Water-Soluble Polyaniline−Polyacrylic Acid Composites as Efficient

Article pubs.acs.org/IECR

Water-Soluble Polyaniline−Polyacrylic Acid Composites as Efficient Corrosion Inhibitors for 316SS Junaid Ali Syed, Shaochun Tang, Hongbin Lu, and Xiangkang Meng* National Laboratory of Solid State Microstructures, College of Engineering and Applied Sciences and Institute of Materials Engineering, Nanjing University, Nanjing, Jiangsu PR China S Supporting Information *

ABSTRACT: Water-soluble polyaniline−poly(acrylic acid) (PANI−PAA) composites with excellent processability and electroactivity were prepared by a one-step in situ polymerization. PAA as a matrix not only improves the solubility of PANI in water but also prevents the formation of macroscopic PANI clusters. The corrosion-inhibition performance of 316 stainless steel (316SS) was evaluated in 0.5 M HCl by electrochemical measurements in the presence of PANI−PAA composites. The results show that PANI−PAA acts as a mixed-type inhibitor, and its inhibition efficiency (IE(R)) increases with inhibitor concentration. The adsorption of the inhibitor on 316SS surface obeys a Langmuir adsorption isotherm. The PANI−PAA composite with an optimized concentration of 200 ppm shows marked increase in IE(R), i.e., 91.68%. The enhanced efficiency is attributed to an insulating interfacial layer formed by the adsorption of PANI−PAA, which obstructs the corrosion reaction at the interface.

1. INTRODUCTION In recent years, inhibitors have attracted much attention because they are believed to be effective and economic agents to protect metals from corrosion.1 As far as corrosion inhibitors are concerned, the most important aspect is to reduce the degradation rate of metals in the processes such as pickling, cleaning, and descaling.2−5 Besides their efficient inhibition performance in acidic medium, it is desired that they be environmentally friendly as well.6−8 Among organic inhibitors, conducting polymer attracts much attention because of its ability to protect metals in aggressive environment.9,10 As a conducting polymer, polyaniline (PANI) is considered an appropriate candidate because of its facile synthesis, environmental stability, redox reversible activity, and controllable electrical conductivity by chemical doping.11,12 PANI is the most promising corrosion inhibitor at very low concentrations because its molecular structure exhibits the extensive delocalization of π electrons.13,14 The organic inhibitors containing the heteroatoms such as N, P, and S,15−17 unsaturated bonds and conjugated systems with aromatic cycles18,19 ensure their adsorption on the metal’s surface. These inhibitors can therefore interact with the metal to form a protective film that hinders the physical contact of aggressive species with the metal surface in the acidic medium.20 Substituted and unsubstituted PANI has been reported as a corrosion inhibitor for metal surfaces in acid chloride solution.21,22 Theoretically, the band gap is affected by the torsion angle between the adjacent repeating units of PANI chains.23 Consequently, the alkyl substituents on PANI that influence the torsion angle by steric effects provide better solubility in organic solvents such as alcohols.24 However, the extremely poor solubility of PANI in aqueous solution limited its application as a corrosion inhibitor in an acidic medium for alloy steel,25 especially in the case of stainless steels including 316SS.26 To © 2015 American Chemical Society

achieve the processability of PANI, several methods such as doping it with functionalized organic sulfonic acids,27,28 incorporation of bulky alkyl chain,29−31 and blending with other processable polymers were employed to obtain its soluble composites.32,33 Soluble doped PANI, such as poly(aniline-co-4amino-3-hydroxy naphthalene-1-sulfonic acid)34 and sulfonated chitosan doped PANI,35 were considered as organic inhibitors to reduce the corrosion rate of steel in acidic medium. The aim of the present work is to develop a series of soluble PANI−poly(acrylic acid) (PAA) composites that are electroactive and act as an efficient corrosion inhibitor for 316SS in strong acidic medium. The properties of the PANI−PAA composite were optimized, and their protection mechanism as active inhibitors was discussed.

