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Heterogeneous corrosion behaviour of carbon steel in water contaminated biodiesel
Corrosion Science 53 (2011) 845–849
Contents lists available at ScienceDirect
Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Short Communication
Heterogeneous corrosion behaviour of carbon steel in water contaminated biodiesel Wei Wang a,b, Peter E. Jenkins c, Zhiyong Ren b,⇑ a Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, Ocean University of China, College of Chemistry and Chemical Engineering, Qingdao 266100, China b 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
a r t i c l e
i n f o
Article history: Received 14 August 2010 Accepted 20 October 2010 Available online 2 November 2010 Keywords: A. Mild steel C. Interfaces
a b s t r a c t Biodiesel has been widely used as an additive to traditional fuel supplies, but the corrosion of metals used in biodiesel infrastructure is becoming an increasing concern. In this study, the influence of water contamination and corrosion behaviour of carbon steel in biodiesel, were characterized using the wire beam electrode (WBE) technique. In situ local current distributions among the electrodes showed a distinct corrosion pattern, with the anodes formed in the area that was exposed to water, and the cathodes formed along the water–biodiesel interface. The anodic current distribution showed a positive correlation with the biodiesel concentration gradient in water. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Biodiesel is an alternative diesel fuel derived from renewable feed stock such as plant or animal lipids. The development of biodiesel as a renewable fuel has attracted considerable attention, as it demonstrated great potential in reducing the world’s dependence on fossil fuels and greenhouse gas (GHG) emission. Biodiesel can be effectively mixed with petroleum diesel and used in diesel engines in blends up to 20%. Though B5 or B20 blends (5% or 20% biodiesel blended with petroleum diesel) have been widely used in Europe, America, and Australia, higher percentages of biodiesel have not been broadly applied due to the concerns of the impact of integrating this alternative fuel into existing infrastructures, such as storage tanks, transportation pipelines, and automotive fuel systems. Biodiesel is considered chemically stable in pure form [1], but it can become more corrosive during storage, transportation, and utilization, when it degrades through moisture absorption, microbial oxidation, and other contaminations [2–6]. For example, water contamination due to condensation is considered as one of the main corrosion factors for storage containers and pipelines. Water contains corrosive ions and promotes microbial growth in the solution and at the biodiesel/water interface, and it may hydrolyze the methyl esters and produce more corrosive fatty acids [2,4]. Although corrosions have been reported by biodiesel manufacturers and users, few studies have provided information on the
⇑ Corresponding author. Tel.: +1 303 556 5287. E-mail address: [email protected] (Z. Ren). 0010-938X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2010.10.020
corrosion mechanisms and prevention and control strategies [2,3,6,7]. Lee et al. conducted some pioneer work on characterizing microbiologically influenced corrosion in biodiesel (B100, 100% biodiesel) and found it had the highest propensity for biofouling [6]. Aktas et al. recently reported that seawater accelerated the corrosion rate in biodiesel due to the reactions between chloride and/ or sulfide from the seawater with the carbon steel electrodes [2]. Other studies also reported corrosion of aluminum, copper, and bronze in biodiesel [3,4]. However, most studies used weight loss measurement and traditional electrochemical methods, such as polarization curve and electrochemical impedance spectroscopy to characterize the overall corrosion rate of plate metals. Such measurements can only provide average electrochemical information on the electrodes, but they are not able to quickly identify the corrosion distribution or distinguish the heterogeneous corrosion mechanisms that occur on metal surfaces at different exposure conditions. Taking water contamination as an example, it is believed that the interface between the biodiesel and formed water drop during condensation is more likely to corrode, but no in situ detection data is available to test the hypothesis. In this study, a unique and simple carbon steel wire beam electrode (WBE) module was developed to directly monitoring local electrochemical responses of the heterogeneous surface conditions. WBE is an electrode array that has been used to characterize local potential and current correlation, biofilm/metal interface, and electrochemistry in other corrosion systems [8–12]. The WBE results documented the formation of anodes and cathodes at locations under different exposure conditions (biodiesel, water, and the interface) and revealed the local corrosion current distribution. These findings provided valuable information on metal corrosion in biodiesel.
