The corrosion kinetics of zinc in sulphate solutions is determined mainly by the mass transport and migration properties of oxide/hydroxide film formed by spontaneous passivation. We have found that the corrosion reaction in 0.5 mol/dm3 Na2SO4 pH = 5 solution is under anodic control, whereas in 0.1 mol/dm3 Na2SO4 solution it is under cathodic control. The electrochemical investigations have cleared up that the SO2- 4-ion concentration influences the rate constant of Zn dissolution due to complex-formation and the porosity of the oxide layer which in turn influences the exchange rate of hydrogen evolution on Zn as well. Subsequent X-ray diffraction, scanning and transmission electronmicroscopy analyses of the formed films have revealed some structural and compositional differences in the corroded Zn surface and at the corrosion layer/solution boundary, respectively. In solution of lower SO2- 4-ion concentration, a depletion space-charge layer is detectable in the ZnO film. In a narrow potential range of the corrosion potential the Mott-Schottky plot of the electrode capacitances-electrode potential data gives a donor density ∼7 × 1019 cm-3 arising from the n-type non-stoichiometry of the ZnO. The less porous corrosion film behaves as a weakly degenerate ZnO semiconductor. The flat band potential in the space charge layer has been found to be ∼ - 1.08 V vs sce.
- Electrochemical impedance spectroscopy
ASJC Scopus subject areas
- Chemical Engineering(all)