Anticorrosion properties of epoxy-nanochitosan nanocomposite coating

Anticorrosion properties of epoxy-nanochitosan nanocomposite coating

Progress in Organic Coatings 113 (2017) 74–81 Contents lists available at ScienceDirect Progress in Organic Coatings journal homepage: www.elsevier...

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Progress in Organic Coatings 113 (2017) 74–81

Contents lists available at ScienceDirect

Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat

Anticorrosion properties of epoxy-nanochitosan nanocomposite coating ⁎

MARK

I.A. Wonnie Ma, Ammar Sh, Ramesh K , Vengadaesvaran B, Ramesh S, A.K. Arof Center for Ionics University of Malaya, Department of Physics, University of Malaya, Kuala Lumpur 50603, Malaysia

A R T I C L E I N F O

A B S T R A C T

Keywords: Nanochitosan Nanocomposite Corrosion Coating Electrochemical impedance spectroscopy

Nanochitosan (NCH) was prepared to act as a nanofiller reinforcing agent that had been incorporated in a protective coating for corrosion protection using high-molecular-weight chitosan. In this study, a series of epoxy resin based nanocomposite coatings were prepared with various of NCH loading ratio and applied on mild steel substrates under ambient conditions. The surface morphology and structural characterization of the NCH and nanocomposite coatings were carried out using Field Emission Scanning Electron Microscope (FESEM), X-ray diffraction (XRD), and Fourier transform infrared (FTIR) spectroscopy. Optical characterization of the nanocomposite specimens was examined by using UV–vis spectroscopy at a range of 300–800 nm in transmission mode. The thermal analysis of the nanocomposite coating was employed using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The corrosion protection performances of the nanocomposite coated mild steel substrates were comparatively studied using electrochemical impedance spectroscopy (EIS). The results showed that all the epoxy resin based nanocomposite coating containing NCH significantly increases the anticorrosion properties.

1. Introduction Chitosan is a naturally abundant organic polymer and a kind of polysaccharide composed of N-Acetylglucosamine and glucosamine units, with many special characteristics such as biocompatibility, biodegradability, nontoxicity, and chemical reaction. Because of these outstanding properties, chitosan can be selected as a useful and effective polymer matrix in composite materials. Due to the presence of hydroxyl and amino groups, the reaction of chitosan will be much more versatile than cellulose and probably beneficial for the homogeneous phases in the chemical crosslinking network [1,2]. Many studies had investigated the chemical reactions of chitosan in an aqueous solution of acids, N-acylation and N-alkylidinations [3–7]. Under an acidic condition, chitosan will be hydrolysed and dissolved in water due to the unstable reaction. This is because of the presence of glycosidic units in chitosan is hemiacetal which makes it unstable in acid. Dutta et al. [8] have discovered that N-acylation of chitosan with acyl halides introduced amido groups at the chitosan nitrogen while N-alkylidination with aldehydes produced N-alkyl chitosan upon hydrogenation resulting in decreased molecular weight and viscosity of chitosan. Accordingly, a decrease in molecular weight of chitosan and the water absorption can also be affected by the increase in the degree of degradation as well as causes an increase in crystallinity index and decomposition temperature [9,10]. In recent years, polymer nanocomposite has attracted research ⁎

attention due to its broad applications and the ability such as degradability, biocompatibility, and nontoxicity [11–17]. Several methods have been developed by researchers for the preparation and characterization of chitosan nanoparticles [18–21]. Huang et al. [22] have degraded chitosan into lower-weight-chitosan using different concentrations of phosphoric acid, and have prepared nanochitosan with concentrations of sodium tripolyphosphate, resulting in the lower potential surface of optimal crosslinking ratio of lower-weight-chitosan and sodium tripolyphosphate. Sudha. et al. [23] have used ionic gelation method for ionically crosslinked chitosan with sodium tripolyphosphate in dilute solution to synthesize nanoparticle. The ionotropic gelation method is an ionic interaction between the negatively charged groups of sodium tripolyphosphate (TPP) and the positively charged primary amino groups, −NH2 of chitosan. TPP is a polyanion which is substantially used as an ion crosslinking agent due to its multivalent properties. It helps the crosslinking process to avoid possible toxicity of chemical or biological reagent and prevent from any other undesirable effect [23–25]. NCH has more polar functions such as N and O which we believe that it can act as an organic inhibitor which is efficient for corrosion protection. For instance, El-Haddad, and Fekry and Mohamed have reported chitosan as an effective corrosion inhibitor for copper and mild steel substrates in acidic medium [26,27]. The inhibition efficiency increased with increasing the concentration of chitosan. Ahmed et al. [28] have studied the stability and corrosion inhibition properties of

