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Submitted for publication May 2003; in revised form, November2003.

‡ Corresponding author. E-mail: rpalou@imp.mx.* Programa de Ingeniería Molecular, Competencia de Química

Aplicada, Instituto Mexicano del Petróleo, Eje Central LázaroCárdenas no. 152, San Bartolo Atepehuacan, 07730, México,D.F., México.

** Departamento de Química Orgánica, Escuela Nacional deCiencias Biológicas, Instituto Politécnico Nacional, Prol. deCarpio y Plan de Ayala, México, D.F., 11340 México.

Evaluation of Corrosion Inhibitors Synthesizedfrom Fatty Acids and Fatty Alcohols Isolatedfrom Sugar Cane Wax

R. Martínez-Palou,‡,* J. Rivera,* L.G. Zepeda,** A.N. Rodríguez,* M.A. Hernández,* J. Marín-Cruz,* and A. Estrada*

ABSTRACT

Saturated waxy fatty acids (WFAc, 3b) and waxy fattyalcohols (WFAl, 4) were isolated from sugar cane wax. Theformer were used for the synthesis of the corrosion inhibitors1-(2-aminoethyl)- (1c) and 1-(2-hydroxyethyl)-2-alkyl-2-imidazolines (1d), and the later were used to prepare waxyphosphate esters (WPE, 6). The inhibiting action of com-pounds 1c and 1d on carbon steel corrosion, using 1 M hy-drochloric acid (HCl) and hydrogen sulfide brine as corrodingsolutions, was evaluated by the weight-loss method and po-larization techniques. In order to know the effect of the alkylchain length on the inhibition efficiency (IE), compounds1c and 1d were compared with the structurally analogousimidazolines (1a and 1b), which were obtained from the com-mercially available fatty acids tall-oil (3a). The imidazolines1c and 1d showed outstanding IE and low corrosion rate(CR) at 250 ppm. Imidazoline 1d showed a synergic effectwith the addition of 6.

KEY WORDS: carbon steel, imidazolines, organic corrosioninhibitors, phosphate esters, sugar cane wax, waxy fattyacids, waxy fatty alcohols

INTRODUCTION

The 1-(2-aminoethyl)-2-alkyl-imidazoline (1a) and1-(2-hydroxyethyl)-2-alkyl-2-imidazoline (1b) arewell-known corrosion inhibitors widely used in theoil field.1-4 The standard procedure to synthesizethem is by condensing diethylentriamine (2a) andaminoethylethanolamine (2b) with tall-oil (T-O, 3a)(Figure 1),5-6 respectively, which is composed mainlyof oleic and linoleic acids.7-8 Both theoretical and ex-perimental studies on 2-alkyl-2-imidazolines showedthat the inhibition efficiency (IE) and the energybinding between the inhibitor and the metal are in-creased along the C8-C18 series.9-11 In the same sense,the efficiency of the inhibition is also dependent ofthe presence of double bonds placed at the alkylchain.12-14 On the other hand, an efficient methodol-ogy for the extraction and purification of WFAc (3b)and WFAl (4) from crude sugar cane (Saccharumofficinarum, L.) wax recently has been described,15

with 1-octacosanoic acid and n-octacosanol (satu-rated C28) being the major compounds. The qualita-tive composition and biological activity of thesemixtures are well known.16-19 Taking advantage ofthose results, this work presents the synthesis ofimidazolines 1c and 1d by condensing the mixture ofWFAc (3b) with 2a and 2b, respectively, while themixture of WFAl (4) was treated with phosphorouspentoxide (5) to afford a mixture of mono- and di-alkylphosphate esters (WPE, 6). To know the effect ofthe alkyl chain size on corrosion inhibition efficiency,

0010-9312/04/000073/$5.00+$0.50/0© 2004, NACE International

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compounds 1c, 1d and 6, and a mixture of 1d:6 (1:1),were evaluated as corrosion inhibitors of carbon steelby weight loss, using 1 M hydrochloric acid (HCl)and brine at pH = 4.5 as corroding solutions, andby potentiodynamic polarization technique. The IEand corrosion rate (CR) were compared with thoseobtained for imidazolines 1a and 1b synthesized fromT-O (3a).

