Corrosion Mechanisms for Zinc Exposed to the Atmosphere

Abstract

Physical and chemical information on zinc corrosion layer formation, evolution, morphology, and chemical makeup is organized and presented, together with information on minerals containing zinc and other crystalline structures that might be expected to be present. The chemical reactions involved in the formation of these constituents during the corrosion process are then surveyed. The pH of the aqueous surface film is found to be crucial to the corrosion process, since it controls the dissolution of the passive oxyhydroxide surface. (The outdoor pH is largely a function of atmospheric SO= and NOx and is low in fogs and some rains, near neutral in dew, and generally lower in winter than in summer; indoors, the surface acidity is controlled by the composition of deposited particles.) The transition from traditional winter maxima in outdoor zinc corrosion to the more recent summer and fall maxima is explained as the juxtaposition of annually-varying rates of bisulfite oxidation by H20= and of atmospheric SO= concentrations. Indoors, most of the surface degradation can be attributed to adsorbed sulfate aerosol particles; the indoor corrosion rates can be constrained to negligible levels by maintaining moderate relative humidities. Zinc is the only one of the common industrial metals for which a product containing carbonate is abundant, but sulfates and chlorides appear as well. In each case, the predominant forms are the slightly soluble hydroxide mixed salts. While full quantitative analyses of atmospheric zinc corrosion are not yet possible, it is clear that the process can only be understood in the context of the anionic surface chemistry of zinc.Zinc is one of the metals regularly exposed to the indoor and outdoor atmosphere. A large proportion of its use is in applications that take advantage of its favorable corrosion properties and its relatively inexpensive price, such as in zinc coating of carbon steel (galvanizing) which results in a product with much better corrosion resistance than that of the underlying carbon steel itself. Among the uses of galvanized steel is the fabrication of electronic equipment frames and housings. In electronics itself, zinc sees extensive use as an alloying element, particularly for copper, with subsequent utilization in contact, connector, and piece part applications.It is a relatively common situation that physical and chemical data on the outdoor corrosion of a metal is relatively abundant, while that for indoor corrosion is virtually or entirely absent. Zinc is one of the few exceptions to this rule, and its study thus serves in part as a template for examining indoor-outdoor corrosion relationships. In the case of outdoor exposures, both surface mineral analyses and extractable surface ion analyses are available. For indoor exposures, extensive studies of the surface ions have been performed. These data permit, at least to some extent, the study of zinc corrosion at different temperatures, humidities, and trace gas and particle burdens.While considerable experience has been ...

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“…Pongamia glabra (Vent); Derris indica (Lamk.)] which belongs to family Leguminosae and sub-family Papilionaceae [2]. It is a medium sized glabrous, perennial tree grows in the littoral regions of South Eastern Asia and Australia [3].…”

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