Corrosion Science 94 (2015) 190–196

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Corrosion Science

journal homepage: www.elsevier .com/locate /corsc i

Corrosion inhibition of biomimetic super-hydrophobic electrodepositioncoatings on copper substrate

http://dx.doi.org/10.1016/j.corsci.2015.02.0090010-938X/� 2015 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +86 431 85095760; fax: +86 431 85095575.E-mail address: liuyan2000@jlu.edu.cn (Y. Liu).

Yan Liu a,⇑, Shuyi Li a, Jijia Zhang a, Jiaan Liu b, Zhiwu Han a, Luquan Ren a

a Key Laboratory of Bionic Engineering (Ministry of Education), Jilin University, Changchun 130022, Chinab Key Laboratory of Automobile Materials (Ministry of Education) and College of Materials Science and Engineering, Jilin University, Changchun 130022, China

a r t i c l e i n f o a b s t r a c t

Article history:Received 10 November 2014Accepted 2 February 2015Available online 21 February 2015

Keywords:A. CopperB. EISB. XPSC. Electrodeposited filmsC. Interfaces

Inspired by the special microstructure of plant leaves such as lotus leaves, we prepared super-hydropho-bic surfaces by a fast, facile, and one-step electrodeposition on copper substrates. The prepared surfacerevealed super-hydrophobicity with a contact angle value of 161.5� ± 2�. Meanwhile, corrosion resistanceof obtained coatings was evaluated by electrochemical measurement in detail. These results indicatedthat electrodeposition coatings provided greater protection against corrosion behavior. Moreover, thesuper-hydrophobicity improved corrosion resistance of the coating. This method can be easily extendedto other conductive materials and time-saving, having a great potential for future application in industrialfields.

� 2015 Elsevier Ltd. All rights reserved.

1. Introduction

Copper is an important material because of its high electricaland thermal conductivity, mechanical workability, fine appearanceand low chemical reactivity [1–3]. Copper is widely used in manyapplications such as conductors in electrical power lines, pipelinesfor domestic and industrial water utilities including seawater, andheat conductors and heat exchangers [4,5]. However, hydroxidesand toxic complexes form on copper in wet environments [6].Thus, the issue of corrosion prevention of copper has attractedthe attention of a number of investigators [7–9]. The corrosionresistance of copper can be significantly improved by reducingthe contact area between water and copper surfaces [10–12],thereby suggesting that transforming bare copper surfaces intosuper-hydrophobic surfaces would improve the corrosion resis-tance of these surfaces.

In nature, a great number of biological materials possess specialsurface wettability [13–15]. For example, lotus leaves exhibitsuper-hydrophobicity and possess self-cleaning property, whichis called ‘‘lotus effect’’ [16]. Due to the high adhesion to water,the rose petal becomes another relatively typical biological proto-type in the super-hydrophobic research [17]. In addition,anisotropic rice leaves, desert beetles and spider silks with watercollecting effects, butterfly wings, mosquito eyes, moth eyes,cicada wings, etc., also have their own special wetting behavior.

Studies on these organisms’ microscopic structure texture revealthe presence of the micro/nano hierarchical structure on the sur-face, and this structure is the main cause for the observed differentwettability [18,19]. Moreover, other special chemical componentsare also important factors in creating the binary structure withself-cleaning effects. Inspired by these creatures, two elementsare needed to realize special wettability on functional surfaces: asurface chemical composition and a rough surface with specialhierarchical micro/nano structures [20,21]. However, a super-hy-drophobic surface with low surface energy and micro/nano binarystructure is difficult to create.

In recent years, various methods have been proposed for the fab-rication of bio-inspired super-hydrophobic surfaces and corrosionresistance properties on the metal materials; these methodsinclude electrochemical modification [12], sol–gel methods[22,23], graft polymerization [24], immersion method [25–27],and self-assembled monolayers (SAMs) [28], etc. However, com-pared with most of these methods, electrodeposition does notrequire such severe conditions as tedious chemical treatments,expensive materials, complex multi-step processing procedures,or expensive equipment, which limit practical applications [29–32]. Electrodeposition technology is cheap, simple, and highlyeffective. Beyond that, by setting different parameter values, thesurface microtopography will also change correspondingly.

