Retardation of oxidation and material degradation in biodiesel: a review

Abstract

In the automobile sector, biodiesel has received considerable attention as a promising diesel substitute because of its enhanced lubricity and reduced emissions. Biodiesel is composed of both saturated and unsaturated fatty acid methyl esters. The unsaturated components of biodiesel make it susceptible to corrosion for automotive components. The corrosiveness of biodiesel and its blend can be attributed to its oxygen moieties, auto-oxidation, high polarity, and hygroscopic nature. These properties may also cause tribological degradation. The present study reviews different aspects of corrosion and tribological degradation of automotive materials in biodiesel. This review will help in understanding the major characteristics and the possible degradation mechanisms of corrosion for both ferrous and nonferrous metals in biodiesel. This article extends an overview of tribological degradation of automotive materials. Remedial measures and related mechanism for both corrosion and tribological degradation have also been emphasized which made this study unique from other reviews.

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1. Introduction

The hike in petroleum diesel consumption in the automobile sector has led to a rapid depletion of fossil fuels.^1,2 This alarming growing demand for fuel has led to the consideration of alternative energy sources. Exhaust emissions after burning of gasoline and diesel also pose a significant threat to the environment.³ Stringent regulations such as EN-590 and ASTM D 975 have been legislated to reduce diesel emissions.^3,4 Hydro-treatment of diesel reduces emission, but its poor contents of sulfur and polar nitrogen compounds cannot provide adequate lubricity. The European Union (EU) has improved the lubricity of diesel by allowing the 5.75% blend of biodiesel into petro-diesel, which also provides minimum emissions. It can provide some technical advantages over petroleum diesel including higher cetane number and flash point, improved lubricity etc.⁵ Thus, biodiesel received widespread acceptance as an anti-wear additive and emission inhibitor for diesel engines.^6,7 The increasing demand for biodiesel established it as a potential alternative fuel.^8–10 However, biodiesel as a substitute must be technically feasible and should exhibit properties similar to those of diesel fuel.^11–14 Pure biodiesel (B100) significantly minimizes polycyclic aromatic hydrocarbon emissions by 19.4%.¹⁵ B100 inherently offers enhanced lubricity because of the high affinity of esters to metals. The polar group of esters forms a protective layer on the tribo-contact surface, which is advantageous in reducing engine wear and friction.^16–20 Conversely, the susceptibility of biodiesel to oxidation is a major drawback because of its unsaturated hydrocarbons, such as oleic acid, linoleic acid, and linolenic acid.^21–24 The presence of double-bonded carbon atoms in mono- (C_nH_2n−1-COOH), di- (C_nH_2n−3-COOH), tri- (C_nH_2n−5-COOH), and poly-unsaturated fatty acids is also responsible for the easy formation of a peroxide (ROO˙) or hydro-peroxide (ROOH) compound. This oxidative nature of biodiesel may lead to oxidative wear in automotive materials.²⁵ The hygroscopic nature of biodiesel leads to the hydrolysis of ester bonds and the formation of free fatty acid molecules.^26,27 These unstable fuel properties of biodiesel and its blends appear more aggravated when fuel comes in contact with automotive materials. At present, automobile companies do not offer a warranty for diesel with >5% blends of biodiesel.²⁸ According to Kousoulidou et al.,^29,30 the maximum blending ratio of saturated biodiesel can reach 30%, which reduces emissions without significantly affecting the vehicle parts. B20 (20% biodiesel and 80% diesel) significantly minimizes polycyclic aromatic hydrocarbons emissions by 13.1%.¹⁵ Low yields of blended biodiesel reduced the knock of residual oxygen in the combustion chamber, particularly for B20. However, B20 slightly influences the power and torque, and this effect is unnoticeable to the user.³¹ Moreover, B20 is the possible reason for the increased corrosion and oxidative wear, which challenges the compatibility of material–fuel interface.³² Several studies on tribology^33–36 and corrosion^37–44 of automotive materials exposed to various biodiesel and their blends have been conducted to quantify the compatibility. These studies are mostly focused on the low percentage of biodiesel blend.⁴⁵ The high percentage of biodiesel has elicited compatibility issues and other technical problems. Therefore, remedial measures are of considerable concern to enhance the performance and life span of automotive materials while using biodiesel. Various synthetic additives to prevent corrosion and tribological degradation have been investigated by different researchers.⁴⁶ For the sake of scope, this paper summarizes the recently published data on the corrosion and tribological degradation studies along with the corresponding solutions.

2. Biodiesel

The use of biodiesel is increasing worldwide. Fig. 1 confirms the rapid growth in biodiesel production compared with crude oil production; the figure clearly indicates the increasing demand of biodiesel. To address the rising demand of biodiesel, the production of feedstocks with high biodiesel yielding ratio should be increased. Among the feedstocks, palm is ranked first, followed by soybean and rapeseed.

Fig. 1 Annual global crude oil and biodiesel production.⁴⁷

Biodiesel is produced from either vegetable oils or animal fats, which comprise triglycerides consisting of three long-chain fatty acids.^48,49 Fatty acids themselves are carboxylic acids with long unbranched aliphatic chains (with 4 to 28 carbon atoms), composed of hydrogen atoms attached to the carbons (Fig. 2).

Fig. 2 Biodiesel (methyl esters) prepared by trans-esterification from vegetable oils (triglyceride) with alcohol at 60 °C to 70 °C in the presence of a catalyst (R_1,2,3: alkyl group).^7,50

Pure biodiesel derived from different feedstocks can significantly affect the unique physical properties. The structural formula of individual fatty acid methyl esters (FAMEs) can significantly influence the biodiesel properties.⁵¹ Properties such as total acid number, viscosity, density, induction period, flash point, and cetane number crucially affect the friction, material surface wear, biodiesel lubricity, performance, and emission.^52–55 Physical properties such as high viscosity, low volatility, and moisture absorption, natural gum (phosphatides) reactivity of unsaturated hydrocarbon chains⁴⁶ poses poor atomization, carbon deposition, ring sticking, injector coking, injector pump failure, lubricating oil dilution, and crank-case polymerization in diesel engine.^22,46,56,57 Considering the disadvantages of these physical properties, researchers proposed the following simple solution after long-term comparative endurance tests: partially substituted mineral diesel with biodiesel can overcome most operational and durability concerns.

Petroleum diesel normally contains 75% aliphatic hydrocarbon and 25% aromatic hydrocarbon with no oxygen. By contrast, biodiesel comprises 40% unsaturated components with the presence of 11% oxygen. Naturally, biodiesel is less stable and more corrosive than diesel. The compositional stability of biodiesel can be affected by its hygroscopic and auto-oxidative nature, microbial growth, interaction with metals, temperature, and so on.^23,58 It is seen in Fig. 3 that saturated fatty acids, with the general formula C_nH_2n+1COOH, consist of single carbon–carbon bonds; hence, saturated fatty acids do not readily accept any additional hydrogen atoms. By contrast, unsaturated fatty acids consist of more than one carbon–carbon double bonds.⁵¹ Double-bonded unsaturated fatty acids in biodiesel are reactive, making biodiesel chemically unstable.^59,60

Fig. 3 Diagram represents a triglyceride, a saturated fat, and an unsaturated fat.⁵¹ Reproduced from Ref. A. Gopinath, K. Sairam, R. Velraj and G. Kumaresan, Effects of the properties and the structural configurations of fatty acid methyl esters on the properties of biodiesel fuel: a review, Proc. Inst. Mech. Eng., Part D, 2015, 229, 357–390 with permission from SAGE PUBLICATIONS LTD, copyright [2016].

Although conventional petroleum diesel is projected to be replaced partially by biodiesel, certain chemical issues hinder its potential commercial applications;⁶¹ these issues will be described later. These limitations can be solved by either material modification or chemical alteration of biodiesel.

