The combination of an anticorrosion layer and cathodic protection is the most cost-effective corrosion control measure. The presence of an anticorrosion layer reduces the need for cathodic protection current, and cathodic protection slows corrosion at the point where the anticorrosion layer breaks down. When the cathodic protection current is insufficient, corrosion is not effectively controlled, but when the cathodic protection current is too large, it leads to overprotection.
The cathodic reaction leads to increased alkalinity near the defective points of the pipeline corrosion protection layer. Most organic binders tend to age and lose adhesion in alkaline environments, leading to peeling of the corrosion protection layer, i.e. cathodic stripping.
As shown in the figure below, the cathodic reaction produces hydrogen atoms, which can move freely in the metal due to their small size. Due to the presence of alloying elements, high-strength steel with cavities or dislocations, hydrogen atoms are easily trapped. When the stranded hydrogen atoms combine to form hydrogen gas, the volume expands and it is difficult to continue to move. When the accumulated hydrogen pressure is too high, it will lead to metal bulging or cracking, called hydrogen cracking. Hydrogen cracking generally does not occur when the pipe stress is less than the yield strength.
To judge whether cathodic protection is under protection or over protection, the judgment index is the disconnection potential of the pipeline, which can not be judged by the energization potential, because the energization potential contains IR drop. Cathodic protection power supply (such as constant potential meter) shows the energization potential, as long as the disconnection potential does not exceed the standard, the energization potential how much does not matter. Although laboratory experiments have proved the phenomenon of hydrogen cracking, there are almost no cases of hydrogen cracking found in engineering practice. Therefore, there is no need to worry too much about hydrogen cracking, especially for steel pipelines below X80, there is no need to consider hydrogen cracking.
The common phenomenon of peeling of 3LPE anticorrosion layer is caused by the lack of quality control during coating in the factory, which has nothing to do with cathodic protection. The prerequisite for cathodic stripping of the anticorrosion layer is the presence of water, and there is no water under the stripped coating, and since the anticorrosion layer is not broken and the cathodic protection current can not reach it, it is not possible for hydroxide ions to be generated and lead to alkalization of the environment.
