The paper gives a new insight into the meaning of local impedances and the related measurement techniques. By numerical simulation it is shown that the only fully relevant quantity is obtained through local values of both the current density and potential. A recently developed equipment based on a commercial SVET device modified for ac polarization is applied to corrosion situations. Under oxygen reduction control of the corrosion rate, serious artifacts are found on the de current density mapping and local impedance. The drawback is explained by the contribution of a chemical term (redox potential) in the probe potential. A model is elaborated accounting for the resulting side effects in both de and ac measurements.