Study of Corrosion Effects of Oxidized EthanolâGasoline Blends on

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Biofuels and Biomass

Study of Corrosion Effects of Oxidized Ethanol-Gasoline Blends on Metallic Materials Lukáš Mat#jovský, Jan Macák, Milan Pospíšil, Martin Staš, Petr Baroš, and Aneta Krausová Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b04034 • Publication Date (Web): 28 Mar 2018 Downloaded from http://pubs.acs.org on March 28, 2018

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Study of Corrosion Effects of Oxidized Ethanol-Gasoline Blends on Metallic Materials Lukáš Matějovský,1 Jan Macák,2 Milan Pospíšil,1 Martin Staš,1* Petr Baroš,1 Aneta Krausová2 1

Department of Petroleum Technology and Alternative Fuels, University of Chemistry and

Technology Prague, Technická 5, 166 28 Prague 6, Czech Republic 2

Department of Power Engineering, University of Chemistry and Technology Prague, Technická

3, 166 28 Prague 6, Czech Republic ABSTRACT. Bioethanol added into gasolines significantly changes the physical and chemical properties of the resulting fuels and can have a considerable influence on their overall thermooxidative stability. During fuel oxidation, different oxidation products such as water, acidic substances and peroxides are formed and these can have corrosive effects on metallic construction materials of the storage and transportation equipment, engines and fuel lines of automobiles, etc. In this work, we tested the laboratory prepared ethanol-gasoline blends (EGBs) E10, E25, E40, E60 and E85, which were artificially oxidized in dependence on their induction period. The oxidized fuels were used to study their corrosion aggressiveness after their thermal load in the presence of oxygen or after the expiry of their shelf life. The corrosion properties of these fuels were tested on steel, copper, aluminum and brass using electrochemical methods such as electrochemical impedance spectroscopy and Tafel curve analysis. For comparison, static immersion tests on copper and brass were performed. The main parameters for the comparison of the corrosive effects were the instantaneous corrosion rate, the polarization resistance and the corrosion rates of copper and brass, which were obtained from the weight losses which occurred during the static tests. The highest corrosion aggressiveness was observed, in most cases, for the 1 ACS Paragon Plus Environment

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oxidized E60 fuel; in this environment, the lowest resistance was observed for brass, at a peroxide content of 250 mg⋅kg-1 already. 1.

INTRODUCTION The International Energy Outlook prognosis assumes a global energy consumption

growth of about 28 % by 2040 in comparison with the consumption in 2015.1 This assumed increase is associated with the expected increase in energy consumption in developing countries. The gradually increasing demand for energy leads to the use of alternative energy sources.2 The current trend is to increase the consumption of fossil fuels as little as possible and preferably use biofuels that are produced from renewable sources. Mobile (transport) applications in Europe produce about 21 % of all greenhouse gas emissions contributing to global warming, and this share is still growing. In order to meet the reduction targets in terms of the Kyoto Protocol, it is necessary to find ways of reducing transport emissions.3 This problem can be partially solved by the gradual implementation of biofuels, on which high demands on quality, material compatibility and stability are given.4 Biofuels in transport applications can be considered as an important source of energy, the importance of which will be increased over a long-term point of view in the framework of the implementation of Euro programs according to the European directives 2003/30/EC on the promotion of the use of biofuels or other renewable fuels for transport and 98/70/EC on the quality of motor fuels in terms of environmental impacts. The biggest users of bioethanol in the world are Brazil (E27, azeotrope E100), Argentina (E12) and the USA (E10, E85), but this fuel is gradually becoming more widespread in the European Union also, especially in Sweden, France, Germany, Poland and Spain.5-6 In the EU, E10 and E85 fuels are widespread. In the Czech Republic, gasolines containing 5 vol. % of

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ethanol (E5) are currently being used together with E85 fuels and ETBE as a high-octane gasoline component. Ethanol is a polar compound with distinctly different physical and chemical properties in comparison with gasolines that can noticeably change the properties of the resulting EGBs. The presence of ethanol in fuels increases their corrosive properties. An absolute ethanol is a hygroscopic compound which is unlimitedly miscible with water. Ethanol present in EGBs can substantially increase their ability to absorb the air humidity which leads to an undesired increase in the water content in fuels. For EGBs with low ethanol contents, contamination with a higher content of water can lead to the separation into two phases (aqueous and organic) and this can negatively influence the engine performance including the extensive corrosion of its components or other kinds of serious damage. Moreover, the addition of ethanol into gasolines has other effects including (i) an increase of the octane number and conductivity of the resulting fuels, (ii) a decrease in hydrocarbon emissions, CO and lower heating value, (iii) a change of polarity, evaporative heat, vapor pressure and compatibility with some materials used for gasolines, etc.7-12 Material compatibility is one of the main problems, especially for cars using fuels with higher ethanol contents. The corrosive effects are visible, especially on metallic parts of fuel systems and on internal parts of engines (fuel tanks, walls of cylinders).13 Bioethanol influences the thermo-oxidative stability of EGBs in dependence on its content.14 EGBs containing 30–60 vol. % of bioethanol exhibit lower induction periods in comparison with fuels containing 10 vol. % of bioethanol and also higher susceptibility to oxidation and formation of oxidation products with corrosive effects.13 The presence of trace amounts of some metallic ions can affect the thermo-oxidative stability of EGBs similarly as some other influences, e.g., an elevated temperature, storage time and composition of the used gasoline. In addition, the presence of metallic ions in fuels is 3 ACS Paragon Plus Environment

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generally undesirable, as they cause corrosion and can be deposited on metallic parts of engines. These ions can participate in combustion reactions and negatively affect the engine performance.15-16 The metallic ions can come from alcohols used for blending with gasolines or from gasolines contaminated in the production, transportation or storage processes.17 It has been proven that ions of copper and iron present in fuels or released from construction materials can mostly affect the thermo-oxidative stability of fuels and the influence is dependent on the concentration of these ions or the storage time of the fuels. Zinc and nickel ions influence the thermo-oxidative stability much less in comparison with iron ions and the influence of these ions is dependent on their concentration only, but not on the storage time of the fuels. The lead ions have a negligible effect on the fuel stability considering both the concentration of the ions and the storage time of the fuels.18-25 Many oxidation products such as acidic substances and peroxides, which are precursors of acidic substances, have corrosive properties. Acidic substances have corrosion effects on steel and aluminum, whereas peroxides have corrosion effects on copper and its alloys, e.g., brass.26 The corrosion aggressiveness of non-aqueous environments and the resistance of metallic materials have been studied and evaluated in many publications.27-49 Electrochemical methods such as corrosion potential measurements, cyclic potentiodynamic polarization and impedance spectroscopy in two- or three-electrode arrangements have mostly been used for such studies.30-49 The main problem of the application of electrochemical methods in corrosion studies of biofuels is their resistivity. The electrochemistry in non-aqueous environments solves this problem usually by using supporting electrolytes (e.g., tetraalkylammonium tetrafluoroborate, perchlorate, etc.) that are soluble in organic environments and increase their conductivity. This is not optimal for corrosion studies, as supporting electrolytes are usually surface-active, influence the corrosion rate, can have inhibitive properties also and this can result in the distortion of the 4 ACS Paragon Plus Environment

