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CHAPTER 9

Anticorrosive Coatings on Metal

Substrate by Sol-Gel Dip Coating Method

Chapter 9 Anticorrosive Coatings on Metal Substrate by Sol-Gel

Dip Coating Method

201

Chapter 9

Anticorrosive Coatings on Metal Substrate by Sol-Gel Dip Coating

Method

9.1 Introduction

There is a current need for alternative coatings that can provide

corrosion resistance to metals or alloy surfaces due to the environmental

hazards posed by conventional coatings. The basic concept of chemical

conversion of metal surfaces is based on deposition of a hydrophobic sol–gel

barrier layer for surface protection and corrosion prevention. The properties of

these organosilica coatings can be tuned by varying the composition of

precursors. The evaluation of hydrophobicity, adhesive strength, and

anticorrosion properties of organically-modified sol–gel derived coatings

suggests their potential utility as technologically-compatible alternatives to

conventional coatings. The deposition of sol-gel coatings on metals is relatively

recent and has been not sufficiently investigated, in spite of its potential

technological interest. Sol–gel-derived coatings have been found to be useful

for several applications mainly due to the ease of solution based processing and

the synthesis flexibility which can be used for forming a wide range of thin

films and coatings [1, 2]. Using the sol–gel process, it is possible to deposit

films with variable thickness from 100 A° to several µm. In addition, the use of

organically-modified precursors provides unique opportunities to tailor the

physical and chemical properties of the final materials. Due to the presence of

an organic component, the organosilica coatings dry evenly and are more

uniform and crack-free as compared to pure silica coatings. While there has

been significant research activity in the use of sol–gel coatings for corrosion

protection [3–5], efficient coating formulations that provide significant

protection as a viable alternative to conventional coatings. One of the critical

issues with sol–gel-derived coatings has been their poor adhesion to the metal

surface due to weak non-covalent binding to the substrate. An additional

concern is their porous nature, which makes them permeable to ions, moisture,


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and other corrosive species. In this context, organosilica sol–gel materials

furnish unique advantages [6]. The properties of sol–gel-derived coatings can

be engineered at the molecular level [7] for optimum physical and chemical

properties such as better adhesion, improved hydrophobicity, low permeability,

as well as texture, morphology, optical properties, and other characteristics.

These materials can also be easily processed in the form of a coating using

inexpensive, environmentally-friendly, and technologically-compatible

methods.

Herein, we used the organosilica sol–gel materials for coating metallic

substrate. Methyltriethoxysilane (MTES) precursor is used to prepare

hydrophobic coatings on copper substrate which not only provide improved

adhesion but also act as a barrier protection layer for minimizing the

permeability of corrosive species. It is found that the coatings are effective at

preventing corrosion of metal substrate. These films are more elastic as

compared to TMOS-derived silica coatings and therefore do not undergo

cracking. These coatings act as barrier layers for metal surfaces for preventing

corrosion. The presence of organic groups also renders these materials

hydrophobic [8] making them impermeable to ions, moisture, and other

hydrophilic species as compared to pristine sol–gel-derived silica coatings.

Thus, by a judicious choice of the precursor, it is possible to impart desired

properties to the final material such as adhesion, water-repellency, and

hydrophobicity. Overall, the strategy presented herein may provide a generic

approach for fabrication of protective coatings on different metallic surfaces.

9.2 Experimental

9.2.1 Preparation of silica films

The hydrophobic silica coating on copper substrates have been prepared

by sol-gel process using dip coating method from an alcoholic solution

containing silica precursor Methyltriethoxysilane (MTES), methanol (MeOH),

and ammonium hydroxide (NH4OH). The chemicals used were

methyltriethoxysilane, (Sigma-Aldrich Chemie, Germany), methanol (s.d.fine-


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chem limited, Mumbai), and ammonia (NH3, sp.gr.0.91, Qualigens fine

chemicals, Mumbai). Double distilled water was used for all the experiments.

