The Use of Electrochemical Impedance Spectroscopy in the Evaluation of Coatings for Outdoor
Bronze
L.A. Ellingson,TJ. Shedlosky, G.P. Bierwagen, E.R. de la Rie and L.B. Brostoff
Electrochemical impedance spectroscopy (EIS) is a technique that quantitatively measures the
amount of corrosion protection provided by a coating to a metal substrate. This article illustrates
the theory of EIS along with its potential for application in the evaluation of protective coatings for
outdoor bronze sculpture and ornamentation. Three coating systems were evaluated on a
monumental cast bronze: a microcrystalline wax; the methyl methacrylate copolymer sold under the
trade name Incralac; and an acrylic/acrylic methane topcoat. Each coating system was aged using
accelerated weathering and evaluated via EIS. This information was then used to calculate the
impedance modulus of the samples, which is displayed in Bode plots and interpreted in terms of
coating performance.
INTRODUCTION
Continuing efforts to protect bronze works of art from the hazards of the outdoor environment are
vital endeavors in the fields of conservation and conservation science [1—10]. The use of clear
coatings is an important option in the protection of bronze. Although protective coatings delay the
onset of corrosion, their effectiveness varies greatly. An effective method to evaluate and compare
the degree of corrosion protection afforded by different coatings is electrochemical impedance
spectroscopy (EIS).
Electrochemical techniques for the quantitative evaluation of coatings on metallic substrates have
been in use for many years [11—14]. EIS has been used extensively in corrosion research
throughout the past two decades, and is a valuable technique for monitoring the performance of
various coatings on bronze [15, 16]. Recently, Carullo et al. described the potential for use of an in
situ EIS instrument that would allow corrosion monitoring of works of art [17]. In anticipation of
such a tool, EIS is shown in this paper to aid in the evaluation of the coating performance of model
samples of coated bronze subjected to accelerated weathering in simulated laboratory conditions.
The following discussion encompasses a detailed introduction to the theory of EIS, as well the
interpretation of EIS data gathered from select samples that were created at the National Gallery of
Art in Washington DC as part of a broader research study into new protective coatings for bronze
[2—4].
ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY
Theory
Electrochemical impedance spectroscopy is a technique that determines a numerical value for the
degree of corrosion protection provided by a coating to a metal substrate. This numerical value is
called the impedance of the coating and is defined as the ability of the coating to resist or oppose
corrosion through a combination of barrier and adhesive properties. The more the coating protects
the underlying metal, the higher the impedance value. Impedance values typically range from 1 x
10° ohms to as low as 100 ohms. The lower limit is defined as the impedance, or resistance, of the
metal substrate and is dependent on the chosen material. Thus the impedance of bronze is not
comparable to the impedance of aluminum. The substrate impedance can be determined by simply
testing an uncoated sample. This value is then defined as the failure point for any coated samples.
The impedance data are usually not illustrated in the form of a bar graph but, for explanation
purposes, Figure 1 displays the information that impedance spectroscopy provides.
The impedance of a sample can be explained through the concepts of current and voltage. Webster's
Dictionary defines current as a 'moving stream of electricity', but more specifically it is the rate of
flow of positive charge. Current (1) passes through an object as shown in Figure 2; current is
expressed in amperes.
Voltage (E) is defined as the potential difference (AE) between two points and is expressed in units
of volts. The potential can be described as the driving force for the electrochemical reactions.
Electrochemical reactions will occur more readily as the potential gradient becomes greater. Figure
3 illustrates the potential difference that exists between two planes within an object where a current,
I, is flowing [18].
Both current and voltage are used to determine the impedance of an electrochemical cell. EIS data
are collected using a three-electrode cell that consists of a reference, counter, and working
electrode. The working electrode is the sample of interest being studied, while the reference
electrode may be, for example, a saturated calomel electrode, as used in this study. No current, or
electricity, flows through the reference electrode. Instead it only serves as a reference with a known
and constant potential. The third element in the electrochemical cell is the counter electrode, to
which the current is allowed to flow.
