corrosion behavior of carbon steel in synthetically produced oil
TRANSCRIPT
Research ArticleCorrosion Behavior of Carbon Steel in SyntheticallyProduced Oil Field Seawater
Subir Paul Anjan Pattanayak and Sujit K Guchhait
Department of Metallurgical and Material Engineering Jadavpur University Kolkata 700032 India
Correspondence should be addressed to Subir Paul spmet4gmailcom
Received 16 September 2014 Accepted 14 November 2014 Published 17 December 2014
Academic Editor Chi Tat Kwok
Copyright copy 2014 Subir Paul et al This is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited
The life of offshore steel structure in the oil production units is decided by the huge corrosive degradation due to SO4
2minus S2minus andClminuswhich normally present in the oil field seawater Variation in pH and temperature further adds to the rate of degradation on steelCorrosion behavior of mild steel is investigated through polarization EIS XRD and optical and SEMmicroscopy The effect of all3 species is huge material degradation with FeSx and FeCl
3and their complex as corrosion products EIS data match the model of
Randle circuit with Warburg resistance Addition of more corrosion species decreases impedance and increases capacitance valuesof the Randle circuit at the interface The attack is found to be at the grain boundary as well as grain body with very prominentsulphide corrosion crack
1 Introduction
The severe corrosion of the submersed structures in the oilfield at the production site and crude oil transportation isunpredictable and is amajor component of the total corrosionloss in oil and gas industries The corrosion species in theaqueous oil field seawater are CO
3
2minus S2minus Clminus SO4
2minus and O(Table 1) [1] which are also influenced by the variation of pHand temperature CO
3
2minus and S2minus are formed from CO2and
H2S of the oil in the aqueous environment And Clminus SO
4
2minusand O are present in the seawater Besides these parametersthere are fluid dynamics of sea water and suspended solidsand sands influencing the erosion corrosion of the marinestructures Crude oil and natural gas can carry various high-impurity products which are inherently corrosive In the caseof oil and gas wells and pipelines such highly corrosivemediaare carbon dioxide (CO
2) hydrogen sulfide (H
2S) and free
water [2]The effect of any individual parameter on corrosion
rate has been studied extensively [3ndash6] But the conjointeffect of the above mentioned parameters and interferingeffects and interactions are complex and are not very wellunderstood The salts and sulfide compounds dissolved incrude oil can provoke the formation of a corrosive aqueoussolution whose chemical composition involves the presence
of both hydrochloric acid (HCI) and hydrogen sulfide (H2S)
[3 4] Corrosion mitigation in the oil field industry hastraditionally been performed by combining methods formeasuring the corrosion rates such as corrosion coupons andregular pipeline inspections with prevention strategies [6]But that required years to get empirical results and couldnot be applied to other geographical locations of differentsea water chemistry All the factors make the corrosionmechanisms in the oil fields very complex with high degreeof interaction among the species Several previous studieshave been performed related to the corrosion process ofiron and steel in H
2S solutions [4 7ndash13] These works
studied the influence of H2S on the corrosion phenomena
at ambient temperature In H2S-containing solutions the
corrosion process of metal may be accompanied by theformation of a sulfide film on the metal surface and leadsto more complicated corrosion behavior Previous researches[14ndash16] have shown that H
2S had a remarkable acceleration
effect on both the anodic iron dissolution and the cathodicevolution in most cases but H
2S may exhibit an inhibitive
effect on the corrosion of iron or steel weld Recently theinfluence of H
2S concentration on the corrosion behavior
of carbon steel at 90∘C has been investigated [15] Physicalmodeling of ships and offshore structures in ocean water byMelchers et al [17ndash19] and Shehadeh andHassan [20] adds to
Hindawi Publishing CorporationInternational Journal of MetalsVolume 2014 Article ID 628505 11 pageshttpdxdoiorg1011552014628505
2 International Journal of Metals
Table 1 Ions present in typical oil field seawater
Species typically found in oil field brines Element Concentration mgLCO2 Dissolved carbon dioxide Barium Ba2+ 31H2CO3 Carbonic acid Boron B 6HCO3
minus Bicarbonate ion Calcium Ca2+ 284CO32minus Carbonate ion Iron Fe3+ 5585
H+ Hydrogen ion Magnesium Mg2+ 2431OHminus Hydroxide ion Phosphorous P3- 1Fe2+ Iron ion Potassium K 50Clminus Chloride ion Sodium Na 4770Na+ Sodium ion Strontium Sr2+ 83K+ Potassium Chloride Clminus 7480Ca2+ Calcium ion Bromide Brminus 20Mg2+ Magnesium ion Sulphate SO4
2minus 21Ba2+ Barium ion Nitrate NO3
minus 050Sr2+ Strontium ion Hydroxyl OHminus 0CH3COOH (HAc) Acetic acid Carbonate CO3
2minus 0CH3COOminus (Acminus) Acetate ion Bicarbonate HCO3
minus 500HSO4
minus Bisulphate ion Dissolved CO2 CO2 924SO42minus Sulphate ion Specific gravity 1014
pH 658Resistivity 04405Ohm
Total dissolved solids 1 3453mg
better understanding of the present investigation Howeverlittle research has been done on the corrosion behavior ofcarbon steel in the presence of both H
2S and NaCl at ambient
and elevated temperatureCorrosion mechanisms in oil field systems are com-
plex and are showing high degrees of interaction betweencorrosion species products and oil field metallurgies Theinteractions of sulfate and chloride are of interest in thiswork since presence of sulfate ions in oilfield producedwater strongly influence corrosion mechanisms While thereare many research works on the effects of CO
2and H
2S on
corrosion of carbon steel those of conjoint effects of S2minusClminus and SO
4
2minus are much less The present investigation aimsto study the conjoint effects of S2minus Clminus and SO
4
2minus alongwith variation of pH and temperature on carbon steel Thecorrosive species included are sulfate chloride hydrogensulfide temperature and pH Sulphate and chloride wereadded asNa
2SO4andNaCl Hydrogen sulfidewas introduced
to the corrosion cell with the following reaction
Na2S +H
2SO4= H2S + FeSO
4 (1)
The effect on corrosion of these species was exam-ined through polarization experimentation using a three-electrode glass corrosion cell and potentiostat Electrochemi-cal AC impedance spectroscopy studies were also carried outfor better understanding of electrochemical effects of corro-sive species on electrical phenomenon occurring at metal-solution interface The corroded and uncorroded substrateswere characterized by XRDThemorphology of the corrodedsurface was investigated by optical microscopy and SEM
2 Experimental Methods
21 Polarization Studies Electrochemical measurementswere conducted using Gamry Potentiostat instrumentcoupled with Echem analyst software controlled by apersonal computer in a conventional three-electrode cellsystemsThe working electrode was carbon steel the counterelectrode was graphite and a saturated calomel electrode(SCE) acted as the reference electrode Experiments wereperformed in different concentrations of Clminus SO
4
2minus and S2minussolutions at preselected pH and temperature to determinethe corrosion potential 119864corr and corrosion current 119894corr Thepotential was scanned between minus15 V and 1V at a scan rateof 1mVs
22 Electrochemical Impedance Spectroscopy (EIS) Theexperimental arrangement was the same as that of polariza-tion studies The electrochemical cell was connected to animpedance analyzer (EIS300 controlled by Echem analystsoftware) for electrochemical impedance spectroscopy Theelectrochemical impedance spectrawere obtained at frequen-cies between 300 kHz and001HzTheamplitude of the sinus-oidal wave was 10mV The following results and informationare obtained from the EIS experiments Polarization resist-ance (119877
119901) electrolyte resistance (119877
119906) double layer capaci-
tance (119862dl) capacitive load or constant phase elementCPE(119884) and 120572 which is defined from the capacitive impe-dance equation 119885 = 1119862(119895119908)minus120572
Capacitors in EIS experiments often do not behaveideally Instead they act like a constant phase element (CPE)The exponent 120572 = 1 for pure capacitance For a constant
International Journal of Metals 3
phase element the exponent 120572 is less than one The ldquodoublelayer capacitorrdquo on real cells often behaves like a CPE insteadof like a pure capacitor
23 X-Ray Diffraction (XRD) Analysis The X-ray diffractiontechnique is used to define the crystalline structure and thecrystalline phases This test was done using a Rigaku UltimaIII X-Ray Diffractometer for recording the diffraction tracesof the samples with monochromatized Cu K
120572radiation at
room temperature the scan region (2120579) ranged from 10∘ to100∘ at a scan rate of 5∘minminus1
24 Scanning Electron Microscope (SEM) Morphology Theelectron micrographs were studied by SEM with acceleratingvoltage 30 kV magnification up to 300000x and resolutionof 35 nm The images of the corroded samples were pho-tographed at low and high magnification
3 Results and Discussions
The effects of Clminus SO4
2minus S2minus pH and temperature ondegradation behavior of carbon steel were studied by poten-tiostatic polarization to determine corrosion current andcorrosion potential The various electrical properties at themetal-solution interface were determined by electrochemicalimpedance spectroscopy (EIS) The presence of differentelements on corroded surface was detected by XRD Themorphology of the degraded surfaces was characterized byoptical microscopy and SEM Before going into the experi-mental findings of the effects of different interfering ions it isworthwhile to discuss the basic electrochemical reactions ofaqueous corrosion of steel in the presence of those ions
The main electrochemical anodic and cathodic reactionsfor the corrosion of carbon steel in aqueous oil fields environ-ments in presences of the ions are as follows
Half Cell Reactions (E versus SCE) Consider
Fe = Fe2+ + 2e (1198640 = minus681) (2)
O2+ 2H2O + 4eminus 997888rarr 4OHminus (1198640 = 0579) (3)
H + e = 12
H2(1198640= minus0241) (4)
In presence of SO4
2minus present reactions are
SO4
2minus+ 2e +H
2O = 2SO
3
2minus+OH (1198640 = minus1177) (5)
2SO3
2minus+ 4e + 3H
2O = 3S
2O3
2minus+ 6OHminus (1198640 = minus099)
(6)
2SO3
2minus+ 4e + 3H
2O = S + 6OHminus (7)
S + 2eminus = S2minus (1198640 = minus0688) (8)
Current density (Acm2)
150
100
050
000
minus050
minus100
minus150
01N Na2SO4
02N Na2SO4
03N Na2SO4
Pote
ntia
l (V
ver
sus S
CE)
1E minus 09 1E minus 07 1E minus 05 1E minus 03 1E minus 01
Figure 1 Potentiodynamic polarization curves of low carbonsteel in different concentration of Na
2SO4solution at pH 6 and
temperature 25∘C
In presence of Na2S or H
2S (Na2S was added in state
of H2S to understand the effect of s2minus)
H2S = H+ +HSminus (9)
HSminus = H+ + Sminus (1198640 = minus0688) (10)
In the presence of Clminus that is (NaClHCl)
Fe2+ + 2Clminus = FeCl2
(11)
FeCl2+ 2H2O = Fe (OH)
2+ 2H+ + 2Clminus (12)
FeCl2+ Clminus = FeCl
3+ e (13)
Fe + 2H+ = Fe2+ (14)
The above equations would help in better understandingof the effects of the corrosion species found in the experimen-tal results
31 Polarization Studies
311 Effect of SO4
2minus Figure 1 shows the potential dynamicpolarization curvewith increasing SO
4
2minus concentration at pH6 The pH of sea water normally varies from 75 to 82 butin the oil fields due to the presence of few acidic substancesnamely carbonic acid H
2S and other organic acids pH may
shift from near neutral towards the acidic side between 6and 4 It is seen here that the corrosion rate increases withincrease in concentration of SO
4
2minus It is seen from (5)ndash(8) that
4 International Journal of Metals
0
05
1
15
000001 001
Pote
ntia
l (V
ver
sus S
CE)
Current density (Acm2)
minus1
minus15
minus05
1E minus 11 1E minus 08
Effect of Na2S
025M Na2SO4
025M Na2SO4 + 01M Na2S025M Na2SO4 + 03M Na2S025M Na2SO4 + 05M Na2S
Figure 2 Effect of S2minus on potentiodynamic polarization curves ofcarbon steel in 025M Na
2SO4solution at pH 6 and temperature
25∘C
the cathodic reduction of SO4
2minus ions produces thiosulfate andsulphide ions both of which are aggressive corrosion speciesand hence degrade the steel surface in sea water in the acidicpH range below the neutral medium The corrosion productin this case should be iron sulphide or thiosulfate
312 Effect of SO4
2minus + S2minus Addition of S2minus to the solutioncontaining SO
4
2minus further enhances the corrosion rate as canbe seen from Figure 2 And the pH has a strong effect on it Itcan be seen from (11) and (12) above that the conjoint effectof SO4
2minus and Sminus is the production of increasing amount of Sminusas well as corrosive sulphur compound
313 Effect of SO4
2minus +Clminus Corrosion rate also increaseswithaddition of Clminus to the solution containing SO
4
2minus (Figure 3)The rate increases with increase in Clminus concentration It isseen from (11)ndash(14) above that theClminus ions attack FeFe2+withthe formation of corrosion products FeCl
2and FeCl
3 FeCl
2
is unstable and may hydrolyse or further react with Clminus ionsto form Fe(OH)
2or FeCl
3 respectively
314 Effect of SO4
2minus + S2minus + Clminus The conjugate of all 3 ionswhich are normally present in the oil field sea water is thedegradation of carbon steel structure at the highest level Itis seen from Figure 4 that 119894corr values have shifted to theright and 119864corr towards the active potential with increasingthe concentration of both S2minus and Clminus
315 Effect of pH There is not any much significant effect ofpolarization curves with change in pH except at pH 11 underalkaline condition (Figure 5) when the corrosion rate is verylow The steel is in the passive region at this pH The pH of
0
Current density (Acm2)
025M Na2SO4
Effect of NaCl
minus02
minus04
minus06
minus08
minus1
minus12
Pote
ntia
l (V
ver
sus S
CE)
025M Na2SO4 + 01N NaCl025M Na2SO4 + 03N NaCl025M Na2SO4 + 05N NaCl
1E minus 09 1E minus 06 1E minus 03 1E + 00
Figure 3 Effect of Clminus on potentiodynamic polarization curves ofcarbon steel in 025M Na
2SO4solution at pH 8 and temperature
25∘C
0
0000001 00001 001 1Current density (Acm2)
1E minus 08
minus01
minus02
minus03
minus04
minus05
minus06
minus07
minus08
minus09
minus1
Pote
ntia
l (V
ver
sus S
CE)
025M Na2SO4
025M Na2SO4 + 03M Na2S + 03N NaCl025M Na2SO4 + 03M Na2S + 01N NaCl025M Na2SO4 + 03M Na2S025M Na2SO4 + 03M Na2S + 05N NaCl
Figure 4 Effect of Clminus + S2minus on potentiodynamic polarizationcurves of carbon steel in 025M Na
2SO4solution at pH 8 and
temperature 25∘C
the oil field water is in the range of 4ndash6 when the corrosionrate is high
316 Effect of Temperature Temperature aggravates thematerial degradation (Figure 6) by increasing diffusion andmass transfer coefficient of the aggressive ions corroding
International Journal of Metals 5
0
05
1
15
minus1
minus15
minus05
Current density (Acm2)
pH 11pH 6pH 8
Pote
ntia
l (V
ver
sus S
CE)
Effect of temperature of 025M Na2SO4
1E minus 09 1E minus 07 1E minus 05 1E minus 03 1E minus 01
Figure 5 Potentiodynamic polarization curves of carbon steel in an aqueous solution of 025MNa2SO4and 03MNa
2S at temperature 25∘C
at different pH
0
05
1
15
0001 01
Pote
ntia
l (V
ver
sus S
CE)
minus1
minus15
minus05
Current density (Acm2)
1E minus 11 1E minus 09 1E minus 07 1E minus 05
Effect of temperature of 025M Na2SO4
Temp 25∘C 025M Na2SO4
Temp 70∘C 025M Na2SO4
Temp 45∘C 025M Na2SO4
Figure 6 Potentiodynamic polarization curves of low carbon steel in 025M Na2SO4solution at (pH = 8) at different temperature
the metallic surface Temperature also increases the 119868119871 the
limiting current density of the concentration polarizationand hence shifts the polarization curves to the right
32 Electrochemical Impedance Spectroscopy (EIS) The phe-nomenon at the interface of the solidmetal and aqueous elec-trolyte is a complex process consisting of a line of positively
and negatively charged ions capacitance due to double layercorroded product or film formation on surfaces polarizationresistance (119877
119901) pore resistance (119877po) and various types of
impedance due to diffusion of ions movement of charge in oraway frommetal surface and adsorption of cation and anionThe whole phenomenon can be represented by an equivalentAC electrical circuit The phenomenon can be interpreted
6 International Journal of Metals
Zm
od(o
hm)
1000
1000
1000
1000
Hz
1000
Hz
1000
Hz
1000
kHz
1000
kHz
1000
kHz
Z curve (025m Na2SO4)Y2-Z curve (025m Na2SO4)
Fit 1 [CPE with diffusion]Y2-fit 1 [CPE with diffusion]
Frequency (Hz)
Y2-Z
phz(∘)
0000
minus2000
minus4000
minus6000
In 025M Na2SO4
Bode plot
Impedance
Phase angle
025M SO42minus
Ru
RE
Rp Wd
WE
Y0120572
empty
CPE with diffusionequivalent circuit
1000
mH
z
1000
mH
z
1000
mH
z
1000
mH
z
1000000
100000
(a)
Zm
od(o
hm)
1000
Hz
1000
Hz
1000
Hz
1000
kHz
1000
kHz
1000
kHz
Fit 1 [CPE with diffusion]Y2-fit 1 [CPE with diffusion]
Frequency (Hz)
Y2-Z
phz(∘)
0000
minus2000
minus4000
minus6000
Bode plot
Impedance
Phase angle
minus8000
In 025M Na2SO4 + 01M Na2S
Z curve (025m Na2SO4 + 01m Na2S)Y2-Z curve (025m Na2SO4 + 01m Na2S)
025M SO42minus + 01M S2minus
Ru
RE
Rp Wd
WE
Y0120572
empty
CPE with diffusionequivalent circuit
1000
1000
1000
1000
mH
z
1000
mH
z
1000
mH
z
1000
mH
z
1000000
100000
(b)
Zm
od(o
hm)
1000
1000
1000
Hz
1000
Hz
1000
Hz
1000
kHz
1000
kHz
1000
kHz
Fit 1 [CPE with diffusion]Y2-fit 1 [CPE with diffusion]
Frequency (Hz)
Y2-Z
phz(∘)
0000
minus2000
minus3000
minus4000
minus1000
Bode plot
Impedance Phase angle
025M Na2SO4 + 01M Na2S + 01N NaCl
Z curve (025m Na2SO4 + 01M Na2S + 01N NaCl)Y2-Z curve (025m Na2SO4 + 01M Na2S + 01N NaCl)
025M SO42minus + 01M S2minus + 01M Clminus
Ru
RE
Rp Wd
WE
Y0120572
empty
CPE with diffusionequivalent circuit
1000
mH
z
1000
mH
z
1000
mH
z
1000
mH
z
(c)
Figure 7 Bode plots of carbon steel in SO4
2minus SO4
2minus + S2minus and SO4
2minus + S2minus + Clminus solution at pH 8 and temperature 25∘C
from the Bode plots which are depicted and discussed in thefollowing section for various corrosive species
Figure 7 displays the Bode plots of carbon steel insolutions of SO
4
2minus SO4
2minus + S2minus and SO4
2minus + S2minus + Clminus Itis to be noticed that in all the solutions EIS data matchthe model of Randle circuit with a Warburg resistance 119882
119889
(given in the inset of each figure) that prevails at the metal-solution interface It is seen that the impedance decreaseswith addition of different aggressive ions compared to thosewith base solution of only SO
4
2minus This decrease in impedanceleads to more current flow across the interface and henceincrease in corrosion rate There is a phase angle shift with
International Journal of Metals 7
Inte
nsity
(cps
)
2120579 (deg)
Fe (110)
Fe (200)Fe (211)
1000
800
600
400
200
040 60 80 100 120
Carbon steel before corrosion
(a)
Inte
nsity
(cps
)
2120579 (deg)
Fe (110) Corroded in Na2SO4 + Na2S
FeS (003)
FeS (003)
FeS (113)
800
700
600
500
400
300
200
100
030 40 50 60 70 80 90 100 110
(b)
Inte
nsity
(cps
)
2120579 (deg)
1000
800
600
400
200
0
40 60 80 100 120
Fe (110)
FeCl3
FeCl3
Corroded in Na2SO4 + NaCl
(c)
Inte
nsity
(cps
)
2120579 (deg)
Fe (110)
FeS (003)
500
400
300
200
100
0
40 50 60 70 80 90 100 110
FeCl3
Corroded in Na2SO4 + Na2S + NaCl
(113)
(d)
Figure 8 X-ray diffraction showing peaks of different corroded phases
frequency The minimum phase angle reaches much lessthan 90 degrees indicating the capacitance in the circuit isnot a pure capacitance but constant phase element CPE(119884)and is given by the capacitive impedance equation 119885 =1119862(119895119908)
minus120572 where 120572 is fraction varying from 0 to 1 the valueless than one indicates that it does not behave ideally as purecapacitance At high frequency the value of impedance119885 modis roughly equal to 119877
119878 while at low frequency the value is
(119877119878+ 119877119875) Both can be determined from the blots Table 1
depicted the computed values of the EIS parameters It isseen that polarization resistance decreases with addition ofmore types of ions which support the polarization resultsof corrosion rates increase as found in Figures 2 3 and 4The increase in corrosion rate is supported by the EIS dataincrease in119884
0which behaves like capacitance and decrease in
polarization resistance (Table 2)The values of 120572 indicate thatthe capacitance behaves like constant phase element ratherthan pure capacitance The Warburg resistance 119882
119889which
signifies resistance to the flow of ions from the solution tocorroded metal surface also decreases with addition of moretypes of ions All the facts confirm the enhanced corrosionrates of carbon steel in the oil field sea water with presence ofSO4
2minus + S2minus + Clminus
33 X-Ray Diffraction The presence of corrosion productsof FeS FeCl
2 and FeCl
3is clearly indicated by the XRD
peak intensities in Figure 8 This supports the corrosionenhancement by SO
4
2minus S2minus and Clminus ions as found inpolarization and EIS studies
8 International Journal of Metals
Table 2 Computed EIS parameters
Sample Corrosive ions 119877119904(ohm) 119877
119901(ohm) 119884
0120583F 120572 119882
119889(ohm)
Carbon steel SO42minus 9443 85000 750 0720 10430
Carbon steel SO42minus + S2minus 8268 3700 5722 0769 4187
Carbon steel SO42minus + S2minus + Clminus 1331 95 9056 0602 2432
(c)
(b)(a)
Figure 9 Optical microscopy images of corroded steel (a) Na2SO4 (b) Na
2SO4+ Na2S and (c) Na
2SO4+ Na2S + NaCl
34 Optical and SEM Microscopy Images The microstruc-tures by optical microscopy clearly show (Figure 9) thecorroded structure of steel with products of corrosion overit The form of corrosion seems to be uniform not localizedSEM images (Figure 10) distinctly reveal how the degree ofdegradation increases with presence of ions in the solutionsfrom SO
4
2minus to SO4
2minus + S2minus to SO4
2minus + S2minus + Clminus It isinteresting to observe from the morphology of SEM image athigher magnification (Figure 11) that the corrosion has takenplace at grain boundary as well as grain body but the attack atgrain boundary is very prominentwith sulphide (S) corrosioncrackThe corrosion products of sulphide compounds as wellas element sulphur are clearly revealedThe structure of SEMimages shows almost catastrophic failure with the presenceof minor and major cracks when the seawater is enrichedwith the presence of all three corrosive ions SO
4
2minus S2minusand Clminus The morphology of SEM image analysis along with
XRD is in complete agreement with the corrosion data of thepolarization experiments and EIS
