Assessment of the effectiveness of cathodic protection based on coupon measurements

This document specifies requirements for the design, installation, positioning, sizing, use and maintenance of coupons for the assessment of the effectiveness of cathodic protection (CP) of buried and immersed metallic structures, such as pipelines, in the case of normal operation as well as AC and DC interference conditions.

Evaluation de l’efficacité de la protection cathodique par mesurages sur coupon

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Status
Published
Publication Date
09-Feb-2020
Current Stage
6060 - International Standard published
Start Date
12-Jan-2020
Completion Date
10-Feb-2020
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ISO 22426:2020 - Assessment of the effectiveness of cathodic protection based on coupon measurements
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INTERNATIONAL ISO
STANDARD 22426
First edition
2020-02
Assessment of the effectiveness of
cathodic protection based on coupon
measurements
Evaluation de l’efficacité de la protection cathodique par mesurages
sur coupon
Reference number
ISO 22426:2020(E)
ISO 2020
---------------------- Page: 1 ----------------------
ISO 22426:2020(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

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Published in Switzerland
ii © ISO 2020 – All rights reserved
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ISO 22426:2020(E)
Contents Page

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

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

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

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

4 Assessment of CP effectiveness ............................................................................................................................................................. 3

5 Application principles .................................................................................................................................................................................... 4

5.1 IR-free potential measurements .............................................................................................................................................. 4

5.2 DC and AC currents and current densities ...................................................................................................................... 4

5.3 Spread resistance .................................................................................................................................................................................. 5

5.4 Corrosion rate measurements ................................................................................................................................................... 5

6 Design considerations .................................................................................................................................................................................... 5

6.1 General ........................................................................................................................................................................................................... 5

6.2 Geometry of the defect ..................................................................................................................................................................... 5

6.3 Dimension of the coupon base plate .................................................................................................................................... 6

6.4 Surface area of the coupon ........................................................................................................................................................... 7

6.5 Other types of coupon geometries ......................................................................................................................................... 7

7 Monitoring purpose — Selection of installation sites ................................................................................................... 7

7.1 General ........................................................................................................................................................................................................... 7

7.2 Detailed and comprehensive assessment of CP effectiveness ........................................................................ 7

7.3 Assessment of CP effectiveness under DC interference conditions ......... .................................................. 8

7.4 Assessment of CP effectiveness under AC interference conditions ........................................................... 9

8 Installation procedures ................................................................................................................................................................................. 9

9 Commissioning of coupons .....................................................................................................................................................................10

9.1 Preliminary checking ......................................................................................................................................................................10

9.2 Start up .......................................................................................................................................................................................................10

9.3 Measurement of the settled parameters ........................................................................................................................11

9.4 Installation and commissioning documents ..............................................................................................................11

9.5 Frequency of coupon measurement ..................................................................................................................................11

Annex A (informative) Special types and procedures of coupons and probes .......................................................12

Annex B (informative) Assessment of the effectiveness of CP under any conditions

including DC and/or AC interferences ........................................................................................................................................15

Annex C (informative) Examples of instant-off and current density measurements on

coupons — Remote monitoring and remote control ...................................................................................................17

Bibliography .............................................................................................................................................................................................................................22

© ISO 2020 – All rights reserved iii
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ISO 22426:2020(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
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INTERNATIONAL STANDARD ISO 22426:2020(E)
Assessment of the effectiveness of cathodic protection
based on coupon measurements
1 Scope

This document specifies requirements for the design, installation, positioning, sizing, use and

maintenance of coupons for the assessment of the effectiveness of cathodic protection (CP) of buried

and immersed metallic structures, such as pipelines, in the case of normal operation as well as AC and

DC interference conditions.
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 15589-1, Petroleum, petrochemical and natural gas industries — Cathodic protection of pipeline

systems — Part 1: On-land pipelines

EN 50162, Protection against corrosion by stray current from direct current systems

3 Terms and definitions
For the purposes of this document, the following terms and definitions 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
complex structure

system comprising the structure (3.13) to be protected connected to one or more foreign electrodes

and/or crossing multiple connected electrodes or passing close or through steel-reinforced concrete

3.2
electrolyte
medium in which an electric current is transported by ions
Note 1 to entry: Electrolyte is synonymous with soil, backfill and water.
3.3
polarization

change of an electrode [e.g. structure (3.13) and/or coupon (3.14)] potential caused by current flow

Note 1 to entry: Current flow results in concentration polarization (3.4) and activation polarization (3.5).

