Formation and Characterization of Model Iron Sul-fide Scales with Disulfides and Thiols on Steel Pipe-line Materials by an Aerosol Assisted Chemical Va-por Method.Ghulam Murtaza,a,b Caroline Nowicka-Dylag,a,† Aleksander A. Tedstone,a,b Steve Orwig,d Kevin R. West,e Chris P. Warrens,e Sai P. Venkateswaran,f Sander Gaemers,g Ming Wei,d Andrew G. Thomas,c Paul O’Brien,a,b* and David J. Lewis.b,c*

aSchool of Chemistry, University of Manchester, Oxford Road, Manchester, United Kingdom M13 9PL.bInternational Centre for Advanced Materials (ICAM, Manchester Hub), University of Manchester, United Kingdom, M13 9PL.cSchool of Materials, University of Manchester, Oxford Road, Manchester, United Kingdom, M13 9PL.dBP Products North America Inc., 150 W Warrenville Road, Naperville, U.S.A. IL 60563.eBP Technology Centre, Whitchurch Hill, Pangbourne, United Kingdom, RG8 7QR. fBP America Inc., 501 Westlake Park Blvd, Houston, U.S.A., TX 77079. gBP International Ltd., Sunbury-on-Thames, Middlesex., United Kingdom, TW16 7LN.

ABSTRACT: An aerosol-assisted chemical vapour method leading to iron sulfide scales of varying phases and morphologies by reaction of pipeline steels with sulfur compounds has been developed. This chemical vapour reaction methodology is useful for generating model iron sulfide scales pertinent to the interaction of sour crude oil with piplelines used in the oil and gas industry.

One of the major challenges faced in the oil and gas industry is the damaging effect of high concen-trations of various sulfur compounds in oils on steel components in process equipment due to sulfidation corrosion. In recent years, sulfur content in crude oil continues to increase and more sour crude oil comes to markets. This provides opportunities and chal-lenges to petroleum refining industry: opportunities due to discounts of high sulfur crudes and chal-lenges due to increasing corrosion risks.

So-called sour oil is defined as having a sul-fur content of greater than 0.5 %, whilst sweet oil has less than 0.5 % sulfur content. In general, the amount of sulfur in crude oil varies from 0.05 – 4 %, and varies between different oil fields.1 On the other hand, the sulfur levels in petroleum products are continuously decreasing due to ever more stringent emission requirements to reduce air pollution. The levels of sulfur in petrol and diesel are 10 and 15 ppm respectively in developed countries and this re-striction is slowly being enforced in the rest of the world.2 This has been coupled to the fact that refin-

ing processes have had to increase the processing severity to meet the challenges from sides of feed-stock and products and to remain profitable to busi-nesses.

For the oil and gas industry, increased sulfur content has presented technical challenges from up-stream discovery to refining downstream. The sulfur containing compounds found in crude oil are typic-ally free sulfur, hydrogen sulfide, thiols, sulfides, di-sulfides and thiophenes.3 These sulfur compounds have different corrosivity. However they have one thing in common: their corrosion reactions form iron sulfide scales on the metal surface. The scale serves as protective layer and slows down the corrosion.

The chemical structure of iron sulfide scales is very complicated, and depends on conditions un-der which they are formed The sequence of reaction products from combining iron and sulfur found by Shoesmith et al.4, 5 is: Mackinawite (Fe1+xS where 0 ≤ x ≤ 0.11) Cubic FeS → Troilite (FeS) → Pyrrhotite (Fe1-xS where 0 < x ≤ 0.20) → Pyrite (FeS2). Many of these FexSy structures are unstable and can convert to others at elevated temperatures. For example Mackinawite can be converted to hexagonal Pyrrhotite by heating in the range 530 – 545 K.6

