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Reduced Graphene Oxide Reinforced Waterborne Soy Alkyd Nanocomposites: Formulation, Characterization and corrosion inhibition analysis. Mohd Irfan, Shahidul Islam Bhat, and Sharif Ahmad ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03349 • Publication Date (Web): 25 Sep 2018 Downloaded from http://pubs.acs.org on September 26, 2018

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ACS Sustainable Chemistry & Engineering

Reduced Graphene Oxide Reinforced Waterborne Soy Alkyd Nanocomposites: Formulation, Characterization and corrosion inhibition analysis. Mohd Irfan1, Shahidul Islam Bhat1 and Sharif Ahmad1* *Corresponding Author Authors Detail 1

Mohd Irfan

Materials Research Laboratory Department of Chemistry, Jamia Millia Islamia (A Central University) New Delhi-110025, India E-mail address: [email protected] Orcid Id: 0000-0001-9784-7213 1

Shahidul Islam Bhat

Materials Research Laboratory Department of Chemistry, Jamia Millia Islamia (A Central University) New Delhi-110025, India E-mail address: [email protected] Orcid Id: 0000-0002-1672-5060 1

Sharif Ahmad*

Materials Research Laboratory Department of Chemistry, Jamia Millia Islamia (A Central University) New Delhi-110025, India Tel no. +91 11 26827508 Fax: +91 11 26840229 E-mail address: [email protected] Orcid Id: 0000-0001-5799-7348 *Corresponding Author

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ABSTRACT The waterborne soya alkyd (WSA) and its RGO dispersed nanocomposites (WSA-RGO) were synthesized via ex-situ polymerization using solvent-less green approach. The soya oil monoglyceride was used as precursor for waterborne alkyd. The synergistic effect of nanofillers and poly melamine co-formaldehyde isobutylated solution modified organic matrix (WSA) on physico-mechanical and corrosion inhibition of these coatings on finally polished carbon steel (CS) was investigated. Their physico-chemical, physico-mechanical and thermal properties were analyzed using standard protocols. The electrochemical corrosion measurements of these coatings were performed using Potentiodynamic Polarization and Electrochemical Impedance Spectroscopy. The aforementioned studies revealed that the nanocomposite coatings exhibit promising corrosion protection performance which is evident from the icorr, Ecorr, impedance and phase angle values of coatings (i.e. icorr 7.736×10-9 Acm-2, Ecorr -0.191 V, impedance ~107 Ω cm2 and phase angle 86o). These results suggest that the proposed waterborne nanocomposite coatings exhibit superior corrosion protection property than those of other such earlier reported systems. Keywords: waterborne; Soy alkyd; RGO; nanocomposite; anticorrosive; phase angle. INTRODUCTION Corrosion is an accelerated form of an ageing, that proceeds via chemical or electrochemical reactions of materials with their surroundings, causing heavy economic losses at the national/international scale.1 It is a thermodynamically sensitive process, which cannot be eradicated but can be reduced to a certain extent by various techniques like surface cleaning, designing, alloying, use of inhibitors, paints and coatings.2–5 Among these, synthetic polymer coatings have been widely used to prevent the corrosion due to their ease of application and periodic protection of industrial infrastructures, that help in reducing the cost of corrosion and its maintenance.

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However, the impact of fossil fuel depletion and emission of volatile


