Nace CP 1, Course Manual

Page: 521 / 535

TM0101-2001
10 NACE International
(d) Determining whether there is physical evidence of
corrosion; and
(e) Recording measurements with reference electrode
placed electrically remote from the tank system, i.e.,
remote earth.
8.2 General
8.2.1 Test Method 1 measures the tank-to-electrolyte
potential as the sum of the polarized potential and any
voltage drops in the circuit.
These voltage drops
include those through the electrolyte and tank coating
from current sources such as impressed current and
galvanic anodes.
8.2.2 Cathodic protection current shall remain “on”
during the measurement process.
This potential is
commonly referred to as the “on” potential.
8.2.3 Because voltage drops other than those across
the tank-to-electrolyte interface may be included in this
measurement, these drops shall be considered, as
discussed in Paragraph 8.6.
8.3 Comparison with Other Methods
8.3.1 Advantages
(a) Requires minimal equipment, and
(b) Less time is required to make measurements.
8.3.2 Disadvantages
(a) The potential measured includes voltage drops
other
than those across the tank-to-electrolyte
interface;
(b) Meeting the requirements for considering the
significance of voltage drops (see Paragraph 8.6) can
result in a need for additional time to assess adequacy
of cathodic protection at the test site; 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.
8.4 Basic Test Equipment
8.4.1 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.
8.4.2 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 for the measured value.
8.4.3 A CSE or other standard reference electrode
may be used. Reference electrodes that may be used
in place of a CSE are described in Paragraph 5.5.
8.5 The following procedure shall be followed when this
test is conducted:
8.5.1 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.
8.5.2 Determine the location of reference electrode
placement for potential measurements. Selection of a
site may be based on:
(a) Accessibility 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.
8.5.3 Make electrical contact between the reference
electrode and the electrolyte at the test site, in a
location that minimizes the voltage gradient from
anodes, other structures, and coating defects (if the
tank is coated).
8.5.3.1 In general, the placement of the reference
electrode as close to the tank surface as possible
and as far from anodes as possible minimizes
many measurement errors associated with voltage
drops.
8.5.3.2 Alternatively, the reference electrode may
be placed at remote earth.
8.5.4 Record the location of the electrode to allow it to
be returned to the same location for subsequent tests.
8.5.5 Connect the voltmeter to the tank and reference
electrode as described in Paragraph 5.6.
8.5.5.1 Evaluate the effect of measurement circuit
resistance on the tank-to-electrolyte potential as
indicated in Paragraph 5.7.
8.5.6 Record the tank-to-electrolyte potential and its
polarity with respect to the reference electrode.
8.5.7 Record a sufficient number of measurements to
determine the level of cathodic protection over the
entire structure.
8.6 Evaluation of Data
8.6.1 The significance of voltage drops can be
considered by comparing historical levels of cathodic