Passivation and Clˉ Induced Depassivation of Cu-Ag Alloys in Borate Buffer Solutions

The electrochemical behaviour of two Cu-Ag alloys, namely (20wt%Cu + 80wt%Ag) and (80wt%Cu + 20wt%Ag) alloys, was studied in 0.15 M borax and 0.15 M boric acid buffer solution, pH =8.45, by means of cyclic voltammetry, potentiodynamic anodic polarization and current/ time transients techniques. SEM and XRD microanalysis were used to examine the changes caused by the electrochemical perturbations. The anodic portion of the voltammogram was characterized by the existence of two potential regions I and II. In the first potential region copper dissolves preferentially and exhibits three anodic peaks A1, A2 and A3. The anodic peak A1 was related to the formation of Cu2O, while the anodic peaks A2 and A3 are related to the oxidation of Cu and Cu2O to CuO and Cu(OH)2, respectively. The preferential dissolution of copper was enhanced and the simultaneous dissolution of silver was retarded on increasing the silver content in the alloy. The potential region II was characterized by the appearance of four anodic peaks A5, A6, A7 and A8 related to the formation of mono-layer, and multi-layers of Ag2O, AgO and Ag2O3, respectively. The cathodic portion of the voltammogram was characterized by the appearance of six cathodic peaks C1, C2, C3, C4, C5 and C6 prior to hydrogen evolution reaction. The addition of small amount of Cl- ion resulted in the appearance of two anodic peaks A/ and A// due to the formation of CuCl2.H2O and AgCl on the electrode surface, respectively. SEM examinations, in presence of Cl- ions, confirmed the existence of pits on the alloy surface. Potentiostatic current/time transients showed that the formation of Cu2O, CuO and Cu(OH)2 involves a nucleation and growth mechanism under diffusion control. Potentiostatic measurements also showed that the overall anodic processes can be described by three stages. The first stage corresponds to the nucleation and growth of a passive oxide layer. The second and the third stages involve pit nucleation and growth, respectively. Nucleation of pit takes place after an incubation time (ti). The rate of pit nucleation, defined as (ti 1), increases with increase in Cl concentration and applied step anodic potential (Es,a).