Comparison of Corrosion Resistance of the AA2524-T3 and the AA2024-T3

The 2XXX Al alloys are characterized by their superior mechanical properties resulting from alloying elements and precipitation hardening treatments. The AA2524-T3 alloy was developed to replace the AA2024-T3 alloy in the aerospace industry. However, both alloys present many intermetallic particles (IMCs) in their microstructure, and this is the main reason for their high susceptibility to localized corrosion (such as pitting and stress corrosion cracking). Despite the similarities between these alloys (e.g., chemical composition and type of intermetallics) the literature comparing their properties is scarce and focuses mainly on their mechanical properties, not their corrosion resistances. In this investigation, the corrosion resistance of the AA2524-T3 alloy was compared to the AA2024-T3 alloy. The microstructure of both alloys was analyzed by Scanning Electron Microscopy before and after immersion in the test electrolyte, and the number and area fraction of intermetallics of each alloy was determined. The corrosion resistance of both alloys was monitored as a function of exposure time by electrochemical impedance spectroscopy and the results were fitted using electrical equivalent circuits. The AA2524-T3 alloy presented not only higher impedance values but also less corroded areas than the AA2024-T3 alloy.


Introduction
The 2XXX Al alloys are largely used in the aircraft industry due to their superior mechanical properties resulting from alloying elements addition, mainly Cu and Mg, and precipitation hardening treatments. The AA2524-T3 alloy is a relatively new alloy (formerly known as C188-T3) developed by ALCOA to replace the AA2024-T3 alloy in fuselage skins. It is currently used in the Boeing 777 aircraft [1,2]. It presents high damage-tolerance and excellent fatigue properties [1]. However, both the AA2024-T3 and AA2524-T3 alloys possess a large amount of intermetallic particles (IMCs) in their microstructure, and this is the main reason for their high susceptibility to localized corrosion such as pitting and stress corrosion cracking [3][4][5][6][7][8]. Localized corrosion in aluminium alloys uses to be caused by microgalvanic effects between IMCs and the matrix [7][8][9][10]. This generally results in pitting corrosion, which is considered to be one of the main microstructure dependent damage mechanisms of high strength aluminium alloys [7]. Particles constituted by the alloying elements present different electrochemical activity from the matrix and, therefore, represent preferential sites for pitting nucleation. In potential-controlled conditions, pitting is characterized by two types of events, metastable and stable, which occur before and after the pitting potential, respectively [11]. The AA2524-T3 alloy has lower amounts of Fe and Si, which are considered as deleterious to the corrosion resistance, and a narrower range for the main alloying elements Cu and Mg [32]. Table 1. Analyzed and nominal chemical composition (wt %) of the AA2024-T3 and AA2524-T3 alloys [39].

Elements
AA2024-T3 AA2524-T3   30 −0.722 [41] AA2524 [42] 445.4 340. 3 19 34 −0.590 [43] Samples of 400 mm 2 were cut from the sheets. Their surfaces were prepared by grinding them with silicon carbide paper up to # 4000, followed by polishing with a diamond paste (up to 0.25 µm) to a mirror surface, degreasing in ethyl alcohol, washing with deionized water, and drying under a hot air stream. The corrosion resistance of both aluminium alloys was monitored in 0.01 mol L −1 NaCl as a function of exposure time by immersion tests and electrochemical impedance spectroscopy (EIS).
Experiments were performed using a Solartron 1287 electrochemical interface coupled to a Solartron 1260 frequency response analyser (FRA), which was used to obtain the electrochemical impedance spectroscopy (EIS) diagrams. The perturbation range was 20 mV (rms). The acquisition rate was 10 points per decade in a frequency range from 10 mHz to 100 kHz for all samples. The electrochemical tests were performed using a three-electrode setup configuration with the aluminium alloys as working electrodes and an exposed area of 1.0 cm 2 ( Figure 1). The reference electrode used was an Ag/AgCl, 3 mol L −1 KCl, and a platinum wire was employed as an auxiliary electrode. The electrochemical experiments were carried out in naturally aerated solution at room temperature. Each test was performed six times to ensure their reproducibility.
The microstructures of the aluminium alloys were analyzed by a Field Emission Gun Scanning Electron Microscope FEI Quanta 650 (FEG-SEM) prior and after different exposure periods to the test electrolyte. The microstructures of the aluminium alloys were analyzed by a Field Emission Gun Scanning Electron Microscope FEI Quanta 650 (FEG-SEM) prior and after different exposure periods to the test electrolyte.

