The passivity of iron in the presence of ethylenediaminetetraacetic acid - II. The defect and electronic structures of the barrier layer

The passive state on iron in EDTA (ethylenediaminetetraacetic acid, disodium salt) containing borate buffer solutions of pH 8.4 to 10.0 at ambient temperature has been explored using potentiodynamic polarization and steady-state techniques, including Mott-Schottky analysis. EDTA effectively suppresses the formation of the outer layer of the passive film thereby rendering the barrier layer amenable to direct examination. It is shown that the barrier layer on iron is a highly disordered, n-type semiconductor. The barrier layer thickness varies linearly with applied potential, whereas the steady-state current density does not depend on the formation voltage. EDTA present in the solution renders the barrier layer of the passive film more defective, as indicated by Mott-Schottky analysis, and the donor density decreases with increasing film formation potential. These findings suggest that the dominant defects in the barrier layer are oxygen vacancies or cation interstitials, or both. No evidence of cation vacancies, which would render the barrier layer p-type and would impart a voltage dependence to the steady-state current was obtained. Thus, if cation vacancies are present, they are not the dominant defect. (C) 2001 The Electrochemical Society.