Y. LI et al.: EFFECT OF ALKALINE SOLUTION CONCENTRATION ON THE PASSIVATION FILM OF Cu-Ni ALLOY
249–255
EFFECT OF ALKALINE SOLUTION CONCENTRATION ON THE
PASSIVATION FILM OF Cu-Ni ALLOY
VPLIV KONCENTRACIJE ALKALNE RAZTOPINE NA NASTANEK
PASIVNEGA FILMA NA ZLITINI Cu-Ni
Yue Li
1
, Shuyan Zhao
2
, Xinyu Zhang
2
, Shuo Zhang
2
, Xiaoliang Wu
2
,
Nianchu Wu
1*
1
School of Mechanical Engineering, Liaoning Petrochemical University, Fushun, 113001, P. R. China
2
Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, P. R. China.
Prejem rokopisa – received: 2023-02-01; sprejem za objavo – accepted for publication: 2023-03-16
doi:10.17222/mit.2023.782
Passive films formed on a Cu-Ni alloy in various concentrations of alkaline environment were investigated by potentiodynamic
polarization, electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy and the Mott–Schottky ap-
proach. The wide passivation range of the copper-nickel alloy was tested in alkaline solutions of three concentrations. The oxide
film on the specimen has a p-type semiconductor property, while the flat band potential (EFB) decreased with increasing solution
concentration. The film resistance of the passive films increased with increasing solution concentration. The pAassive films
showed a duplex structure, including an inner layer of oxide (Cu2O, NiO) and an outer layer of hydroxide (Cu(OH)2, Ni(OH)2).
Keywords: 90Cu-10Ni alloy, corrosion resistance, oxidation film, solution concentration, passivation potential
Raziskovali so nastajanje tanke oksidne plasti na zlitini Cu-Ni v prisotnosti razli~no koncentrirane alkalneraztopine. Raziskave
so izvajali s pomo~jo potenciodinami~ne polarizacije, elektrokemi~ne impedan~ne spektroskopije (EIS), rentgenske foto-
elektronske spektroskopije in Mott-Schottky-jevega pristopa.Pri tem so izbrali tri razli~ne koncentracije alkalne raztopine in
dosegli {irok spekter pasivacije izbrane Cu-Ni zlitine. Oksidni film formiran na povr{ini zlitine je imel lastnosti polprevodnika
tipa p, ploskost pasovnega potenciala (EFB) pa se je zmanj{evala z nara{~ajo~o koncentracijo oz. s pove~anjem alkalnosti razto-
pine. Odpornost pasivnih filmov je nara{~ala z nara{~ajo~o koncentracijo raztopine. Pasivni filmi so imeli dupleks strukturo, ki
je bila sestavljena iz notranje oksidne plasti(Cu2O, NiO) in zunanje plasti hidroksida (Cu(OH)2, Ni(OH)2).
Klju~ne besede: zlitina 90Cu-10Ni, odpornost proti koroziji, oksidni film, koncentracija alkalne raztopine, potencial pasivacije
1 INTRODUCTION
Copper-nickel alloys play an important role in marine
engineering for thermal condensers, seawater desalina-
tion equipment, water supply lines, and other pipeline
applications due to their excellent physical and mechani-
cal properties, resistance to seawater corrosion and bio-
logical fouling.
1
The excellent corrosion resistance of
copper-nickel alloys, on the one hand, is due to the high
standard electrode potential for copper of +0.34 V, which
is difficult to ionize. On the other hand, the Ni
2+
is incor-
porated into the Cu
2
O film, and the ion resistance of the
film is increased. In copper-nickel alloy systems,
70Cu-30Ni and 90Cu-10Ni alloys are the most widely
used. The nickel content of the 70Cu-30Ni alloy is 30 %,
which has better corrosion resistance and is mainly used
for pipelines that withstand larger flow rates. However,
compared with 90Cu-10Ni alloys, due to its higher
nickel content, the cost is higher, so limiting the applica-
tion range. Some issues regarding the main corrosion
protection offered by the copper alloy remain to be stud-
ied further.
2
Typically, the excellent corrosion resistance of a
metal or an alloy is attributed to the passivation oxide
film formed on the surface, possibly only a few nano-
meters thick, that protects the metal surface from the ef-
fects of an aggressive external environment and thus in-
hibits further corrosion.
