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Applied Surface Science 376 (2016) 121–132

Contents lists available at ScienceDirect

Applied Surface Science

jou rn al h om ep age: www.elsev ier .com/ locate /apsusc

nhanced protective properties ofpoxy/polyaniline-camphorsulfonate nanocomposite coating on anltrafine-grained metallic surface

adegh Pour-Ali ∗, Alireza Kiani-Rashid, Abolfazl Babakhani, Ali Davoodiaterials and Metallurgical Engineering Department, Faculty of Engineering, Ferdowsi University of Mashhad, 91775-1111 Mashhad, Iran

r t i c l e i n f o

rticle history:eceived 7 December 2015eceived in revised form 15 March 2016ccepted 16 March 2016vailable online 19 March 2016

a b s t r a c t

An ultrafine-grained surface layer on mild steel substrate with average grain size of 77 nm was pro-duced through wire brushing process. Surface grain size was determined through transmission electronmicroscopy and X-ray diffraction methods. This substrate was coated with epoxy and an in situ synthe-sized epoxy/polyaniline-camphorsulfonate (epoxy/PANI-CSA) nanocomposite. The corrosion behaviorwas studied by open circuit potential, potentiodynamic polarization and impedance measurements.

eywords:ild steel

EMISassivity

Results of electrochemical tests evidenced the enhanced protective properties of epoxy/PANI-CSA coatingon the substrate with ultrafine-grained surface.

© 2016 Elsevier B.V. All rights reserved.

urface nanocrystallization

. Introduction

Ultrafine-grained (UFG) materials have attracted significant sci-ntific interest [1]. These materials are structurally characterized byery fine grain size (nano- and submicron-order) and large amountsf grain boundaries per unit area or volume. UFG materials havenusual and extraordinary mechanical and physical properties thatre fundamentally different from and often far superior to thosef their conventional coarse-grained polycrystalline counterparts2–4]. It is common to produce an UFG surface through severelastic deformation (SPD) processes [5]. The SPD processes includeccumulative Roll-Bonding (ARB) [6,7], Equal Channel Angularressing (ECAP) [8,9], High Pressure Torsion (HPT) [10], Shot Peen-

ng (SP) [11,12], Surface Mechanical Grinding Treatment (SMGT)13], Friction Sliding (FS) [14], Wire Brushing (WB) [15,16], etc.hese processes are successful in forming the ultrafine grains ofean grain sizes smaller than 1 �m in various kinds of metallicaterials [15].

The experimental observations have shown that generally a

rain refinement mechanism occurs during SPD [1,17]. It involveshe formation of dense dislocation walls and dislocation tangles inriginal grains and in the refined cells (under further straining) as

∗ Corresponding author.E-mail address: pourali2020@ut.ac.ir (S. Pour-Ali).

ttp://dx.doi.org/10.1016/j.apsusc.2016.03.131169-4332/© 2016 Elsevier B.V. All rights reserved.

well [11]. Transformations of dislocation walls and dislocation tan-gles into subboundaries with small misorientations form individualcells or subgrains. Thus, total length per unit area of grain bound-aries increases dramatically [10,11]. High strains with high rates arethe prerequisites for mentioned transformations [18]. Among theabove SPD processes, WB is relatively simple, flexible and applica-ble for different classes of materials [16]. In addition, this techniqueis capable to modify the surface microstructure and remove the millscales which, in turn, helps to increase epoxy coating adhesion [19].

Organic coatings have long been used to protect metals againstcorrosion [19]. In recent years, there has been a great inter-est in using electrically conductive polymers to protect alloys[20,21]. Polyaniline (PANI) is the oldest known synthetic electri-cally conductive polymer. PANI is readily synthesized and readilyparticipates in redox reactions. It is thermally stable with highcorrosion resistance [22,23]. PANI can be synthesized by chemicaloxidative polymerization and then be used as corrosion inhibitivepigment in epoxy coatings to protect sensitive alloys. Also, it can bedeposited on the metallic parts through the electropolymerizationof aniline from a suitable medium to limit the dissolution of the sub-strate [21,22]. Epoxy/polyaniline composite coatings are a type ofself-healing coatings which have been used for mitigation of alloys

corrosion [24–26]. From the corrosion standpoint, epoxy coatingscontaining PANI are capable to passivate steel surface beneath thecoating layer in various environmental conditions. The main advan-tage of this type of material is the repassivation of surface in the

1 rface Science 376 (2016) 121–132

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ase of coating damage. In fact, PANI and its dopant can be releasedn the corrosive environment and consequently they can passivatehe metallic surface [24,25]. As a matter of fact, the main problemf this type of coating is that the preparation procedure basicallyontains two separate parts; i) synthesis of PANI and ii) incorpo-ating of the synthesized PANI into epoxy resin. In this way, the

ajor problem is the agglomeration of PANI pigments while addinghem into epoxy matrix so that a rigorous sonication is generallyeeded to prevent this phenomenon. Therefore in our previoustudy, this problem was eliminated by introducing a one-pot proce-ure in which the synthesis process of PANI was performed in epoxyedia and the epoxy/polyaniline-camphorsulfonate (epoxy/PANI-

SA) nanocomposite was prepared without any further treatment25].

