Biofouling and Corrosion Protection of Aluminum Alloys Through Ultrafast Laser Surface Texturing for Marine Applications

Surface biofouling, corrosion, and wettability are important parameters to understand and characterize aluminum alloys to prevent the failure in marine environments. Antifouling technologies predominantly encompass chemical and biocidal approaches with negative environmental consequences. Therefore, this study focuses on a new method of producing non‐toxic and effective antifouling and corrosion‐resistant surfaces. In this study, ultrafast laser texturing is used to modify the surface of an aluminum alloy using a femtosecond laser system. Five different unique texture patterns are designed and fabricated using 3 W laser power, 100 kHz pulse repetition rate, and 4 mm s−1 scanning speed in order to make the aluminum surface antifouling and corrosion resistant. The non‐textured sample has a contact angle of 85° while the textured samples have contact angles of up to 157°. The contact angle increased with time up to 90 days of aging. Biofouling assessment is conducted using marine algae Nitzschia ovalis as a marine fouling test organism. A reduction of biofilm coverage of 79% is recorded from the laser‐produced star pattern texture. This study demonstrates that laser‐textured surfaces have the potential to mitigate the formation of biofilms on the surfaces while also providing a mechanism to control the relative level of corrosion.


Introduction
The buildup of undesirable material on solid surfaces is known as fouling.There are four main types of fouling such as chemical fouling (which includes scaling), biological fouling DOI: 10.1002/admi.202300835(biofouling), deposition (sedimentation or geofouling) fouling, and corrosion fouling. [1]Biofouling has been labeled as the most severe type of fouling among the various types of fouling in marine engineering.Surface biofouling is specifically observed in the marine environment where organisms attach themselves, grow, and secrete substances on various surfaces. [2,3]This problem can be categorized into two types: microbial fouling and macrobial fouling.Microbial fouling involves the formation of biofilms by microorganisms, while macrobial fouling refers to the accumulation of larger organisms such as clams, barnacles, and mussels on submerged surfaces like ship hulls, propellers, and underwater structures. [4,5]The mechanism of fouling in the context of marine biofouling, can be understood through a successional model, which can be described as follows: The initial stage involves the adsorption of organic materials and minerals onto a surface exposed to seawater, leading to the formation of a conditioning film (Stage 1).This conditioning film sets the base for the subsequent stages of biofouling.Following the formation of the conditioning film, primary colonizers, primarily comprising bacteria, begin to populate the surface, leading to the formation of a biofilm layer (Stage 2).This biofilm layer plays a crucial role in facilitating the adherence of subsequent microorganisms, setting the stage for further development.As the biofilm layer matures, it paves the way for the establishment of microfouling communities (Stage 3).These microfouling communities consist of a diverse community of microorganisms, including bacteria and microalgae such as diatoms and other species.Their presence further contributes to the complexity and stability of the biofouling ecosystem.Finally, as the biofouling community becomes more established and mature, macrofoulers, including macroalgae and marine invertebrates, settle onto the surface (Stage 4).These larger organisms become integral components of the biofouling community, adding to its biodiversity and ecological significance, [6,7] This biofouling can cause several issues, including increased hydrodynamic drag, corrosion, reduction in heat transfer rate, and clogging intake and discharge flow systems. [1,8]Therefore, in order to prevent biofouling, ideally it should be prevented at the (Stage 2) where adherence of subsequent microorganisms on surface begins.Hence, to prevent or manage fouling, various strategies are employed such as mechanical cleaning, ultrasonic and electric treatments, and antifouling coatings are applied. [9]The cost of removing and mitigating this biofouling such as recoating and cleaning is estimated to be $56 M per year for the DDG-51 class of US Navy ships. [10]ydrophobicity and super-hydrophobicity of the surfaces are reported to have a direct correlation with the adhesivity of biomolecules.This is because of the interplay between the surface energies of the three phases involved, namely, the surface energy of the foulant, the surface energy of the substrate, and the surface tension of the suspending liquid. [11]These special types of surfaces have nano and micro textures that can be manufactured using different strategies such as chemical modification, [12] coatings, [13][14][15] physical strategies such as lithography, [16] plasma treatment, [17,18] ultrasonic treatment. [19]However, most commercial marine coatings contain fluorine which increases marine toxicity potential and is harmful to the marine environment. [20]ence there is a need to develop a non-toxic environmentfriendly long-lasting solution for fabricating antifouling surfaces. [21]n recent years, laser nano-micro texturing has emerged as a promising approach for antifouling, offering a potential solution to mitigate