Pm

Pa

b

a

ARRAA

KSACEP

1

ecctllocvsLbfidn

nr

oF

0d

Electrochimica Acta 55 (2010) 878–883

Contents lists available at ScienceDirect

Electrochimica Acta

journa l homepage: www.e lsev ier .com/ locate /e lec tac ta

rotection of copper corrosion by modification of dodecanethiol self-assembledonolayers prepared in aqueous micellar solution

eng Wang a, Chenghao Liang a,b,∗, Bo Wu a, Naibao Huang b, Jielan Li b

State Key Laboratory of Fine Chemicals, Department of Chemical Engineering, Dalian University of Technology, Dalian 116012, ChinaDepartment of Materials Science and Engineering, Dalian Maritime University, Dalian 116026, China

r t i c l e i n f o

rticle history:eceived 25 February 2009eceived in revised form 26 June 2009ccepted 30 June 2009

a b s t r a c t

A novel method of preparing 1-dodecanethiol (DT) self-assembled monolayers (SAMs) on copper in aque-ous micellar solution is explored. The resulting SAMs are characterized by XPS, contact angle tests andelectrochemical measurements. It is found that DT dissolved in aqueous micellar solution can adsorbrapidly to the copper surface through strong thiolate bonds to form well-organized SAMs, which have

vailable online 5 July 2009

eywords:AMslkanethiolopper corrosion

properties comparable to those formed in ethanol solution. The electrochemical measurements showthat the inhibition efficiency (IE) increases with an increase in the immersion time of copper in the aque-ous micellar solution. After self-assembly for 1 h, the SAMs are able to effectively protect the underlyingcopper against corrosion in chloride-containing solution by hindering the cathodic process. Overall, theinhibition efficiency can reach 98.94%.

lectrochemical impedance spectroscopyolarization

. Introduction

Copper is an important metal in the chemical and micro-lectronics industries due to its excellent thermal and electricalonductivities [1]. However, it is an active metal that does not resistorrosion well. One effective approach that can be taken to solvehis problem is surface modification using self-assembled mono-ayers (SAMs). In many cases, the SAMs on the active substratesike copper are densely packed monolayer coatings. It was previ-usly shown that the alkanethiol SAMs could effectively inhibit theorrosion of copper in aggressive medium [2–6]. However, the sol-ent used for the preparation of SAMs is organic. The use of organicolvent restricts the application of alkanethiol SAMs in industries.uckily, the long-chain alkanethiols that are insoluble in water cane dissolved within the hydrophobic cores of micelles [7,8]. There-

ore, it is possible to prepare alkanethiols SAMs on copper surfacesn aqueous micellar solution. However, whether the alkanethiolsissolved in the micelles can be delivered to the copper surface

eeds to be further studied.

In this paper, we investigated the feasibility of preparing alka-ethiol SAMs on copper surfaces in aqueous micellar solution. Theesulting SAMs were characterized by XPS, contact angle tests and

∗ Corresponding author at: State Key Laboratory of Fine Chemicals, Departmentf Chemical Engineering, Dalian University of Technology, Dalian 116012, China.ax: +86 411 84727989.

E-mail address: liangch@newmail.dlmu.edu.cn (C. Liang).

013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2009.06.078

© 2009 Elsevier Ltd. All rights reserved.

electrochemical measurements. Furthermore, the SAMs formed inaqueous micellar solution were compared with those assembled inethanol solution.

2. Experimental

2.1. Chemicals

1-Dodecanethiol (DT, Fluka, purity > 97%) and Cetane trimethylammonium bromide (CTAB, Kermal, 99.0%) are commercially avail-able. All the other reagents are of analytical grade.

2.2. Preparation of copper electrodes

The pure copper ≥99.9 wt.% used in this experiment was cutinto rectangular specimens of 1 cm2 area, soldered with Cu-wire forelectrical connections, and mounted into the epoxy resin with oneactive flat surface exposed to the corrosive environment. Successivegrades of SiC-type emery papers up to 2000 grit were employed fol-lowed by polishing in a solution of 68% phosphoric acid to achievea mirror finish. The electrodes were then degreased in double dis-tilled water. Etching of the working electrodes in 7 M HNO3 solutionfor 30 s provided a fresh and oxide-free surface. Finally, the elec-

trodes were rinsed with distilled water [5].

