Nace CP 1, Course Manual

Page: 432 / 535

RP0196-2004
2
NACE International
Driving Voltage: The potential difference between the
galvanic anodes and the tank wall when the CP system is in
operation.
Electrode: A conductor used to establish contact with an
electrolyte and through which current is transferred to or
from an electrolyte.
Electrode Potential: The potential of an electrode in an
electrolyte as measured against a reference electrode. (The
electrode potential does not include any resistance losses
in potential in either the electrolyte or the external circuit. It
represents the reversible work to move a unit of charge
from the electrode surface through the electrolyte to the
reference electrode.)
Electrolyte: A chemical substance containing ions that
migrate in an electric field. For the purpose of this standard,
electrolyte refers to the water, including the dissolved
chemicals, in the tank.
Galvanic Anode: A metal that provides sacrificial
protection to another metal that is more noble when
electrically coupled in an electrolyte. This type of anode is
the electron source in one type of CP.
Holiday: A discontinuity in a protective coating that
exposes unprotected surface to the environment. For the
purpose of this standard, it is a bare spot in a coating that
exposes tank internal metal surface to the stored water.
Impressed Current: An electric current supplied by a
device employing a power source that is external to the
electrode system. (An example is direct current for CP.) IR Drop: The voltage across a resistance in accordance
with Ohm's law.
Polarization: The change from the open-circuit potential as
a result of current across the electrode/electrolyte interface.
Polarized Potential: The potential across the
structure/electrolyte interface that is the sum of the
corrosion potential and the cathodic polarization.
Reference Electrode: An electrode whose open-circuit
potential is constant under similar conditions of
measurement, which is used for measuring the relative
potentials of other electrodes.
Resistivity: (1) The resistance per unit length of a
substance with uniform cross section. (2) A measure of the
ability of an electrolyte (e.g., soil) to resist the flow of electric
charge (e.g., CP current). Resistivity data are used to
design a groundbed for a CP system. For the purpose of
this standard, resistivity is a measure of the ability of the
water to resist the flow of current. Water resistivity is used
in the design of the galvanic anode CP system.
Shielding: (1) Protecting; protective cover against
mechanical damage. (2) Preventing or diverting CP
protection current from its natural path.
Tank-to-Water Potential: The voltage difference between
a submerged metallic portion of the tank and the water
(electrolyte), which is measured with a reference electrode
in contact with the water.
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Section 3: Determination of Need for CP
3.1 Introduction
3.1.1 Steel tank surfaces submerged in natural waters
are subject to corrosion. The methods and procedures
used to control corrosion should be governed by the
rate of corrosion and the cost of maintaining the tank,
including the water quality and planned useful life of the
tank.
3.1.2 Corrosion control by use of coatings alone is
possible only if the coating is continuously maintained
to provide complete coverage of the surface to be
protected. All coatings are subject to damage and
deterioration. CP may not be effective under
disbonded coating.
3.1.3 CP is effective in controlling corrosion only on
submerged metal surfaces during submersion. It does
not reverse structural damage already caused by
corrosion.
3.2 Coated Steel Tanks
3.2.1 CP should be used to prevent corrosion on
submerged, coated steel tank surfaces where holidays
exist—or are anticipated—and where corrosion would
occur as a result of those holidays.
3.2.2 If the water is sufficiently corrosive to justify the
use of a coating, then CP is also justified and provides
a greater degree of protection than when either method
is used alone.
3.2.3 Properly designed and maintained CP systems
extend the useful life of those coatings that are
compatible with CP.
3.2.4 The current required for CP is much lower for
coated steel tanks compared with bare or poorly
coated steel tanks.