Corrosion Science 50 (2008) 2646–2657 0010-938X/$ - see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2008.06.041 Contents lists available at ScienceDirect Corrosion Science journal homepage: www.elsevier.com/locate/corsci 1. Introduction The AA 2024-T3, due to its high-strength/weight ratio, is used in aircraft structures, rivets, hardware, truck wheels, screw machine products, and other miscellaneous structural application [1,2]. This material has a complex microstructure due to the addition of alloying elements and to the presence of impurities [1]. The pre- cipitation sequence of the 2024 alloy consists in the formation of GPB (Guinier–Preston–Bagaryatsky) zones at room temperature, which can be classified as a short-range ordering of the Cu and Mg solute atoms [3]. After artificial ageing, these zones are dissolved and replaced by semi-coherent S0 (Al 2 CuMg) precipitates. Finally, after longer exposure times, stable S (Al 2 CuMg) phase are formed [3–5]. In this alloy, the alloying elements Cr, Mn, and Cu are generally added to control grain structure, weldability and corrosion resistance [6], and specifically Cu is added to improve mechanical resistance. This is achieved by the precipitation of small coherent particles dispersed through the whole matrix [2,7–10]. It is documented that, when in solid solution, Cu increases the localized corrosion resistance of the Al matrix [11]. However, Cu, together with other elements, also precipitates as bigger intermetallics (IMs), which attain high surface densities [12]. The electrochemical activity associated with these IMs is different from the matrix, and gives rise to localized corrosion phenomena [2,9–14]. Moreover, in their vicinity, the passive layer can be weaker and Cu depleted zones can be formed [10] leading to localized attack of the matrix. Although it is generally accepted that the onset of localized cor- rosion of high strength Al alloys is associated with the presence of IMs [9,15], the initial steps of the mechanism is not established yet, mainly due to the IMs’ small size, their heterogeneous dissolution [16–18], and to the fact that their reactivity depend on the electro- lyte [9,16,19,20]. For the 2024 alloy, it is generally accepted that Al–Cu–Mg (S-phase) IMs are initially anodic and become cathodic to the matrix due to selective corrosion of their less noble constit- uents, namely, Al and Mg, leaving nobler Cu-rich remnants that provoke the corrosion of the adjacent matrix [2,10,15,16]. However, some authors also report cathodic dissolution of the matrix near these IMs due to pH rise [21], whereas others state that these IMs are never anodic relatively to the matrix [22]. Regarding the Al–Cu–Fe–Mn IMs, they are always considered nobler than the matrix [16,18,20,22–25] and, consequently, must sustain cathodic reaction. Several authors have described the dis- solution of the matrix in their vicinity [12,18,26,27], and Cu depo- sition on them [12,24], enhancing galvanic activity. However, it has also been reported that they have heterogeneous composition Investigation of the corrosion behaviour of AA 2024-T3 in low concentrated chloride media F.M. Queiroz a , M. Magnani b , I. Costa a , H.G. de Melo c, * a Energy and Nuclear Research Institute, IPEN/CNEN-SP, CCTM, São Paulo, Brazil b Physical Chemistry Department, São Paulo State University, UNESP, São Paulo, Brazil c Chemical Engineering Department, Polytechnic School of the University of São Paulo, CEP 05508-900, São Paulo, SP, Brazil article info abstract Article history: Received 27 March 2008 Accepted 24 June 2008 Available online 8 July 2008 Aluminium alloy (AA) 2024-T3 is an important engineering material due to its widespread use in the aero- space 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 AA 2024-T3 in low concentrated chloride media was investigated using microscopy and electrochemical methods. TEM/EDS observations on non-corroded samples evidenced the heterogeneous composition within the IMs. In addition, SEM observations showed that intermetallics with the same nominal composition present different reactivity, and that both types of coarse IMs nor- mally found in the alloy microstructure are prone to corrosion. Moreover, EDS analyses showed important compositional changes in corroded IMs, evidencing a selective dissolution of their more active constitu- ents, and the onset of an intense oxygen peak, irrespective to the IM nature, indicating the formation of corrosion products. On the other hand, the results of the electrochemical investigations, in accordance with the SEM/EDS observations, evidenced that IMs corrosion dominates the electrochemical response of the alloy during the first hours of immersion in the test electrolyte. © 2008 Elsevier Ltd. All rights reserved. Keywords: A. Aluminium A. Intermetallics B. EIS B. SEM B. TEM * Corresponding author. Tel.: +55 11 3091 2231; fax: +55 11 3031 3020. E-mail address: hgdemelo@usp.br (H.d. Melo).