Superhydrophobic copper stearate/copper oxide thin films by a simple one-step electrochemical process and their corrosion resistance properties

Abstract

Superhydrophobic films were coated on an aluminum alloy surface via a one-step electrochemical modification process in an ethanolic stearic acid (SA) solution containing copper nitrite (Cu(NO₃)₂) under a DC voltage. Various morphologies were obtained when different molar ratios of Cu/SA were used in the solution. The electrochemically modified films were characterized using X-ray diffraction and infrared (IR) spectroscopy; the films consisted of low surface energy copper stearate. This chemical component and the hierarchical rose petal-like micro-nano structure together provided a maximum contact angle of 162° and a minimum contact angle hysteresis of 1.52° with water roll-off properties. Furthermore, the polarization resistance was calculated from the Tafel curve of the superhydrophobic surface obtained from a Cu/SA solution with molar ratio of 0.5, which was 66 times greater than that of a chemically cleaned aluminum alloy substrate. Electrochemical impedance spectroscopy (EIS) revealed an increase in the charge transfer resistance of the chemically cleaned aluminum alloy substrate from 1.56 kΩ cm² to 1130 kΩ cm² for the superhydrophobic surface. Accordingly, the corrosion resistance of the superhydrophobic aluminum alloy surface produced by the one-step electrochemical process was superior to that of the chemically cleaned aluminum substrate.

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1. Introduction

Superhydrophobic surfaces with non-wetting characteristics have a water contact angle (CA) of more than 150° and a sliding angle of less than 5°. The wettability of a solid surface can be governed by its chemical composition and geometric structure.¹ Due to their anti-corrosion, self-cleaning, and icephobic properties, the applications of superhydrophobic surfaces extend from consumer products (car windshields and eye glasses) to engineering components (ship hulls and medical instruments).^2,3 Therefore, superhydrophobic surfaces have garnered increasing attention from scientists and engineers in recent years.^4–6 Numerous artificial fabrication methods have been applied to produce superhydrophobic surfaces.^2,7–9

For example, copper surfaces have been previously rendered superhydrophobic by two step processes: (i) the surface is roughened by chemical etching or by growing nanostructured rough thin films of copper oxide or copper hydroxides and (ii) modifying these surfaces using organic acids such as stearic acid (SA) to minimize their surface energies.^10–14 We have developed a one-step chemical modification process to fabricate superhydrophobic copper surfaces in an ethanolic stearic acid solution by applying a DC voltage between two copper electrodes.^15,16 Similar approach has been used by other researchers to modify copper surfaces using capsaicin molecules and tetradecanoic acid in ethanol to obtain superhydrophobic surfaces.^17,18 This method, however, fails to modify aluminum surface to render them superhydrophobic.¹⁹ On the other hand, aluminum surfaces, due to their light weight and low cost, are generating more interests in the industry such as automotives and aeronautics as well as domestic consumer products. Therefore, it becomes of significant importance to protect these aluminum surfaces from factors such as corrosion. We are actively working on the fabrication of superhydrophobic surfaces on aluminum and other metal surfaces using sol–gel,²⁰ chemical etching,^21,22 chemical bath deposition,²³ surface functionalization techniques,^20,24 as well as direct electrochemical modification of metal surfaces^15,25 and electrophoretic deposition (EPD).²⁶

Corrosion is mostly an unwanted property that occurs on a metal surface due to its greatest affinity to water owing to their very high surface free energies. Corrosion resistant properties of metals can be achieved by fabricating superhydrophobic films on them, as permeability of water into their surfaces can be prevented due to the poor affinity of water to these low energy surfaces. Corrosion current densities (I_corr), polarization resistance (R_p) and the charge transfer resistance (R_ct) are the mostly reported parameters to interpret the corrosion resistance properties. The I_corr, reported in the literature generally varied between 1 and 80 μA cm⁻² for copper surfaces^{12,14,16,18,27} and between 2 and 8 μA cm⁻² for aluminum alloy surfaces.^28–30 The polarization resistance (R_p) of copper and aluminum alloys was reported to be about 2 kΩ cm².^16,30 The charge transfer resistance, (R_ct), of copper was found to vary between 0.2 and 42 kΩ cm².^12,27,31,32 On the other hand, the R_ct for aluminum alloys was found to be around 10 kΩ cm².^29,30,33 Interestingly, superhydrophobic copper surfaces fabricated using stearic acids and other organic acids were reported to have I_corr between 6 × 10⁻³ and 1 μA cm⁻².^{12,14,16–18,27,31} Superhydrophobic thin films fabricated on aluminum alloys surfaces based on stearic acid showed I_corr varying between 6.0 × 10⁻³ μA cm⁻² and 2.5 × 10⁻¹ μA cm⁻².^28–30 The maximum R_p of our copper stearate thin film covered superhydrophobic copper surface fabricated using electrodeposition was 1220 kΩ cm².¹⁶ We also have reported a R_p of 521 kΩ cm² on stearic acid passivated etched aluminum surfaces.³⁰ The R_ct of stearic acid modified superhydrophobic copper substrates was reported to be 16 kΩ cm² (ref. 27) and myristic acid modified superhydrophobic copper surfaces were reported to be 117 kΩ cm².³² The R_ct of superhydrophobic copper surfaces fabricated using other organic molecules was reported to be around 2.5 kΩ cm².^17,31 The R_ct of superhydrophobic thin films fabricated on aluminum substrates were reported to be vary between 13 and 375 kΩ cm².^29,33,34 The large deviation of these corrosion parameters (I_corr, R_p, R_ct) reported in the literature for different superhydrophobic materials was due to the variation of film thickness, surface chemical compositions, reactions with the solvents used for the corrosion studies, time of immersion before performing corrosion studies etc.

