lmti

sstry D

5-9

B. Cyclic voltammetry

5 cpednismes,dem

affectfacturipresenwater

tain aspects of the initial formation of such lms in solutionscontaining low sulde concentrations are difcult to elucidate.The widely accepted initial corrosion mechanism was proposedby Iofa et al. [10]. These authors demonstrated that H2S at lowpH or SH at medium pH forms a monolayer lm through chemi-sorption on iron. This behavior is similar to OH adsorption oniron, which promotes its dissolution [11]. The initial steps in thin

2d (Eq. (3))

form a mackinawite lm (Eq. (4)):

FeHSads H2O! Fe2aq H2S OHaq

FeHSads xFe2aq ! Fe1xSs Haq 4Depending on the pH and electrochemical potential, other spe-

cies of iron sulde may be formed. In sulde solutions with pH val-ues between 6.5 and 8.0 and under open circuit conditions,mackinawite is the major corrosion product formed on the interfa-cial iron [14,15]. At pH values lower than 6.5, mackinawite has

Corresponding author. Tel.: +55 27 33352486; fax: +55 27 33352460.

Corrosion Science 74 (2013) 214222

Contents lists available at

n

.e lE-mail address: marcosbj@hotmail.com (M.B.J.G. Freitas).tion and other properties of the iron lms formed depend on thekinetic mechanism involved in their formation.

Several investigations of iron lm formation have been con-ducted with pure iron and carbon steel in sulde ion solutions. Cer-

FeHSads ! FeHSads 2e

The adsorbed FeHSads species may be dissolve0010-938X/$ - see front matter 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.corsci.2013.04.045and/or

3fer from those of sea water [1]. The presence of sulfur in petroleumand in the existing water in oil wells causes problems such as local-ized corrosion and the formation of various iron anodic lms [25].The iron lms can passivate the steel and may also induce dissolu-tion of the metal either through the anodic lm or in regions whereno iron lm is present [68]. The kinetic mechanism of iron anodiclm formation depends primarily on pH, temperature and suldeconcentration [3,9]. The crystal structure, morphology, composi-

Troilite is a stable monosulde but can be considered metastablecompared to pyrite, a disulde. Iron sulde phases such as macki-nawite, troilite, marcasite and pyrite are formed even at low suldeion concentrations, e.g., parts per million (ppm) [14]. The chemicaland electrochemical processes that occur during the formation ofthese minerals are represented by the following equations [11].

Fes HSaq ! FeHSads 1B. Mssbauer spectroscopyC. Sulphide cracking

1. Introduction

Steel corrosion is a problem thattries, especially the petroleummanution is usually accompanied by thecontent and ionic composition of thes many types of indus-ng industry. Oil produc-ce of water. The solidspresent in oil wells dif-

sulde lm formation may involve the formation of cubic iron sul-de (FeS), tetragonal mackinawite (FeS1x), and hexagonal troilite(FeS) [1214]. The stability of these compounds, in descending or-der, is troilite > mackinawite > cubic iron sulde. Therefore, inprinciple, cubic iron sulde is the most unstable and easily con-verts to mackinawite, which is thermodynamically more stable.A. Carbon steelB. EIS

results that detected pyrite as the major proportion (94%) of the iron species in the corrosion product. 2013 Elsevier Ltd. All rights reserved.Characterization of AISI 1005 corrosion voltammetry of low sulde ion concentra

N. Perini a, P.G. Corradini a, V.P. Nascimento b, E.C. Paa Laboratory of Research and Development of Methodologies for Analysis of Oils, ChemisVitria, Esprito Santo 29075-910, Brazilb Physical Department, Av. Fernando Ferrari 514, Goiabeiras, Vitria, Esprito Santo 2907

a r t i c l e i n f o

Article history:Received 4 December 2012Accepted 24 April 2013Available online 3 May 2013

Keywords:

a b s t r a c t

The mechanism of AISI 100metry, electrochemical im(MS). The proposed mechaby adsorption of HS speciiron. This mechanism was

Corrosio

journal homepage: wwws grown under cyclicons

amani b, M.B.J.G. Freitas a,epartment, Federal University of Esprito Santo, Av. Fernando Ferrari 514, Goiabeiras,

10, Brazil

orrosion in sulde ion solutions has been investigated using cyclic voltam-ance spectroscopy, X-ray diffraction (XRD) and Mssbauer spectroscopyoccurs with the initial formation of oxygenated ferrous species followed

precipitation of iron monosuldes and their partial conversion to bisuldeonstrated by XRD results that revealed Fe-O and Fe-S phases and by MS

SciVerse ScienceDirect

Science

sevier .com/locate /corsc i

FabioHighlight

FabioHighlight

FabioHighlight

FabioHighlight

Sciehigh solubility and Eq. (4) dominates, resulting in enhancement ofthe corrosion rate. The results of some studies using high suldeion concentrations in the presence of cations such as K+, Mg2+

and Ca2+ to simulate eld operation conditions [2,9,16] have beenreported in the literature. These ions may interact with the HxS(x = 1, 2) adsorbate at the interface and produce dubious electro-chemical results.