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organic compounds (VOCs) during their processing have forced the researchers to make efforts for the development of sustainable polymers, using renewable monomers.7 Among these, vegetable oil based polymers of low molecular weight, good fluidity with negligible or no organic solvents find frequent applications in the areas of adhesives, additives, paints, coatings etc.8 Literature reveals that vegetable oil (VO) alkyds have been used in the field of paints and coatings since 1940s. They cover 50% of all the resins used in paints and coatings.9 Odetoye et.al.10 have synthesized Jatropha oil alkyd using a two-stage alcoholysispolyestrification method. However, authors did not provide a clear industrial application of the synthesized resins. Selim etal.11 formulated sunflower oil based γ-Al2O3 nano-rods dispersed alkyd for surface coating applications via solution casting method. However, the corrosion inhibition of synthesized nanocomposites were not analyzed by sophisticated techniques like PDP and EIS. Gamal et.al.12 synthesized graphene oxide and CNT doped PANI alkyd composites, used them as binder for anticorrosive application. The impedance value of these coatings was found to be 105 ohm cm2, which is generally associated with the low performance barrier coatings. Generally, alkyd coatings show satisfactory performance under brine, acid and U.V environments, but fail to stand more severe corrosive conditions. In order to overcome these shortcomings, researchers from time to time have developed various alkyd based nanocomposites through the dispersion of nano-fillers like carbon dot,13 carbon Dot-TiO2,14 MWCNTs,15 Al2O3,16 Fe3O4-SiO2,17 organo-clay,18 and NiO.19 Recently, researchers have reported graphene oxide (GO) and reduced graphene oxide (RGO) nanocomposite coatings with remarkably improved mechanical, thermal and barrier properties.20–22The following properties of these nano-fillers play a significant role in the corrosion protection ability of nanocomposites through (i) inhibition for diffusion of corrosive ions at coating-metal interface, (ii) uniform dispersion of nanofillers within the organic matrices and (iii) induces barrier activity at coating-metal interface.23,24 The



hexagonal 2D sheet structure of RGO25 leads to its uniform dispersion with in the matrix, inducing interaction between RGO and polymer matrix that further enhances hydrophobicity of nanocomposite coatings.20,22 Among various waterborne polymers, waterborne alkyds are found to be of considerable interest, as they possess functionalities like carboxylic, hydroxyl and so forth, which led to electrostatic interaction between them and other similar groups of RGO. Thus the incorporation of RGO nano-fillers as reinforcing/cementing agents in oleo alkyd resulting in the formation of new materials that exhibit superior physico-chemical, physico-mechanical, thermal, anticorrosive, flame retardancy and barrier properties than the individual components. Thus, herein the authors report the synthesis of waterborne soya alkyd (WSA) and RGO dispersed WSA nanocomposites (WSA-RGO) and formulation of PMF cured WSA (WSAPMF-80) and their nanocomposite (WSA-PMF-80-RGO-x) coatings on carbon steel. The structural, morphological, physico-chemical, mechanical, thermal and corrosion inhibition analysis were performed using FTIR, NMR, ASTM laboratory protocols, SEM, TEM, TGA, DSC, PDP, EIS and salt spray techniques. Alkyd nanocomposite coatings exhibit superior corrosion inhibition property as compared to WSA-PMF-80 and other reported coating systems. (Table S3) EXPERIMENTAL SECTION Materials Poly (melamine-co-formaldehyde) isobutylated solution (PMF): [refractive index 1.468, bp 108 °C, mol wt. ~ 1620, density 1g/ml] and phthalic anhydride [C6H4 (CO)2O, mol wt. 148.1 g mol-1,98%] were procured from Sigma Aldrich. Graphite fine powder and p-Toluene sulfonic acid (p-TSA) was purchased from Loba Chemie Mumbai India, while potassium permanganate (KMnO4), ethanol (C2H5OH), glycerol (C3H8O3 98%) and sulphuric acid


