Evaluation of the Effects of Nanoclay Addition on the Corrosion Resistance of Bituminous Coating

Document Type: Research Paper

Authors

1 Department of Technical Inspection, Petroleum University of Technology, Abadan, Iran

2 Technical Faculty, South Tehran Branch, Islamic Azad University, Tehran, Iran

Abstract

In this study, the corrosion resistance of a bituminous coating reinforced with different ratios of nanoclay pigment was studied. To make nanocomposite coatings, 2, 3, and 4 wt.% of clay (Cloisite Na+) were incorporated into water emulsified bitumen. The coatings were applied to steel 37. Optical microscopy and X-ray diffraction (XRD) were used to characterize the nanocomposite structure. In order to investigate the anticorrosion behavior of the coatings, electrochemical impedance spectroscopy (EIS) and direct current polarization techniques were used. The results show that the coatings containing nanoclay have better performance compared to the neat bitumen. Moreover, it was revealed that the corrosion resistance of the nanocomposite increased as the clay loading increased up to 4 wt.%.

Keywords


2.4.
Iranian Journal of Oil & Gas Science and Technology
,
Vol. 4 (2015), No. 2, pp. 15-26
http://ijogst.put.ac.ir
Evaluation of the Effects of Nanoclay Addition on t
he Corrosion Resistance
of Bituminous Coating
Hamid Reza Zamanizadeh
1
, Mohammad Reza Shishesaz
1*
, Iman Danaee
1
, and Davood Zaarei
2
1
Department of Technical Inspection, Petroleum Unive
rsity of Technology, Abadan, Iran
2
Technical Faculty, South Tehran Branch, Islamic Aza
d University, Tehran, Iran
Received
: December 16, 2013
; revised
: January 01, 2014
; accepted
: January 04, 2014
Abstract
In this study, the corrosion resistance of a bitumi
nous coating reinforced with different ratios of
nanoclay pigment was studied. To make nanocomposite
coatings, 2, 3, and 4 wt.% of clay (Cloisite
Na
+
) were incorporated into water emulsified bitumen.
The coatings were applied to steel 37. Optical
microscopy and X-ray diffraction (XRD) were used to
characterize the nanocomposite structure. In
order to investigate the anticorrosion behavior of
the coatings, electrochemical impedance
spectroscopy (EIS) and direct current polarization
techniques were used. The results show that the
coatings containing nanoclay have better performanc
e compared to the neat bitumen. Moreover, it
was revealed that the corrosion resistance of the n
anocomposite increased as the clay loading
increased up to 4 wt.%.
Keywords
:
Bitumen, Nanocomposite Coating, Clay, Corrosion
1. Introduction
Bitumen, obtained from petroleum refinery bottoms,
is a thermoplastic containing bituminous
materials. It has found widespread use in sealants,
binders, waterproof coatings, and paving materials
and it is preferred for its low cost, inherent cohe
sive nature, rheological properties, and thermal
resistance (Çubuk et al., 2009).
Bitumen is a natural polymer of low molecular weigh
t and like all polymers it is viscoelastic (Cheung,
et al. 1997). Over the years, polymeric coatings ha
ve been developed due to their good barrier
properties. However, pristine polymeric coatings ar
e still permeable to corroding agents such as
water, oxygen, and destructive ions like Cl
-
, H
+
, and SO
4
2-
. In order to enhance the barrier properties
of these polymeric coatings, many researchers have
used various kinds of additives such as extenders
and inorganic pigments (Nematollahi, et al. 2010).
The addition of polymers to bitumen is known to
impart enhanced service properties such as improved
thermo-mechanical resistance, elasticity, and
adhesive properties (Collins et al., 1991). However
, polymer modified bitumens are expensive,
difficult to operate, and incompatible (Polaccoa et
al., 2005). Therefore, further efforts have been
made for exploring new modifiers.
