AC impedance study of corrosion films formed on zirconium based alloys

Oxide films formed by water oxidation (360 degrees C) of two Zr alloys, Zir caloy-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 goo d correlation was found between oxide thickness calculated from both IS mea surements at room temperature and weight gain and those estimated by metall ographic examinations. The electrical characteristics of a 2 mu m thick fil m formed on ZrNb(1%)O(0.13%) were investigated as a function of the tempera ture (25-280 degrees C) at constant oxygen partial pressure (0.3 Pa). Core- Cole diagrams suggest a frequency-temperature equivalence: low frequency po ints at low temperatures perfectly superimpose on high frequency points at higher temperatures. This resulting 14 decade 'master' curve so obtained ca n be characterized by the activation energy of the angular frequency omega 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 t he curves obtained at different temperatures with only a parallel resistanc e, the thicknesses of the two layers were estimated to be 1.5 and 0.5 mu m. Finally, both the Arrhenius diagrams of the total conductivity and the dis persion factor are characterized by a breakdown temperature point with two activation energy values. (C) 1999 Elsevier Science Ltd. All rights reserve d.