6th International Conference on Gas Hydrates

PETROLEUM HYDRATE DEPOSITION MECHANISMS: THE INFLUENCE OF PIPELINE WETTABILITY Aspenes, Guro; Høiland, Sylvi; Barth, Tanja; Askvik, Kjell Magne; Kini, Ramesh A.; Larsen, Roar 2008

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Proceedings of the 6th International Conference on Gas Hydrates (ICGH 2008), Vancouver, British Columbia, CANADA, July 6-10, 2008PETROLEUM HYDRATE DEPOSITION MECHANISMS: THEINFLUENCE OF PIPELINE WETTABILITYGuro Aspenes ;1;2, Sylvi H?iland 2, Tanja Barth 1, Kjell Magne Askvik 3,Ramesh A. Kini 4 and Roar Larsen 21 University of Bergen, Department of Chemistry, Bergen, Norway2 SINTEF Petroleum Research, Norway3 StatoilHydro R&D, Bergen, Norway4 Chevron Energy Technology Company, Houston, TX, USAABSTRACTThe mechanisms by which hydrates deposit in a petroleum production-line are likely to be relatedto pipeline surface properties, e.g. pipeline material, surface energy and roughness. In this work,the wettability alteration of pipeline surfaces from contact with oil, as well as the adhesion energybetween water and solid in the presence of oil is investigated. Contact angles are determined as afunction of solid material and oil composition, for both model oils and crude oils. Although contactangles in oil/brine/solid systems have been extensively reported in the literature, the variety of solidsthat may mimic a pipeline is limited. In this study, we include various metal surfaces in addition toglass and a coating. Initial results from using near infrared imaging for collecting contact angle datain non-translucent systems are also presented.Keywords: deposition, adhesion, pipeline, metal surface, wettability, surface energy, hydrateNOMENCLATUREWabc Adhesion energy in a three phase systemq Contact anglegab Interfacial tension between phase a and bINTRODUCTIONPipelines used for petroleum transportation are af-fected over time by the fluids and solids which arecontacting them, giving rise to e.g. corrosion, coat-ing by an oil or wax/asphaltene layer or/and generalwear of the pipeline. The deposition of hydrates ina production line is likely to be affected by the stateof the pipe wall. The molecular forces governing theattraction between two solids, e.g. two hydrate parti-cles or hydrate particle and pipe wall, in the presenceof oil and/or water, may be quantified through theadhesion energy. The adhesion energy depends on Corresponding author: Phone:+47 55 54 39 08 Fax: +4755 54 39 05 Email: guro.aspenes@iku.sintef.nothe interfacial tensions between the involved phases.For solid surfaces in contact with crude oil and wa-ter, the interfacial tensions, and hence the wettabilityof the system, are influenced by adsorption of crudeoil components [1], giving wettability states rang-ing from water- to oil-wet. The formation of oil-wethydrates correlates with non-agglomerating behav-ior and low hydrate plugging tendency [2].For hydrates that form and grow from a waterdroplet stuck on the pipe wall, as well as for flow-ing hydrate particles adhering to the wall, the ad-hesion energy depends on pipeline wettability. Forinstance, a water drop is more likely to adhere to awater-wet than an oil-wet pipe wall. Particle-particleadhesion has been studied to some extent [3], whilethe effect of wettability of the pipeline wall is, tothe best of our knowledge, not studied in the samedegree.In the present work, the effect of crude oil onpipeline wettability has been studied. As long asthe adhesion between hydrate particles and betweenhydrate particles and the wall are low, hydrate de-position and plugging are presumably minor prob-lems, unless particle loadings are high. A goal isto make the hydrates flow with the stream as a dis-persion. It has been shown that acid fractions froma non-plugging oil can be added to a plugging oilto change its hydrate plugging tendency into non-plugging [2, 4]. Some biosurfactants have shown tohave the same effect [4,5].The wettability of a solid can be quantified by theangle q in the three-phase contact point of a liquiddrop in thermal equilibrium on a horizontal surface.The contact angle q is defined here as the angle mea-sured through the aqueous phase, see Figure 1.Figure 1: Sketch of a sessile drop, oil/water/solidsystem.The