2. EXPERIMENTAL SECTION 2.1. Materials and Methods. PAA (powder, average Mr ≈ 450 000) was purchased from Sigma-Aldrich. Aniline (ANI), ammonium persulfate (APS), hydrochloric acid (HCl), and other chemicals (Shanghai Lingfeng Chemicals, China) were of analytical grade and were used as received. The dialysis of PANI−PAA solution was carried out by using dialysis membranes with the molecular weight cutoff of ∼3500 Da. The 316SS plates (15 × 10 × 0.1 cm3) were provided by Senda Decoration Materials Co. Ltd. (Haimen, Nantong, China); their composition is given in Table S1 (Supporting Information). 2.2. Substrate Surface Pretreatment. 316SS plates were cut into (2.5 × 2.5 × 0.1 cm3) coupons. These coupons were ground using 800, 1000, and 1200 SiC papers and polished with aqueous slurry of 3 μm alumina, degreased in hexane and Received: Revised: Accepted: Published: 2950

November 25, 2014 March 3, 2015 March 3, 2015 March 3, 2015 DOI: 10.1021/ie5046395 Ind. Eng. Chem. Res. 2015, 54, 2950−2959

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Figure 1. Formation mechanism of PANI−PAA composites, showing a proposed networklike structure in agreement with their TEM image.

methanol flow, and rinsed with purified water. The coupons were pretreated by a mixture of 98% H2SO4 and 30% H2O2 (7:3, v/v) piranha solution for 1 h and then washed with an exccess of water. Finally, they were sonicated in deionized water for 15 min and dried at room temperature. 2.3. Synthesis of PANI−PAA Composites. PANI−PAA composites were prepared as follows: a series of PAA concentrations (0.45, 0.90, 1.35, 1.80, and 2.25% w/v) were added to 100 mL of 1 M HCl solution and stirred for 30 min until the solution was homogenized. Then, 3.25g (0.014 M) of APS was added. Finally, 1 mL of (0.011 M) ANI was added dropwise to this solution under constant stirring at 0−5 °C. The APS to ANI ratio in the reaction mixture was 1.25:1.0 mM. The reaction mixture was stirred for 6 h, and the resulting green precipitate was filtered (in the cases of pure PANI and 0.45% PANI−PAA composite) and washed with 50−70 mL of 0.1 M HCl and acetone to remove impurities, followed by drying under vacuum at 70 °C. For water-soluble PANI−PAA composites (0.90− 2.25%), the solution was purified by dialyzing it against a 0.1 M HCl aqueous solution to remove the PANI oligomer and impurities. The resulting composites were dried under vacuum at 70 °C. The composites were named according to PAA percentage (w/v) in the mixture, whereas the concentration of ANI monomer was kept fixed. For synthesis of pure PANI, the procedure was the same as used for the composite except for the addition of PAA. As shown in Figure 1, during the initial stage of PANI−PAA synthesis, the small PANI chains attached to PAA matrix through electrostatic interaction. The small PANI chains linked to PAA undergo further polymerization, resulting in the most stable PANI−PAA segments within the matrix. As the reaction goes on, these stable PANI−PAA segments form a globular morphology. Finally, a uniformly distributed PANI−PAA sphere in PAA matrix with a networklike structure was obtained (as shown in the TEM image). 2.4. Instrumentation. The percentage of PANI in the composites was estimated by monitoring the UV−vis band at 390−410 nm by using a Shimadzu UV-3600 spectrophotometer. Fourier transform infrared (FTIR) spectra of pressed pellets of KBr powder were recorded on a Spectrum-GX PerkinElmer spectrometer. Thermogravimetric analysis (TGA) studies were carried out on a NETZSCH STA-409-PC instrument. The morphology of the composites was investigated by using a Hitachi S-4800 to obtain scanning electron microscopy (SEM) images. A JEOL JEM-2100 electron microscope was used to

obtain transmission electron microscopy (TEM) images. The conductivity of the pressed pellets (under 3 ton pressure), having 1 cm diameter and 0.12 ± 0.03 cm thickness, was measured by a four-probe assembly connected with a KEITHLEY nanovoltmeter (model 2400). Cyclic voltammetry (CV) was carried out on Autolab Potentiostat/Galvanostat (Model PGSTAT30) system using a conventional three electrode cell, a platinum foil (2 × 2 cm) working electrode, a platinum counter electrode, and S.C.E as a reference electrode immersed in 1 M HCl solution. A computer-controlled CH1660D electrochemical workstation was used for potentiodynamic polarization and EIS measurements. The measurements were conducted in a usual threeelectrode cell consisting of a Ag/AgCl reference electrode, a platinum counter electrode, and a polished coupon of 316SS as the working electrode. At room temperature, 250 mL of 0.5 M HCl was used as electrolyte solution in the electrochemical experiments, and three samples were used for each test. The exposed area of the working electrode was 4.68 cm2. In the potentiodynamic polarization experiments, the potential sweep started at −0.37 V versus open circuit potential (OCP) with a sweep rate of 10 mV/s. The EIS measurements were carried out at the OCP with a perturbation amplitude of 10 mV, with 10 points per decade, and over the frequency range of 105−10−2 Hz. The software Nova (version 1.1) was used to evaluate the parameters obtained by fitting the impedance data according to a suggested equivalent electrical circuit. A Carl Zeiss Axio Imager M2m Microscope System was used to obtain optical micrographs.

