Evolution of the Electrical Potential for the Cathodic Protection of Pipelines According to the Variation of the Imposed Current

BarhmMohamad 22 views 9 slides Dec 13, 2024
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About This Presentation

This work presents a study on the use of cathodic protection as a measure against corrosion in pipelines. The cathodic protection, compliant with the API 5L standard, is implemented here by applying an impressed current, while carefully considering several essential variables, such as soil character...


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JournalofHarbinInstituteofTechnology(NewSeries)
Received2024-01-15.
∗Correspondingauthor:BarhmMohamad,Ph.D,Lecturer.Email:barhm.mohamad@epu.edu.iq
Citation:ChoukiFarsi,SaidZergane,SalahAmroune,etal.Evolutionoftheelectricalpotentialforthecathodicprotectionof
pipelinesaccordingtothevariationoftheimposedcurrent.JournalofHarbinInstituteofTechnology(NewSeries).DOI:10.11916/
j.issn.1005-9113.2024049
EvolutionoftheElectricalPotentialfortheCathodicProtectionof
PipelinesAccordingtotheVariationoftheImposedCurrent
ChoukiFarsi
1,2
,SaidZergane
1,2
,SalahAmroune
1,2
,BarhmMohamad
3,∗
,AzzedineBenyahia

andMohamedLatrache

(1.MechanicalEngineeringDepartment,FacultyofTechnology,UniversityMohamedBoudiaf-M’sila,M’sila28000,Algeria;
2.LaboratoryofMaterialsandMechanicsofStructures(LMMS),FacultyofTechnology,UniversityMohamedBoudiaf-M’sila,
M’sila28000,Algeria;
3.DepartmentofPetroleumTechnology,KoyaTechnicalInstitute,ErbilPolytechnicUniversity,Erbil44001,Iraq)
Abstract:Thisworkpresentsastudyontheuseofcathodicprotectionasameasureagainstcorrosioninpipelines.The
cathodicprotection,compliantwiththeAPI5Lstandard,isimplementedherebyapplyinganimpressedcurrent,while
carefullyconsideringseveralessentialvariables,suchassoilcharacteristics,thetypeandcolorofthepipelinematerial,
aswellastheplacementandsizeoftheanode.Therefore,itiscrucialtooptimizethelocationandvaluesofanodic
overflowsorgroundresistancestoensureauniformdistributionofpotentialacrosstheentirestructure.Inthismethod,
impressedcurrentprotectionusesanauxiliaryanodeandanexternaldirectcurrentsourcetoinduceacurrentthroughthe
electrolyteandthepipeline,thuscounteringtheresistanceofthesteel.Thisapproachisadvantageousasitallowsforthe
adjustmentofelectricalcharacteristics,particularlycurrentlevels,tomeetspecificneeds.Thefactorsessentialtothe
effectivenessofcathodicprotectionsystems,whichoptimizethedistributionofprotectionpotentialacrossthestructure,
largelydependontheprecisemanagementofgroundresistancesduringanodicdischarge,particularlytheattenuation
coefficient(α).Thesefactorswerestudied,andtheresultsobtainedwerepresentedanddiscussedbasedontheir
influence.
Keywords:cathodicprotection;pipelines;imposedcurrent;electricpotentialdifference;anodeplacement
CLCnumber:TG172   Documentcode:A    ArticleID:1005⁃9113(2024)00⁃0000⁃09
0 Introduction
  Cathodicprotectioniswidelyusedacrossvarious
industries,includingthoseinvolvingburiedpipelines,
condensers,watertanks,andchemicalequipment.
Manymetallicstructures,suchasgasandoilpipelines
andcrudeoilstoragetanks,aresusceptibleto
corrosion,whichresultsfromchemicalor
electrochemicalreactionsthatbreakdownthemetal.
Steelcorrosionisacommonissuemanifestingin
numerouscomplexformsofdamage.Oil
transportationfacilitiesfaceseveralrisks,often
startingwithunintentionaldeformations.These
challengeshighlighttheneedtorepairingand
maintaininginfrastructuretoensuretheoperational
efficiencyofgaspipelinetransportcompaniesandto
minimizeassociatedcosts.
Itiscrucialtodifferentiatebetweenstudying
corrosionproblemsandunderstandingcorrosion
phenomena.Aclearunderstandingofcorrosionevents
isbestachievedthroughanelectrochemicalapproach,
whichprovidesinsightsintowatercorrosion.Given
thatmoistsoilcanactasanelectrolyte,analyzing
metalcorrosioninsoilfromanelectrochemical
perspectiveisvaluable,eventhoughsoildiffersfrom
atypicalelectrolyte.
Thisworkpresentsastudyonprotection
measuresformetallicpipelinessubjectedtosoil
corrosionconstraints.Thesepipelines,oftenusedfor
transportinghydrocarbons,suchasoilandgas,
varyinginlength,diameter,andthetypeof
substancestheycarry.Theyareparticularlyvulnerable
torisksrelatedtomanufacturingdefects,corrosion⁃
induceddegradation,andsurfacecracks
[1]
.To
addressthesechallengesandensurelong⁃term
·1·

