Corrosion Behavior of Steel A3 Influenced by Thiobacillus Ferrooxidans

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1 ACTA PHYSICO-CHIMICA SINICA Volume 24, Issue 9, September 2008 Online English edition of the Chinese language journal Cite this article as: Acta Phys. -Chim. Sin., 2008, 24(9): ARTICLE Corrosion Behavior of Steel A3 Influenced by Thiobacillus Ferrooxidans Songmei Li*, Yuanyuan Zhang, Jianhua Liu, Mei Yu School of Materials Science and Engineering, Beihang University, Beijing , P. R. China Abstract: The electrochemical measurement and surface analysis methods were employed to investigate the corrosion behavior of steel A3 influenced by Thiobacillus ferrooxidans (T.f). Polarization curve results indicated that the presence of Thiobacillus ferrooxidans resulted in higher corrosion potential of the electrode and obviously accelerated the corrosion current density. Atomic force microscope (AFM) results showed that asymmetric biofilms adhered to the surface of steel A3 after 7 days of exposure. The scanning electron microscopy (SEM) results showed that pitting appeared on the surface of steel A3 after 7 days of exposure in T.f solution, which was induced by the metabolism of bacteria and the morphology of the deposit. Pitting holes of steel A3 in T.f solution were deeper after 20 days of exposure. The presence of Thiobacillus ferrooxidans aggravated the localized corrosion of A3 steel. Key Words: Thiobacillus ferrooxidans; Steel; Corrosion It is common that activities of microorganisms may have induced pitting corrosion and brought serious economic losses to the industry in recent years [1,2]. Once the microorganisms adsorb to the material surface, the extracellular polymeric substance (EPS) secreted by bacteria, such as the extracellular polysaccharide, will make the bacteria and its metabolites bond together with the substrate, and then form a kind of complicated biofilm on the metal surface [3]. The heterogeneous biofilm and corrosion product film may change the electrochemical characteristics of the material surface, together with the metabolism of bacteria, the formation of the biofilm may induce serious corrosion, which can decline the service life of the material and equipment, and even bring maximal damage to the material. Thiobacillus ferrooxidans (T.f) is a kind of familiar industrial strain, which was initially separated from the acidic wastewater in the mine by Colmer and Hinkle in 1947, and then gathered great attention to study its acid-production properties. In recent years, T.f has been widely used in the extraction of heavy metals, such as copper, uranium, and gold, and has gradually become one of the main strains for the industrial biological extraction of different metals. T.f is also applied in environmental pollution treatment, especially in the coal desulfurization, the removal of heavy metal from the sewage sludge, and the treatment of the acidic wastewater [4,5]. Of late, a strain of Thiobacillus ferrooxidans, which has a strong corrosive effect on carbon steel, has been separated from the bottom of an oil tank. The microbiological method, electrochemical measurement, atomic force microscopy, scanning electron microscopy, and energy dispersive spectrum analysis have been carried out to investigate the corrosion behavior of steel A3 influenced by Thiobacillus ferrooxidans and explore the law of corrosive effect of the bacillus microbe on the steels, thus providing a theoretical basis for the corrosion control of materials. 1 Materials and methods 1.1 Coupon preparation The steel A3 used in the experiment was composed of C ( 3%), Si ( 1%), Mn ( 4.2%), S ( 2.9%), and P ( 1.9%). A steel A3 sheet was cut into one type of disc shaped specimen with a diameter of 10 mm, and two types of rectangle coupons with dimensions 20 mm 30 mm 2 mm and 10 mm 10 mm 2 mm. The first type of rectangle coupon was used for the hanging sample corrosion test and the surface observation was Received: April 21, 2008; Revised: May 26, *Corresponding author. songmei_li@buaa.edu.cn. Copyright 2008, Chinese Chemical Society and College of Chemistry and Molecular Engineering, Peking University. Published by Elsevier BV. All rights reserved. Chinese edition available online at