In this work, the corrosion of AISI 1005 under low sulde solu-tion concentration was studied in an attempt to elucidate the ini-tial processes of sulde lm formation. This work conciselyelucidates the formation and aging mechanisms of sulde lms.The iron anodic lms were grown using cyclic voltammetric tech-niques. The characterization of the corrosion products was per-formed using electrochemical impedance spectroscopy, scanningelectron microscopy (SEM), energy dispersive X-ray spectroscopy(EDX), X-ray diffraction (XRD) and Mssbauer spectroscopy (MS).

2. Experimental procedures

2.1. Electrochemical experiments

The sulde ion solutions containing 0.013 mmol L1,0.19 mmol L1, 0.38 mmol L1 and 0.83 mmol L1 were preparedwith the reagent Na2S9H2O. The supporting electrolyte solutionwas 0.1 mol L1 Na2SO4. The solution pH was adjusted to 6.5 withdiluted solutions of H2SO4 and NaOH. All solutions were freshlyprepared before each experiment using deionised water. A conven-tional three-electrode cell with a thermostatic bath was used. Thecell consisted of a counter electrode of platinum plate with an areaof 3.7 cm2 and a working electrode of AISI 1005 steel with an areaof 0.20 cm2 and of the following composition (% w/w): 0.063% C,0.009% Si, 0.015% P, 0.060% Al, 0.015% S, 0.302% Mn and balancedFe. The surface was mechanically polished with 320, 400 and 600grit sandpaper. The surface was washed with water, placed in anultrasonic bath with acetone for 10 min, cleaned in water for5 min, dried and immediately immersed in the working solution.The reference electrode contained Ag/AgCl (3.0 mol L1 KCl).In electrode Ag/AgCl used in the electrochemical tests the con-tact with electrolyte was accomplished through a ceramic junc-tion point, which allows low electrolyte ow rate, in orderto hinder the electrolyte solution test contamination with the ref-erence electrolyte and vice versa. For verify the implications ofthe reference Ag/AgCl in the electrochemical tests, the potential ofthe Ag/AgCl used in the electrochemical tests was monitored rela-tive to another Ag/AgCl (not used in these electrochemicalmeasurements). The potential difference between the Ag/AgCl electrode used in this paper vs. other Ag/AgCl electrode (notused in these electrochemical measurements) was always lessthan 4 mV. Indeed, analyzing the several voltammograms obtainedduring this work, all of them (including theirs triplicates) present-ing the anodic and cathodic peaks current at the same potentialvalues, irrespective of sulde concentration Therefore, the lowelectrolyte ow does not affect the results obtained.

All obtained potential values were corrected to the standardhydrogen electrode (SHE).

The cyclic voltammetry experiments consisted of 10 cyclesstarting from an initial potential of590 mVSHE and sweeping until480 mVSHE; the scan direction was then reversed until890 mVSHE was reached at a sweep rate of 10 mV s1.

After these experiments were performed, the corrosion producton the steel electrode surface was washed with distilled water toeliminate the excess of sulde and sulfate ions in the lms and im-mersed in a solution of 0.1 mol L1 Na2SO4 (pH 6.5) to record elec-

N. Perini et al. / Corrosiontrochemical impedance spectroscopy data over a frequency rangeof 104103 Hz with 10 points per decade and a sinusoidal signalof 10 mV amplitude.The stability of iron sulde lms deposited onto carbon steelsurfaces depends on several factors, including the presence of cor-rosive anions, e.g., CN, Cl, H+, F, NO3 , in the solution, tempera-ture and pressure. As a rst step in this work, we decided toperform electrochemical impedance spectroscopy in a solutionwith pH near that of the water found in the petroleum reservoirsof the state of Espirito Santo. We used sodium sulfate as a support-ing electrolyte because it is less aggressive on lms grown on steel.All electrochemical measurements were performed with theAUTOLAB PGSTAT 302N potentiostat/galvanostat.

2.2. Preparation of carbon steel samples for morphological andstructural characterization

Carbon steel samples were machined with silicon carbide (SiC)paper from 320 to 1500 grit, washed with water and acetone, im-mersed for ten seconds in 2.5%v/v HNO3 in ethanol solution andimmediately analyzed by SEM [1719]. The corrosion productsformed on the carbon steel surface in the absence and presence oflow concentrations of sulde ions were analyzed by SEM/EDX toprovide morphological and compositional information on the sur-face lm and on the clean carbon steel surface that remained afterremoval of the corrosion products. The lms formed in the voltam-metry experiments at 0.83 mmol L1 sulde ions were maintainedin an argon atmosphere, removed from the substrate, and thencharacterized by X-ray diffraction and Mssbauer spectroscopy.XRD was performed using a Rigaku diffractometer with Cu Ka radi-ation (k = 1.5418 ), scanning from 10 to 100 with a step scan of0.05 per minute. Mssbauer spectroscopy of 57Fe was performedusing a spectrometer operated in constant accelerationmode undertransmission settings. 57Co in an Rh matrix (25 mCi) at room tem-perature (RT) was used as a radioactive source. The Mssbauer datawere tted using the NORMOS program. The isomer shift values aregiven with respect to metallic iron at RT. Micrographs were col-lected with a scanning electron microscope (Shimadzu SS550)operating at 20 kV at a working distance of 17 mm.

