eis study on pitting corrosion of 7150 aluminum alloy in sodium chloride and hydrocloric acid...
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Materials and Corrosion 2010, 61 No. 9 DOI: 10.1002/maco.200905413 783
EIS study on pitting corrosion of 7150 aluminum alloy insodium chloride and hydrochloric acid solution
G. S Peng, K. H. Chen*, H. C. Fang, H. Chao and S. Y. Chen
The pitting corrosion behavior of 7150 aluminum alloy was studied by
electrochemical impedance spectroscopy (EIS) in the sodium chloride and
hydrochloric acid solution. Based on EIS features and corrosion morphologies as
well as corrosion potential, the process of pitting corrosion could be clearly
divided into four stages: at the first stage, the Nyquist diagramwas composed of
two overlapping capacitive loops at the high-medial frequency and one
inductive loop at the low frequency. At the second stage (metastable pits
developing stage), there existed one small capacitive loop at the high frequency
and one big capacitive loop at the medial frequency. At the third stage (stable
pits developing stage), two time constants were more clearly distinguished,
corresponding to two obvious capacitive loops. At the fourth stage, there
appeared one capacitive loop, attributing to uniform corrosion. An equivalent
circuit was designed to fit EIS, and the experimental results and the fitted results
had good correspondence.
1 Introduction
Al–Zn–Mg–Cu alloy like AA7150 are susceptible to severe
localized corrosion. Pitting corrosion, as one of the localized
corrosion, is responsible for the nucleation of intergranular
corrosion, exfoliation corrosion, further develops to the stress
corrosion cracking. Therefore, much study has focused on the
pitting corrosion of aluminum alloy. Generally, pitting corrosion
is initiated at microstructural inhomogeneities, such as coarse
intermetallic phase [1] or age-harden phase [2–6]. The latter,
which mainly contains h0-MgZn2 and h-MgZn2 in 7000 series
aluminum alloy, is more responsible than coarse intermetallic
phase to pitting corrosion due to its amount and extension.
Smialowska [7] suggested pitting corrosion has four stages.
Firstly, the process occurs at the boundary of the passive film and
the solution. Secondly, the process occurs within the passive film.
Thirdly, metastable pits develop below the pitting potential and
then repassivate. Fourthly, stable pits grow above the pitting
potential.
Electrochemical impedance spectroscopy (EIS) is a powerful
method to characterize the corrosion resistance of aluminum
alloy. The impedance spectra can be fitted to an appropriate
equivalent circuit, which allows obtaining the time dependence of
important properties of the surface layer such as the capacitance
and the resistance of the barrier as well as the porous layers. To
G. S Peng, K. H. Chen, H. C. Fang, H. Chao, S. Y. Chen
State Key Laboratory for Powder Metallurgy, Central South University,
Changsha, 410083 Hunan (P.R. China)
E-mail: [email protected]
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our knowledge, there are plenty of data on pitting corrosion of
aluminum or aluminum alloys in different environments by
EIS, especially those containing chloride ions or EXCO solution
[8–12]. For example, Conde and de Damborenea [8] studied the
pitting corrosion behavior of 8090 alloy in EXCO solution by EIS,
finding that the Nyquist diagram was initially described by a well-
defined capacitive semicircle. With the increase of the immersion
time, the Nyquist diagram exhibited two overlapping semicircles.
They depicted the process of pitting corrosion of 8090 alloy
reasonably combining the Nyquist diagram. Cabot et al. [2] foundthat the depression of the capacitive semicircles was correlated
with the size of the MgZn2 precipitates in heat-treated Al–Zn–Mg
alloy by EIS. In this paper, we studied the pitting corrosion
process of AA7150 in sodium chloride and hydrochloric acid
solution by EIS, finding some unique features of the Nyquist
diagram, further attempting to disclose the relationships between
pitting corrosion and features of EIS.
2 Experimental
2.1 Materials
The major composition (mass fraction%) of the aluminum alloy
is Zn 6.5%, Mg 2.4%, Cu 2.2%, Zr 0.15%, and Al balance. The
specimens were homogenized at 455–470 8C and deformed 85%
at 400 8C, this was followed by subsequent annealing at 200 8Cfor 12 h and solutionized at 480 8C for 2 h, finally aged for
T6 (130 8C, 24 h), which defined an ageing condition close to
peak ageing.
