Corrosion Science 49 (2007) 2905–2919

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Poly(N-methylaniline) coatings on stainless steelby electropolymerization

Aziz Yagan, Nuran Ozcicek Pekmez *, Attila Yıldız

Hacettepe University, Department of Chemistry, 06532 Beytepe, Ankara, Turkey

Received 20 February 2006; accepted 2 January 2007Available online 6 March 2007

Abstract

Poly(N-methylaniline) (PNMA) coatings have been electropolymerized on 304 stainless steel alloyby potentiodynamic, galvanostatic and potentiostatic synthesis techniques from aqueous solutions of0.1 M N-methylaniline (NMA) and 0.3 M oxalic acid. Characterization of PNMA coatings was car-ried out by cyclic voltammetry, UV–Vis and FTIR spectroscopy techniques. Corrosion behavior ofPNMA coated stainless steel electrodes was investigated using linear anodic potentiodynamic polar-ization, Tafel test, chronoamperometry and electrochemical impedance spectroscopy (EIS) tech-niques in 0.5 M aqueous HCl solutions. Corrosion test results showed that PNMA coatingspossessed protection to uncoated stainless steel against corrosion.� 2007 Elsevier Ltd. All rights reserved.

Keywords: A. Stainless steel; A. Acid solutions; B. Cyclic voltammetry; B. EIS; C. Electrodeposited films

1. Introduction

The rate of corrosion of stainless steel can be reduced by a thin oxide layer (passive film)formed naturally on the metal surface [1]. Passive films on stainless steels formed in anaqueous acidic environment are usually very thin, compact and highly rich in Cr [2].The breakdown of passive films on steel surfaces exposed to corrosive environmentscontaining aggressive chloride anions leads to initiation and propagation of localized

0010-938X/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.corsci.2007.01.004

* Corresponding author. Tel.: +90 312 297 79 72; fax: +90 312 299 21 63.E-mail address: npekmez@hacettepe.edu.tr (N. Ozcicek Pekmez).

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corrosion. The stainless steel surfaces destabilize by penetration of chloride anions andundergo active dissolution. The ability of the stainless steel to become repassive afterbreakdown determines its resistance against pitting corrosion. AISI 304 is an inexpensivestainless steel which does not have the corrosion resistance of other stainless steel alloyssuch as 316, 317 [2]. Various attempts have been made to enhance the passivity of stainlesssteel with addition of inhibitors [3] in corrosive solutions, deposition of polyester typecoatings on surface [4].

There has been a lot of interest lately in the use of conducting polymers such as poly-aniline [5–10] and polypyrrole [11,12] obtained by electrodeposition as anti-corrosion coat-ings for prevention of stainless steel corrosion in corrosive medium. It was proven thatconducting polymer coatings on stainless steel lead to improved the corrosion perfor-mance by formation of a stable passive oxide layer between polymer and metal, barriereffect against corrosive ions and electroactive interactions of adhesive polymer layer withcorrosive ions [7,8]. Hermas et al. [9] reported that polyaniline (PANi) enhanced the pas-sivity of stainless steel by increasing the Cr2O3 content in the passive film and inhibitingthe interaction of the oxide with solution. It was also reported that conducting polyanilinecoatings on stainless steel inhibited the formation of pitting corrosion [9]. Huh and co-workers [10] have shown that the performance of corrosion protection provided bypolyaniline to steel depends on the doping type in the polyaniline and the nature of thecorrosion environment. Conducting polymers such as poly(o-methoxyaniline) [13],poly(2,5-dimethoxyaniline)-fluoropolymer blends [14] and poly(m-methoxytoluene) [15]can be also easily electropolymerized on stainless steel surfaces in various electro-lytes. These polymers obtained on stainless steel surfaces showed that substituted con-ducting polymers can be also used as a protective coating against corrosion in neutraland acidic chloride environment. In previous studies from this laboratory PNMA andpoly(N-ethylaniline) (PNEA) coatings on mild steel surfaces were electrodeposited andcharacterized [16,17]. Corrosion behavior of mild steel covered by PNMA and PNEArevealed that the coated steel had slower corrosion rate than bare steel.

The aim of this work is to investigate electrosynthesis conditions of PNMA as a newconducting polymer coating on 304 stainless steel PNMA coatings were obtained by cyclicvoltammetry on the stainless surface. The corrosion protection effect of PNMA coatingsfor stainless steel electrodes was evaluated compared with that for uncoated stainless steelby Tafel test, linear potentiodynamic polarization, chronoamperometry and EIS in 0.5 MHCl solutions.

