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Progress in Organic Coatings
jou rn al h om epage: www.elsev ier .com/ locate /porgcoat
oatings based on electronic conducting polymers for corrosion protection ofetals
.F. Baldissera, C.A. Ferreira ∗
APOL/PPGEM, Universidade Federal do Rio Grande do Sul, BP 15010, 91501-970 Porto Alegre – RS, Brazil
r t i c l e i n f o
rticle history:eceived 27 October 2011eceived in revised form 27 March 2012ccepted 12 May 2012
a b s t r a c t
In this work, corrosion protection of mild steel by a novel epoxy resin (EP)-based coating system contain-ing polyaniline (PAni) as an anticorrosive agent was studied. The corrosion behavior of mild steel samplescoated with an EP/PAni-EB (emeraldine base), EP/PAni-ES (emeraldine salt), EP/SPAN (PAni sulfonated),EP/PAni-fibers, EP/PhoZn (zinc phosphate), EP/ChroZn (zinc chromate) or EP/Charge was investigated
in 3.5% NaCl solution. For this purpose, electrochemical impedance spectroscopy measurements wereutilized. It was found that the addition of three forms of PAni—undoped, sulfonated and fibers—to the EPresin increased its corrosion protection efficiency.
Approximately 30 years ago, the discovery of a new polymeramily, the intrinsically conductive polymers (ICPs), motivated thecientific community because of its enormous potential for appli-ation. These new materials, also called synthetic metals, can reachigh electrical conductivity, very close to the value of some metals.
In recent years, a wide variety of ICP have been tested as cor-osion inhibitors or coatings [1–7]; they are usually depositedhemically or electrochemically in their pure form on the metal.his is an innovative technology, still lacking in Brazil. These poly-ers, when in their doped and conducting condition, are able to
rotect alloys of steel from corrosion by an anodic protection mech-nism (i.e. production of an iron oxide layer with a high protectionbility) and they are also able to regenerate the metal oxide layerf the coating is fractured. Gonc alves et al. [8] utilized Raman spec-roscopy to characterize the alkyd coating containing PAni-EB (PAnindoped), PAni-ES (PAni doped with HCl) and POE-pTSA (poly(o-thoxyaniline) doped with p-toluenesulfonic acid) that underwentumidity chamber experiments. Their results revealed the pres-nce of an oxide layer at the coating/substrate interface, mainlyomposed of Fe2O3 and Fe3O4, demonstrating strong evidence thathe protection provided by the ICP in the experiment was promoted
y the presence of the oxide.
On the other hand, studies on doped conducting polymershowed that corrosion was prevented by the aid of a generated
electric field, which restricted the flow of electrons from the metalto the outside oxidizing species. In contrast, a barrier mechanismbased on a high diffusion resistance against corrosive ions has beenused to explain the corrosion protection imparted by the emeral-dine base of PAni (PAni-EB), which is the non-conductive form ofthis polymer [9].
Because of its low costs, obtainability and excellent chemicalstability [10–12], polyanilines are the most studied EPC. However,these polymers are difficult to process, because they are infusibleand only slightly soluble in industrials solvents, which limit theirapplication, especially in anticorrosive coatings.
PAni-based paints offer coatings with high corrosion resistancefor steel surfaces [13–15]. Wessling [16] established a relation-ship between the corrosion protection of PAni and increased redoxcatalytic activity and amount of conducting polymer, which wasattributed to a metal oxide passive layer formation. Bagherzadehet al. [9] also observed the formation of an iron oxide layer whenassessing composite coatings containing nanopolyaniline as a con-ductive additive. McAndrew et al. [17] described in their patent thatthe EB form of PAni is better than the emeraldine salt (ES) for metalprotection, even though the EB form is the most conductive form ofPAni. Radhakrishnan et al. [18] found that coatings prepared frompolyaniline–nano-TiO2 particles synthesized by in situ polymer-ization exhibited excellent corrosion resistance, much superior toPAni in aggressive environments. The novelty of these coatings liesin the generation of corrosion inhibition by three mechanisms oper-
ating simultaneously, namely: improvement of barrier properties;redox behavior of PANI and formation of p–n junctions preventingeasy charge transport when the coating is damaged by a scratch orscribble.
The polymers used as pigments were analyzed by infrared spec-troscopy. Fig. 1 shows the infrared spectra for PAni-EB, PAni-ES,SPAN, and PAni fibers.
