Facile Approach for Fabrication of Hydrophobic Aluminum Alloy Surfaces Using Fatty Acids

Abstract

Alloys and metals exhibit high sensitivity to corrosion and aggressive environments. Hence, the development of protective treatments through accessible methods with a high degree of protection has become a necessity. This paper presents a method for treating the hydrophilic surface of aluminum alloys using two types of unsaturated fatty acids, thereby increasing the degree of hydrophobicity and protecting the material. The samples were cleaned by a chemical process, followed by immersion in oleic acid (C₁₈H₃₄O₂, 18:1 cis-9) and elaidic acid (C₁₈H₃₄O₂, 18:1 trans-9), and they were then treated at a temperature of 80 °C. Morphological and microstructural analyses were conducted using OM, FE-SEM, EDX, and FTIR to understand the influence of unsaturated monocarboxylic fatty acids on the alloy surfaces. The wettability capacity of the alloys was investigated by measuring the contact angle (CA). The results revealed that the cleaning step and modification treatment with fatty acids are essential steps for increasing the hydrophobic character of the surface. This study can be applied to various types of metallic substrates to enhance their corrosion resistance and long-term chemical stability in aggressive environments, making it adaptable for use in different industrial fields.

1. Introduction

Industrial demands, where different types of metallic surfaces require coverage with protective materials, involve preparing these surfaces by transforming them from hydrophilic to hydrophobic. Generally, metal substrates may be different engineering materials made of aluminum, copper, magnesium, zinc, titanium, iron types, and their alloys, which show their usefulness in the fields of automobile, aircraft, aerospace, transportation, construction, civil engineering, biomedical, and other industries due to their outstanding performance. These subassemblies, metals, and/or alloys exhibit remarkable properties (e.g., wear resistance, mechanical strength, fracture toughness, hardness, fatigue resistance, and biocompatibility, etc.) but are sensitive to corrosion in different environments. Consequently, corrosion presents a serious problem with a negative influence on engineering materials, limiting their applications. Therefore, the development of protection methods is an essential step for many industrial branches [1,2,3,4,5].

The literature presents various approaches to protect aluminum and magnesium surface alloys using multiple methods, including plasma electrolytic oxidation (PEO), laser texturing, electrospinning, layer-by-layer deposition, chemical and mechanical treatments, wet chemical reactions, the plasma fluorination method, thermal/cold spraying, etc. Most of the techniques currently used require advanced technology, strict control of process parameters, time-consuming steps, and the use of strong chemical agents, which, in turn, can cause corrosive processes, thus limiting the protection capacity, performance, and practical applicability. By using different types of processing techniques, clean and uniform surfaces are necessary for further processing to obtain high-quality surfaces. Therefore, the use of accessible, environmentally friendly, and efficient methods represents an opportunity to treat sensitive surfaces with a long-term effect by increasing corrosion resistance, anti-fog, anti-icing, self-cleaning, and antibacterial surfaces and reducing potential environmental and health hazards [6,7,8,9,10,11,12,13,14,15].

Aluminum and its alloys (for example, the 5xxx series) are widely used due to their low density, high strength-to-density ratio, excellent heat and electrical stability, and thermal conductivity, as well as forming properties, design parameters, and reduced costs. Based on its chemical composition (Al95.6 Zn0.15 Mg17–2.4 Mn0.1–0.5 Cu0.15 Fe0.5 Si0.4 Ti0.15 Cr0.15% wt), the A5251 alloy is a medium-strength alloy with good ductility and formability, predominantly anti-erosion properties, and a significant influence on specific properties. However, due to its low sensitivity and corrosion durability, its applicability is limited to aggressive chemical and mechanical environments, temperature variations, etc. [16,17,18,19,20].

