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Electrochemical Assessment of Protection Systems on Metal
Artefacts
P. Letardi
CNR - ISMAR, Genoa, Italy
E.Cano
CENIM-CSIC, Madrid, Spain
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sIntroduction
Basic of EIS
Overview of commercially available
instruments
Electrochemical cell and electrolyte
Data acquisition, representation and
analysis
Test of coatings for cultural heritage
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sBasic of EIS
Energ
y level (
Po
ten
tial E
)
Ref- - - -
Charge flow (Current I)
++ + +
The fundamental approach of
all impedance methods is to
apply a small amplitude
sinusoidal excitation signal to
the system under investigation
and measure the response.
Theoretical Electrochemical
systems are usually
characterised by non-linear I-E
curve .
A low amplitude sine wave
DEsin(wt), of a particular
frequency, is superimposed on
the dc polarization voltage E0.
This results in a current
response of a sine wave
DIsin(wt+f) superimposed on
the dc current I0. The current
response is shifted with
respect to the applied
potential.
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sBasic of EIS
• Perturbation: E(t)=E0 cos(w t)
• System response : I(t)=I0 cos(w t-f )
• Response function: Z (w) = E/I
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sBasic of EIS
Z(w) is a complex quantity with a magnitude and a
phase shift which depends on the frequency of the
signal. Therefore by varying the frequency of the
applied signal one can get the impedance of the
system as a function of frequency.
Typically in electrochemistry, a frequency range of
100kHz – 0.1Hz is used.
The plot of the real part of impedance on the X-axis
and the imaginary part on the Y-axis gives a
Nyquist Plot. While plotting data in the Nyquist
format the real axis must be equal to the imaginary
axis so as not to distort the shape of the curve. The
advantage of Nyquist representation is that it gives
a quick overview of the data and by the shape of
the curve one can make some qualitative
interpretations of the data. The disadvantage is that
one loses the frequency dimension of the data.
The absolute value of impedance (|Z|=Z0) and the
phase shifts are plotted on the Y-axis with log
frequency on the X-axis in two different plots giving
a Bode plot. This is the more complete way of
presenting the data.
Zre
Zim
w0w
|Z|
Nyquist Plot
|Z|
101
102
103
104
105
frequency [Hz]
10-2 10-1 100 101 102 103 104 105
-90
-60
-30
0
Bode Plot
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EIS spectrum can provide
information about
electrochemical processes...
…and interface
properties
C.Gabrielli “Identification of Electrochemical Processes by Frequency
Response Analysis”, (1998)
http://accessimpedance.iusi.bas.bg/vlab2/e-school/tech04.pdf
A.Amirudin, D.Thierry “Application of electrochemical impedance
spectroscopy to study the degradation of polymer-coated metals”,
Prog. Org. Coat. 26 (1995) 1-28
The EIS technique(Electrochemical Impedance Spectroscopy)
Basic of EIS
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sBasic of EIS
Electrochemical Impedance Spectroscopy (EIS): 1. Basic Principles
http://www.ecochemie.nl/download/content/Appl011.pdfElectrochemical Impedance Spectroscopy (EIS): 2. Experimental Setup
http://www.ecochemie.nl/download/content/Appl012.pdf
Basics of Electrochemical Impedance Spectroscopyhttp://www.gamry.com/App_Notes/EIS_Primer/EIS_Primer_2007.pdf
Basics of Electrochemical Impedance Spectroscopy
http://new.ametek.com/content-manager/files/PAR/078.pdf
Electrochemical Impedance Measurements: Instruments and Techniques
http://new.ametek.com/content-manager/files/PAR/079c.pdf
http://www.solartronanalytical.com/Pages/Electrochemistry.htm
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sInstruments and methods
E I
CEWE
Electrolyte
Potentiostat
FRA
REF
PersonalComputer
Commercially available instruments
Electrochemical cell and electrolyte
WE – Working Electrode:
the electrode under investigation
CE – Counter Electrode:
the electrode necessary to close the electrical circuit
REF – Reference Electrode:
the electrode used to determine the potential of the working electrode precisely
Electrolyte:
It ensures electrical conductivity between WE and CE
Potentiostat: it is used to fix the DC value and to
apply a sinusoidal perturbation over it
Frequency Response Analyzer (FRA): controls
the EIS scan and impedance evaluation
Acquisition Software: controls Potentiostat and
FRA, and store data file
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sElectrochemical cell and electrolyte
• 3 electrode
flat plate cell
– WE: coated metal
– RE: Usually SCE or
Ag/AgCl
– CE: platinum,
graphite or
stainless steel
mesh, wire or rod
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sElectrochemical cell and electrolyte
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sElectrochemical cell and electrolyte
• Immersion cell
