Fuel xxx (2012) xxx–xxx

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Fuel

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Analysis of the heavy oil distillation cuts corrosion by electrospray ionization FT-ICRmass spectrometry, electrochemical impedance spectroscopy, and scanningelectron microscopy

S. Freitas, M.M. Malacarne, W. Romão, G.P. Dalmaschio, E.V.R. Castro,V.G. Celante, M.B.J.G. Freitas ⇑Federal University of Espírito Santo, Chemistry Department, Laboratory of Research and Development of Methodologies for Analysis of Oils,Espírito Santo, Av. Fernando Ferrari, 514, 29075-910 Goiabeiras, Brazil

h i g h l i g h t s

" Correlation between the composition of heavy distillation cuts and the corrosion process." Characterization of heavy oil distillation cuts by mass spectrometry." Electrochemical impedance spectroscopy for the determination of the electrical properties of distillation cuts.

a r t i c l e i n f o

Article history:Received 27 December 2011Received in revised form 1 May 2012Accepted 2 May 2012Available online 18 May 2012

Keywords:Crude oilHeavy oilDistillation cutsOil corrosionMass spectrometry

0016-2361/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.fuel.2012.05.003

⇑ Corresponding author. Tel.: +55 27 33352486; faxE-mail address: marcosbj@hotmail.com (M.B.J.G. F

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a b s t r a c t

In this work, the correlation between the composition of heavy distillation cuts and the corrosion processwas analyzed. According to the results of electrospray ionization Fourier transform ion cyclotron reso-nance mass spectrometry (ESI-FT-ICR MS), naphthenic acid species were detected in the seven cuts stud-ied, with m/z and DBE values ranging from 100 to 300 and from 2 to 4, respectively. The relativeabundance of heavy naphthenic acids increase with distillation cut temperature (from 188 to 315 �C).In the residue, naphthenic acids (O2 class) and carbazoles (N class) are detected as polar species. The cor-rosion mechanism for AISI 1020 steel in the heavy oil and distillation cuts was determined by electro-chemical impedance spectroscopy and scanning electron microscopy. The equivalent electrical circuitsproposed for the dehydrated oil, its distillation cuts and the residue are based on observations obtainedfrom the Nyquist and Bode diagrams. Resistance values decrease from oil to cuts in function of increasingdistillation temperature. The metal film resistance values increase with the distillation cut temperatureand are compatible with the formation of passive films. According to the microphotographs, the localizedcorrosion is characterized as alveolar for cuts 2–5 and pitting for cuts 6–7.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Oil is extracted from the field and after a pre-processing step istransferred to a refinery, where it is converted into various products.The first step of the refining process is primary distillation or atmo-spheric distillation, during which the components are separatedaccording to their boiling point. Generally, petroleum hydrocarbonshave boiling points ranging from �160 �C to 600 �C. However, attemperatures of approximately 350 �C, the carbon bonds of theheavier components begin to break, a process known as thermalcracking. Thus, to continue with the distillation process, oil withcomponents that have boiling points higher than 350 �C are trans-

ll rights reserved.

: +55 27 33352460.reitas).

. Analysis of the heavy oil distilcanning electron microscopy. F

ferred from atmospheric distillation chambers to vacuum distilla-tion towers. The worldwide demand for petroleum and itsproducts have forced refiners to dramatically increase their produc-tion rates. Consequently, it has become necessary to characterizeheavy oils to determine which compositions feature high concen-trations of naphthenic acids, carbon dioxide and other constituentsthat contribute to increased rates of corrosion of metallic materialspresent in distillation towers [1–5]. Although petroleum hydrocar-bons consist predominantly of non-corrosive compounds, they con-tain a variety of potentially corrosive impurities in differentconcentrations. The main causes of corrosion in oil plants are thepresence of naphthenic acids, H2S, oxygen, sulfur oxides, amines,cyanides, sulfur compounds and hydrogen gas. Among them,naphthenic acids correspond to the family of acids commonly foundin crude oils, and their concentrations change according to the

lation cuts corrosion by electrospray ionization FT-ICR mass spectrometry,uel (2012), http://dx.doi.org/10.1016/j.fuel.2012.05.003

Table 1Oil cuts physical–chemical characterization.

