# Advances in understanding the initial stages of biomineralisation on AlMg 5083 aluminium alloy in a marine environment: Influence of microbial activity and polarisation

Anthony Diderot, Marie-Line Délia, Régine Basséguy

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Anthony Diderot, Marie-Line Délia, Régine Basséguy. Advances in understanding the initial stages of biomineralisation on AlMg 5083 aluminium alloy in a marine environment: Influence of microbial activity and polarisation. Corrosion Science, 2025, 255, pp.113071. 10.1016/j.corsci.2025.113071 hal-05100977

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# Advances in understanding the initial stages of biomineralisation on AlMg 5083 aluminium alloy in a marine environment: Influence of microbial activity and polarisation

A. Diderot, M.L. Délia, R. Basséguy*

Laboratoire de Génie Chimique, Université de Toulouse, CNRS, INPT, UT, Toulouse, France

## ARTICLE INFO

**Keywords:**

Aluminium alloy

Biomineralisation

Biocorrosion (MIC)

Dioxygen reduction biocatalysis

Marine microorganisms

Dissolution/precipitation mechanisms

## ABSTRACT

This paper investigates the role of microbial activity and polarisation on the first stages of biomineralisation on aluminium alloy 5083 in marine environment. The experiments were carried out in seawater seeded with salt marsh sediments as microbial inoculum. Microbial colonisation was observed on the surface after 10 days immersion at the open circuit potential (OCP). For the first time, the catalysis of dioxygen reduction by marine microbial activity was demonstrated on Al alloy using linear sweep voltammetry. This biocatalytic effect also influenced the corrosion rate (polarisation resistance measurement) and the formation of the biomineralization layer. Surface and cross-section SEM observations showed the existence of this layer, its mass was measured by phospho-chromic stripping. At the OCP, after 10 days' immersion, the layer is 31 % heavier in the presence of microorganisms, demonstrating the involvement of microbial activity in its formation. To understand the mechanism involved, the application of different polarisation potentials highlighted the role of cathodic and anodic reactions, showing that a dissolution/precipitation phenomenon was at the origin of the formation of the mine-ralised layer.

## 1. Introduction

The growth of economic activities linked to the sea—such as maritime transport, marine construction, offshore renewable energies and aquaculture—raises critical issues, particularly regarding the durability of infrastructure exposed to the aggressive marine environment. To address this challenge, research has extensively explored more resistant materials, such as cupronickel, nickel alloys, stainless steel and Inconel [1]. Aluminium alloys offer a promising avenue, as they combine several advantages: they are lightweight, easy to machine and recyclable [2]. Since their introduction in 1940, the development of new aluminium alloys tailored to specific applications has led to the emergence of advanced materials. Environmental concerns, coupled with the rising cost of fossil fuels, have driven manufacturers to seek materials that balance strength and lightness, making the 5XXX and 6XXX series aluminium alloys particularly popular in the maritime transport sector [3].

On the other hand, the marine environment, rich in microorganisms, provides favourable conditions for the development of Microbiologically Influenced Corrosion (MIC), also known as biocorrosion. This

process accelerates the degradation of metallic materials in the presence of microbial activity [4]. Despite the advantages of using aluminium-based materials in marine infrastructures, MIC phenomena affecting them have been relatively understudied (i.e. in field conditions). However, as early as 2013, Vargas and Kannoorpatti highlighted the need to consider microbial activity when assessing the corrosion beha-vior of alloy 5083 in marine settings [5]. This issue is particularly relevant given that marine biofilms can develop on a wide range of materials due to their remarkable adaptability—a capacity demonstrated, for example, in a study involving the immersion of Q235 steel, T2 copper alloy and 1060 aluminium alloy in Hongtang Bay for 30 months [6]. MIC is therefore a widespread concern, affecting not only stainless steels but also all materials capable of electron exchange, including of course aluminium alloys.

In an aerobic environment, it is widely accepted that marine biofilms catalyse oxygen reduction [7–9]. A notable example of this biocatalytic effect was demonstrated in a European research project [10], which found that bacterial colonisation led to a 300–400 mV increase in the corrosion potential of AISI 316 stainless steel samples immersed in various European marine environments: the Mediterranean Sea, Atlantic

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: regine.basseguy@toulouse-inp.fr (R. Basséguy).</footnote>
Ocean, North Sea and Baltic Sea. This potential shift, attributed to the acceleration of oxygen reduction kinetics and referred to by the authors as ‘depolarisation by oxygen reduction’, increases the likelihood stable pitting developing and/or accelerates corrosion kine-tics. Moreover since the 2000s, the concept of electroactive biofilm has been used to describe the ability of single- or multi-species biofilms to exchange electrons with conductive materials, both as acceptors [11] and as donors [12]. Regarding oxygen reduction in the marine environment, studies have shown that biofilm development on stainless steel cathodes can facilitate high current densities [12,13]. Such results can even be reproduced in the laboratory: biofilm grown on stainless steel in a reactor under polarisation at –200 mV/AgAgCl, using microbial inoculum from natural marine biofilm, has been shown to achieve high electroactivity [14,15]. Rimboud et al. also demonstrated that the microorganisms contained in salt marsh catalysed the reduction of dioxygen on carbon felt [16]. Until now, this microbial catalysis of dioxygen reduction on aluminium alloys has never been studied. In fact, biomi-neralisation phenomena have been demonstrated on this material, highlighting the fact that, fortunately, microbial activity does not only have undesirable effects but can also be a potential means of protection against corrosion [17–22]. In the case of aluminium alloys, the biomi-neralisation has been observed on AlMg 5083 during prolonged immersion (15 days to 6 months) in the port of Genoa, Italy, where a protective layer formed on the alloy surface, mitigating corrosion [23, 24]. A comparison between biotic conditions (in situ immersion) and abiotic conditions (immersion in filtered seawater, artificial seawater or NaCl 35 g/L in the laboratory scale) confirmed that microbial activity was responsible for this surface transformation [24,25]. The formation of the protective layer has also been reproduced in the laboratory, in seawater inoculated either with a multi-species consortium such as salt marsh [26,27] or with a pure strain isolated from the China Sea [28,29]. In the case of salt marsh experiments, oxygen was shown to play a crucial role in the formation of a homogeneous protective layer, as it is essential for the survival of aerobic microorganisms that catalyse oxygen reduction [26,30]. This catalysis may contribute to protective layer formation by producing hydroxide ions, which in turn increase local pH and promote the precipitation of ions present in the medium, triggering the biomi-neralisation process. Building on these findings, the primary objective of this study is to validate the hypothesis that microorganisms present in salt marsh catalyse oxygen reduction on AlMg 5083 aluminium alloys.

