Oxidative degradation and corrosiveness of biodiesel

2.1 Photo-oxidation and autoxidation of the biodiesel

Two oxidation mechanisms, autoxidation and photo-oxidation, are possible because the oxygen molecule O₂ exists in two forms, triplet and singlet. The first is the common ground state ³O₂, which is a diradical, ^•O-O^•. The second is the excited singlet form ¹O₂, which is 22.5 kcal/mol more reactive than the triplet form (Knothe, 2007; Porter, Caldwell, & Mills, 1995).

Photo-oxidation is a process that requires exposure to ultraviolet light and a photosensitizer and occurs due to the action of the singlet form. It is more rapid than autoxidation by several orders of magnitude (Knothe, 2007). However, it should not be a significant factor in the manufacture and transport of biodiesel fuels because fuel should be kept in opaque fuel tanks and containers (Knothe, 2007).

The photo-oxidation of unsaturated oils occurs through the absorption of ultraviolet radiation by photosensitive molecules such as chlorophyll and riboflavin, among others, and transfer of the absorbed energy to triplet oxygen generating singlet oxygen, which is about 1500 times more reactive (Labuza, 1971). The singlet oxygen reacts directly with the double bonds by means of addition reaction to form hydroperoxides, which subsequently give rise to shorter-chain compounds such as acids, aldehydes, and alcohols (Ramalho & Jorge, 2006).

Autoxidation reactions are the main mechanisms of oxidation of biodiesel starting from the attack of oxygen molecules from the air to the double bonds present in the alkyl chains of biodiesel. The extent of the auto-oxidation reactions depends on the oil composition, number, position, and geometry conjugation of the double bonds, as well as the presence of antioxidants and storage conditions (Colakoglu, 2007). A single trans unsaturation configuration is more stable than a cis unsaturation. However, conjugated trans unsaturations are more susceptible to oxidation than cis unsaturations. The presence of bis-allylic configurations (-CH=CH-CH₂-CH=CH-) makes chains more susceptible to oxidation by oxygen and leads to polymerization reactions because the central methylene group is activated by two double bonds (Berthiaume & Tremblay, 2006). Carbon chains with unconjugated double bonds are more susceptible to oxidation than a carbon chain with conjugated double bonds because with the delocalization of the π electrons, the conjugated carbon chain becomes more thermodynamically stable (Sanibal & Mancine Filho, 2006).

The auto-oxidation reaction consists of a chain reaction with three stages: initiation, propagation, and termination (Chaithongdee, Chutmanop, & Srinophakun, 2010; Porter et al., 1995; Waynick, 2005).

In the initiation part, hydrogen is abstracted from a carbon next to the double bond to produce a free radical: RH→R^•+H^•.

Once a free radical has been formed, it will combine with a diatomic oxygen in the subsequent reaction to form a peroxy free radical: R^•+O₂→ROO^•.

The peroxy free radical is sufficiently reactive, quickly abstractong a hydrogen atom from another unsaturated molecule to form the hydroperoxide (ROOH) and another carbon-based free radical, thus starting the propagation reaction: ROO^•+RH→ROOH^•+R^•.

The hydroperoxides are unstable and can decompose to form numerous secondary oxidation products, including alcohols, aldehydes, shorter-chain carboxylic acids, and polymers. This process of oxidation leads to changes in the biodiesel properties, mainly viscosity, acid number, and oxidation stability (Bondioli et al., 2003).

The propagation can be followed by termination if the free radicals react with themselves to yield nonactive products. The oxidation process proceeds slowly at first, and then after the initial induction period (IP), it rapidly increases in speed (Berthiaume & Tremblay, 2006; Waynick, 2005). The mechanism of autoxidation of linoleic acid (Teixeira, 2010) is shown in Figure 2.

Figure 2

Mechanism of autoxidation of linoleic acid.

2.2 Hydrolytic reactions

The presence of water during the production, purification, storage, and use of biodiesel causes acceleration of hydrolytic reaction. In addition, the presence of water decreases the heat of combustion of biodiesel and leads to the development of microbe colonies, which can plug the fuel system (Atadashi et al., 2012).

Hydrolytic reactions are catalyzed by acids, bases, and enzymes or by the action of high temperatures and moisture with formation of FFAs. This reaction is reversible and occurs in three stages, forming three molecules of fatty acids (Kusdiana & Saka, 2004).

In the hydrolytic reactions, rupture of ester bonds leads to the formation of FFAs and glycerol (Ramalho & Jorge, 2006). Hydrolytic oxidation (not enzymatic) usually occurs when there are water and high temperatures (Hui, 1996). Enzymatic degradation occurs through the action of lipases present in seeds and oil or those lipases with microbial origin (fermentation processes), which catalyzes the addition of oxygen to the hydrocarbon chain unsaturated fatty acid producing FFAs (Osawa, Gonçalves, & Ragazzi, 2006).

According with DeMello et al. (2007), the rates of hydrolysis are slower than the microbial degradation rates of biodiesels. However, hydrolytic reactions in abiotic conditions could become more relevant where microbial degradation is less ideal.

3.1 Microbial growth and moisture

Microbiological growth is one of the consequences of the biodegradability of biodiesel (Pasqualino et al., 2006). On the one hand, this is an advantage because it renders biodiesel readily biodegradable (Zhang, Peterson, Reece, Haws, & Moller, 1998); on the other hand, oxidation reactions render biodiesel more corrosive than diesel (Fazal et al., 2010). Biodiesel is about 30 times more hygroscopic than diesel, which makes it more corrosive than the fuel oil derivative (Burton, 2008). The absorption of the water by biodiesel seems to be increased with an increase in temperature (Fazal, Haseeb, & Masjuki, 2011b). The absorbed water promotes microbial proliferation (Klofutar & Golob, 2007) and microbial corrosion and directly promotes the corrosion of metals (DeMello et al., 2007).

