Impact of moisture on the corrosion behavior of copper and mild carbon steel in corn biodiesel

1 Introduction

Stringent environmental regulations, scarcity of non-renewable energy sources, and high price of petroleum and other fuels are some of the factors that make biodiesel a feasible fuel to supplement or replace petro-diesel. Although there are many advantages derived from biodiesel utilization, which include biodegradability, lower exhaust emissions, the absence of sulfur, aromatic hydrocarbons, metals, or crude oil residues, and higher flash point when compared to petro-diesel, biodiesel cannot completely replace hydrocarbon fuels because of issues concerning the availability of feed stocks, food vs. fuel debate, the storage stability strongly influenced by oxidation and auto-oxidation, high sensitivity to light, temperature, contaminants, and moisture (Haseeb et al., 2011; Atadashi et al., 2012). Several researchers (Kaul et al., 2007; Haseeb et al., 2010; Fazal et al., 2011a,b, 2012, 2013; Cursaru et al., 2014) already proved that its vulnerability to oxidation and its strong hygroscopic nature are issues, which cause the corrosive nature of biodiesel.

The largest amount of commercial biodiesel is produced from edible or non-edible vegetable oils and animal fats, in particular, through transesterification process (Antolin et al., 2002; Bari et al., 2002; Saeid et al., 2008; Demirbas, 2009; Kapilan et al., 2009).

By transesterification, the biodiesel produced is from the “first generation”. Transesterification is the reaction of triglycerides existing in vegetable oil in the presence of low-molecular weight alcohols such as methanol or ethanol, using either homogeneous catalysts (sodium hydroxide, potassium hydroxide, sulfuric acid, etc.) or heterogeneous catalysts (enzymes, resins, ion exchange resins, etc.) (Refaat, 2010; Yang et al., 2012).

The overall transesterification reaction is:

After transesterification, biodiesel is separated from glycerol by decantation. The unreacted alcohol is removed from biodiesel by vacuum distillation or flash evaporation, and contaminants such as soap, catalysts, or traces of glycerol are eliminated by water wash followed by centrifugation. The residual water in biodiesel is removed by means of an evaporator or vacuum drier (Ferella et al., 2010).

Biodiesel consists of alkyl mono-esters of fatty acids derived from parent oils or animal fats with high levels of unsaturated and polyunsaturated fatty acids, which make it susceptible to oxidation and, as a result, prone to corrosion. In fact, corrosion and oxidation symbiotically coexist and should be approached simultaneously (Ramos et al., 2009; Cursaru et al., 2013, 2014). Moisture and the hygroscopic nature of biodiesel are some of the factors that influence corrosion and, consequently, its quality. The water content accepted by the standard for biodiesel EN14214 is up to 500 ppm, but there are many situations, starting with the biodiesel synthesis and purification steps that require the presence of water. Besides, during storage and utilization in compression ignition engine, it is impossible to avoid the presence of water that remains in the final biodiesel, in dissolved or in a finely dispersed form. According to Maru et al. (2009), Demirbas (2009), and Atadashi et al. (2012), biodiesel has about 30 times higher tendency for water absorption than petro-diesel, which could be a reasonable cause for higher corrosivity of biodiesel than conventional diesel fuel. The presence of water creates difficulties in the transesterification process by reducing the conversion of triglycerides to biodiesel. In addition, it deteriorates the biodiesel quality, decreases the heat of combustion, accelerates the corrosion of fuel system, speeds up the hydrolysis reaction, favors growth of microbe colonies on tanks or injection system of diesel engines, increases the fuel consumption, decreases the engine performances, and reduces the engine life (Zhang et al., 2003; Gomes et al., 2010; Saleh et al., 2010; Atadashi et al., 2012).

For this reason, storage and utilization of biodiesel as fuel for diesel engine are strongly dependent on water content and on the possibility of decreasing the water content from all preliminary synthesis steps, essential for production and refining of biodiesel.

Thus, the main objective of this study is to evaluate the effects of water/moisture in biodiesel, synthesized from corn oil in particular, on the corrosion behavior of typical materials used for fabrication of automotive parts.

