Preliminary Investigation of the Corrosion Behavior of Proprietary Micro-alloyed Steels in Aerated

12 Pages·2017·3.66 MB·English

by  Lawrence Onyeji

JMEPEG (2017)26:5741–5752 (cid:2)The Author(s). Thisarticle isanopenaccesspublication
https://doi.org/10.1007/s11665-017-3031-x 1059-9495/$19.00
Preliminary Investigation of the Corrosion Behavior
of Proprietary Micro-alloyed Steels in Aerated
and Deaerated Brine Solutions
LawrenceOnyeji and GirishKale
(Submitted June28,2017;in revised formSeptember 7,2017;published online October27, 2017)
The corrosion performance of fairly new generation of micro-alloyed steels was compared in different
concentrations of aerated and deaerated brines. Electrochemical polarization, weight loss and surface
analysestechniqueswereemployed.Theresultsshowedathresholdofcorrosionrateat3.5wt.%NaClin
both aerated and deaerated solutions. The average corrosion current density for steel B, for example,
increased from 1.3lAcm22 in 1 wt.% NaCl to 1.5lAcm22 in 3.5wt.% NaCl, but decreased to
1.4lAcm22in10wt.%deaeratedNaClsolutions.Theaeratedsolutionsexhibitedanaverageofover80%
increase in corrosion current density in the respective concentrations when compared with the deaerated
solution. These results can be attributed to the effects of dissolved oxygen (DO) which has a maximum
solubility in 3.5wt.% NaCl. DO as a depolarizer and electron acceptor in cathodic reactions accelerates
anodic metal dissolution. The difference in carbon content and microstructures occasioned by thermo-
mechanical treatment contributed to the witnessed variation in corrosion performance of the steels.
Specifically, the results of the various corrosion techniques corroborated each other and showed that the
corrosionrateof themicro-alloyedsteels canberankedas CR <CR <CR <CR .
Steel A X65 Steel B Steel C
development of new materials with high corrosion resistance
Keywords aeratedsolutions,brine,corrosionbehavior,deaerated
atlowcostasthemosteffectiveoptiontotacklethemenaceof
solution, electrochemical techniques, micro-alloy
steels corrosion in oil and gas industry.
Dissolved oxygen (DO) is one of the major corrosion
enhancing components in crude oil. Liu et al. (Ref 1), Ismail
and Adan (Ref 20) and Jung et al. (Ref 21) reported that the
corrosioneffectofdissolvedoxygenismoreprevalentthanall
1. Introduction other impurities present in crude oil. Liu et al. (Ref 1) studied
theeffectsofchlorides,temperature,pH,carbondioxide(CO )
2
and dissolved oxygen (DO) on the corrosion-induced leakage
Corrosion attack of oil and gas facilities is one of the
of the inner wall of crude oil pipeline. These studies showed
greatest challenges to the operators in the industry. Therefore,
that the effect of DO on the corrosion rate of the pipeline was
assessingtheimpactofcorrosionisofimmediateconcerns.The
dominantwhencomparedwithCO corrosion.IsmailandAdan
effects of corrosion if not controlled can affect production, 2
(Ref20)investigatedthecorrosionbehaviorofAISI1040steel
cause economic loss and create ecological problems with the
in NaCl with different sulfate and chloride contents in the
attendant security risks to life and properties (Ref 1, 2). Crude
presence and absence of oxygen. The results revealed that the
oil and its products are mixtures of hydrocarbon which by
corrosion rate was higher in solutions containing oxygen than
themselvesarenotcorrosive.However,theybecomecorrosive
inallotherdeoxygenatedsolutions.Jungetal.(Ref21)afteran
in the presence of impurities and other components such as
extensive laboratory experiments observed that the corrosion
water, chloride salts, carbon dioxide (CO ), hydrogen sulfide
2 rates for galvanized steel pipe (GSP), carbon steel pipe (CSP)
(H S), elemental sulfur, dissolved oxygen, organic acids and
2 andductilecast-ironpipe(DCIP)decreasedto72,75and91%,
sulfate-reducing bacteria (SRB) (Ref 1, 3-5). Tremendous
respectively,whenDOconcentrationwasreducedfrom9±0.5
efforts have been made to mitigate and control the effects of
to2±0.5 mg/L.Theseresultsareinagreementwiththereports
corrosion.Someoftheseeffortshaveledtothedevelopmentof
of Islam and Farhat (Ref 22), Chilingar et al. (Ref 23) and
various corrosion control techniques such as the use of
Popoola et al. (Ref 24) that DO is a depolarizer and electron
composite materials (Ref 6, 7); various corrosion resistance
acceptor in cathodic reaction and therefore accelerates the
steels (CRS) (Ref 8, 9), inhibitors (Ref 10), electrochemical
anodic dissolution of metals. Also confirming this statement,
protection (Ref 11, 12) and coating techniques (Ref 13-17).
