Chronopotentiometric investigation of the anodic passivation of cadmium in alkaline solutions

In [1], to describe the chronopotentiometric behavior of electrodes whose operation is accompanied by the formation of weakly soluble products on their surfaces, we propose and justify the equation I tau = nF( pi D tau ) exp 0.5 c/2 + Q, (1), where n is the number of electrons in the analyzed electrochemical reaction, F is the Faraday constant, D is the diffusion coefficient, C is the concentration of a weakly soluble product in equilibrium with the solid phase, and Q is the total amount of electricity required for the formation of metal hydroxides and the appearance of a passivating layer. The transient period tau is characterized by the competition of the processes of sedimentation and dissolution of the passivating substance resulting in the formation of a layer of this substance sufficient for an abrupt shift of the potential in the negative or positive direction (depending on the direction of current). Unlike the corresponding term in the Sand equation, the first term on the right-hand side of Eq. (1) contains not the bulk concentration of the depolarizer but the ultimate concentration of the newly formed substance. Equation (1) was deduced and successfully applied to the investigation of the electrode processes in nitrate melts. However, as follows from the statement of the diffusion problem whose solution implies relation (1), it can also be used for the investigation of other processes accompanied by the deposition and dissolution of the products of electrolysis. In the present work, we discuss the possibility of using this equation for the analysis of the anodic dissolution of cadmium in aqueous solutions taking place in the process of discharge of nickel-cadmium cells.