Corrosion Science 50 (2008) 3569–3575
Contents lists available at ScienceDirect
Corrosion Science
journal homepage: www.elsevier.com/locate/corsci
Evaluation of the corrosion resistance of anodized aluminum 6061 using
electrochemical impedance spectroscopy (EIS)
Yuelong Huang a, Hong Shih b,*, Huochuan Huang b, John Daugherty b, Shun Wu b,
Sivakami Ramanathan b, Chris Chang b, Florian Mansfeld a,*
a
Corrosion and Environmental Effects Laboratory (CEEL), The Mork Family Department of Chemical Engineering and Materials Science, University of Southern California,
Los Angeles, CA 90089-0241, USA
b
Lam Research Corporation, 4400 Cushing Parkway, Fremont, CA 94538, USA
a r t i c l e
i n f o
Article history:
Received 9 July 2008
Accepted 5 September 2008
Available online 18 September 2008
Keywords:
A. Aluminum
B. EIS
B. SEM
C. Oxide coatings
a b s t r a c t
The corrosion resistance of anodized Al 6061 produced by two different anodizing and sealing processes
was evaluated using electrochemical impedance spectroscopy (EIS). The scanning electron microscope
(SEM) was employed to determine the surface structure and the thickness of the anodized layers. The
EIS data revealed that there was very little change of the properties of the anodized layers for samples
that were hard anodized in a mixed acid solution and sealed in hot water over a 365 day exposure period
in a 3.5 wt% NaCl solution. The specific admittance As and the breakpoint frequency fb remained constant
with exposure time confirming that the hard anodizing process used in this study was very effective in
providing excellent corrosion resistance of anodized Al 6061 over extended exposure periods. Some
minor degradation of the protective properties of the anodized layers was observed for samples that were
hard anodized in H2SO4 and exposed to the NaCl solution for 14 days.
Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction
The corrosion resistance of aluminum and aluminum alloys can
be greatly increased by forming a thick oxide layer through the
anodizing process in which the aluminum sample is anodically
polarized in an appropriate electrolyte to form an aluminum oxide.
In the anodizing process sulfuric acid is commonly used as the
electrolyte solution to grow the oxide layer to greater thickness
than that of the naturally formed film [1,2]. The oxide layer formed
in this process has a duplex structure consisting of the inner barrier
layer and the outer porous layer. The barrier layer is very thin and
dense. The outer porous layer is a much thicker, porous oxide that
has a close-packed hexagonal cells structure. A sealing process is
necessary to improve the corrosion resistance. The pores in the
outer oxide layer can be sealed in steam and boiling water or in
various cold sealing solutions such as nickel acetate and dichromate [3–5]. A study of the effects of different sealing methods on
the corrosion resistance of anodized Al alloys has been conducted
by Zuo et al. [5]. The authors analyzed the outer oxide layers that
were sealed by boiling water, stearic acid, potassium dichromate
* Corresponding authors. Tel.: +1 213 740 3016; fax: +1 213 740 7797 (F.
Mansfeld), Tel.: +1 510 572 2257/299 0283 (H. Shih).
E-mail addresses: hong.shih@lamrc.com (H. Shih), mansfeld@usc.edu (F. Mansfeld).
0010-938X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.corsci.2008.09.008
or nickel fluoride and concluded that the boiling water sealing
method provided higher corrosion resistance [5].
Electrochemical impedance spectroscopy (EIS) is a powerful
method to characterize the corrosion resistance of anodized Al alloys [6–11]. The impedance spectra can be fitted to an appropriate
equivalent circuit (EC) which allows to obtain the time dependence
of important properties of the anodized surface layers such as the
capacitance as well as the resistance of the barrier and the porous
layers.
Mansfeld et al. [6] studied the protective properties of oxide
layers on Al 2024, 6061 and 7075 that were produced by anodizing
in H2SO4 solutions. Sealing was carried out in hot water, nickel acetate, yttrium acetate or a saturated cerium acetate solution in an
attempt to replace dichromate sealing. The corrosion properties
of the oxide layers for these Al alloys were evaluated by EIS during
exposure in a 0.5 NaCl solution. The results of this study [6]
revealed that the pore resistance Rpo obtained from the EIS data
can be used to determine the corrosion resistance of sealed anodized aluminum alloys. The thicknesses of the barrier layer and the
porous layer can be evaluated using the experimental values of the
capacitance of the barrier layer (Cb) and the porous layer (Cpo),
respectively. Rpo values exceeding 2 105 ohm cm2 were considered to be indicative of properly anodized and sealed samples for
common applications [6]. For the semiconductor industry Rpo
values for anodized aluminum used as a plasma etching chamber
coating should meet or even exceed 5 106 ohm cm2 [12].
