Biocorrosion of AISI 304 Stainless Steel by - American Chemical Society

Ind. Eng. Chem. Res. 2008, 47, 4703–4711

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Biocorrosion of AISI 304 Stainless Steel by DesulfoWibrio desulfuricans in Seawater Thi My Phuc Nguyen, Xiaoxia Sheng, Yen-Peng Ting,* and Simo Olavi Pehkonen Department of Chemical and Biomolecular Engineering, National UniVersity of Singapore, 4 Engineering DriVe 4, Singapore 117576, Singapore

The corrosion behavior of AISI 304 stainless steel (SS304) in a batch system of enriched artificial seawater (EASW) in the presence of a sulfate-reducing bacteria (SRB) species (DesulfoVibrio desulfuricans ATCC 27774) was investigated using analytical techniques including electrochemical impedance spectroscopy (EIS), atomic force microscopy (AFM), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). EIS analysis showed that the polarization resistance of SS304 decreased after immersion in EASW in the presence of the SRB for 14 days compared to the control sample (in the absence of SRB). The corrosion processes were simulated using equivalent circuit models, which provided electrochemical information on the liquid/surface interface for both abiotic and biotic systems. Using AFM, pits and cracks were observed on the stainless steel surface in the presence of SRB, and a thick biofilm produced by SRB was evident in SEM micrographs, which corroborated the EIS results for the explanation of the biocorrosion mechanism. XPS analysis showed changes in the chemical states at the near-surface environment and on the surface of stainless steel. 1. Introduction Anaerobic sulfate-reducing bacteria (SRB), such as DesulfoVibrio sp., are the most common bacteria responsible for microbial corrosion. They reduce sulfate to sulfide and promote the formation of metal sulfides. They have been found on almost all surfaces in seawater, waste/potable water, oil systems, and heat exchangers.1 Seawater, where the concentration of sulfate is typically about 2 g/L, provides an ideal environment for the growth of SRB.2 Because of the role of SRB in microbiologically influenced corrosion (MIC), much research has been focused on determining the mechanisms of SRB-induced corrosion. To date, there is still divergence of views on the mechanisms of MIC by SRB, especially regarding cathodic and anodic depolarization. The corrosion of stainless steel induced by SRB has been reported. Ismail et al.3 found that SRB aggravated the corrosion on 304 stainless steel (SS304) as shown by electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM), while in another study, Werner et al.4 showed that a SRB biofilm did not cause a reduction in the pitting potential of SS304 in comparison to the corrosion behavior in inorganic media but inhibited the corrosion instead. The roles of SRB in MIC are still not fully understood and documented. Stainless steel suffers easily from biofouling because of a slow corrosion rate and fewer corrosion products on the metal surface.5 It is known that stainless steel is prone to localized corrosion in chloride-containing media (i.e., seawater) and is widely accepted that MIC on stainless steel is an important corrosion phenomenon.6 Because of the widespread application of stainless steel in industrial systems, SS304 and 316 stainless steel (SS316), the two popular materials used in both saline and nonsaline systems, have received considerable attention from scientists and engineers. However, review papers suggest that MIC of mild steel by SRB appears to be more frequently researched.7–9 The mechanisms of the SRB-induced corrosion on stainless steel are still not clearly established. * To whom correspondence should be addressed. Tel: +6565162190. Fax: +65-67791936. E-mail: [email protected].

