A MODEL FOR THE AC-IMPEDANCE OF AN ELECTRODE COATED BY A PRECIPITATED SALT FILM

The ac-impedance of an electrode which is coated by a precipitated salt film is described. An important aspect of this model is that it describes the modulation of the film thickness which displays an impedance spectrum typical of that of an RC-series circuit (i.e. a vertical straight line on the Nyquist diagram) at frequencies below 1 Hz, in addition to the spectrum for diffusion through a layer of fixed thickness (i.e. a loop on the Nyquist diagram that is similar in shape to that of convective diffusion). A model for the dissolution of Cu in HCl solutions in the limiting-current region that describes the modulation of the thickness of the precipitated CuCl film is derived and is shown to be a good description of the experimental results.