Hongbo Cong

Use of Coupled Multielectrode Arrays to Elucidate the pH Dependence of Copper Pitting in Potable Water

Natural pitting was investigated using coupled multielectrode arrays (CMEAs) constructed from flush mounted, close-packed UNS C11000 copper wires exposed in chlorinated and aerated Edwards synthetic drinking waters (ESDWs) [J. C. Rushing and M. Edwards, Corros. Sci., 46, 3069 (2004)]. Tests were conducted in ESDWs with pH adjusted from 6 to 10 and containing 5 ppm C1 2 . The CMEA method detected the formation of persistent anodes (pitting) in pH ≥ 7 synthetic waters compared to switchable anodes at pH 6 (uniform corrosion). Local maximum and minimum open-circuit potentials (OCPs) were analyzed before, during, and after pitting events and were compared to pitting (E pit ) and repassivation (E rp ) potentials. Natural pitting was found to occur on a small percentage of electrodes once the OCP rose above E rp determined in upward/downward scans. Pits stopped growing once the OCP dropped below E rp . Pitting factors, determined as a function of pH, increased with pH from 7 to 9. However, a further increase in pH to 10 reduced pitting, which could not be determined from other test methods. This drop in pitting factor could be ascertained from arrays but not from a comparison of OCPs to pitting potentials seen in upward scans. The cathodic capacity of adjacent cathodic sites as a function of water chemistry and the nature of the cathodic sites were factored into this analysis.