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X–ray photoelectron spectroscopy depth–profiling analysis of surface films formed on Cu–Ni (90/10) alloy in seawater in the absence and presence of 1,2,3–benzotriazole

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X–ray photoelectron spectroscopy (XPS) depth–profiling analysis was performed to investigate the chemical composition of the film formed on Cu–Ni (90/10) alloy in seawater and sulphide–polluted seawater in the absence and presence of an inhibitor, 1,2,3–benzotraizole (BTAH). The chemical composition of the film is found to vary at different depths of the film. In seawater environment, the outermost layer is found to consist of CuO, Cu(OH)2 and Cu2O. After sputtering for 12 minutes major constituent of the film is Cu2O. In seawater polluted with sulphide also the outermost layer is found to consist of CuO, Cu(OH)2 and Cu2O without any Cu2S or CuS. After 4 minutes of sputtering only, the peaks due to Cu2S and CuS are detected and the major constituent of the film is still Cu2O. The sulphides of copper are absent after 8 minutes of sputtering. In the presence of BTAH, the XPS of the film showed peaks due to carbon and nitrogen up to 4 minutes of sputtering. This result reveals the presence of [Cu(I)BTA]n complex on the alloy surface in both seawater and sulphide–polluted seawater. The percentage of Cu2O is found to be much less than that found in the absence of inhibitor. In both the stated environments, in the absence of BTAH, the oxygen concentration is reduced to a small value only after 20 minutes of sputtering. Whereas, in the presence of BTAH, the concentration of oxygen is reduced to very small value after sputtering for 12 minutes. These results infer the protective ability of BTAH film on Cu–Ni (90/10) alloy in both seawater and sulphide–polluted seawater even after 30 days of immersion.
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X-ray photoelectron spectroscopy depth-proling analysis of surface
lms formed on CuNi (90/10) alloy in seawater in the absence and
presence of 1,2,3-benzotriazole
B.V. Appa Rao
a,
, K. Chaitanya Kumar
a
, Neha Y. Hebalkar
b
a
Department of Chemistry, National Institute of Technology Warangal, Warangal 506004, Andhra Pradesh, India
b
Centre for Nanomaterials, International Advanced Research Centre for Powder Metallurgy and New Materials (ARCI), Hyderabad 500005, Andhra Pradesh, India
abstractarticle info
Article history:
Received 10 July 2013
Received in revised form 7 February 2014
Accepted 12 February 2014
Available online 20 February 2014
Keywords:
X-ray photoelectron spectroscopy
Depth-proling
Corrosion
CuNi (90/10) alloy
Seawater
1,2,3-Benzotriazole
X-ray photoelectron spectroscopy (XPS) depth-proling analysis was performed to investigate the chemical
composition of the lm formed on CuNi (90/10) alloy in seawater and sulphide-polluted seawater in the ab-
sence and presence of an inhibitor, 1,2,3-benzotraizole (BTAH). The chemical composition of the lm is found
to vary at different depths of the lm. In seawater environment, the outermost layer is found to consist of CuO,
Cu(OH)
2
and Cu
2
O. After sputtering for 12 min, the major constituent of the lm is Cu
2
O. In seawater polluted
with sulphide also the outermost layer is found to consist of CuO, Cu(OH)
2
and Cu
2
O without any Cu
2
SorCuS.
After 4 min of sputtering only, the peaks due to Cu
2
S and CuS are detected and the major constituent of the
lm is still Cu
2
O. The sulphides of copper are absent after 8 min of sputtering. In the presence of BTAH, the XPS
of the lm showed peaks due to carbon and nitrogen up to 4 min of sputtering. This result reveals the presence
of the [Cu(I)BTA]
n
complex onthe alloy surface inboth seawater and sulphide-polluted seawater. Thepercentage
of Cu
2
O is found to be much less than that found in the absence of inhibitor. In both the stated environments, in
the absenceof BTAH, the oxygenconcentration is reduced to a small valueonly after 20 min of sputtering. Where-
as, in the presence of BTAH, the concentration of oxygen is reduced to a very small value after sputtering for 12
min. These results infer the protective ability of the BTAH lm on CuNi (90/10) alloy in both seawater and sul-
phide-polluted seawater even after 30 days of immersion.
