A KINETIC-MODEL OF THE SPONTANEOUS PASSIVATION AND CORROSION OF ZINC IN NEAR NEUTRAL NA2SO4 SOLUTIONS

The corrosion kinetics of zinc in sulphate solutions is determined mai nly by the mass transport and migration properties of oxide/hydroxide film formed by spontaneous passivation. We have found that the corrosi on reaction in 0.5 mol/dm(3) Na2SO4 pH = 5 solution is under anodic co ntrol, whereas in 0.1 mol/dm(3) Na2SO4 solution it is under cathodic c ontrol. The electrochemical investigations have cleared up that the SO 42--ion concentration influences the rate constant of Zn dissolution d ue to complex-formation and the porosity of the oxide layer which in t urn influences the exchange rare of hydrogen evolution on Zn as well, Subsequent X-ray diffraction, scanning and transmission electronmicros copy analyses of the formed films have revealed some structural and co mpositional differences in the corroded Zn surface and at the corrosio n layer/solution boundary, respectively. In solution of lower SO42--io n concentration a depletion space-charge layer is detectable in the Zn O film. In a narrow potential range of the corrosion potential the Mot t-Schottky plot of the electrode capacitances-electrode potential data gives a donor density similar to 7 x 10(19) cm(-3) arising from the n -type non-stoichiometry of the ZnO. The less porous corrosion film beh aves as a weakly degenerate ZnO semiconductor. The flat band potential in the space charge layer has been found to be similar to-1.08 V vs s ee. (C) 1997 Elsevier Science Ltd.