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784 Peng, Chen, Fang, Chao, and Chen Materials and Corrosion 2010, 61 No. 9
2.2 Experimental methods
The surface of specimens for this investigation was ground using
abrasive papers from 600# to 1400#, polished with diamond paste,
degreased using acetone, rinsed with distilled water, and dried in
air. The 5mm-thick specimen for measurement of EIS was
connected to a copper wire, and thenmounted in epoxy resin with
only 1 cm2 surface exposed.
The electrolyte utilized was sodium chloride and hydro-
chloric acid solution. The solution of 30 g/L sodium chlor-
ideþ 10ml/L hydrochloride acid (pH 2) was used and was not
stirred in contact with air, its temperature being maintained at
25 8C and providing the ratio of solution volume to metal surface
of 40ml/cm2.
Electrochemical measurements were carried out by using the
three-electrode system. A saturated calomel electrode was used as
reference electrode, a platinum foil as counter electrode, and the
alloy being studied as the working electrode. Three electrode
systems were connected to CHI 660C Electrochemical work-
station, a product of CH instruments Inc. (Austin, TX, USA) at
the open circuit potential. A sinusoidal potential of 5mV
amplitude and a frequency sweep of 100 kHz–0.01Hz was used
as disturbance signal during the measurement. The equivalent
circuits simulating the electrochemical response of the system
were constructed using ZView2 software. During the above
experiments, the specimen was removed and recorded with
digital camera (SONY DSC-H10) for the corrosion morphologies,
then again immersed in the solution to continue the experiment.
At the immersion of 33, 50, and 104 h, the parallel specimens
were removed and cleaned by using 2% CrO3þ 5% H3PO4 at
80 8C, which corrosion morphologies were observed with JEOL-
2010 SEM, corresponding to Fig. 9b, d, e, and g.
3 Results and discussion
3.1 EIS features
At the initial immersion time, there exist a depressed capacitive
loop at the high-medium frequency and an inductive loop at the
low frequency in the Nyquist diagram (Fig. 1). Though the visual
evaluation of the impedance data above the real axis, which is
obtained at the high-medium frequency, yield only one time
Figure 1. Nyquist plots of 7150 aluminum alloy in sodium chloride and hyd
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constant, the analysis of the data leads to the derivation of at least
two time constants [13]. The number of time constants is
determined by calculating the difference between the fitted and
the experimental data as function of the frequency. When this
difference shows a periodicity, it means that not all the time
constants have been taken into account. Therefore, the visual
depressed capacitive loop above the real axis (Fig. 1) is composed
of two capacitive loops, which concurs with suggestions by other
authors in the literature [8, 14]. Bode phase angle plots exhibit the
existence of a wider maximum with the immersion time, also
suggesting that there are two overlapping maxima. With
increasing immersion time, the radius at the medium frequency
capacitive loop and the angle at the high-median frequency
increase, whereas the inductive loop at the low frequency shrinks
with the immersion time and then disappears at 46 h as shown in
Fig. 2.
Figure 3 illustrates the Nyquist and Bode diagrams observed
for 7150 aluminum alloy during the immersion time of 46–93 h.
It also can be seen the Nyquist diagram consists of two obvious
capacitive loops when the high frequency behavior is magnified.
With the immersion time increasing, the radius of capacitive loop
at medium frequency increases. On the other hand, phase angle
at medium frequency increases and then shifts toward lower
frequency, while high frequency phase angle gets lower, finally
disappears.
When the immersion time reaches 97 h, the Nyquist plot
exhibits one capacitive loop and one Warburg resistance (Fig. 4).
With prolongation of immersion time from 104 up to 115 h, the
Nyquist diagram shows two obvious capacitive loops, correspond-
ing to two obvious time constants at the Bode diagram (Fig. 5).
The radius at median frequency capacitive loop increases with
immersion time. After 124 h, the Nyquist diagram exhibits one
capacitive loop (Fig. 6), corresponding to one time constant.
