Free chlorine consumption induced by cast iron corrosion in drinking water distribution systems

Among the various causes of chlorine consumption in drinking water distribution networks, corrosion has not been so far seriously considered. This work focused on the corrosion study of cast iron, as a representative material of those networks, by both electrochemical methods (steady-state curves, corrosion potential and electrochemical impedance data) and classical photograph observations in the absence and in the presence of chlorine. From impedance results and photographs, a scheme of the different corrosion products layers, that is based on the de Levie's theory, is proposed. By fitting this physical model to experimental impedance diagrams, the anodic charge transfer resistance can be extracted and thus a reliable value of the corrosion rate can be given. The general conclusion is that free chlorine is not electrochemically consumed at the metal surface though it is more easily reducible than oxygen. Therefore, the corrosion of cast iron pipes induces only a chemical decay of chlorine corresponding to its reaction with ferrous ions released by iron dissolution and the total consumption rate is exactly obtained from the corrosion current density.