Observations of intergranular corrosion in AA2024-T351: The influence of grain stored energy
Highlights
► Intergranular attack in AA2024-T3 exposed to 0.1 M NaCl was studied using EBSD. ► EBSD was used to determine the relative grain stored energy which is related to the defect density. ► Intergranular attack was most severe for grains with high grain stored energy.
Introduction
High-strength aluminium alloys such as aluminium alloy 2024-T3 (AA2024-T3) have been used extensively in the manufacture of aircraft [1], [2]. The good mechanical properties of these alloys are obtained through alloying additions which form nano-sized phases, called hardening precipitates and to a lesser extent submicron sized dispersoids, which strengthen the alloy [3], [4]. Unfortunately, these same alloying additions generally reduce the corrosion resistance, not only due to the presence of large, second phase particles from which stable pitting can initiate [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], but also the susceptibility of the grain boundaries due to the formation of nano-sized precipitates in the grain boundary which deplete the matrix in the vicinity of the precipitates [17], [18], [19], [20], [21], [22], [23], [24], [25], [26]. In addition to microgalvanic corrosion associated with the grain boundary and its neighbouring regions, hydrogen embrittlement has been reported to be a contributing factor to the loss of mechanical strength in high strength aluminium alloys [27], [28], [29], [30], [31]. The source of the hydrogen is from hydrolysis reactions in the anolyte solution at the subsurface active corrosion front. The combination of these corrosion susceptible sites, with corrosive environments and cyclic stress regimes experienced during normal operation of airframes can lead to multi-site damage [32], [33] and corrosion accelerated mechanical damage [6], [27], [29], [34], [35], [36], [37]. Mechanical damage, accelerated through intergranular attack (IGA), has been studied extensively in high strength Al-alloys [38], [39], [40], [41]. Some tempers of AA2024, such as the T6 condition, are more susceptible to IGA than the T3 condition, IGA nevertheless occurs in AA2024-T3 [10], [11], [19], [20], [22], [38], [42].
The combination of corrosion pitting and stress regimes can lead to stress corrosion cracking. Mechanical failure due to stress corrosion cracking is generally related to stress raisers in the surface of the alloy [6]. Typical stress raisers include intermetallic (IM) particles where corrosion has occurred. The corrosion of IM particles in AA 2024-T351 has been studied extensively. It is clear from these studies that localized attack around isolated IM metallic particles does not lead to stable pitting [9], [12], [43], [44], [45]. However many studies have concluded that clusters of IM particles lead to stable pitting [7], [9], [10], [12], [18], [43], [44]. In previous papers by the authors it was shown, in some variants of AA2024-T351, that IGA, which occurred within the rings of corrosion product, was observed to penetrate as deep as 60 μm into the surface prior to grain etchout that leads to typical open pitting [10], [11]. This penetration depth exceeds the depth of 20 μm nominated by Wei for the critical size of a pit to act a nucleus for the early onset of fatigue crack growth [6]. In the studies of Boag et al. [9], the rings of corrosion product developed on the surface of AA2024-T351 immersed in 0.1 M NaCl as early as 5 min into the corrosion process. The rings of corrosion product were typically 100–200 μm in diameter. Examples corrosion rings for AA2024-T351 corroded in 0.1 M NaCl for 30 min are shown in Fig. 1. Figs. 1a and b are bright and dark field optical images respectively of a number of corrosion rings. Some of the rings are not that apparent in the bright field, but are clearly seen in the dark field image. As can be seen in Fig. 1 attack is often observed within the rings which contain domes of corrosion product. Intergranular attack was also observed at many of these sites [10]. These features were established at this size scale and their diameter did not increase with time, however, the amount of material deposited onto the corrosion rings themselves did increase with time. Hydrogen evolution was observed early in the corrosion process emanating from within the rings where domes of corrosion product were evident. Typically, the rings contained one or two domes of corrosion product, which were around 20 μm in diameter. The domes were more often not centred within the ring and those in Fig. 1a are often towards the inner edge of the ring (dark features within the ring). While some rings were due to large bubble formation covering the whole site, others only displayed intermittent hydrogen evolution, appearing in bursts that eventually subsided, only to begin again at a later time suggesting stepwise activation followed by deactivation then reactivation. The network of grain boundaries at the surface underwent significant attack within the corrosion rings and this attack extended into the alloy creating an extensive network of IGA beneath the corrosion rings. Further work has revealed that the subsurface IGA can re-emerge elsewhere on the surface [46].
