Biofilms are structured microbial communities embedded in a self-produced matrix of extracellular polymeric substances (EPS) that adhere to material surfaces. On metal surfaces, biofilms can both accelerate and inhibit corrosion, suggesting that understanding their structural and functional properties is critical for effective corrosion control. This review introduces the fundamental characteristics of microbial biofilms, including their EPS composition, life cycle, and adaptive functions in response to environmental cues. Multispecies biofilms are the predominant state of natural environments, exhibiting metabolic cooperation, electron exchange, and structural robustness that is not often observed in laboratory monoculture conditions. The temporal and spatial dynamics of microbial association within complex biofilms are also highlighted.

We further discuss recent advances in biofilm imaging techniques, such as contentious-optimizing confocal reflection microscopy (COCRM), label-free autofluorescence analysis, and microfluidic devices. These tools have enabled non-invasive, real-time observation of living biofilms, revealing their structural plasticity and interactions with metal surfaces. Through these methods, biofilms are now increasingly viewed as dynamic ecosystems rather than static surface deposits.

Finally, we describe various mechanisms of microbiologically influenced corrosion (MIC), including acid production, extracellular electron transfer, and localized differential concentration. The EPS matrix could play a central role in creating microenvironments that induce corrosion.

Understanding biofilms as living communities opens new possibilities for microbiologically influenced corrosion control. Integrating insights from microbiology, materials science, and electrochemistry is essential for the development of sustainable anti-corrosion technologies.

Microbiologically influenced corrosion (MIC) accelerates metal infrastructure corrosion under anaerobic conditions. Sulfate-reducing bacteria (SRB) cause corrosion through Chemical-MIC (CMIC) and Electrical-MIC (EMIC). EMIC has been reported as a phenomenon that promotes cathodic reactions of corrosion by acquiring electrons from solid metals for microbial growth, inducing significant corrosion. However, information about the EMIC mechanism to assess EMIC risk and develop EMIC mitigation strategies is limited. In this study, the corrosion of carbon steel by two SRB strains, Desulfovibrio sp. strain SDB1 and Desulforhabdus sp. strain SDB2 were characterized and compared by electrochemical techniques and microscopic observation. Both isolates increased corrosion of carbon steel by 3.1 or 12.3 times than that in the abiotic control, respectively. Long term corrosion test revealed strain SDB2 has ability to continue corrosion for 1 year. Relative to the abiotic control, linear polarization resistance decreased in both isolates and a 4 and 50.5 folds increase in the corrosion current was noted with the strain SDB1 and strain SDB2, respectively. EIS analysis also showed low resistance of corrosion products formed on carbon steel by strain SDB1 and strain SDB2. Cyclic voltammetry measurements confirmed that electrons could be transferred between the isolates and carbon steel. SEM-EDS indicated semiconductive iron sulfides were included in corrosion products formed on carbon steel. These findings suggested that both SRB promote corrosion by forming different conductive corrosion crusts including biofilms that uptake electrons from corroding metal surfaces. Since the electrochemical profiles of both isolates were distinct from those of previously reported EMIC SRB, it is anticipated that future research will provide further insights into the mechanisms of EMIC.

Fourteen bacterial strains were isolated from sediments collected at a natural gas and iodine recovery facility in Chiba, Japan. Based on 16S rRNA gene sequence similarity, these strains were related to phyla Actinomycetota, Bacillota, Bacteroidota, Spirochaetota, Synerigystota or Thermodesulfobacteriota. When the bacterial strains were grown anaerobically in the presence of an iron (Fe⁰) foil, the Fe⁰ foil in the four cultures of Fusibacter sp. NT003, Proteiniclasticum sp. NT002, Tissierella sp. NT019 and Seleniibacterium sp. NT022, was oxidized in the presence of thiosulfate. The amount of oxidized iron in these cultures was 3.0–10.6 times higher than in the aseptic control. Similarly, the Fe⁰ foil in the three cultures of Actinotalea sp. NT006, Maribellus sp. NT033 and Seleniibacterium sp. NT022 was oxidized in the presence of nitrate. The amount of oxidized iron in these cultures was 5.2–14.2 times higher than in the aseptic control. The Fe⁰ foil in the cultures of the remaining eight strains including Dethiosulfovibrio sp. NT041 was not oxidized in both culture conditions in the presence of thiosulfate or nitrate. This is the first report of microbiologically influenced corrosion by the member of six genera Actinotalea, Tissierella, Fusibacter, Maribellus, Seleniibacterium, and Proteiniclasticum.

Microbiologically influenced corrosion is known to occur on various materials under diverse environmental conditions. However, information on microbial activity during progression of corrosion in actual environments remains limited. Taxonomic insights into microbial community structures, derived from recent advances in molecular biology techniques, alone are insufficient to understand microbial activity in the corrosion processes. In this study, we aimed to estimate microbial activity during corrosion processes by applying bioinformatic analysis to temporal shifts in microbial community structures observed under actual corrosion conditions. We analyzed the microbial communities present in the corrosion products on various steel materials and in biofilms on non-corroded materials from a freshwater industrial water environment. Abundances of functional genes were estimated from these communities and compared with those estimated from the surrounding water samples. Focusing on energy conversion-related sulfur and nitrogen metabolism, we found substantial shifts in inorganic metabolic gene abundance during the corrosion and biofilm formation processes. Notably, there have been no previous reports addressing metabolic transitions between early and late stages of corrosion. Our findings revealed a notable increase in genes associated with nitrogen fixation and ammonia production during the late corrosion stage, providing crucial insight into nitrogen sources supporting microbial proliferation within corrosion products.

