Volume 19 Preprint 59
Polyaniline-Epoxy Coatings for Corrosion Protection of Carbon Steel in Acidic Soil Solution
N.A. Rahman, Mokhtar Che Ismail, and Zakaria Man
Keywords: PANI-epoxy coating, NS4 soil solution, corrosion
This article is providing the information on properties of Polyaniline (PANI)-epoxy as an external coating in acidic soil environment (pH 3 and 5). Epoxy incorporating with PANI at varied weight percentages. The corrosion assessment on coated carbon steel was evaluated by salt spray, cathodic disbonding test, and immersion test. The impedance properties of the coated substrate are estimated by electrochemical impedance spectroscopy (EIS). The increasing of corrosion protection of PANI-epoxy coating was obtained shown that 1.5 wt.% PANI content in epoxy provides an acceptable protection against corrosion in acidic NS4 soil solution. The cross section images by SEM clearly showed that addition of PANI is able to increase the porosity own by neat epoxy coating which supported well that PANI played an important role in anti-corrosion performance for carbon steel in acidic media.
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Polyaniline-Epoxy Coatings for Corrosion Protection of Carbon
Steel in Acidic Soil Solution
N. A. Rahman1, M.C. Ismail2, and Z. Man3
Engineering Department, Universiti Teknologi PETRONAS, 32610 Seri Iskandar,
for Corrosion Research, Universiti Teknologi PETRONAS, 32610 Seri Iskandar, Perak,
Engineering Department, Universiti Teknologi PETRONAS, 32610 Seri Iskandar,
This article is providing the information on properties of Polyaniline (PANI)-epoxy as an
external coating in acidic soil environment (pH 3 and 5). Epoxy incorporating with PANI at
varied weight percentages. The corrosion assessment on coated carbon steel was evaluated
by salt spray, cathodic disbonding test, and immersion test. The impedance properties of
the coated substrate are estimated by electrochemical impedance spectroscopy (EIS). The
increasing of corrosion protection of PANI-epoxy coating was obtained shown that 1.5 wt.%
PANI content in epoxy provides an acceptable protection against corrosion in acidic NS4 soil
solution. The cross section images by SEM were clearly shows that addition of PANI is able
to increase the porosity own by neat epoxy coating which supported well that PANI played
an important role in anti-corrosion performance for carbon steel in acidic media.
Keywords: PANI-epoxy coating, NS4 soil solution, corrosion
In general, the external coating primarily functions as a protection barrier against corrosive
environment. Coating systems are formulated based on both environmental conditions and
desired life of the structure . A foremost application of external coatings is to protect the
metal structure from corrosion attack. Further innovation and coating technology
development is important since corrosion attack still remains a serious problem with a
multiple of governing factors. In underground conditions, the pH plays an important role
and affects the corrosion potential of the pipelines. In underground environment, the pH of
soil sometimes was fall in acidic range, depends on the surroundings factors. The acidic pH
was produced from industrial waste, human activities such as agriculture, acid rains,
mineral leaching and microbiological activity will lead to a serious corrosion risk to buried
structure such as steel, cast, iron and even zinc . To date, most studies on corrosion of
steel was carried out in soil environment with an average pH of 3.5 to 6 – but focusing
on the bare carbon steel substrate. However, the performance of coated carbon steel
structure subject to underground corrosion in acidic soil condition is important to be
For buried pipeline structure, the external coating provides an extra protection against
corrosion in corrosive soil media. Epoxy materials are known to be the most conventional
and superior coating in many aspects when compares to regular paints . However, over
the time, the barrier of the epoxy coating can fail due to surrounding factors that can
influence the integrity of the pipeline protection system. The combination of epoxy and
PANI is given better performance over prolonged exposure compared to neat epoxy paint
system –. PANI also has studied for multiple types of development including flameretardant additives , , bio corrosion , catalyst  and recognizable protecting
properties against corrosion on metal surfaces in the different types of media , –.
