Cathodic Protection 2007[1]

8/9/2019 Cathodic Protection 2007[1]

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Cathodic Protection

System

Operation and Maintenance

Feng Hongchen

HuangHua Risen CorrStop Ltd.

August 2007


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黄骅市瑞晨防腐材料有限公司

HuangHua Risen CorrStop Ltd. 1

www.CorrStop.Com 1

Cathodic ProtectionLevel II Training Course

HuangHua Risen CorrStop Ltd.

December, 2004 Feng Hongchen

www.CorrStop.Com 2

Company Profile

HuangHua Risen CorrStop Ltd. is a professional corrosioncontrol company. Besides cathodic protection materialsproduction , we also produce PVC and FBE powder forcoatings. With technical support from some famous

international companies, we are the first company inChina that used CorrStop Grid for tank bottom cathodicprotection, and completed many cathodic protectionprojects.

Our engineering department provides cathodic protectionsystem design, consultation and training service.


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Corrosion is Disastrous

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Content Level I

Nature of Corrosion

Factors Affecting Corrosion

Cathodic Protection Principle

Cathodic Protection Methods

Pipeline Coatings

Sacrificial Anode CathodicProtection

Impressed Current CathodicProtection

Isolating Joints vs Grounding Cell

Cathodic Protection Criteria

Coating and Cathodic Protection

Reference Electrodes

IR drop and IR Drop FreeMeasurement

Level II

Soil Resistivity

Ground Bed Design

Cathodic Protection Design

Pipeline Cathodic ProtectionDesign

Tank Cathodic Protection Design. Heat Exchanger & Oil Well Casing

Cathodic Protection

Stray Current Interference

Cathodic Protection Shielding

Cathodic Protection Systemmaintenance

Pipeline Coating Survey

Concrete cathodic protection

In Line Inspection, Corrosionmonitoring


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Nature of CorrosionCorrosion Concept and Conditions

What is Corrosion: According to the definition of BS7361, corrosion is the chemical

or electrochemical reaction of a metal with its environment,resulting its progressive degradation ,or destruction.

At atmospheric temperatures the corrosion of metals is anelectro-chemical process.in which, the metal surface is incontact with an electrolyte. The electrolyte may be a filmof moisture containing dissolved salts or may constitutethe whole or part of the surrounding medium, e.g. when

metal is immersed in fresh water, sea water or buried insoil. In the last case the electrolyte is the soil water,containing dissolved salts.

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Nature of CorrosionDefinitions

Anode: The electrode of an electrochemical cell at whichoxidation occurs. (Electrons flow away from the anode in theexternal circuit, which is normally metallic. The anode isusually the electrode where corrosion occurs and metal ionsenter solution.)

Cathode: The electrode of an electrochemical cell at whichreduction occurs.

Electrolyte: A chemical substance containing ions thatmigrate in an electric field, electrolyte refers to the soil orliquid adjacent to and in contact with a buried or submergedmetallic piping system, including the moisture and otherchemicals contained therein.


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currentcathode

eanode

Nature of corrosionCorrosion Concept and Conditions Diagraph

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Conditions for corrosion to occur: There are certain conditions, which must be present before

an electrolytic corrosion cell can function:

There must be an anode and a cathode.

There must be an electrical potential between theanode and cathode.

There must be a metallic path electrically connectingthe anode and cathode.

The anode and cathode must be immersed in anelectrically conductive electrolyte.


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Due to the potential difference existingbetween the anodic and cathodic areas,positively charged metal ions leave the metalsurface at the anode while electrons leave thesurface at the cathodes.

Corrosion takes place at the anodic areaswhere metal ions react with the electrolyte toform the typical corrosion products.

At the cathode area dissolution of metal does

not take place but reactions occur in theelectrolyte.

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Factors Affecting CorrosionSteel surface conditions

If a mill scale or impurity exists on the steel surface,it is the cathode with respect to surrounding baresteel, electrons from the bare steel will flow towardsthe mill scale and corrosion will occur after the

electrons left. The corrosion is usually pit corrosion.

currentcathode

eanode


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Factors Affecting CorrosionEnvironment

The rate of corrosion of steel in soil or water isgoverned by the following factors:

1. Concentration of electrolyte

2. Concentration of oxygen

3. Temperature

In general, the severity of corrosion increases asone of these controlling factors increases, but

because all of the influences are operating at thesame time, their relative importance must beassessed.

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Factors Affecting CorrosionEnvironment

Steel will corrode far more rapidly in well-oxygenatedbrackish water than in normal sea water.

The absence of oxygen, particularly in water-logged soils,may provide a corrosive environment for iron and steel

through the growth of sulphate-reducing bacteria. The most important soil property as regards pipeline

corrosion are salt content and aeration (oxygen content)both of which affect the steel-to-soil potential of buriedpipelines.

The steel-to-soil potential of buried steel is more negativein soils with high salt content than in soils with a low saltcontent. Pipeline corrosion tends to be heaviest


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Factors Affecting CorrosionDifferential Aeration Cells

Steel-to-soil potential of buriedsteel is lower in poorly aeratedsoil (low oxygen content) thanin well aerated soil (highoxygen content).

In this case, the paved roadlowers the oxygen

concentration in the soil aroundthe pipeline, this section ofpipeline becomes anode in thedifferential corrosion cell and

being corroded.

In practice, good aeration andhigh electrical resistivity usuallycorrespond to low moisturecontent.less corrosion.

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Factors Affecting CorrosionSulphate reducing bacteria Corrosion

In anaerobic soils, e.g.clay, sulphate-reducing bacteriamay be active. These micro-organisms, which canexist in active form only in the absence of free oxygen,obtain their energy from the following reaction:

So4-2 +8H = H2S + 2H2O +2OH-1 Bacteria corrosion of iron and steel under anaerobic

conditions is usually rapid and severe. This kind ofattack can often be recognized by the brightappearance of the corroded surface and the rotten-eggs odor.

Since many H atom are eaten by SO4, more electronsare needed to produce H atom,so, more negative ofprotection potential is required.


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Factors Affecting CorrosionNew and Old Pipe

A section of thepipeline has beenreplaced becauseof corrosiondamage.

The new sectionwill fail sooner thanexpected.

Mild steel (rust):

-0.2v- -0.5v Mild steel (clean):

-0.5v- -0.8v

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Factors Affecting CorrosionDissimilar soils

Pipeline going throughtwo electrolytes ofdifferent concentrationsconstitutes a galvaniccell and is often

referred to as aconcentration cell.

Corrosion will occur atanodic section.


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Factors Affecting CorrosionPipeline Embedded in Concrete

Concreteencasement ofpipe will causedifferentialcorrosion cell.

Pipe sectionwithout concreteencasement isanode and being

corroded.

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Factors Affecting Corrosion Anode and cathode ratio

If the anode isrelatively small,corrosion will besevere.

If the anodic area isrelatively largecompared with thecathode, corrosionwill be relativelymild.


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Factors Affecting CorrosionStray Current Corrosion

Interference: Any electrical disturbance on ametallic structure as a result of stray current.

Stray Current: Current through paths otherthan the intended circuit.

Stray-Current Corrosion: Corrosion resultingfrom stray current transfer between the pipeand electrolyte.

