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

Abstract

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 A₃, A₄ and A₅ 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 A₁ and A₂. 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 A₃, A₄ and A₅. The increase in the current density of the anodic peaks A₃ and A₄ 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 A₅ is due to the formation of the soluble SO²⁻ 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 Ag₂S, S and Ag₂O layers involves a nucleation and growth mechanism under diffusion control.