To improve the super-hydrophobicity and corrosion resistanceof copper substrate, we tested the capability of coatings obtainedvia rapid one-step electrodeposition. The relationships among theelectrodeposition voltage, microstructure, and wettability of the

Y. Liu et al. / Corrosion Science 94 (2015) 190–196 191

deposited Ce coating are discussed in detail. We found that theobtained coatings were effective in preventing corrosion of coppersubstrate.

2. Experimental

2.1. Materials

Anhydrous ethanol, myristic acid, and sodium chloride (P96%)of analytical grade were used as received. Cerium chloride(CeCl3�6H2O) was used without further purification.

2.2. Sample preparation

All of the prepared surfaces were produced by the same process.The detailed process is shown as follows. First, two cleaned copperplates with dimensions of 60 mm � 25 mm � 3 mm were abradedwith silicon carbide papers (from 400 to 800 grades), ultrasonicallywashed with distilled water, and dried under atmosphere condi-tion. Second, 0.038 M cerium chloride and 0.1 M myristic acid wereimmersed in ethanol under constantly stirring until a uniform elec-trolyte solution (150 ml) was obtained. Third, the two copperplates were used as cathode and anode in an electrolyte cell, anda direct current (DC) voltage of 20 V was applied to the electrodeswith a distance of 2 cm. After being subjected to electrolysis for30 min, the working electrode was rinsed thoroughly several timeswith distilled water and ethanol and was then dried under atmo-sphere condition. As a result, a cathodic surface was obtained.Samples with various deposition voltages were produced at thesame time.

2.3. Sample characterizations

The surface morphology of the obtained coating was charac-terized by field emission scanning electron microscopy (FESEM,S4800, Japan Electronic) at 15 kV. The corresponding chemicalcomposition was examined by X-ray photoelectron spectroscopy(XPS, SPECS XR50) attached with the SEM. Contact angles were

Fig. 1. SEM images of the as-prepared surfaces at different deposition voltages in the elec(b) 10 V; (c) 15 V and (d) 20 V.

measured by a contact angle meter (JC2000A Powereach, China)at ambient temperature for each surface. Water droplets (2 lL)were dropped on the super-hydrophobic coatings from a dis-tance of 0.2 cm by vibrating the burette. All of the water contactangles were measured at five different points and then averaged.

Electrochemical corrosion behavior was examined by potentio-dynamic polarization tests and electrochemical impedance spec-troscopy (EIS) in a standard three-electrode cell configurationwith a saturated calomel electrode (SCE) as the reference electrode,a platinum electrode as the auxiliary electrode, and the bare Cu orcoated Cu samples as the working electrode. Measurements wereperformed by an electrochemical workstation (GAMRY Refer-ence600, America) at room temperature in the 3.5 wt.% NaCl solu-tion. Before electrochemical experiments, these samples wereimmersed in the NaCl solution for 5 min to get a more stable sys-tem. In the electrochemical tests, samples were mounted with asurface area of 1 cm2 exposed to the corrosive medium. The poten-tiodynamic polarization curves were acquired at a scan rate of10 mV/s. EIS measurement was operated in the frequency rangeof 105–10�2 Hz at the open circuit potential with the amplitudeof the perturbation voltage of 10 mV. EIS results were analyzedby fitting data with the Zsimpwin software. All electrochemical-based experiments were repeated three times, and obtained verygood reproducibility.

