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Progress in Organic Coatings 75 (2012) 386–391

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Progress in Organic Coatings

j ournal homepage: www.elsevier.com/locate/porgcoat

Effect of nano-sized mesoporous silica MCM-41 and MMT on corrosion

properties of epoxy coating

a,b a c c c b,c,∗

Na Wang , Keqi Cheng , Hang Wu , Cheng Wang , Qunchang Wang , Fuhui Wang

College of Materials Science and Engineering, Shenyang University of Chemical Technology, Shenyang 110142, China

College of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, China

State Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, 62 Wencui Road, 110016 Shenyang, China

a r t i c l e i n f o a b s t r a c t

Article history: This study investigated the effect of co-incorporation of two different kinds of nano materials with dif-

Received 12 January 2012

ferent forms, layers (Na-MMT) and mesoporous silica particles (MCM-41), into the polymer matrix on

Received in revised form 8 July 2012

the corrosion performance of epoxy resin. Correspondingly corrosion performance of the coatings was

Accepted 17 July 2012

studied by electrochemical impedance spectroscopy (EIS) in 3.5% NaCl aqueous solution and salt spray

Available online 16 August 2012

test. The X-ray diffraction (XRD) measurement showed that the Na-MMT layers were exfoliated and the

hexagonal framework structure of MCM-41 was retained during and after the composite preparation.

Keywords:

The co-incorporation of Na-montmorillonite (Na-MMT) and MCM-41 into the epoxy coating possessed

Nanocomposites

EIS the best corrosion resistance than incorporating either Na-MMT or MCM-41 particles separately due to

different interfacial structures between the ﬁllers and the matrix. Polymer coatings

1. Introduction transition temperature (Tg) and higher tensile strength through

the incorporation of nano-clay modiﬁed with siloxane [11–13]. The

Epoxy has been widely used as a coating material to protect the improvement of the corrosion resistance of carbon steel with epoxy

steel reinforcement in concrete structures, because of its outstand- resin reinforced with organically modiﬁed clay has been clearly

ing processability, excellent chemical resistance, good electrical demonstrated [14]. More recently study by Hang et al. [15,16] has

insulating properties, and strong adhesion to heterogeneous mate- showed that IBA-modiﬁed clay allows the corrosion performance

rials. However, the major disadvantage of pure epoxy resins is their of epoxy coatings to be improved by an increase of the barrier prop-

brittleness and low fracture toughness. Nonetheless, the successful erties of the ﬁlm and by the inhibitive action of IBA at the carbon

application of epoxy coatings is often hampered by their suscep- steel/coating interface.

tibility to damage by surface abrasion and wear. They also show Numerous publications have been devoted to the preparation

poor resistance to the initiation and propagation of cracks [1–4]. and characterization of the properties of polymer/mesoporous sil-

Incorporation of nano-sized ﬁllers to coatings can help in improv- ica MCM-41 composites. Epoxy resin, polyethylene, polypropylene

ing many properties of the coatings such as UV resistance, corrosion nanocomposites with enhanced thermal stability and mechanical

resistance and mechanical properties like scratch and abrasion. The properties were obtained in previous study [17–22]. Meso-

barrier properties of organic coatings can be improved by inclusion porous materials are used as the reinforcing materials to

of proper ﬁllers. There are various reports concerning improv- enhance the mechanical, thermal properties of polymer mate-

ing corrosion resistance of coatings using nano-particles such as rials, due to its unusual characters, such as extended inorganic

Ti [5,6], TiO2 [7], SiO2 [7], ZrO2 [8], and Zn [9]. Nanoclay is also or inorganic–organic hybrid arrays with exceptional long-range

introduced into epoxy matrix and endowed epoxy/clay composite ordering, highly tunable textural and large surface area properties,

signiﬁcantly improved physical and chemical properties [10]. It was controlled pore size and shape. Also, the reinforcing and toughing

found that the toughness and stiffness were improved, together effect of co-incorporation of two different kinds of nanomaterials

with low water absorption, lower cure shrinkage, moderate glass with different forms, layers (Na-montmorillonite (Na-MMT)) and

particles (MCM-41), into the polypropylene were obtained [19].