Initially the open circuit potential is determined and is defined as the voltage between the reference
electrode and the working electrode. A sinusoidal voltage with an amplitude of 1—10 mV is then
applied to the electrochemical cell with respect to the open circuit potential; this results in a
sinusoidal current response. Whatever current is necessary to maintain the potential difference will
flow through the electrochemical cell between the working electrode and counter electrode. The
sinusoidal current that flows through the cell is out of phase with the sinusoidal potential. In-phase
behavior only occurs in a pure resistor. A 90° lagging of the current only occurs for a pure
capacitor. A capacitor is an electronic circuit element that has the ability to store charge. Because a
coated sample contains both resistive and
Figure 1 Bar graph illustrating information determined by electrochemical impedance
spectroscopy (EIS).
Figure 2 Illustration of current flow.
Figure 3 Illustration of voltage.
capacitive elements, the phase shift of the current and the potential will reside between 0 and 90°.
This sinusoidal behavior is illustrated in Figure 5.
The impedance of a sample is determined from information provided by these two sinusoidal
curves. The sample is scanned over a range of frequencies (ω) and over a period of time (t). The
impedance (denoted as Z) can be determined according to equation 1, shown below:
where |E0| and |I0| are the amplitudes of the AC voltage and current respectively, ft) is the angular
frequency, and θv and θ1 are the phase angles of the voltage and current respectively.
The impedance modulus (denoted as | Z |) is equal to the following:
The impedance modulus, | Z |, incorporates both in-phase and out-or-phase impedance information.
| Z | is defined by using the imaginary forms of impedance as seen in equation 3.
where i = √-l and θ = the phase angle between the real and imaginary axis.
Impedance data are normally illustrated in Bode plots, with log impedance modulus (| Z |) shown
versus log frequency (CO) of the imposed sinusoidal signal. Bode plots are one way of graphing
EIS data to interpret the data further.
Experimental conditions
The samples used in this work were designed and coated at the National Gallery of Art, as part of a
broader research project [2—4]. The elemental composition of the cast, polished, monumental
bronze was 85% copper, 5% lead, 5% zinc and 5% tin. This bronze alloy was chosen because it is
generally representative of metals that have been used in outdoor bronzes [4]. The substrate was
coated in a cleaned, degreased state. Once received at North Dakota State University, initial
impedance measurements were taken, and samples were subsequently cycled weekly between an
ultraviolet weathering chamber and a fog/humidity chamber in accordance with ASTM D 5894-96
Standard Practice for Cyclic Salt Fog/UV Exposure of Painted Metal (Alternating Exposures in a
Fog/Dry Cabinet and a UV/Condensation Cabinet). The ultraviolet weathering chamber (QUV)
consisted of four hours of exposure to 340 nm UV-A light at 60°C. alternated with four hours of
condensation at 50°C.
The use of accelerated weathering chambers has been debated in numerous publications, but
currently this method has been found to correlate fairly well to typical outdoor exposure [19]. It was
shown in a previous study that the cyclic Prohesion corrosion testing correlates well with real-world
testing [2()|. There are a number of advantages to accelerated weathering versus outdoor exposure.
The first obvious advantage is that the experimental time is drastically reduced. The second is that
the weathering scheme can be reproduced. As pointed out by Jacques, no period of natural
weathering can be reproduced precisely to provide equivalent weathering conditions for samples
tested at different time periods [21]. One disadvantage of accelerated aging is that many replicate
specimens are needed to construct a representative data set. Because of variations in sample
preparation, multiple samples should be considered mandatory. A second weakness is that the
corrosion process is only representative of what occurs naturally. Accelerated aging will not
produce all of the same corrosion products found in natural exposures nor can it reproduce all of the
variations of outdoor weathering. The major goal of accelerated weathering is to reproduce and
increase those stresses which have been
Figure 4 Schematic diagram of electrochemical cell used in EIS.
shown to cause most failures in outdoor exposure. Accelerated weathering usually will tell whether
a coating will provide object protection for some finite, but often uncertain, period of time in
outdoor weathering. The following week, samples underwent exposure in the fog/humidity chamber
(called a Prohesion chamber) to an environment that cycled between one hour of salt fog at 25°C
and one hour of no fog at 35°C. The salt fog used for weathering was dilute Harrison solution (0.35
wt% (NH4)2SO4 and 0.05 wt% NaCl in H2O). This solution has been shown to emulate acid rain
exposure [18].
