Cathodic Protection System Design
a topic summary by
Delano P Wegener, Ph.D.
Reference section was revised on 2 JAN 2011 to reflect changes in availability.

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 earth's 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 earth's 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 earth's 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 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.

Disdvantages:

  • Low driving voltage.
  • Limited to use in low resistivity soils.
  • Low maintenance requirement.
  • 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.

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.

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

Any reliable external source of DC current will suffice - the pipe doesn't 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.

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.

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)
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WWW RESOURCES

NACE Glossary of Corrosion Terms:
http://www.nace.org/nace/content/pubsonline/free/GLOSSARY.PDF

NACE Course:
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

Calculators:
http://www.cpdesigncenter.com

Glossaries:
http://www.hghouston.com/a.html

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

Deep Anode Groundbed Design:
http://www.cpdesigncenter.com

Training Opportunities:
http://www.delweg.com/training.htm