Microorganisms have good growth conditions in the barrier between water and biodiesel because removing all water from this fuel is a difficult process. The reason for this is that biodiesel is very hygroscopic (Ambrozin et al., 2009; Passman, 2013). Bacterial and fungal growth in biodiesel also generates wastes that are acidic and can cause metal corrosion. Biodiesel is also susceptible to anaerobic biodegradation, which stimulates biocorrosion because methyl esters of biodiesel are hydrolyzed to the corresponding suite of fatty acids (Schleicher, Werkmeister, Russ, & Meyer-Pittroff, 2009).

Sørensen, Pedersen, Nørgaard, Sørensen, and Nygaard’s (2011) study has shown that a wide variety of microorganisms are involved in fuel degradation and that the introduction of biodiesel affects the types and activities of the microorganisms present in the fuel microcosms. Both aerobic and anaerobic microorganisms have been observed in the incubations despite an initial 84% aeration of the fuel matrix. Sørensen et al. concluded that biodiesel can hold around 20–30 times more water than mineral diesel.

Aktas et al. (2010) studied the anaerobic metabolism of biodiesel and its impact on metal corrosion and suggested that biodiesel can be quite easily hydrolyzed and converted to a variety of fatty acid intermediates by anaerobic microorganisms. They reported that pitting corrosion was observed in carbon steel exposed to biodiesel subjected to anaerobic microorganisms from fresh and marine environments with differing histories of exposure to hydrocarbons, biodiesel, and oxygen. They incubated biodiesel with five anaerobic inocula from fresh and marine environments with differing histories of exposure to hydrocarbons, biodiesel, and oxygen. Results showed that all anaerobic microorganisms were able to biodegrade biodiesel within 1 month. Methyl esters were readily hydrolyzed to the corresponding fatty acids, and the latter were also metabolized. In addition, anaerobic microbial metabolism of biodiesel in coastal seawater samples accelerated the rate of pitting corrosion in carbon steel (Aktas et al., 2010).

The susceptibility to biodeterioration of biodiesel, diesel, and diesel containing 5%, 10%, and 20% biodiesel was evaluated by Bücker et al. (2011) using fungi isolated from contaminated oil systems. Deteriogenic fungal species were able to degrade five common fatty acids found in soy biodiesel; molds and yeasts did not produce surfactants in the assay conditions. They concluded that the tested microorganisms did not cause significant reduction in the pH.

The relationship between corrosion and biodegradation of biodiesel and petroleum-based fuels was evaluated by Aktas et al. (2013) using coupons of unprotected carbon steel in fuel/seawater combinations. They concluded that petroleum-based fuels were similar with biodiesel having the same basic chemistry.

3.2 Effect of temperature and light on biodiesel corrosion and properties

Temperature is an important factor on the deterioration of fuel quality. Many properties of biodiesel are influenced by temperature including oxidation, lubricity, corrosivity, composition, IP, viscosity, density. dos Santos, da Silva, Stragevitch, and Longo (2011) studied unsaturated fatty acid esters at different temperatures. The thermochemical stability order of one double-bond oxidation in the gas phase was determined to be linoleate<γ-linolenate<α-linolenate<oleate<ricinoleate. They concluded that the biodiesel produced from castor oil could be used as an additive to biodiesel produced from other vegetable oil to improve the oxidation stability of the final fuel blend (dos Santos et al., 2011).

The operating temperature of fuel in the fuel system can be as high as 44°C–84°C. Higher temperatures may aggravate metallic corrosion and cause changes in fuel properties. Fazal et al. (2011b) studied the corrosion behavior of mild steel at three different temperatures such as room temperature, 50°C, and 80°C in palm biodiesel by static immersion test. They concluded that the corrosion of mild steel and the degradation of fuel properties in both diesel and biodiesel increase with increase of temperature. The corrosion rate of mild steel in diesel and biodiesel increases 8.7% and 13.5%, respectively, when the temperature increases from room temperature to 80°C. Corrosion attacks for metal surfaces are comparatively more in biodiesel than that in diesel fuel. The exposure of mild steel to biodiesel can increase its oxidation instability. The oxygen content in biodiesel-exposed metal surface increases with an increase in temperature. The corrosion products are composed of iron carbide and iron oxides.

Jain and Sharma (2011b) reviewed the work done so far on the thermal stability of biodiesel and their blends with diesel under different conditions. They concluded that biodiesel is more prone to oxidation in higher temperature due to the formation of oxidation products, such as aldehydes, alcohols, shorter-chain carboxylic acids, gum, and sediment, which are often responsible for fuel filter plugging, injector fouling, and deposit formation during fuel combustion.

Cursaru, Branoiu, Ramadan, and Miculescu (2014) conducted experimental investigations to assess the behavior of automotive materials such as aluminum, copper, and mild carbon steel upon exposure to sunflower biodiesel at room temperature and 60°C. They concluded that at room temperature, corrosion in biodiesel decreases in order: copper>mild carbon steel>aluminum. At 60°C, the corrosion rates decrease in the same order, but the values almost double. Fuel degradation is accelerated by high temperature. The oxidation stability of biodiesel is decreased with the formation of oxidation products such as peroxides and acids, which are further corrosion precursors.

Aquino, Hernandez, Chicoma, Pinto, and Aoki (2012) investigated the influence of light, temperature, and metallic ions on biodiesel degradation and corrosiveness to copper and brass according to ASTM G1 and ASTM G31. The experiments according to ASTM G1 were performed at room temperature and 55°C with and without light. The experiments according to ASTM G31 were performed inside an oven, in the absence of light, at 55°C under constant air bubbling to force the biodiesel degradation. The results of the ASTM G1 tests showed that the thickness loss for both metals is slightly higher when there is light incidence, and these values significantly decrease with highest temperature. In all the studied conditions, the corrosion rates for brass are higher than those for copper samples. Contrary to expectations (i.e., usually, a higher temperature enhances the reaction rate), higher temperatures inhibited metal corrosion, probably because the elimination of the absorbed oxygen in the biodiesel prevented the corrosion of the immersed metals. Biodiesel shows significant degradation in contact with metals as evidenced by increasing water content, higher viscosity, and lower oxidation stability.

