MICROBIAL INFLUENCED CORROSION DUE TO METAL OXIDIZING BACTERIA

Abstract

Corrosion involves the series of chemical, physical, and biological processes and leads to the destruction of materials (metals). Corrosion is a serious problem with societal and economic consequences. Microbial influenced corrosion (MIC) or biocorrosion is the result of electrochemical reactions influenced by metabolic activities of microorganisms. It is one of the major concerns in industries due to huge economic losses the industry has to bear as a result of maintenance, premature failure of the material of construction of its machinery, process equipment, and related factors. Accordingly, it has been a subject of extensive studies over the past decades. Sulfate-reducing bacteria (SRB) are well studied and highly corrosive. However, the MIC mechanism, especially by iron-oxidizing bacteria (IOB), is not yet elucidated completely. In this context, this study presents experiments performed on microbial influenced corrosion due to IOB‗s on mild steel and other allied tests so as to investigate the correlation between the extent of MIC and nature of bacteria, its biofilm through the extracellular polymeric substances (EPS) exuded by IOB strains, nature of corrosion products and the related electrochemical reactions as a key part of the mechanism for MIC of mild steel. In this research, laboratory based studies have been undertaken to investigate the corrosion behavior of mild steel in the presence of iron-oxidizing bacteria. The findings from this research will help to understand the possible mechanism of MIC. Towards this direction, efforts have been made through a series of short and relatively longer-term microbiological corrosion studies. The results from these studies are being presented in SIX (6) chapters of this thesis. Chapter 1: Introduction and review literature - This chapter introduces basic aspects of corrosion followed by details on MIC, different corrosion influencing microbes, and its proposed mechanisms. The literature survey shows various materials and industries affected by MIC. Some case studies related to MIC in different industry sectors have been described briefly. A review of the literature revealed that most of the studies on MIC are related to sulfate-reducing bacteria but very few related to MIC due to other bacteria. Out of them, metal oxidizing bacteria, namely iron-oxidizing bacteria are important bacteria to study for their microbial corrosion effects due to their wide and varied prevalence and deteriorating effect on various metal structures/industrial machinery, etc. as outlined above. v IOB are the dominant part of aerobic bacteria influencing corrosion and their metabolic activities are the main perpetrator of the deterioration of metals. However, not much literature exists related to MIC and EPS constituents of metal oxidizing bacteria. Accordingly, the work done in the present study has been described in the following given Chapters Chapter 2: Materials and Methods with the description of isolating and identifying metal oxidizing bacteria, corrosion experiments, and their set-up, other allied techniques used in the present work for analyzing the results. Chapter 3: To investigate microbial influenced corrosion in Winogradsky media due to two bacterial isolates and test their dependence on the nature of biofilm, etc. produced by them on mild steel. MIC due to the two strains has also been attempted and described (Chapter 3A, 3B, and 3C). Chapter 4: To investigate microbial influenced corrosion in nutrient broth due to two bacterial isolates and test their dependence on the nature of biofilm etc. produced by them on mild steel. MIC due to the two strains has also been attempted and described (Chapter 4A, 4B, and 4C). Chapter 5: To investigate the effect of host media, namely Winogradsky media (inorganic media) and nutrient broth media (organic media), on MIC due to bacterial isolates. Chapter 2: Materials and methods - This chapter deals with the materials and methodology used in the present study. Iron-oxidizing bacteria (IOB), isolated from the rust deposited on the surface of mild steel (ms), exposed to local river water and identified as Pseudomonas sp. strain DASEWM1 and DASEWM2. Mild steel was tested for studying MIC and its performance against corrosion. Winogradsky media and nutrient broth media were used as test solutions for corrosion studies (electrochemical tests and immersion tests). Biofilm formed on the corroded metal surface was analyzed for EPS constituents using UV-Vis spectroscopy. The extent of localized corrosion was determined by optical microscope, and the corroded steel surface was analyzed by Field Emission Scanning Electron Microscope / Energy Dispersive X-Ray Analysis (FESEM)/EDAX) techniques. Corrosion products deposited on the corroded steel surface were identified using XRD and FTIR techniques. vi Chapter 3: The microbial influenced corrosion studies on mild steel in the presence of iron-oxidizing bacteria (Pseudomonas sp. strain DASEWM1) in Winogradsky media are described in part (A) of this chapter and those due to Pseudomonas sp. strain DASEWM2 in Winogradsky media are described in part (B). The comparison of the corrosivity of both iron-oxidizing bacteria Pseudomonas sp. strain DASEWM1 and DASEWM2 in Winogradsky media is discussed in section (C) of this chapter. Part A - Corrosion behavior of steel due to iron oxidizing bacteria (DASEWM1 strain) in Winogradsky media: This chapter describes the corrosion behavior of mild steel due to iron-oxidizing bacteria (DASEWM1 strain) in Winogradsky media (inorganic media). In this work, electrochemical polarization tests, immersion tests, and surface analysis were carried out to estimate the extent of uniform and localized corrosion. Corrosion products deposited on the steel surface were identified using X-Ray Diffraction (XRD) and Fourier Transform Infrared Spectroscopy (FTIR) techniques. Biofilm formed on the corroded metal surface was analyzed by UV- visible spectroscopy for the constituents of extracellular polymeric substances. Results showed that inoculated Winogradsky media accelerate the corrosion of mild steel. Uniform and localized corrosion increases with increased bacterial concentration and EPS constituents of biofilm. Higher corrosion of steel exposed to inoculated media can be attributed to the presence of wider cracks, carbohydrates of EPS, and corrosion product iron sulfite. Whereas fine cracks, absence of EPS, and protective type corrosion product iron hydrogen phosphate on steel exposed to control media may be responsible for lesser corrosion. A mechanism of microbial corrosion has been proposed considering corrosion products and pH of the solution during the corrosion test. (Work published: Sachan, R., and Singh, A.K., 2019. Corrosion of steel due to iron oxidizing bacteria, Anti-corrosion methods and materials, Vol 66 (1) pp.19-26). Part B - Corrosion of steel due to iron-oxidizing bacteria (DASEWM2 strain) in Winogradsky media: In this section, the corrosion behavior of mild steel was investigated in the presence of another IOB strain (DASEWM2) in Winogradsky media. In this part, the same methodology was used, vii as mentioned in part (a). The corrosion rate obtained by Tafel plots was higher when the concentration of sessile cells and constituents of extracellular polymeric substances were high. Lower pitting potential, repassivation potential, and the large area under hysteresis loop were observed in the cyclic polarization curves of steel exposed to inoculated media. These results indicate a higher susceptibility of steel to localized corrosion due to bacteria. Immersion tests also show a higher corrosion rate and large deep pits (open area and under crevice) on the steel surface, indicate a higher degree of corrosion in inoculated media. Corrosion products formed on the corroded steel samples were analyzed using XRD and FTIR techniques. Goethite and lepidocrocite are the main corrosion products in the case of inoculated media. In contrast, lepidocrocite and iron hydrogen phosphate are the main corrosion products in the case of control media. Absence of iron hydrogen phosphate, a protective type of corrosion product, on steel exposed to inoculated media was suggested for the higher degree of corrosion due to bacteria. (Work Published: Sachan, R., and Singh, A.K., Negi, Y.S., “Study of microbial influenced corrosion in presence of iron oxidizing bacteria (Strain DASEWM2)” in Journal of Bio and Tribo corrosion, 6:109). Part C - Comparison of microbial influenced corrosion due to iron-oxidizing bacteria (strains DASEWM1 and DASEWM2) in Winogradsky media: In this segment, a comparison of microbial influenced corrosion due to iron-oxidizing bacteria (strains DASEWM1 and DASEWM2) in Winogradsky media was investigated. It was observed that inoculated media is more corrosive than control media. The results obtained by this study show higher corrosion of mild steel in the presence of DASEWM2 strain than DASEWM1 strain. It may be due to higher carbohydrate concentration and the absence of iron hydrogen phosphates (protective) in the case of DASEWM2 strain. Chapter 4: Part A - Corrosion behavior of steel due to iron-oxidizing bacteria (strain DASEWM1) in nutrient broth media: In this chapter, the corrosion behavior of mild steel was investigated in the presence of IOB strain DASEWM1 in nutrient both media (organic media). In this work, the same methodology was adopted, as in chapter 3rd. Experimental results revealed that the degree of corrosivity depends upon bacterial concentration and the amount of EPS constituents due to which higher viii degree of corrosion is observed in the presence of bacteria. A lesser amount of vivianite (protective in nature) was observed in the case of inoculated media, which might be responsible for increased corrosion. Corrosion products deposited on the steel surface were identified using XRD and FTIR techniques. Corrosion of steel in inoculated media appears to be associated with (i) oxidation of Fe2+ to Fe3+ by bacteria, thereby resulting in a lesser amount of Fe2+ available for the formation of vivianite (ii) some Fe3+ bound to EPS thereby making available some electrons for the oxygen reduction. This overall leads to the increased alkalinity of the media. Considering all these reactions, a possible mechanism of MIC been proposed. [Work communicated (Minor revision): Sachan, R., and Singh, A.K., “Corrosion behavior of Carbon Steel in the presence of Pseudomonas sp. strain DASEWM1” in Environmental sustainability]. Part B - Corrosion behavior of steel due to iron-oxidizing bacteria (strain DASEWM2) in nutrient broth media: In this part, the corrosion behavior of mild steel due to iron-oxidizing