EIS analysis of the protective coatings on the monumental bronze samples was determined through
application of an alternating voltage of 5 mV. The electrochemical cell consisted of a saturated
calomel reference electrode and a platinum mesh counter electrode that were immersed in dilute
Harrison solution. The electrolyte was kept in contact with the working electrode sample by using
an O-ring clamp with an area of 7.0 cm2. A Gamry PC3 potentiostat with CMS 100 software was
used to collect the data over the frequency range of 0.1 to 5000 Hz. Impedance measurements were
made at regular intervals during the weathering regime. The results of the impedance measurements
that are reported in this paper were taken from a single sample. Subsequent work has verified these
results with multiple samples [5].
Figure 5 Sinusoidal potential and current behavior for EIS
Impedance plots
A portion of the data that were collected in this study is exhibited here as an illustration of how EIS
can be effectively used in evaluating various coating systems. The Bode plot in Figure 6 shows the
impedance of the uncoated bronze, which maintains its value of ~300 ohms at low frequency over
the period of 14 days of accelerated weathering exposure. This value thus represents true failure of
a coating over bronze, i.e., the point at which a coating no longer provides corrosion protection.
Higher impedance values of a coated sample would imply that the coating system provides
protection to the metal substrate. However, if corrosion forms underneath the coating, a higher
impedance value will result, as the corrosion acts as an additional barrier layer. Hence the
development of corrosion needs to be monitored as well as the impedance.
Coatings for the protection of bronze are currently in use throughout the world. Two of the most
popular types of coatings used in the United States are waxes and the methyl methacrylate
copolymer coating sold under the name Incralac [2-10]. The latter coating is sometimes applied
with an additional wax topcoat. Waxes are considered to be high-maintenance coatings, since they
should be reapplied or renewed at least once a year [24-26]. Figure 7 shows a Bode plot of
impedance data from a monumental bronze plaque pretreated with the corrosion inhibitor
benzotriazole (BTA) (1.5% in ethanol), rinsed, and brush-coated with a blend of 75% Bareco
Victory Wax (a microcrystalline, synthetic, low melting point wax), plus Bareco Polywax 2000 and
500 (both polyethylene microcrystalline waxes), and Petronauba C (an oxidized polyethene wax)
[2—4]. Two coats of the wax mixture were applied by brush, followed by light buffing. The
measured coating thickness was less than 50 µm [2, 3|. The data indicate that the coating system
initially showed some corrosion resistance but, after only 14 days of accelerated exposure
conditions, the impedance of the coated sample was approximately the same value as the uncoated
bronze. This value was assigned as the point of failure for this system. The rise in impedance after
the first day of exposure is probably due to the fact that the system was not in the steady state at that
time, but came to local equilibrium with the immersion solution by the second day.
The Bode plot in Figure 8 displays impedance data for a bronze sample coated with Incralac and a
topcoat of wax, the same wax mixture as used previously. The Incralac was prepared as a 60 wt%
solution in xylene and sprayed onto the panels as recommended by the manufacturer. The thickness
of the Incralac plus wax topcoat was measured to be 15 x 2.5 µm [2, 3]. The data show that the
impedance of this coating was initially much higher than that of the BTA-pretreated, wax-coated
sample, and remained so over a period of about 112 days of accelerated weathering. Ultimate
failure was approached at 140 days, and is seen as a gradual drop in the impedance modulus. This
drop, which commenced
Figure 6 EIS Bode plot of uncoated bronze sample.
Figure 7 Bode plot of benzotriazole pretreatment + wax over bronze.
after about eight days, corresponds to a gradual decrease in the protective properties of the coating.