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obtained results. Problems connected with low conductivity can be partially minimized by the arrangement of electrodes and the geometry of a measuring cell.32,34,37 In this work, we tested the corrosion resistance of metallic construction materials in the environment of oxidized EGBs after thermal loading and after the expiry of their shelf life. The resistance of metallic materials was monitored considering the contents of water, acidic substances and peroxides, which can be formed in EGBs during their oxidation. The analyses were performed using electrochemical methods without the supporting electrolytes. Also, the possible use of these electrochemical methods was discussed for the measurements in nonaqueous environments such as EGBs. This study complements our previous paper 49. 2.

EXPERIMENTAL SECTION

2.1

Preparation of Ethanol-Gasoline Blends (EGBs) In this study, several groups of EGBs were tested: (i) freshly prepared (unaged,

unoxidized) EGBs with different ethanol contents, (ii) aged (exposed to a free oxidation) EGBs with different ethanol contents, (iii) artificially oxidized EGBs with different ethanol contents and (iv) artificially oxidized EGBs mixed with fresh EGBs to obtain final fuels with 250 mg⋅kg-1 of peroxides, see Table 1. 2.1.1

Fresh EGBs The preparation of fresh EGBs is described in detail elsewhere.49 Briefly, the EGBs used

in this study were prepared from a (i) fermented absolutized ethanol (≥99.9 %, Merck) with a water content of about 700 mg⋅kg-1, and (ii) a gasoline base that was prepared from different petroleum fractions (reformate, isomerate and FCC gasoline) in such a volume ratio so that the contents of saturated, unsaturated and aromatic hydrocarbons met the requirements of the


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ČSN EN 228 standard. The group-type composition of the gasoline base (53 vol % of saturated, 12.1 vol % of unsaturated, and 34.9 vol % of aromatic hydrocarbons) was verified by GC.49 2.1.2

Aged, Artificially Oxidized and Contaminated EGBs The aged EGBs were prepared from the fresh EGBs. The aging (free oxidation) of the

EGBs (1 dm3) was performed in hermetically sealed 2 dm3 GL 45 reagent bottles from clear glass for six weeks. The artificially oxidized EGBs were prepared from the fresh EGBs by oxidation in a tempered pressure reactor with a volume of 2 dm3. The oxidation of the fresh EGBs (1 dm3) was performed at the following conditions: oxygen pressure, 6.5 bar; temperature, 100 °C for 48– 120 hours according to the thermo-oxidative stability (induction period) to almost all oxygen consumption (residual oxygen about 1 bar so that the oxygen consumption during oxidation is the same for all applied fuels). The content of the corrosively aggressive peroxides, acidic substances and water formed during the fuel oxidation was determined by titration, see Table 1. The contaminated fuels were prepared by mixing of the obtained artificially oxidized EGBs with the fresh EGBs (with the same ethanol content) to obtain the final EGBs containing 250 ± 3 mg⋅kg-1 of peroxides (verified by measurement). 2.2

Total Acid Number (TAN) and Peroxide Value (PV) Measurements TAN measurements were performed according to IP 177/96 and ASTM D664-89 on an

automatic DMS TITRINO T50 titrator 215 with potentiometric detection of the point of equivalence. PV measurements were performed according to ČSN EN ISO 27107 on an automatic titrator Mettler Toledo T50 215 with potentiometric detection of the point of equivalence.


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2.3

Water Content The water content was determined by Karl-Fischer titration using a coulometric titrator

WTK (Diram, Czech Republic) according to ČSN ISO 760. Selected Properties of the Freshly Prepared, Aged, Artificially Oxidized and

Table 1:

Contaminated Fuels 2.4

Metallic Materials Used for the Corrosion Studies The metallic materials used for the corrosion studies and their surface treatment prior to

the analyses have been specified elsewhere.49 The composition of the used materials is presented in Table 2. Table 2: 2.5

Elemental Composition of the Metallic Materials Electrochemical Methods and Static Corrosion Test The conditions of the electrochemical measurements and the static corrosion test have

been described in detail elsewhere.49 Electrochemical measurements were performed using a Reference 600 device (Gamry) in two and three electrode arrangements. The two-electrode system consisted of two electrodes from the same metallic material in a planar arrangement. The distance between the electrodes was 1 mm and the surface area of each was 4 × 3 cm (i.e., the total area in contact with fuel was 24 cm2). Measurement of the impedance spectra was performed in a 100 ml cell at amplitudes of 10–70 mV in a frequency range from 1 MHz to 10 mHz. The shape and the evaluation of the obtained spectra is discussed in Section 3.2. The three-electrode system consisted of: (i) a working electrode having the surface area of about 2.6 cm2, (ii) an auxiliary platinum electrode and (iii) a reference calomel electrode


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saturated in a non-aqueous environment containing a bridge with a 1 M solution of lithium chloride in ethanol. Measurement was performed in a cell containing 100 ml of fuel at a scanning range of 0.5 mV⋅s-1 in the range from -0.3 V to 0.3 V relative to the corrosion potential. Tafel curves were evaluated using MS Excel. First, the iR-drop (potential loss) was subtracted from the data characterizing the obtained Tafel curve using resistivities from the obtained impedance spectrum prior the Tafel scan measurement. Then, linear areas of the cathodic and anodic parts of the Tafel curve were evaluated by linear approximation and the βa and βc coefficients were obtained for the calculation of corrosion current resistance (jcorr) according to the Stern-Geary Equation:

݆௖௢௥௥ =

ߚ௔ ∙ ߚ௖ 2,3 ∙ (ߚ௔ + ߚ௖ ) ∙ ܴ௣

where Rp is the polarization resistance evaluated from the impedance spectrum obtained in a three-electrode arrangement prior to the measurement of the Tafel scan. From the corrosion current density, weight losses of materials in time were calculated according to Faraday´s Law and these were then recalculated to the corrosion rate according to the procedure presented elsewhere.49 Static tests were performed in 250 ml hermetically sealed GL 45 reagent bottles containing 160 ml of a tested fuel and two metallic samples (4 × 1 cm) with the total exposed area of 16 cm2.50 The application of methods in individual metal-fuel systems are summarized in Table 3. Table 3:

Overview of the Experiments and Used Metallic Materials


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In the environment of deeply oxidized fuels, it was impossible to perform the electrochemical measurements on copper and brass, as the oxidized fuels contained high amounts of peroxides that caused the quick dissolution of copper and brass. Brass and copper were tested in fuels with lower aggressiveness containing 250 mg⋅kg-1 of peroxides. 3.

