Atmospheric Corrosion Behavior of Typical Aluminum Alloys in Low-Temperature Environment

Abstract

The atmospheric corrosion behavior of type 2024, 5083, 6061, and 7075 aluminum alloys in the Antarctic environment was investigated by outdoor exposure tests and indoor characterization. After one year of exposure to the Antarctic atmosphere, significant differences in surface corrosion states were observed among the specimens. The results revealed that the corrosion rate of the 2024 aluminum alloy was the highest, reaching 14.5 g/(m²·year), while the 5083 aluminum alloy exhibited the lowest corrosion rate of 1.36 g/(m²·year). The corrosion products formed on the aluminum alloys exposed to the Antarctic environment were primarily composed of AlOOH and Al₂O₃. In the Antarctic atmosphere environment, the pits were dominated by a freezing–thawing cycle and salt deposition. The freezing–thawing cycle promotes the wedge effect of corrosion products at the grain boundary, resulting in exfoliation corrosion of high-strength aluminum alloys.

1. Introduction

Aluminum alloys are widely used as structural materials in aerospace vehicles due to their light weight and superior mechanical properties [1,2,3]. However, when exposed to harsh environmental conditions, the corrosion of aluminum alloys can significantly reduce their structural load-bearing capacity, thus posing a threat to the safety of aerospace vehicles [4]. Although corrosion of aluminum alloys has received considerable attention, research on their corrosion behavior in the Antarctic region remains limited [5,6]. The extreme environmental conditions in Antarctica, such as extremely low temperatures, high humidity, salt exposure in marine environments, and intense ultraviolet radiation, can have significant impacts on the corrosion behavior of aluminum alloys, thus increasing their corrosion risk [7]. Therefore, it is of great scientific importance to study the corrosion behavior of aluminum alloys in the Antarctic region. By understanding the mechanisms of environmental influence on aluminum alloys in Antarctica, effective protective measures can be developed to mitigate corrosion damage, ensuring the safe and reliable operation of facilities and equipment in this region [8,9,10,11,12]. Additionally, strengthening research on the corrosion of aluminum alloys in Antarctica will provide valuable insights for the use of aluminum alloys in other extreme environments.

In recent years, significant progress has been made in the study of atmospheric corrosion of aluminum alloys, covering corrosion mechanisms, influencing factors, and protective measures. Morcillo et al. [13] investigated the marine atmospheric corrosion of aluminum alloys at three MICAT test stations in Antarctica and found that the corrosion products formed in this environment exhibited better protective capabilities. The study indicated that the corrosion rate of aluminum alloys decreased rapidly with increased exposure time, and after four years of exposure, the majority of the aluminum surface was corroded, showing significant formation of alumina. Although the metal surface was covered by ice for most of the exposure period, atmospheric corrosion of aluminum remained widespread. Sun et al. [14] studied the atmospheric corrosion behavior of 2024 and 7075 aluminum alloys in three different environments. The results showed a near-linear relationship between the weight loss and exposure time of the alloys in the Wanning test field. Díaz et al. [15] investigated the corrosion mechanisms of 1050 aluminum alloy in pure marine atmospheric environments at varying distances from the coastline. They found that the corrosion rate of aluminum alloys decreased as the distance from the coastline increased. The continuous increase in chloride deposition rates in marine atmospheres did not lead to a corresponding increase in corrosion rates because the corrosion products within the pits made further corrosion much more difficult. With prolonged exposure, both the corrosion and the depth of the formed pits decreased significantly. Chico et al. [16] analyzed the atmospheric corrosion behavior of metals under extreme climatic conditions, highlighting that corrosion mainly depends on the duration of metal surface wetting, the presence of atmospheric pollutants, and environmental temperature. In the Antarctic environment, microscopic images of the aluminum alloy surface showed that the corrosion film cracked due to dehydration during drying cycles of gel-like hydrated basic chlorides formed on the metal surface. The presence of atmospheric pollutants, such as chlorides and sulfur dioxide, accelerates the corrosion process. These pollutants form salts that accumulate at the metal–ice interface, further accelerating the corrosion. Hu et al. [17] explored the corrosion mechanisms and environmental factors affecting aluminum alloys in marine and coastal atmospheric environments. Their findings suggested that the type and extent of corrosion of aluminum alloys were mainly influenced by factors such as atmospheric composition and concentration, exposure conditions and duration, humidity, and wind speed. In marine atmospheric environments, aluminum alloys form multiple layers of corrosion products, including an aluminum oxide film, an oxide corrosion layer, and an outer layer of contaminants. The corrosion products are predominantly sulfides, with chlorides also present in significant amounts.

