The Role of Coarse Intermetallics in the Localized Corrosion Behaviour of AA 2024-T3 in Low Concentrated Chloride Media

Aluminium alloy (AA) 2024-T3 is an important engineering material due to its widespread use in the aerospace industry. However, it is very prone to localized corrosion attack in chloride containing media, which has been mainly associated to the presence of coarse intermetallics (IMs) in its microstructure. In this work the corrosion behaviour of AA2024-T3 in low concentrated chloride media (1 mM) was investigated using microscopy and electrochemical methods. TEM/EDS observations on non-corroded IMs revealed the presence of secondary phases within their microstructure. SEM/EDS analyses showed that IMs with same nominal composition present heterogeneous reaction rates, and that both types of coarse IMs normally found in the AA 2024-T3 microstructure, namely: Al-Cu-Mg (S phase) and Al-Cu-Fe-Mn-(Si) corrode. In such cases, EDS analyses revealed important compositional changes in the corroded IMs, characterized by selective corrosion of their more active constituents and the onset of an intense oxygen peak, irrespective to the IM nature; moreover, corrosion of the surrounding matrix was always verified. On the other hand, the results of the electrochemical investigations, in accordance with the microscopy observations, evidenced that IMs corrosion dominates the electrochemical response of the alloy during the first hours of immersion in the test electrolyte.