cathodic protection 1

8/8/2019 Cathodic Protection 1

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Overview of Overview of Cathodic protectionCathodic protection



History of CPHistory of CP

First First CathodicCathodic protectionprotection waswas usedused byby SirSir..HumphreyHumphrey DavyDavy forfor CopperCopper CladdingCladding of of

shipship ManMan of of War,War, SamarangSamarang

DescribedDescribed byby SirSir DavyDavy asas anan obscureobscurebranchbranch of of Electrochemistry,Electrochemistry, CPCP stillstill

remainsremains difficult difficult toto understandunderstand / / analyzeanalyzebyby analyticalanalytical modelsmodels..



PreamblesPreambles

DesignDesign:: TailorTailor MadeMade withwith referencereference totoconstructionconstruction ,, environment environment etcetc..

Monitor,Monitor, MaintainMaintain andand upgradeupgrade safe,safe,reliablereliable andand effectiveeffective protectionprotectioncontinuouslycontinuously throughout throughout thethe designdesign lifelife

ExperienceExperience:: reliablereliable basisbasis forfor futurefuture designdesign / / EnggEngg andand operationoperation..



CorrosionCorrosion

CorrosionCorrosion isis defineddefined asas thethe degradationdegradation of of aametallicmetallic materialsmaterials becausebecause of of anan electrochemicalelectrochemical

reactionreaction betweenbetween thethe metallicmetallic materialsmaterials andand itsitsenvironment environment..

InIn Nature,Nature, MetalsMetals exist exist asas ores,ores, WhichWhich maymay

bebe oxides,oxides, carbonatescarbonates oror SulphidesSulphides andand metalsmetalshavehave naturalnatural tendencytendency toto revert revert toto itsits originaloriginal

ore ore statestate..



Basic Corrosion CellBasic Corrosion Cell

ConsiderConsider twotwo ironiron (Fe)(Fe) electrodeselectrodes suspendedsuspended inindilutedilute solutionsolution of of commoncommon salt salt.. TheThe electrodeelectrodethat that isis polarizedpolarized positivepositive isis thethe anodeanode andand thethe

electrodeelectrode that that isis polarizedpolarized negativenegative isis thethecathodecathode..

At At thethe anode,anode, thethe metalmetal getsgets oxidizedoxidized

FeFe >> Fe++Fe++ ++ 22ee--

At At thethe cathode,cathode, oxygenoxygen willwill bebe reducedreduced toto formformnegativenegative ionsions..

½½ OO22 ++ HH22O+O+ 22ee-- -->> 22(OH)(OH)--



Galvanic Corrosion CellGalvanic Corrosion Cell

To understand the working of a galvanicTo understand the working of a galvaniccell, it is important to take a look at thecell, it is important to take a look at the

electrochemical series.electrochemical series.

The electrochemical series is a tabulationThe electrochemical series is a tabulationof various elements and is indicative of theof various elements and is indicative of the

extent of their electrochemical activity.extent of their electrochemical activity.



Electrochemical SeriesElectrochemical Series

MgMg

Al Al

ZnZn FeFe

NiNi

PbPb

HH CuCu

Ag Ag

-- 2.38 mV2.38 mV

-- 1.67 mV1.67 mV

--0.76 mV0.76 mV--0.44 mV0.44 mV

--0.23 mV0.23 mV

--0.13 mV0.13 mV

0.000.000.35 mV0.35 mV

0.80 mV0.80 mV



Galvanic Series in sea waterGalvanic Series in sea water

Base endBase end MagnesiumMagnesium

ZincZinc

Aluminium Aluminium

SteelSteel Cast ironCast iron

LeadLead

BrassBrass

CopperCopper

SilverSilver

GoldGold

Noble endNoble end



Old steel in contact with newOld steel in contact with new

steelsteel

+V- +V- +V-

-0.4V -0.65V -0.35V

Old pipe New pipe Old pipe

Cathodic Area Anodic Area Cathodic Area



Corrosion cell due to differentCorrosion cell due to different

soilssoils

A A A AC CC



TYPES OF CP S YSTEMTYPES OF CP S YSTEM

G ALVENIC (Sacrificial) ANODEG ALVENIC (Sacrificial) ANODE

S YSTEMS YSTEM

IMPRESSED CURRENT S YSTEMIMPRESSED CURRENT S YSTEM



Sacrificial CP SystemSacrificial CP System

Sacrificial Anode Pipeline

Current



Advantages of Galvanic Advantages of Galvanicanode systemsanode systems

Low susceptibility to damage by outsideLow susceptibility to damage by outsideinfluencesinfluences