Fazal et al. (2014) investigated the effect of temperature on the tribological performance of palm biodiesel using a four-ball wear machine at 30°C, 45°C, 60°C, and 75°C under a normal load of 40 kg for 1 h at a speed of 1200 rpm. For each temperature, the tribological properties of diesel neat and three biodiesel blends (B10, B20, and B50) were investigated and compared. They concluded that the wear rate and surface deformation at each temperature decrease with increasing of biodiesel concentration because biodiesel or its blend provides better lubricity even at higher temperatures. Friction and wear increase with increasing temperature.

At high temperatures, biodiesel FAME can isomerize to a more stable conjugated structure (Joyner & McIntyre, 1938). Once this isomerization has begun, a cyclohexene ring is formed through a Diels-Alder reaction when a conjugated diene group from one fatty acid chain reacts with a single olefin group from another fatty acid chain (Figure 3). It becomes important at temperatures of 250°C–300°C or above, and the formed products are called dimers (Johnson & Kummerow, 1957; Sonntag, 1979; Wexler, 1964).

Figure 3

Diels-Alder reaction.

3.3 Measurement of corrosion in metals or alloy

The level of corrosion in biodiesel fuel is determined by the copper strip corrosion test and ASTM D 93 specifications (Singh et al., 2012). The test consists of a polished copper strip that is immersed in a specified volume of biodiesel at specific temperature. After a specific time, the copper strip is removed and washed, and its color is then assessed using the ASTM standard. However, this test only provides information on the level of corrosion in biodiesel when copper is present as metal (Hu, Xu, Hu, Pan, & Jiang, 2012). Other metallic materials such as steel, stainless steel, copper-based alloy mild carbon steel, iron-based alloy, gray-cast iron, special-cast iron, aluminum-based alloy, cast aluminum, forged aluminum, sand-cast aluminum, die-cast aluminum, and aluminum fiber are widely used in many parts of diesel engine/vehicle that come into contact with the fuel such as fuel tank, fuel feed pump, fuel lines, fuel filter, fuel pump, fuel injector cylinder, piston assembly, and exhaust system (Singh et al., 2012). Biodiesel has a higher lubricity than diesel fuel (Jakeria, Fazal, & Haseeb, 2014; Tang et al., 2008a,b), and hence, it has a higher tendency to dissolve the metallic parts. Moreover, the impurity content and the degradation of biodiesel also have strong influence on the corrosion of metals (Jakeria et al., 2014). Therefore, it is very important to measure the corrosiveness of biodiesel in other metallic materials.

The main methods adopted for corrosion measurement are weight loss through static emersion tests and electrochemical techniques by electrochemical impedance spectroscopy or on potentiostat/galvanostat. The surfaces of the metal strips in general are analyzed by optical, scanning electron, and atomic force microscopy, which reveal the nature and extent of corrosion (Singh et al., 2012). The corrosion test based on ASTM G32-72 (Laboratory Immersion Corrosion Testing of Metal) consists of a polished metal strip that is immersed in a specified volume of fuel. The tubes containing the fuel and metals are completely immersed into an electric constant-temperature water bath at 43°C for 2 months. After 2 months, the metals are taken out, cleaned with acetone to remove impurities on the surfaces, and dried using a hairdryer, and then, their weights are measured (Hu et al., 2012).

Metallic corrosion may be defined as the deterioration action of the material by chemical, biochemical, or electrochemical environment, coupled or not to mechanical stress. Corrosion causes harmful or unwanted changes in the metal such as wear and chemical or structural changes (Gentil, 2007). Metallic corrosion depends on the oxidation potential of the metal (Gentil, 2007) and the presence of impurities such as water, methanol, free glycerol, FFA, catalyst residues (Munoz, Fernandes, Santos, Barbosa, & Sousa, 2012), and dissolved oxygen in the biodiesel, which can accelerate the oxidation reaction (Zuleta, Baena, Rios, & Calderón, 2012). The presence of residual alcohol in the biodiesel may cause metallic corrosion, which influences the flash point, reduces the cetane number, and decreases the lubricity of the engine (Munoz et al., 2012).

The level of corrosion depends of the type of metal or alloy in contact with the fuel. Corrosion is higher with biodiesel than petrodiesel fuel. Aluminum, copper, copper alloys, and steel, to some extent, were found to be prone to corrosion. However, stainless steel was immune to pitting corrosion. The rate of corrosion is influenced by temperature, water content, microbial growth, and type of feedstock used for the biodiesel synthesis. Feedstock with higher concentrations of unsaturated fatty acids has greater oxidation rates. Copper alloys have been found to be more corrosive than the ferrous alloys (Singh et al., 2012).

Degradation of different automotive materials in palm biodiesel was investigated by Fazal, Haseeb, and Masjuki (2012) using the static immersion test. They concluded that the degradation order of palm biodiesel for different metals is copper>brass>aluminum>cast iron. In all the metals tested, degradation of surfaces exposed to palm biodiesel is comparatively higher than that in diesel. Copper metal is less resistant in biodiesel and also causes more fuel property degradation. For metal-exposed biodiesel, the total acid number (TAN) crosses the standard limit. However, the density and viscosity remain within acceptable range.

Sarin, Arora, Singh, Sharma, and Malhotra (2009) studied the effect of the presence of transition metals likely to be present in the metallurgy of storage tanks and barrels on the oxidation stability of Jatropha methyl ester. They concluded that small concentrations of metal contaminants showed nearly the same effect on oxidation stability as large amounts. Copper showed the strongest detrimental and catalytic effect.