Corresponding author. E-mail addresses: [email protected] (I.A. Wonnie Ma), [email protected] (A. Sh), [email protected] (R. K).

http://dx.doi.org/10.1016/j.porgcoat.2017.08.014 Received 3 July 2017; Received in revised form 19 August 2017; Accepted 26 August 2017 0300-9440/ © 2017 Elsevier B.V. All rights reserved.

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the chitosan-based coating on mild steel substrate under concentrated sulphuric acid medium, which has enhanced the corrosion protection efficiency in acidic medium. Besides, Zheludkevich et al. [29] have used a chitosan-based pre-layer coating to produce a corrosion protective coating with self-healing properties. The result of this study revealed that utilizing inhibitor-doped chitosan resulted in better corrosion protection performance. Moreover, the corrosion inhibiting performance of chitosan in various corrosive environments has been enhanced recently through metals nanoparticles compositing [30–32]. In this study, it has been aimed to investigate the effects of chitosan nanoparticles (NCH) incorporated in nanocomposite coatings on corrosion resistance for mild steel surface in which isophorone diamine is used as curing agent. Nanocomposite coatings have been developed by fixing epoxy/diamine ratio as 4:1 and varying NCH concentration. The performance properties including corrosion protection of all developed coating systems were examined via utilizing field emission scanning electron microscopy, Fourier transform infrared spectroscopy, UV–vis spectroscopy, x-ray diffraction, differential scanning calorimetry, thermogravimetric analysis and electrochemical impedance spectroscopy.

powder (0.5, 1.0, and 1.5 wt.%) to the solution of epoxy resin. The mixture was stirred mechanically by using glass rod for 2 min and was sonicated for 30 s at 60% amplitude for dispersion. The blended solution cannot be held longer in the sonication probe to prevent overheating, thus, stirring and sonicating were repeated 3–4 times alternately. The diamine curing agent was then added to the mixture with an epoxy/hardener ratio of 4:1 and close to vacuum before coated on mild steel by brushing method. The steel panels were abraded with sand blaster followed by acetone degreasing and kept in a desiccator for further use. The free films of the coating were cast on a Teflon® Petri dish and were evaporated at room temperature for several days. Coated samples were then dried and cured for 7 days at room temperature. Samples were labeled as ECH0 as a reference/control followed with ECH0.5, ECH1.0 and ECH1.5 correspond to the NCH loading ratio. Moreover, the thickness of the films are uniform and was measured by using coating thickness gauge elcometer® 456, with a target average thickness of 60 ± 5 μm for further characterization measurement.

2. Experimental

The surface morphologies of the chitosan, NCH, neat epoxy, and nanocomposite sample free films were examined by field-emission scanning electron microscopy (FESEM) (Quanta FEG 450, EDXOXFORD, Eindhoven, Netherland) after being coated with gold by using gold sputter coater (Bio-Rad, Watford, England). The crystalline nature of the chitosan, NCH, and the structural phases present in the nanocomposite samples were determined by X-ray diffraction (XRD) (EMPYREAN, PANalytical, EA Almelo, Netherland) at room temperature. Noting that the nanocomposite free film samples were cast from Teflon® while chitosan and NCH are in powder form. The spectra were recorded using Cu Kα radiation at 40 kV and 30 mA with scanning at 2θ = 10°–50°. Fourier Transform Infrared (FTIR) spectroscopy was employed to determine the functional groups of NCH, epoxy, curing agent and developed coatings. The analyses were performed at ambient atmosphere using an FTIR spectrometer (Perkin Elmer, FTIR-Spectrum 400) and all the spectra were recorded in the transmission mode at a resolution of 4 cm−1 in the range of 400–4000 cm−1. The test has been carried out on the powdered samples of raw materials as well the free films. Ultraviolet-visible (UV–vis) analysis of the nanocomposite films was performed using a UV-3101PC (Shimadzu, Kyoto, Japan) UV–vis spectrometer in transmission mode at a wavelength range of 300–800 nm on medium-speed scanning. The experiments were conducted on glass plate coated with our developed coating materials. The main purpose of UV–vis measurement was to evaluate the percentage changed of the film transparency due to the incorporation of NCH in the epoxy matrix.

