Preparation, degradation of polyaniline doped with organic phosphorus acids and corrosion essays of polyaniline–acrylic blends

Synthetic Metals 156 (2006) 230–238

Preparation, degradation of polyaniline doped with organic phosphorus acids and corrosion essays of polyaniline–acrylic blends Nicoleta Plesu a , Gheorghe Ilia a,∗ , Aurelia Pascariu a , Gabriela Vlase b b

a Institute of Chemistry of Romanian Academy, Mihai Viteazul Bul. 24, 300223 Timisoara, Romania West University of Timisoara, Faculty of Chemistry-Biology-Geography, 16 Pestalozzi Str., Timisoara, Romania

Received 3 May 2005; received in revised form 11 August 2005; accepted 23 November 2005 Available online 6 January 2006

Abstract Preparation and characterization of polyaniline (PANI) doped with anions containing phosphorus was investigated with the help of various technique. The chemical polymerization of aniline was carried out in acid media containing different anions of organic phosphorus acid with the use of ammonium peroxidisulfate as oxidant agent. The highest yield was observed in the case of styrilphosphonic acid. The conductivity increased in the following order: phenylphosphinic acid < 2chloroethylphosphonic acid < styrilphosphonic acid. The solubility tests carried out in DMF, DMSO and NMP showed that the presence of voluminous anions improves the solubility of doped PANI. The synthesized PANI was characterized by density, intrinsic viscosity, solubility, FTIR, UV–vis, NMR, conductivity and TGA measurements. The hardness, elasticity, resistance and protective behavior of acrylic films were determined. The acrylic dispersion based on PANI doped with anions containing phosphorus provides improved corrosion protection of carbon steel. © 2005 Elsevier B.V. All rights reserved. Keywords: Conductive polymers; Polyaniline; Solubility; Thermal stability

1. Introduction Polyaniline (PANI) is known as the only air-stable synthetic metal [1,2]. PANI can be obtained by synthesized either chemically [3] or electrochemically [1,4] as a powder or film. It is known that undoped state of PANI (insulator) is soluble in common organic solvents, but the conducting form (doped form of PANI) is insoluble in almost all solvents, except concentrated sulfuric acid [1–4]. The lower processability is a major disadvantage of PANI. In order to improve the solubility of the conductive form, the polymer chain can be modified with ring or N-substituents [5], by using large anions as dopants (dicarboxilic, camphorsulfonic, p-toluensulfonic acids) [6–10] or by blending it with suitable insulator polymer matrix [11]. Corrosion is a natural process and occurs because metals tend to return to their more stable, oxidized states. The search for an effective organic corrosion inhibitor is required because the use of heavy metals as additives is restricted by their toxicity.

∗

Corresponding author. Tel.: +40 256 49 1818; fax: +40 256 49 1824. E-mail address: [email protected] (G. Ilia).

0379-6779/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2005.11.006

PANI has been used from several years in corrosion protection because of its unique properties. Its structure provides flexibility and allows the existence of three distinguished oxidation states that are leucoemeraldine, emeraldine, and pernigraniline. Leucoemeraldine and pernigraniline forms of PANI are not stable and they will return to the state of emeraldine under the atmospheric environment. Major researches results indicated that PANI immobilized on alloys show a form of anodic protection which stabilized the materials in mineral acids [12–18]. Wei et al. [19–21] evaluated the performance of doped PANI and undoped PANI in various environments. The performance of doped PANI followed by an epoxy topcoat was much better than that of an epoxy topcoat alone [12,18,22]. Even after the doped PANI and epoxy topcoat were removed by scribbling, the doped PANI conferred corrosion resistance to the coated areas. Thus, doped PANI acted as an effective primer against acidic environment. Wessling [23–25] conducted its research on the topic of corrosion prevention of mild steel. Samples of mild steel with undoped PANI and with an epoxy topcoat exhibited corrosion rates slightly slower in 3.5% NaCl solution and 100 times slower in 0.1N HCl solutions than those of samples coated with epoxy alone. McAndrew et al. [22] investigated the