3. Nature of biodiesel

Although biodiesel exhibit properties very similar to those of petroleum diesel, the major concerns of using biodiesel in diesel engines persist. These concerns include corrosiveness, hygroscopic nature, and auto-oxidation.⁵⁸ When the oxidation rate increases, the total acid number increases and can cause corrosion in metal parts.²² Oxidation can indirectly increase wear and reduce the lubricity of engine parts.³⁴ During long-term storage, the compositions can be changed because of the presence of air, sunlight, temperature, or material of storage container. Storage instability can degrade fuel properties such as induction period, acid value, and viscosity. If the storage duration is extended, the acid value, peroxide value, density, and viscosity will increase.⁵⁸

3.1 Oxidation

Biodiesel is chemically degraded by oxidation, which occurs because of aerobic contact during storage. The degradation of biodiesel also occurs by hydrolysis and thermal decomposition. Auto-oxidation is responsible for the formation of hydro-peroxides (ROOH).⁶² The situation becomes more complicated in case of metal-assisted secondary oxidation reaction, which finally results in products such as organic acids, aldehydes, dimers, and polymers.⁶³ The oxidation of biodiesel also depends on the solubility of water in it. During storage, transportation, and application, water can be condensed and interpose a thin layer of water–biodiesel mixture between the biodiesel and the metal surface.⁶⁴ This water reconverts esters into fatty acids via hydrolysis, thereby increasing the degree of unsaturation and causing auto-oxidation.⁶⁵ The oxidation rate of biodiesel can be determined by the number of allylic and bis-allylic methylene moieties adjacent to C=C bonds of oleic acid, linoleic acid, and linolenic acid (Fig. 3).⁶⁶ Relative oxidation susceptibility among the common unsaturated esters are ordered as linolenates > linoleates > oleates.⁶⁷ The OX index determines a clear picture of relative oxidation rate.⁶⁸ This index depends on the coefficients of oleic (O), linoleic (L), and linolenic (Ln) acid methyl ester, which can be determined by the formula of oxidizability (OX = (0.02%O + %L + 2%Ln)) (Fig. 4).⁶⁹

Fig. 4 Common fatty acid methyl ester biodiesel molecules with increasing C=C bonds.²⁵ Reproduced from Ref. Z. Yaakob, B. N. Narayanan and S. Padikkaparambil, A review on the oxidation stability of biodiesel, Renewable Sustainable Energy Rev., 2014, 35, 136–153 with permission from Elsevier, copyright [2016].

Bis-allylic methylene moieties induce greater effects on oxidation than allylic methylene moieties. The presence of poly-unsaturated methyl esters in biodiesel is more vulnerable than monounsaturated methyl esters to oxidation. Oxidation is a three-part process of radical chain reaction, consisting of initiation, propagation, and termination, as shown in Fig. 5. This process initiates with the removal of hydrogen from polyunsaturated fatty acid (bis-allylic acid) to form carbon-based free radicals. These radicals react with dissolved O₂ and forms peroxide (ROO˙) and hydro-peroxide (ROOH) radicals; this reaction is known as the propagation reaction. Compared with hydro-peroxides, peroxide radicals demonstrate higher affinity with O₂; this affinity results in auto-oxidation and increases the percentage of hydro-peroxides. Consequently, this auto-oxidation results in the formation of decomposed compounds, such as acids, aldehydes, esters, ketones, peroxides, and alcohols.⁴⁶ These decomposed compounds are responsible for the high total acid number (TAN) and the formation of insoluble gums and sediments, which reduce the quality of fuel and engine performance.^70,71

Fig. 5 Oxidation mechanism of poly-unsaturated linoleic acid methyl ester.²⁵ Reproduced from Ref. Z. Yaakob, B. N. Narayanan and S. Padikkaparambil, A review on the oxidation stability of biodiesel, Renewable Sustainable Energy Rev., 2014, 35, 136–153 with permission from Elsevier, copyright [2016].

3.2 Hydrolysis

The degradation of biodiesel occurs through hydrolysis and thermal decomposition. The hydrolysis of biodiesel occurs with the presence of moisture, whereas thermal decomposition is caused by elevated temperature.^25,48,72 Hydrolysis is a reaction in which the molecular bonding of esters is broken by the reaction with water. According to ASTM D6751, the maximum allowable amount of water in biodiesel is 0.05%. The hygroscopic and polar nature of biodiesel makes the water molecules attracted to the esters; thus, the water content in biodiesel may become reactive with time. This attraction initiates the hydrolysis of esters, as shown in eqn (1).⁷³ The hydrolysis of esters alters the properties and composition of fuels. Fazal et al.¹¹ indicated that the water content in biodiesel increased significantly after the immersion test; they found that the tendency of biodiesel to absorb water is roughly 30 times higher than that of petroleum diesel. Water found in biodiesel may exist in either dissolved or dispersed form. Phase separation remains often imperfect, and water tends to stay in the final biodiesel product in a finely dispersed form.⁷⁴ The presence of water accelerates its corrosive interactions with the metal surface.

R₁–COOR₂ (ester) + H–OH (water) = R₂–OH (alcohol) + R₁–COOH (fatty acid) (1)

The existence of water in biodiesel also enhances the microbial growth, which in turn may accelerate fuel degradation.⁷⁵ Microbial growth rate is comparatively faster than hydrolysis.⁷⁶ The moisture absorption of biodiesel can reach up to 150 ppm,⁷⁷ which sufficiently decreases the heat of combustion.⁷⁸ This moisture also affects the engine respiratory system via microbe colonization and gel formation, which in turn may lead to fuel system plugging and injector coking. Biodiesel with high amounts of secondary products (mono- and di-glycerides) is more prone to moisture absorption,⁷⁹ whereas water-contaminated biodiesel exhibits enhanced corrosiveness.^43,80 During the distillation and purification of biodiesel, natural antioxidants are destroyed, making biodiesel susceptible to degradation.⁷⁵

3.3 Thermal decomposition

High temperature significantly affects the accelerated oxidation of biodiesel by yielding polymerized compounds and increasing the weight of biodiesel.^72,81 At elevated temperatures, the oxidation temperature decreases along with increasing amounts of bis-allylic hydrogens in biodiesel.⁸² At thermal instability, the poly-unsaturated linoleic acid methyl ester initially forms a cyclohexene ring via isomerization and then forms a dimer via thermal dimerization; this process is known as the Diels–Alder reaction, which is illustrated in Fig. 6.²⁵ In such manner, the elevated temperature and degree of unsaturation lead to thermal oxidation, which may produce aldehydes, ketones, and carboxylic acids, as well as limit the commercial use of biodiesel in automotive applications.⁸³

Fig. 6 Formation of dimer via Diels–Alder reaction.²⁵ Reproduced from Ref. Z. Yaakob, B. N. Narayanan and S. Padikkaparambil, A review on the oxidation stability of biodiesel, Renewable Sustainable Energy Rev., 2014, 35, 136–153 with permission from Elsevier, copyright [2016].

Elevated temperatures considerably influence the alteration of biodiesel properties, including oxidation, lubricity, corrosiveness, composition, induction period, viscosity, and density. According to Horel,⁸⁴ the formation of CO₂ in the presence of O₂ (i.e., respiration) increases along with temperature. Severe corrosion is attributed to the presence of a higher acid number owing to oxidation. Photo-oxidation occurs as a result of exposed duration with intense light. In addition, various impurities, such as chloride ions in water, facilitate the anodic reaction and then degrade themselves and the metal surface in contact.

