BIOFILM FORMATION AND ITS INDUCED BIOCORROSION OF METALS IN SEAWATER SHENG XIAOXIA NATIONAL UNIVERSITY OF SINGAPORE 2007 BIOFILM FORMATION AND ITS INDUCED BIOCORROSION OF METALS IN SEAWATER SHENG XIAOXIA (B.ENG. (Hons.), ZHEJIANG UNIVERSITY) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF CHEMICAL AND BIOMOLECULAR ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2007 Acknowledgements ACKNOWLEDGEMENTS I first would like to express my deepest gratitude and appreciation to my supervisor Prof. Ting Yen Peng, for his constant guidance and inspiration throughout my graduate studies. It was his patience and support through the years which inspired me to preserve in my quest. I also would like to thank my co-supervisor, Prof. Simo Olavi Pehkonen, for providing extremely valuable discussions and suggestions regarding my research. I am very grateful towards Dr. He Jianzhong for helping me conduct the molecular biology experiments, and for her insightful discussions for pointing out the directions to improve my research work. This work has received a great deal of support and assistance from the lab officers Ms. Li Fengmei, Ms. Li Xiang, Ms. Sylvia Wan, Mr. Qin Zhen, and Mr. Boey Kok Hong for their assorted help around the lab. I would like to acknowledge Ms. Samantha Fam for her guidance on the operation of AFM. I also thank Mr. Ng Kim Poi for preparing the metal coupons and making the corrosion cell. Special thanks to my friends Zhao Quangqiang, Zhu Zhen, Wang Yan, Xu Tongjiang, and Xu Ran for their friendship. Their help in my life made my graduate study an enjoyable and exciting experience. I would like to show my greatest appreciation to my husband, Zhang Ning, and my parents for their support and encouragement. This work was supported from Tropical Marine Science Institute (Singapore) National University of Singapore (Research Grant RP-279-000-173-112). i Table of Contents TABLE OF CONTENTS ACKNOWLEDGEMENTS i SUMMARY .v LIST OF FIGURES . vii LIST OF TABLES .xi NOMENCLATURE xii CHAPTER INTRODUCTION 1.1 Biofilm Formation on Metal Surfaces 1.2 Mechanisms of Biocorrosion 1.3 Bacteria Related to Biofilm Formation and Biocorrosion .7 1.3.1 Sulphate-reducing Bacteria (SRB) . 1.3.2 Other Bacteria . 10 1.4 Methods for the Inhibition of Biofilm and Biocorrosion .14 1.4.1 Layer-by-layer (LBL) Polyelectrolyte Multilayer Coating . 14 1.4.2 Organic Inhibitors . 17 1.5 Objectives and Scope of This Work 21 CHAPTER MATERIALS AND METHODS .24 2.1 Metal Coupons .24 2.2 Microorganisms .24 2.3 Isolation and Identification of Strain SJI1 25 2.3.1 Morphological Characterization 25 2.3.2 Physiological Studies 26 2.3.3 16S rRNA Sequence Analysis 28 2.3.4 Phylogenetic Analysis . 28 2.3.5 Nucleotide Sequence Accession Number . 29 2.4 Biofilm Formation .29 2.4.1 Cell Immobilization 29 2.4.2 Zeta Potential (ζ) and Contact Angle Measurements 30 2.4.3 Confocal Laser Scanning Microscopy (CLSM) . 31 2.4.4 AFM Operation of Force Measurement . 31 2.5 Biofilm and Biocorrosion of Stainless Steel AISI 316 and Its Prevention .32 2.5.1 Biofilm and Biocorrosion Experiment Setup . 32 2.5.2 Scanning Electron Microscopy (SEM) . 33 2.5.3 Atomic Force Microscopy (AFM) . 34 2.5.4 Electrochemical Impedance Spectroscopy (EIS) 34 2.6 Preparation of Layer-By-Layer (LBL) Coating 35 2.6.1 Polyelectrolyte Solutions . 35 2.6.2 Layer-by-layer (LBL) Technique . 36 ii Table of Contents 2.6.3 Stability of the PEM on Functionalized SS316 37 CHAPTER ISOLATION, CHARACTERIZATION AND IDENTIFICATION OF A MARINE SULPHATE REDUCING BACTERIA 39 3.1 Cell Morphology 39 3.2 Growth of Desulfovibrio singaporenus Strain SJI1 on Lactate and Acetate .40 3.3 Physiological Properties .44 3.4 16S rRNA Gene Sequence and Phylogenetic Analysis .47 3.5 Summary .51 CHAPTER BIOFILM FORMATION AND FORCE MEASUREMENT .52 4.1 Force Measurement in the Fluid .52 4.1.1 Typical Force Curves . 52 4.1.2 Forces Between the Cell Tip and Different Metal Substrates 55 4.1.3 Cell Tip-Cell Lawn Interactions . 