Although biocorrosion and abiotic corrosion are electrochemical in nature,5 the presence of microorganisms in biocorrosion renders the system much more complex. Various analytical techniques have been used to investigate MIC, including electrochemical and surface analytical techniques, with EIS, atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS) being the most commonly used. EIS has been considered a powerful analytical technique in the study of corrosion systems, particularly in localized pitting corrosion, which usually occurs in the presence of SRB. Additionally, EIS is an important and useful technique to obtain the parameters of electrochemical corrosion after the modeling and the understanding of spectra to determine the physicochemical properties of the metal surface. AFM has been applied to investigate the SRB biofilm and the pits on the SS316 surface with qualitative data.1 In another study, AFM was used to investigate SRBinduced corrosion on mild steel and to quantify the corrosion rate by examining the pit depth.10 AFM and SEM were used to elucidate the role of Leptothrix discophora in causing pitting corrosion of SS316L.11 In addition, XPS has been used to analyze the corrosion products on the metal surface to support the proposed biocorrosion mechanism and to determine the changes in the metal surface under bacterial attack. For instance, Johansson and Saastamoinen12 found the presence of extracellular polymeric substances (EPS), bacteria cells, and trapped ions in the biofilm of Burkhoderia sp. formed on AISI SS304, while in another biocorrosion study, Vinichenko et al. 13 reported the dissolution of a passive film on SS316L and iron surfaces under an attack of mouse fibrosarcoma and human osteosarcoma cells. In the present study, the corrosion behavior induced by SRB on a stainless steel surface was examined using different analytical techniques, i.e., EIS, AFM, SEM, and XPS. The objective was to investigate the corrosion mechanism by modeling the liquid/metal interface based on EIS results with supporting data from AFM, SEM, and XPS.

10.1021/ie071468e CCC: $40.75  2008 American Chemical Society Published on Web 06/18/2008

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Figure 1. Rotating electrochemical cell.

2. Materials and Methods 2.1. Metal. Commercial AISI SS304 was purchased from T.T. Hardware Supplies Co., Singapore. Material was cut into disk coupons with a diameter of 10 mm and a thickness of 0.9 mm. These coupons were polished using polycrystalline oilbased diamond paste 6, 3, and 0.5 µm in succession and degreased using isopropyl alcohol to obtain a mirror surface finish. Finally, the coupons were cleaned using isopropyl alcohol, ethanol, and deionized water, dried in a nitrogen flow, and kept in desiccators until the experiments. 2.2. SRB Culture and Growth Conditions. DesulfoVibrio desulfuricans ATCC 27774 (D. desulfuricans subsp. desulfuricans) was obtained from the American type Culture Collection (ATCC). The culture was grown at 37 °C in a modified Baar’s medium (g/L): MgSO4, 2.0; sodium citrate, 5.0; CaSO4, 1.0; NH4Cl, 1.0; K2HPO4, 0.5; sodium lactate, 3.5; yeast extract, 1.0; Fe(NH4)2(SO4)2, 1.0. When the bacteria concentration reached 108 MPN/mL, 25 mL of the culture was transferred into 500 mL of enriched artificial seawater (EASW) with the composition (g/L): NaCl, 23.476; Na2SO4, 3.917; NaHCO3, 0.192; KCl, 0.664; KBr, 0.096; H3BO3, 0.026; MgCl2 · 6H2O, 10.610; SrCl2 · 6H2O, 0.040; CaCl2 · 2H2O, 1.469; sodium lactate, 3.5; yeast extract, 1; trisodium citrate, 0.5; MgSO4 · H2O, 0.4; CaSO4, 0.1; NH4Cl, 0.1; K2HPO4, 0.05; Fe(NH4)2(SO4)2, 0.1. The SRB was incubated in an anaerobic chamber (Don Whitley, model MASC MG500) containing 80% N2, 10% CO2, and 10% H2. 2.3. Effect of SRB on the Biocorrosion of Stainless Steel. Experiments were carried out in a 500 mL Duran bottle containing EASW with and without SRB. The operation was manipulated in the anaerobic workstation. The growth medium was replenished every 7 days by removing 50% of the spent medium and replacing it with a fresh medium. Sterilized stainless steel coupons were hung in Duran bottles, using a plastic wire and immersed in the test media. All manipulations were performed in the anaerobic chamber. The bottles were well-sealed and incubated at 30 °C for 14 days. The coupons were retrieved for EIS, AFM, and SEM analyses. 2.4. Electrochemical Measurement: EIS Analysis. Electrochemical measurements were performed using a conventional three-electrode cell with platinum as the counter electrode, Ag/ AgCl as the reference electrode, and the AISI SS304 specimens as the working electrodes. The working electrode had a surface area of 0.785 cm2. A frequency response analyzer of Autolab/

Figure 2. Nyquist plot of AISI SS304 after 14 days at 30 °C (a) in sterile EASW and (b) in EASW + SRB media.

