What Does Cathodic Protection Shielding Mean?
Cathodic protection shielding is a term that refers to a method of preventing or diverting the cathodic protection current from its intended path.
Cathodic protection connects the base metal at risk to a sacrificial metal of choice that corrodes instead of the base metal. This technique preserves the metal by providing an active metal that effectively acts as an anode and provide free electrons.
Therefore, cathodic protection shielding is when the amount of protective current that reaches the pipe's metal surface is reduced.
Although it is useful at times for cathodic protection shielding to reduce current level on the pipeline surface, it sometimes becomes a problem when the current level is reduced to a value that is less than what is required to polarize the pipeline surface from the corrosion attack. Properly designed coatings should be utilized for shielding cathodic protection and acting as strong dielectrics.
Cathodic protection shielding is a phenomenon which plays a major role in the corrosion of natural gas pipeline systems. It is often used to reduce, or completely remove, corrosion damage that may occur in active metal surfaces. Cathodic protection has uses in many industries for the protection of pipelines, storage tanks, water treatment plants, ship hulls, offshore production platforms, reinforcement bars in concrete structures and more.
Corrosionpedia Explains Cathodic Protection Shielding
Non-shielding coating is necessary to protect the pipe against external corrosion. During installation, the coating used on pipe for trenchless installation must resist abrasions and other damage possible during installation and non-shielding. To cover the surface quality of the bare pipe, a quality assurance inspection and testing program for the coating must be carried out.
In locations where the coating disbands or electrolyte can enter between the pipe surface and the coating, there is a high risk for corrosion. In situations where coatings allow water to imbue directly, cathodic protection current may flow through the coating to the pipe surface, and for those coatings that do not allow for water to imbue, shielding must be assessed further.
There are some damages that occur as a result of cathodic protection shielding. These are caused by two degradative mechanisms:
- Corrosion: This occurs when the rate is at or less than ordinary corrosion rates in the local groundwater.
- Stress Corrosion Cracking (SCC): This has to do with a rapid failure mechanism but can at times take more than 20 years before SCC initiates and becomes an issue in the structure.
Many times, cathodic protection shielding can exist for decades and will not be detected, recognized or diminished until corrosion occurs. Cathodic protection and coatings keep pipelines free from external corrosion. At coating faults such as pinholes or coating disbondment, cathodic protection serves as a backup to prevent corrosion, however, the first line of protective barrier is the coating. Nevertheless, cracking induced by corrosion and pipeline corrosion have been widely documented in the past.
Either partially or fully, cathodic protection current can be shielded when coatings are disbonded at smaller faults, such as pinholes to reach the disbonding crevice. Cathodic protection cannot protect the area that is in a corrosive environment for this reason.
Designing Cathodic Protection Systems
Properly designed cathodic protection systems have a long life. As such, system design is important. One must:
- Understand the environmental conditions.
- Evaluate the structure requiring protection.
- Review the options for the structure or particular application.
- Select the appropriate system.
- Complete the design, including detailed specifications and drawings using the latest engineering software.
Design engineers who have the right expertise and knowledge of the particular structure requiring corrosion protection should be involved in all phases of system design.