Our developed one-step electrochemical method^15,16 has been further modified by other researchers incorporating an inorganic salt containing metal ions such as Ni²⁺, Ce³⁺, Mn²⁺, etc. in the bath of organic acid to fabricate superhydrophobic metal and alloy surfaces.^34–36 Specifically, Chen et al.³⁵ reported the effect of time on the morphology and wetting properties of superhydrophobic films deposited on copper substrates formed using an ethanolic solution with lanthanum chloride and myristic acid. On the other hand, in another study reported by Chen et al.,³⁶ a superhydrophobic manganese myristate film ((CH₃(CH₂)₁₄COO)₂Mn) was fabricated on a copper surface by a one-step electrodeposition process in an ethanolic myristic acid solution containing manganese chloride. The maximum contact angle on the obtained film was 163° when the electrolysis time was 10 min. However, the relationship between contact angle and morphological features as a function of solution concentration was not clearly illustrated. In a study conducted by Liu et al.,³⁴ the anti-corrosion properties of a superhydrophobic surface on a magnesium alloy, fabricated by electrodeposition of the substrate in an ethanol solution containing cerium nitrate and myristic acid, were explored by potentiodynamic polarization and EIS.

However, using Cu²⁺ ions in the ethanolic stearic acid solution to modify aluminum alloy surfaces by electrochemical modification process has not yet been reported in the literature. In the present study, a superhydrophobic surface was fabricated on a chemically cleaned aluminum alloy substrate sample using a one-step process involving the electrochemical modification of the aluminum alloy substrates in different molar ratios of Cu/SA‡ ethanolic solutions containing stearic acid and copper nitrite salt under the application of DC voltages. The relationship between the molar ratio of Cu/SA in the electrolyte and the morphological, chemical, structural, and water-repellent properties of the films formed on the chemically cleaned aluminum alloy substrate is discussed. Detailed chemical analyses were performed to understand the formation of copper stearate and copper oxide during electrochemical modification process. Furthermore, the corrosion properties of these films were evaluated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS).

2. Experimental

2.1 Preparation of copper stearate films

One by two inch aluminum substrates (AA6061) were cleaned in a soap solution for 15 min and rinsed in distilled water and ethanol. Subsequently, they were etched in 1 mol L⁻¹ NaOH and 10 vol% HNO₃. Finally, the chemically cleaned aluminum alloy substrates were washed with water and ethanol and dried under atmospheric conditions. A pair of cleaned aluminum alloy plates (as the cathode and anode) was immersed in an ethanolic solution of stearic acid containing copper nitrite; 10 V DC was applied for 10 min. The electrodes were separated by a distance of 1.5 cm.

2.2 Characterization techniques

The morphological features of the modified cathodic aluminum surfaces were characterized using scanning electron microscopy (SEM, JEOL JSM-6480LV); elemental analysis was also carried out. X-ray diffraction (XRD) of the modified surfaces was carried out using a Bruker D8 Discover system. The chemical components of the modified surfaces were analyzed using Fourier transform infrared spectroscopy (FTIR, Perking Elmer Spectrum One). Images of the chemically cleaned aluminum substrate coated by electrochemical modification with molar ratio of Cu/SA (0.1, 0.5, and 1.6) in the solution were obtained using Clemex software (CLEMEX JS-2000, PE4.0) to calculate the width of the rose-petal, as well as the diameter and count density of the grape-like cluster. Both static and dynamic contact angles were measured using a First Ten Angstorm contact angle goniometer and a 10 μL deionized water drop. The roughness of the surface was examined using a MicroXAM-100 HR 3D surface profilometer. Electrochemical experiments were carried out using a PGZ100 potentiostat and a 300 cm³ EG&G PAR flat cell (London Scientific, London, ON, Canada); a three-electrode system with a corrosion cell kit composed of a Ag/AgCl reference electrode, platinum counter electrode, and the sample surface as the working electrode was utilized. Potentiodynamic polarization tests were carried out in a 3.0 wt% NaCl solution (natural pH 5.9) at room temperature. EIS experiments were carried out in the frequency range between 0.01 Hz and 100 kHz with a sine-wave amplitude of 10 mV.

3. Results and discussion

3.1 Superhydrophobic surfaces prepared by electrochemical modification

3.1.1 Surface morphology. The morphology of the chemically cleaned cathodic aluminum alloy electrode electrochemically modified in solutions with different molar ratio of Cu/SA, which contained copper nitrite and stearic acid, was evaluated using SEM (Fig. 1). The aluminum alloy electrode was modified using 10 V DC for 10 min. Fig. 1(a) and (b) show the cathode and anode, respectively, following etching with NaOH and HNO₃, followed by electrochemical modification in the ethanolic stearic acid solution. The morphological features of the anode and cathode were the same when the modification was carried out using only stearic acid. Several petal-like nanosheets were present on the chemically cleaned aluminum after modification with Cu/SA solution (molar ratio of 0.0267; Fig. 1(c) and (d)). When the molar ratio of Cu/SA was 0.1, the cathode was completely covered by slightly tilted standing rose petal-like nanosheets; the width of the nanosheets was about 15.9 nm, as shown in Fig. 1(e) and (f). However, when the molar ratio of Cu/SA was increased to 0.5, the morphology transitioned to standing petal-like nanosheets, which formed irregular pores on the modified surface (Fig. 1(g) and magnified SEM image Fig. 1(h)). The width of the nanosheets was increased to 280 nm. Moreover, a series of grape-like micro-sized clusters (Fig. 1(h)) assembled with several tiny nano-sized particles appeared on top of the rose petal-like structures, and formed a second layer. By means of image analysis, the average size of the grape-like cluster was about 1.03 μm and the number density was approximately 6.9 × 10⁵ count per cm². Upon further increase of the molar ratio of Cu/SA to 1.6, the rose petal-like structure and irregular pores disappeared from the aluminum substrate, as shown in Fig. 1(i) and (g). However, the number of grape-like, micro-sized clusters increased with increasing molar ratios of Cu/SA. The average size and number density increased to 1.18 μm and 1.8 × 10⁶ count per cm², respectively. Therefore, the molar ratio of Cu/SA in the solution affected the morphology of the aluminum cathode. Huang et al.¹⁹ obtained superhydrophobic films composed of copper stearate on an aluminum surface via electrodeposition of copper followed by electrochemical modification. The copper stearate formed nanofibers resembling micro sized flower-like particles, which covered the aluminum surface; this morphology was completely different from the morphology obtained in this study. In the present study, the anode and cathode modified by an ethanolic stearic acid solution under the influence of 10 V DC did not show any superhydrophobic properties. A similar phenomenon was reported by Huang et al.¹⁹ regarding aluminum surfaces modified under the same conditions. However, the cathode demonstrated roll-off properties when copper nitrite was added to the ethanolic stearic acid solution in this study.

