Corrosion Behavior and Mechanical Performance of 7085 Aluminum Alloy in a Humid and Hot Marine Atmosphere

This work analyzed changes in the corrosion morphology and mechanical performance of 7085 aluminum alloy after outdoor exposures for different times in a humid and hot marine atmospheric environment. After one month of exposure, a pronounced corrosion of the alloy was observed. The corrosion product was mainly Al(OH)3, and the corrosion features were mainly pitting corrosion and intergranular corrosion (IGC). With the exposure time extended from 6 months to 12 months, the IGC depth increased from 114 μm to 190 μm. After a 1-year outdoor exposure in a humid and hot marine environment, the alloy’s ultimate strength and yield stress remained nearly unchanged, while its elongation and area reduction decreased from the original 6% and 9%, respectively, to 3% and 5%. Moreover, the reasons for IGC and its effect on the mechanical performance was analyzed.


Introduction
7xxx series aluminum alloys have been widely applied in the aviation and aerospace industry due to their excellent properties, such as a low density, high specific strength, good processing performance, and so on [1][2][3]. Among 7xxx series aluminum alloy, 7085 alloy possesses an outstanding and advantageous status because of an extra-good corrosion resistance and low quenching sensitivity [4][5][6]. Thus, many studies have focused on its microstructure and mechanical properties [7][8][9][10]. For aluminum alloy components, they frequently operate in coastal regions, with a high salt spray presence, high humidity, and hot atmosphere. The corrosive effects of such an environment include IGC, pitting corrosion, and stress corrosion of special equipment, significantly deteriorating their service life and safety [11][12][13][14]. Therefore, clarifying the corrosion behavior and mechanical strength variation of the alloy under study in a corrosive environment, especially in an actual marine atmosphere, is quite topical.
Recently, some work on the corrosion behavior of aluminum alloys has been performed [15][16][17][18][19][20]. Zhao et al. studied the atmospheric corrosion behavior and mechanism of 7A85 aluminum alloy in Qingdao industrial-marine atmosphere and concluded that pitting corrosion and IGC caused significant deterioration of the mechanical properties of 7A85 aluminum alloy [17]. Peng et al. studied the corrosion behavior of extruded 6061 aluminum alloy exposed to a simulated Nansha marine atmosphere for 40 days and revealed severe IGC and intragranular corrosion on the alloy sample surface [19]. Song et al. studied the fatigue damage evolution in 7075 alloy and found that the precorrosion in a standard exfoliation corrosion solution leads to a significant decrease of the fatigue property. According to previous studies, it is obvious that aluminum alloys experience serious corrosion in a corrosive environment, resulting in the deterioration of mechanical properties.
Therefore, this work aims to study the corrosion behavior and mechanical performance of 7085 alloy in the South China Sea. Correspondingly, outdoor exposure tests in Wanning,

Experimental Section
The material under this work was 7085 aluminum alloy in the form of a forging billet. Its chemical composition is listed in Table 1. The alloy was solution-treated at 470 • C for 6 h, followed by a 5% cold compression. Finally, it was artificially two-step-aged at 120 • C for 6 h and 160 • C for 10 h and named the T7452 treatment. The tabular samples with a size of 100 mm × 50 mm × 3 mm and dumbbell-shaped tensile samples with a diameter of 10 mm were cut from the forging billet, which was placed in a humid and hot marine atmospheric environment (in Wanning city, Hainan Island, China) for outdoor exposure tests. The average air temperature, relative humidity, and Cl − deposition rate were about 23.9 • C, 87.6%, and 14.5875 mg/(m 2 ·d), respectively [21]. The samples were firmly installed on the exposure test frame and then inclined at 45 • to the horizontal surface, as shown in Figure 1. To observe the cross-section morphology and mechanical performance, the samples were analyzed after exposure for 1, 6, and 12 months. serious corrosion in a corrosive environment, resulting in the deterioration of mechani cal properties. Therefore, this work aims to study the corrosion behavior and mechanical perfor mance of 7085 alloy in the South China Sea. Correspondingly, outdoor exposure tests in Wanning, Hainan Province of China, were designed. After outdoor exposure, the mac roscopic corrosion features, microstructure, and tensile properties were analyzed. Th mechanisms of the 7085 alloy's corrosion in actual humid and hot marine atmosphere and its effects on tensile properties were also discussed.