2. Experimental

N-methylaniline (NMA) supplied from BDH that was doubly distilled under vacuumbefore use. Oxalic acid was received from Merck. In all electrosynthesis experiments, anaqueous solution of the NMA (0.1 M) and oxalic acid (0.3 M) were prepared by usingdoubly distilled water. Electrochemical measurements were carried out in a singlecompartment three-electrode cell with 304 stainless steel disc as working electrode(0.07 cm2), platinum wire as counter electrode and saturated calomel electrode (SCE) asreference electrode. The disc electrode was a stainless steel rod with 3 mm diameter embed-ded in a Teflon holder. Before each experiment, the working electrode was polished with aseries of wet sandings of different grit sizes (320, 400, 600, 800, 1000 and 1200). Afterpolishing, the steel electrode was washed with doubly distilled water and dried at room

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temperature. Electrodeposition was performed by using potentiodynamic, potentiostaticor galvanostatic techniques. The films were coated at a 20 mV s�1 scan rate under poten-tiodynamic conditions. The electroactivity of coatings was tested in 0.3 M oxalic acidsolution (monomer-free) by using the same potentiodynamic synthesis conditions. Thecoatings prepared electrochemically were immersed in distilled water to remove adsorbedelectrolyte, monomer and the soluble oligomers formed during electropreparation of thecoating and then dried at room temperature before electroactivity and corrosion tests.

Anti-corrosion control of uncoated and the coated electrodes was carried out in anaqueous solution of 0.5 M HCl by DC polarization, linear potentiodynamic polarization,chronoamperometry and EIS. The anodic and cathodic polarization curves were recordedby changing the electrode potential from �200 mV to 200 mV with a constant sweep rateof 1 mV s�1. Linear potentiodynamic polarization curves was obtained by sweeping thepotential region between �0.2 and 1.1 V vs. SCE at a 20 mV s�1 scan rate. Chronoamp-erometric measurements were performed at a constant potential of 0.5 V vs. SCE. Electro-chemical impedance measurements were carried out in the frequency range of 105–10�1 Hzwith amplitude of 7 mV at the corrosion potential of PNMA coated and uncoated elec-trodes under unstirred conditions in 0.5 M aqueous HCl solution. All electrochemicalstudies were carried out with CHI 660B electrochemical analyzer under computer control.

PNMA coating obtained under potentiodynamic conditions on the steel substrate wasdissolved in H2SO4 solution for UV–Vis spectrum which was recorded ex situ at roomtemperature in wavelength range between 350 and 900 nm on a Schimadzu UV-2101PC spectrophotometer. The polymer structure was determined using Jasco FT-IR Model400E spectrometer with KBr pellets.

3. Result and discussion

3.1. Electrosynthesis of PNMA coating on stainless steel

Fig. 1 shows (A) first scan (a) in monomer and (b) in monomer-free and (B) full scan ofcyclic voltammograms recorded during electropolymerization of 0.1 M NMA in 0.3 Moxalic acid solution by sweeping the potential region between �0.2 and 1.1 V vs. SCE ata 20 mV s�1 scan rate. Cyclic voltammogram of uncoated stainless steel electrode wasrecorded in monomer-free 0.3 M oxalic acid solution. As can be seen in Fig. 1A(curve a), a dissolution current in the beginning of polarization is observed for stainless steeland the electrode remains in a passive state at a potential region between �0.2 and 0.95 Vvs. SCE. Transpassive dissolution at the forward scan starts at about 0.95 V in monomer-free solution. The evolution of oxygen is very less at this upper potential of 1.1 V vs. SCEdue to the oxygen evolution occurring at above 1.3 V in acidic solutions [18]. During thereverse potential scan, repassivation is observed at about 0.95 V vs. SCE and the currentremains at a low value. However, as can be seen in Fig. 1A (curve b), dissolution currentin the beginning of forward scan is not observed for stainless steel electrode in solution con-taining monomer as a barrier to the steel electrochemical activity by monomer absorptionon the surface. The electrode surface remains in a passive state at a potential region between�0.2 and 0.82 V vs. SCE. As seen in Fig. 1A (curve a), an oxidation wave does not appear,starting at about 0.82 V vs. SCE on the contrary that of a solution containing monomer. Inthe first scan this oxidation wave corresponds to the oxidation of monomer (curve b inFig. 1A). The intensity of this oxidation wave gradually decreases with subsequent scans

Fig. 1. Potentiodynamic polarization curves of stainless steel electrode in 0.3 M oxalic acid solution containing0.1 M NMA in the potential region between �0.20 and 1.10 V vs. SCE (m = 20 mV s�1); (A) first scan ((a) inmonomer-free and (b) in monomer) and (B) full scan in monomer.

