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Some ethoxylated fatty acids as corrosion inhibitors for low carbon steel in formation water
Materials Chemistry and Physics 77 (2002) 261–269
Some ethoxylated fatty acids as corrosion inhibitors for low carbon steel in formation water M.M. Osman∗ , M.N. Shalaby Egyptian Petroleum Research Institute (EPRI), Nasr City, Cairo, Egypt Received 17 May 2001; received in revised form 7 August 2001; accepted 31 October 2001
Abstract A series of ethoxylated fatty acids have been studied in order to evaluate their effect on the corrosion inhibition of low carbon steel. The behavior of these compounds is illustrated by the adsorption isotherms and weight loss measurements. The adsorption isotherms for saturated samples Pa(EO)80 and St(EO)80 exhibit an L-shaped isotherms while the unsaturated samples Ol(EO)80 , Li(EO)80 , and Soy(EO)80 show an S-shaped isotherms. The inhibition efficiency of the first type increases with the decrease in the hydrophobic chain length; and also increases with increasing the number of the double bond present in the hydrophobic chain for the second type. Among all the tested inhibitors, the commercial inhibitor Soy(EO)80 possesses the highest inhibition efficiency. The surface film is characterized by SEM and the morphologies promote the formation of inhibiting film on the steel surface. © 2002 Published by Elsevier Science B.V. Keywords: Corrosion; Low carbon steel; Ethoxylated fatty acids; Adsorption and formation water
1. Introduction The presence of anode, cathode, electrolyte, and an external connection is essentially for electrochemical corrosion process in the oil field. The remove of any one of these factors will be stopped the corrosion process but in fact it is impossible to occur [1]. Corrosion in the oil field appears as leak in tanks, casing, tubing, pipe line, and other equipments. This process changes the base metal to another type of materials. The most corrosive environment in oil field operations is caused by trace amounts of oxygen entering into a sour brine system, as well as the large amounts of carbon dioxide and hydrogen sulfide present in a deep oil-well water (formation water) [2]. These types of environments have destroyed the major items of equipment within six months. This type of corrosion forms a scale which vary from dense and adherent to loose porous, and thick [3,4]. Corrosion inhibitors (surfactants) are widely employed in the petroleum industry to protect iron and steel equipment used in drilling, production, transport, and refining of hydrocarbons [5,6]. The efficiency of the inhibition film depends on the inhibitor concentration and contact time with the metal surface. The work reported here examine some ethoxylated fatty acids and a commercial ethoxylated mixed fatty acid as ∗
Corresponding author. Fax: +20-22-747-433.
0254-0584/02/$ – see front matter © 2002 Published by Elsevier Science B.V. PII: S 0 2 5 4 - 0 5 8 4 ( 0 1 ) 0 0 5 8 0 - 6
corrosion inhibitors for carbon steel in the formation water, to define the most performed one.
2. Experimental 2.1. Formation water Formation water (oil field water) is water that naturally exists immediately in the rocks before drilling. Most oil field water contain a variety of dissolved organic and inorganic compounds. The major elements usually present are sodium, calcium, magnesium, chloride, bicarbonate, and sulfate. The used formation water was given from July (J-69). The cationic analysis for the formation water before and after the immersion of steel are taking place and the value obtained are listed in Table 1 in order to defined what cations are deposited on the steel surface. The ionic analysis takes place by the inductively coupled plasma (ICP).
3. Materials and test solutions Corrosion tests were performed on carbon steel tubing (C, Smax and Pmax = 0.3%), which was cut into coupons (∼1 cm2 exposed surface area). These coupons were polished with different grit emery paper up to 4/0 grade, cleaned with acetone, washed with bidistilled water and finally dried.
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water (1 × 10−2 –10 m mol dm−3 ) for 192 h at 30 ◦ C. The inhibition efficiency of the tested samples is calculated according to the following equation:
Table 1 Cationic analysis of formation water Cations
Na+
K+ Mn2+ Ca2+ Ba2+ Sr2+ Mg2+ Li+ Al3+ Fe2+ Cu2+
Concentration (mg l−1 ) Before immersion
After immersion
82679.000 1649.000 19.670 15355.000 1.030 295.000 3720.000 14.900 0.165 216.000 0.172
82678.300 1235.500 15.350 14855.000 1.028 287.500 2820.000 12.375 0.1648 864.000 0.123
The nonionic surfactants used as inhibitors are: polyoxyethlyene (80) monopalmitate [Pa(EO)80 ], polyoxyethylene (80) monostearate [St(EO)80 ], polyoxyethylene (80) monooleate [Ol(EO)80 ], polyoxyethylene (80) monolinoleate [Li(EO)80 ], and polyoxyethylene (80) monosoyabean fatty acid [Soy(EO)80 ], their molecular formula are listed in Table 2. They were synthesized in the laboratory following the procedure reported elsewhere [7]. The soya bean oil is a mixture of different fatty acids as shown in Table 3. From this table it can be seen that the major fatty acid in this oil is the linoleic acid (50–55%). For each surfactant, the critical micelle concentration (CMC) was determined from the curves of surface tension (measured by Dognan Abribate Tensiometer, Prolabo) versus logarithm the surfactant concentration and their values were collected in Table 2. These curves are used as calibration curves to determine the amount adsorbed by steel surface. The steel coupons were immersed in a 25 ml stagnant surfactant solutions of different concentrations in formation Table 2 The chemical formulae and CMCs values of the used surfactants Surfactant
Chemical formula
CMC × 10−1 (m mol dm−3 )
Pa(EO)80 St(EO)80 Ol(EO)80 Li(EO)80 Soy(EO)80
C15 H31 COO(CH2 CH2 O)80 H C17 H35 COO(CH2 CH2 O)80 H C17 H33 COO(CH2 CH2 O)80 H C17 H31 COO(CH2 CH2 O)80 H RCOO(CH2 CH2 O)80 Ha
1.96 3.33 1.10 0.87 1.62
a
R is a mixture of fatty acids.
