The effect of ageing on cathodic protection shielding by fusion bonded epoxy coatings

Abstract

Cathodic protection (CP) permeable coatings are commonly perceived in the pipeline industry as advantageous in terms of corrosion protection in a coating disbondment scenario, since CP currents through the coating could create an alkaline pH capable of inducing steel passivation. The CP current permeability of fusion bonded epoxy (FBE) coatings in as new condition was recently evaluated and found to be very low. This paper explores the possible effects of ageing and, in particular, water uptake on CP current permeability. FBE films were hydrothermally aged for up to seven months. Water uptake was found to increase drastically over the first three months and then reach a plateau. In addition to weight changes, water uptake and other chemical changes were evaluated through Fourier-transform infrared spectroscopy and differential scanning calorimetry analysis. CP current permeability of aged FBE coatings had a significant increase in relation to unaged samples. However, experiments also suggested that FBE coatings were partially permeable to OH− and/or H+ which limited their ability to produce the high pH environment required for steel passivation. This indicates that the increase in CP current conduction as a result of ageing is not the likely cause for the low corrosion risk observed in service for disbonded FBE coatings. However, low corrosion risk conditions could still be achieved if pinholes are developed in service. The effect of artificial pinholes on FBE coatings was tested and it was found to drastically increase the CP current permeability, allowing a rise of pH to values sufficiently alkaline for steel passivation.