PERGAMON
Electrochimica Acta 44 (1999) 2751±2764
A comparative electrodissolution and localized corrosion
study of 2024Al in halide media
L. Chen 1, N. Myung, P.T.A. Sumodjo 2, K. Nobe *
Department of Chemical Engineering, University of California, Los Angeles, CA 90095-1592, USA
Received 10 August 1996; received in revised form 25 October 1998
Abstract
Electrodissolution and pitting characteristics of 2024Al-T4 rotating disks in di€erent halide solutions have been
investigated by impedance spectroscopy and other electrochemical techniques. In 1 M halide solutions and saturated
AlCl3, the pitting potential (vs. SCE), increased as follows: sat. AlCl3 (ÿ0.75 V) < Cl ÿ (ÿ0.60 V) < Br ÿ (ÿ0.48
V) < I ÿ (ÿ0.30 V). Potentiodynamic measurements from ÿ1.0 to 1.5 V show that above 0.0 V the electrodissolution
rate increased in the order: sat. AlCl3 < I ÿ < Br ÿ < Cl ÿ ; at the end of each experiment the 2024Al-T4 surface was
covered with a black ®lm. The addition of nitrilotrismethylenetriphosphonic acid (NTMP) to chloride solutions had
only a small e€ect on the pitting potential, but alloy dissolution at high anodic potentials was signi®cantly
diminished. Anodic potential step experiments above the pitting potential (Enp) of 2024Al-T4 in Cl ÿ , Br ÿ and I ÿ
for a time duration of 15 min showed that pits were initiated at the periphery of the disk and contained black ®lms;
the number of pits increased as the anodic potential increased. In sat. AlCl3, there was extensive pitting and a black
®lm covered the entire disk surface. This black ®lm is possibly an Al±Cu-oxyhalide mixed salt since analysis
indicated the presence of Cu(II). Black ®lm was not observed in pits on pure AI in these electrolytes under similar
conditions. The presence of tolyltriazole in 1 M NaCl sharply reduced the size and density of pits on 2024Al-T4.
The impedance spectra of 2024Al-T4 in the four electrolytes at anodic potentials above Enp indicate two capacitive
loops in the complex plane plots and two maxima at high frequencies in the corresponding phase angle Bode plots
when pits, which contained black ®lms, were clearly visible. For pure Al and below Enp for 2024Al-T4, the
capacitive loop in the highest frequency region was not perceived. Electrical circuit analogs have been utilized to
simulate the impedance data and satisfactory ®ts were obtained. # 1999 Elsevier Science Ltd. All rights reserved.
Keywords: Aluminum alloys; Impedance spectroscopy; Corrosion; Pitting; Salt ®lm
1. Introduction
Studies of the corrosion and stability of aluminum and
its alloys are still of considerable interest because of their
* Corresponding author. Tel.: +1-310-825-2447; e-mail:
[email protected]
1
Present address: Semitool Inc., Kalispell, MT 59901, USA.
2
Visiting Scholar from Institute of Chemistry, University of
SaÄo Paulo, SaÄo Paulo, Brazil.
technological importance. Although pure Al is too soft
to be used as a heavy duty material for large structures,
high strength Al alloys can be produced by addition of
appropriate alloying elements, such as Cu, Mg and Zn
and by suitable heat treatment procedures [1±4].
However, many of these alloys have lower corrosion resistances than pure Al. Nevertheless, Al and its alloys can
readily undergo various surface treatments, which
increase their corrosion resistance [5]. On the other hand,
alloying elements, impurities and age-hardening treatments can result in the formation of intermetallic precipi-
0013-4686/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved.
PII: S 0 0 1 3 - 4 6 8 6 ( 9 8 ) 0 0 3 9 7 - 1
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L. Chen et al. / Electrochimica Acta 44 (1999) 2751±2764
tates in the alloy, which when exposed to a corrosive environment lead to highly localized attack, such as pitting,
stress corrosion cracking and corrosion fatigue.