All the reagents were used as received.

Prior to the deposition of the hydrophobic films on copper substrates,

the substrates were cleaned in order to get uniform deposition. Pieces of 1 cm ×

5 cm were cut from copper sheet and used as substrates. These substrates were

mechanically polished using zero grade polish paper as an abrasive. This

practice removed the grease and the native oxide layer from the surface of the

copper plate. The coating solution was prepared under basic condition from the

MTES, CH3OH, and H2O in molar ratio of 1:19.1:4.31 respectively with 7M

NH4OH. The MeOH/MTES (M) molar ratio was varied from 9.5 to 19.1. The

coating solution was stirred for approximately 15 minutes.

After substrate preparation and sol preparation, film deposition on the

copper substrates utilized a simple dip-coating process. The substrates were

dipped in the sol at a constant rate of 6 mm/min, immersed in the sol for

approximately 40 minutes, withdrawn at the same constant rate, and then air-

dried for approximately 30 minutes. Following deposition, the substrates were

sintered at 250°C for 3 hours at a heating rate of 2°C/min to ensure

densification of the gel network.

9.3 Results and discussion

9.3.1 Reaction Mechanism

The MTES silicon alkoxide contains one non-hydrolysable methyl

group and three hydrolysable ethoxy groups. Therefore three hydrolysable

ethoxy groups undergo hydrolysis and lead to the formation of monomeric

units of the - Si(OH)3 which are responsible for the formation of silica network.

The hydrolysis and condensation reactions of the MTES are as

per the following chemical reactions:


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Hydrolysis:

(9.1)

Condensation:

(9.2)

9.3.2 Surface Morphological Studies

The two-dimensional morphological study of the water repellent silica

films have been carried out using the SEM micrographs. Figure 9.1 (a) and (b)

shows the surface morphology of the silica films prepared with M = 12.7 and

M = 19.1, respectively. Figure 9.1 (a) shows the irregular shaped silica particles

which are non-homogeneously spread on the copper substrate. In the case of

silica film prepared with M = 19.1 (figure 9.1 (b)), the SEM micrograph shows

spherical silica particles distributed on the copper substrate. The high

magnified SEM micrograph of this film (figure 9.1 (c)) shows very well

spherical shaped silica particles with each having diameter typically ranges

from 11 to 15 µm, distributed on the copper substrate.

+ 3H2O

OC2H5

OC2H5

H3C OC2H5

Si + 3C2H5OH

OH

OH

H3C OH

Si

+ 4H2O

OH

CH3 Si

O

OH

H3C O

Si

O

H3C O

Si

OH

CH3 Si

OH

2

OH

H3C OH

Si

OH

OH

CH3 HO

Si

OH

+


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205

Figure 9.1(a): SEM image of the silica film prepared with M = 12.7.

Figure 9.1(b): SEM image of the silica film prepared with M = 19.1.


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This is due to the fact that the lower dilution of MTES (less M value)

has high catalyst concentration in the sol during the hydrolysis and

condensation reactions. Therefore, there is a rapid clusterification of siloxanes

which give rise to dense and irregular network structure. However, an increase

in the dilution of MTES in methanol reduces the base concentrations and forms

well tailored 3D network structure with bigger well shaped particle sizes.

9.3.3 Atomic Force Microscopy (AFM)

Figure 9.2 (a) and (b) shows the three dimensional atomic force

microscopy images of the silica films prepared with M = 12.7 and M = 19.1,

respectively. The images were recorded at 1×1 µm2 planar in contact mode.

The surface of the films has many dispersed islands that are distributed on the

film surface. The silica film prepared with M = 12.7 and M = 19.1 showed a

RMS roughness value of only 5 and 16 nm, respectively. The increase in

Figure 9.1(c): SEM image of the silica film prepared with M = 19.1 at

magnification of 5000x.


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surface roughness value in the case of the silica film prepared with M = 19.1

contributes higher contact angle.