2.3. Morphology and structure characterization

2.1. Materials Medium viscosity liquid epoxy resin produced from bisphenol A and epichlorohydrin with the unsaturation concentration about 5260–5420 mmol/kg and a molar mass 184–190 wt per equivalent, was supplied by ASA Chemicals (Malaysia) and used as received without any further modification. Isophorone diamine curing agent, 3Aminomethyl-3,5,5-trimethylcyclohexylamine, 99.7 wt.% solute content in water solution with the molar mass 170.30 g/mol and pH value 11.6 were supplied by ASA Chemical (Malaysia). Chitosan obtained from Aldrich Chemistry (USA) is around 75% deacetylated having a molecular weight in the range of 310,000–375,000 Da. It has been used without further purification. Sodium Tripolyphosphate (TPP) was purchased from Sigma-Aldrich (Selangor, Malaysia). 99% acetic acid and 85% phosphoric acid (H3PO4) were purchased from Shikmayu’s Pure Chemical and Friendemann Schmidt, respectively. Solvents of 99.6% absolute ethanol (EtOH) and 0.5% w/v sodium hydroxide (NaOH) were purchased from Fisher Scientific and Bendosen Laboratory Chemicals, respectively. Ultra-pure water (18 MΩ cm, 25 °C) was provided by INFRA Analytical Laboratory. 2.2. Methods 2.2.1. Preparation of nano chitosan (NCH) Chitosan was hydrolyzed by a method based on Huang et al. [22] and followed by the preparation of nanochitosan with acetic acid and TPP. A 5 g of chitosan was added into 100 g of 85% phosphoric acid. The solution was stirred up at 60 °C for 4 h and was precipitated in excess ethanol at 600 rpm for 24 h. The solution was then decanted to remove the unreacted phosphoric acid. Few drops of triethylamine were added to remove salt from hydrolysed chitosan. The mixture was centrifuged with ethanol for several times and then was washed with 1 l of ultrapure water at room temperature for 48 h. For the complete removal of phosphate ion, an aqueous NaOH was added until reached pH 10–11 range and then was further washed with ultra-pure water. The treated chitosan was dried overnight at 30 °C in vacuum. For the preparation of nanochitosan (NCH), 0.5 g of treated chitosan was dissolved in 0.2 wt.% acetic acid and stirred for 30 min. Then, 100 ml of the solution was added to 40 ml of TPP 0.2 g/l, stirred for 2 h at ambient temperature and then centrifuged at high speed. The isolated nanochitosan was rinsed with distilled water, dried and analyzed.

2.4. Thermal analysis Thermogravimetric analysis (TGA) is an analytical technique used to determine the thermal stability of a material and the proportion of its volatile components by monitoring the weight change that occurs when the sample is heated. The thermal stability of nanocomposite samples was analyzed by TGA tests conducted by using Mettler Toledo TGA Q500 runs dynamically 30 °C to 750 °C at a rate of heating equal to 20 °C/min under nitrogen gas with a flow rate of 60 ml/min. Also, TGA is a useful tool for investigating the thermal degradation of the blended polymers. Differential scanning calorimetry (DSC) analyses were performed to investigate the NCH reinforce filler influence on thermal properties of the coating system. DSC tests were carried out via using a TA-Q200 DSC instrument under nitrogen flow. Each sample was heated at a temperature range from −30 °C to 300 °C at a heating rate of 20 °C/min. The thermal behavior of polymers can be studied based on differential scanning calorimetry (DSC) analysis. This technique provides

