On the stress corrosion cracking behaviour of 6XXX series aluminium alloys

The stress corrosion cracking behaviour of 6061 and 6013 sheet materials was investigated in naturally aged and peak-aged heat treatment conditions. Constant deformation, constant load and constant extension rate tests were carried out using aqueous solutions of 0.6 M NaCl and 0.6 M NaCl + 0.06 M NaHCO(3). The microstructure of the alloys was examined using transmission electron microscopy. Potentiodynamic polarisation measurements were performed in aerated and deaerated aqueous chloride and/or bicarbonate solutions to determine corrosion and breakdown potentials. Under constant deformation and constant load conditions, the alloys 6061 and 6013 stressed in transverse direction were resistant to stress corrosion cracking in both T4 and T6 tempers. The constant extension rate testing technique indicated environment-induced cracking with alloy 6061-T4, whereas deterioration in ductility of the other alloys was caused by pitting and intergranular corrosion. Sensitivity to intergranular corrosion did not cause stress corrosion cracking in 6013-T6 sheet.