A novel combination approach for the preparation of superhydrophobic surface on copper and the consequent corrosion resistance

Corrosion Science 110 (2016) 105–113

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A novel combination approach for the preparation of superhydrophobic surface on copper and the consequent corrosion resistance Wei Liu, Qunjie Xu ∗ , Jie Han, Xiaohang Chen, Yulin Min Shanghai Key Laboratory of Materials Protection and Advanced Materials in Electric Power, Shanghai Engineering Research Center of Energy-Saving in Heat Exchange Systems, Shanghai University of Electric Power, Shanghai 200090, China

a r t i c l e

i n f o

Article history: Received 10 November 2015 Received in revised form 28 March 2016 Accepted 12 April 2016 Available online 13 April 2016 Keywords: A. copper B. EIS B. SEM B. XPS C. corrosion fatigue

a b s t r a c t In this paper, superhydrophobic surfaces on copper substrates are prepared by combining the etching and calcination treatment processes. The surface morphologies, wettability, chemical composition and corrosion resistance are characterized by means of scanning electron microscope (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), water contact angle, potentiodynamic polarization and electrochemical impedance spectroscopy techniques. The achieved superhydrophobic surface has a contact angle as high as 157.6◦ and a persistent corrosion resistance in a 3.5 wt.% NaCl aqueous solution. This method could provide an effective route to fabricate superhydrophobic surface with inhibitive and self-cleaning properties for various applications. © 2016 Elsevier Ltd. All rights reserved.

1. Introduction Copper (Cu), due to its good electrical and thermal conductivities, and high mechanical properties [1,2], has been widely used in industrial, military, life and other areas, such as condenser and heat exchanger on the ship, various instruments of the elastic element, tubing, pump, medical equipment, optical instruments, decorative materials, metal artwork and a variety of household appliances [3–5]. However, hydroxides and fluorides could be easily formed on copper in air, some non-oxidizing acid (hydrochloric acid, sulfuric acid, etc.), salt solutions and various organic acids [6]. Thus, the corrosion problems of copper have attracted great attention [7–9]. Superhydrophobic film is very effective to improve the corrosion resistance of copper [10]. The film could increase the copper surface contact angle with water, and prevent the corrosive medium from contacting with the copper surface. In nature, there are many different types of organisms with the special surface wettability [11–13]. For example, lotus leaves exhibit high superhydrophobic and self-cleaning function, which is known as the “lotus effect”. The Micro-nano papillae structure on the surface is responsible for the superhydrophobic proper-

∗ Corresponding author. E-mail address: [email protected] (Q. Xu). http://dx.doi.org/10.1016/j.corsci.2016.04.015 0010-938X/© 2016 Elsevier Ltd. All rights reserved.

ties of lotus leaves [14]. It is expected that special hierarchical micro/nano structures are required to fabricate superhydrophobic surfaces [15,16]. To further improve the quality of superhydrophobic surfaces, low surface energy material modifications to the rough surface may be essential [17]. For the past decade, a lot of methods including template [18], chemical etching [19], oxidation [20], electrodeposition [21,22], and sol-gel methods [23,24], have been used to produce special hierarchical micro/nano structures on metal substrates. For example, Yuan et al. [18] fabricated a regular multi-scale hierarchical structure on a copper foil by combing the template and etching approaching using the back surface of fresh bamboo leaf as the original template. The further treatment by stearic acid resulted in the superhydrophobic property with the water contact angle of 160.0◦ . The corrosion resistance of this sample, however, has not been tested. In this work, a facile and low-cost method is developed to fabricate superhydrophobic surfaces on the copper substrate. It involves chemical etching in an ammonia solution and consequent calcination in air at 340 ◦ C. The surface is then further modified in an ethanol solution containing stearic acid [25]. This superhydrophobic surface shows high corrosion resistance in a 3.5 wt.% NaCl solution.

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Fig. 1. SEM images of bare copper (a) and etched copper with 10 wt.% ammonia solution (b).

