Microbiological (biological, corrosion, microbial) corrosion of stainless steel

Biological corrosion of stainless steel is a process during which stainless metals are destroyed by living organisms such as algae, bacteria, fungi and larger living creatures. Microbiological corrosion of stainless steel is a process of destruction of stainless metal caused by the activity of microorganisms.

Although stainless steel is resistant to corrosion, it can be subject to biological corrosion in certain environments where microorganisms are present. This type of corrosion can cause serious problems, especially in humid environments where microorganisms can thrive.

For example, at sea, the danger is represented by shells clinging to the bottoms of ships and submerged stainless steel structures. Rust is provoked both by the life activity itself and by its residual products.

This type of damage is common in a variety of environments, often found in soil, seawater and the outdoors.

For a long time, such a threat to stainless steel was not considered - aggressive atmospheric influence came to the fore. But today the process has been thoroughly studied and methods for protecting stainless steel from biocorrosion have been prepared. And with the right approach, such a threat can be protected. There are many means that do not allow microorganisms to gain a foothold on the surface of stainless steel.

Loss of materials due to corrosion is inevitable and this issue receives considerable attention at various stages of the processing industry, in particular, the oil, food and leather industries. Due to the significant harmful effects resulting from microbial activity, research into microbial corrosion and methods of combating it has become relevant. About 10% of corrosion damage to metals and non-metals is caused by microbial activity. Microbial corrosion is the result of synergistic contact between bacteria and metals.

Main types of biocorrosion of stainless steel

There are two main types/kinds of biocorrosion:

1. Bacterial. It is caused by intensive reproduction of anaerobic or aerobic bacteria. The most dangerous of them are iron bacteria and sulfur bacteria, the content of which is high in the soil. Such organisms exist even without a constant supply of oxygen, which makes them difficult to destroy. Another problem with this type of biological corrosion is that it occurs in different environmental conditions. Thus, the pH level can vary from 1 to 10.5 points. The standard temperature for starting the process is from 6 to 40 degrees.
2. Mycological. As the name suggests, it is provoked by the products of fungal activity. It is less dangerous than the biological type of damage described above, easier to remove and prevent. But at the same time, the possibility of such a process developing should not be discounted.

It is possible to determine in advance what problem will have to be dealt with, based on the analysis of the soil or water that will come into contact with the stainless metal. But the possibility of introducing microorganisms from outside cannot be ruled out.

Another classification parameter is the division of the influence of microorganisms by the nature of damage. Living beings provoke rusting of stainless metal indirectly or destroy it directly. Among the parameters of division of biological damage are the following:

• Habitat of organisms. They can reproduce in soil, water, organic environment. Air also belongs to the category of habitats of microorganisms.
• Type of damaged material. Depending on the type of bacteria and fungi, they may be interested in different types of materials. And we are not only talking about stainless steel. In addition to alloys, damage is caused to artificial and natural stone, leather, glass, polymer materials that can be structurally combined with stainless steel.
• Biological factor. The material is affected by micro or macroorganisms. If the first category includes the simplest ones, bacteria and fungi, then the second category includes invertebrates, plants and chordates.
• Type of damage to the material. It is destroyed under the influence of the created aggressive chemical environment, stimulation of the occurrence of electrochemical processes, and also directly. A common type of problem is also considered to be complex biocorrosion of stainless metal, when different types of impact can be traced.

The format of destruction can be different - biochemical, physical and combined. In order to avoid material losses, it is necessary to provide protection from all the listed problems.

The nature of microbiological corrosion of stainless steel

Microorganisms release destructive metabolites or organisms harvest electrons from the metal to respire and produce energy. They are responsible for the initiation or enhancement of corrosion caused by pre-existing contaminants such as water and carbon dioxide.

Microbial corrosion occurs when microbes in an aqueous solution come into contact with the surface of a stainless metal. Corrosion occurs in a variety of forms.

Microbes can attack various metals including carbon steel, zinc, copper, and even stainless steel and titanium due to their metabolic activity or products. Microbial biofilms cause microbial impacts through their metabolite-based activities, leading to reservoir acidification and asset degradation. This becomes a major risk issue as industrial assets age, which can lead to accidents.

