Accepted Manuscript
Studies on SiO2-hybrid polymeric nanocomposite coatings withsuperior corrosion protection and hydrophobicity
Sh. Ammar, K. Ramesh, I.A.W. Ma, Z. Farah, B. Vengadaesvaran,S. Ramesh, A.K. Arof
PII: S0257-8972(17)30617-5DOI: doi: 10.1016/j.surfcoat.2017.06.014Reference: SCT 22425
To appear in: Surface & Coatings Technology
Received date: 5 December 2016Revised date: 26 May 2017Accepted date: 5 June 2017
Please cite this article as: Sh. Ammar, K. Ramesh, I.A.W. Ma, Z. Farah, B.Vengadaesvaran, S. Ramesh, A.K. Arof , Studies on SiO2-hybrid polymericnanocomposite coatings with superior corrosion protection and hydrophobicity, Surface &Coatings Technology (2017), doi: 10.1016/j.surfcoat.2017.06.014
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Studies on SiO2 - hybrid polymeric nanocomposite coatings with
superior corrosion protection and hydrophobicity
Sh. Ammar a, K. Ramesh
a,1, I. A. W. Ma
a, Z. Farah
a, B. Vengadaesvaran
a, S.
Ramesh a, A.K. Arof
a
a Center for Ionics University of Malaya, Department of Physics, Faculty of Science,
University of Malaya, Kuala Lumpur, 50603
Abstract
Hybrid polymeric based nanocomposite coating systems (NCs) were fabricated by solution
intercalation method with the use of epoxy (E) and polydimethylsiloxane (PDMS) as the host
hybrid polymeric matrix and different weight ratio of SiO2 nanoparticles, ranging from 2 wt. %
to 8 wt. %, as the reinforcing agents. The effects of introducing PDMS as a modifier to epoxy
resin, as well as, the influence of embedding SiO2 nanoparticles within the developed matrix
were evaluated via utilizing X-ray diffraction (XRD), contact angle measurements (CA), field
emission scanning electron microscope (FESEM), and electrochemical impedance spectroscopy
(EIS). XRD and FESEM results revealed the good dispersion of the nanoparticles within the
polymeric matrix and confirmed the efficiency of the solution intercalation method. CA findings
confirmed the achievement of hydrophobic surface after modifying epoxy resin with PDMS with
CA value at 96º for epoxy- PDMS coating system (ES 0). Also CA results indicated the ability of
SiO2 nanoparticles to participate in developing more hydrophobic surface as CA value up to 132º
was achieved with utilizing 6 wt. % SiO2 loading ratio. The results obtained from EIS studies
1 Corresponding author: Tel: +60379676712; Fax: +60379674146
E-mail addresses: [email protected] (Sh. Ammar), [email protected] (K. Ramesh)
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revealed that the coating system with 2 wt. % had the best anticorrosion performance and have
demonstrated an intact behavior over all the immersion period without any sign of coating
damage or degradation.
Keywords SiO2 nanoparticles, Anticorrosion coatings, hydrophobic surface, EIS, FESEM.
1. Introduction
Degradation of material or corrosion had been quite an issue for industries. When surfaces are
exposed to corrosive environment, degradation of materials occurs and the cost is at the
expensed of billions, both in capitals and lives [1]. X. Pei et al. have pointed out that corrosion
phenomena significantly affect the lifespan, safety, and aesthetics of the infrastructure. That in
turn leads to annual cost at approximately $276 billion just in U. S as a direct cost of metallic
corrosion. In addition, X. Pei et. also reported that Canada spends about $74 billion just to repair
the reinforced concrete structure [2].
Organic coatings are one of the promising approaches in solving the issues. Furthermore, recent
years had shown a tremendous effort in highlighting the performance of these coatings and that
includes the organic coating system and polymeric hybrid coating system that combine the both
the novelty of organic and inorganic functionalities [3-6].