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corrosion prevention of the carbon steels and showed that the undoped PANI performed well as a corrosion resistant coatings while conductive, protonated PANI, showed virtually no effects. In this work we describe the chemical preparation and characterization of PANI doped with anions containing phosphorus. We showed that different organic acid containing phosphorus anions allowed synthesis of PANI that improve the solubility of the resulting doped PANI and the performance in corrosion protection. The resulting polymer has been characterized by FT-IR, UV–vis, NMR, density, inherent viscosity and thermogravimetry. The oxidation state of PANI samples was determined by titration with TiCl3 solution and from UV–vis data. The viscosity and resistivity of acrylic dispersions and hardness, elasticity and protective behavior of coatings were evaluated. Factors that influence the performance of the protective coating, such as the nature of anion used, were investigated from the quantity of Fe(II) release by immersing the bare and coated carbon sample in known solution of 3.5% NaCl and electrochemical measurements. 2. Experimental 2.1. Chemicals Freshly distilled aniline cooled at −4 ◦ C, phenylphosphinic acid (C6 H5 PH(O)OH, APP, Aldrich), styrilphosphonic acid (C6 H5 CH CHPH(O)OH, ASP, prepared in our laboratory), 2-chloroethylphosphonic (2-Cl-C2 H5 PH(O)OH, AClEP, Merk), sulfuric acid (AS, Merck), ammonium peroxidisulfate (Merck), ammonium hydroxide, N,N-dimethylformamide (DMF, Fluka), N-methyl pyrrolidone (NMP, Fluka), dimethylsulfoxide (DMSO, Merck) were used. 2.2. Preparation of PANI doped with anions containing phosphorus PANI was obtained by oxidizing aniline in dilute organic acid with ammonium peroxidisulfate as oxidizing agent. The aniline/oxidant molar ratio was 1 and the aniline/organic acid ratio 0.5. The procedure is the same for all samples: first the aniline salt is formed by the dropwise addition of aniline (0.1 mol L−1 ) in an aqueous solution of organic acid (APP, ASP, AClEP), under continuous stirring and cooling (temperature about 0 ◦ C). In this solution a precooled aqueous solution of ammonium peroxidisulfate (0.1 mol L−1 ) is added dropwise under continuous stirring. After 24 h the stirring is stopped and the PANI is recovered by filtration, washed several times with methanol and distilled water and finally with a dilute acid solution. The resulting PANI was dried in dynamic vacuum for 24 h. The product was converted to PANI-base form with ammonium hydroxide (dedoped form). 2.3. Preparation of acrylic dispersion based on PANI doped with anions containing phosphorus The doped PANI powder was dispersed onto a laboratory dispersing equipment (three rolls machine) till a degree of friction

