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.