3. RESULTS AND DISCUSSION 3.1. UV−Vis and FTIR Spectroscopy. UV−vis spectroscopy was performed to estimate the percentage of PANI in the composites by examining the UV−vis band at 390−410 nm.36 The weighed samples were dissolved in 1 M HCl aqueous solution, and their optical absorption was determined. The concentration of PANI in the dispersion form was calculated as C(g/cm 3) =

Ad εl

(1)

where A is the absorbance at the absorption maximum close to 400 nm, d is the dilution before the measurement, ε is the absorption coefficient of PANI, i.e., 31 500 ± 1700 cm2/g (emeraldine semihydrochloride form in 1 M HCl solution at the wavelength 400 nm),37 and l (= 1 cm) is the thickness of the cell. Table S2 (Supporting Information) shows the list of composites 2951

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The composites have roughly the same UV−vis absorption bands as those presented by PANI. PANI exhibits two bands at 390−400 and 800−900 nm attributed to π to π* transition and poloron band, respectively,39 as shown in Figure S1 (Supporting Information). The spectra of PANI−PAA composites suggest the well-known compact-coil and looplike confirmations.40 FTIR spectra (Figure S2, Supporting Information) shows the bands at 3556 and 1710 cm−1, corresponding to O−H stretching of carbonyl and the (−COO−) group of PAA, respectively. The characteristic absorption peaks of PAA at 1710 and 1455 cm−1 are related to the CO stretching mode in the carboxylic group and bending mode of −CH2−. The peak at 1409 cm−1 is attributed to symmetric stretching of (−COO−) in PAA. The band around 3445 and 2815 cm−1 in PANI and its composites (Figure S2a,b,c, Supporting Information) is attributed to −NH and C−H stretching.41,42 Bands at 1360 and 1615 cm−1 are assigned to (N−B−N) and (N−Q−N) because of nitrogen linkage (where B = benzenoid and Q = quinoid). The peak at 1161 cm−1 is due to the plane-bending vibration of C−H.43 The band at 855 cm−1 is a characteristic of the para-substituted aromatic rings in PANI. During synthesis, acid−base interaction took place between the (−NH3+) of PANI and (−COO−) of PAA matrix, which results in the formation a complex. The diminished band related to the O−H stretching of carbonyl and (−COO−) group of PAA in the composites confirmed the formation of a complex of PANI with PAA matrix. The FTIR results confirm the formation of PANI−PAA composites 3.2. Thermogravimetric Analysis. TGA of PAA (Figure S3, Supporting Information) showed that the weight loss of 5% was observed at about 95−250 °C. At 250−300 °C, the weight loss is about 31%. Furthermore, 21% weight loss at 450 °C was also observed, which indicates that PAA can be considered as thermally stable up to 250 °C. The weight loss in this case was occurring mainly because of the decarboxylation of PAA, whereas polymer chain fission started at 450 °C. PANI (Figure S3b, Supporting Information) shows the loss of 15% because of moisture up to 150 °C. Between 150 and 200 °C, the total loss of 20% accounts for HCl and the degradation of polymer occurring

prepared and their total yield, along with the percentages of PANI and PAA in the solution mixture. The weight percentage of PANI increases with the increasing concentration of PAA in the composite (Figure 2). PANI

Figure 2. Plot showing the percentage of PANI incorporated in different concentration of PAA matrix in the reaction mixture.

percentage increases from 0.169 to 1.609% with the increase in PAA concentration, i.e., from 0.45 to 2.25%. However, at a very high PAA concentration (>2.25%), the percentage of PANI decreases, which should be due to the high viscosity of the PAA solutions that slows the polymerization rate of aniline monomer.38 The PAA matrix facilitates the homogeneous distribution of PANI−PAA globules in the composite (Figure 1). To examine the film-formation property, the reaction mixture was taken in a flat dish and heated to 70 °C in a vacuum oven. The 0.45% PANI−PAA composite cannot be cast into a film, whereas the composites with PAA feed of 0.90% and above provide free-standing films.