biofouling while minimizing environmental impacts, [22,23] The laser irradiance induces nano and micro surface structures on the surface of the material which reduces the surface energy of the substrate surface or enables specific interactions between the surface and the fouling material. [24]29] Researchers have investigated and examined antifouling behavior on metallic surfaces with a view to improving the an-tifouling properties.Lutey et al. conducted an experimental study to tailor both wettability and surface morphology via ultrashort pulsed laser processing which improved the anti-bacterial capabilities of the 316 L stainless steel samples.The strategy involved inducing laser-induced periodic surface structures and nanopillars onto the surface. [30]Rajab et al. produced picosecond lasergenerated surface topographies on titanium alloy Ti6Al4V.These topographies exhibited anti-fouling properties as evidenced by the laser-textured surfaces retaining less bacteria than the control surface. [31]In another recent study conducted by Yusuf et al., the researchers investigated the impact of laser texturing on the material properties of TiO 2 /ZnO ceramic coatings.They employed a picosecond laser system to create dimple-like textures on the surface of the coatings.The presence of these dimples had a significant influence on the antibacterial properties of the TiO 2 /ZnO coatings.The results indicated a reduction in optical densities , lower colony-forming units , and a decrease in live cell bacteria.Moreover, the study also revealed that the increased surface area resulting from the dimples promoted bacterial attachment, thus facilitating the antibacterial effects of ZnO and TiO 2 as agents. [32]In a recent investigation by Nandru et al., the impact of nanosecond laser texturing on surface biofilm formation was examined.The study involved the creation of square pit pattern and triangular pit pattern profiles with dimensions of 100 μm in width and 90 μm in-depth, spaced at 250 μm intervals.The findings revealed that the square pit pattern exhibited a higher contact angle and demonstrated superior inhibition of biofilm formation compared to the triangular pit pattern.This reduction in bacterial attachment could be attributed to the interaction between surface hydrophobicity and the topographical features of the surface. [33]Uniform deposition of polystyrene particles on the laser-textured surfaces is achieved and the deposited material is confined to a smaller area. [34]In a recent research study conducted by McFadden et al., the impact of treating commercially pure titanium (Ti) surfaces using a continuous wave fiber laser under argon shielding on biofilm coverage of various bacterial species was investigated.The five bacterial species tested included both Gram-positive bacteria (Staphylococcus aureus and Staphylococcus epidermidis) and Gram-negative bacteria (Pseudomonas aeruginosa, Escherichia coli, and Proteus mirabilis).The study found that the biofilm coverage of all tested bacterial species exhibited a significant decrease after the treatment of the Ti surfaces with the CW fiber laser under Ar shielding.This reduction in biofilm coverage was attributed to two main factors: micro-roughness of the surface and the formation of an oxide layer resulting from the recrystallization process occurring at the topmost layer of the material. [35]luminum alloy (7075) has become a notable material with its extensive applications in the marine industry due to its special properties, such as high strength low density, satisfactory plasticity, and high corrosion resistance. [36,37]It is significant to investigate algal corrosion behavior and mechanism with continued organic carbon starvation in the marine condition. [23]From this past research, various laser process parameters and effect of surface chemistry and antifouling efficacy have been extensively investigated.However, an area that remains largely unexplored has the potential of different pattern textures which can be fabricated using femtosecond laser in conferring antifouling capabilities.
The literature review also shows that the effects of surface roughness on wetting behaviors have not yet been thoroughly studied.
In this study, we investigate the potential for using femtosecond laser processing in order to modify the surface properties to achieve a highly-hydrophobic surface which has marine antifouling abilities.Specifically, we concentrate on modifying the wetting characteristics of aluminum alloy 7075 material.We explore five unique texture geometries, namely parallel lines, cross hatching, triangles, stars, and rhombus patterns and experimentally test these micro textures for antifouling abilities by rapid biofouling assessment using diatoms (Nitzschia ovalis).Our main goal was to determine the most effective texturing approach that produces algae-resistant surfaces suitable for application in the marine environment.

Material
The material used in this project was commercially available aluminum alloy 7075 T6 (thermally treated, stress-relieved by stretching, and artificially aged), in form of flat plates 30 × 30 mm and 2 mm thick which are supplied by Impact Ireland Metals Ltd.The chemical composition of the aluminum alloy 7075 is shown in Table 1.
The average roughness (Sa) of the bare plate was 400-500 nm before processing as measured by 3D optical profilometer.The surface of the samples was cleaned using isopropyl alcohol before laser processing.

