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 binar y alloys are considered as a consequence of complexation reactions and nucl eophilic substitution of ligands in surface complexes. The problems under d iscussion include the possibility of studying the kinetics of these reactio ns by measurement of the PF induction period and the dependence of the effe ctive activation energy of the process on the solution composition and on t he metal or alloy nature. The role of the electrode potential in the adsorp tion 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 po tential and the effective overvoltage of the reaction leading to PF are ana lyzed. Examples of PF and its inhibition which can only be explained using the complexation concept are considered.