3.4
concentration polarization

portion of an electrode [structure (3.13) and/or coupon (3.14) polarization (3.3)] produced by electrolyte

concentration changes resulting from the passage of a current through an electrolyte (3.2)

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ISO 22426:2020(E)
3.5
activation polarization

change of an electrode [e.g. structure (3.13) and/or coupon (3.14)] potential due to charge transfer

3.6
depolarization

loss of polarization (3.3) of an electrode [e.g. structure (3.13) and/or coupon (3.14)] potential subsequent

to current interruption

Note 1 to entry: Loss of concentration polarization (3.4) of an electrode (e.g. structure or coupon) is > 10 s up

to seconds, hours or days. Only a small fraction of concentration polarization is usually lost within 0,1 s after

current interruption in most cases. The time constant for build-up and depolarization of activation polarization

−4 −3

(3.5) of an electrode is from 10 s to 10 s. Therefore, usually all activation polarization is lost within 0,1 s after

current interruption.
3.7
IR drop

voltage, due to any current, developed in an electrolyte (3.2) such as soil, between the reference

electrode and the metal of the structure (3.13), in accordance with Ohm’s Law (U = I × R)

3.8
IR-free potential
IR free

electrode [e.g. coupon (3.14)] to electrolyte (3.2) potential measured without the voltage error caused

by the IR drop (3.7) due to the protection current or any other current
3.9
instant-off potential
off

electrode [e.g. structure (3.13) and/or coupon (3.14)] to electrolyte (3.2) potential measured very quickly

(typically < 0,3 s) after an interruption of all sources of applied cathodic protection current with the

aim of approaching an IR-free potential (3.8)

Note 1 to entry: The delay between the current interruption and measurement will affect the measured value

and whether there is a decay of concentration polarization (3.4) and/or activation polarization (3.5).

3.10
on-potential

electrode [e.g. structure (3.13) and/or coupon (3.14)] to electrolyte (3.2) potential measured while the

cathodic protection system is energized
3.11
over-polarization
over-protection

achievement of the structure (3.13) to electrolyte (3.2) potentials that are more negative than required

for the control of corrosion and that can damage coatings, increase AC corrosion rate or, particularly for

high yield strength steels, enhance the tendency to crack
3.12
spread resistance

ohmic resistance through a coating defect or coupon (3.14) to remote earth or from the exposed metallic

surface of a coupon towards earth

Note 1 to entry: This is the resistance that controls the DC or AC current through a coating defect or an exposed

metallic surface of a coupon for a given DC on-potential (3.10) or AC voltage. It comprises the metal resistance,

the polarization resistance and the resistance within the coating defect as well as the contribution of the earth

resistance.
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ISO 22426:2020(E)
3.13
structure
metallic structure intended to receive cathodic protection
3.14
coupon

metal sample of defined dimensions and shape made of a metal equivalent to the metal of the

structure (3.13)

Note 1 to entry: For the purpose of this document, the coupon is connected to the external surface of, and

immersed in the electrolyte (3.2) adjacent to, the structure being protected by cathodic protection.

Note 2 to entry: Special kinds of probes (3.15) and coupons (examples of which are given in the annexes) are also

considered part of the coupon definition (hence covered by this document) to the extent that they are intended

to reflect structure coating defects, and thus act as a representative metal sample used to quantify the extent of

corrosion or the effectiveness of applied cathodic protection.
3.15
probe

device incorporating a coupon (3.14) that provides measurements of parameters to assess the

effectiveness of cathodic protection

Note 1 to entry: In this document, the term “coupon” is used as a synonym for both coupons and probes.

3.16
electrical resistance probe
ER probe

device that provides measurements of metal loss by comparison of the calibrated resistance value of a

piece of metal with known physical characteristics
Note 1 to entry: Refer to Annex A for information on ER probes.
3.17
stray current
current flowing through paths other than the intended circuits

[SOURCE: ISO 8044:2020, 4.14, modified — “corrosion” has been deleted from the end of the term, and

“impressed current corrosion caused by” has been deleted from the start of the definition.]

4 Assessment of CP effectiveness

The assessment of the effectiveness of CP in accordance with ISO 15589-1 is based on an IR-free

potential measurement. The determination of the IR-free potential on the cathodically protected

structure is only possible based on combined direct current voltage gradient and close-interval

potential survey measurements. This method is called “intensive measurement” and is described in

EN 13509. This method requires, however, significant measurable voltage gradients associated with

individual coating defects in order to allow for a reliable assessment of their IR-free potential and

demonstrating conformity to ISO 15589-1. As a consequence, the determination of IR-free potential and

demonstrating conformity to ISO 15589-1 is no longer possible on today’s structures with high-quality

coating systems. While it is still possible to determine instant-off potentials on many structures and

use this reading as an approximation to the IR-free potential in certain cases, the increasing level of

AC interference is preventing the separation of the earthing systems connected through decoupling

devices from the cathodically protected structures for safety reasons. Similarly, in the presence of

DC interference conditions, the determination of both IR-free potentials and instant-off potentials is

not possible. As a consequence, on an increasing number of structures neither IR-free potentials nor

instant-off potentials can be determined in order to demonstrate conformity to ISO 15589-1. The only

remaining technology for demonstrating effectiveness is the use of coupons that are connected to the

structure under investigation. The use of coupons is further required by ISO 18086. The determination

of the effectiveness of CP under AC interference is only possible based on a current density measurement

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ISO 22426:2020(E)

on coupons. The validity and accuracy of data obtained on coupons depend on a number of factors, such

as location, geometry and bedding conditions. This document provides guidance on these aspects.

