Effect of nanoparticles on the anticorrosion and

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Jul 8, 2009 - Homogeneous epoxy coatings containing nanoparticles of SiO2, Zn, Fe2O3 and ... properties, and strong adhesion/affinity to heterogeneous materials. ... density: 7.60 g/cm3; chemical composition: C 0.10–0.19%, Cr 0.40– ... Steel substrate preparation: A copper wire was electrically con- .... separated.

Surface & Coatings Technology 204 (2009) 237–245

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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t

Effect of nanoparticles on the anticorrosion and mechanical properties of epoxy coating Xianming Shi a,b,⁎, Tuan Anh Nguyen a, Zhiyong Suo c, Yajun Liu a, Recep Avci c a b c

Corrosion and Sustainable Infrastructure Laboratory, Western Transportation Institute, PO Box 174250, College of Engineering, Montana State University, Bozeman, MT 59717-4250, USA Civil Engineering Department, 205 Cobleigh Hall, Montana State University, Bozeman, MT 59717-2220, USA Imaging and Chemical Analysis Laboratory, Department of Physics, Montana State University, Bozeman, MT 59717, USA

a r t i c l e

i n f o

Article history: Received 10 January 2009 Accepted in revised form 30 June 2009 Available online 8 July 2009 Keywords: Nanoparticle Epoxy coating Corrosion resistance Nanoindentation SEM EIS AFM

a b s t r a c t Homogeneous epoxy coatings containing nanoparticles of SiO2, Zn, Fe2O3 and halloysite clay were successfully synthesized on steel substrates by room-temperature curing of a fully mixed epoxy slurry diluted by acetone. The surface morphology and mechanical properties of these coatings were characterized by scanning electron microscopy and atomic force microscopy, respectively. The effect of incorporating various nanoparticles on the corrosion resistance of epoxy-coated steel was investigated by potentiodynamic polarization and electrochemical impedance spectroscopy. The electrochemical monitoring of the coated steel over 28 days of immersion in both 0.3 wt.% and 3 wt.% NaCl solutions suggested the beneficial role of nanoparticles in significantly improving the corrosion resistance of the coated steel, with the Fe2O3 and halloysite clay nanoparticles being the best. The SiO2 nanoparticles were found to significantly improve the microstructure of the coating matrix and thus enhanced both the anticorrosive performance and Young's modulus of the epoxy coating. In addition to enhancing the coating barrier performance, at least another mechanism was at work to account for the role of the nanoparticles in improving the anticorrosive performance of these epoxy coatings. Published by Elsevier B.V.

1. Introduction Epoxy has been widely used as a coating material to protect the steel reinforcement in concrete structures [1–3], because of its outstanding processability, excellent chemical resistance, good electrical insulating properties, and strong adhesion/affinity to heterogeneous materials. Epoxy coatings generally reduce the corrosion of a metallic substrate subject to an electrolyte in two ways. First, they act as a physical barrier layer to control the ingress of deleterious species. Second, they can serve as a reservoir for corrosion inhibitors to aid the steel surface in resisting attack by aggressive species such as chloride anions. Nonetheless, the successful application of epoxy coatings is often hampered by their susceptibility to damage by surface abrasion and wear [4,5]. They also show poor resistance to the initiation and propagation of cracks [6]. Such processes introduce localized defects in the coating and impair their appearance and mechanical strength. The defects can also act as pathways accelerating the ingress of water, oxygen and aggressive species onto the metallic substrate, resulting in its localized corrosion. Furthermore, being hydrophilic in nature, ⁎ Corresponding author. Western Transportation Institute, Montana State University, P.O. Box 174250, Bozeman, MT 59717-4250, USA. Tel.: +1 406 994 6486; fax: +1 406 994 1697. E-mail address: [email protected] (X. Shi). 0257-8972/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.surfcoat.2009.06.048

epoxy coatings experience large volume shrinkage upon curing and can absorb water from surroundings [7,8]. The pores in the cured epoxy coating can assist in the migration of absorbed water and other species to the epoxy–metal interface, leading to the initiation of corrosion of the metallic substrate and to the delamination of the coating. The barrier performance of epoxy coatings can be enhanced by the incorporation of a second phase that is miscible with the epoxy polymer, by decreasing the porosity and zigzagging the diffusion path for deleterious species. For instance, inorganic filler particles at nanometer scale can be dispersed within the epoxy resin matrix to form an epoxy nanocomposite. The incorporation of nanoparticles into epoxy resins offers environmentally benign solutions to enhancing the integrity and durability of coatings, since the fine particles dispersed in coatings can fill cavities [9–11] and cause crack bridging, crack deflection and crack bowing [12]. Nanoparticles can also prevent epoxy disaggregation during curing, resulting in a more homogenous coating. Nanoparticles tend to occupy small hole defects formed from local shrinkage during curing of the epoxy resin and act as a bridge interconnecting more molecules. This results in a reduced total free volume as well as an increase in the cross-linking density [13,14]. In addition, epoxy coatings containing nanoparticles offer significant barrier properties for corrosion protection [15,16] and reduce the trend for the coating to blister or delaminate.