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(H2SO4) were procured from Merck India. Soy bean oil was obtained from local market. Hydrogen peroxide (H2O2 30 wt. %), Sodium hydroxide (NaOH 97%), Sodium chloride (NaCl 99%) and triethylamine (C6H15N 99%) were supplied by Fisher Scientific (Mumbai, India). These chemicals were used as such. Synthesis of Monoglyceride of Soy Oil (SMG). Soya oil (22.9 g, 0.025 mol) glycerol (4.6 g, 0.05 mol) and NaOH in the ratio of (1:2:0.01 mol) were taken in a three neck round-bottomed reaction vessel attached with a magnetic

stirrer, nitrogen inlet, and thermometer. The reaction was conducted for 1 h under constant stirring in the temperature range of 180-190 °C. The solubility test was used to check the progress of the synthesis of monoglyceride using one part of the reaction product and three parts of methanol (1:3 ratio) and FT-IR spectra were taken at regular intervals of time. The reaction was stopped on achieving the complete solubility of the reaction product (SMG). The FTIR analysis and solubility test confirmed the formation of SMG. The synthesized SMG was subjected to purification before reaction with Phthalic anhydride. The SMG was dissolved in diethyl ether, washed with 20 wt. % solution of sodium sulphate. During the monoglyceride formation the unreacted fatty acids left were removed from the ethereal solution by washing it with sodium bicarbonate 2-3 times and then washed with sodium sulphate solution to remove the soap formed as a result of transesterification. The ethereal solution was finally passed over anhydrous sodium sulphate to remove the moisture if any, while the trapped ether was finally evaporated under vacuum.26 Mechanism of transesterification reaction The synthesis of monoglyceride in presence of NaOH is a base catalyzed reaction.27 The initial step in the transesterification reaction is the reaction of the alcohol (monohydric, dihydric and tri hydric) with the base producing the alkoxide and the protonated form of catalyst. The attack of the alkoxide as a nucleophile at the carbonyl carbon of fatty acid chain 5 ACS Paragon Plus Environment


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leads to a tetrahedral intermediate from which the anion of diglyceride and alkyl ester are formed.28 The anion of the diglyceride deprotonates the catalyst, leading to the regeneration of the active species that reacts with the second alcohol molecule paving the way for another catalytic cycle (Scheme S1).29 Synthesis of Waterborne Soy Alkyd (WSA). The synthesis of waterborne soya alkyd was carried out using the reaction of SMG and phthalic anhydride in (1:0.5 ratio by weight) within three neck round bottom flask fitted with Dean-Stark condenser, nitrogen inlet tube, thermometer and magnetic stirrer. The reaction

setup was heated for 3 h under constant stirring at 190 oC. The acid value determination and

FT-IR spectra of the reaction product were taken at regular interval of time to monitor the progress of the reaction. The reaction was stopped on achieving of acid value 40. The triethylamine and 10 ml of water-ethanol blend in 70:30 ratio were added in reaction adduct to neutralize the free terminal carboxylic groups that resulted in the formation of waterborne soy alkyd. The synthesized resin was washed with 15% aqueous sodium chloride solution followed by distilled water for 2−3 times and dried over anhydrous sodium sulphate. Synthesis of PMF cured Waterborne Soya Alkyd (WSA-PMF-80) The WSA matrix was cured by taking a calculated amount of WSA and different weight percentages (60, 70 and 80 weight % of WSA matrix) of PMF in ethanol (a green solvent)

along with para-toluene sulfonic acid as catalyst. The reaction was performed for 2 h under continuous stirring in the pH range of 5-6 at 120 oC. The progress in the curing reaction checked regularly at the interval of 15 minutes till the constant intensity of the characteristic FTIR peak (–OH and C=O) are obtained. For the curing of WSA resin the different wt. % of PMF were taken in 100:60, 100:70 and 100:80 ratios, among these the WSA-PMF with 100:80 ratio displayed best physico-mechanical properties. Thus WSA-PMF-80 was 6 ACS Paragon Plus Environment

