Thin Nacre-Biomimetic Coating with Super-Anticorrosion Performance

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Thin Nacre Biomimetic Coating with Super-Anticorrosion Performance Yan Zhang, Jinwei Tian, Jing Zhong, and Xianming Shi ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b05183 • Publication Date (Web): 06 Sep 2018 Downloaded from http://pubs.acs.org on September 6, 2018

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Thin Nacre Biomimetic Coating with Super-Anticorrosion Performance Yan ZhangⅠ,Ⅰ, Jingwei TianⅠ, Jing ZhongⅠ,* and Xianming ShiⅠ,Ⅰ, * Ⅰ

Laboratory of Corrosion Science & Electrochemical Engineering, Civil and Environmental Engineering, Washington State University, United States Ⅰ School of Civil Engineering, Harbin Institute of Technology, China Abstract

The rigorous organic/inorganic laminated structure of nacre is developed by millions of years of biological evolution against various external impacts including mechanical loadings and chemical attacks. Nacre-biomimetic materials have been recognized as an effective strategy to achieve high strength and toughness simultaneously. Yet, the understanding of nacre-like structure from the perspective of corrosion protection is still very limited. This work investigates the anticorrosion performance of nacre-biomimetic GO/epoxy (NBGE) coatings with alternating layers. Potentiodynamic polarization measurements indicated that the corrosion rate of steel protected by the NBGE coating with 5 layers of GO and 6 layers of epoxy (5NBGE) and a total thickness of 17 um was 20 times slower than that of steel under the pure epoxy coating twice as thick, in 3.5 wt% NaCl solution. Electrochemical impedance spectroscopy measurements revealed the importance and functions of the GO layers in NBGE coatings. The 5NBGE coating exhibited better performance than carbon-based nanoparticle/epoxy mixed coatings. The superior anticorrosion performance of the NB5G6E coating was supported by photographic observations, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and chloride diffusion measurements. The strong cross-linking layer-by-layer structure of NBGE coatings was proved by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and X-ray diffraction analyses. The anticorrosion mechanism of the NBGE coatings was interpreted by the mitigation of chemical reactions occurring at the steel/coating interface due to the restricted intrusion of O2, H2O, and Cl- through the reduced pores/defects by the intercalated GO layers in the coatings, as shown in the above graphics.

Keywords: biomimetic · graphene oxide · epoxy · corrosion · chloride diffusion Nacre in mollusk shells, featuring a hierarchical structure characteristic of 95 vol.% of oriented aragonite (mainly CaCO3) platelets embedded in 5 vol.% of an organic matrix,1–3 exhibits outstanding mechanical properties.3–5 Nacre with the brick-and-mortar layered structure can 1

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develop high fracture toughness three orders of magnitude higher than that of CaCO3. With the aragonite providing strength and the organic matrix dissipating strain, nacre has inherently high-level resistance to initiation and growth of cracks.2,4 Clearly, such high mechanical performance must intimately relate to the exceptional structure of nacre. Indeed, nacre-biomimetic materials (NBMMs) have attracted great attention in recent decades. A variety of approaches have been adopted to synthesize the nacre-like structure, including layer-by-layer (LBL) deposition, electrophoretic deposition, mechanical approach, freeze-casting, vacuum-assisted filtration assembly, and evaporation-induced self-assembly.1,4 For instance, Kotov’s group systematically investigated the effectiveness of LBL method for the synthesis of NBMMs with montmorillonite clay, polyvinyl acetate, or functionalized carbon nanotubes, all of which exhibited high strength and toughness.6 Ritchie developed free-drying method to yield NBMMs with outstanding mechanical properties in large scale and with high efficiency.2 Studart et al. recently took the advantage of magnetic particles, and controlled the local particle alignment and thus regulated the mechanical properties of NBMMs via external magnetic field.7 The risks of metallic corrosion are well known. In the U.S., this issue was estimated to bear a direct cost of 276 billion dollars per year.8 Globally, a study initiated by the National Association of Corrosion Engineers (NACE) International reported that the estimated global cost of corrosion in 2013 was 2.5 trillion dollars.9 Metallic corrosion not only causes economic loss but also results in potential safety hazard and environmental consequences. Corrosion can negatively impact the life-cycle performance, reliability, and serviceability of motor vehicles as well as infrastructures such as steel bridges and steel-reinforced concrete structures.10–12 Corrosion can also cause accumulation of heavy metals in soil and water,13 thus posing a health risk to humans and animals. An effective way against corrosion is preparing a coating on the metal’s surface. At present, metallic coatings, polymeric coatings, organic-inorganic hydride coatings are frequently used coatings and they have shown varying levels of anticorrosion performance depending on the protection mechanism and the intended metal/environment combination.14 Recent years have seen increased interest in anticorrosion coatings prepared by mixing nanoparticles with polymers.15–18 Graphene and graphene oxide (GO) nanocomposite coatings have exhibited very promising anticorrosion performance, mainly due to the physical barrier effects of graphene nano-platelets. Prasai et al. reported the first experimental study to confirm that the oxidation of copper can be well protected by single layer of graphene grown by chemical vapor deposition (CVD), which was strongly supported by various simulations.19 Interestingly, Schriver el al. showed that the CVD graphene coating could accelerate localized corrosion in the long term, because of the existence of defects.20 Since the defects of coating (local anodes) can dominate its anticorrosion performance, preventing the formation of defects is crucial in the design and fabrication of anticorrosion coatings. In order to make the coating more robust and suitable for practical applications, nanocomposites instead of single layer of graphene coating is preferred. Studies have shown that the addition of graphene or GO fillers significantly benefit the anticorrosion behavior of various polymer coatings.16,21,22 Yet, the desired anticorrosion properties of graphene nanocomposites can be realized only on the condition of uniform dispersion of graphene, because its agglomeration can potentially become the diffusion channel for corrosive agent and accelerate localized corrosion of the underlying metal. Although surface functionalization of graphene can partially improve the dispersion and compatibility of graphene (or its derivatives) with polymer matrix, it often increases the cost and complexity of the synthesis process. Therefore, a general and robust strategy that can fully leverage the physical barrier function of graphene without inducing defects in the coating is 2


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highly desirable. Nacre acts as armor for protecting mollusks from various external impacts, including physical forces (e.g., attack by predators) and chemical attacks induced by complex environmental conditions. For example, shellfishes live in the sea with 3.5 wt% NaCl, and numerous organisms are well protected by their shells, due to the outstanding mechanical properties and anti-permeation performance of nacre. However, the anticorrosion properties of NBMMs remain poorly understood. In this work, nacre-biomimetic GO/epoxy (NBGE) coatings (e.g., Figure 1a) were prepared using spin coating alternatively of epoxy and graphene oxide layers. The anticorrosion performance of such NBGE coatings was systematically examined by potentiodynamic polarization (PDP) measurements and electrochemical impedance spectroscopy measurements, photographic/microscopic observations, energy-dispersive X-ray spectroscopy (EDS) analysis, and chloride diffusion measurements through the coatings. It was discovered that the anticorrosion performance of NBGE coatings improved with the increase in the number of GO layers and epoxy layers. Electrochemical measurements showed that the inhibition efficiency of the NBGE coating with five layers of GO and 6 layers of epoxy (5NBGE) was far greater than that of carbon-based nanoparticle/epoxy mixed coatings. This work aims to shed light on the improved anticorrosion behavior of NBGE coatings due to the laminated organic/inorganic structure.

