Surface morphology and corrosion behavior of pure aluminum and its alloys in aqueous sulfuric acid medium

Aluminum and aluminum alloys are light materials with some of the preferences such as lightweight, high specific strength, good elasticity, and good workability that play an important role in today's modern and industrial world. The economic loss and environmental and safety problems are also the most concerning aspects of these materials. As a result, numerous studies are performed by the researchers to improve the overall environment throughout the materials world. That is why, in this study, the surface morphology and corrosion behavior of pure aluminum and its alloys, such as 7S10 and 7003H, were investigated in aqueous sulfuric acid medium through immersion process at different temperatures. Open‐circuit potential and potentiodynamic polarization techniques were used to evaluate the resistance to corrosion of pure aluminum and its alloys 7S10 and 7003H. The current density increased with increasing temperature in the case of the alloys, and pure aluminum showed the highest corrosion resistance properties. The surface roughness measurement was performed using atomic force microscope to find out the amount of roughness of the used materials before and after the immersion process. Surface roughness was higher on the alloys than in pure Al, which indicates that less corrosion was formed in pure Al than in the alloys. The surface morphology analysis was also carried out using scanning electron microscopic data. The results revealed that the alloy 7003H undergoes more corrosion than pure aluminum and 7S10 in sulfuric acid medium, which clearly indicates that pure aluminum has higher corrosion resistance than the alloys 7S10 and 7003H. The corrosion rate of the test materials decreased with increasing immersion time.

wide area of operations from household to the versatile engineering fucntions. 1,2][10] In addition, these materials are very essential in the area of new and developing fields of sciences, especially fuel and solar cell technology, where Al as a low-work-function metal corrodes and thereby, meaninglessly lowering the assembly efficiency of the above-mentioned cells.On the contrary, the standard reversible potential (E 0 Al∕Al 3+ = −1.66V vs. NHE 11,12 and its high energetic capacity (2980 Ah kg −1 ) of Al and its alloys are excellent qualities when Al is used as an active anodic material in chemical power source technology.][15] Though the production and usage of Al and Al alloys are rapidly increasing, with the accelerated progress of the economy as well as urbanization, the corrosion of these materials is also occurring promptly.Corrosion of these materials has now severe effects on society, health, economy, technology, as well as cultural effects on our community.Thus, it is a serious matter due to the loss of huge materials and their staffs in almost all sectors. 16So, the study of corrosion has become a very important field in order to increase awareness of the necessity to reserve the metallic resources in the world. 17owever, the Al and its alloys, are the materials that are very reactive and are corroded easily due to oxidation. 18Thus, the degradation of different materials by corrosion is a severe issue as well as a field of rapid attention for researchers.It is thought that the yearly losses because of the corrosive properties of different types of materials are about US$350 billion in the USA alone. 19That is why, the corrosion behavior of Al and its alloys has been a topic of prominent investigation because of their great interest in the current advancements in the world.
Al and its alloys are also familiar because they show corrosion-resistant properties in different media, such as the gases in industrial atmospheres, ammonia, phosphoric and chromic acid mixtures, compounds of sulfur, and so forth, owing to the formation of stable Al 2 O 3 on the surface of the materials through exposure to the above media.The properties mentioned above made Al and its different alloys worthy of consideration in different functions, such as structural and packaging materials, transporting and automotive fields, and the aerospace industry. 20It is observed that the application of these materials becomes limited in high acid media because of their faster corroding tendency than expected although Al and its alloys have these types of excellent properties.As a result of the significant applications of Al and its alloys in different sectors, it is really essential to continue the research on their corrosion properties and resistivity in other media.2][23][24][25][26][27][28][29][30][31][32][33] It has also been noticed that no related work has been found on Al and its alloys, such as 7003H and 7S10, in sulfuric acid media at different temperatures.Besides, the Al alloys 7003H and 7S10 are considered more interesting to study here because of their eminent, easily available, and mechanically strong enough properties.Therefore, it is in our great interest to investigate and work on the corrosion-resistant properties of pure Al and its alloys in aqueous sulfuric acid at different concentrations and temperatures.
By considering the remarks pointed out above, it has been attempted here to evaluate the corrosion behavior of commercial pure Al and its alloys such as 7S10 and 7003H through the immersion process in dilute aqueous sulfuric acid media at different temperatures.

