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 (approx =20 A thick) on titanium has been investigated in aqueous Br exp - solutions and in solutions that contain a mixture of Br exp - and anions that inhibit oxide breakdown (i.e., Cl exp - , I exp - , SO sub 4 exp 2- , Fe(CN) sub 6 exp 4- , and Fe(CN) sub 2 exp 3- ). The oxide film is unstable in neutral pH solutions containing only Br exp - , resulting in the formation of stable corrosion pits at relatively low potentials (approx =1.4 V vs. Ag/AgCl). The pitting potential, E sub p , is strongly dependent upon the concentration of Br exp - , and can be modeled using a Langmuir isotherm to describe the adsorption of Br exp - at the oxide film/electrolyte interface. Addition of a second anion inhibits oxide film breakdown, as indicated by a large positive shift in E sub p and a decrease in the number of stable corrosion pits. The dependence of E sub p on the relative concentrations of Br exp - and the inhibitor anion is consistent with competitive adsorption of the anions. Equilibrium adsorption coefficients for I exp - , Br exp - , and Cl exp - are estimated from the dependence of E sub p on anion concentration. The results are used to establish a physical basis for the anomalously low pitting potential for titanium in aqueous Br exp - solutions.