3
The characteristics of the
passive film, including the composition, structure and
thickness, will control the corrosion resistance.
4
Some is-
sues regarding the main corrosion protection offered by
the copper alloy remain to be studied further. Because
passivation plays a crucial role in material applications,
investigations of the passive behavior remain topical is-
sues in corrosion science.
5
The presence, composition
and structure of an oxide overlayer on a metallic sub-
strate is determined by the redox conditions of the inter-
face formed with the environment.
6
In recent years, many
scholars have devoted themselves to the study of the
passivation behavior of copper and copper alloys in
acidic or alkaline solutions. Kose et al.
7
proposed that the
composition of the passivation film formed on Cu-20Zn
and Cu-20Ni alloys in an alkaline borate buffer can be
expressed as ZnO/Cu
2
O/CuO and NiO/Cu
2
O/CuO, re-
spectively. Zaky et al.
8
studied the passivation film of
copper-nickel alloy in NaOH solution by cyclic
voltammetry consisting of Ni(OH)
2
/CuO/Cu
2
O/
Materiali in tehnologije / Materials and technology 57 (2023) 3, 249–255 249
UDK 620.193:669.058.5 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 57(3)249(2023)
*Corresponding author's e-mail:
wunianchu@163.com (Nianchu Wu)

Cu(OH)
2
/Ni
2
O
3
. The surface passivation film of cop-
per-nickel alloy is also related to the metal substrate,
9–13
and the results showed that the addition of Ni to Cu to
form copper–nickel alloys changes the composition of
the surface film; the passivation potential,
14,15
the results
showed that the corrosion resistance increased with in-
creasing potential; i.e., the passivation time.
12
The in-
crease of the immersion time in the solution improved
the alloy stability due to the formation of a stable passive
film, and the solution concentration,
2,9,16,17
significant
passivity was not observed in the un-buffered chloride
solutions at pH 6. The alloys exhibited passivity in the
buffered solutions at pH 10. A large number of studies
were reported to explain the electrochemical behavior of
the passive films on copper. However, little information
about the effect of solution concentration on the compo-
sition of these passive films is available.
In the alkaline solution, by controlling the passivation
potential parameters, the solution composition and con-
centration, pH, temperature and time, substances with ef-
fective protection properties can be obtained. In the pres-
ent work the electrochemical properties of the passive
films formed into Cu-Ni alloy in alkaline solutions were
further studied in the concentrations range
0.01–0.35 mol/L using potentiodynamic polarization,
electrochemical impedance spectroscopy (EIS) and the
Mott–Schottky approach. The chemical composition and
the element state of the passive film were investigated
with X-ray photoelectron spectroscopy (XPS).
2. EXPERIMENTAL
2.1. Materials and solutions
In this study the main chemical composition (w/%) of
90Cu-10Ni alloy is shown in Table 1. The plate thick-
ness of 3 mm is cut into (10 × 10) mm specimens and
then embedded in the epoxy resin. The surface sanded
with sandpaper to 2000# and mirrored with diamond
abrasive paste, and then rinsed with methanol. Three
electrodes were used in this experiment. The saturated
calomel electrode (SCE) was used as the reference elec-
trode and the platinum electrode was used as the counter
electrode. The working electrode is a 90Cu-10Ni speci-
men. The test solutions are 0.01 mol/L NaOH+0.125
mol/L Na
2
B
4
O
7
·10H
2
O, 0.1 mol/L NaOH+0.125 mol/L
Na
2
B
4
O
7
·10H
2
O and 0.35 mol/L NaOH+0.125 mol/L
Na
2
B
4
O
7
·10H
2
O borate buffer solution at 22 °C.
2.2. Electrochemical measurements
In this experiment the electrochemical data were
measured using the CHI660E electrochemical worksta-
tion. Before all the electrochemical experiments, the sur-
face of the sample was initially pretreated cathodically at
–0.8 V
SCE
for 15 min to reduce the air-formed oxide
film.
