Comprehensive evaluation of fungal-induced corrosion in aluminum alloys by Amorphotheca resinae

Abstract

Amorphotheca resinae is a fungus that particularly corrodes aeronautical aluminum alloys, leading to economic issues in various industries. This study aims to investigate the effects of this fungus on the corrosion of four different aluminum alloys, namely 2024, 7075, 5083, and 3105, after 25, 50, and 75 days through scanning electron microscopy (SEM), confocal laser scanning microscopy (CLSM), 3D profilometry, weight loss (%) measurement, energy dispersive spectrometry (EDS), electrochemical impedance (EIS), and pH changes. After 25 days, the 2024 alloy had the highest mycocorrosion rate (1.3417 mpy), while alloy 3105 had an undetectable value on this day. According to the EIS test, the 3105 alloy had the highest level of resistance (1.12 × 10⁵ Ω cm²) to corrosion, while the 2024 alloy was the most susceptible (7138 Ω cm²). The qualitative data from SEM, CLSM, and 3D profilometry also confirmed the quantitative findings where the surface pits on the 2024 alloy were deeper than those of other alloys. Overall, the results showed that the lowest and highest corrosion rates mediated by A. resinae belonged to 3105 and 2024 alloys, respectively. These findings could have significant implications for industries that use aluminum alloys and might help in developing strategies to prevent or control biocorrosion.

Introduction

The ISO 8044 standard defines corrosion as an electrochemical interaction that occurs reciprocally between a metal and its surroundings and results in changes to the metal's properties. When the effect of microorganisms carries out this process, it is assigned to microbial corrosion, a significant issue in many industries and sectors^1,2. According to estimates, microbial corrosion losses account for 20–40% of all corrosion types, causing billions of dollars in economic scathe³. The process of microbial corrosion on metals, including aluminium, iron, copper, etc., and their alloys is carried out by various microorganisms, and fungi are among the significant ones in corrosion⁴. Aspergillus niger, Fusarium spp., Penicillium spp., and Amorphotheca resinae are some of the fungal species that contribute to microbial corrosion; The latter one is the most significant and harmful fungus in corrosion among others as it corrodes aluminum fuel tanks and is resistant to hostile environments thanks to the abundance of spores it produces^5,6.

Among the variety of carbon sources, diesel, and jet fuel could serve as the carbon supply for A. resinae. This fungus typically comes into contact with sediments and the aqueous phase of fuel and water combinations⁷, generating organic acids as a result of hydrocarbon consumption. This is the main cause of its corrosion on metal surfaces that in contrast to alkaline and neutral settings, are more likely to cause metals to corrode^8,9.

Due to characteristics like lightweight, repeatability, ease of work, durability, rust resistance, and electrical conductivity, aluminum, the second most used metal, has become valuable and widely employed¹⁰. Among different alloys of aluminum, 2024, 3105, 5083, and 7075 are widely used in industries. Heat exchangers, hospital equipment, automotive parts, etc. are made of aluminum alloy 3105¹¹. It cannot be heat-treated and shows strong resistance to general corrosion. This resistance is mainly because of the natural formation of a protective oxide layer on its surface. Manganese is included in the alloy's composition, and it boosts its resistance to corrosion by inhibiting the development of large intermetallic particles that can become starting points for corrosion¹². The 2024 aluminum alloy is utilized in the manufacture of airplanes, fuel tanks, auto parts, and other machinery¹³. It has been recognized for its excellent strength and frequently utilized in aerospace industries, is notably prone to corrosion. Its main alloying component is copper, which enhances its mechanical strength but also heightens its susceptibility to corrosion. 2024 aluminum typically experiences the formation of pitting corrosion at the beginning which then start growing rapidly and enlarge over time, resulting in structural deterioration. With extended exposure, the alloy may be susceptible to intergranular corrosion, leading to exfoliation and peeling off of metal layers^14,15.

