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A wide variety of anticorrosive treatments for aluminum alloys that can be employed as ‘‘green’’ alternatives to those based on Cr(VI) are currently under development. This article reports a study of the morphological and anticorrosive characteristics of surface layers formed on the Al– Cu alloy AA2017 by immersion treatment in baths of cerium salt, accelerated by increased temperature and the employment of hydrogen peroxide. Scanning electron microscopy (SEM)/ X-ray energy dispersive spectroscopy (XEDS) studies of the samples treated have demonstrated the existence of a heterogeneous layer formed by a film of aluminum oxide/hydroxide on the matrix, and a series of dispersed islands of cerium over the cathodic intermetallics. The protective efficacy has been evaluated using electrochemical techniques, linear polarizations (LP) and electrochemical impedance spectroscopy (EIS), and salt spray tests. The results obtained indicate that the layer provided good resistance to corrosion in media with chlorides, and the method gives a considerable reduction of the time required for the immersion treatments.
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Protection by Thermal and Chemical Activation with Cerium
Salts of the Alloy AA2017 in Aqueous Solutions of NaCl
MANUEL BETHENCOURT, FRANCISCO JAVIER BOTANA, MARI
´A JOSE
´CANO,
LEANDRO GONZA
´LEZ-ROVIRA, MARIANO MARCOS,
and JOSE
´MARI
´ASA
´NCHEZ-AMAYA
A wide variety of anticorrosive treatments for aluminum alloys that can be employed as ‘‘green’’
alternatives to those based on Cr(VI) are currently under development. This article reports a
study of the morphological and anticorrosive characteristics of surface layers formed on the Al–
Cu alloy AA2017 by immersion treatment in baths of cerium salt, accelerated by increased
temperature and the employment of hydrogen peroxide. Scanning electron microscopy (SEM)/
X-ray energy dispersive spectroscopy (XEDS) studies of the samples treated have demonstrated
the existence of a heterogeneous layer formed by a film of aluminum oxide/hydroxide on the
matrix, and a series of dispersed islands of cerium over the cathodic intermetallics. The pro-
tective efficacy has been evaluated using electrochemical techniques, linear polarizations (LP)
and electrochemical impedance spectroscopy (EIS), and salt spray tests. The results obtained
indicate that the layer provided good resistance to corrosion in media with chlorides, and the
method gives a considerable reduction of the time required for the immersion treatments.
DOI: 10.1007/s11661-011-0858-x
The Minerals, Metals & Materials Society and ASM International 2011
I. INTRODUCTION
THE aluminum-copper alloys are widely employed in
the aeronautical industry for numerous structural com-
ponents because of their excellent ratios of weight to
mechanical properties. Nevertheless, these alloys present
serious problems of localized corrosion, especially in
media that contain chlorides, principally because of the
heterogeneous microstructure of the alloy. This behav-
ior against corrosion in the aggressive medium must be
analyzed and characterized for the subsequent design of
appropriate systems of protection. In a first article, we
describe a study of the corrosion mechanism of the
Al-Cu alloy AA2017-T3 in a solution of NaCl.
[1]
The
results obtained demonstrated that the intermetallics
present in the alloy AA2017 are responsible for its
behavior in a medium with a high concentration of
chlorides. It has been observed that the Al(Cu,Mg)
intermetallics initially present an anodic behavior with
respect to the matrix, giving rise to a process of selective
dealloying of Mg and Al. As a result, there is an increase
in the concentration of Cu in these intermetallics, which
is the cause of their subsequent cathodic behavior. The
process of reduction of oxygen to OH
-
takes place as the
cathodic response. The local increase of the pH causes
the dissolution of the layer of oxide and of the
neighboring aluminum. In contrast, the Al(Cu,Fe,Mn)
intermetallics display a cathodic behavior with respect
to the matrix, and an oxygen reduction reaction takes
place over these. The associated anodic response is the
oxidation of the matrix. At the same time, the local
increase of the pH produces the dissolution of the layer
of oxide and of the matrix that surrounds these
intermetallics. Similar results were obtained by Buchheit
et al.
[2]
and Dimitrov et al.
[3]
in aluminum alloy 2024.
Having identified the behavior of the intermetallics
present in the matrix of the alloy and their influence in
the corrosion processes, this article suggests the design
of a system of protection based on the use of cathodic
inhibitors of the cerium type. Cerium-based conversion
coatings are an effective alternative to hazardous chro-
mate-based systems employed in the treatment of metal
surfaces. However, there is still significant discussion
over the mechanism by which these coatings are formed.
The protection mechanism comprises two stages.
First, an oxide or hydroxide of the lanthanide element
is formed as a consequence of the reaction of the Ce
+3
cation with the OH
-
ions derived from the cathodic
reaction taking place on the cathodic precipitates. The
lanthanide compound formed is highly insoluble.
[4]
In a
second stage, this compound precipitates over the
intermetallic particles, blocking the cathodic reaction.
MANUEL BETHENCOURT, Lecturer, is with the Departamento
de Ciencia de los Materiales e Ingenierı
´a y Quı
´mica Inorga
´nica, Centro
Andaluz de Ciencia y Tecnologı
´a Marinas, Universidad de Ca
´diz,
Polı
´gono Rı
´o San Pedro s/n, 11510-Puerto Real, Ca
´diz, Spain.
Contact e-mail: manuel.bethencourt@uca.es FRANCISCO JAVIER
BOTANA, Professor, and LEANDRO GONZA
´LEZ-ROVIRA,
Researcher, are with the Departamento de Ciencia de los Materiales
e Ingenierı
´a y Quı
´mica Inorga
´nica, Facultad de Ciencias del Mar,
Universidad de Ca
´diz. MARI
´A JOSE
´CANO, Lecturer, is with the
Departamento de Ingenierı
´a Civil, de Materiales y Fabricacio
´n,
Escuela Te
´cnica Superior de Ingenierı
´a Industrial, Univerisad de
Ma
´laga, 29071-Ma
´laga, Spain. MARIANO MARCOS, Lecturer, is
with the Departamento de Ingenierı
´a Meca
´nica, Escuela Superior de
Ingenierı
´a, Universidad de Ca
´diz, C/Chile s/n, 11003-Ca
´diz, Spain.
JOSE
´MARI
´ASA
´NCHEZ-AMAYA, Researcher, is with TITANIA,
Ensayos y Proyectos Industriales, S.L.U. Parque Tecnolo
´gico Tecno-
Bahı
´a Edif. RETSE Nave 4. Ctra. Sanlu´ car Km 7, 11500-El Puerto de
Santa Marı
´a, Spain.
Manuscript submitted April 11, 2011.
Article published online August 30, 2011
182—VOLUME 43A, JANUARY 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
Consequently, the associated anodic reaction rate is also
reduced.
Cerium has been studied widely as an alternative to
the use of Cr(VI) in the protection of aluminum-copper
alloys. Lin et al.
[5]
proposed a method of total immer-
sion for forming protective films of cerium on various
aluminum alloys, including AA2091-T6. This treatment
consisted of the immersion of the material in a solution
containing 1000 ppm of CeCl
3
(2.68 mM) open to the
air and at ambient temperature, for a period of 7 days.
The films obtained were evaluated using electrochemical
impedance spectroscopy (EIS) after immersion in solu-
tions of NaCl for different periods of time. A second
stage was included after the immersion in the baths of
cerium, which consisted of immersion in a solution of
epoxy resin. Despite the good results obtained, the
excessive time required for the formation of the layers
made this method unsuitable for possible industrial
application.
In Reference 6, Garrard studied the influence of
cerium in protection against corrosion of several alloys
including AA2090-T8. In that study, an improvement
was observed in the behavior against pitting corrosion
by treating the samples in solutions containing cerium.
Aldykewicz et al.
[7]
employed scanning electron micro-
scopy (SEM) and X-ray energy dispersive spectroscopy
(EDS) to study the role of cerium as an inhibitor against
corrosion of the alloy 2024-T4 in solutions with chlo-
rides. They showed that the formation of a film rich in
cerium over the cathodic intermetallics decreased the
rate of the cathodic reaction. However, although the
treatments are simple, the excessive time required for
the immersion treatments again meant that they were
not appropriate for industrial use. Another disadvan-
tage is that the protective action of the films formed in
these studies at ambient temperature was limited to
short periods of exposure to NaCl.
For these reasons, it seems that research should be
directed toward devising systems of protection needing
only short treatment periods. With the object of
accelerating the formation of lanthanide films on alu-
minum alloys, various other methods have been pro-
posed. One of the most interesting method consists in
activating the system electrochemically to encourage the
formation of OH
-
ions, so that reaction with the Ln
+3
ions present in the solution would be stimulated.
Hinton et al.
[8]
proposed a method based on galva-
nostatic treatments with current densities ranging from
0.01 to 0.28 mA cm
–2
in solutions containing cerium.
The samples of an Al–1.8 pct Cu alloy treated using this
procedure showed a corrosion rate reduction, in solu-
tions of NaCl, of one order of magnitude. However, the
resistance to pitting corrosion was not as high as that
obtained in immersion treatments. In other words,
although the time required for the treatment was
reduced, the degree of protection obtained was also
reduced, at least for this type of corrosion. The reason
for this tendency to pitting corrosion could be the
presence of small pores in the layer that would cause
the film to lose its protective properties. Similar
treatments were performed employing cerium salts in
organic solvents.