Figure 3. SEM images of (a) PANI, (b) 1.35% and (c) 2.25% PANI−PAA composites. TEM images of (d) 1.35% and (e) 2.25% PANI−PAA composites. 2952

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Industrial & Engineering Chemistry Research after this temperature that continued up to 650 °C (final weight loss of 93%). Similar to that of PANI, the thermal behavior of the composites (Figure S3c,d, Supporting Information) followed the same three stage decomposition. The first weight loss took place around 150−200 °C because of the presence of moisture in the composites. The second weight loss occurred because of HCl between 250 and 400 °C. These two weight losses were not so prominent in the composite, as shown in the derivative weight curve (DTG). The strongest peak in the DTG curve shows prominent weight loss at 550 and 600 °C for 1.35 and 2.25% PANI−PAA composites, respectively, which could be due to the complete degradation and decomposition of the polymer backbone. The difference of temperature in the degradation peak suggests that 2.25% PANI−PAA is more stable than the 1.35% composite. The thermal stability improvement of PANI− PAA composites is similar to that of other PANI composites.44,45 3.3. SEM and TEM Observations. PANI exhibits granular and macroporous morphology, as shown in Figure 3a. As the concentration of PAA increases in the composites, more PANI is homogeneously dispersed in the matrix (as determined by spectrophotometry) and less PANI−PAA globular morphology is observed in the 2.25% PANI−PAA composite (Figure 3c) as compared to the 1.35% composite (Figure 3b). It is evident from SEM images that the PANI−PAA composites are in single phase; this indicates that the interaction between the two components was adequate. These images show morphology that is different from that of previous reports where two distinct phases were found in the composites of PANI with insulating polymers.46 TEM images in Figure 3d,e show that both the composites, i.e., 1.35% and 2.25%, have the same networklike structure containing homogeneously distributed PANI−PAA globules of 100−180 nm in the PAA matrix. A contrast was observed between the center, i.e., the mixture of PANI and PAA, with a PAA-rich outer region of the globule. This is in good agreement with other PANI composites.47,48 These results indicate that the homogeneous PANI−PAA composite with a networklike structure was successfully obtained. 3.4. Conductive and Electroactive Behavior of the Composites. The conductivity data of the pressed pellets and casting films were measured by the four-probe method49 and compiled in Table S3 (Supporting Information). The conductivity decreases with increasing concentration of PAA in the composites (except the 1.80% composite). Pure PANI has a high conductivity of 3.7 S/cm, followed by 1.2, 0.94, and 0.061 S/cm for 0.45, 0.90, and 1.35% PANI−PAA composites, respectively. The higher conductivity exhibited by the 1.80% PANI−PAA composite may be attributed to inorganic impurities and oligomers present in the mixture. The conductivity of the cast film, having a thickness of 0.02 ± 0.006 cm, shows a lower conductivity as compared to their pressed pellet, in the range of 0.035−0.092 S/cm. However, the increase in conductivity of the 2.25% composite film is due to PANI that may be separated as an upper layer in the composite upon drying. The electroactive property of the composites was studied by casting them on Pt electrode to perform CV in 1 M HCl with a scan rate of 50 mV/s. PANI usually experiences two oxidation and reduction processes.50 The intensity of oxidation−reduction peaks decreases with the increasing PAA concentration, i.e., from 1.35 to 2.25% PANI−PAA composite, as shown in Figure 4a,b. The peaks A and A′ are attributed to the oxidation−reduction of leucemeraldine to emeraldine, whereas the B and B′ peaks represent the oxidation of emeraldine to pernigraniline. The breakdown products of PANI as a result of exceeded potential

Figure 4. CVs of (a) 1.35% and (b) 2.25% PANI−PAA composite cast films on a Pt electrode.