3.4 Co-metabolic biodegradation

Co-metabolic biodegradation⁸⁵ refers to the alteration of biodiesel and petroleum diesel by microorganisms, such as bacteria (Pseudomonas fluorescens, Bacillus sp., Bacillus subtilis, Alcaligenes sp., P. aeruginosa, Acinetobacter lwoffi, Flavobacterium sp., Micrococcus roseus, and Corynebacterium sp.), fungi (Amorphoteca, Neosartorya, Talaromyces, and Graphium), and yeast (Candida, Yarrowia, and Pichia).^86,87 Biodiesel is more active than petroleum diesel because of its fatty acid with a hydrocarbon chain and two oxygen atoms. Moreover, fatty acids present great affinity to the abovementioned microorganisms.⁸⁸ The acid is oxidized at the position of β-carbon attached to the functional group and degraded to acetic acid. Afterward, the newly formed fatty acid with two fewer carbons breaks down to CO₂ and H₂O in the presence of microorganisms by a series of reactions.⁸⁹ Biodiesel is more prone to degradation than petroleum diesel because the latter contains aliphatic cyclic hydrocarbon, benzene, and other complex hydrocarbons with strong chemical stability and requiring more energy for microorganisms to penetrate the structure.⁸⁹ By contrast, biodiesel contains oxygenated moieties that can be easily attacked by microorganisms and promotes further degradation.⁹⁰

These factors induce significant effects on the chemical stability of biodiesel and the long-term durability of the automotive component, thereby limiting the commercialization of biodiesel as a transportation fuel. The chemical stability of biodiesel can be improved by applying some preventive measures or alterations, such as modifying or curing with additives.³³

3.5 Remedies

The auto-oxidative stability of biodiesel can be improved by reducing the degree of poly-unsaturation and blending the biodiesel with diesel fuel.²⁵ Auto-oxidation forms hydro-peroxides that decompose and cause secondary oxidation known as polymers.^91,92 Kamalesh et al.⁴⁶ reported that the oxidation stability of biodiesel was inversely proportional to the percentage of linoleic and linolenic acids. Such limitations can be addressed by reducing the affinity of oxygenated moieties to the metal surface.

The deoxygenation of biodiesel is another concurrent remedial measure that poses a high atomic ratio of hydrocarbon with a remarkable reduction in O/C ratio and ensures a higher energy density. Biodiesel without deoxygenation contains oxygenated compounds, such as carboxylic acids (C₇–C₁₀) and esters that threaten the engine components.⁹³ Among the various deoxygenation methods, the application of semi-batch intermediate liquid fraction by using CaO and treated hydrotalcite (MG70) as catalysts at 400 °C achieved 85% to 88% of hydrocarbons.⁹⁴ Bio-oil was deoxygenated by promoting the reactions of decarboxylation and decarbonylation to remove oxygen atoms via the formation of CO₂ and CO byproducts. The successfully deoxygenation-converted biofuel mainly comprised hydrocarbons with a high atomic ratio H/C and a low atomic ratio O/C. With less oxygen content, biodiesel is considered a suitable candidate as a transportation fuel. Moreover, a remarkable reduction in oxygen ensures the high chemical stability of biodiesel, thereby inhibiting its corrosiveness and improves its tribological compatibility against automotive materials. Fuel modification with additives is another popular method for minimizing the auto-oxidation of biodiesels.⁹¹ Among the most commonly used corrosion inhibitors are imidazolines, primary amines, diamines, amino-amines, oxyalkylated amines, naphthaneic acid, phosphate esters, and dodecyl benzene sulfonic.⁴² The use of additives or inhibitors, such as tert-butyl hydroxyl quinine (TBHQ), butylated hydroxytoluene (BHT), butylated hydroxyamisole (BHA), propyl gallate (PrG), pyrogallol (PY), tert-butylamine (TBA), ethylenediamine (EDA), n-butylamine (nBA), and benzotriazole (BTA), achieves chemical stability by procrastinating auto-oxidation. These feasible remedial measures against the auto-oxidation of biodiesel motivate researchers to rethink biodiesel as an alternative fuel in automotive applications.

4. Materials used in automotive engine and fuel system

The automobile application of biodiesel was first tested by Rudolf Diesel in the 18th century.⁹⁵ Numerous endeavors have followed to make biodiesel as a suitable alternative fuel because of its favorable tribological and environmental benefits.¹⁶ The engine and fuel systems in automobiles comprise various static and dynamic components, such as piston, piston ring, cylinder liner, exhaust valve, injector needle, filter plunger, fuel tank, feed pump, fuel filter, and fuel line (Fig. 7). Table 1 describes these components, the materials, and the types of material degradation. All these components are made of either ferrous or nonferrous materials, and their alloys are exposed to biodiesel under different operating conditions.^28,96 When the chemical properties are changed, the biodiesel becomes susceptible to those metals. Therefore, a suitable blend of biodiesel and metal must be selected for both the static and dynamic parts of automobiles.

Fig. 7 Automotive components in engine and fuel systems: (a) fuel tank, (b) fuel pump, (c) fuel line, (d) fuel filter, (e) fuel injector, (f) piston, (g) piston ring, (h) cylinder liner, (i) valve, and (j) fuel filter plunger. Adopted from: http://www.certifiedmastertech.com/wordpress/2011/08/08/automotive-fuel-tank-problems/, http://www.ebay.com/itm/Metal-Fuel-Line-09-10-VW-Jetta-Golf-MK5-TDI-CBEA-Diesel-03L-201-360-F-/351165118882, http://www.agcoauto.com/content/news/p2_articleid/219, http://dieseliq.com/common-reasons-diesel-fuel-injectors-fail, http://www.hotrod.com/how-to/engine/hrdp-9808-piston-ring-science/, http://www.hotrod.com/how-to/engine/hrdp-9808-piston-ring-science/, http://rajkot.all.biz/cylinder-liner-sleeves-g264983#.V06yPmh97b0, http://www.dealernews.com/dealernews/article/honda%E2%80%99s-crf450r-gets-engine-makeover-pro-circuit, http://boschnozzle.blog.163.com/blog/static/18481836020113222451735/.

Table 1 Typical diesel engine and fuel system components, materials, and type of material degradation

Ref.	Component	Types of alloy used	Major type of degradation
37, 46 and 97	Fuel tank	Iron, nickel, manganese, cobalt, copper, leaded bronze	Corrosion
46 and 98	Fuel pump	Aluminum alloy, iron-based alloy, copper-based alloy	Corrosion
28, 83 and 99	Fuel lines	Steel, copper	Corrosion
99 and 100	Fuel filter housing	Aluminum	Corrosion
99 and 101–103	Fuel injector plunger and barrel	18CrNi8 steel: (Cr: 16% to 18%, Ni: 10% to 14%, C: 0.03%, Mn: 2%, Si: 0.75%, P: 0.045%, S: 0.03%, N: 0.10%, and Mo: 2% to 3%)	Tribology
104	Piston	Aluminum alloy: (Si: 11% to 13% Cu: 3% to 5%, Mg: 0.5% to 1.2%, Ni: 1% to 3%, Fe: 0.3%, Mn: 0.3%, Ti: 0.2%, Zn: 0.3%, Zr: 0.2%, V: 0.18%, Cr: 0.05%, and Al: remaining)	Tribology
8	Piston ring	CrMoV18 steel: (C: 0.85% to 0.95%, Si: 1%, Mn: 1%, P: 0.04%, S: 0.015%, Cr: 17% to 19%, Mo: 0.9% to 1.30%, V: 0.07% to 0.12%, and Fe: remaining)	Tribology
8 and 105	Cylinder liner	Gray cast iron: (C: 3.40% to 3.70%, Si: 2.30% to 2.80%, Mn: 0.50% to 0.80%, S: 0.15%, P: 0.25%, and Fe: remaining)	Tribology and corrosion
106	Valve	Austenitic stainless steel: (C: 0.03% to 0.10%, Si: 0.50%, Mn: 0.50%, P: 0.015%, S: 0.015%, Cr: 14.0% to 17.0%, Mo: 0.50%, and Fe: remaining)	Tribology

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Although biodiesel has attracted a very high acceptance in automobile applications because of its environmental and tribological benefits, the corrosiveness of this material remains controversial. In,¹⁰⁷ the automotive components made of ferrous and nonferrous metals were damaged because of the accelerated oxidation or moisture absorption of biodiesel, as shown in Fig. 8. The hygroscopic nature of biodiesel leads to microorganism contamination and enhances the corrosion process.^108,109 Therefore, the corrosive nature of biodiesel poses a serious concern for the long-term durability of automotive components and requires a rigorous investigation. The following sections discuss the degradation of automotive materials exposed to a biodiesel environment.

