EBSD Characterization of 7075 Aluminum Alloy and Its Corrosion Behaviors in SRB Marine Environment

: Aluminum alloy 7075 is an important engineering material for ship structures. However, the corrosion of Al alloys generally exists in various environments, especially in the marine environment. Currently, the corrosion behaviors of Al alloy 7075 in sulfate-reducing bacteria (SRB) marine environment has not been well-addressed. In this paper, the corrosion effect of SRB on 7075 aluminum alloys was studied by adding SRB to real seawater. The microstructure and grain orientation of the super-hardness Al alloy 7075 were studied via the electron back-scattered diffraction (EBSD)technology, and the electrochemical impedance spectroscopy (EIS) test of the electrochemical corrosion behavior of 7075 in a variety of microorganisms, mainly SRB, in real seawater was continuously performed for 21 days. It was concluded that Al alloy 7075 has the strongest texture intensity on the (001), (111), (010), and (0–10) planes, which is 2.565. Adding SRB to real seawater accelerated the corrosion rate, and after corrosion on the 14th day, the protective ﬁlm on the 7075 aluminum alloy surface was completely broken, and the impedance was signiﬁcantly reduced.


Introduction
Aluminum alloy 7075 is an important engineering material, and the corrosion of Al alloy generally exists in various environments, especially in the marine environment [1][2][3][4][5]. The necessity for lowering the weight of ships, increasing the payload, and reducing fuel consumption, has turned shipbuilders towards Al alloys. Compared with low-carbon steels, Al alloys have the potential to reduce the weight of ship structures by up to 50%. However, when exposed to marine environments, overcoming the corrosion caused by salty water poses a great challenge for Al alloys. As a marine structure material, 7075 Al alloy should have a comprehensive corrosion protection design. Corrosion will change the mechanical and physical properties of metal materials, but also cause major accidents, resulting in massive economic losses and pollution of the environment [6][7][8][9][10][11]. In different environments, the corrosion process of aluminum alloys widely varies. External factors that cause aluminum alloy corrosion include the composition of the medium, temperature, pressure, pH value, the stress of the material, etc. Internal factors include the chemical composition, crystal form, structural morphology of the surface, etc. Among all these factors, the marine environment is characterized to be one of the worst ones for Al alloy corrosion. In nature, metals exist in relatively stable forms, such as their oxides, sulfides, or related salts, and their existing forms are relatively stable [12,13]. Pure metals are obtained by different methods such as smelting, electroreduction, etc. However, the metal surface is thermodynamically unstable and, therefore, tends to corrode spontaneously. Most metal corrosion processes occur due to electrochemical corrosion. The tendency of electrochemical corrosion can be judged by the change in Gibbs free energy and the electrode potential [14,15]. In fact, the properties of a metal surface tend to be various different from the metal substrate, such as microstructure, impurities, surface stress, etc. This will lead to electrochemical inhomogeneity on the metal surface, which will form many tiny electrodes on the metal surface, thereby accelerating the corrosion of the metal [16][17][18][19].
Although the corrosion of Al alloys has been well-studied in the literature, few studies have described the sulfate-reducing bacteria (SRB) corrosion on Al alloys [2,19,20]. In the past, most of the SRB corrosion research focused on the steel, especially for the deep-sea pipeline that confronted a fetal microbial corrosion problem [21][22][23]. With the trend that turns shipbuilders towards aluminum alloys, the SRB corrosion on Al alloys should be wellemphasized. Guan et al. reported that the presence of SRB decreases the corrosion resistance of the 5000 series Al alloys [3,24]. Zhang et al. concluded that the marine Aspergillus terreus increases the corrosion rate of 7075 Al alloys in the marine environment [2]. All those studies provide evidence of the disadvantages of microbial to the corrosion of Al alloys. Therefore, microbial corrosion, especially for SRB, should be particularly considered. To our best knowledge, no attempt has been made to examine the corrosion of SRB on 7075 Al alloy in real seawater. Moreover, the correlation of the texture intensity and SRB corrosion on 7075 Al alloy still requires a better fundamental understanding.
In this paper, we studied the microstructure and grain orientation of super-hardness Al alloy 7075 by EBSD. In the corrosion test part, the real seawater with microorganisms was taken in the deep-sea environment of the South China Sea. Then, a large number of SRB were added to the seawater to study the corrosion effect of SRB on Al alloy 7075. The EIS test was continuously performed for 21 days. Finally, the corroded morphologies were characterized with SEM and EDS, and the corroded products were analyzed via XRD to clarify the corrosion mechanisms of different texture and orientation grains in aluminum alloy 7075 in a microbial corrosion environment.

