The protective effect of polypyrrole coating on the corrosion of steel electrode in acidic media

ISSN 2070�2051, Protection of Metals and Physical Chemistry of Surfaces, 2014, Vol. 50, No. 2, pp. 266–272. © Pleiades Publishing, Ltd., 2014.

266

1INTRODUCTION

There are several ways of preventing or reducingcorrosion, such as painting, cathodic protection, add�ing chemical compounds called inhibitors. Corrosioninhibitors are of great importance, being extensivelyemployed in minimizing metal losses in engineeringmaterials [1]. However, most of these compounds aretoxic. Therefore, their usage is limited in practice. Inorder to eliminate hazardous effects from the environ�ment and to protect human health, alternativeapproaches are needed to replace organic coatings thathave limited lifetimes and/or are environmentallyhazardous.

The use of conducting polymers as coating materi�als for corrosion protection of metal/alloy has becomeone of the most exciting new research fields [2–14].Conductive polymers are the ones which conductelectric current without adding a conductive metal.Conductive polymers possess the characteristics ofthree different materials; metals with their mechanicalproperties, semiconductors with their electronic andoptical properties, and of themselves, polymers, withtheir processing properties. It is possible to synthesizesuch polymers both chemically and electrochemically.Identifying the conditions of formation will have greatsignificance in the production of electronic devices,electrical circuits, rechargeable batteries and sensors.Accumulating polymer via electrochemical methodson metals which are not stable thermodynamically inaqueous medium, such as iron, copper and zinc,entails certain problems. The main problem is that themonomers have oxidation potentials and metals havedissolving potentials. Most of the time metal dissolves

1 The article is published in the original.

anodically before the monomer even starts to be oxi�dized. For a successful coating, the dissolving rate of themetal should be slowed down by adjusting the electro�lysis conditions without slowing down the electropoly�merization (by making the metal passive at the poten�tial applied for the oxidation of monomer). In thestudies carried out on this issue, mostly polyaniline[15–21] and polypyrrole [22–29] coatings are exam�ined due to the solubility of the monomers in water.Polypyrrole (PPy) is one of the most promising con�ductive polymers in terms of its high conductivity, sta�bility and ease of synthesis [3, 24].

Considering the above mentioned properties, poly�pyrrole was chosen as conductive polymer in thisstudy. The objective of this study is twofold. First is todetermine the corrosion parameters of steel by coating12% Cr stainless steel with PPy in 1 M HCl, H2SO4

and H3PO4 media and to investigate the protectioneffectiveness and resistance of coating depending onthe immersion time. Second is to show the variation ofthe inhibition efficiency of PPy coating depending onincrease of charge of anions.

EXPERIMENTAL

The chemical composition of the steel employed inthis study is given in Table 1. The 4 mm diameter steelelectrode was cut and embedded in a cold�curing resin

The Protective Effect of Polypyrrole Coating on the Corrosion of Steel Electrode in Acidic Media1

Seval Akpolat and Semra Bilgi *Department of Physical Chemistry, Faculty of Science, Ankara University,

06100 Be evler, Ankara, Turkey*e�mail: [email protected]

Received June 20, 2012

Abstract—In this study, the corrosion parameters of stainless steel containing 12% Cr, have been determinedby Tafel extrapolation method in 1 M HCl, H2SO4 and H3PO4 media. Later, steel was coated with polypyrrolein 0.1 M Pyrrole + 0.3 M Oxalic acid solution by cyclic voltametric method. The corrosion parameters andpercentage inhibition efficiencies of coated electrodes were investigated according to immersion times in thesame media. In all acidic media studied, increases in immersion time, produced increased corrosion densitiesand a decrease in percentage inhibition efficiencies were determined.

