Photoelectrochemical investigation of film formation at titanium and niobium anodes in acid fluoride media.

The properties of the surface oxide films during electrodissolution of titanium and niobium anodes in acid fluoride media are investigated by photoelectrochemical and impedance techniques. In slow voltammetries a linear dependence of photocurrent signal on applied potential is observed, consistent with a constant film formation ratio; an estimate of the latter is given on the basis of capacity data. Phenomena of depassivation observed at the Nb anode during voltammetric and potential step experiments are characterised and attributed to instability of the oxide film. The photocurrent spectra of both surface oxides change in shape under polarisation, showing a limited decrease of the photoresponse for penetrating wavelengths close to the bandgap and a marked decay for the shorter, strongly absorbed wavelengths. The phenomenon may be attributed to formation of a duplex oxide film, with a compact photoactive layer in contact with the metal substrate and an outer layer at the film | solution interface, rich in defects and possibly porous/hydrated, which acts as an optical filter.