The present study investigated the active corrosion protection provided by superhydrophobic cerium stearate coatings. Superhydrophobic cerium stearate was deposited on anodized AA2024-T3 at 40 V with different electrodeposition times using a simple DC electrodeposition technique to know the role of electrodeposition time on surface morphology, hydrophobicity, and corrosion resistance. The structure and morphology of cerium stearate was characterized to understand its formation mechanism. Electrodeposition process at 40 V for 120 min resulted in the formation of dual-scale Allium giganteum like micro/nano hierarchical texture of cerium stearate with a water contact angle of 165±1.6°. The cerium stearate coating obtained for 120 min process time had excellent self-cleaning property and good chemical stability, environmental stability, and mechanical durability acceptable for industrial applications. Electrochemical impedance spectroscopy and scanning vibrating electrode technique were used to investigate the active corrosion protection of cerium stearate coating. The electrodeposited cerium stearate coating showed active corrosion protection based on self-healing ability by releasing cerium (Ce3+) ions.
Aluminum (Al) alloys are widely used in aerospace industries as lightweight alloys due to their excellent mechanical properties resulting from their heterogeneous microstructure, the suitability for surface modifications, good electrical and thermal conductivity, ease of recycling, and low density.1-3 However, they are short of good corrosion resistance and susceptible to localized corrosion when exposed to chloride-containing environments. Chromate-based conversion coatings or organic coatings containing chromium-based pigments are an effective strategy to achieve active corrosion protection to metallic substrates for various applications. Anodization using chromic acid is the preferred pretreatment method used in the aerospace industry for corrosion protection. Another approach used for the corrosion protection of Al alloys is anodization followed by chromic acid rinsing to seal the porous anodic oxide layer. Due to the hazardous and carcinogenic nature of Cr(VI), chromate treatments and coatings containing chromate-based pigments have been progressively prohibited.4 The concerns because of the use of chromate-based coatings have been addressed by using various environmentally friendly alternate approaches such as conversion coatings derived from rare earth salts, sol-gel based thin films, and layer-by-layer assembly of polymer films. Other sealants such as sol-gel coatings, organic acids, and hydrothermal treatment were used and these sealants worked by acting as a barrier against the ingress of aggressive electrolyte through the porous oxide layer.5-9 However, these sealing methods lack active corrosion protection. The focus of the present study is sealing the porous anodic oxide layer using a superhydrophobic surface with active corrosion protection functionality.
The self-cleaning effect exhibited by the lotus leaf inspired many researchers to fabricate superhydrophobic surfaces with a water contact angle (WCA) of over 150° and a sliding angle less than 10° to control corrosion of metallic substrates. Here, corrosion protection is achieved through reducing contact of a metallic surface with water and environmental humidity. This kind of thin superhydrophobic coating would provide satisfactory corrosion protection during the storage of metals and alloys before its use.10-14 Applying a low surface energy material and roughening the surface by creating micro/nano texture are the ways to produce a superhydrophobic surface. Al and its alloys exhibit superhydrophobic functionality by producing an interface that can retain air within the rough surface structure. The air trapped inside the micro/nano texture reduces the contact area of liquid droplets on the surface and provides nonwetting and anticorrosion properties to the alloy surface.15
Various approaches have been used to fabricate superhydrophobic surfaces on Al and its alloys such as spin coating,16 chemical etching,17-23 boiling water treatment,24 sol-gel method,25-26 anodization,27-32 chemical vapor deposition,33 hydrothermal method,34 and laser treatment.35 Vengatesh and Kulandainathan30 reported the fabrication of a hierarchically ordered self-lubricating hydrophobic surface on Al alloy by combining anodization and grafting perfluorinated long chain fatty acids. The corrosion studies revealed that a hydrophobically treated anodized Al alloy surface considerably increased the corrosion resistance compared to hydrophilic oxide surfaces. Ou, et al., proposed a facile method combining the generation of hydroxide or oxide barrier layer by hydrothermal treatment or chemical etching and subsequent surface passivation with low-surface-energy 1H, 1H, 2H, 2H-perfluorooctyltriethoxysilane molecules to fabricate superhydrophobic surface on light alloy substrates.34 Ou, et al., showed that superhydrophobic coatings fabricated on hydrothermally treated light alloy surfaces were more effective in corrosion protection because of the generated oxide/hydroxide barrier layer. The oxide layer imparted micro/nano roughness and enhanced corrosion protection to the light alloys. However, Ou, et al., did not investigate the role of anodization in improving the corrosion resistance of the obtained superhydrophobic surface on Al alloy. Environmentally unfriendly and toxic chemicals containing fluorine such as fluoroalkyl silanes and fluoroacrlylic copolymers are usually used to create low surface energy surfaces on Al alloys.36
Recently, Tong, et al., reported a fluorine free preparation of superhydrophobic surface on an aluminum alloy with anticorrosion performance and mechanical robustness.37 However, their fabrication method of superhydrophobic surfaces includes laser ablation followed by chemical etching and subsequent coating with a coating precursor containing ZnO particles, stearic acid, and commercially available PDMS (poly-dimethylsiloxane). The higher corrosion resistance offered by STA-ZnO-PDMS coatings over STA modified film was because of the PDMS polymer’s superior barrier property. Most of the above-mentioned methods were followed a two-step approach combining the roughening of surface topography and reducing the surface energy by surface modification using low surface energy molecules to fabricate superhydrophobic surfaces. Most of the methods involve multistep fabrication procedures, costly and biologically toxic materials such as fluoroalkyl silanes and octadecyltrichloro silanes, and special experimental conditions and equipment. Therefore, it is essential to have a facile, environmentally friendly, and template-free one-step process for fabricating superhydrophobic surfaces.