Materials and Sample Preparation
Commercial aluminum alloy plates 7075 (wt.% of 6.00 Zn, 2.50 Cu, 2.40 Mg, 0.40 Si, 0.50 Fe, 0.30 Mn, 0.18 Cr, 0.20 Ti, and Al balance) were cut into a size of 10 × 10 × 4 mm and polished using silicon carbide (SiC) paper, with ethanol as a lubricant, starting from 600 and finishing with 1200 grit. The polished samples were ultrasonically cleaned in ethanol and were then dried using a compressed air gun. The real seawater containing marine microorganisms was obtained from Yangjiang, China. Then, 50 mL of SRB medium was added to 200 mL of the real seawater.

Electrochemical Corrosion Test
Electrochemical impedance spectroscopy (EIS) was conducted using an electrochemical workstation (Model CS350, Corrtest, Wuhan, China) for the study of the electrochemical corrosion process in real time. A traditional three-electrode corrosion cell was used for the corrosion test. A saturated calomel electrode (SCE) was used as the reference electrode, and a platinum plate was used as the counter electrode. EIS data were collected at a rate of 7 points per decade over a frequency range of 100 kHz to 10 mHz. EIS data were fitted well using Zview2 software (version 2, Scribner, Southern Pines, NC, USA). More than three independent specimens were used to perform each test.

Surface Examination
The microstructure of samples was examined using a 3D Optical Microscopy (OM, DVM6, LEICA, Wetzlar, Germany), a scanning electron microscope (SEM, MIRA3, TES-CAN, Brno, Czech) equipped with an Oxford electron back-scattered diffraction (EBSD, Oxford Instruments, Oxford, UK) system, and energy-dispersive spectroscopy (EDS, Oxford Instruments, Oxford, UK). The operation voltage of SEM ranged from 5 kV to 20 kV, and the working distance was about 30 mm. The chemical composition of the corroded film was studied via EDS obtained with an operating voltage of 30 kV. Phase analysis was carried out on an X-ray diffraction system (XRD, XPERT-PRO, Malvern Panalytical, Almelo, The Netherlands).

EBSD Characterization of Aluminum Alloy 7075
The performance of a material is determined by its composition and structure. The mechanical and corrosion properties of materials with different structural orientations of the same composition are very different [25][26][27]. Figure 1 shows the EBSD analysis of 7075 aluminum alloy. It can be seen from Figure 1a that the grains of 7075 aluminum alloy are distributed in different directions such as (001), (101), and (111). Grains with different orientations have obvious differences in mechanical properties and corrosion resistance [18,28]. In the seawater environment where SRB exists, microbial electrochemical corrosion of 7075 Al alloy will preferentially occur microbial electrochemical corrosion at the orientation grains with poor corrosion resistance. Al alloys will spontaneously form tens to hundreds of nanometers of Al 2 O 3 film in the air, preventing corrosion and further oxidation of alloy substrate. It has good corrosion resistance under normal environment (near-neutral pH) [29,30]. However, aluminum is an amphoteric metal that can form both acidic and basic oxides. Therefore, Al alloys can exist relatively stable in dry air, but they are prone to electrochemical corrosion in humid or acid-base salt solution environments [31]. Especially for the 7075 Al alloy with super hardness, the secondary phase nanoprecipitation phase is formed due to the addition of a large number of alloying elements such as Zn, Cu, and Mg. During the plastic forming process, the grains and the second phase are continuously strengthened to achieve ultra-high strength and ultra-high hardness. However, the chemical potential of the second phase and grains with different orientations are quite different. In the environment of seawater, especially where microorganisms exist, electrochemical corrosion will preferentially occur in the region with a more negative potential [32][33][34]. As evident from Figure 1b, there are grains at all angles. The inhomogeneity of crystal plane orientation is detrimental to the corrosion resistance of the alloy.   Figure 2b, it can be inferred that the grain size of the alloy is in the range of several hundred nanometers to 30 microns and mainly below 10 microns. This indicates that the alloy is a mixed crystal structure. The fine grains can improve the strength and toughness of the alloy but also the corrosion resistance. However, the structure in Figure 2a shows that the fine-grained structure is mainly composed of deformation twins and second phase precipitation, and there are still a few large grains. Hence, in the seawater environment where SRBs are abundant, localized corrosion will preferentially occur on those sites. Localized corrosion is divided into galvanic corrosion, pitting corrosion, crevice corrosion, intergranular corrosion, selective corrosion, stress corrosion, corrosion fatigue, etc. [35]. The inhomogeneity of the microstructure easily constitutes a microbattery in a seawater environment, which accelerates the local corrosion rate and leads to alloy failure [36,37]. The geometric relationships of crystals are all three-dimensional, but the relationship between different crystal planes and their motion trajectories is difficult to express clearly with three-dimensional diagrams. Consequently, the polaroid projection can be employed to study the relative orientation of all important crystal planes in the crystal. Figure 3 shows the pole figure and inverse pole figure analysis of 7075 aluminum alloy. The (001), (111), (010), and (0-10) surface poles have the strongest texture intensity, which is 2.565. The extreme strength of (001) is due to the rolled plane being the basal plane, while the texture intensity of (111) is due to the face-centered, cubic, closest-packed plane of the aluminum alloy. In contrast to the pole figure, the inverse pole figure can be employed to describe the spatial distribution of the crystal orientation in the crystal coordinates parallel to a certain apparent feature direction of the material in a polycrystalline material. From the inverse pole figure, the texture intensities are strongest at (001) and (111), which is consistent with the result of the pole figure. Additionally, according to Figure 3, the distribution of texture intensity is inhomogeneous, which is detrimental to the corrosion resistance of the alloy.

