Corrosion Science 57 (2012) 215–219
Contents lists available at SciVerse ScienceDirect
Corrosion Science
journal homepage: www.elsevier.com/locate/corsci
Electrochemical corrosion of carbon steel exposed to biodiesel/simulated
seawater mixture
Wei Wang a,b, Peter E. Jenkins c, Zhiyong Ren b,⇑
a
College of Chemistry and Chemical Engineering, Ocean University of China, Qingdao 266100, China
Department of Civil Engineering, University of Colorado Denver, Denver, CO 80217, USA
c
Department of Mechanical Engineering, University of Colorado Denver, Denver, CO 80217, USA
b
a r t i c l e
i n f o
Article history:
Received 22 September 2011
Accepted 18 December 2011
Available online 27 December 2011
Keywords:
A. Mild steel
C. Interfaces
a b s t r a c t
The electrochemical corrosion of carbon steel exposed to a mixture of biodiesel and 3.5% NaCl solution
simulated seawater was characterized using wire beam electrode (WBE) technique. Both optical images
and in situ potential and current measurements showed that all the anodes and most cathodes formed in
the water phase, but the cathodes were mainly located along the water/biodiesel interface. Due to oxygen
concentration gradient and cross-phase ion transfer, low corrosion currents were also detected in biodiesel phase. Further anode reaction was partially blocked by iron rust, but the alkali residual in biodiesel
may interact with corrosion and deteriorate biodiesel quality.
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
Biodiesel has been considered as a viable alternative fuel for
transportation, because it is produced from renewable resources
and reduces the dependence on fossil fuels and green house gas
emission. Biodiesel can be used in pure form (B100) or blended
with petroleum diesel in most diesel engines. The green nature
of biodiesel allows its application not only in ground transportation, but also in marine vessels and airplanes. By 2020, U.S. Navy
plans to have 50% of all its energy resources from alternative fuels,
primarily from biodiesel [1]. Biodiesel is considered chemically stable in pure form [2], but it can become more corrosive during storage, transportation, and utilization. For ships at sea, a series of
tanks are used for fuel storage. To maintain the stability of the ship,
compensated fuel ballast systems automatically draw in seawater
to replace consumed fuel and compensate for ballast weight loss.
When the ballast is refueled, some seawater may remain at the
bottom of the tank, causing a contamination of biodiesel and corrosion of the tank surface exposed to the mixture of biodiesel
and seawater [3]. In addition, condensed moisture in the marine
environment also causes fuel tank corrosion due to promoted
microbial growth and the release of corrosive fatty acids from biodiesel [4].
The corrosion of seawater contaminated biodiesel containers
creates great challenges in marine vessel operations, but very
few studies provide information on the corrosion mechanisms
and prevention strategies. It was recently reported that seawater
⇑ Corresponding author. Tel.: +1 303 556 5287; fax: +1 303 556 2368.
E-mail address: [email protected] (Z. Ren).
0010-938X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.corsci.2011.12.015
could facilitate carbon steel corrosion due to the reactions between
chloride or sulfide on the metal surface, and microbial growth
accelerated biodiesel degradation that caused biofouling [3,4]. Traditional weight loss measurements and classic linear polarization
resistance (LPR) or electrochemical impedance spectroscopy (EIS)
were used to characterize the overall corrosion rate of plate metals
[5,6]. However, such methods can only provide averaged electrochemical information of the whole surface area but cannot reveal
the heterogeneous metal corrosion distribution at different exposure conditions in the multi-phase system. Moreover, it is very difficult to use conventional tools to map the electrochemical
behaviour of metal electrodes in biodiesel due to the high resistance of biodiesel [7]. We recently employed a new electrochemical characterization tool called wire beam electrode (WBE) to
analyze the corrosion behaviour of carbon steel in water contaminated biodiesel and found that the corrosion was closely related to
ion concentration in water [8]. Compared to other local electrochemical techniques, such as scanning kelvin probe (SKP), scanning
vibrating electrode technique (SVET), or local electrochemical
impedance spectroscopy (LEIS), the WBE module consists an array
of wire electrodes (10 10, or 11 11) and is able to provide local
potential and current distributions in situ, making it a suitable
method to analyze complicated electrochemical systems [9–15].
In this study, a WBE platform was used to characterize the effects of simulated seawater on the corrosion of biodiesel fuel tank
surface. The heterogeneous corrosion behaviour of a metal surface
under different exposure conditions (biodiesel, seawater, and
interface) was mapped continuously through potential and current
measurements. The corrosion mechanism and prevention strategies were also discussed.
