ISO 23449:2020

INTERNATIONAL ISO
STANDARD 23449
First edition
2020-10
Corrosion of metals and alloys —
Multielectrode arrays for corrosion
measurement
Corrosion des métaux et alliages — Assemblages multi-électrodes
pour la mesure de la corrosion
Reference number
©
ISO 2020
© ISO 2020
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ii © ISO 2020 – All rights reserved

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 3
4.1 Multielectrode arrays . 3
4.2 Coupled multielectrode array (CMA) . 3
4.3 Multielectrode array with closely packed electrodes for studying spatiotemporal
behaviour of localized corrosion . 4
4.4 Coupled multielectrode array sensor (CMAS) . 5
4.4.1 CMAS for corrosion monitoring . 5
4.4.2 CMAS used without polarization to measure corrosion rate at free
corrosion potential. 6
4.4.3 CMAS used to evaluate the effectiveness of cathodic protection and the
effect of stray current. 6
4.5 Multielectrode arrays for high throughput measurements . 7
4.6 Multielectrode arrays for other applications . 7
5 Instrumentation . 8
5.1 Potential measurement . 8
5.2 Coupling current measurement . 8
5.3 Effective coupling of individual electrodes . 9
5.3.1 Coupling with multichannel ZVA . 9
5.3.2 Coupling with wires and measuring current with a single ZVA .10
6 Fabrication of multielectrode array .10
6.1 Electrode preparation .10
6.2 Number of electrodes .10
6.3 Mounting of electrodes .11
6.4 Surface coating on electrodes for preventing crevice corrosion .11
6.5 Electrode configuration .11
6.6 Size of electrodes .11
6.7 Spacing of electrodes for spatiotemporal studies . .12
6.8 Spacing of electrodes for corrosion monitoring in oil and gas application .12
6.9 Size and spacing of the electrodes for high throughput studies .12
7 Test procedure .12
8 Test report .13
Annex A (informative) Typical results from multielectrode array with closely packed
electrodes for studying spatiotemporal behaviour of localized corrosion .14
Annex B (informative) Typical results from a CMAS for corrosion monitoring .15
Annex C (informative) Example reports.17
Bibliography .18
Foreword
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iv © ISO 2020 – All rights reserved

Introduction
Multielectrode array technology has been used to study electrochemical behaviours and the
[1] to [5]
localized corrosion of metals and alloys since the 1970s . It has been demonstrated that
multielectrode arrays are highly powerful tools for studying the spatiotemporal behaviour of metals
[2] to [16]
in laboratories and for monitoring non-uniform corrosion, especially localized corrosion in
[17]
laboratories and plants . Multielectrode arrays are also used as high throughput probes for studying
[1][18] [19]
the statistical behaviour of metal corrosion and for the evaluation of inhibitors .
This document is designed to outline the requirements and procedures for conducting corrosion
measurements using multielectrode arrays.
The International Organization for Standardization (ISO) draws attention to the fact that it is claimed
that compliance with this document may involve the use of a patent.
ISO takes no position concerning the evidence, validity and scope of this patent right.
The holder of this patent right has assured ISO that he/she is willing to negotiate licences under
reasonable and non-discriminatory terms and conditions with applicants throughout the world. In
this respect, the statement of the holder of this patent right is registered with ISO. Information may be
obtained from the patent database available at www .iso .org/ patents.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights other than those in the patent database. ISO shall not be held responsible for identifying
any or all such patent rights.
INTERNATIONAL STANDARD ISO 23449:2020(E)
Corrosion of metals and alloys — Multielectrode arrays for
corrosion measurement
1 Scope
This document specifies the methodology of using multielectrode arrays for the measurement of
the corrosion, especially localized corrosion, of metals and alloys. It can be used as a powerful tool
for studying the initiation and propagation processes of localized corrosion. It is also a useful tool
for long-term corrosion monitoring in the field, especially for localized corrosion, and for obtaining
high throughput results for the evaluation of metals with different compositions and/or physical
properties in different environments and the screening of a large number of inhibitors. Additionally,
the galvanic coupling current and potential distribution of dissimilar metal parings can be assessed
by multielectrode arrays. Multielectrode arrays can be implemented in full-immersion, thin-film, spray
and alternating wet–dry cycle exposures.
