RP0169-2002
NACE International 13
any purpose, the pipe should be inspected for evidence
of corrosion and/or coating condition.
6.1.3 The criteria in this section have been developed
through laboratory experiments and/or verified by
evaluating data obtained from successfully operated
cathodic protection systems. Situations in which a
single criterion for evaluating the effectiveness of
cathodic protection may not be satisfactory for all
conditions may exist. Often a combination of criteria is
needed for a single structure.
6.1.4 Sound engineering practices shall be used to
determine the methods and frequency of testing
required to satisfy these criteria.
6.1.5 Corrosion leak history is valuable in assessing
the effectiveness of cathodic protection. Corrosion leak
history by itself, however, shall not be used to
determine whether adequate levels of cathodic
protection have been achieved unless it is impractical
to make electrical surveys.
6.2 Criteria
6.2.1 It is not intended that persons responsible for
external corrosion control be limited to the criteria listed
below. Criteria that have been successfully applied on
existing piping systems can continue to be used on
those piping systems. Any other criteria used must
achieve corrosion control comparable to that attained
with the criteria herein.
6.2.2 Steel and Cast Iron Piping
6.2.2.1 External corrosion control can be
achieved at various levels of cathodic polarization
depending on the environmental conditions.
However, in the absence of specific data that
demonstrate that adequate cathodic protection
has been achieved, one or more of the following
shall apply:
6.2.2.1.1 A negative (cathodic) potential of at
least 850 mV with the cathodic protection
applied. This potential is measured with
respect to a saturated copper/copper sulfate
reference electrode contacting the electrolyte.
Voltage drops other than those across the
structure-to-electrolyte boundary must be
considered for valid interpretation of this
voltage measurement.
NOTE: Consideration is understood to mean
the application of sound engineering practice
in determining the significance of voltage
drops by methods such as:
6.2.2.1.1.1 Measuring or calculating
the voltage drop(s);
6.2.2.1.1.2 Reviewing the historical
performance of the cathodic protection
system;
6.2.2.1.1.3 Evaluating the physical and
electrical characteristics of the pipe and
its environment; and
6.2.2.1.1.4 Determining whether or not
there is physical evidence of corrosion.
6.2.2.1.2 A negative polarized potential (see
definition in Section 2) of at least 850 mV
relative to a saturated copper/copper sulfate
reference electrode.
6.2.2.1.3 A minimum of 100 mV of cathodic
polarization between the structure surface
and a stable reference electrode contacting
the electrolyte. The formation or decay of
polarization can be measured to satisfy this
criterion.
6.2.2.2 Special Conditions
6.2.2.2.1 On bare or ineffectively coated
pipelines when long-line corrosion activity is of
primary concern, the measurement of a net
protective current at predetermined current
discharge points from the electrolyte to the
pipe surface, as measured by an earth current
technique, may be sufficient.
6.2.2.2.2 In some situations, such as the
presence of sulfides, bacteria, elevated
temperatures, acid environments, and
dissimilar metals, the criteria in Paragraph
6.2.2.1 may not be sufficient.
6.2.2.2.3 When a pipeline is encased in
concrete or buried in dry or aerated high-
resistivity soil, values less negative than the
criteria listed in Paragraph 6.2.2.1 may be
sufficient.
6.2.2.3 PRECAUTIONARY NOTES
6.2.2.3.1 The earth current technique is often
meaningless in multiple pipe rights-of-way, in
high-resistivity surface soil, for deeply buried
pipe, in stray-current areas, or where local
corrosion cell action predominates.
6.2.2.3.2 Caution is advised against using
polarized potentials less negative than -850
mV for cathodic protection of pipelines when
operating pressures and conditions are
conducive to stress corrosion cracking (see
references on stress corrosion cracking at the
end of this section).
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