Figure 9.2 (a): AFM image of the silica film prepared with M = 12.7.

Figure 9.2 (b): AFM image of the silica film prepared with M = 19.1.


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9.3.4 Fourier Transform Infrared Studies

The chemical composition of the films deposited on copper substrate

was investigated by the FT-IR spectroscopy using the KBr method in

transmission mode. Several characteristic absorption peaks were observed in

the range 450 to 4000 cm-1

indicating the presence of methyl groups in the

sample. The FT-IR spectra of the silica films prepared from M = 12.7 and M =

19.1 are shown in figure 9.3 (a-b), respectively.

The peak at 1074 cm-1

corresponded to the Si–O–Si asymmetric

stretching vibration [9]. The presence of this peak confirms the formation of a

network structure inside the film. The absorption bands observed at around

2950 cm-1

and 1400 cm-1

are due to stretching and bending of C-H bonds and

Figure 9.3: The FT-IR spectra of the silica films prepared from

(a) M = 12.7 and (b) M = 19.1.


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the peaks observed at 847 cm-1

are due to the Si-C bonds [10]. The peak at

around 1600 cm-1

and the broad absorption band at around 3400 cm-1

are due to

the –OH groups [11]. The residual Si-OH groups are the main source of

hydrophilic character. The OH peaks are quite visible for silica films prepared

from M = 12.7. With an increase in M value at M = 19.1, the intensity of the

peak at 1600 cm-1

and the broad OH absorption band at 3400 cm-1

decreased,

whereas the intensities of the C-H absorption peak at around 2950 cm-1

and Si-

C absorption peak at around 840 cm-1

increased. The Si-OH band seen in both

the FT-IR spectra indicates that surface hydroxyls still exist, even though the

materials show the strong hydrophobic properties.

As expected, when organic moiety is removed by a thermal treatment in

air, the hydrophobic character is irreversibly changed to hydrophilic. The

influence of temperature on the water-repellency is systematically investigated

in order to evaluate the thermal stability of the films. Thermal stability tests

were conducted by putting the hydrophobic silica films in a furnace (Vulcan, 3-

550, USA) at various temperatures. In particular, film prepared from M = 19.1

was thermally treated for 5 h to examine the hydrophobic nature against

temperature. The superhydrophobic silica films retained their hydrophobicity

up to a temperature of 310ºC and above this temperature the film became

superhydrophilic; the static water contact angle on the film was smaller than 5°.

This is due to the fact that, above these temperatures the methyl groups get

converted into Si-OH groups leading to the adsorption of water.

9.3.5 Static and dynamic water contact angle measurements

The wetting behavior of superhydrophobic surfaces is governed by both

their chemical composition and geometric microstructure. The influence of

MeOH/MTES molar ratio (M) on static water contact angle, sliding angle and

maximum frictional force to slide the water droplet on film surface is shown in

table 9.1.


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To evaluate the hydrophobic properties of the silica films, the contact

angle (θ) of the water droplet on the films prepared with various M values have

been measured. The water droplet on the silica film prepared with M = 9.5,

adhere on film surface resulting in a water contact angle of 107º and maximum

frictional force required to slide the water droplet on film surface is 81.24 µN

at sliding angle of 56º. At lower M values, the silica film surface is covered

with fewer silicon alkyl groups, leading to less water contact angle and high

sliding angle. However, as the M value is increased, the silica film surface

become more hydrophobic and hence large water contact angle and low sliding

angle is resulted. Although, the maximum frictional force required to slide

water droplet on a film surface is decreased with increasing M value. The water

droplets easily roll off on the silica film surface (M = 19.1) for a small force of

11.94 µN at sliding angle of 7º. This strongly suggests that the contact model of

a water droplet on the film prepared from M = 19.1 is the Cassie-Baxter’s

model. Whereas in the case of the silica film prepared with M = 9.5, satisfies

the Wenzel’s model. The methyl groups enhanced the water repellency of the

surface. Figures 9.4 shows the image of the water droplets on the silica film

prepared on copper substrate from M value of 19.1. All the three water drops

on the superhydrophobic copper substrate shows the same contact angle of

155º, which confirms uniform deposition over the copper substrate.

