CATHODIC PROTECTION

Definition of Cathodic Protection

Cathodic protection is a method to reduce corrosion by minimizing the difference in potential between anode and cathode.

INTRODUCTION.

Cathodic protection is achieved by applying a current to the structure to be protected (such as a pipeline) from some outside source. When enough current is applied, the whole structure will be at one potential; thus, anode and cathode sites will not exist. Cathodic protection is commonly used on many types of structures, such as pipelines, underground storage tanks, locks, and ship hulls.

Cathodic protection is second only to the use of protective coatings as a means of corrosion control. It is widely used for protecting buried and waterfront structures and for protecting the interiors of water storage tanks. In some cases, such as underground pipelines, field experience has shown that cathodic is such an effective means of providing the required levels of safety in the operation of the systems that cathodic protection is required by regulation.

WHEN CATHODIC PROTECTION SHOULD BE CONSIDERED.

Cathodic protection should be considered, possibly in conjunction with other forms of corrosion control, such as protective coatings, wherever the system requiring protection is exposed to an aggressive environment in such a manner that cathodic protection is technically feasible. Cathodic protection is technically feasible when the surfaces to be protected are buried or submerged.

Structures That Are Commonly Protected.

External surfaces of buried metallic structures, surfaces of metal waterfront structures, such as sheet pilings or bearing piles, and the internal surfaces of tanks containing electrolytes, such as water, are applications where cathodic protection is usually technically feasible and cathodic protection is used in protecting such structures. Internal surfaces of small diameter pipelines and other areas where ion flow in the electrolyte is restricted by electrolyte resistance, cathodic protection has limited applicability.

Determining the Need for Protection.

When construction of a new buried or submerged system is being planned, the corrosivity of the environment should be considered as one of the factors in the design of the system. If experience with similar systems in the vicinity of the construction site has shown that the site conditions are aggressive based on leak and failure records, cathodic protection should be considered as a means of controlling corrosion on the new system. Cathodic protection is one of the few methods of corrosion control that can be effectively used to control corrosion of existing buried of submerged metal surfaces. Thus, if leak records on an existing system show that corrosion is occurring, cathodic protection can be applied to stop the corrosion damage from increasing. Cathodic protection can, however, only stop further corrosion from occurring and cannot restore the material already lost due to corrosion.

When Protection Is Required.

In some cases, cathodic protection is required by policy or regulation. Regulations by the Department of Transportation have established standards for transporting certain liquids and compressed gas by pipelines in order to establish minimum levels of safety. These regulations require that these pipelines be protected by cathodic protection combined with other means of corrosion control, such as protective coatings and electrical insulation. These regulations provide excellent guidelines for the application of cathodic protection to buried and submerged pipelines. In addition to these regulations, primarily due to the safety and environmental consequences of system failure, there are an increasing number of federal, state and local governmental regulations regarding the storage and transportation of certain materials that require corrosion control. Many of these regulations either specify cathodic protection as a primary means of corrosion control or allow its use as an alternative method of controlling corrosion.

Source : "Corrosion Control" NAVFAC MO-307 September 1992