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IndianJournalofChemicalTechnology
Vol.3, July1996,pp.212-218
Stability of anodically passivated AI, AI-eu, AI-6061 and AI-7075 in nitric
acid and nitric acid solutions containing chloride
F M AI-Kharafi& WA Badawy
DepartmentofChemistry,FacultyofScience,UniversityofKuwait,P.O. Box5969Safat,13060Kuwait
Received7June 1995;accepted9January1996
The electrochemical behaviour of anodically passivated aluminium and aluminium alloys (AI-Cu,
AI-6061 and AI-7075) has been investigated in nitric acid and nitric acid containing chloride solutions.
The effect of cWoride ion concentration on the stability of the anodic oxide film has been studied.
Electrochemical impedance spectroscopy (EIS) and polarization techniques are employed. The e"perimental impedance data have been fitted to the impedance characteristics of a parallel capacitor/resistor
combination. The anodic oxide film formed on Al or AI-6061 approaches perfect dielectric behaviour,
whereas that formed on AI-7075 or AI-eu deviate from ideal capacitor behaviour. Anodized AI-6061
electrodes are found to be the most stable either in nitric acid or nitric acid containing cWoride solutions. The anodic oxide film consists of two layers-a porous, less stable outer layer and a thin, more
compact stable inner one.
oxide films formed on AI and AI-alloys in nitric
Aluminium and aluminium alloys represent very
acid solutions and to assess the influence of chloimportant industrial materials of high technologiride ions on the stability of these passive films.
cal value. The corrosion and passivation behavThe effects of formation voltage, formation curiour of the metal and its alloys is strongly affected
by the nature of the ambient electrolyte. The elecrent density, ambient electrolyte concentration
trochemical behaviour of these materials during
and alloy composition on the electrochemical
anodic polarization has been extensively studied 1-6. characteristics of ther anodic oxide film have been
studied.
In solutions of aggressive ions such as halide ions,
pitting corrosion and passive film damage can be
initiated at active sites for film-breakdown6-8•
Experimental Procedure
Electrochemical impedance spectroscopy (EIS)
AI and AI-alloys of technical grade were invesprovides information concerning corrosion protigated. The mass spectroscopic analysis of the
materials of interest is presented in Table 1. The
tection by. anodic films such as anodized aluminium or aluminium alIoys9-17.The EIS results obspedmens were prepared as cylindrical electrodes
fitted into glass tubings of appropriate diameter
tained at high and moderate frequencies in neutral solutions containing chloride ions below the
by epoxy resin, leaving a front surface of 0.50,
0.21, 0.20 and 0.21 cm2 for AI, AI- Cu, AI-6061
pitting potential were related to the oxide film
thickness and its internal dielectric relaxation 18. and AI-7075 electrodes respectively, for contact
with the electrolytic solutions. The electrolytic cell
At low frequencies, the relaxation time for the oxconsists of an all glass three electrodes cell with a
ide film dissolution by the formation of aluminilarge sized platinum as a counter electrode and a
um chloride complexes is too high to be exsilver/silver chloride as a reference electrode. The
plained by the impedance results obtained in the
electrolytic solutions were prepared from analytilow frequency range19. The use of nitric acid and
nitric acid/phosphoric acid mixtures in the surface
cal grade reagents .and triple distilled water. All
measurements were carried out at a constant
finishing of AI and AI-alloys especially in the
household industry makes the investigation of
room temperature of 25°C. The EIS measurethese materials in such media important cases
ments were performed using the IM5d-AMOS
worthy of intensive studiesl7,20. The effect of chlosystem (Zahner Elektric GmbH & Co. ~ronach,
ride ions on the stability of anodic films on AI or
Germany). The input signal was usually 10 mV
AI-alloys is not fully understood.