Fig. 1 SEM images (left column) of electrochemically modified the chemically cleaned cathodic aluminum alloy film with the application of 10 V DC voltage for 10 min in Cu/SA solutions with molar ratio of (a) 0, (c) 0.0267, (e) 0.1, (g) 0.5, (i) 1.6. The insets of the images of water drop on the modified surfaces. The high magnified SEM images (right column) (b) chemically cleaned anode aluminum alloy film and (d), (f), (h), (j) corresponding to the left SEM images.

3.1.2 Surface composition. In order to confirm the chemical composition of the modified aluminum substrate, the modified surface was characterized by EDS, as shown in Fig. 2 as well as Table 1. In area 1, distinct peaks corresponding to carbon, oxygen, copper, and aluminum were observed; the atomic percentages were 79.75, 3.13, 1.69, and 15.42%, respectively. It can be deduced that the standing rose petal-like nanosheets composed of low surface energy copper stearate were fabricated on the chemically cleaned aluminum alloy surface. This result was in accord with the XRD results (Fig. 3(a) given below). However, the content of carbon, oxygen, copper, and aluminum atoms in the grape-like clusters in area 2 were completely different from those in area 1 (Fig. 2(b) and Table 1). The aluminum content dropped to 5.97 at%, while the oxygen and copper content sharply increased to 14.51 and 12.96 at%, respectively. The EDS results regarding the considerable amount of copper as compared to aluminum (Fig. 2(b)) were in accord with XRD (Fig. 3(b)) and FTIR results (Fig. 4), confirming the formation of copper oxide (CuO) on the modified aluminum surface. Apart from that, the presence of the element of carbon (the low surface energy methylated components) was apparent in the coating.

Fig. 2 EDS spectra of different areas on the electrochemically modified film prepared in a Cu/SA solution with molar ratio of 0.5 for 10 min: (a) the standing rose petal-like nanosheets structure; (b) grape-like clusters.

Table 1 EDS of the sample subjected to electrochemically modified under 10 V DC with a Cu/SA solution with molar ratio of 0.5, corresponding to Fig. 2

Number	Element (at%)
	C	O	Al	Cu
1	79.75	3.13	15.42	1.69
2	66.56	14.51	5.97	12.96

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Fig. 3 (a) Low angle XRD patterns of the superhydrophobic surface electrochemically modified on chemically cleaned aluminum and film prepared in solutions with different molar ratio of Cu/SA for 10 min; (b) high angle patterns of (a). The inset patterns of (b) show the range of angle from 30° to 38°.

Fig. 4 FTIR spectra of electrochemically modified films prepared in solutions with different molar ratio of Cu/SA for 10 min.

The cathode composition was analyzed using XRD. Fig. 3(a) shows the XRD patterns of the samples obtained from electrochemically modified in Cu/SA solutions with molar ratios of 0.1, 0.5, and 1.6 recorded at a low angle (2θ = 3–28°). The seven distinct peaks presented in Fig. 3(a) were related to the formation of copper stearate ((CH₃(CH₂)₁₆COO)₂Cu),³⁷ which derived from the reaction between copper ions and stearic acid (CH₃(CH₂)₁₆COOH) and deposited on the aluminum substrate under the application of 10 V DC. The intensity of the peaks at 3.71°, 5.57°, 7.44°, and 9.31° increased when the molar ratio of Cu/SA was increased to 0.1; subsequently, the intensity decreased when the molar ratio of Cu/SA was increased from 0.5 to 1.6. The intensity of the peaks at 20.78°, 21.31°, and 22.91° increased when the molar ratio of Cu/SA increased from 0 to 0.5. However, the intensity of the peaks decreased when the molar ratio of Cu/SA reached 1.6. Furthermore, the peaks corresponding to copper stearate in the XRD pattern confirmed that the low surface energy methylated (–CH₃ and –CH₂) component was present on the aluminum substrate. Fig. 3(b) shows the high angle XRD patterns of Fig. 2(a), which demonstrated the angle range from 2θ = 30° to 2θ = 56°. The peaks corresponding to Al(111) and Al(200) at 2θ = 38.78°and 44.69°, respectively, were attributed to the aluminum substrate.³⁸ The intensity of the peak at 2θ = 44.69° from the modified aluminum substrate was less than that of the chemically cleaned aluminum, due to the introduction of the superhydrophobic films. The inset in Fig. 3(b) shows the angle 2θ = 28–38° in the XRD patterns of Fig. 3(b). Two new peaks (2θ = 30.34°, 36.46°),³⁹ stemming from the formation of highly resistive copper oxide (CuO) on the modified cathode were observed when the molar ratio of Cu/SA was increased to 0.5. The peaks of CuO were shifted to higher angles compared to the JCPDS card ([00-080-0076]) due to the residual stress (tension stress) in lattice. The appearance of black grape-like clusters on the deposited films was also noted.⁴⁰ These observations were consistent with the EDS results.

