Catodic Protection Requirements

A CONDENSED DISCUSSION OFCATHODIC PROTECTION REQUIREMENTS

FOR A TYPICALGENERATING STATION

Prepared By

Revision 0Elizabeth J. Smith

January 5, 1983

Revision 1.0Bryan Louque

August 21, 2000

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Contents

1.0 Introduction......................................................................................................1-1

2.0 How Corrosion Occurs .....................................................................................2-1

3.0 Typical Corrosion Cells ....................................................................................3-13.1 Dissimilar Metal Corrosion Cells ..........................................................3-13.2 Dissimilar Environment Corrosion Cells ...............................................3-33.3 Differential Aeration Corrosion Cells ....................................................3-43.4 Bacteriological Corrosion Cells.............................................................3-4

4.0 Cathodic Protection--How It Works..................................................................4-1

5.0 Designing the Cathodic Protection System........................................................5-15.1 Analysis of the Requirements................................................................5-15.2 Selection of the Optimum System .........................................................5-1

5.2.1 Essential Features ................................................................5-15.2.2 Desirable Features ...............................................................5-2

5.3 Selection and Arrangement of the Specific Components........................5-25.3.1 Sacrificial Anode Systems ...................................................5-25.3.2 Impressed Current Systems..................................................5-2

5.4 Avoiding Overprotection and Stray Current ..........................................5-2

6.0 Preferred Methods of Corrosion Control...........................................................6-16.1 Underground Cast Iron and Ductile Iron Pipe........................................6-16.2 Underground Carbon Steel Pipe ............................................................6-16.3 Underground Prestressed Concrete Cylinder Pipe (PCCP).....................6-26.4 Underground Copper or Stainless Steel Pipe .........................................6-26.5 Underground Aluminum Pipe................................................................6-26.6 Underground Steel Tanks......................................................................6-26.7 On-Grade Steel Tanks ...........................................................................6-26.8 Inside Surfaces of Steel Tanks...............................................................6-36.9 Condenser Water...................................................................................6-36.10 Traveling Screens .................................................................................6-46.11 Supplementary Methods of Protection...................................................6-4

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Contents (Continued)

7.0 Maintenance of the Cathodic Protection Systems ..............................................7-17.1 Outline of Routine Performance Testing Procedures .............................7-17.2 Black & Veatch Services.......................................................................7-1

8.0 Basic Design Philosophy Recommendations.....................................................8-18.1 Connection of Pipe to Main Ground Grid ..............................................8-18.2 Soil Resistivity Measurements ..............................................................8-18.3 Multiple Bond Wires at Pipe Joints .......................................................8-28.4 Contact Between Ferrous Pipe and Reinforcing Steel............................8-28.5 Condenser Water Box Protection...........................................................8-2

9.0 Summary..........................................................................................................9-1

Appendix A Process Map of Design, Construction, and Testing ActivitiesAppendix B Typical Cathodic Protection Details

Tables

Table 3-1 Practical Galvanic Series.......................................................................3-2

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1.0 Introduction

The Electric Power Research Institute (EPRI) has estimated the cost assigned tocorrosion in the electric power industry to be 0.24 percent of the gross national product(GNP). This is equivalent to more than 17 billion dollars annually expressed in 1998dollars. Of these costs, EPRI estimates that 15 percent, or 2.5 billion dollars, could havebeen avoided by application of available technology such as equipment design, materialselection, protective coatings and linings, inhibitors and cathodic protection. Experiencehas demonstrated that installation and maintenance of effective cathodic protectionsystems can mitigate corrosion, increase remaining useful life and significantly reducethese costs for all underground, on-grade and submerged metallic structures subject tocorrosion.

This discussion provides a brief introduction into the methods for control ofcorrosion as recommended by Black & Veatch for installations at generating stations withemphasis on corrosion control by cathodic protection. In-depth design recommendationswill not be provided, since there are numerous conditions which may cause corrosion andthe method of control appropriate for each situation must be considered on an individualbasis.