To solve these disadvantages, an increase in the hydrophobic character pursued by performing surface treatments of the substrate with low-surface-energy molecules is considered a potential solution for the problems caused by corrosion. Most studies involving metal substrate protection use long-alkyl-chain thiols, fatty acids, perfluorinated alkyl agents, alkyl or fluorinated organic silanes, polydimethylsiloxane, triazinedithiol compounds, and other combinations. Fatty acids with longer carbon chains (i.e., unsaturated fatty acid, oleic acid-cis-9-octadecenoic acid, OA-C₁₈H₃₄O₂, saturated fatty acid, stearic acid-octadecanoic acid, SA-C₁₈H₃₆O) and aliphatic molecules (dodecanoic, tetradecanoic, and octadecanoic acids) are the most commonly used to obtain hydrophobic surfaces due to their high degree of hydrophobicity, low cost of production, and the possibility of eco-friendly surface modification. As the carbon chain grows, they become more non-polar and their interaction with water decreases, making them ideal for coating different types of substrates [17,21,22,23,24,25,26,27,28].

The use of chemical agents allows for substrate surface hydrophobization but with a harmful effect on health and the environment, limiting their potential applications. In this context, long-chain fatty acids are a beneficial option in the formation of compact layers due to the presence of stable carboxyl groups (-COOH) on the substrate surface. Studies have shown that the use of oleic acid on different types of alloys requires an immersion step in an aqueous solution of N, N-dimethylformamide (DMF) to create a rough surface, followed by chemical modification with the fatty acid to generate superhydrophobic surfaces. Hydrophobic surfaces of Mg alloys were realized through chemical etching in CuCl₂ solution to obtain a rough surface and immersion in oleic acid, which led to the formation of superhydrophobic tails on the surface of the Mg alloy and an increase in the contact angle to 155° [7,29,30].

Regarding the use of stearic acid, a superhydrophobic surface of an aluminum alloy was obtained in an existing study by alkaline corrosion in NaOH solution followed by passivation with stearic acid in ethanol solution, with the formation of a low surface energy of aluminum stearate and a contact angle of approximately 150°. The superhydrophobic properties of the same alloy type were demonstrated by acid corrosion using HCl/H₂O₂ or a mixture of HCl/HF and modification with stearic acid, with the surfaces showing mechanical durability and anti-adhesion properties. A combined process was used to improve the corrosion resistance of various Mg alloys. The first step involved the hydrothermal treatment of Mg(OH)₂ with NaOH solution, followed by modification with stearic acid in a DMF-and-water mixture. The formed hydroxide layer provides anchoring sites for the stearic acid coating, and magnesium stearate formation improves the adhesion between the organic coatings and the substrate surface. A surface with a contact angle of 157° was obtained through this process, indicating the achievement of superhydrophobic surfaces. A hydrophobic film with a hierarchical structure was prepared on a galvanized steel surface using a Cu(NO₃)₂ solution and a galvanic replacement reaction process at ambient temperature. The substrates were rinsed with deionized water to remove the residual salts, followed by immersion in a stearic acid solution. The surface obtained by this method exhibits long-term durability, excellent self-cleaning properties, and good chemical stability [31,32,33,34,35,36].

Based on the published results in the specialized literature, a pretreatment of the surface (e.g., cleaning, degreasing, chemical roughening process, etc.) followed by its activation using different types of fatty acids or long-chain molecules is necessary regardless of the alloy type use. Fatty acids are carboxylic acids with long aliphatic chains of variable length that are either unsaturated and/or saturated. Usually, fatty acid molecules are formed by a long, non-polar hydrocarbon tail with a hydrophobic character and a polar carboxyl (-COOH) group at one end, making it hydrophilic, which helps them to bind more easily to the surface. This dual nature is important to improve the corrosion resistance, self-cleaning, and anti-icing performance of different alloys [19,37,38,39,40,41].

The development of affordable and efficient approaches to achieve hydrophobic surfaces is necessary and important for corrosion protection, improved properties (e.g., self-cleaning, water repellency, frost resistance), and long-term durability, etc.

