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Role of Microstructure on Corrosion Control of AA2024-T3 Aluminium Alloy
[Thesis]. Manchester, UK: The University of Manchester; 2011.
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Abstract
A heterogeneous microstructure is intentionally developed in AA2024-T3 aluminium alloy during solidification and thermomechanical processes to obtain good mechanical properties. As a consequence, the alloy is susceptible to localized corrosion, which is the major nucleus for onset of stress corrosion cracking and fatigue cracking.In this research, electron microscopy was employed to observe intermetallic particles and their periphery and monitor the initiation and development of intermetallic particle induced localized corrosion in AA2024-T3 aluminium alloy. In-situ optical microscopy, energy dispersive X-ray spectroscopy (EDX), X-ray microtomography and atomic force microscopy were also used to provide supportive evidence.Intermetallic particles with different electrochemical natures and geometries were found. The main coarse intermetallic particles are identified as S (Al2CuMg), θ (Al2Cu) and α (Al-Cu-Fe-Mn-(Si)) phases. θ (Al2Cu) and α (Al-Cu-Fe-Mn-(Si)) phases showed a relatively inert nature compared with S-phase particles. It was observed that continuous localized corrosion is associated with large clusters of S-phase and θ-phase intermetallic particles that are buried beneath surface but connected to the alloy surface. Propagating away from the intermetallic particles, corrosion developed preferentially along selected grain boundaries.Electron backscatter diffraction (EBSD) was employed to further investigate the relationship between the grain structure and the intergranular corrosion susceptibility. It was revealed that intergranular attack occurred at the grain boundaries that surround grains of relatively high stored energy. Corrosion was not confined within the region immediately adjacent to the grain boundaries, but had developed into the grains of relatively high stored energy, suggesting that grains with relatively high levels of defects are more susceptible to corrosion.