Interaction of Sulfate-Reducing Bacteria and Carbon Steel Q 235 in

Oct 5, 2011 - ABSTRACT: The effect of sulfate-reducing bacteria (SRB) on corrosion of carbon steels and interaction between SRB and the carbon steel Q...
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Interaction of Sulfate-Reducing Bacteria and Carbon Steel Q 235 in Biofilm Cheng Sun, Jin Xu,* and Fuhui Wang State Key Laboratory for Corrosion and Protection, Institute of Metals Research, Chinese Academy of Sciences, Shenyang 110016, China ABSTRACT: The effect of sulfate-reducing bacteria (SRB) on corrosion of carbon steels and interaction between SRB and the carbon steel Q 235 are investigated in soil-extract solutions (SES). The results show that corrosion rates are smaller in the SES with SRB during growing period of SRB, but bigger during dying period. The procedures of interactions of SRB and the steel are studied by scanning electron microscopy (SEM) and energy-dispersive X-ray analysis (EDXA). SRB do not aggregate on the surface of the steel until an anaerobic space is formed. SRB defend themselves against free oxygen by absorbing substances containing nonfree oxygen.

1. INTRODUCTION Microbiologically influenced corrosion (MIC) of metals has been widely found in buried pipelines, a mine, seaport, lake, dam, power plants, sewage treatment systems, offshore structures, and oil wells since a first report on MIC was published by Garrett in 1892.114 Several theories about sulfate reducing bacteria (SRB) influenced corrosion have been presented, e.g., cathodic depolarization,15 local corrosive cell,16 metabolite induced corrosion,17 and so on. It has been known that SRB are the most important microbes for anaerobic corrosion in soils.18 SRB can utilize hydrogen as an electron donor to obtain energy and thus remove molecular hydrogen from the cathode, leading to cathodic depolarization of the metal surface.19 During the metabolic process sulfate ions are reduced to sulfide ions, which react with dissolved ferrous to form iron sulfide (FeS). Iron sulfide is accumulated on surface of metal and forms a film. Finally, galvanic coupling between iron sulfide and unreacted metal substrate is set, which accelerates the dissolution of the iron.2024 Many research studies have focused on SRB-influenced corrosion of the metals and their alloys. Most of the reports show that SRB accelerated the corrosion of metals. Torres-Sanchez et al.25 have reported that high density and low depth pitting for the stainless steel AISI 304 is formed in presence of SRB, and the corrosion potentials shift toward more negative direction. Lee et al.20 have found that corrosion of mild steel will be enhanced in the presence of biofilm once the iron sulfide particle contacts the metal surface. Sun et al.26,27 have concluded that SRB will accelerate the corrosion rates of zinc in soils. The cathodic protection (CP) efficiency is higher and the average corrosion rate is lower in sterilized soils than those in the inoculated soils at the same potential. The experiment by Li et al.28 showed that the corrosion potential and polarization resistance of copper alloys drastically move toward negative direction as the SRB is in the logarithm phase. However, some researchers have found that sulfide films had protective effects on metal.2931 Syrett et al.3234 has studied the corrosion of the CuNi alloys in sulfide-polluted seawater. The results showed that the mere presence of sulfide does not r 2011 American Chemical Society

cause accelerated attack of the alloys even when the pH is allowed to drop to values as low as 7. The above results mainly focus on the protective effects of the sulfide on the metals in cultured solution or seawater, but soil environments, whether biotic sulfide has protective ability or not, are seldom investigated. The present study is designed to provide further understanding of SRB influenced corrosion for carbon steel Q235 in soils, and the interaction between SRB and the steel.

2. EXPERIMENTAL MATERIALS AND METHODS 2.1. Coupon Preparation. Samples of 15  15  3 mm were cut from plate of carbon steel Q235 with nominal composition (wt%) of 0.30 C, 0.019 P, 0.029 S, 0.01 Si, 0.42 Mn, and balance Fe, then embedded in epoxy resin to prepare test sample with an exposed surface area of 2.25 cm2 for electrochemical measurements. The coupons were abraded with a series of grit papers (200, 400, 600, and 800) followed by cleaning in acetone and alcohol, and dried. 2.2. Soil Solution Preparation. Soils used in this study were taken from Shenyang, China. The soils were dried at 105 °C for 10 h, ground, and then shaken through a sieve with 1-mm diameter openings. The soil-extract solutions (SES) were prepared by filtering the soil solution with the water/soil ratio of 5:1. The analysis results of the solution compositions are given in Table 1. The SES was autoclaved at 121 °C for 20 min and stored at 4 °C for use. 2.3. Microorganisms. To investigate MIC of carbon steel Q235 in soil environments, SRB strains used in this study was isolated from soils. They were anaerobically incubated in an anaerobic bottle, in which there was API RP-38 medium (g/L)35 containing MgSO4 3 7H2O 0.2; ascorbic acid 1.0, NaCl 10.0, KH2PO4 0.5, sodium lactate 4.0, yeast extract 1.0, Received: May 2, 2011 Accepted: October 2, 2011 Revised: September 23, 2011 Published: October 05, 2011 12797

