RELATIONSHIP BETWEEN THE HSAB PRINCIPLE AND EFFECTS OF VARIOUS ANIONS AND A TETRA-N-BUTYLAMMONIUM ION ON PASSIVE FILM OF IRON IN A BORATE BUFFER SOLUTION

Effects of various anions on the passive current density and pitting potential of an iron electrode in a borate buffer solution at pH 8.49 were discussed on the basis of the hard and soft acids and bases principle (HSAB principle). The effect of anion on the passive current density was confirmed to be closely related to the HSAB principle. This result implies that the anion classified as a hard base is adsorbed with less difficulty on the passive film than that of a soft base. A group of anions I-, Br-, Cl-, and S2- at 1 x 10(-2)M of the concentration shifted the pitting potential toward a negative potential, being aggressive upon the passivity of iron. The breakdown of passive film occurs more readily by the action of anion classified as a harder base in this group. Anions F-, ClO4-, CH3CO2-, SO-4(2-), SO-3(2-), SCN-, CN-, and N3-, belonging to another group, scarcely induce the breakdown of a passive film at 1 x 10(-2)M. The anions of the former group were detected near the iron substrate in the passive film, but those of the latter were observed only at the film surface by XPS measurements. A cation, tetra-n-butylammonium ion inhibited the passive current density and the breakdown of passive film in the presence of the added anions.