Currently, there is limited research on the corrosion of aluminum alloys in the Antarctic atmospheric environment. In this study, typical aluminum alloys, specifically grades 2024, 5083, 6061, and 7075, were subjected to exposure in the Antarctic atmospheric environment. The atmospheric corrosion behavior and mechanisms of typical aluminum alloys in the Antarctic environment were investigated by exposure tests, corrosion rate test, and microscopic morphology observation.

2. Experimental

2.1. Preparation and Treatment of Corrosion Samples

The materials used in the outdoor exposure tests in Antarctica included the aluminum alloys 2024, 5083, 6061, and 7075, with their chemical compositions listed in

Table 1. Chemical composition of aluminum alloys used in this work (wt.%, manufacturer’s specifications).

Materials	Si	Fe	Cu	Mn	Mg	Cr	Zn	Ti	Al
2024	0.50	0.50	3.8–4.9	0.30–0.9	1.2–1.8	0.10	0.25	0.15	Bal.
5083	0.40	0.40	0.10	0.40–1.0	4.0–4.9	0.05–0.25	0.25	0.15	Bal.
6061	0.40–0.8	0.7	0.15–0.40	0.15	0.8–1.2	0.04–0.15	0.25	0.15	Bal.
7075	0.40	0.50	1.2–2.0	0.30	2.1–2.9	0.18–0.28	5.1–6.1	0.20	Bal.

. The experimental materials were cut into samples with dimensions of 40 mm × 30 mm × 2 mm using wire electrical discharge machining. The samples were sequentially polished with 400#, 800#, and 1500# sandpaper to obtain a bright finish. The samples were then immersed in ethanol solution and subjected to ultrasonic cleaning for 10 min to remove surface contaminants, followed by drying with an air blower. Four parallel samples were prepared for each material, three of which were used for weight loss measurements, while the fourth was reserved for corrosion product morphology characterization and compositional analysis.

2.2. Weight Loss Testing

The samples were exposed to the Antarctic atmospheric environment for a period of one year and then retrieved. A standard rust removal solution was prepared by mixing 50 mL of phosphoric acid with distilled water to make a total volume of 1000 mL. The exposed samples were immersed in this rust removal solution and subjected to ultrasonic heating at 80 °C for 5–10 min to facilitate the removal of corrosion products. Following this process, the samples were weighed, and the weight loss was calculated using Equation (1):

$$c_{\mathrm{m}}={\displaystyle \frac{(w_0-w_{\mathrm{m}})}{S}}$$

(1)

where w₀ (g) represents the mass of the sample before corrosion, w_m (g) represents the mass of the sample after the corrosion products have been removed, S (m²) is the surface area of the aluminum alloy sample, and c_m (g/m²) is the weight loss per unit area of the aluminum alloy. Therefore, the weight loss rate of aluminum alloys after exposure to the Antarctic atmospheric environment for various durations is obtained using Equation (2):

$$v_{\mathrm{m}}={\displaystyle \frac{c_{\mathrm{m}}}{t}}$$

(2)

where t (year) represents the corrosion time and v_m (g/m²·year) represents the weight loss rate.

