Grain boundary characteristics and their correlation to intergranular corrosion of Al-Mg-Si alloy with trace level content of Cu

The influence of grain boundary misorientation on the intergranular corrosion resistance of Al-Mg-Si at low Cu content (0.04 wt%) was investigated in this work. Intergranular corrosion arises due to the nonuniform chemical composition along the grain boundaries, which leads to differences in potential between grain boundaries and grain interior.

Scanning Electron Microscope (SEM), Electron Backscatter Diffraction (EBSD), Energy Dispersive Spectroscopy (EDS), and Scanning/Transmission Electron Microscopy (S/TEM) are employed to investigate the microstructure and the role of grain misorientation on IGC.

Results obtained in this work are relevant in a better understanding of grain boundary structures in Al-Mg-Si alloys at low Cu content and their susceptibility to IGC. Susceptibility of different categories of grain boundaries to intergranular corrosion was investigated in an aqueous solution containing 10 ml/L concentrated hydrochloric acid and 30g/L NaCl under different immersion times. The results showed dual microstructure of the extruded profile, recrystallized layer on the top and bottom surface, and fully deformed layer in the center. It is also found that misorientation angle has a considerable influence on intergranular corrosion of Al-Mg-Si even at this level of Cu content. High angle grain boundaries are at the highest risk to IGC for immersion time around 24h or less. However, increasing the immersion time for 72 h showed that all types of grain boundaries were subjected to severe intergranular corrosion. Moreover, low angle grain boundaries and coincidence site lattice (CSL) were subjected to a certain degree of ICG even for immersion time less than 24h.