Since PANI has an excellent chemical stability and environmentally friendly properties, it has
been emerging as one of the most promising conducting polymer as a coating against
corrosion. To date, there have been extensive researches, studies on the performance of
PANI as a coating to investigate pipeline corrosion occurring under a different type of
corrosive media. The mechanism protection by external coating is still in debate. Some
authors say that the barrier coating can block the passage of oxygen and water reaching
the metal surface and others think that the protection offered by the organic coating which
offers the high electrical resistance thus preventing current flow between anodic and
cathodic side , , , . Other researchers , ,  were demonstrated the
ability of PANI, acts as a self-healing coatings and improved the corrosion resistance of
carbon steel in corrosive media. Among all the proposed mechanism, the most accepted
possible corrosion protection mechanisms of PANI are the combination of the
electrochemical inhibitor and cathodic protection systems .
To the best of our knowledge, no previous work has been done on the corrosion resistant
assessments of PANI-epoxy coating in acidic of NS4 soil solution media. Since the carbon
steel structure which in contact with soil has the risk to be corroded, the performance of
PANI should also be assessed in this condition. This work aims to use PANI in epoxy paint
system as external coating and to study its anticorrosive properties in the acidic NS4 soil
solution, with pH 3 and 5. The anticorrosion behavior of the PANI-epoxy coating over
carbon steel substrate was examined via salt spray, immersion, and cathodic disbonding
tests. In addition, the electrochemical impedance spectroscopy (EIS) also was performed in
order to investigate the impedance properties of the system during immersion test. EIS is a
powerful technique in order to understand the electrical properties of coating on the
substrate. The cross-section of PANI-epoxy coatings adhere on the carbon steel substrate
was studied by a scanning electron microscopy (SEM).
Substrate and test solution
Carbon steel of composition (weight %): C: 0.07, Si: 0.25, Mn: 1.54, P: 0.013, S: 0.001, Mo:
0.007, Cr: 0.05, Ni: 0.04, Al: 0.041 and Fe remainder was embedded in epoxy cold
mounting, with an exposed surface area of 1.5 cm2 for immersion and cathodic
disbondment test. The substrates were connected to the copper wire in order to establish
the electrical connection during tests (Figure 1) prior to the mounting process. Meanwhile,
for the salt spray test, the substrates with dimension 8.5 cm x 5 cm x 0.8 cm were used.
The substrates were grit blasted to achieve the required surface finish prior to the
application of the coating.
Figure 1. Schematic represent the coated sample for electrochemical measurement
The corrosive environment employed was NS4 soil solution which is widely used as
reference solution to simulate the condition under disbonded pipeline coating in many
types of research –. The soil solution was prepared by dissolving analytic grade
reagents in distilled water with following composition (g/L): 0.122 potassium chloride (KCl),
0.483 sodium bicarbonate (NaOHCO3), 0.181 calcium chloride (CaCl2.2H2O), and 0.131
magnesium sulfate (MgSO4.7H2O). The pH of NS4 soil solution is ranged from pH 8 to 8.5
while 0.1 M of Hydrochloric acid (HCl) was used to achieve soil solution pH of 3 and 5 for
comparison purpose. The conductivity of the solution was measured by digital conductivity
meter model EC600 from Extech Instruments and is shown in Table 1.
Table 1. Conductivity data of test soil solution at ambient temperature
Conductivity, (X 10-6 S cm-1)
NS4 + 1M HCl
NS4 + 1M HCl
PANI-epoxy paint was prepared by direct mixing of PANI into epoxy resin liquefied. PANI in
powder form with a particle size range of 0.3 to 100 µm was commercially obtained from
Sigma-Aldrich. PANI was dissolved in the epoxy resin at different weight percentages which
are tabulated in Table 2. The diglycidyl ether of bisphenol-A epoxy resin (EPIKOTE 828) was
used in this work. The mixture was stirred for 1 hour to obtain a homogeneous solution.
The dispersion of PANI in epoxy coating also was increased by more than 90% by addition
of aromatic solvent into the mixture, in this case, toluene. The rotary evaporator at
temperature 44 oC and 100 RPM rotation was used to remove the solvent from the mixture.