3. Results and discussion

3.1. Surface morphology

Surface morphology is an important parameter of super-hy-drophobic properties, therefore, coatings were characterized byscanning electron microscopy. Fig. 1 shows the SEM images ofthe obtained surface at different deposition voltages. Fig. 1a showsthe SEM image of a cathode surface with electrodeposition DC volt-age of 5 V. This surface looks smooth and there are almost no obvi-ous structures on it. As the voltage increased to 10 V, micro-structures appeared on the substrate surface, and these irregularmicro/nano structures exhibit porous, and are not evenly distribut-

trolyte consisted of 0.038 M cerium chloride and 0.1 M myristic acid 30 min. (a) 5 V;

192 Y. Liu et al. / Corrosion Science 94 (2015) 190–196

ed (Fig. 1b). This phenomenon increases the surface hydrophobicproperty by measuring the static contact angle (Fig. 3). With a fur-ther increase in electrodeposition voltage to 15 V, these originalpore structures disappeared, and the coating surface became com-pletely covered with crystallites (Fig. 1c). Fig. 1d shows that at a DCvoltage of 20 V, papillae of nanometer scale (50–100 nm) appearedon a surface that previously had rough structures, and that, suchpapillae were relatively smaller but more intensive. This suggeststhe surface changes into a surface with hierarchical micro/nanostructures, similar to the microstructure surface of lotus leaves.These findings show that an increase in DC voltage leads to achange in morphology, resulting in the transition of wetting statefrom hydrophobic to super-hydrophobic.

3.2. Chemical composition

In order to investigate the electrodeposition process of thebilayer film, the surface chemical composition was studied bythe X-ray photoelectron spectroscopy (XPS) method. Fig. 2 pre-sents the XPS survey spectrum of the chemical composition onthe as-prepared super-hydrophobic surface with electrodepositiontime of 30 min at 20 V DC voltage. In Fig. 2a, it revealed that ceri-um, carbon and oxygen are the main composition of the film on thecopper substrate. Strong peaks of C 1s at 282 eV, O 1s at 528 eV, Ce3d at 886 eV and 904 eV can be seen. First of all, the appearance ofCe peak confirms the existence of cerium species in the film. Fig. 2bpresents the Ce 3d XPS spectrum of the cerium-implanted speci-men. This region includes several peaks between 880 and 890 eVdue to the 3d5/2 level and another series of peaks correspondingto levels 3d3/2 and 3d5/2 between 890 and 910 eV, therefore it isexpected that the main valence state of cerium on the surface willbe in the form of Ce3+ iron, and the Ce element due to the chemical

Fig. 2. XPS spectrum of super-hydrophobic surface by electrodepositing

substance of CeCl3 [33]. In Fig. 2c and d, we can find that, comparedwith bare copper substrate, the content of C and O elements wasobviously increased on the as-prepared surfaces which were treat-ed with myristic acid, which implied that the surface was coveredwith the film. These results all indicated that, a chemical reactionoccurred between the electrolyte on the copper substrate surface,which resulted in the formation of cerium myristate(Ce½CH3ðCH2Þ12COO�3), which is a low energy material. Accordingto Fig. 1, the obtained surfaces possess the micro/nano hierarchicalstructure and low surface energy at the same time, providing thebasic conditions for super-hydrophobicity.

3.3. Surface wettability

To verify the effect of voltages on surface wettability, the con-tact angles of obtained surfaces were measured, as shown in theFig. 3. In this picture, it can be seen that an increase in voltageresults in a gradual increase at contact angle values. As for the barecopper substrate, the contact angle is only 93.3� ± 2�. As the voltageincreases from 5 V to 20 V, the corresponding contact angle valuesin proper sequence are 142.8�, 153.7�, 156.4�, and 161.5�, respec-tively. The surface hydrophobicity has substantially improved incomparison with the original water contact angle of copper sur-face. Combined with Figs. 1 and 2, it can be proved that both thevariation of surface morphologies and chemical composition haveconsiderable effects on surface wettability.

Fig. 4 shows the schematic of a water droplet on the preparedsurface with a DC voltage of 20 V. According to the Cassie model,the droplet state comes in contact with the surface of the lotusstate, and the air filling with the gaps prevents water drops frominfiltrating the surface. The water droplet is suspended on protru-sions of the surface, thus preventing any contact between water

30 min at 20 V, (a) full-spectrum; (b) Ce 3d; (c) C 1s and (d) O 1s.

