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Brodogradnja/Shipbuilding Volume 65 Number 2, 2014
Tahsin Tezdogan
Yigit Kemal Demirel
ISSN 0007-215X
eISSN 1845-5859
AN OVERVIEW OF MARINE CORROSION PROTECTION WITH A
FOCUS ON CATHODIC PROTECTION AND COATINGS
UDC 629.5(067)
Professional paper
Summary
Corrosion is the gradual deterioration of a material or its properties through a chemical
reaction with its environment. There are several methods of preventing a material from
corroding. Cathodic protection (CP) and coatings are very popular methods for corrosion
protection. Each individual method has its own benefits and drawbacks, whereas experience
has shown that the most effective method of corrosion prevention is a combination of both CP
and coatings. This combination can provide very good protection over a long period of time.
This paper focuses on the combined use of both CP and coatings for ships. Calculation
of a CP design is explained briefly and the factors affecting the choice of the type of CP
system are demonstrated. Then, a sample anode plan of a ship is shown. Finally, the
calculation of a cathodic protection system of a ship is presented using data provided by
coating manufacturers and shipyards.
Key words: Corrosion; cathodic protection; coatings;
1. Introduction
Cathodic protection (CP) is an electrical method used to protect steel structures buried
in soil, or immersed in water, from corrosion. It has been applied to many structures such as
underground storage tanks, lock gates and dams, water treatment facilities, well casings,
rubbish racks, bridge decks, steel pilings, and, of course, ship-wetted hulls [1].
Cathodic protection systems found their earliest use in ships. Sir Humphry Davy
pioneered cathodic protection systems on naval ships in 1824. Sir Davy described a method to
prevent corrosion of the copper-clad wooden hulls of British naval vessels in a series of
papers [2]. He protected copper immersed in sea water by attaching a small amount of iron or
zinc, which acted as a sacrificial metal [3]. Today, Davy’s procedure is still being used to
minimise corrosion damage to steel vessels by installing zinc anodes on ships across the
world [1].
Another remarkable development of the cathodic protection system was seen in the
USA in 1945. In order to keep up with the fast-growing oil and gas industry, thin-walled pipes
An overview of marine corrosion protection T. Tezdogan, Y. K. Demirel
with a focus on cathodic protection and coatings
50
started to be used for transmission purposes. As a result of this, CP systems were installed to
the underground pipelines to protect them from the corrosion damage and to provide a longer
life [2, 3].
Low pressure thicker-walled cast iron pipelines were frequently used in the United
Kingdom. However, the pipes were rarely protected by a CP system until the beginning of the
1950s. In 1952, a CP system was installed to protect a 1000-mile fuel-line network. This
proved to be successful at preventing corrosion. Following this success, the use of CP systems
dramatically increased [3].
The cathodic protection system is the most widely used method of protecting a material
from corrosion, aside from coatings, in a marine environment. Today, many ships and
offshore platforms are protected from corrosion by the aid of a CP system.
Cathodic protection may be used as a single protection method, or it may be used in
conjunction with other protective systems. The specific combination of CP and coatings can
be regarded as the most effective way of corrosion prevention from both practical and
economical aspects [4].
This paper aims to emphasise the significance of cathodic protection methods. Firstly,
corrosion is defined in Section 2. In Section 3, different methods of cathodic protection are
briefly explained, along with their advantageous and disadvantageous. Then, a comparison
between these methods is made in Section 4. Additionally, the costs of various protection
methods for ships are illustrated graphically. In Section 5, the basic principles of paints and
coatings are presented, and their effect on corrosion is explored. Section 6 concerns the
concept behind the combined use of both CP and a coating as a corrosion protection
technique. In Section 7, CP design principles are examined, by looking at the procedure of CP
calculations. Following this, CP design calculations for two different tankers are given in
detail as a real-life case study. Finally, in Section 8, a conclusion and general discussion is
made according to the overall results in the paper.