60 4.1.4 Influence of Nutrient and Ionic Strength on the Cell-Metal Interaction 64 4.1.5 Influence of Solution pH on the Cell-Metal Interaction 68 4.2 Ex-situ Force Measurement .73 4.3 Summary .78 CHAPTER SULPHATE REDUCING BACTERIA BIOFILM AND ITS INDUCED BIOCORROSION OF STAINLESS STEEL AISI 316 80 5.1 AFM Image Analysis .80 5.1.1 Biofilm Investigation . 80 5.1.2 Pits Investigation . 84 5.2 EIS Results 88 5.2.1 Control Coupons in EASW . 88 5.2.2 Coupons in EASW with D. desulfuricans 95 5.2.3 Coupons in EASW with D. singaporenus 97 5.2.4 Comparison of the Coupons with and without SRB . 98 5.3 Summary .100 CHAPTER BIOFILM AND BIOCORROSION INHIBITION USING LAYER-BY-LAYER COATING 102 6.1 Surface Functionalization of SS316 and the Stability of the Multilayers .102 6.2 XPS Analysis of the Functionalized Stainless Steel 104 6.3 Biofilm Viability Study by CLSM 106 6.4 Biofilm and Biocorrosion Study Using AFM 108 6.5 Biocorrosion Study Using Linear Polarization Analysis .110 iii Table of Contents CHAPTER BIOFILM AND BIOCORROSION INHIBITION USING AN ORGANIC INHIBITOR .112 7.1 Evaluation of Organic Corrosion Inhibitor on Abiotic and Biotic Corrosion of Mild Steel 112 7.1.1 XPS Analysis .112 7.1.2 Bacteria Concentration .114 7.1.3 EIS Analysis .115 7.1.4 Linear Polarization Analysis and Potentiodynamic Scanning Curves 118 7.1.5 SEM Analysis 122 7.1.6 AFM Analysis 126 7.1.7 Adsorption Isotherm .128 7.2 Evaluation of Organic Corrosion Inhibitor on Abiotic and Biotic Corrosion of SS316 130 7.2.1 EIS Analysis .130 7.2.2 Linear Polarization Analysis .133 7.2.3 CLSM Analysis 134 7.2.4 AFM Analysis 136 7.2.5 Adsorption Isotherm .138 7.3 Summary .139 CHAPTER CONCLUSIONS AND RECOMMENDATIONS 141 8.1 Conclusions .141 8.2 Recommendations 146 REFERENCES .149 iv Summary SUMMARY Biocorrosion, also termed as microbiologically influenced corrosion (MIC), refers to the electrochemical process where the participation of the microorganisms on a metal surface accelerates the corrosion reaction on the metal surface. An important step of biocorrosion process is the formation of a biofilm, a microbial community which is enveloped by adhered extracellular biopolymer substances (EPS) these microbial cells produce on the surface of a liquid and a surface. In this thesis, several issues related to biofilm and biocorrosion on metals are addressed. These include: (i) the isolation and characterization of a novel marine sulphate-reducing bacteria (SRB) strain from local seawater, (ii) investigating bacteria-metal interactions, (iii) investigating biofilm and its induced biocorrosion of two SRB strains on stainless steel 316 (SS316), and (iv) biofilm and biocorrosion prevention using an organic inhibitor and a layer-by-layer coating on the metal substrate. A novel sulphate-reducing bacterium, designated Desulfovibrio singaporenus strain SJI1, was isolated from seawater near St. John Island, Singapore. The isolate is rod, curved-shaped and motile, and is a typical moderately halophilic and mesophilic strain. Interestingly, D. singaporenus completely oxidizes lactate to acetate via pyruvate as the intermediate during sulphate reduction. Acetate is further partially oxidized to CO2 when it is used as an electron donor. The adhesion of two anaerobic sulphate-reducing bacteria (D. desulfuricans and D. singaporenus) and an aerobe (Pseudomonas sp.) to four polished metal surfaces (i.e. stainless steel AISI 316, mild steel, aluminum, and copper) was examined using a force spectroscopy technique with an atomic force microscopy (AFM). Using a modified bacterial tip, the attraction and repulsion forces (in the nano-Newton range) between the bacterial cell and the metal surface in aqueous media were quantified. Results show that the bacterial adhesion force to aluminum and to copper is the highest and the lowest respectively among the metals investigated. The bacterial adhesion forces to metals are influenced by the surface charges and the hydrophobicity of the metal and bacteria. The cell-cell interactions show that there are v Summary strong electrostatic repulsion forces between bacterial cells. Biocorrosion of SS316 by D. desulfuricans and D. singaporenus was