CORROSION MECHANISMS FOR SILVER EXPOSED TO THE ATMOSPHERE

The physical and chemical phenomena responsible for the atmospheric corrosion of silver are presented. Corrosion layer formation, morphology, and chemical makeup are discussed in the context of silver-containing minerals and other crystalline structures that thermodynamics and kinetics suggest are likely to be present. The constituents that form during the corrosion process are then described, and the formation pathways of acanthite (Ag2S) and chlorargyrite (AgCl), the two minerals most often reported to be present in silver corrosion layers. are shown in schematic diagrams. The presence of these species and the essential absence of sulfate, nitrate, carbonate, or organic salts of silver are shown to be a natural consequence of the thin aqueous layer chemistry that obtains on silver in humid environments. The primary atmospheric agents responsible for the degradation are identified as H2S, COS, particulate chloride, and possibly HCl, all acting in the presence of moderate to high humidity. Gaseous hydrogen peroxide, which is sometimes present, strongly accelerates silver corrosion. Comprehensive kinetic simulations of the corrosion process are desirable, but await laboratory determinations of the rates of dissolution, precipitation, and transformation of silver-containing chemical species.