relationship between interfacial tension andcontact angle was established by Young [6] and isgenerally known as Young?s equationcosq = gso  gswgwo(1)where g is the interfacial tensions between the threedifferent interfaces solid/oil, solid/water and wa-ter/oil. Surfaces with contact angles lower than 90degrees are considered as water-wet, whereas angleslarger than 120 degrees correspond to oil-wet sur-faces. For the intermediate angles, the surfaces haveno preference for one or the other liquid phases.MATERIALS AND METHODSMaterialsThe solid surfaces that have been investigated arestainless steel (AISI 316 L), aluminum (EN AW5052), brass (63% Cu, 37% Zn), glass, quartz andtwo epoxy surfaces coated in two different ways(Epoxy-A and Epoxy B). The surfaces have beenwashed thoroughly with a detergent (sodosil (RM01) from Riedel-de Ha?n) and soaked in distilledwater. The surfaces were then flushed with ethanol(p.a. quality) before drying the surface with nitrogengas directly before use.The model oil consists of petroleum ether (J. T.Baker and Riedel-de Ha?n, boiling range 60-80 ?C)with commercial naphthenic acids (Aldrich) in con-centrations ranging from 0 to 5000 ppm.The aqueous phase is a buffer solution (titrisol fromMerck) of pH 6 (  0.05), consisting of 0.16 mol/lNaOH (aq) and 0.06 mol/l citric acid (aq) (C6H8O7).The crude oil is supplied by StatoilHydro ASA andgiven the name B4c.Metal surfacesThe composition of the different metal alloys wasgiven by the supplier, and has been verified throughx-ray element analysis. All the metal surfaces arecovered with an oxide layer [7].The stainless steel alloy consists of approximately18 % chromium, 12 % nickel, 2 % molybdenium,small traces of carbon and silicon and approximately68 % iron. The surface composition of the stainlesssteel surface is dependent on chemical composition,temperature and pH of the aqueous solution withwhich the solid metal is in contact. At low pH (<6)and in air at room temperature a spontaneous for-mation of a protective surface layer makes the stain-less steel corrosion resistant. The main componentsof the passive layer are insoluble chromium (Cr2O3and/or Cr(OH)3) and traces of iron oxide (mostlyFe2O3) [8?10].The aluminum alloy has approximately 2 % magne-sium and traces of silicon, iron, copper, manganeseand zinc with amounts varying from 0.1 to 0.5 %,giving an amount of aluminum of approximately96 %. Due to the high reactivity of aluminum, withreduction potential of -1.66 volts, the surface con-sists of a thin layer of Al2O3 which protects the sur-face from further reaction and is rapidly self-healingwhen scratched.Both alloying elements of the brass alloy react withoxygen in air giving copper oxide (CuO) and zincoxide (ZnO).Glass surfacesThe quartz cells mainly consist of silicon oxide(SiO2), while optical glass is silicon oxide contain-ing some impurities such as borate. For the modeloil systems glass cells were used, while both quartzand glass were used in the crude oil experiments.Epoxy coated surfacesThe epoxy coating delivered from Jotun AS consistsof two parts that is mixed shortly before use. PartA consists of bis(oxyranylmethyl)ether Bisphenol F(50-100 %), alcyl (c10-c16) glycidyl ether (2,5-10%) and benzyl alcohol (2,5-10 %). Part B consists of3,6-diazaoctanethylenediamin (10-25 %). The finalcomposition consists of 6.5 parts of A and 1 part ofB by volume. Jotun AS state that the coating hasexcellent durability against solvents and chemicalsand very good durability against water.The surfaces were rinsed before coating. The coat-ing was applied to all the different metals (Alu-minum, Brass and Steel). However, the type of metalused showed to have no effect on the measured con-tact angles on the surfaces, since the layer of epoxywas fairly thick. When the two parts A and B weremixed the blend behaved like glue, making it diffi-cult to obtain an even and smooth layer. Two meth-ods were used to coat the metal surfaces, namedepoxy-A and epoxy-B. Epoxy-A was obtained bysimply applying the coating to the surfaces and pol-ish it with sand paper (P80) after hardening, givinga rather smooth surface. Epoxy-B was obtained bydiluting the mixed coating in acetone (proportion  1 g epoxy : 1 ml acetone) and pouring the solutionover