Corrosion Science 67 (2013) 50–59
Contents lists available at SciVerse ScienceDirect
Corrosion Science
journal homepage: www.elsevier.com/locate/corsci
Corrosion mechanism of copper in palm biodiesel
M.A. Fazal, A.S.M.A. Haseeb ⇑, H.H. Masjuki
Department of Mechanical Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia
a r t i c l e
i n f o
Article history:
Received 12 September 2012
Accepted 4 October 2012
Available online 22 October 2012
Keywords:
A. Copper
B. SEM
B. XRD
B. XPS
C. Pitting corrosion
a b s t r a c t
Biodiesel is a promising alternative fuel. However, it causes enhanced corrosion of automotive materials,
especially of copper based components. In the present study, corrosion mechanism of copper was investigated by scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction
(XRD) and X-ray photoelectron spectroscopy (XPS). Compositional change of biodiesel due to the exposure of copper was also investigated. Corrosion patina on copper is found to be composed of Cu2O,
CuO, Cu(OH)2 and CuCO3. Dissolved O2, H2O, CO2 and RCOO radical in biodiesel seem to be the leading
factors in enhancing the corrosiveness of biodiesel.
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
Biodiesel is one of the promising alternative fuels to overcome
the concerns related to the scarcity of petroleum diesel and environmental degradation. It is produced from renewable sources
such as vegetable oils or animal fats and has properties very close
to that of petroleum diesel [1]. However, biodiesel has been found
to be more corrosive to automotive materials than diesel [2–6].
This is more likely to be due to the presence of oxygen moieties,
auto-oxidation, increased polarity of biodiesel and its hygroscopic
nature. Corrosion behavior of different metals such as copper,
brass, bronze, cast iron, carbon steel etc. in diesel and biodiesel
was investigated by several researchers [2–6]. Geller et al. [3] observed that copper alloys were more prone to corrosion in fat based
biodiesel as compared to ferrous alloys. Sintered bronze nozzle was
found to be affected by pitting corrosion after 10 h operation with
biodiesel at 70 °C [4]. In a couple of studies, corrosion of different
metals in diesel and biodiesel was compared both at room temperature [5] and 80 °C [6]. The extent of copper corrosion in biodiesel
was comparatively higher than that of other metals. Biodiesel exposed copper surface at room temperature showed green layer of
corrosion products [5]. But at elevated temperature (80 °C), the
green layer of corrosion products was not visible on the biodiesel
exposed copper surface [6]. The change in color of biodiesel from
colorless to green at elevated temperature (80 °C) suggested that
the corrosion products were dissolved in fuel [6]. In both studies
[5,6], biodiesel was found to degrade metal surfaces comparatively
⇑ Corresponding author. Tel.: +60 3 79675212; fax: +60 3 79675317.
E-mail addresses: [email protected] (M.A. Fazal), [email protected] (A.S.M.A.
Haseeb), [email protected] (H.H. Masjuki).
0010-938X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.corsci.2012.10.006
more than diesel. The extent of copper corrosion in biodiesel was
found to be greater than that of other metals. However, none of
these studies [5–7] was devoted to characterize the corrosion
products as well as to develop the corrosion mechanism of copper
in biodiesel. It has been reported that the ester components [3,4],
moisture absorption and oxidation [5,6] can enhance the corrosion
of copper in biodiesel. In order to control the corrosion in biodiesel,
it is important to understand the corrosion mechanism. In a recent
study [2], corrosion products of both diesel and biodiesel exposed
copper, brass, aluminum and cast iron were investigated after conducting a 2880 h immersion test. But this study [2] does not present changes in corrosion products with respect to immersion time
which is very important to understand the related corrosion
mechanisms.