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of 40 ␮m. The resulted paste contains PANI (38%), acrylic resin SMP 62 (SC. AZUR SA Timisoara), organic solvents (toluene, butanol), dispersing additives and antideponants. Acrylic dispersions with 5.6%, 10.8% and 20% PANI were obtained by dispersing different amounts of prior prepared paste in acrylic resins. 2.4. Characterization The IR spectra were recorded in KBr pellet. UV–vis spectra were carried out in N,N-dimethylformamide and a Specord M42 spectrophotometer was used. 13 C NMR spectra were performed with a Bruker Avance DRX 400 spectrometer in d6 -DMSO at room temperature. The inherent viscosities of PANI samples doped with different organic acid containing anions with phosphorus were determined with Ubbelohde viscosimeter in 97% sulfuric acid 0.01% solution. The density of PANI was determined according to the picnometer method, in decaline. Electrical conductivities were measured on press pellets by twoprobe method. The oxidation state of PANI was determined from the relative proportion between benzenoid and quinoid rings from the polymer chain [27]. The oxidation state was also determined from UV–vis absorbance spectra of the polymer solutions in DMF [26]. The viscosity measurements of acrylic dispersion based on PANI were performed using Brookfield RVT viscosimeter, at room temperature. The hardness of the films was established with the Persoz pendulum (according to SR ISO 15221995); and the Erichsen elasticity of films was determined according to STAS 3046-68. The resistance of acrylic dispersion based on PANI was measured with the Rezistest Cella (Hungary). The protective behavior of obtained acrylic dispersions was evaluated from quantity of Fe(II) released by the coated carbon samples (exposed area 1.0 cm−2 ) immersed for 1 month in 50 mL 3.5% NaCl solutions. The concentration of Fe(II) was determined by the 1,10-phenanthroline spectrophotometric method [37]. The corrosion protection offered by dispersions in acrylic resin (SMP 62) of obtaining PANI samples, coated onto a metal surface (carbon steel) was also studied by recording the linear polarization curves using a Princeton Applied Potentiostat 173. The corrosion cell was equipped with a saturated calomel electrode and graphite counterelectrodes. The working electrode consisted of disc shaped carbon steel electrodes with a diameter of 15 mm. Before coating the substrate was sanded and degreased with methanol and acetone. The coated samples were cured under the ambient environment for 1 day. The thickness of the coating was controlled between 0.03 and 0.05 mm. The samples were embedded in a specimen Teflon holder, leaving exposed a surface area of 1 cm2 . The same electrodes were used to measure the time variation of the open circuit potential. The potentiometric response was measured with a digital voltmeter E0302, in synthetic sea water and 3.5% NaCl solution, against a saturated calomel electrode.

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The corrosion behaviour in time was also studied by immersion tests, using coated plate electrodes with the dimensions of 140 mm × 40 mm × 1 mm. In all experiments the temperature was 25 ± 1 ◦ C. From the polarization curves Tafel plots were constructed. The corrosion parameters (Ecorr and icorr ) were calculated from the Evans diagrams (E–log i). 3. Results and discussion The PANI was obtained in the same conditions (aniline/oxidant molar ratio, aniline/acid molar ratio, temperature); only the nature of acid differs. The degree of protonation increases with the increase of the acid concentration. The conductivity increases with the increase in proton concentration in the aqueous acid media, due to the formation of the doped polymer and to the existence of a small quantity of acid retained by the polymer chain [2–6,29]. The aniline/acid molar ratio was 0.5 and was chosen with respect to the lower solubility of organic acid in water. In all cases the pH was <1.5, so the acidic medium where the aniline exists as an anilinium cation was achieved and an efficient oxidative polymerization took place. It is known that in more acidic media the degradation of polymer is expected and the oxidation process of protonated species is more difficult compared to the unprotonated ones. This is in accordance with Lux’s results [28], i.e. the polymerization is suppressed by more acidic media. This is mainly attributed to the protonation of aniline. Polymerization rate could be assessed by differential method in percent per minute, from %dry mass versus time profile, and was equal with the slope for a particular moment of the polymerization. The polymerization rate decrease in order: AS > AClEP > ASP > APP (Fig. 1) and was explained by the fact that organic acid are more viscous comparative with AS, and diffusion of active species in polymerization process is slower, decreasing the reaction rate. The induction time, the polymerization rate, the yield and finally the inherent viscosity, conductivity and the oxidation state are influenced by the nature of acid. We believe that the differences shown in the induction time and the polymerization rate

Fig. 2.

