DESIGN OF IMPRESSED CURRENT CATHODIC PROTECTION FORSTEEL
IMMERSED IN ABSTRACTImpressed current cathodic protection (ICCP)
and coating give the optimum protection against corrosion for steel
immersed in freshwater. This project presents the results of a
study on the effectiveness of coating, impressed current
cathodicprotection and different environment conditions in
preventing corrosion of steel.Experimental tests were carried out
on coated and bare steel plates with ICCP andwithout ICCP by
immersing in stagnant and flowing freshwater for one month.
Theresults demonstrated that for coated and bare steel with ICCP
and different variableresistance, the values of the potential are
sufficient to protect the bare and the coated steel -840mV to
-875mV.For coated steel without ICCP immersed in stagnantfreshwater
the potential has changed from -702 mV to -630mV, but for the
baresample the change in potential was about -10mV this may be due
to oxide layerformed on the metal surface. For coated steel without
ICCP immersed in flowingfreshwater the drop in potential was about
-50mV and the bare steel with the samecondition was about -100 mV.
A good agreement was observed for corrosion ratebetween weight loss
measurement (4.29 mpy) test and electrochemical test (4.27mpy) for
bare steel in stagnant freshwater. The location of the reference
electrode hassignificant implications for the control the potential
change of ICCP system, thecorrosion potential increases at the top
of the sample (60cm below the water) anddecrease when the sample
was immersed further down to 1 meter in the water level.
11.1Introduction11.2Background of the Study11.3Objectives of the
Study31.4Research Questions31.5Significance of the Study41.6Scopes
of the Study42LITERATURE REVIEW52.1General
Review52.2Electrochemical Nature of Aqueous Corrosion62.3Corrosion
Control92.3.1Design9 viii2.3.2Materials
Selection102.3.3Inhibitors112.3.4Protective Coatings112.3.5Cathodic
Protection112.3.5.1The Principles of
CathodicProtection122.3.5.2Types of Cathodic Protection132.4Current
Sources162.4.1Transformer/Rectifiers162.4.1.1Circuit
Breaker202.4.1.2Transformer212.4.1.3Rectifier Cells212.4.2Rectifier
Efficiency222.4.3Engine Generator Sets232.4.4Batteries, Solar and
Wind Generators232.4.5Thermoelectric Generators242.4.6Closed Cycle
Turbo Generators252.5Anode Materials252.5.1Steel Scrap
Anodes262.5.2Cast Iron Scrap Anodes272.5.3Silicon Iron
Anodes272.5.4Graphite Anodes272.5.5Magnetite Andes282.5.6Lead Alloy
Anodes282.5.7Platinised Titanium Anodes292.5.8Mixed Metal Oxide
Based Anodes292.5.9Zinc Anodes302.5.10Aluminium
Anodes312.6Distributed Anode Cables312.7Protection of Underwater
Structure32 ix3RESEARCH METHOLOGY343.1Introduction343.2Impressed
Current Design353.13.1Physical Dimensions of Structure to
beProtected363.13.2Drawing of Structure to be
Protected363.13.3Electrical Isolation363.13.4Short
Circuits373.13.5Corrosion History of Structures in
theArea373.3Review pH Data373.4Variations in Temperature and
Concentration383.5Current Requirement383.6Coating
Resistance403.7Selection of Anode Material, Weight
andDimensions403.8Calculate Number of Anodes Needed to
SatisfyManufacturers Current Density Limitations423.9Determine
Total Circuit Resistance433.10Calculate Rectifier Voltage to
Determine VoltageOutput of the Rectifier433.11Power Source
Selection443.12Monitoring by Measuring of the
Potential473.13Electrochemical Testing483.13.6Principle of
Measurement483.13.7Preparation of Working Electrode503.14Immersion
Test52 x4RESULTS AND DISCUSSION534.1Chemical Composition of
Materials Used534.2Impressed Current Cathodic
ProtectionCalculations544.2.1For Coated Steel Immersed in
StagnantFreshwater544.2.2For Bare Steel Immersed in
StagnantFreshwater564.2.3For Coated Steel Immersed in
FlowingFreshwater584.2.4For Bare Steel Immersed in
FlowingFreshwater604.3Potential Measurement Results624.3.1Coated
and Bare Steel Immersed inStagnant Freshwater with
ICCP624.3.2Coated and Bare Steel Immersed inStagnant Freshwater
without ICCP644.3.3Coated and Bare Steel Immersed inFlowing
Freshwater with ICCP664.3.4Coated and Bare Steel Immersed inFlowing
Freshwater without ICCP684.4The Effectiveness of the Reference
ElectrodeLocation on The Protection Potrntial
Result704.5Electrochemical Result744.5.1Visual
Inspection744.5.2Polarization Result744.6Immersion Test Results76
xi5 CONCLUSTION AND RECOMMENDATIONSFOR FUTURE
WORK775.1Conclusions775.2Recommendations for Future
work78REFERENCES79APPENDICES81Appendices A - C81-92 xiiLIST OF
TABLESTABLE NO. TITLE PAGE2.1 Comparison between sacrificial anode
system andimpressed current system152.2 Typical consumption rates
of impressed current anodematerials263.1 Current density and types
of environment 293.2 Coated and bare samples immersed in different
conditionsof freshwater443.3 Potentiostatic polarization test
parameters 483.4 Immersion test parameters 524.1 Chemical
composition of low carbon steel 534.2 Electrochemical result 754.3
The result of corrosion rate of samples without ICCP 76 xiiiLIST OF
FIGURESTABLE NO. TITLE PAGE2.1 Shows corrosion of pipeline 62.2
Electrochemical nature of corrosion processes in water 72.3 The
principle of cathodic protection 132.4 (a) Sacrificial anode system
14(b) Impressed current system 142.5 Operation of a single phase
