What Does Non-Shielding Coating Mean?
Non-shielding coating is the coating that, in the event of a disbondment or loss of adhesion, does not divert or prevent the flow of cathodic protection current from its intended path. In most applications, the non-shielding coating provides the main corrosion protection, while the cathodic protection acts as a backup when the coating disbonds. The coating allows the cathodic protection currents to reach the disbonded areas that occur on the pipeline and thereby prevent corrosion.
Corrosionpedia Explains Non-Shielding Coating
Non-shielding characteristics are an important consideration when selecting a pipeline’s coating. Regulations such as the New DOT Part 192.112 require that non-shielding coatings be used to protect pipelines against external corrosion.
The effectiveness of a coating to prevent corrosion in the underground pipeline is based on two primary functions:
1. Resistance of the coating to degradation such as disbondment or loss of adhesion
2. Ability to conduct the cathodic protection (CP) current when the coating fails
All coatings end up failing due to impurities, improper surface preparation, application techniques, soil stress, operational temperature or coating selection criteria that does not consider all possible failure modes. In addition, all coatings have very small openings, micro-fissures, pinholes and other characteristics that allow corrosion-causing compounds such as water and oxygen to migrate through the coating at various rates.
When a non-shielding coating disbonds or loses its adhesion, water may penetrate in the space between the coating and the metal surface. As the coating deteriorates, more corrosion-causing chemicals and substances pass through to the substrate. If the CP had been used in conjunction with the coating, it continues to protect the metal against corrosion, provided that the disbonded coating does not impede the flow of the CP current.
Once the CP currents reach the disbonded area, they raise the pH of the water between the pipe and the disbonded coating to a pH of 9 or higher—a value that does not support corrosion.
Typical non-shielded coatings include:
- Fusion-bonded epoxy
- Mesh-baked tapes
- Two-part epoxies
Most underground pipelines use cathodic protection as part of the corrosion protection systems. However, the CP system often works as a backup, while a suitable non-shielding coating system is used as the primary corrosion protection system. The CP is often designed to begin protection whenever there is a defect or a failure in the coating system.