Corrosion of metals and alloys -- Multielectrode arrays for corrosion measurement

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.

Corrosion des métaux et alliages -- Assemblages multi-électrodes pour la mesure de la corrosion

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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 23449:2020(E)
ISO 2020
---------------------- Page: 1 ----------------------
ISO 23449:2020(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2020

All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may

be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting

on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address

below or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii © ISO 2020 – All rights reserved
---------------------- Page: 2 ----------------------
ISO 23449:2020(E)
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

© ISO 2020 – All rights reserved iii
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ISO 23449:2020(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 156, Corrosion of metals and alloys.

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.
iv © ISO 2020 – All rights reserved
---------------------- Page: 4 ----------------------
ISO 23449:2020(E)
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.
© ISO 2020 – All rights reserved v
---------------------- Page: 5 ----------------------
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.

© ISO 2020 – All rights reserved 1
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ISO 23449:2020(E)
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
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ISO 23449:2020(E)
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

© ISO 2020 – All rights reserved 3
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ISO 23449:2020(E)

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
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ISO 23449:2020(E)

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 ).
© ISO 2020 – All rights reserved 5
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ISO 23449:2020(E)
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
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ISO 23449:2020(E)

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
...

DRAFT INTERNATIONAL STANDARD
ISO/DIS 23449
ISO/TC 156 Secretariat: SAC
Voting begins on: Voting terminates on:
2020-01-07 2020-03-31
Corrosion of metals and alloys — Multielectrode arrays for
corrosion measurement
ICS: 77.060
THIS DOCUMENT IS A DRAFT CIRCULATED
FOR COMMENT AND APPROVAL. IT IS
THEREFORE SUBJECT TO CHANGE AND MAY
NOT BE REFERRED TO AS AN INTERNATIONAL
STANDARD UNTIL PUBLISHED AS SUCH.
IN ADDITION TO THEIR EVALUATION AS
BEING ACCEPTABLE FOR INDUSTRIAL,
This document is circulated as received from the committee secretariat.
TECHNOLOGICAL, COMMERCIAL AND
USER PURPOSES, DRAFT INTERNATIONAL
STANDARDS MAY ON OCCASION HAVE TO
BE CONSIDERED IN THE LIGHT OF THEIR
POTENTIAL TO BECOME STANDARDS TO
WHICH REFERENCE MAY BE MADE IN
Reference number
NATIONAL REGULATIONS.
ISO/DIS 23449:2020(E)
RECIPIENTS OF THIS DRAFT ARE INVITED
TO SUBMIT, WITH THEIR COMMENTS,
NOTIFICATION OF ANY RELEVANT PATENT
RIGHTS OF WHICH THEY ARE AWARE AND TO
PROVIDE SUPPORTING DOCUMENTATION. ISO 2020
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ISO/DIS 23449:2020(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2020

All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may

be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting

on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address

below or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Fax: +41 22 749 09 47
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii © ISO 2020 – All rights reserved
---------------------- Page: 2 ----------------------
ISO/DIS 23449:2020(E)
Contents Page

Foreword ........................................................................................................................................................................................................................................iv

Introduction ..................................................................................................................................................................................................................................v

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

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

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

4 Principle ........................................................................................................................................................................................................................ 2

4.1 Multielectrode arrays ........................................................................................................................................................................ 2

4.2 Coupled multielectrode array (CMA) .................................................................................................................................. 3

4.3 Multielectrode array with closely packed electrodes for studying spatiotemporal

behaviour of localized corrosion ............................................................................................................................................. 3

4.4 Coupled multielectrode array sensor (CMAS) ............................................................................................................. 4

4.4.1 CMAS for corrosion monitoring .......................................................................................................................... 4

4.4.2 CMAS used without polarization to measure corrosion rate at free

corrosion potential......................................................................................................................................................... 5

4.4.3 CMAS used to evaluate the effectiveness of cathodic protection and the

effect of stray current................................................................................................................................................... 5

4.5 Multielectrode arrays for high-throughput measurements ............................................................................. 6

4.6 Multielectrode arrays for other applications ................................................................................................................ 6

5 Instrumentation .................................................................................................................................................................................................... 6

5.1 Potential measurement ................................................................................................................................................................... 6

5.2 Coupling current measurement ............................................................................................................................................... 7

5.3 Effective coupling of individual electrodes ..................................................................................................................... 7

5.3.1 Coupling with multichannel ZVA ....................................................................................................................... 7

5.3.2 Coupling with wires and measuring current with a single ZVA .............................................. 8

6 Fabrication of multielectrode array ................................................................................................................................................. 8

6.1 Electrode preparation ....................................................................................................................................................................... 8

6.2 Number of electrodes ........................................................................................................................................................................ 9

6.3 Mounting of electrodes .................................................................................................................................................................... 9

6.4 Surface coating on electrodes for preventing crevice corrosion .................................................................. 9

6.5 Electrode configuration ................................................................................................................................................................... 9

