Effects of pH Levels on the Surface Charge and Pitting Corrosion Resistance of Fe

Abstract

For pure Fe, solution pH effects on a pitting potential and passive film properties were examined in a borate-phosphate buffer solution using a polarization test, Mott-Schottky analysis, and a galvanostatic test. Combining the results, the relationship between the surface charge and pitting potential was presented. Passive film analyzes revealed that the most protective passive film with the lowest defect density and highest thickness was formed on Fe in the buffer solution at pH 8.5-9. From the dependency of the film thickness on the solution pH, it was concluded that the surface charge of the film was changed by the pH from positive to negative at pH 8.5-9, thus, the film dissolved via an H+-assisted reaction below pH 8.5, and via H2O-assisted reaction above pH 9. Due to the change in the film dissolution reaction, the ion accumulation in the interfacial diffuse layer was altered from cation-selectivity at pH levels lower than 8.5 to anion-selectivity at pH levels higher than pH 9. It was confirmed that the Cl− attack on the passivated Fe surface was significantly limited in the buffer solution with pH levels higher than 9 due to the formation of the anion-selective diffuse layer.