Epoxy and PDMS are two different categories of polymer and both are used as coating systems
for multipurpose including anti-corrosion [5,7,8]. Despite having remarkable resistance towards
many elements, epoxy still fall short with its hydrophilic surface and weakness towards
resistance to crack propagation [9-11]. This could lead to significantly reduced barrier ability
against water, oxygen and other corrosive elements making it a poor choice for anti-corrosion
coating. PDMS on the other hand, has a superhydrophobic surfaces with poor wettability with
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low surface energy. PDMS also possess large free volume that allows it to be used as element for
many other applications [12-14].
While the former is considered to be organic and latter, inorganic, the combination of these two
had proven to be a good anti-corrosion coating due to both properties combined together to form
a superior coating. Ammar et al. had studied the effects of intercalation of PDMS with epoxy and
revealed a significant increase of hydrophobicity of Epoxy-PDMS composite system compared
to neat epoxy [8]. Rath et al. also discovered that PDMS-modified epoxy has lower surface
energy with increases in contact angle measurement [15]. This shows that having both PDMS
and epoxy together made a better anticorrosion coating due to having intercalation of PDMS into
epoxy which lowers the surface energy and increase the hydrophobicity.
Silica or silicon dioxide, SiO2 an inorganic nanoparticle, is often used as additives to enhance the
properties of many organic base resins [16-18]. High mechanical strength, good thermal and
chemical stability and high surface area make SiO2 nanoparticles gain the trust as a useful tool to
enhance the mechanical properties and reduce its thermal degradation at high temperature of the
polymeric films [19,20]. Moreover, many researchers have confirmed the capability of SiO2
nanoparticles in enhancing the barrier properties and the hydrophobicity of the coatings system
by zigzagging the diffusion pathways against the penetrating corrosive agents and producing a
new levels of surface roughness respectively [20]. Islam et al. [17] have successfully reduced the
surface porosity of Ni–P–SiO2 nanocomposite coatings and further enhanced the corrosion
protection ability. Moreover, Pourhashem et al., [11] studied the effect of utilizing SiO2 particles
on the corrosion protection performance of the epoxy coatings via fabricating SiO2 - graphene
oxide (GO) nanohybrids via a facile one-step method. The finding of this study revealed that
utilizing SiO2 nanoparticles along with GO nanosheets led to enhance the overall performance of
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the epoxy coatings via increasing the hydrophobicity of the resulting surface, significantly
improving the adhesion of the epoxy coating to the metallic substrate, and by giving strong
interfacial bonding with the matrix. However, homogenous mixture of silica and organic resin
are considered difficult to be achieved since the nanoparticles aggregates strongly and easily.
This is due to the H-bonding formations between the nanoparticles through the silanol group
presents at the surface [19].
In this paper, an organic-inorganic nanocomposite coating systems consisting epoxy, PDMS and
different loading rates of SiO2 nanoparticles were developed with high hydrophobicity and
superior electrochemical properties. It is worth mentioning that all reported NCs coating systems
were developed through utilizing solution intercalation method which is well documented by
Reddy [21] and Shen et al. [22]. The dispersion of the SiO2 nanoparticles within the polymeric
matrix was evaluated via using XRD and FESEM techniques. The influence of embedding SiO2
nanoparticles within the developed epoxy-PDMS hybrid polymeric matrix on the anti-corrosion
performance and on the wettability of the resulting coating systems was investigated.
