Causes of External Corrosion on Coated and Cathodically Protected Pipelines

Each time a pipeline is exposed and inspected companies should properly evaluate external corrosion to determine the reason why the corrosion occurred.  To many times external corrosion is blamed on lack of cathodic protection (CP), when there are other causes that are not properly evaluated.  Proper evaluation of external corrosion includes using several methods to determine why external corrosion exists even though NACE criteria are met for cathodic protection at the site.

Case histories will be presented that discuss the causes of why coated pipelines with “adequate” cathodic protection continue to have external corrosion.  In other cases, the CP may not seem to be adequate, but no corrosion is found.  These case histories will show some of the proper techniques to evaluate external corrosion and what caused the corrosion to exist.  Each type of coating has properties that can be defined as either CP “shielding” or “non-shielding”.