216
W. Wang et al. / Corrosion Science 57 (2012) 215–219
2. Experimental
2.1. WBE fabrication and material selection
The WBE assay was fabricated by arranging 121 carbon steel
(CS) wires (2.0 mm diameter), (ASTM C1020, McMaster Carr,
USA) into an 11 11 matrix. Each wire electrode was coated with
a thin layer of epoxy resin to guarantee the isolation between the
electrodes [8]. The wires were embedded in epoxy resin, and the
wire electrode surfaces were ground up to 1000 grit silicon carbide
paper and washed with deionized water and ethanol before each
test (Fig. 1). A cubic glass chamber (2.5 2.5 2.5 cm) was glued
with epoxy on the WBE surface to contain biodiesel and salt solution. A soy-based biodiesel (B100, Bertkus Oil Company, USA) was
used in the experiment. Seawater was simulated by dissolving 35 g
NaCl in 1 L DI water to make a 3.5% NaCl solution. In some experiments, a high carbon steel (ASTM C1080) WBE device with the
same configuration was used to perform ionic diffusion characterization under the same mixture condition [8].
2.2. Electrochemical characterization
In order to simulate the formation of biodiesel/seawater mixture in biodiesel storage tank, the WBE was vertically placed to
serve as one container wall, and the other three walls were formed
by gluing glass slides together (Fig. 1). The simulated seawater
containing 3.5% NaCl solution was first added into the container
to cover 5 lines electrodes (Fig. 1c, Lines 7–11). Biodiesel was then
gently filled on top of the seawater layer to form a separate layer
that covered 6 lines of electrodes (Fig. 1c, Lines 1–6). Therefore,
the biodiesel/seawater interface was located between the 6 and 7
line of the WBE assay, as shown in Fig. 1.
The potential and current distribution were measured and
mapped by the WBE monitoring module from modular instruments
(National Instruments Co., USA). Before the electrochemical experiments, all 121 wire electrodes in the WBE were connected together
to allow free electron flow. During potential distribution measurement, all wire electrodes were temporarily disconnected from each
other and the potential on each wire electrode was measured
against an Ag/AgCl reference electrode in sequence. The reference
electrode was immersed in water phase. In the current distribution
test, each individual electrode was disconnected from the other 120
electrodes, and the local current between this single electrode and
other connected electrodes was measured. Once the current data
was collected, the electrode was automatically reconnected to
other electrodes, and the next electrode in the line was separated
from the other electrodes and repeated for the same monitoring
procedure. In current distribution maps, the positive current is defined as the anodic current and the negative current as the cathodic
current. The 121 local potentials were measured in a period of
about 6 s while the interval of current measurements between
two channels was approximately 1 s. The distribution of potential
and current was monitored by a self-designed LabVIEW@ program
and mapped with Surfer 8.0 software. All tests were performed at
room temperature (25 °C) and repeated at least three times.
3. Results
3.1. Potential and current distributions of WBE in biodiesel/simulated
seawater mixture
Fig. 2 shows the changes of potential and current distributions
on the carbon steel WBE in the biodiesel/seawater mixture as a
function of time. At approximately 1 h after the mixture was
formed, the anodic and cathodic currents were detected but
showed a random pattern. No correlation could be found between
the anode/cathode distribution at the biodiesel/water interface
(Fig. 2a, b). However, after 1 d of exposure, such correlation became observable, as most cathodes were found located at the electrodes (line 7) immediately below the water/biodiesel interface,
while the anodes located in the water phase (lines 8–11), forming
a relatively horizontal boundary parallel to the biodiesel/water
interface (Fig. 2c, d). Such a linear cathode/anode distribution became more stable with time, but the potential difference and current density between the cathode and anode increased, indicated
by more distinct color patterns shown in Fig. 2(e, f) for day 10
and (g, h) for day 19. Such electrochemical distributions, where
the cathodes were formed along the biodiesel/water interface
and anodes located in the water phase, are consistent to our previous findings using high carbon steel WBE, but the corrosion occurred faster in this study, presumably due to higher ion
concentration in the seawater [8].
3.2. Images of WBE corrosion in biodiesel/simulated seawater mixture
Fig. 3 shows the corresponding corrosion images of WBE surfaces in biodiesel/seawater mixture at different exposure times.