Corrosion Behaviors of Aluminum Alloy 7075 in SRB Marine Environment
To examine the corrosion resistance of 7075 aluminum alloy against microbial corrosion in SRB-concentrated real seawater, EIS tests were conducted. Figure 4 represents the EIS plot of the obtained 7075 aluminum alloy on SRB seawater. The conventional equivalent circuit for this model is presented in Figure 4 [35,[38][39][40]. The bulk solution resistance is represented by R s , and R p is the pore resistance representing the ohmic resistance through the surface film or the corrosion product layer. Q c is the reactance associated with the surface film or the corrosion product layer. Q dl represents the interfacial reactance that develops due to charge separation at the interface between the metal and the coating, corrosion product layer, and penetrating solution, respectively [41]. The sum of R p and R ct represents the total resistance (R tot ). Typically, a higher total resistance or low frequency (f = 0.01 Hz) impedance means a lower corrosion rate [41,42].  As shown in Figure 4, the resistance of the alloy decreases with the increase in immersion time within the first 4 days. This is caused by the oxide film formed on the surface, which cannot withstand the attack of the SRB-concentrated microorganisms and Cl − in real seawater. It is well-known that the corrosion of aluminum alloy is a process of repairing while corroding. The low-frequency impedance decreased sharply on the 7th day, but it returned to a relatively high value on the 10th day. With the prolongation of immersion time, the protective film of the alloy is constantly repaired while being damaged. However, according to Figure 4, the impedance of the alloy was significantly reduced on the 14th day. Compared with the real seawater without SRB, the low-frequency impedance did not drop on the 14th day, which provides strong evidence that SRB accelerates the corrosion rate of 7075 Al alloy. Fang et al. reported a similar behavior in 5000 series Al alloys [24]. This phenomenon indicated the oxide film formed on the surface of 7075 aluminum alloy completely failed after a long-time immersion.
Microbial corrosion is a long-term and complex process. Generally, in the marine environment, microorganisms first attach to the surface of the hull and then release acidic metabolites to accelerate the localized corrosion. Then, the metabolites form mucosa to create a complete and large-area electrochemical corrosion primary battery to further accelerate alloy corrosion [35,[38][39][40]. According to the EIS results, the seawater environment with SRB-concentrated microorganisms is detrimental to the corrosion resistance of 7075 aluminum alloy. After immersion for 14 days, severe corrosion occurred on the alloy. Figure 5 shows the optical microscopy images of 7075 Al alloy after corrosion in SRB-concentrated real seawater. As shown in Figure 5a, there are obvious corrosion products attached to the alloy surface. By combining the three-dimensional morphologies of Figure 5b,c, it can be confirmed that there was an unevenness of 6.44 µm after 21 days of corrosion. Additionally, obvious corrosion pits were found on the alloy surface. This kind of corrosion pit may be caused by the inconsistent corrosion resistance of different grain orientations or texture intensities on the surface of 7075 aluminum alloy. As the corrosion proceeds for a long time, the active site on the alloy surface breaks down the protective film, and localized corrosion occurs. In particular, in a corrosive environment where microorganisms exist, the corrosion of the alloy is accelerated. In this study, the addition of SRB to the real seawater considerably accelerated the corrosion of the 7075 aluminum alloy.  Figure 6 shows the SEM image and EDS mapping of 7075 aluminum alloy after corrosion in SRB-concentrated real seawater. As seen in Figure 6a,b, there are many scratches on the sample caused by the polishing during the sample preparation process. No obvious localized cracks exist on the surface. At high magnification (Figure 6b), many small particles attach to the surface, which may refer to the corrosion products of 7075 Al alloy. According to the EDS results, the corrosion product of 7075 aluminum alloy after adding SRB in real seawater for 21 days is mainly Al, O, Mg and Si. It is worth noting that, 7075 Al alloy has a high Zn content (6.0%), but strong Zn signals are not seen in EDS results. This indicates a film was already covered on the sample substrate. This film is a mixture of the corrosion product, Al 2 O 3 , and biofilms. Moreover, the X-ray diffraction (XRD) pattern of 7075 aluminum alloy after corrosion was performed. As shown in Figure 7, the surface products of 7075 aluminum alloy after corrosion are mainly ionic crystals, while Al 2 O 3 and SiO 2 are both stable oxides in the natural environment [39]. In addition, there are some intermetallic compounds such as CuAl 2 and CuMg 2 detected on the surface, which are mainly due to the localized corrosion between pre-existing intermetallic compounds and the Al matrix. It has been well-established that CuAl 2 behaves nobler than the Al matrix, but CuMg 2 behaves less noble than the Al matrix [43]. According to the EDS results, a relatively strong Mg signal was obtained, which indicates the corrosion of the more active CuMg 2 . No matter whether the intermetallic particles act as cathodes or anodes, the potential of those intermetallic particles is different from the Al matrix. This phenomenon contributes to the formation of micro-galvanic couples and accelerates localized corrosion on Al alloy. It is worth noting that different grain orientations or texture intensities will similarly build the micro-galvanic couple. All these facts are detrimental to the corrosion resistance of the Al alloy. Therefore, in a real seawater corrosion environment, the area around the intermetallic compounds will act as a weak point to accelerate the corrosion of the Al matrix through electrochemical corrosion [44]. The presence of SRB will further accelerate the corrosion process due to the release of acidic metabolites. It makes micro-galvanic couples on the Al alloy surface become more vulnerable to the attack of localized corrosion [45].