were represented by the peaks C and C′. It has been reported that PANI could act as an advanced corrosion protection material because of its redox catalytic properties that induce a passive layer on the SS surface.51 Therefore, we expect that the PANI−PAA composite exhibits a profound inhibition efficiency similar to that of PANI in the acidic medium.52 3.5. Corrosion Inhibition Tests. 3.5.1. Potentiodynamic Polarization Measurements. The potentiodynamic polarization measurements render information about the corrosion kinetics.53 Figure 5a,b shows the polarization curves of 316SS in the presence of 1.35 and 2.25% PANI−PAA composites, respectively. The corrosion potential (Ecorr) is the intersection of the anodic and cathodic polarization curves that gives the corrosion current density (Icorr) values.54 The corrosion kinetic parameters such as Ecorr, Icorr, inhibition efficiency (IE(I)), and cathodic (βc) and anodic (βa) Tafel slope are summarized in Table 1. It has been observed that Icorr decreases with the increase of inhibitor concentration. This implies that the inhibitor adsorbs on the steel surface and reduces the anodic dissolution by decelerating the evolution of the cathodic hydrogen.55 However, at 300 ppm, further addition of the composites would result in an increased Icorr value that influences the other potentiodynamic parameters as well. This is due to the macroscopic cluster formation of PANI at higher PANI−PAA concentrations.56 It has been reported that the deviation of Ecorr values within the range of 85 mV indicates that the inhibitor acts as a mixed type or else the inhibitor acts as a cathodic or anodic type.57 The displacement in Ecorr values (Table 1) is less than 85 mV and shifts toward the less negative side with an increase of inhibitor concentration. This implies that the inhibitor is a mixed type with a dominant anodic behavior. In addition, Table 1 illustrates that the values of βc and βa changes with the addition of an inhibitor. This suggests that the PANI−PAA composite inhibits the anodic and cathodic reactions. The higher values of βa as compared to βc illustrates that the application of the external current strongly polarizes the anode.58 The adsorption of inhibitor on the steel surface influence the βa values, as a consequence the anodic dissolution was decelerated.1 The inhibition efficiency IE(I) was calculated by the following equation.59 IE(I ) = 2953

0 Icorr − Icorr 0 Icorr

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Figure 5. Potentiodynamic polarization curves of 316SS in the absence and presence of (a) 1.35% and (b) 2.25% PANI−PAA composites.

Table 1. Potentiodynamic Polarization Parameters for 316SS in 0.5 M HCl Solution in the Absence and Presence of 1.35% and 2.25% PANI−PAA Composite with Different Concentrations inhibitor

1.35% PANI−PAA

2.25% PANI−PAA

inhibitor conc. (ppm)

Ecorr (mV)

Icorr (μA)

βa (mV dec−1)

βc (mV dec−1)

IE(R) (%)

blank 20 50 80 100 200 300 20 50 80 100 200 300

−374 −308 −296 −287 −287 −280 −298 −311 −301 −280 −277 −260 −296

241.5 51.18 40.77 36.18 27.06 21.65 55.19 49.66 44.26 18.62 17.27 14.76 58.91

188.44 151.91 174.58 158.34 199.28 188.91 172.32 158.91 183.88 186.43 178.56 188.91 152.23

85.1 77.55 74.59 88.01 84.57 94.97 72.24 66.54 69.08 70.94 68.10 69.18 64.42

78.81 83.12 85.02 88.80 91.04 77.15 79.44 81.67 92.28 92.85 93.88 75.60

Figure 6. (a) Nyquist plots for 316SS in the absence and presence of 2.25% PANI−PAA composite; inset shows the magnified Nyquist plot of 316SS electrode in the absence of inhibitor. (b) Bode plot of 316SS in the absence and presence of 2.25% PANI−PAA inhibitor at different concentrations.

where I0corr and Icorr are the corrosion current density values in the absence and presence of inhibitor, respectively. IE(I) increases with the increase of PANI−PAA inhibitor concentration and follows the order 2.25 > 1.35% PANI−PAA composite. The highest inhibition efficiency is 93.88% for 2.25% PANI−PAA composite. One of the reasons for this significant shift in IE(I) would be that these composites behave as a mixed-type inhibitor

and adsorb on the steel surface, hence blocking the reaction sites.60 3.5.2. Electrochemical Impedance Spectroscopy Measurements. Electrochemical impedance spectroscopy is a powerful tool because it not only provides information about the electrode processes but mechanistic information can also be derived from the impedance profile.61 Figure 6a shows the representative 2954

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transfer resistance (Rct) is employed in the equivalent circuit. The microscopic roughness on the electrode surface affects the CPE behavior and depressed semicircles are observed. This phenomenon would cause a disturbance in solution resistance and in the double-layer capacitance as well.67 The complex impedance of CPE is defined as68