2.2.2. Preparation of epoxy-nanochitosan nanocomposite coating The epoxy paint was formulated by direct addition of dried NCH 75

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information on heat flow and glass transition temperature (Tg) in a controlled atmosphere as a function of time. DSC measurements also provide information on physical changes such as melting, and decomposition of the blended polymers.

matrix, the crosslinking reaction between NCH and epoxide group will lead to a denser embedment of NCH on the epoxy surface. The similar observation has also been observed by Shi et al., where nanoparticles with higher surface area serve actively as better nanofiller due to its advantageous of size enable to fill the small holes and indentation of the epoxy matrix [26]. Moreover, nanofiller also serves as the bridge to both coating matrix and the metal surface. However, in Fig. 1(f) agglomeration of NCH was observed when the loading ratio of NCH increased to 1.5 wt.% in the epoxy matrix. This result clearly indicates that the optimum NCH loading is up to 1.5 wt.%. Nevertheless, many studies have been done on chitosan [26,27,17,38,14] as a compatible inhibitor for corrosion, however, using NCH alone as a reinforced agent for epoxy resin still have widely spaced to be investigated.

2.5. Corrosion measurement The corrosion protection properties of the epoxy coating with and without NCH were evaluated by electrochemical impedance spectroscopy (EIS). To accomplish this, classical three-electrode cell with 3.5 wt% NaCl solution was utilized to carry out these investigations throughout 30 days of immersion. In order to develop the electrochemical cell, a polyvinyl chloride (PVC) tube with a 2.0 cm inner diameter was vertically fixed to the surface of the coated substrate with the help of epoxy adhesive glue. Uncoated part of the metal plate was selected as the working electrode. Saturated calomel electrode (SCE) acted as the reference electrode and a platinum electrode as the counter electrode. All EIS tests were conducted after placing the electrochemical cell in a Faraday cage to reduce the noise during measurement. A Gamry PC14G300 potentiostat (Warminster, PA, USA) with a frequency range of 300 kHz to 10 mHz and an amplitude of the sinusoidal voltage at 10 mV was utilized for data measurement. All of the EIS data were analyzed by the software Gamry Echem Analyst, Version 6.03 (Warminster, PA, USA).

3.2. X-ray diffraction (XRD) spectra Fig. 2 depicts the X-ray diffraction patterns of chitosan, nanochitosan, neat epoxy resin, and nanocomposite epoxy-nanochitosan with different loading ratio of NCH. Chitosan gives a broad amorphous peak at 2θ = 20° and the similar broad amorphous peak was also observed in NCH diffractogram. The disappearance of the peak at 2θ = 20° indicating the increased of its amorphous nature after crosslinking with sodium tripolyphosphate (TPP). For neat epoxy resin, a broad amorphous peak was observed at between 2θ = 18°–23°. The intensity and the broadness of the broad amorphous peak for films that contains epoxy resin blending with NCH with different loading ratio increases with higher intensity. This phenomenon was observed due to the increase in the weight percentage of NCH in a neat epoxy resin matrix. This also confirmed that the intercalation of NCH with epoxy matrix.

