General principles of cathodic protection in seawater

ISO 12473 covers the general principles of cathodic protection when applied in seawater, brackish waters and marine mud. It is intended to be an introduction, to provide a link between the theoretical aspects and the practical applications, and to constitute a support to the other standards devoted to cathodic protection of steel structures in seawater. ISO 12473 specifies the criteria required for cathodic protection. It provides recommendations and information on reference electrodes, design considerations and prevention of the secondary effects of cathodic protection. The practical applications of cathodic protection in seawater are covered by the following standards: - EN 12495, Cathodic protection for fixed steel offshore structures; - ISO 13174, Cathodic protection of harbour installations (ISO 13174); - EN 12496, Galvanic anodes for cathodic protection in seawater and saline mud; - EN 13173, Cathodic protection for steel offshore floating structures; - EN 16222, Cathodic protection of ship hulls; - EN 12474, Cathodic protection of submarine pipelines; - ISO 15589‑2, Petroleum, petrochemical and natural gas industries ? Cathodic protection of pipeline transportation systems ? Part 2: Offshore pipelines. For cathodic protection of steel reinforced concrete whether exposed to seawater or to the atmosphere, ISO 12696 applies.

Principes généraux de la protection cathodique en eau de mer

General Information

Status
Published
Publication Date
09-Oct-2017
Current Stage
9020 - International Standard under periodical review
Due Date
15-Oct-2022
Completion Date
15-Oct-2022
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INTERNATIONAL ISO
STANDARD 12473
Second edition
2017-10
General principles of cathodic
protection in seawater
Principes généraux de la protection cathodique en eau de mer
Reference number
ISO 12473:2017(E)
ISO 2017
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ISO 12473:2017(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2017, Published in Switzerland

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ii © ISO 2017 – All rights reserved
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ISO 12473:2017(E)
Contents Page

Foreword ..........................................................................................................................................................................................................................................v

1 Scope ................................................................................................................................................................................................................................. 1

2 Normative references ...................................................................................................................................................................................... 1

3 Terms, definitions, abbreviations and symbols ................................................................................................................... 1

4 Application of cathodic protection in seawater ................................................................................................................... 5

4.1 General ........................................................................................................................................................................................................... 5

4.2 Galvanic anode method ................................................................................................................................................................... 5

4.3 Impressed current method ........................................................................................................................................................... 6

4.4 Hybrid systems ....................................................................................................................................................................................... 6

5 Determination of level of cathodic protection ....................................................................................................................... 9

5.1 Measurement of protection level ............................................................................................................................................ 9

5.2 Reference electrodes .......................................................................................................................................................................... 9

5.3 Potentials of reference electrodes .......................................................................................................................................... 9

5.4 Verification of reference electrodes ...................................................................................................................................... 9

5.5 Potential measurement ................................................................................................................................................................10

6 Cathodic protection potential criteria ........................................................................................................................................10

6.1 General ........................................................................................................................................................................................................10

6.2 Carbon-manganese and low alloy steels ........................................................................................................................10

6.3 Other metallic materials ..............................................................................................................................................................13

6.3.1 General...................................................................................................................................................................................13

6.3.2 Stainless steels ................................................................................................................................................................13

6.3.3 Nickel alloys ......................................................................................................................................................................14

6.3.4 Aluminium alloys ..........................................................................................................................................................14

6.3.5 Copper alloys ....................................................................................................................................................................14

7 Design considerations .................................................................................................................................................................................14

7.1 Introduction ...........................................................................................................................................................................................14

7.2 Technical and operating data ..................................................................................................................................................15

7.2.1 Design life ............................................................................................................................................................................15

7.2.2 Materials of construction.......................................................................................................................................15

7.3 Surfaces to be protected ..............................................................................................................................................................15

7.4 Protective coatings ...........................................................................................................................................................................15

7.5 Availability of electrical power ..............................................................................................................................................16

7.6 Weight limitations .............................................................................................................................................................................16

7.7 Adjacent structures ..........................................................................................................................................................................16

7.8 Installation considerations ........................................................................................................................................................16

7.9 Current demand ..................................................................................................................................................................................16

8 Effect of environmental factors on current demand ....................................................................................................16

8.1 Introduction ...........................................................................................................................................................................................16

8.2 Dissolved oxygen ................................................................................................................................................................................17

8.3 Sea currents ............................................................................................................................................................................................17

8.4 Calcareous deposits .........................................................................................................................................................................17

8.5 Temperature ...........................................................................................................................................................................................18