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2.0 How Corrosion Occurs

Corrosion of metallic structures is primarily electrochemical (galvanic) in nature.There are four conditions which must exist for electrochemical corrosion to occur. Theseconditions are as follows.

• There must be an anode.• There must be a cathode.• There must be a metallic path electrically connecting the anode and

cathode. (Normally, this will be the structure itself.)• The anode and cathode must be exposed to an electrically conductive

electrolyte, such as soil moisture, or water, surrounding a buried orsubmerged structure.

Wherever these conditions exist, a corrosion cell may be formed. An electriccurrent will flow between the anode and cathode and corrosion of the anode will result.The amount of corrosion will be directly proportional to the amount of current flowingfrom the anode. Therefore, the rate of corrosion will vary proportionately with therelative cathode/anode surface area ratio, the magnitude of the cathode/anode potentialdifference and inversely with the value of the electrolyte resistivity. The figure belowillustrates the relationship between the corrosion rate of a ferrous metal and it'senvironment.

INCREASE DECREASE DECREASE INCREASE

DECREASE INCREASE INCREASE DECREASE

CORROSION

RATE

TEMPERATURE

RESISTIVITY

pH

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Although seldom used in buried or submerged service, amphoteric materials such as leadand aluminum may not react according to the relationships illustrated in the above figure.

Another common form of corrosion is stray current corrosion, frequently referedto as man made corrosion. Stray current corrosion may occur at generating stations usingimpressed current cathodic protection systems and on underground metallic structureswhich are buried in areas where stray direct currents flow through the earth. Straycurrent corrosion occurs when stray direct currents are picked up from the electrolyte bya metallic structure which is so located that it is a preferential path for the stray currentbut has no metallic connection to provide a path back to the source of the stray current.The stray current will protect the metallic conducting structure to which it has strayed atits pickup locations, but unless appropriate protective measures are taken, corrosion willoccur at the locations where it leaves that structure and reenters the electrolyte to returnto its source.

Stray current can also be generated by improper welding operations in agenerating station. Welding machines grounded to a structure allow the welding currentto return to the welding machine through the structure. Welding current discharge fromthe structure, into an electrolyte will cause severe corrosion of the structure at the currentdischarge point.

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3.0 Typical Corrosion Cells

Several types of corrosion cells are frequently encountered at generating stations.Four of the most common types are described here.

3.1 Dissimilar Metal Corrosion CellsDifferent metals are frequently combined into a single system where both metals

are in contact with the electrolyte. This combination will result in corrosion of the anodicmetal, the surface of which will experience metal loss. Some typical examples ofdissimilar metal corrosion cells in generating stations are as follows:.

Anodic Metal Cathodic Metal Buried Structures

Carbon steel pipe Copper pipeNew steel pipe Older steel pipeCoated steel pipe Concrete encased steel pipe

On-Grade StructuresSteel tank bottom Copper grounding conductors

Above Grade Vessels and PipingCarbon steel water piping Copper water pipingCarbon steel water boxes Titanium tubes and tube sheetsCarbon steel tube sheets Stainless steel tubes

The practical galvanic series indicates the relationship of the natural potential(voltage) of several common metals installed in neutral soils and water. This tableindicates which material will experience metal loss (anode) and which will receiveprotection (cathode) when any two dissimilar metals are coupled in such a manner that acorrosion cell is formed. A practical galvanic series is shown below in Table 3-1.

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Table 3-1Practical Galvanic Series

Metal Volts*

Commercially pure magnesium -1.75 Anodic (most active)

Zinc -1.1

Aluminum alloy (5 percent zinc) -1.05

Carbon steel -0.2 to -0.8

Cast iron -0.5

Stainless Steel

Carbon steel in concrete -0.2

Copper, brass, bronze -0.2

Mill scale on steel -0.2

Titanium +0.3 Cathodic (least active)

*Typical potential normally observed in neutral soils and water, measured with respectto a copper sulfate reference electrode.

