Polyurethane

Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Vanadium Pentoxide-Enwrapped Polydiphenylamine/Polyurethane Nanocomposite: High-Performance Anticorrosive Coating Halima Khatoon and Sharif Ahmad* Materials Research Laboratory, Department of Chemistry, Jamia Millia Islamia (A Central University), New Delhi 110025, India

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ABSTRACT: Nanocomposite coatings with synergistic properties hold a potential in long-term corrosion protection for carbon steel. Polydiphenylamine (PDPA) and vanadium pentoxide (V2O5) have rarely been used as a corrosion inhibitor. Moreover, oleo polyurethanes are always demanded in the field of anticorrosive coatings. In view of this, we have synthesized safflower oil polyurethane (SFPU) and their nanocomposites using V2O5-enwrapped PDPA (V2O5-PDPA) as nanofiller. Fourier-transform infrared spectroscopy, X-ray diffraction, nuclear magnetic resonance, scanning electron microscopy, transmission electron microscopy, and thermogravimetric analysis were used to characterize the structural, morphological, and thermal properties of these coatings. Corrosion resistance performance of these coatings in 5 wt % NaCl solution was determined by electrochemical measurements and salt spray tests. These studies exhibited very low Icorr (7.45 × 10−11 A cm−2), high Ecorr (−0.04 V), impedance (1.69 × 1011 Ω cm2), and phase angle (84°) after the exposure of 30 days. An immersion test, in 1 M H2SO4 solution for 24 h, was also performed to investigate the effect of oxidizing acid on the surface of coatings. These results revealed the superior anticorrosive activity of nanocomposite coatings compared to those of plain SFPU and other such reported systems. The superior anticorrosive property of the proposed nanocomposite coatings provides a new horizon in the development of highperformance anticorrosive coatings for various industries. KEYWORDS: safflower oil, polyurethane, polydiphenylamine, vanadium pentoxide, nanocomposite, anticorrosive coatings linoleic acid (71−75%).8 It has a wide scope for applications in the field of medicine, dyes, food and beverages, varnish, paints, and coatings.9 Its polyol contains a higher number of hydroxyl groups that help in the processing of highly cross-linked network of PU, resulting in the formulation of thermally resistance and good adhesive coatings. Although the PUs have good chemical, physical, thermal, and mechanical properties and are in the top list in the field of industrial coating, the research for the enhancement in their anticorrosive performance is still in progress.10 Attempts have been made to develop new generation advanced and modified PU coatings with superior mechanical and anticorrosive properties. Literature reveals that the conducting polymers (CPs) have achieved the path breaking success in the field of corrosion inhibition because of their low cost, easy synthesis, redox reversibility, and good chemical stability and electrical conductivity.11,12 The formation of their protective film ability leads to alter the cathodic or anodic reactions at the metal− coating interface.13 In general, polyaniline (PANI) and their derivatives have been widely used as corrosion inhibitors.

1. INTRODUCTION Corrosion is a natural phenomenon, causing catastrophic deterioration of materials used in thermal power plants, river bridges, automobiles, railways, and other industrial infrastructures that leads to the huge loss of economy and health.1−3 In order to control such losses and to enhance the life of various industrial establishments, the maintenance and protection of such systems is accomplished by synthetic polymer-based anticorrosive paints and coatings. However, depletion of petroleum resources and spiraling rise in their prices have led the researchers to look into some new alternative green, renewable precursors and their application in the processing of sustainable, eco-friendly polymeric coating materials.4 Renewable precursors such as cashew nut shell, latex, chitosan, chitin, lignin, vegetable oil, and so on have been used in the synthesis of sustainable polymers such as alkyds, polyesters, epoxies, polyacrylates, and polyurethanes (PUs).5,6 Compared to other oleopolymers, oleo PUs have fascinated the researchers on account of their good ability for chemical modifications, thermal and excellent corrosion resistance.7 Among various vegetable oils [castor, linseed, soy, palm, neem, pongamia, sunflower, safflower oils (SFOs), etc.] SFO has attracted the attention of paints and coating technologists because of its easy availability, low cost, and highest content of © XXXX American Chemical Society

Received: October 12, 2018 Accepted: December 17, 2018 Published: December 18, 2018 A

DOI: 10.1021/acsami.8b17861 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

monitored by FTIR. After the completion of the reaction, the final product that is the V2O5-PDPA nanoparticle was filtered and washed with distilled water and methanol. Finally, the nanoparticle was dried at 80 °C for 24 h. 2.2. Synthesis of SFO Polyol. SFO polyol (SFPO) was synthesized as per our earlier reported method (Scheme S1b).19 In brief, 40 mL of SFO was taken in a three-necked round bottom flask fitted with a magnetic stirrer and a thermometer. Glacial acetic acid (15 mL) and 2 drops of concentrated sulfuric acid were added into the reaction flask and stirred for 10 min. After which, 45 mL of 50% hydrogen peroxide was added dropwise to the reaction mixture at room temperature under continuous stirring. The reaction temperature was increased up to 90 °C, which was maintained for another 8 h. The obtained viscous solution of the reaction product was washed three times with diethyl ether and sodium chloride solution using a separating funnel by vigorously shaking. This resulted in the formation of two layers (organic and aqueous). The upper organic layer contains pure SFPO, whereas the lower aqueous layer contains unreacted moiety and other contaminants. The aqueous layer was discarded. The upper organic layer was further washed with distilled water. Finally, the liquid SFPO was obtained after drying over the anhydrous sodium sulfate followed by filtration. 2.3. Synthesis of SFPU. The prepared SFPO was treated with treatment demand indicator (TDI) in 1:1 ratio (NCO/OH = 1:1 mole ratio) under continuous stirring. Dibutyltin dilaurate (DBTDL), as a catalyst, was added in the reaction mixture. The reaction was conducted at room temperature for 3 h under a nitrogen environment. The TDI was added dropwise in a SFPO under stirring. The progress of the reaction was monitored by FTIR analysis. 2.4. In Situ Synthesis of V2O5-PDPA/SFPU Nanocomposites. A new approach for the dispersion of nanoparticles in the polymer matrix was adopted, that is in situ polymerization of SFPO within the V2O5-PDPA nanoparticles, which led to the formation of V2O5PDPA/SFPU nanocomposites (Scheme 1). Different wt % (0.5, 1,

