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A COMPARATIVE RESEARCH ON HYBRID AND HYBRID NANO-COMPOSITE PROTECTIVE PRIMARY COATINGS FOR AA2024 AIRCRAFT ALLOY

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Comparison between the barrier ability, durability of hybrid and hybrid nano-composite coatings was performed by Electrochemical Impedance Spectroscopy and subsequent data analysis via an appropriate model equivalent circuit. For completeness, the thicknesses and the superficial morphologies of both coatings were also characterized. The results undoubtedly reveal that the latter coating is twice better than the former, because of the presence of Al 2 O 3 nano-particles. They extend the durability of the hybrid nano-composite coating by formation of a reinforcing phase for the hybrid matrix, and carriers of CeCl 3 as corrosion inhibitor.
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A.A. Salve, S. Kozhukharov, J.E. Pernas, E. Matter, M. Machkova
319
Journal of the University of Chemical Technology and Metallurgy, 47, 3, 2012, 319-326
A COMPARATIVE RESEARCH ON HYBRID AND HYBRID NANO- COMPOSITE
PROTECTIVE PRIMARY COATINGS FOR AA2024 AIRCRAFT ALLOY
A.A. Salve1, S. Kozhukharov2, J.E. Pernas1, E. Matter3, M. Machkova2
1 University of Vigo, Lagoas, Marcosende,
36310 Vigo, Spain
2 University of Chemical Technology and Metallurgy
8 Kl. Ohridski, 1756 Sofia, Bulgaria
3 Department of Chemistry, Faculty of Science, Damanhour
University, 22111 Damanhour, Egypt
E-mail: stephko1980@abv.bg
ABSTRACT
Comparison between the barrier ability, durability of hybrid and hybrid nano-composite coatings was performed by
Electrochemical Impedance Spectroscopy and subsequent data analysis via an appropriate model equivalent circuit. For
completeness, the thicknesses and the superficial morphologies of both coatings were also characterized. The results
undoubtedly reveal that the latter coating is twice better than the former, because of the presence of Al2O3 nano-particles.
They extend the durability of the hybrid nano-composite coating by formation of a reinforcing phase for the hybrid matrix,
and carriers of CeCl3 as corrosion inhibitor.
Keywords: AA2024  aircraft alloy, corrosion protection, EIS, AFM.
Received 05 April 2012
Accepted 12 June 2012
INTRODUCTION
The corrosion as a phenomenon could be
considered as destruction of the solid materials, as a
consequence of their interactions with the surrounding
environment. It could appear in various detrimental
effects, depending on the nature of the corrosion
processes. The brief classification of these processes is
described by Davis [1].
The most widely used approach for protection of
the materials from aggressive chemical compounds in
their environment is the deposition of protective paints
and coatings. Rosero-Navarro [2] mentions that the typical
protection systems involve various coating layers,
including sol-gel or porous anodic films, organic primer
coatings, and top-coats, capable to provide specific passive
and active corrosion protection. The sol-gel approach
for preparing of oxide protective coatings has emerged as
a versatile method [3]. Its application for effective
corrosion protection of aluminium alloys is proposed by
Hamdy and Butt [4]. Furthermore, this approach enables
the synthesis of hybrid materials by combination of organic
and inorganic ingredients, as described elsewhere [5, 6].
These materials form an entire intermediate class [7]
between the organic and inorganic compounds. The ben-
efits of the application of the hybrid materials as primer
coatings have been emphasized by Zheludkevich [8].
Briefly, these benefits are summarized in Fig. 1.
The properties of the hybrid materials could be
additionally improved by involvement of finely dispersed
powders, as described in [8, 9]. The powder material
serves as a reinforcing phase for the hybrid matrix.
Additionally, if the powder material is impregnated with
a corrosion inhibitor, then the former could supply
gradual inhibitor release at the locations of the coating
is damage, as is proposed by Lamaka et al. [10]. Besides
the basic requirements to form dense adherent films,
with appropriate thermo-mechanical properties, the
advanced coating systems should possess also a self-
healing ability. It can be obtained by an ability for self-
reparation of the coating film, or by introduction of
corrosion inhibitors. The former approach can be
achieved by enhancement of the intermolecular
interactions among the ingredients, as described in [11],
or by involvement of capsules with a polymerizing liquid
[12]. The latter method of impregnation with corrosion
Journal of the University of Chemical Technology and Metallurgy, 47, 3, 2012
320
inhibitors in nano-particles is described in [10, 13]. The
present trend for development of advanced coating sys-
tems has motivated the purpose of our research.
The aim of the present work is to compare the
introduction of CeCl3 as inhibitor against AA2024 cor-
rosion in sol-gel derived coatings, either directly, or after
preliminary impregnation of alumina nano-particles.
EXPERIMENTAL
Sample preparation
The samples  object of research in the present
study, were in the form of plates of AA2024 aircraft
alloy, coated by CeCl3 containing hybrid and hybrid,
nano-composite coatings, delivered by the Leibnitz
Institute for New Materials  S==>Hueken (Germany).