Figure 1. a) Fabricating procedure of NBGE coatings on steel; b) SEM image on the fracture profile of the 5NBGE coating; c) Diagrammatic sketch of the penetrating pathways of O2 and Cl- in NBGE coatings; and d) Diagrammatic sketch of the penetrating pathways of O2 and Cl- in 6E coatings

RESULTS and DISCUSSION Fracture profile of NBGE coatings. Spin coating is extensively employed in industry, and it provides very cost-effective method to achieve uniform coating. As can be seen in Figure 1b, alternative layers of GO film and epoxy are distinguishable, with the thickness of approximately 2.5 um and 250 nm, respectively. The GO film consists of thousands of layers GO nanosheets stacked in parallel, featuring naturally formed tortuous two-dimensional channels that have been proven 3


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impermeable even to helium.23 Due to the presence of oxygen-containing functional groups (hydroxyl, epoxide, carboxyl and carbonyl),24 the GO layers have good molecular and interfacial compatibility with the water-borne epoxy layers.25,26 The epoxy curing agent, which consists of small molecules, can in principle penetrate deep into the pre-coated GO film, and crosslink the whole GO film layer. This can reinforce not only the GO film itself, but also the interfacial bond between GO film and epoxy layer.27,28 More importantly, because this coating is formed sequentially, agglomeration of GO nanosheets can be minimized. One challenge of GO/epoxy composite coatings is that, it is difficult to uniformly disperse the GO nano-platelets in the epoxy matrix. This aggregation of GO in epoxy matrix has been reported to induce worse mechanical and anticorrosion performances.29,30 Based on the SEM image in Figure 1b, the GO nano-platelets well overlapped with each other and formed the thin compact GO films between the epoxy layers in the NBGE coating, minimizing the aggregation of GO. The obtained coating has similar structure pattern with nacre, and considering the superior sealer performance of GO films, we expect this NBGE coating free of agglomeration defects to exhibit satisfying anticorrosion properties in aqueous solutions (e.g., 3.5 wt% NaCl). Open circuit potential of coated steel. Figure 2 presents the OCP data of coated steel in 3.5 wt% NaCl solution as a function of exposure time and coating type. The OCP of the coated steel is defined by both the electrical resistance of the coating and the corrosion potential of the steel.15 -0.40

1NBGE 4NBGE

-0.45 OCP (V)

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2NBGE 5NBGE

3NBGE 6E

-0.50 -0.55 -0.60 -0.65 -0.70 1

2

3

4 5 Immersion time (days)

6

7

8

Figure 2. OCP of the steels protected by different coatings after 1-day, 3-day, 7-day and 8-day immersion in 3.5 wt% NaCl solution As shown in Figure 2, all the OCP of the steel protected by the NBGE coatings was higher than that of the 6E coated steel at any time during the 8-day immersion in 3.5 wt% NaCl solution. During the first day of immersion, the OCP of the nacre-biomimetic coatings generally shifted to more noble values with the increase in the number of GO/epoxy layers. Such differences in the initial OCP reading are mainly attributable to the fact that the less Ohmic drop between the steel and the reference electrode, the more negative the OCP reading. In other words, the neat epoxy coating (6E) layer likely featured the lowest electrical resistance whereas the 5NBGE featured the highest one, after one day of immersion in 3.5 wt% NaCl solution. Aside from the influence by the coating’s electrical resistance, the OCP of coated steel can imply the corrosion susceptibility of the carbon steel in the NaCl solution, and a higher OCP typically implies a lower corrosion susceptibility.31 The OCP of the steel protected by the 5NBGE coating was -0.479 VSCE, -0.492 VSCE, -0.439 VSCE and -0.421 VSCE after 1-day, 3-day, 7-day and 8-day immersion respectively, which was the least negative value among all the coated steels and 4


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significantly higher than that of the 6E coated steel (-0.674 VSCE, -0.677 VSCE, -0.661 VSCE and -0.657 VSCE, correspondingly). Most of the OCP of the coated steel decreased with the increase in immersion time (see Figure 2), as a result of the deterioration in both the electrical resistance of the coating layer itself (due to ingress of electrolyte) and the protective oxide layer on the steel surface. The 5NBGE or 6E coated steel exhibited an exceptional behavior, as their OCP steadily increased after three days of immersion, likely due to formation of a dense and less conductive rust layer on the steel surface. Besides, the OCP values of the steel coated with 1NBGE and 2NBGE were higher than that with 3NBGE and 4NBGE at the middle and late periods of the 8-day immersion time, which might be caused by more corrosion products accumulated between steel surface and coating. Potentiodynamic polarization of coated steel. Figure 3 presents the PDP curves of the coated steels after 8-day immersion in 3.5 wt% NaCl solution, from which the corrosion potential (Ecorr), polarization resistance (Rp) and corrosion current density (IPcorr) were determined. The results with their standard deviations (SD) are summarized in Table 1. -4 -5 -6 log(i/A)

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1NBGE 2NBGE 3NBGE 4NBGE 5NBGE 6E

-7 -8 -9 -10 -0.2

-0.4

-0.6 Potential (V)

-0.8

-1

Figure 3. Potentiodynamic polarization curves of the coated steels after 8-day immersion in 3.5 wt% NaCl solution

Table 1. Polarization curve-derived parameters (8-day immersion) of the coated steels in 3.5 wt% NaCl solution, along with the corrosion current density derived from EIS (7-day immersion, IEcorr) 5


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Coating

Ecorr (VSCE)

Rp (kΩ·cm2)

IPcorr (µA/cm2)

SD of IPcorr

IEcorr (µA/cm2)