Surprisingly, polydiphenylamine (PDPA), N-aryl derivative of PANI, has unnoticed and found scanty literature related to their anticorrosive applications.14 This may be due to its porous structures, poor processability, and poor adhesion. These properties can be enhanced by various modifications. Currently, it has been observed that the modification of CP with metal oxides results in the remarkable enhancement in their useful properties. Among various metal oxides, vanadium pentoxide (V2O5) has been widely used as an efficient conversion coatings for the successful replacement of carcinogenic chromium (IV) conversion coatings.15 Several reports are available on vanadium-based conversion coatings. Attar and Motamedi have optimized the chemical surface treatment of ST12 mild steel by using vanadium-based conversion coatings.16 Moreover, the ability of nanostructured V2O5 for the processing of anti-biofouling ship-hull coatings has also been highlighted recently.17,18 In this context, we have exploited the unique property of V2O5 to design anticorrosive coatings. However, the practical application of V2O5 alone in the safflower oil polyurethane (SFPU) matrix does not show a significant performance because of its poor solubility and low electrical conductivity. Such drawbacks can be overcome through the introduction of CP over the surface of these particles, which will improve its stability and interaction ability with the polymer matrix. In view of this, present work reports the synthesis of V2O5wrapped PDPA (V2O5-PDPA) hybrid nanoparticles along with the formulation of SFPU and V2O5-PDPA dispersed SFPU nanocomposite (V2O5-PDPA/SFPU) coatings. Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR) (1H and 13C), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and thermogravimetric analysis (TGA) were used to investigate the structural, morphological, and thermal properties of the prepared coatings. The anticorrosive properties and their protection mechanism were analyzed by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PDP) in a highly corrosive environment (5 wt % NaCl), while the mechanical properties were analyzed using standard ASTM methods. It is believed that we, for the first time, have designed the vanadium pentoxide-enwrapped PDPA/PU (V2O5-PDPA/SFPU) nanocomposite coatings for anticorrosive applications. Furthermore, the nanocomposite coating exhibit superior corrosion inhibition property as compared to SFPU and other reported coating systems (Table S3). The superior anticorrosive property of the proposed coatings provides a new horizon in industrial anticorrosive coatings.

Scheme 1. Schematic Illustration for the Synthesis of the V2O5-PDPA/SFPU Nanocomposite

and 2%) of V2O5-PDPA nanoparticles were dissolved in xylene and sonicated for 4−5 h to get a homogeneous solution. Then, the homogeneous solutions were added dropwise to the SFPO. Simultaneously, the calculated amount of TDI (NCO/OH= 1:1 mole ratio) was added in the reaction adduct, using DBTDL (as a catalyst) and a minimal amount of xylene (used within a permissible limit).20 The reaction was continued for 3 h at room temperature under continuous stirring, resulting in the formation of V2O5-PDPAx/SFPU nanocomposites, where x indicates the wt % of V2O5-PDPA. The progress of the reaction was monitored by FTIR spectra, and on the disappearance of the −NCO peak in the range of (2250−2270 cm−1), the reaction was stopped. 2.5. Preparation of Coatings. Plain carbon steel (CS) specimens were polished with different grades of silicon carbide papers of 200, 400, and 600 mesh. These specimens were washed with double distilled water, degreased with acetone, and finally dried at room

2. EXPERIMENTAL SECTION 2.1. Synthesis of the Vanadium Pentoxide-Enwrapped Polydiphenylamine (V2O5-PDPA) Nanoparticle. The V2O5PDPA nanoparticle was synthesized using a simple one-step in situ chemical oxidative polymerization method (Scheme S1a). Here, DPA monomer polymerizes to PDPA on the surface of V2O5 in the presence of ammonium persulfate (APS). In the process, 0.507 g (0.01 mol) of vanadium pentoxide (V2O5), 1.69 g (0.01 mol) of diphenylamine monomer, and 50 mL of distilled water were transferred in a round bottom flask and were vigorously stirred at 5 °C. The precooled 50 mL aqueous solution of APS (oxidizing agent) was added dropwise in the reaction mixture for a period of 0.5 h under continuous stirring. Then, the reaction was allowed to continue for 24 h at the same temperature. The progress of the reaction was B


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Figure 1. FTIR spectra of (a) SFO and SFPO and (b) SFPU, V2O5-PDPA/SFPU, and V2O5-PDPA.