The aluminum alloy 5083 is frequently used in marine equipment, pressure and storage tanks, constructions, and aircraft sectors, due to its outstanding resistance to corrosion, especially in marine settings. Its behavior under different conditions has been extensively analyzed^16,17. The strong resistance of 5083 aluminum alloy to corrosion in seawater is due to the creation of a passivating oxide layer, which serves as protection against further corrosion. The corrosion resistance of 5083 aluminum alloy is also influenced by its alloying elements, primarily magnesium. The formation of the β-phase (Mg5Al8) can either enhance or reduce corrosion resistance depending on the distribution and morphology of these phases. In natural seawater environments, microorganisms can influence the initial stages of corrosion. The presence of microbial films can alter the electrochemical conditions at the metal surface, sometimes leading to localized corrosion or, conversely, to enhanced protection depending on the microbial species and environmental conditions¹⁶. In situations with a lot of power requirements, 7075 aluminum alloy is employed¹⁸. The aerospace industry and other high-stress applications make extensive use of the this alloy. However, it faces various challenges associated with corrosion. In particular, it demonstrates electrochemical corrosion, especially when exposed to acidic environments. Galvanic corrosion can occur when 7075 aluminum comes into contact with other metals due to the potential difference between the metals, causing the more anodic metal (aluminum in this case) to corrode faster in the presence of an electrolyte¹⁹.

The majority of studies in the field of microbial corrosion on aluminum alloys have exclusively focused on the corrosion of Aspergillus species, A. resinae, and some bacterial genera, including Bacillus and Pseudomonas²⁰. Although studies on A. resinae's ability to corrode aluminum alloy 2024 have been conducted before, to the best of our knowledge, there is no comparative research on the intensity of A. resinae’s mediated corrosion on different kinds of widely used aluminum alloys. Considering the widespread use of these alloys in various industries and their susceptibility to corrosion, having an overview of how this agent will influence these alloys sounds essential. As a result, this study aimed to examine the A. resinae corrosion rates in the aluminum alloys 2024, 7075, 5083, and 3105, analyzing how A. resinae affects their corrosion.

Material and methods

Strain and growth medium

Amorphotheca resinae DSM1203 was obtained from the Research Institute of Petroleum Industry (Iran) and used as the corrosive fungal strain in this experiment. Potato Dextrose Broth (PDB) culture medium (pH 6.5) was used to cultivate A. resinae during the examination process²¹.

Preparation and immersion test

The corrosion rate was calculated using the immersion method test, a graph of weight loss over time was drawn, and the graph's slope was used to determine the corrosion rate. For this examination, A7075, A5083, A2024, and A3105 alloys were obtained in the coupon shape (Produced by Mirab Sanat Rastin Pars in Iran). The composition of the constituent elements of each of the desired alloys are shown in Table 1.

Table 1.

The elemental makeup of the 4 alloys under test^23,24.

Element	7075 (%)	2024 (%)	3105 (%)	5083 (%)
Al	Balance (87.1–91.4)	Balance (90.7–94.7)	Balance (95.85–98.6)	Balance (93.2)
Zn	5.1–6.1	0.25 max	0.25 max	0.25 max
Mg	2.1–2.9	1.2–1.8	0.2–0.8	4.0–4.9
Cu	1.2–2.0	3.8–4.9	0.3 max	0.1 max
Cr	0.18–0.28	0.1 max	–	0.05–0.25
Fe	0.5 max	0.5 max	0.7 max	0.4 max
Si	0.4 max	0.5 max	0.6 max	0.4 max
Mn	0.3 max	0.3–0.9	0.3–0.8	0.4–1.0
Ti	0.2 max	0.15 max	0.1 max	0.15 max
Others (each)	0.05 max	0.05 max	0.05 max	0.05 max
Others (total)	0.15 max	0.15 max	0.15 max	0.15 max

The length, width and thickness of the coupons were 50 mm, 15 mm and 1.5 mm, respectively.

A three-millimeter hole was made on one side of the metals for immersion in the culture medium. The exact diameter and thickness of the coupons were measured using a caliper with an accuracy of 0.01 mm. Since the entire surface of the metals was not exposed to the corrosive environment, the diameter of the disposable area was measured.

Four sample preparation, coupons bearing the numbers 120, 280, 600, and 1200 were polished using silicone sandpaper under a gentle flow of water. The coupon samples were hung into the 250 mL flasks and incubated in 150 mL of the inoculated liquid culture. To mimic the fuel tank's condition, 25 mL of diesel fuel was added to the culture medium which makes a diesel-aqueous two-phasic media. The incubation was carried out in static condition at 30 °C for optimal growth but briefly shaken every few days to inhibit the aqueous phase from becoming anaerobic. After incubation, the physical structural changes of the alloys, corrosion rates, microbial growth, and biofilm thickness were analyzed in samples through different assessments, including pH changes, Scanning Electron Microscopy (SEM), Energy Dispersive Spectrometry (EDS), Electrochemical Impedance (EIS) measurement, 3D profilometry, and Confocal Laser Scanning Microscopy (CLSM). Following that, the samples were cleaned, and the weight loss (%) was assessed²².

