An Extended 1-D Transient Corrosion Model Including Multi-Component Chemical Species

Corrosion in a water distribution system has many adverse impacts including the build-up of scale on the pipe wall, the loss of hydraulic capacity, compromised structural integrity that can lead to the onset and proliferation of leakage, and the deterioration of water quality. Yet, for all its familiarity, pipe corrosion is a complex phenomenon due to the inter-dependent set of reactions that can take place at the pipe wall and with chemicals in the bulk water. The principle objective of this study is to explore the impact of chemical species on an existing but basic 1-D transient model. The research is focused on studying the impact of various chemical species — dissolved oxygen, pH, and the carbonate system (CO2, HCO3–, & CO32–) on the formation of ferrous (Fe2+) and ferric (Fe3+) ions. The associated formation of various iron precipitates is also numerically explored. For the purpose of this study, the 1-D corrosion simulation model developed by Naser and Karney (2005) is extended by adding three sets of chemical reactions including reaction of the pipe wall with the bulk water and dissolved oxygen, reactions involving the carbonate system, and iron precipitation. Each chemical species is tracked using the advection-diffusion-reaction equation (ADRE) coupled to a hydraulic model involving the continuity and momentum equations. The numerical solution of ADRE is computed by the implicit finite difference method and the flow equations are resolved with the method of characteristics (MOC). The model is simulated for various flow conditions both with and without consideration of the carbonate system. Results show that the rate of iron release increases in the axial direction but varies with flow conditions and simulation time. When the carbonate system is neglected, the oxygen concentration steadily decreases, and both iron concentrations and pH increase along the pipe axis with time. However, the inclusion of the carbonate system directly affects the pH and dissolved oxygen concentration in the bulk water which then influences the rate of iron release. In particular, the decrease in pH values and the increase in dissolved oxygen concentrations along the pipe axis, which are observed by the inclusion of the carbonate system into the model, accelerate oxidation of the pipe wall, and a high rate of iron release is observed. This reveals that flow conditions, low pH, high dissolved oxygen concentrations, are the most significant factors that increase the corrosion rate. In a real water distribution system, the rate of internal corrosion is further influenced by other so-called secondary reactions, and also by the physical, chemical and biological characteristics of the water. This paper was presented at the 8th Annual Water Distribution Systems Analysis Symposium which was held with the generous support of Awwa Research Foundation (AwwaRF).

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