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Fuel

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Full Length Article

Influence of stainless steel corrosion on biodiesel oxidative stability duringstorage

S.M. Alvesb,⁎, F.K. Dutra-pereiraa, T.C. Bicudob

a Programa de Pós-Graduação em Engenharia Mecânica, Universidade Federal do Rio Grande do Norte, CEP 59078-970 Natal, RN, Brazilb Laboratório de Ensino e Tecnologia Química, Escola de Ciências e Tecnologia, Universidade Federal do Rio Grande do Norte, CEP 59078-970 Natal, RN, Brazil

G R A P H I C A L A B S T R A C T

A R T I C L E I N F O

Keywords:Biodiesel degradationStainless steel corrosionMicropitting

A B S T R A C T

Biodiesel degradation is the modification of its original composition and properties as a result of fuel aging andmetal corrosion during the storage process. This study examines the linkage between biodiesel degradation andthe corrosion of stainless steel used to store the biodiesel. First, biodiesel was synthesized from soybean oil viamethanolysis and ethanolysis homogeneous base-catalyzed transesterification routes. Next, immersion tests werecarried out at room temperature, with little air turnover and in the dark. AISI 316 coupons were used to evaluatethe corrosion of metal surfaces after contact with the biofuel. Changes in fuel composition were studied usingFTIR and gas chromatography analysis, and the oxidative stability was analyzed by Rancimat and the peroxideindex. The corrosion was evaluated by the gravimetric and SEM/EDS microscopy techniques and XRF analysis.Results revealed little influence of methyl and ethyl esters on metal degradation, indicating that routes have noimportance on corrosion, since a low corrosion rate was observed for both esters, albeit with some surfacemicropitting. On the other hand, the presence of a small amount of metal ions released from the stainless steelsurface during its corrosion promoted oxidation of the biodiesel, changing fuel composition and quality, as wellas reducing its oxidative stability generally.

1. Introduction

Biodiesel has become an alternative for petrodiesel fuel in com-pression ignition engines, mainly due to its renewability, non-toxicity,biodegradability and high cetane number, in addition to being safe for

storage, handling and transporting, and containing no sulfur [1]. An-other important aspect is that it can be locally produced by chemicalprocesses, the most widely used being transesterification, where shortchain alcohols react with triacylglycerides (vegetable oils or animalfats) in the presence of alkali or acid homogeneous catalysts, yielding

https://doi.org/10.1016/j.fuel.2019.03.097Received 7 December 2018; Received in revised form 13 March 2019; Accepted 17 March 2019

⁎ Corresponding author.E-mail address: saletealves@ect.ufrn.br (S.M. Alves).

Fuel 249 (2019) 73–79

Available online 23 March 20190016-2361/ © 2019 Elsevier Ltd. All rights reserved.

T

alkyl esters as the main products. However, some impurities may befound in the biodiesel in cases of incomplete conversion or insufficientwashing (purification). These impurities (glycerol, alcohol, free fattyacids and catalysts) may leave engine deposits and cause corrosion,leading to fuel failure [2]. Moreover, the degree of unsaturation, an-other important issue related to biodiesel stability, is dependent onfeedstock composition [3–5]. Thus, oxidation susceptibility and corro-sivity are properties of interest for establishing biodiesel quality [6–8].

In engine parts, metals such as copper and its alloys, aluminum, castiron, mild steel and stainless steel may be susceptible to corrosion[8–11]. As such, corrosion has become an important issue in biodieselusage. Fazal et al. [12] compared the corrosive behavior of copper,stainless steel, and aluminum in diesel and palm biodiesel. They carriedout immersion tests at 80 °C for 1200 h. In order to determine corrosioncharacteristics, they measured weight loss and morphological changeson the metal surface at the end of the test. The main findings were thatcorrosion damage is more intense with biodiesel than diesel, and thatthe corrosion rate depends on the type of metal. The authors also ob-served that copper and aluminum were susceptible to corrosion bybiodiesel, while stainless steel was not.

The influence of diesel–biodiesel blends has also been investigated[9,13]. The effect of a mixture of palm biodiesel and diesel on thecorrosion characteristics of copper and leaded bronze was evaluated by[9]. They carried out static immersion tests in B0, B50 and B100 fuels atroom temperature for 2640 h. The results showed that adding biodieselto diesel promoted a significant increase in corrosion rate, and thatleaded bronze is less susceptible to corrosion than copper. The authorsfound that biodiesel exhibited higher acidity, free water content andmore oxidation products after immersion tests. Temperature also affectsbiodiesel corrosion rates [11]. Immersion tests in diesel, palm biodieseland a blend of both were performed for 1200 h at room temperature, 50and 80 °C [11]. According to the results, the corrosion rate of mild steelincreased with a rise in temperature.

