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Electrochimica Acta 56 (2010) 517–522

Contents lists available at ScienceDirect

Electrochimica Acta

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abricated super-hydrophobic film with potentiostatic electrolysis methodn copper for corrosion protection

eng Wanga, Ri Qiua,b, Dun Zhanga,∗, Zhifeng Lina,b, Baorong Houa

Shandong Provincial Key Laboratory of corrosion science, Institute of Oceanology, Chinese Academy of Sciences, 7 Naihai Road, Qingdao 266071, ChinaGraduate School of the Chinese Academy of Sciences, 19 (Jia) Yuquan Road, Beijing 100039, China

r t i c l e i n f o

rticle history:eceived 22 June 2010eceived in revised form 4 September 2010ccepted 6 September 2010

a b s t r a c t

A novel one-step potentiostatic electrolysis method was proposed to fabricate super-hydrophobic filmon copper surface. The resulted film was characterized by contact angle tests, Fourier transform infraredspectra (FT-IR), X-ray photoelectron spectroscopy (XPS), Field emission scanning electron microscopy(FE-SEM) and electrochemical measurements. It could be inferred that the super-hydrophobic property

vailable online 21 September 2010

eywords:uper-hydrophobicityotentiostatic electrolysisopper corrosion

resulted from the flower-like structure of copper tetradecanoate film. In the presence of super-hydrophobic film, the anodic and cathodic polarization current densities are reduced for more than fiveand four orders of magnitude, respectively. The air trapped in the film is the essential contributor of theanticorrosion property of film for its insulation, the copper tetradecanoate film itself acts as a “frame”to trap air as well as a coating with inhibition effect. The super-hydrophobic film presents excellent

pper −

olarizationnhibition

inhibition effect to the co

. Introduction

Wettability of a solid surface is very important in manyndustrial processes as well as in our daily life. Especially, arti-cial super-hydrophobic surfaces (water contact angle largerhan 150◦), inspired by the water-repellent nature of lotuseaf, have prompted extensive interests for industrial application1,2].

Generally, it is believed that the higher surface roughness andower surface energy are the two important requirements forhe fabrication of super-hydrophobic surface [3,4]. The traditional

ethods for preparing super-hydrophobic surface on metallicaterial are based on the theory that modifying the rough sur-

ace with hydrophobic material, such as anodic oxidation [5],lectrochemical deposition [6–8], chemical etching [9–12], sol–gelethod [13,14], wet chemical reaction [15] and so on [16,17]. How-

ver, the common weakness of these methods is that the fabricationrocess contains at least two steps, because it is hard to accomplishhe modification with low surface energy material and fabricatingigh surface roughness simultaneously. The complicated fabrica-

ion process will restrain the application of super-hydrophobicurface in industry.

Copper is an important metal in the chemical and micro-lectronics industries due to its excellent thermal and electrical

∗ Corresponding author. Tel.: +86 532 82898960; fax: +86 532 82898960.E-mail address: zhangdun@qdio.ac.cn (D. Zhang).

013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2010.09.017

corrosion and stability in water containing Cl .© 2010 Elsevier Ltd. All rights reserved.

conductivities [18]. However, it is an active metal that does notresist corrosion well [19]. One potential approach to resolve theproblem is to create hydrophobicity or super-hydrophobicity,which can shield surface from attack by moisture contact. However,the low surface energy material film (such as fluorosilane) formedon rough metal surface with traditional method is too thin to be bro-ken down. Moreover, for a larger apparent area, the corrosion ratewill increase when the rough metal surface exposed to corrosivemedium. Thus, there have been only a few reports [16,20–23] aboutthe application of super-hydrophobic surface for metal anticorro-sion until now. Though Liu et al. prepared super-hydrophobic filmon copper surface with one-step immersion method for anticorro-sion, and it can protective the underlying copper effectively, thelong duration (5 days) will still restrain its application in industry[21].

