Book Description

Cast iron is a material that was used for years to fabricate pipe for applications such as water transmission and distribution. About 50 years ago ductile iron pipe (DIP) was introduced as a more economical and better-performing product. DIP is similar to cast iron pipe (CIP) in that it contains several percent carbon distributed as graphite in an iron matrix. However, by the addition of small amounts of magnesium and the controlled annealing of DIP, the graphite becomes distributed as spherical nodules in DIP, whereas it is distributed in the form of flakes in traditional CIP. This microstructure in DIP results in mechanical properties that are superior to those of traditional cast iron. DIP is also centrifugally cast in a steel mold in contrast to the casting of CIP in traditional sand molds with sand cores. As with iron or steel pipes, DIP is subject to corrosion, the rate of which depends on the environment in which the pipe is placed. Corrosion mitigation protocols that depend on the corrosivity of the soil are employed to slow the corrosion process to an acceptable rate for the application. A popular, economical, and often effective corrosion mitigation method for DIP in buried applications is the use of polyethylene encasement (PE), consisting of a sheet of polyethylene wrapped around the pipe or a tube of polyethylene sheeting slipped over the pipe at the time of installation. Another protective method is to use bonded dielectric coatings consisting of various polymers applied as coatings on prepared surfaces or wrap tapes with adhesives on the same prepared surfaces. Bonded dielectric coatings are commonly used on steel pipes but are less commonly used on DIP. Cathodic protection (CP) can also be used to protect metal pipes including DIP. CP electrochemically protects the metal structure either by imposing a direct current (dc) electrical potential to the pipeline or by connecting the pipeline to a sacrificial anode made of a more electrochemically active metal. The decision to use corrosion mitigation systems and the choice of the system to use for buried DIP depend on the corrosivity of the soils in which the pipeline is buried. Many methods of assessing the corrosivity of soils have been developed. All use the resistivity of the soil either as the defining parameter or as one parameter in conjunction with others such as the presence or concentration of specific chemical species that foster corrosion. Soils with low resistivity are more corrosive than soils with high resistivity.