Microbiological Corrosion

Introduction

Corrosion is a destructive process as far as engineering is concerned, and represents a huge economic loss in the order of trillions of dollars. The most accepted and correct concept defines the phenomenon of corrosion as the attack of a material by the surrounding environment with the consequent loss of mass and deterioration of its properties. When considering this definition, the first thing that comes to mind is the destruction of metal under the influence of oxygen and water (aerobic corrosion). However, a large part of the damage caused by corrosion occurs in the absence of oxygen (anaerobic corrosion), such as in the case of the lower area of hydrocarbon storage tanks or other systems where there is stagnant water or with a laminar flow pattern.

It is a fundamental condition for the phenomenon of corrosion (oxidation of the metal) to be achieved that some other chemical compound must be present for the reduction process to take place. In most environments, the substances that are reduced are the dissolved oxygen or hydrogen ions of the acid. Under anaerobic conditions (no oxygen or air present), certain bacteria can thrive and in this environment those microorganisms can provide the chemical compounds that are reduced and allow oxidation of the iron to obtain corrosion reactions. This article presents basic aspects and generalities of the phenomenon of corrosion generated by bacteriological processes.

microbiological corrosion

In this context, under these conditions known as anaerobic, in which there is no oxygen or air in the environment, there are bacteria capable of proliferating and prospering, generating as a by-product of their metabolism the compounds to induce chemical reactions and cause the attack of bacteria. metallic materials. It has been possible to detect processes in which microorganisms capture dissolved hydrogen, thereby allowing the generation of localized potential differences, which in turn have electrochemical effects on the immediate environment and therefore on the metallic material.

Biocorrosion, also known as microbial corrosion and more specifically called microbiologically influenced corrosion or MIC (for its acronym in English, Microbiologically Influenced Corrosion) can be defined as “an electrochemical process that produces the deterioration of a metallic material where microorganisms are involved ( bacteria, fungi or algae) either by initiating, facilitating or accelerating the process of corrosive attack”. And its role is not simply to promote the corrosion process. Its effect can be considered a true catalyst, increasing the rate of corrosion by up to 10 orders of magnitude due to the generation of metabolic by-products such as H ₂ S and certain types of enzymes whose composition contains hydrogen in very high conditions.

Microorganisms and factors that favor biocorrosion

There are several factors that affect the aggressiveness of corrosive bacteria, as biological entities that are: the concentration of oxygen, and other products such as carbon, ammonia, temperature, pH, etc.

Anaerobic microorganisms induce an increase of up to 10 times in the corrosion rate by the production of H ₂ S and the release of specific enzymes (mainly “hydrogenase”). Some species of microorganisms classified as “Hydrogen-dependent” have been identified that use dissolved hydrogen from water in their metabolic processes, causing a potential difference in the surrounding medium.

Aerobic and anaerobic microorganisms participate in biocorrosion mechanisms, which can grow by changing their metabolic processes and each species produces different chemical substances. Each variety of bacteria is responsible for causing a reaction that influences the corrosion process due to the nature of the substance it produces, these reactions are anodic and/or cathodic in nature due to the formation of electrochemical cells.

Among the factors that affect the aggressiveness of corrosive bacteria are:

• Flow type.
• The chemical treatment of the system.
• Oxygen concentration.
• Temperature.
• Total organic carbon concentrations.
• Nitrogen concentrations.
• Ammonia concentration.
• pH.

Types of bacteria involved in the microbiological corrosion process:

• Sulphate-reducing.
• Acid producers.
• metal depositories.
• Exopolymer (gel) formers.

Bacteria and other microorganisms have a tendency to form colonies; these colonies form biofilms on metal surfaces.

This condition makes it difficult to eliminate, which generates operational problems in facilities such as refineries. Another additional drawback is the generation of biomass, whose accumulation also generates corrosion on metal surfaces.

Since these are biological organisms, certain conditions favor them more than others. As found in nature, stagnant waters (which are favorable environments for the appearance of deposits and anaerobic zones), a pH between 4.5 and 9.0, as well as elements or compounds that serve as food for bacterial colonies, such as water, carbon, nitrogen, sulfates and iron compounds. It is interesting to see that the temperature range is wide, varying between 20° and 80°C.

The interaction of microorganisms with each other is important; we must remember that, as biological communities, the symbiotic relationship is also evident in this environment. And this interaction also affects the aforementioned biomass production.

Destruction of protective films on metal surfaces.

• Generation of localized acid environments.
• Creation of corrosive deposits.
• Alteration of anodic and cathodic reactions.
• Interference in protection mechanisms against corrosion.

The morphology of MIC attacks is mainly in the form of stinging or pitting; these are located under the deposits (colonies) of microorganisms that feed on dissolved organic and mineral compounds. It should be noted that the biofilm that forms consists of a protective mechanism for these colonies, and their establishment is one of the factors that acts as a catalyst for corrosive processes, reaching the values mentioned above.

In the seaside refinery environment, it is a widely known fact that biocorrosion is not only caused by microorganisms in submerged environments. Both vertebrate and invertebrate organisms take part in it, and biocorrosion is then generated by the ammonia from its waste products that attack copper and zinc alloys.

An important factor is that these bacterial colonies are capable of using maleate and formate compounds, alcohols such as methanol, ethanol, propanol and butanol as energy sources, excreting fatty acids. In diesel fuel tanks it is common to find microbial activity, as those of us who have some experience also know with internal combustion engines of this type.

Finally, this byproduct of microbial activity, which is the fatty acid, feeds the bacteria called Sulfate-Reducing bacteria, which produce sulfides that are highly harmful to metallic materials, especially steel.

Bibliographic references

O. Medina–Custodio1, A. Ortiz–Prado2, VH Jacobo–Armendáriz3 and R. Schouwenaars–Franssens

About the Author

ing Jose Martinez de Munck. 15 years of experience in Equipment Inspection for the Oil and Gas Industries.

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