Microbiologically influenced or induced corrosion (MIC) refers to the corrosion, which ensuing loss of metal or concrete caused by biological organisms. MIC can occur in any aqueous environment. It is now a commonly occurring phenomenon  for the omnipresent nature of microbes, adequate nutrients, and corrosive byproducts in today's industrial and municipal processes. Protection of MIC has therefore become very critical in many industries including municipal pipeline, marine, storage vessels, sewage treatment facilities and so on.

Wastwater 

These microbes do not "eat" metals such as mild steel, stainless steel, copper alloys or galvanized steels. They produce byproducts that are corrosive to these metals. They are acids, alkalis or reducing agents such as hydrogen sulfides, ammonia and sulfuric and organic acids. These wastes or byproducts also can produce gelatinous deposits, often identified as biomasses, that prevent corrosion inhibitors from reaching the metal surfaces. Commonly, this is referred to as underdeposit corrosion.

In municipal wastewater treatment and collection systems, corrosion problems are quite different than those typically found in industrial systems. In the latter, corrosion results from direct aggression by the chemicals and their reaction products present in various discharge streams serviced by the system. Therefore, corrosion in those industrial systems usually occurs below the water line and tends to be predictable in nature. The most common corrosion encountered  in municipal systems, is above the water line and is typically microbiologically induced. Other chemicals in the waste stream may also cause corrosion below the water line.

For a barrier between wastewater and various substrates such as steel, ductile iron, and concrete, protective coatings are the most common solutions adapted by many municipalities around the world against MIC. The unpredictability in rate of attack referred to above means that virtually every municipality has reasons to be concerned. For metallic structures or pipes, the most common coatings systems used are coal tar epoxy, polyethylene, ceramic epoxy, and recenty used 100% solids polyurethane.(1) For concrete substrates, Polyvinyl Chloride (PVC) and coal tar epoxy have been used extensively since the 1940s. (2,3) There is a great challenge, however, to provide a more economical and also a more reliable coating particularly for concrete substrates.(4)

Since the 1970s 100% solids polyurethane has been extensively used in the wastewater industry. For their impermeability, resistance to microbial attack, excellent adhesion and abrasion resistance, flexibility, and unlimited film build, some of these polyurethanes have accumulated a significant track record of excellent success in the protection of steel, ductile iron, and concrete surface exposed to MIC in wastewater applications. (1,4) In addition to the approach of using protective coatings, various efforts have also been made in order to reduce or eliminate the source of microbiologically influenced corrosion. These efforts include environment alteration, proper design of structures and pipelines, cathodic protection, and the use of anti-microbial agents.

References

• S. Guan and H. Kennedy, "A Performance Evaluation of Internal Linings for Municipal Pipe", NACE International/Corrosion 96, Paper No. 482, Houston, 1996.
• Ameron International, "T-Lock PVC Sheet Liner for Concrete Pipe and Structures R9-96", Brea, 1996.
• TPC 12 (NACE International), "Coal Tar Epoxy Coating – A State  of the Art Review", Houston, 1987.
• Howard Kennedy, "Protection of Reinforced Concrete Pipe Against Microbial Induced Corrosion", NACE International/Corrosion 97, Paper No. 392, Houston, 1997.