Nace CP 1, Course Manual

Page: 520 / 535

TM0101-2001
NACE International 9
6.2.4 Concrete or asphalt-paved areas: Contact
resistance may be reduced by drilling through the
paving to permit electrode contact with the soil.
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Section 7: Voltage Drops Other Than Across the Tank-to-Electrolyte Interface
7.1 Voltage drops present when tank-to-electrolyte
potential measurements are made may occur in the
following:
7.1.1 Measurement Circuit
7.1.1.1 The voltage drop other than across the
tank-to-electrolyte interface in the measurement
circuit is the sum of the individual voltage drops
caused by the meter current flow through:
(a) Instrument test lead and connection
resistances;
(b) Reference electrode internal resistance;
(c) Reference
electrode-to-electrolyte contact
resistance;
(d) Coating resistance;
(e) Tank metallic resistance;
(f)
Electrolyte resistance;
(g) Analog meter internal resistance; and
(h) Digital meter internal impedance.
7.1.1.2 A measurement error occurs if the analog
meter internal resistance or the digital meter
internal impedance is not several orders of
magnitude higher than the sum of the other
resistances in the measurement circuit.
7.1.2 Electrolyte
7.1.2.1 When a tank-to-electrolyte potential is
measured with cathodic protection current applied,
the voltage drop in the electrolyte between the
reference electrode and the tank-to-electrolyte
interface shall be considered.
Measurements
taken close to sacrificial or impressed current
anodes can contain a large voltage drop.
7.1.2.2 Such a voltage drop can consist of, but is
not limited to, the following:
(a) A voltage drop caused by current flowing to
coating holidays when the tank structure is coated;
and
(b) A voltage drop caused by large voltage
gradients in the electrolyte that occur near
operating anodes (sometimes called “raised earth
effect”).
7.1.2.3 Testing to locate anodes by moving the
reference electrode along the tank may be
necessary when the locations are not known.
7.1.3 Coatings
7.1.3.1 Most coatings provide protection to the
tank by reducing contact between the tank surface
and the environment.
While the insulation
provided by a coating reduces the current required
for cathodic protection of a coated tank versus that
required for an uncoated tank, coatings are not
impervious to current flow.
7.1.3.2 Coatings resist current flow because of
their relative ionic impermeability.
Current flow
through the resistance of the coating causes the
voltage drop to be greater than that occurring
when
the tank is bare, under the same
environmental conditions.
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Section 8: Test Method 1—Negative 850-mV Tank-to-Electrolyte Potential of Steel Tanks with Cathodic
Protection Applied
8.1 This section describes the most commonly used test
method to satisfy the criterion stated in NACE Standard
RP0285:
1
“A negative (cathodic) potential of at
least
850 mV with the cathodic
protection applied. This potential is
measured with respect to a saturated
copper/copper
sulfate reference
electrode contacting the electrolyte.
Voltage drops other than those across
the
structure-to-electrolyte boundary
must be considered for valid
interpretation of this voltage
measurement.”
8.1.1 “Consideration” is understood to mean the
application
of sound engineering practice in
determining the significance of voltage drops by such
methods as:
(a) Measuring or calculating the voltage drop(s);
(b) Reviewing the historical performance of the
cathodic protection system;
(c) Evaluating
the physical and electrical
characteristics of the tank and its environment;