Role of anions in the corrosion and corrosion-inhibition of zinc in aqueous solutions

Abstract

The effect of SO 4 2−, Cl −, Br −, I −, ClO 4 −, NO 3 −, NO 2 −, WO 4 2−, H 2PO 4 −, HPO 4 2−, and CrO 4 2− on the corrosion of zinc in aqueous solutions was studied by measuring the potential of a pure zinc electrode in neutral, strongly-aerated solutions of these anions as a function of both time and electrolyte concentration. Steady-state potentials for the first six anions varied with concentration C according to: E = a - b log C where a and b are constants. These anions promote corrosion and the extent of corrosion-promotion is related to their catalytic effect on the reduction of oxygen in solution. At high dilutions NO 2 −, H 2PO 4 −, HPO 4 2− and CrO 4 2− develop potentials which are practically independent of concentration. Corrosion is assumed to occur at a rate equal to that of film repair by oxygen. At a certain salt content ennobling of the potential takes place and inhibition or passivation sets in. The corrosion of zinc in acid solutions of NO 3 −, SO 4 2−, Cl −, ClO 4 −, H 2PO 4 − and CrO 4 2− was also studied by the thermometric method. On the basis of Δ T-normality curves distinction can be made between corrosive NO 3 −, SO 4 2−, Cl − and ClO 4 − anions, on the one hand, and inhibiting H 2PO 2 − and bassivating CrO 4 2− anions, on the other.