2. Experimental procedures
2. 1 Materials and sample preparation
All chemicals were used as received and without any further purification. Epoxy resin
(EPIKOTE 828) representing bisphenol A and epichlorohydrin with an equivalent weight of
184–190and viscosity about 12,000–14,000 cP at 25◦C was utilized as the base and polyamide
(EPI-CURE 3125) with an amine value of 330–360 mg/g as the curing agent were both obtained
from Asachem, Malaysia. Polydimethylsiloxane-hydroxyl-terminated (PDMS) with a viscosity
of 750 cSt and density equal to 0.97 g/ml at 25◦C, used as a modifier and dibutyltindilaurate as a
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catalyst, were both purchased from Sigma Aldrich, Malaysia. 3-Aminopropyltriethoxysilane (3-
APS), the coupling agent was obtained from Shin-Etsu Chemical Co. Ltd, Japan. Silica (SiO2)
nano-particles of 10-20 nm in diameters was obtained from Sigma Aldrich and used without any
further treatment. 90 g of the epoxy resin, 10 g of PDMS3 mL of the 3-APS, and an aliquot of
dibutyltindilaurate were mixed together in a beaker and was stirred constantly for 20 minutes at
80 ºC. Simultaneously, the nanoparticles were dissolved in xylene with weight ratio of 8:2 and
subjected to magnetic stirring at 800rpm for 30 minutes followed by 15 minutes of sonication.
Weight ratios of 2 %, 4 %, 6 % and 8 % of the dissolved SiO2 were added to the epoxy blend (ES
2, ES 4, ES 6, and ES 8, respectively) and mixed together for 20 minutes at 1000 rpm followed
by 60 minutes of sonication prior to the addition of 45 g of the polyamide curing agent. The
mixture was continuously stirred for 5 minutes followed by vacuum degassing to remove any
trapped air bubbles and additional side product (CH3OH which is a side product of the reaction
of PDMS and 3-APS).
The coating systems were applied to both-sides of cold-rolled mild steel panels (0.5 x 50.0 x 75.0
mm) which were used to perform all the tests, except for the XRD test where free films were
casted onto Teflon plates. The steel panels were cleaned using acetone and sandblasted following
the ASTM D609 standard in order to achieve Sa ½ surface profile. Single layer coatings were
applied by brush coating method and the samples were then left to dry for two days under
ambient condition and heat treated at 80 ºC for 24 hours. Elcometer 456 thickness gauge was
used to carry out thickness measurement at five different points per sample once dried. The
compositions of all prepared coating systems with their corresponding designation and the
average of the dry film thickness after been applied on the substrate surface are tabulated in
Table 1.
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2. 2 Physical characterizations
X-ray diffraction (XRD) was run onto casted free films of roughly 2 x 2 cm per sample of the
developed coating systems. The free film samples were analyzed with High Score Plus Ver. 3.0d
(3.0.4) at diffraction angle of 2θ = 5° to 120
°. The test was done in order to study the dispersion
of the nano-particles within the coating and identify the nano-particles within the coating system.
Free flow of silica nano-particles was also run for comparison purposes. The surface morphology
and the nanoparticles dispersion within the matrix were studied through FESEM using FEI
Quanta 450 FEG at 10 kv as the accelerating voltage with the assist of low vacuum (LVSEM).
Moreover, the hydrophobicity state of the resulting coated surfaces was investigated by the
contact angle test using the OCA 15EC (data physics, Germany) instrument. Five water droplets
(5μL/drop) were dropped at five different areas on each sample using a needle located close
enough to the surface to render the kinetic energy of the droplets negligible. The image was
capture immediately to determine the static contact angle. The average of five measurements was
taken as the reported contact angle values with less than 2º measurement error.
2. 3 Electrochemical characterizations
To investigate the electrochemical behavior, anticorrosion performance and the barrier properties
of all the developed coating systems, electrochemical impedance spectroscopy (EIS) technique
was employed. A classical three-electrode cell (Shown in Fig S1) with 3 wt. % NaCl solution
was used to perform these investigations throughout 30 days of immersion. In order to develop
the electrochemical cell, a polyvinyl chloride (PVC) tube with a 2.0 cm inner diameter was
vertically fixed to the surface of the coated substrate with the help of epoxy adhesive glue. With
consideration of the adhesive joint, the sample with a 3.0 cm2 exposed area served as the
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working electrode, a saturated calomel electrode (SCE) acted as the reference electrode, and a
platinum electrode was the counter electrode. All reported EIS studies were performed using
Gamry PC14G300 potentiostat (Gamry Instruments, Warminster, PA, USA) with a frequency
range of 100 kHz to 10 mHz run using a faraday cage, to maintain the minimum noise level
during measuring; and Echem Analyst Version 6.03 analyzer for results evaluation.
