Anticorrosive Coatings Prepared Using EpoxyâSilica Hybrid

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Anticorrosive Coatings Prepared using EpoxySilica Hybrid Nanocomposite Materials Hossein Abdollahi, Amir Ershad-Langroudi, Ali Salimi, and Azam Rahimi Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 17 Jun 2014 Downloaded from http://pubs.acs.org on June 23, 2014

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Industrial & Engineering Chemistry Research

Anticorrosive Coatings Prepared using Epoxy-Silica Hybrid Nanocomposite

1

Materials

2 3

H. Abdollahi1, A. Ershad-Langroudi*1, A. salimi1, A. Rahimi2 1

Color, Resin & Surface Coating (CRSC) Department, Polymer processing faculty, Iran Polymer and Petrochemical Institute (IPPI), 14965/115 Tehran, Iran

2

Polymer Science Department, Faculty of Science, Iran Polymer and Petrochemical Institute

4 5 6 7

(IPPI), 14965/115 Tehran, Iran

8

E-mail: [email protected]

9 10

1 of 36 ACS Paragon Plus Environment


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ABSTRACT: Organic-inorganic nanocomposite protective coatings were prepared by sol-

1

gel method using 3-glycidoxypropyl-trimethoxysilane (GPTMS), Tetramethoxysilane (TMOS)

2

or tetraethoxysilane (TEOS) as silane precursors to compare the effect of two types of

3

alkoxysilane (i.e. methoxy or ethoxy functional group) on aluminum substrate properties. In

4

addition, the TiO2 and AlOOH nanoparticles were derived from tetra-n-butyl titanate and

5

aluminum butoxide, respectively and the protective effect of these nanoparticles on the GPTMS

6

based coatings were investigated. The formation of AlOOH and TiO2 nanoparticles, and the

7

uniform distribution of nanoparticles in the coatings were characterized by dynamic light-

8

scattering (DLS) and different microscopic techniques. Potentiodynamic scanning (PDS) and

9

2000 hours salt-spray testing methods were used to investigate the corrosion resistance of these

10

hybrid sol-gel coatings. The PDS results demonstrated that the corrosion protection of hybrid

11

coatings depends mainly on the silane content, type of the silane precursor and type of the

12

nanoparticles. The coating protective effect improved by increasing polarization resistance (Rp)

13

for about one decade by replacing silane precursors from TEOS to TMOS. In addition, the

14

incorporation of TiO2 in comparison with AlOOH nanoparticles in the GPTMS based coatings

15

showed improving effect on polarization resistance. However, the simultaneous incorporation of

16

TiO2 and AlOOH nanoparticles led to high protective coatings.

17

Keywords: Hybrid coatings, Sol-Gel, silane, nanocomposite, Corrosion protection.

18 19


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1. INTRODUCTION

1

One of the environmental problems of the metals is corrosion which results in decay and loss

2

of material due to chemical attack. The corrosion process of metals is not only of great

3

complexity but also of enormous economic importance. Some metals exhibit strong tendency to

4

be oxidized. Aluminum is an important and reactive metal which is the second metal in

5

production and application after iron and hence its corrosion results in wasting a lot of money 1.

6

Although corrosion of aluminum cannot be avoided completely, it can be retarded or decelerated

7

by employing corrosion control methods. There are several techniques to protect metals from

8

corrosion. One of these techniques is phosphating and chromating of metals which is commonly

9

used for corrosion control of iron and aluminum alloys in the automotive application. Despite the

10

effectiveness of these coatings in corrosion prevention, their toxic side products are regarded as

11

hazardous materials 2. In recent years, the use of polymeric coatings has been actively developed

12

to reduce the possible toxic side products

3–7

. Among these, the environmental friendly sol-gel

13

derived materials and incorporating nanoparticles in the corrosion protective coatings attracted

14

more attention in nano material science 8,9.

15

The sol-gel method is one of the most useful technologies for preparing organic-inorganic

16

hybrid materials and has been successfully applied in large scale coating processes for metal,

17

plastic, and glass

10

. As the name infers, it involves the growth of inorganic networks first by

18

formation of a colloidal suspension (sol) then its conversion to a continuous phase (gel). The

19

precursor for network formation consists of a metal or metalloid element surrounded by various

20

reactive ligands. The chemistry of sol-gel not only offers a low temperature route to ceramics

21

and glasses, but also permits the incorporation of improved or new properties in the system. So,

22



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the technology of sol-gel finds widespread applications in modification of the anticorrosion 9,

1

optical or mechanical properties 11.

2

Herein, metal alkoxides are most popular due to the fact that they react readily with water and undergo various forms of hydrolysis and polycondensation reactions to form final network

12

3

.

4

The most used alkoxides are based on silane such as tetramethoxysilane (TMOS),

5

tetraethoxysilane (TEOS), and 3-glycidoxypropyl-trimethoxysilane (GPTMS)

9,12,13

. The

6

organically modified silicates (ormosils) as a new class of materials, are hybrid organic–

7

inorganic materials formed through the hydrolysis and condensation of organically modified

8

silanes with traditional alkoxide precursors. These systems are of interest for corrosion resistance

9

because they combine the advantages of organic and inorganic polymers 14. In addition to simple

10

inorganic oxidic networks that formed by hydrolysis of metal or silicon alkoxides, the organo-

11

functionalized silanes (e.g. epoxy, vinyl, or methacryloxy organic groups) can be used to

12

incorporate polymerizable organic substituents into the final product. The Si–C bonds in these

13

molecules are stable under the mild conditions of sol-gel process 15.

14

The GPTMS modified ormosil is one of the hybrid materials that can be used to replace

15

organic polymer binders in coating materials exhibiting good corrosion protection 16, anti-scratch

16

17

. The structure of GPTMS provides chemical groups for

17

bonding to the filler surface, while the additional epoxy functionality can be used to develop

18

strong bonds between particles and the matrix. Therefore, many reports have shown the anti-

19

corrosive effect in GPTMS based coatings 14,19–21.

