Inhibitor properties of carboxylates and their adsorption on copper from aqueous solutions

Article abstract of DOI:10.1134/S0036024415060023

Adsorption, protective and passivating properties of several sodium higher carboxylates on a copper surface was studied in a borate buffer solution at pH 7.4. Sodium oleyl sarcosinate is shown to possess the best protective and adsorptive properties. It is established that at E = 0.0 V, the adsorption of oleyl sarcosinate is poly-layered, giving the best protection of copper upon its anodic dissolution. In a moist atmosphere with periodic condensation of moisture on the samples, it exceeded that of 1,2,3-benzotriazole.

Full text of DOI:10.1134/S0036024415060023

ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2015, Vol. 89, No. 6, pp. 1070–1076. © Pleiades Publishing, Ltd., 2015.
Original Russian Text © M.O. Agafonkina, Yu.I. Kuznetzov, N.P. Andreeva, 2015, published in Zhurnal Fizicheskoi Khimii, 2015, Vol. 89, No. 6, pp. 1013–1019.
PHYSICAL CHEMISTRY
OF SURFACE PHENOMENA
Inhibitor Properties of Carboxylates and Their Adsorption on Copper
from Aqueous Solutions
M. O. Agafonkina, Yu. I. Kuznetzov, and N. P. Andreeva
Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Moscow, 119991 Russia
e-mail: kuznetsov@ipc.rssi.ru
Received July 29, 2014
Abstract—Adsorption, protective and passivating properties of several sodium higher carboxylates on a cop-
per surface was studied in a borate buffer solution at pH 7.4. Sodium oleyl sarcosinate is shown to possess the
best protective and adsorptive properties. It is established that at E = 0.0 V, the adsorption of oleyl sarcosinate
is poly-layered, giving the best protection of copper upon its anodic dissolution. In a moist atmosphere with
periodic condensation of moisture on the samples, it exceeded that of 1,2,3-benzotriazole.
Keywords: adsorption, copper, ellipsometry, sodium oleyl sarcosinate, sodium oleate, carboxylates, benzotri-
azole.
DOI: 10.1134/S0036024415060023
INTRODUCTION
The high degree of protection for copper in sulfate
solutions by potassium sorbate was demonstrated in
[14, 15].
The aim of this work was to compare the adsorp-
tion properties of nonoxidizing carboxylates on copper
and estimate their effectiveness in inhibiting the
anodic dissolution of copper in a neutral aqueous
solution and protecting it from corrosion in a moist
atmosphere.
Heterocyclic compounds such as 1,2,3-benzotri-
azole (BTA), benzimidazole, benzotiazole and their
derivatives are known as effective inhibitors of corro-
sion of copper and copper alloys [1–7] in solutions
over a wide range of pH. Use of the most studied
inhibitor BTA for the protection of copper and its
alloys in water-supply facilities suffers from interac-
tions with the chlorine used as a bactericide agent for
water. Researchers therefore seek new inhibitors for
copper that have low toxicity, easy biodegradation,
economic accessibility, and are environmentally
friendly.
Mono- and dinitrobenzoates [8, 9] were recently
used as inhibitors of copper corrosion. They belong
to a group of universal volatile corrosion inhibitors
that have high oxidizing ability and involve organic
cations adsorbed on surfaces of different metals (cop-
per, zinc, aluminum and their alloys). These usually
increase the efficiency of cathode processes, shifting
the metal corrosion potential toward values that
allow the formation of a passivating layer [8]. How-
ever, the high oxidizing ability of these compounds
when used for copper protection leads to the forma-
EXPERIMENTAL
Electrochemical and adsorption investigations
were conducted using a borate buffer solution with
pH 7.40 and prepared on distilled water [16].