5 Application principles
5.1 IR-free potential measurements

The traditional coupon measurement technique has been used to demonstrate conformity of the coupon

polarization, which is taken to be representative of the structure coating defects in accordance with

the requirements of ISO 15589-1. There are several situations where the use of coupons is a feasible

alternative to IR-free potential measurements directly on the structure. In particular, when accurate

measurements directly on the structure itself are problematic. Examples include:
— in areas affected by traction stray currents and telluric currents;
— when dealing with the CP of complex structures;

— interference caused by two or more cathodically protected structures crossing or sharing the same

right-of-way;

— interference between both parts of an isolating joint for a structure protected by two different CP

systems one on each side of the joint;

— effects from equalizing currents from adjacent coating defects: the coupon may be regarded as

one single coating defect exposed in the chemistry of the soil exactly where the coupon has been

buried, whereas measurements on the structure may include a range of coating defects exposed in

varying individual soil chemistries leading to the formation of potential differences and varying

current demand;

— in areas where the CP is applied using several CP sources, and it is not possible or economically

practical to synchronously turn off these CP sources.

EN 13509, EN 50162, ISO 18086 and ISO 15589-1 allow for the use of coupons in such instances.

5.2 DC and AC currents and current densities

The use of coupons allows for an assessment of current densities in order to demonstrate conformity to

ISO 18086 and EN 50162.

The DC current consumed by a coupon is primarily used for assessing the significance of DC stray

current interference. EN 50162 describes a procedure for the demonstration of effectiveness of CP

based on current density. This involves measuring the DC current throughout a period of typically 24 h.

From these measurements, a period is defined in which no interference is present (e.g. hours during

the night when trains do not operate). This period is used as the reference value and the measure of

the reference current under normal CP. Based on the analysis of these currents, an assessment of the

effectiveness of CP under DC interference is performed.

Apart from the risk of corrosion due to DC stray current interference, the DC current density is also

important in the evaluation of effectiveness of CP under AC interference in accordance with ISO 18086.

Excessive cathodic DC current can produce alkalinity near a coating defect to the extent where this

electrolysis (leading to the production of current conducting OH ions) considerably increases the

conductivity of the soil adjacent to the coating defect, thus lowering the spread resistance of this coating

defect and increasing the corrosion rate under AC interference.

The AC current density has become a significant tool in the determination of the effectiveness of CP

under AC interference in accordance with ISO 18086. Essentially, the AC current density associated with

a coating defect with given surface is the result of the AC voltage on the structure at the position of the

coating defect divided by the spread resistance of the coating defect. As the spread resistance and the

AC current density cannot be measured directly at coating defects on structures, ISO 18086 requires

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ISO 22426:2020(E)

a 1 cm coupon for measuring the coupon current and calculating the current density for evaluation of

the effectiveness of CP under AC interference.
5.3 Spread resistance

In relation to coupons, the spread resistance is the ohmic resistance from the exposed metallic surface

of the coupon towards remote earth. This is the resistance that controls the DC or AC current through

a coating defect for a given DC or AC voltage. Determining the spread resistance on coupons allows for

assessing acceptable on-potentials (DC) and AC voltages.
5.4 Corrosion rate measurements

Various types of coupons and probes have been designed for the purpose of quantifying corrosion

and the corrosion rate. Examples are weight loss coupons, perforation probes and ER probes. Refer to

Annex A for more details.
6 Design considerations
6.1 General

The coupon design should reflect the purpose of the coupon measurement. The purpose may be:

— a detailed and comprehensive assessment of the CP effectiveness;
— an assessment of the effectiveness of CP under DC interference;
— an assessment of the effectiveness of CP under AC interference.

The information obtained with coupons depends on the geometry and size of the coupon. In the case of

assessing the effectiveness of CP, the critical aspects are associated with insufficient cathodic current.

In that case, a coupon with a design that results in a highest relative spread resistance, e.g. Figure 1 a),

represents a worst case. In contrast, the most critical conditions in the case of AC and DC interference

occur on small coating defects with a design that results in lowest spread resistance, e.g. Figure 1 c). As

a consequence, these influencing parameters shall be considered. The fundamental concept of a coupon

is the mimicking of a coating defect on the structure. These coating defects can have various shapes

and sizes. Therefore, the coupon geometry should be adapted to an assumed coating defect geometry

and size present on the structure. The relevant parameters are discussed in the following clauses.