846
W. Wang et al. / Corrosion Science 53 (2011) 845–849
2. Experimental 2.1. WBE fabrication and material selection The WBE assay was fabricated by arranging 121 carbon steel ASTM C1080 wires (2.0 mm diameter, McMaster, USA) into an 11 11 matrix. The wires were embedded in epoxy resin for fixation (Fig. 1). The wire electrode surfaces were ground up to 1000 grit silicon carbide paper and washed with deionized water and ethanol before each test [8–11]. A cubic glass chamber (2.5 2.5 2.5 cm) was glued with epoxy on the WBE surface to contain biodiesel. A soy-based biodiesel (B100, Bertkus Oil Company, USA) was used in the experiment. Local tap water was added in some experiments to simulate the ion-containing condensed water in the biodiesel. The tap water sample contained 30 mg/L of Cl , 0.1 mg/L of NO3 , 24 mg/L of Na+, 31 mg/L of Ca2+, 7 mg/L of Mg2+, and 2.8 mg/L of K+.
The current distribution was continuously measured and mapped by the WBE monitoring module, which was constructed with modular instruments (National Instruments Co., USA) and controlled by self-designed programs using the LabVIEWÒ 8.6 software package [8–11]. The local current between each individual wire electrode and all other connected electrodes was measured sequentially. Specifically, one electrode is disconnected from the other 120 connected electrodes, and the current between this single electrode and the other connected electrodes was monitored by a multimeter in the modular device. Once the current data is collected, the electrode is 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. Each current measurement takes less than 1 s and therefore the total time for the 121 current measurements takes about 2 min. A current amplifier (PXI 4022) is employed in the device, which can detect picoampera current level with femtoampere noise [8]. The current distribution maps were drawn using the Surfer 8.0 software. All current data are expressed as Ampere (A).
2.2. Experiment set up and electrochemical measurements Two sets of experiments were conducted to mimic the water contamination in the biodiesel container. In the first test, biodiesel was filled up to the container in which the WBE was horizontally placed as the container bottom. To simulate water drop formation on the bottom of the storage tank, tap water was slowly dripped inside a stainless steel ring (about 1.8 cm in diameter) hung above the WBE without contacting the surface). The ring was used to prevent the water drop from moving around the surface (Fig. 1a). In the second test, the WBE was vertically placed to act as one container wall. Water was first added into the container, and then biodiesel was slowly filled on top of the water layer to form a separate layer of the fuel (Fig. 1b). All tests were performed at room temperature (25 °C) and repeated three times.
3. Results and discussion Fig. 2 shows the current distribution of the horizontal carbon steel WBE with a water drop on the surface surrounded by biodiesel. At approximately 20 min after the water drop was formed, cathodic currents were detected on several wire electrodes that were located along the edge of the water–biodiesel interface, as shown as the light green zones in Fig. 2a. By comparison, only one apparent anodic current was measured at the same time, as indicated by the arrow in Fig. 2a. Fig. 2b shows that more anode and cathode currents were detected within the water drop boundary after 2 h, but in a likely random pattern. This is probably due to the incomplete replacement of water to biodiesel within a short
Fig. 1. Photos and schematics of the WBE set up. (a) Horizontal WBE setting with a water drop inside of a stainless steel ring on the top of the WBE (without contacting). (b) Vertical WBE setting with biodiesel layer on top of the water layer.
847
W. Wang et al. / Corrosion Science 53 (2011) 845–849
a
(A)
(A)
2E-006
2E-006
b
1.5E-006
5E-007 0
22mm Line number
Line22mm number
1E-006
1.5E-006 1E-006 5E-007 0
-5E-007
22 mm Row number
(A) 2E-006
c
-5E-007
22 mm Row number
d
6 mm
22mm Line number
1.5E-006 1E-006 5E-007 0 -5E-007
22 mm Row number Fig. 2. Time-course current distributions of the water drop covered WBE at (a) 20 min (b) 2 h, and (c) 72 h (d) shows the rust covered WBE after 120 h of immersion. The dashed circle in (c) indicates the tendency of cathode distribution at the biodiesel–water interface.