2 abraded to 1000-grit by carbide silicon metallurgical paper, and the other type used for biofilm observation was abraded to 2000-grit and then wet-polished with a polishing cloth to a smooth surface. The disc shaped specimen used for electrochemical measurements was processed into the electrode, the exposed area of which was abraded to 1200-grit. All of these specimens were washed with distilled water and cleaned with 75% ethanol solution and stored in a vacuum desiccator. All chemicals used were AR grade. 1.2 Microbiological source and cultivation Thiobacillus ferrooxidans was found and isolated from the mixed colonies at the bottom of an oil tank. In order to simulate the actual conditions, a mixed medium composed of Leathen and potato dextrose agar (PDA) with a volume ratio of 2:1 was used in the experiment, where the composition of Leathen was as follows: ammonium sulfate ((NH 4 ) 2 SO 4 ) 0.3 g L 1, potassium chloride (KCl) 0.05 g L 1, potassium phosphate dibasic (K 2 HPO 4 ) 0.05 g L 1, magnesium sulfate (MgSO 4 ) 0.5 g L 1, and calcium nitrate (Ca(NO 3 ) 2 4H 2 O) 0.01 g L 1. The ph value of the mixed medium was adjusted to 4.0±0.1. After 3 days of cultivation, the medium containing bacteria changed to a red-brown solution, which was an evidence of the presence of T.f. The T.f bacterium by enrichment cultivation for 10 days served as the inoculum. Each erlenmeyer flask, containing 200 ml of the fresh medium, was inoculated with 10% bacteria and then cultivated on a rotary shaker (ZDP-150, Shanghai Jing Hong Laboratory Instrument Co., Ltd., China) at 37 C. 1.3 Surface observation and biofilm analysis To better understand the formation process of the surface film, coupons after different days of exposure in T.f solution were taken out for SEM (JSM-5800 and S-530, Japan) observation and EDS (Hitachi, Japan) analysis. The rectangle coupons for AFM (NanoscopeIIIa, Instruments Inc., USA) observation were abraded to 2000-grit and then wet-polished on a polishing cloth. After being defatted with acetone and sterilized with the ultraviolet lamp, the coupons were exposed to T.f solution for 7 days, followed by biofilm observation with AFM. A Fourier transform infrared (FTIR) spectroscopy analyzer was also applied to characterize the organic component of the biofilm. The electrochemical tests were carried out with a conventional three-electrode system, done with PARSTAT 2273 potentiostat of America. Steel A3 acted as the working electrode with an exposed area of cm 2, a saturated calomel electrode (SCE) was used as the reference electrode, and the counter electrode was a Pt-plate. After the prescribed exposure time, the electrode was taken out for the Tafel plot measurements, with a scan rate of mv s 1. 2 Results and discussion 2.1 AFM observation and FTIR analysis of T.f biofilm In recent years, atomic force microscope technology has been widely used in the research field of microbiological influenced corrosion (MIC), and it has become a potent tool to investigate the interaction between the biofilm and the substrate [6]. Steele et al. [7] studied the microbiological influenced corrosion behavior of stainless steel 316L by using AFM, and they demonstrated the advantages of AFM in the qualitative evaluation of MIC. In this study, a digital Instrument Nanoscope IIIa AFM with a scanning rate of Hz was applied to observe the biofilm that formed on the surface of steel A3. Nanoprobe silicon nitride (Si 3 N 4 ) cantilevers were purchased from Digital Instruments (Vecco Instruments Inc., USA). AFM images of the T.f biofilm are shown in Fig.1. It was observed that the steel A3 coupon after 7 days of exposure in T.f solution was covered with a layer of biofilm, which consisted of clusters of microbial cells and extracellular polymeric substance [8,9]. The obtained images were analyzed by a multimode digital nanoprobe software. It was manifested that bacilliform bacterial cells with different sizes were observed on the metal surface with relatively high fluctuation, which showed an uneven distribution of the biofilm. In the presence of the extracellular polymeric substance, the bacteria inclined to bond together and form some kind of colonies. The composition of the biofilm could change during the process of aerobic microorganism respiration, thus forming the concentration gradient of oxygen. The selective reproduction and metabolism of the microorganism promoted the formation of the heterogeneous biofilm. The interaction of this biofilm and the corrosive metabolites changed the characteristics of the mate- 1.4 Tafel plot measurements Fig.1 (a) AFM image of biofilms formed on steel A3 exposed in Thiobacillus ferrooxidans (T.f) solution for 7 days, (b) surface fluctuation of steel A3 adhered to T.f biofilms

3 Fig.2 Fourier transform infrared spectrum of biofilms on steel A3 exposed in T.f solution for 7 days Fig.4 EDS spectrum of corrosion products of steel A3 exposed in T.f solution for 20 days rial surface and induced localized corrosion of steel A3. In order to better investigate the biofilm, a Fourier transform infrared spectroscopy analyzer was applied, to characterize the organic component of the biofilm formed on the coupon surface after 7 days of exposure in T.f solution. The biofilm was usually composed of bacterial cells, the extracellular polymeric substance, and the corrosion products, among which the bacterial cells were only 5% 25% of the biofilm volume [10]. As shown in Fig.2, the absorption peaks at , , , and cm 1 were related to O H, C=C, C=O, and C H groups, respectively. Hence the biofilm on the coupon surface contained C=C, C H, and O H groups, which were the components of polyester