They can be described, as follows:
(i)- Sample 1 - a hybrid nanocomposite coating
with addition of 4 % mass of CeCl3, incorporated in 8
% of Al2O3  nanoparticles; thickness 13.99 µm.
(ii) - Sample 2 - a hybrid coating with 4 % mass
addition of CeCl3; thickness 23.6 µm.
The preparation and the subsequent deposition of
the coatings, and the preliminary superficial treatment of
the metallic substrates are described in [3, 14-18], and the
respective procedures are the object of patents [19].
Sample characterization
It was performed by Electrochemical Impedance
Spectroscopy (EIS), and the acquired spectra were
analyzed by further fitting to appropriate model
equivalent circuits. The EIS spectra were recorded with
a Frequency Response Analyser FRA-2, produced by
Eco Chemie, the Netherlands. The EIS was applied
through excitation signals in the frequency range: 105
down to 2 x 10-3 Hz, distributed in 7 steps per decade.
Their amplitude was 50 mV, generally. In some cases,
100 mV were applied, because the insulating-proper-
ties of the coatings disturbed the acquisition of a read-
able spectrum. Circuit zones with 0.64 cm2 of the samples
were exposed to 0.05 M aqueous solutions of NaCl and
served both as model corrosive media and electrolytes,
during the EIS - spectra acquisitions. They were recorded
in respect to Ag/AgCl  3MKCl reference electrodes,
and a platinum counter electrode.
The thickness and roughness of the coatings was
also evaluated. The coating thickness was determined
as an average value of 10 point measurement by
Positector 6000, product of DeFlesko, USA. The
superficial roughness was determined by Atomic Force
Microscope EasyScan-2, Nanosurf, Switzerland. It was
supported by a TAP 190 cantilever, manufactured by
BUDGETSENSORS, Bulgaria.
The AFM observations were performed in square
areas with 49,5 µm linear size, at static regime, forward
mode, with resolution 256 points per line.
REDSULTS AND DISCUSSION
Durability of the coatings
In principle, this measure deals with the ageing
processes of the constructive materials during their
exploitation. In the present work, the durability is
considered to be the period between the first moment
of exposition to the model corrosive medium and the
coating failure. When a corrosion process appears under
the coating, the EIS-spectrum changes entirely its shape.
According to Zheludkevich [13, 20] the spectra of the
coated samples before and after their failure, should be
fitted using different equivalent circuits, described by
the author in the respective publications. The changes
of the spectra, initiated by the coating failure are dis-
cussed in detail below. The durability of the hybrid
nanocomposite coating was registered to be higher than
that of the hybrid one. The former indicated failure after
125 days (3000 hours), while for the latter; it was de-
tected only after 63 days (1512 hours). Additionally, the
spectra of the respective specimens evolve by a different
manner. The spectra of the hybrid composite coating re-
veal a gradual deterioration, whereas those of the other
coating remain almost the same until its failure, as can
be seen in Fig. 2.
EIS  modeling, Equivalent circuits
For the needs of the present work, the obtained
electrochemical impedance spectra were analyzed using
Fig. 1. Schematic presentation of the combination of the
properties rendered by the organic and the inorganic
materials to the resulting hybrid material.
A.A. Salve, S. Kozhukharov, J.E. Pernas, E. Matter, M. Machkova
321
imaginary equivalent circuits, which correspond to the
behavior of the respective real electrochemical systems.
The behavior of the coated systems during their expo-
sure into the corrosive medium was analyzed by appli-
cation of the equivalent circuit depicted in Fig. 3.
The circuit differs from the usually used circuits
for description of coatings. This circuit is more relevant
to the real systems than the circuits composed by three
time constants. According to [8, 21] the hybrid matrix
forms covalent bonds to the surface native oxide layer
of the Al substrate. Taking into account the presence of
these bonds, it was assumed that the hybrid coating and
the oxide layer were indistinguishable, and any clearly
formed interface between them was not represented. This
consideration is supplementaly confirmed by the model
of Bierwagen [22] regarding the conjunction between
the oxide layer and the hybrid primer coating.
In the above equivalent circuit (Fig. 3), two time
constants are used. The first- one is related to the resis-
tance of the electrolyte in the pores and defects R(coat+oxy),
represented in both of the coating and the oxide layer
and the capacitance of the respective layers by them-
selves Q(coat+oxy). The other one, composed by Rct and
Qedl, is related to the capacitance of the electric double
layer and the resistance of the charge transfer reactions
composing the corrosion processes. These reactions pass
between the components of the metal surface and these
species of the electrolyte, which overcome the potential
barrier of the double layer. Before the breakdowns of the
coatings all of the impedance spectra represented only
one depressed semi-circle in Nyquist plots, and only one
large pick in the corresponding Bode plots. It was ac-
cepted that they were composed by two overlapped time
constants. The overlapping is possible only when both
Fig. 2. Bode (a, c) and Nyquist (b, d) plots of hybrid nanocomposite coating (a, b), and hybrid coating (c, d).
