Cathodic protection is a common metal corrosion protection technique that prevents corrosion of metal by applying a cathode current to the surface of the metal being protected, making it a cathode. However, there is also the problem of overprotection in cathodic protection, if the cathodic protection current is too large, it will cause the protected metal surface to be too negative, thus causing a series of hazards.
Over-protection can lead to hydrogen embrittlement on the metal surface, reducing the strength and toughness of the metal, thus affecting the service life of the metal. Overprotection can also lead to calcium deposits on the surface of the metal, which can roughen the surface of the metal, which can affect the appearance and performance of the metal. In addition, overprotection can also lead to the polarization of the metal, which can make the potential of the metal unstable, thus affecting the effectiveness of cathodic protection.
Therefore, cathodic protection overprotection and hazards are a problem that needs to be paid attention to. We need to take measures to prevent overprotection from occurring to ensure the effectiveness of cathodic protection and the lifetime of the metal.
The combination of anti-corrosion layer and cathodic protection is the most cost-effective corrosion control measure, the presence of anti-corrosion layer reduces the need for cathodic protection current, and cathodic protection slows down the corrosion of the corrosion point of the anti-corrosion layer breakage. When the cathodic protection current is insufficient, the corrosion cannot be effectively controlled, but when the cathodic protection current is too large, it leads to overprotection.
The cathodic reaction leads to an increase in alkalinity near the defect point of the anti-corrosion layer of the pipeline. Most organic binders are prone to aging and loss of adhesion in an alkaline environment, resulting in the peeling of the anticorrosive layer, that is, cathodic stripping.
As shown in the figure below, the cathode reaction produces hydrogen atoms, which can move freely in the metal due to their small size. Due to the presence of alloying elements, there are holes or dislocations in high-strength steels, and hydrogen atoms are prone to retention. When the retained hydrogen atoms combine into hydrogen gas, the volume expands, making it difficult to continue moving. When the accumulated hydrogen pressure is too high, it will cause the metal to bulge or crack, which is called hydrogen-induced cracking. When the pipe stress is less than the yield strength, the pipe generally does not undergo hydrogen-induced cracking.
To determine whether the cathodic protection is underprotected or overprotected, the judgment index is the power-off potential of the pipeline, which cannot be judged by the energizing potential, because the energizing potential contains IR drop. The cathodic protection power supply (such as a potentiostat) shows the energizing potential, as long as the power-off potential does not exceed the standard, it does not matter how large the energizing potential is. Although experiments in the laboratory have demonstrated the phenomenon of hydrogen-induced cracking, few cases of hydrogen-induced cracking have been found in engineering practice. Therefore, there is no need to worry too much about hydrogen-induced cracking, especially for steel pipes below x80.
The peeling phenomenon of 3LPE anticorrosive layer common in engineering is caused by the quality is not well controlled during factory coating, and has nothing to do with cathodic protection. The premise of cathodic stripping of the anticorrosive layer is the presence of water, and there is no water under the peeled coating, because the anticorrosive layer is not damaged, and the cathodic protection current cannot reach, so it is impossible to have hydroxide ions to cause environmental alkalinization.