[9,10]
In any case, the need for high
DC potentials (between 20–40 V) and volatile organic
solvents for the formation of the layers made it
difficult for the method to be employed on an
industrial scale.
Cerium has also been employed jointly with hydrogen
peroxide. In this context, Wilson and Hinton
[11]
devel-
oped and patented a system of protection by rapid
deposition of a film, rich in cerium; this system was
known as cerating. The system consisted of immersion in
a solution of 10,000 ppm of CeCl
3
at 323 K (50 C),
which contained 5 pct H
2
O
2
at a controlled pH of 2.7,
for periods of time ranging from 8 to 10 minutes. This
method was applied successfully to the alloy AA7075,
together with a wide range of other alloys, although the
layers produced in this way did not present a satisfac-
tory behavior in normalized assays in a salt fog
chamber. Significant improvements were made subse-
quently to this method that enabled the development of
a new treatment called cerate coating
[1214]
; this has been
applied to the alloy AA2024. This method consisted of
the total immersion of the alloy, which had been
submitted previously to a cleaning and deoxidation
treatment, for 10 minutes in an aqueous solution of
cerium chloride with 0.3 pct by volume of hydrogen
peroxide, at 316 K (43 C) and pH of 1.9. The treatment
times were thus reduced to 2–4 minutes. The protective
capacity of the treatment in solutions of NaCl was
evaluated by studying the percentage decrease of the
rate of corrosion: an optimum value of 95 pct was
obtained. The cerate coating layer survived for up to
336 hours in salt fog chamber assays before any signs
corrosion appeared. The acceleration caused by the
H
2
O
2
may be caused by the rapid increase in the pH
caused by its reduction, which would facilitate the
precipitation of cerium oxide/hydroxide. Another con-
sequence of the introduction of H
2
O
2
is the increased
oxidation of the Ce (III) ions to Ce (IV) in the solution,
which gives rise to a film containing cerium hydroxide,
principally in the form of Ce (IV), as observed in the X-
ray photoelectron spectroscopy (XPS) studies carried
out for the alloy AA2024-T351.
[12]
At the same time, in References 15 and 16 conversions
coatings were produced on 2024-T351 and 6061-T6 Al
alloys by a long treatment for 24 hours in air saturated
with water vapor at 373 K (l00 C), 2 hours in 10 mM
Ce(NO
3
)
3
at 363 K to 373 K (90 Cto100C), 2 hours
in 5 mM CeCl
3
at 363 K to 373 K (90 C to 100 C),
and finally, an anodic polarization for 2 hours in
Na
2
MoO
4
at 500 mV (SCE). These results indicated
that the coating generated on both alloys was predom-
inantly a hydrated aluminum oxide, whereas the cerium
was concentrated locally on the oxide, mainly on
intermetallics particles as Cu
2
(Fe,Mn)Al
7
and CuAl
2
on 2024-T351 and (Fe,Mn,Cr)
3
SiAl
12
on 6061-T6. XPS
data indicated that the cerium was present in a mixture
of Ce (III) and Ce (IV) oxidation states. In contrast,
Mo(VI) was detected around the intermetallic particles.
Subsequently, Dabala
`et al.
[17]
studied treatments
with cerium for the protection against corrosion in
NaCl, among others, of the alloy AA2618-T6, by means
of linear polarizations (LP) and EIS. One of these
treatments consisted of immersion for 10 minutes in a
METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 43A, JANUARY 2012—183
solution, at ambient temperature, of CeCl
3
40 mM and
100 mL/L of H
2
O
2
(33 pct vol), with pH adjusted to 2
using HCl. The layers formed in this way presented a
‘‘cracked’’ morphology that was referred to as ‘‘dried
mud,’’ in which the presence of Ce (IV) could be
appreciated. These layers have been compared with
other treatments such as those described previously;
from these comparisons it has been determined that,
although good resistance to corrosion was obtained, the
treatments did not reach the levels of protection offered
either by chromates or by high-temperature treatments.
Xingwen et al.
[1820]
have proposed a procedure for
the formation of a double conversion layer for the alloy
AA2024. This treatment procedure is based on consec-
utive immersions in solutions of Ce(NO
3
)
3
and
Ce(CO
3
)
2
at controlled pH, 4.0 and 5.0, respectively,
and temperature, 298 K to 303 K (25 Cto30C) and
298 K to 303 K (30 Cto35C), respectively, and time,
120 minutes and 100 minutes, respectively, with small
quantities of H
3
BO
3
and H
2
O
2
. This process comprises
several stages: sanding, chemical polishing, cleaning
with distilled water, air drying, immersion in the
solution for the first conversion layer, cleaning with
distilled water, immersion in the solution for the second
conversion layer, cleaning with distilled water, and air
drying. Studies by SEM/EDS and XPS
[20]
showed the
formation of an amorphous layer of complex structure,
formed by a mixture of cerium oxide and hydroxide, of
both III and IV oxidation states. The EIS studies
showed that the double layer formed provided good
resistance to corrosion in media with chlorides.
Recently, Decroly and Petitjeanse
[21]
studied the effect
of adding catalyzers on the deposition of protective
layers based on CeCl
3
and H
2
O
2
. They found that small
concentrations of a catalyst such as CuCl
2
accelerates
the deposition of cerium as CeO
2
2H
2
O. However, this
catalyst can affect the protective properties of the layer
because it precipitates as metallic copper; this creates
cathodic points that have a notable adverse effect on the
resistance to corrosion. Other authors
[22]
employed
compounds such as cerium dibutylphosphate
[Ce(dbp)
3
]) in the protection of the alloy AA2024-T3
and obtained layers of 500-nm thickness, but by
resorting to immersion treatments lasting 10 days,
which means that the treatment is not viable on an
industrial scale.
Whatever method of deposition is employed on
aluminum alloys rich in copper, the precipitation of
oxides/hydroxides of Ce (III) or Ce (IV) is an electro-
chemical process that takes place as a consequence of
the increase in pH resulting from the cathodic reaction
(reduction of O
2
and/or H
2
O
2
and release of hydrogen).
Therefore, from the first studies made on the deposition
of cerium based on conversion coatings, it has been
necessary to consider the effect of the cathodic interme-
tallics,
[8,23]
which increase the rate of the cathodic
reactions in the area surrounding the intermetallic.
More recently, Campestrini et al.
[24]
emphasized the
importance of studying the influence of the intermetal-
lics in an AA2024 alloy prior to deciding on the most
appropriate method, for that alloy, for the formation of
a conversion coating based on CeCl
3
/H
2
O
2
.
To optimize cerium-based coating process to use in
industry, two methods of protection against corrosion
of the alloy AA2017 in solution of NaCl are proposed in
this work, based on immersion treatments in cerium salt
baths, accelerated by increased temperature and the
employment of hydrogen peroxide.
II. MATERIALS AND EXPERIMENTAL
PROCEDURE
Test pieces of the Al–Cu alloy AA2017, of 3 92.5 9
0.6 cm in size, were prepared; the composition of this
alloy, in percent by mass, is shown in Table I.
Before being treated, the samples were polished on
SiC paper to a finish of 500 grits. Next, the samples were
degreased with ethanol and cleaned carefully with
distilled water.
For the immersion treatments, aerated aqueous solu-
tions of Ce(NO
3
)
3
0.005 M were employed. The process
for depositing the layers consisted of immersing samples
in solutions of Ce
3+
at temperatures ranging from
ambient temperature, 298 K to 363 K (25 Cto90C).
The samples of the alloy were immersed in these solutions
for periods of exposure that varied from 5 to 120 minutes;
the temperature selected was maintained by means of a
thermostatic bath (PRECISTERM S-138 model, from JP
SELECTA [Barcelona, Spain]). In the treatments of
thermal activation with hydrogen peroxide, aqueous
solutions of Ce(NO
3
)
3
0.005 M were employed, at tem-
peratures between 298 K (25 C) and 323 K (50 C), and
concentrations of 0.63, 1.25, and 2.50 mL/L of H
2
O
2
were
added separately to these solutions.
The surface appearance of the deposited layers was
studied by scanning electron microscopy (SEM) using a
field emission microscope (FEM) (Sirion model; Phil-
lips, Amsterdam, The Netherlands). Their composition
was analyzed by means of X-ray energy dispersive
spectroscopy using an EDAX Spectrometer, Phoenix
model, connected to the cited microscope.
The resistance of the treated samples to corrosion in
an aerated solution of 0.59 M NaCl was evaluated using
electrochemical techniques of LP, and polarization
resistance (R
p
); the information obtained using these
techniques was averaged. In previous studies,
[25,26]
it was
proposed to use the parameter passive layer resistance
(R
cp
) for the discriminated evaluation of the protective
layer formed by direct current; this parameter corre-
sponds to the slope on a linear scale of the passive
section of the passive region of the LP. Although this is
also an average term, it gives us information on the
resistance of the layer to polarization.
