Nace CP 1, Course Manual

Page: 522 / 535

TM0101-2001
NACE International 11
protection with physical evidence from the tank to
determine whether corrosion has occurred.
8.6.2 Physical evidence of corrosion is determined by
evaluating items such as leak history data or buried
tank inspection report data regarding locations of
coating failures, localized conditions of more-corrosive
electrolyte, or whether substandard cathodic protection
levels have been experienced.
8.6.3 Cathodic protection shall be judged adequate at
the test site if:
(a) All valid tank-to-electrolyte potential measure-
ments are negative 850 mV, or more negative, with
respect to a CSE; and
(b) The significance of voltage drops has been
considered by applying the principles described in
Paragraph 8.6.1 and Paragraph 8.6.2.
8.7 Monitoring
8.7.1 When the significance of a voltage drop has
been considered at the test site, the measured
potentials
may be used for monitoring unless
significant environmental, structural, coating integrity,
or
cathodic protection system parameters have
changed.
________________________________________________________________________
Section 9: Test Method 2—Negative 850-mV Polarized Tank-to-Electrolyte Potential of Steel Tanks
9.1 This section describes a method that uses an
interrupter(s) to eliminate the cathodic protection system
voltage
drop from the tank-to-electrolyte potential
measurement for comparison with the criterion stated in
NACE Standard RP0285:
1
“A negative polarized potential of at
least 850 mV relative to a saturated
copper/copper
sulfate reference
electrode.”
If direct-connected galvanic anodes that cannot be
interrupted are present, this method is not applicable.
9.2 General
9.2.1 Interrupting the known cathodic protection
current source(s) eliminates voltage drops associated
with
the protective currents being interrupted.
However, significant voltage drops may also occur due
to currents from other sources.
9.2.2 Current sources that can affect the accuracy of
this test method include the following:
(a) Unknown, inaccessible, or direct-connected
galvanic anodes;
(b) Other cathodic protection systems on associated
piping 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; and
(g) Unintentional connections to other structures or
bonds to mitigate interference.
9.2.3 To avoid significant depolarization of the tank,
the “off” period should be limited to the time necessary
to make an accurate potential measurement. The “off”
period is typically less than three seconds long.
9.3 Comparison with Other Methods
9.3.1 Advantages
(a) Voltage drops associated with the protective
currents being interrupted are eliminated.
9.3.2 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
stray currents are present or foreign impressed current
devices are present and cannot be interrupted.
9.4 Basic Test Equipment
9.4.1 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 per volt may be
adequate in certain circumstances in which the circuit
resistance is low.
A potentiometer circuit may be
necessary in other instances.
9.4.2 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.
9.4.3 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.
9.4.4 Sufficient and adequate means to interrupt
cathodic protection current sources simultaneously
such as sacrificial anodes, rectifiers, and electrical
bonds that are influencing the tank.