Volume 6 Preprint 8
The Corrosion Protection of Copper and Copper Alloys using an Electrodeposited Conducting Polypyrrole Coating
C.B. Breslin and A.M. Fenelon
Keywords: Copper, Polypyrrole, Corrosion Protection, Electropolymerization, oxalate
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Volume 6 Paper C012
The Corrosion Protection of Copper and Copper
Alloys using an Electrodeposited Conducting
C.B. Breslin and A.M. Fenelon
Department of Chemistry, National University of Ireland Maynooth,
Maynooth, Co. Kildare, Ireland Cb.Breslin@may.ie
Adherent polypyrrole films were electropolymerized from a near
neutral sodium oxalate solution at pure Cu, CuZn and CuNi electrodes.
The growth of these films was facilitated by the formation of a
pseudo-passive oxalate layer. This layer was sufficiently protective to
minimize dissolution of the substrate, but sufficiently conductive to
enable the electropolymerization of pyrrole at the interface, and the
electropolymerization at the CuNi layer was reduced significantly by
the formation of a nickel-rich oxide phase, however, the presence of
Cu2+ increased the rate of polymer growth, enabling the formation of a
thin polypyrrole layer during the early stages of polymerization.
Likewise, the presence of zinc in the oxalate layer generated at the
CuZn electrode reduced somewhat the rate of polymer formation.
These films exhibited good corrosion protection properties in an
acidified chloride solution.
In recent times, there has been much interest in the possibility of
using conducting polymers, such as polypyrrole, polyaniline or
derivatives of these, in the corrosion protection of various iron and
aluminium-based materials [#ref1-10]. The interest in these polymers
stems from the fact that they can exist in different oxidation states
and can be easily converted between these oxidation states. For
example, the different oxidation states of polypyrrole are shown in
Figure 1. The fully reduced polymer is neutral, but as the polymer is
oxidised, the polymer is doped by anions, A-, to maintain charge
neutrality. It is this property of anion release, as the polymer is
reduced, that makes these polymers attractive as smart coatings in
Figure 1: Oxidation state of polypyrrole: top neutral form, middle
partially oxidised, bottom fully oxidised.
However, there are very few reports devoted to the corrosion
protection properties of these conducting polymers when applied to
copper or copper-based alloys. In the cases where these polymers
have been considered, they have first been synthesized chemically,
and then deposited at the metal surface, for example by spin coating.
Brusic et al. [#ref11] have studied the corrosion protection properties
of polyaniline and its derivatives when spin-coated onto copper as a
function of the applied potential and temperature. It was found that
polyaniline could either enhance the corrosion rate or produce
significant corrosion protection properties depending on the chemical
nature of the polymer backbone and on the oxidation state and extent
and nature of polymer doping.
In this paper results are presented on the electropolymerization of
pyrrole at pure Cu, CuNi and CuZn alloys to generate adherent
polypyrrole coatings. The corrosion protection properties of these
polymers are assessed using electrochemical measurements.
Electrodes were prepared from pure copper (99.99+%), 70Cu30Ni and
63Cu37Zn (total impurities < 6000 ppm). The electrodes were
provided in rod form (with diameters of 5 mm for Cu, 6 mm for CuNi
and 10 mm for CuZn) and as sheets. The rods were embedded in
epoxy resin in a Teflon holder with electrical contact being achieved by
means of a copper wire threaded into the base of the metal sample.
Prior to each test, the exposed surfaces were polished to a smooth
surface finish, using 1200 g SiC, and rinsed with distilled water. In
some experiments, the pure copper electrodes were electropolished in
a 63% phosphoric acid solution at an applied potential of 2.5 V(SCE) to
generate a mirror-like finish.
A standard three-electrode cell was used as the electrochemical cell.
High-density graphite rods were used as the auxiliary electrodes and a
saturated calomel electrode (SCE) was used as the reference electrode.
The electrolytes were prepared using analytical grade reagents and
The electropolymerization solution consisted of 0.1 or 0.2 mol dm-3
pyrrole and a 0.125 mol dm-3 sodium oxalate solution maintained at
a pH of 7.6. A 0.1 mol dm-3 NaCl solution, adjusted to a pH of 3.5
using HCl, was used as the aggressive solution.