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(Fig. 1B). During the first reverse sweep, a reduction peak appears which indicates the for-mation of the oligomers and polymer on stainless steel (compare Fig. 1A (a and b)). Well-defined oxidation and reduction peaks of PNMA between 0.3 and 0.7 V vs. SCE appearafter the first scan, indicating the growth of the polymer (Fig. 1B). The current values ofeach oxidation and reduction peaks are greater than that of a previous cycle which indicatethe built up of an electroactive polymeric material on the stainless steel electrode surface.The growth of the polymer is also evident from the electrical charge values under each suc-cessive oxidation and reduction peaks (Fig. 2a and b). The polymer growth is much less onPt surface than that on stainless steel under the same electrolysis conditions as is seen from

Fig. 2. Variation of (a) the cathodic charge density (b) the anodic charge density vs. cycle number duringelectropolymerization of PNMA on a stainless steel surface in 0.3 M oxalic acid solution containing 0.1 Mmonomer.

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the comparison of two tenth cycle voltammograms on two surfaces as seen in Fig. 3a and balthough the oxidation of the monomer starts at much lower potentials on Pt surface.

The current–potential behavior of uncoated and PNMA coated stainless steel elec-trodes in monomer-free 0.3 M oxalic acid is depicted in Fig. 4 between potential intervalof 0.25 and 0.65 V vs. SCE. The uncoated stainless steel electrode does not give any signaldue to oxidation and reduction (Fig. 4a). When steel electrode is coated by PNMA, oxi-dation and reduction waves appear on first cycle (Fig. 4b). Oxidation and reduction wavecan still be seen after thirty cycles (Fig. 4c) which shows the stability of this electroactivefilm. This electroactivity degrades significantly when the potential scan is extended up to1.1 V vs. SCE where the oxidation peak is shifted to more anodic potentials in each newcycle and cathodic peak decreased gradually (Fig. 5). During the positive potential scan-ning in monomer-free oxalic acid medium, polypernigraniline type-PNMA, known as elec-troinactive, is formed [19]. In addition, it has been found that polyaniline can hold water inits structure in quantities of up to 2.35 molecules of water per aniline ring via hydrogenbonding in aqueous media [20,21]. This hydrogen bonded water may cause decompositionreactions and forms quinone and hydroquinone type products. Loses in electroactivity canbe due to these types of degradation product. In other words, unwanted electrooveroxida-tion causes a decrease in electroactivity of the film in monomer-free solution.

Electrosynthesis of the PNMA carried out under various potentiostatic (1.2 and 1.4 Vvs. SCE) and galvanostatic (1.43; 4.29 and 7.14 mA cm�2) conditions. As the applied cur-rent density was increased during galvanostatic preparation, time required for formation ofpassive interlayer decreases, moreover polymerization potential increases. The coatings ofhomogeneous appearance were obtained at applied current densities above 2.86 mA cm�2.The coatings of homogeneous appearance were obtained at applied potential above 1.2 Vduring potentiostatic preparations. Polymer coatings prepared under potentiostatic or gal-vanostatic conditions did not produce any satisfactory anticorrosive polymeric layers onstainless steel surfaces which was also the case in previous studies polymeric layers on mildsteel surfaces [16,17].

Fig. 3. Potentiodynamic polarization curves concerning (a) first and (b) tenth scans recorded for Pt and stainlesssteel (SS) electrodes in 0.1 M NMA solution containing 0.3 M oxalic acid in the potential region between �0.20and 1.10 V vs. SCE (v = 20 mV s�1).

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3.2. Spectroscopic characterization

Fig. 6 shows the UV–Vis spectra in the wavelength range of 350–900 nm of PNMAcoatings obtained on the steel substrate [11,17]. The broad peak appearing at about420 nm can be attributed to the polaron state of the polymer. The strong absorbance atabout 800 nm shows the existence of bipolaron state which is related to the formationof the quinoid rings, suggesting the presence of extended conjugation in the polymer back-bone [11,17].

Fig. 7 shows that the IR spectrum of PNMA coating on stainless steel which showedsimilar structural properties as the coatings on PNMA deposited on mild steel [14] and

Fig. 4. Cyclic voltammogram of PNMA coated stainless steel electrode in monomer-free 0.3 M oxalic acidsolution in the potential region between 0.25 and 0.65 V vs. SCE (m = 20 mV s�1); (a) uncoated surface (b) firstcycle on coated surface (c) fifth cycle on coated surface.

