Intergranular corrosion and SCC properties of Al-Mg-Si alloy sheets

Abstract

Properties on intergranular corrosion (IC) and stress corrosion cracking (SCC) of three Al–Mg–Si alloys with a fixed Mg content were studied. IC test samples and tensile specimens were prepared from cold rolled sheets of alloy No. 2 (Al–0.7%Mg–0.45%Si), alloy No. 5 (Al–0.7%Mg–1.1%Si) and alloy No. 7 (Al–0.7%Mg–0.45%Si–0.34%Cu), subjected to solution treatment and then aging at 448 K. When immersed in an acid NaCl aqueous solution based on ISO11846–B method, alloys No. 2 and No. 5 were immune to IC, while alloy No. 7 showed a high sensitivity to IC under a wide range from underaging to overaging. Tensile tests by SSRT (slow strain rate technique) in the solution of IC tests were carried out to evaluate the index I of susceptibility to embrittlement as the ratio of reduction in elongation, compared to that in laboratory air. The index I of peak-aged specimens was ranked on the order of alloy No. 5 > No. 2 > No. 7, which disagreed with the tendency of IC susceptibility. Intergranular stresscorrosion cracks occurred in the SSRT tests were attributed to strain-induced APC (active path corrosion) for alloys No. 5 and No. 2, accompanied with preferential breakdown of surface film along grain boundary zones, while for alloy No. 7 to pre-existing APC caused by solute-depleted zone along grain boundaries.