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

The potential-dependent breakdown of the native oxide film (similar to 20 A ngstrom thick) on titanium has been investigated in aqueous Br- solutions a nd 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 ox ide film is unstable in neutral pH solutions containing only Br-, resulting in the formation of stable corrosion pits at relatively low potentials (si milar to 1.4 V vs. Ag/AgCl). The pitting potential, E-p, is strongly depend ent upon the concentration of Br-, and can be modeled using a Langmuir isot herm to describe the adsorption of Br- at the oxide film/electrolyte interf ace. 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 cor rosion pits. The dependence of E-p on the relative concentrations of Br- an d the inhibitor anion is consistent with competitive adsorption of the anio ns. 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 ti tanium in aqueous Br- solutions. (C) 2000 The Electrochemical Society. S001 3-4651(99)04-066-5. All rights reserved.