Electrochemical Impedance Spectroscopy of Hybrid Epoxy Resin Emulsion Coatings

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Journal of Minerals and Materials Characterization and Engineering, 2012, 11, 1012-1019

Published Online October 2012 (http://www.SciRP.org/journal/jmmce)

Electrochemical Impedance Spectroscopy of Hybrid

Epoxy Resin Emulsion Coatings

Rakesh N. Patil, Brihbihari V. Sharma, Prakash A. Mahanwar

Department of Polymer & Surface Engineering, Institute of Chemical Technology, Mumbai, India

Email: [email protected]

Received May 9, 2012; revised June 13, 2012; accepted June 25, 2012

ABSTRACT

A hybrid epoxy resin one pack emulsion with acrylate was synthesized for application in the field of corrosion protec-

tion. Electrochemical impedance tests were applied to steel specimens coated with hybrid epoxy and tested after im-

mersion in 3.5% NaCl solution at room temperature. Results taken from Nyquist and Bode plots as were analyzed by

means of software provided with the instrument. Specimens were examined under scanning electron microscope shows

a clear rupture and degradation in hybrid epoxy coating after prolonged exposer to salt solution.

Keywords: Hybrid; Epoxy; Electrochemical Impedance Spectroscopy; Coatings

1. Introduction

Epoxy resins have excellent characteristic property of

corrosion resistance [1]. They have been used as two

pack systems in combination of different curing agents as

protective coatings [2]. However; they have poor or low

fracture energy, high shrinkage, and show brittle beha-

vior [3]. To overcome this disadvantages hybrid of epoxy

resin was synthesized with acrylate monomers using

emulsion polymerization technique. The hybrid can be

defined as, the system synthesized from two or more par-

ticipating components having superior properties then

both the participants. The hybrid epoxy resin synthesized

emulsion obtained was one pack system with longer shelf

life. For the hybrid to be used as a protective coating,

corrosion resistance properties must be satisfactory.

Electrochemical Impedance Spectroscopy (EIS) has

many advantages in comparison with other electroche-

mical techniques. It is a non-destructive method for the

evaluation of a wide range of materials, including coat-

ings, anodized films and corrosion inhibitors. It can also

provide detailed information of the systems under exami-

nation; parameters such as corrosion rate, electroche-

mical mechanisms and detection of localized corrosion.

Polymer based coatings use barrier technology to protect

substrates from corrosive chemicals and environments,

particularly when in immersion service [4].

This paper reports, results of an investigation of the

corrosion resistance of the hybrid coatings using EIS

techniques. The hybrid one pack epoxy system was syn-

thesized with conventional emulsion polymerization

technique. The effects of coating thickness on water

absorbance and ultimately on corrosion resistance were

also studied. Nyquist and bode plots was analyzed to

predict the corrosion performance of hybrid coatings.

SEM analysis of the specimen was performed after im-

mersion to get a closer view of coating degradation.

2. Experimental

2.1. Materials

Diglycidyl ethers of bisphenol A (DGEBA) epoxy resin

(D.E.R. 336) Kindly provided by Dow Chemical inter-

national, India. n-butylacrylate (BA) and methylmetha-

crylate (MMA) were used as the acrylic monomers.

Acrylic acid (AA) and 2-hydroxyl ethyl methacrylate

(HEMA) were used as the functional acrylic monomers

purchased from Indofil chemicals, Potassium persulfate

(KPS) used as initiator, Ethylene diamine (EDA) solution

used for neutralisation and sodium bicarbonate (SBC) as

a buffer was purchased from Sigma Aldrich, Nonyl phe-

noxy polyethyleneoxy ethanol nonionic surfactant HLB

17.8 (Neoigen DK X 405) nonionic surfactant and sodium

dodecyl benzene sulfonate (Daninol 25P) were used as

anionic surfactant supplied by Dai-Ichi Karkaria. Dis-

tilled and Deionized water was used throughout all

experiments arranged here. All the other chemicals used

in this work were AR grades obtained from a S.D. Fines

Chemicals used without further purification.

2.2. Preparation of Hybrid Epoxy Resin

Emulsion

The synthesis of hybrid emulsion was carried out in a

R. N. PATIL ET AL. 1013

500 mL four necked reaction vessel equipped with a

reflux condenser, nitrogen gas inlet, mechanical stirrer,

addition funnels, and a thermometer placed in a water

bath. Solid content of the emulsion was designed as 45%

by weight. Aqueous phase for emulsion polymerization

was prepared by addition of D.I. Water, surfactants and

buffer to the reactor (weight ratio: D.I. Water/surfactant/

NaHCO3 = 0.96/0.031/0.009), followed by the addition

of the initiator (KPS, 0.8% based on total monomer

mixture) at 70˚C under nitrogen atmosphere. The poly-

merization was performed at 70˚C by adding a mixture of

epoxy resin dissolved in acrylate monomers (weight ratio:

ER/BA/MMA/2-HEMA/AA = 0.60/0.22/0.15/0.015/0.015).