Another way of promoting oxygen reduction is to use a potentiostat to cathodically polarise the electrode. In 1987, Watkins and Davis observed the formation of a deposit on alloy 6351 following cathodic polarisation (potentials between –0.8 and –1.5 V/SCE) during a 7-day immersion in sterilised natural seawater [31]. They attributed this deposit formation to an increase in local pH due to the oxygen reduction facilitated by the applied cathodic potential. Measurements confirmed that pH values near polarised electrode could reach 10–11. Other stu-dies have shown that applying a cathodic potential of –1.25 V/SCE to aluminium alloy 5052 for 24 hours led to partial dissolution of the matrix, particularly around intermetallics in contact with the chloride solution [32]. The release of Al<sup>III</sup> ions (including Al<sup>3+</sup>, AlO<sub>2</sub> depending on pH) into the immersion medium induces the formation of precipitates with different chemical compositions compared to those observed on stainless steel. Specifically, stainless steel immersed in a marine environment under cathodic polarisation typically develops deposits rich in calcium and magnesium, also known as calcium-magnesium deposits [33]. In contrast, aluminium alloy 1100 forms deposits that are aluminium-rich and calcium-free, and their chemical composition is more complex, varying depending on the applied electrode potential [34].

In addition to investigating whether the biocatalysis of oxygen reduction takes place on aluminium alloy 5083, the second objective of this paper is to determine the role of oxygen reduction in the initial stages of protective layer formation on the alloy's surface. The first

series of tests involved immersions at open circuit potential (OCP) in both biologically active and inactive environments in order to monitor the corrosion potential and quantify the mass of the resulting deposits. The kinetics of oxygen reduction in the presence of microorganisms were then monitored using Linear Sweep Voltametry. At the same time, polarisation resistance (Rp)—the inverse of which is proportional to the corrosion current—was used to monitor corrosion kinetics in a biologically active environment. Finally, to gain deeper insight into the reaction mechanisms, anodic and cathodic potentials were applied using chronoamperometry to determine the role of potential in the formation of the mineralisation layer.

## 2. Materials and methods

### 2.1. Immersion media – microorganisms’ source

The experiments consisted of immersions in controlled laboratory conditions, i.e. natural seawater and salt marsh collected from Gruissan, France (43°5'34'' N 3°4'58'' E). The seawater was filtered through a coffee filter to remove larger impurities and then stored in a cold room at 4°C. The salt marsh, which served as a source of microorganisms [26] and whose composition has been analysed previously by our team [16, 35], was stored at room temperature in a canister to allow settling. To prepare reactors (Duran GLS80–500) containing 600 mL of solution, 5 % v/v salt marsh supernatant and 5 % v/v solid salt marsh were added, followed by 90 % v/v seawater. This medium will hereafter be referred to as the biologically active medium.

In order to investigate the role of microbial activity, a biologically inactive medium was also used. This was prepared with 5 % v/v marsh supernatant and 5 % v/v salt marsh sterilised by autoclaving (121°C, 20 min) before adding seawater; the autoclaving process was repeated twice to ensure proper sterilisation. Seawater (90 % v/v) was introduced once the reactor had cooled to room temperature.

All reactors were placed in water baths thermostated at 30°C.

### 2.2. The material of the study

The aluminium alloy used in this study is 5083 H111, specially designed for immersion in marine environments. It comes in the form of 1 mm thick plates measuring 20 × 10 cm. Its composition, measured by WDXRF spectrometry, is given in percent by weight: Al 94.654, Mg 4.350, Mn 0.500, Fe 0.220, Si 0.110, Cr 0.067, Cu 0.065, Zn 0.018, Ti 0.016 [23].

Working electrodes were cut by Electrical Discharge Machining. The first electrode configuration consisted of pieces measuring 2 × 10 cm, with the upper part varnished to delimit a constant immersed surface area of 18.0 cm² (4.5 × 2 cm). A 1 mm² aluminium electric cable was attached to the varnished unimmersed section for connection to the potentiostat. Before immersion, the electrodes were ground using a Mecatech 234 polishing machine with silicon carbide papers (Struers) with a polishing grit of 240 (water flow rate = 5 mL/s, rotation speed of 150 rpm). This surface finish was pre-selected to ensure homogeneous microbial colonisation and maximise protective layer thickness [27].

The second configuration consisted of fully immersible working electrodes with a surface area of 34.8 cm² (3.7 × 4.7 cm). These electrodes were also ground to a 240-grit finish, under the same conditions. They were connected to the potentiostat using 1.2 mm diameter threaded titanium rods, which were covered with heat-shrink tubing to insulate them from the solution.

### 2.3. Experimental setup

The electrochemical setup used was a conventional three-electrode set-up. The 5083 aluminium alloy working electrode was positioned at the centre of the reactor (Fig. 1). The counter-electrode consisted of two plates of DSA-Dimensionally Stable Anode (Magneto, the Netherlands)
<figure><img src="image_3.png" /><figcaption>Fig. 1. Schematic and photo of the three-electrode experimental setup in the reactor containing seawater and salt marsh.</figcaption></figure>

measuring 5 × 2.5 cm, positioned on either side of the working electrode. The reference electrode was a Saturated Calomel Electrode (SCE).

In previous work, it was shown that to obtain a change in surface state on an immersed electrode at OCP, the presence of aerobic microorganisms is essential [26]. Consequently, a continuous supply of oxygen is required to ensure their survival. To maintain a non-limiting oxygen concentration, air was injected into the reactor via an aquarium air pump (NW22, Newa Wind) and passed through a pre-bubbler to minimise evaporation losses. To achieve aerobic conditions with homogeneous oxygen concentrations throughout the reactor volume, custom-designed bubblers were used. These consisted of a ring of Tygon E3603 tubing (Saint Gobin, France), perforated along the entire circumference at regular 1 cm intervals with a needle (1.2 mm diameter). The ring was held on a PVC mesh to keep it elevated above the salt marsh at the bottom of the reactor.

## 2.4. Electrochemical techniques

Electrochemical experiments were conducted using Biologic VMP2 and VSP potentiostats. The following electrochemical techniques (EC-Lab) were employed:

The corrosion potential (OCP) of the working electrode was monitored during immersion, with potential readings taken every minute.

Linear Sweep Voltammetry (LSV) was used to obtain polarisation curves and characterise cathodic reactions on the electrodes after immersion: the potential ramp applied ranges from 0 V down to -0.3 V/ OCP with a sweep rate of 0.139 mV/s. Experiments were carried out in reactors containing only seawater, thermostated at 30°C.

The curves obtained in LSV were also analysed to determine the polarisation resistance (Rp), the inverse of which is proportional to the corrosion rate (Stern-Geary relationship). In order to respect the application conditions of this relationship, only the first points in the vicinity of the OCP (overvoltage not exceeding 20 mV) were used to generate current-potential lines whose slope indicates the value of 1/Rp. A minimum of 120 points were required, and the linear regression of these lines had to have an R² close to 1, which was the case for all our tests, with R² values between 0.989 and 0.999.

Polarised immersion tests (CA Chronoamperometry) were also

conducted. The imposed potentials were as follows: -0.8 V/SCE (anodic), -0.9 V/SCE (close to OCP) and -1.0 and -1.1 V/SCE (cathodic). The current was recorded every minute.

