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Corrosion behaviour of anodized Al-Cu-Li alloy: the role of intermetallic particle-introduced film defects

Y. Ma 1*, H. Wu 1, X. Zhou 2, K. Li 3, Y. Liao 4, Z. Liang 1, L. Liu 1

1 College of Materials Science and Engineering, Chongqing University of Technology, Chongqing 400054, P.R. China

2 Corrosion and Protection Centre, School of Materials, The University of Manchester, Manchester M13 9PL, UK

3 Science and Technology on Power Beam Processes Laboratory, AVIC Manufacturing Technology Institute, Beijing 100024, P. R. China

4 Chongqing University of Education, Chongqing 400067, P. R. China

*Corresponding authors: myl@cqut.

Abstract

The role of intermetallic particle (IMP)-introduced anodic film defects in the initiation and propagation of localized corrosion in an anodized Al-Cu-Li alloy prior to sealing is investigated. Anodizing of IMPs results in cavity defects of micrometer-scale in the anodic film. Corrosion is preferentially initiated in the regions with anodized IMPs at the film/alloy interface. Localized corrosion then propagates in the alloy substrate beneath the anodic film, which ultimately results in cracking of the bulk anodic film. It is suggested that the barrier and porous layers of the anodic film play different roles in different stages of a localized corrosion event.

Keywords: Al-Li alloy; Anodic film; Intermetallic particles; Localized corrosion

1 Introduction

The third generation aluminum-copper-lithium (Al-Cu-Li) alloys have been increasingly used in aerospace industry due to their high specific strength, high specific modulus and relatively low cost compared with carbon fiber reinforced polymer composites [1]. The unique properties of Al-Cu-Li alloys are largely attributed to the addition of lithium (Li), which not only lowers the density but also improves the elastic modulus of the resultant aluminum alloys [2]. Most Li in Al-Cu-Li alloys is present in forms of solid solution and precipitates such as T1 (Al2CuLi), T2 (Al6CuLi3), TB (Al7Cu4Li1) and [pic](Al3Li) phases. However, lithium is also found in some insoluble intermetallic particles (IMPs) [3-5]. The effect of the precipitates [6-16] and IMPs [4, 17, 18] on the corrosion behavior of Al-Cu-Li alloys has been investigated by many researchers.

Like most aerospace aluminum alloys, Al-Cu-Li alloys need to be finished by anodizing to provide the required corrosion resistance under service conditions. It is well known that the typical anodic film formed on pure aluminum is continuous, uniform, and composed of a compact inner layer (barrier layer) and a porous outer layer (porous layer). However, defects such as cavities and inclusions are present in the anodic film formed on aluminum alloys due to different anodizing behavior of IMPs relative to the alloy matrix [17]. It has been generally accepted that IMP-introduced defects in the anodic film decrease the corrosion resistance of the anodized aluminum alloys. For instance, Zhang et al. [19] demonstrated that IMPs in 6060 Al alloy could cause localized dissolution during anodizing and, consequently reduced film growth rate and corrosion resistance; Veys-Renaux et al. [20] found that the impedance of both the porous and the barrier layers of the anodic films formed on different aluminum alloys (1050, 7175 and 2618) in sulfuric acid was reduced in the presence of defects resulting from anodized IMPs; Zhu et al. [21] reported that Si particles in cast Al-Si alloys resulted in thinner and more defective anodic film in the eutectic region, thereby increasing the ease of substrate corrosion attack. Recently, we found that lithium-containing Al-Fe-Mn-Cu IMPs dissolve completely during anodizing, resulting in cavity defects of micrometer-scale in the anodic film; the anti-corrosion performance of the anodic film is associated with the location of the cavities, with those at the film/alloy inference being more likely to cause localized corrosion [18].