3. Results and discussion 3.1. Field scanning electron microscopy (FESEM) The morphology of the chitosan, the prepared NCH, and the epoxynanochitosan nanocomposites were observed by FESEM, as shown in Fig. 1. Nanochitosan (NCH) was prepared using sodium tripolyphosphate (TPP) which carries five negative charges per molecule [5]. In this method, TPP solution is added to acidic chitosan solution in a dropwise manner with a constant stirring for homogeneity mixture. Apparently, spherical NCH particles were formed resulting from the association of the charge stability between TPP and CHI. Worth to note that chitosan contains cationic polysaccharide units which interact with negative charges of TPP [11]. Fig. 1(a) depicts the morphology of chitosan which exhibits irregular membrane-like shape attributes to an amorphous structure. Fig. 1(b) revealed a very homogeneous morphology and spherical shape of chitosan-TPP nanoparticles with the particle size range from 80 to 150 nm. The similar morphology of NCH also obtained by other researchers [33–35] and according to Luo et al. [36] the positive charges of CHI exhibit rough membrane structures due to its ability to form films by casting, while the negative charges of TPP form a smooth film and thus the homogeneity of CHI and TPP have shaped uniform spherical of NCH. Subsequently, with the presence of the glucosic units, NCH tends to agglomerate due to strong hydrogen bond attraction to the water during the dispersion process and swelling still can form in the trend of compactness as shown in Fig. 1(b). The addition of excess TPP to the NCH dispersion possibly achieves a clear aggregation of nanoparticles that tend to aggregate after all surface charges are negated by an excess polyanion. Jonassen et al. [37] have reported the structure of aggregation can be affected by the amount of TPP-to-chitosan ratio employed in the preparation method. However, the chitosan to TPP ratio of 5: 1 was confirmed as a stable and suitable ratio for performing the stability and compositional analysis. On the other hand, Fig. 1(c) demonstrates the structural morphology of the neat epoxy resin. Fig. 1(d) and (e) depicts the surface morphology of epoxy resin consists of NCH with 0.5 wt.% and 1.0 wt.% loading ratio respectively. Fig. 1(d)–(f) clearly demonstrate that when the loading ratio of NCH increases the surface roughness also will increase. This is because as the NCH loading ratio increases in the epoxy

3.3. Fourier transform infrared (FTIR) spectroscopy Fig. 3 shows the FTIR spectrum of chitosan, nanochitosan, neat epoxy resin, and nanocomposite epoxy-nanochitosan with different loading ratio of NCH.The absorption band at 826 cm−1 is attributed to CeOeC stretching and this band shows an increasing trend with increasing NCH loading ratio in the epoxy matrix. The absorbance peak at 915 cm−1 corresponds to the CeO deformation in the oxirane group of epoxy resin [39,40]. This band was only found in epoxy resin and disappeared in ECH0, ECH0.5, ECH1.0, and ECH1.5 showing that crosslinking has been taken place. The band at 1032 cm−1 correspond to the stretching vibration of CeOeC linkages in the ether linkage of the epoxy matrix and glucosamine ring of nanochitosan [41]. The absorption band at 1235 cm−1 corresponds to CH2 wagging. The intensity of this band is low for ECH0 and NCH whereas it increases when the NCH loading ratio increased in epoxy resin, as result confirming the intercalation of NCH in the epoxy matrix. The absorption band at 1500 cm−1 is attributed to CH2 stretching mode in which shows an increase in intensity when the loading ratio of NCH in the epoxy resin increases, firmly further confirming the crosslinking of NCH with epoxide group. Moreover, the absorption band at 1614 cm−1 correspond to C]C stretching mode due to the interaction of NCH and epoxy matrices. 3.4. Optical transmittance of epoxy-nanochitosan nanocomposite coating The UV–vis transmittance spectra of the nanocomposites specimens in the visible wavelength range of 300 − 800 nm are shown in Fig. 4. It is observed that ECH0 (15%, 400 nm) shows translucent characteristic by showing lowest degree of transparency. This is due to the molecular interaction within the epoxy/diamine networks have reduced the light scattering. It is also observed that the incorporation of NCH in the epoxy matrix improves the translucent characteristic of the neat epoxy matrix to become more transparent with the enhanced optical transmission. ECH0.5 records the highest light transmittance approximately 72% at 400 nm which indicates the most transparent specimen than the 76

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Fig. 1. FESEM image of a). chitosan (CHI) and b). nanochitosan (NCH) and epoxy-nanochitosan nanocomposite c). neat epoxy (ECH0), d). 0.5 wt.% (ECH0.5), e). 1.0 wt.% (ECH1.0) and f). 1.5 wt.% (ECH1.5) of NCH.

respectively. The reduction in the transparency percentage for the above mentioned specimens maybe probably due to the agglomeration of nanofillers and the particle size of filler.

other nanocomposite specimens. This observation could be due to crosslinking between the nanofiller and the epoxy matrix, suppressing light transmission through the epoxy-nanochitosan. Shimazaki et al. [42] have reported that the difference in refractive index between nanofiller and resin is small, photon scattering from the nanocomposite is suppressed, leading to the formation of a transparent organic nanocomposite. Moreover, good dispersion of NCH incorporated into epoxy resin can be considered corresponding with the number of NCH amount. The transparency of ECH nanocomposite specimens was reduced with additional of NCH loading ratio as observed in Fig. 4, ECH1.0 and ECH1.5 reduced to 60% and 50% of light transmittance,