EIS was employed with the object of evaluating and
characterizing the mixed layer formed on the samples of
AA2017 in aerated solution of 0.59 M NaCl. All these
Table I. Nominal Composition of Alloy AA2017
(Pct by Mass)
Mg Mn Si Fe Zn Ti Cu Cr Al
0.53 0.59 0.62 0.51 0.096 0.03 3.83 0.12 rest
184—VOLUME 43A, JANUARY 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
measurements were made in a K235 flat cell, form Parc
EG&G (Princeton Applied Research, Oak Ridge, TN),
making use of a Solartron model 1287 potentiostat
coupled to a frequency response analyzer, model SI 1255
from Solartron 1255 (Solartron Analytical, Farnbor-
ough, Hampshire, United Kingdom). The exposed
surface of the working electrode was 1 cm
2
. The zone
of the impedance spectrum studied corresponds to that
in which the responses associated with the intermetallic
precipitates and the passive film can be identified. It has
been established that this range lies between 10 kHz and
0.01 Hz. An Ag/AgCl electrode (Crison Mod. 52-10,
Crison Instruments, SA, Alella, Barcelona, Spain) was
used as the reference electrode (0.207 V vs SHE, KCl
3M).
Finally, the samples were subjected to one of the
evaluation techniques most widely used in industry, the
determination of the resistance of the coatings to neutral
salt spray (NSS). We employed the specification MIL-
DTL-5541F,
[27]
which covers chemical conversion coat-
ings formed by the reaction of chemical conversion
materials with the surfaces of aluminum and aluminum
alloys. For this, a Vostch model VSC/KWT 450 salt spray
cabinet (Vo
¨tsch Industrietechnik, Balingen-Frommern,
Germany) was employed. The period of exposure selected
for the evaluation of the layers was 168 h.
In this case, test pieces of 7.62 cm wide and 25.4 cm
long, with a nominal thickness of 0.06 cm were
prepared. The surfaces of these samples were inclined
6 deg from the vertical.
III. RESULTS AND DISCUSSION
In a previous work, we demonstrate that the inter-
metallics present in alloy AA2017 are responsible for its
behavior in a solution of 0.59 M NaCl.
[1]
It will be
recalled that the two main types of intermetallic present
in alloy AA2017 are as follows: spherical particles, 3 to
5lm in diameter, of Al(Cu,Mg), and a second group of
particles composed of Al(Cu,Fe,Mn), of irregular
appearance, whose dimensions extend from a few
microns up to 15 lm. The behavior of the Al(Cu,Mg)
intermetallics in solutions of NaCl is fairly complex.
Thus, the Al(Cu,Mg) intermetallics initially present an
anodic behavior with respect to the matrix, giving rise to
a process of selective desalting of Mg and Al. As a result
of this, there is an increase in the concentration of Cu in
these intermetallics, which is the cause of their sub-
sequent cathodic behavior. The process of reduction of
oxygen to OH
-
takes place as the cathodic response. The
local increase of the pH causes the dissolution of the
layer of oxide and of the neighboring aluminum. In
contrast, the Al(Cu,Fe,Mn) intermetallics display a
cathodic behavior with respect to the matrix, and an
oxygen reduction reaction takes place over these. The
associated anodic response is the matrix oxidation.
Similarly, the local increase of the pH produces the
dissolution of the oxide layer and of the matrix that
surrounds these intermetallics. Having identified the
corrosion processes that take place when the alloy
AA2017 is exposed to solutions of NaCl, it is reasonable
to believe that an effective system of protection, alter-
native to the use of Cr(VI), can be designed based on the
employment of cathodic inhibitors such as cerium salts.
Presented in Figure 1are SEM images corresponding
to samples of the aluminum alloy AA2017 treated by
thermal activation in a solution of Ce(NO
3
)
3
5mMat
363 K (90 C) for 15 minutes. The islands of high cerium
content present on the surface of the alloys treated can be
observed. The EDS analysis of the substratum confirms
that the cerium precipitates preferentially over the inter-
metallics of cathodic character present in this alloy. The
presence of a peak of oxygen, originating from the
aluminum oxide/hydroxide formed on the surface, can
also be observed in the EDS spectra of the matrix. After
the treatment, cerium is not detected on this surface, as
shown in the EDS spectra in Figure 1(a). As mentioned
previously, the Al(Cu,Mg) intermetallics present in alloy
AA2017 behave initially as anodes but are transformed by
a process of selective desalting into cathodes; this explains
why the cerium is deposited on these intermetallics. The
appearance of these covered intermetallics and their
corresponding EDS spectrum can be appreciated in
Figure 1(b). In contrast, the Al(Cu,Fe,Mn) intermetallics
display a cathodic behavior with respect to the matrix,
and cerium deposited over these can be observed,
Figure 1(c).
With the object of evaluating the degree of protection
afforded by the layers formed in this way, the treated
samples were subjected to linear polarization (LP)
assays in a solution of 0.59 M NaCl. Shown as Figure 2
are the LP curves obtained in the solution of NaCl for
samples of the alloy AA2017, treated by thermal
activation, for different lengths of time, in the solutions
of Ce(NO
3
)
3
5 mM at the temperatures selected. As
references, the LP curve of the untreated sample and the
curve corresponding to 2 days of treatment at ambient
temperature, have been included.
First, a decrease of the corrosion potential E
ocp
with
respect to the value measured for the untreated samples
can be observed in all cases. Second, a reduction of the
density of corrosion current of the samples treated can be
observed in these curves; this indicates a decrease in the
electrochemical activity of the system. To determine the
degree of protection provided by each treatment, two
parameters have been employed. Following the recom-
mendations of Stern,
[28]
applied successfully in previous
studies with the alloy AA5083,
[29]
this activity has been
evaluated through the value of polarization resistance R
p
.
In this case, the degree of protection provided has been
evaluated by comparing the value of this resistance with
that corresponding to an untreated sample, R
p
º, through
DR
p
(Eq. [1]). The values obtained are given in Table II.
DRp¼Rp
R0
p½1
According to the data included in Table II, better
results are obtained the more the temperature is
increased. The maximum degree of protection is pro-
vided by the treatment of immersion in Ce(NO
3
)
3
at
363 K (90 C) for 30 minutes and more; an increase in
METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 43A, JANUARY 2012—185
the polarization resistance by a factor of 20.73 is
obtained at 120 minutes.
As can be observed in all cases in Figure 2, the effect
of the treatment causes a displacement of the cathodic
branch toward lower current densities as well as a
decrease in the corrosion potential of the system with
respect to the corresponding values for the untreated
samples. Both events bring about a reduction in the
activity of the system. In accordance with the previous
observations, it is proposed in
[30,31]
to use the value of
the slope of the passivity section of the LP curves, on a
linear scale, as a measure of the resistance of the mixed
layer R
cp
(Eq. [2]).
Rcp ¼DEpas
Dipas½2
Fig. 1—SEM images and associated EDS spectra of the alloy AA2017 treated by thermal activation at 363 K (90 C) in a solution of Ce(NO
3
)
3
:
(a) Matrix (25,000 times magnification), (b) intermetallics of Al(Cu,Mg) (3000 times magnification), and (c) intermetallics of Al(Cu,Fe,Mn)
(1500 times magnification).
186—VOLUME 43A, JANUARY 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
From its value, the protection from this layer can be
evaluated in terms of its increase over the value
corresponding to that obtained for the untreated sam-
ples, DR
cp
º. The increase of this resistance DR
cp
is
defined in Eq. [3]:
DRcp ¼Rcp
R0
cp ½3
Table II also gives the values of R
cp
and DR
cp
obtained from the curves of Figure 2for the alloy
AA2017. As can be appreciated, in all the cases, an
increase of the resistance of the layer is obtained, and
the greatest degree of protection is reached for 30 to
120 minutes of treatment at 363 K (90 C). This
increase has been quantified as a factor 96.34 at
30 minutes or 112.86 at 120 minutes; that is, a good
improvement was observed with respect to the sample
without protection.
The technique of EIS has been employed to evaluate
the layers formed. Figure 3presents the characteristic
EIS diagrams acquired in a solution of 0.59 M NaCl, in
the range of frequencies at which the response of the
layer is evident for a sample of AA2017 pretreated by
thermal activation, e.g., 10 minutes in Ce(NO
3
)
3
at
323 K (50 C). As reference, the EIS diagram for the
untreated sample has been included in these figures.
In the samples treated, the process associated with the
intermetallics has been minimized, and the only response
observed is in the impedance values associated with the
layer in the range of frequencies selected. In the Nyquist
diagram, for each sample treated, the arc presents a
larger diameter in comparison with that of the untreated
sample, which implies a higher degree of protection.
These data are in good agreement with the results
obtained by linear polarization techniques.
This tendency is also manifested in the higher values
obtained for |Z|, shown in the Bode diagram of
Figure 3(b). In turn, the greater compacity of the layer
covering the alloy is reflected in the Bode diagram for
the phase (Figure 3(c)). As can be observed, the spectra
corresponding to the treated sample present a greater
flattening of the maximum in the Bode diagram for the
phase, h–log(f).
An equivalent circuit like that described in Figure 4
has been employed to evaluate the impedance dia-
grams
[32]
; the response of the intermetallics, minimized
by the presence of cerium, is omitted from this circuit. In
this, the R
c
C
c
loop would represent the response of the
mixed layer formed on the alloy during the treatment.
This response is evident in the zones of the spectrum
corresponding to the higher frequencies. For this reason,
to perform the electrochemical characterization of the
Fig. 2—LP curves recorded in a solution of NaCl for samples of AA2017 treated by thermal activation in the solution of Ce(NO
3
)
3
at the tem-
peratures and periods of time indicated.
Table II. Electrochemical Parameters Obtained from the LP
Curves for Samples of AA2017 Treated by Thermal Activation
in a Solution of Ce(NO
3
)
3
5mM
Tª
[K (C)]
t
(min.)
