Journal of Minerals & Materials Characterization & Engineering, Vol. 4, No. 2, pp 75-84, 2005
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75
Electrochemical Impedance Spectroscopy Of Epoxy-Coated Steel
Exposed To Dead Sea Water
Sami Masadeh
Materials and Metallurgical Engineering Department
Al-Balqa Applied University, Al-Sult JORDAN
Abstract:
Electrochemical impedance tests were applied to steel specimens which were coated by
epoxy and tested after immersion in Dead Sea water for different periods at room
temperature and at 50C. Results taken from Nyquist and Bode plots and as were analyzed
by means of software provided with the instrument. Results were presented as the values
of capacitance and resistance (Cdl and Rct). Results show that degradation occurred
after immersion in the test solution, and were more severe at higher temperature.
Specimens were examined under scanning electron microscope, and micrographs showed
clear rupture and degradation in epoxy coatings.
Key words: Electrochemical impedance spectroscopy, epoxy, Dead Sea water, coatings.
INTRODUCTION
The Dead Sea lies between Jordan and Israel. Its water is unique because it
contains a number of salts with compositions different than any other sea water in the
world. The composition of Dead Sea water is as follows: 14.5% MgCl, 7.5% NaCl, 3.8%
CaCl2, 0.5% MgBr2, 1.2% KCl, and rest is water. Some industrial and tourist structures
lay at sea coast; severe corrosion problems encounter sea water due to high chlorides
content. In many cases, corrosion can not be controlled unless very special alloy steel is
used. In this work, an investigation regarding the effectiveness of epoxy coating in
corrosion control. Electrochemical impedance spectroscopy was used due to its well
known accuracy in testing and efficiency in the evaluation of polymer coated metals and
any change may come over during exposure to corrosive environments [1-6]. Besides the
advantages that can electrochemical impedance spectroscopy offer for the analysis of
adhesion of coatings at coating-metal interface as is worked by many researchers [7-13].
Experimental
Forty specimens in the form of disks having 15 mm diameter and 5 mm thickness
were used in this work. Specimens were ground, cleaned and degreased. Epoxy primer
coating were applied to specimen surfaces and they were left to dry for four hours at 23 C
as recommended by the manufacturer. Epoxy coating then were applied by means of air
spraying with recommended tip range and total output pressure. Steel discs then were left
to dry at 23 C for 8 hours. Coating thickness was measured by a thickness gage meter and
was found to be about 200 micron. Two specimens were tested as soon as they were

76Sami MasadehVol. 4, No. 2
immersed in testing solution and were considered as time zero. Twenty specimens were
immersed in testing solution at room temperature, while the other twenty were immersed
in the solution in a furnace at 50 C. Data were collected for specimens immersed in
solution for 133 day at room temperature and at 50 C. Electrochemical impedance tests
were carried out by using Autolab PGSTAT 30 provided with frequency response
analyzer, FRA2 in the range from 1 Hz to 1 kHz to collect data with a total number of 40
readings for the whole range. Data were collected by means of Frequency Response
Analyzer software developed by Autolab instruments and were in the form of Bode and
Nyquist plots. Tested specimens were washed by distilled water, and then gold coated
and examined under scanning electron microscope.
RESULTS AND DISCUSSION
The equivalent circuit can
be represented of coated
steel by the model shown in
Figure 1. This model was
accepted to be
representative of polymer-
coated metals [1416],
where Cdl and Rct
represent the capacitance
and resistance of the
coating, respectively; w
represents the Warburg
impedance; and Rs is the
resistance of the electrolyte.
It was accepted by some
researchers [17], that the
value of coating resistance,
Rct, is the best for the
measurement of coating
degradation, where can be
found from semi-circle
diameter of the Nyquist
plot. The values of Rct and
Cdl were calculated by the
software (FRA2) provided
with the instrument.
Figures (2-9) represent the
Nyquist plots for the
impedance of specimens tested after immersion in Dead Sea water at different periods at
room temperature. Figure 2 shows the Nyquist plot for the specimen tested at time of
immersion at room temperature, as can be seen, the behavior takes a shape of a part of a
semi circle with very high capacitive and resistive values. The effect of exposure time on
Figure1: Equivalent circuit model of
a polymer-coated metal
.
Figure 2: Nyquist plot of the epoxy-coated sample tested
just after immersion in the testing solution.