SEM and EDS analysis

Following three aforementioned intervals, SEM (VEGA3 TESCAN, Czech Republic) and EDS (DXR-X10P DIGITAL-X-RAY-PRICESSOR) tests were conducted. The alloys were removed from the culture media and fixed with glutaraldehyde for 24 h, then washed in five ethanol dilution series (25, 50, 75, 96, and 100%) in turn (2 h each). The fixed samples were then coated with a thin layer of gold and observed for any sign of corrosion and microbial biofilm formation.

CLSM analysis

CLSM (Leica, Germany) was implemented to trace biofilm on coupon surfaces in 3D resolution. To prepare the samples, the alloys went through the acridine orange staining. This fluorochrome dye attaches to the DNA of cells and shows a green hue under CLSM²⁵.

3D profilometry

3D profilometry analysis was carried out to demonstrate the physical destruction intensity of alloys after mycocorrosion. The size of the pits on the coupon's surface will reveal the depth of microbial invasion and corrosion. The test was done after washing the aluminum coupons in a 3D profilometer (LPM-D2, Iran) and the data was analyzed by Gwyddion v.2.41 software.

EIS analysis

The coupon samples were put into a beaker (as a tube) that had a capacity of around 70 ml, exposing a particular 1.5 cm² region of the samples' surface to the electrolyte. In this investigation, Saturated Calomel Electrodes (SCE) were employed as the reference and the examined alloys served as the working electrodes. A platinum electrode with a surface area of 1 cm² served as the counter electrode. The time constant is represented by the peak that was seen in the phase angle diagram for every sample. The surface and corrosion rates were investigated using the EIS test. In this experiment, a frequency analyzer of the Solarton-SI 1260 type and a potentiostat of the Solarton-SI 1287 type were used. ZsimpWin v.3.60 software was utilized to compare the experiment's outcomes. This test was conducted in accordance with ASTM G31-72 standard.

Coupons weight loss (%) measurement

Loss of the weight is a sign of the physical destruction during corrosion. To measure this parameter, the metals were eradicated from biofilm, the corrosion products, and sediments according to the ASTM G1-03 (American Society for Testing and Materials) standard. Then, the coupons were dehumidified at 50 °C for 4 h and reached a constant dry weight. Finally, the weight loss rate was calculated in mL year⁻¹ by the Eq. 1^26:

$$\text{CR}=\frac{\mathbf{K}\times \mathbf{W}(\mathbf{g}\mathbf{r})}{\mathbf{D}\left(\frac{\mathbf{g}\mathbf{r}}{{\mathit{c}\mathit{m}}^3}\right)\times \mathbf{A}({\mathbf{c}\mathbf{m}}^2)\times \mathbf{T}(\mathbf{h})}$$ 1

where K is the corrosion rate constant, A is the entire exposed area of the coupon, T is the time the coupon samples were exposed to the corrosive solution, W is weight loss, and D is the metal coupon density.

Statistical analysis

All the quantitative tests were performed in triplicate and analyzed with SPSS v.27.0.1 software. The p-value was < 0.001.

Results and discussion

pH variation in the corrosion process of aluminum coupons

Prior to the commencement of the experiment, the pH value of the 2024 alloy was 6.6. Its values were 5.5 on the 25th day of inoculation, 4.85 on the 50th day, and 6.45 on the 75th day. In contrast, the control sample's values ranged from 6.26 to 5.83. The A. resinae fungus is able to synthesize organic acids, including citric, oxalic, succinic, glutaric, and pyruvic acids this ability could be the reason for the pH values decrement on days 25 and 50^6,10. Additionally, it is expected that the fungus will transit from its constant growth phase to the stage of cell death or lysis due to the reduction of carbon resources and mineral components in the culture media, which may be the cause of the increase in pH values on day 75. Alkaline metabolite synthesis is linked to this phase shift²⁷. In several alloys, comparable outcomes were seen. On days 25, 50, and 75, the pH values of the 7075 alloy were 5.54, 4.93, and 6.38, respectively. The pH values of alloy 5083 were likewise 5.70, 5.3, and 6.48, in that order, while for alloy 3105 the values were likewise 5.62, 5.35, and 6.62 in the same periods. It is worth noting, that for each of the four alloys under consideration, the control samples' pH value range was nearly identical.

