Unexpected failures of mortar-lined ductile iron force mains have occurred at unspecified high points due to internal corrosion aggravated by microbiological activity. To avoid unexpected failures in the future, a mechanistic understanding of the microbiologically influenced corrosion and degradation and their rates leading to failure is required. In this work, a detailed failure analysis was performed to uncover the mechanisms and typical rates of internal corrosion at unknown high points in mortar-lined ductile iron pipes (MLDIP). A literature review was conducted to identify existing mechanistic models that could be used in this application to estimate the time to failure considering the composition of wastewater, flow rate, and the size of the air-pocket to be used as a benchmark for future improvement. Laboratory experiments were performed to provide an estimate of the deterioration rates of mortar and corrosion rates of the ductile iron upon exposure to sulfuric acid. The combined results of the failure analysis, literature review, and experimental work were then used to develop degradation curves to estimate the time to failure of MLDIP. This work provides the Hillsborough County Public Utilities Department a much-needed tool that could guide future design and repair or replacement works at wastewater conveyance and treatment facilities that can also be leveraged by other organizations.
Ductile iron pipes have been employed in water and wastewater transport systems for more than four decades. In such applications, the pipe material is usually lined internally with cement or mortar, epoxy, or bituminous paints to protect it from corrosive agents in the flowing fluid. However, in sewage force mains, unexpected corrosion of mortar-lined ductile iron (MLDI) force mains have occurred at certain points.1 Failures from internal corrosion have been attributed to the effect of hydrogen sulfide released intermittently during turbulent flows from the sewage and intensified by the actions of microbial agents present in slimes at the crown of the sewer pipes.2,3 To explain the degradation steps and formulate predictive models, a mechanistic understanding of microbial induced deterioration of the mortar layer as well as the corrosion mechanism and rates of the ductile iron are required.