This document is not intended to be used for measurements of corrosion caused by a non-electrochemical
mechanism.
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 8407, Corrosion of metals and alloys — Removal of corrosion products from corrosion test specimens
ISO 8044, Corrosion of metals and alloys — Vocabulary
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 8044 and the following apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
3.1
uneven general corrosion
corrosion that occurs over the whole exposed area of a metal at different rates across the exposed area
Note 1 to entry: It is a type of general corrosion, as defined in ISO 8044, that produces an uneven or wave-like
[20][21]
surface where the thickness reduction at the more corroded areas is significantly larger than the thickness
reduction at the less corroded areas or the average corroded areas.
3.2
non-uniform corrosion
corrosion that occurs at different rates over a metal surface where there is a localized surplus of net
anodic or net cathodic rates such that a localized area does not exhibit charge neutrality and electrons
flow within the metal from the anodic-dominant areas to the cathodic-dominant areas
Note 1 to entry: Non-uniform corrosion includes both localized corrosion, as defined in ISO 8044, and uneven
general corrosion (3.1). Non-uniform corrosion also includes the type of general corrosion that produces even
surfaces at the end of a large time interval, but uneven surfaces within small time intervals.
3.3
multielectrode array
device consisting of multiple electrodes for corrosion studies and corrosion monitoring
Note 1 to entry: The electrodes in a multielectrode array can either be arranged in an organized pattern on a 2D
plane or packed randomly on a 2D plane or in a 3D space. When the electrodes are randomly packed, the word
“array” in the term means that there are many electrodes in the device.
3.4
zero-voltage ammeter
ZVA
ammeter that imposes a negligibly low voltage drop when inserted into a circuit for measurement
of current
Note 1 to entry: When a ZVA is used to measure the coupling current between two electrodes, the two electrodes
are essentially at the same potential.
Note 2 to entry: Both a zero-resistance ammeter (3.5) and a simple device formed with a shunt resistor and a
voltmeter can be used as the ZVA providing they do not impose a significant voltage drop (< 1 mV) in the current-
measuring circuit.
3.5
zero-resistance ammeter
ZRA
zero-voltage ammeter (3.4) that has a near zero dynamic resistance when inserted into a circuit for
measurement of current
Note 1 to entry: ZRA is usually built with operational amplifiers and may impose a voltage between 50 µV and
2 mV in the current-measuring circuit.
Note 2 to entry: When the measured current is in the nanoampere range or lower as often found in the
multielectrode arrays (3.3), the ZRA’s static resistance determined with Ohm’s Law (ratio of voltage to current)
is usually higher than 50 000 ohm, even though its dynamic resistance (derivative of voltage to current) is near
zero ohm.
3.6
coupled multielectrode array
CMA
multielectrode array (3.3) whose electrodes are coupled together by wires or through the use of a
multichannel zero-voltage ammeter (3.4) between the electrodes and the coupling joint so that all the
electrodes connected to the coupling joint are essentially at the same potential
3.7
coupled multielectrode array sensor
CMAS
coupled multielectrode array (CMA) (3.6) that is used as a sensor for corrosion monitoring
Note 1 to entry: The outputs of a typical CMAS are usually simple parameters such as maximum corrosion rate
and maximum penetration depth, while the outputs of a typical CMA are usually the large number of currents
and/or potentials from all the electrodes.