From (5)ndash(8) it is seen that SO4
2minus is cathodically reducedto give rise to S2minus It is observed from the polarizationcurve (Figure 1) that the increase of SO
4
2minus concentrationdepolarizes the cathodic curves much more compared toanodic ones shifting 119864corr towards the negative potentialThe release of S2minus from the reaction produces corrosionproducts iron sulfides (FeS
119909) on carbon steel surface They
are nonstoichiometric iron sulfide films mainly composed ofmackinawite and pyrrhotite Berner [21ndash26] The black colorof mackinawite phase is also seen as corrosion products inoptical microstructures (Figures 9(a) and 9(b))
The deterioration of metal due to contact with S2minus (H2S)
and moisture is sour corrosion The forms of sour corrosionare uniform (Figure 9(b)) pitting and stepwise cracking(Figures 10(c) and 11(b))
International Journal of Metals 9
(d)(c)
(b)(a)
Figure 10 SEM images of corroded steel (a) before corrosion (b) in Na2SO4 (c) in Na
2SO4+ Na2S and (d) in Na
2SO4+ Na2S + NaCl
The general equation of sour corrosion can be expressedas follows [21]
S2minus + 2H+ = H2S (aq) 119864 = 0097V versus SCE (15)
Hydrogen sulphide dissociates to produce proton andbisulphide as described [22] by the following equations
H2S (aq) = H+ +HSminus 119870 = 91 times 10minus8
HSminus = H+ + S2minus 119870 = 91 times 10minus12(16)
This reaction scheme shows that the presence of hydrogensulfide can contribute to the concentration of sulfide at thesurface by dissociation rather than charge exchange with thesurface The sulfide concentration at the surface is thereforedependent on the concentration of aqueous hydrogen sulfidein the electrolyte as well as the reduction processes of sulfate
H2S exhibits the different role in anodic process of carbon
steel depending on the pH value in the solutions The localsupersaturation of FeS
119909could be formed on the carbon steel
surface via the following reaction with the nucleation andgrowth of one or more of the iron sulfide mackinawite
H2S + Fe +H
2O 997888rarr FeS
119909+ 2H +H
2O (17)
The black corrosion products (FeS119909) formed on the steel
surface in the H2S-containing solutions could be observed
(Figure 9(b)) The addition of chloride to the corrosionsystem did not exhibit any localized corrosion rather it dras-tically increased the 119894corr values and shifted 119864corr towards thenegative potential (Figures 3 and 4) depolarizing the anodicreactionsThe increase in 119894corr with Cl addition seems to havemodified the anodic substrate area (Figures 9(c) and 10(d))and has enhanced the exchange current density for the ironoxidation process
4 Conclusion
All three corrosive ions SO4
2minus S2minus and Clminus have a strongeffect on increasing the corrosion rate of carbon steel
Cathodic reduction of SO4
2minus generates elemental S orS2minus ions in addition to S2minus from H
2S in oil The species
10 International Journal of Metals
(c)
(b)(a)
Figure 11 SEM images of corroded steel (a) in Na2SO4 (b) in Na
2SO4+ Na2S and (c) in Na
2SO4+ Na2S + NaCl
cause major corrosion with formation of FeS119909as corrosion
productsThe attack is found at grain boundarywith sulphidecracking as well as some grain body degradation The effectof addition of Clminus is the increase of 119894corr values by hundredsof times possibly due to enhancement of exchange currentdensity of the anodic reactions
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgments
The authors would like to acknowledge ldquoCOE TEQUIPrdquo inJadavpur University for the support of this work
References
[1] S Nesic ldquoKey issues related to modelling of internal corrosionof oil and gas pipelinesmdasha reviewrdquo Corrosion Science vol 49no 12 pp 4308ndash4338 2007
[2] L T Popoola A S Grema G K Latinwo B Gutti and A SBalogun ldquoCorrosion problems during oil and gas productionand its mitigationrdquo International Journal of Industrial Chem-istry vol 4 aticle 35 2013
[3] BW A Sherar P G Keech andDW Shoesmith ldquoThe effect ofsulfide on the aerobic corrosion of carbon steel in near-neutral
pH saline solutionsrdquo Corrosion Science vol 66 pp 256ndash2622013
[4] B W A Sherar I M Power P G Keech S Mitlin G Southamand D W Shoesmith ldquoCharacterizing the effect of carbon steelexposure in sulfide containing solutions to microbially inducedcorrosionrdquo Corrosion Science vol 53 no 3 pp 955ndash960 2011
[5] F M Song ldquoA comprehensive model for predicting CO2
corrosion rate in oil and gas production and transportationsystemsrdquo Electrochimica Acta vol 55 no 3 pp 689ndash700 2010
[6] M Hairil Mohd and J K Paik ldquoInvestigation of the corrosionprogress characteristics of offshore subsea oil well tubesrdquoCorrosion Science vol 67 pp 130ndash141 2013
[7] J Tang Y Shao T Zhang G Meng and F Wang ldquoCorrosionbehaviour of carbon steel in different concentrations of HClsolutions containing H
2S at 90 ∘Crdquo Corrosion Science vol 53
no 5 pp 1715ndash1723 2011[8] Z A D Foroulis ldquoRole of solution pH on wet H
2S cracking
in hydrocarbon productionrdquo Corrosion Prevention and Controlvol 40 no 4 pp 84ndash89 1993
[9] M A Veloz and I Gonzalez ldquoElectrochemical study of carbonsteel corrosion in buffered acetic acid solutions with chloridesand H
2Srdquo Electrochimica Acta vol 48 no 2 pp 135ndash144 2002
[10] S Arzola and J Genesca ldquoThe effect of H2S concentration on
the corrosion behavior of API 5L X-70 steelrdquo Journal of SolidState Electrochemistry vol 9 no 4 pp 197ndash200 2005
[11] H YMa X L Cheng S H Chen et al ldquoTheoretical interpreta-tion on impedance spectra for anodic iron dissolution in acidicsolutions containing hydrogen sulfiderdquoCorrosion vol 54 no 8pp 634ndash640 1998
International Journal of Metals 11
[12] H Ma X Cheng S Chen C Wang J Zhang and H YangldquoAn ac impedance study of the anodic dissolution of iron insulfuric acid solutions containing hydrogen sulfiderdquo Journal ofElectroanalytical Chemistry vol 451 no 1-2 pp 11ndash17 1998
[13] HMa X Cheng G Li et al ldquoThe influence of hydrogen sulfideon corrosion of iron under different conditionsrdquo CorrosionScience vol 42 no 10 pp 1669ndash1683 2000
[14] H-H HuangW-T Tsai and J-T Lee ldquoElectrochemical behav-ior of the simulated heat-affected zone of A516 carbon steel inH2S solutionrdquo Electrochimica Acta vol 41 no 7-8 pp 1191ndash1199
1996[15] J Tang Y Shao J Guo T Zhang G Meng and F Wang ldquoThe
effect of H2S concentration on the corrosion behavior of carbon
steel at 90 ∘Crdquo Corrosion Science vol 52 no 6 pp 2050ndash20582010
[16] P Smith S Roy D Swailes S Maxwell D Page and J LawsonldquoA model for the corrosion of steel subjected to syntheticproduced water containing sulfate chloride and hydrogensulfiderdquo Chemical Engineering Science vol 66 no 23 pp 5775ndash5790 2011
[17] R E Melchers ldquoDevelopment of new applied models for steelcorrosion in marine applications including shippingrdquo Ships andOffshore Structures vol 3 no 2 pp 135ndash144 2008
[18] M H Mohd D K Kim D W Kim and J K Paik ldquoA time-variant corrosion wastage model for subsea gas pipelinesrdquo Shipsand Offshore Structures vol 9 no 2 pp 161ndash176 2014
[19] R E Melchers and J K Paik ldquoEffect of flexure on rusting ofshiprsquos steel platingrdquo Ships and Offshore Structures vol 5 no 1pp 25ndash31 2010
[20] M Shehadeh and I Hassan ldquoStudy of sacrificial cathodicprotection on marine structures in sea and fresh water inrelation to flow conditionsrdquo Ships and Offshore Structures vol8 no 1 pp 102ndash110 2013
[21] R A Berner ldquoThermodynamic stability of sedimentary ironsulfidesrdquoTheAmerican Journal of Science vol 265 pp 773ndash7851967
[22] R H Hausler L A Goeller R P Zimmerman and R HRosenwald ldquoContribution to the ldquofilming aminerdquo theory aninterpretation of experimental resultsrdquo Corrosion vol 28 no1 pp 7ndash16 1972
[23] P Taylor ldquoThe stereochemistry of iron sulfidesmdasha structuralration for the crysta llization of some metastable phases fromaqueous solutionrdquo American Mineralogist vol 65 pp 1026ndash1030 1980
[24] R A Berner ldquoIron sulfides formed from aqueous solution atlow temperatures and atmospheric pressurerdquo The Journal ofGeology vol 72 pp 293ndash306 1964
[25] D T Rickard ldquoThe chemistry of iron sulphide formation at lowtemperaturesrdquo Stockholm Contributions in Geology vol 20 pp67ndash95 1969
[26] A J Bard R Parsons and J Jordan Standard Potentials inAqueous Solution CRC Press 1st edition 1985
Submit your manuscripts athttpwwwhindawicom
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Nano
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Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal ofNanomaterials
2 International Journal of Metals
Table 1 Ions present in typical oil field seawater
Species typically found in oil field brines Element Concentration mgLCO2 Dissolved carbon dioxide Barium Ba2+ 31H2CO3 Carbonic acid Boron B 6HCO3
minus Bicarbonate ion Calcium Ca2+ 284CO32minus Carbonate ion Iron Fe3+ 5585
H+ Hydrogen ion Magnesium Mg2+ 2431OHminus Hydroxide ion Phosphorous P3- 1Fe2+ Iron ion Potassium K 50Clminus Chloride ion Sodium Na 4770Na+ Sodium ion Strontium Sr2+ 83K+ Potassium Chloride Clminus 7480Ca2+ Calcium ion Bromide Brminus 20Mg2+ Magnesium ion Sulphate SO4
2minus 21Ba2+ Barium ion Nitrate NO3
minus 050Sr2+ Strontium ion Hydroxyl OHminus 0CH3COOH (HAc) Acetic acid Carbonate CO3
2minus 0CH3COOminus (Acminus) Acetate ion Bicarbonate HCO3
minus 500HSO4
minus Bisulphate ion Dissolved CO2 CO2 924SO42minus Sulphate ion Specific gravity 1014
pH 658Resistivity 04405Ohm
Total dissolved solids 1 3453mg
better understanding of the present investigation Howeverlittle research has been done on the corrosion behavior ofcarbon steel in the presence of both H
2S and NaCl at ambient
and elevated temperatureCorrosion mechanisms in oil field systems are com-
plex and are showing high degrees of interaction betweencorrosion species products and oil field metallurgies Theinteractions of sulfate and chloride are of interest in thiswork since presence of sulfate ions in oilfield producedwater strongly influence corrosion mechanisms While thereare many research works on the effects of CO
2and H
2S on
corrosion of carbon steel those of conjoint effects of S2minusClminus and SO
4
2minus are much less The present investigation aimsto study the conjoint effects of S2minus Clminus and SO
4
2minus alongwith variation of pH and temperature on carbon steel Thecorrosive species included are sulfate chloride hydrogensulfide temperature and pH Sulphate and chloride wereadded asNa
2SO4andNaCl Hydrogen sulfidewas introduced
to the corrosion cell with the following reaction
Na2S +H
2SO4= H2S + FeSO
4 (1)
The effect on corrosion of these species was exam-ined through polarization experimentation using a three-electrode glass corrosion cell and potentiostat Electrochemi-cal AC impedance spectroscopy studies were also carried outfor better understanding of electrochemical effects of corro-sive species on electrical phenomenon occurring at metal-solution interface The corroded and uncorroded substrateswere characterized by XRDThemorphology of the corrodedsurface was investigated by optical microscopy and SEM
2 Experimental Methods
21 Polarization Studies Electrochemical measurementswere conducted using Gamry Potentiostat instrumentcoupled with Echem analyst software controlled by apersonal computer in a conventional three-electrode cellsystemsThe working electrode was carbon steel the counterelectrode was graphite and a saturated calomel electrode(SCE) acted as the reference electrode Experiments wereperformed in different concentrations of Clminus SO
4
2minus and S2minussolutions at preselected pH and temperature to determinethe corrosion potential 119864corr and corrosion current 119894corr Thepotential was scanned between minus15 V and 1V at a scan rateof 1mVs
22 Electrochemical Impedance Spectroscopy (EIS) Theexperimental arrangement was the same as that of polariza-tion studies The electrochemical cell was connected to animpedance analyzer (EIS300 controlled by Echem analystsoftware) for electrochemical impedance spectroscopy Theelectrochemical impedance spectrawere obtained at frequen-cies between 300 kHz and001HzTheamplitude of the sinus-oidal wave was 10mV The following results and informationare obtained from the EIS experiments Polarization resist-ance (119877
119901) electrolyte resistance (119877
119906) double layer capaci-
tance (119862dl) capacitive load or constant phase elementCPE(119884) and 120572 which is defined from the capacitive impe-dance equation 119885 = 1119862(119895119908)minus120572
Capacitors in EIS experiments often do not behaveideally Instead they act like a constant phase element (CPE)The exponent 120572 = 1 for pure capacitance For a constant
International Journal of Metals 3
phase element the exponent 120572 is less than one The ldquodoublelayer capacitorrdquo on real cells often behaves like a CPE insteadof like a pure capacitor
23 X-Ray Diffraction (XRD) Analysis The X-ray diffractiontechnique is used to define the crystalline structure and thecrystalline phases This test was done using a Rigaku UltimaIII X-Ray Diffractometer for recording the diffraction tracesof the samples with monochromatized Cu K
120572radiation at
room temperature the scan region (2120579) ranged from 10∘ to100∘ at a scan rate of 5∘minminus1
24 Scanning Electron Microscope (SEM) Morphology Theelectron micrographs were studied by SEM with acceleratingvoltage 30 kV magnification up to 300000x and resolutionof 35 nm The images of the corroded samples were pho-tographed at low and high magnification
3 Results and Discussions
The effects of Clminus SO4
2minus S2minus pH and temperature ondegradation behavior of carbon steel were studied by poten-tiostatic polarization to determine corrosion current andcorrosion potential The various electrical properties at themetal-solution interface were determined by electrochemicalimpedance spectroscopy (EIS) The presence of differentelements on corroded surface was detected by XRD Themorphology of the degraded surfaces was characterized byoptical microscopy and SEM Before going into the experi-mental findings of the effects of different interfering ions it isworthwhile to discuss the basic electrochemical reactions ofaqueous corrosion of steel in the presence of those ions
The main electrochemical anodic and cathodic reactionsfor the corrosion of carbon steel in aqueous oil fields environ-ments in presences of the ions are as follows
Half Cell Reactions (E versus SCE) Consider
Fe = Fe2+ + 2e (1198640 = minus681) (2)
O2+ 2H2O + 4eminus 997888rarr 4OHminus (1198640 = 0579) (3)
H + e = 12
H2(1198640= minus0241) (4)
In presence of SO4
2minus present reactions are
SO4
2minus+ 2e +H
2O = 2SO
3
2minus+OH (1198640 = minus1177) (5)
2SO3
2minus+ 4e + 3H
2O = 3S
2O3
2minus+ 6OHminus (1198640 = minus099)
(6)
2SO3
2minus+ 4e + 3H
2O = S + 6OHminus (7)
S + 2eminus = S2minus (1198640 = minus0688) (8)
Current density (Acm2)
150
100
050
000
minus050
minus100
minus150
01N Na2SO4
02N Na2SO4
03N Na2SO4
Pote
ntia
l (V
ver
sus S
CE)
1E minus 09 1E minus 07 1E minus 05 1E minus 03 1E minus 01
Figure 1 Potentiodynamic polarization curves of low carbonsteel in different concentration of Na
2SO4solution at pH 6 and
temperature 25∘C
In presence of Na2S or H
2S (Na2S was added in state
of H2S to understand the effect of s2minus)
H2S = H+ +HSminus (9)
HSminus = H+ + Sminus (1198640 = minus0688) (10)
In the presence of Clminus that is (NaClHCl)
Fe2+ + 2Clminus = FeCl2
(11)
FeCl2+ 2H2O = Fe (OH)
2+ 2H+ + 2Clminus (12)
FeCl2+ Clminus = FeCl
3+ e (13)
Fe + 2H+ = Fe2+ (14)
The above equations would help in better understandingof the effects of the corrosion species found in the experimen-tal results
31 Polarization Studies
311 Effect of SO4
2minus Figure 1 shows the potential dynamicpolarization curvewith increasing SO
4
2minus concentration at pH6 The pH of sea water normally varies from 75 to 82 butin the oil fields due to the presence of few acidic substancesnamely carbonic acid H
2S and other organic acids pH may
shift from near neutral towards the acidic side between 6and 4 It is seen here that the corrosion rate increases withincrease in concentration of SO
4
2minus It is seen from (5)ndash(8) that
4 International Journal of Metals
0
05
1
15
000001 001
Pote
ntia
l (V
ver
sus S
CE)
Current density (Acm2)
minus1
minus15
minus05
1E minus 11 1E minus 08
Effect of Na2S
025M Na2SO4
025M Na2SO4 + 01M Na2S025M Na2SO4 + 03M Na2S025M Na2SO4 + 05M Na2S
Figure 2 Effect of S2minus on potentiodynamic polarization curves ofcarbon steel in 025M Na
2SO4solution at pH 6 and temperature
25∘C
the cathodic reduction of SO4
2minus ions produces thiosulfate andsulphide ions both of which are aggressive corrosion speciesand hence degrade the steel surface in sea water in the acidicpH range below the neutral medium The corrosion productin this case should be iron sulphide or thiosulfate
312 Effect of SO4
2minus + S2minus Addition of S2minus to the solutioncontaining SO
4
2minus further enhances the corrosion rate as canbe seen from Figure 2 And the pH has a strong effect on it Itcan be seen from (11) and (12) above that the conjoint effectof SO4
2minus and Sminus is the production of increasing amount of Sminusas well as corrosive sulphur compound
313 Effect of SO4
2minus +Clminus Corrosion rate also increaseswithaddition of Clminus to the solution containing SO
4
2minus (Figure 3)The rate increases with increase in Clminus concentration It isseen from (11)ndash(14) above that theClminus ions attack FeFe2+withthe formation of corrosion products FeCl
2and FeCl
3 FeCl
2
is unstable and may hydrolyse or further react with Clminus ionsto form Fe(OH)
2or FeCl
3 respectively
314 Effect of SO4
2minus + S2minus + Clminus The conjugate of all 3 ionswhich are normally present in the oil field sea water is thedegradation of carbon steel structure at the highest level Itis seen from Figure 4 that 119894corr values have shifted to theright and 119864corr towards the active potential with increasingthe concentration of both S2minus and Clminus
315 Effect of pH There is not any much significant effect ofpolarization curves with change in pH except at pH 11 underalkaline condition (Figure 5) when the corrosion rate is verylow The steel is in the passive region at this pH The pH of
0
Current density (Acm2)
025M Na2SO4
Effect of NaCl
minus02
minus04
minus06
minus08
minus1
minus12
Pote
ntia
l (V
ver
sus S
CE)
025M Na2SO4 + 01N NaCl025M Na2SO4 + 03N NaCl025M Na2SO4 + 05N NaCl
1E minus 09 1E minus 06 1E minus 03 1E + 00
Figure 3 Effect of Clminus on potentiodynamic polarization curves ofcarbon steel in 025M Na
2SO4solution at pH 8 and temperature
25∘C
0
0000001 00001 001 1Current density (Acm2)
1E minus 08
minus01
minus02
minus03
minus04
minus05
minus06
minus07
minus08
minus09
minus1
Pote
ntia
l (V
ver
sus S
CE)
025M Na2SO4
025M Na2SO4 + 03M Na2S + 03N NaCl025M Na2SO4 + 03M Na2S + 01N NaCl025M Na2SO4 + 03M Na2S025M Na2SO4 + 03M Na2S + 05N NaCl
Figure 4 Effect of Clminus + S2minus on potentiodynamic polarizationcurves of carbon steel in 025M Na
2SO4solution at pH 8 and
temperature 25∘C
the oil field water is in the range of 4ndash6 when the corrosionrate is high
316 Effect of Temperature Temperature aggravates thematerial degradation (Figure 6) by increasing diffusion andmass transfer coefficient of the aggressive ions corroding
International Journal of Metals 5
0
05
1
15
minus1
minus15
minus05
Current density (Acm2)
pH 11pH 6pH 8
Pote
ntia
l (V
ver
sus S
CE)
Effect of temperature of 025M Na2SO4
1E minus 09 1E minus 07 1E minus 05 1E minus 03 1E minus 01
Figure 5 Potentiodynamic polarization curves of carbon steel in an aqueous solution of 025MNa2SO4and 03MNa
2S at temperature 25∘C
at different pH
0
05
1
15
0001 01
Pote
ntia
l (V
ver
sus S
CE)
minus1
minus15
minus05
Current density (Acm2)
1E minus 11 1E minus 09 1E minus 07 1E minus 05
Effect of temperature of 025M Na2SO4
Temp 25∘C 025M Na2SO4
Temp 70∘C 025M Na2SO4
Temp 45∘C 025M Na2SO4
Figure 6 Potentiodynamic polarization curves of low carbon steel in 025M Na2SO4solution at (pH = 8) at different temperature
the metallic surface Temperature also increases the 119868119871 the
limiting current density of the concentration polarizationand hence shifts the polarization curves to the right
32 Electrochemical Impedance Spectroscopy (EIS) The phe-nomenon at the interface of the solidmetal and aqueous elec-trolyte is a complex process consisting of a line of positively
and negatively charged ions capacitance due to double layercorroded product or film formation on surfaces polarizationresistance (119877
119901) pore resistance (119877po) and various types of
impedance due to diffusion of ions movement of charge in oraway frommetal surface and adsorption of cation and anionThe whole phenomenon can be represented by an equivalentAC electrical circuit The phenomenon can be interpreted
6 International Journal of Metals
Zm
od(o
hm)
1000
1000
1000
1000
Hz
1000
Hz
1000
Hz
1000
kHz
1000
kHz
1000
kHz
Z curve (025m Na2SO4)Y2-Z curve (025m Na2SO4)
Fit 1 [CPE with diffusion]Y2-fit 1 [CPE with diffusion]
Frequency (Hz)
Y2-Z
phz(∘)
0000
minus2000
minus4000
minus6000
In 025M Na2SO4
Bode plot
Impedance
Phase angle
025M SO42minus
Ru
RE
Rp Wd
WE
Y0120572
empty
CPE with diffusionequivalent circuit
1000
mH
z
1000
mH
z
1000
mH
z
1000
mH
z
1000000
100000
(a)
Zm
od(o
hm)
1000
Hz
1000
Hz
1000
Hz
1000
kHz
1000
kHz
1000
kHz
Fit 1 [CPE with diffusion]Y2-fit 1 [CPE with diffusion]
Frequency (Hz)
Y2-Z
phz(∘)
0000
minus2000
minus4000
minus6000
Bode plot
Impedance
Phase angle
minus8000
In 025M Na2SO4 + 01M Na2S
Z curve (025m Na2SO4 + 01m Na2S)Y2-Z curve (025m Na2SO4 + 01m Na2S)
025M SO42minus + 01M S2minus
Ru
RE
Rp Wd
WE
Y0120572
empty
CPE with diffusionequivalent circuit
1000
1000
1000
1000
mH
z
1000
mH
z
1000
mH
z
1000
mH
z
1000000
100000
(b)
Zm
od(o
hm)
1000
1000
1000
Hz
1000
Hz
1000
Hz
1000
kHz
1000
kHz
1000
kHz
Fit 1 [CPE with diffusion]Y2-fit 1 [CPE with diffusion]
Frequency (Hz)
Y2-Z
phz(∘)
0000
minus2000
minus3000
minus4000
minus1000
Bode plot
Impedance Phase angle
025M Na2SO4 + 01M Na2S + 01N NaCl
Z curve (025m Na2SO4 + 01M Na2S + 01N NaCl)Y2-Z curve (025m Na2SO4 + 01M Na2S + 01N NaCl)
025M SO42minus + 01M S2minus + 01M Clminus
Ru
RE
Rp Wd
WE
Y0120572
empty
CPE with diffusionequivalent circuit
1000
mH
z
1000
mH
z
1000
mH
z
1000
mH
z
(c)
Figure 7 Bode plots of carbon steel in SO4
2minus SO4
2minus + S2minus and SO4
2minus + S2minus + Clminus solution at pH 8 and temperature 25∘C
from the Bode plots which are depicted and discussed in thefollowing section for various corrosive species
Figure 7 displays the Bode plots of carbon steel insolutions of SO