Journal of the University of Chemical Technology and Metallurgy, 47, 3, 2012
322
capacitive elements, C(coat+oxy) and Cedl, have values of the
same order of magnitude.
In both equivalent circuits the pure capacity, is
substituted by Constant Phase Element (Q) because the
phase shift (f) is lower than - 90°. For all of the impedance
spectra the phase shift at 105 Hz is superior to -90°. This
fact was rather a consequence of the cells construction,
because the walls of the counter electrode (CE), were per-
pendicular to the plane of the working electrodes (WE).
An other specific feature of this equivalent cir-
cuit is the modeling of the diffusion process by a Con-
stant Phase Element (Qw), instead of an Warburg ele-
ment (W). This substitution is reasonable, because the
diffusion tails in the Nyquist plots possess very sharp
slopes, approaching almost vertical lines. After
prolonged exposure, these tails convert to new time-
constants. Their appearance is an evidence for the failure
of the protective ability of the corresponding coating.
Both these features of the diffusion elements have
predetermined the substitution of W by Qw. Besides,
the conversion of the diffusion tails to new time constants
in the Nyquist plots, and sharp changes in the Bode
plots also appear due to the coating failure. When the
respective coating is already broken, new maxima appear
in the curves of the phase angle (ö  log(f)), in the 1 
10 Hz range. In the corresponding log[Z] - log(f) curves,
additional inflexions arise, as well. These changes of the
spectra can be seen in positions a, and c of Fig. 2. There
the spectra are obtained after the coating failure,
respectively: at 5064 hours the hybrid nanocomposite
coating, and the hybrid one at 1704, 2496, 2016 h, both
have the typical features for a broken coating.
Data fitting to the equivalent circuit
All EIS  spectra for both coatings underwent
fitting to the equivalent circuit depicted in Fig. 3. This
procedure enables data extraction from the spectra for
the capacitance and resistance of each component of
the electrolyte /coating oxide layer / metal system.
Fig. 4 presents the evolution of the resistances R(coat +
oxy) and Rct, acquired by the fitting procedure.
Generally, during their exposure to the corro-
sive medium, the coatings should reveal gradual decrease
of their resistances, because of the gradual penetration
of electrolyte through them. Surprisingly, the analyses
of the EIS  spectra, reveal that until the failure of the
coatings, they maintain relatively high values of their
resistances. Here, it should be mentioned that the sol-
gel derived hybrid coatings obtained by various metallic
alkoxides, are also abundant in hydroxyl moieties. They
are a product of hydrolysis/polymerization processes,
composing the sol-gel synthesis, as described in detail
by Dislish [25]. Despite the finishing annealing
procedures, the obtained by the sol-gel route coatings
possess highly developed aptitude for water uptake,
when they are in contact with aqueous electrolytes, as
described in [23]. Furthermore, Bierwagen [24]
proposes the presence of hydrolysable Metal-(OH)x
bonds in the hybrid matrix, as well. These concepts
presuppose a swelling ability of the hybrid coatings, as
the result of water uptake. Consequently, the swelling
of the hybrid matrixes disturbs the ion mobility inside
the coatings, resulting in maintenance of high R(c oat + oxy )
values. Another reason for this peculiar behaviour
could be the presence of CeCl3 as an active component
of the coatings. This compound is described as a
cathodic inhibitor which forms barrier precipitates of
Ce-oxides/hydroxides, either with OH- anions [26], or
with H2O2 [27] as a product of the cathodic reaction of
the oxygen reduction. Inside the hybrid coating, this
compound could additionally react with the OH-
moieties of the hybrid coating, triggered by electrolyte
penetration. The presence of hydroxyl anions in the
bulk of the coating causes swelling of the hybrid matrix
and the precipitation of the derivative Ce-hydroxide.
Both these phenomena undoubtedly lead to obstruction
of the pathways of electrolyte penetration and
consequently  maintenance of high R(coat + oxy) values.
Additionally, the comparison between positions (a and
b) of Fig. 3 reveals much higher dissipation of the
resistance values for the nano-composite coating. This
fact could be explained taking into account the work of
Rosero-Navarro et al. [9]. They have obtained a hybrid
nano-composite primer coating by polymerization of a
hybrid matrix in a colloidal liquid medium, with dis-
persed SiO2. This technology is actually identical to
Fig. 3. Equivalent circuit used for EIS-data fitting.