Fig. 3. Contact angles of different electrodeposition voltages on the substrate. Theelectrodeposition time is 30 min.

Y. Liu et al. / Corrosion Science 94 (2015) 190–196 193

and solid surfaces. Meanwhile, the liquid–vapor interface hangsfrom the top corners of the pillars. The contact angle in terms ofCassie–Baxter equation is as follows:

cos hr ¼ f 1 cos h0 � f 2 ð1Þ

where f1 and f2 are the fractions of solid surface and air in contactwith liquid, respectively; hr (161.5�) and h0 (93.3�) are the contactangles on the electrodeposition surface of different times and onthe bare copper substrate, respectively. Given that f1 + f2 = 1, thecorresponding f1 is calculated to be 0.0518. The low value of f1 sug-gests only about 5.18% areas of the water drop contacted with thesuper-hydrophobic coating, and the rest 94.82% contacted with

Fig. 4. Schematic illustrations of a water droplet on the super-hydrophobic film: (a)the model of a water droplet on the surface and (b) a drop of water in contact withthe surface with electrodeposition 30 min at 20 V DC voltage.

the air cushion, which further confirmed that super-hydrophobicityresults from the formation of these microstructures.

Fig. 5 presents the graph of the copper substrate covered withsuper-hydrophobic film, with electrodeposition for 30 min at20 V DC voltage, immersed in 3.5 wt.% NaCl solution. It is obviouslythat the surface is exceptionally bright when viewed from the side,suggesting that air can still be trapped in the micro/nano hierarchi-cal structure according to total reflection theory in physics. Air isan optically thinner medium for water. It can be completelyreflected when light transfers from water to the interface of airwith an incidence angle higher than critical angle [34]. This phe-nomenon is also a proof for the point that the solution contactswith the as-fabricated film is the Cassie mode.

3.4. Corrosion resistance

3.4.1. Potentiodynamic polarization measurementsTo investigate the corrosion resistant property of the super-hy-

drophobic surface, the potentiodynamic polarization curves hasbeen used to test the as-prepared super-hydrophobic surface andthe bare Cu substrate between �1.5 V and 0.3 V (vs. Ag/AgCl) ina 3.5 wt.% aqueous NaCl solution. As described in Fig. 6, use theextrapolation of linear Tafel segments of the cathodic curve andthe calculated anodic Tafel lines to obtain the values of Ecorr (vs.

Fig. 5. Digital graph of super-hydrophobic film immersed into 3.5 wt.% NaClsolution captured with oblique angle.

Fig. 6. Potentiodynamic polarization curves measured in 3.5 wt.% NaCl solution forthe bare Cu (a) and the coated Cu (b) by electrodeposited at 20 V for 30 min.

Table 1Corrosion parameters calculated from Tafel plots for the bare Cu and the coated Cu.

Specimen �bc

(mV dec�1)ba

(mV dec�1)�Ecorr (mV/(Ag/AgCl))

icorr

(lA cm�2)gp (%)

Bare Cu 194 ± 5 41 ± 2 330 ± 3 7.096 ± 0.04 –Coated Cu 70 ± 2 62 ± 3 234 ± 2 0.408 ± 0.02 94.3 ± 0.05

Fig. 7. The Nyquist plots of the bare Cu and the coated Cu by electrodeposited30 min at 20 V DC voltage in 3.5 wt.% aqueous NaCl solution. The inset figure is theamplified diagram in the higher frequency range.