SD of IEcorr

1NBGE

-0.679

59

0.85

0.28

8.23

3.73

2NBGE

-0.662

48

0.80

0.13

3.32

4.62

3NBGE

-0.744

114

0.43

0.13

0.37

0.24

4NBGE

-0.717

154

0.32

0.03

0.37

0.13

5NBGE

-0.472

1575

0.06

0.02

0.04

0.02

6E

-0.701

20

1.30

0.65

5.51

2.74

Different from OCP, the measurement of Ecorr is free of influence by the coating’s own electrical resistance and thus more accurately reflect the corrosion condition of the coated steel. The Ecorr of the steels protected by the 6E coating and the 1NBGE ~ 5NBGE coatings were -0.701 VSCE, -0.679 VSCE, -0.662 VSCE, -0.744 VSCE, -0.717 VSCE and -0.472 VSCE, respectively. The Ecorr of the steel protected by the 5NBGE coating was significantly less negative than that of the 6E coating and other NBGE coatings, suggesting that the 5NBGE coating provided the best corrosion protection for the underlying steel. The outstanding anticorrosion performance of the 5NBGE coating was confirmed by the Rp data of coated steels. The steel coated by 5NBGE exhibited the greatest Rp, one or two orders of magnitude higher than the steel under other coatings. The Rp generally increased with the increase in the number of GO/epoxy layers in the NBGE coatings. Compared with the 6E coated steel (20 kΩ•cm2), the Rp of the 5NBGE coated steel increased by 7775% due to intercalation of the additional 5 GO layers between the epoxy layers. Moreover, there was a substantial shift of the Rp from the 4NBGE coating (154 kΩ•cm2) to the 5NBGE coating (1575 kΩ•cm2). Even the Rp of 1NBGE coated steel was about three times greater than that of the 6E coating, confirming that the (0.25 µm) thin GO layer played the most crucial role in enhancing the corrosion resistance of the epoxy coating. Note that the Rp data obtained from the PDP curves of coated steel consisted of two components: the pore resistance of the interface between coating and electrolyte, and the charge transfer resistance of the interface between steel and electrolyte.15 While impossible to differentiate their contribution to Rp, these two resistances will be discussed in the results of electrochemical impedance spectroscopy (EIS) measurements. The IPcorr of the coated steels in 3.5 wt% NaCl solution was subsequently calculated from IPcorr = B/Rp.32 The Stern–Geary constant (B) can be calculated from B = βa*βc/(2.3*(βa+βc)), in where βa and βc were the anodic and cathodic Beta Tafel slopes derived from the PDP curves respectively. Compared with the 6E coated steel, the IPcorr of the 5NBGE coated steel decreased by 95%, i.e., from 1.30 µA/cm2 to 0.06 µA/cm2. The IPcorr generally decreased with the increase in the number of GO/epoxy layers in the NBGE coatings. Moreover, there was a substantial shift of the IPcorr from the 4NBGE coating (0.32 µA/cm2) to the 5NBGE coating (0.06 µA/cm2). Even the IPcorr of 1NBGE coated steel was only 65% that of the 6E coated steel. EIS of coated steel. The Nyquist plots were derived from the EIS measurements which applied the alternating current signal with the frequency from 105 Hz to 0.01 Hz; and they are employed to interpret the role of NBGE coatings on the corrosion protection of the steel. Figure 4S1, Figure 4S2 6


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and Figure 4 present the Nyquist plots of the steel protected by different coatings when exposed to the 3.5 wt% NaCl solution for 1 day, 3 days and 7 days, respectively. All of these feature the characteristics of two capacitive loops: the high frequency loop (Figures 4S1a, 4S2a and 4a) corresponds to the resistance and capacitance of the coating itself; and the low frequency loop (Figures 4S1b, 4S2b and 4b) corresponds to the charge transfer resistance and double layer capacitance of the steel-electrolyte interface.15,17 Generally, a wider diameter of the semicircle in the Nyquist plot suggests better corrosion resistance of a coating, whereas a smaller impedance and semicircle diameter suggest higher corrosion rate.17,33 Regardless of the immersion time, the Nyquist plot of the steel under the 6E coating had the smallest semicircle diameter, compared with the NBGE coatings. Furthermore, the semicircle diameter of the steel under the NBGE coatings became larger with the increase in the number of GO/epoxy layers; and this increase was most prominent from the 4NBGE coating to the 5NBGE coating. These variations agree very well with the results of the PDP measurements. In addition, the semicircle diameter of the Nyquist plot for every coating gradually decreased as the time elapsed, indicating the ingress of the electrolyte into the coatings.15 All the Nyquist plots display the electrochemical process with two-time constants, which can be also clearly observed from the Bode plots as shown in Figures 4S1c, 4S1d, 4S2c, 4S2d, 4c and 4d (c for impedance as a function of frequency and d for phase angle as a function of frequency).

Figure 4. EIS Nyquist and Bode plots for the steel protected by various NBGE coatings or the 6E coating after 7-day immersion in 3.5 wt% NaCl solution. (a) full scale of Nyquist with fitted curves, (b) low scale of Nyquist with fitted curves, (c) impedance of Bode and (d) phase angle of Bode. EECs used to fit the Nyquist plots for different coatings: EEC-a was used for 1NBGE and 2NBGE; and EEC-c was used for 3NBGE, 4NBGE, 5NBGE and 6E Figures 4c and 4d show the impedance and phase angle of the Bode plots for the coated steels immersed in the 3.5 wt% NaCl solution for 7 days, as a function of frequency, respectively. The 7


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Bode plots also display two-time constants, one of which at high frequency represents the dielectric behavior of the coatings and another at low frequency reflects the characteristic of the steel exposed to the pores of the coatings.34 The impedance at low frequency is an indicative of the corrosion resistance of the coatings, and the higher impedance suggests the higher corrosion resistance.35 The impedance at high frequency is the measure of solution resistance.36 On the other hand, the ideal phase angle for a surface coating is -90º, suggesting the coating is a pure non-conductive capacitor. The negative value is used due to that phase angle is the ratio of positive real impedance to negative imaginary impedance. The phase angle approaching to -90º means the highly capacitive behavior of the coating.37 Obviously, the 5NBGE coating displayed the highly capacitive response and the best anticorrosion performance, the 6E coating displayed the worst performance and the other NBGE coatings exhibited the intermediate performance which increased with the number of GO/epoxy layers. This behavior is consistent with that revealed by the PDP curves and the Nyquist plots. In the range of low frequencies, the impedance of the 5NBGE coated steel was more than two orders of magnitude higher than that of the 6E coated steel, indicating that the GO layers between the epoxy layers in the 5NBGE coating effectively prevented the aggressive ions from contacting with the steel surface therefore protected the steel from corrosion. As shown in Figures 4S1d, 4S2d and 4d, the position and the shape of the curve for the 5NBGE coating almost remain unchanged after 7-day exposure time, suggesting the great stability of the coating. To interpret the behaviors of different interfaces of the coated steels and demonstrate the importance of the GO layers between the epoxy layers against steel corrosion, three equivalent electric circuit (EEC) models (as shown in Figures S2a, b and c) with two-time constants were used to fit the EIS data and obtain the EIS parameters (as shown in Figures 5a, b, c and d). The selection of the EEC models was based on the two-time constants and the goodness of fits. In the EEC models, Rs is the solution resistance, which has negligible contribution to the circuit relative to other parameters. Q1 and Q2 are constant phase elements, which represent the capacitance (C1) of the coating and the double layer capacitance (C2) of the steel/electrolyte interface, respectively, in our case.15 R1 and R2 are the resistance of the coating to current flow through the defects/pores and the corrosion resistance (charge transfer resistance) of the steel, respectively.15 W (Table S1) is the Warburg element accounting for linear semi-infinite diffusion,19 which is not discussed in our case. C1 reflects the water absorption as well as the thickness of the coating, and R1 reflects the coating porosity correlated to the defects, pores and air voids in the coating.38 Both are closely related to the dielectric characteristics of the coating. Regardless of immersion time, for the NBGE coatings, the coating capacitance C1 generally decreased and the coating resistance R1 generally increased with the number of GO/epoxy layers (Figure 5). Meanwhile, nearly all the C1 values were lower and all the R1 values were higher than that of the 6E coating. When measured after 1-day, 3-day and 7-day immersion, the C1 of the 6E coating was reduced by 99.98%, 99.49% and 96.43% while its R1 was increased by 255434%, 138826% and 31016%, respectively, by intercalation of the five layers of GO between the epoxy layers. Relative to the 6E coating, the lower C1 of NBGE coatings was likely ascribed to the reduced water/Na+/Cl- absorption of the coating on the steel, due to the barrier effects of the GO layers. The intrusion of water/Na+/Cl- into the coating results in increase of the dielectric constant, thus increasing the C1. The amount of the absorbed water/Na+/Clin the coating can be interpreted by the dielectric constant using Equation 1:39 ϵ=