Laser Surface Texturing
The ultrafast laser surface texturing was conducted on the aluminum alloy 7075 substrates using a laser system which was equipped with a high-energy industrial ultrafast laser source.This laser provided a stable low noise pulsed beam with an average wavelength of 1030 nm and a pulse duration <370 fs.The laser beam surface scanning with different for different pattern designs was conducted with a galvo scanner (Focusshifter-15, Raylase GmbH) and the beam was focused on the sample using an f-theta lens.Gaussian type of beam was present at the focal position which had a full width of the beam at 1/e 2 measured to be 46.4 μm at a working distance of 220 mm from the lens of the galvo scanner.The laser texture pattern was applied across an irradiated area of 8 mm × 8 mm.The laser processing was conducted in the atmospheric environment at 20°C and a relative humidity of 60%.The sample was mounted on the four-axis translation stage (PRO 225 Aerotech Inc) which was calibrated and kept stable with the focal point at the top surface of the sample.In this experiment, the laser process parameters were set as 3 W laser power, 100 kHz pulse repetition rate, and 4 mm s −1 Based on the above parameters the fluence was calculated using the equation: here E p is the laser average pulse energy and A is the laser spot area.Using this equation, the fluence was calculated as 1.77 J cm −2 .Five different laser scan strategies were used to produce five unique types of texture which are as shown in Figure 2.

Morphology
The laser-treated surfaces were evaluated using scanning electron microscopy which was performed using a Jeol JSM-IT 100 InTouch Scope with LaB6 filament, accelerating voltage of 10 kV, and a beam current intensity of 20 pA.The evaluation was done in secondary electron diffraction mode.The images were taken at 100X and 1000X magnification for all the patterns.As the aluminum alloy samples were conductive no extra sample coating was required.

Surface Roughness
The different surface roughness morphology attributes such as S a (arithmetical mean height), S q (root mean square surface roughness), S p (maximum peak height), S v (maximum crest depth), S z (maximum height surface roughness) were measured using a Bruker Contour GT 3D Optical Profilometer as per ISO standard 25178-2:2021. [38]The measurements were made using a 10X lens and focusing on the sample using white light interferometry.The analysis of the surface roughness parameters was measured using inbuilt software Bruker's Vision 64 image analysis on the raw data.

Wettability
The wettability of the surface was determined using static water contact angle.The static water contact angle measurements were performed using a FTA200 Dynamic Contact Angle Analyzer.The sessile drop profile was measured using the non-spherical liquid-vapor curve fit method which was performed with the FTA 200 software.The measurements were taken in ambient conditions (20 °C and 60% relative humidity).For these measurements 10 μL of deionized water was used per applied droplet.The contact angles were taken three times (n = 3) on each sample to ensure repeatability on the same day of the measurement.

Biofouling Experiment
Biofouling assessment was performed using a 10 mL culture suspension of fouling organism, Nitzschia ovalis.Cell numbers An STP of two classes was necessary to start the segmentation.Two classes were used.Class 1 corresponded to the particles to be detected, in this case benthic diatom species, Nitzschia ovalis, appearing red.Class 2 corresponded to the background of the image, appearing green.These classes comprised of sets of pixels that share similar visual characteristics such as color, shape, or size.The segmentation process was carried out using the Fast Random Forest algorithm which employs machine learning using decision trees to classify data based on user-defined patterns for each class.This algorithm was chosen because of its computational efficiency, probabilistic output, ability to handle a wide  variety of image input characteristics, and interactive enhancement based on error handling.In addition, this algorithm avoids over-fitting the data by injecting randomness in the training trees and combining the output of multiple random trees into the final classifier; in the case 200 trees were performed using always in each analysis an out-of-bag error lower than 5% to arrive at the final classifier from which a probability map was obtained.The Random Forest algorithm proved to have robust performance when compared using eight evaluation metrics.The probability map was cropped and processed using an ImageJ plugin to cluster color pixels driven by the user input developed by Biomedical Imaging Group (http://bigwww.epfl.ch/).This ImageJ plugin allowed to segment a color image by pixel clustering.Five red patterns were used for class 1 (corresponding to diatom cells) and five green patterns for class 2 (corresponding to the background).
The percentage area covered by each color was calculated using the K-means clustering algorithm.The analyses were carried out on an individual sample basis.

Electrochemical Corrosion Test
Corrosion behavior of the samples was investigated by open circuit potential (OCP) and cyclic polarization studies using the Gamry Interface 1000-E electrochemical setup.A standard threeelectrode cell configuration was used with a saturated calomel electrode (SCE) as a reference, high-density graphite rod as a counter, and sample as the working electrode.NaCl (3.5 wt%) was used as the electrolyte for corrosion studies at room temperature.Open circuit potential measurements were done prior to cyclic polarization studies for 3600 s.Cyclic polarization was done in the voltage range of −0.3 V to +1.0 V versus E OCP (vs SCE) with the forward and reverse scan rate of 0.166 mV s −1 as per ASTM standard G61-86 and with maximum apex current of 1 mA cm −2 .

