Conduction mechanism of the anodic film on Fe-Cr alloys in sulfate solutions

The passive state of Fe-12% Cr and Fe-25% Cr alloys in 1 M sulfate solutions of pH 0 and 5 was studied with a combination of electrochemical techniques: impedance-spectroscopy, photoelectrochemistry, and de resistance measurements by the contact electric resistance technique. The investigations were supported by studies on pure metal constituents (Fe, Cr) in the same solutions. As a result. the steady-state passive film on the alloys (and on pure Cr) can be described as a thin, essentially insulating layer. Polarization of the steady-state metal/anodic film/electrolyte system to negative and positive potentials away from the potential region corresponding to the highest electronic resistance was concluded to lead to the generation of lower or higher valency defects at the interfaces via solid-state electrochemical reactions. These reactions result in a substantial increase of the conductivity in the first layers adjacent to either the metal/film or the film/electrolyte interface. At very negative (or very positive) potentials the film is transformed into a conductor allowing active (or transpassive) dissolution to take place. A quantitative physical model of the conduction mechanism on the basis of the surface charge approach is proposed and compared with experimental data. (C) 1999 The Electrochemical Society. S0013-4651(98)12-069-4. All rights reserved.