Sodium
C6H5(CH2)10COONа
[3-(CF3)C6H4NH]C6H4COONa
mefenamic [(СН3)2С6Н3NH]C6H4COONa (MEFN)
acids, and sodium oleate
СН3(СН2)7СН=СН(СН2)7СООNa (SOL), and
salts
of
11-phenylundecanoic
(PHUD), flufenamic
(FFN), and
sodium
oleyl
sarcosinate
СН3(СН2)7СН=СН(СН2)7СОN(CH3)СН2СООNa
(SOS) were studied. The first three salts were prepared
tion of soluble copper chloride, which cannot protect via direct neutralization of the corresponding acids
metal from chloride ions [9].
with NaOH solution in an equimolar ratio. SOL (in a
powdered form) and SOS (60% solution) were pur-
chased from BIOKHIM.
The inhibiting properties of carboxylates of nonox-
idizing type have been long recognized. These
include, e.g., salts of aromatic, fatty, hydroxycarbox-
ylic, and unsaturated carboxylic acids, which are able lution currents and promote the spontaneous passiva-
to protect ferrous and nonferrous metals from atmo- tion of copper were studied by means of potentiometry
spheric corrosion [9–11]. These effectively protect while recording the anodic polarization curves. These
copper in water and aqueous solutions of salts [12, 13]. investigations were conducted using an IPC-PRO
The abilities of carboxylates to reduce active disso-
1070

INHIBITOR PROPERTIES OF CARBOXYLATES AND THEIR ADSORPTION
1071
Table 1. Dependence of the pitting potential of copper (B)
on the concentration of inhibitor in neutral borate–chloride
solution.
Pitting potential (Ept) was determined from the
sharp increase in current on the polarization curves
with subsequent visual identification of pitting on the
electrode’s surface. The error in measuring Ept was
0.02 V.
cin,
FFN MEFN PHUD
SOL
SOS
mmol/L
Isotherms of inhibitor adsorption on passive cop-
per were obtained after reduction of the copper surface
at E = −0.60 V with subsequent oxidizing of the sur-
face for 30–40 min in a borate buffer at E = 0.0 V. The
measurements were performed using an ellipsometer
in a cell designed for simultaneous electrochemical
and ellipsometric studies. A helium–neon laser with
λ = 640 nm served as the light source; the angle of light
incidence was 68.5°. The consistency of ellipsometric
parameters after oxidation of the electrode allowed
adsorption measurements on the stable oxidized sur-
face of electrode. A concentrated solution of inhibitor
was introduced into the solution and the time varia-
tions of the ellipsometric angle were recorded. Steady-
state values of ∆ and ψ were used to construct the
dependence of the ellipsometric parameters on the
concentration of adsorbed reagent.
0.0
0.25
0.50
0.80
1.60
3.10
5.80
8.00
10.00
20.00
0.60
0.60
0.60
0.60
0.76
0.79
0.84
0.87
0.95
1.14
0.60
0.67
0.75
0.82
1.04
1.20
1.40
0.55
0.79
0.96
1.08
0.56
0.60
0.69
1.32
0.67
0.71
0.76
0.94
1.18
computer-controlled potentiostat on an M1 copper
electrode with a surface area of 0.75 cm2 in an electro-
chemical cell with widely spaced electrodes. The
working electrode was preliminarily polished and
degreased with acetone. Electrode potentials (E) were
measured with respect to a silver chloride electrode
and recalculated to the normal hydrogen scale. Plati-
num foil served as a counter electrode.
The ellipsometric technique of measuring adsorp-
tion is based on the Drude equation. According to the
Drude equation, variation in the angle of the ellipso-
metric phase shift (Δ) in the range of thinness (up to
10 nm) is proportional to the film thickness:
d = –αδΔ = –α(Δ – Δ0),
(1)
After reducing the oxide film by the cathodic
polarization at E = −0.60 V for 15 min, the potentio-
stat was switched off to establish the free corrosion
potential (Ecorr). A solution of sodium chloride (10
mmol/L) and inhibitor was then added. After estab-
lishing the new value of Ecorr, the anodic polarization
curves were recorded with a potential scanning rate of
0.2 mV/s.
where α is the coefficient of proportionality, Δ0 is the
initial angle characterizing a so-called “pure” surface,
and Δ is the current value of the angle. If angle Δ nar-
rows during the experiment (i.e., δΔ < 0), the film
thickness increases; when δΔ > 0, the film becomes
thinner. In the first case, such variations in angle Δ
indicate an increase in film thickness or the adsorption
of inhibitor; in the second, a reduction of film thick-
ness or the desorption of adsorbate.
i, μА/сm2
The adsorption isotherm was plotted using the
dependence of angle Δ on the concentration of inhib-
itor by assuming that the plateau of the plot corre-
sponded to surface filling θ = 1.0.
