Evaluation of a.c. corrosion likelihood of buried pipelines - Application to cathodically protected pipelines

This Technical Specification is applicable to buried or immersed cathodically protected metallic structures and influenced by a.c. traction systems and/or a.c. power lines.
   In the presence of a.c. interference, the criteria given in EN 12954:2001, Table 1, are not sufficient to demonstrate that the steel is being protected against corrosion.
   This Technical Specification provides limits, measurements procedures and information to deal with long term a.c. interference and evaluating a.c. corrosion likelihood.
   Even though short term interference can cause damage to buried pipelines (e.g. arc fusion), this Technical Specification does not deal with short term interference.

Beurteilung der Korrosionswahrscheinlichkeit durch Wechselstrom an erdverlegten Rohrleitungen - Anwendung für kathodisch geschützte Rohrleitungen

Diese Technische Spezifikation gilt für erdverlegte oder unterirdische bzw. unter Wasser befindliche metallische Anlagen, die kathodisch geschützt sind und durch Wechselstrom-Fahrleitungen und/oder Wechselstrom-Überlandleitungen beeinflusst werden.
Eine erdverlegte Rohrleitung (oder Anlage) im Sinne dieses Dokuments ist eine Rohrleitung (oder Anlage) in Böden oder Wässern, wie in EN 12954 festgelegt.
Bei Anwesenheit einer Wechselstrombeeinflussung sind die in EN 12954:2001, Tabelle 1, gegebenen Kriterien nicht ausreichend, um nachzuweisen, dass der Stahl gegen Korrosion geschützt ist.
Diese Technische Spezifikation gibt Grenzwerte, Messverfahren und Information zum Umgang mit Langzeitwechselstromeinfluss und zur Beurteilung der Wahrscheinlichkeit von Korrosion durch Wechselstrom verursachten Korrosion an.
Obwohl bereits kurze Beeinflussung Schäden an erdverlegten Rohrleitungen bewirken kann (z. B. Verschmelzung durch Lichtbogen), gilt diese Technische Spezifikation nicht für Kurzzeitbeeinflussung.

Evaluation du risque de corrosion des canalisations enterrées occasionné par les courants alternatifs - Application aux canalisations protégées cathodiquement

La présente Spécification technique est applicable aux structures métalliques 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.
Dans le présent document, une canalisation (ou une structure) enterrée est une canalisation (ou une structure) enterrée ou immergée au sens de l’EN 12954.
En présence d’une  influence due aux courants alternatifs, les critères spécifiés dans le tableau 1 de l’EN 12954 sont insuffisants pour démontrer que l’acier est protégé contre la corrosion.
La présente Spécification technique spécifie les limites, les modes opératoires de mesure et les informations relatives à l’influence permanente par les courants alternatifs et à l’évaluation du risque de corrosion engendré par ces courants.
   Bien qu’une perturbation accidentelle puisse provoquer des dommages sur les canalisations enterrées (par exemple une fusion par arc électrique), la présente Spécification technique ne traite pas de ces phénomènes.

Vrednotenje verjetnosti nastanka korozije vkopanih cevovodov – Uporaba pri katodno zaščitenih cevovodih

General Information

Status
Withdrawn
Publication Date
14-Mar-2006
Withdrawal Date
13-Aug-2013
Current Stage
9960 - Withdrawal effective - Withdrawal
Due Date
14-Aug-2013
Completion Date
14-Aug-2013

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SLOVENSKI STANDARD
SIST-TS CEN/TS 15280:2006
01-maj-2006
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Evaluation of a.c. corrosion likelihood of buried pipelines - Application to cathodically

protected pipelines
Beurteilung der Korrosionswahrscheinlichkeit durch Wechselstrom an erdverlegten
Rohrleitungen - Anwendung für kathodisch geschützte Rohrleitungen
Evaluation du risque de corrosion des canalisations enterrées occasionné par les
courants alternatifs - Application aux canalisations protégées cathodiquement
Ta slovenski standard je istoveten z: CEN/TS 15280:2006
ICS:
23.040.01 Deli cevovodov in cevovodi Pipeline components and
na splošno pipelines in general
77.060 Korozija kovin Corrosion of metals
SIST-TS CEN/TS 15280:2006 en