JournalofHarbinInstituteofTechnology(NewSeries)
operation,pipelinesarecontinuouslymonitored.
Amongthemethodsemployedinthisstudyistheuse
ofimpressedcurrentorsacrificialanodes.
Pipelinesaretypicallymadeupofacontinuous
weldedassembly,coatedwithvariousprotective
layersbothinternallyandexternally,andareoften
buriedunderground.Theyofferamorecost-effective
meansofmedium-distancetransportationcomparedto
othermethods.Inthecaseofgaspipelines,they
facilitatetheflowofvariousfluidsbetweenextraction
sitesandconsumptionorexportlocations.Theglobal
networkofgaspipelinesextendstonearlyonemillion
kilometers,morethan25timestheEarth�s
circumference.Inurbanareas,thesepipelinesare
usuallyburiedonemeterdeep,whileindesert
regions,theymaybelaidonthesurface.Their
diametersrangefrom50mm(2inches)to1400mm
(56inches)forlargerpipelines.Duetothedistance
andisolationofextractionsites,underseatransporthas
becomeapreferredoption
[2]

Metalstructuresinpetroleumpipelinesface
challenges,suchasmechanicalstressesandexternal
chemicalattacks,whichcanleadtocorrosion,
cracking,andstructuralinstability.Aspipeline
networksexpandtomeetgrowingfluiddemands,new
technologiesandsolutionsarerequiredtoaddress
consumerdemands.Withoutpipelinetransportation,
costswouldincrease,andproductivitywoulddecrease
comparedtoalternativesliketrucking,railfreight,or
tankers.Currentresearchinpipelinetechnology
anticipatesthefuturedevelopmentofmulti⁃product
pipelines,thoughthiscomeswithenvironmental
considerations.
Foreachapplicationandtypeofproduct
transported,thechoiceofpipelinestypeismade
accordingly.InCanada,thepipelinesystemisrapidly
expanding,makingitoneofthelongestintheworld
(242400km).Corrosionpreventionmustbeginatthe
designphaseandcontinuethroughouttheproject�s
lifespan.Thisinvolvesensuringaspecificlifespan(e.
g.,25years)whileminimizinginvestmentand
maintenancecosts.Additionally,thechosensolution
mustcomplywithenvironmentalprotectionstandards
andallowfortherecyclingordisposalofcomponents
attheendoftheirlife.
Corrosionprotectionforpipelinescanbe
achievedthroughcoatings,cathodicprotection,and
inhibition.Prolongedexposureofpipelinesto
corrosivesoilandaggressivewaterincreasesthe
likelihoodofcorrosioncellforming.Therefore,
implementingprotectivemeasuresagainstcorrosionis
crucial.Preventionmethodsinclude:(a)Coatings:
Theseprovideelectricalinsulationtothepipeline,
preventingdirectcontactwiththeenvironmentand
reducingtheformationofcorrosioncellsonits
surface.(b)Cathodicprotection:Thistechnique
involvesapplyingacontinuouselectriccurrentto
lowerthepotentialofthemetalstructuretoalevel
wherethecorrosionisminimized
[3-5]