1
60
The corrosion experiments were conducted using
plates of М1 copper polished and degreased with ace-
tone. The protective inhibitor film was prepared via
exposure for 5 or 60 min at room or elevated tempera-
tures (60°С) in a borate buffer solution containing car-
40
3
6
2
4
1
5
2
20
0
boxylates or BTA with inhibitor concentration cin
=
2 mmol/L.
After treatment, the samples were dried and placed
in a sealed vessel in which hot water (50°С) was
poured over the bottom. The vessel was then closed.
Upon cooling, abundant water condensed on the ves-
sel walls and samples. Water was replenished and the
samples examined visually every day.
0.1
0.3
0.5
0.7
E, V
Fig. 1. Anodic polarization curves of copper in borate buf-
fer at рН 7.4 containing (1) 10 mmol/L NaCl and PHUD
(mmol/L): (2) 0.4, (3) 0.8, (4) 1.6, (5) 3.1, (6) 4.5.
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AGAFONKINA et al.
Table 2. Dependence of (
°
, kJ/mol) for oxidized copper in borate buffer at рН 7.40 and the logarithm of distribution
−ΔGa
coefficient logD on the nature of the inhibitor.
Inhibitor
SOS*
62
7.00
SOL*
57
7.70
PHUD
30
5.89
FFN
48
2.47
MEFN
43
2.18
BTA*
51
1.34
3
3
2
2
2
3
−ΔGa°
logP
logD
рКа
5.65
3.77
5.10
4.80
3.68
4.78
2.57
4.25
2.29
3.73
1.32
8.38
d, nm
l, nm
0.31 0.04
2.9
0.31 0.04
2.4
0.96 0.14
1.7–1.8
0.5 0.13
1.6
0.7 0.1
1.4
0.31 0.04
0.6
* Value (
) was estimated for the first monolayer of the inhibitor.
−ΔG
°
a
RESULTS AND DISCUSSION
cPHUD < 0.8 mmol/L, the active dissolution current
density of copper was reduced; e.g., adding
0.4 mmol/L PHUD suppressed it fourfold compared
to the background curve, but Ept was unaffected. The
The anodic polarization curves of copper with the
added carboxylates were similar to one another. An
area of active dissolution was observed at low values of
cin; at higher contents of inhibitor, it was followed by a
state of passivity that was disturbed by the chloride
ions in the solution upon a subsequent potential
sweep. Figures 1 and 2 show the typical anodic polar-
ization curves for copper in a borate buffer with pH 7.4
and containing such additives as 10 mmol/L NaCl,
FFN, or SOS. Table 1 gives the values of cin and Ept
that were determined from an analysis of the curves
recorded for all inhibitors. All anodic polariztion
curves of copper start from Ecorr = 0.17 0.03 V.
growth of Ept was observed only when cPHUD
≥
1.6 mmol/L, while at 5.8 mmol/L the value Ept
exceeded the potential of oxygen evolution. Table 1
shows that adding 1.6 mmol/L FFN (another carbox-
ylate) shifted Ept to 0.79 V, and at cFFN = 5.8 mmol/L,
Еpt = 1.08 V. Adding 1.6 mmol/L MEFN was less
effective than FFN: Еpt increased only by 0.07 V and,
even at 10 mmol/L MEFN, by 0.34 V.
Among the investigated sodium salts, SOS (Fig. 2)
and SOL (Table 1) demonstrated the best protective
properties. At cSOS = 0.08 mmol/L, the density of the
passivation current was reduced by a factor of 15 com-
pared to the background curve, and Еpt = 0.67 V.