2.3. Surface Characterization

After retrieving the samples exposed to the Antarctic atmosphere, the surface morphology of the aluminum alloys was analyzed using a Canon 6D camera (Canon, Tokyo, Japan). The samples were then sectioned into smaller specimens (10 mm × 10 mm × 2 mm) using wire electrical discharge machining. The phase composition of corroded samples was analyzed using X-ray diffraction (XRD, D8 Advance, Bruker, Germany) in the range of 10° to 90° with Cu Kα radiation, a voltage of 40 KV, and a scan rate of 5° min⁻¹. The phase analysis of the experimental data was conducted using Jade 6 software. Scanning electron microscopy (SEM, ZEISS Gemini SEM 300, Carl Zeiss AG, Oberkochen, Germany) was used to observe the surface morphology of the samples before and after rust removal, and energy-dispersive X-ray spectroscopy (EDS, Ultim Max 40, Oxford Instrument Technology (Shanghai) Co., Ltd, Shanghai, China) was conducted to analyze the chemical composition of the corrosion product layer cross section. The composition and related characteristics of the corrosion products were analyzed using X-ray photoelectron spectroscopy (XPS, PHI-5000versaprobeIII, ULVAC-PHI, Inc., Kanagawa, Japan). The testing parameters were as follows: the X-ray source was monochromatic Al Kα, operated at a power of 150 W, with a beam spot diameter of 500 μm. Additionally, a laser confocal scanning microscope (CLSM, KEYENCE VK-X250, Keyence Corporation, Osaka, Japan) was used to analyze the three-dimensional surface profile of the samples following rust removal. The analyzed area by the CLSM was 0.015 cm² in each field of view, and four areas were observed in each specimen.

3. Results

3.1. Microstructure and Environmental Conditions of the Exposure Site

The distribution of the second phase was observed by the metallographic microscope. The grains of 2024 and 7075 aluminum alloy are elongated along the longitudinal direction. There are many granular intermetallic compounds (black particles) in the grain and grain boundaries of the four aluminum alloys, as shown in Figure 1. The 7075 aluminum alloy has a smaller second-phase distribution and smaller dimensions compared with the 2024 aluminum alloy (Figure 1a,d). Therefore, the 7075 aluminum alloy is likely to be more resistant to corrosion than the 2024 aluminum alloy. Compared with 6061 and 5083 aluminum alloys, 2024 and 7075 aluminum alloys have more elongated grain shapes, and 5083 and 6061 aluminum alloys have more spherical grain shapes. Aluminum alloy corrosion preferentially occurs at the grain boundaries, where corrosion products result in laminar spallation due to the wedge effect. Therefore, 2024 and 7075 aluminum alloys are more likely to cause exfoliation corrosion caused by the expansion of the corrosion product. Compared to 6061 and 5083 aluminum alloys, the 6061 aluminum alloy has a slightly denser second-phase distribution, but a slightly smaller second phase and grains than the 5083 aluminum alloy (Figure 1b,c).

SEM and EDS results in Figure 1 show that the second phase of 2524, 5083, 6061, and 7075 aluminum alloys are Al₇CuMg, Mg₂Al₃, Mg₂Si, and Al₇Cu₂Fe. The percentage of the field area of 2524, 5083, 6061, and 7075 aluminum alloys in the second stage was calculated using analysis software (Image J, 1.42q), and was 5.78%, 1.55%, 1.96%, and 4.93%, respectively.

The environmental temperature at Zhongshan Station during the exposure period is shown in

Table 2. Temperature variations during the exposure testing at Zhongshan Station.

Month	1	2	3	4	5	6	7	8	9	10	11	12
Average temperature/°C	0.4	−2.6	−8.6	−12.0	−15.2	−15.1	−15.6	−13.1	−13.0	−10.0	−4.3	0.2
Maximum temperature/°C	8.7	3.3	0.8	−3.5	−3.8	−5.2	−2.1	−3.9	−2.8	1.2	6.3	9.6
Minimum temperature/°C	−5.9	−10.5	−22.4	−21.3	−33.7	−33.2	−32.1	−36.4	−26.7	−27.3	−17.4	−6.8
Number of freeze–thaw cycles/day	8	5	-	-	-	-	-	-	-	-	6	9

. According to meteorological data, the highest temperature in the exposure environment is 9.6 °C, while the annual minimum temperature was −36.4 °C, with an average annual temperature of −9.1 °C. Precipitation in Antarctica is extremely sparse, primarily occurring in the form of snow and frost. Additionally, the region is subjected to strong winds throughout the year, particularly in winter, during which wind speed can exceed several tens of kilometers per hour. These strong winds lead to extreme wind chill and blizzard conditions, further exacerbating the harsh climatic conditions in Antarctica. The snow and ice cover have a significant impact on surface temperature, thereby exacerbating the extreme weather patterns in this region.

