Out of Sight, Out of Mind, and Eventually Out of Service
The United States’ wastewater collection and treatment infrastructure is under attack! A corrosion mechanism – termed biogenic sulfide corrosion – is degrading the nation’s concrete sewerage assets. This bacterially mediated corrosion process involves the conversion of sulfates into hydrogen sulfide, which in the gaseous form is a readily detectable and rather malodorous gas. Hydrogen sulfide is then oxidized into sulfuric acid by another genus of bacteria that ultimately causes corrosion of the concrete.
When left unchecked, as often is the case in these environments, biogenic sulfide corrosion has been found to corrode as much as 1 inch of concrete paste in a single year. It becomes rather challenging given the advanced corrosion rates found in these aggressive headspace conditions to meet the anticipated design life of these structures, and recent field experience indicates coating technologies such as polyamide and coal-tar epoxies – long considered standard wastewater coating systems – simply cannot withstand these more aggressive environments.
The high-performance coatings industry recognized this situation and has responded by producing new lining technologies designed to protect against biogenic sulfide corrosion. Since the overall performance is formulation specific, it is important that these products be designed for severe wastewater exposures. Specifically, Tnemec Company has developed a line of 100% solids epoxy products formulated with low permeability to resist H2S and other gasses, as well as sulfuric acid present within these aggressive environments. These epoxy coatings, offered under the Perma-Shield brand name, come in several types: epoxy mortar trowel applied between 125-250 mils; spray applied high-build liner at 60-80 mils; and spray applied fiberglass-reinforced, high-build liner at 100-125 mils. The selection of which epoxy system to use is dependent upon the exposure conditions to elevated hydrogen sulfide gas, impact and abrasive forces and thermal changes within the structure.
Extensive test data is available on these products that will assist specifiers in the selection of these types of wastewater coatings. To request this information, please contact your local Tnemec coating consultant.
Contact Vaughn O'Dea for additional questions or for a copy of the diagram pictured on the left.