133 Coating Failure & External Corrosion of Buried Pipeline-Cathodic Disbondment

Cathodic disbondment is a phenomenon in which the adhesion between a protective coating and the substrate surface is compromised due to cathodic reactions occurring at the metal surface.
Protective coatings are commonly employed alongside Cathodic Protection (CP) by corrosion engineers to prolong the operational lifespan of buried or submerged equipment.
Coatings serve as the primary defense mechanism against corrosion. Despite diligent application, coating discontinuities, known as "holidays," are nearly inevitable due to factors such as application procedures, handling, installation, environmental stresses, and natural aging processes.
Cathodic disbondment, a distinct failure mode associated with cathodic protection, arises when a coating fails to withstand the cathodic potential.
This typically occurs when metal substrates undergo cathodic protection, wherein CP current penetrates through defects in the protective coating, giving rise to a highly alkaline environment at the interface between the metal and coating layers.
In response, CP is utilized to furnish an external current source that counteracts the electrochemical potential driving natural corrosion, thereby safeguarding exposed steel at these holiday sites.
Cathodic disbondment can lead to accelerated corrosion of the underlying metal substrate and compromise the effectiveness of the protective coating system.
Cathodic Shielding coatings: Cathodic shielding coatings are designed to protect underlying materials or substrates from various external factors, such as corrosion, abrasion, and environmental degradation. These coatings act as a barrier, shielding the substrate from harmful elements and preventing deterioration. They are commonly used in industries such as aerospace, automotive, marine, and infrastructure.
All coatings must have the ability to shield CP when properly adhered to the pipe. The problem happens when a disbondment occurs and water penetrates between the coating and the pipe.
This problem exists more for certain types of coatings than for others. Some coatings will shield CP current in some situations and not in others. A key consideration should be "Will the coating shield CP if the bond fails?''
However, all coatings experience some disbondment and, therefore, the behavior of a disbonded coating is important in the overall performance of a coating system. xii Even with adequate cathodic
protection (CP), corrosion can occur under most disbonded coatings.