Experimental Section
The material under this work was 7085 aluminum alloy in the form of a forging bil let. Its chemical composition is listed in Table 1. The alloy was solution-treated at 470 °C for 6 h, followed by a 5% cold compression. Finally, it was artificially two-step-aged a 120 °C for 6 h and 160 °C for 10 h and named the T7452 treatment. The tabular sample with a size of 100 mm × 50 mm × 3 mm and dumbbell-shaped tensile samples with diameter of 10 mm were cut from the forging billet, which was placed in a humid and hot marine atmospheric environment (in Wanning city, Hainan Island, China) for out door exposure tests. The average air temperature, relative humidity, and Cl − deposition rate were about 23.9 °C, 87.6%, and 14.5875 mg/(m 2 ·d), respectively [21]. The sample were firmly installed on the exposure test frame and then inclined at 45° to the horizon tal surface, as shown in Figure 1. To observe the cross-section morphology and mechan ical performance, the samples were analyzed after exposure for 1, 6, and 12 months.  The microstructure was analyzed by optical microscopy (OM), X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The sample etchant for the metallographic analysis was Keller's reagent, con sisting of 95 mL DI water, 2.5 mL HNO3, 1.5 mL HCl (36%), and 1.0 mL HF. The metal lographic microstructure was observed by OM (Leica DFC320) and SEM (JEOL JSM-6390A), and the energy dispersive spectrometer (EDS) of the SEM was used for th compositional analysis. The SEM was executed in secondary electron mode, and it voltage was 20 KV. The varieties of the second phases were analyzed by XRD (D/MAX 2200 PC). The X-ray source was a Cu (Kα) target with a wavelength of λ = 1.5406 Å unde the conditions of 40 kV tube pressure, 30 mA tube flow, 0.02° scan step size, and 10°~90 scanning range. The microscopic structural analysis was performed by TEM (Tecnai F3 G2) with an accelerating voltage of 300 kV. The preparation method is as follows: a mm square piece was cut from the surface of the sample by the wire cutting method The microstructure was analyzed by optical microscopy (OM), X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The sample etchant for the metallographic analysis was Keller's reagent, consisting of 95 mL DI water, 2.5 mL HNO 3 , 1.5 mL HCl (36%), and 1.0 mL HF. The metallographic microstructure was observed by OM (Leica DFC320) and SEM (JEOL JSM-6390A), and the energy dispersive spectrometer (EDS) of the SEM was used for the compositional analysis. The SEM was executed in secondary electron mode, and its voltage was 20 KV. The varieties of the second phases were analyzed by XRD (D/MAX 2200 PC). The X-ray source was a Cu (K α ) target with a wavelength of λ = 1.5406 Å under the conditions of 40 kV tube pressure, 30 mA tube flow, 0.02 • scan step size, and 10 •~9 0 • scanning range. The microscopic structural analysis was performed by TEM (Tecnai F30 G2) with an accelerating voltage of 300 kV. The preparation method is as follows: a 1 mm square piece was cut from the surface of the sample by the wire cutting method. Then, the specimen was polished using 600#~1500# sandpaper to reduce the thickness to less than 0.05 mm, and double-spray electropolishing technology was used to thin the perforation. The double-spray electrolyte was HClO 4 :C 4 H 9 OH:CH 3 OH = 6:34:60 (volume ratio) cooled by liquid nitrogen, and the electrolysis temperature was controlled at approximately −30 • C. The tensile performance at room temperature was tested by a universal testing machine, and the tensile speed was 2 mm/min. Each measured result takes the average of 3 samples. Figure 2 shows the original microstructure of the 7085 alloy. As observed, the microstructure is mainly composed of α(Al) grains and granular secondary phases. The α(Al) grains are equiaxed, and the grain size is small and uniform. According to the SEM image, the second phase particles precipitated continuously and distributed densely at the grain boundaries of the matrix. Moreover, the size of second phase particles is not uniform. The XRD analysis results of the original sample are shown in Figure 3, and the results indicate that the second phase particles belong to the MgZn 2 phase.

