2. Cathodic protection
Corrosion occurs as
a result of an
electrochemical
reaction driven by a
potential difference
between two
electrodes, anode
and cathode,
connected by an
electronic path and
immersed in the
same electrolyte.
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3. High pH (> 9)
Preventing Corrosion
Salt
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4. Cathodic protection
There are various time-proven methods for
preventing and controlling corrosion. One of
these is cathodic protection.
The function of cathodic protection is to reduce
the potential difference between anodes and
cathodes to a neglected value. This reduction is
due to the polarization of cathodes to the
potential of most active anodes. In this way,
corrosion current is mitigated according to
Ohm’s law.
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5. Cathodic protection
There are various time-proven methods for
preventing and controlling corrosion. One of
these is cathodic protection.
Corrosion occurs at anodic areas. If all anodic
area can be converted to cathodic areas the
structure would become cathode and corrosion
stops as there is no corrosion at regions where
current enters from the environment to metal
(cathodic areas).
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6. Cathodic Protection Criteria
In order to achieve adequate Cathodic
Proctection (CP), the protected structure must
be polarized to a certain value. The polarized
potential is measured with respect to a
certain reference electrode. A copper/copper
sulfate reference electrode (CSE) is the most
common electrode used in soil and freshwater.
There are two types of criteria for assessing
cathodic protection. Either one may be used
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7. Types of Cathodic Protection
There are two types of cathodic protection:
– Galvanic anode cathodic protection
– Impressed current cathodic protection
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8. Galvanic anode cathodic
protection
In this type of CP, protection is achieved by
connecting the protected structure to a
sacrificial anode, which is placed close to the
protected structure.
– Sacrificial anodes are made from active metals
such as zinc, aluminium, or magnesium, which are
considered the most active metals according to
the galvanic series.
– CP current is created by the potential difference
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10. Preventing the Corrosion of Iron
(cathodic protection/sacrificial anode)
Sacrificial anode
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11. Impressed current cathodic protection
(ICCP)
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In this type of CP, protection
is achieved by connecting the
protected structure to an
anode bed through
a transformer rectifier (TR).
The anode bed is a series of
buried anodes that are
electrically connected and
surrounded by certain
backfill to reduce their
resistance to the earth. The
anode bed should be placed
remotely from the protected
structure
12. Impressed current cathodic protection
(ICCP)
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The main component of this type of CP is the TR, which
forces the current to flow from the anodes to the
protected structure (cathode). Three types of anodes are
used:
Soluble anodes (aluminium and steel), semi-soluble
anodes (graphite and high silicon cast iron and non-
soluble anodes (platinum, mixed metal oxide, and
polymer).
The type of anodes used depends on the chemical
composition of the electrolyte, to which the substrate is
exposed and the area to be protected.
13. Impressed Current (ICCP)
•
Involves the use of an external power source – metal to be
protected is made cathodic to its surroundings – inert anodes
used which are virtually non-consumable – insulated from
structure
•
Early anodes made from scrap steel but most modern ICCP
systems use lead silver alloy, titanium or niobium
•
Has been used in the protection of steel reinforcement in
concrete
•
The use of modern electronics makes the system self
regulating
•
Very costly to run – mainly used in marine applications – oil
rigs – large anodes placed on sea bed approximately 100m
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14. Impressed Current Cathodic
Protection
Power Supply
Shipboard Power
Controller
Shipboard Power
Insulation
Pt Anode
Reference
Electrode
Paint
Layer
Hull
e-
e
-
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15. Application of Cathodic Protection Systems on
Buried Pipelines
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Pipelines are used for transporting water, petroleum
products, natural gas, and other utilities. There’s a huge
network of piping systems used in every country all over
the world. Pipelines may be onshore or offshore, and are
subject to corrosion in both cases. If corrosion isn't
mitigated, dangerous and expensive damage can be the
result.
16. Applications of Cathodic Protection
•
Galvanized Steel
Zinc coating
•
Sacrificial Anodes
Ship Hulls
Subs (free flooding areas)
Los Angeles Class Sub
Arleigh-Burke Destroyer
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17. Application of Cathodic Protection Systems on
Buried Pipelines
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There are several corrosion control techniques used on
pipelines; cathodic protection is one of them. It can be
applied either to coated pipelines to mitigate the
corrosion attack on areas where coating quality may be
poor. It is also used on bare pipelines. Both types of CP
can be applied to buried pipelines. The application of
either of these types depends on several factors, such as
the required current, soil resistivity, and the area to be
protected.
18. Application of Cathodic Protection Systems on
Buried Pipelines
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CP aims to polarize a pipeline to a minimum potential of
-850 mv, for carbon steel and for adequate CP.
The polarized potential is to be measured through test
stations, which are to be installed at the following
locations along the route of pipeline:
•
At frequent intervals (e.g. < 2 km / 1.24 miles)
•
At crossings with foreign structures
•
At points of electrical isolation
•
At some galvanic anode locations
•
At casings
•
Near sources of electrical interference
•
At the location of stray current discharge to earth
19. Sacrificial anode system in seawater Impressed-current cathodic-protection
system in seawater
4Al => 4AL+++ + 12 e-
3O2 + 12e- + 6H20 => 12OH-
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20. Problems Created by Cathodic Protection
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In large pipeline networks, there are a lot of crossings,
parallelism, and approaches, wherein the pipeline has its
applied CP system. DC interference may occur between
pipelines, accelerating corrosion. In order to overcome
this problem, pipelines can be electrically coupled, either
directly or through resistance.
Cathodic protection is just one method used to prevent
corrosion, not just in pipelines, but in ships, offshore oil
platforms, and other steel structures. Whether it's the
best application for the job, or the only one to be used, is
often specific to the structure being protected
