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Jul 9, 2004 - The corrosion of three metals (aluminum, copper, and austenitic steel (SS 316)) at 80 °C by bio-oil obtained by vacuum pyrolysis of sof...
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Energy & Fuels 2004, 18, 1291-1301

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Corrosion of Metals by Bio-Oil Obtained by Vacuum Pyrolysis of Softwood Bark Residues. An X-ray Photoelectron Spectroscopy and Auger Electron Spectroscopy Study Hans Darmstadt,†,‡ Manuel Garcia-Perez,† Alain Adnot,†,§ Abdelkader Chaala,† Detlef Kretschmer,⊥ and Christian Roy*,† Chemical Engineering Department, CERPIC (Surface Analysis Laboratory), and Mechanical Engineering Department, Laval University, Quebec City, QC, G1K 7P4, Canada Received December 2, 2003. Revised Manuscript Received May 26, 2004

The corrosion of three metals (aluminum, copper, and austenitic steel (SS 316)) at 80 °C by bio-oil obtained by vacuum pyrolysis of softwood bark residue was studied using X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) depth profiling. The bio-oil is very acidic (pH value of 3) and contains significant amounts of water and metal ions that are not present in significant concentrations in the feedstock material (e.g. Cu, Pb, and Fe). These metals were most probably leached from the pyrolysis reactor or peripheral installations. Aluminum and, to a much smaller degree, copper were corroded by bio-oil, whereas SS 316 was not affected. Oxide and/or hydroxide layers were formed on all three metals. However, in the case of aluminum and copper, these layers did not protect the underlying metal against further oxidation. For SS 316, after initial modification of the surface (e.g., leaching of Fe species), subsequent oxidation was prevented by the chromium oxide layer.

1. Introduction There is considerable concern about the widespread use of fossil fuels producing large amounts of carbon dioxide, which is the main contributor to the greenhouse effect. Fuels produced from renewable resources, such as forest residues, do not increase the net amount of carbon dioxide released to the atmosphere. Therefore, oils obtained by pyrolysis of biomass could help satisfy the energy demand in an environmentally friendly way. Biomass, such as bark residue, can be transformed by pyrolysis to charcoal, oil, and gas. Many research groups have studied the use of pyrolytic oils, also known as biooils, as a fuel for burners and gas turbines.1 There are important differences between bio-oils and petroleumderived oils. In comparison to petroleum-derived oils, bio-oils have a lower heating value, a higher oxygen content, and are much more corrosive.2 It was observed that, even at low temperatures, bio-oils strongly corrode aluminum, mild steel, and nickel-based materials, whereas stainless steel, cobalt-based materials, brass, * Author to whom correspondence should be addressed. Telephone: (418) 656-7406. Fax: (418) 656-2091. E-mail address: Christian.Roy@ gch.ulaval.ca. † Chemical Engineering Department. ‡ Present address: Alcan International Limited, Arvida Research and Development Centre, 1955 Boulevard Mellon, PO Box 1250, Jonquie`re, QC, G7S 4K8, Canada. E-mail address: hans.darmstadt@ alcan.com. § CERPIC, Surface Analysis Laboratory. ⊥ Mechanical Engineering Department. (1) Bridgewater, A. V.; Peacocke, G. V. C. Fast Pyrolysis Processes for Biomass. Renewable Sustainable Energy Rev. 2000, 4, 1-73. (2) Oasmaa, A.; Czernik, S. Fuel Oil Quality of Biomass Pyrolysis Oils-State of the Art for the End Users. Energy Fuels 1999, 13, 914921.

and various plastics are much more resistant.3,4 However, practical tests have shown that corrosion-resistant materials for the use of bio-oils in diesel engines can be found (e.g., high-chromium stainless steels).5 The corrosiveness of bio-oils is attributed to the high concentrations of organic acids (e.g., formic, acetic, and propanoic acid). Furthermore, the corrosiveness of bio-oils increases as the temperature and their water content each increase.6 This type of corrosion has important consequences for the construction of heating boilers and power-generation plants, because materials that are suitable for petroleum-derived oils are strongly corroded by bio-oils.7 In the present work, we are only concerned about corrosion in storage tanks and in “relatively cold” portions of combustion plants (such as the feeding systems). Additional corrosion occurs in “hot” portions, (3) Oasmaa, A.; Leppa¨ma¨ki, E.; Koponen, P.; Levander, J.; Tapola, E. Physical Characterisation of Biomass-Based Pyrolysis Liquids. Application of Standard Fuel Oil Analyses; VTT Publications 306; Technical Research Centre of Finland: Espoo, Finland, 1997 (available via the Internet at http://www.inf.vtt.fi/pdf/publications/1997/P306.pdf). (4) Fuleki, D. Bio-fuel System Materials Testing. In Pyrolysis Network Newsletter, March 7, 1999 (available via the Internet at http:// www.pyne.co.uk/pdf/let7.pdf). (5) Jay, D. C.; Rantanen, O.; Sipila¨, K.; Nylund, N.-O. Wood Pyrolysis Oil for Diesel Engines. In Proceedings of the 1995 Fall Technical Conference (Milwaukee, WI, September 24-27, 1995); ASME, Internal Combustion Engine Division: New York, 1995. (6) Aubin, H.; Roy, C. Study on the Corrosiveness of Wood Pyrolysis Oils. Fuel Sci. Technol. Int. 1990, 8, 77-86. (7) Chiaramonti, D.; Bonini, M.; Fratini, E.; Tondi, G.; Gartner, K.; Bridgwater, A. V.; Grimm, H. P.; Soldaini, I.; Webster, A.; Baglioni, P. Development of Emulsions from Biomass Pyrolysis Liquid and Diesel and Their Use in EnginessPart 2: Tests in Diesel Engines. Biomass Bioenergy 2003, 25, 101-111.