Electrochemical experiments were carried out using an EG&G
Potentiostat, Model 263, or a Solartron EI 1287 electrochemical
interface. The polymers were formed using a constant potential of
900 mV(SCE). Once formed, the electrochemical and corrosion-
protection properties of the polymer-modified electrodes were
assessed. The electrochemical tests consisted of cyclic voltammetry,
while the corrosion tests involved anodic polarisation measurements.
Cyclic voltammograms were recorded in a 0.1 mol dm-3 Na2SO4
solution, at a relatively slow scan rate of 5 mV s-1. Anodic polarisation
tests were recorded in the chloride-containing aggressive solutions
from the corrosion potential, at a scan rate of 0.5 mV s-1 in the
anodic direction, until breakdown occurred.
Quartz crystal microbalance studies were performed with a CH1440
potentiostat coupled to a quartz crystal oscillator. The quartz crystal
cell consisted of the quartz crystal, a miniature Ag/AgCl reference and
a platinum counter electrode. The piezoelectric quartz crystal
electrodes consisted of a polished gold surface, 100 nm in thickness
deposited at a 10 nm Cr surface. The resonant frequency of the crystal
was 8 MHz. A thin copper layer (10 nm) was electrodeposited at the
quartz crystal surface prior to the electropolymerization studies in
order to provide a pure copper surface.
Scanning electron micrographs and energy dispersive x-ray analyses
were recorded on a Hitachi S-4700 cold cathode field emission SEM
using a secondary electron detector at an accelerating voltage of 15
kV. The X-ray spectra were obtained using an Oxford Instruments
Inca Energy EDX detector attached to the SEM. The samples were gold
coated prior to imaging using an Emitech K550 sputter coater. A Zygo
White Light Interfereometer was used to obtain information on the
roughness and thickness of the deposited polymer. The thickness of
the polymer was calculated by comparing the height of a coated
section of the surface with an area free from the coating, with a value
averaged over five determinations being taken as the thickness. For
all these measurements an electropolished copper substrate was
Electropolymerization of Pyrrole
It is well known that the electrochemical formation of conducting
polymers at active metals, such as iron, is complicated by the
dissolution of the metal [#ref12]. Relatively high anodic potentials are
required to oxidise the monomer to the radical cation, which is
necessary in the electropolymerization reactions that give rise to the
final polymer. However, these high anodic potentials also result in
dissolution of the metal substrate. Similar complications exist when
copper is used as a substrate, as copper is readily oxidised to Cu2+.
The standard reduction potential for the Cu2+|Cu couple is 340
mV(SHE). For example, it was not possible to form polypyrrole, or
indeed polyaniline, at copper from phosphoric acid, sulphuric acid,
tosylic acid, or neutral sulphate or nitrate solutions as the copper
substrate dissolved too rapidly at the potentials required to oxidise
aniline or pyrrole.
However, it was possible to form polypyrrole at copper from a neutral
oxalate solution as the formation of an initial copper oxalate layer
inhibited the dissolution of copper [#ref13]. This can be seen from the
quartz crystal microbalance data presented in Figure 2(a) which show
the initial mass changes that take place at the copper electrode on
polarising the electrode at 900 mV(SCE) in a 0.125 mol dm-3 oxalate
solution in the presence and absence of pyrrole. The quartz crystal
data were converted to mass changes using the well-known Sauerbrey
equation, Equation 1:
∆f = − o ∆m
where ∆F represents the shift in frequency observed, fo is the resonant
frequency of the fundamental mode of the crystal, ρq is the density of
the crystal, N is the frequency constant for the quartz crystal and ∆m
is the change in mass per unit area.
∆ Mass / µg
Time / s
∆ Mass / µg
∆Frequency / kHz
Figure 2: Quartz crystal microbalance data presented for (a) the initial
stages of oxidation of copper ── in a neutral oxalate solution and
▪▪▪▪▪▪▪▪▪ in a pyrrole-containing oxalate solution; (b) the growth of
polypyrrole from 0.1 mol dm-3 pyrrole in a neutral oxalate solution.
