Recognition of kinetic transient process in corrosion scales of fuel elements by impedance spectroscopy

The presented study concerns with the corrosion kinetics of two zirconium alloys: Zr-Nb-Sn-Fe and Zr-Nb-Fe. Alloy samples were pre-exposed at 360 °C in a LiOH solution containing 70 mg/l of lithium ions. Ex-situ electrochemical impedance spectroscopy (EIS) performed in 0.5 M potassium sulphate solution at 25 °C was used to study the properties of the oxide and kinetic transient effect. Evaluation of the impedance spectroscopy data was based on application of a simple equivalent circuit. The setup of the equivalent circuit conformed to Jonscher´s universal law of dielectric response. The analysis of the impedance data was aimed at estimation of non-dispersive capacitance of the oxide formed during the pre-exposure. Effective values of dielectric constant were calculated using the non-dispersive capacitance and the oxide thickness values, calculated from weight gains. For the pre-transient samples relatively higher values of dielectric constants were obtained. Typical pre-transient dielectric constants for Zr-Nb-Sn-Fe alloy ranged between 20–21, while slightly lower values were obtained for Zr-Nb-Fe alloy. In both alloys steep and significant decrease in effective dielectric constant (e_ef = 9–13) was found for the transient samples. The decrease correlated very well with the drop in percentage of tetragonal oxide determined by Raman spectroscopy and corresponded to the increase of the weight gains of the transient samples. Literature data indicate values of dielectric constants for tetragonal zirconium oxide between 38–46, while those for monoclinic oxide are usually presented between 12–22. The evidenced changes in dielectric constants are therefore in agreement with the expected decrease of tetragonal phase fraction in the oxide layer during the transient. In the Zr-Nb-Sn-Fe post-transient samples values of dielectric constant increased again to 18–20, therefore almost to the pre-transient level. This increase was not evidenced with Raman spectroscopy data, which show constant low content of the tetragonal fraction. Possible explanation of this disagreement is the location of the newly formed post-transient tetragonal oxide presumably at the metal/oxide interface. Oxide thickness of the post-transi-ent samples is 4–7 m and the oxide/metal interface is beyond access of the laser beam of Raman spectrometer. We can conclude that using ex-situ EIS, the transient was observable in both alloys; the change in the ratio of monoclinic and tetragonal phase can be evaluated based on the difference of effective dielectric constant of the two phases. The Zr-Nb-Sn-Fe alloy showed the onset of the transient after the 105th day of pre-exposure, but the change in the ratio of the monoclinic and tetragonal phases was less significant than in the Zr-Nb-Fe alloy, in which, however, the transient could be observed only after 147 days of pre-exposure. The resulting values of the effective dielectric constant of oxides correlated well with the percentage of tetragonal oxide determined by Raman spectroscopy and with the results of the weight gain method.