In the present study, pipe-casing under cathodic protection is modeled by using FEM. The inner and outer surface of the casing and coating defects at the pipe surface is considered as corrosion and protection interface that follows non-linear polarization behavior. The coating of casing, electrolyte resistivity in pipe-casing space and electrical resistivity between pipe-casing are the main variables. FEM model is approved by tens centimeters of the pipe-casing section experimental setup. After proofing the FEM model, it is scaled up to study more realistic dimensions and broader variable ranges.
The FEM simulations show that when the casing is bare and electrically isolated from the pipe, coating defects of the pipe are easily protected in a wide range of solution resistivity in pipe-casing space. If pipe-casing resistance is reduced to lower than 10 ohms, the cathodic protection nullified, and corrosion could happen on the pipe coating defects. The presence of a coating on the inner surface of the casing will only lead to localization of the corrosion on its defects. Lower solution resistivity leads to an overall flattening of the potential distribution to an average value.
A crossing between buried pipelines and transportation arteries such as railways and highways is a common reality. Nowdays it is ordinary practice, and standards are available for such pratctise1, to protect such pipelines by encasing them in a wider pipe, named Casing. The goal is to protect the carrier pipe providing an outer shell capable of withstanding mechanical stresses and eventual corrosion, without leakage risk. In usual conditions this kind of safety measure should not be necessary, still due to the difficult maintenance and monitoring accessibility below railways and highways it becomes a dependable protection method and device.
Cathodic Protection is a further protection applied on this kind of buried structures, to slow down the corrosion process that could endanger the carrier pipe.