Direct current traction systems frequently causeappreciable electric currents to flow in thesurrounding earth, similarly, the impressedcurrent from a cathodic protection system mayalso affect unprotected buried steel structures inthe neighborhood.

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With a poorly coatedpipeline, a straycurrent may enter the

line at a point, travelalong the line and

leave the line atanother defect point.

Where, the currentleaves will becorroded.

Where the currentflows into the pipe willbe protected.


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Factors Affecting CorrosionCorrosion Case


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Cathodic ProtectionPrinciple Cathodic Protection: A technique to control the

corrosion of a metal surface by making that surface thecathode of an electrochemical cell.

The principle is to make the potential of the wholesurface of the structure sufficiently negative withrespect to surrounding medium to ensure no currentflows from the metal to the medium.

This is done by forcing an electric current to flowthrough the electrolyte towards the surface of the metalprotected.thereby, eliminating the anodic area.

Corrosion of steel in normally aerated soils and waterscan be entirely prevented if the steel is maintained at apotential not more positive than –0.85V CSE. Underanaerobic conditions when sulphate-reducing bacteriaare present, it is necessary to depress the potential afurther 100mV, to –0.95V CSE.

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Cathodic Protection MethodGalvanic Anode Cathodic Protection

Galvanic Anode: A metal which, because of its relativeposition in the galvanic series, provides protection tometal or metals that are more noble in the series, whencoupled in an electrolyte.

With this method, current is provided by the galvanic reaction.Thesurface of the structure is made cathodic by connecting it electrically toa mass of less noble metal buried in the common electrolyte, the lessnoble metal is than an anode. Magnesium, Zinc, and Aluminum alloysare used for this purpose.

The anodes are often referred to as sacrificial anodes becauseprotection of the structure is accomplished by the simultaneousconsumption of the anodes by electrochemical corrosion


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Nature of CorrosionGalvanic Series of Metals Against CSE1. Carbon,graphite,coke: 0.30v

2. Platinum: 0 to –0.1v

3. Mill scale on steel: -0.20v

4. High silicon cast iron: -0.20v

5. Copper,brass,bronze: -0.20v

6. Mild steel in concrete: -0.20v

7. Lead: -0.50v

8. Cast iron: -0.50v

9. Mild steel (rusted): -0.20v to –0.50v

10. Mild steel (clean and shiny): -0.50v to –0.80v

11. Commercially pure aluminum: -0.80v

12. Aluminum alloy (5% Zn): -1.05v

13. Zinc: -1.10v

14. Magnesium alloy (6%AL,3%Zn,0.15%Mn): -1.60v

15. Commercially pure magnesium: -1.75v

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Cathodic Protection MethodGalvanic Anode Cathodic Protection

When the pipeline is connected with the Mg, its potentialis lowered down till there are no cathode and anode onthe pipe surface.Since the potential of the pipe is thesame, corrosion will stop.

currentcathode

eanode

current

e


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Cathodic Protection MethodImpressed Current cathodic Protection

Impressed Current: Direct current supplied by acathodic protection system utilizing an external powersource

With this method, the structure is placed in an electriccircuit with a direct-power supply and an anodegroundbed.Current is forced to flow from theelectrolyte to the structure

The system usually consists of AC converter,groundbed,Reference cell and connection cables.

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Cathodic Protection MethodImpressed Current cathodic Protection


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Pipeline CoatingsThe function of external coatings

Coating: A dielectric materialapplied to a structure to separate itfrom the environment.

1. to control corrosion by isolating theexternal surface of the undergroundor submerged piping from theenvironment.

2. to reduce cathodic protectioncurrent requirements, and

3. to improve current distribution.

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Pipeline CoatingsRequirement to Coatings

An effective electrical insulator

1. Effective moisture barrier

Applicability.

1. Ability to resist development of holidays with

time.(soil stress and soil contaminant).

2. Good adhesion to pipe surface.

3. Ability to withstand normal handling,storageand installation.

4. Ability to maintain substantially constantelectrical resistivity with time.

5. Resistance to disbonding. Easy of repair.


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Pipeline CoatingsCoating Selection

Besides the above requirement, followingtypical factors should be considered whenselecting a pipe coating:

1. Type of environment

2. Accessibility of pipeline

3. Operating temperature of pipeline

4. Ambient temperature during application, storage, shipping,

construction and installation.5. Geographical location

6. Pipeline surface treatment and cost.

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Types of Pipeline CoatingCoal tar enamels

Desirable Characteristics:

1. Over 80 years of use

2. Minimum holiday susceptibility

3. Low current requirement

4. Good resistance to cathodic disbondment

5. Good adhesion to steel.

Limitations

1. Limited manufacturer

2. Limited applicators

3. Health and air quality concerns

4. Change in allowable reinforcements.


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Types of Pipeline CoatingFusion bonded epoxy




3. Excellent resistance to cathodic disbondment

4. Excellent adhesion to steel.

Limitations

1. Strict application control

2. Low impact and abrasion resistance

3. High moisture absorption.

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Types of Pipeline CoatingFBE Coating Process


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Types of Pipeline CoatingFBE Coating Process

Fusion bonded epoxy (FBE in short) is applied to thesurface of the pipe by electrostatic spraying.The voltageis about 40kV.

After cleaning by grit blasting to Sa2.5, the pipe is heatedto a temperature of around 180 ℃.

The pipe is fed into an epoxy powder flow-bed, after theadhering to the pipe surface, the epoxy powder will cureand the coating formed.

The pipe is water cooled and flaw checked.

The final thickness is about 0.4mm.

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Types of Pipeline CoatingPolyethylene tapes



2. Minimum holiday susceptibility


4. Easy of application

5. Good adhesion to steel.

Limitations

1. Shielding CP from soil

2. Stress disbondment

3. Handling restrictions.


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Types of Pipeline CoatingPolyethylene Tapes Application Process

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Types of Pipeline CoatingPolyethylene Coating (extruded)


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Types of Pipeline CoatingMulti-layer coating




3. Excellent resistance to cathodic disbondment

4. Excellent adhesion to steel.

5. high impact and abrasion resistance

Limitations

1. Strict application control

2. Possible shielding of CP current

3. High initial coast.

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3-Layer Pipeline CoatingPE or PP is Coated on the FBE Coated Pipe


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Sacrificial Anode Cathodic Protection It is to make use of the potential difference of different

materials.When two dissimilar metals are placed in an electrolyteand joined by a conductor, an electric current tends to flow fromone metal to another via the electrolyte. Such a current flow willincrease the corrosion of the less noble metal and reduce that ofmore noble one.

Protection System

Corrosion Occurs Where The Current

Leaves The anode

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Sacrificial Anode Cathodic Protection

Application:

1. It is by making use of corrosion cell ofdissimilar metals.

Advantage:

1. It can be used flexibly , easy installation

and less maintenance work. Limitations:

1. Small current out put,used in place wherecurrent is small.

2. Used in low soil resistivity area.

Anode materials:

1. Magnesium anode

2. Zinc anode

3. Aluminum anode


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Sacrificial Anode Cathodic Protection

In practice, the theoretical A.h output ofsacrificial anodes is not all available for cathodicprotection, part of it will be consumed by self-corrosion due to the electrolyte action on it.The “anode efficiency” is the ratio of A.hactually supplied to the theoretical A.h outputper unit weight of the metal consumed. That iswhy there is always an “ anode(or current)efficiency” to consider in design.

