PHOTOCORROSION OF PASSIVE FILMS ON TITANIUM IN SULFURIC-ACID

Passive films formed anodically on titanium (Ti) plates in 0.5 M sulfu ric acid (H2SO4) were corroded in aqueous solutions of H2SO4 in the da rk 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 corro sion 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 polarizat ion of the oxide electrode together with the photocurrent. Corrosion o ccurring in the dark decreased very weakly under anodic polarization. Long-term corrosion experiments indicated the initial corrosion rate f or a thick anodic oxide was higher than the later rates, probably beca use of the different oxide quality within the cross section.