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Electrochimica Acta 48 (2003) 1115 /1121
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Precursor sites for localised corrosion on lacquered tinplates
visualised by means of alternating current scanning electrochemical
microscopy
Bernardo Ballesteros Katemann a, Carlota González Inchauspe b, Pablo A. Castro c,
Albert Schulte a, Ernesto J. Calvo b, Wolfgang Schuhmann a,*
b
a
Analytische Chemie-Elektroanalytik & Sensorik, Ruhr-Universität Bochum, Universitätsstr. 150, D-44780 Bochum, Germany
INQUIMAE */Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires Pabellon II, Ciudad Universitaria, AR-1428 Buenos Aires,
Argentina
c
CINI */Centre for Industrial Research, FUDETEC, Dr. J. Simini 250 (2804), Campana, Buenos Aires, Argentina
Dedicated to Professor Dr. Joachim Walter Schultze on the occasion of his 65th birthday.
Abstract
In solutions of low conductivity and at high frequencies the impedance of a SECM tip-auxiliary electrode cell is dominated by the
solution resistance between the tip and counter electrode. Alternating current scanning electrochemical microscopy (AC-SECM)
utilises the effect of an increasing (decreasing) solution resistance as the SECM tip approaches an insulator (conductor) for mapping
domains of different conductivity/electrochemical activity on surfaces immersed into electrolytes. In the present study, we employed
AC-SECM in aqueous solutions to evaluate the integrity of the solid/liquid interface of lacquered tinplates as commonly used in
industry to manufacture, i.e. food cans. Significant differences were determined between the AC response and the phase shift
measured with the SECM tip above the intact coating and above defects where the surface of the steel base is exposed. This allowed
with high lateral resolution to detect and to visualise artificial micro cavities which we consider as an experimental model of
microscopically small precursor sites for localised corrosion.
# 2003 Elsevier Science Ltd. All rights reserved.
Keywords: Scanning electrochemical microscopy; SECM; Electrochemical impedance spectroscopy; Localised corrosion; Lacquered tinplates; ACSECM
1. Introduction
Various types of high-performance coatings have
been developed and applied in industry to metallic
bulk articles such as household appliances, food cans
and automotive components in order to protect them
* Corresponding author. Tel.: /49-234-322-6200; fax: /49-234321-4683; http://www.ruhr-uni-bochum.de/elan.
E-mail address: [email protected] (W.
Schuhmann).
against corrosion, the environmental degradation of
metals and alloys, and to improve their consumers
appeal. Frequently used coatings include for instance
organic paints, enamel, thin layers of corrosion-resistive
metals like tin and chromium, films of passive oxides
and combinations of these systems. Independent on the
nature of the coating, it should be chemically inert,
resistant to mechanical or thermal stress and free of any
cracks and holes to ensure a well-operating protection to
the underlying metal from any corrosive attack. Nonetheless, the coatings can become damaged during
0013-4686/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0013-4686(02)00822-8
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B.B. Katemann et al. / Electrochimica Acta 48 (2003) 1115 /1121
manufacturing procedures and/or continuous use, and
failures in the protecting layer certainly can cause severe
corrosion damage to the products over time. For this
reason, corrosion monitoring is of major interest in
industry not only because of safety concerns and
because of the risk of lost production and/or product
contamination but also on account of the development
of preventive, predictive and corrective maintenance
practices.
Conventional electrochemical techniques such as
electrochemical impedance spectroscopy (EIS) and the
measurement of steady-state polarisation curves [1,2]
have been widely used to study corrosion and provided
important informations about the global properties of
the metal/coating interface and its large-scale corrosion
activity. However, coated surfaces are typically not
corroding uniformly but confined to very small surface
areas (i.e. microscopic pits, cracks and pores) without
any obvious superficial signs of deterioration. Moreover, the appearance of macroscopic (visible) symptoms
of corrosion is only an expression of the final stages of a
complex, dynamic sequence that starts at the microscopic level, making its detection and monitoring a
challenging task. Recognition of the phenomena of
localised corrosion has led to several attempts to
spatially characterise the corrosion activity of microscopic defects in the metal/coating interface with high
lateral resolution. In fact, localised electrochemical
impedance spectroscopy (LEIS) [3], the scanning reference (SRET) and scanning vibrating (SVET) [4] electrode techniques, as well as the scanning Kelvin probe
technique [5], the approach of the scanning droplet cell
[6,7] and scanning electrochemical microscopy (SECM)
[8,9] are amid the micro electrochemical methodologies
currently subjected in a number of laboratories to
investigate the process of localised corrosions.
