Microbiologically Influenced Corrosion (MIC) and its electrical and chemical mechanisms in the degradation of metallic materials

Introduction

Microbiologically Influenced Corrosion (MIC) is a phenomenon that has captured the attention of the scientific and industrial community due to its significant impact on the integrity of metallic structures in various sectors, including the oil, maritime and water treatment industries. This type of corrosion is characterized by the interaction of microorganisms with metal surfaces, which can accelerate deterioration processes and compromise the safety and functionality of equipment and infrastructure.

Microbiologically Influenced Corrosion (MIC) is a complex phenomenon involving the interaction of microorganisms with metallic materials, resulting in their deterioration. This process can manifest itself through electrical and chemical mechanisms, known respectively as EMIC (Electrical Microbially Influenced Corrosion) and CMIC (Chemical Microbially Influenced Corrosion).

Within the scope of MIC, two mechanisms are mainly distinguished: electrical microbiologically influenced corrosion (EMIC) and chemical microbiologically influenced corrosion (CMIC). EMIC involves the direct participation of microorganisms in electrochemical processes that affect electron transfer at the metal surface, while CMIC refers to the production of chemical compounds by microorganisms that indirectly modify the chemical environment and accelerate corrosion.

The purpose of this article is to provide a detailed understanding of the underlying mechanisms of both EMIC and CMIC, highlighting their fundamental differences and their relevance to industrial applications. By delving deeper into these mechanisms, it seeks to provide industry professionals and researchers with conceptual tools that will enable them to identify, prevent and mitigate the effects of microbiologically influenced corrosion in their respective areas of work.

Through this analysis, it is intended to foster greater awareness of the importance of MIC and to promote the development of effective strategies for its control, thus contributing to the extension of the useful life of metallic structures and to the reduction of costs associated with corrosion damage.

Fundamentals of Microbiologically Influenced Corrosion (MIC)

MIC refers to the deterioration of materials due to the activity of microorganisms such as bacteria, fungi and algae. These organisms can alter the electrochemical conditions on the surfaces of materials, facilitating or accelerating corrosive processes. The formation of biofilms, which are microbial communities attached to surfaces, plays a crucial role in this type of corrosion. These biofilms create microenvironments that can differ significantly from the surrounding environment, influencing corrosion reactions.

Microorganisms involved in MIC can be both aerobic and anaerobic, and their metabolic activity can influence corrosion in several ways. For example, sulfate-reducing bacteria (SRB) are anaerobic and can reduce sulfates present in the medium to sulfides, which react with iron to form iron sulfide, a compound that can accelerate steel corrosion. On the other hand, iron-oxidizing bacteria are aerobic and can oxidize metallic iron to iron oxides, also contributing to corrosion.

What is biofilm?

Biofouling (biofouling) is the uncontrolled growth of organic matter in a technical environment. Biofouling develops on existing biofilm and spreads biomass with the current.

Biofilm formation starts with a small impurity, which arrives with water or other fluids in the piping system. The impurity can be bacteria, microbes or algae from a cooling tower. These settle in a place within the pipeline where there is less flow and the biofilm begins to form.

The presence of biofilms can create heterogeneous electrochemical conditions on the metal surface, establishing potential differences that promote the formation of local galvanic cells. These cells can accelerate corrosion in specific areas, leading to forms of localized corrosion such as pitting or crevices. In addition, biofilms can act as physical barriers, trapping corrosive products and maintaining conditions that favor corrosion beneath them.

Electrical Microbiologically Influenced Corrosion (EMIC)

EMIC is characterized by the direct involvement of microorganisms in the electrochemical processes leading to corrosion. Some microorganisms can transfer electrons directly between the metal and their metabolism, altering anodic or cathodic reactions. For example, certain sulfate-reducing bacteria (SRB) can accept electrons from the metal during the reduction of sulfate to sulfide, accelerating corrosion. This mechanism involves a direct interaction between the microorganisms and the metal surface, where the microorganisms act as electrochemical mediators.

In the case of SRBs, these bacteria use molecular hydrogen as a source of electrons for sulfate reduction. Hydrogen can be generated on the surface of the metal as a result of the corrosion reaction, and SRBs can consume this hydrogen, maintaining the corrosion reaction and accelerating the loss of metallic material. This process is known as “cathodic depolarization” and is an example of how microorganisms can directly influence the electrochemical reactions that occur during corrosion.

