October 19, 2009

MIC – a corrosive gas problem?

by Bob Murphy

Many continue to incorrectly refer to the deterioration of concrete sewer pipe as “a corrosive gas problem.” But the real culprit for concrete is MIC, a biogenic process when hydrogen sulfide gas and Thiobacillus bacteria (Thiobacillus is a small rod-shaped bacteria living in sewage or soil and oxidizing sulfur) interact; sulfuric acid is excreted with in sewer systems. Concrete being an alkali substrate, it is attacked vigorously by acids produced. Formation of Gypsum or calcium sulfate CaSO4 is the main product of decomposition by sulfuric acid. The gypsum appears as a white paste-like mass above the waterline in sewers.
Anaerobic bacteria form in raw sewage and naturally produce hydrogen sulfide gas (H2S) in sewer effluent. Factors contributing to rapid bacterial growth are temperature, retention time, high Biological Oxygen Demand (BOD) levels, and turbulence.
The more turbulent the effluent, the more hydrogen sulfide is released. This is stripping out of H2S is most common in lift stations and manholes. The hydrogen sulfide gas collects above the flow line where it combines with carbon dioxide (CO2). Both of these gasses produce a mild, weak acid solution when they dissolve into the sewer’s moist environment.
The (CO2) produces carbonic acid, and the H2S produces sulfuric and polyphonic acid. Both combine with the calcium hydroxide in the concrete to reduce the pH of the surface. As soon as the pH of the concrete falls from its initial levels of pH 11 or pH 12 to around pH 9, biological colonization of Thiobacillus bacteria will occur. Thiobacillus bacteria have the unique ability to convert hydrogen sulfide gas into sulfuric acid (H2SO4) in the presence of oxygen. At this point, it is just a matter of time before serious corrosion and possible collapse can occur.
There are many different species of Thiobacillus that will colonize, each species will reduce the Ph, die out and be replaced by the next (more Aggressive) species.
By living in these narrow ranges of different pH levels, it is difficult to duplicate this environment in the laboratory.

So where do coatings and linings come in? Coatings and linings act as a barrier coat between this environment and the substrate (concrete or steel) In a previous blog, it was discussed the use of coatings and linings in wastewater environments

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