The anode polarization curves for copper in solu-
tions that contained PHUD (Fig. 1) showed that there
was spontaneous passivation of the electrode at cPHUD
≥
0.8 mmol/L with positive variation of Ept (Fig. 1). At
When cSOS = 0.17 mmol/L, SOS was able to passiv-
ate copper even without an increase in Еpt. At cSOS
=
i, μА/сm2
1.6 mmol/L, Еpt rose to 1.04 V; i.e., SOS was consid-
erably more effective than the other carboxylates. At
low concentrations (cSOL < 1.0 mmol/L), SOL turned
out to be only slightly better than SOS in preventing
the local depassivation of copper, but it became more
effective as its concentration cin grew. Logically, the
great protective properties of SOS and SOL with
respect to copper are associated with their high
adsorption capabilities.
70
1
35
1
5
6
4
3
2
Ellipsometric measurements in fact revealed that
the adsorption of SOS anions on the oxidized copper
surface started in the area of low concentrations (cin ≥
0.02 nmol/L), while a poly-layered film started to
form after the formation of a monolayer (shown by the
dashed line in Fig. 3). This is not surprising, since SOS
is known as a colloid surfactant used in a wide range of
industrial applications because of its low toxicity [17].
2
0
0
0.2
0.4
0.6
0.8
1.0
E, V
Fig. 2. Anodic polarization curves of copper in a borate
buffer at рН 7.4 and containing (1) 10 mmol/L NaCl and
SOS (mmol/L): (2) 0.08, (3) 0.17, (4) 0.4, (5) 0.8, (6) 1.6.
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INHIBITOR PROPERTIES OF CARBOXYLATES AND THEIR ADSORPTION
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estimate lnВ, the resulting isotherm was reconstructed in
the coordinates X = θ, Y = ln[θ/(1 – θ)c]. The point
where the tangent intersects with the Y axis in the
medium range of coverage determined lnB. The resulting
−δΔ, deg
5
1.2
1
3
°
lnB was used to calculate
kJ/mol; as it
(−ΔGa ) = 62
turned out, this exceeded the value for BTA, one of the
best known inhibitors of corrosion for copper, by
11 kJ/mol (Table 2).
0.8
0.4
0
2
4
The adsorption of SOL on the oxidized copper
started at logc = –9.00, 1.5 orders of magnitude
greater than the value for SOS. Two SOL layers
formed in the range of logc = –(6.77–5.52) (Fig. 3).
The formation of multilayers is possible in the range of
high concentrations cSOL similar to what was observed
for iron in [18, 19]. This is clear from the kinetic of
variation of angle Δ for cSOL = 0.1 mmol/L (see Fig. 4).
12
10
8
6
4
2
−logc
Fig. 3. Change of the ellipsometric angle ∆ with concen-
tration of (1) SOS, (2) SOL, (3) MEFN, (4) FFN, and (5)
PHUD on a copper surface at Е = 0.0 V. The dashed line
on isotherm (1) shows an arbitrary monolayer.
A slowdown in the growth of (–δΔ) is observed on
its time dependence (the insert in Fig. 4); this can be
attributed to the formation of first monolayer. The
SOL multilayers grow with the time of exposure.
The first layer of such isotherm in the medium cov-
erage range can be described with an accuracy of 95%
by the Frumkin equation:
PHUD turned out to be the least effective of the
investigated inhibitors. This is consistent with the
weak adsorption of PHUD on oxidized copper
(Fig. 3). PHUD anions began to be adsorbed at
cPHUD = 20 μmol/L, three orders of magnitude
Bc = [θ/(1 – θ)]exp(–2aθ),
(2)
where a is a constant of attraction and B is a constant of higher than сFFN. Calculations of the adsorption
adsorption, associated with the free energy of adsorption
characteristics using Eq. (2) confirmed the lower
°
°
°
(
) by the relation B =
. To energy of PHUD adsorption
kJ/mol
(−ΔGa ) = 30
−ΔGa
[exp(−ΔGa /RT )]/55.5
−δΔ, deg
4
3
2
1
0.8
0.4
0
20
t, min
40
0
100
200
300
t, min
Fig. 4. Kinetics of ellipsometric angle variations ∆ on an oxidized copper surface (Е = 0.0 V) upon the addition of SOL (logc =
–4.00). The dashed line in the insert arbitrarily shows the first monolayer.