period, in which the biodiesel film was still partially covered some WBE surface, resulting in cathode area as compared to water covered anode area. Most electrodes that were exposed to water corroded within 72 h, as shown in Fig. 2c, with cathodes that were mostly located at the outer edge of the water drop, i.e., the water–biodiesel interface (the dashed circle), and anodes that were primarily located within the water area (Fig. 2c). Fig. 2d shows that almost all the electrodes under the water drop were covered by rusts after 120 h of operation. This unique electrode distribution pattern in the area exposed to water is believed to relate to the water drop formation and its interaction with the biodiesel container. Water solubility in biodiesel is low, generally less than 0.2%, but during storage or transportation, water can be condensed and suspended in the form of small drops and interpose itself between the biodiesel and the metal surface of the storage tanks or pipes. Under stagnant conditions, the water drop will form a separate phase at the bottom of the biodiesel container. Therefore, the formation of the water drop on the metal surface will push the majority of biodiesel away, but leave a thin layer of water–biodiesel mixture on the metal surface. The water drop itself, in addition to the impurities in the mixture, such as chloride ion, will facilitate the anodic reaction and corrode the metal surface. It may also encourage microbial growth [6]. In contrast, more cathode zones are formed along the outer edge of the water drop, showing a current map similar to the pattern of the ‘‘Evans ring’’ test. Such pattern indicates that most anodic reaction occurs in central part of the water drop due to the high ion promoted corrosion, while the cathodic reaction takes place on the periphery of the water drop due to the higher oxygen concentration as compared to that in bulk water. Considering the low conductivity and higher oxygen solubility in biodiesel, we believe both
reactions occur primarily in the aqueous phase [13,14]. Fig. 3 shows the schematic of the anode–cathode distribution under a water drop on the metal surface, assuming the biodiesel was fully replaced by water on the metal surface. To further testify the hypothesis that more cathodes will be formed at the water–biodiesel interface while anodes will be mainly in the water phase, a second test was performed with a WBE vertically immersed in biodiesel and water solution to serve as a wall of the container. Fig. 4a shows the current distributions after 8 h, and it is clear that the cathodes were formed along the biodiesel–water interface (the fourth row of the WBE), while more anodes were formed in the water phase. Compared to the horizontal WBE test, Fig. 4a also shows a positive correlation between the amount of the anodes (darkness of the blue1 colour in water phase) and the distance from the biodiesel, supporting the hypothesis that the corrosion potential is correlated with biodiesel concentration gradient in water. This is further confirmed by Fig. 4b, which shows the apparent rust formation of the electrodes in the water phase after 72 h, while no corrosion was found in the biodiesel phase. All these findings confirmed that when there is water contamination in biodiesel, corrosion will occur in the water phase, and cathodes will be formed in the water–biodiesel interface. The WBE technique reveals the unique current distribution and corrosion patterns of water contamination in biodiesel. More characteristics of individual wire electrodes at different zones (anode, cathode, transition) can be studied using traditional electrochemical techniques in conjunction with the WBE method. Tap water was
1 For interpretation of color in Fig. 4, the reader is referred to the web version of this article.
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W. Wang et al. / Corrosion Science 53 (2011) 845–849
Fig. 3. Schematic of the electrode distribution under a water drop on the carbon steel surface exposed to biodiesel (not to the real scale).
22 mm
a
2E-007 (A)
Biodiesel
Biodiesel
b
1E-007 0 -1E-007
22mm
Water
-2E-007 -3E-007
Water
-4E-007 -5E-007
6 mm
-6E-007 -7E-007 Fig. 4. (a) Current distributions on the surface of the WBE that vertically immersed in biodiesel–water mixture at 8 h, (b) Images of the rust covered WBE at 72 h. Cathode zone was indicated by the dashed line in (a).
used to simulate the ion-containing water in this study, but the effects of water and the ion species and concentrations in the water need to be further characterized, and more metal materials used in the fuel infrastructure should be tested. 4. Conclusion The in situ local current distribution and corrosion on carbon steel in biodiesel due to water contamination was monitored and mapped for the first time using the WBE technique. While most carbon steel electrodes corroded when exposed to ion-containing tap water, the cathodes were mainly formed at the biodiesel–water interface. The anodic current distribution in water showed a positive correlation with the distance from the biodiesel–water interface and the biodiesel concentration gradient in water. With the fast growth of biodiesel industry, these findings from the WBE method offer new ways to characterize the metal corrosion in biodiesel and provide insights in developing corrosion prevention strategies. The technique can also be used to study the heterogeneous electrochemistry on materials exposed to multiphase systems. Acknowledgements This work was supported by a scholarship from the China Scholarship Council (CSC) and College of Engineering and Applied Science at UCDenver. We thank Dr. Atousa Plaseied for the valuable discussions.
