Exploring the mechanisms of passivity on iron experimental methods for characterizing and developing models to describe nano-oxide growth /

by 1972- Marx, Brian M.

iii In this thesis the passive oxide film formed on iron is studied using many experimental techniques in order to aid in the refinement of a Point Defect Model (PDM), which is capable of predicting and quantifying damage due to the general corrosion of iron. Many older electrochemical methods, like Mott-Schottky analysis, are incorporated with more modern techniques, such as scanning wavelength ellipsometry, electrochemical impedance spectroscopy, and XPS to probe the oxide film. Ellipsometry is used to make in-situ thickness measurements that are performed in unison with potentiostatic and impedance experiments. XPS analysis is used to determine the concentrations of Fe2+ and Fe3+ present in the oxide film formed on iron. The information obtained is used to derive a PDM containing only oxygen vacancies. In addition, a new idea is proposed concerning three distinct regions over the passive range exhibited by iron in a borate buffer solution. Each region corresponds to a change in the electronic behavior of the film as evidenced by basic Mott-Schottky analysis. It is proposed that these different regions represent phase changes for the film. Finally, the oxygen vacancy PDM is applied to implicitly determine if the major defects in the iron oxide film are oxide vacancies. The PDM is also fitted to the experimental data to yield the kinetic parameters and to make predictions concerning the steady-state thickness and current. A Warburg impedance is used in the fitting to obtain diffusivity data for the defects. The kinetic parameters are generated as both a function iv of formation potential and temperature. A PDM, containing only oxygen vacancies, does not fit the data. Therefore, iron interstitials were added to the model, which yielded good fits to the data. A discussion is provided as to the interpretation of the results and the viability of applying the PDM to the iron system.