PGSTAT20 (from EcoChemie BV, Utrecht, Netherlands) was used to perform the EIS scans. All electrochemical measurements were performed after the open-circuit potential was stabilized, which occurred normally after 60 min for the control medium and 90 min for the medium with SRB. A volume of 500 mL of EASW served as the electrolyte. The coupons were taken out and immediately placed in the electrochemical cell for EIS analysis. The media in the bottle as the electrolyte for the EIS analysis was also used in the rotating electrochemical cell (Figure 1). EIS experiments were conducted at frequencies in the range of 105-10-3 Hz and with an alternating current signal of 10 mV. All experiments were duplicated. 2.5. Topology of Surface: AFM Analysis. The AFM (Digital Instruments Nanoscope III) in the contact mode was used to observe pits on the corroded AISI SS304 surface in air after immersion in various media. The biofilm and corrosion products on the coupons were removed by immersing them in an ultrasonic bath for 5-10 min and then in a HNO3 solution (HNO3: H2O ) 1:3 in volume) for 30-50 s. The exposed

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Figure 3. Physical model and its equivalent circuit in fitting the EIS scans for (a) abiotic corrosion and (b) biotic corrosion.

Figure 4. AFM images of stainless steel surfaces after 14 days at 30 °C (a) in sterile EASW and (b) in EASW + SRB.

Table 1. Impedance Parameters of Stainless Steel in EASW and EASW + SRB at 30 °C medium

Rs (Ω) Qdl Rct (Ω) Qpf Rpf (kΩ) Odf

Y0 × 105 (Ω-1 sn) nCPE Y0 × 105 (Ω-1 sn) npf Y0 × 105 (Ω-1 sn) B × 102 (s0.5)

abiotic corrosion (in EASW)

biotic corrosion (in EASW + SRB)

37.9 0.77 0.81 805.6 6.1 0.97 583

41.18 1.226 0.75 600 3.0 0.60 23.45 0.51 5.76

coupons were finally rinsed with sterilized water, cleaned with ethanol, and dried under a nitrogen flow. 2.6. Biofilm Investigation: SEM Analysis. SEM (Jeol JSM5600LV) was applied to observe the biofilm formation on the AISI SS304 surface in the medium with SRB after different exposure times. The bacteria on the coupons were fixed for 4 h in a 2.5% glutaraldehyde solution at 4 °C. It was then dehydrated stepwise in an increasing ethanol concentration series (25%, 50%, 75%, and 100%) for 15 min at each stage. The coupons were dried in desiccators overnight. The samples were finally coated with platinum and observed under SEM.

Figure 5. SEM images of AISI SS304 surfaces exposed to EASW + SRB at 30 °C for 14 days (magnification ×2000).

2.7. Chemical Component Changes: XPS Analysis. XPS analysis was used to investigate the changes in the chemical components at the near-surface environment and on the coupon surface with and without SRB. An XPS wide range for binding energy spectra and XPS highresolution spectra of C 1s, O 1s, and S 2p scans were analyzed to investigate the differences in the chemical composition on the biofilm surfaces after exposure to biotic and abiotic systems

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Figure 7. XPS wide scans of the polished AISI SS304 surfaces exposed to abiotic and biotic media for 14 days at 30 °C.

