Corrosion Science 49 (2007) 1635–1644

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Corrosion protection of steel by bi-layeredpolypyrrole doped with molybdophosphate

and naphthalenedisulfonate anions

Damian Kowalski *, Mikito Ueda, Toshiaki Ohtsuka

Graduate School of Engineering, Hokkaido University, Sapporo 060-8628, Japan

Received 24 July 2006; accepted 11 August 2006Available online 30 October 2006

Abstract

Bi-layered polypyrrole (PPy) coating was investigated for corrosion prevention of steels. The PPycoating consisted of the inner layer doped with PMo12O3�

40 (PMo12) and HPO2�4 ions and the outer

layer doped with naphthalenedisulfonate (NDS) ions. PMo12 ions doped in the inner layer make thepassive oxide film on steel stabilized and the outer layer doped with large organic ions of NDSrestricts decomposition and release of PMo12 ions in the inner layer. Corrosion tests were madein 3.5% NaCl aqueous solution for the steels covered by the single PPy–PMo12, single PPy–NDS,and bi-layered PPy–PMo12/PPy–NDS coatings. The single PPy–PMo12 and PPy–NDS coatingsmaintained the steel in passive state for 34 h and 24 h, respectively. The bi-layered coating keptthe steel in passive state and prevented corrosion of the steels for the longer time period.� 2006 Elsevier Ltd. All rights reserved.

Keywords: A. Polypyrrole; A. Conductive Polymer; C. Corrosion Protection

1. Introduction

During the past decades chromates have been widely used for corrosion prevention ofsteel. Since chromium (VI) is, however, considered as a carcinogen [1], considerable reduc-tion of the amount of Cr (VI) from the previously permissible exposure limit (PEL) is

0010-938X/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.corsci.2006.08.018

* Corresponding author. Tel./fax: +81 11 706 6353.E-mail address: kowalski@cat.hokudai.ac.jp (D. Kowalski).

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required [2]. In environmental view point, chromates should be replaced by nontoxic sub-stitute. Conductive Polymers are thought to be one of the candidates for chromatesreplacement, because their oxidative and catalytic properties induce a stable passive stateof steels [3,4]. Polypyrrole (PPy) is one of the conductive polymers and can be easily pre-pared on steel by electropolymerization from aqueous solutions. In addition, PPy exhibitsgood mechanical properties, thermal stability and high conductivity [5].

During pyrrole (Py) electropolymerization, positive charge is incorporated into PPymatrix and compensated by anions doped from the electrolyte. The kind of counter ionsdoped in the polymer matrix significantly affects properties of the PPy film. There havebeen many reports concerning electrochemical synthesis of PPy films using various typesof dopants, including organic [6–10] and inorganic ions [11–13]. Among organic anions,aliphatic and aromatic sulfonates doped, provide good electrical and mechanical proper-ties and good stability to PPy layer [14]. Inorganic ions, such as ClO�4 or Cl� were alsodoped in PPy; however, they were easily released during reduction and other anions wereincorporated from aqueous solution during the following oxidation [15,16]. The size ofcounter ions plays important role in the exchange of doped anions [17]. When one selectsanions with large size as the dopant, the anions are fixed in the PPy matrix and cations arecontrarily mobile as the charge compensation species. In the previous paper, a PPy coatingto which molybdophosphate ions PMo12O3�

40 , (PMo12), were doped was prepared on steelsto be applied to corrosion prevention of steels [18]. The coating kept the substrate steelpassive state in acidic and neutral NaCl solution for several days, in which appreciablecorrosion was not observed. The protection of the coating may result from formationof stable passivation oxide film between the PPy layer and substrate steel. The stable pas-sive film was assumed to be induced by property of passive-film former of molybdophos-phate ion fixed in the PPy layer and by oxidative property of the PPy matrix.Molybdophosphate ion is characterized with the Keggin structure with relatively largesize. Aqueous solutions containing the analogous hetero- and iso-poly-oxo-anions ofmolybdenum and tungsten have been used for anodic polymerization of PPy and polyan-iline (PNi), and from the solutions, stable PPy layers were prepared to which the poly-oxo-anions were doped [19–24]. The corrosion protection of the PPy coating was graduallydegraded during immersion in aqueous NaCl solution, particularly in neutral solutionin which PMo12 fixed in the PPy layer was decomposed to molybdate and phosphate ionsdue to hydrolysis reactions and then released to the aqueous solution. The inhibition of thedecomposition-release process of PMo12 in the PPy layer is required for more enhance-ment of corrosion protection of the PPy coating.