RESULTS AND DISCUSSION The standard EN 228 prescribes a minimum induction period of 360 minutes for

gasolines. The minimum requirement for the induction period determined according to ASTM D525 was met for all prepared EGBs. The consumption of oxygen during fuel oxidation occurs to a maximum extent after the induction period, i.e., during the induction period, the oxygen consumption and also the formation of oxidation products (acids, peroxides, water, resins) is minimal. However, from the time required for fuel oxidation, see the bottom of Table 1, it is obvious that the lowest induction period was observed for the E40 and E60 fuels (close to 360 min – minimum lifetime), whose oxidation required the shortest time (48 hours); the period of other fuels increases with the increasing time of oxidation.14 For EGBs, a prepared gasoline with induction period of 790 min was used. 3.1

Instantaneous Corrosion Rate The measured values of instantaneous corrosion rates are presented in Table 4. The

corrosion rate was calculated from the weight losses according to the Faraday´s Law and from the current density obtained from the Stern-Geary Equation based on the polarization resistance and the Tafel slopes of the cathodic and anodic part of the Tafel curve.49 For EGBs without oxidation and with ethanol contents of less than 40 vol. %, it was not possible to measure the polarization data due to the high environmental resistance and high compensated iR-drop which was beyond


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the ability of the electrochemical apparatus. The same effect was observed for the oxidized E10 fuel as well. Table 4:

Measured Instantaneous Corrosion Rates of Metallic Materials in Fresh and Oxidized Fuels The fuel oxidation resulted in the formation of acidic substances and water, which

increased the fuel conductivity; thus, it was possible to measure the polarization data for the E25 fuel also, see Table 2. The difficulty of measuring this data is evident from the course of the Tafel curves of the fresh (unoxidized) E40 fuel, see Figure 1a, and the oxidized E25 fuel on mild steel, see Figure 1b, where the curves do not have a smooth course as for fuels with the higher contents of ethanol. Figure 1: Tafel curves of: (a) – mild steel in unoxidized fuels, (b) – mild steel in oxidized fuels, (c) – copper in unoxidized fuels, (d) – brass in fuels containing 250 mg⋅kg-1 of peroxides Due to the oxidation of the fuels, the corrosion potential was shifted toward positive values, see Figures 1b and 1d; for mild steel it was about 500 mV. The corrosion potential of mild steel in an environment of oxidized fuels increased with the increasing ethanol content in the fuel, but only up to 60 vol. % of ethanol. The highest corrosion potential was observed for the oxidized E60 fuel, see Figure 1b. Conversely, the lowest corrosion potential was observed for the oxidized E85 fuel and this value was comparable to that of the oxidized E25 fuel. The corrosion current densities of mild steel measured in the oxidized fuels were in the range of 0.26– 0.47 µA⋅cm-2 which was higher in comparison with the unoxidized fuels; the lowest value of corrosion current density was observed for the oxidized fuel E25, whereas the highest value was observed for the oxidized E60 fuel. For these oxidized fuels, instantaneous corrosion rates in the 10 ACS Paragon Plus Environment

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range of 5.7–10.1 µm⋅year-1 were observed which was higher in comparison with the unoxidized fuels, see Table 4. The oxidized E85 fuel showed a corrosion current density of 0.39 µA⋅cm-2. This indicates that the oxidized E60 fuel will have the highest corrosion aggressiveness on mild steel, which is evidenced by the calculated instantaneous corrosion rates in Table 4. This trend of fuel aggressiveness was similar for copper, brass and aluminum also. In the environment of deeply oxidized fuels, it was impossible to perform the polarization measurements on copper and brass, as the oxidized fuels contained high amounts of peroxides that caused the quick dissolution of copper and brass. Brass and copper were tested in fuels with lower aggressiveness containing 250 mg⋅kg-1 of peroxides that were prepared by adding oxidized fuels into fresh, unoxidized fuels. The corrosion potential of copper and brass in the environment of unoxidized and slightly oxidized (250 mg⋅kg-1) fuels was increasing with the increasing content of ethanol, see Figure 1d. For the corrosion potential of copper and brass, no exception was observed in the environment of the E60 fuel unlike mild steel, see Figure 1b. Brass in unoxidized fuels exhibited lower corrosion potentials than copper. In dependence on the ethanol content in an unoxidized fuel, the corrosion potential of copper varied in the range from -255 to -200 mV, which indicates that the highest corrosion potential was measured on copper in the environment of absolute ethanol. For copper and brass, the fuel corrosion aggressiveness rose with the degree of oxidation. Already, at the mild oxidation of fuels, the corrosion potential of copper and brass were shifted by about 300 mV toward a more positive potential. A mere content of 250 mg⋅kg-1 of peroxides, water and acidic substances, see Table 1, led to (i) the shift of the whole curve toward higher current densities and (ii) to a significant increase of the instantaneous corrosion rates of copper and brass especially for the E60 fuel, see Table 3. Even a moderate increase of 11 ACS Paragon Plus Environment

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the content of peroxides in the fuel can have a strong corrosion effect on copper and brass, which significantly rises with the increasing content of peroxides. Copper and brass exhibited similar behavior in the environment of EGBs and relatively low corrosion rates, which increased with the increasing ethanol content in fuel. Despite this, it is clear that brass will have a significantly lower resistance than copper, especially in the E60 fuel containing 250 mg⋅kg-1 of peroxides, in which, for brass, more than twice higher instantaneous corrosion rate was measured than in comparison with copper. The corrosion rate of aluminum in the oxidized fuels was about four to five times higher than the corrosion rate of mild steel and about three to five times higher than in the unoxidized fuels. The corrosion resistance of mild steel against a local attack is distinctly higher than that of aluminum. It is likely that with an increasing content of acidic substances formed due to fuel oxidation, the instantaneous corrosion rate of aluminum will rise distinctly. 3.2

Electrochemical Impedance Spectroscopy (EIS) Impedance spectra are presented in Figure 2. More details about the shape of the spectra

and their evaluation and measurement conditions are presented elsewhere.49 Figure 2: Nyquist diagram of impedance: (a) – copper in gasoline base in dependence on time, (b) – mild steel in the unoxidized fuels and in the oxidized E25 fuel after 3 hours of exposure, (c) – copper in fuels containing 250 mg⋅kg-1 of peroxides after 6 hours of exposure, (d) – mild steel in the oxidized E10 fuel in dependence on time, (e) – copper in the E10 fuel containing 250 mg⋅kg-1of peroxides in dependence on time, (f) – brass in the E25 fuel containing 250 mg⋅kg-1 of peroxides in dependence on time.