Finally, an aromatic amine hardener was added to the mixture with an epoxy/hardener ratio
of 2:1. The paint was applied on a blasted substrate by using a brush and dried at room
temperature for 24 hours. The dry film thickness (DFT) of the coating was in the range of
165 to 200 µm were measured by coating thickness gauge, model Elcometer 456. In
addition, cross-section morphology of coating on the substrate was observed by Scanning
Electron Microscopy (SEM) once the coating was completely cured.
Table 2. Weight percentages of epoxy resin and PANI
Epoxy resin (wt%)
Cathodic disbondment (CD)
An artificial defect was made in the middle of exposed surface area of the coated substrates
prior to the test. The defect was drilled until the angular cone point of the drill was fully
entered the panels where the cylindrical portion of the drill meets the steel surface. The
coated substrate was drilled by using a 3 mm diameter drill bit. The defect was prepared at
the last moment before cell assembly.
Subsequently, the substrates were immersed for 24 hours in NS4 soil solution at 65 ± 2 ºC
and maintained at a -3.5 volt (V) potential with respect to a silver/silver chloride reference
electrode (Ag/AgCl). The potential of the system was controlled by potentiostatic procedure
equipped by Autolab Metrohm potentiostat/galvanostat instrument using NOVA 1.10
software. An examination is performed immediately at the end of the test. A sharp, thin
knife was used to cut a star pattern onto the coated substrate. An attempt was then made
to peel easily the disbonded coating from the substrate which delaminated radially outward
from the defect site. The CD test was recorded in terms of the radial disbondment (RD) (in
accordance with ASTM G8 ).
This test provides a controlled corrosive environment which has been utilized to produce
relative corrosion resistance information of coated metals exposed in a salt spray chamber.
Every coating system has a pair of coated substrate. The scribed in the form of ‘X’ was
made across one of the coated substrates by using a sharp knife. It served as an artificial
designed to accelerate the corrosion process. The coated substrates were supported
between 150 to 300 from the vertical and parallel, respectively, to the principal direction of
flow of fog through the chamber. The test solution was 5 wt. % NaCl prepared with
deionized water with the pH of the solution in the range from 6.5 to 7.2. The temperature
was maintained at 35 ± 2 ºC in salt spray chamber for 720 hours according to ASTM B117
. The salt solution was sprayed by an atomizer, using compressed air supply, at 0.085
MPa with humidifier temperature at 47 ± 2 ºC. The inspection was done for every week and
the photo of test panels was recorded. The coated substrate was carefully removed from
the chamber at the end of the test before it was gently washed with clean running water to
remove salt deposits from their surface, and immediately dried. Evaluation of rust creepage
for scribed coated substrate was done and the rating was given in accordance with
Procedure A, ASTM D1654-08 .
The anticorrosion properties of the coatings were also evaluated by immersion test in the
NS4 soil solution for a period of 30 days according to ASTM G31. The impedance properties
of each coated substrate were measured by EIS, on day 3 and day 30 of immersion period
by ACM Gill AC potentiostat/ galvanostat equipped with (Ag/AgCl) reference electrode and
graphite as the auxiliary electrode.
RESULT AND DISCUSSION
As noted from the cross-section images, PANI-epoxy coating was able to reduce the
porosity of EP coating. We identified that the compact layer produced by the coating
content PANI acts as a barrier to protect underlying substrate shown in Figure 2. The
content of PANI in the coating has increased the ability to reduce the entry of corrosive
species. This can be attributed by a completely incorporated between PANI and epoxy.
EP with PANI coating
Figure 2. A micrograph of a cross-section coating (a) EP coating and (b) PANI-epoxy coating
Cathodic disbondment (CD) test
The magnitude of RD value in CD test indicates the disbondment area around the holiday.
The coating with a lower RD is better toward CD failure. In both test solution, the average
radial disbondment (RD) value for tested substrate was found to be less than 3.9 mm, 2.9
mm, 2.6 mm, and 2.1mm for EP, 0.5EP, 1.0EP, and 1.5EP respectively in NS4 of pH 3.
Meanwhile the substrate exposed in NS4 of pH 5 having a lower RD value which are 3.8
mm, 2.6 mm, 2.4 mm, and 2.0 mm 1mm for EP, 0.5EP, 1.0EP, and 1.5EP respectively. The
greatest disbondment was observed on the EP substrate as shown in Figure 3.