Since the micrometer-scale IMPs are often inherent to the process employed for production of the Al-Cu-Li alloys, it is either technical impossible or not cost-effective to reduce the number or size of the IMPs. Therefore, it is indispensable to seek for solutions, from the anodizing point of view, to eliminate the impact of IMPs on the performance of the Al-Cu-Li alloys, and this requires mechanistic understanding of the corrosion behavior of the anodized aluminum alloys. However, most of the literature [22-30] is focused on evaluating the anti-corrosion performance of the anodic films which are formed and/or sealed in different conditions, with little attention being paid to the effect of IMP-introduced film defects on the initiation and propagation of localized corrosion in anodized aluminum alloys. Thus, in the present study, a newly-developed, high strength Al-Cu-Li alloy AA2055 (Alcoa 2012) was anodized in a tartaric-sulfuric acid solution and then the corrosion behavior of the anodized alloy was investigated. The anodized alloy prior to post sealing was deliberately selected to avoid the effect of sealing on the film structure and anti-corrosion performance, allowing a focus on the effect of IMPs. The selected alloy, AA2055, is representative for the intended purpose because the alloy is designed to replace conventional 7xxx aluminum alloys used on the upper wing structure of aircraft and contains a large population density of IMPs [31, 32].

2 Experimental

Cold rolled AA2055 alloy plate of 2 mm thickness, provided by CHINALCO, was used, with its composition listed in Table 1. The received alloy was solution treated at 520 °C for 40 min and then quenched in icy water. The formation of major precipitates such as T1, T2, TB and [pic] were effectively inhibited through such heat treatment, leaving constituent IMPs in the alloy matrix [31]. Specimens, of 20 × 20 × 2 mm dimensions, were mechanically ground with 400, 600, 800, 1200 and 2500 grit silicon carbide papers. In order to accurately correlate specific IMPs with characteristic film structures and corrosion events, areas of interest on the mechanically polished specimens were marked with indentations and then the compositions of the IMPs within the marked area were determined using energy dispersive X-ray spectroscopy (EDS). IMPs were exposed to the electron beam for as short time as possible (< 60 s) to reduce the effect of carbon contamination [33].

Anodizing was performed in an aqueous solution containing 0.46 M sulphuric acid and 0.53 M tartaric acid (TSA) [34], under a constant voltage of 14 V vs. the counter electrode, at 37±1 °C, for 5, 180 and 1500 s, respectively. The relatively short anodizing periods (5 and 180 s) were selected to reveal the anodizing behaviour of IMPs in early stages. A large pure aluminium sheet and the specimen were set as cathode and anode, respectively, with the specimen surface facing the pure aluminium sheet cathode. A KR50003-500V/3 DC power supply was used, with the current density-time response recorded. The mechanically polished specimens were masked with epoxy to expose an area of 1cm2 and the epoxy was allowed to cure in air for 24 h. The specimens after anodizing were rinsed in deionized water and dried in a cool air stream. A specimen after anodizing for 1500 s was immersed in a 3.5% NaCl aqueous solution at room temperature (about 20 °C) for up to 84 h to introduce corrosion, with the open circuit potential (OCP) being recorded during the immersion process.

The morphology and composition of IMPs in the alloy were examined using a Zeiss Sigma HD scanning electron microscope, fitted with EDS facilities, and operated at an accelerating voltage of 20 kV. The morphology of the anodized alloy before and after the immersion test was examined using the same microscope and operated at an accelerating voltage of 2 kV to avoid charging effect. The cross sections of the anodized specimens before and after the corrosion test were prepared either through mechanical bending or by ultramicrotomy (Leica Ultracut) using a diamond knife [35]. For mechanical bending, the specimen with a notch on its back side was immersed in liquid nitrogen for 10 min and then bended using two pliers, with the film side opposite to the bending direction. The composition change of the anodic film before and after the immersion test was determined using glow discharge optical emission spectroscopy (GDOES), operated with Argon atmosphere of 635 Pa, radiation frequency of 13.56 MHz, power of 35 W and a sampling time interval of 0.005 s.

