Modeling barrier film growth and dissolution on titanium based on EIS, XPS and photocurrent data

Abstract

The relatively large number of adjustable parameters often precludes the unambiguous interpretation of electrochemical impedance spectroscopic (EIS) measurements in terms of a unique kinetic model. An approach to overcome this ambiguity by parameterization of a model of barrier film growth and dissolution on Ti in fluoride-containing solutions using a combination of in-situ EIS and photocurrent measurements, combined with ex-situ characterization of the oxides by X-ray photoelectron spectroscopy (XPS) is presented. The kinetic model features barrier layer growth via inward transport of oxygen and fluoride mediated by anion vacancies. Dissolution of titanium through the oxide proceeds via two parallel pathways mediated by interstitial cations and cation vacancies, respectively. Outer layer growth is assumed to be limited by the transport of interstitial cations through the barrier layer. Ways to extract information on the rate-limiting steps of the process of passive film formation, growth and restructuring are described and the significance of the obtained parameters for the kinetics of barrier film growth and dissolution is outlined.