EXPERIMENTAL PROCEDURES

The following constituents of T-O, WFAc, andWFAl (Cn:i) were determined by gas chromatography(GC):15

T-O: Oleic acid (C18:1), 51.0%; linoleic acid(C18:2), 45.0%; other fatty acids, 4.0%.

WFAc:Octacosanoic acid (C28:0), 49.7%;triacontanoic acid (C30:0), 28.1%;dotriacontanoic acid (C18:1), 23.2%; otherwaxy fatty acid, 3.0%.

WFAl: n-Hexacosanol (C26:0), 9.5%; n-Octacosanol(C28:0), 73.3%; n-Triacontanol (C30:0),14.1%; other waxy fatty alcohols, 3.1%.

Cn is the number of carbon atoms in the alkylchain, and i is the number of nonsaturated bonds.

T-O (3a) was obtained from manufacturers,8

whereas WFAc (3b) and WFAl (4) were extracted andpurified from refined sugar cane wax.16

Synthesis of Organic CompoundsImidazolines 1c (88%, beige solid, mp 76°C to

79°C) and 1d (90%, beige solid, mp 81°C to 83°C)were prepared by condensing 2a (1.5 mol) and 2b(0.5 mol) with 3b (0.5 mol), respectively. Due to themolecular symmetry for 2a, an excess of this reagentwas used to avoid side products formation. Thus, thecorresponding mixture and 0.5 g of CaO was dis-solved in 50 mL of toluene and placed in a 250-mLthree-necked round bottom flask equipped with amechanical stirrer and a Dean-Stark trap. The reac-tants were refluxed under efficient stirring during2 h. The evolved water was eliminated first by usinga Dean-Stark trap until the intermediate amide wasformed completely. During the cyclization-dehydra-

tion stage, water was removed continuously underreduced pressure (3 mm Hg) while heating at 190°Cuntil reaction completion. Reactions were monitoredby proton nuclear magnetic resonance spectroscopy(1H NMR, 200 MHz, chloroform-d [CDCl3]) at regularintervals. Imidazolines 1a (90%, oil) and 1b (93%, oil)similarly were prepared by condensation with 3a. Allcompounds were fully characterized by spectroscopicmethods (proton and 13-carbon nuclear magneticresonance spectroscopy [1H and 13C NMR], infrareadspectrometry [IR], and gas chromatography-massspectrometry [GC-MS]). Mono- and di-alkylphosphateester WPE (6) were prepared by treating the mixtureof WFAl (4) with phosphorous pentoxide (P2O5) (5) at80°C during 3 h (83%, light beige wax).20

Weight-Loss MeasurementsThis procedure was carried out using 1018 car-

bon steel coupons (0.18%C, 0.35 % Mn, 0.17% Si,0.025% S, 0.03% P, balance Fe) polished with emerypaper grade 600, successively rinsed with distilledwater and ethanol (C2H5OH), and carefully weighedafter drying. The coupons were vertically immersedinto the glass cells containing 250 mL of 1 M HClsolution and 0 (blank), 25, 100, and 250 ppm of eachstudied inhibitor. The cells were closed, and the solu-tions were stirred at room temperature during 24 hat room temperature (20 ± 1°C). The carbon steelcoupons (2.54 by 1.27 by 0.03 cm) were taken out,washed with bi-distilled water, wiped, dried, and ac-curately weighed using an analytic balance (preci-sion: ± 0.1 mg). The CR, measured in mils per year(mpy),21 and IE in percent (%) were calculated fromEquations (1) and (2), respectively:

CR W A t D= × × × ×( . )/ ( )3 45 106 (1)

IE W W W= ×[( – )/ ]0 0 100 (2)

where W0 and W are the weight loss in the absenceand presence of inhibitors, respectively; A is the cou-pon area (cm2), t is time of exposure (h), and D is thedensity of the corroding species (g/cm3).