Gray
[6]
highlightedseveralinstancesofstress
corrosionfailuresinrecentlyconstructedpipelinesin
AustraliaandCanada.Itwasnotedthatindustrial
companieswereconductingmetallurgicaltests,
includinghardnesschecks,tograduallyimprove
externalcoatings.Between1988and1989,in
responsetoa50%increaseinvanadiumprices,
vanadium⁃freesteelwasintroduced,incorporating
elementssuchasmolybdenum(Mo),chromium
(Cr),andTMCP.In1990,advancementsindeep⁃
wateroilandgasexplorationledtotheconstructionof
additionalpipelines,suchasthoseconnectingOman
toIndiaandacrossingtheBlackSea.Duringthis
period,companiesbeganusingthickDouble⁃
SubmergedArcWelded(DSAW)pipes,designedto
withstandpressure⁃inducedbuckling.Additionally,
seamlesssteelpipelineswithastrengthof552MPa
werealsoproduced.
Thegoalofthisresearchistoshowthat
achievinganoptimalpotentialdistributionforcathodic
protectionrequirescarefullymanagingofcritical
factors,includingtheenvironment,currentlevels,
soilcharacteristics,pipematerial,coatingtype,and
anodeplacement.Thisstudyfocusesonusingimposed
currentforcathodicprotectiontoenhancesystem
performanceandtoensureuniformpotential
distributionthroughoutthestructure.
1 Methodology
  Themethodologyadoptedinthisstudyisbased
bothontestingthematerialsusedforthepipelinesand
oncathodicprotectionthroughimpressedcurrent.
1.1 MaterialTesting
  Inthisstudy,theanalyzedpipelinesare
constructedfromAPI5LX60alloysandareequipped
withinternalandexternalprotection,designedto
provideeffectiveinsulationagainstcorrosive
environments.Thesecoatingsareprimarilycomposed
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JournalofHarbinInstituteofTechnology(NewSeries)
ofapolyethylenebase,coveredwithseveral
millimetresofPEpolymersorPPpolypropylene.
Acathodicprotectionsystemisimplementedto
securea24kmpipelinewithanexteriordiameterof
30inches,designedforoiltransportation.Thissystem
appliesthreedistinctlevelsofimpressedcurrentsto
ensurecontinuousprotectionagainstcorrosion.The
pipelinecomplywiththeAPI5Lstandard,established
bytheAmericanPetroleumInstitute,ensuring
adherencetostrictcriteria,particularlyregarding
corrosionresistance.
Soilcorrosivityisacriticalfactorforthe
longevityofundergroundinfrastructures.Itcanbe
assessedbyevaluatingsoilresistanceduringthedesign
phaseofmetallicstructures.Carbonsteels,usedinthe
constructionofpipelinesforcollectnetworksand
pressuremaintenancesystems,arefavouredfortheir
cost⁃effectiveness.Thesesteelsarewidelyemployedin
variousenergysectors,suchasdesalinationnetworks,
oil,andnaturalgasindustries.Theyareoften
categorizedintodifferentclasses,suchasAPI5L
GradeB,withtheirmechanicalchemicalproperties
detailedintheAPI5Lstandard
[7]

