Pf

ND

a

ARRAA

KMZPT

1

[Ttasctc

obtrvabmomafiis

0d

Progress in Organic Coatings 75 (2012) 59– 64

Contents lists available at SciVerse ScienceDirect

Progress in Organic Coatings

jou rn al h om epage: www.elsev ier .com/ locate /porgcoat

reparation of conducting poly-pyrrole layer on zinc coated Mg alloy of AZ91Dor corrosion protection

an Sheng ∗, Toshiaki Ohtsukaivision of Materials Science and Engineering, Faculty of Engineering, Hokkaido University, Kita 13 Nishi 8, Kita-ku, Sapporo, Hokkaido 060-8628 Japan

r t i c l e i n f o

rticle history:eceived 22 February 2012eceived in revised form 8 March 2012ccepted 10 March 2012

a b s t r a c t

Conducting polypyrrole (PPy) film was successfully prepared on the zinc coated magnesium alloy ofAZ91D by electrochemical oxidation under constant current control in sodium tartrate aqueous solu-tion. The oxidation was started from the formation of zinc oxide and zinc tartrate, followed by PPyelectropolymerization. Surface morphology and the layer structure were analyzed by using scanning

vailable online 18 April 2012

eywords:agnesium alloy

inc electroplating layerolypyrrole film

electron microscopy (SEM) and energy dispersive spectroscopy (EDS), Raman spectroscopy and X-rayphotoelectron spectroscopy (XPS). The PPy coating exhibited protection properties of the zinc coatedmagnesium alloy against corrosion in sodium chloride aqueous solution. The PPy coating is assumed toenhance the open circuit potential of the zinc coated alloy and cause the alloy to be passivated.

© 2012 Elsevier B.V. All rights reserved.

artrate

. Introduction

Magnesium and its alloys have been utilized in many fields1–4] such as aerospace components, automobiles, computer, etc.he more widespread application has been, however, disturbed byhe poor corrosion resistance and high chemical reactivity of thelloys [4–6]. New surface finishing for enhancement of the corro-ion resistance is expected to be developed. Conducting polymeroating is one of the promising methods for the corrosion protec-ion of metals and has recently attracted considerable attention fororrosion protection applications [7–11].

Electrochemical synthesis of the conducting polypyrrole (PPy)n oxidizable metals in aqueous medium is not easy [12–14]ecause anodic dissolution of the metals occurs before the oxida-ive polymerization from the pyrrole (Py) monomer. Due to theelatively negative redox potential of Mg2+/Mg at E0 = −2.62 Vs. standard hydrogen electrode (SHE), electric charge anodicallypplied to a bare surface of the magnesium alloys is consumedy the active dissolution before the PPy formation from the Pyonomer [15,16]. Additionally, a loose and thick layer of Mg(OH)2

r MgO is formed on the alloys by the anodic polarization of theagnesium alloys [17–20] and may prevent the formation of the

dhesive PPy coating. Since the conducting PPy layer is thus dif-

cult to form directly on the magnesium alloys, in this paper, we

ntroduce an inactive layer of zinc electroplating on the Mg alloyurface before the PPy formation. Though Turhan et al. [21] reported

∗ Corresponding author. Tel.: +81 11 706 6351; fax: +81 11 706 6351.E-mail address: s nan1983@hotmail.com (N. Sheng).

300-9440/$ – see front matter © 2012 Elsevier B.V. All rights reserved.oi:10.1016/j.porgcoat.2012.03.008

to successfully electropolymerize PPy on Mg alloy of AZ91D froman aqueous medium containing sodium salicylate salt, no resultson the corrosion protection was presented in the paper.

Although zinc is a less noble metal, a stable oxide layer canbe anodically formed. We introduced on the magnesium alloythe zinc electroplating layer on which the PPy layer was subse-quently formed. In this paper, we report successful preparation ofthe PPy layer in sodium tartrate aqueous solution containing 0.5 Mpyrrole monomer on the zinc electroplating on the magnesiumalloy of AZ91D. The microstructure, the composition and adher-ence strength of the coating, consisting of the zinc electroplatingand the electropolymerized PPy layer, are presented. Furthermore,we examined the corrosion protection properties of the coating onthe magnesium alloy in sodium chloride aqueous solution.