Results and Discussion
The surface of both the AA2024-T3 and AA2524-T3 alloys were observed by SEM after 2 h exposure to the 0.01 mol.L −1 NaCl solution. A general view of the microstructures of the AA2024-T3 and AA2524-T3 alloys are shown in Figure 2a (Figure 2e,f)) were identified by EDX, and they are in agreement with the literature [9,10,19]. However, in the AA2524-T3 alloy, the Al-Cu-Mn-Fe-(Si) particles were present in a lower amount and with different morphologies from those found in the AA2024-T3 alloy, whereas the Al-Cu-Mg are more numerous, smaller, and more uniformly distributed. The AA2024-T3 alloy is characterized by a smaller number of IMCs, which, however, are generally larger than those found in the AA2024-T3 alloy ( Figure 3). Consequently, the area fraction of the AA2024-T3 IMC's is higher than the AA2524-T3 (Table 3).

Results and Discussion
The surface of both the AA2024-T3 and AA2524-T3 alloys were observed by SEM after 2 h exposure to the 0.01 mol.L −1 NaCl solution. A general view of the microstructures of the AA2024-T3 and AA2524-T3 alloys are shown in Figure 2a (Figure 2e,f)) were identified by EDX, and they are in agreement with the literature [9,10,19]. However, in the AA2524-T3 alloy, the Al-Cu-Mn-Fe-(Si) particles were present in a lower amount and with different morphologies from those found in the AA2024-T3 alloy, whereas the Al-Cu-Mg are more numerous, smaller, and more uniformly distributed. The AA2024-T3 alloy is characterized by a smaller number of IMCs, which, however, are generally larger than those found in the AA2024-T3 alloy ( Figure 3). Consequently, the area fraction of the AA2024-T3 IMC's is higher than the AA2524-T3 (Table 3). The general analysis of the micrographs ( Figure 4) show that the corrosive attack is more evenly distributed on the AA2524-T3 alloy (Figure 4b), whereas for the AA2024-T3 alloy ( Figure 4a) it is mainly concentrated around larger IMCs, including the dark areas around the bigger Al-Cu-Mg precipitates (indicated by arrows). On both alloys, the Al-Cu-Mn-Fe precipitates showed a rather random behaviour with some of them remaining intact after 2 h of immersion, likely due to a protective oxide film that hinder the cathodic reaction, as suggested by Birbilis, et al. [12] (Figure 4a,b).
After 7 h of testing of the AA2024-T3 alloy, the corrosive attack was mainly concentrated at the interface between the matrix and the larger intermetallic particles. No localized corrosion at the grain boundaries was detected. (Figure 4c). Figure 4d shows that most of the smaller precipitates had been removed from the surface of the AA2524-T3 sample by the corrosive attack, resulting in a "cleaner" surface in comparison with 2 h of test, even though some local activity remained around large IMCs. On the other hand, the corrosive attack of the AA2024-T3 continued to be mainly concentrated at the interface between the matrix and the larger intermetallic particles (Figure 4c), which, as previously showed, were more numerous in this material. For longer immersion times the "cleaning" effect on the AA2524-T3 surface is even more pronounced than that of the AA2024-T3 (Figure 4f,e, respectively). The results showed that IMCs' area fraction of the AA2524-T3 is lower in comparison with the AA2024-T3. More than 94% of these IMCs of the first alloy are smaller than 10 µm, and only 82% of the latter, confirming the theory that the corrosion attack on the AA2524-T3 alloy surface was faster due to its larger number of small particles.