Biodiesel stability after long-term storage is influenced by fatty acidmethyl ester composition, which is related to the feedstock[1,6,8,9,14]. José and Anand [1] investigated karanja and coconutbiodiesel, whose compositions are significantly different. They con-cluded that the rate of degradation is greater for biodiesel with higherunsaturated methyl ester content (karanja biodiesel) when compared tococonut biodiesel. Berrios et al. [15] reported that stainless steel was asuitable material for storage tanks because its effect on stability wasalmost negligible. This is because it exhibits low corrosivity in thepresence of biodiesel [10]. However, container material selectionshould take into account the fact that metal ions may cause biodieseloxidation [8,9,16]. According to Komariah et al. [17], only stainlesssteel and aluminum are metallic materials compatible with biodieseland recommended for its storage. Although aluminum is advised ascontainer materials, some studies [10,12] have demonstrated that it ismore corrosive (up to 13.5 times higher than stainless steel). Moreover,fuel properties such as flash point, viscosity and cetane number can alsobe changed by oxidative instability. A lower cetane number leads toprolonged ignition delay, and an increase in viscosity could result inpoor fuel atomization [18].

Although, there are numerous studies on metal corrosion after ex-posure to biodiesel, none have investigated how it influences biodieselstability during storage. Moreover, an interesting point to evaluate iswhether the short chain alcohol used in the transesterification reactionplays some role in biodiesel corrosivity. In order to bridge the afore-mentioned gap, the present study aimed to investigate the ability ofbiodiesel samples synthesized by methanolysis and ethanolysis of soy-bean oil, to mount a corrosive attack on stainless steel, as well as de-termine the influence of metal corrosion on biodiesel stability.

2. Material and methods

2.1. Transesterification routes for biodiesel production

Fatty acid methyl esters (FAME) and fatty acid ethyl esters (FAEE)were synthesized by methanolysis and ethanolysis routes, respectively,both conducted under a homogeneous base-catalyzed transesterifica-tion reaction of lipids from soybean oil. Dry oil, alcohol, and catalyst(potassium metoxide) at a molar ratio of 1:6:1 were mixed inside amechanical stirrer equipped-reactor for 2 h at room temperature. Afterreaction, esters were removed from the reactional mixture, neutralized,and dried. In addition to FAME and FAEE, a commercial blend B7 (7%of biodiesel in a mixture with diesel) sample was also studied. Thephysicochemical parameters of all the samples were determined ac-cording to the American Society of Testing and Materials (ASTM).

2.2. Corrosion test

The corrosion process of fuel containers was simulated throughimmersion tests in biodiesel. Aluminum and stainless steel are compa-tible metallic materials for biodiesel containers although stainless steelis the most recommended for this purpose [17]. In this study, in orderto understand if metal releasing from the surface during the corrosionprocess causes an influence on biodiesel stability immersion tests ofAISI316 in biodiesel and its blend (B7) were carried out. These testswere performed on AISI 316 stainless steel coupons with a diameter of9.35 cm2, polished by silicon carbide (SiC) paper from grid 200 to 1200,and finished by alumina 1 µm. The coupons were then washed in dis-tilled water, degreased with 0.2mol L−1 hydrochloric acid, commercialethanol, and dried in hot air. Finally, the coupons were weighed.

To evaluate the corrosion process, three-stage mass loss was ob-served in static immersion tests at room temperature in the dark. Beforeimmersion, corroded coupons were treated by rinsing in distilled waterand rubbing the surface with a polymer brush to remove corrosionproducts. In addition, samples were immersed in 0.2mol L−1 hydro-chloric acid solution for 120 s. Exposure time was also analyzed asfollows: initial condition (0 h), stage 1 (720 h), stage 2 (1440 h), andstage 3 (2160 h). The degree of corrosion was determined by measuringthe corrosion rate according to the following equation:

=

×

ν mA tΔ

corr (1)

where νcorr is the corrosion rate (mg cm-2h−1), Δm the percentageweight loss, A the exposed surface area (cm2) and t the immersion time(h).