Aggressive anions, such as the halides, induce breakdown
of the protective passive ®lm [6, 7] and can result in catastrophic failure of the material. There have been extensive investigations on the role of chloride ions in the
breakdown of the passive ®lm, repassivation and initiation of localized corrosion of Al and high-strength Albased alloys. Furthermore, the e€ect of alloy content [7±
10], electrolyte composition [11], heat treatment [12] and
mechanical stress [13] on pitting and repassivation of Albased alloys in halide media have been reported.
It is known that Cu particles and Cu-containing
species, such as intermetallics, on the surface of Al can
promote pitting at adjacent sites [14]. Inaccessible
areas of structures, such as lap joints, should be particularly vulnerable to this type of localized corrosion.
The presence of just a few ppm of Cu + 2 can raise the
corrosion potential of Al in 0.1 M NaCl to the pitting
potential (Enp), as shown by Bohni and Uhlig [15].
Since the 2XXX series of Al alloys has the highest
Cu content as a group, 2024Al (Al±4Cu±1.5Mg±
0.6Mn), which is one of the most widely used high
strength aluminumbased alloys in aircraft, provides a
ready source for Cu-containing inclusions in the protective oxide surface ®lm. Therefore, the present extensive use of this alloy and similar Al alloys with
relatively high Cu content should have serious consequences for the integrity of aircraft structural materials, when the operational service life of aircraft is
extended far beyond its design life.
Benzotriazoles (BTA) and substituted benzotriazoles
may be able to play a role in minimizing the localized
corrosion problems associated with Cu-containing high
strength Al alloys. The more environmentally friendly
BTAs had previously replaced chromates, as corrosion
inhibitors, in automotive cooling systems [16]. In contrast to chromates, the use of the BTAs greatly reduced
water pollution problems because of their minimal toxicity to marine life and biological oxygen demand
(BOD) [17]. BTAs are particularly e€ective in precluding
pitting of iron alloys and AI by the presence of small
amounts of copper in recirculating water systems.
The critical role that salt ®lms play in localized corrosion, such as pitting and stress corrosion cracking,
has been extensively investigated by Beck and a number of other investigators [18±39]. The basis for attributing the presence of salt ®lms as an important
criteria for sustaining localized corrosion is given by
Beck in his reference to the seminal paper of Franck,
who established that pit growth is dependent on the
existence of a resistive salt ®lm [40].
A number of studies on the corrosion and passivation
of Al and Al alloys have been directed to determine the
e€ect of heat treatment on the susceptibility to intergra-
nular corrosion and stress corrosion cracking. In their
investigation of the corrosion fatigue of 2024Al-T3 in
NaCl, Tu et al. attributed the observed changes in polarization to cracks and ¯aws in the surface ®lm [13].
Berrada et al., who studied the e€ect of the temper condition of 2024Al on pitting using SEM and EDX,
reported the dependence of the corrosion behavior on
the alloy microstructure [8]. AFM studies of 2024Al indicate localized corrosion attack by aggressive ions adjacent to intermetallic precipitates and the grain
boundaries [10]. Impedance spectroscopy (EIS) has been
used to study general corrosion processes for many
years, and it has recently been applied to investigate
localized corrosion, such as pitting [12, 41±53].
This paper reports on a study comparing electrodissolution and pitting characteristics of 2024Al-T4 in
neutral (except for sat. AlCl3) halide solutions. The inhibitory action of nitrilotrismethylenetriphosponic acid
(NTMP) on 2024Al-T4 in chloride solutions has also
been studied. NTMP has been considered an e€ective
hydration inhibitor for Al and Al-based alloys in atmospheric environments because of strong chemisorption on aluminum oxide through the phosphonate
group inhibiting degradation of the ®lm by hydration
processes [54]. The results of some preliminary experiments with tolyltriazole (TTA) are also presented.
Potential sweep, potential step and impedance spectroscopy were the electrochemical techniques employed.