Yari(Ref2)reportedthatinabasicorneutralsolution,oxygen
These techniques are either too expensive for effective
reductionreactioninEq 1isthepredominantcathodicreaction
deployment or too difficult to sustain and therefore have not
in competition with hydrogen evolution reaction of Eq 2.
adequately solved the problem of corrosion in the oil and gas
industries. Choi et al. (Ref 18, 19) have proposed the Neutral=basic solutions: O þ2H O þ4e(cid:2) !4OH(cid:2)
2ðgÞ 2 ðlÞ ðaqÞ
ðEq 1Þ
LawrenceOnyejiandGirishKale,SchoolofChemicalandProcess
Engineering, University of Leeds, Leeds LS2 9JT, UK. Contact
e-mails:[email protected]@leeds.ac.uk.
Journal of Materials Engineeringand Performance Volume26(12)December 2017—5741
extract Tafel corrosion parameters such as corrosion current
In hydrogen revolution:2HþðaqÞþ2e(cid:2) !H2ðgÞ ðEq 2Þ density(iCorr),corrosionpotentials(ECorr)andanodic(bA)and
cathodic (b ) constants. The corrosion rate of the steels was
The effect of DO on the corrosion of oil and gas facilities is C
calculated by inserting the values of corrosion current density
significantlyinfluencedbythepresenceofothercomponentsof
(i )obtainedfromTafelextrapolationtechniquesinEq 3
crudeoilstreamssuchaschloridesalts.Accordingtoresearch- Corr
ers (Ref 2, 25-28), the concentration of chloride can influence 3:27(cid:3)10(cid:2)3(cid:3)i (cid:3)E:W:
CR¼ Corr ; ðEq 3Þ
thesolubilityofoxygeninproducedwater.Yari(Ref2)specifi- q
cally reported that the maximum oxygen concentration in sea-
water is obtained in 3.5 wt.% NaCl solution. This work where CR = corrosion rate (mm year(cid:2)1), iCorr = corrosion
thereforeisaimedatinvestigatingtheelectrochemicalbehavior current density (lA cm(cid:2)2), E.W. = equivalent weight (g) and
of three fairly new class of micro-alloyed steels, whose corro- q = density of the sample (g cm(cid:2)3).
sion behavior has not been fully investigated, in differentcon- Attheendofthepolarization,thespecimenswerecarefully
centrationsofaeratedanddeaeratedbrinesolutions. removed from the cell, rinsed with deionized water and dried
with nitrogen gas. Carl Zeiss EVO MA15 Scanning Electron
Microscopy/Energy-Dispersed X-ray (SEM/EDAX) was used
to characterize the corroded surfaces so as to identify their
2. Experimentals
morphology and elemental composition.
2.1 Materials and Material Preparations 2.3 Weight Loss
Three micro-alloyed steels designated as Steel A, Steel B The corrosion rates of the steels were also assessed by
andSteelCwithchemicalcompositionslistedinTable 1were weightlosstechnique.ThesteelswerewetgrindeduptoP1200
usedinthiswork.Thesesteelswhicharemeantforproduction grit with silicon carbide paper, washed with distilled water,
ofpipesarefairlynewinthefamilyofmicro-alloyedsteels,and ultrasonically cleaned in acetone for 4 min and dried in warm
their corrosion propensity has not been satisfactorily under- air. This was followed by careful measurements of the surface
stood. A fourth material, API 5L X65 whose corrosion dimensionswithwhichthesurfaceareaofthespecimenswere
performance in different media has been investigated and computed. The specimens were weighed using Adam PW124
adjudged as the most widely used pipeline material, was used analyticalprecisionweighingbalance,andtheweightrecorded
as reference specimen (Ref 29-31). The specimens were wet as W1. Each of the three specimens were prepared. The
grinded using MetaServ 250 Grinder-Polisher. Silicon carbide specimenswerethenhunginsideabeakercontaining3.5 wt.%
papers of different fineness up to P1200 grit were used. Each NaCl at ambient temperature. One of the specimens was
steel specimen after grinding was rinsed with distilled water, removed from the beaker after 24, 48 and 72 h, respectively.
degreasedwithacetone,driedwithcompressedairandkeptina The corrosion products on the specimens were chemically
desiccator briefly until corrosion setup was ready. The speci- cleaned in specific solution (500 mL of HCl + 500 mL of
mens for OM and SEM/EDX analyses were further polished deionized water + 5 g of hexamethylenetetramine) with vigor-
using 5, 3, and 1 lm abrasive diamond slurry, degreased with ous stirring for 10 min, rinsed with deionized water, dried in
acetone, rinsed with distilled water and dried with compressed warm air and reweighed (W ). The weight loss of the samples
2
air. The final polished surfaces were etched with 2% nital and duetoimmersionfortherespectivetimewasthencalculatedas
analyzed using Zeiss AxioCam MRc 5 Optical Microscopy (W (cid:2)W ), and the corrosion rate computed using Eq 4,
1 2
(OM) and Carl Zeiss EVO MA15 Scanning Electron Micro-
8:76(cid:3)104(cid:3)ðW (cid:2)W Þ
scopy/Energy-Dispersed X-ray (SEM/EDAX). CR¼ 1 2 ; ðEq 4Þ
qAt
2.2 Experimental Setups and Conditions
where CR = corrosion rate (mm/year), q = density (g/cm3),
The experimental setup and conditions are summarized in A= surface are of the sample (cm2) and t = immersion time
Table 2. To ensure reproducibility, each of the four steels was (hours).
