SIST EN ISO 18086:2021
(Main)Corrosion of metals and alloys - Determination of AC corrosion - Protection criteria (ISO 18086:2019)
Corrosion of metals and alloys - Determination of AC corrosion - Protection criteria (ISO 18086:2019)
This document specifies protection criteria for determining the AC corrosion risk of cathodically protected pipelines.
It is applicable to buried cathodically protected pipelines that are 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 document provides limits, measurement procedures, mitigation measures, and information to deal with long-term AC interference for AC voltages at frequencies between 16,7 Hz and 60 Hz and the evaluation of AC corrosion likelihood.
This document deals with the possibility of AC corrosion of metallic pipelines due to AC interferences caused by conductive, inductive 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 document 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).
Korrosion von Metallen und Legierungen - Bestimmung der Wechselstromkorrosion - Schutzkriterien (ISO 18086:2019)
Dieses Dokument legt Schutzkriterien für die Bestimmung des Wechselstromkorrosionsrisikos von kathodisch geschützten Rohrleitungen fest.
Es ist anzuwenden für erdverlegte kathodisch-geschützte Rohrleitungen, die durch Wechsel-strom Fahrleitungssysteme und/oder Wechselstromleitungen beeinflusst werden.
Bei Vorhandensein einer Beeinflussung durch Wechselstrom sind die in ISO 15589 1 angegebenen Schutzkriterien nicht ausreichend, um nachzuweisen, dass der Stahl vor Korrosion geschützt ist.
Dieses Dokument liefert Grenzwerte, Messverfahren, Schutzmaßnahmen und Informationen zum Umgang mit Langzeitbeeinflussung durch Wechselstrom für Wechselspannungen bei Frequenzen zwischen 16,7 Hz und 60 Hz und zur Beurteilung der Korrosionswahrscheinlichkeit durch Wechselstrom.
Dieses Dokument behandelt die mögliche Wechselstromkorrosion von metallischen Rohrleitungen aufgrund der Beeinflussung durch Wechselstrom der durch Wechselstromsysteme verursachten Streuströme durch ohmsche, induktive oder kapazitive Kopplung mit Wechselstromnetzen und die maximal zulässigen Grenz¬werte dieser beeinflussenden Effekte. Es wird die Tatsache berücksichtigt, dass das ein Langzeiteffekt ist, der bei normalen Betriebsbedingungen des Wechselstromsystems auftritt.
Dieses Dokument behandelt nicht die mit Wechselspannungen an Rohrleitungen verbundenen Sicherheitsaspekte. Diese werden in nationalen Normen und Rechtsvorschriften behandelt (siehe z. B. EN 50443).
Corrosion des métaux et alliages - Détermination de la corrosion occasionnée par les courants alternatifs - Critères de protection (ISO 18086:2019)
Le présent document spécifie des critères de protection pour la détermination du risque de corrosion due aux courants alternatifs pour les canalisations protégées cathodiquement.
Il s'applique aux canalisations enterrées qui sont protégées cathodiquement et influencées par des systèmes de traction à courant alternatif et/ou des lignes électriques haute tension en courant alternatif.
En présence d'une influence due aux courants alternatifs, les critères de protection spécifiés dans l'ISO 15589‑1 sont insuffisants pour démontrer que l'acier est protégé contre la corrosion.
Le présent document spécifie les limites, les modes opératoires de mesurage, les mesures d'atténuation et les informations relatives à l'influence en régime de fonctionnement permanent des courants alternatifs pour les tensions de courant alternatif à des fréquences comprises entre 16,7 Hz et 60 Hz, et à l'évaluation du risque de corrosion due aux courants alternatifs.
Le présent document traite de la possible corrosion des canalisations métalliques due à l'influence des courants alternatifs provoquée par le couplage conductif, inductif ou capacitif avec des systèmes électriques à courant alternatif, ainsi que des limites maximales admissibles des effets de cette influence. Il prend en compte le fait qu'il s'agit d'un effet à long terme qui se produit uniquement lors de conditions normales d'exploitation du système électrique à courant alternatif.
Le présent document ne traite pas des problèmes de sécurité liés aux tensions de courant alternatif sur les canalisations. Ceux-ci sont traités dans les normes et réglementations nationales (voir par exemple l'EN 50443).