## 2.5. Scanning electron microscope (SEM) characterisation

After immersion, in order to observe microbial colonisation on working electrodes, several pre-treatment steps were required. The first step involved immersing the electrodes for 1h30 in a solution composed of 33 % v/v 4 % vol glutaraldehyde, 33 % v/v 0.4 molar phosphate buffer with a pH of 7.4, and 33 % v/v distilled water. The electrodes were then washed in a solution of 50 % v/v 0.6 M sucrose, 25 % v/v phosphate buffer (0.4 M stabilised at pH 7.4), and 25 % v/v distilled water. Finally, the electrodes were dehydrated by immersing them in a series of baths, starting with an acetone/water mixture (50/50) for 5 minutes, followed by 30/70 for 5 minutes, and then 100 % v/v acetone for 30 minutes. To complete the drying process, the electrodes were immersed in an isovolumetric acetone/hexamethyldisilazane (HMDS) mixture, and then in pure HMDS, until complete evaporation.

For the observations of the surfaces without microorganisms, the electrodes were cleaned with distilled water and anti-scratch paper, and then dried with pressurised air. They were first photographed with a Canon EOS 70d equipped with a 100 mm macro lens, and then observed with a JEOL JSM 7100 F SEM equipped with an Oxford ASDD X-Max 50 mm² EDX for elemental chemical analysis. A 4 nm thick Pt metal deposit was applied with a Leica EM ACE600 metalliser to ensure good surface conductivity and better observation. All the SEM observations were performed using a 10 kV voltage.

For cross-sectional observation, the electrodes were placed in a 5 cm diameter mould filled with a mixture of epoxy resin (33.5 mL) and hardener (33.5 mL) from the EpoFix Struers kit. After 12 hours of drying, the Mecatech 234 Presi polisher was used to obtain a mirror polish, allowing observation of the electrode edge and the layer formed during immersion. Polishing began with 400-grit silicon carbide paper, followed by 800 and 1200-grit paper (Struers). To achieve the mirror finish, 6 µm, 3 µm and 1 µm diamond pastes were successively applied using the corresponding polishing fabrics: Dur, Mol and Nap (Struers). After polishing, the samples were cleaned with ethanol and water.
## 2.6. Chemical stripping

Following immersion, to determine the mass of the deposit on the electrode surface, the electrodes were immersed in a bath of phosphoric acid and chromic acid (experiments conducted at Mecaprotec Industries, Muret, France). The solution was prepared by mixing 50 mL phosphoric acid (H₃PO₄ ρ = 1.69 g/mL) with 20 g chromium trioxide (CrO₃), then diluting to 1 L with distilled water. The electrodes to be stripped were immersed in this bath, maintained at 80°C, as specified in ISO 8407 [36]. The difference in mass measured before and after stripping was used to determine the mass of the layer, which was then expressed relative to the electrode surface area [g/cm²]. All electrode masses were measured using an Ohaus Adventurer Analytical AX324 precision balance installed on an anti-vibration bench. Preliminary tests determined that the optimum time for dissolving the surface layer was 10 minutes. To ensure that the pickling bath did not affect the aluminium alloy electrode, control electrodes (polished to 240 grit and not immersed in marine environments) were treated with the same protocol.

# 3. Results and discussion

## 3.1. Influence of microbial activity on biomineralisation phenomena

Previous research by the team [26,27,30] demonstrated that microorganisms present in salt marsh promote the formation of a protective layer on AlMg 5083 aluminium alloy during 21-day immersion tests at OCP in a reactor containing seawater plus sediments. To assess the influence of microbial activity on the early stages of this layer's formation at the corrosion potential, the first experiments involved immersing working electrodes for 10 days while continuously monitoring the evolution of OCP (recorded every minute). Two experimental conditions were compared: 1) a biologically active condition in which the seawater was seeded with salt marsh serving as a microbial inoculum, and 2) a biologically inactive condition in which the sediments were autoclaved to eliminate all microbial activity. Two electrode configurations of different sizes (18.0 & 34.8 cm²) were tested (see Section II.1), with at least four replicates per configuration. The OCP monitoring curves for all electrodes, for identical conditions, followed a similar trend, influenced solely by the presence or absence of microbial activity in the immersion medium. Fig. 2 presents a representative curve for each medium.

In the biologically active environment, the electrode potential, initially very cathodic (-1.17 V/SCE), rose very rapidly and stabilised after a few hours, reaching -0.95 V/SCE on Day 2 and then dropping again to a minimum of -1.06 V/SCE on Day 3. After remaining at this minimum for almost a day, the potential increased steadily until Day 10, when it reached at -0.9 V/SCE. This behaviour aligns with previous findings [27], where the passage through a minimum—termed the transition phase—was attributed to the formation of a protective layer

induced by microbial colonisation.

By contrast, in the biologically inactive medium, the electrode potential was initially highly cathodic (-1.1 V/SCE) then increased rapidly, reaching -0.95 V/SCE within a few hours. Unlike in the biologically active conditions, the OCP thereafter continued to rise steadily without exhibiting a transition phase, ultimately stabilising at -0.90 V/SCE by Day 10.

At the end of the 10-day immersion period, the working electrodes were removed from the reactor. The electrodes were dispatched into two subsets: one subset concerned the electrodes removed from the biologically active medium whose surface was observed by SEM directly after the removal. The second subset was for observation and analysis of the surface after cleaning the electrodes.

The electrodes of the first subset underwent a fixation and dehydration treatment to preserve the biological matter present on their surface (see Section II.5). Fig. 3 presents electron microscope observations of the surface along with EDX mapping.

In the SEM image (Fig. 3a), light-grey elements can be seen on the surface of the working electrode, which appears dark-grey. EDX mapping (Fig. 3c) shows that these groups are associated with carbon (red coloration), suggesting that they may correspond to clusters of microorganisms that have colonised the surface. This hypothesis was confirmed by observations at higher magnification (Fig. 3b), which clearly show microorganisms (ovoid particles) organised in clusters in a matrix of an extracellular polymeric substances (filaments).

The electrodes of the second subset, removed from biologically active and inactivate media, were cleaned with distilled water and anti-scratch paper, dried and photographed prior to further characterisation (Fig. 4).

A photo of the bare electrode (Fig. 4a) before immersion is included to highlight changes in surface state. Whatever the immersion conditions, the electrodes undergo a drastic colour change upon immersion. Before immersion, the electrodes are a dull grey with visible polishing streaks (Fig. 4a). After 10 days, electrodes immersed in biologically active media have turned to a homogeneous dark brown-black (Fig. 4b), and those immersed in inactivated media have turned to a homogeneous brown-grey (Fig. 4c).

SEM observation and EDX analysis of the layers formed during the 10-day immersion in biologically active media are presented in Fig. 5.