3. Results and Discussion

Figure 6. (a) AFM image with pits having a rod shape, similar to the shape of SRB. (b) SEM image of the SRB cell. (c) SEM image of the SRB cells and pits on AISI SS304.

at 30 °C for 14 days. The samples were gently removed from the media, dried under a nitrogen flow, and stored overnight in evacuated desiccators before analysis using XPS or energydispersive X-ray/SEM. XPS high-resolution spectra of the elements Ni 2p and Fe 2p were analyzed for the surfaces immersed in the media with and without SRB after removal of the biofilm in deionized water, blown with a nitrogen gas, and stored overnight in evacuated desiccators.

3.1. Electrochemical Characteristics and Physiochemical Models of the SS304 Surface after Corrosion. The EIS data were analyzed to determine the influence of SRB on the corrosion of AISI SS304 in comparison with control coupons, and the results are shown in Nyquist plots in Figure 2. It is evident that the polarization resistance of SS304 was greatly reduced in the presence of SRB. This indicates that the metabolic products of the SRB caused a change in the electrochemical properties of the systems as reported in previous research.14 The introduction of bacterial cells and their products, including corrosive substances and EPSs, may induce changes in polarization resistance.14 The EIS scans were simulated using a Boukamp software program (EQUIVCRT) based on a nonlinear least-squares fit procedure. Two equivalent circuits, Rs(QdlRct)(QpfRpf) and Rs(QdlRct)(Qpf[RpfOd]), were built to describe the electrochemical mechanisms of two different processes: corrosion in the abiotic and biotic systems, respectively (Figure 3a,b). In the presence of SRB, another component (O) was taken into account because the diffusion process becomes more important.3,15 The diffusion is caused by the biofilm, including bacterial cells and their insoluble products, such as metal sulfide in the solution. Figure 2 shows that the simulated data fit well with the experimental data for both the abiotic and biotic corrosion. It is shown in the model as a cotangent-hyperbolic diffusion impedance O appearing in the circuit. The fitting parameters of both abiotic and biotic corrosion are shown in Table 1. These parameters indicate the differences between the abiotic and biotic corrosion. The first difference between the abiotic and biotic corrosion is that the charge-transfer resistance, Rct, is quite large, 805.6 Ω, in the presence of SRB, as compared to 600 Ω in the medium without SRB. In addition, the value of the roughness parameter, n, of the double layer slightly decreases for the SS304 surface in a medium containing SRB: from 0.80 for EASW to 0.75 for EASW + SRB. SRB and their metabolic products not only affect the double layer but also attack the passive film aggressively. The value n (of 0.96) for stainless steel in the sterile

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Figure 8. C 1s spectra of AISI SS304 surfaces (a) before immersion, (b) after immersion in EASW for 14 days at 30 °C, and (c) after immersion in EASW + SRB.

control medium was found to be close to 1, which reflects a relatively homogeneous passive film, while the lower n value for the passive film (of 0.60) in the presence of SRB indicates a rougher passive film layer. Furthermore, the anaerobic biofilm formation in the enriched artificial seawater with sulfatereducing bacteria (EASW + SRB) on stainless steel results in a sharp decrease in the resistance of the passive film, Rpf.