polarized in triplicate using the parameters in Table 2. The
anodic and cathodic branches of Tafel plots were conducted
separately starting with the cathodic scan, followed by 15-min 3. Results and Discussion
open-circuitpotentialbeforetheanodicsweep.Thiswastoavoid
surfacealterationandelectrolytecontaminationasaresultofiron
3.1 Microstructural Characteristics of the Samples
dissolutionduringanodicsweep(Ref32,33).Theguidelinesin
ASTMG102-89(Ref34)andTait(Ref35)wereusedtomerge The optical and SEM micrographs of the as-received steels
the branches. Tafel extrapolation technique was employed to areshowninFig. 1and2,respectively.Thesefiguresshowthat
Table 1 Elemental specifications of the steels (%)
Steels C Si Mn P S Cr V Ti Nb Mo Cu Fe
A 0.12 0.18 1.27 0.008 0.002 0.11 0.057 0.001 0.054 0.17 0.12 Balance
B 0.22 0.032 1.4 0.012 0.001 0.25 0.005 0.023 0.002 0.07 0.03 Balance
B 0.25 0.26 0.54 0.01 0.001 0.99 0.005 0.008 0.022 0.46 0.098 Balance
X65 0.12 0.18 1.27 0.008 0.002 0.11 0.057 0.001 0.054 0.17 0.12 Balance
5742—Volume26(12) December 2017 Journal of MaterialsEngineering andPerformance
Table 2 Experimental parameters and corrosion conditions
Descriptionofitems Parameters
Electrode(working)materials A,B,CandX65
Counterelectrode Platinummesh
Referenceelectrode Silver/silverchloride(Ag/AgCl)-(Sat.KCl)
Workingelectrodesurfacearea,cm2 1
Electrolytes,wt.% NaCl:1.0,3.5,and10.0
Corrosioncell EG&GprincetonappliedresearchmodelK0235flatcell
Electrolytevolumepertest,mL 250
Electrolyteenvironmentalconditions Aeratedanddeaerated
Testpressure,bar 1
Temperature,(cid:3)C Ambient
SolutionpH Unbuffered
TestperiodforLPR,h 6
Potentiostat SolartronanalyticalSI1287electrochemicalinterface
Analyticalsoftware CorewareandCoreView
Electrochemicalmeasurements Potentiodynamicpolarization(LPR,TafelPlot)
LPRsweeprangeversusOCP,mV ±15
LPRsweeprate,mV/S 0.25
Tafelplotscanrange,mV ±250
Tafelplotscanrate,mV/S 0.5
Fig.1 Opticalmicrographsofas-receivedsteels(a)SteelA,(b)SteelBand(c)SteelCand(d)X65
Journal of Materials Engineering andPerformance Volume26(12)December 2017—5743
Fig.2 SEMmicrographsofas-receivedsamples(a)SteelA,(b)SteelBand(c)SteelCand(d)X65
Steels A, B and X65 consist of light and dark zones which in were obtained from carbon steel with the same chemical
the optical micrographs are ferrite and pearlite phases, respec- compositionandviceversadependingonheattreatmentroute.
tively.InSEMmicrographs,thesezonesarecoloniesofpearlite These authors have reported that the precipitation, volumetric
withintheferritematrix.Thelamellaestructuresofthepearlite fraction,uniformityanddistributionofpearliteincarbonsteels
phaseareclearlyvisibleinFig. 2(a).Anotherdistinctivefeature can be attributed to carbon content and thermo-mechanical
of the micrographs of these three specimens is their grain processes (heat treatment routes). Ochoa et al. (Ref 40)
(phase) sizes which within each sample are the same and obtained different microstructures of API 5L X42 using three
uniformly distributed. However, the grain sizes of the steels differentheattreatmentrouteswitheachofthemicrostructures
differ from each other. exhibiting variant corrosion behavior in CO - saturated 0.5 M
2
Thedifference in thegrain sizeof thesespecimens is made NaClsolution.Asiful(Ref43)throughrepeatedquenchingalso
more evident by Fiji-ImageJ analysis and ASTM grain size impacted different microstructures on E34 micro-alloyed steel
number computed according to ASTM E112-12 standard (Ref which showed different corrosion rates in 3.5 wt.% NaCl
36).TheseareshowninTable 3.Thesmallgrainsizenumbers solution. Thus, although Steel A and X65 have the same
of the steels can be attributed to the presence of alloying chemical composition (Table 1), their different microstructural
elements such as Nb, Ti and V which researchers (Ref 29, 37, behaviors (Fig. 2) could be linked to heat treatment processes
38) have observed are good grain refiners. The ferrite–pearlite used.
ratio of Steels A, B and X65 can be ranked as Steel The optical and SEM micrographs of Steel C shown in
B<X65<Steel A as indicated in Table 3. This difference Fig. 1(c)and2(c),respectively,revealedbainiticstructurewith
in microstructures and grain size is linked to the chemical evenly distributed acicular ferrites (Ref 29). Literatures
composition of the steels (Ref 39, 40) and the thermo- revealed that bainitic structures are formed when the decom-
mechanical processes involved in their production (Ref 10, positionofaustenitetoferriteandpearlitesisrestrainedbythe
40, 41). presence of micro-alloying elements (Ref 29, 37, 41, 44, 45).