Mackinawite can also be converted Greigite (Fe3S4) above 373 K,7 and hence oxidation of Mackinawite may be a route to Pyrite via Greigite, though this is still unclear. Inversion twinning in the troilite sys-tem has been described previously by Skala et al.8 We have recently been interested in the formation of pyrite iron sulfide and doped pyrite thin films from single-source precursors9-12 for inexpensive photovol-

taics, though commercially viable power conversion efficiencies in devices remains elusive mainly due to deleterious surface defects.13

The mechanism of scaling in oil-carrying pipelines is currently poorly understood, yet is needed to un-derstand which advanced materials may resist scal-ing be they new steels or protective internal coat-ings for pipelines. It is thought that sulfur directly re-acts with the surface of the metal or alloy to form a sulfur compound which adheres to the surface. This corrosive process usually occurs at temperatures higher than 200°C. The rate of sulfidation increases as the temperature increases. Previous studies have shown that the rate of sulfidation reaches a max-imum at ~455 °C. In addition, when the temperature increases above ~370 °C, H2S decomposes to sulfur which is extremely reactive toward iron14

It is, however, extremely difficult for the oil and gas industry to simulate or produce model iron sulfide scales to study corrosive processes. Current methods used often require long periods in a sealed autoclave,15 high vacuum,16 or the use of toxic di-methyl disulfide,17 carbon disulfide,15 or H2S.18, 19 Formation of scales from aqueous solution is also complicated, especially at low temperatures.20 In

this paper we present a facile method for quickly generating iron sulfide scales on authentic pipeline steel sample coupons by the reaction of various sul-fur molecules, and using an aerosol assisted chem-ical vapour reaction at ambient pressure. The method has great potential for parallel processing. This method provides unique advantages for FeS scale preparation for FeS characterization and re-search: precise control of reaction conditions, minim-isation of safety risk due to the low amount of sulfur compounds needed, and its inherent rapidity and potential for parallel processing.

We generated iron sulfide scales using aero-sols of sulfur compounds streamed at what we shall deem as ‘low’, ‘intermediate’ and ‘high’ temperat-ures which lie in the range 100 to 1000 °C over steel substrate coupons. The various sulfur compounds which are found as major chalcogen components in sour oils, were loaded into the magazine of the ap-paratus: 2 mM of either a tertiary alkyl disulfide (BP1), a tertiary alkanethiol (BP2), a primary alkyl di-sulfide (BP3) or a primary alkanethiol (BP4), all dis-solved in hexane (25 mL). An aerosol of these solu-tions was then generated using a piezoelectric hu-midifier and piped into a Carbolite oven set to the

Fig 1: Secondary electron SEM images of iron sulfide scales gener-ated at high temperature with aliphatic thiols pertinent to sour oil on authentic steel pipeline coupon substrates. Scales generated from: (a) BP1 with carbon steel; (b) BP1 with 9Cr steel; (c) BP2 with car-bon steel; (d) BP2 with 9Cr steel; (e) BP3 with carbon steel; (f) BP3 with 9Cr steel; (g) BP4 with carbon steel; (h) BP4 with 9Cr steel.

Fig. 2: pXRD patterns (Cu Kα source) from iron scales generated at (a) low, (b) intermediate and (c) high temperatures. (1) pXRD patterns of iron sulfide scales from the corrosive action of BP1 on carbon steel coupons, black sticks represent the standard dif-fraction of pyrite (FeS2 ICDD#00-001-1295). (2) pXRD patterns of iron sulfide scales from the corrosive action of BP2 on carbon steel coupons, magenta sticks represent the standard diffraction of pyrite (FeS2 ICDD # 00-001-1295) whilst black sticks repre-sent the standard diffraction pattern of pyrrhotite (Fe1-xS, where 0 > x > 0.2, ICDD # 00-003-1028). (3) pXRD patterns of iron sul-fide scales from the corrosive action of BP3 on carbon steel coupons, black sticks represent the standard diffraction of troilite (FeS FeS ICDD # 01-075-0602). (4) pXRD patterns of iron sul-fide scales from the corrosive action of BP4 on carbon steel coupons,black sticks represent the standard diffraction of troilite (FeS FeS ICDD # 01-075-0602). Asterisks mark substrate peaks on the first spectrum in the series for each panel.