Table 3 The composition of soya bean oil Fatty acid
Composition (%)
Stearic acid Palmitic acid Oleic acid Linoleic acid Linoleulic acid
3–6 6–7 25–39 50–55 2–4
η=
W0 − W × 100 W0
where W0 and W are the weight loss of steel in absence and presence of the used surfactants, respectively. Adsorption isotherms were estimated by determining the concentration of the free surfactant in solutions by surface tension measurements. The corroded surfaces were studied by scan electron microscopy (SEM, JEOLJSM 5400). The studied steel coupons were taken at surfactants concentration around their CMCs for Pa(EO)80 , Li(EO)80 and Soy(EO)80 . In case of Pa(EO)80 , steel coupons immersed in two additional concentrations before and after the CMC were also investigated.
4. Results and discussion In steel/water system, the steel surface is covered with a layer of FeOOH through which the interaction of surfactant molecules takes place. The OH groups on the solid surface are the most important sites for surface interactions; these groups can act as acids or bases. The adsorption process is highly dependent on various parameters such as pH and electrolyte content [8]. The adsorption of the nonionic surfactants Pa(EO)80 , St(EO)80 , Ol(EO)80 , Li(EO)80 , and Soy(EO)80 onto low carbon steel/formation water interface has been studied. It was known that the ethoxylated groups in ethoxylated nonionic surfactants (in aqueous solutions) are present either as cation—active polyoxonium compounds [9], or as an anion—active character [10]. The study of surfactant’s adsorption onto steel surface leads to two different shapes of adsorption isotherms. The saturated Pa(EO)80 and St(EO)80 exhibit an L-shaped isotherms while the unsaturated Ol(EO)80 , Li(EO)80 , and Soy(EO)80 show an S-shaped isotherms. The adsorption isotherms of both Pa(EO)80 and St(EO)80 are shown in Fig. 1. At a very low concentration range, the hydrophilic head group of the surfactant molecule is adsorbed onto the steel surface by the hydrogen bonding between OH group on the surface and O of the oxyethylene group [11] (through the water molecule attracted to it), with the hydrophobic group faced the liquid medium lie flat on the steel surface. The plateau appeared at C eq ≈ 2.5 × 10−2 m mol dm−3 (Fig. 1) means that all the active sites on the surface are occupied by the surfactant molecules forming a complete monomolecular layer on the surface. Further increase in the surfactant concentration leads to a sharp increase in the slope of the isotherms representing the formation of multilayer. This agree with the result given by Kuno [12], he found that the compound with high oxyethylene content show multilayer adsorption.
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Pa(EO)80 (Γ max = 4.46×10−4 m mol g−1 ) in the formation water (calculated from Langmuir equation) than that in sea water (Γ max = 5.56 × 10−4 m mol g−1 ) [16]. The inhibition efficiency increases again with further increase in concentration showing a maximum value around the CMC of the surfactants. By increasing the surfactant concentration, more surfactant molecules overcome the effect of divalent and trivalent cations present in the medium approach to the surface and interact with those already adsorbed on the surface forming a multilayer film, this is the reason of the appearance of maximum efficiency in this concentration range. Another decrease in the efficiency is attained with further increase in the concentration followed by an increase at very high concentration far above the CMC of the surfactant. This alternative behavior is ensured by the electrochemical view where the oxidizing–reducing reaction takes place between the steel and the cations Mz + at the steel surface according to the following equation: Mz+ + Fe M + Fez+
Fig. 1. Adsorption isotherms of Pa(EO)80 (◦) and St(EO)80 (䊉) from formation water onto steel surface after 192 h at 30 ◦ C. The arrows represent the CMCs.
Fig. 1 shows that the effectiveness of adsorption in case of flat orientation decreases with the increase in the chain length. This may be due to the increase in the cross-sectional area of the molecule on the surface and thus saturation of the surface will be accomplished by a smaller number of molecules [13,14]. The adsorption isotherms of Pa(EO)80 and St(EO)80 are in concomitant with the obtained inhibition efficiency data (Fig. 3a). At very low concentration range, the inhibition efficiency increases with concentration due to the adsorption of the surfactant molecules as represented in Fig. 1. The maximum value in this concentration range is due to the complete coverage of the steel surface by a monomolecular layer that is represented by the plateau in the adsorption isotherms as previously discussed. The following decrease in the efficiency is due to the presence of divalent and trivalent cations, e.g., Ca2+ , Mg2+ , and Al3+ in the used formation water, see Table 1, those depressed the surfactant adsorption onto the steel surface [15]. This decrease is ensured by the lower value of the amount adsorbed at the pseudo-plateau of
These cations are present in the formation water with high concentration. As shown in Table 1 the concentration of both Li+ , Mg2+ , K+ , Ca2+ , Cu2+ , Sr2+ and Mn2+ is decreased after the immersion of steel in the water, this is due to their deposition on the steel surface. The increase appears in case of Fe2+ is due to the formation of iron oxide during the corrosion process. The reaction of the deposited cations (Mz + ) on the steel surface may be a reason for the inhibition process to take place. The motion of the cations (Mz + ) in the bulk solution forced the above reaction from the left to the right. The metal (M) formed a protective film on the surface. As a result, a greater amount of the metal (M) deposit on the surface and the formed film thickened quickly, then a part of this film dropped from the steel surface. A corrosion cell formed between the new steel surface and (M) film accelerated the corrosion rate of the steel [17]. From Fig. 3a, the inhibition efficiency decreases with the increase in the hydrocarbon chain length. This may be due to the flat orientation of the adsorbed molecules onto the steel surface as previously discussed. Fig. 2 represents the adsorption isotherms of Ol(EO)80 , Li(EO)80 , and Soy(EO)80 onto steel surface, respectively. The sigmoidal shape of these isotherms reflects the strong interaction between the adsorbed molecules whereby cooperative adsorption occurs [18]. The shape of the isotherm seems to reflect three distinct modes of adsorption. In region I, the surfactant is adsorbed mainly by hydrogen bonding between OH groups on the surface and the oxygen of the oxyethylene groups (through the water molecule attracted to it). The significant increase in adsorption represented in region II resulting from interaction of the hydrophobic chains of oncoming surfactant ions with those of previously adsorbed surfactant and with themselves. This aggregation of the hydrophobic group, which may occur at concentration below the CMC of the surfactant, has been
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Fig. 2. Adsorption isotherms of Ol(EO)80 (), Li(EO)80 (䉱), and Soy(EO)80 (䊐) from formation water onto steel surface after 192 h at 30 ◦ C. The arrows represent the CMCs.
