Corrosion Science 50 (2008) 1538–1545

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Corrosion Science

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Electrosynthesis of adherent poly(3-amino-1,2,4-triazole) films on brass preparedin nonaqueous solvents

M. ELbakri, R. Touir, M. Ebn Touhami *, A. Srhiri, M. BenmessaoudFaculty of Science, Electrochemical Corrosion and Environment Laboratory, BP 133 Kenitra, Morocco

a r t i c l e i n f o

Article history:Received 2 January 2007Accepted 11 February 2008Available online 7 March 2008

Keywords:3-Amino-1,2,4-triazolElectropolymerizationInsulating filmCorrosionInhibition

0010-938X,/$ - see front matter � 2008 Elsevier Ltd.doi:10.1016/j.corsci.2008.02.014

* Corresponding author. Tel.: +212 61 58 63 00; faxE-mail address: mebntouhami@yahoo.fr (M. Ebn T

a b s t r a c t

The kinetics of the electropolymerization of 3-amino-1,2,4-triazole on a brass substrate in alkalinesolution containing methanol was investigated using cyclic polarization, chronoamperometry, electro-chemical impedance techniques and scanning electronic microscopy. The polymeric film was preparedby successive cycles of potential of a 60Cu–Zn electrode between 0 and 1.6 V. During the secondcycle, the oxidation peak of the monomer disappears indicating the formation of the insulating film.We have also shown that the monomer oxidation reaction is essentially irreversible and controlled bya diffusion process. The protective effect of the film formed on brass has been studied in a 3%NaCland 3%NaCl + S2� solution. The results showed important inhibition efficiency, about 96% for 4 h oftesting time.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

Copper and its alloys are widely used in the industrial applica-tion such as cooling water system, thermal station, driven of water,and motor of car. However, the corrosion resistance of these alloysnot only depends on their chemical composition but also dependson the environment medium nature.

The corrosion inhibition by compounds of azole type has beenwidely studied in the last years [1–15]. The pyrazole type com-pounds [11,16–19], triazole [1,2–10,20,21] and tetrazole [22] gavegood results. It is believed that the formation of a chemisorbedpolymeric protective layer [12,20] involves in some cases of film‘‘painting” type.

The polymer deposition produces an effect of surface passiv-ation preventing all other ulterior formation of radicals necessaryfor the development of the polymer and thus consequently thepassage electric current is strongly decreased [23]. Kertit et al.[24] suggested a process of electropolymerization of the 3-ami-no-1,2,4-triazole (ATA) on metallic substrate according to the reac-tion pathway:

All rights reserved.

: +212 37 36 27 32.ouhami).

N

N

N

N H 2

H

H

KOH

-H +

N

N

N

N H-

H

H

N

N

N

N H-

H

H

N

N

N

N H-

H

H

-1 e -

N

N

N

N H

H

H

N

N

N

N H

H

H

N

N

N

N H

H

H

-n e -

Polymer

This compound has already been used like alloy 60Cu–Zn corro-sion inhibitor. But for very long immersion times (>320 h), theyhave noted that the alloy 60Cu–40Zn undergoes a dezincification[25].

The aim of this paper is to study the electropolymerization ofthe 3-amino-1,2,4-triazole in a medium of NaOH 0.1 M dissolvedin pure methanol, and to test the effect of the polymeric film onthe corrosion of the 60Cu–Zn alloy in a 3%NaCl pure and 3%NaCl

Fig. 1. Chemical formula of 3-amino-1,2,4-triazole (ATA).

M. ELbakri et al. / Corrosion Science 50 (2008) 1538–1545 1539

polluted by the sulphurs (2 ppm S2�). The molecule used in thiswork is 3-amino-1,2,4-triazole (ATA) whose formula is shown inFig. 1.