SEM observation at x1500 magnification (Fig. 5a) reveals the existence of a dark-grey cracked layer covering a significant portion of the surface. This cracked appearance is attributed to dehydration of the layer upon removal from the aqueous medium [27]. EDX mapping (Fig. 5b) of the same surface indicates that the smooth, crack-free areas consist primarily of aluminium Al, as shown by the predominant blue co-loration, corresponding to an underlying zone enriched in Al. The blue coloration visible within the cracks also confirms the presence of a layer on the electrode. This cracked layer (represented by the yellow/-green areas) is not fully continuous and contains, in addition to Al, oxygen (O), magnesium (Mg), silicon (Si) and calcium (Ca). The chemical composition of this layer, consistent with previous studies [27], and the absence of carbon (indicated in red on the map) confirm that it is not a microbial matrix.

Cross-sectional observation of the same electrode reveals the existence of a layer approximately 1–2 µm thick (area delimited by the dotted lines in Fig. 6a). Moreover, EDX analysis on one line of the cross-section (Fig. 6b) reveals that the layer starts at 1.0 µm and extends to 4.0 µm, based on the oxygen profile. Within this layer, two zones emerge with different chemical compositions: the inner zone (between 1.0 and 2.9 µm) is richer in Al and Mg, while the outer zone (between 2.9 and 4.0 µm) is enriched in Si and Mg. The crack observed in the cross-sectional image is attributed to the boundary between these two zones. This complex organisation of the deposits in a dual-layer structure was better revealed by TEM in previous works [26,27].

The batch of electrodes immersed for 10 days in a biologically inactivated medium was also observed (Fig. 7 and Fig. 8).

<figure><img src="image_4.png" /><figcaption>**Fig. 2.** Monitoring of the corrosion potential (OCP) of AlMg 5083 electrodes during 10 days of immersion in a biologically active environment (in orange) and a biologically inactive environment (in blue).</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>**Fig. 3.** Microbial colonisation on the surface of an AA5083 electrode after 10 days of immersion at OCP in the biologically active medium, observed by SEM: a) x500 magnification; b) x3500 magnification of the area outlined in image a); and c) EDX mapping of image a) with carbon C shown in red and aluminium Al in blue.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Photograph of 5083 aluminium working electrodes a) before immersion, b) after 10 days of immersion at OCP in the biologically active medium, and c) after 10 days of immersion at OCP in the biologically inactive medium. The electrodes were cleaned with distilled and anti-scratch paper and dried before taking the photo.</figcaption></figure>

On the SEM image of the surface (Fig. 7a), the polishing streaks are visible and the cracked appearance observed on electrodes immersed in biologically active media (Fig. 5) is not present. EDX analysis (Fig. 7b) shows that Al (blue coloration) is uniformly distributed over the entire surface, with significant amounts of O (yellow coloration), some Mg (green coloration) and a few spots of Si (royal blue coloration), showing a distinct distribution of elements compared to the electrodes immersed in an active medium. This finding is confirmed by cross-sectional SEM observations (Fig. 8), which show the presence of a 1–2 µm thick layer more clearly identifiable thanks to its light-grey colour (Fig. 8a) and elemental profiles with different characteristics. EDX analysis (Fig. 8b) on one line shows a decrease in Al and Mg signals, coinciding with peaks of O and Si between 3.9 and 5.6 µm, indicating the presence of the layer

formed on the surface. This layer is thinner than that obtained in the active medium, which aligns with the absence of cracks and lighter coloration. The bi-layer structure observed in the active medium is less apparent in this case, although the profiles show a stall at around 5 µm.

In conclusion, we find that the presence of microorganisms in the immersion medium significantly influences the formation, physical properties (notably thickness) and chemical composition (elemental distribution) of the surface layer during immersion.

To quantitatively assess the influence of microorganisms on the size of the layer formed, the mass deposited on the electrode after 10 days of immersion at OCP (in either active or inactive medium) was measured after the electrode's removal from immersion. The electrode was cleaned (using distilled water and anti-scratch paper), dried, weighed
<figure><img src="image_7.png" /><figcaption>Fig. 5. SEM observation (X1500 – image a) coupled with EDX mapping (image b) of the surface of an electrode immersed in a biologically active environment for 10 days, after cleaning.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>Fig. 6. SEM observation in cross-section of an electrode immersed in a biologically active medium for 10 days. a) X 5000 image with the line indicating where the EDX was performed and b) EDX scan line profile. The dotted lines delimit the area corresponding to the bi-structured layer.</figcaption></figure>

and then dipped in a phospho-chromic stripping bath maintained at 80°C for 10 minutes (see Section II.6). The electrode was then washed with distilled water, dried and weighed again. The difference between the mass of the electrode before and after stripping indicates the mass of the deposit, which is then expressed per unit area (Eq. 1).

$$ \text{mass of deposit} \frac{\text{mg}}{\text{cm}^2} = \frac{\text{mass before stripping} - \text{mass after stripping}}{\text{surface}} \quad (1) $$

Stripping was performed for both electrode sizes (stripped areas of

8.6 and 34.8 cm²). Additionally, in both cases, an unimmersed 240-grit polished control electrode was stripped to confirm that the acid bath did not dissolve the aluminium alloy electrode. The measured deposit masses are shown in Fig. 9.

For each electrode configuration, control stripping shows that a small quantity of surface oxides is solubilised during stripping, validating the methodology. The mass of the deposit is 31 % greater when the electrode is immersed for 10 days in a biologically active medium compared to an inactivated one. The greater mass of the layer in the pre-
<figure><img src="image_9.png" /><figcaption>**Fig. 7.** SEM observation (X1500 – image a) coupled with EDX mapping (image b) of the surface of an electrode immersed in a biologically inactive environment for 10 days, after cleaning.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>**Fig. 8.** SEM observation in cross-section of an electrode immersed in a biologically inactive environment for 10 days a) X 5000 image with the line indicating where the EDX was performed and b) EDX scan line profile. The dotted lines delimit the area corresponding to the layer.</figcaption></figure>

sence of microorganisms shows that microbial activity plays a part in the layer formation process.

On electrodes immersed at OCP, anodic sites (where oxidation reactions take place) and cathodic sites (where reduction reactions, such as oxygen reduction, take place) are uniformly distributed. Oxidation reactions supply electrons to reduction reactions, and the anode current ($I_{ano}$) is equal to the cathode current ($I_{cath}$). On stainless steel, in aerobic conditions, biofilm colonising the electrode surface is known to accelerate oxygen reduction kinetics [12]. It is then observed that the

electrode potential becomes more anodic [10]. The passive layer on the surface of stainless steels limits the rise in anodic current, allowing the corrosion potential to tend towards the pitting potential, thus increasing the probability of electrode pitting [9]. This catalysis of the reduction of dioxygen by electroactive biofilms could also occur on the aluminium alloy. However, the local increase in pH, linked to this reduction, destabilises the passive film naturally present on the aluminium alloy, leaching the alloy exposed to corrosion. The $I_{ano}$ increases, shifting the OCP towards more cathodic potentials during the transition phase.
<figure><img src="image_11.png" /><figcaption>Fig. 9. Mass of deposits per unit area on 5083 aluminium electrodes immersed in biologically active (orange) and inactive (blue) media, as well as an uncoated control electrode (grey), for the two working electrode configurations.</figcaption></figure>

Additionally, the local increase in pH induces the precipitation of chemical elements naturally present in the seawater, contributing to biomineralisation.

