EN ISO 18086:2017

SLOVENSKI STANDARD
01-november-2017
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Corrosion of metals and alloys - Determination of AC corrosion - Protection criteria (ISO
18086:2015)
Korrosion von Metallen und Legierungen - Bestimmung der Wechselstromkorrosion -
Schutzkriterien (ISO 18086:2015)
Corrosion des métaux et alliages - Détermination de la corrosion occasionnée par les
courants alternatifs - Critères de protection (ISO 18086:2015)
Ta slovenski standard je istoveten z: EN ISO 18086:2017
ICS:
77.060 Korozija kovin Corrosion of metals
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EN ISO 18086
EUROPEAN STANDARD
NORME EUROPÉENNE
September 2017
EUROPÄISCHE NORM
ICS 77.060
English Version
Corrosion of metals and alloys - Determination of AC
corrosion - Protection criteria (ISO 18086:2015)
Corrosion des métaux et alliages - Détermination de la Korrosion von Metallen und Legierungen -
corrosion occasionnée par les courants alternatifs - Bestimmung der Wechselstromkorrosion -
Critères de protection (ISO 18086:2015) Schutzkriterien (ISO 18086:2015)
This European Standard was approved by CEN on 23 August 2017.

CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this
European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references
concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CEN
member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by
translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC Management
Centre has the same status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania,
Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Turkey and United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Avenue Marnix 17, B-1000 Brussels
© 2017 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 18086:2017 E
worldwide for CEN national Members.

Contents Page
European foreword . 3

European foreword
The text of ISO 18086:2015 has been prepared by Technical Committee ISO/TC 156“Corrosion of
metals and alloys” of the International Organization for Standardization (ISO) and has been taken over
as EN ISO 18086:2017 by Technical Committee CEN/TC 219 “Cathodic protection” the secretariat of
which is held by BSI.
This European Standard shall be given the status of a national standard, either by publication of an
identical text or by endorsement, at the latest by March 2018, and conflicting national standards shall
be withdrawn at the latest by March 2018.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN shall not be held responsible for identifying any or all such patent rights.
According to the CEN-CENELEC Internal Regulations, the national standards organizations of the
following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria,
Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, Former Yugoslav Republic of Macedonia,
France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta,
Netherlands, Norway, Poland, Portugal, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Turkey and the United Kingdom.
Endorsement notice
The text of ISO 18086:2015 has been approved by CEN as EN ISO 18086:2017 without any modification.

INTERNATIONAL ISO
STANDARD 18086
First edition
2015-06-01
Corrosion of metals and alloys —
Determination of AC corrosion —
Protection criteria
Corrosion des métaux et alliages — Détermination de la corrosion
occasionnée par les courants alternatifs — Critères de protection
Reference number
ISO 18086:2015(E)
©
ISO 2015
ISO 18086:2015(E)
© ISO 2015, Published in Switzerland
All rights reserved. Unless otherwise specified, 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
Ch. de Blandonnet 8 • CP 401
CH-1214 Vernier, Geneva, Switzerland
Tel. +41 22 749 01 11
Fax +41 22 749 09 47
copyright@iso.org
www.iso.org
ii © ISO 2015 – All rights reserved

