Mechanisms of environmentally induced crack initiation in humid air in new generation Al-Zn-Mg-Cu alloys

Recent experience has shown that new-generation 7xxx-series alloys, that have a high Zn content and Zn/Mg ratios, have a greater susceptibility to Hydrogen-Environmental Induced Cracking (H-EIC) on exposure to humid air than more established materials, like AA7050. In this study we report new evidence of the EIC initiation and crack growth behaviour of two new generation alloys, AA7085 and AA7449, when exposed to 50% humidity. In-situ, time lapse, optical imaging over large areas has enabled the exact initiation sites to be identified and investigated with high-resolution fractographic studies, providing evidence for the sequence and mechanisms of initiation and transition to sustained cracking. A consistent behaviour was observed for both alloys. This has revealed that only minute-scale corrosion reactions, involving highly localised condensed water, are necessary for initiation. The preferred initiation sites are metal ligaments between surface-connected pore-clusters and/or intermetallic particles that are subjected to high stress concentration and undergo mechanical damage with associated higher levels of local oxidation. The growth of short proto-cracks from these sites is a distinct stage and displays intermittent arrest markings evidenced by localised corrosion. In contrast, in humid air environments, long cracks in these alloys exhibited relatively constant, higher velocity, with extremely limited corrosion commensurate with oxidation of a free surface in this environment resulting in ~5 nm oxide layer.