Cerium deposition on aluminum alloy 2024-T3 in acidic NaCl solutions

Article abstract of DOI:10.1149/1.1615995

We analyzed the interaction of cerium ions with surface intermetallics on Al 2024-T3 in NaCl media under open circuit conditions at pH 3 using scanning electron microscopic, Auger-electron spectroscopic (AES), and energy dispersive X-ray techniques (EDX), as well as electrochemistry. The results indicate that cerium deposition in a pH 3.0 NaCl solution is strongly surface-site specific. The cerium deposition was the highest on the S-phase particles, lower on the (Cu, Fe, Mn)Al₆ sites, and minimal on the bulk matrix. In addition to the formation of a cerium-rich film on the intermetallics, we found that cerium incorporated into the porous structure of the dealloyed S-phase particles (Cu-Mg-Al intermetallics). Our analyses show that cerium does not prevent the dealloying of the intermetallic particles; in fact, the dealloying is necessary for local cerium deposition. AES and EDX data also revealed significant chloride enrichment on the cerium-rich sites. From our electrochemical measurements we conclude that the cerium-rich layer formed in acidic solution at short exposure times does not inhibit cathodic reaction on the intermetallics.

Full text of DOI:10.1149/1.1615995

B512
Journal of The Electrochemical Society, 150 ͑11͒ B512-B516 ͑2003͒
0
013-4651/2003/150͑11͒/B512/5/$7.00 © The Electrochemical Society, Inc.
Cerium Deposition on Aluminum Alloy 2024-T3 in Acidic NaCl
Solutions
,a
,b
c
,z
A. Kolics,* A. S. Besing,* P. Baradlai, and A. Wieckowski*
Department of Chemistry and Frederick Seitz Materials Research Laboratory, University of Illinois at
Urbana-Champaign, Urbana, Illinois, USA
We analyzed the interaction of cerium ions with surface intermetallics on Al 2024-T3 in NaCl media under open circuit conditions
at pH 3 using scanning electron microscopic, Auger-electron spectroscopic ͑AES͒, and energy dispersive X-ray techniques ͑EDX͒,
as well as electrochemistry. The results indicate that cerium deposition in a pH 3.0 NaCl solution is strongly surface-site specific.
The cerium deposition was the highest on the S-phase particles, lower on the (Cu,Fe,Mn)Al6 sites, and minimal on the bulk
matrix. In addition to the formation of a cerium-rich film on the intermetallics, we found that cerium incorporated into the porous
structure of the dealloyed S-phase particles ͑Cu-Mg-Al intermetallics͒. Our analyses show that cerium does not prevent the
dealloying of the intermetallic particles; in fact, the dealloying is necessary for local cerium deposition. AES and EDX data also
revealed significant chloride enrichment on the cerium-rich sites. From our electrochemical measurements we conclude that the
cerium-rich layer formed in acidic solution at short exposure times does not inhibit cathodic reaction on the intermetallics.
©
2003 The Electrochemical Society. ͓DOI: 10.1149/1.1615995͔ All rights reserved.
Manuscript submitted August 8, 2002; revised manuscript received February 20, 2003. Available electronically September 22,
003.
2
There is an increasing need to find alternatives for chromate in
corrosion protection procedures involving 2000 series aluminum
Responding to these issues experimentally, we used a combina-
tion of microscopic and spectroscopic techniques, such as Auger-
electron spectroscopy ͑AES͒, scanning electron microscopy ͑SEM͒,
and EDX. Additional information on the Al 2024-cerium interaction
is also furnished by electrochemistry. The cerium-intermetallic in-
teractions are highlighted through the analysis of the deposition be-
havior of cerium in acidic solutions, specifically at pH 3.0, as such
1
alloys. Cerium additives appear to be promising candidates for such
a replacement.1 Therefore, the precise inhibition mechanism of ce-
rium needs to be identified. Apparently, in copper containing alumi-
num alloys, especially Al 2024, cerium͑III͒ acts as a cathodic inhibi-
tor by forming a cerium rich layer on the cathodic sites, hence
blocking the electrochemical reactions.2 Using energy dispersive
-4
-4
14
pH usually develops during localized corrosion. Under such con-
2
X-ray analysis ͑EDX͒, Davenport et al. reported that cerium en-
ditions Ce͑III͒ has less tendency to hydrolyze and oxidize to some
1
5
richment occurred almost exclusively on the copper-rich intermetal-
lics with no cerium present on the Fe/Mn rich particles. Extended
X-ray absorption fine structure ͑EXAFS͒ analysis indicated the pres-
ence of Ce͑IV͒, especially above pH 8, after a prolonged exposure
sparingly soluble Ce͑IV͒ compounds.