FIGURE 1. General reaction scheme for imidazolines synthesis.

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Because the very efficient corrosion inhibitors1a and 1b showed high CR values, obtained byweight loss and Tafel extrapolation in 1 M HClsolutions, weight-loss experiments were carried outusing saturated hydrogen sulfide brine at pH = 4.5,following the procedure described in NACE publica-tion 1D-182.22

In this case, corrosive solution was composed by20% kerosene and 80% brine. Brine solution wasprepared by mixing 60,000 ppm of sodium chloride(NaCl), 5,000 ppm of calcium chloride dihydrate(CaCl2·[H2O]2), 10,680 ppm of magnesium chloridedihydrate (MgCl2·[H2O]2), and 3,500 ppm of sodiumsulfide (Na2S) in 1 L of distilled water. The 4.5 pH ofthis solution was reached by adding acetic acid(C2H4O2) and bubbling H2S at 250 mL/min during20 min until saturation. The final concentration ofH2S was 1,140 ppm. The corrosion test was carriedout at 70°C and the bottle was rotated at 30 rpmduring 48 h. The CR and IE were calculated withEquations (1) and (2). All experiments were repeatedthree times. Good reproducibility (±2%) was observedin all cases.

Electrochemical MeasurementsPotentiodynamic polarization tests were carried

out with a potentiostat/galvanostat. Potentiodynamicpolarization measurements were recorded using com-mercial software.23 Graphite and saturated calomelelectrodes were used as auxiliary and reference elec-trodes, respectively. A cylindrical carbon steel elec-trode, with an exposed area of 0.50 cm2 (0.09 in.2),was used as the working electrode (WE). This elec-trode was polished with emery paper grade 600,successively rinsed with distilled water and ethanol,and immersed into the glass cell containing 100 mLof 1 M HCl solution. The scanning rate was achievedat 1 mV/s covering a range from –300 mV to 300 mVvs corrosion potential starting five minutes after im-mersion of the WE into the solution. The CR (mpy)and IE (%) were calculated according to the followingequations:24

CR i EW Dcorr= × ×( . )/0 13 (3)

where icorr is corrosion current density in mA/cm2,obtained graphically using Tafel extrapolation tech-nique, EW is the equivalent weight of corroding spe-cies (g), and D is the density of the corroding species(g/cm3).

IE CR CR CR= ×( – / )0 0 100 (4)

where CR0 and CR are the corrosion rate values with-out and with corrosion inhibitor, respectively. Polar-ization measurements were repeated three times foreach sample.

RESULTS AND DISCUSSION

Table 1 shows icorr, polarization resistance (Rp),weight loss, CR, and IE obtained from gravimetric(A and B methods) and potentiodynamic polarizationtechniques (C method) at different inhibitor concen-trations. The CR and IE determined from the abovemethods follow the same trend. It can be observedthat both parameters obtained from the polarizationmeasurements show good agreement with those ob-tained from weight-loss experiments in 1 M HCl (Aand C methods), denoting a good consistency of theobtained results. The observed deviations are due tothe intrinsic differences of the perturbed system andtesting time, as well as the solution corrosiveness.25