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processed with various wt. % RGO to prepared PMF cured waterborne soya alkyd nanocomposites (WSA-PMF-80-RGO-x where suffix x= 0.5, 1.0, 1.5 denotes the wt. % of RGO dispersed in the WSA matrix). Formulation of PMF cured RGO dispersed Waterborne Soya Alkyd nanocomposite (WSA-PMF-80-RGO-x) Coatings WSA-PMF-80-RGO nanocomposite was synthesized using the solution of WSA and PMF in ethanol was prepared in 100:80 ratio by weight, the p-TSA (p-Toluene sulfonic acid) catalyst was added to the reaction mixture, following the same protocol given in aforementioned unit. In the second step various wt. % of RGO nanoparticle (0.5, 1.0 and 1.5 wt. %) were dispersed in the ethanolic solution of WSA-PMF-80 for 5h under continuous stirring at 50 oC. Thereafter, the complete dispersion of RGO nanoparticles within the WSA-PMF-80 matrix was made with the help of an ultrasonicator (Model No 1.5L 50H, Frequency 40 kHz) through sonication for 2h at 55 oC, which led to the formulation of WSA-PMF-80-RGO-x nanocomposite. Preparation of RGO dispersed WSA-PMF-80 waterborne alkyd nanocomposite coatings (WSA-PMF-80-RGO-x) on CS The 80 wt. % solutions of WSA-PMF-80 and WSA-PMF-RGO-x in ethanol were prepared in a reaction vessel using PMF crosslinker as a curing agent. The solutions of these mixtures were used for the formulation of their coatings in presence of p-TSA as the catalyst. These reaction mixtures were continuously stirred for 2.5 h at 120-130

o

C followed by

ultrasonication for 1h at 50 oC. Before applying the coatings, the surface of carbon steel (CS) was finally polished using SiC papers of 300, 600 and 1000 grade. The CS were further cleaned with distilled water, rinsed ultrasonically with acetone for 10 min, and finally dried by a hot air blower. These solutions were applied by brush techniques on the cleaned surface



of CS specimens of standard sizes viz; 70 mm × 25 mm × 1 mm for physico-mechanical tests, 1 mm × 25 mm × 25 mm and 1 mm × 12 mm × 12 mm were used for electrochemical corrosion and morphological studies respectively. RESULTS AND DISCUSSIONS The synthesis of waterborne Soya alkyd was carried out through a single polycondensation approach between SMG and phthalic anhydride. The reaction of Soya oil and glycerol was carried out in presence of NaOH at 180 °C to synthesize the monoglyceride (scheme 1), which was further reacted with phthalic anhydride for 3 h at 190 °C to form WSA. The WSA matrix was modified with various wt. % of PMF resin as a curing agent to obtain a highly dense, crosslinked structure of WSA-PMF-80 through a reaction between butoxy groups of PMF and hydroxyl (–OH) functionality of WSA leading to elimination of isobutanol. Further, various wt. % of RGO (0.5, 1.0, and 1.5) nanoparticles were dispersed within the crosslinked sites of the s-triazine ring modified (WSA-PMF-80) matrices. The RGO nanoparticles filled in the interstitial and void spaces, leading to the formation of WSA-PMF-80-RGO-x nanocomposite coatings.


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Scheme 1. Reaction scheme for the synthesis of (a) SMG, (b) WSA and (c) WSA-PMF Physico-chemical properties The refractive index, specific gravity, acid value and viscosity of soy oil, SMG, WSA, and WSA-PMF-80-RGO-1.5 (Table S1) shows an increase in specific gravity, refractive index and acid value from soy oil to WSA-PMF-80-RGO-1.5. This can be assigned to the addition 9 ACS Paragon Plus Environment


