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].