Polarization curves for the corrosion of Fe in 1% wt MgCl2 at temperature 30 ◦Canddifferentrotationspeed.. Galvanic current as a function of speed of rotation at different temperature in
Trang 1Journal of Applied Research
www.jart.ccadet.unam.mx Journal of Applied Research and Technology xxx (2017) xxx–xxx
Original
Anees A Khadoma,∗, Baker M. Abodb
aChemical Engineering Department, College of Engineering, University of Diyala, Baquba City, Diyala, Iraq
bTechnical Engineering College, Middle Technical University, Baghdad, Iraq
Received 9 May 2016; accepted 5 October 2016
Abstract
Galvaniccorrosionofsteel–coppercoupleinsalinepetroleumwastewaterintheabsenceandpresenceofcurcumaextractascorrosioninhibitor wasstudiedasafunctionoftemperature,velocity,andinhibitorconcentration.Theelectrochemicalpolarizationtechniquewasusedtoevaluate thecorrosionparameters.Corrosioncurrentsdensitiesincreasewithtemperatureandvelocity,whileitdecreaseswithinhibitorconcentrations.In thisinvestigation,atheoreticalmodelequationwasusedtoanalyzetheshapeofpolarizationcurves.MicrosoftExcelprogramwasusedtofindthe galvaniccurrentandgalvanicpotential.Theoreticalresultsagreedwithexperimentalone
©2017UniversidadNacionalAutónomadeMéxico,CentrodeCienciasAplicadasyDesarrolloTecnológico.Thisisanopenaccessarticleunder theCCBY-NC-NDlicense(http://creativecommons.org/licenses/by-nc-nd/4.0/)
Keywords:Galvanic corrosion; Electrochemical measurements; Numerical solution; Microsoft Excel
1 Introduction
Corrosionisachemicalorelectrochemicalreactionbetween
ametalanditsenvironmentthatproducedegradationofmaterial
There are several kinds of corrosion such as uniform
corro-sion,galvaniccorrosion,crevicecorrosion,pitting,intergranular
corrosion, etc Galvanic corrosion happens when a metal or
alloyiselectricallycoupledtoanothermetalinsamethe
envi-ronment.There are many methods of corrosion control such
as material selection, coatings, inhibitors, and cathodic
pro-tection.Corrosioninhibitorisorganicorinorganiccomponent
thatcanbeaddedinsmallamountstoreducecorrosion
thenaturalmaterialswheretheaimofresearchersthat
concen-tratedonstudyingthe kineticsparameters,such as,activation
parametersandadsorptionbehaviorofthecorrosioninhibition
∗Correspondingauthor.
E-mail address:aneesdr@gmail.com (A.A Khadom).
Peer Review under the responsibility of Universidad Nacional Autónoma de
México.
process(Yaro,Khadom,&Ibraheem,2011a;Yaro,Khadom,&
math-ematicalmodelingwasrarelyused.Mathematicalmodelingisa powerfultoolforincreasingtheavailabilityofelectrochemical dataforanumberofmaterialsandenvironmentalsystemsfor industrialapplicationsthatenablechemicalandmaterials engi-neerstopredictcorrosion potentialsandcorrosionratesusing equationsderivedfromelectrochemicalprinciples(Khadom&
presentworkwasastepinthedirectionofapplicationa mathe-maticalmodelforgalvaniccorrosion–salinewater–inhibitor systematdifferentoperatingconditions
2 Mathematical model
For activation control andto determine the potential of a system, inwhichthereduced andoxidizedspecies are notat unit activity, the familiar Nernest equation can be employed
(1)
http://dx.doi.org/10.1016/j.jart.2016.10.004
1665-6423/© 2017 Universidad Nacional Autónoma de México, Centro de Ciencias Aplicadas y Desarrollo Tecnológico This is an open access article under the
CC BY-NC-ND license ( http://creativecommons.org/licenses/by-nc-nd/4.0/ ).