6.2 Geometry of the defect

The case of a coating defect with vertical side walls is shown in Figure 1 a). This represents the case

where the coating was locally damaged resulting in parallel walls going through the coating. The

resistance of the electrolyte within the defect gives a contribution to the spread resistance and results

in a homogeneous current distribution on the metal surface. This type of coupon, see Figure 1 a), is

least sensitive to the total surface area in the case of large values of z/y. The value y represents the

diameter of the coating defect and z represents the coating thickness. The reason for this is the

parallel current distribution caused by the constrained electrical field. The calculated average current

density is identical to the current density on the edges of the coupon. This configuration represents a

conservative assessment of the effectiveness of CP, since typical coating defects do not have vertical

parallel sides and permit higher average current densities on the steel within the coating defect.

In Figure 1 c), another extreme case of a coating defect represented by a coupon is shown. In this case,

the coupon was constructed with the metal surface flush with the encapsulating coating surface. A large

increase of the current density is observed at the edges next to the coating. This edge effect can result

in a local increase of the current density of up to a factor of 10, compared to the average current density.

This is a result of the non-parallel current distribution and the absence of a constrained electrical field.

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ISO 22426:2020(E)

When high current densities are associated with high corrosion rates (in the case of DC and/or AC

interference), locally increased metal loss is observed resulting in a heterogeneous metal loss. The

corrosion rate is significantly higher with the type of coupon indicated in Figure 1 c) compared to

the one shown in Figure 1 a). The calculated average current density in Figure 1 c) underestimates

the maximum current densities present at the edges of the metal surface. Similarly, using probes that

permit the determination of the average metal loss underestimates the maximum corrosion rate taking

place in the case of Figure 1 c) on the edges. If the structure coating thickness is low (e.g. fusion bonded

epoxy coatings), the coupon type in Figure 1 c) is relatively representative of structure coating defects.

The case in Figure 1 a) is conservative for conventional E measurements for the assessment of the

off

effectiveness of CP (measured values may be less negative than in reality). In contrast, the case in

Figure 1 c) is conservative for AC corrosion investigations, since it results in the lowest possible spread

resistance and highest local current densities; the coupon geometry in Figure 1 c) will indicate higher

AC corrosion rates than expected on a coating defect of the same size on the structure.

Figure 1 b) illustrates a compromise that may be used in all cases based on the avoidance of excessively

conservative data of the geometry in Figure 1 c) in the case of AC and/or DC interference. Similarly, in

the case of an assessment of CP effectiveness, the Figure 1 a) geometry is excessively conservative.

a) Coating defect with b) Coating defect with c) Metal surface flush
vertical side walls angled side walls with encapsulating
coating surface
Key
1 metal plate x insulated coupon surface adjacent to the bare steel surface
2 coating y coating defect diameter
J current density z coating thickness
Figure 1 — Examples of different coupon geometries and the corresponding
current density distribution
6.3 Dimension of the coupon base plate

The lateral dimension of the coupon non-metallic encapsulation mimicking the structure coating

is relevant for the spread resistance and correspondingly for the current density. This is because

the encapsulation (such as the coated structure) restricts the CP current flow in the electrolyte. In

Figure 1 a) to c), x represents the width of insulated coupon surface adjacent to the bare steel surface

and y represents the coating defect diameter. In the case of a defect on a structure, this width x would

correspond to the coating extending around the defect. This value is typically quite large. Detailed

[5]

analysis has shown that the effect of the width x is negligible when x is at least three times y (the

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ISO 22426:2020(E)

diameter of the steel surface of the coupon). If x is smaller, the current density on the coupon will be

increased compared to that of an identical coating defect on the structure.
6.4 Surface area of the coupon

Generally, increasing the coupon size results in smaller average current density since the spread

resistance decreases linearly with increasing y and the current density decreases linearly with the

surface area (i.e. 0,25∙π∙y ). As a consequence, the current density is typically underestimated when the

coupon surface area is chosen larger than the maximum defect size present on the structure. For this

reason, in the case of AC corrosion, the use of 1 cm has been established as a standard dimension in

2 2

ISO 18086. Contrarily, the use of 1 cm to 100 cm coupon surfaces may be indicated for investigating

the effectiveness of CP. The size of the coupon shall be adapted to the coating defects expected on a

given structure. It is important to note that it is not possible to prove the effectiveness of CP on a poorly

coated structure with large coating defects based on a measurement on a 1 cm coupon with a defect

geometry repres
...

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