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

0.000

0.002

0.004

0.006

0.008

i(mA.

cm-2)

E(V/Ag-AgCl)

2. Experimental

The experiments were performed in a classical three-electrodeelectrochemical cell. The working electrode was a rotating diskconsisting of cylindrical samples (Cu 60.61%, Al 0.12%, Zn 39.19%and Si 0.08%) and had a 0.5 cm2 cross-sectional area. The rotatingsweep rate was maintained at 1000 rpm. The samples were firstpolished with emery paper grade 600 and rinsed with acetone.The auxiliary electrode was a large surface area platinum gridand an Ag/AgCl electrode was used as reference. All the potentialspresented are referred to this electrode.

The solution used to deposit polymer was constituted by 3-ami-no-1,2,4-triazole (monomer) 98% – ACROS, sodium hydroxide(supporting electrolyte) 85% – GPR and pure methanol (solvent)99.9% – MERCK.

To evaluate the resistance corrosion of poly(ATA), we used twocorrosive mediums: pure 3%NaCl and 3%NaCl polluted by the sulp-hides (2 ppm S2�).

The experiments were carried out in aerated solution at298 ± 1 K. The voltammograms, I–t transient curves and the elec-trochemical impedance measurements were performed using anEG & G potentiostat (model 6310) driven by a computer. Theamplitude of the a.c. signal was 5 mV (r.m.s) and the frequencyrange analysed was 100 kHz to 0.1 Hz using five points perdecade.

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

0.00

0.50

1.00

1.50

(c)(b)

(a)FE

D

C

BA

C: 0.9V F: 1.4V

B: 0.6V E: 1.3V

A: 0.5V D: 1.1V

i(m

A/c

m2 )

E(V/Ag-AgCl)

Fig. 2. Voltammograms of brass electrode performed in methanol alkaline solutionat a scan rate of 500 mV mn�1 with ATA (10�1 M). (a) First cycle; (b) second cycle;(c) third cycle.

3. Results and discussion

3.1. Electrochemical properties of 3-amino-1,2,4-triazole

3.1.1. Cyclic polarizationIn Fig. 2 typical voltammograms are presented. These are re-

corded on a 60Cu–Zn electrode for potentials ranging from 0 to1.6 V in the presence of ATA. The obtained voltammograms showonly one peak around E = 1.1 V. This peak may be assigned to theoxidation of the monomer. As shown in Fig. 2, the current densitydecreases when the number of cycles increases. The final valueafter three cycles is 0.3 lA cm�2, attributed to the formation ofthe film on the electrode surface. The film efficiency may be eval-uated by the Ip1/Ip2 ratio, where Ip1 and Ip2 are the peaks of the cur-rent for the first and the second cycle, respectively. The efficiencyof the monomer oxidation reaches 96%.

Fig. 3. Voltammograms of brass electrode performed in methanol alkaline solutionat a scan rate of 500 mV mn�1 without ATA.

0 500 1000 1500 2000

0.000

0.500

1.000

1.500

2.000

2.500

1.1V

1.2V 1.3V

1V

0.9V 0.8V

i(m

A/c

m2 )

t(s)

Fig. 4. The current–time curves of brass electrode performed in methanol alkalinesolution at different potentials localized before and after the peak corresponding tothe oxidation of the monomer.

1540 M. ELbakri et al. / Corrosion Science 50 (2008) 1538–1545

For comparison, a polarization curve on brass electrode in 0.1 MNaOH in methanol without ATA (Fig. 3) is also drawn by dot line,this is an oxidation of electrolyte. These results indicate that ATA

0 5 10 15 20 25 300

5

10

15

20

0.1Hz

0.65Hz

4.1Hz

C: 0.9V

0 5000 10000 15000 20000 250000

5000

10000

15000

0.065Hz

0.01Hz6.5Hz

E: 1.3V

0 1 2 3 4 5 60

1

2

3

4

5

6

A: 0.5V

25.87Hz

0.016Hz

Real part ( KΩ.cm2)

Real part ( KΩ.cm2)

