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RESEARCH PAPER
Release studies of corrosion inhibitors from ceriumtitanium oxide nanocontainers
Evaggelos D. Mekeridis • Ioannis A. Kartsonakis •
George S. Pappas • George C. Kordas
Received: 19 March 2010 / Accepted: 7 July 2010 / Published online: 23 July 2010
� Springer Science+Business Media B.V. 2010
Abstract Cerium titanium oxide nanocontainers
were synthesized through a two-step process and then
loaded with corrosion inhibitors 2-mercaptobenzothi-
azole (2-MB) and 8-hydroxyquinoline (8-HQ). First,
polystyrene nanospheres (PS) were produced using
polymerization in suspension. Second, the PS spheres
were coated via the sol–gel method to form a cerium
titanium oxide layer. Finally, the nanocontainers were
made by calcination of the coated PS nanospheres.
The size of the containers was 180 ± 10 nm as deter-
mined by Scanning Electron Microscopy (SEM).
X-Ray Diffraction Analysis (XRD) showed that
the nanocontainers consist of anatase and cerianite
crystalline phases. The presence and loading of the
inhibitors in the nanocontainers was confirmed with
Fourier Transform Infrared Spectroscopy (FT–IR) and
Thermo Gravimetric Analysis (TGA), respectively.
TGA revealed the amount of 10.43 and 4.61% w/w for
2-MB and 8-HQ in the nanocontainers, respectively.
Furthermore, the release kinetics of the inhibitors from
the nanocontainers was studied in corrosive environ-
ment using electrochemical impedance spectroscopy
(EIS) in the presence of aluminum alloys 2024-T3
(AA2024-T3).
Keywords Nanocontainers � Corrosion �Inhibitors � Synthesis � Drug delivery
Introduction
Nanosized materials have been a subject of intensive
investigations in variety of topics from optics and
electronics to biotechnology and medicine (Zheng
et al. 2006; Ding et al. 2006; Pappas et al. 2008).
Materials such as nanoparticles, nanospheres, and
micelles can be used as drug delivery and drug-
controlled release systems. Hollow nanocontainers
are of great interest because of their ability to
encapsulate substances in their hollow inner cavities
and release them at a later stage (Hu et al. 2005).
Recently, it has been recognized that the nanocon-
tainers loaded with corrosion inhibitors when incor-
porated into coatings provide additional protection of
metal alloys, such as AA2024-T3, from corrosion.
AA2024-T3 is mainly used in aeronautical applica-
tions. The methods of delivery of the inhibitors to the
metal surface can influence the efficiency of the
inhibiting action (Lamaka et al. 2007). According to
Raps et al. (2009) and Khramov et al. (2004) the
addition of corrosion inhibitors in sol–gel coatings in
one hand can improve corrosion protection, on the
other hand may deteriorate the barrier properties of the
film. Encapsulating corrosion inhibitors in nanocon-
tainers and then added them to the protective coating
system is an advantageous method to unite the barrier
properties of the coatings with the active action
E. D. Mekeridis � I. A. Kartsonakis �G. S. Pappas � G. C. Kordas (&)
Sol-Gel Laboratory, Institute of Materials Science, NCSR
‘‘DEMOKRITOS’’, 153 10 Agia Paraskevi Attikis,
Greece
e-mail: [email protected]
123
J Nanopart Res (2011) 13:541–554
DOI 10.1007/s11051-010-0044-x
of the corrosion inhibitors (Kartsonakis et al. 2007;
Zheludkevich et al. 2005a, 2007a).
Kartsonakis et al. synthesized hollow titania spheres
using cationic polystyrene lattices which were pre-
pared by polymerization in suspension of styrene using
2.20-azobis (2-methylpropionamidine) dihydrochlo-
ride (AMPA) as an initiator (Kartsonakis et al. 2008).
Zirconia and silica nanoparticles were used as reser-
voirs for the storage and prolonged release of corrosion
inhibitors in silica and silica–zirconia based coatings
by Zheludkevich et al. (2005a, 2007b) and found that
the nanoparticles reinforced the coating and released
inhibitors during contact with moisture. Shchukin et al.
presented halloysite nanotubes with inner voids loaded
by 2-MB and outer surfaces layer-by-layer covered
with polyelectrolyte multilayers as a mean to opti-
mize hybrid sol–gel films (Shchukin et al. 2008).