Thecathodicprotectioncoatingsappliedtothese
pipelinesactasexternalshields,creatinganelectrical
barrierbetweenthemetalanditsenvironment.This
protectionisprovidedbyatwo-layerpolyethylene
coating,designedtomaintaintheintegrityofthe
metalstructure.However,itisimportanttonotethat
thiscoatingdoesnotofferabsoluteandpermanent
protection,asimperfectionsordamagemayoccur
duringinstallationovertime.Thecathodicprotection
systemforthepipelinesinvolvesanegativeelectrical
potentialtothemetal,preventingcorrosionbymaking
itthermodynamicallyimpossiblewhenthemetal
comesintocontactwithanelectrolyte.
1.2 PerformanceMonitoring
  Cathodicprotectionforthepipelineinquestion
involvesreducingthevoltagebetweenthemetaland
thesurroundingenvironmenttobringitintoits
immunityzone.Amongthecommonlyusedmethods,
wehaveselectedcathodicprotectionbyimposed
current.Thisapproachutilizesanexternalgenerator
andanauxiliaryanode,offeringtheadvantageof
adjustingelectricalcharacteristics,particularlythe
current,tomeetspecificrequirements.Thisflexibility
enablestheoptimizationofthesystemandprovides
protectionoverabroadarea
[8-9]
.Theprotection
methodbyimposedcurrent(orcurrentwithdrawal)
employsadirectcurrentelectricalenergysource,
whichcirculatesthroughacircuitasdepictedinFig.1.
Fig.1 Currentconnectionsanddistributions
  Thecathodicprotectionsystemforthepipelineis
setupwiththepositivepoleoftheenergysource
connectedtotheanode,whilethenegativepoleis
connectedtothepipeline.Thecurrentflowsfromthe
anodethroughtheelectrolyteandthenintothe
pipeline,resultinginareductioninpotentialthat
alignswiththemetal’simmunitythreshold.Forsteel,
apotentialoflessthan-850mV/Cu/CuSO4atany
pointalongthepipeistypicallysufficienttoachieve
immunity.However,toaccommodatevarioussoil
conditions,includingbacterialpresence,atarget
potentialofapproximately-1000mV/Cu/CuSO4is
oftensought.
Inthisstudy,API5LX60steelpipelines,used
foroiltransportation,areprotectedwitha100v
cathodicprotectionsystemovera24kmlength.To
assesspotentialdistribution,twelvemeasurement
stationswereinstalledalongthepipeline.Potential
differencemeasurementsweretakenat2kmintervals
todeterminesoilresistivityandensuretheline�s
potentialalignswithstandardprotectionlevels.
Thecriterionforsteelimmunityinan
environmentwithapHbetween4and9isapotential
for850mV,measuredagainstasaturatedcopper
sulfateelectrode.Thismethodofcathodicprotectionis
analogoustotheelementarybattery,whereone
electrodecorrodeswhiletheotherremainsprotected.
Thetechniqueemployedinvolvescurrentwithdrawal
oranimposedcurrentdevice.
Forthe30-inchdiameteroilexportpipeline,
subjectedtoimposedcurrentsof0mA,15mA,and
30mA,itspotentialvaluesvarieddependingonthe
currentapplied.Measuringthepotentialofburied
pipesiscrucialforevaluatingtheeffectivenessof
cathodicprotection.Potentialmeasurements,akinto
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JournalofHarbinInstituteofTechnology(NewSeries)
thosetakenintesttubesimmersedinanelectrolyte,
usereferenceelectrodes.
Close⁃spacepotentialmeasurementswere
performedwithacurrentswitch,amemory
voltmeter,areferenceelectrode,andwiringtocover
longdistances.Continuouselectricalcontactwiththe
pipelinewasmaintainedtoensureaccurateclose⁃space
potentialreadings.Fig.1illustratestheassembly
requiredforconductingclose⁃spacepotentialreadings.
Theprocessbeginswithactivatingthecurrent
switches.Next,identifythenearestjunctionboxor
verificationpointtothemeasurementlocation.
Connectanelectrodetothenegativeterminalofthe
voltmeterandthepositiveterminal(usuallyred)to
thenegativecontactofthestructurearebeing
inspected.Thisnegativecontactcableistypically
availablewithintheverificationpointsorthejunction
box.Placethereferenceelectrodeonthegroundatthe
firstmeasurementpoint,ascloseaspossibletothe
actualmeasurementposition.Readthepotentialvalue
onthevoltmeterscreen,ensuringitsstabilityand
notingitspolarity(positiveornegative),thenrecord
thevalue.Continuetakingpotentialmeasurementsat
subsequentpoints,generallyeverytenmeters,
accordingtothepredefinedmeasurementsteps.When
reachinganewverificationpoint,reconnectthe
negativecontacttoavoidexcessivecablelengths.The
usedequipmentincludesdevicesformeasuring
voltage,alternatinganddirectingcurrentintensityand
electricalresistancewithinternalresistancetypically
aroundR>10MΩ(Megaohms),asdepictedinFig.2.
Fig.2 AC/DCHighresistancecurrentclampmeter
  Thecoppersulfateelectrode,showninFig.5,
consistsofacylindricalplasticreservoirwithaporous
capatitsbase,filledwithasaturatedcoppersulfate
solution.Acopperrodisimmersedinthissolution,
andtheelectrodeservesasareferenceforpotential
measurements,asshowninFig.3.Electrical
conductivityofasoilsolutionisanindicatorofthe
solublesaltcontentinthesoilandapproximately
reflectstheconcentrationofionizablesolutes,orthe
soil�sdegreeofsalinity.Thiselectrochemicalproperty
isbasedontheprinciplethattheconductance(the
inverseofelectricalresistance)ofasolutionincreases
astheconcentrationsofelectricallychargedcations
andanionsrise.Inthisstudy,ElectricalConductivity
(EC)ismeasuredunderstandardizedconditionsata
temperatureof25℃.Theconductanceisdetermined
usingaconductimetriccell,whichconsistsoftwo
electrodesplaced1cmapart,eachwithasurfacearea
of1cm