Surface Chemistry Analysis
The surface chemistry of the irradiated materials was analyzed by X-ray photoelectron spectroscopy (XPS).In this study, a Scienta Omicron XPS system equipped with a monochromatic Al K X-ray source (operated at 280 W and 14 kV) at a base pressure setup of 6 × 10 −7 Pa with a 128 channel Argus CU detector was employed.The kinetic energy of the photoelectrons was determined using an analyzer with a pass energy of 100 eV for the survey and 50 eV for high resolution spectra.The take-off angle between sample's surface normal and the electron optical axis of the spectrometer was 0°.Spectra were referenced to the C-1s peak of adventitious carbon at a binding energy of 284.8 eV.

Morphological Analysis
The resulting patterns from the laser texturing are shown in Figure 3.In the parallel line pattern (S1), straight parallel cuts were made on the surface with a 0.1 mm hatch distance.In the cross hatched pattern (S2), straight parallel and perpendicular cuts were produced.Due to this scan strategy, the surface area increased significantly.The nano and microstructures were formed mostly on the crown region of the surface producing an enhanced surface area.The triangle pattern (S3) resulted in sharp peaks and valleys.SEM imaging depicted a well-defined triangular structure with clear boundaries indicating a surface topology with distinct features that would be found to influence the wetting behavior and surface interactions.Surface pattern (S4) exhibited a starshaped pattern with multiple arms extending from a central point with a high degree of periodicity.The magnified SEM imaging illustrated a star pattern with fine details on the arms and a relatively small untextured area while sample S5 exhibited the rhombus shape pattern according to the planned scan strategy.

Surface Roughness
The surface roughness was measured by Bruker Contour GT 3D Optical Profilometer having white light interferometer with 10X magnification covering a surface area of 100 ×100 μm 2 .All the surface roughness height parameters were measured accordance to ISO standard 25178-2:2021. [38]Figure 1 shows how the five scan patterns altered the 3D topography and surface roughness properties of the Al samples.Samples were prepared with different scan strategies such as parallel hatch pattern (S1), cross hatch pattern (S2), triangle hatch pattern (S3), star hatch pattern (S4), and rhombus hatch pattern are shown in Figure 4b-f respectively.When compared to pristine samples, different laser scan strategies had a substantial impact on increasing the surface area by increasing the surface roughness of the materials.Figure 4a clearly shows that the pristine sample has the lowest surface roughness values as compared to other laser-processed samples.When compared to other scanning procedures, the star hatched pattern (Figure 4e) has the highest Sa and Sq values, while the triangle hatch pattern (Figure 4e) has the highest Sz values.However, it is observed that parallel hatch pattern (Figure 4b) has minimum Sa, Sq, and Sz values.Figure 2 shows the comparison of surface roughness and depth values at different scan strategies.
Figure 5 shows the comparison of surface roughness values (Sa, Sq, Sp, Sv, and Sz) at different scan strategies.From the graph, it is observed that parallel hatch pattern has minimum surface roughness values.
Different laser scanning strategies have been studied to understand their effects on surface morphology in laser surface texturing using ultrafast laser system.Significant variations in dimensional attributes are readily discernible among distinct lasertextured structural configurations with regard to lateral dimensions, topographical characteristics, and geometrical morphology, see Figure 3.This is because metal is partially melted and sputters from the matrix due to the high photonic impact energy action of the ultrashort pulsed laser.The sputtered metal is deposited on the non-laser irradiated area forming nano and micro-scale structures.These nano and micro textures formed adjacent to the micro grooves lead to a local increase in surface area and surface roughness, [39,40] The S a and S p parameters, determined during the roughness analysis, can be regarded as indicators of the quantity of potential contact points for N. ovalis algae cells, based upon their dimensional attributes (see Figure 4).14 μm × 6 μm. [41]Sample S4 had the highest S a and S p values which were 37.4 μm and 13.45 μm respectively.This is directly dependent on the pattern size.When the pattern size is smaller than the cell size, the surface has a better ability to prevent micro algae cell adhesion.These effects are also observed in case of bacterial adhesion by various researchers such as Lutey et al., [31] Rajab et al., [32] and Lu et al. [42] However, if the S a and S p roughness are too low such as in case of S1, S2, and S3 then the cells can adhere and form a biofilm on these surfaces.While these values are solely rooted in geometric considerations and do not accommodate the presence of cellular or surface asymmetry, they give metrics for designing the topological features and establishing laser process parameters.