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Table 1. Compositions of the Soil Used (mg/kg Soil) chemical composition soil type black fluvo-aquic soil

pH 7.75

NO3 46

Cl 31

SO42‑ 48

HCO3 234

Ca2+ 57

Figure 1. Variations of SRB numbers and pH values with time in the SES.

Fe(NH4)2(SO4)2 0.02, for enrichment, and subsequently purified in sterile agar plates using API RP-38 medium by picking up several single black colonies with a sterile inoculation loop. The black colonies were immediately put into the anaerobic bottle containing the mediums, cultured in the constant incubator at 37 °C, and then inoculated mediums were stored at 4 °C for use after the mediums became black. Genomic DNA of the bacterial isolates was extracted according to the phenol-chloroform method. Amplification of geneencoding small-subunit rRNA was carried out using 16S rDNA primers (27f: 50 -AGAGTTTGATC[C/A]TGGCTCAG-30 and 1492r: 50 -TACGG[A/T/C]TACCTTGTTACGACT T-30 ). Polymerase chain reaction (PCR) was performed with 50 μL of a reaction mixture containing 2 μL of DNA as the template, 5 μL of 10  PCR reaction buffer, 4 μL of deoxynucleoside triphosphate (dNTP) at a concentration of 2.5 mmol/L, 1 μL of primer 1 at a concentration of 20 pmol/L, 1 μL of primer 2 at a concentration of 20 pmol/L, 0.25 μL of Taq DNA polymerase at a concentration of 5 U 3 μ/L of, as well as 36.75 μL deionized water. PCR was carried out by using the following program: initial denaturation at 94 °C for 5 min; denaturation at 94 °C for 1 min; annealing (1 min at 55 °C), and extension (3 min at 72 °C); followed by a final extension (at 72 °C for 8 min). The PCR product was measured by agarose (2%) gel electrophoresis. The purified product was sequenced. The 16S rDNA sequence alignment sequence was obtained and was used for initial BLAST searches in Genbank and for phylogenetic analysis. The phylogenetic tree was constructed by using ClustaX. The strain was identified preliminarily as Desulfovibrio desulfuricans (DQ092636) at the similarity level of 98%. Two hundred milliliters of each of the cultures were subsequently transferred into individual aliquots of 800 mL of sterilized SES. The abraded coupons were hung in the medium containing SRB in a sealed jar (1000 mL) for the corrosion experiments. SRB numbers were determined by the three tube multiple most probable number (MPN) method according to the American Society of Testing Materials (ASTM) Standard D 4412-84.

Mg2+ 32

K+ 2

Na+

organic content

whole nitrogen content

total salt content

14

2.26  10

910

464

4

Figure 2. Nyquist plots with time in the SES with and without SRB.

2.4. Electrochemical Impedance Spectroscopy (EIS) Analysis. EIS was used to investigate the electrochemical properties

of the corroded surface after being immersed in SES with SRB over time. All experiments were performed in a three-electrode electrochemical cell, with a platinum electrode used as the counter electrode, and a saturated calomel electrode (SCE) as the reference electrode. The test were operated using the PARSTAT 2273 electrochemical measurement system manufactured by EG&G. The frequency range was from 0.001 Hz to 100 kHz and the amplitude of the sinusoidal voltage signal was 10 mV. The EIS data obtained were modeled and simulated using the Zsimpwin software supplied by the PARSTAT2273. 2.5. SEM and EDXA Analysis. The coupons were extracted from the SES with SRB for the microscopic analyses after immersing for 2 days, 5 days, 10 days, 15 days, and 20 days. The surface appearance of carbon steel Q235 in SES was visualized after preparation using the following procedures. Samples were fixed with 3% glutaraldehyde in a phosphate buffer solution (PBS, pH 7.37.4) for more than 4 h, and then washed with PBS for two changes (5 min each), rinsed with distilled water for another two changes (5 min each), and dehydrated with using an ethanol gradient (at 50%, 75%, 95% and 99% for 10 min) before being finally stored in a desiccator. A scanning electron microscope (XL30-FEG) with the beam voltage at 25 kV was used to visualize the morphology of surface.

