Hydrogen embrittlement susceptibility of AA7075 aluminum alloy subjected to continuous retrogression and re-aging treatment in an aggressive acidic chloride environment

This study investigates the hydrogen embrittlement (HE) susceptibility of AA7075 aluminum alloy subjected to four continuous retrogression and re-aging (RRA) heat treatments in an aggressive acidic environment (3.5 wt% NaCl + 3 g/L NH₄SCN, pH 1.2). Susceptibility was assessed via incremental step-loading tests (ISLT) on hydrogen pre-charged specimens, supported by potentiodynamic polarization, transmission electron microscopy (TEM), selected-area electron diffraction (SAED), and energy-dispersive X-ray spectroscopy (EDS). Among the conditions evaluated, the stepped aging regime of 110 °C/5 h followed by 170 °C/14 h (RRA3) achieved the most favorable property balance, retaining 90% of baseline threshold stress and 97% of fracture elongation after hydrogen exposure — representing a ~ 22% improvement in strength index over the T651 reference. Potentiodynamic polarization confirmed superior electrochemical stability for RRA3, with corrosion current density approximately one order of magnitude lower than the least resistant condition. Microstructural analysis revealed that RRA3 promoted a discontinuous grain boundary precipitate morphology, widened precipitate-free zones, and Zn/Cu-enriched grain boundary phases, collectively suppressing hydrogen diffusion and intergranular decohesion. These results demonstrate that tailored continuous RRA processing can effectively decouple the inverse relationship between strength and hydrogen embrittlement resistance in AA7075.