Peculiarities of the electrical double layer structure on mechanically renewable electrodes of Ag–Sn alloys in aqueous solutions of surface-inactive electrolytes

Abstract

The behavior of electrodes of Ag–Sn binary alloys (Sn content from 3 to 9 at.%; the range of solid solutions of Sn in Ag) renewable by cutting was studied in aqueous NaF solutions by using the impedance method. Upon the electrode surface renewal under potentiostatic conditions in the ideal polarizability range, quick changes in the differential capacitance of the electrical double layer were observed. These changes show that the content of tin atoms in the surface layer increases with the exposure of the renewed surface to the solution, i.e. tin is the surface-active component of the alloy. Within the framework of known phenomenological models of the structure of the electrode ∣ solution interface, a mechanism is put forward based on the analysis of the time effects observed and the experimental results obtained on the behavior of Ag 3Sn intermetallic compound in contact with aqueous NaF solutions. This mechanism allows the processes, which occur in the metal phase during the formation of the equilibrium alloy ∣ electrolyte interface, to be interpreted qualitatively. The mechanism involves two stages: (1) quick diffusion of the surface-active component of the alloy (Sn) to the surface and (2) a slower stage associated with the formation of an intermetallic-like compound Ag 3Sn in the surface layer.