It is believed that the grain refinement affects corrosion resis-ance depending on the type of electrolyte. To be clear, if aoarse-grained sample (especially Fe-based alloys) shows an activeehavior in an electrolyte, grain refinement will likely make theurface “more” active and tend to enhance the oxidation reaction.owever, if a course-grained microstructure is passive in a givenlectrolyte, then grain refinement will likely result in an even moretable and protective passive film [27]. In fact, grain boundariesave distinct properties relative to bulk material in terms of atomicoordination, reactivity and diffusion rates [28]. Consequently, its not unreasonable to expect surfaces with relatively high grainoundary densities to exhibit different electrochemical behaviornamely corrosion rates) than coarser grained surfaces with lowerrain boundary densities. Pisarek et al. [29], Li et al. [30] and Haot al. [31] claimed that the reduction in grain size leads Fe-basedlloys to become more susceptible to general corrosion. Thus, aigh density of grain boundaries increases the reactivity of the sur-

ace through increased electron activity. This can be deduced thathe existing passivating species in the electrolyte can be adsorbed

ore easily onto metallic substrate when the microstructure isltrafine-grained.

As said above, PANI can help the metallic substrate to passi-ate in chloride-containing solutions. The mechanism which haseen proposed is that the corrosion protection ability of PANI isainly attributed to the passivating effect and oxidizing ability of

ts emeraldine state. Also, it has been proposed that with releasinghe sulfonate dopant (existing in PANI-CSA) into environment, theyan form complex compounds underneath the coating and passivehe surface by encouraging the formation of a dense metallic oxidend also complex compounds [32]. Since, iron oxidation is a triggeror PANI reduction reaction [32] and the grain refinement signifi-antly affects the initial oxidation of steel substrate [30], it can beoncluded that the grain refinement of substrate can influence therotective performance of this type of coatings.

Hence, the main target of present work is to study the anticor-osive properties of epoxy coating containing PANI-CSA particlespplied on a metallic substrate (as an example mild steel) withltrafine-grained surface. At first, wire brushing process wassed to achieve a surface layer with ultrafine grains. Transmis-ion electron microscopy (TEM) and X-ray diffraction (XRD) weresed to determine the surface grain size. The epoxy/PANI-CSAanocomposite coating was synthesized through the one-pot pro-edure which has been discussed in experimental section andhen applied onto the prepared substrates. The synthesized coat-ng was characterized using transmission electron microscopyTEM) and Fourier transform infrared spectroscopy (FTIR). Finally,he anticorrosive performance of the epoxy and epoxy/PANI-CSAoatings applied on the mild steel substrates with ultrafine and

oarse surface grains was evaluated by open circuit potential (OCP)easurements, potentiodynamic polarization and electrochemical

mpedance spectroscopy (EIS) during immersion in 3.5 wt.% NaClolution up to 134 days.

Fig. 1. Schematic illustration of surface nanocrystallization processing of mild steelsubstrate through wire brushing. A typical photograph of treated surface is shownin this figure.

2. Materials and methods

2.1. Materials

Epoxy resin was purchased from Ciba (Araldite GY 250) withan epoxide number (EEW) of 183–189 g/equiv. Polyamine hard-ener (D.E.H. 26, aliphatic amine, Tetraethylenepentamine, TEPA)with an amine hydrogen equivalent weight (AHEW) of 27 g/equivand Polyethylene Glycol (PEG) 4000 were provided from Dowchemical company and used as received. Aniline monomers werepurchased from Merck Company (Germany). Ammonium perox-ydisulfate (APS, Merck) and camphorsulfonic acid (CSA, Merck)were also used as initiator and dopant agents, respectively. In orderto purify more, aniline monomers were doubly distilled underreduced pressure. Benzyl dimethylamine (BDMA), used as accel-erator, was obtained from Shijiazhuang Wells Electronic MaterialCo., Ltd, China. Mild steel plates for all studies were purchased fromEsfahan’s Mobarakeh Steel Co., Iran. The chemical composition ofthe mild steel substrates were as follows: 0.17% C, 0.09% Si, 0.36%Mn, 0.12% Cr, 0.10% Mo, 0.30% Ni, 0.01% S, 0.02% P and Fe balance.

2.2. Surface treatment

The annealed mild steel coupons with average grain size of65 �m (Ref substrate) were mechanically ground up to 1000grit emery paper, then washed in deionized water and immedi-ately dried with air. Afterward, the coupons were wire brushed(schematically shown in Fig. 1) at room temperature with the feed-ing speed of 5 mms−1, load of 50 N and rotation speed of 8000 rpm(WB substrate). The diameters of wire and brush parts were 600 �mand 5.8 cm, respectively. In order to obtain a uniform deformation,each wire brushed coupon was processed for two passes, and thecoupon was rotated by 180◦ before the second pass processing.

