Regardless of the type of system used, current flows from the cathodic protection anode through the soil to the structure to be protected. Where this current flows onto a structure from the surrounding electrolyte (soil), the potential of the structure is made more negative. Cathodic protection is achieved when this change in potential is sufficient to arrest corrosion.
It would appear that cathodic protection can be achieved merely by the application of current of sufficient magnitude. Although this statement is true, it is deceptively simple because there are very large differences in the design of cathodic protection systems. These differences result from the infinite variety of structures that are to be protected and from the large assortment of environments in which those structures are located. Because of the large differences in the designs of systems necessary to achieve protection, it is often necessary for existing structures that each system be custom designed for a given location.
In order to prevent corrosion using cathodic protection, current must flow from the electrolyte onto the structure at all locations. If a portion of the structure does not receive current, the normal corrosion activity will continue at that point. If any of the cathodic protection current picked up by the structure leaves that structure to flow back into the electrolyte, corrosion will be accelerated at the location where the current is discharged. As an example, when mechanically coupled piping is used, this can be discontinuous from one pipe section to the next. If a galvanic anode type system is used for protection, it may be necessary to install an anode on each pipe length or to electrically bond across each joint. If one length of pipe is neglected, that length will receive no cathodic protection and the normal corrosion activity will continue. When a rectifier type system is installed on an underground storage tank system, it is even more important that the tank and lines be electrically continuous. If there are non-continuous joints, it is possible for the cathodic protection current to leave the pipe or tank to flow around the electrically discontinuous joint causing corrosion at each point where the current leaves the pipe surface. Similarly, if cathodic protection current is applied to one structure in an area, it is possible for other structures in the neighborhood to be exposed to damage unless proper steps are taken. Potential measurements are used to determine whether such damaging exposure exists. Just as protection is indicated when the potential of a structure is made more negative, stray current corrosion is indicated when the potential of a structure is made less negative as a result of the application of cathodic protection current.
It would appear that cathodic protection can be achieved merely by the application of current of sufficient magnitude. Although this statement is true, it is deceptively simple because there are very large differences in the design of cathodic protection systems. These differences result from the infinite variety of structures that are to be protected and from the large assortment of environments in which those structures are located. Because of the large differences in the designs of systems necessary to achieve protection, it is often necessary for existing structures that each system be custom designed for a given location.
In order to prevent corrosion using cathodic protection, current must flow from the electrolyte onto the structure at all locations. If a portion of the structure does not receive current, the normal corrosion activity will continue at that point. If any of the cathodic protection current picked up by the structure leaves that structure to flow back into the electrolyte, corrosion will be accelerated at the location where the current is discharged. As an example, when mechanically coupled piping is used, this can be discontinuous from one pipe section to the next. If a galvanic anode type system is used for protection, it may be necessary to install an anode on each pipe length or to electrically bond across each joint. If one length of pipe is neglected, that length will receive no cathodic protection and the normal corrosion activity will continue. When a rectifier type system is installed on an underground storage tank system, it is even more important that the tank and lines be electrically continuous. If there are non-continuous joints, it is possible for the cathodic protection current to leave the pipe or tank to flow around the electrically discontinuous joint causing corrosion at each point where the current leaves the pipe surface. Similarly, if cathodic protection current is applied to one structure in an area, it is possible for other structures in the neighborhood to be exposed to damage unless proper steps are taken. Potential measurements are used to determine whether such damaging exposure exists. Just as protection is indicated when the potential of a structure is made more negative, stray current corrosion is indicated when the potential of a structure is made less negative as a result of the application of cathodic protection current.
Source:www.bushman.cc/pdf/corrosion_theory.pdf
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