Modified wire beam electrode: a useful tool to evaluate compatibility between organic coatings and cathodic protection

Abstract

Thick organic coatings are often associated with cathodic protection against the corrosion of metallic structures immersed in seawater. Thus, it is possible to reduce the current demand for the protection by limiting the exposed metal surface and to ensure a protection for the uncoated metal surface parts. Cathodic protection decreases the rates of anodic reactions but increases the rates of cathodic ones, which favors the development of a strong alkalinity, which can lead to a delamination of the coating. It is then necessary to develop an efficient method to evaluate the compatibility between organic coatings and cathodic protection. According to ASTM G-8, cathodic disbonding tests can be driven with an artificial defect within the coating under cathodic polarization; inspection is visual and may be subjective. However, the defect zone is mainly responsible for the electrochemical response and coating properties are difficult to estimate. Then, it can be proposed to develop non-destructive methods which consider the intact part of the coating near the defect. In such conditions, the possibility to get electrochemical response by placing electrodes under the film seems interesting and was first developed using the wire beam electrode. In this work, an experimental probe, constituted of 210 mini-electrodes, named modified wire beam electrode, was prepared in order to follow the processes of the local coating delamination under cathodic protection conditions. Measurements of the galvanic corrosion current between microelectrodes and of the cathodic protection current were realized. Non-uniformity of the coating was easily shown and the delamination rate near the artificial defect was estimated.