Fig. 8 Corroded automotive components in engine and fuel systems: (a) engine block, (b) piston head, (c) cylinder liner, (d) fuel tank, (e) fuel line, and (f) fuel injector. Adopted from: http://www.walshsurveyor.com/yachts/articles/engines/index.htm, http://sharpmagazine.com/2016/04/28/shell-sharp-heres-how-to-protect-your-dream-car-inside-and-out/, http://www.goldeagle.com/STA-BIL360-Performance-Campaign, http://stevedmarineconsulting.com/author/katie/page/2/, http://landroverforums.com/forum/discovery-ii-18/water-corrosion-pistons-sleeves-63360/, http://www.redpowermagazine.com/forums/topic/66527-what-pistons-fit-a-wd-9-take-a-look-at-mine/.

5. Corrosion behavior of automotive materials

Automotive materials mostly comprise ferrous and non-ferrous metals, such as iron, iron-based alloys, steel, copper, copper-based alloys, aluminum, and aluminum-based alloys, which are more or less corrosive in biodiesel. The corrosion compatibility of these materials exposed to biodiesel presents a key research issue. According to Sakae et al.,¹¹⁰ the corrosion of automotive materials can be classified into perforation corrosion, cosmetic corrosion, and edge corrosion, of which perforation corrosion is the most common problem.¹¹⁰ Agarwal et al.¹¹¹ introduced general corrosion, dry corrosion, and wet corrosion when the automotive materials were exposed to ethanol. Ionic impurities (chloride ions and acetic acid) and polar molecules are responsible for general and dry corrosion, respectively, whereas oxidized compounds and azeotropic water are responsible for wet corrosion. Prieto et al.¹¹² introduced galvanic corrosion, which was attributed to the very low conductivity of biodiesel as an electrolyte (4.06 × 10⁻⁹ milliSiemens per cm) and the polarized covalent bond compound in between hydrogen–oxygen and carbon–oxygen bonds. Biodiesel is slightly more polar than diesel fuel because of the electron negativity of oxygen with enhanced corrosion.^113,114 The O₂, H₂O, CO₂, and RCOO⁻ radicals in biodiesel act as leading factors in enhancing the corrosiveness of biodiesel. During storage, the generation of aerobic and anaerobic microorganisms promotes bacterial activities and increases acidity (pH = 3.5).¹¹⁵ Chemical- or microbiological-influenced corrosion may occur in automotive materials, as discussed in the following sections.²⁷

5.1 Corrosion of ferrous metal

Corrosion generally occurs on the sample surface in a uniform or localized manner. For corrosion, Kaul et al. performed a 300-day static immersion test of piston and cylinder liner metal in biodiesel for various feedstocks.¹¹⁶ They found that Salvadora and Jatropha feedstocks were more corrosive than the biodiesel from other feedstocks (Table 2). The biodiesels from Karanja and Mohua feedstocks showed similar corrosion behavior as diesel. Table 2 shows that stainless steel demonstrates the best corrosion resistance among all ferrous metals, which may be attributed to its low carbon content.¹¹⁷ However, the effect of elevated temperature on the corrosion of metal and the degradation of biodiesel remains unknown.

Table 2 Corrosion of ferrous metals under different operating conditions^a

Ref.	Ferrous metal	Biodiesel	Duration h	Temperature °C	Corrosion rate mpy
a Note: B100: palm biodiesel, PB50: 50% biodiesel in diesel, FB80: 80% fat-based bio-fuel blend, and RT: room temperature.
116	Cast iron	B0	7200	RT	0.006
118		B100	1200	RT	0.077
38		B100	2880	RT	0.112
116		Jatropha	7200	40	0.078
		Kanarja, Mahua	7200	40	0.006
		Salvadora	7200	40	0.132
119		FB-80	7200	38	0.192
23	Mild steel	B0	1200	27	0.046
		B100			0.052
23		B0	1200	80	0.05
		B100			0.059
120		B0	720	27	0.0015
		B100			0.004
120		B0	1440	50	0.0014
		B100			0.055
120		B0	2880	80	0.027
		B100			0.08
39	Carbon steel	Rapeseed oil and methanol	1440	43	0.018
44		Sunflower	3000	RT	0.170
		Sunflower	3000	60	0.336
39	Stainless steel	Rapeseed oil and methanol	1440	43	0.001
43		B100	1200	80	0.015

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Tsuchiya et al.¹²¹ studied the corrosion of steel immersed in diesel and 5% FAME blended diesel at 80 °C for 500 h. The pitting corrosion even for 2% FAME biodiesel confirmed that the extremely corrosive free fatty acids, such as formic acid, acetic acid, propionic acid, and caproic acid after oxidation, are responsible for such pitting corrosion. Fazal et al.²³ later investigated the corrosion behavior of cast iron and mild steel at different elevated temperatures in 50% palm biodiesel for 1200 h. Jin et al.¹²⁰ recently investigated ASTM 1045 mild steel at different temperatures and exposure times in palm biodiesel and diesel. Fig. 9 shows the corrosion behavior model of mild steel in diesel and palm biodiesel with the functions of time and temperature. In both kinds of fuels, the corrosion rate increases along with temperature and duration.

Fig. 9 Corrosion behavior of ASTM 1045 mild steel exposed to (a) B100 and (b) B0 with different temperatures and exposure times.¹²⁰

The corrosion of metal and the degradation of fuel both increase along with temperature. An increasing temperature is also accompanied by an increasing presence of oxygen in the form of compounds, such as iron carbide or iron oxides. The subsequent dissolution of these compounds degrades the metal surfaces. Cast iron is highly vulnerable to corrosion mostly in palm biodiesel, followed by mild steel and carbon steel. The higher temperature and relatively higher duration in the metal–fuel interface may also contribute to the increasing corrosion.⁴³ Guthrie et al.¹²² clarified the corrosion mechanism of these ferrous metals, as discussed in the following section.

5.2 Corrosion mechanism of ferrous metal

According to Guthrie et al.,¹²² oxidation generates iron ions that form hydrated oxide with hydroxide ions in the presence of moisture and oxygen. The further reaction of iron hydroxide initiates the corrosion of iron or iron alloy. Cast iron forms a small concentration of Fe₂O₃ and Fe(OH)₂ in diesel, whereas relatively higher amounts of FeCO₃, Fe₂O₃, Fe(OH)₂, and Fe₂(OH)₂CO₃ are formed in biodiesel. Fazal et al.^23,38 investigated the cast iron and mild steel surface degradation when exposed to biodiesel (Fig. 9), and their XRD report ensured the formation of iron compounds (e.g., FeCO₃, Fe₂O₃, and Fe(OH)₂) at higher concentrations. Recently, D. Jin et al.¹²⁰ examined the corrosion behavior of ASTM 1045 mild steel in palm biodiesel and reported that on the basis of the relative area ratio, the quantity of Fe⁺³ in the corrosion product was higher than that of Fe⁺². Such finding is attributed to the presence of higher oxygen in biodiesel, thereby improving the interaction of biodiesel with metal to form metal compounds. For ferrous metals, the compound results from the following chemical reactions (2) (Fig. 10):

Fig. 10 SEM micrographs of cast iron (CI) and mild steel (MS) surface before and after exposure in biodiesel (B100).^23,38 Reproduced from Ref. M. A. Fazal, A. S. M. A. Haseeb and H. H. Masjuki, Effect of temperature on the corrosion behavior of mild steel upon exposure to palm biodiesel, Energy, 2011, 36, 3328–3334 and M. A. Fazal, A. S. M. A. Haseeb and H. H. Masjuki, Degradation of automotive materials in palm biodiesel, Energy, 2012, 40, 76–83 with permission from Elsevier, copyright [2016].

