This mainly depended on the amount of electrons that can be passed through the coating system. This is defined as electrical conductivity. A coating systems that has a very high (electrical) resistance can be defined as an isolating coating system. Examples are asphalt/bitumen and polyethylene (PE) based coatings. On the other hand, there are coating systems that will protect against environmental conditions such as water and oxygen, but will pass electrons. Typically epoxy based systems (e.g FBE-Fusion bond epoxy), can be catagorized under this electrical conductive coating system.
Though pipeline owners doing a good job with cathodically protecting their pipelines, there are still many who misunderstand the use of pipeline coatings in conjunction with cathodic protection. Many companies are still not realizing that there are several types of pipeline coatings that shield cathodic protection current. This can become problematic, when the coating loses adhesion, which allows water to penetrate.
With buried pipelines, water could typically propagate corrosion. Special consideration should be taken with regard to the rapid development of microbial growth and their related corrosion problems (read more about MIC).
The amount and rate of corrosion under disbonded coatings will be dependent on several variables, but most important is the non- permeability of coating system. If a coating system has no good properties concerning very low permeability, water, oxygen and positive ions will pass the coating and accumulate between the coating and the steel surface. This has resulted many times in external corrosion beneath the disbonded coating. So, in spite of the application of cathodic protection, external corrosion on cathodically protected pipelines may occur.