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The following figure illustrates the galvanic series of some commercial metals and alloysin seawater:

Magnesium

Aluminum

Steel

Cast Iron

Chromium Stainless Steel 13% Cr (Active)

Ni-Resist (High Ni cast iron)

18-8 Stainless Steel (Active)

18-8 Mo Stainless Steel (Active)

Tin ACTIVE OR ANODIC

Copper PASSIVE OR CATHODIC

CuproNickels (60-90Cu-40-10Ni)

Monel (70Ni-30Cu)

Inconel (80Ni-18Cr-18Mo) (Passive)

Chromium Stainless Steel 11-30% Cr (Passive)

18-8 Stainless Steel (Passive)

18-8 Mo Stainless Steel (Passive)

Hastelloy C (62Ni-13Cr-7Fe)

Titanium

Platinum

3.2 Dissimilar Environment Corrosion CellsThe natural potential of a structure in one type of electrolyte is usually different

from the natural potential of the same structure in a different type of electrolyte. Thus, apipe passing through two types of soil may experience severe corrosion in one area andno corrosion in another. For example, a pipe which passes through clay into a sandyloam will tend to experience corrosion in the clay area, since the natural potential of thepipe metal to the clay is more negative than the natural potential of the pipe metal to thesandy loam.

Another typical example of a dissimilar environment corrosion cell is a section ofunderground pipe partially encased in concrete and partially surrounded by soil. Thesegment of the pipe surrounded by soil will be anodic to that segment encased in

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concrete. Therefore, the pipe surrounded with soil will corrode, with the most severecorrosion occurring at the soil-concrete interface where the current flow between theanode and the cathode will be greatest.

3.3 Differential Aeration Corrosion CellsDifferential aeration corrosion cells are created when the oxygen concentration

varies between locations on the surface of a metallic structure immersed in an electrolyte.The poorly aerated (oxygen deficient) areas on the structure surface will be anodic tothose areas which are well aerated (oxygen rich). Therefore, the poorly aerated areas willcorrode. The corrosion will be most severe at the interface between the well aeratedareas and the poorly aerated areas. Examples of where this type of corrosion cell willoccur are listed:

• On-grade storage tanks. The base plate nearest the center of the tankbottom has less access to oxygen compared to the base plate near tankperimeter. The base plate near the center of the tank will be anodic to baseplate with unrestricted access to the air near the perimeter.

• Piping soil/air interface. The pipe nearest the bottom elevation of the pipetrench has less access to oxygen compared to the pipe near the soil/airinterface. The pipe at lower elevations in the soil will be anodic to pipewith unrestricted access to the air near grade elevation.

• Underground storage tanks. The underside of underground storage tankshave less access to oxygen compared to tank surfaces at higher elevationsin the tank pit. The tank surfaces at lower elevations in the soil will beanodic to tank surfaces with less restricted access to the air at higherelevations.

3.4 Microbiological Corrosion CellsMicro-organisms may cause changes in the electrolyte surrounding a structure

such that corrosion will occur. The corrosion process results from the metabolicprocesses of anaerobic micro-organisms in locations where an oxygen deficientenvironment can be maintained. Such locations most frequently occur in aqueousenvironments, bogs and water saturated soils. Examples of conditions producing thistype of corrosion cell are as follows:

• Sludge or sediment build-up in vessels or piping.• Use of organic waste and rubble for piping backfill.• Installation in swampy or similar soil which is contaminated.

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4.0 Cathodic Protection--How It Works

Cathodic protection accomplishes reduction or prevention of corrosion of a metalsurface by making it cathodic, for example, by the use of sacrificial anodes or impressedcurrents.

The surface to be protected is made entirely cathodic by creating a corrosion cellconsisting of that surface and an electrically connected structure more anodic than anypart of the surface being protected. The anodic part of the corrosion cell thus created willbe either sacrificial anodes or the anodes of an impressed current system.