To better understand the mechanisms involved, two series of experiments were conducted: the first aimed to demonstrate the bio-catalysis of dioxygen reduction on aluminium alloy throughout immersion (kinetic monitoring), and the second involved controlling the electrode’s potential by the supplying external energy to the system (through polarisation).

## 3.2. Kinetic monitoring of oxygen reduction during immersion

The aim of this series of experiments was to highlight the biocatalysis of oxygen reduction on AA5083 by monitoring the kinetics of what happens in a biologically active environment at OCP. Fig. 10 shows the continuous monitoring of OCP for 10 electrodes immersed for periods ranging from 1 day to 10 days in the presence of microbial activity in 10 independent reactors. Each day an electrode was removed from its reactor.

First, it should be noted that the curves all exhibit a consistent trend, similar to that shown in the previous section (Fig. 2), demonstrating the good reproducibility of results in a biologically active environment. Highly cathodic potentials were recorded at the start of immersion, then rose rapidly and stabilised between -0.9 and -1.0 V/SCE within 2–3 days. This is followed by the transition phase, where a drop occurs, down to a minimum potential. Subsequently, the potential increases again until the end of immersion. The slight variations in timing observed between the curves can be explained by minor differences in

<figure><img src="image_12.png" /><figcaption>Fig. 10. Monitoring of the OCP of 5083 electrodes immersed for periods of between 1 and 10 days in a biologically active environment (seawater plus salt marsh).</figcaption></figure>

the surface state; despite polishing being carried out under identical conditions, the surface state may differ slightly, giving each electrode its own specific electrochemical behaviour.

Each day, a working electrode was removed from its initial immersion medium to characterise the cathodic reaction in a new medium without salt marsh (containing only seawater thermostated at 30°C). To do this, an LSV was performed by applying a potential starting from the OCP down to -0.3 V/OCP. The electrode surface was then cleaned to remove all traces of microbial activity using a Cell Lifter VWR 76036-006 scraper. A new LSV was performed in thermostated seawater. The intensity-potential curves obtained for the so-called colonised electrode (tested immediately after its removal from the immersion medium) and the so-called cleaned electrode were then superimposed on the same graph, as shown in Fig. 11, which corresponds to the electrode removed after 3 days of immersion.

The use of the logarithmic function for current densities highlights the variations in intensity in relation to the imposed potential. For each electrode, a strong curvature appears, corresponding to the oxygen reduction reaction. For the 10 electrodes, whether with or without biological matter, the current-potential diagrams show that this curvature is maximal for potentials between -0.91 and -1.07 V/SCE. These values are in the same range as those found in the literature, which are attributed to the reduction of oxygen on aluminium alloys [31,33]. Fig. 11 shows two non-overlapping curves with offset potentials. It is noticeable that the corrosion potential of the colonised electrode is less cathodic than the potential of the cleaned electrode. This difference in potential reflects that microbial colonisation, which will be removed by cleaning, catalyses the oxygen reduction, such that the colonised working electrode then has a more noble potential. This is the enno-blement previously demonstrated on stainless steel.

The potential difference ΔEcorr (Eq. 2) was calculated for the electrode removed each day, and the graph in Fig. 12 shows its change as a function of time (dots). This trend was then compared with that of the OCP of the electrode that remained immersed for 10 days

$$ \Delta E_{corr} = E_{corr} \text{ colonised electrode} - E_{corr} \text{ cleaned electrode} \quad (2) $$

ΔEcorr increases rapidly from the first day of immersion until the third day. This sharp rise is due to microbial colonisation which, as it increases, catalyses the reduction of oxygen to an ever-greater extent. ΔEcorr decreases from the third to the sixth day of immersion and even becomes negative, meaning that the corrosion potential of the cleaned electrode is more noble than that of the colonised electrode. This drastic change is attributed to a shift in the interaction between the material and the microorganisms. The surface state of the electrode changes after five days of microbial activity during which the reduction of oxygen is catalysed, leading to the formation of the protective layer. From the seventh day onwards, the ΔEcorr increases steadily as microbial colonisation resumes on a modified surface and catalyses the reduction of

<figure><img src="image_13.png" /><figcaption>Fig. 11. Current-potential curves, plotted in seawater thermostated at 30°C, for the electrode removed after 3 days of immersion. The orange curve (orange square) corresponds to the colonised electrode (immediately after removal from the immersion reactor), and the blue curve (blue triangle) corresponds to the electrode after cleaning of the biological matter.</figcaption></figure>
<figure><img src="image_14.png" /><figcaption>**Fig. 12.** Evolution of the OCP of the electrode immersed for 10 days (solid line) and of the ΔEcorr of the electrodes removed daily (dots) as a function of the immersion time in biologically active medium.</figcaption></figure>

dioxygen.

Superimposing the monitoring of ΔEcorr and OCP over the 10-day period (Fig. 12) reveals similar trends in both variables, highlighting the reproducible nature of the bacterial influence. The OCP peaks between Days 2 and 3, while ΔEcorr peaks on Day 3. This is followed by a decrease, with the OCP falling to a minimum between Day 4 and Day 6 and the ΔEcorr reaching its maximum on the seventh day of immersion. The free potential and ΔEcorr rise again until Day 10. Consequently, the variation in OCP over time is closely linked to the biocatalysis of oxygen reduction by the microorganisms from the salt marsh that have colonised the surface.

This catalysis was accompanied by a change in the surface state, which can be monitored by evaluating the corrosion current via measurement of the polarisation resistance (Rp). Using the Stern-Geary relationship, linear regression of measurements obtained during LSV near the OCP (overvoltages < 20 mV) allows the determination of 1/Rp, which is proportional to the corrosion rate.

The evolution of 1/Rp (dots) superimposed on the OCP measured over 10 days (solid line) is shown in Fig. 13. On Day 1, 1/Rp was measured at $0.27 \Omega^{-1}.cm^{-2}$, and it remained high at values of between $0.35$ and $0.45 \Omega^{-1}.cm^{-2}$ for the next four days, indicating a high corrosion rate at the start of immersion. Between the fifth and sixth day, there was a sudden drop in corrosion kinetics, and 1/Rp fell by 64% from 0.34 to $0.12 \Omega^{-1}.cm^{-2}$. This drop is attributed to the change in surface state, which modifies the interaction between the microorganisms and the surface of the working electrode. The superposition of OCP and 1/Rp as a function of time again shows great similarities with the significant drop in 1/Rp during the transition phase. The evolution of 1/

Rp is therefore correlated with variations in OCP. By catalysing the reduction of oxygen, microbial activity induces a local increase in pH, destabilising the alloy's native passive layer and exposing it to the environment. The aggressive nature of the marine environment then corrodes the electrode (uniform corrosion) and releases $Al^{III}$ ions into the immersion environment, which co-precipitate with the ions present in the environment ($Mg^{2+}$, $Si^{2+}$, $Ca^{2+}$) under the effect of the rise in local pH to form the layer described above.