investigated. The biofilm and pit morphology that developed with time were analyzed using atomic force microscopy (AFM). Electrochemical impedance spectroscopy (EIS) results were interpreted with an equivalent circuit to model the physicoelectric characteristics of the electrode/biofilm/solution interface. D. desulfuricans formed one biofilm layer on the metal surface, while D. singaporenus formed two layers: a biofilm layer and a ferrous sulfide deposit layer. AFM images corroborated results from the EIS modeling which showed biofilm attachment and subsequent detachment over time. These results indicate that SRB could directly react with metal surface, and it plays direct role in the biocorrosion. A layer-by-layer coating on SS316 substrate alternately with quaternized polyethylenimine (q-PEI) and poly(acrylic)acid (PAA) to form polyelectrolyte multilayers (PEM) was investigated. The PEM were stable in seawater. The antibiocorrosion ability of PEM on stainless steel was assessed using Pseudomonas sp., D. desulfuricans and D. singaporenus. Compared to the bare stainless steel, the corrosion rates and the pit depths decreased for the PEM functionalized SS316. Biofilm growth on the substrate was inhibited by the antibacterial effect of q-PEI as shown by confocal laser scanning microscopy (CLSM). These results indicate that PEM have potential applications in the inhibition of biocorrosion of metal substrates. Corrosion inhibition of mild steel and SS316 by an organic inhibitor 2-Methylbenzimidazole (MBI) in seawater was also investigated using direct current polarization, XPS, EIS, SEM, CLSM, and AFM. MBI was shown to be an effective inhibitor in controlling abiotic corrosion as well as biocorrosion by D. desulfuricans and D. singaporenus. Tafel plots revealed that MBI predominantly controls the cathodic reaction. The corrosion inhibition effect of MBI on MIC is partially due to the inhibition of the bacterial activity. The adsorption of MBI on the steel surface follows a Langmuir adsorption isotherm model. vi List of Figures LIST OF FIGURES Figure 1.1 Structure of 2-Methyl-benzimidazole (MBI) 20 Figure 2.1 Derivatization of q-PEI 36 Figure 2.2 Layer-by-layer (LBL) coating of q-PEI and PAA multilayer on polished SS316 .37 Figure 3.1 Images of strain SJI1 on a SS316 coupon: (a) a single cell (x10,000); (b) cells growing on SS316 (x5,000); (c) an AFM phase image of an individual cell with a single polar flagellum (scale μm × μm) .40 Figure 3.2 (a) Time course of the growth of strain SJI1 showing increase in cell density (♦) and decrease in sulphate concentration (►); (b) The consumption of lactate (▲) and the production of acetate (●) and pyruvate (■) accompanying bacterial growth. Error bars indicate standard deviation, which are not shown when they are smaller than the symbol. 42 Figure 3.3 Nucleotide sequence of the 16S rRNA gene of strain SJI1 (deposited in the Genbank database on 16th April 2007 under accession number EF178280). .48 Figure 3.4 A phylogenetic tree based on 16S rRNA gene sequences showing the position of strain SJI1 within the genus Desulfovibrio and in relation to other sulphate-reducing bacteria. The tree was calculated using the neighbor-joining method. Bar, 2% sequence divergence. .49 Figure 4.1 A scanning electron microscope image of a silicon nitride tip coated with Pseudomonas sp 52 Figure 4.2 A typical force-distance curve between a Pseudomonas sp. coated tip and SS316. 54 Figure 4.3 Force-distance curves when a Pseudomonas sp. cells coated tip was (a) extended to and (b) retracted from different metal substrates in artificial seawater. .58 Figure 4.4 Force-distance curves when a D. desulfuricans cells coated tip was (a) extended to and (b) retracted from different metal substrates in artificial seawater. .58 Figure 4.5 Force-distance curves when a D. singaporenus cells coated tip was (a) extended to and (b) retracted from different metal substrates in artificial seawater. .59 Figure 4.6 CLSM images of Pseudomonas sp. adhering onto (a) mild steel, (b) copper, (c) aluminum, and (d) on SS316 in artificial