2.3. In situ synthesis of epoxy/PANI-CSA nanocomposite andsample preparation

The epoxy/PANI-CSA nanocomposite was synthesized through aone-pot procedure which is presented briefly here. In this way, 60 gepoxy resin was firstly mixed with 10.6 g PEG and then stirred usingultrasonic homogenizer (Hielscher GmbH) at 60 ◦C until a homoge-nous mixture was obtained (approximately 45 min). Subsequently,the temperature of mixture was decreased to 25 ± 1 ◦C while dis-persing ultrasonically. Parallel to epoxy modification, PANI wasbeing synthesized. For this purpose, 1.7 g of CSA and 11.2 g of aniline

monomers were mixed in 100 mL of double distilled water and theobtained solution was ultrasonically stirred for 3 h at the temper-ature of 3 ± 1 ◦C. In the next stage, this aniline-containing solutionwas added to 70.6 g of PEG modified epoxy resin. This mixture was

S. Pour-Ali et al. / Applied Surface Science 376 (2016) 121–132 123

Table 1Chemical composition of the epoxy and epoxy/PANI-CSA coatings.

Coating Epoxy (g) TEPA (g) PEG (g) BDMA (g) APS (g) CSA (g) Aniline (g) Aniline (wt%)

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tirred with a constant rotation speed (1200 rpm) for 1 h. While stir-ing, high purity N2 flow was purged into the mixture in order to getid of the dissolved oxygen. Afterward, 40 mL pre-cooled aqueousolution containing 22.4 g of APS was added dropwise to the mix-ure during 45 min while stirring being continued at 3 ± 1 ◦C. Afterhe addition process, the stirring was continued for 8 h to ensure theompletion of polymerization. In the final stage of synthesis, 8.7 gf TEPA and 1.5 g of BDMA were added to the mixture. Then, theixture was degassed under vacuum for 45 min. Finally, in order to

educe the viscosity of the mixture and facilitate the coating pro-ess, it was warmed from 0 to 100 ◦C with a heating rate of 3 ◦C/min.t should be noted that the aforementioned procedure was the opti-

ized synthesis condition and the detailed experimental part toeach this point has been previously published [25]. Chemical com-ositions of epoxy and epoxy/PANI-CSA coatings are presented inable 1.

After rinsing the mild steel coupons with distilled water andcetone, dipping method was used to coat all the mild steellates with epoxy and epoxy/PANI-CSA nanocomposite. Subse-uently, the coated plates were dried in oven at the temperature of0 ◦C for 18 h. The dry film thicknesses were estimated by opticalicroscopy. The results showed that the average thickness were in

he range of 114–117 �m.

.4. Surface analysis

The surface microstructure and grain size of the annealed stateas examined by optical microscopy. Surface morphology of the

reated specimen was also characterized by transmission electronicroscopy (TEM, Tecnai G2, Netherlands). The thickness of defor-ation affected zone for wire brushed sample was determined byicrohardness measurement (Buehler hardness tester) and elec-

ron microscopy analysis. X-ray diffraction (XRD) studies werearried out using a Philips X’Pert-Pro instrument operated with Co-� radiation (� = 0.1789 nm) at a scan rate of 0.05◦ s−1 in the rangef 45–130◦ and 0.02◦ step size. Average grain size was estimatedccording to Williamson-Hall equation [33]:

rcos� = (k�/D)+�sin� (1)

here, D is the grain size, k is the Scherer’s factor which is between.89–1.39 depending on the shape and size of the grain (it is usuallyonsidered 0.9), � is representative of strain, � and � are the wave-ength of the irradiated x-ray and Bragg’s angle, respectively. The

r in Eq. (1) is equal to (ˇ02-ˇi2)0.5, in which ˇ0 and ˇi are the widths

f each wave in the half of maximum height in ultrafine grain andoarse grain samples, respectively. With plotting the variation of

r cos� in terms of sin� and fitting the best regression, the aver-ge size of the ultrafine grains formed within the surface layer wasalculated.

.5. Characterization of coatings

The applied coatings were characterized using TEM (CM120,hilips, Netherlands). Size distribution of PANI nanoparticles wasetermined by ImageJ software (http://rsb.info.nih.gov/ij/index.

tml) using Gaussian distribution function. Since there was no colorifference between PANI nanoparticles and epoxy matrix (back-round), nanoparticles were firstly highlighted with red color pixely pixel, afterward the obtained image was scanned, imported in

– – – 0.0022.4 1.7 11.2 9.65

ImageJ, and processed. The area of nanoparticles was quantifiedin ImageJ and assuming a circular shape, diameters of nanoparti-cles were reported as the nanoparticle size. FTIR spectra for epoxyand epoxy/PANI-CSA coatings were recorded using Nexus 670 FT-IRspectrometer series in the wave number range of 4000–400 cm−1.

2.6. Evaluation of the applied coatings

2.6.1. Adhesion of coating to substrateThe adhesion strength of coatings on mild steel substrates was

determined by a direct pull-off standardized procedure (ASTM D4541) using Erichsen Adhesion master 513 MC/525 MC. The adhe-sion measurements were performed prior to exposure in 3.5 wt.%NaCl solution (dry adhesion) and after 1, 7 and 14 days of immer-sion at room temperature (wet adhesion). According to ISO 4624,prior to bonding the dollies with 20 mm diameter were degreasedby acetone. The cyanoacrylate adhesive (Henkel), also named SuperBond Loctite, was employed for doing the pull-off test. After curingof the cyanoacrylate adhesive for 36 h, the epoxy coating was cutaround the dolly and then the dolly was pulled-off vertically withincreasing rate of 100 N s−1.