Further analysis of the modified surface on the chemically cleaned aluminum cathodes was performed using FTIR, as shown in Fig. 4. The intense band centered at 2954 cm⁻¹ was assigned to the asymmetric stretching of –CH₃ (υ_CH), and the peaks at 2918 cm⁻¹ and 2848 cm⁻¹ were attributed to the symmetric and asymmetric stretching modes of the –CH₂ (υ_CH) group of copper stearate (Fig. 4(c)–(f)).¹⁶ As a result, the low surface energy components (–CH₃ and –CH₂) corresponding to methylated moieties on the modified aluminum surface was confirmed. The sharp peaks at 1589 cm⁻¹ and 1467 cm⁻¹ were assigned to symmetric and asymmetric –COO bands, respectively, of copper stearate.¹⁶ Moreover, the presence of the small peak at 1445 cm⁻¹ together with two very weak peaks at around 1421 cm⁻¹ and 1322 cm⁻¹, which may be related to scissoring (δ_CH) –CH₂, as well as asymmetric and symmetric bending (δ_CH) –CH₃, from long chain aliphatic groups^41,42 was noted. The δ_CH absorbance from long –(CH₂)_n– (n ≥ 4) chains at 720 cm⁻¹ was evident in all modified aluminum spectra (Fig. 4(c)–(f)).⁴¹ The intensity of these peaks, as mentioned above, was found to decrease with increasing molar ratios of Cu/SA from 0.1 to 1.6. A new peak at 843 cm⁻¹, which may be due to Cu–O–H vibrations⁴³ was also observed (Fig. 4(d)). Additionally, two new peaks with low intensities appeared at 619 cm⁻¹ and 512 cm⁻¹, due to the formation of CuO on the chemically cleaned aluminum cathode when the molar ratio of Cu/SA was 0.5 (Fig. 4(e)).⁴⁴ The presence of the CuO peak was in accord with the XRD (Fig. 3(b)) and EDS results, which revealed a larger amount of Cu as compared to Al (Fig. 2(b)). Furthermore, the intensity of this peak increased when the molar ratio of Cu/SA was 1.6, indicating that the number of grape-like micro-sized clusters increased with increasing molar ratios of Cu/SA, which was in accord with the SEM image shown in Fig. 1(i).

3.1.3 Surface wetting. Fig. 5(a) and (b) illustrate the variation in surface roughness and water contact angle (CA) as a function of Cu/SA molar ratio. It can be seen from Fig. 5 and Table 2 that the surface roughness and water contact angle of the chemically cleaned aluminum were 0.56 ± 0.05 μm and 54 ± 1°, respectively. The surface roughness of the aluminum substrate after modification with stearic acid at 10 V DC was 0.90 ± 0.08 μm and the contact angle on the surface was 119 ± 4°. The surface roughness and water contact angle increased with increasing molar ratios of Cu/SA from 0.0267 to 0.045. Specifically, the roughness and contact angle of the modified aluminum films increased from 1.36 ± 0.29 μm and 141 ± 1° to 2.49 ± 0.48 μm and 149 ± 4°, respectively. When the molar ratio of Cu/SA was 0.1, the surface roughness was 2.8 ± 0.26 μm and the contact angle was 159 ± 2°. This demonstrated that the superhydrophobicity on the surface was due to the formation of the standing rose petal-like nanosheets, which entrapped air and decreased the affinity of water towards the surface. For the sample prepared with a Cu/SA molar ratio of 0.267, the roughness and contact angle of the modified aluminum surface increased to 3.19 ± 0.39 μm and 160 ± 1°, respectively. Similarly, with a Cu/SA molar ratio of 0.4, the roughness and contact angle increased to 3.25 ± 0.25 μm and 161 ± 1°, respectively. When the molar ratio of Cu/SA was further increased to 0.5, the roughness and contact angle reached the highest values (3.61 ± 0.47 μm and 162 ± 1°), perhaps due to the existence of the standing rose petal-like nanosheets and grape-like micro-sized clusters, which facilitated water roll-off. On the other hand, when the molar ratio of Cu/SA was increased from 0.5 to 1.6, the contact angle decreased to 156 ± 4°, and the surface roughness decreased to 3.02 ± 0.25 μm. The decrease in the contact angle was due to the disappearance of the microporous films formed by the standing rose petal-like nanosheets (Fig. 1(i) and (j)), which decreased the entrapment of the air beneath the water droplets. Nonetheless, the grape-like micro-sized clusters enhanced the contact angle. Therefore, the modified aluminum surface retained the superhydrophobic properties when the aluminum substrate was electrochemically modified in the solution with a Cu/SA molar ratio of 1.6. Recently, Huang et al.¹⁵ demonstrated that superhydrophobic copper stearate films could be fabricated on the surface of anodic copper electrodes in a one-step process by immersing two copper plates in a stearic acid solution under DC voltage. The contact angle of the superhydrophobic surface was 153 ± 2°. Following Huang's work, Li et al.⁴⁵ reported that copper stearate suspensions produced by the reaction between copper acetate and stearic acid in ethanol could be sprayed on various substrates (copper aluminum, steel, glass) to fabricate superhydrophobic surfaces with a maximum contact angle of 160 ± 1°. However, a considerable amount of time was required to prepare the copper stearate suspension.

