Electrochemical studies of copper and thallium
The anodic growth of a Cu2O film on polycrystalline copper surfaces was studied by a potentiostatic method. Plots of anodic current versus time allowed the growth law to be determined and the Cu2O film thickness was found to vary linearly with the cube root of time. The same growth law applied when Cu2O films were formed by aqueous oxygen attack at the copper surfaces. A mechanism has been proposed explaining this law, based on assumptions of Cabrera and Mott for gaseous oxidation of copper. The cyclic voltammetry technique was used to study the growth, dissolution and reduction of Cu2O and the higher copper oxides over the pH range 5.20 to 13.83. Cu2O was the first oxide formed, and depending on the pH, either CuO or Cu(OH)2 was formed over the underlying Cu2O at higher potentials. In NaOH solutions, of concentration 0.10M and greater, presence of the oxide, Cu2O3 was noted at potentials corresponding to oxygen evolution. During cathodic reduction of the films formed anodically, the Cu(II) product was initially reduced to Cu2O, followed by Cu2O reduction to Cu metal. Solid state reactions for the growth, dissolution and reduction processes are included. No explanation for peak formation during anodic film growth by the cyclic voltammetry technique can be found but plateau formation is predicted. The inclusion of chloride in the anodically formed films increased the conductivity of the oxide films greatly. A 0.100M Na2B407 solution containing 9.6 x 10-4M benzotriazole was found to inhibit the formation of films at the copper surface probably by the formation of a Cu(BTA) polymer. The presence of sodium hexametaphosphate had no effect on oxide growth kinetics at pH 9.18. Potentials of specimens of copper piping corroding in Auckland's supply water were measured against a saturated calomel electrode. The values depended largely on the conditions of water flow and stagnation. Illumination and surface preparation of the samples had little effect. The corrosion potentials reached a maximum after approximately 30 days and the high potentials observed were attributed to the resistance of the films of corrosion products covering the metal surface. A mechanism for the corrosion process has been proposed with the electrophoretic deposition of a layer of silica gel over the underlying Cu2O film causing local cell action and the formation of anodic and cathodic areas over the copper surface. The gel allows diffusion of Cu2+ into the solution from the anodic regions and OH-from the cathodic regions, forming a loosely adherent flocc of Cu(OH)2 the metal surface. This flocc is readily scoured from the surface leading to Auckland's green water problem.