3.2. Determination of Corrosion Rate

Figure 2 shows the corrosion rates of aluminum alloys 2024, 5083, 6061, and 7075 exposed at Zhongshan Station. It is evident from the figure that the corrosion rates of 2024 and 7075 aluminum alloys are significantly higher than those of 5083 and 6061 alloys. The relatively rapid corrosion rates exhibited by 2024 and 7075 aluminum alloys are primarily attributed to their high copper (Cu) content. In aluminum alloy systems, Cu tends to form cathodic regions, a characteristic that significantly enhances the alloy’s activity in electrochemical corrosion environments. Specifically, when exposed to corrosive media, Cu forms a tiny galvanic cell, with its surrounding phase accelerating the electron transfer process, thereby facilitating the corrosion reaction. Consequently, the high Cu content in 2024 and 7075 aluminum alloys is one of the key factors contributing to their accelerated corrosion rates. The corrosion rates are 14.5 g/(m²·year) for 2024, 1.36 g/(m²·year) for 5083, 1.5 g/(m²·year) for 6061, and 10.8 g/(m²·year) for 7075. Manuel et al. [13] found that the corrosion rates of aluminum alloys at Antarctic stations were significantly higher than those observed in equivalent marine atmospheres in temperate or tropical climates. Corrosion rates in the Antarctic atmosphere vary depending on the test site and may be related to the salt deposits on the metal surface.

3.3. Corrosion Product Observation

Figure 3 shows the XRD spectra of surface corrosion products for 2024, 5083, 6061, and 7075 aluminum alloys exposed to Antarctic atmosphere. The XRD patterns reveal that the corrosion products of all four aluminum alloys contain both AlOOH and Al₂O₃ phases. Notably, the XRD pattern of the 6061 aluminum alloy exhibits a distinct diffraction peak around 2θ = 18°, which is identified as Al(OH)₃.

Figure 4 illustrates the macroscopic appearance of 2024, 5083, 6061, and 7075 aluminum alloys after exposure to the Antarctic atmospheric environment. The small black dots represent holes drilled prior to sample deployment, serving the purpose of facilitating the differentiation of samples and enabling comparisons with their pre-deployment states. The surface of the 2024 aluminum alloy is completely covered by a layer of black-gray corrosion products, with severe corrosion particularly at the sample edges and perforated areas, where corrosion products accumulate more prominently. This phenomenon is likely associated with the presence of high concentrations of chlorides at the metal–ice interface, which may cause rapid and severe corrosion within a short exposure period. In contrast, the surface of the 5083 aluminum alloy remains relatively smooth, with only a small amount of black-gray corrosion products attached. The corrosion products do not fully cover the alloy surface. The corrosion products on the 6061 aluminum alloy sample are primarily concentrated at the perforations and edges. The corrosion severity of the 7075 aluminum alloy is similar to that of the 2024 alloy, with the entire surface covered by corrosion products that obscure the original appearance and the color being darker than that of the 2024 aluminum alloy. Corrosion is particularly severe at the edges and perforated areas, where the corrosion products are dense and more uniformly distributed. Additionally, the corrosion products are easily detached from the surface, and slight shaking of the sample causes the products to peel off, with more significant delamination occurring at the edges, indicating the occurrence of exfoliation corrosion. These observations further support the hypothesis of a high-concentration saltwater layer existing between the metal and ice layers.

Figure 5 shows the morphology of corrosion products on the surfaces of 2024, 5083, 6061, and 7075 aluminum alloys after exposure in Antarctica. The surface of the 2024 aluminum alloy exhibits numerous cracks and delamination, and the corrosion products do not fully cover the surface, displaying irregular shapes and sizes. In contrast, the 5083 aluminum alloy has fewer corrosion products, with cracks being more widespread and scattered. The corrosion products on the surface of the 6061 aluminum alloy are also minimal, but some delamination is observed. The corrosion of the 7075 aluminum alloy is more severe, with the corrosion product layer covering the surface relatively well, yet cracks are still present and the surface exhibits variations in height, appearing uneven and pitted. Morcillo et al. [13] observed that the surface morphology of aluminum alloys exposed in Antarctica shows a honeycomb-like structure, with the entire surface being corroded and covered by aluminum oxide. The dehydration of aluminum oxide causes localized cracking and peeling, resulting in a distinctive mosaic-like appearance on the corroded surface. Through exhaustive EDS point analysis of the various samples, the signals of Al, C, and O elements in all samples were detected. Notably, the proportion of O elements in the 5083 aluminum alloy was higher than that in the others. Specifically, the high O element content in the 5083 aluminum alloy sample suggests that its corrosion products are primarily composed of aluminum oxides and hydroxides. These corrosion products naturally form when aluminum alloys are exposed to corrosive environments and tend to deposit on the material surface, constructing a protective barrier. The presence of this protective layer plays a crucial role in enhancing the overall corrosion resistance of the material. It effectively isolates the material from corrosive agents in the external environment, thereby slowing or preventing further corrosion processes.

