Nace CP 1, Course Manual

Page: 306 / 535

RP0285-2002
2 NACE International
Electrolyte: A chemical substance containing ions that
migrate in an electric field. For the purposes of this
standard, electrolyte refers to the soil or liquid adjacent to
and in contact with a buried or submerged metallic UST
system, including the moisture and other chemicals
contained therein.
Electrolytic Corrosion: Corrosion caused by an external
source of direct current.
Foreign Structure: Any metallic structure that is not
intended as part of a system under CP.
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.
Galvanic Series: A list of metals and alloys arranged
according to their corrosion potentials in a given
environment.
Groundbed: One or more anodes installed below the
earth’s surface for the purpose of supplying CP.
Holiday: A discontinuity in a protective coating that
exposes unprotected surface to the environment.
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.)
Insulating Coating System: All components comprising
the protective coating, the sum of which provides effective
electrical isolation of the coated structure.
Interference Bond: A metallic connection designed to
control electrical current interchange between metallic
systems.
IR Drop: The voltage across a resistance in accordance
with Ohm’s Law.
Isolation: See Electrical Isolation.
Net Driving Potential: The difference between the
cathode potential and the anode potential in a galvanic
circuit.
Polarization: The change from the open-circuit potential
as a result of current across the electrode/electrolyte
interface. In this standard, polarization is considered to be
the change of potential of a metal surface resulting from
the passage of current directly to or from an electrode.
Polarized Potential: The potential across the
structure/electrolyte interface that is the sum of the
corrosion potential and the cathodic polarization.
Potential Gradient: A change in the potential with
respect to distance, expressed in millivolts per unit of
distance.
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.
Stray Current: Current through paths other than the
intended circuit.
Stray-Current Corrosion: Corrosion resulting from
current through paths other than the intended circuit, e.g.,
by any extraneous current in the earth.
Structure-to-Electrolyte Potential (also structure-to-soil
potential
or pipe-to-soil potential): The potential
difference between a buried metallic structure and the
electrolyte that is measured with a reference electrode in
contact with the electrolyte.
Structure-to-Structure Potential: The difference in vol-
tage between metallic structures in a common electrolyte.
Tank-to-Soil Potential (also tank-to-electrolyte potential):
The potential difference between the tank metallic surface
and the electrolyte that is measured with reference to an
electrode in contact with the electrolyte.
Underground Storage Tank (UST) System: The
equipment and facility constructed, maintained, or used
for underground storage of products including tanks,
piping, pumps, and appurtenances associated with filling,
storage, and dispensing of the stored products.
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Section 3: Cathodic Protection of New UST Systems
3.1 General
3.1.1 There are three basic types of CP available for
new UST systems:
(a) Factory-fabricated galvanic anode systems.
(b) Field-installed galvanic anode systems.
(c) Field-installed impressed current systems.
3.1.2 The recommended practices with respect to
field-installed systems are similar to those for existing
UST systems described in Section 4.