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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SIST-TS CEN/TS 15280:2006
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SIST-TS CEN/TS 15280:2006
TECHNICAL SPECIFICATION
CEN/TS 15280
SPÉCIFICATION TECHNIQUE
TECHNISCHE SPEZIFIKATION
March 2006
ICS 23.040.99; 77.060
English Version
Evaluation of a.c. corrosion likelihood of buried pipelines -
Application to cathodically protected pipelines

Evaluation du risque de corrosion des canalisations Beurteilung der Korrosionswahrscheinlichkeit durch

enterrées occasionné par les courants alternatifs - Wechselstrom an erdverlegten Rohrleitungen - Anwendung

Application pour les canalisations protégées für kathodisch geschützte Rohrleitungen

cathodiquement

This Technical Specification (CEN/TS) was approved by CEN on 8 November 2005 for provisional application.

The period of validity of this CEN/TS is limited initially to three years. After two years the members of CEN will be requested to submit their

comments, particularly on the question whether the CEN/TS can be converted into a European Standard.

CEN members are required to announce the existence of this CEN/TS in the same way as for an EN and to make the CEN/TS available

promptly at national level in an appropriate form. It is permissible to keep conflicting national standards in force (in parallel to the CEN/TS)

until the final decision about the possible conversion of the CEN/TS into an EN is reached.

CEN members are the national standards bodies of Austria, Belgium, Cyprus, Czech Republic, Denmark, Estonia, Finland, France,

Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania,

Slovakia, Slovenia, Spain, Sweden, Switzerland and United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
Management Centre: rue de Stassart, 36 B-1050 Brussels

© 2006 CEN All rights of exploitation in any form and by any means reserved Ref. No. CEN/TS 15280:2006: E

worldwide for CEN national Members.
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CEN/TS 15280:2006 (E)
Contents Page

Foreword ..........................................................................................................................................................3

1 Scope ...................................................................................................................................................4

2 Normative references .........................................................................................................................4

3 Terms and definitions.........................................................................................................................4

4 A. c. interference sources ..................................................................................................................5

5 Simplified description of a.c. corrosion process .............................................................................6

6 Evaluation of a.c. corrosion likelihood..............................................................................................8

6.1 Prerequisite .........................................................................................................................................8

6.2 General ................................................................................................................................................8

6.3 Installation / use of coupons..............................................................................................................9

6.4 Influence of a.c. voltage on the structure..........................................................................................9

6.5 Off potential influence ......................................................................................................................11

6.6 Influence of a.c. current density ......................................................................................................12

6.7 Influence of the on potential toward a.c. corrosion likelihood......................................................13

6.8 Influence of current ratio "I /I "...................................................................................................16

a.c. d.c.

6.9 Influence of soil characteristics on a.c. corrosion likelihood........................................................16

6.10 Assessment of the a.c. corrosion likelihood of the pipeline by using corrosion coupons........17

7 Design consideration........................................................................................................................18

7.1 General ..............................................................................................................................................18

7.2 New interference cases (pipelines/power lines/traction systems in the design phase) -

Condition for calculation..................................................................................................................18

8 Interpretation of data, limits and relevant aspects.........................................................................20

9 Mitigation measures .........................................................................................................................20

10 Monitoring .........................................................................................................................................21

Annex A (informative) Assessment of the corrosion condition by using the electric resistance

technique...........................................................................................................................................22

Annex B (informative) Coulometric oxidation of corrosion products formed by a.c. corrosion............24

Bibliography...................................................................................................................................................25

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CEN/TS 15280:2006 (E)
Foreword

This Technical Specification (CEN/TS 15280:2006) has been prepared by Technical Committee CEN/TC 219

“Cathodic Protection”, the secretariat of which is held by BSI.

Long term a.c. interference on buried metallic pipelines may cause corrosion due to an exchange of

alternating current between the soil and the bare metal at unavoidable coating faults in the structure.

a.c. corrosion is more likely on pipelines which are not cathodically protected. To reduce it, it is advisable to

consider the application of cathodic protection and to follow the present Technical Specification.

Danger to people in contact with the pipeline or connected equipment, malfunction of connected equipment

and other damages to the pipeline or connected equipment are dealt with in relevant CENELEC standards.

This Technical Specification refers to EN 12954 and may be used in place of its Annex A.

According to the CEN/CENELEC Internal Regulations, the national standards organizations of the following

countries are bound to announce this CEN Technical Specification: Austria, Belgium, Cyprus, Czech Republic,

Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania,

Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden,

Switzerland and United Kingdom.
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1 Scope

This Technical Specification is applicable to buried cathodically protected metallic structures and influenced

by a.c. traction systems and/or a.c. power lines.