In recent times, electrodeposition has been become known as a facile method to fabricate superhydrophobic surfaces for metallic substrates owing to its merits such as low cost, ease to fabricate, control over various parameters, ability to coat a large surface area, and scalability.13-14,38-45 Liu, et al., developed a superhydrophobic surface on an Mg alloy surface by Ni electroplating and subsequent surface passivation using stearic acid.42 Su and Yao reported a facile and low-cost fabrication process comprised Ni electrodeposition in traditional Watts bath and heat-treatment in the presence of (heptadecafluoro-1, 1, 2, 2-tetradecyl) triethoxysilane (AC-FAS) to fabricate superhydrophobic surface on Cu alloy.43 Wang, et al., reported the development of superhydrophobic coating on Mg alloy using a three-step process: the electroless Ni deposition and electrodeposition of Cu followed by the final surface passivation using lauric acid. Although the authors claimed that the fabrication method is low cost, large scale, and environmentally friendly, it is difficult to scale up a three-step process industrially.44 Xu, et al., developed a superhydrophobic magnesium alloy surface via a facile electrochemical machining process and subsequently covered it with a fluoroalkylsilane (FAS) film. Though the superhydrophobic surface was fabricated through an electrochemical route, a two-step process and fluoroalkylsilane were used to achieve superhydrophobicity.45 Electrodeposition is a facile method to fabricate uniform coatings on substrate irrespective of its size and shape. Applying a suitable overpotential or current density during electrodeposition leads to morphological instabilities, which provide the rough surface texture required for the hydrophobic state.41,46-48 In the beginning, template-assisted electrodeposition was used to get a patterned layer of superhydrophobic metal deposits. Here, the template fabrication step is the crucial one because the surface texture of the metal deposit heavily depends on the template’s texture. Surface passivation using a low surface energy molecule was carried out to take the wettability to the superhydrophobic regime in many of the reported template-assisted electrodeposition.49-51
The simultaneous attainment of micro/nano surface texture and low surface energy functionality through a one-step electrodeposition method would be a promising strategy for developing cost-effective, simple, and environmentally friendly, nonfluorinated superhydrophobic surfaces. Long-chain carboxylic acids such as hexanoic acid, decanoic acid, hexadecanoic acid, myristic acid, and stearic acid have become cost-effective environmentally friendly alternatives to toxic fluoropolymer to generate superhydrophobic surfaces.27,52-54 These long-chain fatty acids were deposited over the surface engineered metallic substrates by dipping in an ethanolic solution of corresponding fatty acids to provide superhydrophobic character. Alternatively, many researchers reported superhydrophobic surfaces’ fabrication by one-step electrodeposition using an electrolyte solution comprising suitable metal salts and long-chain fatty acids.15,55-62 Liu, et al., and Rasitha, et al., reported the fabrication of superhydrophobic surfaces on Mg and 9Cr-1Mo alloy surfaces, respectively, via one-step electrodeposition using electrolyte solutions comprising Ce, myristic acid, and ethanol.14,38 Both studies reported that the deposited Ce-myristate could impart superhydrophobic functionality and improve corrosion resistance. They attributed the enhanced anticorrosion property of the electrodeposited coatings to hydrophobicity, which efficiently restrains the corrosive solution’s contact to the metallic substrate by trapping air on the hierarchal surface texture. Similarly, Zhang, et al., electrodeposited Ce-stearate using electrolyte solutions comprising Ce, stearic acid, and ethanol on high purity Al tape. The higher impedance value recorded by the electrodeposited Al surface was attributed to the nonwetting behavior of the surface which restrains the penetration of corrosive electrolyte solution.39 None of the aforementioned articles investigated Ce ions’ role in active corrosion protection and any long-term protection offered by the electrodeposited Ce-carboxylic acid salts.
In the present study, a superhydrophobic coating with active corrosion protection on AA2024-T3 is developed using anodization and electrodeposition methods. Anodization was performed in an eco-friendly tartaric/sulfuric acid (TSA) medium to obtain a uniform aluminum oxide barrier layer on the surface of AA2024-T3 to improve the long-term corrosion protection. Even though Ce-stearate coatings were deposited on pure Al substrate, so far, no superhydrophobic coating has been fabricated on AA2024-T3 alloy using electrolyte solutions comprising Ce, stearic acid to the best of our knowledge. The main objective of the present work is to develop a cost effective and environmentally friendly superhydrophobic coating. The electrodeposition was performed at 40 V and the influence of duration of electrodeposition on surface morphology, hydrophobic character, and corrosion resistance was investigated. The role of Ce ions was further investigated in the long-term corrosion protection offered by Ce-stearate deposits besides the nonwettability property of the coatings for corrosion protection. The structure and morphology of the electrodeposited coatings were investigated using scanning electron microscopy, atomic force microscopy, x-ray photoelectron spectroscopy, and laser Raman spectroscopy. The anticontamination performance of the as-prepared electrodeposited coating was evaluated by a self-cleaning test. The durability and stability of the resultant superhydrophobic surface were studied by an abrasion test, and exposing the coated surface in 3.5 wt% NaCl solution. The active corrosion protection and barrier property of the superhydrophobic coatings were studied by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques and scanning vibrating electrode technique (SVET).
Materials and Chemicals
The alloy used in this work was an AA2024-T3 aluminum alloy, with nominal chemical composition (wt%): Cu (4.46), Mg (1.35), Mn (0.64), Fe (0.28), Si (0.18), Ti (0.05), Zn (0.04), Cr (0.01), and Al (balance). The AA2024-T3 sheet was purchased from Virat Aluminum, India and was cut into pieces of 30 mm × 25 mm × 2 mm and polished with SiC emery papers (from 400 to 1000 grit). The chemicals used for anodization were tartaric acid and sulfuric acid, both obtained from Merck, India. Ethanol (Hayman), stearic acid (TCI, India), and cerium nitrate hexahydrate (Alfa Aesar, India) were used to obtain the electrodeposited coatings. Sodium chloride obtained from Loba Cheime, India was used for all corrosion tests and electrochemical studies. Potassium permanganate was purchased from Loba Cheime, India for preparing the pink colored solution for placing droplets for imaging purpose. All of the chemicals used were of analytical grade and were used without further purification. Aqueous solutions for all experiments were prepared with water purified in a three-stage Millipore Milli-Q Plus 185† purification system, having a resistivity of 15 MΩ·cm.
Sample Pretreatment and Anodization
Before anodizing, the AA2024-T3 coupons were alkaline etched in a 10 wt% NaOH for 3 min followed by rinsing in purified water. Subsequently, the AA2024-T3 coupons were dipped in 10 vol% HNO3 for 30 s followed by cleaning with deionized water. Finally, surface pretreated samples were permitted to drying in ambient conditions.
The anodization process was performed using a two-electrode system, in which AA2024-T3 (30 mm × 25 mm × 2 mm) acted as the anode and stainless steel (30 mm × 25 mm × 5 mm) was used as cathode. The distance between the two electrodes was approximately 2 cm. The anodization was performed in a tartaric/sulfuric acid bath (0.53 M C4H6O6 and 0.46 M H2SO4) at 37°C and performed at 14 V for 25 min.4 The constant DC voltage was applied using a DC power supply (Aplab high voltage power supply H0330†). After completing anodization, the specimens were rinsed with deionized water, dried under air conditions at room temperature.
