ISO 22426:2020

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 2020
© ISO 2020
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ii © ISO 2020 – All rights reserved

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
Foreword
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iv © ISO 2020 – All rights reserved

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)
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
−2
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
E
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
E
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
E
on
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.
2 © ISO 2020 – All rights reserved

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 i
...