Microbially Induced Corrosion (MIC)

Corrosion

Physicochemical interactions between a metallic material and its environment can lead to corrosion. Electro-chemical corrosion is a chemical reaction involving the transfer of electrons from zero - valent metal to an external electron acceptor, causing release of the metal ions into the surrounding medium and deterioration of the metal. A series of oxidation (anodic) and reduction (cathodic) reactions of chemical species in direct contact with, or in close proximity to, the metallic surface occurs here.

Anode -generate electrons- negative ions are discharged and positive ions are formed. 

Cathode - receives electrons - positive ions are discharged, negative ions are formed. 

Holes in a protective coating allow metallic iron to be oxidized to Fe2+at the anode. Rust is formed when Fe2+ react with atmospheric oxygen, forming oxide deposits. At the cathode the electron is accepted by oxygen/any other electron acceptor thus reducing it.  The electrochemical interaction between cathodic and anodic sites can cause a large pit to form under the surface, eventually resulting in sudden failure with little visible warning of corrosion

Microbial activity within biofilms formed on surfaces of metallic materials can affect these  cathodic and/or anodic reactions. It can also  modify the chemistry of any protective layers, leading to either acceleration or inhibition of corrosion.

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Microbially Influenced Corrosion

Deterioration of metal due to microbial activity is termed biocorrosion or Microbially Influenced Corrosion (MIC).

"Microbially Influenced Corrrosion (MIC) refers to the influence of microorganisms on the kinetics of corrosion processes of metals, caused by microorganisms adhering to the interfaces (usually called "biofilms"). 

 MIC requires the presence of microorganisms. If the corrosion is influenced by their activity, further requirements are: (I) an energy source, (II) a carbon source, (III) an electron donator, (IV) an electron acceptor and (V) water

MIC or Biocorrosion is a serious problem resulting in reduced water quality, environmental contamination and economic losses. MIC is caused primarily due to microbial film formation on the surface of structures immersed in water such as, water distribution pipes. These microbial films or biofilms provide localized environmental conditions (decreased pH & differential oxygen cells) for initiating and propagating corrosion. 

Biofilms

The term biofilm refers to the development of microbial communities on submerged surfaces in aqueous environments. Biofilm influences the physico-chemical interactions between metal and environment, frequently enhances corrosion, and leads to deterioration of the metal.

Parameters affecting the development of biofilms include:

• Temperature of the system

• Water flow rate

• Nutrient availability

• Surface of the substratum

• pH of water in the system

• Effectiveness of biofouling remedial measures.

     Biofilm formation is the result of an accumulation process that starts immediately after immersion of metal in the aqueous environment. It involves transport of organic and inorganic molecules and microbial cells to the surface, adsorption of molecules to the surface and initial attachment of microbial cells followed by their irreversible adhesion facilitated by production of extracellular polymeric substances (EPS). Once attached, the organisms begin to produce material termed extracellular biopolymer, or ‘slime’. The extracellular polymer that is produced provides a more suitable protective environment for the survival of the organism. The EPS in biofilm consists of lipids, polysaccharides, proteins and nucleic acids. One of the important properties of EPS is their ability to complex with metal ions. It also facilitates further colonisation by bacteria. 


 

    For example, in a marine environment the presence of a biofilm can accelerate corrosion rates of carbon steel by several orders of magnitude.

Biofilms cause maintenance and operational problems including fouling, corrosion, a reduction in flow rate, heat transfer rates etc.

Causes of Corrosion

(1)    Non-uniform (patchy) colonies of biofilm result in the formation of differential aeration cells where areas under respiring colonies are depleted of oxygen compared to the surrounding non-colonised areas. Having different oxygen concentrations at two locations on a metal causes a difference in electrical potential and consequently corrosion. Under aerobic conditions the area under respiring colonies become anodic and the surrounding areas become cathodic.

(2)    Oxygen depletion at the surface of stainless steel can destroy the protective passive film. Since stainless steel require a stable oxide film to provide corrosion resistance, corrosion occurs when the oxide film is damaged or oxygen is kept from the metal surface by microorganisms in a biofilm.

(3) In case of steel and iron pipes, corrosion occur due to different chemical reactions, which establish electrochemical gradients, leading to the loss of metal from pipes due to electrolysis.

