Article Cite This: Energy Fuels 2017, 31, 10880-10889
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Study of Corrosion of Metallic Materials in Ethanol−Gasoline Blends: Application of Electrochemical Methods Lukás ̌ Matějovský,† Jan Macák,‡ Milan Pospíšil,† Petr Baroš,† Martin Staš,*,† and Aneta KrausovᇠDepartment of Petroleum Technology and Alternative Fuels and ‡Department of Power Engineering, University of Chemistry and Technology Prague, Technická 5, 166 28 Prague 6, Czech Republic
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ABSTRACT: Ethanol−gasoline blends (EGBs) can easily absorb large amounts of water because of the presence of ethanol. Acidic compounds and ions can be dissolved in water, and these substances can have corrosive effects on metallic construction materials. With the increasing content of ethanol in fuels, the conductivity and ability of fuel to absorb water increases, and the resulting fuel is becoming more corrosive. In this work, we tested E10, E40, E60, E85, and E100 fuels that were prepared in the laboratory. These fuels were purposely contaminated with water and trace amounts of ions and acidic substances. The aim of the contamination was to simulate the pollution of fuels, which can arise from the raw materials or from the failure to comply with good manufacturing, storage, and transportation conditions. The corrosion properties of these fuels were tested on steel, copper, aluminum, and brass using electrochemical impedance spectroscopy and Tafel curve analysis. For comparison, static immersion tests on steel were also performed. The main parameters for the comparison of the corrosion effects of the tested fuels were the instantaneous corrosion rate; the polarization resistance; and the corrosion rate, which was obtained from the weight loss occurring during the static tests. In most cases, E60 fuel showed the highest corrosion activity.
1. INTRODUCTION Currently, fossil fuels are being gradually substituted by biofuels, which are produced from renewable sources. In 2001, the European Commission adopted a program for the use of alternative fuels for transport.1 According to directive 2003/ 30/EC, 8% of fossil transportation fuels should be substituted by biofuels by 2020. Bioethanol produced from biomass is a promising biofuels. In the United States, E10 and E85 fuels are widespread. These fuels contain 10 and 85 vol % bioethanol, respectively. Bioethanol is mostly used in Brazil, where about 20% of cars burn pure bioethanol (E100), and the rest of the car fleet has been adapted to use E22 and E85 fuels.2 In the European Union, the content of bioethanol in transportation fuels is limited by legislation up to 2.7 wt % oxygen and 5 vol % ethanol.3 In the Czech Republic, E85 fuel and gasolines with up to 5 vol % bioethanol have been used. These fuels have to meet the requirements of standards Č SN EN 228 and Č SN P CEN/ TS 15293. Because of the introduction of pure and mixed fuels, high demands are placed on the quality and material compatibility of these fuels (blends). Ethanol for blending with gasoline (Č SN EN 15376) can contain up to 0.3 wt % water, 0.007 wt % acidic substances (expressed as acetic acid), and 20 mg·kg−1 chlorides. Other properties such as water content or total acid number can also be used as indicators of the corrosive properties of fuels. An increased corrosiveness of ethanol−gasoline blends (EGBs) results from the ability of ethanol to increase the solubility of water in such mixtures.4 Ethanol also strongly influences the dielectric and conductive properties of the resulting EGBs, thereby increasing the risk of galvanic corrosion.28 Especially the metallic parts of fuel systems (fuel tank, cylinder walls)5 are threatened by corrosive effects. Material compatibility in existing vehicles is problematic, especially for fuels with high contents of ethanol. EGBs with © 2017 American Chemical Society
ethanol contents of up to 10 vol % should not exhibit such problems.4 The corrosive properties of different materials have been studied in a large number of publications and patents.6,7 Some patents describe different laboratory tests for simulating material corrosion in a flowing apparatus, in liquids, and in vapors.8 There is also a gravimetric method for the evaluation of corrosion effects.29 Several publications have dealt with the use of electrochemical methods to describe corrosion processes in metal−fuel systems.9−27 For instance, studies on the influences of chlorides, water, pH, and the oxygen content dissolved in ethanol and EGBs on the corrosion of low-alloy steel and aluminum steel have also been published, in which different corrosion potential, cyclic potentiodynamic polarization, and impedance spectroscopy measurements in two- and three-electrode arrangements were used.9−27 Here, we used electrochemical methods to determine the corrosion resistances of metallic construction materials used for fuel systems in the environment of EGBs. The studied blends were purposely contaminated with water and trace amounts of other substances with corrosive properties (chlorides, sulfates, acetic acid). This contamination was intended to simulate the real contamination of EGBs in fuel systems, which can be caused by the failure to comply with good manufacturing, storage, and transport conditions of EGBs or ethanol for blending with gasolines. The application of electrochemical methods is discussed with respect to the options for determining and comparing the corrosion effects of EGBs containing from 10 to 100 vol % ethanol. Received: June 13, 2017 Revised: August 25, 2017 Published: August 28, 2017 10880
