In general microorganisms are responsible for the microbiologically induced corrosion (MIC). Biological organisms fall under two groups based on the type of corrosion they engender

• anaerobic corrosion, and
• aerobic corrosion.

Sulfate reducing bacteria from the genera desulfovibrio are a typical example of anaerobic MIC. In municipal wastewater systems, the SRBs, responsible for anaerobic MIC, are primarily Thiobacilli, which thrive on the sewage environment. System structures are rarely completely filled with sewage; thus, ample space exists above the water line for bacterial growth and gaseous products from the decomposition of sewage to collect. Due to their extremely rapid rate of reproduction, enormous colonies of bacteria are produced in a short time, resulting in a large potential source of corrosive media. 

The MIC mechanism begins with the fermentation of raw sewage that generates methane (CH4) and hydrogen sulfide (H2S) gases. When the wastewater stream is anaerobic, SRBs existing in the slim layer in the submerged area convert the naturally occurring sulfates in the wastewater into H2S. The hydrogen sulfide, methane, and carbon dioxide (CO2) react with water vapor to produce a mild acidic condition which condenses on the structure or pipe surface above the water line. This process lowers the pH of the substrate surfaces to levels favorable for the growth of the Thiobacilli bacteria. The Thiobacilli oxidize H2S and other sulfur bearing materials in the wastewater during their respiration and secrete sulfuric acid as a waste. In turn, each strain of bacteria lowers the pH of the substrate surface to an optimum range for another strain to lodge and reproduce. This process is repeated throughout the chain for all of the Thiobacilli strains involved, creating a very acidic environment, thereby encouraging rapid corrosion.

Factors influencing H2S production
When there are problems in drainage systems caused by H2S, the following factors are usually the main influences in the production of hydrogen sulfide that may lead to the formation of sulfuric acid in sewers.

•  Dissolved sulphide: The sulfide concentration is the limiting factor in the release of hydrogen sulfide to the sewer walls so that corrosion may occur. If metals are present in the sewage stream, a small amount of sulfide is immobilized to form insoluble metal salts. The amount varies from 0.1 to 0.3 milligrams per liter.
• pH: The pH influences dissociation of the sulfide ion species in the sewer. At a pH of 6, more than 90% of the dissolved sulfide is hydrogen sulfide. At a pH of 8, less than 10% is in the form of hydrogen sulfide.
• BIOLOGICAL OXYGEN DEMAND (BOD) AND TEMPERATURE: Temperatures above 15C may contribute to the generation of hydrogen sulfide if all other conditions of sulfide generation are present. BOD is a measure of the oxygen depletion by the decomposition and mineralization of organic matter. In a sewer system, energy is required for the conversion of sulfates to sulfide. The BOD determination is a measure of the energy within the system that will facilitate this conversion. The BOD usually occurs over a 5-day period and has thus become known as the 5-day BOD.
• VELOCITY: Velocity affects the rate of oxygen absorption, the release of hydrogen sulfide to the atmosphere, and the build up of solids. The minimum velocity of the sewer stream should be between 0.61 and 1.07 meters per second to keep solids in suspension. If the velocity causes turbulent flow conditions, increased oxygen may be absorbed into the wastewater, but hydrogen sulfide in wastewater will also be released to the atmosphere. The released hydrogen sulfide may cause corrosion to the wall of the wastewater pipe.
• JUNCTIONS: Junctions are important because the wastewater from tributaries may contain high concentrations of sulfide, lower pH, high BODs, and higher temperatures. All of these factors may affect the hydrogen sulfide production in the main sewer line. Junctions may also affect the type of flow where they enter the main. If the flow is turbulent, more oxygen may be absorbed into the wastewater, or more hydrogen sulfide may be released into the atmosphere. Since the effects of corrosion outweigh the increase in oxygen absorption, the junctions should enter the main in a manner that reduces turbulence.
• FORCE MAINS AND SIPHONS: Special junctions like force mains and siphons, have a similar effect on the quality of the wastewater stream, as do regular junctions. Force mains and siphons may flow at low velocities, or intermittently, allowing the increase of sulfide. Force mains usually flow full, which also facilitates the build-up of sulfides due to the anaerobic conditions in the force main. When force mains and siphons enter the main sewer, the higher concentration of sulfide may cause problems further downstream.
•  VENTILATION: Ventilation is not an effective measure to reduce the corrosion of wastewater pipe because it is difficult to prevent condensation on the walls of the pipe due to temperature variations. The hydrogen sulfide is oxidized in the aerobic layer on the wall of the pipe to form sulfuric acid, which may corrode the pipe as it trickles down the wall of the pipe. If velocities of 0.61 meters per second and oxygen level of 1 milligram per liter and temperatures less than 15oC can be achieved, corrosion in sanitary sewers will not be a problem at any time. Accumulation of solids could be a problem during the three warmest months of the year. During these months, the temperature is sufficiently high to have sewer water temperatures above 15oC. The elevated temperatures would also decrease the dissolved oxygen. Dissolved oxygen is inversely proportional to the temperature of the water. If effective BOD levels are less than 600 milligrams per liter and the effective slope is 0.2 % and flow is 0.085 cubic meters per second, sulfide concentrations will not increase sufficiently to become a problem.