peak to peak in the frequency domain 0.1 to 105
The aim of the present study is to investigate
Hz, since the impedance data were found to be
the dissolution behaviour and stability of anodic
independent of the perturbation signal amplitude
,
I I;
~
I' II~ 11'11
I
i
~I
AL-KHARAFI
& BADAWY:
Table I-Mass
ANODICALLY
PASSIVATED ALUMINIUM AND ITS ALLOYS
213
spectrometric analysis of different electrode materials
0.000
0.007
0.601
0.029
0.001
0.010
0.046
0.015
4.95
1.1797.09
0.93
0.272
2.21
0.038
0.025
0.027
0.003
0.067
0.201
0.193
0.000
0.024
0.499
0.721
0.124
0.164
0.012
0.016
4,80
1.40
999.23
3.43
0.229
0.047
0.006
0.043
0.217
0.248
Sn
Ti
Pb
Mn
Zn
Cr
Ni
Cu
Fe
Si
AI
Mg
in the range 5-15 mY. To check the presence of
another time constant at lower frequencies, some
experiments were conducted over a bandwidth of
1 mHz-105 Hz. At lower frequencies (0.1-100
mHz),
no
reproducible
data
could
be 0
obtainedt8.20. The reaction seems to be much faster than the time required for measurements. Polarization measurements were carried out using an
EG & G (Princeton Applied Research, Model
273 A) potentiostat/galvanostat
interfaced to an
IBM PS/3 computer.
The electrodes were always pretreated and passivated by mechanical polishing of the electrode
surface, firstly with successive grades emery paper
down to 1000 grit and then with smooth cloth
and washed with triple distilled water. In this way,
the electrodes acquired reproducibly bright surfaces. For comparison, some experiments were
carried out after chemical etching of the electrode
surface to be sure that the mechanical polishing
has no effect on the alloy structure and that there
is no pUll-out of precipitate phases, if, present in
the alloy. The electrodes were ehemically etched
in a mixture of 85% phosphoric, 99.5% acetic
and 60% nitric acids (80:15:5 vol. %, respectively)
at 80°C for 5 min before investigation. Anodic
passivation was carried out galvanostatically at
current density 20 mA cm-2 in 0.33 M H3P04 or
0.5 M H2S04, No remarkable difference between
the two anodizing solutions was recorded. After
reaching the desired anodic formation voltage, the
current was interrupted and the passivated electrode was quickly'transferred to the test solution.
The electrochemical measurements were usually
carried out after reaching the steady state ( •••15
min after electrode immersion in the test solution). The measurements were performed at least
twice and a new electrode surface was used for
each run. All potentials were measured against
the {Ag/AgCVCl- (3M KCI)} reference electrode
{Eo=0.1970
V(nhe)}. Details of experimental
procedure are as described elsewhere]3,2].
Results and Discussion
Stability of the anodic oxide film formed on dif
Mass, %
- 170
-130
a
e
30
•.: -70
!i~c•.0.~-100
-30
'0.
Or
>-
\
/ ....•:.--...••..•.•...~....•_ ..•
,. ....•.
'
.....
70
o
30
70 100 130 170 200 230
Real Part ,K.n.
Fig. I-Nyquist
plots of phosphoric acid (0.33 M) anodized
AI-6061 (Vf= 10 V) in 0.1 M HNO, after different time intervals of electrode immersion [(a) 15, (b) 30, (c) 60, (d) 120
and (e) 180 min]
ferent materials and the electronic model of the
oxide/electrolyteinterface- In this series of experiments, the mechanically polished electrodes were
passivated at 20 mA cm-2 to 10 V in 0.33 M
H3P04 and then transferred to the test solution
(0.1 M HNOJ The impedance spectra for each
electrode were recorded after reaching the steady
state ( •••15 min) at different time intervals from
electrode immersion. Typical data of these measurements are presented as Nyquist plots of Al6061 in Fig. 1. For all investigated materials, the
anodic oxide film dissolves upon immersion in
nitric acid solution. At any time interval the Nyquist plot consists of a high frequency semicircle
which is related to the corrosion/passivation processes and hente the electrode impedance is determined by the metal/oxide interface, the oxide
film and the oxide/electrolyte interface2o, and a
low frequency inductive loop which is associated
with the relaxation process in the oxide film
itselfl8. The diameter of the low frequency semicircle is independent of the time of electrode immersion in the test electrolyte. For all specimens
and under polarization conditions at potentials
214
INDIAN 1. CHEM. TECHNOL., JULY 1996
more positive to the pitting potential (- 400 mV
(nhe) for AI), no .reproducible impedance spectra
could be obtained due to surface attack which
was confirmed by cyclic polarization experiments
and microscopic investigations. The attack on the
electrode surface is due to the adsorption of anions (N01 in this case) which leads to oxide film
dissolution22.23. Such attack changes the morphology of the electrode surface as was confirmed by
scanning electron micrography21. The diameter of
the high frequency semicircle decreases as the
time of electrode immersion in nitric acid increases, i.e., it depends on the extent of attack or
in other words on the dissolution process taking
place at the electrode/electrolyte
interface. The
polarization resistance, Rp, of the anodic film
which is the measure of the kinetic facility of the
reaction occurring at the electrode/electrolyte interface24 decreases and the electrode capacitance,
C, which is related to the oxide film thickness22
increases. This means that the anodic oxide film
formed on AI or AI-alloys dissolves continuously
and uniformly in nitric acid solutions.