Hu et al. (2012) compared the corrosion rates of several metals in biodiesel made from rapeseed oil and methanol. The metals studied were copper, mild carbon steel, aluminum, and stainless steel. The results show that the metals corroded with variable degrees as they were immersed in the biodiesel fuel at 43°C for 60 days. The order of their corrosion rates in biodiesel is copper>mild carbon steel>aluminum>stainless steel. Considering the same conditions, the corrosion rates of the metals immersed in diesel fuel were relatively lower. The corrosion rate of copper was approximately six times faster in biodiesel than in diesel. The corrosion rate of mild carbon steel was approximately 12 times faster in biodiesel than in diesel.

Corrosion in engine parts made of metals or alloys in contact with biodiesel occurs because biodiesel easily undergoes oxidation. When biodiesel is oxidized, its long carbon chain is broken to form short-chain fatty acids and aldehydes. In its advanced stages, oxidation causes biodiesel to become acidic, causing fuel system corrosion. Oxidation of biodiesel reconverts esters into different monocarboxylic acids such as formic acid, acetic acid, propionic acid, caproic acid, etc., which are responsible for enhanced corrosion. Also, the water present in the fuel can cause the formation of rust and corrosion owing to the presence of acids and hydroperoxides formed by fuel oxidation. Biodiesel oxidation studies have shown the catalyzing effect of metals such as Cu, Fe, Ni, Sn, and brass (a copper-rich alloy) (Fazal et al., 2012; Knothe, 2007; Pullen & Saeed, 2012; Waynick, 2005). Copper shows the strongest effect in the oxidizability of fatty acid chains. Iron has been shown to be a potent hydroperoxide decomposer, principally at higher temperature. Iron increases the acidity of biodiesel more than copper (Hu et al., 2012).

Haseeb, Masjuki, Ann, and Fazal (2010) performed static immersion tests with coupons of copper and leaded bronze in palm biodiesel at room temperature (25°C–30°C) for 840 h. Results showed that under experimental conditions, pure copper was more susceptible to corrosion in biodiesel compared with leaded bronze. Both copper and leaded bronze showed higher corrosion rate in biodiesel compared with that in diesel. Exposure of biodiesel to copper or leaded bronze has increased its degradation. Oxidized biodiesel was more corrosive compared with as-received biodiesel.

Fernandes et al. (2013) investigated the storage stability and corrosive character of biodiesel through static immersion corrosion tests with coupons of carbon steel and galvanized steel. They show that for a storage period of up to 56 days, both galvanized and carbon steels were compatible with biodiesel even in the absence of an antioxidant.

Díaz-Ballote, López-Sansores, Maldonado-López, and Garfias-Mesias (2009) studied the corrosion process of aluminum in biodiesel and verified that the behavior is similar to that encountered when aluminum is exposed to an aqueous or ethanol alkaline solution. They suggested that corrosion of aluminum can be used as a quantitative indication of the biodiesel purity.

Fazal, Haseeb, and Masjuki (2013) investigated the corrosion mechanism of copper in palm biodiesel and concluded that corrosion increases with increase in immersion time, but after a certain immersion period, formation of oxygenated compounds on biodiesel-exposed copper surface reduces corrosion rate. CuCO₃ was found to be a major constituent of corrosion products after long-term exposure. Dissolved O₂, H₂ O, CO₂, and RCOO^- radical in biodiesel seem to enhance the corrosiveness of biodiesel.

Kamisnki and Kurzydlowski (2008) investigated the corrosion resistance of stainless steel and carbon steel in a diesel oil solution containing different amounts of FAME (as the biocomponent) and microorganisms. Corrosion resistance was examined using impedance methods on samples exposed for up to 26 weeks in an organic phase, water phase, and water-biodiesel interface. They concluded that the acid value of fuel mixtures, invariable in time of exposition, is proportional to the concentration of FAME in the environment and that the presence of bacteria significantly increases its value in environments with high ester content. The authors verified that the presence of microorganisms does not influence changes in biofuel viscosity and that the corrosion rate of steel A 765(IV) is dependent on the concentration of bacteria-degrading fuel. An increase in the corrosion rate is observed in the presence of sulfate reduction bacteria.

Gallina, Stroparo, Cunha, and Rodrigues (2010) investigated the corrosive behavior of 304 austenitic stainless steel in the presence of biodiesel, unwashed and washed, with aqueous solutions of citric, oxalic, acetic, and ascorbic acids, 0.01 mol/l, and compared the results with those obtained for copper (ASTM D130). The assay used for biodiesel corrosion followed the ABNT NBR 14359, but the testing time was extended from 3 to 72 h for samples containing stainless steel blades. An aliquot of 5 ml of biodiesel was collected when the experiment reached the final time that was analyzed by atomic absorption spectrometry (AAS). The AAS results showed a low rate of corrosion for the stainless steel, and among all the alloys elements studied (Cr, Ni, and Fe), the highest rate was observed for chrome in biodiesel with or without washing. Optical microscopy showed that 304 steel, compared with copper, has a low corrosion rate in the 304 steel/biodiesel system.

Wang, Jenkins, and Ren (2011) studied the heterogeneous corrosion behavior of carbon steel in biodiesel contaminated with water. In situ local current distributions among the electrodes showed that the anodes formed in the area exposed to water and the cathodes formed along the water-biodiesel interface.

Wang, Jenkins, and Ren (2012) investigated the electrochemical corrosion of carbon steel using wire beam electrode technique and concluded that carbon steel material corrodes quickly when exposed to a mixture of biodiesel and 3.5% NaCl simulated-seawater solution. Rust covered the majority of the anode surface in the water phase, and an oily substance was formed along the cathode area. Such formations due to corrosion may also affect biodiesel quality.