Considerable amount of work has been done on the corrosion
mechanism of copper in aqueous media and atmospheric conditions. Literature shows that the mechanisms of copper corrosion
largely depend on the environment. In urban atmosphere, the corrosion layers formed on the exposed surface are unstable and partially leached away by rainwater [8–12]. When copper is exposed
to marine atmosphere, enhanced corrosion may occur due to chloride salts, higher concentration of oxygen resulting from water
movement (waves) [13]. Copper corrosion in acidic media has been
studied by Scendo [14]. It has been reported [14] that corrosion is
initiated by oxygen and proceeded in two stages:
2Cu þ 1=2O2 þ 2Hþ $ 2Cuþ þ H2 O
2Cuþ þ 1=2O2 þ 2Hþ $ 2Cu2þ þ H2 O
Metallic copper Cu(II) ions may further enhance the corrosion
process:
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M.A. Fazal et al. / Corrosion Science 67 (2013) 50–59
But in neutral solution, the reaction could be as follows [15]:
2Cu þ 1=2O2 þ H2 O $ 2Cuþ þ 2OH
Additionally, temperature [5,6,16], organic matters [17], gaseous pollutants [18], humidity [19] were also found to influence
the corrosion process and the formation of corrosion products.
Depending on the corrosion media, solid compounds formed on exposed copper surface (patina) could be composed of Cu2O, CuO,
Cu(OH)2, Cu(OH)2.H2O, CuCO3, CuCl2 etc. [20,21]. The present study
aims to investigate the corrosion products that form on copper as a
function of exposure time in palm biodiesel. Based on the results
obtained, a possible corrosion mechanism of copper in palm biodiesel is suggested.
2. Experimental
Corrosion of copper (99.9%, commercially pure) in palm biodiesel (B100) was investigated by immersion test at room temperature (25–27 °C) for different time periods viz., 200, 300, 600,
1200 and 2880 h. The relative humidity at this room temperature
was approximately 82.6%. Palm biodiesel used in this work was
supplied by Weshchem Technology Sdn Bhd, Malaysia.
Test coupons of copper were cut for a size of 0.0172 m diameter
and 0.002 m thickness. A hole of diameter 0.002 m was drilled near
the edge of each specimen for hanging it into the fuels. The samples were then abraded with 400–1200-grit silicon carbide papers,
washed by deionised water and degreased with acetone. Glass
beakers were used for the immersion test. These beakers were kept
open during the test. Two duplicate coupons were immersed in
each of the test fuels. For each immersion time, new duplicate coupons were used. Upon completion of immersion test, the samples
were cleaned carefully in a water stream by using a polymer brash
in order to remove the corrosion products. The weight of each sample was recorded prior to and after immersion test by using a balance with a four decimal accuracy. The obtained data from weight
loss measurement were converted into corrosion rate (lm/y) by
following the Eq. (1).
Corrosion rate ¼
8:76 109 w
DtA
ð1Þ
where corrosion rate is in micrometer per year (lm/y), w is the
weight loss (kg), D is the density (kg/m3), A is the exposed surface
area (m2) and t is the exposure time (h).
The surface morphology and the elemental composition of the
corrosion products were determined by HITACHI S-3400N scanning electron microscopy connected to an energy dispersive
X-ray spectroscope (SEM/EDS). Corrosion products on the biodiesel
exposed metal surface were also examined by using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and
X-ray photoelectron spectroscopy (XPS). The XRD patterns of the
corroded samples were recorded by using a diffractometer (Model:
D5000, SIEMENS) with a Cu Ka radiation (1.5406 1010 m wavelength), operated at 40 kV/40 mA. FTIR spectra of the copper
compounds formed on the biodiesel exposed copper surface were
recorded by Perkin–Elmer Spectrum 100 FTIR Spectrometer in
the region of 4000–400 cm1. The XPS analysis of corroded copper
surface was performed by using AXIS ULTRA DLD spectrometer.
The excitation source was Al Ka (1486.6 eV) radiation. This source
was operated at a total power dissipation of 150 W (10 mA, 15 kV).
The XPS spectra were collected with constant pass energy of
187 eV. Survey scans were acquired from 1200 to 0 eV of photoelectron kinetic energy. Detailed scans were collected for the
relevant peaks of the species detected in the survey scans. The
binding energy (BE) scale was calibrated to compensate for the
electrostatic charging, setting the C1s binding energy of the carbon
contamination to 284.5 eV.