The electrodeposition process was performed using a two-electrode system where the anodized AA2024-T3 acted as the cathode and stainless steel was used as the anode. A constant DC voltage of 40 V was applied using the DC power supply for 1 min to 180 min at room temperature. The electrolyte solution was prepared by mixing an ethanolic solution of 0.2 M stearic acid and 0.1 M cerium nitrate hexahydrate. After electrodeposition, samples were rinsed with ethanol and dried at room temperature. The applied potential was fixed at 40 V, because it was reported that at this potential superhydrophobicity was achieved on metals using similar electrolyte.38
The WCA for all of the electrodeposited coatings was measured by a contact angle goniometer (DataPhysics Instruments, Germany) equipped with software version SCA 20.2.0†. Water droplets of 10 μL volume were deposited on the surface of the samples and the contact angle was obtained by measuring the slope of the tangent to the drop at the liquid/solid interface. In order to assess the homogeneity of the superhydrophobic functionality of the electrodeposited coatings, the water contact angle was calculated by taking measurements at 15 different positions on the sample. Purified water was used in all of the above experiments, and the experiments were performed at room temperature (∼25°C) and constant humidity (∼60%). The structure and surface morphology of electrodeposited samples were characterized by using a field emission scanning electron microscope (FESEM). The topography and surface roughness of electrodeposited samples were determined by an atomic force microscope (AFM). The weight of electrodeposited coatings at different deposition times was measured using a four-digit analytical balance. The chemical state and composition of the electrodeposited samples were examined by x-ray photoelectron spectroscopy and laser Raman spectroscopy. A sand paper-based shear abrasion test was performed on 120 min electrodeposited Ce-stearate coating to get insight into the mechanical durability of the electrodeposited coating. The variation in WCA during the abrasion test as a function of abrasion distance was recorded. The abrasion test was performed by dragging the Ce-stearate coated surface on a 1000 grit SiC emery paper under a load of 2.45 KPa.14,38 To assess the long-term chemical stability of the fabricated superhydrophobic surface, the 120 min electrodeposited Ce-stearate coating was exposed to 3.5 wt% NaCl solution (immersion test) and air (environmental light test) for 840 h and observed the evolution of WCA periodically. The self-cleaning property of the electrodeposited Ce-stearate surface was evaluated by randomly sprinkling some dust contaminant (finely powdered sand) on the 120 min electrodeposited Ce-stearate coating, and then these dust contaminants are cleaned by dropping 10 μL of water.39 The corrosion resistance of the electrodeposited samples was evaluated by using EIS and potentiodynamic polarization studies. These studies were performed using a three-electrode system, with platinum as a counter electrode, Ag/AgCl as a reference electrode, and an electrodeposited sample acting as the working electrode. The exposed area of the working electrode was 1 cm2. EIS and polarization studies were conducted using an aqueous solution of 3.5 wt% NaCl at room temperature. The EIS test was performed with the frequency range from 105 Hz to 10−2 Hz with the amplitude of AC voltage is 10 mV. The potentiodynamic polarization study was performed at a scanning rate of 1 mV/s with the potential range from −0.4 V to +1.2 V. The active corrosion protection based on self-healing ability was investigated using SVET which uses Biologic SCV 470† control unit. Samples prepared for SVET measurement were of dimensions 1 cm × 1 cm and a scan area of 4,000 μm × 4,000 μm and 64 × 48 points on X and Y axis were considered for the measurements. The sample was mounted in a Teflon† holder and the 3.5 wt% NaCl electrolyte solution was added. The initial scan was made immediately after 5 min of exposure to NaCl solution, and the data were collected for various durations of exposure. Each scan consists of 400 data points on a 20 × 20 grid, with an integration time of 1 s per point. A complete scan required 30 min, followed by a 5 min rest period before the next scan. The frequency of the vibrating electrode used was 100 Hz. The current density maps were plotted in 3D format over the scan area, with positive and negative current densities representing anodic and cathodic regions. The measurements were obtained at the open-circuit potential without applying any bias. All of the electrochemical tests were repeated in triplicate and a typical set of data were displayed.
Effect of Electrodeposition Time on Wettability and Morphology and Mechanism of Superhydrophobicity
Electrodeposition on anodized Al samples was performed at 40 V for various time durations from 1 min to 180 min to know the effect of deposition time on the nonwetting property and morphology of the electrodeposited surface. Figure 1 shows the evolution of WCA at different durations of electrodeposited cerium stearate on anodized Al wat 40 V. From Figure 1, it can be observed that hydrophobicity was attained by 1 min electrodeposition of cerium stearate. Simultaneously, 10 min of the electrodeposition process was not sufficient to push the surface into the superhydrophobic regime. Superhydrophobicity was achieved after 30 min of electrodeposition, and when the electrodeposition time was increased from 30 min to 120 min, it can be observed that WCA further increased. The highest average value of the contact angle is obtained for the electrodeposited sample of anodized aluminum at 120 min is 165±1.6°. In some regions, the contact angle value was 179.9° on the anodized surface electrodeposited for 120 min. The average WCA value was significantly reduced to 159.7±2.4° under 40 V for the sample of electrodeposition at 180 min. At the same time, the static WCA for anodized AA2024-T3 was shown in the inset of Figure 1 and it was 71±1°. Hence, it can be stated that electrodeposition was necessary to introduce hydrophobic functionality to anodized AA2024-T3.
In order to gain insights on the effect of electrodeposition process time on the morphology of the electrodeposits and wettability, FESEM images of the anodized and electrodeposited at different electrodeposition times were obtained and presented in Figure 2. The surface morphology of anodized AA2024-T3 surface (Figure 2[a]) and 1 min cerium stearate deposited surface (Figure 2[b]) had similar morphology except for the presence of small cerium stearate particles. As the pores formed after TSA anodization have a very narrow distribution, they are not visible in the present magnification. Even though cerium stearate had deposited on the surface, there was no improvement in the surface roughness to provide superhydrophobicity. However, WCA was increased after 1 min of electrodeposition, and this can be attributed to the low surface energy of cerium stearate. Increasing the electrodeposition time to 10 min resulted in enhanced growth of cerium stearate on the anodized surface. However, the development of small papillae particles was not continuous and homogenous (Figure 2[c]). Prolonging the electrodeposition process time to 30 min resulted in forming a well-defined spherical papillae structure with micro/nano texture (Figure 2[d]). This change in surface texture results in increasing the nonwetting property as the electrodeposited surface showed superhydrophobicity. Further extending the electrodeposition time to 60 min (Figure 2[e]), resulting in the interconnection between the individual papillae structures and agglomeration occurred. Even after 60 min of electrodeposition, the complete coverage of the anodized surface was not achieved as fissures can be seen in Figure 2(e). The complete and uniform coverage of the anodized AA2024-T3 surface was obtained only after performing electrodeposition for 120 min. More interconnections between the individual papillae and agglomeration occurred after 120 min of electrodeposition. The micro/nano texture formed after 120 min of electrodeposition was ideal for trapping air and created air pockets which increased the WCA. The high magnification images (Figures 2[h] and [i]) of individual papillae structure revealed a “nano rice husk” like structures on the spherical papillae and this nanostructured surface imparted higher surface roughness and higher WCA. When the electrodeposition process time was increased to 180 min, the micro/nano surface texture obtained after 120 min of electrodeposition was not maintained, and this resulted in slightly reducing the nonwetting functionality of the electrodeposited surface. This may be the result of the discharge at the surface under the effect of long-time application of the current. Moreover, the cerium stearate deposits’ insulating nature ceases further deposition as saturation of deposition may have occurred.14,38,63
Based on the analysis of the effect of electrodeposition voltage on the morphology and nonwetting character of the deposits, it was revealed that 120 min of electrodeposition was required for obtaining the right micro/nano papillae texture and complete surface coverage. These two things are prerequisites for the electrodeposited coatings to exhibit adequate corrosion protection and superhydrophobicity. Although there were reports of developing good superhydrophobic surfaces using electrodeposited Ce-carboxylic acid salts for Mg alloys and 9Cr-1Mo alloys at 30 V and 40 V, respectively, for a deposition time of 10 min, in this study, it was found that 120 min of electrodeposition time was necessary to achieve complete coverage of the surface and superhydrophobic property.14,38 It was found that superhydrophobicity was not achieved until the electrodeposition process was prolonged to 30 min. Furthermore, the complete surface coverage by cerium stearate was achieved only after 120 min of electrodeposition.
where γSA and γSL are the surface energies of the solid against air and liquid, and γLA is the surface energy of liquid against air.67 Generally, the contact angle of a smooth surface does not surpass 110° to 120°, irrespective of the surface’s nature. Further, Young’s equation is effective only for those solid surfaces which are homogenous, smooth, and isotropic. If a water droplet is in equilibrium on a surface, the surface energy inside and outside the water droplet will be uniform. The same water droplet will be expanded when the energy inside a droplet is lower than that of the outside. Conversely, on a low surface energy surface, the attractive force between the water and the substrate is not strong enough to spread it, hence the surface will tend to be superhydrophobic.