Furthermore, according to traditional theory, it can be assumedthat the hydrophobicity can shield surface from attack by mois-ture/water contact. Reference [16] studied the inhibition abilityof super-hydrophobic and no hydrophobic film to the underlyingcopper. The electrochemical tests showed that the copper cov-ered with super-hydrophobic film exhibited better anticorrosioneffect. It was concluded that hydrophobicity played an importantrole in corrosion behavior instead of the film thickness. How-

ever, there is no direct proof to illuminate the contributions offilm itself and air trapped/hydrophobicity to the anticorrosioneffect of super-hydrophobic film until now. The anticorrosionmechanism of super-hydrophobic film to underlying metal is stillunclear. The understanding of the anticorrosion mechanism of

518 P. Wang et al. / Electrochimica Acta 56 (2010) 517–522

test

sm

swtimetbs

2

2

sCCu

2

ebiagdpeiItc

2

tbimedrTp

SS (Fig. 2), it can be observed that SS is covered with clusters,which are complicated and accumulated. The single cluster withdiameter of 10–20 �m presents flower-like structure composedof micro-sheets. It is clear that there is much vacancy in which aircan be trapped among these sheets. As a result, the real contact

Fig. 1. Contact angle

uper-hydrophobic film from different perspectives will attractore attention.Herein, for the first time, we explored a one-step potentio-

tatic electrolysis method to fabricate super-hydrophobic filmith flower-like structure on copper. The procedure is facile and

ime-saving to operate. With sophisticated technologies, includ-ng FE-SEM, FT-IR, XPS, contact angle test and electrochemical

easurements, the properties of film were characterized. The influ-nces of electrolysis time, acid concentration and anodic potentialo the property of film prepared were also investigated. On theseases, the formation process and anticorrosion mechanism of theuper-hydrophobic film were discussed.

. Experimental

.1. Materials and reagents

Copper (≥99.5 wt.%) foil was cut into 2 cm × 1 cm × 0.1 cm. Aeries of chemicals, including tetradecanoic acid (Shanghai Shanpuhemical Co., Ltd.), sodium chloride (Sinopharm Chemical Reagento., Ltd.) and ethanol (Sinopharm Chemical Reagent Co., Ltd.) weresed as received. Water used was Milli-Q water (Milli Q, USA).

.2. Modification of copper surface

In a typical procedure, the copper specimen was abraded withmery paper to 1500 grade, degreased with ethanol and dried withlower. The growth of copper tetradecanoate film was performed

n a two-electrode cell, in which the copper specimen acted as annode, a platinum wire a cathode. The film was electrochemicallyrown at a constant potential ranging from 2 V to 10 V for designeduration in the ethanol solution of tetradecanoic acid at room tem-erature. Then the copper specimen was brought out, washed withthanol, and dried naturally in nitrogen atmosphere. The potentials control by electrochemical system (CHI 430A, CH Instrumentsnc.). If there is no specified illumination, the film characterized andested is prepared in the ethanol solution of 0.1 mol dm−3 tetrade-anoic acid at a constant potential of 10 V for 5 h.

.3. Surface characterization

The morphology of the film on copper specimen was charac-erized with FE-SEM (JSM-6700F). FT-IR spectrum was obtainedy using a Nicolet iSIO spectrometer. Chemical composition

nformation about the specimen was obtained by XPS. The XPSeasurement was carried out on a Thermo ESCALAB 250 photo-

lectron spectrometer equipped with an Al-anode at a total powerissipation of 150 W (15 kV, 10 mA), and the binding energies wereeferenced to the C 1s line at 284.8 eV from adventitious carbon.he contact angles of 4 �L water droplet on bare and filmed cop-er were measured with contact angle meter (JC2000C1, Shanghai

results of BS and SS.

Zhongchen Digital Technic Apparatus Co., Ltd.) at ambient temper-ature.

