Microbiologically Influenced Corrosion of Copper in Potable Water Systems—pH Effects

Copper tubing used in potable water plumbing systems occasionally experiences reactions that lead to the release of copper corrosion by-products into the water. The main factor controlling microbiologically influenced corrosion (MIC) of copper has been identified as a decrease in pH, which in conjunction with the incorporation of bacterially produced extracellular polymeric substances (EPS) in the copper oxide film, decreases the protective nature of the film. The biofilm (bacteria and EPS) is believed to have a secondary role to the nature of the oxide film in controlling the rate of corrosion. MIC was produced in laboratory reactors containing copper electrodes exposed to simulated potable water in the presence of a biofilm composed of microorganisms isolated from a field site. In the presence of the biofilm, small but significant reductions in pH occurred—from an initial value of 7.5 to between 6.5 and 6.9. Using electrochemical impedance spectroscopy (EIS), it was shown that the presence of a biofilm caused instances of higher corrosion rates similar to those measured in inorganic tests at pH 6.8. Modeling the oxide film as a thin barrier layer covered by a porous layer, EIS data revealed similarities between the oxide structure on samples experiencing MIC to samples exposed in sterile tests at pH 6.8.