solidi
status
Part of Topical Section on
Engineering of Functional Interfaces
physica
Phys. Status Solidi A, 1–7 (2016) / DOI 10.1002/pssa.201532925
a
www.pss-a.com
applications and materials science
Film dissolution kinetics of aluminium
at precisely adjusted pH-values
Sarah Walkner1,2, Martina Hafner1,3, and Achim Walter Hassel*,1,2
1
Institute for Chemical Technology of Inorganic Materials, Johannes Kepler University Linz, Altenberger Str. 69, 4040 Linz, Austria
Competence Centre for Electrochemical Surface Technology, Viktor Kaplan Str. 2, 2700 Wiener Neustadt, Austria
3
Christian Doppler Laboratory for Combinatorial Oxide Chemistry, Institute for Chemical Technology of Inorganic Materials,
Johannes Kepler University Linz, Altenberger Str. 69, 4040 Linz, Austria
2
Received 11 November 2015, revised 10 March 2016, accepted 11 March 2016
Published online 6 April 2016
Keywords aluminium dissolution and restructuring, ICP-MS, precisely adjusted alkaline pH-values
* Corresponding
author: e-mail [email protected], Phone: þ43 732 2468 8701, Fax: þ43 732 2468 8905
This work focuses on the corrosion of aluminium in alkaline
solution particularly at pH 11.0. The pH value plays a key role in
corrosion processes. In a certain pH range, either the metal
dissolves or protective films may be formed. For performing
electrochemical measurements at a precisely adjusted pH value
a setup, the so called ‘impedance titrator’ is used, that allows
adjusting and controlling of pH value very precisely, while
characterising the film properties. Electrochemical impedance
spectroscopy is performed to investigate the formation of
corrosion products. By keeping the pH at a constant value and
simultaneously offering the system a constant hydroxide concentration, film formation takes place under defined conditions
and a full kinetic characterisation becomes possible. This special
system is required as the behaviour of aluminium in alkaline
solution is quite complex and a various number of competing
reactions like oxide film formation and dissolution occur.
ß 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1 Introduction Aluminium is used in many different
fields like nuclear reactors, aerospace, automotive, construction, aluminium alloys, coatings and metal/air
batteries [1–13]. At neutral pH values, a thick oxide film
is formed, which acts as barrier layer [14–16]. In metal/air
batteries, aluminium acts as anode in an alkaline surrounding to ensure conductivity of the electrode. Performance of
these batteries depends on the corrosion behaviour of
aluminium at these pH values [17]. By forming an oxide
film on top of the anode, charge transport is interrupted
and correct performance of the batteries is not provided.
Different studies showed that even at very alkaline
pH values various number of competing reactions occur:
Formation and dissolution of an oxide film like Al2O3 or
Al(OH)3, release of Al3þ into the electrolyte [18–21] and
hydrogen formation are possible [22]. Understanding of
the ongoing processes on the aluminium surface is very
important for battery industry. Different kinds of electrochemical measurements like ring-disk studies, open circuit
potential transients, voltammetric experiments and electrochemical impedance spectroscopy (EIS) were conducted
to understand the behaviour of aluminium in alkaline
solution. These studies revealed that the corrosion of
aluminium is much faster in alkaline solution compared to
acidic pH values, involving different dissolution and oxide
film formation processes [23–28]. This work focuses on
EIS to investigate and explain the different reactions
during alkaline corrosion of aluminium covered by a native
oxide film. Further investigations were done by using
the setup ‘impedance titrator’ that allows controlling and
adjusting the pH value very precisely enabling quantitative
kinetic characterisation of the oxide film formation and
dissolution.
2 Experimental
2.1 Experimental conditions Pure bulk aluminium
(GoodFellow-As rolled 99.999%) was investigated. The
samples were ground, polished and ultrasonically cleaned
before each measurement. After this pre-treatment, the
samples were exposed in air for 24 h to form a native
aluminium oxide layer. The investigated area was 0.79 cm2.
A 0.1 M Na2SO4 (pro Analysis, Merck) solution dearated
with N2 was used, as electrolyte and an aqueous 0.1 M
NaOH solution prepared from a 1 M NaOH standard
ß 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
solidi
status
physica
pss
a
2
(TitriPur, Merck), served as titrating agent, to adjust and
hold pH 11.0.
2.2 Electrochemical measurements All measurements were performed using a special designed setup ‘the
impedance titrator’ (Fig. 1).
