Ayman M. Zaky

Cyclic voltammetric behaviour of – brass in alkaline media

Abstract The electrochemical behaviour of a brasses was studied in NaOH solutions using the cyclic voltammetric technique to establish the effect of such variables as alloy composition, reversal anodic potential, scan rate, NaOH concentration, and temperature. Compounds that formed on the surface during the anodic sweep were analysed using X-ray diffraction analysis (XRD). The data show that the voltammetric behaviour of – brass consists of two potential regions. The first or subcritical potential region was indicated by two anodic peaks designated A1 and A2. Within this potential region, selective dissolution of the less noble component, zinc, occurs (i.e. dezincification). The second potential region consists of three anodic peaks, designated A3, A4, and A5, where the two components of the alloy, namely copper and zinc, dissolve simultaneously. Alloying zinc with copper resulted in a decrease in the dissolution of zinc from the alloy. This behaviour could be explained on the basis that the dissolution is limited by the non-steady state diffusion of zinc atoms from the bulk of the alloy to the alloy solution interface, which implies that the rate determining step of dezincification is the diffusion of zinc atoms. The process of preferential dissolution of zinc leads to the formation of a layer rich in copper and depleted in zinc on the electrode surface.

Cyclic voltammetric behaviour of copper-nickel alloys in alkaline media

Abstract The electrochemical behaviour of two copper nickel alloys; cu-20 wt-%Ni and Cu-80 wt-%Ni, was studied in alkaline solution using the cyclic voltammetric technique. The voltammograms of the alloys were compared with those of the pure copper and pure nickel recorded under the same conditions. The influence of different variables such as switching potential Es, scan rate, NaOH concentration, solution temperature, and repetitive cycling was also studied. The forward sweep was characterised by the appearance of five anodic peaks: A1, A2, A3, A4, and A5, which are related to the formation of Ni(OH)2, Cu2O, Cu(OH)2, CuO, and Ni2O3, respectively. The activity of copper dissolution and, hence, the heights of the corresponding anodic peaks decreased as the nickel content in the alloy increased. X-ray diffraction analysis of the surface of the 80% nickel alloy after polarisation in the anodic direction confirmed the existence of only nickel corrosion compounds. Increasing the nickel content in the alloy retards the dissolution of copper via the formation of a very protective nickel hydroxide passive film. The addition of chloride ions, owing to their ease of adsorption, increases dissolution from the alloy. When the ratio of [Cl-]/[OH-] ex ceeds 1·5, the chloride ions initiate pitting attack. To examine the alloy surface for pit formation SEM was used.

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.

Electrochemical behaviour of copper-nickel alloys in stagnant Na2CO3 solutions

Abstract The electrochemical behaviour of two copper-nickel alloys in solutions containing the carbonate ion has been investigated by cyclic voltammetric, potentiodynamic, and current/time transient techniques. Among the variables studied were alloy composition, stepwise increasing potential, scan rate, carbonate ion concentration and temperature. The composition of the compounds formed on the alloy surface during the anodic polarisation was characterised using X-ray diffraction analysis. The dissolution behaviour of the alloy consists of two potential regions separated by the critical potential Ecrit. The first potential region involves the selective dissolution of the more active constituent, nickel, and the appearance of one anodic peak A1 as a result of the formation of Ni(OH)2on the alloy surface. The second potential region relates to the simultaneous dissolution of copper and nickel. This region was characterised by the appearance of four anodic peaks A2, A3, A4, and A5, which are related to the formation of Cu2O, {CuO and Cu(OH)2}, CuCO3-Cu(OH)2 composite, and NiOOH, respectively. Potentiostatic current/time transients showed that the formation of Ni(OH)2, Cu2O, CuO, Cu(OH)2, CuCO3-Cu(OH)2 and NiOOH layers involves a nucleation and growth mechanism under diffusion control.