Investigation on microstructure and corrosion behaviour of 6XXX series aluminium alloys

Microstructure and corrosion behaviour of 6061 and 6013 sheet material were investigated in the naturally aged and peak-aged heat treatment conditions. Transmission electron microscopy did not reveal strengthening phases in the naturally aged sheet. In the peak-aged temper, beta" precipitates were observed in alloy 6061, whereas both beta" and Q' phases were present in 6013-T6 sheet. Marked grain boundary precipitation was not found. Corrosion potentials of the alloys 6061 and 6013 shifted to more active values with increasing aging. For the copper containing 6013 sheet, the potential difference between the tempers T4 and T6 was more pronounced. When immersed in an aqueous chloride-peroxide solution, alloy 6061 suffered predominantly intergranular corrosion and pitting in the tempers T4 and T6, respectively. On the contrary, 6013 sheet was sensitive to pitting in the naturally aged condition, and intergranular corrosion was the prevailing attack in the peak-aged material. Both alloys 6061 and 6013 were resistant to stress corrosion cracking in the tempers T4 and T6.