Instabilities in electrochemical systems with a rotating disc electrode

polarization curves experimentally obtained in the electro-dissolution of iron in a 1 m h2so4 solution using a rotating disc as the working electrode present a current instability region within the range of applied voltage in which the current is controlled by mass transport in the electrolyte. according to the literature (barcia et. al., 1992) the electro-dissolution process leads to the existence of a viscosity gradient in the interface metal-solution, which leads to a velocity field quantitatively different form the one developed in uniform viscosity conditions and may affect the stability of the hydrodynamic field. the purpose of this work is to investigate whether a steady viscosity profile, depending on the distance to the electrode surface, affects the stability properties of the classic velocity field near a rotating disc. two classes of perturbations are considered: perturbations monotonically varying along the radial direction, and perturbations periodically modulated along the radial direction. the results show that the hydrodynamic field is always stable with respect to the first class of perturbations and that the neutral stability curves are modified by the presence of a viscosity gradient in the second case, in the sense of reducing the critical reynolds number beyond which perturbations are amplified. this result supports the hypothesis that the current oscillations observed in the polarization curve may originate from a hydrodynamic instability.