In this study we introduce new PPy bi-layered coating in which PMo12 are doped in theinner layer and naphthalenedisulfonate (NDS) in outer layer: The inner layer doped withPMo12 functions to form a stable passive film on steels and the outer layer doped withNDS inhibits the decomposition-release process of PMo12.

2. Experimental

Pyrrole monomer (Py) purchased from Kanto Chemical Co. (99% purity) was usedwithout any pretreatment. All the other reagents at analytical grade were obtained fromWako Pure Chemical Indust. Ltd. Solutions used were prepared using double distilledand subsequently deionized Millipore water and deoxygenated by nitrogen bubbling for30 min before use. The carbon steel with the composition of C 0.032, Si 0.013, Mn 0.15,

D. Kowalski et al. / Corrosion Science 49 (2007) 1635–1644 1637

P 0.012, S 0.008, Al 0.018 and Fe 99.767 wt.% was mechanically polished with 800-gradeabrasive paper and degreased in acetone in ultrasonic bath before use. The geometricalsurface of the steel electrodes was about 3 cm2.

A three-electrode cell was used for electrodeposition of PPy on steel substrate. Refer-ence electrode was Ag/AgCl/sat. KCl and the counter electrode was a platinum foil.The electrodeposition was carried out at room temperature.

Three types of the PPy coating was prepared on steel; PPy coating doped with PMo12,PPy coating doped with napthalenedisulfonate (NDS), and bi-layered PPy coating consist-ing of the inner PPy–PMo12 and the outer PPy–NDS layers. Electrosynthesis process waspreceded by passivation of steel in 0.06 M disodium molybdate (Na2MoO4) solution underpotentiostatic condition at 0.8 V for 30 min. After that, the solution was exchanged with0.2 M H3PO4 solution containing 5 mM H3PMo12O40 (H3PMo12) and 0.1 M Py monomer,or with 1,5-naphthalenedisulfonic acid disodium salt (NDS) solution at concentrationsfrom 0.005 M to 0.1 M containing 0.1 M Py monomer. Electropolymerization of PPywas performed by constant current of 1 mA cm�2 with 2.0 C cm�2 electricity. The bi-lay-ered PPy coating was first prepared in the 0.2 M H3PO4 solution containing 5 mMH3PMo12 and 0.1 M Py monomer at 1 mA cm�2 for 1000 s to form the inner layer ofPPy–PMo12 and then in NDS solution at concentrations from 0.005 M to 0.1 M contain-ing 0.1 M Py monomer at 1 mA cm�2 for 1000 s to form the outer layer of PPy–NDS.The total electricity at 2.0 C cm�2 was the same for three PPy coatings.

Thicknesses of the PPy films were measured by Confocal Laser Scanning Microscopy‘‘Laser-Tech 1LD21D’’. Evaluation of the thickness was performed by measuring thedistance difference between coated and uncoated steel surface.

Depth profile of elements in the PPy films was measured by Glow Discharge OpticalEmission Microscopy (GD-OES) ‘‘JY-5000 RF’’ with sampling diameter of 5 mm.

The corrosion tests of the PPy coated steel were carried out at 25.0 ± 0.3 �C in 3.5 wt.%NaCl solution in which the open circuit potential (OCP) was continuously recorded for7d or longer.

3. Results and discussion

3.1. Electrochemical synthesis of bi-layered coating

Since the electropolymerization of Py could not be initiated on non-noble metals with-out pretreatment of the metal substrate, formation of oxide film was preceded to the syn-thesis of polymer. There have been many reports concerning electrosynthesis of PPy onsteels from oxalate solutions [10,17,25] in which iron oxalate was formed during an induc-tion time period and prevent dissolution of steel. The PPy layer was formed on the ironoxalate layer. In this study we introduced a passive film on steel by potentiostatic oxida-tion at 0.8 V in Na2MoO4 solution before PPy polymerization.