Next, the electrodes were immersed into the aqueous micellarsolutions (10−2 M DT and 0.11 M CTAB) at 35 ◦C for the designatedtime. Upon removal, the samples were washed with ethanol toremove the CTAB adsorbed on the copper surface and dried with

imica Acta 55 (2010) 878–883 879

nDd

a

2

2

tat

2

aSspol

2

ceaAtwr1pe−ttw

3

3

wtahtaasw[tt

3

astfS

P. Wang et al. / Electroch

itrogen. CTAB in the solution is used to increase the solubility ofT in water. CTAB provides hydrophobic cores in which DT can beissolved.

For comparison, DT SAMs formed in 10−2 M DT ethanol solutiont 35 ◦C were also prepared.

.3. Characterization of monolayers

.3.1. Test of contact anglesThe contact angles on the bare copper and the Cu/SAM elec-

rodes were measured using a Dataphysics OCA-20 contact anglenalyzer. The drop size of the ultra pure water was 1 �L to avoidhe effects of weight.

.3.2. XPS testIn this study, XPS was used to reveal the surface chemical inter-

ctions that occur between organic thiols and the copper surface.pectra were collected on a Thermo ESCALAB 250 photoelectronpectrometer equipped with an Al-anode at a total power dissi-ation of 150 W (15 kV, 10 mA). The binding energies of the peaksbtained were made referenced to the binding energy of the C1s

ine, set at 285.0 eV.

.3.3. Electrochemical measurementsThe electrodes were put into a conventional three-electrode

ell as soon as the formation of the SAMs ended. For the three-lectrode cell, a platinum wire was used as the counter electrodend a saturated calomel electrode (SCE) was used as a reference.ll the electrochemical experiments were carried out using Prince-

on Applied Research equipment (Parstat 2273). EIS measurementsere carried out at open circuit potential (OCP) in the frequency

ange of 200 kHz to 10 mHz under excitation of a sinusoidal wave of0 mV amplitude. The impedance data were analyzed with Zsim-win impedance analysis software and fitted to the appropriatequivalent circuits. The polarization curves (PC) were obtained from200 mV to +500 mV versus OCP with a scan rate of 0.5 mV/s. For

he above-mentioned tests, a solution of 0.5 M NaCl was used ashe aggressive environment. All the electrochemical measurementsere performed at room temperature.

. Results and discussions

.1. Contact angle measurements

The contact angle formed between the copper sample and theater droplet provides a simple measure of the SAMs [9]. The con-

act angles of the bare copper and SAMs-covered copper in CTABnd ethanol were tested. It is found that the bare copper has aydrophilic surface for the contact angle is 21 ± 5◦. In contrast,he surface of the copper modified in aqueous micellar solutionnd ethanol solution for 1 h are hydrophobic. Their contact anglesre 125.8 ± 3◦ (CTAB) and 131.2 ± 3◦ (ethanol). The measurementsuggest that the SAMs formed in aqueous micellar solution areell oriented, compact, and expose a low-energy methyl surfaces

10–12]. Furthermore, it can be concluded that the SAMs formed inhe aqueous micellar solution have comparable wetting propertyo those formed in ethanol solution.

.2. XPS analysis

Fig. 1(a) presents the survey spectra of the copper modified in

queous micellar solution for 1 h. A take-off angle of 35◦ from theurface was employed. The peaks show that Cu, C, S and O exist onhe copper surface, indicating that DT adsorbed to the copper sur-ace. Fig. 1(b) focuses on the XPS S2p core level corresponding to theAMs-covered copper. Two peaks can be obtained from this figure,

Fig. 1. (a) The survey spectra of the DT SAMs on copper surface. (b) S2p spectra ofDT SAMs formed on the copper surface. (c) N1s spectra of DT SAMs formed on thecopper surface.

corresponding to S2p3/2 and S2p1/2. The single component, withthe S2p3/2 component centered at 162.4 eV and an intensity ratiobetween the S2p3/2 and S2p1/2 components close to the theoreti-cal value of 2, corresponds to the thiolate bonds, Cu–S [13,14]. This

finding confirms the chemical grafting of the DT molecules onto thecopper surface in the aqueous micellar solution.

Fig. 1(c) represents the N1s region and demonstrates a very weakpeak at about 400 eV. The ratio of S/N calculated from the XPS isabout 87/1, proving that very little CTAB exist on the copper surface.