Sacrificial anodes, typically magnesium, zinc, or aluminum alloy, are used whereprotective current and driving potential requirements are relatively small. The naturalpotential (voltage) difference between the sacrificial anodes and the protected surfaceproduces the DC current flow required to make the protected structure cathodic.

An impressed current system is typically used where either the protective currentrequirements or driving potential requirements make such a system cost effective. Animpressed current system consists of a rectifier, impressed current anodes, andinterconnecting electrical conductors. The rectifier is energized from the station auxiliaryelectrical system converting the alternating current supply to direct current for cathodicprotection. The function of the impressed current anodes is to introduce the cathodicprotection current into the electrolyte. The impressed current anodes are of a materialsuch as graphite, high silicon cast iron, platinum-clad titanium or niobium and mixedmetal oxide clad titanium. These anode materials are used because of their lowconsumption rate when discharging current. Although the impressed current anodes areof a material naturally more cathodic than carbon steel, the biasing effect of the rectifierassures that the impressed current anodes will be anodic to the protected structure.

Performance tests are necessary to evaluate the effectiveness of a cathodicprotection system. Therefore, monitoring and measurement facilities are an integral partof most cathodic protection systems. Test stations provide a direct connection to buriedor submerged structure to accomplish the following:

• Measure adjacent structure-to-electrolyte potentials to determine if thestructure has achieved and is maintaining a sufficiently negative potentialto assure protection.

• Measure anode current outputs to determine if each anode is functioningproperly or is becoming depleted and needs to be replaced and to projectanode life expectancy.

• Check insulated flanges to determine if each such flange is properlyinsulated.

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• Check electrical continuity to determine if the structure being protectedcontains any deleterious electrical discontinuities.

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5.0 Designing the Cathodic Protection System

The stages of development for design of a cathodic protection system include thefollowing:

• Analysis of the requirements.• Selection of the optimum system.• Selection and arrangement of the specific components.• Preparation of the drawings and specifications.

5.1 Analysis of the RequirementsTypical factors which must be considered for each structure to be protected

include the following:• Contractual requirements• Materials of construction.• Resistivity (conductivity) of the interfacing medium.• Surface area.• Configuration.• Coating.• Accessibility.• Shielding.• Location in relation to other metallic structures.• Presence of sources of stray current.

5.2 Selection of the Optimum SystemThe cathodic protection system selected should be the simplest and most

cost-effective system considering the following factors.

5.2.1 Essential FeaturesThe following features are essential to an optimum cathodic protection system:

• Adequate protective current and driving potential capacities.• Acceptability of projected service life.• Freedom from uncontrollable stray current effects.• Capability of being constructed in the available space.

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5.2.2 Desirable FeaturesThe following features, while not essential, are extremely desirable if they can be

achieved without compromising the performance of a cathodic protection system:• Maintainability.• Cost-effectiveness.• Simplicity.

5.3 Selection and Arrangement of the Specific ComponentsFollowing the analysis of the requirements and selection of the optimum cathodic

protection system, the design requirements may be calculated. These calculationsdetermine the following information for the type of system selected.

5.3.1 Sacrificial Anode SystemsThe following information must be determined by calculation for a sacrificial

anode system:• Quantity of anodes required.• Size, weight, and material of the anodes.• Life expectancy of the anodes.• Current requirement from each anode.• Location of each anode for maximum effectiveness.

5.3.2 Impressed Current SystemsThe following information must be determined by calculation for an impressed

current system:• DC amperage and voltage rating of the rectifier.• Quantity of anodes required.• Resistance of the ground bed to earth/water.• Required cable size.

• Total anode circuit resistance to earth/water.• Location of the rectifier and anode bed.

5.4 Avoiding Overprotection and Stray CurrentIn the design of a cathodic protection system, the designer must be careful not to

design either inherent overprotection or uncontrollable stray current into the system.