Nyquist plots of 316SS in the absence and presence of PANI− PAA composite in 0.5 M HCl. The semicircles in the Nyquist plot indicate the charge-transfer resistance, double-layer capacitance of the electrode.13,14 Figure 6a shows the depressed semicircles having a center below the real axis, which indicates that the impedance loops exhibit a dispersing effect.62 This dispersing effect is due to the presence of microroughness and irregularities at the electrode/solution interface.63 It is evident from Figure 6a that SS exhibits an impedance behavior that increases with the increase of PANI−PAA concentration in 0.5 M HCl medium. Importantly, the increase of inhibitor concentration does not influence the shape of the impedance profile; however, the PANI−PAA inhibitors exhibit a similar inhibition behavior. The single semicircle in the Nyquist diagram indicates the occurrence of a single charge-transfer reaction in the region of high frequency that is unaffected by the inhibitors. The Nyquist diagrams of 316SS in the presence of PANI−PAA composite indicate that the diameter of the semicircle was larger than the one in the absence of the composite. This marked increase in diameter implies the increase in corrosion resistance of SS because of the adsorption of inhibitor on its surface.60 These results show that in the presence of PANI−PAA composite the 316SS surface exhibits enhanced resistance to corrosion in aggressive environment, as described previously.52 Figure 6b shows the Bode plots for 316SS in 0.5 M HCl in the absence and presence of PANI−PAA inhibitor with different concentrations. For ideal capacitance, the phase angle α and slope S are −90° and −1, respectively.17 However, we observed that the linear region in the Bode impedance modulus plot has a slope and phase angle of −0.81 and −75°, respectively. These values suggest that an adsorbed inhibitor protective film was formed on the surface of SS.64 In a higher frequency range with increasing PANI−PAA concentration, a change in phase angle was observed that is due to the formation of an inhibitor film. This continuous change is related to the surface coverage of SS by PANI−PAA inhibitor.65 The metal degradation mechanism could be studied in detail by using circuit modeling, which provides information about the interface reaction in the corrosive environment.66 The inset in Figure 7 shows the equivalent circuit used to fit the EIS data. To obtain better impedance simulation, the capacitance (Cdl) is replaced with a constant phase element (CPE), and the charge-

ZCPE =

1 Q (Jω)n

(3)

where Q is the CPE, J is the imaginary unit (square = −1), and ω is the angular frequency (ω = 2πf) of the ac voltage applied to the electrolyte cell where n = 1 for an ideal capacitance and n = 0−1 in the case of deviation.69 The experimental data is fitted according to the proposed equivalent circuit, and the obtained equivalent circuit parameters are illustrated in Table 2. The n values are in the range of 0.85−0.98 and close to unity, suggesting that the CPE is related to the capacitance (Cdl).70 Figure 7 shows the Nyquist plot (experimental data, solid circles, and fitting results, solid lines) for 316SS in the presence of inhibitor, which implies that the proposed equivalent circuit satisfactorily fits the impedance data. For both the PANI−PAA inhibitors, it is evident from the data in Table 2 that the Rct values increase whereas the Cdl decreases with the increase of inhibitor concentration. The higher Rct values suggest the formation of an adsorbed inhibitor layer that blocks the mass- and charge-transfer reaction on the surface of SS.71 In contrast, the decreasing Cdl values with the increasing inhibitor concentration indicates the replacement of water molecules at the electrode interface by the adsorption of the organic inhibitor having lower dielectric constant. This implies that the PANI−PAA inhibitor acts as an efficient inhibitor that adsorbs at the metal surface and decreases the extent of the metal dissolution.72 The inhibition efficiency (IE(R)) can be calculated on the basis of Rct values by the equation represented as73 IE(R) =

R ct − R ct0 × 100% R ct

(4)

where R0ct and Rct are the charge-transfer resistance in the absence and presence of inhibitors, respectively. The impedance parameters obtained by fitting the experimental data are summarized in Table 2. With the increase in inhibitor concentration, there is an increase in Rct values with an improved inhibition efficiency of 91.68%. This value is much higher than that of PANI-based copolymer reported as an inhibitor.29 The comparison of IE(R) values obtained by using Rct values of different inhibitors is shown in Figure 8. The studies revealed that the 2.25% PANI−PAA composite shows better corrosion inhibition properties; under same experimental conditions, the 1.35% and 3.15% PANI−PAA composites show inferior corrosion-inhibition efficiency. The data in Table 2 suggest that 200 ppm is the optimum concentration of the composite that decreases the degradation rate of 316SS in 0.5 M HCl. At 300 ppm, the further addition of composite would result a decrease in inhibition efficiency because of the formation of macroscopic PANI clusters.56 In the case of 3.15% PANI−PAA composite, the high content of PAA makes the reaction mixture viscous and decreases the polymerization rate of aniline monomer. This would lower the yield of PANI in the composite; hence, it decreases the corrosioninhibition properties of the composite. Moreover, the decrease in Cdl values may be due to a continuous increase in the thickness of electrical double layer that decreases the local dielectric constant.