However, research about the use of mesoporous silica materials

∗ in paints has not previously been reported. Few papers have pre-

Corresponding author at: College of Materials Science and Chemical Engineer-

sented results concerning the corrosion protection of metals. In this

ing, Harbin Engineering University, Harbin 150001, China. Tel.: +86 24 23915900.

E-mail address: [email protected] (F. Wang). paper, the corrosion resistance of the epoxy coatings modiﬁed by

N. Wang et al. / Progress in Organic Coatings 75 (2012) 386–391 387

different contents of nano-sized mesoporous silica MCM-41 and 3.5% NaCl solution. For the impedance measurement, the coat-

MMT is studied by electrochemical impedance spectroscopy (EIS) ings studied were 30 ± 3 ␮m thick. The area of 9 cm was used for

and salt spray test. testing and the data normalized for 1 cm . Test system consisted

of a three-electrode cell, in which a saturated calomel electrode

(SCE), a stainless steel electrode and a coated coupon were used

2. Experimental

as reference, counter, and working electrodes, respectively. Exper-

iments were performed under the open circuit potential. Three

2.1. Materials

replications were performed to ensure repeatability. Impedance

spectra of coupons in different immersion times were recorded in

In the experiments, commercial available epoxy resin (E44) was

−2 5

10 –10 Hz frequency range, with sinusoidal alternating potential

chosen as main component of the paint. The nano-sized meso-

signal of 10 mV.

porous silica MCM-41 particles were prepared which was reported

Finally, the corrosion performance of the coated specimens was

in the literature [17]. In this study, nano-sized mesoporous MCM-

evaluated in a neutral salt spray test, following the procedure of

41 particles with uniform diameters in the order of 80–100 nm, ◦

ASTM B 117 and employing 5 wt.% NaCl solution at 35 ± 2 C for

spherical shape and mono-disperse were synthesized by our group

500 h. Prior to exposure, the backs and edges of the specimens were

in order to exclude the size and shape effects of ﬁllers on com-

covered with hot melt mixture of beeswax and colophony resin. The

posites. The particle size distribution measured by particle size

specimens were removed from the salt spray chamber after 500 h

analyzer and TEM, IR analysis were reported in previous paper

and representative areas were imaged with a digital camera. The

[17]. MMT was purchased from Fenghong Co. Ltd., Zhejiang. This

images were then used to evaluate the corrosion performance of

clay consisted of a 2/1 ratio of silica to alumina and CEC value was

the coated specimens.

115 mequiv./100 g. Epoxy of bisphenol A type (E-44) used was pur-

chased from WuXi Epoxy Co. Ltd. All other chemicals and solvents

were of analytical grade and used without further puriﬁcation. 3. Results and discussion

3.1. Characterization of epoxy nano-composites

2.2. Preparation of epoxy nanocomposite coatings

3.1.1. XRD

MCM-41 and MMT nanoparticles were kept in a vacuum oven

◦

MCM-41 exhibited XRD patterns with one intense (1 0 0) diffrac-

at 80 C for 1 h to remove physically absorbed moisture and then

tion peak and two additional small reﬂections (1 1 0) and (2 0 0)

directly added to the epoxy resin with butyl alcohol, dimethyl ben-

at low reﬂection angles (Fig. 1(a)), with the characteristics of

zene solution as solvent. The ratio between the epoxy resin and

solvents were 2:1. A ball mill was used as a mixing machine. The

ball mill time was 40 min and the rotation speed was 500 r/min.

As curing agent, polyamine (650#) was used. The content of

the MMT or MCM-41 power in the paint was 0 wt.%, 0.3 wt.%,

0.5 wt.% and 1 wt.%. For MMT/MCM-41 adding, they were 0 wt.%,

0.15 wt.%/0.15 wt.%, 0.25 wt.%/0.25 wt.%, 0.5 wt.%/0.5 wt.%, respec-

tively. The weight ratio of MMT to MCM-41 was 1:1.

