Spectroscopic Characterization of the Passive Film on Iron before and after Exposure to Chloride Ion

Abstract

The ability to use active metals, for example, iron and its alloys, as structural materials in aqueous environments is a result of the growth of passive films on their surfaces.(1) Localized corrosion processes, such as pitting corrosion, crevice corrosion, or stress corrosion cracking, give rise, in many cases, to catastrophic failures in metallic structural components. These corrosion processes are identified with a number of well-defined stages in their development; in the case of pitting corrosion, these include (i) passive film breakdown, (ii) pit nucleation, and (iii) pit termination or propagation.(2) Common to each form of localized corrosion is the presence of an aggressive anion (usually chloride ion) in the environment contacting the metal. The nature of passive films and the breakdown of passivity are central to, and of paramount importance for, the prevention of localized corrosion processes from taking place. Since breakdown is associated with the passive film only, its structure and chemical composition are of great importance. Confidence in the use of existing metals, and in the formulation of new alloys for more aggressive environments, can only be attained by gaining a complete understanding of the interfacial and bulk physical and chemical aspects of passive films.