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In the gasoline base that was used for the preparation of EGBs, it was possible (using EIS) to measure, on all tested metallic materials, the spectrum that contained only the highfrequency part (one half circle). From such spectra, it was not possible to evaluate information about the corrosion of the tested materials. Figures 2b and 2c present the spectra of the E25 fuel at different degrees of oxidation and different gasoline-ethanol ratios that were measured in the two-electrode arrangement. In this case, the spectra had the shape of two relatively well separated half-circles. The high-frequency part of the spectra gives information about the properties of the environment, whereas the lowfrequency capacitive loop is associated with the (i) response of the electric double layer at the phase interface and (ii) parallel polarization resistance, which is the basic corrosion quantity that characterizes the instantaneous corrosion rate. Formation of the double layer at the electrode surface was caused by the presence of oxidation products and by the higher polarity and conductivity of the environment. The equivalent circuits expressing both types of impedance behavior are presented in Figure 3. Figure 3: Equivalent circuits used to evaluate the impedance spectra shown in Figure 2: a) Impedance measured in gasoline and the E10 fuel, with Rfuel representing the resistance of the environment and CPEfuel representing the spatial capacitance of the environment. (b) Impedance measured in the EGB in the presence of contaminants, with Rp representing the polarization resistance and CPEdl representing the capacitance loss of the double layer. Reprinted with permission from ref.49. © 2017 American Chemical Society. In addition, Figure 2c documents the increasing fuel conductivity (the decreasing Rs) with an increasing ethanol content in the EGB. The corrosion products released into the fuel also 13 ACS Paragon Plus Environment

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had a significant influence on the fuel conductivity that increased with the exposure time of material in the fuel – as presented by the impedance spectra of time dependence in Figures 2d-f. The content of water and acidic substances affected the fuel conductivity also, which follows from Table 1 and from the comparison of Figures 2d (Zreal in MΩ⋅cm2) and 2e (Zreal in kΩ⋅cm2). On mild steel for the oxidized E10 fuel, it was possible to measure the low-frequency part of the spectrum and obtain the polarization resistance of the mild steel from the beginning of the experiment, see Figure 2d. In case of the measurement on copper in the E10 fuel containing 250 mg⋅kg-1 of peroxides (which has conductivity about one order of magnitude lower than the E10 fuel as a result of lower contents of acidic substances and water), it was not possible to measure the low-frequency part of the spectrum from the beginning of the experiment. The highfrequency part of the spectrum was measured after a certain time of copper exposure, when the fuel resistivity was reduced as a result of the released copper ions into the fuel in the form of corrosion products, see Figure 2e. This fact demonstrates the influence of the conductivity and the composition of the fuel on the possibility and measurability for the corrosion data of metallic materials in an environment of EGBs using EIS; the E10 fuel with 250 mg⋅kg-1 of peroxides, 0.15 mg of KOH⋅g-1 of acidic substances and about 500 mg⋅kg-1 was practically at the very limit of measurability. The stabilization and rise of polarization resistance with an increasing time of metallic material exposure in the fuel is documented by the compared EIS spectra of brass in the environment of the oxidized E25 fuel in Figure 2f, where a minimum increase of polarization resistance is evident for the last two measurements performed after 24 and 30 hours. The highest values of polarization resistance were measured on stainless steel that with its measured values exhibiting a high resistance against corrosion in the environment of the EGBs. From the dependence of the polarization resistance of stainless steel on the ethanol content in a fuel, a


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different passivation ability of stainless steel is obvious and this can be different in dependence on the composition of an environment. The lower polarization resistance of stainless steel in the E25 and E100 fuels indicates the slower passivation speed of the stainless steel, see Figure 4a. Figure 4: The polarization resistance of metallic materials after (a) 3 hours of exposure in dependence upon the ethanol content in unoxidized fuels and (b) fuels after 6 weeks aging The fresh fuels were left for six weeks-long free oxidation, which resulted in the increase of the content of peroxides, acidic substances and water, see Table 1. Subsequently, EIS measurements were performed. Even mild oxidation resulted in the overall decrease of the polarization resistances, see Figure 4b, for all metallic materials applied as evidenced by the increase of aggressiveness of the fuels with the increasing content of peroxides, acidic substances and water. From the polarization resistances of metallic materials, a dependence on the ethanol content in the fuel is evident, where the lowest values were measured especially in the E60 fuel. Brass in the environment of the unoxidized fuels showed slightly higher polarization resistances than copper. However, this trend changed due to mild oxidation and the polarization resistances of brass were lower in comparison with copper. This was probably caused by the properties of brass which is an alloy of copper and zinc and in an environment of moderately oxidized fuels it loses its resistance due to microcrystallic galvanic corrosion, which can lead to a dezincification of its surface. The oxidation of fuels had an effect not only on the decrease of the polarization resistance of the corrosion materials (increase of corrosion aggressiveness of fuels) in the metalfuel system, but it also influenced the electrochemical properties of fuels, see Figure 5. The increase in the content of peroxides, acidic substances and water in fuels resulted in the increase 15 ACS Paragon Plus Environment

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of their permittivities and conductivities as a result of the decrease of their resistivities. A deeper degree of oxidation had an opposite effect on the permittivity which decreased from the influence of the resulting higher-molecular-weight polymeric oxidation products (resins). Resistivities and permittivities were significantly influenced by the content of ethanol in the fuels. The permittivity of EGBs rose and the resistivity dropped with the increasing ethanol content. Such an increase refers to a low permittivity of the pure gasoline base and a relatively high permittivity of ethanol, see Figure 5. Figure 5: Permittivity and resistance of unoxidized and oxidized fuels and fuels after 6 weeks of aging. Measurements performed with stainless steel electrodes after 3 hours of exposure in a two-electrode planar cell. The polarization resistances in the oxidized fuels were measured in dependence on time – see Figure 6, where the time dependences of the polarization resistance measured for aluminum are compared. Figure 6 shows a significant increase of the polarization resistance of aluminum by 24 h with slight stabilization. It is likely that with the increasing time of exposure of aluminum there would be a further increase in polarization resistance, which would slowly stabilize at high values. This trend demonstrates the ability of moderate passivation of aluminum in the environment of the oxidized fuels. The lowest polarization resistance at 24 hours of exposure was measured for the oxidized E60 and E10 fuels. A high corrosion aggressiveness of the E60 fuel is documented by the measured instantaneous corrosion rate, see Table 3. Based on the trend of dependence of polarization resistance on the ethanol content in the fuel, the E10 fuel should exhibit a lower corrosion aggressiveness than the E25 fuel. Despite this, the oxidized E10 fuel was an exception. The high contents of water and acidic substances (that is similar in comparison with the E60 fuel, see Table 1) can have a high influence on the aggressiveness of 16 ACS Paragon Plus Environment