Trang 2(2) where E is the equilibrium half-cell potential, E o the
standard equilibrium half-cell potential, R is the gas
con-stant (8.314J/Kmol), T is the absolute temperature (K), n is
the number of electrons transferred, F the Faraday constant
(96,487coulomb/equiv.),a redanda oxidareactivitiesor
(concen-trations)ofoxidizedandreducedspecies.Hydrogenionactivity
iscommonlyexpressed,forconvenience,intermsofpH.This
isdefinedas(Uhlig&Revie,1985):
Hence,forthehalf-cellreaction
2H++2e→H2
Tafelslopes (Tafelconstants)aredetermined fromthe
fol-lowingequations(Fontana,1987):
for anode andcathode reaction, respectively, whereα is the
symmetrycoefficient,whichdescribestheshapeoftherate
con-trollingenergybarrier.The relationshipbetweenreactionrate
andovervoltageforactivationpolarizationis:
whereη Aisovervoltage,βasbefore,andiistherateofoxidation
orreductionintermsofcurrentdensity.Thisequationiscalled
the Tafel equation.The reaction rate isgivenby thereaction
currentorcurrentdensity,sothehighfieldapproximationgives
(8) and
|i c| =i o,c exp
(9) Theeffectoftemperatureistochangethevalueoftheexchange
currentdensityi oasfollows(Uhlig&Revie,1985):
R
1
T
(10) Corrosioncurrentforanodicreactionratecanbeobtainedas
(11)
(12)
andthecathodiconeis:
(13)
(14) For the case of diffusion control, the reaction current is givenbyFick’slaw(Liberati,Nogueira,Leonel,&Chateauneuf,
|I| = z c F D A
∂C
∂x
(15)
oritsequivalent
|I| = z c F D A
δ
(16) Thelimitingcurrent,i.e.themaximumcurrentunder diffu-sioncontrolisobtainedwhenC s=0,so
|I L| =z c F D A
δ
(17) or
wherethemasstransfercoefficientisdefinedas
Thecorrosioncurrentisthen
cathodicreactionistheonelikelytobecontrolledbydiffusion
C bsolubilityofoxygeninwater.Thebulkconcentrationof oxy-genchangeswithpressure,forbarometricpressuresotherthan 101.325kPa(sealevel),thebulkconcentrationofoxygencan
becomputedfromthefollowingequation(Truesdale,Downing,
C b= C 101.325 (P T−p)
whereC bisthebulkconcentrationofoxygen,C101.325isa satu-rationvalueat101.325kPa(testedexperimentally,Table1),P T
istotalpressure(kPa),pisthevaporpressureofwater.Themass transfercoefficient(K)inEq.(19)varieswiththefloworrelative speedbetweenmetalandtheenvironment,thegeometryofthe systemandthephysicalpropertiesoftheliquid.Tocalculatethe variationofKindynamicenvironment,dimensionlessgroupare
Trang 3Table 1
Solubility of oxygen in 1% MgCl2.
Temperature ( ◦C) Concentrationof
inhibitor (ppm)
Solubility of oxygen (mg/l)
Allareoftenapplied.Forthesmoothrotatingcylinder
elec-trode, the mass transport correlation is given by Eisenberg,
turbulentregime
ThevalueofKfromEqs.(22)–(25)canbeexpressedas:
D
d
This correlation is valid within the following range:
1000<Re<100000 and850<Sc<11490 Theeffectof
tem-peratureandpressureondiffusioncoefficientisshowninthe
followingequation:
P
T
n
(27) where the exponent n varies from 1.75 to 2.0, T o reference
temperature in K, D o diffusion coefficient at the reference
temperatureandpressure,P o referencepressure.Forgalvanic
corrosion under activation control (Hassan, Abdul Kader, &
I corr system=I a system=I system
And
foronemetal
(31)
(32)
(33)
−α c n c F
(34) fortwometals
orintermsofcurrentdensitiesandareas
i a,1A1+i a,2A2=i c,1A1 + i c,2A2 (36) Or
i a,1f1+i a,2f2=i c,1f1 + i c,2f2 (37)
ifI a.1I a.2,Eq.(31)reduceto
(39)
(40)
−α c F
(41)
fordiffusioncontrol(Hassanetal.,2011):
foronemetal
fortwometals
wheref1andf2aretheanodicandcathodicelectrodearea frac-tions
Eq.(47)became
Forbinarygalvanicsystemunderactivationcontrol(acidic medium)andmasstransfer(diffusion)control(neutralmedium), foronemetal:
(50)
Trang 4i c=i o,c f c exp
−α c n c F
(51)
whentwometalsat(E g):
3 Experimental work
FreshCurcuma longa(turmeric)werewashedunderrunning
water,slicedintosmallpiecesbeforedryinginahotairovenat
50◦Cforabout6h.Dryslicewascollectedandgroundintofine