the surfaces. Epoxy-B was a bit rougher thanEpoxy-A from a visual point of view.MethodsContact angle measurementsAll experiments were performed at room temper-ature, approximately 21 ?C. The metal surfaceswere aged in the oil solution for approximately 24hours. For the model oil systems the surfaces weresubmerged in a glass cell which was filled withpetroleum ether containing naphthenic acid withconcentrations ranging from 0 to 5000 ppm. A waterdrop (buffer) was deposited on the surface.For the experiments with crude oils the cuvettes(quartz and glass, 1x1 cm) were aged in the oil forapproximately 24 hours at approximately 50 ?C toavoid wax precipitation. The same brine used forthe model oil systems (buffer solution, pH 6) wasemployed.The contact angles were determined from imageanalysis. The camera used is a Retiga Exi fast 1394from QIMAGING with a spectral response between400 and 1000 nm. This camera is connected to a mi-croscope (SMZ800) from Nikon. The software usedis ImagePro Plus.Near Infrared imaging has been used for collectingcontact angle data in non-transparent systems. Anoptical filter delivered from Edmund Optics with re-flection below 800 nm (RM-90) was used to takeadvantage of the most transparent region of the oil.The light source used is a Quartz Tungsten Halo-gen lamp delivered from Newport, with a maximumspectral efficiency between 500 and 1500 nm.All angles have been evaluated using an axisymmet-ric drop shape analysis - profile (ADSA-P) methodthat measures the angle from the complete dropshape profile. The contact angle can also be de-termined by manually setting the tangent, but thismethod is associated with some degree of subjectiv-ity [11]. The ADSA-P determines the contact anglefrom combining interfacial tension and gravity prop-erties.Between 8 and 12 parallel measurements were per-formed for each system, and the standard deviationcalculated.Interfacial tensionThe interfacial tensions, g, were measured by thedrop weight method [12], using Harkins-Brownequation [13]g = (VDrg)(2prF) (2)where V is the drop volume, Dr, is the difference indensity of the two phases, g is the acceleration dueto gravity (g = 9,81 m/s2), r is the radius of the nee-dle, and F is a correction factor which is based on theradius of the needle and the volume of the droplet.The densities were determined using an Anton PaarDMS60 densitometer connected to an Anton PaarDMA602HT measuring cell.Surface energyThe state of a solid surface can be quantified throughits surface energy. By approximation, this can be de-termined by using the ?equation of state for interfa-cial tension? (EOS) [14]:gsl =glv +gsv  2pglvgsve b(glv gsv)2 (3)b is a constant that has been determined empiricallyand has an average value of 0.0001247 (mJ/m2) 2[15]. This equation is used for measurements insolid, liquid, vapour (slv) systems, hence the deno-tions. If the equation above is combined with theYoung equation (1), the following relation is ob-tained:cosq = 1+2rgsvglv e b(glv gsv)2 (4)The solid surface energy, gsv, can be determinedfrom this equation using various probe fluids withdifferent surface tensions resulting in different con-tact angles, q. This determination comprises somebasic assumptions such as no interaction betweenthe air and solid surface and that the air can be equal-ized with vacuum. The determination also assumesthat there is no chemical reaction between solid andprobe fluid.One challenge related to this procedure is that thefluid surface tension, glv, must be larger than the sur-face energy, gsv [15]. This can be explained by thespreading coefficient Sls, given by the equation [16]:Sls =gsv  glv  gsl (5)If Sls > 0 the liquid will spread on the solid sur-face. Most of the fluids that were tested in this workspread on the surfaces. The choice of fluids notspreading was limited to DMSO, formamide, glyc-erol and water. This indicates that the surfaces in thiswork are highly energetic, as should be expected.In literature, almost no measurements of surface en-ergies of metals are reported from using this method.Most of the available experimental surface energydata of metals are obtained from surface tensionmeasurements in the liquid state and extrapolated tozero temperature [17]. These values are in the orderof 1000 