The Synergistic Inhibition Effects of Hard Base and Oxidizing Inhibitors on Pit Initiation and Growth of a Passive Film on Iron

Abstract

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 E_pit and the repassivation potential E_rep were mostly shifted in the positive direction by mixtures of OTP^- plus MoO₄^2-, WO₄^2-, NO₃^-, and NO₂^-, these inhibitor mixtures suppressed both pit nucleation and growth. Mixtures of OTP^- plus CrO₄^2- 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 WO₄^2- 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.