seawater. The scale bar is 500 μm for all images. .60 vii List of Figures Figure 4.7 Force-distance curves when bacteria coated tip was extended to the substrate in artificial seawater: (a) D. singaporenus, (b) Pseudomonas sp., and (c) D. desulfuricans .63 Figure 4.8 Force-distance curves when a cells-coated tip was retracted from SS316 in different solutions (a) Pseudomonas sp.; (b) D. desulfuricans; (c) D. singaporenus .66 Figure 4.9 CLSM images of Pseudomonas sp. adhering onto SS316 in (a) DIW; (b) ASW; (c) EASW. 68 Figure 4.10 The adhesion force between cell probe and SS316 in ASW with various pH: (a) Pseudomonas sp.; (b) D. desulfuricans; (c) D. singaporenus. .71 Figure 4.11 XPS measurement of Fe 2p spectra in ASW at various pH: (a) pH 3, (b) pH 5, (c) pH 7, and (d) pH 9. 72 Figure 4.12 A contact mode AFM image of a biofilm on SS316 76 Figure 4.13 Force measurements on the biofilm surface with D. singaporenus: (A—on cell, B—at cell periphery, C—on biofilm substrate, D—on deposit and E—at deposit periphery) .77 Figure 4.14 Force measurements on the biofilm surface with D. desulfuricans: (A—on cell, B—at cell periphery, C—on biofilm substrate, D—on deposit and E—at deposit periphery) .77 Figure 5.1 Atomic Force Microscopy images of stainless steel AISI 316 coupons with D. desulfuricans biofilm; (a) 4-day-immersion; (b) 14-day-immersion; (c) 24-day-immersion; (d) 34-day-immersion; (e) 44-day-immersion. 82 Figure 5.2 Atomic Force microscopy images of SS316 coupons with D. singaporenus biofilm; (a) 4-day-immersion; (b) 14-day- immersion; (c) 24-dayimmersion; (d) 34-day- immersion; (e) 44-day- immersion. 83 Figure 5.3 Two- and three-dimensional images of (a) a single pit, and (b) a D. desulfuricans cell on the SS316 coupons. 85 Figure 5.4 Section analysis on the SS316 coupons: (a) height profile of D. desulfuricans cells; (b) depth profile of a small pit; (c) depth profile of a large pit .86 Figure 5.5 Depth of pits on SS316 at different time of exposure. 87 Figure 5.6 SEM images for biofilm on the SS316 in MASW with (a) D. desulfuricans and (b) D. singaporenus. .87 Figure 5.7 EIS analysis for the samples at 35th day of immersion: (a) control coupon; (b) coupon with D. desulfuricans; (c) coupon with D. singaporenus. 90 Figure 5.8 Equivalent Circuit models: (a) Model of R(Q[R(QR)]) for control coupons; (b) Model of R(Q[R(QR)(QR)]) for control coupons; (c) Model of R(Q[R(QR)(QR)]) for coupons in EASW with D. desulfuricans; (d) Model viii Chapter Conclusions and Recommendations The development of biofilm and the biocorrosion effects of SRB on SS316 were studied. AFM images and EIS modeling results showed that the biofilm formed by D. desulfuricans accumulated with exposure time, while the biofilm formed by D. singaporenus underwent several phases of attachment, growth, subsequent detachment and reattachment. The biofilm induced by different SRB strains showed different morphology and polarization resistance. The biofilm formed by D. singaporenus was compact and crystal-like, whereas the biofilm formed by D. desulfuricans was porous and net-like and thus induced a faster corrosion on the surface of SS316. Pits with a curved-rod shape which is similar to SRB cell morphology suggest a direct role of SRB cells on the localized corrosion. Equivalent circuit models from EIS results bore out the presence of biofilm on the metal surface for both D. desulfuricans and D. singaporenus. However, an additional layer of iron sulfide deposit on the steel surface was formed in the presence of D. singaporenus. In addition, a new method for coating a SS316 substrate with antibacterial chemicals using a layer-by-layer technique was investigated. The SS316 substrate was alternately coated with quaternized polyethylenimine (q-PEI) and poly(acrylic)acid (PAA) to form multilayers on its surface. XPS and contact angle measurements showed the chemical nature of the polyelectrolyte multilayers (PEM). The PEM was found to be stable in seawater. The antibiocorrosion ability of PEM on SS316 was assessed using an aerobic bacterium Pseudomonas