3 Results

Fig. 1 shows the current density-time response recorded during anodizing under a constant voltage of 14 V. The current density dropped rapidly from ~0.02 to ~0.004 A/cm2 in the first ~5 s and then increased to ~0.007 A/cm2 in the next ~35 s. After that, it kept nearly constant, with a mild increasing trend, till the end of the anodizing. The mild increasing trend of the current density may be related to preferential anodizing/dissolution of IMPs during anodizing, which will be demonstrated later. Specifically, the preferential anodizing/dissolution of IMPs led to development of sunken regions in alloy substrate, which resulted in increased surface area for charge transfer and consequently increased current (a constant surface area was used for calculating the current density in Fig. 1). The three stages of anodizing shown in the current density-time curve correspond to barrier layer formation, pore initiation and steady growth of porous anodic film, respectively [36].

Figs. 2a and 2b are backscattered electron micrographs of the alloy before anodizing, showing IMPs with average diameters ranging from several hundred nanometers to several micrometers. The relatively fine IMPs are either broken constituent particles or dispersoids, and their effect on the film structure is much less than the coarse IMPs [17]. Therefore, only coarse IMPs of micrometer-scale were tracked and monitored during the anodizing and the subsequent corrosion process in this work. Twelve particles are labeled in Figs. 2a and 2b, with their chemical compositions determined using EDS and listed in Table 2. The IMPs fall into three groups: high copper-containing Al-Fe-Mn-Cu particle (Particles 1-3) [3], high-copper-containing Al-Cu particle (Particles 4-8) and low-copper-containing Al-Cu particle (Particles 9-12). EDS data taken from the alloy matrix is also provided in Table 2 for comparison. Figs. 2c and 2d are secondary electron micrographs of the same areas as shown in Figs. 2a and 2b, after anodizing for 5 s, revealing distinct features at the sites of Particles 1-12. Fig. 3 shows the morphology of typical particles after anodizing for 5 s at increased magnifications. Oxide flakes are revealed at the site of the high copper-containing Al-Fe-Mn-Cu particle (Fig. 3a, Particle 1). The oxide formed on the high-copper-containing Al-Cu particles protrudes outward, with cracks in the oxide (Figs. 3b and 3c, Particles 5-7). Fine pores in the oxide formed from the high-copper-containing Al-Cu particle (Fig. 3d) suggest a porous nature of the oxide. A sponge-like oxide was formed on the low-copper-containing Al-Cu particles (Figs. 3e and 3f, Particles 9 and 12). According to the current density-time response (Fig. 1), a porous anodic film had not yet formed on the alloy matrix after anodizing for 5 s. Therefore, the distinct features in Fig. 3 suggest preferential anodizing of the IMPs relative to the alloy matrix.

Figs. 4a and 4b compare the backscattered electron micrographs of the same area before (Fig. 4a) and after anodizing for 180 s (Fig. 4b). The regions of the IMPs on the alloy surface become darker after anodizing, suggesting anodizing/dissolution of the IMPs. When imaging the same area after anodizing using secondary electron mode at a reduced accelerating voltage of 2 kV, which is sensitive to surface topography, surface cavities are revealed at the sites of particles (Figs. 4c and 4d). The population density of the surface cavities is estimated to be 3.6~5.8×103 per cm2. The smooth surface of the cavity wall (Fig. 4e) suggests that the oxide initially formed on the particle dissolved completely after anodizing for 180 s. Additionally, localized cracking was occasionally observed in the bulk anodic film (Fig. 4f). Such cracking was caused by volume expansion of the anodized particles that were slightly below the specimen surface. Note that not all anodized IMPs beneath the alloy surface would cause localized cracking because this phenomenon is associated with the size, type and location of anodized IMPs. The surface morphology of the alloy after anodizing for 1500 s (Fig. 5) was similar to that after anodizing for 180 s except that more localized cracking was observed (as indicated by the arrows in Fig. 5a). This is because more IMPs beneath the specimen surface were anodized after 1500 s. The population densities of the surface cavities and localized cracking are estimated to be 2.4~3.9×103 and 3.5~3.7×102 per cm2, respectively. The nanopores on the bulk alloy surface (Fig. 5d) suggest normal growth of a porous anodic film on the alloy after anodizing for 1500 s.