Thus, the presence of microorganisms on metal surface and their metabolic activities results in pitting, de-alloying, crevice corrosion (under deposit corrosion), stress corrosion cracking etc The formation of slime or tuberculation nodules can cause blockages or reduce flow. Low water flow or stagnant conditions make systems more susceptible to microbial growth. The ultimate effect is the premature failure of metal components.

Microbiologically induced corrosion has a potential impact on a wide range of industrial operations,  in oil and gas production, pipelining, refining, petrochemical synthesis, and other industrial sectors. Most of the commercially used metals and alloys such as stainless steels, nickel and aluminium based alloys and materials such as concrete, asphalt and polymers are readily degraded by microorganisms. Protective coatings, inhibitors, oils and emulsions are also subject to microbial degradation.

Microorganisms 

Microbiologically induced corrosion causing organisms are sulphate reducing bacteria (Desulphovibrio, Desulphomonas sp.), iron  bacteria (Gallionella and Ferrobacillus sp.), acid producing bacteria (Pseudomonas, Aerobacter, and Bacillus), and sulphur oxidising bacteria (Thiobacillus sp.)

          Microorganisms and their metabolic products, including enzymes, exopolysaccharides, organic and inorganic acids, and volatile compounds, such as ammonia or hydrogen sulphide, can cause corrosion. Both aerobic and anaerobic organisms play an important role in the initiation, propagation, and inhibition of corrosion. Fungi and algae may also be involved in metal deterioration. In fuel and oil storage tanks, fungal species such as Aspergillus, Penicillium and Fusarium may grow on fuel components and produce carboxylic acids which corrode iron.

•       Regular mechanical cleaning if possible.

• .      Chemical treatment with biocides to control the population of bacteria.

• .      Complete drainage and dry-storage.

How is MIC Different? 

 MIC affects metallic surfaces in a unique manner. Whereas general corrosion affects an entire surface, MIC is localized.

• The microbes initiate the process with a search for a suitable place for habitation. They seek out irregularities on the surface and attach themselves.

• Once attached, they generate by-products such as EPS/slime which retain various organic and inorganic materials.

• These by-products lead to the development of rounded to irregularly shaped nodules; beneath each nodule is a pit. The nodule serves as the habitat for the microbe community.

• When a nodule is developed, it creates conditions that are chemically dissimilar to the surface material to which it is attached. In a typical nodule found in an aerobic environment, microbes live within its exterior layer where they consume oxygen in the water.

• They use up oxygen and reduce the oxygen level within the nodule. This activity creates an environment that allows the underlying anaerobic bacteria to survive and thrive. ·

• This is the beginning of accelerated corrosion. As the microbe community continues to live and develop within the nodule, its by-products eventually lower the pH to acidic levels, which in turn increases the corrosive conditions 

• The acidic conditions promote the growth and development of other acid-producing bacteria whose own acid by-products further reduce the pH to even lower levels. The continuance of the MIC mechanism eventually leads to formation of a mature pit.

Microorganisms Involved in MIC Processes

• Many microorganisms are associated with MIC of metals such as iron, copper and aluminium, and their alloys

• The capability of many bacteria to substitute oxygen with other oxidisable compounds as terminal electron acceptors in respiration, (when oxygen becomes depleted in the environment) permits them to be active over a wide range of conditions conducive for corrosion of metals.

• The ability to produce a wide spectrum of corrosive metabolic by-products over a wide range of environmental conditions makes microorganisms a real threat to the stability of metals.

Microbial Consortia on Biocorrosion

• Biocorrosion occurs in aquatic and terrestrial habitats which- differ in nutrient content, temperature, pressure and pH.

• The presence and physiological activities of microbial consortia on the metallic surfaces play a major role. Mechanisms proposed as cause to biocorrosion is based on the physiological activities carried out by diverse types of microorganisms found within biofilms.

• These mechanisms vary with microbial species. ·

Metal-Depositing Bacteria (MDB)

• Participate in the biotransformation of oxides of metals such as iron and manganese

. • Iron-depositing bacteria (e.g., Gallionella and Leptothrix) oxidize Fe2+ to Fe3+.

