Corrosion-induced hydrogen embrittlement in aluminum alloy 2024

Abstract

Corrosion is a major concern to the structural integrity of aging aircraft structures. The effect of corrosion on the damage tolerance ability of advanced aluminum alloys calls for consideration of the problems associated with the combined effect of corrosion and embrittling mechanisms. The present paper focuses on the observed corrosion-induced embrittlement of 2024 alloy and tries to answer the key question on whether the observed embrittlement is attributed to hydrogen uptake and trapping in the material. The experimental procedures involved: (1) accelerated corrosion testing (EXCO), (2) microstructural investigation of the evolution of corrosion damage (3) hydrogen measurements (4) fractographic analysis of tensile specimens. Corrosion damage in the alloy initiates with pitting and develops to a network of intergranular corrosion leading to exfoliation of material. Hydrogen is produced during the corrosion process and is being trapped in distinct energy states, which correspond to different microstructural sites. These traps are activated and liberate hydrogen at different temperatures. In alloy 2024, four traps T1 to T4 were identified. Trap T1 is considered to be a reversible trap, which liberates hydrogen continuously at low temperatures. Traps T2, T3 and T4 saturate with exposure time and are considered to be irreversible. The hydrogen front advances with the corrosion front, so hydrogen penetrates in the material through the intergranular paths generated by the corrosion process. Then hydrogen diffuses further in the material establishing a hydrogen affected zone beneath the corrosion zone. Removal of the corrosion layer (equal to the depth of attack) leads to complete restoration of yield strength but only partial restoration of ductility. Removal of the corrosion layer and heating above the T4 activation temperature for hydrogen desorption (to activate all traps) leads not only to complete restoration of strength but also to complete restoration of ductility. Fractographic analysis shows the existence of a quasicleavage transition zone between the intergranular corrosion zone and the ductile corrosion-unaffected material. This quasicleavage zone is embrittled by hydrogen diffusion and trapping. These results constitute evidence of hydrogen embrittlement in Al-alloy 2024. Today's aircraft design and maintenance follow the damage tolerance methodology. The present paper sheds light at the degradation of ductility due to the corrosion-induced hydrogen embrittlement, which reduces the damage tolerance of the structure. These findings are particularly important for the so-called "aged aircraft", which has exceeded or is near the operational lifetime, but it is still operated by the airlines. If it is decided to continue the operation of such aircraft, a re-determination of lifetime based on the locally degraded material properties appears essential.