The cross-sectional view of the alloy after anodizing for 1500s is shown in Fig. 6. A uniform and porous anodic film of ~3.7 micrometers thickness was formed on the alloy in the region free of IMPs (Fig. 6a). However, cavities were developed in the anodic film when IMPs was present and anodized; the locations and sizes of the cavities depended on the locations and sizes of the IMPs in the alloy before anodizing (Figs. 6b, 6c and 6d). Due to rapid dissolution of the IMPs and consequent formation of the cavities, sunken regions developed in the alloy substrate below the dissolved IMPs. Fig.6 indicates that anodizing of IMPs could result in cavity defects of micrometer-scale in the anodic film, which significantly compromise the integrity of the anodic film. Therefore, it is necessary to further investigate the impact of the anodized IMPs on the corrosion behavior of the anodized alloy.

The corrosion behavior of the anodized alloy was investigated through immersion test in a 3.5% NaCl solution. Fig. 7 shows open circuit potential-time responses of the bare and anodized AA2055 alloy (after anodizing for 1500 s) during the immersion test. The OCP of the bare alloy increased from -0.89 to -0.57 V in the first 4 min and then kept nearly constant at -0.60 V in the next ~35 h; after that, it fluctuated violently with a decreasing trend. The OCP-time response of the bare alloy suggests passivation, initiation and propagation of localized corrosion in the alloy during the immersion test [37]. The OCP of the anodized alloy fluctuated mildly between -0.56 and -0.62 during the whole process of the immersion test. Compared with the bare alloy, the anodized alloy had no sharp increase of the OCP in initial stage of the immersion test since a thick anodic film was present on the alloy surface. The mild fluctuation of the OCP of the anodized alloy over immersion time suggests initiation of corrosion in the alloy matrix beneath the anodic film; the absence of the violent fluctuation in the late stage of the immersion test suggests that the anodic film had delayed the occurrence of stable/severe localized corrosion.

The surface morphology of the anodized alloy after immersion for different periods of time was examined by SEM. Little evident change of film morphology was observed after immersion for up to 6 h. After immersion for 12 h, fine cracks were occasionally observed in the anodic film. When the immersion time was increased to 24 h, circular cracks started to develop in the anodic film (Figs. 8a and 8b), with their diameters varying from ~15 to ~30 µm and a population density of 1.0~1.2×102 per cm2. After immersion for 84 h, the diameters of the circular cracks varied from ~15 to ~80 µm and their population density reached 1.9~2.1×102 per cm2 (Figs. 8c and 8d). As discussed later, the circular cracks mainly correspond to localized corrosion sites in the alloy substrate beneath the anodic film. The largest diameter of the circular cracks increased from ~30 to ~80 µm when the immersion time was increased from 24 to 84 h, suggesting propagation of localized corrosion with increasing immersion time.

Cross-sectional view of AA2055 alloy after anodizing for 1500 s and subsequent immersion in 3.5% NaCl solution for 24 h is shown in Fig. 9. Cavity defects were found within the anodic film at the location of the circular cracks (Figs. 9a and 9b). Further, detaching of the anodic film from the alloy substrate at the film/alloy interface was observed at the periphery of the cavities. Fig. 9c shows the framed region M in Fig. 9b at increased magnification, revealing corroded alloy substrate below the detached anodic film and different film morphology above the corroded alloy substrate. The anodic film with different morphology must be associated with the corrosion events in the alloy substrate beneath the anodic film and is hereafter referred to as modified anodic film. Fig. 9d shows the cross section of the anodic film that is slightly away from the center of the circular cracks, again, revealing corroded alloy substrate and modified anodic film above the corroded alloy substrate. Note that the modified anodic film is in semiellipse shape. Little evidence of corrosion in the alloy substrate was detected in the region with the cavity defect on the film surface (Fig. 9e) or several hundred nanometers above the film/alloy interface (Fig. 9f). The phenomena observed here are consistent with that observed on AA2099 alloy under the same anodizing and immersion conditions [18].