Microbiological corrosion can be understood in different ways. Most often, this term refers to the influence of microorganisms on the kinetics of metal corrosion processes due to the adhesion of bacteria to the interfaces. For corrosion to work, microorganisms must be present. They need an energy source, a supply of carbon, an electron donor, an electron acceptor, and water if their activity is to cause corrosion. Microbiological corrosion exhibits exceptional action in changing the local chemical composition of the surface by depleting and/or concentrating certain chemical compounds in the environment. Microorganisms can transform certain species present in water into other aggressive forms, such as Fe²⁺ Fe³⁺ , Mn²⁺ Mn⁴⁺ и SO в SO₂ during the metabolic process.

Microbes contribute to corrosion in a variety of ways, both directly and indirectly. Biofilms are thought to be the cause of most corrosion.

Microbiological corrosion can take the form of localized corrosion. It is common in fluid systems and industrial processes due to the vast availability of microorganisms, ample nutrients and aggressive chemicals and can attack metallic and non-metallic materials.

This type of corrosion is associated with a number of species, including bacteria, fungi and archaea.

Microorganisms

The word "microorganism" includes a wide range of living forms, including prokaryotes such as bacteria, archaea and cyanobacteria, as well as eukaryotes such as algae, lichens, yeasts and fungi, and even the simplest. The most important aspect of this group is their size, which is usually between 0.5 and 10 µm. As a result, individual bacteria are rarely visible to the human eye and must be detected and identified using microscopic techniques.

Bacteria, archaea, and fungi are among the microorganisms that cause corrosion. Most published studies focus on bacteria. Sulfate-reducing bacteria have the greatest impact on corrosion within bacteria because sulfate is abundant in anoxic conditions. In harsh environments, such as bodies of water where temperatures can be quite high, archaea become significant. Organic acids are produced by bacteria and fungi that can attack carbon steel. These acids have been observed to attack other metals such as magnesium alloys and zinc.

Bacteria

Bacteria are single-celled microscopic organisms without a true nucleus and are the most important microorganisms involved in microbial corrosion. They are commonly found in soil, water, air, and food, as well as on the skin. Several microbes are involved in the corrosion process. Microorganisms belonging to the sulfate-reducing bacteria group include Desulfovibrionales, Desulfobacterales, etc. Common sulfate-reducing bacteria include the genera Desulfovibrionales, Desulfosporosinus, Desulfotomaculum, Syntropobacterales, Desulfosporomusa, and Desulfobacterales. Iron-oxidizing bacteria that cause corrosion include Gallionella, Sphaerotilus, Leptothrix, and Crenothrix. Iron-reducing bacteria include bacteria of the genera Pseudomonas and Shewanella.

Sulfate-reducing bacteria

Due to their practical uses, sulfate-reducing bacteria are the most studied bacteria in the scientific literature. The valence of the element sulfur varies from 2 to +6. Sulfate-reducing bacteria can use additional sulfur compounds with a valence greater than 2 as terminal electron acceptors in addition to sulfate. Such bacteria are heavy anaerobes, but they can withstand short-term exposure to oxygen without developing.

Nitrate-reducing bacteria

Nitrates are sometimes injected in the oil and gas sector to stimulate the growth of nitrate-reducing bacteria while inhibiting the growth of sulfate-reducing bacteria, in order to limit sulfate reduction and thus facilitate formation souring. On the other hand, microbial-catalyzed iron oxidation with nitrate reduction is more thermodynamically favorable than iron oxidation with sulfate reduction. On AISI 304 stainless steel, Pseudomonas aeruginosa corrosion was reduced by nitrates. As a result, nitrate injection must be precisely dosed to avoid nitrate leakage into the pipes.

Acid-forming bacteria

It has long been known that acid-forming bacteria cause corrosion in biofilms by producing organic acid that lowers the pH. When combined with iron oxidation at a sufficiently low pH, proton attack is thermodynamically favorable. Planktonic cells can promote corrosion in this situation by generating protons to maintain an acidic environment. Organic acids are significantly more corrosive than strong acids such as sulfuric acid at the same pH because they have the ability to buffer protons.