3.5. Thermal analysis of epoxy-nanochitosan nanocomposite coating Thermogravimetric analysis (TGA) was used to investigate the effect of temperature change correspond with mass loss of nanocomposites coating systems. Specifically, TGA provides reliable information on polymer degradation with temperature, decomposition temperature and residual left after exposed under a nitrogen atmosphere. Fig. 5 77

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Fig. 5. TGA thermograms of neat epoxy (ECH0) and NCH nanocomposites coating. Fig. 2. X-ray diffraction patterns of chitosan, nanochitosan and epoxy-nanochitosan nanocomposite.

curves at the range after 200 °C corresponds to degradation and decomposition of the molecular networks epoxy-nanochitosan nanocomposite. As observed in Fig. 5, ECH0 show slight changes in the mass loss at a temperature range of 370–450 °C due to the degradation of epoxydiamine. According to the chemical data sheets, the boiling points of epoxy and isophorone diamine is > 260 °C and 247 °C, respectively. The weight loss above 450 °C is due to the carbonization of the residual samples [43]. Apart from that, TGA thermogram of epoxy-nanochitosan nanocomposite coating system shows the thermal degradation of nanocomposite coatings in a nitrogen atmosphere is a one-step reaction. These results suggest that the entire polymer network consists of epoxy matrix and NCH nanoparticles remained stable without aggregating. Table 1 illustrates initial degradation temperature (TIDT), the temperature at maximum weight loss (Tmax) and the weight% of remaining residual of neat epoxy and epoxy-nanochitosan nanocomposites. From the tabulated Table 1 it is observed that the residual weight (%) is found to be less than 9% due to the dissociation process of NCH molecules in the epoxy resin matrix at a higher temperature. The chemical crosslinking of neat epoxy and nanocomposite coating systems can be further analyzed using differential scanning calorimetric (DSC). In this study, glass transition (Tg) values were determined to evaluate the changes of coating nature with and without NCH. Fig. 6 shows the changed of Tg with different NCH loading ratio. Herein, the glass transition temperature of neat epoxy is about 135 °C and increased to 154 °C with a maximum increased of ∼20 °C was observed at NCH loading of 0.5 wt.%. This result attributes to the highest value of Tg as a function of NCH loading ratio which leads to enhancing in adhesion and optimum crosslinking [44] of epoxy-nanochitosan matrices. Hence, the increase of Tg value indicates the good incorporation of NCH in the epoxy matrix through their mobility interaction physically or chemically which is more pronounced on 0.5 wt.% of NCH loaded. The additional of NCH up to 1.0 wt.% shows a reduction of Tg value attribute to the aggregation of nanoparticle at the epoxy surface.

Fig. 3. FTIR of NCH and epoxy-nanochitosan nanocomposite coating system.

Fig. 4. UV–vis Spectra for nanocomposite specimens. Table 1 Initial Degradation Temperature (TIDT), Temperature at Maximum Weight-Loss Rate (Tmax), And Char Residue At 700 °C Determined From TGA Data.

shows the thermogram curve of neat epoxy resin and nanocomposites film up for heating temperature ranges from 30 °C to 750 °C. The TGA curves in Fig. 5 show that there are about 1% of mass loss at the first stage at 100 °C and below which correspond to the loss of moisture in the neat epoxy resin, and nanocomposite epoxy-nanochitosan with different loading ratio of NCH. The second stage of the TGA 78

Sample

ENC0

ENC0.5

ENC1.0

ENC1.5

TIDT (°C) Tmax (°C) Char residue at 700 °C (wt.%)

365.76 365.86 9.07

380.13 396.79 6.72

369.25 386.55 7.98

364.14 384.32 6.96

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Fig. 6. Glass transition (Tg) values of neat epoxy and nanocomposite determined from DSC thermogram data.