After 85% of the anode weight being

consumed, the anode is considered invalid so ausing factor of 85% is added in anode weightcalculation.

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Sacrificial Anode Calculation

Q Z U t I W

××××= 8766

W= anode weight, kg

I= Current out put (Amps)

t = Design life Yr

U = Using factor

Z = Current capacity

Q = Anode efficiency


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Magnesium Anodes

Materials used are magnesium alloy, aluminum alloyand zinc. Neither magnesium nor aluminum alloysshould be used in situations where sparking maycause explosion.

The potential difference between magnesium alloyand steel is greater than that between zinc oraluminum, enables it to be used economically at arelatively higher soil resistivity(above 30ohm.m) whilealuminum anode is mainly used for offshore

structure.

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Magnesium Anode(High potential Mg anode 1.75V CSE)

Chemical Composition

Aluminum 0.01% max

Manganese 0.50 - 1.3%

Copper 0.02% max

Silicon 0.05% max Iron 0.03% max

Nickel 0.001% max

Others, each 0.05% max

Magnesium Remainder


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Magnesium Anode(low potential Mg anode 1.55V CSE)

Chemical Composition

Aluminum 5.0-7.0% max

Manganese 0.15% min

Copper 0.10% max

Silicon 0.30% max

Iron 0.03% max

Nickel 0.003% max

Others, each 0.30% max

Magnesium Remainder

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Magnesium AnodeElectrical Property

Q = Current efficiency (0.5)

Z = Theoretical Current Capacity (2200 Ah/kg)

U = Anode usage factor (85%)

Open circuit potential: high: -1.75V CSE

low: -1.55V CSE


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Magnesium AnodeElectrical Property

The efficiency of magnesium is usually about 50%. It isalso influenced by the environment. In soil or water with amoderate to low salt content, the efficiency may be lowbecause the current output is low and consequently theanode’s own corrosion may be relatively high. The use ofspecial back fill around the anode gives a higher currentoutput and a better anode efficiency.

At increased temperature, the self-corrosion of theanodes is greater and therefore their efficiencydecreases. For this reason, magnesium anodesshould generally not be used when the temperatureis higher than approximately 30oC in brackish or saltwater or higher than approximately 45oC in freshwater. In sea water their life is too short.

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Magnesium AnodeConfiguration ( Cast Magnesium Anode )

Mg anodes can be used toprotect most of the buriedmetallic structures found in arange of soil resistivities.

Efficiency is enhanced evenfurther when installed in a backfill of 75% gypsum, 20%bentonite, and 5% sodiumsulfate.


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Magnesium AnodeConfiguration (Cast Magnesium Anode)

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Magnesium AnodeExtruded Magnesium Anode


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Zinc Anode Zinc gives a relatively small current output as its

potential difference with protected steel isapproximately 0.25V as compared with 0.7V formagnesium. It is not economical to use it inmedia with resistivity greater than 15ohm.m. Itis mainly used in sea water

Zinc anodes should not be used at temperaturesabove 60 °C and better being used below 40 °C.

At temperatures above 70°C, it may change fromnegative to positive with respect to iron, therebypromoting an attack on steel instead ofprotecting it.

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Zinc AnodeChemical Composition

Al 0.1 - 0.5

Cd 0.02 - 0.07

Fe 0.005 max

Pb 0.006 max

Cu 0.005 max

Zinc Remainder


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Zinc AnodeElectrical Property


Z = Theoretical Current Capacity (827 Ah/kg)


Open circuit potential : -1.1V

Environment temperature: below 50°C

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Zinc Anode Application

It is used in soils with its resistivity below

15ohm.m or offshore structure.


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Zinc Anode Anode Configuration

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Zinc AnodeBracelet Zinc Anode Configuration


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Aluminum Anode

The main use of aluminum anode is in sea water or brackishwater of less than 200ohm.cm resistivity.

They are not suitable for use in soil.

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Aluminum AnodeChemical Composition

Zn 2.8-6.5

Si 0.08-0.21 Max.

In 0.01-0.025

Cu 0.006 Max. Fe. 0.12 Max

Other Each 0.02 Max

Aluminum Remainder


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Aluminum AnodeElectrical property


Z = Theoretical Current Capacity (2000-27(T-20))


T = Environment Temperature ( °C)

Open circuit potential -1.05V CSE

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Aluminum Anode Application

Al anode is mainly used for offshorestructure and internal tank bottom cathodicprotection.


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Aluminum AnodeTank Bottom Application

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Cathodic Protection System InstallationSacrificial Anode Installation

Anodes for the protection of undergroundstructures are buried at intervals along thestructure.

They are installed in an upright position, ifpossible, deep enough to be in permanentmoist soil.

For pipelines, the top of the anodes will beusually be approximately level with thebottom of the pipeline.


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For underground protection, the anode may be packagedin a cotton bag with backfill around the anode.

Alternatively, the backfill may be placed as a slurryaround the anode during burial. The backfill ensures auniform consumption of the anode and promotes ahigher current supply.

Anodes used to provide protection in water should bedistributed as evenly as possible over the surface of thestructure. They are mounted on brackets welded orbolted to the structure, suspended on galvanized cables,or placed on sea bottom alongside the structure.

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The spacing between anodes used toprovide pipeline protection may varyfrom one anode every 3m to one anode

every few miles depending on currentrequired by the pipeline.

The normal distance from the structureat which the anodes are placed isapprox. 1-1.5m.


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Galvanic Ground Bed Installation

The design of agalvanic ground bed issimilar to impressedground bed

The driving voltageavailable to forcecurrent from anode toelectrolyte is the open

circuit potential lessthe polarized pipelinepotential.

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Galvanic Ground BedInstallation

Sacrificial anode isusually placed 1-1.5mfrom the pipeline.Several anodes can beinstalled in a group.


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Anodebackfilled onsite.

The backfillcan fillcompletelyall of thevoids in theauger hole.


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Mg or Zn ribbonanode may beplowed inparallel to thepipeline alongsections of bareof poorly coatedline wherecontinuous local

protection isrequired

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CorrStop


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Galvanic Ground Bed InstallationElectrical Connection

The connection is made by brazing or thermit weldingdirect to the structure or through a test post.

To ensure a god contact, the surface to be connectedmust be thoroughly cleaned and the connection pointbeing insulated.

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Galvanic Ground Bed InstallationBackfill Materials

Anode is usually placed ina backfill which consists of75%gypsum, 20%bentonite, and 5%sodiumsulfate mixture.

The backfill ensures auniform consumption ofthe anode and reducingthe anode groundingresistance so as topromote a higher currentsupply.

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Impressed Current Cathodic Protection Application:

1. Current from out side source is impressedon the pipeline by using of a groundbedand a power source.

Advantage:

1. It can be used in high soil resistivity withlarge current demand..

Limitations:

1. Need power source and moremaintenance work.

2. Corrosion interference can be a problem.

Anode materials:

1. Silicon iron

2. Mixed metal oxide

3. Platinized titanium etc.


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Impressed Current Cathodic Protection

The current is forced to flow to the pipeline.