3. Results and discussion

3.1. Characterization of the carbon steel surface

The SEM image in Fig. 1a shows only scratches caused by thesurface treatment and several black spots that represent structuraldefects and inclusions formed during the production of the AISI1005 carbon steel. Exposure of the carbon steel surface to 2.5%v/v HNO3 alcoholic (ethanol) solution highlighted the grain cores ofthe ferrite phase and also the grain boundaries. In Fig. 1b, no otherphases such as perlite or cementite are observed, conrming theresult of the AISI 1005 carbon steel metallography. Fig. 1c showsone of many inclusions at the steel surface; defects of this typeusually appear in the grain boundary regions. Some authors haveattributed the beginning of the corrosion process to the presenceof inclusions [20,21]. Fig. 1d and e presents the EDX spectra re-corded at the approximate middle of the grain and in the inclusion,respectively. The EDX spectrum recorded in the grain detected onlyiron and carbon, a composition close to that found in the ferritephase. The EDX spectrum recorded in the inclusion detected sulfurand oxygen in addition to iron and carbon; this may have resultedin the presence of oxygenated and sulfured iron species, as will beshown in the results described later in the paper.

3.2. Cyclic voltammetry analysis of AISI 1005 corrosion occurring atlow sulde ion concentrations

nce 74 (2013) 214222 215Fig. 2 displays the voltammetric prole of AISI 1005 steel in asolution without sulde ions (Fig. 2a) and in solutions with sulde

FabioHighlight

FabioHighlight

FabioHighlight

FabioHighlight

FabioHighlight

FabioHighlight

FabioHighlight

FabioHighlight

FabioHighlight

FabioHighlight

FabioHighlight

FabioHighlight

FabioHighlight

FabioHighlight

Scie216 N. Perini et al. / Corrosionion concentrations of 0.013 mmol L1 (Fig. 2b), 0.19 mmol L1

(Fig. 2c), 0.38 mmol L1 (Fig. 2d) and 0.83 mmol L1 (Fig. 2e).Fig. 2a shows that the formation of iron hydroxide lms on the car-bon steel surface started at 390 mVSHE and continued in the posi-tive direction. In the negative direction, a peak current at740 mVSHE can be attributed to the reduction of the iron hydrox-ide lms. The rst two cycles shown in Fig. 2b have similar prolesto those obtained without sulde ion solutions, presenting just onecurrent peak close to 740 mVSHE. From the third cycle onward, itwas possible to distinguish two anodic peaks at 540 mVSHE (a1)and440 mVSHE (a2); these are due to the formation of sulde ironspecies on the steel surface. In the last cycles, two additional catho-dic peaks related to the formation of different iron suldes ap-peared (Fig. 2b). In Fig. 2ce, in which the amount of sulde ionin the solution was 0.19 mmol L1, 0.38 mmol L1 and0.83 mmol L1, respectively, the anodic peaks a1 and a2 appearedafter the rst voltammetric cycle, representing higher denitionand density of the current.

The second cycles for the solutions of different sulde ion con-centrations are plotted in Fig. 3. Fig. 3a displays the entire potentialrange of the voltammograms. It is evident that current density in-creased with the increase in the amount of sulde ions in solution.At potential values more positive than 300 mVSHE, the current

d

Fig. 1. (a) SEM of steel after surface treatment; (b) metallographic analysis by SEM of AISIof the (d) grain and (e) inclusion.nce 74 (2013) 214222density increased for 0.013 mmol L1 and 0.19 mmol L1 and de-creased for 0.38 mmol L1 and 0.83 mmol L1. This is evidence thatthe corrosion process was inhibited in this potential region as aconsequence of the higher amount of sulde present. Fig. 3b repre-sents the anodic process, for which it was observed that the onsetpotential was 640 mVSHE and that peaks a1 (540 mVSHE) and a2(440 mVSHE) increased with increasing sulde ion concentration.In the solution containing 0.013 mmol L1 sulde ions, the onsetpotential was 740 mVSHE, and no anodic peaks were obtained.From these results, it can be inferred that iron sulde compoundsare not formed during the rst cycle.

3.2.1. Mechanism of AISI 1005 corrosion under cyclic voltammetricpolarization at low sulde ion concentrations

No peaks were detected during the rst positive sweep, and theinitial cyclic voltammetric proles appear similar for synthesissolutions with and without sulde ions (Fig. 2). In the presenceof water, iron and steel surfaces are thermodynamically unstable,as suggested by the Pourbaix diagram, and these surfaces immedi-ately form an iron hydroxide layer with low solubility close to neu-tral pH [22]. The formation of iron anodic lms on the steel surfaceis independent of the sulde ion concentration. This process maybe associated with the formation of oxygenated ferrous species, a

e

1005 steel; (c) inclusion on the grain boundary and corresponding EDX composition

FabioHighlight

FabioHighlight

FabioHighlight

FabioHighlight

FabioHighlight

FabioHighlight

FabioHighlight

Sciea

b

N. Perini et al. / Corrosionprocess that begins with the adsorption of OH. During the initialstage, the OHad promotes the dissolution of iron atoms in a poten-tial-determining step reaction, as shown in the following equation[11].

