Chloride Ion Activity and Susceptibility of Al Alloys 7075-T6 and 5083-H131 to Stress Corrosion Cracking

The influence of chloride ion activity on the susceptibility of aluminum alloys 5083-H131 and 7075-T6 to stress corrosion cracking (SCC) was investigated by conducting slow strain-rate tensile tests at a strain-rate of 10⁻⁷ s⁻¹ in naturally aerated aqueous solutions with varying NaCl mass fraction (0.001 to 20 pct) and in a 3.5 pct mass fraction NaCl solution with varying strain-rates (10⁻⁸ to 10⁻⁴ s⁻¹). This study found that both alloys exhibited reduced strengths and failure strains (times) in the solutions compared with laboratory air. The extent of these reductions was greater in alloy 5083 for the conditions examined. The strength and ductility of both alloys decreased with chloride ion activity in a manner that indicates a chemical reaction is responsible. The strength and ductility of both alloys decreased with strain-rate in a sigmoidal manner, but the transition in alloy 7075 occurred at slower strain-rates of approximately two orders of magnitude. It was deduced that the chloride ion interacts chemically with the passivated surface in the potential gradient at the crack tip to cause SCC. While no mechanism of cracking can be eliminated on the basis of these results alone, the results are consistent with the hypothesis that the absorbed hydrogen causes cracking in alloy 7075 while cracking in 5083 is the result of a dissolution mechanism.