The Degradation of Corrosion Resistance for Al 5083 Alloy after Thermal and Superplastic Forming Processes

Abstract

Corrosion behavior, particularly the intergranular corrosion susceptibility of a superplastic Al 5083 alloy (denoted as Al 5083S) and a non-superplastic Al 5083 alloy (denoted as Al 5083N) with various thermal processes and a superplastic forming process, has been systematically evaluated. The nitric acid mass loss test (NAMLT) according to ASTM G 67 indicated that the weight loss of Al 5083S was larger than that of Al 5083N, which was due to the finer grain size in the former alloy. It also showed that superplastically formed specimens of Al 5083S and the specimens of Al 5083S and Al 5083N treated with the same thermal process as the superplastically formed specimens suffered from severe intergranular corrosion. The serious intergranular corrosion of these specimens was attributed to the formation of continuous β (Mg2Al3) precipitates at grain boundaries, i.e., the sensitization effect. Such a detrimental effect can be eliminated by a postforming annealing treatment at 345 °C for 1 h. Furthermore, electrochemical measurements in a 3.5 wt.%NaCl solution also revealed that the sensitized specimens possessed more active corrosion potential (Ecorr), breakdown potential (Eb), and protection potential (Epp), as well as higher corrosion current density (icorr) and passive current density (ip), than those of the as-received specimens. Experimental results also showed that the corrosion resistance of the superplastically formed specimen was the worst among all specimens, which was attributed to the formation of cavities during the superplastic forming in addition to the sensitization effect caused by the thermal processing. The influences of both detrimental effects on the corrosion resistance of the Al 5083 specimens were also discussed.