18
Then the potentiodynamic polarization experi-
ments were performed at a scan rate of 1 mV/s, the scan-
ning range is –0.25 V (vs E
OCPT
) to 1 V. Before the EIS
measurement, the specimens were first passivated for 1 h
at 0.1 V
SCE
, 0.2 V
SCE
, 0.3 V
SCE
, and 0.4 V
SCE
to ensure
that a sufficiently stable film is formed on the specimen.
The EIS experiment is a signal from 10
5
Hz to 10
–2
Hz
with an amplitude signal of 5 mV. ZSimpWin software
was used to fit the EIS experimental data. The
Mott-Schottky plot is obtained by scanning in both the
positive and negative directions at a frequency of
1000 Hz with an amplitude signal of 5 mV, a potential
range of 0 V
SCE
to1V
SCE
, and a potential step of 10 mV.
XPS analyses were performed using a Thermo Scientific
K-Alpha photo-electron spectrometer. Photo-electron
emission was excited by a monochromatic Al K
 source
operated with an initial photo energy of 1486.6 eV. The
C 1s peak at contaminative carbon at 284.8 eV was used
as a reference to correct the charging shifts. The quantifi-
cation of the species in the oxide films was performed
using XPSPEAK software fitting peak.
3 RESULTS AND DISCUSSION
3.1 Cu-Ni alloy thin-film preparation
Figure 1 shows potentiodynamic polarization curves
for the B10 alloy in 0.01 mol/L, 0.1 mol/L and
0.35 mol/L NaOH solutions. All three curves have obvi-
Y. LI et al.: EFFECT OF ALKALINE SOLUTION CONCENTRATION ON THE PASSIVATION FILM OF Cu-Ni ALLOY
250 Materiali in tehnologije / Materials and technology 57 (2023) 3, 249–255
Table 1: Chemical compositions of 90Cu10Ni alloy (w/%)
Cu Ni Fe Mn Zn S P C Pb
substrate 10.55 1.76 0.80 0.031 0.0041 0.004 0.01 <0.005
Figure 1: Potentiodynamic polarization curves for B10 alloy in NaOH
solution

ous passivation ranges. In the 0.35 mol/L solution, the
potential polarization curve has a significant primary
passivation and secondary passivation. The current den-
sities were very small, and its specific value is shown in
Table 2, so exhibiting a better corrosion resistance. Ac-
cording to the potentiodynamic polarization curves,
0.1 V, 0.2 V, 0.3 V and 0.4 V were selected as
passivation potentials.
Table 2: Corrosion potential and current density of B10 alloy in
NaOH solution with different concentrations
Concentrations
(mol/L)
E
corr
(V) I
corr
(×10
–5
A·cm
–2
)
0.01 –0.170 1.21
0.1 –0.169 0.39
0.35 –0.296 0.14
3.2 Evaluation of corrosion resistance
To evaluate the influence of the solution concentra-
tions on the corrosion resistance of the coatings ob-
tained, experiments were carried out using the electro-
chemical impedance spectroscopy (EIS) techniques.
19
EIS has been widely used to study and characterize pas-
sive films formed into pure metals or alloys.
20
According
to the characteristics of the polarization curve, 0.1 V,
0.2 V, 0.3 V and 0.4 V potentials were selected for
passivation for 1200 s, 2400 s and 3600 s, respectively.
The applied potentials were within the passive range. Af-
ter stabilization, the electrochemical impedance was
measured. The equivalent circuit is shown in the Fig-
ure 2. In the circuit, R
s
is the solution resistance, Q
f
is
the constant–phase element (CPE) of the passive films,
R
f
is the passive film resistance, C
dl
is the double–layer
capacitance and R
ct
is the charge-transfer resistance. The
CPE was used as the non-ideal capacitance (n<1)ofthe
passive film caused by surface heterogeneities and
toughness, as given by:
[]
ZY j w
n
CPE
=
−
0
1
() (1)
where Q is the admittance of CPE, n is the CPE expo-
nent, and w is the angular speed. The higher values of
R
ct
and R
f
indicate a better corrosion resistance, thus it
Y. LI et al.: EFFECT OF ALKALINE SOLUTION CONCENTRATION ON THE PASSIVATION FILM OF Cu-Ni ALLOY
Materiali in tehnologije / Materials and technology 57 (2023) 3, 249–255 251
Figure 3: Nyquist diagram of 90Cu-10Ni in NaOH solution: a) 0.1 V, 0.2 V, 0.3 V, 0.4 V passivation for 1200 s, b) 0.1 V, 0.2 V, 0.3 V, 0.4 V
passivation for 2400 s, c) 0.1 V, 0.2 V, 0.3 V, 0.4 V passivation for 3600 s
Figure 2: Equivalent electron circuit used to fit the impedance spectra

can be used to evaluate the corrosion resistance of the
samples.
