Cathodic Protection
303
9.2.1.2 Anode Materials and Backfill
The use of magnesium as a sacrificial anode has already been discussed. For
use with impressed current, auxiliary anodes are usually formed of scrap iron
or graphite. Scrap iron is consumed at a considerably faster rate than graphite
(15 to 20 lb/A-yr vs. 2 lb/A-yr); however, graphite costs more — both initially
and in operating expense. Graphite requires more power than scrap iron. It is
also more fragile, and greater care must be taken during installation. Under
certain conditions, the advantage of the 8 to 10 times longer life outweighs the
added costs, particularly in areas where replacement poses problems.
Platinum-clad or 2% silver-lead electrodes that use impressed current have
been used for the protection of structures in seawater. The latter anodes are
estimated to last 10 years, whereas sacrificial magnesium anodes require
replacement every 2 years. On occasion, aluminum electrodes have been
used in freshwaters.
Because the effective relativity of soil surrounding an anode is limited to
the immediate area of the electrode, the local distance is generally reduced
by using backfill. For impressed current systems, the anode is surrounded
with a thick bed of coke mixed with 3 to 4 parts gypsum to 1 part sodium
chloride. The consumption of the anode is reduced somewhat because the
coke backfill is a conductor and carries part of the current. If the anode is
immersed in a riverbed, lake, or ocean, backfill is not required.
Auxiliary anodes need not be consumed to fulfill their purpose. Conversely,
sacrificial anodes are consumed no less to supply an equivalent current than
is required by Faraday’s law.
For magnesium anodes, backfill has the advantage of reducing the resistance of insulating corrosion-product films as well as increasing the conductivity of the immediate area. A typical backfill consists of a mixture of
approximately 20% bentonite (for retention of moisture), 75% gypsum, and
5% sodium sulfate.
9.2.2 impressed Current Systems
For these systems, the source of electricity is external. A rectifier converts
high voltage to a low-voltage DC current. This direct current is impressed
between buried anodes and the structure is protected.
It is preferable to use inert anodes, which will last for the longest possible
time. Typical materials used for these anodes are graphite, silicon, titanium,
and niobium plated with platinum.
For a given voltage, the current is limited by electrolyte resistivity and by
the anodic and cathodic polarization. With the impressed current system, it
is possible to impose whatever potential is necessary to obtain the current
density required, by means of the rectifier.
Electric current flows in the soil from the buried anode to the underground
structure, to be protected. Therefore, the anode must be connected to the
303
9.2.1.2 Anode Materials and Backfill
The use of magnesium as a sacrificial anode has already been discussed. For
use with impressed current, auxiliary anodes are usually formed of scrap iron
or graphite. Scrap iron is consumed at a considerably faster rate than graphite
(15 to 20 lb/A-yr vs. 2 lb/A-yr); however, graphite costs more — both initially
and in operating expense. Graphite requires more power than scrap iron. It is
also more fragile, and greater care must be taken during installation. Under
certain conditions, the advantage of the 8 to 10 times longer life outweighs the
added costs, particularly in areas where replacement poses problems.
Platinum-clad or 2% silver-lead electrodes that use impressed current have
been used for the protection of structures in seawater. The latter anodes are
estimated to last 10 years, whereas sacrificial magnesium anodes require
replacement every 2 years. On occasion, aluminum electrodes have been
used in freshwaters.
Because the effective relativity of soil surrounding an anode is limited to
the immediate area of the electrode, the local distance is generally reduced
by using backfill. For impressed current systems, the anode is surrounded
with a thick bed of coke mixed with 3 to 4 parts gypsum to 1 part sodium
chloride. The consumption of the anode is reduced somewhat because the
coke backfill is a conductor and carries part of the current. If the anode is
immersed in a riverbed, lake, or ocean, backfill is not required.
Auxiliary anodes need not be consumed to fulfill their purpose. Conversely,
sacrificial anodes are consumed no less to supply an equivalent current than
is required by Faraday’s law.
For magnesium anodes, backfill has the advantage of reducing the resistance of insulating corrosion-product films as well as increasing the conductivity of the immediate area. A typical backfill consists of a mixture of
approximately 20% bentonite (for retention of moisture), 75% gypsum, and
5% sodium sulfate.
9.2.2 impressed Current Systems
For these systems, the source of electricity is external. A rectifier converts
high voltage to a low-voltage DC current. This direct current is impressed
between buried anodes and the structure is protected.
It is preferable to use inert anodes, which will last for the longest possible
time. Typical materials used for these anodes are graphite, silicon, titanium,
and niobium plated with platinum.
For a given voltage, the current is limited by electrolyte resistivity and by
the anodic and cathodic polarization. With the impressed current system, it
is possible to impose whatever potential is necessary to obtain the current
density required, by means of the rectifier.
Electric current flows in the soil from the buried anode to the underground
structure, to be protected. Therefore, the anode must be connected to the