ISO 18086:2015(E)
Contents Page
Foreword .v
Introduction .vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Cathodic protection persons competence . 4
5 Assessment of the AC influence . 5
5.1 General . 5
5.2 Assessment of the level of interference . 5
6 Evaluation of the AC corrosion likelihood. 6
6.1 Prerequisite . 6
6.1.1 General. 6
6.1.2 AC voltage on the structure . 6
6.2 AC and DC current density . 7
6.2.1 General. 7
6.2.2 AC current density . 7
6.2.3 High cathodic DC current density . 7
6.2.4 Low cathodic DC current density . 7
6.2.5 Current ratio “I /I ” . 8
a.c. d.c
6.2.6 Soil resistivity . 8
6.3 Corrosion rate . 8
6.4 Pipeline coatings . 8
6.5 Evaluation of the metal loss . 8
7 Acceptable interference levels . 8
8 Measurement techniques . 9
8.1 Measurements . 9
8.1.1 General. 9
8.1.2 Selection of test sites . 9
8.1.3 Selection of measurement parameter .10
8.1.4 Sampling rate for the recording of interference levels .10
8.1.5 Accuracy of measuring equipment.10
8.1.6 Installation of coupons or probes to calculate current densities.10
8.2 DC potential measurements .10
8.3 AC voltage measurements .10
8.4 Measurements on coupons and probes .11
8.4.1 Installation of coupons or probes .11
8.4.2 Current measurements.11
8.4.3 Corrosion rate measurements.12
8.5 Pipeline metal loss techniques .13
9 Mitigation measures .13
9.1 General .13
9.2 Construction measures .13
9.2.1 Modification of bedding material .13
9.2.2 Installation of isolating joints .13
9.2.3 Installation of mitigation wires .13
9.2.4 Optimization of pipeline and/or powerline route .14
9.2.5 Power line or pipeline construction.14
9.3 Operation measures .14
9.3.1 Earthing .14
9.3.2 Adjustment of cathodic protection level .15
9.3.3 Repair of coating defects .15
ISO 18086:2015(E)
10 Commissioning .16
10.1 Commissioning .16
10.2 Preliminary checking .16
10.2.1 General.16
10.2.2 Coupon AC voltage and current startup .17
10.2.3 Verification of effectiveness.17
10.2.4 Installation and commissioning documents .17
11 Monitoring and maintenance .17
Annex A (informative) Simplified description of the AC corrosion phenomenon .19
Annex B (informative) Coupons and probes .21
Annex C (informative) Coulometric oxidation .26
Annex D (informative) Influence of soil characteristics on the AC corrosion process .27
Annex E (informative) Other criteria that have been used in the presence of AC influence .28
Annex F (informative) Parameters to take into account to choose a DC decoupling device .32
Annex G (informative) Method to determine the reference electrode location to remote earth .34
Annex H (informative) Simultaneous measurement on coupon current densities with high rate .36
Bibliography .38
iv © ISO 2015 – All rights reserved

ISO 18086:2015(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 on the meaning of ISO specific terms and expressions related to conformity
assessment, as well as information about ISO’s adherence to the WTO principles in the Technical Barriers
to Trade (TBT), see the following URL: Foreword — Supplementary information.
The committee responsible for this document is ISO/TC 156, Corrosion of metal and alloys.
ISO 18086:2015(E)
Introduction
This International Standard has incorporated criteria and thresholds together with experience gained
from the most recent data. Various countries have a very different approach to the prevention of
AC corrosion depending primarily on the DC interference situation. These different approaches are
taken into account in two different ways
— either in presence of “low” on-potentials, which allows a certain level of AC voltage (up to 15 V), or
— in presence of “high” on-potentials (with DC stray current interference on the pipeline for instance)
which requires the reduction of the AC voltage towards the lowest possible levels.
This International Standard also gives some parameters to consider when evaluating the AC corrosion
likelihood, as well as detailed measurement techniques, mitigation measures, and measurements to
carry out for commissioning of any AC corrosion mitigation system. Note that Annex E proposes other
parameters and thresholds that require further validation based on practical experiences.
vi © ISO 2015 – All rights reserved

