Anodic dissolution-based mechanism for the rapid cracking, preexposure phenomenon demonstrated by aluminum-lithium-copper alloys

Stress corrosion cracking (SCC) test specimens of Al-Li-Cu alloy that were subjected to fixed-displacement loading and exposed to aerated 3.5 wt% sodium chloride (NaCl) solution for 1 week failed < 24 h after removal into ambient atmospheres. Anodic dissolution-based mechanisms proposed previously for this phenomenon were amended based upon further characterization of the rapid cracking process. Amendments were based on studies of the relative electrochemical behavior of the microstructural elements in the subgrain boundary (SGB) region, time-to-failure SCC testing in a simulated crack solution, evolution of crack potential and pH with time, fractographic examination of failed samples, and x-ray diffraction (XRD) of films passivating crack walls. Results suggested an active path existed along SGB that was composed of the highly reactive T₁ (Al₂CuL₁) precipitate phase and a solute-depleted zone that did not passivate readily when exposed to the crack environment. The matrix phase along crack walls appeared to passivate in the crack environment, thereby confining attack to the SGB region. This active path was enabled when cracks were isolated from a bulk environment, but it was disabled otherwise. Potential and pH conditions required for cracking were reviewed, along with the formation of a hydrotalcite, Li₂[Al₂(OH)₆]₂·CO₃·3H₂O, film that appeared to be responsible for passivating crack walls.