4
2minus SO4
2minus + S2minus and SO4
2minus + S2minus + Clminus Itis to be noticed that in all the solutions EIS data matchthe model of Randle circuit with a Warburg resistance 119882
119889
(given in the inset of each figure) that prevails at the metal-solution interface It is seen that the impedance decreaseswith addition of different aggressive ions compared to thosewith base solution of only SO
4
2minus This decrease in impedanceleads to more current flow across the interface and henceincrease in corrosion rate There is a phase angle shift with
International Journal of Metals 7
Inte
nsity
(cps
)
2120579 (deg)
Fe (110)
Fe (200)Fe (211)
1000
800
600
400
200
040 60 80 100 120
Carbon steel before corrosion
(a)
Inte
nsity
(cps
)
2120579 (deg)
Fe (110) Corroded in Na2SO4 + Na2S
FeS (003)
FeS (003)
FeS (113)
800
700
600
500
400
300
200
100
030 40 50 60 70 80 90 100 110
(b)
Inte
nsity
(cps
)
2120579 (deg)
1000
800
600
400
200
0
40 60 80 100 120
Fe (110)
FeCl3
FeCl3
Corroded in Na2SO4 + NaCl
(c)
Inte
nsity
(cps
)
2120579 (deg)
Fe (110)
FeS (003)
500
400
300
200
100
0
40 50 60 70 80 90 100 110
FeCl3
Corroded in Na2SO4 + Na2S + NaCl
(113)
(d)
Figure 8 X-ray diffraction showing peaks of different corroded phases
frequency The minimum phase angle reaches much lessthan 90 degrees indicating the capacitance in the circuit isnot a pure capacitance but constant phase element CPE(119884)and is given by the capacitive impedance equation 119885 =1119862(119895119908)
minus120572 where 120572 is fraction varying from 0 to 1 the valueless than one indicates that it does not behave ideally as purecapacitance At high frequency the value of impedance119885 modis roughly equal to 119877
119878 while at low frequency the value is
(119877119878+ 119877119875) Both can be determined from the blots Table 1
depicted the computed values of the EIS parameters It isseen that polarization resistance decreases with addition ofmore types of ions which support the polarization resultsof corrosion rates increase as found in Figures 2 3 and 4The increase in corrosion rate is supported by the EIS dataincrease in119884
0which behaves like capacitance and decrease in
polarization resistance (Table 2)The values of 120572 indicate thatthe capacitance behaves like constant phase element ratherthan pure capacitance The Warburg resistance 119882
119889which
signifies resistance to the flow of ions from the solution tocorroded metal surface also decreases with addition of moretypes of ions All the facts confirm the enhanced corrosionrates of carbon steel in the oil field sea water with presence ofSO4
2minus + S2minus + Clminus
33 X-Ray Diffraction The presence of corrosion productsof FeS FeCl
2 and FeCl
3is clearly indicated by the XRD
peak intensities in Figure 8 This supports the corrosionenhancement by SO
4
2minus S2minus and Clminus ions as found inpolarization and EIS studies
8 International Journal of Metals
Table 2 Computed EIS parameters
Sample Corrosive ions 119877119904(ohm) 119877
119901(ohm) 119884
0120583F 120572 119882
119889(ohm)
Carbon steel SO42minus 9443 85000 750 0720 10430
Carbon steel SO42minus + S2minus 8268 3700 5722 0769 4187
Carbon steel SO42minus + S2minus + Clminus 1331 95 9056 0602 2432
(c)
(b)(a)
Figure 9 Optical microscopy images of corroded steel (a) Na2SO4 (b) Na
2SO4+ Na2S and (c) Na
2SO4+ Na2S + NaCl
34 Optical and SEM Microscopy Images The microstruc-tures by optical microscopy clearly show (Figure 9) thecorroded structure of steel with products of corrosion overit The form of corrosion seems to be uniform not localizedSEM images (Figure 10) distinctly reveal how the degree ofdegradation increases with presence of ions in the solutionsfrom SO
4
2minus to SO4
2minus + S2minus to SO4
2minus + S2minus + Clminus It isinteresting to observe from the morphology of SEM image athigher magnification (Figure 11) that the corrosion has takenplace at grain boundary as well as grain body but the attack atgrain boundary is very prominentwith sulphide (S) corrosioncrackThe corrosion products of sulphide compounds as wellas element sulphur are clearly revealedThe structure of SEMimages shows almost catastrophic failure with the presenceof minor and major cracks when the seawater is enrichedwith the presence of all three corrosive ions SO
4
2minus S2minusand Clminus The morphology of SEM image analysis along with
XRD is in complete agreement with the corrosion data of thepolarization experiments and EIS
From (5)ndash(8) it is seen that SO4
2minus is cathodically reducedto give rise to S2minus It is observed from the polarizationcurve (Figure 1) that the increase of SO
4
2minus concentrationdepolarizes the cathodic curves much more compared toanodic ones shifting 119864corr towards the negative potentialThe release of S2minus from the reaction produces corrosionproducts iron sulfides (FeS
119909) on carbon steel surface They
are nonstoichiometric iron sulfide films mainly composed ofmackinawite and pyrrhotite Berner [21ndash26] The black colorof mackinawite phase is also seen as corrosion products inoptical microstructures (Figures 9(a) and 9(b))
The deterioration of metal due to contact with S2minus (H2S)
and moisture is sour corrosion The forms of sour corrosionare uniform (Figure 9(b)) pitting and stepwise cracking(Figures 10(c) and 11(b))
International Journal of Metals 9
(d)(c)
(b)(a)
Figure 10 SEM images of corroded steel (a) before corrosion (b) in Na2SO4 (c) in Na
2SO4+ Na2S and (d) in Na
2SO4+ Na2S + NaCl
The general equation of sour corrosion can be expressedas follows [21]
S2minus + 2H+ = H2S (aq) 119864 = 0097V versus SCE (15)
Hydrogen sulphide dissociates to produce proton andbisulphide as described [22] by the following equations
H2S (aq) = H+ +HSminus 119870 = 91 times 10minus8
HSminus = H+ + S2minus 119870 = 91 times 10minus12(16)
This reaction scheme shows that the presence of hydrogensulfide can contribute to the concentration of sulfide at thesurface by dissociation rather than charge exchange with thesurface The sulfide concentration at the surface is thereforedependent on the concentration of aqueous hydrogen sulfidein the electrolyte as well as the reduction processes of sulfate
H2S exhibits the different role in anodic process of carbon
steel depending on the pH value in the solutions The localsupersaturation of FeS
119909could be formed on the carbon steel
surface via the following reaction with the nucleation andgrowth of one or more of the iron sulfide mackinawite
H2S + Fe +H
2O 997888rarr FeS
119909+ 2H +H
2O (17)
The black corrosion products (FeS119909) formed on the steel
surface in the H2S-containing solutions could be observed
(Figure 9(b)) The addition of chloride to the corrosionsystem did not exhibit any localized corrosion rather it dras-tically increased the 119894corr values and shifted 119864corr towards thenegative potential (Figures 3 and 4) depolarizing the anodicreactionsThe increase in 119894corr with Cl addition seems to havemodified the anodic substrate area (Figures 9(c) and 10(d))and has enhanced the exchange current density for the ironoxidation process
4 Conclusion
All three corrosive ions SO4
2minus S2minus and Clminus have a strongeffect on increasing the corrosion rate of carbon steel
Cathodic reduction of SO4
2minus generates elemental S orS2minus ions in addition to S2minus from H
2S in oil The species
10 International Journal of Metals
(c)
(b)(a)
Figure 11 SEM images of corroded steel (a) in Na2SO4 (b) in Na
2SO4+ Na2S and (c) in Na
2SO4+ Na2S + NaCl
cause major corrosion with formation of FeS119909as corrosion
productsThe attack is found at grain boundarywith sulphidecracking as well as some grain body degradation The effectof addition of Clminus is the increase of 119894corr values by hundredsof times possibly due to enhancement of exchange currentdensity of the anodic reactions
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgments
The authors would like to acknowledge ldquoCOE TEQUIPrdquo inJadavpur University for the support of this work
References
[1] S Nesic ldquoKey issues related to modelling of internal corrosionof oil and gas pipelinesmdasha reviewrdquo Corrosion Science vol 49no 12 pp 4308ndash4338 2007
[2] L T Popoola A S Grema G K Latinwo B Gutti and A SBalogun ldquoCorrosion problems during oil and gas productionand its mitigationrdquo International Journal of Industrial Chem-istry vol 4 aticle 35 2013
[3] BW A Sherar P G Keech andDW Shoesmith ldquoThe effect ofsulfide on the aerobic corrosion of carbon steel in near-neutral
pH saline solutionsrdquo Corrosion Science vol 66 pp 256ndash2622013
[4] B W A Sherar I M Power P G Keech S Mitlin G Southamand D W Shoesmith ldquoCharacterizing the effect of carbon steelexposure in sulfide containing solutions to microbially inducedcorrosionrdquo Corrosion Science vol 53 no 3 pp 955ndash960 2011
[5] F M Song ldquoA comprehensive model for predicting CO2
corrosion rate in oil and gas production and transportationsystemsrdquo Electrochimica Acta vol 55 no 3 pp 689ndash700 2010
[6] M Hairil Mohd and J K Paik ldquoInvestigation of the corrosionprogress characteristics of offshore subsea oil well tubesrdquoCorrosion Science vol 67 pp 130ndash141 2013
[7] J Tang Y Shao T Zhang G Meng and F Wang ldquoCorrosionbehaviour of carbon steel in different concentrations of HClsolutions containing H
2S at 90 ∘Crdquo Corrosion Science vol 53
no 5 pp 1715ndash1723 2011[8] Z A D Foroulis ldquoRole of solution pH on wet H
2S cracking
in hydrocarbon productionrdquo Corrosion Prevention and Controlvol 40 no 4 pp 84ndash89 1993
[9] M A Veloz and I Gonzalez ldquoElectrochemical study of carbonsteel corrosion in buffered acetic acid solutions with chloridesand H
2Srdquo Electrochimica Acta vol 48 no 2 pp 135ndash144 2002
[10] S Arzola and J Genesca ldquoThe effect of H2S concentration on
the corrosion behavior of API 5L X-70 steelrdquo Journal of SolidState Electrochemistry vol 9 no 4 pp 197ndash200 2005
[11] H YMa X L Cheng S H Chen et al ldquoTheoretical interpreta-tion on impedance spectra for anodic iron dissolution in acidicsolutions containing hydrogen sulfiderdquoCorrosion vol 54 no 8pp 634ndash640 1998
International Journal of Metals 11
[12] H Ma X Cheng S Chen C Wang J Zhang and H YangldquoAn ac impedance study of the anodic dissolution of iron insulfuric acid solutions containing hydrogen sulfiderdquo Journal ofElectroanalytical Chemistry vol 451 no 1-2 pp 11ndash17 1998
[13] HMa X Cheng G Li et al ldquoThe influence of hydrogen sulfideon corrosion of iron under different conditionsrdquo CorrosionScience vol 42 no 10 pp 1669ndash1683 2000
[14] H-H HuangW-T Tsai and J-T Lee ldquoElectrochemical behav-ior of the simulated heat-affected zone of A516 carbon steel inH2S solutionrdquo Electrochimica Acta vol 41 no 7-8 pp 1191ndash1199
1996[15] J Tang Y Shao J Guo T Zhang G Meng and F Wang ldquoThe
effect of H2S concentration on the corrosion behavior of carbon
steel at 90 ∘Crdquo Corrosion Science vol 52 no 6 pp 2050ndash20582010
[16] P Smith S Roy D Swailes S Maxwell D Page and J LawsonldquoA model for the corrosion of steel subjected to syntheticproduced water containing sulfate chloride and hydrogensulfiderdquo Chemical Engineering Science vol 66 no 23 pp 5775ndash5790 2011
[17] R E Melchers ldquoDevelopment of new applied models for steelcorrosion in marine applications including shippingrdquo Ships andOffshore Structures vol 3 no 2 pp 135ndash144 2008
[18] M H Mohd D K Kim D W Kim and J K Paik ldquoA time-variant corrosion wastage model for subsea gas pipelinesrdquo Shipsand Offshore Structures vol 9 no 2 pp 161ndash176 2014
[19] R E Melchers and J K Paik ldquoEffect of flexure on rusting ofshiprsquos steel platingrdquo Ships and Offshore Structures vol 5 no 1pp 25ndash31 2010
[20] M Shehadeh and I Hassan ldquoStudy of sacrificial cathodicprotection on marine structures in sea and fresh water inrelation to flow conditionsrdquo Ships and Offshore Structures vol8 no 1 pp 102ndash110 2013
[21] R A Berner ldquoThermodynamic stability of sedimentary ironsulfidesrdquoTheAmerican Journal of Science vol 265 pp 773ndash7851967
[22] R H Hausler L A Goeller R P Zimmerman and R HRosenwald ldquoContribution to the ldquofilming aminerdquo theory aninterpretation of experimental resultsrdquo Corrosion vol 28 no1 pp 7ndash16 1972
[23] P Taylor ldquoThe stereochemistry of iron sulfidesmdasha structuralration for the crysta llization of some metastable phases fromaqueous solutionrdquo American Mineralogist vol 65 pp 1026ndash1030 1980
[24] R A Berner ldquoIron sulfides formed from aqueous solution atlow temperatures and atmospheric pressurerdquo The Journal ofGeology vol 72 pp 293ndash306 1964
[25] D T Rickard ldquoThe chemistry of iron sulphide formation at lowtemperaturesrdquo Stockholm Contributions in Geology vol 20 pp67ndash95 1969
[26] A J Bard R Parsons and J Jordan Standard Potentials inAqueous Solution CRC Press 1st edition 1985
Submit your manuscripts athttpwwwhindawicom
ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CorrosionInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Polymer ScienceInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CeramicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CompositesJournal of
NanoparticlesJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of
Biomaterials
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
NanoscienceJournal of
TextilesHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Journal of
NanotechnologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
CrystallographyJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CoatingsJournal of
Advances in
Materials Science and EngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Smart Materials Research
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
MetallurgyJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
BioMed Research International
MaterialsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Nano
materials
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal ofNanomaterials
International Journal of Metals 3
phase element the exponent 120572 is less than one The ldquodoublelayer capacitorrdquo on real cells often behaves like a CPE insteadof like a pure capacitor
23 X-Ray Diffraction (XRD) Analysis The X-ray diffractiontechnique is used to define the crystalline structure and thecrystalline phases This test was done using a Rigaku UltimaIII X-Ray Diffractometer for recording the diffraction tracesof the samples with monochromatized Cu K
120572radiation at
room temperature the scan region (2120579) ranged from 10∘ to100∘ at a scan rate of 5∘minminus1
24 Scanning Electron Microscope (SEM) Morphology Theelectron micrographs were studied by SEM with acceleratingvoltage 30 kV magnification up to 300000x and resolutionof 35 nm The images of the corroded samples were pho-tographed at low and high magnification
3 Results and Discussions
The effects of Clminus SO4
2minus S2minus pH and temperature ondegradation behavior of carbon steel were studied by poten-tiostatic polarization to determine corrosion current andcorrosion potential The various electrical properties at themetal-solution interface were determined by electrochemicalimpedance spectroscopy (EIS) The presence of differentelements on corroded surface was detected by XRD Themorphology of the degraded surfaces was characterized byoptical microscopy and SEM Before going into the experi-mental findings of the effects of different interfering ions it isworthwhile to discuss the basic electrochemical reactions ofaqueous corrosion of steel in the presence of those ions
The main electrochemical anodic and cathodic reactionsfor the corrosion of carbon steel in aqueous oil fields environ-ments in presences of the ions are as follows
Half Cell Reactions (E versus SCE) Consider
Fe = Fe2+ + 2e (1198640 = minus681) (2)
O2+ 2H2O + 4eminus 997888rarr 4OHminus (1198640 = 0579) (3)
H + e = 12
H2(1198640= minus0241) (4)
In presence of SO4
2minus present reactions are
SO4
2minus+ 2e +H
2O = 2SO
3
2minus+OH (1198640 = minus1177) (5)
2SO3
2minus+ 4e + 3H
2O = 3S
2O3
2minus+ 6OHminus (1198640 = minus099)
(6)
2SO3
2minus+ 4e + 3H
2O = S + 6OHminus (7)
S + 2eminus = S2minus (1198640 = minus0688) (8)
Current density (Acm2)
150
100
050
000
minus050
minus100
minus150
01N Na2SO4
02N Na2SO4
03N Na2SO4
Pote
ntia
l (V
ver
sus S
CE)
1E minus 09 1E minus 07 1E minus 05 1E minus 03 1E minus 01
Figure 1 Potentiodynamic polarization curves of low carbonsteel in different concentration of Na
2SO4solution at pH 6 and
temperature 25∘C
In presence of Na2S or H
2S (Na2S was added in state
of H2S to understand the effect of s2minus)
H2S = H+ +HSminus (9)
HSminus = H+ + Sminus (1198640 = minus0688) (10)
In the presence of Clminus that is (NaClHCl)
Fe2+ + 2Clminus = FeCl2
(11)
FeCl2+ 2H2O = Fe (OH)
2+ 2H+ + 2Clminus (12)
FeCl2+ Clminus = FeCl
3+ e (13)
Fe + 2H+ = Fe2+ (14)
The above equations would help in better understandingof the effects of the corrosion species found in the experimen-tal results
31 Polarization Studies
311 Effect of SO4
2minus Figure 1 shows the potential dynamicpolarization curvewith increasing SO
4
2minus concentration at pH6 The pH of sea water normally varies from 75 to 82 butin the oil fields due to the presence of few acidic substancesnamely carbonic acid H
2S and other organic acids pH may
shift from near neutral towards the acidic side between 6and 4 It is seen here that the corrosion rate increases withincrease in concentration of SO
4
2minus It is seen from (5)ndash(8) that
4 International Journal of Metals
0
05
1
15
000001 001
Pote
ntia
l (V
ver
sus S
CE)
Current density (Acm2)
minus1
minus15
minus05
1E minus 11 1E minus 08
Effect of Na2S
025M Na2SO4
025M Na2SO4 + 01M Na2S025M Na2SO4 + 03M Na2S025M Na2SO4 + 05M Na2S
Figure 2 Effect of S2minus on potentiodynamic polarization curves ofcarbon steel in 025M Na
2SO4solution at pH 6 and temperature
25∘C
the cathodic reduction of SO4
2minus ions produces thiosulfate andsulphide ions both of which are aggressive corrosion speciesand hence degrade the steel surface in sea water in the acidicpH range below the neutral medium The corrosion productin this case should be iron sulphide or thiosulfate
312 Effect of SO4
2minus + S2minus Addition of S2minus to the solutioncontaining SO
4
2minus further enhances the corrosion rate as canbe seen from Figure 2 And the pH has a strong effect on it Itcan be seen from (11) and (12) above that the conjoint effectof SO4
2minus and Sminus is the production of increasing amount of Sminusas well as corrosive sulphur compound
313 Effect of SO4
2minus +Clminus Corrosion rate also increaseswithaddition of Clminus to the solution containing SO
4
2minus (Figure 3)The rate increases with increase in Clminus concentration It isseen from (11)ndash(14) above that theClminus ions attack FeFe2+withthe formation of corrosion products FeCl
2and FeCl
3 FeCl
2
is unstable and may hydrolyse or further react with Clminus ionsto form Fe(OH)
2or FeCl
3 respectively
314 Effect of SO4
2minus + S2minus + Clminus The conjugate of all 3 ionswhich are normally present in the oil field sea water is thedegradation of carbon steel structure at the highest level Itis seen from Figure 4 that 119894corr values have shifted to theright and 119864corr towards the active potential with increasingthe concentration of both S2minus and Clminus
315 Effect of pH There is not any much significant effect ofpolarization curves with change in pH except at pH 11 underalkaline condition (Figure 5) when the corrosion rate is verylow The steel is in the passive region at this pH The pH of
0
Current density (Acm2)
025M Na2SO4
Effect of NaCl
minus02
minus04
minus06
minus08
minus1
minus12
Pote
ntia
l (V
ver
sus S
CE)
025M Na2SO4 + 01N NaCl025M Na2SO4 + 03N NaCl025M Na2SO4 + 05N NaCl
1E minus 09 1E minus 06 1E minus 03 1E + 00
Figure 3 Effect of Clminus on potentiodynamic polarization curves ofcarbon steel in 025M Na
2SO4solution at pH 8 and temperature
25∘C
0
0000001 00001 001 1Current density (Acm2)
1E minus 08
minus01
minus02
minus03
minus04
minus05
minus06
minus07
minus08
minus09
minus1
Pote
ntia
l (V
ver
sus S
CE)
025M Na2SO4
025M Na2SO4 + 03M Na2S + 03N NaCl025M Na2SO4 + 03M Na2S + 01N NaCl025M Na2SO4 + 03M Na2S025M Na2SO4 + 03M Na2S + 05N NaCl
Figure 4 Effect of Clminus + S2minus on potentiodynamic polarizationcurves of carbon steel in 025M Na
2SO4solution at pH 8 and
temperature 25∘C
the oil field water is in the range of 4ndash6 when the corrosionrate is high
316 Effect of Temperature Temperature aggravates thematerial degradation (Figure 6) by increasing diffusion andmass transfer coefficient of the aggressive ions corroding
International Journal of Metals 5
0
05
1
15
minus1
minus15
minus05
Current density (Acm2)
pH 11pH 6pH 8
Pote
ntia
l (V
ver
sus S
CE)
Effect of temperature of 025M Na2SO4
1E minus 09 1E minus 07 1E minus 05 1E minus 03 1E minus 01
Figure 5 Potentiodynamic polarization curves of carbon steel in an aqueous solution of 025MNa2SO4and 03MNa
2S at temperature 25∘C
at different pH
0
05
1
15
0001 01
Pote
ntia
l (V
ver
sus S
CE)
minus1
minus15
minus05
Current density (Acm2)
1E minus 11 1E minus 09 1E minus 07 1E minus 05
Effect of temperature of 025M Na2SO4
Temp 25∘C 025M Na2SO4
Temp 70∘C 025M Na2SO4
Temp 45∘C 025M Na2SO4
Figure 6 Potentiodynamic polarization curves of low carbon steel in 025M Na2SO4solution at (pH = 8) at different temperature
the metallic surface Temperature also increases the 119868119871 the
limiting current density of the concentration polarizationand hence shifts the polarization curves to the right
32 Electrochemical Impedance Spectroscopy (EIS) The phe-nomenon at the interface of the solidmetal and aqueous elec-trolyte is a complex process consisting of a line of positively
and negatively charged ions capacitance due to double layercorroded product or film formation on surfaces polarizationresistance (119877
119901) pore resistance (119877po) and various types of
impedance due to diffusion of ions movement of charge in oraway frommetal surface and adsorption of cation and anionThe whole phenomenon can be represented by an equivalentAC electrical circuit The phenomenon can be interpreted
6 International Journal of Metals
Zm
od(o
hm)
1000
1000
1000
1000
Hz
1000
Hz
1000
Hz
1000
kHz
1000
kHz
1000
kHz
Z curve (025m Na2SO4)Y2-Z curve (025m Na2SO4)
Fit 1 [CPE with diffusion]Y2-fit 1 [CPE with diffusion]
Frequency (Hz)
Y2-Z
phz(∘)
0000
minus2000
minus4000
minus6000
In 025M Na2SO4
Bode plot
Impedance
Phase angle
025M SO42minus
Ru
RE
Rp Wd
WE
Y0120572
empty
CPE with diffusionequivalent circuit
1000
mH
z
1000
mH
z
1000
mH
z
1000
mH
z
1000000
100000
(a)
Zm
od(o
hm)
1000
Hz
1000
Hz
1000
Hz
1000
kHz
1000
kHz
1000
kHz
