ZrO2 nanoparticle encapsulation of graphene microsheets for enhancing anticorrosion performance of epoxy coatings

Accepted Manuscript ZrO2 nanoparticle encapsulation of graphene microsheets for enhancing anticorrosion performance of epoxy coatings

Xinding Lv, Xitao Li, Nan Li, Hechuang Zhang, Yan-zhen Zheng, Jiaojiao Wu, Xia Tao PII: DOI: Reference:

S0257-8972(18)31255-6 https://doi.org/10.1016/j.surfcoat.2018.11.045 SCT 24011

To appear in:

Surface & Coatings Technology

Received date: Revised date: Accepted date:

11 July 2018 14 November 2018 16 November 2018

Please cite this article as: Xinding Lv, Xitao Li, Nan Li, Hechuang Zhang, Yan-zhen Zheng, Jiaojiao Wu, Xia Tao , ZrO2 nanoparticle encapsulation of graphene microsheets for enhancing anticorrosion performance of epoxy coatings. Sct (2018), https://doi.org/ 10.1016/j.surfcoat.2018.11.045

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

ZrO2 nanoparticle encapsulation of graphene microsheets

for

enhancing

anticorrosion

PT

performance of epoxy coatings Xinding Lv a, Xitao Li a, Nan Li a, Hechuang Zhang a, Yan-zhen Zheng a, b,*, Jiaojiao Wu

a

RI

, Xia Tao a, *

State Key Laboratory of Organic-Inorganic Composites, Beijing University of

NU

Chemical Technology, Beijing 100029, China.

Research Centre of the Ministry of Education for High Gravity Engineering &

MA

b

SC

a

D

Technology, Beijing University of Chemical Technology, Beijing 100029, China.

PT E

* Corresponding author. Tel: +86 10 6445 3680. Fax: +86 10 6443 4784

AC

CE

E-mail: [email protected] (Yan-zhen Zheng), [email protected] (Xia Tao)

1

ACCEPTED MANUSCRIPT Abstract Graphene (rGO) is expected to enable an effective anti-corrosion barrier for metals owing to its chemical inertness and impermeability to gases and water. However, previous studies demonstrate limited anti-corrosion property and even corrosion

PT

enhancement of rGO on metal surfaces via micro-galvanic corrosion at the coating

RI

defects. To address the issue, in this work, sandwich-structured GZ microsheets is

SC

prepared by fully-covering the inert nanosized ZrO2 on the double-side of 2D rGO, and then incorporated into epoxy coating as filler for protecting metallic substrate against

NU

corrosion. By virtue of structural, morphological and electrochemical characterizations

MA

and analysis, we find that 0.5 wt.% GZ into epoxy coating enables to form uniform and defect-free anticorrosive coating, and hence improve corrosion-protective properties in

D

3.5 wt.% NaCl solution. The resistance of 0.5 wt.% GZ in the first day immersion is

PT E

81.8 GΩcm2, and the metal substrate is not corroded during immersion. The attached inert ZrO2 shell on rGO effectively eliminates the rGO-metal/rGO connections, and

CE

hence prevents the local micro-galvanic corrosion.

AC

Key words: rGO/ZrO2; anti-corrosion; sandwich structure; epoxy coating

2

ACCEPTED MANUSCRIPT 1. Introduction

Carbon steel is currently used as one of the structural materials in the almost entire industrial filed owing to its outstanding properties. Severe corrosion, which causes

PT

potential safety issues and heavy economic losses, limits the widely commercial application of carbon steel. Much strategies such as metal substrate improvement [1],

RI

cathodic protection [2, 3], protective surface coating [4, 5], corrosion inhibitors [6-8]

SC

etc have been implemented to alleviate the corrosion of carbon steel.

NU

Graphene (rGO), a one-atom-thick two-dimensional sp2 carbon structure, exhibits

MA

superior properties including chemical inertness, high thermal and chemical stability as well as impermeability to all molecules [9-11]. Owing to its prominent barrier property

D

and chemical stability, rGO is believed to be a promising anti-corrosion material for

PT E

metals. Until now, numerous studies have demonstrated that rGO as a corrosioninhibition coating can shield the metal underneath it from chemical reactions [12-14].

CE

A defect-free and continuous rGO coating that completely covered a metal surface can work like tin-plating on metal to prevent uniform corrosion. Besides as an anticorrosive

AC

coating, rGO can also be used as a filler to improve the corrosion protection performance of polymer coatings [15-17]. As comparison with other two dimensional materials such as Al flakes, glass flakes and clay, rGO exhibits a lower solubility to gases, a higher aspect ratio, and hence better corrosion or gas barrier properties of polymer coating with much lower loadings. Furthermore, the incorporated rGO can also endow the polymer coating with exceptionally enhanced and engineering-desired

3

ACCEPTED MANUSCRIPT properties including thermal properties, mechanical properties, wear resistance etc that are hardly compatibly achieved using conventional composites or pristine polymers. However, rGO for barrier coating is a double-edged sword. When rGO used for anticorrosion materials, one disadvantage that is a metal corrosion would accelerate at the

PT

rGO-metal interface, should not to be neglected [18-20]. Once there exists even if minor

RI

defects such as scratches, cracks, or pinholes on the rGO-incorporated coatings during

SC

use, the underlying metal will still suffer from micro-galvanic corrosion at the coating defects, in which rGO acts as the cathode and metal serves as the anode of corrosion

NU

micro-cell. Note that such minor defect-induced local corrosion can weaken the metal

MA

severely even with very low rGO mass loss. It has been proved that the pure rGO covering on the metal surface can only endow the substrate with a short-period

PT E

D

corrosion protective performance [20].