Figure 6 presents the cross-sectional morphology of 2024, 5083, 6061, and 7075 aluminum alloys exposed to the Antarctic atmospheric conditions. In the cross section of the 2024 aluminum alloy, the maximum thickness of the corrosion product is approximately 10 μm, primarily composed of aluminum and oxygen, with small amounts of magnesium, iron, and copper. Due to the relatively high proportion of alloying elements in the matrix, these elements are also distributed in the corrosion product layer, indicating the formation of oxides. The corrosion products on the 5083 aluminum alloy are relatively dense, but cracks are present on the surface. This may be due to the loose nature of the corrosion product layer, allowing the surface solution to penetrate into the layer. During freezing, the solution freezes and expands, causing the corrosion product layer to crack. The thickness of this product layer is about 7 μm, mainly composed of aluminum and oxygen, with small amounts of magnesium. The corrosion products on the 6061 aluminum alloy are fewer and not tightly bonded to the matrix, with a product layer thickness of about 3 μm, consisting mainly of aluminum, oxygen, and magnesium. The corrosion of 7075 aluminum alloy is more severe, with the corrosion product layer between the matrix and epoxy resin showing a mixed state with the aluminum matrix, accompanied by small cracks. In the corrosion products, besides aluminum and oxygen, magnesium and zinc are also present. Under certain conditions, these elements may act as anodic regions, increasing the electrochemical activity of the alloy and promoting localized corrosion.

3.4. XPS Analysis

Figure 7 shows the XPS fitting results of surface corrosion products of four kinds of aluminum alloys exposed to the atmospheric environment at Zhongshan Station in Antarctica. The binding energy of Al 2p peaks at 74.7 eV, 74.2 eV, and 73.8 eV represent Al(OH)₃, Al₂O₃, and AlOOH respectively. XPS results show that the corrosion products of these four kinds of aluminum alloys are composed of AlOOH, Al₂O₃ and Al(OH)₃, where the area of the peak represents its relative content in the corrosion products [18]. The highest content of corrosion products of 2024, 5083 and 7075 aluminum alloys is Al₂O₃, while that of the 6061 aluminum alloy is Al(OH)₃. The formation of alumina reduces the activity of the surface of aluminum alloys, and its inertness enables the oxidation layer to improve the corrosion resistance of aluminum alloys [19]. Al(OH)₃ can also be attached to the aluminum alloys surface to resist the attack of oxygen, moisture, and other corrosive media in the atmospheric environment, and improve the stability of aluminum alloys in corrosive environments.

3.5. Surface Morphology and Topography Observation

Figure 8 shows the surface morphologies of 2024, 5083, 6061 and 7075 aluminum alloys after removal of corrosion products in the atmospheric environment at Zhongshan Station for 12 months. The pits on the 2024 aluminum alloy sample surface are significantly deep and connect. Slight corrosion is observed in the area around the pits. The 5083 aluminum alloy sample surface shows a small diameter and dense pits, but the pits are not connected with each other. The 6061 aluminum alloy sample surface also shows pits, but the density of the pits is small and the sample surface has become uneven. Compared with the above aluminum alloys, corrosion of the 7075 aluminum alloy is the most serious, showing the largest values of pit depth and diameter. Pits with diameters of hundreds of microns can be observed on the sample surface. The exfoliation corrosion phenomenon is obvious, and the stepped layer morphology appears. For the aluminum alloys, the wet–dry cycle effect promotes the growth of the metastable pits, so many small pits can be observed. The characteristic of exfoliation corrosion is that the expansion of corrosion products in the intergranular corrosion leads to the delamination of the aluminum alloy surface and subsurface (Figure 8g,h).