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
Among the copper alloys, CuNi alloys offer good corrosion resis-
tance, when they are used as materials for heat exchangers using sea-
water as coolant [13]. The chemical composition of the surface lm
formed on copper and its alloys due to corrosion in diverse environ-
ments can be analyzed by X-ray photoelectron spectroscopy (XPS).
The thickness of passivation layers formed on CuNi alloy in acidic solu-
tion has been obtained by Druska and Sterhblow [4]. Mathiyarasu et al.
[5] analyzed the passivated lms formed on CuNi (90/10) alloy in a
chloride environment by employing XPS studies. They inferred that
the major component of the corrosion product is Cu
2
O. Kato et al. [6]
employed XPS to study the surface lms and mechanism of corrosion
of CuNi (90/10) alloy in the air-saturated aqueous NaCl solution. The
thickness and chemical composition ofthe surface layers of CuNi alloys
in synthetic sweat were studied by Colin et al. [7] by employing XPS
depth-proling studies.Chauhan and Gadiyar [8] used XPS to character-
ize the passivated lms formed on CuNi (90/10) alloy in unpolluted
and polluted seawater in the absence and presence of FeSO
4
.AnXPS
study of the surface lm formed on this alloy in seawater showed that
the outermost corrosion product is Cu
2
O along with adsorbed water
molecules. The inner lm is composed of nickel ions in two different
valencies and Cl
ions in addition to Cu
2
O. When CuNi alloys are ex-
posed to the seawater environment, several other corrosion products
viz. Cu
2
(OH)
3
Cl, Cu(OH)
2
, CuO, CuCl
2
,Cu
3
(OH)
2
(CO
3
)
2
and CaCO
3
were also found in small quantities in addition to the major corrosion
product of Cu
2
O[911]. In sulphide-polluted seawater, formation of
the protective oxide lm is prevented due to formation of relatively
hard and porous Cu
2
S[12]. Mor and Beccaria [13] found that the lm
formed on CuNi (90/10) alloy in synthetic seawater polluted with 10
ppm sulphide is composed of not only Cu
2
O and Cu
2
(OH)
3
Cl, but also
various amounts of CuS and Cu
2
S.
There was interest on the role of iron added either as ferrous
sulphate to environment or as an alloying element to protect CuNi
(90/10) alloy against corrosion [14,15]. An organic corrosion inhibitor
viz., 1,2,3-benzotriazole (BTAH) is known to be efcient for copper
and its alloys, as it forms a polymeric complex lm of cuprous benzotri-
azolate ([Cu(I)BTA]
n
)[1618]. Aruchamy et al. [19] investigated the
protective lm formed by BTAH on CuNi (90/10) alloy in an alkaline
medium by employing surface enhanced Raman spectroscopy. Allam
Thin Solid Films 556 (2014) 337344
Corresponding author. Tel.: +91 870 2462652.
E-mail addresses: boyapativapparao@rediffmail.com (B.V. Appa Rao),
chaitanyakanukula@gmail.com (K. Chaitanya Kumar).
http://dx.doi.org/10.1016/j.tsf.2014.02.054
0040-6090/© 2014 Elsevier B.V. All rights reserved.
Contents lists available at ScienceDirect
Thin Solid Films
journal homepage: www.elsevier.com/locate/tsf
Author's personal copy
et al. [20,21] studied the performance of BTAH as an inhibitor on the cor-
rosion of CuNi (90/10) alloy in sulphide-polluted salt water by
employing X-ray diffraction, optical microscopy, scanning electron
microscopy and weight-loss studies. The authors of the present study
reported the performance of BTAH as a corrosion inhibitor for CuNi
(90/10) alloy in seawater and sulphide-polluted seawater by employing
electrochemical studies, XPS and weight-loss studies [22]. Nevertheless,
no studies have been reported so far on XPS depth-proling analysis of
CuNi (90/10) alloy in seawater and sulphide-polluted seawater in the
absence and presence of BTAH and hence the present study.
2. Experimental details
2.1. Materials
The CuNi (90/10) alloy specimens were made from the CuNi (90/
10) alloy sheet purchased from Bhumi Impex Pvt. Ltd, India. The
Table 1
Composition of the CuNi (90/10) alloy.
Element Cu Ni Fe Mn Pb Al Others in trace amounts
Composition (wt.%) 88.512 9.882 1.086 0.412 0.046 0.038 0.024
Table 2
Composition of the synthetic seawater.