To circumvent the issue, polymer such as polyaniline, pernigraniline-base is used

CE

to modify rGO for reducing graphene conductivity and preventing graphene from contacting metal substrates [21, 22]. Such rGO/polymer composites are found to be a

AC

good barrier against corrosive media owing to the enhanced electrical resistance of the coatings. However, fabrication of the polymer additives involves complicate polymerization and modification processes, which is fairly difficult to control in industry. Encapsulation of rGO with nanosized inert oxides represents another facile solution to inhibiting the corrosion promotion activity of rGO, in which the inert oxide shell can block the rGO-metal/rGO connections and efficiently avoid the local microgalvanic corrosion. Zirconia nanoparticles with high chemical stability, can be used as 4

ACCEPTED MANUSCRIPT a reinforcement material to improve mechanical and thermal properties [23-25]. Moreover, zirconia nanoparticles are able to interact strongly with rGO sheets to create 3D interconnected composites, which is employed in sensors and capacitors [26-28]. This motivates us to prepare rGO/ZrO2 (denoted as GZ) composite for anticorrosion

PT

coating filler to be compatible with the favorable characteristics of both rGO and ZrO2.

RI

Herein, we use inert ZrO2 nanoparticles to encapsulate the double-side of 2D rGO

SC

microsheets by surface self-assembly and in-situ hydrothermal synthesis process. As expected, such GZ microsheets as a filler in epoxy coating are able to block micro-

NU

galvanic corrosion in the rGO-incorporated coatings and show improved the corrosion-

MA

protective performance of coating.

D

2. Experimental section

PT E

2.1 Synthesis of graphene oxide (GO)

CE

GO was prepared from natural graphite using a modified Hummers method [29]. Typically, 138 mL H2SO4 (98 wt.%) was add to a beaker in an ice-water bath. Then, 3.0

AC

g sodium nitrate and 3.0 g of graphite powder were successively added under stirring for 15 min. Subsequently, 18.0 g KMnO4 was added slowly for reaction at 0 oC under vigorous stirring for 2 h. Afterwards, the reaction system was transferred to a 35 oC water bath and stirred for 1 h, forming a thick paste. Then, 240 mL deionized water was added dropwise to control the reaction system not to boil, and then the mixture was transferred to 98 oC water bath and stirred for 30 min. Another 600 mL of deionized water was added to dilute the solution. Finally, 22.5 mL H2O2 was added to form bright 5

ACCEPTED MANUSCRIPT yellow solution. The dark brown GO was obtained by filtering and rinsing with 1 M HCl and deionized water, followed by freeze-drying.

2.2 Synthesis of GZ microsheets

PT

60 mg GO was added into 120 mL deionized water containing 1.25 g cetyltrimethyl ammonium bromide (CTAB) and 0.05 g NaOH, and then ultrasonically

SC

RI

treated for 30 min. Subsequently, 0.71 g ZrOCl28H2O was added to the above mixture, and the pH was adjusted to ~ 11 using 1 M NaOH. After magnetic stirring for 4 h, the

NU

mixture was transferred into a 100 mL Teflon-lined stainless steel autoclave for

MA

hydrothermal reaction at 180 °C for 15 h. After cooling naturally, 0.2 mL hydrazine hydrate was added, and then the mixture was maintained at 70 oC for 12 h. The GZ

D

sample was finally obtained by rinsing and freeze-drying.

PT E

2.3 Preparation of GZ-embedded coatings

CE

Before mixing with epoxy matrix, the GZ was first modified with aminopropyltriethoxysilane (APTES) to produce GZ-NH2. Typically, 0.1 g of GZ and

AC

2.0 g of APTES were dispersed in 50 mL ethanol under ultrasonication and then stirred at 78 °C for 4 h. 10 mL water was added to the above mixture, followed by stirring for 3 h. Finally, the GZ-APTES was obtained by centrifugation, rinsing and drying in a vacuum oven at 60 oC overnight. Composite epoxy coatings containing GZ-APTES were prepared by the following procedures. A certain amount of GZ-APTES was dispersed into 0.85 mL n-butanol and 0.35 mL xylene under ultrasonicating for 30 min. Then, 1.0 g polyamide hardener and 6

ACCEPTED MANUSCRIPT 2.0 g of epoxy resin were added into the above mixture under continuous stirring for 30 min, yielding a uniform and black viscous solution. The viscous mixture was carefully coated on the metal substrate with a bar coater and dried at room temperature for 72 h. The applied amount of GZ-APTES was 0, 0.0075, 0.015, 0.0225 and 0.03 g,

PT

and the obtained coating is denoted as GZ0, GZ0.25, GZ0.5, GZ0.75 and GZ1, respectively.

RI

For comparison, 0.015 g ZrO2-free rGO-APTES was also embedded into epoxy coating

SC

and then was denoted as G0.5. Note that, Q235 mild steel panels (40 mm×20 mm×1 mm) and copper sheets (80 mm×40 mm×1 mm) were used as the substrates for

NU

electrochemical measurements and morphology together with microstructure

MA

characterizations, respectively. The average thickness of the prepared coatings was

PT E

2.4 Characterizations

D

controlled to be almost identical ca. 0.14 mm.