Table 1 Polarization parameters of bare copper, etched copper with 10 wt.% ammonia solution for 20 h, calcined treatment copper at 340 ◦ C for 10 min in air and superhydrophobic copper modified with an ethanol solution of stearic acid for 3 h in 3.5 wt.% NaCl solution. Samples

Ecorr (mV vs. SCE)

ˇa (mV dec−1 )

icorr (A cm−2 )

p (%)

Bare Etched Calcined Superhydrophobic

−352 −287 −277 −238

104.28 58.2 93.3 52.73

4.318 × 10−6 6.981 × 10−7 1.047 × 10−7 2.354 × 10−8

\ 83.83 97.57 99.45

2. Experiment

trochemical workstation (CHI 660E, CH Instruments Inc.). The workstation was equipped with a standard three-electrode system: the Pt electrode was used as the counter electrode, and the calomel electrode (SCE) and the copper sample were used as the reference electrode and working electrode, respectively. Before electrochemical experiments, these copper samples with an area of 1 cm2 were immersed in the NaCl solution for 1000 s to achieve a stable open circuit potential (OCP vs. Ag/AgCl). The potentiodynamic polarization curves were measured between −0.15 V and 0.15 V (vs. OCP) with the scanning rate of 1 mV/s. The electrochemical impedance spectroscopy (EIS) measurements were conducted in the frequency range of 100 kHz–0.05 Hz at the OCP with the amplitude of voltage of 5 mV.

2.1. Materials Cu (≥99.5 wt.%) foils were purchased from Xiangwei Machinery Co., LTD., YangZhou, China. All chemical reagents were obtained from Sinopharm Chemical Reagent Co., Ltd. and used as received. 2.2. Specimen preparation Copper samples (50 mm × 10 mm × 3 mm) and electrodes (10 mm × 10 mm) were abraded with different grades of emery paper (1#, 3#, 6#), cleaned ultrasonically with alcohol and deionized water respectively, and dried via Blast Oven. The dried copper and copper electrode were etched with a 50 mL ammonia solution (10 wt.%) at room temperature for 20 h, and then the samples and electrodes were washed with alcohol and deionized water, and dried again. After that, these samples and electrodes were calcined in air at 340 ◦ C for 10 min. 2.3. Superhydrophobic surface preparation The copper surfaces which had been etched and calcined were modified in a 0.1 mol/L ethanol solution of stearic acid at room temperature for 3 h. The obtained superhydrophobic copper samples were washed with alcohol and deionized water, and then dried for further characterization. 2.4. Characterization The surface morphologies and chemical compositions of the samples were investigated with a scanning electron microscopy (SEM, SU-1500, Hitachi, Japan), and an X-ray diffraction (XRD, Cu K␣ radiation, Bruker, D8 Advance, Germany). The contact angle (CA) was measured by K100-MK2 Almighty Tension Meter (KRUSS Germany), and the shape of water drops dripping on sample surface was tested with a JC 2000C1 CA system at ambient temperature. The electrochemical corrosion behavior was conducted in a 3.5 wt.% NaCl aqueous solution at room temperature via an elec-

3. Results and discussion 3.1. Surface morphology The geometrical characteristics of the surfaces before and after etching, calcining and modification were investigated by SEM images. Fig. 1a shows the morphology of Cu surface without any treatment. It is very smooth with the CA of 76.5◦ . After the etching treatment (20 h in 10 wt.% ammonia solution), there are irregular shaped cell-like projections with the depth of several micrometers in the surface. This surface shows a very small CA of 21◦ (Fig. 1b). The Cu atoms on the surface may be dissolved in the ammonia solution via reactions (1) and (2): 4Cu + 8NH3 + O2 + 2H2 O = 4 [Cu(NH3 )2 ] OH

(1)

2Cu + 8NH3 + O2 + 2H2 O = 2 [Cu(NH3 )4 ] (OH)2

(2)

