THE INHIBITION OF PIT GROWTH ON AN IRON SURFACE IN A BORATE BUFFER SOLUTION CONTAINING CHLORIDE-ION BY INHIBITORS CLASSIFIED AS SOFT BASES IN THE HSAB PRINCIPLE

The pitting potential E(pit) and the repassivation potential E(rep) were measured for a previously passivated iron electrode in an aerated Cl--containing borate buffer solution (pH 8.49) with and without various inhibitors using potentiodynamic polarization measurements. The values of E(pit) and E(rep) were discussed based on the hard and soft acids and bases (HSAB) principle. Iodide and bromide ions and 1,3-di-n-butyl-2-thiourea, which are classified as soft bases, shifted E(rep) in the positive direction, indicating inhibition of the pit growth process. Electron-probe micro-analyses showed that these soft base inhibitors were adsorbed on the surface of iron substrate acting as a soft acid within a pit. However, inhibition effects of these inhibitors on E(pit) were not observed. Organic anion inhibitors of hard bases, which have been reported to suppress pit formation by repair of defects or pores in the passive film with precipitates of complexes or salts formed by the hard acid-hard base reactions, were ineffective for E(rep) because the hard base inhibitors prefer not to bind to the iron substrate surface within the pit.