of extra functionalities and an increase in the degree of crosslinking with the formation of a crosslinked structures of WSA-PMF-80-RGO-x. However, the decrease in acid value was observed from SMG to WSA, can be due to the consumption of carboxylic acid of phthalic anhydride (acid functionality). The solubility of SMG, WSA, to WSA-PMF was tested in both polar and non-polar solvents. It was observed that SMG, WSA and WSA-PMF exhibit solublity only in polar solvents. This may be due to presence of polar functionalities in their structure. The solubility of WSA and WSA-PMF can be attributed to the formation of COONH+(C2H5)3 salt. FT-IR The FT-IR spectroscopy was employed to confirm the formation of SMG, WSA, and WSAPMF (Figure 1). Presence of broad absorption peak at 3395 cm-1 corresponds to the -OH of SMG, while in case of WSA and WSA-PMF resin the same band is of lower intensity, which confirms the polycondensation reaction taking place between acidic functionalities of phthalic anhydride with hydroxyls of SMG. The band at 1731 cm-1 is due to C=O groups of ester linkages. While the absorption band at 2928 cm-1 and 2846 cm-1 corresponds to the asymmetric and symmetric stretching absorptions of methylene and methyl groups respectively. The presence of band at 1576 cm-1 confirms the transformation of terminal carboxylic groups into carboxylate anions. The absorption band at 1551 cm-1 correspond to the s-triazine ring of poly (melamine-co-formaldehyde) isobutylated introduced into WSA.30 While the absorption band at 1035cm−1 is due to the etherification reaction of WSA with PMF by the release of alcohol.


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Figure 1. FTIR spectra of SMG, WSA and WSA-PMF NMR spectra Figure 2a gives the 1H NMR spectra of the WSA. The peak at δ = 0.79 ppm and 1.21-2.24 ppm correspond to the terminal –CH3 groups and the –CH2 groups of a fatty acid chains. While same band appeared in case of the SMG Figure S1. The peak at δ = 4.17 ppm assigned to the proton of −CH2−O−C=O present in the backbone of the WSA. The protons of unsaturated carbon atoms (C=C) of fatty acid chains exhibits band at δ= 5.30 ppm. Moreover, the peak at δ = 7.30 ppm corresponds to aromatic protons of phthalic anhydride present in WSA. Figure 2b gives the proton NMR spectra of WSA-PMF. The signal at δ = 3.48 ppm corresponds to the −OCH2 groups which can be due to the etherification reaction of WSA with PMF, resulting in the formulation of WSA-PMF resin.



Figure 2c depicts the 13C NMR spectrum of WSA. Peak at 175 ppm corresponds to the C=O carbon atom of ester linkages. The peak at 132 ppm is due to the carbon atoms of phthalic anhydride. Band at 14−49 ppm range ascribed to –CH3 and methylene carbon atom of glyceride chain respectively, the peak at 125 ppm corresponds to CH=CH groups. The band at 165 ppm assigned to the carbon atoms of the s-triazine ring, which confirms the formation of WSA-PMF (Figure 2d).


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Figure. (2a and 2b) 1H NMR spectra of WSA and WSA-PMF, (2c-d) 13C NMR spectra of WSA and WSA-PMF.


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Morphological Studies. TEM analysis HRTEM (Figure 3a, 3b) was employed to assess the dispersion pattern of RGO within the WSA matrix. TEM images of RGO exhibit sheet-like appearance with thick stacking nanostructure ranging from a single layer to a few layers.31 While that of RGO dispersed WSA-PMF exhibit the homogenous dispersion of two-dimensional hexagonal sheet structure of several micrometer dimension with no sign of agglomeration (Figure 3b). The uniform dispersion of RGO within the matrix and chemical interactions between the polar functionalities like –OH and –COOH of RGO with the polar moieties of the matrix, resulted in the improvement of mechanical barrier and other required properties of anticorrosive coatings.32–34 The particle size (0-58 nm) of RGO was determined with the help of Dynamic Light Scattering (Figure S8). SEM Micrograph The SEM micrographs (Figure 3 c-e) of WSA-PMF-80, and nanocomposite (WSA-PMFRGO-0.5 and WSA-PMF-RGO-1.5) exhibit the surface morphology and structure of their coatings. The surface of WSA-PMF-80 coatings (Figure 3 c) exhibits a homogenous, uniform and compact morphology. However, in case of nanocomposite coatings the dispersion of RGO nanosheets in WSA matrix have maintained their 2D morphology and a stacking of RGO nanosheets present in dispersed form within the matrix forming a layered homogenous two phase surface.35,36 The strong interaction between the nanosheet fillers and polymer matrix through their polar functionalities resulted in the formation of more compact uniform layered structure can be seen in SEM micrographs (Figure 3 d-e).37