The alloy after anodizing for 3000 s was also immersed in 3.5% NaCl solution for 24 h for comparison. There were much less circular cracks on the alloy surface anodized for 3000 (Fig. 10a) than the alloy anodized for 1500 s (Fig. 8a) under the same immersion conditions. However, localized corrosion sites of high population densities were observed from the ultramicrotomed cross sections of the alloy anodized for 3000 (Fig. 10b). Scrutiny of the corrosion sites (Figs. 10c and 10d) confirms that the corrosion event was preferentially initiated in the alloy substrate where a cavity defect was present at the film/alloy interface. The thickness of the porous anodic film, measured from the region free of evident defects, was increased from 3.2 µm (1500 s) to 7.4 µm (3000 s) as a consequence of prolonged anodizing. Since the barrier layer structure should be the same for the specimens anodized under the same anodizing voltage, the major difference between the specimens anodized for different periods of time is the total film thickness. Therefore, it is suggested that although the increase of porous layer thickness reduced the number of circular cracks on the anodic film surface, it did not evidently change the initiation of localized corrosion of the anodized alloy.

Fig. 11 shows the distribution of O, S, Al and Cu elements through the film thickness of the anodized alloy after immersion in 3.5% NaCl for up to 84 h (the signal intensity of Cu, O and S is multiplied by a factor of 10, 100 and 100, respectively). For the as-anodized alloy, the relatively strong signals of O and S in the early stage of sputtering are indicative of the anodic film while the relatively strong signals of Al and Cu signals in the late stage of sputtering are indicative of the alloy matrix. The film/alloy interface is determined from the half height of the Al profile in the transitional region between the anodic film and the alloy matrix, as indicated by the dashed-line in Fig. 11a. Assume the effect of the immersion test on the sputtering process is negligible, and then the film/alloy interface of the anodized alloy after immersion for different periods of time can be determined according to the sputtering time (i.e. 120 s). Compared with the as-anodized alloy, there was no evident change of the element profiles after immersion for 1 h (Fig. 11b) and 24 h (Fig. 11c). Interesting, distinct change occurred for all the elements after immersion for 48 h (Fig. 11d), 60 h (Fig. 11e) and 84 h (Fig. 11f).

4 Discussion

Most of IMPs in AA2055 alloy dissolved during anodizing, resulting in micrometer-scale cavity defects in the anodic film. Similar anodizing behavior was previously reported for the high-copper-containing Al-Fe-Mn-Cu particles (HCCPs) in AA2099 aluminum alloy [17, 18]. The high electrochemical activity of the HCCPs was ascribed to the presence of lithium in such particles [4, 5]. In this work, the compositions of the IMPs in AA2055 alloy were examined using normal and windowless EDS. As expected, all major alloying elements in the IMPs except for Li were successfully detected (Table 2). Further effort on detecting Li in the IMPs by using soft X-ray emission spectroscopy (SXES) was also unsuccessful. Due to the unique physical and chemical characteristics of Li (Li hardly displays an emission and it is readily oxidized even under the vacuum conditions of the SEM), it is still a technical challenge to spatially resolve Li in aluminium alloys. However, the anodizing behavior of the IMPs is different from that of the AlFeMnCu IMPs in Li-free alloys, indicates that the IMPs contains very active species, perhaps Li. , Therefore, a profound and systematic investigation of the composition and anodizing behavior of the IMPs in the alloy is necessary in future work.

The immersion tests indicate that the anodic film regions containing cavities at the film/alloy interface are the preferential sites for the penetration of corrosive electrolyte and the initiation of localized corrosion in the alloy substrate (Figs. 8 and 9). Further, it is suggested that the porous layer has little effect on the initiation of localized corrosion in the alloy substrate beneath the anodic film (Fig. 10). Therefore, it is deduced that the barrier layer structure of the anodic film is critical to the initiation of localized corrosion, and the barrier layer structure below the cavities which are at the film/alloy interface should be different from that in other regions.