In previous studies, Gonzalez et al.15 used Rs(QdlRct)(Qpf[Rpf(RQ)]) and Ismail et al.3 applied Rs(QdlRct)(Qpf[RpfWd]) to predict the mechanism of biocorrosion induced by SRB on SS316 and by Pseudomonas fragi and SRB on SS304, respectively. While both of these studies showed similar observations on the passive film, they did not detect any significant change in the double layer (unlike the results presented herein). The obvious change in the electrochemical parameters between abiotic and biotic corrosion can be explained by the accumulation of biofilm on the metal surface in the presence of bacteria,16,17 which, in turn, alters the electrochemical properties of the metal/liquid interface. The decrease in resistance and roughness parameters of both the double layer and the passive film indicates two main points. First, SRB and their products (i.e., sulfide and EPS) may be adsorbed onto the metal surface or even become incorporated into the passive film, changing the electrical structure of the metal/solution interface and the passive film18 and thus making the metal surface easier to corrode. Second, the simulated EIS results predicted that the biofilm played an important role in the metal corrosion. The biofilm is considered as a “house” for the settlement of microorganisms and their metabolic products. 3.2. Surface Topology and Pit Morphology. The AFM height images for the coupons in EASW and EASW + SRB are shown in Figure 4a,b, respectively. The surface exposed to the abiotic medium appears to be different compared to that exposed to the medium with SRB. Some pits and small cracks appeared on the surface exposed to the medium with SRB. This indicates that localized corrosion occurred on the surface of SS304 after 14 days of immersion in EASW + SRB at 30 °C. 3.3. Biofilms on the Exposed Surface. Figure 5 shows a biofilm of D. desulfuricans on the SS304 surface after 14 days of exposure in EASW. It is composed of bacterial cells, EPS, and some water channels. Bacterial cells were observed to be embedded within the biofilm. The EPS acts as a binding material for the adhesion of bacterial cells to the biofilm matrix, which is consistent with previous research that EPS is important in the formation of a SRB microconsortium by holding the discrete cells and metal sulfide particles on the surface of SS304.20 The water channels in EPS allow the distribution and circulation of nutrients as well as the SRB metabolic products within the biofilm,19.20 In the water channels, the microorganisms create a gradient of pH and hydrogen concentration, which induces localized corrosion on the stainless steel surface. This spatial structure of the biofilm has very important effects on metal corrosion. Interestingly, AFM and SEM images show the relationship between pits and SRB. The pits formed by SRB cells on the stainless steel surface can be observed in the AFM image (Figure 6a). It is clear that the shape and size of the pits on the stainless steel surface are similar to those of the SRB cells (Figure 6b). This observation suggests a direct role of SRB on pit initiation on the SS304 surface. Similar results were also detected by Sheng et al.14 that D. desulfuricans could form pits with size and shape similar to those of the SRB on the surface of SS316. 3.4. Differences in the Surface Composition between Abiotic and Biotic Corrosion. XPS was used to analyze the surface composition of SS304 to evaluate the differences between the surfaces after exposure to the abiotic and biotic media for 14 days. The investigation was mainly focused on the differences in the chemical composition of the outermost layer of the stainless steel surfaces after the coupon was collected from the media. On the basis of the XPS results, the influence of the bacterial attachment on corrosion is discussed. The change

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Figure 9. O 1s spectra of AISI SS304 surfaces (a) before immersion, (b) after immersion in EASW for 14 days at 30 °C, (c) after immersion in EASW + SRB, and (d) after removal of the developed SRB biofilm.

in the chemical state of the typical elements on the surface of SS304 (i.e., Fe and Ni) is also assessed. XPS wide-scan results are shown in Figure 7. The coupon in the EASW + SRB medium was covered with a biofilm and showed a more complicated spectrum compared to that in the sterile EASW. The appearance of the elements, such as Na, K, Cl, and S, is clear for the coupon with the biofilm. It reveals that the biofilm can trap the soluble ions and insoluble particles, as well as the SRB metabolic products. A biofilm naturally develops on the stainless steel surface over time in a biotic system. The analysis of the chemical states of biologically derived elements (C, O, and S) is helpful to identify the differences in the chemical composition between surfaces after exposure in the abiotic and biotic systems. Figure 8 shows the C 1s high-resolution scans on the surfaces of the polished SS304 without immersion in any medium and the SS304 coupon immersed in EASW with and without SRB. The binding energies for the components of interest were referenced to the binding energy of the C 1s hydrocarbon peak (C-C) at 284.6 eV. There are three peaks present in the C 1s spectra for the four surfaces analyzed: a C-C bond at 284.6 eV, a C-O bond or a C-N bond at 286.1 eV, and a CdO or OdC-N bond at 288 eV. The C 1s spectrum for the polished SS304 is characteristic of an air-exposed metal.13 It is mainly attributed to a CHn-containing hydrocarbon layer of 0.5-1 nm on the metal surface.21 This may be from the manufacturing process, surface preparation, or contact with the atmosphere.