INTERNATIONAL STANDARD ISO 18086:2015(E)
Corrosion of metals and alloys — Determination of AC
corrosion — Protection criteria
1 Scope
This International Standard is applicable to buried cathodically-protected pipeline that is influenced by
AC traction systems and/or AC power lines.
In the presence of AC interference, the protection criteria given in ISO 15589-1 are not sufficient to
demonstrate that the steel is being protected against corrosion.
This International Standard provides limits, measurement procedures, mitigation measures, and
information to deal with long term AC interference for AC voltages at frequencies between 16,7 and
60 Hz and the evaluation of AC corrosion likelihood.
This International Standard deals with the possibility of AC corrosion of metallic pipelines due to
AC interferences caused by inductive, conductive or capacitive coupling with AC power systems and the
maximum tolerable limits of these interference effects. It takes into account the fact that this is a long-
term effect, which occurs during normal operating conditions of the AC power system.
This International Standard does not cover the safety issues associated with AC voltages on pipelines.
These are covered in national standards and regulations (see e.g. EN 50443).
2 Normative references
The following referenced documents are indispensable for the application 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
ISO 8044, Corrosion of metals and alloys — Basic terms and definitions
IEC 61010-1, Safety requirements for electrical equipment for measurements, control, and laboratory use —
Part 1: General requirements
EN 13509, Cathodic protection measurement techniques
EN 15257, Cathodic protection — Competence levels and certification of cathodic protection personnel
EN 50443, Effects of electromagnetic interference on pipelines caused by high voltage AC electric traction
systems and/or high voltage AC power supply systems
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 8044 and the following apply.
3.1
AC electric traction system
AC railway electrical distribution network used to provide energy for rolling stock
Note 1 to entry: The system can comprise the following:
— contact line systems;
ISO 18086:2015(E)
— return circuit of electric railway systems;
— running rails of non-electric railway systems, which are in the vicinity of and conductively connected to the
running rails of an electric railway system.
3.2
AC power supply system
AC electrical system devoted to electrical energy transmission and includes overhead lines, cables,
substations and all apparatus associated with them
3.3
AC power system
AC electric traction system or AC power supply system
Note 1 to entry: Where it is necessary to differentiate, each interfering system is clearly indicated with its proper
term.
3.4
copper/copper sulfate reference electrode
CSE
reference electrode consisting of copper in a saturated solution of copper sulfate
3.5
AC voltage
voltage measured to earth between a metallic structure and a reference electrode
3.6
interfering system
general expression encompassing an interfering high voltage AC electric traction system and/or high
voltage AC power supply system
3.7
interfered system
system on which the interference effects appear
Note 1 to entry: In this International Standard, it is the pipeline system.
3.8
pipeline system
system of pipe network with all associated equipment and stations
Note 1 to entry: In this International Standard, pipeline system refers only to metallic pipeline system.
Note 2 to entry: The associated equipment is the equipment electrically connected to the pipeline.
3.9
earth
conductive mass of the earth, whose electric potential at any point is conventionally taken as equal to
zero
[SOURCE: IEC 60050 826]
3.10
operating condition
fault-free operation of any system
Note 1 to entry: Transients are not to be considered as an operating condition.
2 © ISO 2015 – All rights reserved

ISO 18086:2015(E)
3.11
fault condition
non-intended condition caused by short-circuit to earth, the fault duration being the normal clearing
time of the protection devices and switches
Note 1 to entry: The short circuit is an unintentional connection of an energized conductor to earth or to any
metallic part in contact with earth.
3.12
conductive coupling
coupling which occurs when a proportion of the current belonging to the interfering system returns to
the system earth via the interfered system or when the voltage to the reference earth of the ground in
the vicinity of the influenced object rises because of a fault in the interfering system and the results of
which are conductive voltages and currents
3.13
inductive coupling
phenomenon whereby the magnetic field produced by a current carrying circuit influences another
circuit
Note 1 to entry: The coupling being quantified by the mutual impedance of the two circuits, and the results
of which are induced voltages and hence currents that depend on, for example, the distances, length, inducing
current, circuit arrangement, and frequency.
3.14
capacitive coupling
phenomenon whereby the electric field produced by an energized conductor influences another
conductor
Note 1 to entry: The coupling being quantified by the capacitance between the conductors and the capacitances
between each conductor and earth, and the results of which are interference voltages into conductive parts or
conductors insulated from earth, these voltages depend, for example on the voltage of the influencing system,
distances, and circuit arrangement.
3.15
interference
phenomenon resulting from conductive, capacitive, inductive coupling between systems and which can
cause malfunction, dangerous voltages, damage, etc.
3.16
disturbance
malfunction of an equipment losing its capability to work properly for the duration of the interference
Note 1 to entry: When the interference disappears, the interfered system starts working properly again without
any external intervention.
3.17
damage
permanent reduction in the quality of service which can be suffered by the interfered system
Note 1 to entry: A reduction in the quality of service could also be the complete cancellation of service.
EXAMPLE Coating perforation, pipe pitting, pipe perforation, permanent malfunction of the equipment
connected to the pipes, etc.
3.18
danger
state of the influenced system which is able to produce a threat to human life
ISO 18086:2015(E)
3.19
interference situation
maximum distance between the pipeline system and AC power system for which an interference is to
be considered
3.20
interference voltage
voltage caused on the interfered system by the conductive, inductive, and capacitive coupling with the
nearby interfering system between a given point and the earth or across an insulating joint
3.21
IR drop
voltage due to any current, developed in an electrolyte such as the soil, between the reference electrode
and the metal of the structure, in accordance with Ohm’s Law (U = I x R)
3.22
IR-free potential
E
IR-free
pipe to electrolyte potential measured without the voltage error caused by the IR drop due to the
protection current or any other current
3.23
off-potential
E
off
pipe to electrolyte potential measured after interruption of all sources of applied cathodic protection
current with the aim of approaching an IR-free potential
Note 1 to entry: The delay before measurement varies according to circumstances.
3.24
on-potential
E
on
pipe to electrolyte potential measured while the cathodic protection system is continuously operating
3.25
spread resistance
ohmic resistance through a coating defect to earth or from the exposed metallic surface of a coupon
towards earth
Note 1 to entry: This is the resistance which controls the DC or AC current through a coating defect or an exposed
metallic surface of a coupon for a given DC or AC voltage.
3.26
coupon
metal sample of defined dimensions made of a metal equivalent to the metal of the pipeline
3.27
probes
device incorporating a coupon that provides measurements of parameters to assess the effectiveness of
cathodic protection and/or corrosion risk
4 Cathodic protection persons competence
Persons who undertake the design, supervision of installation, commissioning, supervision of operation,
measurements, monitoring, and supervision of maintenance of cathodic protection systems shall have
the appropriate level of competence for the tasks undertaken.
EN 15257 or NACE Cathodic Protection Training and Certification Programme constitute suitable
methods of assessing and certifying competence of cathodic protection personnel.
4 © ISO 2015 – All rights reserved