In this document, a buried pipeline (or structure) is intended as buried or immersed pipeline (or structure), as

defined in the Standard EN 12954.

In the presence of a.c. interference, the criteria given in EN 12954, Table 1, are not sufficient to demonstrate

that the steel is being protected against corrosion.

This Technical Specification provides limits, measurements procedures and information to deal with long term

a.c. interference and the evaluation of a.c. corrosion likelihood.

Even though short term interference can cause damages to buried pipelines (e.g. arc fusion), this standard

does not deal with short term interference.
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.
EN 13509:2003, Cathodic protection measurement techniques

EN 12954:2001, Cathodic protection of buried or immersed metallic structures – General principles and

application for pipelines.
3 Terms and definitions

For the purposes of this Technical Specification, the following terms and definitions apply

3.1
a.c. traction system

an a.c. electrical system of a railway, i.e. the electric train units and their feeding and return systems

NOTE The lines used to feed the railway substations (three-phase lines or, sometimes, two-phase lines in case of

16,7 Hz systems) are a.c. power supply systems, and it is suggested to take them into consideration together with the a.c.

traction system.
3.2
cathodic protection system

the entire installation, including active and passive elements, that provides cathodic protection

(See EN 12954:2001 clause 3.2.9)

NOTE It includes for example the following: cathodic protection stations and relevant accessories as remote control

systems, drainages, insulating joints, resistors, diodes, test points, groundings, a.c. discharge / d.c. decoupling devices etc.

3.3
incubation time

period of time before the leakage resistance of exposed metal, at coating faults or coupons, stabilizes due to

electrochemical reactions
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CEN/TS 15280:2006 (E)
3.4
instantaneous measurement
reading of electrical parameters by an operator
3.5
leakage resistance
local resistance to earth of metal exposed to the environment
3.6
long term a.c. interference

interference on a pipeline during normal operating conditions of a.c. power systems (e.g. traction systems or

electricity power lines)
3.7
long term measurements

measurements of electrical parameters taken by an operator having a duration of more than 1 hour using

equipment to store the data
3.8
reference electrical status

electrical status to be used as a reference in subsequent cathodic protection measurements and checks which

conform to protection requirements

NOTE The “electrical status” refers to a well defined cathodic protection system and its electrical configuration

3.9
remote monitoring

supervision of the state of operational equipment by means of telecommunication techniques

3.10
selected test point

test point which allows comparison of present and past parameters to be recorded in the reference electric

status where a cathodic protection system still maintains its efficiency and effectiveness

3.11
short term a.c. interference

interference on a pipeline during a fault condition of a.c. traction systems or electricity power lines

3.12
short measurements

measurements of electrical parameters taken by an operator having a duration of about 5 min using

equipment to store the data
4 A. c. interference sources
Main long term a. c. interfering sources on buried metallic pipelines are:
- a.c. overhead or underground power lines;

- a.c. traction systems (usually fed by a parallel high voltage feeding line which may be 50 Hz or 16,7Hz).

Long term a.c. interference on a buried pipeline may cause corrosion due to an exchange of a.c. current

between the exposed metal of the pipeline and the surrounding electrolyte.

This exchange of current depends on a.c. voltage whose amplitude is related to various parameters such as:

- the configuration of a.c. power line phase conductors and shield wires;
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CEN/TS 15280:2006 (E)
- the distance between the a.c. power line / traction system and the pipeline;
- the current flowing in the a.c. power line / traction system phase conductors;
- the average coating resistance of the pipeline (Ω.m²);
- the thickness of the coating;
- the soil resistivity.
5 Simplified description of a.c. corrosion process

The cathodic protection of pipelines forces current to enter the pipeline through the metal surface in contact

with soil where the coating is damaged. This current prevents corrosion from taking place.

The corrosion reaction is associated with a current leaving the metal surface.

When an a.c. voltage is present on the cathodically protected pipeline, current will flow through the metal

surface at defects in the coating. This current depends on the impedance of the system. During the positive

half wave of the a.c. voltage, the current will leave the metal surface if the a.c. voltage is sufficiently large.

The current leaving the metal surface can cause charging of the double layer capacitance, oxidation of

hydrogen and reduced corrosion products due to the cathodic protection, and oxidation of the pipeline. Since

the current leaving the metal surface is consumed by several non-corrosive processes, generally higher

voltages than between 4V and 10 V are required to result in a significant corrosion attack on the pipeline.