13 C

Fig. 1. The polymerization rate vs. time of aniline at aniline/oxidant molar ratio = 1 and aniline/acid molar ratio is 0.5, in different acid media containing phosphorus anions at 0 ◦ C.

in organic acid are attributed to their hydrophobic effects. Similar observation was made by Choi and Park-Moon [30]. APP ions are the most lyophilic while the sulfate ions are the most hydrophilic. Based on this, APP ions are expected to form the most stable ion pairs with anilinium ions. Since the dissociation of the ion pairs precedes the electron transfer reaction, the kinetics of the following electron transfer is determined by the dissociation of the ion pair. Thus, the kinetics of the polymerization reaction is faster in AS than in other organic acid used. The dissociation of ion pairs affects the initiation, the propagation (growing) and the interruption stage. So, in organic acid due to the slow dissociation of ion pair of growing species, the oligomeric products were favored. It explains the lower yields obtained (the oligomeric species was removed with methanol). Independent by the acid used, the reaction has an induction period, after which the reaction becomes faster. The polymerization of aniline in APP required a long induction period (8–10 times longer) compared to the synthesis in ASP and AClEP and other known acids, mentioned in literature (hydrochloric, sulfuric, picric) [1,3,6–10,29]. The induction time was deter-

NMR chemical shifts for the PANI samples.

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Fig. 4.

Fig. 3.

13 C

NMR spectrum of PANI-APP (ppm/TMS).

mined from temperature–time dependence and represents the time required to reach half of the total increase in temperature during the exothermal step. The induction time seems to be much shorter in APP and suggests the possible formation of the complexes by C6 H5 HP(O)OH and C6 H5 HP(O)O− with the peroxidisulfate ion [31]. Complexes are formed by hydrogen bonds, P H. . .O (from oxidant ions), and the possible concurrent redox reaction could involve the dissociation of H – from the P H bond causing the transfer of two electrons from oxidant (peroxidisulfate) to – O-bridge. The co-existence of the oxidized and reduced units in PANI chains produced eight groups of non-equivalent carbon (Fig. 2), carbon bonded to hydrogen atoms and carbon which does not forms bonds with hydrogen atoms. The number of observed lines significantly exceeds the expected eight lines corresponding to eight non-equivalent carbons. This is due to the introduction of a quinoid ring to the chain, to the possible chain conformation of quinoid–imine ring and the protonation of nitrogen atoms. Assignments of 13 C NMR spectra of samples were based on the model compounds [32,33] and were summarized in Figs. 3 and 4. All 13 C chemical shifts were measured using the XSI scale [34]. The absence of the peak at 147 ppm, associated to the carbon of the benzenoid ring bonded to imine nitrogen (carbon 5: Fig. 2a) and of the peak at 158 ppm, corresponding to the carbon of quinoid ring bonded to imine nitrogen (carbon 8:

13 C

233

NMR spectrum of PANI-SP (ppm/TMS).

Fig. 2a) [32] in the recorded 13 C NMR spectra of doped PANI samples were explained by the protonation of imine nitrogen (carbon 5, 8: Fig. 2b) [35]. The peaks appearing at 142.5 and respectively 138 ppm (Figs. 3 and 4) were assigned to carbons 5 and 8 (Fig. 2b). The presence of amine structure is also confirmed by the IR spectrum, which shows strong absorption at 3382 cm−1 due to νN H and at 1290 cm−1 due to νC H [32]. The yield in PANI, conductivity, inherent viscosity and percent of hydrogen per gram polymer required for the reduction of samples with TiCl3 obtained at aniline/oxidant molar ratio = 1 and aniline/acid molar ratio 0.5, in different acids at 0 ◦ C were summarized in Table 1. The oxidation state of PANI was estimated by calculating the intensity ratio of peaks at 620 and 320 nm (IQ /IB ) in each UV–vis spectrum of the polymer samples and by the hydrogen percentage necessary to reduce PANI samples (Table 1). The ratio of quinoid and benzenoid units (i.e. C N/C N) was calculated on the basis of the following Eq. (1):

(1) The oxidation state (Y) of the obtained polymer was about 0.513 and both methods give appropriated values and indicate that PANI samples were obtained in emeraldine state.

