Pitting Corrosion of Titanium The Relationship Between Pitting Potential and Competitive Anion Adsorption at the Oxide Film/Electrolyte Interface

The potential‐dependent breakdown of the native oxide film (∼20 Å thick) on titanium has been investigated in aqueous solutions and in solutions that contain a mixture of and anions that inhibit oxide breakdown ( i.e. , , , , , and . The oxide film is unstable in neutral pH solutions containing only , resulting in the formation of stable corrosion pits at relatively low potentials (∼1.4 V vs. Ag/AgCl). The pitting potential, , is strongly dependent upon the concentration of , and can be modeled using a Langmuir isotherm to describe the adsorption of at the oxide film/electrolyte interface. Addition of a second anion inhibits oxide film breakdown, as indicated by a large positive shift in and a decrease in the number of stable corrosion pits. The dependence of on the relative concentrations of and the inhibitor anion is consistent with competitive adsorption of the anions. Equilibrium adsorption coefficients for , , and are estimated from the dependence of on anion concentration. The results are used to establish a physical basis for the anomalously low pitting potential for titanium in aqueous solutions. © 2000 The Electrochemical Society. All rights reserved.