Enhancement of the Stress Corrosion Sensitivity of AA5083 by Heat Treatment
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AA5083 aluminum alloy is a medium-strength, non-heat-treatable aluminum alloy that has been widely used in marine structures because of its good corrosion resistance and weldability.[1] However, as illustrated in the phase diagram shown in Figure 1, the microstructure is unstable with respect to precipitation of an ordered intermetallic Mg2Al3 known as b-phase, which precipitates on heterogeneous nucleation sites. The low melting points of aluminum and the principal alloying element Mg creates a situation where substantial diffusion of Mg can be expected at temperatures approaching room temperature. When the expected phase segregation occurs, AA5083 is said to be ‘‘sensitized’’ and becomes susceptible to SCC in aqueous NaCl solutions and sea water. Since b-phase is electrochemically more active than the alloy matrix and will dissolve preferentially in a corrosive environment, it is natural to expect that larger amounts of b-phase precipitation will increase the documented susceptibility to intergranular SCC[2] provided that the b-phase can contact the environment. In this sense, tempers of AA5083 that JIE GAO, Graduate Student, and DAVID J. QUESNEL, Professor, are with the Mechanical Engineering Department, University of Rochester, Rochester, NY 14627. Contact e-mail: quesnel@me. rochester.edu Manuscript submitted October 23, 2009. Article published online August 10, 2010 356—VOLUME 42A, FEBRUARY 2011
promote homogeneous precipitation of b-phase internal to the grains are less sensitive to SCC than those tempers that promote grain boundary precipitation. Surprisingly, intergranular SCC occurs even at the early stages of precipitation of b-phase on grain boundaries, causing Jones et al.[3–5] to suggest that hydrogen embrittlement is responsible for the fracture of b-phase-free portions of grain boundaries between the regions covered by films of b-phase known to fail by anodic dissolution. To examine this in more detail, the present work systematically investigates the initial crack growth rate and the incubation time to help shed more light on the relative contributions of hydrogen embrittlement and anodic dissolution. Before embarking, however, a review of the early literature shows that Dix et al.[7] systematically characterized the correlations among sensitization processes, grain boundary precipitates, and susceptibility to SCC of non-heat-treatable aluminum-magnesium alloys with different magnesium contents. The results from corresponding free corrosion potential measurements indicated that the grain boundary precipitates were highly anodic to the alloy matrix and suggested a dissolution-based mechanism for SCC in aluminum-magnesium alloys. This mechanism was further examined by other researchers from various points of view.[8–16] Still lacking, however, is a comprehensive investigation on the specific role of the developing microstructure in determining the susceptibility of AA5083 alloy to SCC, as well as a detailed SCC METALLURGICAL AND MATERIALS TRANSACTIONS A
mechanism that occurs locally. A single mechanism
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