In this study, SUS304 stainless steel specimen was covered with an agar film containing Na₂S₂O₃ as a quasi-biofilm in a 0.1 kmol·m⁻³ NaCl solution, and the relationship between the Na₂S₂O₃ concentration in the film and the pitting potential of the specimen was determined by the potentiodynamic polarization test. In addition, the Na₂S₂O₃ concentration of the film surface in contact with the specimen was estimated from the pitting potential, and its change with the pre-immersion time was examined. As a result, the following findings were obtained. The Na₂S₂O₃ concentration in the film matched that in the Na₂S₂O₃ solution used in preparing the film from the powder. When the specimens with films containing 0.1 kmol·m⁻³ or more Na₂S₂O₃ were immersed in the NaCl solution, the pitting potential once decreased, then increased, and finally showed a steady value with increasing the immersion time. The Na₂S₂O₃ concentration in the film estimated from the pitting potential decreased rapidly, then slowly, and rapidly again with the immersion time. When a constant potential was applied for a specified period to the specimen covered with a film containing 1.0 kmol·m⁻³ Na₂S₂O₃, then pitting potential was obtained. The relationship between the estimated Na₂S₂O₃ concentration in the film and the potential holding time was similar the relationship between the concentration and the immersion time.

Corrosion was observed in a seawater heat exchanger equipped with duplex stainless steel SUS329J4L tubes. An investigation into the corrosion morphology and operating conditions indicated that the damage was attributable to hot-spot corrosion. The analysis revealed inappropriate flow rate management and insufficient chlorination. Given the concern of microbiologically influenced corrosion associated with insufficient chlorination, the microbial community structure was analyzed. To our knowledge, no previous studies have reported on detail microbial analysis in hot-spot corrosion environments. Although the direct impact of microorganisms on hot spot corrosion remains unclear, their distribution within the heat exchanger tubes was elucidated. Detailed analysis of the damaged areas showed that corrosion frequently occurred at the boundary between the high- and low-temperature regions along the tube length. This suggests the formation of a corrosion cell between these regions. Based on these findings, a mechanism for the onset of hot spot corrosion is proposed.

This technical report presents an engineering analysis of an early case of localized corrosion in SUS304 stainless steel siphon pipes used in an agricultural water facility in Akita Prefecture, Japan. Within only a few years after installation, significant wall thinning and perforation were observed, particularly around welded regions, even though stainless steels are generally expected to provide high resistance under neutral freshwater conditions. Radiographic testing (RT) confirmed corrosion shadows and uneven thinning, and the estimated corrosion rate was several times higher than typical values reported for stainless steels in similar environments. Detailed evaluation based on inspection records, design specifications, and operating history suggests that multiple factors overlapped to accelerate the damage. Multiple overlapping factors are considered responsible, including weld-zone sensitization and stagnant/empty-pipe oxygen limitation with possible local chloride build-up. In this context, microbiologically influenced corrosion (MIC) is treated as a plausible, yet unproven, contributory factor rather than a sole cause. This case highlights that even highly corrosion-resistant alloys may fail to deliver expected performance if site-specific conditions are overlooked. The findings provide important lessons for material selection, corrosion protection, and maintenance strategies in irrigation facilities where stainless steels are widely adopted with expectations of long-term durability.

Microbiologically influenced corrosion (MIC) is increasingly recognized as a key factor affecting the reliability of welded stainless steel structures. Recent studies highlight early-stage MIC under service-like conditions, emphasizing electrochemical–microbial interactions.

Laboratory tests using natural freshwater from industrial facilities have revealed corrosion risks in sensitized austenitic stainless steels such as SUS304. In these materials, the open circuit potential (OCP) often shows time-dependent ennoblement that begins earlier and reaches higher values than in non-sensitized base metal. This behavior is linked to microstructural degradation, notably chromium depletion at grain boundaries, and may contribute to the higher MIC susceptibility of sensitized regions.

Long-term exposure studies across stainless steel grades demonstrate that corrosion morphology—ranging from general to localized or negligible—varies with chromium content and correlates with distinct microbial communities. These findings suggest that microbial populations adaptively localize in response to electrochemical heterogeneity, promoting corrosion initiation and progression.

To probe this effect, weld-like model systems simulating the interface structure between sensitized and non-sensitized stainless steel regions were tested in a three-electrode setup under controlled micro-scale potential gradients. Electrochemical measurements combined with microbial community profiling indicated functional differentiation between anodic and cathodic areas, with certain taxa preferentially colonizing cathodic sites, suggesting the functional localization of microbial activity driven by electrochemical heterogeneity.

Overall, these studies highlight the complex interactions among microstructure, electrochemistry, and microbial distribution in MIC initiation. Such integrated insights provide a basis for improved diagnosis and mitigation strategies of MIC in welded stainless steel structures under realistic environmental conditions.