In Situ Spectroscopic Ellipsometry and Electrochemical Studies of the Barrier Layer on Iron in Borate Buffer Solutions

The growth and properties of the passive films on iron in EDTA (ethylene diamine tetra-acetic acid, disodium salt)-containing borate buffer solution of pH = 8.15 were investigated using in-situ Spectroscopic Ellipsometry (SE), Mott-Schottky analysis, and X-ray photoelectron spectroscopy (XPS). EDTA effectively suppressed the formation of the outer layer of the passive film, thereby rendering the barrier layer amenable to direct examination. It was shown that the barrier layer growth was completed in about 10 s upon stepping the potential in the anodic direction. Contrariwise, thinning of the barrier oxide layer upon stepping the potential in the cathodic direction occurred at a rate that was two orders in magnitude lower than the growth rate. The information obtained above was interpreted in terms of the Point Defect Model (PDM) containing both oxygen vacancies and cation interstitials, with the latter being the dominant defect in the barrier layer.