Recently, the layered silicates have been widely us
ed for the modification of polymers (Ahmed et al.,
2005). Layered silicate is a type of mineral with l
ow cost and abundance. It consists of layers of
* Corresponding Author:
Email: shishesaz@put.ac.ir
16
Iranian Journal of Oil & Gas Science and Technolog
y,
Vol. 4 (2015), No. 2
tetrahedral silicate sheets and octahedral hydroxid
e sheets (Zilg et al., 2001). Polymer chains can
intercalate into the interlayer of clay, which make
s the clay dispersed into the polymer matrix on a
nanometer scale. This leads to significant improvem
ents in the thermal, mechanical, and barrier
properties of polymers (Wanjale et al., 2003).
Montmorillonite (MMT)-modified bitumen composites h
ave been successfully used to improve both
the physical and rheological properties of modified
bitumen (YU et al., 2007). However, previous
research does not report any information on the eff
ect of MMT on the anticorrosion properties of
bitumen. This study investigates the corrosion prot
ection behavior of natural montmorillonite
clay/bitumen nanocomposite coatings.
2. Experimental results
2.1. Materials
Panels of
steel 37 measuring 6.5 cm × 6.5 cm × 0.3 cm were us
ed as the metallic substrates. The
panels were sandblasted to Sa 2 ½ according to ASTM
D609 and were kept in desiccator. Prior to
coating, the panels were degreased with toluene and
acetone. TW315 (TW315 is bitumen emulsified
in water; the max volatile compound and max organic
compound of TW315 are 60 wt.% and1 wt%
respectively and its viscosity measured with Ford c
up number 2 at 23
°
is between 30-60 seconds)
complied with BS3416 type I as waterproof coating a
nd the natural montmorillonite clay (Cloisite Na
+
or Na
+
-MMT) as an additive were provided respectively by
Tiva Company and Sothern Clay Product
Company. Some properties of the latter are shown in
Table 1.
Table 1
Natural montmorillonite nanoclay (Cloisite Na
+
) pigment properties.
Particle size
Color
Density
Moisture content
X-ray
Results (d001)
<25
μ
m
Off-white
2.86 g/cc
4-9%
1.17 nm
2.2. Preparation of nanocomposites
Three sets of nanocomposite samples containing 2, 3
, and 4 wt.% of Cloisite Na
+
were prepared, as
described below, through solvent intercalation tech
nique.
At first the stoichiometric amounts of Cloisite Na
+
were added to 10 ml of distilled water to make 2,
3, and 4 wt.% mixtures. Then, by using a propeller,
the mixtures were mechanically stirred at 1000
rpm for 120 min at room temperature followed by a s
onication process for 90 min in an ice bath. The
ultrasonic lab device UP200H (200W, 24 kHz) with an
amplitude of 100 and a cycle gage of 1 was
used for the sonication purpose. Secondly, the stoi
chiometric amounts of TW315 were added to the
mixtures to make 2, 3, and 4 wt.% of nanocomposite
coatings followed by 45 min mechanical
blending. The coatings were labeled as PNC2, PNC3,
and PNC4 where PNC stands for polymer
nanocomposite and the number indicates the weight p
ercent of clay used in the mixtures. The coatings
were applied to the panels using 100 micrometer bak
er film applicator Elcometer model according to
ASTM D823-95(2012). The thickness of dry film was 6
0±5
μ
m as measured by Elcometer 415.
2.3. Nanocomposite structural characterization
The optical homogeneity of the clay/water dispersio
n and the effects of sonication process on de-
agglomeration of clay aggregates were examined usin
g a BX-50 Olympus optical microscope.
H. R. Zamanizadeh et al. / Evaluation of the Effect
s of Nanoclay Addition ...
17
It is also necessary to analyze the microstructure
of the nanocomposites. XRD is one of the common
techniques to characterize the microstructure of th
e prepared bitumen/clay nanocomposites to find out
the intercalation or exfoliation of clays. The XRD
experiments were performed in the range of 2
θ
=1
°
to 2
θ
=10
°
with X
Pert PRO MPD (PANalytical). The cobalt (Co) radiati
on (
λ
=1.78897 Å) was used
as the XRD source.