DOI: 10.1134/S2070205114020026

c

s

Table 1. The chemical composition (wt %) of studied steelelectrode

C S Cr Si Mn Fe

0.13 0.004 12.4 0.65 0.51 86.306

PHYSICOCHEMICAL PROBLEMS OF MATERIALS PROTECTION

PROTECTION OF METALS AND PHYSICAL CHEMISTRY OF SURFACES Vol. 50 No. 2 2014

THE PROTECTIVE EFFECT OF POLYPYRROLE COATING 267

of methyl methacrylate basis (Technovit, Kulzer,Friedrichsdorf, Germany). Before each test the steelsurface was abraded with a 1200 grit emery paper, thenwashed with bi�distilled water to remove excess reac�tant from the coated steel; the emery paper was notused after the electrosynthesis of polymer coatings.

Electropolymerization and corrosion tests werecarried out in a three electrode cell. The steel was usedas an electrode located in the center of the cell, whilea SCE (Saturated Calomel Electrode) and platinumfoil were used as the reference electrode and counterelectrode respectively, in the other parts of the cell. Bi�distilled water was used to prepare the solutions.Nitrogen was bubbled through the solutions beforeeach experiment for up to 15–20 min to remove oxy�gen from the solution. The experiments were carriedout using the system consisting of Wenking LB 75 Llaboratory model potentiometer, Wenking VSG72 model voltage�scanning generator, YokogawaTechnicorder Type 3077 recorder, BM 101 thermostatand electro�mag mixer. The corrosion parameterswere determined by using the Tafel extrapolationmethod [30].

Electrochemical corrosion measurements wereperformed in 1 M H2SO4 at room temperature. 0.3 Moxalic acid including 0.1 M pyrrole (Py) solution wasused to coat both uncoated and coated�steel with aPPy film. Potential scan rate was initially kept at2.5 mVs–1 until the formation of Fe2C2O4 on the steelsurface.

RESULTS AND DISCUSSION

Figure 1 shows the cyclic voltammogram ofuncoated steel in 0.3 M oxalic acid solution. The firstscan and subsequent ten cycles of current–potentialcurves in 0.1 M pyrrole + 0.3 M oxalic acid can be seenfrom Figs. 2a and 2b.

As can be seen from Figs. 1 and 2a, the passivationpeak in the first scan was disappeared in the secondscan due to the formation of PPy which prevents dis�solution (Fig. 2b). It can be said that the steel surfacecovered with PPy due to the change in the curve and aconsiderable decrease in the current.

PPy coating was characterized by FTIR and theresulting spectra were given in a previous study [13]. Forthe studied three acids, the variation of open–circuitpotentials with immersion time can be seen in Fig. 3.However, these values do not significantly change

20

18

16

14

12

10

8

6

2

1.000.980.780.580.38–0.42

4

0.180.20–0.22

i (m

A c

m–

2 )

E (V)

Fig. 1. Anodic polarization curve of 12% Cr stainless steelin 0.3 M oxalic acid. (Potential scan rate, ν = 20 mV/s).

10

8

6

4

2

1.000.920.720.52–4.8

i (m

A c

m–

2 )

E (V)0.320.12–0.08–2.8

1086

4

2

1.000.920.720.52

i (m

A c

m–

2 )

E (V)0.320.12–0.08–2.8–4.8

(a)

(b)

Fig. 2. a—The first scan curve of the steel in 0.1 M pyrrole +0.3 M oxalic acid medium. (Potential scan rate, ν =20 mV/s); b—The ten scan curves subsequently for steel in0.1 M pyrrole + 0.3 M oxalic acid medium (Potential scanrate, ν = 20 mV/s).

0

–100

–200

–300

–400

–500

120725310–600

Eo

cp (

mV

)

Time, h

1 M HCl1 M H2SO41 M H3PO4

Fig. 3. Open�circuit potentials vs immersion time forPPy–coated steel electrode in 1 M HCl, H2SO4 andH3PO4 media.

268


SEVAL AKPOLAT, SEMRA BILGIÇ

depending on the immersion time. This is consistentwith the results of other researchers [12, 19, 23].