Mucus-producing bacteria

In areas with high overall bacterial concentrations, various types of slime-forming bacteria, usually aerobic, have been found. These bacteria are heterotrophic, meaning they can obtain energy from organic compounds such as alcohols, sugars, and acids. When these bacteria reach critical levels, they can cause clogging or promote corrosion by producing organic acids. These creatures can live in both fresh and salt water, although they are more common in low-salinity environments.

Iron oxidizing bacteria

The following equation describes the ability of iron-oxidizing bacteria to oxidize iron from the divalent to the trivalent state and deposit it as a coating: 4FeCO₃ + O₂ + 6H₂O → 4Fe(OH)₃ + 4CO₂.

In fresh water, this hydroxide layer, along with the accompanying masses of mucus, creates oxygen-concentrating cells that are corrosive and can create an anaerobic environment, but this can also happen in brine. They are aerobic, although they thrive in conditions where oxygen levels are less than 0.5 parts per million, where they contribute significantly to the formation of mucus.

The presence of sodium hypochlorite with iron-oxidizing bacteria in the annular biofilm reactor can enhance the corrosion process, which contributed to the restoration of bacterial growth due to the increase in assimilable organic carbon caused by sodium hypochlorite. In the first stage, it contributes to the formation of a passive film. In the second stage, the growth of iron-oxidizing bacteria and the increase of sodium hypochlorite - increased the corrosion rate, and the predominant effect was biological corrosion.

Most of the iron-oxidizing bacteria used in the treatment are aerobic microbes. They are divided into four main physiological categories such as acidophilic (aerobic iron oxidizers), anaerobic photosynthetic iron oxidizers, neutrophilic (anaerobic iron oxidizers, nitrate-dependent) and neutrophilic (aerobic iron oxidizers). A number of aerobic heterotrophic bacteria oxidizing Fe under microaerophilic conditions have been isolated from Sudek, etc., from the Wailuluu Seamount. Many of them were associated with the genera Pseudoalteromonas and Pseudomonas, which play a role in the precipitation of iron oxides in these marine systems. When bacteria biocatalyze, the rate of Fe ²⁺ oxidation is one hundred times higher than in the abiotic process. Thus, iron-oxidizing bacteria accelerate the dissolution of the metal and the development of localized corrosion.

Sulfur-oxidizing bacteria

Sulfur-oxidizing bacteria are divided into two categories, although they exist in different forms: (1) aerobic (colorless sulfide oxidizers) and (2) anaerobic (colored sulfide oxidizers). The most common sulfur-oxidizing bacteria is Thiobacillus. This bacteria oxidizes sulfur and sulfide to form sulfate and sulfuric acid. They are most often found in storage tanks and platform supports. They cause the most serious damage and provide sulfate-reducing bacteria that can use them.

Mushrooms

Fungi are eukaryotic microorganisms that can be found throughout the world. Fungi have not been well studied in the microbiological corrosion literature. However, in hot and humid climates, they can be a critical factor. Fungi are associated with microbiological corrosion of metals such as copper, carbon steel, stainless steel, and aluminum. Fungal biofilms can consume oxygen in the environment, allowing anaerobic microorganisms such as sulfate-reducing bacteria to thrive underneath. Fungi can dissolve compounds and produce organic acids as a result. These acids can cause corrosion and cracking of pipelines, and also serve as a food source for other corrosion microorganisms such as sulfate-reducing bacteria.

Archaea

Sulfate-reducing archaea reduce the sulfate content of respiration to produce energy. As a result, sulfate-reducing archaea, like sulfate-reducing bacteria, cause corrosion. Sulfate-reducing archaea and sulfate-reducing bacteria are called sulfate-reducing microorganisms in the literature. Since archaea are no longer considered prokaryotes in modern taxonomy, the name sulfate-reducing prokaryote has been gradually abandoned.

Microbes that oxidize metals

It is known that metal-oxidizing microbes play a role in microbiological corrosion. Iron-oxidizing bacteria are known to deposit iron hydroxides extracellularly. This promotes the half-reaction equilibrium of iron oxidation. In addition, aerobic iron-oxidizing bacteria are able to create a localized oxygen-free environment for the development of sulfate-reducing bacteria in the biofilm. It has been demonstrated that a mixed culture of iron-oxidizing bacteria and sulfate-reducing bacteria causes more severe corrosion of iron than a pure strain of iron-oxidizing bacteria or a strain of sulfate-reducing bacteria.

