Free chlorine consumption induced by cast iron corrosion in drinking waterdistribution systems

Among the various causes of chlorine consumption in drinking water distribu tion networks, corrosion has not been so far seriously considered. This wor k focused on the corrosion study of cast iron, as a representative material of those networks, by both electrochemical methods (steady-state curves, c orrosion potential and electrochemical impedance data) and classical photog raph observations in the absence and in the presence of chlorine. From impe dance results and photographs, a scheme of the different corrosion products layers, that is based on the de Levie's theory, is proposed. By fitting th is physical model to experimental impedance diagrams, the anodic charge tra nsfer resistance can be extracted and thus a reliable value of the corrosio n rate can be given. The general conclusion is that free chlorine is not el ectrochemically consumed at the metal surface though it is more easily redu cible than oxygen. Therefore, the corrosion of cast iron pipes induces only a chemical decay of chlorine corresponding to its reaction with ferrous io ns released by iron dissolution and the: total consumption rate is exactly obtained from the corrosion current density. (C) 1999 Elsevier Science Ltd. All rights reserved.