Compound name NaCl KCl MgCl
2
.6H
2
OCaCl
2
MgSO
4
.7H
2
O NaHCO
3
KBr H
3
BO
3
SrCl
2
.6H
2
ONaF
gL
1
27.24 1.40 10.11 2.28 13.92 0.39 0.20 0.026 0.04 0.006
Fig. 1. XPS spectra at differentsputtering times of(a) Cu 2p, (b) O 1s and (c) Ni2p formed
on CuNi (90/10) alloy after 30 day immersion in seawater.
Fig. 2. (a) Relative atomic composition and (b) schematic representation of the surface of
CuNi (90/10) alloy after 30 day immersion in seawater as a function of sputtering time.
338 B.V. Appa Rao et al. / Thin Solid Films 556 (2014) 337344
Author's personal copy
chemical composition as certied by the suppliers was presented as
such in Table 1. The composition of synthetic seawater is given in
Table 2 [23]. All the chemicals used in preparation of synthetic seawater
were of AnalaR grade. A stock solution of seawater containing 1000 ppm
sulphide was prepared. Seawater containing 10 ppm sulphide was pre-
pared by the method of dilution. 1,2,3-Benzotriazole obtained from
Sigma-Aldrich Pvt. Ltd., India was used as such. The CuNi (90/10)
alloy specimens with a dimension of 1.0 × 1.0 × 0.2 cm were used in
all the studies. These specimens were polished with 1/0, 2/0, 3/0 and
4/0 grade emery papers consecutively. Later, these were polished to
mirror nish with slurry of alumina and water on a micro polishing
cloth, which is xed on a rotating disc polishing machine. Then, these
specimens were washed with triple distilled water, degreased with
acetone and dried by blowing N
2
gas for 20 min.
2.2. XPS-depth proling studies
XPS depth-proling studies were performed with an ESCA + spec-
trometer, Omicron Nanotechnology, GmbH, Germany with a base vacu-
um of 1× 10
6
Pa. Al Kαradiation (1486.6 eV) was used for primary
excitation. Survey scan spectra were recorded at a pass energy of
187.9 eV and the individual high resolution spectra were taken at a
pass energy of 20.0 eV with an energy step of 0.1 eV. The XPS studies
of the surface layers of CuNi (90/10) alloy were carried out after
immersing the alloy in seawater and sulphide-polluted seawater in
the absence and presence of BTAH for 30 days. After recording the
survey spectra, the depth-proling was performed with an Ar
+
ion
beam sputtering. An energyof 1.5 keV for 2 min was used for sputtering
to remove carbon, which is a surface contaminant. After removal of the
surface contaminant, the sputtering process was performed using an
energy of 2 keV for 2 min. These conditions resulted in a sputtering
rate of 1 nm/min [24]. The binding energy scale has been referenced
with C 1s (284.6 eV). The peak tting and analysis were performed by
using CASA-XPS software. For Cu 2p, the background subtraction was
carried out by using the Spline Tougard method (before sputtering)
and after sputtering for 2 min onward the Shirley or linear method
was employed. For all other elements, the Shirley method was
employed. The peak shape was analyzed by using the Gaussian/
Lorentzian product formula [GL(30)].
3. Results and discussion
3.1. Interpretation of XPS results
The authorsof the present study reported that the optimum concen-
tration of 1,2,3-benzotriazole was 3.33 mM in order to inhibit general
corrosion of CuNi (90/10) alloy in seawater and seawater polluted
with 10 ppm sulphide [22]. Therefore, 3.33 mM concentration of BTAH
was used in the present study.
X-ray photoelectron survey spectra of the surface lm at different
depths formed on CuNi (90/10) alloy after immersion in synthetic sea-
water for 30 days showed peaks due to copper, oxygen and nickel. The
XPS deconvolution spectra for Cu 2p, O 1s and Ni 2p of the lm at differ-
ent depths are shown in Fig. 1ac respectively. The XPS spectra of the
Fig. 3. XPS spectra at differentsputtering timesof (a) Cu 2p, (b) O 1s, (c)S 2p and (d) Ni 2p formed on CuNi (90/10) alloy after30 day immersion in seawater containing10 ppm sulphide.