3.6. Electrochemical impedance spectroscopy (EIS) To study the influence of NCH nanoparticles on the corrosion protection performance of the EP, EIS measurements were conducted periodically after exposing the coated steel panels to the 3.5% NaCl solution. The obtained results were expressed graphically using Bode plots after 1 day and 30 days of immersion and fitted with two models of equivalent circuits. Figs. 7 and 8 present the Bode plots of the epoxy coating and the coatings reinforced with various amounts of NCH after 1 day and 30 days of immersion, respectively. Electrochemical impedance spectroscopy (EIS) is a powerful tool for Fig. 8. Representative Bode plots in terms of (a) impedance and (b) phase angle of the neat epoxy (ECH0) and nanocomposite coatings after 30 days of immersion.

studying anticorrosion performance and coating degradation on coated surface. When used in conjunction with proper equivalent circuit model, EIS can provide valuable information about the electrochemical behavior of the coating film such as the creation of coating defects and coating delamination during any time of the exposure period. Fig. 9 shows two equivalent circuit models, that is, Model A and Model B. Model A consist of Rs as solution resistance, the coating resistance (Rc), CPEc as the constant phase element of coating capacitance, the charge transfer resistance (Rct) and CPEdl as the constant phase element of double layer capacitance. An extended equivalent circuit of Model A socalled Model B consists of CPEdiff is the constant phase element of diffusion capacitance, and diffusion resistance (Rdiff). Technically, the decrease in coating resistance resulted during the

Fig. 7. Representative Bode plots in terms of (a) impedance and (b) phase angle of the neat epoxy (ECH0) and nanocomposite coatings after 1 day of immersion.

Fig. 9. The equivalent circuits used for fitting the EIS Bode plots.

79

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coating but also reduced the permeable of the coating matrix resulting in a decrement of porosity matrices. Secondly, the deferment of delamination through zigzagged the diffusion path by superior properties of the nanofiller against corrosive agents. As a result, leading to improved barrier performance of the epoxy coating. Last but not least, the inclusion of this spherical nanochitosan improved the of the adhesion of the neat epoxy coating to the underlying substrate and enhanced the physiochemical properties of the coating/metal interface. In comparison, the nanocomposite coating samples could maintain relatively high values of corrosion resistance at low-frequency range throughout the whole period of immersion time. However, it was interesting to notice that after 30 days of immersion time the Bode plots of all prepared nanocomposite coating systems tend to demonstrate better corrosion protection properties comparing to the performance of the same coating systems after 1 day of immersion time. This observation can be attributed due to the vital role of the nanoparticles that assist in filling up the diffusion pathways with the assist of the corrosion product that resulted from the corrosion reaction after some time of exposure. This mechanism could be explained as the electrolyte starts to penetrate within the coating film and at the time it reaches the substrate surface, corrosion reaction started and produced corrosion products. These corrosion products together with the incorporated nanoparticle tend to fill the newly produced channels of the electrolyte penetration and block these pathways against any more transformation of ions which in turn led to stop the corrosion reaction and observing an increment with the resistance values after 30 days of immersion.

Table 2 Fitted Parameter Values of the Equivalent Circuit Elements Along with the Utilised Model after 1 Days of Immersion. System

Rc (Ω cm2)

ENC0 ECH0.5 ENC1.0 ENC1.5

(1.11 (7.53 (1.27 (1.31

± ± ± ±

Rct (Ω cm2)