20

, and multi-functional coatings

18

The incorporating of inorganic particles in the coating matrix generates a synergism of

21

mechanical and chemical characteristics in composite coating. There has been a great number of

22

reports on incorporating of boehmite (AlOOH) 19,22 and anatase (TiO2) 23 nanoparticles as fillers

23


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in the sol-gel coatings (e.g. based on GPTMS). The effect of volume fraction and the type of

1

particles on anticorrosion properties of coatings have been studied for particle filled transparent

2

organic–inorganic coatings on metals 12.

3

As mentioned earlier, the effect of boehmite and anatase on corrosion properties of the

4

GPTMS based coatings has been investigated individually. To the best of our knowledge, the

5

simultaneous effect of boehmite and anatase particles on the corrosion resistance of the sol-gel

6

coatings based on GPTMS have not been reported. The objectives of present study is to

7

investigate the effect of silane content, type of silane precursors (i.e. TMOS & TEOS) and the

8

effects of boehmite and anatase nanofillers on the anticorrosion properties of GPTMS based

9

coating.

10 11

2. EXPERIMENTAL

12

2.1. Materials and Reagents. The alkoxides, 3-glycidoxypropyl-trimethoxysilane (GPTMS,

13

97%), Tetramethylorthosilicate (TMOS, 98%) and Tetraethylorthosilicate (TEOS, ≥98%) were

14

purchased from Alfa Aesar, Fluka and Merck, respectively and were used as received. Tetra-n-

15

butyl titanate (TBT, 97%), Aluminum butoxide (TBA, ≥97%), Sodium chloride and hydrogen

16

chloride were purchased from Merck, Germany. Bisphenol A as an aromatic diol (BPA, 97%,

17

Merck), 1-methylimidazole (1-MI, 99%, Fluka), Ethyl acetoacetate (EAcAc, 97%, Merck), and

18

Ethanol (99%, Merck) were used as solvent without further purification. The deionized water

19

was used for all experiments.

20

2.2. Sol Preparation

21

2.2.1.

Synthesis of Nanoparticle Sols. The sols of the nanoparticles were prepared

22

based on the sol-gel method. Chemical reactivity of the various alkoxides is different for similar

23



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alkoxy groups. Hydrolysis and condensation reactions of the metal alkoxides are very fast and

1

must be controlled. For this reason the chelating agents, which inhibit the condensation reaction,

2

are commonly used for the sol-gel systems’ synthesis depending on the reactivity of the

3

precursors 12.

4

The TBT was used as a TiO2 precursor. The TBT was added to the mixture of EAcAc (as

5

chelating agent) and Ethanol at room temperature. The molar ratio of TBT: EAcAc: Ethanol was

6

1: 1: 20. Hydrolysis reaction was carried out by adding 0.01 M hydrochloric acid (HCl) solution

7

as a second solution at room temperature. The second solution was slowly added into the first

8

solution in 20 minutes under vigorous and constant stirring for 3 hours at room temperature. The

9

prepared sol is refluxed for 8 hours at 80 °C to convert amorphous TiO2 to anatase crystalline

10

phase.

11

The Boehmite sols were prepared in a similar way to TiO2 sols. The TBA was used as a

12

boehmite precursor. TBA was mixed with mixture of EAcAc and ethanol. Then, hydrolysis

13

reaction was carried out by 0.01 M HCl solution which was slowly added into above solution

14

under vigorous stirring for 3 hours at room temperature. The molar ratio of TBA: EAcAc:

15

Ethanol was chosen as 1: 1: 40. The prepared sol was then refluxed for 72 hours at 80 °C to

16

convert amorphous aluminium hydroxide to boehmite crystalline phase. The chelation and

17

hydrolysis reactions 24 are shown in Scheme 1.

18 19


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1

Scheme 1. Reaction schematic of the formation of boehmite nanoparticles.

2 3

2.2.2.

Preparation of The Hybrid Sol. The series of Epoxy-Silica (ES) based hybrid

4

nanocomposite were prepared by mixing of as-prepared TiO2 and AlOOH nanoparticles in

5

GPTMS/TMOS sol followed by stirring vigorously for 2 hours at ambient temperature. Wherein,

6

the GPTMS/TMOS sol was prepared by hydrolysis and condensation of GPTMS and TMOS in

7

ethanol as solvent and 0.01 M HCl solution as the catalyst. The hydrolysis-condensation reaction

8

of the silica based sol with 0.01 M HCl solution was followed by nuclear magnetic resonance

9

(NMR) spectroscopic techniques in our previous paper 25. Then, the BPA as a curing agent of the

10

matrix was added to the above solution and stirred vigorously at a rate of 200 rpm for 4 hours at

11

ambient temperature. The 1-methylimidazol as a catalysis (1 wt.% vs. GPTMS) was used for

12

increasing the rate of ring opening reaction of epoxy groups. After 5 minutes stirring, the

13

solution was used in dip-coating process. Table 1 shows the molar ratio of reagents in the hybrid

14

samples. The thin films of ES hybrid nanocomposites on aluminum alloy 1050 (AA1050)

15

substrate were fabricated by dip-coating at ambient condition. As deposited ES thin films were

16

dried at room temperature for 24 hours, they were then heated in an air-circulating oven at 130

17

°C for about 120 minutes (as seen in scheme 2).

18



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1

2

Scheme 2. Simplified schematic of bonding mechanism between: (a) Epoxy Ring and curing agent BPA; (b) silane molecules and metal surface hydroxide layer.

3 4 5

Table 1. Feed composition ratios of the prepared nanocomposite coatings (based on mol fraction).

