AC impedance study of corrosion films formed on zirconium based alloys

Abstract

Oxide films formed by water oxidation (360°C) of two Zr alloys, Zircaloy-4 (Zy-4) and ZrNb(1%)O(0.13%), were studied by impedance spectroscopy (IS) in gaseous atmospheres. Results show that capacitances are frequency dispersed, in agreement with Jonscher’s law of dielectric relaxation. A good correlation was found between oxide thickness calculated from both IS measurements at room temperature and weight gain and those estimated by metallographic examinations. The electrical characteristics of a 2 μm thick film formed on ZrNb(1%)O(0.13%) were investigated as a function of the temperature (25–280°C) at constant oxygen partial pressure (0.3 Pa). Cole–Cole diagrams suggest a frequency-temperature equivalence: low frequency points at low temperatures perfectly superimpose on high frequency points at higher temperatures. This resulting 14 decade ‘master’ curve so obtained can be characterized by the activation energy of the angular frequency ω close to 0.8 eV. An equivalent circuit based on an association in series of two layers with different dielectric properties was proposed. By fitting the curves obtained at different temperatures with only a parallel resistance, the thicknesses of the two layers were estimated to be 1.5 and 0.5 μm. Finally, both the Arrhenius diagrams of the total conductivity and the dispersion factor are characterized by a breakdown temperature point with two activation energy values.