Electrochemical behaviour of polycrystalline silver in 0·5M NaOH containing sulphide ions

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-, Ag2O and Ag2O2 on the electrode surface. The presence of Na2S 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 Ag2S and S, respectively. The addition of Na2S 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 SO42- 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 Ag2S, S and Ag2O layers involves a nucleation and growth mechanism under diffusion control.