Cerium molybdate nanocontainers were synthesized
and loaded with corrosion inhibitors (8-HQ and 1-H-
benzosulfonic acid) by Kartsonakis and Kordas (2009)
Some of the most effective and environmental
friendly corrosion inhibitors for aluminum alloys are
derived from cerium salts. Nanostructured sol–gel
coatings doped with cerium ions were investigated as
pretreatments for AA2024-T3 (Zheludkevich et al.
2005b). Titania-containing organic–inorganic hybrid
sol–gel films have been developed by Poznyak et al.
as an alternative to chromate-based coatings for
corrosion protection of aluminum alloys (Poznyak
et al. 2008).
8-HQ and 2-MB compounds were studied as
corrosion inhibitors by Lamaka et al. (2007) for
AA2024-T3. They found that these inhibitors provide
anticorrosion protection for AA2024-T3 forming a thin
organic layer of insoluble complexes on the surface of
the alloy. Inhibiting action is the consequence of
suppression of dissolution of Mg, Al, and Cu from the
corrosion active intermetallic zones (Lamaka et al.
2007; Sanyal 1981; Zheludkevich et al. 2007c).
Yasakau et al. examined the addition of 8-HQ at
different stages of the synthesis process to understand
the role of possible interaction of the inhibitor with the
components of the sol–gel system (Yasakau et al.
2008). 2-MB was evaluated by Zheludkevich et al.
(2005c) as corrosion inhibitor for protection of
AA2024-T3 in neutral chloride solutions.
Otsuka-Yao-Matsuo et al. (2004) studied the
photocatalytic behavior of CeTiO4 and CeTi2O6
powders. Keomany et al. prepared thin films of
(CeO2)x–(TiO2)1-x by a sol–gel process involving
two alcoxides (Ce(OBuS)4 and Ti(OBun)4 in BuOH)
were studied by cyclic voltammetry in a lithium-
conducting polymer electrolyte in order to examine
the influence of the structure on the electrochemical
insertion in such films, which are suitable counter
electrode materials for lithium-based electrochromic
windows (Keomany 1995).
In the present study, cerium titanium oxide hollow
nanospheres were synthesized and characterized by
SEM, XRD, TGA, and FT-IR. After that, these
nanocontainers were loaded with corrosion inhibitors
2-MB and 8-HQ to produce an inhibitor delivery
system. Studies were made on the % w/w loading of
the inhibitors by heat treatments and FT-IR spectros-
copy. The release of the inhibitors in a corrosive
environment was tested via EIS. The results suggest
the use of these loaded nanocontainers into coatings
on metal alloys for corrosion protection of metals
used for automobiles, ships, and airplanes.
Experimental
Materials and reagents
All chemicals were of analytical reagent grade.
Titanium tetraisopropoxide (TTIP, Aldrich), polyvi-
nylpyrrolidone (PVP, average molecular weight:
55,000, Aldrich), cerium (III) acetylacetonate (Ce
(acac)3, Aldrich), sodium chloride (NaCl, Aldrich),
sodium hydroxide (Aldrich), 2,20-Azobis (2-methyl-
propionamidine) dihydrochloride (AMPA, Aldrich),
and absolute ethanol (Aldrich), were used without
further purification. Styrene (Aldrich) was double
distilled under reduced pressure prior to use.
Preparation of nanocontainers
Cerium titanium oxide hollow nanospheres were
synthesized through a three-step process. The first
step involves the preparation of positive charged
polystyrene nanospheres. Styrene was polymerized by
polymerization in suspension according to the condi-
tions shown in Table 1. The polymerization process is
described in our previous study (Kartsonakis et al.
2008). In order to eliminate the result of oxygen effect,
the reactions were made in nitrogen atmosphere.
542 J Nanopart Res (2011) 13:541–554
123
Polymerization lasted 12 h. The solution was centri-
fuged and the precipitate was washed with distilled
water. During the second step, the PS nanospheres
were coated via sol–gel method. Sol–gel coating were
prepared with controlled hydrolysis of the alcoholic
solution of TTIP jai Ce(acac)3 in the presence of PS
nanospheres, NaCl, and PVP (Table 2). PVP and NaCl
were added to the mixture reaction to prevent aggre-
gation of the core particles. The positive charged
polystyrene reacts with the negative charged product
of the hydrolysis of TTIP and Ce(acac)3. Monomers or
oligomers of hydrolyzed TTIP and Ce(acac)3 are
condensed on the surface of the polystyrene. Aging of
the solutions at 60 �C, centrifugation, and washing of
the coated nanospheres were followed. The formation
of hollow nanospheres was achieved after heat treat-
ments of the composites at 600 �C with heating rate
10 �C min-1, where the polystyrene cores were
burned off (Kartsonakis et al. 2007, 2008).