Steel substrates (50 mm × 50 mm × 1 mm, UNS G 10190) with

rounded corners and edges were polished with ﬁne emery paper,

washed with acetone and dried for further use. The liquid paints

(pure epoxy and epoxy nano-composites) with 30 ± 3 ␮m were

then applied by using a model XB-120 coater and cured at room

temperature. After solidiﬁcation, the coating thickness was mea-

sured by a Qnix4500 digital meter. The coated samples were kept

in desiccator for a week before testing.

2.3. XRD and DSC test

The identiﬁcation of MCM-41, MMT, and the epoxy nanocom-

posites was carried out by X-ray diffraction (D/max-2500PC, using

◦

Cu Ka radiation at 50 kV and 200 mA with a scanning rate 1 /min

by 0.01 steps).

Differential scanning calorimeter (DSC) was obtained using a

◦

NETZSCHSTA 449C thermal analysis system from −30 to 220 C at

◦

the heating rate of 10 /min under nitrogen atmosphere. The data

was analyzed by Perkin-Elmer 7 series thermal analysis system

to obtain glass transition temperature. Each sample was less than

5 mg.

2.4. Corrosion performance tests

To verify the effect of MCM-41 and MMT nanoparticles on the

corrosion performance of epoxy coating on the mild steel sub-

strates, EIS, salt spray test were carried out. EIS measurements

were performed with 84362 Autolab using ZSimpwin software.

All EIS measurements were carried out at room temperature in Fig. 1. X-ray patterns of all samples. (a) MCM-41; (b) epoxy nano-composite.

388 N. Wang et al. / Progress in Organic Coatings 75 (2012) 386–391

Table 1

well-aligned hexagonal ordering [17]. Pristine MMT showed a

◦ Glass transition temperature, H, and Tonset characteristics of epoxy resin nano-

diffraction peak of the (0 0 1) plane at 4.0 in a 2 value, and its basal

coatings.

spacing was 2.21 nm (Fig. 1(b) curve (a)). Curve b was the result

◦

Sample T H (J/g T ( C)

obtained for epoxy/MMT nano-coating. The MMT layers were inter- g epoxy) onset

calated as indicated by the diffraction peak which started to shift to Epoxy 84.44 5.66 96.10

smaller angle position and became broader. Curves (c) and (d) gave Epoxy/MMT (0.5 wt.%) 85.57 9.58 98.45

Epoxy/MCM-41 89.08 17.50 97.96

typical XRD patterns of epoxy/MCM-41 and epoxy/MMT/MCM-41

(0.5 wt.%)

nano-coating and the appearance of the two curves was very simi-

Epoxy/MMT/MCM-41 93.32 18.33 95.33

lar. Only one reﬂection with a lower intensity remained in the small

(0.25 wt.%/0.25 wt.%)

angle 2 region for the MCM-41 after the composite preparation

(Fig. 1(b) curves (c and d)). The retention of the MCM-41 charac-

teristic diffraction peaks in the XRD pattern of the epoxy/MCM-41

and epoxy/MMT/MCM-41 nano-coating indicated that the hexago-

nal framework structure of MCM-41 was retained during and after

maximum exothermic heat was given off in the combination of

the composite preparation. The diffraction peak of MMT maybe dis-

epoxy and MMT/MCM-41 particles. This may result from the rea-

appear eventually or overlap with that of MCM-41, which indicated

son that the MMT was found to be exfoliated and the formation

that the layers of MMT were exfoliated or intercalated to some

of epoxy chain in the mesopore channels of the MCM-41 (see Sec-

extent (Fig. 1(b) curve (d)), which will be further conﬁrmed by our

tion 3.1.1), thus improving cross-linking extent. However, in case

research in next step. This result was similar to our previous study

of single MMT and single MCM-41 particles adding, epoxy resin

in PP/MMT/MCM-41 [19].