carbohydrate, lipoprotein, and bacterial surface proteins of the cell wall. 2.2 SEM morphologies and EDS analysis of corrosion products Fig.3 SEM morphologies of the corrosion products of steel A3 exposed in T.f solution for 7 days (a) and 20 days (b) Fig.3 shows the SEM images of steel A3 coupons in T.f solution for 7 and 20 days of exposure. As shown in Fig.3a, a heterozygous layer consisted of a few corrosion products, extracellular polymeric substance, and metabolites was clearly observed on the coupon surface after 7 days of exposure, and appeared to be heterogeneous. It is important to point out that this kind of layer often results in the occurrence of localized corrosion [11]. Due to the effect of microbial metabolism, the components of the biofilm change such as the content of oxygen, the ph value, as also the conditions on the material s surface [12]. Different sizes of round deposits, which were compact at the edge and loose in the center, were clearly seen from the SEM images. Therefore, it was difficult for the oxygen to diffuse to the bottom of the substrate surface covered with compact products, thereby a relatively low concentration of oxygen formed, whereas, the clastic and loose deposits in the center were favorable to maintain a higher concentration of oxygen, and as a result, a kind of oxygen concentration cell was formed, which was the key reason for the occurrence of pitting corrosion. Meanwhile, the loose deposits were helpful for the transportation of oxygen for the breed and growth of the aerobic bacteria, which also contributed to the corrosion. After 20 days of exposure in T.f solution, the coupon surface was covered with thick corrosion products (Fig.3b). Fig.4. is the EDS spectrum of the corrosion products of steel A3 exposed in T.f solution for 20 days. EDS analysis of these deposits revealed that the corrosion products mainly contained iron (76.16%), oxygen (23.32%), and silicon (0.51%), etc. Hence, the main component of the deposits was iron oxide, and the silicon could be the impurity introduced by the emery paper during buffing. The peak on the right of the Si was caused by Au after gold sputtering. Under aerobic conditions, T.f generally oxidized ferrous ions and various reducing sulfides to obtain the energy essential for its growth [13,14]. In the ferrous ion-oxidizing system, the electrons produced by T.f during the oxidation process were transferred to the ultimate electron acceptor, oxygen, in an approximately identical way [15], and the ferrous ion could be oxidized to a ferric ion by bacteria as per the following reaction: 4Fe 2+ +O 2 +4H + 4Fe 3+ +2H 2 O. The ferric ion could be reduced to a ferrous ion through the interaction between the ferrous ion and the mineral, and the ferrous ion was oxidized again by the bacteria, the oxidation rate of which was much higher than that of chemical oxidation [16].

4 Fig.5 SEM morphologies of steel A3 exposed in sterile solution for 20 days (a), T.f solution for 7 days (b) and 20 days (c) after removing corrosion products 2.3 Surface analysis after removing corrosion products SEM was carried out to reveal the details of the surface corrosion morphologies of steel A3 exposed in a sterile medium for 20 days and in T.f solution for 7 and 20 days (Fig.5). The SEM analysis indicated that the coupon exposed to the sterile medium exhibited no localized corrosion on the surface after 20 days of exposure (Fig.5a). Although, the coupon exposed in T.f solution for 7 days illustrated several minor corrosion pits on the surface (Fig.5b), after 20 days of exposure, more pitting holes with different sizes appeared (Fig.5c), which were deeper compared to those that appeared after 7 days. The reasons for the pitting behavior of steel A3 induced by T.f could be summarized as follows: (i) the bacteria cells and the metabolites were inclined to combine with the corrosion products in the presence of extracellular polymeric substances (EPS), and the bacteria selectively bred at different areas, which contributed to the unevenness of the biofilm; (ii) under the effect of T.f, the corrosion products with an extremely special morphology appeared on the coupon surface during the initial stage of exposure, and the formation of the oxygen concentration cell was a key factor to the corrosion of steel A3. In conclusion, a strong connection could be confirmed between the MIC of materials and the characteristics of the substrate surface, such as morphology, compactness, and integrity. The special morphology of the corrosion product film combined with the heterogeneous biofilm that developed on the coupon surface in T.f solution apparently stimulated the pitting corrosion of steel A3. electrode surface could be covered with a relatively compact layer composed of corrosion products and organic secretion, which appeared alternately. During the experiment, the electrode was taken out after 20 days of exposure and pitting holes with thick corrosion products