The electrochemical behaviour of silver in 0?5M NaOH solution containing different concentrations (0?0005-0?007M) of sodium sulphide was studied using cyclic voltammetry, potentiodynamic, and current transient techniques. The anodic sweep of the voltammogram in NaOH was characterised by the appearance of three anodic peaks A 3 , A 4 and A 5 that are related to the formation of AgO 2 , Ag 2 O and Ag 2 O 2 on the electrode surface. The presence of Na 2 S in NaOH solution resulted in the appearance of two additional anodic peaks A 1 and A 2 . These two peaks are related to the formation of Ag 2 S and S, respectively. The addition of Na 2 S also increases the heights of the anodic peaks A 3 , A 4 and A 5 . The increase in the current density of the anodic peaks A 3 and A 4 is mainly due to surface enlargement resulting from pitting and the precipitation of sulphur on the electrode surface. Also, the large increase in the current density of the anodic peak A 5 is due to the formation of the soluble SO 22 4 compound. The morphology of the electrode surface was examined by scanning electron microscopy. Characterisation of the corrosion products formed anodically on the electrode surface was undertaken using X-ray diffraction analysis. Potentiostatic current-time transients showed that the formation of Ag 2 S, S and Ag 2 O layers involves a nucleation and growth mechanism under diffusion control.

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composition of the compounds formed on the alloy surface during the anodic polarisation was characterised using X -ray diVraction analysis. T he dissolution behaviour of the alloy consists of two potential regions separated by the critical potential E crit . T he rst potential region involves the selective dissolution of the more active constituent, nickel, and the appearance of one anodic peak A 1 as a result of the formation of Ni(OH) 2 on the alloy surface. T he second potential region relates to the simultaneous dissolution of copper and nickel. T his region was characterised by the appearance of four anodic peaks A 2 , A 3 , A 4 , and A 5 , which are related to the formation of Cu 2 O, {CuO and Cu( OH) 2 }, CuCO 3 -Cu( OH) 2 composite, and NiOOH, respectively. Potentiostatic current/time transients showed that the formation of Ni(OH) 2 , Cu 2 O, CuO, Cu(OH) 2 , CuCO 3 -Cu(OH) 2 and NiOOH layers involves a nucleation and growth mechanism under diVusion control.

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