Corrosion Mechanisms for Aluminum Exposed to the Atmosphere

Upon exposure to the indoor or outdoor atmosphere and during some manufacturing processes, aluminum degrades with the formation of a variety of corrosion products. This paper relates those products to the atmospheric constituents that are their precursors, explores the chemical processes involved in corrosion product formation, and evaluates the magnitudes of the sources for the major corrosive anions. A three‐layer structure is identified: the aluminum itself and its native oxide, the corrosion layer atop the oxidized aluminum, and a surface contamination layer. At low to moderate humidities, this last layer can be pictured as a moist agglomeration of deposited particles and common ionic compounds. At high humidities or in circumstances where active precipitation is falling on the surface, this top layer incorporates a dozen monolayers of water and becomes a slurry of deposited particles in an electrolyte solution. Sulfate species have been found to be the most abundant indoor and outdoor constituents of the corrosion layers; they are shown to arise primarily from sulfur dioxide incorporation and from sulfate is precipitation in the outdoor environment and from sulfate on deposited fine particles indoors. Chloride species are the next most common corrosion product; the indoor sources of chloride onto surfaces are processing residues or reactive gases and airborne particles, and the primary outdoor sources are reactive gases, airborne particles, and precipitation. Three processes are assessed in connection with chemical reactions in the aqueous layer atop the exposed aluminum surface: (i) high acidities are shown to be responsible for the supply of aluminum ions to the surface water layers by dissolution of the lattice; (ii) formation of insoluble amorphous aluminum sulfate is favored outdoors over crystalline aluminum hydroxysulfate formation because of the chemical complexity of the naturally formed crystalline compounds and the freezing out of solution constituents by repeated wet‐dry cycles; and (iii) corrosion products containing chlorine are formed both indoors and out by chloride ion replacement of hydroxide moieties in the aluminum oxyhydroxide surface layer.