Korozija kovin in zlitin - Ugotavljanje nastanka AC korozije - Merila zaščite (ISO 18086:2019)
General Information
Relations
Standards Content (Sample)
SLOVENSKI STANDARD
01-februar-2021
Nadomešča:
SIST EN ISO 18086:2017
Korozija kovin in zlitin - Ugotavljanje nastanka AC korozije - Merila zaščite (ISO
18086:2019)
Corrosion of metals and alloys - Determination of AC corrosion - Protection criteria (ISO
18086:2019)
Korrosion von Metallen und Legierungen - Bestimmung der Wechselstromkorrosion -
Schutzkriterien (ISO 18086:2019)
Corrosion des métaux et alliages - Détermination de la corrosion occasionnée par les
courants alternatifs - Critères de protection (ISO 18086:2019)
Ta slovenski standard je istoveten z: EN ISO 18086:2020
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
December 2020
EUROPÄISCHE NORM
ICS 77.060 Supersedes EN ISO 18086:2017
English Version
Corrosion of metals and alloys - Determination of AC
corrosion - Protection criteria (ISO 18086:2019)
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 18086:2019) Schutzkriterien (ISO 18086:2019)
This European Standard was approved by CEN on 13 December 2020.
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, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,
Poland, Portugal, Republic of North Macedonia, 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: Rue de la Science 23, B-1040 Brussels
© 2020 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 18086:2020 E
worldwide for CEN national Members.
Contents Page
European foreword . 3
European foreword
The text of ISO 18086:2019 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 18086:2020 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 June 2021, and conflicting national standards shall be
withdrawn at the latest by June 2021.
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.
This document supersedes EN ISO 18086:2017.
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, France, Germany, Greece, Hungary, Iceland,
Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Republic of
North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and the
United Kingdom.
Endorsement notice
The text of ISO 18086:2019 has been approved by CEN as EN ISO 18086:2020 without any
modification.
INTERNATIONAL ISO
STANDARD 18086
Second edition
2019-12
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 18086:2019(E)
©
ISO 2019
ISO 18086:2019(E)
© ISO 2019
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
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Published in Switzerland
ii © ISO 2019 – All rights reserved
ISO 18086:2019(E)
Contents Page
Foreword .v
Introduction .vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Cathodic protection persons competence . 5
5 Assessment of the AC influence . 5
5.1 General . 5
5.2 Assessment of the level of interference . 6
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 . 7
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 . 8
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 . 9
7 Acceptable interference levels . 9
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 .11
8.4 Measurements on coupons and probes .11
8.4.1 Installation of coupons or probes .11
8.4.2 Current measurements.12
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 .14
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 18086:2019(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 .18
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 2019 – All rights reserved
ISO 18086:2019(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 metal and alloys.
This second edition cancels and replaces the first edition (ISO 18086:2015), of which it constitutes a
minor revision. The changes compared to the previous edition are as follows:
— references cited informatively (EN 13509 and EN 15257) have been moved from Clause 2 to the
Bibliography;
— in Clause 7, the two instances of the phrase “AC current density” have been changed to “AC average
current density”.
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.
ISO 18086:2019(E)
Introduction
This document 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:
— in the presence of “low” on-potentials, which allows a certain level of AC voltage (up to 15 V);
— in the 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 document 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
the commissioning of any AC corrosion mitigation system. Annex E proposes other parameters and
thresholds that require further validation based on practical experiences.
vi © ISO 2019 – All rights reserved
INTERNATIONAL STANDARD ISO 18086:2019(E)
Corrosion of metals and alloys — Determination of AC
corrosion — Protection criteria
1 Scope
This document specifies protection criteria for determining the AC corrosion risk of cathodically
protected pipelines.
It is applicable to buried cathodically protected pipelines that are 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 document provides limits, measurement procedures, mitigation measures, and information to
deal with long-term AC interference for AC voltages at frequencies between 16,7 Hz and 60 Hz and the
evaluation of AC corrosion likelihood.
This document deals with the possibility of AC corrosion of metallic pipelines due to AC interferences
caused by conductive, inductive 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 document 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 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 8044, Corrosion of metals and alloys — Basic terms and definitions
ISO 15589-1, Petroleum, petrochemical and natural gas industries — Cathodic protection of pipeline
systems — Part 1: On-land pipelines
IEC 61010-1, Safety requirements for electrical equipment for measurement, control, and laboratory use —
Part 1: General requirements
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.