Table 2Nomenclature of anticorrosive paint and pigment used.
Primer Pigment
P1-PhoZn Zinc phosphateP2-ChroZn Zinc chromateP3-PAniEB PAni undopedP4-PAniES PAni doped with HClP5-Blanka Filler
42 A.F. Baldissera, C.A. Ferreira / Progre
The aim of this study was to produce adherent films obtainedrom EP-based paints containing PAni-EB, PAni-ES, SPAN and PAnibers as anticorrosion additives applied onto mild steel, and alsoo investigate their corrosion performance in 3.5% NaCl solution.raoua et al. [19] investigated the importance of the chemical pre-reatment of iron and steel surfaces with dilute nitric acid andbserved the strong adhesion of polypyrrole (PPy) films to the sur-ace. It was shown that iron nitride (Fe N) species, arising fromeaction of HNO3 with the metal, are strongly grafted to the sur-ace and these species may be responsible for the adhesion of PPylms to iron surfaces. In this work, we chose to phosphate the metalurface, a widely used technique that improves the adhesion ofoatings to the substrate.
. Experimental
.1. Synthesis of PAni and undoping
The PAni-ES used in this study was prepared by the polymer-zation reaction of aniline at temperatures between −4 ◦C and 0 ◦C,or 8 h. A solution formed by an oxidizing agent [(NH4)2S2O8] in
M HCl was added slowly, under constant stirring, to a second 1 MCl solution containing the monomer. The PAni-ES obtained wasltered through porous glass funnel #G5, under low pressure topeed up the process. The green powder was washed with distilledater and finally dried in an oven at 60 ◦C for 24 h.
PAni-EB was obtained after treatment of PAni-ES with a.1 mol L−1 NH4OH aqueous solution. After 24 h under constanttirring, the solution was filtered and the dark blue powder wasashed with distilled water. The product was dried in an oven at
0 ◦C for 24 h.
.2. Sulfonation of PAni
To obtain the SPAN, PAni-ES was dispersed in 1,2-ichloroethane (DCE), and heated to 80 ◦C. The chlorosulfoniccid (HSO3Cl) was diluted with DCE (concentration 1.5 M) andlowly dripped in the solution containing PAni. The reactionemained under stirring for 5 h with a reflux system to preventCE loss. The chlorosulfonated polyaniline was filtered andispersed in water, heated to 100 ◦C and maintained at thisemperature for another 4 h to promote hydrolysis. Finally, theroduct was precipitated in acetone, filtered and dried in an ovent 60 ◦C for 24 h.
.3. Synthesis of PAni-fibers
The synthesis was performed by the rapid addition of reactants.olution 1 (6.4 mmol of distilled aniline in 20 mL of 1 M HCl, result-ng in a 0.3 M solution) and Solution 2 (1.6 mmol of (NH4)2S2O8n 20 mL of 1 M HCl, resulting in an aqueous 0.8 M solution) wereuickly mixed and stirred with a magnetic stirrer. After ∼5 minhe resulting solution became green and the reaction continuedor another hour at room temperature.
The fibers were purified by dialysis (dialysis tube,2,000–14,000 MW cutoff, Fisher Scientific) for 1 h, and theesulting solution was filtered through a paper filter.
.4. Preparation of anticorrosive paints – primer
The paints were prepared using suitable equipment for this pur-
ose and the components were weighed according to the basicormulation described in Table 1.
The pigment was completely dispersed in the resin and additivessing a DISPERMAT N1 disperser, with a Cowles disc. After that,
Additives process 0.2
a Initial amount of solvent used in dispersion and grinding of pigments.
zirconium beads were added to the mixture and then pigment-grinding started, in order to achieve the optimum dispersion.During grinding, the speed disperser was increased to approxi-mately 8000 rpm. Solvent was also added during this process. Thesize of dispersed paint particles, evaluated with a fineness of grindgage, was between 6 and 7 Hegman (25 and 15 �m). Finally, solventwas added to reach the desired viscosity.
Seven samples were obtained, each with a different type of pig-ment. Table 2 shows the nomenclature for these paints and thepigment used in each formulation.
2.5. Sample preparation
The specimens for the adherence tests and EIS were preparedfollowing standard procedures. The paints were applied to steelplates SAE 1010 (100 mm × 50 mm × 1 mm), previously degreased,pickled with 3.5 M HCl solution and phosphatized. Thereafter, thesamples were stored in a dust-free envioronment until drying wascomplete. The coating’s thickness was evaluated after the evapora-tion of the solvent at room temperature.