Fig. 5 (a) Surface roughness; (b) water contact angle and (c) contact angle hysteresis of films by electrochemical modification on the chemically cleaned aluminum substrate in the application of 10 V DC voltage in solutions with different molar ratio of Cu/SA for 10 min.

Table 2 Water contact angle and corresponding corrosion current as well as polarization resistance as a function of molar ratio of Cu/SA

The molar ratio of the Cu/SA at 10 V	Water contact angle (°)		Corrosion current density, Icorr (μA cm^−2)	Polarization resistance, Rp (kΩ cm^2)
	Before corrosion	After corrosion
Cleaned Al	54 ± 1	39 ± 2	1.98 ± 0.074	19.86 ± 3.46
0.0267	141 ± 1	96 ± 12	0.11 ± 0.005	399.51 ± 58.7
0.1	159 ± 2	155 ± 1	0.014 ± 0.005	1256.47 ± 316
0.5	162 ± 1	159 ± 1	0.011 ± 0.002	1328.31 ± 420

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Fig. 5(c) shows the contact angle hysteresis under the same conditions as those in Fig. 5(a) and (b). Contact angle hysteresis is the difference between the advancing and receding angles during a relative movement of the droplet; it dropped from 3.88 ± 0.6° on the modified surface obtained with a 0.1 molar ratio of Cu/SA to 1.52 ± 1.3° on the modified surface obtained with 0.5 molar ratio of Cu/SA. In contrast, there was an increase in the contact angle hysteresis from 1.52 ± 1.3° to 5.37 ± 5.7° when the molar ratio of Cu/SA was increased from 0.5 to 1.6. The lower the contact angle hysteresis is, the easier droplets roll off the surface. Therefore, the chemically cleaned aluminum surface electrochemically modified with a Cu/SA molar ratio of 0.5 exhibited the best water roll-off properties.

3.2 Corrosion resistance properties of superhydrophobic surfaces

3.2.1 Analysis of potentiodynamic polarization curves. Fig. 6(a) shows the potentiodynamic polarization curves of the chemically cleaned aluminum hydrophobic film obtained from the solution with a 0.0267 molar ratio of Cu/SA, and superhydrophobic films from solutions with 0.1 and 0.5 molar ratios of Cu/SA. The polarization curves were utilized to determine the corrosion current density (I_corr) and polarization resistance (R_p), which are presented in Fig. 6(b) and (c), respectively. The I_corr was calculated by extrapolation of the cathodic curves within 50 mV around the corrosion potential (E_corr);⁴⁶ R_p was calculated using the Stern–Geary equation,¹⁶ shown in eqn (1):where β_a and β_c are the anodic and cathodic Tafel slopes, respectively.

Fig. 6 (a) Potentiodynamic polarization curves of the chemically cleaned aluminum, hydrophobic film obtained from a Cu/SA solution with molar ratio of 0.0267, superhydrophobic films from Cu/SA solutions with molar ratio of 0.1 and 0.5, variation of (b) the polarization resistant (R_p) and (c) corrosion current density (I_corr) as a function of the molar ratio of the Cu/SA solution in 3.0 wt% NaCl aqueous solution.

It can be clearly seen from Fig. 6(b) and (c), and Table 2 that the I_corr of the chemically cleaned aluminum was 1.98 ± 0.074 μA cm⁻² with a very low R_p of 19.86 ± 3.46 kΩ cm² and the contact angle was decreased to 39 ± 2° after corrosion. However, the hydrophobic film with a water contact angle of 141 ± 1°, prepared using a Cu/SA molar ratio of 0.0267, exhibited a I_corr of 0.11 ± 0.005 μA cm⁻² and R_p of 399.51 ± 58.7 kΩ cm². The contact angle of the hydrophobic films decreased to 96 ± 12° after corrosion. This film exhibited higher corrosion due to the presence of copper stearate on the chemically cleaned aluminum substrate.¹⁶