8.6 Salinity.........................................................................................................................................................................................................18

8.7 pH ....................................................................................................................................................................................................................18

8.8 Marine fouling.......................................................................................................................................................................................18

8.9 Effect of depth .......................................................................................................................................................................................19

8.10 Seasonal variations and storms .............................................................................................................................................19

9 Secondary effects of cathodic protection .................................................................................................................................19

9.1 General ........................................................................................................................................................................................................19

9.2 Alkalinity ...................................................................................................................................................................................................19

9.3 Environmentally-assisted cracking ....................................................................................................................................19

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ISO 12473:2017(E)

9.3.1 General...................................................................................................................................................................................19

9.3.2 Hydrogen embrittlement .......................................................................................................................................20

9.3.3 Corrosion fatigue ..........................................................................................................................................................20

9.4 Chlorine ......................................................................................................................................................................................................21

9.5 Stray currents and interference effects...........................................................................................................................21

10 Use of cathodic protection in association with coatings ..........................................................................................21

10.1 Introduction ...........................................................................................................................................................................................21

10.2 Coating selection ................................................................................................................................................................................22

10.3 Coating breakdown ..........................................................................................................................................................................22

Annex A (informative) Corrosion of carbon-manganese and low-alloy steels .......................................................24

Annex B (informative) Principles of cathodic protection ............................................................................................................28

Annex C (informative) Reference electrodes ............................................................................................................................................31

Annex D (informative) Corrosion of metallic materials other than carbon-manganese and

low-alloy steels typically subject to cathodic protection in seawater ........................................................35

Bibliography .............................................................................................................................................................................................................................37

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ISO 12473:2017(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 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 the following

URL: www.iso.org/iso/foreword.html.

This document was prepared by the European Committee for Standardization (CEN) (as

EN 12473:2014) and was adopted, without modification, by Technical Committee ISO/TC 156,

Corrosion of metals and alloys.

This second edition cancels and replaces the first edition (ISO 12473:2006), which has been technically

revised.
© ISO 2017 – All rights reserved v
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INTERNATIONAL STANDARD ISO 12473:2017(E)
General principles of cathodic protection in seawater
1 Scope

This document covers the general principles of cathodic protection when applied in seawater, brackish

waters and marine mud. It is intended to be an introduction, to provide a link between the theoretical

aspects and the practical applications, and to constitute a support to the other standards devoted to

cathodic protection of steel structures in seawater.

This document specifies the criteria required for cathodic protection. It provides recommendations

and information on reference electrodes, design considerations and prevention of the secondary effects

of cathodic protection.

The practical applications of cathodic protection in seawater are covered by the following standards:

— EN 12495, Cathodic protection for fixed steel offshore structures;
— ISO 13174, Cathodic protection of harbour installations (ISO 13174);
— EN 12496, Galvanic anodes for cathodic protection in seawater and saline mud;
— EN 13173, Cathodic protection for steel offshore floating structures;
— EN 16222, Cathodic protection of ship hulls;
— EN 12474, Cathodic protection of submarine pipelines;

— ISO 15589-2, Petroleum, petrochemical and natural gas industries — Cathodic protection of pipeline

transportation systems — Part 2: Offshore pipelines.

For cathodic protection of steel reinforced concrete whether exposed to seawater or to the atmosphere,

ISO 12696 applies.
2 Normative references

The following documents, in whole or in part, are normatively referenced in this document and are

indispensable for its application. For dated references, only the edition cited applies. For undated

references, the latest edition of the referenced document (including any amendments) applies.

EN 50162, Protection against corrosion by stray current from direct current systems

ISO 8044, Corrosion of metals and alloys — Basic terms and definitions
3 Terms, definitions, abbreviations and symbols

For the purposes of this document, the terms and definitions given in ISO 8044 and the following apply.

NOTE The definitions given below prevail on their versions in ISO 8044.
3.1
acidity
presence of an excess of hydrogen ions over hydroxyl ions (pH < 7)
3.2
alkalinity
presence of an excess of hydroxyl ions over hydrogen ions (pH > 7)
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ISO 12473:2017(E)
3.3
anaerobic condition
absence of free oxygen dissolved in the electrolyte
3.4
calcareous deposits

minerals precipitated on the metallic cathode because of the increased alkalinity caused by cathodic

protection
3.5
cathodic disbondment

failure of adhesion between a coating and a metallic surface that is directly attributable to the

application of cathodic protection
3.6
cathodic protection system
entire installation that provides cathodic protection

Note 1 to entry: It may include anodes, power source, cables, test facilities, isolation joints, electrical bonds.

