Cathodic Protection (CP)

Cathodic Protection (CP) for Corrosion Control

Cathodic protection for corrosion control

Cathodic protection (CP) is a critical component of corrosion mitigation. An electrochemical process, CP is a branch of chemistry dealing with the chemical changes that accompany the passage of an electric current, or a process in which a chemical reaction produces an electric current.

It is used to control the corrosion of a metal surface by transferring the corrosion from the protected structure to a more easily corroded metal. In other words, it controls the corrosion of a metal surface by making it the cathode of an electrochemical cell. It’s accomplished by sending a current into the structure from an external electrode and polarizing the metallic surface in an electronegative direction. This provides protection to the surface and extends the life of the asset.

Examples of structures where cathodic protection systems are employed include underground tanks and pipelines; aboveground storage tanks; water tank interiors; ship hulls; ballast tanks; docks; sheet piling; land and water foundation piles; bridge substructures; tube sheets; oil heater treaters; and reinforcing steel in concrete.

Important considerations for CP management are

  • material selection and design to minimize corrosion, and
  • items that should be addressed in cathodic protection control planning, which affect CPC in design, fabrication and construction, operation and use, and maintenance and sustainability.

Monitoring Cathodic Protection in Pipelines

Onshore pipelines are used all over the world for transporting oil or other fuels, or for transporting water, from one location to another. Because of their importance, many onshore pipelines are protected against corrosion with a CP system. The most obvious reason to monitor pipeline cathodic protection is to make sure the system is functioning correctly and providing adequate corrosion protection. When a structure corrodes, leaks may occur, and the product may be lost. There is also concern over public safety and environmental damage.

For this reason, regulations have been enacted in many industries and countries to make sure structures containing hazardous materials are adequately protected to reduce the risk to the public and environment. Many of these regulatory industries have adopted standards.

Design and Installation of Cathodic Protection Systems

Cathodic protection systems are designed to control the corrosion of a metal surface. While localized damaged surfaces can be patched, ongoing degradation may weaken the integrity of the overall infrastructure. When designed correctly, CP systems can continuously protect infrastructure against corrosion, and techniques such as detailed equipment schematics, system specifications and designs, and field surveys are important when designing a CP system that will provide adequate corrosion protection.

Coatings and Cathodic Protection

Coatings play an important role in CP design. Cathodic protection of uncoated structures can be implemented, but CP of an uncoated structure is usually not cost effective. Coatings alone are not able to prevent corrosion of the metal underneath because they have a finite life, eventually allowing oxygen, water, and chemicals to reach the substrate. Typically coating and CP are used simultaneously, as together, they provide the best technical and economic protection.

Coating inspection is also crucial because it is often desired to install CP to a structure with its coating in the best condition possible. This will ensure the cathodic protection system performs as designed.

Cathodic Protection Interference

Interference is any electrical disturbance on a metallic structure caused by a stray current. Stray current is defined as current flowing on a structure that is not part of the intended electrical circuit. Stray current can be produced by any system conducting an electric current, such as electrified train tracks of overhead power lines that has two or more points of contact with an electrolyte.

Cathodic protection can be designed to mitigate stray current problems. The advantage of CP is that it avoids the use of bonds and the installation and maintenance problems that may accompany them.

Cathodic Protection in Underground and Above-ground Storage Tanks

Corrosion from leaking storage tanks, whether above or below ground, can pollute the environment, threaten public health, and lead to billions of dollars in direct and indirect costs. Fortunately, corrosion prevention technology exists that can protect storage tanks and keep them structurally sound for years to come. Generally, if the steel surface of a storage tank system is in direct contact with an earthen material, it must be protected from corrosion by a CP system.

Government and the public understand the extent to which leaking tanks can damage the environment and threaten public health. To prevent environmental contamination, U.S. federal regulations require those who own or operate underground tanks and the connected piping to have spill, overfill, and corrosion protection mechanisms in place, and many U.S. states have additional tank protection requirements. The owners/operators of tanks who fail to comply with these regulations can be subject to both civil and criminal penalties

Cathodic Protection in Offshore Pipelines and Structures

Many miles of offshore pipelines worldwide are reaching, or have exceeded, the original design life of their cathodic protection systems. CP is routinely used to protect equipment operating in aggressive environments, such as buried pipeline systems and offshore platforms.

In aqueous environments, various chemicals may be introduced to the water or process stream to reduce its corrosivity to the exposed metal. In offshore structures, galvanic CP systems with anodes made of such metals as aluminum, magnesium, and zinc may be used to protect underwater components, and extend the life of offshore pipelines.