Zeta-potentials of the samples in water were measured by a Malvern zeta analyzer

CE

(Nano-ZS 90, Malvern Instrument, UK). The morphologies of as-prepared samples were characterized by scanning electron microscopy (SEM) (JEOL JSM-6701F)

AC

coupling with high-resolution transmission electron microscopy (TEM) (JEOL JEM3010). Powder X-ray diffraction (XRD) analysis was performed on a Rigaku D/max2500 VB2+/PC diffractometer using Cu Kα radiation (λ = 0.15406 nm). Fourier transform infrared (FT-IR) spectrum was measured on a Bruker Vertex 70 v spectrometer. The effect of nanoparticle filler on the corrosion performance of epoxy coated mild steel substrate was verified by electrochemical impedance spectroscopy

7

ACCEPTED MANUSCRIPT (EIS). To seal the back and edges of the samples, hot melt mixture of paraffin and colophony resin was employed. And then a defined area (2 cm×1 cm) of each sample was exposed to the electrolyte. A three-electrode arrangement was used for electrochemical measurements, including a saturated calomel electrode (SCE)

PT

reference electrode, a platinum counter electrode and the exposed sample as the

RI

working electrode, immersed in a 3.5 wt.% NaCl solution. All EIS measurements were

SC

carried out by immersing in 3.5 wt.% NaCl solution for a period of 60 days at open circuit potential, with applied 100 mV sinusoidal perturbation over a frequency range

NU

of 10 mHz to 10 kHz. EIS plot was also collected for bare mild steel specimen within

MA

30 min exposure time.

D

3. Results and discussion

PT E

One big obstacle to the formation of GZ composite is the intrinsic incompatibility between the precursor GO and inorganic Zr-based parts. To address this issue, a surface

CE

self-assembly strategy was adopted via the electrostatically adsorption of Zr-based

AC

groups on the surface of GO, and the schematic shown in Fig. 1. In this work, one precursor i.e. ZrOCl28H2O has a  potential of approximately – 27.5 mV in NaOH solution (Fig. S1). To adsorb Zr-based groups onto the surface of highly negatively charged GO ( = – 29.0 mV) in alkaline solution, cationic surfactant, i.e. CTAB is introduced to firstly electrostatically adsorb onto the surface of GO, following by electrostatically self-assembling negatively charged Zr(OH)4 ( = – 27.5 mV) onto the positively charged CTAB-GO ( = 24.6 mV) (Fig. S1). Afterwards, upon hydrothermal

8

ACCEPTED MANUSCRIPT treatment of Zr(OH)4-CTAB-rGO at 180 oC, GO/ZrO2 is obtained. Finally, GO can be reduced to rGO with aid of hydrazine hydrate, and at the same time give birth to GZ composite. The formation of GZ composite is evidenced by TEM, SEM and XRD, as shown in Fig. 2, Fig. S2 and Fig. 3, respectively. As expected, rGO reduced from GO

PT

that synthesized by a modified Hummers method exhibits a randomly wrinkled flake-

RI

like structure (Fig. 2a), which is similar to the previously observed ones reported by

SC

other groups [30, 31]. A typical TEM image of the as-synthesized GZ composite shows that nanosized ZrO2 particles are homogeneously attached on the surface of rGO

NU

microsheets, indicating that the ZrO2 nanoparticles are successfully interacted with rGO

MA

via electrostatically self-assembly approach to achieve encapsulation of rGO. The magnified TEM images exhibit that the anchoring ZrO2 particles are highly crystallinity

D

and show a size of 5 ~ 10 nm. Compared with SEM images of the blank graphene (Fig.

PT E

S2a), both sides of the GZ surface are rough (Fig. S2c and Fig. S2d). Additionally, nanosized ZrO2 particles coat onto the both surfaces of the GZ. TEM and SEM images

CE

confirm that the nanostructured zirconia is uniformly attached to the graphene surface

AC

to form a sandwich structure. The nanosized ZrO2 particles with excellent insulativity are uniformly attached on the surface of rGO, which is expected to be a good barrier shell to prevent the cross-linking between rGO microsheets and hence result in an extremely low-conductivity of GZ composite. This also indicates that the rGO-metal electrical contacts can also be eliminated completely. The XRD pattern of as-prepared rGO with a peak at 2 = 24.1 o, corresponds to the (002) diffraction of rGO. Additionally, XRD profile of GZ composite is recorded to reveal the formation of ZrO2 on the rGO.

9

ACCEPTED MANUSCRIPT The peaks at 30.0 o, 35.0 o, 50.3 o, 59.9 o, 62.7 o, 74.4 o and 81.7 o can be assigned to the diffractions of the (111), (200), (220), (331), (222), (400) and (331) crystal planes of tetragonal phase in ZrO2, respectively.

PT

In this work, the GZ composite as nanofiller is dispersed into anti-corrosive epoxy coating for further improve its corrosion-protective performance. Before mixing with

RI

epoxy, the GZ is firstly modified with APTES to produce rGO/ZrO2-NH2. FTIR (Fig.

SC

4) is used to investigate the functionalization of the GZ. As compared with unmodified

NU

GZ composite, the newly appearing absorption peaks at 952 cm-1, 1035 cm-1 and 1097 cm-1 are ascribed to the stretch vibration band of Si–O–Zr, Si–O–C and Si–O–Si,

MA

respectively [32-34]. This indicates the successful anchoring of APTES onto the nanosized ZrO2 through chemical bonding. Additionally, the intensity of the absorption

D

peak at 3420 cm-1 representing vibration band of –OH decreases [35], indicating that

PT E

functionalization with APTES effectively reduces –OH on the surface of GZ composite

CE

and consequently favors for better dispersion of the GZ composite in the epoxy matrix.

In order to reflect the better dispersion of APTES-treated GZ composite in the

AC

epoxy matrix, the results for sedimentation test on various sample are shown in Fig. S4. In contrast with the precipitation of untreated sample, APTES-treated GZ composite is still well-dispersed in epoxy resin after stored for 12 h. The ATPES-modified GZ has two active hydrogen groups on the amino group, which can well interact with the epoxy resin. In the coating curing process, solvent vapors and meanwhile produces pores within coating. The tightly interaction of GZ and epoxy greatly slow down the

10

ACCEPTED MANUSCRIPT evaporation of solvent through prolonging the diffusion pathway of solvent molecules in the curing process [36]. Thus the defects and bubbles are eliminated and a uniform and dense protective coating is formed on the substrate surface.