Typical feedstocks for biodiesel production are edible vegetable oils (rapeseed, palm, soy, sunflower, corn, etc.), non-edible vegetable oils (pongamia, jatropha, etc.), animal fat, waste vegetable oils and other feedstocks (algae, biomass, fungi, and bacteria) (Scott et al., 2010; Dantas et al., 2011; Siddharth et al., 2011).

Generally, corn was used as ethanol feedstock and was not considered a viable source of lipids for biodiesel production because of its low lipid content. In the last years; however, new varieties of corn that contain 4–21 wt.% oil were developed and can compete with soybean (15–20 wt.% oil) as feedstock for biodiesel production (Lu et al., 2005; Baker & Zahniser, 2006; Alptekin & Canakci, 2008; Serqueira et al., 2014; Gülüm et al., 2015).

Unfortunately, there is limited literature on the corrosion of automotive materials in corn biodiesel. Most of the researchers focus on determining the different characteristics or oxidation stability of corn biodiesel (Noureddini et al., 2004; Shi et al., 2013).

2 Raw materials for biodiesel production

3 Automotive materials

During utilization in automobile engines, biodiesel or biodiesel blends come into contact with a wide variety of engine parts including pump, gaskets, fuel injector, filters, fuel liners, bearing, piston, piston rings, etc. All these elements of the engine are made of different materials, which behave differently in contact with the fuel. Haseeb et al. (2010, 2011) presented a classification of the typical materials used in the construction of diesel engines, which can be grouped in three categories: (i) ferrous alloys (e.g. steel and cast irons), (ii) non-ferrous alloys (e.g. aluminum alloys and copper alloys), and (iii) elastomers. Several studies are related to the compatibility of different automotive materials such as copper, leaded bronze, aluminum, zinc, brass, and cast iron with biodiesel, and some of them suggest that copper, aluminum, zinc, brass, and bronze are not compatible with biodiesel (Kaul et al., 2007; Haseeb et al., 2010, 2011). Among them, the engine parts like fuel pump, bearings, bushing, etc., manufactured by copper alloys, are mostly affected by the characteristics of the fuel.

Geller et al. (2008) observed that copper alloys are more prone to corrosion by biodiesel compared to ferrous alloys.

4 Effects of water on the quality of biodiesel fuel

Demirbas (2006), Ma et al. (1998), and Atadashi et al. (2012) noted that during the conventional catalytic transesterification of vegetable oils, the presence of water in the feedstocks reduces the conversion of triglycerides to biodiesel fuel because it favors the production of soap, consumes the catalyst and reduces its effectiveness, and results in low conversion and low yield.

One of the major hindrances of biodiesel is its degradability, which is affected by internal and external conditions. The oxidation susceptibility is linked to the presence of the unsaturated fatty acids, which is the main internal condition that affects the degradability of biodiesel. External conditions such as moisture, light, and high temperature can act as a catalyst and accelerate the oxidation of biodiesel and, consequently, its degradation.

Atadashi et al. (2012) revealed that water or moisture is the root of most of the problems encountered during production, refining, or storage of biodiesel. Therefore, the sustainability of biodiesel strongly depends on its purity and quality, and the water content should be minimized, as much as possible, in all the intermediate stages prior to commercialization.

5 Materials and methods

This study aims to investigate the corrosion behavior of copper and mild carbon steel immersed in corn biodiesel. The effects of water content, ranging from 100 ppm up to 700 ppm, from corn biodiesel on corrosion behavior of pure copper and mild carbon steel were investigated at 90°C for 1200 h, by comparing the corrosive products formed on automotive materials after immersion in biodiesel (B100) and B0 (petroleum diesel), respectively. Usually, the engine temperature is higher than 100°C, but the main purpose of the present study is to obtain useful information that may help us understand the corrosion behavior of a fuel system, which usually has an operating temperature range between 40°C and 90°C. Furthermore, the results are expected to help us determine the optimum storage conditions and to select the most compatible materials for diesel fuel systems.

