fter immersion for 4 days. Further immersion
had also a great effect on the behavior of hybrid epoxy
coat- ings. Immersion for 12 days and above, yielded a
double semi-circle, Nyquist plot with an indication about
the failure of hybrid coating and the interaction of metal
surfaces with a solution. Figures 8 and 9 show the values
of Capacitance (Cd) and Resistance (Rc) for specimen E
as a function of immersion time. The capacitance values
of the samples were very low, these values increased gra-
dually as they were immersed in the solution. In the case
of hybrid epoxy coatings, the value of Cd increases after
Figure 1. Equivalent RC circuit for organic coating/metal
systems.
Figure 2. Overlay of Bode plot for hybrid coatings with
different thickness.
Copyright © 2012 SciRes. JMMCE
R. N. PATIL ET AL.
Copyright © 2012 SciRes. JMMCE
1014
Z , Msd.
unknown.txt
Model : R(C(R(CR))) Wgt : Modulus
Z ', ohm-sq. cm
5.50E+055.00E+054.50E+054.00E+053.50E+053.00E+052.50E+052.00E+051.50E+051.00E+055.00E+040.00E+00
- Z '', ohm-sq. cm
3.40E+05
3.20E+05
3.00E+05
2.80E+05
2.60E+05
2.40E+05
2.20E+05
2.00E+05
1.80E+05
1.60E+05
1.40E+05
1.20E+05
1.00E+05
8.00E+04
6.00E+04
4.00E+04
2.00E+04
0.00E+00
Figure 3. Nyquist plot of the epoxy-acrylate tested just after immersion in the testing solution.
Z , Msd.
unknown.txt
Model : R(C(R(CR ))) Wgt : Modulus
Z ', ohm-sq. cm
1.40E+071.30E+071.20E+071.10E+071.00E+079.00E+068.00E+067.00E+066.00E+065.00E+064.00E+063.00E+062.00E+061.00E+060.00E+00
- Z '', ohm-sq. cm
9.00E+06
8.50E+06
8.00E+06
7.50E+06
7.00E+06
6.50E+06
6.00E+06
5.50E+06
5.00E+06
4.50E+06
4.00E+06
3.50E+06
3.00E+06
2.50E+06
2.00E+06
1.50E+06
1.00E+06
5.00E+05
0.00E+00
Figure 4. Nyquist plot of the epoxy-acrylate after 4 days of immersion.
R. N. PATIL ET AL. 1015
Z , Msd.
unknown.txt
Model : R(Q(R(CR ))) Wgt : Modulus
Z ', ohm-sq. cm
3.50E+053.00E+052.50E+052.00E+051.50E+051.00E+055.00E+040.00E+00
- Z '', ohm-sq. cm
2.30E+05
2.20E+05
2.10E+05
2.00E+05
1.90E+05
1.80E+05
1.70E+05
1.60E+05
1.50E+05
1.40E+05
1.30E+05
1.20E+05
1.10E+05
1.00E+05
9.00E+04
8.00E+04
7.00E+04
6.00E+04
5.00E+04
4.00E+04
3.00E+04
2.00E+04
1.00E+04
0.00E+00
Figure 5. Nyquist plot of the sample after 8 days immersion.
Figure 6. Nyquist plot of the sample after 10 days immersion.
Copyright © 2012 SciRes. JMMCE
R. N. PATIL ET AL.
Copyright © 2012 SciRes. JMMCE
1016
Figure 7. Nyquist plot of the sample after 12 days immersion.
15
10
5
0
0510
log Cd
Time in Days
15
surface. This penetration can be through breakdown sites
of the coating. The high viscosity of salt water compared
to water may make water uptake a time consuming
process. The other parameter, (Rc), gives an indication
about the corrosion process at the steel surface. This
parameter can be used to study the effectiveness of some
coating in the protection of metals. Figure 9 shows the
decrease in the value of Rc with respect to immersion
time. Scanning Electron Micrographs taken from speci-
mens tested after 10 days of immersion show a clear
coating degradation in the form of holidays and rupture.