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/
ISO 18086:2019(E)
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;
— 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, which includes overhead lines, cables,
substations and all apparatus associated with them
3.3
AC power system
AC electric traction system (3.1) or AC power supply system (3.2)
Note 1 to entry: Where it is necessary to differentiate, each interfering system (3.6) 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 (3.9) between a metallic structure and a reference electrode
3.6
interfering system
general expression encompassing an interfering high voltage AC electric traction system (3.1) and/or
high voltage AC power supply system (3.2)
3.7
interfered system
system on which the interference (3.15) effects appear
Note 1 to entry: In this document, it is the pipeline system (3.8).
3.8
pipeline system
system of pipe network with all associated equipment and stations
Note 1 to entry: In this document, 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, of which the electric potential at any point is conventionally taken as
equal to zero
[SOURCE: IEC 60050-826]
2 © ISO 2019 – All rights reserved
ISO 18086:2019(E)
3.10
operating condition
fault-free operation of any system
Note 1 to entry: Transients are not to be considered as an operating condition.
3.11
fault condition
non-intended condition caused by a short-circuit to earth (3.9), the fault duration being the normal
clearing time of the protection devices and switches
Note 1 to entry: A 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 that occurs when a proportion of the current belonging to the interfering system (3.6) returns
to the system earth (3.9) via the interfered system (3.7) 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: Coupling is quantified by the mutual impedance of the two circuits. 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: Coupling is quantified by the capacitance between the conductors and the capacitances between
each conductor and the earth (3.9). The results of which are interference (3.15) 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, inductive or capacitive coupling (3.12, 3.13, 3.14) between
systems, which can cause malfunction, dangerous voltages, damage (3.17), etc.
3.16
disturbance
malfunction of a piece of equipment that loses its capability to work properly for the duration of the
interference (3.15)
Note 1 to entry: When the interference disappears, the interfered system (3.7) starts working properly again
without any external intervention.
3.17
damage
permanent reduction in the quality of service that can be suffered by the interfered system (3.7)
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.
ISO 18086:2019(E)
3.18
danger
state of the influenced system that is able to produce a threat to human life
3.19
interference situation
maximum distance between the pipeline system (3.8) and AC power system for which an interference
(3.15) is to be considered
3.20
interference voltage
voltage caused on the interfered system (3.7) by the conductive, inductive or capacitive coupling (3.12,
3.13, 3.14) with the nearby interfering system (3.6) between a given point and the earth (3.9) 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 × R)
3.22
IR-free potential
E
IR-free
pipe to electrolyte potential measured without the voltage error caused by the IR drop (3.21) 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 (3.22)
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 (3.9) or from the exposed metallic surface of a
coupon (3.26) towards earth
Note 1 to entry: This is the resistance which controls the AC or DC current through a coating defect or an exposed
metallic surface of a coupon for a given AC or DC voltage.
3.26
coupon
metal sample of defined dimensions made of a metal equivalent to the metal of the pipeline
3.27
probe
device incorporating a coupon (3.26) that provides measurements of parameters to assess the
effectiveness of cathodic protection and/or corrosion risk
4 © ISO 2019 – All rights reserved
ISO 18086:2019(E)
4 Cathodic protection persons competence
Persons who undertake the design, supervision of installation, commissioning, supervision of operation,
measurements, monitoring and supervision of the maintenance of cathodic protection systems shall
have the appropriate level of competence for the tasks undertaken.
EN 15257 or the NACE Cathodic Protection Training and Certification Programme constitute suitable
methods of assessing and certifying the competence of cathodic protection personnel.
The 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, the
NACE Cathodic Protection Training and Certification Programme, or any other equivalent scheme.
5 Assessment of the AC influence
5.1 General
This document 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 of which the 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.
ISO 18086:2019(E)
5.2 Assessment of the level of interference
Calculations can be carried out (e.g. in accordance with 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.
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 [8]. 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 helps 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 the 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 the 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 the 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.
6 © ISO 2019 – All rights reserved
ISO 18086:2019(E)
Annexes B, C and 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.
6.2 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.
ISO 18086:2019(E)
Annexes A and 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.
Annexes A and 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 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 that it is very difficult in practice to ensure that there are no coating
defects on a pipeline.
8 © ISO 2019 – All rights reserved
ISO 18086:2019(E)
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.
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 ach
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