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Evaluating In-Situ Reactions of Chlorine and Chloramines at the Surface of Copper and Iron using Microelectrodes
Citation:
Lee, W., D. Wahman, D. Lytle, AND J. Pressman. Evaluating In-Situ Reactions of Chlorine and Chloramines at the Surface of Copper and Iron using Microelectrodes. Presented at AWWA Water Quality Technology Conference, New Orleans, LA, November 16 - 20, 2014.
Impact/Purpose:
This presentation will be useful for operators, utilities, and engineers in understanding the interactions between distribution pipe materials and disinfectants
Description:
Corrosion of drinking water plumbing materials is a significant cause of deterioration of treated drinking water quality and a failure to supply safe water to the public. As a result of the Lead and Copper Rule, many water utilities in the US have developed and evaluated corrosion control strategies. A better understanding of the fundamental mechanisms contributing to the rates and magnitude of corrosion at the surface of pipe materials can lead to improved corrosion control strategies, reduction of the costs for distribution system maintenance and conservation of drinking water quality. Although conceptual theories regarding the surface chemistry of corroding metals in drinking water exist, actual measurements of water quality parameters at such surfaces need to be made. The objective of this work was to measure several important water quality parameters at the surface of iron and copper in water using microelectrodes. Specifically, the current research seeks to provide a better understanding of the reaction of drinking water disinfectants with metal pipe materials by measuring in situ free chlorine or monochloramine, pH, and oxygen concentration profiles adjacent to the metal surface using microelectrodes at micro-scale. The results will provide important information on uniform and localized/pitting corrosion and microscopic-kinetics of diffusion-reaction between metal surfaces and potential corrosion causing/inhibiting parameters. Abiotic experiments (32 total sets) were conducted with non-aged fresh ductile iron and copper under various water quality conditions. Conditions included different concentrations of dissolved inorganic carbon (DIC) (10 and 50 mg C/L), pH (7 and 9), and phosphate (0 and 3 mg P/L), and two different water disinfectants (free chlorine and monochloramine) with equivalent chlorine concentrations (4 mg Cl2/L). In general, both metal coupons showed significantly reactive consumptions of free chlorine and monochloramine at their surfaces. Free chlorine was more reactive than monochloramine. Oxygen consumption at ductile iron was greater compared to copper, resulting in more rapid corrosion at the surface. This implies that metal corrosion may be directly related to oxygen consumption, as opposed to disinfectant reactions. However, as corrosion developed, oxygen consumption at ductile iron decreased. Copper showed localized and varied pH distributions at the copper surface, while ductile iron pH generally decreased as it corroded, but increased in uncorroded areas probably because of phosphate effects. At lower pH, more corrosion was observed, indicating greater phosphate passivation at higher pH. Based on the measured concentration microprofiles, various kinetic parameters (i.e., diffusion boundary layer, flux, reaction rate, and surface concentration) were determined to simulate model water distribution systems and assist in evaluating the effect of disinfectants on pipe materials and corrosion.
URLs/Downloads:
http://www.awwa.org/Portals/0/files/education/conferences/wqtc/wqtc14/pdfs/WQTC14FinalProgramWeb.pdf