Low maintenance costs throughout the design lifeLow maintenance costs throughout the design life

Do no require an outside source of energy suchDo no require an outside source of energy suchas commercial poweras commercial power

Fewer system componentsFewer system components

Well suited to the protection of electricallyWell suited to the protection of electrically

isolated/segmented structures (such asisolated/segmented structures (such asmechanically joined pipe sections) in which eachmechanically joined pipe sections) in which eachcomponent is provided with its own galvaniccomponent is provided with its own galvanicanodeanode

Low potential operationLow potential operation--seldom producesseldom producessignificant corrosive interference (stray current)significant corrosive interference (stray current)



Limitations of GalvanicLimitations of Galvanic

anode systemsanode systems Electrical potential available is limitedElectrical potential available is limited

Subject to seasonal variations affectingSubject to seasonal variations affectingoperationoperation

In acid environment, the anode materials areIn acid environment, the anode materials aredirectly attacked by the acid and the life anddirectly attacked by the acid and the life andefficiency are extremely low.efficiency are extremely low.

Maximum services life is dependent on theMaximum services life is dependent on thequantity of anode materialquantity of anode material

Chemistry of the electrolyte (soil) andChemistry of the electrolyte (soil) andtemperature may have and undesirabletemperature may have and undesirable

effect on the anode half cell potentialeffect on the anode half cell potential



Impressed Current CPImpressed Current CP

SystemSystem

Impressed current Anode Pipeline

Current

DC Power

Source

+ -



Criteria for protection.Criteria for protection.

To verify the efficacy of a CP system forTo verify the efficacy of a CP system forburied steel structure.buried steel structure.

It has to meet one or more criteria as set outIt has to meet one or more criteria as set outin NACEin NACE--RP 01RP 01--69.69.

A cathodic potential of at least 850mv with A cathodic potential of at least 850mv withthe CP applied.the CP applied.

A negative polarized potential of at least A negative polarized potential of at least

850mv relative to a CSE.850mv relative to a CSE. A minimum of 100mv of cathodic A minimum of 100mv of cathodic

polarization between the structure surfacepolarization between the structure surfaceand a stable reference electrode.and a stable reference electrode.



Advantages of Impressed Advantages of ImpressedCurrent SystemsCurrent Systems

The electrical potential available throughThe electrical potential available throughthe power supply can be fully adjustablethe power supply can be fully adjustablefrom very low values to quite high valuesfrom very low values to quite high values

The power is limited only by the sourceThe power is limited only by the source A wide variety of materials are available for A wide variety of materials are available for

impressed current anodesimpressed current anodes

Many impressed current anode materialsMany impressed current anode materials

have very low consumption rates and canhave very low consumption rates and canprovide high output with limited sizeprovide high output with limited size

Electronic methods of control andElectronic methods of control andmonitoring are possible.monitoring are possible.



Limitations of ImpressedLimitations of Impressed

Current SystemsCurrent Systems Impressed Current Systems requireImpressed Current Systems require

an outside source of energyan outside source of energy

Regular maintenance is essentialRegular maintenance is essential

Anode lead wires may corrode Anode lead wires may corrode

Operation of the anode at relativelyOperation of the anode at relativelyhigh potentials may causehigh potentials may causeinterference current on otherinterference current on otherstructures.structures.