Original Microstructure
Then, the specimen was polished using 600#~1500# sandpaper to reduce the thickness to less than 0.05 mm, and double-spray electropolishing technology was used to thin the perforation. The double-spray electrolyte was HClO4:C4H9OH:CH3OH = 6:34:60 (volume ratio) cooled by liquid nitrogen, and the electrolysis temperature was controlled at ap proximately −30 °C. The tensile performance at room temperature was tested by a uni versal testing machine, and the tensile speed was 2 mm/min. Each measured result take the average of 3 samples. Figure 2 shows the original microstructure of the 7085 alloy. As observed, the mi crostructure is mainly composed of α(Al) grains and granular secondary phases. The α(Al) grains are equiaxed, and the grain size is small and uniform. According to the SEM image, the second phase particles precipitated continuously and distributed densely a the grain boundaries of the matrix. Moreover, the size of second phase particles is no uniform. The XRD analysis results of the original sample are shown in Figure 3, and the results indicate that the second phase particles belong to the MgZn2 phase.   Figure 4 shows the macroscopic morphology of the 7085 alloy exposed to the hu mid and hot marine atmospheric environment with different exposure times. It is obvi ous that corrosion behaviors were observed on the samples surface after exposure to a humid and hot marine atmospheric environment for one month. Specifically, a large number of clustered gray products (approximately 30% in area) were observed on the sample surface, as shown in Figure 4a. With a prolonged exposure time from 1 month to 6 months and to 12 months, as shown in Figure 4a,b, the area fraction of the corrosion product on the samples' surface did not increase significantly. perforation. The double-spray electrolyte was HClO4:C4H9OH:CH3OH = 6:34:60 ( ratio) cooled by liquid nitrogen, and the electrolysis temperature was controlled proximately −30 °C. The tensile performance at room temperature was tested by versal testing machine, and the tensile speed was 2 mm/min. Each measured resu the average of 3 samples. Figure 2 shows the original microstructure of the 7085 alloy. As observed, crostructure is mainly composed of α(Al) grains and granular secondary phas α(Al) grains are equiaxed, and the grain size is small and uniform. According to t image, the second phase particles precipitated continuously and distributed den the grain boundaries of the matrix. Moreover, the size of second phase particle uniform. The XRD analysis results of the original sample are shown in Figure 3, results indicate that the second phase particles belong to the MgZn2 phase.   Figure 4 shows the macroscopic morphology of the 7085 alloy exposed to mid and hot marine atmospheric environment with different exposure times. It ous that corrosion behaviors were observed on the samples surface after exposu humid and hot marine atmospheric environment for one month. Specifically, number of clustered gray products (approximately 30% in area) were observed sample surface, as shown in Figure 4a. With a prolonged exposure time from 1 m 6 months and to 12 months, as shown in Figure 4a,b, the area fraction of the co product on the samples' surface did not increase significantly.  Figure 4 shows the macroscopic morphology of the 7085 alloy exposed to the humid and hot marine atmospheric environment with different exposure times. It is obvious that corrosion behaviors were observed on the samples surface after exposure to a humid and hot marine atmospheric environment for one month. Specifically, a large number of clustered gray products (approximately 30% in area) were observed on the sample surface, as shown in Figure 4a. With a prolonged exposure time from 1 month to 6 months and to 12 months, as shown in Figure 4a,b, the area fraction of the corrosion product on the samples' surface did not increase significantly.   Figure 5 shows the morphology and XRD map of the corrosion product of the 708 alloy in the humid and hot marine environment. According to Figure 5a, the corrosio product on the specimens' surface was relatively dense, which could protect the matri well during outdoor exposure. This is the reason why the area fraction of the corrosio product did not obviously increase with a prolonged exposure time. The XRD result in dicated that in humid and hot marine environments, the corrosion surface of the presen alloy was mainly composed of an Al matrix, Al(OH)3, and AlCl3. Thus, one can speculat that the corrosion products were mainly Al(OH)3 and AlCl3. As shown in Figure 5a, th EDS result indicated that the main elements of the corrosion product were O and Al, an the atomic percentage was approximately 3:1. It could also be demonstrated that th corrosion product of the 7085 alloy was mainly Al(OH)3. The marine environment, wit high salt fog and a high Clconcentration, will accelerate the corrosion, resulting in th occurrence of AlCl3.