Figure 7. Nyquist plots for 316SS in the absence and presence of 2.25% PANI−PAA composite; experimental data, solid dots, and fit results, orange line. Inset shows the proposed equivalent circuit. 2955

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Table 2. Electrochemical Parameters Obtained by Fitting EIS Data for 316SS in 0.5 M HCl Solution in the Absence and Presence of 1.35% and 2.25% PANI−PAA Composite with Different Concentrations inhibitor

1.35% PANI−PAA

2.25% PANI−PAA

inhibitor conc. (ppm)

Rct (Ω cm2)

Cdl (10−4 Fcm−2)

n

IE(R) (%)

blank 20 50 80 100 200 300 20 50 80 100 200 300

304 516 620 722 1199 2743 719 926 1194 2210 3076 3655 945

3.60 1.42 1.21 1.11 0.99 0.67 0.94 0.82 0.74 0.70 0.54 0.33 1.33

0.87 0.85 0.88 0.86 0.87 0.90 0.89 0.97 0.96 0.98 0.86 0.91 0.95

67.17 74.54 86.24 89.79 91.68 67.83 67.17 74.54 86.24 90.12 91.68 67.83

where C0dl and Cdl are the double-layer capacitance values in the absence and presence of inhibitors, respectively. The surface coverage as a function of inhibitor concentration was plotted and tested by fitting to the Langmuir adsorption isotherm using the equation C 1 = +C θ K ads

(7)

where C is the concentration of inhibitor and K is the equilibrium constant. The surface coverage (θ) was examined graphically to fit an appropriate adsorption isotherm as presented in Figure 9.

Figure 8. Comparison of inhibition efficiency in the presence of different PANI−PAA inhibitors.

Hence, at 200 ppm concentration, a profound increase in inhibition efficiency was observed that indicates that PANI−PAA acts as an efficient corrosion inhibitor for 316SS in 0.5 M HCl. 3.5.3. Adsorption Isotherm and Inhibition Mechanism. The effectiveness and interaction of the inhibitors with the SS surface is mainly dependent on their adsorption ability.74 However, the factors that influence the efficiency and adsorption of these organic inhibitors are the type of the electrolyte and the charge of metal surface and its nature.16 The adsorption of the organic molecule on the metal surface is a quasi-substitution reaction between the inhibitor solution, Organic(sol), and the water, H2O(ads), in contact with the solution/SS interface, which can be expressed as75 Organic(sol) + x H 2O(ads) ⇌ Organic(ads) + x H 2O(sol)

Figure 9. Langmuir adsorption isotherm plots for different PANI−PAA composites in 0.5 M HCl.

The plot of θ versus C yields a straight line with the linear regression coefficient (R2) equal to 0.972 and 0.978 for 2.25% and 1.35% PANI−PAA inhibitors, respectively, and a slope close to 1. Hence, the adsorption of PANI−PAA inhibitor is well-fitted and follows the Langmuir adsorption isotherm. The θ values (Figure 9) show that better surface coverage is obtained in the presence of 2.25% PANI−PAA as compared to 1.35% PANI− PAA. These results manifested that the inhibition property of PANI−PAA was attributed to the adsorption of these composites on 316SS surface. PANI exists as cation in acidic solution and adsorbs on the surface of steel as protonated ions, thus retards the corrosion reaction at the solution/metal interface.59 The higher inhibition

(5)

where Organic(ads) is the adsorbed inhibitor on the surface and x is the amount of water replaced by the inhibitor. The application of the adsorption isotherm is very important because it provides basic information about the interaction of the inhibitor with the steel surface. The surface coverage θ can be evaluated by EIS data using the equation76 θ=

Cdl0 − Cdl Cdl0

(6) 2956

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Figure 10. Optical images of 316SS surface immersed in (a) 0.5 M HCl and (b) in the presence of PANI−PAA composite for 3 h.