2.6. Characterization
Characterization of the polymers was carried out by FTIR anal-ysis (FTIR Spectrometer Perkin Elmer model Spectrum 1000) andelectrical conductivity (Signatone model S-301-6 associated witha source Keithley 2400, using the standard method of four points[20]).
The coatings were evaluated for their adhesion to the metal sub-strate in accordance with ASTM D3359-97 [21] and also throughtesting of electrochemical impedance (potentiostat ECOCHIMIEmodel Autolab 30 with FRA software) to assess the protectivepower corrosion. Film’s thickness was measured using the diffusecurrent method a unit test Byko-7500 (BYK Gardner). The mea-surements were repeated six times and the mean and standarddeviation were calculated.
3. Results and discussion
3.1. FTIR analysis
P6-SPAN SPANP7-PAniFB PAni-fibers
a This primer was used as blank, because it does not have any kind of active pigmentsin the formulation, only filler (talc).
Table 5 shows the values of polarization resistance of the coat-ings for different immersion times in 3.5% NaCl solution, extracted
Table 3Adhesion test classification for coatings.
Primer Classificationa
P1-PhoZn 3BP2-ChroZn 3BP3-PAniEB 3BP4-PAniES 3B
Fig. 1. FTIR spectra of (a) PAni-EB;
The PAni-EB spectrum showed two strong absorption bands at586 cm−1 and 1495 cm−1 attributed to the C C bond strength ofromatic ring groups quinols (Q) and benzenoid (B), respectively.he band at 1309 cm−1 is assigned to the stretching of C N Honds, and that at 1144 cm−1 to NH Q NH bonds [22]. In the PAni-S and PAni fibers spectra these bands move to regions of loweravelength (1560 and 1475 cm−1 and 1559 and 1476 cm−1, respec-
ively) due to the creation of positive charges in the polymer chainhowing a characteristic behavior of doped polymer. These valuesre similar to those previously reported for conducting polymers23,24].
The band present in the PAni-EB spectrum at 1376 cm−1 relatedo the C N stretch of units QBQ disappears completely in the PAni-S and PAni fibers spectra, indicating that the doping process withhe polaron formation (C N+) was very effective [22].
In the SPAN spectrum, the bands at 1580 and 1477 cm−1
ssigned to the C C bond stretching of the aromatic groups Q and are present. The bands at 1070 and 1019 cm−1 are assigned tosymmetric and symmetric deformation of the S O bond, respec-ively. At 813 cm−1 the outside plane deformation of the ringri-substituted at 1, 2 and 4 is shown. The bands at 700 and03 cm−1 are assigned to the S O and C S bonds, respectively [25].
By comparing the FTIR spectra obtained with data from otheruthors [22–25], their similarity can be demonstrated.
.2. Electrical conductivity
The conductivity of the PAni-ES was 39 S cm−1, the SPAN was.4 S cm−1, and the PAni fibers was 48 S cm−1. It was not possi-le to measure PAni-EB conductivity as well as conductivity of alloating films, because of their low conductivity, which cannot beeasured adequately. This may be due to the influence of the other
omponents present in the films such as resin, pigment and fillerhat are electrical insulators and contribute to the samples’ lowonductivity.
Generally, the smaller the size of the acid chain—hence easilyiffusing into the conducting polymer chain—the more effectivehe doping process, and this may explain the conductivity valuesresented by PAni-ES and PAni fibers.
.3. Adherence test
All the steel plates received two coats of anticorrosive primer.he analysis of the results is presented in Table 3.
ni-ES; (c) PAni fibers and (d) SPAN.
Coatings formed from the paints P1-PhoZn, P2-ChroZn, P3-PAniEB, P4-PAniES, P5-Blank, and P6-SPAN displayed a very goodadhesion to metallic substrates, where less than 15% of the coat-ing was removed. The only coating with low adhesion was thatformed from the paint P7-PAniFB, where 15–35% of the coatingwas removed.
3.4. EIS experiments and morphology
All electrochemical experiments were performed in a singlecompartment cell with a three-electrode configuration; the work-ing electrode consisted of a steel plate coated with a dry film ofthe paint, with an exposed surface area of 0.785 cm2. The referenceelectrode was a saturated calomel electrode (SCE) and the counterelectrode was a platinum plate.