The interface between the water droplets is a solid/liquid interface and the surface roughness increases the geometrical area of solid/liquid interface, which in turn results in the penetration of water into the small grooves present on the surface. Therefore, the adhesiveness of the surface increases and the surface might be sticky. When θY is greater than 90°, the dry surface will have lower free energy than that of the wet area on the solid surface and this results in the receding of water droplets from the roughest region. This scenario was explained by Cassie-Baxter equation, where the rough surface is assumed to be a heterogeneous one composed of air trapped between the protrusions on the surface and the solid.68
Electrodeposition Mechanism and Characterization of Superhydrophobic Surface
To confirm the mechanism proposed for electrodeposition, the chemical composition of the electrodeposits was analyzed using XPS and laser Raman spectroscopy. Figure 4 shows the XPS spectra of the as-prepared electrodeposited superhydrophobic surface on anodized AA2024-T3. Figure 4(a) reveals the XPS survey spectrum of 120 min cerium stearate electrodeposited surface. The presence of C, O, and Ce can be observed in Figure 4(a). The high-resolution spectra of each element were deconvoluted further to understand the chemical composition of the as fabricated surface. Figures 4(b) through (d) show the high-resolution XPS spectra of C, O, and Ce, respectively. As a result, the C 1s spectrum (Figure 4[b]) consists of three main peaks. The deconvoluted high-resolution spectra for C 1s shows two peaks, and the peak at a binding energy of 284.17 eV is due to C-C/C-H, and the peak at 287.64 eV is attributed to -C=O-. In O 1s high-resolution spectra (Figure 4[c]), the peak appeared at a binding energy of 531.19 eV due to the presence of –O-Ce- and the peak at 530.61 eV is assigned to the –O-C- bond.14,38,69 The high-resolution deconvoluted spectra for both C 1s and O 1s exhibited additional peaks at lower binding energy sides at 283.52 eV and 530.04 eV, respectively, and this could be due to unfinished charge neutralization or any artifacts arising from surface charging. The deconvoluted high-resolution spectra of Ce 3d (Figure 4[d]) did not have any peak at 917 eV, a signature peak for Ce(VI) state. Different peaks of Ce 3d5/2 and Ce 3d3/2 were observed due to the spin-orbit splitting and the peaks at 881.96 eV and 902.07 eV are assigned to shakedown peaks of Ce 3d5/2 and Ce 3d3/2. The peaks at 885.03 eV and 904.37 eV correspond to Ce 3d5/2 and Ce 3d3/2 parent peaks.38,69 The results obtained from XPS analysis confirm the formation of Ce(III)-stearate at anodized aluminum alloy surface. The absence of Ce(IV) state in the as-fabricated ED can be explained as follows: Owing to the higher effective nuclear charge of Ce4+ ions than that of Ce3+ ions may lead to the initial deposition of Ce(CH3(CH2)16COO)4. Nevertheless, further deposition of Ce4+ ions at the cathodic surface did not occur owing to the insulation behavior of this inner layer. This leads to the reaction between Ce3+ ions available near the cathode surface with stearate anions and deposits Ce(CH3(CH2)16COO)3 as the outer layer of the superhydrophobic surface.38
Figure 5 shows the laser Raman spectra of as-prepared electrodeposited superhydrophobic surface on anodized AA2024-T3 at various electrodeposition process times. The Raman spectrum of stearic acid is also presented for comparison. It can be observed that the characteristic vibrational modes of the stearate group were present in the Raman spectra of electrodeposited superhydrophobic surfaces and the intensity of Raman peaks enhanced as the electrodeposition time increased. The characteristic Raman features for metal-oxygen interactions appear in the wavenumber region of 150 cm−1 to 450 cm−1. This indicates that with increased electrodeposition processing time, increasing cerium stearate is deposited on the surface without any chemical composition change. However, there were no observations of any characteristic Raman bands for Ce-O interactions in the Raman spectra. In an earlier report on the Raman spectra of electrodeposited Ce-myristate, the Ce-O band was observed on the surface obtained after electrodeposition at 50 V and above.38 The Raman band at 1,056 cm−1 and 1,124 cm−1 are due to the vibration of C-C bond stretching. The band at 1,292 cm−1 corresponds to the twisting of CH2 and band at 1,443 cm−1 is due to C=O stretching. The strong band at 2,848 cm−1 and 2,865 cm−1 is indicative of the presence of CH in CH2 and CH3. Interestingly, characteristic vibrational models of stearate groups were not present in the laser Raman spectra after 1 min electrodeposition process. This confirms that 1 min electrodeposition is not sufficient to complete the alloy surface coverage to provide superhydrophobicity as discussed in the Effect of Electrodeposition Time on Wettability and Morphology and Mechanism of Superhydrophobicity section.
Figure 6 depicts the three-dimensional AFM images of anodized AA2024-T3 and the electrodeposited cerium stearate coatings for different electrodeposition times. It can be observed that micro/nano hierarchical papillae-like texture was present after 30 min of electrodeposition process onward and the AFM images were in agreement with surface morphologies observed by FESEM. The root-mean square (rms) values of surface roughness for anodized and electrodeposited coatings were obtained and tabulated in Table 1. Anodized AA2024-T3 had the lowest rms roughness value, and the rms roughness value progressively increased as the time of the electrodeposition process was increased. However, there was a decrease in roughness values when the electrodeposition process time was 180 min. This decrease in roughness value resulted in the reduction of WCA after 180 min of electrodeposition. Figure 7 depicts coating weight measured after different electrodeposition times. It can be observed that coating weight increased with electrodeposition time. The error bars represent the standard deviation of data for different electrodeposition times.
Chemical and Mechanical Stability
Figure 8 shows the variation in WCA during the abrasion test as a function of abrasion distance. The electrodeposited surface was dragged to move on 1000 grit SiC paper in one direction. As shown in a schematic (inset of Figure 8), a load of 2.45 KPa (50 g, 2 cm2) was applied and the superhydrophobic Ce-stearate coating was dragged on 1000 grit SiC emery paper in one direction. Figure 8 shows that the 120 min electrodeposited Ce-stearate coating could maintain a WCA above 150° for the abrasion length 500 mm. WCA reduced to 146.1±0.5° after abrasion distance for 600 mm. This observation can be attributed to the fact that mechanical abrasion might reduce superhydrophobicity after a certain abrasion length due to the load, abrasion surface roughness, and abrasion distance. However, the mechanical stability studies showed that the 120 min electrodeposited surface exhibits good mechanical durability to some extent.
Even though many reports are available in the literature on the fabrication and application of superhydrophobic coatings, the superhydrophobic surface’s mechanical durability and chemical stability is a challenging area. Figure 9 shows the evolution of WCA for 840 h during an immersion test in 3.5 wt% NaCl and exposure in ambient air. While immersed in 3.5 wt% NaCl solution (Figure 9[a]), it was observed that the superhydrophobic functionality was maintained up to 240 h of immersion and the WCA recorded was 150.5±1.4°. Although the 120 min electrodeposited Ce-stearate coating was immersed in an aggressive corrosive solution, it did not lose its nonwetting characteristics even after 10 d of immersion and this indicates that the electrodeposited thin coating was intact. After 35 d of immersion, the WCA reduced to 134.6±0.9°. However, it’s worth noting that it still had nonwetting property. When the electrodeposited Ce-stearate is immersed in NaCl solution, both Na+ and Cl− ions get adsorbed on the surface and as the exposure time increases, more Na+ and Cl− ions are added gets adsorbed on the surface. These adsorbed ions affect the air packets trapped on the surface, which in turn reduces the hydrophobicity by moving from Cassie-Baxter to Wenzel state. Similarly, the coating’s stability under UV light was studied by an environmental light test for 840 h by exposing the 120 min electrodeposited Ce-stearate outdoors and checked its wettability. The WCA was reduced minimally from 162.1±4° to 160.3±0.5°, as shown in Figure 9(b). Significantly, the superhydrophobic property was maintained for 840 h because of the good stability of the 120 min electrodeposited sample.
