A NOVEL ANALYTICAL APPROACH FOR EVALUATING PROTECTIVE COATINGS PERFORMANCE IN WASTEWATER ENVIRONMENTS

摘要

Municipal wastewater collection and treatment systems have historically had corrosive conditions in the aerated headspaces of concrete and metal structures. This biogenic sulfide corrosion has become more severe in recent years due to longer detention times, increased usage of force mains, industrial pretreatment for metals removal and the covering of tanks and channels for odor control purposes. Protective coatings that performed well in wastewater headspaces in the past have repeatedly failed under these newer and more severe exposure conditions. Failure analysis experience has shown that the performance of coatings in these more aggressive environments is closely related to permeability properties. Low permeability of coatings and linings to the corrosive gases and liquids present in the wastewater vapor phase has been shown to substantially increase coating performance. Evaluating coatings that will provide the right barrier properties for long-term performance is currently based on in-service performance histories. This takes too much time for new coating formulations. Until recently, no existing laboratory test methods were available to more expeditiously evaluate coating performance for these newer, more aggressive conditions. Tnemec Company, in collaboration with partners, established a testing project team and decided to develop a novel approach to accelerated laboratory testing for evaluation of coating performance in simulated wastewater headspace conditions. These coatings with superior barrier properties, combined with excellent substrate adhesion, are expected to offer the best performance. A unique laboratory test chamber was designed and built to simulate the contributing exposure conditions pertinent to coating permeability and chemical resistance. Electro-chemical impedance spectroscopy is being used to monitor and evaluate the barrier and permeability properties of coating systems exposed to these replicated conditions. A companion field test program is also been underway at the Deer Island Treatment Plant in Boston, Massachusetts. All coatings tested in the laboratory cabinet have also been exposed to real treatment plant exposure conditions, including intermittently high H2S gas concentrations. This paper describes the simulated exposure conditions and the accelerated parameter capabilities of the cabinet. It further discusses the learnings from nearly two years of laboratory test results. The paper will demonstrate conclusively that permeation resistance is the key factor in the successful performance of coatings in wastewater headspaces. It will further show that this accelerated testing method is the right tool to use when selecting coatings for those environments in the future.