10.1021/ef0340920 CCC: $27.50 © 2004 American Chemical Society Published on Web 07/09/2004

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such as in the furnace. Corrosion there is mainly attributed to the presence of alkali and alkali-earth metals in the bio-oils.8 In the present work, three metals were brought into contact with a bio-oil obtained by vacuum pyrolysis of wood bark. The changes in the metal surface chemistry were followed by X-ray photoelectron spectroscopy (XPS) and by Auger electron spectroscopy (AES). The purpose of this research was to gain a deeper understanding in the oxidation mechanism of metals by bio-oil, with the long-term objective to predict the corrosion resistance of metals for “cold” portions of heating and power-generation installations. The surface chemistry of these metals is also relevant to better understand the catalytic effect that tank walls could have on the bio-oil aging during storage. 2. Experimental Procedures 2.1. Bio-oils. Bio-oils were obtained via the vacuum pyrolysis of bark residues from different softwood trees: 31 vol % balsam fir (Abies balsamea), 55 vol % white spruce (Picea glauca), and 14 vol % black spruce (Picea mariana). The pyrolysis was performed in a process development plant at a temperature of ∼530 °C and a total pressure of 40 kPa (run H67). Under these conditions, the pyrolysis yields were 26 wt % bio-oil, 19 wt % aqueous phase, 27 wt % gas, and 28 wt % charcoal. Details of the pyrolysis conditions9 and the reactor design10-12 can be found elsewhere. After condensation of the vapors and separation of the aqueous phase, the bio-oil consisted of two organic layers.9 Corrosion tests were performed with the upper and the bottom layers, and the whole bio-oil, respectively. 2.2. Corrosion Tests. Prior to the corrosion tests, the surface of the metal strips was cleaned with silicon carbide paper until no impurities could be detected by visual inspection. Final polishing was performed with a 240-grit silicon carbide paper. The corrosion tests were performed with metal strips consisting of aluminum, copper, and stainless steel (SS 316), respectively. The length of the metal strips was 30 mm, and their width was 10 mm, whereas their thickness was 0.6, 1.2, and 1.0 mm for the aluminum, copper, and SS 316, respectively. After the metal strips were weighed, they were separately placed in closed glass containers that were filled with ∼60 mL of bio-oil (upper layer, bottom layer, and the whole bio-oil). The experiments were performed at a temperature of 80 °C for periods in the range of 12-168 h. During these experiments, the weight of the bio-oil decreased slightly, most probably by evaporation of volatile compounds. After 168 h, the weight loss of the bio-oil was ∼0.6 wt %. After the (8) Salmenoja, K.; Kari, K. M. How to Minimise Corrosion Problems in Recovery and Power Boilers. Appita Ann. Gen. Conf. Proc. 2000, 54th (1), 361-367. (9) Ba, T.; Chaala, A.; Garcia-Perez, M.; Rodrigue, D.; Roy, C. Colloidal Properties of Bio-oils Obtained by Vacuum Pyrolysis of Softwood Bark. 1. Characterization of Water-Soluble and WaterInsoluble Fractions. Energy Fuels 2004, 18, 704-712. (10) Roy, C.; Blanchette, D.; Korving, L.; Yang, J.; De Caumia, B. Development of a Novel Vacumm Pyrolysis Reactor with Improved Heat Transfer Potential. In Biomass Conversion; Bridgwater, A. V., Boococks, D. G. B., Eds.; Blakie Academic and Professional: London, U.K., 1997; pp 351-367. (11) Roy, C.; Morin, D.; Dube´, F. The Biomass Pyrocycling Process. In Biomass Gasification and Pyrolysis: State of the Art and Future Prospects. Kaltschmitt, M., Bridgwater, A. V., Eds.; CPL Press: Newbury, U.K., 1997; pp 307-315. (12) Roy, C.; Blanchette, D.; de Caumia, B.; Dube´, F.; Pinault, J.; Be´langer, E Ä .; Laprise, P. Industry Scale Demonstration of the Pyrocycling Process for the Conversion of Biomass to Biofuels and Chemicals. In First World Conference and Exhibition on Biomass for Energy and Industry, June 5-9, 2000, Sevilla, Spain, Vol. II; Kyritsis, S., Beenackers, A. A. C. M., Helm, P., Grassi, A., Chiaramonti, D., Eds.; James & James Science Publishers: London, U.K., 2001; pp 10321035.