Functional surfaces find application in anti-contamination if they have both a high static water contact angle and a low sliding angle. Self-cleaning is one of the significant properties of superhydrophobic coatings. When a water drop sits on a tilted solid surface, the contact angles at the front and back of the droplet relate to the advancing and receding contact angles, respectively, and the difference between advancing and receding contact angles is defined as contact angle hysteresis (CAH). Surfaces with very low CAH have a low water roll-off angle, i.e., the surface’s angle to be tilted for the water drops to roll off.65 Low CAH and low water roll-off angles are essential for the self-cleaning property of the surface. Figure 10 depicts the results obtained from the self-cleaning test. Figure 10(a) shows 120 min Ce-stearate deposited coated surface and Figure 10(b) depicts the sand particles randomly sprinkled on the Ce-stearate coated surface and dropping of water from a micropipette. It can be observed from Figures 10(c) and (d) that sand particles were easily moved away from the coated surface by rolling water droplets. This may be due to less adhesion of the dust particles with 120 min electrodeposited surface. From this result, it is concluded that electrodeposition with cerium stearate at 120 min can protect the surface from pollution. The self-cleaning ability can be explained as follows. The electrodeposition of Ce-stearate provides micro/nano papillae like texture, which results in surface roughness and inhomogeneity. Moreover, the air trapped in these micro/nano protrusions further increases the water repellency and the wettability state of the coated surface was modeled based on Cassie-Baxter state. It was earlier calculated that the water/air interface area was 96.4%, and this reduction in contact area results in a considerable reduction in the adhesion force of the surface. This results in easy bouncing off of the water droplets from the surface. The low surface energy heterogeneous surface results in poor adhesion between the dirt particles and the surface; however, the stronger adhesion forces between the water and dirt result in cleaning the surface.
Figure 11 shows the potentiodynamic polarization curves of bare AA2024-T3, anodized AA2024-T3, and anodized and cerium stearate electrodeposited AA2024-T3 for various duration following 1 h of exposure in 3.5 wt% of NaCl solution. It can be observed from Figure 11 that the anodic arm of the polarization curve is departed from the linear Tafel behavior and this can be attributed to passive film formation and pitting. It was earlier reported that a well-defined experimental anodic branch of polarization curve could not be expected when passivation and dissolution reaction due to pitting occur at the metal surface.70-71 Owing to the deviation from the linearity on the polarization curve’s anodic arm, it is not possible to fit the anodic Tafel slope by Tafel extrapolation. Hence, the electrochemical parameters such as corrosion potential (Ecorr) and corrosion current density (icorr) were determined by Tafel extrapolation of the cathodic arm of the polarization curve. Further, the breakdown potential (Ebr), and ΔE, the difference between Ebr and Ecorr was obtained from the potentiodynamic polarization curve and all of the obtained electrochemical parameters are depicted in Table 2.
It can be observed that AA2024-T3 alloy undergoes pitting corrosion at corrosion potential itself. No passive film was formed, and metal dissolution was evidenced by the anodic arm of the polarization curve. Anodization of AA2024-T3 alloy positively shifted the corrosion potential; however, the anodic arm of anodized AA2024-T3 followed the same pattern of bare AA2024-T3. The electrodeposited AA2024-T3 alloy after anodization showed a different pattern. The corrosion rate was reduced when the anodized AA2024-T3 was electrodeposited for 1 min, however, no passive region was established before pitting. The shifting of the cathodic arm of the polarization curve to lower current densities can be attributed to the suppression of oxygen reduction reaction at the cathodic sites. This demonstrates the cathodic inhibition property of the cerium available in the cerium stearate electrodeposit. As the electrodeposition time increased to 10 min, 30 min, and 60 min, it was observed that corrosion potential shifted to the negative side compared to that of 1 min electrodeposited surface and the passive region was established.
The positive shift of Ecorr of about 100 mV can be due to the deposition of larger quantities of cerium stearate deposits on the anodized AA2024-T3 alloy surface. The SEM micrograph analysis earlier described showed that as the electrodeposition time increased, the coverage of cerium stearate on anodized AA2024-T3 enhanced, however, the coverage was not complete and homogenous. The anodic current densities for 10 min, 30 min, and 60 min cerium stearate electrodeposited AA2024-T3 were lesser than that of 1 min cerium stearate electrodeposited AA2024-T3 surface. This indicates the barrier property of the electrodeposited coating which provides anodic corrosion protection. Even though the increased coverage of the electrodeposited cerium stearate shifted the corrosion potential to the positive side, the presence of pores in the coating due to incomplete coverage might be the reason for the absence of a compact passive barrier layer and small passivation window. The extent of ΔE (Ebr–Ecorr) is considered as the measure of initiation of pits.72-74 The values of ΔE for 10 min, 30 min, and 60 min cerium stearate electrodeposited AA2024-T3 were approximately 70 mV (Table 2), indicating lower corrosion resistance and higher susceptibility for the initiation of pit nucleation.
When the electrodeposition time was increased to 120 min and 180 min, as was already observed in the scanning electron micrographs, the electrodeposited coating coverage was complete and homogenous. This is reflected in the potentiodynamic polarization curve and the separation between corrosion potential and breakdown potential also. ΔE was approximately 1,000 mV for 120 min electrodeposited AA2024-T3 surface. The lowest current density in the anodic arm and corrosion rate was observed for the AA2024-T3 electrodeposited for 120 min. From the analysis of the nonwetting property, it was found out that 120 min electrodeposited sample exhibited the highest WCA, and this water repellency might have contributed to enhanced corrosion protection. Further, the electrodeposited coating obtained after 120 min of deposition had complete coverage and good physical barrier property. Even though the 180 min deposited coating had good coverage, its wetting property was reduced owing to the reduction in the surface roughness of the deposited layer as earlier shown in the AFM analysis. The combination of factors that led to the enhanced corrosion protection of the 120 min electrodeposited AA2024-T3 alloy are (i) cathodic inhibition offered by Ce(III), (ii) thick and complete electrodeposited barrier layer, and (iii) enhanced superhydrophobicity owing to the surface roughness and low surface energy of stearate ions. In order to further understand the corrosion protection offered by the electrodeposited cerium stearate, the coated samples were immersed in 3.5 wt% NaCl solution for 7 d and their impedance response was monitored periodically using EIS.