3. RESULTS 3.1. Variation of SRB Numbers and pH Values. Figure 1 shows the variations of the SRB numbers and the pH values with time in the SES. The numbers of the planktonic SRB smoothly fluctuated in the SES before 6 h, sharply increased to 30 000 cfu/ mL (cfu is colony forming units) by the end of day 2, rapidly decreased to 2 cfu/mL after 10 days, and then remained stable. The pH values of the SES fluctuated with time before 2 days, increased after 2 days, and then remained stable after 10 days. 3.2. EIS Analysis. Figures 2 and 3 show EIS plots of the carbon steel Q 235 with time in the SES with and without SRB. The 12798

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Figure 3. Bode plots with time in the SES with and without SRB.

Table 2. Fitting Results of EIS in the SES with SRB time days

Rs Ω cm2

Rf kΩ cm2

YfmS secn/cm2

2

85.4

49.45

0.3401

0.8878

4.464

5 10

90.15 93.55

42.83 39.37

4.252 3.209

0.8197 0.956

10.920 0.7956

14.46 13.39

0.9248 0.9993

15

70.85

32.25

3.377

0.9564

1.225

12.59

1

20

89.88

28.75

3.791

0.9299

1.059

11.94

0.9824

nf

fitting results are listed in Table 2 and 3, and the equivalent circuits are given in Figure 4. In the electrical analog circuits, Rs represents an electrolyte resistance, Rf and Qf represent a resistance and a capacitance of the film, respectively. Rt and Qdl represent a charge transfer resistance and a double layer capacitance, respectively. RL and L represent a resistance and an inductive reactance of the active pits, respectively. As shown in Figures 2 and 3, the spectra obtained in the SES with SRB had three time constants after 2 days. The time constant at the lowest frequency was due to the presence of the active pits, which had a typical feature of the pitting model.36 Only two time constants were observed in the Bode plots from the fifth day to twentieth day, which indicated that the pitting holes were formed with the early active pits expanding, and no new active pits appear. The time constant at the high frequency might be due to the forming of the biofilm. There was only one time constant in the SES without SRB after 2 days. An inductive loop appeared at the low frequency after 5 days, which was due to the active pits. Two time constants were observed after 10 days. The time constant at the high frequency might be due to the forming of the film of the corrosion products, but the second time constant appeared at the low frequency because the film was not perfect. 3.3. SEM and EDXA Analysis. Five figures, from Figure 5 to Figure 9, show the variations of the microscopic appearances of

Rt kΩ cm2

Ydl mS secn/cm2 0.6327

nt

RL kΩ cm2

L MH cm2

0.9114

22.740

1.23

the surface of the carbon steel Q235 with time in the SES with SRB. As shown in Figure 5, a few loose blowball-shaped products scatteredly distributed on the surface of the steel were observed after 2 days. There were no sessile SRB on the surface of the carbon steel, but lots of planktonic SRB in the SES (Figure 1). The EDXA result showed that the products might be iron phosphide,17 iron hexaphosphate,38 or iron oxide, but elements S were not observed. As shown in Figure 6, after 5 days, the big blowball-shaped products were not observed, but there were many small blowballshaped products, which were different from the products of the second days and also loose. Large amounts of the sessile SRB were observed under the loose products. The planktonic SRB numbers were less at second day than those at fifth day (Figure 1). The above results indicated that SRB prefer living in the regions, which were oxygen-free, between the products and the metals. The EDXA result showed that the products mainly contained elements O, C, and Fe, which might be organic substances, e.g., extracellular polymeric substance (EPS), and iron oxide. The above results showed that biofilms had been formed on the local surface of the steel. Elements P were not observed in the products on the surface of the carbon steel, which might be because the compounds containing elements P, which were formed at the second day, 12799

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Table 3. Fitting Results of EIS in the SES without SRB time days

Rs Ω cm2

2 5 10

48.22

15

50.41

8.726

0.2330

0.9207

12.650

0.3230

0.7216

20

39.09

13.760

0.2290

0.9144

19.680

0.3677

0.9028

Rf kΩ cm2

Rt kΩ cm2

Ydl mS secn/cm2

nt

40.12

2.151

0.3562

0.8164

37.97

2.504

0.4217

0.8281

9.678

0.4569

0.7489

7.848

Yf mS secn/cm2

0.2399

nf

0.907

RL kΩ cm2

L MH cm2

27.080

0.561

Figure 5. Microscopic appearance of the surface of the carbon steel after 2 days.