6.6 Size of electrodes .................................................................................................................................................................................. 9

6.7 Spacing of electrodes for spatiotemporal studies ......... .........................................................................................10

6.8 Spacing of electrodes for corrosion monitoring in oil and gas application .....................................10

6.9 Size and spacing of the electrodes for high throughput studies ...............................................................10

7 Test procedure .....................................................................................................................................................................................................10

8 Test report ................................................................................................................................................................................................................11

Annex A Typical Results from Multielectrode array with closely packed electrodes for

studying spatiotemporal behavior of localized corrosion .....................................................................................12

Annex B Typical Results from coupled multielectrode array sensor (CMAS) for corrosion

monitoring ...............................................................................................................................................................................................................14

Annex C Example Reports ...........................................................................................................................................................................................16

Bibliography .............................................................................................................................................................................................................................17

© ISO 2020 – All rights reserved iii
---------------------- Page: 3 ----------------------
ISO/DIS 23449:2020(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.

International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.

The main task of technical committees is to prepare International Standards. Draft International

Standards adopted by the technical committees are circulated to the member bodies for voting.

Publication as an International Standard requires approval by at least 75 % of the member bodies

casting a vote.

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.

ISO XXX-X was prepared by Technical Committee ISO/TC 156, Corrosion of metals and alloys,

Subcommittee WG 11, Electrochemical test methods.
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ISO/DIS 23449:2020(E)
Introduction

The multielectrode array technology has been used to study electrochemical behaviours and localized

corrosion of metals and alloys since the 1990s 1-2. It has been demonstrated that the multielectrode

arrays are highly powerful tools for studying the spatiotemporal behaviour of metals in laboratories3-13

and for monitoring non-uniform corrosion, especially localized corrosion in laboratories and plants 14.

Multielectrode arrays are also used as high-throughput probes for studying the statistical behaviour of

metal corrosion15 and evaluation of inhibitors16. The goal of this standard is designed to outline the

requirements and procedures for conducting corrosion measurements using multielectrode arrays.

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DRAFT INTERNATIONAL STANDARD ISO/DIS 23449:2020(E)
Corrosion of metals and alloys — Multielectrode arrays for
corrosion measurement
1 Scope

This standard specifies the methodology of using multielectrode arrays for 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 standard is not intended to be used for measurements of corrosion caused by non-electrochemical

mechanism.
2 Normative references

The following documents, in whole or in part, are normatively referenced in this document and are

indispensable for its application. For dated references, only the edition cited applies. For undated

references, the latest edition of the referenced document applies.
ASTM G199, Standard Guide for Electrochemical Noise Measurement

ASTM G217, Standard Guide for Corrosion Monitoring in Laboratories and Plants with Coupled

Multielectrode Array Sensor (CMAS) Technique

ISO 8407, Corrosion of metals and alloys — Removal of corrosion products from corrosion test specimens

ISO 8044, Corrosion of metals and alloys — Basic terms and definitions
ISO 11463, Corrosion of metals and alloys — Evaluation of pitting corrosion

ISO 11845, Corrosion of metals and alloys — General principles for corrosion testing

ISO 14802, Corrosion of metals and alloys — Guidelines for applying statistics to analysis of corrosion data

ISO 17474, Corrosion of metals and alloys — Conventions applicable to electrochemical measurements in

corrosion testing

ISO 17093, Corrosion of metals and alloys — Guidelines for corrosion test by electrochemical noise

measurements

ASTM-G1, Standard Practice for Preparing, Cleaning, and Evaluating Corrosion Test Specimens

3 Terms and definitions

For the purposes of this document, the terms and definitions given in ISO 8044 apply except as

noted below.

3.1 Uneven general corrosion—corrosion that occurs over the whole exposed area of a metal

at different rates across the exposed area. It is a type of general corrosion as defined in ISO 8044 that

17, 18

produces uneven surface or wave-like surface where the thickness reduction at the more corroded

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ISO/DIS 23449:2020(E)

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. Non-uniform corrosion includes both localized corrosion as defined in ISO 8044 and

uneven general corrosion (as defined in 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—A device with multiple electrodes that have similar surface area and are

made of a same metal for corrosion studies and corrosion monitoring.
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 to

simulate the metal surface for studying the spatiotemporal behaviour of localized corrosion. In Figure

1(B), the multielectrode array was buried under sands in a cup to evaluate under deposit corrosion. In

general, the electrodes in a multielectrode array are arranged in regular patterns such as those shown

in Figures 1 and 2, but they may also be arranged in a randomly packed pattern.

Figure 1 — Typical multielectrode arrays with closely-packed electrodes for spatial-temporal

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studies of localized corrosion (A) and a sand cup for evaluation of under deposit corrosion (B )

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

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 neighboring electrodes due to the IR drop can be ignored.

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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 (ZVA) and will be described

in Section 5.2. Figure 2 shows a typical CMA system where all the electrodes are controlled at the same

potential by the potentiostat through a multichannel ZVA box .