In all, assays can be observed that CR and IE areenhanced as inhibitor concentration was increased,meaning that this behavior was more remarkablewhen inhibitors 1c and 1d were used. The structur-ally analogous inhibitors 1a and 1c and 1b and 1dshowed similar effectiveness at 250 ppm; however,imidazolines 1a and 1b were the most efficient at lowconcentrations (50 ppm), especially for the resultsobtained by potentiodynamic polarization measure-ments. This result could be associated with a lowerconcentration of inhibitors 1c and 1d on the metalsurface because of their lower solubility (poor diffu-sion through the aqueous media) compared with in-hibitors 1a and 1b. In other words, inhibitors 1a and1c, or 1b and 1d, have the same heterocyclic system;therefore, it should be expected that each corre-sponding pair possesses the same binding abilitywith the reaction site. However, the larger and satu-rated alkyl chain of compounds 1c and 1d confers onthem a low partition coefficient in aqueous media,making it difficult arrive at the metal surface at con-centrations as low as 50 ppm. On the other hand, itseems that when such compounds are used at higherconcentrations (250 ppm), a critical concentrationcan be attained on the metal surface where a betterinhibition takes place. In fact, according to electro-chemical results (C method), inhibitors 1c and 1dshowed a slight increase in IE in respect to the struc-turally analogous 1a and 1b, respectively, at 250 ppm.At this concentration, the inhibitors 1a and 1b arefairly soluble while 1c and 1d are highly dispersible.It has been established that highly dispersible andstable emulsion-like inhibitor suspensions are moreeffective corrosion inhibitors at higher concentrations(i.e., higher than critical micellar concentration[CMC])26 than completely soluble inhibitors systemsbelow that of CMC.27-28 At, or above, this concentra-tion, the surfactant molecules will form an orderedmolecular arrangement (micelles) in solution, result-ing in the formation of an adsorbed surfactant mono-layer, or bilayer, on the metal surface. At hydrophilicsurfaces (e.g., carbon steel), more than one layer ofsurfactant molecules could form.10,29

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Figure 2 shows the data with error bars (±2%) forgravimetric methods in both 1 M HCl and in saturatedH2S brine, respectively. In spite of the differences incorrosiveness between these mediums, the resultsshow the same trend in both corrosive solutions.

The CR values obtained through B method werelower than those obtained by A and C methods, be-cause a less corrosive solution was used (Entry 1,Table 1). It should be mentioned that this solution isvery similar in composition to that used in the oil in-dustry, in which imidazoline derivatives are widelyused as corrosion inhibitors. This means that com-pounds 1c, 1d, and the mixture 1d/6 could be ofpractical interest in oilfield applications.

Concerning the results obtained by polarizationmeasurements, an examination of the values shownin Table 1 reveals that in all cases the icorr values aredecreased and Rp are increased while inhibitors con-centration is increased. In the same sense, the shift-ing of corrosion potential (Ecorr) into more positivevalues (Table 1, Figure 3 for 1c, 1d and 6 as repre-sentative examples, and Figure 4 for 1d/6) indicatesthe inhibiting effect of this compounds.

Another relevant point should be mentioned. The1-(2-aminoethyl)-imidazolines 1a and 1c are moreefficient inhibitors than their corresponding ana-logues 1-(2-hydroxyethyl)-imidazolines 1b and 1d,

TABLE 1Potentiodynamic Polarization Parameters, Weight Loss, CR, and IE for Different Concentrations of Studied Inhibitors

Conc. Rp icorr Method Method Method Method Method Method Method MethodEntry Sample (ppm) (ohm) (µA/cm2) A(A) B(B) A B C(C) A B C

1 Blank — 91.7 324 0.8320 0.1027 235.0 45.3 299.2 — — —

50 370.0 68 0.1771 0.0107 50.2 4.7 92.4 78.6 89.6 79.02 1a 100 588.2 64 0.0950 0.0025 26.9 1.1 59.0 88.6 97.6 80.2

250 909.1 50 0.0890 0.0007 25.2 0.3 46.2 89.3 99.3 84.6

50 370.0 79 0.2361 0.0109 66.9 4.8 73.0 71.6 89.4 75.63 1b 100 476.2 77 0.1610 0.0020 45.6 0.9 71.0 80.6 98.1 76.2

