Microbial Corrosion: Causes, Consequences, Solutions

Microbial corrosion, also known as biocorrosion or microbiologically influenced corrosion (MIC), is a serious and costly problem that affects various industries and infrastructures. It is caused by the metabolic activities of microorganisms, such as bacteria, fungi, and algae, which colonize metal and non-metal surfaces and produce corrosive substances. Microbial corrosion can lead to material degradation, equipment failure, safety hazards, and environmental damage. In this newsletter edition, I'll explore the causes, consequences, and solutions of microbial corrosion, and provide you with some tips and best practices to detect it and prevent MIC.

Microbiologically influenced corrosion (MIC) is a type of corrosion that is affected by the presence or activity of microorganisms on the surface of corrosion susceptible materials. Microorganisms, such as bacteria, fungi, and algae, can produce corrosive substances, such as acids, sulfides, or enzymes, that can adversely damage metal and non-metal surfaces. MIC can occur in various industries and infrastructures, such as oil and gas, water treatment, chemical processing, nuclear power, and aviation. MIC can be prevented and detected by using various methods, such as mechanical cleaning, chemical treatment, biocides, dry storage, and drainage.

The presence of microorganisms can affect metal surfaces in various ways, depending on the type of microorganisms, the type of metal, and the environmental conditions. Microorganisms can produce corrosive substances, such as acids, sulfides, or enzymes, that can damage metal surfaces. Microorganisms can also form biofilms, which are slimy layers of cells and extracellular substances, that can alter the electrochemical reactions at the metal interface and create localized corrosion sites. Microorganisms can also influence the formation and dissolution of protective oxide layers on metal surfaces, which can affect the corrosion resistance of the metal. Microbial corrosion can cause material degradation, equipment failure, safety hazards, and environmental damage.

The formation and dissolution of protective oxide layers affect metal surfaces by influencing the rate and extent of corrosion due to chemical reactions with the environment. Some metals, such as aluminum, magnesium, and chromium, form oxide layers that are dense, adherent, and insoluble, which protect the metal from further oxidation by blocking the access of oxygen or other corrodents to the metal surface. These oxide layers are called passive films, and they can reduce the corrosion rate significantly. However, if the oxide layer is damaged, dissolved, or removed by mechanical, chemical, or biological factors, the metal surface becomes exposed and vulnerable to corrosion again. Therefore, the formation and dissolution of protective oxide layers depend on the type of metal, the type of oxide, the environmental conditions, and the presence of defects or impurities.

There are several approaches that can be used to detect MIC in pipelines, tanks, vessels, or other systems where corrosion may occur. Some of the common methods include.

• Corrosion monitoring using coupons or probes: This method involves inserting metal samples (coupons) or sensors (probes) into the system and periodically removing them for visual inspection, weight loss measurement, or electrochemical analysis. The coupons or probes can reveal signs of MIC, such as pitting, tubercles, biofilms, or biogenic sulfides. This method can also provide information on the corrosion rate, mechanism, and location.
• Microbiological testing using culture or molecular methods: This method involves collecting samples of water, deposits, or biofilms from the system and analyzing them for the presence and identification of microorganisms that are known to cause or influence corrosion. Culture methods use liquid or solid media to grow the microorganisms under specific conditions, while molecular methods use techniques such as polymerase chain reaction (PCR) or fluorescence in situ hybridization (FISH) to detect and quantify the microorganisms based on their DNA or RNA. This method can provide information on the type, diversity, and activity of the microorganisms involved in MIC.
• Material characterization using microscopy or spectroscopy: This method involves examining the corroded material or corrosion products using various techniques, such as scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), or Fourier transform infrared spectroscopy (FTIR). These techniques can reveal the morphology, composition, and structure of the corrosion products, as well as the presence of biogenic structures, such as extracellular polymeric substances (EPS) or elemental sulfur. This method can provide information on the corrosion mechanism and products associated with MIC.

These are some of the methods that can be used to detect MIC in different systems. However, each of them has its own advantages and limitations, and the choice of the best method depends on the specific case and conditions of the system.

Biocides are substances that can kill or inhibit the growth of microorganisms, such as bacteria, fungi, and algae. Biocides can be classified into two main types: oxidizing and non-oxidizing. Oxidizing biocides, such as chlorine, bromine, and ozone, work by destroying the cell membrane and DNA of microorganisms, thus preventing them from reproducing and forming biofilms. Non-oxidizing biocides, such as formaldehyde, glutaraldehyde, quaternary ammonium compounds, and isothiazolines, work by interfering with the metabolic processes and enzyme activities of microorganisms, thus inhibiting their growth and function.