0.09) × 106 0.06) × 107 0.75) × 107 0.08) × 106

(2.56 (1.11 (1.91 (1.16

± ± ± ±

The equivalent circuit model used in fitting EIS data

0.34) × 107 0.02) × 108 0.26) × 107 0.06) × 107

A A A A

immersion period could be attributed to many reasons such as the diffusion of moisture, ions and corrosive agents through the coating film toward the coating/substrate interface [45]. The EIS results at the first day of immersion, as shown in Fig. 7, revealed the poor corrosion protection ability of all prepared coating systems as the penetration of the electrolyte occurred at the early stage of the immersion, first 24 h of exposure time. The electrochemical responses of all prepared coating systems were recorded in the form of Bode plots and the numerical fitting of all the resistance components of the equivalent circuit was carried out via utilizing model A of the equivalent circuit and the resistance values were tabulated in Table 2. However, this finding could be considered as clear evidence that the coating films have suffered already the initiation of the electrolytes towards the substrate surface but still not yet reached the coating/substrate interface. As the time elapsed, the electrochemical responses of all prepared coating systems were illustrated in Fig. 8. Furthermore, Table 3 highlights the values of all resistance component at this stage on immersion time. Neat epoxy coating (ECH0) demonstrated a significant degradation as its respective Bode plot was found to be fitted perfectly with Model B of the equivalent circuit and the coating resistance (Rc) value drops to approximately 104 Ω cm2. That, in turn, indicates the delamination of the coating upon the interface of coating/metal surface and resulted in blister formation and corrosion initiation [39]. Meanwhile, coating resistance of nanocomposite coating systems increased as shown in Fig. 8 and Table 3, which could be explained as the of the corrosive species were prohibited from reaching the coating/metal interface by the coating film. Also, ECH0.5 and ECH1.0 exhibit more than 1010 Ω cm2 of Rc attributed to the improvement of corrosion protection for the steel substrate probably due to the good dispersion of NCH embedding into the epoxy matrix Nevertheless, the higher loading of nanofiller loading ratio is influential to the anticorrosion performance of protection coating. The Bode plots of ECH1.5 (Fig. 8) after 30 days of measurement resulting in the lowest coating resistance among the nanocomposite coating systems. This is due to the aggregation of the epoxy-nanochitosan network chains which is contributed by the strong hydrogen bonding of nanochitosan leading to weakening the interconnection with epoxy matrix and leading to the creation of small pores at the matrices vicinity. As a result, allowing the diffusion of corrosive agents from the permeable pores to reach the coating/metal interface. The incorporation of nanofiller increased the Rc and the Rct values at shown in Table 3 which in turn contributed to the enhancement of corrosion protection of nanocomposite coating systems. Several advantageous can be listed through the obtained EIS studies. First, chitosan nanofiller not only improved the quality of the cured epoxy

4. Conclusion The obtained results clearly indicate that the use of biopolymers such as the nanochitosan can successfully enhance the anticorrosion performance of the mild steel surface. The incorporation of NCH into epoxy/diamine pre-polymer improved the corrosion resistance properties. These improvements also contribute to the thermophysical characteristics of the biopolymers used. As reported, the preparation of nanochitosan via ionotropic gelation has been done with TPP which then used to develop an organic coating which can sustain in a corrosive environment. The NCH in powder form acted as reinforcement nanofiller in epoxy/diamine pre-polymer solution. The effect of the NCH on the corrosion protection properties of the epoxy/diamine coatings enhanced not only the physical and chemical properties but also in thermal and optical properties. Moreover, the transparency of the film coating decreased with higher nanofiller loadings into the epoxy matrices due to aggregation of nanofiller. These results are worthy of further investigation in future studies. The TIDT and Tg increased as the nanofiller loading ratio increased in the epoxy matrix and this corresponds to the thermal stability of nanocomposite epoxy resin compared to neat epoxy resin. Hence, The NCH incorporated into the epoxy matrices helped to improve their properties physically and chemically. In this regard, epoxy-nanochitosan nanocomposite coatings enhanced the anticorrosion properties. The EIS studies were performed with immersion in a 3.5% NaCl solution up to 30 days to investigate the corrosion protection performance and the barrier properties of all the developed coating systems. The results of EIS studies revealed the ability of nanofiller to alter the

Table 3 Fitted Parameter Values of the Equivalent Circuit Elements Along with the Utilised Model after 1 Days of Immersion. System

Rc (Ω cm2)

ENC0 ECH0.5 ECH1.0 ENC1.5

(4.55 (2.22 (1.21 (3.36

± ± ± ±

0.18) × 104 0.16) × 1010 0.08) × 1010 0.02) × 109

Rct (Ω cm2) (8.69 (0.59 (1.19 (2.49

± ± ± ±

0.23) × 105 0.89) × 1010 0.13) × 1010 0.23) × 109

80

Rdiff (Ω cm2)

The equivalent circuit model used in fitting EIS data

(7.32 ± 0.98) × 105 – – –

B A A A

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