Samples Boilingtemperature(�C)

Weight (%m/m)

Density(g cm�3)

TAN (KOH (mg)/sample (g))

Cut 1 <188 2.7 0.7861 0.4778Cut 2 188–214 2.6 0.8416 1.4371Cut 3 214–246 2.7 0.8602 1.5108Cut 4 246–262 3.7 0.8784 1.3309Cut 5 262–295 3.0 0.8901 1.6412Cut 6 295–304 2.8 0.8923 3.0110Cut 7 304–315 1.9 0.8968 3.6524Crude

oil– 100 0.93 1.8320

Residue >315 79.8 0.9557 1.8432

2 S. Freitas et al. / Fuel xxx (2012) xxx–xxx

source of extraction. Naphthenic acids correspond to a complexmixture of saturated cyclic hydrocarbons (five or six carbons) andcycle-aliphatic carboxylic acids, whose chemical formula is gener-ally CnH2n+zO2, where n indicates the number of carbon atoms andZ the homologous series of naphthenic acid. The value Z dividedby 2 gives the number of rings present in the structure of a naph-thenic acid [6]. These compounds induce corrosion in regions ofthe refineries working at temperatures above 100.0 �C. Under theseconditions, naphthenic acids have the ability to condense along theinner walls of pipes and tanks; they are resistant to the steam ap-plied and form regions of corrosion. Corrosion by naphthenic acidspresent in oils usually removes the material in the regions wherethey are concentrated, forming a uniform structure of pits. In pipingsystems, this corrosion can lead to the leakage of vaporized oil,which is extremely dangerous. The corrosion mechanism of naph-thenic acids has not been fully elucidated and is a subject of interestbecause other chemical processes that influence this reaction arestill unknown. It is known that the corrosive nature of oil is not di-rectly related to the total acid number (TAN) but to the presence of aspecific group of acids and sulfur compounds present in oil [6].Therefore, the study of the molecular composition of distilled cutsof crude oil and their electrochemical properties has greatly at-tracted the interests of researchers and companies in recent years.The physico-chemical properties of oil and its cuts depend stronglyon their composition. Some analytical techniques utilized to charac-terize oil and its cuts include gas chromatography, liquid chroma-tography, nuclear magnetic resonance and mass spectrometry.

Until recently, the compositional complexity of crude oil sam-ples and their heavy distillates exceeded the requisite resolutionand mass accuracy of mass spectrometers, which limited the com-positional information acquired during the direct MS analysis ofcrude oils. This restriction was eliminated with the developmentof atmospheric pressure ionization (API) techniques, most notablyelectrospray ionization (ESI), and the development of ultrahigh-resolution and ultrahigh-accuracy Fourier transform ion cyclotronresonance mass spectrometry (FT-ICR MS).The combination of ESIwith FT-ICR MS has allowed for the direct characterization of crudeoils without pre-separation methods and the unambiguous assign-ment of heteroatom-containing organic components (mainly NSO)of crude oils that contain more than 20000 distinct elemental com-positions [7–10].

Electrochemical impedance spectroscopy (EIS) can provideimportant information about the electrochemical properties of acut, such as electrical conductivity, dielectric constant, resistanceand capacitance [11]. In this work, the cuts of heavy oils and resi-dues obtained using true boiling point distillation curves were ana-lyzed with respect to their chemical and electrochemicalproperties using mass spectrometry (ESI-FT-ICR MS), electrochem-ical impedance spectroscopy and scanning electron microscopy.The correlation between the chemical composition of the cutsand residues and the electrochemical corrosion was analyzed.

2. Experimental

2.1. Petroleum characterization

Samples of oil offshore oil from Espírito Santo, Brazil were col-lected in 2010 and used in this work. The oil was collected in two2-L bottles and transported to LabPetro, where the samples werecharacterized according to the standards of the American Societyfor Testing and Materials (ASTM). Tests were first conducted todetermine the levels of free water, emulsified water and sedi-ment-BSW-(ASTM D4007) [12], density (ASTM D5002-9) [13],API degree (ASTM D1298-9) [14], TAN (ASTM D664-9) [15],kinematic viscosity (ASTM D7042-4) [16], and total sulfur (ASTMD4294) [17]. Because the BSW value was less than 1.0% v/v, it

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was not necessary to dehydrate the samples. The data obtainedfrom the characterization of crude oil are: BSW < 0.05 v/v, den-sity = 0.9355 g cm�3, API degree = 19.5, TAN = 1.832 mg KOH/g,viscosity = 150.45 cSt, at 40 �C, total súlfur = 0.46759% m/m.