The corrosion parameters such as open–circuitpotentials (Eocp), anodic Tafel slopes (ba), corrosioncurrent densities (icorr) and percent inhibition efficien�cies (η%) determined in these media are shown inTable 2.

Percentage inhibition efficiency (η%) and surfacecoverage degree (θ) were calculated by the use of fol�lowing equation

(1�a)

(1�b)

where icorr and are the corrosion current densitiesfor uncoated and PPy–coated steel respectively.

According to the data in Table 2a, the Eocp valuesobtained for uncoated steel in 1 M HCl medium shiftto more anodic potentials immediately after immer�sion. However, no regular variation can be seen for the

η%icorr icorr

'–icorr

�� 100×=

θicorr icorr

'–icorr

��=

icorr'

other immersion times. Corrosion current density hasbeen found to be 1.8 mAcm–2 for uncoated steel whileit has been found to be 0.25 mAcm–2 immediately afterPPy coating. Inhibition efficiency was determined as86.1% immediately after PPy coating. An increase incorrosion current densities and a decrease in inhibi�tion efficiencies have been seen with increasingimmersion time. However, the increase of corrosioncurrent density approximates to the value for theuncoated steel at the end of fifth day. The values ofpercent inhibition efficiencies decrease with increas�ing immersion time, it has found to be 5.5% at the endof this period. Hence, it can be concluded that theprotection effect of the coated steel becomes lower.

As can be seen from Table 2b, the Eocp valuesobtained for uncoated steel in 1 M H2SO4 mediumshift to more negative values immediately after immer�sion and there is no regular change with increasingimmersion time. The corrosion current densitiesincrease and the percent inhibition efficienciesdecrease depending on the immersion time. Higherinhibition efficiency has been determined in 1 M

Table 2. Open circuit potentials (Eocp), anodic Tafel slopes (ba), corrosion current densities icorr and percent inhibition effi�ciencies (η %) obtained for uncoated steel and PPy–coated steel in various immersion time in a—1 M HCl, b—1 MH2SO4 and c—1 M H3PO4

Immersion time (hour) –Eocp (mV) ba (mV) icorr (mA/cm2) η %

a) 1 M HC1 Uncoated immediatelyafter immersion

430 70 1.80 –

380 55 0.25 86.1

1 485 70 0.70 61.1

3 500 80 1.05 41.6

5 520 79 1.30 27.7

72 520 70 1.50 16.6

120 480 110 1.70 5.5

b) 1 M H2SO4 Uncoated immediatelyafter immersion

390 110 6.00 –

466 165 0.28 95.3

1 445 148 0.30 95.0

3 453 163 0.55 90.8

5 445 144 0.75 87.5

72 414 138 1.30 78.3

120 454 160 1.60 73.3

c) 1 M H3PO4 Uncoated immediately after immersion

450485

100125

2.800.125

–95.5

1 475 100 0.215 92.3

3 455 75 1.00 64.3

5 465 100 1.01 43.8

72 455 75 1.59 43.2

120 430 75 2.00 28.5



H2SO4 when compared to the value found for the PPy�coated steel in 1 M HCl. This also indicates that thePPy�coated steel has more protective effect.

Table 2c shows the obtained values of corrosionparameters such as Eocp, ba, icorr and η% in 1 M H3PO4.As can be seen from this table, the values of Eocp do notchange significantly with increasing immersion time.The corrosion current density has been found to be2.8 mA cm–2 for uncoated steel whereas this value is0.125 mA cm–2 immediately after coating. However,an increase in icorr and a decrease in inhibition effi�ciency have been found with increasing the immersiontime. According to this table, PPy�coated steel supplya 28.5% protection effect in 1 M H3PO4. The highestcorrosion current density of uncoated steel electrodehas been found to be in H2SO4 medium which is fol�lowed by H3PO4 and HCl media respectively. η% val�ues vary in the same immersion times as follows: η%

(H2SO4) > η% (H3PO4) > η% (HCl). In Figs. 4–6, thevariation of η% values with immersion time for PPy�coated steel electrode can be seen in 1 M HCl, H2SO4

and H3PO4 media, respectively.