bridge rectifier 192.6 Components of a rectifier 222.7 Typical zinc
anode 302.8 Marine structure anode 323.1 Flow chart of research
methodology 353.2 Schematic of coated and bare samples with and
withoutICCP in45(a) Stagnant freshwater 45(b) Flowing freshwater
453.3 Actual sites in marine technology laboratory 46(a) Stagnant
freshwater side 46(b) Flowing freshwater side 463.4 Wave generator
towing tank 463.5 Silver- Silver chloride reference electrode 47(a)
Schematic 47(b) Real 473.6 Copper- copper sulfate reference
electrode 47(a) Schematic 47(b) Real 473.7 Cell kit setup 49 xiv3.8
Photographs of 50(a) Connection of specimen to copper wire
bybrazing technique50(b) Mounting of samples 503.9 Photographs of
51(a) Working electrode 51(b) Typical surface area of a sample
514.1 Potential measurement of coated and bare samples instagnant
freshwater with ICCP634.2 Samples with ICCP after 1 month immersion
in stagnantfreshwater63(a) Coated sample 63(b) Bare sample 634.3
ICCP anodes after 1 month immersion in stagnantfreshwater for64(a)
Coated sample 64(b) Bare sample 644.4 The potential measurement on
coated and bare samples instagnant freshwater without ICCP654.5
Samples without ICCP after 1 month immersion instagnant
freshwater65(a) Coated sample 65(b) Coated sample 65(c) Bare sample
65(d) Bare sample 654.6 Quantitative analysis of XRD pattern of
corrosionproducts from the bare sample in stagnant freshwater664.7
Potential measurement of coated and bare samples inflowing
freshwater with ICCP674.8 Samples with ICCP after 1 month immersion
in flowingfreshwater67(a) Coated sample 67(b) Bare sample 67 xv4.9
ICCP anodes after 1 month immersion in flowingfreshwater for68(a)
Coated sample 68(b) Bare sample 684.10 Potential measurement of
coated and bare samples inflowing freshwater without ICCP694.11
Samples without ICCP after 1 month Immersion inflowing
freshwater69(a) Coated sample 69(b) Coated sample 69(c) Bare sample
69(d) Bare sample 694.12 Effectiveness of reference electrode
location on thesamples potential in stagnant freshwater with
ICCP714.13 Effectiveness of reference electrode location on
thesamples potential in stagnant freshwater without ICCP714.14
Effectiveness of reference electrode location on thesamples
potential in flowing freshwater with ICCP724.15 Effectiveness of
reference electrode location on thesamples potential in flowing
freshwater without ICCP724.16 Bar chart for samples immersed in
stagnant freshwater 734.17 Bar chart for samples immersed in
flowing freshwater 714.18 (a) A specimen before electrochemical
test 74(b) A specimen after electrochemical test 744.19 Tafel
extrapolation curve for bare steel in freshwater 75 xviLIST OF
APPENDICESAPPENCIX TITLE PAGEA The potential measurement for coated
and bare steel instagnant and flowing freshwater with and without
ICCP81B General properties of low carbon steel 85C Wave generator
towing tank 86 CHAPTER 1INTRODUCTION1.1IntroductionThis section
discuss about the introduction of the study which are backgroundof
the study, purpose and objective of the study, significant of study
and scope ofstudy.1.2Background of the StudyCorrosion can be
defined as destruction or deterioration of the materialbecause of
the reaction with the environment. Most of the materials which
undergocorrosion are metal, so some insist definition of the
corrosion should be specific tothe metal. Mars G. Fontana [1]
suggest that all material including ceramic, polymerand other
non-metallic material which contributes into the corrosion reaction
shouldbe taken care.Corrosion weakens strength and cause failure on
material. Protectionmaterials from undergoing corrosion become
crucial especially tropical country likeMalaysia which has high
humility. Cost of the corrosion in United State is aroundUSD$ 40
billion or RM 140 million annually. Protection need to be done onto
thematerial so that reduce corrosion rate so that less materials
and money being wasted. 2Acidity of water varies over a wide range
because variety of the compositions.Factors affecting acidness of
water is moisture, alkalinity, permeability of air,oxygen, salts,
stray currents, and biological organisms [1].Several methods used
to protect materials from being corrode, for examplecoating,
cathodic and anodic protection. In our research, we will only
concentrateinto impressed current cathodic protection (ICCP) which
is commonly used in bigstructure or component protection. ICCP
systems require the use of an external DCpower supply that is
energized by standard AC current. There are several
importantadvantages for using ICCP systems, for example unlimited
current output capacity,adjustable out capacity and lower cost per
ampere of cathodic protection current [2].Its usually cost
effective to justify the adoption of an ICCP system, forexample it
is much cheaper in term of long term and large structure, for build
anICCP system than to locate and repair the underground structure
leaks. Impressedcurrent cathodic protection (ICCP) system take
advantage of natural electrochemicalreactions of the materials to
minimize corrosion damage. In an ICCP system, anexternal source of
electrons is provided to the metal/electrolyte combination. In
orderto achieve protection from the corrosion the sources of
electrons must be sufficient toraise potential of the structure to
