Mechanism of anodic dissolution and passivation of iron—II. Comparison of the behavior in neutral benzoate and acetate buffer solutions

Abstract

The kinetics of the dissolution and passivation of Fe has been studied in neutral benzoic acid/sodium benzoate buffer solution as a function of the total benzoate buffer concentration and pH. The results have been obtained from potential sweep experiments at 5 mV s−1, which corresponds to quasi steady-state conditions. Two anodic peaks were found in the active dissolution region of the anodic voltammogram. A reaction order of about −1 with respect to benzoate concentration and of about 2 with respect to pH has been found for the Tafel region of the first peak. The corresponding reaction orders were −1 and 0.7 for the second peak. Galvanostatic cathodic reduction experiments were used to characterize the surface film formed during the anodic sweep. Anodic sweep experiments were also performed using mixtures of benzoate and acetate buffers. The behavior of Fe in benzoate and acetate buffer solutions has been compared and contrasted. Benzoate buffer is found to be a much better inhibitor than acetate buffer, and the results can be understood in terms of a strong interaction between the Fe surface and benzoate-containing adsorbates.