DOI: 10.1021/acs.energyfuels.7b01682 Energy Fuels 2017, 31, 10880−10889
Article
Energy & Fuels
where C is the capacitance obtained from the high-frequency part of the impedance spectrum measured in a planar electrode arrangement in n-heptane-metal system, εr is the relative permittivity of n-heptane, and ε0 is the relative permittivity of a vacuum. The cell constant Ks was used for the calculation of the fuel permittivity ε and for the recalculation of the resistivity R according to the equations
2. EXPERIMENTAL SECTION 2.1. Preparation of Ethanol−Gasoline Blends (EGBs). For the preparation of EGBs with ethanol contents ranging from 10 to 85 vol %, we used a gasoline base and ethanol with a water content of 700 mg·kg−1. The gasoline base was prepared from gasoline pool fractions [i.e., reformate, isomerate, and fluid catalytic cracking (FCC) gasoline] that were obtained from Č eská rafinérska (Kralupy nad Vltavou, Czech Republic). The volume ratio of reformate, isomerate, and FCC gasoline was calculated so that the contents of saturated, unsaturated, and aromatic hydrocarbons met the requirements of the standards.30 The group-type composition of the prepared gasoline base (53 vol % saturated, 12.1 vol % unsaturated, and 34.9 vol % aromatic hydrocarbons) was verified by gas chromatography. Because we used high-purity chemicals and gasoline fractions for the preparation of the samples, the chlorine contents in the prepared EGBs can be neglected. For the preparation of contaminated fuels, we used E100 fuel with the following composition: 94 vol % ethanol (p.a.), 6 vol % water, and trace amounts of contaminants (50 mg·L−1 CH3COOH, 3 mg·L−1 NaCl, 2.5 mg·L−1 H2SO4, and 2.5 mg·L−1 Na2SO4). A fuel with the identical composition was also used in other study.9 A stock solution was prepared for the contamination of the fuels. This stock solution was added to the EGBs to achieve the desired water contents and to avoid the separation of the fuels into two phases (aqueous and organic). The water contents in the prepared blends were as follows: 0.5 vol % in E10; 4.3 vol % in E40; and 6 vol % in E60, E85, and E100. 2.2. Metallic Materials Used for Corrosion Studies. The corrosiveness of the model fuels was tested on mild steel (0.16% C, 0.032% P, 0.028% S, balance Fe), aluminum (99.5%, 0.049 Cu, 0.014% Mg, 0.099% Si, 0.011% Mn, 0.057% Zn, 0.301% Fe), brass (34.3% zinc, 0.011% Ni, 0.031% Fe), copper (99.9%, 0.031% Zn, 0.021% Ni, 0.048% Fe), and AISI 304 stainless steel (18.22% Cr, 8.11% Ni, 2.1% Mn, 0.028% C). Prior to the experiments, the surfaces of the metallic samples and the prepared electrodes were polished with sandpaper (1200 grit) under running water, cleaned with demineralized water, and degreased with ethanol and acetone. 2.3. Electrochemical Methods. Measurements were performed using a VoltaLab 40 PGZ 301 radiometer and a reference 600 (Gamry Instruments) potentiostat. All measurements were performed in an grounded Faraday cage. The measuring sequence included the stabilization of the corrosion potential for 60 min and the measurement of the impedance spectrum and polarization characteristics. In environments of high resistivity (gasoline, E10; see Results and Discussion), only impedance spectra were measured after the stabilization of the potential. Electrochemical measurements were performed in both two- and three-electrode arrangements. In the three-electrode arrangement, the working electrode had a cylindrical shape with a diameter of 5 mm and a length of 15 mm. The working electrode was screwed onto a holder sealed with silicon rubber that served as an electrical contact. The total exposed area was 2.6 cm2. A platinum mesh coaxial with the working electrode served as the auxiliary electrode. A saturated calomel electrode (SCE) with a salt bridge containing a 0.1 M solution of potassium nitrate was used as the reference electrode. When clogging of the salt-bridge frit was encountered, a solution of lithium chloride in ethanol was used instead. To determine the dielectric and conductive characteristics of the environments, a symmetric two-electrode planar system was applied. The electrodes were made from metal sheets (3 × 4 cm) equipped with soldered contacts. The soldered areas on the backs of the sheets and the edges were embedded in epoxide resin. The exposed area of each electrode was 12 cm2, and the distance between the electrodes was 1 mm. The cell constant Ks for each electrode couple was determined using n-heptane, which has permittivity of about 1.92, according to the equation
Ks =
εrε0 C
ε=
K sCv ε0
(2)
Rs =
Rv Ks
(3)