This can be seen from Figures 13. The effect of immer-
sion time on the performance of epoxy coating is clear,
and can be seen from the values of Cd and Rc.
Figure 8. The value of Cd versus immersion time for spe-
cimen E.
0.00E+00
5.00E+04
1.00E+05
1.50E+05
2.00E+05
2.50E+05
0510 15
Rc of Coatings
Time in Da
y
s
Water Absorbance Study of Hybrid Coating
The pore resistance, RP, which represents the ability of
coating to protect the substrate, can be determined in the
low frequency region in which the coating impedance is
independent of frequency. The results show that hybrid
coatings have their well-defined pore resistance; Table 1
summarizes the coating impedance at 0.1 Hz obtained in
this investigation as a function of immersion time for
coatings of varying thickness.
Figure 9. Rc versus immersion time for specimen E.
immersion in the electrolyte, reaching a constant value.
The plateau period of Cd indicates the beginning of
detachment of the coating from the substrate due to
adhesion loss. Figure 10 plots the IZI at 0.1 Hz as a function of coat-
ing thickness for a hybrid which are of the same coating
type but at different thicknesses.
In the case of sample E, a low value of Cd were ob-
tained of epoxy coating just after immersion in salt water,
the gradual increase in the value of Cd can be due to
solution penetration between the coating and the steel
The results demonstrated that, the coating impedance
increased with thickness. As shown in Figure 10, the
R. N. PATIL ET AL. 1017
0
50000
100000
150000
200000
250000
0 2040608
Impedance @ (0.1 Hz)
Coating Thickness (µm)
0
Figure 10. Plot of impedance values against coating thick-
ness.
Table 1. Water absorption ω as a function of immersion
time.
Water absorption, ω
Specimen
Coating
thickness
(µm) 4 Day 8 Days 10 Day
A 32 0.00465 0.57519 0.62417
B 40 0.00615 0.21662 1.18781
C 54 0.30368 0.59983 1.28901
D 66 0.15323 0.69160 1.29040
E 75 0.09285 0.36626 1.35465
coating impedance at 0.1 Hz decreased by one order of
magnitude after 8 days of immersion. Further decrease
was observed during further immersion up to 12 days.
During the EIS measurements, no significant corrosion
or coating degradation was observed during 8 days of
testing. The decreases of coating impedance during the
immersion tests is most likely due to the intrusion of
moisture and ions into the structures in the coatings,
which in turn increased the pore conductance. This can
be explained with the structural orientation of hybrid
coatings during film formation.
Figure 11 demonstrates the representative FTIR spec-
tra for the air-facing side and Fig: metal-facing side of
the hybrid containing. The peak at 1508 cm1 for stretch-
ing of paraphenyl of epoxy resin and 1729 cm1 for the
absorption of carbonyl group of acrylates. Absorbance
intensity of 1508 cm1 peak at the metal facing side is
higher than those at air-facing side, suggesting that the
epoxy resin part in emulsion tends to move to the metal
facing side. The driving force of this movement could be
attributed to the difference in the surface free energy be-
tween the epoxy resin and the acrylic copolymer. The
critical surface tensions of poly butyl acrylate, poly-
acrylic acid, poly methyl methacrylate and poly 2-hy-
doxyl ethyl methacrylate are around 31, 11.1, 39 and 37
mN/m, respectively [5], so the critical surface tension of
the acrylic copolymer should be between 11 and 37
mN/m, which is lower than that of the epoxy resin, which
is around 44 mN/m.
Thus, during the process of casting and drying the hy-
brid films, the acrylic-copolymer segments tried to seg-
regate near the air-facing layer and the epoxy segments
moved to the mold-facing side to minimize the surface
energy. This migration is very beneficial because epoxy
resins have excellent adhesion to substrates improving
corrosion resistance, while acrylic copolymers remaining
on the air-facing side have very good weatherability and
appearance.