PT

The dispersion of APTES-treated rGO and GZ composite in cured epoxy resin is also studied by XRD. Fig. S5 shows XRD patterns of pure epoxy, rGO/epoxy, and

RI

GZ/epoxy. In the case of pure epoxy resins, a wide diffraction of 5 to 30 ° caused by

SC

scattering of the cured epoxy molecules can be observed, indicative of its amorphous

NU

nature [37]. Similar XRD patterns are also found for both rGO/epoxy and GZ/epoxy, indicating that GZ or rGO modified by silane coupling agent has excellent dispersibility

MA

in epoxy matrix. Note that upon soaking in 3.5 wt.% NaCl solution for 20 days, GZ composites are still able to well disperse in cured epoxy resin, as confirmed by XRD

PT E

D

pattern.

To show the morphology and microstructure of the coatings, the surface and cross-

CE

section of coatings are characterized by SEM, as shown in Fig. 5&Fig. 6. The fresh GZ0 coating shows very even surface morphology (Fig. 5a) and defects at the cross-sectional

AC

fracture surface (Fig. 6a). These defects are the residual diffusion pathway of solvent molecules (n-butanol and xylene) that rapidly evaporates in the epoxy coating formation period. The G0.5 coating seems non-uniform with some bulges at surface (Fig. 5b). Accordingly, cracks can be observed at the cross-sectional surface owing to the addition of rGO in the epoxy coating (Fig. 6b). From the surface SEM image of GZ0.5 coating, one can see that the coating looks more uniform with less bulges at surface

11

ACCEPTED MANUSCRIPT (Fig. 5c), and no defects or cracks at the cross-sectional surface (Fig. 6c). The defects or cracks can be eliminated by incorporating GZ0.5 to slow down the evaporation of solvent through prolonging the diffusion pathway of solvent molecules in the epoxy

PT

matrix during coating formation.

EIS is an effective method to characterize the anticorrosive performance of

RI

coatings. Nyquist plot obtained from EIS measurements for the adopted bare mild steel

SC

after immersing in 3.5 wt.% NaCl electrolyte for 30 min is shown in Fig. 7a. A distinct

NU

inductive loop at low frequency can be observed in the Nyquist plot, attributable to the adsorption of an intermediate product in the corrosion reaction [25]. Six different

MA

coatings with various contents of nanofiller (GZ0, G0.5, GZ0.25, GZ0.5, GZ0.75, GZ1), are tested during 60 day immersion in 3.5 wt.% NaCl electrolyte, and Nyquist plots are

PT E

D

provided in Fig. 7&Fig. S6. For the substrate coated with pure epoxy (Fig. 7b), when initially soaking in the electrolyte for 1 day, the electrolyte solution penetrates into the

CE

coating through micropores that create during the cure, but yet reaches the surface of the substrate. Accordingly, the impedance response is primarily dominated by the

AC

coating capacitance at high frequencies and the coating resistance at low frequencies with a value of 3.56 GΩ‧ cm2. Noted that the coating resistance is sensitive to the presence of absorbed ions from the environments, and thus it may be an indication to estimate whether the film could be anticipated to be protective or not [38]. The coating resistance of GZ0 decreases rapidly with increasing immersion time, owing to the diffusion and movement of water molecules and ions through the coating film. The continuous ion movement to the coating film would result in more defects in the coating 12

ACCEPTED MANUSCRIPT (Fig. 6d), and consequently increased conductivity. As shown in Nyquist plot measured after immersion for 10 days, the Warburg impedance characteristic appears in the low frequency region, indicating that the electrolyte solution saturates the coating and reaches the metal substrate surface, leading to corrosion of the metal substrate.

PT

Accordingly, the resistance of GZ0 reduces with a value of 117 MΩ‧ cm2.

RI

EIS measurement of G0.5 sample is also conducted for reference and Nyquist plot

SC

is shown in Fig. 7c. The resistance of G0.5 sample in the first day of immersion is ~ 3.62

NU

GΩcm2, a bit higher than that of the pure epoxy resin. After 30 day immersion, the Warburg impedance characteristic appears in the low frequency range. As compared to

MA

pure epoxy, an increased resistance can be obtained by G0.5. Using as the embedded fillers, pure rGO is able to increase the tortuosity of the diffusion pathways for oxygen

PT E

D

and water vapor in the coating matrix, hence improving anticorrosive performance of coating. However, note that the corrosion resistance of G0.5 is much worse than that of

CE

GZ0.5, due to the rGO folding-inducing cracks in the epoxy, which can be observed in cross-sectional SEM image in Fig. 6b. Such cracks would induce local corrosion and

AC

weaken the metal severely.

The GZ serves as a nanofiller to add in the epoxy coating to make up for the ioninduced defects in the coating, and consequently endows the coating with improved anticorrosive performance. From Fig. S6a, it can be seen that the resistive components in the low frequency region increase as compared with neat epoxy coating. For GZ0.25 coating after 1 day immersion, the resistance is calculated around 36.2 GΩcm2, which

13

ACCEPTED MANUSCRIPT is ~ 10 times larger than that of the GZ0 sample. When the immersion period prolongs up to 60 days, two capacitive arcs emerge in Nyquist plot, suggesting that the electrolyte solution reacts with the metal substrate. This means that the content of GZs (0.25 wt.%) is not enough to compensate for the defects in the coating, and the amount of GZs added

PT

in the epoxy coating needs to be further increased.

RI

Nyquist plots (Fig. 7d&Fig. S6b) show that the resistances in the first day

SC

immersion are 81.8 GΩcm2 for GZ0.5 and 36.4 GΩcm2 for GZ0.75, respectively.