Steel A and X65 as indicated in Table 1 have the same Kermani and Morshed (Ref 46) and Kermani et al. (Ref 38)
chemical composition, butFig. 1and2 showedthatthey have identified Cr and Mo as alloying elements that retard decom-
different microstructures. Similar cases have been reported in positionofmartensiteoraustenitetoferritesandcarbides.Steel
the literatures (Ref 40, 42, 43) where different microstructures C as shown in Table 1 contains more Cr (0.99 wt.%) and Mo
5744—Volume26(12) December 2017 Journal of MaterialsEngineering andPerformance
Table 3 ASTM grain size number and ferrite/pearlite ratio (%) for the samples
Samples A X65 B C
ASTMgrainsizeno 3.9 4.5 5.6 5.8
Ferrite/pearlite,% 63.6 57.5 48.2
Averagegrainsize,lm 68.5 46.6 20.6 14.7
(0.46 wt.%)thantheothersteels.Thiscouldhavenecessitated of the specimens hindered increase in corrosion rate (Ref 35,
its bainitic structure. 52). A closer observation of Fig. 3(d-f) shows that the
corrosionrateofthesteelsinthedeaeratedsolutionshasbegun
3.2 Electrochemical Tests to stabilize within the 6-h test. Vera et al. (Ref 29) and El
DesoukyandAboeldahab(Ref53)alsoattributedthedecrease
Linearpolarizationresistance(LPR)andTafelextrapolation
in corrosion rate to the formation of corrosion products on the
techniques were used to assess the corrosion behavior of the
surface of the specimens.
steelsinbothaeratedanddeaeratedaqueoussolutionsofNaCl
The plots of the average corrosion rate for the respective
at ambient temperature. LPRwasconductedfor6 h,whilethe
steels in 1.0, 3.5 and 10.0 wt.% NaCl solutions at ambient
potentiodynamic polarization was performed after 6 h LPR.
temperature and for 6 h are compared in Fig. 4(a) aerated and
This is because LPR technique generally uses only minor
4(b)deaerated.Thesefiguresshowthatthecorrosionattackfor
perturbation around OCP ((cid:4)15mVinthiscaseÞ and so does
all the four specimens was more severe in 3.5 wt.% NaCl
notsignificantlyaffectthesurfaceofspecimen.Thatiswhyitis
solution for both aerated and deaerated solutions than in the
used for assessing long time corrosion propensity of metals
other two concentrations (1.0 and 10.0 wt.%). This is in
(Ref 47). Also, the alternating potential measurement helps to
agreementwiththereportsofYari(Ref2)andFangetal.(Ref
stabilize the OCP after each LPR scan. The solutions were
25). Dissolved oxygen (DO) as an electron acceptor (Ref 21-
stirred throughout the period of the tests. This was to ensure
24)inredoxreactionsandadepolarizer(Ref23)acceleratesthe
thatthesurfaceconcentrationofthespecieswasapproximately
dissolution of metals in chloride environment. The witnessed
equaltothebulkconcentrationandthusminimizestheeffectof
decreaseincorrosionratewithincreaseinNaClconcentration,
diffusion polarization (Ref 47-50). Three different concentra-
accordingtoEliyanetal.(Ref27),canalsobeattributedtothe
tions of NaCl namely 1.0, 3.5 and 10.0 wt.% were used. For
decrease in the solubility of oxygen with increase in NaCl
LPR measurements, the specimens were polarized within the
concentration. Fang et al. (Ref 25) also reported that the
range of±15 mV versus OCP and at a scan rate of
adsorptionofchlorideonthesurfaceofthesteelscontributedto
0.25 mV s(cid:2)1 for 6 h. For the potentiodynamic polarization,
the reduction in corrosion rate with increase in salt concentra-
the Tafel cathodic and anodic branches were scanned sepa-
tion.
rately. However, the effective scan range for the two sweeps
The reports of these authors elucidate the decrease in
was±250 mVversusOCPandatascanrateof0.5 mV/s.The
corrosion rate in 10 wt.% NaCl solution as shown in Fig. 4.
ohmicdropoftheexperimentalsetupwascalculatedtoensure
However,theeffectofoxygencontentonthecorrosionofsteel
itistoominuteandsowasneglected.Thiswasachievedusing
inaqueoussolutionofNaClhasnotbeenclearlyunderstoodas
Eq 5 as reported by Oelssner et al. (Ref 49)
some authors presented divergence results. For instance, El
Z Desouky and Aboeldahab (Ref 53) studied the effect of
R ¼ ; ðEq 5Þ
U 2KA different concentration of NaCl (0.1–2 M) on maraging steel
and reported a continuous increase in corrosion rate with
where Z = the gap between the working electrode and the ref-
increaseinNaClconcentration.InagreementwithElDesouky
erence electrode (mm), K = the electrical conductivity of
and Aboeldahab (Ref 53), Liu et al. (Ref 1) categorized the
3.5 wt.% NaCl solution (mS/cm) (Ref 51) and A= the area
effect of chloride ion on corrosion of carbon steels as smooth
of the working electrode (mm2).
invariant stage, slow rise stage and the rapid rise stage.
