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Fuel 86 (2007) 1162–1168

On the chemical reaction between carboxylic acids and iron,including the special case of naphthenic acid

Omar Yepez *

Department of Engineering and Applied Sciences, Memorial University of Newfoundland, St. John’s, Newfoundland, Canada A1B 3X5

Received 14 March 2006; received in revised form 29 September 2006; accepted 5 October 2006Available online 3 November 2006

Abstract

The reaction between different carboxylic acids and iron was performed by monitoring the amount of iron dissolved in oil through theformation of the iron carboxylate and by the product distribution in the gas phase. It was found that the solubility of the given ironcarboxylate strongly influences the concentration of iron dissolved in oil. At temperatures higher than 300 �C, however, the iron carbox-ylate thermally decomposed and, therefore, the dissolved iron underestimates the acid corrosion potential. On the other hand, the accu-mulated molar amount of hydrogen gas becomes an alternative way to estimate the carboxylic acid corrosion potential.� 2006 Elsevier Ltd. All rights reserved.

Keywords: Naphthenic acid corrosion; Carboxylic acids; Reaction with iron

1. Introduction

Naphthenic acid corrosion has been a problem in the oilindustry for many years. One of the obscure issues regard-ing naphthenic acid corrosion is about the acidity distribu-tion in between the naphthenic acid content of crude oil[1,2]. Since naphthenic acids are a complex mixture of car-boxylic acids with different number of rings, molecularweights and carbon numbers [3–6], the study of the reac-tion between pure carboxylic acids with iron powder mayprovide a better understanding of the naphthenic acid cor-rosion process.

In this sense, there is a report of the influence on the cor-rosion rate, measured by weight lost method on carbonsteel and 5% Cr, with solutions of a family of carboxylicacids in mineral oil, where considerable differences in thecorrosion rates depending on the acid structure, aredetected [7,8].

In this paper, the iron powder test result for a variety ofcarboxylic acid solutions in liquid paraffin is reported.

0016-2361/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.fuel.2006.10.003

* Tel.: +1 580 7672422; fax: +1 580 7674008.E-mail address: Omar.J.Yepez@conocophillips.com

Since a homologous series of carboxylic acids were studied,the influence of the acid strength as a parameter for theaggressiveness of the carboxylic acid was achieved. It wasfound that the solubility of the given iron carboxylaterather than the acidity of the parent acid is what mainlycontributes to the amount of iron dissolved in oil. Thegiven iron carboxylate decomposed at temperatures equalto or higher than 300 �C and therefore, the concentrationof iron in oil at temperatures higher than 300 �C underesti-mates the respective acid aggressiveness. On the otherhand, the molar amount of hydrogen becomes an alterna-tive way to measure the naphthenic acid corrosion processat those temperatures.

2. Experimental

The iron powder test method [9,10] was used to deter-mine the dissolved iron levels of a number of carboxylicacid solutions in liquid paraffin (including naphthenic acidFluka). The method consists in allowing the reactionbetween a stoichiometric excess of iron powder and theacids present in a sample. This is done in a 50 ml closedreactor, where 25 g of the sample and 2.5 g of iron powder

O. Yepez / Fuel 86 (2007) 1162–1168 1163

(0.1 m2/g) are allowed to react for 1 h at 100 rpm, undernitrogen and at different temperatures, namely: 140 �C,180 �C, 220 �C, 260 �C, 300 �C, 340 �C and 380 �C. Afterreaction, the reaction mixture is filtered and the filtrateis sent to measure the dissolved iron by ICP (Induc-tively Coupled Plasma) emission spectroscopy, using theASTM-5708-95. Then a plot of [Fe] in ppm (w/w) vs tem-perature can be done. A four degrees polynomial is fittedwith these data and most of the time, a volcano plotappears. The value of the maximum amount of dissolvediron given by this plot, regardless the temperature of themaximum, is the method final result [11].

All carboxylic acids, were provided by Aldrich and theywere used as received. The total acid number, TAN, of allthe carboxylic acid solutions in liquid paraffin were per-formed by the ASTM-D664. Simulated distillation tech-nique was used for pure naphthenic acid Fluka under theASTM D-proposal November 97. An average molecularweight for this acid mixture was found to be 280 g/mol.The reaction gas products quantification in the chemicalreaction of a given carboxylic acid and iron was performedby gas chromatography using the ASTM-D2505-88 (1998)and ASTM-D2504-88 (1998).