Methanogens

Methanogens have been shown to promote iron corrosion in the absence of oxygen. In a number of studies, they have been linked to pitting corrosion of pipelines. Methanogens often use H₂ as an electron donor during their reduction of CO₂ and respiration. Each of these microbes plays a role in microbial corrosion, and they all share one feature. They produce biofilms on the metal surface.

Biofilms and their impact

When water comes into contact with a concrete surface, it can form a biofilm, which is a general term for biodeposition. Biofilm formation, according to microbiological corrosion, is a critical element in determining the degradation of a solid surface. In marine ecosystems, microorganisms colonize metals and form biofilms. The atmosphere at the biofilm / metal interface is very different from the bulk in terms of pH, dissolved oxygen concentration, and organic / inorganic bacteria. As a result, electrochemical reactions occur that regulate the rate of corrosion. There are different biofilms from different sources. In some cases, the biofilm is formed by the entire microbial population, rather than by a single microorganism.

Biofilm composition

Water (75-90%) is the main component of biofilm. Other components are:

• Microorganisms: bacteria (autotrophic and heterotrophic), fungi, algae, cyanobacteria, the simplest.
• Exopolymer substances: polysaccharides, lipopolysaccharides, humic acids, proteins, nucleic acids, lipids, etc.
• Materials of both organic and inorganic origin.

Biofilm formation

A biofilm is formed when bacteria switch from a free-living planktonic lifestyle to a static sessile lifestyle. Organic molecules begin to aggregate on the wetted surface of a chemically inert substrate almost immediately after immersion in an aquatic environment. Initially, a conditional layer is formed. Adhesion of microorganisms is caused by electrostatic contact and van der Waals forces, which are the same factors that hold the biofilm system together. Functional adhesion between the microbe and the substrate regulates the initial attachment of the cells. This adhesive substance attaches the microorganisms to the soil collectively indefinitely. Using chemical mediators to attract more microorganisms, the biofilm continues to expand and diversify. Over a shorter or longer period of time, more microorganisms gather and create a base. Biofilm formation is a cyclical process (which cannot be stopped). There are five main stages involved in biofilm formation.

1. Adhesion of cells to the surface.
2. Colonization of the surface by cells.
3. Formation of microcolonies of cells.
4. Formation of a monolayer.
5. Maturation of the biofilm with the formation of all its structures.

Mechanism of microbiological corrosion of stainless steel

Oxygen-free microbiological corrosion

This mechanism is also known as microbial corrosion of extracellular electron transfer. Sessile cells within the biofilm use an energetic metal such as elemental iron as an electron donor in this technique. The electron acceptor is a non-oxygen oxidant such as sulfate or nitrate. Since biocatalysis by intracellular enzymes is required, such an oxidant is reduced within the cell cytoplasm. Since the metal matrix is ​​insoluble, oxidation reactions such as iron oxidation occur outside the cell. For the reduction step, extracellular electrons must be delivered to the cytoplasm.

Microbial extracellular electron transport uses a complex electron transport process to transfer electrons across cell walls. Before entering cells to participate in the intracellular respiratory electron transport chain, electrons are transferred from the metal surface to the cytochrome on the cell wall via extracellular metal oxidation. Direct electron transfer and indirect electron transfer are two methods of transferring electrons between a metal surface and a cell wall in extracellular electron transfer. Sessile cell walls directly contact the metal surface in direct electron transfer, or conductive sawtooths are used to connect the cell walls to the metal surface or to transfer electrons. In indirect electron transfer, aqueous electron shuttles collect ions from the metal surface and transport them to the cytochrome on the cell wall.

Metabolite-microbiological corrosion

Microbes secrete corrosive metabolites that cause microbiological corrosion. Microbes can produce organic acids, which leads to the formation of an acidic environment under the biofilm. The reduction of protons, unlike the reduction of sulfates or nitrates, does not require biocatalysis. As a result, metabolite-microbiological corrosion in most cases has an equivalent of abiotic corrosion. Abiotic corrosion by acetic acid is identical to metabolite-microbiological corrosion caused by acetic acid secreted by the biofilm, if the acidity on the metal surface is the same for both abiotic and biotic corrosion. Metabolite-microbiological corrosion can be caused by both fungi and bacteria.