Figure 4. Equivalent circuit of EIS plots.

were repelled away from the surface of the steel when a large number of SRB aggregated on the surface. As shown in Figure 7, the blowball-shaped products had disappeared after 10 days, and a layer of dense biofilm was observed on the surface of the steel. The dense film had already begun to abscise on the local surface of the steel, and the SRB numbers decreased. This indicated that SRB, not only the planktonic SRB but also the sessile SRB, were in the dying stage. The activities of the biofilm and SRB numbers had decreased during the dying stage, which led to the partial abscission of the biofilms. The EDXA result showed that elements S were observed on the surface of the biofilm. As shown in Figure 8, after 15 days, some new blowball-shaped products were observed on the outer layer of the products. The whole surfaces of the carbon steel were almost covered by two different products, including the metabolites of SRB and the

Figure 6. Microscopic appearance of the surface of the carbon steel after 5 days.

corrosion products. Two kinds of products were in symbiotic existence. Few SRB were found both in the SES (Figure 1) and on the surface of the carbon steel (Figure 8). 12800

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Figure 7. Microscopic appearance of the surface of the carbon steel after 10 days.

Figure 9. XPS results of elements P and S in corrosion products in presence of SRB after 15 days.

Figure 8. Microscopic appearance of the surface of the carbon steel after 15 days.

The EDXA results showed that elements P and S were observed in the products of the inner layer, which indicated that this layer might be the biofilm. Elements S were not found in the products of the outer layer. A few elements Ca were observed in the products, which was due to the reason that calcium ions were precipitated with increasing of the pH values of the SES (Figure 1). Figure 9 shows the XPS results of elements P and S in the corrosion products in presence of SRB after 15 days. The XPS analysis indicated that the main components of P and S compounds were FeP3, FeS, and FeS2. The above results showed that with the activities of the biofilms and SRB numbers decreasing, the blowball-shaped

Figure 10. Microscopic appearance of the surface of the carbon steel after 20 days.

products were reabsorbed on the surface of the biofilm due to loss of the negative charges of the biofilms and SRB.36,38 As shown in Figure 10, the surfaces of the carbon steel were entirely covered by a large amount of the loose blowball-shaped products. The EDXA results showed that no elements S were observed in the corrosion products, and the amounts of elements P and Ca were more than those after 15 days, which indicated that the 12801

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Figure 12. Appearances of SRB on the surface of the carbon steel after 5 days.

Figure 11. Morphology of the surface of the steel after removing the corrosion prodcuts.

compounds containing elements P and Ca continuously deposited on the surface of the carbon steel. Figure 11 shows the morphologies of the carbon steel Q235 in the SES with and without SRB after removing the corrosion products. Many pitting holes were obviously observed on the surface of the steel in the SES with SRB, but not without SRB. The surface of the steel still contained the polishing grooves in the SES without SRB. However, these grooves had disappeared in the SES with SRB. The results above indicated that the susceptivity of the steel to corrosion was more in the SES with SRB than that without SRB. The pitting corrosion readily occurred on the surface the steel in the presence of SRB. 3.4. Variation of SRB during the Corrosion Process. Figures 12 and 13 show the SRB appearances on the surface of

Figure 13. Appearances of SRB on the surface of the carbon steel after 10 days.

the carbon steel after 5 days and 10 days, respectively. There were large numbers of SRB on the surface of the carbon steel Q235 after 5 days, as shown in Figure 12, and two morphologies of SRB were observed: one was wrapped by many spherical substances, and the other was not. The EDXA results showed that these spherical substances contained elements Fe and O, which indicated that SRB in the outer surface protected themselves from harmful substances, such as free oxygen, by absorbing the corrosion products, e.g., iron oxide. SRB were still observed on the surface of the steel after 10 days (Figure 13). Several layers of the biofilms could be found, and SRB lived in the regions between different layers. SRB were not 12802

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Table 4. Results of EDXA for the Different Positions on the Surface of the Carbon Steel after 10 Days content/atom% position element

1

2

3

C

52.51

45.76

50.06

O

18.80

24.81

32.96

S Fe

28.69

0.76 28.67

0.59 16.38

wrapped by the spherical substances, which might be due to the reason that the anaerobic space had been formed under the biofilm, and SRB did not need to protect themselves by absorbing other substances. The EDXA results in different positions after 10 days are given in Table 4. There was no element S in the position 1, which was the metallic matrix. Elements S, which decreased with increasing of the distance to the surface of the steel (positions 2 and 3), were observed. The above results showed that SRB lived among the bioflims. Sulfides of metabolic products were absorbed on the inner surface, not on the surface of the steel, and eventually, became a part of the biofilm. Amounts of elements S gradually decreased due to the forming of the corrosion products in the outer layer.

