In a year-long Field Correlation Study of the barrier performance of protective coatings and linings, a 28-day accelerated laboratory testing protocol from Tnemec proved more aggressive than 365-days exposure in the severe headspace environment of a manhole in Texas. The findings are contained in a report prepared by Vaughn O’Dea, director of Sales, Water and Wastewater Treatment, and Remi Briand, vice president, Research & Development.

“The corrosion protection of steel and concrete by a protective coating or lining may be altered by exposure to sewer gases and by the composition of the corrosive reagents found in headspace environments of domestic wastewater conveyance and treatment structures,” according to the study. “Tnemec, in conjunction with a coalition of wastewater experts, protective coatings formulators, and testing agencies, developed an accelerated laboratory testing protocol to quantifiably evaluate the performance qualities of protective coatings and linings recommended for use in severe wastewater headspace environments.”

The laboratory test method, known as The Standard Practice for Rapid Evaluation of Coatings and Linings by Severe Wastewater Analysis Test (S.W.A.T.), simulates a severe wastewater headspace condition. Coated steel and concrete samples are wetted with a corrosive solution and then exposed to hydrogen sulfide (H₂S), which is a gas known as being responsible for altering barrier properties of protective coatings and linings. The test is performed under controlled conditions in an airtight chamber with a constant temperature of 150 degrees Fahrenheit where specimens are immersed into the aqueous solutions three times daily for a period of 15 minutes, then exposed to the sewer gas the balance of the time. The cyclic exposure continues for a period of 28 days.

The field study was performed at seven testing sites, including a manhole located along a drainage ditch in a mixed residential and industrial area of Texas, using similar protective linings and testing methods performed in accordance with the S.W.A.T. protocol. The Texas site was chosen because of its highly-corrosive environment and elevated hydrogen sulfide levels (>500 ppm). Six steel coupons and six concrete cylinder coupons were properly prepared, coated with protective coating systems commonly used for wastewater corrosion protection (see Table), and exposed to the severe headspace environment of the manhole for 365 days.

Researchers used electrochemical impedance spectroscopy (EIS) analysis to test the coated steel samples for permeation resistance. “Any polymer degradation is easily detected by a decrease in the measured impedance,” the study said. “When interpreting permeation resistance using EIS, the higher and more stable the retained impedance following exposure, the better the long-term permeability resistance and, therefore, the better the long-term coating performance.”

Physical testing and visual inspection of samples was also performed by researchers in accordance with various American Society for Testing and Materials (ASTM) methods before and after exposure to S.W.A.T. “Polymers that retain their physical properties, such as adhesion, tensile strength and flexural properties under such corrosive conditions are assumed to offer better substrate protection within severe wastewater environments,” the study stated. “Protective coatings should not blister, check, crack, or allow corrosion of the substrate when exposed to severe wastewater environments. Polymers that retain film quality are assumed to offer better substrate protection.”

Steel panels coated with a polyamide epoxy and a polyamide epoxy coal-tar exhibited blistering and a major drop in EIS readings in both the field study and the S.W.A.T. testing. Panels coated with a two-component, 100 percent solids aromatic polyurethane hybrid and a three-component, 100 percent solids polyamine epoxy mortar exhibited no blistering, visible pinpoint rusting or any other film defect and only a slight drop in EIS readings in both the field study and S.W.A.T.

The steel panel coated with a fiber-reinforced polyamine epoxy exposed to the 28-day S.W.A.T. showed a decrease in permeation at 10 days, followed by a recovery of permeation resistance at 28 days compared to no drop in permeation in the less aggressive field environment. The steel panel coated with a novolac epoxy showed cracking and zero permeation resistance after exposure to the aggressive 28-day S.W.A.T. environment, compared to a minimal loss of film quality and permeation resistance based on results from the field study.

Cross sections of the concrete test samples were microscopically measured by researchers for discoloration of the film, which is an indication of sewer gas penetration. “Permeation by the severe wastewater reagents typically manifests as discoloration when viewed by digitally enhanced optical microscopy,” the study reported. Concrete coupons coated with polyamide epoxy and novolac epoxy exhibited blistering in the field study, which is consistent with what happens with these coatings in S.W.A.T. Panels coated with a two-component, 100 percent solids fiber-reinforced polyamine epoxy, a three-component, 100 percent solids polyamine epoxy mortar and an aromatic polyurethane hybrid exhibited no blistering or any other film defects. A bare concrete coupon exhibited corrosion similar to 28-day S.W.A.T.

The S.W.A.T. has been restricted to simulations that exemplify the application of the “sewer air components” that were known to effect protective coating systems. The “sewer air” was comprised of H₂S due to available data on gases which commonly emanate from septic sewages flowing in typical collection systems. “Until now, analytical data for ‘sewer gases’ appear to have been derived mainly by inference from information surrounding sewage decomposition rather than from studies of air in contact with flowing sewage,” the report added. “The data surrounding the sewer gases collected from this study will facilitate gas modeling and future incorporation into the S.W.A.T. protocol to closer replicate sewer headspace environments.”

Other testing sites in the study included North Central Florida, Northwestern U.S., New England, Rocky Mountain U.S., coastal Virginia and Midwestern U.S. These sites were chosen based on their histories of biogenic sulfide corrosion and their different climates.

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