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Providing more current to a structure than is required for its protection is aproblem primarily associated with impressed current cathodic protection systems. Theeffects of overprotection, as it is commonly called, include the following:

• Disbonding or failure of protective coatings and linings.

• Hydrogen embrittlement of pre-tensioned or alloyed steel. Thisphenomena is most often observed in the catastrophic failure of pre-stressed concrete cylinder pipe (PCCP) and duplex stainless steelstructural members.

• Hydriding of titanium components in condenser or heat exchanger tubesand tubesheets.

• Excessive scaling in scale sensitive applications such as condenser waterboxes.

As discussed previously, stray currents can result in electrolytic corrosion ofunderground metallic structures. Whether the stray current originates with an impressedcurrent cathodic protection system designed by Black & Veatch or from some sourcewith which we are not involved, the effects must be controlled. Control consists ofproviding a path back to the source of the stray current such that the current does notreenter the earth directly from the surface of the structure which is involuntarily receivingthe stray current. The stray current return path may be a metallic bond between thestructures or it may be one or more sacrificial anodes installed as part of the affectedstructure for drainage of the stray current.

Black & Veatch encourages the Owners, for whom we design impressed currentcathodic protection systems, to cooperate with the Owners of foreign structures in thevicinity so that damage to the foreign structures will not occur as a result of stray currentfrom the Black & Veatch designed systems.

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6.0 Preferred Methods of Corrosion Control

As stated in the Introduction, the design criteria which affect cathodic protectionare numerous, and frequently there are several effective solutions to each problem.Identification of the best solution requires knowledge, experience, and good judgment.The following sections describe the principal features of the corrosion control methodsusually recommended by Black & Veatch for several frequently encountered corrosionproblems.

6.1 Underground Cast Iron and Ductile Iron PipeAll pipe should be encased in a polyethylene sleeve in accordance with AWWA

C105 and provided electrical isolation provided at the following locations:

• At transition points between iron pipe and any other type of metallic piperouted below grade.

• At transition points to existing iron pipe.• At locations where the iron pipe transitions aboveground at water storage

tanks.

• At locations where iron pipe enters or exits a structure's foundation.

6.2 Underground Carbon Steel PipeAll pipe should be factory coated, and electrical isolation should be installed at

the following locations:

• At transition points between the carbon steel pipe and any other type ofmetallic pipe.

• At transition points to existing carbon steel pipe.

• At locations where the carbon steel pipe transitions aboveground.Cathodic protection should be provided by either sacrificial anodes or an

impressed current cathodic protection system. Field test stations should be installed atregular intervals along the piping and at each underground electrical isolation joint.

A bonding station and pipe lead wires should be installed at each location wherethe carbon steel pipe transverses another pipe if either pipeline is protected by animpressed current cathodic protection system. The lead wires from the carbon steel pipemay be connected to the lead wires from the foreign pipeline in a bonding station, inorder to control stray current between the two pipes.

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6.3 Underground Prestressed Concrete Cylinder Pipe (PCCP)Bond wires should be installed across all joints and the pipe should be encased in

a polyethylene sleeve wherever the pipe is in an area where stray currents may be present.

6.4 Underground Copper or Stainless Steel PipeThe pipe should be cleaned, primed, and wrapped with polyethylene tape or

acceptable equal bonded insulating tape. Cathodic protection, as described previously forcarbon steel pipe, should be provided.

6.5 Underground Aluminum PipeThe installation of aluminum underground is not recommended. In the absence of

cathodic protection or in the presence of excessive cathodic protection, the service life ofunderground aluminum is unsatisfactory. Cathodic protection creates an alkalineenvironment around a protected aluminum structure. Even if cathodic protection isappropriately applied, aluminum will corrode rapidly in the alkaline environmentwhenever the cathodic protection is out of service.

