19The understanding of microbial influenced corrosion (MIC) in aerobic mixed biofilms 20 benefits from advanced microscopy and microbial ecology characterization of biofilms. Here, 21 the onset of MIC in stainless steel coupons was studied in both natural and artificial seawater. 22 Rapid selection of biofilm-forming microorganisms from natural seawater was observed for 23 field experiments. Potential ennoblement was observed only in natural seawater. A seawater 24 derived mixed microbial consortium enriched in artificial seawater was used to characterize 25 the effect of several parameters on MIC. The concentration of organic carbon was the major 26 determinant of MIC, while shaking speed and polishing played minor roles. The biofilm was 27 preferentially formed at the grain boundaries. These results outline the need for MIC onset 28 characterization with mixed microbial consortia to predict long-term corrosion behaviour of 29 stainless steel in seawater. 30 31 Introduction 32Microbially influenced corrosion (MIC) of metals refers to the involvement of 33 microorganisms in the metal deterioration process. MIC has significant economic 34 consequences for industries such as oil and gas, mining, logistics and waste water treatment, 35 with social and environmental impacts associated with the deterioration of materials 1 . 36 2 Microorganisms affect physicochemical reactions at the metal/liquid interface, either slowing 37 down or accelerating abiotic corrosion processes 1, 2 . Due to their physicochemical 3 and 38 microbiological resistance 4 to metal deterioration and MIC, stainless steels (SS) are used in 39 key marine components. 40 41 MIC mechanisms previously put forth include the effects of differential concentrations of 42 oxygen and nutrients; generation of corrosive metabolites or by-products; alteration of anion 43 ratios and inactivation of corrosion inhibitors 5 . Adsorbed extracellular biofilm matrix 44 molecules such as proteins, lipids, humic acids and polysaccharides change the SS surface by 45 modifying surface charge, wettability or surface energy, thus enhancing or inhibiting MIC 6 . 46The biofilm can also act as a diffusional barrier preventing oxygen and corrosive substances 47 from reaching the metal surface 7 . 48 49 Sulphate-reducing bacteria (SRB) are commonly cited as the primary organisms responsible 50 for MIC under anaerobic and anoxic conditions in seawater through the production of 51 corrosive sulphides. However, aerobic microorganisms have also been increasingly studied, 52 substantiating their role in MIC process. For example, in the presence of the aerobic marine 53 bacterium Pseudomonas sp., SS304 showed a higher corrosion rate and lower resistance of 54 the passive film, indicating localised breakdown of passive film, in contrast with abiotic 55 experiments with stable and passivating Cr-enriched oxide films 8 . Microbial activities can 56 alter the inorganic passive layer and increase metal dissolution. Extensive micro-pitting 57 corrosion was observed underneath biofilms...
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