Corrosion Study of Cast Iron Pipes in Water Distribution Systems by Impedance. Application to Chlorine Consumption

Abstract

Among the various causes of chlorine consumption in potable 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-sate curves, corrosion potential and ac impedance data- and classical photographs observations in the absence and in the presence of chlorine. A process model based on the de Levie theory is used to extract from the experimental impedance data, the value of the charge transfer resistance corresponding to the anodic reaction and by this way the corrosion rate. From impedance and photographs, a structure of the different corrosion layers has been proposed. The general conclusion is that chlorine is not directly consumed electrochemically at the metal though it is more easily reducible than oxygen. Thus, the total consumption of chlorine, induced by the corrosion of cast iron pipes, amounts to the chemical reduction due to ferrous ions released by the metal dissolution. Moreover the rate of chlorine decay is exactly obtained from the corrosion current density.