Material
IMCs Area Fraction (%) AA2024-T3 2.52 ± 0.42 AA2524-T3 1.64 ± 0.14 The general analysis of the micrographs ( Figure 4) show that the corrosive attack is more evenly distributed on the AA2524-T3 alloy (Figure 4b), whereas for the AA2024-T3 alloy ( Figure 4a) it is mainly concentrated around larger IMCs, including the dark areas around the bigger Al-Cu-Mg precipitates (indicated by arrows). On both alloys, the Al-Cu-Mn-Fe precipitates showed a rather random behaviour with some of them remaining intact after 2 h of immersion, likely due to a protective oxide film that hinder the cathodic reaction, as suggested by Birbilis, et al. [12] (Figure 4a,b). The corrosion resistance of both alloys was also evaluated by EIS with immersion time and the results are shown in Figures 5 and 6. It is clearly seen that the impedance modulus associated with the AA2524-T3 alloy was higher than that of the AA2024-T3 during the whole test period. Independently of the alloy brand, EIS diagrams are characterized by a broad high frequency depressed capacitive loop followed by a low frequency capacitive loop that evolves to a diffusion-controlled process after 11 h of exposure to the test electrolyte.
The EIS results for both alloys were fitted with the two/three-time constants equivalent electric circuits (EEC) shown in Figure 7. These ECC have been established in the literature on aluminium alloys EIS fittings and were also tested in previous works from our research group [9,[44][45][46]. Table 4 presents the parameters obtained from the equivalent electrical circuit fitted to the AA2024-T3 and AA2524-T3 EIS data. The EEC of Figure 7a was used to fit the data acquired from the first hour until 9 h of immersion, whereas the diagrams obtained from 11 h until the end of the test were fitted with the EEC of Figure 7b. In this latter case, the low frequency R//CPE element was substituted for a single CPE, meant to simulate a diffusion-controlled process. According to Campestrini, et al. [47], when the resistance associated with the diffusion becomes very large the R//CPE element may be replaced by a simple CPE element with the exponent "α" value of 0.5, which, in an ideal situation, is represented by a Warburg element. In the proposed circuits the pair R ox //CPE ox stands for the capacitance of the oxide film in parallel with conductive pathways associated with defective sites created by the IMCs, which leads to the unprotected metal surface. R ct //CPE dl refers to the charge transfer resistance coupled to the charging of the double layer, whereas CPE cor //R cor is ascribed to the low frequency corrosion processes that kinetically control the alloys' deterioration, which gradually evolves to a diffusion-controlled process and is likely associated with the oxygen reduction reaction taking place in nobler IMCs.  After 7 h of testing of the AA2024-T3 alloy, the corrosive attack was mainly concentrated at the interface between the matrix and the larger intermetallic particles. No localized corrosion at the grain boundaries was detected. (Figure 4c). Figure 4d shows that most of the smaller precipitates had been removed from the surface of the AA2524-T3  Table 4 shows the evolution of the R ox //CPE ox values. For both alloys, CPE ox increased with exposure time. SEM analysis has shown that as the corrosive attack proceeded, holes were formed in the samples surface as a result of the dissolution/detachment of the IMCs particles increasing the effective area exposed to the electrolyte [10]. If pits did not progress in these regions, a fresh oxide layer was formed. However, it should be more defective. Both facts contribute to capacitance increase. In addition, from the beginning of the exposure period, and during the whole test, CPE ox was smaller for AA2524-T3, indicating the presence of a thicker oxide layer in this alloy, in agreement with the results from previous studies in the literature [46,48]. The EIS results for both alloys were fitted with the two/three-time constants equivalent electric circuits (EEC) shown in Figure 7. These ECC have been established in the literature on aluminium alloys EIS fittings and were also tested in previous works from our research group [9,[44][45][46]. Table 4 presents the parameters obtained from the equivalent electrical circuit fitted to the AA2024-T3 and AA2524-T3 EIS data. The EEC of Figure  7a was used to fit the data acquired from the first hour until 9 h of immersion, whereas the diagrams obtained from 11 h until the end of the test were fitted with the EEC of Figure  7b. In this latter case, the low frequency R//CPE element was substituted for a single CPE, meant to simulate a diffusion-controlled process. According to Campestrini, et al. [47], when the resistance associated with the diffusion becomes very large the R//CPE element may be replaced by a simple CPE element with the exponent "α" value of 0.5, which, in an ideal situation, is represented by a Warburg element. In the proposed circuits the pair Rox//CPEox stands for the capacitance of the oxide film in parallel with conductive pathways associated with defective sites created by the IMCs, which leads to the unprotected metal surface. Rct//CPEdl refers to the charge transfer resistance coupled to the charging of the double layer, whereas CPEcor//Rcor is ascribed to the low frequency corrosion processes Concerning R ox , Table 4, it also increased with immersion time, indicating that electrochemically