Figure 2 — A Typical CMA system for electrochemical studies under polarization conditions .

Note that the counter electrode is electrically separated from the multielectrode array.

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 4x4, 5x20, or 10x10 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 initiate and how the localized corrosion propagate on the metal

surface. Annex A shows a typical use of the CMA as shown in Figure 1(A) 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 one-

piece metal and 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 array as a

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function of time, the information of the initiation and propagation of localized corrosion that takes place

naturally on the metal can be obtained.

Figure 3 — Typical characteristics of localized corrosion on a metal surface at its corrosion

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 short a 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. 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 [See ASTM G217-16 for additional information].

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Figure 4 — Typical coupled multielectrode array sensors (CMAS) for high-temperature and

high-pressure (A) and low-pressure (B) 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 at the free corrosion potential. The non-uniform

corrosion rates measured from the CMAS probe corresponds the non-uniform corrosion occurring at

the different anodic sites under the natural condition. Annex B.1 shows some typical corrosion rates in

different environments measured with a CMAS probe made of16 carbon steel electrodes.

Because of the existence of local cathodes (see Figure 5) on each electrode when the CMAS is not

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, so, 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 cathode, 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 polarized such as the cases for monitoring corrosion of

cathodically protected systems or stray-current affected systems (see Section 4.4.3), the local cathode

effect is less important because all the electrodes are 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 in

Section 5.2). Then, the CMAS measure the corrosion under cathodic protection 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.

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 CP safe margin (CPSM) which is defined as the ratio of the

cathodic current from the most corroding electrode to the maximum allowable cathodic current that

corresponds to the evolution of hydrogen, can be used to indicate the degree of cathodic protection

. When the CPSM 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. So, all of the electrodes are fully protected. When the CPSM is 100%, however,

the most anodic electrode starts to experience hydrogen evolution, which should be avoided. Therefore,

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the CPSM should be controlled above zero and below 100%. Annex B.2 shows the typical responses of

the corrosion rate and CPSM from a CMAS when the CP potential varied from the corrosion potential

(about -0.7 V v.s. Cu/CuSO ) to a large negative potential (lower than -1.25 V v.s. Cu/CuSO ).

4 4

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 which requires regular maintenance

and has a short life than the CMAS probe which has only solid components and can last for 10 to 50

years under dry or wet condition.
4.5 Multielectrode arrays for high-throughput measurements

Corrosion behaviours, especially localized corrosion behaviours, of metals are usually stochastic.

For example, localized corrosion rates for carbon steel may vary by 200% to 600% in seawater.

Therefore, statistical approach should be used to characterize the corrosion behaviour of metals

and a large number of samples tested in the same solution under same environmental conditions are

required to derive a statistical conclusion. Multielectrode arrays are highly efficient for such statistical

studies . The development of inhibitors requires the evaluation of the performance of a large number

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. Such applications include:

• Assessing the electrochemical behaviour of a material across various weld zones. As a material is

welded, various zones are created which differ in properties due to the differences in cooling rates

and heat treatment from welding. These differences in properties are spatially dependant and can

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

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 multielectrode array operates in an uncoupled configuration, the open-circuit potential of

each electrode should be measured individually against a reference electrode. When the multielectrode

array is used in a laboratory as a coupled multielectrode array, the coupling joint potential which is a

mixed potential for all the coupled electrodes should be measured against the reference electrode. This

is essential when the array is polarized or one or more groups of selected electrodes are polarized to

different potentials. Because the potential is not needed to derive the corrosion rate, it is not necessary

to measure the potential of the array when the array is used as a CMAS probe for corrosion monitoring.

A multichannel voltammeter or a single channel voltmeter with a multichannel switching mechanism

may be used for measuring the open-circuit potential of the individual electrodes or one or more groups

of selected electrodes against a reference electrode immersed in the same solution. Because the single

electrode surface area of a typical multielectrode array is small (usually less than 1 mm ), the input

impedance of the voltmeter must be high enough so that it does not withdraw a significant amount of

current that would cause the unwanted polarization of the electrodes. Input impedance of more than

100 GΩ is usually acceptable, unless the solution conductivity is extremely low or the kinetics of the

electrode is extremely slow.
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5.2 Coupling current measurement

Figure 5 shows a typical schematic diagram for the measurements of the coupling currents from the

coupled electrodes with or without the polarization. The working electrode (WE) of a potentiostat or

a metal structure under cathodic protection (CP) can be used to the coupling joint to polarize the CMA.

According to the definition for ZVA in Section 4.2, the ZVAs in Figure 5-impose near-zero voltage, so all

the electrodes are at the same potential. A simple device formed with a shunt resistor and a voltmeter

for the measurement of current by applying Ohm's Law is one type of ZVA, as long as they do not impose

a significant voltage drop across in the current-measuring circuit. The zero-resistance amber (ZRA) as

defined
...

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