Darmstadt et al. corrosion tests, the metal strips were washed for 1 h with methanol to remove adsorbed bio-oil. To remove loosely attached organic deposits, the strips then were gently wiped with tissue paper and their weight was determined. The samples that were in contact with the bio-oil for 168 h were analyzed by XPS and AES. 2.3. Surface Chemistry by X-ray Photoelectron Spectroscopy. An ESCALAB MK II-MICROLAB 500 spectrometer (VG Scientific, East Grinstead, U.K.) with non-monochromatized Mg KR radiation was used. The pressure in the analytical chamber was 0.1 1.0 36.0 0.1 0.1 0.7 1.2 610.0 5.0 3.0 3.0 0.1 0.7 5.0 0.8 3.0 20.0 >0.1 2.0 >0.1 >0.1 8.0

4.0 0.1 5.0 100.0 1.0 0.2 1.0 5.0 1500.0 30.0 9.0 9.0 0.2 1.4 16.0 2.0 30.0 85.0 0.2 22.0 0.1 0.1 25.0

two layers. The bottom layer contained more organic oxygen and water. Both layers had the same pH value. However, the concentration of acid groups in the bottom layer was considerably higher than in the upper layer. The upper layer mainly consists of biomass extractive compounds and their derivatives, such as fatty acids, fatty esters, sterols, and paraffins. The bio-oil also contained inorganic compounds, especially in the bottom layer. The most important metals in the bio-oils were iron, calcium, potassium, and lead (Table 2). After pyrolysis, most of the metals present in the feedstock are found in the charcoal and only a small portion ends up in the oil. Thus, the metal concentration in the oils is usually much lower than in the feedstock. This was also the case in the present investigation. However, there were three exceptions. The concentrations of iron, lead, and copper in the oil were higher than in the feedstock. This suggests that most of these three elements was leached by the bio-oil from the pyrolysis reactor or the peripheral installations (e.g., condenser and tubing). Copper is used in sealing rings; iron is the main construction material of the reactor and other installations, whereas lead could originate from welded joints. Visual inspection of the interior of laboratory reactors used for many pyrolysis experiments indicated

Figure 1. Weight change of metals upon contact with whole bio-oil.

indeed severe corrosion of the metal parts. The organic compounds, which were recovered in the condensers as bio-oil, leave the reactor as vapors. Therefore, it seems somewhat surprising that metal compounds found their way into the bio-oil. However, during pyrolysis experiments, it was frequently observed that the current of the vapors flowing from the reactor to the condensers entrained small charcoal particles. These charcoal particles are rich in inorganic compounds. Furthermore, it is also possible that the inorganic compounds were entrained in the form of mineral particles. The presence of metals in the bio-oil is important, because it may influence the corrosion behavior (see below). Additional information on bio-oil properties and composition are reported elsewhere.14 3.2. Weight Change of Strips upon Contact with Bio-oil. Information on the corrosion can be obtained from the mass change of the metal strips when they contact the bio-oil. Chemisorption of oxygen and other gases and vapors present in the atmosphere will initially increase the mass of the metal strips. However, the leaching of metal species will decrease the sample mass. Furthermore, upon contact with bio-oil, organic deposits are formed on the surface of the metal strips (see below) which increases the sample mass. As long as corrosion is not very significant, these effects may compensate each other, to a certain degree. For the three metals studied, the weight changes differed considerably. A significant weight loss was observed for aluminum. After 168 h, the aluminum strip lost ∼5% of its initial weight (Figure 1). A weight loss was also observed for copper. However, in this case, the weight loss was almost 100 times smaller (0.06%). For SS 316, the weight changes were even smaller and the data scattered somewhat; nonetheless, linear regression of the data indicated a very small mass increase for this sample, which totaled ∼0.01% after 168 h. It can be summarized that the bio-oil caused severe corrosion of the aluminum. The copper was also corroded, but to a much smaller degree. The data indicated that no or only little corrosion occurred for SS 316. These results are in agreement with the findings described in the literature.3,4 The weight changes also were dependent on the oil fraction. As with the whole bio-oil, the upper and the (14) Ba, T.; Chaala, A.; Garcia-Perez, M.; Roy, C. Colloidal Properties of Bio-oils Obtained by Vacuum Pyrolysis of Softwood Bark. 2. Storage Stability. Energy Fuels 2004, 18, 188-201.

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Table 3. Elemental Surface Composition (via XPS) of Aluminum Strips before and after Contact with Bio-oila Elemental Content (at. %) aluminum sample

Al

C

Cr

Cu

Fe

N

Na

Ni

O

Pb

prior to contact with bio-oil after contact with the upper layer of the bio-oil after contact with the bottom layer of the bio-oil after contact with the whole bio-oil

26.2 8.7 0.7 1.6

28.3 58.9 76.4 76.5

n.d. n.d. n.d. n.d.

n.d. 0.3 n.d. n.d.

n.d. 0.1 n.d. n.d.

2.4 0.9 0.7 1.1

n.d. n.d. 0.4 0.4

n.d. n.d. n.d. n.d.

43.1 30.7 21.0 20.3

n.d. 0.1 0.4 0.4

a

n.d. denotes that the element was not detected during reasonable acquisition time. Table 4. Elemental Surface Composition (via XPS) of Copper Strips before and after Contact with Bio-oila Elemental Content (at. %) copper sample

Al

C

Cr

Cu

Fe

N

Na

Ni

O

Pb

before contact with bio-oil after contact with the upper layer of the bio-oil after contact with the bottom layer of the bio-oil after contact with the whole bio-oil

n.d. n.d. n.d. n.d.

