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Study of Corrosion of Metallic Materials in EthanolGasoline Blends: Application of Electrochemical Methods Lukáš Mat#jovský, Jan Macák, Milan Pospíšil, Petr Baroš, Martin Staš, and Aneta Krausová Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01682 • Publication Date (Web): 28 Aug 2017 Downloaded from http://pubs.acs.org on September 6, 2017

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STUDY OF CORROSION OF METALLIC MATERIALS IN ETHANOL-GASOLINE BLENDS: APPLICATION OF ELECTROCHEMICAL METHODS

Lukáš Matějovský a, Jan Macák b, Milan Pospíšil a, Petr Baroš a, Martin Staš a,*, Aneta Krausováb

a

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

Technology Prague, Technická 5, 166 28 Prague 6 b

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

Technická 3, 166 28 Prague 6 E-mail: [email protected]

Keywords: corrosion, static immersion test, bioethanol, ethanol-gasoline blends, biofuels, material compatibility, instantaneous corrosion rate, electrochemical methods

ABSTRACT. Ethanol-gasoline blends (EGBs) can easily absorb large amounts of water due to the presence of ethanol. In water, acidic compounds and ions can be dissolved and these substances can have corrosive effect on metallic construction materials. With the increasing content of ethanol in fuels, the conductivity and ability of fuel to absorb water increases and such fuel is becoming more corrosive. In this work, we tested E10, E40, E60, E85 and E100 fuels that were prepared in laboratory. These fuels were purposely contaminated by water and trace amounts of ions and acidic substances. The aim of the contamination was to simulate pollution of fuels, which may come from raw materials or from failure to comply with good manufacturing, storage and transportation conditions. Corrosion properties of these fuels were tested on steel, copper, aluminum and brass using electrochemical impedance spectroscopy and

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Tafel curves analysis. For a comparison, static immersion test on steel was performed. The main parameters for the comparison of corrosion effects of the tested fuels were instantaneous corrosion rate, polarization resistance and corrosion rate which was obtained from the weight losses during the static test. 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, European Commission adopted the program for the utilization of alternative fuels for transport [1]. According to the directive 2003/30/EC, 8 % of fossil transportation fuels should be substituted by biofuels by 2020. Bioethanol produced from biomass is one of the promising biofuels. In the US, E10 and E85 fuels are widespread. These fuels contain 10 and 85 vol. % of bioethanol, respectively. Bioethanol is mostly used in Brazil, where about 20 % of cars burn pure bioethanol (E100) and the rest of the car-fleet is adapted to use E22 and E85 fuels [2]. In the EU, the content of bioethanol in transportation fuels is limited by legislation up to 2.7 wt. % of oxygen and 5 vol. % of ethanol [3]. In the Czech Republic, E85 fuel and gasolines with up to 5 vol. % of bioethanol have been used. These fuels have to meet the requirements of ČSN EN 228 and ČSN P CEN/TS 15293 norms. Due to the introduction of pure and mixed fuels, high demands are given on quality and material compatibility of these fuels (blends). Ethanol for blending with gasoline (ČSN EN 15376) can contain up to 0.3 wt. % of water, 0.007 wt. % of acidic substances (expressed as acidic acid) and 20 mg·kg-1 of chlorides. Other properties such as water content or total acid number can also be used as indicators of 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


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influences dielectric and conductive properties of the resulting EGBs and hereby, a risk of galvanic corrosion increases [28]. Especially 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 a high content of ethanol. EGBs with the ethanol content up to 10 vol. % should not exhibit such problems [4]. There is a high number of publications and patents, in which corrosive properties of different materials have been studied [6,7]. Some patents describe different laboratory tests to simulate material corrosion in a flowing apparatus, in liquids or in vapors [8]. There is also a gravimetric method for the evaluation of corrosion effects [29]. Several publications deal with the utilization of electrochemical methods to describe corrosion processes in metal-fuel systems [9-27]. For instance, there were also published some studies of influence of chlorides, water, pH and content of oxygen dissolved in ethanol and EGBs upon the corrosion of low-alloy steel and aluminum steel, for which different corrosion potential measurement, cyclic potentiodynamic polarization and impedance spectroscopy in two- and three-electrode arrangements were used [9-27]. Here, we used electrochemical methods to determine the corrosion resistance of metallic construction materials of fuel system in the environment of EGBs. The studied blends were purposely contaminated by water and trace amounts of other substances with corrosive properties (chlorides, sulfates, acidic acidic). This contamination should have simulated a real contamination of EGBs in fuel systems, which may be caused due to 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 determination and comparison of the corrosion effects of EGBs containing from 10 to 100 vol. % of ethanol.