Susceptibility to IGA for members of the 2xxx series of aluminium alloys, which have some of the highest copper levels in aluminium alloys, is often attributed to compositionally different features in the grain boundary. For example, one mechanism attributes intergranular attack to the decoration of the grain boundary network with precipitates including S (Al2CuMg) or θ (Al2Cu) phases [19], [23]. The precipitation of these particles at the grain boundary results in depletion of alloying elements from the surface of adjacent grains which increases the susceptibility to IGA. The particles are both compositionally and electrochemically different to both the adjacent grains and the depleted zone. The electrochemistry of these phases has been studied extensively on macroscopic IM materials [47], [48], [49], [50] as well as in situ [25], [51], [52]. Within the grain boundary, compositions typical of the S-phase, i.e. Al2CuMg, are anodic with respect to the adjacent grains and can be expected to undergo preferential dissolution leading to acidification of the anolyte solution through hydrolysis of aluminium ions in the intergranular crevice and also to copper enrichment on the surface of the grain crevice behind the active anodic head at the point of attack. On the other hand compositions typical of the θ-phase particles i.e. Al2Cu, are cathodic with respect to the matrix [53] and might be expected to promote dissolution of the adjacent depleted zone.
In addition to the possibility of chemically and electrochemically heterogeneously different regions within the grain boundary network there are structurally different regions as well. Studies on misorientation angles in steels, for example, show that corrosion susceptibility can depend on the orientation difference between grains when they meet at the grain boundary since the grain boundary energy is determined by grain boundary structure [54]. In terms of the co-incident site lattice (CSL) approach there are special grain boundaries e.g. coherent twin boundaries (Σ3) that have a higher resistance to intergranular corrosion. Indeed the studies of Kim et al. [55], [56] have shown that these special grain boundaries demonstrate resistance to corrosion in high purity aluminium. However, the CSL approach only partly explains the susceptibility of low angle grain boundaries [57], [58]. In any case the grain boundaries of interest in this study are random, high angle grain boundaries. The energy of a low-angle boundary increases with the degree of misorientation between the neighbouring grains. The situation in high-angle boundaries is more complex. High angle grain boundaries possess certain low energy configurations. Generally, however, high energy grain boundaries are more susceptible to IGA compared with low energy grain boundaries.
Our previous work examining the nature of IGA beneath the corrosion rings in AA2024-T3 showed that severe IGA penetrated up to 60 μm into the surface within 120 min of immersion in 0.1 M NaCl solution at ambient temperature [10]. Similar rapid intergranular corrosion (IGC) penetration has been reported by others [35], [38], [39], [56], [59], [60]. While around 15% of the grain boundaries were decorated with θ-phase there were many grain boundaries subject to IGA, where no θ-phase was evident [46]. Furthermore, no depletion zones were observed in the grain boundary network and the distribution of grain misorientation angles for the attacked grain boundaries was similar to those of the parent material, indicating that grain misorientation was not responsible for the IGA [11]. In light of these observations, the IGA was attributed simply to the pH gradient at the “head” of the IGA attack compared to solution in the intergranular crevice behind the active front.
Since that report, further research has provided some insight into the nature of the IGA which is reported here. Scanning electron and transmission electron microscopies in conjunction with electron backscatter diffraction (EBSD) have been used to characterise grain boundary attack in AA2024-T351. It has been found that the grain stored energy, which reflects the defect density in individual grains, may play a crucial role in the development of intergranular corrosion as explored below.
Section snippets
Materials and preparation
Discs, with a thickness of 1.6 mm, were pressed from AA2024-T351 alloy sheet. Inductively coupled plasma – atomic emission spectroscopy (ICP-AES) indicated the following composition: 4.45 wt.% Cu, 0.28 wt.% Fe, 1.34 wt.% Mg, 0.54 wt.% Mn, 0.065 wt.% Si, 0.14 wt.% Zn, 15 ppm Ni, 120 ppm V and 20 ppm Zr (ppm by weight). The discs were ground on silicon carbide papers to P1200, then polished using 6, 3, 1 and 0.25 μm diamond paste and given a final wash in ethanol.
Polished surfaces were immersed horizontally
Results
Fig. 2 shows secondary and backscatter electron images of a typical corrosion ring as well as corrosion events within a corrosion ring on AA2024-T351, (Fig. 2a) after 18 h of immersion of the alloy in 0.5 M NaCl. The development of the corrosion rings was discussed in the introduction. The areal view of the corrosion event (Fig. 2a and b) shows nodules of corrosion product within a region displaying extensive grain boundary attack. The white line in Fig. 2b shows the location of a section made
Discussion
Intergranular corrosion is generally attributed to some degree of heterogeneity in the grain boundary structure associated with second phase precipitates. The best understood mechanism of IGA in aluminium–copper alloys is associated with the precipitation of Al2Cu particles at the grain boundaries which depletes the adjacent solid solution of copper leaving it more anodic and therefore prone to corrosion [19], [22], [62]. As a result of precipitation and depletion, a narrow band forms on either
Conclusions
IGA of AA2024-T351 was studied using electron microscopy and EBSD. Extensive IGA was observed to propagate well into the alloy prior to the development of any substantial pits. Examination of the grain boundaries in the parent material as well as the corroded material indicated that while there was some grain boundary decoration with θ-phase precipitates these were not observed ahead of the IGA in the corroded specimens. Additionally, the extent of attack on the grain boundaries far exceeded
Acknowledgements
The authors would like to acknowledge the UK – ESPRC LATEST2 Programme Grant and the CSIRO for co-sponsorship of Dr. Luo’s Doctor of Philosophy studies undertaken at the University of Manchester.
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