A.A. Salve, S. Kozhukharov, J.E. Pernas, E. Matter, M. Machkova
323
the applied for the hybrid nanocomposite coating in
[15-19], and the present work. This method of synthe-
sis is possible, because the nanoparticles are dispersed
in a colloidal system during the sol-gel synthesis. In
these conditions, the nanoparticles should be sur-
rounded by OH- adsorption layers, in order to avoid
whatever coagulation. Consequently, the resulting hy-
brid nano-composite coating is much more abundant
in OH- moieties. Nevertheless, even at strong stirring
and addition of detergents, the agglomeration of the
nano-particles is unavoidable. Additionally, in the
present case, the pores of the nano-particles, together
with the cavities in their agglomerates are filled by
solid CeCl3. When electrolyte penetrates inside the
coating, it dissolves the cerium salt, which reacts with
the OH- ions, present in the coating. In contrast, a part
of the dissolved CeCl3 releases the occupied space,
enabling easier access of the electrolyte towards the metal
surface. As a result, the hybrid nano-composite coating
possesses lower stability (i.e: higher rate of dissipations) of
the R(coat+oxy) values. These processes are related to the
capacitances of the coatings, as well. Fig. 5 illustrates the
evolution of the Q-values of the coatings within the
exposure time, obtained by fitting of the EIS spectra by the
equivalent circuit (Fig. 3).
The capacitances of the coatings evolve by the same
manner, as their resistances. They maintain relatively low
values, corresponding to high capacitive resistances. Un-
til the failure of the coatings, in both cases, the capaci-
tance Q(coat + oxy) remains about 0.50 up to 0.95 nF/cm2,
whereas the values of Qedl are with one order of magni-
tude higher. The combination of high resistances and low
capacitances demonstrate almost excellent barrier abili-
ties for both coatings. The comparison between the posi-
tions a and b of Fig. 5 reveals higher dissipation of
the values for the hybrid nano-composite coating, like
for the corresponding resistances, shown in Fig. 4.
These dissipations reveal that the equilibrium
between the penetration of the electrolyte and the re-
spective obstructions is more dynamic, compared to
those of the hybrid coating. It could be assumed that
the agglomerates promote easier access of the electrolyte
to the metallic surface. However, even insignificant
quantities of corrosion products could cause a remark-
able contribution by supplemental obstruction to the
electrolyte penetration. The supplemental obstruction,
caused by AlnCl3n-m (OH)m polyaluminum complexes is
described in detail in previous works [17]. In this ar-
ticle was mentioned that the aluminum hydroxyl-chlo-
rides form Keggin-like structures, which contributed for
additional retarding of the electrolyte penetration. The
values of the exponential argument n for Q(ct) for both
of the coating shift down to 0.8, even to 0.7, approach-
ing the n values for Qw. They convert the meaning of
the respective Constant Phase Elements from pure ca-
pacitances to diffusion expressing elements. Neverthe-
less, the n-values are too high and do not permit the
substitution of the respective CPE, by Warburg diffu-
sion expressing element (W). It was established that the
Q elements could be substituted by W elements only
in the case of n equal to 0.5. This fact confirms that
the diffusive penetration of the electrolyte through the
coatings is strongly suppressed by the obstruction phe-
nomena, related to: (i) - swelling of the hybrid matrix,
(ii) precipitation of derivative Ce hydroxides, as
consequence of the interaction between dissolved CeCl3
and OH- -moieties of the coating; (iii)  precipitation of
Ce-oxides/hydroxides due to interactions with corro-
sion products of the oxygen reduction, as described by
Yasakau, (iv)  obstruction, caused by AlnCl3n-m(OH)m
Fig. 4. Evolution of the resistances R(coat + oxy) and Rct of the samples: a – for hybrid nano-composite coating; b – for hybrid
coating.
Journal of the University of Chemical Technology and Metallurgy, 47, 3, 2012
324
species. All of these factors contribute simultaneously
for the maintenance of high barrier abilities of both
coatings in relatively extended periods of exposure to
aggressive corrosive medium.
Influence of the thickness and roughness of the coat-
ings over their capacitances
The results shown in Fig 5 reveal that the
capacitances Q(coating + oxy) and Qct possess rather similar
values for both coatings, despite the differences in their
thicknesses, compositions and structures. The weak
dependence of the capacitance (which is almost equal
for both of the coatings), and the thickness (which is
twice higher for the hybrid coating) is explained in [18].
Obviously, the capacitances of the coatings are related
to their roughness, as well. Fig. 6 represents the AFM
superficial images for both coatings.
Both surfaces look similar to each other. It seems
that no excessive agglomeration among the nanoparticles
has appeared. The quantitative assessment of the
roughness has revealed that the average roughness of
the hybrid nano-composite coating (Sa = 48.07 nm) is
twice higher than this of the hybrid one (Sa = 28.19
nm). Nevertheless, the coatings are generally smooth
enough, and their roughness does not influence
considerably their capacitances.
Principally, the higher roughness of the coatings
provides larger specific surface, predetermining higher
access of aggressive species from the electrolyte to the
coating. Consequently, the remarkable smoothness of
both coatings renders additional contribution for the
remarkable durability of both coatings. Furthermore,
the insignificant difference between their roughnesses
is confirmed by the almost equal values of the respec-
tive Qedl  capacitance as shown in Fig. 5.
