UFC 3-570-06
JANUARY 31 2003
relatively high, or the location or spacing of the anodes makes it impossible to measure
the structure potential remote from the anodes, other criteria should be used or
excavations made to properly place the reference electrode to obtain a valid potential
measurement. For very small and well-coated structures (such as valves, elbows, and
tie downs), use the 100 mV polarization criterion. For all sacrificial systems, if the
sacrificial system is designed to allow interrupting the current from all anodes
simultaneously, the -100 mV polarization criterion could be used. The -0.85 instant-
OFF criterion is usually not attainable in most soil conditions with sacrificial anodes,
unless the native potential of the structure is very high and/or the soil resistivity is very
low. The -0.85V instant-OFF criterion should not be used for sacrificial CP systems
except in rare cases; use the 100 mV shift criterion or the -0.85 ON criterion
(considering IR).
7-3.1.2
Impressed Current Cathodic Protection System. The first consideration
for determining the criteria to use with impressed current CP systems is the type of
anode bed used.
For distributed anode impressed current systems, the -0.85 instant-OFF or
the 100 mV polarization criterion should be used; the -0.85 ON criterion
should not be used. For structures with a high dielectric strength coating,
the -0.85 instant-OFF criterion may be the easiest to use, although the
100 mV polarization criterion can be used. For structures which are bare,
poorly coated, or have a deteriorated coating, the 100 mV polarization
criterion should be used.
For remote anode impressed current systems, all criteria, or a mixture of
criteria may be used. If the anodes are truly remote from the test point,
the electrolyte resistivity is low, the dielectric strength of the coating is
high, and the circuit resistance of the CP system is low, the -0.85 ON
criterion (considering IR error) is sufficient. Accounting for voltage drops
other than those across the structure-to-electrolyte boundary is usually
accomplished by placing the reference electrode as near to the structure
as possible (directly over the pipeline or tank). This, combined with
knowledge of the dielectric strength or the structure coating, size of the
structure, the electrolyte resistivity, the distance and voltage at the
anodes, the rectifier output voltage, and the rectifier output current, is
usually enough to determine the adequacy of the CP applied to the
structure. If in doubt, or when potential readings are questionable, test the
location using another criterion, or excavate to locate a temporary
reference electrode (or install a permanent reference electrode) as close
as practical to the structure to further minimize any possible IR error. For
structures with a high dielectric strength coating, regardless of the
electrolyte resistivity, distance from the anodes, or the CP system circuit
resistance, the -0.85 instant-OFF criterion may be the easiest to use,
7-8
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