R
p
(kX)DR
p
R
cp
(kXcm
2
)DR
cp
273 (0) 0 5.26 1 0.93 1
298 (25) 2 days 9.25 1.76 5.94 6.39
323 (50) 5 12.37 2.35 2.99 3.22
10 12.98 2.47 3.02 3.25
15 26.20 4.98 8.57 9.22
30 18.60 3.54 6.70 7.20
60 16.58 3.15 1.60 1.72
120 17.65 3.36 1.55 1.67
348 (75) 5 30.60 5.82 10.46 11.25
10 49.08 9.33 20.86 22.43
15 26.54 5.05 5.06 5.44
30 24.12 4.59 5.42 5.83
60 17.09 3.25 4.45 4.78
120 13.61 2.59 1.92 2.06
363 (90) 5 56.55 10.75 43.38 46.65
10 59.28 11.27 45.15 48.54
15 51.13 9.72 47.82 51.42
30 105.86 20.13 90.15 96.34
60 106.72 20.29 92.01 98.94
120 109.02 20.73 104.96 112.86
METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 43A, JANUARY 2012—187
mixed layer, it is necessary only to analyze the zone of
the spectrum between 10 kHz and 1 Hz.
[26,30]
Table III presents the values associated with the layer,
after the evaluation of the EIS spectra. What is notable
in this table is that, as indicated, the samples protected
present a higher R
c
in the Nyquist diagrams for all
the cases; this is indicative of the greater degree of
protection.
The protective effect has been evaluated in terms of
the resistance of the layer R
c
, adjusted for the EIS data.
It should be noted, in this case, two different surfaces are
being compared because the presence of the intermetal-
lic particles in the untreated samples is being ignored.
For this reason, it is interesting to analyze the improve-
ment on average, comparing the values of R
c
obtained
for treated and untreated samples R
c
º. The increase of
this resistance, DR
c
, is defined in Eq. [4]:
DRc¼Rc
R0
c½4
This value has been applied successfully in previous
studies with the alloy AA5083.
[30]
In this work, the
authors showed the beneficial effect of the temperature
in cerium baths with the formation of a layer of oxide of
sufficient thickness and compacity. In these treatments,
not only the cathodic sites of the alloy are blocked and a
layer of aluminum oxide/hydroxide is formed on the
metal matrix, but also the dielectric properties of this
layer are modified; this makes the layer act as a better
insulating barrier.
As can be appreciated, for the case of AA2017, the
best results are obtained after 30 minutes or more of
immersion in Ce(NO
3
)
3
at 363 K (90 C): An increase is
obtained in the average resistance by a factor of 15.66.
However, good values for protection have been obtained
for treatment of only 15 minutes at 363 K (90 C).
By way of summary, it has been established that the
treatments of aluminum alloys by thermal activation in
solution of Ce(NO
3
)
3
allow the formation, in relatively
short periods of time that can be considered attractive
for industrial application, of a mixed layer constituted
by islands of cerium deposited over the intermetallics
and a film of aluminum oxide that covers the metal
matrix.
The treatments by thermal activation are carried out
in industrially acceptable periods of time, although the
thermal energy required suggests that a better compro-
mise between time and temperature needs to be found.
In the actual cerium conversion coatings processes, a
modification has been made in to the thermal activation
treatment, consisting of the incorporation of small
quantities of H
2
O
2
to the solution of cerium. This has
enabled the temperatures previously employed to be
reduced. After validating the design of the protection
systems based on the combined action of temperature
and H
2
O
2
in solutions of cerium on the Al-Mg alloy
AA5083,
[29,31]
these treatments have been applied to the
alloy under study here, which is AA2017.
Presented in Figure 5are SEM images corresponding
to samples of the aluminum alloy AA2017 treated by
Fig. 3—Nyquist diagram (a), Bode diagram for |Z| (b), and Bode diagram for the phase (c), acquired in NaCl for samples of alloy AA2017 trea-
ted by thermal activation in solution of Ce(NO
3
)
3
at the temperatures indicated. Surface of the working electrode: 1 cm
2
.
Fig. 4—Equivalent circuit of the system based on Ref. 31.
188—VOLUME 43A, JANUARY 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
Table III. Electrochemical Parameters Obtained from EIS for AA2017 Treated in a Solution of Ce(NO
3
)
3
5 mM,
at the Temperatures and for the Times Indicated
T[K (C)] t(min.) R
e
(Xcm
2
)C
c
(lFcm
–2
)u
c
R
c
(kXcm
2
)DR
c
273 (0) 0 17.16 19.01 0.923 5.09 1.00
298 (25) 2 days 15.89 20.46 0.876 5.39 1.06
323 (50) 5 17.41 10.50 0.920 24.14 4.74
10 15.18 12.79 0.876 10.11 1.99
15 14.05 8.19 0.861 14.47 2.84
30 17.51 17.71 0.891 6.78 1.33
60 16.53 18.50 0.857 10.83 2.13
120 15.24 11.11 0.879 25.05 4.92
348 (75) 5 24.66 51.66 0.813 10.30 2.02
10 14.67 36.78 0.815 19.41 3.81
15 17.23 36.63 0.800 9.76 1.92
30 14.03 34.25 0.801 9.83 1.93
60 9.66 15.67 0.836 8.38 1.65
120 14.34 37.67 0.810 8.14 1.60
363 (90) 5 7.53 21.47 0.827 8.07 1.59
10 14.21 17.26 0.836 8.88 1.74
15 15.77 17.27 0.837 16.54 3.25
30 16.49 33.78 0.813 79.69 15.66
60 15.83 28.32 0.793 89.23 17.53
120 17.77 35.52 0.779 112.39 22.08
Surface of the working electrode: 1 cm
2
.
Fig. 5—SEM image (4000 times magnification) and EDS corresponding to a sample of the alloy AA2017 treated by thermal activation at 323 K
(50 C) in a solution of Ce(NO
3
)
3
and 1.25 mL/L H
2
O
2
, for 15 min.
METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 43A, JANUARY 2012—189
thermal activation in a solution of Ce(NO
3
)
3
5mMand
1.25 mL/L of H
2
O
2
at 323 K (50 C) for 15 minutes.
As can be appreciated in Figure 5, protective layers
are formed similar to those obtained by thermal
activation showed in Figure 1(b) and (c). In both cases,
cerium ions act as cathodic inhibitors at active sites, i.e.,
appropriate intermetallic particles, through precipita-
tion of insoluble cerium oxide/hydroxide at local regions
of high pH. The oxygen reduction reaction results in
generation of OH
ions at cathodic intermetallic,
leading to a local increase in pH. The OH
ions react
with the cerium ions present in the solution, resulting in
the formation of rare earth oxide/hydroxide islands. The
islands block the cathodic sites, reducing the cathodic
current and, therefore, the overall corrosion rate. The
appearance of the covered intermetallics is similar in
both cases, and they can be distinguished one from
another by EDS. The islands detected are formed by a
compound of cerium that is deposited over the cathodic
intermetallics, Al(Cu,Mg) or Al(Cu,Fe,Mn), dispersed
on the surface of the alloy. It has even been possible to
identify Ce in the composition of the matrix, a finding
that was not appreciated by means of the previous
treatments at high temperature (Figure 1(a)). This may
be associated with the fact that the addition of small
quantities of H
2
O
2
facilitates the oxidation of Ce (III) to
Ce (IV), which enables the more uniform deposition of
the lanthanide element over the surface of the sample. In
accordance with these observations, the mixed layer
formed by thermal activation in the solution with H
2
O
2
added would also be expected to afford protection
against corrosion.
To analyze the state of oxidation of the cerium
present in the mixed layers formed by thermal activation
with and without H
2
O
2
, XPS was used; the region of the
XPS spectrum analyzed was that corresponding to Ce3d
(Figure 6). This region has three differentiated zones.
The first zone corresponds to the peaks that appear in
the range of binding energies of 880 and 890 eV because
of Ce 3d
5/2
; the second corresponds to the zone where
the spectra of Ce 3d
3/2
and 3d
5/2
overlap, for energies of
890 and 910 eV; and the third is caused by the
emergence of a u’’’ satellite peak associated with Ce
3d
3/2
at an energy of 917 eV, only because of the
presence of Cerium (IV).
[33]
These zones can be identi-
fied in Figure 6, corresponding to the theoretical spectra
of Ce (III) and Ce (IV).
In Reference 34 it was demonstrated that the percent-
age of Ce (IV) could be calculated as the ratio between the
integral area of the u’’’ peak and the total area of the
spectrum of Ce 3d. When this method is used to calculate
the percentage of the state of oxidation for the Ce present
in this type of protective layer on AA2017, it is found that
Ce is in state IV. However, if the state of oxidation of the
Ce is analyzed for samples treated by immersion both at
ambient temperature and with thermal activation, it can
be confirmed that the cerium in these layers is present as
both Ce (III) and Ce (IV).
So, as can be observed in Figure 7, the u’’’ peak
appears in the spectrum of Ce3d corresponding to
samples treated at 323 K (50 C) in a solution of
Ce(NO
3
)
3
5 mM and 2.50 mL/L of H
2
O
2
; this is
indicative of the presence of Ce (IV). Therefore, the
results would indicate that, in the mixed protective layer,
the islands of cerium are formed, on the one hand, by a
mixture of cerium in states (III) and (IV) for the samples
Fig. 6—Theoretical XPS spectrum for Ce (III) and Ce (IV).
Fig. 7—High resolution spectrum of the Ce region for the treatments indicated.