339B.V. Appa Rao et al. / Thin Solid Films 556 (2014) 337344
Author's personal copy
elements before sputtering of the surface lm are rst interpreted. The
XPS deconvolution spectrum of Cu 2p, exhibits the Cu 2p
3/2
peak at
933.9 eV along with a shake-up satellite at 943.3 eV. These peaks are
normally assigned to the presence of Cu(II) species [2527]. However,
in the present case, there must be some amount of Cu(I) also as the ox-
idation of Cu(0) to Cu(I) occurs rst, followed by oxidation of Cu(I) to
Cu(II). It may also be noted that after 2 min of sputtering, the major
composition of the surface lm is Cu
2
O. The O 1s deconvolution spec-
trum exhibits a high intensity peak at 530.6 eV and a low intensity
one at 531.5 eV. The former peak is assigned to mainly CuO and small
amount of Cu
2
O while the latter one is assigned to Cu(OH)
2
[28].From
these results it can be inferred that the surface lm after corrosion of
the alloy in synthetic seawater is composed of CuO, Cu(OH)
2
and
Cu
2
O. The C 1s peaks at 283.9 eV, 284.6 eV and 288.0 eV are due to con-
taminant carbon, which is likely due to cracking of vacuum oil used dur-
ing the operation of XPS instrument [29]. Therefore, after 2 min of
sputtering itself the C 1s peaks disappeared.
XPS analysis of the surface lm after sputtering for 2 min is interest-
ing. A low intensity peak at 933.9 eV along with a shake-up satellite at
943.4 eV is detected. These results inferthe presence of CuO [27].Inad-
dition to these peaks, there is a peak of high intensity at 932.4 eV, which
infers the presence of Cu
2
Oinhighabundance[27]. An intense O 1s
peak at 530.9 eV is assigned to the presence of Cu
2
O[30,31]. Another
peak at 529.5 eV with less intensity is assigned to the presence of CuO
[27]. Thus, after 2 min of sputtering the lm is composed of a greater
percentage of Cu
2
O along with a small percentage of CuO. Similar
peaks are observed in the surface lms up to 10 min of sputtering. No
nickel is found in the lm even after sputtering for 10 min. The high
intense Cu 2p
3/2
peak at 932.4 eV after 12 min of sputtering is attributed
to the presence of Cu(0) [32]. The less intense peak at 932.6 eV is as-
cribed to Cu(I) [27]. The presence of the O 1s peak at 531.4 eV infers
the presence of Cu
2
O. That means the surface lm at this depth is com-
posed of elemental copper along with some amount of Cu
2
O. The inten-
sity of oxygen is found to decrease with the increase in sputtering time.
After sputtering for 20 min, it is found to be minimum. The binding
energy of Cu 2p
3/2
is 932.4 eV only in all the spectra after 14 min of
sputtering. This value is in good agreement with the binding energy of
copper in elemental form, i.e., 932.5 ± 0.2 eV [27,32].Thereisan
increase in the amount of elemental copper from 14 min of sputtering
onward. After 20 min of sputtering, the surface lm is composed of a
major percentage of elemental copper along with a small amount of
cuprous oxide.
It is worth mentioning that Ni 2p signals are detected only after
14 min of sputtering of the lm. Ni 2p overlay spectra at different
depths of the lm are already shown in Fig. 1c. The deconvolution spec-
trum of Ni 2p
3/2
exhibits a peak at 854.5 eV along with a small shake-up
satellite at 861.2 eV. This peak is assigned NiO [27,33]. The spectrum
shows a less intense peak at 856.2 eV, which reveals the presence of
Ni
2
O
3
[27,33,34]. In addition to these peaks, the Ni elemental peak
with a binding energy of 852.4 eV is also detected [27].TheO1s
deconvolution spectrum exhibits a peak at 529.9 eV, which is ascribed
to the presence of both NiO and Ni
2
O
3
[3436].Thelm at this depth is
also composed of Cu
2
O as evidenced by the presence of Cu 2p
3/2
peak at
932.5 eV and an O 1s peak at 531.4 eV. A peak at 932.3 eV in the XPS
spectrum of Cu 2p
3/2
infers the presence of copper in elemental form.
These observations inferred that the lm is composed of Cu, Ni and
Cu
2
O along with NiO and Ni
2
O
3
in minor abundance. With the
increase in sputter time, the intensities of NiO and Ni
2
O
3
peaks are
decreased and the intensity of elemental Ni peak is increased. After
20 min of sputtering, the intensity of the Ni 2p
3/2
peak at 854.4 eV is
much less and the intensity of the Ni 2p
3/2
peak at 852.3 eV is much
higher.