Real part ( KΩ.cm2)

Imag

inar

y pa

rt (K

Ω.c

m2 )

Imag

inar

y pa

rt (K

Ω.c

m2 )

Imag

inar

y pa

rt (K

Ω.c

m2 )

Fig. 5. Electrochemical impedance diagrams obtained on the brass at different potentials(A) 0.5 V; (B) 0.6 V; (C) 0.9 V; (D) 1.1 V; (E) 1.3 V; (F) 1.4 V.

is oxidized to form an insulating poly-3-amino-1,2,4-triazolep(ATA) and this barrier film covers the electrode surface to preventfurther oxidation.

0 5000 10000 15000 20000 25000 300000

5000

10000

15000

20000

0.1Hz

0.65Hz6.5Hz

D: 1.1V

0 5000 10000 150000

5000

10000

15000

20000

10.3Hz

0.01Hz

0.065Hz

F: 1.4V

0 500 1000 1500 20000

500

1000

1500

0.1HZ0.25Hz0.65Hz

41Hz

B: 0.6V

Real part ( KΩ.cm2)

Real part ( KΩ.cm2)

Real part ( KΩ.cm2)

Imag

inar

y pa

rt (K

Ω.c

m2 )

Imag

inar

y pa

rt (K

Ω.c

m2 )

Imag

inar

y pa

rt (K

Ω.c

m2 )

localized before and after the peak corresponding to the oxidation of the monomer.

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

0.00

0.50

1.00

1.50

2.00

2.50 20m V/S

15m V/s

10m V/s

5m V/S

i(m

A/c

m2 )

E( V/Ag-AgC l)

Fig. 6. The cyclic voltammograms of brass in electrodeposition at given scan rates,mV s�1.

Table 2The effect of scan rate on Ep and Ip

Scan rate (mV/s) Ep (V/Ag–AgCl) Ip (mA cm�2)

5 0.995 0.85110 1.254 1.69015 1.385 2.34520 1.439 2.550

1 2 3 4 5

0.5

1.0

1.5

2.0

2.5

3.0

I p(mA

/cm

2 )

Vb1/2(mV/s)1/2

Fig. 7. The linear dependence of Ep on log(V1/2) for brass in electrodepositionmedium.

M. ELbakri et al. / Corrosion Science 50 (2008) 1538–1545 1541

3.1.2. Current–time curvesFig. 4 shows the variation of the current density with time at

different potentials localized before and after the peak correspond-ing to the oxidation of the monomer. A smooth current was ob-served while a thick polymer coating was formed. This improvedthe formation of the isolated organic layer on the electrode surface.The low value of the current obtained at the end of the electrolysisis attributed to the oxidation of the monomer through the defect ofthe organic layer.

Current–time curves show that the homogeneous and adhesivefilm depends on the electrolysis potential value. At higher voltages,the fissured film is due to the oxidation of the electrolyte duringthe process of film formation [26]. At lower voltages, the film isformed but this requires a long time.

3.1.3. Electrochemical impedanceThe impedance diagrams shown in Fig. 5 were obtained for a

60Cu–Zn electrode at different potentials localized before and afterthe peak corresponding to the oxidation of the monomer (Fig. 2).Two capacitive loops were observed. The capacitance value forthe high frequency (HF) loop varies between 1.5 and 3 lF cm�2;this value is too low to correspond to a double layer (�50 lF cm�2).The associated parameters presented in Table 1 show that theproduct RHF � I (where RHF is the diameter of the capacitive timeconstant in the HF range and I is an anodic current density) isnot constant, which makes it difficult to ascribe this loop to thecharge transfer process.

These observations led to the fact that the first time constantobtained on the impedance diagrams may be attributed to thecharge transfer process coupled with a second process, whichmay correspond to the mass transfer. For lower frequencies, the va-lue of capacity is lower than 1 mF cm�2. This result could be ex-plained either by the fact that the measured capacitance doesnot have the physical character of an interfacial one and/or bythe presence of dissolution products on the metallic surface.