2.3. Characterization of chemical modifications

To observe the chemical changes in biofuel properties in each im-mersion stage, 0.3mL of the sample was collected and its peroxidenumber was analyzed according to AOCS Cd 8-53. Sample degradationwas also monitored by FTIR (Fourier Transform Infrared) measure-ments in a BRUKER FT-IR VERTEX 70 spectrometer, with 16 scans, at aresolution of 4 cm−1, in the range of 4000–400 cm−1. After last im-mersion stage, the fuels were also analyzed by X-ray fluorescence (XRF)to verify the presence of soluble corrosion products in fuel.

Fatty acid methyl and ethyl esters profile of samples in differentstages of oxidation were evaluated by gas chromatography coupled to amass spectrometer (GC/MS) Agilent Technologies GC, model GC-7890A/MS-5975C, equipped with a Column HP-5MS (30mlength×250 μm diameter× 0,25 μm film thickness). The sampleswere filtered using syringe filters with 0.45 μm of porosity. The con-ditions of analysis included: 1.0 μL injection volume; drag gas flow of1.5 mLmin−1; injector temperature of 250 °C; interface temperature of300 °C; ionization source at 250 °C; ionization mode by electrons usingenergy of 70 eV for fragmentation and scanning acquisition mode of

S.M. Alves, et al. Fuel 249 (2019) 73–79

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compounds with m/z between 35 and 500. The temperature gradientstarted with the oven at 80 °C for 5min, and after that, it was raised at arate of 5 °Cmin−1 to 260 °C; from this point, the rate was 10 °Cmin−1

up to 280 °C when the temperature was held constant for 5min. Thecompounds identification was performed by comparing data obtainedto the databases of the NIST libraries versions 8.0.

The oxidation stability of each sample was determined by theRancimat method (according to EN15751) in a Metrohm Rancimatmodel 843. For every sample, 7.5 g ± 0.01 g of mass was submitted toa constant temperature of 110 °C in a reaction vessel with 25mm lengthand to a moisture-free air with a flow rate of 10 L h−1. The inductionperiod (IP) was set as the inflection point in the conductivity (μS cm−1)versus time (h) curve.

2.4. Stainless steel surface analyze

The surface morphology of corroded coupons was characterized bySEM/FEG Zeiss Auriga 40 connected to an energy dispersive X-rayanalyzer (SEM/EDS), enabling surface chemical composition to be ex-amined. In addition, X-ray fluorescence (XRF) was used as compli-mentary analysis.

3. Results and discussion

3.1. Corrosion analysis after immersion tests

The chemical composition of esters in biodiesel is responsible fortheir main properties and depends on their feedstock. In this study,soybean oil was used as raw material for biofuel production and anelevated content of unsaturated components was expected in the sam-ples. This contributes to biodiesel oxidation, which increases the acidnumber and water content, enhancing its corrosivity [19–21]. Biodieselcorrosivity can also be affected by several factors including acidnumber, water content, metals and unsaturated acids [19]. After im-mersion, coupons started losing weight, probably due to oxidativecontact with the fuel samples, indicating ions released from the metalsurface to the fluid. B7 caused major weight loss in AISI 316 stainlesssteel when compared to the others (Fig. 1), possibly due to its slightlyhigher acid number [19,20] (Table 1).

The corrosion rate for stainless steel was very low, as expected, withvalues ranging from 1.5×10−4 up to 7.5× 10−4 mpy. Similar resultswere observed by [10,12]. Nevertheless, the influence of expositiontime and fuel composition was verified. The corrosion rate can alsodepend on the alcoholic moiety of ester chains, which are composed ofethyl and methyl groups in FAME and FAEE samples, respectively. This

probable composition effect on corrosion is illustrated in Fig. 1, whereAISI 316 stainless steel was more susceptible to FAME than to FAEE,suggesting an increase in corrosivity for that sample, in line with itslower acid number. The effect of methyl and ethyl alcohol moiety ismainly relevant in the first immersion stage (720 h), at the onset ofcorrosion (Fig. 1). In this stage, corrosion is governed by fuel properties,because the oxidation process is in the initial phase. The corrosion ratedoes not depend on ester composition. Corrosion can be triggered byacid compounds in biodiesel, after which the chemical identity of theesters, especially characterized by their polarity imparting oxygenatoms, acts as an inhibitor of metal degradation. The functional groupof esters has been related to lubricity improvement of ULSD (ultra lowsulfur diesel) [5]. Sundus et al. [21] observed the contradictory natureof biodiesel based on tribo-corrosion statements. Regardless of biodieselis prone to oxidation and highly corrosive, several tests have shown thatit can decrease wear and friction at specific conditions (short-term use,for instance) [20]. A common agent for biodiesel oxidation are themetal ions released from the metal surface that are able to modify someof the fuel properties, especially oxidative stability [11,16,19,22].