Various additional parameters influence this process, such as leakage resistance of the defect, soil

composition, cathodic protection level etc.

The processes taking place are schematically illustrated in Figure 1. During the positive half wave the bare

metal surface is oxidized resulting in the formation of a passive film. This is due to the current that leaves the

metal surface. During the negative half wave, when the current enters the metal surface this passive film is

reduced to iron hydroxide. In the following anodic cycle a new passive film grows. Upon reduction of the

passive film the amount of iron hydroxide is increased. Hence every a.c. cycle results in some oxidation of the

metal. In the long term this can result in a significant metal loss.

For comparison, the situation without a.c. interference is shown in Figure 2 In this case no metal loss is

observed, since the current always enters the metal surface.
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CEN/TS 15280:2006 (E)
23 4
key
1 Current
2 Metal
3 Passive film (eg. Fe O )
3 4
4 Iron hydroxide (eg. Fe(OH) )
t Time

Figure 1 - Graph of cathodic protection current with a.c. influence - Schematic description of the a.c.

corrosion process with cathodic protection
23 4
Key
1 Current
2 Metal
3 Passive film (eg. Fe O )
3 4
4 Iron hydroxide (eg. Fe(OH) )
t Time
Figure 2 – Graph of cathodic protection current without a.c. influence
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CEN/TS 15280:2006 (E)
6 Evaluation of a.c. corrosion likelihood
6.1 Prerequisite

The a.c. voltage on a pipeline is considered as the most important parameter to be taken into account when

evaluating the adversely influences by a.c. system.

Therefore, before beginning any evaluation of the a.c. corrosion likelihood, the a.c. voltage of the pipeline

should be lowered if necessary, according to 6.4.4.
6.2 General
The factors which mainly influence the a.c. corrosion phenomena are:
- induced a.c. voltage;
- a.c. current density on the exposed metal;
- d.c. polarisation;
- size of coating faults;
- local soil resistivity;
- local soil chemical composition.

The corrosion likelihood due to a.c. voltage can be evaluated by taking into account different factors which are

generally used in conjunction, such as:
- a.c. voltage on the structure; (See 6.4)
- pipe to soil off potential; (See 6.5)
- a.c. current density; (See 6.6)
- On potential; (See 6.7)
- a.c./d.c. current ratio; (See 6.8)
- soil characteristics; (See 6.9)
- corrosion condition of coupons. (See 6.10)

Other techniques are available to evaluate corrosion. Some of these techniques are described in the annexes

A and B.

In the following, the conditions for evaluating the likelihood of corrosion, either by measurements or by

calculation, are clarified.
For existing pipelines, measuring procedures are also suggested.

Routine direct short measurements on pipelines rarely reflect worst case conditions while calculation gives

more reliable information on the a.c. interference.

For this purpose, measurements shall be related to worst established conditions during normal operation of

interfering systems.

As a first approach, the general evaluation of the likelihood of a.c. corrosion can be by measurement of the a.c.

voltage of the structure (see 6.4).
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SIST-TS CEN/TS 15280:2006
CEN/TS 15280:2006 (E)

To evaluate the different factors influencing the a.c. corrosion, the use of coupons is necessary (see EN

13509:2003 Clause 4, and EN 12954). For coupons, an incubation time has to be considered when evaluating

measurement results.

The evaluation of more than one factor is necessary to better assess the a.c. corrosion likelihood. The more

factors considered, the better the assessment.
6.3 Installation / use of coupons

Due to the fact that many electrical and electrochemical parameters cannot be measured directly on the

pipeline itself, the use of coupons is particularly recommended for evaluating the likelihood of a.c. corrosion.

A.c. corrosion coupons are usually made of a steel plate having a known bare surface area (preferably 1 cm

round), simulating a coating fault. They are buried close to the pipeline and connected to it through a test post.

These coupons can either be used for measuring and/or for verifying local protection conditions. In this last

option, three coupons may be installed at the same place (it is recommended to keep at least one meter

between each coupon). The coupons should be excavated at the same time for the purpose of statistical

examination.

Coupons have to be used where the worst conditions have been found in accordance with the calculation or

the measurement described in 7.2.

A reference electrode can either be used during the measurement or permanently installed.