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the E10 fuel. A similar course of polarization resistance in oxidized fuels was also observed on mild steel and stainless steel. Figure 6: The time dependence of polarization resistance of aluminum measured in the oxidized fuels As a result of very low polarization resistance of copper and brass, it was possible to measure the high-frequency part of the impedance spectra only which gave information about the properties of the environment and the changes of the environment due to releasing of ions of dissolving metal. Therefore, the influence of fuels on copper and brass was studied in fuels with 250 mg⋅kg-1 of peroxides (model fuels simulating a low degree of oxidation). A significantly lower aggressiveness of these fuels is shown in Table 1, which evidences the lower content of water and acidic substances than in the deeply oxidized fuels. Figure 7: The time dependence of polarization resistance of brass in fuels with 250 mg⋅kg-1 of peroxides The stabilization of the polarization resistance and its dependence on the ethanol content in the fuel with 250 mg⋅kg-1 of peroxides exhibited a similar course on copper and brass. Hence, Figure 7 presents the time dependence of the polarization resistance of brass in the E10–E85 fuels only. The lowest polarization resistance was measured for the E60 fuel containing 250 mg⋅kg-1 of peroxides. The polarization resistance of brass rose with the decreasing ethanol content in fuel. According to expectations, the highest polarization resistance was measured in the E10 fuel with 250 mg⋅kg-1 of peroxides. The polarization resistance in this fuel was measured after a longer exposure time. This fact is related to the low conductivity of the fuel as illustrated in Table 3 which shows a significantly higher resistivity for the E10 fuel with 250 mg⋅kg-1 of


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

peroxides in comparison with the deeply oxidized E10 fuel and the E25 fuel with 250 mg⋅kg-1 of peroxides, where the polarization resistance was measured from the beginning of the experiment. Table 5: Resistivities of Fuels after 3 Hours of Exposure in a Planar Electrode Arrangement The resistivities of fuels, see Table 5, measured on metallic materials differed from each other in dependence on the surface properties of the metallic materials. This difference is caused by the different resistance of materials against corrosion and the different properties of oxidic layers on the surfaces of the metals. A corrosion-resistant steel minimally dissolves in the environment and no significant changes of conductivity and permittivity occur as a result of the dissolved ions. On the surface of stainless steel, a very thin passive layer of oxides was formed which was conductive and protected the metal significantly more against corrosion than other metallic materials; these values will be, thus, least affected by a measurement error.49 The highest polarization resistance was measured in the unoxidized fuels and especially in the unoxidized E60 fuel, see Figure 8a. After six weeks of free oxidation of the E25, E40 and E60 fuels, the values of polarization resistance were reduced by more than a half due to the increase of the content of peroxides, acidic substances and water. The drop of polarization resistance of these fuels informs one about a significant increase of corrosion rates within 6 weeks. For the E85 fuel only, another trend was observed – no significant change of the polarization resistance was observed for mild steel after 6 weeks of a free oxidation. The E85 fuel contained the highest amount of water, but the lowest amount of acidic substances, see Table 1, which are corrosion agents for mild steel. It is likely that due to the higher content of water in the E85 fuel-mild steel system, a better coverage of mild steel with oxidic layer will occur and the metal will be better protected against corrosion in comparison with other fuels, or a lower drop in polarization resistance will occur. The lowest value of TAN refers to the lower corrosion 18 ACS Paragon Plus Environment

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Energy & Fuels

aggressiveness of the E85 fuel which is in accordance with the highest polarization resistance of mild steel. This fact is documented by the results obtained for mild steel in an environment of artificially oxidized fuels, see Table 1. The lowest polarization resistance of mild steel was measured in the E60 fuel, for which the highest instantaneous corrosion rate was measured after the oxidation. The high polarization resistance measured in the E25 fuel did not change significantly with the oxidation degree, as shown in Figure 8b. It is likely that for the E25 fuel, the passivation of aluminum occurred regardless of the increase of TAN and water content. Even with a high degree of the oxidation of the E25 fuel, no saturation of fuel with water (and thus separation of the aggressive water-ethanol phase) occurred. Apparently, the ethanol content was not so high as to cause the increase of dry corrosion which can occur on aluminum. Dry corrosion is related to the low nobility of metals, where, in an environment of anhydrous ethanol, alcoholates are formed on the surface of such less noble metals and such alcoholates are soluble in an environment and the metal is not protected against corrosion by a passive surface layer.51 The results obtained by EIS in fresh fuels can give information about the course of the possible dry corrosion. In fresh (unoxidized fuels), a decrease of polarization resistance with an increasing content of ethanol was observed and the lowest value was observed for the E85 fuel. Aluminum in the E85 fuel exhibited an increase in polarization resistance with the degree of oxidation – most likely due to the increasing water content which can support passivation and inhibit dry corrosion. Free oxidation had much lower influence on the change of polarization resistance of aluminum in comparison with mild steel. A significant change occurred in the artificially oxidized E40 and E60 fuels with a high degree of oxidation, where the lowest polarization resistance of aluminum was measured.


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Figure 8: The comparison of polarization resistances of mild steel (a), aluminum (b), stainless steel (c), brass (d), and copper (e) that were measured by EIS after 3 hours of exposure in unoxidized and artificially oxidized fuels The results of the polarization resistance presented in Figure 8c confirm the very good passivation ability of stainless steel with an increasing content of ethanol. Despite this, the passivation rate of stainless steel decreased with the increasing degree of oxidation. The lowest polarization resistances of stainless steel were measured in the environment of the deeply oxidized fuels and especially, in the E60 fuel, where the highest corrosion aggressiveness was measured on other metallic materials also. The highest change with the degree of fuel oxidation was observed especially for the E40, E60 and E85 fuels, which corresponded to the highest increase in corrosion aggressiveness already with a low increase of water content, PV and TAN. The results measured on brass and copper in dependence on the degree of oxidation (and the content of peroxides) of the fuel presented in Figures 7d and 7e are similar. The lower resistance with the degree of oxidation (increasing content of peroxides) was demonstrated on brass, where a significantly higher drop in polarization resistance was measured in comparison with copper. The lowest polarization resistance in copper and brass was measured in the E60 fuel containing 250 mg⋅kg-1 of peroxides. It is likely that the corrosion of copper and brass in this fuel will occur with the highest corrosion resistance in comparison with other fuels. Also, it is obvious that the corrosion rate of these materials will rise with an increasing content of peroxides, as documented by the obtained instantaneous corrosion rates and the results of the static tests. 3.3

Static Immersion Test on Copper and Brass Peroxides form complexes with copper and support its dissolution in fuels and corrosion

of copper-containing materials. Although such materials cannot be used in modern fuel systems, 20 ACS Paragon Plus Environment