sp. and two anaerobic bacteria, D. desulfuricans and D. singaporenus. Compared to the pristine SS316 substrate, the 144 Chapter Conclusions and Recommendations corrosion rates of the functionalized SS316 measured by EIS and linear polarization decreased in the presence of PEM coating. The depths of the pits caused by the biocorrosion also decreased significantly as quantified by AFM. Biofilm growth on the metal substrate was inhibited by the antibacterial effect of PEI as evidenced by CLSM using LIVE/DEAD Baclight bacterial viability kits. These results indicate that PEM shows potential application in the inhibition of biocorrosion of metal substrates. Finally, an organic inhibitor, MBI, was examined for its effectiveness in controlling abiotic corrosion and biotic corrosion induced by SRB. MBI was found to be an effective inhibitor in the control of mild steel and SS316 corrosion in sterile seawater, and in seawater in the presence of D. desulfuricans or D. singaporenus. Tafel plots revealed that MBI predominantly inhibits the cathodic reaction. The corrosion inhibition by MBI at 0.1-2.5 mM is more effective for the corrosion caused by D. desulfuricans than that caused by D. singaporenus, with a corrosion inhibition efficiency of 85% and 93.96% for mild steel and stainless steel, respectively, in the presence of D. desulfuricans. The corrosion inhibition of MIC is partly due to the inhibition of bacterial activity, as well as the reduction of bacterial attachment on the steel surface. The adsorption of MBI on both mild steel and stainless steel surfaces obeys the Langmuir adsorption isotherm. 145 Chapter Conclusions and Recommendations 8.2 Recommendations We have isolated a new marine SRB strain and tested its corrosion behavior to metals, which is particularly helpful in understanding local biocorrosion problem of marine facilities in Singapore. We have performed experiments with AFM force spectroscopy using a cell probe, and have quantified the bacteria-metal forces in a nano-level. Contribution has been made to the understanding of the initial biofilm formation in the marine environment. However, the work so far is relatively limited because the reasons why the strain could use acetate and which gene and enzymes contribute to this property remain to be investigated. Further work is necessary to identify the exact gene and enzymes that are related to the acetate oxidation. Moreover, this work is limited to the initial bacteria-metal interaction; as such it does not address the factors influencing the biofilm accumulation. It is important to explore the biofilm development after the initial formation of first bacterial layer on metal surfaces. The AFM images of the pits which mirror the bacterial morphology suggest that the SRB directly interact with SS316, but the questions on the mechanism of the direct reaction of SRB with the metal surface, and the electron flow among the metal, bacteria and biofilm remain unanswered. Therefore, more work is required to shed light on these critical issues, and further studies are needed to extend the contributions made in this thesis. Some recommendations are proposed for future research: 146 Chapter Conclusions and Recommendations Study of the acetate metabolism pathway of D. singaporenus As acetate oxidation with sulphate reduction by D. singaporenus is a unique capability of this organism, it would be interesting and important to research in depth the metabolic pathway of acetate oxidation. So far some researchers have proposed the acetate metabolism pathway of several acetate utilizing SRB, such as the citric acid cycle with the synthesis of pyruvate from acetyl CoA and carbon dioxide as an anaplerotic reaction (Thauer, 1982), and the direct breakdown of C-C bond in activated acetate (Schauder et al., 1986). Study on the enzymes and particular gene participating in the acetate oxidation would be useful to explore the unique behavior for bacteria survival in the natural environment. Application of force spectroscopy technology in coating strategies The development of cell