40.7 61.1 77.9 73.9

n.d. n.d. n.d. n.d.

8.1 11.6 1.7 4.4

n.d. n.d. n.d. n.d.

n.d. n.d. n.d. n.d.

n.d. n.d. n.d. n.d.

n.d. n.d. n.d. n.d.

51.2 27.3 20.4 21.7

n.d. n.d. n.d. n.d.

a

n.d. denotes that the element was not detected during reasonable acquisition time. Table 5. Elemental Surface Composition (via XPS) of SS 316 Strips before and after Contact with Bio-oila Elemental Content (at. %) SS 318 sample

Al

C

Cr

Cu

Fe

Mn

N

Na

Ni

Si

O

Pb

before contact with bio-oil after contact with the upper layer of the bio-oil after contact with the bottom layer of the bio-oil after contact with the whole bio-oil

n.d. n.d. n.d. n.d.

59.8 71.3 60.5 66.8

1.5 1.4 4.8 2.9

n.d. 0.3 n.d. n.d.

4.2 n.d. 1.0 0.7

>0.1 >0.1 >0.1 >0.1

n.d. n.d. n.d. n.d.

n.d. 0.8 n.d. n.d.

2.3 0.9 0.8 0.3

>0.1 >0.1 >0.1 >0.1

32.7 23.2 32.8 28.4

n.d. 0.1 n.d. n.d.

a

n.d. denotes that the element was not detected during reasonable acquisition time.

bottom layers caused a significant weight loss of aluminum, a much smaller weight loss of copper, and a very small weight gain of SS 316 (not shown). However, the weight changes were much more pronounced after contact with the bottom layer, as compared to the upper layer. For aluminum and SS 316, the weight changes after contact with the bottom layer were approximately twice as large, as compared to the upper layer. This difference was even larger for copper. In this case, the weight loss caused by the bottom layer was five times larger as compared to the upper layer. This behavior can be explained by the concentration of acid groups in the bottom layer, which was considerably higher than in the upper layer (see Table 1). In the next sections, the chemical nature of the metal surface before and after contact with the oil fractions is discussed. 3.3. Changes of the Surface Chemistry of the Metal Strips upon Contact with Pyrolytic Oil. 3.3.1. Elemental Composition. On the surface of the aluminum strip before contact with the pyrolytic oil, in addition to aluminum, carbon, oxygen, and nitrogen were detected (Table 3). It is well-known that, in air, metallic aluminum reacts with oxygen and a thin Al2O3 layer is formed on its surface,15 often preventing further oxidation of the metal. Furthermore, unless special measures are taken, after contact with the atmosphere, even “clean” metal surfaces are covered with organic compounds. Thus, the carbon, oxygen, and nitrogen were assigned to the Al2O3 layer and to organic compounds deposited on it. Prior to contact with the biooil, on the surfaces of copper and SS 316, carbon and oxygen were detected as well. However, in contrast to aluminum, no nitrogen was observed (Tables 4 and 5). (15) Rotole, J. A.; Sherwood, P. M. A. X-ray Photoelectron Spectroscopic Studies of the Oxidation of Aluminium by Liquid Water Monitored in an Anaerobic Cell. Fresenius J. Anal. Chem. 2001, 369, 342-350.

Upon contact of the three metals with the bio-oil, the concentration of carbon increased significantly, indicating the formation of strongly adsorbed organic deposits that were not removed by solvent washing and the gentle mechanical cleaning that was performed prior to the XPS experiments (see Section 2.2). The formation of the deposits was also evidenced by pronounced darkening of the metals upon contact with the bio-oil. The chemical nature of these organic deposits will be discussed in Section 3.3.2. After contact with the bio-oil, on some of the aluminum and SS 316 samples, in addition to the elements found on the “clean” strips, copper, iron, lead, and sodium were detected. Of these elements, only sodium is usually present in significant concentrations in biomass.16 The concentrations of copper, iron, and lead are generally in the parts per million (ppm) range.17,18 This was also the case for the softwood pyrolyzed in the present investigation (Table 2). However, as already discussed previously, apparently the bio-oil leached copper, iron, and lead from the reactor and peripheral installations. The amount of metal compounds deposited on the strips was dependent on the bio-oil fraction. On the SS 316 strips, copper and lead were detected only after contact with the upper layer of the bio-oil (see Table 5). Similarly, copper and iron were detected on the aluminum strips only after the strips were in contact with the upper layer (see Table 3). The presence of these metals on the aluminum and SS 316 strips can have important corrosion implications. (16) Oasmaa, A.; Kuoppala, E. Fast Pyrolysis of Forestry Residue. 3. Storage Stability of Liquid Fuel. Energy Fuels 2003, 17, 1075-1084. (17) Devi, T. G.; Kannan, M. P. Gasification of Biomass Chars in AirsEffect of Heat Treatment Temperature. Energy Fuels 2000, 14, 127-130. (18) Oikari, R.; Aho, M.; Hernberg, R. Demonstration of a New OnLine Analyzer for the Measurement of Vaporized Toxic Metal Compounds in a Fluidized Bed Combustor. Energy Fuels 2003, 17, 87-94.