ISO 18086:2015(E)
Competence of cathodic protection persons to the appropriate level for tasks undertaken should be
demonstrated by certification in accordance with prequalification procedures such as EN 15257, NACE
Cathodic Protection Training and Certification Programme, or any other equivalent scheme.
5 Assessment of the AC influence
5.1 General
This International Standard is applicable to all metallic pipelines and all high voltage AC traction systems
and high voltage AC power supply systems and all major modifications that can significantly change the
AC interference effect.
The effects are the following:
— danger to people who come in direct contact or contact through conductive parts with the metallic
pipeline or the connected equipment;
— damage of the pipeline or to the connected equipment;
— disturbance of electrical/electronic equipment connected to the pipeline.
Electrical/electronic systems installed on a pipeline network shall be chosen, such that they will neither
become dangerous nor interfere with normal operating conditions because of short-term voltages and
currents, which appear during short circuits on the AC power system.
Long term AC interference on a buried pipeline can cause corrosion due to an exchange of AC current
between the exposed metal of the pipeline and the surrounding electrolyte.
This exchange of current depends on an AC voltage whose amplitude is related to various parameters
such as the following:
— configuration of AC power line phase conductors;
— presence and configuration of the earthing conductor;
— distance between the AC power line/traction system and the pipeline;
— current flowing in the AC power line/traction system phase conductors;
— average coating resistance of the pipeline;
— thickness of the coating;
— soil resistivity;
— presence of earthing systems;
— voltage of the AC railway system or the AC power line system.
5.2 Assessment of the level of interference
Calculations can be carried out (e.g. according to EN 50443) by mathematical modelling to determine
the earthing requirements necessary to maintain touch voltages within acceptable safe levels. Their
results can also be used to determine voltages necessary to reduce the AC corrosion likelihood.
During the design phase of new influencing systems (electricity power line or railway line) or a new
influenced system (pipelines), an estimation of the level of AC voltage on the pipeline should be calculated.
Calculations can be carried out by mathematical modelling to determine the level of voltage produced
on the pipeline. In the case of existing structures, field measurements can also be used as an option to
calculation.
ISO 18086:2015(E)
According to the results of calculations or field measurements, relevant mitigation measures should be
installed on the influencing systems and/or the influenced system to achieve the relevant AC voltage to
reduce the AC corrosion likelihood (see Clause 7.).
Guidance on calculating the AC voltage on a structure caused by an AC power system was published in
Reference [6] The algorithm determines the worst case conditions for the input parameters used for the
calculation.
Due to inconsistent load demands on AC power systems, the magnitude of operating currents in power
lines varies. The fluctuations depend on daily and seasonal changes. Input data for calculation purposes
should be based on the realistic operating conditions or the maximum power load of the influencing
system.
NOTE Carrying out calculations with input data based on both approaches is a help to estimate the range
between both results and to choose the right method.
6 Evaluation of the AC corrosion likelihood
6.1 Prerequisite
6.1.1 General
The AC voltage on a pipeline is the driving force for the AC corrosion processes taking place on the steel
surface at coating defects. Among other things, corrosion damage depends on AC current density, level
of DC polarization, defect geometry, local soil composition and resistivity (see Annex D).
Basically, there are three different approaches to prevent AC corrosion: to limit the AC current flowing
through a defect, to control cathodic protection level, and to ensure that any coating remains defect free.
These approaches are not mutually exclusive.
The evaluation of AC corrosion likelihood should be performed by evaluation of some or all of the
following parameters:
— AC voltage on the structure;
— on-potential;
— IR-free potential;
— AC current density;
— DC current density;
— AC/DC current density ratio;
— soil resistivity;
— corrosion rate.
Annex B, Annex C, and Annex E provide further information.
6.1.2 AC voltage on the structure
The acceptable AC voltage thresholds (see Clause 7 and Annex E) depend on the chosen strategy to
prevent AC corrosion. Hence, a given interference situation on the pipeline can influence the decision
regarding the applicable strategy.
6 © ISO 2015 – All rights reserved

