MIC in Sewers
Microbiologically Influenced Corrosion is wellknown in sewers as BAC, Biological Acid Corrosion. As a result of anaërobe conditions sulphates in the sewer water is transited into hydrogen sulphide (H2S). The Sulphate Reducing Bacterius Desulfovibrio desulfuricans is active under such conditions. Other bacteria active in anaërobe conditions and that transpose proteins in volatile sulphur compositions are the bacteria Veillonella, Clostrida and Proteus. The sewer atmosphere above the water will contain a high concentration of hydrogen sulphide.
On the cooler concrete wall water condensation occurs and the hydrogen sulphide dissolves in the water droplets. But because of the presence of air in the atmosphere the sulphide oxidises into elemental sulphur. Thiobacilles will transpose the sulphur into sulphuric acid.
Various thiobacilles are active dependent on the pH value of the environment. The “thiobacillus infermedius” and the “thiobacillus novellus” decrease in number when the pH value of the sulpuric acid has been decreased down to 6. Then the “thiobacillus neapolitanus” becomes active down to a pH of ca 5. At pH’s lower than 5 the “thiobacillus thiooxidans” takes over the process and at last a pH value of 1 to 2 can be achieved, a rather corrosive condition.
MIC in Dung Silos
During the fermentation and the anaërobe decomposition of the dung hydrogen sulphide is formed by the reduction of sulphates. Dung for itself is not corrosive for steel because it is neutral or light alkaline. Open on the air there exist a rather high smell problem. Therefore in the years ‘80 open silos should be covered by law. However this changes the environment on the walls of the silo considerably.
Because of the relative high temperature of the dung water condensation will occur on the cooler non-isolated walls above the dung level. Hydrogen sulphide dissolves in the water film and droplets and in presence of the “thiobacilles” sulphuric acid is formed. As described before under influence of the “thiobacillus thiooxidans” the pH value can be decrease down to 1.
When the dung level is fluctuating or in an older silo the wall will have been contaminated with chlorides too and a still more corrosive environment is present: a mixture of sulphuric and hydrochloric acid. This was a disaster for the existing silos made of coextruded thin steel plates of carbon steel and stainless steel (AISI 304 and AISI 316). Since then the construction of the silos has been changed to inner sheets of rubber. Application of steels or alloys resistant to such corrosive conditions would be too expensive and that was not an option.