GTA, University of Central Florida
The Flint (MI) water crisis has brought national attention to the lead leaching problems in drinking water distribution systems. G alvanic joints which contain lead can also contribute to the elevation of lead concentration in the water service lines, resulting in noncompliance with the Lead and Copper Rule (LCR). Historically, numerous field and lab tests were performed primarily with bulk water analysis using inductively coupled plasma mass spectrometry (ICP-MS), electrochemical methods, and others. These studies illustrate the fundamental water chemistry between different water conditions and parameters, such as pH, chlorine residual , corrosion inhibitor, and chloride-to-sulfate ratio (CSMR). However, most studies have relied on bulk monitoring in drinking water pipelines, while liquid-metal interfacial chemical species dynamics in micro-environment was underappreciated due to the lack of the experimental tools for direct measurement. The current study was to apply multiple microelectrodes for real time in situ direct measurements of pH, dissolved oxygen (DO), and free chlorine concentrations near the metal surfaces (50 µm above), and the effects of water flow patterns, pHs, dissolved inorganic carbon (DIC ), phosphate and chlorine residual in micro-environment. These parameters were evaluated using an epoxy mounted brass-solder-brass galvanic joint coupon (2×1 cm). 13 groups of experiments conducted with different pH (7 and 9), free chlorine concentration (2 and 4 mg Cl2/L), alkalinity (DIC 10 and 50 mg C/L), and flow condition (2 mL/min and stagnation). Maximum pH variation of 7.5 unit (ΔpH=7.5) between brass (pH 10.3) and solder surface (pH 2.8) was found at neutral condition with less than 8 hours of stagnation. The average pH variation exceeded 4 pH unit under different stagnant conditions. However, under flow condition, especially with high pH (pH 9) or high alkalinity condition (50 mg C/L), the pH was more evenly distributed with their buffer pH. And no obvious surface oxygen (< 0.3 mg/L) decrease was found. Data also showed that with addition of 3.0 mg P/L of orthophosphate as corrosion inhibitor, the pH diversity on coupon surface was greatly decreased under both flow and stagnant condition. The novelty of this research can build a solid foundation chemistry in micro-environment, and future corrosion inhibition investigation.
Abstract: Novel dual potentiometric microsensor probe has been developed for the simultaneous detection of Zn2+ concentration and pH distributions in the Scanning Electrochemical Microscopy investigation of corroding galvanized steel. The individual sensors show nearly theoretical behavior over a wide concentration range. The applicability of this probe is first demonstrated on a Fe-Zn galvanic couple, as it shows excellent performance in these simultaneous model experiments. In addition, linear scans recorded over a cut edge of coated galvanized steel evidences the complementary information gathered on the electrochemical behavior of the corroding sample, and adumbrates promising and feasible applications of multi-barrel microelectrodes in corrosion research.
Pub.: 21 Oct '16, Pinned: 19 Jun '17