The atomic percentage of each element identied at different depths
of CuNi (90/10) alloy in seawater in the absence of BTAH is shown in
Fig. 2a. Before sputtering, the atomic percentage of copper is found to
be as low as 3.51%. After sputtering for 2 min, there is an increase in
the atomic percentage of copper, which continues to increase with the
increase in sputter time. After 20 min of sputtering, it is found to be
73.99%. The intensity of oxygen is found to be 41.38% before any
sputtering. After 20 min of sputtering it is reduced to a low value of
16.71%. Nickel is identied after 14 min of sputtering. All these results
lead to the schematic representation of the chemical composition of
the surface layers at different depths of CuNi (90/10) alloy as given
in Fig. 2b.
The X-ray photoelectron survey spectra of the surface lms at differ-
ent depths, formed on CuNi (90/10) alloy after immersion in synthetic
seawater polluted with 10 ppm sulphide showed peaks due to copper,
oxygen, sulphur and nickel. The XPS deconvolution spectra for Cu 2p,
O 1s, S 2p and Ni 2p at different sputtering times of the surface lm
are shown in Fig. 3ad respectively. The results after sputtering for
4 min of the lm only are discussed as all other results are the same as
those observed in the case of the lm formed in seawater without any
sulphide.
The XPS deconvolution spectrum of S 2p exhibits S 2p
3/2
peak at
161.5 eV, which corresponds to Cu
2
SorCuS[37,38]. The computer
deconvolution spectrum of Cu 2p
3/2
at this depth exhibits two different
peaks. The peak at 932.4 eV with high intensity is due to the presence of
Cu(I) and the one at 934.4 eV along with a small shake-up satellite at
946.2 eV of low intensity is due to the presence of Cu(II). These results
infer the presence of Cu
2
S and CuS. Similar peaks are also observed
after 6 min of sputtering with reduced intensities. The S 2p peaks, how-
ever, disappeared after sputtering for 8 min.
The atomic percentage of each element identied at different depths
of CuNi (90/10) alloy in seawater polluted with 10 ppm sulphide in the
Fig. 4. (a) Relative atomic composition and (b) schematic representation of the surface of
CuNi (90/10) alloy after 30 day immersion in seawater containing 10 ppm sulphide.
340 B.V. Appa Rao et al. / Thin Solid Films 556 (2014) 337344
Author's personal copy
absence of BTAH is shown in Fig. 4a. Before sputtering, the atomic per-
centage of copper is found to be 10.76%. It increased with sputtering
time and after sputtering for 20 min, it is found to be 74.05%. The inten-
sity of oxygen is found to be 58.96% before sputtering. After sputtering
for 20 min, it is reduced to a low value of 15.84%. Nickel is identied
after 14 min of sputtering. Sulphur is detected only after 4 min of
sputtering with a percentage of 20.53. Sulphur is also found after
sputtering for 6 min and is totally absent after 8 min of sputtering. The
schematic representation of chemical composition of the surface layers
at different depths of CuNi (90/10) alloy is given in Fig. 4b.
The X-ray photoelectron survey spectra at different depths of the
surface lm formed on CuNi (90/10) alloy after immersion in synthetic
seawater in the presence of 3.33 mM BTAH showed peaks due to cop-
per, carbon, nitrogen, oxygen and nickel. The XPS deconvolution spectra
for Cu 2p, C 1s, N 1s and O 1s at differentsputtering times of the surface
lm are shown in Fig. 5ad. Before any sputtering, the XPS exhibited
peaks due to N 1s and C 1s, which infer the presence of BTAH in the
surface lm. The Cu 2p
3/2
peak at 932.4 eV infers the presence of Cu(I)
species. The C 1s spectrum shows three peaks, one at 284.9 eV with
high intensity and the other two at 283.9 eV and 287.7 eV with less
intensity. The high intense C 1s peak at 284.9 eV is characteristic of
carbon present in the aromatic ring of BTAH [39]. The less intense C 1s
peaks at 283.9 eV and 287.7 eV are due to contaminant carbon [29].
The N 1s peak at 399.7 eV is due to delocalization of the three nitrogen
atoms present in the ring, which provides a negative charge to the ring
[40]. This peak is ascribed to nitrogen in the [Cu(I)BTA]
n
complex. Sim-
ilar peaks of C 1s (284.9 eV) and N 1s are also observed after 2 and 4 min
of sputtering. After sputtering for 6 min, both the C 1s andN 1s peaks are
absent. The O 1s peak at 530.8 eV is due to the presence of Cu
2
O. Thus,
the outermost surface lm consists of Cu
2
O and [Cu(I)BTA]
n
complex,
which is responsible for protection of the alloy from corrosion in the
seawater environment. No nickel 2p peak is identied even up to
10 min of sputtering.
