6.5 Anode spacing.- After determining the
number of anodes required in an installation,
the success or failure of the system is
dependent primarily upon the proper location
and installation of the galvanic anodes.
Locating galvanic anodes along a
comparatively short bare pipe line in
homogenous soil is quite simple as the
required number of anodes can be equally
spaced along the pipeline and connected to
the line with insulated wires. The same thing
is true of a long-coated pipeline under
identical conditions. However, the problem is
not usually that simple as the proper spacing
along a continuous structure depends upon
the varying physical condition of the structure
surface and the surrounding soil. For
example, an evenly-coated pipeline located in
soil that changes form mildly corrosive (5,000
to 10,000 ohm-centimeters) to very corrosive
(1,000 ohm-centimeters or less), the spacing
of the anodes along the line would vary.
Closer spacing or anodes along the pipeline
would be required for the part of the pipe in
the very corrosive soil to afford the same
protection being received by the pipeline
section in the mildly corrosive soil with greater
distances between the anodes. The same
closer spacing of anodes is required if the soil
conditions are found to be constant, but it is
known that the condition of the protective
coating varies. A closer grouping of the
anodes is required where the protective
coating is inferior. In the case of a bare pipe
of considerable length, it is usually not
economical by protect the entire pipeline.
However, a bare pipeline located in the
above-mentioned soil condition can usually be
protected economically by protecting with
sacrificial anodes the pipe sections in the very
corrosive soils. This type of protection is
referred to as "hot spot" protection and is
economically justifiable in that the useful life of
the entire pipeline has been extended by the
cathodic protection of the severely corroding
areas. Sacrificial anodes should be placed
around the structure symmetrically to provide
good current distribution and to increase
anode efficiency. Some structures, however,
are very irregular and care must be taken to
distribute the anodes to provide adequate
protection to as much of the metal work as
possible. Installation of the anodes should be
made at a distance of 3.1 m (10 to 30 ft) from
the structure.
number of anodes required in an installation,
the success or failure of the system is
dependent primarily upon the proper location
and installation of the galvanic anodes.
Locating galvanic anodes along a
comparatively short bare pipe line in
homogenous soil is quite simple as the
required number of anodes can be equally
spaced along the pipeline and connected to
the line with insulated wires. The same thing
is true of a long-coated pipeline under
identical conditions. However, the problem is
not usually that simple as the proper spacing
along a continuous structure depends upon
the varying physical condition of the structure
surface and the surrounding soil. For
example, an evenly-coated pipeline located in
soil that changes form mildly corrosive (5,000
to 10,000 ohm-centimeters) to very corrosive
(1,000 ohm-centimeters or less), the spacing
of the anodes along the line would vary.
Closer spacing or anodes along the pipeline
would be required for the part of the pipe in
the very corrosive soil to afford the same
protection being received by the pipeline
section in the mildly corrosive soil with greater
distances between the anodes. The same
closer spacing of anodes is required if the soil
conditions are found to be constant, but it is
known that the condition of the protective
coating varies. A closer grouping of the
anodes is required where the protective
coating is inferior. In the case of a bare pipe
of considerable length, it is usually not
economical by protect the entire pipeline.
However, a bare pipeline located in the
above-mentioned soil condition can usually be
protected economically by protecting with
sacrificial anodes the pipe sections in the very
corrosive soils. This type of protection is
referred to as "hot spot" protection and is
economically justifiable in that the useful life of
the entire pipeline has been extended by the
cathodic protection of the severely corroding
areas. Sacrificial anodes should be placed
around the structure symmetrically to provide
good current distribution and to increase
anode efficiency. Some structures, however,
are very irregular and care must be taken to
distribute the anodes to provide adequate
protection to as much of the metal work as
possible. Installation of the anodes should be
made at a distance of 3.1 m (10 to 30 ft) from
the structure.
(FIST 4-5) 12
6.6 Anode installation.- Anodes should be
buried a minimum of 0.6 m (2 ft) into a low-
resistivity material. This may necessitate deep
holes to reach moist soil. Clays are common,
low-resistivity materials. In order to assure
minimum electrical resistance between the
anode and ground, a chemical backfill is used.
Anodes are sometimes supplied prepacked,
with the chemical backfill in a cloth bag
around the magnesium. If prepacked anodes,
which are preferred, are used, no additional
chemical backfill is required. If bare anodes
are used, chemical backfill should be tamped
around the bare anodes as shown in
buried a minimum of 0.6 m (2 ft) into a low-
resistivity material. This may necessitate deep
holes to reach moist soil. Clays are common,
low-resistivity materials. In order to assure
minimum electrical resistance between the
anode and ground, a chemical backfill is used.
Anodes are sometimes supplied prepacked,
with the chemical backfill in a cloth bag
around the magnesium. If prepacked anodes,
which are preferred, are used, no additional
chemical backfill is required. If bare anodes
are used, chemical backfill should be tamped
around the bare anodes as shown in
The anode and backfill shall be placed in a
water-filled hole and tamped. The anode
leads should be buried a minimum of 0.5 m
(18 in) and the free end should be attached to
the structure by thermosetting resin, welding,
or brazing, if resin is used, care must be taken
to ensure a metal-to-metal contact. This
connection should be protected by a suitable
protective coating. Another lead should be
connected to the structure in a similar manner
and the other end brought to the surface to
terminate in a test structure as shown in
water-filled hole and tamped. The anode
leads should be buried a minimum of 0.5 m
(18 in) and the free end should be attached to
the structure by thermosetting resin, welding,
or brazing, if resin is used, care must be taken
to ensure a metal-to-metal contact. This
connection should be protected by a suitable
protective coating. Another lead should be
connected to the structure in a similar manner
and the other end brought to the surface to
terminate in a test structure as shown in
slack to facilitate testing This lead may be
used by a corrosion engineer to determine (by
a pipe-to-soil potential test) whether the
cathodic protection system is working properly
and whether the anodes have been
consumed. The anode-to-structure lead
should be constructed of No. 10 or 8 AWG
type TW copper wire. Splice connections
should be made using a split, bolt-type
electrical connector of the proper size. The
connection should be wrapped with three
layer of plastic electrician's tape, followed by
three layers of self-vulcanizing, rubber
insulating tape and the joint encased in a
suitable electrical waterproofing compound.
In low resistivity soils, a resistor is often
required in the anode lead to reduce the
current supplied to the structure to the amount
necessary to maintain the proper protection.
used by a corrosion engineer to determine (by
a pipe-to-soil potential test) whether the
cathodic protection system is working properly
and whether the anodes have been
consumed. The anode-to-structure lead
should be constructed of No. 10 or 8 AWG
type TW copper wire. Splice connections
should be made using a split, bolt-type
electrical connector of the proper size. The
connection should be wrapped with three
layer of plastic electrician's tape, followed by
three layers of self-vulcanizing, rubber
insulating tape and the joint encased in a
suitable electrical waterproofing compound.
In low resistivity soils, a resistor is often
required in the anode lead to reduce the
current supplied to the structure to the amount
necessary to maintain the proper protection.
7. IMPRESSED CURRENT (RECTIFIER)
SYSTEMS
SYSTEMS
solved by cathodic protection using an
impressed current or rectifier system. Such a
impressed current or rectifier system. Such a