to 2000 mJ/m2 at approximately 300 Kelvin.This gives quite different values than from determin-ing surface energy from solid state with contact an-gle measurements.Adhesion energyThe contact angle can be used to determine the ad-hesion energy, which is calculated from rewritingYoung?s equation into the Young-Dupr? equation[18]:Wswo =gwo(1+cosq) (6)The adhesion energy, Wswo, gives the adhesion en-ergy per unit area of a solid surface (s) and water (w)adhering in oil (o), and thus comprises both interfa-cial tension, gwo, between the brine and oil phase,and contact angle, q, into one parameter.RESULTS AND DISCUSSIONSurface energyAs mentioned, surface energy showed to be diffi-cult to measure for such highly energetic surfacesas metal surfaces and glass, due to spreading con-ditions for most of the probe fluids. Fluids such ashexane, heptane, decane and benzene with surfacetensions between 18 and 28 mJ/m2 spread on all thesurfaces while the fluids with higher surface tensionthan 45 mJ/m2 could be used on some of the sur-faces. An example of surface energy determination(brass) is shown in Table 1.Table 1: Surface energy determined for brass.Probe Surface tension Angle Surface energyfluids g(mJ/m2) q(degrees) g(mJ/m2)Hexane 18.5* Spreading -Heptane 20.3* Spreading -Decane 23.9* Spreading -Benzene 28.9* Spreading -DMSO 45.1 20.6   2 42.3Formamide 59.2 45.8   2 42.9Glycerol 65.7 47.5   2 46.4Water(dist) 73.0 53.2   3 46.9Water(buffer) 73.6 55.4   3 45.6Average 45   2*Values from literature [15].The ?equation of state for interfacial tension? (EOS)method, see Equation 4, was used for determiningthe surface energy for all the surfaces , i.e. stain-less steel, glass, aluminum and epoxy coated sur-faces. The average values and standard deviationsfrom these experiments are given in Table 2 and Fig-ure 2. The variation between some of the valuesobtained was rather large and obvious outliers wereeliminated from the data set.Table 2: Surface energies for all the solids.Solid Surface energy Probe fluidssurface g(mJ/m2) usedEpoxy-A 24   2 3Epoxy-B 29   2 4Brass 45   2 5Aluminum 59   1 2Stainless steel 64   5 2Glass 65 1Figure 2: The different surface energies studied inthis work.Podgornik et al. [19] recently reported surface en-ergy for steel determined with contact angle mea-surements and probe fluids, giving a value of ap-proximately 30 mJ/m2. This is somewhat lower thanthe value determined in this work, but it should benoted that our material is stainless steel.The surface energies for the solids range from 24 to65 mJ/m2. For glass, only one probe fluid could beused. Therefore, the value given for glass has nostandard deviation.Model oilsAs mentioned above, it has been shown that acidfractions can change the hydrate plugging tendencyby making the hydrate surface less water-wet [4].A wettability alteration of the pipeline towardsless water-wet behavior from adsorption of e.g.petroleum acids, is likely to reduce the possibility ofhydrate deposition, and thus the plugging tendency.The results from the contact angle measurements arepresented in Table 3 and Figures 3 and 4. The fig-ures show the contact angles as a function of acidconcentration. The results are presented in two sep-arate graphs, Figure 3 and 4, because the acids seemto have different adsorption behavior on the differ-ent surfaces. Figure 3 illustrates the metal surfaces,while Figure 4 illustrates the glass and epoxy coatedsurfaces.Figure 3: Contact angles of water drops on metalsurfaces in petroleum ether with different concen-tration of naphthenic acids.Figure 4: Contact angles of water drops on glassand coated surfaces in petroleum ether with differ-ent concentration of naphthenic acids.From the graphs we can see that the angles increasewith increasing concentration of acids, i.e. towardsmore oil-wet conditions. Thus, the experiments inthis work show that petroleum acids are able tochange the wettability of the pipe surface, in a sim-ilar manner as it can change the wettability of theTable 3: Measured angles of buffer drops on different surfaces with different concentrations of naphthenicacids in petroleum ether.Concentration Epoxy-A Epoxy-B Brass Aluminum Steel GlassNaphthenic acids Angle Angle Angle Angle Angle Angleppm q (degrees) q (degrees) q (degrees) q (degrees) q (degrees) q (degrees)0 145  2 133  4 119  8 71  3 73  9 29  3500 - 132  4 - 128  11 - -700 157  4 - 119  5 - 77  9 30  31000 158  3 141  2 135  4 150  4 76  10 20  31500 158  4 140  2 133  3 - 113  9 36  42000 163  4 148  3 131  4 152  2 111  4 36  45000 174  2 145  5 129  3 142  8 111  6 40  3hydrate surface.The change in wettability is likely to be caused byadsorption of acids onto the surfaces. The surfacesseem to get saturated with acid molecules at dif-ferent concentrations. The metal surfaces have sat-uration limits at approximately 1000-1500 ppm ofacids, while the other surfaces have a rather smoothincrease from 0 to 5000 ppm of acids. The alu-minum surface is most influenced by the acids, whilebrass is the least affected surface as far as the metalsurfaces are concerned. The standard deviations ofthe measurements are rather large, which is true formost contact angle measurements, in general [15].The two different epoxy coated surfaces give differ-ent contact angles even though they are made of thesame material. The two curves are virtually the samebut only with an upward shift. The difference canbe due to different physical structure of the coating,such as surface roughness.The interfacial tension between petroleum ether,with different concentrations of naphthenic acids,and buffer solution is shown in Figure 5. This canbe combined with the contact angles to give an adhe-sion energy, as defined in Equation 6. The value ob-tained indicates the adhesion energy between brine(buffer) and a solid surface in the presence of oil.The adhesion energies are given in Table 4.Adhesion energy as a function of acid concentrationis shown in Figures 6 and 7.Figure 6 illustrates the metal surfaces, while Fig-ure 7 illustrates the glass and epoxy coated surfaces.The curves are similar to the results from the con-tact angle measurements, only inverted. The resultsindicate that the adhesion of water to a solid surfacedecrease with increasing concentrations of acids inFigure 5: Interfacial tension between buffer solutionand petroleum ether with different concentrations ofnaphthenic acids.an oil/brine/solid system.In order to fully understand what actually happensfor each of the surfaces the surface-acid interactionsare important. This is beyond the scope of thispresent paper. However, some brief considerationsare given below.The interaction is believed to be driven by a LewisBr?nsted type acid-base chemisorption between thereactive carboxylic acid head and specific surface re-active sites on the metal surfaces [20?23].Different chemical reactions results in different ori-entation of acids on the surfaces [23]. The differenteffect of acids on the contact angles on metal sur-faces has been suggested to occur due to differencein reactivity [24], where aluminum is a more reac-tive metal than the other metal samples tested here.There seems to be a correlation between adhesionTable 4: Adhesion energy between buffer solution and different surfaces, in different concentrations of naph-thenic acids in petroleum ether.Concentration Epoxy-A Epoxy-B Brass Aluminum Steel GlassNaphthenic acids Adhesion Adhesion Adhesion Adhesion Adhesion Adhesionenergy energy energy energy energy energyppm mJ/m2 mJ/m2 mJ/m2 mJ/m2 mJ/m2 mJ/m20 5   1 9   1 14   3 36   1 36   4 52   1500 - 8   1 - 9   4 - -700 2   1 - 12   2 - 29   4 44   11000 2   0 5   1 7   1 3   1 28   4 44   01500 2   1 5   1 7   1 - 14   3 40   12000 1   0 3   1 7   1 3   0 14   1 39   15000 0   0 4   1 8   1 4   2 13   2 36   1Figure 6: Adhesion energy between buffer solutionand metal surfaces in petroleum ether with differentconcentrations of naphthenic acids.Figure 7: Adhesion energy between buffer solutionand glass and coated surfaces in petroleum etherwith different concentrations of naphthenic acids.energy and solid surface energy. This is shown inFigure 8. The adhesion of water to a solid surfacein oil is lower on surfaces with low surface energies,meaning that the aqueous phase has less tendencyto stick to these surfaces. The graph also shows thedecrease in adhesion energy as the concentration ofacids is increased. The surfaces have become moreoil-wet.Aluminum deviates somewhat from the other sur-faces, most likely because of surface