6 Results and discussion

The corrosion behavior of metals and susceptibility to oxidation or autoxidation are the most important conditions imposed to commercial biodiesel. Prior to its utilization in diesel engines, biodiesel is very sensitive to oxidative and thermal degradation, mainly due to the presence of the unsaturated fatty acids. During the oxidation of biodiesel, there appears a variety of secondary oxidation products, such as lower molecular organic acids, aldehydes, and ketones that increase the total acidity and the risk of corrosion in vehicle and fuel-handling system. Therefore, it is essential to distinguish between the cause of poor oxidation stability and corrosivity of biodiesel, especially because there are different mechanisms of action.

The degree of unsaturation (DU) was calculated based on the fatty acid composition of corn biodiesel presented in Table 7 and in equation (1).

Low oxidation stability is determined mainly by the unsaturated fatty acid methyl esters (85.1 wt.%), which include oleic (C18:1), linoleic (C18:2), and linolenic (C18:3).

The oxidation stability decreases with the increase in the content of polyunsaturated methyl esters; thus, high values of the DU indicate poor oxidation stability for biodiesel. The result calculated for corn biodiesel (128.9) indicates poor oxidation stability, which does not correlate with the total number of double bonds, but with the total number of bis-allylic sites, those that react with oxygen to form peroxide or hydroperoxide (Mc Cormick et al., 2007).

Table 5 shows the physico-chemical properties of biodiesel. The quality properties evaluated for corn biodiesel met the minimum or maximum limits on the EN 14214, except for oxidation stability. The oxidation stability was determined by the Rancimat method according to the EN 14214, showing 1.95 h, smaller than the lower limits of 6 h.

On the other hand, moisture and the hygroscopic nature of biodiesel may promote microbial growth and corrode the fuel system. Tables 5 and 6 show that the water content from corn biodiesel, which is 100 ppm and, for as-received diesel fuel, 94 ppm. These values are below the minimum limit imposed by the standards.

In order to evaluate the effect of water on the corrosion behavior of copper and mild carbon steel, 100, 500, and 700 ppm of water were added in corn biodiesel. The results were compared to those recorded after static immersion in commercial diesel fuel (B0).

Figure 1 shows the corrosion rates of copper and mild carbon steel exposed to biodiesel with 100, 500, and 700 ppm of water concentration at 90°C and 1200 h. The specimens were named B100_1 (corn biodiesel with 100 ppm of water), B100_2 (corn biodiesel with 500 ppm of water), and B100_3 (corn biodiesel with 700 ppm of water).

Figure 1:

Corrosion rate for copper (Cu) and mild carbon steel (MCS) after immersion in B0 (diesel fuel), B100_1 (corn biodiesel with 100 ppm of water), B100_2 (corn biodiesel with 500 ppm of water), and B100_3 (corn biodiesel with 700 ppm of water) at 90°C for 1200 h.

The results indicate that corrosion rates are lower in diesel fuel than that in corn biodiesel, which also demonstrate that corn biodiesel is more corrosive than diesel fuel. These observations are not surprising because previous studies had ascribed higher stability of diesel fuel to the saturated hydrocarbons from its composition (Knothe, 2007; Hu et al., 2012; Cursaru et al., 2014). For both metals, it was found that corrosion rates increase with the increase in water concentrations in biodiesel, which indicate an acceleration of corrosion for higher water concentrations. In the same experimental conditions, corrosion of copper is more severe than that of mild carbon steel.

In order to investigate the degradation of fuels, prior to and after immersion investigations, fuels were characterized by measuring the total acid number (TAN) according to EN 14104. The degrees of unsaturation along with the acid value give important information about the oxidation stability of biodiesel and its susceptibility to corrosion. The acid number was used to determine the level of the free fatty acids and other acid types, responsible for biodiesel degradation. The oxidation products formed during biodiesel degradation, such as aldehyde, ketones, alcohols, and volatile low-molecular organic acids (i.e. acetic acid and formic acid), are highly corrosive to the metal part of the engines.

