Failure analysis in buried ductile iron pipelines: A study of leakage in drinking water distribution systems

Severely external corrosion has occurred on the ductile iron pipelines tube, which caused leakage and affected the drinking water supply of numerous people. The tube was made up of QT400 ductile iron with external and internal protective coatings. The service environment was the underground of a riverbed close to residential areas. In this study, this failure and its mechanism were studied through field investigations and laboratory experiments. The penetration rate of the localized corrosion is twice higher than that of uniform corrosion. It is perceived that DC stray current causes the localized corrosion, which results in smooth and black-deposits-covered morphology. Microorganisms, especially sulfate-reducing bacteria (SRB) in the soil plays an auxiliary role in the failure process. Additionally, the micro-galvanic corrosion between graphite and matrix promotes the failure. This study shows that an in-depth investigation on the stray-current corrosion in buried cast iron pipes is critical, although sacrificial anodes have been adopted.