Prior to and after immersion test, fuels were analyzed by FTIR
(in the region of 4000–400 cm1), Gas chromatography mass spectroscopy (GCMS) and TAN (Total acid number) analyser in order to
investigate the changes in acidity, functional groups and compositions of biodiesel. For GCMS analysis, the used column was SGE
054101. The GC oven was programmed from 50 to 250 °C for different stages. The mass spectrometer was operated in a full scan
mode with an ion source temperature of 250 °C. ASTM (American
society for testing and materials) standard D664 was followed to
measure the TAN number of different fuels.
3. Results
Fig. 1 shows the corrosion rate of copper upon exposure to palm
biodiesel for different test periods ranging from 200 to 2880 h. It is
seen that the corrosion rate initially increases, reaching a maximum at approximately 600–1200 h. It then decreases gradually.
Corrosion rates obtained from the duplicate test coupons show
consistent results.
Fig. 2 shows the appearance of test coupons before and after
exposure to biodiesel for different periods. A blue-greenish layer
is seen to appear at the edge of the coupon (Fig. 2b) and this gradually increases with the increase of immersion time. Ultimately it
covers the whole surface. As seen in Fig. 2b, only the edge of the
coupon is blue greenish while the rest is reddish in color. It is further observed that the color of corrosion product layer changes
with immersion time from blue-greenish to green at an exposure
period of about 600 h (Fig. 2c). Corrosion product layer of the coupon exposed for 2880 h shows only green color (Fig. 2d). This demonstrates the conversion of copper compounds on the exposed
surface with time. With the increase of immersion time, thickness
of corrosion product seems to increase.
The exposed coupons were then cleaned and SEM micrographs
were taken for further investigations (Fig. 3). A number of small
pits are found to form randomly on the 200 h biodiesel exposed
copper surface (Fig. 3b). These pits increased in size with the increase of immersion time. Propensity of the pit is also seem to increase with time. Elemental analysis (Fig. 4) of the exposed surface
shows the presence of oxygen. This indicates that even after cleaning, the oxygenated compounds are still present.
Fig. 5 shows the FT-IR spectra of corrosion layer, formed on the
copper surface after immersion in palm biodiesel for 300 h at room
20
Corrosion rate (µm/y)
Cu2þ þ Cu $ 2Cuþ
16
12
Coupon 1 (from each set)
Coupon 2 (from each set)
8
4
0
500
1000
1500
2000
2500
3000
Immersion time (h)
Fig. 1. Corrosion rate of copper in palm biodiesel for different immersion time.
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M.A. Fazal et al. / Corrosion Science 67 (2013) 50–59
Fig. 2. Photographs of copper surfaces (0.0172 m dia) upon exposure to palm biodiesel for different immersion time.
Fig. 3. SEM micrographs of copper surface before (a) and after (b-f) exposure in palm biodiesel for different immersion time.
M.A. Fazal et al. / Corrosion Science 67 (2013) 50–59
53
Fig. 4. EDS graphs for elemental analysis of copper surfaces after exposure to palm biodiesel for (a) 1200 h and (b) 2880 h.
100.5
651.2
99.9
3323.72
99.6
Absorbance
100.2
99.3
4400
3400
2400
1400
400
Wave number/cm-1
Fig. 5. FTIR spectra of corrosion layer, formed on copper surface upon exposure to
palm biodiesel for 300 h at room temperature (25–27 °C).
temperature. The peaks at the region of 540, 610 and
3323 cm1 seem to indicate the presence of CuO, Cu2O and hydroxyl group (OH) respectively. The peaks at 1020, 1434, 1740 cm1
regions are more likely attributed to carboxylate (COO) anion. OH
group seems to indicate the presence of Cu(OH)2, while the presence of carboxylate anion (COO) suggests the film to be composed
of metal carboxylate corrosion compounds, e.g. CuCO3.
The peaks at 2060, 2180, 2298, 2850, 2920 cm1 are
attributed to different functional groups of biodiesel. The peaks
at 2850, 2920 cm1 are assigned to stretching vibration of CH2,
while 2100–2290 cm1 indicates (–C@C–) alkenes.
