Among the various causes of free chlorine consumption in drinking water distribution networks, the internal corrosion of pipes has not been so far seriously considered. This work focused on the corrosion study of cast iron, as a representative material of those networks, in non-chlorinated and chlorinated waters, by both electrochemical impedance spectroscopy and classical photographic observations. The results collected with cast iron rotating disk electrodes show that the behavior of cast iron depends only on the presence or absence of free chlorine and not on its concentration. Moreover, they lead to a scheme of different corrosion products layers based on the porous electrode theory of de Levie. This physical model describes the behavior of cast iron with and without free chlorine and at any time of immersion. The experimental impedance diagrams were at first fitted by the measurement model developed by Orazem et al. in order to identify the stochastic error and to delete non-stationary points. Then the regression of the physical model to the data provided the value of the anodic charge transfer resistance and, in this way, the value of the corrosion current. As the anodic charge transfer resistance was found to have the same order of magnitude as the polarization resistance, a simpler technique than the electrochemical impedance can be used to assess the corrosion rate.