The dissolution process obey the empirical formula:
. . . (1)
where 0 is the oxide film thickness at any time interval t, 0° the initial oxide film thickness (at the
moment of electrode immersion in the test solution), and f3 is the rate coefficient of oxide film
dissolution 7.22.The oxide film thickness, 0, was
calculated from the impedance data using the
equations:
6 ~"' •...
..
4
c~ I ', ....
<.J:>
E
---
---
812
4
2
ol_
0
1112
.
, mm
Fig.
Variation of the oxide film thickness with time of immersion in 0.1 M HN03 of the investigated materials [(0) AI6061, (e) AI, (~) AI-7075 and (.) AI-eu]
2-
I II
Ii
c= 1/2
njZim
C=Auo/d
(2)
(3)
where C is the electrode capacitance, f is the frequency in Hz, Zim is the. electrode impedance, A
is the electrode area, EO is the permittivity of free
space (8.85 x 10 - 14Fcm - I), and E = 8.4 is the dielectric constant of the oxide film2s. The variation
of the oxide film thickness with immersion time in
0.1 M HN03 solution for the different investigated materials is presented in Fig. 2. For all electrodes, the 0 vs t I/2 relation consists of two linear
parts. In both parts, the oxide film thickness decreases linearly with ti/2, i.e., the rate of oxide film
dissolution changes at a certain time ( •••120 min).
The same was observed with AI and AI-Si
alloys21. The first segment of the 0 vs t1/2 relation
has greater slope than the second one, which indicates that the rate of the oxide film dissolution
decreases after the point of inflection and that the
anodic oxide film formed on Al or its alloys consists of two layers. The outermost layer is porous
and dissolves faster than the inner one, which is
compact and more stable26.27.The duplex nature
of the anodic oxide film formed on aluminium or
aluminium alloys was also observed earliers.7.2s-27.
The anodic oxide film formed on AI or its alloys
can be represented by an equivalent circuit consisting of a parallel combination of resistor Rp,
and capacitor C, in series with a resistor Rs,
equivalent to the electrolyte resistance2I. Other
equivalent circuit models including capacitive features and inductive features are successful in describing the electrochemical behaviour of Al or its
alloys in the very low frequency regions, (F< 0.1
Hz)6.2R.Since it is necessary to compare the electrochemical behaviour of Al and the investigated
alloys, it is useful to reduce the number of components of the suggested model to those which
describe the dielectric properties of the passive
film:. The capacitor/parallel resistor model, investigated at high frequencies (F> 0.1 Hz) is suitable
for this purpose. At high frequencies the resistance of the inductive features becomes included
in the polarization resistance Rp. The capacitive
features of the high frequency semicircle is related
to the barrier layer itself2°. The impedance data
of the anodized electrode were correlated to the
suggested model and then subjected to a procedure of data fitting. For data fitting procedures,
Bode plots are always recommended as standard
plots, since all experimental data are equally represented and the phase angle (), is a veJ)'liensitive
parameter for indicating the presence· of additional time constants in the impedance spectra2S. The
I
1'111'11111
AL-KHARAFI
215
& BADAWY: ANODICALLY PASSIVATED ALUMINIUM AND ITS ALLOYS
90
300
75
100
5K f-Ox-"(.
IK
'tl
~ 500
cu
•••
45
U
:£
~ -300
va.
200
1II
10
a.
