Protection: Reduction of corrosion rate by shifting the corrosion
potential of the electrode toward a less oxidizing potential by applying
an external electromotive force.
A metal which, because of its relative position in the galvanic series,
provides sacrificial protection to metals that are more noble in the
series, when coupled in an electrolyte.
Protection System: A cathodic protection system in which the external
electromotive force is supplied by a galvanic anode.
Cathodic Protection System: A cathodic protection system in which
the external electromotive force is provided by an external DC power
One or more anodes installed below the earth's surface for the purpose
of supplying cathodic protection.
A device which converts alternating current to direct current.
Groundbed: A group of anodes installed remote (300 feet or more)
from the structure and spaced on 15 to 30 foot centers.
Anode Groundbed: A group of anodes installed close (5 to 20 feet)
to and along a structure to be protected and spaced on 25 to 500 foot
Deep Anode Groundbed:
One or more anodes installed vertically at a nominal depth of 50 feet
or more below the earth's surface in a drilled hole.
Groundbed: One or more anodes installed vertically at a nominal
depth of 50 feet or less below the earth's surface.
of the type of cathodic protection system to be installed, two simple
field tests should be conducted prior to beginning the system design.
Soil resistivity should be determined for the specific area where the
groundbed is to be installed. Even small differences in location can
cause large differences in soil resistivity. Soil resitivitiy may be
determined using any one of:
- Soil box procedure
- Wenner (4-pin)
- Single rod
Whenever possible, a trial and error process using a temporary groundbed
and a portable power supply should be used to determine the current
required to protect the structure.
the above process is not feasible, make an assumption about current density
requirements and calculate current requirement for the area of the structure
to be protected. Use typical current densities for your area, or more
general current densities as found in the technical section of http://www.mesaproducts.com/
- Set up a temporary
groundbed with ground rods and a temporary power supply.
- Energize the
- Perform an
on-off survey over the structure to be protected.
- Increase the
current and repeat the survey.
- Repeat Steps
3 and 4 until the structure is protected according to established
anodes are most efficiently used on electrically isolated coated structures.
The current output of a galvanic anode installation is typically much
less than that which is obtained from an impressed current cathodic protection
Galvanic anode installations tend to be used mostly on underground structures
in applications where cathodic protection current requirements are small
and where earth resistances are acceptably low.
Magnesium anodes are available in a variety
of shapes and sizes, bare or prepackaged with the most popular being
the 17 lb. prepackaged anode. As a general guideline, one may assume
magnesium anodes to be acceptable where soil resistivities are between
1,000 ohm-cm and 5,000 ohm-cm. Short chunky shapes are suitable for
low resistivity areas, but long slender shapes should be employed in
higher resistivity areas.
Zinc anodes are also available in many shapes and sizes. They are appropriate
in soils with very low resistivities (750 ohm-cm to 1500 ohm-cm). Favorable
environments are sea water and salt marshes. Short chunky shapes are
suitable for low resistivity areas, but long slender shapes should be
employed in higher resistivity areas.
Aluminum anodes are not commonly used in earth burial applications.
Some proprietary aluminum alloy anodes work well in a sea water environment.
so no external power source is required.
- Easy field
- Low maintenance
- Less likely
to cause stray current interference problems on other structures.
- When the current
requirement is small, a galvanic system is more economical than
an impressed current system.
- Low driving
- Limited to
use in low resistivity soils.
- Low maintenance
- Not an economical
source of large amounts of CP current.
- Very Little
capacity to control stray current effects on the protected structure.
CATHODIC PROTECTION SYSTEM
current system is used to protect large bare and coated structures and
structures in high resistivity electrolytes. Design of an impressed current
system must consider the potential for causing coating damage and the
possibility of creating stray currents, which adversely affect other structures.
to a variety of applications
- Current output
may be controlled
- Not constrained
by low driving voltage
- Effective in
high resistivity soils
- Increased maintenance
- Higher operating
- May cause interference
on other structures
Location should be determined early in the design process because
its location may affect the choice of groundbed type. The following
factors should be considered when choosing a groundbed location.
are normally used to distribute protective current over a broad area
of the structure to be protected. These are frequently called remote
groundbeds because the structure is outside the anodic gradient of the
groundbed caused by the discharge of current from the anodes to the
- Soil Resistivity
- Soil Moisture
with other Structures
of Power Supply
or other Damage
of the Goundbed
of Right of Way
Distributed Anode Groundbeds are used to reduce the potential
for interference effects on neighboring structures. They are used to
protect sections of bare or poorly coated structure. They are used in
congested areas where electrical shielding might occur with other groundbeds.
Deep Anode Groundbeds are remote to the structure by virtue of
the vertical distance between anode and structure. Deep anode groundbeds
therefore achieve results similar to remote surface groundbeds. A deep
anode groundbed is an appealing choice when space is not available for
a conventional groundbed or when surface soil has high resistivity and
deeper strata exhibit low resistivities.