From the analyses in the present research work
it can be concluded that both of coatings possess remark-
able corrosion protective capability, as a combination of
significant barrier protective ability and extended dura-
bility. Surprisingly, the barrier properties, expressed in
high resistance to the electrolyte which is subsequently
entrapped in the cavities of the coatings, and the electric
capacitances of the coatings are almost equal despite the
remarkable differences in their thicknesses. Additionally,
the durability of the nanocomposite coating is twice higher
(3000 hours) than of the other one (1512 hours), regard-
less of the fact that the former is twice thinner. Conse-
quently, the Al2O3 nano-particles have significant con-
tribution to the coating strength, acting as a reinforcing
phase in its structure.
Furthermore, the direct addition of CeCl3 during
the synthesis of the hybrid coating could possess a serious
detrimental effect, which could be revealed in different
aspects: (i) It could disturb the cross-linking processes
during the hydrolysis/polymerization processes in the
sol-gel system. Undoubtedly, the Ce-ions from the
dissolved CeCl3 could terminate the polymerization
processes, hindering the gel-formation of the system.
(ii)  The abundance of OH- ions as a product of the
hydrolysis/polymerization processes during the synthesis
leads to inactivation of the CeCl3 and even posterior
agglomeration of Ce-hydroxides in the hybrid matrix.
Both of these undesirable processes are removed
by introduction of CeCl3 after its preliminary encapsulation
inside Al2O3-nanoparticles. As a matter of fact, these
particles reveal a significant effect as a reinforcing phase
for the hybrid matrix, and as carriers of the corrosion
inhibitor. This technological approach leads to efficient
preservation of the cerium compound by its prevention
from undesirable reactions during the sol-gel synthesis.
Fig. 5. Evolution of the capacitances Q(coat + oxy) and Qct of the samples: a – for hybrid nano-composite coating; b – for hybrid coating.
A.A. Salve, S. Kozhukharov, J.E. Pernas, E. Matter, M. Machkova
325
As a result, during the exposition of the respec-
tive coating to the aggressive NaCl containing aqueous
solution, the active CeCl3 prevents the hydrolysis of the
hybrid matrix, reacting with the aggressive species (i.e:
dissolved O2, OH-, and Cl- ions) from the liquid me-
dium. Additionally, the insoluble Ce-oxides/hydroxides
cause supplemental obstruction for further electrolytes
uptake. By that manner, the preliminary encapsulated
active CeCl3 efficiently contributes for the remarkable
extension of the durability of the respective coating. The
Al2O3 nano-particles do not cause almost any change of
the barrier abilities of the coating, and its electrochemical
parameters (R(coat + oxy), Rct, Q(coat + oxy) and Q(edl) ) are al-
most equal to these of the hybrid coating.
CONCLUSIONS
The characterization of the coatings and the
subsequent analyses of the data lead to the following
conclusions:
The introduction of CeCl3 after its preliminary
encapsulation in Al2O3 nano-particles, promotes
remarkable extension of the durability of the respective
coating, preventing any undesirable processes during the
sol-gel synthesis.
Al2O3 nano-particles enlarge the durability of the
respective coating, serving as a reinforcing phase for
the hybrid matrix, and as carriers of intact CeCl3, but
they do not affect the barrier abilities of the respective
coating.
The electrochemical properties of the coatings
do not suffer any significant changes during the expo-
sure of the respective samples to the corrosive medium.
This fact is a consequence of the obstruction of the elec-
trolyte penetration, due to: (i) swelling of the hybrid
matrix, (ii) precipitation of derivative Ce-hydroxides ei-
ther inside the coating, or as a result of corrosion inhi-
bition effect, (iii) formation of Keggin-like structures,
by the corrosion products.
Both the hybrid and the hybrid nanocomposite
coatings possess similar electrochemical characteristics,
despite the differences in their thicknesses. The second
coating shown twice higher durability, regardless of its
lower thickness.
Acknowledgements
The authors thank to the EC ERASMUS office
and the Bulgarian NSF, for contract DVU-02/102,
particularly to Gustavo Pelaez. The work team of Leibnitz
Institute for New Materials (Germany) is also highly
appreciated.
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... BDD has a number of advantages over conventional electrodes made from silver, platinum or glassy carbon [15]. It has a wide potential window, from À1 to +1.8 V, allowing detection of redox species that would normally fall outside the operating range of conventional electrodes, including heavy metals (mercury [60], cadmium, lead, nickel [61], arsenic [62]), polycyclic aromatics (PCAs) and pesticides [63], hormones and estrogenic compounds, explosives [64], neurotransmitters such as dopamine [65], drugs (paracetamol, cyanides, narcotics, pharmaceuticals) [66], and water-soluble nerve agents [64]. Within this operating window, the response is flat, so there is no background, making the BDD electrodes highly sensitive. ...
... Following the rule of thumb regarding the simplification of the MEC mentioned elsewhere [62,63], it was composed by only one modified time constant (RC unit), associated with the solution resistance (R sl ). The MEC configuration used for the present research is a simplified version of that applied in previous works [4-6, 57, 59]. ...
