Effects of stress on intergranular corrosion and intergranular stress corrosion cracking in AA2024-T3

High strength AA2024-T3 alloy has been widely used in aircraft structure application because of its high ratio of strength to weight and uniform corrosion resistance. However, the pronounced susceptibility to intergranular corrosion (IGC) and intergranular stress corrosion cracking (IGSCC) in this alloy seriously limits their lifetime as engineering materials. The propagation kinetics of cracks along the grain boundaries and relationship of IGC and IGSCC are critical to predict the structure lifetime. Conventional investigation methods constrained a single crack per specimen can not characterize and evaluate multiple IGC cracks exist in real structures. In this study, a number of specially designed approaches have been developed to allow better understanding of the effects of applied and residual tensile and compressive stresses on growth kinetics of IGC and IGSCC in AA2024-T3. The stressed foil penetration technique and microfocal x-ray radiography along with electrochemical measurements are primarily used in this study.; The breakdown potential was lower and the passive current density was higher under tension compared to unstressed samples. Tensile stress had a larger effect on penetration rate at potentials above the critical breakdown potential than below the breakdown potential, indicating that pitting is less susceptible to the effects of stress than a properly-oriented IGC crevice. The effects of tensile stress on the penetration rates in various orientations were strongly linked to the anisotropic microstructure. A smooth transition in penetration associated with applied stress suggests that the mechanisms of IGC and IGSCC are similar.; Microfocal x-ray radiography analysis resulted in IGSCC kinetics similar to those generated by the foil penetration method. This technique characterized the morphology of individual cracks and generated the kinetics of localized IGC growth, which is not limited to the formation of a single crack and the competition between SCC cracks can be studied. The crack that led to the final failure was often not the fastest growing crack at the early stages of the experiment. Tensile stress along the transverse direction promotes linking of the IGC on the edges of the elongated grains to form a fracture surface that is nominally perpendicular to the transverse stress. Arrest marks were observed on the fracture surface associated with the transition from IGC to IGSCC. The current oscillations suggest that the linking process is discontinuous in nature. (Abstract shortened by UMI.)...