On the Electrochemical and Quasi In Situ Corrosion Response of the Q-Phase (Al_xCu_yMg_zSi_w) Intermetallic Particle in 6xxx Series Aluminum Alloys

The electrochemical response and corrosion associated with the Q-phase (Al_xCu_yMg_zSi_w) intermetallic compound was studied. Q-phase intermetallics are the principal strengthening phase in a number of Cu-containing 6xxx series (Al-Mg-Si) alloys, and Q-phase has not previously been uniquely studied in regard to its influence on localized corrosion in detail. Herein, quasi in situ scanning transmission electron microscopy was utilized in understanding the localized corrosion response associated with nanoscale Q-phase precipitates in a Cu-containing 6xxx series Al alloy sheet with a composition (in wt%): 97.3Al-0.9Si-0.74Mg-0.84Cu-0.08Fe-0.14Mn. Furthermore, the Q-phase compound was also produced on the microscale within a bulk-synthesized alloy. The electrochemical behavior of the microscale Q-phase was studied using a micro-electrochemical capillary cell and by potentiodynamic polarization. Quasi in situ scanning electron microscopy was also performed on the bulk alloy synthesized to contain microscale Q-phase, revealing that Q-phase was comparatively noble relative to the matrix and other intermetallic phases present—even following extended immersion in 0.1 M NaCl. The work herein also indicates that Q-phase undergoes incongruent dissolution/dealloying, with fine nanoscale Cu particles, containing multiple twins, were observed upon Q-phase following a period of corrosion.