Fit 1 [CPE with diffusion]Y2-fit 1 [CPE with diffusion]
Frequency (Hz)
Y2-Z
phz(∘)
0000
minus2000
minus4000
minus6000
Bode plot
Impedance
Phase angle
minus8000
In 025M Na2SO4 + 01M Na2S
Z curve (025m Na2SO4 + 01m Na2S)Y2-Z curve (025m Na2SO4 + 01m Na2S)
025M SO42minus + 01M S2minus
Ru
RE
Rp Wd
WE
Y0120572
empty
CPE with diffusionequivalent circuit
1000
1000
1000
1000
mH
z
1000
mH
z
1000
mH
z
1000
mH
z
1000000
100000
(b)
Zm
od(o
hm)
1000
1000
1000
Hz
1000
Hz
1000
Hz
1000
kHz
1000
kHz
1000
kHz
Fit 1 [CPE with diffusion]Y2-fit 1 [CPE with diffusion]
Frequency (Hz)
Y2-Z
phz(∘)
0000
minus2000
minus3000
minus4000
minus1000
Bode plot
Impedance Phase angle
025M Na2SO4 + 01M Na2S + 01N NaCl
Z curve (025m Na2SO4 + 01M Na2S + 01N NaCl)Y2-Z curve (025m Na2SO4 + 01M Na2S + 01N NaCl)
025M SO42minus + 01M S2minus + 01M Clminus
Ru
RE
Rp Wd
WE
Y0120572
empty
CPE with diffusionequivalent circuit
1000
mH
z
1000
mH
z
1000
mH
z
1000
mH
z
(c)
Figure 7 Bode plots of carbon steel in SO4
2minus SO4
2minus + S2minus and SO4
2minus + S2minus + Clminus solution at pH 8 and temperature 25∘C
from the Bode plots which are depicted and discussed in thefollowing section for various corrosive species
Figure 7 displays the Bode plots of carbon steel insolutions of SO
4
2minus SO4
2minus + S2minus and SO4
2minus + S2minus + Clminus Itis to be noticed that in all the solutions EIS data matchthe model of Randle circuit with a Warburg resistance 119882
119889
(given in the inset of each figure) that prevails at the metal-solution interface It is seen that the impedance decreaseswith addition of different aggressive ions compared to thosewith base solution of only SO
4
2minus This decrease in impedanceleads to more current flow across the interface and henceincrease in corrosion rate There is a phase angle shift with
International Journal of Metals 7
Inte
nsity
(cps
)
2120579 (deg)
Fe (110)
Fe (200)Fe (211)
1000
800
600
400
200
040 60 80 100 120
Carbon steel before corrosion
(a)
Inte
nsity
(cps
)
2120579 (deg)
Fe (110) Corroded in Na2SO4 + Na2S
FeS (003)
FeS (003)
FeS (113)
800
700
600
500
400
300
200
100
030 40 50 60 70 80 90 100 110
(b)
Inte
nsity
(cps
)
2120579 (deg)
1000
800
600
400
200
0
40 60 80 100 120
Fe (110)
FeCl3
FeCl3
Corroded in Na2SO4 + NaCl
(c)
Inte
nsity
(cps
)
2120579 (deg)
Fe (110)
FeS (003)
500
400
300
200
100
0
40 50 60 70 80 90 100 110
FeCl3
Corroded in Na2SO4 + Na2S + NaCl
(113)
(d)
Figure 8 X-ray diffraction showing peaks of different corroded phases
frequency The minimum phase angle reaches much lessthan 90 degrees indicating the capacitance in the circuit isnot a pure capacitance but constant phase element CPE(119884)and is given by the capacitive impedance equation 119885 =1119862(119895119908)
minus120572 where 120572 is fraction varying from 0 to 1 the valueless than one indicates that it does not behave ideally as purecapacitance At high frequency the value of impedance119885 modis roughly equal to 119877
119878 while at low frequency the value is
(119877119878+ 119877119875) Both can be determined from the blots Table 1
depicted the computed values of the EIS parameters It isseen that polarization resistance decreases with addition ofmore types of ions which support the polarization resultsof corrosion rates increase as found in Figures 2 3 and 4The increase in corrosion rate is supported by the EIS dataincrease in119884
0which behaves like capacitance and decrease in
polarization resistance (Table 2)The values of 120572 indicate thatthe capacitance behaves like constant phase element ratherthan pure capacitance The Warburg resistance 119882
119889which
signifies resistance to the flow of ions from the solution tocorroded metal surface also decreases with addition of moretypes of ions All the facts confirm the enhanced corrosionrates of carbon steel in the oil field sea water with presence ofSO4
2minus + S2minus + Clminus
33 X-Ray Diffraction The presence of corrosion productsof FeS FeCl
2 and FeCl
3is clearly indicated by the XRD
peak intensities in Figure 8 This supports the corrosionenhancement by SO
4
2minus S2minus and Clminus ions as found inpolarization and EIS studies
8 International Journal of Metals
Table 2 Computed EIS parameters
Sample Corrosive ions 119877119904(ohm) 119877
119901(ohm) 119884
0120583F 120572 119882
119889(ohm)
Carbon steel SO42minus 9443 85000 750 0720 10430
Carbon steel SO42minus + S2minus 8268 3700 5722 0769 4187
Carbon steel SO42minus + S2minus + Clminus 1331 95 9056 0602 2432
(c)
(b)(a)
Figure 9 Optical microscopy images of corroded steel (a) Na2SO4 (b) Na
2SO4+ Na2S and (c) Na
2SO4+ Na2S + NaCl
34 Optical and SEM Microscopy Images The microstruc-tures by optical microscopy clearly show (Figure 9) thecorroded structure of steel with products of corrosion overit The form of corrosion seems to be uniform not localizedSEM images (Figure 10) distinctly reveal how the degree ofdegradation increases with presence of ions in the solutionsfrom SO
4
2minus to SO4
2minus + S2minus to SO4
2minus + S2minus + Clminus It isinteresting to observe from the morphology of SEM image athigher magnification (Figure 11) that the corrosion has takenplace at grain boundary as well as grain body but the attack atgrain boundary is very prominentwith sulphide (S) corrosioncrackThe corrosion products of sulphide compounds as wellas element sulphur are clearly revealedThe structure of SEMimages shows almost catastrophic failure with the presenceof minor and major cracks when the seawater is enrichedwith the presence of all three corrosive ions SO
4
2minus S2minusand Clminus The morphology of SEM image analysis along with
XRD is in complete agreement with the corrosion data of thepolarization experiments and EIS
From (5)ndash(8) it is seen that SO4
2minus is cathodically reducedto give rise to S2minus It is observed from the polarizationcurve (Figure 1) that the increase of SO
4
2minus concentrationdepolarizes the cathodic curves much more compared toanodic ones shifting 119864corr towards the negative potentialThe release of S2minus from the reaction produces corrosionproducts iron sulfides (FeS
119909) on carbon steel surface They
are nonstoichiometric iron sulfide films mainly composed ofmackinawite and pyrrhotite Berner [21ndash26] The black colorof mackinawite phase is also seen as corrosion products inoptical microstructures (Figures 9(a) and 9(b))
The deterioration of metal due to contact with S2minus (H2S)
and moisture is sour corrosion The forms of sour corrosionare uniform (Figure 9(b)) pitting and stepwise cracking(Figures 10(c) and 11(b))
International Journal of Metals 9
(d)(c)
(b)(a)
Figure 10 SEM images of corroded steel (a) before corrosion (b) in Na2SO4 (c) in Na
2SO4+ Na2S and (d) in Na
2SO4+ Na2S + NaCl
The general equation of sour corrosion can be expressedas follows [21]
S2minus + 2H+ = H2S (aq) 119864 = 0097V versus SCE (15)
Hydrogen sulphide dissociates to produce proton andbisulphide as described [22] by the following equations
H2S (aq) = H+ +HSminus 119870 = 91 times 10minus8
HSminus = H+ + S2minus 119870 = 91 times 10minus12(16)
This reaction scheme shows that the presence of hydrogensulfide can contribute to the concentration of sulfide at thesurface by dissociation rather than charge exchange with thesurface The sulfide concentration at the surface is thereforedependent on the concentration of aqueous hydrogen sulfidein the electrolyte as well as the reduction processes of sulfate
H2S exhibits the different role in anodic process of carbon
steel depending on the pH value in the solutions The localsupersaturation of FeS
119909could be formed on the carbon steel
surface via the following reaction with the nucleation andgrowth of one or more of the iron sulfide mackinawite
H2S + Fe +H
2O 997888rarr FeS
119909+ 2H +H
2O (17)
The black corrosion products (FeS119909) formed on the steel
surface in the H2S-containing solutions could be observed
(Figure 9(b)) The addition of chloride to the corrosionsystem did not exhibit any localized corrosion rather it dras-tically increased the 119894corr values and shifted 119864corr towards thenegative potential (Figures 3 and 4) depolarizing the anodicreactionsThe increase in 119894corr with Cl addition seems to havemodified the anodic substrate area (Figures 9(c) and 10(d))and has enhanced the exchange current density for the ironoxidation process
4 Conclusion
All three corrosive ions SO4
2minus S2minus and Clminus have a strongeffect on increasing the corrosion rate of carbon steel
Cathodic reduction of SO4
2minus generates elemental S orS2minus ions in addition to S2minus from H
2S in oil The species
10 International Journal of Metals
(c)
(b)(a)
Figure 11 SEM images of corroded steel (a) in Na2SO4 (b) in Na
2SO4+ Na2S and (c) in Na
2SO4+ Na2S + NaCl
cause major corrosion with formation of FeS119909as corrosion
productsThe attack is found at grain boundarywith sulphidecracking as well as some grain body degradation The effectof addition of Clminus is the increase of 119894corr values by hundredsof times possibly due to enhancement of exchange currentdensity of the anodic reactions
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgments
The authors would like to acknowledge ldquoCOE TEQUIPrdquo inJadavpur University for the support of this work
References
[1] S Nesic ldquoKey issues related to modelling of internal corrosionof oil and gas pipelinesmdasha reviewrdquo Corrosion Science vol 49no 12 pp 4308ndash4338 2007
[2] L T Popoola A S Grema G K Latinwo B Gutti and A SBalogun ldquoCorrosion problems during oil and gas productionand its mitigationrdquo International Journal of Industrial Chem-istry vol 4 aticle 35 2013
[3] BW A Sherar P G Keech andDW Shoesmith ldquoThe effect ofsulfide on the aerobic corrosion of carbon steel in near-neutral
pH saline solutionsrdquo Corrosion Science vol 66 pp 256ndash2622013
[4] B W A Sherar I M Power P G Keech S Mitlin G Southamand D W Shoesmith ldquoCharacterizing the effect of carbon steelexposure in sulfide containing solutions to microbially inducedcorrosionrdquo Corrosion Science vol 53 no 3 pp 955ndash960 2011
[5] F M Song ldquoA comprehensive model for predicting CO2
corrosion rate in oil and gas production and transportationsystemsrdquo Electrochimica Acta vol 55 no 3 pp 689ndash700 2010
[6] M Hairil Mohd and J K Paik ldquoInvestigation of the corrosionprogress characteristics of offshore subsea oil well tubesrdquoCorrosion Science vol 67 pp 130ndash141 2013
[7] J Tang Y Shao T Zhang G Meng and F Wang ldquoCorrosionbehaviour of carbon steel in different concentrations of HClsolutions containing H
2S at 90 ∘Crdquo Corrosion Science vol 53
no 5 pp 1715ndash1723 2011[8] Z A D Foroulis ldquoRole of solution pH on wet H
2S cracking
in hydrocarbon productionrdquo Corrosion Prevention and Controlvol 40 no 4 pp 84ndash89 1993
[9] M A Veloz and I Gonzalez ldquoElectrochemical study of carbonsteel corrosion in buffered acetic acid solutions with chloridesand H
2Srdquo Electrochimica Acta vol 48 no 2 pp 135ndash144 2002
[10] S Arzola and J Genesca ldquoThe effect of H2S concentration on
the corrosion behavior of API 5L X-70 steelrdquo Journal of SolidState Electrochemistry vol 9 no 4 pp 197ndash200 2005
[11] H YMa X L Cheng S H Chen et al ldquoTheoretical interpreta-tion on impedance spectra for anodic iron dissolution in acidicsolutions containing hydrogen sulfiderdquoCorrosion vol 54 no 8pp 634ndash640 1998
International Journal of Metals 11
[12] H Ma X Cheng S Chen C Wang J Zhang and H YangldquoAn ac impedance study of the anodic dissolution of iron insulfuric acid solutions containing hydrogen sulfiderdquo Journal ofElectroanalytical Chemistry vol 451 no 1-2 pp 11ndash17 1998
[13] HMa X Cheng G Li et al ldquoThe influence of hydrogen sulfideon corrosion of iron under different conditionsrdquo CorrosionScience vol 42 no 10 pp 1669ndash1683 2000
[14] H-H HuangW-T Tsai and J-T Lee ldquoElectrochemical behav-ior of the simulated heat-affected zone of A516 carbon steel inH2S solutionrdquo Electrochimica Acta vol 41 no 7-8 pp 1191ndash1199
1996[15] J Tang Y Shao J Guo T Zhang G Meng and F Wang ldquoThe
effect of H2S concentration on the corrosion behavior of carbon
steel at 90 ∘Crdquo Corrosion Science vol 52 no 6 pp 2050ndash20582010
[16] P Smith S Roy D Swailes S Maxwell D Page and J LawsonldquoA model for the corrosion of steel subjected to syntheticproduced water containing sulfate chloride and hydrogensulfiderdquo Chemical Engineering Science vol 66 no 23 pp 5775ndash5790 2011
[17] R E Melchers ldquoDevelopment of new applied models for steelcorrosion in marine applications including shippingrdquo Ships andOffshore Structures vol 3 no 2 pp 135ndash144 2008
[18] M H Mohd D K Kim D W Kim and J K Paik ldquoA time-variant corrosion wastage model for subsea gas pipelinesrdquo Shipsand Offshore Structures vol 9 no 2 pp 161ndash176 2014
[19] R E Melchers and J K Paik ldquoEffect of flexure on rusting ofshiprsquos steel platingrdquo Ships and Offshore Structures vol 5 no 1pp 25ndash31 2010
[20] M Shehadeh and I Hassan ldquoStudy of sacrificial cathodicprotection on marine structures in sea and fresh water inrelation to flow conditionsrdquo Ships and Offshore Structures vol8 no 1 pp 102ndash110 2013
[21] R A Berner ldquoThermodynamic stability of sedimentary ironsulfidesrdquoTheAmerican Journal of Science vol 265 pp 773ndash7851967
[22] R H Hausler L A Goeller R P Zimmerman and R HRosenwald ldquoContribution to the ldquofilming aminerdquo theory aninterpretation of experimental resultsrdquo Corrosion vol 28 no1 pp 7ndash16 1972
[23] P Taylor ldquoThe stereochemistry of iron sulfidesmdasha structuralration for the crysta llization of some metastable phases fromaqueous solutionrdquo American Mineralogist vol 65 pp 1026ndash1030 1980
[24] R A Berner ldquoIron sulfides formed from aqueous solution atlow temperatures and atmospheric pressurerdquo The Journal ofGeology vol 72 pp 293ndash306 1964
[25] D T Rickard ldquoThe chemistry of iron sulphide formation at lowtemperaturesrdquo Stockholm Contributions in Geology vol 20 pp67ndash95 1969
[26] A J Bard R Parsons and J Jordan Standard Potentials inAqueous Solution CRC Press 1st edition 1985
Submit your manuscripts athttpwwwhindawicom
ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CorrosionInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Polymer ScienceInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CeramicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CompositesJournal of
NanoparticlesJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of
Biomaterials
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
NanoscienceJournal of
TextilesHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Journal of
NanotechnologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
CrystallographyJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CoatingsJournal of
Advances in
Materials Science and EngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Smart Materials Research
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
MetallurgyJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
BioMed Research International
MaterialsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Nano
materials
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal ofNanomaterials
4 International Journal of Metals
0
05
1
15
000001 001
Pote
ntia
l (V
ver
sus S
CE)
Current density (Acm2)
minus1
minus15
minus05
1E minus 11 1E minus 08
Effect of Na2S
025M Na2SO4
025M Na2SO4 + 01M Na2S025M Na2SO4 + 03M Na2S025M Na2SO4 + 05M Na2S
Figure 2 Effect of S2minus on potentiodynamic polarization curves ofcarbon steel in 025M Na
2SO4solution at pH 6 and temperature
25∘C
the cathodic reduction of SO4
2minus ions produces thiosulfate andsulphide ions both of which are aggressive corrosion speciesand hence degrade the steel surface in sea water in the acidicpH range below the neutral medium The corrosion productin this case should be iron sulphide or thiosulfate
312 Effect of SO4
2minus + S2minus Addition of S2minus to the solutioncontaining SO
4
2minus further enhances the corrosion rate as canbe seen from Figure 2 And the pH has a strong effect on it Itcan be seen from (11) and (12) above that the conjoint effectof SO4
2minus and Sminus is the production of increasing amount of Sminusas well as corrosive sulphur compound
313 Effect of SO4
2minus +Clminus Corrosion rate also increaseswithaddition of Clminus to the solution containing SO
4
2minus (Figure 3)The rate increases with increase in Clminus concentration It isseen from (11)ndash(14) above that theClminus ions attack FeFe2+withthe formation of corrosion products FeCl
2and FeCl
3 FeCl
2
is unstable and may hydrolyse or further react with Clminus ionsto form Fe(OH)
2or FeCl
3 respectively
314 Effect of SO4
2minus + S2minus + Clminus The conjugate of all 3 ionswhich are normally present in the oil field sea water is thedegradation of carbon steel structure at the highest level Itis seen from Figure 4 that 119894corr values have shifted to theright and 119864corr towards the active potential with increasingthe concentration of both S2minus and Clminus
315 Effect of pH There is not any much significant effect ofpolarization curves with change in pH except at pH 11 underalkaline condition (Figure 5) when the corrosion rate is verylow The steel is in the passive region at this pH The pH of
0
Current density (Acm2)
025M Na2SO4
Effect of NaCl
minus02
minus04
minus06
minus08
minus1
minus12
Pote
ntia
l (V
ver
sus S
CE)
025M Na2SO4 + 01N NaCl025M Na2SO4 + 03N NaCl025M Na2SO4 + 05N NaCl
1E minus 09 1E minus 06 1E minus 03 1E + 00
Figure 3 Effect of Clminus on potentiodynamic polarization curves ofcarbon steel in 025M Na
2SO4solution at pH 8 and temperature
25∘C
0
0000001 00001 001 1Current density (Acm2)
1E minus 08
minus01
minus02
minus03
minus04
minus05
minus06
minus07
minus08
minus09
minus1
Pote
ntia
l (V
ver
sus S
CE)
025M Na2SO4
025M Na2SO4 + 03M Na2S + 03N NaCl025M Na2SO4 + 03M Na2S + 01N NaCl025M Na2SO4 + 03M Na2S025M Na2SO4 + 03M Na2S + 05N NaCl
Figure 4 Effect of Clminus + S2minus on potentiodynamic polarizationcurves of carbon steel in 025M Na
2SO4solution at pH 8 and
temperature 25∘C
the oil field water is in the range of 4ndash6 when the corrosionrate is high
316 Effect of Temperature Temperature aggravates thematerial degradation (Figure 6) by increasing diffusion andmass transfer coefficient of the aggressive ions corroding
International Journal of Metals 5
0
05
1
15
minus1
minus15
minus05
Current density (Acm2)
pH 11pH 6pH 8
Pote
ntia
l (V
ver
sus S
CE)
Effect of temperature of 025M Na2SO4
1E minus 09 1E minus 07 1E minus 05 1E minus 03 1E minus 01
Figure 5 Potentiodynamic polarization curves of carbon steel in an aqueous solution of 025MNa2SO4and 03MNa
2S at temperature 25∘C
at different pH
0
05
1
15
0001 01
Pote
ntia
l (V
ver
sus S
CE)
minus1
minus15
minus05
Current density (Acm2)
1E minus 11 1E minus 09 1E minus 07 1E minus 05
Effect of temperature of 025M Na2SO4
Temp 25∘C 025M Na2SO4
Temp 70∘C 025M Na2SO4
Temp 45∘C 025M Na2SO4
Figure 6 Potentiodynamic polarization curves of low carbon steel in 025M Na2SO4solution at (pH = 8) at different temperature
the metallic surface Temperature also increases the 119868119871 the
limiting current density of the concentration polarizationand hence shifts the polarization curves to the right
32 Electrochemical Impedance Spectroscopy (EIS) The phe-nomenon at the interface of the solidmetal and aqueous elec-trolyte is a complex process consisting of a line of positively
and negatively charged ions capacitance due to double layercorroded product or film formation on surfaces polarizationresistance (119877
119901) pore resistance (119877po) and various types of
impedance due to diffusion of ions movement of charge in oraway frommetal surface and adsorption of cation and anionThe whole phenomenon can be represented by an equivalentAC electrical circuit The phenomenon can be interpreted
6 International Journal of Metals
Zm
od(o
hm)
1000
1000
1000
1000
Hz
1000
Hz
1000
Hz
1000
kHz
1000
kHz
1000
kHz
Z curve (025m Na2SO4)Y2-Z curve (025m Na2SO4)
Fit 1 [CPE with diffusion]Y2-fit 1 [CPE with diffusion]
Frequency (Hz)
Y2-Z
phz(∘)
0000
minus2000
minus4000
minus6000
In 025M Na2SO4
Bode plot
Impedance
Phase angle
025M SO42minus
Ru
RE
Rp Wd
WE
Y0120572
empty
CPE with diffusionequivalent circuit
1000
mH
z
1000
mH
z
1000
mH
z
1000
mH
z
1000000
100000
(a)
Zm
od(o
hm)
1000
Hz
1000
Hz
1000
Hz
1000
kHz
1000
kHz
1000
kHz
Fit 1 [CPE with diffusion]Y2-fit 1 [CPE with diffusion]
Frequency (Hz)
Y2-Z
phz(∘)
0000
minus2000
minus4000
minus6000
Bode plot
Impedance
Phase angle
minus8000
In 025M Na2SO4 + 01M Na2S
Z curve (025m Na2SO4 + 01m Na2S)Y2-Z curve (025m Na2SO4 + 01m Na2S)
025M SO42minus + 01M S2minus
Ru
RE
Rp Wd
WE
Y0120572
empty
CPE with diffusionequivalent circuit
1000
1000
1000
1000
mH
z
1000
mH
z
1000
mH
z
1000
mH
z
1000000
100000
(b)
Zm
od(o
hm)
1000
1000
1000
Hz
1000
Hz
1000
Hz
1000
kHz
1000
kHz
1000
kHz
Fit 1 [CPE with diffusion]Y2-fit 1 [CPE with diffusion]
Frequency (Hz)
Y2-Z
phz(∘)
0000
minus2000
minus3000
minus4000
minus1000
Bode plot
Impedance Phase angle
025M Na2SO4 + 01M Na2S + 01N NaCl
Z curve (025m Na2SO4 + 01M Na2S + 01N NaCl)Y2-Z curve (025m Na2SO4 + 01M Na2S + 01N NaCl)
025M SO42minus + 01M S2minus + 01M Clminus
Ru
RE
Rp Wd
WE
Y0120572
empty
CPE with diffusionequivalent circuit
1000
mH
z
1000
mH
z
1000
mH
z
1000
mH
z
(c)
Figure 7 Bode plots of carbon steel in SO4
2minus SO4
2minus + S2minus and SO4
2minus + S2minus + Clminus solution at pH 8 and temperature 25∘C
from the Bode plots which are depicted and discussed in thefollowing section for various corrosive species
Figure 7 displays the Bode plots of carbon steel insolutions of SO
4
2minus SO4
2minus + S2minus and SO4
2minus + S2minus + Clminus Itis to be noticed that in all the solutions EIS data matchthe model of Randle circuit with a Warburg resistance 119882
119889
(given in the inset of each figure) that prevails at the metal-solution interface It is seen that the impedance decreaseswith addition of different aggressive ions compared to thosewith base solution of only SO
4
2minus This decrease in impedanceleads to more current flow across the interface and henceincrease in corrosion rate There is a phase angle shift with
International Journal of Metals 7
Inte
nsity
(cps
)
2120579 (deg)
Fe (110)
Fe (200)Fe (211)
1000
800
600
400
200
040 60 80 100 120
Carbon steel before corrosion
(a)
Inte
nsity
(cps
)
2120579 (deg)
Fe (110) Corroded in Na2SO4 + Na2S
FeS (003)
FeS (003)
FeS (113)
800
700
600
500
400
300
200
100
030 40 50 60 70 80 90 100 110
(b)
Inte
nsity
(cps
)
2120579 (deg)
1000
800
600
400
200
0
40 60 80 100 120
Fe (110)
FeCl3
FeCl3
Corroded in Na2SO4 + NaCl
(c)
Inte
nsity
(cps
)
2120579 (deg)
Fe (110)
FeS (003)
500
400
300
200
100