Finally, naturalization was carried out with ethylene dia-

mine solution and emulsion was stored in sealed con-

tainer.

2.3. Preparation of Specimen for Testing

Specimens in the form of mild steel strips with 1000 ×

700 × 0.5 mm size were used for analysis. Specimens

were cleaned and degreased before coating application.

Hybrid epoxy coatings were applied to specimen surfaces

and they were left to dry for 24 hours at 28˚C. Dry film

thickness was measured by a thickness gage. Two speci-

mens were tested as soon as they were immersed in test-

ing solution and were considered as time zero. Data were

collected from specimens immersed in solution for 12

days at room temperature.

2.4. Methods

Electrochemical Impedance Tests

The corrosion behavior of the specimens was monitored

using electrochemical impedance spectroscopy (EIS)

during immersion in 3.5% NaCl solution open to air and

at room temperature for up to 12 days. A three-electrode

set-up was used to record corrosion potential of coating.

A saturated calomel electrode (SCE) was used as the re-

ference electrode. It was coupled capacitively to a Pt

wire to reduce the phase shift at higher frequencies. Ele-

ctrochemical impedance tests were carried out by using

Versa STAT 3 provided with frequency response analy-

zer, frequency in the range from 1 Hz to 1 kHz to collect

data with a total number of 40 readings for the whole

range. The amplitude of the sinusoidal voltage signal was

50 mV. Data were collected by means of Frequency

Response Analyzer software developed by Princeton

Applied Research instruments and were in the form of

Nyquist plots. Tested specimens were washed with dis-

tilled water, and then gold coated to examined under

scanning electron microscope.

2.5. Results & Discussion

The equivalent circuit for polymer coated steel can be

represented by the model shown in Figure 1. Where, RS

is the solution resistance, RP is the pore resistance of the

coating, RT is the charge transfer resistance of the metal/

solution interface, Cdl is the double layer capacitance,

and Cc is the coating resistance.

Figures 3-7 represent the Nyquist plots for the impe-

dance of specimens tested after immersion in 3% Nacl

solution at room temperature for a period of 12 days.

Figure 3 shows, the Nyquist plot for the specimen tested

at the time of immersion, the behavior takes a shape of a

part of a semicircle with high capacitive and resistive

values. The overlay of bode plots for hybrid coating with

different thickness are presented in the Figure 2. From

which it can be predicted that hybrid coating possesses

good corrosion resistance properties.

The effect of exposure time on the impedance beha-

vior of hybrid epoxy coating can be seen from Figures 8

and 9, a severe change in capacitance and resistance

values just after 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.

R. N. PATIL ET AL.

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.

R. N. PATIL ET AL.

1016

Figure 7. Nyquist plot of the sample after 12 days immersion.

‐15

‐10

‐5

0510

log Cd

Time in Days

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

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

50000

100000

150000

200000

250000

0 2040608

Impedance @ (0.1 Hz)

Coating Thickness (µm)

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 cm−1 for stretch-

ing of paraphenyl of epoxy resin and 1729 cm−1 for the

absorption of carbonyl group of acrylates. Absorbance

intensity of 1508 cm−1 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].





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



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.

R. N. PATIL ET AL.

1018

Table 2. Coating impedance, IZI, at 0.1 Hz as a function of immersion time.

Impedance @ 0.1Hz (Ω 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

85. 5

88. 5

91. 5

94. 5

EXP - 39 - 2

(a)

80010001200140016001800200024002800320036004000 1/cm

72.5

77.5

82.5

87.5

EXP - 39

(b)

Figure 11. FTIR-ATR of hybrid (a) air facing, (b) metal facing.

R. N. PATIL ET AL. 1019

0.5

1.5

0510 15

Water absorbance (ω)

Time in Days

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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the Presence of Poly (Methyl Methacrylate) and Polysty-

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doi:10.1002/app.20242

[4] H. Nadia, C. Hacene, G. Gildas, B. Kamel, “The Corro-

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NaCl Solution.” Advances in Chemical Engineering and

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[5] J. Brandrup and E. H. Immergut, “Polymer Handbook,”

2nd Edition, Wiley, New York, 1975.

[6] L. Hartshorn and E. Rushton, “Society of Chemical

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