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2.

EXPERIMENTAL

2.1

Preparation of ethanol-gasoline blends (EGBs) For the preparation of EGBs with the ethanol content ranging from 10 to 85 vol. %,

we used a gasoline base and ethanol with the water content of 700 mg·kg-1. The gasoline base was prepared from gasoline pool fractions (i.e. reformate, isomerate and FCC gasoline) that were obtained from Česká rafinérska (Kralupy nad Vltavou, Czech Republic). A volume ratio of reformate, isomerate and FCC gasoline was calculated so that the content of saturated, unsaturated and aromatic hydrocarbons meets the requirements of the standards [30]. The group-type composition of the prepared gasoline base (53 vol. % of saturated, 12.1 vol. % of unsaturated and 34.9 vol. % of aromatic hydrocarbons) was verified by gas chromatography. In the prepared EGBs, chlorine content can be neglected, because we used high-purity chemicals and gasoline fractions for the preparation of samples. For the preparation of contaminated fuels, we used E100 fuel with the following composition: 94 vol. % of ethanol (p.a.), 6 vol. % of water and trace amounts of contaminants (50 mgl-1 of CH3COOH, 3 mgl-1 of NaCl, 2.5 mgl-1 of H2SO4 and 2.5 mgl-1 of Na2SO4). A fuel with the identical composition was also used in other study [9]. A stock solution was prepared for the contamination of fuels. This stock solution was added to EGBs to achieve the desired water content and to avoid separation of fuel into two phases (aqueous and organic). 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 Corrosiveness of the model fuels was tested on mild steel (0.16 % C, 0.032 % P, 0.028

% S, bal. 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 % of zinc, 0.011 % Ni, 0.031 % Fe), copper (99.9 %, 0.031 % Zn, 0.021 % Ni, 0.048 % Fe) and stainless steel AISI 304 (18.22 % Cr, 8.11 % Ni, 2.1 % Mn, 0.028


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% C). Prior to the experiments, surfaces of metallic samples and prepared electrodes were polished by sandpaper (1200 grit) under running water, cleaned by demineralized water and degreased by ethanol and acetone.

2.3

Electrochemical methods Measurements were performed using VOLTALAB 40 PGZ 301 (Radiometer) and

Reference 600 (Gamry) potentiostat. All measurements were performed in an earthed Faraday cage. A measuring sequence included the stabilization of corrosion potential for 60 minutes and measurement of impedance spectrum and polarization characteristics. In an environment of high resistivity (gasoline, E10, see Results and Discussion), only impedance spectra were measured after the stabilization of potential. Electrochemical measurements were performed both in a two- and three-electrode arrangement. In the three-electrode system, the working electrode had cylindrical shape with a diameter of 5 mm and length of 15 mm. The working electrode was screwed onto a holder sealed by silicon rubber that served as an electrical contact. A total exposed area was 2.6 cm2. Platinum mesh coaxial to working electrode served as an auxiliary electrode. A saturated calomel electrode (SCE) with a salt bridge containing 0.1 M solution of potassium nitrate was used as a reference electrode. When clogging of the salt bridge frit was encountered, a solution of lithium chloride in ethanol was used instead. To determine dielectric and conductive characteristics of environments, a symmetric two-electrode planar system was applied. Electrodes were made from metal sheets (3x4 cm) equipped with soldered contact. Soldered area back of the sheets and edges were embedded into epoxide resin. The exposed area of each electrode was 12 cm2 and the distance between them was 1 mm. The cell constant Ks for each electrode couple was determined on n-heptane, which has permittivity of about 1.92, according to the following equation:


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𝐾𝑠 =

𝜀𝑟 𝜀0

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(1)

𝐶

where C is capacitance obtained from the high-frequency part of the impedance spectrum measured in a planar electrode arrangement in n-heptane-metal system; εr is relative permittivity of n-heptane and ε0 is relative permittivity of vacuum. Cell constant Ks was used for the calculation of fuel permittivity ε and for the recalculation of resistivity R according to the following equations:

𝜀=

𝐾𝑠 𝐶𝑣

𝑅𝑠 =

(2)

𝜀0 𝑅𝑣

(3)

