peak increased, which implies that dissolved Nb act as another electron transition source.
Repassivation behaviors of Zr-alloys are also examined by using the rapid scratching electrode techniques under the potentiostatic condition. Repassivation on the scratched surface of the alloy was analyaed in terms of three step; at the initial stage(within 10 msec), passive film grew according to the place exchange model in which logi(t) is linearly proportional to q(t) with a slope of 1/K , and thereafter repassivation after 25 msec occurred according to the high field conduction model in which logi(t) is linearly proportional to 1/q(t) with a slope of cBV. from 10 msec to 25 msec, passive film grew according to both of these model.
The value of 1/K, reported that it was directly proportional to the activation energy for the place exchange process of a metal -oxygen pair, could be used as a measure of passive film protectiveness and erosion resistance of the alloy. An alloy/environment system with a higher value of 1/K exhibited faster repassivation rate with a formation of more protective and erosion-resistant passive film.
With an addition of Nb into the Zr alloy, 1/K decreased and cBV increased. It means that the protectiveness and erosion resistance of the alloy decreased by Nb alloying. Also, as film formation potential and solution temperature increased, the protectiveness and erosion resistance of the alloy decreased. Using the experimental value of 1/K obtained in relative low temperature, the activation energy for the place exchange of metal-oxygen pair$(W_o)$ in high temperature, in which zirconium alloy is practically used in nuclear reactor, could be predicted.