2.6.2. Corrosion evaluationElectrolyte for all electrochemical evaluations was 3.5 wt.% NaCl

solution and the temperature was approximately 25 ◦C. An EG&Gpotentiostat (Model 273A) and a Solartron frequency response ana-lyzer (Model SI 1255) were employed to run the corrosion tests in aconventional three electrode configuration. Coated mild steel as theworking electrode (exposed area: 13.4 cm2), a saturated calomelreference electrode and a platinum auxiliary electrode were usedto conduct electrochemical test. OCP values were recorded for var-ious types of coating up to 14 days of immersion. The impedancemeasurements were performed in the frequency range of 10 kHzto 10 mHz using an AC overpotential with amplitude of 10 mVaround OCP in different immersion times up to 20 weeks. Theobtained impedance data were analyzed employing ZView soft-ware. Potentiodynamic polarizations were carried out at a sweeprate of 1 mV s−1. Latter measurement was performed according tothe procedure which has been described within the our previousstudy [25].

3. Results

3.1. Coatings characterization

3.1.1. FTIR analysisFTIR evaluation was carried out to confirm the formation of

PANI-CSA in epoxy matrix. Fig. 2 shows FTIR spectra obtained fromepoxy and epoxy/PANI-CSA nanocomposite. It has been reportedthat the characteristic absorption bands of PANI are about 560 cm−1

(C N C bonding mode of aromatic ring), 625 cm−1 (C C, C Hbonding mode of aromatic ring), 891 cm−1 (C H out of planebending in benzenoid ring), 1030 cm−1 (S O bonding for cam-phorsulfonic acid), 1304 cm−1 and 1505 cm−1 (C N stretching ofbenzenoid ring), 1563 cm−1 (C N Stretching of quinoid ring) and

3295 cm−1 (N H Stretching) [34–36]. Another remarkable peak,is approximately located at 3211 cm−1, corresponds to the O Hbonding in PEG. Also, the presence of the benzenoid and quinoidunits is considered as an indication of the emeraldine structure

124 S. Pour-Ali et al. / Applied Surface S

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Fig. 2. FTIR spectra of the epoxy and epoxy/PANI-CSA coatings.

f PANI [36]. Thus, FTIR results obviously verify the formation ofANI-CSA particles in epoxy matrix.

.1.2. TEM analysisTEM analysis provides a direct proof of nanoparticles formation

nd size distribution in nanocomposite systems [35]. Fig. 3 depictsEM images and the estimated particle size distribution of PANI-

SA nanoparticles in epoxy matrix. As expected for epoxy coating,o PANI-CSA particle is observed in the TEM images. As known,ufficient numbers and uniform distribution of PANI nanoparticlesn a matrix is the basic requirement to ensure that the nanocom-

ig. 3. TEM images of epoxy and epoxy/PANI-CSA coatings, taken with a Philips CM120 Tn epoxy matrix is obvious for epoxy/PANI-CSA nanocomposite.

cience 376 (2016) 121–132

posite can be served as an appropriate protective coating [24]. Ascan be seen in Fig. 3, this fact is clearly visible in epoxy/PANI-CSAcoating so that a narrow distribution in particle size (approximatelybetween 30 and 50 nm) is detected by TEM images.

3.2. Morphology of wire brushed surface and hardnessmeasurement

Fig. 4 displays SEM images from the surface and cross sectionof WB substrate. Surface appearance and the thickness of deforma-tion affected zone for different areas of WB are almost similar. Asobviously seen in Fig. 1, this treated coupon shows a bright metal-lic luster. Wire brushing waves can be also observed on the surfaceof WB. High magnification microscopy with TEM reveals that thewire brushing produced an ultrafine-grained surface with a nearlyuniform distribution (see Fig. 5). According to TEM analysis, theaverage grain size for WB is estimated to be 80 nm. This value isin agreement with the results obtained by Tao et al. [11] and Songet al. [16].

Fig. 6 shows the XRD patterns of the surface of WB substrateand the annealed steel coupon as reference (Ref). The broadeningof the peaks in wire brushed state can be an indication of grainsrefining to an ultrafine size [37]. The result of Williamson-Hall curveof treated sample, presented in Fig. 7, is reasonably in agreementwith TEM observations. In this way, the average surface grain sizeof WB is calculated to be 74 nm. Thus, these results obviously verifythe formation of ultrafine grains on the surface of wire brushed mildsteel coupon.

