The correlation of some characteristics of the zirconium oxide films with the environment chemistry

It is known that zirconium alloys are extensively used as structural materials in the primary circuit of a CANDU PHWR (Pressurized Heavy Water Reactor). The main types of these alloys are: zircaloy-2, zircaloy-4 and Zr-2.5%Nb. In normal operation conditions, the oxidation behavior of these zirconium alloys in several aqueous environments may be described as an initial 'pretransition' period in which the weight gains follow a nearly cubic rate law as a function of exposure time. When the oxide thickness is of about 2-5 mm, the 'post-transition' period appears, in which the oxidation rate is higher and approximately linear. Therefore, the lifetime and the reliability in operation of zirconium alloys are often limited by the thickness and uniformity of the oxide at the waterside of these components. Corrosion of zirconium components in PHWR can be, in certain conditions, a dangerous process able to limit significantly their service life. Our experimental data were obtained testing the following zirconium alloys: Zircaloy-2, Zircaloy-4 and Zr-2.5 % Nb autoclavized in some specific conditions to primary circuit, i.e. in aqueous solutions of several pHs at high temperature and pressure in the presence of different gases dissolved in the working environments and in solutions contaminated with F- (HF). The obtained data have been used to represent the kinetic curves weight gain as a function of time. On the autoclavized samples impedance measurements were performed to determine some of the characteristics of zirconium oxides with the aim: - to detect failures in films; - to correlate the oxidation law of the zirconium alloys with the main characteristics of films; - to find out models of zirconium oxide/solution interfaces. Based on the impedance curves - Bode, Nyquist and phase angle - some correlations between the impedance spectra, oxidation kinetics and the protective/nonprotective character of zirconium oxides and the physico-chemical characteristics of autoclavizing environment were established. (authors)