2.2. True boiling point distillation

A sample of crude oil (�2.62 kg or 2.8 L) was distilled in accor-dance with ASTM D 2892 (Standard Test Method for Distillation ofCrude Oil) [18] at LabPetro-UFES-Brazil. At the end of the distilla-tion process, seven cuts and a final residue with a boiling pointabove 315 �C were obtained. Material balance was performed bymass distillation by weighing the oil and residual cuts and deter-mining the percentage of the cuts masses and that of the final res-idue. Cuts 2, 3, 6 and 7 were extracted according to their masspercentages (corresponding to a total recovery mass of 20.2 wt%)in the oil source to perform electrochemical impedance analysisdue to the reduced volume obtained for these cuts. The cuts werealso analyzed with respect to total acid number, density and ESI-FT-ICR MS. The results obtained from the distillation of the crudeoil are shown in Table 1.

2.3. Characterization of the oil and distillation cuts by ESI-FT-ICR MS

Petroleum samples were analyzed by ESI(-)-FT-ICR MS. Briefly,each sample was diluted to �0.4 mg mL�1 in 50:50 (v/v) toluene/methanol (with 0.1% m/v of NH4OH). The resulting solution was di-rectly infused at a flow rate of 5 ll min�1 into the ESI source. Themass spectrometer (model 9.4 T Solarix, Bruker Daltonik, Bremen,Germany) was set to operate in the negative ion mode (ESI(-))and in a mass range of m/z 100–1200. The ESI source conditionswere as follows: nebulizer gas pressure of 14.5 psi, capillary volt-age of 4.0 kV, and transfer capillary set to 250 �C. The ion accumu-lation time in the hexapole for 0.7 s was followed by transportingto the analyzer cell through the electrostatic lens system. Eachspectrum was acquired by accumulating 200 scans of time-domingtransient signals in the length of 4 mega-point time-domain datasets. Front and back trapping voltages in the ICR cell were of�0.60 V and �0.65 V, respectively. All mass spectra were exter-nally calibrated by using a NaTFA solution (m/z from 200–1200),after they were internally recalibrated by using a set of most abun-dant homologous alkylation series for each sample. A resolvingpower (m/Dm50% � 400000, in which Dm50% is the mass spectralfull peak width at half-maximum peak height) at m/z 400 and massaccuracy (<1 ppm) provided the unambiguous molecular formulaassignment for singly charged molecular ions. Mass spectra wereacquired and processed using the software package Compass DataAnalysis (Bruker Daltonics, Bremen, Germany). With a customalgorithm developed specifically for petroleum data processing(Composer software), the MS data were handled and the elemental

lation cuts corrosion by electrospray ionization FT-ICR mass spectrometry,uel (2012), http://dx.doi.org/10.1016/j.fuel.2012.05.003

Table 2Estimated molecular formulas for the measured m/z values from ESI(-)-FT-ICR MSresults for petroleum distillation cuts.a

DBE Molecular Theoretical m/z Measured m/z Error

S. Freitas et al. / Fuel xxx (2012) xxx–xxx 3

compositions of the compounds were determined by the measure-ment of m/z values. To help summarize, visualize, and interpret theMS data, classical plots of carbon number (O2 and N classes) versusthe double bond equivalent (DBE) values were constructed.

formula values values (ppm)

2 [C8H13O2–H]� 141.09210 141.09212 0.1418[C9H15O2–H]� 155.10775 155.10777 0.1289[C10H17O2–H]� 169.12340 169.12343 0.1774[C11H19O2–H]� 183.13905 183.13908 0.1638[C12H21O2–H]� 197.15470 197.15476 0.3043[C13H23O2–H]� 211.17035 211.17042 0.3315[C14H25O2–H]� 225.18600 225.18598 �0.0888[C15H27O2–H]� 239.20165 239.20175 0.4181[C16H29O2–H]� 253.21730 253.21733 0.1185[C17H31O2–H]� 267.23295 267.23298 0.1123