ISO 18086:2015(E)
6.2 AC and DC current density
6.2.1 General
The AC and DC current density on a coating defect controls both the cathodic protection level and
AC corrosion process. Therefore, it is a more reliable parameter for the evaluation of the AC corrosion
likelihood than the on-potential or the AC voltage. However, in contrast to the voltages present on
the pipeline, the current density cannot be readily determined. In principle, the current density can
be calculated from the spread resistance and the geometry of the coating defect and the AC voltage.
This calculation is generally not possible since the geometry of the coating fault and its surface area
are generally not known. Moreover, the application of cathodic protection can significantly change the
spread resistance and therefore, the current density at a given voltage.
The current density can only be estimated by means of coupons or probes. When evaluating the
AC corrosion likelihood by means of a coupon or probe, it is important to consider the limitations of
this technique. The calculation of the current density based upon the metallic coupon or probe surface
area and on the current measured on a coupon or probe, the current is averaged over the entire coupon
or probe surface. However, the current distribution on the coupon or probe can vary depending on its
geometry. Typically, current densities at the edges of the coupon or probe are larger than the current
averaged over the entire surface. Moreover, the often observed formation of chalk layers can decrease
the effective coupon or probe surface area. Again, this effect results in an under estimation of the current
density.
6.2.2 AC current density
The AC current density results in anodic and cathodic charge transfer. A detailed explanation of the
charge transfer process is given in Annex A. This current can be consumed in charging of the double
layer capacitance at the steel surface, in the oxidation of hydrogen (resulting in a decreasing pH), in the
oxidation of corrosion products, and in the oxidation of the metal. The oxidation of the metal results in
corrosion. Generally, an increasing AC current density results in a larger amount of metal oxidation and
higher corrosion rates. However, the anodic current is not the only current that can affect the corrosion
process. Cathodic current can reduce oxide layers formed and increase the pH on the metal surface.
High AC current densities do not necessarily cause AC corrosion if the charge passed through the metal
surface can be consumed in reactions other than metal oxidation and oxide film reduction. This is
the case in the presence of low cathodic DC current densities. As a consequence, the judgment of the
AC corrosion likelihood based on the AC current density requires the additional consideration of the
cathodic DC current density.
Nevertheless, there is an empirically determined lower limit for the AC current density below which the
probability for AC corrosion is extremely low (see Clause 7).
6.2.3 High cathodic DC current density
A high DC current density results in more negative cathodic protection levels and the formation of a
high pH at the pipeline surface. However, the formation of a high pH-value, the decrease of the spread
resistance, and the increased reduction of surface oxide films can result in an acceleration of the
corrosion rate under simultaneous AC interference. Nevertheless, a sufficiently high DC current density
can prevent any anodic metal oxidation and therefore, the occurrence of AC corrosion.
Annex A and Annex E give detailed explanations about this process.
6.2.4 Low cathodic DC current density
A low DC current density results in a limited increase of the pH value at the metal surface, does not
significantly change the spread resistance, and has less reductive effect on metal oxides on the pipeline
surface. Therefore, the AC corrosion likelihood significantly decreases with decreasing DC current
densities. However, low DC current densities can result in an insufficient level of cathodic polarization
of the metal surface as stated in ISO 15589-1.
ISO 18086:2015(E)
Annex A and Annex E give detailed explanations about this process.
6.2.5 Current ratio “I /I ”
a.c. d.c
High DC current densities, depending on the AC current density, can result in both high and low
AC corrosion rates. Hence, the ratio of the two current densities may be used to assess the corrosion
likelihood. As long as the ratio is below a certain threshold (see Annex E), no AC corrosion can occur
since metal oxidation in the anodic half wave is prevented. The key advantage of using the ratio as an
indicator of corrosion likelihood is that the uncertainties regarding the condition of the metal surface
(e.g. formation of a chalk layer) are eliminated since the precise metal surface area is not required for
the calculation.
6.2.6 Soil resistivity
The AC corrosion process is controlled by the current density on a steel coating defect, which depends
on the voltage at the location and the spread resistance. The spread resistance is influenced by the soil
resistivity. The following soil resistivity parameters have been determined by experience in terms of AC
corrosion risk:
— below 25 Ω.m: very high risk;
— between 25 Ω.m and 100 Ω.m: high risk;
— between 100 Ω.m and 300 Ω.m: medium risk;
— above 300 Ω.m: low risk.
For further guidance on the effect of soil composition on AC corrosion risk, Annex D gives a more detailed
information.
6.3 Corrosion rate
A direct way of evaluating the AC corrosion likelihood is by determining the corrosion rate on a probe
(see 8.4.3). This allows complex interference situations to be assessed on the basis of the actual measured
corrosion rate. The principles of the Electrical Resistance (ER) probe concept are described in Annex B.
6.4 Pipeline coatings
AC corrosion can only take place on metal surfaces that are in contact with the surrounding soil. The
AC current passing through the metal/soil interface results in oxidation of the metal. By providing a
holiday-free coating, the risk of AC corrosion is greatly reduced.
NOTE This method is limited by the fact it is very difficult in practice to ensure that there are no coating
defects on a pipeline.
6.5 Evaluation of the metal loss
Metal loss measurement tools, such as internal inspection, can be used to verify the effectiveness of the
applied mitigation measures on new pipelines and to identify if any external metal loss has occurred on
existing pipelines without mitigation.
NOTE The resolution in terms of width and depth of the In-Line Inspection (ILI) tool is a crucial parameter to
be considered to detect metal loss (such as AC corrosion).
7 Acceptable interference levels
The design, installation, and maintenance of the cathodic protection system shall ensure that the levels
of AC voltage do not cause AC corrosion. Since the conditions vary for each situation, a single threshold
value cannot be applied.
8 © ISO 2015 – All rights reserved

