Inhibition of Passive Film Breakdown and the Hard and Soft Acids and Bases Principle
Inhibition of passive film breakdown on iron in a borate buffer solution containing Cl- by various inhibitors is reviewed on the basis of the hard and soft acids and bases principle. Since Fe3+ is equivalent to a hard acid according to the principle, inhibitors classified as hard bases suppress the pit nucleation by repairing defects in the passive film with deposits of their complexes with Fe3+. The pit growth is inhibited with inhibitors acting as soft bases by adsorption on the surface of substrate iron, a soft acid within the pit. Both pit nucleation and growth processes are suppressed by some mixtures of the hard and soft base inhibitors. Oxidizing inhibitors, some of which can change the substrate surface to an oxidized one, inhibit both processes by hard acid-hard base interaction.