2. Experimental

Pyrrole monomer (Py) purchased from Kanto Chemical Co.(purity 99%) was used without purification. For the preparation ofelectrolyte, analytical grade reagents and pure water provided fromthe Milli-Q purification system were used. The electrolytes weredeoxygenated by passing nitrogen for longer than 45 min beforeuse.

The die-cast AZ91D alloy (8.77 wt.% Al, 0.74 wt.% Zn) was cut to25 mm × 15 mm × 4 mm, and then polished by #220 to #1500 abra-sive papers, rinsed with distilled water, degreased in acetone, and

dried in air. The magnesium alloy was covered by zinc electroplat-ing. The zinc plating was formed in an aqueous solution containingzinc sulfate at concentration of 30 g L−1 and potassium pyrophos-phate at 100 g L−1 by constant current at 20 mA cm−2 for 30 min at

6 in Organic Coatings 75 (2012) 59– 64

6ce

t0siAt

6e6ulttJte

iow

FtTieu

3

3

zflwta

3

cdb3t−tcwtiawiatza

Fig. 1. SEM micrograph of (A) electrodeposited zinc surface and (B) the cross-

4

2

3

2

20 mA⋅cm-2

1

215 mA⋅cm

-2

10 mA⋅cm-2

0 3

0.6

0.9

-1

0

5 mA⋅cm-2

Po

ten

tia

l, E

/ V

13.4 13.6 13 .8

0.0

0.3

181614121086420

-2

0 N. Sheng, T. Ohtsuka / Progress

0 ◦C. After electroplating of zinc, the alloy was covered by a sili-on resin except for the surface exposed to aqueous solution. Thexposure surface area of the Mg alloy was about 3 cm2.

The conducting PPy layer was formed by constant current oxida-ion at 5–20 mA cm−2 in 0.2 M sodium tartrate solution containing.5 M pyrrole (Py) monomer at room temperature on the magne-ium alloy treated as described above. Py monomer was addednto the deoxygenated sodium tartrate solution by a syringe. Ag/AgCl/saturated KCl electrode and a platinum foil were used for

he reference electrode and counter electrode, respectively.The surface morphology was observed by FE-SEM, JEOL JSM-

500F. The thickness and the depth profile of the PPy coating werevaluated from the cross-sectional view by SEM-EDS, JEOL JSM-510LA. A Raman spectrometer (Bunko-Keiki model BRM-300) wassed with excitation of an YVO4 laser at 532.0 nm wavelength laser

ight. The laser power for the excitation was controlled to be lowerhan 5 mW to avoid damage of the black PPy film. The X-ray pho-oelectron spectroscopy (XPS) analysis was carried out by JEOLPS-9200 with X-ray source of Mg K� at 1253.6 eV. The calibra-ion of the binding energy was performed by taking the Au 4f7/2lectron peak (Eb = 84.00 eV).

The corrosion protection by the PPy layer was examined bymmersion of the magnesium alloy at 25 ◦C in 3.5 wt.% NaCl aque-us solution (pH = 5.6) in which the open circuit potential (OCP)as continuously monitored.

The adhesion test was based on the standard sellotape test.irstly, the surface PPy layer was cut into 1 mm × 1 mm size squares,hen sticking the 3M Scotch clear tape on, and finally, stripping it.he adherence intensity was evaluated from the ratio of the remain-ng number to the total number of the squares. For adhesion of zinclectroplating layer, a pull-off adhesion tester (Elcometer 106) wassed.

. Results

.1. Electroplating of zinc

Fig. 1(A) and (B) shows the SEM photographs of the electroplatedinc surface and the cross-section. The surface was covered with aake-like zinc crystal. From the cross-sectional view, the surfaceas homogeneous and smooth. Adhesion strength of the zinc elec-

roplating layer to the AZ91D substrate, measured by the pull-offdhesion tester, was large enough at about 8 MPa.