6

Sample code

GPTMS

TMOS

TEOS

Al(C4H9O)3

Ti(O-n-Bu)4

Organic content

ES1

1

-

-

-

-

100

ES2

1

0.25

-

-

-

80

ES2-E

1

-

0.25

-

-

80

ES3

1

0.25

-

-

0.125

66.67

ES4

1

0.25

-

0.0625

0.0625

66.67

ES5

1

0.25

-

0.125

-

66.67

ES6

-

0.25

-

-

-

0

(%)a

All mole fraction are based on 1 mole of GPTMS Organic content (%) =((mol GPTMS) / (mol GPTMS+ mol TMOS+ mol nanoparticles))×100

a


7 8 9

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2.3. Substrate Preparation and Film Deposition. AA1050 substrate was used in this work.

1

The AA1050 substrate was polished by the 400 and 600 grit sand paper. Each substrate was

2

initially rinsed with distilled water. Then, it was immersed in 5 wt.% solution of alkaline cleaner

3

(NaOH) and 0.01 M HCl solution, respectively. The substrates were rinsed thoroughly with

4

methanol and hexane solution to remove all possible dirties and greases, prior to coating

5

application.

6

The sol-gel films were prepared by dip-coating method. The clean substrate was immersed in

7

the final hybrid sol solution for 2 minutes, followed by controlled withdrawal at a speed of 20

8

cm/min. After coating application, the samples were air-dried for 24 hours and placed in an oven

9

to cure at 130 °C for 120 minutes.

10 11

2.4. Characterization. The attenuated total reflectance infrared spectroscopy, ATR-IR,

12

technique was recorded in a Bruker (IFS 484, Germany) microscope. The average size of the

13

TiO2 and AlOOH sols particles was determined by dynamic light-scattering measurement, DLS

14

(SEMA Tech 39 chem du terron 06200 NICE-France). Furthermore, the transmission electron

15

microscopy, TEM, (PHILIPS, Model CM120, Netherlands) at an accelerating voltage 120 kV

16

was used to investigate the average particle size. In order to investigate thermal stability and the

17

amount of inorganic content of hybrid nanocomposites, the thermo gravimetric analysis, TGA,

18

(TGA/DSC1, Mettler toledo, Switzerland) were examined under nitrogen atmosphere at a

19

heating rate of 10 °C/min. The coating adhesion on the AA1050 substrate was tested by the

20

Erichsen Scratch–Adhesion Test (Neurtek, Spain). The standardized cross-cut test provides a

21

straight forward method of establishing the coating adhesion quality according to the ASTM D

22

3359B-02. The water contact angles on the coatings surface were measured by G10 KRUSS at

23



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ambient condition. With five replicates of water droplets placed at three different positions of one specimen. The surface free energy was calculated using Owens and Wu methods

26

1

. The

2

morphological properties of surface and fractured surface of coated substrates were investigated

3

by scanning electron microscopy, SEM, (VEGA\\TESCAN, Czech Republic) at an accelerating

4

voltage of 1500 kV. The distributions of Al, Ti and Si atoms in thin film were studied by Energy-

5

Dispersive X-ray spectroscopy, EDX mapping (INCA Penta FET×3, Oxford, England). Atomic

6

force microscopy (AFM) was employed to observe the surface morphology and the RMS

7

roughness of the ES based hybrid nanocomposite films. Topographic AFM images of the

8

samples were obtained by an instrument (Dual scope Ds 95-200E/DME/) using non-contact

9

mode at a scan area of 15×15µm2.

10

2.5. Electrochemical Analysis. Electrochemical analysis was performed to evaluate corrosion

11

resistance of the prepared coatings quantitatively. The analysis was done under extreme

12

environmental conditions, consisting of an aqueous of 5 wt.% sodium chloride solution. To

13

measure the electrochemical corrosion performance of sample-coated AA1050 electrodes, a

14

series of ES were first coated onto the AA1050 coupons by dip-coating technique. A 1.0×1.0 cm2

15

area at the center of each specimen was exposed to the solution during testing. In order to

16

prevent premature corrosion along the edges of the substrate, the remained area of the specimen

17

was carefully sealed with bees wax prior to the evaluation. The electrodes were kept in the

18

working solutions for at least 60 minutes to reach steady potential. The potential was increased at

19

a rate of 5 mV/s, starting from 500mV below the open circuit potential (OCP). Electrochemical

20

analysis of bare and coated substrates was done using an Auto lab PGSTAT30 potentiostat which

21

works using a conventional standard three-electrode electrochemical test system. Three

22

electrodes are a saturated calomel electrode (SCE, Ag/AgCl/Cl−, 0.222 V) as a reference, a

23


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platinum counter electrode and sample electrode. Polarization curves of the films were drawn by

1

corrosion analysis software program at room temperature and the needed data were exported. All

2

the analyses were repeated three times and the results were reproducible.

3

2.6. Corrosion Resistance Test. Another corrosion resistance analysis was applied to confirm

4

the electrochemical analysis results qualitatively. Salt spray tests was applied in this method. the

5

test of the coated aluminum alloy substrates were done by exposing the samples to a salt fog

6

atmosphere generated by spraying 5 wt.% aqueous NaCl solution at 35±1°C for 2000 hours in

7

accordance with ASTM B117 specifications. After removal from the salt fog chamber, all

8

specimens were rinsed with distilled water to remove any residues and finally the images of the

9

samples were investigated.

10 11

3. RESULTS AND DISCUSSION

12

3.1. Nanoparticle formation. Morphology and particle size analyses were used to show

13

nanoparticles formation. TEM images (Figure 1) of the samples ES3 and ES5 which contain the

14

TiO2 and AlOOH nanoparticles, respectively show that the formed nanoparticles were uniform

15

with diameters of 40-50nm and 50-60nm, respectively. Furthermore these images reveal that the

16

AlOOH nanoparticles were further agglomerated in comparison to TiO2 nanoparticles (Figure

17

1b). The higher agglomeration of the AlOOH nanoparticles can be attributed to higher surface

18

energy induced by hydroxyl groups

27

. To confirm the above results particle size analysis was

used to confirm formation of these nano size particles.

19 20

Titanium dioxide and boehmite sol particle sizes were measured with a laser diffraction

21

particle size (DLS) analyzer. Particle size distributions of the both sols are shown in Figure 2.