3.2 Discussion
The EIS data above the real axis at the different immersion time
were analyzed with ZView2 program using the equivalent circuit
as shown in Fig. 7, which is based on the comprehension of the
pitting corrosion and interpreted different physicochemical
properties in different stages of pitting corrosion. It can be seen
that there is good compatibility between the experimental and
rochloric acid solution measured at different immersion time of 1–33 h
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Materials and Corrosion 2010, 61 No. 9 EIS study on pitting corrosion 785
Figure 2.Nyquist plots of 7150 aluminum alloy in sodium chloride and hydrochloric acid solutionmeasured at different immersion time of 33–46 h
Figure 3.Nyquist plots of 7150 aluminum alloy in sodium chloride and hydrochloric acid solution measured at different immersion time of 46–93 h
Figure 4. Nyquist plots of 7150 aluminum alloy in sodium chloride and hydrochloric acid solution measured at 97 h
Figure 5.Nyquist plots of 7150 aluminum alloy in sodium chloride and hydrochloric acid solutionmeasured at different immersion time of 104–115 h
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786 Peng, Chen, Fang, Chao, and Chen Materials and Corrosion 2010, 61 No. 9
Figure 6. Nyquist plots of 7150 aluminum alloy in sodium chloride and hydrochloric acid solution measured at 124 h
simulated data in above figures (Figs. 1, 3, and 5, solid line
represents fitting line). In all cases, simulation of the impedance
spectra is improved by replacing the capacitance C, by a constantphase element (CPE), which is expressed as an exponent affecting
the imaginary component of complex impedance:
ZCPE ¼ 1
CðjvÞn (1)
where C is a constant, j is the complex operator (�1)1/2, v is the
angular frequency, and n is an empirical exponent. As n¼ 1, ZCPE
represents an ideal capacitance; n¼ 0, a resistance; n¼�1, an
inductance; and n¼ 0.5, a Warburg impedance. The presence of a
ZCPE often has been explained by dispersion effects that can be
caused by microscopic roughening of a surface, and which, in
turn, have been related to surface preparation or localized
corrosion [15].
Figures 8 and 9 show the corrosion potential and the typical
corrosion morphologies at different immersion time, respec-
tively. Based on the Nyquist diagrams and the corrosion
morphologies as well as the corrosion potential of the sample,
the process of pitting corrosion of the alloy in sodium chloride
and hydrochloric acid solution can be clearly divided into four
stages. A schematic model of the process of pitting corrosion is
obtained as shown in Fig. 10.
Stage 1
From the beginning to 33 h, copious hydrogen bubbles evolve on
the surface of aluminum alloy rapidly. The EIS contains three
parts, high frequency capacitive loop and medial frequency
Figure 7. Equivalent electrical circuit for all immersion of 7150
aluminum alloy in sodium chloride and hydrochloric acid solution
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capacitive loop along with a low frequency inductive loop.
According to the view by Brett [16], the corrosion process in
passive systems is controlled by the reactions in the interface
oxide/electrolyte, producing corrosion product. Thus, in such
systems the oxide/electrolyte response predominates at medial
frequency. Hence, the high frequency loop must be associated
with the electron transfer process in the interfaces metal/oxide.
As to the inductive loop, Keddam et al. [17] and Cao et al. [18]
indicated that it is more likely promoted by the weakening of the
protectively effectiveness of the aluminum oxide layer. In this
case, it is believed that the inductive component is associated with
the weakening of the protective oxide layer. The simulated
parameter values are displayed in Table 1.
