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

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 MoO42− ions and of ferric molybdate in solutions of NaCl, KCl, MgCl2, CaCl2, BaCl2, AlCl3, and FeCl3. The hydrous ferric oxide membrane was only permeable to Cl− ions in chloride solutions, whereas the membrane with adsorbed MoO42− 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 MoO42− 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 MoO42− ions would be reduced in the presence of multivalent cations.