Application of Impedance Spectroscopy and Surface Analysis to Obtain Oxide Film Thickness

Abstract

The corrosion behavior of ASTM A416 steel in alkaline electrolyte was investigated by electrochemical and surface analysis approaches, including X-ray photoelectron spectroscopy (XPS) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The power-law model was used to extract values for oxide film thickness from constant-phase-element (CPE) parameters obtained as functions of operating conditions. Calibration experiments showed that, despite different silicon content in nominally identical steels, different film thicknesses as observed by HAADF-STEM, and different impedance responses, three samples yielded a common value for ρδ, an important parameter in the power-law model. Application of Monte Carlo simulations showed that values of both ρδ and film thickness δ followed log normal distributions. Application of the power-law model allowed extraction of film thicknesses, yielding 2–6 nm for silicon-rich steel and 1–2 nm for silicon-poor steel.