The formation of different compounds on the biodiesel exposed
copper surface as a function of immersion time was also investigated by XRD (Fig. 6). It is observed that initially (200 h) a small
amount of Cu2O, CuO, Cu(OH)2, CuCO3 are formed (Fig. 6a). With
the increase of immersion time (from 200 to 300 h), the amount
of CuCO3 increased as indicated by the peak height. Peaks of metallic copper decreased in height with the increase of immersion time.
The change in the peak height of copper or different copper compounds with respect to immersion time is shown in Fig. 7. It is seen
that the presence of larger amount of CuCO3 at the 2880 h biodiesel
exposed copper surface completely hides the peaks of the base metal copper. As a result, no peak for metallic copper was found. After
cleaning the exposed surface, small peaks for CuO, Cu2O and
CuCO3Cu(OH)2 were still visible (Fig. 8). The inner layer for Cu2O
probably keeps CuO from being reduced by base copper metal. Further oxidation of these oxides may lead to the formation of other
copper compounds (e.g. CuCO3, CuCO3Cu(OH)2) which are loosely
adherent and mostly removed after cleaning.
XPS analysis was carried out to confirm further the nature of
corrosion products formed on the biodiesel exposed surface.
Fig. 9 shows the wide scan XPS spectrum taken on the copper surface exposed to palm biodiesel for 300 h at room temperature. The
Cu LMM Auger peak and the N 1s, C 1s and O 1s XPS peaks are ob-
served on the surface of the corrosion layer. Details of the copper,
oxygen, carbon and nitrogen peaks are shown in Fig. 10. It is observed that the Cu-based corrosion product formed on the biodiesel exposed copper surface is Cu2O containing a minor amount of
Cu-, O- and C-based compounds.
Fig. 11 shows the change in color of copper exposed biodiesel
with respect to as-received biodiesel. It is seen that the as-received
biodiesel is colorless but the copper exposed biodiesel after 2880 h
exposure is greenish in color. The copper exposed biodiesel visually seen to start changing color at about 300 h of immersion.
The copper exposed biodiesel turned into more greenish color with
the increase of immersion time.
Fig. 12 shows the FTIR peaks identified for as-received biodiesel
and biodiesel after exposure to copper at room temperature for
300 h. The major peaks as seen in Fig. 12a occur at 3455,
1740, 2920, 1170 cm1 which seem to indicate the presence
of O–H, C@O, CH2 and stretching ester C–O–C groups respectively.
The increase of the peak height (Fig. 12b) in the region of
3455 cm1 (O–H groups) after exposure of copper for 300 h indicates the presence of H2O to a greater extent.
Table 1 shows the compositional difference as obtained by
GCMS analysis between as-received biodiesel and biodiesel after
exposure to copper at room temperature for 300 h. It is seen that
the percentages of methyl palmitate, methyl oleate are reduced
from 40.43 to 31.28 and 36.07 to 27.72, respectively. Different
types of alcohol (heptanol), aldehyde (octanal, nonanal), ketone
(2-octanone), carboxylic acids (e.g. myristic acid, palmitic acid),
short chain esters (e.g. methyl 12-oxo-9 undecenoate) etc. are
formed when copper is immersed in biodiesel. These compositional changes of biodiesel are likely to degrade the fuel properties.
Fig. 13 shows the change in TAN number of the copper exposed
biodiesels with the increase of immersion time. It is seen that the
TAN numbers of the as-received biodiesel is below the limit set by
the ASTM D6751 standard. The rest crosses this limit.
4. Discussion
The maximum corrosion rate (17.74 lm/y) of copper in biodiesel is obtained within the immersion period of 600–1200 h.
The corrosion rate then gradually decreases with time. Increased
corrosion rate as a result of increased weight loss includes two
stages: formation of corrosion product layers (such as blue-greenish) on biodiesel exposed metal surface and dissolution of formed
layers. Appearance of 200–600 h biodiesel exposed coupon seems
to indicate the subsequent formation and dissolution of corrosion
products in palm biodiesel. Dissolution of corrosion product after
1200 h exposure seems to decrease and thereby weight loss is also
relatively decreased with the immersion time. The growth of the
product layer more likely reduces the reaction between metal
and biodiesel by providing a protective barrier. This may cause
the reduction of corrosion rate after crossing the maximum limit.