Vol. 4, No. 2. Electrochemical Impedance Spectroscopy Of Epoxy-Coated Steel77
Exposed To Dead Sea Water
Figure 4: Nyquist plot of the sample after 23 days immersion.
impedance behavior
of epoxy coating
can be seen from
Figure 3, a severe
change in
capacitance and
resistance values
just after immersion
for 7 days. Further
immersion had also
great effect on the
behavior of epoxy
coating. Immersion
for 46 days and
above, yielded a
double semi-circle
Nyquist plot with an
indication about the
failure of epoxy
coating and an
interaction between
steel surface and
solution, this is
illustrated in
Figures (5-6). The
Nyquist plots
obtained for
samples tested at 50
C are shown in
Figures (10-14).
Plots show that
there were two semi
circles or parts of
semi circles which
is the case of failed
coating. The
comparison of
results obtained at
different
temperatures show
that, degradation
took place at shorter
periods at higher
temperatures.
Figures (15-16)
show the values of
Figure 3: Comparison of Nyquist plots of specimens tested
after immersion for 7 and 14 days.
Figure 5: Nyquist plot of the sample after 67 days immersion

78Sami MasadehVol. 4, No. 2
Figure 6: Nyquist plot of the sample after 90 days immersion
Figure 7: Nyquist plot of the sample after 97 days immersion
Figure 8: Nyquist plot of the sample after 119 days immersion
Cdl and Rct as a
function of immersion
time respectively at
room temperature and
at 50 C. The
capacitance values of
the samples were very
low, these values
increased gradually as
they were immersed in
the solution at both
temperatures. The
dependence of water
uptake by coatings on
Cdl was studied by
many authors (18-19).
In case of epoxy
coatings, the value of
Cdl increases after
immersion in the
electrolyte, reaching a
constant value after
some hours (20). The
plateau period of Cdl
indicates the beginning
of detachment of the
coating from the
substrate because of
adhesion loss (20). In
the case of our
samples, a high value
of Cdl were obtained
of epoxy coating iust
after immersion in
Dead sea water, the
gradual increase in the
value of Cdl can be
due to solution
penetration between
the coating and steel
surface. This
penetration can be
through breakdown
sites of the coating.
The high viscosity of
Dead Sea water

Vol. 4, No. 2. Electrochemical Impedance Spectroscopy Of Epoxy-Coated Steel79
Exposed To Dead Sea Water
Figure 9: Nyquist plot of the sample after 133 days immersion
Figure 10:
Figure 11:
compared to water
may make water
uptake a time
consuming process.
The other
parameter, (Rct),
gives an indication
about corrosion
process at steel
surface. This
parameter can be
used to study the
effectiveness of
some coating in the
protection of
metals. Figure 16
shows the decrease
in the value of Rct
with respect to
immersion time.
Scanning Electron
Micrographs taken
for specimens tested
at both temperatures
show a clear
coating degradation
in form of holidays
and rupture. This
can be seen from
Figures (17-22).
The effect of
temperature on the
performance of
epoxy coating is
clear, and can be
seen from the
values of Cdl and
Rct. The scanning
electron
micrographs
showed degradation
after immersion for
10 days, Figure 21.

80Sami MasadehVol. 4, No. 2
Figure 13:
Figure 14:
Figure 12:

Vol. 4, No. 2. Electrochemical Impedance Spectroscopy Of Epoxy-Coated Steel81
Exposed To Dead Sea Water
Figure 15: The value of Cdl Versus Immersion time two different temperatures
Figure 16: Rct as a function of time and at different temperatures
Figure 17:

82Sami MasadehVol. 4, No. 2
Figure 18:
Figures (19-20): Two Scanning electron micrographs of epoxy coating after
immersion for 126 days in Dead Sea water at room temperature
.
Figure 21: Scanning electron micrograph of epoxy coating after
immersion for 10 days in Dead Sea water at 50 C

Vol. 4, No. 2. Electrochemical Impedance Spectroscopy Of Epoxy-Coated Steel83
Exposed To Dead Sea Water
CONCLUSIONS
1. Epoxy coating degradation was observed after 46 days of immersion at room
temperature.
2. Degradation of epoxy coating was observed after 10 days at 50 C.
3. Resistance of epoxy coating in Dead Sea water was better at room temperature
4. Corrosion of substrate by the ingress of ionic species through coating, increases
disbanding between coating and substrate, which promotes the degradation of coating
by the dual action of chemicals and mechanical processes.
5. Electrochemical Impedance Spectroscopy (EIS) was a good technique to characterize
the electrical properties of organic coatings and their adhesion to metal surfaces.
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Figure 22: Scanning electron micrograph of epoxy coating after
immersion
f
or 50 da
y
s in Dead Sea water at 50
C

84Sami MasadehVol. 4, No. 2
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