Nonstationary Dependences during the Polarization of Liquid Metal Electrodes in Molten Salts with Circulation Cell Structures

When liquid metal electrodes are polarized in dilute melts, dissipative structures appear due to natural and interfacial (Marangoni effect) convection. The type of the structures determines the form of polarization dependence. During cathodic polarization of liquid electrodes made of lead, bismuth, cadmium, or aluminum in chloride melts in the range from the current-free potential to the zero-charge potential, structures in the form of vortices, namely, circulation cells (CCs), are predominant at the interface due to the Marangoni microeffect. The stationary polarization dependences obtained under potentiostatic conditions have a characteristic current density maximum, and the time dependences of overvoltage η and current density i for a steplike setting of current under galvanostatic conditions or potential under potentiostatic conditions (nonstationary curves) have characteristic current or overvoltage extrema decreasing in magnitude until a stationary state forms. In other words, the system evolves from the moment of circuit closure according to a law of damped oscillations, with a period depending on time. The values of η and i in the first extremum always have the highest amplitude. At the time τext of reaching the first current density (iext) or overvoltage (ηext) extremum, CCs become visible at the interface. No movements at the interface are visually observed before τext. The time interval from circuit closure to τext (0.1–1 s) is a transition period during CC formation. The following two conditions should be simultaneously met during this period: (a) the diffusion front should move away from the electrode to a sufficient distance, and a diffusion layer with a concentration gradient normal to the interface should form; (b) the difference between the concentrations at the electrode surface and in the electrolyte volume should reach a certain critical value. In the form of a criterion equation, we obtain a relation between τext, the properties of the system, and the electrode sizes. The values of τext calculated by this equation satisfactorily coincide with the experimental values. The energy of CC formation is shown to be determined by the properties of the salt and metal phases, to depend on the current density or the potential, and to change over a wide range (0.3–68 J/m2).