Does the resistance of the coating whether it is high or low has an impact on cathodic protection (CP) in terms of protection and shielding of CP current?
Dear Elsadig
I may say it actually has an impact. The higher the coating resistance is, the lower passage of corrosive species (including water, oxygen, and ions) would occur. Moreover, you should take it into account that the presence of more corrosive materials on the metal surface leads to higher dissolution of the metal. Therefore, in such a situation you need higher external current (more electrons) for the cathodic reaction to ensure prevention of anodic reaction (i.e. dissolution of metal). Accordingly, it seems quite obvious that coating resistance play a significant role in the magnitude of external current in cathodic protection.
dear Elsadig Eltai ;
During the service life of the pipeline, the combined use of Cathodic Protection and Coatings both are required for economic reasons. Cathodic Protection alone will not completely stop corrosion and Coatings alone also cannot protect base metal because of the pores or damaged areas effects. Therefore, the coating needs to be a good electrical/electrolytic insulator to isolate the metal substrate from direct contact with the electrolyte, and have an excellent electrical resistance to prevent any electrochemical reactions on the metal surface of the pipeline. you can find more details in this reference;
http://www.materialsperformance.com/articles/cathodic-protection/2016/04/using-pipeline-coatings-with-cathodic-protection
Dear Elsadig, In response to your question, the lower the resistance of the coating, the higher the impressed current you need to obtain effective protection. It should be also noted that too high cathodic protection currents may lead to cathodic disbonding of the coating.
Best regards, Andreas
Thanks Sajjad, Mahmood and Andreas for your kind responses.
What about impact of cathodic protection on coating which may resulted in shielding of CP current?
Does the CP current has an impact on coating resistance, in other words Does CP current negatively affect the resistant of the coating by pushing more water and corrosive species towards the metal surface? and if yes what is the mechanism?
Elsadig Eltai - In my experience, a higher CP voltage in the range of hydrogen evolution led to a greater drop in coating impedance measured using EIS. Electroosmosis is said to be the most common mechanism. I also came across a concept of a drag force on the water molecules due to the movement of the ions in the direction of the CP current. The research by W W Kittleberger et al on 'electroendoosmosis' are quite illuminating on this concept.
Regarding corrosive species, anions such as chlorides are moved away from the cathode to the anode by cathodic protection. This has a further indirect protective effect.
In fact the primary reason for a coating being applied is to reduce the current needed to Cathodically polarize and protect the structure. The higher electrical resistance from the coating translates to a lower current being imposed. Therefore, when a low resistance coating (such as say a water soaked bitumen layer) is considered, then a lot of current leakage happens and the rectifier draws maximum current or sacrificial anodes perish fast.
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