The estimated ohmic drop was about 0.1 lX. This is very
To further buttress the effect of NaCl concentration on the
negligible and hence was neglected in all the tests.
corrosion behavior of the steels, the corrosion rates in both
3.2.1 Linear Polarization Resistance Techniques. Fig-
media were plotted against NaCl concentrations as shown in
ure 3 shows the plots of average LPR corrosion rate against
Fig. 5.Thisfigureclearlyshowedthattheseverityofcorrosion
time for the four specimens in three different concentrations
attackwashighestforallthesteelsin3.5 wt.%Naclsolutionof
(1.0, 3.5 and 10.0 wt.%) of aerated and deaerated NaCl
both conditions. A sharp increase in corrosion rate can be
solutionsatambienttemperatureandfor6 h.Itcanbeobserved
observedastheconcentrationincreasedfrom1.0 wt.%NaClto
fromFig. 3(a-c)thattheplotsofcorrosionrateversustimefor
3.5 wt.% NaCl, while a mild decrease was observed as NaCl
allthefourspecimensinaeratedmediaincreasedwithtime,but
concentration increased from 3.5 wt.% NaCl to 10.0 wt.%
forsteelsinFig. 3(c).Thisincreaseincorrosionrateaccording
Nacl. This is in corroboration with the profiles of the results
to Davis (Ref 52) and Taits (Ref 35) can be ascribed to the
shown in Fig. 4.
porosity of the air-formed oxide film on the surface of the
AccordingtoHanet al.(Ref 54),NaCl concentrationsplay
specimens which could not protect the steels from corrosion.
three major roles in corrosion kinetics of carbon steels which
Ontheotherhand,Fig. 3(d-f)showeddecreasingcorrosionrate
includeacceleratingthecorrosionprocessbyforminginterme-
withtimeforallthesteelsinthedeaeratedmedia.Thissignified
diate corrosion species, decreasing the solution pH which also
that thedissolutionoftheair-formed oxidefilm onthesurface
acceleratesthecorrosionprocessandreducingthesolubilityof
Journal of Materials Engineering andPerformance Volume26(12)December 2017—5745
Fig.3 Average LPRcorrosion ratesof the steels in aeratedand deaerated brine solutions atambient temperature andfor 6h: (a)1.0wt.% aer-
ated,(b)3.5wt.%aerated,(c)10.0wt.%aerated,(d)1wt.%deaerated,(e)3.5wt.%deaerated,(f)10wt.%deaerated
Fig.4 Average LPR corrosion rates of steels in different concentration NaCl solutions: (a) deaerated and (b) aerated at ambient temperature
andfor6h
the dissolved oxygen with the attendant decrease in corrosion
rate. The net effect of increase in NaCl concentration is the Anodic reaction: FeðsÞ !Feð2aþqÞþ2e(cid:2) ðEq 6Þ
reduction in corrosion rate. This means that the effect of
reductioninDOsolubility(saltingout)dominatestheothertwo Cathodic reaction:O2ðgÞþ2H2OðlÞþ4e(cid:2) !4OH(cid:2)ðaqÞ ðEq 7Þ
competingeffects.ThiswasalsoreportedbyYari(Ref2)thatin
a basic or neutral solution, oxygen reduction reaction is the Total reaction: 2Fe þO þ2H O !2Fe2þ þ4OH(cid:2)
ðsÞ 2ðgÞ 2 ðlÞ ðaqÞ ðaqÞ
predominant cathodic reaction in competition with the hydro-
ðEq 8Þ
genevolutionreaction.Caceresetal.(Ref55)alsoreportedthat
themajorreactiongoverningcorrosioninmostpracticaloilfield
Fe2þ þ2OH(cid:2) !FeðOHÞ ðEq 9Þ
application is the reduction reaction of oxygen present in ðaqÞ ðaqÞ 2ðsÞ
solution. Vera et al. (Ref 29) summarized the electrochemical
4FeðOHÞ þO þ2H O !4FeðOHÞ ðEq 10Þ
and chemical reactions that occur during metal dissolution in 3ðsÞ 2ðgÞ 2 ðlÞ 3ðsÞ
aqueous solutions as shown in Eq 6-11.
5746—Volume26(12) December 2017 Journal of MaterialsEngineering andPerformance
Fig.5 AverageLPRcorrosion ratesvs.aeratedanddeaeratedNaCl Fig.7 Corrosion rate by weight loss of specimens in 3.5wt.%
concentrationatambienttemperatureandfor6h NaClatambienttemperature
siveness of sodium chloride solution with aerated condition
being the most corrosive. This is attributed to the effect of
sodium chloride concentration on the solubility of oxygen in
aqueous solution, which is reported to be highest at 3.5 wt.%
NaCl (Ref 2, 25).
3.2.2 Weight Loss. The results of the weight loss exper-
imentsareshowninFig. 7.Thisfigurerevealedthattherewasa
decrease in corrosion rate with immersion time. Like in the
LPR experiments, this is due to the accumulation of corrosion
products that partially block the surface of the steels with
increase in immersion time (Ref 40, 57). The steels also
exhibited the same trend of behavior as observed in the LPR
experiments attesting to the fact that the corrosion rate of the
steels with ferrite–pearlite microstructures is lower than the
Steel C with bainitic structure (Ref 40, 58).