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Impressed Current Cathodic Protection

When impressed current is used for protection ofthe buried structure, the anodebed is buried atsome distance from the structure. The positiveterminal of the power source is connected to the

anodebed and the negative terminal connected tothe structure. The resulting current is from anodethrough the soil to the structure.

Transformer/rectifier is usually used to supply thedirect current.


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Impressed Current Cathodic ProtectionComponent

T/R: convert AC to DC and supply power tothe CP system.

Anodebed: Transfer the current to theenvironment:

Reference cells and cables.

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Impressed Current Cathodic ProtectionTransformer/rectifier

The apparatus is usually asilicon rectifier.

Its output voltage depends onelectrical resistance of thecathodic protection circuit.

According to requirements, itcan work under constantvoltage; constant current,potential controlled output.


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Impressed Current Cathodic Protection Anode materials

Any current conducting materials can be usedfor anode, but for reasons of economy, thematerials which cost least will be used.

Following materials are usually used due totheir low consumption rate.

Mixed Metal Oxide Anode

High Silicon Cast Iron Platinized Titanium

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Anode MaterialsMixed metal oxide anode

The anode is made withtitanium substrate coatedwith mixed metal oxidecatalyst . The catalyst is

thermally applied to thetitanium to form anextremely chemical resistantbond. It is small, light weight.

Operating current density: soil and fresh water, 100A/sqm;

sea water,500A/sqm.

Consumption rate: less than1.0mg/A.yr.


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Anode MaterialsMixed metal oxide anode

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Anode MaterialsSilicon anode

Silicon anodes have beenused for decades and it isproven to be one of themost reliable anode

materials.

Anode consumption rate:0.45kg/A.Yr.

Operating current density:10A/sqm.

Wide applicationenvironment.


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Anode MaterialsSilicon anode

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Anode Materialsothers

Graphite anode

Scrip iron anode

Platinizedniobium/Titanium anode,it is used for steelvessel internal cathodicprotection.


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Isolating Joint Function To make a cathodic protection

system work properly, the currentmust be confined on the section ofthe protected pipeline.

Electrical Isolation: Thecondition of being electricallyseparated from other metallicstructures or the environment.

Short contact with other grounding

structure must be eliminated. Mostly used isolating device is

isolating joint or isolating flange.

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Isolating Joint Diagraph


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Isolating Joint Specification

Test performed before and after the hydrotestIsolanelastomer

SECOND SEALING

- 100% Visual & Dimensional checkSilicone ADHESIVE

SEALANT

- 100% Dielectric strength test : >5 KV x 1 min. (50 Hz A.C.) *

- 100% Electrical Resistance test >25 Mohm (1000 V D.C.) *

Cold cured

Epoxy resin

INSULATINGFILLER MATERIAL

- 100% MP examination on welds [F] ASME VIII UW 53 App. 6

- 100% Hydrostatic test pressure : 1.5 times the DP for 15 minutes

Epoxy Resin

200 m DFT

INTERNAL

LINING

- 100% RT examination on butt welds [W] ASME VIII UW 51Epoxy resin

200 m DFT

EXTERNAL COAT.

Twopack solventless

Inspection & Testing Acrylonitrile el.NBR 70

SEALING RING" O-RING "

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Isolating Joint Usage

Much of current is drained away by the bare tank and poorlycoated branch pipeline which render the main pipeline under-protected.


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If the branch pipeline is completely isolated fromthe main line, stray current from the groundbedmay still reach the tank, depending on thelocation of the anodebed, flow from the branchline to the point near the main line and return tothe main line through the soil. This would causeheavy corrosion at the point where the currentleaves the branch line.

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If this is not prevented by proper groundbedlocation, a resistance bond is recommended acrossthe isolating joint to permit sufficient current to betaken by the branch line and tank to preventserious corrosion to themselves but will leave themain line fully protected.

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Grounding Cell

Alternating current, lightning strikes may break throughthe isolating joints, to protect the isolating joint fromdamage, grounding cell is used to discharge the currentfrom one side to another.

The composition of the Zn inside which forms the groudingcell is the same with Zn anode.


Grounding Cell

6mm铁芯


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Grounding Cell for Isolating Joints

6mm Iron Core

Backfill：Gypsum75%、Bentonite20%、Sodium Sulfate5%

Cotton Sack

Zn Rod：35x35x1000mm

T ap e Wo od S pa ce r


Cathodic Protection Potential

The criteria mostly used involves the measuring thepotential between pipeline and earth. This criteria isused to evaluate the change in structure potentialwith respect to its environment that are caused by CP

current flowing to the structure from the surroundingsoil or water.

Pipe-to-Electrolyte Potential: The potential differencebetween the pipe metallic surface and electrolyte thatis measured with reference to an electrode in contactwith the electrolyte.


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Cathodic Protection Potential


Cathodic Protection Potential As a normal practice, the

polarization potential of thestructure should reach –0.85V CSEminimum.

Polarization potential:

1. the potential across thestructure/electrolyte interface thatis the sum of corrosion potentialand the cathodic polarization.

2. It can be regarded as the off-potential.

For a proper designed CP system,the polarization potential is keptbetween –0.85V- 1.15V CSE.


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Criteria for Cathodic Protection

Polarization: The deviation from the corrosionpotential of an electrode resulting from the flow ofcurrent between the electrode and the electrolyte

Polarized Potential: The potential across thestructure/electrolyte interface that is the sum ofthe corrosion potential and the cathodicpolarization

Reference Electrode: A reversible electrode witha potential that may be considered constant under

similar conditions of measurement. (Examples:saturated copper/copper sulfate, saturatedcalomel, and silver/silver chloride.)


V1: Corrosion potential (natural potential), V2: Cathodic Polarization,

V1+V2: Polarization Potential, (Off-Potential),

V=V1+V2+IR: On-potential,

Criteria for Cathodic Protection


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Criteria for cathodic Protection

Three primary criteria for CP of underground orsubmerged steel or cast iron piping are listed inSection 6 of NACE RP-169-96:

-850mV CSE with CP applied without IR drop.

A polarized potential of -850mV CSE

100mV of polarization


Criteria for cathodic Protection-850mV CSE with CP applied This criteria is the mostly used for determining if

a buried pipeline has attained an acceptablelevel of CP. The IR drop must be consideredwhen using this criteria.

During measurement, the reference cell is placedas close as possible to the structure. But formajority of coated structures, most of the IRdrop is across the coating and the measurementis less affected by the reference electrodeplacement.

The IR drop can be eliminated by taking theinstantaneous off potential.


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Criteria for cathodic ProtectionPolarized Potential of -850mV CSE

Adequate protection is achieved with a negativepolarized potential of at least -850mV CSE.

Polarized potential is the potential across thestructure/electrolyte interface that is the sum ofcorrosion potential and the cathodic polarization.

The polarized potential is measured directly after theinterruption of all current sources and is oftenreferred as instant off-potential.

The difference in potential between native potential

and off-potential is the amount of polarization thathas occurred as a result of CP application.

The difference between the on-potential and the off-potential is the IR drop in the electrolyte.


Criteria for cathodic Protection100mV of polarization criteria

Adequate protection is achieved with a minimum of100mV of cathodic polarization between the structuresurface and a stable reference electrode contactingthe electrolyte. The formation or decay of polarizationcan be measured to satisfy this criteria.

