Nace CP 1, Course Manual

Page: 437 / 535

RP0196-2004
NACE International 7
(IR) drops other than those across the tank/water
boundary, the presence of dissimilar metals, structural
components, and other conditions that may interfere
with valid interpretation of potential measurements.
6.4.2 In order to observe the polarized tank-to-water
potential, the IR drop shall be eliminated from the
measurement. Among the methods used to eliminate
IR drop are (1) measuring the tank-to-water potential
with the protective current instantaneously interrupted
and (2) interrupting the electrical connection between
the tank and a submerged coupon(s) and immediately
measuring the tank-to-water potential of the coupon.
6.4.3 A sufficient number of potential measurements
shall be taken to determine that adequate protection is
being achieved throughout the steel tank, especially at
the most difficult-to-protect locations. In practice,
potential profiles of the submerged steel at intervals
along the wall and floor during initial adjustment shall
be taken.
6.5 Alternative Reference Electrodes
6.5.1 While CSE is the standard reference electrode
for use in water storage tanks, other reference
electrodes that may be substituted for CSE are listed
below along with their potential equivalent to -850 mV
vs. a CSE.
(a)
vs. Zn: +250 mV, and
(b)
vs. saturated silver/silver chloride electrode: -735
mV.
Regular calibration of reference electrodes is
recommended.
6.6 Special Considerations
6.6.1 Abnormal conditions in which protection is
ineffective or only partially effective sometimes exist.
Such conditions include elevated temperatures,
disbonded coatings, shielding, bacterial attack, and
unusual contaminants in the water.
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Section 7: Operation and Maintenance
7.1 This section covers the procedures necessary for the
operation and maintenance of an effective galvanic anode
CP system.
7.2 Operation
7.2.1 After the system has been allowed to reach a
stable polarization level, the system shall be tested
and adjusted to obtain the potentials indicated in
Paragraph 6.3.
7.2.2 For a galvanic anode CP system to control
corrosion effectively, it must be kept in continuous
operation and provide the desired tank-to-water
potential.
7.3 Maintenance
7.3.1 Maintenance should be conducted under the
direction of qualified personnel as described in
Paragraph 1.8 of this standard.
7.3.2 Annual Testing
7.3.2.1 Under normal conditions, testing of
galvanic anode systems shall be performed
annually. This frequency could vary depending on
changes in parameters listed in Paragraph 4.2.1.
7.3.2.2 Tests shall be conducted to determine
whether the criteria set forth in Paragraph 6.3 are
being achieved. The potential and current output
shall be monitored and the circuit resistance
adjusted so that the criteria in Paragraph 6.3 are
achieved.
7.3.2.3 Reference Electrodes
7.3.2.3.1 With current off, stationary
reference electrodes installed in the tank
should be compared with a recently calibrated
portable electrode of identical composition to
ensure proper levels of protection. If it is not
possible to interrupt the current, the stationary
reference electrode shall be removed from the
tank and tested in a nonmetallic container.
The two reference electrodes should have a
potential difference of less than ±10 mV.
When the potential difference exceeds ±20
mV, the stationary electrode shall be
replaced.
7.3.2.3.2 Placement of the stationary
reference electrode in the tank shall be
evaluated to determine whether it
corresponds to the most representative of
portable reference electrode potentials of the
tank. Readings made with a portable
reference electrode moved to several different
locations shall be compared with readings
from the stationary reference electrode to
ensure the stationary reference electrode
values are representative of the entire tank.
7.3.2.3.3
For installations in which a
stationary reference electrode is not installed
or is not functioning, a calibrated portable