3.8
cathodic protection effectiveness margin
cathodic protection margin of effectiveness
CPEM
degree of cathodic protection derived from the current of a coupled multielectrode array sensor (3.7) that
has a value of 0 % when the cathodic protection starts to be adequate in terms of acceptable corrosion
rate (e.g. 0,01 mm/a or 0,0 mm/a), and a value of 100 % when excessive hydrogen evolution starts
2 © ISO 2020 – All rights reserved

4 Principle
4.1 Multielectrode arrays
One of the characteristics of non-uniform corrosion, especially localized corrosion, on a metal surface is
that there are some small areas that are more anodic and some small areas that are less anodic or that
are cathodic. Multielectrode arrays, as shown in Figure 1, are highly effective tools for studying non-
uniform corrosion. In Figure 1 a), the electrodes of the multielectrode array were closely packed in a
[10]
5 × 20 pattern to simulate the metal surface for studying the spatiotemporal behaviour of corrosion .
In Figure 1 b), the multielectrode array was buried under sands in a cup to evaluate under deposit
[22]
corrosion . In general, the electrodes in a multielectrode array for spatial-temporal studies are
arranged in regular patterns on a 2D plane, such as those shown in Figures 1 and 2, and this type of
[2][13]
multielectrode arrays are also called “wire beam electrodes” . The multielectrode arrays may also
[1][18]
be arranged randomly on a 2D plane or 3D space . In this case, the word “array” in the term means
that there are many electrodes in the device.
a)  With closely packed electrodes for b)  With a sand cup for evaluation of
[10] [22]
spatial-temporal corrosion studies under-deposit corrosion
Key
1 5 × 20 electrodes flush-mounted in epoxy 3 24 electrodes
2 sand-holding cup 4 heating device
Figure 1 — Typical multielectrode arrays
The currents of the electrodes composing the array can be measured individually. It is possible to
measure the potentials of each individual electrode, of a selected group of electrodes, or the totality of
the electrodes if they are coupled (see below). It is at times technically feasible to polarize one or more
electrodes using a single-channel potentiostat or a multi-channel potentiostat in order to evaluate the
[10]
effects of polarization on the neighbouring electrodes . Because of the small size of the electrodes
in the array, the polarization currents are usually very small (less than 1 µA) and their effects on the
measurements of the potential of the neighbouring electrodes due to the IR drop can be ignored.
4.2 Coupled multielectrode array (CMA)
If all the electrodes or a selected number of electrodes in a multielectrode array are coupled together
by wires or through the use of multichannel ammeters that impose near-zero voltages between the
electrodes and the coupling joint so that all the electrodes connected to the coupling joint are essentially
at the same potential, such multielectrode array is called a “coupled multielectrode array (CMA)”. The
ammeters that impose near zero voltage are called “zero-voltage ammeters (ZVAs)” and are described
in 5.2. Figure 2 shows a typical CMA system where all the electrodes are controlled at the same potential
[3]
by the potentiostat through a multichannel ZVA box .

Key
A multichannel data acquisition system 4 multichannel ZVA
B potentiostat 5 bottom view and electrode ID of the array
1 electrochemical cell 6 reference electrode
2 counter electrode
3 CMA
NOTE 1 All electrodes are at the same potential.
NOTE 2 The counter electrode is electrically separated from the multielectrode array.
[3]
Figure 2 — Typical CMA system for electrochemical studies under polarization conditions
4.3 Multielectrode array with closely packed electrodes for studying spatiotemporal
behaviour of localized corrosion
4.3.1 If the electrodes are arranged in an organized pattern such as 4 × 4, 5 × 20 or 10 × 10 and the
electrodes are closely packed and their size are small (typically < 1 mm in diameter), such multielectrode
array may be used to study the spatial and temporal behaviour of corrosion on a metal surface, e.g. when
and where localized corrosion first initiates and how the localized corrosion propagates on the metal
surface. Annex A shows a typical use of the CMA for studying the spatial and temporal behaviour of
corrosion.
4.3.2 The CMA may also be used at its corrosion potential without any polarization. In a typical case
of localized corrosion where there is clear separation of anodes and cathodes, the array simulates a one-
piece metal section for which the electrodes that have net anodic currents simulate the anodic areas and
the electrodes that have net cathodic currents simulate the cathodic areas on the metal surface as shown
in Figure 3. By measuring the electron flow from the anodic electrodes to the cathodic electrodes on the
4 © ISO 2020 – All rights reserved

array as a function of time, the information of the initiation and propagation of localized corrosion that
takes place on the metal under freely corroding conditions can be obtained.