.2. Electropolymerization of PPy

Fig. 2 shows the potential transient as a function of electricharge during galvanostatic electropolymerization of PPy at currentensities (CD) from 5 to 20 mA cm−2 on the AZ91D alloy coveredy zinc electroplating. The duration of galvanostatic oxidation was600 s at 5 mA cm−2 and 900 s at 10, 15, and 20 mA cm−2. The poten-ial initially exhibited a plateau at the relatively low potential of0.9 V to −0.6 V, which depended on CD, and then sharply rose

o a peak at about 2.5–3.5 V, and finally gradually decreased to aonstant value of 1.3–2.4 V. The potentials of the plateau and peakere increased with the increase of CD and the electric charge to

he potential peak greatly depended on the CD. The small plateaun the potential transient was observed in Fig. 2, depending on CDt 0.3 V by 5 mA cm−2 and at 1.8 V by 20 mA cm−2, if the plot wasidened in horizontal direction. It is assumed that the zinc oxide

s formed during the initial plateau and the zinc tartrate is formed

round the small plateau. The formation of the zinc compounds inhe initial stage is discussed later. Nucleation of PPy is started on theinc oxide compounds at around the peak potential. After the peak,

black PPy layer was observed. After the nucleation, the potential

sectional view.

gradually felt down and was kept constant. In this period, the PPylayer continuously grew on the whole surface.

Fig. 3 shows the electric charge of the initial potential plateau (Q)as a function of CD in logarithmic scale. The electric charge sharplyincreased with decrease of CD to 13.3 mC cm−2 at 5 mA cm−2.

Elec tric charge, Q / C . cm

Fig. 2. Potential transient during constant current oxidation at 5, 10, 15, and20 mA cm−2 in 0.2 M sodium tartrate solution containing 0.5 M Py monomer as afunction of electric charge applied.

N. Sheng, T. Ohtsuka / Progress in Org

12

14

8

10

-3C

·cm

-2

2

4

6

Q /

10

1.31.21.11.00.90.80.7

0

log ( i / 1 0-3A·cm-2 )

Fig. 3. Electric charge of the initial potential plateau as a function of CD in logarith-mic scale.

Fig. 4. FE-SEM micrograph of PPy layer forme

anic Coatings 75 (2012) 59– 64 61

3.3. Surface morphology of PPy coating

The surface morphology of the PPy layer on the AZ91Dalloy covered by zinc electroplating was observed by FE-SEM. The morphology is given in Fig. 4. The PPy layerformed at 20 and 15 mA cm−2 (Fig. 4(A) and (B)) exhibitedcauliflower-like appearance, in which small spherical grainswith a few �m diameter conglomerated. For the PPy layerformed at 10 mA cm−2 (Fig. 4(C)), however, small sphericalgrains with 1 �m diameter were homogeneously distributedand the conglomerate feature was not observed. The size ofthe cauliflower-like conglomerate was a function of CD. How-ever, the size of the small spherical grains did not depend onCD.

d at (A) 20, (B) 15, and (C) 10 mA cm−2.

62 N. Sheng, T. Ohtsuka / Progress in Organic Coatings 75 (2012) 59– 64

FM

3

btFTtaizb

3

abto[on

apatfcaa

1581125

1414

1328

PPy film

934

990

Inte

nsi

ty, I /

cp

s

1048

2100180015001200900600300

Ranman shift, Δν / cm-1

ig. 5. (A) SEM micrograph and (B) EDS elemental analysis for the cross-section ofg alloy/Zn/PPy coating.

.4. Depth profile of the Mg/Zn/PPy coating

The cross-sectional view of the zinc coated AZ91D alloy coveredy PPy was observed by SEM and the depth profile of elements inhe cross-section was measured by EDS. The results are shown inig. 5 in which the PPy layer was formed at 15 mA cm−2 for 900 s.he EDS result was an average at the width of 3.6 �m indicated byhe arrow shown in the SEM view (Fig. 5(A)). From the SEM viewnd the elemental depth profile, the PPy layer containing C and Ns seen to cover the zinc electroplating layer. The thickness of theinc and PPy layers was evaluated from the cross-sectional view toe about 10 �m.

