IMPEDANCE OF ANODICALLY PASSIVATED FE80B20 GLASSY ALLOY IN SOLUTIONS OF VARIOUS PH

Anodic passive films grown on the glassy alloy Fe80B20 under strongly controlled conditions are characterized by an ac impedance technique i n solutions of different pH (7.4-12.1), and potentials covering passiv e and beginning of the oxygen evolution reaction (OER) regions. Modell ization of the passive film/solution (F/S) interface region is propose d on the basis of shapes of Bode plots and Mott-Schottky dependencies. Semiconductor behaviour due to the oxygen nonstoichiometry with Nerns tian pH dependence is obtained over almost the whole passive potential region. Passive current densities independent of the electrolyte pH i ndicate that the corrosion current is determined by a pH independent e lectric field strength within the space-charge layer. At higher anodic potentials and higher electrolyte pH, due to the surface deprotonatio n reaction, there is an increased density of basic (MOH(-)) surface gr oups acting as surface states, and reactants in the first step of OER. Further increase in anodic potential facilitates surface oxidation of (MOH(-)) to (MOH(0)). In the OER potential region, the passivated Fe8 0B20 electrode acts as a classical oxygen electrode. Comparison of the data with those obtained on the pure Fe passivated under the same con ditions, shows the detrimental effect of B on either the corrosion pro perties of the passive film in the passive potential region, or on the parameters of OER. (C) 1997 Published by Elsevier Science Ltd.