To get the polarization criteria,, first measure thenative potential and than the off-potential, if thedifference is large than 100mV, you can see the100mV criteria is satisfied.

Another method of assessing the formation ofcathodic polarization is to measure the on-potentialimmediately after energizing the CP system and thanre-measure the on-potential after a few hours to daysof operation.The cathodic shift in the cathodicdirection should be larger than 100mV.


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Criteria for cathodic Protection100mV of polarization criteria

100mV criteria is used mainly when thecoating is poor or the structure is bare.Inmany cases, 100mV of polarization can beachieved where the off-potential is lessnegative than –0.85V CSE. The 100mVcriteria has the advantage of minimizingcoating degradation and hydrogenembitterment, both of which can occur as aresult of over protection.

-0.85V CSE criteria is mainly used for newerpipeline system


Criteria for cathodic ProtectionCP system energized


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Cathodic Protection PotentialIR Drop

IR Drop: The voltage acrossa resistance in accordancewith Ohm's Law

When we take the potentialmeasurement, the referencecell will be placed somedistance from the structure.

Since the current is flowing

from the electrolyte, there willbe a voltage drop caused bythe electrolyte resistance. It iscalled IR drop.


Cathodic Protection PotentialIR drop in Pipeline CP

When current is flowing from anodebed to the buriedpipeline, there will be a voltage drop on the soil resistance,which will impose error in cathodic protection measurement.


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Cathodic Protection PotentialIR drop in Pipeline CP


Cathodic Protection PotentialIR-free Potential Measurement

Since the IR drop will add an error to the reading,to get a valid interpretation, the IR drop will haveto be eliminated from the measurement result.

To get the IR free potential, the current will beswitched off at the moment of the measurement.Since the polarization potential decays relativelyslow, the polarization potential can be measured.


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Cathodic Protection PotentialIR drop free measurement

To eliminate the IR drop in the measurement, themost often used method is to take the off-potential ofthe protected structure. That is to say the potential istaken within 0.5 second of the power turning off.

Although other method is also be used, but it is notrealistic in site survey.

During the off-potential measurement, be sure that all

of the power source on this section of pipeline beingturned on and off simultaneously.


Criteria for cathodic ProtectionCP system de-energized When the CP system is de-energized,

the pipe-to-soil potential undergoesan instantaneous positive shift as aresult of elimination of the IR drop.The potential measured at thismoment is referred to as the off-potential.

There may be a spike in thepotential reading immediately after

the interruption of the CP system,which is a result of inductive effectof the pipeline and the CP systemand last for hundred of millisecond,the off-potential is typicallymeasured 200-500 ms after theinterruption.


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No coating can be made perfect. While most of thecorrosion protection work is done by coating, cathodicprotection is used to protect points where the coating is

damaged or has a flaw. Pit corrosion on a coated pipeline without CP will occur

earlier.

Excess cathodic protection will cause coating damage.



Although it is technically possible to protectuncoated buried structures by cathodicprotection, the cost is usually prohibitive.

And it is difficult to arrange the anodes soas to provide a uniform current distribution.

Coating supplemented with cathodicprotection is the optimum corrosionprotection method.


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Coating and Cathodic ProtectionOver Protection

The success of the the corrosionprotection depends on proper cathodicprotection potential.

Too negative a CP potential will damagethe coating in two ways.

Alkalinity caused by CP may cause the coatingdeterioration.

Hydrogen produced at flaws in a coating may progressively detach the coating from the surfaceof the metal.



While the hydrogen isreleased, the surplus OH willmake the solution near themetal surface alkaline.

Hydrogen gas releasing willdetach the coating at thecoating flaws.

Cathodic Disbondment: Thedestruction of adhesionbetween a coating and thecoated surface caused byproducts of a cathodicreaction


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Experiment has resulted in the followingconclusion:

Hydrogen evolution is initiated at an off-potential of –1.12V CSE andbecome vigorous at off-potential of –1.17V –1.22V CSE

The most negative off-potential obtainable is –1.22V CSE, the off-potentialcan’t be made more negative than this value even with a substantialincrease in applied current.

An increase in the current applied to a specimen at an off-potential of –

1.22 V resulted in increased hydrogen evolution and an increase in thenegative on-potential, but the off-potential remain unchanged.

The off-potential could not be directly related to the on-potential, andtherefore, the on-potential is not considered to be a valid indicator ofhydrogen evolution.


Cathodic Protection PotentialReference Electrodes

It is a reversible electrode used formeasuring the potentials of otherelectrodes:

Easy to use and maintain Stable potential over time

Potentials varies little with current flow

Not easily contaminated

Doesn't contaminate what is beingmeasured.


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Cathodic Protection PotentialReference Electrodes

Reference electrodes include:Silver/silver chloride,calomel andcopper/copper sulfate electrode.

Hydrogen electrode is rarely usedbecause of the difficulty in constructingand maintaining.

Relative to standard hydrogen electrode,other electrodes potential is as following:

1. Copper/copper sulfate (CSE): 0.300v

2. Calomel (saturated KCI)(SCE): 0.241v

3. Ag/AgCi(saturated KCI): 0.250v

4. Zinc(sea water): -0.80v


Cathodic Protection PotentialCopper/copper sulfate electrode

It is mainly used in soil and fresh waterenvironment


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Cathodic Protection PotentialCopper/copper sulfate electrode structure


Cathodic Protection PotentialSilver/silver chloride electrode

It is mainly used in seawater.


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Cathodic Protection PotentialZinc Electrode

It ismade ofpure zincandmainlyused insea water.


Cathodic Protection PotentialPotential Comparison for -0.85v CSE

In practical engineeringonshore, we normally useCSE electrode .

If the potential of theprotected structure reaches

–0.85V CSE, we regard thestructure being properlyprotected.

Relative to this criteria, otherreference electrodepotentials are as the rightcolumn.

Copper/copperelectrode:

-0.85V

Silver/silverelectrode:

-0.8V.

Zinc electrode:

+0.25V.


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Potentials of Common ReferenceElectrodes

1. Copper/Sulfate (Saturated) [CSE] 0.300

2. Calomel (Saturated KCl) [SCE] 0.241

3. AgC/AgCl (Saturated KCl) 0.196

4. AgC/AgCl [seawater] 0.250

5. Standard Hydrogen Electrode [SHE] 0.000

6. Zinc (Seawater) 0.800


Permanents Electrode InstallationPipeline

Permanent referenceelectrode installed forpotential monitoring of

buried pipelines. It is installed near the

pipeline surface.


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Permanents Electrode InstallationFuel Tank

Reference electrodefor under groundfuel tanks


Permanents Electrode InstallationTank bottom

The reference electrodeis buried beneath thetank bottom.

The backfill package issoaked beforeinstallation.


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Permanents Electrode InstallationWater tank

For water tank cathodicprotection, thereference cell isinstalled 2-3 cm fromthe inside tank wall.

Zn reference cell maybe used at this situation.


Permanents Electrode InstallationWater treatment equipment

The reference cell isinstalled near the tankwall.