Microscopic Corrosion Features
To reveal the corrosion mechanism of the 7085 alloy, its microscopic corrosion fea tures were analyzed. Figure 6 shows the cross-sectional morphology of the 7085 allo after outdoor exposure. In a humid and hot marine environment, the 7085 alloy exhibi ed significant pitting corrosion and IGC. Meanwhile, pitting corrosion was mainly o the surface of the section, and IGC was mainly in the interior of the alloy. However, IGC and pitting corrosion were connected to each other. Moreover, IGC spread along th grain boundaries of fine equiaxed Al grains, and the corrosion was mainly intergranula corrosion. With the exposure time extended from 6 months to 12 months, as shown i Figure 6c, the IGC depth increased from 114 μm to 190 μm. Accordingly, the averag   Figure 5a, the corrosion product on the specimens' surface was relatively dense, which could protect the matrix well during outdoor exposure. This is the reason why the area fraction of the corrosion product did not obviously increase with a prolonged exposure time. The XRD result indicated that in humid and hot marine environments, the corrosion surface of the present alloy was mainly composed of an Al matrix, Al(OH) 3 , and AlCl 3 . Thus, one can speculate that the corrosion products were mainly Al(OH) 3 and AlCl 3 . As shown in Figure 5a, the EDS result indicated that the main elements of the corrosion product were O and Al, and the atomic percentage was approximately 3:1. It could also be demonstrated that the corrosion product of the 7085 alloy was mainly Al(OH) 3 . The marine environment, with high salt fog and a high Clconcentration, will accelerate the corrosion, resulting in the occurrence of AlCl 3 . Figure 4. Macroscopic morphology of the 7085 alloy exposed to a humid and hot marine atm pheric environment: (a) outdoor exposure for 1 month; (b) outdoor exposure for 6 months and outdoor exposure for 12 months Figure 5 shows the morphology and XRD map of the corrosion product of the 7 alloy in the humid and hot marine environment. According to Figure 5a, the corros product on the specimens' surface was relatively dense, which could protect the ma well during outdoor exposure. This is the reason why the area fraction of the corros product did not obviously increase with a prolonged exposure time. The XRD result dicated that in humid and hot marine environments, the corrosion surface of the pres alloy was mainly composed of an Al matrix, Al(OH)3, and AlCl3. Thus, one can specu that the corrosion products were mainly Al(OH)3 and AlCl3. As shown in Figure 5a, EDS result indicated that the main elements of the corrosion product were O and Al, the atomic percentage was approximately 3:1. It could also be demonstrated that corrosion product of the 7085 alloy was mainly Al(OH)3. The marine environment, w high salt fog and a high Clconcentration, will accelerate the corrosion, resulting in occurrence of AlCl3.

Microscopic Corrosion Features
To reveal the corrosion mechanism of the 7085 alloy, its microscopic corrosion tures were analyzed. Figure 6 shows the cross-sectional morphology of the 7085 a after outdoor exposure. In a humid and hot marine environment, the 7085 alloy exhi ed significant pitting corrosion and IGC. Meanwhile, pitting corrosion was mainly the surface of the section, and IGC was mainly in the interior of the alloy. However, I and pitting corrosion were connected to each other. Moreover, IGC spread along grain boundaries of fine equiaxed Al grains, and the corrosion was mainly intergranu corrosion. With the exposure time extended from 6 months to 12 months, as shown Figure 6c, the IGC depth increased from 114 μm to 190 μm. Accordingly, the aver