active sites on both alloys' surfaces become less numerous and/or were less easily reachable. As already demonstrated in the SEM analysis (Figure 4), the selective attack near the IMCs results in surface "cleaning", leading to a lower number of active sites. In addition, deposition of corrosion products was frequently found above large IMCs. These features would hinder the access of aggressive species to the matrix surface and can contribute to the R ox increase. Accordingly, as the surface of AA2524-T3 progressively became "cleaner" than AA2024-T3, the rate of the R ox increase was much faster for the former alloy. Therefore, it is proposed that the attack of the small particles and their removal from surface are the main reasons for the R ox increase, as this reduces the number of defective sites where electrochemical reactions could take place. However, blocking of active site by aluminium hydroxide precipitation may also have contributed to the R ox increase. that kinetically control the alloys' deterioration, which gradually evolves to a diffusioncontrolled process and is likely associated with the oxygen reduction reaction taking place in nobler IMCs.   As mentioned earlier, the R ct //CPE dl pair is associated with the charge transfer reactions (R ct ) occurring at the interface of the matrix at the defective sites of the oxide (near active IMCs). The evolution for the components R ct and CPE dl with exposure time to the 0.01 mol L −1 NaCl electrolyte is shown in Table 4. For AA2524-T3, R ct behaviour was significantly oscillating during the whole immersion period, which must has been a consequence of the detachment and outbreak of new IMCs as corrosion proceeded. For this sample, CPE dl remained almost constant during the first 15 h, indicating that the electrochemically active surface area remained almost unchanged. However, from this period onwards the decrease in CPE dl might be attributed to the formation of corrosion products on some active areas, clearly indicated on the AA2524-T3 surface, that partially blocked these sites.
Conversely, for AA2024-T3, after up to 40 h of testing, R ct decreased continuously. This tendency may be explained by the enrichment in the more noble components of the remaining active IMCs. Therefore, even though in smaller number, due to detachment, enhanced galvanic activity would be expected between the matrix and the Cu-enriched IMCs. For longer immersion periods, an increase of R ct was observed. This can be ascribed to the precipitation of corrosion products above the IMCs and their vicinity, which provided a barrier between the electrolyte and the metallic surface. Accordingly, CPE dl initially increased, indicating that the electrochemically active surface augmented. However, for longer exposure times, it started to decrease due to active surface area reduction as corrosion products precipitated.
Finally, the low frequency time constant, corresponding to the rate-controlling step of the corrosion process, was mainly represented by CPE cor and its "α cor ", and R cor were important only up to nine hours of exposure. During the initial immersion period, α cor values ranged between 0.69 and 0.83, suggesting a mixed controlled process for both alloys, corresponding to the period where the EIS diagrams were fitted with the EEC of Figure 6. For longer immersion times, "α cor " varied between 0.4 and 0.6, showing the main contribution of diffusive processes likely through the precipitated corrosion products.
Higher capacitance values were associated with the AA2024-T3 alloy comparatively to the AA2524-T3 (Table 4) after the first hour of immersion, although there was much larger variation in the CPE cor values for this latter alloy, likely due to the stronger contribution of partially soluble corrosion products on active sites at its surface which were periodically removed.
The electrochemical results demonstrated that the corrosion resistance of the AA2524-T3 was higher than the AA2024-T3.

Conclusions
In this study, the corrosion resistances of the AA2024-T3 and the AA2524-T3 alloys were compared. The differences between these alloys are the number and the precipitates' area fraction. The AA2024-T3 alloy has a smaller number of IMCs, which, however, are larger than those found in the AA2524-T3 alloy. Therefore, the IMCs area fraction of the former is higher than the latter. The SEM results suggest that the rapid removal of active sites from the AA2524-T3 surface (smaller IMCs) promotes a cleaner surface.
The hypothesis that defective areas prone to electrochemical reactions were removed from AA2524-T3 was confirmed by the EEC fitting to EIS results, as AA2524 R ox increases and CPE ox decreases with immersion time. Additionally, the corrosion process of AA2524, represented by CPE cor , "α cor " and R cor , are significant only up to nine hours of exposure and confirm the higher contribution of active sites of the surface, caused by partially soluble corrosion products.
The electrochemical tests also showed that the localized corrosion resistance of the AA2524-T3 was higher than the AA2024-T3 during the whole test period. This was indicated by the higher impedance values associated with the first alloy and confirmed by surface observation after various immersion periods in the tested electrolyte. On the other hand, SEM analysis showed that the corrosion attack on the AA2524-T3 alloy surface was faster due to its larger number of small particles.