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Permanents Electrode InstallationFresh water docks


Permanents Electrode InstallationPipeline immersed in fresh water


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Permanents Electrode InstallationWharf sheet piling water side


Permanents Electrode InstallationOffshore oil platform


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Soil Resistivity MeasurementWinner four pin method

The winner method uses four pinsdriven into the ground along astraight line, equidistant from eachother, causing an alternatingcurrent to flow through the soiland measure the voltage drop.

The meter will then represents aresistance reading and the soilresistivity is computed from aformula.

P=2x3.14xAxR: where:

1. A: distance between pins2. R:soil resistance presents by

meter

3. P: soil resistivity at the depth of A.


Soil Resistivity MeasurementWinner four pin method


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Soil Resistivity Approximation

In order to determine the anodebedgrounding resistance, the soil resistivitywill be tested first. According to experience:

Sea water: 20 ohm.cm

Sea mud: 40-100 ohm.cm

Clay: 4000-8000 ohm.cm

Wet sand: 10000 ohm.cm

Dry sand: 40000 ohm.cm


Current Density Requirement

To meet the potentialrequirement,estimated current densityof bare steel in various environment:

1. Soil: 5-30mA/m2

2. Fresh water: 10-30mA/m2

3. Moving fresh water: 65mA/m2

4. Sea water: 45-55mA/m2

5. Moving sea water: 160mA/m2

6. Sea mud: 10-30mA/m2


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Ground Bed DesignNear-surface anodebed

This kind of anode iseasy to install.

It is used where thesurface soil resistivityis low.

There may becorrosion interference

problem.


Ground Bed DesignDeep Anodebed

Deep anode bed is usedwhere the surface soilresistivity is high or theground surface space is

limited.

It is Usually 15-20m belowthe ground surface.whilemaximum depth can reachas deep as 150m.

It has less corrosioninterference problem.


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Ground Bed DesignSite Selection

In selecting groundbed sites, the most influencing factoris the soil resistivity.Other considerations include:

1. Are there underground metallic structure within the areaof influencing.

2. Is the ground bed site within the right of way.

3. Is there a power line present

4. Is the site reasonably accessible for construction andmaintenance.

5. Are there any future constructions.

6. Location of sacrificial anode are easier to select since itcan be placed within the right of way, independent ofpower supply and relatively free of interference withother structures.


Ground Bed DesignRemote ground bed

Current discharging from aground bed will cause voltagedrops in the earth between points along lines radiating fromthe ground bed.

As one walks away from the ground bed, the voltage dropper unit distance becomes less and less until a point is

reached, beyond which, no further significant voltage dropcan be observed. This is the edge of the ground bedinfluence. If the pipeline is located out side of this point, theground bed is called remote ground bed.

In this case, current flow into a general mass of the earth,which may be considered a resistance-less conductor.Current will flow from this infinite conductor to the pipelineand cause a voltage drop across the resistance between thepipeline and the infinite conductor.


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Ground Bed DesignRemote ground bed

With current flowing in an infinite conductor,the resistance of the pipeline it self may limitthe length of pipeline that can be protectedfrom one ground bed.(large diameterpipeline can have a longer section beingprotected).

A limitation near the ground bed is the need

to maintain the pipe-to-soil polarizedpotential less negative than –1.1V CSE toavoid coating damage and hydrogen effectsin susceptible steels.


Ground Bed Design Anodebed Position


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Pipeline is usually located outside of the voltagefield of the groundbed.

The formula can be used to calculate thedistance between a pipeline and the groundbed. Since the ground position is influenced bymany factors, the result can only be referred.

We usually regard that when the potential at apoint reaches 0.1v-0.5V with respect to remote

earth, the ground bed can be regarded asremote ground bed.



I: Anode Current A

P: Soil resistivity ohm.cm

Vx:Potential at point X V

X:Distance from the anodebed m

Vx

IP

X

00159.0

=


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Ground Bed DesignEquivalent Circuit


Ground Bed DesignClose Ground Bed

The pipeline will pass through the areaof influence surrounding a ground bed.Only a short section of the pipeline can

be protected. The region of pipeline protection by a

single anode is like a flashlight beamshined on a wall.As the flash light ismoved to the wall, the area illuminateddecreases but the light intensityincreases.


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Protective Current Impressed on aPipeline by a Close Ground Bed


Ground Bed DesignBackfill of the Anodes

The soil resistivity willhave to be got beforethe design can start.

The carbonaceous

backfill serves twofunctions:

1. Increasing the anodesize and reducinganodebed groundingresistance.

2. The materialsconsumption takesplace at the out edges

of the backfill column.


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Ground Bed DesignDetermination of Current Density

According to NACE RP169 andour experience, for a sufficientcathodic protection, 10mA/ m2

current is needed for bare steelburied in soils.

For new pipeline design, acoating break down factor orcurrent density is assumed andthan calculate the total currentneeded.

For existing structure, thecurrent is determined better bysite test or according toexperience.

For the commoncoatings used today,following currentdensity is assumed.

Coal Tar Enamel Asphalt ;FBE;PE:

0.01-0.05mA/m2

3-layer Coating:

0.01 mA/m2


Ground Bed Design Anode Materials Selection

For small structure in low soil resistivityarea, sacrificial anode can be used.

In areas with high soil resistivity or large

current is needed, impressed currentcathodic protection will be used.

According to current and soil resistivity,determine the type and number of anodematerials.


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Ground Bed DesignSingle Anode Grounding Resistance

⎟⎟ ⎠

⎞⎜⎜⎝

⎛ −⎟

⎠

⎞⎜⎝

⎛ ⋅⋅⎟ ⎠

⎞⎜⎝

⎛ ⋅⋅

= 14

ln2 r

L

L Ra

π

ρ

Ra = Grounding resistance(ohms)

ρ = Soil resistivity (ohm-m)

L =Anode length (m)

r = Anode radius （m）


Ground Bed DesignMulti-anodes Grounding Resistance

P: Soil resistivity in ohm-cm

N: Number of anodes in parallel

L: Length of anode in meters

D: Diameter of anode in meters

S: Anode spacing in meters

R: Resistance of vertical anodes in parallel to earth in

ohms

⎟ ⎠

⎞⎜⎝

⎛ +−= N LnS

L

D

L Ln

NL

P R 656.0

21

800159.0


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Ground Bed DesignTotal Circuit Resistance

Anode bed grounding resistance

Pipeline resistance to earth

1. The resistance can be got from pipeline voltage changewhen powered divided by the current supplied.

Cable resistance from pipeline to power source and frompower source to anodebed.

Back voltage between ground bed and pipeline

1. Back voltage is that which exists between the anodes andpipeline in opposition to the applied voltage.

2. For ground bed with carbonaceous backfill, the value is2.0V.

3. It can be measured in practice by taking the voltagereading between negative and positive rectifier terminalafter switching the rectifier power off.


Ground Bed DesignCables

For cathodic protection work, it is usually to usepolyethylene insulated, PVC or polyethylenesheathed copper cables. Cable sizes required forvarious current load can refer the following:

16 mm210-25

10 mm25-10

4.0 mm21-5

1.0 mm20-1

Cross area of copperconductor

Current in Ampere


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Cathodic Protection Installation


Cathodic Protection InstallationTransformer/Rectifier Installation

Various standards forrectifier installationare used dependingon local conditions

and regulations. None-explosive proof

unit is usuallyinstalled in the meterroom.