3. Results and discussion
3. 1 X-ray Diffraction (XRD)
X-ray diffraction was deployed to study the structural characterization of nano-SiO2, neat epoxy
and the hybrid nanocomposites coating free film using CuKα (λ = 1.54 Å) as the radiation
source. Braggs law equation (1) were utilized to calculate the d-spacing of relevant data
Braggs’ Law: λ = 2d sin θ (1)
Where λ = wave length, d = spacing between layers of atom and θ = angle between the incident
rays and the surface of the crystal.
Fig. 1 shows a broad peak of SiO2 spectra at 2θ = 21.9º and the peaks are compared to
JCPDS file. The d-spacing of the nanoparticles were calculated to be 4.4. The result
corresponded to the literature value (JCPDS No 47-1144) [23,24]. The diffraction pattern also
confirms all the films consist of SiO2 shows a very broad peak confirming amorphous nature.
Apart from that, the neat epoxy resin free film also shows the similar broad peak
confirming that it is also in amorphous nature. The absence of a peak at 2θ = 21.9º confirmed
there is no nano-silica powder in the epoxy resin. The ES 0 free film shows a broad peak at 2θ =
17.9º with d-spacing of 4.9 corresponds to the presence of the silicone group, Si-O-Si, from
PDMS which was blended with neat epoxy resin. This corresponds to literature value (JCPDS
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No 27-1402) [25]. A diffraction shoulder at 2θ=12° attributes to the phenomena of PDMS
grafting the epoxy surface and the similar pattern has been observed by Ferreira et. Al [26].
However, this peak fade of when SiO2 nanoparticles were added into the hybrid solution which
leads to increase of Si-O-Si bonds. The existence of broad band at 2θ = 40 and 2θ = 50° due to
amorphous nature of the neat epoxy resin. In ES 0, this broad band shows a shift which
corresponds to the intercalation of PDMS with the epoxide group in epoxy resin. This similar
observation also has been observed in all other nanocomposite coating free films (ES 2 – ES 8)
with the incorporation of nano-silica.
Moreover, Figure 2 depicts the 2θ position and corresponding d-spacing with nano-SiO2
wt. %. The peak shift between 2θ = 19.5º and 2θ = 26.0º corresponds to the presence of nano-
silica in the form of SiO2 within the nanocomposite coatings matrices indicating intercalation of
epoxy resin penetrated the crystal plane of the nano silica [27]. The increase of d-spacing of SiO2
in ES 2 – ES 8 systems shows that the nano-silica are relatively dispersed within the matrix since
the increase of d-spacing corresponds to the increasing distance between layers of atoms. This
confirms the epoxy structure intercalates with nano-SiO2 until the lattice structure is affected.
When the loading ratio of SiO2 becomes 6 wt. %, the 2θ peak at 19.6º has shifted to a lower
value due to agglomeration of SiO2 nanoparticle and was further confirmed by the high d-
spacing of 4.5 in ES 6 as compared to ES 8 and ES 4. At the low loading ratio of 2 wt. % SiO2,
the 2θ value is observed to be low as well at 19.5º. This is due to very low loading ratio and that
the matrix could not intercalate within the plane of the nano-silica properly and this results in the
high d-spacing between the planes of silica dioxide.
3. 2 Contact angle measurement (CA)
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In order to study the wetting ability of the coating systems, contact angle measurements were
utilized. If the contact angles measured are less than 90º, the surface is hydrophilic; if the contact
angles are between 90 º and 150
º, the surface will be classified as hydrophobic and if the angles
exceed 150 º, then the surface will be categorized as superhydrophobic surface. The values of CA
for all prepared coating systems were recorded and illustrated in Fig. 3. E 0 coating system
shows hydrophilic behavior with contact angle of 65º. As depicted in Fig. 3, the CA results
revealed that modifying neat epoxy resin with PDMS has increased the hydrophobicity of ES 0
with increasing the CA from 65º to 96
º. While, further shifting toward more hydrophobicity was
observed after the addition of 2 wt. % of SiO2 nanoparticles where ES 0 coating system shows a
tremendous increase regarding the contact angle from 96º to 122
º. The angle increases gradually
until the addition of 6 wt. % at 132º but dropped to 121
º with the application of ES 8 coating
system.