After the calcining treatment, a more rough structure can be formed on the substrate. Vertically oriented peak-like projections appears to be the results of the etching process (Fig. 2a). These peaklike projections show nearly uniform size with approximate 600 nanometers in thickness and two micrometers in width (Fig. 2b). The Cu ion diffusion through the grain boundaries results in more irregular hump of Cu layers in the role of high-temperature calcination stress in the copper film, which could also support the compressive stress relaxation mechanism. Once the surfaces were modified in an ethanol solution of stearic acid for 3 h, catkin-like structure with a size of several micrometers in the surface can be observed on the copper surface (Fig. 3a). In a magnified image (Fig. 3b), catkin-like structure with about 100 nanometers in thickness and 30 micrometers in width can be cleared observed. This unique surface showed a CA of 157.6◦ . It is the kind of copper carboxylate [23,24], which copper oxides on the etched surface could rapidly be reacted with stearic acid. Since the modified reaction belongs to the Metathesis reaction [25], the mod-

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Fig. 2. SEM images of copper after calcination treatment in air at 340 ◦ C for 10 min: (a) low magnification; (b) high magnification.

Fig. 3. SEM images of superhydropobic copper surface modified in an ethanol solution of stearic acid for 3 h: (a) low magnification; (b) high magnification.

ification time could be decreased relatively. It can only spend 3 h to form micro-nano catkin-like structure.

934 eV) is not detected [28,29]. These results indicate that film on Cu essentially consists of Cu(II) coordinated with stearic acid. 3.3. Surface wettability

3.2. Chemical composition XRD analysis is carried out to study the composition and crystal structure on the copper substrate. Fig. 4 shows the XRD patterns of the bare, etched and etched-calcined treatment copper substrates respectively. The sharp peaks located at 2␪ = 74.18◦ is assigned to the (2 2 0) diffraction of copper. The peak located at 2␪ = 50.52◦ and 43.37◦ are attributed to (2 0 0) and (1 1 1) planes of copper, respectively. The results confirm that the structures of copper substrate before and after etching maintain constant without formation of Cu2 O. Interestingly, the height of PWH of intensity of (1 1 1) facets of the bare copper are stronger than the etched copper and the height of PWH of intensity of (2 2 0) facets of the etched copper is very smaller than that of the pure copper, which means that the copper basal surface is exposed with the oriented (1 1 0) and (2 2 0). The (1 1 1) diffraction of Cu2 O appears in the XRD pattern of etched-calcined copper. It indicates that the micro-nano peak-like structures are mainly Cu2 O crystals, consistent with SEM images. In order to investigate the nature of the superhydrophobic film, the surface composition was characterized by the X-ray photoelectron spectroscopy (XPS). Fig. 5a shows the XPS survey spectrum of the as-prepared superhydrophobic copper surface (etching + calcination + stearic acid treatment). Oxygen, carbon, and copper are the main compositions of the film. The existence of carbon and oxygen suggests that stearic acid exists in the film. From the Cu 2p spectrum of filmed copper (Fig. 5b), the main Cu 2p3/2 and 2p1/2 can be observed at the binding energies around 934.6 and 954.6 eV, respectively. It indicates that Cu(II) is the main valence state of Cu surface [26,27]. CuO is absent because its characteristic feature (Cu 2p3/2 at binding energy ranging from 933.6 eV to

To understand the effect of etching time on surface wettability, contact angles of as-prepared surfaces were measured, as shown in Fig. 6. As for the bare copper substrate, the contact angle is only 76.5◦ . As the etching time increased from 5 h to 25 h, the corresponding contact angle values can be varied to 132.7◦ , 148.2◦ , 150.3◦ , 157.6◦ and 152.4◦ , respectively. The contact angle of the copper surface could be reached to maximum value after 20 h of etching. Fig. 7 shows the schematic of a water droplet on the superhydrophobic copper surface. According to the Cassie model [30], the droplet would be in contact with the surface of the lotus state. The gaps filling with air could prevent water drops from wetting the surface. As a result, the water droplet is suspended on protrusions of the surface without direct contact with the solid Cu substrate. According to the Cassie-Baxter equation [31]: cosc = f1 cos1 + f2 cos2