Accurate descriptions of the setup are given elsewhere [29, 30]. A typical three electrode configuration was
used for performing electrochemical measurements. A
commercially available Ag/AgCl electrode (Methrom;
Ag,AgCl 0–80 8C) placed in a luggin capillary filled with
3 M KCl solution served as reference electrode (RE) and a
gold wire as counter electrode (CE). The aluminium sample
was placed in a specially designed holder, contacted with
a brass rod and served as working electrode (WE).
Electrochemical measurements were performed with a
potentiostat (IVIUM CompactStat). Prior to performing EIS
the open circuit potential (OCP) was recorded, impedance
spectra were measured with a bias of the previously
measured OCP. The amplitude of the AC signal was 10 mV
in the frequency range from 1 kHz to 10 mHz. Electrochemical impedance spectra and pH measurements were
recorded hourly after stirring and bubbling of the electrolyte
during this hour, to remove charges from the sample surface
and to reach a steady state of the electrochemical system.
EIS was applied to examine the ongoing corrosion processes
and formation of precipitates.
S. Walkner et al.: Film dissolution kinetics of aluminium
without arising irregularities or drifts. Standard solutions
for the calibration of the ICP-MS were prepared from Al
standard stock solution (100.12 0.70 mg ml1, Inorganic
Ventures, USA). Figure 2 shows the performed analysis
of the aluminium sample during and after the corrosion
process.
OCP and EIS measurements are performed during the
ongoing corrosion process. Additionally, to the OCP/EIS
measurements, corresponding electrolyte samples were
taken within the same time frame, which ensures coherence
between electrochemical results and electrolyte composition. To investigate the sample surface after performing
measurements at pH 11.0, scanning electron microscopy
(SEM) and energy-dispersive X-ray spectroscopy (EDX)
are used.
3 Results and discussion
3.1 Electrochemical measurements Generally at
pH 11.0 aluminium undergoes a dissolution process.
Possible reactions are [31]:
Al þ 4OH ! ½AlðOHÞ4 þ 3e ;
ð1Þ
Al OHÞ3 þ OH ! Al OHÞ4 ;
ð2Þ
Al þ 3OH ! AlðOHÞ3 þ 3e ;
ð3Þ
Al þ 2H2 O ! AlO2 þ 4Hþ þ 3e ;
ð4Þ
2AlO2 þ2 Hþ ! Al2 O3 þH2 O:
ð5Þ
2.3 Further investigations of dissolution
process As film formation and dissolution often occur
simultaneously, the electrolyte was also investigated by
means of inductively coupled plasma-mass spectroscopy
(ICP-MS). ICP-MS (THERMO SCIENTIFIC iCAP Q) was
used for analysing the dissolved Al. The ICP-MS contains
a quadrupole analyser combined with a collision cell.
Time resolved measurements were performed to obtain an
extended probing interval and to observe the signal regime
in order to ensure an accurate evaluation of the signal
Reactions (1)-(2) are preferred at pH 11.0. Simultaneously, although only to a minor extent, a restructuring of
the native oxide film, aluminium hydroxide, as well as
alumina formation and dissolution (reactions (3)-(5)) can be
Figure 1 Schematic of the used setup ‘the impedance titrator’.
Figure 2 Performed investigation methods during the corrosion
of bulk aluminium at pH 11.0.
ß 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.pss-a.com
Original
Paper
Phys. Status Solidi A (2016)
3
Figure 3 OCP transient on bulk aluminium sample measured
at pH 11.0.
observed. Figure 3 shows the changing of the open-circuit
potential during the experiment.
During the first 2 h, the OCP drops rapidly towards the
cathodic region. This steep decline is followed by a period of
slightly increasing OCP. This shows that at the beginning of the
experiment, the native aluminium oxide layer is restructured
and dissolves to a certain extent. In the next time period
aluminium dissolution continues, but some new aluminium
hydroxide is also formed. Impedance spectra (Fig. 4) reveal that
this hydroxide layer is not protective and does not cover the
sample completely as the film resistance decreases.
For each impedance spectrum two time constants can be
observed. After the first 3 h, the appearance of the phase
shift changes drastically, which means that two different
processes become visible in the spectra. At the beginning of
the experiment, the spectra show the natively grown
aluminium oxide. After the first 3 h, the native grown
oxide has changed its appearance and pores were closed. In
the following experimental time, the oxide film starts to
dissolve and is no longer covering the whole surface and,
therefore, losing its protective properties. Al(OH)3 is
deposited on the surface leading to a fluctuation of Rfilm
and Cfilm values during the experiment. All spectra were
fitted using the software ‘ZView’ from Scribner Assoc. The
equivalent circuit that was used is shown in Fig. 5.