2. Experimental
Rotating disk electrodes were fabricated from
2024Al-T4 rod (Reynolds Aluminum). The 0.60-cm
cylinders were ®t into a Te¯on holder; only the base of
the cylinder (cross-sectional area of 0.28 cm2) was
exposed to the solution. Some experiments were performed with pure aluminum (99.99%, Alcan Co.) of
the same diameter as the alloy electrodes. Prior to the
experiments the electrodes were mechanically polished
with aloxite paper of various grits, (#120, 240, 400 and
600) soaked in deionized water, washed in acetone and
®nally rinsed thoroughly with deionized water. The
electrochemical cell was a 1.5-l beaker with a side compartment separated from the main chamber by fritted
glass to accommodate the platinum gauze counter electrode. The reference electrode was a saturated calomel
electrode (SCE) with a luggin capillary probe placed
below the disk. All potentials are reported relative to
SCE. The disk electrodes were rotated at 1500 rpm,
unless speci®ed otherwise, all experiments were performed at room temperature with the solutions
exposed to air. The aqueous solutions were as follows:
(a) 0.5 M NaCl, (b) 1.0 M NaCl, (c) 0.5 M NaCl + 20
mM NTMP, (d) 1.0 M NaCl + 20 mM NTMP, (e) 1.0
M NaBr, (f) 1.0 MKI and (g) saturated AlCl3 (3.11
L. Chen et al. / Electrochimica Acta 44 (1999) 2751±2764
M) [55]. The solutions were prepared from analytical
grade reagents and deionized water.
A potentiosat (EG&G PAR model 273) and a frequency response analyzer (Solartron model 1260) were
the electrochemical instrumentation used in this work
to perform linear sweep voltammetry (LSV), potential
step and impedance measurements. Linear potential
sweeps were performed at 1 mV s ÿ 1 from ÿ1.0 to 1.5
V. A few LSV experiments were extended beyond 1.5
V. The current transients at di€erent potentials were
recorded by stepping the electrode from the rest potential to speci®c anodic potentials. Frequencies were
between 100 kHz and 0.05 Hz. Ten frequencies per
decade were scanned using sinusoidal potential amplitudes of 5 and 10 mV. Impedance measurements were
initiated after maintaining the anodic potential for approximately 5 min, unless speci®ed otherwise.
3. Results and discussion
3.1. Potential sweep experiments (LSV)
Fig. 1 gives the anodic polarization behavior of
2024Al in di€erent halide media. The pitting potential
2753
(Enp), which is the potential at which the current density abruptly departs from the passive current (see
inset) is estimated from the polarization curves [56, 57].
Enp is strongly dependent on the nature of the electrolyte and follows the sequence: sat. AlCl3 (ÿ0.75
V) < Cl ÿ (ÿ0.60 V) < Br ÿ (ÿ0.48 V) < I ÿ (ÿ0.30 V).
This indicates that 2024Al is more vulnerable to pitting
attack in chloride-containing solutions than in bromide
and iodide solutions, as reported previously for pure
Al [11, 57±59]. The lower pitting potential in sat. AlCl3
is due to the higher content of free-Cl ÿ and a more
acidic solution.
A pitting potential of ÿ0.71 V obtained for pure Al
in 1 M NaCl (data not shown) is in agreement with
other work [15]. The 110 mV di€erence between pure
Al and 2024Al-T4 indicates that addition of alloying
elements and heat treatment has a marked e€ect on
the pitting potential.
It has been proposed that pit initiation of aluminum
involves three consecutive stages: adsorption of halide
ions on the oxide-covered surface, chemical reaction of
the adsorbed halide ions with aluminum ions in the
oxide ®lm and thinning of the oxide ®lm by the dissolution of the complexes formed [60]. There is competitive adsorption between the halide ions and water
Fig. 1. Anodic potentiodynamic behavior of 2024Al: (a) 1 M NaCl, (b) 1 M NaBr, (c) 1 M KI, (d) AlCl3(sat.) Inset shows
expanded polarization plot near Enp.