Weight loss in the corrosion process of aluminum coupons

The results of the corrosion rate ratio in the presence and absence of A. resinae are shown in Table 2. The findings demonstrated that the samples' corrosion rates in A. resinae culture are noticeably higher than that of the uninoculated medium (control) with increasing exposure duration, revealing that the presence of this fungus can speed up alloy corrosion²⁸. The samples with the highest and lowest rates of corrosion were 2024 and 3105, respectively. The ratio of the corrosion rate for all the samples on day 50 was at its highest value and for the entire test period, the 2024 sample's corrosion rate was reduced by 1.6, 2.2 and 2.4 times more than that of the control. Samples 5083 and 3105 did not show measurable corrosion on the 25th day, but the ratios after 50 and 75 days, were gradually increased by 1.8 and 7.1 times in sample 5083 and 2 and 9.1 times in sample 3105 (Table 2). The corrosion rates of the 2024 alloy were 1.3417, 0.6655, and 0.8393 on days 25, 50, and 75, respectively. This is significantly greater than the corrosion rates of the alloy 3105, with 0.2442, and 0.1493 on days 50 and 75. On day 75, alloy 2024 also lost 7 mg of its weight, which is far greater than alloy 3105's with 2 mg of weight reduction.

Table 2.

The weight loss and corrosion rate of aluminum alloys in the test period of 25–50–75 days.

Time (days)	Alloy code	Initial weight (g)		Second weight (g)		Weight loss (mg)		Corrosion rate (mpy)		Control to test ratio
		Control	Test	Control	Test	Control	Test	Control	Test
25	2024	2.983	3.281	2.982	3.279	1	2	0.2104	1.3417	1.6232
	3105	0.573	1.246	0.573	1.246	0	0	0	0	0
	5083	2.508	2.684	2.508	2.684	0	0	0	0	0
	7075	1.733	2.212	1.732	2.211	1.222	1	0.1601	0.2181	1.3547
50	2024	2.327	2.567	2.325	2.562	2	1	0.2994	0.6655	2.2228
	3105	1.145	1.071	1.144	1.070	1	1	0.1195	0.2442	2.04355
	5083	2.136	2.668	2.135	2.665	1	3	0.1941	0.3496	1.8011
	7075	1.748	1.876	1.746	1.873	2	3	0.2718	0.4691	1.7259
75	2024	2.282	2.437	2.279	2.430	3	7	0.3532	0.8393	2.3763
	3105	0.893	1.200	0.892	1.198	1	2	0.0795	0.1493	1.8779
	5083	2.615	2.035	2.613	2.032	2	3	0.2263	0.3873	1.7321
	7075	1.899	1.838	1.897	1.834	2	4	0.2319	0.5284	2.7563

In general, the results indicated that the tendency to corrosion and the corrosion rate increased gradually²⁹. This could be because of the gradual production of organic acids, reaching their maximum concentration between days 25 and 50³⁰. Compared to the 75th day which is based on the identical results of Little et al.²⁷, entering the fungus to death and lysis phase, the pH is increased and corrosion is reduced. Also, the resistance of aluminum in neutral and alkaline pH environments to corrosion could be another factor that lowers the corrosion rate on day 75³¹. Amorphotheca resinae destroys the oxide film layer that is naturally formed on the aluminum surface which has a protective effect against corrosion. This will cause more corrosion of the samples²⁹. While in sterile environments due to the existence of this oxide layer, the tendency of aluminum to corrode is very weak^{8,32, 33}. The obtained results showed that the 2024 and 3105 alloys had the lowest and highest corrosion resistance, respectively which is consistent with the reports of Imo et al.’s³⁴ and Sun et al.’s³⁵ studies.