Fig.3 Schematicrepresentationofacopper/
sulfatereferenceelectrode
  Thesaturatedpasteextracttechnique,proposed
bytheU.S.SalinityLaboratoryin1954,isusedto
measuresoilsalinity.Thismethodinvolvesdryingand
grindingthesoilsample,sievingitto2mm,andthen
saturatingitwithdistilledwatertocreatea
characteristicpaste.Thepasteisthensubjectedto
centrifugationorvacuumaspirationtoobtaina
solution,fromwhichelectricalconductivityis
measuredatthestandardtemperatureof25℃.This
techniqueaimstoapproximate,inastandardized
manner,theconditionsofthesoilinitsnaturalstate.
Thesample,afterbeingdriedinambientair,is
graduallysaturatedwithdemineralizedwateruntilit
reachesitsliquiditylimit,asdefinedbytheAtterberg
limits.Afterallowingthesoiltorestfor24h,the
liquidisextractedformeasurementusingthesaturated
pasteextractmethod,developedbyresearchersatthe
U.S.SalinityLaboratoryandhasbeeninternationally
recognized.Thismethod,knownforitsaccuracy,
measureselectricalconductivity,abbreviatedas
CEe
[9]

Theregionunderstudy,fromapedological
·4·

JournalofHarbinInstituteofTechnology(NewSeries)
perspectiveandaccordingtotheFrenchsoil
classification,ischaracterizedbyrawmineralsoils,
poorlyevolvedsoils,halomorphicsoils,and
hydromorphicsoils.Thesesoilsarepredominantly
sandy,withanalkalinepHandhighsalinity.The
mineralfractionisalmostentirelycomposedof
minerals,whiletheorganicfractionisverylow,
typicallylessthan1%.ThepHofthesoilvaries
dependingonthedepthandlocationwherethepipeis
buried.Insandyareas,thepHvaluesmeasuredover
22kmrangefrom4to9,withanaveragevalueof6,
indicatingaweaklyacidicpH.
Pourbaixdiagrams,alsoknownasE⁃pHor
potential⁃pHdiagrams,areutilizedtopredictthemost
stablestatesofametal,includingitscorrosion
productsandassociatedionsinanaqueoussolution
undervaryingconditionsofpotentialandpH.The
PourbaixdiagramfortheFe⁃H2Osystemat25℃,
showninFig.4,illustratestheboundariesforinsoluble
corrosionproductsofdissolvedmetalsandthe
concentrationlimitsoffreemetalions.
Fig.4 Iron⁃waterE⁃pHequilibriumcurveat
25℃,TakenfromPourbaix
[15]
  Inthecontextofundergroundpipecorrosion,an
increaseincorrosionratecanoccurunder
thermodynamicallyfavorableconditions.Whensteel
reachesanelectropositivepotentialinanalkaline
electrolyte,itcanleadtotheformationofsolid
complexessuchasFe
2O
3orFe
3O
4Thesecomplexes
canformaprotectivelayeronthesurfaceoftheiron,
mitigatingfurthercorrosion
[10-16]