There are mainly two factors that have been recognized that can affect the wetting ability of a
coated surface. Chemical composition and surface roughness of the final system can affect the
wetting ability of the surfaces [28,29]. The effect chemical composition could be used to explain
the increased contact angle of ES 0 coating system as PDMS has naturally hydrophobic behavior
and low surface energy. Furthermore, it is an elastomer with weak intermolecular forces and the
intercalation of the PDMS into the neat epoxy affects the pre-existing surface energy of the
epoxy and thus increases the hydrophobicity of the coating [15]. In addition, introducing a
second phase to the system will increase the surface roughness and ultimately increase the
hydrophobicity of the surface coating [23]. The addition of nano silica to a coating system will
introduce a second phase dimension to the surface and roughen the surface morphology. The
increase in surface roughness affected the surface hydrophobicity through the formation of air
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pockets between the water droplets and the surface, leading to a composite solid-liquid-air
interface system which explained the increase of contact angle for ES 2, ES 4 and ES 6.
However, the drop in ES 8 is due to the aggregation of the nano silica since it has a high
inclination to aggregate at high volume and thus affected the hydrophobicity of the surface [30].
3. 3 Morphological studies by FESEM
In order to evaluate the dispersion state of the utilized SiO2 nanoparticles within the
fabricated hybrid polymeric matrix, FESEM was utilized to analyze the surface morphology of
the coating system after been applied to the substrate surface and cured. FESEM images for all
NCs coating systems were depicted in Fig. 4 (a) – (d). Besides investigating the nanoparticles
distribution within the surface area, the surface properties and phase distribution can be
evaluated via FESEM technique. From the obtained micrographs, an acceptable dispersion of
SiO2 nanoparticles within the hybrid polymeric matrix could be confirmed without any presence
of cracks or phase separation. Furthermore, it was interesting to notice that as the incorporation
of SiO2 nanoparticles resulted in achieving higher values of CA and developing surfaces that
characterized with more hydrophobicity. FESEM images of All NCs have revealed a good
dispersion of the nanoparticles which could ensure the creation of more rough surface with
heterogeneous morphology which was found in strong agreement with CA results.
Moreover, FESEM findings could obtain an evidence regarding the competence of the
sonication process in developing a good level nanoparticles dispersion within the polymeric
matrix. Under the same conditions of the sonication process, increasing the loading ratio of the
nanoparticles resulted in increasing the amount of the nanoparticles in the area unit. Hence, the
ability of the particles to be attracted to each other enlarged, thus producing relatively larger
agglomerated particles. It was clearly observed that ES 6 coating system demonstrated the best
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nanoparticles distribution which could revealed that 6 wt. % of SiO2 could be considered the
optimum loading ration that can ensure that the nanoparticles can cover all the surface area
without forming large agglomerated particles. In fact, it is worth to be mentioned that there are
many parameters involving in determining the optimum loading ratio of the nanoparticles for the
coating system (i.e. particles size, sonication time, sonication power). However, increasing the
loading ratio of the SiO2 nanoparticles up to 8 wt. % led to observe some agglomerations of the
nanoparticles as depicted in Fig. 4 (c) and (d). That can be explained by the fact that the distance
between the nanoparticles become smaller as the number of the particles increases within the
area unit and also due to the tendency of the nanoparticle to agglomerate at the high loading rates
[30,31].
