Ion Selectivity of Precipitate Films Affecting Passivation and Corrosion of Metals

Abstract Ion selectivity in precipitate films of hydrated iron (III) and iron (III) molybdate has been studied by measuring the membrane potential and polarization property as a function of solution pH and ion valency. The hydrated iron (III) oxide membrane is found to be anion selective in 1-1 electrolyte solutions in the pH region lower than a specific pH. Beyond this pH, the membrane turns to be cation selective. This specific pH, at which the membrane potential is identical to the liquid junction potential, is named the point of iso-selectivity pHpis, and is evaluated in KCI solutions to be 10.3. The hydrated iron (III) oxide membranes with adsorbed divalent anions such as MoO42− ions are cation selective in monovalent electrolyte solutions even in the pH region lower than pHpis. This cation selectivity can be attributed to the strong adsorption of the divalent anions forming the negative fixed charges on the membranes. Under an applied membrane potential, the ionic current is rectified through the bi...