Fes OHaq ! FeOHaq 2e 5When the FeOHaq solubility is exceeded, iron (II) hydroxide is

precipitated, according to the following equation:

FeOHaq OHaq ! FeOH2s 6Considering that the HS species has a higher adsorption con-

stant on the iron atoms compared with OH, it follows that dis-placement of OH by HS can occur according to the followingequation [23]:

FeOH2s HSaq ! FeHSads 2OHaq 7After the rst cycle, two anodic peaks were detected for the

solutions with 0.19 mmol L1, 0.38 mmol L1 and 0.83 mmol L1

sulde ion (for 0.013 mmol L1, two peaks were detected afterthe fourth cycle). These two anodic peaks, a1 (540 mVSHE) anda2 (440 mVSHE), could be attributed to the formation of ironmonosulde and iron bisulde, respectively. The density charge

d

Fig. 2. Cyclic voltammetric proles of AISI 1005 steel in solutions of 0.1 mol L1 Na2SO4w0.83 mmol L1 of sulde ion concentration, at 10 mV s1.c

nce 74 (2013) 214222 217throughout the interface increased with cycle number and reacheda plateau for the peak a2 (440 mVSHE). At higher sulde ion con-centrations, 0.38 mmol L1 and 0.83 mmol L1, the peak a1(540 mVSHE) was not detected after the sixth cycle. The formationof the sulde lm, at least initially during the corrosion process,may inhibit the dissolution of iron at more positive potential val-ues, as was shown in a previous section.

It is widely accepted that the chemical step represented in Eq.(7) is followed by a mackinawite formation phase, which can berepresented by:

FeHSads xFe2ads ! Fe1xSs Haq 8The mackinawite phase is very unstable and is quickly con-

verted into iron monosulde, FeS [24]. The latter species may reactwith itself or with aqueous HS at the interface to produce bisul-de iron species, as suggested by the following equations below.

FeSs FeSs ! Fe2aq FeS2aq 2e 9

FeSs HSaq ! FeS2aq Haq 2e 10Based on the results shown in Figs. 2 and 3, it seems reasonable

to assume that the process that occurs at potentials near

e

ith (a) no sulde, (b) 0.013 mmol L1, (c) 0.19 mmol L1, (d) 0.38 mmol L1 and (e)

3.2.2. Structural characterization of the iron lms by XRD andMssbauer spectroscopy

Fig. 4 presents the X-ray diffraction pattern for the iron lmformed on the carbon steel surface at 0.83 mmol L1 of suldeion solution during cyclic voltammetric experiments (nal corro-

1

Science 74 (2013) 214222a

218 N. Perini et al. / Corrosion540 mVSHE is exhausted after a few cycles, while the process thatoccurs at potentials near 440 mVSHE increases until saturation. Incontrast, at lower concentrations of sulde ions, i.e.,0.013 mmol L1, the two oxidation processes coexist throughoutthe test cycles. X-ray diffraction and Mssbauer spectroscopy weretherefore conducted as complementary methods to investigate theproposed mechanism for the most interesting sample, i.e., the sam-ple in which the sulde ion concentration was 0.83 mmol L1.

sion product obtained at 0.83 mmol L of sulde ion solution).The X-ray diffraction pattern of this sample is complex and clearlyshows the presence of several crystalline phases. According to theresults reported in the JCPDS database, it was possible to identifythe presence of the following crystalline phases: (a) solid sulfur(S) with Bragg peaks at 2h = 32.0, 33.8, 48.7 and 54.6 (card 8-0247); (b) iron bisuldes identied as marcasite (FeS2), with peaksat 2h = 33.8, 38.5, 39.2, 54.6, 68.4 and 71.2 (card 37-475)) andpyrite (FeS2), with peaks at 2h = 28.7, 57.2, 61.5, 78.9 and 88.2(card 42-1340). In addition to the Fe-S phases, iron hydroxide III(Fe(OH)3), with peaks at 2h = 18.5, 35.0, 43.0, 62.7 (card 22-0346), lepidocrocite (FeOOH, 2h = 27.0 and 68.4, card 44-1415)and iron sulfate hydrate (FeSO4nH2O), with peaks at 2h = 26.9,

b

c

Fig. 3. (a) Second cycle for AISI 1005 steel in a solution of 0.1 mol L1 Na2SO4containing different concentrations of sulde. (b) Region of the anodic peaks a1 anda2. (c) Region of the cathodic peaks c0, c1 and c2.32.0, 38.5, 44.5 and 48.7 (card 22-0633) could also be identiedin the X-ray pattern [25]. The presence of the sulfur can be attrib-uted to oxidation of the iron monosuldes [26].