The presence of C-O and CdO peaks may result from the polishing and cleaning solutions. For the stainless steel coupons exposed to EASW and EASW + SRB, the peaks at 286.1 and 288 eV may also be due to the presence of C-N and OdC-N, which are from the proteins in the media or on the bacterial cells. The intensity of these peaks was greater than the peaks for the polished SS304. This indicates the adsorption of organic substances from the solution to the surface. For the SS304 surface exposed to the medium with SRB, the (Od)C-N bond may also account for the presence of the peptide linkages of protein compounds in bacteria; the increase in the intensity of C-C bonds indicates the accumulation of the long aliphatic chains in bacterial cells12 to the stainless steel surface; the CdO bond may be assigned to an intracellular storage lipid; the C-O bond may be from the extracellular polymers.22 The C 1s spectra of the surface in EASW + SRB revealed two additional peaks: carboxylate groups (OdC-O) at 289.2 eV and aromatic saturated bonds at 292.6 eV.23 It is evidence of the accumulation of organic acids (OdC-O), the common bacterial metabolites entrapped within the biofilm, and the appearance of the aromatic saturated bonds, which may be from EPS. O1s spectra (Figure 9) show the O 1s peaks from four surfaces: the polished SS304 without immersion in any medium, the SS304 coupons immersed in EASW with and without SRB, respectively, and the SS304 surface, on which the developed SRB biofilm was removed. The main O 1s spectra include three components: metal oxides at 529.5 eV and hydroxides at 531.4

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Figure 10. S 2p spectra of AISI SS304 surfaces after immersion for 14 days at 30 °C (a) in EASW and (b) in EASW + SRB.

eV. The peak at 533.0 eV is attributed to the CdO bond or the adsorbed H2O.24 The above three peaks were also found in the O 1s spectra of the stainless steel surface with biofilm. However, the peak intensity was different from the surface immersed in EASW, the surface in EASW + SRB (after the biofilm removal), and the control surface (i.e., the polished surface without immersion). The decrease in the intensity of metal oxides relative to metal hydroxides (or C-O bonds) is as follows: a polished surface > a surface in EASW > a surface in EASW + SRB (after the biofilm removal). This indicates the dissolution of a metal oxide layer under corrosive conditions (i.e., in the presence of SRB), which, in turn, results in a fast corrosion rate. Therefore, the coupon immersed in EASW + SRB resulted in more severe corrosion in comparison to other cases. This result is in agreement with the findings of Vinnichenko et al.,13 in which the corrosion of AISI SS316 was induced by the mouse fibrosarcoma and human osteosarcoma cell cultures. In addition, O-CdO bonds at 534.8 eV and N-O-S at 535.8 eV appeared on the coupon surface exposed to the EASW + SRB medium. The first peak confirms the presence of carboxylate groups as discussed earlier, while the second peak may be from the sulfides produced by SRB. The S 2p spectra of the abiotic and biotic surfaces are shown in Figure 10a,b, respectively. As discussed above, a remarkable difference between the biotic sample exposed to SRB and the abiotic sample is the distinct sulfur (S 2p) spectrum. A very

Figure 11. Ni 2p spectra of AISI SS304 surfaces after immersion for 14 days at 30 °C (a) in EASW and (b) in EASW + SRB.