Taken together, these results provide a framework for understanding aluminium alloy/microorganism interactions, particularly with respect to oxygen reduction (Fig. 14).

Immersed in a biologically active marine environment, the electrode is colonised (1) by electroactive microorganisms that facilitate the reduction of oxygen [13] such as *Marinobacter* sp. which was found on both AlMg surfaces immersed 10 days in seawater with salt marsh [30] and carbon felt cathodes formed in the presence of salt marsh [16]. This biocatalysis (2) accelerates oxygen reduction kinetics and locally increases pH values through the release of hydroxide ions. As the passive film of aluminium alloys is unstable above pH 8, the working electrode undergoes dissolution [37]. Furthermore, studies have shown that dissolution rates in the presence of bases whose $OH^-$ anions are associated with $Na^+$ cations have led to maximum dissolution kinetics on AA1100 alloys [3]. We can therefore assume that this will be the case in seawater, which is rich in $Na^+$. The dissolution (3) of the electrode releases $Al^{III}$ ions into the medium, potentially exceeding the maximum solubility concentration locally [37], leading to precipitation phenomena. In industrial effluent treatment processes, the addition of aluminium ions or an aluminium sacrificial anode (releasing $Al^{III}$ ions) is a common method for significantly reducing concentrations of cations such as $Mg^{2+}$, $Ca^{2+}$, $K^+$ and $Na^+$ [38]. This process, known as electrocoagulation, is used in applications such as seawater desalination, with the addition of $Al^{3+}$ ions precipitating the silica present in solution to prevent clogging of the reverse osmosis membranes [38]. The release of $Al^{III}$ ions into the marine environment therefore leads to the precipitation of the various chemical elements initially present in solution. Oxides/hydroxides, rich in Al, Mg, Ca and Si, form on the electrode surface, contributing to the formation of the protective layer. This dissolution/precipitation process (3), which can be observed during the transition phase between the fifth and sixth day of immersion when monitoring ΔEcorr (Fig. 12) and 1/Rp (Fig. 13), is responsible for the change in surface state observed on removal from the immersion reactors (Fig. 3). Microorganisms can then colonise the modified surface, grow and catalyse again oxygen reduction, as indicated by the increase in ΔEcorr from Day 7 onwards (Fig. 12).

<figure><img src="image_15.png" /><figcaption>**Fig. 13.** Evolution of the OCP of the electrode immersed for 10 days (solid line) and of 1/Rp of the electrodes removed daily (dots) as a function of the immersion time in biologically active medium.</figcaption></figure>

<figure><img src="image_16.png" /><figcaption>**Fig. 14.** Interactions between microorganisms, oxygen reduction and the surface state of aluminium alloy 5083 in a marine environment.</figcaption></figure>
### 3.3. The role of polarisation in biomineralisation phenomena

In order to determine the respective contributions of oxygen reduction and oxidation reactions to the change in surface state, polarisations were applied to the electrodes immersed in biologically active and inactive environments for 10 days. The choice of polarisation potentials was guided by previous results: monitoring the OCP of the electrodes (Fig. 2 and Fig. 10) indicates that the potential reaches a maximum value close to -0.9 V/SCE under all conditions. Accordingly, an anodic potential (-0.8/SCE), a potential near the OCP (-0.9 V/SCE) and two cathodic potentials (-1.0 and -1.1 V/SCE) were selected. The current densities measured over the 10-day polarisation period are presented in Fig. 15.

In the biologically active environment (Fig. 15a), for the applied potential of -0.8 V/SCE, the current densities remained strictly positive throughout the entire immersion period. After peaking at 0.016 A/m² on Day 1, the current rapidly decreased and stabilised at 0.002 mA/cm² by the end of immersion. For -0.9 V/SCE, the current density was positive for the first days, reaching a maximum of 0.013 mA/cm² at the very beginning, and then decreasing rapidly. After the fourth day, it became cathodic and stabilised at -0.001 mA/cm². The currents of the electrodes polarised at -1.0 and -1.1 V/SCE behaved differently: an increase in the reduction current was observed for both potentials, indicating oxygen reduction. For -1.0 V/SCE, the current density rapidly reached -0.012 mA/cm² (maximum cathodic current) before slightly and continuously decreasing (in absolute value) until Day 10, reaching -0.005 mA/cm². For -1.1 V/SCE, the current density rapidly attained a maximum reduction current of -0.013 mA/cm² on the first day and remained relatively stable until the end of immersion (-0.012 mA/cm²).

In the biologically inactive condition (Fig. 15b), the chroanoamperograms follow a similar trend to those in the active medium for each potential, with differences only in the maximum and stabilised current values at the end of immersion. These values are lower in absolute value than those in the active medium for potentials of -0.8 V/SCE (maximum anodic current of 0.004 mA/cm², stabilised at 0.0003 mA/cm² on Day 10) and -1.0 V/SCE (maximum cathodic current of -0.009 mA/cm², stabilised at -0.003 mA/cm² on Day 10). For potentials of -0.9 V/SCE (maximum anodic current of 0.014 mA/cm², stabilised at 0.0003 mA/cm²) and -1.1 V/SCE (maximum cathodic current of -0.014 mA/cm², stabilised -0.013 mA/cm²), the values were very similar in both media (active and inactive).

This initial analysis of the polarisation results shows that microbial activity has an impact primarily at the anodic potential of -0.8 V/SCE and especially at the cathodic potential of -1.0 V/SCE, for which the reduction current is higher thanks to biocatalysis.

From the current densities $i(t)$, it is possible to calculate the total electrical charge $Q$ exchanged for each potential imposed during the 10-day immersion period $t$ (Eq. 3).

$$Q = \int_{0}^{t} i(t) dt \qquad (3)$$

The total charges exchanged as a function of the potentials imposed in the biologically active and inactive conditions are shown in Fig. 16.

Regardless of the immersion medium (biologically active or inactive), the total electrical charge (Fig. 16) was positive for potentials of -0.8 V and -0.9 V/SCE, indicating that oxidation reactions predominate, whereas the total charge was negative for potentials below -1.0 V/SCE.

At the most anodic potential (-0.8 V/SCE), the total charge in the active medium was greater than in the inactive medium, demonstrating that microbial activity increases the quantity of electrons exchanged. At -0.9 V/SCE, where the imposed potential is close to the OCP, the total electrical charges were the lowest recorded, making it difficult to determine the influence of microbial activity on this parameter. At -1.0 V/SCE, the total charge exchanged was 1.6 times greater in the presence of microbial activity, reaching 6.3 C/cm². At -1.1 V/SCE, the total charges exchanged were similar, with or without microbial activity.

After 10 days of immersion under polarisation, the working electrodes were cleaned with distilled water and anti-scratch paper, dried and then photographed (Fig. 17).