Based on an early work from Horrocks et al. [10], we
recently developed alternating current scanning electrochemical microscopy (AC-SECM) as a novel tool for
measuring local interfacial impedance properties with
high lateral resolution [11]. AC-SECM combines electrochemical impedance measurements with SECM. In
brief, a high-frequency alternating voltage is applied
between the SECM tip and a counter electrode, the AC
current response to the voltage perturbation is measured
using a lock-in amplifier or frequency response analyser,
and, using phase-sensitive demodulation, the impedance
of the 2-electrode arrangement is determined. This
impedance can be described by a simple RC equivalent
circuitry consisting of a series combination of the
solution resistance between the two electrodes (Rsol)
and the double-layer capacitance of the tip electrode
(Cd). In electrolytes of sufficiently low conductivity and
at high frequencies the impedance of the circuitry is
dominated by Rsol which was found to be strongly
depending on the tip-to-sample distance and the nature
of the sample material. In fact, the impedance increased
(decreased) with the tip electrode approaching an
insulator (conductor). Approach curves (plots of the
AC response as a function of distance) were used to
position the SECM tip at an appropriate working
distance in a similar way as earlier described [11,12]
and imaging in the AC mode of SECM was achieved by
scanning the tip electrode across the sample surface (x ,
y -plane) and monitoring either the AC response or its
phase shift as a function of tip location. This approach
of local AC-response measurements allowed clearly to
identify and to visualise microscopic domains of different conductivity/electrochemical activity present on an
array of Pt-band microelectrodes immersed in a mediator-free electrolyte.
In the present study, AC-SECM was used to examine
in-situ the metal/coating interface of lacquered tinplates
which are commonly used in industry to manufacture
metal food containers. It is demonstrated, that ACSECM allowed to visualise microscopic cracks and holes
in the coating of the lacquered tinplates with high lateral
resolution. As a matter of fact, these local defects in the
corrosion-resistive tin/polymer coating are leading to an
exposure of the underlying steel and have to be
considered as potential precursor sites for localised
corrosion.
2. Experimental
2.1. Chemicals, samples and microelectrode preparation
The electrolytes were prepared using double-distilled
water and the impedance measurements performed in
inert solutions of either 1 /10 mM sodium or potassium
chloride (Sigma-Aldrich, Deisenhofen, Germany) or
aqueous solutions of 1.5% sodium citrate (SigmaAldrich) and 1.0% sodium chloride with no added redox
mediator. The tips for the AC-SECM studies were glassinsulated, disk-shaped Pt microelectrodes (12.5 mm disk
radius) which were fabricated from 25 mm diameter Ptwires (Goodfellow, Bad Nauheim, Germany) following
a procedure as previously described [13]. Lacquered
tinplates were provided by the iron and steel industry
(Siderar, Argentina). They consist of a low-carbon steel
covered with a thin layer of electrodeposited tin and
additionally lacquered with a film of an organic solventbased epoxyphenolic varnish (film thickness 9/15 mm).
Artificial micro cavities such as holes and cracks (see
insets in Figs. 2 and 4A) were introduced in some of the
samples by mechanically puncturing or scraping their
B.B. Katemann et al. / Electrochimica Acta 48 (2003) 1115 /1121
polymer coating using the sharp end of an electrochemically etched tungsten STM tip.
2.2. Instrumentation
Conventional electrochemical impedance spectroscopy on lacquered tinplates was carried out in a 1compartment electrochemical cell, in 3-electrode configuration with a Pt-mesh counter and a Ag/AgCl reference
electrode and using a frequency response analyser (S5720C, NF Corporation, Yokohama, Japan) and a
potentiostat/galvanostat (Model IMP 88 PG, Jaissle
Elektronik GmbH, Waiblingen, Germany) for the
measurements of impedance spectra.
