International Journal of Electrochemical Science (Aug 2007)
Electrochemical investigations on formation and pitting susceptibility of passive films on iron and iron-based alloys
Abstract
This paper reviews recent research results on the formation and the pitting susceptibility of passive films formed on iron and iron-based alloys in neutral solutions, obtained in authors’ groups with electrochemical techniques including linear sweep voltammetry, alternative current impedance, electrochemical nose, and scan reference electrode techniques. At first, the formation process of the passive film on iron in neutral aqueous is described. The process involves the formations of Fe(OH)2, Fe3O4 and Fe2O3. The content of Fe2+ in a passive film reflects the stability of the passive film. Then, the electronic properties and the pitting susceptibility of passive films formed on iron and an iron-based alloy and their relation are considered. The passive film formed on iron or iron-based alloy has a structure with point defects that consist of metal interstitials, metal vacancies and oxygen vacancies. The nature of the passive film depends on the point defects that can be represented by the donor density of the passive film. Mott-Schottky relation can be used to determine the electronic properties of the passive film. The relationship between reciprocal square capacitance and potential meeting the Mott-Schottky behavior is limited at a narrow range of low potentials. At higher potentials, the reciprocal capacitance not reciprocal square capacitance is proportional to potential, which is ascribed to the increasing thickness of the passive film with potential. The pitting susceptibility of a passive film is proportional to the donor density of the passive film and influenced by the formation potential of the passive film, inhibitors and microstructures of base metals. The stability of a passive film increases with increasing formation potential of the passive film. In the case of the formation of passive films by inorganic inhibitors, the stability of the passive film formed by chromate is best, followed by nitrite and then bicarbonate. The passive film formed on ferrite is most unstable, followed by martensite and then hypoeutectoid. There exists interaction between different microstructures, which results in pitting initiation and propagation on the microstructure with a weaker passive film. Pitting corrosion takes place on ferrite when coupling ferrite with hypoeutectoid or matensite, and on martensite when coupling martensite with hypoeutectoid.