2.4. Electrochemical experiments

The electrochemical experiments, including polarization curvesand open circuit potential (Eoc)–t curves, were obtained with acomputer-controlled electrochemical system (CHI 604D, CH Instru-ments Inc.) at ambient temperature in 3.5 wt.% NaCl solution. Theseexperiments were performed in a three-electrode cell with a plat-inum electrode as counter electrode, bare copper specimen/filmedcopper specimen as working electrode, and a silver/silver chlo-ride (Ag/AgCl) electrode as reference electrode. Polarization curveswere recorded at a sweep rate of 1 mV s−1. Every electrochemicaltest was repeated for more than three times to make sure a goodrepeatability of experiment result.

3. Results and discussion

3.1. Morphology and wettability

Fig. 1 shows the contact angles of the bare specimen (BS)and super-hydrophobic film coated specimen (SS). It is foundthat the BS surface is hydrophilic with water contact angle of52 ± 3◦. In contrast, the SS presents super-hydrophobic propertywith water contact angle of 154.3 ± 3◦. From the morphology of

Fig. 2. FE-SEM imagine of SS.

P. Wang et al. / Electrochimica Acta 56 (2010) 517–522 519

ah

3

tI±Taoab1gto(aCcw

tFipstapbaoXC

stc

C

Tma

C

020040060080010001200

0.0

5.0x104

1.0x105

Cu 3p

Inte

nsity

/ cp

s

Binding Energy / eV

C1s

O1s

Cu 2p

280282284286288290292294

0.0

5.0x103

1.0x104

1.5x104

2.0x104

2.5x104

Inte

nsity

/ cp

s

Binding Energy / eV

288.4 eV

284.6 eV

9309409509609702x103

3x103

4x103

954.6 eV

939 - 945 eV

934.6 eV

Inte

nsity

/ cp

s 959 - 965 eV

a

c

b

Fig. 3. FT-IR of (a) tetradecanoic acid and (b) super-hydrophobic film.

rea between water droplet and solid surface is reduced, and theydrophobicity of SS is dramatically intensified.

.2. Formation mechanism of super-hydrophobic film

Fig. 3 shows the FT-IR spectra of tetradecanoic acid (a) andhe super-hydrophobic film (b) in the range of 4000–400 cm−1.n the two cases, five peaks are observed at similar wavelengths2 cm−1, corresponding to the resolution of the spectrometer.hey are related to asymmetric vibration of CH3 at 2959 cm−1,symmetric vibration of CH2 at 2918 cm−1, symmetric vibrationf CH2 at 2853 cm−1 , bending of vibration of CH2 at 1476 cm−1

nd rocking vibration of CH2 at 723 cm−1 [24,25]. However, a newand corresponds to the coordinated COO moieties appears at587 cm−1 [17], whereas these bonds related to the free carboxylroup of tetradecanoic acid all disappear after modification, such ashe stretching vibration of C O (1702 cm−1), stretching vibrationf C–O (1098 cm−1), both in-plane (1412 cm−1) and out-of-plane944 cm−1) vibration of OH. Furthermore, there is no obvious char-cteristic band of the compound containing copper (such as Cu2O,uO and Cu(OH)2) in the IR spectrum of film. All these draw us toonclude that the film is essentially consisted of the copper complexith tetradecanoic acid through carboxyl group.

From the XPS survey spectra (Fig. 4(a)), it can be obtainedhat oxygen, carbon and copper are the main composition of film.ig. 4(b) is the C1s spectra of SS. The two obvious peaks correspond-ng to the alkyl (284.6 eV) and coordinated COO group (288.4 eV)rove that copper complex with tetradecanoic acid is the soleource of carbon in film. From the Cu 2p spectra of SS (Fig. 4(c)),he main Cu 2p3/2 and Cu 2p1/2 can be seen in the binding energyround 934.6 and 954.6 eV, respectively, with a spin-orbit cou-ling energy gap of 20 eV. Both of these peaks are accompaniedy intensive shake-up satellite features in the ranges of 939–945nd 959–965 eV. It is indicated that Cu2+ are the main valence statef copper in film [26,27]. Combining the analysis of both IR andPS results, it can be concluded that chemical composition of SS isu(CH3(CH2)12COO)2.