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they were used as appropriate materials to describe the corrosion influenced by peroxides in dependence on the ethanol content in fuels. Figure 9: The time dependence of the weight loss relative to the area of the copper and brass samples during the static immersion tests in the E40, E60 and E85 fuels containing 250 mg⋅kg-1 of peroxides. Graph on the bottom – the initial course of the experiment. From the results presented in Figure 9, it is obvious that the chosen time period (1200 hours) was sufficient enough to reach metal-fuel equilibrium in static immersion tests. From a comparison of weight losses in fuels, it follows that the equilibrium was reached in different times from the beginning of the experiment. The highest increase in brass loss in the early stage of exposure (50 hours) for the E60 fuel is especially noteworthy, see Figure 9a. The gradual increase in copper loss in the E60 and E85 fuels is also noteworthy, where a significant increase of weight loss was observed after 100 hours, Figure 9b. A very low (almost zero) corrosion rate was measured for the E40 fuel on copper for 600 hours. The corrosion rate of copper in the environment of the E40 fuel, then stabilized at about 1200 hours on the lowest measured value, see Table 4. This can be explained (i) by the coverage of the surface of the metal by a layer of oxides which can initially protect the metal against corrosion and also (ii) by the low content of ethanol in fuel, as ethanol content can significantly influence the solubility of copper and the products of surface corrosion. This effect was not observed on brass and the weight losses were measured in the E40 fuel from the beginning of the experiment already. The corrosion rate of brass in the environment of the E40 fuel stabilized on a value that was higher in comparison with the value of the corrosion rate of copper in the E85 fuel, see Table 6. From this, it is obvious that brass exhibited (due to its alloy character) a distinctly lower corrosion resistance in the EGBs in comparison with copper. For brass, microcrystalline galvanic corrosion can be supported by 21 ACS Paragon Plus Environment

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peroxides, acidic substances and water in fuels; microcrystalline galvanic corrosion is related to a higher corrosion rate and a lower resistance. The corrosion rate of both materials significantly depended on the content of ethanol in fuel and a similar trends were observed for both materials. The highest corrosion rates were for both materials measured in the E60 fuel. Table 6:

The Corrosion Rate of Brass and Copper in Fuels Containing 250 mg⋅kg-1 of Peroxides The high aggressiveness of the E60 fuel containing 250 mg⋅kg-1 of peroxides is also

demonstrated by the visual comparison of fuels after the static test in Figure 10, which shows the coloration of fuels caused by the release of copper ions as a consequence of corrosion. Figure 10: The coloring of fuels: E40 (left), E60 (middle), E85 (right) containing 250 mg⋅kg-1 of peroxides tested on copper The surface of brass that was exposed to the tested fuels in the static tests showed no significant color shades, see Figure 11. For copper, a darker shade was observed in the E60 fuel, which corresponded to the highest corrosion rate. Figure 11: The comparison of the surfaces of the samples of copper (A) and brass (B), which were exposed to fuels containing 250 mg⋅kg-1 of peroxides during the static immersion tests: (1) – E40, (2) – E60, (3) – E85 It was shown that the E60 fuel can act as a significantly aggressive environment. The solubility of oxygen in fuels is dependent on the (i) composition of the used gasoline and (ii) ethanol content; oxygen can significantly support the corrosion of metals, as it contributes to depolarization reactions during corrosion. The solubility of oxygen is significantly higher in gasolines than in ethanol, from which it follows that the solubility of oxygen will decrease with 22 ACS Paragon Plus Environment

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the increasing content of ethanol in mixtures with gasolines.52 Unlike this, the solubility of water increases with the increasing content of ethanol in the mixtures.53-54 Water can be formed in higher amounts during fuel oxidation and it can act as a corrosion agent on many metallic materials. An obvious reason of the considerable aggressiveness of the oxidized E60 fuel can be the complementary effect of the sufficient solubility of water and oxygen and also the low thermos-oxidative stability of this fuel which results in the formation of aggressive products such as peroxides, acidic substances, water, etc. The oxidized fuels exhibited significantly lower aggressiveness on mild steel and aluminum in comparison with the contaminated fuels.49 For copper and brass, an opposite trend was observed. Copper and brass resisted corrosion more in the contaminated fuels than in fuels already with a mild degree of oxidation and low content of peroxides. 4.

CONCLUSIONS It is well known that gasolines with higher contents of bioethanol significantly influence

the corrosion of ferrous and nonferrous metallic materials that come into contact with such fuels during storage, transportation, etc. Bioethanol can affect the thermo-oxidative stability of fuels and shorten their shelf-life. After the expiration of the storage life of the fuels or under their thermal stress, significant amounts of oxidation products can be generated which can have a corrosive effect on different metallic materials. In this work, we demonstrated the influence of even a low degree of fuel oxidation on the corrosion aggressiveness of the fuel on mild steel, aluminum, copper and brass. In addition, our results indicate that with the increasing degree of oxidation, the corrosion rate of metallic materials significantly increases. The highest corrosion rate was observed for mild steel, copper and brass in the oxidized E60 fuel. For ferrous metals, acidic substances demonstrably act as corrosion agents. We observed an increase of the corrosion 23 ACS Paragon Plus Environment

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Page 24 of 50

rates of steel and aluminum with the increasing TAN of fuels. For copper and its alloys, acidic substances were weaker corrosion agents than peroxides, which are precursors of acidic substances and even a low amount of peroxides caused a significant corrosion effect. The content of 250 mg⋅kg-1 of peroxides can cause (i) an increase of the instantaneous corrosion rate by up to two orders of magnitude, (ii) a significant decrease of polarization resistance and (iii) an increase of the solubility of material in fuels. It is obvious that with the increasing PV of fuels, the corrosion rate of copper and its alloys increases significantly. The oxidation products can influence the conductivity of fuels and also the course of corrosion. Both applied electrochemical methods (polarization, EIS) were proven to be complementary and convenient to measure the properties of the metal-fuel systems based on ethanol. EIS performed in a planar electrode arrangement is a method applicable even in low-conductive environments such as petrol and fuels with lower contents of ethanol that do not exhibit properties of an electrolyte. Despite this, this method makes it possible to obtain information on the transmission properties of gasoline and non-ethanol fuels. In some cases, corrosion information can be obtained for highly oxidized fuels containing 10 vol. % of ethanol, e.g., on mild steel or after a longer period of exposure on copper or brass. 5. 1.

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ABBREVIATIONS EGBs

ethanol-gasoline blends

EIS

electrochemical impedance spectroscopy

ETBE

ethyl tert-butylether

FCC

fluid catalytic cracking

PV

peroxide value

TAN

total acid number

AUTHOR INFORMATION

Corresponding author Telephone: +420220444238. Fax: +420220444321. E-Mail: [email protected] ORCID Lukáš Matějovský: 0000-0002-0248-4809 Martin Staš: 0000-0002-3106-8601


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ACKNOWLEDGEMENT

This research was funded from the institutional support for the long-term conceptual development of the research organization (company registration number CZ60461373) provided by the Ministry of Education, Youth and Sports, the Czech Republic and the “National Program of Sustainability” (NPU I LO1613, MSMT-43760/2015).