reaction withthe acids.Figure 8: Correlation between solid surface energyand adhesion energy of brine in petroleum ether withdifferent concentrations of acids, 0 and 5000 ppm.The relevance of this fundamental work to real-istic pipeline surfaces, being rough and corroded,might be discussed, but the surfaces employed inthis work are nevertheless representative for materi-als frequently used in laboratory equipment. Hence,the results are of direct value for understanding theinfluence of pipeline wettability on hydrate deposi-tion in e.g. wheel tests and bench scale flow loops.From experimental observations during wheel tests,a water drop stuck on the pipe wall has shown toconvert to hydrate when entering the relevant P, Tdomain. Furthermore, the tests will be able to revealfundamental knowledge of a generic kind.In general, the results from this work suggest thathydrates will stick less to surfaces with low surfaceenergy and when the surrounding oil has a high con-centration of acids.Experiments with model systems can also be usefulfor explaining results obtained in experiments withcrude oils, as shown in the next section.Real oil systemsExperiments from a crude oil system are presentedin Table 5. The oil tested here gives a contact an-gle larger than any of the model oil experimentsperformed on glass (see Table 3). The angle andthe resulting adhesion energy are compared withthe model oil system containing 5000 ppm of acids,which was presented in Tables 3 and 4. The contactangle is larger for the crude oil system compared tothe model oil system. However, due to different in-terfacial tensions of the systems, the adhesion ener-gies for the systems are quite similar. Hence, the sur-face is rendered more oil-wet from contact with thecrude oil, but the brine does not adhere any strongerto it.Table 5: Contact angles and adhesion energies ofbrine on glass in crude oil, and in a model oil with5000 ppm acids.Oil phase Contact Interfacial Adhesionangle tension energyq (degrees) mJ/m2 mJ/m2Crude oil 86   6 33 35   3Model oil 40   3 20 36   1The amount of extracted acids of the B4c oil is de-termined to be 11.8 mg/g oil [25]. This is a con-centration of 11 800 ppm of acids. At this point noexperiments has been performed on higher concen-trations than 5000 ppm. However, the contact an-gle will probably remain unchanged after a certainconcentration from the surface sites being filled, asshown for the metal surfaces.CONCLUSIONSThe wettabilty of pipeline surface material has beenmeasured after exposure to oil. Petroleum acids arefound to render all the surfaces more oil wet.The effect of acids on the adhesion in abrine/oil/solid system has also been tested. The re-sults show that:? The adhesion energy between brine and solidsurface in oil depends on the solid surface ma-terial. The adhesion energies can be correlatedto the initial solid surface energies of the sur-faces.? Acids affect the adhesion in the sense that anincrease in acid concentration leads to a reduc-tion in adhesion and more oil-wet conditions.? Preliminary experiments with epoxy coatedsurfaces indicate that surface roughness canhave an effect on the adhesion in brine/solid/oilsystems.? It is indicated that hydrates will stick less to thepipeline surfaces when acids are present in theoil.ACKNOWLEDGMENTSStatoilHydro and Chevron ETC are acknowledgedfor funding and permission to publish data. TheNorwegian Research Council, the Petromaks pro-gram, is thanked for funding.The HYADES project group consists of experiencedresearch personnel from both university, research in-stitution and industry, that actively participates inplanning of activities and discussion of results. Thein-kind contributions from both university and in-dustry partners are of essential value for the qualityof the project, and are thus highly appreciated. Theproject group consists of the following persons:? SINTEF Petroleum Research: Roar Larsen,David Arla, Sylvi H?iland, and Jon HaraldKaspersen.? University of Bergen: Tanja Barth, Alex Hoff-mann, Pawel Kosinski, Anna E. Borgund, GuroAspenes (PhD student), Boris Balakin (PhDstudent), Ziya Kilinc (MSc student), and H? onPedersen (MSc student).? Chevron ETC (Houston): Ramesh Kini, LeeRhyne, and Hari Subramani.? StatoilHydro: Per Fotland and Kjell M. Askvik.Arta AS is acknowledged for providing the metalsamples.REFERENCES[1] H?iland Standal S. Wettability of solid surfacesinduced by adsorption of polar organic compo-nents in crude oil. Dr.Scient. (PhD equivalent)thesis, Department of Chemistry, University ofBergen, Norway, 1999.