5.3 Corrosion of non-ferrous metal

After a 10-month immersion test at 38 °C, Geller et al.^119,123 observed that copper and brass demonstrated the highest and second highest weight loss because of pitting corrosion.¹²⁴ Fazal et al. performed a similar study on palm biodiesel^23,38 and reported that copper was not corroded in biodiesel but also acted as a catalyst to degrade fuel properties. Table 3 shows that copper is very much aggressive in rapeseed methyl ester (0.915 mpy corrosion rate) compared with palm and sunflower. Palm and sunflower show almost similar corrosion behavior for copper and aluminum. Chew et al.⁴⁰ conducted a similar experiment on magnesium, which was 35% lighter than aluminum yet exhibited almost similar strength. Magnesium yielded a significantly higher corrosion rate than aluminum and copper. Magnesium, copper, and copper-based alloys are the most vulnerable to corrosion among all non-ferrous metals, whereas leaded bronze shows the best corrosion resistance capability among all copper-based alloys. Aluminum and its alloys demonstrate higher corrosion resistance than all non-ferrous materials in all kinds of biodiesel tests and can be used as benchmarks for the corrosion of other materials.¹¹⁶ The compatibility of different ferrous and non-ferrous materials has been reported, and the following section will discuss the corrosion mechanism for non-ferrous materials as clarified by Amin et al.¹²⁵ and Fazal et al.³⁸

Table 3 Corrosion of non-ferrous metals under different operating conditions^a

Non-ferrous metal	Ref.	Biodiesel	Duration sec	Temperature °C	Corrosion rate mpy
a Note: B100: palm biodiesel, PB50: 50% biodiesel in diesel, FB80: 80% fat-based biofuel blend, RME 100: 100% rapeseed methyl ester.
Copper	39	Rapeseed oil and methanol	1440	43	0.023
	37	B100	2640	RT	0.042
	38	B100	2880	RT	0.392
	43	B100	1200	80	0.586
	37	B100 (oxidized)	840	60	0.053
	123	FB80	7200	38	0.557
	126	RME 100	600	80	0.915
	44	Sunflower	3000	RT	0.323
				60	0.640
Leaded bronze	37	B100	2640	RT	0.018
		B100 (oxidized)	840	60	0.023
Aluminum	40	B100	720	RT	0.123
			1440		0.052
	38	B100	2880	RT	0.173
	39	Rapeseed oil and methanol	1440	43	0.003
	43	B100	600 to 1200	80	0.202
	126	RME 100	600	80	0.35
	44	Sunflower	3000	RT	0.162
				60	0.316
Aluminum alloy	116	Jatropha	7200	40	0.011
		Kanarja, Salvadora	7200	40	0.005
		Mahua	7200	40	0.012
Magnesium	40	B100	720	RT	3.091
			1440		2.656
Brass	38	B100	2880	RT	0.209

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5.4 Corrosion mechanism of non-ferrous metal

The surface of the material exposed to biodiesel was characterized via SEM to illustrate clearly the corrosion attack (Fig. 11). Fazal et al.^38,41 investigated the corrosion mechanisms for copper, brass, and aluminum. Their rigorous study on the corrosion mechanism of Cu clearly illustrated the corrosion phenomenon, whereas their XRD pattern on the copper surface indicated the presence of comparatively higher concentrations of CuCO₃ along with other copper compounds, such as CuO, Cu₂O, Cu(OH)₂, and CuCO₃·Cu(OH)₂. These results suggest that for copper and copper-based alloys, copper carbonate (CuCO₃) and cuprite oxide (Cu₂O) are the dominant compounds formed in biodiesel and diesel, respectively. According to eqn (3), the O₂ and CO₂ dissolved from the atmosphere¹²⁷ or the RCOO˙ radical generated from esters play important roles in the formation of copper compounds. Fazal et al.⁴¹ concluded that the ion concentration in biodiesel and the Cu acted as catalysts for enhanced corrosion. Amin et al.¹²⁵ investigated the degradation of aluminum exposed to biodiesel and found that a trihydroxide film was formed when the aluminum was exposed to OH⁻ ion. They added that an increase in pH would accelerate the corrosion by damaging the film.

Fig. 11 SEM images of Cu, BS, and Al surfaces before and after exposure to biodiesel (B100).³⁸ Reproduced from Ref. M. A. Fazal, A. S. M. A. Haseeb and H. H. Masjuki, Degradation of automotive materials in palm biodiesel, Energy, 2012, 40, 76–83 with permission from Elsevier, copyright [2016].

5.5 Remedial measures for corrosion of automotive materials

Most studies show that metals are more corrosive in biodiesel than in diesel. Material susceptibility always follows an order of corrosion rate, where magnesium > copper > brass > cast iron > mild carbon steel > aluminum > stainless steel.³⁹ Hu et al. argued that copper, cast iron, and mild carbon steel were significantly more susceptible than aluminum and stainless steel because of their higher presence of carbon. Although stainless steel is the best corrosion-resistant automotive material in biodiesel, this material remains corrosive in biodiesel and is not economically viable to use for automotive components.

Some characteristics of biodiesel, such as oxygen-containing functional groups (moieties), free fatty acids, unsaturation degree, and hygroscopic nature, challenge the compatibility of the material–fuel interface. Therefore, the retardation of engine material part degradation and biodiesel corrosiveness is a main topic of concern that can be mitigated by corrosion inhibitors.¹²⁸ The inhibitor is not used to prevent corrosion but to extend the time of corrosion.¹¹⁴ According to Yıldırım et al.,¹²⁹ the effectiveness of corrosion inhibitors depends on their adsorption capabilities on reactive sample surfaces. As shown in Fig. 12a–c, the adsorption on the metal surface inhibitor successively forms a protective layer over the metal surface and prevents the direct contact between the reactive metal and corrosive biodiesel.

Fig. 12 Possible orientation of the inhibitor on the metal stripe surface exposed to biodiesel.

Yıldırım et al. also added that among all corrosion inhibitors (i.e., bearing nitrogen (N), oxygen (O), or sulfur (S) heteroatoms), amine-based inhibitors showed the best corrosion inhibition because the adsorption of molecules on the metal surface via heterocyclic moiety increased the interaction of N atoms (protonated amine) much higher than that of other heteroatoms that prevented the direct contact between the active metal and fuel. Amine-based corrosion inhibitors, such as TBA, EDA, nBA, and BTA, demonstrate excellent corrosion prevention performance through adsorption on the metal surface. Such adsorption may be classified into two major types, namely, (1) electrostatic attraction between the charged molecules and charged metal (spontaneous physical adsorption) and (2) interaction of unshared electron pairs in the molecule, the π-electrons of the nitrogen atom, or the metal (chemisorption).¹³⁰ Fazal et al.¹¹⁸ investigated the effects of EDA, TBA, and nBA on the corrosion of copper, brass, aluminum, and cast iron and found that 100 ppm TBA-doped biodiesel was an effective corrosion inhibitor with 49.41% inhibition efficiency. The existence of a new phase (Fe(NO₃)₃·9H₂O) on the metal surface that may be formed via TBA contributes to corrosion retardation. Nevertheless, further studies with different additive concentrations and elevated temperatures must be conducted to identify the inhibition action and adsorption type of additives on different automotive materials. Deyab^131,132 investigated the adsorption of ascorbyl palmitate (AP) and butylated hydroxytoluene (BHT) on exposed carbon steel, ethanol-blended gasoline, and blended biodiesel by using the weight-loss method and electrochemical impedance spectroscopy (EIS). They confirmed that the concentration inhibition efficiency increases at a specific temperature, whereas the temperature inhibition efficiency decreases at a specific concentration. M. A. Deyab^131,132 also justified the adsorption isotherm model of inhibitors with the help of thermodynamic functions, such as activation energy (E_a), free Gibbs energy (ΔG_addi), and standard enthalpy (ΔH_addi). The following section discusses the inhibition mechanism in detail (Table 4).

