PITTING CORROSION OF TITANIUM

The breakdown of native and anodically grown oxide films on Ti electrodes is investigated by scanning electrochemical microscopy (SECM), video microscopy, transmission electron microscopy, and voltammetry. SECM is used to demonstrate that the oxidation of Br- on Ti occurs at microscopic surface sites (10 to 50 mum diam, 30 sites/cm2) that are randomly positioned across the oxide surface. After determining the position of the active sites for Br- oxidation, breakdown of the oxide is initiated by increasing the electrode potential to more positive values. Direct correspondence is observed between the location of the electroactive sites and corrosion pits, indicating that oxide breakdown is associated with a localized site of high electrical conductivity. The potential at which pitting is observed in voltammetric experiments is found to be proportional to the average oxide thickness, for values ranging between 20 and 100 angstrom, indicating that breakdown is determined either by the magnitude of the electric field within the oxide or by the interfacial potential at the oxide/Br-solution interface. Pitting occurs at significantly lower potentials in Br- solutions than in Cl- solutions, suggesting a strong chemical interaction between the TiO2 surface and Br-. A mechanism of oxide breakdown is proposed that is based on the potential-dependent chemical dissolution of the oxide at microscopic surface sites.