Chapter
One of the most important tendencies for the development of modern chemistry is related to the new trends in biomedicine and pharmacy. Today, significant advances have been made in the development of biodegradable and biocompatible polymer materials and the synthesis of corresponding polymer nano-micelles. In this work we aimed to synthesize new biodegradable and biocompatible block copolymers based on poly(lactide-co-glycolide) (PLGA) and poly (ethylene glycol) (PEG) with linear (ABA-type) architecture, to investigate their ability to form nanosized micelles and its aptitude to acts as carriers of combretastatin-like antitumor agents. The synthesis of linear PLGA1000-PEG1000-PLGA1000 block copolymer was carried out in line with standard procedures. The size and morphology of the obtained micelles were determined by transmission electron microscopy (TEM) and dynamic light scattering (DLS). The capability of obtained micelles to adsorb and deliver combretastatin-like antitumor agents into living cells was also demonstrated. We showed that the obtained copolymers formed nanosized micelles with the combretastatin-like antitumor agent 16Z. In vitro biocompatibility results denote that all tested blank-nano-micelles are devoid of cytotoxic effects and may be used as non-toxic drug carriers to target cells. Cellular uptake of 16Z-loaded PLGA1000-PEG1000-PLGA1000 micelles by HepG2 cells showed continuous uptake up to 72 h and a delay in the cytotoxic effect of 16Z. In an attempt to design new biomimetic analogs of natural combretastatin A-4 (CA-4) and its synthetic amino-derivatives, we selected benzothiazolone heterocycles as a scaffold for a bioisosteric replacement. In silico, drug-likeness and toxicity predictions showed better properties of benzothioazolone CA-4 analogs. The obtained results are very promising and need future detailed investigations.
... Many studies have been carried out to demonstrate the influence of nanoparticles on corrosion resistance of coatings on steel [13][14][15][16][17][18][19] or light metals [20][21][22]. There are many examples in the literature of the use of a range of the incorporation of fillers to improve coating properties. ...
Article
Full-text available
Although corrosion resistance and mechanical properties of sol-gel coatings have been studied independently, there are limited studies that consider both collectively. However, since any form of mechanical damage could impair the protective function of the coating, it is prudent to consider the mechanical durability of coatings as well as their corrosion resistance. The present work considers the impact of silica nanoparticles on the morphology and mechanical properties of a sol-gel derived coating. The relationships between the results obtained from tests such as atomic force microscopy (AFM), nanoindentation or erosion test with previously reported corrosion results obtained via salt spray or electrochemical impedance spectroscopy (EIS) are discussed. Results show that reinforcing a sol-gel coating with silica nanoparticles and, particularly, functionalised silica nanoparticles led to coatings with improved mechanical properties and enhanced erosion impact resistance. The role of nanoparticles on improving mechanical properties and corrosion resistance, which is of importance within the coating industry, is discussed.
Book
Book description This handbook is about theory, phenomenon and state-of the-art research results in corrosion science, written by accomplished, internationally revered scientists and engineers on the fields. The numerous illustrations are related to subtle ideas. Furthermore, experimental data are based on original research results. Figures summarise delicate achievements besides well-elaborated examples and case studies. All the representations serve clear and rapid understanding. Discussed material originates partially from international literature of leading groups and advanced proprietary results of the authors. This handbook is intended for college and university students striving for B.Sc. and M.Sc. degrees. Furthermore, the one is also targeted who plans to pursue further training or post-gradual study and perform high-quality research. This compilation with seven chapters is devoted to young scientists with an interest in making the first move to corrosion science and to acquaint with many of basics and necessary terms. There are many examples of remarkable findings and recognised standard practices followed by engineers in the related industry. Three main topics are covered in this material such as aluminium and titanium alloys, thin films and composite coatings as well as microbially influenced corrosion. If anyone finds hard to differentiate multiple phenomenon, to understand the general terms, to define reasons for using specific investigation methods and struggles with comprehension of this very multidisciplinary science, then this publication is unanimously a good material to help them. Readers, who are relatively new to corrosion and protection, are anticipated to gain aspiration for further study and to make endeavour in this field, too. When chapters are perused by beginners, then overview on basics and clear insight to precious details will develop in readers’ mind. After careful reading this compilation, the outcome delineates as rapid recognition of many of the corrosion phenomenon in the everyday and professional life, identifying their root-causes, proposing investigation methods to obtain further appropriate knowledge in many aspects and the ability to recommend remedy measures to forgoing integrity failure. The editor, who worked on various fields of chemistry and authored highly interested, one of the most read and downloaded papers in the subjects of corrosion protection with graphene films and graphene based composite coatings as review papers, as well as zinc-rich paint coatings containing alumina-carrier based core-shell type nano-sized particles combined with either physically modified or chemically functionalised multi-walled carbon nanotubes based on his own research results, wishes all the book readers instructive, useful and enjoyable times whilst reading the chapters.