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Cathodic Protection of Buried PipelinesChoice of system

An impressed current system is usual for the cathodicprotection of buried pipelines.Depending on the price andavailability of electric current, terrain through which thepipeline passes, sacrificial anodes may sometimes beconsidered.

The use of sacrificial anodes is preferred when one or moreof the following conditions apply:

1. Lack of trained personnel to maintain the ICCP system.

2. Pipeline route is not suitable for installing equipment forICCP.

3. A current supply is not available.4. Cathodic protection is to be applied only at “ hot spot” (at

pockets of low resistance soil in an otherwise generally highresistance soil).

5. Pipeline is in highly congested area where ICCP will causeinterference with other buried steel structures.


Cathodic Protection of Buried Pipelines

Design Example:

Impressed current cathodic protection is desired for the 159mmwelded fuel oil pipeline. Since there is no other undergroundstructure in the area, so, a surface groundbed using prepackagedmixed metal oxide anode are used.

Design Data:1. Soil resistivity in area where groundbed is desired is 2000 ohm.cm

2. Pipe is 159 mm in diameter3. Pipeline length is 38km

4. Design life of the system is 15 years.

5. Design current density is 10mA per square meter of bare pipe

6. The pipe is coated with fusion bonded epoxy coating.

7. Assume 99% coating efficiency based on experience with this type ofcoating.Coating resistance 2500 ohm-m2

8. The pipeline is isolated from the pump house and the tan with isolating joints.

9. We have decided that the cathodic protection system circuit resistanceshould not exceed 2.0 Ohms.


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Cathodic Protection of Buried PipelinesDesign Example

Computations1. External surface area of the pipeline : =38000x3.14x0.159=16975

m2.

2. Current requirement: =16975x10x1%=1697mA.

3. Calculate the number of the anodes:for 25 cm by 150cm packaged canisterswith 4 mm by 120cm MMO anode rod, current output of such an anode is 1.2A.

According to the current capacity, two anodes are sufficient to supply therequired current.

4. Calculate the grounding resistance of a single anode:R=6.1 ohm.

5. To meet Max. 2.0 ohm resistance requirement, 4 no anodes will be needed.

6. Consider the anode mutual interference among the anodes, calculate the

resistance again with the formula for multi-anodes, it works out that 5 anodeswill be needed while their spacing is 3m.The actual grounding resistance is1.71 ohm.

7. This is quite close to the required 2.0 ohm resistance, so, 6 anodes areselected for variation in soil resistivity, wire and pipeline to soil resistance.

L DS ××=π

CbCd S I ××=



With 6 anodes, the grounding resistance is 1.49 ohm.

Groundbed location: To ensure a uniform current distribution,the groundbed should be located some distance from thepipeline, the higher the soil resistivity, the further away thegroundbed should be. According to experience, groundbed is

usually placed 50-300m from the pipeline. Since the soilresistivity is quite low in this case, the groundbed is placed50m from the pipeline.

Anode cable resistance: Select 10mm2 , its resistance is0.8ohm/100m , 6 anodes will have a lead resistance of0.14ohm.

Nowadays, it has become a normal practice if the groundbedis not very far away from the T/R, each anode wire will rundirectly to the T/R without splices.


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Pipe to soil resistance:2500/16975=0.15ohm

Total resistance = 1.49+0.14+0.15=1.78 ohm, it is inline with the design requirement.

Calculate the T/R voltage: V=1.7x1.78x120%+2=5.6V.(when using coke backfillmaterials, 2v back voltage will be added to thecalculation)

Select T/R: based on the voltage and currentrequirement, the nearest standard capacity availablefrom commercial market is 10V/5A.




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Cathodic Protection with Mg Anodes

Sacrificial anode cathodic protection is desired for the 159mmwelded fuel oil pipeline. Since there is other undergroundstructure in the area.

Design Data:

1. Soil resistivity in area where groundbed is desired is 2000 ohm.cm

2. Pipe is 159 mm in diameter

3. Pipeline length is 38km

4. Design life of the system is 10 years.

5. Design current density is 10mA per square meter of bare pipe

6. The pipe is coated with fusion bonded epoxy coating.

7. Assume 99% coating efficiency based on experience with this type ofcoating.Coating resistance 2500 ohm-m2

8. The pipeline is isolated from the pump house and the tank with isolating joints.


Cathodic Protection DesignPipeline

Computations1. External surface area of the pipeline : =38000x3.14x0.159=16975 m2.

2. Current requirement: =16975x10x1%=1697mA.

3. Calculate the weight of the anodes per current capacity: W=160kg according to thefollowing formula: If 7.7kg anode is selected. 21 no. will be needed.

4. Check the anode number per grounding resistance. Single anode groundingresistance is calculated according to the above formula: If the packaged dimension ofthe anode is 760mmx250mm, the grounding resistance of each anode is 9.2 ohm.

Assume the polarized potential of the pipeline is –0.9v, the open circuit potential ofthe Mg anode is –1.75V CSE, the driving voltage will be 0.85v. Single anode currentout put is 0.85/9.2=92mA. To achieve a current out put of 1697mA,18 pieces ofanodes will be needed, which is less than the number calculated from current capacity.So, 21 no. anodes will be used.

5. The anodes will be distributed along the pipeline evenly.

L DS ××=π

CbCd S I ××=

Q Z U

t I W

××××

=8766

⎟⎟ ⎠

⎞⎜⎜⎝

⎛ −⎟

⎠

⎞⎜⎝

⎛ ⋅⋅⎟ ⎠

⎞⎜⎝

⎛ ⋅⋅

= 14

ln2 r

L

L Ra

π

ρ


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Tank Bottom Cathodic Protection For tank bottom cathodic

protection, the API651:Cathodic Protection of

Above Ground PetroleumStorage Tanks is generallyfollowed.

Principles of pipeline cathodicprotection can be applied totank protection.

The anode can distribute alongthe premier of the tank or

placed beneath the tankbottom. A new development isby using Grid anode


Tank Bottom Cathodic ProtectionMagnesium Ribbon Anode

Mg anoderibbon is oftenused for smalltank bottom

cathodicprotection.


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Cathodic Protection DesignOil Tank External

Project: product oil tank bottom external protection:

Reference Standards:

1. BS7361 – 1991, cathodic protection part 1 code of practice for landand marine applications.

2. NACE RP0169-1996, control of external corrosion on under ground orsubmerged metallic piping system.

3. API 651 cathodic protection of above ground petroleum storage tank

Design data

1. Tank diameter: 22m

2. Environmental temperature: 25 degree.

3. Soil Resistivity: 8000 ohm. Cm4. Coating breakdown factor: 50%

5. Current density: 10mA per square meter

6. Design life: 10 years.


Cathodic Protection DesignOil Tank External

Calculation: Protected area:=3.14x112=380 m2

Protected bare area: 190m2,

Currents needed is :1900mA

Q Z U

t I W

××××

=8766


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Cathodic Protection InstallationOil Tank External

W=180kg

Ribbonlength:=180/0.36=500m.

Groundingresistance=0.42ohm.

Mg potential: -1.75V CSE.

Polarization potential -0.90V CSE.

