EIS Technique in Passivity Studies: Determination of the Dielectric Properties of Passive Films
Abstract
Different descriptions of oxide layers were presented in the literature, including the ion-migration mechanism,1 the generalized
high field model,2 and, more recently, the point defect model (PDM) introduced by Digby MacDonald.3,4 The PDM describes the
main properties of the oxide layer in terms of kinetics and transport processes, but application of the model is complicated by the
large number of parameters. Generally, the experimental electrochemical impedance for oxide-covered metals is very simple and
can be reduced to one loop which looks like a depressed semicircle. This leads D. D. Macdonald to use a CPE in the equivalent
circuit to represent the geometrical capacitance of the barrier layer.5 However, even if a CPE is well suited for a fitting purpose,
it lacks physical meaning.
In the present chapter, the oxide layer is considered to be a dielectric layer characterized by its resistivity and permittivity. The
impedance of an oxide-covered electrode is dominated by the oxide layer under anodic potentials; whereas, for a cathodic
potential, a parallel impedance must be taken into account. The impedance of the CPE introduced by D. D. Macdonald5 is thus
considered here to be the impedance of the oxide layer when the potential is anodic.