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This work examines the influence of nanoparticles, including SiO2, Zn, Fe2O3 and halloysite clay, on the surface morphology, anticorrosion behavior and Young's modulus of epoxy coatings. It is expected to shed more light on the fundamental mechanisms through which nanoparticles interact with the epoxy matrix and thus provide guidance for the design of high-performance epoxy coatings used for corrosion protection of steel.

Inc.) and sonication (Model 50 T, VWR, West Chester, PA) for 10 min. After that, the hardener-acetone solution was added to the mixture, followed again by stirring and sonication for 10 min. The steel substrate was dipped into the finally obtained mixture for one time and then kept in a dry place at room temperature for 7 days to allow full curing, which led to the formation of a uniform coating for the anticorrosion and surface indentation tests in this work.

2. Materials and methods

2.3. Morphological study of coatings

2.1. Materials

The surface morphology and thickness of the obtained nanocomposite coatings were studied using Field Emission Scanning Electron Microscopy (FESEM). The films were removed from the steel coupon surface, and then sputter-coated with a very thin Iridium layer (approximately 1–2 nm) to avoid the charging effect caused by the nonconductive nature of epoxy coatings and to get high resolution with this virtually grain-free coating material. The surface morphologies and the cross-section were analyzed by a Zeiss Supra 55VP PGT/ HKL system, which offers an ultra-high resolution at a relatively low voltage.

The epoxy resin and its hardener used in this research were obtained from Phoenix Resins Inc. (Cinnaminson, NJ, USA), commercially known as MAS epoxies-FLAGTM. The liquid epoxy resin was a blend of multifunctional low molecular weight diluents and the diglycedal ether of bis-phenol-A, whereas the hardener was based on adduction reaction chemistry of aliphatic amines. The weight ratio of the epoxy resin to the hardener was 2:1. Zn nanoparticles with a mean diameter of 35 nm, a specific surface area of 30-50 m2/g and faceted morphology were purchased from Accumet Materials Co. (Ossining, NY, USA). Spherical SiO2 and Fe2O3 nanoparticles, obtained from MTI corp. (Richmond, CA, USA), have a mean diameter of 15 and 20 nm and a specific surface area of 440 and N30 m2/g, respectively. Halloysite nanoclay (Al2Si2O5(OH)4·2H2O + SiO2) featuring a hollow cylindrical structure was purchased from Reade Advanced Materials (Reno, NV, USA). The steel coupons purchased from Metal Samples (Munford, AL, USA) were of Cor-ten B type (UNS number K11430; density: 7.60 g/cm3; chemical composition: C 0.10–0.19%, Cr 0.40– 0.65%, Cu 0.25–0.40%, Fe 97.0–98.2%, Mn 0.90–1.25%, P ≤ 0.04%, Si 0.15–0.30%, S ≤ 0.05%, V 0.02–0.10%) with a surface area of 2 cm2. Sodium chloride (NaCl) and acetone were purchased from Fisher Scientific (Pittsburgh, PA, USA). 2.2. Nanocomposite preparation Steel substrate preparation: A copper wire was electrically connected to one surface of each cylindrical steel coupon, and then this surface and all the other surfaces except the one exposed to electrolyte for corrosion testing were sealed with a thick bulk epoxy resin. After epoxy curing, the unsealed coupon surface was polished on silicon carbide (SiC) papers down to a grid size of 1000 with the aid of a metallographic grinding disc. After polishing, the sample surface was rinsed with tap water, sonicated in de-ionized water and then rinsed with acetone. Coating preparation: Epoxy composites are usually prepared by dispersing nanoparticles into the epoxy matrix either with a solvent or through a heating process. However, the latter process is prone to clustering or agglomeration of nanoparticles, resulting in poor dispersion. The use of solvent is beneficial for dispersal of nanoparticles in the resin, but the curing agents are usually added to the mixture after the solvent is removed by vacuum evaporation, which deteriorates the homogeneity of the nanocomposites after curing, especially for a high nanoparticle loading. To solve this problem, the curing agent can be added to the mixture before removing the solvent, which is expected to improve the dispersion of nanoparticles in the coating. In addition, the slurry can be directly applied on the surface of metallic substrates to form a uniform thin barrier coating. In this work, acetone was chosen as the solvent, since the analyses by Fourier transform infrared spectroscopy (FTIR) and FT-Raman [17] indicated that the sonication processing in acetone did not induce chemical change in the epoxy network. Before mixing, both resin and its hardener were diluted separately by acetone with a 1:1 weight ratio. Nanoparticles, which account for 1 wt.% of the total weight of resin and hardener, were added to the resin-acetone solution, followed by stirring at speeds up to 1550 rpm (Model 14-503, Fisher Scientific,

