Passivity and Pit Stability Behavior of Copper as a Function of Selected Water Chemistry Variables

The electrochemical pitting behavior of UNS C11000 copper was investigated in a synthetic potable water found to cause pitting. Tests were also conducted in several other HCO3-, SO42-, and Cl- containing waters with systematic variations in concentrations of these species. Studies of the effect of water chemistry on passivity, uniform corrosion, and pitting were accomplished using the cyclic voltammetry method complemented by various diagnostic methods. Certain water chemistry concentrations promote pitting. Critical pitting potentials (E-Pit) for copper pitting are decreased by certain water chemistry variables. High [SO42-]/[OH-], [SO42-]/[HCO3-], and Cl-/[HCO3-] ratios lower pitting potentials while an increase in alkalinity (increasing [OH-] or [HCO3-]/[CO32-]) improves passivity and raises pitting potentials. HCO3-/CO32- can protect copper surfaces by forming carbonate containing minerals. However, carbonated species are less beneficial toward passivity compared to OH- with respect to passivation efficiency. Empirical equations that forecast pitting and repassivation potentials as a function of selected water chemistry variables were developed by linear regression analysis based on experimental pitting and repassivation potential trends with HCO3-, Cl-, and SO42- content. The origins of the trends with water chemistry variables are discussed.