Statistically, no matter when the anodizing process is terminated, there always exist anodized IMPs at the film/alloy interface. If an IMP is anodized and the oxide formed from the IMP just dissolves completely, the barrier layer should be absent in the region; if an IMP is partially anodized, the barrier layer formed from it is defective in this region due to the loose and cracked nature of the oxide (Fig. 3). In both cases, the regions containing the anodized/dissolved IMP at the film/alloy interface are vulnerable locations for the corrosive electrolyte to reach the alloy matrix and initiate localized corrosion. As for the cavities which are not at the film/alloy interface, they should have little effect on the initiation of localized corrosion in the anodized alloy because the barrier layer structure below them should be compact, similar to that of the normal barrier layer formed from alloy matrix.

The immersion test clearly showed that localized corrosion in the anodized alloy propagated after the initiation. It is believed that there two stages for the propagation of localized corrosion in the anodized alloy. In the early stage of propagation, the circular cracks have not yet developed in surface of the anodic film, and the corrosion process, to an extent, is controlled by the inward diffusion of the corrosive electrolyte and the outward diffusion of the corrosion products through the anodic film. In this stage, the effect of the porous layer thickness is significant because the diffusion distance increases with the increase of the porous layer thickness, especially for the corrosion products. Consequently, it would take longer time to develop the circular cracks in the relatively thick anodic film (Fig. 10). In the later stage of propagation, the circular cracks begin to develop on the surface of the anodic film, which allows direct access of the corrosive electrolyte to the alloy matrix below the cracked anodic film and, consequently rapid propagation of the localized corrosion. In this case, the barrier effect of the anodic film to the electrolyte and corrosion products deteriorates rapidly and the corrosion resistance of the anodized alloy is mainly determined by the nature of the alloy matrix.

Fig. 12 schematically illustrates the corrosion process of the anodized AA2055 alloy exposed to corrosive electrolyte. Suppose a cavity is formed from a IMP just before the termination of the anodizing process (Fig. 12a). When the anodized alloy is exposed to corrosive environment, the electrolyte will penetrate the porous layer and then reach the barrier layer of the anodic film. Due to the absence or defectiveness of the barrier layer below the cavity, the electrolyte reaches the alloy substrate in this region preferentially, initializing localized corrosion (Fig. 12b). The initiated localized corrosion then propagates in the peripheral region along the film/alloy interface, together with corrosion products diffusing via pores in the anodic film (revealed as modified anodic film in Fig. 12c). The corrosion event starts earlier in the central region of the cavity than the peripheral region, therefore the corrosion products diffuse further in the central region than the peripheral region. From a 3D point of view, the modified anodic film should be in the form of a solid spherical cap, which is revealed as semiellipse shape from the cross-sectional view. With further propagation of the localized corrosion, film cracking begins to develop locally (Fig. 12d) due to the lifting effect of the corrosion products at the film/alloy interface and the volume change in the modified anodic film. Once open-to-surface cracks emerge on the surface of the anodic film, the process for substances/ species exchange between the corrosion fronts and the outside environment will be accelerated due to the presence of macro-cracks in the anodic film and, consequently, more severe localized corrosion will be expected.

5 Conclusions

1) Anodizing of IMPs in AA2055 Al-Cu-Li alloy resulted in cavity defects of micrometer-scale in the anodic film. When the anodized IMPs were present at the film/alloy interface, localized defects were present in the barrier layer of the anodic film.

2) Localized corrosion was preferentially initiated in the regions with the cavity defects at the film/alloy interface.

3) The initiated localized corrosion then propagated in the alloy substrate beneath the anodic film, which was controlled by a diffusion process. The propagation of the localized corrosion ultimately resulted in localized cracking of the anodic film and direct access of the electrolyte to the alloy below the cracked anodic film.

