ELEKTRO INDONESIA Edisi ke Dua, April 1996
ELEKTRONIKA
Cathodic Protection System
(Summary)
I. Introduction
One of the main problem with equipments and structures which are made of steel is corrosion.
The proper corrosion problem handling may increase the life time of the equipment or structures
and on the other hand the unproper handling will reduce the life time or even will cause an
unexpected damage to the structures. Therefore corrosion problem handling is an important part
of the industrial design. One of the most popular and proven corrosion protection system is
cathodic protection. Basically corrosion process is analogical to electrolysis process which happen if the following components are present :
- Anode
- Cathode
- Electrolyte
- Metallic path between the anode and cathode
If part of the steel become more positive than the other, which meant anode and cathode is
present, and if the steel is immersed in the water or located underground (water/soil as
electrolyte) then the four components of corrosion cell is exist. Part of the steel may become more
positive than other part due to the following conditions :
- Dissimilar Metal Cells
Part of the steel which has more positive natural potential tend to become anode (see EMF
series).
- The steel is placed in the soil with different composition
Part of the steel which located at lower resitivity soil tend to become anode.
- The stell have parts with different temperature
Part of the steel which higher temperature tend to become anode.
- The stell have parts with different stress
Part of the stell with higher stress tend to become anode.
- The steel have parts with different age
The new steel tend to become anode.
Corrosion take place at the place where the electric current go out
of the steel (anode area) and
corrosion will not take place at the place where the current come
into the steel (cathode area).
If all surface of the steel become cathode area there
will be no corrosion, this condition can be
achieved by using cathodic protection system.
Table .1 - Electromotive Series of Metals
Metal on Formed Potential
Lithium Li +2.96
Rubidium Rb +2.93
Potassium K +2.92
Strontium Sr +2.92
Barium Ba +2.90
Calcium Ca +2.87
Sodium Na +2.71
Magnesium Mg +2.40
Alumunium Al +1.70
Berylium Be +1.69
Manganese Mn +1.10
Zinc Zn +0.76
Chromium Cr +0.56
Iron (ferrous) FE +0.44
Cadmium Cd +0.40
Indium In +0.34
Thallium Tl +0.33
Cobalt Co +0.28
Nickel Ni +0.23
Tin Sn +0.14
Lead Pb +0.12
Iron (ferric) Fe +0.04
Hydrogen H 0.00
Antimony Sb -0.10
Bismuth Bi -0.30
Arsenic As -0.30
Copper (cupric) Cu -0.34
Copper (cuprous) Te -0.56
Tellurium Te -0.56
Silver Ag -0.80
Mercury Hg -0.80
Palladium Pd -0.82
Platinum Pt -0.86
Gold (auric) Au -1.36
Gold (aurous) Au -1.50
II. Principle of Cathodic Protection
Cathodic protection is a corrosion protection method in which
the corrosion cell is overcomed by
providing external anode and let direct current to flow from that
external anode on to all surface of
the steel or to make all surface of the steel to be protected become
the cathode. To stop the
corrosion process, the cathdic protection driving potential must
be greater then the corrosion cell.
The driving potential sufficiently can be monitored by measuring the pipe to earth potential. The pipe to earth potential of -0,85 v with reference to copper/copper sulfatere reference electrode
indicate sufficient cathodic protection driving potential.
- Galvanic Cathodic Protection System
In galvanic cathodic protection system the driving potential comes
from the natural potential /emf
different between the anode and the steel. The anode materials must
have high enough natural potential in order to provide enough driving
potential.
In galvanic cathodic protection system the anode is connected directly
to the steel to be protected
through the anode lead wire without electric power supply.
- Impressed Current System
In impressed current system the driving potential comes from a direct current power source call
as transformer rectifier. In this system the potential different between the anodes and the steel
can be raised or or lowered by adjusting the transformer rectifier. In impressed current system
the anode will be conneted to the positive terminal of transformer rectifier and the steel will be conneted to negative terminal of transformer rectifier.
III. Cathodic Protection Equipment
- Transformer rectifier
Transformer rectifier is a DC power source for impressed current cathodic
protection system.
Basically it is a step down transformer which has rectifier in the output
side to rectify the AC current into the DC current. The transformer
should have wide output voltage range for example from 2 V to 20 V (with 2 V increament). The
wide output range adjustment is required in order to get the right driving potential, if it is to low the
cathodic protection will not function properly and if it is to high it can damage the coating of the protected steel.
- Anodes
The anodes type to be used depend on the cathodic protection system and the location of the
steel to be protected. For onshore galvanic cathodic protection system magnesium anodes are
normally used. For onshore impressed current system graphite anodes and high silicion iron are
normally used. For off shore galvanic cathodic protection system alluminium or zinc anodes are
normally used.