After sputtering for 2 min, the Cu 2p
3/2
computer deconvolution
spectrum exhibits a peak at 932.6 eV, which is an evidence for the exis-
tence of Cu(I). The presence of the C 1s peak at 284.9 eV is characteristic
of carbon present in the aromatic ring of BTAH [39]. The binding energy
value of the N 1s peak is altered to 398.6 eV after 2 min of sputtering.
Similar result on reduction of the binding energy value of N 1s after
sputtering for 2 min has been reported in the literature [41] in the XPS
studies of the benzotriazole lm formed on copper. Similar peaks of
both C 1s and N 1s peaks are found after 4 min of sputtering also. Both
the peaks are diminished after sputtering for 6 min. These results infer
that the [Cu(I)BTA]
n
complex lm formed on the alloy surface is very
thin.
After 8 and 10 min of sputtering, both the Cu
2
O and Cu peaks are
identied. After sputtering for 12 min, the computer deconvolution Cu
2p spectrum exhibits a peak at 932.4 eV, which indicates the presence
of elemental copper. The intensity of the O 1s peak at 530.8 eV is
found to be very less, which infers a small amount of Cu
2
O at this depth.
Ni 2p peaks are identied only after 12 min of sputtering. The pres-
ence of the Ni 2p
3/2
peak at 852.1 eV infers the presence of Ni in elemen-
tal form [27]. The less intensity peak at 852.8 eV along with a shake-up
satellite at 858.2 eV is due to the presence of NiO [27,33]. The less in-
tense O 1s peak at 529.7 eV is due to the presence of a trace amount
of NiO [3436]. It may be noted that in the presence of inhibitor, the sur-
face lm is composed of mainly copper and nickel in their elemental
forms after 12 min of sputtering. It is worthwhile to compare with the
Fig. 5. XPS spectra at different sputtering times of (a) Cu 2p, (b) C 1s, (c) N 2p and (d) O 1s formed on CuNi (90/10) alloy after 30 day immersion in seawater in the presence of BTAH.
341B.V. Appa Rao et al. / Thin Solid Films 556 (2014) 337344
Author's personal copy
results in the absence of inhibitor in which case copper and nickel are
found in their elemental forms in the lm only after 20 min of
sputtering.
The atomic percentage of each element identied at different depths
of CuNi (90/10) alloy in seawater containing 3.33 mM BTAH is shown
in Fig. 6a. Before sputtering, the atomic percentage of copper is found to
be 2.22%. After six minutes of sputtering it is found to be 82.15%. Nitro-
gen and carbon are found up to 4 min of sputtering. After sputtering for
6 min, both nitrogen and carbon are diminished. The intensity of oxygen
is found to be 12.8% before any sputtering. After sputtering for 12 min it
is reduced to a very low value of 8.71%. Nickel is identied after 12 min
of sputtering. A schematic representation of the chemical composition
at different depths of the surface layers formed on CuNi (90/10) alloy
is given in Fig. 6b.
The X-ray photoelectron survey spectra at different depths of the
surface lm formed on CuNi (90/10) alloy after immersion in synthetic
seawater polluted with 10 ppm sulphide in the presence of 3.33 mM
BTAH showed peaks due to copper, carbon, nitrogen and oxygen
(Fig. 7ad). The results are similar to those found in case of seawater
containing 3.33 mM BTAH. However, before sputtering, the Cu 2p com-
puter deconvolution spectrum exhibits Cu 2p
3/2
peak at 934.5 eV along
with a shake-up satellite at 942.1 eV, which infers the presence of Cu(II)
species also on the alloy surface. This result infers the presence of CuO or
Cu(OH)
2
or both. In addition to this peak, another Cu 2p
3/2
peak at
932.4 eV infers the presence of the [Cu(I)BTA]
n
complex. The presence
of the O 1s peak at 531.3 eV indicates the presence of CuO or Cu(OH)
2
and Cu
2
O.
