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Effects of Water Quality and Orthophosphate on Surface Characteristics of Cu Corrosion in Drinking Water using Atomic Force Microscopy
Citation:
Daniels, S. L., D. A. LYTLE, AND J. Garno. Effects of Water Quality and Orthophosphate on Surface Characteristics of Cu Corrosion in Drinking Water using Atomic Force Microscopy. Presented at Water Quality and Technology Conference, Seattle, WA, November 15 - 19, 2009.
Impact/Purpose:
To inform the public.
Description:
Since the passage of the Lead and Copper Rule (LCR) in 1991, researchers have examined the effects of water chemistry on the solubility of copper to establish best approaches for reducing copper levels. Despite recent developments, important gaps still exist regarding copper corrosion including pitting corrosion and copper release. There remains a need to study copper corrosion particularly at the fundamental level to advance our understanding of copper corrosion and our ability decrease copper levels in drinking water. The objective of this study was to use a novel approach, atomic force microscopy (AFM), to investigate the influence of water chemistry on the properties of copper corrosion by-products that form on copper during early stages of corrosion in water. The application of AFM in the drinking water field is relatively new, will permit the study of copper corrosion on the nanometer scale and may be particularly useful in understanding non-uniform corrosion. Specifically, the impacts of water chemistry including pH and corrosion inhibitors (e.g., phosphates), and time on corrosion by-product growth and development in drinking water will be examined. The uniformity of corrosion deposits as well as their thickness of corrosion by products will be reported and compared. For this study pure copper was cut into 1 x 1 in2 test sections, and cleaned and allowed to air dry before imaging. The experiment was conducted in a reaction cell containing 1L of Milli-Q water and treated with sodium bicarbonate, sodium sulfate, sodium chloride. The pH was controlled (6.5 and 9) by titrating hydrochloric acid and sodium hydroxide in the solution. Water conditions for the test solution 120 mg/L sulfate, 3 mg/L Cl2, 10 DIC and 60 mg/L chloride and 6 mg/L orthophosphate when used. The Cu coupons were suspended in the test water and agitated slowly in a breaker. Samples were remove from the water after 6 and 24 hrs and allowed to dry. AFM images were acquired in air with AC-mode. Additional solids analysis approaches such as X-ray diffraction were used to characterize copper surfaces. The impact of water chemistry and time on the development of corroding copper surface in water will be discussed. Results from this study demonstrated that AFM can provide a direct view into the initial on-set of corrosion on copper surfaces in water. For example, dramatic differences in surface morphology were observed with high resolution AFM when the orthophosphate was present in the water. Specifically AFM topography images revealed at pH 6.5 and pH 9 without the addition of the orthophosphate large cubic crystalline structures (likely cuprite) with a height of approximately 300 nm after 24 hrs of exposure to water. However, when orthophosphate was present, crystalline features were lost and the height was significantly reduced to 40 nm and 60 nm, respectively. It can be concluded that the addition of orthophosphate retards and mitigates the growth and formation of some corrosion deposits on the copper surface. Results will be provided in detail and the potential practical implications of the observations to the drinking water field will be discussed.