NU

Subsequently, the coating resistance decreases to some extent with increasing the immersion time up to 60 days, and reaches 2.78 GΩcm2 for GZ0.5 and 1.26 GΩcm2 for

MA

GZ0.75, respectively. Such resistance values of the both samples are significantly higher than the result obtained by GZ0 samples, indicating that GZ has a positive effect on the

PT E

D

barrier properties and resistance of the nanocomposite coating samples. For GZ0.5, only one apparent time constant is observed during 60 day immersion, meaning that the

CE

coating resistance behavior is only a single capacitive loop screening in Nyquist plot. As seem from SEM images in Fig. 5, a lot of corrosion holes can be clearly observed

AC

on the surface of GZ0, and more seriously the number of defects at the coating increases after 20 day immersion. Similarly, corrosion pits also appear on the surface of G0.5, and an increase in number of defects is found at the coating, which is attributed to the diffusion and movement of water molecules and the movement of ions in the coating. Surprisingly for GZ0.5 coating, although the surface roughens after immersion, no corrosion pit or defect can be observed at the coating. This further verifies the high corrosion resistance and improved anticorrosion property of GZ0.5. The high resistance 14

ACCEPTED MANUSCRIPT of GZ0.5 and its constancy over a long period of immersion in electrolyte confirm the effectiveness of GZ in enhancing barrier protection of coatings.

By further increasing the amount of GZ in the coating (Fig. S6c), the resistance of

PT

GZ1 is calculated to be ~ 14.1 GΩcm2 after 1 day immersion. After 60 day immersion, the Warburg impedance appears at the end of the low frequency region, and accordingly

RI

the resistance drops rapidly from 1.16 GΩcm2 to 26.6 MΩcm2. This is because that

SC

the large amount of GZs will destroy the internal structure of the coating and result in

NU

augmenting defects in the coating.

MA

Bode plots of all the coating during 60 day immersion in 3.5 wt.% NaCl electrolyte, are also provided in Fig. 8&Fig. S7. In general, the Bode-phase peak locating at the

D

high frequency is ascribed to the response of coatings, while the one locating at

PT E

medium/low frequency is assigned to the responses of corrosion that is indicative of the coating failure. After 1 day immersion in the electrolyte, only one obvious peak at

CE

high/medium frequency can be observed for all the samples, due to the responses of coatings. A phase angle of about  90 ° in the high frequency region is observed by the

AC

Bode plots of all the samples, indicating that all the samples have good barrier properties in the early stages of immersion. For the pure epoxy resin (Fig. 8a), a peak indicative of the response of corrosive appears at low frequencies ranging from 100 to 10 mHz, indicating that the coating has lost its protective ability after 30 day immersion. For G0.5 (Fig. 8b), the low-frequency peak characteristic of the responses of steel corrosion cannot be distinguished until 60 days, indicating that the G0.5-incorporated

15

ACCEPTED MANUSCRIPT coating exhibits better anticorrosive performance as compared with the neat epoxy coating. As for GZ0.5, Bode-phase plots exhibit only one obvious peak at high/medium frequencies during the immersion period (Fig. 8c), suggesting that the anticorrosive property is further improved. It should be noting that although the peak width gradually

PT

narrows, the corrosion and coating defect responses cannot be detected during the

RI

whole immersion period. This suggests that the substrate coated with these samples

SC

cannot be corroded after 60 day immersion, owing to elimination of the coating defects by the adding GZ filler as observed in Fig. 6c. Moreover, the Bode-modulus curve

NU

shows that the modulus of GZ0.5 at 10 mHz is greater than those of other samples.

to block the corrosion of substrate.

MA

Larger modulus means more resistant to corrosion. Thus, GZ0.5 is the optimal coating

PT E

D

The Nyquist plot is further analyzed by Zsimperwin software and the EIS results are fitted using different equivalent circuit models, as provided in the Fig. 9. At the

CE

beginning of the immersion, the water does not penetrate to reach the coating/base metal interface, the fitting circuit is shown in Fig. 9a. In the middle of the immersion,

AC

the electrolyte solution completely penetrates into the coating, reaching the interface of the coating/substrate metal and forming a corrosion reaction micro-galvanic in the interface region (Fig. 9b). But the macroscopic pores cannot be found on the surface of the coating at this period. At the end of the immersion, rust spots or macroscopic pores can be observed on the surface of the organic coating. The original concentration gradient disappears in the organic coating, and meanwhile a new diffusion occurs in the interface region due to the accelerated corrosion reaction rate of the metal layer (Fig. 16

ACCEPTED MANUSCRIPT 9c). These models represent electrolyte resistance, Rs, coating resistance, Rf, coating capacitance, CPEf, charge transfer resistance, Rct, double layer capacitance, CPEct, and Warburg resistance, Zw, respectively.

PT

The curve of the fitting Rf as a function of immersion periods are exhibited in Fig. 10, showing that Rf gradually decreases with increased immersion time. After 60 day

RI

immersion, the Rf of G0.5 is slightly higher than that of GZ0, while the Rf of GZ0.5 is

SC

about 2 orders of magnitude higher than that of GZ0. The increase in Rf is attributable

NU

to the intercalation of fillers. Note that the Rf of GZ0.5 is 15 times greater than that of G0.5, because the GZ have a rigid structure which makes it more difficult to fold in the

MA

coating matrix than rGO.