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Energy & Fuels

Table 1:

Selected Properties of the Freshly Prepared, Aged, Artificially Oxidized and Contaminated Fuels Mass fraction

Fuel

PV (mg⋅⋅kg-1)

E25

TAN (mg KOH⋅⋅g-1) 0.0082

22.5

Water content (mg⋅⋅kg-1) 360

E40

0.0083

17.2

520

E60

0.0096

9.2

550

E85

0.0120

3.8

756

E100

0.0160

–

774

E25 after 6 weeks

0.0547

110

431

E40 after 6 weeks

0.0698

127

595

E60 after 6 weeks

0.0666

151

690

E85 after 6 weeks

0.0412

147

1020

E10/96 hours

5.40

1997

9318

E25/72 hours

4.74

2651

9866

E40/48 hours

4.63

4616

11066

E60/48 hours

5.55

4235

13111

E85/120 hours

0.99

945

2981

-1

0.1420

252

475

-1

0.1350

250

610

-1

E40 + 250 mg⋅kg PV

0.2304

252

1122

E60 + 250 mg⋅kg-1 PV

0.5672

249

2073

0.3023

251

2350

E10 + 250 mg⋅kg PV E25 + 250 mg⋅kg PV

-1

E85 + 250 mg⋅kg PV

Remark

freshly prepared unoxidized fuels

6 weeks aged unoxidized fuels

artificially oxidized fuels

(contaminated) fuels containing 250 mg⋅kg-1 PV

Table 2:

Elemental Composition of the Metallic Materials

Material

Elemental composition

Mild steel

0.16 % C, 0.032 % P, 0.028 % S, balance Fe

Stainless steel AISI 304

71.4 % Fe, 18.22 % Cr, 8.11 % Ni, 2.1 % Mn, 0.028 % C

Aluminum

99.5 %, 0.049 Cu, 0.014 % Mg, 0.099 % Si, 0.011 % Mn, 0.057 % Zn, 0.301 % Fe

Copper

99.9 %, 0.031 % Zn, 0.021 % Ni, 0.048 % Fe

Brass

34.3 % Zn, 0.011 % Ni, 0.031 % Fe


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Table 3:

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Overview of the Experiments and Used Metallic Materials Method/material

Fuel

Polarization + EIS, threeelectrode arrangement

EIS – planar, twoelectrode arrangement

fresh fuels

mild steel, stainless steel, Al, Cu, brass


–

aged fuels

–


–

mild steel, stainless steel, Al

mild steel, stainless steel, Al

–

Cu, brass

Cu, brass

Cu, brass

artificially oxidized fuels contaminated fuels

Table 4:

Static test

Measured Instantaneous Corrosion Rates of Metallic Materials in Fresh and Oxidized Fuels Corrosion rate (µ µm⋅⋅year-1)

Fuel

Mild steel

Aluminum

Copper

Brass

E40

3.8

7.3

5.2

3.7

E60

4.4

9.2

5.4

4.5

E85

6.8

10.3

7.2

4.7

E100

2.4

2.9

7.7

27.1

E25/72 hours

5.7

18.2

–

E40/48 hours

7.8

30.5

–

– –

E60/48 hours

10.1

52.5

–

–

E85/120 hours

8.4

38.2

–

–

E40 + 250 mg⋅kg-1 PV

–

–

40.9

53.4

-1

–

–

84.3

211.5

-1

–

–

47.0

22.2

E60 + 250 mg⋅kg PV E85 + 250 mg⋅kg PV

Table 5:

Resistivities of Fuels after 3 Hours of Exposure in a Planar Electrode Arrangement

Fuel

Resistivity (kΩ Ω⋅m) Mild steel

Stainless steel

Aluminum

Copper

Brass

E10/96 hours

1146

1058

519

–

–

E25/72 hours

73.50

70.70

35.40

–

–

E40/48 hours

17.25

17.44

8.82

–

–

E60/48 hours

8.34

8.58

5.60

–

–

E85/120 hours

7.83

7.17

5.12

–

–


Page 33 of 50 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

–

–

–

8350

6340

E25 + 250 mg⋅kg PV

–

–

–

153.4

97.90

E40 + 250 mg⋅kg-1 PV

–

–

–

9.77

13.34

-1

–

–

–

5.01

7.56

-1

–

–

–

3.58

5.17

E10 + 250 mg⋅kg-1 PV -1

E60 + 250 mg⋅kg PV E85 + 250 mg⋅kg PV

Table 5:

The Corrosion Rate of Brass and Copper in Fuels Containing 250 mg⋅kg-1 of Peroxides Fuel

Corrosion rate (µ µm⋅⋅year-1) Brass

Copper

E40

12.0

4.0

E60

17.8

15.6

E85

10.6

8.6


Energy & Fuels

2

10

E40

E60

E100

1

10

0

-2

i (µ A.cm )

10

-1

10

-2

10

-3

10

-4

10

(a) -0.5

-0.4

-0.3

-0.2

-0.1

0.0

0.1

0.2

E vs. SCE (V)

2

10

oxE25/72 hours oxE60/48 hours


1

10

0

10 -2

i (µ A.cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 50

-1

10

-2

10

-3

10

-4

10

0.0 0,0

0.1 0,1

0.2 0,2

0.3 0,3

0.4 0,4

E vs. SCE (V) 34 ACS Paragon Plus Environment

0.3 0.5 0,5

0.6 0,6

(b) 0.7 0,7

Page 35 of 50

2

10

E40

E60

E85

E100

1

10

0

-2

i (µ A.cm )

10

-1

10

-2

10

-3

10

-4

10

(c) -0.5 2

10

-0.4

-0.3

-0.1

-0.2

E vs. SCE (V)

-1

0.0

0.1

0.2

-1

E40 + 250 mg.kg PV

E60 + 250 mg.kg PV

-1

E85 + 250 mg.kg PV 1

10

0

10

-2

i (µ A.cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

-1

10

-2

10

-3

10

(d) 0.0 0,0

0.1 0,1 E vs. SCE (V) 35 ACS Paragon Plus Environment

0.2 0,2

0.3 0,3

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1: Tafel curves of: (a) – mild steel in unoxidized fuels, (b) – mild steel in oxidized fuels, (c) – copper in unoxidized fuels, (d) – brass in fuels containing 250 mg⋅kg-1 of peroxides


Page 36 of 50

Page 37 of 50

600

1 hour

6 hours

24 hours

0.1 Hz

2

-jZimag (kΩ .cm )