– Testing of wax
coatings
C. Price et al, Proceedings of Metal 95,
James&James, London (1997) 233-241
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sElectrochemical cell and electrolyte
• Immersion cell
– Testing of oil
coatings
D. Hallam et al, Proceedings of Metal 04, National
Museum of Australia, Canberra (2004) 388-399
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sElectrochemical cell and electrolyte
a contact probe setup measurement method has been developed to make measurements on
art object also
CE
REF
Cloth soaked with mineral water
Contact
probe
WE
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ST Contact Probe
CE: Stainless Steel
AISI 316
REF: Stainless Steel
AISI 316
inserted in PTFE
= 1.5 cm
A= 1.77 cm2REF
CE
REF cable
CE cable
Letardi P., in: P.A.Vigato (Ed.), The Science of Art, Outdoor bronze protective coatings: characterisation by a new contact-probe Electrochemical Impedance measurements technique, Libreria Progetto di Padova, 2002, pp. 173-178
Instruments and methodsElectrochemical cell and electrolyte
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sElectrochemical cell and electrolyte
• Integrated cell+EIS electronics
E. Angelini et al, IEEE Transactions on Instrumentation and Measurement 55 (2006) 436-441
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sElectrochemical cell and electrolyte
• Electrolyte: as close as possible to the
real environment of the object or
specially aggressive towards de
metal/coating
– NaCl 0.1M
– Harrison (0.35% (NH4)2SO4+0.05% NaCl)
– Mineral water (Letardi)
– K2SO4 0.25M (Hallam)
– Acetic acid (Dowsett)
P. Letardi et al., Proceedings of Metal 98, James&James, London (2001) 303-308
D. Hallam et al, Proceedings of Metal 04, National Museum of Australia, Canberra (2004) 388-399
M. Dowsett et al., Proceedings of Metal 07, Rijkmuseum Amsterdam (2007) 26-31
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sOverview of commercially available instruments
http://www.ecochemie.nl/?pag=7
http://www.gamry.com/Products/Potentiostats.htm
http://www.princetonappliedresearch.com/products/potentiostats.cfm
http://www.solartronanalytical.com/Pages/Electrochemistry.htm
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sOverview of commercially available instruments
• Transportable instrumentation
•Gamry FAS1 Femtostat
•Portable instrumentation
•Gamry Ref600
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sData acquisition and representation
Bode plot Nyquist plot
E. Cano, D. Lafuente, D.M. Bastidas, J. Solid State Electrochem. 14
(2010) 381-391
Do not miss phase angle graph!
Same scale!
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sBode vs. Nyquist
C. Gabrieli, “Use and Application of Electrochemical Impedance
Techniques TECHNICAL REPORT NUMBER 24. Solartron Analytical (1997)
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sData analysis
• Physio-chemical model from first principles
• Equivalent circuits (EC)
• Evaluation of single parameters (|Z| at low
frequency, C coating, etc.)
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sEquivalent circuits
• EC= analogues that reproduce the
electrical properties of our system
• Pasive elements: resistors, capacitors,
inductances, and other special elements
(CPE, Warburg Impedance, etc.)
• Many circuits can be used to model the
same data
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sEquivalent circuits
Gamry Instruments (2007) “Basics of electrochemical impedance spectroscopy”
http://www.gamry.com/App_Notes/EIS_Primer/EIS_Primer_2007.pdf
A good fitting does not mean you choosed a good model!
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sThe basic EC
• Intact coating
10-3
10-2
10-1
100
101
102
103
104
105
-90
-80
-70
-60
-50
-40
-30
-20
-10
0
100
101
102
103
104
105
106
107
108
109
1010
(b)
Ph
ase
An
gle
f
Frequency / Hz
(a)
Imp
ed
an
de
Mo
du
lus |
Z|
/
cm
2
20 40 60 80 100
-20
-40
-60
-80
-100
-Z"
/ M
c
m2
Z' / M cm2
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sThe basic EC
• Damaged coating
10-3
10-2
10-1
100
101
102
103
104
105
-90
-80
-70
-60
-50
-40
-30
-20
-10
0
100
101
102
103
104
105
106
107
108
109
1010
(b)
Ph
ase
An
gle
f
Frequency / Hz
(a)
Imp
ed
an
de
Mo
du
lus |
Z|
/
cm
2
20 40 60 80 100
-20
-40
-60
-80
-100
-Z"
/ k
c
m2
Z' / k cm2
2 4
-2
-4
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sThe basic EC
• Damaged coating
(CPE)
20 40 60 80 100
-20
-40
-60
-80
-100
-Z"
/ k
c
m2
Z' / k cm2
2 4
-2
-4
10-3
10-2
10-1
100
101
102
103
104
105
-90
-80
-70
-60
-50
-40
-30
-20
-10
0
100
101
102
103
104
105
106
107
108
109
1010
(b)
Ph
ase
An
gle
f
Frequency / Hz
(a)
Imp
ed
an
de
Mo
du
lus |Z
| /
c
m2
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sThe basic EC
• Damaged coating
(CPE)
wjY
ZCPE
0
1
α =1 CPE=C
α =0 CPE=R
α =-1 CPE=L
α =0.5 CPE=W
Usually, CPE is interpreted as an imperfect capacitor
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sThe basic EC
• Difussion
(Warburg Imped.)