D

The coating capacitance of all samples for 60 day immersion time are shown in

PT E

the Fig. 11. The CPEf is used to study water penetration into the coating. From Fig. 11, it can be seen that the CPEf of GZ0 increases with prolonging immersion time,

CE

indicating that water readily penetrates the coating. The CPEf of G0.5 fluctuates during the entire immersion period. The water uptake can result in swelling of the coating and

AC

subsequently blistering that may occur at some thin or defect areas in the coating. The osmotic pressure due to the swelling of the coating may break through the blisters to form direct paths for corrosion species to access the metal substrate, consequently resulting in corrosion reaction. The CPEf of GZ0.25 initially increases in the 30th day, indicating that GZ0.25 gradually lose anticorrosion protective effect from 30 to 60 day immersion. As for GZ1, the CPEf gradually increases over the entire immersion time,

17

ACCEPTED MANUSCRIPT but still less than that of GZ0, suggesting that water is more difficult to penetrate into the coating than GZ0. It is worth pointing out that the CPEf of GZ0.5 and GZ0.75 almost keeps unchange during the whole immersion period, indicating that GZ0.5 and GZ0.75 possess a stable coating/metal interface without any evidence of corrosion. This means

PT

that fillers fill the gaps within the epoxy resin to inhibit the diffusion and movement of

RI

water molecules and ions in the coating.

SC

In order to study the corrosion behavior at the coating defects, a ~ 3 mm scratch

NU

was made on the surface of each sample. The scratched specimens were then immersed in a 3.5 wt.% NaCl solution for 3 days. For G0.5, when the coating breaks down, the

MA

underlying metal would still suffer from micro-galvanic corrosion at the coating defects, in which rGO and metal respectively act as the cathode and the anode of corrosion

PT E

D

microcell, and the metal corrosion accelerates at the rGO-metal interface. As for GZ0.5 coating, ZrO2 as an inert oxide shell effectively reduces the conductivity of rGO, hence

CE

decreasing the potential difference between the composite and the metal. Thus, microgalvanic corrosion can be largely suppressed.

AC

As shown in the Fig. 12, the corrosion rate of G0.5 is much faster than GZ0. For GZ0.5, ZrO2 acts as an inert oxide shell that prevent rGO-metal/rGO connections and effectively avoid local microcurrent corrosion. It is note that the etching path of GZ0 and GZ0.5 are the same, and the filler has a very weak effect on improving the anticorrosion performance of the coating when the coating is scratched. Therefore, GZ0.5 and GZ0 exhibit similar corrosion behavior (Fig. 12a), and the charge transfer

18

ACCEPTED MANUSCRIPT resistances of GZ0.5 is much higher than that of G0.5 (Fig. 12b). The scratch experiment and results show that rGO has a significant role in promoting corrosion, and GZ effectively inhibits this effect.

PT

4. Conclusions

self-assemble

and

in-situ

hydrothermal

synthesis

method.

SC

electrostatically

RI

To summarize, ZrO2 encapsulated rGO microsheets were obtained via

Incorporation of 0.5 wt.% GZ as nanofiller in the epoxy coatings is found to greatly

NU

improve the barrier protection ability of the coating. APTES promotes the dispersion

MA

of GZ in epoxy resin, connecting GZ and epoxy resin through chemical bonds. The improvement in anticorrosion property is originating from increased tortuosity of the

D

diffusion pathways for oxygen and water vapor in the coating matrix owing to the

PT E

existence of well-dispersed GZ. Moreover, when the coating breaks down, the fullycovering the inert nanosized ZrO2 on the double-side of 2D rGO effectively suppresses

CE

corrosion-promoting effect that usually occurs in the pure rGO coating. The reason for

AC

the inhibition phenomenon is that the inert nanosized ZrO2 serving as an insulating spacer, completely eliminates the rGO-metal/rGO connections and effectively prevents local microcurrent corrosion. This work provides a feasible way to inhibit the corrosionpromotion activity of rGO via inert oxide encapsulation to directly prevent the contact between metal and rGO.

19

ACCEPTED MANUSCRIPT Acknowledgements

The work was supported by the National Natural Science Foundation of China (Nos.

AC

CE

PT E

D

MA

NU

SC

RI

PT

21476019, 21377011).

20

ACCEPTED MANUSCRIPT References [1] T. Bellezze, G. Roventi, R. Fratesi, Electrochemical characterization of three corrosion-resistant alloys after processing for heating-element sheathing, Electrochim. Acta, 49 (2004) 3005-3014.

PT

[2] S. Li, J. Fu, Improvement in corrosion protection properties of TiO2 coatings by

RI

chromium doping, Corros. Sci., 68 (2013) 101-110.

SC

[3] H. Yun, J. Li, H.-B. Chen, C.-J. Lin, A study on the N-, S- and Cl-modified nanoTiO2 coatings for corrosion protection of stainless steel, Electrochim. Acta, 52 (2007)

NU

6679-6685.

MA

[4] I. Kartsonakis, A. Balaskas, E. Koumoulos, C. Charitidis, G. Kordas, Incorporation of ceramic nanocontainers into epoxy coatings for the corrosion protection of hot dip

D

galvanized steel, Corros. Sci., 57 (2012) 30-41.

PT E

[5] H. Wei, Y. Wang, J. Guo, N.Z. Shen, D. Jiang, X. Zhang, X. Yan, J. Zhu, Q. Wang, L. Shao, Advanced micro/nanocapsules for self-healing smart anticorrosion coatings, J.

CE

Mater. Chem. A, 3 (2015) 469-480.

AC

[6] P. Okafor, X. Liu, Y. Zheng, Corrosion inhibition of mild steel by ethylamino imidazoline derivative in CO2-saturated solution, Corros. Sci., 51 (2009) 761-768. [7] B. Xu, W. Gong, K. Zhang, W. Yang, Y. Liu, X. Yin, H. Shi, Y. Chen, Theoretical prediction and experimental study of 1-butyl-2-(4-methylphenyl) benzimidazole as a novel corrosion inhibitor for mild steel in hydrochloric acid, J. Taiwan Inst. Chem. E., 51 (2015) 193-200. [8] K.-C. Chang, M.-H. Hsu, H.-I. Lu, M.-C. Lai, P.-J. Liu, C.-H. Hsu, W.-F. Ji, T.-L.

