Inorganic Anions and Copper Pitting

Copper (Cu) pitting was examined in oxygenated solutions containing bicarbonate (HCO3–), chloride (Cl– ), nitrate (NO3–), perchlorate (ClO4–), or sulfate (SO42–) ions at pH values of 5.5, 7.0, 8.5, and 10.0. While short-term (a few hours) cyclic polarization experiments at anion concentrations (or ionic strengths) typical of drinking waters were inconclusive, potentiostatic testing provided insight into Cu pitting behavior. Cl– exhibited a time-dependent dual nature, accelerating anodic reactions in the short term and causing passivation over longer periods. Passivation occurred only at pH 10.0 in the systems containing NO3– and ClO4– and did not occur in the presence of SO42–. Results supported the notion that SO42– or NO3– and not Cl–, (as commonly believed), are key to Cu pitting in potable water distribution systems.