Microscopic Corrosion Features
To reveal the corrosion mechanism of the 7085 alloy, its microscopic corrosion features were analyzed. Figure 6 shows the cross-sectional morphology of the 7085 alloy after outdoor exposure. In a humid and hot marine environment, the 7085 alloy exhibited significant pitting corrosion and IGC. Meanwhile, pitting corrosion was mainly on the surface of the section, and IGC was mainly in the interior of the alloy. However, IGC and pitting corrosion were connected to each other. Moreover, IGC spread along the grain boundaries of fine equiaxed Al grains, and the corrosion was mainly intergranular corrosion. With the exposure time extended from 6 months to 12 months, as shown in Figure 6c, the IGC depth increased from 114 µm to 190 µm. Accordingly, the average weight loss increased from 0.0231 g to 0.0294 g. Thus, after outdoor exposure for 12 months, the corrosion rate of 7085 alloy was calculated, and the value was 2.7013 g/(m 2 a). months, the corrosion rate of 7085 alloy was calculated, and the value was 2.7013 g/(m 2 a).   Figure 7a, the second phase was densely distributed at the grain boundary, which was consistent with the SEM observation. Meanwhile, a large number of dispersed second phases were also found in the Al matrix, and their size was rather small at the nanoscale. The electron diffraction pattern and high-resolution analysis in Figure 7b-d revealed that the second phase at the grain boundary was the η (MgZn2) phase with a close-packed hexagonal structure, while the fine second phase in the grain was a metastable (MgZn2) phase with a simple hexagonal structure. Herein, the lattice parameters of the η phase    Figure 7a, the second phase was densely distributed at the grain boundary, which was consistent with the SEM observation. Meanwhile, a large number of dispersed second phases were also found in the Al matrix, and their size was rather small at the nanoscale. The electron diffraction pattern and high-resolution analysis in Figure 7b-d revealed that the second phase at the grain boundary was the η (MgZn 2 ) phase with a close-packed hexagonal structure, while the fine second phase in the grain was a metastable (MgZn 2 ) phase with a simple hexagonal structure. Herein, the lattice parameters of the η phase were The corrosion behavior of alloys is known to be closely related to their microstructure. When grain boundary precipitates of the alloy distribute continuously, the alloy has a higher sensitivity to IGC [22][23][24]. In 7xxx series aluminum alloys, the potentials of the η (MgZn 2 ) phase and Al matrix were −0.86 and −0.68 V, respectively [25]. Hence, relative to the Al matrix, the η phase easily became the anodic dissolved phase, which was preferentially corroded in the humid and hot marine environment. Therefore, the continuous corrosion of the grain boundary η phase provided the conditions for IGC. A further analysis of the microscopic characteristics of the 7085 alloy is shown in Figure 8. A high, geometrically necessary dislocation density exists in the grain boundary and the area of the intergranular crack, indicating that there was a stress concentration in the grain boundary, which would promote IGC. In summary, there are two reasons for the IGC of the 7085 alloy. One is that there is a continuous anodic dissolution of the η phase at the grain boundary. During the corrosion process, the η phase is continuously corroded and consumed, and the intergranular crack expands. The other reason is the high stress concentration in the α-Al crystal boundary, which promotes the progress of IGC. Materials 2022, 15, x FOR PEER REVIEW 6 of 9 The corrosion behavior of alloys is known to be closely related to their microstructure. When grain boundary precipitates of the alloy distribute continuously, the alloy has a higher sensitivity to IGC [22][23][24]. In 7xxx series aluminum alloys, the potentials of the η (MgZn2) phase and Al matrix were −0.86 and −0.68 V, respectively [25]. Hence, relative to the Al matrix, the η phase easily became the anodic dissolved phase, which was preferentially corroded in the humid and hot marine environment. Therefore, the continuous corrosion of the grain boundary η phase provided the conditions for IGC. A further analysis of the microscopic characteristics of the 7085 alloy is shown in Figure 8. A high, geometrically necessary dislocation density exists in the grain boundary and the area of the intergranular crack, indicating that there was a stress concentration in the grain boundary, which would promote IGC. In summary, there are two reasons for the IGC of the 7085 alloy. One is that there is a continuous anodic dissolution of the η phase at the grain boundary. During the corrosion process, the η phase is continuously corroded and consumed, and the intergranular crack expands. The other reason is the high stress concentration in the α-Al crystal boundary, which promotes the progress of IGC.  The corrosion behavior of alloys is known to be closely related to their microstructure. When grain boundary precipitates of the alloy distribute continuously, the alloy has a higher sensitivity to IGC [22][23][24]. In 7xxx series aluminum alloys, the potentials of the η (MgZn2) phase and Al matrix were −0.86 and −0.68 V, respectively [25]. Hence, relative to the Al matrix, the η phase easily became the anodic dissolved phase, which was preferentially corroded in the humid and hot marine environment. Therefore, the continuous corrosion of the grain boundary η phase provided the conditions for IGC. A further analysis of the microscopic characteristics of the 7085 alloy is shown in Figure 8. A high, geometrically necessary dislocation density exists in the grain boundary and the area of the intergranular crack, indicating that there was a stress concentration in the grain boundary, which would promote IGC. In summary, there are two reasons for the IGC of the 7085 alloy. One is that there is a continuous anodic dissolution of the η phase at the grain boundary. During the corrosion process, the η phase is continuously corroded and consumed, and the intergranular crack expands. The other reason is the high stress concentration in the α-Al crystal boundary, which promotes the progress of IGC.  Figure 9 shows the tensile performance of the 7085 alloy after outdoor exposure for different times in humid and hot marine environments. The original ultimate strength and yield stress were 510 MPa and 462 MPa, respectively. After exposure for 12 months, it is obvious that the ultimate strength and yield stress of the 7085 alloy did not change significantly. However, after one year of exposure, the alloy's elongation and area reduction began to decrease (from the original 6% and 9%, respectively, to 3% and 5%). This pattern differed from that of the 7085 alloy fatigue strength, which decreased more than 90% after 3 months of outdoor exposure in a marine environment [26]. Additionally, the ultimate strength variation of the present alloy was slower than that of 2xxx aluminum alloys, which decreased by more than 10% after outdoor exposure in a marine environment [27]. Combined with the analysis of the corrosion features, the main corrosion mechanism of the 7085 alloy was IGC, which formed a large number of intergranular cracks inside the microstructure. With a prolonged corrosion time, the IGC depth increased. The cracks were also observed on the corrosion surface and fracture surface, as shown in Figure 10. However, the tensile performance, especially the strength, did not change significantly, indicating that the tensile performance of the 7085 alloy was not sensitive to the IGC behavior and that only when the IGC reached a certain depth would it lead to a tensile performance reduction.

