Nace CP 1, Course Manual

Page: 518 / 535

TM0101-2001
NACE International 7
5.8 The tank-to-electrolyte potential measurement as
described above is a result of the voltage drop created by
current flowing through the electrical resistances of the
items listed in Paragraph 5.7. For a coated tank, coating
deterioration should be considered.
5.9 All readings shall be taken with reference electrodes
that are in contact with the electrolyte. Readings shall not
be taken through concrete or asphalt. Soil contact may be
established through at-grade openings by drilling a small
hole in the concrete or asphalt, or by contacting a seam of
soil between concrete and asphalt.
5.10 The following conditions should be taken into
consideration when tank-to-electrolyte potential
measurements are made to determine the level of cathodic
protection at the test site:
(a) Effectiveness of coatings, particularly those known or
suspected to be deteriorated or damaged;
(b) Bare sections of tank structure being protected;
(c) Bonds to mitigate interference;
(d) Parallel coated tanks, electrically connected and
polarized to different potentials;
(e) Shielding;
(f)
Effects of other structures on the measurements;
(g) History of corrosion leaks and repairs;
(h) Location and depth of anodes;
(i)
Existence of isolation devices, including high-
resistance pipe connections and compression couplings;
(j)
Chemical composition of electrolytes, such as unusual
corrosives, chemical spills, presence of hydrocarbons in
soil, extreme soil resistivity changes, acidic waters, and
contamination from sewer spills;
(k) Possible sources of DC interference currents, such as
welding equipment, foreign rectifiers, mining equipment,
and electric railway or transit systems;
(l)
Contacts with other metals or structures;
(m) Areas of construction activity during the tank history;
(n) Underground metallic structures close to or crossing
the tank;
(o) Other appurtenances; and
(p) Electrolyte pH.
5.11 The effect of voltage drops other than those across
the tank-to-electrolyte interface shall be considered for valid
interpretation of tank-to-electrolyte potential measurements
made to satisfy a criterion. Measurement errors should be
minimized to ensure reliable tank-to-electrolyte potential
measurements.
5.12 The effect of voltage drops on a tank-to-electrolyte
potential measurement can be determined by interrupting
all significant current sources before taking the potential
measurement. This measurement must be taken without
delay after the interruption of current to avoid loss of
polarization. This measurement is referred to as an instant-
off potential, and is considered to be the “polarized
potential” of the tank at that location. This measurement
does not account for voltage drops across the tank-to-
electrolyte interface, which is part of the protection potential.
NOTE: The current interruption may cause a voltage spike.
This spike shall not be recorded as the instant-off potential.
The magnitude and duration of the voltage spike can vary;
however, the duration is usually within 0.5 second.
5.13 The following are examples of situations in which it
may not be practical to interrupt all current sources to make
the instant-off potential measurement:
5.13.1 Galvanic anodes are connected directly to the
tank without benefit of aboveground connections.
Interruption of this kind of system requires excavation
of the connections.
5.13.2 Interference from CP devices on foreign
structure or electrical continuity with foreign structure.
5.13.3 Manmade sources of DC stray currents, such
as other cathodic protection systems, mass transit, DC
welding, or mining operations are nearby.
5.14 If voltage drops have been evaluated at a test location
and the tank-to-electrolyte potential found to be satisfactory,
the “on” tank-to-electrolyte potential value may be used for
monitoring until significant environmental, structural, or
cathodic protection system parameters change.
5.14.1 Significant environmental, structural, or cathodic
protection system parameter changes may include:
(a) Replacement or addition of tank components and
systems;
(b) Addition, relocation, or deterioration of cathodic
protection systems;
(c) Failure of electrical isolating or bonding devices;
(d) Changes in the effectiveness of coatings;
(e) Influence of foreign structures; and
(f)
Modification of the environment.
5.15 After a cathodic protection system is operating, time
may be required for the tank to polarize. This should be
taken into consideration when measuring the potential at a
test site on a newly protected tank or after re-energizing a
cathodic protection device.