Microbial corrosion ,also called microbiologically influenced corrosion or MIC, is caused by the presence and activities of microbes. This corrosion can take many forms and can be controlled by biocides or by conventional corrosion control methods. Microbes are the first living creatures that have appeared on the earth. They have flourished on the earth for at least 3.8 Ga (Ga= 109 years) of its 4.5 Ga existence. They formed the first biosphere and continue to carry out a multitude of essential geochemical activities that prepared the Earth for the eventual evolution of macroscopic life. Microorganisms are found everywhere in nature, owing to the existence of extremophiles, and adapted to generally hostile environments.
Extremophiles abundant in deep hydrothermal vents, sub seafloor sediments, hypersaline lagoons, methane seeps, deserts, geysers and beneath the surface of rocks. Some of the microbes are known to survive longer duration in vacuum, or to be unusually resistant to radiation. It is estimated that 50% of the living protoplasm on this planet is microbial. Microorganisms are the basis for the all ecosystems processes and often well associated with themselves and higher organisms. Due to their relationship with all plant and animal groups and their abundance (in terms of biomass) in virtually all habitats, man has long exploited this metabolic wealth for food production and preservation, management of pests and pathogens, generating biofuels, monitoring pollutants, cleaning up of oil spills, industrial wastewater treatment and largely used for the development of health care applications (i.e. produce of medicinal compounds).
There are a number of mechanisms associated with this form of corrosion, and detailed explanations are available at the web sites listed at the bottom of this section. Most MIC takes the form of pits that form underneath colonies of living organic matter and mineral and biodeposits. This biofilm creates a protective environment where conditions can become quite corrosive and corrosion is accelerated.
Many engineers continue to be surprised that such small organisms can lead to spectacular failures of large engineering systems. The microorganisms of interest in Microbiologically Influenced Corrosion (MIC) are mostly bacteria, fungi, algae and protozans.
Bacteria are generally small, range in size from 0.1 to 15 micron, with some "giants" that may reach half a millimeter. They make up the most metabolically diverse group of living organisms and tend to live and grow under wide ranges of temperature, pH and oxygen concentration. Carbon molecules represent an important nutrient source for bacteria.. Although some are parasitic to animals and plants, the majority of bacteria are free-living, having either a neutral or beneficial relationship with humans and other animals and plants. Their metabolic versatility is incredible; while most are heterotrophs, using either light or chemical energy. One of their most remarkable characteristics is their ability to multiply rapidly, with generation times usually ranging between minutes to hours. Bacteria also include cyanobacteria, a specific group of microorganisms capable of oxygenic photosynthesis.
Fungi are a large and diverse group of eukaryotic, non-photosynthetic, spore-forming organisms. They have rigid cell walls. Respiration takes place in bodies called mitochondria in the cytoplasm. Fungal cells have an elaborate arrangement of internal membranes. Fungi can be divided into two broad groups: filamentous fungi (including moulds and macrofungi) and yeasts. Fungi can be separated into yeasts and molds. Corrosion damage to aircraft fuel tanks is one of the well-known problems associated with fungi. Fungi tend to produce corrosive products as part of their metabolisms; it is these by-products that are responsible for corrosive attack. Furthermore, fungi can trap other materials leading to fouling and associated corrosion problems.
Protozans are predators of bacteria and algae and therefore potentially mitigate microbial corrosion problems.
Protozoa are a large group of eukaryotic, single celled organisms, which lack a rigid cell wall and usually chloroplasts. They vary widely in size, cell structure and form, ranging from Amoeba with its very fluid shape and simple internal organization and few specialised organelles through to Paramecium with its fixed shape, complex internal organisation and many specialised organelles.
Algae are a diverse group of eukaryotic organisms that contain chlorophyll and carry out photosynthesis. Some contain other photosynthetic pigments which gives them their characteristic colour - most algae are green, but some are red. They occur in a wide range of forms from microscopic to macroscopic e.g. seaweeds, some of which are up to 30 metres long and are not considered to be micro-organisms. Microscopic algae exist either as single cells e.g. chlorella, in colonies e.g. Volvox or in filaments e.g. Spirogyra.
Archaebacteria have a wide range of shapes: spheres, rods, spirals, lobed, flat rectangular or irregular. Some exist as single cells, other form filaments or clusters. Some are motile. They are often called extremophiles because they are found in extreme conditions in the environment, such as in hot springs, or salt crystallizing pans the depths of the ocean.
Viruses are very small, ranging between 0.01 and 0.03 ?m, and only we visualized under electron microscope. They cannot live independently, and only multiply inside the cells of other organisms. However, their demand for a host is fairly specific. For example, it is unlikely that a crustacean virus will attack humans or fish. Viruses are also the simplest of all organisms and are made of nucleic acid (either DNA or RNA), frequently coated with a protein layer.
MIC is responsible for the degradation of a wide range of materials. Bacteria can exist in several different metabolic states. Those that are actively respiring, consuming nutrients, and proliferating are said to be in a "growth" stage. Those that are simply existing, not growing because of unfavorable conditions, are said to be in a "resting" state.
Some strains, when faced with unacceptable surroundings, form spores that can survive extremes of temperature and long periods without moisture or nutrients, yet produce actively growing cells quickly when conditions again become acceptable.
The latter two states may appear, to the casual observer, to be like death, but the organisms are far from dead. Cells that actually die are usually consumed rapidly by other organisms or enzymes. When looking at an environmental sample under a microscope, therefore, it should be assumed that most or all of the cell forms observed were alive or capable of life at the time the sample was taken.