ENVIRONMENTAL INDUCED CRACKING in Al-Li-Cu-Mg-Zr ALLOYS of PEAK AGED and RETROGRESSED and REAGED TEMPERS UNDER APPLIED POTENTIALS

Abstract

Stress corrosion cracking (SCC) behaviour of 8090 and 1441 Al-Li-Cu-Mg-Zr alloys of peak aged T8, over aged T7 and retrogression and reaged (RRA) T77 tempers has been studied by slow strain rate test (SSRT) in 3.5 % NaCl and in 3.5 % NaCl + 0.1M LiCl + 0.7 % H2O2 environments. The results show that a small addition of LiCl and H2O2 to NaCl makes the media susceptible to SCC. T8 temper has been found to be the most SCC susceptible, T7 temper the least, and RRA T77 tempers are intermediate to T7 and T8 tempers. SSRT under applied potentials in 3.5 % NaCl solution shows that the alloy tempers are immune to SCC at free corrosion potential and at small cathodic overvoltages, but at higher cathodic potentials, hydrogen embrittlement (HE) sets in. The applied anodic potentials are found to be severely damaging, and local anodic dissolution (LAD) is believed to be associated mechanism for the SCC damage. RRA tempers are found to be more prone to HE compared to T8 temper and this has been explained on the basis of microstructural variation observed by TEM, XRD and DSC.