Passivation and Cl¯ Induced Depassivation of Cu-Ag Alloys in Borate Buffer Solutions (original) (raw)

2008, International Journal of Electrochemical Science

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 A 1 , A 2 and A 3. The anodic peak A 1 was related to the formation of Cu 2 O, while the anodic peaks A 2 and A 3 are related to the oxidation of Cu and Cu 2 O 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 A 5 , A 6 , A 7 and A 8 related to the formation of mono-layer, and multi-layers of Ag 2 O, AgO and Ag 2 O 3 , respectively. The cathodic portion of the voltammogram was characterized by the appearance of six cathodic peaks C 1 , C 2 , C 3 , C 4 , C 5 and C 6 prior to hydrogen evolution reaction. The addition of small amount of Clion resulted in the appearance of two anodic peaks A / and A // due to the formation of CuCl 2 .H 2 O and AgCl on the electrode surface, respectively. SEM examinations, in presence of Clions, confirmed the existence of pits on the alloy surface. Potentiostatic current/time transients showed that the formation of Cu 2 O, 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 (t i). The rate of pit nucleation, defined as (t i −1), increases with increase in Cl − concentration and applied step anodic potential (E s,a).