Figure 3. TEM micrographs of (a) RGO, (b) WSA-RGO and SEM micrographs of (c) WSAPMF-80 (d) WSA-PMF-80-RGO-0.5 (e) WSA-PMF-80-RGO-1.5 Raman Spectral Analysis The Raman spectra of WSA-PMF-80-RGO-0.5 and WSA-PMF-80-RGO-1.5 (Figure S2). The peak at 1833 cm-1 is assigned to the C=O group present in matrix and reduced graphene 16 ACS Paragon Plus Environment

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oxide. The Raman band in 1625-1680 cm-1 range corresponds to C=C, While the peak at 1400-1450 cm-1 is assigned to symmetric and asymmetric stretching of the −CH2 and −CH3. Moreover, the bands in between 800-950 cm-1 are assigned to C-O-C, which are due to the etherification reaction. Further the Raman band at 3412 cm-1 corresponds to the hydroxyl groups of the matrix. The presence of these characteristic peaks in Raman spectra (Figure S2) confirmed the formation of homogenous dispersed RGO alkyd matrix. 38,39 Thermal Studies The thermal stability of WSA-PMF-80 and WSA-PMF-RGO nanocomposites were measured using TGA to determine the effect of concentration of RGO and its dispersion in WSA under N2 atmosphere at 10°C/min (Figure 4). The onset of the first degradation in WSA-PMF-80 is found to be at 229 °C, which is due to the loss of adsorbed solvent molecules, while in case of nanocomposite WSA-PMF-80-RGO-1.5 the first degradation is observed at 302 °C. The 10% weight loss of WSA-PMF-80 occurred at 284 °C, while the same was recorded at 336 o

C in case of WSA-PMF-80-RGO-1.5. The weight loss occurred between 290-300 °C due to

the decomposition of ether and ester moieties of matrix and filler present in the coating. Further, these thermograms reveal a steep decomposition up to 521 oC. In case of plain coating (WSA-PMF-80) a higher weight loss observed with the char residue (CR) of 11.94 %. On the other hand the nanocomposite coatings exhibits higher thermal stability and lower weight loss with the higher char residue of 26.33 % at 521 oC. The higher CR of nanocomposite further confirms the strong physico-chemical interactions between RGO and matrix, which provide sufficient barrier effect that inhibits the transfer of metal ions across the coatings. The DSC thermograms of nanocomposite coatings (Figure S3) revealed that the increase in the glass transition temperature occur with the increases loading of RGO nanofillers. The increase in glass transition value in case of nanocomposite is due to the cementing effect of the 2D RGO nanofillers, which fill the voids and the interstitial spaces of



WSA-PMF-80 matrix and restricts the motion of the polymer chains, resulting in the higher value of Tg.

Figure 4. TGA thermograms of WSA-PMF-80 and WSA-PMF-80-RGO-1.5 Physico-mechanical properties Table S2 summarizes the value for various physico-mechanical parameters (impact test, scratch hardness, bend test, cross hatch test, MEK test, gloss at 45o and coating thickness) of WSA-PMF, WSA-PMF-80-RGO-0.5, WSA-PMF-80-RGO-1.0 and WSA-PMF-80-RGO-1.5. The extent of adhesion of nanocomposite coating was evaluated by subjecting the scratch hardness test of coatings. The values of scratch hardness show an increasing trend from WSA-PMF to WSA-PMF-RGO-1.5. The increasing trend in scratch hardness values suggest that the presence of pendent polar groups in filler and matrix of composite coatings developed strong physico-chemical interactions between composite coating and metal surface interface.40 The increased high cross link density of composite coatings after the loading of 18 ACS Paragon Plus Environment

