ISO/TR 5604:2024

Technical
Report
ISO/TR 5604
First edition
Electrochemical noise measurement
2024-02
for assessing the protection of metal
afforded by organic coatings
Mesurage du bruit électrochimique pour l'évaluation de la
protection du métal induit par des revêtements organiques
Reference number
© ISO 2024
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ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principles . 2
4.1 Organically coated metal exposed to a corrosive environment – how a resistance
measurement can indicate protection .2
4.2 Generation of electrochemical noise from bare and coated metal .3
4.3 E valuation criteria .3
5 Apparatus . 3
5.1 Test cell .3
5.2 Field or site work .3
5.3 Reference electrode .4
5.4 Working electrode .4
5.5 Electrolyte .4
6 Description of the measurement system . 4
7 Electrode configurations . 5
7.1 General .5
7.2 Bridge (laboratory use) .5
7.3 Single substrate (laboratory use) .5
7.4 No connection to the substrate as used in the laboratory.6
7.5 Field measurement .7
8 Dummy cell and checking/calibration . 7
9 Measurement . 7
10 Data analysis and data treatment . 8
11 Summary . 8
Annex A (informative) Description of an experiment with resistors simulating a paint system . 9
Annex B (informative) Data analysis and data treatment.11
Annex C (informative) Comparison of noise data from bare metal compared with organically
coated metal .12
Bibliography . 14

iii
Foreword
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General test methods for paints and varnishes.
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iv
Introduction
There are several test methods for assessing the protection afforded by organic coatings on metal,
particularly on low carbon steel, using the available electrochemical measurements. The most commonly
used technique is electrochemical impedance spectroscopy (EIS), which is detailed in the ISO 16773 series,
and is well suited for laboratory use. However, there is a strong need for a non-intrusive technique that can
be used on site to monitor or for quality control that is quick and is relatively simple to use and interpret.
This document gives the current state of the art for such a technique, which is electrochemical noise
measurement (EN). The developments described in this document suggest that electrochemical noise
measurement can be used as an alternative to and potentially even as the preferred process for field or on-
site use.
For further information, a detailed description of the EN methodology when applied to bare metal can be
found in ISO 17093. The differences associated with using the technique for organic coatings are described
in this document, which is intended to complement ISO 17093.
The EN approach has been directly compared with methodologies such as EIS and DC resistance, with good
[3][4]
agreement.
Furthermore, electrochemical noise applied to assess coatings has been reviewed in References [5] and [6],
showing the potential for this technique.

v
Technical Report ISO/TR 5604:2024(en)
Electrochemical noise measurement for assessing the
protection of metal afforded by organic coatings
1 Scope
This document describes the principle of electrochemical noise measurement (EN), specifically focusing
on the application of the technique to indicate the level of protection provided by an organic coating to
the underlying metal. It discusses the principles behind the measurement method but also the type of
electrochemical apparatus, the experimental set-up and electrodes configurations, the presentation of
measured data, and analysis of results that have been used in the work done so far and reported in the
references. This work has mainly been done in the laboratory, and there are some examples of work
conducted in the field.
NOTE At the time of publication of this document, electrochemical noise measurement for coatings is mainly
used by a number of academic groups. This document aims to encourage more general application of this method,
particularly to field work.
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 16773-1, Electrochemical impedance spectroscopy (EIS) on coated and uncoated metallic specimens — Part
1: Terms and definitions
ISO 17093, Corrosion of metals and alloys — Guidelines for corrosion test by electrochemical noise measurements
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 16773-1, ISO 17093 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
electrochemical noise
EN
fluctuation typically at low frequencies (≤1 Hz) and low amplitude in current and potential, generated by
electrochemical reactions and other processes on the surface, for example, bubble evolution
3.2
electrochemical potential noise
fluctuation in potential of an electrode relative to a reference electrode or fluctuation in potential between
two similar electrodes
Note 1 to entry: The electrochemical potential noise is expressed in microvolts (µV) or millivolts (mV).