Electrochemical measurements
Electrochemical impedance spectroscopy (EIS) is a n
ondestructive useful technique in studying,
measuring, and estimating coating durability (Soer
et al., 2009). The EIS measurements were
performed using Auto lab PGSTAT 302N coupled with f
requency response analyzer (FRA) 1260 over
a frequency range of 100 kHz to 1 mHz with the 10-m
V amplitude of sinusoidal voltage of open
circuit potential. Fitting of experimental impedanc
e spectroscopy data to the proposed equivalent
circuit was done by a written least square code bas
ed on Marquardt method for the optimization of
functions and Macdonald weighting for the real and
imaginary parts of the impedance (Danaee, 2011;
Macdonald, 1984). Three conventional electrode cell
s were used for the electrochemical
measurements. A 3.5 wt.% NaCl solution was employed
as the electrolyte. Coated panels acted as
working electrode with an exposed area of 2.009 cm
2
. A Pt electrode as the counter electrode and a
saturated Ag/AgCl as the reference electrode were e
mployed. The setup of the cell was placed in a
Faraday cage.
The polarization measurements were obtained in 3.5
wt.% NaCl after 30 days of immersion. The data
were recorded from below 200 mV to above 200 mV of
the open circuit potential (
E
ocp
) at a scanning
rate of 1 mV.s
-1
. The polarization resistance (
R
p
) was calculated from Tafel plots according to Ster
n-
Geary equation (Danaee et al., 2013; Ghasemi et al.
, 2013):
.
1
2.303(
)
a
c
p
a
c
corr
R
I
β β
β
β
=
×
+
(1)
where,
β
a
,
β
c
, and
I
corr
are anodic Tafel slope, cathodic Tafel slope, and
corrosion current density
respectively. The corrosion rate was also calculate
d through the following equation (Poorqasemi et
al., 2009):
0.0032
corr
I
MW
CR
nd
=
(2)
where,
I
corr
,
MW
,
n
, and
d
are corrosion current density (
μ
A.cm
-2
), molar mass (g.mol
-1
), charge
number, and density (g.cm
-3
) of the tested metal respectively.
3. Results and discussion
3.1. Optical microscopy
Optical microscopy was used to ensure the dispersio
n of nanoclay in water before adding theme to the
bitumen (TW315). First of all, the dispersion was m
ade in the water containing 4 wt.% clay. Figure 1
presents the optical micrographs of 4 wt.% clay/wat
er suspensions after 120 min of mechanical
agitation and 60 min and 90 min of sonication proce
ss respectively in sections a, b, and c.
Agglomerates were formed due to cohesive forces bet
ween clay stacks during the wetting process of
clay with water. As shown in Figure 1(a), the mecha
nical agitation is a weak technique to overcome
18
Iranian Journal of Oil & Gas Science and Technolog
y,
Vol. 4 (2015), No. 2
these cohesive forces; thus a lot of clay is still
agglomerated. Applying the sonication process and
increasing the sonication time decreased the size a
nd quantity of agglomerates (Figures 1(b) and 1(c))
,
which shows that sonication can cause the penetrati
on of water molecules into the space between the
sheets of clay. Therefore, the sonication process i
s an effective process to de-agglomerate clay stack
s
in the clay/water dispersion. Once clay dispersed i
n the 4 wt.% clay/water mixture, the
abovementioned operations were also carried out for
the other mixtures (the clay/water mixtures of 2
and 3 wt.%).
Figure 1
Optical micrographs of 4 wt.% clay/water suspension
s: a) 120 min mechanical mixing; b) 60 min sonicati
on; c)
90 min sonication.
a)
b)
c)
H. R. Zamanizadeh et al. / Evaluation of the Effect
s of Nanoclay Addition ...