Wettability and Its Transition
The surface wettability which is one of an important properties to determine the adhesion of the surface of the laser-ablated samples was assessed by measuring their static water contact angles (WCAs).Each recorded value was obtained as an average from three distinct locations on the samples.The increase in WCA values signifies a reduction in the wetting capability of the surfaces over time.The pristine aluminum alloy sample did not show any significant change in water contact angle.The pristine (without textures) sample had the water contact angle of 85 ± 4°.The maximum water contact angle achieved was on sample S5 which was 157.11°the transition is shown in Figure 6.
After 30 days it can be observed that the water contact angle was increased in all the laser-textured samples, especially in the S4 and S5 the samples were converted to hydrophobic (WCA > 90°).
The observed change of time-dependence of WCAs for the surface textures created by laser texturing is particularly seen in case of aluminum alloy, when they were exposed to ambient air.The same effect is observed in previous literature by Yang et al., which supports this theory of evolution of the surface water contact angle. [43]After 60 days from the day of texturing of ambient aging all the textures were converted to hydrophobic (WCA> 90°).After 90 days from the day of laser texturing, the samples S3, S4, and S5 showed highly-hydrophobic surfaces (WCA> 150°).However, after 120 days from the day of laser texturing there were no changes observed on any samples indicating that a steady state surface condition was reached.Achieving this steady state is different for different materials and different types of laser systems and texture designs.This state is also dependent on the ambient conditions. [44]The wetting behavior of the laser textured aluminum samples was evaluated by measuring the static water contact angle over a period of 120 days.In Figure 6a, the evolution of the static water contact angle with time is reported for the samples fabricated using the laser surface texturing method.It can be observed that on the day of texturing, the samples initially became hydrophilic.This is attributed to the level of surface oxide which is lowest initially which is also related to the energy applied on the aluminum surface during the laser texturing. [42]After 60 days it can be seen that the contact angle produced on the samples reached near to the water contact angle of the pristine sample of 85 ±4°.After 90 days the water contact angle reached the steady state (as there was no change observed even after 120 days) and two samples S3 and S4 became highly-hydrophobic (WCA > 150°).The highest water contact angle of 157°was achieved on S4 which had the star pattern.Similar findings have been reported by Cardoso et al., specifically on the aluminum alloy material. [45]t can also be attributed that the ambient conditions also affect the wettability transition and the stabilization of contact angle after particular days as shown by previous literature by Hauschwitz et al. [46] Antifouling and highly-hydrophobic surface are closely related concepts as these surfaces have low surface energy and micro-nano dual structure which were found to be unsuitable for the attachment of fouling organisms, making them effective in preventing biofouling.

Effect on Biofilm Formation Experiments
The prevention or delay of early settling and biofilm development is widely acknowledged as a means to avoid or postpone later fouling occurrences.The inhibition of biofilm growth holds significant potential to substantially mitigate the extensive damage inflicted by the subsequent increase of biological fouling populations.The study aimed to investigate the antifouling efficacy of various textures to assess their potential utility as antibiofouling techniques within the marine industry.
Biofouling assessment was performed using a 10 mL culture suspension of fouling organism, Nitzschia ovalis and the cell area coverage was assessed by image processing.Figure 7 shows the result of the biofouling assessment on all the surface textures.In Figure 7a the segmented color image by pixel clustering is presented which shows diatoms in red color and the surface in green color.The percentage area covered by each color was calculated using the K-means clustering and the area coverage is presented in Figure 7b.
It has been observed that laser-textured surfaces can prevent or decrease the formation of biofilms.The laser-textured surfaces were exposed to the 10 mL culture suspension of fouling organism, Nitzschia ovalis.The behavior of test organisms on the  candidate micro-textures was observed after 3 h exposures of textures.In the context of surface attachment of diatoms, the behavior of diatoms was found to be significantly influenced.The biofouling assessment revealed noteworthy differences in the N. ovalis cell area coverage among the various samples.The mean biofouling cover is shown in the plot above in Figure 7b.Three trials were conducted on each type of texture in order to get repeatability of the results.The pristine aluminum alloy surface showed an average cell coverage of 21.6%.On a smooth, flat surface, diatoms exhibit a propensity to settle readily, establishing multiple attachment points, as illustrated in Figure 7a Pristine.This result serves as a baseline for comparison with the laser-textured samples.Conversely, where surfaces have micro-textures, two distinct outcomes are possible for diatoms.Sample S1, characterized by a particular laser texture, displayed an average cell area coverage of 24.32%.The higher percentage of cell coverage on S1 indicated that this particular laser texturing strategy or pattern is not as effective in inhibiting N. ovalis adhesion.The topography plays an important role here.If the size of the microtexture of the surface exceeds the scale of the diatom, it may settle within the recesses, forming multiple attachment points, and benefiting from protection against hydrostatic forces.This is due to the N. ovalis organisms having more surface area with the parallel line pattern in S1 to settle on, attach to and cover the surface, similar to colonization by S. aureus as reported in previous literature by Epperlein et al., on LIPSS surfaces with straight parallel lines. [47]owever, when the surface topography is smaller than the diatom, the diatom can settle on top of the surface features.The organisms have less surface area of their shell (frustule) composed of biogenic silica, in contact with the surface, therefore reducing the amount of attachment points that it can form, leading to weak attachment.Therefore, in case of sample S2 which has cross hatched pattern exhibited superior biofouling resistance, with an average cell coverage of only 10.30%.This finding suggests that the laser texture applied to S2 successfully hindered N. ovalis attachment, resulting in a significantly lower biofouling coverage compared to the pristine surface.One of the potential reasons for lesser attachment could be air entrapment which reduced the adhesion between the textures. [48]Sample S3, featuring a different laser texture with triangular pattern, showed an average cell area coverage of 13.68%.Although not as effective as S2, the laser texturing in S3 still provided moderate protection against N. ovalis adhesion.
Sample S4 with the star pattern showcased exceptional biofouling resistance, with an average cell coverage of merely 4.63%.This result indicates that the unique laser texture applied to S4 exhibited potent anti-biofouling properties.The potential reason for low biofilm formation on the star pattern (S4) is that there is a low surface energy and lower number of points for attachment of the diatoms where the N. ovalis can settle and form colonies, as evidenced by the higher S p values for this surface.This finding is similar to the findings by Rajab et al., on colony of Escherichia coli on TiO 2 surface. [31]Also, this sample has lowest adhesion properties as it is highly-hydrophobic surface as observed in previous section of wettability.Sample S5, with a rhombus pattern, displayed an average cell area coverage of 13.57%.
In a recent study by Nanduru et al., on laser textured Ti6Al4V alloy surface which had square pattern and 134°contact angle, a Table 2. Open circuit potential (E OCP ), Corrosion potential (E corr ), and corrosion current density (I corr ) of pristine, laser textured, and laser textured and environmentally aged aluminum samples after 2 h of immersion in 3.5 wt% NaCl solution.reduction in biomass coverage from 50% to 20% was recorded. [33]owever, our results show better biofilm inhibition on sample S4 on which a reduction in the biomass coverage from 22% to 4% was recorded.

