Sources of error in the use of electrochemical impedance spectroscopy for the investigation of coatings and other materials

This document describes the main sources of error in the use of electrochemical impedance spectroscopy for the investigation of coatings and other materials. The sources of error listed here include all process steps from the set-up of the sample with the measuring cell right through to evaluation. NOTEÂ Â Â Â Â Â The sources of error discussed here do not represent a complete list.

Sources d'erreur dans l'utilisation de la spectroscopie d’impédance électrochimique pour l'étude des revêtements et autres matériaux

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Publication Date
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TECHNICAL ISO/TR
REPORT 5602
First edition
2021-11
Sources of error in the use of
electrochemical impedance
spectroscopy for the investigation of
coatings and other materials
Sources d'erreur dans l'utilisation de la spectroscopie d’impédance
électrochimique pour l'étude des revêtements et autres matériaux
Reference number
ISO/TR 5602:2021(E)
© ISO 2021
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ISO/TR 5602:2021(E)
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© ISO 2021 – All rights reserved
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ISO/TR 5602:2021(E)
Contents Page

Foreword ..........................................................................................................................................................................................................................................v

Introduction .............................................................................................................................................................................................................................. vi

1 Scope ................................................................................................................................................................................................................................. 1

2 Normative references ..................................................................................................................................................................................... 1

3 Terms and definitions .................................................................................................................................................................................... 1

4 Error in the make-up of the measuring cell ............................................................................................................................ 1

4.1 Roughness of the surface .............................................................................................................................................................. 1

4.2 O-ring — Considerations about the precise determination of the exposed area ........................ 3

4.3 Faulty cell make-up ............................................................................................................................................................................ 7

4.3.1 Optically detectable leaks .......................................................................................................................................... 7

4.3.2 Optically non-detectable causes ........................................................................................................................... 7

4.4 Reference electrodes ......................................................................................................................................................................... 9

4.4.1 General information on the distance between the reference and working

electrodes ................................................................................................................................................................................. 9

4.4.2 Shielding ................................................................................................................................................................................. 11

4.4.3 Air bubble in the reference electrode ........................................................................................................... 11

4.4.4 Poisoning of the reference electrode ............................................................................................................. 11

4.4.5 Bleeding of the reference electrode ............................................................................................................... 11

4.5 Counter electrodes ........................................................................................................................................................................... 11

4.5.1 Relative sizes ...................................................................................................................................................................... 11

4.5.2 Reactive counter electrodes .................................................................................................................................. 11

4.6 Gas inclusions in the measuring cell ................................................................................................................................ 11

5 Faults caused by electronics incl. shielding .........................................................................................................................12

5.1 Faraday cage ..........................................................................................................................................................................................12

5.2 Extended cable (without active shielding) ................................................................................................................. 15

5.3 Cable breaks ........................................................................................................................................................................................... 16

5.4 Contact resistances between metallic contacts and the working electrode/counter

electrode ................................................................................................................................................................................................... 17

5.5 Inductivities ........................................................................................................................................................................................... 18

5.6 Measurement range switching .............................................................................................................................................. 19

5.7 Scattering signals in power supply ................................................................................................................................... 20

5.8 Insufficient signal-to-noise ratio ......................................................................................................................................... 22

5.9 Influence of peripheral devices ............................................................................................................................................ 22

6 Parameter selection, measurement range limits ..........................................................................................................24

6.1 Open-lead test....................................................................................................................................................................................... 24

6.2 Note on dummy cells – ISO 16773-3 ................................................................................................................................. 24

6.3 Unsuitable amplitude .................................................................................................................................................................... 24

6.4 Insufficient frequency range ...................................................................................................................................................26

6.5 Repetition rate for subsequent measurements ...................................................................................................... 27

7 Non-stationary measurement conditions ..............................................................................................................................28

7.1 General ........................................................................................................................................................................................................28

7.2 Temperature fluctuations .......................................................................................................................................................... 29

7.3 Electrolytic conductivity ............................................................................................................................................................ 31

7.4 Swelling ...................................................................................................................................................................................................... 31

7.5 Drifting OCP ........................................................................................................................................................................................... 31

7.6 Corroding working electrode .................................................................................................................................................33

7.7 Reactive counter electrodes .................................................................................................................................................... 33

7.8 Gas formation at the counter electrode ......................................................................................................................... 33

8 Design and selection of equivalent circuit diagrams.................................................................................................34

8.1 Constant phase element ..............................................................................................................................................................34

8.2 Multiple possibilities for the selection of equivalent circuits .................................................................... 35

iii
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ISO/TR 5602:2021(E)

8.3 Warburg impedance ....................................................................................................................................................................... 37

9 Significance of measurement values from equivalent circuits .......................................................................37

9.1 Measurement uncertainty ........................................................................................................................................... .............. 37