3 [C10H15O2–H]� 167.10775 167.10778 0.1795[C11H17O2–H]� 181.12340 181.12343 0.1656[C12H19O2–H]� 195.13905 195.13908 0.1537[C13H21O2–H]� 209.15470 209.15482 0.5737[C14H23O2–H]� 223.17035 223.17053 0.8066[C15H25O2–H]� 237.18600 237.18606 0.2530[C16H27O2–H]� 251.20165 251.20173 0.3185[C17H29O2–H]� 265.21730 265.21728 �0.0754

4 [C12H17O2–H]� 193.12340 193.12357 0.8803[C13H19O2–H]� 207.13905 207.13926 1.0138[C14H21O2–H]� 221.15470 221.15473 0.1357[C15H23O2–H]� 235.17035 235.17079 1.8710[C16H25O2–H]� 249.18600 249.18622 0.8829

a Molecular formulas are estimated from error calculated, error = [(m/zmeasured�m/ztheoretical)/m/ztheoretical] � 106. Generally, the error obtained is lower than 1 ppm.

2.4. Electrochemical measurements

The cell consisted of two identical platinum electrodes with apurity of 99.99% w/w, length of 2.0 cm, width of 1.75 cm andthickness of 0.2 cm. The reference electrode was connected to ashort counter electrode. To study the metal interfaces, AISI 10200.16 cm2 was used as a working electrode. The distance betweenthe steel and Pt electrodes was 1.2 mm. The cell with two elec-trodes was used due to the high impedance of the oil studied.Similar conditions are used to analyze ceramic materials andpolymers. To carry out the impedance measurements, the systemfollowed the principles of linearity, stationary, and causality andis validated by Kramers and Kronig transforms. The potentialand current were measured until constant values were reachedfor each oil or cut distillation. Measurements were performed inthe frequency range 10 kHz to 1.0 mHz, measuring ten pointsper decade, with an amplitude of 350 mV DC. A Faraday cagewas used to minimize the noise; the experiments were conductedat temperatures of 25.0 ± 0.1 �C. We used an Autolab PGSTAT 100potentiostat/galvanostat with an ECHO CHIMIE ECD module forlow currents (on the order of pA).

Fig. 1. ESI(-)-FT-ICR MS for petroleum distillation cuts (cuts 2–7). The molar mass distribution broadens and shifts to higher mass with increasing boiling point. The insertshows naphthenic acids structures with its respective m/z and DBE values.

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4 S. Freitas et al. / Fuel xxx (2012) xxx–xxx

2.5. Characterization of samples by scanning electron microscopy

To characterize the type of corrosion present in AISI 1020steel, samples were washed with acetone, polished with120- to 1220-grit sandpaper, and immersed in a bath withacetone under ultrasonication for 10 min. The AISI 1020 steelsamples were then immersed in containers with the originaloil, distillation cuts, and residue. After a period of 144 h, thespecimens were washed with acetone and kerosene and left inan ultrasonic bath for 10 min. Immediately after drying, imageswere taken in a scanning electron microscope (JEOL JSM6610 LV) without metallization.

3. Results and discussion

3.1. Characterization of the oil and distillation cuts by massspectrometry

Fig. 1a–f shows the ESI(-)-FT-ICR MS results for cuts 2–6 ex-tracted by petroleum distillation, where the naphthenic acids spe-cies are detected as deprotonated molecules: [M–H]� ion.

Briefly, 24 naphthenic acid species were detected in the cutsthat were analyzed, with m/z and DBE values ranging from 127to 267 and from 2 to 4, respectively (see the insert shown inFig. 1, where possible structures are illustrated). Additionally,predicted molecular formulas, measured and theoretical m/z val-ues, DBE and error (generally, lower than 1 ppm) are shown inTable 2. Note that as the distillation cut temperature increased,the center of the molecular weight distribution shifted to higherm/z values. Additionally, the increase in TAN (Table 1) with theboiling point suggests that the amount of O2 species or naph-thenic acids increases with distillation cut temperature. The re-sults observed are consistent with those reported by Marshallet al. [9]. Fig. 2a and b shows the ESI(-)-FT-ICR spectra withm/z ranges from 200–1000 for primary crude oil (Fig. 2a) andthe respective residue distillation (Fig. 2b). The expansionaround m/z 400 shown in the ESI(-)-FT-ICR MS spectra allowsfor the comparison of the relative intensity of the two mainspecies identified: naphthenic acids and carbazoles. Note thatthe relative abundance of heavy naphthenic acids increases withresidue distillation (Fig. 2b) due to the migration of light speciesduring the distillation process from the crude oil studied to thecuts produced.