called hemimicelle or cooperative adsorption [19]. In region III, the slope of the isotherm is reduced and the adsorption is usually complete when the surface is covered with the surfactant molecules [20]. In many cases this occurs in the neighborhood of the CMC [21]. Fig. 3b illustrates the dependence of inhibition efficiency on the surfactant concentration. It was shown that the efficiency decreases with increasing the concentration in the first region of the curve due to the vertical orientation of the adsorbed molecules on the steel surface. It is noteworthy that the adsorption in the second region of the isotherm does not lead to increase the inhibition efficiency but the reverse is true. This is because the adsorption in this region is a result of the formation of what is known by hemimicelle or admicelle as previously discussed. So, these adsorbed ions do not occupy a new sites on the surface and, as a result, the bare areas on the steel surface present at the end of region I still as it is up to the end of region II. The following increase
in the inhibition efficiency starts with the beginning of the third region of the isotherm at which the surface approach to a complete coverage with surfactant molecules. The maximum inhibition efficiency takes place at a concentration around the CMC where the adsorption process is leveled off and all the active surface sites are covered by a complete layer of surfactant molecules. The following decrease in the efficiency is due to the oxidizing–reducing reaction which leads to a drop of the deposited metal film as discussed before. At very high concentration far above, the CMC of the tested surfactants, the effect of these cations is overcome by the large number of surfactant ions approach to the surface and the inhibition efficiency shows a steady state. Fig. 3a and b shows two different trends of the inhibition efficiency toward increasing surfactant concentration in the very low concentration region. The isotherms of the two samples Pa(EO)80 and St(EO)80 , discussed previously, follow the Langumuirian type (Fig. 1). The initial curvature
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Fig. 3. The dependence of the inhibition efficiency (η, %) on the concentration of: (a) Pa(EO)80 (◦) and St(EO)80 (䊉); (b) Ol(EO)80 (), Li(EO)80 (䉱), and Soy(EO)80 (䊐) in formation water after 192 h at 30 ◦ C. The arrows represent the CMCs.
shows that as more sites in the substrate are filled it becomes increasingly difficult for a bombarding solute molecule to find a vacant site available. This implies that the adsorbed solute molecule is not vertically oriented [22]. Consequently, the flat orientation causes more coverage surface area at low concentration as well as the more inhibition of this surface toward corrosion as shown in the first region of the curves in Fig. 3a. In case of Ol(EO)80 , Li(EO)80 , and Soy(EO)80 , their isotherms exhibit a sigmoidal S-shaped (Fig. 2). It must be noted that all these samples contain one or more double bonds in their hydrophobic chain. The presence of these bonds increase the chance of intermolecular attraction between the adjacent hydrophobic tails of the adsorbed
molecules which takes place by dispersion and van der Waal forces. This interaction pushes the adsorbed molecule to be oriented perpendicular to the surface from the beginning of the adsorption process and is represented by the curvature and convex curve towards the concentration axis in region I (Fig. 2). Clint referred the formation of this isotherm shape to the onset of adsorbate–adsorbate interactions [5]. This behavior takes place at a very low concentration, hence, there is no sufficient molecules in the medium to adsorb onto the steel and blocking the active sites present on its surface. For this reason, the inhibition efficiency toward corrosion of these samples decreases in spite of raising the surfactant concentration as shown in Fig. 3b.
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At very high concentration far above the CMC, the inhibition efficiency increases with the surfactant concentration in case of the saturated sample while the unsaturated samples show a steady state. As discussed previously, the saturated samples lie flat on the surface and at the very high concentration the hydrophobic tail may be oriented to be tilted to the solid surface which permit adsorption of another molecules. The molecules of the unsaturated samples are arranged perpendicular to the surface and covered it completely, beside the interaction which takes place between the adjacent hydrophobic tails. These reasons prevent the adsorption of new molecules in this high concentration region. In general, the inhibition efficiency of the saturated samples (those exhibit the L-type) are lower than those of the unsaturated samples which exhibit the S-shape. This may be due to the presence of double bond in the hydrophobic chain of the latter which facilitates the complete coverage of the steel surface and leads to more inhibition of the corrosion. Fig. 3b shows that the efficiency increases (as a result of the increase in adsorption) with the presence of double bond in the hydrophobic chain. The Li(EO)80 contains two double bonds exhibits higher adsorption and inhibition efficiency than Ol(EO)80 which contains one double bond. The Soy(EO)80 (which their hydrophobic moiety consists of a mixture of different fatty acids having one and/or more double bonds) possesses the highest inhibition efficiency among all the tested surfactants. This result is in agreement with the previous work [23,24]. From these results we can conclude that the commercial Soy(EO)80 has a good performance to use as a corrosion inhibitor for low carbon steel in the formation water. So, as the soya bean oil consists mainly of linoleic acid (Table 3), one can recommend that the commercial inhibitor Soy(EO)80 (of low cost) can be used instead of Li(EO)80 in the steel protective process. 4.1. Sample characterization The micrographs of the specimens using scanning electron microscopy are shown in Figs. 4–9. Fig. 4a and b shows the micrograph of the steel surface in formation water without any additives. Fig. 4a clarifies a strong damage of the surface with different types of corrosion including: pitting illustrated by a black points, scales which represented by the light central area in the micrograph, and a moderate corrosion appears as grey areas. Also a very deep corrosion surrounded by a moderate one are appeared on another section of the original surface (Fig. 4b). By adding surfactants this corrosion partially inhibited with different degrees depending on the concentration and the nature of the used surfactants. This fact is confirmed by the micrographs of steel in present of additives as shown in Figs. 5–9. Figs. 5–7 represent the micrographs of steel in Pa(EO)80 at three different concentrations. At a concentration 0.05 m mol dm−3 below the CMC of Pa(EO)80 , two types of corrosion are observed: extreme pitting (Fig. 5a)
Fig. 4. (a, b) SEM micrographs of low carbon steel immersed in formation water (without inhibitor) after 192 h at 30 ◦ C.