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RGO and inclusion of PMF ring in WSA also help in enhancing the adhesion at coatingmetal interface. The cross hatch test of these coatings also reveal the strong adhesion between the coating and CS surface since no peeling of 1mm square of cross hatch are observed after tap removal (Figure S4). The optical images of the scratched surfaces of these coatings showed only fine scratch lines and no spalling of coatings. This suggests that coatings possess strong adhesion and good plasticity that induces higher stress bearing ability to the coatings (Figure S4).41 In addition all the nanocomposite coatings with different ratios of RGO are found to pass the impact resistance and 1/8 inch conical mandrel bend test due to the presence of flexible moieties like ether linkages and long aliphatic chains of soy oil.42 Further the values of MEK double rub test for all coated CS were found >450. The drying time (curing rate) of WSA-PMF-80, WSA-PMF-80-RGO-.5%, WSA-PMF-80-RGO-1%, and WSA-PMF-RGO-80-1.5% composites decreased drastically compared to that of pristine WSA-PMF, which can be due to the incorporation of RGO and introduction of s-triazine moiety in the backbone of WSA. Electrochemical Corrosion Studies The deterioration of metals by electrochemical means is largely governed by their contact with acidic, alkaline and saline medium. Among them the highly corrosive medium (alkaline medium) is predominantly encountered by industries like household, petrochemical, automobile, ships, aeronautics, fertilizer plants etc. Thus it has become necessary to understand the basic corrosion mechanism and corrosion protection performance by employing various sophisticated electrochemical analysis techniques (PDP, EIS and Salt spray). PDP Studies PDP studies were employed to perform the quantitative analysis of the corrosion protection performance of coated and uncoated CS in 3.5 wt. % NaCl for 9 days immersion time. The 19 ACS Paragon Plus Environment


Tafel plots were made as a function of immersion time for CS, WSA-PMF-80, WSA-PMF80-RGO-x, coatings depicted in Figure 5. The electrochemical corrosion measurements i.e. icorr (corrosion current density), Ecorr (corrosion potential), Rp (polarization resistance) were measured for these system under 3.5 wt. % NaCl solution. These studies exhibit higher corrosion protection performance of nanocomposite (WSA-PMF-80-RGO-x) coatings as per the values given in (Table 1). The point of intersection of the Tafel plots provides information about both Ecorr and icorr.43,44 A positive shift in corrosion potential (Ecorr) in case of PMF cured plain alkyd (WSA-PMF-80) and nanocomposite coatings (WSA-PMF-80-RGO-x) as compared to that of bare CS after the 216 immersion time was observed in the order of (i.e. 0.523, ˂-0.476, ˂-0.388, ˂-0.267, -0.191 V) similarly.45,46 While a negative shift in icorr of the same was observed as (8.529×10-5 ˂1.767×10-7 ˂ 1.473×10-7 ˂ 7.755×10-7 ˂ 7.736×10-9 Acm-2) respectively. The increase in Ecorr and the decrease in icorr values in case of composite coatings suggest that the nanacomposite coating provide remarkable protection to CS through physical barrier mechanism. The decrease in icorr values can be corroborated to the impermeable nature of RGO reinforced nanocomposite coatings act as physical barrier, which also induces the torturous path creating hindrance for the diffusion of corrosive moieties at coating-metal interface.47 Further the thermodymic feasibility for corrosion reaction can also be explained by Ecorr and icorr values.48 The higher value of Ecorr indicate superior barrier property ability of the formulated coatings. Ecorr values of bare CS, WSA-PMF-80, WSA-PMF-80-RGO-0.5, WSAPMF-80-RGO-1.0 and WSA-PMF-80-RGO-1.5 are compared (-0.523, ˂ -0.476, ˂-0.388, ˂0.267, -0.191V) predict the role of RGO dispersion in the corrosion protection performance of the RGO dispersed nanocomposite coatings, which revealed the higher positive shift in Ecorr of nanocomposite coatings. This also suggest promising corrosion protection behavior of nanocomposite coatings is well in agreement with that of Faraday’s law,49 which says that the 20 ACS Paragon Plus Environment