EXPERIMENTAL
The commercial Al (99+%, Nilaco) and two Al alloys, namely 7003H and 7S10, were used as the test specimens for the experiments.The test specimens of the dimensions of pure Al were 30 mm × 10 mm × 1 mm and alloys were 40 mm × 12 mm × 3 mm, were used for immersion process.The pure aluminum was cut into pieces with the sizes and alloys used in their as-received/commercial sizes mentioned above.All the test materials were polished up to #2000 with emery paper, rinsed with acetone, and dipped into around 90 cm 3 of test solutions containing PTFE cells.The immersion process was carried out for 24-96 h at 80 • C using aqueous sulfuric acid media at very low concentration (5 × 10 −6 M) as the test solutions with the reagent grade of H 2 SO 4 .The schematic diagram of the immersion test is shown in Figure 1.Very low concentrated or dilute H 2 SO 4 was used in this study especially in the immersion process as it is considerably less hazardous without the oxidative and dehydrating properties and concentrated H 2 SO 4 is very reactive, corrosive, and can dissolves metals easily.
The weight of the prepared test samples was taken before and after the immersion using a weight measuring balance with ±0.1 mg resolution and recorded.For the most accurate results, the test samples were utilized at least three times, and their average values were taken.The mass loss was found from the weight difference that was taken before and after the immersion.The corrosion rate (C R ) of the test samples was calculated using Equation (1) mentioned below 34 : where C R represents the corrosion rate in mg cm −2 h −1 , m symbolizes the mass loss of the test samples in mg, A denotes the surface area in cm 2 , and t denotes the immersion time in hours.
The measurements of open circuit potential (OCP) and potentiodynamic polarization test were completed to study the corrosion resistance of pure Al and its alloys such as 7003H and 7S10 in 0.05 M H 2 SO 4 + 2 ppm F − solutions at different temperatures using a PTFE (polytetrafluoroethylene) 3-electrode electrochemical test system.The reference electrode was the saturated calomel electrode (SCE), the counter electrode was the Pt sheet, and the samples worked as the working electrode using the Ivium software as a potentiostat or galvanostat, which was controlled by a computer.The measurement of the potentiodynamic polarization was carried out at 1.0 mV/s of the scan rate and ±1.5 V of the potential range.
The microscopic pictures were taken using the OLYMPUS BX60M optical microscope, Japan, and surface roughness was measured by an atomic force microscope (AFM, NX-10), respectively, before and after the immersion process to check the surface of the test materials.The surface morphology and elemental analyses of the test samples were also performed by scanning electron microscopy (SEM) and energy dispersive x-ray (EDX) spectroscopy data, respectively, before and after the immersion process using FE-SEM (JSM-7800F).Ten millimeters of working distance and 10 kV of accelerating voltage were maintained at the time of measuring SEM and EDX data.

RESULTS AND DISCUSSION
The test specimens, such as pure Al, alloy 7S10, and 7003H, were characterized and investigated through several experimental techniques after the immersion process in the test solution.The mass of the test specimens was measured after immersion for a duration of 24-96 h. Figure 2 shows the images of pure Al and its alloys 7S10 and 7003H in different forms, such as bare/as-received, as-polished, and after immersion in 5 × 10 −6 M H 2 SO 4 solution for 96 h at 80 • C. Through the observation of the images, it was preliminarily considered that more corrosion occurred in the case of alloy 7003H compared to pure Al and alloy 7S10.In addition, the chemical composition of the alloys 7S10 and 7003H is shown in  1, which indicates the greater strength in the case of 7S10, which consists of a higher percentage of some elements such as Zn, Mg, and Cr than that in 7003H.

Optical microscopic images
The optical microscopic images of the test samples were taken before and after immersion in the test solutions, as shown in Figure 3.The images show that some light spots appeared on the surface of pure Al after the immersion in the solution.
On the contrary, some smaller pit shaped pores were present in the surfaces of Al alloys 7S10 and 7003H.In addition, pit-shaped pores were a little larger in the case of 7003H compared to 7S10.Thus, it is considered that the corrosion products were very low and smaller on the surfaces of pure Al than its alloys, which indicates that pure Al is more corrosion-resistant than the above-mentioned alloys.

OCP test
OCP measurement is a common and nondestructive method to find out the corrosion potential and understanding corrosion activity of a material where a counter electrode, a reference electrode, and the test materials as the working electrode are used. 35The OCP measurement of pure Al and its alloys such as 7S10 and 7003H that were performed for 900 s in 0.05 M H 2 SO 4 + 2 ppm F − aqueous solution at 80 • C as shown in Figure 4.It shows that the OCP of 7003H was the lowest among the test materials.The OCP of pure Al was literally the highest, but surprisingly, the OCP of 7S10 crossed the potential of pure Al in around 390 s.The background of this surprising behavior of 7S10 is unknown to us.After all, the OCP results revealed that the pure Al is considered as the highest corrosion-resistant material among the test materials in this study.

Potentiodynamic polarization test with temperature effect
In order to evaluate the corrosion resistance and observe the temperature effect in the solution, the potentiodynamic polarization tests of pure aluminum and its alloys 7S10 and 7003H were performed in 0.05 M H 2 SO 4 + 2 ppm F − aqueous solution at different temperatures, such as 30, 50, and 80 • C, as shown in Figure 5.The curves clearly show that the current density was highest at 80 • C. For 30 and 50 • C, the current density was higher at 30 • C up to +200 mV but increased at 50 • C after +200 mV.In addition, the current density increased as the temperature increased for both alloys.Thus, based on the results found in potentiodynamic tests, it is suggested that pure Al has the highest corrosion resistance properties among the test materials used in this study.