6.6 Underground Steel TanksCarbon steel underground steel tanks should be procured with an STI-P3

corrosion control system. Corrosion control regulations governing the operation ofunderground steel tanks can be established by federal, state and local agencies. Verifythat STI-P3 underground steel tanks satisfy requirements for corrosion control prescribedby the governing regulatory agency.

6.7 On-Grade Steel TanksOn-grade steel tanks may be constructed directly on the following bases:• Sand• Concrete

• AsphaltAll on-grade tanks, with the exception of water tanks, installed on the sand or soil basematerials should be cathodically protected with an impressed current system.

On-grade steel water tanks installed on sand bases are typically grounded withcopper rods and a dedicated grounding ring. Black & Veatch recommends that acathodic isolator be installed to isolate the dedicated tank grounding system from theplant main ground grid in the absence of fault conditions. Sacrificial anodes should be

082100 6-3

installed at each tank grounding pad. The sacrificial anode will provide neutralizingcurrent to the ground rod-to-tank bottom corrosion cell that will develop. The anodeattached to the copper ground rod will sacrifice itself to both the steel tank base plate andthe copper ground rod to prevent corrosion of the tank bottom.

On-grade tanks installed on concrete pads or asphalt bases shall not becathodically protected, as the concrete or asphalt material will effectively shield the steelsurface from the protective effect of the cathodic protection system. Tanks requiringcathodic protection, such as fuel oil tanks, should not utilize asphalt, concrete, oiled sandor oil impregnated materials in the foundation.

6.8 Inside Surfaces of Steel TanksTanks containing liquids such as fuel oil with entrained salt water should be

cathodically protected with zinc anodes welded to the inside bottom of the tank. The zincwill be sacrificial to the steel and prevent corrosion.

6.9 Condenser Water BoxesSacrificial anodes or mounting bosses for future installation of anodes are

recommended for installation inside fresh water condenser water boxes. If corrosioncannot be controlled with sacrificial anodes, an impressed current system may be used.An impressed current system will provide effective protection for the water boxes andtubesheets. An impressed current cathodic protection system and permanent referenceelectrodes for monitoring the water box-to-water potential is typically recommended foreach water box.

The following figure may be used to determine the condenser cathodic protectionrequirements based on circulating water type and physical characteristics from thecondenser to be protected.

CirculatingWater

START

SeaWater

ImpressedCurrent

CP System

GalvanicCP System

WaterBox

FreshWaterTube/

Tubsheet

Bare

Coated

DissimilarMetals

TubeProtrusion

More Than 0.25” Past Tubesheet

SimilarMetals

Less Than 0.25” Past Tubesheet

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Black & Veatch recommends the interior surfaces of the water box wall be cleaned andcoated with a good quality coating. Refer to Black & Veatch Technical Guide, CoatingSystem Data Sheets, for information on selection and application of condenser waterboxcoating systems.

Some tube/tubesheet materials are sensitive to hydrogen embrittlement orhydriding in the presence of excessive levels of cathodic protection. These materialshave a history of acceptable performance under impressed current cathodic protection solong as the cathodic protection system is properly designed, installed and operated.Special care must be taken when certain combinations of tube and tubesheet materials areinvolved (e.g., hydrogen embrittlement of duplex stainless steel materials or hydriding oftitanium tube/tubesheet material is reportedly possible).

6.10 Traveling ScreensCathodic protection is not required for traveling screens in fresh water. Properly

coated structure frames and screens of corrosion-resistant materials will ordinarilyprovide adequate service.

In a salt water environment, all metal surfaces of the traveling screen, racks andguides, except those fabricated with stainless steel, should be cleaned and coated with agood quality coating. This coating will aid in the prevention of corrosion to those partsof the screen which are out of the water and exposed to the salt water in the splash zone.Refer to Black & Veatch Technical Guide, Coating System Data Sheets, for informationon selection and application of travelling screen coating systems.

A cathodic protection system should be provided and should consist of animpressed current system using anodes and reference electrodes installed in each screenintake bay in an evenly distributed pattern.