194 Y. Liu et al. / Corrosion Science 94 (2015) 190–196

Ag/AgCl), icorr and cathodic and anodic Tafel slopes (bc and ba),which were obtained by extrapolating the linear portions of theanodic and cathodic branches to their intersection. Parametersassociated with Tafel polarization curves are summarized inTable 1. Polarization curve of bare Cu can be separated into threedistinct regions. First is an apparent Tafel region extending to peakcurrent density because of Cu dissolution into Cu+. Second is aregion of decreasing current density as a result of CuCl formation.Third is a region of increasing current density as a result of CuCl2

�

formation [34]. With regard to the coated Cu, these three regionsof anodic dissolution of Cu disappear, and both anodic and cathodicpolarization current densities decreased by almost 1 order of mag-nitude. This result indicates the excellent inhibitory effect of super-hydrophobic film on Cu substrate. In addition, as depicted in Tafelpolarization curves, compared to bare Cu substrate (�330 mV), thecorrosion potential of coated copper, Ecorr (vs. Ag/AgCl), slightly

Fig. 8. The Bode plots of the bare copper substrate and the coated Cu by

shifted in the positive direction to �234 mV, indicative of the pro-tective nature of super-hydrophobic coating on the substrate sur-face. Consequently, corrosion resistance of super-hydrophobicfilm is intensified. The corresponding corrosion inhibition efficien-cy was symbolized as gp and calculated with using following equa-tion [35]:

gp ð%Þ ¼i0corr � icorr

i0corr

� 100 ð2Þ

where i0corr and icorr are the corrosion current density values without

and with the inhibitor (coating), respectively. These values indicat-ed that, copper dissolution and Cl� transports are stronglyrestrained by the protective capability of super-hydrophobic film.It confirmed that, the air/liquid interface in the obtained surfaceacted an important role in the corrosion protection performance,which resulted in the formation of super-hydrophobic surface withbetter corrosion inhibitive property than the bare Cu substrate.

3.4.2. EISAs a powerful, complementary electrochemical technique, elec-

trochemical impedance spectroscopy was also used to evaluate thecorrosion inhibitive properties of the prepared super-hydrophobicsurface. EIS experiments on bare copper and surface-functionalisedcopper coupons were conducted by immersion in 3.5 wt.% aqueousNaCl solution. In Fig. 7, the diameter of the Nyquist loop in the cop-per surface covered with super-hydrophobic coating is significant-ly larger than that of bare copper, indicating the polarizationresistance has been greatly enhanced with the presence of super-hydrophobic coating of cerium myristate. The inset figure is theenlarged diagram in the higher frequency range.

With regard to the bare Cu, we can observe that it includes acapacitive loop at high to medium frequencies and a straight line(Warburg impedance) at low frequencies, indicating that the diffu-sion process is controlled by the cathodic diffusion of dissolvedoxygen from bulk solution to the electrode surface and the anodicdiffusion of soluble Cu species from electrode surface to bulk solu-tion, such as CuCl2

� [2,36]. From the Bode-phase angle versus fre-quency plots (Fig. 8a), in the testing frequency range, bare Cushows only one time constants as for the formation of the corro-sion layer at a higher frequency (103–102 Hz). In this case, theEIS result for bare copper substrate can be analyzed with theequivalent circuit shown in Fig. 9a, and lists the specific value inTable 2, where Rs is the electrolyte resistance; Rf is the resistanceof film (corrosion product); W is Warburg impedance; and CPEf isthe constant phase element of the corrosion layer. The CPE ismathematically expressed as [37]:

electrodeposited 30 min at 20 V DC voltage in 3.5 wt.% NaCl solution.

Fig. 9. Equivalent circuit model used for fitting of EIS dates of bare Cu (a) andcoated Cu (b).

Y. Liu et al. / Corrosion Science 94 (2015) 190–196 195

ZCPE ¼ Y�10 ðixÞ

�n ð3Þ

where Y0 is a proportionality factor and ‘n’ has the meaning of phaseshift. The value of ‘n’ represents the deviation from the ideal behav-ior and it lies between 0 and 1.