EIS was used for evaluating the protection offered by electrodeposited cerium stearate for different deposition times, and the results are presented in Figure 12. Figure 12(a) shows the Bode plots for 1 min cerium stearate deposited anodized AA2024-T3 during 7 d of immersion in 3.5 wt% NaCl solution. As expected, the electrodeposition resulted in the enhancement of total impedance with respect to anodized and bare AA2024-T3 (Figure S1, the EIS bode plots for bare and anodized AA2024-T3 are provided in the Supplemental Material). It can be observed that a time constant corresponding to electrodeposited (ED) Ce-stearate was present in the Bode plots until 24 h of immersion in 3.5 wt% NaCl solution. The resistance offered by the Ce-stearate ED layer was significantly reduced even during the initial 24 h of immersion as evidenced by the Bode phase angle plot. This decrease in impedance response of the ED Ce-stearate could be attributed to the incomplete coverage after 1 min electrodeposition process. It can be interpreted that only two time constant was observed for 1 h immersed specimen. The time constant that appeared at the high-frequency region (105 Hz to 103 Hz) corresponds to the electrodeposited layer and the time constant ranges from mid-frequency to low frequency correspond to the porous layer. The physical meaning of the processes occurring after 24 h of immersion in 3.5 wt% NaCl can be explained using the electronic equivalent circuit (EEC) of three time constants. The first one appearing at the high-frequency region corresponds to the ED layer together with the pore walls, followed by the porous layer at mid-frequency and the final time constant at lower frequency refers to the compact anodized layer at the pore bottom.
No corrosion-related activities were observed in the EIS spectrum following 24 h of immersion. From 48 h of immersion onward, no physical barrier protection was offered by ED cerium stearate layer, as evidenced by the presence of a resistive plateau in the impedance modulus at high frequency. The time constant observed in the frequency region of 103 Hz to 101 Hz appeared on 1 min ED cerium stearate surface during 48 h to 100 h of immersion in NaCl solution could be attributed to the porous layer containing cerium hydroxide/oxide. The second relaxation process centered at 1 Hz corresponds to the anodized layer, and the final time constant appeared at the low-frequency region relates to pitting corrosion occurred at the metal/oxide layer interface. As the immersion time increased further to 144 h and 168 h, the first two time constants became wider, and shifted to lower-frequency ranges and the phase angle value increased. This behavior can be attributed to the pore widening and possible progressive pore blocking by an auto-sealing process that occurs throughout the pore depth due to the dissolution of anhydrous alumina from the pore walls and its precipitation. Furthermore, the relaxation process due to pitting corrosion at 10−2 Hz becomes more pronounced after 168 h of immersion.
Figure 12(b) shows the Bode plots for 10 min cerium stearate deposited anodized AA2024-T3 following immersion in 3.5 wt% NaCl solution from 1 h up to 168 h. Three relaxation processes corresponding to the ED layer, porous oxide layer, and compact anodic oxide layer were observed after 1 h immersion in 3.5 wt% NaCl solution. It is further observed that a time constant corresponds to ED Ce-stearate layer was present on anodized AA2024-T3 surface even after 144 h of immersion in 3.5 wt% NaCl solution. This observation indicates that 10 min ED Ce-stearate could offer barrier protection to the chloride ions and water molecules’ infiltration and enhance the total impedance compared to that of 1 min ED cerium stearate. As observed from the phase angle plot, the theta value decreased at the high-frequency region due to the destruction of the ED layer and theta (θ) value increased in the mid- and low-frequency relaxation processes due to the pore widening and progressive pore blocking by dissolved aluminum oxide from the pore walls.
Figure 12(c) shows the Bode plots for 30 min cerium stearate deposited anodized AA2024-T3 following immersion in 3.5 wt% NaCl solution from 1 h up to 168 h. The evolution of EIS response for 30 min ED Ce-stearate surfaces are different from that of 1 min and 10 min electrodeposited surface. The impedance response from the ED layer was present even after 168 h of immersion. This indicates that as the deposition time increases, more Ce-stearate was deposited and the coverage was enhanced and, hence, the superior barrier protection offered by ED layer for 30 min deposited surface. It can be observed from the |Z| curve that resistance offered by the ED layer decreased continuously as the immersion time increased. This is due to the infiltration of water and chloride ions through the ED layer’s pores, which shows that as the immersion time increased, the physical barrier property of the ED layer deteriorated. Similarly, the relaxation process occurring at the mid-frequency region gets widened as the immersion time increased from 24 h to 168 h and this widening of the mid-frequency time constant (103 Hz to 10−1 Hz) can be attributed to the combined effect of pore widening and pore blocking owing to the deposition of cerium oxide/hydroxide and anhydrous alumina deposition from the pore walls.
Figure 12(d) shows the Bode plots for 60 min cerium stearate deposited anodized AA2024-T3 following immersion in 3.5 wt% NaCl solution from 1 h up to 168 h. It can be observed from Bode modulus and phase angle plots that the barrier property of the ED layer increased as the immersion time increased. This could be attributed to the deposition of the cerium oxide/hydroxide layer at the pore mouth, indicating possible active corrosion protection offered by the electrodeposited cerium stearate layer. Furthermore, up to 100 h of immersion, only two time constants appeared on the Bode plots, and this indicates that the coating could offer good protection up to 100 h by not allowing the electrolyte to infiltrate and reach the anodic oxide layer. However, a time constant corresponding to the compact barrier oxide layer appeared after 144 h of immersion, and it gets more expressive after 168 h of immersion in 3.5 wt% NaCl solution.
Figure 12(e) shows the Bode plots for 120 min cerium stearate deposited anodized AA2024-T3 following immersion in 3.5 wt% NaCl solution from 1 h up to 168 h. The 120 min cerium stearate electrodeposited AA2024-T3 showed the highest total impedance compared to that of other electrodeposited cerium stearate coatings for various electrodeposition process times. Moreover, during the entire 7 d of immersion, only two time constants were appeared on the EIS spectra corresponding to the ED layer in the high-frequency region (105 Hz to 101 Hz) and from the porous layer in the remaining frequency region (101 Hz to 10−2 Hz). From the Bode phase angle plot, it can be observed that the theta value was increased from 1 h immersion to 168 h immersion in 3.5 wt% solution indicating an increase in the capacitive behavior of the coating. The increase in the total impedance value and nonoccurrence of any relaxation process even from the compact anodic oxide layer indicates the superior corrosion protection offered by 120 min Ce-stearate deposited anodized AA2024-T3 during 7 d of immersion in 3.5 wt% NaCl solution. This can be attributed to the deposition of cerium oxide/hydroxide layer at the mouth of the porous layer, complete coverage of the surface by the micro/nano textured cerium stearate electrodeposits, and the highest superhydrophobicity offered by the 120 min electrodeposited Ce-stearate layer. It was found from the investigation of nonwetting property that the increase in deposition time from 120 min to 180 min did not enhance the coating’s superhydrophobicity due to the decrease in surface roughness.
The evolution EIS response of 180 min Ce-stearate deposited anodized AA2024-T3 during 7 d of immersion in 3.5 wt% NaCl solution is depicted in Figure 12(f). A direct comparison of the evolution of the Bode plots for 120 min and 180 min of electrodeposition establishes the role of superhydrophobicity on long-term corrosion protection performance of the coatings. Even though the total impedance values did not lower below 106 Ω·cm2 after 168 h of immersion for 180 min electrodeposited surface, a relaxation process corresponding to the anodic oxide layer appeared at the low-frequency region. This indicates that the reduction in surface roughness owing to the changes in the micro/nano texture during 180 min electrodeposition results in decreased superhydrophobic functionality and, hence, the corrosion protection performance. A detailed analysis of the impedance Bode phase angle and modulus plots shows three time constants corresponding to three relaxation processes occurring at the surface after 72 h of immersion onward. The third time constant appears at low frequency and relates to the anodic oxide layer, and the obtained results showed that water could access the oxide layer after 72 h of immersion.
