Anion effects on the stress corrosion cracking behaviour of aluminium alloys

The stress corrosion cracking behaviour of plate material of the aluminium alloys 2024‐T351, 8090‐T8171, 7475‐T651, and 7075‐T7351 was investigated performing constant load tests. Short transverse tensile specimens were permanently immersed in aerated aqueous 0.6 M Na₂Cl solutions with additions of Na₂SO₄, NaNO₃, NaHCO₃, NH₄HCO₃, Na₂HPO₄, Na₂SO₃ or Na₂CO₃. The concentration of the added salts was 0.06 M. The applied stress was 100 MPa, except with 7075‐T7351 specimens, which were loaded at 300 MPa. Environment induced failure was not observed in neutral 0.6 M NaCl solution. The various salts added promoted intergranular stress corrosion cracking with the alloys 2024‐T351, 8090‐T8171, and 7475‐T651. Threshold stresses were generally below 100 MPa. For 8090‐T8171 exposed to chloride containing electrolytes with additions of sulfate, hydrogen phosphate, or sulfite, threshold stresses were approximately 100 MPa or higher. Similar results were obtained for 7475‐T651 plate when immersed in chloride‐hydrogen phosphate and chloride‐carbonate solutions. Alloy 7075‐T7351 was resistant against intergranular stress corrosion cracking. Specimens suffered pitting corrosion during immersion in the corrosive environments. Failure observed with 7075‐T7351, in particular when exposed to the chloride‐nitrate solution, was associated with reduction of cross‐sectional area due to pitting and transgranular stress corrosion cracking.