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A Visual Insight into the Degradation of Metals Used in Drinking Water Distribution Systems Using AFM
Citation:
Daniels, S. AND D. Lytle. A Visual Insight into the Degradation of Metals Used in Drinking Water Distribution Systems Using AFM. Presented at AWWA Annual Conference, Boston, MA, June 08 - 12, 2014.
Impact/Purpose:
To inform the public on the degradation of metals in drinking water distribution systems.
Description:
Evaluating the fundamental corrosion and passivation of metallic copper used in drinking water distribution materials is important in understanding the overall mechanism of the corrosion process. Copper pipes are widely used for drinking water distribution systems and although it is durable and resist to oxidation, when in contact with aggressive environment, copper can corrode. Corroding pipes and leaching of the metal into drinking water has been associated structural failure and gastrointestinal problems. It is widely agreed the formation of copper by-products are influenced by water chemistry which impacts the acceleration or migration of the corrosion of copper pipes. Characterization of surface changes on metals used in drinking distribution at the molecular level is an important piece of the puzzle in understanding the corrosion mechanism. Atomic force microscopy (AFM) is a surface characterization technique based on scanning a tip across the sample surface to generate a 3-D image with nanometer resolution. The novelty of AFM for analyzing materials used for distributing drinking water affords the opportunity to visualize the morphological changes in the early stages of the degradation and/or passivation of the metal at the micro/nano-scale. The purpose of this study was to use AFM to show surface changing in the formation and reduction of corrosion by products layers on metallic copper surfaces as a function of drinking water chemistry such as pH, time and corrosion inhibitor (orthophosphate and polyphosphate). To prepare the samples, copper coupons exposed to water mimicking drinking water found at the tap for 6 h and 24 h. Water was prepared in a l L reaction cell to a composition of 10 mg C/L dissolved inorganic carbon (DIC), 60 mg/L chloride, 120 mg/L sulfate, and 3 mg/L free chlorine. Formation and reduction of the samples were compared without and with orthophosphate (6 mg/L) as the inhibitor. Our AFM results showed the morphology change (shape and size) of copper by-products after 6 and 24 h over a pH range of 6.5-9. As the immersion time was increased to 24 h for pH 6.5 and 7, dense, angular structures formed on coupon surfaces. At pH 8 and 9 after 24 h, the morphology changed into clustered, grainy morphologies. In the presence of orthophosphate, smaller deposits were observed for each pH range. Additional analysis with XRD and NEXAFS identify the by-products as cuprite (Cu(II)) for samples without orthophosphate and copper coupons treated with the inhibitor produce a mixed oxidation state with the primary state being the pure metal, Cu(0).
URLs/Downloads:
http://www.awwa.org/portals/0/files/education/conferences/ace/conference-program/2014/resources/90.htm