22

Particle size of the TiO2 and AlOOH sols were 40 nm with polydispersity index of 1.28 and

23



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55nm with poly dispersity index of 1.53, respectively. As it was observed TiO2 particles were

1

smaller than boehmite and particle size distribution of particles in TiO2 sol is narrower than that

2

of in AlOOH sol. These results also indicated the higher tendency of the AlOOH nanoparticles to

3

agglomeration compared to TiO2 nanoparticles.

4

5

Figure 1. TEM images of (a) ES3 and (b) ES5 hybrid films.

6

7

Figure 2. Size distribution of TiO2 and AlOOH nanoparticles.

8 9

3.2. ATR-IR Characterization. ATR-IR spectroscopy first was used to show the formation of

10

the nano particles from the precursors and then to confirm the curing reaction of the epoxy-silica

11

coatings.

12


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The FT-IR spectra of the TBT (Ti(O-n-Bu)4) and the TiO2 sol after hydrolysis reaction were

1

investigated in our previous research in which all OR groups of the TBT were converted to OH

2

groups via hydrolysis reaction

28

. Figure 3 presents the ATR-IR spectrum of the Al(C4H9O)3

3

(TBA) and the AlOOH (boehmite) sol after hydrolysis reaction. A comparison of these spectra

4

indicates that almost all of the initial aluminum butoxide (TBA) was converted to the AlOOH

5

during the hydrolysis reaction according to scheme 1. In Figure 3b, a broad band between 3200-

6

3600 cm-1 is assigned to the fundamental stretching vibration of different O–H (free or bonded)

7

groups attributed to boehmite particles and water

29

. Water incorporation is confirmed by

8

absorption in the 1622 cm-1. Characteristic absorption peaks for the OR groups of Aluminum

9

alkoxide, which is the precursor of the sols, occurs at 915, 988, 1056 and 1100 cm-1

. Since

10

there is no absorption peak in the boehmite particle spectrum (Figure 3b), it may be concluded

11

that all three OR groups of TBA were replaced by OH groups from water. An additional band

12

appearing in 1065 cm-1 may be referred to the boehmite structure 30.

13

30

The cross-linking of organo-silica networks via chemical coupling of Bisphenol A and epoxy

14 15 16 17

functionalities (scheme 2a) was demonstrated by ATR-IR spectroscopy. Figure 4 (spectra a and

18

b) show ATR-IR spectra for ES1 hybrid films prepared before and after adding Bisphenol A as

19

Figure 3. FT-IR curve for (a) TBA and (b) AlOOH.



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cross-linking agent, respectively. The most resolved bands were attributed to several vibrational

1

frequencies of the GPTMS structural fragments, including oxirane methylene bending at 1480

2

cm−1, epoxide ring breathing band at 1250 cm−1, and antisymmetric epoxide ring deformation

3

bands at 750 and 906 cm−1. The band of the antisymmetric epoxide ring deformation seemed to

4

be almost intense and allowed monitoring the chemical reaction of coupling of epoxy

5

functionalities of GPTMS with Bisphenol A as cross-linker. As it can be seen in Figure 4

6

(spectra b), this band almost disappeared in ATR-IR spectra of the cured ES1 coating indicating

7

formation of a cross-linked network of silica-epoxy (scheme 2).

8

Figure 4 ATR-IR spectra of ES1 organic-inorganic hybrid films (a) before and (b) after adding cross-linking agent, (c) hybrid film coated on AA1050 and (d) bare AA1050 substrate.

9 10 11 12

Also ATR-IR spectra of ES1 hybrid–AA1050 interface is presented in Figure 4. The Al-O-Si

13

bond (scheme 2b) was observed at 939 cm-1 which represented the formation of networks

14

between oxide layer on AA1050 substrate and silicates

12

. This observation was indirectly

15

supported by the SEM cross-section images of the hybrid coating on the AA1050 substrate

16


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which showed good adhesion of film to AA1050 substrate which will be discussed later (see

1

Figure 9).

2

3.3. Surface characteristics of the coatings. The formed nanoparticles have been

3

incorporated in the organic-inorganic hybrid matrix to prepare a nanocomposite coating. Hence

4

the surface characteristics of the filled and unfilled coatings are important to be investigated.

5

Figure 5 shows the SEM images of the surface of the hybrid films coated on AA1050

6

substrates. As it can be observed, all of the samples (except ES6) have a crack free surface and

7

the coatings without filler (ES1 and ES2) showed smoother surface than the coatings with filler

8

(ES3, ES4, and ES5). These observations indicate that the nanoparticles may result in

9

enhancement of the surface roughness. However, the hybrid coatings containing TiO2 and

10

AlOOH nanoparticles represent uniform, homogeneous, crack free and protective film with high

11

adhesion on the AA1050 substrates (Figure 5c-e).

12

An advantage of the organically modified hybrid systems is the possibility of preparing thick, crack-free films

12

13

. So the pure inorganic coatings from TMOS (i.e. ES6) showed distinctive

14

cracks on the surface (Figure 5f). This can be related to the characteristics of the inorganic

15

materials. Although inorganic component contributes to the increase of scratch resistance,

16

durability and adhesion to the metal substrate, it is so brittle which results in crack initiation and

17

propagation in ES6 (include 100% silane content).

18



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Page 16 of 36

1

Figure 5. SEM images of the different hybrid nanocomposite sample films on AA1050 substrate: (a) sample ES1 (b) sample ES2, (c) sample ES3 (d) sample ES4, (e) sample ES5 and (f) sample ES6.

2 3 4 5

Another observation is increasing surface roughness of the hybrid coating with increasing

6

silane content (Inorganic component). Sample ES2 with 80% organic content and the sample

7

ES3 with 66.67% organic content were different in smoothness. This result has been confirmed

8

by AFM analysis.

