ISO 12473:2017
(Main)General principles of cathodic protection in seawater
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
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Standards Content (Sample)
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)
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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.
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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.
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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
fc
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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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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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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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)
22
−−
22Cl →+Cl e (2)
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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