scope:
General
A CP coupon may be used to determine the level of CP of a buried
or submerged metallic structure. CP coupons are installed in the
electrolyte near the structure and are then connected to it through
a test station. This allows the CP coupon to be connected to the CP
system on the structure, thus simulating a similar-sized bare area
of the structure's surface, such as at a holiday in the coating.
The CP coupon may be disconnected from the circuit during periodic
testing, and its instant-disconnect potential measured. The
potential of the CP coupon may then continue to be monitored and
the depolarization calculated. These measurements represent the
polarized and depolarized potentials of the structure in the
vicinity of the CP coupon. They also allow the IR drop in the
electrolyte to be calculated for use in conventional potential
measurements made from grade level. A second, freely corroding
(native) coupon may be installed at the same location as the CP
coupon to measure the free-corrosion potential of the structure in
open-circuit conditions.
NACE Standard RP01691 includes criteria for determining the CP
status of a buried or submerged structure. For voltage measurements
that are made when CP current is applied, IR drops other than those
across the structure-to-electrolyte boundary must be considered.
NACE Standard RP0169 includes a number of ways this may be done and
NACE Standard TM04972 includes a number of test methods used for
these criteria. CP coupons may also be used to evaluate compliance
with CP criteria, including considering the IR drop. The practices
described in this standard must be followed with careful evaluation
of the specific situation in which the coupons are to be used.
CP coupons have several advantages. Structure-toelectrolyte
potentials that have the IR drop considerably reduced or eliminated
may be obtained without interrupting multiple CP sources. CP
coupons may also be used on buried structures with direct-connected
galvanic anodes, which must not be interrupted. Using CP coupons,
depolarization testing may be performed in most cases without
de-energizing the CP system. An additional advantage is the ability
to record accurate structure-toelectrolyte potentials on structures
affected by stray currents.
When CP coupons are used, there may be differences between
polarized potentials of the CP coupon and the structure. This is
because the polarized structure-toelectrolyte potential measured at
grade is usually the combined potential of the structure over a
rather large area, including holidays in the coating and locations
where the electrolyte or other conditions that affect the potential
of a structure may vary. Errors caused by these variations are
included in a potential measured at any given point along a
structure and may be significant. These errors generally do not
occur with coupons because of their small size and uniform
conditions. Coupons located in areas where these variables are
different can provide a good representation of the CP effectiveness
on a structure.
A typical problem in measuring a structure-toelectrolyte
potential is the effect of IR drops from uninterruptible current
sources. By design, CP coupons may be disconnected from the
structure and CP system, thereby eliminating the IR drop
attributable to these current sources. Even when all current
sources have been interrupted, long-line currents can still affect
the structure-toelectrolyte potential readings measured at grade on
a pipeline. Because the effective reference point of a CP coupon is
very close to the CP coupon surface, IR drops caused by long-line
currents are minimized.