and scale in another section of the steel which is shown by a light area in Fig. 5b. This result is in agreement with the decrease in the inhibition efficiency at this concentration region (Fig. 3a). At the CMC of this surfactant (1.96 × 10−1 m mol dm−3 ), pits begin to form on a separated areas of the steel surface as in Fig. 6a and b. This behavior is due to depression of the corrosion rate by the more adsorption of the surfactant molecules onto the tested surface. The specimen is observed to undergo a little and smaller pits with increasing the concentration far above CMC, i.e., 10 m mol dm−3 (Fig. 7a and b). This is due to further increase in the molecules adsorbed onto the surface as the surfactant concentration increases which reflecting the inhibition effect of Pa(EO)80 . Figs. 8 and 9 represent the micrographs of the steel in the presence of Li(EO)80 and Soy(EO)80 at 0.87 × 10−1 and 1.62 × 10−1 m mol dm−3 (at their CMCs), respectively. Figs. 8a and 9a show very little damage in the steel surface. This is due to the formation of a complete layer of surfactant molecule on the steel surface as previously discussed. The scan of another section of the steel surface (Figs. 8b and 9b) are closely similar to each other. These results insure the probability of using the commercial Soy(EO)80 as a good cheaper inhibitor to protect the oil field pipe line.
M.M. Osman, M.N. Shalaby / Materials Chemistry and Physics 77 (2002) 261–269
Fig. 5. (a, b) SEM micrographs of low carbon steel immersed in Pa(EO)80 at a concentration far below the CMC after 192 h at 30 ◦ C.
Fig. 6. (a, b) SEM micrographs of low carbon steel immersed in Pa(EO)80 at a concentration around the CMC after 192 h at 30 ◦ C.
Fig. 7. (a, b) SEM micrographs of low carbon steel immersed in Pa(EO)80 at a concentration far above the CMC after 192 h at 30 ◦ C.
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Fig. 8. (a, b) SEM micrographs of low carbon steel immersed in Li(EO)80 at a concentration around CMC after 192 h at 30 ◦ C.
Fig. 9. (a, b) SEM micrographs of low carbon steel immersed in Soy(EO)80 at a concentration around CMC after 192 h at 30 ◦ C.
5. Conclusion
References
The corrosion of low carbon steel in formation water was inhibited by using some ethoxylated fatty acids. Pa(EO)80 and St(EO)80 follow Langmuirian model in their adsorption onto steel surface, while Ol(EO)80 , Li(EO)80 and Soy(EO)80 show an S-shaped isotherm. The adsorption and the inhibition efficiency of the tested inhibitors follow the following order for saturated and unsaturated, respectively:
[1] A.G. Ostroff, Introduction to Oil Field Water Technology, National Association of Corrosion Engineers, Houston, TX, 1979. [2] F.N. Speller, Corrosion Causes and Prevention, McGraw-Hill, New York, 1951. [3] O.A. Tomas, P.R. Alan, Production Operations, Vol. 2, Oil and Gas Consultants International, Inc., Tulsa, 1985. [4] Corrosion Control in Petroleum Production, An Official NACE Publication, 1979. [5] J.H. Clint, Surfactant Aggregation, Blackie, Glasgow and London, Chapman & Hall, New York, 1992. [6] A.J. McMahon, Coll. Surf. 59 (1991) 187. [7] A.N. Wrightly, F.D. Smith, A.J. Stirton, J. Am. Oil Chem. Soc. 34 (1957) 39. [8] G.M. Forland, T. Rahman, H. Holand, K.J. Borve, J. Coll. Interf. Sci. 182 (1996) 348. [9] B.Z. Wurzschmitt, Anal. Chem. 130 (1950) 105. [10] A. Chivala, A. Martina, Textil-Rundshou 2 (1947) 147. [11] H. Rupprecht, H. Liebl, Kolloid Z. Z. Polym. 250 (1972) 719.
St(EO)80 < Pa(EO)80 , Ol(EO)80 < Li(EO)80 < Soy(EO)80 The amount adsorbed and the inhibition efficiency are affected by the length of the hydrocarbon chain and with the number of double bond present in the molecule. Adsorption isotherm, weight loss, beside SEM ensure that the commercial Soy(EO)80 can be used as a good inhibitor of low cost.
