Poly(o-anisidine) coatings on copper: synthesis, characterization and evaluation of corrosion...

Poly(o-anisidine) coatings on copper: synthesis, characterizationand evaluation of corrosion protection performance

Sonal Patila, S.R. Sainkarb, P.P. Patila,*

aDepartment of Physics, North Maharashtra University, P.O. Box 80, Jalgaon 425001, Maharashtra, IndiabNational Chemical Laboratory, Pune 411008, India

Received 7 March 2003; received in revised form 26 August 2003; accepted 6 October 2003

Abstract

The poly(o-anisidine) (POA) coatings were synthesized on copper (Cu) by electrochemical polymerization (ECP) of

o-anisidine (OA) and their corrosion protection performance in an aqueous solution of 3% NaCl was investigated by

potentiodynamic polarization technique. The ECP of OA was carried out under cyclic voltammetric conditions from an

aqueous solution of sodium oxalate to generate strongly adherent and smooth POA coatings on Cu substrates. These

coatings were characterized by cyclic voltammetry (CV), UV-vis absorption spectroscopy, Fourier transform infrared

(FTIR) spectroscopy, scanning electron microscopy (SEM) and X-ray diffraction (XRD) measurements. The optical

absorption spectroscopy study reveals the formation of the mixed phase of pernigraniline base (PB) and emeraldine salt

(ES) form of POA. The potentiodynamic polarization curves were used to evaluate the ability of the POA coatings to

protect the Cu surface. The potentiodynamic polarization curves show that the POA coating increases the corrosion

potential and drastically reduces the corrosion rate of copper. The corrosion rate of POA coated Cu is �100 times lower

than that observed for bare Cu.

# 2003 Elsevier B.V. All rights reserved.

Keywords: Corrosion-resistant coatings; Conducting polymer coatings; Poly(o-anisidine) coatings; Cyclic voltammetry; Copper

1. Introduction

Copper is widely used in microelectronics for

wiring, electromagnetic interference (EMI) shielding,

electrostatic dissipation, etc. Despite the fact that

copper is noble, it readily corrodes in a variety of

environments. Particularly, it is more susceptible to

the presence of chlorides, sulfur and applied fields [1].

It has been shown that the use of benzotriazole (BTA)

significantly improves the corrosion resistance of Cu

[2,3]. However, it is toxic and it does not provide the

corrosion protection at high potentials and elevated

temperatures. As a result, several attempts [4–8] have

been made to formulate new and environmently favor-

able polymeric coatings. Guenbour et al. [4] have

investigated the corrosion protection of Cu by poly-

aminophenol coatings synthesized by the ECP of

2-aminophenol from an alkaline hydroalcoholic

solution. Cieileo et al. [5] have shown that the oxime

compounds result in the formation of a polymeric

Cu(II) inhibitor complex which exhibits strong in-

hibition.

The use of conducting polymers as coating materi-

als for corrosion protection of metals/alloys has

Applied Surface Science 225 (2004) 204–216

* Corresponding author. Tel.: þ91-257-252187;

fax: þ91-257-252183.

E-mail address: [email protected] (P.P. Patil).

0169-4332/$ – see front matter # 2003 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2003.10.050

become one of the most exciting new research fields

in most recent times [9–14]. A number of reports on

the use of conducting polymers for the corrosion

protection of metals have appeared in the literature

in the mid-1990s. In view of the ability of the con-

ducting polymer coatings to protect the metal surfaces,

there is a growing interest particularly during the

past 3–4 years to synthesize these coatings on iron,

aluminum and their alloys and to evaluate the corro-

sion protection offered by them [9–16]. However,

only few studies [17] have been carried out on the

corrosion protection of copper by conducting polymer

coatings inspite of its use in wide range of technolo-

gical applications. In most of these studies, the con-

ducting polymers were first synthesized chemically

and then deposited on the metal surface. Brusic et al.