190—VOLUME 43A, JANUARY 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
treated in the absence of H
2
O
2
and, in contrast, by Ce
(IV) exclusively for the samples treated with H
2
O
2
.
In References 35 and 36, the E-pH diagram for the
Ce-H
2
O system was revised and showed as an essential
tool to understand and predict the factors in the cerium
precipitation in aqueous systems with or without H
2
O
2
.
In the revised diagram in the Ce
3+
/Ce (IV)-O
2
system,
the Ce
3+
ion is stable in the presence of O
2
in the low
pH range (<2) because the oxygen line is located in Ce
3+
stable region. So, at pH <2 it is not possible to oxidize
Ce
3+
to Ce (IV). However, at higher pH values, the
oxygen line could be located in Ce (IV) stable region,
and in this case, Ce
3+
may be oxidized to Ce(OH)
4
by
O
2
. In contrast, the pH for precipitation is decreased as
the Ce
3+
concentration increases.
For Ce
3+
/Ce (IV)-H
2
O
2
system the E-pH diagram
shows that at lower pH values, H
2
O
2
tends to be a
reducing agent for Ce (IV), whereas at higher pH values,
it tends to be an oxidizing agent for Ce
3+
. In the last
case, H
2
O
2
is considered to be a stronger oxidizing agent
than Ce (IV). In conclusion, at higher pH values, Ce
(IV) is the most stable form, particularly in the presence
of stronger oxidation agents as H
2
O
2
.
According with these works, Scholes et al.
[37]
used
titrations of cerium-based conversion coating solutions to
simulate the coating deposition process and to model the
reactions that occur at the metal–solution interface
during coating, with a particular emphasis on investigat-
ing the role of hydrogen peroxide (H
2
O
2
). The titration
curves obtained support the proposed formation of Ce
(III) peroxo complexes such as Ce(H
2
O
2
)
3+
as an initial
step, followed by deprotonation, oxidation, and precip-
itation to form peroxo-containing Ce (IV) species such as
Ce
(IV)
(O
2
)(OH)
2
. A correlation between the results
obtained for titrations and for coated panels of Al alloy
AA2024-T3 confirmed that this model was valid.
Continuing with the procedure described for the
evaluation of the layers formed with thermal activation,
an analysis of the terms of R
p
and R
cp
obtained from the
linear polarization assays in NaCl has been made. A
comparison of the parameters obtained for the unpro-
tected sample of the alloy has also been made; therefore,
the degrees of protection have been determined using the
values of DR
p
and DR
cp
. Thus, Figure 8presents the
characteristic LP curve for the alloy AA2017 protected
by means of this treatment, combining Ce(NO
3
)
3
and
H
2
O
2
in the immersion bath at 323 K (50 C). As can be
appreciated in this figure, it has been possible to displace
the cathodic branch to values of current density that are
up to 2 orders of magnitude lower. Similarly, it has been
possible to define a greater passive range in the anodic
branch.
Table IV gives the values of R
p
and DR
p
obtained
from the analysis of these curves.
The best results are obtained with 15 minutes of
immersion in the baths of Ce(NO
3
)
3
5mM and
1.25 mL/L of H
2
O
2
at 323 K (50 C). The polarization
resistance is increased by a factor of 55.16; in other
words, the activity of the system is decreased with
respect to the untreated alloy.
The EIS technique has also been employed to evaluate
the layers formed by the combination of thermal
activation and addition of H
2
O
2
. Figure 9shows Bode
diagrams, acquired in the solution of NaCl, that are
characteristic for this alloy after being treated.
Equally, these EIS diagrams have been fitted to the
equivalent circuit of Figure 4. The parameters related to
the layer calculated in this fit have been included in
Table V.
As can be observed in these tables, the same conclu-
sions can be drawn as those already obtained by LP.
As can be appreciated from these results, by reducing
the temperature to 323 K (50 C) and adding small
quantities of hydrogen peroxide, results are obtained
that may be even better than those obtained from the
treatments by thermal activation at temperatures up to
363 K (90 C), with a better performance of the system.
Thus, by immersing samples in a solution of
Ce(NO
3
)
3
and 1.25 mL/L of H
2
O
2
for 5 minutes at
323 K (50 C), results are achieved that are of the same
order as those from immersion in a solution of
Ce(N
3
O
3
) for 30 minutes, at 363 K (90 C).
Fig. 8—LP curves recorded in a solution of NaCl for samples of the
alloy AA2017 treated by thermal activation at 323 K (50 C) in solu-
tion of Ce(NO
3
)
3
and 1.25 mL/L H
2
O
2
, for 15 min.
Table IV. Electrochemical Parameters Obtained
from the Linear Polarization Curves for Samples of AA2017
Treated in Ce(NO
3
)
3
5 mM and H
2
O
2
, at 323 K (50 °C)
[H
2
O
2
]
mL/L
ttreat.
(min)
R
p
(kX)DR
p
R
cp
(kXcm
2
)DR
cp
0 0 5.26 1.00 0.93 1.00
0 5 12.37 2.35 2.99 3.22
15 26.20 4.98 8.57 9.22
30 18.60 3.54 6.70 7.20
0.63 5 16.42 3.12 10.21 10.98
15 70.68 13.44 35.09 37.73
30 88.38 16.80 38.65 41.56
1.25 5 17.81 3.39 10.71 11.52
15 290.16 55.16 114.45 123.06
30 32.25 6.13 17.24 18.54
2.50 5 16.57 3.15 1.70 1.83
15 42.73 8.12 9.11 9.80
30 33.86 6.44 7.49 8.05
Surface of the working electrode: 1 cm
2
.
METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 43A, JANUARY 2012—191
The industrial application of an anticorrosive surface
treatment requires, in addition to demonstrated eco-
nomic viability, compliance with specific standards or
requirements for its applicability to be validated. Thus,
with the aim of verifying the protective character of the
treatments proposed, the samples treated by thermal
activation in the solution of Ce(NO
3
)
3
were subjected to
assays of 168 hours in salt fog chamber in accordance
with the standard described in Reference 27. It should
be taken into account that the behavior of the different
systems of protection in this assay should not be taken
as a direct indication of the relative resistance to
corrosion of these coatings in service. However, these
assays provide a means for checking that the quality of
the coating is maintained
Presented in Figure 10 are macrographs of samples of
AA2017, both untreated and treated in Ce(NO
3
)
3
5mM
at 363 K (90 C), after 168 hours in salt spray cabinet.
No signs of corrosion could be appreciated in the
samples treated. The treatment shows an improvement
under NSS conditions, and the results obtained demon-
strate that these treatments in which the thermal
activation is combined with the addition of H
2
O
2
to
the immersion bath can be considered a viable alterna-
tive for application on an industrial scale.
IV. CONCLUSIONS
In this study, the alloy AA2017 (Al–Cu) was treated
by:
1. Complete immersion in solutions of cerium salts by
thermal activation
2. Thermal activation combined with hydrogen perox-
ide activation
In the first treatment, the thermal activation to 363 K
(90 C) of the solution of Ce (III) led to developed a
layer of oxide with sufficient thickness and compacity
and a greater coverage of the mixed layer, providing a
higher level of protection than treatments at room
temperature. In this case, cerium oxi/hydroxide islands
blocked the cathodic activity in the vicinity of cathodic
intermetallics.
In the second treatment, the thermal activation to
323 K (50 C) of the solution of Ce (III) led to faster
growth of the oxide layer, while the addition of small
Fig. 9—Bode diagrams (a) for the phase and (b) for |Z|, acquired in NaCl, for samples of the alloy AA2017 treated by thermal activation in
solution of Ce(NO
3
)
3
5 mM with and without H
2
O
2
. Surface of the working electrode: 1 cm
2
.
Table V. Values of the Elements of the First Loop of the
Equivalent Circuit for AA2017 in a Solution of NaCl, After
Being Treated at 323 K (50 °C), in Ce(NO
3
)
3
and H
2
O
2
[H
2
O
2
]
(mL/L)
t
(min)
R
e
(Xcm
2
)
C
c
(lFcm
–2
)u
c
R
c
(kXcm
2
)DR
C
0 0 17.16 19.01 0.923 5.09 1.00
0 5 17.41 10.50 0.920 24.14 4.74
15 14.51 17.84 0.863 7.77 1.53
30 17.51 17.71 0.891 6.78 1.33
0.63 5 17.20 12.99 0.850 10.26 2.02
15 15.37 11.99 0.878 34.99 6.87
30 17.66 11.26 0.870 35.82 7.04
1.25 5 15.82 19.49 0.839 14.14 2.78
15 20.34 11.39 0.889 101.51 19.94
30 17.98 16.95 0.840 39.19 7.70
2.50 5 14.06 18.96 0.847 10.47 2.06
15 13.42 20.61 0.821 15.42 3.03
30 16.65 16.04 0.840 41.20 8.09
Surface of the working electrode: 1 cm
2
.
192—VOLUME 43A, JANUARY 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
amounts of H
2
O
2
accelerated the formation of the
cerium islands. The cerium oxi/hydroxide islands also
blocked the cathodic sites, but the best results were
obtained as a consequence of incorporating more cerium
in the protective layer, which itself is a result of the
increased concentration of OH
-
caused by the presence
of H
2
O
2
in the immersion bath.
The results obtained by both methods showed that
these treatments are potentially usable by the industry
for corrosion protection of Al-Cu alloys.