Comparing the electrodes before and after immersion, a change in surface colour is observed, from metallic grey (before immersion) to a grey partially covered with brown or entirely brown. Under both biologically active and inactive conditions, the more cathodic the potential, the greyer the electrodes and the smaller the surface occupied by brown areas. Specifically, electrodes immersed at -0.8 V/SCE exhibit a darker

<figure><img src="image_17.png" /><figcaption>Fig. 16. Total electrical charge measured during the ten days of immersion in biologically active (orange) and inactive media for the different applied potentials.</figcaption></figure>

<figure><img src="image_18.png" /><figcaption>Fig. 15. Chronoamperograms for imposed potentials of -0.8 V (violet), -0.9 V (grey), -1.0 V (yellow) and -1.1 V (green) /SCE in a) biologically active and b) biologically inactive media.</figcaption></figure>
<figure><img src="image_19.png" /><figcaption>**Fig. 17.** Photograph of 5083 aluminium working electrodes before immersion (a), after 10 days of immersion in the biologically active medium polarised at –0.8 V/SCE (b), –0.9 V/SCE (c) –1.0 V/SCE (d) and –1.1 V/SCE (e), and after 10 days of immersion in the biologically inactive medium at –0.8 V/SCE (f), –0.9 V/SCE (g), –1.0 V/SCE (h) and –1.1 V/SCE (i).</figcaption></figure>

hue than those polarised at –1.1 V/SCE. These colour changes reflect the surface modifications linked to the anodic/cathodic reactions occurring at the electrode surface. At –0.8 V/SCE, a potential re-latively close to the OCP, oxygen reduction persists despite the application of an anodic potential, whereas at –1.1 V/SCE the anodic contribution is significantly diminished, allowing reduction reactions to dominate.

At the same imposed potential, the electrodes immersed in a biologically active medium consistently appear browner than those immersed in a biologically inactive medium. However, since the chemical composition of both immersion media is identical, the only variable is the existence of microbial activity. Even under polarisation, microbial activity influences the formation of the oxide layer, demonstrating the role of biocatalytic dioxygen reduction in the formation of the protective layer. The surface state (in terms of colour and homogeneity) of electrodes immersed in an active medium appears at potentials 0.1 V less cathodic than for the electrodes immersed in a biologically inactive medium. This 100 mV switch can only be explained by microbial activity. The biocatalysis of oxygen reduction under polarisation reduces the external energy required for oxide layer formation.

SEM observation of electrodes immersed in a biologically active medium and polarised at –0.9 V/SCE for 10 days is presented in Fig. 18.

The SEM observation (Fig. 18a) reveals that the electrode is covered by a relatively homogeneous, cracked grey layer, similar to that observed at the OCP. The EDX analysis of this surface (Fig. 18b) shows that this layer is composed of aluminium (Al), oxygen (O), magnesium (Mg), silicon (Si) and calcium (Ca). The aluminium rich underlying zone is visible at the bottom of the cracks (blue colour), confirming the presence of the layer on the electrode's surface. SEM observation of the electrode cross-section and EDX analysis (not shown here for brevity) were similar to those made on the electrode formed at the OCP in a

biologically active medium (Fig. 6).

Observation of electrodes polarised at –1.1 V/SCE for 10 days in a biologically active medium is presented in Fig. 19.

The SEM image at x1500 magnification (Fig. 19a) reveals a grey layer with a granular appearance and minimal cracking. This surface differs from those observed on the electrodes after immersion at OCP (Fig. 5), –0.8 V/SCE (Figure 18) and –1.0 V/SCE (not shown here for brevity). EDX mapping (Fig. 19b) reveals that this homogeneous layer covers the entire electrode. The predominant green coloration indicates that this layer is rich in magnesium Mg (green), with additional contributions of aluminium Al (blue) and oxygen O (yellow). The few visible cracks, appearing in blue, indicate the underlying aluminium rich zone (blue). The coverage, chemical composition and surface appearance of this layer, which differ markedly from those observed on other electrodes, clearly indicate that it is distinct from those formed under other conditions.

The cross-sectional observation of this electrode reveals a layer with a thickness between 2 and 4 µm (area delimited by the dotted lines in Fig. 20a). A vertical crack passing through the thickness of the is also clearly visible. EDX analysis along a line in the cross-sectional view (Fig. 20b) indicates that the layer starts at 0.5 µm and ends at 4.0 µm (as indicated by the O and Al profiles). The concentration profiles within the layer reveal that it is very rich in oxygen and aluminium, with oxygen levels higher than previously observed. The amount of carbon present is also notable and differs from the line scans observed in earlier figures (Fig. 6) and (Fig. 8). The chemical composition of the layer observed in section reveals that it is markedly different from the layers observed under other conditions.

All of these observations show that the potential applied during immersion plays a crucial role in the formation of the layer. The

<figure><img src="image_20.png" /><figcaption>**Fig. 18.** SEM observation (X1500 – image a) coupled with EDX mapping (image b) of the surface of an electrode immersed in a biologically active environment for 10 days under polarisation at –0.9 V/SCE.</figcaption></figure>
<figure><img src="image_21.png" /><figcaption>**Fig. 19.** SEM observation (X1500 – image a) in section coupled with EDX scan line (image b) of the surface of an electrode immersed in a biologically active medium for 10 days under polarisation at –1.1 V/SCE.</figcaption></figure>

<figure><img src="image_22.png" /><figcaption>**Fig. 20.** SEM observation in cross-section of an electrode immersed in a biologically active medium for 10 days under polarisation at –1.1 V/SCE a) X 5000 image with the line indicating where the EDX was performed and b) EDX scan line profile. The dotted lines delimit the area corresponding to the layer.</figcaption></figure>

influence of potential during deposit formation on aluminium alloy 6351 has also been demonstrated under polarisation in sterilised seawater [31], with varying applied potentials leading to differences in the amount of material deposited on the surface of the electrode.

The deposit masses were determined according to the protocol outlined in Section II.6. Fig. 21 presents the deposit masses for each polarisation potential (-0.8, -0.9, -1.0, -1.1 V/SCE) under both active and biologically inactive conditions.

The mass of deposits formed in biologically active conditions is

greater than that formed in biologically inactive conditions for potentials between –0.8 V and –1.0 V/SCE. However, this trend is reversed at –1.1 V/SCE. In the previous batch of experiments (Section III.2), it was noted that the curvature indicative of dioxygen reduction was maximal between –0.91 and –1.07 V/SCE, corresponding to optimal range for oxygen reduction at the imposed potentials of –0.9 V and –1.0 V/SCE. Despite this, the mass of deposits at these potentials is greater under biologically active conditions than under inactive conditions. This observation suggests that the formation of the layer is linked to
<figure><img src="image_23.png" /><figcaption>**Fig. 21.** Deposit mass (histogram) and ratio of deposit mass to total electric charge (dotted line) as a function of the potential imposed during polarisation in a biologically active (orange) and inactive (blue) environment.</figcaption></figure>

microbial activity, which, by catalysing dioxygen reduction, provides additional energy to that supplied by the potentiostat. Notably, the greatest deposit mass occurs at –0.9 V/ESC in the presence of microbial activity, despite the minimal total exchanged charge (Fig. 16). The proximity of this potential to the OCP value explains the low charge measurement, as the anodic and cathodic charges effectively cancel each other out.

