PITTING POTENTIAL AND PROTECTION POTENTIAL OF CARBON-STEEL FOR CHLORIDE-ION AND THE EFFECTIVENESS OF DIFFERENT INHIBITING ANIONS

The pitting potential, E(p), and protection potential, E(pp), have been determined for carbon steel using a potentiodynamic method. Increasing Cl- concentrations shift these potentials to more active values. The pitting potential, E(p), and protection potential, E(pp), depend on the concentration of Cl- according to the equations: E(p) = a + b log [Cl-] and E(pp) = a + b log [Cl-]. The pitting potential is shifted to more noble values by the presence of other anions which do not themselves cause pitting, e.g. HPO42-, benzoate, CO32-, acetate, citrate. Sufficient concentrations of these anions act as pitting inhibitors. The following relationship has been determined between the minimum anion concentration [I] necessary to inhibit pitting of carbon steel and Cl- ion concentration: log [Cl-] = a + b log [I]. From these equations obtained for the anions, the effectiveness of the inhibitors used in preventing pitting corrosion was in the order as listed. These results are interpreted in terms of competitive adsorption of Cl- and other anions for sites on the metal surface.