Sr.

No.

MeOH/MTES

Molar ratio (M)

Water contact

angle (θθθθ)

Water

sliding angle

Maximum

frictional force

fmax (µµµµN)

1. 9.5 107º 56º 81.24

2. 12.7 123º 43º 66.83

3. 15.92 137º 32º 51.93

4. 19.1 155º 7º 11.94

Table 9.1: Change in static and dynamic water contact angle values and

maximum frictional force with increase in M values.


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211

To evaluate the mechanical properties of the silica films, the copper

substrates deposited with M = 19.1 was bent more than 90°. The contact angle

of the water droplet on the bent copper substrate was measured which shows

the almost same contact angle as on the flat film. Figures 9.5 shows the image

of the water droplet on the bent (>90°) copper substrate deposited with M =

19.1.

Figures 9.4: The image of the water droplets on the silica film prepared

on copper substrate from M value of 19.1.


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Thus, the superhydrophobic coatings prepared on copper substrate

shows good mechanical strength. The coatings demonstrated excellent

adhesion and flexibility, which could be attributed to the formation of chemical

bonding at the interface and the incorporation of organic components,

respectively.

9.3.6 Effect of humidity and chemical aging tests on the wetting properties

of the silica films

For artificial superhydrophobic surfaces, the water repellent capability

gradually degrades during long-term outdoor exposure and accumulation of

contamination. The effect of humidity on the wetting properties of silica film

prepared on copper substrate with M = 19.1 was carried out at relative humidity

of 95% at 35ºC temperature over 90 days. It was observed that there was no

Figures 9.5: The image of the water droplet on the bent (>90°) copper

substrate deposited with M = 19.1.


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213

any significant effect on the superhydrophobicity of the silica films. This

reveals that the silica films prepared on copper substrate with M = 19.1 are

strongly durable against humidity. The chemical aging tests were conducted by

immersing the samples prepared with M = 19.1 into 4% concentration of

sulfuric acid solution at room temperature for 12 hours. Prior to the contact

angle measurement; the samples were taken out from the solution and

thoroughly cleaned with double distilled water. The film revealed a static water

contact angle of 150º showing the strong durability against acid environment.

9.4 Conclusions

Silica based coating, prepared by a single step sol-gel process using

methyltriethoxysilane as a precursor was found uniform and relatively dense.

The results indicate that it is possible to tailor the composition to modify the

properties of these coatings such as hydrophobicity, wettability, adhesion,

mechanical stability and corrosion prevention. The coatings also demonstrated

excellent adhesion and flexibility, which could be attributed to the formation of

chemical bonding at the interface and the incorporation of organic components,

respectively. Precise selection of precursor and sol–gel composition yielded

coatings that were found to be adhesive, water-repellant, and effective at

preventing corrosion of coated copper substrate. Taken together, these coatings

provide better corrosion protection through (a) providing a water repellent

surface for reduced interaction of water with metal surface and (b) chemically

modifying the surface of a metal to make it more inert. As such, the strategy

can be used to prepare adhesive, stable, chemically resistant, inert, long lasting

coatings for efficient prevention of corrosion. Finally, the approach outlined

herein presents a novel alternative technology which may be easily adapted for

commercial and mass production of anticorrosion coatings for different

metallic surfaces.


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[4] T. L. Metroke, R. L. Parkhill, E.T. Knobbe, Prog. Org. Coat. 41 (2001) 233.

[5] A. Conde, A. Duran, J. de Damborenea Prog. Org. Coat. 46 (2003) 288.

[6] P. Judeinstein, C. Sanchez, J. Mater. Chem. 6 (1996) 511.

[7] B. C. Dave, Mater. Technol. 14 (1999) 115.

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