𝐾𝑠

where Cv is capacitance obtained from the high-frequency part of the impedance spectrum measured in the metal-fuel system; Rv is measured resistivity of environment for the given metal-fuel system. For impedance measurements, an amplitude range was chosen to be from 5.0 to 70.0 mV in dependence upon the conductivity of studied environment and corresponding total cell impedance. For higher values of amplitude, the linearity of response was verified. Measurements were performed mostly in the frequency range of about 1 MHz to 1 mHz. For the measurement of polarization curves, the polarization range was from -0.3 V to +0.3 V relative to corrosion potential and polarization rate was 0.5 mV·s-1. Post-run IR drop compensation was performed with polarization characteristics using ohmic resistance values estimated by high frequency impedance measurements. Tafel coefficients were estimated from the anodic and cathodic parts of polarization curves. From the Tafel coefficients, polarization resistance and electrode area, corrosion current density was calculated according to the Stern-Geary equation:

𝑗𝑐𝑜𝑟𝑟 =

𝐵

(4)

𝑅𝑝


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𝐵=

𝑏𝑎 .𝑏𝑘

(5)

2,3(𝑏𝑎 +𝑏𝑘 )

where jcorr is corrosion current density, ba and bk are Tafel constants and Rp is polarization resistance estimated from EIS measurements. Alternatively, corrosion currents were found as the intersection of extrapolated Tafel lines. Subsequently, from Faraday law, corrosion rate was calculated from corrosion current density as follows:

m  A I t

(X)

𝑀

𝐴 = 𝑧𝐹

(X)

where m is mass of substance, I is current, t is time, A is proportionality constant designated as the

electrochemical

equivalent

of

substance,

measured

in

kgC-1, F is

Faraday

constant (9.6485104 Cmol−1), z is number of electrons needed to exclude one molecule

2.4

Static corrosion test Static tests were performed according to the modified method that is described in

ASTM D130-04. This method has been used as a complementary and comparative method to electrochemical measurements. The method is based on the exposure of metallic material for a certain time in a studied sample at a given ratio of the (i) volume of studied sample and (ii) surface area of metallic material. The ratio was chosen to be 9 ml of fuel on 1 cm2 of surface area of metallic material in order not to achieve the stabilization of corrosion rate too soon. Static tests were performed for a long-term period (1200 hours) in a closed 250 ml bottle at ambient temperature without oxygen access; the bottle contained 200 ml of the corrosion environment and three testing samples of carbon steel having the same surface area. The metallic samples were in certain time periods weighed. From the time dependence of weight losses, corrosion rate was calculated according to equations 6 and 7.

𝑣𝐿𝑟 = 8,76

𝑣𝑃𝑚

(6)

𝜌


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𝑣𝑃𝑚 =

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∆𝑚

(7)

𝑇.𝑆

where vPm is corrosion rate in g.m-2.h-1, ρ is density of metallic material in g.cm-3, m is average weight loss in g, S is surface of metallic material in m2 and T is period (in hours) from the beginning of the test to removal of the metal plate

After the experiment, the appearance of surfaces of metallic samples tested in fuels were compared visually.

2.5

Total acid number (TAN) measurement A TAN measurement was performed to compare the aggressiveness of the fuels

amongst each other. The measurement was performed according to ASTM D664-89 on an automatic titrator DMS TITRINO 716 with the potentiometric detection of the point of equivalence. The TAN of EGBs without contamination increased in the range of 0.008– 0.016 mg KOHg-1 on the content of ethanol in fuel from 10–100 % vol. The results are presented in Table 1. Table 1: TAN of the prepared contaminated EGBs TAN (mg KOHg-1)

Fuel E10 + 0.5 % H2O

0.0155

E40 + 3.4 % H2O

0.0196

E60 + 3.4 % H2O

0.0210

E60 + 6 % H2O

0.0251

E85 + 3.4 % H2O

0.0242

E85 + 6 % H2O

0.0312

E100 + 6 % H2O

0.0334


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3.

RESULTS AND DISCUSSION

3.1.

Instantaneous corrosion rate Figure 1 shows Tafel curves of mild steel in the non-contaminated E40, E60 and E100

fuels. Corrosion potential of mild steel increased with the increasing content of ethanol in fuel. In these fuels, current density was measured to be 0.18–0.32 Acm-2. The lowest value of current density (0.18 Acm-2) was measured in the E100 fuel, to which corresponds the lowest 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 Tafel curve, where no increase of current density with the increasing anodic potential can be observed (see Figure 1, blue curve); moreover, anodic current was lower than cathodic current. E40

10

-2

i (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

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E60

E100

0

10

-1

10

-2

10

-3

-0,5

-0,4

-0,3

-0,2

-0,1

0,0

0,1

0,2

E vs. SCE (V)

Figure 1: Tafel curves of mild steel measured in pure fuels E40, E60 and E100.