If not stated otherwise, AC-SECM measurements
were performed in a two-electrode configuration, with
the SECM tip as working and the sample (the tinplate)
as counter electrode. The electrochemical cell with the
tinplate fixed to its bottom was mounted on the threeaxis translation stage of a home-built SECM (for details
of its design see Ref. [14]) driven by computer-controlled
stepper motors having a nominal resolution of 0.6 mm
per half step in each direction. A frequency response
analyser (same as mentioned above) was used to apply a
sinusoidal voltage perturbation (typically 10/100 mVpp
at frequencies of 1/10 kHz) to the electrodes held at
open circuit potential. The AC current response of the
system was measured with a current amplifier providing
a current sensitivity of 104 /1011 V A 1 (Model 427,
Keithley Instruments, Cleveland, USA). The output of
the current amplifier was fed into the input of the
frequency response analyser which in turn provides the
modulus Z (under the experimental conditions representing the solution resistance between the tip and
counter electrode) and the phase shift (u ) of the AC
signal of the electrode arrangement. Both, Z and u are
parameters which are dependent on the tip-to-sample
distance and the sample’s conductivity and in principle
can be used for imaging with a SECM operated in the
AC mode [11]. However, in this study, Z was chosen as
a means to monitor the tip-to-sample distance for
recording of approach curves and for tip positioning,
and, in contrast to recent results by Wipf et al. [12], even
as a function of the x - and y -tip position for AC-SECM
imaging. A PC in combination with a Windows software
programmed in Microsoft VISUAL BASIC 3.0 (Microsoft,
Redmont, USA) was used to control all system parameters and for data acquisition.
Alternatively, AC-SECM was performed using a lockin amplifier (PAR 5210, Perkin Elmer, Bad Wildbad,
Germany) instead of the frequency response analyser in
order to generate the sinusoidal voltage perturbation
and to analyse the systems AC response. In these
experiments, a highly sensitive potentiostat (PG100,
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Jaissle Elektronik GmbH, Waiblingen, Germany) was
used to operate the electrochemical cell in 3-electrode
configuration, with the SECM tip as working, a Pt-wire
as counter and a Ag/AgCl as pseudo-reference electrode.
The lock-in amplifier provided the current magnitude R
of the AC current response with respect to the reference
signal. Approach curves (R vs. d ) were recorded and
used for tip positioning whereas monitoring R as a
function of x and y tip position allowed imaging.
3. Results and discussion
The lacquered tinplates were studied first using
conventional (global) electrochemical impedance spectroscopy [15]. Impedance spectra were measured on
samples immersed into an aqueous solution of 1.5%
sodium citrate and 1.0% sodium chloride as a function
of the immersion time. A significant decrease in the
impedance modulus was observed with an increasing
time of exposure, especially at lower frequencies (see
Fig. 1). This drop in impedance is most likely due to a
swelling of the organic film induced by an uptake of
water [16]. In addition, impedance spectra were recorded
on lacquered tinplates with an increasing number of
artificial micro cavities in their polymer coating which
were considered as experimental models for microscopically small precursor sites for localised corrosion. As
expected, the impedance spectra displayed appreciable
differences in that with an increasing number of defects
the impedance modulus notably decreased (see Fig. 2).
These two experiments are a good proof that measuring
global impedance spectra offers valuable information
Fig. 1. Impedance spectra (Bode plots) of a lacquered tin plate
(exposed area: 12 cm2) immersed into an aqueous solution of 1.5%
sodium citrate and 1.0% sodium chloride and recorded at different
times of exposure to this electrolyte. The amplitude of the voltage sign
wave used for perturbation was 50 mVpp with frequencies ranging from
0.1 Hz to 10 kHz.
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Fig. 2. Impedance spectra (Bode plots) of lacquered tin plates (exposed area was 12 cm2) with an intact coating and with a various number of
artificial micro cavities (pin holes) introduced by mechanically puncturing the protecting polymer film. The EIS measurements were conducted in an
aqueous solution of 1.5% sodium citrate and 1% sodium chloride. The amplitude of the voltage sign wave used for perturbation was 50 mVpp with
frequencies ranging from 0.1 Hz to 10 kHz. Inset: Optical image presenting four of the pin holes in the coating of the tinplate.
about the condition of a polymer-coated surface. However, these measurements are not capable of providing
details about local properties of the surface of the
sample. For instance, global impedance spectra do not
allow to gain an insight in the number and size of defects
possibly be present in the coating due to the fact that the
signal is an averaged response of the entire surface of the
sample exposed to the electrolyte. Thus, no difference
between a low density of large pores and a large density
of small pores in the coated metal can be observed. This
limitation of global impedance spectroscopy gave good
reason for applying the approach of AC-SECM to the
lacquered tinplates with the aim to detect and visualise
individual microscopic defects in their organic coating
with high lateral resolution.
In a typical AC-SECM experiment, a sinusoidal
voltage perturbation with frequencies not smaller than
1 kHz was applied in a diluted solution of KCl (1 /10
mM) between the SECM tip and the counter electrode
(sample), causing a small alternating current to flow. As
already mentioned above, Z , the impedance modulus of
such a two-electrode arrangement, is in solutions of low
ionic strength and at high enough frequencies dominated by the solution resistance Rsol and was found to be
strongly dependent on the tip-to-sample distance (d)
and the chemical nature of the sample material.
