Formation and electrochemical behaviour of poly(pyrrole) coatings on steel substrates

Abstract

Abstract This paper focuses on the deposition process and the semi‐conductive properties of PPy layers on steel. The polymerisation of PPy was performed galvanostatically on steel in an aqueous solution of 0.1 M pyrrole and 0.1 M oxalic acid (H 2 C 2 O 4 ) using a current density of 1 mA/cm 2 . The build‐up of a ferrous oxalate (FeC 2 O 4 ) layer, which seals the seel substrate surface, is initially observed. PPy layers made by this method are always doped with oxalate anions. A mechanism is proposed in which PPy deposits at sites where ferrous oxalate is dissolved. PPy is a semiconductor, just like oxide films formed naturally on some metals. Mott‐Schottky diagrams for 9.0, 11.6 and 15.0 kHz suggests a frequency dependence of the dielectric constant of polypyrrole (ε PPy ). The flatband potential E FB was about − 0.39 V vs. NHE and the charge carrier density was estimated to be about 5 · 10 20 m −3 . Also, in earlier work by Rammelt et al. and by Miller and Bockris (using different electrolytes and substrate materials) a frequency dependence has been found for a constant phase element that was used to describe the space charge capacitance C SC . The frequency dependence observed can partly be ascribed to surface roughness, which seems experimentally supported by SEM‐research.