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 89 No. 6 2015

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AGAFONKINA et al.
logD = logP – log[1 + 10pH – pKa].
(3)
Table 3. Time before the occurrence of the first spot of cor-
rosion (in days) on the M1 plates with and without passiva-
tion in the solution of inhibitor (cin = 2.0 mmol/L) at differ-
ent temperatures in cells with 100% moisture and daily con-
densation of moisture on the samples.
Our calculations yielded the highest values logD =
5.65 and 5.10 (our results are given in Table 2) for SOL
and SOS salts, respectively. They therefore had the
highest surface activity and were able to reduce the
currents of the active dissolution of copper even at low
values of cin and stabilize the state of passivation.
The least hydrophobic compound, MEFN, is
capable of inhibiting copper in a borate–chloride
solution and preventing local depassivation at rather
high values of cMEFN: 10.0–20.0 mmol/L, which
exceed by 12 and 4 times those of hydrophobic SOL
and SOS compounds, respectively. However, PHUD
anions are highly hydrophobic and are adsorbed to a
lesser degree than the other carboxylate-type inhibi-
tors that were studied. Interactions with the copper
Inhibitor
I
II
III
Without inhibitor
SOS
1.2 0.3
21 0.5
17 0.5
10 0.5
23 0.5
18 0.5
22 0.5
13 0.5
18 0.5
SOL
7
0.5
PHUD
FFN
–
–
6
7
0.5
0.5
BTA
12 0.5
15 0.5
Symbols: I and II, exposed at room temperature for 5 and 60 min, surface are apparently due not only to the hydrophobic
respectively; III, exposed at 60°С for 5 min.
properties of a molecule but also to the presence of
heteroatoms (particularly nitrogen) in its composition
or multiple bonds that can form both additional inter-
°
relative to those of FFN
kJ/mol and
(−ΔGa ) = 48
molecular bonds between particles of an adsorbate
such as SOS and other bonds with the protected metal
surface.
°
MEFN
kJ/mol (Table 2).
(−ΔGa ) = 43
The variations in angle Δ during adsorption were
used to determine the thicknesses of the inhibitor
monolayers. At Е = 0.0 V, an oxide film with refrac-
tion index ≈ 2.2 0.2 [20] and proportionality coeffi-
cient from the Drude equation α = 0.8 0.1 nm/deg
formed on the copper surface. Comparing the thick-
nesses of conventional monolayers d and correspond-
ing molecular sizes l calculated on the basis of bond
lengths presented in Table 2, we may assume that the
adsorbed SOS and SOL molecules were arranged hor-
izontally (flat) on the copper surface. It should be
noted that both inhibitors were adsorbed on iron sur-
face [19] in the same range of concentrations and in a
vertical orientation. Interactions between the inhibi-
tors and iron surface occurred via the carboxyl groups
of the inhibitor molecules, while the hydrocarbon rad-
icals were oriented toward the solution. SOL and SOS
molecules contain alkyl, giving them hydrophobic fea-
tures and surface activity in water. However, SOS mol-
ecules include not only carboxyl groups but nitrogen
that can form coordination bonds with copper and are
probably responsible for the better surface adsorbabil-
ity of SOS molecules.
Analysis of the results from adsorption and polar-
ization measurements demonstrates the important
role of anion hydrophobicity in the ability to inhibit
copper dissolution. Hydrophobicity is estimated using
the logarithm of the distribution coefficient derived
from the data on the distribution of chemical com-
pounds in the octanol–water system [21, 22]. Hydro-
phobic properties can be estimated using logarithm
logР of the distribution coefficient of the investigated
compound in the octanol–water system of two immis-
Results obtained by other investigators [23–26]
favor this version. These investigations demonstrated
the impact of the tail groups of SOS molecules on
interactions with metal surfaces. For example, the role
of amide bonds in the self-organization of surfactants
on a gold surface was revealed in [23]. Microbalancing
was used to show that the ability to form hydrogen
bonds resulted in more closely-packed arrangements
on hydrophobic surfaces. Sarcosine derivatives form
chelate compounds with a metal surface that can
induce the polymolecular adsorption of SOS mole-
cules.