0
40 50 60 70 80 90 100 110
FeCl3
Corroded in Na2SO4 + Na2S + NaCl
(113)
(d)
Figure 8 X-ray diffraction showing peaks of different corroded phases
frequency The minimum phase angle reaches much lessthan 90 degrees indicating the capacitance in the circuit isnot a pure capacitance but constant phase element CPE(119884)and is given by the capacitive impedance equation 119885 =1119862(119895119908)
minus120572 where 120572 is fraction varying from 0 to 1 the valueless than one indicates that it does not behave ideally as purecapacitance At high frequency the value of impedance119885 modis roughly equal to 119877
119878 while at low frequency the value is
(119877119878+ 119877119875) Both can be determined from the blots Table 1
depicted the computed values of the EIS parameters It isseen that polarization resistance decreases with addition ofmore types of ions which support the polarization resultsof corrosion rates increase as found in Figures 2 3 and 4The increase in corrosion rate is supported by the EIS dataincrease in119884
0which behaves like capacitance and decrease in
polarization resistance (Table 2)The values of 120572 indicate thatthe capacitance behaves like constant phase element ratherthan pure capacitance The Warburg resistance 119882
119889which
signifies resistance to the flow of ions from the solution tocorroded metal surface also decreases with addition of moretypes of ions All the facts confirm the enhanced corrosionrates of carbon steel in the oil field sea water with presence ofSO4
2minus + S2minus + Clminus
33 X-Ray Diffraction The presence of corrosion productsof FeS FeCl
2 and FeCl
3is clearly indicated by the XRD
peak intensities in Figure 8 This supports the corrosionenhancement by SO
4
2minus S2minus and Clminus ions as found inpolarization and EIS studies
8 International Journal of Metals
Table 2 Computed EIS parameters
Sample Corrosive ions 119877119904(ohm) 119877
119901(ohm) 119884
0120583F 120572 119882
119889(ohm)
Carbon steel SO42minus 9443 85000 750 0720 10430
Carbon steel SO42minus + S2minus 8268 3700 5722 0769 4187
Carbon steel SO42minus + S2minus + Clminus 1331 95 9056 0602 2432
(c)
(b)(a)
Figure 9 Optical microscopy images of corroded steel (a) Na2SO4 (b) Na
2SO4+ Na2S and (c) Na
2SO4+ Na2S + NaCl
34 Optical and SEM Microscopy Images The microstruc-tures by optical microscopy clearly show (Figure 9) thecorroded structure of steel with products of corrosion overit The form of corrosion seems to be uniform not localizedSEM images (Figure 10) distinctly reveal how the degree ofdegradation increases with presence of ions in the solutionsfrom SO
4
2minus to SO4
2minus + S2minus to SO4
2minus + S2minus + Clminus It isinteresting to observe from the morphology of SEM image athigher magnification (Figure 11) that the corrosion has takenplace at grain boundary as well as grain body but the attack atgrain boundary is very prominentwith sulphide (S) corrosioncrackThe corrosion products of sulphide compounds as wellas element sulphur are clearly revealedThe structure of SEMimages shows almost catastrophic failure with the presenceof minor and major cracks when the seawater is enrichedwith the presence of all three corrosive ions SO
4
2minus S2minusand Clminus The morphology of SEM image analysis along with
XRD is in complete agreement with the corrosion data of thepolarization experiments and EIS
From (5)ndash(8) it is seen that SO4
2minus is cathodically reducedto give rise to S2minus It is observed from the polarizationcurve (Figure 1) that the increase of SO
4
2minus concentrationdepolarizes the cathodic curves much more compared toanodic ones shifting 119864corr towards the negative potentialThe release of S2minus from the reaction produces corrosionproducts iron sulfides (FeS
119909) on carbon steel surface They
are nonstoichiometric iron sulfide films mainly composed ofmackinawite and pyrrhotite Berner [21ndash26] The black colorof mackinawite phase is also seen as corrosion products inoptical microstructures (Figures 9(a) and 9(b))
The deterioration of metal due to contact with S2minus (H2S)
and moisture is sour corrosion The forms of sour corrosionare uniform (Figure 9(b)) pitting and stepwise cracking(Figures 10(c) and 11(b))
International Journal of Metals 9
(d)(c)
(b)(a)
Figure 10 SEM images of corroded steel (a) before corrosion (b) in Na2SO4 (c) in Na
2SO4+ Na2S and (d) in Na
2SO4+ Na2S + NaCl
The general equation of sour corrosion can be expressedas follows [21]
S2minus + 2H+ = H2S (aq) 119864 = 0097V versus SCE (15)
Hydrogen sulphide dissociates to produce proton andbisulphide as described [22] by the following equations
H2S (aq) = H+ +HSminus 119870 = 91 times 10minus8
HSminus = H+ + S2minus 119870 = 91 times 10minus12(16)
This reaction scheme shows that the presence of hydrogensulfide can contribute to the concentration of sulfide at thesurface by dissociation rather than charge exchange with thesurface The sulfide concentration at the surface is thereforedependent on the concentration of aqueous hydrogen sulfidein the electrolyte as well as the reduction processes of sulfate
H2S exhibits the different role in anodic process of carbon
steel depending on the pH value in the solutions The localsupersaturation of FeS
119909could be formed on the carbon steel
surface via the following reaction with the nucleation andgrowth of one or more of the iron sulfide mackinawite
H2S + Fe +H
2O 997888rarr FeS
119909+ 2H +H
2O (17)
The black corrosion products (FeS119909) formed on the steel
surface in the H2S-containing solutions could be observed
(Figure 9(b)) The addition of chloride to the corrosionsystem did not exhibit any localized corrosion rather it dras-tically increased the 119894corr values and shifted 119864corr towards thenegative potential (Figures 3 and 4) depolarizing the anodicreactionsThe increase in 119894corr with Cl addition seems to havemodified the anodic substrate area (Figures 9(c) and 10(d))and has enhanced the exchange current density for the ironoxidation process
4 Conclusion
All three corrosive ions SO4
2minus S2minus and Clminus have a strongeffect on increasing the corrosion rate of carbon steel
Cathodic reduction of SO4
2minus generates elemental S orS2minus ions in addition to S2minus from H
2S in oil The species
10 International Journal of Metals
(c)
(b)(a)
Figure 11 SEM images of corroded steel (a) in Na2SO4 (b) in Na
2SO4+ Na2S and (c) in Na
2SO4+ Na2S + NaCl
cause major corrosion with formation of FeS119909as corrosion
productsThe attack is found at grain boundarywith sulphidecracking as well as some grain body degradation The effectof addition of Clminus is the increase of 119894corr values by hundredsof times possibly due to enhancement of exchange currentdensity of the anodic reactions
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgments
The authors would like to acknowledge ldquoCOE TEQUIPrdquo inJadavpur University for the support of this work
References
[1] S Nesic ldquoKey issues related to modelling of internal corrosionof oil and gas pipelinesmdasha reviewrdquo Corrosion Science vol 49no 12 pp 4308ndash4338 2007
[2] L T Popoola A S Grema G K Latinwo B Gutti and A SBalogun ldquoCorrosion problems during oil and gas productionand its mitigationrdquo International Journal of Industrial Chem-istry vol 4 aticle 35 2013
[3] BW A Sherar P G Keech andDW Shoesmith ldquoThe effect ofsulfide on the aerobic corrosion of carbon steel in near-neutral
pH saline solutionsrdquo Corrosion Science vol 66 pp 256ndash2622013
[4] B W A Sherar I M Power P G Keech S Mitlin G Southamand D W Shoesmith ldquoCharacterizing the effect of carbon steelexposure in sulfide containing solutions to microbially inducedcorrosionrdquo Corrosion Science vol 53 no 3 pp 955ndash960 2011
[5] F M Song ldquoA comprehensive model for predicting CO2
corrosion rate in oil and gas production and transportationsystemsrdquo Electrochimica Acta vol 55 no 3 pp 689ndash700 2010
[6] M Hairil Mohd and J K Paik ldquoInvestigation of the corrosionprogress characteristics of offshore subsea oil well tubesrdquoCorrosion Science vol 67 pp 130ndash141 2013
[7] J Tang Y Shao T Zhang G Meng and F Wang ldquoCorrosionbehaviour of carbon steel in different concentrations of HClsolutions containing H
2S at 90 ∘Crdquo Corrosion Science vol 53
no 5 pp 1715ndash1723 2011[8] Z A D Foroulis ldquoRole of solution pH on wet H
2S cracking
in hydrocarbon productionrdquo Corrosion Prevention and Controlvol 40 no 4 pp 84ndash89 1993
[9] M A Veloz and I Gonzalez ldquoElectrochemical study of carbonsteel corrosion in buffered acetic acid solutions with chloridesand H
2Srdquo Electrochimica Acta vol 48 no 2 pp 135ndash144 2002
[10] S Arzola and J Genesca ldquoThe effect of H2S concentration on
the corrosion behavior of API 5L X-70 steelrdquo Journal of SolidState Electrochemistry vol 9 no 4 pp 197ndash200 2005
[11] H YMa X L Cheng S H Chen et al ldquoTheoretical interpreta-tion on impedance spectra for anodic iron dissolution in acidicsolutions containing hydrogen sulfiderdquoCorrosion vol 54 no 8pp 634ndash640 1998
International Journal of Metals 11
[12] H Ma X Cheng S Chen C Wang J Zhang and H YangldquoAn ac impedance study of the anodic dissolution of iron insulfuric acid solutions containing hydrogen sulfiderdquo Journal ofElectroanalytical Chemistry vol 451 no 1-2 pp 11ndash17 1998
[13] HMa X Cheng G Li et al ldquoThe influence of hydrogen sulfideon corrosion of iron under different conditionsrdquo CorrosionScience vol 42 no 10 pp 1669ndash1683 2000
[14] H-H HuangW-T Tsai and J-T Lee ldquoElectrochemical behav-ior of the simulated heat-affected zone of A516 carbon steel inH2S solutionrdquo Electrochimica Acta vol 41 no 7-8 pp 1191ndash1199
1996[15] J Tang Y Shao J Guo T Zhang G Meng and F Wang ldquoThe
effect of H2S concentration on the corrosion behavior of carbon
steel at 90 ∘Crdquo Corrosion Science vol 52 no 6 pp 2050ndash20582010
[16] P Smith S Roy D Swailes S Maxwell D Page and J LawsonldquoA model for the corrosion of steel subjected to syntheticproduced water containing sulfate chloride and hydrogensulfiderdquo Chemical Engineering Science vol 66 no 23 pp 5775ndash5790 2011
[17] R E Melchers ldquoDevelopment of new applied models for steelcorrosion in marine applications including shippingrdquo Ships andOffshore Structures vol 3 no 2 pp 135ndash144 2008
[18] M H Mohd D K Kim D W Kim and J K Paik ldquoA time-variant corrosion wastage model for subsea gas pipelinesrdquo Shipsand Offshore Structures vol 9 no 2 pp 161ndash176 2014
[19] R E Melchers and J K Paik ldquoEffect of flexure on rusting ofshiprsquos steel platingrdquo Ships and Offshore Structures vol 5 no 1pp 25ndash31 2010
[20] M Shehadeh and I Hassan ldquoStudy of sacrificial cathodicprotection on marine structures in sea and fresh water inrelation to flow conditionsrdquo Ships and Offshore Structures vol8 no 1 pp 102ndash110 2013
[21] R A Berner ldquoThermodynamic stability of sedimentary ironsulfidesrdquoTheAmerican Journal of Science vol 265 pp 773ndash7851967
[22] R H Hausler L A Goeller R P Zimmerman and R HRosenwald ldquoContribution to the ldquofilming aminerdquo theory aninterpretation of experimental resultsrdquo Corrosion vol 28 no1 pp 7ndash16 1972
[23] P Taylor ldquoThe stereochemistry of iron sulfidesmdasha structuralration for the crysta llization of some metastable phases fromaqueous solutionrdquo American Mineralogist vol 65 pp 1026ndash1030 1980
[24] R A Berner ldquoIron sulfides formed from aqueous solution atlow temperatures and atmospheric pressurerdquo The Journal ofGeology vol 72 pp 293ndash306 1964
[25] D T Rickard ldquoThe chemistry of iron sulphide formation at lowtemperaturesrdquo Stockholm Contributions in Geology vol 20 pp67ndash95 1969
[26] A J Bard R Parsons and J Jordan Standard Potentials inAqueous Solution CRC Press 1st edition 1985
Submit your manuscripts athttpwwwhindawicom
ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CorrosionInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Polymer ScienceInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CeramicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CompositesJournal of
NanoparticlesJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of
Biomaterials
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
NanoscienceJournal of
TextilesHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Journal of
NanotechnologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
CrystallographyJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CoatingsJournal of
Advances in
Materials Science and EngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Smart Materials Research
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
MetallurgyJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
BioMed Research International
MaterialsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Nano
materials
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal ofNanomaterials
International Journal of Metals 5
0
05
1
15
minus1
minus15
minus05
Current density (Acm2)
pH 11pH 6pH 8
Pote
ntia
l (V
ver
sus S
CE)
Effect of temperature of 025M Na2SO4
1E minus 09 1E minus 07 1E minus 05 1E minus 03 1E minus 01
Figure 5 Potentiodynamic polarization curves of carbon steel in an aqueous solution of 025MNa2SO4and 03MNa
2S at temperature 25∘C
at different pH
0
05
1
15
0001 01
Pote
ntia
l (V
ver
sus S
CE)
minus1
minus15
minus05
Current density (Acm2)
1E minus 11 1E minus 09 1E minus 07 1E minus 05
Effect of temperature of 025M Na2SO4
Temp 25∘C 025M Na2SO4
Temp 70∘C 025M Na2SO4
Temp 45∘C 025M Na2SO4
Figure 6 Potentiodynamic polarization curves of low carbon steel in 025M Na2SO4solution at (pH = 8) at different temperature
the metallic surface Temperature also increases the 119868119871 the
limiting current density of the concentration polarizationand hence shifts the polarization curves to the right
32 Electrochemical Impedance Spectroscopy (EIS) The phe-nomenon at the interface of the solidmetal and aqueous elec-trolyte is a complex process consisting of a line of positively
and negatively charged ions capacitance due to double layercorroded product or film formation on surfaces polarizationresistance (119877
119901) pore resistance (119877po) and various types of
impedance due to diffusion of ions movement of charge in oraway frommetal surface and adsorption of cation and anionThe whole phenomenon can be represented by an equivalentAC electrical circuit The phenomenon can be interpreted
6 International Journal of Metals
Zm
od(o
hm)
1000
1000
1000
1000
Hz
1000
Hz
1000
Hz
1000
kHz
1000
kHz
1000
kHz
Z curve (025m Na2SO4)Y2-Z curve (025m Na2SO4)
Fit 1 [CPE with diffusion]Y2-fit 1 [CPE with diffusion]
Frequency (Hz)
Y2-Z
phz(∘)
0000
minus2000
minus4000
minus6000
In 025M Na2SO4
Bode plot
Impedance
Phase angle
025M SO42minus
Ru
RE
Rp Wd
WE
Y0120572
empty
CPE with diffusionequivalent circuit
1000
mH
z
1000
mH
z
1000
mH
z
1000
mH
z
1000000
100000
(a)
Zm
od(o
hm)
1000
Hz
1000
Hz
1000
Hz
1000
kHz
1000
kHz
1000
kHz
Fit 1 [CPE with diffusion]Y2-fit 1 [CPE with diffusion]
Frequency (Hz)
Y2-Z
phz(∘)
0000
minus2000
minus4000
minus6000
Bode plot
Impedance
Phase angle
minus8000
In 025M Na2SO4 + 01M Na2S
Z curve (025m Na2SO4 + 01m Na2S)Y2-Z curve (025m Na2SO4 + 01m Na2S)
025M SO42minus + 01M S2minus
Ru
RE
Rp Wd
WE
Y0120572
empty
CPE with diffusionequivalent circuit
1000
1000
1000
1000
mH
z
1000
mH
z
1000
mH
z
1000
mH
z
1000000
100000
(b)
Zm
od(o
hm)
1000
1000
1000
Hz
1000
Hz
1000
Hz
1000
kHz
1000
kHz
1000
kHz
Fit 1 [CPE with diffusion]Y2-fit 1 [CPE with diffusion]
Frequency (Hz)
Y2-Z
phz(∘)
0000
minus2000
minus3000
minus4000
minus1000
Bode plot
Impedance Phase angle
025M Na2SO4 + 01M Na2S + 01N NaCl
Z curve (025m Na2SO4 + 01M Na2S + 01N NaCl)Y2-Z curve (025m Na2SO4 + 01M Na2S + 01N NaCl)
025M SO42minus + 01M S2minus + 01M Clminus
Ru
RE
Rp Wd
WE
Y0120572
empty
CPE with diffusionequivalent circuit
1000
mH
z
1000
mH
z
1000
mH
z
1000
mH
z
(c)
Figure 7 Bode plots of carbon steel in SO4
2minus SO4
2minus + S2minus and SO4
2minus + S2minus + Clminus solution at pH 8 and temperature 25∘C
from the Bode plots which are depicted and discussed in thefollowing section for various corrosive species
Figure 7 displays the Bode plots of carbon steel insolutions of SO
4
2minus SO4
2minus + S2minus and SO4
2minus + S2minus + Clminus Itis to be noticed that in all the solutions EIS data matchthe model of Randle circuit with a Warburg resistance 119882
119889
(given in the inset of each figure) that prevails at the metal-solution interface It is seen that the impedance decreaseswith addition of different aggressive ions compared to thosewith base solution of only SO
4
2minus This decrease in impedanceleads to more current flow across the interface and henceincrease in corrosion rate There is a phase angle shift with
International Journal of Metals 7
Inte
nsity
(cps
)
2120579 (deg)
Fe (110)
Fe (200)Fe (211)
1000
800
600
400
200
040 60 80 100 120
Carbon steel before corrosion
(a)
Inte
nsity
(cps
)
2120579 (deg)
Fe (110) Corroded in Na2SO4 + Na2S
FeS (003)
FeS (003)
FeS (113)
800
700
600
500
400
300
200
100
030 40 50 60 70 80 90 100 110
(b)
Inte
nsity
(cps
)
2120579 (deg)
1000
800
600
400
200
0
40 60 80 100 120
Fe (110)
FeCl3
FeCl3
Corroded in Na2SO4 + NaCl
(c)
Inte
nsity
(cps
)
2120579 (deg)
Fe (110)
FeS (003)
500
400
300
200
100
0
40 50 60 70 80 90 100 110
FeCl3
Corroded in Na2SO4 + Na2S + NaCl
(113)
(d)
Figure 8 X-ray diffraction showing peaks of different corroded phases
frequency The minimum phase angle reaches much lessthan 90 degrees indicating the capacitance in the circuit isnot a pure capacitance but constant phase element CPE(119884)and is given by the capacitive impedance equation 119885 =1119862(119895119908)
minus120572 where 120572 is fraction varying from 0 to 1 the valueless than one indicates that it does not behave ideally as purecapacitance At high frequency the value of impedance119885 modis roughly equal to 119877
119878 while at low frequency the value is
(119877119878+ 119877119875) Both can be determined from the blots Table 1
depicted the computed values of the EIS parameters It isseen that polarization resistance decreases with addition ofmore types of ions which support the polarization resultsof corrosion rates increase as found in Figures 2 3 and 4The increase in corrosion rate is supported by the EIS dataincrease in119884
0which behaves like capacitance and decrease in
polarization resistance (Table 2)The values of 120572 indicate thatthe capacitance behaves like constant phase element ratherthan pure capacitance The Warburg resistance 119882
119889which
signifies resistance to the flow of ions from the solution tocorroded metal surface also decreases with addition of moretypes of ions All the facts confirm the enhanced corrosionrates of carbon steel in the oil field sea water with presence ofSO4
2minus + S2minus + Clminus
33 X-Ray Diffraction The presence of corrosion productsof FeS FeCl
2 and FeCl
3is clearly indicated by the XRD
peak intensities in Figure 8 This supports the corrosionenhancement by SO
4
2minus S2minus and Clminus ions as found inpolarization and EIS studies
8 International Journal of Metals
Table 2 Computed EIS parameters
Sample Corrosive ions 119877119904(ohm) 119877
119901(ohm) 119884
0120583F 120572 119882
119889(ohm)
Carbon steel SO42minus 9443 85000 750 0720 10430
Carbon steel SO42minus + S2minus 8268 3700 5722 0769 4187
Carbon steel SO42minus + S2minus + Clminus 1331 95 9056 0602 2432
(c)
(b)(a)
Figure 9 Optical microscopy images of corroded steel (a) Na2SO4 (b) Na
2SO4+ Na2S and (c) Na
2SO4+ Na2S + NaCl
34 Optical and SEM Microscopy Images The microstruc-tures by optical microscopy clearly show (Figure 9) thecorroded structure of steel with products of corrosion overit The form of corrosion seems to be uniform not localizedSEM images (Figure 10) distinctly reveal how the degree ofdegradation increases with presence of ions in the solutionsfrom SO
4
2minus to SO4
2minus + S2minus to SO4
2minus + S2minus + Clminus It isinteresting to observe from the morphology of SEM image athigher magnification (Figure 11) that the corrosion has takenplace at grain boundary as well as grain body but the attack atgrain boundary is very prominentwith sulphide (S) corrosioncrackThe corrosion products of sulphide compounds as wellas element sulphur are clearly revealedThe structure of SEMimages shows almost catastrophic failure with the presenceof minor and major cracks when the seawater is enrichedwith the presence of all three corrosive ions SO
4
2minus S2minusand Clminus The morphology of SEM image analysis along with
XRD is in complete agreement with the corrosion data of thepolarization experiments and EIS
From (5)ndash(8) it is seen that SO4
2minus is cathodically reducedto give rise to S2minus It is observed from the polarizationcurve (Figure 1) that the increase of SO
4
2minus concentrationdepolarizes the cathodic curves much more compared toanodic ones shifting 119864corr towards the negative potentialThe release of S2minus from the reaction produces corrosionproducts iron sulfides (FeS
119909) on carbon steel surface They
are nonstoichiometric iron sulfide films mainly composed ofmackinawite and pyrrhotite Berner [21ndash26] The black colorof mackinawite phase is also seen as corrosion products inoptical microstructures (Figures 9(a) and 9(b))
The deterioration of metal due to contact with S2minus (H2S)
and moisture is sour corrosion The forms of sour corrosionare uniform (Figure 9(b)) pitting and stepwise cracking(Figures 10(c) and 11(b))
International Journal of Metals 9
(d)(c)
(b)(a)
Figure 10 SEM images of corroded steel (a) before corrosion (b) in Na2SO4 (c) in Na
2SO4+ Na2S and (d) in Na
2SO4+ Na2S + NaCl
The general equation of sour corrosion can be expressedas follows [21]
S2minus + 2H+ = H2S (aq) 119864 = 0097V versus SCE (15)
Hydrogen sulphide dissociates to produce proton andbisulphide as described [22] by the following equations
H2S (aq) = H+ +HSminus 119870 = 91 times 10minus8
HSminus = H+ + S2minus 119870 = 91 times 10minus12(16)
This reaction scheme shows that the presence of hydrogensulfide can contribute to the concentration of sulfide at thesurface by dissociation rather than charge exchange with thesurface The sulfide concentration at the surface is thereforedependent on the concentration of aqueous hydrogen sulfidein the electrolyte as well as the reduction processes of sulfate
H2S exhibits the different role in anodic process of carbon
steel depending on the pH value in the solutions The localsupersaturation of FeS
119909could be formed on the carbon steel
surface via the following reaction with the nucleation andgrowth of one or more of the iron sulfide mackinawite
H2S + Fe +H
2O 997888rarr FeS
119909+ 2H +H