On the one hand, according to Figure 3, the Nyquist
diagram of 90Cu-10Ni in NaOH solution, with an in-
crease of the applied potential, the radius of capacitive
resistance arc increases. And the R
p
of passive film fit-
ting data shows in Figure 4 that as the concentration in-
creases, R
p
and R
ct
increase. So, the corrosion resistance
increases. This can be explained as follows: the applied
potential is the driving force for oxide formation, the
growth and crystallisation of the passive film. There was
a competition between nucleation and growth; at lower
applied potential, there were fewer nucleation sites, and
the grains grew slowly to a large scale, whereas at higher
applied potential, more susceptible nucleation sites were
activated, and the grains tended to have a refined struc-
ture. On the other hand, there was a formation of a pro-
tective film on the copper alloy surface when it was im-
mersed in the alkaline solution. In the alkaline solution,
increases in the solution concentration depend on the
concentration of hydrogen ions. And, this is known as
one of the factors affecting the corrosion rate. The
changes in concentration can either induce or modify
electrochemical reactions.
21
The increases in solution
concentration relate to the increases in stability of the
formed corrosion product, most likely as an oxide. These
oxides can form protective films, slowing down the an-
odic activity.
22
The corrosion resistance of the
passivation film increases with an increase of the solu-
tion concentration, while the passivation time and the
passivation potential remain unchanged. This is because
the inner layer of the passivation film is mainly copper
oxide and a small amount of nickel oxide, and the outer
layer is copper oxide and hydroxide. Generally, the outer
layer is relatively loose and porous, and has strong pro-
tection without the inner layer.
3.3 Semiconductor performance of passive film
The semiconductor property of the passivation film
can be described with the Mott-Schottky theory. The lin-
ear region of the plots is due to the variation on the width
of the space-charge layer of the oxide film on the speci-
men with the applied potential, according to:
• p-type:
12
2
C eN
EE
kT
e
=− − −
⎛ ⎝ ⎜ ⎞ ⎠ ⎟ 
 A
FB
(2)
• n-type:
12
2
C eN
EE
kT
e
=− − −
⎛ ⎝ ⎜ ⎞ ⎠ ⎟ 
 D
FB
(3)
where E is the applied potential (V
SCE
), E
FB
is the
flat-band potential (V
SCE
),  0
is the permittivity of free
space (F·cm
–1
), is the relative dielectric constant, N
D
is
the donor density (cm
–3
), N
A
is the accept density
(cm
–3
), A is the surface area of the sample (cm
2
), k is the
Boltzmann constant (1.38 × 10
–23
J/K), T is the absolute
temperature and e is the charge of the electron (1.6  10
–19
C).
Table 3: Accept densities N
A
of passive films at different solution:
a) 1200 s, b) 2400 s, c) 3600 s
(a)
NA(10
20
cm
–3
) 0.1V 0.2V 0.3V 0.4V
0.01mol/L 5.27 5.59 5.5 3.4
0.10 mol/L 3.72 4.65 3.7 3.3
0.35 mol/L 7.48 8.86 5 6.4
(b)
N
A
(10
20
cm
–3
) 0.1V 0.2V 0.3V 0.4V
0.01mol/L 5.05 4.9 3.8 4.7
0.10 mol/L 3.44 3.23 2.2 1
0.35 mol/L 6.31 4.64 9.1 5.5
(c)
NA(10
20
cm
–3
) 0.1V 0.2V 0.3V 0.4V
0.01mol/L 2.18 4.2 4.2 0.4
0.10 mol/L 2.8 2.2 4.2 3.4
0.35 mol/L 6.09 4 14 9.8
From Equation (2) the slope of the plot of the experi-
mental C
–2
as a function of E can determine N
A
, and the
extrapolation of the linear portion to C
–2
= 0 can obtain
E
FB
. The data of N
A
is shown in Table 3. Different types
of defects exist within the barrier layer, including anion
vacancies, cation interstitials, and cation vacancies, re-
Y. LI et al.: EFFECT OF ALKALINE SOLUTION CONCENTRATION ON THE PASSIVATION FILM OF Cu-Ni ALLOY
252 Materiali in tehnologije / Materials and technology 57 (2023) 3, 249–255
Figure 4: Electrochemical impedance spectroscopy fitting yields R
p
. Passivation time is: a) 1200 s, b) 2400 s, c) 3600 s

sulting in different types of electronic, semiconducting
films.