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Y. Huang et al. / Corrosion Science 50 (2008) 3569–3575
Dasquet et al. [10] and Moutarlier et al. [11] indicated that EIS was
an important technique for evaluating the corrosion resistance of
anodized Al alloys. The results of Moutarlier’ study [11] showed
that the thickness of the barrier layer obtained from the EIS method was the same as that determined from transmission electron
microscopy (TEM).
The extent of degradation of the anodized layers during exposure to a corrosion solution can be monitored as a function of exposure time by simple methods that do not require the recording of
an entire impedance spectrum in a wide frequency range. These
methods include recording of the breakpoint frequency which is
defined as fb = 1/(2pCpoRpo) at the phase angle U = 45° in the frequency region in which the impedance is determined by Cpo [6,13–
15].
A single frequency impedance test for anodized aluminum is
described by ASTM B 457 [16]. The specific admittance As, which
is the inverse of the impedance at 1000 Hz normalized to the surface area, can be used as a qualitative measure of the coating thickness and an indicator of an oxide layer that was properly sealed
[16,17]. Low values of As indicate poor sealing quality of the porous
layer [16]. Otero et al. [17] used EIS to evaluate aging of cold-sealed
aluminum oxide films formed on pure aluminum. They also determined As values and conclude that As = 10 lS/cm2 was indicative of
a porous layer that was properly sealed in hot water [17]. For a porous layer with e = 36 a value of As = 10 lS/cm2 corresponds to a
porous layer thickness of 20 lm according to As = 200/dpo (lS/
cm2) [17].
Although many studies have been conducted to improve the
corrosion resistance of anodized aluminum alloys, the protective
properties of the oxide layer still need to be further improved to
meet the challenges faced by the semiconductor industry. In order
to improve the long-term corrosion resistance of anodized Al 6061T6, two different hard anodizing (type III) processes have been
evaluated.
2. Experimental methods
2.1. Materials
Samples
of
Al
6061-T6
with
dimensions
of
10 cm 10 cm 0.3 cm were degreased by soaking in a detergent
for 10 min followed by deionized (DI) water (2 Mohm-cm) rinsing
for 2 min and hot DI water rinsing at 45 °C for 2 min.
Three different types of anodized Al 6061 samples were prepared by Lam Research Corporation: three production tank samples (PT), two R&D tank samples (RDT) and two sulfuric acid
hard anodized (SAHA) samples.
2.2. Anodizing procedures
The oxide layers of hard anodized samples are usually produced
at high current densities in a tank that contains sulfuric acid near
0 °C. The hard anodizing (type III) [18] process produces porous
oxide layers that are thicker than 25 lm. The anodizing procedures
used for the PT and RDT samples followed the hard anodizing process but used a mixed acid solution that mainly contained sulfuric
acid (H2SO4) and oxalic acid (H2C2O4). The ramping of the anodizing voltage for the PT and RDT samples was different from that
used for anodizing of the SAHA samples. The final voltage of the
anodizing process stopped at about 106 V, while it stopped at
60 V for the SAHA samples. The three PT samples were anodized
in a production tank which is a large size tank and allows optimization of anodizing parameters. The two RDT samples were anodized in the R&D tank which is a small size tank using the same
anodizing procedure that was used for anodizing of the PT samples.
The R&D tank does not allow complete optimization of tank condition control [19]. The two SAHA samples were produced following
the standard type III anodization process. After anodizing, the samples were first rinsed by cool DI water for 5 min (2 Mohm-cm DI
water), hot DI water at 45 °C for 5 min (2 Mohm-cm DI water),
and then high purity DI water (18 Mohm-cm) for 10 min.
2.3. Sealing procedures
After complete rinsing, the samples were immersed in a hot
high purity DI water tank for sealing. The sealing time was about
3 min per lm in oxide thickness. The bath temperature was controlled at 98 °C or higher and the pH was maintained between
5.7 and 6.2. After hot DI water sealing, the samples were moved
into a class 100 cleanroom. Isopropyl alcohol (IPA) was used to
wipe the samples with class 100 cleanroom wipes. High purity
DI water (18 Mohm-cm) was used for additional rinsing for
5 min followed by drying with nitrogen gas that was filtered with
a 0.1 lm filter. The samples were then baked in a class 100 cleanroom compatible oven at 110 °C for 30 min, removed from the
oven and cooled down to room temperature [19,20].