3. Results and discussion

3.1. Acid strength versus iron carboxylate solubility

From Fig. 1 it can be observed that, as the molecularweight of a homologous series of carboxylic acids (all arelinear and saturated carboxylic acids) increases, its corre-spondent acid constant, Ka, diminishes. In other words,butanoic acid (butyric, C4) is stronger than hexanoic(caproic acid, C6) and this one is stronger than octanoic

0.E+00

2.E-06

4.E-06

6.E-06

8.E-06

1.E-05

1.E-05

1.E-05

2.E-05

2.E-05

2.E-05

50 100 150M.W

C8

C6

C4

C2

Ka

(M)

Fig. 1. Acid dissociation constant versus molecular weig

(caprilic acid, C8) and so on. These acids, as a group, areten times stronger than octadecanoic acid (stearic, C18).The latter and 9-octadecenoic acid (oleic acid, C18) sharevery similar structures and molecular weights, the only dif-ference being an unsaturation (double bond) in the carbonnumber 9 of oleic acid. This difference seems to be enoughto produce different acid constants, i.e., 1.78 vs 2.82 ·10�6 M, respectively.

From Fig. 2, it can be observed that the strongest acid ofthe family of acids studied, butanoic acid is not dissolvingiron. Later and besides diminishing the acid strength, theacids: hexanoic (C6), octanoic (C8) and decanoic (C10)are dissolving more iron per TAN unit, progressively. Thisfact of dissolving more iron as the acid constant is dimin-ishing, indicates that the iron carboxylate solubility andnot the acid strength is controlling the amount of dissolvediron.

In Ref. [12] the water solubility of the acids studied isshown. Butanoic acid is completely soluble in water and,as the carbon chain increases: C6, C8 and C10, such solu-bility diminishes up to octadecanoic acid (C18) which iswater insoluble. If these results are used backward, an ideaof the solubility of the acids in oil and therefore, the solu-bility of its iron carboxylates can be obtained. In this sense,decanoic acid is more soluble in oil than octanoic, up tobutanoic acid, which is the less soluble in oil. Because itis expected that the respective iron carboxylate will behavein the same way, the iron butanoate is less soluble in the oilused as solvent, and that is why this acid does not dissolveiron. In other words, butanoic acid has the highest acidityto attack iron but dissolved iron will not be detectedbecause the iron butanoate solubility in oil is very low.Hexanoic (C6) did show iron dissolved and octanoic (C8)showed more iron in comparison with the latter. In this

200 250 300. (g/mol)

C18,octadecanoic acid

C18, 9-octadecenoic acid

ht, for linear carbon chain carboxylic acids [15,16].

0

50

100

150

200

250

300

350

400

80 100 120 140 160 180 200 220 240 260 280 300

M.W. (g/mol)

C4 C6

C8

C10C12

C14

C16C18

Naphthenic9-octadecenoic

[Fe]

/TA

N (

ppm

*g/m

g K

OH

)

Fig. 2. Relationship between the maximum of dissolved iron per TAN unit versus molecular weight. All acids were 0.14 M, except for dodecanoic acid(C12, 0.1 M), butadecanoic acid (C14, 0.28 M) and hexadecanoic acid (C16, 0.07 M). At 280 g/mol, there are octadecanoic acid (C18, 0.07 M),9-octadecenoic acid (rhombus) and naphthenic acid (square).

1164 O. Yepez / Fuel 86 (2007) 1162–1168

case, however, this happens with acids having very similaracidities (see Fig. 1). This can be easily explained if the ironoctanoate has higher solubility in oil in comparison withthe iron hexanoate, as it has been suggested.

As one continues in the homologous series from C12,C14, C16 and C18 in Fig. 2, however, a decrease in theamount of dissolved iron per TAN unit occurs, up toapproach zero with the acids: hexadecanoic (C16) andoctadecanoic (C18). This goes in line with a decrease inthe acid strength observed in Fig. 1. Surprisingly, the otherC18 acid, the 9-octadecenoic acid shows an amount of dis-solved iron per TAN unit higher than the amount foundfor the naphthenic acid. This fact indicates two things:(1) C18 acids are strong enough to attack iron and (2)the solubility of the iron carboxylate rather than the acidstrength is ruling the production of the respective ironcarboxylate.