In petroleomic MS, Kendrick [19] and van Krevelen diagrams[20] are classical diagrams used to visualize trends in crude oilcomposition. Another useful plot is that in which DBE is plottedversus carbon number; such plots were therefore constructed(Fig. 2c–f) from the ESI(-)-FT-ICR MS data of the primary crudeoil and residue distillation. Fig. 2c–f shows plots of DBE versuscarbon number for O2 and N classes of crude oil (Fig. 2c–e)and residue distillation (Fig. 2d–f). Between the two classes ana-lyzed, the O2 class is the main class affected by the atmosphericdistillation process. Initially, the high abundance hydrocarbonhomologous series of polar markers ranged from C14 (Fig. 2c,crude oil) to C28 (Fig. 2d, residue distillation). In both cases,the DBE values are constant in aromaticity, DBE � 4. These re-sults are in good agreement with the ESI(-)-FT-ICR spectra ofthe cuts obtained (Fig. 1), which naphthenic acids with carbonnumbers ranging from 8 to 17 were distillated and dispersedalong with cuts at a distillation temperature between 188 and315 �C. Heavy oil distillation cuts were analyzed by electro-chemical impedance spectroscopy (EIS). With EIS, it is possibleto analysis specific parts of the oil and cuts based on theirrelaxation frequencies.

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3.2. Analysis of AISI 1020 steel corrosion in heavy oil distillation cutsby electrochemical impedance spectroscopy and scanning electronmicroscopy

Electrochemical impedance spectroscopy and scanning electronmicroscopy were used to analyze AISI 1020 steel corrosion in hea-vy oil distillation cuts. Electrochemical impedance spectra can beseparated into two regimes: high and medium frequencies(10 kHz to 10 mHz) and low frequencies (10 mHz to 1.0 mHz).The volume phase of the oil or distillation cuts is typically analyzedin the high and medium-frequency range. At low frequencies, pro-cesses such as diffusion, the adsorption and desorption of organiccompounds, and charge-transfer reactions that occur at the me-tal-oil interface were analyzed. In the Bode diagram (Fig. 3a andb), the variation of the phase angle is characteristic of a non-idealcapacitor. Extrapolation to the range of low frequencies gives thepolarization resistance RM/O (Fig. 3b).The polarization resistancecan be correlated with metal corrosion. Fig. 3c shows the Nyquistdiagrams for AISI 1020 steel in the distillation cuts, oil and residue.The distortion of these semi-circles can be attributed to the pres-ence of more than one system with different relaxations times. Inthis case, the systems are the volume phase and the AISI 1020interface. The equivalent electrical circuits proposed for the dehy-drated oil, its distillation cut and the residue (Fig. 3c highlights) arebased on observations obtained from the Nyquist and Bodediagrams. A simulation was performed to obtain the circuit(ROQO)(RM/OQM/O), where RO and QO are, respectively, the resistanceand constant phase element of the volume phase and RM/O and QM/

O are, respectively, the polarization resistance and constant phaseelement of the metal-oil interface.

The values of these circuit elements are presented in Table 3.When a constant phase element CPE is present in an electric cir-cuit, Eq. (1) is used to calculate the capacitance:

C ¼ ðQRn�1Þ1=n ð1Þ

where C is the capacitance, Q is the constant phase element andn = 1 for an ideal capacitor.