With further increase of the molar ratio of Cu/SA to 0.1 and 0.5, the I_corr values of the superhydrophobic films decreased to 0.014 ± 0.005 μA cm⁻² and 0.011 ± 0.002 μA cm⁻², and the R_p values increased to 1256.47 ± 316 kΩ cm² and 1328.31 ± 420 kΩ cm², respectively. The contact angles of these superhydrophobic surfaces were similar before and after corrosion. The contact angles of these two superhydrophobic decreased to 155 ± 1° and 159 ± 1° after corrosion, respectively. Polarization resistance is used to estimate the anti-corrosion performance of various materials. Larger polarization resistance results in better protection against corrosion. In Fig. 6(a), the cathodic current of the superhydrophobic surface obtained from the solution with a Cu/SA molar ratio of 0.5 was reduced by about two orders of magnitude as compared to that of the chemically cleaned aluminum. On the other hand, the anodic current of the surface decreased by one order of magnitude, and it dropped significantly at about −0.5 V because the superhydrophobic film was broken. The reduced current was attributed to air trapped on the superhydrophobic surface, acting as a capacitor, which prevented the contact between the chemical ions (Na⁺ and Cl⁻) and aluminum substrate thus restricting the electron transfer between the electrolyte and aluminum substrate.⁴⁷ This reduction was also due to a restricted supply of oxygen.³⁰ The I_corr values of the superhydrophobic surfaces were much lower than that of the chemically cleaned aluminum, and the R_p values of the superhydrophobic surfaces were much greater than that of the chemically cleaned aluminum. The enhanced polarization resistance was attributed to the superhydrophobic properties as well as an increased the higher thickness of the copper stearate films,¹⁶ the formation of highly resistive copper oxide (CuO),⁴⁸ as evidenced by the increase in the XRD peak height of copper stearate (Fig. 3(a)), the appearance of copper oxide peaks (Fig. 3(b)), as well as the increased FTIR peak intensity of low surface energy components (–CH₃, –CH₂) from copper stearate (Fig. 4). In a study conducted by Liu et al.,⁴⁹ a micro-nano flower-like superhydrophobic coating based on cerium myristate was fabricated by a one-step electrodeposition in a mixture of myristic acid and cerium chloride at 50 V for 10 min on a brass substrate. They reported that the current density of the superhydrophobic surface was 6.94 ± 0.347 μA cm⁻². However, they did not provide the polarization resistance, which is required to analyse the corrosion properties. The R_p of the superhydrophobic cerium myristate coating was calculated to be 7.81 kΩ cm²,³⁰ which was 170 times less than that of our superhydrophobic copper stearate surface. Huang et al.¹⁶ also used a one-step electrochemical modification procedure to produce copper stearate-based superhydrophobic films on a copper substrate in an ethanolic stearic acid solution. The copper stearate-based superhydrophobic film exhibited a current density of 0.01 μA cm⁻² and a polarization resistance of 1220 kΩ cm² in a 3.5 wt% NaCl aqueous solution. In the present study, as 3.0 wt% NaCl was less reactive, the maximum polarization resistance of the superhydrophobic copper stearate surface was 1328 kΩ cm², which was slightly higher than that reported in Huang's study.

As can be seen from the potentiodynamic polarization curves, the corrosion potential shifted towards negative values with increasing molar ratios of Cu/SA. Similarly, a few groups, e.g., Huang et al.¹⁶ and Momen et al.,⁵⁰ demonstrated that the corrosion potential of superhydrophobic surfaces was more negative than that of as-received substrates. Nonetheless, in most previous publications, the corrosion potential of superhydrophobic surfaces exhibited the opposite behavior of as-received aluminum substrates.^47,51,52 In our case, this phenomenon may be due to the formation of a new composition on the aluminum surface such as copper stearate. The superhydrophobic copper stearate surface electrochemically modified on aluminum substrates, as a physical barrier to retard electrolyte penetration, prevented the oxidation of the aluminum substrates in corrosive environments.

In order to obtain the exact polarization resistance, a second method was used to calculate R_p using Ohm's law (slope of the linear potential–current (E–I) curves),³⁰ as shown in eqn (2):

where E and I are potential (±10 mV variation around the E_corr) and current, respectively.

The linear E–I of the films obtained with solutions with Cu/SA molar ratios of 0, 0.0267, 0.1, and 0.5 are shown in Fig. 7(a). The slope of the linear E–I of the superhydrophobic copper stearate surface was greater than that of the chemically cleaned aluminum. Specifically, the values of Ohm's R_p were 18.53 kΩ cm², 455.38 kΩ cm², 1340.99 kΩ cm², and 1316.95 kΩ cm², respectively. Fig. 7(b) shows the R_p calculated using the Stern–Geary equation (R_p1) versus the R_p calculated using Ohm's law (R_p2). An acceptable correlation between the values was obtained (the value of the slope of the fit was about 1). The R_p values calculated using the Stern–Geary equation were used here and are shown in Table 2.

Fig. 7 (a) The potential–current curves of the surfaces for the R_p calculated by Ohm's law; (b) the value of R_p calculated by Stern–Geary equation (R_p1) versus the value of R_p calculated by Ohm's law (R_p2) obtaining the fitting line.

3.2.2 Analysis of surface morphology and chemical composition after corrosion. It was significant to investigate the reason behind the modified aluminum alloy substrate maintaining superhydrophobic properties in the corrosive NaCl solution (3.0 wt%). Changes in the morphology and chemical composition of the chemically cleaned aluminum surface, hydrophobic surface obtained from the solution with a Cu/SA molar ratio of 0.0267, and superhydrophobic surfaces obtained from the solution with a Cu/SA molar ratio of 0.5 were analyzed using SEM and FTIR after the corrosion tests (Fig. 8).

Fig. 8 SEM images of (a and b) chemically cleaned Al surfaces before and after corrosion, (c and d) the surface obtained from Cu/SA solutions with molar ratio of 0.0267 and 0.5 after corrosion, respectively. The inset images show the water drops on the respective surfaces. The (e and f) shows the FTIR spectra of the aluminum coating from 0.0267 and 0.5 Cu/SA before and after corrosion, respectively.

Newly formed corrosion pits are marked by black arrows as shown in Fig. 8(b). The insets in Fig. 8(a) and (b) reveal that the contact angle on the chemically cleaned aluminum surface was reduced from 54 ± 1° to 39 ± 2° after the corrosion test. As shown in Fig. 8(c), the hydrophobic surface also demonstrated corrosion pits after corrosion, which were similar to those on the chemically cleaned aluminum substrate. Nonetheless, the number of corrosion pits on the hydrophobic surface was much lower and the diameter of the pits was smaller than those on the chemically cleaned aluminum substrate. Additionally, the contact angle of the hydrophobic surface decreased from 141 ± 1° to 96 ± 12° after corrosion, as shown in the inset images of Fig. 1(c) and 8(c). The sharp decrease in the contact angle may be attributed to the loss of water-repellent properties on the hydrophobic aluminum surface. Fig. 8(f) shows the FTIR spectrum of the hydrophobic surface before and after corrosion. The intensities of the –CH₃, –CH₂, and –COO bands decreased after corrosion, which was in accord with the decreased contact angle of this surface following corrosion.