880 P. Wang et al. / Electrochimica Acta 55 (2010) 878–883

FT

Giscdooaai

3

3

NsaccttistfEF

electrode and Rct is the charge transfer resistance of the correspond-

ig. 2. (a) The Nyquist plots of bare copper measured in the 0.5 M NaCl solution. (b)he Bode plots of bare copper measured in the 0.5 M NaCl solution.

enerally, CTAB can adsorb to the copper surface via static, whichs typical of physical adsorption [15]. CTAB adsorbed on the copperurface can be easily replaced by the chemical adsorption of DT toopper. It is reasonable to infer that the CTAB can only exist in theefect or the surface of DT SAMs. We can infer that the formationf DT SAMs occurs in several steps. CTAB and DT gradually adsorbnto the copper surface until nearly all the active sites of the surfacere covered by DT and CTAB. Then, DT begins to replace the CTABdsorbed on the surface. At the end, nearly all the active sites arenvaded by DT and a well-organized monolayer is formed.

.3. Effect of self-assembly time on the film properties

.3.1. Electrochemical impedance spectroscopyFig. 2 represents the EIS results for bare copper in a 0.5 M

aCl solution. The Nyquist plots of bare copper measured in NaClolution (Fig. 2(a)) display a capacitive loop at high frequenciesnd a straight line (Warburg impedance) at low frequencies. Theapacitive loop is attributed to the relaxation time constant of theharge-transfer resistance (Rct), whose value is approximately equalo the diameter of the capacitive loop and the double-layer capaci-ance (Cdl) at the copper/electrolyte interface [16–18]. The Warburgmpedance reflects the anodic diffusion process of soluble copperpecies (CuCl−2 ) from the surface of the electrode to the bulk solu-

ion as well as the cathodic diffusion process of dissolved oxygenrom the bulk solution to the surface of the electrode [19,20]. TheIS results of the bare copper can be analyzed with the circuit inig. 3.

Fig. 3. The equivalent circuits for bare copper in the 0.5 M NaCl solution.

In order to investigate the influence of self-assembly time on theprotective properties of the films, the DT SAMs were prepared inaqueous micellar solution containing CTAB for times ranging from10 s to 1 h. As shown in Fig. 4, the Nyquist plots of SAMs-coveredelectrodes differ from those of the bare one in shape and size(Fig. 4(a)). The continuous increase in the diameter of the Nyquistsemi-circles with an increase in the immersion time suggests thatthe SAMs greatly but gradually changed the corrosion kinetics ofthe copper surface. In the cases of short immersion time (10 s and30 s), the Warburg impedance observed in Fig. 2(a) can be found inthe Nyquist plots (Fig. 4(a)). A new peak appears at about 1000 Hzin the phase angle plots (Fig. 4(c)), which means that the formationof SAMs changes the electrode interfacial structure and results inan extra time constant. It is reasonable to analyze these plots withthe equivalent circuit in Fig. 5(a). In the cases of long immersiontime (more than 30 s), the Warburg impedance observed in Fig. 2(a)disappears from the Nyquist plots, reflecting that the SAMs havegood inhibition effect and they can prevent the diffusion processesof the ions such as CuCl−2 (to some extent) [21]. These EIS resultscan be analyzed with the circuit in Fig. 5(b). It is notable that thelog |Z| versus log frequency plots of the copper modified for 1 h(Fig. 4(b)) demonstrate linear variation in the logarithm of elec-trode impedance. A slope very close to −1 was found in the middlefrequency range, which means the spectra contain only one capac-itive contribution. Thus, the spectra can be analyzed with a simplemodel of ohmic resistance composed of an R–C parallel combinationin series (Fig. 5(c)) [4].

In the equivalent circuits shown in Figs. 3 and 5, Rs stands for thesolution resistance between the working electrode and referenceelectrode, Rct stands for the charge-transfer resistance corre-sponding to the corrosion reaction at the metal substrate/solutioninterface, W represents Warburg impedance attributed to masstransport in the corrosion reactions, Rsam stands for the transferresistance of electrons through the monolayers, and Qdl and Qsam

respectively represent constant phase elements (CPEs) modelingthe double-layer capacitance (Cdl) and the capacitance of the SAMs(Csam). The admittance and impedance of a CPE are respectivelydefined as

YQ = Y0(jω)n (1)

and

ZQ = (jω)−n

Y0(2)

where subscript Q represents a CPE, Y0 the modulus, ω the angularfrequency and n the phase [22].