2O (17)
The black corrosion products (FeS119909) formed on the steel
surface in the H2S-containing solutions could be observed
(Figure 9(b)) The addition of chloride to the corrosionsystem did not exhibit any localized corrosion rather it dras-tically increased the 119894corr values and shifted 119864corr towards thenegative potential (Figures 3 and 4) depolarizing the anodicreactionsThe increase in 119894corr with Cl addition seems to havemodified the anodic substrate area (Figures 9(c) and 10(d))and has enhanced the exchange current density for the ironoxidation process
4 Conclusion
All three corrosive ions SO4
2minus S2minus and Clminus have a strongeffect on increasing the corrosion rate of carbon steel
Cathodic reduction of SO4
2minus generates elemental S orS2minus ions in addition to S2minus from H
2S in oil The species
10 International Journal of Metals
(c)
(b)(a)
Figure 11 SEM images of corroded steel (a) in Na2SO4 (b) in Na
2SO4+ Na2S and (c) in Na
2SO4+ Na2S + NaCl
cause major corrosion with formation of FeS119909as corrosion
productsThe attack is found at grain boundarywith sulphidecracking as well as some grain body degradation The effectof addition of Clminus is the increase of 119894corr values by hundredsof times possibly due to enhancement of exchange currentdensity of the anodic reactions
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgments
The authors would like to acknowledge ldquoCOE TEQUIPrdquo inJadavpur University for the support of this work
References
[1] S Nesic ldquoKey issues related to modelling of internal corrosionof oil and gas pipelinesmdasha reviewrdquo Corrosion Science vol 49no 12 pp 4308ndash4338 2007
[2] L T Popoola A S Grema G K Latinwo B Gutti and A SBalogun ldquoCorrosion problems during oil and gas productionand its mitigationrdquo International Journal of Industrial Chem-istry vol 4 aticle 35 2013
[3] BW A Sherar P G Keech andDW Shoesmith ldquoThe effect ofsulfide on the aerobic corrosion of carbon steel in near-neutral
pH saline solutionsrdquo Corrosion Science vol 66 pp 256ndash2622013
[4] B W A Sherar I M Power P G Keech S Mitlin G Southamand D W Shoesmith ldquoCharacterizing the effect of carbon steelexposure in sulfide containing solutions to microbially inducedcorrosionrdquo Corrosion Science vol 53 no 3 pp 955ndash960 2011
[5] F M Song ldquoA comprehensive model for predicting CO2
corrosion rate in oil and gas production and transportationsystemsrdquo Electrochimica Acta vol 55 no 3 pp 689ndash700 2010
[6] M Hairil Mohd and J K Paik ldquoInvestigation of the corrosionprogress characteristics of offshore subsea oil well tubesrdquoCorrosion Science vol 67 pp 130ndash141 2013
[7] J Tang Y Shao T Zhang G Meng and F Wang ldquoCorrosionbehaviour of carbon steel in different concentrations of HClsolutions containing H
2S at 90 ∘Crdquo Corrosion Science vol 53
no 5 pp 1715ndash1723 2011[8] Z A D Foroulis ldquoRole of solution pH on wet H
2S cracking
in hydrocarbon productionrdquo Corrosion Prevention and Controlvol 40 no 4 pp 84ndash89 1993
[9] M A Veloz and I Gonzalez ldquoElectrochemical study of carbonsteel corrosion in buffered acetic acid solutions with chloridesand H
2Srdquo Electrochimica Acta vol 48 no 2 pp 135ndash144 2002
[10] S Arzola and J Genesca ldquoThe effect of H2S concentration on
the corrosion behavior of API 5L X-70 steelrdquo Journal of SolidState Electrochemistry vol 9 no 4 pp 197ndash200 2005
[11] H YMa X L Cheng S H Chen et al ldquoTheoretical interpreta-tion on impedance spectra for anodic iron dissolution in acidicsolutions containing hydrogen sulfiderdquoCorrosion vol 54 no 8pp 634ndash640 1998
International Journal of Metals 11
[12] H Ma X Cheng S Chen C Wang J Zhang and H YangldquoAn ac impedance study of the anodic dissolution of iron insulfuric acid solutions containing hydrogen sulfiderdquo Journal ofElectroanalytical Chemistry vol 451 no 1-2 pp 11ndash17 1998
[13] HMa X Cheng G Li et al ldquoThe influence of hydrogen sulfideon corrosion of iron under different conditionsrdquo CorrosionScience vol 42 no 10 pp 1669ndash1683 2000
[14] H-H HuangW-T Tsai and J-T Lee ldquoElectrochemical behav-ior of the simulated heat-affected zone of A516 carbon steel inH2S solutionrdquo Electrochimica Acta vol 41 no 7-8 pp 1191ndash1199
1996[15] J Tang Y Shao J Guo T Zhang G Meng and F Wang ldquoThe
effect of H2S concentration on the corrosion behavior of carbon
steel at 90 ∘Crdquo Corrosion Science vol 52 no 6 pp 2050ndash20582010
[16] P Smith S Roy D Swailes S Maxwell D Page and J LawsonldquoA model for the corrosion of steel subjected to syntheticproduced water containing sulfate chloride and hydrogensulfiderdquo Chemical Engineering Science vol 66 no 23 pp 5775ndash5790 2011
[17] R E Melchers ldquoDevelopment of new applied models for steelcorrosion in marine applications including shippingrdquo Ships andOffshore Structures vol 3 no 2 pp 135ndash144 2008
[18] M H Mohd D K Kim D W Kim and J K Paik ldquoA time-variant corrosion wastage model for subsea gas pipelinesrdquo Shipsand Offshore Structures vol 9 no 2 pp 161ndash176 2014
[19] R E Melchers and J K Paik ldquoEffect of flexure on rusting ofshiprsquos steel platingrdquo Ships and Offshore Structures vol 5 no 1pp 25ndash31 2010
[20] M Shehadeh and I Hassan ldquoStudy of sacrificial cathodicprotection on marine structures in sea and fresh water inrelation to flow conditionsrdquo Ships and Offshore Structures vol8 no 1 pp 102ndash110 2013
[21] R A Berner ldquoThermodynamic stability of sedimentary ironsulfidesrdquoTheAmerican Journal of Science vol 265 pp 773ndash7851967
[22] R H Hausler L A Goeller R P Zimmerman and R HRosenwald ldquoContribution to the ldquofilming aminerdquo theory aninterpretation of experimental resultsrdquo Corrosion vol 28 no1 pp 7ndash16 1972
[23] P Taylor ldquoThe stereochemistry of iron sulfidesmdasha structuralration for the crysta llization of some metastable phases fromaqueous solutionrdquo American Mineralogist vol 65 pp 1026ndash1030 1980
[24] R A Berner ldquoIron sulfides formed from aqueous solution atlow temperatures and atmospheric pressurerdquo The Journal ofGeology vol 72 pp 293ndash306 1964
[25] D T Rickard ldquoThe chemistry of iron sulphide formation at lowtemperaturesrdquo Stockholm Contributions in Geology vol 20 pp67ndash95 1969
[26] A J Bard R Parsons and J Jordan Standard Potentials inAqueous Solution CRC Press 1st edition 1985
Submit your manuscripts athttpwwwhindawicom
ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CorrosionInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Polymer ScienceInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CeramicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CompositesJournal of
NanoparticlesJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of
Biomaterials
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
NanoscienceJournal of
TextilesHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Journal of
NanotechnologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
CrystallographyJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CoatingsJournal of
Advances in
Materials Science and EngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Smart Materials Research
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
MetallurgyJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
BioMed Research International
MaterialsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Nano
materials
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal ofNanomaterials
6 International Journal of Metals
Zm
od(o
hm)
1000
1000
1000
1000
Hz
1000
Hz
1000
Hz
1000
kHz
1000
kHz
1000
kHz
Z curve (025m Na2SO4)Y2-Z curve (025m Na2SO4)
Fit 1 [CPE with diffusion]Y2-fit 1 [CPE with diffusion]
Frequency (Hz)
Y2-Z
phz(∘)
0000
minus2000
minus4000
minus6000
In 025M Na2SO4
Bode plot
Impedance
Phase angle
025M SO42minus
Ru
RE
Rp Wd
WE
Y0120572
empty
CPE with diffusionequivalent circuit
1000
mH
z
1000
mH
z
1000
mH
z
1000
mH
z
1000000
100000
(a)
Zm
od(o
hm)
1000
Hz
1000
Hz
1000
Hz
1000
kHz
1000
kHz
1000
kHz
Fit 1 [CPE with diffusion]Y2-fit 1 [CPE with diffusion]
Frequency (Hz)
Y2-Z
phz(∘)
0000
minus2000
minus4000
minus6000
Bode plot
Impedance
Phase angle
minus8000
In 025M Na2SO4 + 01M Na2S
Z curve (025m Na2SO4 + 01m Na2S)Y2-Z curve (025m Na2SO4 + 01m Na2S)
025M SO42minus + 01M S2minus
Ru
RE
Rp Wd
WE
Y0120572
empty
CPE with diffusionequivalent circuit
1000
1000
1000
1000
mH
z
1000
mH
z
1000
mH
z
1000
mH
z
1000000
100000
(b)
Zm
od(o
hm)
1000
1000
1000
Hz
1000
Hz
1000
Hz
1000
kHz
1000
kHz
1000
kHz
Fit 1 [CPE with diffusion]Y2-fit 1 [CPE with diffusion]
Frequency (Hz)
Y2-Z
phz(∘)
0000
minus2000
minus3000
minus4000
minus1000
Bode plot
Impedance Phase angle
025M Na2SO4 + 01M Na2S + 01N NaCl
Z curve (025m Na2SO4 + 01M Na2S + 01N NaCl)Y2-Z curve (025m Na2SO4 + 01M Na2S + 01N NaCl)
025M SO42minus + 01M S2minus + 01M Clminus
Ru
RE
Rp Wd
WE
Y0120572
empty
CPE with diffusionequivalent circuit
1000
mH
z
1000
mH
z
1000
mH
z
1000
mH
z
(c)
Figure 7 Bode plots of carbon steel in SO4
2minus SO4
2minus + S2minus and SO4
2minus + S2minus + Clminus solution at pH 8 and temperature 25∘C
from the Bode plots which are depicted and discussed in thefollowing section for various corrosive species
Figure 7 displays the Bode plots of carbon steel insolutions of SO
4
2minus SO4
2minus + S2minus and SO4
2minus + S2minus + Clminus Itis to be noticed that in all the solutions EIS data matchthe model of Randle circuit with a Warburg resistance 119882
119889
(given in the inset of each figure) that prevails at the metal-solution interface It is seen that the impedance decreaseswith addition of different aggressive ions compared to thosewith base solution of only SO
4
2minus This decrease in impedanceleads to more current flow across the interface and henceincrease in corrosion rate There is a phase angle shift with
International Journal of Metals 7
Inte
nsity
(cps
)
2120579 (deg)
Fe (110)
Fe (200)Fe (211)
1000
800
600
400
200
040 60 80 100 120
Carbon steel before corrosion
(a)
Inte
nsity
(cps
)
2120579 (deg)
Fe (110) Corroded in Na2SO4 + Na2S
FeS (003)
FeS (003)
FeS (113)
800
700
600
500
400
300
200
100
030 40 50 60 70 80 90 100 110
(b)
Inte
nsity
(cps
)
2120579 (deg)
1000
800
600
400
200
0
40 60 80 100 120
Fe (110)
FeCl3
FeCl3
Corroded in Na2SO4 + NaCl
(c)
Inte
nsity
(cps
)
2120579 (deg)
Fe (110)
FeS (003)
500
400
300
200
100
0
40 50 60 70 80 90 100 110
FeCl3
Corroded in Na2SO4 + Na2S + NaCl
(113)
(d)
Figure 8 X-ray diffraction showing peaks of different corroded phases
frequency The minimum phase angle reaches much lessthan 90 degrees indicating the capacitance in the circuit isnot a pure capacitance but constant phase element CPE(119884)and is given by the capacitive impedance equation 119885 =1119862(119895119908)
minus120572 where 120572 is fraction varying from 0 to 1 the valueless than one indicates that it does not behave ideally as purecapacitance At high frequency the value of impedance119885 modis roughly equal to 119877
119878 while at low frequency the value is
(119877119878+ 119877119875) Both can be determined from the blots Table 1
depicted the computed values of the EIS parameters It isseen that polarization resistance decreases with addition ofmore types of ions which support the polarization resultsof corrosion rates increase as found in Figures 2 3 and 4The increase in corrosion rate is supported by the EIS dataincrease in119884
0which behaves like capacitance and decrease in
polarization resistance (Table 2)The values of 120572 indicate thatthe capacitance behaves like constant phase element ratherthan pure capacitance The Warburg resistance 119882
119889which
signifies resistance to the flow of ions from the solution tocorroded metal surface also decreases with addition of moretypes of ions All the facts confirm the enhanced corrosionrates of carbon steel in the oil field sea water with presence ofSO4
2minus + S2minus + Clminus
33 X-Ray Diffraction The presence of corrosion productsof FeS FeCl
2 and FeCl
3is clearly indicated by the XRD
peak intensities in Figure 8 This supports the corrosionenhancement by SO
4
2minus S2minus and Clminus ions as found inpolarization and EIS studies
8 International Journal of Metals
Table 2 Computed EIS parameters
Sample Corrosive ions 119877119904(ohm) 119877
119901(ohm) 119884
0120583F 120572 119882
119889(ohm)
Carbon steel SO42minus 9443 85000 750 0720 10430
Carbon steel SO42minus + S2minus 8268 3700 5722 0769 4187
Carbon steel SO42minus + S2minus + Clminus 1331 95 9056 0602 2432
(c)
(b)(a)
Figure 9 Optical microscopy images of corroded steel (a) Na2SO4 (b) Na
2SO4+ Na2S and (c) Na
2SO4+ Na2S + NaCl
34 Optical and SEM Microscopy Images The microstruc-tures by optical microscopy clearly show (Figure 9) thecorroded structure of steel with products of corrosion overit The form of corrosion seems to be uniform not localizedSEM images (Figure 10) distinctly reveal how the degree ofdegradation increases with presence of ions in the solutionsfrom SO
4
2minus to SO4
2minus + S2minus to SO4
2minus + S2minus + Clminus It isinteresting to observe from the morphology of SEM image athigher magnification (Figure 11) that the corrosion has takenplace at grain boundary as well as grain body but the attack atgrain boundary is very prominentwith sulphide (S) corrosioncrackThe corrosion products of sulphide compounds as wellas element sulphur are clearly revealedThe structure of SEMimages shows almost catastrophic failure with the presenceof minor and major cracks when the seawater is enrichedwith the presence of all three corrosive ions SO
4
2minus S2minusand Clminus The morphology of SEM image analysis along with
XRD is in complete agreement with the corrosion data of thepolarization experiments and EIS
From (5)ndash(8) it is seen that SO4
2minus is cathodically reducedto give rise to S2minus It is observed from the polarizationcurve (Figure 1) that the increase of SO
4
2minus concentrationdepolarizes the cathodic curves much more compared toanodic ones shifting 119864corr towards the negative potentialThe release of S2minus from the reaction produces corrosionproducts iron sulfides (FeS
119909) on carbon steel surface They
are nonstoichiometric iron sulfide films mainly composed ofmackinawite and pyrrhotite Berner [21ndash26] The black colorof mackinawite phase is also seen as corrosion products inoptical microstructures (Figures 9(a) and 9(b))
The deterioration of metal due to contact with S2minus (H2S)
and moisture is sour corrosion The forms of sour corrosionare uniform (Figure 9(b)) pitting and stepwise cracking(Figures 10(c) and 11(b))
International Journal of Metals 9
(d)(c)
(b)(a)
Figure 10 SEM images of corroded steel (a) before corrosion (b) in Na2SO4 (c) in Na
2SO4+ Na2S and (d) in Na
2SO4+ Na2S + NaCl
The general equation of sour corrosion can be expressedas follows [21]
S2minus + 2H+ = H2S (aq) 119864 = 0097V versus SCE (15)
Hydrogen sulphide dissociates to produce proton andbisulphide as described [22] by the following equations
H2S (aq) = H+ +HSminus 119870 = 91 times 10minus8
HSminus = H+ + S2minus 119870 = 91 times 10minus12(16)
This reaction scheme shows that the presence of hydrogensulfide can contribute to the concentration of sulfide at thesurface by dissociation rather than charge exchange with thesurface The sulfide concentration at the surface is thereforedependent on the concentration of aqueous hydrogen sulfidein the electrolyte as well as the reduction processes of sulfate
H2S exhibits the different role in anodic process of carbon
steel depending on the pH value in the solutions The localsupersaturation of FeS
119909could be formed on the carbon steel
surface via the following reaction with the nucleation andgrowth of one or more of the iron sulfide mackinawite
H2S + Fe +H
2O 997888rarr FeS
119909+ 2H +H
2O (17)
The black corrosion products (FeS119909) formed on the steel
surface in the H2S-containing solutions could be observed
(Figure 9(b)) The addition of chloride to the corrosionsystem did not exhibit any localized corrosion rather it dras-tically increased the 119894corr values and shifted 119864corr towards thenegative potential (Figures 3 and 4) depolarizing the anodicreactionsThe increase in 119894corr with Cl addition seems to havemodified the anodic substrate area (Figures 9(c) and 10(d))and has enhanced the exchange current density for the ironoxidation process
4 Conclusion
All three corrosive ions SO4
2minus S2minus and Clminus have a strongeffect on increasing the corrosion rate of carbon steel
Cathodic reduction of SO4
2minus generates elemental S orS2minus ions in addition to S2minus from H
2S in oil The species
10 International Journal of Metals
(c)
(b)(a)
Figure 11 SEM images of corroded steel (a) in Na2SO4 (b) in Na
2SO4+ Na2S and (c) in Na
2SO4+ Na2S + NaCl
cause major corrosion with formation of FeS119909as corrosion
productsThe attack is found at grain boundarywith sulphidecracking as well as some grain body degradation The effectof addition of Clminus is the increase of 119894corr values by hundredsof times possibly due to enhancement of exchange currentdensity of the anodic reactions
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgments
The authors would like to acknowledge ldquoCOE TEQUIPrdquo inJadavpur University for the support of this work
References
[1] S Nesic ldquoKey issues related to modelling of internal corrosionof oil and gas pipelinesmdasha reviewrdquo Corrosion Science vol 49no 12 pp 4308ndash4338 2007
[2] L T Popoola A S Grema G K Latinwo B Gutti and A SBalogun ldquoCorrosion problems during oil and gas productionand its mitigationrdquo International Journal of Industrial Chem-istry vol 4 aticle 35 2013
[3] BW A Sherar P G Keech andDW Shoesmith ldquoThe effect ofsulfide on the aerobic corrosion of carbon steel in near-neutral
pH saline solutionsrdquo Corrosion Science vol 66 pp 256ndash2622013
[4] B W A Sherar I M Power P G Keech S Mitlin G Southamand D W Shoesmith ldquoCharacterizing the effect of carbon steelexposure in sulfide containing solutions to microbially inducedcorrosionrdquo Corrosion Science vol 53 no 3 pp 955ndash960 2011
[5] F M Song ldquoA comprehensive model for predicting CO2
corrosion rate in oil and gas production and transportationsystemsrdquo Electrochimica Acta vol 55 no 3 pp 689ndash700 2010
[6] M Hairil Mohd and J K Paik ldquoInvestigation of the corrosionprogress characteristics of offshore subsea oil well tubesrdquoCorrosion Science vol 67 pp 130ndash141 2013
[7] J Tang Y Shao T Zhang G Meng and F Wang ldquoCorrosionbehaviour of carbon steel in different concentrations of HClsolutions containing H
2S at 90 ∘Crdquo Corrosion Science vol 53
no 5 pp 1715ndash1723 2011[8] Z A D Foroulis ldquoRole of solution pH on wet H
2S cracking
in hydrocarbon productionrdquo Corrosion Prevention and Controlvol 40 no 4 pp 84ndash89 1993
[9] M A Veloz and I Gonzalez ldquoElectrochemical study of carbonsteel corrosion in buffered acetic acid solutions with chloridesand H
2Srdquo Electrochimica Acta vol 48 no 2 pp 135ndash144 2002
[10] S Arzola and J Genesca ldquoThe effect of H2S concentration on
the corrosion behavior of API 5L X-70 steelrdquo Journal of SolidState Electrochemistry vol 9 no 4 pp 197ndash200 2005
[11] H YMa X L Cheng S H Chen et al ldquoTheoretical interpreta-tion on impedance spectra for anodic iron dissolution in acidicsolutions containing hydrogen sulfiderdquoCorrosion vol 54 no 8pp 634ndash640 1998
International Journal of Metals 11
[12] H Ma X Cheng S Chen C Wang J Zhang and H YangldquoAn ac impedance study of the anodic dissolution of iron insulfuric acid solutions containing hydrogen sulfiderdquo Journal ofElectroanalytical Chemistry vol 451 no 1-2 pp 11ndash17 1998
[13] HMa X Cheng G Li et al ldquoThe influence of hydrogen sulfideon corrosion of iron under different conditionsrdquo CorrosionScience vol 42 no 10 pp 1669ndash1683 2000
[14] H-H HuangW-T Tsai and J-T Lee ldquoElectrochemical behav-ior of the simulated heat-affected zone of A516 carbon steel inH2S solutionrdquo Electrochimica Acta vol 41 no 7-8 pp 1191ndash1199
1996[15] J Tang Y Shao J Guo T Zhang G Meng and F Wang ldquoThe
effect of H2S concentration on the corrosion behavior of carbon
steel at 90 ∘Crdquo Corrosion Science vol 52 no 6 pp 2050ndash20582010
[16] P Smith S Roy D Swailes S Maxwell D Page and J LawsonldquoA model for the corrosion of steel subjected to syntheticproduced water containing sulfate chloride and hydrogensulfiderdquo Chemical Engineering Science vol 66 no 23 pp 5775ndash5790 2011
[17] R E Melchers ldquoDevelopment of new applied models for steelcorrosion in marine applications including shippingrdquo Ships andOffshore Structures vol 3 no 2 pp 135ndash144 2008
[18] M H Mohd D K Kim D W Kim and J K Paik ldquoA time-variant corrosion wastage model for subsea gas pipelinesrdquo Shipsand Offshore Structures vol 9 no 2 pp 161ndash176 2014
[19] R E Melchers and J K Paik ldquoEffect of flexure on rusting ofshiprsquos steel platingrdquo Ships and Offshore Structures vol 5 no 1pp 25ndash31 2010
[20] M Shehadeh and I Hassan ldquoStudy of sacrificial cathodicprotection on marine structures in sea and fresh water inrelation to flow conditionsrdquo Ships and Offshore Structures vol8 no 1 pp 102ndash110 2013
[21] R A Berner ldquoThermodynamic stability of sedimentary ironsulfidesrdquoTheAmerican Journal of Science vol 265 pp 773ndash7851967
[22] R H Hausler L A Goeller R P Zimmerman and R HRosenwald ldquoContribution to the ldquofilming aminerdquo theory aninterpretation of experimental resultsrdquo Corrosion vol 28 no1 pp 7ndash16 1972
[23] P Taylor ldquoThe stereochemistry of iron sulfidesmdasha structuralration for the crysta llization of some metastable phases fromaqueous solutionrdquo American Mineralogist vol 65 pp 1026ndash1030 1980
[24] R A Berner ldquoIron sulfides formed from aqueous solution atlow temperatures and atmospheric pressurerdquo The Journal ofGeology vol 72 pp 293ndash306 1964
[25] D T Rickard ldquoThe chemistry of iron sulphide formation at lowtemperaturesrdquo Stockholm Contributions in Geology vol 20 pp67ndash95 1969
[26] A J Bard R Parsons and J Jordan Standard Potentials inAqueous Solution CRC Press 1st edition 1985
Submit your manuscripts athttpwwwhindawicom
ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CorrosionInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Polymer ScienceInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CeramicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CompositesJournal of
NanoparticlesJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of
Biomaterials
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
NanoscienceJournal of
TextilesHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Journal of
NanotechnologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
CrystallographyJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CoatingsJournal of
Advances in
Materials Science and EngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Smart Materials Research
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
MetallurgyJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
BioMed Research International
MaterialsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Nano
materials
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal ofNanomaterials
International Journal of Metals 7
Inte
nsity
(cps
)
2120579 (deg)
Fe (110)
Fe (200)Fe (211)
1000
800
600
400
200
040 60 80 100 120
Carbon steel before corrosion
(a)
Inte
nsity
(cps
)
2120579 (deg)
Fe (110) Corroded in Na2SO4 + Na2S
FeS (003)
FeS (003)
FeS (113)
800
700
600
500
400
300
200
100
030 40 50 60 70 80 90 100 110
(b)
Inte
nsity
(cps
)
2120579 (deg)
1000
800
600
400
200
0
40 60 80 100 120
Fe (110)
FeCl3
FeCl3
Corroded in Na2SO4 + NaCl
(c)
Inte
nsity
(cps
)
2120579 (deg)
Fe (110)
FeS (003)
500
400
300
200
100
0
40 50 60 70 80 90 100 110
FeCl3
Corroded in Na2SO4 + Na2S + NaCl
(113)
(d)
Figure 8 X-ray diffraction showing peaks of different corroded phases
frequency The minimum phase angle reaches much lessthan 90 degrees indicating the capacitance in the circuit isnot a pure capacitance but constant phase element CPE(119884)and is given by the capacitive impedance equation 119885 =1119862(119895119908)
minus120572 where 120572 is fraction varying from 0 to 1 the valueless than one indicates that it does not behave ideally as purecapacitance At high frequency the value of impedance119885 modis roughly equal to 119877
119878 while at low frequency the value is
(119877119878+ 119877119875) Both can be determined from the blots Table 1
depicted the computed values of the EIS parameters It isseen that polarization resistance decreases with addition ofmore types of ions which support the polarization resultsof corrosion rates increase as found in Figures 2 3 and 4The increase in corrosion rate is supported by the EIS dataincrease in119884
0which behaves like capacitance and decrease in
polarization resistance (Table 2)The values of 120572 indicate thatthe capacitance behaves like constant phase element ratherthan pure capacitance The Warburg resistance 119882
119889which
signifies resistance to the flow of ions from the solution tocorroded metal surface also decreases with addition of moretypes of ions All the facts confirm the enhanced corrosionrates of carbon steel in the oil field sea water with presence ofSO4
2minus + S2minus + Clminus
33 X-Ray Diffraction The presence of corrosion productsof FeS FeCl
2 and FeCl
3is clearly indicated by the XRD
peak intensities in Figure 8 This supports the corrosionenhancement by SO
4
2minus S2minus and Clminus ions as found inpolarization and EIS studies
8 International Journal of Metals
Table 2 Computed EIS parameters
Sample Corrosive ions 119877119904(ohm) 119877
119901(ohm) 119884
0120583F 120572 119882
119889(ohm)
Carbon steel SO42minus 9443 85000 750 0720 10430
Carbon steel SO42minus + S2minus 8268 3700 5722 0769 4187
Carbon steel SO42minus + S2minus + Clminus 1331 95 9056 0602 2432
(c)
(b)(a)
Figure 9 Optical microscopy images of corroded steel (a) Na2SO4 (b) Na
2SO4+ Na2S and (c) Na
2SO4+ Na2S + NaCl
34 Optical and SEM Microscopy Images The microstruc-tures by optical microscopy clearly show (Figure 9) thecorroded structure of steel with products of corrosion overit The form of corrosion seems to be uniform not localizedSEM images (Figure 10) distinctly reveal how the degree ofdegradation increases with presence of ions in the solutionsfrom SO
4
2minus to SO4
2minus + S2minus to SO4
2minus + S2minus + Clminus It isinteresting to observe from the morphology of SEM image athigher magnification (Figure 11) that the corrosion has takenplace at grain boundary as well as grain body but the attack atgrain boundary is very prominentwith sulphide (S) corrosioncrackThe corrosion products of sulphide compounds as wellas element sulphur are clearly revealedThe structure of SEMimages shows almost catastrophic failure with the presenceof minor and major cracks when the seawater is enrichedwith the presence of all three corrosive ions SO
4
2minus S2minusand Clminus The morphology of SEM image analysis along with
XRD is in complete agreement with the corrosion data of thepolarization experiments and EIS
From (5)ndash(8) it is seen that SO4
2minus is cathodically reducedto give rise to S2minus It is observed from the polarizationcurve (Figure 1) that the increase of SO
4
2minus concentrationdepolarizes the cathodic curves much more compared toanodic ones shifting 119864corr towards the negative potentialThe release of S2minus from the reaction produces corrosionproducts iron sulfides (FeS
119909) on carbon steel surface They
are nonstoichiometric iron sulfide films mainly composed ofmackinawite and pyrrhotite Berner [21ndash26] The black colorof mackinawite phase is also seen as corrosion products inoptical microstructures (Figures 9(a) and 9(b))
The deterioration of metal due to contact with S2minus (H2S)
and moisture is sour corrosion The forms of sour corrosionare uniform (Figure 9(b)) pitting and stepwise cracking(Figures 10(c) and 11(b))
International Journal of Metals 9
(d)(c)
(b)(a)
Figure 10 SEM images of corroded steel (a) before corrosion (b) in Na2SO4 (c) in Na
2SO4+ Na2S and (d) in Na
2SO4+ Na2S + NaCl
The general equation of sour corrosion can be expressedas follows [21]
S2minus + 2H+ = H2S (aq) 119864 = 0097V versus SCE (15)
Hydrogen sulphide dissociates to produce proton andbisulphide as described [22] by the following equations
H2S (aq) = H+ +HSminus 119870 = 91 times 10minus8
HSminus = H+ + S2minus 119870 = 91 times 10minus12(16)
This reaction scheme shows that the presence of hydrogensulfide can contribute to the concentration of sulfide at thesurface by dissociation rather than charge exchange with thesurface The sulfide concentration at the surface is thereforedependent on the concentration of aqueous hydrogen sulfidein the electrolyte as well as the reduction processes of sulfate
H2S exhibits the different role in anodic process of carbon
steel depending on the pH value in the solutions The localsupersaturation of FeS
119909could be formed on the carbon steel
surface via the following reaction with the nucleation andgrowth of one or more of the iron sulfide mackinawite
H2S + Fe +H
2O 997888rarr FeS
119909+ 2H +H
2O (17)
The black corrosion products (FeS119909) formed on the steel
surface in the H2S-containing solutions could be observed
(Figure 9(b)) The addition of chloride to the corrosionsystem did not exhibit any localized corrosion rather it dras-tically increased the 119894corr values and shifted 119864corr towards thenegative potential (Figures 3 and 4) depolarizing the anodicreactionsThe increase in 119894corr with Cl addition seems to havemodified the anodic substrate area (Figures 9(c) and 10(d))and has enhanced the exchange current density for the ironoxidation process
4 Conclusion
All three corrosive ions SO4
2minus S2minus and Clminus have a strongeffect on increasing the corrosion rate of carbon steel
Cathodic reduction of SO4
2minus generates elemental S orS2minus ions in addition to S2minus from H
2S in oil The species
10 International Journal of Metals
(c)
(b)(a)
Figure 11 SEM images of corroded steel (a) in Na2SO4 (b) in Na
2SO4+ Na2S and (c) in Na
2SO4+ Na2S + NaCl
cause major corrosion with formation of FeS119909as corrosion
productsThe attack is found at grain boundarywith sulphidecracking as well as some grain body degradation The effectof addition of Clminus is the increase of 119894corr values by hundredsof times possibly due to enhancement of exchange currentdensity of the anodic reactions
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgments
The authors would like to acknowledge ldquoCOE TEQUIPrdquo inJadavpur University for the support of this work
References
[1] S Nesic ldquoKey issues related to modelling of internal corrosionof oil and gas pipelinesmdasha reviewrdquo Corrosion Science vol 49no 12 pp 4308ndash4338 2007
[2] L T Popoola A S Grema G K Latinwo B Gutti and A SBalogun ldquoCorrosion problems during oil and gas productionand its mitigationrdquo International Journal of Industrial Chem-istry vol 4 aticle 35 2013
[3] BW A Sherar P G Keech andDW Shoesmith ldquoThe effect ofsulfide on the aerobic corrosion of carbon steel in near-neutral
pH saline solutionsrdquo Corrosion Science vol 66 pp 256ndash2622013
[4] B W A Sherar I M Power P G Keech S Mitlin G Southamand D W Shoesmith ldquoCharacterizing the effect of carbon steelexposure in sulfide containing solutions to microbially inducedcorrosionrdquo Corrosion Science vol 53 no 3 pp 955ndash960 2011
[5] F M Song ldquoA comprehensive model for predicting CO2
corrosion rate in oil and gas production and transportationsystemsrdquo Electrochimica Acta vol 55 no 3 pp 689ndash700 2010
[6] M Hairil Mohd and J K Paik ldquoInvestigation of the corrosionprogress characteristics of offshore subsea oil well tubesrdquoCorrosion Science vol 67 pp 130ndash141 2013
[7] J Tang Y Shao T Zhang G Meng and F Wang ldquoCorrosionbehaviour of carbon steel in different concentrations of HClsolutions containing H
2S at 90 ∘Crdquo Corrosion Science vol 53
no 5 pp 1715ndash1723 2011[8] Z A D Foroulis ldquoRole of solution pH on wet H
2S cracking
in hydrocarbon productionrdquo Corrosion Prevention and Controlvol 40 no 4 pp 84ndash89 1993
[9] M A Veloz and I Gonzalez ldquoElectrochemical study of carbonsteel corrosion in buffered acetic acid solutions with chloridesand H
2Srdquo Electrochimica Acta vol 48 no 2 pp 135ndash144 2002
[10] S Arzola and J Genesca ldquoThe effect of H2S concentration on
the corrosion behavior of API 5L X-70 steelrdquo Journal of SolidState Electrochemistry vol 9 no 4 pp 197ndash200 2005
[11] H YMa X L Cheng S H Chen et al ldquoTheoretical interpreta-tion on impedance spectra for anodic iron dissolution in acidicsolutions containing hydrogen sulfiderdquoCorrosion vol 54 no 8pp 634ndash640 1998
International Journal of Metals 11
[12] H Ma X Cheng S Chen C Wang J Zhang and H YangldquoAn ac impedance study of the anodic dissolution of iron insulfuric acid solutions containing hydrogen sulfiderdquo Journal ofElectroanalytical Chemistry vol 451 no 1-2 pp 11ndash17 1998
[13] HMa X Cheng G Li et al ldquoThe influence of hydrogen sulfideon corrosion of iron under different conditionsrdquo CorrosionScience vol 42 no 10 pp 1669ndash1683 2000
[14] H-H HuangW-T Tsai and J-T Lee ldquoElectrochemical behav-ior of the simulated heat-affected zone of A516 carbon steel inH2S solutionrdquo Electrochimica Acta vol 41 no 7-8 pp 1191ndash1199
1996[15] J Tang Y Shao J Guo T Zhang G Meng and F Wang ldquoThe
effect of H2S concentration on the corrosion behavior of carbon
steel at 90 ∘Crdquo Corrosion Science vol 52 no 6 pp 2050ndash20582010
[16] P Smith S Roy D Swailes S Maxwell D Page and J LawsonldquoA model for the corrosion of steel subjected to syntheticproduced water containing sulfate chloride and hydrogensulfiderdquo Chemical Engineering Science vol 66 no 23 pp 5775ndash5790 2011
[17] R E Melchers ldquoDevelopment of new applied models for steelcorrosion in marine applications including shippingrdquo Ships andOffshore Structures vol 3 no 2 pp 135ndash144 2008
[18] M H Mohd D K Kim D W Kim and J K Paik ldquoA time-variant corrosion wastage model for subsea gas pipelinesrdquo Shipsand Offshore Structures vol 9 no 2 pp 161ndash176 2014
[19] R E Melchers and J K Paik ldquoEffect of flexure on rusting ofshiprsquos steel platingrdquo Ships and Offshore Structures vol 5 no 1pp 25ndash31 2010
[20] M Shehadeh and I Hassan ldquoStudy of sacrificial cathodicprotection on marine structures in sea and fresh water inrelation to flow conditionsrdquo Ships and Offshore Structures vol8 no 1 pp 102ndash110 2013
[21] R A Berner ldquoThermodynamic stability of sedimentary ironsulfidesrdquoTheAmerican Journal of Science vol 265 pp 773ndash7851967
[22] R H Hausler L A Goeller R P Zimmerman and R HRosenwald ldquoContribution to the ldquofilming aminerdquo theory aninterpretation of experimental resultsrdquo Corrosion vol 28 no1 pp 7ndash16 1972
[23] P Taylor ldquoThe stereochemistry of iron sulfidesmdasha structuralration for the crysta llization of some metastable phases fromaqueous solutionrdquo American Mineralogist vol 65 pp 1026ndash1030 1980
[24] R A Berner ldquoIron sulfides formed from aqueous solution atlow temperatures and atmospheric pressurerdquo The Journal ofGeology vol 72 pp 293ndash306 1964
[25] D T Rickard ldquoThe chemistry of iron sulphide formation at lowtemperaturesrdquo Stockholm Contributions in Geology vol 20 pp67ndash95 1969
[26] A J Bard R Parsons and J Jordan Standard Potentials inAqueous Solution CRC Press 1st edition 1985
Submit your manuscripts athttpwwwhindawicom
ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CorrosionInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Polymer ScienceInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CeramicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CompositesJournal of
NanoparticlesJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of
Biomaterials
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
NanoscienceJournal of
TextilesHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Journal of
NanotechnologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
CrystallographyJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CoatingsJournal of
Advances in
Materials Science and EngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Smart Materials Research
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
MetallurgyJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
BioMed Research International
MaterialsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Nano
materials
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal ofNanomaterials
8 International Journal of Metals
Table 2 Computed EIS parameters
Sample Corrosive ions 119877119904(ohm) 119877
119901(ohm) 119884
0120583F 120572 119882
119889(ohm)
Carbon steel SO42minus 9443 85000 750 0720 10430
Carbon steel SO42minus + S2minus 8268 3700 5722 0769 4187
Carbon steel SO42minus + S2minus + Clminus 1331 95 9056 0602 2432
(c)
(b)(a)
Figure 9 Optical microscopy images of corroded steel (a) Na2SO4 (b) Na
2SO4+ Na2S and (c) Na
2SO4+ Na2S + NaCl
34 Optical and SEM Microscopy Images The microstruc-tures by optical microscopy clearly show (Figure 9) thecorroded structure of steel with products of corrosion overit The form of corrosion seems to be uniform not localizedSEM images (Figure 10) distinctly reveal how the degree ofdegradation increases with presence of ions in the solutionsfrom SO
4
2minus to SO4
2minus + S2minus to SO4
2minus + S2minus + Clminus It isinteresting to observe from the morphology of SEM image athigher magnification (Figure 11) that the corrosion has takenplace at grain boundary as well as grain body but the attack atgrain boundary is very prominentwith sulphide (S) corrosioncrackThe corrosion products of sulphide compounds as wellas element sulphur are clearly revealedThe structure of SEMimages shows almost catastrophic failure with the presenceof minor and major cracks when the seawater is enrichedwith the presence of all three corrosive ions SO
4
2minus S2minusand Clminus The morphology of SEM image analysis along with
XRD is in complete agreement with the corrosion data of thepolarization experiments and EIS
From (5)ndash(8) it is seen that SO4
2minus is cathodically reducedto give rise to S2minus It is observed from the polarizationcurve (Figure 1) that the increase of SO
4
2minus concentrationdepolarizes the cathodic curves much more compared toanodic ones shifting 119864corr towards the negative potentialThe release of S2minus from the reaction produces corrosionproducts iron sulfides (FeS
119909) on carbon steel surface They
are nonstoichiometric iron sulfide films mainly composed ofmackinawite and pyrrhotite Berner [21ndash26] The black colorof mackinawite phase is also seen as corrosion products inoptical microstructures (Figures 9(a) and 9(b))
The deterioration of metal due to contact with S2minus (H2S)
and moisture is sour corrosion The forms of sour corrosionare uniform (Figure 9(b)) pitting and stepwise cracking(Figures 10(c) and 11(b))
International Journal of Metals 9
(d)(c)
(b)(a)
Figure 10 SEM images of corroded steel (a) before corrosion (b) in Na2SO4 (c) in Na
2SO4+ Na2S and (d) in Na
2SO4+ Na2S + NaCl
The general equation of sour corrosion can be expressedas follows [21]
S2minus + 2H+ = H2S (aq) 119864 = 0097V versus SCE (15)
Hydrogen sulphide dissociates to produce proton andbisulphide as described [22] by the following equations
H2S (aq) = H+ +HSminus 119870 = 91 times 10minus8
HSminus = H+ + S2minus 119870 = 91 times 10minus12(16)
This reaction scheme shows that the presence of hydrogensulfide can contribute to the concentration of sulfide at thesurface by dissociation rather than charge exchange with thesurface The sulfide concentration at the surface is thereforedependent on the concentration of aqueous hydrogen sulfidein the electrolyte as well as the reduction processes of sulfate
H2S exhibits the different role in anodic process of carbon
steel depending on the pH value in the solutions The localsupersaturation of FeS
119909could be formed on the carbon steel
surface via the following reaction with the nucleation andgrowth of one or more of the iron sulfide mackinawite
H2S + Fe +H
2O 997888rarr FeS
119909+ 2H +H
2O (17)
The black corrosion products (FeS119909) formed on the steel
surface in the H2S-containing solutions could be observed
(Figure 9(b)) The addition of chloride to the corrosionsystem did not exhibit any localized corrosion rather it dras-tically increased the 119894corr values and shifted 119864corr towards thenegative potential (Figures 3 and 4) depolarizing the anodicreactionsThe increase in 119894corr with Cl addition seems to havemodified the anodic substrate area (Figures 9(c) and 10(d))and has enhanced the exchange current density for the ironoxidation process
4 Conclusion
All three corrosive ions SO4
2minus S2minus and Clminus have a strongeffect on increasing the corrosion rate of carbon steel
Cathodic reduction of SO4
2minus generates elemental S orS2minus ions in addition to S2minus from H
2S in oil The species
10 International Journal of Metals
(c)
(b)(a)
Figure 11 SEM images of corroded steel (a) in Na2SO4 (b) in Na
2SO4+ Na2S and (c) in Na
2SO4+ Na2S + NaCl
cause major corrosion with formation of FeS119909as corrosion
productsThe attack is found at grain boundarywith sulphidecracking as well as some grain body degradation The effectof addition of Clminus is the increase of 119894corr values by hundredsof times possibly due to enhancement of exchange currentdensity of the anodic reactions
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgments
The authors would like to acknowledge ldquoCOE TEQUIPrdquo inJadavpur University for the support of this work
References
[1] S Nesic ldquoKey issues related to modelling of internal corrosionof oil and gas pipelinesmdasha reviewrdquo Corrosion Science vol 49no 12 pp 4308ndash4338 2007
[2] L T Popoola A S Grema G K Latinwo B Gutti and A SBalogun ldquoCorrosion problems during oil and gas productionand its mitigationrdquo International Journal of Industrial Chem-istry vol 4 aticle 35 2013
[3] BW A Sherar P G Keech andDW Shoesmith ldquoThe effect ofsulfide on the aerobic corrosion of carbon steel in near-neutral
pH saline solutionsrdquo Corrosion Science vol 66 pp 256ndash2622013
[4] B W A Sherar I M Power P G Keech S Mitlin G Southamand D W Shoesmith ldquoCharacterizing the effect of carbon steelexposure in sulfide containing solutions to microbially inducedcorrosionrdquo Corrosion Science vol 53 no 3 pp 955ndash960 2011
[5] F M Song ldquoA comprehensive model for predicting CO2
corrosion rate in oil and gas production and transportationsystemsrdquo Electrochimica Acta vol 55 no 3 pp 689ndash700 2010
[6] M Hairil Mohd and J K Paik ldquoInvestigation of the corrosionprogress characteristics of offshore subsea oil well tubesrdquoCorrosion Science vol 67 pp 130ndash141 2013
[7] J Tang Y Shao T Zhang G Meng and F Wang ldquoCorrosionbehaviour of carbon steel in different concentrations of HClsolutions containing H
2S at 90 ∘Crdquo Corrosion Science vol 53
no 5 pp 1715ndash1723 2011[8] Z A D Foroulis ldquoRole of solution pH on wet H
2S cracking
in hydrocarbon productionrdquo Corrosion Prevention and Controlvol 40 no 4 pp 84ndash89 1993
[9] M A Veloz and I Gonzalez ldquoElectrochemical study of carbonsteel corrosion in buffered acetic acid solutions with chloridesand H
2Srdquo Electrochimica Acta vol 48 no 2 pp 135ndash144 2002
[10] S Arzola and J Genesca ldquoThe effect of H2S concentration on
the corrosion behavior of API 5L X-70 steelrdquo Journal of SolidState Electrochemistry vol 9 no 4 pp 197ndash200 2005
[11] H YMa X L Cheng S H Chen et al ldquoTheoretical interpreta-tion on impedance spectra for anodic iron dissolution in acidicsolutions containing hydrogen sulfiderdquoCorrosion vol 54 no 8pp 634ndash640 1998
International Journal of Metals 11
[12] H Ma X Cheng S Chen C Wang J Zhang and H YangldquoAn ac impedance study of the anodic dissolution of iron insulfuric acid solutions containing hydrogen sulfiderdquo Journal ofElectroanalytical Chemistry vol 451 no 1-2 pp 11ndash17 1998
[13] HMa X Cheng G Li et al ldquoThe influence of hydrogen sulfideon corrosion of iron under different conditionsrdquo CorrosionScience vol 42 no 10 pp 1669ndash1683 2000
[14] H-H HuangW-T Tsai and J-T Lee ldquoElectrochemical behav-ior of the simulated heat-affected zone of A516 carbon steel inH2S solutionrdquo Electrochimica Acta vol 41 no 7-8 pp 1191ndash1199
1996[15] J Tang Y Shao J Guo T Zhang G Meng and F Wang ldquoThe
effect of H2S concentration on the corrosion behavior of carbon
steel at 90 ∘Crdquo Corrosion Science vol 52 no 6 pp 2050ndash20582010
[16] P Smith S Roy D Swailes S Maxwell D Page and J LawsonldquoA model for the corrosion of steel subjected to syntheticproduced water containing sulfate chloride and hydrogensulfiderdquo Chemical Engineering Science vol 66 no 23 pp 5775ndash5790 2011
[17] R E Melchers ldquoDevelopment of new applied models for steelcorrosion in marine applications including shippingrdquo Ships andOffshore Structures vol 3 no 2 pp 135ndash144 2008
[18] M H Mohd D K Kim D W Kim and J K Paik ldquoA time-variant corrosion wastage model for subsea gas pipelinesrdquo Shipsand Offshore Structures vol 9 no 2 pp 161ndash176 2014