Encapsulation and release of inhibitors
The obtained cerium titanium nanocontainers were
loaded with the corrosion inhibitors 8-HQ and 2-MB.
The loading procedure included first the preparation
of a saturated solution of the inhibitor in acetone.
After that, an amount of cerium titanium oxide
nanocontainers was placed in a sealed container. The
nanocontainers were placed in a vacuum system to
draw out the air inside them. Then, the saturated
solution of the inhibitor in acetone was inserted in the
sealed container and the whole mixture was stirred at
room temperature for 12 h. Finally, the cerium
titanium oxide nanocontainers loaded with the inhib-
itor were collected through centrifugation and were
dried under vacuum overnight.
The release of 8-HQ and 2-MB from nanocon-
tainers was studied via EIS. A typical three electrode
cell was used in a Faraday cage. For this purpose,
solutions of 0.01, 0.05, and 0.1% w/v concentration
of nanocontainers loaded with inhibitors in a corro-
sive environment (0.05 M NaCl) were prepared.
Furthermore, solutions of pure inhibitors 8-HQ and
2-MB with the same concentration were also studied
for comparison reasons. Panels of AA2024-T3 were
used as the working electrode, a platinum sheet as the
counter while a saturated calomel electrode (SCE)
served as reference electrode. The panels had been
previously cleaned, under specific conditions. The
AA2024-T3 panel cleaning includes the insertion of it
into 2% w/w solution of NaOH for 3 min at 40 �C.
After that, the panel is rinsed with distilled water and
is inserted into 4.33 M solution of HNO3 for 1 min at
room temperature. Finally, it is rinsed with distilled
water. EIS measurements were taken after 3, 6, 24,
48, 72 h of exposure in 0.05 M NaCl solution. The
exposed geometric area was 2 cm2 for all the
experiments. All the samples were in vertical
position; the experiments were carried out at room
temperature. For every result, a minimum of three
repetition measurements were taken.
Instrumentation
The average nanocontainer size and the morphology of
the substrate after 72 h of exposition in NaCl, were
determined by SEM using a PHILIPS Quanta Inspect
(FEI Company) microscope with W (tungsten) filament
25 kV equipped with EDAX GENESIS (AMETEX
PROCESS & ANALYTICAL INSTRUMENTS). The
phase of the nanocontainers was examined by XRD
using a powder diffractometer (SIEMENS D-500
equipped with a Cu Ka lamp with wavelength
1.5418 A). Temperature treatments such as Thermo
Gravimetric Analysis (TGA) were made using a Perkin
Elmer (Pyris Diamand S II) analyzer at the heating rate
Table 1 The conditions used in the preparation of polystyrene
latex at 80 �C
Material Quantity (g)
Styrene 9.06
AMPA 1.3
Water 900
Nanospheres’ size (nm) 195 ± 10a
a Determined by scanning electron microscopy analysis
Table 2 Conditions of preparation of coated spheres
Material Quantity (g)
Ethanol (ml) 800
PVP (g) 8.0
NaCl 5 mM (ml) 20
Polystyrene (g) *9
TTIP (ml) 9.0
Nanospheres’ size (nm) 215 ± 10a
a Determined by scanning electron microscopy analysis
J Nanopart Res (2011) 13:541–554 543
123
of 10 �C min-1 in air. Fourier Transform Infrared
Spectroscopy (FT-IR) was made using a BRUKER
EQUINOX 55-S spectrometer. Nitrogen adsorption
experiments and pore size measurements were per-
formed using a volumetric static sorption apparatus
(Autosorb-1 MP, Quantachrome Instruments). The
release of inhibitors from nanocontainers was studied
via Impedance analyzer (Solartron Sl 1260 Impedance/
gain-phase analyzer) connected to a Solartron PGstat
(Solartron Sl 1470 Electrochemical interface).
Results and discussion
Scanning electron microscopy analysis
As shown in Fig. 1, the polymerization process leads
to polystyrene nanospheres with uniform size with an
average diameter of 195 ± 10 nm. Figure 2 shows
that after calcinations, the cerium titanium oxide
hollow nanospheres exhibit an average diameter of
180 ± 10 nm. The EDX analysis shows that tita-
nium, cerium, and oxygen constitute the spectrum of
the spheres. Gold appears due to the gold coating that
was applied to the spheres in order to be conductive
for the SEM analysis (Fig. 3).