×

×

(1)

where ϵ is the dielectric constant of the coating, C1 is capacitance of the coating, d is the thickness 8


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of the coating, ϵ0 is the permittivity of vacuum and A is the area of the coating. The absorbed water/Na+/Cl- into the 5NBGE coating was 0.02%, 0.55% and 3.86% of that into the 6E coating with 1-day, 3-day and 7-day immersion time respectively, determined by Equation 2:39 A% =

× 100% (2)

where ϵ5NBGE is the dielectric constant of the 5NBGE coating and ϵ6E is that of the 6E coating. This provide direct evidence that most of the pores/defects in the 6E coating were mitigated by the intercalated five GO layers. Therefore, the number of pathways to ionic flow in the coating was significantly reduced. The improved coating resistance suggests improved performance of the coating as a physical barrier against the ingress of the electrolyte, likely due to the reduction of microcracks, pores and air voids in the coating. Conceptually, a small amount of GO nanoplatelets entered the pores of epoxy layers during the spin coating process. Over the time of immersion, the C1 increased for the 3NBGE, 4NBGE and 5NBGE coatings and the R1 decreased for all the coatings, indicating the entry of the electrolyte into the coatings.15,18,35,40,41 The observed decreases in the C1 value for the 1NBGE, 2NBGE and 6E coatings and the increases in the R1 value for the 6E coating may be attributable to the formation of the non-conductive corrosion product film at the interface between the coating and the steel.39

Figure 5. EIS-derived parameters for the NBGE coatings (#1-#5) and the 6E coating (#6) after 1-day, 3-day and 7-day immersion in 3.5 wt% NaCl solution The charge transfer resistance R2 and the double layer capacitance C2 for the coatings exhibit similar trends as R1 and C1. Compared with the steel under the 6E coating, the C2 under the 5BGGE coating was reduced by 99.99%, 99.99% and 99.28% while its R2 increased by 88542%, 50747% and 15301%, when measured after 1-day, 3-day and 7-day immersion, respectively. This is ascribed to the presence of the GO layers in the 5NBGE coating. Double layer capacitance is associated with corrosion activity.42 The significant decrease in C2 for the 5NBGE coating indicates the much lower 9


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corrosion activity of the steel, which resulted from the superior physical barrier performance of the GO layers. The significant increase in R2 confirmed that the corrosion resistance of the steel at the steel/electrolyte interface was improved.15 It is noteworthy that even the R1 and R2 of the 1NBGE coating were significantly greater than that of the 6E coating, implying the importance of the GO layers in the NBGE coatings. For all the coated steel systems, the variations of R1 and R2 were consistent with the Rp variations from the PDP measurements. To investigate the efficiency of the GO layers between the epoxy layers in the 5NBGE coating for the corrosion protection of the steel, the inhibition efficiency (IE) was calculated using Equation 3:43 IE% =

CR

!" #$

− CR

CR

!" #$

#$

× 100% (3)

Where CRcwithout GO and CRcwith GO is the corrosion rate (CR) of the coating without and with GO modification, respectively. The CR of the coated steels can be determined from CR = Icorr*K*EW/(ρ*A),19 in where K was the corrosion rate constant, EW was equivalent weight for the steel, ρ was the density for the steel and A was the sample area. All the steel samples had same values for K, EW, ρ and A items. The IEcorr of the coated steels in 3.5 wt% NaCl solution was calculated from IEcorr = B/R2, assuming B was 26 mV for steel corrosion.44 R2 from EIS reflects the “true” corrosion resistance (charge transfer resistance) of a coating whereas Rp from PDP measurements represents the total resistance (pore resistance and charge transfer resistance) of a coated sample. The calculation of the Icorr from Rp could be misleading if a coating has very high pore resistance but relatively low R2. In addition, EIS measurements feature the use of a smaller polarization (disturbance) of the steel than PDP measurements, and thus better reflect the natural condition of the coated steel in the electrolyte. In light of the above considerations, IEcorr derived from R2 rather than Rp calculated for the steel under the various coatings after 7-day immersion (Table 1) and used to calculate IE for the coatings. IE can clearly demonstrate the effect of GO or graphene (G) on the anticorrosion performance of epoxy coatings, by cancelling the influence of epoxy type or metal type, and the data from some relevant studies are summarized in Table 2. The calculated IE of the 5NBGE coating (vs. the 6E coating) was 99.3%. According to Equation 1, the IE of the 0.5 wt% G/waterborne epoxy composite coating with a thickness of 50 µm was 94.4% after immersion in 3.5 wt% NaCl solution for 2 days,18 and the G/epoxy composite coating with the hydrophobic surface and the thickness of 110 µm had the IE of 93.3% after immersion in 3.5 wt% NaCl solution for 30 min.35 Remarkably, related to these two G/epoxy coatings, the 5NBGE coating with lower thickness (25 um) and longer immersion time (7 days) exhibited much greater IE, even when the coating surface was untreated. To our knowledge, the three comparable anticorrosion epoxy coatings would be the functionalized fullerene (C60)/epoxy composite coating with the thickness of 30 µm, the nanochitosan/epoxy composite coating with the thickness of 60 µm and the nanocellulose/epoxy composite coating with the thickness of 60 µm, which were reported to have a comparable IE (approximate 100%) after immersion in 3.5 wt% NaCl solution for 1 day, 30 days and 30 days respectively.17,45,46 In another study, the IE of 99.7% was achieved by the 0.1 wt% GO/epoxy composite coating after 2-hour immersion in 3.5 wt% NaCl solution, but the thickness of 150 µm was almost nine-fold as the 5NBGE coating.16 Note that a thicker epoxy coating is more prone to cracking over time due to stress concentration in the coating. The 5NBGE coating is designed to feature high resistance to 10


