Nace CP 1, Course Manual

Page: 329 / 535

RP0176-2003
NACE International
7
4.4.1.5 Ag/AgCl (saturated KCl): -0.76 V (or more
negative for protection).
4.5 Methods of Measuring and Evaluating CP
4.5.1 The most common method of evaluating the CP
level is to measure electrical potential between the
structure and a suitable reference electrode.
4.5.1.1 Potential measurements should be made
with the reference electrode located in the water
as close as practicable to the structure to minimize
included voltage drops. Measurements taken with
the electrode close to a structure member but dis-
tant from anodes and in areas of greatest shield-
ing should be given major consideration in evalu-
ating the protective level of a structure.
4.5.1.2 The most frequently used method of mea-
suring potentials involves suspending the refer-
ence electrode freely in the water from a design-
nated location on the structure. The electrode is
lowered to a specified series of depths for read-
ings, and the process is repeated at other appro-
priate points on the structure. The location of the
electrode may not be known because of drift
resulting from water currents. This method is use-
ful for determining the general condition of the CP
system, but may not define problem areas on mar-
ginally protected structures.
4.5.1.3 The reference electrode may be carried by
a diver or a remotely operated vehicle (ROV). This
method provides excellent knowledge of electrode
location and can result in a potential survey in any
desired degree of detail. Diver safety must be
considered during evaluation of impressed current
protected structures. This may involve limiting the
output from or de-energizing at least a part of the
system for the duration of the measurements. If
the system is de-energized, the effect of the re-
duction on the level of protection should be con-
sidered during the evaluation of the potential mea-
surements.
4.5.1.4 The reference electrode may be run down
a guide wire for better control of its location. The
guide wire may be permanently installed on the
structure, or it may be temporarily installed with the
aid of a heavy weight to anchor it at the bottom. If
temporarily installed, the wire (if metallic) should
be electrically isolated from the structure. The
guided electrode is capable of providing more ac-
curate potential information than the freely sus-
pended electrode, depending on the proximity of
the guide wire to the members of the structure.
4.5.1.5 A number of permanent reference elec-
trodes may be mounted on a structure. While the
exact electrode locations are known, the informa-
tion obtained from these electrodes is limited to
the adjacent structure surfaces.
Although this limitation holds true for any potential
measurement, this method can provide a repro-
ducible basis for comparing potentials at different
times. The accuracy of permanent electrodes
should be periodically checked against another
electrode. Dual reference electrodes that combine
zinc and silver-silver chloride references into a
single installed permanent unit also help to detect/
reduce malfunctions.
4.5.1.6 In addition to reference electrodes, some
structures have been equipped with permanent
monitors to measure current density and current
output from representative galvanic anodes.
These devices are particularly useful when dealing
with new structure designs, or new environments
in which precise CP design criteria are not avail-
able. These devices typically use calibrated
shunts to arrive at the current output or current
density value; signals are usually transmitted top-
side using hard-wired connections.
4.5.2 In addition to potential surveys, current density
surveys can produce valuable data. Current density
surveys utilize specially designed reference electrode
arrays that measure the voltage gradient in the sea-
water around the structure. Although these surveys do
not determine the level of protection present on the
structure, they may be useful in determining current
distribution and predicting remaining anode life.
4.5.3 Visual inspection is sometimes used to gain de-
tailed knowledge not attainable by other means.
4.5.3.1 Divers may be employed to inspect visu-
ally or by feel if turbidity or light conditions so dic-
tate.
4.5.3.2 Visual inspection using suitable instru-
ments may include physical measurements such
as the crack length and depth, wall thickness, or
pit depth, as well as anode size and condition.
4.5.3.3 Underwater photography may be used to
provide a permanent record of conditions.
4.5.3.4 Underwater television can provide a run-
ning topside view if decisions must be made while
underwater work is in progress. Videotapes can
provide a permanent record of the underwater
conditions.
4.5.4 Steel coupons with metallurgy similar to that of
the structure can be placed on a structure for later re-
moval to determine the effectiveness of corrosion con-
trol measures. Coupons might be especially useful in
areas suspected of being comparatively inaccessible to
CP current.