Amorphotheca resinae is aerobic and consumes large amounts of dissolved oxygen in the culture medium. This phenomenon is significant in deep layers of the biofilm structure, leading to the creation of an anaerobic niche over there. This area also accumulates enormous amounts of organic acids by trapping them into rigid layers of biofilm structure. These events create two separated regions with different gradients of oxygen; The aerated area which acts as the anode and the non-aerated (anaerobic) area as the cathode³⁶. By creating an oxygen-concentrated region, the aluminum oxide layer is destroyed and causes localized corrosion and the release of Al³⁺³⁷. As explained in the results of weight loss (Table 2), with the increase of the test time, the number of holes on the surface of the samples in the biotic solution gradually increases, which shows that the corrosion of aluminum alloys by the A. resinae is mainly the result of localized cavities. Because some of the holes will link to form larger ones, aluminum alloys' mechanical qualities and lifetime of use are decreased²⁹.

As per the obtained results, sample 3015 has the strongest corrosion resistance and the lowest corrosion rate, corresponding to Imo et al.³⁴ and Sun et al.³⁵; alloy 2024 has the lowest corrosion resistance.

Corrosion evaluation by SEM

To examine how fungus adhered to metal surfaces and to spot corrosion, SEM micrographs were obtained after the test period of 25, 50, and 75 days (see Fig. 1). In images a2 to d2, the biofilm and fungal hyphae on the coupons surfaces are visible, which in the case of sample 7075 is very compact and sticky. Figure 2c3 shows the gathered hyphae and spores inside the cavity on the surface of sample 5083 and in a3 to d3 micrographs, intense local holes are obvious. The average diameter of holes was 8.08, 2.46, 9.05, and 5.13 µm in 2024, 7075, 3105, and 5083 alloys, respectively (Fig. 1a4–c4). Although the 3105 sample had the highest average diameter of holes, the corrosion of this sample was insignificant. That may be the reason for surficial corrosion in which the holes of corrosion were not very deep. The SEM images of aluminum alloys revealed that the shape and diameter of the corrosion holes in the fungal environment varied. This difference may be attributed to the non-uniform growth of the fungus in the initial stages of development, as well as the accumulation of biofilm in distinct regions of the aluminum samples, resulting in uneven distribution of the corrosion holes. Some corrosion products were observed on the surface of samples a2, b3, c2, and d3, and with an increasing incubation period, more intense fungal hyphae and spores on the aluminum surface were obvious and expected. This intensity could cause local corrosion which is shown in Fig. a4–d4. The results obtained from the preliminary weight loss and SEM analysis revealed that alloy 2024 has a high sensitivity to corrosion even when there is no fungus.

Figure 1.

SEM images of corroded and uncorroded aluminum alloys. a1–d1 control samples without A. resinae. a2–d2 samples after 25 days in the presence of A. resinae. a3–d3 samples after 50 days in the presence of A. resinae. a4–d4 samples after 75 days in the presence of A. resinae.

Figure 2.

Diagrams of resistance of four different alloys to corrosion. a Nyquist diagram, b Bode diagram, c Phase angle diagram. The diameter of each semicircle shows the corrosion resistance of each sample. d The recommended EIS test equals circuit.

Corrosion evaluation by EDS

The proportion of elements in a sample may be determined using the EDS method. Based on these results, Mg, O, C, and Al elements were detected in EDS spectra (see Supplementary Fig. S1 online) of the cultivated aluminum alloys, while Cl, S, Na, and Cu were present at lower intensity. As Zhang et al.⁸ demonstrated, the presence of O and C elements can be attributed to fungal hyphae and spores, which can confirm the growth of fungal biofilm on the surface of the samples. According to the results of EDS (Table 3), the relative weight percentage (Wt.%) of Al in the corrosion products was the highest among the other compounds. The reason could be quested in the presence of A. resinae that accelerates the dissolution of aluminum due to biofilm adhesion. The longer the fungus is present, the more aluminum is deposited as Alekhova et al.³⁸ and Fan et al.²⁹ mentioned in their reports. Some aluminum alloy elements such as Mg may increase the growth of the fungus to some extent³⁹. Of the elements that comprise the 2024 alloy, Cu has the highest concentration. According to the findings of Buchheit et al.⁴⁰, the presence of Cu ions is common in the vicinity of corrosion-mediated holes. So, the high degree of corrosion is directly related to the presence of the Cu element in the corrosion products, which are discharged along with the alloy. Spherical compounds observed near the holes are assigned to copper deposits from the dealloying process, and chlorine is also observed along with them⁴⁰.

Table 3.

Quantitative results of EDS analysis and available elements for tested alloys per unit (wt.%).