Figure 9 shows the 3D corrosion morphology of 2024, 5083, 6061 and 7075 aluminum alloys after removing corrosion products. The 3D profile information of these four kinds of aluminum alloys is consistent with the SEM morphological results, with the 5083 and 6061 aluminum alloys exhibiting better pit resistance and smaller pit depth and pit surface area. The 2424 aluminum alloy is more prone to uniform corrosion and has the largest pit surface area, while the alloy pit diameter and pit depth are the largest on the 7075 aluminum alloy.

Figure 10 shows the pit parameters obtained from the 3D topography results of the four kinds of aluminum alloys. For the 7075 aluminum alloy, the average pit depth and the maximum pit depth are the largest, and the maximum pit depth of the 5083 aluminum alloy is the smallest. The average pit depth of the 6061 aluminum alloy is the smallest. For 5083 and 6061 aluminum alloys, the pit depth is mainly concentrated within 30 µm. For the 2024 aluminum alloy, the pit depth is mainly concentrated within 60 µm. For the 7075 aluminum alloy, the pit depth is mainly concentrated within 80 µm. Cl⁻ deposition rate and TOW are the necessary conditions for pit initiation and growth. The 7075 aluminum alloy shows serious pitting, which is related to the highest content of impurity element composition, especially Zn (Table 1). The formation of pits on the 2024 aluminum alloy is related to the content of impurity elements composition and typical Al₂CuMg phase. The average pit depth of the 6061 aluminum alloy is the smallest, as it contains the least impure element composition. Compared with the 6061 aluminum alloy, the 5083 aluminum alloy contains more impurity elements, especially Mg (Table 1), which promotes the initiation of pits, resulting in greater pit density and average pit depth than the 6061 aluminum alloy.

4. Discussion

4.1. Effect of Aluminum Alloy Composition on Atmospheric Corrosion Behavior

The composition of the aluminum alloys has an important influence on the atmospheric corrosion. Different alloy compositions can change the chemical properties of an aluminum alloy and subsequently affect its corrosion behavior when exposed to the environment. Aluminum alloys are formed by mixing aluminum with other elements such as iron, silicon, copper, magnesium, manganese, and zinc, adding these elements to improve strength, workability, electrical conductivity, and other properties. Copper is added to the 2xxx aluminum alloy composition. In the atmospheric environment, corrosion of the aluminum alloy produces a large number of Al³⁺ and a small number of Cu²⁺, and the Cu²⁺ that enter the solution are adsorbed onto the aluminum alloy surface to form the cathode of the electrochemical reaction. The reduction efficiency of the copper ions and the oxygen can be increased, thus increasing the corrosion rate of the 2xxx aluminum alloy [20]. The 5xxx aluminum alloy belongs to the Al-Mg series of aluminum alloys. Its second phase (Mg₂Al₃) is inert more than other solid solutions in aluminum alloys, and the 5xxx aluminum alloy is less susceptible to corrosion. As a result, the corrosion resistance of the 5xxx aluminum alloy is better in atmospheric environments than other three kinds of aluminum alloys. The 6xxx aluminum alloy has certain mechanical properties and excellent corrosion resistance. The Mg₂Si phase is the basis of precipitate hardening. However, compared with Al, the Mg₂Si phase is an anode with higher electrochemical activity, and ionic compounds are generated after corrosion, reducing the electrochemical activity of the aluminum alloy surface and slows the rate of electrochemical corrosion [21,22]. The 7xxx Al alloy is composed mainly of zinc–magnesium or magnesium–copper, which exist in the typical second phase form in aluminum alloys. The presence of these alloying elements increases the local corrosion sensitivity of aluminum alloys in environments containing Cl⁻ [14,23].