Experimental
3
of Al 2024 alloy to the aerated Ce͑III͒ containing solutions. Nev-
2
Samples were made of Al 2024-T3 alloy of 0.6 cm geometric
surface area. Prior to the experiments, the samples were wet pol-
ished with SiC emery paper down to no. 2400 followed by a pol-
ishing down to no. 4000 using glycol lubricant. After polishing, the
samples were ultrasonicated in isopropyl alcohol for 10 min and
washed briefly with Millipore water before introduction to the mea-
ertheless, Ce͑III͒ enhances the cathodic activity on Al Cu phase as
2
reported recently.5
While there is a consensus as to the formation of cerium contain-
ing layers on the copper-rich intermetallics, it is not known how the
stability of intermetallics is affected by cerium ions in a corrosive
medium. Previous studies have shown that, in inhibitor-free NaCl
solution, the S-phase particle ͑Cu-Mg-Al intermetallics͒ undergoes
suring
cell
for
electrochemical
characterization.
The
Ϫ
Ag/AgCl(͓Cl ͔ ϭ 3 M) half-cell and a platinum gauze were used
as reference and counter electrodes, respectively. All potentials
quoted in this paper are given vs. the Ag/AgCl reference, which is
0.20 V negative to the standard hydrogen electrode ͑SHE͒. Prior to
chronoamperometric analysis, the electrode was held at open circuit
potential for 45 min in cerium containing or cerium-free sodium
chloride solution. After 45 min, the electrode potential was changed
to a value more negative than the open circuit potential ͑OCP͒ and
the current was measured as a function of time. The supporting
electrolyte was 0.1 M NaCl solution, pH 3.0, with the solution pH
adjusted to 3.0 by HCl addition. All solutions were prepared from
analytical reagent grade quality chemicals using Millipore water.
Ultrahigh vacuum ͑UHV͒ measurements were conducted using a
Physical Electronic Industries, PHI model 660 AES spectrometer. In
AES and scanning Auger microscopic ͑SAM͒ studies, a 10 kV elec-
tron beam energy with an emission current of 40 nA was applied.
The samples used in the UHV program were pretreated in the same
manner as those in electrochemical experiments except that polish-
ing was continued down to 1 ␮m with diamond slurry in glycol
lubricant in order to minimize the effect of surface roughness in the
spectroscopic analysis. Further experimental steps were the same as
extensive dealloying through the preferential dissolution of alumi-
num and magnesium.6-11 The dealloying in acidic solutions also re-
sults in major morphological changes. Specifically, most of the
S-phase particles exhibit a porous structure7
,8,10
and are often sur-
rounded by a groove, indicating extensive dissolution of the imme-
diately neighboring matrix. This transformation creates highly reac-
tive local cathodic sites on the surface.6-13 The other most common
phase, the (Cu,Fe,Mn)Al intermetallic, also enriches in copper, es-
6
pecially in acidic solution, while it develops a thick iron oxide cor-
rosion product layer in near neutral media.1
0-11
The bulk matrix also
undergoes dealloying and develops a rough surface structure in both
acidic and alkaline solutions.1
0,12
Recently, it was shown that the presence of chromate ions can
significantly inhibit the dealloying of S-phase particles and the cor-
rosion of Fe-rich intermetallics.10 Because the intermetallics play a
critical role in the corrosion susceptibility of the alloy, it is of special
interest to see not only how the intermetallics affect cerium deposi-
tion but also how cerium influences the localized reaction, i.e., the
dealloying of the particles.
