The dissolution kinetics and salt film precipitation of Zn and Fe in chloride solutions: Importance of the common-ion effect and diffusivity

Abstract

The lead-in pencil electrode technique was used to study the pit solution chemistry of pure Zn and Fe in the presence and absence of a common cation. It was found that during 1-D pitting the relative importance of the common-ion effect on the dissolution kinetics, vis-à-vis that of the diffusion coefficient, is strongly related to the tendency of the dissolving metal to form chloro-complexes. The dissolution kinetics were characterized in terms of the limiting current density, salt film dissolution potential, supersaturation of the pit solution and pit growth rate. The dissolution kinetics decreased with increasing bulk Cl− concentration for both metals, irrespective of the presence of a common cation. The reduced dissolution kinetics of Zn with increasing bulk Cl− concentration was found to be associated with a lowered diffusion coefficient rather than the common-ion effect (which would reduce metal salt solubility). On the other hand, Fe dissolution was affected by both the common-ion effect and reduced diffusivity. These differing observations appear to be due to the different tendencies for Zn and Fe to form chloro-complexes, as well as their different chloride salt solubilities.