2.4. Electrochemical characterization of coatings Electrochemical measurements were conducted using a threeelectrode system. The epoxy-coated steel coupon served as the working electrode, while the counter electrode and the reference electrode used were a platinum grid and a saturated calomel electrode (SCE) respectively. The coatings evaluated in the electrochemical measurements had similar thickness as those used in the morphological study since they were prepared following the same procedures. The corrosive solutions tested included 0.3 wt.% and 3 wt.% aqueous NaCl solutions. Two methods were used to test the anticorrosive performance of these nanocomposite coatings: electrochemical impedance spectroscopy (EIS) and potentiodynamic weak polarization. Over the 28-day immersion of the coated steel, the EIS measurements were carried out periodically using a Gamry ECM8 Multiplexer. The steel was polarized at ±10 mV around its open circuit potential (OCP) by an alternating current (AC) signal with its frequency ranging from 10 kHz to 10 mHz (10 points per decade). In the potentiodynamic weak polarization tests, the steel was polarized around its corrosion potential (−30 mV to 30 mV/SCE vs. OCP by a direct current (DC) signal at a scan rate of 0.2 mV/s. Polarization resistance (Rp) is defined by the slope of the potentialcurrent density plot at the corrosion potential. Corrosion current (Icorr) is calculated from Icorr = B/Rp, assuming B = 26 mV for the steel corrosion. 2.5. Characterization of mechanical property The mechanical properties of nanocomposite coatings were measured using the nanoindentation method based on Atomic Force Microscopy (AFM) [18]. For each sample, 256 pairs of force vs. displacement curves were obtained over the surface from a 5 × 5 μm2 area by subdividing the area into 16 × 16 equal-sized pixels and acquiring one pair of curves from the center of each pixel. Each pair of curves includes a loading and an unloading curve. The tip velocities for these measurements varied between 0.3 and 0.5 μm/s. The stiffness of each sample was evaluated by fitting the corresponding loading curves to the Hertzian model which approximates the tip geometry as a hemisphere [16]. The curves are fitted in such a way that only the initial part of the indention (up to 1000 nN loading force) was selected for fitting so that the indentation depth is limited to be less than 100 nm, which is comparable to the tip radius. Such a small indentation depth relative to the coating thickness, usually more than 10 µm, is in consistence to the hemispherical tip geometry approximation, and also minimizes the possible interference from the

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substrate contribution. Fitting the indentation data to the Hertzian model in some cases does not give very good results, but the obtained Young's modulus still remains close to the values obtained by other techniques within 50% accuracy [19]. 3. Results and discussions 3.1. Effect of nanoparticles on the morphology of epoxy coating Typical top-view FESEM images of the control coating (plain epoxy, containing no nanoparticles) and the nanocomposite coatings are shown in Fig. 1a–h. The average thickness of the epoxy coatings was estimated from their cross-sectional view (as shown in Fig. 1f–h). We used freshly prepared epoxy-acetone solutions and followed identical sample preparation procedures to prepare all the coating samples (with temperature, epoxy concentration, and pulling speed of the steel coupon out of coating solution stayed the same). It was observed that the plain epoxy coating and the epoxy coatings modified by nanoparticles of Zn, Fe2O3 and halloysite clay had a similar thickness of approximately 40 μm, whereas the nano-SiO2 modified epoxy coating had a thickness of approximately 10 μm, according to the FESEM cross-sectional imaging of the coatings (Fig. 1f–h). This is mostly due to the significant reduction in the viscosity of the epoxy-acetone solution induced by the addition of SiO2 nanoparticles. Fig. 1a indicates that the cured plain epoxy coating has a relatively homogeneous morphology. The epoxy coating modified by SiO2 nanoparticles with a high specific surface area of 440 m2/g was observed to be much denser than the plain epoxy coating and showed no sign of nanoparticle agglomeration (Fig. 1c), partly attributable to its reduced internal stress inherent in the reduced coating thickness. The epoxy coating modified by Zn nanoparticles with a lower specific surface area (30–50 m2/g) was also denser than the plain epoxy coating, but had some agglomeration of nanoparticles (Fig. 1b). In the case of the epoxy coating containing Fe2O3 nanoparticles with a even lower specific surface area (≥30 m2/g), even more agglomeration of nanoparticles was observed along with aggravated microcracks (as shown in Fig. 1d). Based on the top-view and cross-sectional view of the coating, such microcracks were found to be localized near the top surface of the coating and no crack was observed across the entire thickness of the coating layer or near the coating-steel interface. We speculate that the nanoparticles with higher specific surface area not only served as better nano-fillers for the epoxy matrix, but also more actively participated in the epoxy-curing process (possibly acting as nuclei for the growth of cross-linking epoxy-amine networks). The small size of the nanoparticles is also advantageous since it enables their penetration into ultra-small holes, indentation and capillary areas both in the coating matrix itself and at the metallic substrate. In the presence of halloysite clay, the nanocomposite epoxy coating exhibited a textural structure with little agglomeration (see Fig. 1e), owing to the hollow cylindrical structure characteristic of the hoalloysite nanoparticles. 3.2. Effect of nanoparticles on the corrosion resistance of the coated steel The corrosion potential, corrosion current, polarization resistance and instantaneous corrosion rate were estimated from the measured potentiodynamic polarization curves of epoxy-coated steel. Fig. 2a and b shows the temporal evolution of instantaneous corrosion rate of the steel coated by various epoxy coatings, during the 28-day immersion in 0.3 wt.% and 3 wt.% NaCl solutions, respectively. The incorporation of a small amount of nanoparticles (1% by total weight of resin and hardener) into the epoxy coating significantly reduced the corrosion rate of the epoxy-coated steel in both electrolytes, while the beneficial effect of nanoparticles was more pronounced in 0.3 wt.% NaCl solution than in 3 wt.% NaCl solution.