As the observed oxide film is quite porous the equivalent
circuit consists of the film resistance and film capacitance
(Rfilm, CPEfilm). The charge transfer resistance and double
layer capacitance (Rct, CPEdl) can be observed too. Figure 6
shows the Bode plot of measured and fitted impedance spectra
received by using the equivalent circuit shown in Figure 5.
Figure 7 represents the calculated values for the
different impedance elements (Rfilm, CPEfilm) explaining
the corrosion behaviour of aluminium at pH 11.0.
By using a CPE, many factors have to be considered
such as adsorption reactions, electrode porosity, surface
roughness and heterogeneities [32, 33]. It is assumed that
the decrease of the exponent from the CPE (CPE-P) during
measuring time indicates an increase of porosity of the
www.pss-a.com
Figure 4 Impedance spectra recorded in the first 15 h of the
experiment.
formed corrosion products. In this way porosity is linked
to the change in the exponent of CPE. Furthermore, film
capacitance increases during the whole experiment. An
increase of film capacitance (Cfilm) corresponds to a
decrease of the oxide layer thickness or to an increase in
porosity.
This means, that at the beginning of the experiment, the
thickness of the native developed oxide layer gets thinner.
Figure 5 Equivalent circuits for fitting the recorded impedance
spectra.
ß 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
solidi
status
physica
pss
a
4
S. Walkner et al.: Film dissolution kinetics of aluminium
The native oxide dissolves and is restructured,
aluminium dissolves and new aluminium hydroxide is
formed. Previous studies also showed this behaviour of
aluminium in alkaline solutions. First Al(OH)3 develops
electrochemically followed by a chemical dissolution
induced by OH attack of the hydroxide layer [16, 17].
Figure 6 Bode plot of the recorded impedance spectrum (dotted
line) and fitted spectrum (continuous line) after 1 h of the
experiment.
With ongoing experimental time, the sample surface
covered with corrosion products decreases due to increasing
porosity of formed precipitates. This finding matches well
with the behaviour of the exponent from the CPE and with
previous investigations [34]. Within the first 3 h, the film
resistance shows a slight increase, afterwards the value
drops rapidly. The subsequent decline of Rfilm corresponds
to the dissolution of the former protective layer, which
means that it takes 3 h until the native grown aluminium
oxide gets dissolved after restructuring. This is a kinetic
effect as according to the Pourbaix diagram of aluminium
at pH 11.0, the oxide should be dissolved. After the first 3 h,
Al dissolution and Al(OH)3 formation can be observed.
From the results of the impedance spectra one can see that
congruent reactions occur at the aluminium surface.
3.2 Aluminium dissolution and repassivation
To ensure thermodynamically defined conditions throughout the entire measurement, pH 11.0 is precisely adjusted by
adding 0.1 M NaOH in a highly controlled and reproducible
way. Figure 8 shows the deviation of the pH value in the
electrolyte and consumption of titrating agent. The added
volume of NaOH solution shows a linear behaviour during
the whole experiment except for the first 2 h. This is in
agreement with the recorded impedance spectra which
showed the restructuring of native grown oxide during this
time period.
Based on the added volume of NaOH solution amount of
dissolved aluminium in the electrolyte can be calculated as
follows:
For example for the first measurement value after 1 h
0.22 ml of NaOH are consumed corresponding to 2.2 105
mol OH as a 0.1 M NaOH titrating agent is used. During
aluminium dissolution according to reaction (1), four
hydroxide ions are consumed. Aluminium gets dissolved
as aluminate ions which means 4 OH are consumed to form
Al(OH)4. By considering the cell volume (80 ml) and the
molar mass of aluminium (26.98 g mol1), the concentration
of dissolved aluminium according to reaction (1) can be
calculated.
nðAlðOHÞ4 Þ ¼ 5:5 106 mol;
ð6Þ
cðAlðOHÞ4 Þ ¼ 6:875 105 mol l1 :
ð7Þ
Figure 9 shows the change of the aluminium
concentration in the electrolyte during the whole experiment. The concentration measured with ICP-MS and
calculated from the titration results are in the same range.
Figure 7 Change of the film resistance Rfilm, capacitance Cfilm of
CPEfilm and exponent of the constant phase element CPE-P with
time taken from subsequently recorded impedance spectra.