3.2. Surface morphology and chemical changes

The corrosion morphologies and mechanisms of metal surfaces wereanalyzed using SEM and EDS. SEM images and EDS mapping of thesurface exposed to biodiesel are displayed in Fig. 2. The SEM micro-graphs reveal that after exposure to biodiesel, corrosion caused a slightchange in the metal surface. A number of micropits (dark spots) arefound on the metal surface, pitting density is lower than that observedby [9,10], and little influenced by transesterification routes. Fig. 2shows that the size and distribution of the pits are different and seem todepend on fuel characteristics. The metal surface immersed in B7 ex-hibited more and smaller pits than those immersed in biodiesel.Stainless steel resists corrosion via a thin protective film that forms onthe surface. However, this steel is susceptible to pitting associated withthe dissolution and regeneration of passive film, as corroborated bySEM and EDS. After immersion (2160 h), oxides such as Fe2O3 andCr2O3 were formed on the stainless steel surface, as demonstrated byEDS mapping and XRF analysis (Table 2). Small cavities caused bycoupon roughness form active regions where micropits occurred as aresult of iron reactions.

According to Brandão et al. [23], micropitting is mainly defined bythe small size of its pits, typically around 10 µm wide and deep. Vo-giatzis et al. [24] proposed a model to describe micropitting formation.After immersion in fuel, the corrosion products form a relatively stablelayer on the disc surface and micropits formed. This layer behaves as a“protective film” that decreases the corrosion rate (see Fig. 1). Theoxide layer is observed mainly for B7, as confirmed by EDS analysis ofoxygen distributed over the entire surface, but more intensely in mi-cropitting. Surfaces immersed in biodiesel behave differently, becausethis layer is predominantly found in micropitting (black dots on themetal surface). With respect to the transesterification route, biodieselfrom ethanol exhibited fewer signs of corrosion than the methyl route.The XRF results corroborate this observation, because a larger amountof Fe2O3 and Cr2O3 is found on the surface exposed to B7 and FAME.

In addition to chemical and morphological metal surface analysis,Fig. 1. Corrosion rates of stainless steel at different immersion times.

Table 1Physicochemical parameters of FAME, FAEE and B7 samples.

Parameter FAME FAEE B7 Limits Method

Density (g cm−3) 879.8 879.9 836.1 860–900 ASTM D4052Viscosity (cSt) 4.8 4.5 2.6 2.0–6.0 ASTM D445Water content (%) 0.045 0.03 0.02 0.05 ASTM D6304Peroxide index (meq kg−1) 30.0 22.3 27.0 – AOCS Cd 8-53Acid number (mg KOH g−1) 0.3 0.2 0.4 0.5 ASTM D664pH 7 6 7 1–14 –

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XRF was also used to verify the presence of metals-containing corrosionproducts in the fuels (Table 3). Iron oxide (Fe2O3) was detected in allfuel samples, while molybdenum oxide (MoO3) was observed only at

diesel fuel (B7), suggesting either higher solubility of MoO3 in B7 or theinfluence of the container metal composition where B7 was stored sinceit was obtained from local trading. The presence of these oxides even atlow concentrations may cause some modification at the chemicalidentity of fuels. Jain and Sharma [25] simulated the effect of metalcontamination on oxidative stability of biodiesel and verified that smallconcentration (around 0.5 ppm) of iron promoted a reduction in oxi-dative stability.

3.3. Oxidative degradation of samples

Infrared spectroscopy has been used to determine the presence orabsence of chemical entities by changing the height/area of specificpeaks [14,26–29]. In order to observe the degradation process of fuelsamples exposed to the coupons, pre-immersion FTIR spectra are de-picted in Fig. 3. Typical biodiesel regions can be observed in FAME andFAEE samples, as well as a significant decline in O-CH3 stretching andcarbonyl absorption in the spectrum of B7. Saturated aliphatic estersexhibit strong absorption in the range of 1735–1750 cm−1 [29] (car-bonyl absorption region). These absorptions at lower frequencies couldindicate the presence of more than one type of carbonyl-containingcompound in addition to ester chains. Organic compounds, such ascarboxylic acids, ketones and aldehydes, can absorb in the range of1700–1740 cm−1 [26] and may be oxidation products resulting fromoxidative processes originating in the fatty esters.

