Corrosion Mechanism Of Cast Iron In Occluded Water In Drinking Water Distribution Systems

Corrosion in drinking water distribution systems is a serious problem and increasingly attracts attentions from water industries, governments, and research institutes. Corrosion will not only shorten the service life of pipeline, but also cause the secondary pollution of drinking water quality and water leakage in drinking water distribution systems. Water leakage causes huge losses of treated water and great socio-economic losses. Therefore, control of water leakage is a major objective for water industries. Water leakage is closely related to the corrosion, and occluded water plays an important role in the corrosion process. Therefore, this paper characterized the water quality of occluded water, studied the accumulation and migration mechanism of chemicals in occluded water, and analyzed the corrosion behavior of cast iron in occluded water using the simulated occluded cell device. Finally, a conceptual model of iron corrosion in the occluded water was proposed. This work extends the knowledge of corrosion mechanism in drinking water distribution systems, provides the technical support for the water industries in terms of water leakage control.Through the in-depth analysis of water quality characteristics in corroded drinking water pipes, three types of waters were identified firstly: bulk water, steady water and occluded water. Steady water acted as a transition layer. Occluded water had an acid environment, contained very high concentration of dissolved iron and also suspended Fe oxides(hydroxides), and was enriched of chloride, sulfate, nitrate, and manganese. The corrosion rate of cast iron in occluded water was much higher than that in the bulk water.In order to study the corrosion behavior of cast iron in occlueded water, an occluded cell device was designed to simulate the local corrosion. The mechanism of migration of anion from flowing water to occluded water was studied using the simulated occluded cell device. The migration of chloride, sulfate and nitrate was not only competitive, but also collaborative. Flowing water quality parameters affected the migration rate of anions. However, chloride and sulfate were enriched in occluded water.The corrosion behavior and influencing factors of cast iron in occluded water were studied by electrochemical measurements. The corrosion rate was increased with time. But it can be controlled by adjusting the flowing water quality parameters. In order to control the local corrosion of cast iron, it should adjust the pH of flowing water at about 8.50, reduce the chloride concentration, increase the nitrate concentration, and appropriately adjust the sulfate concentration.Based on the above study and the analysis of autocatalytic process, a conceptual model of iron local corrosion was proposed. The dissolution of iron metal in occluded water was the predominant anodic reaction, and the reduction of oxygen in the exterior of the tubercles was the predominant cathodic reaction. Occluded water was anoxic, acid, and enriched of anion. The autocatalytic effect accelerated the corrosion rate of cast iron.