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M.A. Fazal et al. / Corrosion Science 67 (2013) 50–59
Fig. 6. XRD pattern of copper surface after immersion in biodiesel for different immersion time.
Coupons, exposed for 2880 h showed comparatively more uniform,
thick and deep green corrosion product layer covering the entire
copper surface. This suggests that the green copper compounds become protective at increased immersion time leading to a decrease
in corrosion rate.
The SEM micrographs reveal that both the pit size and pitting
density are found to be dependent on exposure time. By visual
observation, these micrographs demonstrate that both the size
and propensity of the pits increase with immersion time. Though
the exposed surface was cleaned before taking SEM and EDS spectra, the elemental analysis shows the presence of oxygen, carbon
with base metal, copper. The obtained EDS results demonstrate
that the concentration of oxygen increases with the increase of
immersion time. The presence of higher oxygen suggests greater
concentration of oxygenated copper compounds adherent to the
metal surface. Due to corrosive attack, pit may form on base metal
as well as due to breaking down of oxygenated compounds.
To investigate the corrosion products, several analytical techniques such as FTIR, XRD and XPS were conducted on biodiesel exposed copper surface before cleaning. The absorptions from CuO
and Cu2O were identified in the FTIR spectrum at 540 and
610 cm1, respectively. These oxides were also observed in the
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M.A. Fazal et al. / Corrosion Science 67 (2013) 50–59
similar regions by Lefez et al. [22]. The peaks at 1020, 1434 cm1
are due to carboxylate anion (COO) bands. Presence of COO anion (1020, 1434 cm1) and OH group (3340 cm1) indicates the formation of carbonate species CuCO3 and hydroxide Cu(OH)2 species
respectively. Similar results for the composition of copper film
were also observed by Armstrong and Hall, [23].
XRD analysis shows that upon 200 h immersion, a small
amount of Cu2O, CuO, CuCO3 and Cu(OH)2 were formed. Formation
of CuCO3 gradually increases with the immersion time and after
2880 h, the exposed surface was mostly covered by it. A set of possible reaction for the formation of CuCO3 from both metallic copper
and copper compounds has been suggested in later section.
Cleaned surface after 2880 h immersion shows a compact layer
of Cu2O of red color. Small peak for CuO is also found. The inner
of Cu2O probably keeps CuO from being reduced by Cu0 [20].
Further confirmation about the corrosion products was done by
XPS analysis. The curve fitting analysis reveals different components occurring at different binding energies. The peaks detected
at a binding energy of 932.3 eV (Cu 2p3/2) and 952.1 eV (Cu 2p1/2)
represent Cu(I) compounds (Cu2O) or metallic Cu. The Cu (II)
Fig. 7. The change in peak height for different copper compounds obtained in XRD
pattern after immersion in palm biodiesel for different periods.
(a) Cu/2880 h
(c) XRD on biodiesel exposed cleaned copper surface
Cu
1200
CuO
300
Cu
(b)
Cu
600
Cu2O
Cleaned
CuCO3.Cu(OH)2
Cu
900
0
30
40
50
60
70
80
90
100
Fig. 8. (a) Biodiesel exposed copper surface (0.0172 m dia) after immersion in palm biodiesel for 2880 h, (b) biodiesel exposed cleaned surface, (c) XRD pattern of cleaned
copper surface.
Fig. 9. The XPS survey spectra measured on the copper surface upon exposure to palm biodiesel for 300 h.
56
M.A. Fazal et al. / Corrosion Science 67 (2013) 50–59
Fig. 10. XPS spectra for the regions of (a) Cu 2p, (b) O 1s, (c) C 1s and (d) N 1s collected from the corrosion layer formed on biodiesel exposed copper surface.
species peaks include CuO (933.5 eV) and CuCO3 or Cu(OH)2
(935 eV). Dissolved water in biodiesel may easily react with Cu2O
to form Cu(OH)2. Existence of carbonates is indicated by the feature
at 287.3 eV in the C1s region as reported by Squarcialupi et al. [24].
