In situ characterization of Zircaloy-4 oxidation at 500 degrees C in dry air

The in situ oxidation of Zircaloy-4 at 500 degreesC in dry air was investig ated by thermogravimetric analysis (TGA) and electrochemical impedance spec troscopy (EIS). The coating of the alloy by a platinum film as electrode ma terial was observed as not to modify the oxidation kinetic properties. Afte r an initial cubic rate law, a transition to a quasi-linear curve occurs. T he independence of the oxidation behavior to the Pt coupling is compatible with oxygen diffusion as the rate-determining step. During the pre-transiti on step, the rest potential of the cell Pt/oxide/Zy-4, the color of the oxi de and the modulus of the single EIS signature indicate the high non-stoich iometry of the oxide. The kinetic transition was proposed to be correlated to the degradation of the film into a partially porous layer. This alterati on of the oxide is associated to the appearance of a 1.2 V constant rest po tential and the modification of the impedance diagrams in two high modulus contributions. The Cole-Cole representation has been used to demonstrate th at the time variation of impedance spectra is related to the oxide growth. An equivalent circuit including two RC loops in series, whose capacitances are frequency dispersed, was proposed to be related to the film structure. Fitted data show that the thickness of the assumed protective layer of the film, close to the metal-oxide interface, is time independent in agreement with a constant oxidation rate. Finally, electrical properties of this inne r layer were found to be quite different in pre- and posttransition stage. (C) 2001 Elsevier Science B.V. All rights reserved.