4) The initiation of localized corrosion in the anodized alloy (without sealing) was mainly controlled by the barrier layer structure of the anodic film while the propagation of the localized corrosion was controlled by the substances/species exchange between the corrosion fronts and the outside environment.

Acknowledgements

The authors wish to gratefully acknowledge the following projects: Basic and Frontier Research Program of Chongqing (Grant No. cstc2016jcyjA0490, cstc2017jcyjAX0285) and Research Foundation of AVIC Manufacturing Technology Institute (No. KS911608114).

Data availability

The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.

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Figure captions

Fig. 1 Current density-time response recorded during anodizing of AA2055 alloy in an aqueous solution containing 0.46 M sulfuric acid and 0.53 M tartaric acid, under 14 V, at 37 °C, for 1500 s.

Fig. 2 (a, b) Backscattered electron micrographs of AA2055 alloy, showing distribution of intermetallic particles; (c, d) corresponding secondary electron micrographs after anodizing for 5 s, showing preferential anodizing of intermetallic particles.

Fig. 3 Secondary electron micrographs of AA2055 alloy after anodizing for 5 s, showing morphologies of anodized intermetallic particles: (a) Particle 1; (b) Particle 5; (c) Particles 6 and 7; (d) the framed area in (b) at increased magnification; (e) Particle 9; and (f) Particle 12.

Fig. 4 (a) Backscattered electron micrograph of AA2055 alloy before anodizing; (b) backscattered and (c) secondary electron micrograph of the alloy after anodizing for 180 s; and (d-f) the framed areas A, B and C at increased magnifications.

Fig. 5 Secondary electron micrographs of AA2055 alloy after anodizing for 1500 s: (a) surface morphology at low magnification; (b) a surface cavity; (c) localized cracking in the anodic film; and (d) nanopores in the anodic film.

Fig. 6 Cross-sectional view of the anodic film formed on AA2055 alloy after TSA anodizing for 1500 s: (a) uniform film region; (b-d) regions containing cavities in the anodic film. The cross sections were prepared by mechanical bending.

Fig. 7 Open circuit potential-time responses of bare and anodized AA2055 alloy recorded during immersion in 3.5wt. % NaCl solution at 20 °C for 84 h.

Fig. 8 Secondary electron micrographs of AA2055 alloy after anodizing for 1500 s and subsequent immersion in 3.5% NaCl solution for (a) 24 h and (c) 84 h; (b) and (d) are the framed regions K and L at increased magnifications.

Fig. 9 Secondary electron micrographs of the cross sections of AA2055 alloy after anodizing for 1500 s and subsequent immersion in 3.5% NaCl solution for 24 h, showing: (a-c) localized corrosion sites; (d) the framed region M at an increased magnification; (e) the region containing a surface cavity; and (f) the region containing a cavity above the film/alloy interface.

Fig. 10 Secondary electron micrographs of AA2055 alloy after anodizing for 3000 s and subsequent immersion in 3.5% NaCl solution for 24 h: (a) surface morphology; (b) cross-sectional morphology; (c) and (d) the framed regions R and S at increased magnifications.

Fig.11 Glow discharge optical emission spectroscopy (GDOES) analysis of AA2055 alloy after anodizing for 1500 s (a) and subsequent immersion in 3.5% NaCl for (b) 1 h, (c) 24 h, (d) 48 h, (e) 60 h and (f) 84 h, showing distribution of Al, Cu, O and S elements through the film thickness. The dashed-line indicates the film/alloy interface.

Fig. 12 Schematic diagrams showing initiation and propagation of localized corrosion in anodized AA2055 alloy exposed to the corrosive electrolyte: (a) formation of a cavity at the film/alloy interface due to anodizing/dissolution of an intermetallic particle; (b) penetration of the electrolyte through the anodic film and initiation of localized corrosion in the alloy substrate below the cavity; (c) propagation of the localized corrosion and modification of the anodic film surrounding the cavity; and (d) localized cracking of the anodic film and consequently accelerated corrosion propagation.

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