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Fundamentals of Corrosion
positive pole of the rectifier, and the structure to the negative pole. All cables
from the rectifier to the anode and to the structure must be electrically insulated. If not, those from the rectifier to the anode will act as an anode and
deteriorate rapidly, while those from the rectifier to the structure may pick
up some of the current, which would then be lost for protection.
9.2.2.1 Current Requirements
The specific metal and environment will determine the current density
required for complete protection. The applied current density must always
exceed the current density equivalent to the measured corrosion rate under
the same conditions. Therefore, as the corrosion rate increases, the impressed
current density must be increased to provide protection.
Factors that affect current requirements are:
1. The nature of the electrolyte
2. The soil resistivity
3. The degree of aeration
The more acidic the electrolyte, the greater the potential for corrosion and the
greater the current requirement. Soils that exhibit a high resistance require a
lower cathodic current to provide protection. In an area of violent agitation or
high aeration, an increase in current will be required. The required current to
provide cathodic protection can vary from 0.5 to 20 mA/ft 2 of bare surface.
Field testing may be required to determine the necessary current density
to provide cathodic protection in a specific area. These testing techniques
will only provide an approximation. After completion of the installation,
it will be necessary to conduct a potential survey and make the necessary
adjustments to provide the desired degree of protection.
9.2.2.2 Anode Materials and Backfill
Although it is generally preferred to use inert anodes, it is possible to use
scrap iron. Scrap iron is consumed at a considerably faster rate than graphite
or other inert anode material. The advantage of scrap iron is a lower initial
cost and lower operating cost because its power requirements are less. In
areas where replacement poses a problem, the cost of using the more inert
anodes outweighs the reduced cost of the scrap iron.
Platinum-clad or 2% silver-lead electrodes have been used for the protection of structures in seawater and are estimated to last 10 years, whereas
sacrificial magnesium anodes have a life of 2 years.
Because the effective resistivity of the soil surrounding an anode is limited
to the immediate area of the anode, this local resistance is usually reduced
using backfill. The anode is usually surrounded by a thick bed of coke mixed
Fundamentals of Corrosion
positive pole of the rectifier, and the structure to the negative pole. All cables
from the rectifier to the anode and to the structure must be electrically insulated. If not, those from the rectifier to the anode will act as an anode and
deteriorate rapidly, while those from the rectifier to the structure may pick
up some of the current, which would then be lost for protection.
9.2.2.1 Current Requirements
The specific metal and environment will determine the current density
required for complete protection. The applied current density must always
exceed the current density equivalent to the measured corrosion rate under
the same conditions. Therefore, as the corrosion rate increases, the impressed
current density must be increased to provide protection.
Factors that affect current requirements are:
1. The nature of the electrolyte
2. The soil resistivity
3. The degree of aeration
The more acidic the electrolyte, the greater the potential for corrosion and the
greater the current requirement. Soils that exhibit a high resistance require a
lower cathodic current to provide protection. In an area of violent agitation or
high aeration, an increase in current will be required. The required current to
provide cathodic protection can vary from 0.5 to 20 mA/ft 2 of bare surface.
Field testing may be required to determine the necessary current density
to provide cathodic protection in a specific area. These testing techniques
will only provide an approximation. After completion of the installation,
it will be necessary to conduct a potential survey and make the necessary
adjustments to provide the desired degree of protection.
9.2.2.2 Anode Materials and Backfill
Although it is generally preferred to use inert anodes, it is possible to use
scrap iron. Scrap iron is consumed at a considerably faster rate than graphite
or other inert anode material. The advantage of scrap iron is a lower initial
cost and lower operating cost because its power requirements are less. In
areas where replacement poses a problem, the cost of using the more inert
anodes outweighs the reduced cost of the scrap iron.
Platinum-clad or 2% silver-lead electrodes have been used for the protection of structures in seawater and are estimated to last 10 years, whereas
sacrificial magnesium anodes have a life of 2 years.
Because the effective resistivity of the soil surrounding an anode is limited
to the immediate area of the anode, this local resistance is usually reduced
using backfill. The anode is usually surrounded by a thick bed of coke mixed