Exploring the Mechanisms of Passivity on Iron: Experimental Methods for Characterizing and Developing Models to Describe Nano-oxide Growth

Open Access
Author:
Marx, Brian M.
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
November 04, 2005
Committee Members:
  • Digby D Macdonald, Committee Chair
  • Howard W Pickering, Committee Member
  • Long Qing Chen, Committee Member
  • Thomas E Mallouk, Committee Member
Keywords:
  • Point Defect Model
  • XPS
  • Impedance
  • Iron Oxide
  • Iron
  • Passive layer
Abstract:
In this thesis the oxide 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 and XPS to probe the oxide film. Ellipsometry is used to obtain in-situ thickness measurements that are performed in unison with potentiostat and impedance experiments. XPS analysis is used to determine the concentrations of Fe2+ and Fe3+ present in an 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 measurements for the defects. The kinetic parameters are generated as both a function of formation potential and temperature. A discussion is provided as to the interpretation of the results and the viability of applying the PDM to the iron system.