Investigation of the mechanisms affecting corrosion susceptibility of wrought aeronautical aluminium alloys Al-Cu-Li (AA2198) and Al-Cu-Mg (AA2024) for different pre-stretching levels

Abstract

Aluminium (Al) alloy sheets are usually stretched to manufacture aircraft structures with complex geometries. The corrosion susceptibility of AA2198 Al alloy is examined as a function of the extent of pre-stretching, using a wide range of advanced microscopy, electrochemical techniques, and tensile mechanical testing. Intergranular corrosion attack manifested in sub-surface secondary cracking for the 1.5 % pre-stretching level. A mechanism of deformation-induced chemical heterogeneity is believed to be responsible for the resulting intergranular corrosion, in which δ′-phase nucleation following pre-stretching allows for the removal of matrix Li within highly deformed grains, creating a micro-galvanic coupling between neighbouring grains. At pre-stretching levels of 4.0 % and higher, a transition to transgranular corrosion was observed, whilst the electron backscattered diffraction results indicated that recovery occurs at these pre-stretching levels. At 7.5. % pre-stretching level, the charge transfer resistance values were essentially increased due to the segregation of Li to sub-grain boundaries, allowing for more matrix Li removal and, hence, to the more rapid transgranular attack. On the contrary, the level of corrosion degradation of the tensile properties of AA2024-T3, although being more severe than that of the AA2198 alloy, is not essentially influenced by varying the extent of pre-stretching.