cannot wet their surfaces sufﬁciently. Shi et al. [7] reported that Tg

(glass transition temperature) of high solid epoxy coatings is closely

related to the cross-linking degree: the higher cross-linking is, the

3.1.2. DSC measurement

higher T is. The shift of the glass temperature to a higher tem-

The exothermic peaks in DSC analysis of different epoxy nano- g

perature of epoxy/MMT/MCM-41 was probably due to the strong

coatings reveal the possibilities that chemical bonding at interface

interaction between MMT/MCM-41 and epoxy resin. Moreover,

with epoxy matrix may be formed when curing the composites. For

combining two nano-materials with different shapes may generate

the precise investigation into the exothermic peaks, MMT, MCM-41

more effectively enhanced effect due to the expected synergistic

and MMT/MCM-41 were mixed stoichiometrically with polyamine

effect [19]. More compact structure of coatings can be obtained

to obtain the data such as the onset temperature (Tonset), Tg (glass

through improving cross-linking extent, which leads to better bar-

transition temperature) and the heat of reaction (H) evolved in

rier performance against water permeation.

the curing analyzed by system software, as shown in Table 1. The

Fig. 2. Nyquist plots of epoxy nanocomposites coatings immersion in 3.5% NaCl electrolyte (a) epoxy varnish coating; (b) epoxy coating containing MMT; (c) epoxy coating

containing MCM-41; (d) epoxy coating containing MMT/MCM-41.

N. Wang et al. / Progress in Organic Coatings 75 (2012) 386–391 389

3.2. Corrosion performance tests

3.2.1. EIS study

Nyquist plots were displayed in Fig. 2 for the coatings with MMT,

MCM-41 and MMT/MCM-41 nano-particles after various immer-

sion times in 3.5% NaCl electrolyte.

Fig. 2(a) showed the electrochemical impedance spectra of the

coating without nano-particles (varnish coating). The impedance

response revealed initial behaviour that was dominated by the

coating capacitance at high frequencies and coating resistance

in the low frequency region with a resistive component greater

6 2

than 1.8 × 10 cm after 24 h immersion in the electrolyte. With

increasing immersion time (168 h), the resistance value decreased,

due to the penetration of water and movement of ionic species

through the coating layer, increasing the coating conductivity [23].

With additional immersion time up to 408 h, the second semicir-

cle at low frequencies immerged in the EIS spectra and the barrier

properties of the coating decreased further. It can be ascribed to

increasing of corrosion rate, possibly through the presence of fur-

ther pores in the coating or an increase in the area exposed at the

base of the existing pores or ﬂaws [24].

Fig. 3. Equivalent electrical circuits.

However, the spectra of the coatings with 0.5 wt.% MMT power

was different from that of the varnish coating (Fig. 2(b)). In the

immersion time of 408 h, the resistance value remained above after 408 h immersion in the electrolyte. The impedance values

6 2

2 10 cm , almost the same as vanish epoxy coating after 24 h recorded for these coatings were clearly higher than impedance

immersion time. The capacitive arc changed as a semicircle, and data obtained for adding only MMT or MCM-41 nano-particles to

the resistance of the coating was detected. At the same time, there the epoxy coatings, indicating the barrier properties and high ohmic

was a short line appearing at the end of the semicircle, which did resistance of co-incorporation of MMT and MCM-41. After 960 h

not disappear until 624 h when the tail transformed to be a semi- immersion, there was an arc appearing at the end of the semicir-

circle. After 624 h, there are two time constants, the corrosion of cle, which implied that there were two time constants. The metal

the substrate was occurring [25]. substrate began to react with the corrosive aqueous media [27].

The spectra of the coatings with 0.5 wt.% MCM-41 power was From the facts mentioned above, there were two time con-

different from that of the varnish coating and the coating with stants in the spectrum of the coating for varnish epoxy after 408 h

0.5 wt.% MMT (Fig. 2(c)). After 24 h immersion, there was only immersion in the NaCl solution. However, in the spectra of the coat-

one capacitive arc, which implied that the coating acted as an ing with 0.5 wt.% MMT, 0.5 wt.% MCM-41 and co-incorporation of

intact capacitor prohibiting permeation of corrosive species such as 0.25 wt.% MMT and 0.25 wt.% MCM-41, the second time constant

water, oxygen, and other ions towards the surface of the metal sub- appeared after immersion for 624 h, 624 h and 960 h, respectively.

strate [26]. The coating capacitance at high frequencies and coating This indicated effectiveness of MCM-41 and MMT nano-particles

resistance in the low frequency region were with a resistive com- for improving barrier properties of coating layer. MCM-41 and MMT

