PSI - Issue 42

Christina Margaraita Charalampidou et al. / Procedia Structural Integrity 42 (2022) 1708–1713 Charalampidou et al./ Structural Integrity Procedia 00 (0000) 000 – 000

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1. Introduction During the last decades, Al-Cu-Li alloys have a key role in the aerospace industry, e.g., Rioja et al. (2012), due to their lightweight characteristics and their improved mechanical properties. Several research works of the last decades are focused to the mechanical and corrosion behaviour of third generation Al-Cu-Li alloys, e.g., Alexopoulos et al. (2013), Moreto et al. (2012), Donatus et al. (2018), Huang et al. (2016), Zou et al. (2018) etc. These materials have shown improvements in mechanical properties, damage tolerance as well as corrosion resistance, e.g., Dursun et al. (2014). Nevertheless, they were found to be highly susceptible to intragranular, e.g., Milagre et al. (2019), trangranular as well as exfoliation corrosion attack, e.g., Araujo et al. (2020), due to their complex precipitation hardening system, e.g., Ma et al. (2015) and Zhang et al. (2016). Cu, Mg and Li alloying elements are added in Al-Cu-Li alloys to improve their mechanical properties, as referred to Li et al. (2003). The addition of these elements introduces precipitation hardening due to the formation of several micro- and nanometric intermetallic particles. The main strengthening nanometric particles in Al-Cu-Li alloys include δ ΄ (Al 3 Li), θ ΄ (Al 2 Cu) and T 1 (Al 2 CuLi). T 1 is considered as the major strengthening phase of these alloys, according to Blankenship et al. (1992), Nie et al. (1996) and Araullo et al. (2014) which nucleate preferably in dislocations, sub grain, and grain boundaries. These phases are formed by applying appropriate heat treatments, such as artificial ageing, and influence the mechanical and corrosion behaviour of the alloys, e.g., Deschamps et al. (2012). Ma et al. (2015) investigated the effects of microstructure on the corrosion resistance of AA2099-T83 and showed that corrosion attack caused by constituent particles was superficial when compared with that caused by T 1 phase dissolution, referred as severe localised corrosion. Additionally, Li et al. (2008) proposed another corrosion mechanism associated with the T 1 phase, where they suggested that during exposure to corrosive environments, the anodic T 1 phase initially exhibits selective dissolution of Li and Al and afterwards becomes cathodic with respect to the matrix due to Cu-enrichment that leads to the attack of the surrounding matrix. In the present work, investigation of corrosion mechanisms of AA2198-T8 under three different corrosive solutions of various aggressiveness and the effect on mechanical properties degradation is performed. Investigation on the corrosion behaviour of AA2198 alloy is of major importance while identifying the corrosion-induced degradation mechanism is of crucial interest in the aircraft maintenance protocols. 2. Materials and experimental procedure 2.1. Materials Material used in the present work was wrought AA2198-T8 which was received in sheet form of 3.2 mm nominal thickness. The weight percentage chemical composition of AA2198 is 2.9-3.5% Cu, 0.8- 1.1% Li, ≤ 0.35% Zn, ≤ 0.5% Mn, 0.25-0.8% Mg, 0.04- 0.18% Zr, ≤ 0.08% Si, 0.1 - 0.5% Ag, ≤ 0.01% Fe and Al rem. , according to sheet manufacturer. The T8 temper corresponds to under-ageing condition for AA2198 since it exhibits moderate yield stress value that is significantly lower that the respective values at the peak-ageing condition. Tensile specimens with dimensions of 57 mm * 12.5 mm * 3.2 mm in the reduced cross-section and 155 mm total length were machined from the longitudinal (L) rolling direction. Prior to corrosion exposure, specimens were ground with SiC papers up to 1200 grit, then rinsed with deionized water and acetone, and eventually dried with cool flowing air. 2.2. Methods 2.2.1 Corrosion exposure Different tensile specimens were exposed for several hours each to the various laboratory corrosion solutions: (a) exfoliation corrosion solution (hereafter called EXCO) according to ASTM G34 standard, (b) 3.5 wt. % NaCl (hereafter called NaCl solution) according to the ASTM G44 standard and (c) Harrison’s (diluted version) corrosion environment according to ASTM D5894 standard. EXCO solution consisted of sodium chloride (4.0 M NaCl), potassium nitrate (0.5 M KNO 3 ) and nitric acid (0.1 M HNO 3 ) diluted in 1 l distilled water. Concentration of NaCl solution consisted of 3.5 g NaCl for each 96.5 ml distilled water, while H arrison’s solution consisted of ammonium sulphate (0.35 g (NH 4 )2SO 4 ) and sodium chloride (0.05 g NaCl) for each 96.5 ml distilled water. The solution volume was calculated per exposure area of the specimens and was constant at approximately 20 ml/cm 2 for all corroded