a level at which negligible corrosion occurs [3]. 31.3Objectives of
the StudyThe objectives of this study are:1.To design an ICCP model
for steel structure immersed in freshwater.2.To compare between
impressed current cathodic protection for a steelstructure immersed
in stagnant freshwater and impressed currentcathodic protection for
a steel structure immersed in flowing freshwater.3.To measure the
potential of steel with and without impressed currentcathodic
protection and determine the effectiveness of impressedcurrent
cathodic protection design.4.To determine the effect of coating and
ICCP protection on corrosionbehavior of carbon steel.5.To determine
the effectiveness of the location of the reference electrodeon the
protection potential.1.4Research QuestionsThe research questions
are1.How to build an effective laboratory scale impressed current
cathodicprotection setup for a structure immersed in water?2.How to
improve current impressed cathodic protection system?3.How to
control parameters of the ICCP for example current, selectedanode
etc. 41.5Significance of the StudyThe findings of this study are
important to understand theory of the ICCPsystem. In the current
project, an effective laboratory scale ICCP system has
beendesigned. Comparison of results for laboratory ICCP system and
real application canbe done for further understanding the effect of
parameters upon ICCP system.1.6Scopes of the StudyThe scopes of the
study include the following;1.Literature review on corrosion
principles.2.Design an impressed current cathodic protection for
steel immersed infreshwater by calculating the current required,
selecting an anodematerial, number of anodes, circuit resistance
and power sourceselection.3.Determine the effectiveness of coating
and ICCP protection oncorrosion behavior of carbon steel by
measuring the potential for steelin different freshwater
conditions.4.Determine the effectiveness of the locations of the
reference electrodeon the protection potential by measuring the
potential at differentpositions of the samples. CHAPTER 2LITERATURE
REVIEW2.1General ReviewCorrosion is defined as destruction or
deterioration of a material, because it isa form of destructive
attack of a metal by chemical or electrochemical reaction withits
environment. In the most common use of the word, corrosion means a
loss ofelectrons of metals reacting with water and oxygen. In the
other way, some of thescientists think that deterioration by
physical cause is not belong to corrosion, but isdescribed as
erosion, galling, or wear [1]. Suggest that some of the chemical
attackwill accompanies physical deterioration physical
deteriorations, for examplecorrosion erosion, corrosive wear, or
fretting corrosion, included both destructionand deterioration into
the concept of corrosion [2].Corrosion is an electrochemical
process in which a current leaves a structureat the anode site,
passes through an electrolyte, and reenters the structure at
thecathode site as Figure 2.1 shows. For example one small section
of a pipeline may beanodic because it is in a environment with low
resistivity compared to the rest of theline. Current would leave
the pipeline at that anode site, pass through theenvironment, and
reenter the pipeline at a cathode site. Current flows because of
apotential difference between the anode and cathode. That is, the
anode potential ismore negative than the cathode potential, and
this difference is the driving force forthe corrosion current. The
total systemanode, cathode, electrolyte, and metallic 6connection
between anode and cathode (the pipeline in Figure 2.1) is termed
acorrosion cell [4].Figure 2.1Corrosion of a Pipeline Due to
Localized Anode and Cathode(Source: Technical manual, Headquarters
Department of The US Army Washington,1985)2.2Electrochemical Nature
of Aqueous CorrosionIn our societies, water is used for a wide
variety of purposes, from supportinglife as potable water to
performing a multitude of industrial tasks such as heatexchange and
waste transport. The impact of water on the integrity of materials
isthus an important aspect of system management. Nearly all
metallic corrosionprocesses involve transfer of electronic charge
in aqueous solutions. Thus, tounderstand the electrochemical nature
of aqueous corrosion it is necessary to start thediscussion with
the electrochemical reactions. Basically all environments
arecorrosive to certain degree, thus we take an example of
corrosion of a metal M with2+ as the oxidation number in HCl acid
for discussion on the electrochemicalreactions as shown in Figure
2.2.
7Figure 2.2Simple Model Describing The Electrochemical Nature of
CorrosionProcesses in HCl[5]Metal ions go into solution at anodic
areas in an amount chemicallyequivalent to the reaction at cathodic
areas. In the cases of iron-based alloys, thefollowing reaction
usually takes place at anodic areas: [5]M + 2HClMCl2+ H2(2.1)Metal
reacts with acid solution forming soluble metal chloride and
liberatinghydrogen bubbles on the surface. In ionic form the
reaction isM + 2H++ 2ClM+2+ 2Cl+ H2(2.2)
8Eliminating Cl from both side of the reaction givesM + 2H+M+2+
H2(2.3)Reaction (2.3) can be separated as followsMM+2+ 2e (Anodic
reaction) (2.4)2H++ 2e-H2(Cathodic reaction) (2.5)In deaerated
solution, the cathodic reaction is shown in equation (2.5).