The values of the elements of the equivalent circuits obtainedby fitting are given in Table 1. The listed surface coverage (�) listedis calculated with the following formula:

1 − ϑ = R0ct

Rct(3)

where R0ct is the charge transfer resistance of the bare copper

ing SAMs-covered electrodes. For the copper modified for 1 h, thesurface coverage is calculated with Rsam.

From the data of Table 1, it is clear that the Rct of bare copper isvery low. When the electrode surface is covered with DT SAMs, the

P. Wang et al. / Electrochimica Acta 55 (2010) 878–883 881

Fig. 4. The EIS results of the bare copper and the copper modified by different immer-sion times. (a) Nyquist plots; (b) Bode log |Z| versus log frequency plots; (c) Bode �versus log frequency plots.

Table 1Values for the circuit parameters and coverage of SAMs.

SAMs Ydl (�−1 cm−2 sn 10−6) ndl Rct (k� cm2)

Blank 542.9 0.66 1.310 s 38.4 0.70 5.930 s 23.0 0.69 10.55 min 1.13 1 83.215 min 1.20 1 108.41 h – – –

Fig. 5. Equivalent circuits for SAMs-covered copper in the 0.5 M NaCl solution.

surface coverage increases quickly with the immersion time. It canreach 98.44% within 5 min, proving that the formation of monolay-ers on fresh copper surfaces is a fast process. When the electrode isimmersed for more than 5 min, the nsam value is close to 1, so theimpedance of the CPE represents a pure capacitor and Ysam repre-sents the monolayer’s capacitance. Ysam is constant in these cases,suggesting that the film thickness does not change with the immer-sion time [4]. In other words, the thickness of DT SAMs reachesa maximum within 5 min. Furthermore, the very high resistance(702 k� cm2) and surface coverage (99.81%) of the copper modi-fied for 1 h indicates that the densely packed and homogeneousSAMs have formed on the copper surface. Therefore, the copper iswell separated from the contacting aqueous phase and effectivelyprotected against corrosion [4].

3.3.2. Polarization curvesFig. 6 presents the polarization curves of the bare copper and the

copper modified by different immersion times in aqueous micellarsolution. The anodic polarization curve for the naked copper elec-trode has a linear region. The apparent Tafel slope indicates mixedmass-transfer and kinetic control [19].

From Fig. 6, it can be seen that the anodic current densities of thecopper electrodes are reduced significantly when the immersiontime is short (between 10 s and 30 s). In the cases of long immer-sion time (more than 5 min), both the anodic and cathodic current

densities of the SAMs-covered copper electrodes are reduced signif-icantly. The hindering of the cathodic process is much greater thanthat of the anodic process. The suppression of the cathodic processincreases with an increase in assembly time. After self-assembly

Ysam (�−1 cm−2 sn 10−6) nsam Rsam (k� cm2) � (%)

– – – –7.27 0.89 0.93 77.973.46 0.92 2.28 87.620.74 0.97 94.6 98.440.66 0.97 140.8 98.800.66 0.98 702.0 99.81

882 P. Wang et al. / Electrochimica Acta 55 (2010) 878–883

Fig. 6. Polarization curves of the bare copper and the copper modified by differentimmersion times.

Table 2Electrochemical parameters for the naked copper and SAMs-covered electrodesobtained from the polarization curves.

SAMs Ecorr (mV) icorr (�A/cm2) IE (%)

Blank −237 2.448 –10 s −217 0.539 77.9830 s −212 0.290 88.15511

fm

(oTt

I

bttW9mistt

sp9

TV

S

BCE

min −278 0.120 95.105 min −269 0.077 96.85h −262 0.026 98.94

or 1 h, the cathodic current densities are reduced by two orders ofagnitude.

From the polarization curves, the corrosion current densityicorr) can be calculated by extrapolating the linear Tafel segmentsf the anodic and cathodic curves to the corrosion potentials (Ecorr).he inhibition efficiency (IE) is obtained with the following equa-ion:

E% = i0corr − icorr

i0corr× 100% (4)

where i0corr and icorr are the corrosion current densities of theare and filmed copper, respectively. The calculated results fromhe equation are listed in Table 2. The results show that the inhibi-ion efficiency of copper assembled for 5 min increases to 95.10%.

hen the electrode is immersed for 1 h, the IE reaches a value of8.94%. The high IE of DT SAMs against the corrosion of copperay be attributed to non-conducting property and hydrophobic-

ty of the alkanes in the densely packed monolayers on the copperurface [23]. The former retards electron transfer across the elec-

rode interface and the latter provides an effective barrier againsthe intimate contact of water to the underlying copper surface [24].