References [1] J.A. Melero, G. Calleja, A. Garcia, M. Clavero, E.A. Hernandez, R. Miravalles, T. Galindo, Storge stablilty and corrosion studies of renewable raw maerials and petrol micrures: A key issue for their co-processing in refinery units, Fuel 89 (2010) 554–562. [2] D.F. Aktas, J.S. Lee, B.J. Little, R.I. Ray, I.A. Davidova, C.N. Lyles, J.M. Suflita, Anaerobic metabolism of biodiesel and its impact on metal corrosion, Energ. Fuel 24 (2010) 2924–2928. [3] L. Díaz-Ballote, J.F. López-Sansores, L. Maldonado-López, L.F. Garfias-Mesias, Corrosion behaviour of aluminum exposed to a biodesel, Electrochem. Commun. 11 (2009) 41–44. [4] A.S.M.A. Haseeb, H.H. Masjuki, L.J. Ann, M.A. Fazal, Corrosion characteristics of copper and leaded bronze in palm biodiesel, Fuel Process. Technol. 91 (2010) 329–334. [5] X.Y. Lou, P.M. Singh, Role of water, acetic acid and chloride on corrosion and pitting behaviour of carbon steel in fuel-grade ethanol, Corros. Sci. 52 (2010) 2303–2315. [6] J.S. Lee, R.I. Ray, B.J. Little, An assessment of alternative diesel fuels: microbiological contamination and corrosion under storage conditions, Biofouling 26 (2010) 623–635. [7] S. Kaul, R.C. Saxena, A. Kumar, M.S. Negi, A.K. Bhatnagar, H.B. Goyal, A.K. Gupta, Corrosion behaviour of biodiesel from seed oils of Indian origin on diesel engine parts, Fuel Process. Technol. 88 (2007) 303–307. [8] X. Zhang, W. Wang, J. Wang, A novel device for the wire beam electrode method and its application in the ennoblement study, Corros. Sci. 51 (2009) 1475–1479. [9] W. Wang, X. Zhang, J. Wang, The influence of local glucose oxidase activity on the potential/current distribution on stainless steel: A study by the wire beam electrode method, Electrochem. Acta 54 (2009) 5598–5604. [10] W. Wang, Y.H. Lu, Y. Zou, X. Zhang, J. Wang, The heterogeneous electrochemical characteristics of mild steel in the presence of local glucose oxidase – a study by the wire beam electrode method, Corros. Sci. 52 (2010) 810–816. [11] D.L. Zhang, W. Wang, Y. Li, An electrode array study of electrochemical inhomogeneity of Zinc in Zinc/steel couple during galvanic corrosion, Corros. Sci. 52 (2010) 1277–1284.
W. Wang et al. / Corrosion Science 53 (2011) 845–849 [12] Y.J. Tan, N.N. Aung, T. Liu, Novel corrosion experiments using the wire beam electrode. (I) Studying electrochemical noise signatures from localized corrosion processes, Corros. Sci. 48 (2006) 23–38. [13] G.A. Zhang, Y.F. Cheng, Electrochemical corrosion of X65 pipe steel in oil/water emulsion, Corros. Sci. 51 (2009) 901–907.
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[14] H.Q. Becerra, C. Retamoso, D.D. Macdonald, The corrosion of carbon steel in oilin-water emulsions under controlled hydrodynamic conditions, Corros. Sci. 42 (2000) 561–575.