[19] R E Melchers and J K Paik ldquoEffect of flexure on rusting ofshiprsquos steel platingrdquo Ships and Offshore Structures vol 5 no 1pp 25ndash31 2010
[20] M Shehadeh and I Hassan ldquoStudy of sacrificial cathodicprotection on marine structures in sea and fresh water inrelation to flow conditionsrdquo Ships and Offshore Structures vol8 no 1 pp 102ndash110 2013
[21] R A Berner ldquoThermodynamic stability of sedimentary ironsulfidesrdquoTheAmerican Journal of Science vol 265 pp 773ndash7851967
[22] R H Hausler L A Goeller R P Zimmerman and R HRosenwald ldquoContribution to the ldquofilming aminerdquo theory aninterpretation of experimental resultsrdquo Corrosion vol 28 no1 pp 7ndash16 1972
[23] P Taylor ldquoThe stereochemistry of iron sulfidesmdasha structuralration for the crysta llization of some metastable phases fromaqueous solutionrdquo American Mineralogist vol 65 pp 1026ndash1030 1980
[24] R A Berner ldquoIron sulfides formed from aqueous solution atlow temperatures and atmospheric pressurerdquo The Journal ofGeology vol 72 pp 293ndash306 1964
[25] D T Rickard ldquoThe chemistry of iron sulphide formation at lowtemperaturesrdquo Stockholm Contributions in Geology vol 20 pp67ndash95 1969
[26] A J Bard R Parsons and J Jordan Standard Potentials inAqueous Solution CRC Press 1st edition 1985
Submit your manuscripts athttpwwwhindawicom
ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CorrosionInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Polymer ScienceInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CeramicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CompositesJournal of
NanoparticlesJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of
Biomaterials
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
NanoscienceJournal of
TextilesHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Journal of
NanotechnologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
CrystallographyJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CoatingsJournal of
Advances in
Materials Science and EngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Smart Materials Research
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
MetallurgyJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
BioMed Research International
MaterialsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Nano
materials
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal ofNanomaterials
International Journal of Metals 9
(d)(c)
(b)(a)
Figure 10 SEM images of corroded steel (a) before corrosion (b) in Na2SO4 (c) in Na
2SO4+ Na2S and (d) in Na
2SO4+ Na2S + NaCl
The general equation of sour corrosion can be expressedas follows [21]
S2minus + 2H+ = H2S (aq) 119864 = 0097V versus SCE (15)
Hydrogen sulphide dissociates to produce proton andbisulphide as described [22] by the following equations
H2S (aq) = H+ +HSminus 119870 = 91 times 10minus8
HSminus = H+ + S2minus 119870 = 91 times 10minus12(16)
This reaction scheme shows that the presence of hydrogensulfide can contribute to the concentration of sulfide at thesurface by dissociation rather than charge exchange with thesurface The sulfide concentration at the surface is thereforedependent on the concentration of aqueous hydrogen sulfidein the electrolyte as well as the reduction processes of sulfate
H2S exhibits the different role in anodic process of carbon
steel depending on the pH value in the solutions The localsupersaturation of FeS
119909could be formed on the carbon steel
surface via the following reaction with the nucleation andgrowth of one or more of the iron sulfide mackinawite
H2S + Fe +H
2O 997888rarr FeS
119909+ 2H +H
2O (17)
The black corrosion products (FeS119909) formed on the steel
surface in the H2S-containing solutions could be observed
(Figure 9(b)) The addition of chloride to the corrosionsystem did not exhibit any localized corrosion rather it dras-tically increased the 119894corr values and shifted 119864corr towards thenegative potential (Figures 3 and 4) depolarizing the anodicreactionsThe increase in 119894corr with Cl addition seems to havemodified the anodic substrate area (Figures 9(c) and 10(d))and has enhanced the exchange current density for the ironoxidation process
4 Conclusion
All three corrosive ions SO4
2minus S2minus and Clminus have a strongeffect on increasing the corrosion rate of carbon steel
Cathodic reduction of SO4
2minus generates elemental S orS2minus ions in addition to S2minus from H
2S in oil The species
10 International Journal of Metals
(c)
(b)(a)
Figure 11 SEM images of corroded steel (a) in Na2SO4 (b) in Na
2SO4+ Na2S and (c) in Na
2SO4+ Na2S + NaCl
cause major corrosion with formation of FeS119909as corrosion
productsThe attack is found at grain boundarywith sulphidecracking as well as some grain body degradation The effectof addition of Clminus is the increase of 119894corr values by hundredsof times possibly due to enhancement of exchange currentdensity of the anodic reactions
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgments
The authors would like to acknowledge ldquoCOE TEQUIPrdquo inJadavpur University for the support of this work
References
[1] S Nesic ldquoKey issues related to modelling of internal corrosionof oil and gas pipelinesmdasha reviewrdquo Corrosion Science vol 49no 12 pp 4308ndash4338 2007
[2] L T Popoola A S Grema G K Latinwo B Gutti and A SBalogun ldquoCorrosion problems during oil and gas productionand its mitigationrdquo International Journal of Industrial Chem-istry vol 4 aticle 35 2013
[3] BW A Sherar P G Keech andDW Shoesmith ldquoThe effect ofsulfide on the aerobic corrosion of carbon steel in near-neutral
pH saline solutionsrdquo Corrosion Science vol 66 pp 256ndash2622013
[4] B W A Sherar I M Power P G Keech S Mitlin G Southamand D W Shoesmith ldquoCharacterizing the effect of carbon steelexposure in sulfide containing solutions to microbially inducedcorrosionrdquo Corrosion Science vol 53 no 3 pp 955ndash960 2011
[5] F M Song ldquoA comprehensive model for predicting CO2
corrosion rate in oil and gas production and transportationsystemsrdquo Electrochimica Acta vol 55 no 3 pp 689ndash700 2010
[6] M Hairil Mohd and J K Paik ldquoInvestigation of the corrosionprogress characteristics of offshore subsea oil well tubesrdquoCorrosion Science vol 67 pp 130ndash141 2013
[7] J Tang Y Shao T Zhang G Meng and F Wang ldquoCorrosionbehaviour of carbon steel in different concentrations of HClsolutions containing H
2S at 90 ∘Crdquo Corrosion Science vol 53
no 5 pp 1715ndash1723 2011[8] Z A D Foroulis ldquoRole of solution pH on wet H
2S cracking
in hydrocarbon productionrdquo Corrosion Prevention and Controlvol 40 no 4 pp 84ndash89 1993
[9] M A Veloz and I Gonzalez ldquoElectrochemical study of carbonsteel corrosion in buffered acetic acid solutions with chloridesand H
2Srdquo Electrochimica Acta vol 48 no 2 pp 135ndash144 2002
[10] S Arzola and J Genesca ldquoThe effect of H2S concentration on
the corrosion behavior of API 5L X-70 steelrdquo Journal of SolidState Electrochemistry vol 9 no 4 pp 197ndash200 2005
[11] H YMa X L Cheng S H Chen et al ldquoTheoretical interpreta-tion on impedance spectra for anodic iron dissolution in acidicsolutions containing hydrogen sulfiderdquoCorrosion vol 54 no 8pp 634ndash640 1998
International Journal of Metals 11
[12] H Ma X Cheng S Chen C Wang J Zhang and H YangldquoAn ac impedance study of the anodic dissolution of iron insulfuric acid solutions containing hydrogen sulfiderdquo Journal ofElectroanalytical Chemistry vol 451 no 1-2 pp 11ndash17 1998
[13] HMa X Cheng G Li et al ldquoThe influence of hydrogen sulfideon corrosion of iron under different conditionsrdquo CorrosionScience vol 42 no 10 pp 1669ndash1683 2000
[14] H-H HuangW-T Tsai and J-T Lee ldquoElectrochemical behav-ior of the simulated heat-affected zone of A516 carbon steel inH2S solutionrdquo Electrochimica Acta vol 41 no 7-8 pp 1191ndash1199
1996[15] J Tang Y Shao J Guo T Zhang G Meng and F Wang ldquoThe
effect of H2S concentration on the corrosion behavior of carbon
steel at 90 ∘Crdquo Corrosion Science vol 52 no 6 pp 2050ndash20582010
[16] P Smith S Roy D Swailes S Maxwell D Page and J LawsonldquoA model for the corrosion of steel subjected to syntheticproduced water containing sulfate chloride and hydrogensulfiderdquo Chemical Engineering Science vol 66 no 23 pp 5775ndash5790 2011
[17] R E Melchers ldquoDevelopment of new applied models for steelcorrosion in marine applications including shippingrdquo Ships andOffshore Structures vol 3 no 2 pp 135ndash144 2008
[18] M H Mohd D K Kim D W Kim and J K Paik ldquoA time-variant corrosion wastage model for subsea gas pipelinesrdquo Shipsand Offshore Structures vol 9 no 2 pp 161ndash176 2014
[19] R E Melchers and J K Paik ldquoEffect of flexure on rusting ofshiprsquos steel platingrdquo Ships and Offshore Structures vol 5 no 1pp 25ndash31 2010
[20] M Shehadeh and I Hassan ldquoStudy of sacrificial cathodicprotection on marine structures in sea and fresh water inrelation to flow conditionsrdquo Ships and Offshore Structures vol8 no 1 pp 102ndash110 2013
[21] R A Berner ldquoThermodynamic stability of sedimentary ironsulfidesrdquoTheAmerican Journal of Science vol 265 pp 773ndash7851967
[22] R H Hausler L A Goeller R P Zimmerman and R HRosenwald ldquoContribution to the ldquofilming aminerdquo theory aninterpretation of experimental resultsrdquo Corrosion vol 28 no1 pp 7ndash16 1972
[23] P Taylor ldquoThe stereochemistry of iron sulfidesmdasha structuralration for the crysta llization of some metastable phases fromaqueous solutionrdquo American Mineralogist vol 65 pp 1026ndash1030 1980
[24] R A Berner ldquoIron sulfides formed from aqueous solution atlow temperatures and atmospheric pressurerdquo The Journal ofGeology vol 72 pp 293ndash306 1964
[25] D T Rickard ldquoThe chemistry of iron sulphide formation at lowtemperaturesrdquo Stockholm Contributions in Geology vol 20 pp67ndash95 1969
[26] A J Bard R Parsons and J Jordan Standard Potentials inAqueous Solution CRC Press 1st edition 1985
Submit your manuscripts athttpwwwhindawicom
ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CorrosionInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Polymer ScienceInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CeramicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CompositesJournal of
NanoparticlesJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of
Biomaterials
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
NanoscienceJournal of
TextilesHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Journal of
NanotechnologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
CrystallographyJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CoatingsJournal of
Advances in
Materials Science and EngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Smart Materials Research
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
MetallurgyJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
BioMed Research International
MaterialsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Nano
materials
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal ofNanomaterials
10 International Journal of Metals
(c)
(b)(a)
Figure 11 SEM images of corroded steel (a) in Na2SO4 (b) in Na
2SO4+ Na2S and (c) in Na
2SO4+ Na2S + NaCl
cause major corrosion with formation of FeS119909as corrosion
productsThe attack is found at grain boundarywith sulphidecracking as well as some grain body degradation The effectof addition of Clminus is the increase of 119894corr values by hundredsof times possibly due to enhancement of exchange currentdensity of the anodic reactions
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgments
The authors would like to acknowledge ldquoCOE TEQUIPrdquo inJadavpur University for the support of this work
References
[1] S Nesic ldquoKey issues related to modelling of internal corrosionof oil and gas pipelinesmdasha reviewrdquo Corrosion Science vol 49no 12 pp 4308ndash4338 2007
[2] L T Popoola A S Grema G K Latinwo B Gutti and A SBalogun ldquoCorrosion problems during oil and gas productionand its mitigationrdquo International Journal of Industrial Chem-istry vol 4 aticle 35 2013
[3] BW A Sherar P G Keech andDW Shoesmith ldquoThe effect ofsulfide on the aerobic corrosion of carbon steel in near-neutral
pH saline solutionsrdquo Corrosion Science vol 66 pp 256ndash2622013
[4] B W A Sherar I M Power P G Keech S Mitlin G Southamand D W Shoesmith ldquoCharacterizing the effect of carbon steelexposure in sulfide containing solutions to microbially inducedcorrosionrdquo Corrosion Science vol 53 no 3 pp 955ndash960 2011
[5] F M Song ldquoA comprehensive model for predicting CO2
corrosion rate in oil and gas production and transportationsystemsrdquo Electrochimica Acta vol 55 no 3 pp 689ndash700 2010
[6] M Hairil Mohd and J K Paik ldquoInvestigation of the corrosionprogress characteristics of offshore subsea oil well tubesrdquoCorrosion Science vol 67 pp 130ndash141 2013
[7] J Tang Y Shao T Zhang G Meng and F Wang ldquoCorrosionbehaviour of carbon steel in different concentrations of HClsolutions containing H
2S at 90 ∘Crdquo Corrosion Science vol 53
no 5 pp 1715ndash1723 2011[8] Z A D Foroulis ldquoRole of solution pH on wet H
2S cracking
in hydrocarbon productionrdquo Corrosion Prevention and Controlvol 40 no 4 pp 84ndash89 1993
[9] M A Veloz and I Gonzalez ldquoElectrochemical study of carbonsteel corrosion in buffered acetic acid solutions with chloridesand H
2Srdquo Electrochimica Acta vol 48 no 2 pp 135ndash144 2002
[10] S Arzola and J Genesca ldquoThe effect of H2S concentration on
the corrosion behavior of API 5L X-70 steelrdquo Journal of SolidState Electrochemistry vol 9 no 4 pp 197ndash200 2005
[11] H YMa X L Cheng S H Chen et al ldquoTheoretical interpreta-tion on impedance spectra for anodic iron dissolution in acidicsolutions containing hydrogen sulfiderdquoCorrosion vol 54 no 8pp 634ndash640 1998
International Journal of Metals 11
[12] H Ma X Cheng S Chen C Wang J Zhang and H YangldquoAn ac impedance study of the anodic dissolution of iron insulfuric acid solutions containing hydrogen sulfiderdquo Journal ofElectroanalytical Chemistry vol 451 no 1-2 pp 11ndash17 1998
[13] HMa X Cheng G Li et al ldquoThe influence of hydrogen sulfideon corrosion of iron under different conditionsrdquo CorrosionScience vol 42 no 10 pp 1669ndash1683 2000
[14] H-H HuangW-T Tsai and J-T Lee ldquoElectrochemical behav-ior of the simulated heat-affected zone of A516 carbon steel inH2S solutionrdquo Electrochimica Acta vol 41 no 7-8 pp 1191ndash1199
1996[15] J Tang Y Shao J Guo T Zhang G Meng and F Wang ldquoThe
effect of H2S concentration on the corrosion behavior of carbon
steel at 90 ∘Crdquo Corrosion Science vol 52 no 6 pp 2050ndash20582010
[16] P Smith S Roy D Swailes S Maxwell D Page and J LawsonldquoA model for the corrosion of steel subjected to syntheticproduced water containing sulfate chloride and hydrogensulfiderdquo Chemical Engineering Science vol 66 no 23 pp 5775ndash5790 2011
[17] R E Melchers ldquoDevelopment of new applied models for steelcorrosion in marine applications including shippingrdquo Ships andOffshore Structures vol 3 no 2 pp 135ndash144 2008
[18] M H Mohd D K Kim D W Kim and J K Paik ldquoA time-variant corrosion wastage model for subsea gas pipelinesrdquo Shipsand Offshore Structures vol 9 no 2 pp 161ndash176 2014
[19] R E Melchers and J K Paik ldquoEffect of flexure on rusting ofshiprsquos steel platingrdquo Ships and Offshore Structures vol 5 no 1pp 25ndash31 2010
[20] M Shehadeh and I Hassan ldquoStudy of sacrificial cathodicprotection on marine structures in sea and fresh water inrelation to flow conditionsrdquo Ships and Offshore Structures vol8 no 1 pp 102ndash110 2013
[21] R A Berner ldquoThermodynamic stability of sedimentary ironsulfidesrdquoTheAmerican Journal of Science vol 265 pp 773ndash7851967
[22] R H Hausler L A Goeller R P Zimmerman and R HRosenwald ldquoContribution to the ldquofilming aminerdquo theory aninterpretation of experimental resultsrdquo Corrosion vol 28 no1 pp 7ndash16 1972
[23] P Taylor ldquoThe stereochemistry of iron sulfidesmdasha structuralration for the crysta llization of some metastable phases fromaqueous solutionrdquo American Mineralogist vol 65 pp 1026ndash1030 1980
[24] R A Berner ldquoIron sulfides formed from aqueous solution atlow temperatures and atmospheric pressurerdquo The Journal ofGeology vol 72 pp 293ndash306 1964
[25] D T Rickard ldquoThe chemistry of iron sulphide formation at lowtemperaturesrdquo Stockholm Contributions in Geology vol 20 pp67ndash95 1969
[26] A J Bard R Parsons and J Jordan Standard Potentials inAqueous Solution CRC Press 1st edition 1985
Submit your manuscripts athttpwwwhindawicom
ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CorrosionInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Polymer ScienceInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CeramicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CompositesJournal of
NanoparticlesJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of
Biomaterials
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
NanoscienceJournal of
TextilesHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Journal of
NanotechnologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
CrystallographyJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CoatingsJournal of
Advances in
Materials Science and EngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Smart Materials Research
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
MetallurgyJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
BioMed Research International
MaterialsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Nano
materials
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal ofNanomaterials
International Journal of Metals 11
[12] H Ma X Cheng S Chen C Wang J Zhang and H YangldquoAn ac impedance study of the anodic dissolution of iron insulfuric acid solutions containing hydrogen sulfiderdquo Journal ofElectroanalytical Chemistry vol 451 no 1-2 pp 11ndash17 1998
[13] HMa X Cheng G Li et al ldquoThe influence of hydrogen sulfideon corrosion of iron under different conditionsrdquo CorrosionScience vol 42 no 10 pp 1669ndash1683 2000
[14] H-H HuangW-T Tsai and J-T Lee ldquoElectrochemical behav-ior of the simulated heat-affected zone of A516 carbon steel inH2S solutionrdquo Electrochimica Acta vol 41 no 7-8 pp 1191ndash1199
1996[15] J Tang Y Shao J Guo T Zhang G Meng and F Wang ldquoThe
effect of H2S concentration on the corrosion behavior of carbon
steel at 90 ∘Crdquo Corrosion Science vol 52 no 6 pp 2050ndash20582010
[16] P Smith S Roy D Swailes S Maxwell D Page and J LawsonldquoA model for the corrosion of steel subjected to syntheticproduced water containing sulfate chloride and hydrogensulfiderdquo Chemical Engineering Science vol 66 no 23 pp 5775ndash5790 2011
[17] R E Melchers ldquoDevelopment of new applied models for steelcorrosion in marine applications including shippingrdquo Ships andOffshore Structures vol 3 no 2 pp 135ndash144 2008
[18] M H Mohd D K Kim D W Kim and J K Paik ldquoA time-variant corrosion wastage model for subsea gas pipelinesrdquo Shipsand Offshore Structures vol 9 no 2 pp 161ndash176 2014
[19] R E Melchers and J K Paik ldquoEffect of flexure on rusting ofshiprsquos steel platingrdquo Ships and Offshore Structures vol 5 no 1pp 25ndash31 2010
[20] M Shehadeh and I Hassan ldquoStudy of sacrificial cathodicprotection on marine structures in sea and fresh water inrelation to flow conditionsrdquo Ships and Offshore Structures vol8 no 1 pp 102ndash110 2013
[21] R A Berner ldquoThermodynamic stability of sedimentary ironsulfidesrdquoTheAmerican Journal of Science vol 265 pp 773ndash7851967
[22] R H Hausler L A Goeller R P Zimmerman and R HRosenwald ldquoContribution to the ldquofilming aminerdquo theory aninterpretation of experimental resultsrdquo Corrosion vol 28 no1 pp 7ndash16 1972
[23] P Taylor ldquoThe stereochemistry of iron sulfidesmdasha structuralration for the crysta llization of some metastable phases fromaqueous solutionrdquo American Mineralogist vol 65 pp 1026ndash1030 1980
[24] R A Berner ldquoIron sulfides formed from aqueous solution atlow temperatures and atmospheric pressurerdquo The Journal ofGeology vol 72 pp 293ndash306 1964
[25] D T Rickard ldquoThe chemistry of iron sulphide formation at lowtemperaturesrdquo Stockholm Contributions in Geology vol 20 pp67ndash95 1969
[26] A J Bard R Parsons and J Jordan Standard Potentials inAqueous Solution CRC Press 1st edition 1985
Submit your manuscripts athttpwwwhindawicom
ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CorrosionInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Polymer ScienceInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CeramicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CompositesJournal of
NanoparticlesJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of
Biomaterials
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
NanoscienceJournal of
TextilesHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Journal of
NanotechnologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
CrystallographyJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CoatingsJournal of
Advances in
Materials Science and EngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Smart Materials Research
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
MetallurgyJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
BioMed Research International
MaterialsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Nano
materials
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal ofNanomaterials
Submit your manuscripts athttpwwwhindawicom
ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CorrosionInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Polymer ScienceInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CeramicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CompositesJournal of
NanoparticlesJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of
Biomaterials
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
NanoscienceJournal of
TextilesHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Journal of
NanotechnologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
CrystallographyJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CoatingsJournal of
Advances in
Materials Science and EngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Smart Materials Research
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
MetallurgyJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
BioMed Research International
MaterialsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Nano
materials
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal ofNanomaterials