Chapter
This chapter describes the basic concepts related to the application of cerium compounds as a main alternative to the already restricted approaches of the use of toxic and environmentally unacceptable compounds. The chapter begins with a brief description of the importance of aluminium alloys for the aircraft industry and the basic corrosion forms and damages typical for these alloys. Besides the indispensability of the coating procedures for providing long-term corrosion protection, the basic multilayered coatings systems are also discussed. Following this, the basic stages and factors of deposition of cerium conversion coatings (CeCCs) as primer coating layers are described. Subsequently, various methods that involve cerium compounds as active components in upper and finishing coating layers are proposed based on the literature analysis. Furthermore, some alternatives of the cerium compounds as environmentally friendly active coating ingredients, such as organic corrosion inhibitors, are also proposed. The chapter finishes with the recent and the most actual directions for further improvement of coatings. Finally, the development of dense, self-healing, sun-light-protected, hydrophobic coatings are described.
Article
Full-text available
The electrochemical behavior of sol-gel prepared, hybrid nanocomposite coatings was investigated via electro-chemical methods. It was accomplished in respect of proving their corrosion protective capabilities. Amorphous ZrO 2 (size 5nm) or crystalline CeO 2 (size 10nm) nanoparticles were embedded into the sol-gel matrices with solid content from 8wt% to 20wt%. The sol-gel coatings were deposited on aluminum alloy AA2024, used as a substrate. In addition, selected sol-gel-coated samples were covered by two extra layers by an industrial painting system. The measured thicknesses of the prepared single sol-gel coatings and the top-coated samples varied from 13 to 19 µm, and from 40 to 78 µm, respectively. The corrosion protection performance of the samples obtained was studied by means of electrochemical measurements, including voltammetry and impedance spectroscopy.
Article
Full-text available
One of the most important applications of sol-gel technology is the fabrication of coatings. This is because of the possibility of applying oxide coatings with practically all types of chemical compositions at low ambient temperatures on many substrates of various shapes through the use of liquid solutions. Both oxides and different types of organic-inorganic hybrid coatings have been reported. Both oxides and hybrid coatings are usually amorphous at ambient temperatures but some oxides can be converted to the crystalline phase with heating. Regardless of the intended applications of the coatings their physical properties are always of importance. For instance, an anti-reflective coating for an automobile mirror is of little practical value unless it is fairly scratch-resistant. In this review which covers published information in the past fifteen years, some of the more important results of physical properties of sol-gel derived coatings are discussed firstly for oxides and then for organic-inorganic hybrids. It appears that properties such as the hardness of oxide coatings are inadequate unless the heat-treatment temperatures are in excess of about 400°C. The hybrid coatings, especially when they contain a dispersed phase of a hard solid like colloidal silica, can be processed at temperatures below about 150°C and can improve the performance of organic plastics such as the polycarbonates. There is insufficient scientific understanding of the relationship between physical properties and other interdependent variables such as processing conditions, chemistry and coating thickness. More research in this area will undoubtedly contribute to the availability of better and new coatings via the sol-gel approach.
Article
Hybrid polymers (ORMOCER®s, Ceramers) are composites with inorganic and organic nanobuilding blocks linked via stable covalent bonds based on organically modified silicon alkoxides and/or functionalized organic oligomers/polymers. A method often used is the formation of the inorganic network by polycondensation reactions (e.g. Si-O-Si bond formation) in a first step, followed by the organic crosslinking (thermal curing at 80-180 °C or UV-processing). Another type uses the silylation of organic polymers, with subsequent hydrolysis and polycondensation reactions of the silanized units. The paper focuses on the selection of precursors for various applications and the chemistry involved in processing. Basic properties and the application potential of ORMOCER®s as coatings, fibers and composites are presented. Property/composition relationships are shown for mechanical and permeation/barrier properties.
Article
The corrosion protective ability of hybrid oxy silane nano-composite coatings deposited on AA2024 by sol–gel technique was studied. The coatings are developed as an environmentally friendly alternative of the toxic chromium containing coatings on aluminium. A cerium salt, Ce2(SO4)3, was used as inhibitor of the corrosion process. Two methods were applied to introduce the salt in the hybrid matrix: directly in the matrix, or by porous Al2O3 nano-particles preliminary loaded by the salt. Atomic force microscopy (AFM) was used to evaluate the superficial morphology of the coatings, while their layer structure was studied by means of scanning electron microscopy (SEM). Linear voltammetry (LVA) and electrochemical impedance spectroscopy (EIS) were used for assessment of the barrier ability. The hybrid matrix was found to possess remarkable barrier ability which was preserved even after prolonged exposure of the coatings to a model corrosive medium of 0.05 M NaCl. In all cases, the cerium salt involved either directly or by Al2O3 nano-particles proved to deteriorate the protective properties of the coatings and to accelerate pitting nucleation. The experimental results have shown that cerium sulphate, introduced in the by the both manners in the hybrid matrix did not efficiently inhibit the corrosion of AA2024, unlike the reported inhibiting properties of other cerium salts.