It is known that capacitance is defined by the following
equation:
C ¼ "0"S
d(2)
where e0 is the permittivity of vacuum, e is the relative permittivity
of the film, S is the area of surface or interface, and d is the
thickness of the film or the double-layer. As can be seen from
Table 1, Cd and Rp, which represent the interfaces metal/oxide,
Figure 8. Relationship between corrosion potential and immersion
time
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Materials and Corrosion 2010, 61 No. 9 EIS study on pitting corrosion 787
Figure 9. Typical corrosionmorphologies of specimen at different immersion time (a) and (b) 33 h, (c)–(e) 50 h, (f) and (g) 104 h, and (h) 124 h, among
which (a, c, f, and h) obtained by digital camera and (b, d, e, and g) obtained by SEM
Figure 10. A schematic model of the process of pitting corrosion
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fluctuate with the immersion time. It can be explained that the
thickness of oxide layer varies with the time due to hydroxy ion
(origins from the reduction of proton and hydroxy ion to be left
behind in the passive layer) competes with aggressive chloride ion
on the oxide film to form Al(OH)p–mClm [7, 19–21]. Finally, the
increasing trend of Cd and Rp is attributed to the thickness of
oxide layer gets thinner and chloride ion attacking alloy surface
gets more difficult for being obstructed by the corrosion products,
respectively. In addition, Clf and Rt increase with the time due to
the area of corrosion product and further producing corrosion
product resistance increase, respectively. The angle of high-
median frequency, especially obvious to median frequency, shifts
toward the higher angle, indicating the surface of alloy is covered
by corrosion product gradually with the immersion time, as can
be seen from Fig. 9a. In addition, no corrosion pits can be found
in the surface of alloy (Fig. 9b), indicating no pitting corrosion
occurs in this stage. Corrosion potential keeps unchanged at
about �0.74V.
Stage 2
From 33 to 97 h, the electrode surface becomes darker than in
stage 1 due to being covered by corrosion products (Fig. 9c). Some
hydrogen bubbles adsorb on the electrode surface. Compared to
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788 Peng, Chen, Fang, Chao, and Chen Materials and Corrosion 2010, 61 No. 9
Table 1. Simulated EIS information for pitting corrosion in sodium chloride and hydrochloric acid solution in stage 1
Immersion time (h) Cd 10�3 [V�1 cm�2 S�n] n1 Rp [V cm2] Clf 10
�2 [V�1 cm�2 S�n] n2 Rt [V cm2]
1 5.75 0.60 3.311 1.42 0.77 4.925
5 7.67 0.62 4 1.39 0.78 4.826
9.5 5.65 0.65 3.243 1.86 0.67 7.596
21 8.70 0.63 5.381 1.94 0.75 9.442
24 4.95 0.74 3.826 2.27 0.70 11.34
33 9.32 0.66 5.714 2.23 0.76 11.63
stage 1, there exist two distinct capacitive loops at the high-
median frequency. It is well known that precipitates, especially
MgZn2 phase, are projecting out from the surface plan promoting
a microscopic roughness (Fig. 9d). As a consequence of the
heterogeneity, the oxide layer above MgZn2 phase is likely to be
thinner due to easier attack. Since the dissolution of the original
oxide layer above MgZn2 phase is the most intense process after
the passive layer gets thinner, the capacitive loop at the high
frequency is attributed to the dissolution of the original oxide
layer above MgZn2 phase.Wloka and Virtanen [3] and Cabot et al.[6] also observed the electrochemical response resulting from
MgZn2 phase. On the other hand, the capacitive loop at the
medial frequency represents the interface oxide/electrolyte,
which cause has been described in stage 1. The simulated
parameter values are displayed in Table 2. As can be seen, with the
immersion time increasing, Cd and Rp, which reflects the original
oxide layer dissolution above MgZn2 phase, fluctuate due to
MgZn2 phase distribution inhomogeneity. For the corrosion
product layer is thicker, Clf decreases whereas Rt increases. When
the immersion time is 97 h, there exists an obvious Warburg
resistance in Nyquist diagram (Fig. 4), and the corrosion potential
sharply increases to �0.478V (Fig. 8). The results illustrate the
corrosion rate gets slower with the immersion time. Since the
Nyquist diagram at the immersion time 97 h is the transition
from stage 2 to stage 3, not the typical pitting corrosion process,
we will not simulate by the equivalent circuits. Figure 9e shows
the cross-sectional view of corrosion sample when the immersion
time reaches 50 h. The area A and B, represent the original oxidelayer being dissolved and having been dissolved above MgZn2
able 2. Simulated EIS information for pitting corrosion in sodium chloride and hydrochloric acid solution in stage 2
mersion time (h) Cd 10�5 [V�1 cm�2 S�n] n1 Rp [V cm2] Clf 10
�2 [V�1 cm�2 S�n] n2 Rt [V cm2]
6 4.58 0.61 3.40 2.57 0.54 28.16
9 0.17 0.37 105.5 2.24 0.59 27.17
8 0.32 0.37 87.66 2.31 0.61 36.45
5 4.80 0.17 151 2.16 0.67 59.80
2 1.05 0.25 115.9 2.06 0.68 65.35
3 5.86 0.23 95.17 1.93 0.75 79.20
T
Im
4
4
5
7
8
9
Table 3. Simulated EIS information for pitting corrosion in sodium chlori
Immersion time (h) Cd 10�4 [V�1 cm�2 S�n] n1 Rp
104 2.84 0.57
115 2.63 0.56
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phase, respectively. The stage 2, therefore, is defined as an
incubation period for pitting development.