Figure 2. (a) 1H NMR and (b) 13C NMR spectra of SFPU. temperature. SFPU and x-V2O5-PDPA/SFPU nanocomposites were applied on finally polished and cleaned CS specimens of standard size that is 75 mm × 25 mm × 1 mm and 25 mm × 25 mm × 1 mm using a brush technique. The coatings on earlier sized specimens were used for an electrochemical corrosion test while those of later size were used for morphological studies.

thesized using the in situ polymerization reaction, which involves a negligible amount of solvents as compared to other conventional methods.21 Moreover, the in situ approach results in the reaction of −OH group of SFPO with the NCO group of TDI and −NH group of V2O5-PDPA with −CO group of urethane at once, which resulted in the formation of high cross-linked V2O5-PDPA/SFPU nanocomposites. These hybrid nanofillers induce cementing effects by occupying the interstitial sites of polymer matrix, resulting in the formation of more a compact structure (Scheme S1c).22 3.1. Fourier-Transform Infrared Spectroscopy. FTIR spectroscopy of SFO, SFPO, SFPU, V2O5-PDPA, and V2O5PDPA/SFPU nanocomposites was performed to confirm the formation of final products by identifying the functional group as represented in Figure 1a,b. In the spectra of SFO (Figure 1a), the characteristic peaks at 2927 and 2898 cm −1

3. RESULTS AND DISCUSSION The formulation of V2O5 enwrapped with PDPA was carried out via in situ chemical oxidative polymerization resulting in the formation of hybrid nanofillers containing both organic and inorganic phases within one component, while the synthesis of SFPU was carried out using an addition polymerization reaction between the SFPO and TDI (NCO/OH = 1:1 mole ratio) in the presence of DBTDL (as a catalyst). Furthermore, V2O5-PDPA/SFPU nanocomposites were synC


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Figure 3. XRD patterns for (a) V2O5 and V2O5-PDPA nanoparticles, (b) SFPU and V2O5-PDPA/SFPU nanocomposites.

peak of V2O5-PDPA at 1172 cm−1 (−CN stretching vibration) and 806 cm−1 (−C−H out of plane) appears in V2O5-PDPA/ SFPU nanocomposite. All the remaining peaks are in consistent with the SFPU backbone. Thus, the occurrence of these peaks confirmed the interaction of V2O5-PDPA with the SFPU matrix and formation of V2O5-PDPA/SFPU nanocomposite with the broad structural features. 3.2. Nuclear Magnetic Resonance. 1H NMR and 13C NMR spectra of SFPU are shown in Figure 2a,b. In the 1H NMR spectra of SFPU (Figure 2a), the signal at δ = 0.9 and δ = 1.3−1.7 ppm represents the terminal −CH3 and internal −CH2 group of SFO fatty acid chain present in the backbone of SFPU, respectively. The peak at δ = 2.1−2.8 ppm is due to the protons attached to the carbonyl group, while the peak at δ = 3.5−4.9 ppm ascribed to proton directly attached to the oxygen atom.31 Peak at δ = 5.4 ppm is due to the proton linked to urethane functional group. The peak for aromatic protons of toluene diisocyanate appeared at δ = 7.0−7.5 ppm.32 13C NMR spectrum of SFPU has shown in Figure 2b. The peak at δ = 19−35 ppm corresponds to methyl and methylene carbon of fatty acids. The signal at 173 ppm is due to the presence of ester linkage of SFO present in the backbone of SFPU chain.31 Furthermore, the chemical shift at δ = 38 ppm is raised due to the carbon atom attached next to the oxygen atom. The unsaturated carbon of TDI appears at δ = 125−137 ppm, while the signal at δ = 144 ppm is corroborated with the −CO group attached to the −NH (−NHCOO) group of urethane.33 These observations confirmed the formation of SFPU. 3.3. X-ray Diffraction. The variation in the degree of crystallinity and effect of concentration of V2O5-PDPA hybrid nanoparticles on the structural changes in the V2O5-PDPA/ SFPU nanocomposite were discussed by using X-ray diffraction (XRD) pattern (Figure 3b). XRD patterns for V2O5 and V2O5-PDPA nanoparticle (Figure 3a) justify the wrapping of PDPA over the surface of V2O5. Figure 3a shows orthorhombic phase of V2O5 with high crystalline nature. However, in the case of V2O5-PDPA, the characteristic peak follows almost the same pattern like V2O5 with low intensity and no shift was observed. The decreased intensity without any characteristic shift in peaks revealed that the structure of V2O5 has not been destroyed and the formation of PDPA has occurred on the surface of V2O5.34 On the other hand, the diffraction peak for SFPU that appear at 2θ = 20° directs the amorphous structure of SFPU.35 While in the case of V2O5PDPA/SFPU nanocomposite, the same diffraction peaks appeared at 2θ value, indicating that the introduction of small amount of V2O5-PDPA does not change the structure of SFPU but contribute to change the intensity of the peak. It can