~ 100
30
~
III
>
w
-500
d
-700
50
-900
20_
100m
-100
fi
'3
Q
•••
.g
>
60
G
-10
o
I
2
5 10 30 100 300
Frequency
lK
3K
10K 30K lOOK
, Hz
Fig. 3-Computer
fitted data of R, = 26.0 Q, Rp = 8.24 kQ
and C=0.51 fJ-F (x) to experimental Bode plot of AI-6061
(lOV) after 180 min of electrode immersion in 0.1 M HNO)
-9
-8
...,1
-6
!larE'(!. A Icm2
-5
IOn
Fig. 4-Currenl
density-potential curves during polarization
of (a) AI-Cu, (b) AI-7075 (c) AI-6061 and (d) AI in 0.1 M
HNO)
(0)
Table 2-Deviation
of absolute impedance and phase shift
from the theoretical model after 15 and 180 min of electrode
immersion in 0.1 M HNO, solution
AI-Cu
AI-7075
AI
AI-6061
Deviation after 15 min Deviation after 180 min
Table 3- Values of Rp, iwr and E" for the phosphoric acid
(0.33 M) anodized electrodes (VF= 10 V) measured in 0.1 M
HNO) after 15 min of electrode immersion
Electrode
+29
5.79cm2 icorr,fJ-Acm-2
1.74
-52
1.14
-396
-586
0.47
0.78
45.00
14.84
49.55
E,,,mV
~,kQ
1.10
2.60
1.30
1.60
Electrode Z,%
1.10
1.90
2.0
1.9
() Z,%
5.2
3.0
1.3°
6.3
5.4
1.6
1.4
experimental Bode plots of AI-6061 electrodes,
after 3 h immersion in 0.1 M HN03 solutions,
were fitted to the computer generated data of
Rs = 26.0 Q, Rp = 8.22 kQ and C= 0.518 JlF and
presented in Fig. 3. The procedure of data fitting
was applied to impedance spectra of all investigated materials after anodic polarization and at different time intervals of electrode immersion in the
nitric acid solution. The mean error in the absolute impedance and the mean deviation of the
phase angle were calculated and presented in
Table 2. It is clear from the data presented in this
table that AI gives the best fitting to the proposed
parallel capacitor/resistor model. The small deviations of the absolute impedance and phase angle
obtained by this electrode indicate that the oxide
film on AI approaches perfect dielectric behaviour. The passive film on other electrodes show
larger deviation compared to pure AI especially
after 15 min of electrode immersion in the nitric
acid solution (d. Table 2).
The effcct of the alloying elements on the cor-
rosion behaviour and the stability of the anodic
oxide film in nitric acid solution is reflected clearly on the polarization characteristics of the different electrodes. Fig. 4 presents the polarization
curves of AI, AI-6061, AI-7075 and AI-Cu electrodes in 0.1 M HN03 solution. The corresponding values of the polarization resistance Rp, corrosion current icorr,and steady state potential Ess
are presented in Table 3. From Fig. 4 and Table
3, it is clear that the anodic passive film formed
on AI-Cu is the least stable one whereas that
formed on AI-6061 represent the most stable passive film against corrosion in nitric acid. According to their stability in nitric acid solutions the anodized electrodes can be arranged in the followingorder:
AI-6061 > AI> AI-7075 > AI-Cu
The stability order of the investigated electrodes
is independent of the concentration of the test solution. It remained the same in 1, 0.1 and 0.01 M
HN03 solutions as ambient electrolytes. The
same order of stability was confirmed by impedance measurements in 1 M HN03 solutions. The
impedance and polarization data emphasize the
high stability of {he passive film formed on AI-
216
INDIAN 1. CHEM. TECHNOL., JULY 1996
6061. The polarization experiments are in good
agreement with the impedance data and the microscopic investigation of the electrode surface21 •
It is clear that the presence of the small amount
of Mg as an alloying element in AI-6061 (1.4%)
improves the corrosion resistance of the alloy.
The mass-spectrometric investigation of the alloy
has shown that it contains 1.40% Mg and 0.60%
Si. Such a combination in a heat treatable
wrought alloy leads to the formation of Mg2Si
phase, which is the basis for precipitation hardening. Either in solid solution or as submicroscopic
precipitate, Mg2Si has a negligible effect on electrode potential. The alloy is normally used as heat
treated, therefore, no detrimental effects derive
from the major alloying element or from the minor components like Cr, Zn or Zr which are
usually added to control the grain structure. To
minimize effects on corrosion resistance, copper
additions which increase strength in the alloy are
limited to very small amounts, i.e., 0.2% in Al6061. Increasing the copper content decreases the
corrosion resistance of the alloy as can be seen
for AI-7075 (1.17% Cu) and AI-Cu (4.80% Cu)
(d. Table 3 and 4).