ISO 18086:2015(E)
This is achieved by reducing the AC voltage on the pipeline and current densities as specified below.
— As a first step, the AC voltage on the pipeline should be decreased to a target value, which should
be 15 V rms or less. This value is measured as an average over a representative period of time
(e.g. 24 h).
— As a second step, effective AC corrosion mitigation can be achieved by meeting the cathodic
protection potentials defined in ISO 15589-1:2015, Table 1 and
— maintaining the AC current density (rms) over a representative period of time (e.g. 24 h) to be
2 2
lower than 30 A/m on a 1 cm coupon or probe, or
— maintaining the average cathodic current density over a representative period of time (e.g. 24 h)
2 2 2
lower than 1 A/m on a 1 cm coupon or probe if AC current density (rms) is more than 30 A/m ,
or
— maintaining the ratio between AC current density (J ) and DC current density (J ) less than
a.c. d.c.
5 over a representative period of time (e.g. 24 h).
NOTE Current density ratios between 3 and 5 indicate a small risk of AC corrosion. However, in order to
reduce the corrosion risk to a minimum value, smaller ratios of current density than 3 would be preferable (see
Annex E).
Further information is provided in Annex E.
Effective AC corrosion mitigation can be also demonstrated by measurement of corrosion rate.
8 Measurement techniques
8.1 Measurements
8.1.1 General
This clause covers techniques related to the measurements
...