2. What is corrosion?
Roberge states that, “corrosion is the destructive attack of a material by reaction with its
environment” [5]. Rust is the most well-known corrosion product, which is yielded when steel
and iron involve in the corrosion process. As this paper largely focuses on the economical
aspects of corrosion, the chemistry of the corrosion process will not be explained in detail.
However, Figure 1 gives a basic schematic of a corrosion cell.
Fig. 1 A corrosion cell [4]
As corrosion damage costs a lot of money to repair, methods which are able to prevent
corrosion are given great importance. It is stated that between 3 and 5 percent of the gross
national product (GNP) of industrialised countries is associated with corrosion damage.
Corrosion of metals causes the U.S. economy to loose approximately $300 billion each year.
An overview of marine corrosion protection T. Tezdogan, Y. K. Demirel
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Incredibly, 33 % of this cost could be prevented by the selection and use of an appropriate
corrosion protection system.
3. Methods of cathodic protection
There are two main ways of achieving cathodic protection. These are the impressed
current cathodic protection (ICCP) system and the sacrificial anodes cathodic protection
(SACP) system.
3.1 The impressed current cathodic protection system
The impressed current system generates electrons from an external DC power source.
As illustrated in Figure 2, an ICCP system consists of a rectifier, anodes, reference electrodes
and a controlling unit. The required positive current is provided by the rectifier, and is
delivered by the anodes to the structure to be protected. During this process, the reference
electrodes track the protection level and the controlling unit regulates the produced current
accordingly. Eventually, the metal structure becomes negatively charged, which ultimately
leads to decrease of potential below a certain threshold value [4]. This threshold value is
traditionally accepted that the steel is cathodically protected when it has a potential of -800
mV, or more negative. As it is written in NORSOK standards, regarding cathodic protection
(M-503), “The CP system shall be capable of polarizing all submerged steel of the
installations to a potential between -800 mV and -1100 mV vs. the Ag/AgCl/seawater
reference electrode, and to maintain the potential in this interval throughout the design life of
the installations” [6].
The ICCP system has been commonly used because it provides remarkable protection
against corrosion in all types of ships and offshore platforms, pipelines, ports, and steel piles,
etc. In spite of all the benefits of an ICCP system, it does have the following drawbacks [7]:
− Skilled workers are required.
− A continuous power supply must be sustained.
− The current must always be connected in the right direction.
− If permanent anodes are used, then current shields are required.
3.2 The sacrificial anodes cathodic protection system
There are a considerable number of older ships, with only short in-service lifetimes left,
for which the installation of an ICCP system would not be desirable. Installing an SACP
system is preferred for these ships as this would avoid the high initial costs of installing an
ICCP system. SACP is mainly used for short term operation due to its low cost.
The basis of the SACP system is that the potential difference between the steel to be
protected and a second metal in the same environment causes the driving voltage. If no
anodes were attached to a ship’s hull, then over time the steel would begin to interact with
electrolytes and oxygen dissolved in seawater. Eventually the steel would undergo corrosion
to revert back to a naturally-occurring ore, such as iron oxide. Sacrificial anodes can be used
to prevent a hull from corroding in this manner. During installation, the anodes are either
clamped or welded to the steel surface of the hull, to ensure permanent contact between the
two types of metal. The two metals will have different electrochemical potentials, meaning a
galvanic cell is generated between the two metals due to their difference in voltage. In this
cell, the steel of the hull acts as a cathode, as a partner to the sacrificial anode. A redox
reaction can then occur between the two metals, with electron transfer occurring from the
anode to the cathode, dictated by their difference in electrochemical potentials. The cathode
undergoes reduction, becoming more negatively-charged due to electron donation from the
An overview of marine corrosion protection T. Tezdogan, Y. K. Demirel
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anode, and the anode undergoes oxidation, with positively-charged metal ions (cations)
forming at its surface. These cations will undergo reactions with dissolved oxygen in
seawater, leading to the formation of metal oxides (corrosion) at the surface of the anode. The
favourable difference in electrochemical potential between the sacrificial anode and the ship’s
hull avoids corrosion occurring on the hull surface itself - instead, only the attached anode
undergoes corrosion, and thus it is given the term “sacrificial anode” [2, 4].
