Intergranular corrosion and stress corrosion cracking of an aged AlCu alloy in 1 N NaCl solution

The susceptibility of an aged pure AlCu alloy with 4 wt% Cu to intergranular corrosion (IGC) and to stress corrosion cracking (SCC) has been studied in 1 N NaCl solution. For this purpose the time to failure of thin flat tensile samples under constant load has been determined in dependence of the electrode potential and compared with the electrochemical behaviour of unstressed samples. Below the pitting potential of pure aluminum in 1 N NaCl solution the tensile specimens never failed. Above this potential, failure did occur, and time to failure decreased with increasing electrode potential. Unstressed samples showed intergranular attack at potentials at least 40 mV higher than the pitting potential of pure aluminum. Metallographic examinations showed that mechanical stress accelerates the intergranular attack by a factor 10 to 100. Current densities calculated from the velocity of penetration under stress are in good agreement with values known from the pitting of pure aluminum. From the results obtained in this work the conclusion is that SCC of the aged AlCu alloy can be regarded as stress accelerated IGC.