1

10

0

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-2

10

10

-1

10

-2

10

-3

steel, E60 steel, E60 + 6 % H2O Al, E60 Al, E60 + 6 % H2O

-1,0

-0,8

-0,6

-0,4

(a) -0,2

0,0

E vs. SCE (V)

-2

i (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

i (A.cm )

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10

1

10

0

10

-1

10

-2

10

-3

Cu, E60 Cu, E60 + 6 % H2O brass, E60 brass, E60 + 6 % H2O

-0,4

-0,2

0,0

(b) 0,2

E vs. SCE (V)

Figure 2: Tafel curves: steel and aluminum (a) and copper and brass (b) in pure E60 fuel and contaminated E60 fuel with 6 % H2O

For EGBs without contamination and with the ethanol content 40 vol. %, it was not possible to measure polarization data due to a high environment resistance. The difficulty of data measurement for the E40 fuel is also evident from the course of Tafel curve, which is not as smooth as other curves that were measured in blends with a higher ethanol content.


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From the comparison of Tafel curves of copper, brass, aluminum and mild steel in the non-contaminated and contaminated E60 fuels (Figure 2), a very low corrosion potential of aluminum (from -900 to -700 mV) is evident. The corrosion potential of brass and mild steel in an environment of the non-contaminated E60 fuel varies in significantly higher values (from 300 to -150 mV). The contamination of E60 fuel had effect on the increase of the corrosion potential and current density of metallic materials, for which also significantly higher corrosion rates were observed. For copper and brass, contamination of fuel resulted in the shift of the corrosion potential towards positive values; higher value of corrosion potential was observed for copper due to its higher nobility in comparison to brass. The highest current densities in the contaminated E60 fuel were observed for mild steel and brass (see Figure 2). Instantaneous corrosion rates of mild steel and aluminum in the non-contaminated fuels is raised with the increasing content of ethanol up to 85 vol. %; the lowest corrosion rate was measured in the E100 fuel (see Table 2). For copper and brass, corrosion rate raised with the increasing content of ethanol in the whole range (up to E100). A different behavior of mild steel and aluminum was probably caused due to their passivation ability in an environment of the absolute ethanol (E100); copper and brass do not have such ability. Such low values also indicate the low influence of dry corrosion of absolute ethanol on mild steel and especially on less noble aluminum. Dry corrosion is associated with the low nobility of metals, when in an environment of dry ethanol, an alcoholate is formed on the surface of less noble metal. The alcoholate is soluble in the environment and the metal is not protected against corrosion by a passive surface layer [31]. Despite this, an important influence of dry corrosion cannot be ruled out especially for a long-term period. The corrosion data of the contaminated fuels on the studied materials indicate that the contaminated E60 fuel containing 6 vol. % of water had the most aggressive effect on all tested metallic materials. In the contaminated fuels, corrosion rates were four to ten times higher in comparison to the non-contaminated fuels. It is obvious that


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water and ion contents had a significant influence upon the aggression of fuels. Corrosion rate of materials increased with the water content as well as with the ethanol content, but only up to 60 vol. % of ethanol.

Table 2:

Measured instantaneous corrosion rates of metallic materials in pure and contaminated fuels calculated from the Stern-Geary equation Corrosion rate (myear-1)

Fuel Steel

Aluminium

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

E40+3.4 % H2O

14.5

38.0

20.7

21.9

E60+3.4 % H2O

19.1

16.5

28.2

25.0

E60+6 % H2O

45.7

48.0

40.4

40.6

E85+3.4 % H2O

12.7

27.0

33.5

34.4

E85+6 % H2O

42.0

34.2

37.9

37.3

E100+6 % H2O

3.8

31.7

44.5

10.1

3.2.

Electrochemical impedance spectroscopy Impedance spectra are presented in Figure 3 in the form of Nyquist diagrams. The

impedance spectra had in the complex plane in all cases the shape of one or two half circles with the center below the real axis. Evaluation of half circles was performed by approximation of experimental data by impedance of parallel connection of resistance and constant phase element (CPE), which can be related by the following equation: 12 ACS Paragon Plus Environment

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𝑍=

𝑅 1 + 𝑅𝑄(𝑗𝜔)𝑛

(8)

where R denotes parallel resistance, Q is CPE coefficient, n is CPE exponent,  is angular frequency and j is imaginary unit.