3.7
coating breakdown factor

ratio of cathodic current density for a coated metallic material to the cathodic current density of the

bare material
3.8
copper/copper sulphate reference electrode

reference electrode consisting of copper in a saturated solution of copper sulphate

3.9
dielectric shield

alkali resistant organic coating applied to the structure being protected in the immediate vicinity of

an impressed current anode to enhance the spread of cathodic protection and minimize the risk of

hydrogen damage to the protected structure in the vicinity of the anode
3.10
driving voltage

difference between the structure/electrolyte potential and the anode/electrolyte potential when the

cathodic protection is operating
3.11
electro-osmosis

passage of a liquid through a porous medium under the influence of a potential difference

3.12
environmentally assisted cracking

cracking of a susceptible metal or alloy due to the conjoint action of an environment and stress

3.13
groundbed

system of immersed electrodes connected to the positive terminal of an independent source of direct

current and used to direct the cathodic protection current onto the structure being protected

3.14
hydrogen embrittlement

process resulting in a decrease of the toughness or ductility of a metal due to absorption of hydrogen

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ISO 12473:2017(E)
3.15
hydrogen stress cracking
HSC

cracking that results from the presence of hydrogen in a metal and tensile stress (residual and/or

applied)

Note 1 to entry: HSC describes cracking in metals which may be embrittled by hydrogen produced by cathodic

polarization without any detrimental effect caused by specific chemicals such as sulphides.

3.16
isolating joint (or coupling)

electrically discontinuous joint or coupling between two lengths of pipe, inserted in order to provide

electrical discontinuity between them
3.17
master reference electrode

reference electrode, calibrated with the primary calibration reference electrode, used for verification

of reference electrodes used for field or laboratory measurements
3.18
over-polarization

occurrence in which the structure to electrolyte potentials are more negative than those required for

satisfactory cathodic protection

Note 1 to entry: Over-polarization provides no useful function and might even cause damage to the structure.

3.19
pitting resistance equivalent
PREN

indication of the resistance of a corrosion resistant alloy to pitting in the presence of water, chlorides

and oxygen or oxidation environment, accounting for the beneficial effects of nitrogen

Note 1 to entry: For the purposes of this document, PREN is calculated as follows: PREN = % Cr + 3,3[(% Mo) +

0,5 (% W)] + 16 (% N).
3.20
potential gradient
difference in potential between two separate points in the same electric field
3.21
primary calibration reference electrode

reference electrode used for calibration of master reference electrodes is the normal hydrogen

electrode (N.H.E.)

Note 1 to entry: The official reference electrode, standard hydrogen electrode (S.H.E.), which considers

the fugacity coefficient for hydrogen gas and the activity coefficient for H ions, is practically impossible to

manufacture.
3.22
protection current

current made to flow into a metallic structure from its electrolytic environment in order to achieve

cathodic protection of the structure
3.23
reference electrode

electrode having a stable and reproducible potential that is used as a reference in the measurement of

electrode potentials

Note 1 to entry: Some reference electrodes use the electrolyte in which the measurement is carried out. Their

potential varies according to the composition of this electrolyte.
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ISO 12473:2017(E)
3.24
resistivity (of an electrolyte)

resistivity is the resistance of an electrolyte of unit cross section and unit length

Note 1 to entry: It is expressed in ohm.metres (Ω.m). The resistivity depends, amongst other things, upon the

amount of dissolved salts in the electrolyte.
3.25
saturated calomel reference electrode

reference electrode consisting of mercury and mercurous chloride in a saturated solution of potassium

chloride
3.26
silver/silver chloride reference electrode

reference electrode consisting of silver, coated with silver chloride, in an electrolyte containing a known

concentration of chloride ions

Note 1 to entry: Silver/silver chloride/ saturated KCl electrodes are electrodes currently used in the laboratory

and for master reference electrode.

Note 2 to entry: Silver/silver chloride/seawater (Ag/AgCl/seawater) electrodes are electrodes currently used for

field measurements in seawater.
3.27
slow strain rate test

test for evaluating susceptibility of a metal to environmentally assisted cracking (3.12 in this document)

that most commonly involves pulling a tensile specimen to failure in a representative environment at

−5 −1

a constant displacement rate chosen to generate nominal strain rates usually in the range 10 s to

−8 −1
10 s

Note 1 to entry: Slow strain rate testing may also be applied to other specimen geometries, e.g. bend specimens.