Effect of formation voltage-The electrodes under investigation were passivated at 20 mA cm - 2
in 0.33 M H3P04 to formation voltage of 4, 7 and
10 V versus Ag/ AgCl/Cl- electrode. The anodically passivated electrodes were then investigated
in nitric acid solutions of different concentrations.
The oxide film thickness was calculated according
to Eqs (2) and (3) and plotted against tl/2 according to Eq. (1). The data for AI-6061 are presented in Fig. Sa, which shows clearly the change in
the dissolution rate after 120 min of electrode immersion in the nitric acid solution for different
anodization voltage. Almost the same trend was
obtained with the other electrode materials. In all
cases, the duplex nature of the anodic oxide film
was observed2s-27• The initial oxide film thickness
0°, was extrapolated (at t= 0) at different anodization potentials up to .10 V and plotted against the
formation voltage, VF, (Fig. 5b). The linear rela~
tion between 0° and VF gives a slope of 0.6 nm
V-I representing the anodization constant of AI60G1.
Effect of concentration of nitric acid-The
dissolution behaviour of the anodically passivated
electrodes was traced over 3 h of electrode immersion in nitric acid solutions of different concentrations. The polarization resistance and electrode capacitance for the investigated electrodes
after 15 min of electrode immersion in nitric acid
solutions are calculated and presented in Table 4.
The measured impedance spectra show that the
polarization resistance decreases as the concentration of nitric acid increases. Also, a gradual decrease of electrode impedance with the immersion
time in any of tbe nitric acid solutions was also
observed with the investigated materials. This
means that the oxide film undergoes a dissolution
process in the ambient electrolyte and the fate of
dissolution changes after approximately 120 min
indica~ng the duplex nature of the anodic oxide
film formed on AI or its alloys as reported before.
The polarization resistance of the anodic oxide
film is very sensitive to the concentration of the
ambient electrolyte, e.g., it increases for AI-GOG1
electrodes from 3.16 kQ cm2 in 1 M HN03 to
374.3 kQ cm2 in 0.01 M HN03 solution. It was
also noted that the thickness of the anodic oxide
film remained on the electrode surface after 180
min immersion in 0.01 M HN03 is approximately
4.-5 times that remained after immersion in 1 or
0.1 M HN03 solutions for the same time. This is
due to the fact that dilute solutions of HN03
( ~ 0.01 M) are not aggressive enough to ensure
complete dissolution of the porous oxide layer21•
In dilute solutions diffusion phenomena take
place and a flat pattern in the phase presentation
appears.
Effect of chloride ions-In this series of experiments, the effect of chloride ions on the dissolution behaviour of the anodic film formed on AI,
AI-Cu, AI-6061 and AI-7075 electrodes was in-
Table 4- Values of polarization resistance and electrode capacitance of the phosphoric acid anodized electrodes ( V F = lOV) after
15 min of electrode immersion in nitric acid solutions of different concentrations
1.54
x36.04
\0
1.7
4.5~
5.44
~.86
4.lJ3
3.36
I.lJ
5.5
~7.50
49.54
45.00
10-"
14.~4
10
\0
10-"
0.537
0.6lJ8
10-"
3.16
(,"(,(,' 1M
O.IM
5.66
4.40
1.5
xxxRp,
374.3
10'
10
3.11
0.01 M
10-5
32.22
5.7lJ
CFcm
CFcm'
kQcm'
Rp,
kQcm'
Rp,kQcm'
,I
I~;
,I
I
iii
11111 11 11I1
11I1
"
I
i
AL-KHARAFI
& BADAWY:
Q
6
,,
ANODICALLY
o
10V
•
7V
217
PASSIVATED ALUMINIUM AND ITS ALLOYS
-20
-17
b. 4V
< -13
~-10
1::
~ -7
>-
t;
c:
'0.
n~/
"
",
.•.
.•.
,/~
~'....- ...
_-
"
---------
.....•....
" "a
\,,
,,
--". ~..
\
-3 t
to'
c
1
'
.•....
.I
,I
I
,
--,,,
"
--
.•....
2
o
4
8
12
1'I2,min
5
10 13 17 20 23
Real Pari, K .n..