Electrochemical Corrosion Test
The corrosion test was conducted on the pristine sample and sample with star pattern (S4) texture which shows antifouling abilities.Both environmentally aged and sample produced on the same day were considered for the corrosion test.The test was conducted using three three-electrode electrochemical setups with electrolyte as 3.5% NaCl solution.Potentio-dynamic polarization studies were performed and Tafel exploration was used to calculate corrosion parameters.
Figure 8a shows the open circuit potential of base and processed specimens.Potential spikes observed in the V-t curve for non-aged and pristine specimens suggest the occurrence of activation and re-passivation processes.Pristine samples reported the lowest OCP, at −723 mV, compared to non-aged and old specimens, which recorded OCP values of −772 mV and −809 mV, respectively.Cyclic polarization curves (Figure 8b) were used to obtain the E corr (corrosion potential) and I corr (corrosion current) of all the tested specimens.Table 2 shows the E ocp , and I corr of the specimens calculated using Tafel exploration.E corr was found to be lowest for the pristine sample when compared to other specimens, following the trend of OC.In contrast, aged specimens exhibited enhanced corrosion resistance, as seen by a reduced corrosion current of 4.1 μA cm −2, as compared to both the non-aged and pristine samples, which measured 4.1 and 5.22 μA cm −2 respectively.
An important evaluation factor for surfaces that will be employed in demanding marine conditions is their ability to resist corrosion.The electrochemical characterization of the base and processed specimens was therefore performed in salt water (3.5 wt.% NaCl).
Potential spikes observed in the V-t curve (see Figure 8a) for non-aged and pristine specimens suggest the occurrence of activation and re-passivation processes.These processes correspond to the initiation and healing of metastable pits.In contrast, the E-t curve of the aged specimen exhibits only a limited number of smaller spikes in magnitude.It signifies that a relatively stable passive layer was formed on the aged specimen compared to pristine and non-aged specimens.However, the lower E OCP recorded on the aged specimen might be due to the formation of ZnO as well as carbonaceous layer formed over time which leads to increased number of active sites created by laser processing, which could potentially hasten corrosion.
The aged specimen shows slightly lower corrosion current compared to the non-aged and pristine specimens.Thus, suggesting superior corrosion performance (25% increase) compared to the other specimens.However, the passivity breakdown is apparent by the significant rise in the anodic current.In addition, all the tested specimens attained the maximum permissible current during anodic polarization.Therefore, the results suggest that the specimens exhibit weak resistance to pitting corrosion.The corrosion results suggest that the aged specimen exhibits superior corrosion resistance compared to both the pristine and non-aged specimens.Furthermore, the combination of laser processing and aging appears to enhance the corrosion performance of the aluminum alloy.