Fig. 5 displays the 57Fe Mssbauer spectrum recorded at RT forthe 0.83 mmol L1 sample. This spectrum basically consists of twocomponents, one sextet and one doublet. However, it was ttedassuming three components (two doublets D1 and D2 and one sex-tet S). The second doublet was used to account for the absorptionline asymmetry observed at velocities close to 3 mm/s when com-pared with the line at approximately 3 mm/s. The doublet D1covers most of the area of the spectrum and is the component withthe major fraction of the Fe-phases. It has a relatively broadabsorption resonance line width, suggesting either an atomic dis-ordered effect or the presence of multiple Fe-phases with similarisomer shift values (d) and quadruple splitting (DEQ) hyperneparameters. The hyperne parameters of the doublet D1 obtainedby tting are: 85% of the total relative area; d value of0.33 0.06 mm s1; and DEQ equal to 0.69 0.06 mm s1. Itshould be stressed that these hyperne parameter values are com-monly found in Fe3+ states. Because the absorption Mssbauer lineof the doublet D1 is relatively broad, the doublet may be associatedwith the presence of the different Fe3+ phases observed in the X-ray pattern of this sample. However, it should also be emphasizedthat these hyperne values are relatively comparable (withinexperimental error) to those found in disulde iron phases, i.e.,FeS2 (d = 0.32 0.01 mm s1 and DEQ = 0.61 0.01 mm s1)[2729]. Therefore, because we do not have sufcient energyFig. 4. X-ray diffraction patterns for the corrosion products formed on the surfaceof AISI1005 steel during cyclic voltammetry in solutions containing0.83 mmol L1sulde.

Scieresolution to separate the subspectra of the Fe3+ phases observedin XRD and because the temperature dependence of the Mssbauerexperiments could not be determined in our setup, we attributethe doublet D1, according to its hyperne parameters, basicallyto disulde iron phases. On the other hand, doublet D2 has a rela-tive total area of 5%. Its d and DEQ values, also obtained from thetting, are 1.87 0.06 mm s1 and 1.92 0.06 mm s1, respec-tively. These values are similar to those found in Fe2+ phases. Thus,according to the X-ray data obtained using this sample, it can beassociated with the hydrated ferrous sulfate phase.

Finally, the magnetic component observed in this spectrum wastted with a broad sextet S. The hyperne parameters of the S-component are similar to those of Fe-rich Fe-C phases (a-Fe-likephase). Therefore, it is attributed to the a-Fe-like phase obtainedwhen we removed the sample powder from the steel plate treatedin 0.83 mmol L1 sulde ion solution. It should be mentioned thatthe S-component represents 10% of the total contribution to thespectrum. Thus, after subtracting the a-Fe like contribution, itcan be estimated that the disulde phases represent approximately94% of the iron-containing corrosion products and that ferrous sul-fate hydrate represents approximately 6%. Almost all of the mono-

Fig. 5. Room temperature 57Fe Mssbauer spectrum of the corrosion productformed on the surface of AISI 1005 carbon steel after cyclic voltammetry insolutions containing 0.83 mmol L1 sulde ions. The dark points are the experi-mental data, while the grey line is the tting using NORMOS program. The darklines represent the subspectra used to t the experimental data.

N. Perini et al. / Corrosionsulde produced during voltammetry experiments was convertedto bisulde. The physical characterization of the lms formed insulde solutions corroborates the mechanism proposed in Section3.2.1.

3.2.3. Morphological characterization of the lms formed by cyclicvoltammetry

Fig. 6 shows the SEM/EDX characterization of iron lms formedduring cyclic voltammetry from 890 mVSHE to 110 mVSHE with 10cycles at a scan rate of 10 mV s1. The left panel of Fig. 6 showsmicrographs of these iron lms. The central panel presents theEDX spectra of the iron lms, and the right panel shows the elec-trode surfaces after removal of the iron lms. In the left panel(Fig. 6be) can be seen the iron lms grown in the presence of0.013 mmol L1, 0.19 mmol L1, 0.38 mmol L1 and 0.83 mmol L1

sulde ions. The electrode coverage increases with increasing sul-de content of the solution. The iron lm formed in solution con-taining 0.83 mmol L1 sulde ions has a compact form andexhibits some cracks on the surface. The EDX spectra of lmsformed in solutions lacking sulde have high relative amounts ofiron compared to oxygen; due to the low lm coverage, the ironof the steel surface is still detected. Some sulfur, which was pre-sumably present due to the use of a (Na2SO4) support electrolyteand which led to the formation of iron sulfate, was detected (seemodel to the experimental data.The CPE element is written as follows:

ZCPE 1Yojxa

11

where 0 < a < 1.As shown in Figs. 8 and 9, the tted curves and the experimen-

tal data are in good agreement. This is further conrmed by the er-ror values for each element listed in Table 1, which shows thecalculated values of the circuit elements. Because the same supportelectrolyte was used in each case, Rs was approximately 5.0X cm2

for all sulde ion concentrations tested. The Clm formed in thethe Mssbauer spectra in Fig. 5). As the sulde ion concentrationwas increased to 0.013 mmol L1, the surface lm coverage in-creased, and the relative amounts of iron and oxygen becamenearly equal. At 0.19 mmol L1 and 0.38 mmol L1, the sulfur/ironratio was almost one, and at 0.83 mmol L1 it was approximatelytwo. These results are consistent with the results presented inthe previous sections.

The micrographs in the right panel, which show the iron surfaceafter removal of the lms, show that the corrosion is localized forsolutions without sulde ions and that it is intergranular for lmsformed in solutions containing 0.013 mmol L1, 0.19 mmol L1,0.38 mmol L1 and 0.83 mmol L1.

