Device to Study Electrochemistry of Iron at a Defect of Protective Coating of Electronic Conducting Polymer

摘要

Protective effectiveness of electronic conducting polymers such as polyaniline and polypyrrole (PPy) are attributed to their high oxidation-reduction potential that allows maintaining the passive state of the substrate metal. However, when the polymer layer is damaged by a scratch or a hole, the protective actions of the conducting polymer remain unclear. For certain authors, the substrate metal becomes passive and for others it remains active. Therefore, it is important to know the electrode potential at the defect hole and the current coupling the polymer film and the damaged area. To determine them experimentally, a disk-in-ring electrode configuration was devised with a bare iron disk of 0.25 mm diam and a PPy coated large ring (5 mm o.d. and 0.4 mm i.d.). When the disk and the ring electrodes were connected, in 0.1 M K_2SO_4 acidified at pH 4, the iron disk received a charge of 30 mC cm~(-2) and became passive. After that, the coupling current increased progressively, and when one-quarter of the charge initially present in the PPy coating was consumed, the iron electrode became active. During these periods, the disk and the ring electrode potential were identical. The loss of protective effectiveness of PPy appeared, therefore, to be due, not to the charge exhaustion, but presumably to the increase of the internal electrical resistance of PPy film.