[17] have deposited the films of polyaniline and its

derivatives on Cu by spin coating technique and

studied the corrosion protection properties as a func-

tion of the applied potential and temperature. It was

observed that these films could either provide signifi-

cant corrosion to Cu exposed to water or enhance the

corrosion rate. They found that the chemical nature of

the polymer backbone, oxidation state and the extent

and nature of polymer doping significantly affects the

corrosion protection properties.

In the studies reported in this paper, we have

synthesized the POA coatings on Cu substrates by

ECP of OA and examined the ability of these coatings

to serve as protective coatings against the corrosion of

Cu. The objectives of the present study are (i) to

synthesize strongly adherent POA coatings on Cu

substrates; (ii) to investigate suitability of aqueous

sodium oxalate medium for the ECP of OA on Cu

substrates; (iii) to identify the oxidation state of POA

synthesized from sodium oxalate medium on Cu

substrates and (iv) to examine the possibility of using

a substituted derivative, POA for corrosion protection

of Cu.

The choice of OA monomer is due to the following

reasons: (a) The incorporation of the substituents in

the polymer skeleton is a common technique to

synthesize polymers having improved properties.

The OA is a substituted derivative of aniline with

methoxy (–OCH3) group substituted at ortho-position.

This study therefore explores the possibility of utiliz-

ing the POA as alternative to polyanilines for corro-

sion protection of Cu. (b) The monomer OA is

commercially available at low cost. (c) The conversion

of monomer to polymer is straightforward. (d) More

recently, our group [18,19] has investigated the ECP

of OA on low carbon steel substrates from aqueous

solution of oxalic acid. (e) Hardly any attempt

has been made to synthesize the POA coatings on

Cu substrates. In this work, cyclic voltammetry (CV)

is used to synthesize the POA coatings on Cu sub-

strates. These coatings were characterized by CV,

FTIR spectroscopy, UV-Vis absorption spectroscopy,

SEM and XRD measurements. The extent of corrosion

protection offered by these coatings to Cu in aqueous

solution of 3% NaCl was examined by the potentio-

dynamic polarization measurements and CV.

2. Experimental

Analytical reagents (AR) grade chemicals were

used throughout the present study. The OA monomer

was double distilled prior to its use. The aqueous

solution of sodium oxalate (C2O4Na2) was used as

the supporting electrolyte. The concentrations of

sodium oxalate and OA were kept constant at 0.2

and 0.1 M, respectively.

The Cu (99.98% purity) substrates (size �1 cm�1:5 cm and 0.5 mm thick) were cut from a piece of Cu

plate. The substrates were polished with a series of

emery papers, followed by thorough rinsing in acetone

and double distilled water and dried in air. Prior to any

experiment, the substrates were treated as described

and freshly used with no further storage.

The POA coatings were synthesized by ECP of OA

on Cu substrates under cyclic voltammetric condi-

tions. The ECP was carried out in a single compart-

ment three electrode cell with Cu as working

electrode (1.5 cm2), platinum as counter electrode

and saturated calomel electrode (SCE) as refer-

ence electrode. The cyclic voltammetric conditions

were maintained using a SI 1280B solartron electro-

chemical measurement system (UK) controlled by

corrosion software (CorrWare, Electrochemistry/

Corrosion Software, Scribner Associates Inc. sup-

plied by Solartron, UK). The synthesis was carried

out by cycling continuously the electrode potential

between �500 and 1500 mV at a potential scan rate

of 20 mV/s. The number of cycles was varied from 5

to 25. After deposition the working electrode was

S. Patil et al. / Applied Surface Science 225 (2004) 204–216 205

removed from the electrolyte and rinsed with double

distilled water and dried in air.

The corrosion protection performance of these coat-

ings was evaluated at room temperature in aqueous

solution of 3% NaCl having pH 6.23 by using elec-

trochemical techniques. The electrochemical techni-

ques include CV and potentiodynamic polarization

experiments. For these measurements, a Teflon holder

was used to encased the POA coated Cu substrates so

as to leave an area of 0.4 cm2 exposed to the solution.