Key
1 metal 4 corrosion products and electrolyte
2 electrons 5 corrosive electrolyte (liquid, thin, film or wet deposits)
3 anodic sites 6 cathodic sites
NOTE The electrons flow randomly in metal from anodic sites to cathodic sites. Cathodic reactions such as
- - - +
O +4e +2H O = 4OH occur at the cathodic sites. Anodic reactions such as Fe-2e +2H O = Fe(OH) + 2H occur at
2 2 2 2
the anodic sites.
Figure 3 — Typical characteristics of localized corrosion on a metal surface at its corrosion
[4]
potential — Electrons flow from anodic areas to the cathodic areas within the metal
In a typical non-uniform general corrosion case, there is no clear separation of anodes and cathodes, but
some areas corrode more and some areas corrode less and all areas are anodic, at least for a short duration
during corrosion process. The electrodes on the array that have net anodic currents simulate the more
corroding areas and the electrodes that have net cathodic currents simulate the less corroding areas.
4.4 Coupled multielectrode array sensor (CMAS)
4.4.1 CMAS for corrosion monitoring
If a CMA is used as a sensor for corrosion monitoring, such a CMA is called a “coupled multielectrode
array sensor (CMAS)”. Figure 4 shows some typical CMASs. Unlike a CMA for spatial studies, a CMAS
for field applications usually has fewer electrodes [see Figure 4 a)] and the electrodes can be randomly
packed [see Figure 4 b)]. There is no need for a plant operator to know all the individual currents and
create a corrosion map for a sensor in the fields. It often suffices for the operator to know the maximum
corrosion rate, at the worst corroding area, and the associated maximum penetration depth, without
needing to know where exactly these worst areas are. The outputs of the CMAS probes are often those
two simple parameters: maximum corrosion rate (calculated from the most anodic current, which is
from the worst or most corroding electrode) and maximum penetration depth (calculated from the
most corroded electrode). The operators can apply their corrosion mitigation measures (e.g. by adding
a corrosion inhibitor) based on the maximum corrosion rate. On the other hand, the engineers may
decide how often the plant equipment should be inspected based on the maximum penetration depth
[36]
(see ASTM G217-16 for additional information ).
a)  CMAS with 8 electrodes for high- b)  CMAS 16 electrodes packed
[4] [31]
temperature and high-pressure systems randomly
Key
1 insulator 4 fitting for mounting to pressure vessel
2 electrodes 5 electrical connector
3 probe body
Figure 4 — Typical CMAS with randomly packed electrodes for field applications
4.4.2 CMAS used without polarization to measure corrosion rate at free corrosion potential
CMAS probes are often used without any polarization. In this case, all the electrodes that are coupled
together simulate the behaviour of a one-piece metal section at the free corrosion potential. The non-
uniform corrosion rates measured from the CMAS probe corresponds to the non-uniform corrosion
occurring at the different anodic sites under the freely corroding condition. B.1 shows some typical
corrosion rates in different environments measured with a CMAS probe made of 16 carbon steel
electrodes.
Because of the existence of local cathodes (see Figure 5) on each electrode when the CMAS is not
sufficiently polarized, the corrosion current measured by the ZVA in the external circuit may
underestimate the corrosion rate on each electrode. This is especially true for the case of uniform
corrosion, therefore the non-polarized CMAS probe is not suitable for uniform corrosion. For non-
uniform corrosion cases, however, the worst corroding electrode, when its potential is raised
significantly by the other less corroding electrodes, usually does not have significant local cathodic
current, and the maximum corrosion rate of a CMAS probe that is calculated from the worst corroding
electrode is often close to the corrosion rate occurring on this electrode. When the CMAS electrodes are
sufficiently polarized, such as the case when monitoring corrosion of cathodically protected systems
or stray-current affected systems (see 4.4.3), the local cathode effect is less important because all the
electrodes are sufficiently polarized from their corrosion potentials.