Compared with the surface coverage obtained from the EIStudy, the values of IE are almost identical with that of �. For exam-le, the IE and � values obtained after self-assembly for 1 h are8.94% and 99.81%, respectively.

able 3alues for the circuit parameters and coverage of SAMs.

AMs Ydl (�−1 cm−2 sn 10−6) ndl Rct (k� cm2)

lank 542.9 0.66 1.3TAB – – –thanol – – –

Fig. 7. EIS results of DT SAMs formed in aqueous and ethanol solutions.Nyquist plots; (b) Bode plots.

3.4. Comparison of the anticorrosion properties of the DT SAMsprepared in different solutions

In order to investigate the differences between the DT SAMsformed in aqueous micellar solution and those formed in ethanolsolution, copper electrodes are assembled in the two kinds ofsolutions for 1 h. The electrodes are tested in 0.5 M NaCl using elec-trochemical methods.

3.4.1. EIS measurementsFig. 7 shows the EIS results of the two copper electrodes assem-

bled in different solutions. From the Nyquist plots (Fig. 7(a)), twonearly regular semicircles with similar diameter are observed. Thespectra for both SAMs are almost overlapped in the Bode plots(Fig. 7(b)), which means that they can be analyzed by the equiv-alent circuit shown in Fig. 5(c). Table 3 shows the values for thecircuit parameters. It is clear that the surface coverage of the twoelectrodes is almost the same, which means that the SAMs formed

in different solutions present comparable anticorrosion proper-ties. Since the nsam value is close to 1, the impedance of the CPErepresents a pure capacitor and Ysam represents the monolayer’scapacitance. The consistency of Ysam for the two SAMs shown in

Ysam (�−1 cm−2 sn 10−6) nsam Rsam (k� cm2) � (%)

– – – –0.66 0.98 702.0 99.810.66 0.97 819.0 99.84

P. Wang et al. / Electrochimica

Fig. 8. Polarization curves of the bare copper and the copper modified in aqueousand ethanol solutions.

Table 4Electrochemical parameters for the naked copper and SAMs-covered electrodesobtained from the polarization curves.

SAMs Ecorr (mV) icorr (�A/cm2) IE (%)

BCE

Tst

C

tdvoεiaa

3

tistcercE

4

b

[

[[

[[

[[[

108.[22] M. Hosseini, S. Mertens, M. Ghorbani, M. Arshadi, Mater. Chem. Phys. 78 (2003)

lank −237 2.448 –TAB −262 0.026 98.94thanol −271 0.022 99.10

able 3 indicates that the monolayer’s capacitances are almost theame. Using a Helmholtz model of the interface, the film capaci-ance is related to its thickness by the following equation:

sam = ε0εsamA

dsam(5)

where ε0 is the permittivity constant, εsam the relative permit-ivity of DT, A the geometrical surface area of the electrode andsam the thickness of the monolayer. Since the content of CTAB isery low, the effect of CTAB can be ignored. Therefore, the valuesf εsam in Eq. (5) for the two SAMs are the same and the values of0εsamA for the two SAMs can also be regarded as the same. Thus,t is reasonable to conclude that the thicknesses of the two SAMsre almost the same, which suggests that the film formed in thequeous micellar solution is a monolayer.

.4.2. Polarization curvesFig. 8 represents the polarization curves of the bare copper and

he copper modified by the two kinds of solution. From Fig. 8, its clear that the polarization curves of the two modified copperamples almost overlap. Therefore, both of the SAMs can hinderhe cathodic process of corrosion effectively. Table 4 shows theorrosion current density and inhibition efficiency. The inhibitionfficiencies of the two SAMs are 98.94% (CTAB) and 99.10% (ethanol),espectively, which means that the two SAMs have comparable anti-orrosion abilities. These results agree with that obtained from theIS studies.

. Conclusions

DT can chemisorb on copper surfaces through strong thiolateonds to form a well-organized self-assembled monolayer in aque-

[

[

Acta 55 (2010) 878–883 883

ous micellar solution. The formation of DT SAMs occurs in severalsteps. First, CTAB and DT gradually adsorb to the copper surfaceuntil nearly all the active sites of the surface are covered. Then,DT begins to replace the CTAB that is physically adsorbed to thecopper surface. Finally, all the active sites are invaded by DT and awell-organized monolayer is formed. In the SAMs, very little CTABis detected. Therefore, it is inferred that the CTAB exists in the defector the surface of the DT SAMs.

