  Hydrogen embrittlement (HE) affects all major high-strength structural
materials and as such is a major impediment to lightweighting e.g. vehicles and
help reduce carbon-emissions and reach net-zero. The high-strength 7xxx series
aluminium alloys can fulfil the need for light, high strength materials, and
are already extensively used in aerospace for weight reduction purposes.
However, depending on the thermomechanical and loading state, these alloys can
be sensitive to stress-corrosion cracking (SCC) through anodic dissolution and
hydrogen embrittlement. Here, we study at the near-atomic-scale the intra- and
inter-granular microstructure ahead and in the wake of a propagating SCC crack.
Moving away from model cases not strictly relevant to application, we performed
an industry-standard test on an engineering Al-7XXX alloy. H is found
segregated to planar arrays of dislocations and to grain boundaries that we can
associate to the combined effects of hydrogen-enhanced localized plasticity
(HELP) and hydrogen-enhanced decohesion (HEDE) mechanisms. We report on a
Mg-rich H-rich amorphous oxide on the corroded crack surface and evidence of
Mg-related diffusional processes leading to dissolution of the strengthening
eta-phase precipitates ahead of the crack. We show ingress of up to 1 at% O,
i.e. well above the solubility limit of O in Al, near the oxide-metal
interface, while no increased level of H is found in the matrix. We provide an
array of discussion points relative to the interplay of structural defects,
transport of solutes, thereby changing the resistance against crack
propagation, which have been overlooked across the SCC literature and prevent
accurate service life predictions.