Thisequation is rapid in most media, as shown by the lack of
pronounced polarizationwhen metal is made an anode employing an
external current. When metal corrodes,the rate is usually
controlled by the cathodic reaction, which in general is muchslower
(cathodic control).The most important basic principle of corrosion
is during metallic corrosion,the rate of oxidation equals to the
rate of reduction. In some corrosion reactions, theoxidation
reaction occurs uniformly on the surface while in other cases it is
localizedand occurs at specific areas.Generally, corrosion form can
be represented by the equation of (2.4).Simplest equation of
reaction is in acidic deaerated solution, while aerated acidic
andalkaline solution will be represented by the equations (2.6) and
(2.7) 9O2+ 2H2O + 4e-4OH (aerated alkaline solution) (2.6)andO2+
4H++ 4e-2H2O (aerated acidic solution) (2.7)In the absence of all
other reduction reactions, water will be reduced by2H2O + 2e-H2+
2OHThe equation is equivalent to reaction (2.5), assuming
dissociation of water toH+ and OH- and subtracting OH- from both
sides of the reaction [5].2.3Corrosion ControlThere are five
popular methods to control corrosion2.3.1DesignAs an old adage
says, corrosion prevention must start at the blackboard, at
thedesign stage. A good design at the blackboard is no more costly
than a bad design, abad design is always more expensive than a good
design in reality. Technical designincludes the aspects of design
that directly bear on the proper technical functioningof the
product attributes that describe how it works and how it is made.
Designconfiguration has a critical role to play in the service life
of components. Theimportant point is that the designers must have
an understanding and awareness of 10corrosion problems. Corrosion
is, however, only one of the several parameters withwhich the
designer is concerned and it may not be, however, important to a
designerto give consideration to corrosion unless dictated by a
requirement. In manyinstances, corrosion is incorporated in design
of an equipment only after itspremature failure. More often, more
attention is paid to the selection of corrosionresistant materials
for a specific environment, and a minimal consideration is givento
design, which leads to equipment failure. For instance, even a
material, like 90-10coppernickel may fail prematurely as a
condenser tube material, if the flow velocityof salt water or
seawater is not given a due consideration for a smooth flow in
thetube design. This has been a common observation in desalination
plants in the Gulfregion. This chapter would highlight how
corrosion could be prevented by adoptinggood design practices
[8].2.3.2Materials Selection.The world of materials comprises of
polymers, metals, ceramics, glasses,natural materials and
composites. Revolutionary developments have taken place inrecent
years because of the highly competitive materials market and
emergence ofnew materials and new processing techniques. selecting
a corrosion resistant alloywould be the answer to corrosion
problems.However, corrosion resistance is not the only property to
be considered whenselecting a material. Cost dictate the selection
of materials [8]. 112.3.3InhibitorsA corrosion inhibitor is a
substance which when added in a small quantities toa corrosive
environment reduces the corrosion rate of the metal by action at or
nearthe metal surface.Whether a substance is an inhibitor or not
depends on the nature of both themetal and environment.It is
convenient to classify inhibitors according to which electrode
reactionthey affect: anodic or cathodic [8].2.3.4Protective
CoatingsThe objective of a coating is to provide a barrier between
the metal and theenvironment. Another advantage of protective
coatings is that it is possible tocombine the protective function
with aesthetic appeal. Coating can be classified intoMetallic and
Non Metallic coatings [8].2.3.5Cathodic ProtectionCathodic
protection is a method to reduce corrosion by minimizing
thedifference in potential between anode and cathode. This is
achieved by applying acurrent to the structure to be protected
(such as a pipeline) from some outside source,or current can be
passed between the cathode and the anode due to the different
inpotential When enough current is applied, the whole structure
will be at one 12potential; thus, anode and cathode sites will not
exist. Cathodic protection iscommonly used on many types of
structures, such as pipelines, underground storagetanks, locks, and
ship hulls.2.3.5.1The Principles of Cathodic ProtectionThe
principle of cathodic protection is in connecting an external anode
to themetal to be protected and the passing of an electrical dc
current so that all areas ofthe metal surface become cathodic and
therefore do not corrode. The external anodemay be a galvanic
anode, where the current is a result of the potential
differencebetween the two metals, or it may be an impressed current
anode, where the currentis impressed from an external dc power
source. In electro-chemical terms, theelectrical potential between
the metal and the electrolyte solution with which it is incontact
is made more negative, by the supply of negative charged electrons,
to avalue at which the corroding (anodic) reactions are stifled and
only cathodicreactions can take place. The current density and the
potential are quite high and afterapplying ICCP the potential
decrease with decreasing the current density as shown inFigure 2.3.
