Page 614 - Corrosion Engineering Principles and Practice

576 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 577

No deflection
Deflection toward Deflection toward
defect defect

No signal
(when directly
above defect)
Increasing signal Decreasing signal
strength (when strength (when
approaching defect) leaving defect)

X Buried pipeline

X Location of coating defect Equipotential lines

FIGURE 13.40 Schematic description of the DCVG methodology.

13.9.4 Corrosion Coupons
Cathodic protection coupons are now being used as an alternative
method to make potential measurements that may be substantially
free of voltage-drop error. A CP coupon is a small piece of metal that
is electrically connected to the structure at a test station. The potential
of a coupon will closely approximate the potential of any exposed
portion of the structure (holiday) located in the vicinity of the
coupon. By disconnecting the coupon from the structure at the test
station, an “instant-off” potential measurement can be made on the
coupon without having to interrupt any other current sources.
However, these measurements are still not completely free of voltage-
drop error. Any voltage drop occurring in the electrolyte in the
distance between the reference electrode and the coupon surface will
still be incorporated into measurements. Placing a reference electrode
as close as possible to a coupon can minimize this error. However,
the reference electrode must not be placed so close that it shields the
coupon [16].
Perhaps the most important consideration in the installation of
corrosion coupons is that a coupon must be representative of the
actual pipeline surface/defect. The exact metallurgical detail and
surface finish as found on the actual pipeline are therefore required