Nace CP 1, Course Manual

Page: 340 / 535

RP0176-2003
18
NACE International
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Section 8: Dielectric Shields
8.1 Introduction
8.1.1 This section recommends practices for the sel-
ection and use of dielectric shields, including shield
configurations and materials, for CP of structures.
8.2 General
8.2.1 The objective of dielectric shields, and coatings
used as dielectric shields, is to prevent extremely high
current densities and current wastage in the vicinity of
the anodes. This serves to promote more uniform pro-
tective current distribution.
8.3 Use of a Dielectric Shield
8.3.1 Use of a dielectric shield depends on the CP de-
sign (see Section 5). Typically, dielectric shields are
used in conjunction with an impressed current system
because higher voltage gradients are generally used in
such systems. A dielectric shield may also be used
with a high-potential galvanic anode such as magnes-
ium.
8.3.2 Specific uses of dielectric shields, including coat-
ings used as dielectric shields, are as follows:
(a) to cover cathode surfaces adjacent to anodes,
(b) to provide a nonconducting extension to steel pipe
conduits for impressed current anodes,
(c) to afford a backing for flush-mounted galvanic
anodes,
(d) to minimize current demand by structure members
in highly congested areas, and
(e) to restrict anode current output, i.e., reduce
wastage of anode current to the structure in the
immediate vicinity of the anode.
8.3.3 A general underwater coating may be consid-
ered as a type of dielectric shield and can be used
either to reduce the required current or to extend the
life of a CP system.
8.4 Types of Dielectric Shields
8.4.1 Various dielectric shields have been used in
structure CP. These include yard-applied coatings,
prefabricated plastic or elastomeric sheets, plastic pipe
sleeves, and coated standoffs or supports for im-
pressed current anodes.
8.5 Design Considerations
8.5.1 Electrochemical reactions at the anode and
cathode produce corrosives and gases that may attack
the dielectric shield or cause disbonding by gas forma-
tion under the dielectric shield. Materials selected
should be suitable for the intended service.
8.5.2 For proper performance, the design of the di-
electric shield (radius around the anode or distance
from the anode to the nearest bare cathode surface)
must involve considerations of the anode current out-
put, the water resistivity, the geometry of the structure,
and the location of other anodes.
8.5.3 Mechanical damage from environmental factors
such as wave forces, water currents, floating debris,
ice, and marine organisms must be considered. The
design shall also include allowances for possible hand-
ling damage prior to structure placement.
8.5.4 The anticipated life or general deterioration of
any dielectric shield material should be considered in
the CP design (see Paragraph 5.8.3.4).
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Section 9: Operation and Maintenance of Cathodic Protection Systems
9.1 Introduction
9.1.1 This section designates procedures for energiz-
ing and maintaining continuous, effective, and efficient
operation of CP systems.
9.2 Electrical Measurements
9.2.1 Electrical measurements and inspections are
necessary to determine that protection has been est-
ablished according to applicable criteria and that each
part of the CP system is operating properly. Conditions
that affect protection are subject to change with time.
Corresponding changes are required in the CP system
to maintain protection. Periodic measurements and in-
spections are necessary to detect performance
changes in the CP system.
9.2.2 Care should be exercised in selecting the loca-
tion, number, and type of electrical measurements
used to determine the adequacy of CP (see Section 4).