FT-infrared spectroscopy analysis
Figure 4 shows the FT-IR spectrum of the nano-
spheres, before and after calcination. This spectrum
verifies the formation of inorganic shells and the
complete removal of the organic components. The
FT-IR spectrum in Fig. 4a of the nanospheres prior to
calcinations reveals well-defined bands of the phenyl
group (703, 750, 1445, 1494, and 3022 cm-1) in
polystyrene. The peak at 2,919 cm-1 is due to the
CH2 group. The peaks at 1590, 1150, and 1163 cm-1
are the band characteristic of PVP, indicating that
PVP has not been well removed during the experi-
mental process. It can be seen that the characteristic
peaks of polystyrene are missing from the spectrum
after calcination.
The spectra in Fig. 4b, is the one after calcination;
the band characteristic of the polystyrene latex have
been disappeared, indicating that polystyrene latex
has been well removed from the core/shell composite
particles by calcination at 600 �C.
Both FT-IR spectra for as-prepared and after
calcination samples show absorption peaks at the wave
number region between 400 and 1,000 cm-1. This
region contains bands typical of metal oxygen bonding.
The absorption peaks of TiO2 are at 470, 525, 540, 579,
690, 700, 790 cm-1 (Kartsonakis et al. 2008;
Zheludkevich et al. 2005c; Keomany 1995; Verma
et al. 2004; Mc Devitt and Baun 1964). For CeO2, the
characteristic peaks are at 425 cm-1, 525 cm-1,
540 cm-1 (Kartsonakis et al. 2007; Zheludkevich
et al. 2005c; Keomany 1995; Verma et al. 2004). It is
mentioned that the FTIR spectra after calcinationsFig. 1 SEM images of polystyrene nanospheres
Fig. 2 SEM images of cerium titanium oxide hollow
nanospheres
544 J Nanopart Res (2011) 13:541–554
123
depict broad band at the region between 400 and
1,000 cm-1 and the above peaks can be distinguished.
The broad bands in the range of 3,200–3,500 cm-1
and at 1,652 cm-1 correspond to stretching vibration
of O–H bond of the physically adsorbed water in the
sample (Verma et al. 2004).
Thermogravimetric and differential thermal
analysis
Figure 5 shows the TGA–DTA diagrams of cerium
titanium oxide nanocontainers. The first weight loss is
observed in the range of 30–150 oC which can be
attributed to desorption of physically adsorbed water
(free and physisorbed water) (Kartsonakis et al. 2007).
The second weight loss in the range of 150–230 oC can
be attributed to the chemisorbed water; the monolayer
of H2O molecules which directly interact with the
solid surface such as cerium and titanium cations and
hydroxyls and to the dehydroxylation (release of OH
from the structure) (Kartsonakis et al. 2007; Kartso-
nakis and Kordas 2009). It is observed from the TGA
diagram that polystyrene is burned off between 290
and 400 �C (the third sharp weight loss). The fourth
weight loss between 400 and 450 �C is attributed to
the burn off of polyvinylpyrrolidone (Jablonski et al.
2008). Hence, calcination at 600 �C in air removed the
polystyrene core particles completely.
DTA diagram shows an exothermic peak between
260 and 340 �C. This peak is due to the condensation
of hydroxyl groups. The exothermic peak between
365 and 420 �C is due to crystallization of amorphous
cerium and titanium oxides into crystalline (Raps
et al. 2009). This was confirmed by the XRD pattern
shown in Fig. 6 of the sample treated at 600 �C.
The sudden decrease of temperature at 400 �C is
due to the accuracy of the instrument and depends on
the heating rate (10 �C min-1) and the organic
content (PS) of the sample which are both very high.
X-ray diffraction analysis
Crystalline phases were identified according to the
JCPDS (Joint Committee on Powder Diffraction
Standards) file numbers 21-1271 and 43-1002 for
Fig. 3 EDX analysis of
cerium titanium oxide
hollow nanospheres
Fig. 4 FT-IR spectra of: a cerium titanium oxide nanospheres
(before calcination), b Cerium Titanium oxide hollow nano-
spheres (after calcination)
J Nanopart Res (2011) 13:541–554 545
123
anatase and cerianite, respectively. Figure 6 shows
the XRD pattern of the sample after calcination. The
peaks at 2h = 25.5� (101), 38.1� (004), 48.2� (200),
55.15� (105), 55.05� (211), and 62.9� (204) represent
to tetragonal anatase. According to the JCPDS
Library, the above peaks are slightly moved to higher
values of 2h. The presence of cerium identified from
the main peak at 2h = 28.6� (111), which is charac-
teristic of cerianite, causes stress to the crystal
structure of anatase leading to the increase of peak
positions that mentioned above (Verma et al. 2007).