.5. Raman spectroscopy and XPS measurement

Fig. 6 shows the Raman spectra of the PPy film that was formedt 10 mA cm−2 for 900 s on the zinc electroplating. The Ramanands of PPy film at 1581, 1414, 1328 and 990 cm−1 are assignedo the conjugated C C stretching, C N stretching, C N stretchingr ring stretching of Py ring, and the ring deformation, respectively22–24]. The bands at 1048 and 934 cm−1 are the C H in-plane andut-of-plane deformation, respectively [22–24]. In Fig. 6 there wereo Raman bands corresponding to the doping by tartrate anions.

In order to acquire information on the elementary compositionnd confirm tartrate anions doped in the PPy layer, XPS analysis waserformed. On the survey spectrum, we found signals of nitrogennd oxygen, which indicated the presence of PPy and tartrate onhe surface, respectively. Fig. 7 shows XPS spectra of the PPy film

−2

ormed at 10 mA cm for 900 s. The carbon signal of C 1 s (Fig. 7(A))omprised three peaks at 285.1, 286.9 and 288.7 eV. The main peakt 285.1 eV corresponds to the C C carbon in the Py ring [25–27],nd the second peak at 286.9 eV is attributed to both carbons of

Fig. 6. Raman spectra of PPy layer formed at CD of 10 mA cm−2 in 0.2 M sodiumtartrate containing 0.5 M Py monomer.

C N in the Py ring and to the C OH in the doped tartrate anion[27]. The third peak at 288.7 eV corresponds to carbon of O C Oin tartrate [25–27].

The nitrogen signal of N 1s (Fig. 7(B)) also comprised three peaksat 398.0, 400.2 and 402.1 eV. All three peaks originated from nitro-gen of PPy. The main peak centering at 400.2 eV is attributed tothe neutral nitrogen atoms in the Py ring, NH [25,28]. The smallshoulder peak at the lower binding energy of 398.0 eV is assignedto C N [28–30], which may be a structural defect in the form ofimine-like nitrogen. Another shoulder at the higher binding energyat 402.1 eV is assigned to the positively charged nitrogen [25].

The oxygen signal of O 1s (Fig. 7d) comprised three peaks at532.0, 532.6 and 533.6 eV. The presence of oxygen atoms is an evi-dence of the doping by tartrate anions in the PPy film. The strongestpeak at 532.6 eV is attributed to the oxygen in the C O band,and the other two at 532.0 and 533.6 eV are the oxygen atomsof carboxylic acid groups in the tartrate anion, C O and C O,respectively [30,31].

Since little signals were observed for Zn 2p1/2 and Zn 2p3/2, itwas concluded that no zinc existed on the PPy surface.

3.6. Corrosion test in 3.5 wt.% NaCl solution

The AZ91D alloy covered by zinc and the electropolymerized PPywas immersed in the 3.5 wt.% sodium chloride solution at 25 ◦C inorder to evaluate the corrosion protection of PPy coating. Duringthe immersion, the surface of the alloy was visually observed forthe rust generation. The transient of OCP during the immersion isshown in Fig. 8, in which the PPy layer was formed on zinc coatedAZ91D magnesium alloy by constant current control with 10, 15,and 20 mA cm−2 for 900 s and the thickness of the PPy layer wasestimated to be about 8.5, 10, and 30 �m from the cross-sectionalSEM view, respectively.

When the potential was higher than −1.0 V no corrosion prod-ucts were observed. When the potential decayed lower than −1.0 V,white corrosion products immediately appeared. Thus, the timeperiod before the potential decays to −1.0 V is assumed as a pro-tection time, in which the PPy coated substrate was possibly keptin the passive state. The protection time was found from Fig. 8 todepend greatly on the thickness of the PPy layer, increasing withthe thickness of the PPy layer covering the alloy.