6.11 Supplementary Methods of ProtectionMaterial selection, coating, encasement, joint bonding and insulated flanges or

any combination thereof, are supplementary methods of protection which may be used inconjunction with, or in place of, cathodic protection.

6.12 Alternate Corrosion Control MethodsChemical treatment is an important corrosion control method normally used in

closed cycle cooling water systems, steam systems and other water systems.

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7.0 Maintenance of the Cathodic Protection Systems

Owners are often unfamiliar with the benefits of proper maintenance of cathodicprotection systems. An arrangement for direct communication between a knowledgeablerepresentative of the Owner and the Black & Veatch materials application section shouldbe made, on each project where cathodic protection will be provided, to discussmaintenance procedures.

The Owner should be made aware of the capabilities and limitations of eachcathodic protection system and should be provided with basic information on how torecognize problems as they occur.

7.1 Outline of Routine Performance Testing ProceduresThe Owner should be provided with an outline of the recommended procedures

for routine performance testing of each cathodic protection system. Included in theperformance testing outline should be a recommended testing schedule.

7.2 Black & Veatch ServicesThe Owner should be advised that Black & Veatch is capable of performing many

of the recommended routine tests and to correctly interpret the results of such tests.Black & Veatch will recommend remedial measures when test results indicate theirnecessity and, if retained to do so, will oversee modifications and perform tests to verifythat any cathodic protection system deficiencies have been corrected.

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8.0 Basic Design Philosophy Recommendations

The following are the recommendations of the materials application sectionconcerning the basic design philosophy of cathodic protection for generating stations.

8.1 Connection of Pipe to Main Ground GridShould the underground piping network at a generating station be physically

connected to the main ground grid, or should it be totally isolated?Electrical isolation of underground piping from the ground grid is preferred;

however, the underground piping network has been connected to the ground grid on someprojects at the request of the client.

Underground ferrous metals (such as steel, ductile iron, and cast iron) will beanodic to the copper ground grid if they are connected together. To avoid acceleratedcorrosion of the ferrous metal, it is necessary to provide sufficient protective current toraise the potential of all metal surfaces to the same level relative to the surrounding soil.Although design variations make a meaningful cost analysis impractical, directconnection of underground piping to the station grid may be expected to increase the totalcost of a cathodic protection system for underground station piping by an order ofmagnitude. The actual cost will vary depending on the size of the project, the amount ofunderground ferrous pipe, and the resistivity of the soil surrounding the pipe and groundgrid.

The alternatives for cathodic protection of underground station piping (listed inorder of preference) are as follows:

• Cathodically protect the underground ferrous pipe and structures withsacrificial anodes and electrically isolate the buried, protected structuresfrom the ground grid, the building steel, the reinforcing steel, and theaboveground piping.

• Install an impressed current system that will be sufficient to protect theunderground ferrous pipe, structures, ground grid, building steel, andreinforcing steel. Insulated flanges are not required.

8.2 Soil Resistivity MeasurementsField measurements of soil resistivity using the Wenner Four-Electrode Method

should be performed by Black & Veatch in accordance with ASTM G57.Soil resistivity data obtained during site geological surveys should obtain

adequate information for cathodic protection system design.

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8.3 Multiple Bond Wires at Pipe JointsShould one or two bond wires be installed across each pipe joint for electrical

continuity?If electrical continuity is to be achieved along a section of slip-fitted pipe, a wire

is exothermally welded across the joint. If the exothermal weld at either end isimproperly made, continuity will be interrupted resulting in loss of protection for aportion of pipe. The possibility of a single inferior weld will increase with the length ofpipe and the subsequent number of welds. Black & Veatch recommends the installationof two parallel bond wires across each such joint. If the Owner's decision is to use onebond wire instead of two, the contractor must maintain strict quality control during theexothermal welding process and backfilling of the pipe trench.