This component can be ascribed to copper carbonate (CuCO3) due to
the reaction between copper and RCOO. This can also be formed
from the reaction of cupric oxide (CuO) with the CO2 of the atmosphere [24]. CO2 or COO radical may also react with Cu(OH)2 to
form hydroxycarbonate. The blue-greenish color of the outer layer
seems to indicate the presence of carbonate and hydroxyl based
compounds. These results fully comply with the results obtained
from FTIR and XRD analysis. The fitting procedure applied to O 1s
peak revealed the presence of at least three different oxygen containing compounds. The O 1s peaks located at a binding energy of
530.8, 530.4 eV are attributed to O2 while that at 531.5 eV is
attributed to OH species. The position of the peaks agrees well
with the literatures [25,26] and we can therefore reasonably
suggest that the surface layer is composed of Cu2O covered with
carbonates (CO3) and hydroxyls (OH). To confirm the chemical
state of different elements, further study could be done by analyzing the half width of the resolved peak.
Results from XRD also show that the concentration of carbonate
increases with the increase of immersion time (200–2880 h) and
after 2880 h exposure, the outer surface is almost completely covered with carbonate species. Upon exposure to palm biodiesel, copper initially (<300 h) forms CuO, Cu2O, Cu(OH)2 and CuCO3
compounds and finally (after 2880 h exposure) carbonates species,
CuCO3 of green color is found to be formed as major copper compound. The steps for the formation of different compounds have
been seen schematically in Fig. 14. It is showed that Cu2O of red
color along with small amount of green CuCO3 and blue greenish
Cu(OH)2 form on 200 h biodiesel exposed copper surface. Carbonate layers formed may reduce metal-biodiesel interaction and decrease weight loss. This suggests that carbonate compound
seems to act as protective layer. It is thought that O2, H2O molecule
and COO radical or dissolved CO2 in biodiesel approach towards
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M.A. Fazal et al. / Corrosion Science 67 (2013) 50–59
103
87
2922.72
71
Absorbance
(a)
55
4400
3400
2400
1400
400
Wave number/cm-1
103
(a) As-received
(b) 300 h
(b)
2922.62
1741.84
71
Absorbance
87
55
4400
3400
2400
1400
400
Wave number/cm-1
Fig. 12. FT-IR spectra of biodiesel (a) before and (b) after exposure to copper at
room temperature for 300 h.
(c) 600 h
(d) 2880 h
Fig. 11. Color of biodiesel before (a) and after (b–d) exposure of copper for different
immersion time. (For interpretation of the references to color in this figure legend,
the reader is referred to the web version of this article.)
copper surface to interact and thereby form different compounds,
as supported by experimental results obtained from FTIR, XRD
and XPS analysis. It is assumed that the following possible reaction
occurs on copper surface upon exposure to palm biodiesel:
Formation of oxides:
2Cu þ 1=2O2 ! Cu2 O
ð2Þ
Cu2 O þ 1=2O2 ! 2CuO
ð3Þ
Formation of carbonates:
Cu2þ þ 2RCOO ! CuCO3 þ R R þ CO
ð4Þ
Cu2 O þ 2CO2 þ 1=2O2 ! 2CuCO3
ð5Þ
CuO þ CO2 ! CuCO3
ð6Þ
Formation of hydroxides and carbonates:
2Cu þ H2 O þ CO2 þ O2 ! CuðOHÞ2 CuCO3
ð7Þ
2CuðOHÞ2 þ CO2 ! CuðOHÞ2 CuCO3 þ H2 O
ð8Þ
2Cu þ O2 þ 2H2 O ! 2CuðOHÞ2
ð9Þ
CuO þ H2 O ! CuðOHÞ2
ð10Þ
According to Hernandez et al. [11], in the presence of oxygen,
copper can lead to the formation of cuprous oxide (Cu2O) by reaction (2). But Cu2O is unstable and rapidly it turns to the stable species, CuO [27] by following reaction (3). The presence of dissolved
water, CO2, RCOO etc. in biodiesel causes the formation of carbonate and hydroxyl based copper compounds (reactions (4)–(10)).
