Influence of the applied potential and pH on the steady-state behavior of the iron oxide

Numerous studies on the formation of passive oxide films on metals have been made, all aiming to understand and quantify how the thickness of the oxide film depends on the electrolyte solution's pH and on the applied potential. A one dimensional electrochemical model is presented considering the passive film on a rotating disk electrode (RDE) in contact with electrolyte solutions with the pH in the interval [8.4–11.2]. The mobile species in the oxide are oxygen vacancies, iron interstitials Fe2+, Fe3+ and iron vacancies. Reactions at the interfaces involving the defects in the oxide are combined with equations providing species and potential distributions, both in the oxide film and in the electrolyte solution. This approach offers new ways to link the film–solution interface with locally changing electrolyte composition. The passive film is described in terms of mass balance equations together with Poisson's equation. The results of our numerical simulations based on the finite element method show that Fe3+ interstitials and oxygen vacancies are the species responsible for the transport of mass in the oxide. These results are discussed and compared with experimental data and other numerical results.