Nace CP 1, Course Manual

Page: 210 / 535

Installing C.P. Components 8:11
CP 1 – Cathodic Protection Tester Course Manual
©
NACE International, 2000
07/01/04
Electric resistance or so called “smart” coupons have a probe, again located
near the structure and connected to it and to a test instrument receptacle in
the test station head. The probe is wired in such a way that the instrument
can measure the probe’s electrical resistance. If the probe corrodes, its
resistance increases; the increase in resistance is measured by the
instrument as a rate of corrosion in mils per year (mpy) [mm/year]. If the
structure is adequately protected, the corrosion rate will be zero.
Coupon test stations are used mainly on pipelines. They should be installed
so that the foot of the test station (containing the coupons) is within the
pipeline backfill and at the depth of the bottom of the pipe. The tube of the
test station should be about 6 inches (15 cm) from the side of the pipe to
prevent shielding of cathodic protection current from the pipe. Backfill
around the pipe carefully and compact the backfill thoroughly.
To provide a low resistance path up to a reference electrode that may be
inserted in the tube, a low resistivity backfill is used within the tube. Often
this is a mixture of sand and bentonite.
Figure 8.11 shows a coupon station.
Coupon
Element
Current
Interrupt
Switch
Reference Cell
Shunt
Pipe Lead
Pipeline
Reference
Tube
Figure 8.11 Typical Coupon Test Station