Effect of Selected Water Chemistry Variables on the Pitting Behavior of Copper under Driven and Natural Conditions

The pitting behavior of UNS C11000 copper was investigated in a synthetic potable water found to cause pitting using both driven electrochemical methods and under natural conditions at OCP. The latter was monitored with coupled multi-electrode arrays (CMEA). Tests were also conducted in several HCO₃^-, SO₄^2- and Cl^- containing-waters with systematic variations in the concentrations of these species. Empirical equations that forecast pitting (E_Pit) and repassivation (E_Rp) potential as a function of selected water chemistry variables were developed. OCP was a strong function of water chemistry, pH and time. Maximum OCPs were found to increase with both pH and Cl₂. Empirical equations that forecast the OCP as a function of pH and Cl₂ were also developed. Natural pitting was tested in different pH synthetic waters (6-10) at various Cl₂ levels (0-5 ppm). The CMEA method successfully detected the formation of persistent anodes (pitting sites) in pH ≥7 synthetic waters compared to formation of switchable anodes at pH 6. A pitting factor was determined as a function of water chemistry. Pit events were statistically analyzed and potential excursions were compared to E_Pit and E_Rp. Natural pitting events occurred on a percentage of electrodes once the OCP rose above E_Rp and this could be rationalized based on the statistical spread in E_Pit. Pits stopped growing once the OCP dropped below E_Rp Pitting severity increased with Cl₂  level and pH. The cathodic capacity of adjacent cathodic sites as a function of water chemistry and the nature of the cathodic sites was also factored into this analysis.  Lastly, the possible roles of deposited aluminum solids in pit initiation were also examined using several diagnostic methods.