G ALVANIC CP IS APPLIED G ALVANIC CP IS APPLIED

FORFOR

WELL CO ATED PIPELINEWELL CO ATED PIPELINE

SHORT PIPELINESHORT PIPELINE

STRO AGE T ANKSTRO AGE T ANK

HOT SPOTSHOT SPOTS



FACTORS AFFECTING FACTORS AFFECTING

DESIGNDESIGN NATURE OF ENVIRONMENTNATURE OF ENVIRONMENT

NATURE OF CO ATINGNATURE OF CO ATING

POLARISATION LEVELPOLARISATION LEVEL

LINEAR ELECTRICAL RESIST ANCELINEAR ELECTRICAL RESIST ANCE

EXTENT OF NEIGHBOURING EXTENT OF NEIGHBOURING

STRUCTURESTRUCTURE AVAILABIILITY OF POWER AVAILABIILITY OF POWER

STRA Y CURRENTSSTRA Y CURRENTS



Steps in designing CPSteps in designing CPinstallation Sacrificialinstallation Sacrificial

SystemSystem1.1. Establish soil resistivityEstablish soil resistivity

2.2. Estimate current requirementEstimate current requirement

3.3. Electrical continuity of structureElectrical continuity of structure4.4. Consider requirement for electrical isolationConsider requirement for electrical isolation

5.5. Select suitable Anode materialSelect suitable Anode material

6.6. Calculate total mass of metal for required lifeCalculate total mass of metal for required life

7.7. Select individual anode shape to satisfy currentSelect individual anode shape to satisfy currentrequirementrequirement

8.8. Check the total anode weight determine by (6)Check the total anode weight determine by (6)satisfy requirement of (7)satisfy requirement of (7)

9.9. Consider facilities for monitoring performanceConsider facilities for monitoring performance

10.10.Proper project drawing & specificationsProper project drawing & specifications



teps n es gn ngteps n es gn nginstallation Impressedinstallation Impressed

CurrentCurrent1.1. Establish soil resistivityEstablish soil resistivity

2.2. Estimates total current requirementEstimates total current requirement

3.3. Establish electrical continuity of structureEstablish electrical continuity of structure

4.4. Consider requirement for electrical isolationConsider requirement for electrical isolation

5.5. Consider the no. & disposition of anode/ground bedConsider the no. & disposition of anode/ground bed6.6. Select suitable anode materialSelect suitable anode material

7.7. Calculate Anode/ground bed size, shape, configrnCalculate Anode/ground bed size, shape, configrn

8.8. Calculate circuit resistance & system DC VoltsCalculate circuit resistance & system DC Volts

9.9. Design the DC wiring system for the most economical cableDesign the DC wiring system for the most economical cable

size & total IR drop in the circuit.size & total IR drop in the circuit.10.10. Select rectifier voltage & current outputSelect rectifier voltage & current output

11.11. Design electrical circuit, fittings & switchgearDesign electrical circuit, fittings & switchgear

12.12. Select location of CP test stationsSelect location of CP test stations

13.13. Consider facilities required for control monitoringConsider facilities required for control monitoring

14.14. Proper project drawing & specificationsProper project drawing & specifications



AD VANT AGES OF GOOD AD VANT AGES OF GOOD

CO ATING FOR CPCO ATING FOR CP REQUIRED LOW CURRENT DENSITYREQUIRED LOW CURRENT DENSITY

LOW POWER CONSUMPTIONLOW POWER CONSUMPTION

LOW WEAR OF ANODESLOW WEAR OF ANODES

LARGER SPACING BETWEEN CPLARGER SPACING BETWEEN CPST ATIONST ATION

INTERFERENCE PROBLEMS AREINTERFERENCE PROBLEMS AREMINIMISEDMINIMISED



Coating & CathodicCoating & Cathodic

ProtectionProtection

Cost

Coating

Cathodic

Protection

0 % 100 %

Total Cost



CURRENT REQUIREMENTS FOR STEELCURRENT REQUIREMENTS FOR STEEL

ENVIRONMENT URR ENT DENSITY IN

mA/m2

WELL-AER ATE NEUTR AL OIL 21.5 ± 32.3

RY, WELL-AER ATE OIL 5.4 ± 16.1

WET, OIL, MO ER ATE/ EVERE

ON ITION

26.9 ± 64.6

HIGHLY ACI OIL 53.8 ± 161.4

TATIONARY FRE H WATER 53.8

MOVING FRE H WATER 53.8 ± 64.6

WELL COATE I ELINE IN

OIL

0.01



Protective currentProtective current

requirementrequirement

2Io = 2 T D J L

Io = Protective current from one side of the CP Station

D = Diameter of the pipeline (m).