(3)

where f1 and f2 are the fractions of solid and air of the solid/water interface,  c (157.6◦ ) and  1 (76.5◦ ) are the contact angles on the superhydrophobic and bare copper substrate, respectively.  2 (180◦ ) is the intrinsic contact angle of the solid surface and the air cushion. Given that f1 + f2 = 1, the corresponding f1 is calculated to be 0.0611. The low value of f1 suggests that only about 6.11% of areas of the water drop could be contacted with the superhydrophobic film and the rest (93.89%) is contacted with the air cushion. These results further confirm that the superhydrophobicity of the surface results from the formation of unique microstructures. Fig. 8 presents the water droplets on the copper substrate covered with superhydrophobic film with 50 mL ammonia solution

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(111)

Cu O 2

etched&calcined

Intensity

etched

(111)

(200) (220)

bare

20

25

30

35

40

45

50

55

60

65

70

75

80

85

90

2theta(degree ) Fig. 4. XRD patterns of bare copper surface, etched copper surface with 10 wt.% ammonia solution and etched-calcined treatment copper surface after calcination treatment at 340 ◦ C for 10 min in air.

Fig. 5. XPS spectra of superhydrophobic copper modified in an ethanol solution of stearic acid for 3 h: (a) Survey; (b) Cu 2p spectra.

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Fig. 6. Contact angle of copper hydrophobic surface at different etching time.

Potential (V,vs Ag/AgCl )

-0.20 4 -0.25

3

2

-0.30

1 2 3 4

-0.35

-0.40 0

Fig. 7. Schematic showing a water droplet on the prepared superhydrophobic copper surface.

1

bare etched calcined superhydrophobic 200

400

600

Time (s)

800

1000

Fig. 9. The open circuit potential versus time of bare copper, etched copper with 10 wt.% ammonia solution for 20 h, calcined treatment copper at 340 ◦ C for 10 min in air and superhydrophobic copper modified with an ethanol solution of stearic acid for 3 h in 3.5 wt.% NaCl solution.

3.4. Corrosion resistance

Fig. 8. Photograph of the water droplets on the superhydrophobic copper surfaces modified in a 0.1 mol/L ethanol solution of stearic acid for 3 h.

(10 wt.%) etched at room temperature for about 20 h and calcination treatment in air at 340 ◦ C for 10 min. It is obviously that the combination approach can get the high superhydrophobic surface with large contact angle on copper substrate.

3.4.1. Open circuit potential Fig. 9 shows the open circuit potential (OCP vs. Ag/AgCl) versus time of bare copper, etched copper with 10 wt.% ammonia solution for 20 h, calcined treatment copper at 340 ◦ C for 10 min in air and superhydrophobic copper modified with an ethanol solution of stearic acid for 3 h in 3.5 wt.% NaCl solution. OCP of bare copper is stable at −352 mV vs. Ag/AgCl. Meanwhile, OCP of superhydrophobic copper is approximately 114 mV more positive than that of bare copper. This finding implies that superhydrophobic film in solution can reduce susceptibility of underlying copper to corrosion. The potentiodynamic polarization and EIS plots were measured at the corresponding open circuit potential. 3.4.2. Potentiodynamic polarization measurements To investigate the corrosion resistant property of the superhydrophobic surface, potentiodynamic polarization measurements were conducted on various surfaces between −1.5 V and 0.15 V (vs.