Based on the discussion above, the formation mechanism ofuper-hydrophobic film is proposed as follows: During the elec-rolysis process, Cu2+ irons are spontaneously released from theopper surface into solution at high anodic potential according to:

u → Cu2+ + 2e (1)

2+

hese released Cu ions can be captured by tetradecanoic acidoiety in solution immediately to form copper tetradecanoate

ccording to the reaction:

u2+ + 2CH3(CH2)12COOH → Cu[CH3(CH2)12COO]2 + 2H+ (2)

Binding Energy / eV

Fig. 4. (a) The survey spectra of the SS, (b) C 1s spectra of SS and (c) Cu 2p spectraof SS.

With the electrolysis progresses, more and more copper tetrade-canoate are formed in solution. It is certain that a concentrationgradient of copper tetradecanoate forms from surface of copperto bulk solution for the effect of mass transfer, and the concen-tration of copper tetradecanoate surrounding the copper electrodeis the highest in solution. Thus, copper tetradecanoate is prone toprecipitate on copper electrode surface when it is supersaturated.

3.3. Effects of treating conditions to the wetting property offilmed copper

In order to get more information about the super-hydrophobicfilm formation process, we have studied the effects of treating

520 P. Wang et al. / Electrochimica Acta 56 (2010) 517–522

8642040

60

80

100

120

140

160

180C

onta

ct a

ngle

/°

t/h

10 V

5 V

2 V

a

b

8642040

60

80

100

120

140

160

180

Con

taca

ang

le/ °

t/h

0.02 M

0.05 M

0.1 M

Fig. 5. The influences of treating conditions to the wetting property of filmed cop-per. (a) The contact angle changes of filmed copper as a function of treating timeattt

ca

actcnpsttdicpup

tt(Flohcf

-0.25

-0.20

-0.15

-0.10

-0.05

0.00

0.05

E v

s (A

g/A

gCl)/

V

SS

DS

BS

t different anodic potentials in tetradecanoic acid solution of the same concentra-ion (0.1 mol dm−3); (b) the contact angle changes of filmed copper as a function ofreating time upon treating in tetradecanoic acid with different concentrations athe anodic potential 10 V.

onditions (including electrolysis time, acid concentration andnodic potential) to the wetting property of filmed copper.

Fig. 5(a) presents the contact angle changes of filmed copper asfunction of treating time at different anodic potentials in tetrade-anoic acid solution of the same concentration (0.1 mol dm−3). Inhe case of applying anodic potential 2 V, the contact angle of theopper maintains around 110◦ within 8 h treating time, and there iso obvious film formed on copper after treatment. When the anodicotential increases to 5 V, the contact angle maintains about 110◦ inhort electrolysis time (3 h). However, if a long time (5 h) is adopted,he contact angle can reach more that 150◦. It should be noticedhat the minimum time needed to obtain super-hydrophobic filmecreases to 1 h when the anodic potential is 10 V. As the more pos-

tive anodic potential can result in higher releasing rate of Cu2+, weonclude that the copper tetradecanoate formed surrounding cop-er specimen can reach its saturated concentration within less timender more positive anodic potential. As a result, the formationrocess of super-hydrophobic film is shortened.