In addition to SRBs, other microorganisms, such as iron-oxidizing bacteria, can participate in EMIC. These bacteria can oxidize metallic iron to iron ions, releasing electrons that can be used in cathodic reactions. This process can establish galvanic cells on the surface of the metal, accelerating corrosion in specific areas.

Chemical Microbiologically Influenced Corrosion (CMIC)

In contrast, CMIC involves the production of corrosive chemical compounds by microorganisms, which indirectly accelerate corrosion. For example, some sulfur-oxidizing bacteria can produce sulfuric acid, lowering the local pH and promoting acid corrosion of the metal. Other microorganisms can generate organic acids or ammonia, creating chemical conditions that promote corrosion. In this case, the microorganisms do not interact directly with the metal surface in electrochemical terms, but modify the chemical environment, facilitating corrosive processes.

An example of CMIC is the activity of sulfur-oxidizing bacteria, such as Thiobacillus thiooxidans, which can oxidize sulfur compounds to sulfuric acid. This acid can lower the pH of the surrounding environment, creating highly corrosive conditions for metals. Sulfuric acid can react with the metal, forming metal sulfates and releasing hydrogen, resulting in accelerated corrosion.

Another example is the production of organic acids by certain fungi and bacteria. These acids can chelate metal ions, forming soluble complexes that facilitate metal dissolution and accelerate corrosion. In addition, the production of ammonia by some bacteria can increase the local pH, which can lead to the formation of corrosive compounds that damage the metal.

Key differences between EMIC and CMIC

Action mechanism

• EMIC: Direct involvement of microorganisms in electrochemical reactions, altering the transfer of electrons on the metal surface.
• CMIC: Production of corrosive chemical compounds that modify the chemical environment, indirectly accelerating corrosion.

Types of microorganisms involved

• EMIC: This type of corrosion is mainly associated with microorganisms that can interact directly with the metal surface, altering the electrochemical reactions. A prominent example is sulfate-reducing bacteria (SRB), which can accept electrons directly from the metal during their metabolism, thus accelerating corrosion.
• CMIC: In this case, corrosion is influenced by microorganisms that produce corrosive chemical compounds as part of their metabolism. For example, sulfur-oxidizing bacteria generate sulfuric acid, lowering the local pH and promoting acid corrosion of the metal.

Effect on the metal surface

• EMIC: The direct interaction of microorganisms with the metal surface can result in the formation of local galvanic cells, leading to localized and accelerated corrosion in specific areas.
• CMIC: The production of corrosive chemical compounds can lead to more widespread corrosion, affecting larger areas of the metal surface due to the diffusion of these compounds into the surrounding environment.

Mitigation strategies

• EMIC: Mitigation strategies may include the use of coatings that inhibit microbial adhesion and biofilm formation, as well as the application of electrochemical techniques to monitor and control microbial activity on the metal surface.
• CMIC: Mitigation can focus on the control of environmental conditions that favor microbial activity, such as the reduction of available nutrients and the application of biocides to reduce the microbial population and, therefore, the production of corrosive compounds.

Conclusion

Microbiologically Influenced Corrosion (MIC) remains a complex phenomenon with multiple mechanisms yet to be elucidated. The relative contribution of chemical (CMIC) and electrical (EMIC) corrosion in the degradation of metallic structures is an area of active research, especially with regard to the role of sulfate reducing bacteria (SRB). Understanding these processes in greater depth is critical to developing more effective prevention and control strategies in industrial and natural environments.

References

1. Uhlig, H. H. (1985). Corrosion and Corrosion Control. This classic book provides a comprehensive introduction to the science and engineering of corrosion, addressing the fundamental thermodynamic and electrochemical principles that cause corrosion, as well as practical methods of protection and prevention.
2. Inspenet (2022). “Microbiologically Influenced Corrosion: Common Mistakes”. This article provides key insights into MIC, highlighting common mistakes in its understanding and management, and offers valuable information for professionals in the field.
3. Predictiva 21. (2019). “Understanding biocorrosion: what is it and how to mitigate it?”. This resource analyzes how microorganisms can influence corrosion through the production of corrosive compounds and other effects, and discusses strategies for mitigating biocorrosion in industrial settings.
4. SciELO Spain (2005). “Corrosion of microbiological origin: looking to the future”. This review describes the current status of research on biocorrosion and biofouling of metals and alloys for industrial use, and presents current trends in monitoring and control strategies to mitigate the detrimental effects of biocorrosion.