EQCN study of anodic dissolution and surface oxide film formation at Au in the presence of Cl− or Br− ions: A model process for corrosion studies

The anodic dissolution of an Au electrode and associated thin-layer oxide film formation in aq. H2SO4 in the presence of Cl− or Br− ions at various concentrations provides a model process for metal corrosion. In the present work such processes were investigated using cyclic voltammetry and chronoamperometry, with complementary nanogravimetry measurements using the EQCN. The results clearly indicate that in 0.5 M H2SO4 electrolyte, containing 1 mM Br− or Cl−, Au dissolves over the potential range 1.0 – 1.45 V(RHE) through a 3e oxidation process involving Au complex-ion formation that can be followed in situ by means of UV spectroscopy. The linear relationship between mass changes and reciprocal square-root of sweep-rate and between anodic currents in cyclic voltammetry at ca. 1.20 V for Br− (1.39 V for Cl−) and square-root of sweep-rate/or electrode rotation rate indicated quantitatively that the dissolution process is diffusion-controlled. It was interesting to find that electrode rotation in the presence of Cl− ions has little effect on the anodic formation of surface oxide, while, on the contrary, with Br− ions present, currents for oxide film reduction are not observed at rotation rates > ca. 400 rpm.