engleski

Breakdown of passive film on copper in bicarbonate solutions containing sulfate ions: breakdown models evolution

The formation, breakdown, and composition of the surface films formed on a copper rotating disk electrode at 25°Cwere studied by potentiodynamic and potentiostatic pulse measurements. The present results explain the nature of the pitinitiation process, especially some characteristic values of pitting phenomena such as the critical breakdown potential andthe induction time for pit nucleation on passive copper in contact with bicarbonate solution, pH = 8.3, containing aggressivesulfate ions. It was found that the critical breakdown potential and the inhibition potential depend on the concentration of the aggressive anion A ([SO]) and inhibitor anion I ([HCO]). The critical potentials are explained by the adsorption of sulfate anionson the metal surface and their competition with the inhibitor. On the basis of the experimentally obtained dependenciesfor the critical breakdown potential and induction time on halide concentration and potential, respectively, the varioustheories for pit initiation on a passive metal in contact with a solution containing aggressive ions were tested. Theexperimental data are analyzed in terms of the point defect model, the 2-D nucleation and growth model, and the sulfatenuclei model for passivity breakdown. The validity of the particular models is discussed. Numerical analysis enabled theevaluation of several unknown parameters which are characteristic for the models proposed.

copper rotating disc electrode ; surface film ; inhibition

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