Fig. 8 depicts the variation of microhardness (indenting load of25 g) from surface to the bulk material for Ref and WB substrates.The results indicate that near the surface of treated coupon, hard-ness is significantly increased. In the case of WB, the nanostructured

EM. The presence of synthesized PANI-CSA particles and their uniform distribution

S. Pour-Ali et al. / Applied Surface Science 376 (2016) 121–132 125

Fig. 4. SEM micrographs of the surface (a and b) and the cross section (c and d) of wire brushed substrate.

ace of wire brushed substrate, taken with a Tecnai G2 TEM.

spsuBt

rawr

Table 2Surface roughness parameters of Ref and WB substrates.

Sample Ra (�m) Rt (�m)

Fig. 5. TEM images from two different sites of the surf

urface layer leads to an increase in hardness up to 1.65 times com-aring to the its core or coarse-grain substrate (Ref). Because audden jump is observed in hardness, it can be claimed that theltrafine surface grain layer is not thick (less than 50 �m) [38].ased on electron microscopy (Fig. 4) and hardness measurements,he mean thickness of this layer is about 20 �m.

The values of Ra (arithmetic mean deviation of the assessed

oughness profile) and Rt (total height of roughness profile) forll samples are presented in Table 2. As can be seen, through theire brushing process, the Ra value, which can be considered as the

epresentative parameter of surface roughness, is increased from

Ref 0.24 1.15WB 14.43 18.59

0.24 �m (Ref) to 14.43 �m (WB). Thus, in addition to grain refine-ment, sharp increase in roughness, is also obtained. This increasein the surface roughness can promote the dry adhesion strength ofcoatings [19].

126 S. Pour-Ali et al. / Applied Surface S

Fig. 6. XRD patterns of annealed (Ref) and wire brushed (WB) mild steel substrates.

Fig. 7. Williamson-Hall analysis of WB substrate. Fit to data, the grain size isextracted from the y-intercept of the fit.

Fig. 8. Microhardness profiles for the annealed and wire brushed coupons from thesurface towards the depth (25 g).

cience 376 (2016) 121–132

3.3. Coating adhesion

One of the most important functions of organic coatings is toprevent the spreading of corrosion from the initial site of electrolytepenetration. This function requires adequate coating adhesion inthe absence (dry adhesion) or presence of water and/or electrolyte(wet adhesion). Nonetheless dry adhesion has been recognized fora long time as a poor predictor of coating performance [22,39]. Asknown, the most important factors in corrosion prevention by acoating are the processes occur at the metal/coating interface dur-ing exposure to a corrosive environment. This phenomenon cantypically result in the coating delamination. Thus, pull-off test wasperformed on all coated coupons after 1, 7 and 14 days immersedin 3.5 wt.% NaCl solution and the results are given in Table 3. Itshould be noted that the dry adhesion was also carried out and theresults for all coatings were cohesive (dolly could not completelyseparate the coating from the surface of mild steel), pointed to anappropriate adhesion to carbon steel substrates. As can be seen inTable 3, after 14 days of immersion, an intense drop is observed inthe adhesion of epoxy coating. In the case of this coating, ultrafine-grained surface leads to a low adhesion strength, so that WB + epoxysample has remarkably less wet adhesion strength than Ref + epoxyone. It can be hypothesized that with decreasing grain size due towire brushing, total length of grain boundaries as high energy sitesincreases and surface reactivity strongly alters. As a result, whencorrosive species reach the metal surface, grain boundary atoms,which have a large volume fraction, are more prone to firstly partic-ipate in the corrosion reaction. In these cases, formation of furthercorrosion products can assist the separation of coating from thesubstrate. This is consistent with the report by Li et al. [30].

On the other hand, this situation is quite different forepoxy/PANI-CSA coating (Table 3). Compared with epoxy coatedcoupons, in the same conditions, epoxy/PANI-CSA coating hasalways greater adhesion strength. In addition, for epoxy/PANI-CSA coated mild steel with ultrafine-grained surface, superior wetadhesion strength is observed. So that after 14 days, wet adhe-sion strength for WB and Ref samples is 1.97 and 0.88 Nmm−2,respectively. Apparently, this situation is caused by the presenceof PANI-CSA particles in this coating. According to previous stud-ies [32,40,41], two arguments can be presented. First, it can beproposed that in the presence of PANI-CSA particles, the numberof microleakages are decreased (i.e. PANI-CSA acts as polymericfiller in epoxy coating) [40,41]. Second, with reaching the corro-sive species the steel surface, the PANI particles may reduce thelocal corrosion rate through passivating the surface and conse-quently preventing more formation of the bulky corrosion productswhich facilitate the disbonding of the coating [32]. In the case ofWB substrate which has ultrafine-grained surface, initial generalcorrosion begins more severe than Ref sample (shown in Section3.4.2). These conditions can encourage the releasing of dopantanions on the whole surface of substrate [25]. It has been proposedthat with releasing the sulfonate dopants existing in PANI-CSA,they can form complex compounds underneath the coating andpassivate the mild steel surface [24,36]. Thus, increased releaseof sulfonate dopants and iron cations for epoxy/PANI-CSA coatedsubstrate which has ultrafine surface grains can facilitate the for-mation of an ideal passive film. Such film can be responsible for thementioned improved adhesion strength.