ACKNOWLEDGMENTS
This work was supported by the Comisio
´n Intermin-
isterial de Ciencia y Tecnologı
´a, Project MAT2001-
3477.
REFERENCES
1. M. Bethencourt, F.J. Botana, M.J. Cano, M. Marcos, J.M. Sa
´n-
chez-Amaya, and L. Gonza
´lez-Rovira: Corrosion Sci., 2009,
vol. 51 (3), pp. 518–24.
2. R.G. Buchheit, R.P. Grant, P.F. Halva, B. Mckenzie, and G.L.
Zender: J. Electrochem. Soc., 1997, vol. 144 (8), pp. 2621–28.
3. N. Dimitrov, J.A. Mann, M. Vukmirovic, and K. Sieradzki: J.
Electrochem. Soc., 2000, vol. 147 (9), pp. 3283–85.
4. K. Aramaki: Corrosion Sci., 2001, vol. 43 (11), pp. 2201–15.
5. S. Lin, H. Shih, and F. Mansfeld: Corrosion Sci., 1992, vol. 33 (9),
pp. 1331–49.
6. W.N. Garrard: Corrosion Sci., 1994, vol. 50 (3), pp. 215–25.
7. A.J. Aldykewicz, H.S. Isaacs, and A.J. Davenport: J. Electrochem.
Soc., 1995, vol. 142 (10), pp. 3342–50.
8. B.R.W. Hinton, D.R. Arnott, and N.E. Ryan: Mater. Forum,
1986, vol. 9 (3), pp. 162–73.
9. B.R.W. Hinton, N.E. Ryan, and D.R. Arnott: Mater. Australiasia,
1987, vol. 19, pp. 18–20.
10. J. Stoffer, T.J. O’Keefe, X. Lin, E. Morris, P. Yu, and S. Hayes:
Collection of Technical Papers – 40th AIAA/ASME/ASCE/AHS/
ASC Structures, Structural Dynamics and Materials Conference
and Exhibit, AIAA, St. Louis, MO, 1999, pp. 903–17.
11. L. Wilson and B.R.W. Hinton: Patent WO 88/06639, 1988.
12. A.E. Hughes, R.J. Taylor, B.R.W. Hinton, and L. Wilson: Surf.
Interface Anal., 1995, vol. 23, pp. 540–50.
13. B.R.W. Hinton, A. Hughes, R. Taylor, M. Henderson, K. Nelson,
and L. Wilson: Proceedings of the 13th International Corrosion
Congress (ICC), vol. 3, Melbourne, Australia, 1996, pp. 337-1,
337-7.
14. A.E. Hughes, K.J. Nelson, R.J. Taylor, B.R.W. Hinton, M.J.
Henderson, L. Wilson, and S.A. Nugent: International Patent
Application No. PCT/AU94/00539, International Patent No.
WO95/08008, 1995.
15. A.E. Hughes, J.D. Gorman, and P.J.K. Paterson: Corrosion Sci.,
1996, vol. 38 (11), pp. 1957–76.
16. J.D. Gorman, S.T. Johnson, P.N. Johnston, P.J.K. Paterson, and
A.E. Hughes: Corrosion Sci., 1996, vol. 38 (11), pp. 1977–90.
17. M. Dabala
`, L. Armelao, A. Buchberger, and I. Calliari: Appl. Surf.
Sci., 2001, vol. 172, pp. 312–22.
18. Y. Xingwen and C. Chunan: Trans. Nonferreous Met. Soc. China,
2000, vol. 10 (5), pp. 580–84.
19. Y. Xingwen, C. Chunan, Y. Zhiming, Z. Derui, and Y. Zhongda:
Mater. Sci. Eng. A, 2000, vol. A28, pp. 456–63.
20. Y. Xingwen, C. Chunan, Y. Zhiming, Z. Derui, and Y. Zhongda:
Corrosion. Sci., 2001, vol. 43, pp. 1283–94.
21. A. Decroly and J.P. Petitjeanse: Surf. Coat. Tech., 2005, vol. 194
(1), pp. 1–9.
22. D. Ho, N. Brack, J. Scully, T. Markley, M. Forsyth, and B.
Hinton: J. Electrochem. Soc., 2006, vol. 153 (9), pp. B392–01.
23. A.J. Aldykewicz, H.S. Isaacs, and A.J. Davenport: J. Electrochem.
Soc., 1996, vol. 143, pp. 147–54.
24. P. Campestrini, H. Terryn, A. Hovestad, and J.H.W. de Wit: Surf.
Coat. Tech., 2004, vol. 176 (3), pp. 365–81.
25. A. Aballe, M. Bethencourt, F.J. Botana, M.J. Cano, and M.
Marcos: Mater. Corros., 2002, vol. 53, pp. 176–84.
26. M. Bethencourt, F.J. Botana, M.J. Cano, and M. Marcos: Appl.
Surf. Sci., 2002, vol. 189, pp. 162–73.
27. MIL-DTL-C-5541, Military Specification: Chemical Conversion
Coatings on Aluminum and Aluminum Alloys, Revision F, 2006.
28. M. Stern: Corrosion Sci., 1958, vol. 14, pp. 329–32.
29. M. Bethencourt, F.J. Botana, M.J. Cano, R.M. Osuna, and M.
Marcos: Mater. Corros., 2003, vol. 54, pp. 77–83.
30. M. Bethencourt, F.J. Botana, M.J. Cano, M. Marcos, J.M. Sa
´n-
chez-Amaya, and L. Gonza
´lez Rovira: Corrosion. Sci., 2008,
vol. 50 (5), pp. 1376–84.
31. A. Aballe, M. Bethencourt, F.J. Botana, M.J. Cano, and M.
Marcos: Mater. Corros., 2002, vol. 53, pp. 176–84.
Fig. 10—Images of samples of the alloy AA2017 (a) untreated and (b) treated by thermal activation at 363 K (90 C) in Ce(NO
3
)
3
, after
168 hours in salt fog chamber.
METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 43A, JANUARY 2012—193
32. M. Bethencourt, F.J. Botana, M.J. Cano, and M. Marcos: Appl.
Surf. Sci., 2004, vol. 238, pp. 278–81.
33. M.F. Montemor, A.M. Simo
˜es, and M.G.S. Ferreira: Prog. Org.
Coat., 2001, vol. 43, pp. 274–81.
34. D.R. Mullins, S.H. Overbury, and D.R. Huntley: Appl. Surf. Sci.,
1998, vol. 409, pp. 307–19.
35. S.A. Hayes, P. Yu, M.J. O¢Keefe, and O. Stoffer: J. Electrochem.
Soc., 2002, vol. 149 (12), pp. C623–630.
36. P. Yu, S.A. Hayes, T.J. O’Keefe, M.J. O’Keefe, and O. Stoffer: J.
Electrochem. Soc., 2002, vol. 152 (1), pp. C74–79.
37. F.H. Scholes, C. Soste, A.E. Hughes, S.G. Hardin, and P.R.
Curtis: Appl. Surf. Sci., 2006, vol. 253, pp. 1770–80.
194—VOLUME 43A, JANUARY 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
... Fe-rich IMPs usually display high cathodic potential difference with the matrix, in particular in AA7075 due to the high Mg and Zn content of the alloy. S-phase is very complex and can initially undergo selective dissolution of Mg and later behave as a cathodic intermetallic particle [38]. Other strengthening particles like MgZn 2 and θ-phase (Al 2 Cu) are very small and most likely are less important in the deposition process of Ce species. ...
... In other words, a cathodic mechanism of protection was observed. This result is in agreement with others obtained in several aluminum alloys, including AA2017 [15,21,38,41]. ...
Article
Full-text available
The selection of appropriate surface pretreatments is one of the pending issues for the industrial application of cerium-based chemical conversion coatings (CeCC) as an alternative for toxic chromate conversion coating (CrCC). A two-step surface pretreatment based on commercial products has been successfully used here to obtain CeCC on AA2024-T3 and AA7075-T6. Specimens processed for 1 to 15 min in solutions containing CeCl3 and H2O2 have been studied by scanning electron microscopy coupled with energy-dispersive X-ray analysis (SEM-EDX), glow discharge optical emission spectroscopy (GDOES), potentiodynamic linear polarization (LP), electrochemical impedance spectroscopy (EIS), and neutral salt spray (NSS) tests. SEM-EDX showed that CeCC was firstly observed as deposits, followed by a general coverage of the surface with the formation of cracks where the coating was getting thicker. GDOES confirmed an increase of the CeCC thickness as the deposition proceed, the formation of CeCC over 7075 being faster than over 2024. There was a Ce-rich layer in both alloys and an aluminum oxide/hydroxide layer on 7075 between the upper Ce-rich layer and the aluminum matrix. According to LP and EIS, CeCC in all samples offered cathodic protection and comparable degradation in chloride-containing media. Finally, the NSS test corroborated the anti-corrosion properties of the CeCC obtained after the commercial pretreatments employed.
... The protective effect was also evaluated in terms of resistance and pseudo-capacitance of the conversion layers; adjusted for the EIS data, the results, as functions of the RE concentration (0.01,0.1, 0.5, and 1 g L −1 ) for the bath temperature and immersion time that yielded the highest electrochemical performance, are shown in Figure 11 a-b. The average improvement from comparison of the polarization resistance of treated (Rpcoat ) and untreated (Rp film ) samples has been defined in previous studies for different aluminum alloys as [55,56]: ...
