Nace CP 1, Course Manual

Page: 502 / 535

TM0497-2002
22 NACE International
in 0.3-m (1-ft) increments for about 1.5 to 3.0 m (5 to
10 ft) laterally to the pipe and through the “peak.” The
maximum potential will occur a few feet to the side of
the pipe if the peak is due to a galvanic anode.
Moreover, if the pipe location is known with certainty
and a galvanic anode is present, the potentials will be a
minimum over or to the side of the pipe opposite where
the maximum occurs.
The closer the transverse
measurements are to the anode, the more the location
of the minimum will be shifted away from the side of the
pipe opposite the location of the maximum.
B5.5 Stray current flowing to a pipe from sources such
as foreign rectifiers and electrified railroads cause the
pipe at that location to have more-negative potential
and may be misinterpreted as an anodic condition.
Stray current discharging from a pipe can cause a less-
negative potential and be misinterpreted as a cathodic
condition.
B6 Cathodic Protection Using the Net Protective Current
Technique
B6.1 Cathodic protection should be applied to the
anodic area(s).
B6.2 It is necessary to wait until polarization has
stabilized before making a detailed evaluation of the
net current protective level. Polarization of bare pipe
may require a relatively long time ranging up to several
months.
B6.3 When an impressed current source is used, the
side drain potential (potential gradient lateral to the
pipe longitudinal direction) should be measured at the
predetermined anodic condition with the protective
current applied. Relative to the reading directly over
the pipe, a higher (more-negative) reading with the
reference electrode lateral to the pipe indicates that
current is being conducted to the pipe at this point.
The amount of current flow indicated by this method
may not be enough to control small local corrosion
cells.
B6.4 Galvanic anodes are usually installed at or near
the location of the anodic areas. Caution shall be used
when interpreting the results of pipe-to-electrolyte
potential measurements made close to an anode.
B6.5 Monitoring of cathodic protection can be
simplified by establishing test points and recording the
pipe-to-electrolyte potential exhibited when the side
drain measurements indicate a net current flow to the
pipe. These potentials may then be used to monitor
the level of cathodic protection.