Taking into account the evolution of the total electric charge, when analysing the mass variations of the deposits, provides additional insight into the effect of microbial catalysis (Fig. 21 dotted lines). For polarisation potentials –0.8, –1.0 and –1.1 V/SCE, the ratio of deposit mass to total electric charge is similar in both biologically active and inactive conditions. However, at –0.9 V/SCE, this ratio is greater in biologically active conditions. This means that, for the same total electric charge, the deposit mass is 2.5 times greater in biotic environments than in biologically inactive ones. This maximum occurs at a potential within the oxygen reduction domain (–0.91 and –1.07 V/SCE) for 5083 aluminium alloy immersed in a marine environment, confirming once again the link between formation of the protective layer and the biocatalysis of oxygen

reduction. This result clearly illustrates the effect of microbial catalysis, which reduces the external energy required for layer formation.

## 4. Summary of results

The three experimental series presented in this study enable the proposal of mechanisms for the formation of the protective layer (Fig. 22) under different polarisation conditions.

The application of a cathodic potential (Fig. 22 I) promotes reduction reactions (1) on the surface of the working electrode, while suppressing the anodic oxidation reaction (2) of the material. The more cathodic the applied potential, the lower the contribution of the anodic reaction (2) to the overall current and the greater the increase in pH. High concentrations of hydroxide ions attack the native passive film, which is sensitive to pH variations, leading to chemical dissolution of the electrode and the release of Al³⁺ ions into the medium [31,32]. At –1.0 and –1.1 V/SCE, in both biologically active and inactive environments, it was clearly observed that the total electric charge (Fig. 15) and current densities (Fig. 16) were cathodic. At –1.0 V/SCE, the current density (Fig. 15a) and total electric charge (Fig. 16) were higher in the presence of microorganisms. This microbial activity directly increases the energy available for dioxygen reduction, resulting in a greater rise in interfacial pH, which further promotes the attack on the passive film.

Anodic polarisation (Fig. 22 III) promotes the oxidation of the material by releasing Al³⁺ ions (anodic reaction 3), whereas dioxygen reduction (reaction 4), through a minority process, generates hydroxide ions that may further accelerate the dissolution of the matrix. The contribution of dioxygen reduction becomes all the more important as the imposed potential approaches the OCP. The anodic current densities and total positive charge measured during anodic polarisation at –0.8 V/SCE indicate that oxidation reactions (3) dominate on the electrode surface. In the presence of microbial activity at –0.8 V/SCE, both the current densities (Fig. 15a) and total charges (Fig. 16) are higher. By increasing the number of exchanged electrons, the microorganisms enhance the oxidation of the material, thereby contributing to the release of Al³⁺ ions into the medium.

The OCP represents a mixed potential, where both oxidation and reduction reactions occur simultaneously, with anodic and cathodic

<figure><img src="image_24.png" /><figcaption>**Fig. 22.** Proposed mechanisms for the formation of the protective layer during anodic, cathodic or OCP polarisation.</figcaption></figure>
sites naturally distributed across the surface of the electrode. At the OCP (Fig. 22 II), the oxidation of the material (reaction 6) produces electrons that are used in the reduction of dioxygen (reaction 5). This electrochemical oxidation can also be coupled with the chemical dissolution of the material, as the release of hydroxide ions destabilises the passive film, which is sensitive to pH variations. The oxidation of the electrode (reaction 6) and dissolution of the oxide layer lead to the release of Al<sup>III</sup> ions. The presence of microbial activity, which catalyses dioxygen reduction (Fig. 11), results in an increased number of exchanged electrons, increasing the oxidation of the material and the local rise in pH and, consequently, further intensifying the dissolution of the substrate.

Regardless of the electrochemical conditions (anodic, cathodic or OCP polarisation), immersion of the AlMg 5083 electrode in a marine environment results in the release of Al<sup>III</sup> ions. When the maximum solubility concentration of Al<sup>III</sup> ions is exceeded near the electrode, the ions precipitate, which in turn induces the precipitation of other metal ions naturally present in the environment, such as Mg<sup>2+</sup>, Ca<sup>2+</sup> and silicon ions carried by salt marsh. This process, commonly used in industrial effluent treatment, is called electrocoagulation. Moreover, the increase in local pH due to hydroxide ions promotes further precipitation, leading to the formation of a protective layer (reaction 7) on the electrode surface, as shown in Fig. 3 and Fig. 17. SEM observations reveal that these layers are oxygen-rich, validating their formation in a basic environment.

Microbial activity plays a significant role in the mechanisms involved. Under cathodic polarisation at -1.0 V/SCE, microbial activity accelerates dioxygen reduction, resulting in a larger deposit. Conversely, under anodic polarisation at -0.8 V/SCE, microbial activity boosts electronic exchanges (Fig. 16), facilitating material oxidation and the release of Al<sup>III</sup> ions, which precipitate and form a thicker layer in biologically active conditions. At OCP, the presence of microorganisms catalysing dioxygen reduction accelerates both the electrochemical oxidation and chemical dissolution of the material, leading to larger deposit mass (Fig. 9).

By colonising the surface of AA5083 electrodes immersed in a marine environment, in all the conditions tested, electroactive microorganisms shift reaction equilibria, driving mineralisation processes on the electrode surface.

## 5. Conclusion

All these experiments carried out in natural seawater inoculated with salt marsh have highlighted, for the first time, the catalysis of dioxygen reduction by marine microbial activity on 5083 aluminium alloy. This biocatalysis leads to the formation of a mineralisation layer in greater quantity, which a recorded increase of 31 % in the deposit mass in the presence of microbial activity. Furthermore, the tests under anodic and cathodic polarisation demonstrate that the mass of the deposits formed under polarisation is always greater in biologically active conditions than in a biologically inactivated environment, except at the -1.1 V/SCE potential. This more cathodic potential induces a change in the composition of the layer, characterised by increased oxygen, carbon and aluminium content.

For potentials within the oxygen reduction domain (between -0.91 and -1.07 V/SCE), we find that microbial activity reduces the energy required to form the layer, lowering it by 100 mV compared to the inactivated medium. Finally, polarisation at -0.9 V/SCE, near the OCP, results in the formation of a layer similar to that formed at the OCP.

This work contributes to a deeper understanding of the initial stages of biomineralisation on AlMg 5083 aluminium alloy in the presence of marine microbial activity. Ongoing research is focused on investigating the variations in interfacial pH as a consequence of the catalysis of oxygen reduction and on exploring the influence of microbial activity.

## Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper

## Acknowledgements

This work was financially supported by the MESRI (Ministère de l'Enseignement Supérieur, de la Recherche et de l'Innovation) funding of the French government. This work was included in the MICOATEC project (ANR-19-CE08-0018) financially supported by the ANR. The authors would like to thank Luc Etcheverry, engineer at LGC, for technical support and Mecaprotec Industries (Muret-France) in particular Benoit Fori for his contribution during the phospho-chromic stripping.

## Data availability

Data will be made available on request.

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