Both traces are similar during the first 20 s of polarisation and are
characterized by an initial mass loss, then a rapid mass increase,
which is consistent with the formation of a copper oxalate layer at the
copper surface. Finally a more gradual mass loss, which is consistent
with dissolution of the copper substrate, is observed. This latter mass
loss clearly shows that the copper oxalate layer is not very protective,
but inhibits sufficiently the dissolution of copper to enable the growth
of an adherent polypyrrole layer. Growth of the polypyrrole layer can
be seen following 20 s of polarisation and is marked by the mass
increase after approximately 22 s. Further growth of the polypyrrole
layer can be seen from the data presented in Figure 2(b), which show
the mass change and the accompanying frequency shift as a function
of the polymerization period in a 0.1 mol dm-3 pyrrole solution. A
rapid increase in the mass is seen during the first 100 s of
polymerization, this is then followed by a slow growth of the polymer
at times greater than 1000 s. This final slower growth period is
consistent with the transition of the polymer growth from three
dimensions to a two-dimensional growth phase.
In Figure 3(a) and (b) typical current-time transients are shown for the
electropolymerization of pyrrole at copper, copper-zinc and the
copper-nickel alloy. In Figure 3(a), data are shown for pure copper and
copper-zinc during the electropolymerization of pyrrole. The currenttime behaviour during the first 20 s is dominated by the dissolution of
the substrate [#ref13] and the formation of a complex oxalate layer,
being consistent with the data presented in Figure 2. However, once
this period has elapsed, deposition of polypyrrole at the electrode
surface dominates the electrochemical response. It is interesting to
compare the traces recorded for the pure copper and copper-zinc
electrode; the currents are a factor of four greater for the pure copper
substrate indicating a much higher rate of electropolymerization at the
pure copper surface. In the case of the CuNi substrate, the application
of high potentials, such as 900 mV(SCE), which are needed to initiate
the electropolymerization reactions, also gave rise to increased rates
of the formation of the nickel hydroxide, which in turn inhibited the
formation of the polypyrrole layer. In order to increase the rate of the
electropolymerization reaction, a Cu2+ solution was added to the
electropolymerization medium. Likewise, electroformation of the
nickel-rich oxide was inhibited through the deposition of a thin copper
layer at the CuNi electrode prior to electropolymerization. These data
are presented in Figure 3(b), in which plots are shown for CuNi
polarised in the pyrrole-containing solution, CuNi polarised in the
pyrrole solution containing Cu2+ and a copper-coated CuNi electrode
polarised in the polymerization solution.
Current /A cm
Current /A cm
Figure 3: Current-time plots recorded at 900 mV(SCE) for (a) ▪▪▪▪▪▪▪▪▪
copper in a 0.2 mol dm-3 pyrrole solution, ── CuZn in a 0.2 mol dm-3
pyrrole solution; (b) - - - - CuNi in a 0.2 mol dm-3 pyrrole solution,
▪▪▪▪▪▪▪▪▪ CuNi in the pyrrole solution containing 0.01 mol dm-3 CuSO4 and
── copper-modified CuNi in the pyrrole solution containing 0.01 mol
It is clear from these data that the presence of Cu2+, or modification of
the electrode by a thin Cu layer prior to the electropolymerization
reactions, gives rise to an increase in the rate of electropolymerization.
This increase in polymer growth in the presence of copper cations has
been documented previously by Millar et al. [#ref14] who have shown
that the presence of copper cations promotes oxidation of the polymer
units due to the strong oxidising power of the Cu2+ cation. Also, Rivas
et al. [#ref15] have reported an increase in the polymer yield in the
presence of copper cations.
These electropolymerization procedures gave rise to the deposition of
highly adherent polypyrrole layers on each of the three substrates. In
fact, the polymer layers could only be removed by mechanical
polishing of the electrodes. The thickness of the polypyrrole layers
grown on pure copper for a 30-min electropolymerization period in a
0.1 and 0.2 mol dm-3 pyrrole solution was measured as 5.0 and 9.0
Characterization of the polypyrrole deposits
A Typical SEM micrograph showing the morphology of the polypyrrole
coating on the copper substrate is shown in Figure 4. These data were
recorded following dehydration of the polymer. The polymers were
formed and then exposed to the atmosphere at 25 oC for seven days.