Impedance data were measured periodically as the open circuitpotential in 3.5% NaCl solution, and a frequency range of 106 Hz to0.1 Hz using an amplitude of sinusoidal perturbation at 10 mV AC.A Faraday cage was used during EIS experiments.
Simulations were performed using Nova 1.7 software and it wasobserved that the equivalent circuits to model the impedance dataare the classic polymer coatings on metal substrates (Fig. 2). War-burg impedance (Fig. 2b) appears at the circuit after the capacitivearc formation, i.e., after the electrolyte has penetrated the coatingand the metal surface has been reached.
Table 4 shows the paints used in each plate prepared for thistest as well as the dry film thickness.
The Nyquist diagrams are shown in Figs. 3–6 for different timesof exposure to a 3.5 wt.% NaCl solution.
P5-Blank 4BP6-SPAN 3BP7-PAniFB 2B
a5B, 0% area removed and 0B, more than 65% of area removed
Fig. 2. Equivalent circuits before (a) and after (b) the capacitive arc formation.
AniES, P5-Blank, P6-SPAN and P7-PAniFB after 24 h of exposure to 3.5% NaCl.
do
R
wqt
atpc
TP
Fig. 3. Nyquist plots recorded for P1-PhoZn, P2-ChroZn, P3-PAniEB, P4-P
irectly from the Nyquist diagrams by the method of extrapolationf the capacitive arc from the relationship:
p = Zr,ω→0 − Zr,ω→∞
here: Rp, polarization resistance; Zr,ω→0, impedance when the fre-uency tends to zero; and Zr,ω→∞, impedance when the frequencyends to infinity.
Nyquist diagrams and the resistance values of the films obtainedfter the formation of the capacitive arc showed that after elec-
rolyte penetration into the coating, all samples displayed minimalolarization resistance, except paint P2-ChroZn (Fig. 4). After theorrosive attack, the film resistance increases again, probably due
able 4aints used on steel plates and dry film thickness.
Fig. 5. Nyquist plots recorded for paints after exposure to 3.5% NaCl: P3-PAniEB –1344 h, P4-PAniES – 696 h, and P5-Blank – 504 h.
Fig. 6. Nyquist plots recorded for paints after exposure to 3.5% NaCl: P6-SPAN –1464 h and P7-PAniFB – 936 h.
Fig. 7. Micrographs of PAni.
Table 5Polarization resistance values for the coatings.
Time (h) 24 504 696 840
Primer Resistance (� cm2)a
P1 ∞ ∞ ∞ 3.0 ×P2 ∞ ∞ ∞ ∞
P3 2.7 × 108 – – –
P4 ∞ ∞ 4.5 × 107 –
P5 ∞ 6.0 × 106 – –
P6 ∞ ∞ ∞ ∞
P7 ∞ ∞ ∞ ∞
a The films resistance decreases after electrolyte penetration in the coating and are rep
Fig. 8. Surface of the coating before (left) and after (right) the corrosion experi-ments: P1-PhoZn (a) and (b); P2-ChroZn (c) and (d).
to the formation of corrosion products that block the pores andthe ion transport becomes more difficult, which was observed inprevious studies [26].
The highest resistance of coatings depended on the type ofsample. Paint P1-PhoZn showed a resistance of 3.0 × 107 � cm2
after 840 h of immersion in 3.5% NaCl solution (Fig. 4). Thecoating formed from paint P3-PAniEB showed a resistance of2.7 × 108 � cm2 after only 24 h of immersion in 3.5% NaCl solu-tion (Fig. 3). In a previous study [26], it was demonstrated thatcoatings prepared from paints containing chlorinated rubber andPAni-EB showed excellent corrosion protection when evaluated inthe same test conditions. Therefore, the low resistance detected canbe attributed to defects (porosity) on the coating surface, as seen inFig. 9a. The resistance of this coating reached 9.0 × 106 � cm2 after1344 h of immersion (Fig. 5), the lowest value among all samplescontaining pigments.
Paint containing PAni-ES (P4-PAniES) showed a resistance of4.5 × 107 � cm2 after 696 h testing (Fig. 5). The coating formedfrom paints P6-SPAN and P7-PAniFB showed a resistance of9.0 × 107 � cm2 and 8.5 × 108 � cm2 after 1464 and 936 h ofimmersion in 3.5% NaCl solution, respectively (Fig. 6).