... From Figure 11 and equation 7, it was computed that the average improvements of Rp for the CeCCs are 11.10 ± 0.54, 70.75 ± 3.58, 29.12 ± 1.45, and 50.55 ± 1.45 times, using an ion concentration of 0.01, 0.1, 0.5, and 1.0 g L −1 , correspondingly; whereas for the same concentration for lanthanum, the In summary, it has been established that treatments of AA6061 aluminum alloys by thermal activation up to 70°C in a RE(NO 3 ) x ion solution with a concentration lower than 0.5 g L −1 allow the formation of an oxide/hydroxide layer over the intermetallic and aluminum oxide composite film that covers the metal matrix [56] with sufficient thickness and uniformity to provide a higher level of protection than the room temperature treatments. As a consequence, R p is increased to large values depending on an adequate combination of ion concentration, and bath immersion time, although this last variable above 600 min exhibits a small contribution to the barrier properties. ...
Article
The present work is aimed to investigate the corrosion resistance of rare earth (RE) protective coatings deposited by spontaneous deposition on AA6061 aluminum alloy substrates. Coatings were deposited from water-based Ce(NO3)(3) and La(NO3)(3) solutions by varying parameters such as rare earth solution concentration, bath temperature and immersion time. The values of the Tafel slopes indicate that the cathodic process is favored by concentration polarization rather than activation polarization.
... Na2SO4 0,001 mol/L Na2SO4 [14] 0,1 mol/L Na2SO4 [93,142] 0,3 mol/L Na2SO4 [143] 0,5 mol/L Na2SO4 [58,[144][145][146] 1 mol/L Na2SO4 [147] K2SO4 0,5 mol/L K2SO4 [148,149] Na2SO4/NaCl 0,1 mol/L Na2SO4 / 0,001 mol/L NaCl [14] 0,1 mol/L Na2SO4 / 0,05 mol/L NaCl [150,151] 0,1 mol/L Na2SO4 / 0,1 mol/L NaCl [93] 0,5 mol/L Na2SO4 / 0,01 mol/L NaCl [146] NaCl 0,005 mol/L NaCl [152] 0,05 mol/L NaCl [93,[153][154][155][156][157][158][159] 0,1 mol/L NaCl [160] 0,5 mol/L NaCl [2,45,165,166,111,113,149,155,[161][162][163][164] 0,6 mol/L NaCl [92,93,173,143,161,[167][168][169][170][171][172] 1 mol/L NaCl [174] Il est à noter que deux types d'ions sont utilisés : les sulfates SO4 2et les chlorures Cl -. ...
Thesis
Dans le contexte aéronautique actuel, le phénomène de corrosion est une problématique majeure et c'est pourquoi des traitements de surface sont nécessaires. Les traitements de conversion sont très utilisés mais mettent en oeuvre actuellement du chrome hexavalent ou trivalent. Or, une réglementation environnementale (REACh), qui entrera en vigueur en 2024, vise à interdire l'utilisation de substances à base de chrome hexavalent qui sont toxiques. Dans une problématique de développement durable, les industriels du secteur sont en train d'investiguer des solutions totalement exemptes de chrome pour ce type d'application. Ces travaux de thèse s'inscrivent dans ce cadre et portent sur la formulation de nouvelles solutions de traitements de conversion sur alliage d'aluminium 2024-T3 ainsi que l'étude paramétrique associée. Plusieurs inhibiteurs de corrosion connus du CIRIMAT ou recensés dans la littérature ont été étudiés tels que le cérium, les tungstates et les molybdates. La première étude a consisté à discriminer ces inhibiteurs par détermination de leurs propriétés électrochimiques, principalement par chronopotentiométrie et spectroscopie d'impédance électrochimique, et de leur microstructure, la couche de référence restant celle à base de chrome trivalent. Deux couches de conversion (à base de Ce3+ et de WO4 2-) ont été sélectionnées et ont fait l'objet de caractérisations microscopiques approfondies ainsi que d'une analyse exhaustive par spectroscopie d'impédance électrochimique via l'utilisation de circuits électriques équivalents. Les résultats ont démontré qu'il existe une bonne corrélation entre la structure d'une couche de conversion (couche interfaciale, couche interne et couche externe) et ses propriétés électrochimiques. Ainsi, la couche interfaciale confère au système la meilleure résistance à la corrosion via un effet barrière très marqué. La couche interne protège cette couche interfaciale vis-à-vis de l'électrolyte par la formation d'une couche passivante. Enfin, la couche externe joue le rôle de réservoir d'inhibiteur de corrosion qui peut cicatriser le système lors d'un endommagement local. Finalement, pour des raisons de facilité de mise en œuvre à l'échelle industrielle, la couche de conversion à base de tungstate a été privilégiée. Dans le but d'optimiser ses performances en anticorrosion, l'influence de paramètres tels que la concentration en inhibiteur, la durée de conversion, le pH et la durée de stockage a été évaluée. La solution optimisée a ensuite été complétée par un post-traitement et a démontré de bonnes propriétés anticorrosion après analyse électrochimique. Ces résultats ont conduit à une seconde étape qui est le transfert à l'échelle pilote chez l'industriel, Mecaprotec Industries, coordinateur du projet FUI NEPAL dans lequel plusieurs industriels sont partenaires.
... Los intermetálicos Al(Cu,Mg) inicialmente presentan un comportamiento anódico. Posteriormente, como se ha observado mediante SEM-EDX, durante el tratamiento en Smut Go sufren desaleación de Mg, y el consiguiente enriquecimiento en Cu de los mismos modifica su comportamiento a catódico (Aballe et al., 1998, Obispo et al., 2000Zhu y Van Ooij, 2003;Lacroix et al., 2008;Bethencourt et al., 2009;Bethencourt et al., 2012). Por su parte, los intermetálicos Al(Cu,Fe,Mn,Si) presentan un comportamiento catódico desde el comienzo. ...
Article
Full-text available
Se han investigado los efectos de un pretratamiento superficial empleado en el sector aeroespacial sobre la aleación 2024-T3 Al-Cu antes de la generación de recubrimientos por conversión química. Estos pretratamientos influyen en las fases metálicas, que desempeñan un papel clave en el desarrollo de nuevos recubrimientos de conversión sin cromo y en la susceptibilidad a la corrosión localizada en medios que contienen cloruros. El pretratamiento estudiado consta de dos etapas alcalinas y una ácida. La microscopía electrónica de barrido reveló que después del proceso completo, las fases Al(Cu,Mg) se eliminaban parcial o totalmente mediante desaleación y su posterior enriquecimiento de cobre, mientras que solo se atacó la matriz de aluminio que rodeaba las fases Al(Cu,Fe,Mn,Si). El análisis electroquímico reveló el viraje a catódico de las fases Al(Cu,Mg) que aún permanecen en la superficie mientras que la fases Al(Cu,Fe,Mn,Si) presentaron un mayor potencial de corrosión que la matriz de aluminio. Por el contrario, ninguna de estas fases se vio afectada cuando se emplearon únicamente las dos etapas alcalinas. Identificados los procesos que tienen lugar cuando la aleación es sometida a un pretratamiento superficial, es posible diseñar sistemas de protección alternativos a los cromatos.
... It is known that the formation of cerium-based conversion coatings proceeds over and around intermetallic particles, both anodic and cathodic against the aluminium matrix, although via different mechanisms depending on the electrochemical behaviour of the intermetallic [31]. It has been also described that the coating forms on other electrochemically active areas such as grain boundaries or rolling marks made during manufacturing [11,43,44]. In our work, on one hand, microanalysis by SEM-EDS of samples CeCC-1 and CeCC-2 with 15 min and only 3 min of cerium treatment confirmed that Al(Cu,Mg) intermetallic and Al-Cu-Fe-Mn-(Si) phases are seeds for the deposition of the coating, as described in Figures 6-8. ...
Article
Full-text available
A standard three-step surface pretreatment employed in the aerospace sector for Al alloys have been investigated prior to the generation of cerium conversion coatings (CeCC) on aluminiumcopper alloy 2024. Two pretreatments were analysed, one without final acid etching (Pretreatment 1) and another with this step (Pretreatment 2). Both pretreatments affect the alloy intermetallic phases, playing a key role in the development of the CeCC, and also in the susceptibility to localised corrosion in NaCl medium. Scanning electron microscopy coupled with energy-dispersive X-ray analysis (SEM-EDX) revealed that after Pretreatment 2, Al(Cu,Mg) phases were partially or totally removed through dealloying with their subsequent copper enrichment. Conversely, none of these intermetallic phases were affected when the final acid step was not employed (Pretreatment 1). Meanwhile, Al-Cu-Fe-Mn-(Si) phases, the other major Al–Cu alloys intermetallics, suffers minor changes through the whole pretreatments chain. The protective efficiency of CeCC was evaluated using electrochemical techniques based on linear polarisation (LP) and electrochemical impedance spectroscopy (EIS). Samples with CeCC deposited after the Pretreatment 1 gave higher polarisation resistance and impedance module than CeCC deposited after Pretreatment 2. SEM-EDX and X-ray photoelectron spectroscopy analysis (XPS) indicate that the main factors explaining the corrosion resistance of the coatings is the existence of Al(Cu,Mg) intermetallics in the surface of the alloy, which promote the deposition of a cerium-based coating rich in Ce4+ compounds. These Al(Cu,Mg) intermetallics were kept in the 2024 alloy when acid etching was not employed (Pretreatment 1).