The data show clearly that the polymer is homogenous and is crack
and defect-free despite being dehydrated over an extended period,
which indicates that the metal-polymer arrangement is stable. The
characteristic microspheroidal grains, or aggregates, of polypyrrole,
with sizes up to 2 µm are observed. In Figure 5, the surface roughness
of a 9.0 µm thick polypyrrole layer at pure copper is shown. These
data were recorded using white light interfereometry and show
polymer peaks reaching heights close to 3.0 µm indicative of a rough
surface. The presence of these polypyrrole peaks is in agreement with
the two-dimensional growth phase evident from the quartz crystal
data, Figure 2.
Figure 4: SEM micrograph of polypyrrole deposited at copper.
Figure 5: White-light interfereometry of polypyrrole deposited at
The data in Figure 6 show the electrochemical behaviour of the
polypyrrole-coated Cu electrode in 0.1 mol dm-3 Na2SO4. The polymer
was deposited at 900 mV(SCE) from a 0.1 mol dm-3 pyrrole solution.
These data were recorded at a scan rate of 5.0 mV s-1. Before cyclic
polarisation, the sample was polarised at 0.4 V(SCE) to enable
oxidation of the polymer. These data are typical of pure polypyrrole
cycled in a sulphate solution [#ref13]. A reduction peak centred at –
625 mV(SCE) and an oxidation peak at –195 mV(SCE) is observed,
showing that the polypyrrole deposited at Cu using the relatively high
potential of 900 mV(SCE) exhibits typical redox activity.
Current /A cm
Potential /(V vs. SCE)
Figure 6: Cyclic voltammogram recorded for polypyrrole-coated Cu in
a 0.1 mol dm-3 Na2SO4 solution at a scan rate of 5 mV s-1.
Corrosion Protection Properties
The corrosion-protection properties of the polypyrrole films deposited
at pure copper, copper-zinc and copper-nickel are shown in Figures 7
Anodic polarisation data are presented in Figure 7 for pure
copper, copper coated with a 5.0 µm polypyrrole film and copper
coated with a 9.0 µm polypyrrole film in an acidified, pH 3.0, chloride11
containing solution. The plots depicted for pure copper are consistent
with dissolution of copper, with corrosion potentials close to –130
mV(SCE) and high anodic currents, exceeding 1.0 mA cm-2, being
observed at potentials higher than –50 mV(SCE). The anodic peak
visible at potentials in the region of 0 V(SCE) is probably associated
with the formation of copper-chloride complexes, such as CuCl2. The
behaviour of the polymer-coated electrodes is significantly different.
The corrosion potentials adopted by the 5.0 µm and 9.0 µm polymer
films are 0 and 290 mV(SCE) respectively. But, more significantly, the
polymers remain protective at much higher potentials. A clear
breakdown potential at 470 mV(SCE) can be seen for the 5.0 µm
polymer, but the conducting properties of the 9.0 µm polymer make it
difficult to mark the onset of copper dissolution as the currentpotential response is governed mainly by the activity of the polypyrrole
and not the copper substrate.
Potential /(V vs SCE)
Current/ A cm
Figure 7: Anodic polarisation plots recorded in a pH 3.5, 0.1 mol dm-3
NaCl solution for ▪▪▪▪▪▪▪ uncoated Cu, ── Cu coated with a 5.0 µm
thick polypyrrole layer, and - - - - Cu coated with a 9.0 µm thick
As can be seen from the cathodic polarisation data shown in Figure 8,
these noble corrosion potentials observed for the polymer-coated
electrodes are associated largely with an increase in the rate of the
reduction reactions. Much higher cathodic currents are measured on
polarisation of the polypyrrole-coated electrode in the cathodic
direction. These cathodic currents can be attributed to reduction of the
polymer, particularly the increased currents in the vicinity of –0.37
V(SCE), and to an increase in the rate of the oxygen reduction reaction
at the polymer solution interface. Indeed, Kinlen et al. [#ref16] and
Tallman et al. [#ref3] have found that the conducting polymer,
polyaniline, can mediate oxygen reduction at the polymer electrolyte
Potential /(V vs. SCE)
Current /A cm
Figure 8: Cathodic polarisation plots recorded in a pH 3.0, 0.1 mol dm3
NaCl solution for - - - - pure copper and ▪▪▪▪▪▪▪ polypyrrole-coated
The evolution of the open-circuit potentials as a function of time for
the polypyrrole-coated copper electrode immersed in the acidified
chloride solution can be seen in Figure 9. The open-circuit potential of
the uncoated copper electrode remains constant, independent of time
at a value of approximately –140 mV(SCE). The open-circuit potentials
of the polypyrrole-coated electrodes are significantly higher. For
example, the open-circuit potentials of the polymer-coated electrodes
reach values of approximately 240 mV in the early stages of
immersion and then decay to reach values close to 0 V(SCE) after a 14-
hr immersion period for the 9.0 µm thick polymers and extends
between 0 and –40 mV(SCE) for the 5.0 µm thick polymers. Such a
decay in the corrosion potential is normally associated with a loss in
the corrosion protection properties of the coating [#ref1]. Indeed,
some loss in the corrosion protection properties of the coating is seen
with extended immersion times. However, this is not very significant.