J = Protective current density (A/m2).

L = One side Spread of protection (m)



Pipeline Spread ofPipeline Spread of

protectionprotection

L = Spread of protection in m

(UL = Potential difference between drain point & at

end (x = L).D = Diameter of the pipeline (m).

J = Protective current density (A/m2).

R' = Resistance of the pipe (ohm/m).

8

(U

D J R

L

T'

2 =



Choice of Sacrificial Anodes forChoice of Sacrificial Anodes forEarth ApplicationEarth Application

Anode Anodematerialmaterial

Anode Anodecorrosioncorrosionpotential mVpotential mV

Anode Anodepotentialpotentialdifferencedifference

mV (anodemV (anode 850 mV)850 mV)

Kg/AKg/A --yearyear

ZincZinc

MagnesiuMagnesiumm

--11001100

-- 16001600

--250250

--750750

1212

88



CALCULATION OF ANODE LIFECALCULATION OF ANODE LIFE

A Galvanic Anode useful life can be calculated when the A Galvanic Anode useful life can be calculated when thefollowing parameters are known:following parameters are known:

Anode output current Anode output current I (A)I (A)

Anode weight Anode weight m (kg)m (kg) EfficiencyEfficiency E (%)E (%)

CapacityCapacity C (Ah/kg)C (Ah/kg)

T

he lifetime L in hours can be calculated fromT

he lifetime L in hours can be calculated fromE . C . mE . C . m

L =L = ------------------------ (hours)(hours)

II



CP HEALTH MONITORING SURVE YCP HEALTH MONITORING SURVE Y

EFFECTIVENESS OF CP S YSTYEMEFFECTIVENESS OF CP S YSTYEM

Close Interval Potential SurveyClose Interval Potential Survey

CORROSION PROTECTIVE CO ATINGCORROSION PROTECTIVE CO ATING

Pearson SurveyPearson Survey

CAT SurveyCAT SurveyDCVG SurveyDCVG Survey



RecommendationsRecommendations

GO VT OF INDIA (MOP&NG GO VT OF INDIA (MOP&NG OISD)OISD)

CPL survey once in 5 year orCPL survey once in 5 year orwhenever inadequate CP is observedwhenever inadequate CP is observed

which ever is earlier.which ever is earlier.

Pearson survey , DCVG survey, CAT Pearson survey , DCVG survey, CAT

survey to elaborate the coating defectssurvey to elaborate the coating defectsas and when required.as and when required.



Continuous Potential Logging (CPL)Continuous Potential Logging (CPL)

Pipe to soil potential is only criteriaPipe to soil potential is only criteriaused for determination of corrosionused for determination of corrosionactivities/condition & effectiveness ofactivities/condition & effectiveness ofcorrosion protective coating &corrosion protective coating &cathodic protectioncathodic protection

CPL survey is an essential survey.CPL survey is an essential survey.



PEARSON SURVE YPEARSON SURVE Y

To find outTo find out

the location & depth of pipeline.the location & depth of pipeline.

Exact locations of coating defects.Exact locations of coating defects.

Location of electric contacts.Location of electric contacts.



Current attenuation test (CAT) surveyCurrent attenuation test (CAT) survey


To find out the location & depth ofTo find out the location & depth ofpipeline.pipeline.

Exact locations of coating defects.Exact locations of coating defects.

This is a user friendly & modernizedThis is a user friendly & modernizedversion of Pearson Surveyversion of Pearson Survey



Direct current voltage GradientDirect current voltage GradientDCVG surveyDCVG survey


the location of Electric contacts.the location of Electric contacts.may be by exp. Operator.may be by exp. Operator.

locations of coating defects.locations of coating defects.



THANK THANK

YOU YOU

cathodic protection 1

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