13Figure 2.3The Principle of Cathodic Protection2.3.5.2Types of
Cathodic ProtectionThere are two main types of cathodic protection
systems; there are impressedcurrent and sacrificial anode. Both
types of cathodic protection have anodes, acontinuous electrolyte
from the anode to the protected structure, and an externalmetallic
connection (wire). These items are essential for all cathodic
protectionsystems.(a) Sacrificial Anode Cathodic ProtectionA
sacrificial anode cathodic protection system in fig 2.4 (a) makes
use of thecorrosive potentials for different metals. Without
cathodic protection, one area of thestructure exists at a more
negative potential than another, and results the occurrence
14of corrosion on the structure. On the other hand, if a
negative potential metal, such asMg is placed adjacent to the
structure to be protected, such as a pipeline, and ametallic
connection is installed between the object and the structure, the
object willbecome the anode and the entire structure will become
the cathode. New additionobject will be sacrificially corrodes to
protect the structure. Thus, this protectionsystem is called a
sacrificial anode cathodic protection system because the
anodecorrodes sacrificially to protect the structure. Anodes
materials in this system areusually made of either Mg or zinc
because of these metals higher potential comparedto steel
structures [7].(b) Impressed Current Cathodic
ProtectionImpressed-current systems in Figure 2.4 (b) employ inert
(zero or lowdissolution) anodes and use an external source of DC
power (rectified AC) toimpress a current from an external anode
onto the cathode surface [7].Figure 2.4(a) Sacrificial Anode System
(b) Impressed Current System
15Table 2.1:Comparison Between Sacrificial Anode System and
Impressed CurrentSystemSacrificial Anode System Impressed Current
SystemIt requires no external source External power is essentialIt
can be easily installed and maintained More complicated system for
installationIt can be used in areas where the soilresistivity is
lowLimited to use below a soil resistivity of3000 ohms-cmIt is
economical Less economical for small structureFor small structures
For big structuresIn addition to the structure to be protected and
the electrolyte (soil, water,etc.), impressed current cathodic
protection systems consist of the following
essentialcomponents:1.The current source, such as
transformer/rectifiers, solar generators, etc.2.The impressed
current anodes, buried in soil or immersed in water.3.The
interconnecting cables [7].An ICCP uses a rectifier (an electrical
device for converting alternatingcurrent into direct current) to
provide direct current through anodes to the metal tank,piping, or
other underwater components to achieve corrosion protection.The
system may also be provided with a current control circuit to
regulate theprotection level. Such regulation is particularly
useful when different structures areprotected by the same current
source.Impressed current cathodic protection (ICCP) is widely
employed inconjunction with surface coatings to control the
corrosion of the underwaterstructures. The potential static ICCP
systems normally fitted employ closed loopcontrol in which the
current output from a DC. power supply is controlled via areference
electrode (RE) which measures surface potential in its vicinity.
This
16potential is compared with the required protection value (set
potential), typically 800or 850 mV vs silver/silver chloride or
copper/copper sulfate System current output isthen varied, via the
driving voltage of the power supply, to maintain a zero errorsignal
and hence a constant potential at the RE. Current output is thus
controlledautomatically in response to the operational conditions
and the system is, therefore,demand-responsive. The processes
involved in cathodic protection are essentiallyelectrochemical
phenomena at the interfaces between the water and the
cathodicstructure (and the anodic surfaces). ICCP system current
output, as determined viathe maintenance of the set potential in
the vicinity of the RE(s), will be affected by anumber of factors,
such as surface condition, coatings and the presence or of
flow[6].2.4Current
Sources2.4.1Transformer/RectifiersTransformer/rectifiers are the
most economical and usually most reliablecurrent sources for
impressed current cathodic protection. They shall be of a
specialdesign for cathodic protection service and able to operate
under the prevailingservice and weather
conditions.Transformer/rectifier units can be either oil- or
air-cooled. For installationoutdoors in hot climates, oil-cooled
units are preferred. Units with a high currentrating are often
oil-cooled although modern semiconductor technology allowsincreased
current capacities for air cooled units. Air-cooled units are
usually smallerand less expensive than oil cooled units with the
same capabilities.AC power for transformer/rectifier units can be
either single-phase or three-phase. Especially for high power
units, three-phase units are preferred because they 17normally
provide a smoother DC output than single-phase units unless
sophisticatedsmoothing circuits are installed.AC sources able to
accelerate the corrosion of mild steel even though they
arecathodically protected in both the media [11].The
transformer/rectifier shall be provided with an isolator or Moulded
CaseCircuit Breaker (MCCB) on its incoming circuit and, where
applicable, on its ACsub-circuits. Additionally, suitably sized
fuses shall be installed on thetransformer/rectifier's phase AC
sub-circuits and negative DC output circuits.The rectifying
elements shall be constructed with high current density
silicondiodes, so arranged as to provide full wave rectification.
To prevent damage tooverload or short spikes in the supply, the
current rating of the diodes shall be morethan 125 % of the maximum
current rating of the rectifier and have a minimum peakinverse
voltage of 1200 V.The unit shall be able to withstand a short
circuit at the output terminals of upto 15 s duration without
damage to the circuits.The output RMS ripple shall not exceed 5 %
of the DC output currentbetween 5 % and 100 % of the rated current
output. This is particularly important forcertain anode types such
as platinised titanium.The output voltage shall be adjustable from
zero to the maximum rated outputwhen on load. A stepless
(continuous) adjustment is preferred. If tapping switchesare used,
these shall be front mounted switches with a step-size of maximum 3
% ofmaximum output. Transformer tapping should not be done by
relocating jumpersunless changes in operating conditions are
expected to be infrequent (e.g. whensubsequent potential or current
control is used). Electronic voltage and/or currentcontrol may be
used, e.g. in combination with automatic potential control 18For
low current applications such as for well-coated structure, a
ballastresistor may be required to provide a minimum load for good
operation of therectifier.The transformer/rectifier shall be
provided with approximately 70 mmdiameter or similarly sized square
pattern meters to read the output voltage andcurrent. The measuring
accuracy shall be better than 2 % of full scale.The polarity of the
DC terminals and AC supply terminals shall be clearlymarked. AC and
DC cables shall be physically separated e.g. by an insulating
panel.A built-in timer unit may be required. The timer unit may be
mechanical orelectronic and shall be capable of switching the full
output current in a sequence of50 s on and 10 s off. If more than
one transformer/rectifier are protecting a singlestructure, all
transformer/rectifier timer units should be provided with a
facility forsynchronous switching. During normal operation, the
timer shall be bypassed.If a transformer/rectifier is oil-cooled,
the incoming cables shall terminate inseparate non-oil filled cable
boxes and penetration into the tank shall be via bushingsabove oil
level. A sight glass and thermometer shall be provided [7].The
three-phase bridge is the most common circuit for rectifiers
operatedfrom a three-phase AC power line. Each phase of a
three-phase AC current is spaced120 electrical degrees apart and
therefore the voltage of each secondary windingreaches its peak at
different times.Figure 2.6 shows the operation of a single phase
bridge rectifier. Thedirection of flow reverses 60 times per second
for 60 cycles AC. In a positive half-cycle (diagram A), current
originates at T2 on the secondary winding. It is blockedby D3
(silicon diode). The current, therefore, flows through direction
D1, follows thepath (3) and through diode D4 it enters the negative
terminal T2. In the next half- 19cycle (1/120th) of a second later,
polarities at T1 and T2 are reversed (see diagramB). The current is
blocked by diode D4 and flows through D2, follows the path
(3)through D3 in the same direction as before. The load RL thus
receives energy in theform of pulses at 120 per second.Although
three-phase rectifiers are used as mentioned before, each
singlebridge shares a pair of diodes with one of the other bridges.