Interpretation of R and Q depends upon (i) the type of corrosive environment and (ii) fact whether it is high-frequency or low-frequency part of the spectra. Figures 3a and 3b can be used for an illustration. The measurement presented in Figure 3a was performed using very high amplitude of perturbation signal (70 mV) in a planar, two-electrode system on copper in pure gasoline that was used for the preparation of EGBs. The gasoline base had a high environment resistance, which was several orders higher than that of EGBs. Naturally, the gasoline base does not have properties of electrolyte and at the phase boundary, no electrical double layer is formed. Spectrum consisted of a single half circle and its high frequency tail started in the coordinate origin of the complex plane. Elements P and Q are to be interpreted as resistance of environment (gasoline) and non-ideal environment capacitance (gasoline bulk capacitance). From their values, information relating solely to the medium (fuel resistivity and permittivity) can be obtained. Figure 3b presents spectra measured in a two-electrode system for different gasolineethanol ratios with addition of water and corrosive contaminants. In this case, electrolytic properties of the environment are exhibited due to the presence of ions and higher polarity of the environment and the spectrum has shape of two relatively well separated half circles. The high frequencies capacitive loop refers about the properties of an environment whereas the lowfrequency one expresses the impedance response of electrical double layer capacitance at phase boundary and parallel polarization resistance. Polarization resistance is a basic corrosion


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variable that characterizes the instantaneous corrosion rate. Equivalent circuits expressing both types of impedance behavior are presented in Figure 4.

-Zimag (k.cm2)

600 1 hour 6 hours 24 hours

0.1 Hz

400

1 Hz

200

0.01 Hz

0

(a)

0

200

400 Zreal (k.cm2)

600

E40

400

800

E60

E85

2

-Zimag (k.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

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0,01 Hz

200 1 Hz 100 Hz

(b)

0 0

200

400

600

2

Zreal (k.cm )

Figure 3: Nyquist diagram of impedance of copper in a gasoline base in dependence upon time (a) and of mild steel in fuels with 3.4 vol. % of water and 40–85 vol. % of ethanol (b).

Rfuel

CPEfuel

14

Rfuel

Rp

CPEfuel

CPEdl

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Figure 4: Equivalent circuits used to evaluate the impedance shown in Figures 3a and 3b a) impedance measured in

gasoline and E10 fuel, Rfuel – resistance of

environment, CPEfuel – spatial capacitance of environment b) impedance measured in EGB in the presence of contaminants, Rp – polarization resistance, CPEdl – loss capacitance of double layer

Time dependences of polarization resistances measured in contaminated fuels (presented in Figure 5a) reveal a relatively fast stabilization (within 24 hours). The progression of value of polarization resistance predicates the corrosion rate, which in an environment of E40–E85 fuels with a water content of 3.4 vol. % proceeds at constant rate until equilibrium metal–fuel occurs and layer of corrosion products (that protects metal) is created. The measurements also revealed that the contaminated E60 fuel was the most aggressive environment, in which the lowest polarization resistance (the highest instantaneous corrosion rate) was observed.

1000

E40

E50

E60

E85

800

2

Rp (k.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

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600

400

200

0 0

5

10 15 time (hours)

20


25

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40000

E40

E60

E85

30000

2

Rp (k.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

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20000

10000

0 0

5

10 15 time (hours)

20

25

Figure 5: Time dependences of polarization resistance of mild steel (a) and stainless steel AISI 304 (b), exposed in the contaminated fuels with 3.4 vol. % of water

Stainless steel (see Figure 5b) shows very good properties and high resistance to the aggressiveness of the contaminated fuels containing ethanol. The polarization resistance of stainless steel after 24 hours was two orders of magnitude higher than that that of mild steel and grew linearly for 24 hours without any indication of an occurrence of metal-fuel balance, as observed for mild steel (see Figure 5a). It is likely that the polarization resistance of the stainless steel will continue to rise in time until it will stabilize at a very high value. This time course of the polarization resistance of stainless steel indicates its very good passivation capability in contaminated fuels. From the values of polarization resistance presented in Table 3, it is obvious that contaminated E10 fuel was the least aggressive one with the lowest conductivity (see Table 4). E10 was able to absorb the lowest amount of water, which acts as a strong corrosive agent and increases the aggressiveness of fuels. For mild steel in the contaminated E10 fuel, it was not possible to measure polarization resistance even after 24 hours of stabilization. For the contaminated E10 fuel, we reached the limit of feasibility of electrochemical measurements when measuring corrosion in an environment of the EGBs. The highest polarization resistance 16 ACS Paragon Plus Environment

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was observed for stainless steel, which completely resists the corrosive effects of contaminated EGBs.