The water absorbance of coatings due to the presence
of hydrophilic acrylate monomers affects the capacitance
of coatings. The measurement of the water absorption
using EIS techniques is based on the determination of the
changes of coating capacitance. The coating capacitance
can be calculated from the EIS data C = 1/IZI at the fre-
quency of 1/2Π Hz. The water absorption can be calcu-
lated by the formula given by Hartshorn et al. [6].
2
t0 HO
logCClog





where ω is the volume fraction of the absorbed water, Ct
is the coating capacitance at time t, C0 is the capacitance
at t = 0, and 2
HO
is the dielectric constant of water. In
this investigation, the coating capacitance measured im-
mediately after immersion is taken as C0. The calculated
ω using the capacitance values obtained in this work are
summarized in Table 1.
The plote of water absorbance aginst immersion time
for specimen A and E are shown in Figure 12. A close
examination of the data in Tables 1 and 2 re- veals that
correlation between IZI and ω during the im- mersion
tests, indicating that the decrease of IZI was due mainly
to the water absorption in the coatings. For the thin hy-
brid epoxy coatings tested in this investigation, the rapid
water absorption in the first four days of im- mersion
could be best explained by the capillary action in the
micro pore/defect structures, which was followed by a
slow water dissolution in the coatings. The water ab-
sorption for thicker coatings during 10 days of testing
suggested that, the rates of homogeneous water dissolu-
tion into the coatings were slow. The high performance
of this type of coating has been reflected by EIS data
measured during 10 days of immersion. Figure 13 repre-
sents SEM micrograph for specimen after immersion test.
Micrograph showing the degradation of hybrid coatings
after 10 days of immersion test.
2.6. Conclusions
Hybrid one pack epoxy coating was found to be effe-
ctive as a protective coating against corrosion resi-
stance.
Corrosion of substrate by the ingress of ionic species
through coating, increases disbonding between coat-
ing and substrate, which promotes the degradation of
coating by the dual action of chemicals and mecha-
nical processes.
Copyright © 2012 SciRes. JMMCE
R. N. PATIL ET AL.
Copyright © 2012 SciRes. JMMCE
1018
Table 2. Coating impedance, IZI, at 0.1 Hz as a function of immersion time.
Impedance @ 0.1Hz ( cm2)
Specimen Coating thickness (µm) 0 Day 4 Day 8 Days 10 Day
A 32 7.401841e+03 7.253213e+03 6.026863e+02 4.867542e+02
B 40 1.046800e+04 1.019086e+04 2.692158e+03 5.894301e+01
C 54 4.012673e+04 1.067432e+04 2.934141e+03 1.453298e+02
D 66 2.079440e+05 1.066014e+05 1.019086e+04 1.067342e+03
E 75 2.127438e+05 1.419121e+05 2.873426e+04 3.860423e+02
80010001200140016001800200024002800320036004000 1/cm
84
85. 5
87
88. 5
90
91. 5
93
94. 5
%T
EXP - 39 - 2
(a)
80010001200140016001800200024002800320036004000 1/cm
70
72.5
75
77.5
80
82.5
85
87.5
90
%T
EXP - 39
(b)
Figure 11. FTIR-ATR of hybrid (a) air facing, (b) metal facing.
R. N. PATIL ET AL. 1019
0
0.5
1
1.5
0510 15
Water absorbance (ω)
Time in Days
A
E
Figure 12. Plot of water absorbance ω against immersion
time.
Figure 13. SEM micrograph of hybrid coating (E) after 10
days of immersion.
For the hybrid coating impedance increased with in-
creasing coating thickness.
For the hybrid epoxy coatings, the quick water absor-
ption at early immersion stage could be explained by
the structure of coatings.
It is concluded that EIS data will be useful in predict-
ing lifetime expectancy of coatings in immersion ser-
vice.
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Copyright © 2012 SciRes. JMMCE