Table 4 Corrosion inhibition in diesel and biodiesel

Corrosion inhibitors	Structural formula	Medium	Concentration	Material	Corrosion rate	Inhibition efficiency	Ref.
Tert-butylamine (TBA)		B100	100 ppm	Cast iron	0.039 mpy	49.41%	118
Ethylenediamine (EDA)		B100	100 ppm	Cast iron	0.053 mpy	31.38%	118
n-Butylamine (nBA)		B100	100 ppm	Cast iron	0.068 mpy	12.06%	118
Ascorbyl palmitate (AP)		Ethanol-blended gasoline	120 mg l^−1	Carbon steel	1.6 × 10^−5 mg cm^−2 h^−1	98.8%	131
Butylated hydroxytoluene (BHT)		20% wost cooking-oil-blended diesel	150 mg l^−1	Carbon steel	1.38 × 10^−5 mg cm^−2 h^−1	94.4%	132

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5.6 Corrosion inhibition mechanism

The inhibition mechanism of additives through adsorption can be investigated by the weight loss of degraded materials. The measured weight loss can be converted into corrosion rate and inhibition efficiency as follows:^41,131andwhere corrosion rate (C_R) is expressed in micrometer per year (μm year⁻¹); W, D, t, and A denote weight loss (kg), density of material (kg m⁻³), immersion time (h), and exposed area (m²), respectively. In eqn (3), C_R and C_Ri represent the corrosion rates in biodiesel and additive-doped biodiesel, correspondingly.

5.6.1. Adsorption isotherms for the inhibition system. The performance of the additive was investigated using the recently proposed adsorption isotherms.^131–135 Previous research provides basic information on the interaction between the additive and metal surface. The best-fitted experimental data for the isotherm model are determined and can be used to develop a suitable model of adsorption. The Langmuir model is the easiest and widely used isotherm.¹³⁶ Langmuir hypothesized that a surface contains a certain number of sites where species can adhere through physisorption or chemisorption. Surface coverage (θ) is needed to determine the characteristic curve of the adsorption isotherm. For different concentrations of the additive, θ can be calculated using the inhibition efficiency equation (θ = η/100). The calculated values help in plotting a graph and finding a suitable adsorption isotherm. The Langmuir mathematical relationship is used to fit the θ values¹³⁷ as follows:

C_addi/θ = 1/K_eqi + C_addi, (6)

where C_addi is the concentration of the additive, and K_eqi is the equilibrium constant of the adsorption and denotes the strength between the adsorbate (biodiesel) and adsorbent (material surface). The reciprocal of the intercept of the isotherm line represents the equilibrium constant of the adsorption. High values of K_eqi indicate the stronger adsorption of the inhibitor.¹³⁴ The K_eqi value is employed to determine the separation factor (K_L) and energy of the adsorption as follows:¹³⁸

K_L = [1 + K_eqiC_addi]⁻¹ (7)

The above equation identifies the type of isotherm under the following conditions: (i) favorable adsorption if 0 < K_L < 1 and (ii) unfavorable adsorption if K_L > 1.¹³⁸ The standard free energy of adsorption (ΔG_addi) must be determined to identify whether the adsorption process is spontaneous. The ΔG_addi of the inhibitor on the material surface can be calculated as follows:¹³⁹

ΔG_addi = −RT ln(55.5 × 10⁶K_eqi) (8)

where ΔG_addi, R, T, and K_eqi denote the adsorption energy of the inhibitor (kJ mol⁻¹), the universal gas constant (8.314472 J mol⁻¹ K⁻¹), the operating temperature (K), and the equilibrium constant of the adsorption process, respectively. Depending on the value of ΔG_addi, the adsorption process can be classified as (i) chemical adsorption (if ΔG_addi < −40  kJ mol⁻¹), (ii) physicochemical adsorption (if −40 ≤ ΔG_addi ≤ −20  kJ mol⁻¹), or (iii) physical adsorption (if ΔG_addi > −20  kJ mol⁻¹).¹³⁴

5.6.2. Thermodynamic functions of the corrosion inhibition mechanism. The effect of elevated temperature on corrosion inhibition can be identified by several thermodynamic functions, such as activation energy (E_a) and standard enthalpy (ΔH_addi), for adsorption processes. These functions help define the adsorption nature of different concentrations of additives on the metal surface that is exposed to biodiesel at elevated temperatures as well as the nature of adsorption reaction. The following equation computes the activation energy (E_a) of the corrosion process in different additive concentrations:¹⁴⁰where C_R@RT and C_R@ET denote the inhibited corrosion rates at room temperature (T_RT) and elevated temperature (T_ET) (K), respectively. The endothermic or exothermic process in adsorption can be distinguished by considering another thermodynamic parameter, namely, the standard enthalpy of adsorption (ΔH_addi), which can be calculated by the linear regression between ΔG_addi/T and 1/T of the following Gibbs–Helmholtz equation:¹⁴¹

In the exothermic process, the absolute value of ΔH_addi is lower than 40 kJ mol⁻¹. However, in the endothermic process, the absolute value of ΔH_addi approaches 100 kJ mol⁻¹ or above.¹⁴² This process for biodiesel requires further study.

6. Tribological degradation

In automobiles, 48% of the total energy that is developed by an engine is lost because of friction.¹⁴³ The United States Department of Energy reported that reducing the friction and wear in the tribological components of engine can save US$ 120 billion per year.¹⁴⁴ Therefore, the importance of reducing tribological degradation has been increasingly acknowledged. Several studies have also been conducted to understand the lubricity of biodiesel for automotive materials.^33–36

6.1 Wear and friction

Friction and wear play important roles in defining the durability of mating components against tribological degradation where the sliding contact takes place. Diesel engine and its fuel systems comprise numerous dynamic tribo-pairs, such as piston and piston ring to cylinder liner, valve to valve guide, injector needle to injector inner chamber, and filter plunger to plunger guide. Most of these tribo-pairs are directly exposed to fuel, which solely provides the lubricity of these mating components.⁹⁶ Biodiesel shows excellent lubricity by reducing friction and wear through its unsaturated hydrocarbon content.^23,46,145 Several experiments using reciprocating motion, rotating motion with four-ball, pin-on-disk, and ball-on-plate arrangements were performed to evaluate the wear behavior in the metal and biodiesel interface. For example, Xu et al.¹⁴⁶ investigated the effect of different biodiesels on automotive materials in relative motion. They found a better lubricity yet a rough surface texture in surface morphology because of the oxidation of biodiesel. The main wear mechanism was identified as oxidative wear. In,¹²¹ a black corrosive organic product was observed adjacent to the worn surface because of the oxidation of biodiesel, thereby creating corrosive acid at a higher temperature.^147,148 However, this oxidation creates an inorganic oxide layer or lubrication film and provides less abrasive wear,¹⁴⁹ which can be attributed to the higher oxidation rate of fresh biodiesel, which in turn forms relatively higher molecular weight compounds and increases viscosity or lubricity.¹⁵⁰ In their analysis of the tribological performance of biodiesel (Jatropha), Kumar et al.¹⁴⁸ found high coefficients of friction and wear at a high temperature and load because of the accelerated oxidation of biodiesel at high temperature as shown in Fig. 13. These findings indicated that temperature and load individually and collectively affected the tribological performance of biodiesel. Blends with lower concentrations of biodiesel showed a higher frictional coefficient because of the non-uniformity of heteroatom.³⁴ Abrasive wear, which is observed for pure diesel fuel and lower concentrations of biodiesel, can be identified by the particle sizes of extruded debris of less than 20 μm.¹⁵¹ These third body abrasions may exist if the debris size ranges from 1 μm to 4 μm with less than 20 μm clearance. In actual practice, the debris rolls within the clearance and grooves, thereby resulting in less friction and no further abrasive wear.