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L. Chen et al. / Electrochimica Acta 44 (1999) 2751±2764
molecules or hydroxyl ions; the former would tend to
depassivate, while the latter would tend to passivate
the aluminum surface. Any e€ects which enhance
adsorption of halide ions would promote pitting. The
chemical reaction step also a€ects pitting and is
strongly dependent on the stability of the complexes
formed between aluminum and halide ions. The bond
strengths reported for Al±Cl, Al±Br and Al±I are 494,
444 and 368 kJ, respectively [61]. The increase in pitting potential of Cl ÿ < Br ÿ < I ÿ follows the order of
decreasing Al±X bond strength.
As shown in Fig. 1, current continued to increase
with increasing anodic potential for all four electrolytes
with the electrodissolution rate increasing in the order:
sat. AlCl3 < I ÿ < Br ÿ < Cl ÿ . A lower electrodissolution rate was obtained for 2024Al in saturated AlCl3
than other electrolytes, but the anodic current continued to increase well beyond 4 V in contrast to Beck's
report for Al that the current dropped to a plateau
above 4 V, which he attributed to complete surface
coverage of an anhydrous AlCl3 ®lm [55]. The likely
reason is the di€erent electrode con®gurations. Beck
used an anode facing up, which readily enabled the deposition and build up of the AlCl3 salt ®lm on the disk
inhibiting dissolution. On the other hand, in the present work, the rotating disk electrode, which is facing
down, sweeps any AlCl3 salt ®lm o€ the surface. At
the end of the potentiodynamic experiments for
2024Al in all four electrolytes, the electrode surfaces
were recessed and completely covered with a thin black
®lm.
The e€ect of chloride concentration and the addition
of NTMP on electrodissolution of 2024Al are shown
in Fig. 2. As the chloride concentration is decreased,
the pitting potential increased to ÿ0.55 V, and electrodissolution decreased, as expected. Fig. 2 (see inset)
also shows that the presence of 20 mM NTMP had
only a small e€ect on the pitting potential but reduced
the electrodissolution rate signi®cantly at high anodic
potentials in both 0.5 and 1 M NaCl solutions. In the
absence of NTMP, visual inspection of the electrode
surface after the potentiodynamic measurements
revealed a recessed surface covered with a thin black
®lm as described above. On the other hand, in the presence of NTMP, the surface was completely covered
with a white ®lm as a potential of 1.0 V was
approached. After 1.0 V, thickening of this surface
®lm is indicated, further inhibiting dissolution [62].
3.2. Potential step experiments
Comparison of typical current transients of 2024Al
in 1 M NaCl with (dashed lines) and without (solid
lines) NTMP is shown in Fig. 3. In the absence of
NTMP, current transients for potentials, Enp < ÿ0.45
V, show an initial fast current decay followed by an
increase to a steady value. The initial current decay is
ascribed to growth of the oxide ®lm, followed by an
increase in current after the minimum due to localized
breakdown of the oxide and the development of
pits [11]. Degradation of the protective ®lm formed on
2024Al in chloride solutions readily occurred even at
Fig. 2. Anodic potentiodynamic behavior of 2024Al in NaCl: (a) 1 M NaCl, (b) 1 M NaCl + 20 mM NTMP, (c) 0.5 M NaCl and
(d) 0.5 M NaCl + 20 mM NTMP. Inset shows expanded polarization plot near Enp.
L. Chen et al. / Electrochimica Acta 44 (1999) 2751±2764
2755
Fig. 3. E€ect of NTMP on current transients of 2024Al in 1 M NaCl at various anodic potential steps; none (solid line), 20 mM
NTMP (dashed line).
the low anodic potential of ÿ0.55 V, which is above
the pitting potential (see inset of Fig. 1). At higher potentials (ÿ0.30 V) initial oxide ®lm growth did not
occur, as indicated by the absence of a current minimum, and electrodissolution and pitting increased. The
pits that could be observed visually contained black
®lms. On the other hand, in the presence of NTMP,
smaller currents are observed due to the formation of
a protective surface ®lm (white).
At anodic potentials below Enp, the current transients of 2024Al in 1 M Br ÿ and I ÿ exhibited a monotonic decay due to the growing oxide layer (Fig. 4).
