Microstructure and Corrosion Behaviour of Aerospace Aluminium Alloys

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Abstract

In the present work, the relationship between the microstructure and the corrosion behaviour of Al-Cu-Mg and Al-Cu-Li alloys with various thermomechanical conditions has been studied.The microstructural characterization of the AA2024-T351 Al-Cu-Mg alloy revealed that the constituent intermetallic particles, assigned as S-phase (Al2CuMg), θ-phase (Al2Cu) and α-phase (Al-Cu-Fe-Mn-(Si)), are present individually or together as clusters. Further, precipitates along with Mg / Cu segregations were detected along the grain boundary network. It was also revealed that the cold working to obtain T351 temper resulted in the heterogeneous distribution of grain-stored energy.It was revealed that selective dissolution of Mg from the S-phase particle results in the copper-rich S-phase remnant, which contributes to the conversion of its electrochemical property. As a result, the micro-galvanic coupling between the S-phase remnant and the alloy matrix leads to the development of trenching at its adjacent. Trenching was also found in the periphery of the θ-phase particle and the α-phase particle, due to their more positive electrode potentials relative to that of the alloy matrix. Further, selective dissolution also occurred in the θ-phase and α-phase particles, resulting in the development of porous banding structure along certain orientation. Localized corrosion in the AA2024-T351 aluminium alloy preferentially propagated in the form of intergranular corrosion at the early stage of the exposure to sodium chloride solution. The distribution of grain-stored energy significantly affects the development of intergranular corrosion. With prolonged exposure time, localized corrosion propagated selectively into grain interior, resulting in the development of crystallographic pits. In the corrosion front area, the necessary chemical condition (low pH and chloride rich) was generated and maintained. Meanwhile, a copper-enriched layer beneath the corrosion product layer acted as an effective cathode to support the anodic dissolution. Therefore, the anodic dissolution of aluminium at the corrosion front is self-supported.The 2A97 Al-Cu-Li alloys in different thermomechanical conditions exhibited different amounts and distributions of T1 phase (Al2CuLi) precipitates since the pre-ageing cold working and the ageing condition significantly affect the T1 phase precipitation in Al-Cu-Li alloys. Further, the different thermomechanical histories also resulted in significantly different grain structures in the alloys.The corrosion morphology of the 2A97 alloy is closely associated with the distribution of T1 phase precipitates. In the T3 condition, attacked grain boundaries are the dominant corrosion features since T1 phase precipitates distribute along selective grain boundaries. In the T4 condition, localized corrosion propagated in the form of intergranular corrosion and developed into the grain interior with prolonged immersion time, related to the distribution of T1 phase precipitates. In contrast, crystallographic pits are evident in the grain interior of 2A97-T6 alloy due to the high population density of matrix T1 phase precipitates. Both grain interior and grain boundary were selectively attacked in the 2A97-T8 alloy during the immersion testing, which is consistent with the T1 phase precipitates distribution. Further, it was also found that the selective corrosion behaviour in the 2A97 Al-Cu-Li alloys is closely associated with the heterogeneous distribution of grain-stored energy, with high localized corrosion susceptibility corresponding to high level of stored energy.

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