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cracking, not only because of its small thickness, but also its nacre biomimetic structure that effectively mitigates the propagation of cracks. It can be inferred the GO or G/epoxy coating with the nacre-like structure is more effective and efficient to protect steel or iron against corrosion than other forms of GO or G/epoxy coatings. Table 2. Comparison of IE between the NBGE coating and other forms of G or GO-polymer coatings under different experimental conditions Coating

5NBGE coating

IE

99.3%

Solvent based GO-epoxy composite 99.7% coating Functionalized C60-epoxy 100.0% composite coating Nanochitosan-epoxy 100.0% composite coating Nanocellulose-epoxy 99.9% composite coating Hydrophobic G/epoxy composite 93.3% coating G-epoxy composite coating

94.4%

Thickness (µm)

Metal

Electrolyte

Immersion time

Reference #

17

Q235 carbon steel

3.5 wt% NaCl

7 days

This study

150

Mild steel

3.5 wt% NaCl

2 hrs

16

30

Cast irons

3.5 wt% NaCl

24 hrs

17

Mild steel Mild steel

3.5 wt% NaCl 3.5 wt% NaCl

30 days

45

30 days

46

110

Carbon steel

3.5 wt% NaCl

30 min

35

50

Q235 carbon steel

3.5 wt% NaCl

48 hrs

18

60 60

Photographic/microscopic observations and EDS analysis. Photographic observations for the surfaces of the coated and coating-exfoliated steels were carried out to support the results of electrochemical measurements. The 6E coating was used as control to contrast its steel corrosion condition in 3.5 wt% NaCl, relative to the 5NBGE coating. The 1NBGE coating was selected because of its better anticorrosion performance than that of the 6E coating and the worst anticorrosion performance among the NBGE coatings, based on the electrochemical measurements. Figure 6 illustrates the 5NBGE coating had better anticorrosion performance than the 6E coating and the 1NBGE coating. Figure 6a, b and c (before immersion) and Figure 6d, e and f (after 7-day immersion) display the surface conditions of the 6E coating, the 1NBGE coating and the 5NBGE coating respectively, while Figure 6g, h and i show the corrosion conditions on their corresponding coating-exfoliated steel surfaces. All the three coatings (Figures 6a, b and c) had smooth surfaces before exposure to 3.5 wt% NaCl solution. However, after 7-day immersion time, the steel coupons with the 6E coating (Figure 6d) and the 1NBGE coating (Figure 6e) exhibited signs of severe corrosion, whereas the steel under the 5NBGE coating (Figure 6f) exhibited no sign of corrosion and looked as good as its original condition. In addition, after removing the coating layer, it can be clearly seen that obvious corrosion products existed on the steel surfaces under the 6E coating and 11


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the 1NBGE coating (Figures 6g and h); in contrast, there was no visible corrosion product on the steel surface under the 5NBGE coating (Figure 6i).

Figure 6. Digital photographs of coatings and steel surfaces in 3.5 wt% NaCl solution. From left to right: before immersion, after 7-day immersion, and after 7-day immersion and then removal of coating. From top to bottom: 6E coating, 1NBGE coating and 5NBGE coating To further demonstrate that the 5NBGE coating provided the best corrosion protection for steel as suggested by photographic observations, the SEM micrographs of the three coating-exfoliated steel surfaces after 7-day immersion time were obtained (as shown in Figure 7). The chemical composition on each of coating-exfoliated steel surfaces was then analyzed by EDS. The images from top to bottom display the morphologies of the steel surfaces after the 6E coating, the 1NBGE coating and the 5NBGE coating were exfoliated, respectively. Obviously, serious corrosion occurred on the steel surfaces protected by the 6E coating and the 1NBGE coating whereas the steel surface protected by the 5NBGE coating was smooth and featured uncompromised integrity even at the magnification level of 20000×, which again confirms that the 5NBGE coating had the best corrosion resistance among the three examined coatings.

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Figure 7. SEM images on coating-exfoliated steel surfaces after 7-day immersion in 3.5 wt% NaCl solution. From top to bottom: 6E coating, 1NBGE coating and 5NBGE coating. From left to right: magnification levels of 1000× and 20000× Table 3 presents the results of EDS analysis on each of coating-exfoliated steel surfaces. In this case, corrosion was mainly caused by the electrochemical reaction of steel with oxygen in air and water molecules in the electrolyte.47 Thus, oxygen and iron are the two primary elements that need to be considered for the analysis of chemical composition on coating-exfoliated steel surfaces, whereas carbon is ignored. 3.7 wt% oxygen element was detected while 91.5 wt% iron element was found on the exfoliated steel surfaces protected by the 5NBGE coating, confirming that the steel surface was still dominated by iron element and not compromised by corrosion. On the other hand, 41.4 wt% oxygen element and 50.6 wt% iron element for the 6E coating and 26.2 wt% oxygen element and 66.9 wt% iron element for the 1NBGE coating suggest the formation of corrosion products on the steel surfaces. The anticorrosion performance of the 1NBGE coating was better than the 6E coating, which is a good agreement with the results of electrochemical measurements.

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Table 3. Surface chemical composition on coating-exfoliated steel surfaces Element

6E coating

1NBGE coating

5NBGE coating

C (wt%)

8.0

6.9

4.8

O (wt%)

41.4

26.2

3.7

Fe (wt%)

50.6

66.9

91.5

FTIR analysis. The bonding interactions between the GO layers and the epoxy layers in the 5NBGE coating was investigated by FTIR spectroscopy. Figure 8 (a-d) represent the spectrums for the GO nanoparticles, the 6E coating, the 5NBGE coating and the GO/epoxy mixture with the GO content of 6 wt.%. The spectrum of the GO/epoxy mixture was used to compare with that of the 5NBGE coating, to examine any difference deriving from the two different structures.