Figures 7 and 8 represent SEM image and EDSdiagram for uncoated steel, respectively. PPy coating,though not for a long period, protects steel in HCl,H2SO4 and H3PO4 media. In all three of the studiedacidic media, the coating efficiency immediately afterthe coating process has been high whereas efficiencydecreases as the immersion time increases. However,coating loses its protection properties after 5 days in allthree acidic media. Among the studied acidic media,corrosion current density of steel electrode is highestin H2SO4, which is followed by H3PO4 and HClmedia, respectively. The medium where the PPy coat�ing shows the highest inhibition efficiency is againH2SO4 medium. In our study, highest corrosion cur�

706050

403020

100

72 120531

η %

Time, h

Fig. 4. The η% values vs immersion time for PPy–coatedsteel electrode in 1 M HCl.

70605040302010

072 120531

η %

Time, h

8090

100

Fig. 5. The η% values vs immersion time for PPy–coatedsteel electrode in 1 M H2SO4.

70605040302010

072 120531

η %

Time, h

8090

100

Fig. 6. The η% values vs immersion time for PPy–coatedsteel electrode in 1 M H3PO4.

10 µm

Fig. 7. SEM image of uncoated steel.

270



rent density with uncoated steel and highest inhibitionefficiencies with PPy�coated steel are both obtained inH2SO4 medium, and therefore SEM images and EDSdiagrams for this medium only are presented. SEMimage and EDS diagram of uncoated steel electrodeare given in Fig. 9 and Fig. 10, respectively.

Uncoated steel was sanded with waterproof emerypaper from thick to thin, and the surface was cleansedwith 6 μm diamond paste to take SEM image. SEMimage shows the significant alteration of the surfacestructure which is evidence of corrosion. The consid�erable drop in Fe peak seen in the EDS diagram con�firms that iron dissolves with corrosion and passes intothe solution. SEM image and EDS diagram of PPy�coated steel after immersion of 1 h in H2SO4 mediumare given in Fig. 11 and Fig. 12, respectively. The cau�liflower�like structure in the image is due to the forma�tion of PPy coating [31]. Very high carbon peak andsignificant decrease in iron peak in EDS results maylead to the conclusion that the steel has been coatedwith PPy thoroughly (Figs. 11 and 12).

The corrosion inhibition effect of chloride ions isdue to their adsorption on the uncoated steel surface.Since the total Gibbs free energy of water�electrode,ion�electrode, ion�water interactions for the adsorp�tion of Cl– is, ΔG = –8.9 kJ mol–1, Cl– adsorbs on steelsurface by contact adsorption [32]. Therefore, the cor�rosion rate is lower in HCl medium (Table 2a). As isknown, the corrosive effect of these ions occurs underthe coating. The higher corrosion rate of sulphateions, however, can be explained by considering thatsulphate ions are much bigger than chloride ions (Å)[33], hardly adsorb on the surface and consequentlyhave less effect (Table 2b).

The protection of phosphate ions on the surface israther through the formation of iron phosphate com�pounds. Because of its size (Å), the iron�

phosphate compound which is

partially unstable in the acidic medium, provideshigher protection than that of sulphate (Table 2c).This protective effect can be due to the buffering char�

rPO4

3– 2.38=

rSO4

2– 2.30= , rCl

– 1.67=

3020100

Fe

F

Mn

Cr

C Si

Fe

Fe

Fe

FeFeMn

Cr

Cr

Cr

Cr

Si Mn

Fig. 8. EDS diagram of uncoated steel.

10 µm

Fig. 9. SEM image of corroded uncoated steel in H2SO4medium. (Immersion time = 1 h).

3020100

Fe

MnCr

40

V

Cr

Cr

Cr

Fe

Fe

VV

MnMn

MnVV

Fig. 10. EDS diagram of corroded uncoated steel inH2SO4 medium. (Immersion time = 1 h).