However, unlike the hydrophobic aluminum surface, there was no extensive variation in the intensities of the –CH₃, –CH₂, and –COO bands from the superhydrophobic copper stearate aluminum surface before and after corrosion (Fig. 8(f)). Furthermore, the morphology of the superhydrophobic surface stayed similar before and after corrosion, as shown in of Fig. 1(g) and 8(d). In contrast, a minor change in morphology in the high magnified SEM inset image of Fig. 8(d1) can be observed that several of the grape-like micro-sized clusters disappeared from the superhydrophobic copper stearate aluminum surface following corrosion (marked with a circle) (Fig. 1(h)). In Fig. 1(g) and 8(d), the inset images of the water droplet on the superhydrophobic surface indicated the wetting properties were maintained both before and after corrosion. These results were in accord with the polarization curves, revealing that the electrochemically modified aluminum surfaces exhibited superior anti-corrosion properties as compared to the chemically cleaned aluminum; this property was enhanced on the superhydrophobic copper stearate aluminum surface as compared to the hydrophobic aluminum surface. In most previous reports, the properties of superhydrophobic surfaces were reported before corrosion.^53–55 However, in our reports,^16,30,56 variations in the morphology, composition, and wettability of superhydrophobic surfaces after corrosion were noted. Specifically, Huang et al.¹⁶ reported the corrosion resistance properties of superhydrophobic copper stearate films on a copper substrate, and demonstrated that water drops easily rolled off these surfaces following corrosion. In addition, the SEM images of these surfaces after corrosion revealed a small change in the morphological features, and the number of flower-like micro-nano structures decreased slightly.

3.2.3 Electrochemical impedance spectroscope (EIS). Apart from the polarization curves, electrochemical impedance spectroscopy (EIS) was also utilized to investigate the electrochemical degradation of metals and their superhydrophobic films. The EIS data of the chemically cleaned aluminum and superhydrophobic surface derived from 0.5 molar ratios Cu/SA solution after 15 h of exposure in a 3.0 wt% sterile NaCl solution. Fig. 9 illustrated the Nyquist and Bode plots of the fitted data from the EIS data analyzed by ZView2 program based on the equivalent electrical circuit. Table 3 summarized the fitted impedance parameters (both the original and fitting EIS plots in the ESI† document).

Fig. 9 (a) Nyquist plots, (b) Bode modulus plots as well as (c) Bode phase angle plots for chemically cleaned aluminum alloy substrate and superhydrophobic surface aluminum alloy substrate. The inset image in (a) demonstrated the magnitude of the high frequency range of the plots. (d) Equivalent circuits of the studied system (d1) chemically cleaned aluminum alloy substrate (d2) superhydrophobic surface.

Table 3 The electrochemical model of impedance parameters from EIS simulations

Sample	Rs (Ω cm^2)	Cf (μF cm^−2)	Rf (Ω cm^2)	n1	Cdl (μF cm^−2)	Rct (Ω cm^2)	n2	Cw (μF cm^−2)	Rw (Ω cm^2)	n3
Chemically cleaned Al	6.87				19.94	1.56 × 10^3	1	3.72	3.69 × 10^3	0.49
Superhydrophobic surface	225.6	0.041	2.56 × 10^4	0.78	0.053	1.13 × 10^6	0.77	95.53	3.19 × 10^6	0.61

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The inset in Fig. 9(a) shows that the Nyquist plot of the chemically cleaned aluminum consisting of a small semi-circle at high frequency and a straight line at low frequency. Furthermore, only one time constant was found in the Bode phase plot of the chemically cleaned aluminum, due to appearance of one peak as shown in Fig. 9(c). On the other hand, two semi-circles and a straight line at low frequency were observed in the Nyquist plot of the superhydrophobic surface. The presence of two time constants was observed in the Bode phase plot shown in Fig. 9(c). The very high frequency time constant, at around 2 × 10⁴ Hz, was associated with the capacitance of the superhydrophobic copper stearate film on the aluminum substrate. The medium frequency, around 2 × 10¹ Hz, was assigned to the capacitance of the double layer. Moreover, the time constant of the superhydrophobic surface moved to a lower frequency as compared to that of the chemically cleaned aluminum near the frequency of 7 × 10¹ Hz, due to the good barrier properties of superhydrophobic film, which inhibited the penetration of Na⁺ and Cl⁻ through the pores in the films to the sub-aluminum substrate^57,58 (Fig. 9(c)). Additionally, the line in the low frequency range was related to the Warburg diffusion of the ions in the NaCl solution, which represented the electrochemical degradation governed mainly by ionic diffusion.

Fig. 9(b) shows the Bode plot in terms of the modulus of impendence (|Z|) versus frequency for the chemically cleaned aluminum (bottom part) and superhydrophobic surface (top part). The EIS plot of the chemically cleaned aluminum alloy substrate was obtained only within a narrow frequency range, because this plot was messy and illogical at low frequencies.⁵⁹ The |Z| value of the superhydrophobic surface was shifted up by nearly three orders of magnitude as compared to that of the chemically cleaned aluminum (Fig. 9(b)). Therefore, the superhydrophobic surface possessed greater anti-corrosion properties than the chemically cleaned aluminum. Liu et al.²⁹ reported that the corrosion resistance of graphene coated aluminum was an order of magnitude larger than that of an uncoated aluminum substrate. Similarly, in a study by Huang et al.,³⁰ the corrosion resistance of the 24 min-passivated NaOH-etched aluminum alloy substrate was two orders of magnitude greater at low frequencies than an as-received aluminum alloy substrate. It is well known that higher |Z| values at lower frequencies lead to better anti-corrosion properties of metal substrates.⁶⁰ Therefore, the corrosion resistance of the superhydrophobic copper stearate aluminum substrate was much better than that of the graphene coated aluminum substrate and the 24 min-passivated NaOH-etched aluminum alloy substrate.