Figure 8. FTIR spectra of a) GO, b) neat epoxy, c) 5NBGE coating and d) GO/epoxy mixture As shown in Figure 8a, the main characteristic peaks of the GO particles appeared at 1098 cm-1 (C-O epoxide groups), 1621 cm-1 (C=C skeletal vibrations from unoxidized graphitic domains), 2927 cm-1 (C-H stretching groups) and 3424 cm-1 (O-H hydroxyl groups).16,48 The transmittance variations of the four peaks in the spectrum of the 5NBGE coating (Figure 8c) reflected the bonding interactions between the GO layers and the epoxy layers, compared to the spectrum of the 6E 14


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coating (Figure 8b). One the one hand, the intensity ratio of the absorption bands around 1098 cm-1, 1621 cm-1 and 3424 cm-1 increased, suggesting that the contents of C-O, C=C and 0-H bonds increased due to the incorporation of the GO layers in the 6E coating. The C-O, C=C and O-H bonds from the GO nanoparticles overlapped with the bonds from the epoxy, and they crosslinked with each other by the physical forces (van der Waals forces) such as dipole-dipole forces, and hydrogen bonding, etc. On the other hand, the significant increment for the intensity ratio of the absorption band around 2900 cm-1 attributed to the formation of more C-H stretching bonds via the GO-epoxy chemical reactions. Both the physical and chemical crosslinks of the functional groups on the GO nanoparticles with the epoxy could explain for the formation of the tight nacre-like structure of the 5NBGE coating as shown in Figure 1b. Moreover, the spectrums of the 5NBGE coating and the GO/epoxy mixture (Figure 8d) had the similar shapes and the similar intensity ratios of the absorption band at 2900 cm-1 (C-H stretching groups), indicating the GO layers chemically bonded with the epoxy layers in the 5NBGE coating as the GO nanoparticles existed in the mixture. The obvious increment for the intensity ratios of the absorption bands at 1615 cm-1 and 3400 cm-1 in the spectrum of the mixture might be due to the excessive GO nanoparticles, compared with the spectrum of the 5NBGE coating. XRD and XPS analyses. The chemical structure between the GO layers and the epoxy layers in the 5NBGE coating was further investigated by XRD and XPS analyses. The XRD patterns of the GO, the 5NBGE coating and the neat epoxy coating are shown in Figure 9. The neat epoxy coating had the peak with a high intensity at 2θ=18.5º, indicating its amorphous structure. For the 5NBGE coating, the peaks appeared at 2θ=6º, 8º and 18.5º. Typically, the XRD pattern of GO has a strong peak at 2θ=11º.49,50 The peak of GO at 2θ=11º was shifted to the peaks at 2θ=6º and 8º appeared in the 5NBGE coating, suggesting the spacing increment in the GO layers. This implies that the epoxy chains intercalated into the GO layer, thus forming the strong cross-linking between the GO layer and the epoxy layer. The peak at 2θ=18.5º of the 5NBGE coating indicates presence of crystalline phases in the cured epoxy.

Figure 9. XRD spectra of the GO, the 5NBGE coating and the neat epoxy coating Figure S3a presents the C1s core-level XPS spectrum at the surface of the GO layer exfoliated with the epoxy layer in the 5NBGE coating. The C1s core-level spectrum at the surface of the GO layer could be fitted into five peaks including C=C species at binding energy of 284.5 eV, C-O species at binding energy of 285.0 eV and 286.6 eV, C=O species at binding energy of 286.0 eV and 15


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O-C=O species at binding energy of 288.9 eV. Figure S3b is the C1s core-level XPS spectrum for the GO powder produced from the same laboratory,51 where the GO was prepared and used in this study. The C1s core-level spectrum of the GO powder was fitted into the four peaks: C=C, C-O, C=O and O-C=O. The similarities of the four common peaks and their relative intensities between the two XPS spectra suggest that any reduction of the GO during the formation of the NBGE structure can be ignored. More importantly, the appearance of the new peak (C-O species at binding energy of 286.6 eV) in the spectrum of the GO layer suggests chemical reaction between the GO layer and the epoxy layer, thus providing the distinct evidence that strong crosslink was formed between these two types of layers. Chloride diffusion of NBGE membrane. To verify the function of GO layers in NBGE coatings, the 5NBGE membrane having the best anticorrosion performance was selected for chloride permeability measurements, and the 6E membrane was prepared as control. Figure S4 presents the temporal evolution of electrical conductivity of the solution in the right tube (initially with DI water). For the two membranes during the 11 days of monitoring, the chloride concentration of the solution as shown in Figure S6 was obtained by converting the electrical conductivity through the linear fitting in Figure S5. Obviously, the chloride diffusion rate of the 5NBGE membrane was much lower than that of the 6E membrane. The chloride concentration of the solution for the 6E membrane reached to 0.23 mol/L after 11 days, whereas for the 5NBGE membrane it was 0.11 mol/L. This indicates that the GO layers in NBGE coatings effectively inhibited the penetration of chloride ions through the neat epoxy coating. Anticorrosion mechanism of NBGE coatings. The anticorrosion mechanism of the NBGE coatings can be explained by the variations of the chemical reactions occurring at the steel/coating interface. Anodic half reaction (Equation 4) and cathodic half reaction (Equation 5) are the two dominant half reactions accounting for the corrosion at the interface: Fe(s) → Fe+, (aq) + 2e0 (4) 1 H+ O(l) + O+ (g) + 2e0 → 2(OH)0 (aq) (5) 2 Ultimately, the corrosion product was formed by the irreversible reaction expressed as Equation 6: Fe+, (aq) + 2OH0 (aq) → Fe(OH)+ ↓ (6) Besides, the corrosion process has been accelerated due to the incorporation of chloride ions as the catalyst of the reactions, which can be expressed as Equation 7 and 8: Fe+, (aq) + 2Cl0 (aq) → FeCl+ (7) FeCl+ + 2OH 0 → Fe(OH)+ ↓ +2Cl0 (aq) (8) Therefore, it can be inferred that the availability of H2O, O2 and Cl- at the interface were the critical factors affecting the entire corrosion process. The 5NBGE coating has substantially slowed down the intrusion of O2 H2O, and Cl- with the extra 5 GO layers among epoxy layers, relative to the 6E coating, as conceptually illustrated in Figure 1c. The large amount of micropores in the 6E coating provided various pathways to transport O2, H2O, and Cl- from the aggressive solution to the steel surface, allowing the occurrence of steel corrosion. With addition of the GO layers with less-porous and highly-tortuous compact layer-by-layer structure, the ingress of O2, H2O, and Cl- was only possible through the gaps at where the interlayer space was larger than the size of these chemical species or through the defects in each of the GO layers. This may help explain the relatively unsatisfying anticorrosion performance of the 1NBGE – 4NBGE coatings, which had more gaps in the GO layers due to the simple assembling/air-dry process. Fortunately, such gaps were compensated as the number of the 16


