Anodic behavior of copper in weakly alkaline solutions

The anodic polarization behavior of copper was studied in the pH range 9.4 to 10.8, where pH was controlled (buffered) by additions of either borate, bicarbonate/carbonate, ammonia, or sodium hydroxide. Several solutions contained either 1M sulfate, 1M chloride, or 5M chloride. Passivation effects were observed in all solutions, except those containing 5M chloride, and were accompanied by two anodic current peaks involving formation of Cu sub(2)O films and films containing Cu(OH) sub(2) and CuO. Passivity breakdown occurred when the Cu super(11) state was present in the film and occurred over a potential range whose highest and lowest values differed by -0.7 V. The prime factor controlling the breakdown potential was the concentration of the buffer species, independent of the type of buffer, and not the pH of the bulk solution. It was concluded from the results and subsequent discussion that film breakdown was determined primarily by the loss of pH control at the electrode surface, due to the generation of protons accompanying film growth. A simple mass-transfer model defined the general relationship between buffer concentration, anodic passive current density, and loss of pH control. Secondary factors contributing to film breakdown, such as the formation of soluble complexes, were also considered.