powder using a high-speedblender.The dry, Curcumalonga
waspacked inaplasticbag, sealed,andkeptuntil used.The
slice(25g)wasblendedwithdistilledwater(250ml)inareflux
heaterwithconicalflax(500ml)for3hat70◦C.Theconicalflax
wassuppliedwithmixerforhomogenoussolutionanduniform
temperaturedistribution.Thesolutionisthencooled,followed
byfiltrationtoextractsolidparticlesfreeinhibitorsolution.Each
1cm3ofextractyieldapowder0.0045gofsolidmaterial
Atotalof 90testrunswascarried outinthe presenceand
absenceofinhibitoratdifferentexperimentalconditionsusing
potentiodynamicpolarizationtechnique.Testswerecarriedout
using a beaker of 500ml The cell containing working
elec-trode, luggingcapillary probe,thermometer,counter graphite
electrode.Allpotentialvaluesweremeasuredinreferencetoa
saturatedcalomelelectrode(SCE).Theluggingcapillaryprob
wasadjustedsuchthatitwasatadistancenotmorethan1mm
fromtheworkingelectrode.Theworkingelectrodewere2
sam-plesfromcarbonsteeltype(ASTMA106/A)andcoppertype
(ASTMB-111-443).Theworkingelectrodewas(2.4cmoutside
diameterx 1.35cm long)carbonsteel(typeASTMA106/A)
and copper (type ASTM B-111-443) cylinder; this cylinder
wasfixedon brasszone ontheshaft Graphiteelectrodewas
usedas acounterelectrodehasadimensionof(9.5cm
diam-eter×8cm long),two wires were connectedtoacylindrical
concentric graphiteelectrode.The chemicalcompositions(%
wt)ofworkingelectrodeswereforsteelalloy0.25%C,0.5%Mn,
0.025%P,0.025%S,0.1%Si,0.4%Cr,0.15%Mo,andthebalance
isFe.Thecopperalloycompositionsare70–73%Cu,0.007%Pb,
0.0006%Fe,0.001%Sb,0.0.009%Sn,andthebalanceisZn.Mild
steelandcopperspecimenswerecleanedusingemerypaperof
gradenumber220,320,400,and600,thenwashedwithrunning
tapwater followedbydistilledwater, thendriedwithaclean
tissue,degreasedwithbenzene,dried,degreasedwithacetone,
dried,andfinallyleftindesiccatoroversilicagel.Theelectrode
wasmounteddirectlytotheworkingelectrode.(SCE)wasused
asareferenceelectrode.ToensurethatKClsolutionwas
satu-rated,asmallamountofKCl(solid)waskeptinthesolutionof
(SCE)aslongasthetest.Thecathodicpolarizationiscarriedout beginningfromthehighestnegativepotentialof−900mVuntil reachingthecorrosionpotential.Thepotentialwaschangedin scanrate of10–15mV/min,thenthecurrent isrecorded.The anodic polarizationreadingsstartof apotentialresulting ina zerocurrentdensityandisincreasedinastepof10–15mVwith recording of the current at each step for oneminute interval untilapotentialofabout−100mV.Thegalvaniccorrosionrate
ofsteel–coppercoupleintheabsenceandpresenceofinhibitor concentrationof0,90,180,270and360ppm,arearatioof cath-odetoanode1:1,atdifferenttemperature30,35,and40◦Cwas
evaluatedinaerated1%(wt)MgCl2andpH6
4 Results and discussion
The corrosion behaviorof Feand Cu in1% MgCl2 solu-tionwithandwithoutinhibitorwasstudiedusingpolarization techniques.Theinhibitorwastestedindifferentconditions of temperature,velocityandinhibitorconcentration.Atotalof90 test runs is carried out.Electrochemical parametersfor each metal individually were calculated using polarization curves similartoFigures1and2that obtainedforFeandCuin1% MgCl2solution.Whilegalvanicparameterswerecalculatedby superimposingtheanodicbranchofthelessnoblematerial(mild
–800 –700 –600 –500 –400 –300
1 0
–1 –2
–3 –4
log(I) (mA /cm 2 )
200 rpm
50 rpm
0 rp m
Fig 1 Polarization curves for the corrosion of Fe in 1% wt MgCl2 at temperature
30 ◦Canddifferentrotationspeed.
–800 –700 –600 –500 –400 –300 –200 –100 0
0 –1
–2 –3
–4
log(I) (mA /cm 2 )
200 rpm
50 rpm
0 rp m
Fig 2 Polarization curves for the corrosion of Cu in 1% wt MgCl2 at temper-ature 30 ◦Canddifferentrotationspeed.
Trang 5–800
–700
–600
–500
–400
–300
–200
–100
0
1 0
–1 –2
–3 –4
log(I) (mA /cm2)
Cu Fe
Fig 3 Polarization curves for the corrosion of Fe–Cu coupling at speed of
rotation 50 rpm, temperature 40 ◦Cand180ppminhibitorconcentration.