19
3.2. X-ray diffraction (XRD)
The state of intercalation or exfoliation of nanocl
ay structure in bitumen matrix was studied using lo
w
angle X-ray diffraction (SAXS). Figure 2 shows the
XRD patterns of 2, 3, and 4 wt.% clay-bitumen
nanocomposites and pure Cloisite Na
+
used in this research. As displayed in Figure 2, p
ure Cloisite
Na
+
has one peak at 2
θ
=8.6095° according to Bragg’s law. This peak is rel
ated to the d-spacing of
11.91694 Å of clay layers (this value is in agreeme
nt with the data sheet presented in Table 1 with a
negligible difference). For the nanocomposite speci
mens containing 2 and 3 wt.% of nanoclay, there
is no peak on their SAXS patterns, which confirms a
great exfoliation of all the nanoclay in bitumen
matrix for these two nanocomposites. However, in th
e case of the 4 wt.% clay-bitumen
nanocomposite, one peak with a low intensity appear
ed on SAXS pattern at 2
θ
=4.74195, which is
shifted to a lower angle compared with that of the
pure Cloisite Na
+
. This result shows that a
considerable intercalation (according to Bragg’s la
w
d
= 21.62158 Å) has occurred for some clays in
the 4 wt.% clay-nanocomposite and the other clays e
xfoliated in the bitumen matrix. The high
exfoliation of clays shows the nanoscale manner.
Figure 2
XRD patterns of pure Cloisite Na
+
and bitumen/clay nanocomposites
3.3. Electrochemical impedance spectroscopy (EIS)
Figures 3 to 6 show the Nyquist plots of coated ste
el at different immersion times and clay contents i
n
a 3.5 wt.% NaCl solution. The plots show a depresse
d capacitive loop for all the coatings at low
immersion times, which arises from the time constan
t of the resistance and capacitance of the
coatings. The equivalent circuit compatible with th
e Nyquist diagram is depicted in Figure 7a. To
obtain a satisfactory impedance simulation, it is n
ecessary to replace the capacitor (
C
) with a constant
phase element (CPE)
Q
in the equivalent circuit. Constant phase elements
Q
c
,
R
s
, and
R
c
correspond to
20
Iranian Journal of Oil & Gas Science and Technolog
y,
Vol. 4 (2015), No. 2
coating layer capacitance, solution resistance, and
coating resistance respectively. The most widely
accepted explanation for the presence of CPE behavi
or and depressed semicircles on solid electrodes
is microscopic roughness, causing an inhomogeneous
distribution in the solution resistance as well as
in the double layer capacitance (Danaee and Nikneja
d Khomami et al., 2013; Danaee et al., 2011). To
corroborate the equivalent circuit, the experimenta
l data are fitted to the equivalent circuit and the
circuit elements are obtained. Table 2 shows the eq
uivalent circuit parameters for the impedance
spectra at different clay loadings and immersion ti
mes. These results show that the addition of
nanoadditives improves the coating resistance and t
he loop size decreases as the immersion time rises
(Figures 3 and 4).
Table 2
Impedance data of nanocomposite coating at differen
t contents and immersion times in a 3.5 wt.% NaCl
solution.
I mme r s io n t i me
Sa mp le
3 0 mi n.
1 4 Da y s
2 1 Da y s
6 0 Da y s
Neat bitumen
R
c
(ohm)
1.8×10
6
4.2×10
5
3.3×10
5
1.7×10
5
Q
c
(F)
1.5×10
-6
5×10
-6
2×10
-8
1.5×10
-8
n
1
0.75
0.6
0.65
0.65
R
ct
(ohm)
3.4×10
6
1.2×10
6
Q
dl
(F)
2×10
-6
1.3×10
-5
n
2
0.63
0.65
PNC2 (2 wt.%)
R
c
(ohm)
2.3×10
6
6.3×10
5
5.3×10
5
2.5×10
5
Q
c
(F)
3×10
-6
3×10
-6
4×10
-6
1.5×10
-8
n
1
0.75
0.6
0.63
0.6
R
ct
(ohm)
1.3×10
6
Q
dl
(F)
1.5×10
-6
n
2
0.6
PNC3 (3 wt.%)
R
c
(ohm)
3.1×10
6
1.1×10
6
7.6×10
6
4.8×10
5
Q
c
(F)
2.1×10
-6
2×10
-6
3×10
-6
8×10
-6
n
0.75
0.75
0.7
0.76
PNC4 (4 wt.%)
R
c
(ohm)
5.1×10
6
1.4×10
6
1.1×10
6
6.9×10
5
Q
c
(F)
1.2×10
-6
1.9×10
-6
2×10
-6
4×10
-6
n
0.75
0.83
0.72
0.74
H. R. Zamanizadeh et al. / Evaluation of the Effect
s of Nanoclay Addition ...