M.M. Osman, M.N. Shalaby / Materials Chemistry and Physics 77 (2002) 261–269 [12] [13] [14] [15] [16]
H. Uno, R. Abe, Kolloid Z. 177 (1961) 40. H. Kölbel, K. Hörig, Angew. Chem. 71 (1959) 691. H. Kölbel, P. Kuhn, Angew. Chem. 71 (1959) 211. O. Ofor, J. Coll. Interf. Sci. 179 (1996) 323. M.N. Shalaby, M.M. Osman, A.A. El-Feky, Anti-Corr. Meth. Mater. 46 (1999) 254. [17] G.N. Mu, T.P. Zhao, M. Liu, T. Gu, Corrosion 52 (1996) 853. [18] J. Zhao, W. Brown, J. Phys. Chem. 100 (1996) 3775. [19] T. Wakmastsu, D.W. Fuerstenau, in: W.J. Weber Jr., E. Matizevic (Eds.), Adsorption from Aqueous Solution, American Chemical Society, Washington, DC, 1968;
[20] [21]
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C.H. Giles, A.P. D’Silva, I.A. Easton, J. Coll. Interf. Sci. 47 (1974) 766. D.J.M. Robb, A.E. Alexander, Soc. Chem. Ind. (London), Monogr. 25 (1967) 292. R.C. Goot, in: Proceedings of the Fifth International Congress on Surface Active Substances, Barcelona, Spain, September, Vol. 2, 1968. C.H. Giles, T.H. Ewan, S.N. Nakhwa, D. Smith, J. Chem. Soc. 3973 (1960). T. Szauer, A. Branol, Electrochim. Acta 26 (1981) 1219. M.M. Osman, Ph.D. Thesis, Faculty of Science, Ain Shams University, 1989.
Some ethoxylated fatty acids as corrosion inhibitors for low carbon steel in formation water M.M. Osman∗ , M.N. Shalaby Egyptian Petroleum Research Institute (EPRI), Nasr City, Cairo, Egypt Received 17 May 2001; received in revised form 7 August 2001; accepted 31 October 2001
Abstract A series of ethoxylated fatty acids have been studied in order to evaluate their effect on the corrosion inhibition of low carbon steel. The behavior of these compounds is illustrated by the adsorption isotherms and weight loss measurements. The adsorption isotherms for saturated samples Pa(EO)80 and St(EO)80 exhibit an L-shaped isotherms while the unsaturated samples Ol(EO)80 , Li(EO)80 , and Soy(EO)80 show an S-shaped isotherms. The inhibition efficiency of the first type increases with the decrease in the hydrophobic chain length; and also increases with increasing the number of the double bond present in the hydrophobic chain for the second type. Among all the tested inhibitors, the commercial inhibitor Soy(EO)80 possesses the highest inhibition efficiency. The surface film is characterized by SEM and the morphologies promote the formation of inhibiting film on the steel surface. © 2002 Published by Elsevier Science B.V. Keywords: Corrosion; Low carbon steel; Ethoxylated fatty acids; Adsorption and formation water
1. Introduction The presence of anode, cathode, electrolyte, and an external connection is essentially for electrochemical corrosion process in the oil field. The remove of any one of these factors will be stopped the corrosion process but in fact it is impossible to occur [1]. Corrosion in the oil field appears as leak in tanks, casing, tubing, pipe line, and other equipments. This process changes the base metal to another type of materials. The most corrosive environment in oil field operations is caused by trace amounts of oxygen entering into a sour brine system, as well as the large amounts of carbon dioxide and hydrogen sulfide present in a deep oil-well water (formation water) [2]. These types of environments have destroyed the major items of equipment within six months. This type of corrosion forms a scale which vary from dense and adherent to loose porous, and thick [3,4]. Corrosion inhibitors (surfactants) are widely employed in the petroleum industry to protect iron and steel equipment used in drilling, production, transport, and refining of hydrocarbons [5,6]. The efficiency of the inhibition film depends on the inhibitor concentration and contact time with the metal surface. The work reported here examine some ethoxylated fatty acids and a commercial ethoxylated mixed fatty acid as ∗
Corresponding author. Fax: +20-22-747-433.
0254-0584/02/$ – see front matter © 2002 Published by Elsevier Science B.V. PII: S 0 2 5 4 - 0 5 8 4 ( 0 1 ) 0 0 5 8 0 - 6
corrosion inhibitors for carbon steel in the formation water, to define the most performed one.
2. Experimental 2.1. Formation water Formation water (oil field water) is water that naturally exists immediately in the rocks before drilling. Most oil field water contain a variety of dissolved organic and inorganic compounds. The major elements usually present are sodium, calcium, magnesium, chloride, bicarbonate, and sulfate. The used formation water was given from July (J-69). The cationic analysis for the formation water before and after the immersion of steel are taking place and the value obtained are listed in Table 1 in order to defined what cations are deposited on the steel surface. The ionic analysis takes place by the inductively coupled plasma (ICP).
3. Materials and test solutions Corrosion tests were performed on carbon steel tubing (C, Smax and Pmax = 0.3%), which was cut into coupons (∼1 cm2 exposed surface area). These coupons were polished with different grit emery paper up to 4/0 grade, cleaned with acetone, washed with bidistilled water and finally dried.