correspond to −CH and −CH2 stretching frequency, while the peak at 1750 cm−1 is due to −CO stretching of the conjugated carbonyl group in the SFO. The absorption peak at 1655 cm−1 is due to the presence of −CC group present in SFO.23 Moreover, the peaks at 1220, 1165, and 1095 cm−1 represent triester (−C−CO) linkages. Figure 1a shows the FTIR spectra of SFPO which exhibits a broad absorption peak at 3450 cm−1 due to the stretching vibrations of the hydroxyl group (−OH).24 The disappearance of the peak at 1655 cm−1 confirms the transformation of all the double bonds of SFO into the hydroxyl group, which further indicates the completion of the reaction and the formation of SFPO. In the FTIR spectra of SFPU (Figure 1b), a progressive decrease of −OH stretching vibration at 3450 cm−1 and the shifting of stretching vibration signal to the lower frequency region to 3300 cm−1 are observed upon the addition of TDI, ascribing the formation N−H stretching vibration of the urethane bond in SFPU.25 The peak appears due to the hydrogen bonding between the −OH group of SFPO and the −NCO group of TDI. The other peaks due to the development of urethane linkage at 1735 cm−1 (−C−O of the soft segment), 1608 cm−1 (−CO), 1538 cm−1 (−NH− CO), 1228 cm−1 (−C−O of hard segment), and 1052 cm−1 (−C−CO stretching vibration) further confirm the successful formation of SFPU.26 The FTIR spectra of V2O5-PDPA (Figure 1b) represent the peaks at 3380 cm−1 (−NH stretching vibration bands), 1590, 1491, and 1316 cm−1 for C−H stretching in quinoid, phenyl, and benzenoid, respectively.27 Moreover, peaks at 806 and 743 cm−1 are assigned to C−H out of plane aromatic and 1,4 substituted aromatic rings, respectively. An intense peak appeared at 1016 cm−1 can be attributed to the shifting of VO peak of V2O5 to higher frequency compared to their reported value that is 990 cm−1.28 This revealed that there is σ−π interaction between V2O5 and PDPA. Here, the molecular orbital of PDPA overlaps the empty d-orbital of metal ions and forms σ-bond, while the π* molecular orbital of PDPA overlaps the d-orbital of metal ions to form the π-bond. Additionally, the hydrogen bonding between PDPA and oxygen atoms on the surface of metal oxide (V2O5) in the composites leads to the enwrapping of V2O5 by PDPA.29,30 On the other hand, the FTIR spectra of V2O5-PDPA/SFPU (Figure 1b) exhibit a broaden peak of −NH stretching at 3338 cm−1, which is due to the in situ interaction of −NCO group of TDI with the −OH of SFPO and −NH group of V2O5-PDPA with the −CO group of urethane that led to the formation of cross-linked V2O5-PDPA/SFPU nanocomposite. A small D


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ACS Applied Materials & Interfaces be seen that the intensity of the amorphous segment decreases in V2O5-PDPA/SFPU. This can be attributed to the fact that the V2O5-PDPA is covalently linked to the SFPU backbone resulting in the increase in chain stiffness, which restricted the movements of the chain segments and thus decreases the intensity. Furthermore, it is shown that the order of peak intensity follows a sequence: SFPU > 2V2O5-PDPA/SFPU > 1V2O5-PDPA/SFPU > 0.5V2O5-PDPA/SFPU. An increase in peak intensity for 2V2O5-PDPA/SFPU nanocomposite is attributed to the crystalline nature of V2O5-PDPA. Moreover, it is interesting to mention that no additional peak of V2O5PDPA was observed, implying the uniform dispersion instead of aggregation within the SFPU matrix.36 3.4. Physicochemical and Physicomechanical Properties. With the loading of V2O5-PDPA nanoparticles in SFPU, an increase in viscosity, specific gravity, and refractive index (Table S1) is observed. On the other hand, physicomechanical properties such as the scratch hardness test values increased from SFPU to V2O5-PDPA/SFPU (Table S2) because of the fact that V2O5-PDPA nanoparticles (i) interact electrostatically with the matrix and (ii) fill empty space of matrix.37 Moreover, homogeneously dispersed V2O5-PDPA nanoparticles act as a rigid obstacle that restricts the movement of the tip of scratch tester, thus enhancing the scratch hardness values.38 The increased loading of V2O5-PDPA in the SFPU matrix improved the impact resistance of coatings that induces the crack healing property through the controlled chain flexibility (Figure S1). Bend test showed good flexibility for all the coatings, as no cracks or ruptures were seen on the surface. The excellent bending ability of nanocomposite and SFPU coatings is mainly due to the long fatty acid chain present in the polymeric backbone, which is known to act as an internal plasticizer.22 The digital picture (Figure 4a) and optical micrograph (Figure

Figure 5. TGA thermograms of SFPU and V2O5-PDPA/SFPU-x nanocomposites.

Figure 4. (a) Digital pictures and (b) optical micrograph of coated samples before and after cross cut tape test.

loss due to the removal of trapped solvents. The first weight loss appeared in the range of 250−383 °C due to the decomposition of labile urethane group of the SFPU and the aromatic moiety of TDI.41 However, initially a slight difference was observed in the decomposition temperatures of SFPU and nanocomposite. This can be assigned to the fact that the metal oxide (V2O5) has a low linear coefficient of thermal expansion than that of the SFPU matrix.42 Owing to this behavior, the heat absorbed is used to increase the interatomic distance rather to increase the temperature of polymer, which ultimately exhibits chain scission behavior and caused lower thermal stability of nanocomposite. Furthermore, the second decomposition in the temperature range of 383−537 °C with major loss of 80 wt % was seen due to the breakdown of long fatty acid chain present in the SFPU and V2O5-PDPA/SFPU.32 After this, a clear difference in the wt loss can be seen (Figure 5, inset), describing the higher thermal stability of nanocomposite than that of plain SFPU and the stability increases with the loading of V2O5-PDPA nanoparticle into the SFPU matrix. The char residues at 800 °C are clearly depicted and found to be highest (∼10%) for 2V2O5-PDPA/SFPU nanocomposite. The higher char residue of nanocomposite revealed that the V2O5-PDPA hinders the composite to volatilize on further thermal degradation.43 Moreover, the homogeneous dispersion and strong electrostatic interaction between V2O5PDPA and SFPU reduces the chain mobility in nanocomposite and eventually retard its degradation.44 3.6. TEM Analysis. To further confirm the structure of V2O5-PDPA and its dispersion into the SFPU matrix, TEM analysis was performed and is displayed in Figure 6a. A twophase structure that is dark tubular due to the presence of V2O5 and bright globular for PDPA was observed with varying size 10−25 nm, where the bright organic phase seems to wrap the dark inorganic phase. In the TEM image of nanocomposite (Figure 6b), the morphology of V2O5-PDPA nanoparticle was

