PHOTOCORROSION OF PASSIVE FILMS ON TITANIUM IN SULFURIC-ACID

Passive films formed anodically on titanium (Ti) plates in 0.5 M sulfuric acid (H2SO4) were corroded in aqueous solutions of H2SO4 in the dark and under illumination of a 250-W mercury (Hg) lamp. The corrosion depth was determined by calculating the thickness of the oxide layers from interference patterns of reflection spectra in the visible region. Corrosion was observed at pH less than or equal to 3, with the corrosion rate increasing exponentially with decreasing pH and achieving a maximum value at pH similar to 1. Photocorrosion generally was quicker than corrosion occurring in the dark in all cases (i.e., open-circuit, short-circuit, bias conditions) and increased under anodic polarization of the oxide electrode together with the photocurrent. Corrosion occurring in the dark decreased very weakly under anodic polarization. Long-term corrosion experiments indicated the initial corrosion rate for a thick anodic oxide was higher than the later rates, probably because of the different oxide quality within the cross section.