The potential change versus electropolymerization time during the constant currentsynthesis at 1 mA cm�2 of the PPy coating on the passivated steel is presented in Fig. 1.It is seen that electropolymerization of Py in the PMo12 solution is characterized by an ini-tial spike of potential and the following steady potential. The spike is probably ascribed tonucleation of polymer on steel surface. After that, potential reveals constant potential at0.58 V at which uniform growth of the PPy film takes place. Exchange of the solution with0.005 M NDS solution cause potential increases to 1.20 V and gradually decreases to

Fig. 1. Potential change with time during polymerization of pyrrole (Py) at constant current of 1 mA cm�2. Theinitial step was made in 5 mM H3PMo12O40 + 0.2 M H3PO4 + Py for 1000 s, and the second step in (—) 0.005 MNDS + 0.1 M Py, (� � �) 0.01 M NDS + 0.1 M Py, (Æ - Æ -) 0.05 M NDS + 0.1 M Py, (Æ Æ -Æ Æ -) 0.1 M NDS + 0.1 M Pyfor 1000 s.

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0.96 V. The polymerization potential for the inner layer is close to that reported for PPydoped with heteropoly oxometalate anions reported [19]. The growth of the inner layerdoped with PMo12 showed relatively low polymerization potential as compared with thatin the NDS solution. The difference in steady polymerization potentials between thegrowth of PPy–PMo12 and PPy–NDS films is around 0.32 V. The polymerization of outerlayer was made in NDS solutions at concentrations from 5 mM to 0.1 M, and the potentialwas also plotted in Fig. 1. It is seen in Fig. 1 that increase of concentration of NDS from5 mM to 0.05 M cause decrease of polymerization potential. This dependence may bemainly attributed to the simultaneous doping process of electrons and anions dependenton the concentration of the anions in aqueous solution. The lower value of polymerizationpotential in PMo12 solution may be due to the lower pH at pH = 1.34, in contrast to thepH = 5.76 of the NDS solution.

The steel was uniformly covered by the smooth bi-layered coating of PPy–PMo12/PPy–NDS. The coatings were strongly adhesive to the steel substrate. The thickness of the PPylayer was evaluated by observation of confocal laser microscopy. The results are summa-rized in Table 1 in which the ratio of thickness to electricity for polymerization and den-sities (the ratio of mass to thickness) of the PPy layer are presented. The outer layer dopedby NDS shows a lower film density at 2.1 g cm�3. The changes in physical properties of the

Table 1Ratio of thickness to electricity, d/Q, for polymerization and density, q, (ratio of mass to thickness) of the PPylayers

PPy film d/Q (lm cm2/C) q (g/cm3)

PPy–PMo12 2.1 3.4PPy–NDS 2.7 2.1PPya 1.2 5.5

a The PPy layer was formed in pure water containing 0.1 M Py monomer.

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films were associated with the kind of counter anion incorporated in the polymer. It couldbe also related to doping ratio which depends on the solution for polymerization.

In addition, we prepared a PPy film by electropolymerization from pure water contain-ing Py monomer without any supporting electrolyte. The density of this film was muchhigher at 5.5 g cm�3. Probably the dense PPy film prepared from pure water is low dopingconcentration of counter anions. This dense PPy film is attractive and will be discussed inthe other paper.

The single layers of PPy–PMo12 and PPy–NDS were also prepared. The potential tran-sients during the formation of PPy–NDS on steel were almost the same as that in the outerlayer growth in Fig. 1.

The SEM micrographs of the single layers of PPy–PMo12 and PPy–NDS covering thesteels are shown in Figs. 2 and 3, respectively. The layers show relatively homogeneousand globular morphology with particles size from 1 to 5 lm and from 2 to 10 lm diameterfor the PPy–PMo12 and PPy–NDS layers, respectively. The finer structure of the PPy–PMo12 layer may be related with the higher film density of PPy–PMo12 as previouslyshown.

Fig. 3 shows SEM images for the PPy–NDS film prepared from NDS solutions withdifferent concentrations. No significant change in surface morphology is seen betweenthe layers formed in the solution at different NDS concentrations.