Surface Chemistry Analysis
The surface chemistry was analyzed by XPS.The penetration depth of XPS varies with the material and the binding energy of the peak being examined, however typically ≈90% of the signal may be expected to come from within 5 nm of the surface.As such, the measurements are highly surface sensitive, and can be strongly influenced by surface conditions such as roughness or surface contamination.Charging of samples under bombardment may lead to shifting of peaks.To account for the effect of surface charging samples were grounded, calibrated to the adventitious carbon peak, and checked against expected results.As hydrophobicity is a surface phenomenon, the surface sensitivity of XPS may also provide greater insight than bulk composition characterization.
The surface compositions of the samples, calculated using the RSF-adjusted XPS peak areas, are presented in Table 3.The variation in composition is relatively small, and the carbon and oxygen signals may be, in part or total, contributions from surface adsorbates.Comparing the ratio of Zn to Al, samples 1, 2, 3, 4, and 5 have ratios of 0.29, 0.27, 0.32, 0.32, and 0.26, respectively.
While some of the oxygen signals may derive from surface adsorbates rather than oxygen incorporated into the samples, extension into lower binding energies for the O 1s peak can indicate the presence of metal oxides.The unprocessed sample shows a peak centered at 532.0 eV.The peak was best fit with two components, at 531.9 and 533.6 eV, consistent with C═O and C─O from organic adsorbates, respectively.The laser-processed samples all exhibit extension into lower binding energy compared to the unprocessed metal (see Figure 9).Deconvolved components were fit using Gaussian-Lorentzian functions, with additional components only included when they reduced the RMS of the fit.All processed samples were best fit with two components, a dominant higher binding energy component (531.2-532.0eV) and a smaller lower binding energy component (527.5-529.2eV).This strongly indicates the presence of some metal oxide(s) in all the processed samples.Sample 4 shows greater extension into lower binding energy, which may indicate greater oxidation in this sample.
The C 1s peak for the unprocessed metal was a strong, single component peak at 284.8 eV, indicating the dominance of adventitious carbon over any other adsorbates.The C 1s peak for the laser-processed samples shows a similar broad shape and position.All processed samples fit best with three components (see    tively.The large, clear change in the carbon peak between the unprocessed material and the laser-processed samples suggests a mechanism for the increased hydrophobicity.However, just looking at the results from the laser-processed samples, the small variations in specific carbon adsorbate types between the individual samples did not show a clear correlation with the hydrophobicity or anti-fouling performance.The middle component position corresponds with adventitious carbon (C─C) which is expected at ≈284.8 eV binding energy, the higher binding energy component is likely COO − carboxyl or ester groups (expected at ≈288.5 eV), and the lower binding energy component is likely dehydrogenated carbon bonded to the aluminum (expected at ≈282 eV). [49,50]CO compounds (expected ≈286 eV) or ═O (expected at ≈287 eV) may be present but could not be distinguished or fit in these data.[52] As the hydrophobicity of the laser-processed samples was seen to increase over time, this is the likely mechanism in this work.The XPS characterization was carried out after 90 days of aging, with the broad carbon peak (compared to the unprocessed sample) indicating the increase in other adsorbed organic compounds after processing and aging and suggesting that these other compounds play a significant role in the enhanced hydrophobicity.These results support immersion treatments in chemical solutions after texturing as a key avenue in achieving the best hydrophobicity in the shortest timeframe. [53]or the Al 2p peak, the binding energy for metallic Al (72.6 eV) and Al oxides (74.6-75.6 eV) are reasonably distinct.The Al 2p peaks for all processed samples were centered at higher binding energies (73.5-75.0eV, compared to a literature value for metallic Al of ≈72.6 eV for the 2p 3/2) consistent with Al oxide (see Figure 9).The higher binding energy component contributed 76, 74, 78, 76, and 80% to the full peak for samples 1, 2, 3, 4, and 5, respectively.These results suggest that the aluminum at the surface is significantly oxidized for all samples, with sample 4, which peaks at a visibly lower binding energy, exhibiting the highest oxidation.The unprocessed metal has a peak position of 74.7 eV, suggesting the material is initially oxidized and the laser processing may create a more Al-rich metallic surface.
Zinc oxide may also be contributing to the indications of oxide, however, due to the close binding energies of Zn and ZnO 2p peaks (1021.7 and ≈1022 eV), making it is difficult to distinguish.The Zn 2p peak position was 1022.1, 1021.9, 1022.8,1021.1, and 1021.9 for samples 1, 2, 3, 4, and 5, respectively, see Figure 10.However, the Zn LMM Auger peak has a significantly different position of 494.7 eV for metallic zinc and 498.4 eV for ZnO.This peak was positioned at about 498.5 eV for all samples and is consistent with the presence of ZnO, see Figure 11.For the unprocessed metal, the Zn peaks were not detectable.This could be due to a higher concentration of zinc near the surface created by the laser processing.An increased concentration of ZnO at the surface may contribute to the wetting behavior, as ZnO thin films have established hydrophobic properties. [54,55]However, due to the low kinetic energy of the Zn 2p peak, it would also be strongly attenuated by a thicker surface layer.It is not possible to conclude that this mechanism is present, with the adsorbed carbon species described above being the most likely dominant contributor to hydrophobicity.