All coatings, except that formed by paint P3-PAniEB, showed ananticorrosive performance superior to that obtained by paint P5-Blank, a formulation that contains no active pigment, only filler.
6 2
This coating showed a resistance of 6.0 × 10 � cm after 504 h ofimmersion (Fig. 5).
From these results, it was observed that paint prepared withpigment zinc chromate in the formulation (P2-ChroZn) showed
246 A.F. Baldissera, C.A. Ferreira / Progress in O
Fig. 9. Coating surface before (left) and after (right) the corrosion experiments: P3-Pa
ttiwt
medium.
AniEB (a) and (b); P4-PAniES (c) and (d); P5-Blank (e) and (f); P6-SPAN (g) and (h);nd P7-PAniFB (i) and (j).
he best anticorrosive performance when analyzed by EIS, even for
he experiments maintained for a long time (>1500 h). For coat-ngs obtained from paints prepared with conducting polymer, it
as observed that P6-SPAN and P7-PAniFB maintained a high elec-rical resistance for longer compared to the coating formed by
rganic Coatings 75 (2012) 241– 247
P1-PhoZn, a formulation containing commercial zinc phosphatepigment. Therefore, it can be stated that PAni showed a very goodcorrosion protection in a 3.5% NaCl solution.
The toxicity of hexavalent chromium is well-known [27]and other alternatives have been studied to replace this metal.The intrinsically conducting polymers are an alternative tometal-based anticorrosion pigments. In this sense, our resultsratify the viability of use of different forms of PAni (undoped,doped and sulfonated) in the preparation of epoxy-based pro-tective paints displaying a performance similar to other resinsystems.
Sample P7-PAniFB has been prepared with PAni in the form offibers. Its morphological analysis with the polymer in powder wasperformed by scanning electron microscopy and the micrographsare presented in Fig. 7. It is possible to observe the presence offibrous material inside and outside the clusters. The fiber diameterwas ∼250 nm.
Figs. 8 and 9 show the aspects of the coatings, at the place wherethe IES test has been done, before and after undergoing the exper-iments.
As all paints displayed approximately the same fineness, theirmorphology was similar before samples were exposed to the cor-rosive solution (Figs. 8a, 8c, 9c, 9e and 9i). Only the paints preparedwith PAni-EB and SPAN (Fig. 9a and g) showed a different morphol-ogy, containing holes whose diameters are higher than the othersamples.
The experiments lasted more than 70 days, and larger poreswere observed in all coatings after the tests, indicating the coatingdegradation and exposing the metal substrate.
It is observed that paint P2-CroZn (Fig. 8c and d), which showedthe best electrochemical performance in the test, was the samplethat presented the least degradation after these tests. After theexperiment, the coating showed a large amount of pores; how-ever, it showed little exposure of the metallic substrate. The notablebehavior of this paint in the tests can be explained by a dense coat-ing [27] and the possible formation of a passive layer containing amixture of Fe2O3 and Cr2O3 oxides [28].
4. Conclusions
Epoxy resin has been chosen due to the fact that paints for-mulated with this resin are widely used in marine and industrialmaintenance. Many of the formulations containing conventionalprotective metal-based pigments are currently subject to envi-ronmental restrictions and their use may be not allowed in thefuture. The main objective of this study was to demonstratethat the addition of a conducting polymer to a coating based ona classic polymer resulted in improved corrosion protection ofsteel.
Coatings prepared from paints containing EP and different formsof PAni, and subject to an aggressive assay were able to offer abetter protection of mild steel than EP resin alone. The coatingcontaining SPAN was found to have the best performance in theprotection of the metal among all the other samples containingPAni and the coating containing zinc phosphate (a commercialpigment widely used for this purpose). Only Paint 3, containingPAni-EB, showed an anticorrosion protection lower than the othercoatings. This poor performance could be ascribed to the highporosity of the coating film, because this sample showed largerholes in the surface even before being dipped into the corrosive
Based on the satisfactory results obtained so far, paints preparedwith EP and PAni are able to be used as protective coating to metals,even those exposed to very aggressive environments such as themarine atmosphere.
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A.F. Baldissera, C.A. Ferreira / Progre
cknowledgements
The authors would like to thank CNPq for financial support andhe postdoctoral fellowship (PDJ).
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