Control of Corrosion
The five general methods used in the control of corrosion are coating, cathodic protection, material selection, environmental modification, and design practices. Control of underground corrosion is primarily achieved by two methods: coating and cathodic protection. An effective external coating can provide corrosion protection to over 99% of the exposed pipe surface. The protective coating is usually applied to the pipe or tank before burial. The coating serves to electrically insulate the metal from the soil. If the metal could be completely isolated, then the establishment of corrosion cells would be prevented and no corrosion current would flow. However, no coating can be considered a perfect coating. Damage to the coating as a result of handling, transportation, installation, thermal stresses, and soil stresses will eventually create defects or “holidays” that expose the underlying steel to the environment.
Cathodic protection is an electrochemical technique for preventing corrosion of a metal exposed to an electrolyte. The process involves application of DC electrical current to the metal surface from an external source. By forcing the metal surface to accept current from the environment, the underground metal becomes a cathode and protection occurs. The external source can use outside AC power through a rectifier and groundbed or by attaching sacrificial metals such as magnesium or aluminum to the structure to be protected. It is used extensively in preventing corrosion to underground and submerged steel structures; such as pipelines, production well casings, and tanks.
Effective application of cathodic protection can provide complete protection to any exposed areas for the life of the structure. The combination of an external coating and cathodic protection provides the most economical and effective choice for protection of underground and submerged pipelines. For bare or ineffectively coated existing pipeline systems, cathodic protection often becomes the only practical alternative for corrosion protection.
Cathodic protection is a mandated requirement of federal and state regulations governing underground transmission pipelines, gas distribution systems, and underground fuel tanks. These requirements include installation, monitoring, and maintenance of cathodic protection systems.
