Modeling of the cast iron/drinking water system by electrochemical impedance spectroscopy

Among the various causes of free chlorine consumption in drinking water dis tribution networks, the internal corrosion of pipes has not been so far ser iously considered. This work focused on the corrosion study of cast iron, a s a representative material of those networks, in non-chlorinated and chlor inated waters, by both electrochemical impedance spectroscopy and classical photographic observations. The results collected with cast iron rotating d isk electrodes show that the behavior of cast iron depends only on the pres ence or absence of free chlorine and not on its concentration. Moreover, th ey lead to a scheme of different corrosion products layers based on the por ous electrode theory of de Levie. This physical model describes the behavio r of cast iron with and without free chlorine and at any time of immersion. The experimental impedance diagrams were at first fitted by the measuremen t model developed by Orazem et al, in order to identify the stochastic erro r and to delete non-stationary points. Then the regression of the physical model to the data provided the value of the anodic charge transfer resistan ce and, in this way, the value of the corrosion current. As the anodic char ge transfer resistance was found to have the same order of magnitude as the polarization resistance, a simpler technique than the electrochemical impe dance can be used to assess the corrosion rate. (C) 1999 Elsevier Science L td. All rights reserved.