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M.M. Osman, M.N. Shalaby / Materials Chemistry and Physics 77 (2002) 261–269
water (1 × 10−2 –10 m mol dm−3 ) for 192 h at 30 ◦ C. The inhibition efficiency of the tested samples is calculated according to the following equation:
Table 1 Cationic analysis of formation water Cations
Na+
K+ Mn2+ Ca2+ Ba2+ Sr2+ Mg2+ Li+ Al3+ Fe2+ Cu2+
Concentration (mg l−1 ) Before immersion
After immersion
82679.000 1649.000 19.670 15355.000 1.030 295.000 3720.000 14.900 0.165 216.000 0.172
82678.300 1235.500 15.350 14855.000 1.028 287.500 2820.000 12.375 0.1648 864.000 0.123
The nonionic surfactants used as inhibitors are: polyoxyethlyene (80) monopalmitate [Pa(EO)80 ], polyoxyethylene (80) monostearate [St(EO)80 ], polyoxyethylene (80) monooleate [Ol(EO)80 ], polyoxyethylene (80) monolinoleate [Li(EO)80 ], and polyoxyethylene (80) monosoyabean fatty acid [Soy(EO)80 ], their molecular formula are listed in Table 2. They were synthesized in the laboratory following the procedure reported elsewhere [7]. The soya bean oil is a mixture of different fatty acids as shown in Table 3. From this table it can be seen that the major fatty acid in this oil is the linoleic acid (50–55%). For each surfactant, the critical micelle concentration (CMC) was determined from the curves of surface tension (measured by Dognan Abribate Tensiometer, Prolabo) versus logarithm the surfactant concentration and their values were collected in Table 2. These curves are used as calibration curves to determine the amount adsorbed by steel surface. The steel coupons were immersed in a 25 ml stagnant surfactant solutions of different concentrations in formation Table 2 The chemical formulae and CMCs values of the used surfactants Surfactant
Chemical formula
CMC × 10−1 (m mol dm−3 )
Pa(EO)80 St(EO)80 Ol(EO)80 Li(EO)80 Soy(EO)80
C15 H31 COO(CH2 CH2 O)80 H C17 H35 COO(CH2 CH2 O)80 H C17 H33 COO(CH2 CH2 O)80 H C17 H31 COO(CH2 CH2 O)80 H RCOO(CH2 CH2 O)80 Ha
1.96 3.33 1.10 0.87 1.62
a
R is a mixture of fatty acids.
Table 3 The composition of soya bean oil Fatty acid
Composition (%)
Stearic acid Palmitic acid Oleic acid Linoleic acid Linoleulic acid
3–6 6–7 25–39 50–55 2–4
η=
W0 − W × 100 W0
where W0 and W are the weight loss of steel in absence and presence of the used surfactants, respectively. Adsorption isotherms were estimated by determining the concentration of the free surfactant in solutions by surface tension measurements. The corroded surfaces were studied by scan electron microscopy (SEM, JEOLJSM 5400). The studied steel coupons were taken at surfactants concentration around their CMCs for Pa(EO)80 , Li(EO)80 and Soy(EO)80 . In case of Pa(EO)80 , steel coupons immersed in two additional concentrations before and after the CMC were also investigated.
4. Results and discussion In steel/water system, the steel surface is covered with a layer of FeOOH through which the interaction of surfactant molecules takes place. The OH groups on the solid surface are the most important sites for surface interactions; these groups can act as acids or bases. The adsorption process is highly dependent on various parameters such as pH and electrolyte content [8]. The adsorption of the nonionic surfactants Pa(EO)80 , St(EO)80 , Ol(EO)80 , Li(EO)80 , and Soy(EO)80 onto low carbon steel/formation water interface has been studied. It was known that the ethoxylated groups in ethoxylated nonionic surfactants (in aqueous solutions) are present either as cation—active polyoxonium compounds [9], or as an anion—active character [10]. The study of surfactant’s adsorption onto steel surface leads to two different shapes of adsorption isotherms. The saturated Pa(EO)80 and St(EO)80 exhibit an L-shaped isotherms while the unsaturated Ol(EO)80 , Li(EO)80 , and Soy(EO)80 show an S-shaped isotherms. The adsorption isotherms of both Pa(EO)80 and St(EO)80 are shown in Fig. 1. At a very low concentration range, the hydrophilic head group of the surfactant molecule is adsorbed onto the steel surface by the hydrogen bonding between OH group on the surface and O of the oxyethylene group [11] (through the water molecule attracted to it), with the hydrophobic group faced the liquid medium lie flat on the steel surface. The plateau appeared at C eq ≈ 2.5 × 10−2 m mol dm−3 (Fig. 1) means that all the active sites on the surface are occupied by the surfactant molecules forming a complete monomolecular layer on the surface. Further increase in the surfactant concentration leads to a sharp increase in the slope of the isotherms representing the formation of multilayer. This agree with the result given by Kuno [12], he found that the compound with high oxyethylene content show multilayer adsorption.
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Pa(EO)80 (Γ max = 4.46×10−4 m mol g−1 ) in the formation water (calculated from Langmuir equation) than that in sea water (Γ max = 5.56 × 10−4 m mol g−1 ) [16]. The inhibition efficiency increases again with further increase in concentration showing a maximum value around the CMC of the surfactants. By increasing the surfactant concentration, more surfactant molecules overcome the effect of divalent and trivalent cations present in the medium approach to the surface and interact with those already adsorbed on the surface forming a multilayer film, this is the reason of the appearance of maximum efficiency in this concentration range. Another decrease in the efficiency is attained with further increase in the concentration followed by an increase at very high concentration far above the CMC of the surfactant. This alternative behavior is ensured by the electrochemical view where the oxidizing–reducing reaction takes place between the steel and the cations Mz + at the steel surface according to the following equation: Mz+ + Fe M + Fez+
Fig. 1. Adsorption isotherms of Pa(EO)80 (◦) and St(EO)80 (䊉) from formation water onto steel surface after 192 h at 30 ◦ C. The arrows represent the CMCs.