3.2.3 Potentiodynamic Polarization (Tafel Plots). Po-
Fig.6 Comparison of corrosion rates of steels in aerated and
tentiodynamic polarization measurements were conducted in
deaeratedNaClsolutionsatambienttemperatureandfor6h
bothaeratedanddeaerated1.0 wt.%NaCl,3.5 wt.%NaCland
10.0 wt.% NaCl at ambient temperature. Figure 8 shows the
Tafelplotsoftheexperimentinaeratedanddeaerated3.5 wt.%
2FeðOHÞ !Fe O (cid:5)H O þ2H O ðEq 11Þ
3ðsÞ 2 3 2 ðsÞ 2 ðlÞ NaCl solutions, respectively. The Tafel extrapolation parame-
The results of this work also showed that the rate of corro- ters for all the steels in aerated and deaerated solution for the
sion of the specimens was more severe in aerated solutions three NaCl concentrations are listed in Table 4
than in deaerated solutions as is clearly shown in Fig. 6. This It can be observed from Fig. 8 that the anodic current
means that all the specimens exhibited higher susceptibility to increasedcontinuouslywithincreaseincorrosionpotentialsfor
corrosion attack in aerated solutions than in deaerated solu- all the specimens in aerated and deaerated 3.5 wt.% NaCl
tions. Many researchers (Ref 1, 2, 20, 21, 54-56) have ob- solutions. This signified a continuous dissolution of iron in
served similar results concerning the corrosion effect of agreement with the trend of the LPR results. Also, the anodic
dissolved oxygen in crude oil. The decrease in corrosion rate branchesshowsharpincreaseinslopesexhibitingwell-defined
of the specimens in deaerated solutions when compared with Tafelian regions within the low over-potentials. This is an
the aerated solutions was about 89% in 1.0 wt.% NaCl, 93% indication that passive films were not formed (Ref 40). The
in 3.5 wt.% NaCl and 92% in 10.0 wt.% NaCl. This is in cathodic branch for all the steels in the aerated solutions
agreement with the report of Jung et al. (Ref 21) who re- showed a mixed charge and mass transfer control reactions
ported a decrease of 91% when the concentration of dissolved depicting well-pronounced current plateaus. This can be
oxygen was decreased from 9 to 2 mg/L. ascribed to the mass-transport limitations of oxygen reduction
Theeffectofdissolvedoxygen(DO)onthecorrosionofoil reactionssincereductionindissolvedoxygenasshowninEq 1
and gas facilities is significantly influenced by the presence of is the dominant reaction in aerated conditions (Ref 2, 29, 57).
other components of crude oil streams. One of such compo- On the other hand, under deaerated condition, the cathodic
nentsischloridesalts.Mamengetal.(Ref56)investigatedthe reduction reaction shown in Eq 1 with a corresponding small
effectsofpHandoxygenonthecorrosionresistanceofcarbon rate of hydrogen evolution is the main reaction. This low
steel in highly concentrated sodium chloride and reported that hydrogen evolution resulted to the witnessed low corrosion
oxygen content is an important factor influencing the corro- rate.
Journal of Materials Engineering andPerformance Volume26(12)December 2017—5747
Fig.8 Tafelplotsofspecimensinaeratedanddeaerated3.5wt.%NaClsolutionsatambienttemperature
Table 4 Tafel extrapolation parameters of the specimens in aerated and deaerated solutions of three NaCl concentrations
NaCl(wt.%) Sample E ;mV i ,lA/cm2 b ,mV/dec b ,mV/dec
Corr Corr a c
1.0Aerated SteelA (cid:2)637 6.00 38 145
SteelB (cid:2)656 7.50 60 175
SteelC (cid:2)502 11.00 38 120
X65 (cid:2)611 6.50 65 120
1.0Aerated SteelA (cid:2)733 1.19 54 94
SteelB (cid:2)732 1.25 53 96
SteelC (cid:2)726 1.37 57 122
X65 (cid:2)730 1.22 49 105
3.5Aerated SteelA (cid:2)590 21.00 65 449
SteelB (cid:2)591 24.00 69 411
SteelC (cid:2)581 28.00 55 530
X65 (cid:2)580 21.50 64 450
3.5Deaerated SteelA (cid:2)742 1.40 53 99
SteelB (cid:2)740 1.50 55 117
SteelC (cid:2)738 1.60 55 108
X65 (cid:2)738 1.40 53 111
10.0Deaerated SteelA (cid:2)607 17.00 40 80
SteelB (cid:2)634 20.00 65 265
SteelC (cid:2)600 23.00 55 155
X65 (cid:2)621 18.00 50 120
10.0Deaerated SteelA (cid:2)758 0.90 54 86
SteelB (cid:2)741 1.35 51 90
SteelC (cid:2)742 1.4 56 101
X65 (cid:2)731 1.16 40 64
solution to (cid:2) 758 mV in deaerated 10% NaCl solution. This
O þH O þ4e(cid:2) !4OH(cid:2) ðEq 12Þ
2ðgÞ 2 ðlÞ ðaqÞ cathodic shift in corrosion potential is an indication that the
corrosionprocesswasmainlyacceleratedbycathodicreactions
2H2OðlÞþ2e(cid:2) !H2ðgÞþ2OH(cid:2)ðaqÞ ðEq 13Þ (Ref 59). This gave a corresponding decrease in corrosion
current density from 17 lA cm(cid:2)2 in the aerated solutions to
The Tafel plots for other environmental conditions (aerated 1
