Determination of the pH of iso-selectivity of the interfacial diffusion layer of Fe
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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 (pHpis) at which the nature of the diffuse layer is changed from the anion-permeable at pH<pHpis to the cation-permeable at pH>pHpis. The pHpis 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 Fe3+ ion at pH≤8.5 to that producing FeO₂ - at pH≥9, suggesting that the pHpis of Fe is about pH 8.5~9. In addition, the passive film formed at pH 8.5~9, pHpis, 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≤8.5 due probably to the formation of anion permeable diffuse layer, but it was almost constant at pH≥9 irrespective of Cl- concentration due primarily to the formation of cation permeable diffuse layer on the film, confirming again that pHpis of Fe is 8.5~9.
Abstract
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 (pHpis) at which the nature of the diffuse layer is changed from the anion-permeable at pH<pHpis to the cation-permeable at pH>pHpis. The pHpis 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 Fe3+ ion at pH≤8.5 to that producing FeO₂ - at pH≥9, suggesting that the pHpis of Fe is about pH 8.5~9. In addition, the passive film formed at pH 8.5~9, pHpis, 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≤8.5 due probably to the formation of anion permeable diffuse layer, but it was almost constant at pH≥9 irrespective of Cl- concentration due primarily to the formation of cation permeable diffuse layer on the film, confirming again that pHpis of Fe is 8.5~9.
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