Moreover, we have also revealed the relationship betweenhe contact angle of filmed copper and treating time uponreating in tetradecanoic acid with different concentrations0.02–0.1 mol dm−3) at the anodic potential 10 V. As shown inig. 5(b), it is impossible to reach the super-hydrophobic state at

−3

ow concentration (0.02 mol dm ). However, with the increasef concentration from 0.05 mol dm−3 to 0.1 mol dm−3, the super-ydrophobicity can be obtained within 4 h and 1 h, respectively. Itan be concluded that higher concentration of tetradecanoic acid isavorable to the formation of super-hydrophobic film in the ranges

180012006000t/s

Fig. 6. Eoc–t curves of BS, SS and DS in 3.5% NaCl solution.

of treating conditions selected. On one hand, within the designedtreating time, more Cu2+ can be released from copper surface in thesolution with higher tetradecanoic acid concentration for its higherelectric conductivity. On the other hand, more Cu2+ can be cap-tured by tetradecanoic acid to form copper tetradecanoate due tothe higher concentration of tetradecanoic acid. In this case, coppertetradecanoate surrounding copper surface can reach its saturatedconcentration and precipitate on copper surface within less time.

3.4. Anticorrosion mechanism

Impressively, when the SS is immersed in 3.5% NaCl solution,the surface is exceptionally bright when viewed from the side. Thisphenomenon suggests that air can still be trapped in rough surfaceaccording to the theory of total reflection in physics. In details, air isoptically thinner medium for water, when light transfers from thewater to the interface of air with an incidence angle higher thancritical angle �c, it will be reflected totally.

In order to investigate the inhibition effect of super-hydrophobic film to the underlying copper, as well as thecontribution of air trapped to the anticorrosion property. Elec-trochemical experiments of BS, SS and deaerated SS (DS) arecarried out in 3.5% NaCl solution. The deaeration process of super-hydrophobic film is designed as follows: Firstly, the SS is immersedinto ethanol for 5 min. For higher surface energy of tetradecanoicacid complex to ethanol than to water, the surface of SS can bewetted by ethanol, in other words, the air adsorbed on surface canbe replaced by ethanol molecule spontaneously. It can be believedthat the air trapped in film is all driven out by ethanol after beingwetted by ethanol. After that, the wetted SS by ethanol is trans-ferred into 3.5% NaCl solution immediately. The ethanol adsorbedon the surface can be gradually replaced by water for their com-pletely mutual solubility to each other and the extremely smallerquantity of ethanol than water. For the sealing effect of water, aircannot be trapped into film any more. It can be regarded that the airtrapped in film is driven out after this process. As an apparent phe-nomenon, the surface is no longer as bright as before when viewedfrom the side.

Fig. 6 shows the Eoc–t curves of BS, SS and DS in 3.5% NaCl solu-tion. It is found that the Eoc of BS is stable at −0.21 V vs (Ag/AgCl).The Eoc of DS is relatively stable at −0.15 V vs (Ag/AgCl), which isslightly more positive than that of BS. In contrast to the two speci-

mens, the Eoc of SS vibrates within the potential range from −0.05 Vto −0.08 V vs (Ag/AgCl), implying the poor electro-conductive prop-erty of super-hydrophobic film for the air trapped. Although the Eoc

values do not provide any direct information on the corrosion kinet-ics, the increase of Eoc suggests both copper tetradecanoate film

P. Wang et al. / Electrochimica Acta 56 (2010) 517–522 521

-0.5

0.0

0.5

1.0E

vs

(Ag/

AgC

l)/V

SS

DS

BS

ic

NtepdCd

C

C

C

cficipcptfifiCrriltc

3

cFttT4ddt

2015105080

100

120

140

160

Con

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ang

le/°

t/d

-1-2-3-4-5-6-7-8-9-10-11-12-0.5

0.0

0.5

1.0

E v

s (A

g/A

gCl)/

V

log(j/A cm-2)

BS

0 day

3 days

10 days

20 days

a

b

-1-2-3-4-5-6-7-8-9-10-11

log(j/A cm-2)

Fig. 7. Polarization curves of BS, SS and DS in 3.5 wt.% NaCl solution.

tself and the air trapped can reduce susceptibility of the underlyingopper corrosion [28].