9

AFM technique was used to monitor the surface roughness and distribution of particles in the

10

coated surface. The three-dimension AFM images of the ES1, ES2 and ES3 organic-inorganic

11

nanocomposites coatings are shown in Figure 6. As it is observed the surface is smoother for the

12

ES1 than other samples. The sample ES2 (with 80 percent of organic content) show higher

13

roughness (Figure 6b). The ES3 coatings exhibit much rougher surfaces than ES2 film, which is

14

due to the presence of the TiO2 nanoparticles and 66.67% organic content in the ES3 film. From

15

the micrographs, it can be seen that TiO2 particles are more homogeneously dispersed in the

16


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film. The surface roughness values of the ES1, ES2 and ES3 nanocomposite coatings are

1

estimated to be about 0.834, 1.13, and 1.25nm for the root mean square (RMS) roughness (Rq),

2

and about 0.902, 1.89, and 1.92nm for the average surface roughness (Ra), respectively, over a

3

length span of 15µm. These results are related to the small size of the particle on these surfaces

4

and are in agreement with contact angle results.

5 6

7

Figure 6. AFM topographies of the (a) ES1, (b) ES2 and (c) ES3 coatings.

8 9

The TiO2 and AlOOH nanoparticles distributions in the organic-inorganic hybrid matrix were

10

also studied using an SEM mapping technique. A series of SEM elemental mapping images of

11

the as-synthesized samples showed that the TiO2 and AlOOH particles were evenly dispersed

12

and distributed throughout the composite samples. Series of SEM mapping images and the EDS

13

pattern of ES4 nanocomposite are shown in Figure 7. Well distributions and dispersion of TiO2,

14

AlOOH and silane network were observed in Ti, Al and Si mapping of sample ES4 which

15

contains all components. Furthermore, EDS analysis was carried out at the surface of sample

16

ES4 (Figure 7-b). The uniform dispersion of the TiO2 nanoparticles and AlOOH particles

17

suggests a successful incorporation of TiO2 nanoparticles and AlOOH particles into hybrid

18

matrix.

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1

The wettability and surface tension of the coating film was evaluated by contact angle

2 3 4 5 6

measurements using water and methylene iodide (MI) as probe liquids. The contact angles of the

7

coating film with different amounts of inorganic content and type of nanoparticles is shown in

8

Table 2. It is observed from Table 2 that the lowest contact angle belongs to the sample ES1

9

(with lowest inorganic content) and sample ES5 (see Table 1, with AlOOH nanoparticles).

10

Figure 7. The microscopic analysis of sample ES4 (a) SEM surface micrographs, (b) EDS elemental pattern, (c) Ti mapping, (d) Si mapping and (e) Al mapping.

11 12

Table 2: The result of contact angle and surface energy measurements of the specimens. Sample code

θWa

θMIb

γp (mj/m2)

γd (mj/m2)

γ (mj/m2)

XP= γp/γ

ES1

75.2

49.2

11.37

35.80

47.18

0.24

ES2

77.6

48.5

10.23

36.11

46.34

0.22

ES2-E

76.8

47.2

10.35

36.08

46.67

0.22


13

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ES3

89.3

52.1

5.64

34.39

40.03

0.14

ES4

79.9

52.7

9.63

34.07

43.70

0.22

ES5

66.6

55.5

16.31

32.68

48.99

0.33

-

-

-

-

ES6 a

-

-

Among the effective factors on contact angle and surface tension are surface roughness 31 and

1 2 3 4

surface functional groups 32. According to the Cassie model 33 roughness enhances contact angle

5

which this phenomenon was observed here; increasing inorganic nanoparticles content from 0

6

mol% (ES1) to 20 mol% (ES2) caused roughness increment and hence contact angle

7

enhancement.

8

b

Contact angle with Water Contact angle with methylene iodide (MI)

Based on the obtained results not only inorganic particle content but also the type of the

9

inorganic particle affects on the contact angle. Replacing TiO2 nanoparticles with AlOOH

10

nanoparticles resulted in a decrement in contact angle. This can be attributed to higher polarity of

11

the AlOOH nanoparticle-containing surface compared to TiO2 nanoparticle one. The polarity can

12

be related to higher amount of hydroxyl group on AlOOH than TiO2 nanoparticles surfaces.

13

To get more information on the surface energy of the hydrophilic coating, the contact angle

14

measurements were carried out by using probe liquids (MI) rather than water on the samples of

15

hybrid nanocomposite coatings. The properties of the liquids have been collected in Table 2. The

16

values of the polar (γp) and dispersive (γd) components of the surface tension together with the

17

polarity parameter XP (defined as XP=γp/γd) were obtained by the Wu equations

. The results

18

of the energy calculation by this method on the coating surface are presented in Table 2.

19

According to listed results in this Table, addition of inorganic content (silane, TiO2 and AlOOH

20

nanoparticles) had a remarkable effect on the wetting behavior of the specimens. For example

21

the XP values of the samples ES2 and ES3 is lower than ES1. This can be due to increment of

22


26


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Page 20 of 36

surface roughness and decrement of surface energy because increasing inorganic filler content

1

led to higher surface roughness which was confirmed by AFM analysis. While XP of the sample

2

ES5 (ES2+AlOOH) is approximately 50% higher than that of the coating without AlOOH (i.e.

3

ES2). One possible explanation is the incorporation of hydrophilic group namely hydroxyl on the

4

AlOOH nanoparticles that caused the induced hydrophilicity to the surface. Hydroxyl groups can

5

decrease the interfacial tension of condensed water droplets which leads to water film

6

condensation on the surface 34.

7

3.4. Thermal properties. The thermal stability and the amount of inorganic content of the

8

hybrid coatings were evaluated by thermogravimetric analysis (TGA). The variations in weight

9

loss of the samples were shown in Figure 8 ,from ambient temperature to ~610°C under nitrogen

10

atmosphere. The values of the initial degradation temperature for 5% weight loss (Td5%), the

11

major decomposition temperatures (Tmd) and the residual weight percent at 610°C (R610) were

12

also presented in Figure 8.