Microbalancing and surface plasmon resonance
were used in [24] to continue investigations of the
adsorption of surfactants—SOS and sodium salts of
other amino acids (N-lauryl sarcosinate, N-lauryl
aminomalonic, N-lauryl aspartate)—from an aqueous
solution on gold and quartz surfaces. All of these com-
pounds contain polar tail groups with carboxyls that
cling to gold surfaces and form monolayers.
Fourier spectroscopy, ellipsometry, and XPS were
used to show that such inhibitors as SOS act as com-
plexing reagents on steel surface [25]. Long hydropho-
bic fragments of molecules are oriented vertically to
the surface and keep the corrosive components of the
medium away from the metal. This was confirmed by
the experimental data in [26]. The adsorption of SOS
on steel proceeds via the formation of a complex of the
molecule’s amino acid fragments and the surface of
the metal. When this happens, the hydrocarbon frag-
ments of SOS molecules are oriented almost vertically.
Molecules of SOL and SOS inhibitors thus gener-
cible liquids [21], helping us calculate the characteris- ate poly-layered films on copper that are bound firmly
tic magnitude of anion hydrophobicity corrected for to the surface and can resist the effects of aggressive
acid dissociation [22]:
agents. It is important that SOS molecules are
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INHIBITOR PROPERTIES OF CARBOXYLATES AND THEIR ADSORPTION
1075
adsorbed on copper to a much greater extent than
°
kJ/mol, indicating that the chemisorp-
(−ΔGa ) = 62
those of BTA, the well-known inhibitor of corrosion
for this metal and its alloys that was investigated in
[20]. The data from Table 2 show that the values
tion of oleyl sarcosinate is possible.
While important, the hydrophobic properties of
carboxylates are not the only features responsible for
the stable adsorption bonds between an inhibitor and a
copper surface and their protection properties. The
composition and structure of the functional (“head”)
group in an inhibitor molecule are important for gen-
erating possible bonds with the protected surface. The
generation of a polymolecular inhibitor film and a
complex compound of inhibitor with metal cations
enhances the protection of copper. Corrosion testing
of copper passivated in an aqueous solution of carbox-
ylates at room temperature and 60°C demonstrated
the high efficiency of SOS inhibitor for protecting
copper in a moist atmosphere, compared to the other
investigated compounds. A rise in temperature length-
ens the time before the occurrence of first corrosion
spots even at relatively short times of passivation.
°
(
) for BTA are much less than those for SOS and
−ΔGa
SOL, but they are considerably higher than those for
MEFN and especially FFN. At the same time, BTA is
less hydrophobic than all of the other investigated
higher carboxylates. This confirms our belief that
while hydrophobic properties are important, they are
not the only features responsible for the stability of
adsorption bonds between an inhibitor and a copper
surface, or its protective properties. Since BTA can
generate poly-layered films on copper due to the
polymerization of its complex with copper cations [3,
20], the formation of poly-layered coatings can be
considered as a general and valuable feature of such
different (in terms of chemical structure) inhibitors as
SOS and BTA.
ACKNOWLEDGMENTS
CONCLUSIONS
This work was supported by the Russian Founda-
tion for Basic Research, project no. 13-03-00188.
Corrosion testing of copper plates treated with
diluted solutions of the investigated carboxylates at
room temperature revealed that SOS exceeded other
studied inhibitors in protecting copper from corrosion
in a moist atmosphere (Table 3). From the time to the
occurrence of the first corrosion spot in a moist atmo-
sphere, we may conclude that the efficiency of SOS
was 3–3.5 times higher than those of FFN or PHUD.
The protection of copper by SOS and SOL was likely
accompanied by self-organization in adsorption lay-
ers, since effectiveness grew substantially along with
the duration of the passivating treatment of copper. It
should be noted that the effectiveness of treating cop-
per with SOS and SOL exceeded that of BTA [20].
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Translated by E. Khozina
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Vol. 89
No. 6
2015