Chew, Haseeb, Masjuki, Fazal, and Gupta (2013) investigated the corrosion of aluminum and magnesium in palm biodiesel through immersion test at room temperature for 1440 h. They concluded that magnesium exhibits a much higher corrosion rate compared with aluminum. The surface morphology of biodiesel-exposed magnesium coupon changed significantly, whereas aluminum did not undergo any significant changes. Any corrosion product that might have formed on the surface of both metals is not thick enough to be detected by the X-ray diffraction technique. Biodiesel undergoes significant degradation upon exposure to metals. It crosses the TAN limit of 0.5 mg KOH/g. The study suggests that magnesium catalyzes the oxidation and polymerization reaction in the biodiesel to form gum on the magnesium surface.

Kaul et al. (2007) have reported than bronze, brass, copper, lead, tin, and zinc may oxidize diesel or biodiesel and create sediments, whereas aluminum and stainless steel are compatible with biodiesel.

3.4 Types of feedstock

There are more than 350 oil-bearing crops identified as potential sources for biodiesel production (Atabani et al., 2012). The corrosiveness of biodiesel depends on the type of feedstock used in its production. The chemical composition of the feedstock can show various degrees of corrosiveness. Kaul et al. (2007) studied the corrosivity of biodiesel from different oil sources by static immersion tests using metallic (aluminum alloy) piston for 300 days at 15°C–40°C. The authors observed that the corrosion rate varied with the chemical composition of each feedstock. The corrosion tests were performed with biodiesel of non-edible oils such as Jatropha curcas, Pongamia glabra (karanja), Madhuca indica (mahua), and Salvadora oleoides (pilu), with the last showing the highest corrosion rate.

Pantoja, Conceição, Costa, Zamian, and Rocha Filho (2013) examined the influence of fatty acid composition on the physical-chemical properties of biodiesels. The elevated kinematic viscosity values, cold filter plugging point, and oxidative stability were due to the presence of saturated fatty acids.

In general, feedstock with higher concentrations of unsaturated fatty acids has greater oxidation rates. Biodiesel produced from vegetable oils that have higher percentage of polyunsaturated fatty acids are, in general, more susceptible to oxidation. The oxidative stability of the blends decreases when the total content of linoleic (two double bonds) and linolenic acids (three double bonds) increases (Meira et al., 2011a,b; Singh et al., 2012; Zuleta et al., 2012). Sunflower with higher content of linoleic acid is more reactive than soybean biodiesel (Maru et al., 2009).

4.1 Antioxidants

An antioxidant is a substance that delays the start or slows the rate of lipid oxidation reaction (Knothe, 2007; Ramalho & Jorge, 2006). Antioxidants only extend the IP until the antioxidant is exhausted, after which oxidation starts. The use of antioxidants affects autoxidation but not photo-oxidation. Antioxidants can be classified into primary antioxidants, synergists, oxygen removers, biological antioxidants, chelating agents, and mixed antioxidants (Ramalho & Jorge, 2006).

Primary antioxidants are phenolic compounds that promote inactivation of free radicals through the donation of hydrogen atoms to these molecules, thus stopping the reaction chain (Simic & Javanovic, 1994). Synthetic polyphenols, such as 2-tert-butyl-4-hydroxyanisole and 3-tert-butyl-4-hydroxyanisole (BHA), 2,6-bis(1,1-dimethylethyl)-4-methylphenol (BHT), 2-(1,1-dimethylethyl)-1,4-benzenediol or tert-butylhydroquinone (TBHQ), and propyl gallate (PG), and natural tocopherols are the main and best-known antioxidants of this group. Tocopherols can also be classified as biological antioxidants. Synergists are substances with little or no antioxidant activity that may increase the activity of an antioxidant (Ramalho & Jorge, 2006).

Some primary antioxidants, when used in combination, may act synergistically. Oxygen removers are compounds that act by capturing the oxygen present in the medium through chemical reactions, making them stable and thus unavailable to act as propagators of autoxidation. Oxygen scavengers or reducing agents are used as oxidation inhibitors. These include ascorbic acid, which can regenerate spent antioxidant, and β-carotene, which age as a singlet oxygen quencher (Knothe, 2007). Biological antioxidants include various enzymes such as glucose oxidase, catalase, and superoxide dismutase. Other antioxidants are chelating agents, such as ethylenediamine tetraacetic acid (EDTA), salts of EDTA, citric acid and its salts, phosphoric acid, phosphates, or amino acids, which help in removing metal ions, mainly copper and iron, that catalyze lipid oxidation. An unshared pair of electrons in their molecular structure promotes the action of complexation (Knothe, 2007; Ramalho & Jorge, 2006).

Unrefined vegetable oils, which still contain their natural levels of antioxidants, such as tocopherols, sterols, and tocotrienols, usually have improved oxidative stability compared with refined oils (Knothe, 2007). The level of natural antioxidants can diminish during the oil-refining process or biodiesel production process (Knothe, 2007; Tang et al., 2008a,b) and during biodiesel purification, making it susceptible to degradation (Jakeria et al., 2014; Sharma, Singh, & Upadhyay, 2008). Therefore, the use of synthetic antioxidant additives for biodiesel can be considered a necessity (Chaithongdee et al., 2010; Sharma et al., 2008). However, the efficiency of a given antioxidant depends on the feedstock used for the production of the biodiesel (Mittelbach & Schober, 2003).

The most commonly used antioxidants for biodiesel, including some additives developed for petroleum fuels, are BHT, BHA, PG, TBHQ, and pyrogallol (PY) (Karavalakis, Stournas, & Karonis, 2010).

Phosphorylated compounds are secondary antioxidants. Phosphorous stabilizers are considered hydroperoxide decomposers, which also react with peroxy and oxyradicals by a nonradical reaction (Kriston et al., 2009).

Some studies proved the antioxidant capacity on mineral oils and polymers of the constituents of cashew nut shell liquid, a by-product of cashew (Anacardium occidentale L.) nut industrial processing (Alexander & Thachil, 2010; Amorati et al., 2011; Rios, Santiago, Lopes, & Mazzetto, 2010).