Current transients for the alloy in Br ÿ and I ÿ for potential steps near Enp are also shown (Fig. 4b and d),
respectively, and indicate the initiation of pitting.
These pits also contained black ®lms. At these anodic
potentials, repassivation after pit initiation was not evident.
Examination of the electrode surface immediately
after extended higher potential step experiments for
2024Al in Cl ÿ , Br ÿ and I ÿ solutions showed a black
ring covering the outer perimeter of the rotating disk
surface re¯ecting current distribution e€ects in the
breakdown of the oxide layer. The oxide layer at the
center appeared to be intact.
Fig. 5 shows the current transients for 2024Al in
saturated AlCl3. At the low anodic potentials of ÿ0.70
and ÿ0.65 V, the current increased monotonically with
time to a steady value. The alloy surface after these
measurements was pitted and partially covered with a
black ®lm. At higher potentials (rÿ0.60 V) the current abruptly increased to a maximum and then
decreased to a lower steady state value. At this point
Fig. 4. Current transients of 2024Al in 1 M NaBr (a and b) and in 1 M KI (c and d) at various potential steps.
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L. Chen et al. / Electrochimica Acta 44 (1999) 2751±2764
in the experiment the surface was completely covered
with the black ®lm. Since the steady current is relatively high, the ®lm should be quite porous.
The LSV and potential step experiments establish
the following points when anodic polarization of
2024Al in halide media exceeded the pitting potential.
The LSV experiments show that for extended high
anodic polarization, 2024Al rotating disks in Cl ÿ ,
Br ÿ , I ÿ and sat. AlCl3 solutions undergo extensive
electrodissolution of Cu as well as Al and results in the
complete coverage of the disks with a black salt ®lm.
On the other hand, for potential step experiments of
2024Al in Cl ÿ , Br ÿ , and I ÿ at relatively moderate
anodic polarization, pits and black ®lms were observed
only around the periphery of the disk. However, for
sat. AlCl3 solutions, pits and black ®lms were observed
over the entire rotating disk due to the much higher
halide concentrations than 1 M Cl ÿ , Br ÿ , and I ÿ solutions. The black salt ®lm is possibly an Al±Cu oxychloride species since analysis indicated the presence of
Cu(II). Such black ®lms did not form on pure Al in
these electrolytes under similar conditions.
3.3. Open circuit TTA experiments
Preliminary experiments of 2024Al-T4 in 1 M NaCl
under stationary and open circuit conditions showed
that after one week the surface appearance was
unchanged in the presence of tolyltriazole (5.5 mM).
On the other hand, in the absence of tolyltriazole pits
with black ®lm were observed at the edge of the disk
after two days; after one week pitting with black ®lms
extended toward the center of the disk with increased
pit density at the edge. After 11/2 months, there were
a few small pits with black ®lm in the presence of
TTA, but many more, much larger pits were seen in
Fig. 5. Current transients of 2024Al in saturated AlCl3 at various potential steps.
the absence of TTA. The experiments indicate that
benzotriazole derivatives in NaCl can sharply diminish
pitting and the exposure of Cu at the surface of Cucontaining high strength Al alloys.
3.4. Impedance spectroscopy experiments
Complex plane and corresponding impedance Bode
plots for 2024Al in solutions of two NaCl concentrations at the rest potential after an immersion time
of 75 min are shown in Fig. 6. Measurements were
performed for both stationary and rotating (1500 rpm)
disk electrodes. A capacitive semicircle is clearly evident in the complex plane plot, and the polarization
resistance was determined from its diameter. Fig. 6
shows that the polarization resistance at the rest potential is dependent on both the concentration of NaCl
and the hydrodynamic conditions at the electrode. An
increase in the NaCl concentration decreased the
polarization resistance of both stationary and rotating
electrodes. Convection decreased the polarization resistance signi®cantly by increasing mass transport of
the aggressive ions to the electrode and removing corrosion products from the surface.