9.2 Plausibility analysis ........................................................................................................................................................................38

10 Interpretation of the measurement values of various coating systems ................................................39

10.1 Pre-treatment ....................................................................................................................................................................................... 39

10.2 Film thickness and measurement surface ..................................................................................................................40

10.3 Number of layers ................................................................................................................................................................................ 41

10.4 Conditioning........................................................................................................................................................................................... 45

10.5 Generic type of binder ........................................................................................................................................... ........................ 45

11 Presentation of data ......................................................................................................................................................................................45

Annex A (informative) Calculation of the coating capacitance ............................................................................................48

Annex B (informative) Further information on the influence of the double-layer capacitance .......49

Annex C (informative) Estimation of the order of magnitude of an apparent capacitance

caused by corrosion .......................................................................................................................................................................................50

Bibliography .............................................................................................................................................................................................................................52

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ISO/TR 5602:2021(E)
Foreword

ISO (the International Organization for Standardization) is a worldwide federation of national standards

bodies (ISO member bodies). The work of preparing International Standards is normally carried out

through ISO technical committees. Each member body interested in a subject for which a technical

committee has been established has the right to be represented on that committee. International

organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.

ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of

electrotechnical standardization.

The procedures used to develop this document and those intended for its further maintenance are

described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the

different types of ISO documents should be noted. This document was drafted in accordance with the

editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).

Attention is drawn to the possibility that some of the elements of this document may be the subject of

patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of

any patent rights identified during the development of the document will be in the Introduction and/or

on the ISO list of patent declarations received (see www.iso.org/patents).

Any trade name used in this document is information given for the convenience of users and does not

constitute an endorsement.

For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and

expressions related to conformity assessment, as well as information about ISO's adherence to

the World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see

www.iso.org/iso/foreword.html.

This document was prepared by Technical Committee ISO/TC 35, Paints and varnishes, Subcommittee

SC 9, General test methods for paints and varnishes.

Any feedback or questions on this document should be directed to the user’s national standards body. A

complete listing of these bodies can be found at www.iso.org/members.html.
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ISO/TR 5602:2021(E)
Introduction

Electrochemical impedance spectroscopy is described in detail in ISO 16773-1 to ISO 16773-4. It

became apparent during use of these standards that sources of error and measurement artefacts that

lead to incorrect interpretations are not dealt with comprehensively. This document supplements the

ISO 16773 series of standards to deal with this issue.
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TECHNICAL REPORT ISO/TR 5602:2021(E)
Sources of error in the use of electrochemical impedance
spectroscopy for the investigation of coatings and other
materials
1 Scope

This document describes the main sources of error in the use of electrochemical impedance spectroscopy

for the investigation of coatings and other materials. The sources of error listed here include all process

steps from the set-up of the sample with the measuring cell right through to evaluation.

NOTE The sources of error discussed here do not represent a complete list.
2 Normative references

The following documents are referred to in the text in such a way that some or all of their content

constitutes requirements of this document. For dated references, only the edition cited applies. For

undated references, the latest edition of the referenced document (including any amendments) applies.

ISO 4618, Paints and varnishes — Terms and definitions

ISO 16773-1, Electrochemical impedance spectroscopy (EIS) on coated and uncoated metallic specimens —

Part 1: Terms and definitions
3 Terms and definitions

For the purposes of this document, the terms and definitions given in ISO 4618, ISO 16773-1 and the

following apply.

ISO and IEC maintain terminology databases for use in standardization at the following addresses:

— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
limit impedance

minimum or maximum impedance that can be measured using the impedance spectrometer

3.2
limit frequency
minimum or maximum frequency that can be set on the impedance spectrometer
4 Error in the make-up of the measuring cell
4.1 Roughness of the surface

A wet and rough surface could conduct stray currents to a scratch or artificial defect, see Figure 1. This

could yield in a spectrum showing a much lower resistance than in reality. Examples of spectra are

shown in Figure 2.
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ISO/TR 5602:2021(E)
Key
1 without UV-irradiation
2 after UV-irradiation
PMMA tube
seal
coating
electrolyte
steel

Figure 1 — Conductive path from counter electrode to scratch due to surface roughness

The rough surface was measured on the unscratched area. Although the rough surface was dried with

a tissue, the residual amount of water was sufficient to produce a conductive path via the scratch to the

substrate. As result, the spectrum of the sample resulted in the incorrect identification of a defective

coating. After 2 h of continuous immersion in the cell, the surface outside the cell had dried and the

conductive path was interrupted, which resulted in a typical spectrum of an intact coating.