For example, the breakdown potential of copper coated by a 5.0 µm
polypyrrole layer was 250 mV(SCE) following eight days of immersion
in the acidified chloride solution compared to –150 mV(SCE) for the
uncoated electrode. Although the breakdown potential at +250
mV(SCE) is clearly lower than the value of +470 mV(SCE) recorded for
the freshly immersed polymer, the polymer coating clearly has
protective properties following eight days of immersion in this highly
Potential /(V vs. SCE)
Figure 9: Open-circuit potential-time plots recorded in a pH 3.5, 0.1
mol dm-3 NaCl solution for ▪▪▪▪▪▪▪ uncoated Cu, ── Cu coated with a
5.0 µm thick polypyrrole layer, and - - - Cu coated with a 9.0 µm thick
The synergistic interaction of the polypyrrole coating and the wellknown corrosion inhibitor benzotriazole, BTAH, can be seen from the
polarisation data presented in Figure 10. For these experiments, a 5.0
µm polypyrrole coating was employed. These results show the
combined effect of the polymer and the inhibitor having breakdown
potentials of about 250 mV higher than either the benzotriazole
system or the polymer film alone. It appears that the polypyrrole,
which is in an oxidised state at these applied potentials, Figure 1, is
doped by the benzotriazole anions, BTA-, which in turn are capable of
passivating the copper surface.
Potential /(V vs. SCE)
Current /A cm
Figure 10: Anodic polarisation plots recorded in a pH 3.5, 0.1 mol dm3
NaCl solution for ── uncoated Cu, - - - uncoated Cu in the
presence of BTAH (0.01 mol dm-3) ▪▪▪▪▪▪▪ polypyrrole-coated Cu in the
presence of BTAH and
polypyrrole-coated Cu in the absence of
Representative plots, showing the anodic polarisation behaviour of the
polypyrrole-coated and uncoated CuZn electrode in the acidified 0.1
mol dm-3 NaCl solution, are shown in Figure 11. For comparative
purposes, the anodic polarisation behaviour of the oxalate-coated
electrode is shown. The anodic polarisation behaviour of CuZn and the
oxalate-coated surface in the acidified solution is consistent with
active dissolution. The anodic current increases steadily at potentials
beyond the corrosion potential, which lies at –130 mV(SCE) for the
uncoated and –190 mV for the oxalate-coated surface, to reach
currents on the mA scale at approximately -30 mV(SCE). The
polarisation behaviour of the polymer-coated CuZn electrode is
significantly different, representing a much more corrosion resistant
system. The corrosion potential lies at a much more noble potential,
60 mV(SCE) and the current recorded during polarisation of the
electrode to high anodic potentials remains low, increasing slightly
with increasing potential, until potentials exceeding 500 mV(SCE) are
reached. A more rapid increase in the anodic current is observed at
520 mV(SCE) indicating dissolution of the underlying substrate.
Dissolution of the substrate was confirmed by adding the indicator,
Erichrome Black T, to the test solution. This indictor, which is sensitive
to the presence of zinc cations, changed colour from blue to a pinkviolet colour, characteristic of the presence of Zn2+, at potentials in the
region of the breakdown potential. However, the copper-sensitive
indicator, Murexide, did not undergo any colour change at the
breakdown potential, indicating that these breakdown events involve,
mainly, the dissolution of the zinc component.