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GO layers increased, like in the case of the 5NBGE coating. Figure S7 presents the diagrammatic sketch of the variations of Ecorr and Icorr relative to the content of O2 participating in the anodic and cathodic half-reactions. The solid curves reflect the anodic half-reactions, whereas the dash curves account for the cathodic half-reactions. For the steel protected by the 5NBGE coating, because the cathodic half-reaction was suppressed by the low availability of H2O and O2, the anodic half-reaction was accordingly suppressed, and the steel maintained in a passive state, which was due to the lack of electron sink. For the steel with the 6E coating, the anodic reaction showed an active state, which resulted from the high availability of H2O and O2 in the cathodic half-reaction. Due to the intrusion of plenty of H2O and O2, the cathodic half-reaction for the steel with the 6E coating had a relatively low electrode potential determined based on the Nernst equation (Equation 9),52 resulting in the low Ecorr and the high Icorr. For the steel with the 5NBGE coating, most of H2O and O2 were impeded by the GO layers, thus giving rise to a relatively high electrode potential for the cathodic reaction. This high electrode potential led to the higher Ecorr and the lower Icorr compared to that of the 6E coating, which results are consistent with that of OCP, PDP and EIS measurements. RT ABCD E = E; + ln @ G (9) => AEF where E is the half-cell reduction potential, E0 is the standard half-cell reduction potential, R is the universal gas constant, T is the temperature in kelvins, n is the number of electrons transferred in the half reaction, F is the Faraday constant, aRed is the activity coefficient of the reduced agent and aOx is the activity coefficient of the oxidized agent.

CONCLUSION NBGE coatings synthesized by the simple spinning-coating method significantly enhance the anticorrosion performance of the neat epoxy coatings due to intercalation of the multilayers of GO between epoxy layers. The corrosion resistance of NBGE coatings improves with the increase of the number of GO layers and epoxy layers. The ultra-thin NBGE coatings consisting of 5 layers of GO and 6 layers of epoxy have more benefits than carbon-based nanoparticle/epoxy mixed coatings for corrosion protection. The superior anticorrosion performance of the NBGE coating stems from: its compact laminated structure and possible cross-linking between GO layers and epoxy layers; exceptional properties of GO layers as physical barrier and micro-pore filler; good electrical insulation of epoxy layers against the electrical conductivity of GO layers. The results of FTIR, XRD and XPS analyses have supported that strong cross-linking structure formed between the GO layers and the epoxy layers in NBGE coatings. The anticorrosion mechanism of the NBGE coatings was explained by the mitigation of chemical reactions occurring at the steel/coating interface because of the limited intrusion of O2 H2O, and Cl- through the reduced pores/defects by the intercalated GO layers in the coatings. NBGE coatings have shown promising potential to serve as corrosion-inhibiting coatings for metals in service.

EXPERIMENTAL Synthesis of graphene oxide suspension. The GO was synthesized with the modified Hummer’s method.53,54 20 g of 325 mesh natural graphite powder were mixed with the 80 ºC mixture consisting of 30 mL of concentrated sulfuric acid, 10 g of potassium persulfate and 10 g of 17


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phosphorus pentoxide. The mixture stayed at room temperature for 6 hours, then the mixture was filtered and thoroughly washed with deionized (DI) water. After the pre-oxidized graphite was dried in an oven at 60 ºC overnight, it was mixed with 460 mL of 0 ºC concentrated sulfuric acid. 60 g of potassium permanganate was gradually added into the mixture with stirring, and the temperature was controlled to be lower than 20 ºC. 920 mL of DI water was then added into the mixture, followed by stirring at 35 ºC for 2 hours. The mixture of 2.8 L of DI water and 50 mL of 30% hydrogen peroxide was added in 15 min. The GO slurry was obtained by centrifuging the mixture, and it was washed with 5 L of diluted hydrochloric acid (1:10 ratio) and then DI water, followed by dialysis for further removing impurities. 2 mg/mL GO suspension was prepared from the GO slurry, followed by sonication for 30 min. Preparation of NBGE coating. Q235 carbon steel (10 mm × 10 mm × 1.5 mm) was used as substrate. A copper wire was electrically connected with steel coupon. All metal parts were sealed in epoxy resin except a surface area with 10 mm × 10 mm exposed to air. Prior to coating, steel coupons were polished with silicon carbide papers using grit sizes from 100 to 1500, washed with tap water, sonicated in DI water, rinsed with acetone and dried in air. The epoxy resin and hardener were purchased from Hanzhoung Shanghai Chemical Co., Ltd. The waterborne epoxy consisted of epoxy resin, hardener and DI water with the ratio of 1:2:3. To prepare NBGE coatings, the GO suspension and the waterborne epoxy were coated on steel coupons through spin-coating method with 3000 rpm and 30-second spinning time for each of layers. NBGE coatings were composed of single/multiple layers of GO and multiple layers of epoxy. Each of coatings had a top epoxy layer. Other layers between the top layer and the steel surface consisted of one or multiple two-layer structures: a GO layer seated on an epoxy layer. 8-hour curing time of epoxy layer at room temperature was needed before GO was coated on, and 4-hour curing time of GO layer was needed before epoxy was coated on. Figure 1a displays the fabricating procedure of the NBGE coatings. To simplify, the coating containing one two-layer structure is called 1-cycle nacre-biomimetic GO/epoxy (1NBGE) coating. Thus, for the coating in Figure 1a, it is called 5-cycle nacre-biomimetic GO/epoxy (5NBGE) coating. 6-layer epoxy (6E) coatings as control were prepared as well. Anticorrosion performance of nacre-biomimetic GO/waterborne epoxy coating. The anticorrosion performance of nacre-biomimetic GO/waterborne epoxy coatings on steel coupons in 3.5 wt% NaCl solution were evaluated through the electrochemical measurements performed using Electrochemical Analyzer/Workstation CHI660E. The electrochemical cell was a three-electrode system containing the coated steel coupons served as the working electrode, a platinum mesh served as the counter electrode and a saturated calomel electrode (SCE) served as the reference electrode. Open circuit potential (OCP) was recorded for 400 s before starting electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PDP) measurements. The EIS measurements were conducted on the sample polarized at ±10 mV around its OCP by an alternating current signal in the frequency range from 105 Hz to 0.01 Hz with 10 points per decade after 1-day, 3-day and 7-day immersion time. The PDP measurements were carried out on the sample polarized at ±250 mV around its OCP by a direct current signal with a scan rate of 0.2 mV/s after 8-day immersion time. All the tests were triplicated to obtain reliable data. Characterization. Scanning electron microscope (SEM, HELIOS NanoLab 600i, FEI Co., Ltd, USA) and energy-dispersive x-ray spectroscopy (EDS) were used to evaluate the corrosion conditions on the interface between the steel surfaces and the bottom layers of the nacre-biomimetic coatings according to localized morphologies and element distributions at microscopic scale. The 18