Driving voltage:=0.85V. Actual current

output:=0.85/0.42=1.9A.

2.02>1.9. Ok


Tank Bottom Cathodic ProtectionGrid Anode Installation

The anodegrid is formedby MMOribbon

connected byTitaniumconductor bar.

They are spotwelded at theintersection


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Tank Bottom Cathodic ProtectionGrid Anode Installation

It can provide avery uniformcurrent to thetank bottom

No interferencecurrent will beproduced with

this kind ofanode bed.


Tank Bottom Cathodic ProtectionGrid System Installation


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Tank Bottom Cathodic Protection

It has alongdesign life of over40 years.

The groundingsystem of thetank need to beretrofitted.

The siteinstallation is verysimple and the

quality is easy tobe guaranteed.


Oil Tank Bottom Cathodic ProtectionInternal

Oil tank bottom is usuallyprotected with Al anodes sincethere is always a depth of salinewater accumulated at the bottom.

The recommended currentdensity for bar metal is 100mAper square meter and the coatingbreak down factor is 10%.

Anode quantity can be computedaccording to design life andcurrent requirement and theanodes will be distributed evenlyon the tank bottom.


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Cathodic Protection DesignOil Tank Internal

Crude oil tank: 60m

Temperature: 45 degree

Tank wall protected height is 1.5m.

Design life 10 years:

Coating break down factor 10%.

Current density:160mA/m2

Tank bottom area:S=3.14x302=2826m2


Cathodic Protection DesignOil Tank Internal

S2=3.14x60x1.5=282m2

Total area: 282+2826=3088m2.

Current:=160x3088x10%=4941mA, 5 A.

Current efficiency:2000-27(T-20)=1325 Anode weight:=432kg

If we choose 4.5 AL anode,96 pieces will be used.

The anodes will be distributed evenly on the tankbottom.


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Water Tank Cathodic ProtectionInternal

For tanks holds freshwater, current densityis usually 25mA /m2.

If the impressedcurrent method is used,coating break downfactor is 20%.asmaximum.

If using sacrificial

anode, coating breakdown factor is 10% asaverage.


Water Tank Cathodic ProtectionInternal

By using theMMO wire anode,the system canbe easily installed.

Pay muchattention to thecable connectionseal.


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Cathodic Protection of Underground Tanks

Under ground tanks can beprotected by either sacrificialanode or impressed current .

For single or small number oftanks, they can be protected bysacrificial anodes which sitedseveral meters from them, Tryto make the anodes distributioneven.

For large tank or tank farms,

impressed current is often adoptto meet the large currentrequirement. The anodes should

be evenly distributed.


Cathodic Protection of Underground TanksBy Galvanic Anode Protection by sacrificial anodes

1. Anodes should preferably be sited on a line normal to thelong axis of the tanks at a distance of about 4.5m from theout side surface of the tank. If two anodes are used, oneshould be positioned on each side of the tank. For a well

coated tank, the sitting of the anodes is not critical and theymay be sited to suit conditions, at a distance of approx. 3-6m from the tank.

2. The cable can be connected at vent pipe, lifting lugwherever convenient.

3. The cable from the tank and cable from the anode shouldbe connected through a measuring box, a wire from thetank is preserved to easy the current/potentialmeasurement.


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Cathodic Protection of Underground TanksICCP

Protection by impressed current system

1. For a large group of tanks, an impressed currentsystem is most suitable.

2. An even spread of protection must be ensured bysuitable placing of the anodes.

3. Insulating joints should be installed in the line fromthe tank. The insulating joint should be close to thetank to avoid high current consumption by badlycoated lines or contacts of lines with other metalstructures.


Cathodic Protection of Heat Exchangers General

1. Because of the high water temperature and the heavycurrent requirement, ICCP is recommended for cathodicprotection of box coolers, although the space available foranodes is small and their number and dimensions must be

limited.

2. The high water temperature prevailing in box coolers, oftenover 40 degree which makes the self-corrosion of Mg anodetoo high to be used satisfactorily.

3. Zn anodes are unsatisfactory because they supply too lowcurrent at a high temperature and their polarity with respectto steel may be reversed over 60 degree.


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Cathodic Protection of Heat ExchangersCurrent Determination Current Requirement

1. With sea water as the cooling medium and the watertemperatures up to 45 degree, the current density requiredfor cathodic protection of steel is 110mA-220mA/m2.

2. The wall of the cooler box are usually gunited and thecurrent required for protection is then negligible.a calculationusing the data given above is sufficient for currentcalculation.

3. Because of the close proximity of the anodes to thecathodically protected tubes, tube to water potentialsshould be used with caution, and the points where thesecheck measurements are taken should be as remote aspossible from the anodes.

4. The tube to water potential should be maintained at – 0.85VCSE.


Cathodic Protection of Heat Exchangers

Anode materials1. As stated above, since the space available is small,

platinized titanium anodes are particularly suitablefor use in box coolers with either fresh or salt

water.

2. Lead-alloy anode are unsuitable for use in freshwater.

3. Facilities for measuring current in the leads toanode should be provided.


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Cathodic Protection of Oil Well Casing

General:1. The cause of casing corrosion may be sulphate

reducing bacteria, acidic water, or corrosion cellsset up between formations containing water ofdifferent salt content or between casing and flowlines.

2. Cathodic protection prevents corrosion only on the

outside surface of the casing.


Cathodic Protection of Oil Well CasingCurrent Determination

Current requirement:

1. The current density required for protection variesfrom 5 to 30 mA/m2 of casing surface. Averagevalues of protection currents for wells of different

depth are as following:

302400

51500

2900

Current (A) Approx. Depth (m)


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Cathodic Protection of Oil Well CasingCurrent Determination The current required for protection of a well is determined by

measuring the potential of the well head with respect to a CSE,applying the current in steps.

The required current is determined from the potential/currentrelationship.

1. A graph is made showing the steel to soil potential against thelogarithm of the impressed current density. The relationship is astraight line with a slight inclination at a low current; after a breakpoint the curve continues as a straight line with a sharper rise at ahigher currents. The break point indicates the current densityrequired to provide cathodic protection.

2. As an check, electrical current in the casing can be measured using

a tool equipped with spring-loaded contacts spread at 7.5mintervals, which is run inside the casing. From the potentialdifference between these contacts, the direction and magnitude ofelectric current flowing in the casing can be derived by means ofOhm’s law.




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Test procedures1. Measure the native potential of the well head.

2. Increase the current with 0.1A at every two to threeminutes.

3. Measure the instant off potential.

4. The current interruption should last no more than 2seconds, then a higher current applied onto the casing.

5. Casing to soil potential and current applied should beplotted on a semilogrithmic graph paper.

6. The current required is taken at the intersection point.7. A simple way in practice is to maintain the well head off

potential at – 1.15V CSE. Current density falls into therange between 5-30mA/m2.


Cathodic Protection of Oil Well CasingSystem Selection

Impressed current system:1. With the ICCP system, the anode bed is located 30-60m

from the well, or in a central position when more wells areprotected with one ICCP system.

2. The well head should be insulated from the flow line toprevent loss of current to other structures and to preventstray current.

3. When several wells are protected with one ICCP system,each should be connected via a resisto

Documents

Cathodic Protection 2007[1]