CORROSION MECHANISMS FOR IRON AND LOW-ALLOY STEELS EXPOSED TO THE ATMOSPHERE

Despite extensive study over the years, the chemical processes involved in the atmospheric corrosion of iron and its alloys remain poorly understood. Most conceptual studies have ignored the chemical influence of the trace anions (Cl−, NO3, SO42−, CO2H−, etc.) present in the atmosphere and in precipitation. This review, presented from the perspective of atmospheric chemistry and mineralogy, provides an analysis of rust layer formation, evolution, morphology, and composition, together with information on iron-containing minerals and other crystalline structures that are likely to be present. The chemical reactions involved in the formation of these constituents during the corrosion process are then presented. The reactions are not spatially homogeneous, but favor pits, voids, and crevices in the metal surface. It is demonstrated that (i) the pH of the moisture on the surface is crucial to the corrosion process, since it controls the dissolution of the passive oxyhydroxide surface; the pH is largely controlled by atmospheric SO2 and NOx dissolved in the moisture or by fog or rain deposited on the surface; (ii) the extant data suggest that the rate of iron oxyhydroxide formation is slow; hence, the presence of reactive anions generally results in their blending into mixed hydroxy-anion products; (iii) the interactive chemistry of readily available hydrogen peroxide and bisulfite ion in the aqueous surface film can either enhance or impede the rate of corrosion; (iv) photon-driven reactions can promote the corrosion of iron and its alloys. This analysis unifies the analytical information, as well as the data on kinetic processes, and provides the basis for a full understanding of the atmospheric corrosion of iron and low alloy steels. © 1990, The Electrochemical Society, Inc. All rights reserved.