In Fig. 8b, the impedance of the sample with super-hydrophobiccoating is approximately three orders higher than that of the baresample. The log |Z| versus log frequency plots of the coated Cudemonstrate the linear variation with a slope very close to �1 inthe higher frequency range. These phenomena indicate that thesuper-hydrophobic film provides high resistance to corrosivemedium. Thus, the equivalent circuit model for the coppersurface with super-hydrophobic film was shown in Fig. 9b. InECC, the Rct||Cdl represented the impedance with the interface reac-tion between the super-hydrophobic film and the copper substrate,as well as the extent of ionic conduction through a coating in anelectrolytic environment, and was commonly used a standard for

Table 2Electrochemical impedance parameters of bare Cu and the coated Cu.

Sample Rs (X cm2) Rf (kX cm2) CPEf

Y0 (lX sn

Bare Cu 136.2 ± 6.13 2.462 ± 0.013 32.16Coated Cu 83.64 ± 3.25 52.54 ± 0.21 3.6

estimating the extent of corrosion protection derived from theorganic coatings. In this equivalent circuit, Rs is the electrolyteresistance, CPEf is constant phase element modeling super-hydrophobic surface, Rf is applied to describe the impedance ofthe super-hydrophobic film, Cdl is used to denote electrical doublelayer capacitance, and Rct is the charge transfer resistance, respec-tively [38]. The obtained surface was covered with the micro/nanohierarchical structure, and the existence of air/liquid interfacereduces the corrosion of copper, which agrees well with the resultsof Tafel and SEM. In other words, both Nyquist plot and Bodediagram results indicate that a super-hydrophobic surface cansignificantly improve the corrosion resistance property of the Cusample in contact with NaCl solution.

3.4.3. Corrosion inhibitive mechanismBased on the SEM images, the corrosion test results, and Fig. 4, it

can be proposed the super-hydrophobic film inhibitive mechan-ism. The unique structures on the super-hydrophobic surface,which contain binary hierarchical structure, trapped a largeamount of air, thus preventing the wetting between liquid andsolid surface, and significantly reducing the actual contact areabetween the liquid and the substrate. A layer of protective air cush-ion forms on the surface, thereby hindering the formation of aclosed loop circuit and ultimately inhibiting corrosion effectively.The air captured by micro/nano papillae on the super-hydrophobicsurface form the capillary system. When the corrosion mediumtouches the surface, it not only penetrates into the material’s sur-face, but may also be expelled. The cooperation between air cush-ion and capillary effect is essential to improve the corrosioninhibitive property of copper, suggesting that hydrophobicitygreatly enhances the corrosion protection performance of coppersubstrate.

4. Conclusions

The corrosion inhibitive and super-hydrophobic coatings withmicro/nano structures on the cathodic copper substrate were suc-cessfully fabricated by a rapid one-step electrodeposition method.The coating electrodeposited at 20 V for 30 min exhibited super-hydrophobicity with a contact angle of 161.5� ± 2�. The super-hy-drophobicity of the coating resulted from the formation of specialhierarchical micro/nano structures and cerium myristate with lowsurface energy. At the same time, the super-hydrophobic surfaceexhibited better corrosion inhibitive properties in comparison withbare Cu substrate, as demonstrated by measurement results frompotentiodynamic polarization curves (Tafel curves) and EIS tech-niques in 3.5 wt.% aqueous NaCl solution. Therefore, it can beassumed that the corrosion resistance of the coating was consis-tent with its hydrophobicity. In the current research, this elec-trodeposition process is relatively facile, which offers an effectivestrategy and promising industrial applications for the fabricationof functional surfaces with super-hydrophobic and corrosioninhibitive properties on various metallic materials.

Cdl (lF cm�2) Rct (kX cm2)

cm�2) n

0.587 – –0.625 0.0067 ± 0.00015 14.01 ± 0.64

196 Y. Liu et al. / Corrosion Science 94 (2015) 190–196

Acknowledgements

This work was supported by the National Natural Science Foun-dation of China (Nos. 51275555, 51325501 and 51201068) andBasically Science Research Foundation of Jilin University (No.2013ZY09)

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