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When the pipeline is subjected to cathodic current and its negative potential increases to a certain level, hydrogen gas evolves on the pipeline surface due to the reduction of H? at the cathode. This hydrogen evolution phenomenon can weaken or even destroy the adhesive force of the anti-corrosion coating, a process commonly referred to as cathodic disbondment, which accelerates the aging of the anti-corrosion coating. Different anti-corrosion coatings have different hydrogen evolution potentials. For asphalt anti-corrosion coatings, hydrogen evolution begins when the applied potential is lower than -1.20V (relative to the saturated copper sulfate electrode), and significant hydrogen evolution occurs when the potential reaches -1.50V (relative to the saturated copper sulfate electrode). Therefore, the maximum protective potential for asphalt anti-corrosion coatings is set at -1.20V (relative to the saturated copper sulfate electrode). For other types of anti-corrosion coatings, the maximum protective potential should be determined experimentally.

3) Protective Current Density

In GB/T 10123, protective current density is defined as: "the current density flowing into or out of an electrode surface maintained at a constant potential within the protective potential range." This definition applies to both cathodic protection and anodic protection, though for cathodic protection, it specifically refers to "flowing into." Protective current density is influenced by factors such as the nature of the metal, medium composition, concentration, temperature, surface condition (coating status), medium flow, and surface cathodic deposits. For soil environments, it may also be affected by seasonal factors.

Since protective current density is not a fixed value, it is generally not used as a control parameter for cathodic protection. It is only employed as a control parameter when potential measurement is not feasible. For example, in the protection of oil well casings, current density is an important parameter and can be used as a control criterion.

Due to the difficulty in determining the specific locations and areas of the cathode and anode of corrosion cells in practical applications, the data listed in tables are typically calculated based on the entire surface area of the protected metal in contact with the electrolyte. Such experimental data are suitable for smaller metal structures, such as tank bottoms or platform piles. However, for long-distance pipelines with significant variations in soil resistivity and coating quality along the route, these data often show considerable deviations. Therefore, for pipeline cathodic protection, the minimum and maximum protective potentials are commonly used as evaluation criteria.

3. Conditions and Auxiliary Facilities for Cathodic Protection of Pipelines

1) Basic Conditions for Cathodic Protection of Pipelines

(1) Electrical Insulation of the Pipeline.

① Insulating Joints. Pipeline insulating joints come in various types, including flange-type, monolithic-type (buried), and union-type. In recent years, developed monolithic buried insulating joints feature an integrated structure, direct burial capability, and high insulation performance. They overcome the drawbacks of insulating flanges, such as poor sealing performance, assembly affecting insulation quality, inability to be buried, and susceptibility to dust accumulation at the outer edges, making them ideal insulating connection devices for pipelines.

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