2.4. Test methods
Impedance spectra were collected at the open-circuit potential
(OCP) in a three-electrode cell in which the test sample was placed
on the bottom with an exposed area of 20 cm2. A stainless steel
electrode was used as the counter electrode and a SCE as the reference electrode. The impedance spectra were obtained with a BSAZahner IM6 unit using a frequency range between 1 MHz to 1 mHz
and an ac signal amplitude of 10 mV. Seven samples (three PT, two
RDT and two SAHA) have been tested by EIS during exposure to
3.5% NaCl for 14 (RDT and SAHA samples) or 365 days (PT
samples).
The surfaces of exposed and unexposed samples have been
evaluated using SEM (Cambridge Model Stereoscan 360). Observations of the cross sections of the sample were also made to determine the thickness of the oxide layer.
3. Results and discussion
Fig. 1 shows some of the impedance spectra that were obtained
for the three types of anodized Al 6061 samples for an exposure
time of 365 days or 14 days in the Bode plot format in which the
logarithm of the impedance modulus |Z| and the phase angle U
are plotted vs. the logarithm of the frequency f of the applied ac
signal. Very little difference can be detected in the spectra for the
PT and RDT samples and there was hardly any change of the
impedance for each sample with exposure time. The spectra for
the SAHA samples differed from those determined for the other
samples since a third time constant appeared at intermediate frequencies. The third time constant might be due to fine cracks and
defects in the oxide layers that extend into the base metal. The
spectra for the PT and RDT samples are representative for samples
that have been properly anodized and sealed in hot water. The very
high impedance values at intermediate frequencies for the PT and
the RDT samples indicate that the outer oxide layer is very well
sealed. The spectra for all PT and RDT test panels for a given treatment were almost identical and did not change much with exposure time. These results indicate that the anodizing process was
very reproducible and that the sealing process was very effective.
Figs. 2 and 3 show the open-circuit potential (OCP) values for
the three types of samples. The OCP for the three PT samples increased in the first 14 days of exposure and then remained very
stable at values close to zero V vs. SCE (Fig. 2). The OCP values
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Y. Huang et al. / Corrosion Science 50 (2008) 3569–3575
a
b
9
9
8
8
7
7
6
6
5
RDT
5
PT
4
3
2
1
-4
-3
-2
-1
1d
2d
3d
5d
7d
9d
11d
14d
4
1d
7d
40d
142d
260d
365d
3
2
1
0
1
2
3
4
5
6
-90
-90
-80
-80
-70
-70
-60
-60
-50
-50
-40
-40
-30
-30
-20
-20
-10
-10
-3
-2
-1
0
1
2
3
4
5
6
-3
-2
-1
0
1
2
3
4
5
6
0
0
-4
-3
-2
-1
0
c
1
2
3
4
5
6
9
SAHA
1d
2d
3d
5d
7d
9d
11d
14d
8
7
6
5
4
3
2
1
(c)
-3
-2
-1
0
1
2
3
4
5
6
-3
-2
-1
0
1
2
3
4
5
6
-90
-80
-70
-60
-50
-40
-30
-20
-10
0
Fig. 1. Bode-plots for PT (a) RDT (b), and SAHA (c) samples for an exposure period of 14 days and 365 days respectively in 3.5 wt% NaCl solution.
for the two RDT and two SAHA samples were stable with exposure
time of 14 days indicating the high corrosion resistance of these
anodized surfaces (Fig. 3). For the SAHA samples the OCP values
were close to 0.6 V (Fig. 3) which is similar to the OCP of bare
3572
Y. Huang et al. / Corrosion Science 50 (2008) 3569–3575
0.6
Cpo (nF/cm 2)
0.1
E ocp (V)
0
-0.1
0.5
0.4
PT1
PT1
PT2
PT2
PT3
PT3
0.3
0
-0.2
0
100
200
300
50
100
150
200
250
300
350
Time (days)
400
Time (day)
0.7
Fig. 2. Open-circuit potential (OCP) as a function of exposure time for three PT
samples.
PT1
PT2
Cb ( µ F/cm 2)
0.6
0.2
RDT1
RDT2
0
SAHA1
E ocp (V)
SAHA2
PT3
0.5
0.4
-0.2
-0.4
0.3
0
50
100
150
200
250
300
350
250
300
350
250
300
Time (days)
-0.6
7.5
-0.8
4
6
8
10
12
14
Time (days)
Fig. 3. Open-circuit potential (OCP) as a function of exposure time for two RDT and
two SAHA samples.