Table 1 The yield, conductivity, inherent viscosity and density of PANI and percent of hydrogen per gram polymer in function of nature of dopant Synthetic

Acid

Yield (%)

Conductivity (S cm−1 )

Inherent viscosity (dL g−1 )

IQ /IB (R)

%H/g dry polymer

1 2 3 4

ASP AClEP APP AS

60.18 59.19 57.78 75.62

1.39 1.58 1.10 3.27

0.603 0.625 0.523 0.720

1.68 1.51 1.12 1.48

0.567 0.589 0.562 0.552

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Table 2 The solubility data of doped PANI samples obtained at 0 ◦ C at aniline/oxidant molar ratio = 1 and aniline/acid molar ratio 0.5, with different acids Dopant

AS APP ASP AClEP

Solvent NMP (g/l)

DMF (g/l)

DMSO (g/l)

– 0.521 0.587 0.502

0.042 0.106 0.268 0.217

0.006 0.232 0.401 0.320

The conductivity and inherent viscosity of PANI samples were higher in the case of AS. The lower value of the inherent viscosity of the samples obtained in ASP, AClEP and APP reveals the presence of shorter polymer chain (lower molar masses) in PANI samples. It is known that the conductivity increases with the increase in proton concentration in the aqueous acid media, due to the formation of the doped polymer and to the existence of a small quantity of acid retained by the polymer chain and with the increase in polymer chain length [1,2,11]. In this case the same synthesis parameters were used (the same aniline/acid molar ratio) and so, the slightly decrease of conductivity may be attributed to the shorter polymer chain obtained in organic acid. The solubility tests in solvents like NMP, DMF, DMSO, indicate an enhanced solubility of doped PANI samples compared with the samples obtained in sulfuric acid (AS) (Table 2). The increase in solubility was attributed to the presence of organic voluminous anions which are able to give much stronger polymer–solvent interaction than polymer–polymer. The PANIASP; PANI-AClEP; PANI-APP; samples were dispersed more easily in acrylic resins till a degree of friction of 40 ␮m comparatively with PANI-AS. In the same manner, the procesability of doped PANI samples with anions containing phosphorus was improved. The TGA curves recorded for doped and dedoped PANI samples are presented in Figs. 5 and 6 and show a decrease of the thermal stability of doped PANI samples synthesized in acids containing phosphorus anions compared to the PANI samples synthesized in inorganic acid.

Fig. 6. The thermal stability of dedoped PANI samples.

The thermal stability of doped form decreases in order: PANIASP < PANI-AClEP < PANI-APP < PANI-AS and it is in accordance with the inherent viscosity of doped PANI samples and with the thermal behavior of the organic acids used in synthesis. In general the thermal behavior of PANI samples shows a characteristic “three step” weight loss. During heating up to 120 ◦ C, the weight losses is generated by the residual water presented in all PANI samples, water molecules are able to occupy sites instead of dopants in polyaniline so, the residual water cannot be assumed to be humidity, because all the samples were dried before TGA measurements. This process was also observed by other authors [4,7,11,19–21]. The weight loss in the temperature range 200–400 ◦ C is related to dopant loss and degradation of the oligomer chains formed. The weight loss became higher if the polymer chain is shorter. The weight loss increases also due to the presence of dopant content in the polymer samples. The dedoped PANI samples showed a better thermal stability compared with the doped one (Fig. 5). The characteristics of obtained acrylic dispersions based on synthesized PANI samples are presented in Table 3. Table 3 Physical properties of films based on acrylic dispersion of doped PANI with anions containing phosphorus Sample type

PANI content (%)

Hardness (s)a

Elasticity (mm)

Resistance ()b

PANI-APP

5.6 10.8 20.0

163 166 171

2.1 2.0 1.9

2.1 × 10−5 1.8 × 10−5 1.1 × 10−5

PANI-ASP

5.6 10.8 20.0

157 162 165

2.5 2.3 2.3

1.06 × 10−5 0.83 × 10−5 0.68 × 10−5

PANI-AClEP

5.6 10.8 20.0

119 128 132

3.1 3.1 3.3

0.57 × 10−5 0.47 × 10−5 0.29 × 10−5

PANI-AS

5.6 10.8 20.0

141 143 152

2.7 2.6 2.7

0.38 × 10−4 0.23 × 10−4 0.03 × 10−5

a

Fig. 5. The thermal stability of doped PANI samples.

b

The pellicle presented drying type E. For dispersion.