3.1.4. Scan rateThe influence of the scan rate on the cyclic voltammetry behav-

iour of brass in the electropolymerization solution is shown inFig. 6. The effect of scan rate (Vb) on Ip and Ep characteristic ofthe oxidation monomer peak can be quantitatively analysed (Table2). Fig. 7 shows the relationship between Ip and (Vb)1/2. The plot isin the form of a straight line with the ordinate at its origin equal tozero. Additionally, it is also found that the Ep value for the peakshows a linear variation with logarithmic scan rate (Fig. 8). Thesefindings suggest that the anodic processes, in the monomer oxida-tion potential range, are under diffusion control, and the corre-sponding system is completely irreversible [27].

The peak current Ip is related to the scan rate with relation[30,31]

Ip ¼ ð2:99 � 105ÞnðanaÞ1=2ACD1=20 V1=2

where A, D0, C, n, na, a and V represent the electrode area, the diffu-sive species coefficient, the concentration of the diffusive species,the number of exchanged electrons, the apparent number of elec-

Table 1The effect of anodic polarisation on HF parameters

Points RHF (KX cm2) fHF (Hz) CHF (lF cm�2) 2.3RHF � I (mV)

A 2.5 25.87 2.2 703.8B 1.2 41 3.2 579.6C 10.5 6.5 2.3 21530.3D 15.7 6.5 1.5 45163.3E 13.7 6.5 1.9 7907.4F 11.5 10.3 1.3 3969.2

trons transferred, the transfer coefficient and the scan rate,respectively.

The expression of the current of the peak becomes [27,28]

Ip ¼ 0:227nFAC0k0 exp½�ðanaF=RTÞðEp � E0Þ

This equation predicts that log(Ip) is linear vs. Ep with a slope equalto �(anaF/RT), the electron transfer rate constant (k0) can be calcu-lated from the intercept (Fig. 9). We found that E0 = 318 mV,ana = 0.55, k0 = 0.48 � 10�5 cm2 s�1 and D0 = 1.27 � 10�2 cm2 s�1.

3.1.5. Sweep rateThis study was carried out for a 0.1 M monomer concentration

(support electrolyte concentration 0.1 M NaOH) with a 500 mV/min of scan rate. The voltammograms I = f(E) are given in Fig. 10,and the characteristics of the curves I = f(E) are represented in (Ta-ble 3).

0.35 0.40 0.45 0.50 0.55 0.60 0.650.9

1.0

1.1

1.2

1.3

1.4

1.5

E p(V/A

g-A

gCl)

log(V1/2)(mV/s)1/2

Fig. 8. The linear dependence of Ip on V1/2 for brass in electrodeposition medium.

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

0

1

2

3

4

5

6

7

i(mA

/cm

2 )

E(V/Ag-AgCl)

0 tr/mn 100 tr/mn 500 tr/mn 1000 tr/mn 1500 tr/mn

Fig. 10. The I = f(E) curves of ATA oxidation at various sweep rate on alloy 60Cu–40Zn electrode.

Table 3The parameters curves recorded at various sweep rates

Sweep rate (rpm) Epik (V/Ag–AgCl) Ipik (mA cm�2)

0 1.098 1.25100 1.035 3.43500 1.015 4.891000 1.02 5.391500 1.03 6.16

1542 M. ELbakri et al. / Corrosion Science 50 (2008) 1538–1545

This table shows that the values of current density peaks corre-sponding to ATA oxidation increase with increasing sweep ratework electrode. This result indicated that the process is controlledby diffusion.

3.2. The evaluation of corrosion resistance of poly(ATA) films in3%NaCl and 3%NaCl + 2 ppm S2� solutions

The purpose of this study was to improve our understanding ofthe mechanism of protection offered by poly(ATA). Impedancemeasurements were performed as described earlier.