13

In all the samples, the initial weight loss was observed in temperature range of ca 200°C

14

which is related to the low molecular evaporation. For samples ES3, ES4 and ES5, the initial

15

weight loss was greater than the samples ES1 and ES2, due to a great amount of solvent and

16

EAcAc used in the sample preparation.

17

Based on Figure 8, the Tmd of the coatings containing nanoparticles (i.e. samples ES3, ES4

18

and ES5) occur at lower temperature than coatings without nanoparticles. This behavior may be

19

better discussed via changes in network cross link density. The samples without nanoparticle (i.e.

20

ES1 and ES2) have higher cross link density and smaller chains

35

, hence exhibiting highest

21

thermal stability. Therefore, the Tmd of the ES2 reduced from 450°C to 402, 375 and 370°C for

22

sample ES3, sample ES4 and sample ES5, respectively. The difference in Tmd reduction of the

23


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samples ES3, ES4 and ES5 may be attributed to difference in thermal conductivity of the

1

nanoparticles used, i.e., TiO2 and AlOOH. The thermal conductivity of the AlOOH (30W/mk) 36

2

is higher than TiO2 nanoparticles (11.7W/mk)

37

resulting to lower thermal stability of the

samples containing AlOOH nanoparticles.

3 4

At temperatures exceeding 500 °C, almost all the organic part of the sample was destroyed,

5

resulting to remaining inorganic weight of sample as char yield. The residual weight percent at

6

610°C (R610) for ES1, ES2, ES3, ES4 and ES5 was measured at about 35.25, 39.49, 51.83, 49.72

7

and 46.79 wt.%, respectively. Due to lack of nanoparticles, the ES1 sample showed the lowest

8

char yield (28.21%), as shown in Figure 8. Whereas, the samples ES3, ES4 and ES5 showed

9

higher percent of inorganic phases (silane network, AlOOH and TiO2 nanoparticles) with

10

approximately the same char yields. On the other hand, the R610 of the sample is different as

11

shown by the following: ES3>ES4>ES5. This effect is due to the differences of the molecular

12

weights of the nanoparticles (MWTiO2=80gr/mol and MWAlOOH=60gr/mol), because similar molar

13

ratios of nanoparticle precursors were selected. So when TiO2 was replaced with AlOOH

14

nanoparticles the R610 value was reduced from 51.83% (ES3) to 46.79% (ES5).

15 16



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Page 22 of 36

1

Figure 8. The variations in the weight losses of different hybrid coating samples.

2 3

3.5. Adhesion Quality. As the final application of these nanocomposites is coating, therefore

4

surface and interfacial surface morphologies are the key parameters to show their performance.

5

Interfacial surface adhesion was investigated by SEM cross-section micrograph of the ES5

6

hybrid coating on the AA1050 substrate (Figure 9). A good adhesion was observed, as no crack

7

and gap were formed in the interface between the coating and AA1050 substrate which can be

8

related to the formation of Al–O–Si bonds between silica particles of the coating and AA1050

9

substrate that latterly confirmed by ATR-IR analysis. The thickness of the films was evaluated to

10

be ~20µm.

11

12

Figure 9. SEM cross-section micrograph of the sample ES5 on the AA1050 substrate. 22 of 36 ACS Paragon Plus Environment

13

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The strength of adhesion of hybrid films coated on AA1050 substrate was measured by cross-

1

cut test according to the ASTM D 3359B-02. The SEM images of the samples after cross-cut test

2

are shown in the Figure 10. The yellow arrows on the SEM micrographs indicate the removed

3

area of the coating from AA1050 substrate during by flaking step by tape. The results are

4

specified as a 5B class (the highest adhesion strength and percent of removed area is 0%) for the

5

ES2, ES3, ES4 and ES5 series and 4B (percent of removed area is less than 5%) for the ES1 and

6

ES6 specimens. According to the results of cross-cut test and SEM cross-section, a high level of

7

adhesion was observed between the coatings and the AA1050 substrate. The adhesion of the

8

films on AA1050 surface is based on the condensation of hydroxyl groups of the AA1050

9

surface (Al-OH) and the hybrid coating sols (Si-OH) that redound to formation of Al-O-Si bond

10

(as explained by ATR-FTIR analysis). Among these films the samples ES1 and ES6 showed

11

lower level of adhesion in comparison to the other samples. This is due to the absence of one of

12

the organic or inorganic phases which in ES1 with 100 mol.% organic content resulted in low

13

Al-O-Si bonds and in ES6 with 100% inorganic content caused the brittleness of inorganic film

14

and hence low adhesion on the AA1050 substrate. According to the cross-cut results, ES2

15

(GPTMS/TMOS: 4/1) composition was selected for investigation of AlOOH and TiO2 effect on

16

corrosion protection of the GPTMS based coating and composition of other samples (i.e. ES1

17

and ES6) were not studied further for this purpose due to the inability to obtain a good adhesion.

18

Adhesion measurement on ES3 (ES2+TiO2), ES4 (ES2+TiO2+AlOOH) and ES5 (ES2+AlOOH)

19

shows the TiO2 and AlOOH nanoparticles have good compatibility in GPTMS based hybrid

20

films with 5B class of adhesion quality for these specimens.

21 22



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Page 24 of 36

1

Figure 10. The SEM images of the different sample films after the cross-cut test according to the ASTM D 3359B-02. Removed areas during flaking step, are indicated by the yellow arrows on the SEM micrographs.

2 3 4 5

3.6. Corrosion Protection of the Coatings. Corrosion resistance properties of hybrid coatings

6

were investigated using potentiodynamic scan (PDS) method. By this method, corrosion

7

protection of sample-coated AA1050 coupons can be observed from the values of corrosion

8

potential (Ecorr), polarization resistance (Rp), corrosion current (icorr) and corrosion rate (Rcorr).

9

Figure 11 shows the typical polarization curves (Tafel plots) of bare and hybrid sol-gel coated

10

AA1050 substrates. Electrochemical parameters obtained from above Tafel plots are listed in

11

Table 3. The values in the Table 3 are not strictly Rp and Rcorr parameters (Eq. 1 and Eq. 2,

12

respectively), however comparisons between samples are still valid since the sample geometry

13

was the same for all nanocomposites coatings and all samples were tested under the same

14

conditions.

