Optimum trace copper levels for SCC resistance in a Zn-modified Al-5083 alloy

As β-phase formation has long been held accountable for stress-corrosion cracking (SCC) susceptibility, studies have been carried out in an effort to determine if minor additions of elements can potentially modify the characteristics of the grain boundary β-phase that forms. A series of alloys were cast that contained various levels of zinc, copper, and silver, in order to classify the nature of the grain boundary phase in the presence of these additional elements. In order to evaluate any measurable benefit to SCC resistance, tensile tests were carried out on alloys that were subjected to an extensive sensitization treatment, under slow strain rate conditions, in an aqueous NaCl environment. Under these conditions, it has been shown that zinc modifications alone are severely degrading to SCC resistance, while minor copper additions along with zinc can have a beneficial effect. The benefits exist in a very narrow window, however, as larger copper additions can have the undesirable additional effect of precipitating as corrosion-prone S-phase.