The acid number measured for corn biodiesel was 0.22 mg KOH/g and for diesel fuel, 0.15 mg KOH/g, values lower than that specified by EN 14214 standards (max: 0.5 mg KOH/g). Figures 2 and 3 show the TAN values of different fuels before and after the immersion tests.

Figure 2:

TAN values of different fuels before immersion tests at 90°C for 1200 h. B0 (diesel fuel), B100_1 (corn biodiesel with 100 ppm of water), B100_2 (corn biodiesel with 500 ppm of water), and B100_3 (corn biodiesel with 700 ppm of water) at 90°C for 1200 h.

Figure 3:

TAN values of different fuels after immersion tests: (A) copper, (B) MCS.

These results show that the TAN values of diesel fuel exposed to copper and mild carbon steel are similar to diesel fuel prior to immersion tests, which indicates higher stability for diesel fuel in experimental conditions. It was also found that the TAN values of biodiesel increase with water content upon exposure to both metals, but the values recorded after exposure to copper are higher (with about 12%) than those observed for biodiesel exposed to mild carbon steel. Even if the water concentration in B100_2 sample did not exceed the upper limit for water imposed by EN ISO-12937 standard, from Figure 3A and B, it is seen that after exposure to both metals, the TAN value exceeded the upper limit for acid value imposed by the standard for biodiesel.

It was found that due to exposure of copper, TAN values rise, indicating that copper is weaker in biodiesel compared to mild carbon steel. These results are in accordance with those of Fazal et al. (2012) and Kaul et al. (2007).

Figure 4 shows the XRD patterns of corrosion products on the copper and mild carbon steel exposed to diesel fuel and corn biodiesel with 100, 500, and 700 ppm water concentration at 90°C for 1200 h.

Figure 4:

XRD pattern of copper (Cu) after immersion in B100_1 (A), B100_2 (B), B100_3 (C), B0 (D), and of MCS after immersion in B100_1 (E), B100_2 (F), B100_3 (G), B0 (H).

The XRD patterns (Figure 4A–D) mainly indicate the presence of the base metal copper (Cu). CuO and Cu(OH)₂ were detected on the surface of the copper strips after immersion in diesel fuel, while CuO, Cu(OH)₂, Cu(HCOO)₂, Cu₂O, CuCO₃·Cu(OH)₂, and Cu(OH)₂H₂O were found on the surface of copper immersed in corn biodiesel.

After the immersion of mild carbon steel in diesel fuel (Figure 4H), the concentration of iron compounds (e.g. FeO, Fe₂O₃, FeCO₃, and FeO(OH)) are comparatively less than those in biodiesel (e.g. FeO, Fe₂O₃, FeO(OH), FeCO₃, and C₂H₂FeO₄·2H₂O) (Figure 4E–G). These results are in agreement with those reported by Fazal et al. (2011a,b, 2013) and Karavalakis et al. (2011). In the presence of water and oxygen, copper- and iron-exposed surfaces indicate the formation of the compounds emphasized by XRD, which are precursors for corrosion. In our previous study, a reaction mechanism that might occur on copper and mild carbon steel surfaces upon exposure to sunflower biodiesel was proposed (Cursaru et al., 2014). According to this mechanism, the metal reacts with dissolved oxygen and forms different metallic oxides. In the presence of dissolved oxygen and water from biodiesel, copper leads to the formation of Cu(OH)₂, while iron leads to FeO(OH). Formation of copper carbonate or iron carbonate may be initiated by the formation of the RCOO˙ radical generated from the decomposition of the esters.

