Rosen 9704935 Sulfide corrosion of concrete in wastewater collection systems occurs when microorganisms in anaerobic films below the water surface produce hydrogen sulfide. The sulfide evaporates into the atmosphere and redissolves in moisture that has condensed on the portions of the pipe above the wastewater. Microorganisms in this environment convert the sulfide to sulfuric acid. The acid reacts with calcium hydroxide and calcium carbonate in the cement, producing sulfate minerals and causing the concrete to corrode. The investigators develop a mathematical model of the corrosion process that is tuned, tested, identified and validated via field data and laboratory experiment. The effort includes the analysis of the model equations (which take the form of a system of coupled partial differential equations with moving or free boundaries) from the point of view of well-posedness (existence, uniqueness, regularity of weak, strong, local, global solutions, etc.) and the sensitivity of solutions with respect to parameters. It also includes the development, analysis and testing of efficient, convergent and robust numerical integration schemes for numerical simulation, parameter estimation and control. In addition to using available field data to verify the model, a laboratory test-bed to provide additional data for model verification and parameter estimation is designed and fabricated. Electrochemical techniques applied to instrumented concrete samples are used to (i) characterize the concrete structures using electrochemical impedance spectroscopy (EIS), and (ii) sense the diffusion profiles of corrosive species. Correlation of the results of these two studies allows investigation of relationships between concrete microstructure and diffusion processes. Computational schemes are developed to use the field and laboratory data to identify unmeasurable parameters that appear in the model, and to test corrosion slowing control schemes based upon the model. Sulfide corrosion causes millions of dollars in damage in the U.S. each year. Attempts have been made to control sulfide corrosion in sewers. Most try to prevent the release of hydrogen sulfide into the pipe atmosphere. Strong base has been used to kill the biofilm, and cleaning has been used to remove it. The addition of trace metals precipitates the sulfide, preventing its release to the atmosphere. However, the biofilms are tenacious, and grow back rapidly, requiring additional treatment. Any anti-biofilm treatment is constrained because it must not damage the similar microorganisms that are the basis of the wastewater treatment system. Chemical precipitation requires continuous treatment. Any effort involving chemical addition must contend with the very large volumes of water flowing through wastewater collection systems: no matter how cheap the chemical, the colossal amounts that must be purchased and fed into the system are expensive. Thus an effort to first understand and to then interrupt (i.e. control) the corrosion process is needed, and may provide the only possible remedy for some cases. The results of the study should yield a deeper understanding of the corrosion process, and provide a powerful tool for engineers to use in the design of concrete wastewater systems and in the formulation, testing, tuning and evaluation of corrosion abatement procedures. The model and results should guide improvements in protection techniques where sulfide corrosion is a problem, and provide insight into the corrosion processes of other concrete structures and the substantial damage caused. They could also potentially reduce maintenance costs. The modeling ideas developed as a part of this project should be transferable to other corrosion problems, and the analytical and computational techniques that result should be of use in the area of moving boundary problems, their analysis, numerical solution, identification and control.