ß 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 8 Stability of the pH value during the experiment and
volume of titrated 0.1 M NaOH to adjust pH 11.0.
www.pss-a.com
Original
Paper
Phys. Status Solidi A (2016)
5
thickness of the formed hydroxide film assuming covering
the entire sample results in
hðAlðOHÞ3 Þ ¼
V
¼ 1:2 mm;
A
ð10Þ
with h(Al(OH)3) being the thickness of the formed
aluminium hydroxide and A the area which is in contact
with the electrolyte (0.79 cm2).
Figure 10 shows the calculated aluminium hydroxide
thickness within the experiment.
The formed aluminium hydroxide shows a final
thickness of 4 mm.
SEM investigations of the aluminium surface were
performed before starting the experiment (Fig. 11).
Figure 9 Calculated concentration of aluminium in the electrolyte
from both ICP-MS and titrating results.
It is clearly observable that the concentration obtained from
the impedance titrator shows a steeper increase, which is due
to the formation of Al(OH)3 shown in reaction (3).
For the formation of Al(OH)3 hydroxide ions have to be
titrated to the system although aluminium does not dissolve.
By its formation the obtained values of aluminium in the
electrolyte are higher by calculation of titrated values than
those received by ICP-MS measurements. Aluminium
hydroxide is formed during the whole process but what
can be assumed from the two lines is that with ongoing time
more and more aluminium hydroxide is formed. These
results once more make clear that in the first hours of the
experiment, the native oxide is restructured and only a small
amount of new aluminium hydroxide is formed. Later with
increasing difference between titration and mass spectrometric results, aluminium hydroxide growth is enhanced,
which is, however, still not fully protective. The formation
of new aluminium hydroxide shows a linear growth. The
concentration difference of titrating (nAl-Titration) and ICPMS (nAl-ICP-MS) measurements gives the opportunity to
calculate the thickness of the formed aluminium hydroxide
layer by the following equations calculated for measurements values after 1 h:
nðAlðOHÞ3 Þ ¼ nAlTitration nAlICPMS
¼ 2:94 106 mol;
VðAlðOHÞ3 Þ ¼
nM
¼ 9:5 105 cm3 :
r
ð8Þ
ð9Þ
With V(Al(OH)3) as the volume of formed aluminium
hydroxide on the sample in cm3, n amount of Al(OH)3
in mol, M molar mass of Al(OH)3 (78 g mol1) and r density
of Al(OH)3 (2.42 g cm3). With respect to the defined area
that is in contact with the electrolyte calculation of the
www.pss-a.com
Figure 10 Calculated aluminium hydroxide thickness from the
difference of impedance titrator and ICP-MS results during the
whole experiment.
Figure 11 SEM images of the aluminium surface before
electrochemical measurements.
ß 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
solidi
status
physica
pss
a
6
S. Walkner et al.: Film dissolution kinetics of aluminium
gratefully thank the company voestalpine for their support. The
financial support by the Austrian Federal Ministry of Economy,
Family and Youth and the National Foundation for Research,
Technology and Development in the frame of the Christian
Doppler Laboratory for Combinatorial Oxide Chemistry (COMBOX) is gratefully acknowledged.
References
Figure 12 SEM images of the formed aluminium hydroxide
films.
SEM measurements after measurements with the
impedance titrator revealed that the corrosion products
are extremely porous and do not cover the whole sample as
already seen in the impedance spectra (Fig. 12).
These images once more confirm the former results
obtained from OCP, EIS, and ICP-MS measurements.
4 Conclusions The behaviour of bulk aluminium
covered with a native oxide layer at pH 11.0 was studied.
Different ongoing surface processes were quantitatively
investigated such as dissolution and restructuring of the
native oxide film, dissolution of aluminium and formation of
Al(OH)3 on the surface. Results showed that the native
oxide does not protect the aluminium from further corrosion
and a voluminous hydroxide film that is not protective forms
at the sample.
Acknowledgements Financial support of the Austrian
Research Promotion Agency (FFG) within the COMET framework
and financial support of Lower Austria is appreciated. The authors
ß 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
[1] G. E. Totten and D. S. MacKenzie, Handbook of Aluminium,
Alloy Production and Materials Manufacturing (Marcel
Dekker, New York, 2003).
[2] R. C. Dorward and T. R. Pritchett, Mater. Design 9, 63
(1988).
[3] E. A. Starke, Jr. and J. T. Staley, Prog. Aerospace Sci. 32, 131
(1996).
[4] J. P. Immarigeona, R. T. Holta, A. K. Koula, L. Zhaoa, W.