Corrosion of Metals by Bio-oil from Bark Residue

Energy & Fuels, Vol. 18, No. 5, 2004 1295

Table 6. Relative Area of the C 1s Peaks Relative Peak Area (%) sample

C1, C-C

C2, C-OH

C3, CdO

C4, -CO2H

aluminum, prior to contact with bio-oil aluminum, after contact with upper layer aluminum, after contact with bottom layer aluminum, after contact with whole bio-oil

60 68 70 67

20 19 19 23

12 6 9 8

1 7 2 2

copper, prior to contact with bio-oil copper, after contact with upper layer copper, after contact with bottom layer copper, after contact with whole bio-oil

56 69 59 61

24 15 30 27

13 11 9 10

7 5 2 2

SS 316, prior to contact with bio-oil SS 316, after contact with upper layer SS 316, after contact with bottom layer SS 316, after contact with whole bio-oil

78 66 64 58

11 23 22 28

8 7 10 11

3 4 4 3

Figure 2. C 1s XPS spectra of metal strips before and after contact with bio-oil. The spectra were normalized to the maximum intensity.

Metal ions of higher electrode potential oxidize metals of lower electrode potential (e.g., 3Cu2+ + 2Al f 3Cu + 2Al3+), which may accelerate corrosion of the less-noble metal. This point will be discussed below. 3.3.2. Chemical Nature of the Organic Deposits on the Metal Strips. It was already mentioned that, even on the “clean” metals prior to contact with the bio-oil, significant amounts of carbon were detected. For the aluminum and copper strips, the elemental concentration of carbon increased considerably upon contact with the bio-oil (see Tables 3 and 4), whereas for SS 316, the increase was much more moderate (see Table 5). Information on the chemical nature of the organic deposits can be obtained from the C detail spectra. These spectra were fitted to four peaks: one peak for C atoms with bonds to C and H atoms (C1, C-C, BE ) 285 eV), and three peaks for C atoms with one, two, and three bonds to O atoms (C2, C-OH, BE ) 286.5 eV; C3, CdO and C-O-C, BE ) 288.0 eV; and C4, -COOH, BE ) 289.5 eV,19 respectively). For all three metals, the most intense peak in the spectra was the C1 peak. However, all spectra also showed significant peaks of C atoms with bonds to O atoms (Figure 2). After contact with the different layers of the bio-oil, the total peak areas of the peaks assigned to C atoms with bonds to O atoms were comparable. These peaks accounted for approximately one-third of the total area (see Table 6). As already mentioned, upon (19) Briggs, D. In Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy; Briggs, D., Seah, M. P., Eds.; Wiley: Chichester, U.K., 1983; p 359.

contact with the bio-oil, the color of the metal strips changed to gray and/or black, indicating the formation of organic deposits. The bio-oils are very acidic; therefore, the deposits may have formed by acid-catalyzed condensation reactions (e.g., R-OH + HO-R′ f R-OR′ + H2O). Such reactions were observed when the biooils were stored at temperatures as low as 9 °C.16 The observed C spectra are in agreement with oxygen-rich high-molecular-weight compounds formed by condensation reactions. The presence of significant amounts of other classes of compounds such as aromatic hydrocarbons in the deposits must be excluded. The biomass feedstock material contained lignin, which, on pyrolysis, yields aromatic compounds.1 However, in the C XPS spectra, no π f π* peaks were observed, which are found in the spectra of aromatic compounds. These peaks are usually shifted by ∼6.5 eV to higher BE values from the C1 peak.20 Therefore, our results indicated that no or only small amounts of these aromatic compounds (such as pyrolytic lignin) participated in the formation of deposits on the metals. Additional information on the organic deposits and on the nature of inorganic oxygen compounds on the metals can be obtained from the O detail spectra. These spectra were fitted to four peaks: one peak for O atoms in inorganic oxides (O1, oxide), two peaks for O atoms in an organic environment with two and one bonds to C atoms, respectively (O2, CdO and C-O-C, BE ) 531.5 eV; and O3, C-OH, BE 533.0 eV), and finally one peak assigned to strongly adsorbed water (O4, H2O, BE ) 534.6-535 eV21). In the case of the copper strips, the O2 peak also may contain a contribution from Cu(OH)2.22 In the case of aluminum, the BE value of the oxide peak was 531.0 eV, whereas its BE value was 530.0-530.5 eV in the case of copper and SS 316, respectively. These BE correspond to the literature values for Al2O3 and the oxides of copper, iron, chromium, and nickel, respectively.22 The spectra of aluminum and copper prior to contact with the bio-oil were dominated by the peaks assigned (20) Gardella, J. A.; Ferguson, S. A.; Chin, R. L. pi* r pi Shake-up Satellites for the Analysis of Structure and Bonding in Aromatic Polymers by X-ray Photoelectron Spectroscopy. Appl. Spectrosc. 1986, 40, 224-232. (21) Xie, Y.; Sherwood, P. M. A. X-ray Photoelectron Spectroscopic Studies of Carbon Fiber Surfaces. 11. Differences in the Surface Chemistry and Bulk Structure of Different Carbon Fibers Based on Poly(acrylonitrile) and Pitch and Comparison with Various Graphite Samples. Chem. Mater. 1990, 2, 293-298. (22) National Institute of Standards and Technology XPS and Auger Databank, U.S. Department of Commerce, Washington, DC (available via the Internet at http://srdata.nist.gov/xps/index.htm).