Fig. 2 A simple ICCP system [2] Fig. 3 A simple SACP system [2]
These anodes are generally made of aluminium, zinc or magnesium alloys, which are
anodic with respect to steel materials [1]. A schematic illustration of a typical SACP system is
given in Figure 3.
Determining the anode type is an important consideration. Anode selection depends on
both economic factors and engineering calculations. Guidance to aid the selection of the
optimum anode alloy is given in Section 4.
It is recommended that 15% - 20% of the sacrificial anodes should be installed to the
stern and rudder area of the ship [7]. A typical installation of sacrificial anodes around the
stern of a ship is shown in Figure 4. Calculating a cathodic protection system will be
explained in Section 7.1.
Fig. 4 A typical installation of anodes around the stern of a ship [8]
The SACP system has some benefits: in addition to requiring no power supply to be
installed, the sacrificial anode technique is also very simple to maintain and use. However, it
is more expensive than an ICCP system for long term operation, although it has low initial
costs.
4. Comparison of CP system features
It is very important to decide which CP system ensures the most efficient solution for
the vessel. The decision depends on several factors. The most significant factors are the type
of structure, and safety with regards to the environment and the cargo. The impressed current
systems are generally employed to external areas of simple geometric structures because a
complicated ICCP system is more expensive than a simple SACP system even over a long
period. However, this does not pose a problem for a ship hull as it has a large plane surface.
An overview of marine corrosion protection T. Tezdogan, Y. K. Demirel
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The bullet points below list some of the biggest advantages of both systems [4]:
The ICCP system is used to protect hulls for these reasons:
− Smooth surface, no extra drag
− Flexibility
− Light for large displacement vessels
− Long life time
− No welding requirements
− Fully automatic
The SACP system is used to protect hulls for these reasons:
− Simple installation
− Maintenance-free between dry dockings
− Worldwide availability
− Low cost for short term operation
The SACP system is particularly very appropriate for internal use and complex
structures since the protection of the entire structure is provided by distributing small anodes.
For that reason, tanks in vessels are often protected using sacrificial anodes.
If it is concluded that the SACP system is the best option, then a decision should be
made regarding which metal alloy to use. Several metals may be preferred, but zinc (Zn) or
aluminium (Al) anodes are commonly used in the sector. Magnesium is not used in sea water
since it releases a large volume of hydrogen gas due to a self-corrosion process.
Al is more expensive than Zn in terms of kilo price, while the consumption rate is 1/3 of
the weight of an equivalent zinc anode. Consequently, distributing solely aluminium anodes
over a ship’s hull costs roughly half that of zinc anodes. Hence, Al anodes are advantageous
over Zn anodes based on overall costs [7]. On the other hand, it should be borne in mind that
“there are also restrictions in the use of aluminium anodes inside and adjacent to cargo tanks
carrying cargoes with a low flash point. The reason being the danger of generating sparks if
the anodes loosen and fall down” [4].
A life cycle cost for a Panamax with wetted area circa 9000 m2 is illustrated in Figure 5.
The costs (in USD) of using different CP systems or materials are shown in the figure with
respect to service years.
As evidenced in Figure 5, the initial cost is slightly higher for ICCP, whereas the cost
after 15 years is of the order of 7 to 8 times higher for SACP. Furthermore, if a comparison
between sacrificial anode alloys is undertaken, the figure clearly demonstrates that an Al alloy
is more cost effective than a Zn alloy.
Figure 6 is a demonstration of cost comparison between the submerged steel without
any CP technique, with SACP, and with ICCP. As expected, applying no CP system results in
the highest cost to the vessel in question (nearly 46,000 USD after 20 years). On the other
hand, CP and corrosion repairs cost 10,000 USD to a ship with ICCP after 20 years in service.