10 µm

Fig. 11. SEM image of PPy–coated steel in H2SO4medium. (Immersion time = 1 h).



acteristic of phosphate at the narrow region of thenearest surface. This situation can counteract the pro�tection conferred by sulphate through extending thelife of the formed iron�phosphate compound.

For the PPy�coated electrode however, the Cl– inthe polymer is not effective in the control of surfacestructure. It is possible for pores to be open constantlydue to the continuous motion of Cl– ions in the poly�mer pores. Moreover, the polymer enhances the cor�rosive effect of chloride by forming a sub�coating.Though PPy is formed even in the chloride solution, itis not too passive since Cl– is aggressive and causes pit�ting corrosion by settling under the formed passivefilm. These pits are rendered more stable by the iron�oxalate precipitate; the film is decomposited throughthe motion of oxalate anions [34]. There is not enoughrapid O2 formation for the maintenance of decompos�ited passive film in chloride medium. This finding hasalso been proved by Klimartin et al. [35]. Sulphate ionslower the rate of corrosion development and preventthe contact of metal with solution either by penetra�tion or closing the polymer pores at the polymer/solu�tion interface. Hermas et al. [28] proposed that thePPy film keeps the potential of stainless steel more sta�ble and at higher positive values in sulfuric acidmedium. Our findings for H2SO4 medium also con�firm this conclusion. During the electropolymeriza�tion of PPy on the steel surface, the corrosion ofH2SO4 and H3PO4 can be prevented partially due tothe formation of stable oxide layer; that is, PPy pre�vents the dissolution of steel. This is also reported byKralzic et al. [20]. Our findings also confirm thesefindings. The effect of phosphate should be close tothat of sulphate however, due to the inclination ofphosphate ions in the pores to form iron�phosphate,they could have caused the decomposition of polymerfilm rather than the protection. In this case, protectiveeffect of phosphate is likely to be lower than sulphate.

CONCLUSIONS

The conclusions of this study can be drawn as fol�lows:

1. Steel surfaces can be coated with PPy film viaelectropolymerization. This coating protects steelagainst corrosion in HCl, H2SO4 and H3PO4 media,but this is valid only for a short span of time.

2. The protective effect of PPy coating formed onsteel surfaces exists mostly in H2SO4 medium, and thisis followed by H3PO4 and HCl media, respectively.

3. The inhibition efficiencies of PPy coatingdepend on the nature and charge of the anions of thestudied acids.

ACKNOWLEDGMENT

The authors acknowledge Dr. Gökhan Gece(Bursa Technical University) for his technical assis�tance.

