An XPS and AFM study of polypyrrole coating on mild steel

We have characterized electrically conductive polymer, polypyrrole (Ppy), films on mild steel with atomic force microscopy (AFM) and x-ray photoelectron spectroscopy (XPS). The Ppy films were polymerized electrochemically from sodium p-toluene sulphonate (ToS) water-ethanol solution. A very good corrosion protective effect of those films on mild steel has been observed previously. After drying in air, thin Ppy hms (similar to 200 nm) were highly adhesive, whereas thicker films (similar to 7.5 mu m) could easily be peeled off from the metal. All films had a nodular surface structure. Thin films were very smooth and their roughness increased with their thickness. The AFM study of the interface between the metal and the polymer showed areas of different morphology, which suggests that adhesive and cohesive failure of the polymer film occurs. A strong variation of the chemical composition at the interfacial layer was observed. A sulphur-rich layer was formed at the interface, indicating that the counter-anions (ToS) were accumulated on the surface of the metal High-binding energy iron and imine-type nitrogen were detected in a thin Ppy film. Moreover, thin films were substantially overoxidized. The number of anions incorporated into the Ppy matrix varied as a function of film thickness: the 'doping level' was about 30% for thicker films and only 12% for thin ones. Nitrate counter-anion-containing Ppy films were also studied in some cases for comparison. (C) 1997 by John Whey & Sons, Ltd.