Nace CP 1, Course Manual

Page: 527 / 535

TM0101-2001
16 NACE International
10.3.2 General
10.3.2.1 Steel tanks may be adequately protected
if, from the “off” potential, applying cathodic
protection causes a polarized potential that is at
least 100 mV more negative.
10.3.2.2 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.
10.3.3 Comparison with Other Methods
10.3.3.1 Advantages
(a) This method is especially useful for a bare or
ineffectively coated tank; and
(b) This method is advantageous if corrosion
potentials could be low (for example, 500 mV or
less negative) or the current required to meet a
negative 850-mV potential criterion would be
considered excessive.
10.3.3.2 Disadvantages
(a) Additional equipment is required;
(b) Additional time may be required to set up
equipment and to make the tank-to-electrolyte
potential measurements; and
(c) Test results are difficult or impossible to
analyze if foreign impressed currents are present
and cannot be interrupted or if stray currents are
present.
10.3.4 Basic Test Equipment
10.3.4.1 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.
10.3.4.2 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.
10.3.4.3 Sufficient and adequate means to
interrupt cathodic protection current sources such
as sacrificial anodes, rectifiers, and electrical
bonds that are influencing the tank simultaneously.
10.3.4.4 A CSE or other standard reference
electrode may be used. Reference electrodes that
can be substituted for the CSE are described in
Paragraph 5.5.
10.3.5 The following procedure shall be used when this
test is conducted:
10.3.5.1 Before the test, verify that cathodic
protection equipment has been installed but is not
operating.
10.3.5.2 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.
10.3.5.3 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).
10.3.5.4 Record the location of the electrode to
allow it to be returned to the same location for
subsequent tests.
10.3.5.5 Connect the voltmeter to the tank and
reference electrode as described in Paragraph
5.6.
10.3.5.6 Measure and record the tank-to-
electrolyte potential and its polarity with respect to
the reference electrode.
10.3.5.6.1 This potential shall be the value
from which the polarization formation is
calculated.
10.3.5.7 Apply the cathodic protection current.
Sufficient time should be allowed for the tank
potentials to reach polarized values.