Contents lists available at ScienceDirect
Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Short Communication
Heterogeneous corrosion behaviour of carbon steel in water contaminated biodiesel Wei Wang a,b, Peter E. Jenkins c, Zhiyong Ren b,⇑ a Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, Ocean University of China, College of Chemistry and Chemical Engineering, Qingdao 266100, China b 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
a r t i c l e
i n f o
Article history: Received 14 August 2010 Accepted 20 October 2010 Available online 2 November 2010 Keywords: A. Mild steel C. Interfaces
a b s t r a c t Biodiesel has been widely used as an additive to traditional fuel supplies, but the corrosion of metals used in biodiesel infrastructure is becoming an increasing concern. In this study, the influence of water contamination and corrosion behaviour of carbon steel in biodiesel, were characterized using the wire beam electrode (WBE) technique. In situ local current distributions among the electrodes showed a distinct corrosion pattern, with the anodes formed in the area that was exposed to water, and the cathodes formed along the water–biodiesel interface. The anodic current distribution showed a positive correlation with the biodiesel concentration gradient in water. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Biodiesel is an alternative diesel fuel derived from renewable feed stock such as plant or animal lipids. The development of biodiesel as a renewable fuel has attracted considerable attention, as it demonstrated great potential in reducing the world’s dependence on fossil fuels and greenhouse gas (GHG) emission. Biodiesel can be effectively mixed with petroleum diesel and used in diesel engines in blends up to 20%. Though B5 or B20 blends (5% or 20% biodiesel blended with petroleum diesel) have been widely used in Europe, America, and Australia, higher percentages of biodiesel have not been broadly applied due to the concerns of the impact of integrating this alternative fuel into existing infrastructures, such as storage tanks, transportation pipelines, and automotive fuel systems. Biodiesel is considered chemically stable in pure form [1], but it can become more corrosive during storage, transportation, and utilization, when it degrades through moisture absorption, microbial oxidation, and other contaminations [2–6]. For example, water contamination due to condensation is considered as one of the main corrosion factors for storage containers and pipelines. Water contains corrosive ions and promotes microbial growth in the solution and at the biodiesel/water interface, and it may hydrolyze the methyl esters and produce more corrosive fatty acids [2,4]. Although corrosions have been reported by biodiesel manufacturers and users, few studies have provided information on the
⇑ Corresponding author. Tel.: +1 303 556 5287. E-mail address: [email protected] (Z. Ren). 0010-938X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2010.10.020
corrosion mechanisms and prevention and control strategies [2,3,6,7]. Lee et al. conducted some pioneer work on characterizing microbiologically influenced corrosion in biodiesel (B100, 100% biodiesel) and found it had the highest propensity for biofouling [6]. Aktas et al. recently reported that seawater accelerated the corrosion rate in biodiesel due to the reactions between chloride and/ or sulfide from the seawater with the carbon steel electrodes [2]. Other studies also reported corrosion of aluminum, copper, and bronze in biodiesel [3,4]. However, most studies used weight loss measurement and traditional electrochemical methods, such as polarization curve and electrochemical impedance spectroscopy to characterize the overall corrosion rate of plate metals. Such measurements can only provide average electrochemical information on the electrodes, but they are not able to quickly identify the corrosion distribution or distinguish the heterogeneous corrosion mechanisms that occur on metal surfaces at different exposure conditions. Taking water contamination as an example, it is believed that the interface between the biodiesel and formed water drop during condensation is more likely to corrode, but no in situ detection data is available to test the hypothesis. In this study, a unique and simple carbon steel wire beam electrode (WBE) module was developed to directly monitoring local electrochemical responses of the heterogeneous surface conditions. WBE is an electrode array that has been used to characterize local potential and current correlation, biofilm/metal interface, and electrochemistry in other corrosion systems [8–12]. The WBE results documented the formation of anodes and cathodes at locations under different exposure conditions (biodiesel, water, and the interface) and revealed the local corrosion current distribution. These findings provided valuable information on metal corrosion in biodiesel.
846
W. Wang et al. / Corrosion Science 53 (2011) 845–849
2. Experimental 2.1. WBE fabrication and material selection The WBE assay was fabricated by arranging 121 carbon steel ASTM C1080 wires (2.0 mm diameter, McMaster, USA) into an 11 11 matrix. The wires were embedded in epoxy resin for fixation (Fig. 1). The wire electrode surfaces were ground up to 1000 grit silicon carbide paper and washed with deionized water and ethanol before each test [8–11]. A cubic glass chamber (2.5 2.5 2.5 cm) was glued with epoxy on the WBE surface to contain biodiesel. A soy-based biodiesel (B100, Bertkus Oil Company, USA) was used in the experiment. Local tap water was added in some experiments to simulate the ion-containing condensed water in the biodiesel. The tap water sample contained 30 mg/L of Cl , 0.1 mg/L of NO3 , 24 mg/L of Na+, 31 mg/L of Ca2+, 7 mg/L of Mg2+, and 2.8 mg/L of K+.