Biocides are usually applied to the system where microbial corrosion occurs, either continuously or intermittently, depending on the severity and type of the problem. Biocides can be injected into the water stream, sprayed onto the metal surface, or added to the coating or paint of the metal. The effectiveness of biocides depends on various factors, such as the concentration, contact time, temperature, pH, water quality, and type of microorganisms present. Biocides should be used with caution, as they may have negative impacts on the environment and human health, as well as cause resistance and adaptation of microorganisms. Therefore, it is important to select the appropriate type and dosage of biocides for each specific case of microbial corrosion.

Some examples of industries that use biocides to prevent microbial corrosion are:

• Oil and gas operations: Biocides are used to control the growth of sulfate-reducing bacteria (SRB) and other microorganisms that can cause corrosion and souring in pipelines, wells, and storage tanks.
• Water treatment: Biocides are used to disinfect drinking water, wastewater, and cooling water systems from harmful microorganisms that can cause corrosion, biofouling, and health risks.
• Food and beverage: Biocides are used to preserve food quality and safety by preventing microbial spoilage and contamination in food processing and packaging equipment.
• Healthcare: Biocides are used to sterilize medical devices, instruments, and surfaces from microorganisms that can cause infections and biocorrosion.
• Chemical processing: Biocides are used to prevent microbial corrosion and biofouling in reactors, pipelines, tanks, and heat exchangers that handle corrosive chemicals and fluids.
• Nuclear power: Biocides are used to control microbial corrosion and biofouling in nuclear reactors, cooling systems, and fuel storage facilities.
• Aviation: Biocides are used to prevent microbial corrosion and biofouling in aircraft fuel tanks, engines, and hydraulic systems.

There are alternatives to using biocides for treating microbial corrosion. Some of them are:

• Cathodic protection: This is a technique that uses an external direct current to make the metal surface act as a cathode, thus preventing the oxidation reaction that causes corrosion. Cathodic protection can be applied by using sacrificial anodes, such as zinc or magnesium, that corrode preferentially instead of the metal, or by using impressed current systems, such as rectifiers or solar panels, that supply the required current to the metal. Cathodic protection can be effective against both aerobic and anaerobic microorganisms, as it reduces the availability of oxygen and electron acceptors on the metal surface.
• Beneficial bacterial biofilms: This is a strategy that uses non-corrosive or corrosion-inhibiting bacteria to form a protective layer on the metal surface, thus preventing the attachment and growth of corrosive bacteria. Beneficial bacterial biofilms can also produce substances that inhibit the activity or metabolism of corrosive bacteria, such as hydrogen peroxide, nitric oxide, or biosurfactants. Beneficial bacterial biofilms can be applied by inoculating the metal surface with selected bacterial strains, or by modifying the environmental conditions to favor their growth.
• Protective coatings: This is a method that uses polymers, paints, or other materials to cover the metal surface, thus creating a physical barrier that prevents the contact between the metal and the corrosive agents, such as water, oxygen, or microorganisms. Protective coatings can also incorporate additives that enhance their anticorrosive properties, such as corrosion inhibitors, biocides, or nanoparticles. Protective coatings can be applied by spraying, dipping, brushing, or electroplating the metal surface with the desired coating material.

These are some of the alternatives to using biocides for treating microbial corrosion. However, each of them has its own advantages and disadvantages, and the choice of the best option depends on the specific case and conditions of the system.

Let me close out this Coatings INSIGHT edition on biologically influenced corrosion by stating the following.

Microbial corrosion is a complex and costly problem that impacts various industries and infrastructures. It is caused by the presence and activity of microorganisms that can produce corrosive substances, form biofilms, or alter the electrochemical reactions on metal surfaces. Microbial corrosion can be detected by using various methods, such as corrosion monitoring, microbiological testing, or material characterization. Microbial corrosion can be prevented or treated by using various strategies, such as biocides, cathodic protection, beneficial bacterial biofilms, or protective coatings. However, each of these methods has its own advantages and limitations, and the choice of the best option depends on the specific case and conditions of the system. Therefore, it is important to understand the causes, consequences, and solutions of microbial corrosion, and to apply the appropriate measures to protect the materials and equipment from this serious threat.

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