400

1 Hz

0.01 Hz

200

(a)

0 0

4000

200

400 2 Zreal (kΩ.cm )

600

800

freshly prepared unoxidized fuels 6 weeks aged unoxidized fuels oxidized fuels

3000

2

-jZimag (kΩ .cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

2000

1000

0

(b) 0

1000

2000

3000 2

Zreal (kΩ.cm )


4000

5000

Energy & Fuels

2000

-1

E40 + 250 mg.kg PV -1 E60 + 250 mg.kg PV -1 E85 + 250 mg.kg PV

1500

2

-jZimag (kΩ.cm )

1000

500

0

(c) 0

500

1000

1500

2000

2500

2

Zreal (kΩ.cm ) 15

1 hour

6 hours

24 hours

30 hours

10 2

-jZimag (MΩ.cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 50

5

0

(d) 0

5

10

2

Zreal (MΩ.cm ) 38 ACS Paragon Plus Environment

15

20

Page 39 of 50

80

1 hour

6 hours

24 hours

48 hours

2

-jZimag (kΩ .cm )

60

40

20

0

(e) 0

20

40

60

80

2

100

120

Zreal (kΩ.cm ) 6000

1 hour

3 hours

6 hours

24 hours

30 hours

5000

4000 2

-jZimag (kΩ.cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

3000

2000

1000

0

(f) 0

2000

4000

6000

2

Zreal (kΩ.cm )


8000

10000

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 50

Figure 2: Nyquist diagram of impedance: (a) – copper in gasoline base in dependence on time, (b) – mild steel in the unoxidized fuels and in the oxidized E25 fuel after 3 hours of exposure, (c) – copper in fuels containing 250 mg⋅kg-1 of peroxides after 6 hours of exposure, (d) – mild steel in the oxidized E10 fuel in dependence on time, (e) – copper in the E10 fuel containing 250 mg⋅kg-1of peroxides in dependence on time, (f) – brass in the E25 fuel containing 250 mg⋅kg-1 of peroxides in dependence on time.

Rfuel a)

Rfuel

Rp

CPEfuel

CPEdl

b)

CPEfuel

Figure 3: Equivalent circuits used to evaluate the impedance spectra shown in Figure 2: a) Impedance measured in gasoline and the E10 fuel, with Rfuel representing the resistance of the environment and CPEfuel representing the spatial capacitance of the environment. (b) Impedance measured in the EGB in the presence of contaminants, with Rp representing the polarization resistance and CPEdl representing the capacitance loss of the double layer. Reprinted with permission from ref.49. Copyright 2017 American Chemical Society.


Page 41 of 50

30000

mild steel

304ss

Al

Cu

brass

(a)

25000

2

Rp (kΩ .cm )

20000

15000

10000

5000

0

25

40

60

85

100

ethanol (vol. %)

30000

mild steel

304ss

Al

Cu

(b)

brass

25000

20000 2

Rp (kΩ .cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

15000

10000

5000

0

25

40

60

ethanol (vol. %)


85

Energy & Fuels

Figure 4: The polarization resistance of metallic materials after (a) 3 hours of exposure in dependence upon the ethanol content in unoxidized fuels and (b) fuels after 6 weeks aging

7

25

10

εr, fresh

εr, 6 weeks

εr, oxidation

R

R

R

S, fresh

20

S, 6 weeks

6

10

S, oxidation

2

Rs (kΩ .cm )

5

10 15

4

10

εr

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 42 of 50

10

3

10

2

10

5

1

10 0

0

10 0

20

40

60

80

ethanol (vol. %)

Figure 5: Permittivity and resistance of unoxidized and oxidized fuels and fuels after 6 weeks of aging. Measurements performed with stainless steel electrodes after 3 hours of exposure in a two-electrode planar cell.


Page 43 of 50

40000

oxE10/96 hours oxE40/48 hours oxE85/120 hours


30000

2

Rp (kΩ .cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

20000

10000

0 0

5

10

15

20

25

time (hours)

Figure 6: The time dependence of polarization resistance of aluminum measured in the oxidized fuels


Energy & Fuels

E10

E25

E40

10

15

20

E60

E85

5

10

4

10 2

Rp brass (kΩ .cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 44 of 50

3

10

2

10

1

10

0

5

25

30

time (hours)

Figure 7: The time dependence of polarization resistance of brass in fuels with 250 mg⋅kg-1 of peroxides


Page 45 of 50

14000

(a)


2

Rp mild steel (kΩ.cm )

12000

10000

8000

6000

4000

2000

0

14000

12000

E25

E40

E60

E85


(b)

10000 2

Rp Al (kΩ .cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

8000

6000

4000

2000

0

E25

E40

E60


E85

Energy & Fuels

30000

(c)


20000

2

Rp 304ss (kΩ .cm )

25000

15000

10000

5000

0

12000

10000

E25

E40

E60

E85

freshly prepared unoxidized fuels 6 weeks aged unoxidized fuels -1 fuels containing 250 mg.kg PV

(d)

8000

2

Rp brass (kΩ .cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 46 of 50

6000

4000

2000

0

E25

E40

E60


E85

Page 47 of 50

12000

10000

freshly prepared unoxidized fuels 6 weeks aged unoxidized fuels -1 fuels containing 250 mg.kg PV

(e)

8000 2

Rp Cu (kΩ .cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

6000

4000

2000

0

E25

E40

E60

E85

Figure 8: The comparison of polarization resistances of mild steel (a), aluminum (b), stainless steel (c), brass (d), and copper (e) that were measured by EIS after 3 hours of exposure in unoxidized and artificially oxidized fuels


Energy & Fuels

30

brass, E85 Cu, E85

brass, E60 Cu, E60

brass, E40 Cu, E40

25

-2

weight loss (g.m )

20

15

10

5

0 0

200

400

600

800

1000

1200

1400

time (hod)

10 9 8 7 -2

weight loss (g.m )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 48 of 50

6 5 4 3 2 1 0 0

50

100

150

200

time (hod)


250

300

Page 49 of 50 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Figure 9: The time dependence of the weight loss relative to the area of the copper and brass samples during the static immersion tests in the E40, E60 and E85 fuels containing 250 mg⋅kg-1 of peroxides. Graph on the bottom – the initial course of the experiment.

Figure 10: The coloring of fuels: E40 (left), E60 (middle), E85 (right) containing 250 mg⋅kg-1 of peroxides tested on copper


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(A)

(1)

Page 50 of 50

(B)

(2)

(3)

(1)

(2)

(3)

Figure 11: The comparison of the surfaces of the samples of copper (A) and brass (B), which were exposed to fuels containing 250 mg⋅kg-1 of peroxides during the static immersion tests: (1) – E40, (2) – E60, (3) – E85


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