This study investigates the wetting capacity of an aluminum alloy type A5251 surface using two types of monounsaturated fatty acids: oleic acid (OA, cis-9–18: 1) and elaidic acid (EA, trans-9–18: 1), after preliminary metal surface treatment to increase compatibility, develop surfaces that meet the criteria of corrosion resistance in different environments, and expand the range of practical applicability in the industry (e.g., antibacterial, antifouling, self-cleaning, etc.). The use of elaidic acid is little discussed in the literature, and for a better analysis of the results, we chose to compare its properties with those of oleic acid, for which it is known that the action of fatty acids on metallic surfaces is determined by the carboxyl terminal group of the molecule (hydrophilic part), as well as the hydrophobic long hydrocarbon chain. The absorption of fatty acid molecules on the alloy surface establishes a good interaction between the functional groups of the aluminum hydroxide and the carboxylic acid molecule [39,42].

The impact of the roughening and hydrophobization steps on the structural, morphological, and wetting properties of the aluminum substrate was investigated. This work is justified by the need to improve the performance of aluminum alloys by attaching fatty acids to their surface using a simple, accessible, and efficient method to offer additional protection against corrosive environments.

2. Materials and Methods

2.1. Materials

Aluminum alloy type A5251 was purchased from Sigma-Aldrich (St. Louis, MO, USA), and cut in dimensions of 15 × 15 mm. The chemical composition of the A5251 alloy plate includes Mg (2%), Mn (0.3%), and Al (97.7%). The main raw materials used for substrate processing are acetone (C₃H₆O), ethanol (C₂H₆O), sodium hydroxide (NaOH), nitric acid (HNO₃), oleic acid (C₁₈H₃₄O₂, 18:1 cis-9), and elaidic acid (C₁₈H₃₄O₂, 18:1 trans-9; isomer of oleic acid). Fatty acid solutions were prepared by diluting oleic acid (OA) and elaidic acid (EA) with ethanol to a concentration of 50 mM.

2.2. Preparation of the Hydrophobic Coatings

Surface degreasing was performed to remove grease traces, dust, and other organic contaminants. The aluminum alloy samples were ultrasonically cleaned in a mixture of acetone and ethanol (in a volume ratio of 1:1) for 30 min. The substrates were chemically etched by immersion in an alkaline solution (5% NaOH) for 3 min and rinsed in deionized water. The black layer of smut that formed at the alloy surface after NaOH attack solution treatment as well as intermetallic particles, insoluble oxides, and residual solutes left on the surface were removed in acid solution (VHNO₃:VH₂O = 1:1, volumetric ratio) using an immersion time of 120 s. Following surface preparation by chemical roughening, the metallic particles are preferably displaced or dissolved to form the alloy’s rough surface. These steps require suitable process parameters and optimum conditions to obtain the best result. After this step, the aluminum samples were rinsed in deionized water to remove the by-products of the reaction and dried in an airflow. The roughened aluminum alloy samples were immersed in oleic acid (OA) and elaidic acid (EA) solutions, respectively, being maintained at room temperature for 3 h. Samples from the fatty acid solutions were immersed in ethanol and ultrasonically treated to remove unattached acids from the surface. After this stage, the samples were finally dried in an oven at a temperature of 80 °C for 1 h, or they dried at room temperature for a longer period before being characterization.

The use of unsaturated monocarboxylic fatty acids, such as oleic acid (OA) and elaidic acid (EA), allows for a decrease in surface energy, and the long-chain alkyl (by hydrophobic tails) forms chemical bonds by strongly attaching to the alloy’s aluminum surfaces. The molecules were adsorbed on the surface of the aluminum alloys by a simple approach, allowing for the interaction between the aluminum hydroxide surface and the carboxylic acid functional groups. The formation mechanism of hydrophobicity on the aluminum alloy surface is presented schematically in Figure 1.

2.3. Characterization

To prove the applicability of the aluminum alloy, the samples were investigated after chemical etching and their modification with various types of fatty acids both morphologically, structurally, and from the point of view of their hydrophobic behavior.

Structural assessment, chemical bonds, and interactions between fatty acid molecules and the aluminum alloy surface were investigated using Fourier transform infrared spectroscopy (Tensor 27, Bruker Optics, Ettlingen, Germany), equipped with an ATR Platinum holder, in the spectral range of 4000–400 cm⁻¹, by averaging 64 scans with a resolution of 4 cm⁻¹ at room temperature.