Alloy	Al	C	O	Na	Mg	Cu	Cl	S
2024	55.20	19.97	16.20	–	1.07	6.79	0.77	–
3105	49.36	30.54	14.33	1.95	1.52	–	–	2.29
5083	46.86	34.93	16.86	–	0.69	–	0.67	–
7075	48.43	37.96	10.68	0.34	0.68	–	–	1.90

Corrosion evaluation by EIS

Corrosion resistance and corrosion rate were checked using the EIS test. The structure of the fungal biofilm, the activity of the fungus, and its metabolites can all have an impact on the electrochemical reaction that drives the microbial corrosion process in aluminum samples⁴¹. A semicircular diagram forms as a consequence of the growth of fungi and the production of fungal acid metabolites, followed by microbial adhesion to the sample surface and the build-up of corrosion products. It can be seen in Fig. 2 that samples 3105, 5083, 7075, and 2024 had the highest to lowest corrosion resistance, respectively. Sample 2024 had a resistance of 7138 Ω cm², a lower value than the other samples’ resistance, while 3105 alloys had a resistance value of 1.12 × 10⁵ Ω cm², which was greater than that of the others. Additionally, it is evident that the magnitude of load transfer resistance (Rct), which indicates the value of metal resistance against metal ion was released which in the case of sample 3105 was about 15.6 times higher than that of 2024 alloy The EIS test's recommended equal circuit demonstrated in Fig. 2d. Also according to Table 4, C_dl in 2024 alloy was more than 3105 alloy. In the Nyquist diagram, the diameter of each semicircular represents the corrosion resistance of each sample. According to this plot (Fig. 2a), the semicircles' diameters were around 4000 Ω cm² in 2024 alloy, 11,000 Ω cm² in 7075, 32,000 Ω cm² in 5083, and 40,000 Ω cm² in 3105 alloys. From these findings, it can be concluded that 3105 and 5083 alloys are more resistant to the dissolution of metal ions in the microbial environment, causing their resistance to corrosion in the same condition. As can be demonstrated, there is a respectable level of agreement between the results and the outcomes of the other tests, which shows that alloy 2024 has the highest and alloy 3105 has the lowest corrosion rates. According to the results of Fan et al., 2021, The Nyquist diagram's decreasing ring diameter during the incubation time indicates that A. resinae is adhered to the sample surface and generates acidic metabolites²⁹. The number of loops (peaks) shows the number of time constants. High-frequency rings are created due to the capacity of the two electric layers formed between the oxide layer on the surface of the alloy and the solution, and these two electric layers are also called capacitive rings. Also, medium-frequency rings can be attributed to the loosening process of the oxide film. The results of the device are presented as point values with a negative slope (Bode plot, Fig. 2b), and as can be seen in Fig. 2c, there is only one peak that shows the dissolution of the oxide layer on the alloys.

Table 4.

Parameters and values that were measured in the EIS test (mean value of EIS parameters ± SD).

Sample	Rs (Ω cm2)	Rct (kΩ cm2)	ndl	Qdl (Ω−1 sn cm−2)	Cdl (µF cm−2)	χ2 × 10−3
3105	336.21 ± 0.92	112.10 ± 0.92	0.81 ± 0.001	3.59E−5	9.19	1.3
5083	259.35 ± 1.17	88.65 ± 1.29	0.80 ± 0.002	4.72E−5	10.66	3.4
7075	264.11 ± 0.98	31.43 ± 0.26	0.80 ± 0.001	5.31E−5	10.87	2.5
2024	196.74 ± 0.54	7.14 ± 0.03	0.79 ± 0.001	8.19E−5	11.21	1.8

Rs: the solution resistance. Rct: the charge transfer resistance. The constant phase element (CPE) values C_dl, n_dl, and Q_dl represent the double layer capacitance, which is utilized to describe non-ideal capacitive behavior and frequency-dependent capacitance of the electrochemical double layer at the electrode/electrolyte interface and, respectively. χ² × 10⁻³: Fit criterion of experimental data and model.