4.4.3 CMAS used to evaluate the effectiveness of cathodic protection and the effect of stray
current
As mentioned in 4.3.2, a CMA can be used with or without polarization. When the coupling joint where
all the electrodes are coupled to is connected to a metal structure under cathodic protection (CP), all
the electrodes are polarized to essentially the same CP potential as the metal structure (see Figure 5).
Then, the CMAS measures the corrosion under CP conditions. When the CP is insufficient, one or more
of the electrodes will undergo corrosion and the maximum corrosion rate calculated from the most
corroding electrode is a good indicator for the insufficiency of the CP. When the corrosion rate as
indicated from the most corroding electrode is near zero, all the electrodes are protected by the CP.
6 © ISO 2020 – All rights reserved

Because the lowest corrosion rate is zero, it cannot be used to indicate the degree of protection by
the CP, except for showing that the CP is effective, when the corrosion rate from the most corroding
electrode is zero. Another parameter, the “cathodic protection margin of effectiveness” or “cathodic
protection effectiveness margin (CPEM)”, which is defined as the ratio of the cathodic current from
the most corroding electrode to a maximum allowable cathodic current that corresponds to excessive
[23]
hydrogen evolution, can be used to indicate the degree of CP .
When the CPEM is larger than zero, the current from the most corroding electrode is cathodic (has
the same sign as the maximum allowable cathodic current) and there is no corrosion from any of the
electrodes on the CMAS. Therefore, all of the electrodes are fully protected. When the CPEM is 100 %,
however, the most anodic electrode starts to experience excessive hydrogen evolution, which should
be avoided. Therefore, the CPEM should be controlled above zero and below 100 %. B.2 shows the
typical responses of the corrosion rate and CPEM from a CMAS when the CP potential varies from the
corrosion potential (about −0,7 V versus Cu/CuSO ) to a large negative potential (lower than −1,25 V
versus Cu/CuSO ).
If the CP is considered adequate when the corrosion rate is lower than a predetermined acceptable level
(e.g. 0,01 mm/a), the CPEM may also be defined such that its value is 0 % when the corrosion rate from
the CMAS is at this acceptable level and 100 % when excessive hydrogen evolution occurs. Because
the CPEM is related to how safely a CP system protects the metal structure, it is also called “cathodic
[23]
protection safe margin (CPSM)” .
Compared with the CP criteria based on the instant-off potential measurements, the method provided by
the CMAS does not require the use of the reference electrode. The reference electrode usually requires
regular maintenance and has a shorter life than the CMAS probe, which has only solid components and
can last for 10 to 50 years under dry or wet conditions.
4.5 Multielectrode arrays for high throughput measurements
Corrosion behaviours, especially localized corrosion behaviours, of metals are usually stochastic.
[24]
For example, localized corrosion rates for carbon steel may vary by 200 % to 600 % in seawater .
Therefore, a statistical approach should be used to characterize the corrosion behaviour of metals and
a large number of samples tested in the same solution under the same environmental conditions are
required to derive a statistical conclusion. Multielectrode arrays are highly efficient for such statistical
[18]
studies . The development of inhibitors requires the evaluation of the performance of a large number
[19]
of formulations and multielectrode arrays can also be useful for such evaluations .
4.6 Multielectrode arrays for other applications
Multielectrode arrays can also be used in many other applications, including the following.
— Assessing the electrochemical behaviour of a material across various weld zones. As a material is
welded, various zones are created that differ in properties due to the differences in cooling rates
and heat treatment from welding. These differences in properties are spatially dependant and can
[25]
be assessed with multielectrode arrays .
— Understanding the galvanic throwing power of sacrificial anodes by measuring the coupling current
and potential distributions between the sacrificial anode and the surrounding electrodes being
[26]
cathodically protected .
— Studying the effect of corrosion inhibitors on reducing the anodic or cathodic reaction rates at an
electrode interface, especially for spatial-temporal release from inhibitor containing coatings.
5 Instrumentation
5.1 Potential measurement
When the m
...