Voltammetric and Raman microspectroscopic studies on artificial copper pits grown in simulated potable water

Artificial copper pits were prepared by electrochemically oxidising 60-80 μm diameter copper wires embedded in an epoxy resin over periods of 12-14 h. The electrolyte matrix consisted of various combinations of approximately 40 ppm unbuffered solutions (pH = 6-8) of sodium salts of Cl-, HCO₃- and SO₄^2- that are similar in concentration to what are found in potable water supplies in many metropolitan areas throughout the world. It was found that in the concentrations used for the study, HCO ₃^- and to a lesser degree Cl^- had a positive affect on preventing pit growth under potentiostatic control, with both anions causing passivation of the copper metal. On the other hand, SC₄ ^2- was found to be very aggressive to copper dissolution and led to the formation of relatively deep pits (about 0.5 mm). Raman microspectroscopic analyses were performed on the freshly prepared undried caps that formed at the top of the pits and allowed the identification of several corrosion products by a comparison with standard copper mineral samples. The most complicated cap structure was observed in the presence of all three anions with distinct regions of the pit corresponding to cuprite (Cu₂O), eriochalcite (CuCl₂ · 2H₂O), atacamite and/or botallackite [Cu₂Cl(OH)₃] and brochantite [Cu₄(SO ₄)(OH)₆].