Coatings Used in Conjunction with Cathodic Protection - Shielding vs Non-shielding Coatings

When using cathodic protection on coated pipelines, end users must consider the problems that exist if the coating disbonds (loses adhesion).  Many in the pipeline industry assume cathodic protection will solve their external corrosion problems without truly understanding the relationship between the coating and cathodic protection.  Cathodic protection (CP) current is very effective when it actually has a path to the pipe metal.  Many times external corrosion on pipelines is caused by disbonded coatings that shield CP, not lack of CP. 

When disbondment or blistering occurs, most coating types divert current from its intended path, therefore, CP current can not adequately protect the external surfaces of a pipe.  These are called “shielding” pipeline coatings.  There are certain types of pipeline coatings that will allow the CP current to effectively protect the pipe if disbondment occurs and water penetrates between the coating and the pipe.  These are called “non-shielding” pipeline coatings.  This paper will discuss the differences in the two types of coating systems and how CP works with these coatings.