On the severe localized corrosion susceptibility of the AA2198-T851 alloy
Introduction
The interest in the potential properties of Al-Li alloys started in the 50’s when the first generation Al-Li alloys were used in specialized applications. However, they were not found proper for aero-structural parts due to their low short-transverse fracture toughness, poor corrosion resistance, and poor thermal stability [1,2]. Efforts were made to overcome these challenges in the second generation Al-Li alloys by changing the alloy composition and thermomechanical processing routes, but the second generation Al-Li alloys were still associated with high anisotropy of tensile properties [3].
The third generation Al-Li alloys (with lower Li contents compared with the previous generations) were mainly developed for military and space applications. The aim of developing the third generation Al-Li alloys was to meet the demands of future commercial airframes [4]. This generation, apparently, has overcome the main drawback of mechanical anisotropy, and it is gaining increasing interest and growing usage in aerospace applications due to their remarkable lightweight. The alloys are currently being used in modern aircrafts [[5], [6], [7]]. According to literature, the third generation Al-Li alloys show improved mechanical properties relative to the conventional 2xxx and 7xxx series [3,8].
Despite the improvements in mechanical properties, the third generation of Al-Li alloys are still susceptible to localized forms of corrosion which is associated with the high reactivity of Li and the resultant Li-containing intermetallic phases [9]. The localized corrosion resistance of these types of alloys has been of great concern, and much research work has been carried out on the alloys of the third generation [[10], [11], [12], [13], [14], [15], [16], [17], [18]].
The corrosion properties and performance of Al-Li alloys are dependent on alloy composition and microstructural features. Addition of Cu and Li into the aluminum alloy leads to precipitation of phases such as T1 (Al2CuLi) and δ’ (Al3Li). The T1 phase is the main strengthening phase in these alloys but it has also been strongly associated with the severe localized corrosion of the Al-Li alloys [[10], [11], [12], [13], [14], [15], [16], [17], [18]].
Two types of corrosion attack associated with T1 phase has been reported in literature: (i) dissolution of this phase near a precipitation free zone resulting in the formation of small cavities; and (ii) selective dissolution of the T1 phase along with the dissolution of the adjacent aluminum matrix, leaving large pits [13]. It has also been proposed that the T1 phase might either undergo selective dissolution along with the dissolution of the adjacent Al matrix forming large pits [15], or dissolve near to the precipitate free zone forming small cavities [16].
Ma et al. [17] studied the corrosion behavior of the AA2099-T83 alloy in 3.5 wt.% NaCl solution and observed two types of localized corrosion. The first type was identified as a type of attack associated with the constituent particles containing aluminum, copper, iron and manganese. This type of attack was also related to the distribution of the constituent particles in the alloy. As these types of particles were cathodic to the matrix, the attack occurred at the matrix surrounding them. The second type of localized corrosion was called severe localized corrosion and it was characterized by hydrogen gas evolution and corrosion rings around the attacked areas [18]. This type of localized corrosion has also been previously reported in literature for the AA2024-T3 [[19], [20], [21], [22], [23]].
According to Ma et al. [17], the origin of severe localized corrosion in the AA2099-T83 alloy is related to selective dissolution of the T1 phase. This phase precipitates preferentially at grain/subgrain boundaries and dislocations within grains and preferential attack occurs at grains that were exposed to great plastic deformation when subjected to cold stretching prior to aging and, consequently, presented increased density of T1 phase after aging. Due to the high electrochemical activity of the T1 phase, corrosion attack occurred preferentially in the more deformed grains.
Pitting corrosion resistance of Al-Li alloys, including the AA2198 alloy, has been reported in literature [[24], [25], [26]]. Alexopoulos et al. [27] studied the mechanical properties degradation of the AA2198 alloy due to corrosion exposure. According to the authors, this alloy is more resistant to corrosion than the AA2024 alloy in EXCO solution. Moreto et al. [26] studied the localized corrosion behavior of the AA2198-T851 and compared the results with that of the AA2524-T3 in naturally aerated 0.6 mol L−1 NaCl solution by localized electrochemical techniques. The authors concluded that the AA2198-T851 presented higher pitting resistance when compared with the AA2524-T3 and suggested that the first alloy could be a potential replacement for the conventional AA2524-T3 alloy. However, in another work, Moreto et al. [28] showed that the 2198 alloy is more susceptible to corrosion compared with conventional alloys, such as 2524 and 7050, based on corrosion potential and pitting potential values. These clearly show that already reported results on the localized corrosion resistance of the AA2198 alloy are controversial. To buttress this point, previous results obtained in our laboratory [29] that compared the localized corrosion resistance of the alloys AA2198-T851, AA2524-T3 and AA2024-T3, showed that the first alloy presented the highest susceptibility to localized corrosion among the studied ones. Also, in a very recent report by Donatus and co-workers [30] it was revealed that the AA2198-T851 alloy is more susceptible to corrosion compared with the AA2024-T3 alloy. However, the corrosion mechanisms in the two alloys are different. While the predominant corrosion mechanism in the AA2198-T851 is intragranular, that in the AA2024-T3 is intergranular.
Thus, based on the conflicting results in literature and the fact that works on the corrosion mechanism of the AA2198-T851 alloy are still very rare, there is need for further research in order to advance the understanding of localized corrosion behavior of the AA2198-T851 alloy. Therefore, the aim of this study is to investigate the localized corrosion resistance of this alloy by means of monitoring the corrosion development on the alloy during immersion tests using a combination of scanning vibrating electrode technique and microscopic observation after various periods of exposure to 0.01 mol L−1 sodium chloride solution.
Section snippets
Experimental
The chemical composition obtained from Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) and the nominal composition of the AA2198-T851 alloy used in this study are presented in Table 1.
Microstructural characterization
An optical micrograph showing the microstructure of the AA2198-T851 alloy is presented in Fig. 1. Slip bands in single grains are highlighted by red circles. Slip bands are due to the development of inhomogeneous plastic strains in high stacking fault energy materials in which cross-slip is eased [31]. They are characterized by the presence of dislocation accumulation in bands separated by relatively dislocation-free regions [32]. Slip bands in aluminum alloys have been reported in literature [
Conclusions
The AA2198-T851 investigated in this study showed high susceptibility to severe localized corrosion which was observed from the first hours of exposure to 0.01 mol L−1 NaCl solution. The severe localized corrosion was intragranular but the propagation pathway was crystallographic. The crystallographic propagation was associated with the slip bands in the grains of the alloys. The dislocation-rich zones in the slip bands favored preferential precipitation of the active T1 phase and,
Acknowledgments
The authors are grateful to FAPESP (Proc. 2013/13235-6) for financial support for this research, CAPES (Capes/Cofecub N°806-14) for the grants of M. X. Milagre, FAPESP (2017/03095-3) for the grants of U. Donatus and to LNANO for the provision of FEG/SEM and TEM analysis.
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