Fig. 3a shows that yellow rust corrosion products began to accumulate on the electrode surface submerged in water after 4 h.
The rust coverage expanded significantly by day 4 (Fig. 3b), covering almost all the electrode surface in the water phase except line
7. At day 20, the liquid mixture was removed from the container to
allow the direct exposure of the electrode array. It was found that
though the majority of the wire electrodes submerged in water
were covered by a thick layer of yellow rust, the electrode line right
below of the biodiesel/water interface (line 7) was not covered by
rusts but instead by a layer of oily substance. The location of this
oily layer directly correlated with the cathode distribution as
shown in Fig. 2(g, h).
3.3. Current density difference in water, biodiesel, and the interface
Biodiesel has very low conductivity as compared to seawater, so
no apparent corrosion was observed on the electrodes exposed to
Fig. 1. Schematic diagram of the WBE (a), photo of the side view of the glass container containing solution mixture (b) and the WBE array surface (c).
W. Wang et al. / Corrosion Science 57 (2012) 215–219
217
Fig. 2. Potential (a, c, e, and g) and current (b, d, f, and h) distributions of carbon steel WBE exposed to biodiesel/simulated seawater mixture. (a, b) at 1 h, (c, d) at 1 d, (e, f) at
10 d, and (g, h) at 19 d.
biodiesel. However, though the majority of the cathodes were
found located at the water phase around the biodiesel/water interface, weak cathodic currents were also monitored in biodiesel close
to the interface (line 6). The cathodic currents along electrode line
6 ranged from 10 8 to 10 7 A on day 19, which was much lower
than the cathodic current in water phase (10 6 A) but still orders
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W. Wang et al. / Corrosion Science 57 (2012) 215–219
Fig. 3. Images of the WBE surface exposed to biodiesel/simulated seawater mixture: (a) 4 h; (b) 4 d; and (c) 20 d, right after the liquid removal from the container. The
biodiesel/water interface is indicated by the dashed line and the cathodic zone is indicated by arrows.
of magnitudes higher than the measured current in pure biodiesel
phase (10 10 A). This phenomenon has been observed in repeated
experiments.
4. Discussion
confirm this hypothesis. One glass beaker was filled with biodiesel
and 3.5% NaCl plus 0.1 M NaOH solution, while another beaker was
filled with the same mixture without NaOH. Similar oily substance
formed in the beaker with NaOH within 2 h, but no such substance
was observed in the beaker without NaOH after 3 weeks. This finding suggests that the formation of impurities due to electrochemical corrosion may deteriorate the quality of biodiesel.
4.1. Mechanisms of electrochemical distribution of carbon steel
WBE in biodiesel/simulated seawater mixture
4.2. Cathodic current measured in biodiesel
Both in situ electrochemical distribution monitoring and visual
observation on the WBE electrode array showed that corrosion occurs when carbon steel is exposed to the mixture of biodiesel and
seawater. While most electrodes corroded when exposed to simulated seawater containing high salt concentration, the cathodes
were mainly formed at the biodiesel/seawater interface, similar
to that cathode is located at the three-phase-line zone in atmospheric corrosion. Such corrosion behaviour is believed to be due
to a combination of several mechanisms. Biodiesel has different
physiochemical properties than seawater, as it has very low conductivity, high dissolved oxygen (DO) concentration, and limited
solubility in water. During the potential mixing of biodiesel and
seawater in ballast tanks or other systems, two concentration gradients were created across the biodiesel/water interface during a
two-way diffusion – the DO gradient from biodiesel to seawater,
while the ion or conductivity gradient from seawater to biodiesel.
This makes the interface a unique environment to create an oxygen
concentration cell that leads to corrosion, because studies showed
that along the interface, the DO concentration can be as twice as
that in water, so oxygen acts as the electron accepter, while the
conductivity in seawater (55–61 ms/cm) is much higher than it
in biodiesel (almost zero) to facilitate electrochemical reactions.
Therefore, the cathodes were mainly formed in water phase along
the biodiesel/seawater interface, but weak cathodic current were
also detected in the biodiesel phase due to much higher resistance
in biodiesel.
