Performance of Ductile Iron Pipes. I: Characterization of External Corrosion Patterns

Ductile iron pipes have been used in North America since the late 1950s. This paper describes research that endeavored to gain a thorough understanding of the geometry of external corrosion pits and the factors (e.g., soil properties, appurtenances, and service connections) that influence this geometry. This understanding is subsequently used in the companion paper to devise a sampling scheme and to infer the condition of ductile iron buried water mains. Soil corrosivity is not a directly measurable parameter, and pipe external corrosion is primarily a random phenomenon. The literature is replete with methods and systems that attempt to use soil properties (e.g., resistivity, pH, and redox potential) to quantify soil corrosivity and subsequently predict pipe corrosion. In this research, varying lengths of ductile iron pipes were exhumed by several North American water utilities. The exhumed pipes were cut into short sections, sandblasted, and tagged. Soil samples were also obtained at discrete locations along the exhumed pipe. Pipe sections were scanned for external corrosion using a specially developed laser scanner. Scanned corrosion data were processed using specially developed software to obtain information on pit depth, pit area, and pit volume. Statistical analyses were subsequently performed on these three geometrical attributes. Various soil characteristics were investigated to determine their impact on the geometric properties of the corrosion pits. Subsequently, a method is proposed to assess the condition of a ductile iron pipe, based on the geometry of corrosion pits of several samples extracted along the pipe. This paper describes the pipe exhumation, data preparation, and statistical analysis of corrosion pits. The companion paper describes a sampling scheme to infer pipe condition of ductile iron buried water mains.