Electrochemical study of the passive behaviour of Ni-Cr alloys in a beratesolution - a mixed-conduction model approach

This paper discusses the application of the recently introduced mixed-condu ction model (MCM) to describe the electrochemical behaviour of anodic films formed on Ni-Cr alloys. The MCM emphasises the coupling between the spatia l distribution of ionic point defects in the film and the electronic conduc tivity of the film. The basic concepts of the MCM are introduced, and equat ions for the concentration profiles of ionic defects, for the conductivity and for the ac response of the film are derived. The experimental part of t his work consist of characterising the anodic behaviour of Ni-Cr alloys, pu re Ni and pure Cr in 0.1 M Na2B4O7 solution (pH 9.3) at ambient temperature using rotating disc and cyclic voltammetry, contact electric resistance (C ER) and electrochemical impedance spectroscopic (EIS) techniques. The exper iments indicate that the film on a Ni-20%Cr alloy resembles that on pure Cr , whereas similarities can be found between the film on Ni-10%Cr alloy and pure Ni. The conductivity of the passive film formed on Ni-Cr alloys can be concluded to be based on the spatial and potential distribution of the val ence states of Ni and Cr in the film. In spite of several simplifying assum ptions used in the simulations, a qualitative agreement between the theory and experimental data was achieved. (C) 2001 Elsevier Science B.V. All righ ts reserved.