On the mechanisms for surface and bulk embrittlement of AA2024 and AA2198 aluminium alloys, following hydrogen exposure

In order to investigate the mechanism of embrittlement of two high-strength aluminium alloys, AA2024-T3 and AA2198-T8, a number of tensile and compact tension specimens were subjected to two hours of immersion in an exfoliation corrosion (EXCO) solution. Some of these samples had previously been subjected to different levels of pre-stretching, before immersion. It was found that, for the Al-Cu-Li alloy AA2198, a maximum of tensile embrittlement was observed at a low pre-stretching level of 1.5 %. At this level, a maximum was also observed with regard to the surface crack density observed, following testing. The surface cracking was shallow and intergranular, but the primary fracture surfaces were found to be ductile with multi-void coalescence. Scanning electron microscopy work demonstrated that, following EXCO exposure, a thin (~ 100 µm) subsurface layer of intergranular attack was present. A degassing heat treatment significantly reduced the diffusible hydrogen, but the intergranular surface cracking remained. A mechanism is proposed to explain the embrittlement observed both on the surfaces and in the bulk of the alloys.