The surface analysis of the processed samples was conducted using an Optical Microscope, with an Olympus EP-L3 acquisition camera (Leica Microsystems, Zurich, Switzerland), with 20× and 50× objective lenses, operating in bright field mode. Image capturing and data acquisition were performed using the advanced imaging Leica Application Suite version 4.9.0 software. The morphological and compositional analysis determination of the aluminum sample surface after chemical etching and modification with fatty acids was achieved through a Scanning Electron Microscope (Nova NanoSEM 630, FEI Company, Hillsboro, OR, USA), with a Through-the-lens Detector (TLD) at a magnification of 150 kX at an acceleration voltage between 10 and 15 kV, equipped with an Energy Dispersive X-ray Spectroscopy (EDX) system (Smart Insight AMETEK, Inc., Berwyn, PA, USA) with an acceleration voltage of 15 kV.

The hydrophobic character of the substrate surface was determined by measuring the contact angle using a goniometer (Theta Optical Tensiometer, KSV Instruments, Helsinki, Finland). Small volumes of water (1.5 µL) were deposited on each analyzed surface, and the contact angle was measured at least three times. The average value was used for the analysis.

3. Results and Discussions

3.1. Functional Group Analysis (FTIR)

FTIR spectroscopy revealed the interaction between the fatty acids and the alloy surface, which enhanced the hydrophobic character. The FTIR spectra of the main components, i.e., the chemically roughened aluminum alloy A5251, the fatty acid raw materials (oleic acid—OA; and elaidic acid—EA) used for the wetting surface (hydrophobic character), and the alloys modified with the specified acids, are shown in Figure 2a–e.

The roughened A5251 alloy spectrum (Figure 2a) indicates a lack of specific impurities, confirming the beneficial action of the chemical etching solution and surface preparation process. Also, the bands below 800 cm⁻¹ are attributed to M-O vibrational modes, which suggests that the surface of the alloy is affected by the solutions used. The presence of the OH groups necessary to favor the anchoring of the fatty acid molecules onto the surface of the alloys is confirmed by the existence of the low-intensity bands in the spectral range of over 3000 and 1650–1500 cm⁻¹.

Figure 2b shows the typical spectrum for oleic acid, characterized by absorption bands that can be attributed to the symmetrical and asymmetrical vibration mode of the CH bonds in the CH₂ groups (at 2923 and 2855 cm⁻¹), CO in the carboxylic groups of type C=O (at 1706 cm⁻¹) and C-O (at 1288 and 1090 cm⁻¹), as well as OH in-plane and out-of-plane bands (at 1455 and 935 cm⁻¹). The existence of the double bond can be observed in the spectrum because of the appearance of a low-intensity peak at about 3006 cm⁻¹, a peak that can be associated with the bond =C-H. The oleic-acid-modified aluminum (Figure 2c) sample spectrum confirmed the presence of fatty acids on the substrate surface, with a slight shift toward smaller wavelengths. In the spectrum, bands associated with the vibration mode of the associated C-H (2923 and 2853 cm⁻¹) and =C-H (3005 cm⁻¹), C=O (1709 cm⁻¹), and C-O (1283 and 1085 cm⁻¹) bands were detected. Although most of the peaks in the sample spectrum are smaller than the control sample of OA, the intensity of the central peak at 1085 cm⁻¹ is higher, confirming fatty acid anchoring onto the surface of the substrate of A5251.

The specific spectrum of elaidic acid (Figure 2d) is characterized by peaks that can be attributed to CH (from CH₃ and CH₂ centered at 2955, 2917, and 2847 cm⁻¹ and =CH at 3035 cm⁻¹), CO (from C=O centered at 1712 cm⁻¹ and C-O, in the spectral range 1250–1000 cm⁻¹), and OH (at 1435 and 962 cm⁻¹). The spectrum of the aluminum sample modified with EA (Figure 2e) shows a behavior similar to the samples treated with OA, slightly displaced. In the spectrum of the modified sample, specific peaks of the acid chain were found, which can be attributed to C-H (2916 and 2849 cm⁻¹) and =C-H (3047 cm⁻¹), C=O (1702 cm⁻¹), and C-O (1299 and 1067 cm⁻¹). Also, the band centered at 1067 cm⁻¹ is wider and more intense than in the case of the pure acid, confirming the anchoring on the surface of interest. From a structural perspective, the obtained results revealed the existence of vibrational bands characteristic of the fatty acids used, and in the case of the acid-modified aluminum samples, it is confirmed that long-chain branches of the hydrocarbon base were attached to the surface of the aluminum samples [43,44].