Porosity measurements
The samples of the empty nanocontainers were
degassed at 300 �C for 18 h before the measurement.
The specific area was calculated with the B.E.T
method in the range of relative pressure 0.05–0.3 P/P0
and was found to be 129 m2 g-1. The pore size
distribution was calculated through the B.J.H method
at desorption isotherm and the mean pore radius
found to be 1.6 nm with a pore volume 0.503 cc g-1.
The hysteresis through desorption (Fig. 8) is charac-
teristic for curves of type IV (IUPAC). This fact
indicates the presence of mesopores in the sample.
The observation of steps at the adsorption isotherm
clearly denotes the presence of different size of pores
in the sample. This result is both presented in the pore
size distribution diagram and in the axis of relative
pressure (through hysteresis that come near to 0.2)
(Figs. 7, 8).
The loaded nanocontainers with 8-HQ and 2-MB
were degassed at 25 �C for 18 h. The measurement of
the 2-MB-filled sample showed some interesting
results compared to the hollow nanospheres. The total
amount of the adsorbate is less in the filled sample. The
specific area decreased significantly from 129 m2 g-1
for the hollow nanospheres to 17 m2 g-1 for the
nanospheres filled with 2-MB. Also the pore size
distribution, calculated from desorption branch
through B.J.H method, showed a decrease in the pore
volume from 0.503 to 0.301 cc g-1 indicating the
filling of the pores and the hollow structure with the
inhibitor. The pore size distribution of the filled sample
did not show any pores with radius below 4 nm. The
measurement of the sample filled with 8-HQ was
impossible, due to evaporation of the inhibitor under
the preparation conditions (high vacuum, leak test of
the instrument).
Encapsulation and release of inhibitors
TGA diagrams of pure 8-HQ, 2-MB, and cerium
titanium oxide nanocontainers loaded with 8-HQ or
2-MB are shown in Fig. 9. Pure 8-HQ began to
degrade at 120 �C until 212 �C where no residue left.
The diagram of cerium titanium nanocontainers
loaded with 8-HQ shows a first weigh loss between
30 and 130 �C corresponding to acetone and physi-
cally adsorbed water (free and physisorbed water)
(Takeuchi et al. 2005), a second weight loss between
130 and 170 �C due to 8-HQ that is on the surface of
the nanocontainers and finally a third and forth weight
loss from 170 to 850 �C correspond to oxidative
degradation of encapsulated 8-HQ. Pure 2-MB is
completely burned off between 180 and 330 �C. The
diagram of loaded nanoconainers with 2-MB depicts a
sharp weight loss from 200 to 330 �C corresponding to
Fig. 5 TGA and DTA curves of cerium titanium oxide hollow
nanospheres
Fig. 6 XRD pattern of cerium titanium oxide hollow
nanospheres
546 J Nanopart Res (2011) 13:541–554
123
the oxidative degradation of the inhibitor that is on the
surface of the nanocontainers, a second weight loss
between 330 and 850 �C due to the oxidative degra-
dation of the inhibitor that are enclosed into the
nanocontainers.
Comparing the TGA diagrams, it is observed that
pure 8-HQ is burned off at higher temperatures in the
samples of cerium titanium nanocontainers loaded
with this inhibitor. This retardation (roughly 200 �C
higher than pure inhibitor) is attributed to the
protection provided by the shell of nanocontainers.
This result indicates that 8-HQ is encapsulated into
nanocontainers. The same analysis can be made for
the nanocontainers loaded with 2-MB. The burn off
2-MB between 330 and 850 �C corresponds to
inhibitor that is inside the shell of the nanocontainers.
The weight losses observed by the TGA measure-
ments were used to determine the amount of inhibitors
loaded into the nanocontainers. First, we consider the
sample of nanocontainers loaded with 8-HQ. We take
the weight of the sample at 170 �C (G1) and at 850 �C
(G2), we then subtract G2 from G1 (DG = G1 - G2)
and last we divide DG by G1 to obtain the ratio mass
loss, rm = DG*100/G1 due to 8-HQ. In the case of
Ce–Ti nanocontainers loaded with 8-HQ, rm is about
4.37% w/w. In the case of Ce–Ti nanocontainers
loaded with 2-MB, we take G1 at 330 �C because the
2-MB burns off at this temperature. Above this
temperature, the weight loss corresponds to mass
encapsulated in the nanocontainer. G2 is taken at
900 �C. We estimate rm about 25.36% w/w for the
Ce–Ti nanocontainers loaded with 2-MB.