Based on the experimental data, we assume that the copper compounds such as Cu2O, CuO, CuCO3, Cu(OH)2 are the major constituents for 200 h biodiesel exposed surface, while only CuCO3 is
main product for 2880 h exposed copper. The observation provides
that dissolved O2, H2O, CO2 and RCOO in biodiesel are associated
with the formation of different copper compounds. Increased concentration of carbonate species with immersion time can be attributed to RCOO and CO2. Further investigation should be done to
understand the individual effect of O2, H2O, CO2 and RCOO for
the formation of different copper compounds.
The band concentrated in the region of 3000–3500 cm1 [28]
corresponds to stretching of O–H groups (3455 cm1). The dominant features at 3000–2800 and 1820–1660 cm1 are attributed to
the symmetric and asymmetric stretching vibrations of CH2 and
C@O respectively [29,30]. So, in the present study, the band at
1740 cm1 arises predominantly from the ester C@O groups.
The peaks in the region 650–750 and 1300–1000 cm1 correspond
to ester C–O–C stretching vibrations and –CH@CH– bonds respectively. Broadening of the OH peak at 3455 cm1 in the copper exposed biodiesel could be attributed to the presence of H2O at
higher concentration. Increasing intensity of the peak at
1740 cm1 (arising from carbonyl C@O groups) suggests a higher
concentration of aldehyde and ketone.
Analysis of compositional change of palm biodiesel before and
after copper exposure shows that the major reduction with respect
to as-received condition occurs for methyl palmitate (from 40.43%
to 31.28%) and methyl oleate (from 36.07% to 25.72%). This may be
attributed to reactions resulting from metal contact or absorption
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M.A. Fazal et al. / Corrosion Science 67 (2013) 50–59
Table 1
GCMS analysis revealing the compositional differences of biodiesel before and after exposure to copper at room temperature for 300 h.
Commercial name/formula
% of Area
Commercial name/formula
As-received
B100
B100 /Cu/
300 h
Hexane (C6H14)
Methyl caprylate CH3(CH2)6COOCH3
Methyl laurate CH3(CH2)10COOCH3
Methyl myristate CH3(CH2)12COOCH3
Palmitate CH3(CH2)14COOCH3
Palmitoleate CH3(CH2)5CH@CH(CH2)7COOCH3
Methyl heptadecanoate CH3(CH2)15COOCH3
1.41
0.12
2.04
5.97
40.43
1.22
0.18
4.00
0
0.9
3.36
31.28
0.82
0.38
Methyl stearate CH3(CH2)16COOCH3
11.46
11.36
Methyl oleate CH3(CH2)7CH@CH(CH2)7COOCH3
Methyl linoleate
CH3(CH2)4CH@CHCH2CH@CH(CH2)7COOCH3
Methyl arachisate CH3(CH2) l8COOCH3
Caprylic acid C8H16O2
36.07
0.11
27.72
0.74
Heptene C7H16
0.236
0
1.06
0.51
0
0.17
% of Area
As-received
B100
B100 /Cu/
300 h
Octanal CH3(CH2)6CHO
Nonanal C9H18O
Methyl caprate CH3(CH2)8COOCH3
Heptanol CH3(CH2)6OH
2-Octanone C8H16O
Methyl 9-oxononanoate C10H18O3
Methyl 12-oxo-9-dodecenoate
C13H22O3
Methyl 11-oxo-9-undecenoate
C12H20O3
Pentadecanol C15H32O
Myristic acid C14H28O2
0
0
0
0
0
0
0
0.13
0.11
0.37
3.2
1.07
1.47
0.31
0
0.48
0
0
0.91
0.88
Palmitic acid C16H32O2
Methyl 9,10-epoxyoctadecanoate
C19H36O3
Methyl linolenate C19H32O2
0
0
0.03
4.18
0
0.22
TAN number (mgKOH/g)
3
Limit: ASTM D6751
for biodiesel
Copper coupon
2
Cu2O
CuO
Cu(OH)2
CuCO3
1
200 h exposed copper
0
Cu2O
CuO
Cu(OH)2
CuCO3
Cu(OH)2
CuCO3
Fig. 13. Change in total acid number (TAN) of biodiesel before and after exposure to
copper at room temperature (25–27 °C) for different immersion periods.