Article
The paper analyses the corrosion behaviour of naturally and artificially aged AA2024 alloy in NaCl solution and in the presence of an environment‐friendly corrosion inhibitor, CeCl3. On the basis of the values of polarisation resistance and corrosion current density, the corrosion resistance of the protective inhibitor film is established as well as the general corrosion resistance of this aluminium alloy. Resistance to pit formation is determined based on the difference in pitting and corrosion potentials while resistance to pit growth is determined based on the amount of charge consumed during pit growth. A scanning electron microscope is used to examine the morphology of the pits formed during the pitting corrosion testing, as well as to determine the cerium content on intermetallic particles and the matrix AA2024 alloy. The corrosion behaviour of AA2024 alloy is investigated after different test periods in NaCl solution and in the same solution with the CeCl3 inhibitor. The corrosion resistance of both tempers of AA2024 alloy is more than one order of magnitude higher in the presence of CeCl3. An explanation of the observed differences in the corrosion behaviour of the naturally and artificially aged AA2024 alloy is proposed. Different corrosion behaviour of the alloy after different test periods is also explained. The manuscript considers the relation between the microstructure and the corrosion properties of AA2024 aluminium alloy tempers in the presence of CeCl3. It is shown that corrosion resistance of naturally and artificially aged temper is considerably higher in the presence of cerium. For both tempers, polarisation resistance decrease over time, while resistance to pit formation increases, which is explained with cerium layer morphology on cathodic inclusions.
Article
Ce(NO3)3 and Na2MoO4 are adopted to form (MoCe) composite corrosion inhibitor in allusion to the corrosion problem of steel in acidic conditions. The experimental results showed that the anticorrosion effects were enhanced and the inhibition efficiencies were increased by (NH4)2S2O8. The reason of enhancement is the increase of coordination bonds amount between Ce4+ and , the augment of combining sites of interface between anti-corrosion film and steel, and the reinforce of adsorption caused by the transformation of Ce3+ to Ce4+ by oxidants. The process and conditions for transformation of Ce3+ to Ce4+ and formation of complexes are discussed. The related thermodynamic and kinetic parameters are calculated and the possibility for (NH4)2S2O8 to improve the performance of MoCe corrosion inhibitor is proved.
Article
Cerium compounds have been identified as leading candidates to replace hexavalent chrome as conversion coatings on aluminum alloys to improve corrosion resistance. Cerium also shows promise for use as an inhibiting pigment in paint systems. The cerium conversion coatings can be deposited using either spontaneous or nonspontaneous electrolytic processes. In both cases the protective cerium oxide film forms by a precipitation mechanism that is very dependent on electrochemical potential and pH. The oxidation state and phase of the condensed cerium has been shown to be an important aspect of the corrosion protection properties provided by the film. Because of the strong influence of the solution chemistry and operating parameters on film performance, a basic knowledge of the system stability is essential. Toward this end, a revised E-pH diagram was developed for the Ce-H2O system. The Ce-H2O-HClO4 system was chosen as an example system in which hydroxy ions are the only significant complexing species for the Ce ions. A stability diagram was constructed using more recent thermodynamic data on cerium species not available for the original diagram published in Pourbaix's atlas. Significant differences were noted between the previously published Ce-H2O diagram and the one presented here. Precipitation tests were carried out to verify the trends indicated in the new diagram. The importance of the updated E-pH diagram in understanding the formation processes of cerium conversion coatings are discussed. (C) 2002 The Electrochemical Society.
Article
Scanning Electron Microscopy (SEM), X-Ray Photo-electron Spectroscopy (XPS) and Scanning Auger Electron Spectroscopy (AES) have been used to study the surface chemistry of a multistep process for the generation of cerium- and molybdenum-containing conversion coatings on aluminium alloys. After standard pretreatment, the conversion coating was produced on 2024-T351 and 6061-T6 Al alloys by (i) treatment for 24 h in air saturated with water vapour at 100 °C, (ii) 2 h in 10 mM Ce(NO3)3 at 90–100 °C, (iii) 2 h in 5 mM CeCl3 at 90–100 °C and (iv) anodic polarisation for 2 h in Na2MoO4 at 500 mV (SCE). XPS indicated that the coating generated on both alloys was predominantly a hydrated aluminium oxide. It appeared from SEM and Scanning AES that the cerium was concentrated locally on the oxide. XPS indicated that the cerium was present in a mixture of Ce(III) and Ce(IV) oxidation states. Many of these cerium rich areas were associated with the constituent particles (large intermetallics). Mo(VI) was detected with XPS and there was evidence from SEM and AES to suggest that there was local enrichment of Mo around the intermetallic particles.
Article
The process of the double layer rare earth metal (REM) conversion coating on aluminum alloy LY12 (2024) was introduced in this paper. The corrosion resistance of REM conversion coating was examined by electrochemical impedance spectroscopy. The results showed that the coating increased the corrosion resistance (Rp) of the alloy surface, thus reducing the driving force of corrosion. The morphologies of the coating were studied by scanning electron microscope. The results revealed that the coating consisted of layers of spherical particles with different size, which grew gradually to cover the surface of aluminum alloy. The X-ray diffraction analysis revealed that the film was amorphous. The results of X-ray photoelectron spectroscopy indicated that the film consisted of cerium(III and IV) oxide and cerium(III and IV) hydroxide.
Article
An epoxy coating and a combination of chemical passivation in rare earth metal chlorides with the polymer coating provide excellent corrosion resistance for Al 6061, Al/SiC, Al/graphite and Al-Li 2091-T6. An accelerated corrosion test for polymer coatings based on electrochemical impedance spectroscopy (EIS) and a model for the impedance of polymer coated samples with an artificial defect have been developed to evaluate the quality of coatings and to predict the lifetime of coated metals in a short time.
Article
This paper reports work aimed at finding an alternative treatment to chromate conversion coatings (CCC) that could meet all the requirements of industrial applications. A first step was made by carrying out the deposition of films of CeO2·2H2O on aluminium alloys in a few minutes at room temperature with or without a catalyst. Different types of pretreatment (acidic or alkaline) led to drastically different results. A model for the catalytic deposition mechanism is suggested which is consistent with scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) observations. The anticorrosive properties of these coatings were investigated by potentiokinetic curves measurements and by electrochemical impedance spectroscopy (EIS). Encouraging results were obtained though the performances still do not equal those of CCC. Some of the coating features (adhesion, sealing of the anodic sites, etc.) which should still be improved are highlighted.
Article
Cerium-based conversion coatings are progressing as an effective alternative to hazardous chromate-based systems used in the treatment of metal surfaces. However, there is still considerable debate over the mechanism by which these coatings are formed. Here, titrations of cerium-based conversion coating solutions were carried out in order to model the reactions that occur at the metal–solution interface during coating, with a particular emphasis on investigating the role of hydrogen peroxide (H2O2). The titration curves obtained support the proposed formation of Ce(III) peroxo complexes such as Ce(H2O2)3+ as an initial step, followed by deprotonation, oxidation and precipitation to form peroxo-containing Ce(IV) species such as Ce(IV)(O2)(OH)2. The precipitates resulting from titrations were characterised by Raman spectroscopy, X-ray diffraction and thermogravimetric analysis, confirming the presence of peroxo bonds, and nano-sized CeO2 crystallites that decreased in size with increasing H2O2 concentration. Characterisation of cerium conversion coatings on aluminium alloy surfaces confirmed the presence of peroxo species in the coatings, thereby supporting the titration model.
Article
There are a variety of procedures, described in the bibliography, for producing conversion coatings using salts of lanthanide elements, and the coatings obtained by means of some of these procedures show acceptable levels of protection. Nevertheless, the principal limitation usually presented by these procedures is the excessively prolonged treatment times required to achieve such levels of protection. This therefore limits the applicability of these treatments on an industrial scale.Coatings on the alloy AA5083 obtained by using Ce(III) are characterised by having a mixed or heterogeneous nature, being composed of a layer of alumina covering the matrix, together with islands of cerium formed over the cathodic intermetallics that are present on the surface of the alloy. The results obtained indicate that, once these precipitates have been covered, the level of protection provided is conditional upon the thickness of the layer of alumina.In this study, a procedure is proposed for obtaining conversion coatings on the alloy AA5083 based on immersion in solutions of Ce(III) at temperatures higher than ambient. By this means, coatings can be produced in only a few minutes, and of such quality that salt fog tests of 168h duration are successfully passed. Furthermore, studies conducted employing electrochemical techniques of linear polarisation indicate that the degree of protection provided by these coatings is several orders of magnitude superior to that achieved with other treatments.
Article
Alternative pretreatments are currently under development in order to avoid the environmental impact produced by using surface finishing processes based on chromates. Some of the environmentally friendly alternatives proposed involve the use of lanthanide based compounds. In this study, conversion coatings on AA5083 (Al-Mg) samples developed using full immersion pretreatments in 500 ppm CeCl3 aqueous solutions have been investigated. Their microscopic and compositional features have been analyzed using SEM images and EDS spectra. From this analysis it has been determined that this layer over the surface of the samples is of a heterogeneous composition. This coating is formed by an alumina coating covering the aluminum matrix and dispersed cerium-rich islands deposited over the cathodic sites of the alloy. A characterization methodology is proposed based on the utilization of different electrochemical techniques, such as Open Circuit Potential monitoring (OCP), Linear Polarization (LP) and Electrochemical Impedance Spectroscopy (EIS).
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.