However, compared to diesel fuel, biodiesel-exposed mild carbon steel surface shows a comparatively higher concentration of iron oxides and hydroxides such as FeO, Fe₂O₃, and FeO(OH) and the appearance of iron formate hydrate C₂H₂FeO₄·2H₂O (Figure 4A–C). The concentration of iron compounds formed on mild carbon steel exposed to corn biodiesel is higher than that in diesel fuel and increases with water concentration in biodiesel. Similar findings were observed for copper. Copper formate Cu(HCOO)₂ was detected on the copper strip surface after exposure to corn biodiesel. The reason why the chemical species C₂H₂FeO₄·2H₂O and Cu(HCOO)₂ were detected in corn biodiesel-exposed metal is more likely due to the high level of water from biodiesel, O₂, and CO₂ absorbed from air and high immersion temperature. These results are interesting because there are only a few research studies that mention the presence of formic acid as a corrosion product, although they suggested that a variation of biodiesel color, during immersion tests, from reddish yellow to pale green, is an indicator of the transformation of copper into copper carbonate (Tsuchiya et al., 2006; Haseeb et al., 2010). According to Scott (2002), the copper formate is a patina component on important statues (i.e. Statue of Liberty), and the neutral formate is freely soluble in water and is the usual product formed from the direct reaction between copper and formic acid. It is more likely to assume that formic acid and copper formate are responsible for increasing the TAN number and corrosion rate rather than copper carbonate. In order to assess the concentration of formate from all corrosion compounds, XRD measurements were coupled with Rietveld quantitative analysis. These investigations reveal that the concentration of copper formate increases from 1.45 wt.% (for copper strip immersed in B100_1) to 1.58 wt.% (for copper strip immersed in B100_3), while the iron formate concentration increases from 0.41 (for mild carbon steel strip immersed in B100_1) to 1.15 wt.% (for mild carbon steel strip immersed in B100_3). TAN and XRD coupled with Rietveld quantitative analysis proved that the presence of water in biodiesel has a strong influence on the corrosion rate.

Increasing the water content in biodiesel caused an intensification of the oxidation products such as oxides, peroxides, and acids as a result of decomposition of peroxides and salts, contributing to increase in the corrosion rate.

Further investigations of the metal surface morphologies were observed by scanning electron micrographs. Representative SEM images of each test coupon for both metals are shown in Figures 5 and 6. Figure 5 presents the SEM micrographs of copper (Cu) after immersion in diesel fuel (B0) and biodiesel (B100) with 100, 500, and 700 ppm water concentration at 90°C for 1200 h.

Figure 5:

SEM micrographs of copper (Cu) after immersion in diesel fuel (B0) and corn biodiesel (B100) with 100, 500, and 700 ppm water concentration at 90°C for 1200 h.

Figure 6:

SEM micrographs of MCS after immersion in diesel fuel (B0) and biodiesel (B100) with 100, 500, and 700 ppm water concentration at 90°C for 1200 h.

After 1200-h exposure, metal strips exhibit obvious corrosion attack. It is observed that corrosion attack on diesel fuel-exposed metal surface is minor than that on biodiesel-exposed surface. Copper strips in biodiesel are subjected to higher corrosion attack.

By increasing the water content in biodiesel, it is seen as an intensification of corrosion attack on metallic surfaces. Copper strips immersed in biodiesel with the lowest water concentration show a few and small pits that seems to represent the beginning of pit formation; by increasing the moisture, an amplification of the pit formation was observed, the copper strip immersed in the biodiesel sample with 700 ppm of water being entirely corroded (the corrosion pits are highlighted by red arrows in Figure 5).

Figure 6 presents the SEM micrographs of mild carbon steel (MCS) after immersion in diesel fuel (B0) and biodiesel (B100) with 100, 500, and 700 ppm water concentration at 90°C for 1200 h.

The corrosion effects of diesel fuel on mild carbon steel were minor. After exposure to biodiesel with 100, 500, and 700 ppm of water concentration, the SEM images indicate that mild carbon steel is prone to pitting corrosion, and higher water concentrations accelerate the formation of pits on the metal surface (the corrosion pits are highlighted by red arrows in Figure 6). However, for both metals, the degradation of the surfaces exposed to corn biodiesel is comparatively higher than that in diesel fuel, but copper is less resistant in biodiesel and more prone to corrosion.

These observations are in agreement with XRD and corrosion rate results, which indicate that copper is more susceptible to corrosion than ferrous alloys.

In order to understand the appearance of the corrosion products on the metal surfaces and changes in the corrosion products due to the increase in the water content in corn biodiesel, a possible reaction mechanism to form the different compounds of copper (reactions 3–16) and iron is proposed (reactions 17–21).