Stage 3
It belongs to stable pit growth, which is indicated by the
generation of hydrogen bubbles at a few fixed spots on the
exposed surface and the corrosion product layer stops getting
thick. In this stage, as depicted in Fig. 10, MgZn2 phase in the
cavities is exposed and dissolved gradually resulting from it is
more anodic than the matrix. Corrosion morphology is shown in
Fig. 9f and g. The physicochemical properties on the two
capacitive loops at high and median frequency (Fig. 5) are
consistent with Conde’s view [8, 20], that is, Cd and Rp represent
the original oxide layer dissolution whereas Clf and Rt reflects the
pitting corrosion process due to MgZn2 dissolution. The fitted
results are listed in Table 3.
It is found thatCd decreases with immersion time, indicating
the decrease of the original oxide layer area. A trend that Clf
increases with the immersion time can be found because the area
of the pitting corrosion increases. Rp and Rt increase with the
immersion time, suggesting the pitting corrosion rate is slow
gradually due to corrosion process is hindered by corrosion
product.
Stage 4
After long time corrosion, the corrosion condition in the sodium
chloride and hydrochloric acid solution is not aggressive, the
de and hydrochloric acid solution in stage 3
[V cm2] Clf 10�2 [V�1 cm�2 S�n] n2 Rt [V cm2]
41.66 1.70 0.76 60.88
41.78 1.75 0.70 81.54
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Materials and Corrosion 2010, 61 No. 9 EIS study on pitting corrosion 789
surface of the corroding electrode begins self-preparation,
which will result in only one capacitive loop in the Nyquist
plot (Fig. 6) and corrosion morphology shown in
Fig. 9h, indicating the beginning of general corrosion. As the
stage does not belong to the pitting corrosion stage, we will not
discuss it in this paper.
4 Conclusions
A four-stage evolution of pitting corrosion has been found on the
surface of the 7150 aluminum alloy during being immersed in
sodium chloride and hydrochloric acid solution based on the
analysis of corrosion morphology and EIS features as well as
corrosion potential. At the first stage, there exist two capacitive
loops and one inductive loop, indicating that the original
oxide layer gets thinner and corrosion products generates
gradually by the chloride ion attack. At the second stage
(metastable pits developing stage), the Nyquist diagram shows
two capacitive loops, which represent the oxide layer dissolves
above the MgZn2 phases and corrosion products generates,
respectively. At the third stage (stable pits developing stage), the
Nyquist diagram exhibits two obvious capacitive loops, attributing
to the original oxide layer area gets smaller and the area covered by
pit corrosion gets bigger, respectively. At the fourth stage, Nyquist
diagram has one capacitive loop, corresponding to general
corrosion stage.
Acknowledgements: The authors wish to acknowledge the
financial support of National Basic Research Program of China
(No. 2005CB623704), National Natural Science Foundation of
China (Grant No. 50471057), and Creative research group of
National Natural Science Foundation of China (Grant No.
50721003). National Key Technology Reporting and Developing
Program of China (2007BAE38B06).
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(Received: May 31, 2009)
(Accepted: June 22, 2009)
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