The three phase bridge islike three single-phase bridges, with each
bridge sharing a pair of diodes with one ofthe other bridges [7]. A
rectifier consists of three important components circuitbreaker,
transformer and rectifying elements (stacks). Brief details are
given inFigure 2.5.Figure 2.5Operation of a Single Phase Bridge
Rectifier. Arrows ShowConventional (positive) Current Flow
Direction
202.4.1.1Circuit BreakerThese are basically switches with an
internal mechanism which opens theswitch when the current exceeds a
prescribed designed limit. They also serve as onand off switches.
There are two types of switches: (1) magnetic and (2) thermal.
Thecircuit breaker protects equipment from over loading.In the
magnetic type, a coil is woven around a brass tube and a magnetic
fieldis set up by a current flowing in the coil. The magnetic slug
is held at one end of atube by a spring. The magnetic field
attracts the slug, but at or below the ratedcurrent the slug does
not move. At overload, the magnetic field pulls the slug into
thecoil. When the slug is drawn to the opposite end of the tube,
the circuit is completedfor the trip mechanism and the breaker
switch trips. The movement of the magneticflux is slowed down and a
time delay is provided. The breaker can trip on to 101125% of the
rated current. Overloadsof ten times the rated currents can be
sustained. The dropping is very fastwhen the overload is ten
times.In thermal magnetic breakers, the thermal tripping is caused
by the flowingcurrent through the resistor close to the bimetallic
strip. When the current exceedsthe rated value, the bimetallic
element trips the breaker and a long time delay isinvolved before
the breaker can be closed [7]. 212.4.1.2TransformerThis consists of
two coils of wire wound around an iron core. The coils arenot
connected electrically, but the core provides a magnetic link
between them. ACvoltage is applied to one coil (primary), the
changing magnetic field crosses to theother coil (secondary) and
induces a voltage in it. The changing field induces the ACvoltage
in the secondary coil that is proportional to the turns ratio
between the twocoils [7].P S =P S 2.4.1.3Rectifier CellsThe change
of AC power to DC is done by rectifying elements. They act
likecheck valves by offering low resistance to current flow in one
direction and highresistance in the other direction. The function
of the rectifying element is to allow thecurrent to flow readily in
one direction and to block current flow in the oppositedirection
fig 2.6. The Selenium cell is the most common rectifier cell.
Selenium isapplied to one side of an aluminum base plate which has
been nickel plated. A thinmetallic layer is applied over the
selenium layer. This layer acts as counter electrode.It collects
the current and provides low resistance to the contact surface.
These cellsmay be arranged in stacks or parallel to produce the
desired voltage and currentrating [7].
22Figure 2.6Components of a Rectifier2.4.2Rectifier
EfficiencyThis is the ratio between the DC power output and AC
power input.Rectifiers are used as a source of DC power. Rectifiers
convert the AC current (60cycles) to DC current through rectifier
operated at maximum efficiency at the fullrated loads.Overall
rectifier efficiency =DC 100%An efficiency filter can be used to
minimize the ripples.