Variation in Mechanical Performance
it is obvious that the ultimate strength and yield stress of the 7085 alloy did not change significantly. However, after one year of exposure, the alloy's elongation and area reduction began to decrease (from the original 6% and 9%, respectively, to 3% and 5%). This pattern differed from that of the 7085 alloy fatigue strength, which decreased more than 90% after 3 months of outdoor exposure in a marine environment [26]. Additionally, the ultimate strength variation of the present alloy was slower than that of 2xxx aluminum alloys, which decreased by more than 10% after outdoor exposure in a marine environment [27]. Combined with the analysis of the corrosion features, the main corrosion mechanism of the 7085 alloy was IGC, which formed a large number of intergranular cracks inside the microstructure. With a prolonged corrosion time, the IGC depth increased. The cracks were also observed on the corrosion surface and fracture surface, as shown in Figure 10. However, the tensile performance, especially the strength, did not change significantly, indicating that the tensile performance of the 7085 alloy was not sensitive to the IGC behavior and that only when the IGC reached a certain depth would it lead to a tensile performance reduction.   it is obvious that the ultimate strength and yield stress of the 7085 alloy did not change significantly. However, after one year of exposure, the alloy's elongation and area reduction began to decrease (from the original 6% and 9%, respectively, to 3% and 5%). This pattern differed from that of the 7085 alloy fatigue strength, which decreased more than 90% after 3 months of outdoor exposure in a marine environment [26]. Additionally, the ultimate strength variation of the present alloy was slower than that of 2xxx aluminum alloys, which decreased by more than 10% after outdoor exposure in a marine environment [27]. Combined with the analysis of the corrosion features, the main corrosion mechanism of the 7085 alloy was IGC, which formed a large number of intergranular cracks inside the microstructure. With a prolonged corrosion time, the IGC depth increased. The cracks were also observed on the corrosion surface and fracture surface, as shown in Figure 10. However, the tensile performance, especially the strength, did not change significantly, indicating that the tensile performance of the 7085 alloy was not sensitive to the IGC behavior and that only when the IGC reached a certain depth would it lead to a tensile performance reduction.

Conclusions
The results obtained made it possible to draw the following conclusions:

1.
After exposure to a humid and hot marine environment for one month, Al alloy 7085 exhibited an obvious corrosion behavior, and the corrosion product was relatively dense, mainly Al(OH) 3 . With a prolonged exposure time, the corrosion product did not increase significantly.

2.
In a humid and hot marine atmospheric environment, the main corrosion feature of the 7085 alloy was IGC, resulting from the continuous anodic dissolution of the η (MgZn 2 ) phase and the stress concentration at the α-Al grain boundary. With the exposure time extended from 6 months to 12 months, the IGC depth increased from 114 µm to 190 µm. Accordingly, the average weight loss increased from 0.0231 g to 0.0294 g.

3.
After a 1-year outdoor exposure of the 7085 alloy in a humid and hot marine atmospheric environment, its ultimate tensile strength and yield stress values did not change significantly. However, the elongation and area reduction decreased from the original 6% and 9%, respectively, to 3% and 5%. The variation of the tensile performance is mainly ascribed to intergranular cracks.