1
0
discussed previously.
*
Electrochemical Society Active Member.
Sample imaging by SEM and the EDX analyses were performed
using a Hitachi S-4700 microscope equipped with a field emission
electron gun. The accelerator voltage for imaging was set to 10 kV,
while 20 kV was used for quantitative analysis. The sample was not
tilted for imaging or qualitative/quantitative analyses. In some cases,
a
Present address: Blue29 Corporation, Sunnyvale, CA 94089.
Present address: United Technologies Research Center, East Hartford, CT 06108.
On leave from: Department of Radiochemistry, University of Veszpr e´ m, Veszpr e´ m,
H-8201 Hungary.
E-mail: andrzej@scs.uiuc.edu
b
c
z
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Journal of The Electrochemical Society, 150 ͑11͒ B512-B516 ͑2003͒
B513
Figure 2. AES analysis of features on Al2024-T3 alloy exposed to
.1 M NaCl ϩ 10 mM CeCl , pH 3.0 solution for 90 min. ͑A͒ bulk matrix,
0
3
͑
B͒ (CuFeMn)Al intermetallics, and ͑C͒ S-phase particles.
6
Figure 1. AES analysis of features on Al2024-T3 alloy exposed to
.1 M NaCl ϩ 10 mM CeCl , pH 3.0 solution for 45 min. ͑A͒ bulk matrix,
0
3
͑
B͒ (CuFeMn)Al intermetallics, and ͑C͒ S-phase particles.
6
as revealed by our SEM analysis. Nevertheless, AES analyses indi-
cated significant cerium enrichment ͑Fig. 1B͒, especially after 90
min of exposure ͑Fig. 2B͒. AES depth profiling data evidence a ca.
30 nm thick Ce-rich layer and a 150 nm thick oxide layer on the
intermetallics ͑Fig. 3͒. As seen from the depth profiling results, ce-
rium was enriched only in the top layers of the oxide film. As a
comparison, we note that the oxide film thickness on the same type
of intermetallic was only ca. 15 nm thick after 45 min exposure of
the substrate to 0.1 M NaCl ϩ 10 mM Na2CrO4 , pH 3.0 solution.10
It is also noteworthy that cerium did not change the relative ratio of
the alloying elements in the surface oxide film. Specifically, the
Al:Cu:Fe ratio was the same in 0.1 M NaCl and 0.1 M NaCl
ϩ 10 mM CeCl3 solutions. Comparing these data with our depth
to avoid possible charging effects, a thin Au-Pd layer was deposited
on the sample before analysis.
Special care was taken for the EDX analyses, especially at the
S-phase particles, since the EDX sampling volume is usually larger
than the size of the particle. In such cases, the bulk matrix contri-
bution to the spectrum cannot be neglected. This contribution is,
however, strongly dependent on the size of the particle, which can-
not be judged by simple SEM analysis. Therefore, EDX analyses
were performed on the same surface sites before and after exposure
to aqueous solutions. This was achieved by making reference marks
on the surface to easily locate the same surface site after solution
treatment.
profiling results performed on unexposed (Cu,Fe,Mn)Al ͑Fig. 4͒,
6
1
0
we concluded that, unlike chromate, cerium does not inhibit the
surface dealloying of the (Cu,Fe,Mn)Al intermetallics and, conse-
quently, the surface enrichment of copper. Note that the dealloying
6
Results and Discussion
AES analyses were performed on Al 2024-T3 substrates exposed
of (Cu,Fe,Mn)Al intermetallics is significantly lower than that of
6
to 0.1 M NaCl ϩ 10 mM CeCl , pH 3.0 solution for 45 or 90 min.