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After 28 days of immersion, the nanoparticles reduced the corrosion rate of epoxy-coated steel by 638–2365 times in 0.3 wt.% NaCl solution and by 11–910 times in 3 wt.% NaCl solution. For the steel protected by the nanocomposite epoxy coatings, its corrosion rate increased by 20– 1263 times when the chloride concentration increased from 0.051 M (0.3 wt.% NaCl) to 0.51 M (3 wt.% NaCl). Fig. 3a and b shows the temporal evolution of polarization resistance (Rp) of the steel coated by various epoxy coatings, in 0.3 wt. % and 3 wt.% NaCl solutions, respectively. The incorporation of nanoparticles into the epoxy coating significantly enhanced the polarization resistance of the epoxy-coated steel in both electrolytes, while the beneficial effect of nanoparticles was more pronounced in 0.3 wt.% NaCl solution than in 3 wt.% NaCl solution. It should be noted that the measured Rp consisted of a component characteristic of the coating-electrolyte interface inside the coating (indicating coating porosity/compactness) and another component characteristic of the steel-electrolyte interface (indicating charge transfer resistance). For the nonconductive coatings investigated, the Rp of the coated steel in the relatively less corrosive 0.3 wt.% NaCl solution (Fig. 3a) can be used to estimate the relative void fractions within the coating matrix, assuming an inverse proportional relationship between the coating compactness and the measured Rp. On the other hand, the Rp of the coated steel in the relatively more corrosive 3 wt.% NaCl solution (Fig. 3b) can be used to estimate the relative corrosion resistance of the steel at the steel-electrolyte interface. Fig. 4a and b presents the Nyquist diagrams of the steel coated by various epoxy coatings, after 7-day immersion in 0.3 wt.% and 3 wt.% NaCl solutions, respectively. As shown in both figures, the Nyquist diagrams derived from the EIS measurements featured two capacitive loops, with the high-frequency loop (on the left) and the lowfrequency loop (on the right) attributed to the resistance and capacitance of the coating and of the steel-electrolyte interface respectively. These experimental EIS curves represent the electrochemical process with two time constants, which are well separated. Equivalent electric circuit is generally used to interpret the EIS data. Equivalent electric circuit with two time constants had been used in [20] for epoxy coated steel with nano Ag pigment in 3.5 wt.% NaCl. In this research, the fitting of all EIS data was performed using a simple equivalent electric circuit model (Fig. 5) with two time constants well separated. The obtained parameters are given in Tables 1 and 2, where R1 and C1 are the resistance and capacitance of coating characteristic of its pore network structure (the coating-electrolyte interface inside the coating), and R2 and C2 are the corrosion resistance of the steel and the double layer capacitance on the steel surface (the steelelectrolyte interface) respectively. If the coating was intact, an equivalent electric circuit with only one time constant (instead of two time constants as shown in Fig. 5) would be needed for analyzing the EIS data. R0 is the solution resistance between the reference electrode and the working electrode (nanocomposite coated steel), which depends not only on the resistivity of electrolyte (ionic concentration, type of ions, temperature and so on) but also on the geometry of the area in which current is carried. R0 is not a property of the coating. Therefore, it is not technically or theoretically important in the analysis of coating performance. The incorporation of a small amount of nanoparticles (1% by total weight of resin and hardener) into the epoxy coating greatly increased the coating resistance R1 by 6–1295 times and reduced the coating capacitance C1 by 1–112 times, indicating reduced coating porosity and improved barrier performance for corrosion protection of the steel substrate. It was also observed that over the time of immersion in the corrosive electrolytes R1 decreased and C1 increased, indicating the entry of electrolyte into the epoxy coatings, which is consistent with previous research [21,22]. The incorporation of nanoparticles into the epoxy coating also significantly increased the charge transfer resistance R2 by 3–186 times and reduced the double layer capacitance C2 by 1–484 times, indicating enhanced corrosion resistance of the steel at the steel-

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Fig. 1. SEM images of epoxy coatings. a): plain epoxy; b): with Zn nanoparticles; c): with SiO2 nanoparticles; d): with Fe2O3 nanoparticles; e): with halloysite clay; all of which were top view at magnification level of approximately 100,000 times; f): cross-sectional view of the plain epoxy coating indicating a thickness of 37.5 μm; g) cross-sectional view of the nano-Zn modified epoxy coating indicating a thickness of 36.6 μm; and h) cross-sectional view of the nano-SiO2 modified epoxy coating indicating a thickness of 9.7 μm.