6.4 Influence of a.c. voltage on the structure
6.4.1 General

The a.c. voltage on a structure subjected to interference by a.c. systems should be considered as the most

important parameter to be taken into account when evaluating the likelihood of corrosion on buried pipelines.

The a.c. voltage on a structure subjected to interference by a.c. systems can either be calculated or directly

measured on the structure itself. To evaluate if a calculation or measurements are needed, see 6.4.2.

6.4.2 Voltage calculation

People’s safety and the malfunction of apparatus should be calculated in accordance with CIGRE Technical

Brochure N°95 published in 1995 “Guide on the Influence of High Voltage A.C. Power Systems on Metallic

Pipelines”.

The same algorithms can be used to calculate the pipeline a.c. voltage by taking into account the worst case

under normal operating conditions of the interfering systems.
The limits indicated in 6.4.4 apply.
6.4.3 A.c. voltage measurements
6.4.3.1 General

A.c. interference can be determined by measurement of the a.c. voltage on the pipeline.

While taking measurements on an existing pipeline, the following should be taken into account:

- high voltage power line; variations during time, according to the charge;
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CEN/TS 15280:2006 (E)

- a.c. traction systems; variations during time in accordance with the power absorbed by the traction

system and along the pipeline.

A.c. measurements should be performed on pipelines or sections of them where unacceptable a.c.

interference is suspected or may be expected based on map observation, calculation, or routine

measurements.

The a.c. voltage measurements should be performed in a first instance on all test points, CP stations,

insulating joints and casings, wherever an accessible measuring cable is connected to the pipeline or section.

The following should be included in these measurements:
- instantaneous measurement of a.c. voltage;

- verification of the possible industrial frequencies (mainly 16,7 Hz and 50 Hz).

At a later stage, these measurements may be restricted to a few particular positions, chosen along the most

influenced areas.

NOTE These can be short measurements, provided that they are related to a known electrical behaviour of the

pipeline during a 24 h duration under the influence of the interfering source.
Measurements should be made with the following procedure:

- for routine measurements, short duration a.c. voltage measurements are sufficient and are

carried out between the metal structure and an electrode (which may be a copper / copper

sulfate reference electrode, or any metallic low resistance in contact with the soil) placed over

the pipe;

- for more correct measurements, the electrode should be located at the “remote earth”.

6.4.3.2 Procedure for a.c. voltage measurements

A high input impedance a.c. voltmeter is connected with one pole to a reference electrode (copper/copper

sulfate), which is placed on the soil above the pipeline, and with one pole to the pipeline at a test station. The

voltmeter indicates the instantaneous rms (effective) value of the induced a.c. voltage on the pipeline.

NOTE For long-term evaluation, a data recorder instead of a voltmeter should be used.

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SIST-TS CEN/TS 15280:2006
CEN/TS 15280:2006 (E)
key
1 Soil
2 a.c voltmeter or recorder
3 Reference electrode
4 Pipe
Figure 3 - Measurement of induced a.c. voltage along the pipeline
6.4.4 a.c. voltage limits on buried pipelines

a.c. corrosion likelihood is linked to the a.c. current density at the coating fault and the consequent flowing

between the metallic pipeline and the environment.

The driving force for this is the pipeline a.c. voltage. Therefore, a.c. corrosion likelihood can be primarily

mitigated by reducing the a.c. voltage.

To reduce the a.c. corrosion likelihood on a buried pipeline, the pipeline a.c. voltage, measured at selected

test points according to the procedure described in 6.4.3.2, should not exceed at any time:

- 10 V where the local soil resistivity is greater than 25 Ω.m;
- 4 V where the local soil resistivity is less than 25 Ω.m.

These values should be considered as the threshold limits which significantly reduce a.c. corrosion likelihood;

they are based on long term practical experience of European operators.
6.4.5 Evaluation of a.c. voltage influence on corrosion likelihood

The a.c. voltage values measured or calculated in accordance with 6.4.2 and 6.4.3 shall be compared with the

threshold limits specified in 6.4.4.

To better evaluate a.c. corrosion likelihood on buried pipelines, due consideration is also to be given to other

factors (defined in 6.2).
6.5 Off potential influence
6.5.1 General

The ordinary on / off measurement techniques (rectifier switching) are not appropriate to evaluate a.c.

corrosion likelihood because they do not allow for elimination of a.c. and d.c. interferences.

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SIST-TS CEN/TS 15280:2006
CEN/TS 15280:2006 (E)

To evaluate the a.c. corrosion likelihood, IR free potential measurements on coupons are necessary.