This amount reaches 25,000 USD for a vessel with SACP after the same length of time in
service. It should be concluded that the cost after 20 years with sacrificial anodes is circa 2.5
times higher than the vessel with ICCP.
An overview of marine corrosion protection T. Tezdogan, Y. K. Demirel
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Fig. 5 Life cycle cost for a Panamax Fig. 6 Cost comparison of cathodic protection
with wetted area circa 9000 m2 [7] (per 100 m² of underwater steel) [9]
5. Paints and coatings
Paint is any liquid, liquefiable, or mastic product which contains pigments and which is
converted to an opaque film after application to a substrate in a thin layer. The film provides
protective and/or decorative features to the substrate.
The main components of paint are a binder, colour pigment, extender (filler), solvent
and additives (auxiliary substances).
Paints are mainly discussed from a corrosion protection point of view in the following
subsection.
5.1 Corrosion prevention by paints
Paint provides protection from corrosion in three ways; providing a barrier effect,
providing an inhibitor effect, and providing a galvanic effect.
As the name suggests, the barrier effect generates a barrier between the material and the
environment. There are no rust –inhibiting pigments inside the paints providing only a barrier
effect. Most paints, many primers, all intermediate coats, and top coats appear in this group.
In most applications, aluminium and glass flakes are used in primers to enhance the barrier
effect.
Paints with the inhibitor effect contain inhibiting pigments such as zinc phosphate. Such
pigments are only used in primers. It should be mentioned that these paints are not suited to
under-water use.
Paints which use a galvanic effect contain pure zinc pigments and are used only as
primers. The basic idea behind the galvanic effect is that the zinc forms a metallic contact
with the steel; hence it can behave as an anode. Even if the paint coating cracks or flakes, the
steel will still be protected by the zinc pigments acting in the manner of a cathode [4].
An impervious coating serves as an inert barrier to protect a material’s surface from
corrosion. A simple illustration of an impervious coating system is shown in Figure 7.
In order to achieve the satisfactory application of a protective coating to a ship, the
following fundamental requirements must be met [7]:
− Surface preparation
− Surface pre-treatment
− Anticorrosive or barrier coating application
− Antifouling coating application
Surface preparation to grade Sa 2½ according to the ISO 8501-1 standard prior to
coating application is suggested to ensure a good adhesion. High pressure water jetting can
also be performed since it provides a desired substrate with a surface more tolerant to paints
[4].
An overview of marine corrosion protection T. Tezdogan, Y. K. Demirel
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Fig. 7 An impervious coating system [4]
6. Combined use of both CP and coatings for ships
A painted coating is likely to break down over time. After a certain period of time in
service, the properties of the system often fall below the limiting criterion, unless it is
mended. This eventually leads to a high cost of renovation. Cathodic protection, however, can
provide an improvement to the protection of the system.
Anodes can be installed at two main time points within the life of a vessel. One option
involves installing the anodes during the construction of the vessel itself. The second option is
to install the anodes after a certain service period.
From experience, installing the anodes at the ship building stage gives the best result for
corrosion protection. “The calcareous deposit formed will precipitate on uncorroded steel at
once, should a defect occur. If the anodes are installed after corrosion has propagated for a
while the resulting layer at the steel surface will consist of a mixture of the already existing
rust and the calcareous deposit. A layer like this will protect the base material, but will have a
somewhat reduced protective property compared with the above case” [4].
It should also be emphasised that an excellent coating application to a hull involves
almost no consumption of the anodes. In this situation, the anodes will only provide a
guarantee for the construction and begin to work immediately after the coating deteriorates.
An illustration of the influence of using cathodic protection as a support for the coating
is shown in Figure 8. The installation of a CP system after coating breakdown is a temporary
and expensive solution. On the other hand, as it is evidently seen from the figure, the
combination of CP and coating from the construction stage is the most effective and
economical corrosion protection technique.