23
The slope of the straight line is negative, show-
ing p-type semiconductor characteristics. It is suggested
that the passive oxide layers formed on the Cu-Ni alloy
depend on the predominant defect present in the passive
film. The cation vacancy was the dominant defect in the
film. According to the electron band theory of solids, if
the number of holes in the valence band is more than that
of electrons in the conduction band of oxides, the oxides
are considered as a p-type semiconductor such as Cu
2
O,
Ni
2
O
3
and NiO.
Figure 5 is the Mott–Schottky diagram of the sample
in different concentration solutions. It can be seen from
the comparison of the flat-band potential (E
FB
) in all fig-
ures that the value of the flat band potential (E
FB
)d e -
creases with the increase of the solution concentration.
In other words, the breaking potential (E
b
) increases with
the decrease of the flat band potential (E
FB
), and the cor-
rosion resistance of the metal matrix increases accord-
ingly. This conclusion is consistent with that of the elec-
trochemical impedance.
3.4 XPS Analysis of the Surface Layers
The XPS spectra of the 90Cu10Ni alloy after
passivation at 0.4 V for 3600 seconds in different solu-
tions are shown in Figure 6. The fitting of the peaks ob-
tained from the XPS results showed that the metallic and
oxidized states of Cu 2p, Ni 2p and O 1s appeared in the
passivation film. This was to obtain high-resolution spec-
tra, in order to study the composition of the passivation
film more carefully.
Figure 6 shows XPS spectra of the passivation film
formed on the surface of the 90Cu-10Ni alloy in differ-
ent concentrations of NaOH solution after 0.4 V
passivation for 3600 s. Cu species were detected in the
passivation film. The high-resolution spectra of Cu 2p is
shown in the Figure 6a, the Cu 2p spectra can be
deconvoluted into three peaks, the metallic state
(Cu(met)) peak, the cuprous species peak and the biva-
lent (Cu 2p) species peak. It is observed that the solution
concentrations play a crucial role in the composition ox-
ide species, they will be altered at different solution con-
centrations. Cu (0) was detected in the XPS spectrum of
the passivation film formed by the 0.01 mol/L solution.
The relative peak heights of Cu (I) and Cu (II) indicate
that Cu (II) is the primary oxidized species or hydroxyl
in the passive film. The relative peak intensity of Cu (I)
increased in the XPS spectra of 0.10 mol/L solution
passivation film. Compared with the passivation film
formed in the 0.01 mol/L solution, the passivation film is
more compact. The relative peak intensity of Cu (I)i n
the passivation film increases with the increase of the so-
lution concentration. For the passivation film formed in
Y. LI et al.: EFFECT OF ALKALINE SOLUTION CONCENTRATION ON THE PASSIVATION FILM OF Cu-Ni ALLOY
Materiali in tehnologije / Materials and technology 57 (2023) 3, 249–255 253
Figure 5: Mott–Schottky plots of Cu90Ni10 alloy in NaOH solution: a) 0.1 V, 0.2 V, 0.3 V, 0.4 V potentiostatic passivation for 1200 s, b) 0.1 V,
0.2 V, 0.3 V, 0.4 V potentiostatic passivation for 2400 s, c) 0.1 V, 0.2 V, 0.3 V, 0.4 V potentiostatic passivation for 3600 s

0.35 mol/L solution, Cu mainly exists in the form of Cu
(I) oxide. At this time, the compactness of the
passivation film is further strengthened.