4. DISCUSSION 4.1. Relation of SRB Numbers and pH Values. At the beginning of the experiment, SRB are in adaptation period. The growth of SRB is inhibited as a result of low oxygen concentrations in the SES, but SRB can not be killed out by low concentrations of oxygen because SRB are facultative anaerobic bacteria.13,14 As shown in Figure 1, the numbers of SRB do not increase in this period. There is hardly any change in the pH value of the SES. As known, the cathodic reaction is oxygen-absorption for the corrosion of carbon steel in neutral solutions. The oxygen concentrations decreased with the cathodic reaction increasing, and the survived SRB begin to propagate after the accommodation to the new environment, which results in the increasing of the SRB numbers and the amounts of metabolites. During the metabolic process of the SRB, the following reaction occurrs (the SES gradually became black after 2 days, which indicated that that FeS had formed):

4Fe þ SO4 2 þ 4H2 O f FeS þ 3FeðOHÞ2 þ OH From the above equation, it can be seen that the result of the whole reaction led to the increasing of pH value in the solutions because of activity of SRB during the growing periods. In the late period, nutrients that are limited in the SES are rather depleted during the metabolism of SRB, which results in the rapid decreasing of the SRB numbers. The amounts of metabolites sharply decrease with the SRB numbers decreasing during the dying periods, which leads to the sharply decreasing of the concentrations of OH ion newly formed. The above results are the reasons for little change of pH value of the SES. 4.2. Analysis of Charge Transfer Resistance. A charge transfer resistance, Rt, is considered as a standard for assessing the corrosion rate of the metal.39 As shown in Figure 14, for the SES with SRB, Rt increases with time after immersing for 2 days,

Figure 14. Variation of Rt with time in the SES.

then decreases, and remains stable, which is accordance with the variation of the SRB numbers on the surface of the steel (Figure 5Figure 8, Figure 10, Figure 12, Figure 13). This indicates that the corrosion rates have a negative correlation with the growth of the sessile SRB. At the beginning of the experiment, the corrosion rate decreases with the SRB number on the surface increasing. This is due to the reason that the biofilms, containing EPS and bacteria, formed on the surface are negatively charged and compact.36,38 The film with negative charges has a repulsion to corrosive anion, which enhances corrosion-resistance. The compact film also protects the metal from corrosion induced by corrosive environments. With dying of SRB, activity of the biofilms and electronegativity disappear, which results in part exfoliation and decreasing of protective ability of the biofilm. Rt increases with time in the SES without SRB after immersing for 2 days, which was due to the forming of the film of the corrosion products. Rt is bigger before 5 days, and smaller after 10 days in the SES with SRB than that without SRB, i.e., The corrosion rate is smaller in the SES with SRB in the early period of the experiment than that without SRB. This may be attributed to the protective ability of the biofilms which are negatively charged and compact. This result corroborates Sheng and Gonzalez’s works.30,31 They have concluded that the morphology of the biofilm has a significant influence on the corrosion effect, and a compact biofilm with metal sulfides may in some instances act as a protective film on the metal surface. The protective ability of the biofilm stems from the accumulation of FeS and Fe(OH)2 inside the biofilm pores. The corrosion rate is bigger in the SES with SRB in the later period than that without SRB, which is due to the forming of a microcell with a big cathode and a small anode. With the decreasing of the biofilm activity and the adhesion of the films, the films partially exfoliate. The free metal acts as the anode and the films act as the cathode, which leads to the forming of the microcell. Finally, the corrosion is enhanced. 4.3. Procedure of Interaction between SRB and Carbon Steel. Figure 15 shows the schematic diagram of interaction between SRB and the carbon steels, which is an idealistic and simple diagram. From the results of the EDXA and the SEM, the whole procedure of interaction between SRB and the carbon steel is straightforwardly described as follows: At the beginning, as shown in Figure 15a, the amounts of SRB sharply increase with time in the SES (Figure 1). Amounts of metabolites, iron oxides, and some phosphorus compounds, e.g., phosphide17 and hexaphosphate,37 are formed in the SES. Oxides and phosphorus compounds are deposited or absorbed on the 12803