Fig. 5. Cyclic voltammogram of PNMA coated stainless steel electrode in monomer-free 0.3 M oxalic acidsolution in the potential region between �0.20 and 1.10 V vs. SCE (m = 20 mV s�1).

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on inert electrodes such as Pt and ITO [22,23]. The IR spectrum of PNMA coating onstainless steel which showed similar structural properties as PNMA coatings depositedon mild steel and on inert electrodes such as Pt and ITO [11,14,22–24]. A broad band cen-tered at 3432 cm�1 is assigned to the N–H stretching vibration of an aromatic amine. Sym-metric and asymmetric stretching vibrations of aliphatic C–H due to the ethyl group of theNMA units in the PNMA appear at 2860, 2915 cm�1 [22,23]. The peaks at 1651 and

Fig. 7. The FT-IR spectrum for PNMA deposited on stainless steel electrode under potentiodynamic conditionsin a potential region between �0.20 and 1.10 V vs. SCE (m = 20 mV s�1), in 0.3 M oxalic acid solution containing0.1 M NMA.

0.16

0.27

0.2

0.25

350 900400 600 800

Abs

Wavelength [nm]

Fig. 6. UV–Vis spectra of PNMA dissolved in H2SO4 solution (PNMA was deposited on stainless steel electrodeunder potentiodynamic conditions in a potential region between �0.20 and 1.10 V vs. SCE, in 0.3 M oxalic acidsolution containing 0.1 M NMA).

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1581 cm�1 are due to the quinoid rings of the PNMA. The absorption peaks at 1492–1442 cm�1 are associated with the benzenoid rings. The band at 1331–1365 cm�1 maybe interpreted as being due to the C–N asymmetric stretching vibration. The observationof the bands at 1253 cm�1 suggests the presence of carboxyl groups of the dopants in thePNMA film. The bands at about 1160 and 900 cm�1 are assigned to the in-plane and out-of-plane C–H bending motions of the aromatic rings, respectively. The peak at 792 cm�1 ischaracteristic of para-disubstituted aromatic rings that indicate the polymer formation.

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The band at 577 cm�1 which is due to the quinoid (quinonimine) segments in the PNMA[14].

4. Anticorrosive properties of PNMA coated stainless steel

The homogeneous, dark green and adhesive electroactive PNMA coatings obtained bypotentiodynamic synthesis technique were used for corrosion tests.

4.1. Tafel test

Fig. 8 shows the Tafel curves recorded for uncoated and PNMA coated stainless steelimmersed in 0.5 M HCl solution. As can be seen the current density of cathodic Tafel lineof PNMA coated stainless steel decreases significantly compared to that of uncoated steel.Anodic polarization curve (Fig. 8b) shows that the current density increases due to disso-lution of electrodes at the beginning of anodic polarization and then, current densitydecreases due to passivation of the electrode. After breakdown of passive layer the currentdensity starts to increase significantly until the end of the polarization curve. The criticalpassivating current density is much less for coated stainless steel surface at the beginningof anodic polarization. The currents after passivation (between �0.1 and 0.1 V vs. SCE)are very similar for uncoated and coated surfaces. However, the recent results [25] showthat at the cathodic reaction on the PANi is not simply the reduction of dissolved oxygen,and that dissolved oxygen has very little contribution to the reduction reaction of thePANi film, especially when the polarization potential is above 0.1 V. The electrochemicaldoping/dedoping process of PANi in the acidic solution provides the necessary passivecurrent for stainless steel. The increase of the current after breaking of passivation,

Fig. 8. Tafel curves of (a) uncoated (b) PNMA coated stainless steel obtained which was obtained underpotentiodynamic conditions with a potential sweep between �0.20 V and 1.10 V vs. SCE (m = 20 mV s�1), in0.5 M HCl solution.

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on the other hand, is much more intense towards the end of anodic polarization due to thecontribution originating from the electroactivity of conducting PNMA coating and/orinsertion of anions as counter ions in the polymer structure, compared to uncoated stain-less steel [9]. According to above discussion, Tafel test is actually an insufficient method forunderstanding of protective properties against stainless steel corrosion of conducting poly-mer coatings. This is due to electroactive interactions of adhesive polymer layer with cor-rosive ions. It can be consider as a characterization technique. It is noted that thecorrosion potential is shifted to cathodic direction, when stainless steel is coated byPNMA. This behavior has been also observed in other reports [16,17].