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fracture surfaces of the coatings were also observed using SEM. Fourier Transform infrared spectroscopy (FTIR, Agilent Cary 660), X-ray photoelectron spectroscopy (XPS, ESCALAB 250 XI, Thermo Scientific) and X-Ray Diffraction (XRD, Rigaku D/max-rB) were carried out to analyze the chemical structure of the 5NBGE coating. The samples were scanned from 5º to 80º for XRD analysis. Chloride diffusion measurements. To verify the effectiveness of GO layers in NBGE coatings to prevent chloride ions passing through, the chloride diffusion measurements of the coatings were performed. The coatings were formed on 47 mm hydrophilic polyvinylidene fluoride (PVDF) membrane with a 0.22 µm pore size, a 125 µm thickness and a 70% porosity, by the spin-coating method. The water flow rate of the PVDF membrane was greater than 1 mL/min×cm2. The PVDF membrane was purchased from MilliporeSigma company in the United States, and the information was provided by the vendor. Then the membrane was assembled with two plastic tubes and fixed with bolts and nuts. A certain volume of 3.5 wt% NaCl solution was added into the left tube, while the same volume of DI water was poured into the right tube. The diagrammatic sketch of the system is shown in Figure S1 (Supplementary document). Starting from the injection of DI water, the electrical conductivity of the solution in the right tube was monitored with the electric conductivity meter (DDS-11A) up to 11 days at room temperature. The test was triplicated to obtain reliable data. ASSOCIATED CONTENT Supporting Information. Figure S1. Diagrammatic sketch of a specimen for chloride diffusion measurements. Figure S2. Diagrammatic sketch of equivalent electric circuits. Figure S3. High resolution of the C1s core-level XPS spectra (a) at the surface of the GO layer exfoliated with the epoxy layer in the 5NBGE coating and (b) for GO powder. Figure S4. Variation of the electrical conductivity of the solution in the tube initially ﬁlled with DI water. Figure S5. Relationship of the electrical conductivity and the concentration for standard NaCl solutions. Figure S6. Temporal evolution of the NaCl concentration of the solution in the tube initially ﬁlled with DI water. Figure 5S1. EIS Nyquist diagrams for the steels protected by the nacre-biomimetic coatings and the 6E coating after 1-day immersion in 3.5 wt% NaCl solution. (a) full scale, and (b) low scale. Figure 5S2. EIS Nyquist diagrams for the steels protected by the nacre-biomimetic coatings and the 6E coating after 3-day immersion in 3.5 wt% NaCl solution. (a) full scale, and (b) low scale. Table S1. Warburg element values by fitting the EIS data of different coated steels using three equivalent electric circuit models. This material is available free of charge via the Internet at http://pubs.acs.org. Conflict of Interest. The authors declare no competing financial interest. AUTHOR INFORMATION Corresponding Authors. Jing ([email protected]).

Zhong

([email protected])

and

Xianming

Shi

Author Contributions. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Y. Zhang conducted most of the experiments and wrote the draft manuscript; J. Tian conducted exploratory experiments with coating proportioning and assisted in the XRD and XPS experiments; J. Zhong and X. Shi designed the experiments and provided guidance throughout this study, with X. Shi focusing on more on the electrochemical tests. 19


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ACKNOWLEDGMENT This work was supported by National Natural Science Foundation of China (Project 51278390) and Simpson-Strong Tie, Inc. (WSU Excellence Fund). REFERENCES (1) Yuan, B.; Bao, C.; Qian, X.; Song, L.; Tai, Q.; Liew, K. M.; Hu, Y. Design of Artificial Nacre-Like Hybrid Films as Shielding to Mitigate Electromagnetic Pollution. Carbon 2014, 75, 178-189. (2) Munch, E.; Launey, M. E.; Alsem, D. H.; Saiz, E.; Tomsia, A. P.; Ritchie, R. O. Tough, Bio-Inspired Hybrid Materials. Science 2008, 322, 1516-1520. (3) Cheng, Q.; Jiang, L.; Tang, Z. Bioinspired Layered Materials with Superior Mechanical Performance. Acc. Chem. Res. 2014, 47, 1256-1266. (4) Zhao, H.; Yue, Y.; Guo, L.; Wu, J.; Zhang, Y.; Li, X.; Mao, S.; Han, X. Cloning Nacre’s 3D Interlocking Skeleton in Engineering Composites to Achieve Exceptional Mechanical Properties. Adv. Mater. 2016, 28, 5099-5105. (5) Espinosa, H. D.; Rim, J. E.; Barthelat, F.; Buehler, M. J. Merger of Structure and Material in Nacre and Bone-Perspectives on De Novo Biomimetic Materials. Prog. Mater. Sci. 2009, 54, 1059-1100. (6) Tang, Z.; Kotov, N. A.; Magonov, S.; Ozturk, B. Nanostructured Artificial Nacre. Nat. Mater. 2003, 2, 413-418. (7) Erb, R. M.; Libanori, R.; Rothfuchs, N.; Studart, A. R. Composites Reinforced in Three Dimensions by Using Low Magnetic Fields. Science 2012, 335, 199-204. (8) Koch, G. H.; Brongers, M. P. H.; Thompson, N. G.; Virmani, Y. P.; Payer, J. H. Corrosion Cost and Preventive Strategies in the United States. 2002. (No. FHWA-RD-01-156) (9) Koch, G.; Varney, J.; Thompson, N.; Moghissi, O.; Gould, M.; Payer, J. International Measures of Prevention, Application and Economics of Corrosion Technologies Study. NACE International IMPACT Report 2016. (10) Shi, X.; Fay, L.; Yang, Z.; Nguyen, T. A.; Liu, Y. Corrosion of Deicers to Metals in Transportation Infrastructure: Introduction and Recent Developments. Corros. Rev. 2011, 27, 23-52. (11) Shi, X.; Liu, Y.; Mooney, M.; Berry, M.; Hubbard, B.; Nguyen, T. A. Laboratory Investigation and Neural Networks Modeling of Deicer Ingress into Portland Cement Concrete and Its Corrosion Implications. Corros. Rev. 2011, 28, 105-154. (12) Knudsen, O.; Forsgren, A. Corrosion Control Through Organic Coatings, Second Edition; CRC Press, 2017. (13) Ikechukwu, E. E.; Pauline, E. O. Environmental Impacts of Corrosion on the Physical Properties of Copper and Aluminium: A Case Study of the Surrounding Water Bodies in Port Harcourt. Open Journal of Social Sciences 2015, 03, 143. (14) Qian, Y.; Li, Y.; Jungwirth, S.; Seely, N.; Fang, Y.; Shi, X. The Application of Anti-Corrosion Coating for Preserving the Value of Equipment Asset in Chloride-Laden Environments: A Review. Int. J. Electrochem. Sci. 2015, 10, 10756-10780. (15) Shi, X.; Nguyen, T. A.; Suo, Z.; Liu, Y.; Avci, R. Effect of Nanoparticles on the Anticorrosion and Mechanical Properties of Epoxy Coating. Surf. Coat. Technol. 2009, 204, 237-245. 20


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