Fig. 1 shows that the effectiveness of adsorption in case of flat orientation decreases with the increase in the chain length. This may be due to the increase in the cross-sectional area of the molecule on the surface and thus saturation of the surface will be accomplished by a smaller number of molecules [13,14]. The adsorption isotherms of Pa(EO)80 and St(EO)80 are in concomitant with the obtained inhibition efficiency data (Fig. 3a). At very low concentration range, the inhibition efficiency increases with concentration due to the adsorption of the surfactant molecules as represented in Fig. 1. The maximum value in this concentration range is due to the complete coverage of the steel surface by a monomolecular layer that is represented by the plateau in the adsorption isotherms as previously discussed. The following decrease in the efficiency is due to the presence of divalent and trivalent cations, e.g., Ca2+ , Mg2+ , and Al3+ in the used formation water, see Table 1, those depressed the surfactant adsorption onto the steel surface [15]. This decrease is ensured by the lower value of the amount adsorbed at the pseudo-plateau of
These cations are present in the formation water with high concentration. As shown in Table 1 the concentration of both Li+ , Mg2+ , K+ , Ca2+ , Cu2+ , Sr2+ and Mn2+ is decreased after the immersion of steel in the water, this is due to their deposition on the steel surface. The increase appears in case of Fe2+ is due to the formation of iron oxide during the corrosion process. The reaction of the deposited cations (Mz + ) on the steel surface may be a reason for the inhibition process to take place. The motion of the cations (Mz + ) in the bulk solution forced the above reaction from the left to the right. The metal (M) formed a protective film on the surface. As a result, a greater amount of the metal (M) deposit on the surface and the formed film thickened quickly, then a part of this film dropped from the steel surface. A corrosion cell formed between the new steel surface and (M) film accelerated the corrosion rate of the steel [17]. From Fig. 3a, the inhibition efficiency decreases with the increase in the hydrocarbon chain length. This may be due to the flat orientation of the adsorbed molecules onto the steel surface as previously discussed. Fig. 2 represents the adsorption isotherms of Ol(EO)80 , Li(EO)80 , and Soy(EO)80 onto steel surface, respectively. The sigmoidal shape of these isotherms reflects the strong interaction between the adsorbed molecules whereby cooperative adsorption occurs [18]. The shape of the isotherm seems to reflect three distinct modes of adsorption. In region I, the surfactant is adsorbed mainly by hydrogen bonding between OH groups on the surface and the oxygen of the oxyethylene groups (through the water molecule attracted to it). The significant increase in adsorption represented in region II resulting from interaction of the hydrophobic chains of oncoming surfactant ions with those of previously adsorbed surfactant and with themselves. This aggregation of the hydrophobic group, which may occur at concentration below the CMC of the surfactant, has been
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Fig. 2. Adsorption isotherms of Ol(EO)80 (), Li(EO)80 (䉱), and Soy(EO)80 (䊐) from formation water onto steel surface after 192 h at 30 ◦ C. The arrows represent the CMCs.
called hemimicelle or cooperative adsorption [19]. In region III, the slope of the isotherm is reduced and the adsorption is usually complete when the surface is covered with the surfactant molecules [20]. In many cases this occurs in the neighborhood of the CMC [21]. Fig. 3b illustrates the dependence of inhibition efficiency on the surfactant concentration. It was shown that the efficiency decreases with increasing the concentration in the first region of the curve due to the vertical orientation of the adsorbed molecules on the steel surface. It is noteworthy that the adsorption in the second region of the isotherm does not lead to increase the inhibition efficiency but the reverse is true. This is because the adsorption in this region is a result of the formation of what is known by hemimicelle or admicelle as previously discussed. So, these adsorbed ions do not occupy a new sites on the surface and, as a result, the bare areas on the steel surface present at the end of region I still as it is up to the end of region II. The following increase
in the inhibition efficiency starts with the beginning of the third region of the isotherm at which the surface approach to a complete coverage with surfactant molecules. The maximum inhibition efficiency takes place at a concentration around the CMC where the adsorption process is leveled off and all the active surface sites are covered by a complete layer of surfactant molecules. The following decrease in the efficiency is due to the oxidizing–reducing reaction which leads to a drop of the deposited metal film as discussed before. At very high concentration far above, the CMC of the tested surfactants, the effect of these cations is overcome by the large number of surfactant ions approach to the surface and the inhibition efficiency shows a steady state. Fig. 3a and b shows two different trends of the inhibition efficiency toward increasing surfactant concentration in the very low concentration region. The isotherms of the two samples Pa(EO)80 and St(EO)80 , discussed previously, follow the Langumuirian type (Fig. 1). The initial curvature
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Fig. 3. The dependence of the inhibition efficiency (η, %) on the concentration of: (a) Pa(EO)80 (◦) and St(EO)80 (䊉); (b) Ol(EO)80 (), Li(EO)80 (䉱), and Soy(EO)80 (䊐) in formation water after 192 h at 30 ◦ C. The arrows represent the CMCs.
shows that as more sites in the substrate are filled it becomes increasingly difficult for a bombarding solute molecule to find a vacant site available. This implies that the adsorbed solute molecule is not vertically oriented [22]. Consequently, the flat orientation causes more coverage surface area at low concentration as well as the more inhibition of this surface toward corrosion as shown in the first region of the curves in Fig. 3a. In case of Ol(EO)80 , Li(EO)80 , and Soy(EO)80 , their isotherms exhibit a sigmoidal S-shaped (Fig. 2). It must be noted that all these samples contain one or more double bonds in their hydrophobic chain. The presence of these bonds increase the chance of intermolecular attraction between the adjacent hydrophobic tails of the adsorbed
molecules which takes place by dispersion and van der Waal forces. This interaction pushes the adsorbed molecule to be oriented perpendicular to the surface from the beginning of the adsorption process and is represented by the curvature and convex curve towards the concentration axis in region I (Fig. 2). Clint referred the formation of this isotherm shape to the onset of adsorbate–adsorbate interactions [5]. This behavior takes place at a very low concentration, hence, there is no sufficient molecules in the medium to adsorb onto the steel and blocking the active sites present on its surface. For this reason, the inhibition efficiency toward corrosion of these samples decreases in spite of raising the surfactant concentration as shown in Fig. 3b.