4.2. Linear potentiodynamic polarization

The detailed information about the resistance to pitting corrosion of stainless steel byPNMA coating can be obtained by linear potentiodynamic polarization technique inacidic chloride medium. Fig. 9 compares linear potentiodynamic polarization curves ofuncoated (a) and PNMA coated (b) stainless steel surfaces in 0.5 M HCl medium.Uncoated stainless steel electrode is seen somewhat increase in current at interval �0.30and �0.15 V due to anodic dissolution (Fig. 9). Beyond this potential the steel surfaceis covered with enrichment of chromium-rich passive oxide film due to slower dissolutionof chromium oxide when compared to iron oxide in acid chloride solution [24]. It is gen-erally accepted that a passive film on a stainless steel consists of oxides of iron and alloyingelements in the substrate [21]. When the polarization potential reaches a certain criticalhigher potential (Epit), the current rises suddenly due to breakdown of the passive layer(transpassivation). At Epit (pitting potential), the chloride anions displace the adsorbed

Fig. 9. Linear potentiodynamic polarization of (a) uncoated and (b) coated stainless steel which was obtainedunder potentiodynamic conditions with a potential sweep between �0.20 V and 1.10 V vs. SCE (m = 20 mV s�1),in 0.5 M HCl solution.

Table 1Epit (pitting potential) and Imax (dissolution current) values obtained from linear anodic potentiodynamicpolarization of uncoated and PNMA coated stainless steel electrodes in 0.5 M HCl solution

Stainless steel Epit/V Imax/mA cm�2

Uncoated

First cycle 0.52 1.08Second cycle 0.50 1.04Fifth cycle 0.48 0.72

PNMA

First cycle 0.36 0.72Second cycle 0.52 0.59Fifth cycle 0.52 0.45

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passivating species at some locations and accelerate local anodic dissolution [26]. Trans-passivation is a process of some chemical species in the original passive film turning intohigher valence and more soluble products [27]. The ratio of higher valency oxides overlower valency oxides in the film normally increases with potential [27]. Epit shifts to morenegative direction for coated stainless steel during first cycle compared to that of uncoatedsteel (Table 1). On the other hand, Epit shifts to more positive direction during the subse-quent cycle for coated surface. It is clear that the current density values are much lowereven after pitting potential at each cycle for coated surface indicating better protectionagainst corrosion. It also needs to take into account in that the contribution originatingfrom the electroactivity of PNMA coating to Imax. Conducting PNMA coating on stainlesssteel lead to improved the corrosion performance by barrier effect against corrosive ionsand electroactive interactions of adhesive polymer layer with corrosive ions [7,8].

4.3. Chronoamperometry

The more detailed information about the resistance of pitting corrosion of uncoatedand coated steel electrodes can be obtained from chronoamperometric measurements per-formed at a constant potential of 0.5 V that is higher than that necessary to initiate pitting(0.2–0.3 V) [28]. Typical chronoamperometric responses of uncoated stainless steel andcoated steel are presented in Fig. 10. PNMA coated stainless steel exhibit low anodic cur-rent densities at 0.5 V compared to uncoated stainless steel. It is apparent from Fig. 10 thatgood protective properties by PNMA coating is characterized by low anodic current den-sities at a constant applied potential due to formation of a stable passive oxide layer [9].This behavior is reflected to improved the corrosion performance by increasing the Cr2O3

content in the passive film between polymer and metal, barrier effect against corrosiveions. Similar protective properties have been also observed in the case of polyaniline basedfilms [28]. It can be concluded from chronoamperometric test results that conductingPNMA coatings on stainless steel inhibited the formation of pitting corrosion.

4.4. Electrochemical impedance spectroscopy

Fig. 11 shows the Nyquist plots of uncoated and PNMA coated stainless steel elec-trodes recorded after exposure time of 0.5 h in 0.5 M HCl solution, respectively. Imped-ance measurements provide information on both the resistive and capacitive behavior of

Fig. 10. Chronoamperometric responses recorded for (a) uncoated and (b) PNMA coated stainless steels at 0.5 Vin 0.5 M HCl solution.