3D Profilometeric analysis

According to the results obtained from previous tests, 2024 and 3105 alloys had the highest and lowest corrosion rates, respectively (See Supplementary Fig. S2 online for the obvious corrosion pits on the surface of the alloys). Based on these results, CLSM and 3D profilometry tests were performed on these two samples to compare the efficiency of corrosion on them. Using a 3D profilometer, the configuration of the pits on the coupon surface was assessed. The results indicated that the pits in the 2024 alloy are deeper than in 3105 where the maximum corrosion depth in the 2024 alloy was equal to 480 µm compared to 61 µm in 3105. The pits' structure, depth, and width were also analyzed through this method, showing that in contrast to alloy 2024, which had long, narrow corrosion pits, alloy 3105 had shallower, rounder, and wider corrosion pits. The dippest pit was observed after 50 days of incubation in alloy 2024. These outcomes prove the results of weight loss measurement and EIS analysis. The values of the results are shown in Fig. 3, where the 3D figures and diagrams of the corroded areas of the surface of the samples are shown.

Figure 3.

The 3D profilometer graph and diagrams of the corroded areas of the surface of aluminum alloy coupons with the structure of pits obtained from fungal biofilm corrosion after 50 days. 3D graph and Depth of a 2024 alloy and b 3105 alloy. The diagrams of the corroded areas of the surface of c 2024 alloy and d 3105 alloy.

The presence and percentage of some elements in the aluminum alloy component can be definitive to the severity of corrosion or in other words, the resistance of the metal to corrosion. For example, the amount of Mg in the 2024 alloy is less than 7075 and 5083 samples, and according to the investigation of Wen et al.⁴², corrosion resistance increases with a higher amount of Al–Mg combination^42,43. Also, the present and changing value of Cu can lead to a different corrosion rate. 2024 alloy has the highest amount of Cu. The results obtained by Hong et al. show that the negative effect of pitting corrosion is increased by the presence of this element^44,45 which is consistent with the results of this experiment.

Corrosion evaluation by CLSM

The CLSM technique was used to examine fungal biofilms, as well as how much biofilm was present in various areas of the surface after 50 days. In this test, the green color denotes the presence of fungal biofilm, and its intensity over the surface reveals how strongly the biofilm is adhered to the surface. As could be seen in Fig. 4, alloy 2024 had a more intense green color in its CLSM micrograph, while the irradiance was visibly lower in alloy 3105, indicating a thicker and more intense biofilm complex on the former alloy. These results are consistent with earlier research and indicate that alloy 2024 is more disposed to biofilm formation, which in turn increases the alloy's rate of corrosion.

Figure 4.

CLSM and 3D micrograph from the surface of the samples. The intensity of the fluorescence indicates the intensity of the formed biofilm. 3D micrographs of a 2024 alloy and b 3105 alloy. CLSM micrographs of c 2024 alloy and d 3105 alloy.

Conclusions

Previous studies have shown that A. resinae can affect the corrosion of aluminum alloys which is related to the ability of this fungus to produce a wide range of metabolites and organic acids under aerobic conditions. These corrosive metabolites can change the chemical nature of the environment and the electrochemical properties of the metal, leading to metal corrosion. The present study has focused on comparing the rate, severity, and depth of the mycocorrosion developed by A. resinae on three aluminum alloys in 25-day intervals. Comparing the corrosion rate and resistance of the aluminum alloys, this experiment showed that alloys 2024 and 3105 had the highest and lowest corrosion rate, respectively. From the electrochemical analysis of impedance, it was found that the presence of fungus affected the metal's resistance to corrosion, and the resistance values of the samples in alloys 2024, 7075, 5083, and 3105 were respectively the lowest to the highest. These values were supported by outcomes of the microscopic observations in the CLSM analysis in which the intensity of biofilm formation on the 2024 alloy's surface was obviously higher than the most resistant alloy of the experiment (3105). The findings also confirmed that the intensity and depth of corrosion increases over time, as does the release of corrosion by-products, and the thickness and hardness of the biofilm. The observations and comparison results of this study can provide a better view to aluminum-using industries in order to accurately select the type of alloy depending on the corrosion resistance of each one.

Author contributions

H.M. designed the project. A.H.S performed the experiments, A.H.S. and M.G. wrote the paper, M.E., M.G., and H.T. edited the manuscript. All the authors read and approved the final manuscript.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Data availability

All data are included in the manuscript and additional information, and further queries about sharing data can be directed to the corresponding author.

Competing interests

The authors declare no competing interests.

Supplementary Information

The online version contains supplementary material available at 10.1038/s41598-024-70150-x.