  Theareabetweenlines(a)and(b)inFig.4
representsthezoneofthermodynamicstabilityfor
watermolecules,wherebothpotentialandpHremain
relativelyconstant.Approachingtheboundariesofthis
regionmayleadtothedissolutionofwater.Foreach
2kmsectionofthepipeline,anodebedsareinstalled
everymeters.
  Fig.5illustratesthearrangementofanodesin
relationtotheburiedpipeline,positionedevery3
metersapart.Theeffectivenessofananodeis
influencedbyitsplacementrelativetothestructure
anditsenvironment.Optimalperformanceisachieved
whentheanodeisinstalledcentrallyalongthe
structuretoensurecomprehensivecoverage,
positionedmorethan3metersawayfromthepipeand
anyotherburiedmetalstructures,andlocatedinthe
mostsaturatedsoilavailable,withlowelectrical
resistivity,lessthan30Ω·mforzincanodesandless
than50Ω·mformagnesiumanodes.Anodebedsare
typicallyinstalledatregularintervalsalongthe
pipelinetoofferprotectionagainstcorrosion.The
spacingbetweenthesebedsdependsonvarious
factors,includingsoiltype,pipelinedimensions,and
technicalspecificationsofthecathodicprotection
system.Generally,anodebedsarespacedseveral
kilometersapart,butthisdistancemaybeadjustedto
ensureeffectivelong⁃termprotection.
Fig.5 Diagramrepresentingtheelectricalcircuit
andtheflowofcurrentthroughthepipe
  Theanodesusedconsistofacylindricalplastic
reservoirfilledwithasaturatedcoppersulfate
solution,withacopperrodimmersedinside.This
setupservesasareferenceelectrodeforpotential
measurements.Theweir(anodicbed)distributesthe
protectioncurrentfromthepositivepoleofa
transformer⁃rectifierstationintotheground.The
currentisdiffusedfromthehorizontallyinstalled
anodesatthebottomofthetrench,connectedviaa
conductivecable.Theanodebed,whichbehaveslike
ananode,issubjecttooxidationandmustbe
constructedfrommaterialsthatensurealifespanof
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JournalofHarbinInstituteofTechnology(NewSeries)
approximately15to30years.Anodebedstypically
consistofanodes,anodeconnectioncables,and
bypasskits.
ThenumberofanodesisobtainedfromEq.(1)
[10]

N=
Ma
ma
(1)
Todeterminethenumberofanodes(N)
required,giventhetotalmass(Ma)inkilogramsand
theunitmassofeachanode(ma)inkilograms,the
calculationisperformedtomeetacurrentrequirement
of21.86Amps.Theanodemassnecessarytoachieve
thiscurrentrequirementiscalculatedasinEq.(2).
Ma=Ca.T.I (2)
Thenumberofanodes(N)required,giventhe
totalanodemass(Ma)of327.9kg,andassuminga
unitmassofeachanode(ma),thecalculationis
performedtomeetacurrentrequirementof21.86Amps.
Theanodeconsumption(Ca)is0.5kg/(A·a),
withalifespanTof30years,andIrepresentsthe
current(A).
Thismethodofprotection,commonlyreferredto
asprotectionbywithdrawal,involvesconnectingthe
structuretobeprotectedtothenegativepoleofa
directcurrentsource,whilethepositivepoleis
connectedtoaconductivepart(metalorgraphite)
buriedatacertaindistance.Thecurrentleavesthis
part,calledaspillway,crossestheground,whichis
capturedbythepipe,andreturnstothegenerator
throughthemetalofthepipe.Thismakesthepipe
negativeinrelationtotheground.Ifthepotential
criterionismetinallrespects,thepipelineis
cathodicallyprotected.Connectionsaremadeby
cathodicweldingforeachsection
[11]

