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
Standards Content (Sample)
Assessment of the effectiveness of
cathodic protection based on coupon
Evaluation de l’efficacité de la protection cathodique par mesurages
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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
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INTERNATIONAL STANDARD ISO 22426:2020(E)
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.
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/
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
medium in which an electric current is transported by ions
Note 1 to entry: Electrolyte is synonymous with soil, backfill and water.
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).
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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change of an electrode [e.g. structure (3.13) and/or coupon (3.14)] potential due to charge transfer
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
(3.5) of an electrode is from 10 s to 10 s. Therefore, usually all activation polarization is lost within 0,1 s after
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)
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
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).
electrode [e.g. structure (3.13) and/or coupon (3.14)] to electrolyte (3.2) potential measured while the
cathodic protection system is energized
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
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
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metallic structure intended to receive cathodic protection
metal sample of defined dimensions and shape made of a metal equivalent to the metal of the
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.
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.
electrical resistance 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.
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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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
— 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
— 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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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
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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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
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
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
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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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
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