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ISO/TR 5602:2021(E)
Key
X frequency, f, in Hz
Y1 impedance, Z, in Ω
Y2 phase angle, φ, in degrees
2 h drying
wet
Figure 2 — EIS spectra of the initially wet coating and 2 h after drying
4.2 O-ring — Considerations about the precise determination of the exposed area

If an O-ring is used to seal the cell, the exposed area is smaller than the theoretically assumed area

because the O-ring will be compressed, and therefore, the exposed area will be reduced (see Figure 3).

a) Ideal situation, uncompressed
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ISO/TR 5602:2021(E)
b) Real situation, compressed
Key
R radius of the uncompressed O-ring
a difference in the radius of the O-ring due to compression
Figure 3 — Uncompressed and compressed O-ring

This behaviour can be visualized easily by using two transparent PMMA (poly methylene methacrylate)

plates which were compressed with 4 screws. The screws were gently tightened only by hand and

without any tools.

Figure 4 shows the set-up and Figure 5 and Figure 6 show the compressed O-rings of 1,2 cm and 5 cm

diameter, respectively.
Figure 4 — Compression of O-ring using 4 screws
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ISO/TR 5602:2021(E)
Figure 5 — Compressed O-ring of 1,2 cm diameter
Figure 6 — Compressed O-ring of 5 cm diameter
The exposed area can be calculated as illustrated in Figure 7.
SR=π ·

a) O-ring not compressed — Contact surface of the specimen with testing solution

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ISO/TR 5602:2021(E)
SR=−π · a
b) O-ring compressed — Contact surface of the specimen with testing solution
2 2
ΔπSS=−SR=⋅ −⋅π Ra−
01 0 0
c) Reduction of contact surface of specimen due to O-ring compression
Key
S geometric area with the O-ring uncompressed
S exposed area with the O-ring compressed
ΔS difference S − S
0 1
R radius of the uncompressed O-ring
a difference of the radius of the O-ring due to compression
Figure 7 — Calculation of the exposed area

The error dS between exposed area S and geometric area S can be approximated depending on the

1 0
O-ring radius, R , and the measured contact, 2a, using Formula (1):
2 ⋅⋅aR ⋅ a
d S =⋅ 100= ⋅ 100 (1)

Some examples for calculation of the error of the exposed area are shown in Table 1.

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ISO/TR 5602:2021(E)

Table 1 — Approximate error estimation of contact surface of specimens in corrosion cells

Geometric area
Difference in Exposed area
Radius of the with the O-ring
the radius of the with the O-ring Error of the
uncompressed uncompressed
O-ring due to compressed exposed area
O-ring (theoretical
compression (real surface)
surface)
R a R − a S S dS
0 0 0 1
2 2
mm mm mm mm mm %
6 0,8 5,2 113 85 25
12 0,8 11,2 452 394 13
24 0,8 23,2 1 809 1 690 7
30 0,8 29,2 2 826 2 677 5
6 1 5 113 79 31
12 1 11 452 380 16
24 1 23 1 809 1 661 8
30 1 29 2 826 2 641 7
6 1,25 4,75 113 71 37
12 1,25 10,75 452 363 20
24 1,25 22,75 1 809 1 625 10
30 1,25 28,75 2 826 2 595 8
4.3 Faulty cell make-up
4.3.1 Optically detectable leaks

Optically detectable leaks in the measuring cell are obvious and are not dealt with here.

4.3.2 Optically non-detectable causes

The behaviour shown in Figure 8 was observed in a non-reproducible manner for a very well-

documented coating (cathodic e-coat) that is in familiar use in measurement technology. This behaviour

occurred with varying amounts of pressure on the measuring cell at different locations on the same

test panels; however, a direct relationship was not detected.

If the behaviour shown in Figure 8 is observed in a measuring cell, the measuring cell is not suitable.

Generally, every measurement set-up is tested for errors with a familiar system before this measuring

cell is used on an unfamiliar system.
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ISO/TR 5602:2021(E)
a) Magnitude of impedance as a function of frequency
b) Phase angle as a function of frequency
Key
X logarithm of the frequency, log f, in Hz
Y1 logarithm of the modulus of the impedance, log |Z|, in Ω⋅cm
Y2 absolute value of the phase angle, φ, in degrees
wrong sealing
sealing correct
wrong sealing
sealing correct
Figure 8 — Possible influence of a faulty cell set-up
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ISO/TR 5602:2021(E)
4.4 Reference electrodes

4.4.1 General information on the distance between the reference and working electrodes

RE1, RE2 and RE3 as shown in Figure 9 are the various positions where the reference electrode RE can

be placed to measure the potential. At distances very close to the working electrode, the equipotential

lines are close together and small variations in the position of the reference electrode can lead to

large variations in the ohmic drop. This applies in particular to uncoated samples. In some cases, it

is preferable not to use a Luggin capillary, and instead to place the reference electrode far from the

working electrode and measure and compensate for the ohmic drop.
Key
1 working electrode (WE)
2 reference electrode 1 (RE 1)
3 reference electrode 2 (RE 2)
4 reference electrode 3 (RE 3)
5 equipotential lines
Figure 9 — Equipotential lines shown at close proximity to the working electrode