Approach curves (Z vs. d ) were recorded with a 12.5mm-radius Pt micro disk electrode approaching a
lacquered tinplate or a Au surface which was used for
simplicity reasons as a model of the conducting steel
surface underneath the coating of the tinplates. As can
be seen in Fig. 3A, at d values of a few times the tip
diameter, the surface of the insulating lacquer starts to
hinder the current flow leading to an increase of the
modulus Z . In contrast, with the tip getting closer to the
Au surface, Z decreased because the proximity to the
conducting Au-surface enhances the current flow and
lowers Rsol (see Fig. 3B).
The nature of these approach curves is the principle
prerequisite for imaging domains of different conductivity on the surface of the coated tinplates by means of
AC-SECM. Obviously, the increase (decrease) in Z as
the SECM tip is getting close to an insulator (conductor)
follows a similar dependence as that of the tip current
B.B. Katemann et al. / Electrochimica Acta 48 (2003) 1115 /1121
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sample surface (at distances of just about the radius of
the SECM tip, see Fig. 3A). Imaging in the AC mode of
SECM was achieved by scanning the SECM tip in
constant height in the x, y-plane above the tinplates and
monitoring Z (R ) as a function of tip location. Using
this approach, we scanned lacquered tinplates with
artificial scratches or pinholes of microscopic dimensions in its protecting polymer layer. As demonstrated in
the Figs. 4 and 5, the lateral variations in the electrical
conductivity between regions with an intact coating and
regions with failures were clearly detected because of the
remarkable differences in the observed AC signal and
hence allowed to visualise the defects with a high spatial
resolution.
Single line scans (see Fig. 4A) were used to position
the tip electrode either right above the centre of a
scratch at an area where the steel base of the tinplates is
Fig. 3. Comparison between approach curves (normalised modulus Z
as a function of tip-substrate separation) obtained on (A) a lacquered
tinplate and (B) a gold plate, both measured in 10 mM KCl with a 12.5
mm radius Pt micro disk electrode. The amplitude of applied sine
voltage used for perturbation was 10 mVpp at a frequency of 10 kHz.
obtained with SECM in the (amperometric) feedback
mode in solutions containing an appropriate redox
mediator [10]. AC-SECM offer the advantage over
SECM in the feedback mode in that the measurements
can be performed in solutions free of any redox
mediator avoiding perturbations of the chemistry at
the metal/coating interface otherwise at risk because of
undesirable reactions between the electrochemically
active species and the metal.
Prior to imaging the surface of lacquered tinplates,
approach curves (either Z or R , the current magnitude
vs. d ) were measured and used for positioning the
SECM tip at an appropriate working distance within the
feedback range. Moving towards an area of intact
coating, the approach was typically stopped when the
values for Z (R ) changed by about 50% with respect to
their values with the tip in infinite distance from the
Fig. 4. (A) A single line-scan displaying the modulus Z (normalised to
values Zo measured above the intact coating) as a function of the tip
position in x -direction at the same time as the tip moves in constant
height across a microscopic scratch in the coating of a lacquered
tinplate. The measurement was performed using a 12.5 mm radius Pt
micro disk electrode as the SECM tip, 10 mM KCl as the electrolyte,
and a perturbation of 20 mV at 5 kHz frequency. Inset: optical image
of the scratch. (B) 3D-image of the scratch shown in (A) obtained with
AC-SECM in a constant-height mode and using multiple line scans
(same parameters as described in A).
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B.B. Katemann et al. / Electrochimica Acta 48 (2003) 1115 /1121
Fig. 6. Local electrochemical impedance spectra with the SECM tip
electrode kept in fixed position either above the intact region of the
coating of a lacquered tinplate (%) or above the centre of a scratch (m)
as for example shown in Fig. 4. The spectra (100 Hz to 50 kHz) were
measured in 10 mM KCl with a disk-shaped 12.5-mm-radius Pt
microelectrode and a 20 mV perturbation.
Fig. 5. AC-SECM images of microscopically small pin holes in the
coating of lacquered tinplates. (A) 2-D image of an array of pin holes
(overview). (B) 3-D image of one pin hole at higher magnification.