The ability of the POA coating to protect the Cu

against dissolution was studied by recording the cyclic

voltammograms in an aqueous solution of 0.2 M

sodium oxalate in the potential range between �500

and 1500 mV at a scan rate of 20 mV/s. Before

polarization the substrates were immersed into the

solution and the open circuit potential (OCP) was

monitored until a constant value was reached. The

polarization resistance measurements were performed

by sweeping the potential between �250 and 250 mV

from OCP at the scan rate of 2 mV/s. All the measure-

ments were repeated at least four times and good

reproducibility of the results was observed. The corro-

sion potential (Ecorr) and corrosion current density

(Icorr) were obtained from the slopes of linear polar-

ization curves. The corrosion rate (CR) was calculated

by using the following expression:

CR ðmm=yearÞ ¼ 3:268 � 103 IcorrEW

r

where Icorr is the corrosion current density (A/cm2),

EW the equivalent weight of copper (g) and r the

density of Cu (g/cm3).

The FTIR transmission spectra of POA coating

were recorded in horizontally attenuated total reflec-

tance (HATR) mode in the spectral range 4000–

400 cm�1 using a Perkin-Elmer spectrometer, 1600

Series II, USA. The optical absorption studies of these

coatings were carried out ex situ at room temperature

in the wavelength range 300–1100 nm using micro-

processor controlled double beam UV-Vis spectro-

photometer (Hitachi, Model U 2000). The structural

properties were investigated using XRD technique.

The X-ray diffractograms were recorded with a

Rigaku diffractometer (Miniflex Model, Rigaku,

Japan) having Cu Ka (l ¼ 1:542 A). SEM was

employed to characterize the surface morphology with

a Leica Cambridge 440 Microscope (UK).

3. Results and discussion

3.1. Electrochemical behavior of Cu in an aqueous

solution of sodium oxalate

The Cu electrodes were first polarized under cyclic

voltammetric conditions in 0.2 M sodium oxalate

solution in order to understand the different processes

that occur at the electrode surface. The first, second

and 10th scans of cyclic voltammogram of the Cu

electrode polarized in a 0.2 M sodium oxalate solution

are shown in Fig. 1. The first positive cycle (Fig. 1a)

shows an increase in the anodic current density from

��177 mV which indicates that dissolution of Cu

Fig. 1. Cyclic voltammogram scans (a) first, (b) second and (c)

10th recorded during the polarization of Cu electrode in 0.2 M

sodium oxalate solution. Scan rate: 20 mV/s.

206 S. Patil et al. / Applied Surface Science 225 (2004) 204–216

begins at this potential. On polarizing further in the

anodic direction, a broad anodic peak (A) at �190 mV

is observed, which is attributed to the more intense

dissolution of Cu. As can be seen, the anodic current

density again increases between �531 and 739 mV

and beyond 750 mV it remains almost constant. Dur-

ing the negative cycle, the high value of the anodic

current reveals that the dissolution initiated during

positive cycle continues until the electrode potential

reduced to low values in the region of 0 mV. Thus, in

the potential range �177 to 500 mV, the Cu electrode

is active and undergoes anodic dissolution which

produces Cu2þ ions in its vicinity. These ions interact

with the oxalate electrolyte to form insoluble copper

oxalate, which adheres to the electrode surface thereby

forming a copper oxalate interphase. This interphase is

perhaps produced according to the following two

reactions:

Cu ! Cu2þ þ 2e�

Cu2þ þ ðC2O4Þ2� þ H2O ! CuC2O4 H2O

On repetitive cycling, the anodic peak A is still

observed. In addition to this, a new peak at

��100 mV is seen, which is assigned to the formation

of copper oxide. It is clearly seen that the position of

peak A is shifted in anodic direction and the current

density corresponding to this peak decreases gradually

with the number of scans. These observations suggest

stabilization of the Cu electrode as a result of the

formation of CuC2O4H2O on the electrode surface.

Thus, the anodic peak A is attributed to passivation

of the Cu electrode surface in the sodium oxalate

medium via the formation of CuC2O4H2O interphase.

This interphase appears to be sufficiently protective to

decrease the rate of Cu dissolution.