Figure 13 shows the evolution of EIS response after 1 h immersion in 3.5 wt% NaCl solution for cerium stearate electrodeposited AA2024-T3 alloy with different electrodeposition time. It can be observed that only two time constants appeared on the EIS spectra when the Ce-stearate electrodeposition time was 60 min, 120 min, and 180 min. These two time constants’ physical meaning can be interpreted as the response from the ED layer and the porous oxide layer. Conversely, when electrodeposition was performed for 30 min or the lesser duration (1 min and 10 min), the coating could not offer sufficient corrosion protection even during 1 h of immersion as a third time constant corresponding to the compact barrier oxide layer or pitting corrosion. Figure 14 shows the evolution of the EIS response after 168 h immersion in 3.5 wt% NaCl solution for Ce-stearate electrodeposited AA2024-T3 alloy with different electrodeposition time. When electrodeposition time was 1 min, there was no evidence for the presence of ED layer as EIS modulus plot shows only a resistive plateau at the high frequency corresponding to the electrolyte resistance. Further, the total impedance at low frequency was in the range of 104 Ω·cm2, indicating that pitting corrosion had commenced on the AA2024-T3 surface. An electrodeposition process time of 30 min at 40 V was required to get the total impedance value of more than 106 Ω·cm2. It is a generally accepted fact that a total impedance value north of 106 Ω·cm2 is required for adequate corrosion protection in 3.5 wt% NaCl solution.
The value of |Z| at 10−2 Hz is a good measure of active corrosion protection. Figure 15 depicts the evolution of |Z| at 10−2 Hz for cerium stearate deposited anodized AA2024-T3 at 40 V with different electrodeposition times; 1 min, 10 min, 30 min, 60 min, 120 min, and 180 min. The obtained results indicate the synergistic effect of active corrosion protection offered by cerium ions and the passive physical barrier protection provided by the superhydrophobic Ce-stearate layer. As far as the corrosion protection is concerned, both 1 min and 10 min electrodeposited Ce-stearate layer could not offer adequate corrosion protection, though they can provide active corrosion protection by inhibiting oxygen reduction at cathodic sites. Both active and passive protection is necessary for sufficient long-term corrosion protection. The incomplete surface coverage might be the reason for their low corrosion resistance. Even though 30 min and 60 min electrodeposited layer could offer better corrosion protection compared to that of 1 min and 10 min ED layers, as immersion time increased total impedance value gradually decreased. However, when electrodeposition time was increased to 120 min and 180 min, the ED layers could provide excellent corrosion protection in 3.5 wt% NaCl solution due to the combined effect of superhydrophobicity, thick and compact ED layer and active corrosion protection provided by cerium ions present in the ED layer. Further, it can be observed that for the 120 min ED layer, |Z| value at 10−2 Hz almost remains unchanged during the 168 h of immersion period after registering an increase from 1 h to 24 h immersion period.
The obtained Bode plots were fitted using EECs as immersion time progresses, different phenomena appear on the surface of the coated specimen. Several EECs were tried and tested to fit the experimentally obtained impedance data to evaluate corrosion protective properties quantitatively. Only those electrical equivalent circuits which had the lowest chi-squared values and minimum error were selected. Figure 16 illustrates the EEC models used to fit the EIS measurements of different electrodeposited test samples, and others have already used similar EECs; however, the physical meaning may vary.52-53 Constant phase elements (CPE) were used instead of capacitance due to the nonideal capacitive behavior of the electrode/electrolyte interface. Rsol represents the solution resistance. Qed and Red represent the CPE and resistance of the electrodeposited layer, respectively. Qp and Rp correspond to the CPE and resistance of porous anodized layer which might be filled by cerium stearate during electrodeposition. Qox and Rox correspond to the compact aluminum oxide layer. Qcor and Rcor stand for the CPE of double layer and polarization resistance of metal substrates. Based on the obtained impedance values and the frequency region where the time constants have appeared, these EECs were used for samples electrodeposited for various time.
The evolution of Red as a function of immersion time (Figure 17[a]) shows the effectiveness of the hydrophobic cerium stearate ED layer on corrosion protection performance of the entire protective system. Further, the obtained fitting parameter indicates the importance of the electrodeposition process duration for enhanced corrosion protection. Red arises from the resistance offered by the hydrophobic cerium stearate electrodeposited layer. Though a thin layer of nonuniform deposition of cerium stearate occurred on the anodized AA2024-T3 substrate for 1 min electrodeposition, it could not offer corrosion protection after 24 h of immersion. Although the electrodeposition of hydrophobic cerium stearate takes place promptly, the absence of complete and uniform coverage results in insufficient corrosion protection during immersion. A minimum electrodeposition time of 10 min is required for improved corrosion protection during immersion. The Red values were continuously decreased during immersion in NaCl solution for 10 min electrodeposited specimen. This indicates that even though the ED layer obtained after 10 min electrodeposition could provide some protection for 144 h of immersion, it could not cease the ingress of chloride ions during the whole immersion time. As the electrodeposition time increased to 30 min and 60 min, both the coverage and nonwetting property of the ED layer enhanced. However, complete coverage was not achieved when electrodeposited for 30 min, resulting in a continuous decrease of Red values as a function of immersion time. Red values for both 60 min and 180 min electrodeposited were always higher than 105 Ω·cm2, indicating nonwetting characteristics and uniform coverage. However, the Red values for 120 min electrodeposition were always higher than 106 Ω·cm2 owing to the complete coverage of the anodized AA2024-T3 surface and superior nonwetting property. This further indicates that the ingress of chloride ions was minimal during the entire immersion time. The surface roughness owing to the micro/nano texture created after 120 min of electrodeposition might have played a role in sustaining the superhydrophobicity during the entire immersion period.
Rp arises from the second time constant in the medium-frequency range, and it corresponds to the porous structure of the anodic oxide layer, which contains cerium stearate deposited during the electrodeposition process. The values of Rp are significant because any changes in the impedance values owing to the deposition of cerium hydroxide/oxide or alumina deposition reflect in Rp. As the immersion time increases, the Rp values for 1 min, 10 min, 30 min, and 60 min electrodeposited specimens gets decreased, indicating that nonwetting property was insufficient in blocking the infiltration of electrolyte-containing chloride ions through the hydrophobic ED layer as well as porous layer (Figure 17[b]). However, for both 120 min and 180 min electrodeposited coatings, the Rp values were higher than 106 Ω·cm2 during most of the immersion time. This can be attributed to the superior nonwetting property and active corrosion protection offered by the possible deposition of cerium oxide/hydroxide in the porous anodic oxide layer. Rp values increased as a function of time for 120 min electrodeposited coatings indicating its enhanced active corrosion protection during immersion.