3.2. Depth profile of the PPy film

The PPy layer consisting of PPy matrix and doped anion has been analysed by varioustechniques like XPS, IR, and Raman spectroscopy [9,19,26]. Qualitative identification ofthe counter anions incorporated in polymer will be relatively easy, while quantitative anal-ysis is not easily achieved. GD-OES was applied in this study to analysis of the PPycoatings, because the compositional depth profile of the coatings was obtained by glow-discharged sputtering. The GD-OES depth profile of the bi-layered PPy coating withPPy–PMo12/PPy–NDS is shown in Fig. 4. The result reveals the presence of C, N, S,

Fig. 2. SEM micrographs of PPy–PMo12 layer electrodeposited with electricity of 1 C cm�2 in H3PMo12O40 +H3PO4 + Py solution.

Fig. 3. SEM micrographs of PPy–NDS layer electrodeposited with total electricity of 1 C cm�2 in solutions (a)5 mM NDS + Py, (b) 0.05 M NDS + Py.

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Mo, P and O all of which constitute the polymer matrix and the doping anions. From theprofile three regions are distinguishable. The first region corresponds to the outer PPylayer doped with NDS the presence of which was identified by the signal of S. The intensityof the S signal was found to be proportial to the NDS concentration in the PPy layer andprobably depends on concentration of the polymerization solution. However, the compo-sitional ratio, i.e., doping ratio NDS to PPy was not determined due to no information oncalibrated intensity of S in NDS. The second region of the profile is associated with theinner PPy layer doped with PMo12 and HPO2�

4 . For quantitative analysis of ratio of dopedspecies, PMo12 to HPO2�

4 the calibrated intensity of P, and Mo was evaluated from mea-surement by GD-OES of a thin layer of H3PMo12 which was coated on steel plate fromacetonic solution in which H3PMo12 was dissolved with acrylic lacquer. The ratio betweenPMo12 and HPO2�

4 was found to be 1:2.2 from this calibration. Although the concentra-tion of H3PO4 in polymerization solution was an order of magnitude higher than PMo12,the doping level for HPO2�

4 was found to be rather low. This suggests that PMo12 anionsare preferentially doped in the PPy layer during the polymerization. The C and N

Fig. 4. GD-OES depth profile of the bi-layered PPy coating on steel. The inner PPy layer was prepared inH3PMo12O40 + H3PO4 + Py solution with electricity of 1 C cm�2 and the outer layer in NDS + Py solution with1 C cm�2.

D. Kowalski et al. / Corrosion Science 49 (2007) 1635–1644 1641

intensities are higher in the inner PPy layer than that in the outer layer. It may be relatedwith the higher density of the inner layer. The third region of the profile corresponds tosteel substrate in which the Fe signal is observed.

3.3. Corrosion tests

Corrosion tests of the PPy coated steels were performed in 3.5% NaCl aqueous solu-tion. The open circuit potential (OCP) was measured versus time during the immersionof the coated steels. Fig. 5 shows the OCP of the steels coated by PPy–NDS and PPy–PMo12 single layers electrodeposited with electricity of 2 C cm�2. The coated steel initiallyexhibits potentials around 0.12 V and 0.15 V for PPy–NDS and PPy–PMo12, respectively.

Fig. 5. Open Circuit Potential as a function of immersion time in 3.5 wt.% NaCl solution for the steel covered by($) PPy–NDS, (h) PPy–PMo12 single layers electrodeposited with electricity of 2 C cm�2.

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This relatively high potential indicates that the steels are passivated by the oxidative PPylayers, while the potential of the bare steel was �0.64 V in the active dissolution region.The difference in potential at the initial stage between both coatings is approximately0.03 V. During the time period at which the potential is maintained in the passive region,no appreciable corrosion takes place. The corrosion starts after the abrupt decrease ofpotential to �0.35 V. The protection time is thus determined by the time at which thepotential abruptly changed from the passive to the active region. The coated steel wasmaintained in the passive and protective state for 24 and 33 h for the PPy–NDS andPPy–PMo12 coatings, respectively.

The change in OCP for the bi-layered coating of PPy–PMo12/PPy–NDS electrodepos-ited with electricity of 2 C cm�2 is shown in Fig. 6. The coated steel exhibits potential atabout 0.18 V in the passive region during the initial immersion for 43 h. The potential thendecreased to �0.3 V, followed by gradual decrease to the active potential region of steel.