220 200 180 160 140 120 100 80 60 40 20
0
–20
Rotational speed (rpm)
40
60
80
100
120
140
160
-6 A/cm
2 )
30ºC
35
40
Fig 4 Galvanic current as a function of speed of rotation at different temperature
in absence of inhibitor.
steel)tothecathodicbranchofthemostnoblematerial(copper
alloy)asshowninFigure3.Similarfiguresareobtainedatall
operating conditions From polarization curves, the free
cor-rosioncurrent I corr,corrosion potentialE corr,limiting current
density,i L,andgalvaniccorrosion,I gwereobtained.Aplateau
correspondingtoalimitingcurrentdensity(i l)isobservedinthe
cathodicregionofthe polarizationcurvesbetween−800 and
−600mV SCE Therefore,the cathodicreaction seemstobe
controlledbydiffusion.ItwasshownthatthevaluesofI corrand
I gincreasedwithtemperature andvelocity,whileitdecreased
withinhibitorconcentration.Maximuminhibitorefficiencywas
around 90% at 360ppm and 30◦C. The average values of
inhibitor efficiency were approximately 55, 72,76, and83%
atinhibitorconcentrationof90,180,270,and360ppm
respec-tively.Theincreasingincorrosionprotectionwithadditionof
inhibitormaybeattributedtoincreaseinmetalsurfacecoverage
Increasinginhibitor concentrationbeyond380ppmmayyield
furtherreduction in galvaniccorrosion current,but
economi-calconsiderationshavetobetakenintoaccount.Figures4and5
showthevariationofgalvaniccorrosionasafunctionofdifferent
operatingconditions
28 30 32 34 36 38 40 42
Temperature (ºC)
0 10 20 30 40 50 60 70 80 90
-6 A/cm
2 ) C = 0 ppm
90 180 270 360
Fig 5 Galvanic current as a function of temperature at different inhibitor con-centration.
ordinarygalvaniccorrosionmodelandfluiddynamicsanalysis model.Thecalculationisdividedintotwotypes.Activation con-trolandmasstransfercontrol.Simplificationsleadingtoanalytic solutionsofthemodelequationsaresocomplex,sonumerical solutionsmustbeattempted.Asanexample,anumericalmethod
algorithmareasfollows:
1 Estimateequilibriumpotentialsformetalsandforhydrogen fromEq.(1)atTof30,35and40◦C.ForpHvaluesuseEq.
(3)tocalculatehydrogenionconcentrations
2 Tafel slopes for anodicandcathodic reactionsare estab-lishedfromEqs.(4)and(5)atα=0.5andTof30,35and
40◦C.
3 Theexchangecurrent densityiscalculatedfromEq (10) forthreevaluesoftemperatures(30,35and40◦C).
4 Bulkconcentrationofoxygeninwateriscalculatedfrom
Eq.(21)atdifferenttemperatures30,35and40◦C,byusing,
5 ThevalueofoxygendiffusivityisestimatedfromEq.(27)at differenttemperatures30,35and40◦C.Themasstransfer
coefficientKiscalculatedbyusingEq.(26)
6 ThelimitingcurrentisestimatedfromEq.(20)atdifferent temperatures30,35and40◦C.
7 Itisnecessarytorealizethatthegalvaniccorrosion poten-tials(E g)ofthereactionsinvolvedarechosenbetweenthe morenegative(orlesspositive)equilibriumpotentialofthe metalsandtheequilibriumpotentialofhydrogenevolution
8 ThevaluesofE eq,β,i o,E g (=E a=E c)aresubstitutedinEqs (11)and(13)todetermineanodicandcathodiccurrents
9 Foractivationcontrol:
10 The summationsof the anodicandcathodic currents are comparedtodeterminetheabsolute valueof their differ-ence
Trang 6Table 2
Galvanic corrosion of Cu/Fe couple versus temperature under the following conditions: [Fe 2+ ] = [Zn 2+ ] = 10 −6M,fFe=0.5,fZn=0.5,alphaofH2=0.5,alphaof
Cu = Fe = 0.5.