The passive films also detected the Ni species. The
three graphs in Figure 6b show the spectra of Ni 2p in
solutions of three concentrations. The Ni 2p spectra can
be deconvoluted into three peaks, the metallic state
(Ni(met)) peak, the bivalent peak and trivalent species
peak. The peak at 554.3 eV is attributed to the NiO.
24
The XPS spectra of the passivation film Ni 2p formed in
three solutions were compared, the relative intensity of
the NiO peak increased with the increase of the solution
concentration. The main composition of the passivation
film is changed from nickel hydroxide to oxide. This
makes the passivation film more corrosion resistant. In
addition, the peak at 855.6 eV is Ni(OH)
2
.
The O 1s spectra can also be split into three compo-
nents, i.e., O
2–
,O H
–
and H
2
O. It can be seen that OH
-
(531.8 eV)
17
is the primary constituent of the passive
film in 0.01 mol/L and 0.10 mol/L solution, which corre-
sponds to the formation of Cu(OH)
2
and Ni(OH)
2
.Atthe
same time, O
2–
(530.2 eV)
24,25
is the primary constituent
of the passive film in the 0.35 mol/L solution, which cor-
responds to the formation of the Cu
2
O, CuO, NiO and
Ni
2
O
3
species. The peak at 533 eV represents H
2
Ointhe
passive film.
17
The peak at 533 eV represents H
2
Ointhe
passive film. In comparison with the O
2–
and OH
–
, it can
be concluded that the OH
–
is the primary species in the
passive film in the 0.01 mol/L and 0.10 mol/L solution.
However, as the concentration increases to 0.35 mol/L,
the O
2–
became the primary constituent of the passive
film. So, the main components of the passivation film are
NiO, Cu
2
O and other protective substances.
According to the XPS data, the physical structure and
ion-transfer process of the passive film formed by
90cu-10ni in a NaOH solution are shown in Figure 7.
The passivation film shows a double-layer structure. The
inner layer is relatively dense Cu
2
O and NiO, and the
outer layer is porous hydroxide and CuO. The oxide film
of the inner layer plays a major role in protection. The
content of oxide produced in 0.35 mol/L solution is
higher. Therefore, with an increase of the solution con-
centration, the protection of the passivation film to the
body increases. This conclusion is consistent with the re-
sults of electrochemical impedance and the Mott-
Schottky analysis.
4 CONCLUSIONS
The passive films on a Cu-Ni alloy in alkaline solu-
tions were investigated in the concentration range from
0.01 mol/L to 0.35 mol/L using potentiodynamic curves,
electrochemical impedance spectroscopy (EIS), X-ray
photoelectron spectroscopy and Mott–Schottky plots.
The following conclusions can be drawn:
1). There is a wide passivation range of copper nickel
alloy in alkaline solutions of three concentrations. The
passive films have p–type bipolar semiconductors char-
acteristics. The flat band potential (E
FB
) decreased with
an increasing solution concentration. This indicates that
the passivation film has better corrosion resistance.
2). The film’s resistance value increased when in-
creasing NaOH concentration and the applied potential,
due to the increase in the Cu dissolution of 90Cu–10Ni
alloy. The passivation film forms a stable oxide and is
protective.
3). The passivation film is composed of stable oxides
and hydroxides, which are protective. With an increase
of the solution concentration, the content of Cu
2
O and
NiO in the film increases, and the formation ability of
passive film increases in 0.35 mol/L solution.
Y. LI et al.: EFFECT OF ALKALINE SOLUTION CONCENTRATION ON THE PASSIVATION FILM OF Cu-Ni ALLOY
254 Materiali in tehnologije / Materials and technology 57 (2023) 3, 249–255
Figure 7: Formation of passive film on Cu-Ni alloy in solution
Figure 6: XPS spectra of the passivation film formed on the surface of
90Cu-10Ni alloy in NaOH solution after 0.4 V passivation for 3600 s

Acknowledgements
This work was supported by Natural Science Founda-
tion of Liaoning Province under Grant No.
2022-MS-364; Educational Commission of Liaoning
Province of China under Grant No. L2020021; Fushun
Revitalization Talents Program under Grant No.
FSYC202107011.
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