3.4. Corrosion evaluation

3.4.1. OCP measurements

Fig. 9 shows the variation of the OCP in 3.5 wt.% NaCl solution

for different samples. Typical plots are reported by intentionallyintroducing a coating defect using a 3 mm pointed needle. Eachpoint in this diagram is the average value of the measured poten-

S. Pour-Ali et al. / Applied Surface Science 376 (2016) 121–132 127

Table 3The adhesion strength of the epoxy and epoxy/PANI-CSA coatings applied on Ref and WB steel substrates after different exposure time in 3.5 wt.% NaCl.

Substrate Epoxy coating Epoxy/PANI-CSA coating

1 days (N/mm2) 7 days (N/mm2) 14 days (N/mm2) 1 days (N/mm2) 7 days (N/mm2) 14 days (N/mm2)

Ref 1.11 0.48 0.29 cohesive 1.55 0.88WB 1.23 0.32 0.16 cohesive cohesive 1.97

Fig. 9. Variation of open circuit potential of epoxy and epoxy/PANI-CSA coatings onannealed and wire brushed mild steel coupons in NaCl solution.

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In order the corrosive behavior of the coatings to be better stud-

ig. 10. Potentiodynamic polarization curves of the Ref and WB mild steel couponsoated by epoxy and epoxy/PANI–CSA nanocomposite.

ials for three identical samples. The initial OCP of all coated mildteels is found to be about −210 mV vs. SCE and after prolong-ng the immersion, the OCP shifts toward more cathodic regionnd reaches an almost constant value. At early stages of immer-ion, an intense OCP drop is observed which is very significantor pure epoxy coated coupons. Afterward, the OCP of PANI-CSAontaining coatings gradually rises and stabilizes. Initial OCP dropnd subsequent potential increasing (for PANI-CSA containing coat-ng) are related to diffusion of electrolyte through coating defectsnd passivity of steel surface after PANI particles and sulfonateons are released in environment, respectively [24,25,32]. As cane seen, grain refinement has almost no effect on coating with-ut any PANI-CSA particles. In fact, in these samples, continuationf active corrosion leads to a continuous decrease in OCP values.

n the other hand, for the substrate with ultrafine-grained surface,ANI-CSA containing coating performance is completely different.

n this condition, the sample shows a nobler behavior so that the

Fig. 11. Nyquist (a) and Bode (b and c) plots of Ref and WB substrates coated withepoxy and epoxy/PANI-CSA coatings after 10 days of immersion in NaCl solution.The electrical circuit used to simulate the EIS data is shown as the inset.

OCP value of WB reaches −247 mV after 10 days. According to thisresult, it can be suggested that an ultrafine structure increases thepassivity capability of epoxy/PANI-CSA coating.

3.4.2. Potentiodynamic polarization test

ied and the protectiveness ability to be evaluated, potentiodynamicpolarization test was performed using an especial procedure. Forthis purpose, after 28 days of immersion in 3.5 wt.% NaCl solution,

128 S. Pour-Ali et al. / Applied Surface Science 376 (2016) 121–132

y (lef

aatai2FsostbstHosn

Fig. 12. Nyquist and Bode plots of Ref and WB substrates coated with epox

3 mm diameter hole was intentionally drilled in the coatings tottain the bare metal surface. Then, typical three-electrode elec-rochemical cell was set up and the NaCl solution was poured to

glass tube. This procedure has been presented in Fig. 10 as thenset. Before this test, the working electrodes were immersed for

h in solution to obtain a steady state open circuit potential (OCP).ig. 10 shows the polarization curves of coated coupons. As can beeen, for epoxy coated Ref and WB samples no passivity region isbserved on the anodic branch and the polarization curves showimilar behavior to the well-known bare mild steel in NaCl solu-ion. Comparing the anodic curves of epoxy coated samples, it cane seen that the shape of the anodic curves of two samples are theame in spite of the difference of their surface structures. It meanshat the anodic reaction processes of these samples did not change.

owever, at a fixed anodic potential, the anodic current densitiesf these two samples are different: the larger the grain size, themaller the anodic current density. In fact, with decreasing in theumber of active sites (grain boundaries), dissolution tendency of

t) and epoxy/PANI-CSA (right) after 80 days of immersion in NaCl solution.

iron atoms also decreases. This result is consistent with previousstudies [29–31]. On the other hand, for epoxy/PANI-CSA coatedcoupons, the anodic part shows a passivity region with nearly lowcurrent density which decreases for the substrate with ultrafine-grained surface layer. According to Fig. 10, it can be concluded thatfor WB + epoxy/PANI-CSA sample, iron dissolution is increased atfirst (increment in critical current density) but simultaneous releas-ing of PANI and sulfonate ions leads to form a passive film on thesurface of steel. In fact, since the oxidation reaction of metal acts asa trigger for the reduction of PANI in epoxy, uniform and exhaus-tive distribution of oxidation sites throughout the surface of WBcan cause an ideal, intact and dense film on the substrate surfaceof WB + epoxy/PANI-CSA. This result provides an appropriate evi-dence to confirm the mechanism by which PANI affords protection

[32]. Thus, in the presence of a substrate with ultrafine-grainedsurface (like WB), passivation effect of PANI will be remarkablyincreased.