Lomonaco et al. (2012) reported the synthesis of phosphorylated compounds derived from cardanol, a phenolic by-product of the cashew (A. occidentale L.) industry and its application as antioxidants for biodiesel. They used the phosphorylated compounds in biodiesel samples in three different concentrations (500, 1000, and 2000 ppm) and showed that biodiesel with new antioxidants added is more resistant to the thermo-oxidative process.

Obadiah, Kannan, Ramasubbu, and Kumar (2012) investigates various synthetic phenolic antioxidants on the oxidation stability and storage stability of the Pongamia (karanja) biodiesel. The antioxidants used were BHA, BHT, TBHQ, PY, and gallic acid (GA). Results showed that IP increased substantially with the addition of certain antioxidants (BHA, BHT, TBHQ, PY, and GA) for the Pongamia biodiesel. PY was the best antioxidant at concentrations between 500 and 3000 mg/l. Mittelbach and Schober (2003) related the influence of a series of antioxidants on the oxidation stability of biodiesel produced from rapeseed oil, sunflower oil, used frying oil, and beef tallow, both undistilled and distilled. The synthetic antioxidants PY, PG, TBHQ, and BHA produced the greatest enhancement of the induction period, whereas BHT was not very effective. They founded a good correlation between the improvement of the oxidation stability and the fatty acid composition.

Das, Bora, Pradhan, Naik, and Naik (2009) studied the effect of the addition of the different antioxidants BHT, BHA, PY, PG, and TBHQ in the stability of the karanja biodiesel. They verified that PG is the best antioxidant, followed by BHA and BHT.

Dunn (2002) investigated the effects of oxidation under controlled accelerated conditions on fuel properties of methyl soyate (SME). The authors concluded that reaction time affects kinematic viscosity (ν) and that antioxidants TBHQ and α-tocopherol (α-T) showed beneficial effects on retarding oxidative degradation of SME.

Karavalakis and Stournas (2010) studied the impact of various synthetic phenolic antioxidant additives on the oxidation stability of several neat biodiesels obtained from different origins and their blends using the modified Rancimat method (EN 15751). All biodiesel samples were treated with 1000 mg/kg of each additive and then blended with diesel fuel. They concluded that oxidation stability improved with all antioxidants tested. The efficiency of the antioxidants depends upon the types of biodiesel. Among the antioxidants evaluated, BHT and BHA displayed the lowest effectiveness in neat methyl esters, whereas their use in biodiesel blends showed a greater stabilizing potential. It was also found that additives that significantly improve the stability of biodiesel may act as pro-oxidants in biodiesel blend.

Karavalakis et al. (2010) studied the impact of various synthetic phenolic antioxidants on the oxidation stability of biodiesel blends using the modified Rancimat method. Experimental results revealed that TBHQ, PG, and PY were the most effective additives in neat methyl ester, whereas BHT and BHA were the least effective.

Chen et al. (2011) investigated the oxidative stability of Jatropha biodiesel in the presence of various synthetic antioxidants. They concluded that the effectiveness of the antioxidants was in the following order: PY>PG>Ethanox4760E>N,N′-di-sec-butyl-p-phenylenediamine>2,2′-methylene-bis-(4-methyl-6-ter-butylphenol)>BHA>TBHQ∼ BHT>2,5-di-tert-butyl-hydroquinone>α-T.

Kivevele, Mbarawa, Bereczky, Laza, and Madarasz (2011) investigated the effectiveness of three antioxidant additives, PY, PG, and BHA, on the oxidation stability of biodiesel produced from Croton megalocarpus oil. The result showed that the effectiveness of these antioxidants was in the following order: PY>PG>BHA.

Pantoja et al. (2013) examined the influence of BHA, PG, and TBHQ antioxidants on linseed biodiesels at concentrations from 500 to 4000 ppm. PG the most efficient antioxidant for the studied biodiesels. The PG antioxidant was shown to be more efficient than BHA and TBHQ. When antioxidants are added to methyl esters with low oxidation rates, oxidative stability sharply increases.

Maia et al. (2011) investigated the oxidative stability of soybean biodiesel B100 in the presence of three synthetic antioxidants, BHA, TBHQ, and BHT, pure or in mixture. They concluded that soybean biodiesel presented IP over 6 h, at 110°C, in accordance with the specifications of the norm EN1412 when the three oxidants were used separately or in ternary mixture. BHA and TBHQ presented higher efficiency as antioxidants of the B100 biofuel.

Schober and Mittelbach (2004) investigated the potential of 11 different synthetic phenolic antioxidants to improve the oxidation stability of biodiesel prepared from different feedstock. They concluded that the antioxidants 2,5-di-tert-butyl-hydroquinone (DTBHQ), IONOX 220, Vulkanox ZKF, Vulkanox BKF, and Baynox were able to significantly improve the oxidation stability of biodiesel.

Du Plessis, de Villiers, and Van der Walt (1985) studied the stability of methyl and ethyl fatty acid esters obtained from sunflower seed oil. Storage of esters in contact with air, especially above 30°C, resulted in significant increases in oxidation parameters. However, exclusion of air retarded oxidation at all temperature levels. Addition of TBHQ prevented oxidation of biodiesel samples stored under moderate conditions.

DeGuzman, Tang, Salley, and Simon Ng (2009) examined the synergisms of blends of primary antioxidants from combinations of BHA, PG, PY, and TBHQ to increase oxidative stability. They conclude that binary antioxidant formulations TBHQ/BHA, TBHQ/PG, and TBHQ/PY were most effective at 2:1, 1:1, and 2:1 weight ratio, respectively, in both distilled soybean oil- and distilled poultry fat-based biodiesel. The best synergistic effect was observed with the TBHQ/BHA pair, whereas the best stabilization factors were achieved using the TBHQ/PY blends.

