How does the use of internal coatings affect the cathodic protection current demand for a pipeline?

The use of internal coatings significantly reduces the cathodic protection (CP) current demand for a pipeline because the coating acts as a barrier between the steel and the corrosive environment inside the pipe. Cathodic protection works by applying a direct current to the pipeline, making it the cathode in an electrochemical cell and preventing corrosion. Without an internal coating, the entire internal surface of the pipeline is exposed to the corrosive fluid, requiring a significant amount of CP current to maintain adequate protection. An effective internal coating significantly reduces the area of steel exposed to the corrosive environment, therefore decreasing the amount of CP current required to protect the pipeline. The coating acts as an electrical insulator, preventing the flow of current between the steel and the fluid, except at any holidays (defects) in the coating. The CP system then only needs to provide current to these small areas of exposed steel, dramatically reducing the overall current demand. The degree of reduction in CP current demand depends on the quality and integrity of the coating. A well-applied coating with few holidays will result in a greater reduction in current demand than a poorly applied coating with many holidays. Even with an internal coating, CP is still typically required to provide protection at holidays and to account for any future degradation of the coating over time. However, the CP current demand will be much lower than for an uncoated pipeline. The use of internal coatings can also extend the life of CP systems by reducing the consumption rate of anodes in sacrificial anode CP systems or reducing the power consumption of impressed current CP systems. Overall, internal coatings are an effective means of reducing the CP current demand for pipelines, leading to significant cost savings and improved corrosion protection.