4.2. Corrosion Mechanism Analysis of the Aluminum Alloys in Antarctic Environment

To analyze the corrosion mechanism of aluminum alloys in the Antarctic atmosphere, it is necessary to consider the influence of the peculiar Antarctic marine atmospheric conditions on the corrosion behavior. Figure 11 shows the corrosion mechanism of aluminum alloys exposed to the Antarctic atmosphere. The Antarctic marine atmospheric environment is significantly different from that of temperate and tropical environments. According to the definition of ISO 9223 [24] for the time of wetting (TOW) of the metal surface, the temperature in some polar regions is lower than 0 °C throughout the year, the TOW is 0, and corrosion cannot occur.

Our previous work proved that temperatures below 0 °C alone are not enough to eliminate corrosion behavior, and even at temperatures of −20 °C, the corrosion electrochemical process is still ongoing [25]. Brass et al. [26] found that the presence of salt allows corrosion to occur at −50 °C (the eutectic point of the CaCl₂-H₂O system is −55 °C). Song et al. [27] believed that even completely frozen NaCl solution still showed electricity and that there were three conductive paths in the ice layer to induce corrosion. Two important factors affecting the corrosion rate of the Antarctic atmosphere are aerosol chloride and TOW (Figure 11a). Aerosol chlorides come from salt-rich aerosol and snow in marine environments. Other meteorological factors such as extreme wind speeds and precipitation patterns in the Antarctic environment can lead to the formation of ice crystals and snow on aluminum alloy surfaces, which can affect the corrosion rate. In addition, due to climate change in recent years, the ambient temperature has been rising. The surface temperature of the aluminum alloy is not sufficiently high to evaporate the deposited snow/ice, but it is high enough to keep the melted snow/ice wet for a long time, which leads to the formation of ice/snow layers of different thickness on the aluminum alloy surfaces. Prolonged exposure to sunlight in summer can also cause the surface temperature of the metal sample to exceed the ambient temperature, resulting in an electrolyte film covering the aluminum alloy surface, which in turn leads to a longer TOW.

At the initial stage of exposure, the aluminum alloy matrix surface spontaneously forms a natural oxide film with excellent adhesion and corrosion resistance, which can isolate the aluminum matrix from the aggressive environment [28]. With prolonged exposure, the oxide film is converted to AlOOH (Al₂O₃·H₂O) by a hydration reaction [29,30]. Although a stable oxide film is formed on the surface of the aluminum alloy, the active anion preferentially attaches to the oxide film from the active site through the oxide film to the aluminum alloy matrix due to the strong corrosive chloride ions in the deposited salt particles in the Antarctic atmospheric environment (Figure 11b). The presence of a thin liquid film below the ice layer on the aluminum alloy surface allows Cl⁻ to accumulate in large quantities and continuously and steadily enter the aluminum alloy surface, where Cl⁻ acts as a catalyst in the pit process. The pits first occur at the active sites on the oxide film surface, where a large area of relatively complete film acts as the cathode. This active site acts as the anode when the film is damaged by Cl⁻ deposited in salt particles on the aluminum alloy surface, and the fresh aluminum alloy is exposed to the external environment. The corrosive solution adsorbed on the aluminum alloy surface acts as the electrolyte, and its electrochemical reaction is as follows:

$$\mathrm{Al}-3{\mathrm{e}}^{-}\to \mathrm{A}{\mathrm{l}}^{3+}$$

(3)

$${\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O}+4{\mathrm{e}}^{-}\to 4\mathrm{O}{\mathrm{H}}^{-}$$

(4)

Al³⁺ combines with OH⁻ to form Al(OH)₃. The hydrolysis of Al³⁺ produces H⁺, leading to local acidification [31,32,33], which further promotes the corrosion process.

$$\mathrm{A}{\mathrm{l}}^{3+}+3\mathrm{O}{\mathrm{H}}^{-}\to \mathrm{A}\mathrm{l}{(\mathrm{O}\mathrm{H})}_3$$

(5)

$$\mathrm{A}{\mathrm{l}}^{3+}+{\mathrm{H}}_2\mathrm{O}\to \mathrm{A}\mathrm{l}\mathrm{O}{\mathrm{H}}^{2+}+{\mathrm{H}}^{+}$$

(6)

$$\mathrm{A}{\mathrm{l}}^{3+}+{\mathrm{H}}_2\mathrm{O}+{\mathrm{e}}^{-}\to \mathrm{A}\mathrm{l}\mathrm{O}{\mathrm{H}}^{+}+{\mathrm{H}}^{+}$$