weak signal of sulfur was observed from the XPS S 2p spectra of the polished and abiotic surfaces, while the sulfate and sulfide signals were evident on the SRB surface. Sulfate is considered to be from the adsorption of soluble sulfate salts from the solution to the metal surface (at 168.8 eV), while sulfite (at 166.6 eV) was the intermediate product of sulfate reduction. The precipitated FeS (at 161 eV) and FeS2 (at 162.9 eV) are the corrosion products of the iron substrate in EASW.25 Moreover, organic sulfide was also present in the biofilm because of the presence of the doublets at 163.9 eV.25,26 Generally, the organic sulfides are considered to be components of the cell membrane and the EPS in the biofilm. It is wellknown that sulfur is an important component in proteins, and it determines their structure and activity, especially for the enzymes in bacterial cells. For SRBs, iron-sulfur proteins, such as hydrogenase of D. desulfuricans, play a critical role in the biological electron-transfer process and in many enzymatic reactions.25 Therefore, the presence of the organic sulfides in the SRB biofilm indicates that SRB may play an important role in biocorrosion by accelerating the cathode reaction of metal corrosion. Evidently, there are clear changes in the chemical states of the biolayer for the spectra of C 1s, O 1s, and S 2p among the different samples. It was found that surface modification of the SS304 after incubation in EASW or EASW + SRB makes it

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Johanson and Saastamoinen12 in that the iron oxides on the austenitic SS304 surface were dissolved under the attack by Burkhoderia species. Therefore, the XPS high-resolution spectra of Fe and Ni exhibited the influence of the enriched seawater and SRB toward the passive layer of AISI SS304 after 14 days of immersion. The XPS analysis indicates that the main reason for corrosion induction is the changes in the chemical states of the near metal surface due to SRB biofilm accumulation. 4. Conclusions The surface analysis by different analytical techniques, such as EIS, AFM, SEM, and XPS, showed the differences in the corrosion in the abiotic and biotic systems. EIS is a conventional method to evaluate the corrosion behavior and to set up the electric circuit model of the biofilm/metal interface. This analysis revealed diffusion in the presence of SRB in EASW. SEM and AFM images showed the morphology of the SRB cells and pits on the stainless steel surface. The pits with shape and size similar to that of SRB indicate the direct role of SRB on pit initiation on the stainless steel surface. The biofilm on the surface is heterogeneous over the SS304 surface after 14 days of immersion. Furthermore, the XPS spectra revealed the effect of SRB cells on the dissolution of metal oxides. This indicates that SRB accelerated the corrosion rate of AISI SS304 by localized corrosion (i.e., pitting).

Literature Cited

Figure 12. Fe 2p spectra of AISI SS304 surfaces after immersion for 14 days at 30 °C (a) in EASW and (b) in EASW + SRB.

different in the organic layer adsorption. The most noticeable feature is the changes in the spectra of O 1s and C 1s peak positions on the abiotic and biotic surfaces and the appearance of a S 2p signal for the biotic surfaces. The difference in the chemical states is one of the main results from the corrosion in the abiotic and biotic systems. Furthermore, the XPS spectra showed changes in the SS304 surface under an attack of SRB based on the basic three elements in SS304: Fe, Cr, and Ni. It was noticed that there were some differences in the Ni 2p and Fe 2p spectra between the abiotic surface and the surface attacked by SRB (Figures 11 and 12). The signal of Ni0 was very weak on the SS304 surface immersed in EASW, while the metallic Ni was detected at 852 eV24 on the stainless steel surface after the removal of the biofilm. This implies that the outermost oxide layer on the SS304 surface may not contain much Ni27 and the dissolution of the oxide layer made additional metallic Ni exposed to the outermost layer. On the other hand, the Fe 2p spectra (Figure 12) showed a decrease in the oxide peaks of the stainless steel surface after corrosion by SRB compared to the surface after the abiotic corrosion. It is obvious that there was more elemental Fe on the SRB corroded surface. This indicates that the passive film becomes thinner under the SRB-influenced corrosion24 by the dissolution of the metal oxide into the bulk solution. This is consistent with the O 1s spectra discussed above in that fewer oxides were observed on the stainless steel surface after the removal of the biofilm. Similar results were also reported by

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ReceiVed for reView October 30, 2007 ReVised manuscript receiVed March 6, 2008 Accepted May 6, 2008 IE071468E