S-phase particles; therefore, the porosity of the Cu-Mg-Al interme-
3
1
0,11
The bulk matrix exhibited no major compositional change after 45
min of exposure ͑Fig. 1A͒; however, 90 min of exposure resulted in
a copper enrichment on the surface ͑Fig. 2A͒. These data imply that
cerium deposition is not instantaneous on the surface in acidic solu-
tion and is, therefore, likely to be connected to the local pH increase
induced by the cathodic reactions on the copper-rich areas.
tallics will be considerably higher.
This is in line with our AES
observation because notable incorporation of cerium deep into the
(Cu,Fe,Mn)Al intermetallic structure was not found.
6
The most significant cerium deposition was detected on the
CuMgAl intermetallics ͑Fig. 1C and 2C͒. The AES depth profiling
2
analyses performed on S-phase particles show significant amounts
of oxygen and cerium even at a depth of 1 ␮m ͑Fig. 5͒. The particle
was also enriched in copper in the entire analysis depth and shows
The surface morphology of (Cu,Mn,Fe)Al6 intermetallics was
the same in absence and presence of cerium in 0.1 M NaCl, pH 3.0,
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B514
Journal of The Electrochemical Society, 150 ͑11͒ B512-B516 ͑2003͒
Figure 3. AES depth profiling analysis of (CuFeMn)Al intermetallic after
6
9
0 min exposure of Al 2024-T3 substrate to 0.1 M NaCl ϩ 10 mM CeCl3 ,
Figure 5. AES depth profiling analysis of S-phase particles after 90 min
pH 3.0 solution. ͑1͒ oxygen, ͑2͒ aluminum, ͑3͒ cerium, ͑4͒ copper, ͑5͒ iron.
exposure of Al 2024-T3 substrate to 0.1 M NaCl ϩ 10 mM CeCl , pH 3.0
3
solution. ͑1͒ copper, ͑2͒ oxygen, ͑3͒ cerium, ͑4͒ aluminum.
no sign of magnesium, which is indicative of extensive dealloying.
These results are also in line with the present ͑Fig. 6͒ and previous
EDX analyses.10 Namely, S-phase particle exhibited a strong mag-
bulk matrix, (Cu,Fe,Mn)Al6 intermetallics and S-phase particles,
respectively ͑Fig. 8A-C͒. These data imply a possible connection
between cerium and chloride deposition. Although such coupled sur-
1
0
nesium peak in the EDX spectrum before solution exposure, while
the same particles exposed to acidic NaCl solutions showed little or
no magnesium in the particle. SEM imaging provided further evi-
dence on the inability of cerium to prevent dealloying ͑Fig. 7͒.
Namely, the S-phase particles exhibited a porous structure similar to
that observed in the absence of cerium. Therefore, it is likely that
cerium detected within the intermetallics was due to cerium deposi-
tion within the pores of the particle. Imaging analysis by SEM as
well as elemental mapping by Auger indicated that, typically, the
cerium deposit did not cover the entire particle.
2
,4
face accumulation was found earlier, no attention was given to
this relationship before. We attributed the high chloride content of
the deposit to the formation of a sparingly soluble Ce(OH) Cl
x
3Ϫx
16
compound ͑where x ϭ 2.33 and log K ϭ Ϫ17.7 ͒, which depos-
s
ited together with Al(OH) and Ce(OH) . The cerium compounds
3
3
started to deposit at around the same pH, i.e., pH 7.7, under the
given concentrations of cerium and chloride ions in the solution.
Such a coupled deposition was even more evident when the local
cathodic reactions are further enhanced, i.e., by cathodic polariza-
tion ͑see the section on Electrochemical data, below͒. EDX analysis
of an S-phase particle after cathodic polarization indicated enhanced
chloride and cerium concentrations ͑Fig. 9͒ as compared to that
observed after exposure under open circuit conditions ͑Fig. 6͒. With
cathodic polarization, the cathodic reaction was accelerated, induc-
Another interesting feature of cerium deposition was its chloride
deposition enhancing effect. Specifically, the relative concentration
of chloride is 0.3 Ϯ 0.1%, 0.8 Ϯ 0.2%, and 1.3 Ϯ 0.5% on the
Figure 4. AES depth profiling analysis of (CuFeMn)Al intermetallic on Al
6
2024-T3 after polishing. ͑1͒ oxygen, ͑2͒ aluminum, ͑3͒ copper, ͑4͒ iron, ͑5͒
Figure 6. EDX analysis of an S-phase particle on Al2024-T3 alloy exposed
manganese.
to 0.1 M NaCl ϩ 10 mM CeCl , pH 3.0 solution for 90 min.