4b) before and after cross cut tape test exhibited the strong adhesion of the coatings with the CS surface as no squares were peeled off after the removal of tapes.39 The strong adhesive strength of the nanocomposite coating is mainly attributed to the interlocking effect between V2O5-PDPA and SFPU matrix that provides a cross-linked network structure.40 3.5. Thermal Stability. The thermal stability of SFPU and V2O5-PDPA/SFPU nanocomposites was investigated using TGA. As shown in Figure 5, all the composites and plain SFPU are stable up to 250 °C exhibiting a slight or negligible weight

Figure 6. TEM images of (a) V2O5-PDPA nanoparticle and (b) V2O5-PDPA/SFPU nanocomposite. E


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Figure 7. (a) CA and (b) water uptake behavior for SFPU and V2O5-PDPA/SFPU nanocomposites.

hydrogen gas were evolved, due to the reaction of CS with acidic medium that evolves hydrogen gas.26 From the images, it was observed that the 2V2O5-PDPA/SFPU exhibits excellent acid resistance because of the presence of well-dispersed V2O5PDPA nanoparticle of low surface energy within the SFPU matrix. These results demonstrate that the 2V2O5-PDPA/ SFPU composite coating could offer an enhanced corrosion protection for CS substrate in a harsh corrosive environment. The surface topography and the structural changes occurred on the surface of the coating, after the immersion in 1 M H2SO4 solution, were analyzed by the high-precision atomic force microscopy (AFM) technique. The two-dimensional (2D) and three-dimensional (3D) AFM images of SFPU and V2O5-PDPA/SFPU-coated samples were taken after the immersion test are shown in Figure 8a−b′. It is clearly

clearly seen, which confirms the well dispersion of V2O5-PDPA within the SFPU matrix. Black globules and tubes exemplify that the nanoparticles (10−80 nm) were uniformly dispersed in the SFPU matrix (gray in color) without any accumulation. The results confirm the formation of well-dispersed regular V2O5-PDPA/SFPU nanocomposite.45 3.7. Contact Angle and Water Uptake. Furthermore, to elucidate the surface characteristics, that is surface wettability of these coatings, the contact angles (CAs) were measured (Figure 7a). It was found that the SFPU coating reflected a hydrophilic character, as the CA for this is 68°, while for the nanocomposite the CA value increases with the increased loading of nanoparticle and reached to 96° for 2V2O5-PDPA/ SFPU coating exhibiting a hydrophobic character. This could be attributed to the increase in the roughness value at the nanoscale on the surface due to the homogeneous dispersion of V2O5-PDPA nanoparticles within the SFPU matrix.46 Moreover, the water uptake was obtained via weighing the mass of the coated samples (10 mm × 10 mm × 1 mm) via a microbalance at 24 h of time intervals. The samples were immersed in the distilled water and weighed after drying with tissue paper and by blowing the nitrogen gas. The water uptake (Qt) was calculated according to the following equation.47 Q = [(mt − mo)/mo] × 100

(1)

where, mo and mt are the mass of sample before immersion and immersed at time t. The water absorption behavior of SFPU and their nanocomposites is shown in Figure 7b. It is inferred that the maximum water absorption takes place for SFPU and increases with immersion time. However, the water absorption behavior decreases with increase in the concentration of nanoparticles in SFPU and a negligible amount of water uptake takes place even after 144 h. The decrease in water absorption corresponds to the increase in water CA, which resists the penetration of water. 3.8. Corrosion Resistance Performance of the SFPU and V 2 O 5 -PDPA/SFPU Nanocomposite Coatings. 3.8.1. Immersion Test. An immersion test for the formulated coatings was conducted to investigate the anticorrosive efficiency. For this, the plain SFPU and 2V2O5-PDPA/SFPUcoated samples were immersed in 1 M H2SO4 for 24 h. The CS sample was also immersed in the same solution condition for comparison. Figure S2a,b shows the digital images of the coated and uncoated samples immersed in 1 M H2SO4 solution and after the immersion, respectively. It was observed that as we put the steel sample in the solution, many bubbles of

Figure 8. (a,b) 2D and (a′,b′) 3D AFM images after the immersion test in 1 M H2SO4 for SFPU and 2-V2O5-PDPA/SFPU, respectively.

shown in Figure S2b that mild steel sample is badly damaged due to the acid attack on the surface. However, in the case of SFPU, the average roughness value (450 nm) is found to be more than that of V2O5-PDPA/SFPU (300 nm) as shown in Figure 8a′,b′, which can be attributed to the growth of sulfides and oxides on the surface of the SFPU coating.26,48 On the other hand, the lower value of roughness for V2O5-PDPA/ SFPU reveals that V2O5-PDPA/SFPU protects the CS substrate more efficiently than SFPU and the 1 M H2SO4 solution did not alter the surface of the coating after 24 h of immersion. F


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ACS Applied Materials & Interfaces 3.8.2. Salt Mist Test. Furthermore, the corrosion inhibition performance and the surface morphology of the SFPU and V2O5-PDPA/SFPU nanocomposite coatings, exposed to the salt mist test (5 wt % NaCl, 90% humidity) for 7 days, were investigated by SEM analysis. Figure 9a,b shows SEM images of SFPU and 2V2O5-PDPA/ SFPU-coated sample surfaces, respectively, after exposure to

Figure 10. PDP curves for bare CS, plain SFPU, and V2O5-PDPA/ SFPU coated CS in 5 wt % NaCl.