8.4 Contact Between Ferrous Pipe and Reinforcing SteelWhen a relatively small area of an underground ferrous pipe is encased in

concrete, such as where it enters a building, is it important to prevent contact betweenthe ferrous pipe and the reinforcing steel in the concrete?

The natural potential of reinforcing steel in concrete is approximately the same asthat of copper, therefore, when ferrous pipe is attached to the reinforcing rods, adissimilar metals corrosion cell will exist and the ferrous pipe will corrode. Even whenthe underground ferrous pipe is intentionally connected into the main copper ground grid,as previously discussed, it is essential that the contractor avoid any direct contact betweenthe reinforcing steel and the pipe.

8.5 Condenser Water Box ProtectionWhat is Black & Veatch's basic cathodic protection design for condenser water

boxes?The basic cathodic protection design for condenser water boxes depends largely

upon the type of metals used for the tubes, tube sheets, and water box walls, and whetherthe water inside the water box is fresh water or salt water.

The following steps may be used as a basic guide for the preliminary cathodicprotection design. By observing the installation order, the desired level of protection maybe provided while avoiding costly and perhaps damaging overprotection.

8.5.1 Galvanic Anode Application

• Install anode mounting bosses inside of the water box wall as close to thetube sheet face as practical, typically 18" from the tubesheet.

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8.5.2 Impressed Current Anode Application• Design and install an impresseed current system using mixed metal oxide

or platinized titanium probe-type anodes and an automatic potentialcontrol rectifier.

• Install permanent reference electrodes to measure the degree of protectionachieved.

• Adjust the rectifier so that the protective current will be sufficient tocontrol corrosion but not in excess to cause hydrogen embrittlement orhydriding of tube/tubesheet.

• Monitor and adjust the system on a routine basis.

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9.0 Summary

Developments in corrosion control technology, since the early 1800s, haveprovided feasible and cost-effective methods for limiting, and in some cases eliminating,the corrosion of most underground and submerged metallic pipe and structures. With thecost of materials and installation constantly increasing, the design of effective cathodicprotection systems is becoming a recognized necessity for both new and existinginstallations of materials which are subject to galvanic and electrolytic corrosion.

The fact that corrosion is affected by numerous variables, makes the design of aneffective cathodic protection system unique. In some situations, the cost of effectivelong-term cathodic protection may prohibit its use or, continuing maintenance maynecessitate periodic inspection and adjustment which the Owner may not desire or beprepared to perform. For this reason, it is important that the cathodic protection designer,the mechanical and structural engineers, and the Owner all work together with regard tomaterial selection, component arrangement, installation procedures, and collection of testdata to develop a coordinated system which will provide acceptable service at the lowestcost.

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Appendix AProcess Map of

Design, Construction, and Testing Activities

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Appendix AProcess Map of

Design, Construction, and Testing Activities

DesignRequirements

• Review scopedocuments:contract, SDS,PDM, schedule

• Review codeand standardrequirements

Design Data• Soil resistivity• Water

resistivity• Vendor

equipment data• Underground

utilities data• P&ID's• Pipeline list• Plant

arrangements• Piping detail

drawings• Foundation

drawings

Design• CP system type• Anodeselection• Quantity ofanodes• Rectifiercapacity• Anode spacing• Equipment

interfacerequirements

DevelopConstruction

Drawings

DevelopProcurement& InstallationSpecifications

IssueConstructionDocuments

Field Services• Resident

engineering• System testing

andcommissioning

• Provide as-built drawings

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Appendix BTypical Cathodic Protection Details

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Appendix BTypical Cathodic Protection Details

List of Typical Cathodic Protection Details

Cathodic Protection - Underground Pipe

Cathodic Protection - Water Storage Tanks

Cathodic Protection - Insulated Fitting Details

Cathodic Protection - Fuel Oil Storage Tanks

Cathodic Protection - Condenser (Impressed Current Anode)

Cathodic Protection - Condenser Sacrificial Anode and Mounting Stud Arrangement