3.7. Adhesion of the PPy layer

The PPy layer strongly adheres to the zinc electroplating layeron the magnesium alloy. When the PPy layer formed on the

N. Sheng, T. Ohtsuka / Progress in Organic Coatings 75 (2012) 59– 64 63

800

1000

1200

C 1s

(A) C-C

(285

C-N

/ C-O

O=

C-O

200

400

600

5.1

eV)

OH

(286.9

eV)

O (2

88

.7 eV

)

280282284286288290292294

0

Binding Energy / eV

(B) -

200

300

400 N 1s

-N-H

(40

0.2

eV)

=N

-(3

98.0

e

-NH

+(4

02.1

eV)

396398400402404406

0

100

Inte

ns

ity

/ c

ou

nts

Inte

ns

ity

/ c

ou

nts

Inte

ns

ity

/ c

ou

nts

Binding Ener gy / eV

V)

)

800

1000

1200

1400

O 1s

o

C-O

C-O

(53

2.6

eV

C=

O (5

32

(C)

0

200

400

600

800o (53

3.6

eV)

V) .0 eV

)

528530532534536538

Binding Energ y / eV

zno−ai

4

4

fot

0.0

-2

-0.8

-0.420mAcm

(30 μm)

15mAcm-2

(10 μm)

10mAcm-2

(8.5 μm)

3.02.52.01.51.00.50.0-1.6

-1.2

Po

ten

tia

l, E

/ V

vs

. A

g/A

gC

l/S

at.

KC

l

Time , t / h

Fig. 8. Open circuit potential (OCP) of zinc coated AZ91D magnesium alloy coveredby PPy layer during exposure in 3.5 wt.% sodium chloride aqueous solution. The PPy

Fig. 7. XPS spectra of PPy–tartrate film: (A) C 1s, (B) N 1s, and (C) O 1s.

inc electroplating at various CDs underwent the sellotape test,o squares were peeled off. It was found that the adherencef PPy layer was large enough. After the potential decreased to1.5 V during the immersion in the sodium chloride solution, thedhesion of the PPy layer was lost. For the PPy layer after themmersion, 100% squares were easily removed.

. Discussion

.1. Role of the zinc electroplating layer

Zinc electroplating may be a necessary process for the PPy layerormation on the magnesium alloy. When the magnesium alloy wasxidized in the tartrate solution containing Py monomer withouthe zinc electroplating layer, the potential did not rise high enough

layers were formed at CD of 10, 15, and 20 mA cm−2 for 900 s in 0.2 M sodium tartratesolution containing 0.5 M Py monomer.

to electropolymerize the Py monomer and remained in the regionof active dissolution of magnesium. The presence of the zinc layeron the magnesium alloy prevents the dissolution significantly andenables PPy to be electropolymerized on the surface.

4.2. Structure of the PPy layer

When the zinc coated magnesium alloy is oxidized under gal-vanostatic condition, in sodium tartrate solution containing pyrrolemonomer, initially the potential was kept constant without any PPyformation. The plateau potential was higher with increase of the CDapplied (Fig. 2) and the charge density of the plateau increased withdecrease of CD (Fig. 3). During the plateau, we assumed that zincoxide and tartrate are formed on the zinc electroplating layer withsimultaneous dissolution of zinc. In order to inspect the formationof zinc compounds before the PPy polymerization, Raman spec-tra were gathered at specified points during the constant currentelectropolymerization. A potential transient during the PPy elec-tropolymerization at 10 mA cm−2 is shown in Fig. 9(A), in which twoRaman spectra of the surface were taken at −0.5 V and at 0.8 V atwhich the galvanostatic oxidation was stopped. The Raman spectraare shown in Fig. 9(B). The Raman peaks at 812 cm−1, 929 cm−1 and1007 cm−1 were identified as those of tartrate compounds [32,33].The large peak at 1420 cm−1 may be also a Raman band for O C Osymmetric stretching in tartrate, although the relative intensity istoo large, as compared with reference data of sodium tartrate takenin this study which was not shown. We can assume that the tartratecompound is zinc tartrate. The Raman peak at 565 cm−1 indicatesthe presence of zinc oxide, ZnO on the surface [34,35]. Two sharppeaks at 322 and 1560 cm−1 were assumed to correspond to thoseof the Py monomer adsorbed or absorbed in the surface compound.When Raman peak intensities of the tartrate and zinc oxide werecompared, it was seen that the intensity of zinc oxide decreasesfrom −0.5 V to 0.8 V, while the intensities of Raman bands of zinctartrate increased or remained constant. The relative ratio of theintensities means that the zinc oxide was formed at the more neg-ative potential and then was gradually replaced with zinc tartrate.Since the thickness of the zinc oxide and zinc tartrate formed beforethe PPy polymerization is considered to increase with the chargeof the plateau shown in Fig. 3, the thicker layer of zinc oxide andtartrate may be formed with the lower CD.

