The role of some organic anions in promoting or inhibiting the corrosion of zinc

Abstract

The corrosion behaviour of zinc in solutions of the sodium salts of formate, acetate, propionate, citrate, tartrate, oxalate or benzoate anions was studied by determining the steady state potentials. The first five anions behave unifunctionally, promoting the corrosion of zinc. The extent to which corrosion is promoted depends on the type, the concentration and the dissociation constants of the corresponding acids. The oxalate ion behaves bifunctionally, promoting corrosion of zinc at high dilutions and inhibiting it at higher concentrations. The benzoate ion shifts the steady state potential of zinc in the noble direction, indicating the occurrence of corrosion inhibition. Chloride ions destroy the passive film formed on a zinc electrode previously equilibrated in solutions of benzoate anions. The concentration C inh of the inhibitive anion required to prevent the corroding action of the aggressive anion at a concentration C agg can be obtained from the relation log C inh = A + n log C agg where A is a constant and n is approximately the ratio of the ionic charges of the inhibitive and aggressive anions.