Delano P. Wegener, Ph.D. e-mail: del@delweg.

com
4507 Shalom Creek URL: http://www.delweg.com
Spring, TX 77388 Phone: (281) 353-4862
_________________________________________________________________

CATHODIC PROTECTION SYSTEM DESIGN

The 28th Biennial Corrosion Control Conference
The East Texas Section of NACE International
Tuesday, MAY 9, 2000

Galvanic and Impressed Current Systems

GroundBed Types

Materials

Interactive Design Program on the WWW

Groundbed Design Calculations

References
DEFINITIONS
Cathodic Protection: Reduction of corrosion rate by shifting the corrosion potential of the
electrode toward a less oxidizing potential by applying an external electromotive force.

Galvanic Anode: 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.

Galvanic Cathodic Protection System: A cathodic protection system in which the external
electromotive force is supplied by a galvanic anode.

Impressed Current Cathodic Protection System: A cathodic protection system in which the
external electromotive force is provided by an external DC power source.

Groundbed: One or more anodes installed below the earths surface for the purpose of
supplying cathodic protection.

Rectifier: A device which converts alternating current to direct current.

Conventional Groundbed: A group of anodes installed remote (300 feet or more) from the
structure and spaced on 15 to 30 foot centers.

Distributed 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 centers.

Deep Anode Groundbed: One or more anodes installed vertically at a nominal depth of 50 feet
or more below the earths surface in a drilled hole.

Shallow Vertical Groundbed: One or more anodes installed vertically at a nominal depth of 50
feet or less below the earths surface.

REQUIRED FIELD TESTS

Regardless 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:
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) procedure
Single rod test procedure

Current Requirement:
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.
Set up a temporary groundbed with ground rods and a temporary power supply.
Energize the system
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 criteria.

If 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

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 densities for your area, or more general current densities as found in the technical section of
http://www.mesaproducts.com/

GALVANIC CATHODIC PROTECTION SYSTEM

Galvanic 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 system.

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.

Materials:
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.

Advantages:
Self-powered so no external power source is required.
Easy field installation.
Low maintenance requirement.
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.

Disadvantages:
Low driving voltage.
Limited to use in low resistivity soils.
Not an economical source of large amounts of CP current.
Very Little capacity to control stray current effects on the protected structure.

IMPRESSED CURRENT CATHODIC PROTECTION SYSTEM

An impressed 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.

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Advantages:
Flexibility
Applicable to a variety of applications
Current output may be controlled
Not constrained by low driving voltage
Effective in high resistivity soils

Disadvantages:
Increased maintenance
Higher operating costs
May cause interference on other structures

Groundbeds:
Groundbed 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.
Soil Resistivity Soil Moisture
Interference with other Structures Availability of Power Supply
Accessibility Vandalism or other Damage
Purpose of the Goundbed Availability of Right of Way

Conventional Groundbeds 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 surrounding soil.

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 limited.

Power Supplies:
Rectifiers 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:

Enclosure Type Cooling Type Control Type

Rectifying Element Circuit Type AC Input
DC Volts DC Amperes Options

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.

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.

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 Generators (engine, wind, or turbine powered) are used in special circumstances.

Any reliable external source of DC current will suffice the pipe doesnt care about the power
source.

Anodes:
Scrap 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.

Carbon Backfill:
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.

The primary 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 groundbed
installations.

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 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:

Resistivity, 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 compact
settling.
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 shaped particles.

ON-LINE DEEP ANODE GROUNDBED DESIGN PROGRAM

A demonstration of the design program available at http://www.lidaproducts.com

CALULATIONS FOR GROUNDBED DESIGN PROGRAM

Almost all of those references listed at the end of this paper which discuss groundbed design,
provide step-by-step instructions with examples. What follows here does not repeat those
discussions, but is instead a list of the required steps as seen from the eyes of the computer.
This is what the program does.

It is important to keep in mind that this is a program which designs groundbeds for LIDA anodes
and therefore involves some proprietary constants and formulas. Calculations are based on data
provided by the user and data provided by the manufacturer.

Major program steps are listed to illustrate the internal process. Details of the calculations are not
presented.

The calculations use the following:

Dwights Formula
Sunde Formula
Ohms Law
Proprietary Formula
Resistance in parallel formula
Resistance in series formula
Area of geometric shapes
Volume of geometric solids
Unit conversions
General Algebraic Manipulations

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Step 1: Calculate required number of anodes.

Step 2: Calculate maximum number of anodes per string.

Step 3: Calculate minimum number of strings

Step 3b: Calculate the number of holes

Step 4: Calculate backfill length

Step 4a: Calculate minimum backfill length

Step 4b: Adjust the minimum backfill length

Step 4c: Calculate the length of spare backfill

Step 5: Calculate backfill-to-soil current density

Step 6: Calculate cable lengths

Step 6a: Calculate total cable length

Step 6b: Calculate cable length in a string

Step 6c: Calculate length of cable embedded in backfill

Step 7: Calculate Resistance

Step 7a: Calculate anode-to-backfill resistance

Step 7b: Calculate backfill-to-soil resistance

Step 7c: Calculate cable resistance

Step 7d: Calculate total groundbed resistance

Step 8: Calculate rectifier operating voltage

Step 9: Calculate required volume of backfill

Step 9a: Calculate volume of anodes

Step 9b: Calculate volume of vent pipe embedded in backfill

Step 9c: Calculate volume of cable embedded in backfill

Step 9d: Calculate total volume of backfill column

Step 9e: Calculate required volume of backfill

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HARDCOPY RESOURCES
AUCSC, Basic Course, Morgantown, WV, Appalachian Underground Corrosion Short
Course, 1985

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)

WWW RESOURCES
NACE Glossary of Corrosion Terms:
http://www.nace.org/naceframes/RESOURCES/GLOSSARY.PDF

NACE Course:
Cathodic Protection - Design I - Participants taking this five day course will develop an
appreciation for cathodic protection design and it's complexities. The course stresses the
principles, methodology, and financial advantages in designing a system to include cathodic
protection. For more information point your browser to:
http://www.nace.org/naceframes/Education/edgenindex.htm

Companies which Provide Cathodic Protection Services:

http://www.delweg.com/cp/company1.htm

Calculators:
http://www.delweg.com/library/exhibit/exbmain.htm
(over 10,760 on-line calculators listed)
http://www.lidaproducts.com/calculator/gbdmain.htm

Glossaries:
http://www.delweg.com/library/exhibit/exbmain.htm
http://www.corrosionsource.com/handbook/glossary/
http://www.hghouston.com/glossary.html

Technical Information:
http://www.mesaproducts.com/

Deep Anode Groundbed Design:

http://www.lidaproducts.com/technical/techmain.htm

Training Opportunities:
http://www.delweg.com/cp/trainpos/cptrnmain.htm