The compatibility of the paint system with the cathodic protection must also be taken
into consideration in the CP design; otherwise protection cannot be properly ensured. The
compatibility of the coatings is normally appraised by standard laboratory tests such as
ASTM G8.
Fig. 8 Corrosion vs. time curves in different corrosion protection methods [4]
An overview of marine corrosion protection T. Tezdogan, Y. K. Demirel
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7. CP design principles
Corrosion leads to metal loss, surface roughness, and thus an increase in frictional
resistance, which in turn affects fuel consumption. Hence, a cathodic protection system should
be designed to maximise protection of the vessel from corrosion. Two basic groups of data are
required for the design: background and dimensioning data [4].
The background data can be listed as follows:
− A general plan of the structure
− A statement of the current condition (paint break-down, corrosion damage …)
− The materials to be protected
− Information about the surrounding environment (cargo, external environment, etc)
− The cargo or ballast level in the tank
− The expected damage
Designing a CP system depends on the numbers and locations of sacrificial anodes.
These calculations are performed with respect to following parameters:
− The area (m2) to be protected
− Percent breakdown of the coating
− Mean current density (mA/m2)
− The lifetime of the protection in years (y)
− Individual current output of the anode material (A/kg)
− The rate of consumption of the anode (kg/A.y)
Each vessel/structure requires a different current density. It varies depending on the type
of vessel and condition of the hull. Table 1 gives a general idea about the current density
requirements of different vessels in two different conditions. The first column lists the current
density requirements of newly built vessels, and the second lists those for vessels in service
[10]. The current density of a construction is widely affected by the quality of the coating
system. The current density requirement for an unpainted material surface may increase up to
180 mA/m2 [4]. Thus a CP method with paint system is strongly recommended since coating
lowers the current density requirement of the structure to be protected.
Table 1 Current density requirements for a range of vessels [10]
Vessel Type New
building
mA/m2
In
service
mA/m2
Ocean-going ships
(coated) 10 15
Other ocean going
ships 12 15
Coasters 14 20
Ro-Ro ferries 14 20
Trawlers 22 24
Kort nozzle tugs 22 24
Dredgers 24 27
Ice breakers 25 30
Tugs 18 22
An overview of marine corrosion protection T. Tezdogan, Y. K. Demirel
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7.1 CP design calculations
Cathodic protection design of a ship is carried out in accordance with very simple
calculations. Firstly the total net anode weight requirement of the system is determined (Eq.
1), and then the number of anodes is found (Eq. 2). The formulae for these calculations are
given below [10]:
1000
xx x
A
iCT
W= (1)
where
W= Total net anode weight (kg)
A= Area to be protected (m2)
i= Current density of the structure (m.A/m2)
C= Anode consumption rate (kg/A/y)
T= Design life (y).
And the number of anodes is easily computed by
NW
w
= (2)
in which
N= Number of anodes, and
w= Individual net weight of anodes (kg).
7.2 Real case studies
The calculations given in Section 7.1 have been applied to an oil product tanker which
has a 1835 m2 wetted surface in the underwater hull, 15 m2 in the sea chests, and 24 m2 in the
rudder blade. It is desired to achieve a 3-year protection by an SACP system. Aluminium
anodes are chosen to be used. After the calculations, it is concluded that the vessel needs a
total of 293 kg of anodes to provide external protection for 3 years.
Figure 9 shows the anode distribution around the stern of the ship where the anodes are
installed in 3.750 metres separations based on the calculations performed.
Fig. 9 Anode distribution of the vessel around the stern
A different and more detailed example is an 8400 DWT chemical tanker. Data of
protected wetted areas of the ship is given below:
− Hull: 3310 m2
− Rudder: 40 m2
− Thruster Tunnels 18 m2
− Sea Chests 15 m2
The ship has an epoxy paint system, so it does not need as much current density in the
hull.
