1998 Volume 47 Issue 7 Pages 460-467
The synergistic inhibition effects of a hard base, octylthiopropionate ion C8H17S(CH2)2- COO-(OTP-) and various oxidizing inhibitors on pit nucleation and growth were investigated for a passivated iron electrode in an aerated borate buffer solution (pH 8.49) containing an aggressive anion Cl- by polarization measurements. Because the pitting potential Epit and the repassivation potential Erep were mostly shifted in the positive direction by mixtures of OTP- plus MoO42-, WO42-, NO3-, and NO2-, these inhibitor mixtures suppressed both pit nucleation and growth. Mixtures of OTP- plus CrO42- and VO3- inhibited pit initiation but stimulated pit propagation, resulting in the formation of large pits. The hard bases, OTP- and oxidizing inhibitors suppressed pit nucleation by repairing defects in the passive film with deposits of their salts or complexes. After pit initiation, since oxidizing inhibitors like WO42- and NO3- oxidized the iron substrate surface at the bottom of a pit to change a soft acid to a hard one, they inhibited pit growth by precipitation of oxides within the pit. Inhibition of pit nucleation and growth on the passive iron surface was closely related to the hard and soft acids and bases (HSAB) principle.