uniformly distributed globular microscopic PANI−PAA composites. The composites achieve solubility in water when the PAA concentration is above 0.45% in the reaction mixture. These composites are environmentally stable with excellent processability, even at higher PAA concentrations. The potentiodynamic polarization curves indicate that the inhibitors act as a mixed type that affects both the anodic and cathodic reactions by blocking the active metal sites and did not change the corrosion mechanism of 316SS in 0.5 M HCl medium. Impedance data specify that dissolution of 316SS was prevented by the adsorption of PANI−PAA on its surface and were found to follow the Langmuir adsorption isotherm. The inhibition efficiency of PANI−PAA composites increases with increasing inhibitor concentration, and their inhibition ability follows the order 3.15% < 1.35% < 2.25%. The marked increase in inhibition efficiency IE(R), i.e., 91.68%, was observed at the inhibitor concentration of 200 ppm. The PANI−PAA networklike structure not only avoids the sudden breakdown of the protective PANI−PAA layer, but also forms an insulating layer by the adsorption process on 316SS surface. The PANI−PAA composites are promising and efficient inhibitors in an acidic medium.

property of PANI was attributed to its adsorption mechanism on 316SS surface. There are two proposed possible mechanisms for the adsorption of inhibitor on SS. First, adsorption may be through conjugated π electrons,20 aromatic rings of PANI, and their interaction with 316SS surface. Second, the adsorption is through electrostatic interaction between the positively charged (NH3+) of PANI and the 316SS surface. On the basis of the assumptions of Rowerder et al., the macroscopic network of conducting polymers would result in the fast reduction of conducting polymer and fail to provide protection against corrosion.56,77,78 However, we have overcome this problem by using PAA as a matrix that contains globular microscopic PANI−PAA uniformly distributed in it (as shown by SEM and TEM images in Figure 3). This morphology would avoid the formation of conducting-polymer-based macroscopic clusters. In the presence of PAA matrix, PANI shows mild behavior and decreases quick reduction by fast cation mobility in the PANI network. Therefore, the corrosion protection ability of the composites was enhanced. Another problem is the interfacial corrosion caused by the formation of a galvanic element because of the direct contact of PANI clusters with the metal surface. This problem was resolved by the formation of an insulating oxide layer at the interface in the presence of PANI−PAA composite. When corrosion started at the interface of adsorbed PANI− PAA/SS, the redox reaction would occur between PANI and steel surface. This would result in the formation of an interfacial oxide layer at the interface.78 The degraded PANI, i.e., the completely reduced form, would also assist in the formation of an insulating interfacial layer. These are the possible mechanisms that would result in the improved inhibition efficiency and decelerates the corrosion reaction at the solution/SS interface in chloride containing environment. 3.5.4. Surface Morphology. The surface morphology of the system was examined by immersing 316SS coupons in 0.5 M HCl solution for 3 h in the absence and presence of inhibitors. The images show that in the absence of inhibitor the 316SS surface was strongly damaged by the chloride environment, resulting in pitting corrosion (Figure 10a). In dramatic contrast, the 316SS surface showed no visual evidence of corrosion (Figure 10b) even after 3 h exposure in the presence of PANI−PAA inhibitor. These observations revealed that there is a protective layer of adsorbed PANI−PAA inhibitor on the 316SS surface that was responsible for improved corrosion protection.

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ASSOCIATED CONTENT

S Supporting Information *

UV−vis (Figure S1) and FTIR (Figure S2) spectra of the pure PANI and different PANI−PAA composites. Thermogravimetric analysis of different PANI−PAA composites (Figure S3). Composition of 316SS plates (Table S1). The percentage of PANI and PAA with their total yield and absorption maxima (Table S2). Conductivity of PANI and different PANI−PAA composites (Table S3) This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: (+86) 25 8368 5585. Fax: (+86) 25 8359 5535. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors appreciate the assistance of Dr. Zhenhua Cao, Mr. Yongguang Wang, and Mr. Yan Lei in experimental work and for the use of the four-probe assembly connected with the KEITHLEY nanovoltmeter instrument. This work was jointly supported by the PAPD (no. 50831004), the Natural Science Foundation of Jiangsu Province (no. 2012729), the Innovation

4. CONCLUSIONS Soluble and electroactive PANI−PAA composites were synthesized by an in situ polymerization method. PAA as a matrix prevents the macroscopic cluster formation of PANI and leads to 2957

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Fund of Jiangsu Province (no. BY2013072-06), the National Natural Science Foundation of China (nos. 51171078, 11004098, and 11374136), and the State Key Program for Basic Research of China (no. 2010CB631004).

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