STEEL AND DUCTILE IRON
Structural steels, mild carbon steels, ductile iron etc., a number of metals tend to corrode generally over the entire surface in the absence of crevices or galvanic effects. Corrosion is determined in these cases, by the rate at which dissolved oxygen can be delivered to the metal surface. Biological organisms present in the aqueous medium often have the potential to increase or decrease oxygen transport to the surface; consequently, these organisms play a role in increasing or decreasing general corrosion. Most MIC, however, manifests as localized corrosion because most organisms do not form in a continuous film on the metal surface. Microscopic organisms tend to settle on metal surfaces in the form of discrete colonies or at least spotty, rather than continuous films.

The classic mechanism for MIC of steel and iron was proposed by Von Wolzgen Kuhr (in 1934), is based on the idea that the rate-limiting step in corrosion is the dissociation of hydrogen from the cathodic site. It is thought that SRB consume hydrogen through the action of their hydrogenase enzymes, and thus depolarize the cathode, accelerating corrosion. Some investigators still believe that this mechanism is the important one for MIC of iron and steels, despite the fact that numerous experiments using SRB in pure culture gave corrosion rates far less than those seen at field sites and less than those measured in experiments using MIC communities.

Most coatings systems used for protecting a metal from MIC have been working well. Even after being coated with polymeric coatings, however, a metal may still be open to biologically influenced attack either through biodeterioration of the coating or through MIC of the underlying metal. Microorganisms are frequently implicated both in the corrosion of metals and in the deterioration of complex polymeric coatings. Deterioration may involve blistering, delamination, and changes in porosity, selective leaching of material components and changes in material properties by contamination from microbial metabolites. Corrosion of the substrate metal may be directly or indirectly influenced by the presence of organisms.

CONCRETE
Wherever there is a serious septic sewage problem, corrosion of concrete surfaces occurs. Although having an initial alkalinity as high as a pH of 13 due to the formation of lime in the hydration of dicalcium and tricalcium silicates (Portland cement components), concrete surfaces could have a pH as low as 0.6 because of microbiologically induced attack. The sulfuric acid (H2SO4) content on the exposed areas has been measured to as high as 5% or even 10%. This concentration of sulfuric acid corresponds to a pH value less than 1.0. The sulfuric acid generated due to MIC directly attacks the underlying substrate and causes destruction of the infrastructure. In many instances, microbiologically induced corrosion has damaged concrete structures to the point where major rehabilitation was required in as few as four years, or total collapse occurred within six years.

Many coatings have been applied to such concrete sewer structures and piping. Almost all have resulted in a complete failure. There are a few coatings systems that would withstand sulfuric acid, however, the actual coating of concrete is a more complicated and difficult procedure than providing the same type of protection to steel. Steel is a smooth, impervious, uniform, dense surface, while concrete is soft, brittle, subject to cracking, filled with air and water pockets, porous, water permeable, and chemically active. A coating over concrete surfaces is therefore subject to imperfections, and even a minute imperfection will cause the concrete to disintegrate under the coating.

References
D. Pope and E. Morris, "Mechanisms of Microbiologically Induced Corrosion (MIC)", Materials Performance, Vol. 34, No. 5, NACE International, May 1995, p. 24.
D.B. Mitton, et al., "Microbially Influenced Corrosion of Polymer-Coated Metallic Substrates", 1st Mexican Symposium and 2nd International Workshop on METALLIC CORROSION, University of South Florida, 1997.
C.G. Munger, "Corrosion Prevention by Protective Coatings", NACE International, 1984, p.302.