Nace CP 1, Руководство по эксплуатации

Страница: 368 / 535

RP0388-2001
2 NACE International
Coating: A liquid, liquefiable, or mastic composition that,
after application to a surface, is converted into a solid
protective, decorative, or functional adherent film.
Conductivity: A measure of the ability of a material to
carry an electric current. In water, this depends on the total
concentration of the ionized substances dissolved and the
temperature at which the measurement is made. It is the
reciprocal of resistivity and is usually expressed in µS/cm
(µmhos/cm).
Corrosion: The deterioration of a material, usually a metal,
that results from a reaction with its environment.
Current Density: The current to or from a unit area of an
electrode surface.
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 purposes 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 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.)
Impressed Current Anode: An anode, usually composed
of substantially inert materials, that is supplied with
impressed current.
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.
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: A measure of the specific resistance of a
material to the passage of electric current. It is usually
expressed in ohm-centimeters (ohm-cm) and is the
reciprocal of conductivity.
Tank-to-Water Potential: The voltage difference between
a submerged metallic portion of the tank and the electrolyte
(water), which is measured with a reference electrode in
contact with the electrolyte.
________________________________________________________________________
Section 3: Determination of the Need for Cathodic Protection
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 hydraulic and aesthetic effects of
corrosion debris in the tank and piping.
3.1.2 All coatings are subject to damage and
deterioration. Therefore, corrosion control by use of
coatings alone on the submerged surfaces of a steel
water storage tank is usually not possible.
3.1.3 Cathodic protection is effective in controlling
corrosion only on the submerged metal surfaces.
3.2 Cathodic Protection of Coated Tanks
3.2.1. In almost all cases, natural waters are sufficiently
corrosive to require the use of protective coatings and
cathodic protection for corrosion control. Protective
coatings and cathodic protection are synergistic with
the combination of the two methods, providing a
greater degree of corrosion protection than either
method used alone.
3.2.2. Properly designed and maintained, cathodic
protection systems can extend the useful life of the
water tank and its coating system.
3.2.3 The current required for cathodic protection is
lowered significantly when coatings are also used.
3.2.4 The coating system should be compatible with
cathodic protection.