ELECTROCHEMICAL-BEHAVIOR OF THIN ANODIC OXIDE-FILMS ON ZIRCALOY-4 - ROLE OF THE MOBILE DEFECTS

The first stages of the electrochemical oxidation of Zircaloy-4 are in vestigated using simple electrochemical tests and modeling the passive film modifications occurring as a result of contact with the electrol yte. Variations in electrode potential (open-circuit conditions) or cu rrent density (potentiodynamic scans) can be simply explained by a hig h field (F similar to 10(6) V/cm) assisted passive film growth. Under open-circuit conditions, this field does not vary with exposure time ( in the 2 h to 48 h range). The minimum electric field for the onset of high-field behavior is also evaluated and found smaller than the theo retical value which can be explained by a variation in the concentrati on of mobile defects throughout the film. Measurements of the electrod e potential decay after a potentiodynamic scan confirm this model, all owing interpretation of the film modification as a combination of two separate phenomena: film growth under a high electric field and point defect annihilation.