Fig. 7 presents the time evolution of the polarization
resistance in 1 M NaCl determined from impedance
measurements at the rest potential (Fig. 6). As
expected, the polarization resistance decreased with
time for both stationary and rotating electrodes, indicating an increased corrosion rate with immersion
time. Larger polarization resistances were always
observed for stationary electrodes.
Fig. 8 shows the e€ect of NTMP on the EIS characteristics of 2024Al in 1 M NaCl at anodic potentials
above Enp. The anodic potentials were maintained for
approximately ®ve minutes before the EIS measurements were initiated. In the absence of NTMP a depressed capacitive semicircle and an inductive loop are
clearly seen in the complex plane plots. Further, a
close examination of both complex plane and Bode
plots at high frequencies indicates a possible additional
capacitive loop, which is more evident at higher anodic
potentials. The ®rst, barely perceived capacitive loop in
the high frequency region in Fig. 8 may re¯ect the
sparse coverage of pits with black ®lm on the disk; the
second, larger one at lower frequencies is associated
with the oxide ®lm. When the anodic potential (e.g.
ÿ0.55 V) was maintained for 2 h before EIS measurements, the capacitive loop in the highest frequency
region becomes quite distinct, and it is also clearly evident in the phase angle Bode plot (Fig. 9). There was a
considerable increase in the number of pits containing
black ®lm. Pure Al in 1 M NaCl did not exhibit the
high frequency capacitive loop (Fig. 10); furthermore,
black ®lms in the pits were not observed. It should be
noted that Ea ÿ Enp = 0.05 V for both the 2024Al and
L. Chen et al. / Electrochimica Acta 44 (1999) 2751±2764
Fig. 6. Complex plane and Bode plots of 2024Al in 0.5 M
and 1 M NaCl at the rest potential after 75 min immersion.
Stationary electrodes-open points; rotating electrodes-closed
points.
Fig. 7. Time dependence of the polarization resistance of
2024Al in 1 M NaCl at the rest potential.
2757
pure Al results shown in Figs. 9 and 10. The low frequency inductive loops in Figs. 8±10 have been
ascribed previously for Al in NaCl to localized
corrosion [41, 52]. The diameter of the large capacitive
loop decreased with increasing anodic potential, as
shown in Fig. 8, due to the breakdown of the oxide
®lm and increased rate of electrodissolution.
In NTMP-chloride solutions EIS behavior of 2024Al
at potentials above Enp (Fig. 8) shows two clearly distinct capacitive semicircles at high frequencies and an
inductive loop at low frequencies. As shown in the
inset of Fig. 2, only a slight increase in Enp (010 mV)
was observed due to the presence of NTMP. However,
the capacitive loop at the highest frequencies is much
more pronounced in the presence than in the absence
of NTMP and is enhanced further as the anodic potential is increased. A white ®lm is formed and it is likely
a surface ®lm involving NTMP. Furthermore, NTMP
increased the total impedance of the system in accord
with the inhibiting e€ects observed in the potential
step measurements (Fig. 3). The corresponding phase
angle Bode plots in Fig. 8 provide an additional basis
for these comparisons, as shown by the two capacitive
time constants at high frequencies in the presence of
NTMP. It should be noted that the capacitance loop
in the highest frequency region associated with the
NTMP ®lm appears to be distinct from that related to
the black ®lms in pits in NTMP-free Cl ÿ , Br ÿ , I ÿ
and sat. AlCl3 solutions.
EIS plots for 1 M NaBr below and above the pitting
potential (ÿ0.48 V) are shown in Fig. 11. At ÿ0.55 V,
which is below the pitting potential of ÿ0.48 V, there
is one capacitive semicircle. However, at ÿ0.50 V there
is, in addition, an inductive loop, indicating that pits
had developed during the potential step experiments at
this potential. This inductive loop has been interpreted
as a manifestation of pitting by Metikos-Hunkovic [41]
and Bessone et al. [52], as well as others for Al in
NaCl. The high frequency capacitance loop, which is
associated with black ®lms in the pits, is observed
when the anodic potential is raised to ÿ0.45 V, in addition to the larger one at lower frequencies and the
inductive loop at low frequencies. The two maxima in
the phase angle Bode plot at high frequencies re¯ect
the two capacitive loops.
