LOCAL CHEMISTRY OF STRESS-CORROSION CRACKING IN Al-Li-Cu-Mg ALLOYS

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Abstract
The initiation of stress-corrosion cracking in Al-Li-Cu-Mg alloys is subtly affected by the chemistry of the environment. Aerated 3.5% NaCl solution is unable to initiate cracks under total immersion conditions, but removal from the solution after a couple of days can lead to a rapid failure. This "pre-exposure" effect is consistent with absorption of CO2 into alkaline intergranular fissures, creating a critical balance between active dissolution and passivation by lithium aluminate. The proposed local environment has been simulated macroscopically and shown to cause rapid SCC in simple immersion tests. Sulphate additions inhibit the intergranular corrosion seen in NaCl solutions, and can induce SCC at very low concentrations (e.g. 0.012M in 0.6M NaCl). In dilute chloride solutions, simulating natural fresh waters, either bicarbonate or sulphate at an appropriate level is required for SCC initiation. In all these solutions, both acidic and alkaline crack chemistries are possible, depending on the degree of separation of anode and cathode. The alkaline chemistry is unique to alloys containing lithium

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