Effects of selected water chemistry variables on copper pitting propagation in potable water

The pit propagation behavior of copper (UNS C11000) was investigated from an electrochemical perspective using the artificial pit method. Pit growth was studied systematically in a range of HCO3−, SO42− and Cl− containing-waters at various concentrations. Pit propagation was mediated by the nature of the corrosion products formed both inside and over the pit mouth (i.e., cap). Certain water chemistry concentrations such as those high in sulfate were found to promote fast pitting that could be sustained over long times at a fixed applied potential but gradually stifled in all but the lowest concentration solutions. In contrast, Cl− containing waters without sulfate ions resulted in slower pit growth and eventual repassivation. These observations were interpreted through understanding of the identity, amount and porosity of corrosion products formed inside and over pits. These factors controlled their resistive nature as characterized using electrochemical impedance spectroscopy. A finite element model (FEM) was developed which included copper oxidation kinetics, transport by migration and diffusion, Cu(I) and Cu(II) solid corrosion product formation and porosity governed by equilibrium thermodynamics and a saturation index, as well as pit current and depth of penetration. The findings of the modeling were in good agreement with artificial pit experiments. Malachite, bronchantite, cuprite, nantokite and atacamite corrosion products were both observed in experiment and predicted by the model. Stifling and/or repassivation occurred when the resistance of the corrosion product layer became high enough to lower the pit bottom potential and pit current density such as 10-5 A/cm2 could be attained with thick and dense layer. The ramifications of these findings towards pit propagation characteristics in potable waters will be discussed with improved insight into the roles of Cl− and SO42− ions.

► The effects of water composition on pit propagation kinetics on Cu were separated from pit initiation and stabilization using the artificial pit method in a range of dilute HCO3-, SO42- and Cl--containing waters. ► The effective polarization and Ohmic resistance of pits were lower in SO42--containing solutions and greater in Cl--containing solutions. ► Relationship between the solution composition and the corrosion product identity and morphology were found. ► These, in turn controlled the corrosion product Ohmic resistance and subsequently the pit growth rate.