Figure 3. The RD values on different pH of NS4 soil solution
They are less disbondment of substrate coated was found on the coated substrate with
increasing PANI amount in the paint system. This implies that PANI and epoxy work
together to reduce the water entry during CD test while improving the performance of the
coating in acidic soil solution. It is notice that the effect becomes minimal when reducing
the PANI concentration, since the differences in PANI content in epoxy paint system were
minimal to each other. Theoretically, the disbondment was low resistance against water
penetration from the artificial defect. 1.5 EP showing the lowest RD value compared to
others. The different performance of coating system was due to how fast PANI reduction
starting from the defect site will progress into the intact coating. Obviously, in a good
PANI in the vicinity of the defect can be activated and stop corrosion . Hence, based on
CD test, it is hypothesized that 1.5 EP was given an outstanding coating system for
corrosion protection coating with reduction in almost 50% and 46 % of RD value in pH 3 and
5 respectively compared to EP. In the absence of PANI (EP coating), a low interdiffusion
bonding causes molecules to migrate easily across the interface. The reason for the
different corrosion protection provided is complicated by the reaction between epoxy and
PANI. Generally, PANI affects the nature of the oxide layer forming at the coating/metal
interface and at the exposed defect. Sathiyanarayanan  was successfully demonstrated
the effectiveness of PANI pigmented coating to protect steel in acidic media by passivation
mechanism of pin holes. In these cases, it is believed that more PANI loaded in epoxy based
system was found to be more resist to CD.
Salt spray test
The scribed and unscribed test panels are shown in Figure 4 (a) and (b) respectively.
Basically, all test panels clearly show the sign of corrosion after 30 days of ASTM B117
exposure. It can be seen that the performance of a developing coating system was limited.
EP test panel corroded the most compared to the other test panels. The delamination and
rusting phenomena on EP test panel are worst compared with the condition of the coated
test panel in the presence of PANI. 1.5 EP test panels showed the least corroded,
delaminated and rusted areas.
Figure 4. Test panels images after exposed in ASTM B117 condition (a) scribed test panel
coated by EP, 0.5 EP, 1.0 EP, and 1.5 EP coating system in order and (b) unscribed test panel
coated by coated by EP, 0.5 EP, 1.0 EP, and 1.5 EP in order
Other than the visual analysis, evaluation of rust creepage for scribed test panel was done
and the rating was given in accordance with Procedure A, ASTM D1654-08. The rating
values are tabulated in Table 3.
Table 3. Salt spray test evaluation
Creepage Size (mm)
Over 5 to 7
Over 2 to 3
Over 2 to 3
Over 1 to 2
According to Protective coatings and lining, QP-CS1, the acceptance criteria for passing
the salt spray exposure belong to the coated substrate which was rated at more than 4-5.
Based on this standard, all test panels were passing the acceptance criteria except the test
panels coated with EP coating system. In accelerated corrosive environment, the creepage
was spread which evidently confirm there are none of the self-healing reaction on the
coated substrate. There were a significant amount of rusts and blistering on all the tested
panels. The corrosion products formed during the corrosion process are mainly localized in
the scribe. However, the presence of PANI was able to reduce the delamination of coating,
up to 43% compared to EP. For unscribed test panels, after 30 days exposure, the
conditions for every test panels were observed to be almost the same. It has been found
that the coating is intact with a lesser degree of blister or delamination. No apparent
change in gloss or texture was observed except the discoloration of the coating after long
exposure in the salt spray chamber. In this test, the ability of all coated test panels to
withstand in the corrosive-humid atmosphere can be acquired. According to available
results obtained in this test, it can be proposed that the test panels coated with PANI-epoxy
coating provide better corrosion protection than EP. Basically, the relationship between the
spreading of creepage and adhesion between coating/substrate is a reversible relationship.
If the adhesion strength between coating/substrate is high, supposedly there will be less
delamination and damage. However, the delamination could be due to the low hydrolysis of
the bonds in wet conditions, experienced by the test panels in the salt spray chamber.