The current distribution was continuously measured and mapped by the WBE monitoring module, which was constructed with modular instruments (National Instruments Co., USA) and controlled by self-designed programs using the LabVIEWÒ 8.6 software package [8–11]. The local current between each individual wire electrode and all other connected electrodes was measured sequentially. Specifically, one electrode is disconnected from the other 120 connected electrodes, and the current between this single electrode and the other connected electrodes was monitored by a multimeter in the modular device. Once the current data is collected, the electrode is 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. Each current measurement takes less than 1 s and therefore the total time for the 121 current measurements takes about 2 min. A current amplifier (PXI 4022) is employed in the device, which can detect picoampera current level with femtoampere noise [8]. The current distribution maps were drawn using the Surfer 8.0 software. All current data are expressed as Ampere (A).
2.2. Experiment set up and electrochemical measurements Two sets of experiments were conducted to mimic the water contamination in the biodiesel container. In the first test, biodiesel was filled up to the container in which the WBE was horizontally placed as the container bottom. To simulate water drop formation on the bottom of the storage tank, tap water was slowly dripped inside a stainless steel ring (about 1.8 cm in diameter) hung above the WBE without contacting the surface). The ring was used to prevent the water drop from moving around the surface (Fig. 1a). In the second test, the WBE was vertically placed to act as one container wall. Water was first added into the container, and then biodiesel was slowly filled on top of the water layer to form a separate layer of the fuel (Fig. 1b). All tests were performed at room temperature (25 °C) and repeated three times.
3. Results and discussion Fig. 2 shows the current distribution of the horizontal carbon steel WBE with a water drop on the surface surrounded by biodiesel. At approximately 20 min after the water drop was formed, cathodic currents were detected on several wire electrodes that were located along the edge of the water–biodiesel interface, as shown as the light green zones in Fig. 2a. By comparison, only one apparent anodic current was measured at the same time, as indicated by the arrow in Fig. 2a. Fig. 2b shows that more anode and cathode currents were detected within the water drop boundary after 2 h, but in a likely random pattern. This is probably due to the incomplete replacement of water to biodiesel within a short
Fig. 1. Photos and schematics of the WBE set up. (a) Horizontal WBE setting with a water drop inside of a stainless steel ring on the top of the WBE (without contacting). (b) Vertical WBE setting with biodiesel layer on top of the water layer.
847
W. Wang et al. / Corrosion Science 53 (2011) 845–849
a
(A)
(A)
2E-006
2E-006
b
1.5E-006
5E-007 0
22mm Line number
Line22mm number
1E-006
1.5E-006 1E-006 5E-007 0
-5E-007
22 mm Row number
(A) 2E-006
c
-5E-007
22 mm Row number
d
6 mm
22mm Line number
1.5E-006 1E-006 5E-007 0 -5E-007
22 mm Row number Fig. 2. Time-course current distributions of the water drop covered WBE at (a) 20 min (b) 2 h, and (c) 72 h (d) shows the rust covered WBE after 120 h of immersion. The dashed circle in (c) indicates the tendency of cathode distribution at the biodiesel–water interface.