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comparable to the documented values of 20–560 MPa for organic coatings obtained using similar methods [24]. The epoxy coating modified with SiO2 nanoparticles showed a significantly enhanced Young's modulus of ∼2.5 GPa, which coincided with the much smaller deformation hysteresis in Fig. 6c relative to other samples (Fig. 6a, b, d &e). A smaller increase, ∼30%, in Young's modulus was observed for the nano-Zn modified epoxy coating. As shown in Fig. 7, the modification with nanoparticles does not always enhance the stiffness of the epoxy coatings: the nanoclay and nano-Fe2O3 modified samples showed ∼30% and ∼25% decrease relative to the unmodified epoxy coating, respectively. The mechanical properties of nanocomposites, represented by Young's modulus in this paper, depend heavily on the integrity and internal properties of the coating surface, since under mechanical stress the micro-voids between the nanoparticles or between the polymer matrix and the nanoparticles may become the origin of cracks. For the epoxy coating modified by SiO2 nanoparticles with a high specific surface area of 440 m2/g, the distinct improvement in its stiffness may be ascribed to the following mechanisms. First, the nanoparticles tend to occupy holidays such as pinholes and voids in the thin-film coating and serve as the bridges in the interconnected matrix, causing a reduction of the total free volume and an enhancement of the cross-linking density of the cured epoxy [13,14]. As such, the cured nanocomposite coating has reduced chain segmental motions and improved stiffness. Second, the SiO2 nanoparticles may act to prevent epoxy disaggregation during curing and result in a more homogenous coating [13,14]. Finally, the SiO2 nanoparticles resulted in a reduced viscosity of the epoxy solution in acetone and thus led to a thinner coating layer on the steel, which diminished the internal stress of the cured coating.

Fig. 2. Temporal evolution of corrosion rate of epoxy-coated steel in (a) 0.3 wt.% NaCl solution, and (b) 3 wt.% NaCl solution, as a function of nanoparticles.

electrolyte interface [23]. It should be noted that the charge transfer resistances R2 obtained by fitting EIS were lower than the polarization resistances Rp measured by potentiodynamic polarization curves. A possible explanation is that surface area for steel-electrolyte interface, where steel was directly in contact with electrolyte, was lower than a whole surface area of steel coupon. 3.3. Effect of nanoparticles on the Young's modulus of epoxy coating As the indentation depth in AFM experiments is less than 100 nm and the coating thickness is above 10 μm, the contribution of steel substrate to the final results is negligible. Fig. 6 presents the forcedisplacement curves for the control coating (plain epoxy, containing no nanoparticles) and the nanocomposite coatings, which were obtained using the AFM-based nanoindentation technique. The upper (black) curve and the lower (red) curve in each diagram correspond to the loading and the unloading processes of the indentation, respectively. A hysteresis between the loading and unloading curves was observed for all the tested samples, indicating a plastic deformation of the epoxy coatings upon the indentation. The nearly linear slope of the unloading curves in Fig. 6 indicates that the tip indentation resulted in very little elastic deformation and thus the loading curve was fitted to obtain the mechanical properties of the coating. In this work we presume that all the coatings have identical mechanical properties over the whole thickness and the Young's modulus obtained by fitting only the initial part of the loading indentation process to the Hertzain model were presented in Fig. 7 as the representative value of the whole coatings. All the coatings, except the one modified with SiO2 nanoparticles, showed Young's modulus ranging from ∼ 60 to ∼350 MPa, which are

Fig. 3. Temporal evolution of polarization resistance of epoxy-coated steel in (a) 0.3 wt.% NaCl solution, and (b) 3 wt.% NaCl solution, as a function of nanoparticles.

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Fig. 4. EIS Nyquist diagrams for nanocomposite-epoxy-coated steel after 7 days (a) in 0.3 wt.% NaCl solution, and (b) in 3 wt.% NaCl solution. Their impedance components were plotted at full scale (left side) and low scale (right side).

The addition of other nanoparticles did not affect the Young's modulus of the epoxy coating as prominent as the SiO2 nanoparticles. We tentatively correlate this result with the lower specific surface areas of Zn and Fe2O3 nanoparticles (30–50 m2/g and ≥ 30 m2/g, respectively) relative to SiO2. In the case of Fe2O3, the agglomeration of nanoparticles in the cured nanocomposite coating led to aggravated microcracks on the coating surface, as evidenced by FESEM imaging (Fig. 1d), which further weakened the coating and resulted in the smallest Young's modulus among all the tested samples. This highlights the importance of good dispersion of nanoparticles in delivering desirable mechanical properties of nanocomposite epoxy coatings. Unlike the other nanoparticles used this work that possess an approximately spherical geometry, the halloysite clay nanoparticles feature a nanotubular structure with an averages diameter of 30 nm and