Conclusion
The design and production of micro scaled topography, as discussed, on aluminum alloy surfaces are important in order to be able to prevent, in an environmentally friendly manner, biofouling and corrosion for marine applications.This paper focused on the ultrafast laser texturing of aluminum 7075 which is used in structural and moving components in marine applications which are subjected to corrosion and biofouling.Five surface texture patterns of parallel, cross-hatched, triangle, star, and rhombus were produced on the aluminum alloy 7075 samples.The main conclusions of this study are noted below: • Designed changes in surface morphology and surface roughness were produced and characterized on the aluminum alloy specimens.
• Femtosecond laser texturing also led to a change in wettability and made the samples hydrophilic.The variations in contact angle over time up to 120 days were recorded.Notably, the triangle and star patterns exhibited a gradual transformation to highly-hydrophobic states, culminating in contact angles of 150°and 157°respectively.• Biofouling assessment employing N. ovalis marine algae demonstrated a notable reduction in biofilm coverage, from 21% on pristine to 4.63% on the star texture (S4).This achievement marked a very high level of 79% reduction in biofilm formation.
• The corrosion studies revealed that the aged star pattern texture (S4) exhibited a reduced propensity for corrosion in comparison to non-aged and pristine counterparts.This result suggested a significant enhancement in corrosion performance by 25% relative to pristine samples.• XPS analysis showed an increase in the formation of zinc oxide, as shown in Figure 10, on the surface of the aluminum alloy 7075 samples which imparted a notable boost to fouling and corrosion resistance.
Moreover, this study offers substantial evidence supporting the assertion that femtosecond laser surface texturing represents a viable and effective procedure for enhancing the preparedness of aluminum surfaces intended for the demanding conditions of marine environments.

Figure 1 .
Figure 1.Laser surface texturing setup a) schematic and b) picture.

Figure 2 .
Figure 2. Schematic of the scan strategies used for the five different texture types produce (S1 to S5).

Figure 3 .
Figure 3. SEM images of the laser-processed aluminum alloy surface showing the different morphologies of different textures at 100× and 500× magnification (S1) shows parallel pattern, S2 shows crosshatched pattern, S3 shows triangular pattern, S4 shows star star-hatched pattern, S5 shows rhombus hatched pattern.

Figure 4 .
Figure 4. Surface profiles and scanning depth of a) pristine and b-f) samples fabricated using different scan strategies.

Figure 5 .
Figure 5. Distribution of different surface roughness attributes Sa, Sq, Sp, Sv, and Sz for different textures fabricated by femtosecond laser texturing.Error bars shown are 95% CIs.

Figure 6 .
Figure 6.a) Static water contact angle measurements (10 μL deionized water droplets) were taken with time on the untreated reference and substrates and treated using femtosecond laser having five unique different patterns S1-S5.Error bars shown are the 95% CIs.b) Wettability transition of laser textured sample S5 from highly hydrophilic (43.5°) to highly-hydrophobic (157.1°) in 90 days of environmental aging after laser processing.

Figure 7 .
Figure 7. a) Cropped Segmented images of N. ovalis biofilm on pristine sample and laser fabricated surface textures S1-S5; and b) N. ovalis coverage on different samples immersed in cell suspension for 3 h.Error bars show the 95% CIs with (n = 3).

Figure 8 .
Figure 8. a) Open circuit potential measurements (Potential, E vs time, t) obtained for 3600 s; b) Cyclic polarization curves of the tested specimens in 3.5 wt% NaCl electrolyte at room temperature using standard three-electrode electrochemical setup.

Figure 10 .
Figure 10.XPS spectra of the C 1s peak for the samples, with fitted components.

Figure 11 .
Figure 11.a) XPS spectra of the Zn LMM Auger peak for the laser processed samples, b) XPS spectra of the Zn LMM Auger peak for the laser processed sample 5.

Table 1 .
Chemical composition of aluminum alloy substrate 7075.

Table 3 .
Atomic composition of the samples as determined by XPS.