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Table 7. Relative Area of the O 1s Peaks Relative Peak Area (%) sample

O1, oxide

O2, CdO

O3, C-OH

O4, -H2O

aluminum, prior to contact with bio-oil aluminum, after contact with upper layer aluminum, after contact with bottom layer aluminum, after contact with whole oil

8 0 3 2

61 23 25 20

29 73 71 77

2 4 1 1

copper, prior to contact with bio-oil copper, after contact with upper layer copper, after contact with bottom layer copper, after contact with whole

12 7 0 4

62 82 36 47

21 10 64 44

4 0 0 5

SS, prior to contact with bio-oil SS, after contact with upper layer SS, after contact with bottom layer SS, after contact with bottom layer

44 6 22 15

41 38 49 43

13 46 28 37

2 10 1 5

Figure 3. O 1s XPS spectra of the metal strips before and after contact with bio-oil. The spectra were normalized to the maximum intensity.

to O atoms in an organic environment (Figure 3). The oxide peak only accounted for ∼10% of the total peak area (Table 7). Upon contact with the bio-oil, the relative area of the oxide peak decreased, whereas the areas of the peaks assignment to organic deposits increased. This indicates that the organic deposits formed from the bio-oil covered at least a portion of the oxides. However, for copper, this conclusion cannot be drawn from the O spectra. The BE value of O atoms in Cu(OH)2 (531.1 eV) falls into the range of that of the O2 peak, whereas the BE values of O atoms in Cu2O (530.5 eV) and CuO (529.8 eV) are lower.22 For the copper sample prior to contact with the bio-oil, an oxide peak at 530.4 eV was found, indicating the presence of Cu2O. After contact with the upper layer and the whole bio-oil, the oxide peak was observed at 530.7 eV, suggesting that a portion of the Cu2O was transformed to Cu(OH)2. Finally, after contact with the bottom layer, no oxide peak was detected. The corresponding Cu 2p spectrum, however, clearly confirmed the presence of Cu(OH)2 (see below). Interestingly, on SS 316, larger oxide peaks than on the other two metals were observed. There are two possible explanations for this observation. First, the layer of organic compounds deposited on SS 316 could be thinner, in comparison to the other metals, or, second, the oxide layer on SS 316 could be thicker than that on the other metals. The first possibility is very unlikely, because, prior to contact with the bio-oil, considerably more carbon was found on SS 316 than on the other metals (see Tables 3-5), indicating that there

were more organic compounds adsorbed on SS 316 than on the other metals. Thus, the large oxide peaks in the spectrum of SS 316 suggests that the oxide layer was thicker than that on the other metals. This is important, with respect to corrosion, because oxide layers can protect the underlying metal against oxidation.23 For example, in Cr2O3 layers, the diffusion of metal and O ions is very slow, which significantly decreases the corrosion of austenitic steels (such as SS 316). In the present work, the AES depth profiles of SS 316 indicated indeed the formation of a thick, chromium-rich oxide layer (see below). The oxygen spectra before and after contact with the bio-oil differed considerably. Significant differences were also found between the three metals (see Figure 3). On the apparently clean metals, most probably smallmolecular-weight organic compounds were adsorbed. As suggested by the C spectra, after contact with the biooils, the metals were covered with high-molecularweight organic deposits most probably formed by condensation reactions. The nature of the oxygen groups differs for these two groups of compounds, which explains the observed differences in the O spectra. It is reasonable to assume that small-molecular-weight organic compounds contained predominantly C-OH, R-CH2-CHO, and -COOH groups, whereas in the high-molecular-weight organic deposits, mostly C-OC, R-CH2-CHdCH-CHO-R, and -CO-O-R groups were present. Unfortunately, XPS is not a very suitable method to differentiate between these functional groups, because the BE value of O atoms in different chemical environments is very similar.24 Thus, the only conclusion that can be drawn from the O spectra concerning the organic deposits is that the chemical nature of the oxygen groups in these deposits is strongly dependent on the chemistry of the metal substrate, which will be discussed next. 3.3.3. Chemical Nature of Aluminum Strips before and after Contact with Bio-oil. The Al 2p spectra showed two doublets, at 72.8 eV (Al 2p3/2, not corrected for charging) and at 74.4 eV, which were assigned to metallic aluminum and to Al2O3, respectively.22 In all spectra, the Al2O3 doublet was much more intense than the doublet of metallic aluminum (Figure 4). The most intense (23) Tanga, J. E.; Halvarssona, M.; Astemanb, H.; Svenssonb, J.-E. The Microstructure of the Base Oxide on 304L Steel. Micron 2001, 32, 799-805. (24) Beamson, G.; Briggs, D. High-Resolution XPS of Organic Polymers. The Scienta ESCA300 Database; Wiley: Chichester, U.K., 1992.

