During pitting of iron and stainless steels the potential drop across the interface of the dissolving surface within the pit is in the region of active metal dissolution, even when the potential of the specimen surface is controlled at potentials as high at +1.4 (SHE), as well as during natural corrosion. This conclusion is supported by measurements of the potential of the electrolyte within pits and by the observation that hydrogen gas is produced within the pits. From calculations of the concentration and potential gradients within the electrolyte in a pit it is further concluded that existing models of pit growth are inconsistent with these data. A modified model of pit growth is presented which involves growth by active dissolution and includes a high resistance path resulting from a constriction caused by a hydrogen bubble. This model of pit growth is also believed to apply to the propagation of crevices and, in some instances, of intergranular attack.
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 (CI-, NO~ , 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.
Electronic materials and devices corrode in the same ways as automobiles, bridges, and pipelines, but their typically small dimensions make them orders of magnitude more susceptible to corrosion failure. As elsewhere, the corrosion involves interactions with the environment. Under control, these interactions can be put to use, as in the formation of protective and functional oxide films for superconducting devices. Otherwise, they cause damage, as in the electrolytic dissolution of conductors, even gold, in the presence of humidity and ionic contamination from atmospheric particles and gases. Preventing corrosion entails identifying the damaging interactions and excluding species that allow them to occur.
The anodic corrosion of gold has been studied in concentrated chloride solutions as a function of chloride concentration and pH. Results are discussed in terms of five potential regions: active, prepassive, active‐to‐passive transition, passive, and transpassive. The existence of HAuCl2 with a dissociation constant of the order of 1 to 10 and a mechanism for the active dissolution of gold are postulated. The relation between the breakdown of the passivating film in the transpassive region and the gold dissolution and chlorine evolution reactions is discussed.
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