3.2.1. 3%NaCl solution and 3%NaCl solution + 2 ppm S2�

Fig. 11 presents Nyquist plot of impedance spectra of 60Cu–Znalloy in aerated 3%NaCl solution with and without 2 ppm of sul-phide at corrosion potential (Ecorr) after 1 h of stabilisation time.

The impedance spectra show only one capacitive loop. Its diam-eter decreases from 1311 X cm2 in the absence of sulphide to689 X cm2 in the presence of sulphide. This variation confirmedthat the sulphide ions accelerate the corrosion rate, and the corro-sion products of the film formed on the electrode surface becomeless protective compared to the one formed in 3%NaCl solution.

0.7 0.8 0.9 1.0 1.1 1.2

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

Ln(I p)

(Ep-E0)

Fig. 9. The relationship between ln(Ip) and (Ep–E0) for brass in electrodepositionmedium.

The value of capacity increases from 77 lF cm�2 to 366 lF cm�2

(table 4). It is worth to recall that Cu2S is an electronic conductingspecies present in the corrosion products layer, and thus gives riseto a double layer capacity [29]. These values of capacity wereattributed to the double layer capacity associated to the chargetransfer resistance.

3.2.2. Polymer coated metalMost authors agree that the simple AC impedance shown in

Fig. 12 can be used for the analyses of impedance data for polymercoated metals that have been exposed to corrosive media [30–35].At low frequencies, we would expect to find the impedance re-sponse owing to the contact between the aqueous solution (atthe bottom of the defects) and the metal. This concept of filmporosity has been used recently by Walter [36].

0.0 0.5 1.0 1.50.0

0.5

1.0

3% NaCl 3% NaCl + 2ppm S2-

0.631

0.0631

0.0631

1.585Hz3.981Hz

Imag

inar

y pa

rt (k

Ohm

.cm

2 )

Real part (kOhm.cm2)

Fig. 11. The electrochemical impedance diagrams of brass at Ecorr after 1 h of im-mersion in 3%NaCl solution with and without 2 ppm of S2�.

Table 4The impedance parameters for brass in 3%NaCl solution and 3%NaCl solution + 2 ppmS2� obtained at Ecorr after 1 h of immersion

Medium Rp (X cm2) f (Hz) C (lF cm�2)

3%NaCl 1311 1.585 773%NaCl + 2 ppm S2� 689 0.631 366

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 150

1

2

3

4

5

6

7

8

9

10

Real part (kOhm.cm2)

2.512 Hz

0.398 Hz

0.06 Hz0.63 Hz

10 Hz

1h4h24h

Imag

inar

y p

art

(kO

hm

.cm

2 )

Fig. 13. The electrochemical impedance diagrams of brass with poly(ATA)coating in3%NaCl solution determined for a different immersion time.

Table 5The effect of immersion time for brass polymer coated electrode in 3%NaCl solutionand 3%NaCl solution + 2 ppm S2�

Solution Immersion time (h) Rp (X cm2) C (lF cm�2) E%

3%NaCl 1 6500 61 804 17,900 56 8724 30,000 35 92

3%NaCl + 2 ppm S2� 1 71,000 3 994 68,000 9 9924 8500 118 88

0 10 20 30 40 50 60 70 80 900

10

20

30

40

50

60

Real part (kOhm.cm2)

1Hz

1Hz 0.1Hz

0.1Hz

0.01Hz

0.01Hz

0.01Hz

1h

4h

24h

Imag

inar

y p

art

(kO

hm

.cm

2 )

Fig. 14. The electrochemical impedance diagrams of brass with poly(ATA) coatingin 3%NaCl solution + 2 ppm S2� determined for a different immersion time.