Article
Sol–gel derived films exhibit a high potential as substitutes for the environmentally unfriendly chromate metal-surface pre-treatment methods. Inorganic sol–gel derived films offer good adhesion between metals and organic paint. However, they cannot provide adequate corrosion protection due to their high crack-forming potential. Introduction of an organic component to an inorganic sol–gel system leads to the formation of thicker, more flexible and functionalized films with enhanced compatibility to different organic top coatings. Incorporation of nanoparticles in the hybrid sol–gel systems increases the corrosion protection properties due to lower porosity and lower cracking potential along with enhancement of the mechanical properties. Furthermore, the incorporation of inorganic nanoparticles can be a way to insert corrosion inhibitors, preparing inhibitor nanoreservoirs for “self-repairing” pre-treatments with controlled release properties.
Article
The protective ability of hybrid nano-composite oxysilane coatings, deposited via sol–gel method on AA2024-T3 – aluminium alloy, were studied by linear voltammetry (LVA) and electrochemical impedance spectroscopy (EIS) methods in 0.05M solution of NaCl. Cerium chloride (CeCl3) was incorporated as an inhibitor into a sol–gel hybrid matrix in two different routes: directly and via filled porous Al2O3 nano-particle aggregates with diameters up to 500nm. The influences of the inhibitor concentration, as well as the influence of nano-particles on the barrier properties and the susceptibility against corrosion, were evaluated and EIS spectra were fitted by appropriated equivalent circuits. The values for Ccoat, Rcoat, Coxy and Roxy were achieved and their evolution over time was investigated. The investigated coatings possess highly expressed barrier properties (106 to 107Ωcm2). Despite of the chloride ions inside of the matrix, some samples illustrated a significant durability of over 4000h during exposure to the corrosion medium before first signs of corrosion appeared. The electrochemical results were compared with the neutral salt spray test. Thus, it was proved that the potential of these coatings is to be used as anticorrosive protective materials and are candidate to replace Cr(VI)-based anti-corrosion coatings.
Article
Functional, nanocomposite, sol-gel based hybrid coatings were the object of a study in respect to their corrosion-protective capabilities of aluminum alloy (AA2024) and steel (DC01). Commercially available coating system consisting of primer and top coat has been applied on the sol-gel coatings as an additional protection. Amorphous ZrO 2 (5nm) and crystalline CeO 2 (10nm) nanoparticles were embedded in the sol-gel matrices with solid contents between 8wt% to 20wt%. The thicknesses of the prepared three single sol-gel coatings and the four top-coated coating systems varied between 13 to 19 µm and 40 to 78 µm, respectively. The coating systems were determined by means of electrochemical, hit and vibration tests as well as under corrosive conditions in a salt-spray chamber. Selected mechanical properties have been investigated, as well. The nano-hybrid coatings obtained showed good mechanical and corrosion protective capabilities in the cases of presence as well as in of absence of an industrial topcoat system.
Article
Active corrosion protection of AA2024-T3 alloy has been provided by an environmental-friendly, well adhering pre-treatment system consisting of an inhibitor-loaded titanium oxide porous layer and a sol–gel based thin hybrid film. A novel approach aimed at developing a nanoporous reservoir for storing of corrosion inhibitors on the metal/coating interface has been proposed. The nanostructured porous TiO2 interlayer was prepared on the aluminium alloy surface by controllable hydrolysis of titanium alkoxide in the presence of template agent. The morphology and the structure of the TiO2 film were characterized with TEM, EDS, SEM, and AFM techniques. Different ways of loading of the inhibitor in the pre-treatment coating were discussed. In contrast to direct embedding of the inhibitors into the sol–gel matrix, the use of the porous reservoir eliminates the negative effect of the inhibitor on the stability of the hybrid sol–gel matrix. TiO2/inhibitor/sol–gel systems show enhanced corrosion protection and self-healing ability confirmed by EIS and SVET measurements.
Article
The deposition of cerium-rich films on copper under cathodic polarization was studied as a model system for understanding the mechanism of corrosion inhibition of copper-containing aluminum alloys. Deposition was also studied on gold and iron for comparison with copper. Inhibition of corrosion of the aluminum alloys is achieved by deposition of a cerium-rich film on the copper-containing intermetallics that blocks the cathodic reduction of oxygen at these sites. X-ray absorption near-edge structure measurements show that cerium-rich films precipitated from aerated solutions are in the tetravalent state. Thermodynamically, the Pourbaix diagram predicts that under these conditions cerium should be in the trivalent state. This indicates that cerium chemistry is determined by processes in the solution rather than the potential of the electrode. Cerium-rich film formation is dependent on reduction of oxygen which influences the oxidation of Ce(III) to Ce(IV) in solution and precipitation of the film by changing the local pH at the electrode. The generation of hydrogen peroxide by oxygen reduction is considered to enhance cerium-rich film formation by oxidizing Ce(III) to Ce(IV) in solution. This was confirmed by addition of hydrogen peroxide to the solution.