In order to justify the formation of CuC2O4H2O

interphase, we have performed the XRD studies on the

Cu electrode polarized in 0.2 M sodium oxalate solu-

tion. The XRD results of bare and polarized Cu

electrode (after 10 scans) are shown in Fig. 2. The

XRD pattern of bare Cu electrode (Fig. 2a) shows the

presence of diffraction peaks at 2y values of 43.48,50.68, 74.28, 89.88 and 95.08 corresponding to the Cu

substrate. The XRD pattern of polarized Cu electrode

(Fig. 2b) exhibits significant changes with respect to

the bare Cu. Apart from the characteristic peaks of Cu,

the XRD pattern indicates the presence of diffraction

Fig. 2. XRD pattern of (a) bare Cu, (b) polarized Cu substrate and (c) POA coated Cu. The (*) and (~) indicate the diffraction peaks due to

CuC2O4H2O and Cu2O, respectively.


peaks at 2y values of 24.88, 36.28, 37.08, 41.88, 45.48,49.08 and 54.88 corresponding to CuC2O4H2O phase.

In addition, it also shows the diffraction peaks at 2yvalues of 30.28, 41.88 and 74.28 corresponding to

Cu2O.

Thus, the CVand XRD results clearly reveal that the

polarization of the Cu electrode in an aqueous solution

of sodium oxalate leads to the formation of the inter-

phase which mainly comprised of polycrystalline

CuC2O4H2O.

The SEM micrograph of the copper electrode polar-

ized in a 0.2 M sodium oxalate solution is shown in

Fig. 3. It indicates relatively uniform coverage and

formation of needle shaped crystals at the surface of

copper electrode.

3.2. Electrochemical polymerization of OA on Cu

from an aqueous solution of sodium oxalate

The first, second and 10th scans of the cyclic

voltammogram recorded during the ECP of OA on

the Cu electrode from 0.2 M sodium oxalate solution

are shown in Fig. 4. The main features of the first scan

(Fig. 4a) are not affected by the presence of OA

monomer. The anodic peak A with an emergence of

shoulders at �460 and 676 mV is observed. As dis-

cussed earlier, the peak A corresponds to the dissolu-

tion of the Cu electrode. The visual observation of the

working electrode during the first positive cycle shows

that the color of the Cu surface slowly changes to dark

black from the potential �500 mV. This observation

implies that the deposition of the POA begins during

the first positive cycle. Therefore, the emergence of

the shoulders at �460 and 676 mVare attributed to the

oxidation of OA monomer and thus its actual poly-

merization. The deposition of a thin layer of polymer

coating during the first cycle may be responsible for

the protection of Cu against dissolution and hence

negligibly small anodic current densities are observed

at high potentials (>797 mV) and during the reverse

cycle.

During the next scan, the broad anodic peak (A0) is

observed and rest of the features are similar to that of

the first scan. The peak A0 is centered at �453 mVand

it is significantly different from that observed in the

first scan. As the deposition of the POA coating begins

during the first positive cycle, the peak A0 may be

attributed to the polymerization process rather than to

the dissolution of Cu. On repetitive cycling, the vol-

tammograms identical to that of second scan are

obtained. However, the current density corresponding

to the peak A0 decreases gradually with the number

of scans.

The observation of negligibly small anodic current

densities at all potentials in the 10th scan suggests

the formation of electroinactive POA coating.

Fig. 3. SEM micrograph of interphase formed during the polarization of Cu substrate in 0.2 M sodium oxalate solution.


The visualization of the Cu electrode after 10th scan

reveals the formation of a dark black colored POA

coating. The coating is uniform, compact and strongly

adherent.

The FTIR spectrum of POA coating synthesized on

Cu under cyclic voltammetric conditions (5 cycles)

recorded in HATR mode is shown in Fig. 5. This

spectrum exhibits the following spectral features: (i)

a broad and weak band centered at �3324 cm�1 due to

the characteristic N–H stretching vibration suggests

the presence of –NH– groups in OA units [20,21]. (ii)

The band at �1600 cm�1 is an indicative of stretching

vibrations in quinoid (Q) rings [20,23].

(iii) The band �1510 cm�1 represents the stretching

vibrations of the benzoid (B) rings [20–23].