0.9 lA cm(cid:2)2 in the deaerated solutions of Steel A. These
and 10.0 wt.% and deaerated 1.0 and 10.0 wt.%) displayed
values are comparable with the report of other authors (Ref 3,
similar profiles, but with a mild current plateaus in the deaer-
30, 31). Since corrosion current density i is proportionally
ated solutions which is due to virtually absence of DO. Corr
relatedtocorrosionrate(CR)(Ref59,60)andfromthevalues
TheTafel plotsoftheexperiments inaerated anddeaerated
of i in Tables 4, the corrosion rates of the steels in both
1.0 wt.% NaCl, 3.5 wt.% NaCl and 10.0 wt.% NaCl solutions Corr
aerated and deaerated solutions can be ranked as
shown in Fig. 9 vividly revealed that the corrosion current
CR <CR <CR <CR . This trend of corrosion
density in aerated media is more than in the deaerated media. SteelA X65 SteelB SteelC
rate corroborated the linear polarization resistance results and
This is in corroboration with the results of LPR experiments
canbeattributedtothechemicalcompositionsandmicrostruc-
and with the report of Jung (Ref 21).
tures of the samples.
It is evident from Fig. 9 and Table 4 that there was a
Figure 10 shows the SEM micrographs of the specimens
cathodicshift(i.e.,inthenegativedirection),whichforSteelA
corroded in aerated and deaerated 1 wt.% NaCl. A closer
(for instance) increased from (cid:2) 607 mVin aerated 10% NaCl
5748—Volume26(12) December 2017 Journal of MaterialsEngineering andPerformance
Fig.9 Comparison of Tafel plots of specimens in aerated (a) and deaerated (DA) (a) 1wt.% NaCl, (b) 3.5wt.% NaCl and (c) 10wt.% NaCl
solutions
Fig.10 SEM micrographs of the samples corroded in 1wt.% aerated and deaerated brine solutions at ambient temperature: (a) Steel A in aer-
ated,(b)SteelAindeaerated,(c)SteelBinaerated,(d)SteelBindeaerated,(e)SteelCinaerated,(f)SteelCindeaerated,(g)X65inaerated,
(h)X65indeaerated
examination of this figure showed that the steels corroded in corrosion rate. Figure 2 reveals that Steels A and X65 have
aerated condition exhibited more severe attack than those ferrite–pearlite microstructures with Steel A having more
corroded in deaerated solution as reported by Jung et al. (Ref ferritic phase and larger grain size (Table 3). On the other
21).The presence, amount and distribution of various hand, X65 has more pearlites and smaller grain sizes.
microstructural phases have considerable influence on the This difference in microstructure is dependent on the
corrosion behavior of steels offering significant protective and chemical composition and the thermo-mechanical processes
adherent properties to the corrosion product. The variation in involvedinproduction.Thishasbeenreportedintheliteratures
pearlite/ferrite ratio and the grain size of the specimens shown (Ref 40, 42) that carbon steels with the same chemical
in Table 2 could have contributed to the witnessed trend in composition can exhibit different microstructures and vice
Journal of Materials Engineering andPerformance Volume26(12)December 2017—5749
versadependingonheattreatmentroute.Thiscansignificantly carbon content formed carbides with Cr (Ref 38, 66–68)
affect the corrosion behavior (protective and adherent proper- leading to increased cathodic site and therefore increased
tiesofcorrosionfilm)ofcarbonsteel.Whenafreshlypolished corrosion rate (Ref 29, 41).
carbon and low-alloyed steel with ferrite–pearlite microstruc- The work of Kermani et al. (Ref 38) also showed that
tures is immersed inan electrolyte, selective dissolution ofthe vanadium (V) had a major beneficial effect in reducing
ferrite phase takes place leaving the pearlite unaffected. The corrosion rate and that Vin the excess of 50% stoichiometric
undissolved pearlite forms cementite platelets. The adherence ratio of carbon is needed to obtain the best combination of
andprotectivenessofthecorrosionproductfilmisrelatedtothe corrosion resistance. Thus, using simple charge/mass balance
presence of these cementite platelets which strengthen and calculation,onlyabout 0.005 wt.% Cisrequired to precipitate
anchor the films to the metal substrate (Ref 42, 45). Some as carbide with the available V in Steel C (Table 1). Conse-
authors (Ref 32, 41, 42, 61) have also reported that cementite quently,theexcess/balanceCcontentwillprecipitatewithCras
acts as cathode, while the ferrite acts as the anode in ferrite– chromiumcarbide.Thisalsocontributedtoincreasedcorrosion
pearlite microstructures. It follows that the specimen with rate of Steel C.
