Aquatic Pollution and Coastal Management

Unit 7 : Microbial pollution

7.6.Biocorrosion
Biocorrosion processes at metal surfaces are associated with microorganisms, or the products of their metabolic activities including enzymes, exopolymers, organic and inorganic acids, as well as volatile compounds such as ammonia or hydrogen sulphide. These can affect cathodic and/or anodic reactions, thus altering electrochemistry at the biofilm/metal interface. Various mechanisms of biocorrosion, reflecting the variety of physiological activities carried out by different types of microorganisms, are identified and recent insights into these mechanisms have been reviewed. Many modern investigations have centered on the microbially-influenced corrosion of ferrous and copper alloys and particular microorganisms of interest have been the sulphate-reducing bacteria and metal (especially manganese)-depositing bacteria. The importance of microbial consortia and the role of extracellular polymeric substances in biocorrosion have been emphasized.

Mechanisms of biocorrosion
Microbiologically influenced corrosion (MIC) does not invoke any new electrochemical mechanisms of corrosion; rather, it is the result of a microbiologically-influenced change that promotes the establishment or maintenance of physico-chemical reactions not normally favoured under otherwise similar conditions. Various mechanisms of biocorrosion, which reflect the variety of physiological activities carried out by different types of microorganisms, have been identified; however, it must be remembered that, in nature, these microbial processes do not act in isolation, but in concert with the chemical and electrochemical forces in the particular environment.

Activities of microorganisms as the driving force for biocorrosion
Microorganisms implicated in biocorrosion of metals such as iron, copper and aluminium and their alloys are physiologically diverse. Their ability to influence the corrosion of many metals normally considered corrosion resistant, in a variety of environments, makes microorganisms a real threat to the stability of those metals.
The main types of bacteria associated with corrosion failures of cast iron, mild and stainless steel structures are sulphte-reducing bacteria, sulphur-oxidising bacteria, and iron-oxidising/reducing bacteria, manganese-oxidizing bacteria, and bacteria secreting organic acids and exopolymers or slime. These organisms can coexist in naturally occurring biofilms, often forming synergistic communities (consortia) that are able to affect electrochemical processes through co-operative metabolism not seen in the individual species. Much recent research activity has centered on the role of "quorum sensing" molecules, such as acylhomoserine lactones, in control of microbial activities in biofilms, with the aim of using this knowledge to reduce problematical biofilm formation in industry.