Non-linear dynamics of the passivity breakdown of iron in acidic solutions

Breakdown of the iron passivity in acid solutions accompanied by current oscillations was investigated by using electrochemical techniques, which reveal the non-linear dynamical response of the system in the current–potential (I–E) and current–time (I–t) planes. Current oscillations of the Feelectrolyte electrochemical system were studied in the (a) absence and (b) presence of chlorides. In case (a) two oscillatory regions were distinguished; one at low potentials associated with the formation–dissolution of a ferrous salt and another at higher potentials associated with the formation-breakdown of the oxide film. Chaotic oscillations appear in the former region whereas periodic oscillations of a relaxation type appear in the latter region. In case (b), complex periodic and aperiodic oscillations are induced by small amounts of chlorides due to pitting corrosion. Pitting corrosion is a multistage localized process of a great technological importance. It consists of a local breakdown of the passive oxide film and formation of individual areas of metal active dissolution. It was shown that oscillatory phenomena might be used for the characterization of pitting corrosion. Examples of the application of the non-linear dynamical response to pitting corrosion were discussed at early and late stages of the pit growth.