Because different electrochemical parameters were used to model the relaxation process occurring at the coating/metal interfaces based on experimentally obtained impedance values and the frequency range of the time constants, Rtotal was calculated and its evolution as a function of immersion time is depicted in Figure 17(c). The total resistance Rtotal includes the resistance of the electrodeposited layer, porous anodic layer, compact oxide layer, and the polarization resistance (Rtotal = Red + Rp + Rox + Rcor). Rtotal was obtained and compared to validate the corrosion protection efficiency as a higher value of Rtotal reveals superior corrosion resistance. Rtotal was always less than 106 Ω·cm2 for 1 min electrodeposited coating, indicating that it could not give adequate corrosion protection during immersion. When the total impedance offered by a coating is more than 106 Ω·cm2, it can be stated that the coating could offer satisfactory corrosion protection in the testing environment. It was observed that 10 min electrodeposited coating also could not provide enough resistance to the penetration of electrolyte-containing chloride ions. This study proved that for adequate corrosion protection, 30 min of electrodeposition is required, as it could provide hydrophobicity and better surface coverage. Further, it can be observed that Rtotal was progressively increased as a function of immersion time for 120 min and it increased in the order of 107 Ω·cm2, indicating possible active corrosion protection offered by the cerium stearate electrodeposits. The enhancement in corrosion resistance can be attributed to the fact that the presence of sustained superhydrophobicity ensures that the chloride ions did not attack the compact oxide barrier layer during the entire immersion time of 168 h. Further, the deposits of cerium oxide/hydroxide in the porous layer ensured that the barrier layers remain undamaged and durable corrosion protection possibly by active corrosion protection.
In order to get more insights into the active corrosion protection offered by cerium stearate, SVET analysis was performed on 120 min electrodeposited Ce-stearate coating. The local corrosion activity that occurred on Ce-stearate coating was compared to that of anodized AA2024-T3 surface under a freely corroding condition in 3.5 wt% NaCl solution and the results are presented in Figures 18 and 19. Aqueous corrosion of metals and alloys is an electrochemical process involving oxidation of metal at anodic sites and reduction of species from solution at cathodic sites on the metal surface at separated areas. SVET, in general, is a noninvasive technique and the results obtained are self-explanatory. As shown in Figure 18, anodic currents due to the oxidation of metal are represented by the red color, while the blue color represents cathodic currents by the reduction of oxygen at the metal surface. The green-colored regions correspond to null current or minimal current, which cannot be detected, and this represents areas that are usually associated with the absence of electrochemical activity at the surface. For the anodized AA2024-T3 specimen after 5 min of immersion itself, anodic and cathodic activities were observed on the surface (Figure 18[a]). The observed current was in microamperes, and this clearly showed that corrosion started immediately after immersion in 3.5 wt% NaCl solution. As the exposure time in NaCl solution increased, more corrosion spots were observed on the surface randomly. After 48 h of exposure (Figure 18[d]), the intensity of anodic peaks was slightly reduced by the auto sealing process throughout the pore depth due to the dissolution of anhydrous alumina from the pore walls its precipitation as hydrated alumina resulting in pore blocking.52
It can be observed from Figure 19 that the current density was in nano amperes for the electrodeposited specimen when exposed in 3.5 wt% NaCl solution and this can be attributed to the nonwetting property resulting from the presence of air bubbles entrapped inside the micro/nano protrusions of the compact layer of Ce-stearate electrodeposits. Both anodic and cathodic sites can be observed during the initial scan itself (Figure 19[a]), however, much smaller anodic current densities than anodized AA2024-T3 surface. As the exposure time increased to 12 h (Figure 19[b]), more anodic and corresponding cathodic activities can be seen on the surface. However, after 24 h of exposure in NaCl solution, the scanned area appeared in green, indicating that no corrosion spots were present on the surface (Figure 19[c]). This can be attributed to the active corrosion protection provided by cerium present in the coating, indicating the self-healing ability of the Ce-stearate coating at the functional level. The functional self-healing is achieved by restoring the corrosion protection functionality of the coating as the Ce ions suppress the oxygen reduction reactions at the cathodic sites. The protection from pitting corrosion was ensured even after 48 h of immersion in 3.5 wt% NaCl solution (Figure 19[d]). SVET results suggest that Ce-stearate electrodeposited coatings could decrease the current activities and conductive pathways after extended NaCl solution exposure. Such a decrease in corrosion current with exposure time results from filling the pores in the coating and passivation of active corrosion sites on the surface.75
The oxygen reduction reactions at cathodic sites result in the development of alkaline pH and favors the dissolution of the oxide layer and Al matrix around the intermetallic inclusions as given below:
The Ce(OH)4 may further dehydrates to cerium oxide.
The study revealed that as the electrodeposition time increased, a greater amount of was Ce-stearate deposited on anodized AA2024-T3 surface (Figure 7). The electrodeposited Ce-stearate formed at longer process time (120 min and 180 min) on anodized AA2024-T3 could act as a compact passive barrier layer and provided cathodic inhibition by Ce present in the coating. However, a greater amount of Ce-stearate deposition was not the only factor providing superior corrosion protection. The nonwetting property of the electrodeposited surface also played a crucial role in inhibiting corrosion. This was proved by the better corrosion protection offered by 120 min Ce-stearate electrodeposited surface compared to that of 180 min Ce-stearate electrodeposited surface, though the latter had more coating weight. The decrease in surface roughness of 180 min Ce-stearate electrodeposited surface resulted in the reduction of its nonwetting property and this in turn reflected in its reduced corrosion resistance.
A superhydrophobic coating with active corrosion protection on AA2024-T3 was developed using anodization and electrodeposition methods in the present study. Anodization was carried out in an eco-friendly tartaric/sulfuric acid medium to obtain a uniform aluminum oxide barrier layer on the surface of AA2024 to improve the long-term corrosion protection. A superhydrophobic Ce-stearate coating was deposited under the applied voltage of 40 V, and its nonwetting property and active corrosion performance was investigated. Comprehensive electrochemical studies of the electrodeposited Ce-stearate superhydrophobic coatings were performed to understand the corrosion protection mechanism. Several points can be concluded from this study.
Anodization of AA2024-T3 in TSA medium prior to the electrodeposition of Ce-stearate superhydrophobic coating significantly enhanced the corrosion protection efficiency.
Electrodeposition process time of 30 min was necessary to achieve superhydrophobicity, however, the complete and uniform coverage of Ce-stearate coating was achieved only after 120 min of electrodeposition. Further, when the electrodeposition time was increased to 180 min, the nonwetting property was decreased owing to uneven growth and partial damage of structures for long electrodeposition time.
Electrodeposition process at 40 V for 120 min resulted in the formation of dual scale Allium giganteum like micro/nano hierarchical texture with a WCA of 165±1.6° and the as fabricated surface was mainly composed of Ce(CH3(CH)2COO)3.
The electrodeposited Ce-stearate coating obtained for 120 min process time had excellent self-cleaning property and good chemical stability, environmental stability, and mechanical durability acceptable for industrial applications.
The electrodeposited Ce-stearate coating demonstrated active corrosion protection based on self-healing ability by releasing Ce3+ ions from the coatings and subsequently diminishing the cathodic oxygen reduction reaction by the precipitation of cerium hydroxide salts on the cathodic sites.
The enhanced corrosion protection provided by Ce-stearate electrodeposited coatings could be due to the combination of following three factors: (i) cathodic inhibition offered by Ce(III), (ii) thick and complete electrodeposited barrier layer, and (iii) enhanced superhydrophobicity owing to the surface roughness and low surface energy of stearate compound.
The authors acknowledge the Central Instrumentation Facility (CIF) of CSIR-CECRI for obtaining FESEM images, and LRS and XPS spectra.