lengths between 0.5 and 10 µm [25]. It has been reported that the addition of montmorillonite nanoclay which has a layered structure ((Na,Ca)0.33(Al,Mg)2(Si4O10)(OH)2·nH2O), enhanced the modulus of epoxy coatings [26–28]. However, our results indicated that the nanoclay modified epoxy coating exhibited a loss in the Young's modulus compared with the plain epoxy coating. Although currently it remains unclear whether the weakening of the nanoclay-epoxy composite in our experiments is associated with the hollow cylindrical structure of the halloysite clay nanoparticles, such a difference suggests that the nanostructures of clay materials may play a significant role in the mechanical properties of epoxy composite coatings. 3.4. Role of nanoparticles in enhancing the anticorrosive performance of epoxy coating According to the EIS data after 7-day immersion in both 0.3 wt.% and 3 wt.% aqueous NaCl solutions (Tables 1 and 2), the incorporation

Table 1 Parameters of the equivalent circuits after 7 days in 0.3 wt.% NaCl solutions.

Fig. 5. Schematic drawing of the equivalent circuit. R0 is associated with the electrolyte resistance. R1 and C1 are the resistance and capacitance of coating, respectively. C2 is the capacitance of the double layer. R2 is the charge transfer resistance at the steelelectrolyte interface.

Coating samples

R0 (Ω∙cm2) R1 (Ω∙cm2) R2 (Ω∙cm2) C1 (F∙cm−2) C2 (F∙cm−2)

Plain epoxy Epoxy + nano-Zn Epoxy + nano-SiO2 Epoxy + nanoclay Epoxy + nano-Fe2O3

373.7 362.1 128.8 238.7 498.1

1.44E + 05 4.26E + 07 1.24E + 08 3.70E + 06 4.96E + 07

7.29E + 05 8.68E + 07 9.07E + 07 4.47E + 07 1.36E + 08

1.11E−09 5.78E−10 4.85E−10 5.89E−10 6.64E−10

3.58E−06 1.08E−07 1.25E−08 1.50E−07 7.37E−09

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Table 2 Parameters of the equivalent circuits after 7 days in 3 wt.% NaCl solution. Coating samples

R0 (Ω∙cm2) R1 (Ω∙cm2) R2 (Ω∙cm2) C1 (F∙cm− 2) C2 (F∙cm− 2)

Plain epoxy Epoxy + nano-Zn Epoxy + nano-SiO2 Epoxy + nanoclay Epoxy+nano-Fe2O3

200.1 366.4 765.9 254.2 200.3

352.5 4.24E + 04 2.51E + 03 3.60E + 04 4.57E + 05

2.42E + 04 9.63E + 04 7.28E + 05 3.11E + 06 9.03E + 05

6.64E−09 3.35E−09 5.85E−11 1.78E−09 7.61E−10

1.64E−05 7.06E−06 7.39E−07 6.86E−06 4.31E−07

of nanoparticles increased the coating resistance R1 and the charge transfer resistance R2 while reducing the coating capacitance C1 and the double layer capacitance C2. This suggests that at least two possible mechanisms contributed to the enhanced corrosion protection of nanocomposite epoxy coating. First, nanoparticles improved the quality of the cured epoxy coating, reduced the porosity of the coating matrix, and zigzagged the diffusion path available by deleterious species, leading to improved barrier performance of the epoxy coating. Second, nanoparticles improved the adherence of the

Fig. 7. Young's modulus of epoxy coatings doped with different nanoparticles. For each sample, the data were averaged from four nanoindentation curves randomly selected from 256 curves obtained.

Fig. 6. Curves of force vs. displacement of a) plain epoxy; epoxy doped b) with Zn nanoparticles; c) with SiO2 nanoparticles; d) with Fe2O3 nanoparticles; e) with halloysite clay nanoparticles.