7 Conclusions

Nowadays, there is a strong tendency to replace the conventional diesel fuel with more environmentally friendly fuels, for example, biodiesel. Unfortunately, corrosion and a high susceptibility to oxidation or autoxidation are the main concerns associated to biodiesel compatibility issues.

The standard for biodiesel EN 14214 has been developed based on the standard for petro- diesel; therefore, the specific method to investigate the corrosive proclivity is the same for the diesel, biodiesel, and biodiesel blends. The standard for biodiesel imposes as a method to measure the corrosion of biodiesel by strip tarnish test according to ASTM D130. This test method covers the detection of corrosiveness of copper to biodiesel, biodiesel blends, or diesel fuel. Unfortunately, the results of this test indicate only a slight tarnish for diesel fuel and biodiesel, which usually associate these fuels to the first corrosion class. This method indicates only marginal corrosion and cannot make a measurable difference between diesel fuel and different concentrations of biodiesel in blends.

It was already proved that biodiesel (because of its hygroscopic nature and the content of polyunsaturated methyl esters) is more prone to corrosion; therefore, in order to have a complete overview on the corrosiveness of biodiesel, it is not enough to examine the copper strip corrosion results, but also the water content, the acid value, the oxidation stability, and polyunsaturated methyl ester content.

The experimental aspects of this paper were to investigate the corrosion behavior of copper and mild carbon steel used as automotive materials, after exposure to corn biodiesel. The effect of water from corn biodiesel on the corrosion behavior of metal materials was studied by comparing corrosive products formed on the copper and mild carbon steel after static immersion in corn biodiesel and commercial diesel fuel.

1. The corrosion attacks on diesel-exposed metal surfaces are comparatively less than that on corn biodiesel-exposed metal surfaces.

2. Upon exposure to corn biodiesel, copper is subjected to higher corrosion attack followed by mild carbon steel.

3. The corrosion attacks on the surface of metal strips become more serious with increasing water content in corn biodiesel. An increase in the TAN value after immersion test indicates intensification of the oxidation and further corrosion.

4. The corrosion mechanism should be associated to the chemical corrosion. The corrosion products were mainly oxidation products such as CuO, Cu(OH)₂, CuCO₃, and also, a small amount of copper formate was detected on the metal surface.

5. On mild carbon steel surface, Fe₂O₃, Fe(OH)₂ and FeCO₃ are the main oxidation products detected, but a small amount of iron formate was found to be formed on the metal surface exposed to corn biodiesel.

6. The concentration of the copper or iron compounds formed on the metal surfaces exposed to diesel fuel are comparatively less than that to biodiesel, and the degradation of the metal surfaces exposed to corn biodiesel is higher than that in diesel. The presence of water in biodiesel seems to accelerate the corrosion of the metal surfaces, as well as the degradation of fuel characteristics.

Further study should be done to understand the appearance of the cooper and iron formate and their role in the corrosion attack on the metal surfaces.

Biodiesel is not a “young” product; it celebrates over 100 years of existence. It came back into light in the 1980s, after the world oil crisis of the 1970s, as a result of the struggle to discover renewable energy sources, alternative fuels to fossil fuel. Despite the many benefits related to biodiesel utilization, there are still key issues that need to be overcome so that biodiesel gets mature and can be, in the near future, a viable substitute for petro diesel. In order to make biodiesel a sustainable biofuel, it is imperative to find alternative raw materials, with production that does not use arable land and does not compete with the food industry. It is also mandatory to increase the economic perspectives, by reducing the production costs, especially the costs for raw materials, to decrease the amount of the byproduct – glycerol, or to find alternative methods for diversification of the products derived from glycerol. Issues as low oxidation stability or high susceptibility to oxidation or autoxidation should be overcome using antioxidant additives, by selecting raw materials with a low content of polyunsaturated fatty acids and by avoiding the contact between biodiesel and moisture or other impurities, which can promote oxidation and further corrosion.