The impedance spectra of 2024Al in 1 MKI solutions are presented in Fig. 12. Consistent with the potential step results, a capacitive semicircle with a large
polarization resistance was obtained between ÿ0.55
and ÿ0.40 V where passive conditions prevailed
(Enp0ÿ 0.30 V). The absence of the inductive loop is
attributed to the absence of pits and con®rm the LSV
and potential step measurements. At higher anodic potentials where pits and black ®lms were observed, two
capacitive loops at high frequencies and an inductive
loop at low frequencies are indicated by the complex
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L. Chen et al. / Electrochimica Acta 44 (1999) 2751±2764
Fig. 8. Complex plane and Bode plots of 2024Al in 1 M NaCl in the absence and presence of NTMP at di€erent anodic potentials.
Experimental (points) and simulated (lines). Phase angle Bode plots-closed points; impedance plots-open points.
L. Chen et al. / Electrochimica Acta 44 (1999) 2751±2764
2759
Fig. 9. Complex plane and Bode plots of 2024Al in 1 M NaCl at Eÿ Enp = 0.05 V and 2 h immersion. Same symbols as in Fig. 8.
plane and Bode plots, as discussed above for both Cl ÿ
and Br ÿ solutions.
Fig. 13 presents the complex plane and Bode plots
obtained for 2024Al in saturated AlCl3 at potentials
above Enp (ÿ0.75 V). At ÿ0.70 and ÿ0.65 V one large
capacitive loop and two low frequency inductive loops
are evident. However, the capacitive loop in the high
frequency region is not readily discerned at these
anodic potentials. The second inductive loop, which is
possibly due to adsorbed hydrogen [63, 64], disappeared at higher anodic potentials (rÿ 0.60 V). It is at
these two low anodic potentials (< ÿ 0.60 V) where the
potential step measurements in Fig. 5 show the current
increasing monotonically to a steady value. At higher
potentials (rÿ 0.60 V) where the current transients
exhibited maxima, the capacitive semicircle in the highest frequency region becomes quite pronounced in
both the complex plane and phase angle Bode plots,
and complete surface coverage of the 2024Al disk by a
black ®lm was observed.
The electrical circuit analogs, or equivalent circuits
(EC), in Fig. 14 were utilized with the Boukamp program to simulate the impedance characteristics of
2024Al-T4 and Al in halide media during anodic
polarization, as shown in Figs. 8±13. Within the passive region where only one semicircle in the complex
plane plots and one peak in the phase angle Bode plot
are seen, the EC in Fig. 14 with the parallel inductive
and resistive elements omitted, ®t satisfactorily the
impedance spectra (Figs. 10 and 11). Further, the complete EC in Fig. 14a was able to simulate the impedance spectra exhibiting one capacitive loop and one
inductive loop for Al and 2024Al in Figs. 10 and 11,
respectively. Other investigators had applied this EC
previously to simulate the impedance characteristics of
Al above the pitting potential (e.g. Ref. [41, 45]). For
Fig. 10. Complex plane and Bode plots of Al in 1 M NaCl at Eÿ Enp = 0.05 V and 2 h immersion. Same symbols as in Fig. 8.
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L. Chen et al. / Electrochimica Acta 44 (1999) 2751±2764
Fig. 11. Complex plane and Bode plots of 2024Al in 1 M NaBr at di€erent potentials. Same symbols as in Fig. 8.
2024Al in this work, the spectra simulated by the EC
in Fig. 14a represent the initial stages of pitting.
For more extensive pitting of 2024Al (greater time
duration and/or higher anodic polarization) when
black salt ®lms in the pits were clearly evident, a second capacitive loop (complex plane plots) and a second maximum (phase angle Bode plots) in the highest
frequency region are seen, and the EC in Fig. 14b was
utilized to simulate the impedance spectra, as shown in
Figs. 8, 9 and 11±13. Although the second capacitive
loop at high frequencies in the case of 2024Al in 1 M
NaCl anodically polarized for about 5 min at ÿ0.55 V
is not distinct (Fig. 8), polarization for 2 h lead to
more pits, and black ®lm in the pits were observed.