required temperature which contained either carbon steel or 9Cr steel coupons, using an argon carrier gas at a flow rate of 200 cm3 min-1The experiments were run for specified time and then the apparatus was unloaded after cooling to below 50 °C. The so-called aerosol assisted chemical vapour deposition (AACVD) apparatus used in this study has previously been described in detail,21-27 and the reader is di-rected toward the excellent review by Parkin, Car-malt and co-workers that describes the development of the method for materials fabrication.28

Post-reaction, we investigated the surface morphology of the coupons by secondary electron scanning electron microscopy (SEM). It was found that scales with a crystalline appearance had formed at high temperature on all steel types and for all sul-fur compounds investigated (Figure 1), with subtle differences in the morphologies presented. At the lowest temperatures with all sulfur compounds, the substrates appeared bare, and similar in appearance

to control samples, and FeS scale was not observed with SEM, with the exception of the film formed from BP1 on carbon steel at the intermediate temperat-ure. Energy dispersive X-ray (EDX) spectroscopy (vide infra) confirmed that these scales were iron sulfide (FexSy) species.

Powder X-ray diffraction (pXRD) was used to in-vestigate the phase of the iron sulphide formed. The scales produced at the high temperature from the reaction of BP2 on carbon steel and 9Cr steel coupons could be indexed unambiguously to troilite (FeS) (Figure 2). Troilite was also found to be the predominant phase for the scales produced at high temperature from the action of BP2 on 9Cr steel and for BP3 on both pipeline materials. In contrast, scales produced at the high temperature by the ac-tion of BP1 on carbon and 9Cr steel produced pre-dominantly pyrite (FeS2), whilst the reaction of BP2 on carbon steel produced a mixture of pyrite and the non-stoichiometric iron-sulfide variant, pyrrhotite

Fig. 3: Onset of sulfidation with increasing temperature of carbon steel (black points) and 9Cr steel (red points) pipeline coupons with various aliphatic thiols relevant to sour oils assessed by EDX spectroscopy using the intensity ratio of the S and Fe Kα lines, ([S at%] / [Fe at%]) as a measure of the extent of sulfidation. (a) onset of sulfidation of carbon steel and 9Cr steels with BP1. (b) onset of sulfidation of carbon steel and 9 Cr steels with BP2. (c) onset of sulfidation of carbon steel and 9 Cr steels with BP3. (d) onset of sulfidation of carbon steel and 9 Cr steels with BP4. Error bars on all plots assume a ±2 at% error on elemental quantification of Fe and S by EDX spectroscopy.

(Fe1-xS, where 0 > x > 0.2). This suggests that under the conditions studied that both the steel type and chemical structure of the thiol are important to the phase of the iron sulfide scale produced. It is inter-esting that both end compounds of the FexSy series, pyrite and troilite can be produced; by tuning of the conditions it should be possible to ‘select’ an iron sulfide scale of interest to study by judicious choice of substrate and aliphatic thiol. In all cases, diffrac-tion reflections associated with scales could only be recorded for scales generated at the high temperat-ure; no diffraction peaks except those associated with the steel substrates were observed at reactions run at the low temperature. It is worthwhile noting that in a review by Bonis of 125 distinct field cases of sour weight loss corrosion in pipelines located at 45 different oilfields worldwide, all of the iron scale phases we produced here have been observed.29 Using EDX spectroscopy at 20 kV, it was possible to use the ratios of intensities of the Fe and S Kα lines to give a figure of merit for the onset of the sulfida-tion of the coupons (Figure 3). At the low temperat-ures, the Fe:S ratio (defined here as [S at%] / [Fe at%]) is also low (ca. 0 – 0.05), indicating that little or no scale formation by sulfidation was observed, mir-roring the pXRD results. However, at the high tem-perature the surface corrosion chemistry is switched on, and the Fe:S ratio rises sharply, in the range ca. 0.8 – 2.0. These values depend on the stoichiometry of the phase formed; pyrite forming thiols generally display [S at%] / [Fe at%] > 1.0, whilst troilite and pyrrhotite forming species have [S at%] / [Fe at%] < 1.0. It is worth noting that in this case the EDX spec-troscopy data cannot give quantitative information on elemental stoichiometries due to the interaction of the electron beam with the underlying substrate; rather we use it in a qualitative manner here to show the onset of film formation. It is clear from this data that 9Cr steel tends to always resist sulfidation better than carbon steel at high temperature.