The influence of the reference electrode distance is negligible for measurements on coated (high-

resistance) samples. See Figure 10.
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ISO/TR 5602:2021(E)

a) Three |Z| curves are shown here that were recorded with different distances between the

reference electrode and working electrode. The distances to the coated substrate are 37 mm

(top), 15 mm (middle) and 2 mm (bottom).

b) Three phase angle curves are shown here that were recorded with different distances be-

tween the reference electrode and working electrode. The distances to the coated substrate are

37 mm (top), 15 mm (middle) and 2 mm (bottom).
Key
X1 frequency, f, in Hz
X2 frequency, f, in Hz
Y1 modulus of the impedance, |Z|, in Ω⋅cm
Y2 absolute value of the phase angle, φ, degrees
top (mostly invisible due to overlayings with the middle and bottom data points)
middle
bottom
Figure 10 — Spectra of coated (high-resistance) samples
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ISO/TR 5602:2021(E)
4.4.2 Shielding

If the distance between the reference electrode and working electrode is too small, the electrical field

is shielded, and this leads to an undefined state. Shielding of the working electrode by the reference

electrode can only occur as a problem with the Haber-Luggin capillary, as there is a very small distance

between the capillary opening and working electrode in this case.
4.4.3 Air bubble in the reference electrode

An air bubble in the reference electrode leads to undefined potentials and can result in strong

oscillations in the phase.
4.4.4 Poisoning of the reference electrode

Poisoning of the reference electrode occurs when ions or molecules diffuse into the electrode and react

with the reference material. This results in a potential shift. This effect is less significant with a coated

electrode than with a metallic electrode.

Poisoning of the reference electrode can largely be prevented by hydrostatic effects and by correctly

positioning the electrode.

The use of a Haber-Luggin electrode can be effective in order to delay poisoning of the reference

electrode.
4.4.5 Bleeding of the reference electrode

Diffusion of ions from the reference electrode into the electrolyte is referred to as bleeding of the

reference electrode. These ions can alter the reaction at the working electrode.

The use of a Haber-Luggin electrode can be effective in order to delay bleeding of the reference

electrode.
4.5 Counter electrodes
4.5.1 Relative sizes

For coated samples, there are no known artefacts that can be ascribed to an unsuitable choice of relative

sizes of the working electrode and counter electrode. This can be explained by the very low currents

in the measurement of these high-resistance systems. However, the area of the counter electrode is

chosen as large as possible.
4.5.2 Reactive counter electrodes

As regards the make-up of measuring cells, the counter electrode consists of a material that is inert to

the electrolyte over the period of measurement. If there are doubts as to the suitability of the counter

electrode material, comparative measurements are carried out on different materials with a known

reference sample. Exposure experiments are also carried out.
4.6 Gas inclusions in the measuring cell

Despite the influence of gas bubbles in the measuring cell that is to be expected theoretically, this could

not be verified in a test experiment with gas bubbles on the counter electrode, see Figure 11. A single-

layer test coating was measured.
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ISO/TR 5602:2021(E)
Key
X logarithm of the frequency, log f, in Hz
Y1 logarithm of the modulus of the impedance, log |Z|, in Ω⋅cm
Y2 absolute value of the phase angle, φ, in degrees
with air bubbles
without air bubbles

Figure 11 — Bode plots for two measurements with and without gas bubbles on the counter

electrode

The reduction of the effective area caused by gas-bubble absorption at the working electrode has an

influence on the measurement values of the layer capacitance and layer resistance.

5 Faults caused by electronics incl. shielding
5.1 Faraday cage

Any discontinuities in the phase angle plot at 50 Hz are probably due to insufficient shielding. See

Figure 12.
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ISO/TR 5602:2021(E)
Key
X frequency, f, in Hz
Y1 modulus of the impedance, |Z|, in Ω⋅cm
Y2 absolute value of the phase angle, φ, in degrees
t = 0 h
t = 10 h
t = 20 h
t = 40 h
t = 70 h
Figure 12 — Dramatic grid frequency influence visible on a Bode plot

Figure 12 shows a dramatic example for grid frequency influences, whereas Figure 13 shows negligible

disturbance.
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ISO/TR 5602:2021(E)
Key
X logarithm of the frequency, log f, in Hz
Y1 logarithm of the modulus of the impedance, log |Z|, in Ω⋅cm
Y2 absolute value of the phase angle, φ, in degrees
phase angle
impedance
Figure 13 — Negligible grid frequency influence visible on a Bode plot

Figure 14 shows a magnified representation of the phase plot; the phase plot reacts particularly

sensitively t
...

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