2 ResultsandDiscussions
2.1 MeasurementsofPotentialDifferences
AlongtheEntireLengthofthePipeline
  Themeasurementspotentialdifferencesalongthe
entire24kmlengthofthepipelinearepresentedin
Table1.Thesemeasurements,takenevery2km,
maintaintherequiredthecathodicprotectionlimitof
-850mV.Each2kmsectionofthepipelinewas
subjectedtothreedifferentimposedcurrentlevels:
0mA,15mA,and30mA,asoutlinedinTable1.
Theimpactoftheseimposedcarrentvaldesonthe
potentialdifferencesmeasuredinmillivolts(mV),is
illustrated.
TheresultspresentedinTable1indicatethatthe
averagepotentialdifferencesforeach10kmsegment
decreasedforallimposedcurrent(0mA,15mAand
30mA)fromthestarttotheendofthe24km
pipeline.Thistrendisfurthervisualizedbythecurves
inFig.4.Theobservedchangesinpotentialare
attributedtothevariationsinsoilresistivityasthe
pipelinelengthincreases.
TheconnectionbetweenTable1andthegraphs
inFig.6isshownthroughbytheaverageelectrical
potentialdifferencesacrossthepipelinesections,
plottedagainsttheimposedcurrents.Thiscomparison
highlightshowthepotentialdifferencesevolvewith
increasingpipelinelengths,startingfromanimposed
currentof0mAandrisingto30mA.Thecurvesin
Fig.6showtheelectricpotentialvaluesalignwitheach
appliedcurrent,comparedtotheconstantcurrent
requirement,representedbytheblueline.
Table1 Potentialdifferencemeasurementforeachpipeintervalindicated(therequiredlimitpotentialis-850mV)
Numberofeach
pipelineinterval
NumberofTest
station(km)
Potentialdifference(-mV)withalimitoftherequiredpotential-850mVandanimposedcurrentequalto:
  0mA   15mA   30mA
01 0to2 -400
02 2to4 -630
03 4to6 -560
04 6to8 -400
05 8to10 -560
06 10to12 -530
07 12to14 -570
08 14to16 -620
09 16to18 -550
10 18to20 -650
11 20to22 -550
12 22to24 -750
Averageof
24km
-564.16(mV)
-1170
-1105
-1110
-1100
-950
-900
-1010
-1100
-1150
-1120
-950
-1130
Averageof
24km
-1066.25(mV)
-1530
-1500
-1450
-1300
-1370
-1400
-1250
-1360
-1270
-1230
-1210
-1300
Averageof
24km
-1347.5(mV)
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JournalofHarbinInstituteofTechnology(NewSeries)
  Table1displaysthepotentialmeasurementsin
millivolts(mV)foreachsectionofthepipeline,
whichspansatotallengthof24kmanddividedinto
12sectionsof2kmeach.Thesemeasurements
correspondtodifferentimposedcurrentsinmilliamps
(mA).Thetablerevealsthatthepotentialvaluesvary
witheachimposedcurrent.Thehighestoverallaverage
potentialfortheentire24kmis-564.16mVatan
imposedcurrentof0mA,whilethelowestaverage
potentialis-1347.5mVatanimposedcurrentof30
mA.Astheimposedcurrentincreases,thepotential
valuesdecrease.Thesevariationsareattributedtothe
changingphysicalandchemicalpropertiesofthesoil,
suchasconductivity,pH,andhumidity,asthe
distancefromtheinitialsectionofthepipeline
increases.
2.2 FluctuationsinElectricalPotential
Differences
  ThegraphicalrepresentationsinFig.6,which
illustratethefluctuationsinelectricalpotentials
associatedwithcathodicprotectionasdelineatedin
Table1,arederivedfromstatisticalcomputations
basedoncorrelationcoefficientsfromexperimental
data.Thesecurvesrevealclear:asthelevelsof
imposedcurrentsincrease,thediscrepanciesin
protectiveelectricalpotentialsgraduallydecrease.This
decreasecontinuesuntilthepotentialsexceed
-1500mVatthemidpointofthe12kmpipelines.
Afterthispoint,thevaluesstarttorisemoresharply
towardstheendofthe24kmstudysections.Thisshift
isprimarilyduetothevaryingresistivityofthesoil
surroundingthepipelines.Inaccordancewiththe
constructionspecificationsforouroxygenatedcarbon
steelpipelines,therecommendedpotentialissetat
-850mVwithintheCu/Cu⁃SO
4soles.
  Analyzingtheseresultshighlightsthebenefitsof
curvesthatapproximatethestraightblueline,which
representstherequiredconstantpotentialdifferencesof
-850mV.Notably,theredandgreencurves
corresponding0and15mArespectively,showa
closeralignmentwiththisidealline,indicatinga
higherlevelofacceptability.Ontheotherhand,the
purplecurve,whichdeviatessignificantlyfromthe
desiredpotential,isconsideredlessfavorable.These
observationsemphasizetheimportanceofcarefully
controllingandadjustmentofimposedcurrentsin
cathodicprotectionsystemstoensurethatthe
protectiveelectricalpotentialsremainwithinthe
desiredrange.Visualizingthesetrendsthrough
graphicalrepresentationshelpsevaluatethe
effectivenessandefficiencyofthecathodicprotection
measuresemployed.Moreover,thesefindings
underscorethesignificanceofunderstandingand
accountingforvariationssoilresistivity,asthey
directlyaffecttheperformanceandefficacyofcathodic
protectionsystemsusedinpipelinesandsimilar
structures.
Fig.6 Curvesrepresentingthevariationin
potentialdifferencevaluesmeasuredat
each2kmintervalalongthe24km
pipeline
Table2 Presentsthegenerallyrecommendedpotentialsfordifferentmetalsinvariousenvironments
Metal
Recommendedareasofprotection
Inthesoil
(electrodeCu/Cu-SO
4)
Inseawater
(electrodeAg/Ag-Cl)
Steelandcarbon:
1)Aerobicenvironment <-0.85V <-0.80V
2)Anaerobicenvironment <-0.95V <-0.90V
Copperalloys <–0.50Vto–0.65V <–0.45Vto–0.60V
Aluminum Between–0.95Vand–1.20V Between–0.90Vand–1.15V
Lead Between–0.60Vand1.50V Between–0.55Vand–1.45V
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JournalofHarbinInstituteofTechnology(NewSeries)
  ReviewingTable2offersasummaryofthe
generallyrecommendedpotentialsforvariousmetalsin
differentenvironmentstoensureadequatecathodic
protectionandpreventcorrosion.Forsteelburiedin
soil,theoptimumpotentialtypicallyrangesfrom-850
to-1100millivolts(mV)relativetoacopper/copper
sulfate(Cu/CuSO
4)referenceelectrode
[16]
.In
freshwater,therecommendedpotentialforsteelis
around-800to-1000mVcomparedtoaCu/CuSO