(iv) The presence of Q and B bands clearly show that the

POA coating is composed of amine and imine units. It is

known that the ideal form of PANI contains roughly

equal amounts of Q and B units. The relative intensity

(1600/1510) of these bands is greater than unity. There-

fore, it can be concluded that the formation of perni-

graniline base (PB) is predominant in the coating. (v)

The bands at 1265 and 1658 cm�1 are attributed to

the presence of carboxyl groups of sodium oxalate in

the POA coating [24]. (vi) The bands at 1120 and

1031 cm�1 are attributed to the 1–4 substitution on

the benzene ring. (vii) The band at �1386 cm�1 is due

to the stretching of N–N bands. The presence of this

band indicates the formation of POA coating via head to

head coupling. (viii) The band at �1174 cm�1 is con-

sidered as a measure of the degree of delocalization of

Fig. 4. Cyclic voltammogram scans (a) first, (b) second and (c)

10th recorded during the synthesis of POA coating on Cu substrate

under cyclic voltammetric conditions. Scan rate: 20 mV/s.

Fig. 5. FTIR spectrum of the POA coating synthesized on Cu

substrate under cyclic voltammetric conditions. Scan rate: 20 mV/s.


electrons on POA and is referred to as the electronic-

like band. It is a characteristic peak of emeraldine salt

(ES) phase of POA [22–25]. The presence of this band

suggests the formation of ES phase of POA in the

coating along with the PB. (ix) The weak band at

�1454 cm�1 is assigned to the C–N stretching vibra-

tions in quinoid imine units. (x) The non-observance of

the bands around 900 and 800–700 cm�1 reveals that

1,2- and 1,3-substitutions do not occur to a significant

extent, although the deposition of POA takes place at a

relatively high potential [25]. Thus, the FTIR spectro-

scopic study reveals that the ECP of OA has occurred

and results into the deposition of POA coating on the

Cu electrode surface.

It is surprising to observe that the XRD pattern of

the POA coated Cu (Fig. 2c) exhibits the diffraction

peaks corresponding to Cu. It does not show the

diffraction peaks corresponding to copper oxalate

interphase. Also, as the POA coating has a highly

disordered structure, it does not indicate any diffrac-

tion peak. Thus, the XRD result reveals that the copper

oxalate intephase does not exit at the electrode surface

which supports the FTIR spectroscopic study.

The optical absorption spectrum of POA coating

(Fig. 6) shows a broad peak at about 560 nm and a

shoulder at about 720 nm. The peak at 560 nm is

attributed to the presence of PB phase of POA. The

PB is the fully oxidized form of POA and is insulating

in nature [26]. The shoulder at �720 nm is a signature

of the ES phase of POA, which is the only electrically

conducting phase of POA [26]. The simultaneous

appearance of 540 nm peak and a shoulder at

Fig. 6. Optical absorption spectrum of POA coating in DMSO solution.


800 nm clearly reveals the formation of mixed phase

of PB and ES phase of POA. Thus, the optical absorp-

tion spectroscopy results are in well agreement with

the CV and FTIR spectroscopy results.

The SEM micrograph of the POA coating synthe-

sized on Cu is shown in Fig. 7. It clearly reveals that

the POA coating is relatively homogeneous, compact

and featureless. Moreover, it does not indicate the

presence of the copper oxalate crystals on the elec-

trode surface which is in agreement with the XRD

result. The quality of the coating is so excellent that no

crack or detachment of the coating is observed. The

formation of the homogeneous and crack-free coating

is attributed to the presence of the Cu2þ ions in the

vicinity of the electrode during the ECP of OA [27,28].

The XRD and SEM (cf. Figs. 2c and 7) results

clearly reveal that the copper oxalate interphase does

not exist at the electrode surface. Therefore, it may be

thought that the copper oxalate formed during the

early stages of the growth decomposes into soluble

species and diffuses into the electrolyte and the ECP

of OA occurs on the Cu substrate from where it was

decomposed. As a consequence, the Cu substrate

surface still remains in the passive state. It seems

that the decomposition of the interphase generates a

suitable surface for the deposition of the POA.