Fig. 13 Tribological behavior of steel exposed to (a) Jatropha and (b) B0 with different temperatures and load.¹⁴⁸

The lubricity of diesel or biodiesel protects a tribo-pair from friction and wear. The reduction of friction and wear reduces fuel consumption with increased torque, improves durability, increases reliability with a low maintenance cost, and extends the life service interval.¹⁵² Given the poor lubricity of conventional diesel, the pure biodiesel or its blends may present promising alternatives that can address the challenges in lubricity. To improve lubricity, ASTM-D7467 has mandated biodiesel blends, such as B10 (10% biodiesel and 90% diesel) and B20 (20% biodiesel and 80% diesel).²⁹ The standard mixture of biodiesel (B20) can reduce particulate emissions by 15% to 20%.¹⁵³ The presence of sulfur in diesel provides a favorable lubricity, but the high sulfur content of diesel negatively affects the thermal and thermo-oxidative stability as well as the SO₂/SO₃ emission.¹⁵⁴ Conversely, biodiesel offers better lubricity than diesel fuel with extremely low sulfur content.¹⁵⁵ Such better lubricity can be attributed to the elements in biodiesel (i.e., unsaturated components, free fatty acids, oxygenated molecules, and long chain molecules), the polarity, moderate viscosity, and oxidative nature of ester molecules, and the contaminants (i.e., glyceride and triglycerides).^33,34 The lubricity of biodiesel mainly depends on methyl esters and mono-glycerides, whereas free fatty acids, diglycerides, and triglycerides almost have no effects on lubricity. Drown et al.¹⁵⁶ argued that long chain fatty acids and the residual glycerin rate of oleic acid in biodiesel can behave favorably with better lubricity. Sharma et al.¹⁵⁷ explained that each molecule of the aliphatic fatty acid (C_nH_2n+1COOH) contained a polar long covalently bonded hydrocarbon chain. The long chain ends have high affinity to partially positive-charged metal surfaces and align themselves normally to the surface to create a monolayer film for the sliding surface. This sliding layer provides a favorable lubricity because of the absence of direct metal-to-metal contact and the reduced thermal energy.⁵⁷ Minami and Mitsumune¹⁵⁸ reported that the anti-wear properties of biodiesel depended on the amount of peroxide, which can decompose the anti-wear additives and introduce higher wear. The presence of oleic acid rate in biodiesel also has a vital role on the lubricity property.¹⁵⁹

6.2 Metal surface degradation

The fuel properties of biodiesel are affected during auto-oxidation. Fig. 14 shows that the changes in fuel properties can lead to degradation or damage the automotive components, thereby affecting the lubricity of biodiesel. Some products, such as hydro-peroxides and organic acids, are formed during oxidation, create an insoluble residue, and decrease lubricity. Table 5 presents the results of the biodiesel lubricity tests on HFRR (ASTM D6079) and four ball wear machine (ASTM D4172) under different biodiesel blends. These tests reveal that temperature friction and wear both increase along with decreasing lubricity,^26,34,148 which may be attributed to the reduced thickness of the film created at the contact surface as a result of the increased temperature. This phenomenon may also be explained as the fragmentation of film through polymerization, cross linking, and decomposition at elevated temperatures.^57,160,161 Generally, the tables indicate that all blends of different feedstocks behave better than diesel fuel with better lubricity, less friction, and less wear without engine modifications. From the tribological perspective, coconut oil achieves the best results in the rigorous test followed by sunflower and palm biodiesel because a higher percentage of saturated fatty acid can ensure better lubricity by preventing the reconversion of unsaturated fatty acid. These results may also be attributed to the fact that the carbon chains of fatty acids in coconut oil are longer than those of fatty acids in sunflower and palm oil.

Fig. 14 Tribological degradation of automotive components: (a) cylinder liner, (b) piston, (c) cam, and (d) crank shaft. Adopted from: http://dev.pceo.com/ViscosityModifierPart2.html, http://www.zmax.com/about_zmax/testing/, http://garage.grumpysperformance.com/index.php?threads/cast-roller-cams-and-high-spring-pressures-don-t-work-well.1489/, http://vintagetractorengineer.com/2012/12/mf35-perkins-3a-152-strip-down/.

Table 5 Wear scar diameter and friction coefficient at various operation parameters

Ref.	Biodiesel	Blends	Tribo-pair	Load (N)	Speed/frequency/rotation	Temp (°C)	WSD (mm)	COF	Duration h
162	Coconut	B100	CI	1226	50 km h^−1	—	0.54	0.07	2
34	Palm	B0	Cr-alloy steel	392	1500 rpm	75	0.95	0.09	1
		B10					0.927	0.0892
		B20					0.848	0.08775
		B50					0.82	0.086
		B100					0.76	0.0857
148	Jatropha	B0	Steel	588	1500 rpm	45–75	1	0.08–0.1	1
		B20/B40/B100					0.5–0.7	0.055–0.075
163	Soybean	B0	Low C-steel	100	—	60	0.4	0.11	1
		B20					0.27	0.10
158	Sunflower	B100	Steel	392	1200 rpm	75	0.3	—	1
154	Rapeseed	B100	Steel	2	50 Hz	60	0.217	0.125	1.5

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Although additive-free biodiesel lubrication film is superior to additive-free diesel, the former has a higher wear rate than commercial lubricants.¹⁶² The hydrodynamic and/or boundary lubrication film thickness can be reduced by the high temperature, load, and frequency, as well as the low viscosity and concentration.^161,164 Oxidation instability, as a consequence of temperature, moisture absorption, and reactivity of the unsaturated hydrocarbon chain, has negative effects on the lubricity and wear for different blended fuels.^19,22 Moreover, oxidation increases viscosity and demonstrates better lubricity in the short-term test, but the degradation of fuel reduces lubricity in the long-term study.¹²¹ The degradation of fuel enhances corrosion and subsequently leads to the rapid tribological degradation of automotive materials. Therefore, the remedial measures with inhibitors must be investigated to improve the lubricity of biodiesel and its blends.

6.3 Remedial measures for wear reduction

Antioxidants are naturally present in triglyceride yet is neutralized through transesterification, thereby deteriorating oxygen stability.²⁵ In tribology, the oxidative deterioration of biodiesel introduces oxidative wear.¹⁴⁶ Some other factors, such as moisture absorption, deposit formation, corrosiveness, contaminants, elevated temperature, and reduced viscosity, have negative effects on biodiesel lubricity.^26,34 Biodiesel oxidation cannot be fully eradicated yet can be significantly minimized by using antioxidants that can retardate the reconversion of free fatty acids to triglycerides. Therefore, antioxidants are needed to improve the oxidation stability of biodiesel and reduce oxidative wear. Small percentages of hydroxylated and unsaturated components can be used to improve the lubricity of low-sulfur diesel fuels.¹⁶⁵ However, a higher percentage of biodiesel is currently used as an alternative to petro diesel to reduce emission. Such percentage must be free from hydroxylated and unsaturated components to prevent oxidation, which can lead to poor atomization, sticking, and coking. Therefore, researchers must search for a suitable antioxidant to improve lubricity. Antioxidants are generally divided into hydro-peroxide decomposers and chain breakers. Phenolic types are most commonly used as chain-breaking antioxidants. The active hydroxyl group in phenolic antioxidant provides protons that intercept the peroxide radical (RCOO˙) to prevent the creation of another radical as illustrated in Fig. 15. Therefore, the chain reaction of auto-oxidation is hampered as follows:

Radical tapping stage: R–COO˙ + AO = R–COOH + A˙

Radical termination stage: A˙ + A˙ = A–A or non-radical material

Fig. 15 Retardation mechanism of the chain reaction of auto-oxidation.

