Electrochemical behavior of mild steel having different surface roughness in polymerization solution containing oxalic acid and pyrrole monomer were studied using potentiodynamic polarization technique. Curves for converting polymerization potential to current density were obtained and different potential stages in these curves were characterized. In order to define the mechanisms of coating formation in different potentials and determining various stages, electrochemical impedance spectroscopy tests were performed at various potentials. Surface morphology and structure of specimens prepared in different potentials were investigated using scanning electron microscopy and x-ray diffraction techniques respectively. Results obtained revealed that in each potential range, as potential increases, the following mechanisms are prevailed: (a) dissolution of passive layer without oxidation of monomer, (b) oxidation of monomer to oligomers with high solubility without coating formation, (c) formation of planar polypyrrole coating with crystalline nucleates, (d) formation of semiplanar polypyrrole coating, (e) polypyrrole coating with cauliflower morphology. Infrared spectroscopy studies of coatings prepared at various potential ranges were also carried out. X-ray diffraction pattern of polypyrrole coatings revealed that the crystallinity of semicrystalline polypyrrole is 68%. Elemental analysis of coatings revealed that the ratio of pyrrole to oxalate ions in amorphous polypyrrole is 1 to 5 while in crystalline one is 1 to 4. Surface topography of coatings was studied using atomic force microscopy. Corrosion behavior of polypyrrole coated mild steel with various morphologies was investigated using potentiodynamic polarization techniques. Results showed that corrosion behavior of specimens with semicrystalline polypyrrole coating was superior to amorphous one.