An overview of marine corrosion protection T. Tezdogan, Y. K. Demirel
with a focus on cathodic protection and coatings
58
The current density requirements of various areas of the vessel are listed as follows:
− Hull: 10 mA/m2
− Rudder: 100 mA/m2
− Thruster Tunnels 150 mA/m2
− Sea Chests 40 mA/m2
The aluminium anode alloy consumption rate is 3.39 kg/A in each year (given by the
CP supplier).
The external CP system calculations have been performed in accordance with equations
(1) and (2). The following values are the results, giving the total net anode weight requirement
of the vessel in order to ensure a 5-year protection.
− Hull: 561.0 kg
− Rudder: 67.8 kg
− Thruster Tunnels 45.8 kg
− Sea Chests 10.2 kg
It is decided that 30% of the anodes in the hull should be mounted in the stern hull, and
the rest should be mounted in the remain hull.
The number of anodes, and the net and gross weights of each anode are shown in Table
2. Different anode types are distributed in various areas on the ship’s external surface in this
example.
Table 2 Anode distributions in the external ship
ANODE DATA ANODE ANODE WEIGHT(KG
)
AREA TYPE NET GROSS
STERN HULL A-141 12.5 14.1 168.3 14 197.4
REMAIN HULL A-141 12.5 14.1 392.7 32 451.2
RUDDER A-98 8.6 9.8 67.8 8 78.4
THRUSTER TUNNELS A-55 4.6 5.5 45.8 10 55.0
SEACHESTS A-32 2.5 3.2 10.2 10 32.0
Total net
we i g ht Number of
anodes Total gross
weig ht (k g)
The calculations and results given in this section for both examples provide a general
idea to engineers about an external CP system design. The calculations can easily be adapted
to any ship as long as the current density requirements are known.
Current density requirement tests are carried out by applying a current using a
temporary test setup, and adjusting the current from the power source until convenient
protective potentials are attained [11].
8. Conclusions
A general review of cathodic protection methods has been presented. The two main
methods are: ICCP and SACP, which have been described in the earlier sections.
The comparison of both methods suggests that in spite of the fact that the ICCP system
has a slightly higher initial cost, it is more economical than SACP in the long term.
Nonetheless, the SACP system is easy to be installed and apparently very convenient for
internal use such as in ballast and cargo tanks.
The use of coatings for corrosion prevention was also examined. It is clearly shown in
the paper that coatings provide the most effective protection when they are used in
combination with a proper CP system.
An overview of marine corrosion protection T. Tezdogan, Y. K. Demirel
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59
The CP calculation procedure has also been explained and as an example, two real cases
are considered. It was shown that cathodic protection calculations greatly depend on the area
to be protected and the current density requirement of the structure.
It is of note that there has been a great effort to develop a novel, environmentally
friendly and cost effective marine antifouling and anticorrosive coating, as reported in [12]. It
is believed that these efforts will lead to the long term effective prevention of fouling and
corrosion for all types of marine structures, while minimising the need for maintenance and
repair.
Acknowledgements
The corresponding author gratefully acknowledges the sponsorship of Izmir Katip
Celebi University in Turkey, where he has been working as a research assistant, for giving the
Council of Higher Education PhD Scholarship to fully support his PhD research at the
University of Strathclyde, Glasgow. Additionally, the authors would like to thank Miss Holly
Yu for her help with the final proofreading.
This paper is based on the study presented at the International Conference on Marine
Coatings, which was held on April 18, 2013 in London.
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[12] Y. K. Demirel, M. Khorasanchi, O. Turan, A. Incecik: On the Importance of Antifouling Coatings
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Submitted: 25.08.2013.
Accepted: 26.02.2014.
Tahsin Tezdogan
Yigit Kemal Demirel
University of Strathclyde Department of Naval
Architecture, Ocean and Marine Engineering
Henry Dyer Building, 100 Montrose Street
Glasgow G4 0LZ, United Kingdom