EIS Study on Pitting Corrosion of 7150 Aluminum Alloy in Sodium Chloride and Hydrocloric Acid...

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.

� 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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


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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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


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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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



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


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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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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5 References

[1] K. Jafarzadeh, T. Shahrabi, M. G. Hosseini, J. Mater. Sci.Technol. 2008, 24, 215.

[2] P. L. Cabot, J. A. Garrido, E. Perez, A. H. Moreira, P. T. A.Sumodjo, W. Proud, Electrochim. Acta 1995, 40, 447.

[3] J. Wloka, S. Virtanen, Surf. Interface Anal. 2008, 40, 1219.[4] T. Ramgopal, P. Schmutz, G. S. Frankel, J. Electrochem. Soc.

2001, 148(9), B348.[5] A. H. Moreira, A. V. Benedetti, P. T. A. Sumodjo,

J. A. Garrido, P. L. Cabot, Electrochim. Acta 2002, 47, 2823.[6] P. L. Cabot, J. A. Garrido, E. Perez, J. Appl. Electrochem. 1995,

25, 781.[7] Z. Szklarska-Smialowska, Corr. Sci. 1999, 41, 1743.[8] A. Conde, J. de Damborenea, Electrochim. Acta 1998, 43, 849.[9] Z. Zhang, J. Q. Zhang, H. B. Shao, J. M. Wang, C. N. Cao,

Trans. Nonferrous Met. Soc. China 2001, 11, 748.[10] A. Aballe, M. Bethencourt, F. J. Botana, M. Marcos,

R. Osuna, Mater. Corros. 2001, 52, 185.[11] J. X. Su, Z. Zhang, Y. Y. Shi, F. H. Cao, J. Q. Zhang, Mater.

Corros. 2006, 57, 484.[12] J. F. Li, Z. Q. Jia, C. X. Li, N. Birbilis, C. Cai, Mater. Corros.

2008, 59, 1.[13] D. H. Van Der Weijde, E. P. M. VanWesting, J. H. W. deWit,

Corr. Sci. 1994, 36, 643.[14] F. H. Cao, Z. Zhang, J. F. Li, Y. L. Cheng,Mater. Corros. 2004,

55, 18.[15] K. Juttner, Electrochim. Acta 1990, 35, 1501.[16] C. M. A. Brett, Corr. Sci. 1992, 33, 203.[17] M. Keddam, C. Kuntz, H. Takenouti, D. Schuster, D. Zuili,

Electrochim. Acta 1997, 42, 87.[18] C. N. Cao, J. Wang, H. C. Lin, J. Chin. Soc. Corros. Prot. 1989,

9, 261.[19] P. W. Kai, C. A. Richard, J. Electrochem. Soc. 1990, 137, 3010.[20] A. Conde, J. J. de Damborenea, Mater. Corros. 1999, 50, 447.[21] L. Tomcsanyi, K. Varga, L. Bartik, G. Horanyi, E. Maleczki,

Electrochim. Acta 1989, 34, 855.

(Received: May 31, 2009)

(Accepted: June 22, 2009)

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EIS Study on Pitting Corrosion of 7150 Aluminum Alloy in Sodium Chloride and Hydrocloric Acid...

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