The release of the inhibitors was studied via EIS. If
the inhibitors are released from the nanocontainers,
they should provide corrosion protection of the
AA2024-T3 panels. This can be observed by EIS.
The first experiment was accomplished using pure
inhibitors in the solution (e.g. 2-MB 0.1% w/v in a salt
solution) in order to clarify the extend of corrosion
protection attributed to the inhibitors (Figs. 11, 13).
The second experiment was carried out using filled
nanocontainers with inhibitors (8-HQ Fig. 11, 2-MB
Fig. 13) in order to observe the effect of protection due
to the inhibitor release from the nanocontainers. The
third experiment was done using different amounts of
loaded nanocontainers in the solution (e.g. 0.01, 0.05,
and 0.1% w/v) shown in Figs. 10 and 12 for 8-HQ and
2-MB, respectively. One can perceive an influence of
the concentration on the corrosion protection attrib-
uted to the release of the inhibitors from the
nanocontainers. The best results against corrosion
were obtained for 0.1% w/v, and correspond to the
Fig. 7 BJH pore
distribution of cerium
titanium oxide hollow
nanospheres
Fig. 8 Isotherms of cerium titanium oxide hollow nanospheres
and loaded nanospheres with 2-MB
J Nanopart Res (2011) 13:541–554 547
123
protection provided by the same concentration of pure
inhibitor (0.1% w/v of 8-HQ and 2-MB). According to
TGA measurements, the concentration of 2-MB in
0.1% w/v nanocontainers is 86.1 mM and the con-
centration of 8-HQ is 17.2 mM. The impedance at low
frequencies corresponds to the polarization resistance
of the AA2024-T3 electrode and, therefore, can be
used to estimate the corrosion protection (Lamaka
et al. 2007). In the low frequency region, it can be seen
that the total value of impedance is about one order of
magnitude higher for the specimens immersed in the
NaCl solution containing the nanocontainers loaded
with inhibitors (Figs. 10, 11, 12, 13). It can be clearly
seen that both chemical compounds worked as
Fig. 9 TGA curves of:
pure 8-HQ, pure 2-MB, and
cerium titanium oxide
nanocontainers loaded with
8-HQ and 2-MB
Fig. 10 Bode diagrams of AA2024 after 72 h of immersion in
0.05 M NaCl with 0.01, 0.05, 0.1% w/v of nanocontainers
loaded with 8-HQ and without nanocontainers
Fig. 11 Bode diagrams of AA2024 after 72 h of immersion in
0.05 M NaCl with a 0.1% w/v of nanocontainers loaded with
8-HQ, b pure inhibitor 8-HQ, and c bare AA2024
548 J Nanopart Res (2011) 13:541–554
123
corrosion inhibitors comparing to the solution without
loaded nanocontainers. Two time constants revealed at
5 and 0.01 Hz for the solution with nanocontainers.
The higher frequency time constant can be assigned to
the capacitance of the double layer on the surface of
the alloy. The low frequency time constant is related
to a diffusion limitation of the corrosion process
(Lamaka et al. 2007).
Instead of capacitances, Constant Phase Elements
(CPE) were used in all fittings procedures because the
phase angle of the capacitor is different from -90�.
The impedance of the CPE depends on frequency
according to the following equation (Kartsonakis
et al. 2007; Zheludkevich et al. 2005b). The imped-
ance of the CPE depends on frequency according to
the following equation
1
Z¼ QðjxÞn ð1Þ
where Z is the impedance, Q a parameter equals to (1/
|Z|) at x = 1 rad s-1, x is the frequency and n B 1 a
power coefficient calculated as ratio of phase angle at
maximum of corresponding time constant to -90�.
The capacitance of the inhibitor is calculated by the
following equation:
Cinh ¼ QinhðxmaxÞðninh�1Þ ð2Þ
xmax is the frequency at which the imaginary
impedance reaches a maximum for the respective
time constant.
Rsol is the resistance of the solution, Rox is the
resistance of the native oxide layer, Rinh is resistance
of the inhibitor layer and Rpol is the polarization
resistance. Cox and nox are the parameters of constant
phase element (CPE) describing the capacitance of
the oxide layer, Cinh and ninh are the parameters of the
CPE describing the capacitance of the inhibitor layer.
