The effect of molybdate anion on the ion-selectivity of hydrous ferric oxide films in chloride solutions

Abstract

The ion-selective property of hydrous ferric oxide precipitate films has been investigated by measuring membrane potentials which arise across precipitate membranes of hydrous ferric oxide with and without adsorbed MoO 4 2− ions and of ferric molybdate in solutions of NaCl, KCl, MgCl 2, CaCl 2, BaCl 2, AlCl 3, and FeCl 3. The hydrous ferric oxide membrane was only permeable to Cl − ions in chloride solutions, whereas the membrane with adsorbed MoO 4 2− ions was permeable to cations in NaCl and KCl solutions, and to both Cl − and cations in the presence of multivalent cations. The ferric molybdate membrane was permeable to Cl − and cations in NaCl and KCl solutions, and only to Cl − ions in the presence of multivalent cations. It is suggested that in chloride solutions, the corrosion of iron covered with a precipitate film of hydrous ferric oxide is accelerated by enrichment of Cl − ions under the film, which may decrease the local pH and introduce a positive diffusion potential in the film. The adsorption of MoO 4 2− ions on the oxide changes the ion-selectivity of the precipitate film from the anion-selective to the cation-selective in solutions of NaCl and KCl. This cation-selectivity of the film may inhibit the corrosion of iron, because of H + ions diffusing out of the film. The inhibitive effect of MoO 4 2− ions would be reduced in the presence of multivalent cations.