Measurements were performed in 1 mM NaCl and in a 3-electrode
configuration with a Pt counter and a Ag/AgCl pseudo reference
electrode. The images were obtained with AC-SECM in a constantheight mode operating a disk-shaped, 12.5 mm radius Pt microelectrode
(working electrode) with a perturbation of 100 mV at 1 kHz frequency.
exposed to the electrolyte or above the intact coating
just next to the defect. Microelectrode impedance
spectra with frequencies ranging from about 100 Hz to
50 kHz were then recorded at these discrete locations
with the tip held in fixed position. Although the
impedance values in spectra as shown in Fig. 6 actually
are a measure of the solution resistance, they still
indirectly reflect the specific nature of the surface
beneath the sensing surface of the SECM tip since
obviously the solution impedance is modulated by the
properties of the sample material. In fact, the impedance
moduli were found to be significantly higher with the tip
above the intact coating as compared with the ones
obtained with the tip above the micro cavities. These
differences were largest in the higher frequency domain
of the impedance spectra. For this reason, frequencies
not smaller than 1 kHz have been used for imaging
tinplate samples in the AC mode of SECM.
In conclusion, AC-SECM was used to characterise the
solid/liquid interface of lacquered tinplates in contact
with aqueous solutions. We provided evidence that ACSECM has the capability to distinguish between regions
on the samples surface with an intact coating and with
failures in the protecting polymer film. Furthermore,
microscopic domains of a varying conductivity/electrochemical activity on surface artificial micro cavities such
as cracks and pinholes used as a representative model of
precursor sites for localised corrosion were clearly
identified and properly visualised at a high spatial
resolution.
Acknowledgements
We gratefully acknowledge the financial support of
this work by the Deutsche Forschungsgemeinschaft
(DFG) in the framework of the SFB 459, project A5.
The exchange of scientists between Germany and
Argentina was supported by PROALAR grant jointly
funded by the DAAD (Germany) and ANPCyT (Argentina). CGI gratefully acknowledges a postdoctoral
fellowship under the industrial scheme of CONICET
(Argentina) and FUDETEC (Technit. Corp.).
References
[1] P.R. Roberge, Handbook of Corrosion Engineering, McGrawHill, New York, USA, 1999.
[2] F. Mansfeld, M. Kendig, Werkstoffe Korrosion 36 (1985) 473.
[3] F. Zhou, D. Thierry, H.S. Isaacs, J. Electrochem. Soc. 144 (1997)
1957.
[4] J.W.H. de Wit, D.H van der Weijde, A. de Jong, F. Blekkenhorst,
S.D. Meijers, Electrochemical Methods in Corrosion Research
VI, Pt. 1 and 2, 289-2, 1998, pp. 69.
B.B. Katemann et al. / Electrochimica Acta 48 (2003) 1115 /1121
[5] M. Stratmann, R. Feser, A. Leng, Electrochim. Acta 39 (1994)
1207.
[6] M.M. Lohrengel, A. Moehrig, M. Pilaski, Fresenius J. Anal.
Chem. 367 (2000) 334.
[7] T. Suter, H. Böhni, Electrochim. Acta 42 (1997)
3275.
[8] J.V. MacPherson, P.R. Unwin, in: A.J. Bard, M.V. Mirkin (Eds.),
Scanning Electrochemical Microscopy, Marcell Dekker, New
York, USA, 2001, p. 521.
[9] A. Schulte, S. Belger, W. Schuhmann, Mater. Sci. Forum 394 /
395 (2002) 145.
[10] B.R. Horrocks, D. Schmidtke, A. Heller, A.J. Bard, Anal. Chem.
65 (1993) 3605.
1121
[11] B. Ballesteros Katemann, A. Schulte, M. Koudelka-Hep, E.J.
Calvo, W. Schuhmann, Electrochem. Commun. 4 (2002) 134.
[12] M.A. Alpuche-Aviles, D.O. Wipf, Anal. Chem. 73 (2001) 4873.
[13] C. Kranz, M. Ludwig, H.E. Graub, W. Schuhmann, Adv. Mater.
7 (1995) 38.
[14] A. Hengstenberg, C. Kranz, W. Schuhmann, Chem. Eur. J. 6
(2000) 1547.
[15] M.J.L. Gines, P.A. Castro, T. Perez, H. Rissone, J.L. Zubimendi,
W. Egli, Evaluation of Tinplate/Lacquer Systems by Electrochemical Methods: Effect of Can-Forming and Lacquer Composition. Metal Bulletin/ITRI’s 7th International Tinplate
Conference, Amsterdam, Netherlands, 2 /4 October 2001.
[16] F. Mansfeld, J. Appl. Electrochem. 25 (1995) 187.