Wallacea, and J. C. Beddoesa, Mater. Charact. 35, 41 (1995).
[5] W. S. Millera, L. Zhuanga, J. Bottemaa, A. J. Wittebrooda, P.
De Smetb, A. Haszlerc, and A. Viereggec, Mater. Sci. Eng. A
280, 37 (2000).
[6] G. S. Cole and A. M. Sherman, Mater. Charact. 35, 3 (1995).
[7] S. Sch€urz, M. Fleischanderl, G. H. Luckeneder, K. Preis, T.
Haunschmied, G. Mori, and A. C. Kneissl, Corros. Sci. 51,
2355 (2009).
[8] S. Sch€urz, G. H. Luckeneder, M. Fleischanderl, P. Mack, H.
Gsaller, A. C. Kneissl, and G. Mori, Corros. Sci. 52, 3271
(2010).
[9] M. Voith, A. I. Mardare, and A. W. Hassel, Phys. Status
Solidi A 210, 1000 (2013).
[10] Q. F. Li and N. J. Bjerrum, J. Power Sources 110, 1 (2002).
[11] A. I. Mardare, M. Kaltenbrunner, N. S. Sariciftci, and S.
Bauer, Phys. Status Solidi A 209, 813 (2012).
[12] S. Yang and H. Knickle, J. Power Sources 112, 162 (2002).
[13] M. L. Doche, F. Novel-Cattin, R. Durand, and J. J. Rameau, J.
Power Sources 65, 197 (1997).
[14] D. S. Keir, M. J. Pryor, and P. R. Sperry, J. Electrochem. Soc.
114, 777 (1967).
[15] J. Albert, M. A. Kulandainathan, M. Ganesan, and V. Kapali,
J. Appl. Electrochem. 19, 547 (1989).
[16] Y. Jeliazova, M. Kayser, B. Mildner, A. W. Hassel, and
D. Diesing, Thin Solid Films 500, 330 (2006).
[17] S. Pyun and S. Moon, J. Solid State Electrochem. 4, 267
(2000).
[18] H. B. Shao, J. M. Wang, Z. Zhang, J. Q. Zhang, and C. N.
Cao, J. Electroanal. Chem. 549, 145 (2003).
[19] K. C. Emreg€ul and A. A. Aks€ut, Corros. Sci. 42, 2051 (2000).
[20] A. Mozalev, A. Poznyak, I. Mozaleva, and A. W. Hassel,
Electrochem. Commun. 3, 299 (2001).
[21] A. Mozalev, A. J. Smith, S. Borodin, A. Plihauka, A. W.
Hassel, M. Sakairi, and H. Takahashi, Electrochim. Acta 54,
935 (2009).
[22] H. Z. Wang, D. Y. C. Leung, M. K. H. Leung, and M. Ni,
Renew. Sust. Energy Rev. 13, 845 (2013).
[23] J. Bernard, M. Chatenet, and F. Dalard, Electrochim. Acta 52,
86 (2006).
[24] I. Boukerche, S. Djerad, L. Benmansour, L. Tifouti, and K.
Saleh, Corros. Sci. 78, 343 (2014).
[25] G. T. Burstein and C. Liu, Corros. Sci. 37, 1151 (1995).
[26] S. Adhikari and K. R. Hebert, Corros. Sci. 50, 1414 (2008).
[27] F. J. Martin, G. T. Cheek, W. E. OGrady, and P. M.
Nathishan, Corros. Sci. 47, 3187 (2005).
www.pss-a.com
Original
Paper
Phys. Status Solidi A (2016)
[28] S. M. Moon and S. I. Pyun, Electrochim. Acta 44, 2445
(1999).
[29] C. Fenster, M. Rohwerder, and A. W. Hassel, Mater. Corros.
11, 60 (2009).
[30] S. Walkner and A. W. Hassel, Electrochim. Acta 131, 130 (2014).
[31] E. Deltombe, C. Vanleugenhagen, and M. Pourbaix, Atlas
of Electrochemical Equilibria in Aqueous Solutions
www.pss-a.com
7
(National Association of Corrosion Engineers, Houston,
1974), p. 168.
[32] J.-B. Jorcin, M. E. Orazem, N. Pebere, and B. Tribollet,
Electrochim. Acta 51, 1473 (2006).
[33] S. Bonk, M. Wicinski, A. W. Hassel, and M. Stratmann,
Electrochem. Commun. 6, 800 (2004).
[34] T. Hurlen and A. T. Haug, Electrochim. Acta 29, 1133 (1984).
ß 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim