ANODIC GROWTH OF PASSIVE FILMS ON TITANIUM

The title subject is studied by slow galvanostatic and potentiostatic transients and dc capacitance determinations on potentiostatically well stabilized (20 h) passive titanium electrodes in acidic sulphate (pH 1.4) and neutral acetate (pH 7.2) solutions at 25-degrees-C. Combined with film thickness data and model studies, the results reveal information on the roughness factor of the electrodes used, on the kink-site dependent metal-ion transfer at the metal-oxide interface and on the ionic transport, the dielectric constant, the space charge and the electric field in the passive film. The interfacial metal-ion transfer appears to occur by four-valent titanium ions and the ionic transport in the film mainly by fully ionized oxygen vacancies. Under steady-growth conditions, the surface density of kinks on the metal (s) and the volume density of oxygen vacancies in the passive film at the metal-oxide interface (N0) respectively obey d log s/d log j almost-equal-to 1/2 and d log N0/d log j almost-equal-to 3/4 where j is the current density of metal oxidation.