Characterization of 6111-like aluminum alloys using electrochemical techniques and electron microscopy

Aluminum and aluminum alloys are susceptible to localized corrosion in corrosive environments, particularly in halide-containing solutions. However, the mechanism(s) through which corrosion occurs on Al-Mg-Si alloys with or without Cu addition remained unclear. This dissertation reports on the investigation of pitting and IGC susceptibilities of three 6111-like aluminum alloys with 0%, 0.68%, and 1.47%Cu. The electrochemical behavior of the alloys was evaluated using open circuit exposure, conventional polarization techniques, and electrochemical noise technique. Scanning electron microscopy (SEM) was used to characterize the corrosion morphologies of the alloys. Transmission electron microscopy (TEM) was employed to characterize the alloys' microstructures. Scanning transmission electron microscopy (STEM) and EDS nano-profiling were used to study the grain boundary characteristics of the three alloys.; All three alloys are susceptible to pitting and IGC attack in chloride-containing electrolytes. Electrochemical noise measurements revealed that the induction time for localized corrosion to initiate was often on the order of hours if not less, even in solution containing merely 5 mM NaCl. Regardless the heat treatment conditions and degree of polarization, the 6111-like alloy with 0%Cu exhibited crystallographic tunneling morphology. Over-aging treatment transformed the nature of intragranular attack on the Cu-containing 6111-like alloys from the hemispherical micropitting to a mixture of hemispherical micropitting and crystallographic tunneling. STEM/X-ray microanalysis yielded some significant findings on the three as-received alloys, which were never reported before. 30∼70 run wide Mg and Si depleted regions and 10∼40 nm wide Cu-depleted regions along some high angle grain boundaries (GB) have been detected in the Cu-free and Cu-containing 6111-like alloys, respectively. The presence of an Al-Cu-Mg-Si phase at some high angle GBs of the Cu-containing alloys has been observed unequivocally for the first time. Such findings have experimentally confirmed for the first time some of the assumptions made by others regarding the IGC susceptibility of 6XXX aluminum alloys, which was based on the noble GB precipitates and adjacent solute depleted regions. As a result, a direct link between the GB elemental profiles and the alloys' IGC susceptibility has been established for the first time on Al 6XXX alloys.