The effect of NDS concentration for the PPy preparation on the corrosion protectionwas examined. The result is given in Fig. 7 in which the outer PPy layers were preparedin the solutions at NDS concentration of 0.005, 0.01, 0.05, and 0.1 M. The OCP duringthe initial immersion for 43 h in NaCl solution was almost the same for the four coatings.The change appeared after decrease from the passive region. After the decrease, the poten-tial was kept at about �0.35 V. The time period at the plateau depends on the coatings.Finally the potential reaches �0.58 V in the active region.

Degradation of the protection ability of the PPy layer in the NaCl solution may beexplained as the following way. In the first time period, passive oxide film underneaththe PPy layer is stable enough to maintain the steel in the passive state. Oxidation and cor-rosion of steel gradually takes place which is accompanied by reduction of PPy backboneand release of counter anions, simultaneously. In addition, penetration of electrolyte intothe PPy layer gradually takes place during the immersion, and PMo12O3�

40 anions aredecompose to MoO2�

4 and HPO2�4 by hydrolysis reaction (1) with water penetrated [18,27].

PMo12O3�40 þ 12H2O! 12MoO2�

4 þHPO2�4 þ 23Hþ ð1Þ

Fig. 6. Open Circuit Potential as a function of immersion time in 3.5 wt.% NaCl solution for the steel covered bybi-layered coating of (h) PPy–PMo12/PPy–NDS. The OCP of the steel coated by the single ($) PPy–NDS coatingwas plotted for comparison. The outer PPy–NDS layer was formed from 0.005 M NDS solution.

Fig. 7. Open Circuit Potential as a function of immersion time in 3.5 wt.% NaCl solution for the steel covered bybi-layered coating of PPy–PMo12/PPy–NDS. The inner layer was prepared in H3PMo12O40 + H3PO4 + Pysolution with electricity of 1 C cm�2. The outer layer was prepared in solutions of (h) 0.005 M NDS + 0.1 M Py,($) 0.01 M NDS + 0.1 M Py, (}Þ 0.05 M NDS + 0.1 M Py, (O) 0.1 M NDS + 0.1 M Py in the second step ofpolymerization with electricity of 1 C cm�2.

D. Kowalski et al. / Corrosion Science 49 (2007) 1635–1644 1643

The release rate of the decomposed forms is assumed to be higher than that of PMo12O3�40

because of the smaller molecular size. After keeping at the passive region, potential rapidlydecreases to around �0.3 V at which the passive layer is broken and some amount of elec-trolyte reaches the steel surface. The potential is further maintained at �0.3 V for a fewtens of hours. In this stage, PPy is assumed to be not completely in reduced state, and someamount of counter anions still existed in polymer matrix. It is assumed that corrosion ofsteel is inhibited by PPy coating even after potential drop to about �0.3 V, though it donot correspond to the passive state of steel. Potential gradually decreases during furtherimmersion to the active potential region of steel. In this stage, PPy is in reduced stateand loses protection ability of steel.

The steel covered by double layer coating was maintained in the passive state by 17 hlonger than the PPy–NDS single layer and by 10 h longer than the PPy–PMo12 layer.The better corrosion protection of the bi-layered coating may be explained as the follow-ing way; the NDS counter anions with relatively large size have lower mobility than PMo12

and its decomposed anions, thus the outer PPy–NDS layer inhibits the release ofPMo12 anions from the inner part of the coating to aqueous solution. It is assumed thatPMo12 anions and/or their decomposed forms remains in the inner layer of PPy for longertime and stabilizes the passive oxide film on steels more than the single layer PPy coatings.

4. Conclusions

(1) Bi-layered polypyrrole (PPy) coating was prepared on carbon steels. In the coating,the inner layer was doped with molybdophosphate, PMo12O3�

40 (PMo12) ions andphosphate ions and the outer layer with naphthalenedisulfonate (NDS) ions.

(2) The single PPy–PMo12 coating 4.2 lm thick and the PPy–NDS coating 5.4 lm thick,maintained the steel in the passive state for 24 h and 34 h, respectively, in 3.5 wt.%NaCl aqueous solution.

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(3) The bi-layered coating 4.8 lm thick consisting of the inner PPy–PMo12 and the outerPPy–NDS layers kept the steel in the passive state for the longer time period. Thebetter corrosion prevention may be explained as the following way; PMo12 ionsincorporated in the inner layer make the passive oxide film on the steel stabilizedand the outer layer doped by the large size organic ions of NDS restricts decompo-sition and release of PMo12 ions in the inner layer.

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