The relationship between resistance, conductivity and ionmobility is given by

R ¼ lAk¼ l

AFPj

izCuð2Þ

where k is the conductivity, l the mobility, z the charge, F the Far-aday constant, C the concentration,

A the area and l the electrode distance.An increase in the concentration of the ionic and polar com-

pounds in the distillation cuts causes an increase in the conductiv-ity as a result of a decrease in resistance. Therefore, a decrease inthe mobility also causes a decrease in resistance. The values ofthe resistance Ro decrease with the increasing distillation temper-ature of the cut, Table 3. This behavior is in full agreement with theESI (-)-FT-ICR MS measurements, which detected an increase in theO2 species amount in the distillation cuts with increasing cuttingdistillation temperature’’. Cuts 2 and 3 have low-boiling aliphatichydrocarbons and generally low molecular weight. In these cuts,the concentration of ionic conductors and their mobilities aresmall. This is evidenced by the results of ESI(-)-FT-ICR MS, whichdetected the presence of naphthenic acids species with lower car-bon chains and m/z ranges from 120 to 220. Naphthenic com-pounds with higher molecular weights are found when thedistillation cut temperature increases, as observed for cuts 4, 5, 6and 7, consequently, the resistances values are lower than the vol-atile cuts (cuts 2 and 3). When compare the RO results obtained forresidues, they should exhibit values lower than those of the cuts

lation cuts corrosion by electrospray ionization FT-ICR mass spectrometry,uel (2012), http://dx.doi.org/10.1016/j.fuel.2012.05.003

C14 DBE 4

C28 DBE 4

C23 DBE 15

C23 DBE 15

Fig. 2. ESI(-)-FT-ICR MS for (a) crude oil and (b) residue distillation; plot of DBE vs carbon number for O2 and N classes of (c–e) crude oil and (d–f) residue distillation. Thecarbon number abundance distribution maximum (red arrow) shift from �C14 (crude oil, 2c) to �C28 (residue distillation, 2d) for O2 class. The DBE values remained constantin aromaticity, DBE � 4. For N class, the abundance distributions maximum values remained constant, C23 and DBE 15. (For interpretation of the references to color in thisfigure legend, the reader is referred to the web version of this article.)

S. Freitas et al. / Fuel xxx (2012) xxx–xxx 5

and closer to those of the original oil. However, this behavior is dueto samples contain more complex compounds such as naphthenicacids, asphaltenes, resins, and solid particles with low ionic mobil-ity. According to Eq. (2), when the mobility decreases, the resis-tance increases, as shown in the Nyquist and Bode diagrams inFig. 3a–c.

The RM/O resistances at the metal-oil interface exhibit the oppo-site behavior with respect to the resistance of the volume phase

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(Ro). The RM/O values increase with the distillation cut temperature.The RM/O values are compatible with the formation of passive filmson metal surfaces. These passive films are formed due to theadsorption of compounds containing sulfur, nitrogen and oxygenon metal surfaces. Due to the higher concentration of asphaltenes,resins and naphthenic acids, the resistance of the residue was ex-pected to be greater than that of the cuts. However, this was notobserved. To resolve this contradiction, we performed scanning

lation cuts corrosion by electrospray ionization FT-ICR mass spectrometry,uel (2012), http://dx.doi.org/10.1016/j.fuel.2012.05.003

-3 -2 -1 0 1 2 3 4 5

0

20

40

60

80

100

Crude oil Cuts 2 and 3 Cut 4 Cut 5 Cuts 6 and 7 Residue distillation Fiting

- Pha

se a

ngle

(Deg

rees

)

Log (f)-3 -2 -1 0 1 2 3 4 5

4

5

6

7

8

9

10

11

12

Crude oil Cuts 2 and 3 Cut 4 Cut 5 Cuts 6 and 7 Residue distillation Fiting

Log

(Z)

Log (f)

10KHz

10mHz

1mHz

Crude oil Cuts 2 and 3 Cut 4 Cut 5 Cuts 6 and 7 Residue distillation Fiting

(a) (b)

(c)

0 3x1010 6x1010 9x1010 1x1011 2x1011

Z'(Ohm)

0

3x1010

6x1010

9x1010

1x1011

2x1011

-Z"(

Ohm

)

Fig. 3. EIS experiments in oil and subsequent cuts. (A) Bode plot (log z), (B) bode plot (phase angle), (C) Nyquist plot, where (o) reffers to original oil, (D) cuts 2 and 3, (r) cut4, (I) cut 5, (}) cuts 6 and 7 and (h) residue. (-) Reffers to fitting.

Table 3EIS parameters of oil and distillation cuts.