(Ecorr = −0.59) of CS substrate were observed that can be attributed to the initiation of pitting corrosion just after the immersion. Owing to this delamination, thus the corrosion protection analysis of plain CS was not performed for further days. In the case of plain SFPU, a remarkable decrease in Icorr (1.49 × 10−9 A cm−2) was observed revealing its good barrier property. A drastic shift in Ecorr to anodic region, with respect to CS, was also observed that is from −0.595 to −0.147 V. On the other hand, the V2O5-PDPA/SFPU nanocomposite exhibits very low Icorr value 7.45 × 10−11A cm−2, which is about 2 and 6 orders of magnitude lower than the SFPU and plain CS, respectively. However, Ecorr for nanocomposite was measured to be 0.518 V, which is more positive than that of SFPU and CS substrate. A positive shift in Ecorr for nanocomposite coatings is due to the presence of conducting V2O5-PDPA that ennobles them, thus revealing their amended corrosion resistance property. The much higher Ecorr and lower Icorr values of V2O5-PDPA nanocomposite than that of SFPU and CS are because of the hydrophobic nature, excellent mechanical properties, large specific surface area, and higher redox potential of V2O5-PDPA in the SFPU matrix.51 Furthermore, the rate of corrosion for these coatings is also in consistency with the corrosion current densities that is 0.18934, 1.64 × 10−5, and 8.66 × 10−7 mpy for CS, SFPU, and nanocomposite coatings, respectively. The higher Ecorr value of nanocomposite coatings even after 30 days is estimated to be about −0.04 V, which corresponds to the formation of a protective passive oxide layer. This passive layer is continuously forming due to the redox nature of V2O5-PDPA and presence of −NH group in the polymeric chain of PDPA that absorbs Fe2+ and Fe3+ ions and possibly passivate the metal in the passive region.52 Thus, the nanocomposite coating exhibits prolonged corrosion resistance property for the CS with extremely higher corrosion PE (99.999%) even after 30 days. These studies reveal the robust and long-term anticorrosion performance of nanocomposite coatings that can be attributed to the superior physical barrier and adhesion property that repels the corrosive ions. 3.10. Electrochemical Impedance Spectroscopy. Figure 11a shows Nyquist plots for the plain CS, obtained after the exposure in 5 wt % NaCl solution for 1 day, which shows a single depressed semicircle. Figure 11b−d shows the Nyquist plots for SFPU and nanocomposite after 10 days interval of immersion time. Initially (1st day), the Nyquist plot of SFPU consisted of a single semicircle at high and low frequency (inset, Figure 11b). However, after 10 days an

Figure 9. (a,b) SEM micrograph after salt mist and (c,d) enlarged view for SFPU and 2V2O5-PDPA/SFPU respectively.

salt mist in 5 wt % NaCl aqueous solution for 7 days. In the case of 2V2O5-PDPA/SFPU-coated sample, relatively smooth with few signs of corrosion products can be observed. While for the SFPU coated sample, relatively more signs (spread all over the surface, Figure 9a) of corrosion products were seen. The enlarged versions of SEM images for SFPU (Figure 9c) and 2V2O5-PDPA/SFPU (Figure 9d) have also been shown here to have a better insight into the corrosion behavior. As shown in Figure 9c, it is revealed that many corrosion products (salt deposited) in granular shape were formed on their surfaces. On the other hand, Figure 9d shows the enlarged versions of SEM images for 2V2O5-PDPA/SFPU exhibiting less salt deposition than that of SFPU in the form of rocklike structure. 3.9. PDP Analysis. The quantitative measurements for the corrosion protection efficiency (PE) of SFPU and V2O5PDPA/SFPU nanocomposite coatings were conducted under the aggressive corrosive environment (5 wt % NaCl) for 30 days using PDP study. The polarization curves for coated and uncoated CS samples in the given environment are presented in Figure 10, and the values for their kinetic corrosion (Tafel) parameters such as corrosion current density (Icorr), corrosion potential (Ecorr), corrosion rate, and PE are summarized in Table 1. The corrosion PE (%) values of all the coatings were calculated using the obtained Icorr values as per the eq 249 PE (%) = (Icorr,0 − Icorr,c/Icorr,c) × 100

(2)

where Icorr,0 and Icorr,c are the corrosion current of bare CS and coated CS, respectively. Icorr and Ecorr are the important parameters that measure the corrosion protective performance and tendency of a substrate to corrode, as the rate of corrosion is directly proportional to the Ecorr value and inversely proportional to Icorr values.50 Thus, the coating having higher Ecorr and lower Icorr provides superior anticorrosive performance. The higher corrosion current density (Icorr = 1.63 × 10−5) and lower corrosion potential G


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Table 1. Tafel Parameters for Coated and Uncoated Carbon Steels after Immersion in 5 wt % NaCl Solution after 30 Days sample

Icorr (A cm−2)

Ecorr (V)

corrosion rate (mpy)

polarization resistance (Ω)

PE (%)

CS SFPU 2V2O5-PDPA/SFPU

1.63 × 10−5 1.41 × 10−9 7.45 × 10−11

−0.594 −0.147 −0.040

0.18934 1.64 × 10−5 8.66 × 10−7

1628 1.00 × 108 1.58 × 108

99.991 99.999

Figure 11. Nyquist plots for (a) CS, (b) SFPU, (c) 0.5 V2O5-PDPA/SFPU, and (d) 2V2O5-PDPA/SFPU.