(7)

$$\mathrm{A}{\mathrm{l}}^{3+}+3{\mathrm{H}}_2\mathrm{O}\to \mathrm{A}\mathrm{l}{(\mathrm{OH})}_3+3{\mathrm{H}}^{+}$$

(8)

Although Al(OH)₃ has excellent stability in an acidic environment, a small amount of Cl⁻ can directly enter the corrosion product layer and compete with OH⁻ (within Al(OH)₃) for adsorption [34,35]. With the increase in exposure time, Cl⁻ gradually replaces OH⁻ (within Al(OH)₃), forming AlCl₃, which accelerates the corrosion process, as shown in the formula below:

$$\mathrm{A}\mathrm{l}{(\mathrm{O}\mathrm{H})}_3+\mathrm{C}{\mathrm{l}}^{-}\to \mathrm{A}\mathrm{l}{(\mathrm{O}\mathrm{H})}_2\mathrm{C}{\mathrm{l}}^{-}+\mathrm{O}{\mathrm{H}}^{-}$$

(9)

$$\mathrm{A}\mathrm{l}{(\mathrm{O}\mathrm{H})}_2\mathrm{C}\mathrm{l}+\mathrm{C}{\mathrm{l}}^{-}\to \mathrm{A}\mathrm{l}(\mathrm{O}\mathrm{H})\mathrm{C}{\mathrm{l}}_2+\mathrm{O}{\mathrm{H}}^{-}$$

(10)

$$\mathrm{A}\mathrm{l}(\mathrm{O}\mathrm{H})\mathrm{C}{\mathrm{l}}_2+\mathrm{C}{\mathrm{l}}^{-}\to \mathrm{A}\mathrm{l}\mathrm{C}{\mathrm{l}}_3+\mathrm{O}{\mathrm{H}}^{-}$$

(11)

In the cold Antarctic climate, thin liquid films on the surface of aluminum alloys form ice sheets due to low temperatures, which then melt under the action of sunlight. The active sites (defect or second-phase particle location) are the origin of corrosion initiation when aluminum alloys are exposed to freeze–thaw cycles for a long time. The concentration of Cl⁻ in the thin liquid film increases due to the formation of the ice layer, which accelerates the penetration of aggressive ions and further promotes the diffusion of dissolved oxygen [36]. As the corrosion progresses, the corrosion product gradually covers the aluminum alloy surface. Since the activity of the grain boundaries is higher than that of the grain interiors, corrosion develops rapidly along the grain boundaries, and aluminum alloys exhibit intergranular corrosion (Figure 11c) [37]. When the temperature is lowered, the water in the corrosion product freezes and the corrosion product at the grain boundaries creates an expansion and wedge force, which further promotes the exfoliation corrosion of the aluminum alloy (Figure 6d and Figure 11d).

5. Conclusions

In this study, the atmospheric corrosion behavior and mechanisms of four typical kinds of aluminum alloys in the Antarctic outdoor environment were analyzed by exposure tests and indoor characterization tests. The main conclusions are as follows.

(1)

The corrosion rate of the 2024, 5083, 6061, 7075 aluminum alloys exposed to the Antarctic environment was 14.5 g/(m²·year), 1.36 g/(m²·year), 1.5 g/(m²·year), and 10.8 g/(m²·year), respectively.

(2)

The corrosion products formed on the 2024, 5083, 6061, and 7075 aluminum alloys exposed to the Antarctic environment were primarily composed of AlOOH and Al₂O₃.

(3)

For the 7075 aluminum alloy, the average pit depth and the maximum pit depth were the largest, and the maximum pit depth of the 5083 aluminum alloy was the smallest.

(4)

In the Antarctic environment, the initiation and growth of aluminum alloy pits are facilitated by freeze–thaw cycles and salt deposition. Due to peculiar alloy composition and microstructure, the grain boundary of aluminum alloys has high activity and grain boundary corrosion occurs. The freeze–thaw cycle causes the corrosion products at the grain boundary to expand, and aluminum alloy exfoliation corrosion occurs due to the wedge effect.