15

The corrosion rate (Rcorr, in mm/year) was evaluated from the following equation 38: R mm⁄year

⁄ . / . !"

# 3270


(Eq. 1)

16 17

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where icorr is the corrosion current (A/cm2) determined by an intersection of the linear portions

1

of the anodic and cathodic curves, M is the molecular weight (g/mol), D is the density (g/cm3),V

2

is valence and 3270 is constant.

3

The polarization resistances (Rp) were calculated from the polarization curves (Tafel plots), according to the Stearn–Geary equation 39: R(

4 5

)* ) +.,-,)* .)

(Eq. 2)

Here, ba and bc are anodic and cathodic Tafel slopes (E/log i), respectively.

6 7

8

Figure 11. Polarization curves of the coated and uncoated AA1050 substrate. Comparison effect of (a) silane precursor, (b) silane content and (c) type of nanoparticles on the corrosion resistance of the as-prepared hybrid coatings.


9 10 11 12 13 14


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Page 26 of 36

Table 3. Electrochemical parameters obtained from Tafel plots.

a

Sample code

Ecorr(V)

Barea

1

icorr

Rp

Rcorr

ba

bc

(A/cm2)

(Ω)

(mm/year)

(V/dec)

(V/dec)

-1.42

6.61×10-4

1.32×102

5.93×100

-1.10

0.17

ES1

-0.95

2.68×10-8

4.61×106

2.41×10-4

0.70

0.48

ES2

-1.27

1.73×10-7

2.49×105

1.55×10-3

0.58

0.12

ES2-E

-1.17

1.52×10-6

3.70×104

2.11×10-2

0.62

0.30

ES3

-0.45

4.30×10-9

6.63×107

3.86×10-5

1.48

1.19

ES4

-0.62

2.20×10-10

1.26×108

2.02×10-6

0.14

0.12

ES5

-1.32

1.29×10-8

1.33×106

1.16×10-4

0.17

0.06

ES6

-1.34

3.51×10-5

1.82×103

0.32

0.18

0.71

2

Pristine AA1050 used for test.

3

The corrosion protection properties of sol-gel derived coatings are strongly dependent on the

4

several factors such as aging, type of nanofillers, curing temperature and the percent of organic

5

content 12. In this research, firstly, the corrosion protection of the GPTMS based hybrid coating

6

with TEOS (ES2-E) was determined in order to compare this with the corrosion protection of the

7

TMOS one (ES2) and then the effect of silane content or silane network on corrosion resistance

8

of epoxy-silica hybrid coatings was studied. Furthermore the effects of the type of nanofillers

9

(i.e. TiO2 and AlOOH nanoparticles) on corrosion protection properties of the hybrid coatings

10

were investigated.

11

The effect of the type of silane precursors (i.e. TEOS (sample ES2-E) and TMOS (sample

12

ES2)) on the corrosion protection behavior of the GPTMS based hybrid coatings can be observed

13

in Figure 11a. According to Tafel curves of these samples, the TMOS precursor resulted in

14

higher corrosion resistance of the coatings in comparison to TEOS. The corrosion resistance of

15

ES2 and ES2-E were 2.49×105 Ω and 3.70×104 Ω, respectively. This phenomenon may be

16

related to the trapped alcohol molecules in the network during silane network formation 40. Small

17


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molecules of methanol which is formed in TMOS network evaporates and separates rapidly from

1

the sol during post curing at 130°C while the formed ethanol molecules in TEOS are larger and

2

hence difficult to diffuse which cause pore and non homogeneity formation in the network.

3

Explanation of lower corrosion protection of ES2-E sample compared to ES2 sample could be

4

found by morphology study of the surface after the PDS analysis. The SEM images of the ES2-E

5

and ES2 samples after the PDS analysis are shown in the Figure 12. As seen in this Figure, the

6

surface of the sample ES2-E has higher porous structure than sample ES2. The yellow arrows on

7

the SEM micrographs of the ES2-E surface show the few created pours during PDS analysis.

8

This phenomena indicate the corrosive species easily diffuse through the sample ES2-E resulting

9

to more pores on sample surface. Therefore, the samples containing TEOS were not studied

10

further due to the inability to obtain a good corrosion protection.

11 12

13

Figure 12. The SEM images of the samples ES2 and ES2-E after the PDS analysis.

14 15

We prepared three-hybrid coatings (ES1, ES2 and ES6) with 1:0, 1:0.25 and 0:1 molar ratio

16

of GPTMS/TMOS to study of the silane content effect. It is found that silane content have a

17

significant influence on the corrosion protection of the sol-gel coatings. Figure 11b shows the

18

polarization curves of ES1, ES2 and ES6 hybrid coatings. The polarization curves of the sol-gel

19



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Page 28 of 36

coated electrodes were markedly different from that of the bare AA1050 substrate. First, the

1

open circuit potential (OCP) of the hybrid coatings was significantly higher than that of the

2

uncoated AA1050 substrate. Secondly, the open circuit current density of the created samples

3

(2.68×10−8 A/cm2 for ES1) was far smaller than that of the bare AA1050 substrate (6.61×10−4

4

A/cm2). These results indicate that the epoxy-silica hybrid film is a true barrier coating and

5

implied that the sol-gel coating provided a physical barrier for blocking the electrochemical

6

process. As shown in Scheme 3(a), the corrosion protection mechanism of the epoxy-silica

7

hybrid coatings includes the formation of high strength and continuous intermediate layer and

8

good physical barrier property of the coating.