3. 4 Electrochemical impedance spectroscopy (EIS)
The EIS measurements were conducted periodically (every five days) after exposure to 3% NaCl
solution and the results were expressed graphically using Nyquist and Bode plots as illustrated in
Fig. 5 and 6 after 1 and 30 days of immersion time, respectively. According to the
electrochemical behavior of the various prepared coating systems, three different models of the
equivalent circuit were proposed in the regard of obtaining the best numerical fitting along all
study periods. From Fig. 5 it can be seen that along all prepared coating systems, just the neat
epoxy, E 0, coating system has demonstrated a sign of electrolyte penetration initiation as a full
semi-circle observed in the Nyquist plot and an obvious bend in the low-frequency region with
Bode plot. This observation further confirms the fact that pure organic resins cannot behave as
intact coating layers with superior barrier performance due to the existence of free volume and
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porosity which directly act as a diffusion pathway for the electrolyte toward the coating - metal
interface [32,33]. At this stage of immersion, model B of the equivalent circuit perfectly describe
the electrochemical behavior of E 0 coating system. This model consists of the following
elements: a resistor Rs, solution resistance, which does not have any important technical or
theoretical information about the coating system thus was not included in this study [33]. A
resistor Rpo and a constant phase element CPEpo which represents the electrolyte resistance
through the pores, and its associated CPEpo. In addition, this model contains a parallel
combination of a constant phase element and resistor which in turn represent the parameters of
the corrosion reaction namely, constant phase element of double layer capacitance (CPEdl) and
charge transfer resistance (Rct). However, in contrary, ES 0 and all prepared NCs demonstrated a
capacitive behavior with one capacitive loop in Nyquist plot and straight line in the Bode plot
with a slope of -1 which is a clear evidence about the efficiency of these coating systems in
preventing any penetration of the electrolyte with complete isolation of the metal surface from
the surrounding corrosion medium. As there was no any charge transfer or diffusion of the
electrolyte within the coating films, model A with just Rs, Rpo and CPEpo were used to fit the EIS
diagrams at this stage of immersion time.
As the exposure time elapsed, the most significant degradation was found with the substrate
coated with a E 0 coating system. As illustrated in Fig. 6, two full semi-circuit in Nyquist plot
with Bode plot of two times constant which indicate the poor corrosion resistance of the epoxy
resin. As the penetration of the 3% NaCl solution leads to initiate the corrosion process, model C
of the equivalent circuit with new components namely, constant phase element of diffusion
capacitance (Cdiff) and diffusion resistance (Rdiff), was suggested in order to describe the
diffusion process of the corrosion products within the coating film.
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On the other hand, the incorporation of PDMS with the epoxy resin results in a remarkable
enhancement in the anti-corrosion performance as the EIS results of the ES 0, after 30 days of
immersion, were found with one semi-circle in Nyquist plot and Bode plot with one time
constant as the straight line bend in the low-frequency region. However, this observation
revealed the ability of PDMS in enhancing the corrosion protection performance which was in
accordance with the findings reported in the literature [8,34,35]. However, as the impedance
plots of ES 0 coating system were demonstrated one time constant and been fitted with model B
of the equivalent circuit, it can be understood that even the hybrid polymeric matrix could suffer
a degradation of the barrier properties after long time of immersion and demonstrate a limitation
in performing as intact coating film against corrosion. Among all prepared NCs coating systems,
ES 2 system with 2 wt. % SiO2 nanoparticles demonstrated the best performance with almost no
changes observed with Nyquist and Bode plot with one capacitive loop in Nyquist plot and a
straight line in the Bode plot with a slope of -1 which represent the total capacitive behavior of
the coating film. This is due to the effective barrier role of the nano-sized SiO2 particles within
the hybrid polymeric matrix which led to close up the pores and zigzag the diffusion pathways
against the corrosive agents. That, in turn, force these agents to travel longer distance to reach
the substrate, therefore, gaining more stability against corrosion [30,36]. It is worth mentioning
that no further protection was developed as the SiO2 loading ratio increased up to 4 wt. %.