Fig. 7 shows the polarization curves of BS, SS and DS in 3.5 wt.%aCl solution. It is obvious that three distinct regions can be iden-

ified from the polarization curve of BS: (I) apparent Tafel regionxtending to the peak current density due to the dissolution of cop-er into Cu+ according to Eq. (3), (II) a region of decreasing currentensity until a minimum value is reached due to the formation ofuCl according to Eq. (4), and (III) a region of increase in currentensity as a result of CuCl2− formation according to Eq. (5) [29].

u → Cu+ + e (3)

u+ + Cl− → CuCl (4)

uCl + Cl− → CuCl2− (5)

In the presence of super-hydrophobic film, the anodic andathodic polarization current densities are reduced for more thanve and four orders of magnitude, respectively, and the polarizationurrent density maintains less than 10−7 A cm−2 within the test-ng potential range. It is proven that the super-hydrophobic filmresents excellent inhibition effect to the corrosion of underlyingopper. To our surprise, the film still presents super-hydrophobicroperty (with water contact angle of 153.1 ± 3◦) after polariza-ion, which suggests an excellent stability of super-hydrophobiclm in water containing Cl−. The polarization current density oflmed copper increases conspicuously after the air is driven out.ompared with the BS, the anodic current density of DS can onlyeduced for about two orders of magnitude, and the cathodic cur-ent density decreases slightly. It is indicated that the air trappedn the surface plays an essential role in the anticorrosion of under-ying copper for its insulation. In addition to acting as a “frame” torap air, the copper tetradecanoate film can inhibit the corrosionopper to some extent.

.5. Durability of super-hydrophobic film

The durability of super-hydrophobic film was investigated usingontact-angle measurements in parallel with polarization curves.ig. 8(a) presents the changes of contact angle of SS as a func-ion of immersion time in 3.5 wt.% NaCl solution. It can be foundhat SS can maintain its super-hydrophobic property within 3 days.

hough its contact angle falls to below 150◦ after an immersion ofdays, it can maintain around 140◦ even after an immersion of 20ays. It is indicated that the film is stable in NaCl solution, and theecrease of contact angle with immersion time might be related tohe microstructure changes of film. Fig. 8(b) shows the polarization

Fig. 8. Changes of the performances of SS as a function of immersion time in 3.5 wt.%NaCl solution. (a) The contact angle changes of filmed copper as a function of immer-sion time in NaCl solution; (b) the polarization curves of filmed copper as a functionof immersion time in NaCl solution.

curves as a function of immersion time in 3.5 wt.% NaCl solution. Itshould be noticed that both anodic and cathodic polarization cur-rent densities of SS increase with the immersion time, implyingthe degrading of protection ability of film with immersion time.However, the anodic polarization current density of SS can main-tain less than 10−5 A cm−2 within test potential range even after animmersion of 20 days, indicating the film is still compact withinimmersion time. Combined with contact angle testing results, itcan be inferred that SS can present excellent stability and protec-tive ability to underlying copper for a long period of immersion inNaCl solution.

4. Conclusions

In summary, a novel one-step potentiostatic electrolysis methodhas been proposed to fabricate super-hydrophobic film on copper.It is time-saving and facile to operate. Both positive anodic potentialand high concentration of tetradecanoic acid can accelerate the for-mation process of super-hydrophobic film. The super-hydrophobicfilm with high water contact angle results from the flower-likestructure of copper tetradecanoate film. The super-hydrophobicfilm presents excellent inhibition effect to the copper corrosion

−

and stability in water containing Cl . The air trapped in super-hydrophobic film is the essential contributor of the excellentanticorrosion property for its insulation, the copper tetradecanoatefilm itself acts as a “frame” to trap air as well as a coating with inhi-bition effect. This method is time-saving and facile to operate, and

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cknowledgement

This work was supported by National Key Technology R&D Pro-ram of China (Grant No. 2007BAB27B01).

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