9

According to polarization curves in Figure 11b and corrosion parameters in Table 3, the

10

sample with lower silane content had shown better corrosion protection properties than the

11

substrate with the higher silane content. The polarization resistance decreased from 4.61×106Ω

12

(ES1) to 2.48×105Ω (ES2) and 1.82×103Ω (ES6), respectively. Also, corrosion rate was

13

increased from approximately 2.41×10−4 mm/year to 1.55×10−3 mm/year (for specimen ES2) and

14

3.15×10−1 mm/year (for specimen ES6), respectively. This observation can be related to the

15

formation of porous coating in higher silane content. Higher silane content led to formation of

16

larger linear silica chains and stronger gel network. Upon evaporation of ethanol during drying,

17

the chains would be more resistance to the capillary-driven collapse of the gel network, resulting

18

in the formation of a more porous structure 41.

19

Figure 11c shows the electrochemical polarization curves for the AA1050 coated with ES3,

20

ES4 and ES5 nanocomposites with ∼20µm thicknesses, observed by SEM image, in 5% NaCl

21

solution at room temperature. Electrochemical parameters obtained from Tafel plots have been

22

listed in Table 3. All films show a similar polarization behavior. Based on the electrochemical

23


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parameters, the nanoparticle containing coatings (i.e. specimens ES3, ES4 and ES5) show better

1

corrosion resistant than the specimen without filler (i.e. ES2).

2

The nanocomposite-coated AA1050 (i.e. ES3) shows a polarization resistance (Rp) value of

3

6.63×107Ω in 5 wt.% NaCl, which is about several times greater than polarization resistance of

4

the ES2-coated AA1050. Enhanced corrosion protection effect of nanocomposite materials might

5

be resulted from dispersing the nanoparticles in epoxy-silica hybrid matrix which increases the

6

tortuosity of diffusion pathway of water and corrosion agents (e.g., chloride and oxygen) and

7

appears as an efficient barrier 38, as shown in Scheme 3(b).

8

9

Scheme 3. Schematic diagram of (a) mechanism of the corrosion protection by epoxy-silica hybrid coatings and (b) diffusion pathway of corrosion agent in the as prepared hybrid coating.

10 11 12

For all coatings containing nanoparticle the same trend was observed. But the corrosion

13

resistance of the sample containing AlOOH nanoparticles was lower than TiO2 nanoparticles

14



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Page 30 of 36

one. This can be attributed to the hydroxyl groups on the AlOOH nanoparticle surface which

1

induce hydrophilicity and hence easier diffusion of moisture and corrosive materials throughout

2

the coating. Effect of the nanoparticles on the contact angle was discussed earlier which confirms

3

this interpretation. The interesting observation is higher corrosion resistance of the coating

4

containing both TiO2 and AlOOH nanoparticles which shows synergistic effect of these

5

nanoparticles. Although alumina sol has more hydroxyl and organic groups than TiO2 sol which

6

resulted in lower corrosion resistance in ES5 compared to ES3 containing TiO2 particles, low

7

amount of alumina in specimen ES4 causes an increase in corrosion resistance due to the

8

difference in energy levels 42.

9

Figure 13 shows the results of 2000 hours salt spray test of bare and hybrid sol-gel coated

10

AA1050 substrates according to ASTM B117. Removal of residual hydroxyl and organic groups

11

by environment leads to some defects in the microstructure of thin film which can propagate

12

crack formation. Therefore the increase in corrosion rate can be due to the development of

13

defects (like pores and cracks) in the hybrid nanocomposite film when subjected to the corrosive

14

environment

43

. The occurrence of cracks in the structure of a ES5 hybrid coating, due to long-

15

term (2000 hours) exposure to the corrosive media (NaCl 5% solution), is observed in SEM

16

image of Figure 13. These results show that the hybrid sol-gel film is a true barrier coating.

17

Moreover, the corrosion protection properties of sol-gel derived coatings are strongly dependent

18

on the silane content and the type of the nanofillers. The results of the salt spray test have good

19

agreement with electrochemical measurement results and confirmed them.

20


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1

Figure 13. 2000 hours salt spray tests for bare and hybrid sol-gel coated AA1050 substrates.

2 3



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Page 32 of 36

4. CONCLUSIONS

1

A compact and uniform hybrid coating was successfully prepared on the AA1050 substrates

2

by the sol-gel method. PDS and salt spray investigations showed that the corrosion protection

3

properties of sol-gel derived coatings are strongly dependent on the type of silane precursors (i.e.

4

TMOS & TEOS), silane content and the type of nanoparticles. The SEM observations showed

5

the formation of a uniform, homogeneous, crack free and highly adherent protective film on the

6

AA1050 substrates. The SEM images of the samples after the PDS analysis showed that the

7

TEOS induce higher porous structure to hybrid coating than TMOS. The cross-cut results

8

showed that the adhesion of hybrid film on aluminum substrate improved by adding the silane

9

content but the corrosion protection was reduced. In addition, the effect of synthesized TiO2 and

10

AlOOH nanoparticles on the corrosion protection of the GPTMS based were investigated. The

11

PDS results showed that the coating with TiO2 nanoparticles exhibits higher corrosion resistance

12

than coating with AlOOH nanoparticles. However, using both of them in coating composition

13

led to a synergistic effect on the corrosion resistance. TEM images indicated that the synthesized

14

AlOOH and TiO2 particles had uniformity in particle sizes with diameters of 50-60nm and 40-

15

50nm, respectively. The Al, Ti and Si mapping micrographs confirmed a uniform distribution of

16

nanoparticles in the coating matrix.

17

The hydrophilicity of the surface depends on the inorganic content in GPTMS based coatings

18

and it is reduced by increasing the inorganic content. However, the AlOOH nanoparticles in

19

hybrid composition showed a different effect by increasing hydrophilicity of the coating surface

20

as a result of its hydrophilic groups (namely hydroxyl).

21 22


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(1)

REFERENCES

1

Fontana, M. G. Corrosion Engineering. Tata McGraw-Hill Educ.: New York, 2005.

2

(2) Bastos, A. C.; Ferreira, M. G.; Simões, A. M. Corrosion inhibition by chromate and phosphate extracts for iron substrates studied by EIS and SVET. Corros. Sci. 2006, 48, 1500.

3 4

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