However, as higher loading ratio of the SiO2 nanoparticles utilized up to 8 wt. %, the corrosion
properties of the coating systems declined. As a full semi-circle observed in the Nyquist plot and
an obvious bend in the low-frequency region with Bode plot were observed with ES 6 and ES 8
coating systems. That could be attributed to the tendency of the nanoparticles to form
agglomerations at the high loading ratio as the number of the nanoparticles increases within the
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unit area. Thus forming a larger particles size will affect the barrier properties of the utilized
reinforcing particles will face some difficulties in achieving the will dispersion within the
polymeric matrix and fill up all the pores. Moreover, all the values of all parameters that have
been obtained by fitting the EIS data according to the proposed equivalent circuit models after 1
day and 30 days of immersion are tabulated in Tables S 1 and S 2 in Supplementary material.
Investigating the stability of the coating films against been damaged by the electrolyte and
evaluate the state of the interfacial adhesion bonds at the coating - substrate interface over the
different immersion periods were carried out via using EIS method in the Bode plot format
(impedance and phase angle as a function of the frequency). With the assist of breakpoint
frequency values, which is the frequency at - 45º phase angle, the region below the Bode plot
could be divided into capacitive and resistive regions. After that, recording and analyzing the
changes that could happen to these regions for each coating system over the whole period of
immersion could help to obtained full understanding and detailed information about the
electrochemical behavior, barrier properties, coating delamination and electrolyte diffusion state
[37,38]. Fig. 8 (a) and (b) illustrate the Bode plots of E 0 coating system after 1 and 30 days of
immersion, respectively. As it can be clearly seen that E 0 coating system demonstrated two
regions under the Bode plot since the first day of immersion namely, a resistive region at low-
frequency domain and a capacitive region at high-frequency domain. However, after 30 days of
immersion, an increase with the resistive region and corresponded decrease in the capacitive
region indicate the diffusion of the electrolyte within the coating film, thus, coating delamination
and loss of adhesion occurred at the substrate surface. In contrary, as it can be clearly observed
in Fig. 9 (a) and (b), that showing the Bode plots of ES 0 coating system after 1 and 30 days of
immersion, respectively, the incorporation of PDMS within the epoxy resin results in enhancing
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the barrier properties as lower breakpoint frequency was recorded, therefore, a relatively smaller
resistive region at the low-frequency domain and larger capacitive region among the neat epoxy
coating system was recorded.
However, the efficiency of utilizing SiO2 nanoparticles to enhance the corrosion protection
performance and obtain more stability against the exposure to the electrolyte was confirmed by
plotting the Bode plots after 1 and 30 days of immersion as shown in Fig. 10 (a) and (b),
respectively. A capacitive behavior over the whole tested frequency ranges indicated the
superiority of ES 2 coating system in acting as intact coating film without any damage or
electrolyte penetration up to 30 days of immersion.
Moreover, coating damage index which as a good indicator to study the integrity of the coating
systems was obtained from EIS studies and was used to classify the developed coating system
and confirm the ability of SiO2 nanoparticles to guarantee the development of intact organic-
based coating system. Coating damage index (D2) and could be obtained from the following
equation [37]:
𝐷2 = (𝐴2
𝐴1) × 100 (2)
Where A1 is the area of the capacitive region and A2 is the area of the resistive region. Fig. 11
illustrates the coating damage index of all prepared coating systems after 1 day and 30 days of
immersion. The highest coating damage was recorded for neat epoxy coating system, E 0, after 1
day of immersion. As PDMS was incorporated within epoxy resin, lower damage index was
achieved. However, no damage was observed for all NCs coating systems at this stage of
immersion. After 30 days of immersion, all prepared coating systems showed a damage at
different levels except ES 2 coating system which demonstrated a superior intact behavior and
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completely prohibited the corrosive agent and the electrolyte to be penetrated within the
polymeric matrix and reach the substrate surface.
4. Conclusions
Neat epoxy and hybrid epoxy - PDMS coatings were successfully fabricated and investigated.
