Intergranular corrosion and Mg enrichment of AA 5083

Mg is commonly added to increase the strength of lightweight non-heat-treatable Al-Mg alloys. However these alloys become highly susceptible to grain boundary precipitation of β-Al 3 Mg 2 phase and enrichment of magnesium oxide on the metal surface induced by the alloy fabrication process and/or by the service conditions (i.e. thermal cycles). Transmission electron micrographs (TEM) of specimens sensitized at 200oC revealed elongated Mg-rich precipitates along the grain boundary line, which are preferred sites for anodic oxidation, giving rise to intergranular corrosion (IGC). Auger electron spectroscopy (AES) revealed increased yield of Mg and O, suggesting enrichment of Mg oxide on the alloy surface. Thus, the electrochemical behavior of the surface becomes affected strongly by the presence of Mg. In addition the presence of precipitates at grain boundaries also contributes to increase the number of defective areas on the protective film, giving rise to different open circuit potential (OCP) behaviors and surface morphologies for varied sensitizing conditions. Intergranular corrosion and Mg surface enrichment are discussed in the light of three main aspects: oxidation behavior and properties of the resultant oxide layer, characteristics of the grain boundaries, and grain boundary precipitation.