3.2.4. Electrochemical impedance spectroscopy of iron lms formed bycyclic voltammetry

The iron lms still adhered on the carbon steel were analyzedby electrochemical impedance spectroscopy after the cyclic vol-tammetric experiments, and the open circuit potential was mea-sured. The results are displayed in Fig. 7. After immersion of theiron electrodes in the solution, the potential increased until theequilibrium of the iron lm in contact with the solution wasreached. Because the solution is not saturated with iron ions, thepotential difference at the lm/solution interface depends on theoxygen activity in the oxide and the concentration of protons inthe solution. The potential for iron lm became more positive insolution without sulde and in solutions with low content(0.013 mmol L1) of sulde ions; in these solutions, the steadystate was reached in 10 min. At the other tested concentrationsof sulde ions, the steady state was reached in approximately100 min. The electrochemical impedance spectroscopy data wererecorded in the stationary potential for each case. Fig. 8 presentsthe electrochemical impedance spectroscopy data as a Nyquist dia-gram. All of the curves have similar shapes. At high frequencies,distorted semicircles appear. At low frequencies, straight lines thatshow a tendency to form semicircles at the lowest frequencies areobserved. This behavior was very clear for lms formed in0.013 mmol L1 sulde. The semicircle diameter decreased forlms formed at intermediate concentrations of sulde(0.19 mmol L1, 0.38 mmol L1) and increased again at0.83 mmol L1. Fig. 9 shows a Bode diagram in which phase angledata are presented in Fig. 9a and impedance module data are pre-sented in Fig. 9b. The electrochemical impedance spectroscopyparameters were obtained by tting the experimental data to theequivalent circuit shown in Fig. 10, which includes the solutionresistance (Rs), the lm resistance (Rlm) and capacitance (Clm), aconstant phase element of the double layer (CPEdl), the chargetransfer resistance (Rct) and a diffusional element (Zd). The param-eter values together with their respective error values are summa-rized in Table 1. The ZView software version 3.3 (written by DerekJohnson, Scrinbner Associates, INC) was used to t an electrical

nce 74 (2013) 214222 219presence of sulde is greater than that of lm formed in blank solu-tion because of an increase in the dielectric constant or in thethickness of the lm. The resistances attributed to the lms (Rlm)

FabioHighlight

FabioHighlight

FabioHighlight

FabioHighlight

(a

Scie220 N. Perini et al. / Corrosionformed in the presence of sulde were slightly smaller than thosefound in the blank solution. The CPE parameters increased with thesulde ion concentration, most likely due to decreasing lmroughness or/and porosity.

(b

(c

(d

(e

Fig. 6. SEM and EDX of the carbon steel surface corrosion products formed during cyclic vabsence of sulde or the presence of (b) 0.013 mmol L1, (c) 0.19 mmol L1, (d) 0.38 mm)

nce 74 (2013) 214222The changes in Rct and Zd may be related primarily to the lmthickness and composition. The lm diffusional resistance, Zd,decreased for lms formed in the presence of greater amounts ofsulde. This result is related to the high conductivity of pyrite

)

)

)

)

oltammetry (left side) and the surface after removal of the lm (right side) in (a) theol L1 and (e) 0.83 mmol L1 sulde.

aScience 74 (2013) 214222 221N. Perini et al. / Corrosioncompared to that of iron hydroxides, which promotes high masstransport between the lm and the solution.

The Rct increased from 197X cm2 for lms formed in the ab-sence of sulde to 284X cm2 for lms formed in the presence of0.013 mmol L1 sulde. This low concentration of sulde was notsufcient to achieve high coverage of the iron sulde compounds.Additionally, the charge transfer resistance increased due to thelow conductivity of the iron hydroxide lm, which presented ahigher coverage as indicated by the EDX results shown in Fig. 6b;the relative quantities of iron and oxygen are very similar. TheSEM image on the left side of Fig. 6 shows low coverage by the lm;in this gure, it is still possible to observe the scratches on the steelsurface that occurred during its pre-treatment.

In the case of lms formed in solutions with sulde contents of0.19 mmol L1 and 0.38 mmol L1, the Rct values decreased to113X cm2 and 71X cm2, respectively. These results can be attrib-uted to the high coverage of the steel surface by the iron suldelms compared to lms formed at 0.013 sulde or no sulde.

Fig. 7. Open circuit potential for the lms formed on the surface of AISI 1005 steelimmersed in 0.1 mol L1 Na2SO4.

Fig. 8. Nyquist diagram for the lms formed on the surface of AISI 1005 steel duringcyclic voltammetry at different sulde concentrations in a solution containing0.1 mol L1 Na2SO4 at pH 6.5.bThe EDX shows that the relative amounts of iron and sulfur aresimilar (Fig. 6c and d). For lms formed at 0.83 mmol L1 sulde,the Rct increased to 142X cm2, and EDX indicated a large amountof sulfur relative to iron (Fig. 6e). As indicated by cyclic voltammet-ric experiments and corroborated by the XDR and MS data, the in-crease in pyrite may act as an inhibitor and cause the lm tobecome more resistive under formation conditions involving0.83 mmol L1 of sulde ions. This behavior shows that at0.83 mmol L1 the corrosion rate decreases signicantly and mostlikely indicates that iron lms containing sulfur species passivatethe carbon steel.

