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 Cl(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. (C) 2009 The Electrochemical Society. [DOI:10.1149/1.3251288] All rights reserved.