The inhibition efficiency of phenolic antioxidants can be determined by the number of hydroxyl/phenolic groups that are found in the molecular structure. The inhibition efficiency of several additives can be explained based on the number of hydroxyls and OH groups that are attached to the aromatic ring. TBHQ, PG, and PY contain two OH groups, whereas BHT and BHA only contain one OH group that is attached to the aromatic ring. Based on theory of electro-negativity, TBHQ, PG, and PY provide more locations to react with the free radical, thereby terminating the auto-oxidation chain reaction without forming any acidic products.⁵⁸ Moreover, decomposer antioxidants (i.e., sulfides and phosphides) convert hydro-peroxides into alcohols and become stable after transforming into an oxidized form.

Table 6 shows the commonly used synthetic antioxidants, with each antioxidant behaving differently for various feedstocks and fatty acid ester compositions.¹⁶⁸ TBHQ, AP, PrG, and PY have more than one hydroxyl groups that is attached to the aromatic ring; therefore, these antioxidants are more effective than BHT and BHA. TBHQ is considered a more active antioxidant for palm biodiesel, soybean, and sunflower because this antioxidant can rapidly form a complex bond with the free radicals of these feedstocks than those of other feedstocks. By contrast, PY and PrG have also been reported as the best antioxidants because of their molecular structures, which have a larger number of hydroxyl groups attached to their rings.¹⁶⁹ Table shows that PY is the most effective antioxidant for biodiesel, whereas TBHQ is the best antioxidant for palm biodiesel. Several studies have attempted to quantify the inhibition effectiveness of specific biodiesels as shown in Table 6. However, the inhibition mechanism of additives on the tribological degradation of different automotive materials has attracted limited research. The following section discusses the materials and methods for identifying the retardation mechanism of degradation because of wear and friction.

Table 6 Antioxidants used for wear reduction

Ref.	Name	Structural formula	Suitable medium
166	Tert-butyl hydroxyl quinine (TBHQ)		Palm biodiesel, soybean, sunflower
58	Butylated hydroxyamisole (BHA)		Castor oil, methyl soyate
72	Butylated hydroxytoluene (BHT)		Soyabean oil ethyl este, Tallow
167	Propyl gallate i.e. 3,4,5-trihydroxybenzoate (PrG)		Soyabean
46	Pyrogallol i.e. 1,2,3-trihydroxybenzene (PY)		Rapeseed oil, cooking oil, tallow, Karanja oil, Jatropha oil, croton oil

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6.4 Retardation mechanism of tribological degradation

The lubrication regime must be determined to characterize the retardation mechanism, whereas the minimum film thickness to surface ratio must be identified to determine the lubrication regime or specific lubrication film thickness.

6.4.1. Film thickness. The specific lubrication film thickness can be determined as follows:¹⁷⁰where λ is a specific lubrication film thickness, h_min is the minimum film thickness (m), and R_a1 and R_a2 denote the average roughness for the interface materials. The corresponding minimum film thickness for isoviscous lubricant can be predicted by using the well-known Hamrock and Dowson formula as shown in eqn 3.^171,172 This equation is widely applied in many tribo-pairs with varying contact geometries, including the point contact of hemisphere (ball) on a flat surface (plate).^173,174 The minimum film thickness equation is expressed as follows:where R_C, V, η₀, L, and E denote the radius of curvature,¹⁷⁵ surface velocity (m s⁻¹), dynamic viscosity at atmospheric pressure of the lubricant (Pa s)—which can be obtained by multiplying the measured kinematic viscosity (m² s⁻¹) by density (kg m⁻³)—normal load (N), and modules of elasticity (Pa), respectively. Therefore, we obtain the following:where ν and E denote the Poisson's ratio and young modulus of the interfaced materials, respectively.

6.4.2. Adsorption of lubricant on the metal surface. The adsorption isotherm can be derived from the coefficient of friction. This isotherm shows the relationship between the concentration of additives in biodiesel and its coverage on the tribo-interface surfaces. The friction-derived adsorption isotherm can be obtained from the fractional surface coverage at a specific concentration as follows:^176,177where μ_blend is the coefficient of friction (COF) of biodiesel blend, μ_ac is the COF at a specific additive concentration, and μ_bc is the COF at the concentration above which the COF no longer increases or decreases. The Langmuir isotherm is the simplest adsorption model that provides useful insights into the concentration dependence surface adsorption.¹⁷⁸ K_eqi is the equilibrium constant of the adsorption and is required to yield the ΔG_ads values of the additives on the rubbing surfaces. The equilibrium constant can be defined as follows in terms of surface coverage (θ) and additive concentration (C):¹⁷⁸

The slope of the θ⁻¹ versus C⁻¹ isotherm line represents the equilibrium constant of the adsorption (K_eqi).¹⁷⁸ K_eqi is related to the Gibbs free energy for the adsorption process. The negative value of ΔG_addi indicates the spontaneous adsorption of the inhibitor on the sample surface.¹³⁴ The numerical value of ΔG_addi determines the type of adsorption as physical, chemical, or physicochemical.^134,179 The ΔG_addi of the inhibitor on the material surface can be calculated as follows:¹³⁹

ΔG_addi = −RT ln(55.5 × 10⁶K_eqi) (17)

where ΔG_addi, R, T, and K_eqi denote the adsorption energy of the inhibitor (kJ mol⁻¹), the universal gas constant (8.314472 J mol⁻¹ K⁻¹), the operating temperature (K), and the equilibrium constant of the adsorption process, respectively.

7. Conclusions

This study reveals that the oxidation stability of pure biodiesel depends on the degree of saturated fatty acids, which varies across different feedstocks. The changes in the physical properties and chemical instability of biodiesel initiate the corrosion and tribological degradation of automotive materials, such as iron, iron-based alloy, steel, copper, copper-based alloy, aluminum, and aluminum-based alloy. Even when blended with diesel, biodiesel remains highly prone to corrosion and tribological degradation. Therefore, the degradation of the material and stability of biodiesel must be hampered by altering or modifying the fuel or metal surface. The use of antioxidants or corrosion inhibitors for fuel alteration is more feasible than material surface modification. Antioxidants delay auto-oxidation and retain the oxidation stability of biodiesel, whereas corrosion inhibitors form a protective oxide layer through the physical, chemical, or physicochemical adsorption processes. Such processes can be justified through different adsorption models and thermodynamic functions to quantify the effectiveness of corrosion inhibitors. Although many studies have investigated the corrosion inhibition of the fuel–metal interface by using different inhibitors, few studies have investigated the inhibition mechanism. Moreover, considering that palm biodiesel with different additive concentrations and elevated temperatures has never been investigated, the inhibition action and adsorption types of additives remain unknown. Therefore, future studies must investigate the inhibition mechanism of palm biodiesel against corrosion and the tribological degradation of its material. Improving the inhibition mechanism in any operating condition may allow the large-scale replacement of petrodiesel and the commercialization of biodiesel.

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