Page 602 - Corrosion Engineering Principles and Practice

564 C h a p t e r 1 3 C a t h o d i c P r o t e c t i o n 565

provided, and all splices and connections must be insulated perfectly.
Any defect in the cable insulation system may become a point of
current discharge that will corrode the wire until it separates, thus
breaking the electrical connection between the rectifier and all or part
of the groundbed installation.
By contrast, all buried wire connecting self-powered galvanic
anodes to a protected structure are subject to current collection from
the environment and are free of corrosion. Insulation on such wires is
used to prevent their picking up unnecessary current producing and
associated loss of anode efficiency. This is also true of the insulated
cable extending from the negative terminal of the impressed current
power source to the protected structure.

13.6 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.

* The expression “remote-earth” used means the infinite conductor with practically
no resistance which is the earth’s mass.