where Cv is the capacitance obtained from the high-frequency part of the impedance spectrum measured in the metal−fuel system and Rv is the measured resistivity of the environment for the given metal−fuel system. For impedance measurements, the amplitude range was chosen to be from 5.0 to 70.0 mV depending on the conductivity of the studied environment and the corresponding total cell impedance. For higher values of the amplitude, the linearity of the response was verified. Measurements were performed mostly in the frequency range from about 1 MHz to 1 mHz. For the measurement of polarization curves, the polarization range was from −0.3 to +0.3 V relative to the corrosion potential, and the polarization rate was 0.5 mV·s−1. Postrun iR drop compensation was performed with polarization characteristics using the ohmic resistance values estimated from high-frequency impedance measurements. Tafel coefficients were estimated from the anodic and cathodic parts of the polarization curves. From the Tafel coefficients, polarization resistance, and electrode area, the corrosion current density was calculated according to the Stern−Geary equation as
jcorr =
B=
B Rp
bab k 2.3(ba + b k )
(4)
(5)
where jcorr is the corrosion current density, ba and bk are Tafel constants, and Rp is the polarization resistance estimated from electrochemical impedance spectroscopy (EIS) measurements. Alternatively, corrosion currents were found as the intersections of extrapolated Tafel lines. Subsequently, from Faraday’s law, the corrosion rate was calculated from the corrosion current density as follows
m = AIt
(6)
M zF
(7)
A=
where m is the mass of the substance; I is the current; t is the time; A is the proportionality constant designated as the electrochemical equivalent of the substance, measured in kg·C−1; F is the Faraday constant (9.6485 × 104 C·mol−1); and z is the number of electrons needed to exclude one molecule 2.4. Static Corrosion Tests. Static tests were performed according to the modified method described in ASTM D130-04. This method has been used as a complementary and comparative method for electrochemical measurements. It is based on the exposure of a metallic material for a certain time in a studied sample at a given ratio of the (i) volume of the studied sample and (ii) the surface area of the metallic material. The ratio was chosen to be 9 mL of fuel for 1 cm2 of surface area of metallic material so that the stabilization of the corrosion rate would not be achieved too soon. Static tests were performed for a long period (1200 h) in a closed 250 mL bottle at ambient temperature without oxygen access; the bottle contained 200 mL of the corrosion environment and three test samples of carbon steel having the same surface area. The metallic samples were weighed
(1) 10881
DOI: 10.1021/acs.energyfuels.7b01682 Energy Fuels 2017, 31, 10880−10889
Article
Energy & Fuels after certain time periods. From the time dependence of the weight losses, the corrosion rate was calculated according to the equations v vLr = 8.76 Pm ρ (8)
vPm
Δm = TS
Table 2. Measured Instantaneous Corrosion Rates of Metallic Materials in Pure and Contaminated Fuels Calculated by the Stern−Geary Equation corrosion rate (μm·year−1)
(9)
where vPm is corrosion rate in g·m−2·h−1, ρ is the density of the metallic material in g·cm−3, Δm is the average weight loss in g, S is the surface area of the metallic material in m2, and T is time (in hours) from the beginning of the test to the removal of the metal plate for measuremeent. After the experiments, the appearances of the surfaces of the metallic samples tested in the fuels were compared visually. 2.5. Measurement of Total Acid Numbers (TANs). TAN measurements were performed to compare the aggressiveness of the fuels. The measurements were performed according to standard method ASTM D664-89 on a DMS Titrino 716 automatic titrator with potentiometric detection of the point of equivalence. The TANs of EGBs without contamination increased in the range of 0.008− 0.016 mg KOH·g−1 for ethanol contents in the fuel of 10−100% vol. The results are presented in Table 1.
fuel
steel
aluminum
copper
brass
E40 E60 E85 E100 E40 + 3.4% H2O E60 + 3.4% H2O E60 + 6% H2O kE85 + 3.4% H2O E85 + 6% H2O E100 + 6% H2O
3.8 4.4 6.8 2.4 14.5 19.1 45.7 12.7 42.0 3.8
7.3 9.2 10.3 2.9 38.0 16.5 48.0 27.0 34.2 31.7
5.2 5.4 7.2 7.7 20.7 28.2 40.4 33.5 37.9 44.5
3.7 4.5 4.7 27.1 21.9 25.0 40.6 34.4 37.3 10.1
Table 1. TANs of the Prepared Contaminated EGBs fuel
TAN (mg of KOH·g−1)
E10 + 0.5% H2O E40 + 3.4% H2O E60 + 3.4% H2O E60 + 6% H2O E85 + 3.4% H2O E85 + 6% H2O E100 + 6% H2O
0.0155 0.0196 0.0210 0.0251 0.0242 0.0312 0.0334
3. RESULTS AND DISCUSSION 3.1. Instantaneous Corrosion Rate. Figure 1 shows Tafel curves of mild steel in the uncontaminated E40, E60, and E100
Figure 2. Tafel curves of (a) steel and aluminum and (b) copper and brass measured in pure E60 fuel and contaminated E60 fuel containing 6% H2O.
corrosion rate (see Table 2). This low corrosion rate and current density can be explained by the passivation ability of mild steel in absolute ethanol. This is also evident from the course of the anodic part of the Tafel curve, where no increase in current density with increasing anodic potential can be observed (see Figure 1, blue curve); moreover, the anodic current was lower than the cathodic current. For EGBs without contamination and with ethanol contents of