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Table 4 The content of PANI and thickness of pellicle deposited onto carbon steel Sample type

Sample

PANI contenta (%)

Quantity of coating (g)b

Approximate content of PANI (g)c

Thickness of pellicle (mm)d a

b

PANI-APP

C1 C2 C3

5.6 10.8 20

0.031 0.032 0.038

0.0018 0.0035 0.0077

0.031 0.032 0.038

0.032 ± 0.30 0.035 ± 0.19 0.036 ± 0.23

PANI-ASP

C4 C5 C6

5.6 10.8 20

0.039 0.032 0.036

0.0022 0.0035 0.0073

0.039 0.032 0.036

0.036 ± 0.25 0.032 ± 0.13 0.032 ± 0.21

PANI-AClEP

C7 C8 C9

5.6 10.8 20

0.031 0.033 0.040

0.0017 0.0037 0.0081

0.030 0.033 0.039

0.031 ± 0.28 0.030 ± 0.24 0.034 ± 0.25

PANI-AS

C10 C11 C12

5.6 10.8 20

0.296 0.280 0.311

0.0166 0.0302 0.0622

0.029 0.027 0.030

0.029 ± 0.14 0.023 ± 0.14 0.025 ± 0.24

a b c d

Content of PANI in acrylic dispersion. Quantity of PANI acrylic dispersion applied on the plate (before drying). Content of PANI on the plate sample. (a) Calculated from quantity of coating and plate profile, and (b) Measured.

The decrease of the elasticity and increase of the hardness of resulting films were attributed to the presence of rigid phenyl ring in the anion structure of doping acid. The hardness of coatings also increases with the increase of polyaniline content. The value of the hardness is a measure of the solidity and firmness of the coating. So, the hardness of samples shows a good resistance of obtained coatings under a static load or to scratching and a good cohesion of the particles on the substrate. The semiconductive acrylic dispersions were obtained as a result of resistance value. The protective behavior against metal dissolution of films based on acrylic dispersion of doped PANI with anions containing phosphorus coated on carbon steel was evaluated. The characteristics of the coated steel samples are presented in Table 4. It is known that coatings, with PANI in ES state (doped form), protect substrate from corrosion by stabilizing the oxide layer formed on the metal surface and thus prevent the metal dissolution process [12,18,22]. Preliminary corrosion assay in all cases, after exposure time in 3.5% NaCl shows that the quantity of Fe(II) decrease (Fig. 7) for the coated steel samples comparatively with the corresponding control samples (C0 ). With the increase of PANI content in dispersion the concentration of released Fe(II) decreases. The catalytic function of PANI is represented by the possibility of formation of the passivating oxide layer between substrate and PANI coating. Also, with the increase of PANI content in dispersion is in agreement with the fact that PANI coatings became denser, and the hardness of pellicle is higher (Table 3). At same concentration of PANI in dispersion the concentration of released Fe(II) depends on anion nature. Experimental data shows that the protective behavior decrease in order ASP > APP > AClEP > AS. For all samples the thickness of films are almost the same, only the nature of dopant and percentage of PANI differ. Therefore, the chemical nature of anions influences the behavoiur as a barrier effect against the aggressive medium.

Also, the penetrating rate (Rp ) for all coated sample and control sample can be calculated using the following expression: Rp =

m Atρ

(2)

where the m is the weight loss for each sample, A the exposed area in cm2 , t the time of immersion in years, and ρ is the density of metallic substrate in g cm−3 . The uncoated sample (C0 ) presents the higher penetrating rate. With the increases of PANI content in coatings the penetrating rate decreases. The lower values for the Rp were observed for the PANI doped with ASP (Fig. 8). Coatings, especially the doped ones with ASP, shift the open circuit potential of the substrate to more positive values, establishing a passivation-like condition. Catalytic function of PANI is represented by the possibility of formation of the passivating oxide layer between substrate and polyaniline coating. Coatings, with PANI in ES state (doped form), protect substrate from corrosion by stabilizing the oxide layer formed on the metal surface

Fig. 7. Concentration of Fe(II) after 1 month for steel and steel coated samples.