Figure 12. Bode plots for (a) CS, (b) SFPU, (c) 0.5V2O5-PDPA/SFPU, and (d) 2V2O5-PDPA/SFPU.

high and low frequency range decreases with the increase in immersion time. After 30 days of immersion, SFPU exhibited high Rpore (3.29 × 106 Ω cm−2), Rct (7.18 × 107 Ω cm−2), Zw (4.57 × 10−7 Ω cm−2), and CPE2 (4.41 × 10−10 F cm−2), implying its remarkable corrosion protective efficiency. While the Nyquist plots for 0.5 V2O5-PDPA/SFPU nanocomposite coating showed a single semicircle over the whole immersion time and the radius of semicircle decreased with the immersion time. This suggests a complete capacitive and good barrier

additional element in the form of Warburg resistance appeared (Figure S3) due to the initiation for diffusion of corrosive ions through the coatings.53 After this, until 30 days, the Nyquist plots show the same behavior having a semicircle at high frequency (solution resistance) and a tail at the low frequency region (sum of charge transfer resistance and Warburg resistance). This behavior implies that the chloride ions start to diffuse slightly into the coating through pinholes after 10 days of immersion. Furthermore, the radius of impedance at H


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Figure 13. Optical micrographs of (a,b) CS, (c,d) SFPU, (e,f) 0.5V2O5-PDPA/SFPU, and (g,h) 2V2O5-PDPA/SFPU before and after corrosion studies.

Figure 14. Corrosion protection mechanism of SFPU and V2O5-PDPA/SFPU nanocomposite coatings.

of plain SFPU representing the ability of the coatings to lock up the current between the cathodic and anodic areas. The values of |Z|0.01 Hz for all the coated substrate decrease with the increase in immersion time (Figure S4) in the sequence SFPU < 0.5V2O5-PDPA/SFPU < 2V2O5-PDPA/SFPU, which evidenced that the stability of coatings toward the corrosive ions increases with the increased loading of V2O5-PDPA nanoparticle. For SFPU, the impedance value was found to be decreased by 2 fold (4.46 × 1010 to 1.96 × 108 Ω cm2), whereas 2V2O5-PDPA/SFPU nanocomposite coatings displayed higher value of impedance (1.69 × 1011 Ω cm2) in the early immersion and a slight decrease in the value (2.68 × 1010 Ω cm2) was found even after 30 days of immersion. These results highlight the superior corrosion protective performance of 2PDPA-V2O5/SFPU coatings. Furthermore, the phase angle plot is also a deterministic parameter that directs the damage or delamination of the anticorrosive coatings. The bare CS showed only one time constant that appears at low frequency revealing the delamination of the CS with a minimum phase angle, suggesting that the CS undergo easily in corrosion reaction under the saline environment (Figure 12a). It was observed from the phase angle plots (Figure 12b) that the SFPU coating showed high phase angle value at mid and high frequency (two-time constant) in the early immersion time (1st day), which shows less delamination of the coating. However, after 10th days of immersion it much slight delamination, which is

property even at low loading of V2O5-PDPA nanoparticle. However, the Nyquist plot of 2V2O5-PDPA/SFPU nanocomposite coating (Figure 11d) exhibits a linear relationship with the real and imaginary parts of impedance due to the uniform dispersion and high content of nanofiller within the SFPU matrix acting as a strong secondary physical barrier. A decrease in the capacitive loop was recorded with the increase in immersion time. This suggests that the nanocomposite coatings exhibit a higher value of Rpore (3.29 × 107 Ω cm−2), Rct (1.34 × 1010 Ω cm−2), and CPE2 (5.59 × 10−11 F cm−2) than that of plain SFPU resulting in its superior anticorrosive behavior. The long term (after 30 days) anticorrosive efficiency of this coating is due to the fact that the V2O5-enwrapped PDPA locks the paths providing more complexity and tortuosity for the diffusion of ions, resulting in the adjourning of chloride ions penetration. Furthermore, the Bode plots for the coated CS, SFPU, and V2O5-PDPA nanocomposite coatings are shown in Figure 12a−d. Generally, the peak of Bode plot curve at high frequency (i.e. 103 to 105 Hz) responds to the protection of coatings while that of at middle to low frequency (i.e. 10−2 to 103) indicates the strong barrier effect for corrosive ion at the coating−metal interface. Figure 12a exhibits very low impedance value for CS after 1 day of immersion due to the occurrence of corrosion process on the metal surface. While for SFPU and V2O5-PDPA nanocomposite coatings, the modulus at low frequency (|Z|0.01 Hz) is very high as compared to that I


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continues until the PDPA has the ability to undergo charge transfer reaction at the metal−coating interface and enhances the corrosion protection property. In addition, the anticorrosive property of V2O5-PDPA/ SFPU coatings was compared with other similar reported coating systems (Table S3). It is worth to mention here that many literatures are available on modified CP (wrapped or intercalated) based anticorrosive composite coatings, while scant literature is available on metal oxide-CP/PU anticorrosive coatings. Dong et al. investigated the improvement in corrosion inhibition and barrier property of PU by reinforcing polydopamine-wrapped carbon nanotubes (PDA@CNTs). They inferred that PDA@CNTs improves the adhesion as well as the corrosion resistant property of PU by increasing the compactness and decreasing the defects in PU.56 Cai et al. found that the anticorrosive performance of waterborne PU significantly enhanced when reduced graphene oxide/PANI content is 0.75 wt %.57 Another study reports the anticorrosive performance of boron nitride wrapped PANI nanohybrid dispersed polyvinyl alcohol (PVA). They have investigated that the synergistic effect of PANI and boron nitride nanoparticles with PVA, enhanced PE (99.30) of coating.51 Thus, the current nanocomposite coating system exhibits comparable anticorrosive performance in few cases and superior performance in most of the cases having a low Icorr, and corrosion rate, high Ecorr, PE, and |Z|0.01 value (>1011).

