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Microelectrodes Based investigation of the Impacts of Water Chemistry on Copper and Iron Corrosion
Liggett, J., D. Lytle, J. Pressman, AND D. Wahman. Microelectrodes Based investigation of the Impacts of Water Chemistry on Copper and Iron Corrosion. Presented at AWWA WQTC Conference, New Orleans, LA, November 16 - 20, 2014.
To inform the public.
The effect of bulk drinking water quality on copper and iron pipe corrosion has been extensively studied. Despite past research, many have argued that bulk water quality does not necessarily reflect water quality near the water-metal interface and that such knowledge is necessary to fully understand corrosion mechanisms and control metal release. Microelectrode technology is a tool that can be used to obtain such information. Although microelectrodes have been commonly used in neuroscience and ecology fields, their application to drinking water research has been limited to studying biofilm. A major advantage of microelectrodes is their small size which enables them to be used without damaging the materials being investigated. This proves crucial in corrosion research as pipe disturbance can create undesired metal release. Furthermore, microelectrodes can examine water chemistry conditions within microns of the pipe surface. These conditions may differ from that of the bulk water thereby revealing additional information on the processes of metal corrosion. The objective of this work was to determine the impact of bulk water quality on pH, dissolved oxygen (DO), and free chlorine profiles at the surface of copper and ductile iron coupons over three months using microelectrodes. Specifically, the impact of bulk water pH, dissolved inorganic carbon and orthophosphate on profiles was assessed. Once per month, pH, DO, and free chlorine microprofiles were collected and analyzed on each of the metals. All aqueous samples were submitted for inorganic water quality analysis and solid formations on corroded metal surfaces were analyzed for mineralogy, crystal structure, and elemental analysis. Changes in pH were minimal at the surface layer of the copper coupons but varied significantly near the surface of the ductile iron coupons. These pH changes may provide insight into localized corrosion events at the iron pipe surface. DO concentrations remained relatively stable across the surface layer of the copper coupons but decreased more significantly across the iron coupons. This is an interesting result because DO concentrations can affect not only metal release but also the function of phosphate as a corrosion inhibitor. Free chlorine concentrations were found to decrease in the surface layers of both metals. Maintaining chlorine residual in the bulk water and near solid surfaces is required for continuous disinfection and understanding the effects of chlorine decay near pipe surfaces may provide insight into these localized reactions. The application of microelectrodes can be greatly expanded in drinking water research. Further applications include determining the effects of other disinfectants such as chloramines or studying corrosion on other metal coupons such as lead or brass. The information gathered using microelectrodes can further our understanding of corrosion near pipe surfaces and foster the creation of innovative control strategies.