Slow Strain Rate Testing of Aluminum Alloy 7050 in Different Tempers Using Various Synthetic Environments

The slow strain rate testing (SSRT) technique was used to investigate the stress corrosion cracking (SCC) behavior of aluminum alloy Al 7050 (UNS A97050) in different tempers in various electrolytes at the free-corrosion potential. Smooth tensile specimens were strained dynamically in the short transverse direction under permanent immersion conditions. Strain rates were from 5 x 10^–8/s to 1 x 10^–4/s. Using substitute ocean water, Al 7050 was found sensitive and immune to environmentally assisted cracking in the peak-aged temper T651 and in the overaged temper T7351, respectively. In the less-overaged heat treatment T7651, fracture energy data revealed a large scatter. An aqueous solution of 0.5 M sodium perchlorate (NaClO₄) was not conducive to environmentally assisted cracking. SSRT performed in an aqueous solution of 0.1 M sodium chloride (NaCl) + 0.05 M sodium sulfate (Na₂SO₄) + 0.05 M sodium nitrate (NaNO₃) + 0.01 M sodium bicarbonate (NaHCO₃) at pH 3.5 indicated SCC susceptibility for Al 7050-T651. The latter electrolyte did not promote SCC with the alloy in the heat treatments T7651 and T7351. Scatter was observed in the fracture energy data of Al 7050-T7351 specimens dynamically strained in the mixed salt solution. Deterioration was attributed to pitting attack, as supported by fractography.