Electrochemical Impedance Spectroscopy (EIS) measurements
The coated substrates were evaluated based on the EIS measurement during immersion
test, is expressed graphically by Nyquist plot and Bode plot as shown in Figure 6 and
Figure 7 respectively.
Figure 6. EIS data for coated substrate immersed in NS4 soil solution of pH 3 shows as
Bode plot for; (a) 0.5 EP, 1.0 EP and 1.5 EP, (b) EP and uncoated substrate, and Nyquist plot
for; (c) 0.5 EP, 1.0 EP and 1.5 EP, and (d) EP and uncoated substrate
Figure 7. EIS data for coated substrate immersed in NS4 soil solution of pH 5 shows as
Bode plot for; (a) 0.5 EP, 1.0 EP and 1.5 EP, (b) EP and uncoated substrate, and Nyquist plot
for; (c) 0.5 EP, 1.0 EP and 1.5 EP, and (d) EP and uncoated substrate
Changes in impedance parameters were monitored when corrosion began and subsequently
coating properties become degraded . The Nyquist plots are characterized by a
semicircle inductive loop which attributed to the adsorption of an intermediate product in
the corrosion reaction . The higher diameter of semicircle arc, representing the lower of
the charge transfer resistance (the corrosion rate). In Bode plots, the impedance spectrum
exhibited two-time constants. The first time constant, which is at higher frequencies, HF
was associated with the properties of the coating, while the second at lower frequencies, LF
was related to the degree of delamination and/or corrosion . The coating behaves like
capacitance at HF while resistance at LF . According to the literature –, the
impedance modulus at 0.1 Hz (|Z|0.1Hz) is an appropriate parameter for characterization of
the protective properties of the coatings. The high values of |Z|0.1Hz are associated with a
high resistance behavior of the coating.
The output data from EIS spectra were analyzed using NOVA 1.0 software from Metrohm
Autolab, using the fitting procedure to a Rs (Qc [Rpore (Qdl Rp)]) model circuit for uncoated
substrate and EP coating system and a Rs (Cc [Rpore (Cdl Rp)]) model circuit for PANI in epoxy
coating systems. Rs was the solution resistance, Rp coating resistance, Rpore pore resistance,
Cdl the capacitance at the area under the coating, and Cc was related to the coating
capacitance. Instead of capacitance (C), a constant phase element (Q) combine with
resistance, R was used to represent the non-ideal behavior of the coating which gave a
better fitted model for uncoated substrate and EP coating system. The components in the
equivalent circuit are divided into certain regions to represent the process occurred on each
of the layers. For example ‘the coating regions’ labeled in the schematic diagram shown in
Figure 5a and b, both equivalent circuits represent the activity denoted by the coating,
particularly during immersion period. Furthermore, the RpCdl or RpQdl combinations which
are in series to Rpore represented the electrochemical corrosion process at the metal
interface. In addition, Rp is considered to be due to the formation of ionically conducting
paths in the polymer . The impedance values consist of Rs, Qdl, Cdl, Qc, Cc, Rp, and Rpore
obtained from the fitting procedure, are shown in Table 4 and 5. Such information permit
to evaluate the coating properties in each of NS4 soil solution pHs initially and at the end of
Table 4. Electrical properties for substrate immersed in NS4 soil solution of pH 3
Table 5. Electrical properties of substrate immersed in NS4 soil solution of pH 5
Immersion in NS4 soil solution of pH3
The Nyquist plots recorded in Figure 6 (b and d), shows the diameter of the semicircles arc
for 30 days immersion is smaller than 3 days of immersion. This suggests the water
adsorption could be the reason behind this response. However, the higher semicircle arc for
EP and uncoated substrate were much smaller than those obtained for 0.5 EP, 1.0 EP and
1.5 EP. Unfortunately, the higher the semicircle arc is found on the substrate which having a
reducing amount of PANI content in the epoxy coating system. According to the Bode plots,
every substrate immersed in NS4 with pH 3 exhibit an obvious decreasing of |Z|0.1Hz values
from days 3 to days 30 in the range of 3% to 68%. Initially, the 0.5 EP exhibits lower metal
dissolution followed by 1.0 EP, 1.5 EP, EP and finally the uncoated substrate. However, as
the immersion period was up to day 30, the value of |Z| decreased sharply for all
substrates. The decreasing value of |Z|0.1Hz for PANI in the epoxy coating was higher at the
