anticorrosive coatings on metal substrate by sol...
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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,
Chapter 9 Anticorrosive Coatings on Metal Substrate by Sol-Gel
Dip Coating Method
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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-
Chapter 9 Anticorrosive Coatings on Metal Substrate by Sol-Gel
Dip Coating Method
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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:
Chapter 9 Anticorrosive Coatings on Metal Substrate by Sol-Gel
Dip Coating Method
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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
+
Chapter 9 Anticorrosive Coatings on Metal Substrate by Sol-Gel
Dip Coating Method
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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.
Chapter 9 Anticorrosive Coatings on Metal Substrate by Sol-Gel
Dip Coating Method
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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.
Chapter 9 Anticorrosive Coatings on Metal Substrate by Sol-Gel
Dip Coating Method
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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.
Chapter 9 Anticorrosive Coatings on Metal Substrate by Sol-Gel
Dip Coating Method
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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.
Chapter 9 Anticorrosive Coatings on Metal Substrate by Sol-Gel
Dip Coating Method
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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.
Chapter 9 Anticorrosive Coatings on Metal Substrate by Sol-Gel
Dip Coating Method
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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.
Chapter 9 Anticorrosive Coatings on Metal Substrate by Sol-Gel
Dip Coating Method
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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.
Chapter 9 Anticorrosive Coatings on Metal Substrate by Sol-Gel
Dip Coating Method
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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.
Chapter 9 Anticorrosive Coatings on Metal Substrate by Sol-Gel
Dip Coating Method
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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.
Chapter 9 Anticorrosive Coatings on Metal Substrate by Sol-Gel
Dip Coating Method
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