Role of the Complexation Concept in the Present Views on the Initiation and Inhibition of Metal Pitting

The initiation and inhibition of the pit formation (PF) on metals and binary alloys are considered as a consequence of complexation reactions and nucleophilic substitution of ligands in surface complexes. The problems under discussion include the possibility of studying the kinetics of these reactions by measurement of the PF induction period and the dependence of the effective activation energy of the process on the solution composition and on the metal or alloy nature. The role of the electrode potential in the adsorption of anions on passive iron is emphasized; this adsorption is described by the Frumkin isotherm in the case of efficient PF inhibitors belonging to the arylcarboxylate or dialkylphosphate classes. The concepts of the PF potential and the effective overvoltage of the reaction leading to PF are analyzed. Examples of PF and its inhibition which can only be explained using the complexation concept are considered.