After that, the developed hybrid polymeric blend was utilized as the host matrix and has
reinforced with four different weight rates of SiO2 nanoparticles. The development of these
nanocomposite coating systems was carried out through the employment of solution intercalation
method with the assistance of sonication process. The integrity of introducing the inorganic nano
fillers, the achievement of good dispersion and the resultant morphology were examined via
XRD and FESEM techniques. The water repellence of the developed coated surface was
evaluated by CA measurements. Moreover, EIS studies were carried out up to 30 days of
immersion in 3% NaCl solution to investigate the corrosion protection performance and the
barrier properties of all developed coating systems. Furthermore, with the excellent intercalated
structure of the nanocomposite coating evinced on the ability of the incorporated nano SiO2
nanoparticles to alter the overall coating performance through forming a hydrophobic character
with superior barrier properties was obtained via CA and EIS findings. FESEM images revealed
good dispersion of the nanoparticles at low loading rates and giving a sign about the initiation of
agglomerations as the loading ratio of SiO2 nanoparticles exceed 6 wt. %. EIS studies revealed
that after 30 days of immersion time the best anticorrosion properties were observed for ES 2
coating system with significant stability over the entire immersion time. Besides, the
investigation on the damage index further confirmed the superior intact behavior of this system
which loaded with 2 wt. % nano SiO2.
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Acknowledgement
We would like to thank University of Malaya for supporting this project with the grants PG001 –
2016A and BKS027-2017. We also thank Ministry of Education for the FRGS grant FP12-
2015A.
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Table 1: Composition and nomenclature of all prepared coating systems
System Epoxy wt. % PDMS wt. % Nano SiO2 wt.
%
Code Dry film
thickness
1 100.0 0.0 0.0 E 0 71.2 ± 3.2
2 90.0 10.0 0.0 ES 0 73.4 ± 1.7
3 90.0 10.0 2.0 ES 2 72.0 ± 2.5
4 90.0 10.0 4.0 ES 4 71.9 ± 2.6
5 90.0 10.0 6.0 ES 6 74.3 ± 1.9
6 90.0 10.0 8.0 ES 8 71.1 ± 3.7
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Fig. 1. XRD patterns of nano-SiO2, neat epoxy and nanocomposites coating systems
Fig. 2. The peak position of 2θ and respective d-spacing of NCs coating (ES 0 – ES 8)
Fig. 3. Representative the contact angle values of all prepared coating systems
Fig. 4: FESEM images of (a) ES 2, (b) ES 4, (c) ES 6 and (d) ES 8 nanocomposite coating
systems
Fig. 5. Representative (a) Nyquist and (b) Bode plots for all prepared coating systems after 1 day
of immersion.
Fig. 6. Representative (a) Nyquist and (b) Bode plots for all prepared coating systems after 30
days of immersion.
Fig. 7. Representative Bode plots of E 0 coating system after a) 1 day and b) 30 days of
immersion along with the determining the breakpoint frequency and the correspond capacitive
(A1) and resistive (A2) regions.
Fig. 8. Representative Bode plots of ES 0 coating system after a) 1 day and b) 30 days of
immersion along with the determining the breakpoint frequency and the correspond capacitive
(A1) and resistive (A2) regions.
Fig. 9. Representative Bode plots of ES 2 coating system after a) 1 day and b) 30 days of
immersion along with the determining the breakpoint frequency and the correspond capacitive
(A1) and resistive (A2) regions.
Fig. 10. Coating damage index of all prepared coating systems after 1 and 30 days of immersion.
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Fig. 1.
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Fig. 2.
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Fig. 3.
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Fig. 4.
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Fig. 5.
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Fig. 6.
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Fig. 7.
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Fig. 8.
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Fig. 9.
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Fig. 10.
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Highlights
1. Nano silica reinforced epoxy/PDMS hybrid coatings were developed.
2. Contact angle has been improved from 65o to 132o.
3. The lowest coating damage index exhibited by coatings with 2 wt.% SiO2 nanoparticles.
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