3.28
specified minimum yield strength
SMYS

minimum yield strength prescribed by the specification under which steel components are

manufactured, obtained through standard analysis and representing a probabilistic value

Note 1 to entry: It is an indication of the minimum stress steel components may experience that will cause plastic

(permanent) deformation (typically 0,2 %).
3.29
stray currents
current flowing through paths other than the intended circuits
3.30
structure to electrolyte potential

difference in potential between a structure and a specified reference electrode in contact with the

electrolyte at a point sufficiently close to, but without actually touching the structure, to avoid error

due to the voltage drop associated with any current flowing in the electrolyte
3.31
sulphate reducing bacteria
SRB

group of bacteria that are found in most soils and natural waters, but active only in conditions of near

neutrality and absence of oxygen and that reduce sulphates in their environment, with the production

of sulphides and accelerate the corrosion of structural materials
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ISO 12473:2017(E)
3.32
telluric currents

electrical currents induced by time varying changes in the earth's magnetic field

Note 1 to entry: They are able to flow in metallic conductors laid in the soil or in the sea.

3.33
zinc reference electrode

electrode consisting of pure zinc or zinc alloy specific for anodes in contact with the electrolyte in

which the measurements are carried out

Note 1 to entry: Zinc reference electrodes are currently used for measurements in seawater carried out at

permanent locations.
4 Application of cathodic protection in seawater
4.1 General

Metallic materials in aqueous environments such as seawater are susceptible to corrosion produced by

electrochemical reactions. General information on corrosion of carbon-manganese or low alloy steels is

given in Annex A.

Cathodic protection is an electrochemical corrosion prevention system based on the decrease of

corrosion potential to a level at which the corrosion rate of the metal is significantly reduced (ISO 8044).

For industrial structures, residual corrosion rates less than 10 µm/yr are typically achieved with a

fully effective cathodic protection system.

Cathodic protection is achieved by applying a voltage able to supply sufficient current to the metallic

surface to lower the potential. General information on the principles of cathodic protection is given in

Annex B.

There are two methods whereby the protection current can be supplied to polarize the surface:

a) galvanic anode systems in which the current for protection is provided by a metal of more negative

corrosion potential than the item to be protected i.e. aluminium, zinc and magnesium alloys for

steel and iron for copper and copper based alloys,

b) impressed current systems in which direct current (normally produced from alternating current

by a transformer rectifier) is used in conjunction with relatively inert anodes such as graphite, thin

coatings of platinum or activated mixed metal oxides on metals such as titanium or niobium, lead

alloys, silicon-iron, etc; in some cases a consumable anode such as scrap iron or steel is used.

4.2 Galvanic anode method

If two dissimilar metals are connected in the same electrolyte, a galvanic cell is produced. The open

circuit voltage is the natural potential difference which exists between the two metals. If the circuit

is closed, the potential difference drives an electrical current. The more negative electrode behaves

as an anode and it releases electrons to the circuit and dissolves more rapidly while the more positive

electrode behaves as a cathode and dissolves less readily. The use of galvanic anodes in cathodic

protection is based on this phenomenon.

Assuming the structure to be protected is made of steel, aluminium or zinc alloy galvanic anodes can

be used to form the cell, because these alloys are less noble (more electronegative) than steel. Anode

attachment to the structure is made through a steel core on to which the anode material is cast. Thus

the structure is in metallic contact with the anode material and also in electrolytic contact with it once

the structure is immersed. This is represented in Figure 1, where it is seen that the electrons released

by the dissolution of metal atoms are consumed in the cathodic reduction of oxygen on the structure

and hydroxyl ions are produced at the structure surface.
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ISO 12473:2017(E)
4.3 Impressed current method

Most impressed current anodes are of a type that do not dissolve readily on anodic polarization but

sustain alternative reactions which involve decomposition of the aqueous environment, or oxidizing of

dissolved chloride ions in it, i.e:
24H OO→+ He+4 (1)
22Cl →+Cl e (2)

Figure 2 represents an impressed current cathodic protection system using an inert anode in seawater

where in the secondary reactions hydrogen and chlorine are evolved.

The advantages of the impressed current system are that it is possible to have a large adjustable driving

voltage so that relatively few anodes need to be installed even to protect large uncoated structures in

comparatively high resistivity environments. A comparison of galvanic and impressed current anode

systems is given in Table 1.
4.4 Hybrid systems

These comprise a combination of galvanic anodes and externally powered impressed cu

...

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