Coating ageing and its impact on CP conduction

The conduction of cathodic protection (CP) current through coatings is often seen as an advantage in terms of corrosion protection of buried steel structures such as pipelines. As coatings undergo in-service degradation their barrier property may become compromised enabling conduction of CP currents and sometimes promoting a protective alkaline environment. Water uptake in organic coatings that contain hydrophilic groups such as epoxies is believed to play a key role in increasing the conduction of CP currents. This paper explores the effect of ageing and water uptake on CP current permeability. Fusion bonded epoxy films (FBE) were hydrothermally aged for several months. The degradation of the films due to ageing was evaluated through thermos-gravimetric analysis and differential scanning calorimetry analysis. A major increase in water uptake was found over the first three months of ageing which exhibited an increased CP current permeability.
However, the alkalinity levels achieved in the experiments simulating corrosion under disbonded coatings were below the ranges required for steel passivation. Whereas the presence of a macroscopic pinhole on FBE was found to radically increase the conduction of CP currents assisting the development of a high pH environment required for steel passivation. Experimental results indicate that the development penetrating defects during service could be a more likely cause for the low corrosion risk condition frequently observed in FBE disbonded coatings, rather than the conduction of CP currents through the polymer network resulting of water absorption.