S. Pour-Ali et al. / Applied Surface Science 376 (2016) 121–132 129

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ig. 13. The changes in: (a) coating capacitance, (b) pore resistance, (c) double layer cime in 3.5 wt.% NaCl solution.

.4.3. EIS measurementsTo gain insight into the corrosion behavior of coated ultrafine

rained surface, impedance measurements were carried out at thepen circuit potential up to 134 days. In this way, Figs. 11 and 12resent the results after 10 and 80 days of immersion, respec-ively. Like common interpretation for imperfect organic coatings24,25,42], impedance diagrams show two time constants: one atigh frequency part which is related to the performance of organicoating and the second one at low frequency part that correspondso the reactions occurring on the metal through defects and poresn the coating. The total impedances at 0.01 Hz, |Z|0.01 Hz, revealhat the most effective inhibition performance has been obtainedor WB + epoxy/PANI-CSA system. The values of |Z|0.01 Hz for thisample are about one order of magnitude and two orders of mag-itude higher than that of Ref + epoxy/PANI-CSA and WB+epoxynes, respectively. This phenomenon could be elaborated form twospects. The first one is the performance of PANI-CSA nanopar-icles in the coating as filler. Presence of PANI-CSA nanoparticlesn the epoxy matrix with uniform distribution may fill the exist-ng holes and diffusion pathways [25]. Thus, the diffusion of ionicpecies through epoxy coating will be retarded and/or suppressed.his can be considered as a reason for the higher corrosion resis-ance of epoxy/PANI-CSA coating compared with the pure epoxyne. Another aspect to take into account is the trigger effect of sur-ace nanocrystallization on the PANI redox reactions. Based on theorrosion protection mechanism of doped PANI [32,36], this iteman be much more important than the first one. Oguzie et al. [43]ound that the surface nanocrystallization of mild steel increases

ts active area which in turn leads to enhanced electrogenerationf Fe2+ in corrosive medium. Electrochemical corrosion behaviorf WB + epoxy sample obeys this rule (see Figs. 11 and 12). In theresence of PANI-CSA nanoparticles in epoxy coating, a contro-

tance and (d) polarization resistance of different samples as a function of immersion

versial behavior is observed. In these conditions, the severe irondissolution of surface nanocrystallized substrate can activate thePANI redox reactions which shown in Fig. 14a. The remarkableincrease in the diameter of time constant at low frequencies forWB + epoxy/PANI-CSA system (see Figs. 11 and 12d) suggests thatthe enhanced releasing of sulfonate ions by PANI (as a result ofenhanced generation of Fe2+) can elevate the polarization resis-tance through ideal passivating of the nanocrystallized surface.

Using the equivalent electrical circuit shown in Fig. 11a, variouselectrochemical parameters were extracted from the Nyquist andBode plots. This circuit reveals electrolyte resistance, Rs, constantphase element of coating, CPEc , pore resistance, Rc , constant phaseelement of double layer, CPEdl , and polarization resistance, Rp. Theconstant phase elements consist of Y0 and n which are the admit-tance of the CPE and deviation parameter, respectively. The valuesof coating capacitance (Cc) and double layer capacitance (Cdl) werecalculated by means of latter parameters and following equations[25,44].

Cc =(Y0,c × R1−n

c

)1/n(2)

Cdl =(Y0,dl × R1−n

p

)1/n(3)

where Y0,c and Y0,dl are the admittance of CPEc and CPEdl , respec-tively. The evolution of Cc and Rc during 134 days of immersion in3.5 wt.% NaCl solution are depicted in Fig. 13a and b, respectively.The Cc variations for epoxy coating demonstrate a monotonic risingwith increasing in the immersion time which is attributed to theelectrolyte uptake in the coating. This process involves the first step

of electrolyte uptake via penetration through coating pores; then,as immersion time increases, the coating begins to become satu-rated and more aggressive species reach the steel surface [24,45].Epoxy/PANI-CSA coating display lower values for Cc , which may be

130 S. Pour-Ali et al. / Applied Surface Science 376 (2016) 121–132

F d (b)

e

aiPocea

ig. 14. (a) Protection reactions in the presence of doped PANI in epoxy matrix anpoxy and epoxy/PANI-CSA coatings.

ttributed to the low pore content and/or low diffusivity of coat-ng for electrolyte molecules which in turn confirms the role ofANI-CSA nanoparticles as fillers in the epoxy coating. As a result

f diffusion of electrolyte molecules and ionic species through theoating pores, a sharp decline in the Rc values is observed at thearly stages of immersion which is followed by a low reduction ratefter four days. The rate of decrease in Rc for WB + epoxy/PANI-CSA

schematic representation of the effect of grain refinement on the performance of

sample is more subtle which is an indication of higher corrosionresistance for this system.