• Also capable of oxidizing manganous ions to manganic ions with the deposition of manganese dioxide.

• Promote corrosion reactions by the deposition of ferric and manganic oxides and the local consumption of oxygen caused by bacterial respiration in the deposit. 

Metal-Reducing Bacteria (MRB)

• Promote corrosion of iron and its alloys through reactions which dissolve corrosion-resistant oxide films on the metal surface.

• Protective passive layers on  stainless steel surfaces can be lost or replaced by less stable reduced metal films that allow further corrosion to occur.

• Numerous types of bacteria including those from the genera of Pseudomonas and Shewanella are able to carry out manganese and/or iron oxide reduction.

Acid-Producing Bacteria (APB)

• Produce inorganic or organic acids as metabolic by-products-  HNO₃, H₂SO₄, HNO₂ and carbonic acid H₂CO₃.

• Sulphurous acid and sulphuric acid - bacteria of the genera Thiobacillus. Other bacteria, such as Thiothrix and Beggiatoa spp. Thiobacilli are extremely acid tolerant and can grow at a pH value of 1.

• Nitric acid and nitrous acid - bacteria belonging to the groups of ammonia- and nitrite-oxidising bacteria.

• sulphuric and nitric acid corrosion - Lowering of the pH and formation of protective deposits on the surface, e.g., calcium carbonate. These salts are water-soluble and, hence, a formation of a protective corrosion product layer is not possible. - 

• Some bacteria e.g. Pseudomonas aeruginosa produce extracellular acidic polysaccharides such as alginic acid, during biofilm formation on metal surfaces.

• These acids can be highly concentrated at the metal-biofilm interface

• Microsensors (ultramicroelectrodes) which have been used to probe the pH gradients within microbial biofilms, revealed both horizontal and vertical variations in pH values at the biofilm / metal interface.

Enzymes and Bio corrosion

• Enzymatic activities are readily detected in biofilms, but the importance of reactions mediated by these enzymes has only recently been considered as relevant to biocorrosion.

• These enzymes are involved in oxygen reduction, therefore, in principle, they might facilitate corrosion by accelerating the overall cathodic reaction. ·

Sulphate-Reducing Bacteria (SRB)

• SRB are a group of ubiquitous, diverse anaerobes that reduce oxidised sulphur compounds, such as sulphate, sulphite and thiosulphate, as well as sulphur to H₂S.

• Although SRB are strictly anaerobic (obligate anaerobes), some can tolerate oxygen and are even able to grow at low oxygen concentrations.

• The activities of SRB in natural and man-made systems are of great concern in oil, gas and shipping industries, which are seriously affected by the sulphides generated by SRB.

•Sulphide production leads to health and safety problems, environmental hazards and severe economic losses due to reservoir souring and corrosion of equipment. 

    These bacteria have a major role in pitting corrosion of various metals and their alloys in both aquatic and terrestrial environments under anoxic as well as oxygenated conditions.

• Several models have been proposed to explain the mechanisms by which SRB can influence the corrosion of steel. These have included accelerated cathodic and anodic reactions, production of corrosive iron sulphides, release of exopolymers capable of binding Fe-ions, sulphide-induced stress-corrosion cracking, and hydrogen-induced cracking etc. The main mechanism is, the sulfate reducing activity. The hydrogen sulfide produced (biogenic sulphide) is found to be more corrosive than the chemically-derived sulfides.

·Fungi

• Well-known producers of organic acids, and therefore capable of contributing to MIC (e.g., Aureobasidum pullulans).

• Biocorrosion of aluminum and it alloys is caused by the fungi Cladosporium (utilises the hydrocarbons of diesel fuel to produce organic acids), Aspergillus spp., Penicillium spp. and Fusarium spp.

• Selectively dissolve  the copper, zinc and iron at the boundaries of aircraft aluminum alloys-Form pits which persist under the anaerobic conditions established under the fungal mat.

Control

Microbiologically influenced corrosion, or microbial corrosion or biological corrosion can be prevented through a number of methods:

 1.     Regular mechanical cleaning if possible.

2.     Chemical treatment with biocides to control the population of bacteria.

3.     Complete drainage and dry-storage.