Development and Application of Microelectrodes to Study in situ Disinfectant Loss and Corrosion on Metal Surface

The primary objective of this research was to develop, fabricate, evaluate and utilize microelectrodes to metal coupons in artificial water system. In the brass experiment, it presented profiles of direct measurements of free chlorine/monochloramine, oxygen and pH to brass and cement based coupons. In monochloraminated water, brass showed a much faster corrosion process under observation. Profiles showed a less monochloramine consumption with as high as 7% greater oxygen utilization comparing to the brass in free chlorine solution, reflecting oxygen could be a major part of the corrosion initiation process. While cement showed less reactive characteristics to disinfectants and oxygen compared to the brass profiles, however, pH showed a significant rise for cement coupon under monochloramine condition. In galvanic experiment, the developed lead micro-ISE (100 µm tip diameter) showed excellent performance toward soluble lead (Pb2+) with the sensitivity of 22.2 ± 0.5 mV decade-1 and limit of detection (LOD) of 1.22×10-6 M (0.25 mg L-1). The response time was less than 10 seconds with a working pH range of 2.0 – 7.0. Using the lead micro-ISE, lead concentration microprofiles were measured from the bulk to the metal surface over time. Combined with two-dimensional (2D) pH map, this work clearly demonstrated that lead leaching at the metal surface is non-uniform and lower surface pH leads to higher lead leaching from the surface. Once significant pH variation (ΔpH: 6.0) was developed across brass-lead joint coupon, even a small pH change (ΔpH: 0.6) within the Pb/Sn alloy resulted in 4 times different surface lead concentrations (42.93 vs. 11.61 mg L-1) and 5 times different fluxes (18.5×10-6 vs. 3.5×10-6 mg cm-2s-1). Continuous surface lead leaching monitoring and surface characterization found that free chlorine is the primary contributor to lead leaching