probes and its use in force spectroscopy to study the bacteria-metal interaction is useful, particularly in the application of anti-biofilm strategies. A common technique to prevent the biofilm attachment is to deposit a layer of coating on the surface. The usual way to detect the effectiveness of the coating is the observation of biofilm formation over a period of time. A study on the application of the force spectroscopy using cell probes would be useful to investigate the effectiveness of the coatings for biofilm and biocorrosion inhibition. Biofilm formation and biocorrosion in a flow-through system In the present work, the experiments were operated in a static system in the study of biofilm formation and biocorrosion on the metal surfaces. However, natural systems are usually flowing systems, such as the piping lines and heat exchangers. 147 Chapter Conclusions and Recommendations The flow rate of the fluid has a significant effect on the microorganisms’ attachment and accumulation on the metal substrates. Slow flow rate would facilitate the biofilm growth and accumulation, while fast flow rate would result in the easy detachment of biofilm from substrates. Hence, it is important to investigate the relationship between the flow rate and the biofilm formation as well as the biocorrosion effect on metals. Biocorrosion of metals in a mixed culture Numerous microorganisms are known to cause biocorrosion in the natural environment. A mixed culture or a consortium of microorganisms may give rise to more severe biocorrosion than in pure culture systems. In general, there is a cooperative role between the aerobic and anaerobic bacteria, which would lead to more intense corrosion. 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Sci. 46 (2004) 3031-3040. 158 Publications PUBLICATIONS Journal Publications ♦ Sheng X., Ting Y.P., Pehkonen S.O., Direct Force Measurement of Bacteria Adhesion on Metal in Aqueous Media, Water Science and Technology 54 (2006) 17-25. ♦ Sheng X., Ting Y.P., Pehkonen S.O., The influence of sulphate-reducing bacteria biofilm on the corrosion of stainless steel AISI 316, Corrosion Science 49 (2007) 2159-2176. ♦ Sheng X., Ting Y.P., Pehkonen S.O., Force measurements of bacterial adhesion on metals using a cell probe atomic force microscope, Journal of Colloid and Interface Science 310 (2007) 661-669. ♦ Sheng X., Ting Y.P., Pehkonen S.O., Evaluation of an organic corrosion inhibitor on abiotic corrosion and microbiologically influenced corrosion of mild steel, Industrial & Engineering Chemistry Research 46 (2007) 7117-7125. ♦ Sheng X., He J.Z, Ting Y.P., Pehkonen S.O., Isolation and characterization of a novel marine sulphate-reducing bacterium from seawater, Applied and Environmental Microbiology, 2007, submitted. ♦ Sheng X., Ting Y.P., Pehkonen S.O., The influence of ionic strength, nutrients and pH on bacterial adhesion to metals, Journal of Colloid and Interface Science, 2008, submitted. Conference Proceedings ♦ Sheng X., Ting Y.P., Pehkonen S.O., Direct measurements of interactive forces on the components of biofilm using atomic force microscopy, 17th International Congress of Chemical & Process Engineering, Prague, Aug. 2006. ♦ Sheng X., Ting Y.P., Pehkonen S.O., Inhibition of microbiologically influenced corrosion of mild steel and SS316 by an organic inhibitor, 17th International Biohydrometallurgy Symposium, Frankfurt, Sep. 2007. 159 [...]... investigate in depth the impact of biofilm formation and the mechanisms of biocorrosion (in particular modeling the metal /biofilm/ bulk fluid interface The specific objectives of are: i To isolate and identify a strain of SRB from local seawater, and characterize the morphological, physiological, and phylogenetic properties ii To investigate the driving force of the initial biofilm formation on metals in seawater. .. consumption of hydrogen or 2 Chapter 1 Introduction oxygen (cathodic reactants) in the metal -biofilm interface Therefore, it is important to study the mechanism of the biofilm and biocorrosion In this chapter, a review will be given on