Impedance of passive oxide films with graded thickness: Influence of the electrode and cell geometry

The impedance of cylindrical electrodes and of rectangular sheet electrodes coated by passive films with either a constant or a graded thickness has been calculated using a finite element method for solving Laplace's equation. The comparison of different electrode/cell geometries characterized by homogenous (on cylindrical electrodes) or inhomogeneous distributions of primary current (on rectangular sheet electrodes) and by homogeneous or graded thickness has allowed to clarify the origin of frequency dispersion phenomena. Calculated curves were compared with experimental impedance plots obtained with Nb and Ta sheet electrodes and with Nb cylindrical electrodes. The results of the finite element calculations for rectangular sheet electrodes are in better agreement with the experiments than those of a previous analytical calculation developed under a simplifying assumption (i.e. identical local electrolyte resistance at each point of the electrode surface) which was equivalent to neglecting current distribution induced by electrode and cell geometry.