3
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Journal of The Electrochemical Society, 150 ͑11͒ B512-B516 ͑2003͒
B515
Figure 7. SEM images of S-phase particles on Al2024-T3 alloy after expos-
ing the substrate to 0.1 M NaCl ϩ 10 mM CeCl , pH 3.0 solution for 90
3
min. ͑Images B and D are higher magnification SEM micrographs of par-
ticles in image A and C, respectively.͒
ing a more pronounced pH increase and, consequently, a higher
extent of cerium hydroxide or cerium-hydroxo-chloride deposition.
The presented results indicate that cerium deposition actually
proceeded on all surface sites, although to varying degrees, even in
acidic solution. The increasing amount of surface cerium with ex-
posure time is related to the dealloying rate of the bulk matrix,
(
Cu,Fe,Mn)Al6 intermetallics, and S-phase particles. Namely, all
surface sites became enriched in copper to a certain degree. The
transformation was the fastest and most complete on the S-phase
particles. The (Cu,Fe,Mn)Al intermetallics and the bulk matrix got
6
more dealloyed as aluminum dissolves from the matrix. Therefore,
cerium, in contrast to chromate, does not prevent the dealloying of
the intermetallics.
In fact, extensive dealloying of the Al2024 surface is a prerequi-
site for cerium deposition in acidic media. The effect of copper
enrichment on cerium deposition was indirect because the primary
reason for the surface accumulation of cerium is the increased local
pH vs. bulk solution pH. The higher local pH developed due to the
cathodic reactions, such as hydrogen ion or oxygen reduction, oc-
curring preferentially on the copper-rich sites. This favors the hy-
drolysis of cerium͑III͒, which later leads to the formation of a ce-
rium oxide/hydroxide film. The first step in the process is
preferential dealloying of magnesium and aluminum at S-phase par-
ticles. As the dealloying process continued, the relative copper con-
tent of the particles increased, changing the originally anodic par-
ticle ͑vs. bulk matrix͒ to cathodic. The cathodic reactions, which
proceeded after extensive dealloying, induced the local pH increase
and the hydrolysis of solvated cerium ions.
Figure 8. ͑A͒ SEM and AES ͑B͒ Ce, ͑C͒ Cl elemental map from the surface
of Al2024-T3 alloy after exposing the substrate to 0.1 M NaCl
ϩ 10 mM CeCl , pH 3.0 solution for 90 min.
3
in the absence and presence of cerium͑III͒ ions in 0.1 M NaCl
solution. These results indicate that there is no substantial difference
in the OCP change in the two solutions during the first 90 min. The
OCP after 90 min was Ϫ0.89 Ϯ 0.06 V and Ϫ0.96 Ϯ 0.06 V in the
absence and presence of 10 mM CeCl , respectively. Specifically,
3
the electrode potential did not shift more negative in the presence of
cerium, which would indicate the inhibition of cathodic reactions.
Therefore, we suggest that the cerium-rich film formed on the
surface does not provide a sufficient barrier layer to block electro-
chemical processes. This hypothesis is further supported by cathodic
chronoamperometric analysis. In this case, after 45 min of exposure
at OCP, we held the electrode potential at Ϫ1.2 V, i.e., ca. 0.2 more
negative than the OCP, and measured the current decay ͑Fig. 10͒. In
this case, the cerium-containing solution indicated higher cathodic
activity. This is maintained at electrode potentials negative to
Judging from the relative atomic concentration of cerium on the
surface and the thickness of the cerium-rich layer, the extent of
cerium deposition increased in the following order: bulk matrix
Ͻ (Cu,Fe,Mn)Al intermetallics Ͻ S-phase particle. However, we
6
note that the relative atomic concentration of cerium varied signifi-
cantly, i.e., from 0 to 6% on the (Cu,Fe,Mn)Al intermetallics and 0
6
to 22% on the S-phase particles after 45 min exposure. The large
spread in the concentration indicates that the electrochemical activ-
ity, such as the rate of hydrogen ion reduction, differed markedly
even at intermetallics of the same kind. This is in contrast to our
findings reported with chromate, because the total chromium, i.e.,
Cr(III) ϩ Cr(VI), concentration varied only by Ϯ1.5%.