AISI 1020 steel parameters

Samples RO CPEO RM/O QM/O

G (X) Q1 (10�12 X�1 cm�2 s�1) n1 C1 (pF) s1 (ms) G (X) Q2 (10�12 X�1 cm�2 s�1) n2 C2 (pF) s2 (ms)

Crude Oil 4.27 3.17 0.99 1.94 8.28 0.04 3510 0.85 3.77 0.89Cuts 2 and 3 102 3.67 0.99 2.18 222.1 0.39 21.90 0.95 1.93 4.69Cut 4 20.0 5.03 0.99 3.04 60.87 3.97 3.97 0.96 0.49 12.16Cut 5 15.8 7.26 0.99 4.42 69.83 5.94 3.57 0.97 0.74 27.61Cuts 6 and 7 6.14 11.89 0.99 7.34 45.09 24.34 3.70 0.99 2.19 332.6Residue distillation 46.1 3.49 0.98 1.23 56.90 0.10 24.3 0.99 15.47 9.90

6 S. Freitas et al. / Fuel xxx (2012) xxx–xxx

electron microscopy measurements. The microphotographs of AISI1020 steel with and without contact with the crude oil, distillationcut and the residue are shown in Fig. 4a–g. Fig. 4a shows that therewas no corrosion on the AISI 1020 steel surface after mechanicalpolishing and cleaning in an ultrasonic bath; there was only agroove caused by mechanical polishing. Fig. 4b–e shows evidenceof corrosion. According to the microphotographs, the localized cor-rosion is of the alveolar type, in which the diameter of the erodedarea is greater than its depth. The micrographs of the AISI 1020steel surface after contact with cuts 6 and 7 are shown in Fig. 4f.According to the microphotographs, the diameter of the erodedarea is lower than its depth; therefore, the localized corrosion isof the pitting type. The TAN is higher for cuts 6 and 7 than for otherdistillation cuts (Table 1). However, only this information does not

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provide the corrosivity of crude oil. For corrosion analysis it isimportant to have a reliable understanding of the thermodynamicsof hydrogen sulfide and iron-sulfide systems. In the presence ofH2S, many types of iron sulfides may form as corrosion productsin carbon steels, such as mackinawite (Fe1+xS), cubic ferrous sulfide(FeS), smythite (Fe3+xS4), greigite (Fe3S4), pyrrhotite (Fe1�xS),troilite(FeS), and pyrite (FeS2), which have different crystal structures,oxidation states, and Fe and S stoichiometries [21]. Regarding themechanism for the formation of iron-sulfide films, it is proposedthat a mackinawite layer is initially formed on the steel surfaceby a solid-state reaction, which is then easily cracked. This typeof film is porous, has no passivating properties and allows for thedissolution of the metal. Subsequently, mackinawite (Fe1+xS) filmsundergo an aging process, which consists of a change in the films’

lation cuts corrosion by electrospray ionization FT-ICR mass spectrometry,uel (2012), http://dx.doi.org/10.1016/j.fuel.2012.05.003

S. Freitas et al. / Fuel xxx (2012) xxx–xxx 7

structure and composition. Increasing concentrations of sulfurcompounds lead to the formation of passive FeS films. The sulfurcontent increases as function of the boiling point, following a pat-tern that has already been described elsewhere. The area of theelectrode coated with an iron-sulfide film (i.e., FeS) further pre-vents the dissolution of iron. The area of naphthenic acid corrosionis reduced and is where the formation of pits occurs. The naph-thenic acid corrosion (NAC) and sulfur process can be divided intofour steps: (i) the diffusion of the naphthenic acid and sulfur mol-ecules from the volume phase toward the metal surface, (ii) theabsorption of the naphthenic acid and sulfur molecules at the ac-tive spots on the metal surface, (iii) charge transfer reaction atthe active spots on the metal surface, and (iv) the dispersion or dis-solution of corrosion products. The steps of the AISI 1020 steel cor-rosion process in the presence of naphthenic corrosion and sulfurcompounds in distillation cuts are described below, where R de-notes the naphthenic acid radical and Fe(RCOO)2 the corrosionproduct that is soluble in the naphthenic acid or oil medium.

