Microbial corrosion, also known as microbiologically influenced corrosion (MIC), is a complex process where microorganisms accelerate the degradation of materials, primarily metals. This hidden phenomenon silently causes damage within various industrial systems, contributing to significant economic losses and safety concerns across numerous sectors. It is distinct from traditional chemical corrosion due to the direct involvement of living organisms in altering the material’s environment.
Understanding Microbial Corrosion
Microbial corrosion involves a complex interplay between biological activity and electrochemical reactions, setting it apart from purely chemical corrosion processes. Unlike purely chemical corrosion, living organisms actively modify conditions at the material’s surface. These microorganisms, including various types of bacteria, archaea, and fungi, attach to surfaces and form communities called biofilms.
Biofilms are crucial because they create unique microenvironments that alter local chemistry and electrochemical potential at the material-surface interface. Within these areas, microbes can concentrate corrosive substances, deplete protective agents, or participate in electron transfer. This biological activity accelerates material degradation across a wide range of materials, from metals to concrete.
The Mechanisms of Microbial Corrosion
Microorganisms accelerate corrosion through several distinct mechanisms. One primary way is through the production of corrosive metabolites. Acid-producing bacteria (APB) secrete organic acids that lower local pH, dissolving protective films and accelerating degradation. Sulfate-reducing bacteria (SRB), under anaerobic conditions, produce hydrogen sulfide (H2S), a highly corrosive compound that reacts with metal surfaces to form metal sulfides, promoting corrosion.
Another mechanism involves biofilms, communities of microbes encased in a protective extracellular polymeric substance (EPS). These biofilms create differential aeration cells on the metal surface by establishing oxygen concentration gradients. Areas beneath the biofilm become oxygen-depleted (anodic), while surrounding areas remain oxygen-rich (cathodic), leading to localized pitting corrosion. The EPS matrix also traps corrosive ions and can hinder corrosion inhibitors.
Some microbes also directly influence corrosion through electron transfer. Iron-oxidizing bacteria (IOB) oxidize ferrous iron (Fe2+) to ferric iron (Fe3+), extracting electrons directly from the metal surface, forming rust tubercles and contributing to localized pitting. The exact nature of direct electron transfer from metal to microbe is still under investigation, but it involves the microorganism directly accepting or donating electrons, bypassing intermediate hydrogen molecules.
Environments Prone to Microbial Corrosion
Microbial corrosion is a widespread problem affecting numerous industries and infrastructure due to the ubiquitous presence of microorganisms. Pipelines, especially those transporting oil, gas, or water, are susceptible due to stagnant or low-flow conditions where water and nutrients accumulate, fostering biofilm growth. Up to 70% of internal pipeline leaks have been attributed to MIC.
Storage tanks, particularly those holding hydrocarbons, are common sites for microbial corrosion. Water often settles at the bottom, creating an ideal interface for microbial proliferation and the degradation of tank walls and associated piping.
Marine structures, including offshore platforms and ship hulls, face risks from constant exposure to seawater. Seawater provides diverse microbial communities and nutrients, leading to extensive biofilm formation and accelerated corrosion. Wastewater treatment plants, with their high organic content and diverse microbial populations, are also prone to degradation of metal and concrete infrastructure. Medical implants can be affected, as bodily fluids provide a suitable environment for microbial growth.
Controlling Microbial Corrosion
Effective management of microbial corrosion involves detection and control strategies. Detection often begins with visual inspections for slime, pitting, or rust tubercles. More advanced methods include microbial testing, such as ATP assays or gene sequencing, and electrochemical monitoring.
Control strategies aim to eliminate or inhibit microbial activity or protect the material. Material selection, like using corrosion-resistant alloys (e.g., stainless steels or titanium), offers increased resistance. Protective coatings, such as epoxies, act as physical barriers and can incorporate antimicrobial agents.
Biocides, chemical agents designed to kill or inhibit microbial growth, are widely used in closed systems. These include oxidizing biocides like chlorine or non-oxidizing types such as glutaraldehyde. Environmental modifications include deoxygenating water to inhibit aerobic microbes, adjusting pH, or implementing water treatment like filtration or UV sterilization to create unfavorable conditions for corrosive microorganisms. An integrated approach, combining several methods, is generally most effective.