Al 6061. Apparently the substrate metal was exposed to the corrosive solution through fine cracks that run through the anodized
layers. Additional differences in the measured OCP values for the
three types of anodized Al 6061 samples might be due to different
amounts of MgSi2 and MgSiFe second phases present in the anodized layers [19,20].
The impedance spectra were fitted to the equivalent circuit (EC)
shown in Fig. 4 using the software ANODAL [6,21]. Cb and Cpo are
the capacitance of the inner barrier layer and the outer porous
layer, respectively and Rb is the resistance of the barrier layer.
Zpo = K(jx)n, where K and n are fit parameters and x = 2pf, is a constant phase element that is used to account for the variations of the
properties of the pores in the outer porous layer such as pore diameter, pore depth and degree of sealing [22]. The fit parameter K is
used as a measure of the pore resistance Rpo.
Cpo
Cb
PT1
PT2
LogRpo (ohm.cm 2)
2
PT3
7
0
50
100
150
200
Time (days)
12
PT1
PT2
LogRb (ohm.cm 2)
0
PT3
11
10
9
0
Rs
50
100
150
200
350
Time (days)
Zpo
Rb
Fig. 4. Equivalent circuit for the analysis of the impedance spectra for different
anodized Al samples.
Fig. 5. Time dependence of fit parameters as a function of exposure time for three
PT samples.
3573
Y. Huang et al. / Corrosion Science 50 (2008) 3569–3575
Cpo (nF/cm 2)
1
0.5
SAHA1
SAHA2
RDT1
RDT2
0
0
2
4
6
8
10
12
14
Tim e (day)
1
The fit parameters Cb, Cpo, Rb and Rpo for three PT samples are
plotted in Fig. 5. Cpo had values between 0.3 and 0.6 nF/cm2 and
Cb remained between 0.4 and 0.6 lF/cm2. The barrier and porous
layer thickness d can be estimated based on
0.5
ð1Þ
SAHA1
SAHA2
RDT1
RDT2
0
0
2
4
6
8
10
12
14
10
12
14
12
14
Time (day)
7
LogRpo (ohm.cm 2)
where x = po or b; eo = 8.85 1012 F/m and A is the exposed area.
For an average value of Cb = 5 107 F/cm2 and e = 10 [23] the
thickness db of the barrier layer is estimated to be 18 nm. The thickness of the anodized layer obtained by the SEM image of the cross
section for the PT sample was 63 lm (Fig. 6). For an average value
of Cpo = 4.6 1010 F/cm2 the dielectric constant e of the porous
layer of the PT sample is estimated to be 33. The observed oxide
thickness values can be compared with the thickness of the porous
layer of 20 lm and the thickness of the barrier layer of 20 nm for Al
samples anodized in H2SO4 and sealed in hot water (SA/HWS) [6].
The very high and stable values of Rb and Rpo (Fig. 5) indicate that
the sealing process was very effective for the mixed acid anodizing
method. Rpo values exceeding 200 kohm cm2 are considered to be
indicative of properly anodized and sealed samples [17].
The spectra for the two RDT samples are shown in Fig. 1b. Cpo had
final values between 0.3 and 0.5 nF/cm2, while the final values of Cb
were between 0.3 and 0.5 lF/cm2 (Fig. 7). Since the two RDT samples
were anodized using the same anodization procedure as the two PT
samples, the dielectric constant of the porous layer e = 33 can be
used to estimated the thickness of the porous layer. The calculated
average values of dpo and db were 68 lm and 22 nm, respectively.
The very high and constant values of Rpo indicate that the sealing
process was very effective (Fig. 7).The fit parameters for the two
SAHA samples are also shown in Fig. 7. The Rb and Rpo values for these
anodized surfaces were similar for those observed for the RDT samples, however a slow decrease of Rpo was found for the SAHA samples. Cpo and Cb for the two SAHA samples remained more or less
constant during the entire exposure time. For sample SAHA1 Cpo
and Cb had average values of about 7 1010 F/cm2 and
8.2 107 F/cm2, respectively. The dpo and db values were estimated
as 46 lm and 11 nm, respectively. Cpo and Cb for the SAHA2 sample
had average values of about 6.3 1010 F/cm2 and 8.2 107 F/cm2,
respectively corresponding to values of dpo = 50 lm and db = 11 nm.