Step 1: Naphthenic acid and sulfur diffusion process

RCOOHðsolubleÞðvolume-phaseÞ!RCOOHðsolubleÞðmetal-interfaceÞ ð3Þ

SðcompoundsÞðvolume-phaseÞ ! SðcompondsÞðmetal-interfaceÞ ð4Þ

Step 2: Adsorption process2RCOOHðsolubleÞ þ FeðsolidÞ ! FeðRCOOHÞ2ðadsorbedÞ ð5Þ

SðcompoundsÞ þ FeðsolidÞ ! FeðSÞðadsorbedÞ ð6Þ

Step 3: Charge-transfer reactionFeðRCOOHÞð2ÞðadsorbedÞ ! FeðRCOOÞ2ðsolubleÞ þH2ðgÞ ð7Þ

SðadsorbedÞ þ FeðsolidÞ ! FeSð1�xÞðsolidÞ ð8Þ

Fig. 4. Microphotografs for AISI 1020 steel corrosion tests. In (a) is blank test, (b) in origiresidue.

Please cite this article in press as: Freitas S et al. Analysis of the heavy oil distilelectrochemical impedance spectroscopy, and scanning electron microscopy. F

Step 4: Diffusion of the corrosion productFeðCOOÞ2ðsolubleÞðmetal-interfaceÞ! FeðRCOOÞ2ðsolubleÞðvolume-phaseÞ ð9Þ

4. Conclusion

ESI(-)-FT-ICR MS data for cuts 2–6 obtained by petroleum distil-lation showed the presence of naphthenic acids species with m/zand DBE values ranging from 127 to 267 and from 2 to 4, respec-tively. The relative abundance of heavy naphthenic acids andTAN increases with distillation cut temperature. Consequently,the O2 class is the main class affected by the atmospheric distilla-tion process, where plots of DBE versus carbon number showedthat the carbon number abundance distribution maximum shiftedfrom �C14 (for crude oil) to �C28 (for residue distillation). The DBEvalues remained constant in aromaticity, DBE � 4.

The corrosion mechanism for AISI 1020 steel in the heavy oiland distillation cuts was determined by electrochemical imped-ance spectroscopy and scanning electron microscopy. The Nyquistdiagrams for AISI 1020 steel in the distillation cuts, oil and residueshow distorted semi-circles, which are attributed to the volumephase and AISI 1020 interface. In the Bode diagram, the variationof the phase angle is characteristic of a non-ideal capacitor. Theequivalent circuit proposed for oil, its distillation cut and residuewas (ROQO)(RM/OQM/O), where RO e QO are, respectively, the resis-tance and constant phase element of the volume phase and RM/O

and QM/O are, respectively, the polarization resistance and constantphase element of the metal-oil interface. The values of the resis-tance Ro decrease with the increasing distillation temperature ofthe cut. The RM/O resistances at the metal-oil interface show theopposite behavior with respect to the resistance of the volumephase (Ro). The RM/O values increase with the distillation cut

nal oil, (c) fractions 2 and 3, (d) fraction 4, (e) fraction 5, (f) fractions 6 and 7 and (g)

lation cuts corrosion by electrospray ionization FT-ICR mass spectrometry,uel (2012), http://dx.doi.org/10.1016/j.fuel.2012.05.003

8 S. Freitas et al. / Fuel xxx (2012) xxx–xxx

temperature. The RM/O values are compatible with the formation ofpassive films on metal surfaces, which are formed due to theadsorption of compounds containing sulfur, nitrogen and oxygenon metal surfaces. The microphotographs of AISI 1020 detail thecorrosion process. According to the microphotographs, the local-ized corrosion is of the alveolar type, where the diameter of theeroded area is greater than its depth for cuts 2–5. The micrographsof the AISI 1020 steel after contact with fractions 6 and 7 show thatthe localized corrosion is of the pitting type. The sulfur content in-creases as the boiling point increases. The area of the electrodecoated with an iron-sulfide film (i.e., FeS) further prevents the dis-solution of iron. Additionally, the area of naphthenic acid corrosiondecreases and the formation of pitting occurs in this area. Duringthe corrosion of AISI 1020 steel in the presence of sulfur com-pounds in distillation cuts 2–5, a mackinawite layer is initiallyformed on the steel surface. This type of film is porous, has no pas-sivating properties and allows for the dissolution of the metal. ForAISI 1020 steel corrosion in the presence of sulfur compounds indistillation cuts 6 and 7, an increase in the concentration of sulfurcompounds leads to the formation of passive FeS films.

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