Fig. 9. Bode (a) phase angle; (b) module of impedance for the lms formed on thesurface of AISI 1005 steel during cyclic voltammetry at different sulde concen-trations in a solution of 0.1 mol L1 Na2SO4 at pH 6.5.

Fig. 10. Equivalent circuit for the lm formed on the surface of AISI 1005 steel in asolution of 0.1 mol L1 Na2SO4 at pH 6.5.

Table 1Electrochemical impedance spectroscopy parameters obtained by equivalent circuit tting as represented in Fig. 8.

222 N. Perini et al. / Corrosion Science 74 (2013) 2142224. Conclusions

The effect of dilute solutions of sulde ions on carbon steel cor-rosion has been investigated using various electrochemical tech-niques and by ex situ physical characterization. Variation of thesulde concentration changed the type of anodic lm formed onthe steel surface. Using cyclic voltammetry techniques, no peakswere detected for the rst cycle during the positive sweeping. Afterthe rst cycle, two oxidation processes, the formation of monosul-de iron (540 mVSHE) and the formation of bisulde iron(440 mVSHE), were found to coexist. Nearly all of the monosuldeproduced during the voltammetry experiments was converted tobisulde and pyrite because the rst observed process ceased tooccur. XRD characterization indicated the presence of bisuldespecies. MS results on the corrosion product formed in0.83 mmol L1 sulde solution indicated that the major com-pounds formed were pyrite and iron sulfate. The lms with lowcoverage of iron sulde exhibited higher charge transfer resistance.With increased coverage of iron sulde, this parameter decreased;it then increased again for lms formed at higher sulde ion con-centrations (0.83 mmol L1). This effect can be related to the pres-ence of the pyrite phase as a major Fe species of the formedsupercial lm. Based on these results, the initial stages of AISI1005 carbon steel corrosion at low sulde concentrations are char-acterized by the formation of oxygenated ferrous species, highadsorption of HS species, precipitation of monosulde and, nally,interconversion of monosulde to a bisulde iron phase.

Acknowledgments

The authors acknowledge PETROBRAS, CNPq, UFES, FAPES andCAPES for their nancial support.

References

[1] A. Permanyer, G. Mrquez, J.R. Gallego, Compositional variability in oils andformation waters from the Ayoluengo and Hontomn elds (Burgos, Spain).Implications for assessing biodegradation and reservoir compartmentalization,Org. Geochem. 54 (2013) 125139.

[2] Z.F. Yin, W.Z. Zhao, Z.Q. Bai, Y.R. Feng, W.J. Zhou, Corrosion behavior of SM 80SStube steel in stimulant solution containing H2S and CO2, Electrochim. Acta 53(2008) 36903700.

[3] T. Hemmingsen, F. Fusek, E. Skavs, Monitoring of the corrosion process on

[S2] Rs Clm Rlm

mmol L1 X cm2 mF cm2 X cm2

No sulde 5.78(0.02) 0.08(0.01) 6.3(0.2)0.013 7.66(0.01) 0.20(0.06) 3.1(0.2)0.19 4.21(0.01) 1.15(0.08) 0.6(0.4)0.38 4.01(0.01) 11.0(0.3) 5.0(0.4)0.83 4.76(0.01) 9.6(0.4) 2.9(0.2)sulde lm formation with electrochemical and optical measurements,Electrochim. Acta 51 (2006) 29192925.

[4] A. Hernandez-Espejel, M.A. Dominguez-Crespo, R. Cabrera-Sierra, C.Rodriguez-Meneses, E.M. Arce-Estrada, Investigations of corrosion lmsformed on API-X52 pipeline steel in acid sour media, Corros. Sci. 52 (2010)22582267.

[5] P. Smith, S. Roy, D. Swailes, S. Maxwell, D. Page, J. Lawson, A model for thecorrosion of steel subjected to synthetic produced water containing sulfate,chloride and hydrogen sulde, Chem. Eng. Sci. 66 (2011) 57755790.

[6] E. Kuzmann, M.L. Varsnyi, A. Vrtes, W. Meisel, Inuence of sulphite on thepassivation of iron, Electrochim. Acta 36 (1991) 911913.[7] C. Avendano-Castro, R. Galvan-Martinez, A. Contreras, M. Salazar, R. Orozco-Cruz, E. Martinez, R. Torres-Sanchez, Corrosion kinetics of pipeline carbon steelweld immersed in aqueous solution containing H2S, Corros. Eng. Sci. Technol.44 (2009) 149156.

[8] J.W. Tang, Y.W. Shao, J.B. Guo, T. Zhang, G.Z. Meng, F.H. Wang, The effect of H2Sconcentration on the corrosion behavior of carbon steel at 90 C, Corros. Sci. 52(2010) 20502058.

[9] A. Hernandez-Espejel, M. Palomar-Pardave, R. Cabrera-Sierra, M. Romero-Romo, M.T. Ramirez-Silva, E.M. Arce-Estrada, Kinetics and mechanism of theelectrochemical formation of iron oxidation products on steel immersed insour acid media, J. Phys. Chem. B 115 (2011) 18331841.