The atomic percentage of each element identied at different depths
of CuNi (90/10) alloy in sulphide-polluted seawater containing
3.33 mM BTAH is shown in Fig. 8a. Before sputtering, the atomic per-
centage of copper is found to be 0.81%. After 12 min of sputtering it is
found to be 83.24%. Nitrogen and carbon are found up to 4 min of
sputtering. These results infer the formation of a polymeric [Cu(I)
BTA]
n
complex on the surface of the alloy. After sputtering for 6 min,
both the nitrogen and carbon are diminished. The intensity of oxygen
is found to be 14.69% before any sputtering. After 12 min of sputtering,
it is reduced to a low value of 8.965%. Nickel is identied after 12 min of
sputtering. A schematic representation of the chemical composition at
different depths of the surface layers formed on CuNi (90/10) alloy is
given in Fig. 8b.
The depth at which the intensity of oxygen decreases to half in the
absence of BTAH, or the depth at which the intensity of nitrogen dimin-
ishes in the presence of BTAH was taken as a measure of lm thickness
[24,42]. In seawater, in the absence of BTAH, theintensity of oxygen de-
creases to half after 14 min of sputtering. From this observation, the
thickness of the oxide lm is calculated to be 14 nm. Whereas, in sul-
phide-polluted seawater, the thickness of the oxide lm is calculated
to be 12 nm. In the presence of BTAH, in both seawater and sulphide-
polluted seawater, the intensity of nitrogen is diminished after 4 min
of sputtering. The thickness of [Cu(I)BTA]
n
is considered to be approxi-
mately 4 nm.
3.2. Mechanism of corrosion and its inhibition
The oxidation of CuNi (90/10) alloy occurs in seawater via dissolu-
tion of copper and nickel resulting in formation of Cu
2
O, CuO, Cu(OH)
2
,
NiO and Ni
2
O
3
whichareshowninEqs.(1)(5) respectively [4345].
2Cu þH2OCu2Oþ2Hþþ2eð1Þ
Cu2OþH2O2CuO þ2Hþþ2eð2Þ
Cu2Oþ2H2OCu OHðÞ
2þ2Hþþ2eð3Þ
Ni þH2ONiO þ2Hþþ2e
ð4Þ
2NiO þH2ONi2O3þ2Hþþ2e
ð5Þ
In seawater polluted with sulphide, hydrosulde (HS
) ions are
known to promote corrosion of alloy [46,47].
HS
þCu þ1=2O2CuS þOH
ð6Þ
HS
þ2CuCu2SþHþð7Þ
In the presence of BTAH, there is chemisorption of BTAH on the alloy
surface followed by the formation of the polymeric complex [20],as
showninEqs.(8) and (9) respectively,
BTAH aqðÞ
þCuBTAH :Cu chemisorptionðÞ ð8Þ
nBTAH þnCuþCu 1ðÞBTA½
n
þnHþPolymeric complex formationðÞð9Þ
Fig. 6. (a) Relative atomic composition and (b) schematic representation of the surface of
CuNi (90/10) alloy after 30 day immersion in seawater in the presence of BTAH.
342 B.V. Appa Rao et al. / Thin Solid Films 556 (2014) 337344
Author's personal copy
4. Conclusions
XPS depth-proling studies inferred the formation of a protective
polymeric complex, [Cu(I)BTA]
n
, which protects the alloy from corro-
sion even after an immersion period of 30 days. This protective [Cu(I)
BTA]
n
polymeric complex is found even after 4 min of sputtering of
the lm. The thickness of this lm is found to be approximately 4 nm.
In seawater and sulphide containing seawater, in the absence of BTAH,
the outermost layer of the lm consists of CuO, Cu(OH)
2
and Cu
2
O.
After sputtering the lm for 14 min, the surface layer is found to consist
of Cu
2
O, NiO and Ni
2
O
3
. The thickness of the oxide lm in the absence of
an inhibitor is found to be approximately 14 nm in seawater and 12 nm
in sulphide-containing seawater. In sulphide-containing seawater, the
presence of sulphur is detected in intermediate layers only. In the pres-
ence of BTAH in both the environments, the percentage of Cu
2
O is found
to be very less after 12 min of sputtering, whereas in the absence of
BTAH, it is found up to 20 min of sputtering.
Acknowledgment
The authors are grateful to the Naval Research Board (NRB) and the
Rajiv Gandhi National Fellowship (RGNF), UGC, Govt. of India for the
nancial assistance.
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presence of BTAH.
344 B.V. Appa Rao et al. / Thin Solid Films 556 (2014) 337344
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