Very recently, a similar process has been reported

by our group [18,19] for the synthesis of POA coatings

on low carbon steel (LCS) substrates from the aqueous

solution of oxalic acid. It has been shown that the

formation of the passive iron oxalate interphase and its

subsequent decomposition is necessary for the ECP of

OA to occur on the LCS substrates. By analogy with

this, it may be argued that the copper oxalate inter-

phase formed during early stages decomposes and

facilitates the ECP of OA on Cu.

3.3. Measurement of porosity in POA coatings

The coating porosity is one of the important para-

meters, which strongly governs the anti-corrosive

behavior of the coatings. Therefore, measurement of

the coating porosity is essential in order to estimate the

overall corrosion resistance of the coated substrate.

In this work, the porosity in POA coatings on Cu

substrates was determined from potentiodynamic

polarization resistance measurements [29]. The poten-

tiodynamic polarization curves for bare Cu, copper

oxalate coated on Cu (polarized Cu substrate in 0.2 M

sodium oxalate in the potential range between �500

and 1500 mV at a scan rate of 20 mV/s) and POA

coated on Cu in aqueous solution of 3% NaCl are

shown in Fig. 8. The values of the corrosion potential

(Ecorr), corrosion current density (Icorr), Tafel con-

stants (ba and bc), polarization resistance (Rp) and

corrosion rate obtained from these curves are given in

Fig. 7. SEM of POA coating synthesized on Cu substrate under cyclic voltammetric conditions (10 cycles, scan rate: 20 mV/s).


Table 1. The porosity in POA coating was calculated

using the relation [29]

P ¼ Rps

Rp

� �10ð�jDEcorrj=baÞ

where P is the total porosity, Rps the polarization

resistance of the bare Cu, Rp the measured polarization

resistance of coated Cu, DEcorr the difference between

corrosion potentials and ba the anodic Tafel slope for

bare Cu substrate. The porosity in POA coating (15

cycles) was found to be �0.23%.

In order to investigate the influence of the coating

thickness on the porosity and corrosion protection

properties of the POA coatings, we have synthesized

the coatings by 5, 10, 15 and 20 cycles from �500 to

1500 mV at a scan rate of 20 mV/s and the potentio-

dynamic polarization resistance measurements were

performed in an aqueous solution of 3% NaCl. As can

be seen from Table 1, the porosity in coatings does not

change significantly when the deposition time (i.e., the

number of cycles) is varied. This suggests that the

thickness of the POA coating may not significantly

influence the porosity in the coating. The lower values

of the porosity in POA coatings permit an improve-

ment of the corrosion resistance by hindering the

access of the electrolyte to the Cu substrates.

Fig. 8. Potentiodynamic polarization curves for (a) bare Cu, (b) copper oxalate coated on Cu and (c) POA coated on Cu in aqueous solution of

3% NaCl.

Table 1

Potentiodynamic polarization resistance measurement results

Sample Ecorr (mV) Icorr (mA/cm2) ba (mV/dec) bc (mV/dec) Rp (O/cm2) CR (mm/year) P (%)

Bare Cu �234 24.06 72.0 110.0 784.05 0.28 –

Cu oxalate coated on Cu �182 69.50 85.0 130.0 321.10 0.80 –

POA 5 cycles �174 00.52 54.0 340.0 38911.60 0.006 0.29

POA 10 cycles �177 0.56 48.0 160.0 28537.19 0.006 0.44

POA 15 cycles �183 0.26 50.0 180.0 65583.99 0.003 0.23

POA 25 cycles �187 0.41 54.0 210.0 45311.65 0.004 0.38


3.4. Corrosion protection performance of the

POA coating

The potentiodynamic polarization curve (Fig. 8a)

for bare Cu is consistent with dissolution of Cu with

corrosion potential (Ecorr) close to ��234 mV and

high anodic current (>1 mA/cm2) is observed at poten-

tials higher than �125 mV. The broad anodic peak

observed at �125 mV is probably associated with the

formation of copper chloride complexes. It is reported

[30] that in chloride media (<1 M) the formation of

cuprous chloride species is dominant and the electro-

dissolution of Cu is represented by the two step

reaction:

Cu þ Cl� ¼ CuCl þ e�

CuCl þ Cl� ¼ CuCl2�

In order to support this, we have carried out the

XRD measurements on Cu after the potentiodynamic

polarization measurements and the corresponding

XRD pattern is shown in Fig. 9. Apart from the

characteristic diffraction peaks of Cu, the XRD pattern

indicates the presence of diffraction peaks at 2y values

of 23.48, 29.08 32.08 and 38.68 due to the CuCl2. The

CuCl2 is brown yellowish in color and hence the broad

anodic peak may be attributed to the formation of

CuCl2 during potentiodynamic polarization measure-

ments. The visual observation of the electrode surface

after the potentiodynamic polarization measurements

shows that the Cu surface is covered with a layer of

brown yellowish colored coating.

The SEM micrograph of the POA coating after the

potentiodynamic polarization measurements in an

aqueous solution of 3% NaCl is shown in Fig. 10.

It exhibits relatively uniform coverage and the overall

texture is ‘pumice-like’.

The potentiodynamic polarization curve for copper

oxalate coated Cu is shown in Fig. 8b. Although the

Ecorr is shifted in the positive direction until �183 mV,

the Icorr is higher than that observed for bare Cu. This

observation reveals that the copper oxalate coating

provides poor corrosion protection to Cu.

The potentiodynamic polarization curve for POA

coated Cu (15 cycles) is shown in Fig. 8c. It is clearly

observed that the corrosion current density (Icorr)

decreases from 25 mA/cm2 for bare Cu to 0.26 mA/

cm2 for POA coated Cu. The Ecorr increases from

�234 mV for bare Cu to �183 mV for POA coated

Cu. The positive shift of 51 mV in Ecorr indicates the

protection of the Cu surface by the POA coating. The

corrosion rate of Cu is significantly reduced as a result

of the reduction in the Icorr. The corrosion rate of Cu is

found to be �0.003 mm/year which is �100 times

lower than that observed for bare Cu. These results

reveal that the POA acts as a protective layer on Cu and

improves the overall corrosion performance.

It is found that the corrosion rate does not change

significantly (cf. Table 1) when the deposition time (i.e.,

the number of cycles) is varied. Thus, it seems that the

thickness of the coating does not significantly affect the

corrosion protection properties of the POA coatings.

The outstanding corrosion protection offered by

POA coating to Cu may be due to the fact that the

Fig. 9. XRD pattern of Cu recorded after the potentiodynamic polarization resistance measurement in aqueous solution of 3% NaCl. The (*)

indicates the diffraction peaks due to CuCl2.


deposited polymer is strongly adherent and uniformly

covers the entire electrode surface as evident by the

SEM. Furthermore, the delocalized p-electrons in this

polymer facilitate its strong adsorption on the Cu

surface leading to the outstanding corrosion inhibi-

tion. In addition, the oxygen of the methoxy group

may facilitates the complexation to the Cu surface and

helps to enhance the adhesion of the POA coating to

the Cu surface [17].

In order to further reveal the corrosion protection

ability of the POA coating, we have studied the elec-

trochemical behavior of the POA coated Cu electrode

in a 0.2 M aqueous solution of sodium oxalate (in the

absence of monomer). The CVs recorded for bare and

POA coated Cu in the potential range between �500

and 1500 mV at a scan rate of 20 mV/s in a 0.2 M

aqueous solution of sodium oxalate are shown in

Fig. 11. The first scan of the CV recorded for bare

Cu (Fig. 11a) is similar to that shown in Fig. 1a. As

discussed earlier, it reveals the dissolution of Cu and

formation of copper oxalate interphase. The first scan

of the CV recorded for POA coated Cu (Fig. 11b) is

significantly different from that for bare Cu. The

absence of the anodic peak A and the observation of

negligibly small current densities at all potentials indi-

cates that the dissolution of the underlying Cu substrate

is inhibited due to the POA coating. To support this, we

have added a copper-sensitive indicator, Murexide, to

the electrolyte and monitored the color change at the

polymer/electrolyte interface. This indicator is violet in

the absence of Cu2þ but changes to orange on com-

plexation with Cu2þ. It was observed that the color of

the electrolyte remains violet even after recording 15

scans for POA coated Cu which indicates the absence

of substantial amount of Cu2þ ions in the vicinity of

polymer/electrolyte interface. Thus, the POA coating

appears to have outstanding protective ability to inhibit

the dissolution of the Cu.