Conclusions
The necessity for lowering the weight of ships, increasing the payload, and reducing fuel consumption has turned shipbuilders toward aluminum alloys. However, according to the study of the EBSD characterization of 7075 Al alloy and its corrosion behaviors in the SRB marine environment, the application of 7075 Al alloy in the marine environment still has a great hidden danger of corrosion. The following conclusions can be drawn: 1. According to the EBSD characterization, (001), (111), (010), and (0-10) surface poles have the strongest texture intensity, which is 2.565. The extreme strength of (001) is due to the rolled plane being the basal plane, while the texture intensity of (111) is due to the face-centered, cubic, closest-packed plane of the aluminum alloy. The inhomogeneous distribution of texture intensity may be detrimental to the corrosion resistance of the alloy.
2. The corrosion of 7075 Al alloy is a process of repairing while corroding. According to the EIS results, the low-frequency impedance (f = 0.01 Hz) of the alloy on the 14th day was significantly reduced, which indicates that the oxide film formed on the surface of 7075 aluminum alloy completely failed. Compared with the real seawater without SRB, the low-frequency impedance did not drop on the 14th day, which provides strong evidence that SRB accelerates the corrosion rate of 7075 Al alloy.
3. In the SRB-concentrated real seawater corrosion environment, the area around the intermetallic compound acts as a weak point, becoming more vulnerable to corrosion.