Cdl and ndl are the parameters of CPE which
characterize the capacitance of the double layer
capacitance (Lamaka et al. 2007).
For the system with the nanocontainers, two time
constants are observed at high frequencies which
can be attributed to the native aluminum oxide layer
(at about 1 Hz) and to the layer of adsorbed
inhibitor (at about 50 Hz). In the case of 2-MB,
the time constant attributed to the presence of the
adsorbed layer is better observed at 72 h in NaCl
with 0.05 and 0.1% w/v nanocontainers. In the case
of 8-HQ, a wide time constant is observed consisted
of the two phases that are mentioned above.
Figure 14 shows the equivalent circuit used to fit
the experimental data. Tables 3 and 4 summarize
the parameters obtained after fitting.
Figures 15, 16, and 17 present the evolution of the
capacitance of the inhibitor layer as a function of the
time for different nanocontainer concentrations. One
Fig. 13 Bode diagrams of AA2024 after 72 h of immersion in
0.05 M NaCl with a 0.1% w/v of nanocontainers loaded with
2-MB, b pure inhibitor 2-MB, and c bare AA2024
Fig. 12 Bode diagrams of AA2024 after 72 h of immersion in
0.05 M NaCl with 0.01, 0.05, 0.1% w/v of nanocontainers
loaded with 2-MB and without nanocontainers
J Nanopart Res (2011) 13:541–554 549
123
can perceive a dependence of the capacitance from
the thickness of the layer that is formed from the
dielectric constant and the resistance of the inhibitor
after exposure of AA2024-T3 in 0.05 M NaCl
solution with nanocontainers (Lamaka et al. 2007).
It is observed that each inhibitor affects the time
evolution and the extend of the capacitance in a
different fashion. 2-MB loaded nanocontainers exhibit
much higher values than 8-HQ loaded nanocontainers
(Figs. 15, 16, 17). Furthermore, the parameters of
capacitance and resistance of 8-HQ do not change
significantly as the time goes by compared to 2-MB.
This may be attributed to the fast formation of a stable
layer of the inhibitor on the aluminum surface. A
prolonged decrease of the capacitance is observed for
the solution with 0.1% w/w concentration of nano-
containers which can be attributed to the increase of
thickness of the inhibitor layer. Both inhibitors form a
dense layer on the surface of the aluminum alloy
2024-T3.
Figures 15, 16, and 17 present the direct effect of
the concentration of the 8-HQ-loaded nanocontainers
to the resistance of the aluminum oxide layer, Rox.
Low values of Rox are obtained for concentration
0.01% w/w. The values of Rox at 0.1% w/w are one
order of magnitude higher than for concentration of
0.01% w/w. In the case of 2-MB loaded nanocon-
tainers, these shifts are also of an order of magnitude
with much higher absolute values, though. One can
highlight at this position, that the curves of the theta
versus frequency match for the inhibitor solution and
loaded nanocontainer solution. This observation is a
strong support of the assumption of the release of the
inhibitors from the nanocontainers. This result dem-
onstrates that the inhibitors are not trapped perma-
nently in the nanocontainers.
SEM images and optical observation
Figures 18 and 19 show an optical visualization of
the protection of the AA2024-T3 surface provided
by the loaded nanocontainers after exposure for 72 h
in 0.05 M NaCl solution. Figure 18 demonstrates
the degradation of the bare sample exposed in the
corrosive environment without the presence of
nanocontainers. One can perceive a fully corroded
sample with many pits. On the other hand, the
addition of the nanocontainers has decreased the
number of the pits on the surface of AA2024-T3.
This result is in agreement with the respective EIS
measurements.
