Nace CP 1, Руководство по эксплуатации

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TM0101-2001
NACE International 13
potential, the tank potential becomes less negative
by 100 mV or more.
10.2.2.2 Interrupting the known cathodic protection
source(s) eliminates voltage drops associated with
the protective current(s) being interrupted.
10.2.2.3 Other current sources that can affect the
accuracy of this test method include the following:
(a) Unknown, inaccessible, or direct-connected
galvanic anodes;
(b) Cathodic protection systems on associated
tank systems or foreign structures;
(c) Electric railway systems;
(d) Galvanic, or bimetallic, cells;
(e) DC mining equipment;
(f)
Adjacent tanks, electrically connected and
polarized to different potentials;
(g) Unintentional connections to other structures
or bonds to mitigate interference; and
(h) DC welding equipment.
10.2.3 Comparison with Other Methods
10.2.3.1 Advantages
(a) This method is especially useful for bare or
ineffectively coated tanks; and
(b) This method is advantageous in places where
corrosion potentials may be low (for example, 500
mV or less negative) or the current required to
meet a negative 850-mV polarized potential
criterion would be considered excessive.
10.2.3.2 Disadvantages
(a) Additional equipment is required;
(b) Additional time may be required to set up
equipment
and to make tank-to-electrolyte
potential measurements; and
(c) Test results are difficult or impossible to
analyze if foreign impressed current devices are
present and cannot be interrupted or if stray
currents are present.
10.2.4 Basic Test Equipment
10.2.4.1 A voltmeter with adequate input
impedance. Commonly used digital instruments
have a nominal impedance of 10 megohms. An
analog instrument with an internal resistance of
100,000 ohms/volt may be adequate in certain
circumstances in which the circuit resistance is
low. A potentiometer circuit may be necessary in
other instances.
10.2.4.1.1 Recording voltmeters can be useful
for recording polarization decay.
10.2.4.2 Meter leads with insulated wire and
terminal connections suitable for making reliable
electrical contact with the tank and reference
electrode. Color-coded meter leads are suggested
to avoid confusion of polarity of the measured
value.
10.2.4.3 Sufficient and adequate means to
interrupt cathodic protection current sources such
as sacrificial anodes, rectifiers, and electrical
bonds that are influencing the tank simultaneously.
10.2.4.4 A CSE or other standard reference
electrode may be used. Reference electrodes that
may be substituted for the CSE are described in
Paragraph 5.5.
10.2.5 The following procedure shall be used when this
test is conducted:
10.2.5.1 Before the test, verify that cathodic
protection equipment has been installed and is
operating properly.
Sufficient time should be
allowed for the tank potentials to reach polarized
values.
10.2.5.2 Provide means for current interruption in
all DC current sources influencing the tank at the
test site. The “off” interval should be kept as short
as possible but still long enough to read a
polarized tank-to-electrolyte potential after any
“spike” (see Figure 2a) has collapsed.
10.2.5.3 Determine the location of reference
electrode placement for potential measurements.
Selection of a site may be based on:
(a) Location accessible for future monitoring;
(b) Other protection systems, structures, and
anodes
that may influence the structure-to-
electrolyte potential;
(c) Electrical
midpoints between protective
devices;
(d) Known location of an ineffective coating if the
tank structure is coated; and
(e) Location of a known or suspected corrosive
environment.
10.2.5.4 Make electrical contact between the
reference electrode and the electrolyte at the test
site in a location that minimizes the voltage
gradient from other structures, and coating defects
(if the tank is coated).
10.2.5.5 Record the location of the electrode to
allow it to be returned to the same location for
subsequent tests.
10.2.5.6 Connect the voltmeter to the tank and
reference electrode as described in Paragraph
5.6.