Biocorrosion

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Biocorrosion

	Biocorrosion - Biocorrosion is one of the direct consequences of microbial film formation on the surface of water distribution pipes. It is one of the major contributor of water quality and environmental contamination. Biocorrosion causes severe economic losses in water distribution systems. Corrosion of metal pipings and other distribution systems occurs by biochemical reactions mediated by microorganisms and is influenced by the following factors.
- differential oxygen concentrations on a biofilm surface results in the formation of anodic and cathodic sites. - differential concentration of metal ions in biofilm stimulate corrosion by equilibrium changes. - any metabolic process may influence corrosion rates by providing either an electron sink or an electron source. - production of organic and inorganic acids increases the solubility of metals. - other corrosive metabolites, like hydrogen and sulfide, produced by microorganisms.
The most common manifestation of biocorrosion is the formation of tubercles. These result from chemical or biological deposition of iron and manganese oxides to form deposits or concretions on a metal surface. Biological deposition may result from intracellular or extracellular interactions with iron and manganese. In addition, enzymatic oxidation of the metal may occur, with concurrent energy gains to the bacterium. A mass of bacterial cells and minerals is formed that is bound together by bacterial exopolymers.
These limit the diffusion of oxygen and nutrients into and out of the tubercle resulting in steep chemical gradients. Outer layers of tubercle remain aerobic with a near neutral pH and the inner layers become reduced and are acidic due to organic acid production. Thus a complex system of aerobes and anaerobes coexist within the same tubercle. Gallionella, an iron-oxidising bacterium and Pedomicrobium manganicum, a manganese-oxidising bacterium are the important microorganisms associated with biocorrosion in distribution systems. Other bacterial genera, which are metal-depositing microorganisms, found in tubercles include Hyphomicrobium, Sphaerotilus, Crenothrix, Leptothrix and Siderocapsa.
	Like biocorrosion of steel piping systems, biocorrosion of concrete systems may also occur resulting in serious economic losses. Damage to the sewerage systems by acid deterioration is often known as sewer crown corrosion. Concrete degradation depends on the sulfur cycle and the microorganisms are mainly sulfur oxidising Thiobacilli. Examples are Thiobacillus thioparus and Thiobacillus thiooxidans.

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