M. ELbakri et al. / Corrosion Science 50 (2008) 1538–1545 1543

Impedance data were interpreted using the equivalent electricalcircuit of Fig. 12, where Cfilm, Rfilm and fc(film) represent the coatingcapacitance, the resistance related to the electrolyte passingthrough the coating defects and the characteristic frequency atthe maximum of the imaginary part, respectively. According toWong et al. [37] Cdc is the double layer capacitance, and Rct is thecharge transfer resistance.

In the present work, EIS of the polymer film formed on 60Cu–Znelectrode was performed as a function of immersion time.

3.2.2.1. 3%NaCl solution. In the presence of the polymer, the spec-trum shows the presence of two time constants (Fig. 13). At lowfrequency loop, it is found that the resistance value increased inthe presence of polymer, indicating the inhibiting effect of thispolymer (Table 4 and Table 5). The inhibition efficiency (% E) wascomputed from following equation:

E% ¼ 100 � ½ðRp � R0pÞ=Rp�

where R0p and Rp are the values of polarisation resistance of brass

after immersion in aggressive solution without and with poly(ATA)coating, respectively.

In the presence of the polymer, it is found that the resistancevalue increased with the immersion time, whereas the doublelayer capacity value found to be decreased (Table 5). This showedthat the action of polymer is reinforced with time immersion.

3.2.2.2. 3%NaCl solution + 2 ppm S2�. In the case of 3%NaCl solutioncontaminated by sulphide, it is found that the polarization resis-tance (Rp) value at law frequency increases in the presence of poly-mer (Fig. 13), whereas the capacity value found to be decreasedfrom 366 lF cm�2 to 3 lF cm�2 (Tables 4 and 5). The decrease incapacity value means that the polymer film formed by electropoly-merization of ATA on the metal surface is more protective com-pared to the behaviour observed in the absence of polymer(Fig. 11). However, for longer times the resistance decreases grad-ually as the film becomes more penetrated by the aggressive ionsas S2� (see Fig. 14).

3.3. Surface composition analysis

The electrode surface was examined by SEM after two days ofimmersion in 3%NaCl and 3%NaCl + 2 ppm S2� without and withpolymer. The micrograph (Figs. 15a and 16a) shows that in the ab-

Rfilm

Re Cdc

Cfilm

RctRe Re + Rfilm Re + Rfilm + Rdc

Film

Double Couche

-ZRe (ko hm.cm2)-ZIm

(ko

hm.c

m2 )

Fig. 12. Nyquist impedance spectrum of Cu–Zn/coating/NaCl 3% and the equivalent electrical circuit.

Fig. 15. The SEM pictures of brass electrode surface after 48 h immersion in 3%NaCl solution: (a) without polymer coating; (b) with polymer coating.

Fig. 16. The SEM pictures of brass electrode surface after 48 h immersion in 3%NaCl solution + 2ppm S2�: (a) without polymer coating; (b) with polymer coating.

1544 M. ELbakri et al. / Corrosion Science 50 (2008) 1538–1545

sence of polymer, the surface is covered by a heterogeneous layerof products. The studies on the corrosion of 60Cu–Zn alloy showedthat for short immersion times, there is a selective dissolution ofZinc and that this dissolution probably involves the formation ofCuCl�2 complex [38,39] and oxides such as ZnO and Cu2O [40].

In the presence of polymer, the state of the surface improvesdistinctly (Figs. 15b and 16b) indicating the inhibiting effect of thisfilm.

4. Conclusion

The I = f(E) and I = f(t) permitted us to carry out the polymericfilm properties as well as the optimal conditions of its formation.In this work, we have also studied the kinetics of electropolymer-ization of ATA in methanol and alkaline solution by the EIS mea-surements. The protective effect of organic coatings submitted to3%NaCl solution and 3%NaCl solution + 2 ppm S2� has been evalu-ated. The results showed that the inhibition efficiency was about87% for 4 h of testing time and increased with immersion time incase of 3%NaCl solution, and it is very high about 99% and decreasedwith immersion time in case of 3%NaCl solution + 2 ppm S2�.

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