T( ◦C) rpm E g(mV) ICu(A/cm 2 ) IFe ( A/cm 2 ) IH2/Cu (A/cm2 ) IH2/Fe (A/cm2 ) I Limiting( A/cm 2 )
11 A newvalue of E g isassumed as instep 8 andthe
pro-gramisexecutedagain,showingthedifferencebetweenthe
summationoftheanodicandcathodiccurrentstodecrease
12 Step11isrepeateduntilaminimumdifferencecurrent is
found.Theminimumwillbedetectedwhenthesweeping
proceduregoesbeyondthetruegalvanicpotentialvalueas
thedifferencestartsincreasing.Theprecisionwillbegreater
thesmallerthepotentialstepwhiletheprocessingtimewill
increaseaccordingly
13 Formasstransfercontrol:
currentsandlimitingcurrents,Eq.(42),arecalculatedand
compared todeterminetheabsolutevalue oftheir
differ-ence
15 A newvalue of E g isassumed as instep 8 andthe
pro-gramisexecutedagain,showingthedifferencebetweenthe
summationoftheanodicandlimitingcurrentstodecrease
16 Step11is repeateduntil aminimumdifference isfound
Theminimumwillbedetectedwhenthesweeping
proce-duregoes beyondthe truegalvanicpotential valueasthe
difference startsincreasing.Theprecisionwill begreater
thesmallerthepotentialstepwhiletheprocessingtimewill
increaseaccordingly
17 Forcathodereactionunderactivationcontrolcomplicitwith
masstransfer:
18 The differencebetweenthesummationof theanodicand
cathodiccurrentsandlimitingcurrentsEq.(49),are
calcu-latedandcomparedtodeterminetheabsolutevalueoftheir
difference
19 A newvalue of E g isassumed as instep 8 andthe
pro-gramisexecutedagain,showingthedifferencebetweenthe
summationoftheanodicandcathodiccurrentsandlimiting
currentstodecrease
20 Step11is repeateduntil aminimumdifference isfound
Theminimumwillbedetectedwhenthesweeping
proce-duregoes beyondthe truegalvanicpotential valueasthe
difference startsincreasing.Theprecisionwill begreater
thesmallerthepotentialstepwhiletheprocessingtimewill
increaseaccordingly
AprogramwritteninMicrosoft Excel 2010forfreecorrosion
rateofsinglemetalandbinarygalvanicsystemunderactivation
control (acidicmedium) andmass transfer(diffusion)control
0 20 40 60 80 100 120 140 160 180
250 200
150 100
50 0
2 )
rpm
Theoretical Polarization
Fig 6 Comparison of the galvanic current density obtains from polarization curve and theoretical calculation at temperature 30 ◦Cdifferentrpm.
0 20 40 60 80 100 120 140 160 180
250 200 150 100 50
0
2 )
rpm
Theoretical Polarization
Fig 7 Comparison of the galvanic current density obtains from polarization curve and theoretical calculation at temperature 35 ◦Cdifferentrpm.
(neutral medium)andalsotocalculatethegalvanic corrosion rate whenthesystemisunderbothactivationandmass trans-fercontrol.Table2thatshowstheeffectoftemperatureonthe corrosion currentand corrosion potential of copperandiron, limiting current was calculatedfrom Eq (44),corrosion cur-rent ofmetalswerecalculatedfromEq.(53).Figures6and7 showacomparisonofthegalvaniccurrentdensityobtainfrom polarizationcurveandtheoreticalcalculation.Figures8and9 showacomparisonofthegalvaniccurrentdensityobtainfrom polarizationcurve,weightlossandtheoreticalcalculation
Trang 760
40
20
0
30
Temperature ºC Theoretical Polaeization curve
2)
Fig 8 Comparison of the galvanic current density obtains from polarization
curve, weight loss and theoretical calculation at different temperature.
180
160
140
120
100
80
60
40
20
0
Temperature ºC
2 )
Theoretical at 200 rpm
Polarization at 200 rpm
Theoretical at 50 rpm Polarization at 50 rpm
Fig 9 Comparison of the galvanic current density obtains from polarization
curve and theoretical calculation at different temperature and rpm.
5 Conclusion
Galvaniccurrentandthecorrosioncurrentdensitiesincrease
with increasing temperature and velocity, and decrease with
increasing inhibitor concentrations The addition of inhibitor
reducedthegalvaniccorrosioncurrent.Amathematicalmodel
equationwasveryeffectivetooltoanalyzetheshapeof
polariza-tioncurves.Theoreticalresultsagreedwithexperimentalone
Conflict of interest
Theauthorshavenoconflictsofinteresttodeclare
Acknowledgment
TheauthorswouldlikethanksProf.Dr.AprealS.Yaro (Bagh-dad University – Chemical Engineering Department) for his continuousencouragementandassistance
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