As it was mentioned, the action of the nanocon-
tainers is based on the formation of chelate com-
plexes on the aluminum surface that are difficult
to be dissolved. This can be proved by the EIS
measurements with the appearance of two time
Fig. 14 Equivalent circuit used for fitting experimental EIS
spectra
Table 3 Calculated values for EIS data obtained at different
immersion times for NaCl solution with 0.01, 0.05 and 0.1% w/v
Ce–Ti oxide nanocontainers loaded with 2-MB
Parameter 0.01% 0.05% 0.1%
Rsol (X cm2) 124 132 130
Rinh (X cm2) 3,106 1,541 1,677
Qinh (S cm-2) 1.01E-4 5.84E-5 7.25E-5
Ninh 0.760 0.810 0.820
Cinh (F cm-2) 6.49E-5 3.61E-5 4.23E-5
Rox (X cm2) 1,310 5,612 8,307
Rpol (X cm2) 1,747 3,866 8,322
Table 4 Calculated values for EIS data obtained at different
immersion times for NaCl solution with 0.01, 0.05 and 0.1% w/v
Ce–Ti oxide nanocontainers loaded with 8-HQ
Parameter 0.01% 0.05% 0.1%
Rsol (X cm2) 141.4 145.8 131.4
Rinh (X cm2) 552.6 305.7 69.16
Qinh (S cm-2) 5.53E-4 5.67E-4 9.84E-5
Ninh 0.790 0.720 0.780
Cinh (F cm-2) 3.75E-4 2.45E-5 7.64E-5
Rox (X cm2) 554.8 351 1796
Rpol (X cm2) 1,015 1,414 8,204
550 J Nanopart Res (2011) 13:541–554
123
Fig. 15 Evolution of capacitance and resistance of inhibitor film after exposure for 72 h in 0.05 M NaCl solution containing Ce–Ti
nanocontainers loaded with 2-MB
Fig. 16 Evolution of resistance of aluminum oxide layer after exposure for 72 h in 0.05 M NaCl solution containing Ce–Ti
nanocontainers loaded with: 2-MB, 8-HQ
Fig. 17 Evolution of capacitance and resistance of inhibitor film after exposure for 72 h in 0.05 M NaCl solution containing Ce–Ti
nanocontainers loaded with 8-HQ
J Nanopart Res (2011) 13:541–554 551
123
constants for the solutions with the nanocontainers
assigned to the aluminum oxide layer and to the
adsorbed layer of the corrosion inhibitor or to the
products of the interaction of Al3?, Mg2?, or Cu2?
that are formed through the initial stages of
corrosion. The layer that is formed by the chelate
complexes blocks the penetration of chloride ions to
the native oxide layer leaving it intact (Lamaka
et al. 2007; Yasakau et al. 2008).
The formation of the chelate complexes on the
surface of aluminum and to the active regions of
2024-T3 alloy stops the evolution of corrosion and is
the cause of locally active protection Fig. 19. The
totally corroded AA2024-T3 is presented in Fig. 20. On
the other hand, the formation of the inhibitor layer can
be observed using SEM as shown in Fig. 21 and Fig. 22.Fig. 18 Visual photograph of AA2024-T3 sample after
exposure for 72 h in 0.05 M NaCl
Fig. 19 Visual
photographs of AA2024-T3
panel after exposure for
72 h in 0.05 M NaCl with
a 0.01%, b 0.05%, c 0.1%
w/v of nanocontainers
loaded with 8-HQ and
d 0.01%, e 0.05%, f 0.1%
w/v of nanocontainers
loaded with 2-MB
Fig. 20 SEM images of
AA2024-T3 panel after
exposure for 72 h in 0.05 M
NaCl solution without
nanocontainers
a magnification 1,000,
b magnification 50,000 and
c EDX analysis
552 J Nanopart Res (2011) 13:541–554
123
Conclusion
Cerium titanium hollow nanocontainers were
synthesized. Their size was characterized by SEM
measurements and was 170 ± 10 nm. XRD analysis
showed that the nanospheres consist of anatase and
cerianite crystalline phases. Thermal treatments with
TGA and DTA proved that hollow nanospheres are
produced due to burn off of polystyrene cores. The
synthesized nanocontainers have different size of
pores. Moreover, these nanocontainers were loaded
with the corrosion inhibitors 8-HQ and 2-MB.
Thermal treatments with TGA and DTA proved
that nanocontainers were loaded with 4.37% w/w of
8-HQ and 25.36% w/w of 2-MB, respectively. Fur-
thermore, the introduction of nanocontainers loaded
with inhibitor to a corrosive environment to which an
AA2024-T3 is exposed; shows that as the concen-
tration of loaded nanocontainers is increased, a more
effective protective layer is formed on the surface of
the metal alloy, through the release of the inhibitor
from the nanocontainers. These nanocontainers can
be used in a vast field of implementations such as
additives in corrosion protective coatings.
Acknowledgments This project was supported by FP7
Collaborative Project ‘‘MUST’’. The abbreviation ‘‘MUST’’
stands for ‘‘Multi-Level Protection of Materials for Vehicles by
‘‘SMART’’ Nanocontainers’’ (EC Grant Agreement Number
NMP3-LA-2008-214261).
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