The only difference between octadecanoic (a linear andcompletely saturated carbon chain) and 9-octadecenoicacid is a double bond in the carbon number 9 of the latter,which opens the possibility of a cis structure in its carbonchain (both C8 chains are on the same side of the doublebond). As a consequence, this acid is not linear like octade-canoic acid. This characteristic probably makes the iron9-octadecenoate being more soluble in oil than its counter-part. This cis structure can also happen in naphthenic acid.Although naphthenic acid is a mixture of acids, as can beappreciated from Fig. 3, one of their basic characteristicsis that they have a carbon ring in their structure [5,6,13].This ring is equivalent to an insaturation (double bond)and this open the possibility that the naphthenic acid

may have a structure that resembles more the 9-octadece-noic acid, i.e. a cis structure rather than a linear one, allow-ing a good dissolution of its respective iron salt.

This increase follows by a decrease in the acid corrosiv-ity, as the acid strength increases, and has been observed incarbon steel coupon weight lost experiments, performedwith different cyclohexyl carboxylic acids, in which the car-bon chain between the cyclohexyl ring and the carboxylgroup is increasing in order to change the acid strength[7]. The same results observed in Fig. 2, an increase fol-lowed by a decrease in the net attack toward the couponsas the number of carbon in the chain increases, is observed.Since these experiments involve the measure of the weightloss of a steel coupon, and not the amount of dissolvediron, it could be concluded that as in the results ofFig. 2, is the iron carboxylate solubility and not the acidstrength what is determinant in naphthenic corrosion andthis is why both methods produces similar results.

3.2. Product distribution in the reaction between iron and

carboxylic acids

In Figs. 4 and 5, the amount of dissolved iron and theamount of molecular hydrogen and CO2 produced as afunction of the temperature, due to the reaction betweenhexanoic and octanoic acid with iron, respectively, isshown. In both cases, the amount of dissolved iron is highat low temperatures and then, after 200 �C, it decreases asthe temperature increases. On the other hand, the amountof molecular hydrogen, in the reaction gas mixture, isincreasing up to 300 �C, where it is stabilized. Finally,

Fig. 3. Simulated distillation curve (distillation point against time) for the naphthenic acid used in this paper.

0

50

100

150

200

250

300

350

100 150 200 250 300 350 400

[Fe]

(pp

m)

0

10

20

30

40

50

60

70

80% molar

H2

CO2

H2

CO 2

T (ºC)

Fig. 4. Dissolved iron (squares) and relative amount of products vs.temperature, for the reaction between hexanoic acid 0.14 M and iron.

0

200

400

600

800

1000

1200

1400

1600

1800

100 150 200 250 300 350 400

[Fe]

(pp

m)

0

10

20

30

40

50

60

70

80

H2

CO2

% molar

H2

CO 2

T (ºC)

Fig. 5. Dissolved iron (squares) and relative amount of products vs.temperature, for the reaction between octanoic acid 0.14 M and iron.

O. Yepez / Fuel 86 (2007) 1162–1168 1165

CO2 is produced at 250 �C, arriving to 20% of the reactiongas mixture just at 380 �C. The decrease in the amount ofdissolved iron occurs because the iron hexanoate and octa-noate are thermally decomposing [14]. However, such acidsalready attacked the iron, since the amounts of molecularhydrogen produced (a direct product of the reactionbetween the acid and iron) increases as the temperatureincreases. By calibrating the molecular hydrogen with thecorresponding dissolved iron produced at lower tempera-tures, an estimate of the mole amount of CO2 could bedone. By this way, it was determined that only 8% of hex-anoic acid (Fig. 4) and just 6% of the octanoic acid (Fig. 5)initially present in the reaction decomposed at 380 �C. Thiscorroborates that the iron carboxylate is decomposing,

since the other way to explain the decrease in the amountof dissolved iron is by the disappearance of the correspond-ing acid through thermal decomposition, which has beenfound to be very low.

Given the activity observed with the carboxylic acidstudied, a more detailed study of the reaction of naphthenicacid with iron was done. Fig. 6 shows the dissolved ironagainst the temperature, measured as the concentrationof the naphthenic acid in liquid paraffin, is increased. Asthe naphthenic acid concentration increases, the saidamount of dissolved iron, also increases, up to a maximumwhich occurs between 260 �C and 300 �C. Then, a decreasein the amount of dissolved iron occurs up to a minimum at380 �C. From these results of dissolved iron at each

0,071 M

0,14 M0,28 M

4003503002502001501000

2000

4000

6000

8000

10000

12000

T (ºC)

0.07 M0.14 M0.28 M

[Fe]

(pp

m)

Fig. 6. Dissolved iron as a function of temperature, for naphthenic acidsolution: 0.07 M, 0.14 M and 0.28 M. TAN values are 4.6, 11.6 and 22.2mgKOH/g, respectively.