Immersion test in dilute sulfuric acid media
The corrosion rate is an important parameter to evaluate the loss of the materials used where corrosion rate mostly depends on local environment and the composition of the metallic alloys.Hence, in order to assess the corrosion amount and rate, the immersion test was carried out following the standard guide NACE TM0169/G31-12a. 36The changes that occurred in the test samples of pure Al and its alloys 7S10 and 7003H after immersion in 5 × 10 −6 M H 2 SO 4 solution at 80 • C are presented in Figure 6. Figure 6 shows that the amount of mass loss increased with increasing immersion time for all the samples used in this study.The mass losses of alloy 7003H were larger than those of pure Al and alloy 7S10.Furthermore, it was seen that the mass loss rate of 7S10 alloy was higher than that of pure Al at corresponding times.Thus, the results found from the immersion test indicate that the corrosion rate of pure Al was lower than that of its alloys 7S10 and 7003H, which confirms the suitability of using pure Al as a better candidate in different sectors than the alloys mentioned in this study.The corrosion rate (C R ) of the test samples of pure Al and its alloys 7S10 and 7003H was found to be 1.95, 2.12, and 4.64 μg cm −2 h −1 , respectively, after immersion in 5 × 10 −6 M H 2 SO 4 aqueous solution at 80 • C.

Surface roughness measurement
8][39][40] The AFM images (scan range 90 mm × 90 mm) of pure Al and its alloys 7S10 and 7003H are shown in Figure 7 before and after the immersion test.The images clearly show that the surface roughness was increased in the case of alloy 7003H.In addition, the surface roughness was higher in the case of 7S10 than in pure Al.The amount of surface roughness of pure Al and its alloys 7S10 and 7003H before and after immersion was found from the AFM data and mentioned in Table 2. Thus, the results in Figure 7 and Table 2 indicate that less corrosion occurred in pure Al than in its alloys 7S10 and 7003H.

Surface morphology analyses
SEM is an analytical tool that aims to investigate the surface morphology of the test materials.The corrosion morphologies of the test samples, pure Al and its alloys 7S10 and 7003H, were examined by SEM before and after the immersion test in 5 × 10 −6 M H 2 SO 4 solution for 96 h at 80 • C, as shown in Figure 8.The SEM images of the test materials before exposure to the test solution show a repeated pattern of Al and other atoms due to polishing up by emery paper, as shown in Figure 8A,C,E.The SEM images of immersed samples after exposure to the test solution are given in Figures 8B,D,F which show the degradation and some crack patterns of grain and surface erosion with more or less uniform attack in the test solution due to the corrosion effects of the test samples.The corrosion products appeared more on the surfaces of the test samples 7003H and 7003H than pure Al, which shows a coincidence with the results found in the polarization as well as other tests described above in this study.However, the corrosion that occurred in the samples of pure Al and its alloys was relatively mild, as a very low-concentrated sulfuric acid solution was used for the study.

CONCLUSIONS
Aluminum and its alloys represent a vital group of materials because of their immense scientific and industrial demand and large number of appliances.The present research investigated the corrosion behavior of commercial pure aluminum and its alloys, such as 7S10 and 7003H, in aqueous sulfuric acid medium at different temperatures.The findings in this study can be concluded as pure aluminum is more corrosion-resistant than its alloys (7S10 and 7003H).The surface roughness as well as the amount of corrosion are lower in pure Al than in its alloys.With the increase in immersion time in the test solution, there is a decrease in the corrosion rate for all the test materials.The morphological characteristics found from the SEM images indicate that more corroded products were formed in Al alloys than in pure Al.Finally, it can be recommended that pure Al be considered a more suitable material than its alloys, such as 7S10 and 7003H, for applications in different fields.

F I G U R E 1
Schematic diagram of the immersion test in different conditions used for this study.

F I G U R E 3 F I G U R E 4
Optical microscopic images of pure aluminum and its alloys 7S10 and 7003H before and after immersion in 5 × 10 −6 M H 2 SO 4 solution for 96 h at 80 • C. Evolution of OCP for pure aluminum and its alloys in 0.05 M H 2 SO 4 + 2 ppm F − aqueous solution at 80 • C.

F I G U R E 6
Mass loss versus immersion time of pure aluminum and its alloys 7S10 and 7003H after immersion in 5 × 10 −6 M H 2 SO 4 solution at 80 • C.

F I G U R E 7
Surface roughness of pure aluminum and its alloys 7S10 and 7003H before and after immersion in 5 × 10 −6 M H 2 SO 4 solution for 96 h at 80 • C.

8
SEM images of pure aluminum and its alloys 7003H and 7003H (A, C, E) before and (B, D, F) after immersion in 5 × 10 −6 M H 2 SO 4 solution for 96 h at 80 • C.

Table
Pure Al and its alloys 7S10 and 7003H images in different forms: bare/as-received, as-polished, and after immersion in 5 × 10 −6 M H 2 SO 4 solution for 96 h at 80 • C. TA B L E 1 Chemical compositions of the alloys (mass%).
Average surface roughness of the test specimens before and after immersion in the test solutions for 96 h at 80 • C.
TA B L E 2