This surface condition is clearly re¯ected in both the
complex plane and phase angle Bode plots (Fig. 9).
The interesting feature of the two inductive loops
for 2024Al in sat. AlCl3 at the low anodic potentials of
ÿ0.70 and ÿ0.065 V (Enp0ÿ 0.75 V) is simulated by
addition of a second parallel L4±R4 circuit elements in
series with parallel L2±R2 in Fig. 14a. Although a
good ®t was obtained for the low frequency impedance
spectra at ÿ0.65 V, the ®rst inductive loop at ÿ0.70 V
could not be simulated with this EC. As mentioned
above, this part of the impedance spectra may possibly
be due to adsorbed hydrogen atoms [63, 64], whose
presence diminishes and ®nally becomes negligible as
the anodic potential is increased.
In some cases, simulation of the impedance spectra
was improved by replacing the capacitance, C1, by a
constant phase element, CPE = [1/T( jo)n], where n = 1
represents an ideal capacitance. However, the fre-
L. Chen et al. / Electrochimica Acta 44 (1999) 2751±2764
Fig. 12. Complex plane and Bode plots of 2024Al in 1 M KI at di€erent potentials. Same symbols as in Fig. 8.
2761
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L. Chen et al. / Electrochimica Acta 44 (1999) 2751±2764
Fig. 13. Complex plane and Bode plots of 2024Al in saturated AlCl3 at di€erent potentials. Same symbols as in Fig. 8.
L. Chen et al. / Electrochimica Acta 44 (1999) 2751±2764
2763
ducted to ascertain the e€ect of tolyltriazole (TTA) on
the surface appearance. The electrodes were examined
after 1 and 2 days, 1 week and 11/2 months. The presence of TTA sharply reduced pitting and the appearance of the black ®lm.
Acknowledgements
This work was supported by the US Air Force/
Oce
of
Scienti®c
Research
(F49620-93-10320P00004). PTAS acknowledges a fellowship grant
from the Brazilian Research Council, CNPq.
Fig. 14. Equivalent circuits employed to simulate impedance
spectra.
quency response exponents (n) were close to one, approximately 0.9 in these cases.
4. Conclusion
Electrodissolution and pitting behavior of 2024AlT4 rotating disk electrodes in Cl ÿ , Br ÿ , I ÿ and sat.
AlCl3 solutions have been investigated by linear sweep
voltammetry, potential step and impedance spectroscopy. Pitting potentials (Enp) increased in the
order: AlCl3 (sat., ÿ0.75 V vs SCE) < Cl ÿ (1 M, ÿ0.60
V) < Br ÿ (1 M, ÿ0.48 V) < I ÿ (1 M, ÿ0.30 V). The
presence of Cu(II) in the black ®lms formed during
electrodissolution has been determined. Initial pitting
at low anodic polarization occurred at the disk periphery; black ®lm was observed in the pits. Pitting was
more extensive in sat. AlCl3. With increase in anodic
polarization, pitting extended toward the disk center.
For extensive and prolong anodic polarization, pits
and black ®lm covered the entire disk surface.
Within the passive region for 2024Al-T4, impedance
measurements showed a single capacitive loop in complex plane plots. In the pitting region where black ®lm
in the pits was perceived, a second capacitive loop
appeared at higher frequencies, as well as an inductive
loop at low frequencies. Two maxima at high frequencies were exhibited in the corresponding phase angle
Bode plots. Since pure Al exhibited only a single capacitive loop in the complex plane plots and one maximum in the phase angle Bode plots at high
frequencies, the second capacitive loop at higher frequencies is attributed to the presence of the Cu-containing black ®lm.
Preliminary experiments of 2024Al-T4 in 1 M NaCl
at open circuit and stationary conditions were con-
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