Liang et al. (2006) studied the effect of natural and synthetic antioxidants on the oxidative stability of palm diesel. They verified that crude palm oil methyl esters containing not less than 600 ppm of vitamin E exhibit oxidative stability of more than 6 h, conforming to the specification of the European standard for biodiesel (EN 14214), whereas distilled palm oil methyl esters antioxidants failed to meet the specification. They concluded that the synthetic antioxidants, namely BHT and TBHQ, were more effective than natural antioxidant, α-T. Rancimat IP of distilled palm oil methyl esters increases drastically with small increments of the amount of TBHQ used (<0.1%).

Damasceno et al. (2013) investigated the addition of different antioxidants, caffeic acid, ferulic acid, and tert-hydroquinone into soybean biodiesel during storage period as well as evaluation of its stability by accelerated techniques such as Rancimat and others. They concluded that the three antioxidants were effective in retarding the oxidation processes at the initial time of storage. The efficiency of the antioxidant took the following efficiency order: caffeic acid>ferulic acid>tert-butylhydroquinone. The efficiency of caffeic acid was noticeable in maintaining the IP values at 6.66 h during the entire storage period of 90 days, fulfilling the limit specified by EN 14214 (Damasceno et al., 2013).

Serrano, Bouaid, Martínez, and Aracil (2013) studied the effectiveness of three commercial synthetic and one natural antioxidants in improving the oxidation stability of various biodiesel fuels produced from different vegetable oils, soybean methyl ester, rapeseed methyl ester (RME), high-oleic sunflower methyl ester, and palm methyl ester, and prepared by two different purification steps, using distilled water and acidified distilled water (0.1 m citric acid solution) wash methods. PG- and tocopherol-based antioxidants were identified as the most effective substances in delaying the oxidative stability of the samples tested. Results for tocopherol-based antioxidant did not agree with bibliographical results. It has been concluded that pure tocopherols are less effective. The high IP values measured may be due to other antioxidant substances in the commercial formula. When citric acid was used as washing agent, the IP values of all samples improved and fulfilled the European standard EN 14214.

Tang, Guzman, Simon Ng, and Salley (2010) investigated the effectiveness of various individual and binary antioxidants to improve the storage stability of soybean oil and distilled soy biodiesel. The antioxidants studied were α-T, BHA, BHT, TBHQ, DTBHQ, ionol BF200, PG, and PY. The authors concluded that the IP of untreated biodiesel significantly decreased with the increasing storage time, whereas the IP values of biodiesel with TBHQ added remained constant for up to 30 months. TBHQ was the most effective antioxidant in improving the storage stability of soybean biodiesel (Tang et al., 2010).

Xin, Imahara, and Saka (2009) studied the oxidation stability of safflower biodiesel using the Rancimat method with addition of PG from 0 to 5000 mg/l at temperatures from 100°C to 120°C. They concluded that the IP of biodiesel increases with the increase in antioxidant concentration and decreases with increase in temperature.

Dinkov, Hristov, Stratiev, and Aldayri (2009) tested three commercially available antioxidants, 2,6-di-tert-butyl-1,2-dihydroxybenzene (A), butylated phenol (B), and 2,6-di-tert-butyl-phenol (C), on biodiesel/diesel blends stability. They concluded that the addition of antioxidant B to biodiesel/diesel blends led to a reduction in the formation of insoluble gums. The antioxidant increases the blends’ resistance toward the formation of secondary oxidation products.

Chuck, Parker, Jenkins, and Donnelly (2013) produced guaiacol, a potential antioxidant, and other oxygenated phenolics and short-chain esters on the ozonolysis of lignin. Guaiacol, a potential antioxidant, was found to be completely miscible with low-sulfur petrol, diesel, aviation kerosene, and RME.

Moser (2012) tested the efficacy of gossypol, a naturally occurring polyphenolic aldehyde that is toxic to humans and animals, as an antioxidant additive in biodiesel prepared from soybean oil, waste cooking oil, and technical-grade methyl oleate. The study concluded that gossypol was effective as an exogenous antioxidant for the FAMEs investigated, especially in FAMEs containing a high percentage of endogenous tocopherols.

Jain and Sharma (2011a) studied the effect of five antioxidants, namely BHT, TBHQ, BHA, PG, and PY, in biodiesel of J. curcas oil and blends with diesel. PY was found to be the best antioxidants among all five antioxidants in the pure biodiesel of J. curcas oil and its blends with diesel.

İleri and Koçar (2013) investigated the effect four antioxidants on the oxidation stability of canola biodiesel. The order of efficiency of antioxidants was TBHQ>BHA>BHT>2-ethylhexyl nitrate (EHN).

4.2 Corrosion inhibitors

Corrosion inhibitors are substances added to the medium to prevent the occurrence of metallic corrosion in the gaseous phase, the aqueous phase, or in oil. The protection offered by the inhibitors depends on the type of metal or metal alloy as well as the corrosive medium (Rangel, 2009). The inhibitors cannot totally prevent the corrosion but only extend the time before corrosion starts to occur in the metal exposed to corrosive medium (Singh et al., 2012).

The inhibitors can be organic or inorganic, and the former are generally more effective in inhibiting electrochemical corrosion. Some organic molecules can adsorb at the metal-solution interface, which inhibits the corrosion on the metal surface (Yooa, Kima, Chunga, Baika, & Kimb, 2012). In general, organic corrosion inhibitors contain multiple bonds and heteroatoms (centers of nucleophilic organic inhibitor) such as oxygen, nitrogen, sulfur, and phosphorus, which have free electron pairs and are available to be shared with metals (Figure 4) (Bentiss et al., 2002; Lebrini, Lagrenée, Venzin, Gengembre, & Bentiss, 2005; Quraishi & Ansari, 2006; Rafiquee, Kahn, Saxena, & Quraishi, 2007; Ranel, 2009; Yooa et al., 2012).