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the interface and makes possible to evaluate the performance of a PNMA coating as aprotective layer against metal corrosion [29]. As can be seen Fig. 11a, the impedancediagram is in the shape of a depressed semicircle and Rct value is equal to the diameterof the semicircle which is 28.4 X cm2. Uncoated steel exhibited a low polarization resis-tance due to the easy attack of the corrosive chloride ions on steel surfaces [30].Fig. 11b shows that PNMA coated steel exhibited much higher impedance value(2259.3 X cm2), which indicates the protective effect of coating in corrosive medium.The slightly suppressed nature of the semicircle may indicate that the interfacial imped-ance results from more than one electrochemical process possible related to the polymericfilm and to the changes of the double layer [29]. Bode plots show that phase angle isgreater for PNMA coated steel (Fig. 12b) in a wider frequency range compared touncoated steel (Fig. 12b). This is related to the resistance of coating against penetrationof corrosive chloride anions [30].

The data analysis is estimated before the standard fitting procedure using software. Themechanism of impedance behavior of PNMA coated stainless steel electrode is representedby an equivalent circuit in Fig. 13. Rs, Rct and CPE are the solution resistance, the chargetransfer resistance and the constant phase element, respectively. CPE is employed toinstead of the double layer capacitance (Cdl) reflected an ideal capacitor to describe thenon-homogeneities in the system. The Rct values refer to charge transfer resistance ofthe steel/electrolyte interface and the PNMA coating resistance. The CPE comprises thecapacitance of both double layer and the conductive film.

CPE ¼ ½QðjxÞa��1

Q is the frequency independent constant, and x is the angular frequency. a values are thecorrelation coefficients for the CPE (0 < a < 1). The corrosion kinetic parameters obtaineduncoated and PNMA coated stainless steel in 0.5 M HCl solution and inhibition efficiency

Fig. 11. Impedance spectra of (a) uncoated (b) PNMA coated stainless steel electrodes which was obtained underpotentiodynamic conditions with a potential sweep between �0.20 V and 1.10 V vs. SCE (m = 20 mV s�1), in0.5 M HCl solution.

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(IE) are given in Table 2. The inhibition efficiency was evaluated from the measured chargetransfer resistance Rct values as

EIð%Þ ¼ ð1=R0ctÞ � ð1=RctÞð1=R0

ct� 100

where R0ct and Rct are the charge transfer resistance values for uncoated and PNMA coated

stainless steel, respectively.Rct and CPE jointly belong to the electrochemistry of corrosion at the polymer-metal

interphase after coating penetration by corrosive anions [29–32]. Effective corrosion resis-tance is associated with high Rct and low CPE values [29,32]. As can be also seen in Table 2,Rct value of PNMA coated stainless steel is higher than those of the uncoated steel, sincethe penetration on electrode surface of corrosive chloride ions is inhibited by conductingpolymer coating. The CPE value of PNMA coated stainless steel than those of the

Fig. 12. Bode plots of (a) uncoated (b) PNMA coated stainless steel electrodes obtained in which was obtainedunder potentiodynamic conditions with a potential sweep between �0.20 V and 1.10 V vs. SCE (m = 20 mV s�1),in 0.5 M HCl solution.

Fig. 13. Equivalent circuit representing impedance behavior of the PNMA coated stainless steel.

Table 2The Rs, Rct, CPE and IE% values of uncoated and PNMA coated stainless steel obtained by EIS in 0.5 M HClsolution

Uncoated PNMA coated

Rs (X cm2) 0.13 0.66Rct (X cm2) 28.4 2259.3CPE (lF cm�2) 393.4 2.64EI (%) – 92.8

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uncoated steel is significantly lower due to inhibition of electron transfer to polymer frommetal. These show protective property of the coating. PNMA provides protection of about93% in corrosive medium. The corrosion resistance of coated PNMA on electrochemicallystainless mild steel surfaces is due to homogeneous polymer coated surface, stability ofelectroactivity and/or high conductivity of coatings and occurrence of a homogeneouspolymer/metal interphase during electrosynthesis.

5. Conclusions

The electroactive PNMA has been electrodeposited on 304 stainless steel electrode fromaqueous oxalic acid solution. Electroactive and strongly adherent coatings can be obtainedunder potentiodynamic conditions during sequential scanning of the potential regionbetween �0.2 and 1.10 V, at a scan rate of 20 mV s�1. The corrosion behavior of the

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PNMA coated stainless steel samples under immersion in highly aggressive HCl solutionwas studied by linear anodic potentiodynamic polarization, chronoamperometry, Tafeltest and EIS techniques. The results show that the PNMA as a conducting polymer coat-ing exhibited effective protective behavior in highly corrosive media containing acidic chlo-ride anions and decreased the corrosion rate of stainless steel.

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