in evidence from the increase in breakpoint frequency and decrease in phase angle. The value of breakpoint frequency (f b) gradually increases, 8.04, 11.63, 15.52, and 17.38 Hz at 1st, 10th, 20th, and 30th days of immersion, respectively. On the other hand, for V2O5-PDPA/SFPU nanocomposite coatings (Figure 12c,d) showed a single time constant with high phase angle (≥80) over the whole immersion period. The angle for 2V2O5-PDPA/SFPU nanocomposite coatings reached to 84° with no breakpoint frequency showing no delamination of the coating till 20th day. On the 30th day, it showed a little delamination with a very small value of f b (0.01 Hz). However, 0.5V2O5-PDPA/SFPU coatings showed a decrease in the phase angle with a relatively higher value of f b that is 0.02 Hz at the 1st day and 0.08 Hz after 30 days of immersion. To further evaluate the structure and texture of the coated and uncoated CS before (Figure 13a,c,e,g) and after (Figure 13b,d,f,h) the EIS and PDP analysis in 5 wt % NaCl, the optical micrographs were taken. Figure 13b shows that the uncoated CS substrate was completely corroded, while the SFPU (Figure 13d) and 0.5 V2O5-PDPA/SFPU (Figure 13f) coated substrate were affected partially. However, the CS coated with 2V2O5-PDPA/SFPU remained unaffected (Figure 13h). 3.11. Corrosion Protection Mechanism. The corrosion protection mechanism for the SFPU and V2O5-PDPA/SFPU nanocomposite coatings on steel is shown in Figure 14. In the case of SFPU coatings, hydrocarbons, urethane groups, and free −NCO groups help in the formation of well adhered uniform and compact coatings structure and provide the sufficient corrosion resistance ability. However, the plain SFPU can provide good corrosion resistant performance as long as the coating acts as a barrier and when the diffusion of ions takes place it reaches the metal substrate easily/directly due to the absence of any nanoparticles. In contrast, a different mechanism has been proposed for the corrosion protection (inhibition) provided by SFPU nanocomposites. Among these, “ennobling” is one of the mechanisms used to explain the anodic protection and passivation of the metal through oxidation by the CPs. Because of the redox nature, CP stabilizes the potential of metal in passive region by reducing itself. Passive layer formation, barrier effects, and adhesion are the key factors that govern the protection mechanism.13,50 In the case of V2O5-PDPA/SFPU nanocomposite coatings, the steel substrate is protected by all these factors. Furthermore, the wrapping of PDPA over the surface of V2O5 strengthens the barrier property to the V2O5-PDPA/SFPU nanocomposite coatings, which can be due to the synergistic effect of V2O5 and PDPA. Here, V2O5-PDPA acts as an anion storage, offers electrical barrier for electron transport, and slows down the metallic corrosion by retarding the electrochemical reactions.51,54 This consequently reduces the structural damages and penetration of corrosive species into the coating. However, after the prolonged corrosive attack, the barrier effect of the coatings starts to degrade and allows the penetration of ions to the coating, but these ions will not reach to the metal−coating interface directly. This can be attributed to the homogeneous dispersion of V2O5-PDPA nanoparticles, which hinders the penetration of corrosive species by providing a more tortuous path that lead to the increase in penetration time.55 Eventually, after following a long path, it reaches the steel substrate and forms a passive Fe2O3 layer. An oxidation of Fe to Fe2+ or Fe3+ takes place through the passive oxide film by the reduction of PDPA-emeraldine salt to PDPA-emeraldine base. This process

4. CONCLUSIONS Conductive V2O5-PDPA/SFPU nanocomposite anticorrosive coatings were successfully prepared for the first time. FTIR, TEM, and TGA were used to characterize the structure, morphology, and thermal stability of the nanocomposite. The coating demonstrated a remarkable improvement in physicomechanical properties. The corrosion performance of coated and uncoated CS was carried out by PDP and EIS analysis under 5 wt % NaCl solution for 30 days. It was found that 2V2O5-PDPA/SFPU nanocomposite coating exhibited highest impedance modulus (1.69 × 1011 Ω cm2) at 0.01 Hz (| Z|0.01 Hz), lowest corrosion current density (7.45 × 10−11 A cm−2), maximum PE (99.999%), and very low corrosion rate (8.66 × 10−7 mpy). The wrapping of vanadium pentoxide with PDPA resulted in the formation of strong secondary barrier (providing dual protection) that helps in the enhancement of anticorrosive activity of PU nanocomposite coatings. These studies revealed that the proposed PU nanocomposite coatings have a potential scope for their application in the field of paints and coatings.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b17861. Materials and characterization methods, schematic representation for the synthesis of V2O5-PDPA, SFPO and V2O5-PDPA/SFPU nanocomposite, digital images after physicomechanical characterization, table for physicochemical and physicomechanical properties, digital images for immersion test, electrochemical equivalent circuit, impedance at low frequency (|Z|0.01 Hz), and table of comparison for corrosion resistance performance with other reported systems (PDF) J


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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +91 11 26827508. Fax: +91 11 26840229 (S.A.). ORCID

Sharif Ahmad: 0000-0001-5799-7348 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS One of the author H.K. is highly thankful to UGC-Maulana Azad National Fellowship for providing financial assistance under the award letter no. F1-17.1/2014-15 MANF.

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