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 I week failed < 24 h after r emoval into ambient atmospheres. Anodic dissolution-based mechanisms p roposed 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 microstruc tural elements in the subgrain boundary (SGB) region, time-to-failure SCC testing in a simulated crack solution, evolution of crack potentia l and pH with time, fractographic examination of failed samples, and x -ray diffraction (XRD) of films passivating crack walls. Results sugge sted an active path existed along SGB that was composed of the highly reactive T-1 (Al(2)CuL(i)) precipitate phase and a solute-depleted zon e that did not passivate readily when exposed to the crack environment . The matrix phase along crack walls appeared to passivate in the crac k environment, thereby confining attack to the SGB region. This active path was enabled when cracks were isolated from a bulk environment, b ut it was disabled otherwise. Potential and pH conditions required for cracking were reviewed, along with the formation of a hydrotalcite, L i-2[Al-2(OH)(6)](2) . CO3 . 3H(2)O, film that appeared to be responsib le for passivating crack walls.