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TM0101-2001

NACE International

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

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 test
method, electrolyte refers to the soil or liquid, including
contained moisture and other chemicals, next to and in
contact with a buried or submerged metallic tank system.)

Foreign Structure:

Any metallic structure that is not

intended as part of a system under cathodic protection.

Free Corrosion Potential: See

Corrosion Potential.

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 cathodic protection.

Groundbed:

One or more anodes installed below the

earth’s surface for the purpose of supplying cathodic
protection.

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 cathodic
protection.)

“Instant-Off” Potential:

A measurement of a tank-to-

electrolyte

potential

made

without

perceptible

delay

following the interruption of cathodic protection.

Interference:

Any electrical disturbance on a metallic

structure as a result of stray current.

Isolation: See

Electrical Isolation.

“Off” or “On”:

A condition whereby cathodic protection

current is either turned off or on.

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.

Potential Gradient: A change in the potential with respect
to distance, expressed in mV per unit of distance.

Potentiometer:

A device for the measurement of an

electromotive force by comparison with a known potential
difference.

Protection Potential: A measured potential meeting the
requirements of a cathodic protection criterion.

Reference Electrode:

An electrode whose open-circuit

potential

constant

under

similar

conditions

measurement, which is used for measuring the relative
potentials of other electrodes.

Remote Earth: A location of the earth far enough from the
affected

structure

that

the

soil

potential

gradients

associated with currents entering the earth from the affected
structure are insignificant.

Resistance to Electrolyte: The resistance of a structure to
the surrounding electrolyte.

Reverse-Current Switch:

A device that prevents the

reversal of direct current through a metallic conductor.

Shielding: Preventing or diverting the cathodic protection
current from its intended path to the structure to be
protected.

Sound Engineering Practices:

Reasoning exhibited or

based on thorough knowledge and experience, logically
valid,

and

having

technically

correct

premises

that

demonstrate good judgment or sense in the application of
science.

Stray Current:

Current through paths other than the

intended circuit.

Structure: Tanks, piping, and associated equipment that
may/may not be under the influence of a cathodic protection
system.

Tank 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.

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. This measurement is commonly called tank-to-
soil (T/S).

Tank-to-Soil

Potential:

See

Tank-to-Electrolyte

Potential.

Telluric Current: A current flowing through the ground due
to natural causes, such as the earth’s magnetic field or
auroral activity.