Incralac coatings, as typically applied to bronze in conservation treatments, are generally considered
to provide adequate protection for about three to five years [6, 10, 27]. Therefore, the 140-day
period of accelerated weathering may correlate roughly to this time-frame in natural outdoor
exposure. Although this coating may not be thought of as a high-performance coating, the impedance data show that the Incralac coating greatly outperforms the wax coating as applied and
weathered in this study.
In order for a coating to be categorized as 'high performance' it should maintain high impedance
values for a longer duration of time than seen with the Incralac on bronze samples. Data for one
coating that fulfilled this requirement are shown in Figure 9. This novel coating is a three-part
system: Nikolas acrylic 11565,
Figure 8 Bode plot of Incralac + wax coating over bronze.
Figure 9 Bode plot of Nikolas acrylic 11565 + Nikolas acrylic urethane 9778 + wax coating over
bronze.
followed by Nikolas acrylic urethane 9778, and top-coated with a wax [2—4]. This higher
performance coating was chosen because of its chemistry and potential for a longer lifetime and
better protection. The un-thinned acrylic was sprayed onto the surface in a single coat, on top of
which a 50% solution of the acrylic urethane in a recommended thinner was then spray-applied. The
resultant coating thickness was measured to be 40 x 7.5 u.m [2, 3]. Failure of this coating did not
appear to commence until about 98 days of exposure. Corrosion was evident after that period and is
shown by the gradual decrease in the impedance modulus to failure approached at about 252 days of
accelerated exposure.
The low frequency impedance values shown in Figures 6—9 are also plotted in a bar graph format
(Figure 10) in order more easily to compare the overall behavior of the three coatings on bronze as
studied by EIS. This figure shows that the Incralac and acrylic/ acrylic urethane coatings initially
showed good barrier
Figure 10 Bar graph illustrating impedance variations between samples.
properties, whereas the wax coating initially provided only limited resistance. The bar graph also
indicates that the overall time to ultimate failure for the three coatings was significantly different:
the durability of the acrylic/acrylic urethane coating was almost double that of the Incralac coating,
and almost 20 times greater than the wax coating. This indicates that the Acrylic/ acrylic urethane
coating, as applied in this study, provided the greatest corrosion protection of the coatings studied
by EIS. EIS is thus shown to be an enormously useful tool in the ranking, as well as in the relative
service-life prediction, of coatings on a metallic substrate.
CONCLUSIONS
Electrochemical impedance spectroscopy is a valuable method for quantitatively determining the
overall protective ability of various coating systems. Three coatings on bronze, as analyzed by EIS
and the resulting Bode plots, show various degrees of progressive failure as the impedance values
approached that of an uncoated substrate over different periods of time. The quantitative
information obtained from these plots illustrates a significant difference in protection provided by
wax, Incralac + wax, and an acrylic/acrylic urethane coating on monumental bronze. Quantitative
information such as this can be used to aid in the investigation of new protective coatings for
outdoor bronze, as well as to aid decisions about existing coating options in outdoor bronze
conservation. The value of this application could furthermore be greatly enhanced in the future by
the development of commercially available, in situ electrochemical impedance spectroscopy
instruments for use in the field.
ACKNOWLEDGEMENTS
This work was made possible by funding from the National Center for Preservation Technology and
Training fund MT-2210-9-NC-20.
MATERIAL SUPPLIES
[ncralac was obtained from StanChem, Inc., 401 Berlin Street, East Berlin, CT 06023, USA.
Nikolas Acrylic 11565 and Nikolas acrylic urethane 9778 is produced by G.J. Nikolas & Co.. Inc.,
2800 Washington Blvd. Bellwood, IL 60104, USA. The wax used in this study is a typical recipe
blend used by American conservators.
REFERENCES
1 Van Zelst, L., and Lachevre, }., 'Outdoor bronze sculpture: problems and procedures of protective
treatment', Technology and Conservation 8 (1983) 18-24.
2 Brostoff, L., and de la Rie, R., 'Research into protective coating systems for outdoor bronze
sculpture and ornamentation. Phase IV, PTT Publications No. 1999-23, National Center tor
Preservation Technology and Training, Natchitoches, Louisiana (1999).