Otero et al. [17] used the specific admittance As as a qualitative
measure of coating thickness and an indicator of an oxide layer
that was properly sealed in hot water. A value of As = 10 lS/cm2
was considered to be indicative of an oxide layer properly sealed
in hot water. As = 10 lS/cm2 corresponds to an oxide layer thickness of 20 lm according to As = 200/dpo lS/cm2 for an oxide layer
with e = 36 [17].
SAHA1
SAHA2
RDT1
RDT2
6.5
0
2
4
6
8
Time (day)
10
LogRb (ohm.cm 2)
C x ¼ o x A=dx
Cb (µ F/cm 2)
Fig. 6. SEM images of the cross section of a PT sample.
9
SAHA1
SAHA2
RDT1
RDT2
8
0
2
4
6
8
10
Time (day)
Fig. 7. Time dependence of fit parameters as a function of exposure time for two
SAHA and two RDT samples.
3574
Y. Huang et al. / Corrosion Science 50 (2008) 3569–3575
500
3
SAHA1
SAHA2
400
A s (µ S/cm 2)
RDT1
fb (Hz)
RDT2
300
200
PT1
PT2
100
PT3
2
0
0
50
100
150
200
250
300
350
0
2
4
6
8
10
12
14
Time (days)
Time (days)
Fig. 8. Time dependence of specific admittance As as a function of exposure time for
three PT samples.
Fig. 10. Time dependence of break point frequencies fb as a function of exposure
time for two RDT and two SAHA samples.
6
80
SAHA2
fb (Hz)
As (µ S/cm 2)
60
SAHA1
4
RDT1
RDT2
40
PT1
20
PT2
PT3
2
0
2
4
6
8
10
12
14
0
Time (days)
0
Fig. 9. Time dependence of specific admittance As as a function of exposure time for
two RDT and two SAHA samples.
Since jZ j ¼ 1=xC po
It follows that
which for
As ¼ 1=Z 1000 A ¼ xC po =A ¼ 2pf o =dpo
¼ 36 leads to
2
As ¼ 200=dpo ðlS=cm Þ
ð2Þ
ð3Þ
ð4Þ
where Z1000 is the impedance at 1000 Hz, Cpo is the capacitance of
the porous layer and A = 20 cm2 is the exposed area. Based on the
calculated value e = 33 for the PT sample, As = 183/dpo (lS/cm2).
Figs. 8 and 9 show the time dependence of As for the 7 samples.
These data suggest that the porous layer has a thickness of about
71 lm for the RDT and PT samples that did not change much with
time. This thickness value is in general agreement with the thickness of the porous layer calculated from the Cpo data. For the two
SAHA samples an average value of dpo = 40 lm is determined from
the average As values.
The time dependence of the properties of the anodized layers
during exposure to a corrosive solution can also be estimated from
the impedance spectra using the concept of the breakpoint frequency which is defined as fb = 1/(2pCpoRpo) [6,13–15]. For the
two samples treated in the SAHA process fb slightly increased with
exposure time (Fig. 10) most likely due to the slow decrease of Rpo
with exposure time (Fig. 7) which suggests that some conductive
paths and defects had developed in the porous layers during the
exposure period [24,25]. For the PT and RDT samples fb values were
stable for exposure time of 365 or 14 days, respectively indicating
that there was very little change of the porous layers (Figs.10 and
11). Figs. 10 and 11 confirm the conclusions based on the analysis
of the impedance spectra that the anodized layers for the five sam-
50
100
150
200
250
300
350
400
Time (days)
Fig. 11. Time dependence of break point frequencies fb as a function of exposure
time for three PT samples.
ples that were hard anodized in the mixed acid (R&D tank and the
production tank) were very stable and provided excellent corrosion
resistance over extended exposure periods.
4. Summary and conclusions
EIS has provided valuable information concerning the properties of the inner barrier layer and the outer porous layer of Al
6061 that was hard anodized using two different processes and
their changes during exposure to a corrosive solution. The impedance spectra obtained for samples that were anodized in a mixed
acid process and sealed in hot water were in very good agreement
with spectra that are normally observed for samples that had been
properly anodized and sealed [6,17]. There were very little changes
of the spectra with exposure time to 3.5% NaCl for the individual
samples which suggests that these surfaces were very corrosion
resistant. There were also very little differences in the spectra for
the different samples in one group, i.e. the three PT samples, which
were treated in the same manner. The thickness of the porous layers was higher than that commonly found for Al alloys that were
anodized in H2SO4 and sealed in hot water [6,17]. The impedance
spectra as well as the As and fb values for the three PT samples were
very stable for an exposure time of 365 days showing that these
surfaces were very corrosion resistant. The very high and stable
values of Rpo for these three samples indicate that the newly developed anodizing and sealing process was very effective.