X-ray photoelectron spectroscopy was used to probe the topmost surface (ca. 10 nm probe depth) of the scales formed at high temperature (Support-ing Information). XPS Spectra were recorded using a monochromated Ka X-ray source (hn = 1486.6 eV) and Specs Phoibos 150 NAP electron energy anal-yser. All spectra are aligned on the binding energy scale to the C 1s peak of adventitious hydrocarbon at 285 eV. Survey scans were recorded at 60 eV pass energy and high resolution Fe and S 2p at 20 eV pass energy. Peak fitting was carried out using CASAXPS Software using 70 % Gaussian:30 % Lorentzian peak shapes to account for instrumental broadening and natural line widths, respectively. All of the survey spectra are dominated by atmospheric contamination peaks signified by the C 1s (284.5 eV) and O 1s (530 eV) peaks in the spectra.

High resolution S spectra revealed peaks sepa-rated by 1.1 eV which are due to the spin-orbit split doublets of the S 2p levels. Spectra from the various samples require 4 or 6 peaks to obtain a satisfactory fit, indicating two or three different chemical envi-ronments. All spectra contain a significant amount of sulfate, which is likely to be due to oxidation of the topmost surface layers of the FeS. Fe 2p3/2 spectra

were also recorded for the samples. All of the spec-tra are dominated by a peak centred at ~ 711 eV. Peak fitting to Fe 2p spectra is complicated by multi-plet splitting due to the high spin d-electron configu-ration. However, using the parameters developed by Grosvenor et al. for high-spin Fe(II) compounds,30 a good fit to the peak centred at ~711 eV is obtained. In addition to the three multiplet split peaks, a satel-lite peak is also present at a binding energy of 714.6 eV for all samples. This, too, is consistent with Fe in a +2 oxidation state. This appears to rule out the formation of Fe2O3 or Fe3O4. The presence of Fe3+ species can also be ruled out since no satellite fea-tures are observed at energies of around 722 eV.

ConclusionsIn summary, we have used a processing route

based on a chemical vapour reaction of sulfur com-pounds with pipeline steels to produce iron sulfide scales pertinent to the oil and gas industry. This presents a step-change in the way that iron sulfide scales are generated; the method does not require autoclaves, electrolysis or the use of CS2 or H2S. The method could be used for the study of the critical factors important to the formation of iron sulfide scales in pipeline materials, as well as being used as a method to rapidly produce FexSy scale for ad-vanced characterization and whislt enabling the in-vestigation of FexSy scale formation chemistry. There may also be the potential to adapt this methodology to the formation of inexpensive solar absorber ma-terials based on FexSy.

AUTHOR INFORMATIONCorresponding Author*to whom correspondence should be addressed: Pro-fessor Paul O’Brien CBE FRS FREng email:paul.o’bri-en@manchester.ac.uk. Dr. David J. Lewis MRSC, email: david.lewis-4@manchester.ac.uk. ORCid: 0000-0001-5950-1350.Present Addresses†Current address: University College London, Gower Street, London, United Kingdom. WC1E 6BT

NotesThe authors declare no competing financial inter-ests.

ACKNOWLEDGMENT We acknowledge the funding and technical support from BPthrough the BP International Centre for Advanced Materials(BP-ICAM) which made this research possible.

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