Fig. 6-Effect of chloride ions on the Nyquist plots of phosphoric acid anodized AI-6061 after ISO min of electrode immersion in OJ M HNO 3 containing different concentration of
Cl- [(-) 0.35 M, (..... ) 35 mM and (----) 3.5 mM]
,
Fig. Sa-Variation of d with (1/2 of phosphoric acid anodized
--.-.-..••..
d AI -eu". "
AI-6061 at different formation voltages after immersion in
c AI-7075
08.
--4
."
..
Q.
c
l'-_....
b
0
a
AI-6061
E
(0) 0
10V, (e) 7V and (A)
0.1 M HN03 ..>0
.. 4V]
0-2
'0.
-6 b AI
:0::
~
2
b
-
27 30
"
,,
.....
,-
, "'~ '.
•...
'.
....
h; )
6
E ,.
c
2.
,
/
o
,,
/
4
048
o
vF,V
Fig. 5b-lnitial
oxide film thickness, dO, of phosphoric acid
anodized AI-6061 (JOV) as a function of fonnation voltage
vestigated. The different electrodes were passivated at 20 mA cm-z in 0.33 M H3P04 solution to
10 V and then investigated in 0.1 M HN03 solution containing different concentrations of NaCl
ranging between 3.5 mM and 0.35 M. Typical example of these investigations is given as Nyquist
plots of Al-6061 in Fig. 6. As discussed before,
each Nyquist plot consists of two semicircles. The
high frequency semicircle, which is due to the interaction occurring at the electrode/electrolyte interface, is affected greatly by the presence of
chloride ions. In comparison to naturally passivated electrodeszl,
the presence of the anodic film
has increased the corrosion resistance of the elec-
2
4
6
8
Real Part, Kn.
Fig. 7-Effect of chloride ions on the behaviour of phosphoric acid anodized alloys after 180 min of electrode immersion
in 0.1 M HN03 containing 0.35 M NaCI [(a) AI-6061, (b) AI,
(c) AI-7075 and (d) AI-Cu]
trode and the extent of attack decreases. No lateral spreading of CI- ions beneath the protective
film can occur and the only process is the localized attack on the oxide film surface which leads
to oxide film dissolution. The dissolution process
was found to obey Eq. (1), and the 0 vs. tl12 relation shows also the same inflection indicating the
duplex nature of the anodic oxide film.
The rate of oxide film dissolution (- do/dt) is
given by:
-doldt=
icorr(MlnpF)
... (4)
218
INDIAN J. CHEM. TECHNOL., JULY 1996
where M is the molecular mass of the anodic oxide film (Alz03 = 102 gmol- I), n is the metal stoichiometric factor = 3, p is the density of the oxide = 3.79 g em - 3 and F is the Faraday's constant = 96500 coulomb.
I.e.,
-doldt=9.3XlO-5xicorrcms-'
icorris the corrosion current density in A em -
:?
The dissolution rate depends on the concentration of CI- which attack the passive film at defective areas30-32. The extent of attack differs from an
alloy to the other. This is clearly reflected on the
Nyquist impedance plots of the anodized electrodes after 180 min of electrode immersion in
0.1 M HNO.1 containing 0.35 M Cl- solution presented in Fig. 7. It is clear that AI-Cu and AI7075 are much affected by the presence of chloride ions and a remarkable decrease of the corrosion resistance of both alloys with the increase of
chloride ion concentration was recorded. AI-6061
shows the highest corrosion resistance against
chloride. This means that the presence of Mg as
an alloying element with Al in Al-6061 has improved the corrosion characteristics of the alloy
as discussed before29• On the other hand, the
presence of Cu and/or Zn in AI-Cu and AI-7075
alloys increases the affinity of the alloy towards
attack by Cl- which can be attributed to the natural tendency of Cu and Zn to form oxyhalide
complexes which enhances the dissolution of the
barrier layer and increases the rate of corrosion
of the alloy.
COAciusions
Anodized AI-6061 which contains mainly 1.4%
Mg represent the most stable AI-alloy in nitric acid or nitric acid containing CI- solutions. The
thickness of the passive film formed on the electrode surface depends on the electrode matedal.
Acknowledgement
The financial support of the Kuwait University,
Research Grant No. SC060 is gratefully acknowledged.
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•
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