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.
The Bureau of Reclamation (commonly called Reclamation or BOR) of the U.S. Department of the Interior has from its inception fostered in the western United States water transmission projects designed to provide water for agricultural and municipal uses in areas otherwise deficient in local water supplies. In doing so, Reclamation has specified the types of materials appropriate for particular applications and the type of corrosion mitigation systems appropriate for each pipeline material depending on the corrosivity of the soils. As technology has been developed further and as experience has been gathered, Reclamation has changed the specifications for corrosion mitigation systems from time to time, most recently in 2004.
The Bureau of Reclamation has chosen a relatively simple means of classifying the corrosivity of soils for pipelines installed under its control. For any section of pipeline to be designed, the in situ soil resistivity is measured at multiple positions along the length of the pipeline. A computer program develops a layered model of true resistivities and forms an ordered probability plot developed from these resistivities; the resistivity at the 10th percentile of probability is selected. In other words, 10 percent of the true resistivities derived from the in situ resistivity measurements have lower resistivity values than this characteristic resistivity, and 90 percent have higher resistivity values. If this characteristic resistivity is greater than or equal to 3,000 ohm-cm, the soil is considered to be in the least-corrosive category. If it is between 2,000 and 3,000 ohm-cm, the soils are considered to be of moderate corrosivity. If the characteristic resistivity is less than or equal to 2,000 ohm-cm, the soils are considered to be highly corrosive.
When Reclamation issued modified corrosion protection requirements in its technical memorandum, TM 8140-CC-2004-1,1 polyethylene encasement (PE) was specified as the corrosion mitigation method for DIP in soils of low corrosivity. For DIP used in moderately corrosive soils, PE and CP were specified. For highly corrosive soils, Reclamation specified that bonded dielectric coatings and CP were needed for adequate corrosion protection of DIP. It is this latter specification that