Effect of Solution Aggressiveness on the Crack Growth Resistance and Cracking Mechanism of AA2024-T3

Abstract

Aluminum alloy 2024-T3 was examined using a range of microscopy techniques at the early stages of corrosion attack to investigate the corrosion-induced cracking mechanism. Two different corrosive environments—exfoliation corrosion (EXCO) and 3.5 wt% NaCl—were used for the exposure of tensile and prenotched compact-tension C(T) specimens of AA2024-T3. Different embrittlement mechanisms are noticed for the two investigated corrosive environments. Significant intergranular corrosion (IGC) and grain boundary embrittlement are evident in the specimens exposed to EXCO solution, whilethis was not the case for the milder solution comprising of 3.5 wt% NaCl. With regard to the milder solution, the corrosion attack is not only restricted to the grain boundary, but evolves transgranularly to the neighboring grains of the IGC attacked region and, consequently, the grain boundary strength in the direct vicinity is not notably affected. The extent of secondary cracks, after the exposure of C(T) specimens to EXCO solution and the subsequent crack-growth resistance evaluation, were found to correlate with the diameter of the plastically affected zone (≈3.78±0.04 mm). Additionally, the depth of these cracks was found to correlate well with the thickness of the intergranular fracture surface, giving evidence that the secondary cracks form due to grain boundary embrittlement; probably attributed to hydrogen embrittlement phenomena.