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Figure 15. Procedure of the interaction between SRB and the carbon steel.

surface. Simultaneously, sulfate-reducing bacteria protect themselves from harmful substances, e.g., free oxygen, by absorbing the iron oxide. At the second stage, as shown in Figure 15b, large amounts of SRB aggregate on the surface of the steel with time, and SRB begin to decrease in the SES (Figure 1). The phosphorus compounds are repelled away from the surface of the steel, which may be due to an incompatibility between SRB and the

compounds of P. At the same time, iron oxides originally absorbed on the surface of SRB begin to be desorbed, and become a barrier to the free oxygen, and form a new anaerobic room. At the third stage, as shown in Figure 15c, few SRB are observed in the SES. Biofilms, which contain EPS and cells, are formed on the surface of the steel with the increasing of the metabolites, e.g., extracellular polymer substances (EPS). The 12804

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Industrial & Engineering Chemistry Research ball-shaped iron oxides are not observed on the surface of SRB under the biofilm (Figure 11). At the fourth stage, as shown in Figure 15d, the biofilms begin to exfoliate with the activity of the biofilm and the SRB numbers decreasing. The phosphorus compounds redeposit on the surface of the steel, and small amounts of calcium hydroxide are also precipitated on the surface due to the increasing of pH value of the surface of the steel. At last, as shown in Figure 15e, the products containing the phosphorus compound and calcium hydroxide increase with time, and a thick film of the products is formed on the outer surface of the steel. The sulfides, which are produced by metabolism of SRB, are not any more formed with dying of SRB, but the corrosion of the steel still continues to occur. Sulfides are covered by a layer of corrosion products, e.g., ferrous oxides. The above results only simply describe the process of the interaction between SRB and the carbon steel during the period of immersing, which is an idealistic description.

5. CONCLUSION The corrosion rate of the carbon steel Q235 is smaller in the SES with SRB than that without SRB in the early experimental period, however, the rate is bigger in the SES with SRB than that without SRB in the late period. SRB can protect themselves from the harmful substances by absorbing oxides during the course of experiment. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel.: (+86) 24 2391 5867. Fax: (+86) 24 2391 5867.

’ ACKNOWLEDGMENT The project was financially supported by the National Natural Science Foundation of China (Grant 50971128, 51131001), the National R&D Infrastructure and Facility Development Program of China (Grant 2005DKA10400-CT-2-02) ’ REFERENCES (1) Li, S. Y.; Kim, Y. G.; Jeon, K. S.; Kho, Y. T. Microbiologically influenced corrosion of underground pipelines under the disbonded coatings. Met. Mater. 2000, 2000 (6), 281–286. (2) Maruthamuthu, S.; Kumar, B. D.; Ramachandran, S.; Anadkumar, B.; Palanichamy, S.; Chandrasekaran, M.; Subramanian, P.; Palaniswamy, N. Microbial corrosion in petroleum product transporting pipelines. Ind. Eng. Chem. Res. 2011, 50, 8006–8015. (3) Chen, G.; Clayton, C. R. The influence of sulfate-reducing bacteria on the passivity of type 317L austenitic stainless steel. J. Electrochiem. Soc. 1998, 145, 1914–1922. (4) Angell, P.; Luo, J. S. Microbially sustained pitting corrosion of 304-stainless-steel in anaerobic seawater. Corros. Sci. 1995, 37, 1085–1096. (5) Xu, J.; Wang, K. X.; Sun, C.; Wang, F. H.; Li, X. M.; Yang, J. X.; Yu, C. K. The effects of sulfate reducing bacteria on corrosion of carbon steel Q235 under simulated disbonded coating by using electrochemical impedance spectroscopy. Corros. Sci. 2011, 53, 1554–1562. (6) Torres, S. R.; Garcia, V. J. Corrosion of AISI 304 stainless steel induced by thermophilic sulfate reducing bacteria (SRB) from a geothermal power unit. Mater. Corros. 2001, 52, 614–618. (7) Nivens, D. E.; Nichols, P. D.; Henson, J. M. Reversible acceleration of the corrosion of AISI-304 stainless steel exposed to seawater

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