Table 3:

Polarization resistance of metallic materials in an environment of the contaminated fuels after 6 hours of exposure estimated from the impedance measured in a planar electrode arrangement. Polarization resistance (k.cm2) Fuel Steel

Stainless steel

Copper

Brass

Aluminum

E10+0.5 % H2O

--

15700

6590

3170

9870

E40+3.4 % H2O

543

10529

976

402

2065

E60+3.4 % H2O

251

6953

899

486

2503

E60+6 % H2O

234

8686

388

283

1810

E85+3.4 % H2O

719

9773

575

343

2438

E85+6 % H2O

478

10325

332

246

2890

E100+6 % H2O

1699

6394

900

491

3312

The higher polarization resistance of stainless steel in E85 fuel with a higher water content refers to the increasing passivation ability of stainless steel with the increasing water content in fuel. When comparing the polarization resistances and corrosion rates of mild steel and aluminum, it is obvious that the relatively high polarization resistances of aluminum (see Table 3), which indicate lower corrosion rates, do not correspond to the measured values of instantaneous corrosion rate which were relatively higher, see Table 2. The impedance measurements were performed on electrodes made from aluminum sheet. For the polarization measurements, a cylindrical electrode made from the aluminum bar was used. It is likely that 17 ACS Paragon Plus Environment

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Page 18 of 31

the different results between the polarization resistance of aluminum and its instantaneous corrosion rate (calculated on the basis of the polarization characteristics) can be considerably influenced by the different technology of the production of logs and sheets resulting in different corrosion resistance and surface properties of aluminum. For mild steel, copper and brass, substantially lower polarization resistances were observed in comparison to stainless steel. Polarization resistance of brass and copper decreased with the increasing content of water and ethanol up to 85 vol. %. The lowest value of polarization resistance of mild steel was observed for the E60 fuel, where the most extensive pitting corrosion was observed after the experiment (see Figure 6). Relatively high polarization resistance of mild steel in the contaminated E100 fuel was due to the initial passivation of mild steel, which is indicated by the measured instantaneous corrosion rate (see Table 2) and also by the result of static test on mild steel (see below). For the contaminated E100 fuel, corrosion attack was less extensive in comparison to other fuels. After the experiment, several large pits with a low surface density were observed on the surface of electrode (see Figure 6). On the other hand, for the contaminated E85 fuel, we observed several huge clusters of tiny pits and corrosion spots that localized on part of the surface. Yet, whole area of the electrode was not covered unlike for the E60 fuel with 3.4 vol. %. In this case shallow pits covered most of the surface. In Figure 6 (3) that presents the twelve times zoomed-in part of surface of electrode exposed in the E60 fuel, we can see that corrosion nuclei were formed uniformly in corrosively most active spots – i.e. grooves that were formed by grinding with sandpaper during the treatment of electrode surface prior to the experiment. From this, it follows that a low polarization resistance of mild steel refers to a poor passivation ability and possible development of intensive corrosion as a result of the attack by chlorides in an environment of the contaminated EGBs with 40–60 vol. % of ethanol.


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(1)

(3)

(2)

Figure 6: Comparison of surface of electrode from mild steel: (1) 24 hours of exposition in E100 fuel with 6 vol. % of water, (2) 48 hours of exposition in E85 with 6 vol. % of water, (3) 24 hours of exposition in E60 fuel with 3.4 vol. % of water (zoomed in 12x).

Resistivity of fuels (see Table 4) measured on various metallic materials differed from

(1)

(3)

each other in dependence upon the surface properties of the metallic material. This difference was caused by the resistance of material to corrosion and properties of passive oxide layer on the surface of metal. Stainless steel was minimally soluble in the environments and no significant changes of conductivity and permittivity of environment occurred due to the release of ions. On the surface of stainless steel, a very thin, passive layer of oxides was formed, which was conductive and protected the metal substantially more against corrosion in comparison to other metallic materials and these values were thus least affected by an error.


