Transition from Localized Corrosion to Stress Corrosion Cracking in an Al-Li-Cu-Ag Alloy

The early stages of stress-assisted corrosion and stress corrosion crack (SCC) initiation from machine-notched aluminum alloy (AA) 2096 (UNS A92096) specimens, lacking prefatigue cracks, were investigated. Double-notched tensile specimens of an extremely SCC-susceptible underaged (i.e., 4-h artificially aging at 160°C) temper of AA2096 were exposed in an alternate immersion (A/I) 0.6 sodium chloride (NaCl) environment. Focused ion beam (FIB) sectioning/secondary electron imaging methods were used to study and document the earliest stages of pit, intersubgranular corrosion, corrosion fissure, and SCC development starting from LT surfaces. SCC in machine-notched AA2096 involved several stages: trenching of the alloy matrix along bands of constituent particles, constituent particle-induced pit or trench coalescence, elongated corrosion fissure growth initially by general dissolution on the length scale of subgrains and the transition to intersubgranular attack, followed by the initiation/transition to intergranular SCC. In this work, FIB sectioning to surgically produce high-precision cross sections between constituent particles in stressed alloys revealed that early trenching between particles did not produce intersubgranular attack. Intersubgranular attack occurred within 50-μm to 100-μm deep corrosion fissures after 100 h of A/I exposure in conjunction with considerable general, intragranular dissolution. Both intersubgranular and high-angle boundary SCC, lacking evidence of substantial dissolution, were observed within well-developed (i.e., >100 h A/I exposure) corrosion fissures. These results point to the need for a critical corrosion fissure depth, externally applied stress, susceptible alloy temper, and possible critical localized corrosion site chemistry to produce sharp intersubgranular cracking that enabled transition to intergranular SCC in this alloy.