Y. Huang et al. / Corrosion Science 50 (2008) 3569–3575
The impedance spectra and the parameters As and fb for the two
RDT samples were very stable during 14 days exposure. The high
values of Rpo reflect the high corrosion resistance and the effectiveness of the hard anodizing and hot water sealing processes.
The impedance spectra for the two SAHA samples suggest that
the oxide layers formed by hard anodizing in H2SO4 were more
complex than those for the other samples. The OCP data and the
slow decrease of Rpo suggest that the base metal was exposed to
the corrosive test solution through fine cracks and other defects.
Acknowledgment
Y. Huang and F. Mansfeld acknowledge financial support by the
Lam Research Corporation.
References
[1] S.D. Cramer, B.S. Covino Jr, Aluminum Anodizing ASM Handbook 13A
Corrosion: Fundamentals Testing and Protection, vol. 736, Materials Park,
2003.
[2] L.L Sheir, R.A. Jarman, G.T. Burstein, Coatings Produced by Anodic Oxidation,
Corrosion, Corrosion Control, vol 2, third edition, Butterworth-Heinemann, 15
(3) 2000.
[3] S. Wernick, R. Pinner, P.G. Sheasby, The Surface Treatment and Finishing of
Aluminum and its Alloys, sixth ed., vol. 2, ASM International, 2001.
[4] M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, National
Association of Corrosion Engineers, 1974.
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
[22]
[23]
[24]
[25]
3575
Y. Zuo, P. Zhao, J. Zhao, Surf. Coat. Technol. 166 (2003) 237.
F. Mansfeld, G. Zhang, C. Chen, Plat. Surf. Finish. 84 (Dec.) (1997) 72.
J. Hitzig, K. Juettner, W.J. Lorenz, W. Paatsch, Corros. Sci. 24 (1984) 945.
F. Mansfeld, M.W. Kendig, Corrosion 41 (1984) 490.
F. Mansfeld, M.W. Kendig, J. Electrochem. Soc. 135 (1988) 828.
J.-P. Dasquet, D. Caillard, E. Conforto, J.-P. Bonino, R. Bes, Thin Solid Films 371
(2000) 183.
V. Moutarlier, M.P. Gigandet, B. Normand, J. Pagetti, Corros. Sci. 47 (2005)
937.
H. Shih, D. Outka, J. Daugherty, Specification for Hard Anodized Aluminum
Coatings Using Mixed Acids for Critical Chamber Components, Lam Research
Confidential Specification, January 17, 2006.
F. Mansfeld, C.H. Tsai, Corrosion 47 (1991) 958.
J.R. Scully, J. Electrochem. Soc. 136 (1989) 979.
F. Mansfeld, L.T. Han, C.C. Lee, G. Zhang, Electrochim. Acta 43 (1998) 2933.
ASTM B 457, Standard test Method for Measurement of Impedance of Anodic
Coatings on Aluminum ASTM Annual Book of Standards, vol. 192, Springer,
2003.
E. Otero, V. Lopez, J.A. Gonzales, Plat. Surf. Finish. 83 (1996) 50.
J. Edwards, Coating and Surface Treatment Systems for Metals, vol. 38, ASM
International, Finishing Publications Ltd, 1994.
H. Shih, Private Communications, April and May, 2008.
H. Shih, T.C. Huang, J. Daugherty, Lam Research Confidential Technical Report,
October 10, 2006.
F. Mansfeld, H. Shih, H. Greene, C.H. Tsai, ASTM STP 1188 (1993) 37.
F. Mansfeld, Analysis and interpretation of EIS data for metals and alloys,
Schlumberger Technical Report 26 (1993).
J. Hitzig, K. Juttner, W.J. Lorenz, W. Paatsch, Corros. Sci. 24 (1984) 945.
J. Hubrecht, J. Vereecken, M. Piens, J. Electrochem. Soc. 131 (1984) 2010.
J.D. Scantlebury, A. Guiseppi-Elie, D.A. Eden, L.M. Callow, Corrosion 39 (1983)
108.