Role of Cl− in breakdown of Cu–Ag alloys passivity in aqueous carbonate solutions

The electrochemical behaviour of two Cu–Ag alloys was studied in 0.1 M Na2CO3 solution containing different concentrations of Cl− ions using linear polarization and current/time transients under the effect of different variables of Cl− ions concentration, scan rate and applied anodic potentials. In Cl− free solutions, the anodic voltammogram consists of two potential regions I and II. The potential region I exhibits three anodic peaks A1, A2 and A3 that correspond to the formation of Cu2O, Cu(OH)2 and CuO, respectively. The potential region II exhibited four anodic peaks A4, A5, A6 and A7 due to the formation of AgO−, Ag2O, Ag2CO3 and Ag2O2. In the presence of Cl− ions, the anodic voltammograms depends considerable on the concentration of Cl− ions. Increasing the amounts of Cl− ions up the 0.02 M (alloy I) or 0.006 M (alloy II) the heights of all the anodic peaks were decreased and their peak potentials were shifted to less negative values. The existence of pitting was confirmed by SEM micrograph. The pitting potential Epit was shifted towards more active potential values as the concentration of Cl− ions in the solution was increased. When the scan rate is high, initiation of the pitting can be noticed only at more positive potentials, corresponding to a sufficiently short pit incubation time. The potentiostatic current/time transients show that the incubation time decreases with increasing the applied anodic potential and the Cl− ion concentration and the pitting corrosion can be described in terms of instantaneous three-dimensional growth under diffusion control.