1166 O. Yepez / Fuel 86 (2007) 1162–1168

temperature and knowing the initial amount of acid pres-ent, a reaction yield could be obtained. In Fig. 7, such yieldversus acid concentration as a function of the temperatureis shown. At 220 �C, a noticeable increase in the yield as afunction of the concentration is observed, i.e. an increase inthe naphthenic acid attack.

In Fig. 8, the amount of dissolved iron and the amountof molecular hydrogen and CO2 produced as a function ofthe temperature, due to the reaction between naphthenicacids with iron, is shown. The blank was liquid paraffinand iron without naphthenic acid, which produced depre-

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

100.0

0 0.05 0.1[RC

140°C180°C220°C 260°C

Rea

ctio

n Y

ield

%

Fig. 7. Reaction yields for the naphthenic acid reaction with high area ir

ciable amounts of such gases. The amount of iron at260 �C is, approximately, 3100 ppm, which represents1.4 mmol. Since this number of moles is 1 :1 with theamount of hydrogen, according to

2RCOOH + Fe! (RCOO)2Fe + H2 ð1Þ

It could be said that 49% of the gas mixture is 1.4 mmol(see Fig. 8); therefore, the total gas moles are 2.8 mmol.CO2 is 20% of the gas mixture at 380 �C, which gives0.6 mmol. The initial amount of naphthenic acid was3.5 mmol, therefore, this CO2 indicates that only 17% ofthe acid decomposed, and that is happening at very hightemperature, 380 �C. Therefore, as in Figs. 5 and 6, the de-crease of the dissolved iron is not due to acid decomposi-tion. But it could be explained, as a consequence of thethermal decomposition of the iron naphthenate, which be-gan at 300 �C (the thermal decomposition of fatty acidmetallic salts begins at 200 �C [14]). Moreover, since themain source of hydrogen is the reaction between the naph-thenic acid and iron, the molecular hydrogen could be usedas a measure of the progress of the naphthenic acid attack.In this sense, from Fig. 9, it is observed that naphthenicacid attack increases at 300 �C, it is maintained at 340 �Cand only falls when the acid begins to thermally decom-pose. This new maximum due to hydrogen production rep-resents 5740 ppm of dissolved iron and it is �2 times morethan the corrosion potential described by the iron dis-solved. The naphthenic acid is still attacking the iron attemperatures as high as 340 �C, producing hydrogen andthe iron naphthenate which undergoes decomposition atthat temperature.

0.15 0.2 0.25 0.3OOH]

on powder, as a function of the acid concentration and temperature.

0

500

1000

1500

2000

2500

3000

3500

130 180 230 280 330 380

T (°C)

0

10

20

30

40

50

60

70

80

90

100%molar

H2

CO2

[Fe]

ppm

Fig. 8. Dissolved iron (squares) and relative amount of products vs. temperature, for the reaction between naphthenic acid 0.14 M and iron.

T (ºC)

0

1000

2000

3000

4000

5000

6000

100 150 200 250 300 350 400

[Fe]

(pp

m)

Fig. 9. Equivalent dissolved iron (triangles), derived from the hydrogen gas amount detected at a given temperature, in the reactions of Fig. 8 and thecorresponding amount of dissolved iron (squares).

O. Yepez / Fuel 86 (2007) 1162–1168 1167

4. Conclusions

Iron carboxylate solubility and not its parent acidstrength, seems to direct the efficiency of the chemical reac-tion between iron and naphthenic acid. The molecularhydrogen detected is the product of the reaction betweennaphthenic acid and iron, whereas the CO2 detected isthe product of the acid thermal decomposition. It wasfound that the measurement of the amount of dissolvediron underestimates the corrosion rate that the naphthenic

acid may produce at temperatures over the thermal decom-position of the iron naphthenate.

Acknowledgements

The author is deeply thankful to L. Torres and J. Haufor fruitful discussions as well as to A. Gonzalez, H. Jaspeand R. Lorenzo for iron powder test experiments. Theassistance in the publication cost of this article by the Inco

1168 O. Yepez / Fuel 86 (2007) 1162–1168

Innovation Centre at Memorial University of Newfound-land is also highly appreciated.

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[12] Budavari S, editor. Merck index. 11th ed. New Jersey (USA): Merck& Co. Inc.; 1989. p. 265. 266, 1079, 764, 1386, 867, 576, 242.

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