The Py polymerization is then started on the zinc electroplatinglayer that is covered by zinc oxide and zinc tartrate. Finally the thickPPy layer is formed on the inner layer of zinc oxide and zinc tartrate.

64 N. Sheng, T. Ohtsuka / Progress in Org

(A)

1

2

3

V

-1

0

-0.5 V

Po

ten

tia

l, E

/ V

0.8

(B)

0 20 0 40 0 60 0 80 0

Time, t / s

14162.5

1560

100

79

29

V

Inte

nsit

y, I / cp

s

0.8 V

82

1

322

ZnO

500100015002000

-0.5

Raman shift, Δν / cm-1

F1

4

PtTitmtovaptptp

wacitTt

[

[

[

[[[

[

[[

[[[

[

[[

[

[

[

[

[

[

[

[

ig. 9. (A) Potential change with time during polymerization of Py at CD of0 mA cm−2. (B) Raman spectra of the film formed at point (a) −0.5 V and (b) 0.8 V.

.3. Corrosion protection

The AZ91D alloy covered by the zinc electroplating and thePy layer exhibited relatively high OCP at about 0.0 V in the ini-ial immersion in 3.5% sodium chloride solution, as shown in Fig. 8.he high potential indicates that the zinc electrolating on the alloys passivated by the inner layer of zinc oxide and zinc tartratehat were formed in the potential plateau before the PPy poly-

erizartion; in other words, the inner layer of zinc oxide and zincartrate works as a passivating layer against corrosion. When theuter PPy layer did not exist on the inner layer, however, the passi-ation was immediately lost by dissolution of the inner zinc oxidend zinc tartrate. The outer PPy layer can work as a barrier thatrevents contact between the inner layer and the aqueous solu-ion. We can assume that the oxidative property of the PPy layerlays a more efficient role in which the oxidative property keepshe zinc electroplating in the passive state and maintains the innerassivating layer stable.

When the alloy covered by zinc electroplating and the PPy layeras immersed for long time in sodium chloride solution, chloride

nions may penetrate in the outer PPy layer with water and finallyontact of the inner layer. The chloride anions and water entering

nduces dissolution and breakdown the inner zinc oxide and zincartrate, and the zinc electroplating loses the passivating layer.he loss of the passivating layer results in potential decrease tohe active potential region and the appreciable corrosion of the

[[[

anic Coatings 75 (2012) 59– 64

magnesium alloy is started. The loss of the passive film underneaththe PPy layer may also result in the loss of adhesion strength ofthe PPy layer.

5. Conclusions

A conducting poly-pyrrole (PPy) layer was successfully preparedon magnesium alloy AZ91D in aqueous sodium tartrate solutionby introducing a zinc electroplating layer between the magnesiumalloy and the PPy layer. The PPy coating formed is assumed to havethe layer structure of Mg alloy/Zn electroplating/zinc oxide and Zntartrate/PPy layer doped with tartrate.

The PPy coating maintained the passive state in 3.5% sodiumchloride solution for 1–2 h, depending on the thickness of the PPylayer. In the passive state, no corrosion products were observed,indicating that the PPy layer works as an effectively protectivelayer.

References

[1] A.M. Fekry, M.Z. Fatayerji, Electrochimica Acta 54 (2009) 6522–6528.[2] S. Toros, F. Ozturk, I. Kacar, Journal of Materials Processing Technology 207

(2008) 1–12.[3] D. Gastaldi, V. Sassi, L. Petrini, M. Vedani, S. Trasatti, F. Migliavacca, Journal of

the Mechanical Behavior of Biomedical Materials 4 (2011) 352–365.[4] T.M. Pollock, Science 328 (2010) 986–987.[5] M. Jönsson, D. Persson, C. Leygraf, Corrosion Science 50 (2008) 1406–1413.[6] A. Eliezer, E.M. Gutman, E. Abramov, Y. Unigovski, Journal of Light Metals 1

