Effect of electrochemical deposition parameters on the synthesis, structure and properties of polyaniline-polypyrrole composite coatings on steel

Polyaniline-polypyrrole composite coatings were formed on low carbon steel using oxalic acid as electrolyte by aqueous potentiostatic electrodeposition. Potentiostatic method is a powerful technique that can force simultaneous polymerization of both pyrrole and aniline. A passive layer of iron (II) oxalate is deposited on the steel surface prior to the formation of composite coatings. The electrochemical deposition process shows three distinct regimes—dissolution of steel, formation of passive layer and formation of polymeric composite coatings. These three regimes have been studied in depth using spectroscopic techniques and electron microscopy. Quantitative analysis of the Current-time transient (I-t) curves show that the nucleation and growth of the passive layer occur by diffusion controlled 3-D instantaneous nucleation.; It has also been shown that the morphology and the chemical structure of the composite coatings depend upon the electrochemical deposition (ECD) parameters. The ECD parameters that affect the formation of the coatings are the applied potential; molar feed ratio of monomers and the reaction time. The development of the composite coatings on steel was studied in depth using Infrared Spectroscopy, Scanning Electron Microscopy and X-ray Photoelectron Spectroscopy. For equimolar feed ratio of monomers (aniline and pyrrole), it was shown that polypyrrole starts to form on the steel surface prior to the incorporation of polyaniline.; Corrosion resistance and adhesion strength of the coatings were evaluated using DC polarization tests and Lap Joint tests respectively. It was shown that the electrochemical deposition parameters (molar feed ratio of monomers, applied potential and reaction time) influence the corrosion and adhesion performance of the coatings. In general, polyaniline-polypyrrole composite coatings show much better performance than the homopolymers. Especially, the coatings formed using equimolar feed ratio of monomers showed better results than those formed at all other feed ratios.