3 Brostoff, L., Shedlosky, T., and de la Rie, R., 'Research into protective coating systems for
outdoor bronze sculpture and ornamentation. Phase III', PTT Publications No. 2000-05, National
Outer for Preservation Technology and Training, Natchitoches, Louisiana (2000).
4 BrostofF, L.B., and de la Rie, E.R., 'Research into protective coating systems for outdoor bronze
sculpture and ornamentation' in 1.0. MacLeod, S.L. Pennec and L. Robbiola (eds), Metal 95, James
& James, London (1995) 242-244.
5 Bierwagen, G.P., Ellingson, L., and Shedlosky, T., "Development and testing of organic coatings
for the preservation of outdoor bronze sculpture from air-pollutant enhanced corrosion', PTT
Publications No. 2001-08, National Center for Preservation Technology and Training,
Natchitoches, Louisiana (2001).
6 Scott, D., Copper and Bronze in Art Corrosion, Colorants, Conservation, Getty Publications,
Los Angeles (2002).
7 Montagna, D., 'Caring for outdoor bronze sculpture', Cultural Resources Management 18 (1995)
26-28.
8 Riss, LX, 'Managing the care of outdoor metal monuments by the National Park Service: Some
past experience and future direction' in V.N. Naude (ed.). Sculptural Monuments in an Outdoor
Environment, Pennsylvania Academy of the Fine Arts. Philadelphia (1983) 29-38.
9 Kipper. P.V., The Care of Bronze Sculpture, 2nd edn, Path
Publications and Rodgers & Nelsen, Loveland (1998). 10 Weil, P.0., 'The conservation of outdoor
bronze sculpture: a review of modern theory and practice' in Preprints of Papers Presented at the
Eighth Annual Meeting, San Francisco, 1980, American Institute for Conservation of Historic and
Artistic Works, Washington DC (1980) 129-140.
1 I Bierwagen, G.P., Jeffcoate, C, Li, J., Balbyshev, S., Tallman, D.T., and Mills, D.J., 'The use of
electrochemical noise methods (ENM) to study thick, high impedance coatings', Progress in
Organic Coatings 29 (1996) 21-29.
12 Bierwagen, G.P., Tallman, D.E., Li, J., Balbyshev, S., and Zidoune, M., 'Electrochemical noise
studies of aircraft coatings Over A12024-T3 in accelerated exposure testing' in Corrosion 2000,
NACE International, Houston (2000) paper 427.
13 Hunter, C.N., Osborne, J.H., and Taylor, S.R., 'Electrochemical screening test for corrosion
protective aerospace coatings for A12024-T3', Corrosion 56 (2000) 1059-1070.
14 Bertocci, U., and Huet, F., 'Noise analysis applied to electrochemical systems'. Corrosion
51(2) (1995) 131-144.
15 Mansfeld, F., Kendig, M.W., and Tsai, S., 'Recording and analysis of AC impedance data for
corrosion studies II.Experimental approach and results', Corrosion 38 (1982) 570-580.
16 Nishikata, A., Ichihara, Y., and Tsuru, T., 'An application of electrochemical impedance
spectroscopy to atmospheric corrosion study', Corrosion Science 37 (1995) 897-911.
17 Carullo, A., Ferraris, F., Parvis, M., Vallan, A., Angelini, E., and Spinelli, P., 'Low-cost
electrochemical impedance spectroscopy system for corrosion monitoring of metallic antiquities
and works of art', Institute of Electrical and Electronics Engineers, Inc., Transactions on
Instrumentation and Measurement 9 (2000) 371-375.
18 Oldham, K.B., and Myland, J.C., Fundamentals of Electrochemical Science, Academic Press,
San Diego (1994).
19 Skerry, B.S., and Simpson, C.H., 'Accelerated test method for assessing corrosion and
weathering of paints for atmospheric corrosion control', (Avrosiou 49 (1993) 663-674.