3.3
electrochemical current noise
fluctuation in current to one electrode or between two electrodes
Note 1 to entry: The electrochemical current noise is normally expressed in nanoamperes (nA).
3.4
electrochemical noise resistance
R
n
resistance obtained by dividing the standard deviation of potential noise by the standard deviation of
current noise from the time record
3.5
working electrode
WE
coated metal in contact with the electrolyte, made out of the investigated material(s)
3.6
pseudo-working electrode
PWE
working electrode (3.5) other than coated metal
Note 1 to entry: A silver/silver chloride electrode (Ag/AgCl) in laboratory work or a noble metal like silver, platinum or
gold can be used as part of a sensing probe in field work. PWEs are needed for configurations other than the standard
bridge.
3.7
pseudo reference electrode
P
Ref
electrode used as a reference electrode in field work
Note 1 to entry: The metal itself (for single substrate configuration), platinum wire or sheet, copper, and silver.
3.8
electrode configuration
arrangement by which the electrodes are connected to the measuring device, i.e. bridge (Bridge), single
Substrate (SS), no connection to the substrate (NOCS)
Note 1 to entry: The configuration used depends on the particular circumstances of the measurement.
3.9
zero resistance amperemeter
ZRA
electronic circuit which measures current but has itself no significant impedance
Note 1 to entry: For coatings, a ZRA capable of measuring low levels of current down to picoamperes (pA) is usually
needed.
4 Principles
4.1 Organically coated metal exposed to a corrosive environment – how a resistance
measurement can indicate protection
Water and oxygen penetrate through an organic coating leading to electrochemical reactions occurring at
[7]
the interface between the metal and the coating. In aqueous corrosion, discrete anodic and cathodic areas
arise, and these allow corrosion to proceed whenever there is an ionically conductive pathway between the
substrate and the surrounding environment. However, if a high ionic resistance is introduced into the circuit
between anodes and cathodes, e.g. by the coating, the rate of corrosion will decrease to a low value. This
is one accepted mechanism by which the coatings can operate to prevent corrosion. Another mechanism,
which can take place concurrently, is interface reactions when aided by an oxidizing agent, e.g. oxygen,

hydrogen peroxide or chemicals within the coating. These can result in the formation of a passive coherent,
protective, chemically stable oxide layer which slows the corrosion rate. For this “passivity” to pertain for
any length of time, the coating acts to prevent the arrival of incoming aggressive ions. In both mechanism
routes, a high resistance (R) between the anodes and cathodes prevent aggressive ions arriving at the
interface and the rate of the corrosion process is largely determined by the through film ionic resistivity
of the coating system. Overall, the resistance whether measured by a DC technique as R , by EIS as R ,
dc 0,1Hz
or by electrochemical noise resistance as R , is believed to provide a quantitative measure for the extent to
n
which the coating is protecting the underlying metal. However, the influence of, for example, the binder type
on coating resistance, is not fully understood. Therefore under certain circumstances, it is possible that the
above statement on coating resistance is not fully justified.
4.2 Generation of electrochemical noise from bare and coated metal
A metal in contact with any solution generates both current noise and potential or voltage noise, due to
[8]
small random fluctuations which arise stochastically during electrochemical processes. The voltage
noise can be easily measured with respect to a stable reference electrode. In the case of current noise, it is
important to simulate the situation within the metal by using two identical but separated metal specimens
and measuring the current in between using a zero resistance amperemeter (ZRA). The current noise is
[5]
the level of fluctuation on the current value. It has been theoretically shown that the standard deviation
of the voltage values, i.e. voltage noise, divided by the standard deviation of the current values, i.e. current
noise, gives rise to a parameter called electrochemical noise resistance (R ). In the case of bare metal, R has
n n
been shown to be equivalent to the resistance obtained using the linear polarization technique.
When a coating is present, R can be attenuated in the case of current noise and amplified in the case of
n
potential noise, through the coating. The level of both potential and current noise that is sensed by the
instrument can be converted to R . The largest value of resistance R is very likely the resistance of the
n n
coating system to the movement of ions (R ) and this in turn has a direct bearing on the rate of corrosion,
paint
as described in 4.1. An exception would be when the coating has broken down and corrosion is occurring,
and in that case a low measured value will be indicative of failure.
4.3 Evaluation criteria
The measurement of the electrochemical noise resistance (R ) provides a manner of assessing the protection
n
capability of the coating.
8 2 6 2 8 2
When R > 1 × 10 Ω·cm indicates good protection, 1 × 10 Ω·cm to 1 × 10 Ω·cm is the borderline and
n
6 2 [9]
R < 1 × 10 Ω·cm affords poor protection. The rate of reduction of R with time will indicate the rate at
n n
which the coating system is breaking down.
5 Apparatus
5.1 Test cell
The test cell is a leak proof cell constructed of material that will not corrode, deteriorate or contaminate the
solution. The test cell was used to contain the electrolyte. The electrode holder and mounting material for
the electrodes were mounted so as to have no influence on the measurement.
5.2 Field or site work
Although in laboratory work leak proof cells can be permanently attached, in field work a different approach
is used since a dismountable probe is needed. One solution is to hold the electrolyte solution in a filter paper
pad cut to the size of the pseudo reference electrode (P ) or pseudo working electrodes (PWE), whose area
Ref
defines the measurement area.
5.3 Reference electrode
In the field a robust electrode, such as a silver/silver chloride (Ag/AgCl) or copper/copper sulfate (Cu/CuSO )
was used.
5.4 Working electrode
In almost all the work described in the references, the configurations were not just the standard bridge (see
6.2) where two nominally identical and separate coated samples are needed, and the two working electrodes
will be the metal under the coating in each sample. The two most common alternatives are single substrate
(see 6.3) configuration, where the reference becomes th
...