After the pipeline is buried in the ground, affected by the nature of the soil along the pipeline and environmental differences, there are differences in the potential of each point on the surface of the pipeline. The location of the more negative potential for the anode, corrosion; potential more positive parts of the cathode, corrosion inhibition. The current between the cathode and anode is called corrosion current.

  Among the many anodes on the surface of the pipe, there will always be a location where the potential is most negative. If the potential of the other parts can be negatively shifted, all reach that potential, there will be no potential difference between the points on the pipe surface, no anode, no more corrosion current. The most negative anodic potential on the metal surface is usually defined as the minimum protection index for cathodic protection.

  Since the potential of each part of the pipeline has to change (polarization) after it is buried in the ground, it is impossible to measure the potential of a certain anode or a certain cathode from the ground surface, and what can be measured is the combined potential of many cathodes and anodes, which is called the natural potential or corrosion potential.

  The natural potential of the metal structure and the most negative anode potential will change with the environment. After years of research and practice, it is found that the most negative anodic potential of a carbon steel structure will not be more negative than -0.85Vcse regardless of the environment, so -0. 85Vcse is defined as the minimum protection potential of a carbon steel structure. It is also found through the study that the difference between the natural potential of the metal and its most negative anodic potential is less than 100mV, that is to say, from the basis of the natural potential, the potential is negatively shifted by 100mV, and the potential of the structure is already more negative than the most negative anodic potential. Therefore, 100mV cathodic polarization index is also the minimum cathodic protection index often used in engineering. Generally, the current required to meet the 100mV cathodic polarization index is less than the current required to meet the -850mVcse minimum index.