Electrochemical behaviour of poly(pyrrole) coatings on steel

The electrochemical and corrosion protective properties of electrochemically deposited thin films of polypyrrole (PPy) have been studied. Two systems have been investigated: (1) Oxalate-doped PPy; and (2) a dual layer system consisting of PPy-oxalate and PPy-poly(styrene sulfonate). Large anions like PSS are interesting, because they possibly prevent ingress into the layer by halide ions like chloride. Open circuit potential measurements and impedance spectra of both systems are compared. For intact coating systems the impedance spectra of the four systems investigated can be described by an equivalent circuit model consisting of an RfQf circuit, due to the polypyrrole film itself, and a modified Randles circuit in series, to account for the double layer at the polypyrrole/ electrolyte interface and the polypyrrole reaction. In 0.1 M NaCl the time to failure for the dual layer is almost 6-fold that of the single layer. This indicajes that PSS is indeed capable of preventing the ingress of chloride ions. In 0.1 M Na2SO4 the time to failure for the dual layer is less than double that of the single layer.