The anodic polarisation plots recorded for the polypyrrole coated CuNi
electrode, the uncoated CuNi and the copper-modified CuNi electrode
in the acidified chloride solution are shown in Figure 12. The currentpotential profiles recorded for the uncoated electrodes are consistent
with breakdown and dissolution of the electrodes. The copper-
modified CuNi electrode appears to be more active than the
unmodified CuNi electrode. This is consistent with the passivating
properties of the nickel alloying addition. Again, very different
behaviour is seen with the polymer-coated substrate. The corrosion
potential is increased by almost 400 mV and the potential at which the
current adopts a 1.0 mA cm-2 value, by over 650 mV compared to the
uncoated electrodes. Shown also in this figure are data recorded for
the polypyrrole-coated electrode following a 14-hr immersion period
in the aggressive halide solution. Although this plot clearly shows a
loss in the corrosion protection properties with continued immersion,
it can be seen that this polymer-coated electrode can be polarised to
relatively high anodic potentials before high anodic currents are
measured. Indeed, some of the anodic current measured for these
polymer-coated electrodes is due to the electroactivity and oxidation
of the polypyrrole and not due to dissolution of the substrate [#ref
13]. It is also interesting to note that although the open-circuit
potential adopted by the polypyrrole-coated electrode following the
14-hr immersion time is lower than that measured for the uncoated
electrode in the acidic solutions, this polymer continues to exhibit
corrosion protection properties on polarisation of the electrode.
Potential /(V vs SCE)
Current /A cm
Anodic polarisation plots recorded in a pH 3.5, 0.1 mol
dm-3 NaCl solution for ── uncoated CuZn, ▪▪▪▪▪▪▪ polypyrrole-coated
CuZn and - - - oxalate-coated CuZn.
Potential /(V vs. SCE)
Current /A cm
Figure 12: Anodic polarisation plots recorded in a pH 3.5, 0.1 mol dm3
NaCl solution for ── uncoated CuNi, ▪▪▪▪▪▪▪ polypyrrole-coated CuNi
and - - - polypyrrole-coated CuNi following 14 hrs immersion and
Cu modified CuNi.
It can be seen from these results that adherent polypyrrole coatings,
with good corrosion protection properties, can be deposited onto
copper, copper-zinc and copper-nickel alloys in the presence of
oxalate anions at near neutral pH values. The ease of formation of
these polypyrrole layers appears to be connected with the fact that a
pseudo passive-like layer, or film, forms on the electrodes under the
electropolymerization conditions employed. It is well know that
oxalate anions form stable complexes with copper cations [#ref17]. In
particular, oxalate anions (Ox2-) form complexes with Cu2+ to generate
copper oxalate complexes with a 1:1 and 1:2 stoichiometry, Equations
2 and 3. The formation of this layer can be seen clearly form the
quartz crystal data presented in Figure 2.
Cu2+ + Ox2- ⇌ Cu(Ox)
Cu2+ + 2Ox2- ⇌ (Cu(Ox)2)2-
This layer inhibits dissolution of the substrate and facilitates the
formation of polypyrrole. However, this layer does not inhibit
completely the dissolution of the substrate, as evident from the data
presented in Figure 2. Therefore, it is likely that the polymer contains
cations of the substrate material, such as Cu2+, Zn2+ and Ni2+. Indeed,
an SEM-EDX cross section analysis of the polypyrrole layer deposited
at copper revealed the presence of copper distributed throughout the
polymer. However, it has been reported that oxidised copper can be
used to enhance the conductivity and stability of polypyrrole [#ref
14,15]. This has been attributed to electron transfer between copper
and the N+ of polypyrrole to form a stable Cu-polypyrrole complex,
thus preventing nucleophilic attack on the positively charged nitrogen.
Consequently, the presence of oxidised copper within the polypyrrole
coating may have beneficial effects.
It is evident form the electropolymerization studies presented in Figure
3 that the substrate plays an important role in the rate at which the
polypyrrole is deposited.