Assessment of variations of Cdl and Rt yields valuable infor-

mation regarding the corrosion reactions occurring at the metalsubstrate beneath the coating. The higher the Cdl , the more the dis-bonded area at the interface of coating/metal [42]. As presentedin Fig. 13c, WB + epoxy system demonstrate larger disbonded area

rface S

toietWocsPo

4

aicftiiwtebldsssi[ocmoeftsWwctsaimhrtselimtn

5

wc

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S. Pour-Ali et al. / Applied Su

han the other three systems with higher value of Cdl after 134 daysf immersion. Variation of Rp as a function of immersion time

s also shown in Fig. 13d. Rp values decreases monotonically forpoxy coating during immersion, which corresponds to the con-inues corrosion process underneath this coating. In the case of

B + epoxy/PANI-CSA system, Rp values reach a level of 1 and 3rder of magnitude higher than Ref + epoxy/PANI-CSA and epoxyoated ones, respectively. It is interpreted that the nanocrystallizedurface can effectively stimulate the passivation effect of dopedANI and as a result of formation an ideal passive film, more Rp isbtained.

. Discussion

The results reported in this work show that when a failure (suchs cracks, pinholes, scratches, etc.) occurs in the epoxy based coat-ng, depending on the surface microstructure of substrate and theoating composition, coating performance will be absolutely dif-erent. In the case of pure epoxy coated samples, iron dissolution inhe wire brushed sample (with ultrafine-grained surface) is morentense than the annealed one (surface grain size ∼65 �m). Thiss consistent with some early published studies [29–31]. In fact,

ire brushing as a severe plastic deformation dramatically reduceshe average grain size i.e. total length of grain boundaries (highnergy sites) remarkably increases. Increased number of the grainoundaries as high energy sites caused by wire brushing process

eads to an increased corrosion rate in saline solution. These con-itions are completely different for epoxy/PANI-CSA coated mildteels. As it has been widely reported [24,25,32,40,41], PANI is amart conducting polymer which passivates the surface of the steelubstrates by formation of complex compounds and Fe2O3. Accord-ng to protection mechanism proposed by Sathiyanarayanan et al.32], concentration and distribution of Fe2+ ions (as a componentf protective reaction) can severely affect the performance of PANIontaining coating. One of the most important factors in the for-ation of Fe2+ and its distribution is the density and configuration

f grain boundaries on the surface. Based on our results, these highnergy sites are more uniformly distributed throughout the sur-ace after the mentioned wire brushing process. In addition, theotal length per unit area of surface grain boundaries in WB sub-trate is much greater than the annealed one. Thus, in the case of

B substrate which has ultrafine surface grains, iron dissolutionill be more uniform at the different sites of surface. Hence, it

an be claimed that for ultrafine-grained surface, during the ini-ial corrosion, Fe2+ ions are released throughout the surface andubsequently react with the sulfonate dopant anions released as

consequence of polymer reduction and finally form an insolubleron-dopant salt which uniformly and completely passivates the

etal surface. In other words, for WB + epoxy/PANI-CSA sample,igh concentration of Fe2+ ions which participate in the protectiveeaction can help to the formation of dense passive film underneathhe epoxy/PANI-CSA coating. Such mechanism is schematicallyhown in Fig. 14. According to this proposed mechanism, it can bexpected that a dense and uniform distribution of high energy siteseads to a much improved performance for epoxy/PANI-CSA coat-ng on mild steel. Finally, it should be remarked that the proposed

echanism for WB + epoxy/PANI-CSA coated mild steel coupons inhis work is an alternative tool to confirm the protection mecha-ism of doped PANI.

. Conclusions

In this research, the surface nanocrystallization of mild steelas achieved by using wire brushing process. The electrochemi-

al corrosion behavior of mild steel substrate with ultrafine surface

[

[

cience 376 (2016) 121–132 131

grains coated with epoxy and epoxy/PANI-CSA coatings was inves-tigated for the first time. Based on our results, the corrosionresistance of epoxy coated mild steel in NaCl solution was severelydecreased in the presence of a substrate with ultrafine-grainedsurface. This behavior is a direct result of the increase of ener-getic sites (grain boundaries) on the surface of substrate. On theother hand, for epoxy/PANI-CSA coated sample, mentioned ener-getic sites can serve as triggers for PANI redox reaction. In fact,enhanced and almost uniform initial corrosion of a substrate withultrafine-grained surface leads to release more sulfonate anions atdifferent sites of the coating. These conditions provide the prereq-uisites for the reaction between Fe2+ and sulfonate ions which canform a dense and uniform passive layer beneath the coating. For-mation of such passive film leads to high polarization resistancefor ultrafine-grained surface so that for wire brushed substratewith average grain size of 77 nm coated with epoxy/polyaniline-camphorsulfonate, an ideal protective behavior is obtained after134 days of immersion.

Acknowledgments

The authors would like to acknowledge Mr. Mohsen Norooziwho supplied the electron microscopy services from the surfaceand cross section of wire brushed substrate in Hohai University. Theauthors also thank Dr. Navid Ramezanian, Mr. Mahdi Kamali, Dr.Saeed Abbasizadeh, Mr. Ramin Dallal-Moghaddam, Mr. Ali Kosari,Mr. Hashem Teimourinejad and Mr. Afshin Nazarnejatizadeh forvaluable discussions.

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