Cathodic protection shielding of pipeline coatings is largely debated among coatings specialists and suppliers. This ambiguous term for non-shielding coatings can designate i) coating expected to intrinsically let the cathodic protection (CP) current passing through or ii) coating system with a failure mode (loss of adhesion, cracks, etc.) that allows a distribution of CP current to the metal substrate though coating defects. In the standards and industrial recommendations, for new applied coatings, relatively high electrical resistivities are defined as acceptation criteria. These resistivity levels depend upon the coating nature and the application technology. In the literature, Fusion Bonded Epoxy (FBE) coatings are probably those for which the CP shielding ambiguity is particularly present.
In this, study, the shielding character of traditional field coatings, such as single and dual layers FBE and liquid epoxy was assessed. Different thicknesses were investigated in the range from 350 to 1000 µm. An experimental setup was designed to assess the ability of a disbonded coating to allow the CP currents to protect the steel underneath. Conditions at low and high oxygen contents were considered below this model disbonded coating. This setup aims at reproducing blistering conditions with and without renewal of the confined electrolyte. For this purpose, substrate free coating samples were used (membrane type samples). Additional experiments were performed to measure the CP currents passing through similar coatings on steel substrates during 12 months. Finally, 30 years old FBE disbonded coating, collected from the field and presenting a low shielding behavior, was similarly characterized. The obtained results allow discussing the ability of tested coating (presenting low and high shielding properties) to protect the steel under blistering conditions.
Cathodic protection, coating, shielding, non-shielding, electrochemical impedance spectroscopy