Microbiologically influenced corrosion remains a pervasive challenge for industries reliant on metals exposed to marine environments, such as the maritime, offshore renewable energy, and energy sectors. This form of corrosion is primarily driven by microorganisms that can accelerate the deterioration of metals, leading to significant operational and structural issues. The interaction between biofilms, microbial communities that form on surfaces,and corrosion is particularly complex, and understanding this relationship is crucial to developing effective strategies for corrosion prevention and management. A novel study led by Liam Jones, Maria Salta, Torben Lund Skovhus, and their colleagues, published in npj Materials Degradation, introduces an innovative approach to studying this interaction using a dual anaerobic reactor model to investigate biofilm development and its effects on carbon steel, a material commonly used in marine and offshore structures.
MIC is a multifaceted process in which both abiotic (non-living) and biotic (microbial) factors contribute to the degradation of metal surfaces. Abiotic corrosion, driven by factors like temperature, pH, and mechanical stress, often occurs in isolation, but in many real-world environments, it interacts with the biotic corrosion driven by microbial activity. This makes understanding and predicting MIC particularly difficult. Marine environments, in particular, provide a fertile ground for the development of biofilms, dense microbial communities that adhere to metal surfaces and are encased in extracellular polymeric substances. These biofilms not only protect microorganisms from environmental stressors, but they also create a microenvironment that accelerates corrosion. Within these biofilms, electroactive bacteria, such as sulfate-reducing bacteria (SRB) and iron-reducing bacteria (IRB), play a particularly important role by mediating electron transfer between the metal surface and their microbial environment, directly contributing to corrosion processes.
The challenge in studying MIC has been the lack of experimental models that accurately replicate the complex conditions found in marine and offshore settings. Traditional laboratory studies often rely on simplified or static conditions that fail to capture the dynamic and multifactorial nature of corrosion in real-world environments. To address this gap, the researchers developed a dual anaerobic biofilm reactor model, which simulates anaerobic, or anoxic, conditions commonly found in marine environments, where oxygen is limited or absent. This novel reactor was designed to allow for the simultaneous study of electrochemical corrosion of UNS G10180 carbon steel in two different environments: one with artificial seawater (abiotic) and the other with both biotic and abiotic conditions (biotic). This setup, which uses a complex microbial consortium derived from marine littoral sediments, provides a more accurate and realistic representation of the microbial and environmental interactions that contribute to corrosion in offshore settings.
One of the critical findings of the study was the identification of electroactive microorganisms as key contributors to biofilm formation and corrosion. Through DNA extraction and 16S rRNA amplicon sequencing, the researchers were able to map the microbial communities present in the biofilms and identify key players, particularly SRB and IRB. These bacteria are well known for their ability to reduce sulfate and ferric ions, respectively, through a process called extracellular electron transfer. In the case of SRB, these bacteria oxidize organic compounds or hydrogen gas to reduce sulfate to hydrogen sulfide, which reacts with ferrous ions released from the corrosion of the metal to form iron sulfides. These corrosion products can accelerate the corrosion process by creating localized galvanic cells on the surface of the metal, further increasing the corrosion rate. Meanwhile, IRB utilize ferric ions as electron acceptors, leading to the dissolution of iron oxide passivating layers, which normally protect steel from corrosion.
The dual reactor model also allowed the researchers to observe how mixed-species biofilms behave under anaerobic conditions. The interactions between different bacterial species within the biofilm play an important role in the corrosion process. Biofilms composed of multiple species often exhibit synergistic relationships that make them more resilient and harder to eradicate compared to single-species biofilms. The study demonstrated that SRB and IRB not only contributed to the electrochemical activity that accelerated corrosion, but their interaction with other microorganisms in the biofilm enhanced their ability to mediate electron transfer. This collective behavior within the biofilm is a key factor in the accelerated corrosion rates observed in real-world scenarios, where microbial communities are diverse and highly dynamic.
Moreover, the researchers found that biofilms, due to their EPS matrix, provide physical protection to the microorganisms inside, shielding them from external environmental pressures such as antimicrobial agents. The biofilm's structure also allows for the efficient accumulation of nutrients, which supports microbial growth and activity. This makes biofilms a persistent threat in industrial systems, as they can harbor high concentrations of microorganisms that constantly contribute to corrosion, even under harsh environmental conditions. The ability of microorganisms to exchange genetic material within biofilms through horizontal gene transfer further complicates the situation, as new strains of bacteria with enhanced corrosion capabilities can emerge, increasing the overall threat to metal structures.
Another crucial aspect of the study was the focus on the electrochemical interactions between biofilms and metals. The team used advanced electrochemical techniques to measure the corrosion rates of carbon steel in both biotic and abiotic environments. The data showed that biofilm formation significantly increased the corrosion rates of carbon steel compared to abiotic conditions. This finding underscores the importance of considering microbial activity in corrosion studies and highlights the need for more accurate models to predict the impact of biofilms on metal degradation in marine environments.
The study also has significant practical implications for the offshore and maritime industries, which must regularly address the issue of MIC in the maintenance and protection of metal structures. The findings suggest that traditional methods of controlling corrosion, such as the use of coatings or cathodic protection, may need to be reevaluated in light of the complex interactions between biofilms and corrosion. The dual anaerobic reactor model developed in this study provides a valuable tool for testing different anti-corrosion strategies, including biocides and corrosion inhibitors. The researchers hope that their work will lead to the development of more effective treatment protocols for biofilm-mediated corrosion, ultimately improving the longevity and safety of offshore infrastructure.
In addition to advancing scientific understanding, the study contributes to ongoing efforts to develop standardized protocols for evaluating and mitigating MIC. There are currently no internationally recognized standards for testing the efficacy of biocides and other treatments against biofilm-related corrosion. By developing a reproducible experimental setup that mimics real-world conditions, this study offers a pathway toward the creation of standardized testing methods. The researchers' approach, which integrates electrochemical, microbiological, and molecular analyses, represents a more holistic approach to studying MIC. The aim is to use these insights to inform industry standards, such as those being drafted by the Association for Materials Protection and Performance, which will guide future research and industrial practices in mitigating biofilm-related corrosion.
This comprehensive study represents a significant step forward in the understanding of biofilm-mediated corrosion and offers valuable tools for both researchers and industry professionals. By addressing the complexities of microbial interactions in biofilms and their effects on metal surfaces, the research provides new insights into the mechanisms of MIC and paves the way for more effective strategies to combat this costly and persistent problem. As industries continue to rely on carbon steel and other metals in challenging marine environments, the need for robust, real-world experimental models to study corrosion and biofilm interactions will become even more pressing. The dual anaerobic biofilm reactor developed in this study is a promising tool that can help meet these challenges and guide the development of more effective corrosion control strategies in the future.