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Fig. 8. Penetration rate (mm/year) for coated and uncoated steel samples in 3.5% NaCl.

and thus prevent the metal dissolution process. In all cases, after exposure time in 3.5% NaCl protonated PANI samples exhibited corrosion potentials more noble than the corresponding control samples. The smallest shift in corrosion potential with exposure time was observed for the samples coated with acrylic resin and PANI doped with ASP. The measurement of the open circuit potential (OCP) in 3.5% NaCl solution is presented in Fig. 9. The corrosion current densities and potentials of the investigated coated steel samples were calculated from the polarization curves (Fig. 10). The polarization curves recorded in 3.5% NaCl solution show a positive potential shift and a reduction in the corrosion when the carbon steel is coated with PANI-APP and PANI-ASP. The data were summarized in Table 5. The carbon steel became nobler when coated with PANI-APP and PANI-ASP, as indicated by the measurement of Ecorr of different samples. As a result, doped PANI passivates the surface of carbon steel and

Fig. 10. Polarization curves in 3.5% NaCl: 1, carbon steel; 2, carbon steel coated with 10.8% PANI-AS; 3, carbon steel coated with 10.8% PANI-AClEP; 4, carbon steel coated with 10.8% PANI-APP; 5, carbon steel with 10.8% PANI-ASP.

the effectiveness of PANI as a passivating corrosion inhibitor is of electrochemical nature, i.e. there is charge transfer between PANI and the steel. For all samples the thickness of films are almost the same, only the nature of dopant and percentage of PANI differ. The potential measurements show a positive potential shift when the carbon steel is coated with doped PANI with phosphorus anions. These anions contain phosphorus–carbon (P C) bonds, contrary to the phosphorus–oxygen (P O) bonds in the common inorganic phosphates inhibitors. The P C bonds are much more resistant to conversion into orthophosphates than P O bonds in inorganic phosphates. It is known that inorganic phosphates present some disadvantages like: sensitivity to hydrolysis, temperature and pH fluctuations, as well as high residence time in industrial water [35]. Organic phosphonates can act as corrosion inhibitors leading to better stabilizing effects and lower hydrolysis at high pH and temperature ranges. Therefore, we think that the nature of dopant anions used can improve the corrosion protection presented by PANI. It means the steel is nobler Table 5 The corrosion potentials and corrosion current for uncoated carbon steel and coated with acrylic dispersion based on doped PANI

Fig. 9. Open circuit potential–time dependence for the prepared electrodes.

Sample

Ecorr (mV vs. SCE)

icorr (␮A cm−2 )

Carbon steel Coating with 5.6 wt.% PANI-APP Coating with 10.8 wt.% PANI-APP Coating with 20 wt.% PANI-APP Coating with 5.6 wt.% PANI-ASP Coating with 10.8 wt.% PANI-ASP Coating with 20 wt.% PANI-ASP Coating with 5.6 wt.% PANI-AClEP Coating with 10.8 wt.% PANI-AClEP Coating with 20 wt.% PANI-AClEP Coating with 5.6 wt.% PANI-AS Coating with 10.8 wt.% PANI-AS Coating with 20 wt.% PANI-AS

−632 −523 −502 −497 −510 −492 −481 −538 −520 −512 −590 −562 −551

9.7 6.22 5.89 4.24 3.62 2.76 1.85 8.32 7.86 7.13 9.42 8.27 7.90

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Fig. 11. The corrosion test effectuated in synthetic sea water: P5, acrylic dispersion 20% PANI-AClEP (left, initial; right, after 144 h); P6, acrylic dispersion 5.6% PANI-AS (left, initial; right, after 144 h); P7, acrylic dispersion 5.6% PANI-ASP (left, initial; right, after 144 h).