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Table 4:

Page 20 of 31

Resistivity of the contaminated fuel after 6 hours of exposure in a planar electrode arrangement Resistivity (k.m) Fuel Steel

Stainless steel

Copper

Brass

Aluminum

E10+0.5 % H2O

9100

3000

4300

6380

5010

E40+3.4 % H2O

11.85

10.63

7.15

9.56

8.67

E60+3.4 % H2O

7.13

7.15

5.32

5.61

6.49

E60+6 % H2O

2.65

3.30

2.65

2.93

2.66

E85+3.4 % H2O

3.69

3.85

4.52

4.93

4.11

E85+6 % H2O

2.89

2.99

2.59

2.46

2.35

E100+6 % H2O

3.74

3.23

1.97

3.03

1.99

From the dependence of permittivity and resistivity of the EGBs measured with stainless steel presented in Figure 7, it is obvious that the permittivity increased and the resistivity decreased with the increasing ethanol content in fuel. This also corresponds to a low permittivity of pure gasoline base (relative permittivity of gasoline compounds: pentane 1.84; toluene 2.38) and relatively high permittivity of ethanol (24.3). The highest relative permittivity was measured in an environment of the contaminated fuels, in which also lowest resistivity were observed. This agrees with the fact that with the increasing water content in EGBs, relative permittivity will increase and resistivity of fuel will decrease.


Page 21 of 31

7

30

10 

r, pure



r, cont.

R

S, pure

R

S, cont. 6

10

25

5

4

10 15

Rs (k..m)

10

20

r

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

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3

10 10

2

10 5

1

10 0

0

10 0

20

40

60

80

100

ethanol (vol. %)

Figure 7: Relative permittivity and resistivity of the contaminated fuels and fuels without contamination measured for stainless steel in a planar electrode arrangement

Capacitance of electrical double layer was another corrosion parameter that informs about the surface of metal and course of corrosion processes. In Figure 8, the time dependences of capacitances of metallic materials are compared, which were measured in the contaminated E60 fuel with 3.4 vol. % of water. From the course of dependences, it can be assumed that the stabilization occurred within six hours from the start of experiment for all tested materials. The highest capacitance was measured for mild steel. The capacitance increase for mild steel can substantially be related to the development of local corrosion and increasing concentration of dissolved ionic corrosion products in the environment. The highest capacitance change from the start of experiment was observed for stainless steel. This drop may be related to the increase of polarization resistance in a very short time and formation of passive film on the surface. The


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lowest values of capacitance were measured for copper and brass and these values did not change substantially in time.

30 mild steel

304ss

aluminium

copper

brass

25

20 -2

Cdl (F.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 22 of 31

15

10

5

0 0

5

10 15 time (hours)

20

25

Figure 8: Time dependences of double layer capacitances of metallic materials measured in E60 fuel with 3.4 vol. % of water and trace amounts of impurities

3.3.

Static immersion test on mild steel Time dependences of corrosion decreases in the contaminated fuels that were tested

by the static method are presented in Figure 9. The calculated corrosion rates of steel are presented in Table 5.


Page 23 of 31

20

E100 + 6 % H2O

E60 + 6 % H2O

E85 + 6 % H2O

E60 + 3,4 % H2O

E85 + 3,4 % H2O

E40 + 3,4 % H2O

E10 + 0,5 % H2O

-2

W (g.m )

15

10

5

0 0

400

800

1200

time (hours)

6

-2

W (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

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4

2

0 E100 + 6 % H2O

E60 + 6 % H2O

E85 + 6 % H2O

E60 + 3,4 % H2O

E85 + 3,4 % H2O

E40 + 3,4 % H2O

E10 + 0,5 % H2O

0

20

40

60

time (hours)


80

100

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Figure 9: Time dependence of the weight loss of mild steel samples during the static immersion test. Figure bellow: first 100 hours of exposure

From the results presented in Figure 9, it is obvious that the chosen time period (1200 hours) was sufficient to reach the equilibrium metal-fuel in the static immersion tests. From the comparison of the course of time dependences of corrosion decreases in fuels, it can be assumed that the equilibrium occurred in different times from the start of experiment. Noteworthy is especially the highest increase of steel loss in the early stage of exposure (80 hours) for the E60 fuel with 6 vol. % of water. Noteworthy is also the gradual increase of steel loss with the increasing time in an environment of the contaminated E100 fuel, in which the corrosion rate was very low for the first 50 hours (Figure 9 bellow) and its stabilization occurred after about 1000 h of exposure. This opposite course in comparison to the E60 fuel with 6 vol. % of water evidences the initial passivation, for which almost no steel loss was observed. The subsequent gradual increase in the corrosion rate can be caused by the deterioration of passive layer due to the influence of chlorides, water and corrosion products, which can have catalytic effect on corrosion of steel in a contaminated ethanol. This fact is also evidenced by the measured low instantaneous corrosion rate and high polarization resistance of mild steel in this metal-fuel system. A similar initial course as for E100 fuel was also observed for E85 fuel with 3.4 vol. % of water. For this fuel, the stabilization of corrosion rate of steel occurred in a shorter time and at substantially lower corrosion rate. A significant influence of water on the corrosion rate and its evolution over time can be observed for E85 fuel with the water contents of 3.4 and 6 vol. %, respectively. In case of 6% of water, almost two-times higher corrosion rate of steel was measured and no initial passivation occurred. Generally, corrosion rate increased with the water content and depended significantly also upon the content of ethanol in fuels.