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Table 8. Position of the Cu XPS and Auger Signals XPS, Binding Energy, Cu 2p3/2 (eV) sample

Peak 1

aluminum, after contact with upper layer SS 316 after contact with upper layer copper, before contact with bio-oil copper, after contact with upper layer copper, after contact with bottom layer copper, after contact with whole bio-oil

933.2a 932.7a 933.1a 932.6a

copper Cu2O CuO Cu(OH)2 (GeO2)0.65 (Na2O)0.3 (CuO)0.05

932.6b

a

AES, Kinetic Energy, LMM (eV)

Peak 2

Peak 1

Peak 2

933.4 932.7

919.4 916.8

913.8

935.0 334.6 934.6 934.8 918.6b 916.7b 919.5b

932.4b 933.6b 934.8b 932.7c b

914c c

Not corrected for charging. Average value of all data for this compound listed in ref 22. Data taken from ref 26.

Figure 5. Cu 2p XPS spectra of the aluminum and SS 316 strips after contact with the upper layer. The spectra were normalized to the maximum intensity.

Figure 4. Al 2p XPS spectra of the aluminum strips before and after contact with bio-oil. The spectra were normalized to the maximum intensity.

doublet of metallic aluminum was found for the aluminum strip before contact with the bio-oil, whereas smaller doublets of metallic aluminum were observed after contact with the bio-oil. It was already mentioned that the metallic aluminum was covered by an Al2O3 layer, which itself was covered by organic deposits. The presence of Al doublets after contact with the bio-oil indicates that the combined thickness of these two layers was smaller than the XPS analysis depth (∼5 nm). On high-purity aluminum, the typical thickness of the Al2O3 layer is 3 nm.15 Because the bio-oil strongly corroded the aluminum strips (see Figure 1), it can be concluded that the Al2O3 layer did not protect the underlying metallic aluminum against oxidation by the bio-oil. The Al2O3 layer was etched away, and the very reactive metal was exposed to the oxidizing bio-oils. After contact with the bottom layer, no doublet of metallic aluminum was observed. In this case, the surface concentration of aluminum was very low (0.7%, see Table 3), indicating that almost the entire volume probed by XPS consisted of organic deposits. Beneath this thick layer and the Al2O3 layer, metallic aluminum

could not be detected. However, it is assumed that, as in the other cases, the thickness of Al2O3 layer was on the order of a few nanometers. As already mentioned previously, after contact with the bio-oil, on the aluminum strips, several metals that were not present on the “clean” aluminum were detected. These were copper, iron, and lead after contact with the upper layer and lead after contact with the bottom layer and the whole bio-oil (see Table 3). All these metals have a higher standard potential than aluminum25 and may have a significant influence on the corrosion of aluminum. Thus, for further analysis, their detail spectra were recorded. After contact with the upper layer, the Cu 2p spectrum showed a doublet at BE ) 933.4 eV (Cu 2p3/2 peak, Figure 5). Such a BE value is typical for Cu2+, such as in CuO (Table 8). The corresponding Cu Auger spectrum showed two peaks, at KE values of 919.4 and 913.8 eV, respectively (Figure 6). The first KE value is typical for CuO, whereas the second KE value was reported for mixed oxides that contained CuO (see Table 8). All these signals clearly indicated the presence of Cu2+ species on the Al strips. However, the Cu 2p spectrum (Figure 5) did not show any satellites, which are usually found for Cu2+ species shifted by 5-10 eV to higher BE values from the principal Cu 2p signals.27,28 (25) Gra¨fen, H.; Horn, E.-M.; Schlecker, H.; Schindler, H. Corrosion. In Ullmann’s Encyclopedia of Industrial Chemistry, Release 2003, 7th Edition; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany. (26) Hussain, Z.; Salim, M. A.; Khan, M. A.; Khawaja, E. E. X-ray Photoelectron and Auger Spectroscopy Study of Copper-SodiumGermanate Glasses. J. Non-Cryst. Solids 1989, 110, 44-52.

1298 Energy & Fuels, Vol. 18, No. 5, 2004

Darmstadt et al.

Figure 6. Cu LMM Auger spectra of the aluminum and SS 316 strips after contact with the upper layer. The spectra were normalized to the maximum intensity.

Figure 8. Pb 4f XPS spectra of the aluminum and SS 316 strips after contact with bio-oil. The spectra were normalized to the maximum intensity. Figure 7. Fe 2p XPS spectrum of the aluminum strip after contact with the upper layer.

