Coupling of Chemical and Hydrodynamic Instabilities at the Electrochemical Dissolution of Metals

Abstract

The conditions for genesis of chemically induced hydrodynamic convection flow at resting and slowly rotating disk electrodes were studied. On the basis of these studies, the convection flow patterns beneath the electrode could be directly correlated with the etched patterns in the surfaces of the corresponding electrodes. It was not possible to capture images during experiments at high rotational speeds (1000 - 6000 rpm) on account of the very fast movements in the solution as well as the strong light absorption of concentrated iron(III) chloride solution. However, the hydrodynamic flow emerges as an etched spiral pattern in the electrode surface, thus enabling detailed investigation of the structures after each experiment. These spiral-like patterns follow a logarithmic rule and feature an invariant curvature, even under different experimental conditions. This invariant behaviour of the spiral pattern formation can be explained physically, and a fixed ratio of tangential to radial flow of 2:sup:-½:/sup: was found for the curvature of the spirals generated. Besides the formation of a topographically structured surface, the system exhibits galvanostatic potential oscillations. In addition to classical electrochemical oscillations, a new type of oscillation was detected. These superimposed oscillations could be correlated directly with the circular height profile of the topographically structured surface. It could be shown that this new type of oscillation is caused by the interaction between hydrodynamic vortex patterns in the boundary layer, on the one hand, and the electrochemical dissolution process, on the other hand. The experiments and theoretical interpretations shown in this work regarding the pattern formation at non-, slow and fast rotating disk electrodes under dissolving conditions provide a fundamental contribution to understanding the coupling between electrochemical processes and hydrodynamic flow at dissolving disk electrodes.