IN-SITU CHARACTERIZATION OF PASSIVE FILMS ON AL-TI ALLOY BY PHOTOCURRENT AND IMPEDANCE SPECTROSCOPY

The anodic behaviour of an Al-Ti alloy (Ti-48Al-1V, atomic %) was inve stigated in different aqueous electrolytes. In all cases the alloy was passive owing to the growth of a barrier-like oxide film according to the high field mechanism. The study of the growth curves suggests par tial dissolution of the film. during the formation process in acidic s olution. The kinetic parameters for film Formation have been estimated in neutral solutions and the dielectric constant of the passive layer was roughly estimated. The in-situ characterization of the passive fi lm revealed a n-type behaviour only for very low thicknesses, whilst t hicker films showed insulator-like characteristics. The analysis of th e photocurrent spectra suggests that the composition of the surface ox ide does not change in a large thickness interval since the very thin initial layers formed by simple immersion of the alloy in solution. Fr om the values of the threshold energy derived from both anodic and cat hodic spectra recorded in different conditions it is postulated thar t his passive film consists of a mixed oxide of the main components of t he alloy. A more complex behaviour was observed at higher film thickne sses, which reveals some modification in the properties of the surface layer. (C) 1998 Elsevier Science Ltd. Ail rights reserved