Nace CP 1, Course Manual

Page: 444 / 535

RP0290-2000
2 NACE International
Sign conventions for potential and current density as well as
conventions for graphical presentation of data should be in
accordance with ASTM G 3.
8
2.2 Situations in which physical conditions effectively
preclude full implementation of sampling programs derived
from ASTM E 105 and ANSI/ASQC Z1.4 may exist.
Examples of these cases include asphalt or other
nonconductive overlays, conductive asphalt cathodic
protection systems, and situations in which access to the
cathodically protected surface is difficult and limited. In
these situations embedded reference electrodes should be
installed. Additionally, “most anodic” sites may be
constructed. Typically, this involves fully exposing a short
length of existing reinforcement. The excavation is then
patched with concrete containing an amount of chloride well
above that found in existing conditions. Attention should be
given to the location of “most anodic” sites to avoid making
the sites exceptionally favorable cathodic protection current
paths. A reference electrode may also be installed. It is
important that the reinforcing steel adjacent to the
embedded reference electrode be left in the original
concrete. If this is not done, the measured potentials will
not be representative of the reinforcing steel in undisturbed
concrete. If “most anodic” sites are to be the basis for
cathodic protection system performance judgment, the
number of sites constructed should reflect the complexity of
the structure.
2.3 NACE Work Group T-11-1a developed these criteria
through empirical evaluation of data obtained from
successfully operated impressed current cathodic protection
systems. NOTE: Persons using this standard should review
data made available after this standard’s publication to
determine whether more effective criteria have been
established. It is not intended that persons responsible for
corrosion control be limited to these criteria if it can be
demonstrated by other means that corrosion control has
been achieved. A combination of criteria may be used for a
single structure.
2.3.1 100-mV Polarization Development/Decay
2.3.1.1 A minimum of 100 mV of polarization
should be achieved at the most anodic location in
each 50 m
2
(500 ft
2
) area or zone, or at artificially
constructed anodic sites, in accordance with
Paragraphs 2.1 and 2.2, provided its corrosion
potential or decayed off-potential is more negative
than -200 mV
cse
(versus a copper/copper sulfate
reference electrode [CSE]). If the corrosion
potential or decayed off-potential is less negative
than -200 mV
cse
, then the steel is passivated and
no minimum polarization is required.
2.3.1.2 When the polarization decay method is
used, the decay should be determined by
interrupting the protective current and monitoring
the reinforcement’s potential measured relative to
a stable reference electrode. When the current is
interrupted an immediate voltage shift that is the
result of eliminating the IR drop occurs. This shift
should not be included in the polarization
measurement. The potential of the steel
immediately after the shift (instant-off potential
measured between 0.1 and 1.0 second after
interrupting the current flow) shall be used as the
initial value from which to measure polarization.
Figure 1 depicts a typical polarization decay curve.
The polarization equals the instant-off potential
subtracted from the reinforcement’s final
“depolarized potential.”
2.3.1.3 Polarization development should be
determined by measuring the potential
immediately before applying current to the
reinforcing steel and measuring the instant-off
potential at intervals of time during the polarization
of the steel reinforcement.
The polarization equals the initial reinforcement
potential (static potential) before applying current
subtracted from the reinforcement’s “final instant-
off potential.” Figure 2 depicts a typical
polarization development curve using “instant-off”
values only.