bacteria (DASEWM2 strain) in nutrient broth media was investigated. Experimental results showed that the corrosion rate (obtained by Tafel plots) was higher when bacterial (sessile cells) concentration and EPS constituents were high. In the case of inoculated media, lower pitting potential, and repassivation potential was observed with increasing incubation time. Higher corrosion rate and deep pits on the carbon steel surface were observed in the case of inoculated media. Corrosion of steel in inoculated media appears to be associated with (i) oxidation of Fe2+ to Fe3+ by DASEWM2 strain, thereby resulting in a lesser amount of Fe2+ available for the formation of vivianite (ii) more anionic functional groups of EPS (higher concentration of carbohydrates) bind with ferric ions and making available some electrons for the oxygen reduction. Part C - Comparison of microbial influenced corrosion due to iron-oxidizing bacteria (strains DASEWM1 and DASEWM2) in nutrient broth media: Part c of this chapter describes the comparison of microbial influenced corrosion due to iron- oxidizing bacteria (strains DASEWM1 and DASEWM2) in nutrient broth media. This study presented the co-relation of extracellular polymeric substances exuded by IOB strains as a key mechanism for MIC of mild steel. A comparative study using surface analysis, electrochemical tests, and immersion tests in nutrient broth media revealed a higher degree of corrosion in inoculated media (strain DASEWM1<DASEWM2) than control media. These results were also ix supported by electrochemical impedance spectroscopy (EIS) and FESEM of corroded steel surfaces. Moreover, electrochemical and EPS constituent‗s analysis, the correlation between the corrosion rate to the EPS constituents were also studied. (Work published: Sachan, R., and Singh, A.K., 2020. Comparison of microbial influenced corrosion in presence of iron oxidizing bacteria (strains DASEWM1 and DASEWM2), Construction and Building Materials, 256, p.119438) Chapter 5 - Effect of host media on the microbial influenced corrosion in the presence of Pseudomonas sp. (DASEWM1 strain and DASEWM2 strain): In this chapter, the effect of host media on microbial influenced corrosion due to iron-oxidizing bacteria (strain DASEWM1 and DASEWM2) was investigated. Results revealed higher corrosion rate (obtained by Tafel curves), lower pitting potential, and repassivation potential in the case of inoculated inorganic media (Winogradsky media) than organic media (nutrient broth media). Higher corrosion rate and deep pits in the mild steel surface (open surface area and under crevice) were observed in inoculated Winogradsky media in comparison to nutrient broth media after the immersion test. Results show (i) higher corrosion in inoculated media than control media (both media) (ii) inoculated inorganic media is more corrosive than inoculated nutrient broth media (iii) higher carbohydrate concentration in EPS of bacterial strains were observed when the strains grow in inorganic media. Chapter 6: Conclusion and Recommendations for future work: This chapter discusses the conclusions and future recommendations of the thesis. Conclusion  Experiments performed on corrosion due to iron-oxidizing bacteria (DASEWM1 and DASEWM2 strains) show that corrosivity of the solution increases with (i) the addition of bacteria (ii) time. Maximum corrosive conditions were observed when bacteria were most active and produced a maximum concentration of EPS constituents. The corrosivity increases with bacterial concentration and concentration of EPS constituents.  The corrosivity and nature of biofilm due to the two isolated bacteria belongs to Pseudomonas sp. (DASEWM1 and DASEWM2 strains) have been compared. DASEWM2 strain observed to be more corrosive than DASEWM1 strain. A higher x fraction of carbohydrate in biofilm and less amount of protective corrosion products appear to be responsible for observance of the higher degree of corrosion attack on the steels exposed to inoculated media with DASEWM2 strain.  The studies on the effect of host media indicate that inorganic media (Winogradsky media) is more corrosive than organic media (nutrient broth media). Bacterial growth and concentration of EPS constituents were observed to be higher in the case of inoculated inorganic media than organic media. The tendency of EPS to bind metal ions appears to be responsible for the extent of corrosion attack. Recommendations for future work In the present work, an attempt has been made to see the dependence of biofilm and it‘s EPS on microbial influenced corrosion. However, to get a detailed idea about the mechanism of MIC, one needs to synthesize EPS and its various components and do the necessary tests. It is also important to know metal contents in the biofilm, enzymes involved in the corrosion process, etc. One needs to determine the corrosivity of other bacteria, even to predict the MIC in the natural environment. From the industrial application point of view, it is essential to investigate the effect of the flow rate of media on MIC. Work on establishing control of MIC by the development of inhibitor biofilm will pave the way for an environment-friendly approach to MIC without the use of chemical inhibitors. A study on the physiology of bacteria to investigate the genes responsible for corrosion reactions may help in controlling MIC by gene modification.