20 Austin, M J., 'An evaluation of the Prohesion corrosion test cabinet', European Polymers Paint
Colour Journal 182(4317) (1992) 570-572.
21 Jacques, L.F.E., 'Accelerated and outdoor/natural exposure testing of coatings', Progress in
Polymer Science 25 (2000) 1337-1362.
22 Steele, J., and Steele, C.J., 'Performance criteria for coatings on concrete in chemical
exposures', Journal of Protective Coatings & Linings (July 1999) 29-35.
23 Odeno-Alego, V., Hallam, D., Viduka, A., Heath, G., and Creagh, D., 'Electrochemical
impedance studies of the corrosion resistance of wax coatings on artificially patinated bronze' in W.
Mourey and L. Robbiola (eds), Metal 98, James & James, London (1998) 315-319.
24 Price, C, Hallam, D., Heath, C, Creagh, D., and Ashton.J., 'An electrochemical study of waxes
for bronze sculpture' in I.D. MacLeod, S.L. Pennec and L. Robbiola (eds), Metal 95, James &
James, London (1995) 233-241.
25 Moffett, D.L., 'Wax coatings on ethnographic metal objects: justification for allowing a tradition
to wane', Journal of the American Institute for Conservation 35 (2000) 1—7.
26 Otieno-AIego, V., Hallam, D., Heath, G., and Creagh, D., 'Electrochemical evaluation of the
anti-corrosion performance of waxy coatings tor outdoor bronze conservation' in W. Mourey and
L. Robbiola (eds). Metal 98, James & James, London (1998) 309-314.
27 Beale, A., and Smith, R., 'An evaluation of the effectiveness of various plastic and wax coatings
in protecting outdoor bronze sculpture exposed to acid deposition: a progress report' in
Conservation of Metal Statuary and Architectural Decoration in Open-Air Exposure, ICCROM,
Rome (1986) 99-124.
AUTHORS
LISA A. ELLINGSON is a 2001 graduate from North Dakota State University with a MS degree in
polymers and coatings science. She also earned her BS degree from NDSU in 1999 in chemistry
with an option in Polymers and Coatings Science. She was a member of
the corrosion group at NDSU from 1997 to 2001 where she studied the corrosion of outdoor
bronzes and aluminum alloys. Lisa is currently employed at CIMA Labs, Inc. in Minneapolis, MN
as a research associate in the Research and Development Formulations Department. Address: 3324
Carnation Avc N, Brooklyn Park, MN 55443, USA. Email: lisah@ximalabi.com
TARA J. SHEDLOSKY graduated from Colgate University in 1997 with a BA in chemistry. She began
working on the subject of protective coating for outdoor bronze sculpture at the National Gallery of
Art in Washington DC and is currently continuing this research while pursuing a PhD in the
Polymers & Coatings Department at North Dakota State University. Address: Department of
Polymers & Coatings, North Dakota State University, 1735 NDSU Research Park Drive, Fargo,
ND 58105-5376, USA. Email: tara,shcdlosky@ndsu.nodak.edu
GORDON P. BIERWAGEN received his BS in chemistry and mathematics from Valparaiso University
and his PhD in physical chemistry from Iowa State University. After post-doctoral work in
chemical engineering at the University of Minnesota, he worked in the paint and coatings industry
until 1989, when he joined the Department of Polymers & Coatings at North Dakota State
University in Fargo, as professor. His research interests have been in the physical chemistry of
coatings and coating materials, pigment dispersion and dispersion stability, pigment particle
packing in coating films, corrosion control by coatings, and coating lifetime prediction and its
testing. Since 1999 he has been Chair of Polymers & Coatings, and editor-m-chief of Progress in
Organic Coatings since 1995. Address as for Shedlosky. Entail:
gordon.bierwagen@ndsu.nodak.edu
E. RENE HE LA RIE received MS and PhD degrees in chemistry from the University of Amsterdam,
The Netherlands. He has been head of the scientific research department at the National Gallery of
Art in Washington DC since 1989, where artists' methods and materials as well as materials used in
the treatment of works of art are being studied. Before coming to the National Gallery of Art, he
held positions at the Metropolitan Museum of Art, New York, and at the Training Program for
Conservators and the Central Research Laboratory for Objects of Art and Science, both in
Amsterdam. He has held adjunct positions at the University of New York and the University of
Amsterdam. He has been an editor for Studies in Conservation since 1994. Address: Scientific
Research Department, National Gallery of Art, 6th and Constitution Avenue NW, Washington, DC
20565, USA. Email: rdelarie@fsi.com
LYNN B. BROSTOFF received her Ali from Vassar College, MA in art history and Certificate of Conservation from the Institute of Fine Arts at New York University, MS in polymer materials science
from the University of Cincinnati, and PhD in chemistry (conservation science) from the
University of
Amsterdam, The Netherlands. After leaving the scientific research department at the National
Gallery of Art in Washington, DC in 1998, she held a position as senior staff scientist at the
Research Center on the Materials of the Artist and Conservator (Carnegie Mellon University), and
is currently a staff scientist at the Smithsonian Center for Materials Research and Education.