end of the test compared to EP and the uncoated substrate. However, a comparison of the
overall impedance, the |Z|0.1Hz values are still higher in magnitude for PANI in the epoxy
coating system, especially for 1.5 EP. Such behavior can be further explained with the
increasing of the resistance (Rpore). It evolves with immersion time for all substrates with 1.5
EP exhibited the highest value compared to the others. The resistance found to increase in
the presence of corrosion product. In addition, a significant decreasing of Rp value is
observed for all substrate. For example, the Rp values of the uncoated substrate and 0.5 EP
are 0.7 kΩ and 748 kΩ on day 3, decreased to 0.6 kΩ and 313 kΩ respectively at the end of
immersion period. These results indicate that the poor barrier property owns by the EP
coating can be drastically increased up to 98% when added 0.5% concentration of PANI on
it. The degree of protection is increased in addition higher concentration of PANI in EP
Immersion in NS4 soil solution of pH5
Figure 7 depicts the Nyquist plot and Bode plot of the substrates immersed in NS4 soil
solution at pH 5. All the impedance spectra exhibit two depressed semicircles arc for
substrate 0.5 EP, 1.0 EP and 1.5 EP. The resistive behavior with high impedance is
decreased towards the end of immersion period for all substrates. However, the
corresponding Nyquist plot for 1.5 EP exhibits the higher impedance compare to the others.
Such behavior is shown that PANI-epoxy coating act as an effective physical barrier against
the penetration of corrosive ions and protect the underlying metal surface in this particular
media In addition, the impedance values at LF denote that the 1.5 EP own the best
corrosion protection as it had the highest |Z|0.1Hz values throughout the immersion period.
The less corrosion protection was carried out by the coating consists of lower PANI content.
At the end of immersion period, |Z|0.1Hz values of all coated substrate were decreased in less
than one order of magnitude compare to their initial |Z|0.1Hz values except for uncoated
substrate. Such behavior is consistent with the decreasing of Rpore values throughout the
immersion test. As the time elapsed, the solutions gradually penetrated to the coating
system and consequently decrease the impedance value due to porosity especially on the EP
coating. However in the presence of PANI a lower porosity was obtained resulted a higher
Rpore values at the end of immersion period. Furthermore, Rp values of all substrate also
decreased rapidly at the end of immersion period. Besides, the Cc values are lower about
10-10 F cm2, indicative of purely capacitive coating and low ion permeation owned by the
coating systems. The corrosion resistance of these coating enhanced by synergistic
interaction between PANI and epoxy. In fact, the coating resistance was increased in the
increasing of the PANI content in the coating.
Summary and conclusions
Following conclusions were drawn from this study. The presence of PANI in epoxy resin act
as an effective physical barrier against the penetration of corrosive ions and protect the
underlying metal surface regardless the pH of the NS4 soil solution. In addition to this, PANI
in powder form is able to provide the mechanical integrity to the epoxy coating by filling up
the porous characteristics of epoxy. The homogenous dispersion of PANI in epoxy mixture
was successfully reducing the porosity of EP coating. This behavior was clearly observed in
the cross-section images, discussed previously. Therefore, PANI-epoxy coating delays the
start of corrosion on the underlying surface, and the optimum protection is increased by
increasing the amount of PANI in the coating.
Accelerated corrosion studies such as immersion, CD, and salt spray test was successfully
adopted in this work. PANI-epoxy coating is able to improve the overall anticorrosion
performance of the coating in comparison with the neat epoxy coating. Furthermore, EIS
showed that the effective physical barrier against the penetration of corrosive ions and
protect the underlying metal surface with 1.5 wt. % of PANI content. In addition, the coating
properties demonstrate excellent performance in terms of resistant against long-term
immersion period. In CD test, it was found that the coating is intact with a lesser degree of
blister or delamination when increasing the PANI content in the epoxy coating.
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