Biodegradation-microbiological corrosion

Microbes in biodegradation-microbial corrosion secrete enzymes that decompose organic materials such as plasticizers and polymers, allowing them to access tiny organic molecules as food. The biodegradation of polymer insulation in electrical systems is controlled by this corrosion. Biodegradation-microbial corrosion can produce anaerobes or aerobes depending on the type of bacteria involved in the biodegradation.

Microbiological corrosion in the presence of oxygen

Microbiological corrosion caused by metal oxidizing bacteria

Iron-oxidizing bacteria and manganese-oxidizing bacteria are the two main categories of aerobic metal-oxidizing bacteria. Aerobic iron-oxidizing bacteria are corrosive bacteria that promote microbial corrosion and are commonly found in oil field water. To produce energy, these organisms use oxygen as the final electron acceptor in respiration to catalyze the oxidation of ferrous iron to ferric iron. The ability to catalyze the oxidation of Mn(II) to Mn(IV) and precipitate manganese dioxide distinguishes metal-oxidizing bacteria.

Oxygen concentration cell

Aerobic slime-producing bacteria can be found in a variety of environments, including marine habitats. The biofilm that these bacteria form on metal surfaces is unevenly distributed. Aerobic bacteria use respiration to remove oxygen from areas beneath the biofilms, resulting in areas of low oxygen concentration. As a result, these areas become anodic compared to oxygen-rich areas, leading to localized oxygen corrosion. Areas with less dense biofilm coverage or no biofilm coverage, which act as cathodic sites to reduce oxygen by consuming electrons, have higher oxygen concentrations.

Prevention and reduction of microbiological corrosion

Protective coatings, corrosion inhibitors, polymers, anodic and cathodic protection, and corrosion-resistant stainless steels and alloys are among the most commonly used corrosion control technologies today.

There are a number of methods to prevent and reduce microbiological corrosion:

1. Choosing the right type of stainless steel:

Different types of stainless steel have different resistance to microbiological corrosion. For environments with a high risk of microbiological corrosion, it is recommended to use stainless steel with a high molybdenum content (e.g. AISI 316, AISI 316L or AISI 904L).

2. Maintaining cleanliness:

Regular cleaning and disinfection of stainless steel surfaces will help prevent biofilm formation. Various cleaning methods can be used, such as mechanical cleaning, chemical cleaning or ultrasonic cleaning.

3. Environment control:

Reducing humidity and temperature can help slow down the growth of microorganisms and reduce the risk of microbiological corrosion. It is also important to control the pH of the environment, as some microorganisms thrive in acidic or alkaline conditions.

4. Use of antimicrobial coatings:

Antimicrobial coatings can be applied to the surface of stainless steel to help prevent the growth of microorganisms. These coatings may contain silver, copper, or titanium dioxide ions.

5. Cathodic protection:

Cathodic protection is a method used to protect metals from corrosion by applying an electric current to them. This method can be effective in protecting stainless steel from microbiological corrosion in some cases.

6. Other methods:

There are other methods that can be used to prevent and reduce microbiological corrosion, such as biocides, ozonation and ultraviolet irradiation.

It is important to select the method or combination of methods that is most appropriate for the specific situation. It is important to note that there is no universal method for preventing microbiological corrosion. The best approach will depend on the specific environment and application.

Chromium near the surface of stainless steel oxidizes in air, forming a layer of chromium oxide that protects the underlying iron from direct contact with oxygen in aerobic environments and also blocks protons and microbial metabolites that promote iron oxidation. Electroactive microbes that can directly extract electrons from metal surfaces can attack stainless steel, but microbes that use H₂ to transfer electrons between Fe⁰ and the microbe are ineffective in corroding high-quality, high-chromium stainless steels.

Stainless steel, despite its strong corrosion resistance, can be susceptible to microbiological corrosion. According to numerous studies, microorganisms are able to form biofilms on the surface of some types of stainless metal, creating pits and destroying the metal. The development of secondary metabolites has also been found on the surface and biofilms, which increases the deterioration. Therefore, choose the right grade of stainless steel for your future projects.

Important: this information is provided for general guidance only and is not a substitute for consultation with a corrosion specialist.