Address; SCMRE, 4210 Silver Hill Road, Suitland, MD 20746, USA. Email: brostoffl@scmre.si.edu
Résumé — La spectroscopic d'impédance élcctrochimiquc (SIE) est une technique qui mesure
quantitativement le niveau de protection contre la corrosion d'un substrat métallique fourni par un
revêtement. L'article illustre la théorie de la SIE et décrit son application pour évaluer les
revêtements protecteurs des sculptures et ornements en bronze exposés à l'extérieur. Trois types de
revêtements ont été étudiés sur un bronze coulé monumental : une cire microcristalline, un
copolymère de méthacrylate de méthyle vendu sous le nom commercial de lncralac, et un
revêtement de surface à base d'acrylique/ méthane acrylique. Chaque type de revêtement a été
soumis à un vieillissement accéléré et évalué par SIE. L'information a ensuite été utilisée pour
calculer le module d'impédance des échantillons, représenté dans des diagrammes de Bode et
interprété en termes de performance protectrice.
Zusammenfassung — Mit Hilfe der Elektrochemischen Impedanz Spektroskopie (EIS) kann man
den Korrosionsschutz durch ein Überzugsmaterial auf einem Metall quantitativ bestimmen. In
dieser Arbeit wird einerseits die Methode theoretisch beschrieben, andererseits werden die
Möglichkeiten von EIS anhand von Schutzüberzügen auf Außenbronzen und Ornamenten
ausgelotet. Drei Schutzüberzüge wurden evaluiert: ein Mikrokristallines Wachs, ein
Acryl/Methacrylat Copolymer, das unter dem Namen Incar/ack gehandelt wird, sowie ein
Acryl/Acrylurethanüberzug. Jedes Schutzsystem ivurde durch beschleunigte Bewitterung gealtert
und mit Hilfe von EIS bewertet. Diese Information ivurde zur Bestimmung des Impedanzmoduls der
Proben genutzt. Das lmpcdanzmoduls wurde in Bode Diagrammen aufgetragen und hinsichtlich der
Schutzwirkung interpretiert.
Resumen — La espectroscopia electroquímica de impedancia (EIS) es una técnica que mide
cuantitativamente el nivel de protección contra la corrosión que suministra una capa de
recubrimiento a un substrato metálico. Este artículo ilustra primeramente los aspectos teóricos de
la EIS, se evalúa además su potencial para evaluar la efectividad de las capas de protección en
esculturas y ornamentaciones de bronce en exteriores. Se evaluaron tres sistemas de recubrimiento
en un bronce monumental: una cera microcristalina, un copolímero de metilmetacrilato vendido
bajo el nombre de lncralac y un acabado acrilico/acrílico de metano. Cada sistema de
recubrimiento fue envejecido aceleradamente de modo artificial y evaluado mediante EIS. La
información obtenida fue después usada para calcular los módulos de impedancia de las muestras,
los cuales se representan en gráficos Bode y se interpretan en términos del desempeño de los
recubrimientos.