For onshore application the anodes can be ordered prepacked in backfilled
(the anode has already packed with low resistivity backfilled material),
in order to get better current output. For
offshore pipeline application the anodes is bracelet type to suit
the shape of the pipe. For bracelet
type the thickness of the anode shall be lower than the thickness of
the concrete coating of the
pipe to be protected.
- Anodes Distribution Box
Anodes distribution box is junction box which is used to connect each anodes lead wire to one
cable, before they are connected to the pipe or transformer rectifier terminal. The purpose of
using this box is to provide facilities to check each anode current output, so it can be cetected if
one anode is not working.
- Test Station
Test station or test box is a junction box consixt of a terminal block, to which the cable from the
pipe (welded to the pipe) is terminated. The test station are installed along thepipe line which is
protected by cathodic protection, to allow the pipe to earth potential to be tested along the pipe
line roates, to check wether the cathodic protection function properly in all part of the pipe line.
- Bonding Box
Bonding box is a juntion box complete with adjusting resistor, which is used to connect two
cathodic protection system to avoid interference.
- Thermo Weld
Thermo weld consist of the mould and the powder, it is normally used
to connect the cable to the
protected steel. There are many type of mould to suit the aplication
such at cable to pipe, cable
to horizontal flat steel, cable to vertical flat steel, cable to
cable etc, for cathodic protection system,
thermo weld is also used for cable to cable connection. It is recommended
to house to cable
connection in cable splicing kit with resin pouring in order to avoid
current leakage.
- Cathodic Protection Multimeter
The cathodic protection multimeter & reference electrode is required to measure the pipe to earth
natural potential. To measure the pipe to earth natural potentialone cable from the pipe (welded to
pipe) connected to one terminal of the multimeter and one other cable from multimeter terminal is
connected to reference electrode). Connection to the pipe can be taken from test station.
- Insulating flange or insulating joint
To separate the protected pipe with the other steel structure or equipment insulating flange or
insulating joint is used. The purpose to separate the protected steel with other structure is to avoid
current leakage.
- Soil Resistivity Meter
Soil resistivity is one important factor in cathodic prtection design. The soil resistivity will determine
the Cathodic protection system to be used and the amount of anode required, and the location of
the anodes groundbid.
Nilson four pin soil resistivity meter is normally used, with this meter soil resistivity meter at certain
defth can be measured by using the certain spacing of the pin.
IV. Cathodic Protection Survey
- Soil Resistivity Survey
Soil resistivity survey shall be performed prior to the design. The most popular method is to used
Nilson soil resitivity meter. The other way is by using soil box especially to measure the soil in
deepwell groundbed system., where the soil resistivity must be measured at various defth, by
taking the soil sample during drilling.
- Current Requirement survey
If the structure to be protected has already been installed at site, beside from calculation, the
more exact current requiment can be found by using current requiment survey. This survey can
be done by using a DC source such as car battery, variable DC source, ampermeter and
temporary anodes. Temporary anodes can be made from any kind of steel available at side. The
temporary anode shall be connected to the positive terminal of the battery and negative terminal
of the battery shall be connected to the steel.
When the pipe to earth potential reach - 0,85 v with reference to copper/copper sulfate reference
electrode, the current requiment can be read through the ampermeter. This current requiment can
be used to determine the anodes quantity and also to determine the transformer rectifier rating for
impressed current system.
- Interference Survey
If teo structure is located close to the steel protected by cathodic there may be an interference
problem. Interference can cause damage to foreign structure and can cause cathodic protection
system do not work properly. To monitor wether there is interference or not the pipe to earth
potential of both structure shall be measured.
If there is no change in pipe to earth potential in one system while the other cathodic protection
system is switch on and off, that's mean no interference.
This criteria is also used to avoid
interference by bonding the two structure through bonding box.
The resitance at the bonding box
is adjusted untill there is no change in pipe to earth potential
of one system while the other
cathodic protection system is switched on and off.
The same criteria is also used to monitor the insulating flange, where
the other side of the flange potential to earth shall not change while
the cathodic protection system at one side is switched on off.
V. Cathodic Protection Design
- Cathodic Protection System Selection
Cathodic protection system selected base on technical and economical reason such as :
- soil resistivity value
- pipe size and length (for short length or small soze of pipe galvanic system is more economical)
- availability of electric power
- Anodes Quantity Calculation
Anodes quantity can be calculated base on the following tecnical data :
- soil resistivity
- pipe/steel structure size and length
- lifetime
- Selection of Anodes Type
Anodes shall be selected base on the location such as onshore or offshore and current output
requirement.
- Selection of Anodes Installation Location
Anodes location is selected based on the following :
- soil resistivity
- distence from the pipe (for impresed current)
- uniform distribution (for galvanic system)
- Selection of transformer Rectifier Rating
Transformer rectifier rating is determined based on the available power source the soil resistivity
and the current requirement. The current requirement will determine the output DC current of the
transformer rectifier, while the output DC voltage can be calculated based on the soil resistivity,
cable resistivity , anodes to earth resistivity and pipe for earth resistivity.
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