3.2. Optical Microscopy (MO)

After roughening and modification with fatty acids (OA and EA), the control surface of the aluminum alloy substrate (A5251) was visually inspected using optical microscopy (MO). Figure 3a–d presents microstructural images at 20× magnification and detail at 50× magnification to observe the porosity and micro-cracks from the surface of the analyzed samples.

The microstructure analysis of the A5251 substrate (Figure 3a) revealed the presence of some randomly oriented scratches distributed on the alloy surface. The rough surface analysis (Figure 3b) indicates a porous structure, with pits of varied dimensions and irregularly distributed over the entire surface, the details of the structure being acquired at a magnification of 50×. Treatment with chemical etching solutions removes contamination and impurities and dissolves the surface intermetallic particles, causing a certain degree of porosity. The formed pits become active centers for better fatty acid film anchorage. Visual assessment of the surface alloys modified with oleic acid (Figure 3c) and elaidic acid (Figure 3d) revealed the insertion of acid molecules into the pits on the alloy surfaces [45].

3.3. SEM-EDX Analysis

The microstructural analysis of the A5251 alloy samples is shown in Figure 4a–d. The SEM image of the A5251 control alloy (Figure 4a) shows a very small porosity with many defects and irregularly sized pores with an uneven distribution on the surface. A porous surface with high roughness and evenly distributed pits with irregular shapes and dimensions varying between 5 and 10 nm was observed in the roughened aluminum samples (Figure 4b). Etching in an alkaline solution creates roughness on the aluminum surface. The pore structure is related to the composition and microstructure, and their presence can be associated with the removal of intermetallic particles from the surface because of the local dissolution of the surrounding matrix [37,46,47].

From the SEM images of the samples modified with fatty acids, we observed that the morphology of the alloy surfaces changes depending on the type of acid used. Thus, the modification with oleic acid alloys (Figure 4c) shows the formation of a thin layer of acid, which does not fill the pits formed using the roughening process. Analyzing the samples modified with elaidic acid (Figure 4d), a thin and homogeneous layer was deposited on the alloy surface, completely covering the pits, forming a strongly hydrophobic surface.

These images come to support the characterization data presented below, namely the hydrophilic character observed by measuring the contact angle for the control A5251 alloy and the roughened sample with uniform roughness on the surface and the possibility for the formation of hydrophobic surfaces after modification with OA (contact angle of 109°) and EA (contact angle of 132°) as a result of partial or total filling the pits with fatty acids.

To evaluate the chemical composition at the atomic level, Figure 5a–d present the EDX spectra for the A5251 alloy samples. In Figure 5a, the control alloy shows an atomic percentage composition of 96.18 at %Al, 2.85 at %Mg, and 1.97 at %Mn, indicating the main elements from the analyzed zones of the alloy. The EDX spectrum of a representative sample of the roughened substrate (Figure 5b) shows peaks that can be attributed to the atoms present in the alloy (Al (K) at 1.49 keV, Mn (L) at 0.637 keV, Mg (K) at 1.254 keV), as well as oxygen atoms (O (K) at 0.52 keV). Also, the chemical composition was found to be 95.18 at %Al, 1.41 at %Mg, and 0.48 at %Mn; the composition of the surface did not change significantly, and the main elements are kept at close values to the control alloy. The presence of oxygen atoms suggests the possibility of the presence of an aluminum hydroxide film formed due to the attack of hydroxide ions (HO-) on the metal surface, which facilities easy anchoring of fatty acid [48,49].

In the case of the A5251 substrate samples (Figure 5c,d), the presence of the characteristic alloying atoms as well as the C (C (K) atoms at 0.277 KeV) and O (O (K) at 0.52 keV) is observed, confirming the presence of the fatty acid molecules (OA and EA) on the surface of the treated aluminum samples.

