system is physically comparable to the sacrifi-
cial anode system in that an anode is
installed in the electrolyte (soil or water) and
is metallically connected to the corroding
structure which is made the cathode.
However, rather than rely for protection on
the current which results from the anode-
cathode couple, an artificial source of current
is introduced into the circuit as shown in
cial anode system in that an anode is
installed in the electrolyte (soil or water) and
is metallically connected to the corroding
structure which is made the cathode.
However, rather than rely for protection on
the current which results from the anode-
cathode couple, an artificial source of current
is introduced into the circuit as shown in
First, the galvanic potential of the anode is no
longer relevant, and almost any electrode
material may be used. Scrap iron, abandoned
structures, driven steel anodes, etc., among
sacrificial materials will suffice; or
nonsacrificial materials, such as high silicon
iron, graphite, or platinum, may be selected
as anodes. Second, a more powerful and
flexible system can be designed because the
artificial current source makes available
higher voltages and currents which can be
manipulated to advantage. For instance,
anodes can be located considerable
distances from a pipeline and sufficient
current supplied to protect the lines for as
much as an 80.5-km (50-mi) length. Also,
high enough voltages can be obtained to sup-
ply necessary currents for protection in high-
resistance soils where sacrificial anodes are
ineffective. Third, the power potential in a rec-
tifier system carries with it the danger that at
excessive current densities, coatings on the
structure may be damaged or destroyed or
that accidental reversal of the polarity of the
impressed current source may cause highly-
accelerated corrosion of the structure instead
of protecting it.
longer relevant, and almost any electrode
material may be used. Scrap iron, abandoned
structures, driven steel anodes, etc., among
sacrificial materials will suffice; or
nonsacrificial materials, such as high silicon
iron, graphite, or platinum, may be selected
as anodes. Second, a more powerful and
flexible system can be designed because the
artificial current source makes available
higher voltages and currents which can be
manipulated to advantage. For instance,
anodes can be located considerable
distances from a pipeline and sufficient
current supplied to protect the lines for as
much as an 80.5-km (50-mi) length. Also,
high enough voltages can be obtained to sup-
ply necessary currents for protection in high-
resistance soils where sacrificial anodes are
ineffective. Third, the power potential in a rec-
tifier system carries with it the danger that at
excessive current densities, coatings on the
structure may be damaged or destroyed or
that accidental reversal of the polarity of the
impressed current source may cause highly-
accelerated corrosion of the structure instead
of protecting it.
7.2. General installations procedure.- Im-
pressed current systems are appropriate for
protection of larger structures and are more
effective in handling the more complicated
corrosion problems than are sacrificial
anodes. The correct installation of such
systems ordinarily requires a preliminary field
survey of the structure and surrounding
terrain to obtain soil resistivities and other
information and data. A temporary anode
ground bed may be installed and a temporary
source of direct current such as a welding
machine used to supply current to determine
current and other system requirements
necessary to assure correct distribution. After
the permanent system has been designed
and installed, follow up measurements should
be made to assure that adequate protection
has been supplied where required and that
pressed current systems are appropriate for
protection of larger structures and are more
effective in handling the more complicated
corrosion problems than are sacrificial
anodes. The correct installation of such
systems ordinarily requires a preliminary field
survey of the structure and surrounding
terrain to obtain soil resistivities and other
information and data. A temporary anode
ground bed may be installed and a temporary
source of direct current such as a welding
machine used to supply current to determine
current and other system requirements
necessary to assure correct distribution. After
the permanent system has been designed
and installed, follow up measurements should
be made to assure that adequate protection
has been supplied where required and that
no excessive voltages occur. The design and
installation of an impressed current system
calls for specialized knowledge and
considerable experience in this field. The
operation and maintenance of a rectifier
system is more complex than for a sacrificial
anode system, and field personnel will usually
require instruction to obtain the best results.
installation of an impressed current system
calls for specialized knowledge and
considerable experience in this field. The
operation and maintenance of a rectifier
system is more complex than for a sacrificial
anode system, and field personnel will usually
require instruction to obtain the best results.
8. Isolation and sectionalization
8.1. Purposes.- Two purposes can be served
by the use of insulated joints in cathodic pro-
tection, that of isolation and that of
sectionalization. Isolation is the application of
insulated joints to prevent a galvanic cell from
being formed with a portion of the structure
being the sacrificial metal or to insulate a
structure being protected from others that
would add an excessive drain on a cathodic
protection system. Sectionalization is the
dividing of a group of structures into smaller
units for cathodic protection because of
different current requirements or merely to
simplify a system. Although both purposes
are mentioned here, the most common
application in powerplant work would be that
of isolation.
by the use of insulated joints in cathodic pro-
tection, that of isolation and that of
sectionalization. Isolation is the application of
insulated joints to prevent a galvanic cell from
being formed with a portion of the structure
being the sacrificial metal or to insulate a
structure being protected from others that
would add an excessive drain on a cathodic
protection system. Sectionalization is the
dividing of a group of structures into smaller
units for cathodic protection because of
different current requirements or merely to
simplify a system. Although both purposes
are mentioned here, the most common
application in powerplant work would be that
of isolation.
8.2. Examples using insulating joints.- A
typical example of an isolation application is
shown in
typical example of an isolation application is
shown in
connecting of the dissimilar metals provides
an ideal path for the flow of current from the
iron to the copper through the ground
(electrolyte). The iron in this case is anodic,
or the sacrificial metal. The same conditions
are provided in
an ideal path for the flow of current from the
iron to the copper through the ground
(electrolyte). The iron in this case is anodic,
or the sacrificial metal. The same conditions
are provided in
ground mat is connected directly to the steel
pipe, and both are in a common electrolyte,
the ground. The flow of current in this case is
from the steel pipe to the copper ground mat.
The steel pipe is anodic, or the sacrificial
metal. The condition depicted in
pipe, and both are in a common electrolyte,
the ground. The flow of current in this case is
from the steel pipe to the copper ground mat.
The steel pipe is anodic, or the sacrificial
metal. The condition depicted in
from the copper ground mat by the installation
of an insulated joint. Under this condition, the
only corrosion action is local, caused by
inhomogeneity in the metal or contacting
solution. Therefore, in
of an insulated joint. Under this condition, the
only corrosion action is local, caused by
inhomogeneity in the metal or contacting
solution. Therefore, in
will still permit the flow
(FIST 4- 5)
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