Metallurgical Investigation of Buried Ductile Cast Iron Sewerage System Pipeline Premature Failure

Premature failure of two upward-inclined underground pipes carrying sewage caused sinkhole formation in a busy urban road. Metallurgical failure analysis revealed that the pipes were made of ductile cast iron. External and internal coatings were appropriate to protect the pipes from external soil and internal sewage slurry corrosion. Partial filling of sewage slurry during the pumping recess allowed turbulent flow in the empty section during pumping, and the impact of turbulence eroded the internal coating, i.e., mortar lining, introduced erosion-corrosion and progressively reduced the pipe wall thickness in the upper half of the pipes resulting to weakening. Intermittent turbulence led to some localized corrosion fatigue-induced fracture in the weakened pipe wall. Erosion-corrosion with the assistance of corrosion fatigue led to the premature failure of the ductile cast iron pipes.