In relation to sample degradation, some peaks display slightly dif-ferent height or area, which could indicate chemical changes. In thecarbonyl region (Fig. 4a) two events are noteworthy. The first is relatedto the peak height of 1741 cm−1, which decreased as exposure timeincreased. This could can be attributed to oxidation reactions that canconvert esters to other non-carbonylated substances [26,28], causingthe C]O absorption band to decrease. The second event is the emer-gence of a shoulder at about 1718, 1720 and 1724 cm−1, for 720, 1440and 2160 h of degradation respectively, which may be related to theformation of oxidation products (aldehydes, ketones, fatty acids) from

Fig. 2. SEM Images and EDS analysis of the exposed surface: a) B7, b) FAMEand c) FAEE.

Table 2Chemical analysis of surfaces by XRF.

Fuel Oxide compound (µg/kg)

Fe2O3 Cr2O3 NiO MoO3 MnO SO3

FAEE 670.835 176.145 87.065 24.41 14.14 19.24FAME 675.255 173.615 88.085 24.97 14.255 19.395B7 677.245 175.955 89.11 24.615 14.235 18.405

Table 3Chemical analysis by XRF of fuels after immersion test (stage 3).

Fuel Oxides (ppm)

Fe2O3 MoO3

FAEE 0.517 –FAME 0.650 –B7 0.458 0.533

Fig. 3. FTIR spectra from 4000 to 400 cm−1 for FAME, FAEE and B7.

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hydroperoxide breakdown [26] and hydrogen bonding to carbonyloxygen [29], and another broad signal at about 1695 cm−1, probablydue to conjugation of the carbonyl group with a double-bond [26].

Stretching vibrations of the CeO bond from the O-CH3 group inlong-chain methyl esters resulted in three absorption bands at 1170,1195 and 1244 cm−1 (Fig. 4b) [29]. Furlan et al. [26] suggested thatdecreases in peak intensity in this region and in carbonyl absorptionmay result from oxidative breakage of methyl-ester linkages (O-CH3).

The chemical composition of FAME and FAEE is strongly related totheir feedstock characteristics. Double bonds containing raw materialsyield unsaturated fatty chain products after transesterification. Doublebonds are active sites for biofuel oxidation processes [18] and re-sponsible for its poor storage stability. The presence of unsaturation ishighlighted by FTIR absorptions. Double bonds in a cis configurationabsorb at 710 and 3009 cm−1 (Fig. 5). However, these peaks decreasedwith oxidation time, suggesting a loss of this type of bond due to theoxidation processes taking place. A decline in cis double bonds is fol-lowed by an increase in trans configuration, as observed between 900and 1000 cm−1, where the bending vibrations of trans double bondsbecome more intense. The primary oxidation products are hydroper-oxides, which may have undergone cis/trans isomerization, as displayedin Fig. 6 [20,26].

The decomposition patterns of FAEE were similar to those of FAME,and FTIR spectra were plotted to compare fuel degradation after 2160 h(as shown at Fig. 7). Fig. 7 demonstrates that FAEE peaks are less in-tense, suggesting more intense degradation for ethyl biodiesel.

Although B7 induced major surface corrosion, its chemical degradationwas milder than that of biofuels, probably due to the absence of doublebonds in its carbon chain.

Besides FTIR spectra, the modifications in chemical composition asconsequence of fuel degradation were observed in the fatty acid profilesobtained by GC/MS investigation. The content of main fatty acid esterslike palmitic (C16:0), stearic (C18:0), oleic (C18:1), and linoleic (C18:2)were quite reduced with degradation time (Fig. 8) denoting possibleconversion of these chemical entities into oxidation products, corro-borating with FTIR results.

The physicochemical properties analyzed (Table 1) and corrosionresults are in line with FTIR data discussed here regarding the occur-rence of oxidative events under metal exposure to biofuels. These in-dicate the formation of polymer compounds able to cause engine pro-blems such as pump and fuel line clogging [26,30]. The increase in theperoxide index shows the evolution of oxidation processes in the fuelsamples, since it is related to the amount of peroxide and hydroperoxideresulting from oxidative attacks on the biofuel [16,19]. For the samplesstudied here, peroxide values rose with immersion time (Fig. 9), re-vealing the presence of oxidation products. Peroxide index resultscorroborate the previous finding and the literature [31], namely thatFAEE is the most degraded sample, followed by FAME and B7.