[10] Z.A. Iofa, V.V. Batrakov, B. Cho Ngok, Inuence of anion adsorption on theaction of inhibitors on the acid corrosion of iron and cobalt, Electrochim. Acta9 (1964) 16451653.

[11] K.E. Heusler, G.H. Cartledge, The inuence of iodide ions and carbon monoxideon the anodic dissolution of active iron, J. Electrochem. Soc. 108 (1961) 732740.

[12] R.D. Medicis, Cubic FeS, a metastable iron sulde, Science 170 (1970) 11911192.

[13] R.A. Berner, Tetragonal iron sulde, Science 137 (1962) 669.[14] D.W. Shoesmith, P. Taylor, M.G. Bailey, D.G. Owen, The formation of ferrous

monosulde polymorphs during the corrosion of iron by aqueous hydrogen-sulde at 21-degrees-C, J. Electrochem. Soc. 127 (1980) 10071015.

[15] T. Hemmingsen, H. Lima, Electrochemical and optical studies of sulde lmformation on carbon steel, Electrochim. Acta 43 (1998) 3540.

[16] X.B. Huang, Z.F. Yin, H.L. Li, Z.Q. Bai, W.Z. Zhao, Corrosion of N80 tubing steel inbrine at 1.2 MPa CO2 containing trace amounts of H2S, Corros. Eng. Sci.Technol. 47 (2012) 7883.

[17] ASTM, E3 Standard Guide for Preparation of Metallographic Specimens, ASTMInternational, West Conshohocken, Pennsylvania, US, 2011.

[18] J.W. Stewart, J.A. Charles, E.R. Wallach, Iron-phosphorus-carbon system Part 3 metallography of low carbon iron-phosphorus alloys, Mater. Sci. Technol. 16(2000) 291303.

[19] T.K. Kandavel, R. Chandramouli, P. Karthikeyan, Inuence of alloying elementsand density on aqueous corrosion behaviour of some sintered low alloy steels,Mater. Des. 40 (2012) 336342.

[20] J. Stewart, D.E. Williams, The initiation of pitting corrosion on austeniticstainless steel: on the role and importance of sulphide inclusions, Corros. Sci.33 (1992) 457474.

[21] D.E. Williams, Y.Y. Zhu, Explanation for initiation of tting corrosion ofstainless steels at sulde inclusions, J. Electrochem. Soc. 147 (2000) 17631766.

[22] H. Tamura, The role of rusts in corrosion and corrosion protection of iron andsteel, Corros. Sci. 50 (2008) 18721883.

[23] K. Schwabe, U. Ebersbac, G. Kreysa, On the kinetics of anodic passivation ofiron, cobalt and nickel, Electrochim. Acta 12 (1972) 927938.

[24] J.E. Thomas, W.M. Skinner, R.S.t.C. Smart, A comparison of the dissolutionbehavior of troilite with other iron(II) suldes; implications of structure,Geochim. Cosmochim. Acta 67 (2003) 831843.

[25] N. Nava, E. Sosa, J.L. Alamilla, C. Knigth, A. Contreras, Field sludgecharacterization obtained from inner of pipelines, Corros. Sci. 51 (2009)26522656.

[26] E. Sosa, R. Cabrera-Sierra, Marina E. Rincn, M.T. Oropeza, I. Gonzlez,Evolution of non-stoichiometric iron sule lm formed by electrochemical

CPEdl Rct Zd

Y0 (103) n X cm2 X cm2

1.7(0.3) 0.64 197(5) 350(30)2.0(0.1) 0.71 284(1) 128(4)

11.6(0.9) 0.78 113.3(0.5) 34(2)18.0(0.2) 0.87 71(1) 21(2)23.9(0.3) 0.88 142(1) 32(4)oxidation of carbon steel in alkaline sour, Electrochim. Acta 47 (2002) 11971208.

[27] B.J. Evans, R.G. Johnson, F.E. Senftle, C.B. Cecil, F. Dulong, The Fe-57 mossbauerparameters of pyrite and marcasite with different provenances, Geochim.Cosmochim. Acta 46 (1982) 761775.

[28] M.R. Antonio, G.B. Karet, J.P. Guzowski, Iron chemistry in petroleumproduction, Fuel 79 (2000) 3745.

[29] N. Nava, E. Sosa, J.L. Alamilla, C. Knight, A. Contreras, Field sludgecharacterization obtained from inner of pipelines, Corros. Sci. 51 (2009)26522656.

Characterization of AISI 1005 corrosion films grown under cyclic voltammetry of low sulfide ion concentrations1 Introduction2 Experimental procedures2.1 Electrochemical experiments2.2 Preparation of carbon steel samples for morphological and structural characterization

3 Results and discussion3.1 Characterization of the carbon steel surface3.2 Cyclic voltammetry analysis of AISI 1005 corrosion occurring at low sulfide ion concentrations3.2.1 Mechanism of AISI 1005 corrosion under cyclic voltammetric polarization at low sulfide ion concentrations3.2.2 Structural characterization of the iron films by XRD and Mssbauer spectroscopy3.2.3 Morphological characterization of the films formed by cyclic voltammetry3.2.4 Electrochemical impedance spectroscopy of iron films formed by cyclic voltammetry

4 ConclusionsAcknowledgmentsReferences