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Compared to the bare copper substrate, the corrosion potential and current density of the superhydrophobic surface changed from −352 mV vs. Ag/AgCl and 4.318 × 10−6 A cm−2 to −238 mV vs. Ag/AgCl and 2.354 × 10−8 A cm−2 , respectively. The lower corrosion potential and current density suggest a better corrosion resistance of the superhydrophobic surface. The corresponding corrosion inhibition efficiency (p ) is calculated following the equation [35]:

Potential (V, vs.Ag/AgCl)

0.0

-0.1

1 2 3 4

bare etched calcined superhydrophobic

-0.2 4

-0.3

2

-0.4

p (%) =

3

-0.5 -10

-9

0 icorr

× 100

(4)

where i0 corr and icorr are the corrosion current densities without and with the inhibitor (film), respectively. The very high p (99.45%) of the superhydrophobic surface further could confirm its superior corrosion resistance.

1

-11

0 icorr − icorr

-8

-7

2

-6

-5

-4

-3

log(i, A/cm ) Fig. 10. Potentiodynamic polarization curves of bare copper, etched copper with 10 wt.% ammonia solution for 20 h, calcined treatment copper at 340 ◦ C for 10 min in air and superhydrophobic copper modified with an ethanol solution of stearic acid for 3 h in a 3.5 wt.% NaCl solution.

OCP) in a 3.5 wt.% NaCl solution. In a typical polarization curve, a lower corrosion current density (icorr ) or a higher corrosion potential (Ecorr vs. Ag/AgCl) corresponds to a lower corrosion rate and higher corrosion resistance [32–34]. As shown in Fig. 10, all the values of Ecorr (vs. Ag/AgCl), icorr and anodic Tafel slopes (␤a) were obtained by extrapolating the linear portions of the anodic and cathodic branches to their intersection and summarized in Table 1.

3.4.3. EIS EIS experiments on various copper surfaces were conducted in a 3.5 wt.% aqueous NaCl solution at corresponding OCPs. Fig. 11 shows EIS plots and their fitting results of bare copper, etched copper, etched and calcination treated copper without modification in ethanol, and superhydrophobic copper in a 3.5 wt.% NaCl solution. The Nyquist impedance diagram of the bare copper (Fig. 11a) consists of a depressed semicircle at the high frequency and a straight line at the low frequency. The semicircle caused by the charge transfer process is related to the time constant of the charge-transfer resistance (Rct ) and the double-layer capacitance (Cdl ) at the interface of copper/NaCl aqueous solution [36]. When the semicircle is depressed, it can be accredited to the frequency dispersion that exerts by the inhomogeneity and roughness of the substrate surface

Fig. 11. Nyquist plots measured at the corresponding open circuit potential in 3.5 wt.% NaCl aqueous solutions: (a) bare copper; (b) etched copper with 10 wt.% ammonia solution for 20 h; (c) calcined treatment copper at 340 ◦ C for 10 min in air; (d) superhydrophobic copper modified with an ethanol solution of stearic acid for 3 h.

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Fig. 12. EIS results measured at the corresponding open circuit potential in 3.5 wt.% NaCl aqueous solutions about bare copper, etched copper with 10 wt.% ammonia solution for 20 h, calcined treatment copper at 340 ◦ C for 10 min in air and superhydrophobic copper modified with an ethanol solution of stearic acid for 3 h. (a) Bode log |Z| versus log (f/Hz) plots, (b) Bode-phase angle versus log (f/Hz) plots.

[37,38]. The straight line typically known as Warburg impedance can be ascribed to the anodic diffusion process of copper chloride compounds from the electrode surface to the bulk solution [39]. The presence of Warburg impedance indicates that diffusion rather than charge transfer process dominates under this circumstance. For etched (Fig. 11b), etched and calcined (Fig. 11c), and superhydrophobic copper surfaces (Fig. 11d), there is no obvious Warburg impedance. This suggests that the electrochemical reactions on these three samples are controlled by the charge transfer instead of the diffusion process, which is different from the bare copper surface. The much larger diameter of the capacitive loop for the superhydrophobic copper surface than other surfaces (Fig. 11d) demonstrates a better corrosion inhibition of the former. Fig. 12a shows Bode plots and their fitting results of various copper surfaces in a 3.5 wt.% NaCl solution. It is clear that the total impedance follows the trend of superhydrophobic > etched and calcinated > etched > bare copper surface. For the bare copper, one