Fig. 10. SEM of Cu recorded after the potentiodynamic polarization resistance measurement in aqueous solution of 3% NaCl.

Fig. 11. Cyclic voltammograms of (a) bare Cu and (b) POA coated

Cu recorded in 0.2 M sodium oxalate solution. Scan rate: 20 mV/s.


It is pointed out that the POA coating is soluble in

DMSO solution, whereas the copper oxalate inter-

phase is insoluble. The copper oxalate interphase

remains as it is even after keeping it for 30 min in

DMSO solution. As discussed earlier, the copper

oxalate interphase formed during the early stages

of the growth decreases the rate of Cu dissolution.

However, it does not prevail at the electrode surface

and it seems that it undergoes the dissolution and

facilitates the ECP of OA. The further evidence in

support of this was obtained by comparing the corro-

sion behavior of the POA removed Cu substrates with

that of bare Cu. The POA coating was removed

carefully by dissolving it in DMSO and the potentio-

dynamic polarization measurements were performed

in the aqueous solution of 3% NaCl. The potentio-

dynamic polarization curves for the bare Cu and the

POA removed Cu are shown in Fig. 12. Interestingly,

the Ecorr and the corrosion rate for both the samples

are very similar. This suggests that the passive copper

oxalate interphase formed during the early stages of

the growth does not remain at the electrode surface

and the protection of the Cu is mainly due to the POA

coating.

We have also performed the potentiodynamic polar-

ization resistance measurements by using the POA

coated Cu substrates after storing them in air at 25 8Cfor 7 days and the corresponding polarization curve is

shown in Fig. 13b. The higher shift (�70 mV) in Ecorr

as compared with the freshly prepared and dried

coating reveals the increase in the corrosion resistance

offered by the POA coatings to Cu. This may be

attributed to the dehydration of the polymer during

the storage process. Thus, POA coating shows the high

chemical as well as physical stability because the

coating keeps its adherence to Cu substrate even after

the storage.

4. Conclusions

The ECP of OA in an aqueous solution of sodium

oxalate results into the deposition of uniform, compact

Fig. 12. Potentiodynamic polarization curves for (a) bare Cu, (b) POA removed Cu recorded in aqueous solution of 3% NaCl.

Fig. 13. Potentiodynamic polarization curves for POA coated Cu

(a) freshly prepared and (b) after storing in air for 7 days at 25 8C.


and strongly adherent POA coatings on Cu substrates.

The formation of POA coatings occurs after the

passivation of the Cu substrate via formation of

CuC2O4H2O interphase. The XRD measurement

clearly reveals the formation of polycrystalline

CuC2O4H2O interphase. It is found that the copper

oxalate interphase formed during early stages of the

synthesis decomposes and facilitates the ECP of OA

on Cu. Our investigations clearly reveals that the

sodium oxalate is a suitable medium for the ECP of

OA on the Cu substrate and it favors the formation of

the mixed phase of PB and ES forms of POA.

The POA coatings exhibit significant corrosion pro-

tection properties in an aqueous solution of 3% NaCl.

The superior corrosion protection properties of the POA

may be due to its strong adhesion and uniform coverage.

The corrosion rate of POA coated Cu is found to be

�100 times lower than that observed for bare Cu. It is

observed that the coatings remain stable chemically as

well as physically even after storing them in air for 7

days. Thus, the results of the present investigation

clearly points out the remarkable capability of the

POA to protect Cu against corrosion and dissolution.

Acknowledgements

The work was supported by All India Council for

Technical Education (AICTE), New Delhi and the

Department of Atomic Energy (DAE), India.

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