period, in which the biodiesel film was still partially covered some WBE surface, resulting in cathode area as compared to water covered anode area. Most electrodes that were exposed to water corroded within 72 h, as shown in Fig. 2c, with cathodes that were mostly located at the outer edge of the water drop, i.e., the water–biodiesel interface (the dashed circle), and anodes that were primarily located within the water area (Fig. 2c). Fig. 2d shows that almost all the electrodes under the water drop were covered by rusts after 120 h of operation. This unique electrode distribution pattern in the area exposed to water is believed to relate to the water drop formation and its interaction with the biodiesel container. Water solubility in biodiesel is low, generally less than 0.2%, but during storage or transportation, water can be condensed and suspended in the form of small drops and interpose itself between the biodiesel and the metal surface of the storage tanks or pipes. Under stagnant conditions, the water drop will form a separate phase at the bottom of the biodiesel container. Therefore, the formation of the water drop on the metal surface will push the majority of biodiesel away, but leave a thin layer of water–biodiesel mixture on the metal surface. The water drop itself, in addition to the impurities in the mixture, such as chloride ion, will facilitate the anodic reaction and corrode the metal surface. It may also encourage microbial growth [6]. In contrast, more cathode zones are formed along the outer edge of the water drop, showing a current map similar to the pattern of the ‘‘Evans ring’’ test. Such pattern indicates that most anodic reaction occurs in central part of the water drop due to the high ion promoted corrosion, while the cathodic reaction takes place on the periphery of the water drop due to the higher oxygen concentration as compared to that in bulk water. Considering the low conductivity and higher oxygen solubility in biodiesel, we believe both
reactions occur primarily in the aqueous phase [13,14]. Fig. 3 shows the schematic of the anode–cathode distribution under a water drop on the metal surface, assuming the biodiesel was fully replaced by water on the metal surface. To further testify the hypothesis that more cathodes will be formed at the water–biodiesel interface while anodes will be mainly in the water phase, a second test was performed with a WBE vertically immersed in biodiesel and water solution to serve as a wall of the container. Fig. 4a shows the current distributions after 8 h, and it is clear that the cathodes were formed along the biodiesel–water interface (the fourth row of the WBE), while more anodes were formed in the water phase. Compared to the horizontal WBE test, Fig. 4a also shows a positive correlation between the amount of the anodes (darkness of the blue1 colour in water phase) and the distance from the biodiesel, supporting the hypothesis that the corrosion potential is correlated with biodiesel concentration gradient in water. This is further confirmed by Fig. 4b, which shows the apparent rust formation of the electrodes in the water phase after 72 h, while no corrosion was found in the biodiesel phase. All these findings confirmed that when there is water contamination in biodiesel, corrosion will occur in the water phase, and cathodes will be formed in the water–biodiesel interface. The WBE technique reveals the unique current distribution and corrosion patterns of water contamination in biodiesel. More characteristics of individual wire electrodes at different zones (anode, cathode, transition) can be studied using traditional electrochemical techniques in conjunction with the WBE method. Tap water was
1 For interpretation of color in Fig. 4, the reader is referred to the web version of this article.
848
W. Wang et al. / Corrosion Science 53 (2011) 845–849
Fig. 3. Schematic of the electrode distribution under a water drop on the carbon steel surface exposed to biodiesel (not to the real scale).
22 mm
a
2E-007 (A)
Biodiesel
Biodiesel
b
1E-007 0 -1E-007
22mm
Water
-2E-007 -3E-007
Water
-4E-007 -5E-007
6 mm
-6E-007 -7E-007 Fig. 4. (a) Current distributions on the surface of the WBE that vertically immersed in biodiesel–water mixture at 8 h, (b) Images of the rust covered WBE at 72 h. Cathode zone was indicated by the dashed line in (a).
used to simulate the ion-containing water in this study, but the effects of water and the ion species and concentrations in the water need to be further characterized, and more metal materials used in the fuel infrastructure should be tested. 4. Conclusion The in situ local current distribution and corrosion on carbon steel in biodiesel due to water contamination was monitored and mapped for the first time using the WBE technique. While most carbon steel electrodes corroded when exposed to ion-containing tap water, the cathodes were mainly formed at the biodiesel–water interface. The anodic current distribution in water showed a positive correlation with the distance from the biodiesel–water interface and the biodiesel concentration gradient in water. With the fast growth of biodiesel industry, these findings from the WBE method offer new ways to characterize the metal corrosion in biodiesel and provide insights in developing corrosion prevention strategies. The technique can also be used to study the heterogeneous electrochemistry on materials exposed to multiphase systems. Acknowledgements This work was supported by a scholarship from the China Scholarship Council (CSC) and College of Engineering and Applied Science at UCDenver. We thank Dr. Atousa Plaseied for the valuable discussions.
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