REFERENCES

1. Lui, G.Q., Zhu, Z.Y., Ke, W., et al., Corrosion, 2001,vol. 57, no. 8, p. 730.

2. Collins, W.D., Weyers, R.E., and Al�Qadi, I.L., Corro�sion, 1993, vol. 40, no. 1, p. 74.

3. Skotheim, T.A., Handbook of Conductive Polymers,N.Y.: M. Dekker, 1986.

4. Su, W. and Iroh, J.O., Electrochim. Acta, 1999, vol. 44,no. 13, p. 2173.

5. Zarras, P., Anderson, N., Webber, C., et al., Rad. Phys.Chem., 2003, vol. 58, nos. 3–4, p. 387.

6. Rammelt, U., Nguyen, P.T., Plieth, W., Electrochim.Acta, 2003, vol. 48, no. 9, p. 1257.

7. Tan, C.K. and Blackwood, D.J., Corrosion Sci., 2003,vol. 45, no. 3, p. 545.

8. Tüken, T., Yazici, B., and Erbil, M., Surf. Coat. Tech�nol., 2006, vol. 200, nos. 12–13, p. 4802.

9. Rohwerder, M., and Michalik, A., Electrochim. Acta,2007, vol. 53, no. 3, p. 1300.

10. Shinde, V., Gaikwad, A.B., and Patil, P.P., Surf. Coat.Technol., 2008, vol. 202, no. 12, p. 2591.

11. Chaudhari, S., Gaikwad, A.B., and Patil, P.P., Curr.Appl. Phys., 2009, vol. 9, no. 1, p. 206.

12. Martins, J.I., Reis, T.C., Bazzaoui, M., et al., CorrosionSci., 2004, vol. 46, no. 10, p. 2361.

13. Hasanov, R. and Bilgic, S., Prog. Org. Coat., 2009,vol. 64, no. 4, p. 435.

14. Hasanov, R., Bilgic, S., and Gece, G., J. Solid StateElectrochem., 2011, vol. 15, no. 5, p. 1063.

15. DeBerry, D.W., J. Electrochem. Soc., 1985, vol. 132,no. 5, p. 1022.

16. Shah, K. and Iroh, J., Synth. Met., 2002, vol. 132,no. 1, p. 35.

17. Fenelon, A.M. and Breslin, C.B., Surf. Coat. Technol.,2005, vol. 190, nos. 2–3, p. 264.

18. Hür, E., Bereket, G., and ahin, Y., Curr. Appl. Phys.,2007, vol. 7, no. 6, p. 597.

19. Mirmohseni, A. and Oladegaragoze, A., Synth. Met.,2000, vol. 11, no. 2, p. 105.

20. Kralji , M., Mandi , Z., and Dui , Lj., Corrosion Sci.,2003, vol. 45, no. 1, p. 181.

S

c � c � c �

8640

FeMn

Cr

C

FeS

2

Cr

O

N

S

SS

S

Fe FeFeFeMn

CrCr

Fig. 12. EDS diagram of PPy–coated steel in H2SO4medium. (Immersion time = 1 h).

272



21. Martyak, N.M., Material Science and Engineering A,2004, vol. 371, nos. 1–2, p. 57.

22. Reut, J., Opik, A., and Idla, K., Synth. Met., 1999,vol. 102, no. 1, p. 1392.

23. Tüken, T., Yazici, B., and Erbil, M., Prog. Org. Coat.,2004, vol. 51, p. 152.

24. Inzelt, G., Pineri, M., Schultze, J.W., andVorotyntsev, M.A., Electrochim. Acta, 2000, vol. 45,nos. 15–16, p. 2403.

25. Ocon, P., Cristobal, A.B., Herrasti, P., and Fatas, E.,Corrosion Sci., 2005, vol. 47, no. 4, p. 649.

26. Tüken, T., Surf. Coat. Technol., 2006, vol. 200, no. 16–17, p. 4713.

27. Machnikova, E., Pazderova, M., Bazzaoui, M., andHackerman, N., Surf. Coat. Technol., 2008, vol. 202,no. 8, p. 1543.

28. Hermas, A.A., Nakayama, M., and Ogura, K., Electro�chim. Acta, 2005, vol. 50, no. 18, p. 3640.

29. Herrasti, P. and Ocon, P., Appl. Surf. Sci., 2001,vol. 172, nos. 3–4, p. 276.

30. McCafferty, E., Corrosion Sci., 2005, vol. 47, no. 12,p. 3202.

31. Iroh, J.O. and Levine, K., Eur. Polymer J., 2002,vol. 38, no. 8, p. 1547.

32. Bockris, J.O.M. and Reddy, A.K.N., Modern Electro�chemistry, N.Y.: Plenum Rosetta Ed., 1977.

33. Huheey, J., Inorganic Chemistry Principles of Structureand Reactivity, N.Y.: Harper & Row Publishers, 1972.

34. Hien, N.T.L., Garcia, B., Pailleret, A., and Deslouis, C.,Electrochim. Acta, 2005, vol. 50, nos. 7–8, p. 1747.

35. Kilmartin, P.A., Trier, L., and Wright, G.A., Synth.Met., 2002, vol. 131, no. 1, p. 99.

Documents

The protective effect of polypyrrole coating on the corrosion of steel electrode in acidic media