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

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 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.

Full text

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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