3.4. Wetting Capacity (Contact Angle)

Wetting capacity was evaluated by measuring the contact angle values using the sessile drop method between the surface of the aluminum sample and the water droplet. The average value of the contact angle obtained for the surfaces of alloy 5251 and the images of the distilled water droplets are shown in Figure 6a–d. The surfaces are described as hydrophilic, exhibiting a low contact angle with water (θ < 90◦), and the surface wettability increases with the decrease in θ. At high contact angles (θ > 90◦), the surface becomes hydrophobic, with water repellency and low adhesion [49].

It can be seen that the control alloy (Figure 6a) before treatment has a hydrophilic surface with a water contact angle of 74°. Based on the results presented in Figure 6b, the surface of the roughened aluminum alloys led to a hydrophilic character, which presented a value of the contact angle equal to 60°. The morphological structure of the surface alloy through the steps of processing has high porosity and decreases the average value of the contact angle. By surface modification with fatty acids and improvement of the wettability, and implicitly a hydrophobic character was observed. This behavior is observed when using oleic acid (Figure 6c), where the value of the contact angle increases to about 109°. When using elaidic acid (Figure 6d), the surface is strongly hydrophobic, with high water repellence and an increase in the contact angle to 132°.

In both cases, the drops of water deposited on the surface were almost a round sphere, indicating that the surfaces are hydrophobic. In our research, chemical modification treatment and a reduction in surface energy with fatty acids led to the formation of a hydrophobic surface. The increase in the water contact angles of the modified samples was a result of the adsorption of organic molecules and the formation of a monolayer. The behavior of oleic acid is more different due to the cis-double bond having a bent arrangement, which limits side-chain interactions. In the case of elaidic acid, due to the alkyl group arranged in a trans-geometry across the C=C double bond, it can adopt an almost linear configuration and thus form a close-packed monolayer structure with the substrate. Trans-isomers are more thermodynamically stable compared to cis-isomers due to double bond stiffness and the presence of long chains being more spatially distant than in cis-isomers. The difference between the two fatty acids is reflected in the value of the contact angle and the drop shape of the water on the alloy surface [50,51,52].

4. Conclusions

This paper presents the process of obtaining surfaces based on strongly hydrophobic aluminum using oleic acid and elaidic acid as unsaturated monocarboxylic fatty acids. The A5251 aluminum alloy surfaces exhibit high hydrophobicity.

The FTIR spectra of the alloy after chemical treatment proved the presence of OH groups that favor the attachment to the surface of the fatty acids used. The spectra of the samples modified with the acids show peaks with a slight displacement characteristic of the raw materials, and the central band at 1085 cm⁻¹ associated with the CO bond (from OA) and the band from 1067 cm⁻¹ (from EA) confirm the anchoring of the acid to the surface of the A5251 substrate.

Morphological studies showed that by alloy roughening, pits were formed that allowed the fatty acids to be easily attached to the sample surface. The pits were covered with a thin layer of the acids on the surfaces with fatty acid modification, leading to the growth of the hydrophobic character. Structural analysis confirmed the presence of the main elements, C and O atoms from the fatty acids, on the alloy surface.

The analysis of the contact angles between the water droplets and the roughened surfaces shows a decrease in value compared to the contact angle determined for the surface of the standard alloy. The A5251 alloys after the chemical treatment step show a hydrophilic behavior, and through surface modification with fatty acids, an improvement in the wetting capacity was found, showing an increasing in the contact angle to 109° in the case of oleic acid and 132° in the case of elaidic acid. From the analysis of the results, it was shown that the best option for obtaining hydrophobic surfaces with superior properties is through the usage of elaidic acid (the trans-isomer of oleic acid).

The obtained results indicate that a simple and low-cost approach can be used to create hydrophobic aluminum surfaces with a high contact angle and with self-cleaning, anti-icing, and water-repellant properties. Although the results were obtained on aluminum alloys, this approach can be applied to other types of alloys, extending the scope of application in various sectors of interest.