Adsorption Behavior of Ions Related to Corrosion Phenomena and the Hard and Soft Acids and Bases Principle
Specific adsorption of anions on surfaces of metals and oxidized metals is reviewed in relation to the hard and soft acids and bases principle. Anions classified as soft bases are readily chemisorbed on the bare metal surface equivalent to a soft acid, resulting in suppression and stimulation of metallic corrosion in an acid solution with inhibitive and aggressive anions, respectively. Anions acting as hard bases are adsorbed on the oxidized surface, a hard acid without difficulty. Adsorption of aggressive anions on a passive surface of iron in a borate buffer is associated with passive film breakdown. The formation of protective films with metallic cations on the oxidized surface in a neutral solution is also related to the principle.