Page 25 of 31

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Table 5:

Steady-state corrosion rates of the steel in the contaminated fuels measured by the static immersion tests Corrosion rate (myear-1)

Fuel E10 + 0.5 % H2O

0.5

E40 + 3.4 % H2O

2.4

E60 + 3.4 % H2O

3.9

E60 + 6 % H2O

6.1

E85 + 3.4 % H2O

2.5

E85 + 6 % H2O

4.8

E100 + 6 % H2O

7.8

It was shown that the fuel with 60 vol. % of ethanol can act as a significantly aggressive environment. The solubility of oxygen is substantially higher in gasoline in comparison to ethanol [32], which implies that it will decrease with the content of ethanol in a blend with gasoline. On the other hand, with the increasing content of ethanol, the possibility of water dissolution increases. E60 fuel is able to dissolve ca. 6 vol. % of water at the ambient temperature [33,34], and high ability to simultaneously dissolve water and oxygen can be the reason of the relatively higher aggressiveness of the contaminated E60 fuel. The aggressiveness of E60 fuel in comparison to other fuels is presented in Figure 10.


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(A)

(1)

(2)

(3)

(4)

(5)

(B)

(E60)

(3)

(E85)

(6)

(4)

(7)

Figure 10: Comparison of surface of the steel, in dependence upon the: (A) ethanol content in fuel and (B) decreasing water content in E60 and E80, after the static immersion tests: (1) E10 + 0.5 vol. %. H2O, (2) E40 + 3.4 vol. % H2O, (3) E60 + 6 vol. % H2O, (4) E85 + 6 vol. % H2O, (5) E100 + 6 vol. % H2O, (6) E60 + 3.4 vol. % H2O, (7) E85 + 3.4 vol. % H2O

4.

CONCLUSION Gasolines containing higher amounts of bioethanol significantly influence corrosion

of ferrous and nonferrous metals. The highest corrosion rate was measured for mild steel, copper and brass in the contaminated E60 fuel. This can be related to the content of water and acidic substances and a solubility of oxygen in fuel, which decreases with the increasing content 26 ACS Paragon Plus Environment

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of ethanol. An initial passivation of the mild steel in the contaminated E100 fuel was demonstrated. With the increasing time a gradual increase in corrosion rate was observed. Very high polarization resistances of stainless steel in all contaminated fuels proved a high resistance of this material to corrosion and passivation ability in high dissolved oxygen concentrations. Thus, stainless steel seems to be a suitable material for the production of fuel system components for ethanol-gasoline fuels with a high ethanol content. The applied electrochemical methods proved to be suitable to measure the properties of metal-fuel interaction. Impedance spectroscopy performed in a planar arrangement was applicable even in extremely lowconductive environments as gasoline and fuels with a low content of ethanol, which did not exhibit properties of electrolyte. In such case charge transfer properties of the fuel (as resistivity and permittivity) can be obtained from impedance data.



ACKNOWLEDGEMENTS

This work was supported from National Program of Sustainability (NPU I LO1613, MSMT43760/2015).

 1.

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R. Battino, T. R. Rettich, T. Tominaga: The Solubility of oxygen and ozone in liquids, Journal of Physical and Chemical Reference Data 1983, 12, 163. 33. Z. Mužíková, M. Pospíšil, V. Polák: Vlastnosti lihobenzínových směsí s vyšším obsahem ethanolu. 13. Ročník konference Reotrib 2007: Kvalita paliv a maziv, Milovy, 16. – 17. 2007, 143-163, ISBN: 978-80-7080-011-9 34. T. M. Letcher, C. Heyward, S. Wootton, B. Shuttleworth: Ternary phase diagrams for gasoline-water-alcohl mictures, Fuel 1986, 65, 891-894


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