when coated with coating with PANI doped with phosphorus anions, as indicate by the measurement of Ecorr of different samples. The values of Ecorr and icorr for carbon steel are −632 mV and 9.7 ␮A, respectively. With the increase of PANI content in dispersion the Ecorr and icorr decreases. At same concentration of PANI in dispersion the Ecorr and icorr depends on anion nature. When coated with acrylic dispersion based on PANI-AS, the values of Ecorr and icorr are changed to −590 mV and 9.42 ␮A, respectively. The value of Ecorr is increased to −492 mV and icorr is reduced to 2.76 ␮A for coating with 10.8 wt.% PANIASP. When the doped PANI concentration in pellicle is higher, the corrosion current is lower. This is in agreement with the fact that PANI coatings became denser, and the hardness of pellicle is higher. The protective behavior of acrylic coatings based on doped PANI was also studied by immersion tests, using coated plates with the dimensions of 140 mm × 40 mm × 1 mm (coating thickness 0.05 mm). The salt water immersion test was used to assess the adhesion of prepared coatings. Immersion tests were performed in synthetic sea water [36] prepared in our laboratory. Visual observations made on the samples after 144 h (6 days) at room temperature. The carbon steel samples coated with acrylic dispersion based on PANI doped with different anions containing phosphorus show different behavior. No sign of corrosion was observed in the case of coating with PANI-ASP and PANI-APP even at low concentration of PANI in dispersion. These coatings presented the highest value of hardness. Acrylic dispersions with lower content of PANI-AClEP (5.6 and 10.8%) provide a lower protection. A mild protection was observed for acrylic dispersion with higher content of PANIAClEP (case of 20% PANI-AClEP). Severe corrosion was observed for coating with acrylic dispersions with PANI-AS even at higher PANI content (Fig. 11). The results of the preliminary laboratory study regarding the protective behavior indicate that PANI doped with anions

containing organic phosphorus acids performed corrosion protection, especially from ASP. 4. Conclusions The experimental data showed that PANI could be synthesized in different organic acids containing anions with phosphorus with a yield of 57–60%. The polymerization process in APP requires an induction time 8–10 times longer than in ASP, AClEP and AS. The UV–vis and titration with TiCl3 data showed that the oxidation state of the obtained polymer was about 0.513. The presence of organic anions containing phosphor reduces the thermal stability of PANI. Coatings with doped PANI form protect substrate from corrosion by stabilizing the oxide layer formed on the metal surface and thus prevent the metal dissolution process. Among the PANI, the presences of organic phosphorus anions in the coatings add a supplementary corrosion inhibition leading to a better stability. PANI doped with different anions containing phosphorus presents a better protection compared with PANI doped with inorganic acid (AS) so, they can be considered like an effective organic corrosion inhibitor against neutral and basic environments. An optimum formulation of acrylic coatings based on PANI with 5.6 wt.% PANI-ASP exhibits good behavior in the immersion test. References [1] H.S. Nalwa, Handbook of Conductive Molecules and Polymers, vol. 2, John Wiley & Sons Ltd., London, 1997. [2] A.G. MacDiarmid, J.C. Chiang, A.F. Richter, Synth. Met. 18 (1987) 285. [3] D.W. Hatchett, M. Josowicz, J. Janata, J. Phys. Chem. B. 103 (1999) 10992. [4] P.M. Beadle, Y.F. Nicolau, E. Banke, P. Ronnon, D. Djurado, Synth. Met. 95 (1998) 29. [5] W. Gazotti, M.-A. de Paoli, Synth. Met. 80 (1996) 263. [6] E. Erdem, M. Karakisla, M. Sacak, Eur. Polym. J. 40 (2004) 785.

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