biofilm formation, biocorrosion mechanisms, bacteria species associated with the biofilm and biocorrosion of metals, as well as methods for the inhibition of biofilm and biocorrosion. .. bacteria inhibition, has never been tested for the control of biocorrosion Thus a test of the new layer-by-layer coating is desirable to reveal the biocide and anti -biocorrosion efficiency of the coating 20 Chapter 1 Introduction 1.5 Objectives and Scope of This Work The aims of this thesis are to examine the role of microorganisms in biofilm formation and its induced corrosion of metals, and to investigate... approximately 20% of all corrosion damage of metals is induced by biocorrosion (Flemming, 1996) Financial cost associated with the repair and replacement of equipment resulting from the damage of biofilm and biocorrosion problem run into millions of dollars annually Brennenstuhl et al (1992) reported that biocorrosion caused a damage of approximately US $ 55 million in stainless steel exchangers within 8 years... Related to Biofilm Formation and Biocorrosion Microorganisms associated with biocorrosion of metals such as iron, aluminum, copper and their alloys are diverse in the natural environment Their ability to influence the corrosion of metals by changing the corrosion resistance in the environment makes the microorganisms deleterious to the metals The main types of bacteria involved in biocorrosion of metal... microbiologically influenced corrosion (MIC), refers to the influence of microorganisms on the kinetics of corrosion processes of metals, induced by microorganisms adhering to the interfaces, i.e on the biofilm Biocorrosion is not a new corrosion mechanism but it integrates the role of microorganisms in the corrosion processes It occurs directly and indirectly as a result of the activities of living microorganisms... monitoring and changes in design Therefore, it is important to study the biocorrosion behavior of metals and its corrosion mechanisms in the marine environment There are usually several mechanisms involved in biofilm induced corrosion A biofilm not only entraps deleterious metabolites secreted by bacteria, but also creates gradients of pH, dissolved oxygen, nutrient, and chloride Over time, this alters and. .. from above, substantial research had been done on the biocorrosion of metals induced by SRB, but, understanding of the biocorrosion mechanisms is far from complete, and in particular, there is considerable margin in the study of SRB biofilm interactions with metals in the seawater Although the concept that SRB can directly react with metal was proposed and the fact that SRB can grow with iron as the only... ferric and manganic oxides and the local consumption of oxygen by bacterial respiration in the deposit (Beech and Gaylarde, 1999) It has been shown that IOB/MOB can promote the ennoblement of metals (i.e a change to more positive values of pitting potential) and pitting corrosion 10 Chapter 1 Introduction Comparisons in the chemistry of microbially and electrochemically induced pitting of 316L stainless... electrochemically induced pitting (Shi, et al., 2006) These findings suggest a possibility that the microorganisms were directly involved in pit initiation Chromium, manganese and iron are dissolved in the passive layer and manganese-containing deposit was formed on the metal surface during the pitting process of Leptothrix discophora, while only manganese and iron are dissolved in the passive layer in the anodic . sulphate-reducing bacteria (SRB) strain from local seawater, (ii) investigating bacteria-metal interactions, (iii) investigating biofilm and its induced biocorrosion of two SRB strains on stainless. mechanism of the biofilm and biocorrosion. In this chapter, a review will be given on biofilm formation, biocorrosion mechanisms, bacteria species associated with the biofilm and biocorrosion of metals, . for the inhibition of biofilm and biocorrosion. 1.1 Biofilm Formation on Metal Surfaces Biofilm is composed of microorganisms (including bacteria, fungi, algae and protozoa) adhering to