21

ACCEPTED MANUSCRIPT Chuang, Y. Wei, J.-M. Yeh, Room-temperature cured hydrophobic epoxy/graphene composites as corrosion inhibitor for cold-rolled steel, Carbon, 66 (2014) 144-153. [9] S. Böhm, Graphene against corrosion, Nat. Nanotechnol., 9 (2014) 741. [10] J.S. Bunch, S.S. Verbridge, J.S. Alden, A.M. Van Der Zande, J.M. Parpia, H.G.

PT

Craighead, P.L. McEuen, Impermeable atomic membranes from graphene sheets, Nano

RI

Lett., 8 (2008) 2458-2462.

oxidation, Phys. Rev. B, 85 (2012) 155445.

SC

[11] M. Topsakal, H. Şahin, S. Ciraci, Graphene coatings: an efficient protection from

NU

[12] N. Kirkland, T. Schiller, N. Medhekar, N. Birbilis, Exploring graphene as a

MA

corrosion protection barrier, Corros. Sci., 56 (2012) 1-4. [13] D. Prasai, J.C. Tuberquia, R.R. Harl, G.K. Jennings, K.I. Bolotin, Graphene:

D

corrosion-inhibiting coating, ACS nano, 6 (2012) 1102-1108.

PT E

[14] S. Chen, L. Brown, M. Levendorf, W. Cai, S.-Y. Ju, J. Edgeworth, X. Li, C.W. Magnuson, A. Velamakanni, R.D. Piner, Oxidation resistance of graphene-coated Cu

CE

and Cu/Ni alloy, ACS nano, 5 (2011) 1321-1327.

AC

[15] S. Liu, L. Gu, H. Zhao, J. Chen, H. Yu, Corrosion resistance of graphene-reinforced waterborne epoxy coatings, J. Mater. Sci. Technol., 32 (2016) 425-431. [16] Y. Li, Z. Yang, H. Qiu, Y. Dai, Q. Zheng, J. Li, J. Yang, Self-aligned graphene as anticorrosive barrier in waterborne polyurethane composite coatings, J. Mater. Chem. A, 2 (2014) 14139-14145. [17] K.-C. Chang, W.-F. Ji, M.-C. Lai, Y.-R. Hsiao, C.-H. Hsu, T.-L. Chuang, Y. Wei, J.-M. Yeh, W.-R. Liu, Synergistic effects of hydrophobicity and gas barrier properties

22

ACCEPTED MANUSCRIPT on the anticorrosion property of PMMA nanocomposite coatings embedded with graphene nanosheets, Polym. Chem., 5 (2013) 1049-1056. [18] M. Schriver, W. Regan, W.J. Gannett, A.M. Zaniewski, M.F. Crommie, A. Zettl, Graphene as a long-term metal oxidation barrier: worse than nothing, ACS nano, 7

PT

(2013) 5763-5768.

RI

[19] Y.-P. Hsieh, M. Hofmann, K.-W. Chang, J.G. Jhu, Y.-Y. Li, K.Y. Chen, C.C. Yang,

SC

W.-S. Chang, L.-C. Chen, Complete corrosion inhibition through graphene defect passivation, ACS nano, 8 (2013) 443-448.

NU

[20] C. Cui, A.T.O. Lim, J. Huang, A cautionary note on graphene anti-corrosion

MA

coatings, Nat. Nanotechnol., 12 (2017) 834.

[21] C.-H. Chang, T.-C. Huang, C.-W. Peng, T.-C. Yeh, H.-I. Lu, W.-I. Hung, C.-J.

D

Weng, T.-I. Yang, J.-M. Yeh, Novel anticorrosion coatings prepared from

PT E

polyaniline/graphene composites, Carbon, 50 (2012) 5044-5051. [22] W. Sun, L. Wang, T. Wu, Y. Pan, G. Liu, Synthesis of low-electrical-conductivity

CE

graphene/pernigraniline composites and their application in corrosion protection,

AC

Carbon, 79 (2014) 605-614. [23] A.R. Puigdollers, F. Illas, G. Pacchioni, Structure and properties of zirconia nanoparticles from density functional theory calculations, J. Phys. Chem. C, 120 (2016) 4392-4402. [24] S. Shibli, F. Chacko, C. Divya, Al2O3–ZrO2 mixed oxide composite incorporated aluminium rich zinc coatings for high wear resistance, Corros. Sci., 52 (2010) 518-525. [25] M. Behzadnasab, S. Mirabedini, K. Kabiri, S. Jamali, Corrosion performance of

23

ACCEPTED MANUSCRIPT epoxy coatings containing silane treated ZrO2 nanoparticles on mild steel in 3.5% NaCl solution, Corros. Sci., 53 (2011) 89-98. [26] A.E. Vilian, S.-M. Chen, M.A. Ali, F.M. Al-Hemaid, Direct electrochemistry of glucose oxidase immobilized on ZrO2 nanoparticles-decorated reduced graphene oxide

PT

sheets for a glucose biosensor, RSC Adv., 4 (2014) 30358-30367.

RI

[27] S. Giri, D. Ghosh, C.K. Das, Growth of vertically aligned tunable polyaniline on

SC

graphene/ZrO2 nanocomposites for supercapacitor energy‐ storage application, Adv. Funct. Mater., 24 (2014) 1312-1324.

NU

[28] M. Kaur, K. Pal, An investigation for hydrogen storage capability of zirconia-

MA

reduced graphene oxide nanocomposite, Int. J. Hydrogen Energy, 41 (2016) 2186121869.

PT E

Soc., 80 (1958) 1339-1339.