232.4.3Engine Generator SetsWhere AC power is not available to
supply rectifiers and the required poweris high, engine generator
sets may be used to provide the electrical supply needed.If a
remote survey unit with alarms cannot be installed, a
two-generatorsystem shall be used (one running, one on standby)
with an automatic changeoversystem.Remote generator units are prone
to failure and vandalism and requirefrequent maintenance. For
critical systems, alternatives such as solar power may be abetter
option [7].2.4.4Batteries, Solar and Wind GeneratorsIf the AC mains
suffer frequent power failures, the use of batteries, chargedby
mains powered battery chargers, may be used instead of
transformer/rectifiers.Batteries may also be charged by means of a
wind-powered generator or bysolar cells. The batteries should be
charged on a regular basis to provide a continuoussource of
cathodic protection current.Cathodic protection systems using
batteries shall be provided with suitableoutput voltage and/or
current control equipment and a load cut off system to avoiddamage
to the batteries due to a complete discharge. 24Battery chargers
and generators shall be provided with regulators to ensurethat the
recommended charging rates are applied and shall be equipped with
aprotection system to prevent overcharging of the batteries.The
design of wind and solar generators shall be based on extensive
localweather reports, stating average and minimum sun and/or wind
periods and intensityduring all seasons, generally a one-year
period, to determine the capacity of thesystem. The battery
capacity shall be based on the required autonomy during
theprevailing maximum time without sun or wind.Wind and solar
generators shall be rated to recharge the batteries in less than48
hours from a partially discharged state due to an extended period
of no wind/sun.In tropical areas the generators and batteries shall
be designed to operate inhigh ambient temperatures. Solar
generators should be designed to maintain thedesign capacity at the
highest ambient temperature [7].2.4.5Thermoelectric
GeneratorsThermoelectric generators are based on the thermocouple
principle.Heating one side of a stack of thermocouples, sized to
provide the required DCpower, generates power. Heating of the unit
is normally accomplished by means ofgas from the gas line that is
protected by the unit.Thermoelectric units are economical but their
reliability depends largely onthe quality of the supply gas. Dust
and liquids transported with the gas may block theburner system and
extinguish the flames. This can be avoided by using
additionalpressure control systems or filters but this makes these
units less competitive. 25Thermoelectric units tend to operate more
efficiently in cold climatescompared to hot (tropical) climates
[7].2.4.6Closed Cycle Turbo GeneratorsA closed cycle turbo
generator consists basically of a combustion system, avapour
generator, a turbo alternator, an air-cooled condenser, a
rectifier, alarms andcontrols housed in a shelter. It can supply
200 to 3,000 Watt of filtered DC power.The gas supply is normally
provided from the structure or from a separate supplysystem. The
units are manufactured by specialized companies. Like
thermoelectricgenerators their reliability probably depends on the
gas quality and cleanliness [7].2.5Anode MaterialsAny
current-conducting material could be used for the anodes or
groundbeds,but for reasons of economy and required service life,
the material should have a lowconsumption rate at an acceptable
cost. Materials used for groundbed constructioncan be carbon steel
scrap, cast iron scrap, graphite cylinders, special alloy rods
ornoble materials plated with inert materials such as platinum or
mixed metal oxides.A description of the various materials is given
below and approximate currentdensities and consumption rates are
given in Table (2.1) 26Table 2.2:Typical Consumption Rates of
Impressed Current Anode MaterialsImpressedcurrent
anodeMaterialMaximumcurrent density,A/mWorkingcurrentdensity,
A/mConsumptionrateSteel - 0.5 10 kg/A.yrAluminium 10 4.8 2
kg/A.yrGraphite 25 2.5 to 10 0.25 kg/A.yrSilicon Iron 50 5 to 25
0.1 kg/A.yrMagnetite 200 115 0.02 kg/A.yrLead Alloy 300 50 to 150
0.085 kg/A.yrPlatinised Titanium 2000 250 to 700 8
mg/A.yrPlatinisedTantalum orPlatinised Niobium2000 500 to 1000 8
mg/A.yrMMO on Titanium 1000 500 to 100 1 mg/A.yr2.5.1Steel Scrap
AnodesIn some cases, steel scrap is used as an impressed-current
anode. This may befor temporary protection or for economical
reasons. Abandoned steel-lined oil orwater wells can be quite
suitable. The sections are thin, however, and early failure
islikely. Another weakness is the anode cable connection, which
should preferably notcontact the soil. For long term protection of
critical installations, the use of scrapmetal is not recommended
[7].
272.5.2Cast Iron Scrap AnodesCast iron scrap generally has the
advantage of being thick in section and ofsuch form that any one
piece will be in soil of more or less uniform resistivity.Moreover,
a graphite surface is left exposed as the outer iron is consumed,
so that theremaining iron with its graphite surface acts as a
graphite anode, thus reducing therate of iron consumption. Old
engine blocks are examples. The anode cableconnection remains the
weak point [7].2.5.3Silicon Iron AnodesHigh silicon cast iron has
been found to be a suitable anode material. It isrelatively
inexpensive and it is used on quite a large scale for groundbeds.
It issuitable both in soil and water. In soil applications, it is
normally surrounded by acarbonaceous backfill. Current densities
can be high and consumption rates are lowtaking into account the
high mass per anode. The anodes come in different sizes
anddifferent cable attachments. They are quite brittle and shall be
handled carefully. Forseawater applications the silicon iron is
usually alloyed with about 5 % chromium toresist pitting
[7].2.5.4Graphite AnodesGraphite anodes have a low rate of
consumption. The choice betweengraphite and silicon iron often
depends on availability in a given area.Graphite anodes are
generally cylindrical in shape, though other forms areavailable.