× 6 2

ponent greater than 6 10 cm after 168 h immersion in the nano-particles tended to occupy small hole defects formed from

electrolyte. local shrinkage during curing of the epoxy resin and acted as a

For the systems included with 0.25 wt.% MMT and 0.25 wt.% bridge interconnecting more molecules. This resulted in a reduced

MCM-41 nano-particles together, it was clearly seen that only one total free volume as well as an increase in the cross-linking den-

apparent time constant was observed for 408 h of immersion as sity. This novel nano-network composite with the fully exfoliated

shown in Fig. 2(d). It was characterized by a single capacitive loop MMT and dispersed MCM-41 was seen in Scheme 1. This result was

representative of resistance of coating. The coating capacitance coincided with our previous DSC results.

at high frequencies and coating resistance in the low frequency Analyses of Nyquist plots suggested that different equivalent

6 2

region were with a resistive component greater than 6 10 cm circuit models were required to ﬁt the results (Fig. 3), which

Scheme 1. Model analogy novel nano-network composite.

390 N. Wang et al. / Progress in Organic Coatings 75 (2012) 386–391

immersion time at ﬁrst, and then attained a plateau. It was

also clearly observed that the coating resistance of epoxy coat-

ings containing single MMT, single MCM-41 or co-incorporation

of MMT/MCM-41 was higher than for the neat epoxy coated

specimen. This face may be attributed to the higher barrier prop-

erties and ionic resistance of single MMT, single MCM-41 or

co-incorporation of MMT/MCM-41 nano-particles embedded in the

epoxy coating samples.

For single MMT and single MCM-41 ﬁlling epoxy coating, it can

be found that the coating resistance with 0.5 wt.% MMT alone and

with 0.5 wt.% MCM-41 alone was the best among the others. For

the epoxy coating with single MMT, as the XRD results indicated,

the layers of MMT were only intercalated (Fig. 1(b) curve (b)). The

improvement in coating resistance may be presumably due to the

interaction between the epoxy matrix and the clay layers.

For single nano-sized MCM-41 ﬁlling, also as the XRD result indi-

cated, the chain of epoxy was formed in the mesopore channels

of the MCM-41 which restrained the agglomeration of MCM-41

particles on one hand. The epoxy phase in the nano-sized pores

extending along the channels to the openings could enhance the

interaction through the entanglement and inter-diffusion between

the matrix and the particulate on the other hand.

The coating resistance of the co-incorporation of MCM-41 and

MMT ﬁller was one order of magnitude higher than either the pure

nano-sized MCM-41 ﬁller or MMT ﬁlled epoxy coating. The MMT

and MCM-41 was found to be exfoliated and the formation of epoxy

chain in the mesopore channels of the MCM-41, respectively as

revealed by the X-ray diffraction measurements (Fig. 1(b) curve

(d)). The co-incorporation of MMT and MCM-41 improved interfa-

cial interaction in the nano-coatings owing to the inter-diffusion

and entanglement between the epoxy chains and the nano mate-

rials. Normally, exfoliated nanocomposite should exhibit superior

anti-corrosion properties because of the stronger interfacial inter-

action between the matrix and the exfoliated clay platelets [14].

Because of the different shapes of MCM-41 and MMT, they together

caused a reduction of the total free volume and an enhancement of

the cross-linking density of the cured epoxy. For the epoxy coat-

ing with MMT/MCM-41, the MCM-41 particles dispersed well in

the epoxy matrix because of the formation of epoxy chain in the

nano-sized pores. The dual nano-structured MCM-41 played an

important role as the bridges in the interconnected matrix. Addi-

tion, the MCM-41 and MMT nanoparticles may act to prevent epoxy

disaggregation during curing and result in a more homogenous

coating.

In addition, epoxy coatings containing MMT/MCM-41 offered

signiﬁcant barrier properties for corrosion protection and reduce

the trend for the coating to blister or delaminate.

Fig. 4. Time dependence of coating resistance for epoxy nano-coatings containing

different contents of MMT, MCM-41, MMT/MCM-41 nanoparticles.