Figure 4

Corrosion inhibitors.

The efficiency of an organic compound in the inhibition of electrochemical corrosion generally depends their molecular structure, planarity and of the presence of free electrons on heteroatoms that can form bonds with the metal surface by electron transfer, wherein the metal is the electrophile and the organic inhibitor is the nucleophile. These characteristics determine the adsorption capacity of these molecules on the metal surface to form a cohesive film that protects the metal surface. This film consists of the chelate or complex formed with the metal surface. The efficiency of the inhibitor depends on the stability of the chelate (Rangel, 2009).

The use of corrosion inhibitors in oil, diesel or different acid solution has long been studied. The common corrosion inhibitors in oil and gas media are imidazolines (Ramachandran & Jovancicevic, 1999; Ramachandran et al., 1996; Villamizar, Casales, Martinez, Chacon-Naca, & Gonzalez-Rodriguez, 2008; Wang et al., 1999), benzaldehydes (Emregül & Hayvali, 2004), furans (Machnikova, Whitmire, & Hackerman, 2008), isoxazolidines (Ali, Al-Muallem, Saeed, & Rahman, 2008; Ali, Saeed, & Rahman, 2003), triazoles (Bentiss, Lagrenee, Traisnel, & Hornez, 1999; Hassan, Abdelgahani, & Amin, 2007; Khaled, 2008; Lebrini, Traisnel, Lagrenée, Mernari, & Bentiss, 2008; Rammelt, Koehler, & Reinhard, 2008), thiadiazole (Bentiss, Lebrini, Vezin, & Lagrenée, 2004; Lebrini, Bentiss, Venzin, & Lagrenee, 2006), oxadiazoles (Bentiss et al., 2002; Bentiss, Traisnel, & Lagrenee, 2000), pyridines (Abd El-Maksoud and Fonda, 2005), primary amines, diamines, amino-amines, oxyalkylated amines, naphthaneic acid, phosphate esters, dodecyl benzene sulfonic acids (Martínez et al., 2009; Videla, Saravia, Guiamet, Allegreti, & Furlong, 2000), benzoic acid, sulfides, and thiophenes (Bentiss et al., 2002).

Corrosion inhibitors can be used in diesel, such as carboxylic acids, amines, and amine salts of carboxylic acids such as alkyl- or polyalkyl-succinics, and their esters, dimeric acids, and amine salts (Haycock & Thatcher, 2004), being amine-based compounds (including primary amines, diamines, aminoamines, and oxyalkylated amines), are more effective (Muthukumar, Maruthamuthu, & Palaniswamy, 2006; Rajasekar, Maruthamuthu, Palaniswamy, & Rajendran, 2007). However, as the composition of biodiesel (mono-alkyl esters of fatty acids) is different from that of mineral diesel (hydrocarbons), the type of corrosion inhibitors will be different for these two fuels (Singh et al., 2012).

There are few studies showing the effect of inhibitors on the corrosion of metals in biodiesel. Antioxidant compounds may also act as a corrosion inhibitor (Almeida et al., 2011), but the mechanism of action of corrosion inhibitors is the formation of a persistent adsorbed monolayer film at the metal/solution interface (Ranel, 2009).

Hancsok, Bubalik, Beck, and Baladincz (2008) developed multifunctional additives based on rapeseed oil methyl ester by applying radical initiation in a more environmentally friendly and energy-economic manner through the widely used thermal synthesis method for the production of polyisobutylene (PIB)-succinimide-type additives. These synthesized additives showed corrosion inhibition and lubricity in diesel fuel, 5% biodiesel containing diesel fuel, and 100% biodiesel with 20 ppm succinimide derivative (SID) (from PIB, maleic anhydride, and RME).

Fazal, Haseeb, and Masjuki (2011c) studied three different types of amine-based inhibitors such ethylenediamine (EDA), n-butylamine (nBA), tert-butylamine (TBA) on the corrosion of cast iron in palm biodiesel. The investigated amines showed different levels of inhibition on the corrosion of cast iron. Fuel properties owing to the addition of EDA significantly degraded, although it reduced the corrosion rate comparatively higher than others. TBA was found as the most effective corrosion inhibitor, which ensures applicable fuel properties.

Almeida et al. (2011) studied the behavior of the antioxidant tert-butylhydroquinone on the storage stability and corrosive character of biodiesel. The oxidation stability of biodiesel was considerably affected after 24 h of the static immersion test using copper coupons. Similar result was verified for the TBHQ-doped biodiesel after 24 h of immersion, which indicated that the antioxidant did not retard biodiesel degradation under the same conditions. Meanwhile, copper release was less intense in the TBHQ-doped biodiesel, which provided evidence that the antioxidant retarded the corrosion process acting as a corrosion inhibitor through the formation of a protective film layer (partial coverage).

Fernandes et al. (2013) investigated the storage stability and corrosive character of soybean biodiesel through static immersion corrosion tests with coupons of carbon steel and galvanized steel immersed in soybean biodiesel with and without TBHQ (500 mg/kg) for 12 weeks. The presence of TBHQ mitigated biodiesel deterioration even after 12 weeks of the immersion test with both types of steel. Galvanized steel and carbon steel have been shown to be compatible with biodiesel over a storage period of 56 days because biodiesel properties were not affected. The addition of TBHQ reduced the corrosion of galvanized steel because zinc was not detected in biodiesel even after 12 weeks of the experiment, which suggests the corrosion inhibitor effect of TBHQ.

Yooa et al. (2012) investigated the corrosion inhibition behavior of biodiesel-based 2-(2-alkyl-4,5-dihydro-1H-imidazol-1-yl) ethanol derivatives (A-DIEs). They concluded that the presence of double bonds and functional groups in the A-DIE chains and the sufficiently long chain length enhanced the attachments of A-DIEs on the metal surface, leading to high efficiency in the inhibition of the corrosion.