The graphite is impregnated with wax or resin, which reduces
flaking, or 28disintegration of the anodes as the graphite is
consumed. The anodes are suppliedwith terminal connections, and
with cables if required. When installed in soil,impregnated
graphite anodes are generally used with a backfill of
carbonaceousmaterial such as coke breeze. In soil and seawater,
current densities of up to 10 A/m2may be employed, but in fresh or
brackish water, the current densities should notexceed 2.7 A/m2in
fresh water or 5.4 A/m2in brackish water. At higher outputs,
thesurface of the graphite deteriorates excessively due to the
formation of gas.Graphite anodes are brittle and require careful
handling during transport,storage, and installation. Long graphite
cylinders may be broken by subsidence ofsurrounding soil
[7].2.5.5Magnetite AnodesMagnetite (Fe3O4) anodes are made by means
of a proprietary process. Themagnetite is plated onto metal (copper
alloy) cylinders, which provide the electricalconnection. They are
light in weight but brittle. Current output and consumption rateare
favorable. Because of single-source supply, they are used less
often than otheralloys [7].2.5.6Lead Alloy AnodesAn alloy of lead,
silver, and antimony (1 % of silver, 6 % of antimony) hasbeen used
in salt water. At a current density of 108 A/m2, the annual
consumption isabout 85 g/A. The alloy has good mechanical
properties and can be cast or extrudedto any desired shape.
Platinised titanium or MMO anodes have largely replaced thistype of
anode [7]. 292.5.7Platinised Titanium AnodesThese anodes are used
for salt water or fresh water where the conductivity isvery low.
Titanium develops an adherent oxide layer of high electrical
resistance.The oxide layer prevents corrosion by acting as a
barrier. Titanium acts as an inertsupport for the platinum.
Platinum can withstand very high current density and it isgenerally
applied to a small area only. The platinum layer is normally 2.5
microns inthickness and it has an estimated life expectancy of 10
years. Titanium sheets, 12mm thick with a platinum coating of
2.55.0m, can be loaded to 10 A/dm2or overa period of years. Rod
anodes of 1025 mm diameter are used frequently forprotection of
vessels, pipes, condensers, heat oil terminals, etc. [7].2.5.8Mixed
Metal Oxide Based AnodesThese anodes are the latest technology in
anode material and have largelyreplaced other anode types, both
onshore and offshore. They consist of a proprietarymixture of
(noble) metal oxides plated on a titanium or niobium substrate.
This typeof anode has the same advantages (and some limitations) as
platinised anodes but isgenerally cheaper. They can be made in
various shapes such as ribbons, rods, wires,mesh etc. Ribbon shapes
are often used as distributed anodes for localised protectionof
structure or under structure bottoms. Applicable current densities
are high andconsumption rate is low [7]. 302.5.9Zinc AnodesZinc
anodes are frequently used for protection of submarine pipelines.
Theyare commercially available in weights from 5 to 60 lb. They
have a driving potentialof 1.10V compared to a CuCuSO4reference
electrode. The details of zinc anodesare shown in Figure 2.7.Figure
2.7Shows Typical Zinc AnodeCorrosion products insulate the anodes
and the anodes are, therefore, installedbelow the water table in
soils with no free carbonate or phosphate so that passivitydoes not
occur [8].
312.5.10Aluminium AnodesThese are mostly employed for seawater
applications. The base metalcontains 9899% of aluminum. Aluminum
anode has some characteristics which are:1.The cost is low and they
are light in weight.2.The corrosion products do not contaminate the
water.3.The rate of consumption varies between 7 and 9 lb/A-year.
Theefciency varies between 87 and 95%.4.The anodes are easily
passivated and must be rinsed with NaCl toreactivate. Backll must
be used with aluminum anodes [8].2.6Distributed Anode
CablesDistributed anode cables consist of a copper core sheathed by
a conductivepolymer that allows passage of cathodic protection
current to the water. The currentdensity of the anode is usually
low, and such cables are mainly used for localisedprotection of
structure. They have also been used successfully for the protection
ofcoated buried tanks and vessels and for the protection of coated
external tankbottoms. These anodes require a specialized design and
should not be operated abovetheir rated current density.
Consumption rates or anode life can be obtained from theSupplier
[7].The cathodic protection current decreases with the time of the
immersion, andattains stable value after approximately 15 days,
probably due to the solidification of 32the coating and/or the
accumulation of the corrosion products in the coating
pores[10].Cathodic protection current density increases with
increasing distancebetween cathode and anode [9].2.7Protection of
Underwater StructureStructures in seawater are protected by
so-called bracelets (annular anodes) asshown in Fig. (2.9). In
marine structures, corrosion is at maximum at a small distancebelow
the water line and decreases with depth. Corrosion is less severe
in mud.. Inthe impressed current system non-consumable graphite
anodes are required, whereasin the galvanic system a magnesium
anode is the best material. Zinc anode is alsoused as galvanic
anodes, but the cost is high [9].Figure 2.8Marine Structure
Anode
33The potential necessary to protect buried steel is0.85 V,
however, in thepresence of sulfates, reducing bacteria a minimum
potential of 0.95V with respectto copper sulfate electrode would be
necessary. Approximately 10 mA/m current isneeded for protection of
bare steel in sluggish water. In rapidly moving water, 30mA/m for
bare steel in a flowing water would be necessary. Current
requirements invarious environments can be found abundantly in the
literature as well as cathodicprotection specifications [7].In ICCP
design its difficult to know the expected potential distribution
overthe underwater structure that leads to reliance to current
density measurement as amean of assessment. The corrosion
influenced by the environment factors such asvelocity and pH.
Accordingly, when ICCP system is designed, various
protectionfactors need to reflect in accord one with the underwater
environment. The currentdensity increases with increasing velocity,
but it decreases with increasing pH [6].For coated steel containing
defect under appropriate CP potentials, cathodicreaction is
dominated by reduction of oxygen. Mass-transfer of oxygen
throughsolution layer and the defect with a narrow [12].