In order to accurately analyze the EIS results, two equivalent electrical circuit models are presented in Fig. 9 (d) and the corresponding fitted electrochemical parameters are shown in Table 3. As shown in Fig. 9(d1), the equivalent circuit model, representing the electrochemical behavior of the chemically cleaned aluminum alloy substrate, demonstrated one time constant. In this circuit, R_s is the solution resistance; R_ct (1.56 × 10³ Ω cm²) can be interpreted as the charge transfer resistance of the double layer fabricated at the interface between the aluminum surface and the salt solution. The capacitance elements (C) in the electrical equivalent circuits employed are all replaced with the constant phase elements (CPE). (CPE_dl, C_dl) represents the double layer capacitance. The impedance of CPE is defined by the following equation:⁵⁹

where Y_o is the frequency-independent real constant, ω is angular frequency, j is imaginary unit and n is the CPE exponent (0 ≤ n ≤ 1).

For chemically cleaned aluminum alloy substrate, the CPE_dl was pure capacitance due to an n₂ value equal to 1. The C_dl value was 19.94 μF cm⁻². W (the diffusional impedance) with an n₃ value close to 0.5 represents the Warburg diffusion, which was mainly controlled by ionic diffusion. The corrosion reaction occurred on the surface of the chemically cleaned aluminum alloy substrate. For the superhydrophobic coating on the aluminum alloy substrate, the equivalent circuit displayed two time constants according to the impedance spectrum presented in Fig. 9(d2). Specifically, R_f (2.56 × 10⁴ Ω cm²) was related to the resistance of the superhydrophobic film, which was controlled by the pore dimensions. The constant phase element (CPE_f, C_f) was attributed to the capacitance of the superhydrophobic film (which was associated with the film thickness; detect structure etc.³²). The C_f value was 0.041 μF cm⁻². The semi-circle (the capacitance of the superhydrophobic film) was compressed, which resulted from an increased roughness as well as inhomogeneities of the electrode surface. In addition, the diffusion impedance (W) with an n₃ value of 0.61 was obtained. The n₃ value was higher than 0.5, due to either the superhydrophobic surface or the formation of passivation film which restrained ion diffusion. It can be seen from Table 3 that the R_ct value of the superhydrophobic surface was approximately 1.13 × 10⁶ Ω cm². The constant phase element (CPE_dl, C_dl) with an n₂ value close to 1.0 characterized the double layer capacitor with slight porosity.³² The C_dl value was 0.053 μF cm⁻². It is noted from both the EIS data and the equivalent circuit model for the superhydrophobic surface that the corrosive solution containing chemical ions (Na⁺ and Cl⁻) reached the sub-aluminum alloy substrate though the micro-pores on the superhydrophobic films after immersion in the salt solution for 15 h. These results were in agreement with the potentiodynamic polarization curves, which revealed that the anodic current of the surface dropped significantly at about −0.5 V due to the broken superhydrophobic surface (Fig. 9(a)). This result was comparable to those presented by Yin et al.,³³ where two time constants were observed on the equivalent circuit for where two time constants were observed on the equivalent circuit for superhydrophobic aluminum surfaces obtained from anodization followed by myristic acid modification. Based on the equivalent circuit, the electrochemical model impedance parameters for the superhydrophobic surface included a R_ct value of 3.376 × 10⁵ Ω cm² and a C_dl value of 0.078 μF cm⁻². Therefore, our superhydrophobic surface exhibited greater anti-corrosion properties than that reported by Yin et al.³³ As compared to the chemically cleaned aluminum, the higher R_ct and lower C_dl indicated superior anti-corrosion properties of the superhydrophobic cooper stearate aluminum surface. The anti-corrosion properties significantly depended on the air trapped in the micro–nano structures, which prohibited the salt solution with chemical ions (Na⁺ and Cl⁻) from penetrating the film and corroding the sub-aluminum alloy substrate.^27,61

4. Conclusions

A simple one-step electrochemical modification process was utilized to fabricate a superhydrophobic surface on an aluminum alloy substrate using ethanolic stearic acid containing copper nitrite and applying a DC voltage. The surface morphology, roughness, and wetting properties varied according to the molar ratio of Cu/SA. The optimum surface was comprised of a rose petal-like microstructure made of copper stearate as well as grape-like micro-sized clusters of copper oxide, as noted by SEM, FTIR, and XRD, which demonstrated that the chemically cleaned aluminum substrate was transformed into a superhydrophobic surface. The corresponding roughness and contact angle of the surface was 3.61 μm and 162°, respectively. This surface displayed a polarization resistance of 1328 kΩ cm², which was 66 times greater than that of the chemically cleaned aluminum alloy substrate. Additionally, EIS revealed an increase in the charge transfer resistance of the chemically cleaned aluminum alloy substrate from 1.56 kΩ cm² to 1130 kΩ cm² for the superhydrophobic surface. This implies that the superhydrophobic copper stearate-coated aluminum alloy has better anti-corrosion properties than the chemically cleaned aluminum alloy substrate in corrosive environments.

Acknowledgements

The author would like to thank the China Scholarship Council (CSC) for the financial support provided over the course of this study covering expenses in Canada. We also acknowledge the financial support on the research expenses provided by the Natural Science and Engineering Research Council of Canada (NSERC).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra01944g

‡ Cu/SA means molar ratio of Cu²⁺ : molar ratio of SA.

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