referenceelectrode.Whensteelimmersedinseawater,
theacceptablepotentialrangeisusuallybetween-800
and-1050mVrelativetoaCu/CuSO
4reference
electrode.
Foraluminumburiedinsoil,theidealpotentialis
between-750and-950mVrelativetoaCu/CuSO

referenceelectrode.Infreshwater,therecommended
potentialforaluminumtypicallyrangesfrom-700to
-900mVcomparedtoaCu/CuSO
4reference
electrode.Foraluminuminseawater,thesuggested
potentialisapproximately-700to-950mVrelative
toaCu/CuSO
4referenceelectrode.
Copperburiedinsoilshouldideallymaintaina
potentialbetween-600and-800mVcomparedtoa
Cu/CuSO
4referenceelectrode.Infreshwater,the
recommendedpotentialforcoppergenerallyranges
from-550to-750mVrelativetoaCu/CuSO

referenceelectrode.Forcopperinseawater,the
advisedpotentialistypicallybetween-550to
-800mVcomparedtoaCu/CuSO
4reference
electrode.
Thesepotentialguidelinesarecrucialforensuring
effectivecathodicprotectionandcorrosionprevention.
However,adjustmentsmaybenecessarydependingon
specificenvironmentalconditionsandrequirements.
Regularmonitoringandperiodicadjustmentsareoften
neededtomaintainadequateprotectionagainst
corrosion.
3 Conclusions
  Insummary,ourstudyhighlightsthatachieving
effectivecathodicprotectionrequiresmeticulous
controlovervariousenvironmentandtechnicalfactors.
Keyelementsincludesoilcomposition,pipeline
materialquality,coatingtypeused,aswellasthe
characteristics,dimensions,andplacementofanodic
overflows.Eachofthesefactorsplaysacrucialrolein
theeffectivenessofcathodicprotectionsystems.
Optimizingthedistributionofprotectionpotential
acrossthestructureislargelydependentontheprecise
managementofearthresistancesatanodicdischarge,
especiallytheattenuationcoefficient(α).This
coefficient,influencedbyinsulationresistanceandthe
conditionofthecoating,isfundamentaltothe
effectivenessoftheprotection.Highinsulation
resistancelowerstheattenuationcoefficient,thereby
increasingtheeffectiverangeofprotection.
Conversely,ahighattenuationcoefficientreducesthe
effectiverange,necessitatinganincreaseinthe
numberofdischargestationsalongthepipeline,
particularlywheninsulationresistanceislow.This
underscoresthecriticalimportanceofcontinuous
monitoringofcoatingconditionsandprotection
potential.
Whileourstudyhasdemonstratedtheimportance
ofcontrollingtheseparameterstoensureeffective
protectionpotentialdistribution,furthervalidation
throughpracticalapplicationsisneededtoconfirm
thesefindingsandrefinetheprotectivestrategies.
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