Self-validating electrochemical methodology for quantifying ionic currents through pipeline coatings

Abstract

An enhanced electrochemical method for accurately measuring cathodic protection (CP) currents through protective coatings has been developed as a means of quantifying the permeability and ‘cathodic shielding’ characteristics of nominally intact coating films. This method takes advantage of the fact that cathodic currents could significantly affect the alkalinity of areas under disbonded coatings to self-validate the correctness of current density measurements. In addition, the method provides a high level of accuracy and control over the environment. The method was evaluated using a series of ideally CP-shielding and non-CP-shielding materials, as well as, commercial pipeline coatings. A robust correlation of the measured current with generation of hydroxyl ions and the local alkalinity was established in all cases. For some commercial field joint epoxy pipeline coatings, results suggest that low CP levels could allow sufficient ionic current through a coating to promote a high-alkalinity local environment. However, this may not be the case for fusion-bonded epoxy coatings, which shielded CP and maintained a near-neutral pH under the coating.