3.2.2. Salt spray test

The corrosion resistance of various epoxy coating specimens

was evaluated by the rusts and blistering along the coating’s sur-

represented initial times of corrosion process, water saturated face on the mild steel substrate. The aspects of epoxy resin, epoxy

epoxy coating on the mild steel substrate, and ﬁnally accumulation resin with MMT and with MCM-41 and with MMT/MCM-41, after

of corrosion products at metal/coating interface (diffusion process) exposure in salt fog for 500 h, were shown in Fig. 5. For the var-

[28–30]. During the initial period of immersion, Fig. 3(a) was used to nish epoxy resin, serious rusting appeared along the surface for

ﬁt the impedance data, where Rs was the solution resistance, Cc and neat epoxy coating with diameter of rusts almost 2–4 mm was

Rc were the coating capacitance and the coating resistance, respec- observed (Fig. 5(a)). A few rusts with diameter 1–2 mm could be

tively. As the immersion time increases, EIS data can no longer be observed for MMT incorporated epoxy resin (Fig. 5(b)), while very

satisfactorily ﬁtted with the model in Fig. 3(a). Therefore, the long few rusts could be observed for MCM-41 incorporated epoxy resin

time degradation of polymer-coated metals may be described by (Fig. 5(c)). However, no apparent rusting along the surface was

the general circuit shown in Fig. 3(b). Here, Cd1 is the capacity of observed on the MMT/MCM-41 incorporated epoxy resin surface

double layer, and Rt is the charge transfer resistance [31,32]. after salt spray for 500 h (Fig. 5(d)). The corrosion results coincided

The coating resistance of all the coatings studied was plot- with the impedance spectra, implying that the co-incorporation of

ted as a function of immersion time in Fig. 4. It was showed MMT and MCM-41 can effectively prevent the epoxy resin from

clearly that the coating resistance always decreased with increasing blistering and delamination.

N. Wang et al. / Progress in Organic Coatings 75 (2012) 386–391 391

Fig. 5. Aspects of (a) epoxy varnish, (b) varnish with MMT (0.5 wt.%), (c) varnish with MCM-41 (0.5 wt.%), (d) varnish with MMT/MCM-41 (0.25 wt.%/0.25 wt.%) after exposure

in salt spray for 500 h.

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4. Conclusions 237–245.

[10] W.G. Ji, J.M. Hu, L. Liu, J.Q. Zhang, C.N. Cao, Surf. Coat. Technol. 201 (2007)

4789–4795.

EIS results showed that the co-incorporation of MMT

[11] H.W. Tian, W.H. Li, C.Z. Zong, B.R. Hou, J. Chin. Soc. Corros. Prot. 29 (2009)

0.25 wt.%/MCM-41 0.25 wt.% signiﬁcantly improved the corrosion 365–370.

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resistance of epoxy varnish via increasing barrier properties, which

2753–2763.

made water and ion species hard to transport and reduced the trend

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3683–3688.

MMT particles contributed to the compact nanocomposite coatings,

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Acknowledgements

[21] N. Wang, N. Gao, Q.H. Fang, E.F. Chen, Mater. Des. 32 (2011) 1222–2122.

[22] N. Wang, Z.X. Shi, J. Zhang, L. Wang, J. Compos. Mater. 42 (2008) 1151–1157.

[23] Y. Gonzalez-Garcia, S. Gonzalez, R.M. Souto, Corros. Sci. 49 (2007) 3514–3526.

The authors gratefully acknowledge the ﬁnancial support

[24] M. NIknahad, S. Moradian, S.M. Mirabedini, Corros. Sci. 52 (2010) 1348–1357.

of the National Natural Science Foundation of China (Grant

[25] B. Ramezanzadeh, M.M. Attar, M. Farzam, Surf. Coat. Technol. 205 (2010)

Nos.: 51103086 and 51173110), Distinguished Young Scholars 874–884.

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of Liaoning Province Higher Growth Plans, China (Grant No.:

[27] M. Barletta, L. Lusvarghi, F.P. Mantini, G. Rubino, Surf. Coat. Technol. 201 (2007)

LJQ2011040), and China Postdoctoral Science Foundation (Grant

7479–7504.

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