600 h exposed copper
of moisture, O2, CO2 etc. [6,16,31]. Different types of alcohol (heptanol, pentadecanol), aldehyde (octanal, nonanal], ketone (2-octanone), carboxylic acids (caprylic acid, pamitic acid, acetic acid
etc.) and also some other components like methyl 9, 10 epoxyoctadecanoate, methyl 9-oxononanoate are found to be produced after
exposure of metal. These are in good agreement with other studies
[31–33] dealing the effect of H2O, O2, atmosphere, metal contact
etc. on the degradation of saturated and/or unsaturated components of biodiesel.
Few long chain molecules presented in Table 1 are only available after exposure of copper in biodiesel. Those were not found
in as-received biodiesel. Those newly produced molecules could
be formed by conjugations of radicals (reaction 11). Similarly,
few small chain radicals can be formed by breaking down of long
chain molecules (reaction 12). These newly formed radicals may
react with metal and may form different metal compounds as
shown in the above reaction (4).
R þ R ! R R
ð11Þ
R1 COOR ! R1 COO þ R
ð12Þ
In addition to the change in composition of biodiesel, the corrosion products or metal contact may change the fuel properties. Increased TAN number indicates the increased level of oxidation of
biodiesel [34]. Due to exposure to copper, different types of
mono-carboxylic acids such as octanoic acid, myristic acid,
Cu2O
CuO
1200 h exposed copper
Cu2O
CuO
Cu(OH)2
CuCO3
2880 h exposed copper
Fig. 14. Schematic sketching for the formation of different corrosion compounds on
biodiesel exposed copper surface.
palmitic acid etc. are found to have formed. Increased TAN number
can be attributed to the formation of acidic components after exposure of copper.
From the results exposed in this work, it can be demonstrated
that copper forms different types of copper compounds upon exposure to biodiesel for different immersion time. Dissolved O2, H2O,
CO2 and RCOO radical in biodiesel seem to be the leading factors
M.A. Fazal et al. / Corrosion Science 67 (2013) 50–59
for the formation and dissolution of these compounds. Such understanding is expected to form a basis which can be used to develop
kinetic model as well as to reduce corrosion effectively.
5. Conclusions
This study suggests the following conclusions:
1. Corrosion of copper in biodiesel increases with the increase
of immersion time. But after a certain immersion period, formation of oxygenated compounds on biodiesel exposed copper surface reduces corrosion rate.
2. The corrosion patina identified by XRD is composed of CuO,
Cu2O, CuCO3, Cu(OH)2. These copper compounds were further confirmed by XPS analysis.
3. After long term exposure, CuCO3 was found to be as a major
constituent of corrosion products. A compact layer of Cu2O
between the outer film and the copper substrate probably
keeps CuO from being reduced by Cu0 and CuCO3 seems to
be formed from both metallic copper and copper compounds. Dissolved O2, H2O, CO2 and RCOO radical in biodiesel seem to be the leading factors for the enhanced
corrosiveness of biodiesel.
4. The major functional groups identified for as-received biodiesel are O–H groups, carbonyl C@O groups, stretching ester
C–O–C. Prominence of the peaks for O–H groups after a certain period of immersion indicates the presence of H2O to a
greater amount.
5. Upon exposure of copper, few components such as pentadecanol, octanoic acid, myristic acid, palmitic acid, heptanol,
octanoic acid methyl ester, heptanoic acid methyl ester etc.
formed at different concentrations. The acidic components
seem to be the cause of increased TAN number and this
can also enhance the corrosion of copper.
Acknowledgement
The authors would like to acknowledge the financial support
provided by the Ministry of Science, Technology & Innovation
(MOSTI) under the science Fund by Grant No. 03-02-03-SF3073,
University of Malaya Research Grant (UMRG) by Project No.
RG137-12AET and by the Institute of Research Management and
Consultancy, University of Malaya (UM) under the PPP Fund Project No. PS110/2010A.
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