Potential to Environment

The criteria used for the control of a CP system also provide a direct method to monitor the efficiency of that system and determine how well a structure supposedly under CP is actually protected against corrosion. Potential measurements are used most commonly as a criterion of protection. The basis for this is that if current is flowing onto a protected structure, there must be a change in the potential of the structure with respect to the environment. This is because the current flow causes a potential change, which is a combination of the voltage drop across the resistance between the protected structure and the environment, and the polarization potential developed at the structure surface. 

The resistance between the protected structure and the environment includes the resistance of any electrically insulating paint or coating on the structure. This is illustrated in Fig. 13.31. detailing the various resistive components when a buried pipe is polarized by an ICCP system. In this example, the desirable place to measure potential would be across the interface between the pipe and the environment, as is represented by the terminals marked polarization potential on the equivalent circuit of Fig. 13.31. 

Actually, in practice, this is rarely feasible on buried structures, and it becomes necessary to resort to measuring the potential between the structure and the environment surface directly above or at a point nearest to the structure. The measured potential now includes the polarization potential plus a portion of the voltage drop across the structure-to-remote-earth resistance, as shown by the potential measured between the structure and point A on Fig. 13.31. The potential measured between the structure and remote earth, point C, is not affected by the CP current lines and therefore provides a true estimate of the polarization shift produced on the structure by a CP system. 

Point A on earth above pipe Resistance between pipeline and remote earth 

Equivalent circuit 1 Pipeline within area of influence of groundbed 

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