FACTORS INFLUENCING THE BREAKDOWN SUSCEPTIBILITY OF THE PASSIVE FILM ON CU-NI ALLOY

Cyclic voltammetry and current transients, complemented with Auger electron analysis in conjunction with argon ion sputtering, were used to establish the mechanism of passivity breakdown on 9OCu-10Ni alloy in borate buffer, pH = 9.25, containing chloride ions. The incubation time, t(i), and the induction time, tau, appear as characteristic time parameters, indicating that different stages are involved in the overall breakdown process. A model of passivity breakdown with the onset of pitting corrosion is proposed, taking into consideration a duplex structure of the passive film, which is composed of an inner Cu2O layer and an outer CuO layer. Auger depth analysis revealed that the extent of Cl- incorporation into the oxide film is highly dependent upon previous anodic treatment, as well as upon time of exposure to the chloride-containing solution. The results are discussed in terms of improving resistance toward breakdown with increasing perfection of the passive film.