IMPEDANCE MEASUREMENTS OF A TIN ELECTRODE IN H2SO4 SOLUTIONS

The impedance response of a tin electrode in 4.5 and 6 M H2SO4 Solutions has been studied at the rest potential, as well as during anodic polarization in the active dissolution and passivation regions. The validity of a model for the active dissolution of tin advanced earlier is proven and extended to explain the mechanism of the open circuit corrosion processes. In the passivation range, the impedance spectra observed are complex, and very dependent on the positive potential and the acid concentration. The high frequency response is ascribed to a barrier-like film formed on the electrode surface, through which ions are transported by high field-assisted migration. The low frequency response suggests an intricate coupling between the passivation and activation-dissolution processes at the film-solution interface. A qualitative explanation of the electrode response is presented.