D

[29] W.S. Hummers Jr, R.E. Offeman, Preparation of graphitic oxide, J. Am. Chem.

[30] H. Wang, J.T. Robinson, X. Li, H. Dai, Solvothermal reduction of chemically

CE

exfoliated graphene sheets, J. Am. Chem. Soc., 131 (2009) 9910-9911.

AC

[31] D. Long, W. Li, L. Ling, J. Miyawaki, I. Mochida, S.-H. Yoon, Preparation of nitrogen-doped graphene sheets by a combined chemical and hydrothermal reduction of graphene oxide, Langmuir, 26 (2010) 16096-16102. [32] H. Di, Z. Yu, Y. Ma, C. Zhang, F. Li, L. Lv, Y. Pan, H. Shi, Y. He, Corrosionresistant hybrid coatings based on graphene oxide–zirconia dioxide/epoxy system, J. Taiwan Inst. Chem. E., 67 (2016) 511-520. [33] Y. Lin, J. Jin, M. Song, Preparation and characterisation of covalent polymer

24

ACCEPTED MANUSCRIPT functionalized graphene oxide, J. Mater. Chem., 21 (2011) 3455-3461. [34] Z. Yu, H. Di, Y. Ma, Y. He, L. Liang, L. Lv, X. Ran, Y. Pan, Z. Luo, Preparation of graphene oxide modified by titanium dioxide to enhance the anti-corrosion performance of epoxy coatings, Surf. Coat. Technol., 276 (2015) 471-478.

RI

aminopropyltriethoxysilane, J. Mater. Sci., 43 (2008) 33-37.

PT

[35] J. Kathi, K. Rhee, Surface modification of multi-walled carbon nanotubes using 3-

SC

[36] W. Sun, L. Wang, T. Wu, M. Wang, Z. Yang, Y. Pan, G. Liu, Inhibiting the corrosion-promotion activity of graphene, Chem. Mater., 27(2015) 2367-2373.

NU

[37] B. Ramezanzadeh, S. Niroumandrad, A. Ahmadi, M. Mahdavian, M.M.

MA

Moghadam, Enhancement of barrier and corrosion protection performance of an epoxy coating through wet transfer of amino functionalized graphene oxide, Corros. Sci., 103

D

(2016) 283-304.

PT E

[38] M. Behzadnasab, S. Mirabedini, M. Esfandeh, Corrosion protection of steel by epoxy nanocomposite coatings containing various combinations of clay and

AC

CE

nanoparticulate zirconia, Corros. Sci., 75 (2013) 134-141.

25

ACCEPTED MANUSCRIPT

NU

SC

RI

PT

Figure Captions

AC

CE

PT E

D

MA

Fig. 1 Schematic illustration of synthesis of sandwich-structured GZ microsheets through an electrostatic adsorption (step 1) and a hydrothermal technique (step 2).

Fig. 2 TEM image (a) of rGO, TEM (b) and HRTEM images (cd) of GZ.

26

AC

CE

PT E

D

MA

Fig. 3 XRD patterns of rGO and GZ.

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

Fig. 4 FTIR spectra of untreated (a) and APTES-treated (b) GZ nanoparticles.

27

SC

RI

PT

ACCEPTED MANUSCRIPT

AC

CE

PT E

D

MA

NU

Fig. 5 SEM images of the coating surface: (a&d) GZ0, (b&e) G0.5 and (c&f) GZ0.5. (ac): before soaking, and (d-f): after soaking for 20 days.

Fig. 6 Cross-sectional SEM images of the coating: (a&d) GZ0, (b&e) G0.5 and (c&f) GZ0.5. (a-c): before soaking, and (d-f): after soaking for 20 days. Inset: magnified crosssectional SEM images of the coating (scale bar = 5 μm).

28

MA

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

AC

CE

PT E

D

Fig. 7 Nyquist plots of different coatings during 60 day immersion in 3.5 wt.% NaCl electrolyte (a: bare mild steel, b: GZ0, c: G0.5, d: GZ0.5).

29

AC

CE

PT E

D

MA

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

Fig. 8 Bode modulus and phase plots of different coatings during 60 day immersion in 3.5 wt.% NaCl electrolyte (a: GZ0, b: G0.5, c: GZ0.5).

30

PT

ACCEPTED MANUSCRIPT

AC

CE

PT E

D

MA

NU

SC

RI

Fig. 9 Equivalent circuit models used for numerical simulation of the EIS measurements of the coatings at different stages (a: Initial corrosion process; b: water complete penetration in the epoxy coating; c: accumulation of corrosion products at metal/coating interface).

Fig. 10 The fitting results (coating resistance) of the collected EIS results of different coatings during 60 day immersion in 3.5 wt.% NaCl electrolyte.

31

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

AC

CE

PT E

D

MA

Fig. 11 The fitting capacitance of the six coatings (GZ0, G0.5, GZ0.25, GZ0.5, GZ0.75, GZ1) during 60 day immersion in 3.5 wt.% NaCl electrolyte.

Fig. 12 Scratch test for the GZ0, G0.5 and GZ0.5 coatings. (a) Bode modulus plots of scratched samples at different immersion days (green: GZ0, red: GZ0.5, blue: G0.5). (b) The fitting results of charge transfer resistance (Rct).

32

ACCEPTED MANUSCRIPT

AC

CE

PT E

D

MA

NU

SC

RI

PT

Graphical Abstract

33

ACCEPTED MANUSCRIPT Highlights Nanosized zirconium dioxide is used to encapsulate graphene.



The zirconium dioxide shell can avoid the local micro-galvanic corrosion.



This composite nanofiller can improve corrosion resistance of epoxy coating.

AC

CE

PT E

D

MA

NU

SC

RI

PT



34