On the other hand, the electrodes submerged in seawater
served as the anode due to low DO exposure in water. Moreover,
as the anodes were gradually covered by corrosion rust, they were
further blocked by the rust from oxygen supply. After the experiment, the loose layer of yellow rust was removed and a firm layer
of black corrosion was observed. It was believed that the upper yellow layer rust was Fe(OH)2 and the inner black layer was Fe3O4
(black magnetite), and Fe3O4 was formed due to less oxygen supply
at the electrode surface [16]. In contrast, the oily mixture formed
on the cathodes below the biodiesel/water interface allowed higher DO diffusion to the electrode surface that promoted oxygen
reduction (Fig. 3c) [17]. Such a mixture is hypothesized to be related to OH generated during oxygen reduction and released from
biodiesel, because alkali has been used as main catalyst during
transesterification [18]. A simple control test was conducted to
Compared to a much higher cathodic current in the water phase
along with the biodiesel/seawater interface, weak currents were
detected on the low carbon WBE electrodes in the biodiesel phase
around the interface. Though biodiesel is considered stable and not
conductive in its pure form, such a low current may be due to its
mixing with seawater, which led the diffusion of salts into biodiesel [8]. The salts increased local conductivity and facilitated cathodic reaction with the presence of high DO concentration in
biodiesel. To further test this hypothesis, a high carbon steel
WBE device with the same configuration was used to conduct a
similar experiment. High carbon steel was chosen here because
the higher carbon content facilitates the corrosion rate in the
experimental condition, and therefore faster observation and characterization can be made without mechanism changes [19]. After
2 d exposure to the biodiesel/seawater mixture, apparent corrosion
marks were observed on the electrodes in biodiesel phase, confirming the presence of electrochemical reaction. Furthermore,
the closer the electrode was to the biodiesel/water interface, the
more wire electrodes showed corrosive activity. The number of
wire electrodes with corrosion marks increased from 1 to 10 from
line 4 to line 6, and the size of the corrosion marks also increased
accordingly (Fig. 4).
The corrosion marks on electrodes submerged in biodiesel were
not observed in our previous high carbon steel WBE study, in
which tap water with minimum ion concentration was used to
mix with biodiesel [8]. We believe this phenomenon difference further confirms that the high ion concentration in seawater will significantly facilitate corrosion, not only in the water phase, but also
in the biodiesel phase. Such observation also agrees with previous
findings that corrosion did occur on carbon steel submerged in biodiesel [3]. No microbial growth was observed during experiments,
so no impact of microbial caused corrosion or fouling was
considered.
Based on the above results, we believe the electrochemical distribution of the carbon steel exposed to biodiesel/seawater mixture
can be drawn as the graph shown in Fig. 5. The cathode area should
be located along the biodiesel/seawater interface, but primarily in
water phase. Weak cathodic current could be detected in biodiesel
phase along the interface, but the corrosion will be much less severe due to the low conductivity of biodiesel. Anodes will be
W. Wang et al. / Corrosion Science 57 (2012) 215–219
219
detected in the biodiesel phase along the interface, but the current
distribution in biodiesel showed a positive correlation with the distance from the biodiesel/water interface and the ion concentration
gradient in biodiesel. Rust covered the majority of the anode surface in the water phase, and an oily substance was formed along
the cathode area. Such formations due to corrosion may also affect
biodiesel quality. As biodiesel becomes a more dominant fuel
source for transportation, these findings offer new ways to quickly
characterize metal electrochemical corrosion and provide insights
for developing corrosion prevention strategies.
Acknowledgements
This work was partially supported by the Office of Research
(ONR) under award N000141110813 (Ren). Wang was supported
by a scholarship from the China Scholarship Council (CSC) and College of Engineering and Applied Science at UCDenver.
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Fig. 4. Image of the high carbon steel WBE exposed to the biodiesel/seawater
mixture (line 1–6 in biodiesel, and line 7–11 in seawater).
Fig. 5. Schematic diagram of corrosion current distribution on carbon steel surface
exposed to biodiesel/seawater mixture in lab scale WBE. The diffusion of ions and
biodiesel are indicated by arrows.
located in the water phase and show a positive correlation with the
distance from the interface and the biodiesel gradient in water.
5. Conclusion
Wire beam electrode electrochemical analysis shows that carbon steel material corrodes quickly when exposed to a mixture
of biodiesel and 3.5% NaCl solution simulated seawater. A distinctive corrosion pattern was detected using potential and current
measurements as well as observed by optical images. While all
the anodes located in the water phase due to low DO exposure,
most of cathodes were formed in the water phase along the
water/biodiesel interface. Relatively low corrosion current were
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