This procedure is called “off potential” because it refers to a test coupon disconnected from the pipeline, thus

without interference from a.c. and/or d.c. currents on the coupon itself.

All the measurements indicated in the following shall be related to worst case conditions, in location and on

the time, during normal operation of the interfering systems.
6.5.2 Coupon off potential measurements
Off potential shall be measured in accordance with EN 13509.
6.5.3 Interpretation of coupon off potential

As a.c. corrosion phenomena are linked to the switching, according to the pH, between immunity, and

passivity condition, and vice versa (see Clause 5), the coupon-to-soil off potential should be more negative

than, but as close as possible to, the limiting critical potential (which is E = –850 mV for iron or steel in

aerobic or –950 mV in anaerobic soil containing sulfate reducing bacteria).

To fully evaluate the a.c. corrosion likelihood on buried pipelines, due consideration is also to be given to other

factors (defined in 6.2).
6.6 Influence of a.c. current density
6.6.1 Determination of a.c. current density
The use of coupons is described in 6.3.

A period of time needed for the outbreak of a.c. corrosion, usually in the order of some months, should be

taken into consideration before taking these measurements.

The a.c. current in the connection wire between the pipeline and the coupon is measured and divided by the

area of the test coupon.

However, the evaluation of the measurement results is uncertain for the following reasons:.

a) due to uncertain contact between the coupon and the soil and its representativity compared with the

contact between the pipeline and the soil.

b) in some types of soils having a high lime content, the coupon may be partially covered by isolating

calcareous deposit, which changes the recorded a.c. current density.

Due to electrochemical process at the surface of the coupon which leads to a change of composition of the

medium, the a.c. current density calculated may be significantly different from the current density measured

on coupon.

6.6.2 Evaluation of a.c. corrosion likelihood by using the current density parameter

The pipeline is considered protected from a.c. corrosion if the rms a.c. current density (J ) is lower than 30

a.c.
A.m .

In practice, the evaluation of a.c. corrosion likelihood is on a broader basis as follows:

- J lower than 30 A.m : no or low likelihood;
a.c.
-2 2
- J between 30 A.m and 100 A.m : medium likelihood;
a.c.
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SIST-TS CEN/TS 15280:2006
CEN/TS 15280:2006 (E)
- J higher than100 A.m : very high likelihood.
a.c.

To fully evaluate the a.c. corrosion likelihood on buried pipelines, due consideration is also to be given to other

factors (defined in 6.2).
6.7 Influence of the on potential toward a.c. corrosion likelihood
6.7.1 General

The intensity of the CP current able to reach and polarize the steel surface at the bottom of a coating fault

depends on the driving potential (∆E = difference of the on potential “E “ and IR-free-potential “E “) and

on IR-free

on the total circuit resistance according to the Ohm’s law. Eon has to be measured versus a reference

electrode located at usual measuring points (at the surface of the soil).

This circuit resistance is the sum of the polarization resistance of the bare steel at the bottom of the coating

fault, of the resistance of the medium located in the coating fault and of the leakage resistance in the soil (see

Figure 4).
Most of the coating faults can be considered as pores (circular coating fault).
Key
1 R leakage in the soil
2 R pore
3 R polarisation
4 Coating
5 Steel
d pore diameter
t depth of the pore (coating thickness)
Figure 4: Location for leakage, pore and polarisation resistances
6.7.2 Determination of the current intensity in a coating fault

The following elements and definitions are necessary for the determination of the current intensity in a pore:

− Definition of the driving voltage for cathodic
∆E = E −E [V]
on IR−free
protection:
− Bare steel surface area of a coating fault: S [m ]
− Resistance of coating fault: [Ω]
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SIST-TS CEN/TS 15280:2006
CEN/TS 15280:2006 (E)
− Diameter of the pore (typical coating fault)/ d (m)
− Specific polarization resistance: r [Ω.m ]
pol
− Soil resistivity: ρ [Ω.m]
soil
− Leakage resistance:
soil
R = [Ω] (1)
leak
(usual practical equation)
− Pore medium resistance:
ρ ⋅t
pore
R = [Ω] (2)
pore
πd² / 4
− Pore medium resistivity: ρ [Ω.m]
pore
− Polarization resistance of the bare steel at
pol

the bottom of the coating fault: R = [V] (3)

pol
− Current I into a coating faul
...

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