inside could be observed with a naked eye. The electrode exposed in sterile solution was confirmed to experience an even corrosion with comparatively fewer corrosion products. Fig.6 also shows that the I corr value of the electrode in T.f solution is apparently higher than that in sterile solution after 20 days of exposure. The propagation of bacteria and the oxidation of the ferrous ion reduced the soluble oxygen in the solution. The reductive sulfide produced by T.f during the oxidation process could be oxidized to sulfate through chemistry or by the effect of thiobacillus, and sulfuric acid could be produced in a certain condition [17,18], which decreased the ph value of the solution and destroyed the material. The erosive substances generated during the microorganism metabolism process greatly changed the membrane resistance of the material, although a relatively compact biofilm formed on the surface, which, to a certain extent, served as a protective film during the initial stage of immersion. The heterogeneous biofilm could still result in the localized corrosion of material [19], thereby, the corrosion current increased and the corrosiveness of steel A3 was aggravated. However, the shapes of polarization curves did not change noticeably in T.f solution compared to the sterile one, which indicated that the microor- 2.4 Polarization curve measurements In order to determine the changes in the corrosion current density (I corr ) and the corrosion potential (E corr ) with exposure time, the polarization curve measurement was applied for steel A3 electrodes after 20 days of exposure in sterile and T.f solution, respectively. As shown in Fig.6, the E corr of steel A3 in T.f and sterile solutions were 621 and 652 mv (vs SCE), respectively. A relatively higher E corr was detected in T.f solution, which could probably be interpreted by the inhibitive effect of the protective corrosion product film formed on the electrode surface. With the prolonging of exposure time, the Fig.6 Polarization curves of steel A3 electrodes exposed in sterile and T.f solutions for 20 days

5 ganism did not change the nature of its anodic and cathodic processes, but increased the rate of the anodic and cathodic reactions, which initiated or accelerated the corrosion of material [20]. 3 Conclusions (1) AFM images showed that the T.f biofilm formed on the steel A3 surface, changed the microscopic condition of materials, and after being exposed in T.f solution for 7 days the uneven film resulted in the initiation of pitting corrosion. (2) In the presence of T.f, the special morphology of corrosion products urged the bacteria to grow and metabolize selectively under the deposits. The oxygen concentration cell directly resulted in the localized corrosion of steel A3. (3) SEM results indicated that pitting corrosion was observed on the surface in the presence of bacteria after 7 days of exposure, the corrosiveness of steel A3 was aggravated in T.f solution with the prolonging of time. (4) Polarization curves, after 20 days of exposure, showed an increase in E corr and I corr in T.f solution, indicating that the presence of microorganisms accelerated the rate of anodic and cathodic reactions. The corrosion behavior of steel A3 can be related to the metabolism of the microorganism, the integrity, and the compact density of the biofilm and corrosion product film on the material surface. References 1 Yuan, S. J.; Choong, A. M. F.; Pehkonen, S. O. Corrosion Science, 2007, 49: Xu, C, M.; Zhang, Y. H.; Cheng, G. X.; Zhu, W. S. Materials Science and Engineering, 2007, 443: Fu, Y. B. Materials Development and Application, 2006, 21: 34 4 Acharya, C.; Kar, R. N.; Sukla, L. B. Fuel, 2001, 80: Chao, S. H.; Zhang, X. G. Guangdong Chemical Industry, 2005, 8: 30 6 Beech, I. B.; Smith, J. R.; Steele, A. A.; Penegar, I.; Campbell, S. A. Coll. Surf. B: Biointerfaces, 2002, 23: Steele, A.; Goddard, D. T.; Beech, I. B. Int. Biodeterio. Biodegrad, 1994, 34: 35 8 Crundwell, F. Mineral Engineering, 1996, 9: Johnson, L. R. Journal of Theoretical Biology, 2008, 251: Caldwell, D. E.; Korber, D. R.; Lawrence, J. R. Microb. Methods, 1992, 2: Dexter, S. C.; Duquette, D. J.; Siebert, O. W.; Videla, H. A. Corrosion, 1991, 47: Sheng, X. X.; Ting, Y. P.; Pehkonen, S. O. Corrosion Science, 2007, 49: Breed, R. E.; Gibbons, N. E. Bergey s manual of determinative bacteriology. 8th ed. Trans. Institute of Microbiology, Chinese Academy of Sciences. Beijing: Science Press, 1984: Shen, L.; Zhang, Z. L.; Jia, X. S. Alta Scientiae Circumstantiae, 2006, 26: Zhang, C. G.; Xia, J. L.; Qiu, G. Z. The Chinese Journal of Nonferrous Metals, 2006, 16: Xu, S. X.; Zhang, Y. K.; Chen, N.; Liang, Y.; Li, W. Q. Sichuan Chemical Industry, 2006, 9: Lai, C. X. Total Corrosion Control, 2001, 15: Li, H. X. Trans. Nonferrous Met. Soc. China, 2006, 16: Lin, J.; Yan, Y. G.; Chen, G. Z.; Liu, G. Z.; Li, Q. F. Electrochemistry, 2006, 12: Enos, D. G.; Taylor, S. R. Corrosion, 1996, 52: 831