BREAKDOWN OF PASSIVE FILM ON COPPER IN BICARBONATE SOLUTIONS CONTAINING SULFATE-IONS

The formation, breakdown, and composition of the surface films formed on a copper rotating disk electrode at 25-degrees-C were studied by potentiodynamic and potentiostatic pulse measurements. The present results explain the nature of the pit initiation process, especially some characteristic values of pitting phenomena such as the critical breakdown potential and the induction time for pit nucleation on passive copper in contact with bicarbonate solution, pH = 8.3, containing aggressive sulfate ions. It was found that the critical breakdown potential, E(b)o, and the inhibition potential, E(I) depend on the concentration of the aggressive anion A ([SO42-]) and inhibitor anion I ([HCO3-]) according to the equations: E(b)o = a + b log [SO42-] (b < 0) and E(I) = a' + b' log ([HCO3-]/[SO42-]) (b' > 0). The critical potentials are explained by the adsorption of sulfate anions on the metal surface and their competition with the inhibitor. On the basis of the experimentally obtained dependencies for the critical breakdown potential and induction time on halide concentration and potential, respectively, the various theories for pit initiation on a passive metal in contact with a solution containing aggressive ions were tested. The experimental data are analyzed in terms of the point defect model, the 2-D nucleation and growth model, and the sulfate nuclei model for passivity breakdown. The validity of the particular models is discussed. Numerical analysis enabled the evaluation of several unknown parameters which are characteristic for the models proposed.