Although B7 showed a higher corrosion rate (Fig. 1), the presence ofmetal ions did not promote fuel degradation due to their chemicalnature. Silva et al. [32] evaluated soybean biodiesel stability duringstorage in the dark in amber glass flasks. After 90 days of immersion,they found that the peroxide index increased from 26.57meq kg−1 to70meq kg−1. This led us to conclude that metal ions accelerate bio-diesel degradation, since the peroxide index for FAME in the presentstudy was around 230meq kg−1 after 90 days. In addition, Fernandeset al. [33] studied the influence of metal ions from galvanized steelcorrosion, observing that the peroxide index rose by 1100% after84 days, while in the absence of these particles, the increase was only5%. Thus, given that stainless steel, a suitable storage tank material,was used in the tests, and that the rise in peroxide index was 650%, itcan be inferred that even localized corrosion can accelerate biodieseldegradation.

Fuel degradation was also evaluated by oxidative stability mea-surements. The Rancimat method provides the induction period (h) thatis related to the time when oxidation takes place. The analysis revealeda reduction of IP as immersion time increases (Table 4), suggesting fueldegradation. Comparing with literature, this behavior was expected,[25] verified that only the presence of 0.5 ppm of iron (Fe) reduced the

Fig. 4. FTIR spectra from 1800 to 1700 cm−1 (a) and 1400 to 1100 cm−1 (b).

Fig. 5. Degraded and non-degraded FAME spectra in the 3150–2700 cm−1

(before break) and 1050–600 cm−1 range (after break).

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IP in 18%, also according to [34] after four months of storage in carbonsteel container, the IP of soybean biodiesel decreased around 50%.Corroborating the FTIR and Peroxide index results, the more degradedfuel was FAEE, followed by FAME and B7.

4. Conclusion

The corrosivity of biodiesel and diesel towards AISI 316 stainlesssteel and a resulting influence on fuel stability were studied consideringbiodiesel obtained from different transesterification routes. The corro-sivity of FAME and FAEE towards AISI 316 stainless steel appear to besimilar and relatively low in intensity when compared to their corro-sively towards other metals. AISI 316 was more susceptible to B7,probably due to its acidity. SEM images, and EDS and FRX analysesrevealed the protective oxide film formation on micropitting, especiallyfor coupons immersed in B100 samples. More and smaller micropitswere observed after the B7 immersion test. As this film breaks, metalions dissolve into the biodiesel (around 0.5 ppm of iron oxide), in-creasing fuel degradation. However, the results demonstrated that evena mild corrosive attack promotes biodiesel degradation due to the re-lease of metal ions from the metal surface. The FTIR and GC showingchange in biodiesel chemical composition, indicating that the

occurrence of oxidation reactions and degradation of fuel. Moreover,the increase of peroxide index and reduction of induction period in-dicated the fuel degradation caused by the occurrence of oxidationprocesses resulting from the release of metal ions from the stainlesssteel surface after contact with biofuel samples. B7 is less susceptible todegradation by metal ions, because the hydrocarbons are more stable to

Fig. 6. Propagation stage of biodiesel oxidation [17].

Fig. 7. FTIR spectra from FAME, FAEE and B7 in the last stage of degradation.

0 720 1440 21600.00

9.50x105

1.90x106

2.85x106

3.80x106

4.75x106

Peak

are

a / m

m2

Stage / h

C16:0 C18:0 C18:1 C18:2

a)

0 720 1440 21600.0

5.0x105

1.0x106

1.5x106

2.0x106

Peak

are

a / m

m2

Stage / h

C16:0 C18:0 C18:1 C18:2

b)

Fig. 8. Chemical changes in fatty acid profiles from GC/MS analysis in eachoxidation stage for (a) FAME and (b) FAEE.

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oxidation.

Acknowledgments

This study was financed in part by the Coordenação deAperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) -Finance Code 001.

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Fig. 9. Peroxide index of FAME, FAEE and B7 at different immersion times.

Table 4Induction period (h) from Rancimat of fuels in every degradation stage.

Stage Time (h) IP (h)

FAME FAEE B7

Initial 0 5.54 4.36 23.461 720 4.44 2.83 20.552 1440 3.13 2.81 19.943 2160 2.02 1.23 15.84

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