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cured epoxy coating to the underlying substrate and altered the physiochemical properties of the coating-steel interface, the specific pathway of which is dependent on the type of nanoparticles as described below. Table 3 provides the open circuit potential (OCP) data of epoxycoated steel in salt solutions, as a function of nanoparticle type, NaCl concentration and exposure duration. It should be cautioned that the OCP reading of the coated steel was contributed both by the corrosion potential of the steel itself and by the electrical resistance of the coating layer. Incorporating 1 wt.% of nano-Zn particles with a mean diameter of 35 nm into the epoxy coating reduced the corrosion rate of the epoxycoated steel by 739 and 11 times, respectively, after 28-day immersion in 0.3 wt.% and 3 wt.% aqueous NaCl solutions. In the less corrosive electrolyte (0.3 wt.% NaCl), the OCP of the steel protected by the nanoZn modified epoxy coating was 0.052 VSCE and −0.229 VSCE at 24 h and 672-hours respectively, which were significantly higher than that of the plain-epoxy-coated steel (− 0.457 VSCE and −0.584 respectively, as shown in Table 3). We speculate that due to the low dosage of Zn in epoxy coating, nano-Zn nanoparticles were quickly consumed to form ZnO, which worked as both an anodic-type inhibitor and a good nano-filler to significantly inhibited corrosion of bare steel. This noble shift in the OCP induced by the Zn nanoparticles decreased over time in 0.3 wt.% NaCl and diminished in the more corrosive electrolyte (3 wt.% NaCl), likely due to the cathodic dissolution of ZnO. It merits further investigation to see whether a higher dosage of ZnO nanoparticles can provide better long-term anti-corrosive performance for the epoxy coating. Zinc-rich primers have been extensively and successfully used for corrosion protection in heavy-duty environments, often involving much higher loading of zinc powder with mm and μm grain sizes [29,30]. Therefore, we also compared the performance of an epoxy coating with ordinary Zn particles (b150 μm, 1% by total weight of resin and hardener) and that of the nano-Zn modified epoxy coating. In the first 24 h of exposure to 3 wt.% NaCl solution, the steel protected by the nano-Zn-modified epoxy coating showed a significantly lower corrosion rate (and similar OCP) than the steel protected by the ordinary-Zn-modified epoxy coating. Yet over time such nano-effect diminished and by the 9th day of immersion the two Zn-modified coatings offered comparable corrosion protection for the underlying steel. It merits further investigation whether increasing the loading of nano-Zn particles in the epoxy coating would significantly improve its long-term anticorrosive performance. Incorporating 1 wt.% of nano-Fe2O3 particles with a mean diameter of 20 nm into the epoxy coating reduced the corrosion rate of the epoxy-coated steel by 2365 and 910 times, respectively, after 28-day immersion in 0.3 wt.% and 3 wt.% aqueous NaCl solutions. In addition to enhancing the coating barrier performance, Fe2O3 nanoparticles served as anodic-type corrosion inhibitor to significantly reduce the corrosion of the epoxy-coated steel in both electrolytes. In the less corrosive electrolyte (0.3 wt.% NaCl), the OCP of the steel protected by

Table 3 Open circuit potential of epoxy-coated steel in salt solutions, as a function of nanoparticle type, NaCl concentration and exposure duration. Coating on the steel

Plain epoxy Epoxy + nano-Zn Epoxy + nano-SiO2 Epoxy + nanoclay Epoxy + nano-Fe2O3

OCP of coated steel in 0.3 wt.% NaCl (V, vs. SCE)

OCP of coated steel in 3 wt.% NaCl (V, vs. SCE)

24-hour

672-hour

24-hour

672-hour

− 0.457 0.052 − 0.597 − 0.170 0.060

− 0.584 − 0.229 − 0.548 − 0.237 − 0.049

− 0.603 − 0.598 − 0.551 − 0.583 − 0.437

− 0.396a − 0.613 − 0.597 − 0.507 − 0.336

a The OCP of this plain-epoxy-coated steel generally increased over time with significant fluctuations and this high OCP reading may be resulted from the formation of corrosion product on steel surface.

the nano-Fe2O3 modified epoxy coating was 0.060 VSCE and − 0.049 VSCE at 24 h and 672-hours respectively, which were significantly higher than that of the plain-epoxy-coated steel (−0.457 VSCE and −0.584 respectively, as shown in Table 3). This noble shift in the OCP induced by the Fe2O3 nanoparticles decreased over time in 0.3 wt.% NaCl and was much less apparent in the more corrosive electrolyte (3 wt.% NaCl). It should be noted that the nano-Fe2O3 particles did not disperse very well in the epoxy coating (as shown in Fig. 1d) and better anti-corrosive performance of the nanocomposite coating can be expected once better dispersion of the nanoparticles is achieved. It is interesting to note that Fe2O3 nanoparticles were previously reported to alter the magnetic properties of epoxy resin as well [31]. Traditionally, Fe2O3 pigment with mm and μm grain sizes was used in protective paint as corrosion inhibitor, and its protective mechanism was considered physical rather than chemical. By mechanically strengthening the paint film, reducing moisture permeation through the film, and screening out destructive UV radiation, Fe2O3 pigments was excellent auxiliary pigments in metal primers and top coats [30]. With improved dispersion in the epoxy matrix, the nano-Fe2O3 particles are expected to provide long-term anticorrosive performance of the epoxy primers. Incorporating 1 wt.% of nano-SiO2 particles with a mean diameter of 15 nm into the epoxy coating reduced the corrosion rate of the epoxy-coated steel by 983 and 32 times, respectively, after 28-day immersion in 0.3 wt.% and 3 wt.% aqueous NaCl solutions. This is consistent with a previous study [32], where the incorporation of 1 wt.% nano-SiO2 particles improved the anticorrosive performance of the epoxy coating on 2024-T3 aluminum alloy. The OCP data in Table 3 suggest that in both electrolytes the corrosion protection offered by the SiO2 nanoparticles had more to do with the improvement in the coating pore network than any modification of the coating-steel interface. The SiO2 nanoparticles tend to occupy holidays in the thinfilm coating and serve to bridge more molecules in the interconnected matrix, leading to increased cross-linking density of the cured epoxy as well as improved corrosion protection for the steel substrate. Incorporating 1 wt.% of halloysite clay nanoparticles into the epoxy coating reduced the corrosion rate of the epoxy-coated steel by 638 and 614 times, respectively, after 28-day immersion in 0.3 wt.% and 3 wt.% aqueous NaCl solutions. In the less corrosive electrolyte (0.3 wt.% NaCl), the OCP of the steel protected by the halloysite clay nanoparticles modified epoxy coating was −0.170 VSCE and −0.237 VSCE at 24 h and 672-hours respectively, which were significantly higher than that of the plain-epoxy-coated steel (−0.457 VSCE and −0.584 respectively, as shown in Table 3). The experimental results suggest that halloysite nanoclay caused the noble shift in the OCP of the epoxy-coated steel and significantly inhibited its corrosion, in the case of 0.3 wt.% NaCl. In the more aggressive electrolyte (3% NaCl), even though the halloysite clay still provided strong corrosion protection for the steel substrate, the noble shift in the OCP was no longer evident, likely due to the cathodic dissolution of aluminum oxide or hydroxide. Our findings are consistent with previous research in terms of beneficial effect of nanoclay on the anticorrosive performance of coatings. By incorporating 2–6 wt.% montmorillonite nanoclay into the polyurethane coating, Chen-Yang et al. [33] demonstrated that the corrosion rate of coated stainless steel in 5 wt.% NaCl solution was reduced by about 10–30 times. Yeh et al. [34] reported that the corrosion rate of coated steel in 5 wt.% NaCl solution could be reduced by about 10 times when incorporating 1 wt.% montmorillonite nanoclay into the siloxane-modified epoxy coating. In our study, the halloysite nanoclays were found to drastically enhance the polarization resistance (Rp) of the coated steel, which was 15 MΩ∙cm2 after 1 h in 3 wt.% NaCl. This value is much higher than Rp of the steel coated by polyurethane/montmorillonite nanocomposite coatings (300 KΩ∙cm2 after 30 min in 5 wt.% NaCl, [33], and Rp of the stainless steel coated by siloxane-modified epoxy/montmorillonite nanocomposite coatings (400–600 KΩ∙cm2 after 5 h in 5 wt.% NaCl, [34]).

