Determination of the pH of Iso-Selectivity of the Interfacial Diffusion Layer of Fe

Passive metal forms an interfacial diffuse layer on the surface of passive film by its reaction with $H^+$ or $OH^-$ ions in solution depending on solution pH. There is a critical pH, called pH point of iso-selectivity ($pH_{pis}$) at which the nature of the diffuse layer is changed from the anion-permeable at pH<$pH_{pis}$ to the cation-permeable at pH>$pH_{pis}$. The $pH_{pis}$ for a passivated Fe was determined by examining the effects of pH on the thickness of passive film and on the dissolution reaction occurring on the passive film under a gavanostatic reduction in borate-phosphate buffer solutions at various pH of 7~11. The steady-state thickness of passive film formed on Fe showed the maximum at pH 8.5~9, and further the nature of film dissolution reaction was changed from a reaction producing $Fe^{3+}$ ion at $pH\leq8.5$ to that producing $FeO_2{^-}$ at $pH\geq9$, suggesting that the $pH_{pis}$ of Fe is about pH 8.5~9. In addition, the passive film formed at pH 8.5~9, $pH_{pis}$, was found to be the most protective with the lowest defect density as confirmed by the Mott-Schottky analysis. Pitting potential was decreased with increasing $Cl^-$ concentration at $pH\leq8.5$ due probably to the formation of anion permeable diffuse layer, but it was almost constant at $pH\geq9$ irrespective of $Cl^-$ concentration due primarily to the formation of cation permeable diffuse layer on the film, confirming again that $pH_{pis}$ of Fe is 8.5~9.