Conducting polymer/polyimide-clay nanocomposite coatings for corrosion protection of AA-2024 alloy

Corrosion of metals is a major problem in the aerospace and automobile industry. The current methods of corrosion protection such as chromate conversion coatings are under increased scrutiny from the Environmental Protection Agency (EPA) due to their carcinogenic nature. Intrinsically conducting polymers (ICPs) like polyaniline and polypyrrole have been considered as a potential replacement for chromate conversion coatings and have been under investigation since past decade.The goal of this study is to replace the chromate conversion coating by an environmentally friendly organic coating. Poly (N-ethyl aniline) coating was electrodeposited as the primer layer and polyimide-clay nanocomposite was solution cast as the barrier layer on AA-2024 alloy. This study will provide a better understanding of the corrosion protection mechanism of the conducting polymer coating. Various characterization techniques such as infrared spectroscopy, cyclic voltammetry and scanning electron microscopy were used to study the formation, chemical structure and morphology of the coatings. Electrodeposition parameters like monomer concentration, applied current density and the reaction time were varied in order to optimize the properties of the conducting polymer coating. The corrosion performance of the primer coating was evaluated by DC polarization studies. It was found that poly (N-ethyl aniline) reduces from emeraldine to leucoemeraldine form reducing the rate of cathodic reaction, which reduces the rate of corrosion of AA-2024 alloy.Polyimide-clay nanocomposite coating was solution cast on the conducting polymer primer layer for enhancing the barrier and corrosion properties of the coating system. The concentration of polyimide (10--25 vol%) and clay (0.1 and 1 wt%) were varied in the coating formulation to optimize the barrier properties of topcoat. X-ray diffraction showed that the intergallery clay distance decreased from 17.2 A to 11.79 A after immidization of polyimide-clay nanocomposite coating and infrared spectroscopy suggested that there was hydrogen bonding interaction between clay and polyimide chains. DC polarization study, electrochemical impedance spectroscopy and scanning vibrating electrode technique were used to evaluate the corrosion property and model the coating degradation in corrosive medium. It was found that the corrosion property were dependent on the thickness of the barrier coat and concentration of clay in the polyimide coating.The results obtained from the above mentioned test suggest that poly (N-ethyl aniline)/polyimide-clay nanocomposite coatings system is a potential candidate to replace the traditionally used and environmentally unfriendly chromate conversion coating.