(2001) 179–186.[7] D. Kowalski, M. Ueda, T. Ohtsuka, Corrosion Science 49 (2007) 1635–1644.[8] D. Kowalski, M. Ueda, T. Ohtsuka, Corrosion Science 50 (2008) 286–291.[9] D. Kowalski, M. Ueda, T. Ohtsuka, Corrosion Science 49 (2007) 3442–3452.10] M. Sabouri, T. Shahrabi, H.R. Faridi, M.G. Hosseini, Progress in Organic Coatings

64 (2009) 429–434.11] J.I. Martins, S.C. Costa, M. Bazzaoui, G. Gonc alves, E. Fortunato, R. Martins, Jour-

nal of Power Sources 160 (2006) 1471–1479.12] T. Ohtsuka, M. Iida, M. Ueda, Journal of Solid State Electrochemistry 10 (2006)

714–720.13] M. Takakubo, Synthetic Metals 16 (1986) 167–172.14] S. Tsuchiya, M. Ueda, T. Ohtsuka, ISIJ International 47 (2007) 151–156.15] J.I. Martins, M. Bazzaoui, T.C. Reis, S.C. Costa, M.C. Nunes, L. Martins, E.A. Baz-

zaoui, Progress in Organic Coatings 65 (2009) 62–70.16] J.I. Martins, T.C. Reis, M. Bazzaoui, E.A. Bazzaoui, L. Martins, Corrosion Science

46 (2004) 2361–2381.17] J.E. Gray, B. Luan, Journal of Alloys and Compounds 336 (2002) 88–113.18] J.H. Nordlien, S. Ono, N. Masuko, K. Nisancioglu, Corrosion Science 39 (1997)

1397–1414.19] A. Bakkar, V. Neubert, Corrosion Science 49 (2007) 1110–1130.20] R. Udhayan, D.P. Bhatt, Journal of Power Sources 63 (1996) 103–107.21] M.C. Turhan, M. Weiser, M.S. Killian, B. Leitner, S. Virtanen, Synthetic Metals

161 (2011) 360–364.22] J. Duchet, R. Legras, S. Demoustier-Champagne, Synthetic Metals 98 (1998)

113–122.23] F.e. Chen, G. Shi, M. Fu, L. Qu, X. Hong, Synthetic Metals 132 (2003) 125–132.24] M.J.L. Santos, A.G. Brolo, E.M. Girotto, Electrochimica Acta 52 (2007)

6141–6145.25] F. Samir, E. Benseddik, B. Corraze, C. Bernede, M. Morsli, A. Bonnet, A. Conan, S.

Lefrant, Synthetic Metals 69 (1995) 341–342.26] E.A. Bazzaoui, M. Bazzaoui, J. Aubard, J.S. Lomas, N. Félidj, G. Lévi, Synthetic

Metals 123 (2001) 299–309.27] M. Bazzaoui, L. Martins, E.A. Bazzaoui, J.I. Martins, Journal of Electroanalytical

Chemistry 537 (2002) 47–57.28] J.F. Marco, J.R. Gancedo, H. Nguyen Cong, M. del Canto, J.L. Gautier, Solid State

Ionics 177 (2006) 1381–1388.29] A. Madani, B. Nessark, R. Boukherroub, M.M. Chehimi, Journal of Electroanalyt-

ical Chemistry 650 (2011) 176–181.30] L. Ruangchuay, J. Schwank, A. Sirivat, Applied Surface Science 199 (2002)

128–137.31] P. Goldberg-Oppenheimer, S. Cosnier, R.S. Marks, O. Regev, Talanta 75 (2008)

1324–1331.32] N. Kaneko, M. Kaneko, H. Takahashi, Spectrochimica Acta Part A: Molecular

Spectroscopy 40 (1984) 33–42.33] V. Ramakrishnan, J.M.T. Maroor, Infrared Physics 28 (1988) 201–204.34] T. Ohtsuka, M. Matsuda, Corrosion 59 (2003) 407–413.35] S.J.A. Hugot-Le Goff, B. Saïdani, R. Wiart, Journal of Electroanalytical Chemistry

263 (1989) 127–135.