[2] H?iland S, Askvik KM, Fotland P, Alagic E,Barth T, Fadnes F. Wettability of freon hydratesin crude oil/brine emulsions. J. Coll. Int. Sci.2005;287:217-225.[3] Taylor CJ, Dieker LE, Miller KT, Koh CA,Sloan Jr. ED. Micromechanical adhesion forcemeasurements between tetrahydrofuran hy-drate particles. J. Coll. Int. Sci. 2007;306:255-261.[4] H?iland S, Borgund AE, Barth T, FotlandP, Askvik KM. Wettability of freon hydratesin crude oil/brine emulsions: The effects ofchemical additives. In: Proceedings of the5th International Conference on Gas Hydrate.2005;4:1151-1161.[5] York JD, Firoozabadi A. Comparing effective-ness of rhamnolipid biosurfactant with qua-ternary ammonium salt surfactant for hy-drate anti-agglomeration. J. Phys. Chem. B2008;112:845-851.[6] Young T. Miscellaneous Works, Vol. 1. Lon-don: Murray, 1855. p. 418.[7] Marcus P. Surface science approach ofcorrosion phenomena. Electrochimica Acta1998;43:109-118.[8] Hashimoto K, Asami K, Kawashima A,Habazaki H, Akiyama E. The role ofcorrosion-resistant alloying elements inpassivity. Corros. Sci. 2007;49:42-52.[9] Pardo A, Merino MC, Coy AE, Viejo F, Arra-bal R, Matykina E. Effect of Mo and Mn ad-ditions on the corrosion behavior of AISI 304and 316 stainless steel in H2SO4. Corros. Sci.2007;in press.[10] Robin R, Miserque F, Spagnol V. Correla-tion between composition of passive layerand corrosion behavior of high Si-containingaustenitic stainless steels in nitric acid. J. Nuc.Mater. 2008;in press.[11] Askvik KM, H?iland S, Fotland P, Barth T,Gr?nn T, Fadnes FH. Calculation of wettingangles in crude oil/water/quartz systems. J.Coll. Int. Sci. 2005;287:657-663.[12] Adamson AW. Physical chemistry of surfaces.fifth ed. John Wiley & Sons, New York, 1990.[13] Gunde R, Kumar A, Lehnart-Batar S, M ?derR, Windhab EJ. Measurement of the surfaceand interfacial tension from maximum vol-ume of a pendant drop. J. Coll. Int. Sci.2001;244:113-122.[14] Spelt JK, Li D. The equation of state approachto interfacial tensions. In: Neumann AW, SpeltJK, editors. Applied Surface thermodynamics.New York: Marcel Dekker, 1996. p. 239-292.[15] Kwok DY, Neumann AW. Contact angle mea-surements and contact angle interpretation.Adv. Coll. Int. Sci. 1999;81:167-249.[16] Padday JF. Apparatus for measuring thespreading coefficient of a liquid on a solid sur-face. J. Sci. Instruments 1959;36:256-257.[17] Tyson WR, Miller WA. Surface free energiesof solid metals: Estimation from liquid surfacetension measurents. Surf. Sci. 1977; 62:267-276.[18] Israelachvili J. Intermolecular and surfaceforces. With applications to colloid and bio-logical systems. Academic Press, 1985. p. 149-151.[19] Podgornik B, Zajec B, Strnad S, Stana-Kleinschek K. Influence of surface energy onthe interactions between hard coatings and lu-bricants. Wear 2007;262:1199-1204.[20] Chen PJ, Wallace RM, Henck SA. Thermalproperties of perfluorinated n-alkanoic acidsself-assembled on native aluminum oxide sur-faces. J. Vac. Sci. Technol. A 1998;16:700-706.[21] van der Brand J, Blajiev O, Beentjes PCJ,Terryn H, de Wit JHW. Interaction of anhy-dride and carboxylic acid compounds with alu-minum oxide surfaces studied using infraredreflection absorption spectroscopy. Langmuir2004;20:6308-6317.[22] Allara DL, Nuzzo RG. Spontaneously or-ganized molecular assemblies. 2. Quantita-tive infrared spectroscopic determination ofequiibrium structures of solution-adsorbed n-alkanoic acids on an oxidized aluminum sur-face. Langmuir 1985;1:52-66.[23] Chen SH, Frank CW. Infrared and fluores-cence spectroscopic studies of self-assembledn-alkanoic acid monolayers. Langmuir1989;5:978-987.[24] M ?ller B. Corrosion inhibition of differentmetal pigment in aqueous alkaline media. Cor-ros. Sci. 2001;43:1155-1164.[25] Borgund AE, Erstad K, Barth T. Fractionationof crude oil acids by HPLC and characteriza-tion of their properties and effects on gas hy-drate surfaces. Energy & Fuels 2007;21:2816-2826.

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