higher cathode–anode (pearlite–ferrite) ratio should be more
susceptible to corrosion attack (Ref 23, 24, 60). Pearlite phase
hasalsobeenobservedtoincreasewithcarboncontent(Ref62, 4. Conclusion
63). Therefore, the specimen with higher carbon content has
morepearlitephaseandconsequentlygreatercathodetoanode Thecorrosionbehaviorofthreefairlynewgenerationsteels
ratio resulting in higher corrosion rate. This phenomenon was in the family of micro-alloyed steel with varying chemical
exhibited by the three steels with ferrite–pearlite microstruc- compositions and microstructures and whose corrosion char-
tures (Steels A, B and X65) used in this work. acteristics have not been properly understood were compared
Another distinctive characteristic of these micro-alloyed with API 5L X65 carbon steel in different concentrations of
steels is their grain size as shown in Table 3. Asiful et al. aerated and deaerated brine solutions using linear polarization
(Ref 43) noted that small grain microstructures have more resistance, potentiodynamic polarization and weight loss tech-
surface area and so are more prone to corrosion attack. It niques.Thecorrosionproductsonthesurfaceofthesteelswere
then means that the combine effect of these two factors characterized using SEM/EDAX analysis. From the results of
(cathode–anode ratio and grain size) majorly contributed to the experiments, the following conclusions can be drawn:
Steel A exhibiting a better corrosion performance than X65. The three fairly new class of micro-alloyed steels exhibited
Thus, although Steel A and X65 have the same chemical comparablecorrosionpropertiesofmicro-alloyedsteelshaving
composition (Table 1), their different microstructural behav- presented similar corrosion characteristics with the reference
iors (Fig. 2) could be due to heat treatment processes used. specimen (API 5L X65). The potentiodynamic polarization
This in turn contributed to the witnessed variation in their plotsofthesesteelsshowedawell-definedanodicTafelregion,
corrosion performance. Steel B also consist of ferrite–pearlite but the cathodic behavior indicated a mixed charge and mass
phase with finer grain size and globular structure leading to transfer control reactions with a more pronounced limiting
porous and flaky corrosion film with poor adhesion as shown currentplateausintheaeratedsolution.Thiswasascribedtothe
in Fig. 10(c) and (d) and hence exhibiting a much lower mass-transport limitations of oxygen reduction reactions occa-
corrosion resistance than Steel A and X65. This corroborated sioned by the presence of dissolved oxygen in the aerated
the report of Islam and Farhat (Ref 22). The protruding solutionsleadingtoabout80%highercorrosionratethaninthe
laminar cementite can be clearly seen in Fig. 10 for steels A deaerated solutions. The corrosion rate of these proprietary
(Fig. 10a and b) and X65 (Fig. 10g and h), suggesting that steelsincreasedwithincreaseinNaClconcentrationdisplaying
the ferrite phases have suffered dissolution, while the a threshold in 3.5 wt.% NaCl solutions due to decrease in the
cementite appeared unaffected (Ref 64). solubilityofoxygen(saltingout)andtheadsorptionofchloride
SteelCconsistsofbainiticstructureswithdispersedacicular on the surface of the steels. Also, the specimens demonstrated
ferrite. This formed a sludge-like corrosion product which variationsincorrosionratewhich waslinkedtotheirchemical
according to Zhao (Ref 39) renders the specimen unprotected composition and microstructures whereby the specimens with
against corrosion attack. These sludge-like corrosion products ferrite–pearlitemicrostructuresaswellassmallergrainsizeand
allowtheingressofcorrosionspeciestothesubstrateleadingto lesscarboncontentpresentedbettercorrosionresistanceinboth
severecorrosionspallation(Ref64)asshowninFig. 10(e)and aerated and deaerated solutions. In general, the results of the
(f). Steel C also has high Cr and Mo content. These elements variouscorrosiontechniquesandthesurfaceanalysisemployed
improvecorrosionresistancebyfavoringpassivity(Ref29,38, corroborated each other and showed that the corrosion rate
41,46,65).Kermanietal.(Ref38)andKermaniandMorshed (CR) of the specimens can be ranked as CR <CR <
SteelA X65
(Ref 46) observed that an optimum Cr content, which is CR <CR . Thus, within the limit of experimental
SteelB SteelC
subjectedtootheralloyingconstituentsandheattreatment,had error,onlySteelAshowedabettercorrosionperformancethan
a significant beneficial role on the CO2 corrosion performance the reference specimen (API 5L X65).
ofthesteels.Uedaetal.(Ref65)showedthatbelow60 (cid:3)C,the
effect of Cr addition in enhancing corrosion performance was
effectivewithCrcontentmorethan1 wt.%.Otherauthors(Ref
Acknowledgment
62, 63) have equally reported that the corrosion resistance of
carbon steels deceases with increasing carbon content. It then Wewishtoacknowledgeandappreciatethesponsorshipofthis
means that due to high carbon content and low Cr content work by Petroleum Technology Development Fund (PTDF),
(< 1 wt.%), the effect of Cr in enhancing corrosion perfor- Abuja, Nigeria. For supplying the micro-alloyed steels used in
manceofsteelCwasnotpronounced.Thisisbecausethehigh thiswork,we say thankyouto ProfessorB. Kermani.
5750—Volume26(12) December 2017 Journal of MaterialsEngineering andPerformance