Shallow Vertical Groundbeds are commonly used where space is
are the most common power source for cathodic protection systems. Each
of several manufacturers offer a dizzying array of options, most commonly
in the following areas:
If a manufacturer
were to offer each of the options listed in one reference, then that
manufacturer would be offering 8,957,952 variations, assuming
the choices are independent.
- Cooling Type
- Control Type
- Circuit Type
- AC Input
- DC Volts
- DC Amperes
Solar Cells can provide an dependable power supply in certain
parts of the world. Great inefficiencies may result if the entire CP
system is not designed with the power supply in mind.
Generators (engine, wind, or turbine powered) are used in special
Any reliable external source of DC current will suffice - the
pipe doesn't care about the power source.
iron is sometimes used as an anode simply because it is available.
Non-uniform consumption, high rate of consumption, and discoloration
of surrounding structures are distinct disadvantages.
Graphite anodes are one of the most commonly used anodes for
impressed current systems. Most common applications are to protect underground
structures. Graphite anodes are suitable for deep, shallow vertical,
or horizontal ground beds with carbonaceous backfill.
High Silicon Cast Iron anodes are widely used in underground
applications in both shallow and deep groundbeds. Specially formulated
high silicon cast iron anodes are also used in seawater. Although the
performance is improved with coke breeze; its use is not critical.
Platinized Titanium anodes take advantage of the low consumption
rate and high current density. Voltages in excess of 10 Volts will result
in severe pitting of the titanium core causing premature failure.
Platinized Niobium/Tantalum anodes also take advantage of the
properties of platinum, but avoid the low driving voltage restriction
of platinized titanium anodes. Breakdown of the niobium oxide film occurs
at approximately 120 Volts. Thus these anodes are used where high driving
voltage is required.
Magnetite anodes are quite expensive but have an extremely long
life. They are therefore an economical choice for some applications.
Mixed Metal Oxide anodes consist of a high purity titanium substrate
with an applied coating consisting of a mixture of oxides. The titanium
serves as a support for the oxide coating. The mixed metal oxide is
a crystalline, electrically-conductive coating that activates the titanium
and enables it to function as an anode. When applied on titanium, the
coating has an extremely low consumption rate, measured in terms of
milligrams per year. As a result of this low consumption rate, the tubular
dimensions remain nearly constant during the design life of the anode
- providing a consistently low resistance anode.
The carbon backfill
serves as a sacrificial buffer between the anode and the reaction
environment. Carbon backfill is used to accomplish three major goals:
- Maintain stability
of the excavation (hole).
- Serve as the
primary anodic reaction surface.
- Lower resistance-to-earth
of the system.
objective of the carbon backfill is to electronically conduct the current
discharged from the anode surface to the carbon-earth interface where
the electrochemical reaction can occur with least impact on the anode.
Metallurgical Coke is low in carbon content, porous and therefore
low in specific gravity, and high in ash and volatiles content. These
three characteristics cause metallurgical coke to have a relatively
high resistivity. Metallurgical coke is not suitable for deep anode
Petroleum Coke must be calcined (heat treated). Prior to calcination,
petroleum coke is non conductive and is therefore not suitable for backfill.
Backfill Selection should be based on a consideration of the
following coke characteristics:
or more significantly in-situ bulk resistivity determines how well
the objective of the carbon backfill is achieved.
- Specific Gravity
affects compact settling. A high specific gravity helps to insure
- Carbon Content
of the backfill material determines the anode system life.
- Particle Sizing
determines the amount of contact between anode and backfill. For
optimum contact, particle size should be small relative to the anode
diameter. Very small (less than 7.5 microns) particles should be
avoided because they are high in ash content.
- Particle Shape
affects how well the backfill settles and the tendency for the backfill
to trap gases. A spherical shape is preferred over flat, irregularly
Course, Morgantown, WV, Appalachian Underground Corrosion Short Course,
AUCSC, Intermediate Course, Morgantown, WV, Appalachian Underground Corrosion
Short Course, 1986
AUCSC, Advanced Course, Morgantown, WV, Appalachian Underground Corrosion
Short Course, 1993
Peabody, A.W., Houston, TX: NACE International, 1967
Lewis, T.H., Deep Anode System - Design, Installation and Operation, Hattiesburg,
MS: Loresco International, 1997
Morgan, J., Cathodic Protection, Houston, TX: NACE International, 1987
Schrieber, C.F., "Deep Anode Groundbed Design and Installation Guidelines",
Chardon, OH: ELTECH Systems, Inc., 2000 (available at http://www.lidaproducts.com)
(I am trying to get permission to reprint the article)
of Corrosion Terms:
Designing for Corrosion Control: The Designing for Corrosion Control Course reviews the principles of corrosion and corrosion control and provides a systematic method for applying the technology of corrosion prevention to the design process. It offers an overview of the steps involved in materials selection common to many industries. It also covers the economic considerations of including corrosion control in system design and the financial principles used in evaluating alternative materials and designs.
Designing for Corrosion Control
Deep Anode Groundbed