Ϫ1.2 V. We note that the observed behavior is in alignment with the
5
electrochemical results of Kendig et al. obtained on CuAl phase at
2
pH 5.4.
In summary, neither the electrochemical nor the surface analyti-
cal results evidenced a corrosion inhibition effect by cerium in
Electrochemical data.—We measured the OCP change vs. time
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B516
Journal of The Electrochemical Society, 150 ͑11͒ B512-B516 ͑2003͒
a cerium-rich coating may not be adequate to prevent a rapidly
progressing localized attack because the buildup of a protective film
is a slow process, requiring a local pH buildup to initiate cerium
deposition.
Conclusions
The accumulation of cerium on Al 2024-T3 was analyzed in
acidic NaCl media. Our data indicate that the cerium deposition
increased in the order: bulk matrix Ͻ (Cu,Fe,Mn)Al6
intermetallics Ͻ dealloyed S-phase particles. We conclude that het-
erogeneous cerium distribution on the Al 2024-T3 surface was due
to an inhomogeneous pH distribution, which is the result of local
cathodic reactions. Because the existence or enhancement of ca-
thodic reactions is a result of dealloying, we conclude that cerium
does not inhibit the dealloying of intermetallics and the bulk matrix.
In fact, at this pH, the dealloying is a prerequisite for the cerium
deposition. As a result, the deposition of cerium in acidic solution is
not instantaneous.
Our data also implies that cerium penetrated and filled up the
porous structure of the dealloyed S-phase particles. The observed
codeposition of chloride with cerium was most likely due to the
formation of a cerium-hydroxo-chloride precipitate. This precipitate,
together with the cerium ions in the solution, failed to exhibit any
inhibiting effect on the cathodic reaction at short exposure time in
acidic solution.
Figure 9. EDX analysis of the deposit formed above the Cu-rich interme-
tallics on Al2024-T3 alloy in 0.1 M NaCl ϩ 10 mM CeCl , pH 3.0 at E
3
ϭ Ϫ1.20 V.
Acknowledgments
This material is based upon work supported by the U.S. Depart-
ment of Energy, Division of Materials Sciences under award no.
DEFG02-91ER45439, through the Frederick Seitz Materials Re-
search Laboratory at the University of Illinois at Urbana-
Champaign. Assistance from Nancy Finnegan ͑AES͒ and Vania
Petrova ͑SEM/EDX͒ from the Frederick Seitz Materials Research
Laboratory is acknowledged. AES and SEM/EDX studies were con-
ducted in the Center for Microanalysis of Materials, University of
Illinois at Urbana-Champaign, which is partially supported by the
U.S. Department of Energy under grant DEFG02-91-ER45439.
acidic media after short, i.e., 90 min, exposure time. These results
17
are supported by the findings of Hinton et al. who reported sig-
nificant corrosion-inhibiting effects of cerium on other aluminum
alloys only after 100 h exposure in near neutral solutions. Based on
these assessments, it is likely that a local release of cerium ͑III͒ from
The University of Illinois at Urbana-Champaign assisted in meeting the
publication costs of this article.
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Figure 10. Chronoamperometric analysis of Al2024-T3 alloy at
ϭ Ϫ1.2 V after its exposure to pH 3.0 solutions of 0.1 M NaCl ͑ ͒ and
.1 M NaCl ϩ 10 mM CeCl ͑ ͒ for 45 min at OCP.
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3
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