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At very high concentration far above the CMC, the inhibition efficiency increases with the surfactant concentration in case of the saturated sample while the unsaturated samples show a steady state. As discussed previously, the saturated samples lie flat on the surface and at the very high concentration the hydrophobic tail may be oriented to be tilted to the solid surface which permit adsorption of another molecules. The molecules of the unsaturated samples are arranged perpendicular to the surface and covered it completely, beside the interaction which takes place between the adjacent hydrophobic tails. These reasons prevent the adsorption of new molecules in this high concentration region. In general, the inhibition efficiency of the saturated samples (those exhibit the L-type) are lower than those of the unsaturated samples which exhibit the S-shape. This may be due to the presence of double bond in the hydrophobic chain of the latter which facilitates the complete coverage of the steel surface and leads to more inhibition of the corrosion. Fig. 3b shows that the efficiency increases (as a result of the increase in adsorption) with the presence of double bond in the hydrophobic chain. The Li(EO)80 contains two double bonds exhibits higher adsorption and inhibition efficiency than Ol(EO)80 which contains one double bond. The Soy(EO)80 (which their hydrophobic moiety consists of a mixture of different fatty acids having one and/or more double bonds) possesses the highest inhibition efficiency among all the tested surfactants. This result is in agreement with the previous work [23,24]. From these results we can conclude that the commercial Soy(EO)80 has a good performance to use as a corrosion inhibitor for low carbon steel in the formation water. So, as the soya bean oil consists mainly of linoleic acid (Table 3), one can recommend that the commercial inhibitor Soy(EO)80 (of low cost) can be used instead of Li(EO)80 in the steel protective process. 4.1. Sample characterization The micrographs of the specimens using scanning electron microscopy are shown in Figs. 4–9. Fig. 4a and b shows the micrograph of the steel surface in formation water without any additives. Fig. 4a clarifies a strong damage of the surface with different types of corrosion including: pitting illustrated by a black points, scales which represented by the light central area in the micrograph, and a moderate corrosion appears as grey areas. Also a very deep corrosion surrounded by a moderate one are appeared on another section of the original surface (Fig. 4b). By adding surfactants this corrosion partially inhibited with different degrees depending on the concentration and the nature of the used surfactants. This fact is confirmed by the micrographs of steel in present of additives as shown in Figs. 5–9. Figs. 5–7 represent the micrographs of steel in Pa(EO)80 at three different concentrations. At a concentration 0.05 m mol dm−3 below the CMC of Pa(EO)80 , two types of corrosion are observed: extreme pitting (Fig. 5a)
Fig. 4. (a, b) SEM micrographs of low carbon steel immersed in formation water (without inhibitor) after 192 h at 30 ◦ C.
and scale in another section of the steel which is shown by a light area in Fig. 5b. This result is in agreement with the decrease in the inhibition efficiency at this concentration region (Fig. 3a). At the CMC of this surfactant (1.96 × 10−1 m mol dm−3 ), pits begin to form on a separated areas of the steel surface as in Fig. 6a and b. This behavior is due to depression of the corrosion rate by the more adsorption of the surfactant molecules onto the tested surface. The specimen is observed to undergo a little and smaller pits with increasing the concentration far above CMC, i.e., 10 m mol dm−3 (Fig. 7a and b). This is due to further increase in the molecules adsorbed onto the surface as the surfactant concentration increases which reflecting the inhibition effect of Pa(EO)80 . Figs. 8 and 9 represent the micrographs of the steel in the presence of Li(EO)80 and Soy(EO)80 at 0.87 × 10−1 and 1.62 × 10−1 m mol dm−3 (at their CMCs), respectively. Figs. 8a and 9a show very little damage in the steel surface. This is due to the formation of a complete layer of surfactant molecule on the steel surface as previously discussed. The scan of another section of the steel surface (Figs. 8b and 9b) are closely similar to each other. These results insure the probability of using the commercial Soy(EO)80 as a good cheaper inhibitor to protect the oil field pipe line.
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Fig. 5. (a, b) SEM micrographs of low carbon steel immersed in Pa(EO)80 at a concentration far below the CMC after 192 h at 30 ◦ C.
Fig. 6. (a, b) SEM micrographs of low carbon steel immersed in Pa(EO)80 at a concentration around the CMC after 192 h at 30 ◦ C.
Fig. 7. (a, b) SEM micrographs of low carbon steel immersed in Pa(EO)80 at a concentration far above the CMC after 192 h at 30 ◦ C.
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Fig. 8. (a, b) SEM micrographs of low carbon steel immersed in Li(EO)80 at a concentration around CMC after 192 h at 30 ◦ C.
Fig. 9. (a, b) SEM micrographs of low carbon steel immersed in Soy(EO)80 at a concentration around CMC after 192 h at 30 ◦ C.
5. Conclusion
References
The corrosion of low carbon steel in formation water was inhibited by using some ethoxylated fatty acids. Pa(EO)80 and St(EO)80 follow Langmuirian model in their adsorption onto steel surface, while Ol(EO)80 , Li(EO)80 and Soy(EO)80 show an S-shaped isotherm. The adsorption and the inhibition efficiency of the tested inhibitors follow the following order for saturated and unsaturated, respectively:
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St(EO)80 < Pa(EO)80 , Ol(EO)80 < Li(EO)80 < Soy(EO)80 The amount adsorbed and the inhibition efficiency are affected by the length of the hydrocarbon chain and with the number of double bond present in the molecule. Adsorption isotherm, weight loss, beside SEM ensure that the commercial Soy(EO)80 can be used as a good inhibitor of low cost.
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