Pitting corrosion of titanium - The relationship between fitting potential and competitive anion adsorption at the oxide film/electrolyte interface

The potential-dependent breakdown of the native oxide film (similar to 20 Angstrom thick) on titanium has been investigated in aqueous Br- solutions and in solutions that contain a mixture of Br- and anions that inhibit oxide breakdown (i.e., Cl-, I-, SO42-, Fe(CN)(6)(4-), and Fe(CN)(6)(3-)). The oxide film is unstable in neutral pH solutions containing only Br-, resulting in the formation of stable corrosion pits at relatively low potentials (similar to 1.4 V vs. Ag/AgCl). The pitting potential, E-p, is strongly dependent upon the concentration of Br-, and can be modeled using a Langmuir isotherm to describe the adsorption of Br- at the oxide film/electrolyte interface. Addition of a second anion inhibits oxide film breakdown, a indicated by a large positive shift in E-p and a decrease in the number of stable corrosion pits. The dependence of E-p on the relative concentrations of Br- and the inhibitor anion is consistent with competitive adsorption of the anions. Equilibrium adsorption coefficients for I-, Br-, and Cl- are estimated From the dependence of E-p on anion concentration. The results are used to establish a physical basis for the anomalously low pitting potential fur titanium in aqueous Br- solutions. (C) 2000 The Electrochemical Society. S0013-4651(99)04-066-5. All rights reserved.