X. Shi et al. / Surface & Coatings Technology 204 (2009) 237–245

245

4. Conclusions

Acknowledgements

Nanoparticles of Zn, SiO2, Fe2O3 and halloysite clay were successfully dispersed into epoxy resin matrix at a concentration of 1% by the total weight of epoxy resin and its hardener. The electrochemical monitoring of the coated steel over 28 days of immersion in both 0.3 wt.% and 3 wt.% NaCl solutions suggested the beneficial role of nanoparticles in significantly improving the corrosion resistance of the coated steel, with the Fe2O3 and halloysite clay nanoparticles being the best. The potentiodynamic weak polarization test revealed that after 28 days of immersion the nanoparticles reduced the corrosion rate of epoxy-coated steel by 638–2365 times in 0.3 wt.% NaCl solution and by 11–910 times in 3 wt.% NaCl solution. The EIS measurements indicated that the incorporation of nanoparticles increased the coating resistance R1 and the charge transfer resistance R2 while reducing the coating capacitance C1 and the double layer capacitance C2. In addition to enhancing the coating barrier performance, at least another mechanism was at work to account for the role of the nanoparticles in improving the anticorrosive performance of these epoxy coatings. The epoxy coating modified with SiO2 nanoparticles showed a significantly enhanced Young's modulus of ∼ 2.5 GPa. Nonetheless, the modification with nanoparticles does not always enhance the stiffness of the epoxy coatings. A ∼30% increase in Young's modulus was observed for the nano-Zn modified epoxy coating, whereas the nanoclay and nano-Fe2O3 modified samples showed ∼30% and ∼25% decrease relative to the unmodified epoxy coating, respectively. For future research, it would be important to investigate the combined use of Fe2O3, SiO2 and halloysite clay nanoparticles and examine the potential synergy between them to allow simultaneous improvements in both anticorrosion and mechanical properties of epoxy coating. A nano-Fe2O3 and/or nanoclay modified epoxy primer with a nano-SiO2 pigmented topcoat would be an interesting system to explore. It is also necessary to investigate ways to improve dispersion of the nanoparticles in the coating matrix and make the transition from solvent-based to waterborne epoxy. More in-depth study of the effect of nanoparticles on epoxy-curing dynamics and kinetics would further advance the knowledge base of such nanocomopsite coating systems. To enable long-term anticorrosive performance of the nanocomposite coatings, one should also investigate the potential application of the nanoparticles as reservoirs for the storage and prolonged lease of corrosion inhibitors. For instance, corrosion inhibitors can be absorbed by the nanoparticles and released only during contact with moisture, thus endowing the coating with self-healing properties [16].

This work was supported by the Research and Innovative Technology Administration under U.S. Department of Transportation through the University Transportation Center research grant. We would like to specially thank the ICAL facility at Montana State University for their help with the use of AFM and FESEM.

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