250 476.2 70 0.1342 0.0009 37.9 0.4 64.6 83.9 99.1 78.4

50 179.9 165 0.3560 0.0179 100.9 7.9 152.4 57.2 82.6 49.04 1c 100 212.8 131 0.1091 0.0136 30.9 6.0 121.0 86.9 86.8 59.6

250 625.0 38 0.1061 0.0081 30.1 3.6 35.0 87.2 92.1 88.3

50 175.4 167 0.2270 0.0225 64.3 9.9 154.2 72.7 78.1 48.55 1d 100 256.0 109 0.2012 0.0159 56.9 7.0 100.6 75.8 84.5 66.3

250 555.5 58 0.1690 0.0116 47.9 5.1 53.6 79.7 88.7 82.1

50 96.2 274 0.6371 0.0762 180.6 33.6 253.0 23.4 25.8 15.46 6 100 108.7 264 0.5940 0.0696 168.4 30.7 243.8 28.6 32.2 18.5

250 121.9 240 0.4702 0.0567 133.2 25.0 221.6 43.5 44.8 26.0

50 140.8 145 0.1680 0.0122 47.6 5.4 133.8 79.8 88.1 55.27 1d/6 100 500.3 67 0.1340 0.0111 38.0 4.9 61.8 83.9 89.2 48.4

(1:1) 250 588.3 46 0.1301 0.0014 36.8 0.6 42.4 84.4 98.6 85.8

(A) Gravimetric method with 1 M HCl.(B) Gravimetric method NACE ID-182.(C) From Tafel slopes.

Weight Loss (g) CR (mpy) IE (%)

denoting a clear dependence on structural proper-ties. This behavior is conclusive enough, since it iswell known that amines are better than alcohols toprotect carbon steel from acidic corrosion. Therefore,the amino groups of compounds 1a and 1c makeavailable a better binding site to form a five-mem-bered coordinated ring with an iron atom from themetal surface, improving its CR and IE.30-33

Concerning the mixture of mono anddialkylphosphates (6) from Table 1, its low efficien-cies as inhibitors can be observed when they areused alone. However, a 1:1 mixture of 1d (the less-efficient inhibitor of the studied imidazolines) andthe alkylphosphates 6 (Table 1, Entry 7) showedhigher efficiencies as those encountered when 1d isindividually administered (Figure 4). This mixtureshowed a well-defined synergic effect at all concen-trations, as was observed in structural analogues ofimidazolines and phosphate esters bearing shorteralkyl chains.34

CONCLUSIONS

❖ The 2-alkylsubstituted imidazolines 1c and 1d,synthesized from WFAc, showed an outstanding pro-tection against carbon steel corrosion in 1 M HClaqueous solution at 250 ppm concentration. Con-

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trary to its structural analogues 1a and 1b, the effi-ciency of compounds 1c and 1d diminishes notice-ably at low concentrations, probably because of itslower solubility. The mixture of phosphate derivatives6 and inhibitor 1d showed a synergic effect on inhibi-tion efficiencies regarding the individual effect of 1d.These results clearly show that sugar cane wax canbe a source of long chains fatty acid and waxyalcohols for the preparation of corrosion inhibitors

FIGURE 2. Weight loss data with error bars for 1a, 1b, 1c, 1d, and 1d/6: (a) A method and (b) B method.

(a) (b)

FIGURE 3. Potentiodynamic polarization curves for carbon steel in1 M HCl at 250 ppm of 1c, 1d, and 6.

FIGURE 4. Potentiodynamic polarization curves for carbon steel in1 M HCl at different concentrations of the mixture 1d/6 (1:1).

1c, 1d, and 6, respectively, which could be of par-ticular interest for application against acid corrosionin, for instance, the oil field.

ACKNOWLEDGMENTS

The authors acknowledge the Instituto Mexicanodel Petróleo for financial support (project D00328). J.Marín-Cruz acknowledges CONACyT for scholarship.

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