20 40 60 80 100
-20
-40
-60
-80
-100
-Z"
/ M
c
m2
Z' / M cm2
10-3
10-2
10-1
100
101
102
103
104
105
-90
-80
-70
-60
-50
-40
-30
-20
-10
0
100
101
102
103
104
105
106
107
108
109
1010
(b)
Ph
ase
An
gle
f
Frequency / Hz
(a)
Imp
ed
an
de
Mo
du
lus |Z
| /
c
m2
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sOther EC
• Multiple paralel R-C
S-J. Lee and S-I Pyun, J Solid State Electrochem 11 (2007)
829-839
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sOther EC
• Multiple nested R-C
M. Evesque, M. Keddam and H. Takenouti, Electrochim.
Acta 49 (2004) 2937-2943
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sSingle EIS parameters
• |Z| max or |Z| low f
• C coat
• C DL
• f min
• Θ high f
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sEIS analysis software
• Zview (Scribner)
• EIS300 (Gamry)
• PowerSine (PAR)
• EIS Spectrum
Analyser (freeware!!!)
– http://www.abc.chemistry.bsu.by/vi/analyser/
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sTest of coatings for cultural heritage
UN MN UA
◊ Natural ◊ 80 years urban◊ 20 µm◊ brochantite|Z|10mHz = 69 ± 18 K
◊ Natural ◊ 1 year marine ◊ 30 µm◊ Atacamite |Z|10mHz = 5 ± 2 K
◊ Artificial ◊ Pichler process◊ 80 µm
◊ brochantite|Z|10mHz = 29 ± 4 KUrban artificial UA Substrate
Treatment
none Tref T1S T2S T3S
|Z| 1
0m
Hz [K
]
100
101
102
103
104
105
UA substrate
Urban Natural UN Substrate
Treatment
none Tref T1S T2S T3S I1S Iref
|Z| 1
0m
Hz [K
]
100
101
102
103
104
105
UN substrate
UN substate (1)
Marine natural MN Substrate
Treatment
none Tref T1S T2S T3S I1S Iref
|Z| 1
0m
Hz [K
]
100
101
102
103
104
105
MN substrate
Patina plays a
relevant role
on coating
behaviour
• composition
• thickness
• texture
• porosity
• …
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sTest of coatings for cultural heritage
[C]
[B
]
[D]
[A] [F]
[E]
[A] - Soter 201LC
[B] - R21
[C] - Tromm TeCe 3534F
[D] - Incralac
[E] - Incralac+Soter
[F] - Incralac+R21
Test on monuments could be prone to several limitations:• accessibility
• outdoor measurements
• variables out of control
• authorisations
• …
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sTest of coatings for cultural heritage
Test on monuments could be prone to several limitations:• …
• previous restoration treatments are also to be considered carefully
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Coupons for testing:
• alloy composition
• dimension
• patina
• natural (new)
• natural (old)
• artificial
• characterisation
Coating application:
• method
• thickness
• characterisation
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wheatering:
Multiple coupons for statistical reasons
• naturalEN ISO 8565 – Metals and alloys – Atmospheric corrosion testing – General requirements for field
tests.
• artificialEN ISO 11341 - Paints and varnishes. Artificial
weathering and exposure to artificial radiation. Exposure to filtered xenon-arc radiation
EN ISO 9227 - Corrosion tests in artificial
atmospheres. Salt spray tests
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Because of the large number of variables
involved, one need several coupons in order
to check the behaviour
Statistical methods such as Experimental
Design could be of great help in order to
reduce the number of experiments required
http://www.itl.nist.gov/div898/handbook/pri/section3/pri3.htm
http://www.chemometrics.se/images/stories/pdf/aug2002.pdf
J. A. Bishopp, M. J. Parker and T. A. O’Reilly, The use of statistical experimental design procedures in the development and “robust” manufacture of a water-based, corrosion-inhibiting adhesive primer, International Journal of Adhesion and Adhesives 21,(2001), 473-480
M.J. Anderson, P.J. Whitcomb, Design of experiments for coatings, in A.A. Tracton, Coatings
Technology Handbook, Third Edition, 3rd ed., CRC Press, 2005
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coatings for metallic cultural heritage: a review. Journal of Solid State Electrochemistry, 1-11.
Loveday D, Peterson P, Rodgers B (2004) Evaluation of organic coatings with electrochemical
impedance spectroscopy: Part 1: Fundamentals of electrochemical impedance spectroscopy. JCT
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Loveday D, Peterson P, Rodgers B (2005) Evaluation of organic coatings with electrochemical
impedance spectroscopy. JCT CoatingsTech 2, 22-27.
Loveday D, Peterspm P, Rodgers B (2004) Evalution of organic coatings with electrochemical
impedance spectroscopy part 2: Application of EIS to coatings. JCT CoatingsTech 1, 88-93.
Mansfeld F (1995) Use of electrochemical impedance spectroscopy for the study of corrosion protection
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Canberra, 2004, pp. 379-387
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