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rate of corrosion is inversely proportional to icorr values and directly to that of Ecorr values. Thus the lowest icorr and highest Ecorr values (Table 1) of composite coatings confirm the higher corrosion inhibition performance, this can be due to the homogenous dispersion of layered 2D-RGO nanofillers in WSA matrix. Besides the presence of higher cross-linked matrix which act as a strong physical barrier, inhibits the diffusion of aqueous corrosive ions and molecules at coating-metal interface,2 The order of corrosion protection activity of coated and uncoated CS systems based on PDP studies is as CS < WSA-PMF-80 < WSA-PMF-80RGO-0.5 < WSA-PMF-80-RGO-1.0 < WSA-PMF-80-RGO-1.5 (Table 1).



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Figure 5. Tafel plots of (a) bare CS (b) WBSA-PMF-80 (c) WBSA-PMF-80-RGO-0.5 (d) WBSA-PMF-80-RGO -1.0 and (e) WBSA-PMF-80-RGO-1.5 in saline medium (3.5 wt. % NaCl) Table 1. The PDP and EIS studies for bare CS, WSA-PMF-80 and WSA-PMF-80-RGO-x coated CS under saline medium (3.5 wt.% NaCl) System

Medium

Ecorr

Icorr

Rp

(V)

(Acm-2)

(Ω cm2)

Corrosion rate (mpy)

Rpore

Cc

(Ω cm2)

(farad)

CS

3.5 wt.% NaCl

-0.523

8.529×10-5

11666

1.099

2.3×103

1.7×10-6

WSA-PMF-80

do

-0.476

1.767×10-7

213490

0.009

7.37×104

4.7×10-7

WSA-PMF-80RGO-0.5

do

-0.388

1.473×10-7

3.03×105

0.002

1.54×105

2.23×10-8

WSA-PMF-80RGO -1

do

-0.267

7.755×10-7

4.37×105

0.001

2.81×105

1.37×10-10

WSA-PMF-80RGO -1.5

do

-0.191

7.736×10-9

1.05×107

0.0008

3.26×106

6.67×10-10

EIS Studies EIS is a non-destructive and highly sensitive technique used to analyze the electrochemical corrosion mechanism, to check the metal ion dissolution and coating resistance for the flow of corrosive ions at coating-metal interface.50 The EIS experiments were carried out on WSAPMF-80 and WSA-PMF-80-RGO-x coated and uncoated CS under 3.5 wt. % NaCl for 216 h to investigate their corrosion inhibition performance of these coatings and long term barrier effect of RGO on nanocomposite coatings using Nyquist and Bode plots. This study confirmed the promising corrosion inhibition activity of nanocomposite coatings. The results obtained by EIS studies are well in agreement with that of PDP. The small EIS sinusoidal


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wave was applied to the bare and coated CS. The impedance modulus |Z| for the same was calculated as a function of applied frequency (ω). The impedance frequency behaviour seems to be more helpful in the determination of corrosion protection mechanism of nanocomposite coatings. The nyquist plots (Figure 6) show the high-frequency and low frequency intercept. The high-frequency intercept is used for solution resistance while the sum of Rpore and Rs is presented by low frequency intercept.51 The best fitted equivalent circuit for coatings with single time constants in the given corrosive media (Figure S5) consist of Rs (solution resistance) Rpore (pore resistance) and Q (constant phase elements). The deviation in their values indicate a change from ideal capacitance value which can be due to the dispersion effect.41 The extent of deviation can be expressed by n. If the value of n=1, Q, gets converted into C that indicates the formation of double layer capacitance between solution and the electrode, when n approaches to zero, it starts to act as a resistor.52 However, it is worth to mention here that if n becomes closer to 1 (i.e. 0

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