time constant was observed in the phase angle plot (Fig. 12b). New time constant appears at the low frequency region and the original one shifts to higher frequencies after calcination. Compared to the calcined treatment copper specimen, new time constants of superhydrophobic copper appear at the low frequency and the original ones shift to higher frequencies (Fig. 12b). The significant increase in the total impedance and the appearance of an additional time constant imply the formation of a superhydrophobic film with a high corrosion resistance [40]. The EIS result for bare copper substrate can be analyzed with the equivalent circuit shown in Fig. 13a, and the simulated results are listed in Table 2, where Rs is the electrolyte resistance, Rf is the resistance of the film, W is the Warburg impedance, and CPEf is the constant phase element of the corrosion layer. The CPE is mathematically expressed as [41]: ZCPE =

1 Y0 (iω)

n

(5)

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Table 2 Electrochemical parameters obtained from simulation of EIS results of bare copper, etched copper with 10 wt.% ammonia solution for 20 h, calcined treatment copper at 340 ◦ C for 10 min in air and superhydrophobic copper modified with an ethanol solution of stearic acid for 3 h. Samples

Bare Etched Calcined treatment Superhydrophobic

Rs ( cm2 )

9.324 9.192 9.591 9.352

Cf Y0 (␮ sn cm−2 )

n

32.16 30.15 5.79 3.54

0.534 0.583 0.639 0.668

Rf (k cm2 )

Cdl (␮F cm−2 )

Rt (k cm2 )

2.522 11.58 17.48 65.75

\ 58.48 1.51 0.0017

\ 17.93 50.14 68.02

close to −1 in the larger frequency range. This result indicates that the superhydrophobic film could provide a high resistance to corrosive. A corresponding equivalent circuit model for the copper surface with a superhydrophobic film is proposed in Fig. 13b. In this equivalent circuit, Rs is the electrolyte resistance, CPEf is the constant phase element modeling the superhydrophobic surface, Rf is applied to describe the impedance of the superhydrophobic film, Cdl is used to denote electrical double layer capacitance and Rct is the charge transfer resistance, respectively [42,43]. 4. Conclusions In this study, the superhydrophobic film was fabricated on the copper substrate via a simple method by combining chemical etching and thermal oxidation processes. The micro-nano catkin-like structure formed on the copper substrate showed a contact angle of 157.6◦ after etching in an ammonia solution for 20 h, calcination in air at 340 ◦ C for 10 min and immersion in an ethanol solution containing stearic acid for 3 h. The superhydrophobic film on the copper substrate exhibited a good corrosion resistance a in 3.5 wt.% NaCl aqueous solution with potentiodynamic polarization and EIS measurements. Acknowledgments This work was financially supported by National Natural Science Foundation of China (No. 21553001), Innovation Program of Shanghai Municipal Education Commission (No. 14ZZ152) and Science and Technology Commission of Shanghai Municipality (No. 14DZ2261000). References

Fig. 13. Equivalent circults for copper substrates in 3.5 wt.% NaCl aqueous solution: (a) bare copper, (b) etched copper with 10 wt.% ammonia solution for 20 h, calcined treatment copper at 340 ◦ C for 10 min in air and superhydrophobic copper modified with an ethanol solution of stearic acid for 3 h.

where Y0 is a proportionality factor and ‘n’ has the meaning of phase shift. The value of ‘n’ represents the deviation from the ideal behavior and it lies between 0 and 1. In Fig. 11d, the impedance of the sample with the superhydrophobic film is approximately three times larger than that of the pure sample. The log |Z| versus log frequency plot of the superhydrophobic copper shows a linear relationship with a slope very

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