21
Figure 3
Nyquist diagrams of clay/bitumen coated samples aft
er 30 min of immersion.
Figure 4
Nyquist diagrams of clay/bitumen coated samples aft
er 14 days of immersion.
22
Iranian Journal of Oil & Gas Science and Technolog
y,
Vol. 4 (2015), No. 2
Figure 5
Nyquist diagrams of clay/bitumen coated samples aft
er 21 days of immersion.
Figure 6
Nyquist diagrams of clay/bitumen coated samples aft
er 60 days of immersion.
Twenty one days after immersion, neat bitumen and P
NC2 started to create their second loop, which
indicated that the corrosion began at the interface
of the coatings and their substrates. Figure 7(b)
shows the equivalent circuit compatible with these
Nyquist diagrams, for which the corresponding
parameters are tabulated in Table 2. Constant phase
elements
Q
dl
and
R
ct
correspond to double layer
capacitance
1
n
n
dl
dl
Q
R C
=
(Danaee et al., 2010) and charge transfer resistan
ce respectively.
H. R. Zamanizadeh et al. / Evaluation of the Effect
s of Nanoclay Addition ...
23
Due to the higher stability of the coatings, only o
ne capacitive loop was observed in samples PNC3
and PNC4 even after 60 days of immersion. For 60 da
ys of immersion, sample PNC4 shows the
highest
R
c
(R
c
= 6.9×10
5
) and thereby the best performance compared with th
e other coatings. By
increasing the immersion time,
Q
c
increases (e.g. Q
c
varies between 1.2×10
-6
F to 4×10
-6
F for PNC4),
which is indicative of increased water penetrated i
nto the coating, because water has a higher
dielectric constant compared to the polymeric coati
ngs (Deflorian et al., 1999).The conclusion is that
adding nanoclay to bitumen forces corroding agents
to travel a longer tortuous path to reach the
substrate (Sun et al., 2008).
Figure 7
Equivalent circuits used for the numerical fitting
of impedance plots obtained for the different immer
sion times;
(a) before electrolyte reaches the metallic substra
te and (b) after the initiation of corrosion proces
s due to
electrolyte penetration.
3.4. Potentiodynamic measurements
Figure 8 shows the Tafel polarization curves of the
coated samples obtained after 30 days of
immersion in a 3.5 wt.% NaCl solution. Tafel calcul
ations are listed in Table 3, where
E
corr
,
I
corr
,
CR
,
β
a
,
β
c
, and
R
p
are the corrosion potential, corrosion current den
sity, corrosion rate, anode Tafel
constant, cathode Tafel constant, and polarization
resistance respectively. The corrosion current
density decreases while the corrosion potential ris
es as the amount of nanoclay increases. PNC4 has
the most positive value of
E
corr
(-0.482 mv), the highest value of
R
p
(1.334 M
cm
-2
), and the lowest
value of
I
corr
(0.02485
μ
A.cm
-2
), which confirms its enhanced corrosion protection
properties. The
enhancement of corrosion protection effect is relat
ed to the increase in the tortuosity of the diffusi
on
pathways of corroding agents due to the presence of
the dispersed silicate nanolayers (nanoclay).
a)
b)
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