Higher anodic currents, consistent with
higher rates of electropolymerization, are obtained for the pure copper
substrate. A somewhat lower rate of electropolymerization is seen for
the CuZn electrode. This is probably connected with the presence of
zinc in the initial pseudo-passive layer that forms during the
electropolymerization reactions. Indeed, Morales et al. [#ref18] have
shown, using XPS, that the passive layer formed on various brasses in
a pH 9 solution consists of a complex ZnO.xH2O/Cu2O-CuO layer. The
ZnO electroformation results in a dezincification process so that a thin
copper-rich layer resides at the metal/oxide interface. In the presence
of the oxalate species, it is likely that a ZnC2O4 phase is formed, as
ZnC2O4 is sparingly soluble with a pKs of 7.9 [#ref19] (where Ks refers
to the solubility product). Taking the oxalate concentration of 0.125
mol dm-3 and using the pKs value, it is seen that the solubility product
is exceeded at a Zn2+ concentration of 1 x 10-7 mol dm-3, making this
a likely phase in these experiments. It is interesting to note that
polypyrrole layers did not form at pure zinc under these experimental
conditions. This may suggest that the zinc oxalate complex is not
sufficiently conducting to facilitate the electropolymerization reactions
and may explain why the rate of the electropolymerization reaction is
reduced four-fold at the CuZn compared with the pure copper
interface. As already highlighted, the formation of a nickel-rich oxide
electropolymerization reactions. Furthermore, as the electrode is
polarised in the polymerization solution, the applied potential of 900
mV(SCE) facilitates the growth of the nickel-rich oxide phase. When
the rate of the electropolymerization is increased by the addition of
Cu2+ to the solution, a polypyrrole layer is nucleated at the surface
during the early stages of polarisation. This enables polymer growth
and prevents the electroformation of the nickel-rich phase.
There is clear evidence from the corrosion data that the polypyrrolecoated electrodes exhibit corrosion protection properties in the
formation. It is also interesting to note that there is very little
difference in the corrosion protection properties of the polypyrrole
coatings on the three substrates, as evident from a comparison of the
data presented in Figures 7, 11 and 12, even though breakdown at the
CuZn coated electrode is associated largely with oxidation of the zinc
component. There is a clear ennoblement in the open-circuit
potentials, for all three substrates. This effect has been reported
previously for polyaniline-coated iron electrodes [#ref1]. For this iron
system, the corrosion-protection properties are often gauged by
measuring the period elapsed until the open-circuit potential of the
polymer-coated electrode drops to that of the uncoated electrode.
This signifies loss of corrosion protection and typically lasts from
minutes to hours. However with the polymer-copper system, Figure 9,
the open-circuit potentials reach a steady state value that is always
more noble than the potential of the bare electrode, and even after 8
days of immersion remains at this steady-state value. This difference
between the iron and copper systems may be due to the much higher
corrosion susceptibility of iron. Nevertheless, the acidified chloride
solution used in these studies gives rise to the corrosion of copper.
The data presented in Figure 10 show that higher breakdown
potentials are recorded when the polymer is doped by benzotriazole,
highlighting the importance of the doping species. Indeed, the loss in
the protection properties of the polypyrrole coatings with immersion
time may be associated with doping of the polymer by chloride anions
and the transport of these anions to the copper interface.
The authors gratefully acknowledge the support of this work by
Enterprise Ireland. The authors would also like to thank Dr. Liam
Carroll, Biomedical Research Centre, NUI Galway for carrying out the
!ref1 ‘Protection of iron against corrosion using a polyaniline layer - II.
phosphoric/metanilic solution’ M.C. Bernard, S. Joiret, A. Hugot-
Le-Goff and P.D. Long, J. Electrochem. Soc., 148, ppB299-B303,
!ref2 Electroactive conducting polymers for corrosion control - Part 2.
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Tallman, J. Solid State Electrochem., 6, pp85-100, 2002.
!ref3 ‘Conducting polymers and corrosion: Part 2 - Polyaniline on
aluminum alloys’ D.E. Tallman, Y. Pae and G.P. Bierwagen,
Corrosion, 56, pp401-410, 2000.
!ref4 ‘Corrosion protection by ultrathin films of conducting polymers’
U. Rammelt, P.T. Nguyen and W. Plieth, Electrochim. Acta., 48,
pp 1257-1262, 2003.
aluminium: electrochemical activity and corrosion protection
properties’ K.G. Conroy and C.B. Breslin, Electrochim. Acta., 48,
!ref6 ‘Development of polyaniline-polypyrrole composite coatings on
steel by aqueous electrochemical process’ R. Rajagopalan and
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