According to the electrochemical measurements, the formationof DT SAMs in aqueous micellar solution is a fast process. Within5 min, the thickness of the SAMs reaches a maximum and theIE increases markedly to 95.10%. After self-assembly for 1 h, theSAMs are able to greatly hinder the cathodic oxygen reductionreaction. The good inhibition effect can be attributed to the non-conducting properties and hydrophobicity of the alkanes in thedensely packed monolayers on the copper surface.The propertiesof the DT SAMs formed in aqueous solution are compared to thoseformed in ethanol solution. The thicknesses of two SAMs are almostthe same, and the two SAMs have comparable wetting and anti-corrosion properties. All of the results suggest that the proposedmethod used to prepare DT SAMs is simple and environmentallyfriendly. Therefore, it has the potential to replace the conventionalmethod.

In the future, modification of different metal surfaces with DTSAMs prepared in different aqueous micellar solutions may be con-sidered. These future studies may accelerate the application of DTSAMs in the industry.

References

[1] G.K. Jennings, J.C. Munro, T.-H. Yong, P.E. Laibinis, Langmuir 14 (1998)6130.

[2] H.Y. Ma, C. Yang, S.H. Chen, Y.L. Jiao, S.X. Huang, D.G. Li, J.L. Luo, Electrochim.Acta 48 (2003) 4277.

[3] P.E. Laibinis, G.M. Whitesides, J. Am. Chem. Soc. 114 (1992) 9022.[4] Zeljka Petrovic, Mirjana Metikos-Hukovic, Ranko, Babic, Prog. Organ. Coat. 61

(2008) 1.[5] B. Lussem, L. Muller-Meskamp, S. Karthauser, R. Waser, Langmuir 21 (2005)

5256.[6] A.Hutt David, ChangqingLiu, Appl. Surf. Sci. 252 (2005) 400.[7] M.S. Diallo, L.M. Abriola, W.J. Weber, Environ. Sci. Technol. 28 (1994)

1829.[8] DongYan, Jeremy A. Saunders, G.Kane Jennings, Langmuir 18 (2002)

10202.[9] P.E. Laibinis, G.M. Whitesides, D.L. Allara, Y. Tao, A.N. Parikh, R.G. Nuzzo, J. Am.

Chem. Soc. 113 (1991) 7152.10] C.D. Bain, E.B. Trough, Y. Tao, J. Evall, G.M. Whitesides, R.G. Nuzzo, J. Am. Chem.

Soc. 111 (1989) 321.[11] S.H. Chen, C.W. Frank, Langmuir 5 (1989) 978.12] S.R. Cohen, R. Naaman, J. Sagiv, J. Phys. Chem. 90 (1986) 3054.13] F. Sinapi, S. Julien, D. Auguste, L. Hevesi, J. Delhalle, Z. Mekhalif, Electrochim.

Acta 53 (2008) 4228.[14] Z. Mekhalif, F. Sinapi, F. Laffneur, J. Delhalle, J. Electr. Spec. Relat. Phenom. 121

(2001) 149.15] H. Ma, S. Chen, B. Yin, Corros. Sci. 45 (2003) 867.16] H. Ma, X. Cheng, G. Li, S. Chen, Z. Quan, S. Zhao, L. Niu, Corros. Sci. 42 (2000)

1669.[17] O.E. Barcia, O.R. Matoos, N. Pebere, B. Tribollet, J. Electrochem. Soc. 140 (1993)

2825.[18] O.E. Barcia, O.R. Matoos, Electrochim. Acta 35 (1990) 1601.19] H.P. Lee, K. Nobe, J. Electrochem. Soc. 133 (1986) 2035.20] Y. Feng, K.S. Siow, W.K. Teo, K.L. Tan, A.K. Hsieh, Corro. 53 (1997) 389.21] W.J. Guo, S.H. Chen, B.D. Huang, H.Y. Ma, X.G. Yang, Electrochim. Acta 52 (2006)

800.23] Y. Feng, W.-K. Teo, K.-S. Siow, Z. Gao, K.-L. Tan, A.-K. Hsieh, J. Electrochem. Soc.

144 (1997) 55.24] G.K. Jenings, J.C. Munro, T.-H. Yong, P.E. Laibinis, Langmuir 14 (1998) 6130.