The Fe 2p spectra showed two doublets, at BE (2p3/2 peak) values of 711.9 and 716.9 eV, respectively (Figure 7). The BE of the first doublet is typical for Fe2O3,22 whereas the second doublet was assigned to a satellite. Finally, the Pb 4f spectra showed doublets at BE ) 139.2 eV (Pb 4f7/2, Figure 8), indicating the presence of Pb2+ species.22 It can be summarized that the metal species found on the aluminum strip after contact with the bio-oil were not present in their metallic state. However, this does not rule out the possibility that these metals influenced the oxidation of aluminum. It is possible that metal ions were first reduced by reaction with metallic aluminum (e.g., 3Pb2+ + 2Al f 3Pb + 2Al3+) and then were re-oxidized by oxygen (e.g., Pb + 1/ O f PbO). In this proposed mechanism, the more2 2 noble metal would act as a corrosion catalyst. Copper and lead were also detected on SS 316 after contact with the upper layer. As for the aluminum strips, the spectra of SS 316 indicated the presence of nonmetallic copper and lead species. The signals in the Cu 2p XPS and Cu Auger spectra had energies that were typical for Cu2O (see Table 8), whereas the BE value of the Pb 4f7/2 peak (138.6 eV) indicated Pb2+ species (see Figure 8). 3.3.4. Chemical Nature of Copper Strips before and after Contact with Bio-oil. The Cu 2p spectra showed doublets at ∼933.0 eV (Cu 2p3/2 peak, not corrected for charging, D1) and 934.8 eV (D2). These signals were assigned to metallic copper and Cu(OH)2, respectively (see Table 8). However, in the O 1s spectra, after contact (27) McIntyre, N. S.; Sunder, S.; Shoesmith, D. W.; Stanchell, F. W. Chemical Information from XPSsApplication to the Analysis of Electrode Surfaces. J. Vac. Sci. Technol. 1981, 18, 714-721. (28) Perrin, C.; Simon, D.; Mollimard, D.; Bajard, M. T.; Baillif, P.; Bardolle, J. Etude par Spectroscopie d′E Ä lectrons ESCA de Composes Sulfures et Oxydes du Cuivre, Cine´tique de Sulfuration et Identification des Composes Formes lors de la Contamination de Cuivre dans Diffe´rents Atmosphe`res. J. Chim. Phys. 1984, 81, 39-47.

Figure 9. Cu 2p XPS spectra of copper strips before and after contact with bio-oil. The spectra were normalized to the maximum intensity.

with the upper layer and the whole bio-oil, respectively, peaks indicating Cu2O were detected (see Figure 3). Thus, in addition to metallic copper and Cu(OH)2, smaller amounts of Cu2O most probably were present on these copper strips. Additional peaks in the Cu XPS spectra at higher BE values were assigned to satellites, which are often found in Cu 2p spectra. In all spectra, the D2 doublet was more intense than the D1 doublet (Figure 9). Concerning the corrosion resistance of copper, it is important to recall that copper oxide/hydroxide layers are porous. Therefore, they do not protect the underlying metal against oxidation.29 This explains the

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Energy & Fuels, Vol. 18, No. 5, 2004 1299

Figure 10. Fe 2p XPS spectra of the SS 316 strips before and after contact with bio-oil. The spectra were normalized to the maximum intensity.

observed corrosion of the copper strip by the bio-oil (see Figure 1). The standard potential for the oxidation of copper (0.35 V, Cu/Cu2+) is considerably higher, in comparison to aluminum (-1.66 V, Al/Al3+),25 which is most probably the reason copper was much less corroded than aluminum. 3.3.5. Chemical Nature of SS 316 Strips before and after Contact with Bio-oil. 3.3.5.1. XPS Spectra. SS 316 typically contains, in addition to iron, 16-18 wt % chromium, 10-14 wt % nickel, 2-3 wt % molybdenum, 2 wt % manganese, and 1 wt % silicon. All these elements were detected in the XPS survey spectrum (not shown). However, only the detail spectra of the major components (iron, chromium, and nickel) were recorded. The Fe 2p spectra showed two doublets, at 707.0 (Fe 2p3/2 peak, not corrected for charging) and 710.3 eV (Figure 10). The first doublet was assigned to metallic iron, whereas the BE value of the second doublet is between the BE values of Fe3O4 and Fe2O3.22 Prior to contact with the bio-oil, the oxide doublet was more intense than the doublet of metallic iron, whereas the opposite was observed after contact with bio-oil. The AES results (see below) indicated that iron species were leached on contact with the bio-oil. The strong reduction of the iron oxide doublet suggests that this leaching was more pronounced for iron oxide species than for metallic iron. After contact with the upper layer, no iron at all was observed (see Table 5). This can be explained by a layer of organic deposits that was thicker, in compared to that on the other samples. After contact with the upper layer, the carbon concentration was indeed higher than that on the other samples (see Table 5). However, the absence of iron could also be related to the presence of copper and lead, which were only detected after contact with the upper layer. As discussed previously, these metals may catalyze the oxidation and subsequent leaching of iron. Next, the chemical nature of chromium on SS 316 is discussed. The Cr 2p spectra showed two doublets, at 537.7 eV (Cr 2p3/2 peak, not corrected for charging) and (29) Wong, A. S. W.; Gopal Krishnan, R.; Sarkar, G. X-ray Photoelectron Spectroscopy and Auger Electron Spectroscopy Investigation on the Oxidation Resistance of Plasma-Treated Copper Leadframes. J. Vac. Sci. Technol. A 2000, 18, 1619-1631.

Figure 11. Cr 2p XPS spectra of the SS 316 strips before and after contact with bio-oil. The spectra were normalized to the maximum intensity.

576.8 eV (Figure 11). These signals were assigned to metallic chromium and to Cr2O3, respectively.22 In all spectra, the Cr2O3 doublet was much more intense than the doublet of metallic chromium, which accounted for